Thermoelectric properties of nanostructured half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01 were characterized before and after 2.5 MeV proton irradiation. A unique high-sensitivity scanning thermal microprobe was used to simultaneously map the irradiation effect on thermal conductivity and Seebeck coefficient with spatial resolution less than 2 μm. The thermal conductivity profile along the depth from the irradiated surface shows excellent agreement with the irradiation-induced damage profile from simulation. The Seebeck coefficient was unaffected while both electrical and thermal conductivities decreased by 24%, resulting in no change in thermoelectric figure of merit ZT. Reductions in thermal and electrical conductivities are attributed to irradiation-induced defects that act as scattering sources for phonons and charge carriers

Chinnathambi, Karthik

Jaques, Brian J.

English

Boise State University - ScholarWorks

Boise State UniversityScholarWorksMaterials Science and Engineering FacultyPublications and Presentations Department of Materials Science and Engineering6-11-2018Proton Irradiation Effect on ThermoelectricProperties of Nanostructured N-Type Half-HeuslerHf0.25Zr0.75NiSn0.99Sb0.01Karthik ChinnathambiBoise State UniversityBrian J. JaquesBoise State UniversityCopyright (2018) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of theauthor and the American Institute of Physics. The following article appeared in:Kempf, N., Karthik, C., Jaques, B.J., Gigax, J., Shao, l., Butt, D.P.,... Zhang, Y. (2018). Proton Irradiation Effect on Thermoelectric Properties ofNanostructured N-Type Half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01. Applied Physics Letters, 112(24), 243902.and may be found at doi: 10.1063/1.5025071Proton irradiation effect on thermoelectric properties of nanostructuredn-type half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01Nicholas Kempf,1 Chinnathambi Karthik,2 Brian J. Jaques,2 Jonathan Gigax,3 Lin Shao,3Darryl P. Butt,4 Ran He,5 Dezhi Wang,5 Zhifeng Ren,5 and Yanliang Zhang1,a)1Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame,Indiana 46556, USA2Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725-2090, USA3Department of Nuclear Engineering, Texas A&M University, College Station, Texas 77843-3128, USA4College of Mines and Earth Sciences, University of Utah, Salt Lake City, Utah 84112, USA5Department of Physics and the Texas Center for Superconductivity, University of Houston, Houston,Texas 77204, USA(Received 6 February 2018; accepted 25 May 2018; published online 13 June 2018)Thermoelectric properties of nanostructured half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01 were character-ized before and after 2.5MeV proton irradiation. A unique high-sensitivity scanning thermal micro-probe was used to simultaneously map the irradiation effect on thermal conductivity and Seebeckcoefficient with spatial resolution less than 2 lm. The thermal conductivity profile along the depthfrom the irradiated surface shows excellent agreement with the irradiation-induced damage profilefrom simulation. The Seebeck coefficient was unaffected while both electrical and thermal conduc-tivities decreased by 24%, resulting in no change in thermoelectric figure of merit ZT. Reductionsin thermal and electrical conductivities are attributed to irradiation-induced defects that act as scat-tering sources for phonons and charge carriers. Published by AIP Publishing.https://doi.org/10.1063/1.5025071Thermoelectric materials have promising applications inenvironments with high radiation flux, such as those innuclear power plants and in space. For instance, in-pile ther-moelectric devices can generate electricity from thermalenergy to enable self-powered sensors for in-situ monitoringof critical parameters of the nuclear reactor pressure vesseland nuclear fuel assembly. In dry storage casks, thermoelec-tric devices can be used to power wireless sensor networksto actively monitor temperature and weld stability withoutcostly power and data cable installation.1 In space, radioiso-tope thermoelectric generators can provide steady power fordecades and have been used on Mars and on deep-spaceprobes.2 