1

POLITECNICO DI MILANO

School of Industrial and Information Engineering

Master of Science in Materials Engineering and Nanotechnology

SYNTHESIS OF Nb-Sn COATINGS BY ELECTROCHEMICAL

DEPOSITION

Supervisors: Silvia FRANZ

Massimiliano BESTETTI

Master Thesis Author:

Ceren BAYKAL

Matricola 836300

Academic year 2016- 2017

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INDEX

1. STATE OF ART .................................................................................................................... 10

1.1. Superconductive Materials ........................................................................................ 10

1.1.1. Superconductivity ............................................................................................... 10

1.1.1.1. Classification ............................................................................................... 11

1.1.1.2. Properties of Superconductors ................................................................... 12

1.1.1.3. Superconductors for use in Magnets .......................................................... 14

1.1.2. Properties of Nb3Sn ............................................................................................ 15

1.1.3. Manufacture of Superconducting Nb3Sn strands .............................................. 17

1.1.3.1. Bronze Route ....................................................................................... 18

1.1.3.2. Internal Tin (IT) .................................................................................... 19

1.1.3.3. Powder in Tube .................................................................................... 20

1.1.3.4. Restacked Rod Process ........................................................................ 20

1.1.3.5. Modified Jell Process ........................................................................... 21

1.1.4. Thin Film Production of Nb3Sn ........................................................................... 22

1.1.5. Influence of Copper on Nb3Sn ............................................................................ 23

1.1.6. Diffusional Phase Growth of Nb-Sn System ....................................................... 26

1.2. Electrodeposition ....................................................................................................... 28

1.2.1. Copper Electrodeposition ......................................................................... 29

1.2.2. Tin Electrodeposition ................................................................................ 30

2. Materials & Methods ........................................................................................................ 32

2.1. Etching and Cleaning ................................................................................................. 32

2.2. Electrochemical Setup ............................................................................................... 33

2.3. Electrolytes ................................................................................................................ 34

2.3.1. SolderonTMMHS-W ............................................................................................. 34

2.3.2. Copper Strike Electrolyte.................................................................................... 35

2.3.3. Copper Barrier Electrolyte .................................................................................. 35

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2.4. Heat Treatments ........................................................................................................ 37

2.5. Sample Characterization ............................................................................................ 38

2.5.1. X-Ray Analysis ..................................................................................................... 38

2.5.2. SEM Analysis ....................................................................................................... 38

3. Experimental Results ........................................................................................................ 39

3.1. Electrodeposition of Copper Strike on Nb ................................................................. 40

3.2. Electrodeposition of Sn on Cu/Nb ................................................................................. 44

3.3 Electrodeposition of Copper Barrier Layer .................................................................. 44

3.4. Optimization of Etching Procedure ............................................................................... 48

3.4.1 Second Generation Samples .................................................................................... 51

3.5 Thermal Treatments .................................................................................................. 56

4. Conclusions and further works ......................................................................................... 63

Bibliography .............................................................................................................................. 65

ACKNOWLEDGEMENT .............................................................................................................. 67

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FIGURE INDEX

Figure 1-1: Critical temperature of superconductor compared to normal metal ................... 10

Figure: 1-2 Diagram of Meissner effect.................................................................................... 11

Figure 1-3: Bc-Tc diagrams for Type I and Type II superconductors ........................................ 12

Figure 1-4 Diamagnetism ......................................................................................................... 13

Figure 1-5: Flux pinning and vortex .......................................................................................... 14

Figure 1-6: Binary phase diagram of the Nb-Sn System ........................................................... 16

Figure 1-7 Schematic representation of Nb3Sn A-15-unit cell ................................................ 17

Figure 1-8 General scheme for most common manufacturing techniques of Nb3Sn strands 18

Figure 1-9 Schematic view of internal tin process ................................................................... 19

Figure 1-10 Restacked Rod Process (RRP), by Oxford Instruments Superconducting

Technology (OI-ST). .................................................................................................................. 21

Figure 1-11: Jelly roll process ................................................................................................... 22

Figure 1-12 Isothermal section from the Cu-Nb-Sn system at 700 C ....................................... 24

Figure 1-13 Cu- Sn Phase diagram ............................................................................................ 25

Figure 1-14: Nb-Sn phase diagram ........................................................................................... 26

Figure 1-15 A simple setup of electrodeposition ..................................................................... 28

Figure 2-1: Electrochemical cell ............................................................................................... 33

Figure 2-2: Carbolite Furnace at Polimi .................................................................................... 37

Figure 3-1: Samples preparation, B: Back of samples F: Front of samples .............................. 40

Figure 3-2: a) An example of pre-samples Sn on Cu/Nb structure b) Cu on Sn/Cu/Nb structure

.................................................................................................................................................. 44

Figure 3-3: General scheme of samples ................................................................................... 45

Figure 3-4: Corrosion of Samples ............................................................................................. 45

Figure 3-5: SEM analysis on red part of corroded sample at different magnifications: a)

element detected on surface (261 X), b) surface morphology (3.00 KX) ................................. 46

Figure 3-6: SEM analysis on black part of corroded sample at different magnifications: a)

element detected on surface (261 X), b) surface morphology (3.00 KX) ................................. 47

Figure 3-7: Thermal Treatment at Mechanical Engineering Department ............................... 56

Figure 3-8: Heat Treatment profile A for sample 43 ................................................................ 58

Figure 3-9: Sample 43 after Heat Treatment ........................................................................... 59

Figure 3-10: Heat Treatment Profile B for Sample 36 .............................................................. 59

Figure 3-11: Sample 36 after heat treatment .......................................................................... 60

Figure 3-12: XRD Pattern of Sample 36 .................................................................................... 60

Figure 3-13: Heat Treatment Profile C for sample 45 .............................................................. 61

Figure 3-14: Sample 45 after heat treatment .......................................................................... 61

Figure 3-15: XRD pattern of sample 45 .................................................................................... 62

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TABLE INDEX

Table 2-1: Etching solutions ..................................................................................................... 32

Table 2-2: Features of Nb Foils. ................................................................................................ 33

Table 2-3: Suggested Operational Parameters ........................................................................ 34

Table 2-4: Used parameters for tin plating .............................................................................. 34

Table 2-5: Copper Strike Solution............................................................................................. 35

Table 2-6: Working Parameters for Copper Strike Layer Plating ............................................. 35

Table 2-7: Copper Barrier Layer ............................................................................................... 36

Table 2-8: Working Parameters of Copper Barrier Layer Deposition ...................................... 36

Table 3-1: Copper Strike Electrolytes ....................................................................................... 41

Table 3-2: Pre-samples for calibration of Cu on Nb substrates ............................................... 42

Table 3-3: Pre-samples for Calibration Cu on Nb substrates 2 ................................................ 43

Table 3-4: Different Approaches to Optimize Etching Procedure ........................................... 48

Table 3-5: First generation samples (Step by Step) ................................................................. 49

Table 3-6: Effect of aging on first-generation samples ............................................................ 50

Table 3-7: Properties of Nb foils used for second generation samples ................................... 51

Table 3-8:Images of sample before and after roughening ...................................................... 51

Table 3-9: Optimized etching procedure of second generation samples for copper strike

layer Electrodeposition step .................................................................................................... 52

Table 3-10: Second generation samples step by step .............................................................. 53

Table 3-11: Thicknesses of layers of second generation samples ........................................... 54

Table 3-12: Images of second generation samples after aging ............................................... 55

Table 3-13: Thermal treatment profiles ................................................................................... 57

Table 3-14: General information about heat treated samples ................................................ 57

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ABSTRACT

This thesis aimed to produce a Nb-Sn coating by electrochemical deposition method

following a collaboration between the Department of Chemistry, Materials and

Chemical Engineering at Politecnico di Milano and the Technical Division of the Fermi

National Accelerator Laboratory in Chicago (USA). The Nb3Sn phase is produced by

electrodeposition from aqueous solutions of Cu/Sn/Cu multilayers onto Nb

substrates, followed by thermal treatment in inert atmosphere.

The thesis includes an extensive investigation on electrochemical nucleation and

growth of copper onto Nb substrates, which can be considered the main issue of the

whole process. Chemical stability of the samples was also addressed. The thermal

treatments were tuned in order to obtain Nb-Sn phases. The resulting coatings, both

at the intermediate stages and at the end of the process, were evaluated by visual

observation. Morphology was observed by scanning electron microscopy,

composition and thickness was assessed by electron dispersive X-ray spectrometry

and X-Ray fluorescence, crystal phases were determined by X-ray diffraction.

Optimized electrodeposition of copper onto Nb lead to copper strikes having good

homogeneity and adhesion and chemically stable Cu/Sn/Cu multilayers. After thermal

treatment, the electrodeposited coatings showed Nb-Sn phases, namely Nb3Sn and

NbSn2, and Cu-Sn phases.

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Estratto

Questa tesi mira a produrre rivestimenti in lega Nb-Sn mediante metodi

elettrochimici ed è nata da una collaborazione tra il Dipartimento di Chimica,

Materiali e Ingegneria Chimica del Politecnico di Milano e la Divisione Tecnica del

Fermi National Accelerator Laboratory di Chicago (Stati Uniti d'America ). I

rivestimenti in lega Nb-Sn sono stati ottenuti mediante elettrodeposizione da

soluzioni acquose di multistrati Cu/Sn/Cu e successivo trattamento termico. La tesi

include una procedura di ottimizzazione di questo approccio sinstetico con particolare

riferimento alla fase di nucleazione e crescita di rame su substrati di niobio ed alla

stabilità chimica dei multistrati ottenuti. Lo studio inoltre include l’identificazione dei

parametri di trattamento termico ottimali per l’ottenimento di leghe Nb-Sn. La

morfologia dei campioni è stata valutata mediante microscopia ottica e microscopia

elettronica, la struttura cristallina è stata indagata mediante diffrazione di raggi X, la

composizione e lo spessore dei singoli strati sono stati misurati mediante

spettroscopia a raggi X a dispersione di elettroni e fluorescenza a raggi X. I campioni

ottenuti dimostrano la presenza di fasi Nb-Sn (in particolare Nb3Sn a NbSn2) e Cu-Sn.

