Shock-Wave Consolidation of Nanostructured BismuthTelluride Powders

JAN BECK,1 MANUEL ALVARADO,1 DAVID NEMIR,1,5

MATHEW NOWELL,2 LAWRENCE MURR,3 and NARASIMHA PRASAD4

1.—TXL Group, Inc., 2000 Wyoming Ave., El Paso, TX 79903, USA. 2.—EDAX Inc., 392 E. 12300South, Suite H, Draper, UT 84020, USA. 3.—Department of Metallurgical and Materials Engi-neering, University of Texas at El Paso, 500 W. University Ave., El Paso, TX 79968, USA.4.—NASA Langley Research Center, Mail Stop 468, Hampton, VA 23681-2199, USA. 5.—e-mail:david@txlgroup.com

Nanostructured thermoelectric powders can be produced using a variety oftechniques. However, it is very challenging to build a bulk material from thesenanopowders without losing the nanostructure. In the present work, nano-structured powders of the bismuth telluride alloy system are obtained inkilogram quantities via a gas atomization process. These powders are char-acterized using a variety of methods including scanning electron microscopy,transition electron microscopy, and x-ray diffraction analysis. Then the pow-ders are consolidated into a dense bulk material using a shock-wave consoli-dation technique whereby a nanopowder-containing tube is surrounded byexplosives and then detonated. The resulting shock wave causes rapid fusingof the powders without the melt and subsequent grain growth of other tech-niques. We describe the test setup and consolidation results.

Key words: Thermoelectric, explosive, consolidation, shock wave,nanopowder, atomization, compaction

INTRODUCTION

Thermoelectric generation is the solid-state con-version of heat energy to electrical energy andarises from the intercoupled electrical and thermalcurrents in a material. Multielement thermoelectricdevices are built from multiple, electrically seriesconnected n-type and p-type semiconductor ther-moelements that are disposed in thermal parallelbetween a heat source and a heat sink. The keymaterial transport properties that characterizethermoelectric performance are the thermopower,S, which expresses the magnitude of the voltageproduced from a thermal gradient, the electricalconductivity, r, and the electronic and lattice ther-mal conductivities, je and jl, respectively. These areoften lumped into a single thermoelectric figure ofmerit Z, where

Z ¼ rS2

je þ jl: (1)

The thermoelectric efficiency increases withincreasing Z, so anything that can be done toincrease Z is desirable. One approach is to usenanostructuring to reduce the lattice component ofthermal conductivity.1,2

Nanostructured materials are polycrystalline sol-ids having a submicron crystallite size (generally lessthan 100 nm) and, consequently, a high density ofgrain boundaries. Nanostructured bulk materialsare generally made by consolidating nanopowders.Such nanopowders, having nanosized crystallites,can be produced through a variety of well-docu-mented processes including ball milling, cryomilling,chemical synthesis, melt-spinning, and gas atom-ization. A common procedure is to take those nano-powders and press a solid using an axial press. Thiscompact is often then sintered to ensure goodmechanical and electrical bonding between particles.(Received July 22, 2011; accepted December 17, 2011;

published online February 1, 2012)

Journal of ELECTRONIC MATERIALS, Vol. 41, No. 6, 2012

DOI: 10.1007/s11664-011-1878-4� 2012 TMS

1595

However, the temperatures applied during sinteringcan cause undesirable grain growth of the nano-structured material. When a nanostructured endproduct is desired, the challenge is to obtain highdensity and good interparticle bonding while pre-serving small (nanosized)grains. Thispaperdescribesa two-step procedure to make large quantities of abulk nanostructured thermoelectric material, first byusing gas atomization to produce a nanopowder andthen by taking that powder and converting it into abulk solid using shock-wave consolidation. Gasatomization is an industrially available, high-throughput, inexpensive process for making nano-structured and amorphous powders for the aerospaceindustry. The explosive consolidation process is wellsuited to volume production, so the combination ofatomization for nanopowder production followed byshock-wave consolidation to make a bulk material ispotentially attractive for low-cost manufacturing.