Thermoelectric material performance is determinedby the thermoelectric figure of merit ZT¼ a2rT/(je þ jl),where a is the Seebeck coefficient, r is the electrical conduc-tivity, T is the absolute temperature, and je and jl are theelectronic and lattice components of thermal conductivity,respectively.While there have been some studies of the irradiationeffect on thermoelectric materials in nuclear reactors, mosthave been performed on long-established bulk materialssuch as germanium telluride, bismuth telluride, and lead tel-luride.3–5 The combination of thermal and fast neutron irra-diation at relatively low temperature (<200 C) created pointdefects in these materials, resulting in reduced carrier mobil-ity and lower ZT.Nanostructured half-Heusler alloys are promising thermo-electric materials for nuclear applications due to their environ-mental friendliness, high-temperature mechanical and thermalstability, and increased ZT compared to their bulk counter-parts.6–9 In addition to significant ZT enhancement realizedthrough nanostructuring, these materials have potentiallyimproved radiation tolerance due to high-density nanostruc-tures and grain boundaries.7–18 Nevertheless, there have beenno reports of the irradiation effect on state-of-the-art nano-structured thermoelectric materials.Protons and other ions are often used as surrogates forneutrons as they offer relatively high damage rates and supe-rior control over dose and energy without rendering the mate-rial radioactive. However, the depth of damage caused by ionirradiation is typically limited to a few hundreds of nano-meters for heavy ions and up to several tens of microns forlight ions like protons. Since the irradiated layer is so thin,surface-sensitive methods must be used to characterize mate-rial properties in the damaged region. Additionally, the dam-age profile from ion irradiation is not uniform within this thinlayer. For light ions, damage in the leading region is relativelyuniform but is followed by a peak near the end of the cascadewhere substantial damage accumulates.19,20 To date, all ionirradiation studies on thermoelectric materials have imple-mented bulk property measurements which fail to uncover thedamage profile in the thin irradiated region.21–25 In this work,microscale thermal conductivity and Seebeck coefficient aremapped as a function of depth from the irradiated surface.The damage profile is compared to the irradiation-inducedvacancy concentration calculated with Transport of Ions inMatter (TRIM) simulation.26The effect of 2.5MeV proton irradiation on nanostruc-tured half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01 is reportedherein. Thermal conductivity, electrical conductivity, andSeebeck coefficient were characterized before and afterproton irradiation. A unique scanning thermal microprobetechnique with unprecedented sensitivity27,28 was usedto simultaneously characterize thermal conductivity anda)Email: yzhang45@nd.edu. Telephone: þ1-5746316669.0003-6951/2018/112(24)/243902/4/$30.00 Published by AIP Publishing.112, 243902-1APPLIED PHYSICS LETTERS 112, 243902 (2018)Seebeck coefficient with<2lm spatial resolution. In con-junction, standard measurement techniques were used tocharacterize and corroborate the macro- and microscale ther-moelectric properties before and after irradiation. X-ray dif-fraction (XRD) and transmission electron microscopy(TEM) were used to compare the microstructure before andafter irradiation.First, thermal diffusivity was measured by laser flash androom-temperature thermal conductivity was found to be5.31W/mK for the non-irradiated sample. In preparation forirradiation, a 1mm  17mm bar was cut from the disk andmasks were used to block protons in selected regions of thebar. Samples were irradiated with 2.5MeV protons to a flu-ence of 2 1016/cm2 with 100 nA current—an energy anddose intermediate of those reported in other proton irradiationstudies on thermoelectric materials.24,25 After irradiation,scanning thermal microscopy (SThM) was performed on boththe irradiated and non-irradiated surfaces at the same time,eliminating uncertainties associated with measurements takenat different times due to potential surface