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Part I

State of Art

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1. STATE OF ART

1.1. Superconductive Materials

1.1.1. Superconductivity

Superconductivity is a characteristic of complete vanishing of electrical resistance in

certain materials which might be an element, inter-metallic alloy or compound, when

they are cooled below a specific temperature which is called transition temperature or

critical temperature. Resistance produces energy losses flowing through materials thus

it is avoided in concerning situations. The phenomenon of superconductivity was first

observed by Dutch physicist Heike Kamerlingh Onnes in 1911. When he cooled

mercury below 4.2 K, he observed that mercury lost suddenly it’s all electrical

resistance. This discovery was followed by observation of several other metals which

shows zero resistance when their temperatures were lowered below their critical

temperature. [1]

Figure 1-1: Critical temperature of superconductor compared to normal metal

In 1933, another remarkable development in this field was achieved by German

scientists Walther Meissner and Robert Ochsenfeld. They discovered that a

superconducting material will repel a magnetic field. A magnet moving by a conductor

induces currents in the conductor. This is the principle on which the electric generator

operates. But, in a superconductor the induced currents exactly mirror the field that

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would have otherwise penetrated the superconducting material - causing the magnet

to be repulsed. This phenomenon is known as strong diamagnetism and is today often

referred to as the "Meissner effect". The Meissner effect is so strong that a magnet can

be levitated over a superconductive material. [2]

Figure: 1-2 Diagram of Meissner effect

The critical current Ic, the critical field Bc and critical temperature Tc are the crucial

independent parameters to define the critical surface of superconducting state. The

critical current refers to the highest value of current which can go through a

superconductor material which might be used to correlate with the term current

density Jc. The critical field Bc stands for the maximum magnetic field that can be

applied on superconducting material. When both critical values of Ic and Bc are exceed,

material starts to resist and heat as a normal metal. For some materials, there are two

different critical field values because of vortices.

1.1.1.1. Classification

Superconductive materials can be divided into two group according to their magnetic

behavior and character of transition when they encounter with high magnetic field

which is destroyable for their superconductivity and resulting normal conducting state.

The specific features of Type I superconductors are zero electrical resistivity under a

critical temperature, zero internal magnetic field which refers to Meissner effect and a

critical magnetic field above which superconductivity terminates. BCS theory

(Bardeen–Cooper–Schrieffer theory) which was the first microscopic theory of

superconductivity. BSC theory is based on electron pairs coupled by lattice vibration

interactions. Significantly, the best conductors like gold, silver, copper do not become

superconducting at all at room temperature. They have the smallest lattice vibrations,

so their behavior correlates well with the BCS Theory [3]. Type I has a limitation in

practice since their critical magnetic field values are so small and their superconductive

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feature vanishes suddenly at that temperature. Type I superconductors are named

‘’soft’’ superconductors meantime Type II ones are called as ‘’hard’’ superconductors.

Type II superconductors has different response to an applied magnetic field. Their

superconducting state is conserved at higher magnetic field and higher temperatures

thanks to resulting two different critical field Bc1 and Bc2 in the case of increasing field

from zero. At the first critical field point Bc1, applied field starts to partially penetrate

the interior of material even though their superconductivity is remained. Above the

second critical field point Bc2, which is much higher, superconductivity starts to

disappear and applied field still partially penetrates the superconductor hence

Meissner effect is not valid anymore.[1] This makes them useful for technological

applications requiring high magnetic fields such as high electromagnets that is made of

superconducting wires. For instance, wires made from Nb3Sn have a Bc2 as high as 24.5

Tesla – in practice it is lower[1]. NbTİ is another example of Type II superconductor

which is used in construction of high field electromagnets, it’s critical field value is 15

T. Type II superconductors usually exist in a mixed state of normal and

superconducting regions. This is sometimes called a vortex state, because vortices of

superconducting currents surround filaments or cores of normal material [4].

Figure 1-3: Bc-Tc diagrams for Type I and Type II superconductors

1.1.1.2. Properties of Superconductors

The main properties of superconductors as zero resistance and Meissner effect are

already explained however, in detailed, their specific behavior can be explained by flux

pinning and ideal diamagnetism. Besides they exhibit no thermoelectric effect which

means no Seebeck, Peltier and Thomson effect is detectable.

The orbital motion of electrons creates tiny atomic current loops and resulting

production of magnetic fields. These current loops will arrange their self and align to

opposite direction of applied field. It can be related with atomic version of Lenz’s law.

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Materials which have only magnetic response in this kind of situation are called

diamagnetic.

All materials are inherently diamagnetic, but if the atoms have some net magnetic

moment as in paramagnetic materials, or if there is long-range ordering of atomic

magnetic moments as in ferromagnetic materials, these stronger effects are always

dominant. Diamagnetism is the residual magnetic behavior when materials are neither

paramagnetic nor ferromagnetic.

Any conductor will show a strong diamagnetic effect in the presence of changing

magnetic fields because circulating currents will be generated in the conductor to

oppose the magnetic field changes. A superconductor will be a perfect diamagnet

since there is no resistance to the forming of the current loops

Figure 1-4 Diamagnetism

Flux pinning is another important phenomenon which describes

superconductive materials properties. It refers to a small and flat superconductor

pinned in space above a permanent magnet also called quantum levitation or quantum

locking. Stable and robust levitation can occur only with type II superconductors. As

mentioned before, type II superconductors has different behavior for an applied

magnetic field between two critical values. There is a partial penetration which is in

the form of regular array of normal conducting regions. These normal conducting

regions are the reason of partial penetration through superconductor in the form of

thin filaments generally called vortices or flux lines. When an external applied

magnetic field that is bigger that the lower critical field, vortex regions run through the

sample. If the external field increases till upper critical point value, the number of

filaments increase and gather together thus the whole sample change into normal

conductor. Real materials for instance high Tc superconductors have some impurities,

missing or displaced atoms resulting defects and many crystal boundaries. These

crystal boundaries and defects hinder the movement of flux lines, called flux pinning.

Flux pinning and Meissner effect can be related but they have a one crucial difference:

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Meissner effect protects the superconductor from all magnetic field with repulsion

while pinned state of superconductor and superconductor in place.

Figure 1-5: Flux pinning and vortex

1.1.1.3. Superconductors for use in Magnets

In the grand scheme of things, superconductors find a wide range application area

specifically in electricity, medical applications and electronics. They are used in

laboratories, especially in particle accelerators, in astrophysics with the use of

bolometers, in ultrasensitive magnetic detectors called SQUIDs, and in

superconducting coils to produce very strong magnetic fields. Type II superconductors

such as niobium-tin and niobium-titanium are used to make the coil windings for

superconducting magnets. These two materials can be fabricated into wires and can

withstand high magnetic fields.

A superconductive magnet is used to keep the charged particles confined around a

circular orbit, and requires both bending and focusing forces generated by

electromagnetic fields. The Lorentz force is given by:

𝐿 = 𝑒(�⃗� + �⃑�×�⃗⃑�) (1)

the magnetic term that does not generate work can only be used for bending.

Although they do not increase the particle energy, magnetic field are very effective in

bending the trajectory. Magnetic dipole fields perpendicular to the plane of the

particle trajectory are used to bend the beams. Quadrupole fields around the beam

axis focus the particles, and longitudinal electric fields are used to accelerate them. A

noticeable difference to be considered in comparing a conventional and a

superconducting magnet is that in the former the field is present almost only in the

iron sector, while in the latter the field surrounds the entire space around it. This

configuration significantly constraints the choice of the structural materials. Typical

construction of the coils is to embed many fine filaments (20 µm diameter) in a copper

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matrix. The solid copper gives mechanical stability and provides a path for the large

currents in case the superconducting state is lost. These superconducting magnets

must be cooled with liquid helium. Superconducting magnets can use solenoid

geometries as do ordinary electromagnets. Most high energy accelerators now use

superconducting magnets [5]. The circular proton accelerator at Fermilab (Tevatron),

uses 774 superconducting magnets in a ring of circumference 6.2 kilometers. They

have also found wide application in the construction of magnetic resonance imaging

(MRI) apparatus for medical imaging.

1.1.2. Properties of Nb3Sn

Nb3Sn is an intermetallic compound which has a high performance so that has a wide

usage on applications such as Nuclear Resonance Magnetic devices, high field

laboratory magnets, fusion and accelerator magnets. Nb3sn belongs to A-15

compounds family. Depending on the value of x, temperature and pressure, chemical

composition of A3−x B1+x crystallize into many different structures. Many A15

compounds exhibit the phenomenon of superconductivity at relatively high

temperatures about 20 K (-424°F) and in high magnetic fields on the order of several

tens of teslas[6]. One of the important one is with the formula A3B where A might be

Nb, V, Ta, Zr and B might be Sn, Ge, Al, Ga, Si, has the structure of beta-tungsten

designated in crystallography by the symbol A-15, and is superconducting. V3Si was the

first discovered A-15 superconductor by Hardy and Hulm[7] . After one year of that,

superconductivity in Nb3Sn was discovered Matthias in 1954 and have found wide

interest because of its capability of carrying very large current and become an

alternative to NbTi which was the most commonly used for large scale applications

[8].Intermetallic Nb-Sn is based on the superconductor Nb, which exists in a BCC Nb

structure (Tc ∼ = 9.2 K), or a metastable Nb3Nb A15 structure (Tc ∼ =5.2 K). When

alloyed with Sn and in thermodynamic equilibrium, it can form either Nb1−βSnβ (about

0.18 ≤ β ≤ 0.25) or the line compounds Nb6Sn5 and NbSn2 according to the generally

accepted binary phase diagram reported in Figure 1.6.