GAS ATOMIZATION OF Bi2Te3

AND Bi2Te2.7Se0.3 MATERIAL

Using gas atomization, nanopowders of a p-typethermoelectric material with stoichiometry Bi2Te3

and nanopowders of an n-type material with stoi-chiometry Bi2Te2.7Se0.3 were prepared. In the gasatomization process, melts having the correct stoi-chiometry are forced through an orifice into super-sonic streams of inert gas. Cooled particles arecollected and the resulting powders analyzed forparticle size and grain distribution.

There are a variety of process parameters thatcan be adjusted when using gas atomization tomake powder. For our study, the focus was on twothermoelectric compounds (Bi2Te3 and Bi2-

Te2.7Se0.3), atomized with two types of gases, argonand helium. Powders were prepared using a 10-kgHermiga gas atomizer using process parametersgiven in Table I. We analyzed the resulting powderswith x-ray diffraction (XRD), transmission electronmicroscopy (TEM), electron beam backscatter dif-fraction (EBSD), and scanning electron microscopy(SEM) to determine the nature of nanostructuringin the powder and the variations with choice ofatomizing gas. XRD was first carried out to confirmthat the desired material alloy was obtained (itwas). Once the alloy was determined to be of theright crystal structure, we proceeded to examine thepowders in the SEM. For all four combinations ofchemistry and atomization gas, we noted a widerange of particle size. For both alloys, the shape andmakeup of the powders was related to the choice ofatomization gas. For both gases, particles of all sizesare easily found, but the helium atomized particlesconsistently show more jagged surfaces andattachment of smaller particles to the surfaces.Figure 1 shows the general character of argonatomized Bi2Te3, and Fig. 2 shows the generalcharacter of helium atomized Bi2Te3.

With all four combinations of material and gas wefound nano- and microsized grains ranging from

amorphous (no grain) to 1 lm. Areas of some pow-der particles were amorphous as determined byelectron diffraction in TEM. Figures 3 and 4 show a

Table I. Process parameters used in gasatomization

AlloyAtomizing

GasPressure

(bar)

MeltTemp.

(�C)

MassFlow

Gas/Powder

Bi2Te3 Helium 30 800 0.72Bi2Te3 Argon 40 800 3.86Bi2Te2.7Se0.3 Helium 45 800 4.88Bi2Te2.7Se0.3 Argon 30 850 0.22

Fig. 1. TEM image of argon atomized Bi2Te3.

Fig. 2. TEM image of helium atomized Bi2Te3.

Beck, Alvarado, Nemir, Nowell, Murr, and Prasad1596

particle with amorphous character and its electrondiffraction pattern.

To examine the smallest powder particles moreclosely, TEM was used to determine if there aregrains that are in the size range of interest. We alsotook diffraction patterns to estimate the crystallin-ity of the individual grains. We found that, espe-cially for particles below about 50 nm, the particlemakeup was highly irregular. Some particles werecompletely amorphous. Grains of any size could beeasily found. It is difficult to determine the actualdistribution of shapes, but from manually scanningthrough the samples, we determine that, for bothtypes of powder, many more of the very small par-ticles are produced when using helium as an atom-ization gas. This is a reasonable result since thethermal conductivity of helium is higher than thatof argon and so the cooling rate of the atomized meltis greater.

To analyze the larger particles, EDAX Corpora-tion performed detailed electron backscatter dif-fraction (EBSD) studies. This method allows directimaging of the grains, their sizes, and their relativespatial orientations. Since we already know that thesmall particles can have almost arbitrarily smallgrain size, we are interested in the grain sizes,shapes, size distribution, and orientation of thelarger particles. Figure 5 depicts an EBSD analysisof Bi2Te2.7Se0.3 powder that was helium atomized.The step size was set to 200 nm. The grain orien-tation for each pixel is color coded, showing at aglance the inner grain structure of the particles.Black areas are areas for which the resolution isinsufficient to make a determination. Consequently,these areas could be voids, very small grains, oreven amorphous areas such as the bonding resin

used to prepare a sample from powder. We find that,independent of magnification level, the picture lookssimilar, with grains ranging from very large tosmall enough to hit the limit of the instrumentresolution. Below the EBSD resolution, as noted inFig. 4, grains exist down to the amorphous limit.This, then, shows a very wide grain size distributionfor our gas atomized powders.