oxidation and con-tamination. Figure 1 shows SThM results from the incidentsurface of the selectively irradiated sample. The irradiatedregions show a 14% reduction in thermal conductivity com-pared with the non-irradiated regions. On the other hand, thereis no statistically significant difference in the Seebeck coeffi-cient between the irradiated and non-irradiated regions.Next, the bar was cut and polished to expose a cross sec-tion containing both the irradiated region near the incidentsurface and the bulk, non-irradiated region. Figure 2 showsthe SThM results mapping thermal conductivity and Seebeckcoefficient as a function of depth from the irradiated surfacealong with the damage profile calculated using TRIMsimulation.There is a clear reduction of thermal conductivity in thedamaged region to a depth of 40 lm, with the apparent peakin damage occurring at a depth of 34 lm—an excellentagreement with the damage profile calculated using TRIM.The profile of property change is a characteristic shape withtwo distinct regions: (1) a relatively uniform reduction inthermal conductivity up to 30lm from the surface and (2) asudden decrease in thermal conductivity from 31 to 34 lm.Beyond 35 lm, the thermal conductivity quickly recovers tothe non-irradiated value as the irradiation damage decays tonaught. Thermal conductivity decreased from 5.4W/mK inthe non-irradiated region to 4.1W/mK in the irradiatedregion up to 30 lm deep. There is no statistically significantchange in Seebeck coefficient, in agreement with the inci-dent surface SThM results.Based on the damage depth measured with SThM, a filmof 35 lm thickness was prepared so that the entire depth ofthe film was irradiated. The bulk electrical conductivity andSeebeck coefficient of the film were measured before andafter irradiation (shown in Fig. 3).The decrease in electrical conductivity with increasingtemperature is due to the high level of doping which rendersthe semiconducting nature of this material degenerate.Similarly, the increase in Seebeck coefficient with increasingtemperature is characteristic of highly doped n-type halfHeusler materials.8,9,16 Electrical conductivity decreased by24% at room temperature and 17% at 200 C. The Seebeckcoefficient changed negligibly at room temperature andincreased by 1.5% at 200 C; however, this small change iswell within measurement uncertainty. This co-validates theSeebeck coefficient obtained from the incident surface andcross-sectional SThM.XRD was performed on non-irradiated and irradiatedsections of the selectively irradiated bar. Figure 4(a) showsno conspicuous difference between the diffraction patternsof irradiated and non-irradiated sections, indicating nodetectable phase change due to the proton irradiation.Additionally, the diffraction patterns match closely with thecataloged phases, indicating excellent phase purity of theHf0.25Zr0.75NiSn0.99Sb0.01 sample. However, when comparedto those of the non-irradiated region, the two largestFIG. 1. Thermal conductivity and Seebeck coefficient of the selectively irra-diated Hf0.25Zr0.75NiSn0.99Sb0.01 surface obtained using SThM. Dashed linesindicate boundaries between irradiated and non-irradiated regions.FIG. 2. (a) Thermal conductivity and (b) Seebeck coefficient as a functionof depth from the irradiated surface of the Hf0.25Zr0.75NiSn0.99Sb0.01 sampleobtained using SThM on the cross section of the irradiated bar. The redcurve represents the concentration of irradiation-induced vacancies calcu-lated using TRIM simulation assuming a displacement energy of 25 eV foreach atom.29 TRIM predicts 1 replacement collision for every 49 vacanciesgenerated.FIG. 3. Electrical conductivity and Seebeck coefficient of theHf0.25Zr0.75NiSn0.99Sb0.01 film before and after proton irradiation.243902-2 Kempf et al. Appl. Phys. Lett. 112, 243902 (2018)diffraction peaks from the irradiated region show a 1.3% and1.8% increase in full width at half maximum and a 14% and18% decrease in integrated intensity at 2h ¼ 29:3 and41:8, corresponding to 200ð Þ and 220ð Þ, respectively. Onthe other hand, there is no measureable shift in the peak posi-tion. Considering that the XRD patterns were obtainedsequentially from adjacent