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Figure 1-6: Binary phase diagram of the Nb-Sn System

Both the line compounds at β=0,45 and β=0,67 are superconducting with Tc<2,8 K for

Nb6Sn5 and Tc<2,68 K for NbSn2 and thus are of negligible interest for practical

applications. The Nb-Sn phase of interest occurs from β≈0,18 to 0,25. It can be formed

either above 930°C in the presence of a Sn-Nb melt, or below this temperature by solid

state reactions between Nb and Nb6Sn5 or NbSn2. Some investigations suggest that the

nucleation of higher Sn intermetallic is energetically more favorable for lower

formation temperatures, as indicated by the dashed line within the Nb1-βSnβ stability

range. At low temperatures (T≈43 K) and at 0,245 < β < 0,252 it can undergo a shear

transformation, resulting in a tetragonal structure. This transformation is schematically

depicted in the inset in the phase diagram.

Nb3Sn is a brittle compound having a crystal structure with a cubic cell as showed in

Figure 1.7. Sn atoms form a body centered cubic lattice and each cube is bisected by

orthogonal Nb chains which is related with high critical temperature of A-15

compounds. The distance between Nb atoms in BCC Nb is longer than the distance

between Nb atoms in A-15 cubic structure.

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It is suggested that this reduced Nb distance in Nb chains results in a narrow peak in

the d-band density of states, resulting in a very high density of states near the Fermi

level. This is, in turn, believed to be responsible for the high Tc in comparison to BCC

Nb [8].

Figure 1-7 Schematic representation of Nb3Sn A-15-unit cell

1.1.3. Manufacture of Superconducting Nb3Sn strands

Nb-Ti is a widely-used superconductor material. However, its use is limited to

applications of magnetic field up to 8T.74At present, Nb3Sn intermetallic compound

with A15 structures considered to be one of the most suitable superconductors for the

applications where field requirements go beyond the limit of Nb-Ti superconductors.

However, intermetallic com-pounds are in general brittle and cannot be drawn as wire.

To circumvent this problem, different manufacturing technologies have been

developed for Nb3Sn, such as bronze method, internal tin process, powder metallurgy

route, Jelly roll processes technique, and so forth [9].

Nb3Sn was produced essentially in the form of tapes by heating Nb foils in a melted Sn

bath at high temperatures. Recently it is produced in the form of a round strand which

composite of Nb, Sn and Cu and can be used to produce cables. A-15 compound is

formed at the interface of Nb and Cu-Sn matrix by performing a thermal treatment at

temperatures about 700 ◦C. Certain steps at low temperatures are needed to have

homogenous Cu-Sn matrix before the heat treatment. Estimating the critical current

density JC is complex as a consequence of presence of bronze matrix between wires

accomplished from the diffusion of Sn into Cu. Critical current density JC is evaluated in

the part of strand where there is no Cu but overall average value is determined on the

whole strand cross section.

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Several producing methods have been tested and developed in the last years by

different companies. The most significant ones are: bronze route, internal tin (IT),

modified jelly roll (MJR) and powder in tube process (PIT) and nowadays Restacked

Rod Process is becoming popular.

Figure 1-8 General scheme for most common manufacturing techniques of Nb3Sn strands

1.1.3.1. Bronze Route

The bronze route method, since it is reliable, cost efficient and reproducible process,

has been used for long time for mass production of superconducting Nb3Sn strands.

Pure Nb or Nb containing small fraction of Tantalum or Titanium bars are put together

in the bronze matrix following by extrusion and drawing to their final size. The Sn

content in the matrix should be less than 13.5wt% which is a limit value for ductile

bronze alloy[10]. Bronze matrix preferably is quite big to provide enough Sn to Nb

rods. The first billet is made by putting hundreds of Nb rods and it is drawn into

hexagonal rods and then cut and reassembled into a second billet. Then the second

billet is extruded, annealed and drawn to its final wire size. Oxygen free high thermal

conductivity copper is used to stabilize the wires against flux jumping and should be

protected from the diffusion of bronze-tin by a Nb barrier or Ta barrier. When Nb is

used for achieving this goal, it can cause field distortions which is not preferable in

some applications like particle accelerator magnets, even there is a cost reduction.

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Thus, Ta barrier is used in the case of particle accelerator magnets. The stabilizing

copper can be incorporated internally with up to 27% of the wire cross section or

externally with a copper part of 30-60%[10]. The bronze route method necessitates

multiple annealing steps because bronze hardens quickly.

1.1.3.2. Internal Tin (IT)

Internal Tin process was raised from the idea to make much more tin quantity in

contact with Nb with respect to bronze route, because higher Sn concentration would

give higher critical current density.

The Sn content limitation is eliminated by internal thin process. Even though it does

not require annealing hence there is no filament distortion because of in- process,

there are two negative aspects of internal Tin method. Firstly, a nonuniform

deformation occurs by means of big variation on strength of Cu and Sn. Secondly, after

thermal treatment large voids form in filament which weakens the mechanical

properties through the medium of large quantity of Sn with respect to others [11].

Figure 1-9 Schematic view of internal tin process

The process starts with elemental niobium filaments and a tin core (with titanium as

ternary addition) in a copper matrix surrounded by a tantalum-niobium diffusion

barrier, forming bundles. Several bundles distributed in concentric rings in a pure

copper matrix constitute the wire. The soft starting metals allow extrusion, swaging

and drawing to the final diameter without the need for multiple anneals as would be

required in the bronze process. After final deformation, the wires undergo a multi-

stage heat treatment, in which tin diffuses through copper, forming initially Cu-Sn

phases, and finally superconducting A15 Nb3Sn by reactive diffusion into the niobium

filaments.

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1.1.3.3. Powder in Tube

The powder-in-tube (PIT) process has appeared as one of the most promising and

economically feasible techniques to produce long lengths high-Tc oxide based

superconducting wires. The PIT method involves multi-pass wire drawing followed by

rolling and heat treatment.[12]

Powder in tube (PIT) process is commonly applied for fabrication of wires and tapes

and wire shaped bulk samples can be obtained. When it is compared to other

production techniques, it is shown that it has more basic steps of production which is

mixing all the starting materials once, packing the powders in a metal tube and finally

sintering. Safety and simplicity are the main worth of this method[13].

In detail, the Powder in Tube process for producing superconducting Nb3Sn wire is

based on Nb tubes which contains intermetallic compound NbSn2 powder. Nb tubes

are stacked in a copper matrix in agreement with a pattern which selected before.

Different layouts can be used for instance, 36 ,192 and 492 and then they are drawn

and twisted to their final wire shape excluding any intermediate thermal treatment.

Generally, after coil winding a short heat treatment at a temperature about 675 °C

should be done to have a Nb3Sn phase from the inner radius of Nb tube outward. Sn

from the powder and Nb reacts with each other by diffusion and a Nb3Sn layer grows

until about 2 /3 of the Nb wall has reacted and rest of the Nb tube material plays a role

as a natural barrier to prevent contamination of Cu matrix. Instead of single stack in

the wire, by using double stacking technique, number of filament and the percentage

of Cu can be increased (above 60 %), thus critical current is decreased but does not fit

with the requirements for PIT type of Nb3Sn strands.[14]

1.1.3.4. Restacked Rod Process

Although Nb3Sn is getting popular and widely used for any kind of large magnet

systems thanks to their high performance, every technology needs its own

requirements and resulting modify of the strand. Material made for High Energy Physic

(HEP) accelerator applications now have 12 T, 4.2 K Jc values of 3000 A/mm2, but

require further development to reduce low field flux jump instabilities. Recently, OI-ST

(Oxford Instruments-Superconducting Technology) is working on different methods to

decrease

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the effective filament diameter, including subdividing the sub-elements, and restacking

a larger number of sub-elements in the final restack billet. Restacked rod process (RRP)

is one of these approaches and is used for manufacturing internal Sn strands by using

Nb rod extrusion. RRP performance and yield is better when it is compared with

Modified Jelly Roll (MJR) process which is a relatively old technique so, RRP is more

suitable for large productions [10].

Particularly, sub-elements with Nb alloy filaments in a copper matrix and a Sn alloy

core are restacked in this process. The Nb alloy content is maximized by minimizing the

copper fraction while also providing enough Sn for complete conversion to Nb3Sn.

During reaction, the small filaments expand and merge together to act as a single

effective filament per sub-element. As a result of minimizing the copper content, the

bronze has high tin content, which accelerates the growth of Nb3Sn.This promotes full

reactions while retaining fine grain size for final reaction times of 48 hours. In addition,

each sub-element is surrounded by a Nb barrier which also partially converts to nb3sn

and contributes to the critical current density [15].

Figure 1-10 Restacked Rod Process (RRP), by Oxford Instruments Superconducting Technology (OI-ST).

1.1.3.5. Modified Jell Process

The Modified Jelly Roll (MJR) is an alternative way of the internal tin method. It

consists of two parallel sheets of niobium and copper rolled around a solid tin rod

(Figure 1.11). In this way, the niobium part in the non-copper area is increased to 35%.

The roll is inserted in a copper tube to form a billet. During drawing the niobium cross

section is reduced by ∼ 750 times and shaped as a hexagonal rod. A second billet is

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then assembled using the hexagonal rods, and drawn to the final wire size. No

annealing is required during the process. Protecting barriers must be inserted between

the tin core and the niobium sheet, between the niobium and copper sheets, and

before the external stabilizing copper. For this purpose, tantalum, vanadium, and

niobium are used. The same factors as in the IT method characterize the conductor

properties. Like the IT process, this technology also suffers from filament bridging.