The EBSD software can extract the grain sizedirectly and plot the distribution. Figure 6 shows

Fig. 3. TEM picture of a large particle. Fig. 4. Diffraction pattern from particle in Fig. 3 showing an amor-phous characteristic.

Fig. 5. EBSD for grain orientation in Bi2Te2.7Se0.3 gas atomizedpowder, 200 nm step size.

Shock-Wave Consolidation of Nanostructured Bismuth Telluride Powders 1597

the grain size distribution corresponding to theregion depicted in Fig. 5. It is notable that the countof the grains at smaller size is increasing as thegrain size decreases. We find that this trend con-tinues as we decrease the step size for the EBSDanalysis further. For our samples, down to about100 nm, for any arbitrary grain size, smaller grainsoutnumber bigger grains. When using a step size of200 nm, the calculated average grain size by EBSDis 854 nm. When using a step size of 50 nm, we getan average of about 200 nm. So even when exclud-ing the very small particles, the average particlesize of the powders is well below 1 lm. EBSD can beperformed down to a step size of 10 nm.

The preceding analysis indicates that the powderscontain significant proportions of various grain sizesdown to amorphous areas. There is much room inthe manufacturing method to decrease the averagegrain size. The atomization gas can be precooled,and we are exploring ways to control the yield ofnanopowder grain sizes.

SHOCK-WAVE CONSOLIDATIONTO MAKE A BULK MATERIAL

Making a bulk nanostructured material can bechallenging. When bulk material is prepared from amelt, there can be significant grain growth duringcooling. Likewise, hot pressing or sintering a greencompact will cause grain growth, particularly if it isfor long times or at high temperature. Explosiveshock-wave consolidation to convert powders to abulk material is an alternative. Using explosives forpowder compaction was described at least as earlyas a 1958 patent filing.3 Using a cylindricalarrangement for hard and difficult-to-consolidateceramic powders was reported in the 1980s.4 Morerecently, explosives have been used to consolidateAl-7.5%Mg nanopowders in order to produce adense, high-strength nanostructured bulk materialwith minimal grain growth.5

We are using explosive consolidation to producethe mechanical bonding of bismuth telluride nano-powders. In this process, nanopowder is loaded intoa tube and precompacted. Explosives are then usedto consolidate this powder into a solid. The explosivethat we use is an ammonia nitrate/fuel oil (ANFO)mixture which is 6% fuel oil by weight. This is astandard mixture that is used in commercial miningand blasting. A C3 Primasheet� is used as a boosterto detonate the ANFO. Consolidation occurs in amatter of microseconds with the resulting highpressures causing the powders to bond without themelt/recrystallization that leads to grain growth.The setup shown in Figs. 7, and 8 depicts theprocedure.

The explosive consolidation has its own processparameters which have to be determined for eachcandidate material. The powder particle size dis-tribution, density, and green compaction density areimportant. Equally important is the geometry of theexplosion, the ratio of the mass of the explosives tothe mass of the material to be consolidated, and thedetonation velocity of the explosive. The process isiterative and consists of a series of experiments,with the results from each experiment guidingparameter adjustment. From the standpoint ofmechanical bonding, features that one looks for areso-called mach stems, cracking, incomplete consoli-dation, and melting. Each of these features points tothe need for an adjustment of the explosive setup.

The primary questions to be answered in thisphase of the research were:

1. Does the bismuth telluride material systembehave as typical powders do with respect toexplosive consolidation? If so, then well-estab-lished procedures can be used to tune themethod.