regions near the center of theselectively irradiated bar with no difference in grain sizebetween regions, the peak broadening observed in the irradi-ated diffraction pattern indicates some degree of non-uniform microstrain not present in the non-irradiatedregion,30 which is congruent with the theory that the protonirradiation generated point defects such as vacancies andinterstitials.TEM was performed on non-irradiated and irradiatedspecimens prepared from the same sample. Specimens weremechanically thinned to<20 lm and then ion milled withArgon to<100 nm. The TEM images correspond to a depthof approximately 20 lm from the irradiated surface (in thenearly constant region of the damage profile). Figure 4(b)shows a typical high-resolution TEM image and selectedarea electron diffraction (SAED) pattern of an irradiatedspecimen. TEM images and SAED patterns before and afterirradiation are fundamentally identical, revealing a highdegree of crystallinity with no indication of local amorphiza-tion or transformation of the crystal structure, confirmingthat no extended defects were generated by the protonirradiation.Seebeck coefficient is sensitive to the carrier concentra-tion, as indicated by the expression for Seebeck coefficientin heavily doped semiconductorsa ¼ 8p2k2B3eh2mTp3n 2=3: (1)Here, kB is Boltzmann’s constant, e is the carrier charge, h isPlanck’s constant, m is the carrier effective mass, and n is thecarrier concentration.7,32 Since electrical conductivity decreasedwhile Seebeck coefficient remained unchanged over the entiretemperature range of measurement (Fig. 3), carrier mobilitywas markedly diminished by irradiation while changes in car-rier concentration were insignificant.To determine the relative influence of proton irradiationon lattice and electronic thermal conductivity, the Lorenznumber was taken to be the Sommerfeld value. Using roomtemperature thermal conductivity from cross-section SThM,the electronic and lattice components of thermal conductivitywere 1.11 and 4.25W/mK before irradiation, respectively.After irradiation, the values were 0.83 and 3.25W/mK,resulting in an 24% reduction in both electronic and latticethermal conductivity.In an unlikely coincidence, the precise irradiation doseand energy resulted in nearly uniform mobility suppressionof phonons and charge carriers in nanostructuredHf0.25Zr0.75NiSn0.99Sb0.01. Irradiation-induced point defects,such as vacancies, substantially reduce charge carrier mobil-ity via local charge disorder.33 At the same time, suchdefects create mass and strain fluctuations which scatter pho-nons and reduce the phonon mean free path.34 While thisnanostructured half-Heusler features nanograins which scat-ter long wavelength phonons,8,35 the comparably high den-sity of irradiation-induced point defects effectivelysuppresses phonons of shorter wavelength, further decreas-ing lattice thermal conductivity.In summary, 2.5MeV room temperature proton irradia-tion of nanostructured Hf0.25Zr0.75NiSn0.99Sb0.01 resulted ina 24% reduction in electrical and thermal conductivities withno change in Seebeck coefficient. The proton irradiationcaused no extended defects but introduced point defects thatact as scattering sources for phonons and charge carriers,lowering mobility. The combined effect of these propertychanges yielded no change in the thermoelectric figure ofmerit. Nevertheless, these property changes must be consid-ered when using this material in radioactive environments,as the change in conductivities alters the material’s in-situthermal and electrical profiles.This material is based upon work supported under anIntegrated University Program Graduate Fellowship. Thiswork was funded by the U.S. Department of Energy, Officeof Nuclear Energy, under Award No. DE-NE0008255. N.K.acknowledges financial support from U.S. DOE NEUPfellowship support.1D. A. Clayton, W. H. J. Andrews, and R. Lenarduzzi, ORNL, Report No.ORNL/TM-2012/442 (2012), p. 442.2T. C. Holgate, R. Bennett, T. Hammel, T. Caillat, S. Keyser, and B.Sievers, J. Electron. Mater. 45, 6044 (2016).3J. C. Danko, G. R. Kilp, and P. V. Mitchell, Adv. Energy Convers. 