Figure 1-11: Jelly roll process

1.1.4. Thin Film Production of Nb3Sn

Pisoni [16]was able to deposit a thin (less than 1 µm) niobium layer on copper,

stainless steel 304 and glass with a sputter deposition technique at 400 ◦C, but oxygen

impurities were present. He could not deposit Sn through sputter deposition since it

could easily contaminate the walls of the sputtering chamber and fatally damage the

instrument. At Cornell Laboratory and in Jefferson National Accelerator Facility the

direct deposition of Sn on a Nb substrate by vapor diffusion technique was studied [17]

[18]. In those experiments, there were two insist issues that had to be dealt with. The

first was the formation of Sn droplets on the surface, that have poor superconducting

properties, and the second was the absence of complete coating on Nb surface. Those

issues were partially solved by first annealing the Nb substrate, to prevent its reaction

with the condensating Sn, allowing the accumulation of a continuous Sn film before

diffusion into Nb begins.

Agatsuma, Tateishi and Arai [19] succeded to make Nb3Sn thin films on MgO

substrate by using a sputtering target which consist of Nb3Sn single phase and made

from the reacted Nb3Sn powder of Nb and Sn mixture. The nb3sn grown film

23

sputtered on MgO substrate heated at 1123 K exhibited superconductivity below

about 8.5 K.

A Nb3Sn thin film process through CVD or electrodeposition, if it can be developed,

would allow films of this material to be studied easily with a view to solving the

materials problems now limiting the maximum gradients in Nb3Sn cavities. A strong

incentive is the prediction that gradients almost twice as high as for niobium could be

obtained. Although there are other ways to produce this A15 compound, once the

materials problems are understood and solved, CVD and electrodeposition would be

inexpensive ways to mass produce cavities for a long accelerator.

Film-based cavities have not yet accepted as an alternative to conventional bulk

niobium cavities. These kinds of film based cavities still remain their mystery for

physicist. In theory, a lot of money can be saved, since 40 nm penetration depth

means that only 1.5 microns of superconductor are really needed. The most promising

film-based cavities are the ones produced at CERN, which have reached accelerating

gradients in excess of 20 MV/m[20] .Some limitations of film-based cavities can include

following aspects:

Many defects are occurred in the production of thin films at low temperatures and

they will cause oxygen and hydrogen trapping which will be significant at high

gradients.

Their grains size is much more smaller than the grain size of conventional bulk niobium

cavities which result grain boundary diffusion and oxygen and hydrogen trapping since

they are more faster than impurity diffusion in bulk niobium

1.1.5. Influence of Copper on Nb3Sn

Wires have always a ternary phase because of presence of Cu thus they are different

than pure binary Nb-Sn system. Presence of copper is required to obtain

superconductive Nb3Sn wires most efficiently by performing solid diffusion at higher

temperatures. Nb3Sn phase is obtained at 650 °C or higher temperatures. In a Nb-Sn

system Nb3Sn is formed above 930 °C where the only stable phase is Nb3Sn. Non-

superconducting phases of Nb-Sn system such as NbSn2 and Nb6Sn5 are stable at

temperatures under 845 °C too and they also grow at the interface [20]. On the other

hand, in a ternary system with presence of copper only stable phase is Nb3Sn even at

low temperatures. The diffusion path from the Cu–Sn solid solution to the Nb–Sn solid

solution passes through only the A15 phase field, destabilizing the formation of the

non-superconductive phases NbSn2 and Nb6Sn5 [21]. In conclusion, A15 compound

24

formation temperature is decreased by addition of Cu resulting a higher grain

boundary density and required for pinning and limiting grain growth.

Superconductor wires efficiency is related with microstructural defects like grain

boundaries or pores, grain size distribution, possible layer thickness differences and

the variation of composition over the cross section. When thermodynamics and

kinetics of system are combined, formation of the structure can be determined. To

understand diffusion process and calculate thermodynamic behavior ternary phase

diagram of Cu-Nb-Sn system was calculated at 700 °C which is shown in Figure1.12.

[9].Although ternary phase diagrams of Cu-Nb-Sn system are constituted generally at a

fixed temperature, they are lack of some details and these details cause contradiction

in regions and unclarity on role of Cu.

Figure 1-12 Isothermal section from the Cu-Nb-Sn system at 700 C

Very low temperature A15 formation has been recorded at temperatures down to

about 450°C in a ternary system containing 5.4 at. % Cu. Although Cu can be detected

in the A15 layers, it is generally assumed to exist only at the grain boundaries and not

to appear in the A15 grains. The absence of Cu in the A15 grains might explain why the

binary A15 phase diagram can often be used to qualitatively interpret compositional

analysis in wires. Also, to first order, the addition of Cu does not dramatically change

the superconducting behavior of wires as compared to binary systems [22]. In the case

of bronze matrix in wires, when the binary phase diagram of Cu-Sn (Fig.1.13) is

evaluated, it can be seen that only phases can form by diffusion at the interface of Cu

and Sn are η, ε, δ, γ phases below 580 °C.

25

Sn richest phases are Cu6Sn5 (η phase), Cu3Sn (ε phase) and Cu41Sn11 (δ phase) and

their composition range is very attenuate and this permits to consider a constant

concentration of Sn along the intermetallic layer during the phase growth by diffusion.

The maximum allowed Sn quantity in the copper matrix of the bronze process is 13.5

wt.%. This is because a ductile phase is needed to extrude wires and α phase is the

only ductile one, but it is not stable at higher Sn concentration in the range of working

temperatures.

Figure 1-13 Cu- Sn Phase diagram

26

1.1.6. Diffusional Phase Growth of Nb-Sn System

Three intermetallic phases Nb3Sn, Nb6Sn5 and NbSn2 are formed in the niobium-tin

system. Nb6Sn5 and NbSn2 appear to be stoichiometric with narrow homogeneity

range, but Nb3Sn can exist over a wide composition range from about 73 to 83 at. %

niobium although the niobium”-rich compositions are not formed on annealing below

1400 °C. Nb6Sn5 and NbSn2 form at 930 and 845 °C respectively. The three phases

appear to be stable to low temperatures, but Nb3Sn and Nb6Sn5, are slow to form

below about 800 °C. The solubility of niobium in liquid tin is small at temperatures

below 1000 °C and the solid solubility of tin in niobium decreases from about 9 at. %

tin at the peritectic temperature of Nb3Sn to about 1 at. % tin at 1495 °C and is

negligible below 1000 °C. An equilibrium diagram, shown in Figure1.14, is constructed

from the present data and from other published information.[23]

Figure 1-14: Nb-Sn phase diagram

27

In his master thesis Reginato [24] determined the diffusional parameters by

considering a parabolic growth rate which was derived from first Fick’s Law which can

be described by following formula:

𝐿 = √2𝐷𝑇 (2)

in which L represents the thickness of the Nb3Sn phase formed at the interface

between Sn and Nb, t is the duration time of the heat treatment performed at a

constant temperature, and D is the inter-diffusion coefficient between Sn and Nb and

presents an Arrhenius Temperature dependent behavior where D0 represents the

diffusion frequency, R is the gas constant, and T is the temperature of the heat

treatment:

𝐷 = 𝐷0𝑒−

𝑄

𝑅𝑇 (3)

Activation energy for diffusion Q0 was calculated about 202 kJ/mol which was lower

than that measured for Nb surrounded by a bronze matrix. Finally, thermal profile was

decided as follows: Samples were kept for 72 h at 210 °C to favor the relaxation of

internal stresses, for 10 h at 450 °C and finally for 24 h at 700 °C to produce the

superconductive phase and obtain a fine-grained structure which improves flux

pinning which gave the desired result.

28

1.2. Electrodeposition

Electrodeposition is the process of depositing materials such as simple metals but also

alloys and semiconductors onto a conducting surface from a solution which contains

ionic species. It is sometimes called as electroplating.

Electrodeposition technique is widely used to create thin films of materials on surfaces

and give different properties to the external surface. A simple setup for

electrodeposition is shown in the figure. There are working, reference and counter

electrodes which are connected to a potentiostat instrument and placed in a vessel

containing liquid which consist ionic species. While counter electrode is used to

complete electric circuit, the working electrode is the material to be plated. When an

electric field is applied, uncharged elements or compounds forms and adhere to the

surface of working electrode. The strength of the electric field or the potential is

measured versus the reference electrode, but the actual current flows between the

working and counter electrodes.

Figure 1-15 A simple setup of electrodeposition

The adhesion onto substrate is the most important requirements of electrodeposition

method which depends on cleaning of substrate, alloy formation by the inter-diffusion

between substrate and deposited material and intermetallic compound which is not

desirable.

Mechanical properties should be considered during electrodeposition method.

Generally, electrodeposition causes internal stresses related to coalescence of three

dimensional, epitaxial crystallites, dislocation configurations or other factors.

29

1.2.1. Copper Electrodeposition

Copper is the most widespread metal to be plated and it finds wide range of usage

area in plating on plastics, zinc die casting, automotive bumpers, printed wiring boards

etc. Copper electrodeposition is used for engineering and decorative purposes which

needs good mechanical and physical properties. Since it covers even minor

imperfections in the substrate, it can be used as underplate material. Copper deposits

also play a role as a thermal expansion barriers by absorbing the stress produced when

metals with different thermal expansion coefficients undergo temperature changes

especially suitable with plastic substrates. Alkaline cyanide and pyrophosphate

complex ion systems and acid sulfate and fluoborite simple ion systems are the most

important commercially used plating systems for copper electrodeposition. Other kind

of plating system solutions are unstable or do not result good deposit features.

