NATIONAL INSTITUTE OF NUCLEAR PHYSICSPADOVA UNIVERSITY

Camacho A., Rossi A., Palmieri V.

Sixth International Workshop on Thin Films and New Ideas for RF Superconductivity

Preparation of Nb3(Ga,Al)

superconductors by Electromagnetic

Induction Heating

Outline• Introduction of the common techniques used to prepare binary and

ternary superconducting A15 compounds.

• Electromagnetic Induction Heating Technique.

• Experimental procedure:

• Samples preparation.

• Heat Treatment Performed.

• Validation of the technique according superconducting properties and

quality of A15 phase on samples.

• Application of our technique on 6 GHz niobium cavities.

• Conclusions.

Introduction

• Common techniques used to prepare A15 superconducting phase:

Arc-melting process, chemical vapor deposition (CVD), sputtering,

etc.

• More specialized techniques such as:

• Melt-spin quenching technique: Nb3Ga 20.0K, V3Ga

15.0K, and Nb3Al 18.4K reported by Clemente [1]

[1] Clemente,“Superconducting properties of A15 compounds prepared by melt-spin quenching“

EM- Induction Heating Rapid heating

High temperatures during annealing process (~3000 °C)

Vacuumless

Self-heating of the sample

Short time of treatment

Clean quartz chamber

Economic system

Application on 6 GHz niobium Cavities

A15 compounds by EM-IH technique

6 GHz Nb Cavities

EM- Induction Heating System

Work head

15 KW

Powersupply

Pyrometer

Quartz tube

Flange

Flange

Argon or

Helium

Low overpressure

(250-3000)ºC

Exhaust gases

Coil

Input gas

Cavity or Sample

Experimental Procedure

Before annealing the samples

Chemical TreatmentBCP solution:

HF/HNO3/H3PO4 = 1:1:2

(20x10x5) mm

Experimental Procedure

How?

• Binary compounds Configuration I: Liquid Gallium (99.9% pure) Aluminum Foil (99% pure)

• Ternary compounds Configuration I and II: Paste: liq. Ga+ Al foil

Configuration I

Configuration II

Heat TreatmentChanging the voltage

and time

Rapid heating, quenching and transformation

Validation of the EM-IH TechniqueMaterials Number of samples Total

Nb-Ga 10

61Nb-Al 6

Nb-Al-Ga 45

Inductive

Measurement

Tc

Binary Compounds

Heat treatment for 10 minutes, changing the temperature from 1500 °C up to 1800 °C

Binary Compounds

• All the Tc are near to 12 K, an average of 3 K above of niobium transition

(9 K).

• The difficult to synthetize binary A15 compounds (Nb3Ga and Nb3Al) is

related to the competition from more stable phases, such as s phases

(Nb2Al and Nb5Ga3) and a-Nb phases (solid solution in bcc structure of

niobium).

• Samples annealed that not reach temperatures higher than 1500ºC, only

Nb superconducting transition was evidenced.

• The results suggest that the annealing process for 10 minutes at high

temperatures degrades the superconducting phase initially formed

Nb3Ga Nb3Al

Lattice parameter of 5.1809 Å, very close to the standard lattice parameter, 5.1800 Å

Lattice parameter of 5.2141 Å, much higher than the standard value, 5.1780 Å.

From our first attempt of A15 phase, we concluded...

1. Critical temperature results suggest higher diffusion of gallium atoms than aluminum atoms on niobium samples at the same heating conditions (corrosive property of gallium)

2. Wettability problems with liquid gallium which make the preparation of the samples before the heat treatment difficult

3. Very short time of heat treatment is necessary

TERNARY COMPOUNDS!!! Nb-Al-Ga

Ternary Compounds

Heat treatment for ~1 minute, changing the temperature from 1420 °C up to 2000 °C

Tc= (18±0.35)K

1

• Direct transformation of A15 phase from high temperatures of

niobium samples.

• Ternary compound seems to stabilize the A15 phase.

High Tc and sharp superconducting transition

Ternary Compounds

Heat treatment for ~1 minute, changing the temperature from 1420 °C up to 2000 °C

2

Broad superconducting transitions

• Gallium/ aluminum evaporate.

• Less control of the stoichiometry.

• Further studies are required in order toestablish a relation of the quantity of aluminum/gallium evaporated.

(210)

(200)

(310)

(320)(211)

X-ray diffraction in the planes: • (321) Nb3Ga• (110) Nb3Al

were not observed

Lattice parameter: 5.1904 Åa-Nb3Ga< a-Nb3(Al,Ga)< a-Nb3Al

Profile temperature vs time Cooling rate:

50 [˚C/s]

Heating rate: 90 [˚C/s]

Results Microstructure of Nb-Ga-Al_1 sample

Niobium Nb-Al-Gax

Niobium Nb-Al-Ga

Crack

x

Element wt% At.%Nb 82±1 66±1Ga 11,3±0,9 12,1±0,9O 1,9±0,1 9,0±0,6Al 4,7±0,2 12,9±0,4

73% at. Nb, 13.3% at. Ga and 14.2% at. Al. A15 and A2

Results

Nb-Al-GaNiobium

Microstructure of Nb-Ga-Al_1 sample

Nb-Al-Ga SamplesResults

Niobium Nb-Al-Gax

Microstructure of Nb-Ga-Al_1 sample

x

Niobium Nb-Al-Ga

Nb-Al-Ga Niobium

Results

Mapping that shows the interface between niobium and superconducting layer

Total Counts X-RaysElement Color Smin Smax

O K Red 11 99Ga L Green 17 405Al K Blue 19 296Nb L Yellow 151 2805Ga K Purple 21 366

Nb (L) Ga (K) Al (K)

Ga (L)-Nb (L) Al (K)-Nb (L)O (K)- Nb (L)

Ga (L)-Al (K) Ga (L)-Al (K)-Nb (L) Ga (L)-Al (K)-Nb (L)- O (K)

6 GHz niobium Cavities

Experimental Procedure

Before Coating: 1. Centrifugal Tumbling

Experimental Procedure

Before Coating: 2. Chemical Treatment, BCP solution

Before Coating: 3. High Pressure Water Rising

Experimental Procedure

Rotator

Liquid Gallium

Experimental Procedure

Yttria- stabilized Zirconium oxide

Inside Liq. gallium or paste with

Cavities Measurements

Sample Time [min.] Annealing Temperature, Max. [°C] Notes

1 1,3 2000 Melted

2 14,4 1731 Cavity with a small hole.

3 2,2 1770 Melted

4 3,0 1200 Normal conductor

510,0 1091 Normal conductor

66,1 2031 Normal conductor

71,4 1830 Normal conductor

Experimental Procedure

Direct transition of superconducting phase from high temperaturesusing E-M induction heating.

The temperature of the samples are very sensitive by changing voltageand time.

Nb+Al+Ga stabilized the A15 superconducting phase.

The best configuration is niobium+aluminum/gallium+niobium whichavoid the evaporation of gallium and aluminum at high temperatures more control of the stoichiometry SHARP HIGH SC TRANSITION

Conclusions

Thanks for your attention