2, 79(1962).4J. Vandersande, J. McCormack, and A. Zoltan, Intersociety EnergyConversion Engineering Conference (1990), p. 392.5M. Idnurm and K. Landecker, Br. J. Appl. Phys. 18, 1209 (1967).6Y. L. Zhang, M. Cleary, X. W. Wang, N. Kempf, L. Schoensee, J. Yang,G. Joshi, and L. Meda, Energy Convers. Manage. 105, 946 (2015).7S. Chen and Z. F. Ren, Mater. Today 16, 387 (2013).8S. Chen, K. C. Lukas, W. S. Liu, C. P. Opeil, G. Chen, and Z. F. Ren,Adv. Energy Mater. 3, 1210 (2013).9H. Zhang, Y. M. Wang, K. Dahal, J. Mao, L. H. Huang, Q. Y. Zhang, andZ. F. Ren, Acta Mater. 113, 41 (2016).10J. Yang, G. P. Meisner, and L. Chen, Appl. Phys. Lett. 85, 1140 (2004).11Y. T. Liu, H. H. Xie, C. G. Fu, G. J. Snyder, X. B. Zhao, and T. J. Zhu,J. Mater. Chem. A 3, 22716 (2015).12Q. Shen, L. Chen, T. Goto, T. Hirai, J. Yang, G. P. Meisner, and C. Uher,Appl. Phys. Lett. 79, 4165 (2001).13S. Sakurada and N. Shutoh, Appl. Phys. Lett. 86, 082105 (2005).14G. Joshi, T. Dahal, S. Chen, H. Z. Wang, J. Shiomi, G. Chen, and Z. F.Ren, Nano Energy 2, 82 (2013).FIG. 4. (a) Results of XRD showing the diffraction pattern of irradiated(top/orange) and non-irradiated (middle/blue) Hf0.25Zr0.75NiSn0.99Sb0.01along with the cataloged half-Heuslers (bottom/red). The small peak indi-cated by a diamond is associated with the half-Heusler phase, reported asunknown.31 (b) High-resolution TEM image along the [001] zone axis ofproton-irradiated Hf0.25Zr0.75NiSn0.99Sb0.01 with the inset showing theselected area electron diffraction (SAED) pattern.243902-3 Kempf et al. Appl. Phys. Lett. 112, 243902 (2018)15L. H. Huang, Q. Y. Zhang, B. Yuan, X. Lai, X. Yan, and Z. F. Ren, Mater.Res. Bull. 76, 107 (2016).16G. Joshi, X. Yan, H. Z. Wang, W. S. Liu, G. Chen, and Z. F. Ren, Adv.Energy Mater. 1, 643 (2011).17A. Bhardwaj, D. K. Misra, J. J. Pulikkotil, S. Auluck, A. Dhar, and R. C.Budhani, Appl. Phys. Lett. 101, 133103 (2012).18H. H. Xie, H. Wang, Y. Z. Pei, C. G. Fu, X. H. Liu, G. J. Snyder, X. B.Zhao, and T. J. Zhu, Adv. Funct. Mater. 23, 5123 (2013).19M. J. Swenson and J. P. Wharry, J. Nucl. Mater. 467, 97 (2015).20K. Nordlund, J. Keinonen, M. Ghaly, and R. S. Averback, Nature 398, 49(1999).21G. S. Fu, L. Zuo, J. Lian, Y. Q. Wang, J. Chen, L. T. Jon, and Z. G. Xiao,Nucl. Instrum. Methods Phys. Res., Sect. B 358, 229 (2015).22T. Chang, J. Kim, M. J. Song, and W. Lee, AIP Adv. 5, 057101(2015).23J. W. Roh, D. H. Ko, J. Kang, M. K. Lee, J. H. Lee, C. W. Lee, K. H. Lee,J. S. Noh, and W. Lee, Phys. Status Solidi A 210, 1438 (2013).24P. Chaudhari and M. B. Bever, J. Appl. Phys. 37, 4181 (1966).25A. Saji, S. Ampili, S. H. Yang, K. J. Ku, and M. Elizabeth, J. Phys.:Condens. Matter 17, 2873 (2005).26J. F. Ziegler, M. D. Ziegler, and J. P. Biersack, Nucl. Instrum. MethodsPhys. Res., Sect. B 268, 1818 (2010).27N. Kempf and Y. Zhang, “A robust high-sensitivity scanning thermalprobe for simultaneous microscale thermal and thermoelectric propertymapping” (unpublished).28Y. L. Zhang, C. L. Hapenciuc, E. E. Castillo, T. Borca-Tasciuc, R. J.Mehta, C. Karthik, and G. Ramanath, Appl. Phys. Lett. 96, 062107(2010).29A. Y. Konobeyev, U. Fischer, Y. A. Korovin, and S. P. Simakov, Nucl.Energy Technol. 3, 169 (2017).30G. K. Williamson and W. H. Hall, Acta Metall. 1, 22 (1953).31R. V. Skolozdra, E. E. Starodynova, and Y. V. Stadnyk, Ukr. Fiz. Zh. 31,1258 (1986).32G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008).33C. Uher, J. Yang, S. Hu, D. T. Morelli, and G. P. Meisner, Phys. Rev. B59, 8615 (1999).34L. Andrea, G. Hug, and L. Chaput, J. Phys.: Condens. Matter 27, 425401(2015).35A. N. Gandi and U. Schwingenschlogl, Phys. Chem. Chem. Phys. 18,14017 (2016).243902-4 Kempf et al. Appl. Phys. Lett. 112, 243902 (2018)

Proton Irradiation Effect on Thermoelectric Properties of Nanostructured N-Type Half-Heusler Hf\u3csub\u3e0.25\u3c/sub\u3eZr\u3csub\u3e0.75\u3c/sub\u3eNiSn\u3csub\u3e0.99\u3c/sub\u3eSb\u3csub\u3e0.01\u3c/sub\u3e

https://scholarworks.boisestate.edu/cgi/viewcontent.cgi?article=1345&amp;context=mse_facpubs

Proton Irradiation Effect on Thermoelectric Properties of Nanostructured N-Type Half-Heusler Hf\u3csub\u3e0.25\u3c/sub\u3eZr\u3csub\u3e0.75\u3c/sub\u3eNiSn\u3csub\u3e0.99\u3c/sub\u3eSb\u3csub\u3e0.01\u3c/sub\u3e

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