Nowadays, some alkaline systems have been accomplished by replacing cyanide with

no cyanide solutions. [25]

In the latest years, Cu has been a good alternative to aluminum as a material for

interconnects in the electronic industry. Although there are so many ways such being

CVD, PVD and sputtering to make copper thin film deposition onto substrates,

electrodeposition method is considered as the most economic, productive and reliable

one. Two kind of approaches have been studied to do copper electrodeposition: acidic

and basic. In the Basic chemical system requires buffering and some complexing agents

like amines. By adding reagents such as chelating and brightening, morphology of

copper surface is modified.[26]

Operating conditions such as temperature, current density, agitation, ultrasonic

agitation, filtration and anodes are important parameters to obtain a good copper

electrodeposition. Temperatures are in the range of 20 to 60 °C but typically 32 to 43

°C is common because it can be obtained more economical way with small amount of

heating or even no heating and no cooling process. Increasing temperature induces

higher conductivity and smaller cathode anode polarization. Temperatures below 30 C

is suggested for brightness in acid solutions to achieve good leveling power.[27]

An increase in current density in either the sulfate or the fluoborite solution results in

increased cathode polarization (but not to the extent noted for many other solutions).

Cathode films become more depleted in Cu(II) ion and more concentrated in sulfate

ion when the current density is increased. [28]. Current density and agitation must be

balanced in order to obtain deposits having the desired properties.

30

1.2.2. Tin Electrodeposition

Tin is a kind of ductile, soft and bright color metal where it can be used as a finish on

surfaces by electrodeposition. Tin electroplating is widely used in microelectronics for

instance, printed circuit boards, in automotive industry or in jewelry production for a

decorative purpose. In addition to that, tin is one of the very few metals that is

convenient for cooking equipment. Surfaces coated with pure tin are resistive to air

oxidation and corrosion and result improved solderability. Besides that, bright tin

coatings for decorative purposes might be pleasing aesthetically and be resistive to

environment. Electroplating of tin has been known for more than one hundred and

fifty years. The electrolytes used for tin electrodeposition can be alkaline stannate

based or acidic stannous salt based solutions according to desired properties of

deposit or operational parameters. Important parameters may consist the range of

applied current density, electrolyte throwing power, cathode current efficiency,

surface morphology, texture and appearance, deposit uniformity, speed of deposition,

use of bath additives and electrolyte stability, solubility of tin compounds, stability of

electrolytes, solderability and possible reflow of deposits on heating [29].

Alkaline stannate based electrolytes should be heated over 60 °C and should have low

current density range and they are generally hydroxide based compositions. They can

be operated without any additives because stannate ion appears as a soluble complex.

Acidic divalent stannous based solutions use up less electricity but they are complex

baths because they need additives for electrolyte stability, adhesion and improved

deposit morphology which is their major disadvantage. These additives which are

organic chemicals might be gelatin, cresylic acid, cresol sulphonic acid, an aromatic

hydroxyl compound or combination of them[30].

In acidic solutions, Sn(II) and Sn(IV) are equally stable and Sn(II) can be easily oxidized

to Sn(IV) by atmospheric and anodic oxidation. The loss of stannous tin, if not correct,

causes loss in the deposit rate and productivity. Most of the proprietary tin plating

processes contain antioxidants to minimize the oxidation of the divalent tin by

atmospheric oxygen. Another way to minimize Sn(IV) formation is to add a piece of tin

sponge in the plating tank. The most commonly encountered tin electroplating

chemistry are based on fluoboric acid, hydrochloric/hydrofluoric acid, sulfuric acid and

Phenolsulfonic acid. These acidic baths have number of limitations: the sulfuric acid

and fluoboric acid baths have problems related with their corrosive nature and sulfuric

acid causes itself also anode passivation at high current densities and oxidation of Sn

(II) to Sn(IV). Phenolsulfonic acid causes sludge formation resulting tin loss, decrease in

current efficiency and finally obtaining poor quality deposit[15]. In addition to that

31

these acidic baths need expensive waste treatment since they consist of many

additives.

.

32

2. Materials & Methods

In this chapter, all chemicals used and methods to perform different experiments are

described with their basic features. In particular, sample preparation, electrochemical

setup, electrodeposition working parameters and heat treatments were explained.

2.1. Etching and Cleaning

An effective etching and cleaning processes is needed to get rid of niobium oxides and

obtain a good adherent coating prior to electrodeposition. Different kind of etching

solutions are tried to have the best results which are reported in Table 2.1.

Solution Composition Time (sec)

1 0,5 M Sulphuric acid + HF 10 % W 10

2 2 Parts of 80 mL 85% H3PO4 1 Part of 80 mL 49% HF 1 Part of 80 mL70% HNO3

60

3 2 Parts of 100mL 85% H3PO4 2 Parts of 100 mL 49% HF 1 Part of 100 mL 70% HNO3

120

4 2 Parts of 90 mL 85% H3PO4 1,5 Part of 90 mL 49% HF 1 Part of 90 mL 70% HNO3

120

Table 2-1: Etching solutions

While solution 1 did not work out, solution 2 and 3 worked better and amount of

hydrofluoric acid was reduced from 2 parts to 1.5 parts after observing it is extreme

aggressive effect. The substrate preparation was performed just before the beginning

of the experiments, and the drying was performed with nitrogen to avoid a possible

oxidation.

33

2.2. Electrochemical Setup

The deposition cell was an open-air cell and first constructed with a 200 mL Pyrex

beaker, copper anode and a metallic cathode, both connected electrically to the circuit

with the use of a crocodile connector. During production of second generation

samples, more smaller beakers 75 or 100 mL were used. The temperature was

controlled by a heater connected to a thermometer and the agitation of the solution

was provided by a magnetic stirrer. No reference electrode was needed since only

Galvano-static depositions were performed. Niobium was used as a functional

substrate, active in the formation of the superconductive phase and several types of

anodes were used as titanium, niobium or copper, according to the experiment to be

performed. The properties of Nb foils are described in Table 2.2.

Nb foils Value /Name

Thickness 50, 250, 1000 µm

Supplier Goodfellow

Purity 99.9%

Temper Annealed

Table 2-2: Features of Nb Foils.

Figure 2-1: Electrochemical cell

34

2.3. Electrolytes

Different aqueous electrolytes were used to deposit tin and copper on niobium

substrates.

2.3.1. SolderonTMMHS-W

Solderon TM MHS-W is a commercial bath which is used to perform tin plating. MHS-W

is a high-speed, non-foaming electroplating process for the rapid deposition of fine-

grained, matte tin coatings from an organic sulfonate electrolyte, providing excellent

thickness distribution [31]. Recommended working parameters are seen in Table 2.2.

During electrodeposition, bath temperature was kept 50 °C and current density was 50

mA /cm2 according to last experiments optimized operational data [24] which gave a

smooth and adherent coating and good thickness distribution.

Parameter Suggested

Temperature 20- 65 °C

Cathode Current Density 5- 200 A/ dm2

Anode to Cathode Ratio 3:1 minimum

Agitation Dynamic solution with cathode movement

Cathode efficiency 95- 100%

Table 2-3: Suggested Operational Parameters

Operation parameters which was used in the experiments are reported at Table 2.4.

Operation parameters

Values

Temperature 50 °C

Current density 50 mA / cm2

Agitation No

Table 2-4: Used parameters for tin plating

35

2.3.2. Copper Strike Electrolyte

First copper strike layer on niobium substrates were performed with acidic electrolyte

which is prepared with copper sulphate and sulfuric acid in deionized water.

Composition is shown Table 2.4. The copper electrolyte which was used in Reginato ‘s

thesis work [24] is modified using hydrofluoric acid instead of hydrochloric acid .

Besides working parameters are changed to fit better to this condition which are

reported in the Table 2.5.

Chemicals Purity Supplier/product

code

Concentration

CuSO4 ACS reagent ≥

98.0 %

Sigma-Aldrich 0,24 M

H2SO4 95- 97 % Sigma-Aldrich 2,04 M

HF 48% Sigma- Aldrich 1,098 M

Table 2-5: Copper Strike Solution

Table 2-6: Working Parameters for Copper Strike Layer Plating

2.3.3. Copper Barrier Electrolyte

To perform last layer of the samples, a copper barrier electrolyte is used which is

composed by copper pyrophosphate trihydrate, sodium nitrate and sodium

pyrophosphate in deionized water. Concentrations are shown in Table 2.6 and working

parameters are in Table 2.7. The important parameter for this electrolyte solution is

pH. It must have pH between 8 to 8,5 above this Ph value, it does not work properly.

Operation

parameters

Values

Temperature 40 °C

Current

density

30 mA/cm2

Agitation No

Anode Copper

Cathodic

efficiency

˜100%

36

Pyrophosphate promotes adequate dissolution of anode and inhibits the formation of

insoluble salts and finally ammonium salts and nitrates are added to enhance the

deposition rate and adherence and smoothness of coating.

Chemicals Purity Supplier/Product code Concentration (g/L)

Cu2P2O7 Copper 33% ÷ 37%

P2O5 36% ÷ 40%

Pure Galvanic

Codex /284621 26

NaNO3 >99.0 % Fluka Chemika /52999 5

Na4P2O7 180

Table 2-7: Copper Barrier Layer

Operation

Parameters

Values

Temperature 50 °C

Current Density 20 mA/cm2

Agitation Magnetic

Stirrer

pH 8,5

Table 2-8: Working Parameters of Copper Barrier Layer Deposition

37

2.4. Heat Treatments

Different heat treatments were performed in the Politecnico di Milano labs in

Chemistry, Material and Chemical Engineering Department and Mechanical

Department.

The heat treatments at Polimi was performed Carbolite Tubular furnace which is more

suitable for small dimension samples and has simple structure shown in Figure 2.5. The

quartz glass inside the oven was connected to gas bars through a flowmeter which was

controlling the flow rate. The sealing of the tube and bars were done carefully to avoid

any contamination. Exhausting the argon gas was not effective since samples were

oxidized after the heat treatment. The quartz tube is 90 cm long and has˜ 5 cm

diameter. Samples put on silica glass were placed inside the tube with a help of a metal

stick which has a hook at the end of its length. Titanium coil was placed also in the

entrance of the gas in the tube to absorb oxygen.