2. Is there a difference between the microstruc-tured powders and the nanostructured powderswhen it comes to consolidation parameters?

Fig. 6. Grain size distribution. Fig. 7. Inner tube holds nanopowder; outer tube holds explosive.

Beck, Alvarado, Nemir, Nowell, Murr, and Prasad1598

To be able to answer question 2, we also preparedexplosive consolidations of ball-milled (rather thangas atomized) powders that were sorted to 100 meshsize.

The standard method for explosive consolidationis to start with some sample explosions and thenmap the result to the graph in Fig. 9.6 This willyield the desired mechanical consolidation. Theexplosion parameters may be fine-tuned in a secondoptimization step after good mechanical bondinghas been achieved.

Intuitively, one may think more explosive is bet-ter. However, this is not so, as it can result in adestroyed sample. In the process of consolidatingthe material, there is plastic deformationof the solids, that is, particle deformation pre

compaction and bulk material deformation postcompaction. A shock wave is a traveling wave thatconsists of a leading compressive region followed bya tensile region. The shock wave meets in the centerof the pipe and is reflected back out. As the suddenpulse is applied to the powder particles, it causesparticle surface heating, allowing interfacial melt-ing while the particle interiors remain relativelycool. The trick is to design the explosives so that inthe compressive wave, the particles receive suffi-cient energy to bond but there is not so much energythat the following tensile wave tears them apart.Even with the right amount of explosive we can finda mach stem (hole) running down the center of thepipe, indicating that we have a nonuniform pressurewave in the pipe with the pressure increasingtowards the center, leading to the deposition of toomuch energy and resulting in particle melting and

Fig. 9. Standard map of explosive consolidation parameters withconsolidated Bi2Te3 sample outcomes corresponding to their placeon the map.6Fig. 8. Shock-wave consolidation procedure.

Fig. 10. Succession of explosive Bi2Te3 nanopowder consolidations.

Shock-Wave Consolidation of Nanostructured Bismuth Telluride Powders 1599

crystal growth upon cooling. All of these phenomenaand their causes are well known and can beadjusted; for example, reducing the mass of explo-sive can remove the mach stem but still yieldexcessive melting and cracking. A further reductionin explosive mass can reduce the melting altogether,and reduction in explosive velocity can eliminatethe cracking of the material, yielding a very strongsolid slug. When the process is adjusted to removethese problems and provide mechanically solidsamples, the microstructure can finally be analyzedwith the usual methods (SEM, etc.). Good consoli-dation (good mechanical bonds) does not guaranteegood interparticle bonding. Further adjustments ofthe process are needed to achieve that. After suc-cessive iterations of explosive consolidation, wewere able to get a good mechanical bond for allbismuth telluride powders. Figure 10 shows a suc-cession of experiments that yielded a mechanicallysound sample. For the sample on the far left, thereis incomplete bonding. For the middle sample, thereis a significant mach stem and grain growth. Therightmost sample illustrates good consolidationwith minimal grain growth.

CONCLUSIONS

The question of whether the bismuth telluridesystem behaves predictably with respect to explo-sive consolidation has been answered in the affir-mative. We can be confident that the same processof successive refinement that allows for consolida-tion of other powders will yield a parameter set that

will consolidate bismuth telluride nanopowderswithout excessive grain growth. Since we have foundthe explosive parameters for good mechanical con-solidation, the next step is to fine-tune theseparameters to get good intergrain bonding as well,something that is critical for establishing good elec-trical conductivity and homogeneous thermoelectricproperties. The procedures to do this are well knownand researched; for example, metallic glasses havebeen successfully bonded with explosive consolida-tion while maintaining their amorphous structure.7

The method is not limited to bismuth telluride, andwork on transition-metal chalcogenides and silicidesis in the beginning stages.

ACKNOWLEDGEMENTS

This work was supported by NASA under Con-tracts #NNX09CF76P and NNX10CB69C.

REFERENCES

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(1960).4. M.A. Meyers, N.N. Thadhani, and L. Yu, Shock Waves for

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