Figure 2-2: Carbolite Furnace at Polimi

38

2.5. Sample Characterization

2.5.1. X-Ray Analysis

XRD and XRF analysis were performed on the samples. FISCHERSCOPE XRAY XAM was

used to perform XRF analysis. The X-Ray tube and the detector system are located

underneath the measuring stage. This results in a bottom to top measurement

direction. It is suited for measuring small components with varying geometry. The

surface area to be tested can be placed directly on the measuring stage. The

measurement spot is automatically set by the instrument at the correct distance.

XRD is a nondestructive, widely used technique performed to determine the

crystalline microstructure of materials. It is generally exercised in a combination of

known crystalline structure’s information and proper match algorithm.

XRD analysis were performed using a PW 1830 generator and a PW 3020 goniometer,

with a Cu Kα1 emission line. Cu-Sn multilayer samples were analyzed in powder mode.

2.5.2. SEM Analysis

Scanning electron microscope analysis were performed using a Carl Zeiss EVO 50 EP

microscope. The used acceleration Voltage was 20 kV and the measurements were

performed in high vacuum. The samples analyzed were metallic, and didn’t need to be

metallized.

39

3. Experimental Results

In his master thesis work Reginato[24] was able to obtain Nb3Sn phase thickness of

about 13 µm by electrodeposition of 10-20 µm thick Sn layers and Cu intermediate

layers onto Nb substrates and followed by high temperature diffusion in argon

atmosphere. First Cu layer prior to Sn layer was aimed to have better adhesion

between Nb substrates and Sn, also to have the aimed phase Nb3Sn through Cu- Sn

alloys which formed during thermal treatments. The second intermediate Cu layer was

the last layer on the Nb/Cu/Sn structure in order to hinder Sn leakages and act as

barrier layer. Electrodeposition was performed from aqueous solutions at current

densities in the 20–50 mAcm−2 range and at temperatures between 40°C and 50°C.

Heat treatments were performed as follows: the samples were kept for 72 hours at

210 °C to favor the relaxation of internal stresses, for 10 h at 450 °C to form a liquid Sn

phase and start Nb- Cu diffusion and obtain a Cu3Sn (Ɛ) bronze phase and finally for 24

h at 700 °C in order to produce the superconductive phase and obtain a fine-grained

structure which improves flux pinning. Presence of cubic Nb3Sn phase (A15 structure)

was proved with X-ray diffraction patterns and superconductive behavior with

superconductive set up mechanism.

In this chapter, similar path to achieve Nb3Sn phase is followed with some different

approaches particularly in the electrodeposition and heat treatment part of the whole

experiment. The requirement for better adhesion in electrodeposition and different

issues in heat treatments led us to develop a multilayered structure made of Nb-Sn-Cu

with different chemicals and methods.

Since electrodeposition is reliable, basic and cheap process and able to provide

pure and adherent Sn metal deposits on Nb substrates, coating on complex shaped

magnets might become more cost effectively with this procedure described above.

Besides, this approach can be applied either in classical wire processes such as Jelly

Roll, Powder in Tube, Restacked Rod Process, or in direct production of

superconducting surfaces.

40

3.1. Electrodeposition of Copper Strike on Nb

The most important key to have good, compact and non-oxidized multilayer

structure of samples is related to electrodeposition of Cu on Nb substrates. The biggest

problem was raised from presence of niobium oxides on the surface. As they weaken

the adhesion of electrodeposited metal layers, they affected samples before heat

treatments and hindered the formation of superconductive phase after heat

treatment, since they behave as a barrier for inter-diffusion of subsequent layers.

Thus, etching and cleaning of Nb substrates and processing electrodeposition right

after chemical treatment of Nb without losing time is important in this experiment;

consequently, niobium foils were treated with acidic solution and acetone to reduce

the presence of niobium oxides which will be explained in detailed in the following

sections.

Electrodeposition tests were carried out on niobium foils of 2cm× 6 cm having

thickness of 250 μm as show in Figure3.1. Thickness of foils were changed to thicker

foils after the trials of first heat treatments because they became curly and so fragile.

Besides that, after completing all layers, when the kapton tape was tried to take off,

surface was not continuous and compact; thus, sample’s covering was changed again

in the following steps.

Figure 3-1: Samples preparation, B: Back of samples F: Front of samples

Several kinds of anodes were used for electrodeposition such as copper, titanium,

graphite and niobium. Although niobium laminar anodes were used in previous thesis

work, final preference was copper anodes since they give better results and extend

bath life; have less metallic contamination; produce parts without roughness; have less

sludge; and require less maintenance.

41

Copper Strike Electrolyte Composition

A CuSO4 0,24 M H2SO4 2,04 M HCI 1,098 M

B A+ NaF 0,5 M

C A+ NaF 0,5 M + HF 0,5 M

D A+ HF 0,5 M

E CuSO4 0,24 M H2SO4 2,04 M HF 1,098 M

Table 3-1: Copper Strike Electrolytes

Different copper strike electrolytes were tried to figure out the best condition which

are shown Table 3.1. With electrolyte A, which was used before in thesis work of

Reginato [24], nucleation of Cu on Nb was achieved but there were no homogenous

and adherent coating of copper as it can be seen in Table 3.2 ; in addition, with

electrolyte B, C and D the coating was tried to be modified. As pre-sample 9 gives the

best result, the following samples were produced according to that. During

electrodeposition bath temperature was kept 40 °C, current density was 20 mA /cm2

and etching time 120 seconds based on experimental results in Table 3.2 and Table

3.3.

Before the electrodeposition, foils were submerged in etching solution specified in

Section 2.1 and cleaned with deionized water and sink directly to electrolyte solution

without drying.

42

Table 3-2: Pre-samples for calibration of Cu on Nb substrates

Sample Name Area (cm²) Electrolyte Current ( mA/cm² ) Temperature (C°) Etching Time (sec) Duration Time (min) Stirr XRF (Medium) Standart deviation Picture

sample 1 2 A 30 40 roughening-10 30 on

sample 2 2 A 15 40 roughening-10 30 on

sample 3 2 A 5 40 roughening-10 30 on

Rsample 1 2,4 A 30 36 15 8 off

Rsample 2 2 A 30 40-42 ranged 15 12 off

Rsample 3 2 A 30 38-42 ranged 15 15 off

Sample 4. 1. 1,7 A 30 40 -42 ranged 15 15 off

Sample 4. 2. 1,7 A 30 40 60 15 off

Sample 4. 3. 1,7 A 30 40 60 15 on

43

Table 3-3: Pre-samples for Calibration Cu on Nb substrates 2

Sample Name Area (cm²) Electrolyte Current ( mA/cm² ) Temperature (C°) Etching Time (sec) Duration Time (min) Stirr XRF (Medium) Standart deviation Picture

Sample 4. 4. 1,7 B 30 42-39 ranged 60 15 off no image

Sample 4.exp. 1 1,7 C 30 41- 40 ranged 60 20 off

Sample 4.exp. 2 1,7 D 30 40 60 20 off no image

Sample 4.exp. 3 1,7 E 30 40 120 20 off 32,22 7,033

Sample 3.exp. 4 1,6 E 30 42 60 20 off 11,58 1,483

Sample 2.exp. 5 1,56 E 20 40 60 20 off 5,036 1,021

Sample 1.exp 1 1,45 E 10 38-40 90 20 off 4,24 0,169

Sample 5. exp 1 1,48 E 20 40 60 25 off no no

Sample 5.exp 2 1,36 E 20 40-42 75 25 off 9,209 0,375

Sample 6. exp3 1,5 E 20 40 90 25 off 6,569 0,432

Sample 7. exp4 1,28 E 20 40 120 25 off 9,394 1,122

Sample 8 1,75 E 20 40 120 10 off 4,49 4,4

Sample 9 2,58 E 20 40 120 5 off 2,207 0.187

44

3.2. Electrodeposition of Sn on Cu/Nb

During electrodeposition bath temperature was kept 50 °C and current density was 50

mA /cm2 according to previous experiments optimized operational data [25]. Solderon

TM MHS-W: aqueous tin plating solution was used. To reach 15-micron thickness,

duration was calculated as 8 minutes. Parameters were optimized easily after 10 trials

and good results were obtained, shown in Figure 3.2(a). Etching on Cu was performed

with diluted HCI and acetone.

a) b)

Figure 3-2: a) An example of pre-samples Sn on Cu/Nb structure b) Cu on Sn/Cu/Nb structure

3.3 Electrodeposition of Copper Barrier Layer

During electrodeposition of copper barrier layer, temperature was kept at 50 °C and

current density 20 mA/ cm². Temperature and pH of electrolyte were tried to keep

stable sensitively because electrolyte would not work out of 8 to 8,5 pH range and

would precipitate immediately below 50 °C. After few trials, to have enough thickness,

duration time was fixed at 75 minutes and homogeneous and adherent copper layer

was obtained on Sn/Cu/Nb structure as shown in Figure 3.2(b).

Furthermore, taking the all above into account, first generation samples were

produced as shown in Figure 3.3.

45

Figure 3-3: General scheme of samples

However, some problems started to occur during production of samples after fixing all

parameters and having complete multilayer structure samples as in shown Figure 3.4.

The biggest problem was the corrosion of samples after 3 days by turning into a black

color. During these period, some practical deductions were done:

There is an influence of changing chemicals, particularly using a new bottle of acids

which are used for etching processes can be too aggressive and resulting bad

adherence of coating.

The electrolyte copper strike layer has 2- 3 days aging time before it is used.

The electrolyte for copper barrier layer must have pH between 8- 8,5 sensitively, above

or below that pH range, it gives a dark green sludge instead of a homogeneous red

color copper coating on the top of multilayers.

Electrochemical cell configuration where 2 anodes were copper sheets and cathode

was a Nb foil was more effective that using just one copper anode.

The samples which faced with corrosion problems are shown in Figure 3.5.

Figure 3-4: Corrosion of Samples

Side A - after aging Side B- after aging

46

a)

b)

Figure 3-5: SEM analysis on red part of corroded sample at different magnifications: a) element detected on surface (261 X), b) surface morphology (3.00 KX)

47

a)

b)

Figure 3-6: SEM analysis on black part of corroded sample at different magnifications: a) element detected on surface (261 X), b) surface morphology (3.00 KX)

To investigate the reasons of corrosion on samples, SEM examination was performed

resulting the SEM images shown in Figure 3-6 and 3-7. SEM imagining shows that in

the corroded part of sample, Cu, O, Na and P were detected on the surface which

means that washing of the etching solution was the reason to cause corrosion. After

that step, optimization of etching procedure was needed.

48

3.4. Optimization of Etching Procedure

Some controlled experiments were performed on more small niobium pieces to

investigate the best conditions for etching and cleaning process which is shown at

Table 3.4. the composition of the etching solution was described in Section 2.1. In the

last step of 3 cycles of etching duration was decreased to 1 minute because the

samples became so thin with aggressive behavior of etching solution.

Sample Etching Procedure

D1 Etching (2 min) -washing- drying with paper- electrodeposition- no covering Etching (2 min) -washing- drying with paper- electrodeposition- no covering Etching (1 min)-washing- drying with paper- electrodeposition- adherent covering

D2 Etching (2 min)-washing- drying with paper- washing Etching (2 min)-washing- drying with paper- washing- electrodeposition- no covering Etching (1 min)-washing- drying with paper-electrodeposition- adherent covering

D3 Etching (2 min) –washing-drying-waiting for 10 minutes Etching (2 min)- washing-drying-electrodeposition-no covering Etching (2 min)-washing-drying-electrodeposition- no covering Etching (1 min)- ultrasonic washing for 2 minutes- drying- electrodeposition- adherent covering

D4 Etching (2 min)-ultrasonic washing for 6 minutes-drying Etching (2 min)-ultrasonic washing for 6 minutes- drying-electrodeposition-no covering Etching (1min)-ultrasonic washing for 2 minutes-drying-electrodeposition- adherent

Table 3-4: Different Approaches to Optimize Etching Procedure

Previous samples were produced without drying with a paper. Observation was made

after 3 days to see how they would react to environment and result a corrosion on the

surface. Sample D1 gave the best results and most stable appearance. After that point

samples were produced according to Sample D1 production path. After that point,

samples had less corrosion on the surface which were the samples sent to Fermilab

49

which can be seen at Table 3.5 and Table 3.6 with information about their production

date, their thicknesses and their appearance 3 days after production date.

Table 3-5: First generation samples (Step by Step)

Sample code Step 1 Step 2 Step 3

11

12

13

14

15

16

17

18 no picture

50

Table 3-6: Effect of aging on first-generation samples

Sample Code Production Date Cu1 thickness (µm) Sn thickness (µm) Cu2 thickness (µm) Side A - after aging Side B- after aging

sample 11 27 June 2016 2,364 ± 0,330 18,21 ±1,086 16 ± 1,6

sample 12 29 June 2106 2,521 ± 0,331 17,6 ± 0,389 16,45 ± 3,5

sample 13 13 July 2016 2,096 ± 0,143 16,61 ± 0,966 17,05 ± 2,27

sample 14 18 July 2016 2,260 ± 0,207 18,79 ± 0,92 20,52 ± 1,644

sample 15 19 July 2016 2,193 ± 0,234 15,88 ± 0,437 23,93 ± 2,23

sample 16 19 July 2016 2,239 ± 0,323 15,13 ± 1,264 18,515 ± 1,202

sample 17 20 July 2016 2,112 ± 0,159 13,07 ± 1,415 18,71 ± 1,377

sample 18 20 July 2016 2,301 ± 0,162 13, 034 ± 1,564 14,13 ± 1,868

51

3.4.1 Second Generation Samples

Since they First generation samples did not give satisfied results, there was a need of

production of second generation samples by modifying electrodeposition of copper

barrier layer, particularly etching and cleaning procedure. Since Sn electrodeposition

and copper barrier layer deposition technique remained same, they are not explained

for both first and second generation samples.

Firstly, niobium foils thicknesses were increased because first produced samples

became so curly and fragile and after heat treatment at Fermilab. Properties of Nb foils

which were used for production of second generation samples are described in Table

3.7.

Nb foils Value /Name

Thickness 1 mm

Supplier Goodfellow

Purity 99.9%

Temper Annealed

Table 3-7: Properties of Nb foils used for second generation samples

Secondly, etching procedure was changed to avoid oxidation of samples: samples were

treated with a diluted solution of HF, % 30 in weight, for three minutes and cleaned

with acetone and deionized water then before electrodeposition, samples surfaces

were roughened with a sandy paper to help the nucleation of copper on substrate.

Table 3-8:Images of sample before and after roughening

surface before

roughneing

surface after

mechanical

roughneing

52

One more time dipping in etching solution for 3 minutes was followed by cleaning with

acetone and finally electrodeposition was performed. Etching procedure for copper

strike layer can be seen detailed in Table 3.9.

Step Actions

1 Dipping in etching solution

2 Cleaning with acetone +deionized water

3 Roughening with grid paper

4 Dipping in etching solution

5 Cleaning with acetone

6 Copper strike electrodeposition

Table 3-9: Optimized etching procedure of second generation samples for copper strike layer Electrodeposition step

In this way, so smooth and adherent copper coating on niobium substrates were

achieved. Between all steps of electrodeposition of layer, an ultrasonic washing was

performed for 2 minutes and drying before passing the next step. All electrodeposition

steps gave adherent, homogenous coating as shown in Table 3.8. The pictures of

samples were taken right after the electrodeposition steps without aging time.

53

Table 3-10: Second generation samples step by step

54

Finally, thicknesses of copper strike layer were increased to have enough copper at the

interface of Nb and Sn. Tin coating thicknesses were ranged from 8 µm to 18 µm and

copper barrier layer thicknesses were ranged from 9 µm to 17 µm, as shown in Table

3.11, which were determined with XRF analysis.

Table 3-11: Thicknesses of layers of second generation samples

As it can be seen in Table 3.12, second generation samples were observed three days

after their production date to see the effect of aging on corrosion of samples. They

were maintained in plastic bags. They kept their chemical structure stable without

aggressive corrosion on the surfaces for longer times compared with first produced

samples. Only sample 40, 41 and 42 exhibited pitting corrosion on their surfaces but

they were not as aggressive as observed in the first produced samples that half of the

sample were corroded.

Sample Code Production Date Nb Substrate Thickness (µm) Cu (1) Thickness (µm) Sn(2) Thickness (µm) Cu(3) Thickness (µm)

34 22.11.2016 500 2.66 ±0.88 10.05 ±5.03 9.71 ±1.24

35 22.11.2016 50 2.89 ±0.3 11.01 ±3.21 10.03 ±0.96

36 23.11.2016 500 2.603 ± 0.24 14.85 ±0.35 7.72 ±0.504

37 24.11.2016 500 2.82 ±0.23 16.59 ±7.02 11,14 ±0,802

38 24.11.2016 500 2.6 ±0.2 16.53 ±2.02 16.43 ±1.8

39 28.11.2016 50 3.11 ±0.14 17.65 ±5.03 16.83 ±3.32

40 28.11.2016 500 2.58 ±0.32 12.99 ±1.65 15.55 ±1.58

41 29.11.2016 500 2.81 ±0.5 18.40 ±1.43 17.65 ±0.9

42 1.12.2016 500 2.4 ±0.27 13.08 ±3.8 15.06 ±0.6

43 1.12.2016 500 2.09 ±0.2 8.14 ±2.4 16.97 ±0.54

44 2.12.2016 500 2.9 ±0.5 14.96 ±1.12 16.01 ±2.45

45 5.12.2016 500 2.15 ±0.163 16.67 ±0.41 15.76 ±4.1

46 6.12.2016 500 2.13 ± 0.43 16.67 ±2.1

55

Table 3-12: Images of second generation samples after aging

Sample Code First Apparence Aging

34

35

36

37

38

39

40

41

42

43

44

45

46

56

3.5 Thermal Treatments

The thermal treatment was performed in argon atmosphere to prevent oxygen and

water contamination inside the oven, thereby formation of oxides at the surface of

samples as mentioned in Section 2.4.

The first step was practiced at a temperature around 210 °C which should be lower

than Sn melting point (231.9 °C) for 72 or 14 hours to achieve a relaxation of internal

stresses between the metal layers and to start the diffusion between Cu and Sn. During

first step 3 µm thick Cu6Sn5 (η) phase should form according to literature [32]. The

second intermediate step was performed at 430 °C or 450 °C for 10 hours. The aim of

second step is forming a liquid Sn phase and starting diffusion between Nb and Cu and

eventually forming a Cu3Sn (Ɛ) bronze phase. It is also needed to avoid Kirkendall effect

which means the motion of the boundary layer between two metals that occurs as a

consequence of the difference in diffusion rates of the metal atoms. In addition to

that, higher temperatures would cause higher Sn pressures on Cu barrier layer

resulting Sn leakages on the surface. Final step of thermal treatment was carried out at

700 °C for 24 hours to form aimed Nb3Sn superconducting phase. At the end of 24

hours, the oven was turned off, and the sample removed when the internal

temperature was lowered enough.

First attempt was performed at Mechanical Engineering Department of Politecnico di

Milano laboratory which was not successful because of the technological limit of the

oven related with exhausting system of argon. Sample 38 and 40 were oxidized as

shown in Figure 3.7.

Figure 3-7: Thermal Treatment at Mechanical Engineering Department

Second attempt was carried out in the labs of Chemistry, Material and Chemical

Engineering Department as mentioned above at Section 2.4.

57

1. Step 2. Step 3. Step

Thermal

Treatment

profile

T (C°) Time (h) T (C°) Time(h) T (C°) Time(h)

A 210 14 450 10 700 24

B 190 14 430 10 700 24

C 220 72 430 10 700 24

Table 3-13: Thermal treatment profiles

Three different thermal treatment profiles were used with different temperatures and

duration, shown in Table 3.13.

The flowing rate of Argon was chosen 10 cc/min for all heat treatment profiles and a

titanium coil was placed inside of the quartz tube as an oxygen trapper.

Table 3-14: General information about heat treated samples

Sample Production Date Nb (µm) Cu (1)(µm) Sn(2) (µm) Cu(3)(µm) Before thermal treatment

Heat treatment

profile After thermal treatment

36 23.11.2016 1000 2.603 ± 0.24 14.85 ±0.35 7.72 ±0.504 B

37 24.11.2016 1000 2.82 ±0.23 16.59 ±7.02 11,14 ±0,802 C

43 1.12.2016 1000 2.09 ±0.2 8.14 ±2.4 16.97 ±0.54 A

45 5.12.2016 1000 2.15 ±0.163 16.67 ±0.41 15.76 ±4.1 C

46 6.12.2016 1000 2.13 ± 0.43 16.67 ±2.1 C

58

General information of samples such as production date, thicknesses, how they looked

before and after heat treatments and their treatment profile are summarized in Table

3.14.

Sample 43 was treated with thermal profile A, while Sample 36 with profile B and

sample 45,46 and 37 with profile C. With this kind of oven, which is used generally to

form carbon nanotubes, cleaning of the quartz tube and sealing the oven all around

the tubes are crucial to avoid any kind of contaminations. The problem was raised

from technologic limit of oven: since it’s working mechanism is related to shutting

down the machine when it reaches the target temperature and there is no automatic

cooling mechanism, there will be some fluctuations until temperature gets stable.

Thus, with profile A at target temperature 210 °C, we had overshooting of the sample

presumably so, for sample 43, heat treatment was not successful there was no

adhesion and the formed layer was delaminated from surface as shown in Figure 3.9.

Besides, first step was decreased to 14 hours assuming that it is enough time necessary

for relaxation of internal stresses but it was not sufficient time. XRD analysis could not

be done since the layer was delaminated from surface.

Figure 3-8: Heat Treatment profile A for sample 43

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00 50,00 55,00 60,00

Tem

per

atu

re [

°C]

Time [h]

Heat Treatment Profile A

59

Figure 3-9: Sample 43 after Heat Treatment

Figure 3-10: Heat Treatment Profile B for Sample 36

Small Sn islands were observed on the surface of Sample 36 (Figure 3.11) which was

treated with heat profile B, presumably because of the insufficient thick of copper

barrier layer.

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00 50,00 55,00 60,00

Tem

per

atu

re [

°C]

Time [h]

Heat Treatment Profile B

60

Figure 3-11: Sample 36 after heat treatment

Presence of tin oxides and niobium oxides and NbSn2 phase were detected from XRD

patterns of Sample 36: SnO2 (26.484°, 33.78°), Nb2O5(22.63°,28.41°), NbSn2 (31.861°,

36.66°), NbO (30.05°, 42.93°), Sn (30.65°, 62.55°), Cu(43.31°), Cu6Sn5 (35.16°) as

shown in Figure 3.12.

21 28 35 42

-200

0

200

400

I (a

.u.)

Theta (°)

I

Cu

Cu10Sn3

Cu6Sn5

Nb2O5

NbO

NbSn2

Sn

SnO2

Sample 36

Figure 3-12: XRD Pattern of Sample 36

61

Figure 3-13: Heat Treatment Profile C for sample 45

To overcome the problems with these thermal treatments, Sample 45 was treated

with longer in the first step about 72 hours, shown in Figure 3.13. In addition to that,

to reach first target time from room temperature ramping time of the oven was

decreased to 1°C/min to avoid overshooting of the sample.

Figure 3-14: Sample 45 after heat treatment

62

The XRD pattern (Figure 3.15) of Sample 45 which was the second-generation sample

and treated with last heat treatment profile C shows the reflection of a Nb3Sn phase

(A15) structure. Other reflections can be attributed to oxides, Sn and Cu-Sn phase:

NbO2 (25.98°,35.20°,52.02°), Sn (30.62°), Nb3Sn (38.0°, 41.78°), Cu6Sn5(43.29°),

SnO2(65.81°), Nb (55.36°, 69.61°) and Cu3Sn (57,37°).

40

0

80

160

I (a

.u.)

Theta (°)

I

Cu6Sn5

Nb3Sn

NbO2

Sn

SnO2

Cu3Sn

Nb

Sample 45

Figure 3-15: XRD pattern of sample 45

63

4. Conclusions and further works

This thesis is a product of the collaboration between Politecnico di Milano and Fermi

National Accelerator Laboratory in Chicago, on the electrochemical synthesis of Nb-Sn

alloys. Nb-Sn alloys coatings were fabricated by combining electrochemical techniques

and heat treatments. Samples were produced by electrodeposition of a thin Cu strike

layer (2-3 µm) onto Nb substrates with different thicknesses (250 µm-500 µm).

Deposition of a Sn layer (10-20 µm) and a Cu barrier layer followed the Cu seed layer

on substrates and Nb/Cu/Sn/Cu multilayer structures were obtained. Considering the

bronze matrix in wires production, presence of copper strike layer was accepted as a

necessity because it lowers the A15 formation temperature, consequently limiting

grain growth and retaining a higher grain boundary density required for pinning. In

addition to that, the copper barrier layer was needed as a barrier layer to preserve the

integrity of the samples. It was understood that the thicknesses were not enough thick

was causing the droplets on the surface of substrates. The first part of the experiment

was focused on the electrodeposition of layers, particularly Cu strike coating features.

Since Cu electrodeposition on Nb substrate is hard to obtained because of the

presence of niobium oxides which can form easily, weaken the adhesion and act as a

barrier layer during thermal treatments, many different etching and cleaning

procedures were developed. The best results in the sense of corrosion resistivity and

stability with aging were obtained with special etching procedure: dipping in etching

solution which is a diluted solution of HF for three minutes, cleaning with acetone and

deionized water, roughening with sandy paper, dipping in etching solution, cleaning

with acetone and finally performing electrodeposition in a row. The electrodeposition

of Cu was carried out using a copper sulphate based electrolyte prepared in laboratory,

while coating of Sn was achieved by using a sulfonate based commercial bath

electrolyte.

The second part of the experiment was centered upon to investigate the most suitable

thermal treatment profiles to perform solid state diffusion between layers and obtain

Nb-Sn alloys. The optimized thermal treatment profile is described as follows:

Nb/Cu/Sn/Cu multilayer structured samples were kept for 72 hours around 210 °C to

remain just under the melting point of Sn to achieve relaxation of internal stresses, for

10 h at 430 °C to compose a liquid Sn phase and initiate diffusion between Nb-Cu and

Cu-Sn resulting Cu3Sn (Ɛ) bronze phase and ultimately for 24 hours at 700 °C to

produce Nb3Sn superconductive A15 structured phase. The XRD patterns exhibited

both the presence of Nb3Sn and NbSn2 crystalline phases and of Cu-Sn phases. In

addition to that, there were many different oxides of Niobium such as Nb0, Nb02,

64

Nb2O5, and tin oxides detected on the surfaces by means of XRD analysis. The SEM and

EDX which fulfilled on longitudinal cross sections of the samples confirmed the

presence of oxides which was the either because of oxides formed during production

of samples and interacting with environment or because of the technological limit of

the ovens that were used for the experiments.

These multilayer structured thin films could be used for optimizing the properties of

Nb3Sn Superconductors as an alternative way to commercial bulk wires and cables

production. Electrochemical deposition techniques require much lower capital cost

compared to vacuum deposition techniques and allows the deposition of metals and

alloys on complex shapes which is not the case for Nb3Sn with classical metallurgical

techniques because of the brittle behavior of it.

Concerning the all procedure has been through, further work is suggested to have a

deeper knowledge and understanding of the approaches of electrochemical synthesis

and thermal treatments. Modifying the multilayer structure by using different etching

and deposition procedure would be worth to investigate more detailed. Most

especially, heat treatments should be optimized by trying different profiles and be

investigated more detailed sample by sample. In the interest of having

superconductive Nb3Sn layers which continuous along the substrates and uniform in

thickness, the better study must be done with demonstration with superconductivity

tests or magnetization measurements.

65

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67

ACKNOWLEDGEMENT

I would first like to thank my thesis advisor Assistant Professor Silvia Franz of the

Department of Chemistry, Material and Chemical Engineering "G. Natta" at Politecnico

di Milano. The door to Prof. Franz office was always open whenever I ran into a trouble

spot or had a question about my research or writing. She consistently allowed this

paper to be my own work, but steered me in the right the direction whenever she

thought I needed it.

I would also like to thank the experts who were involved in the validation survey for

this research project: Massimiliano Bestetti and Emanuale Barzi, their passionate

participation and input, the validation survey could not have been successfully

conducted.

I would also like to acknowledge Mazdak Hashempour, I am gratefully indebted to for

very valuable help on this thesis.

Finally, I must express my very profound gratitude to my parents and to my friends for

providing me with unfailing support and continuous encouragement throughout my

years of study and through the process of researching and writing this thesis. This

accomplishment would not have been possible without them. Thank you.

Ceren Baykal