In a multi component nanocrystalline material, crystallites and interfacial nanostructures form a disordered morphology. In such a disordered nanostructured material, the energy disruption for both electrons and phonons can happen in a time scale that is smaller or comparable to the energy relaxation time. Therefore, the charge and heat carriers can maintain an energetically non-equilibrium distribution that can result in fundamentally different electrical and thermal properties than in the crystalline materials.

The concept of multi-phase nanocrystalline (or nanocomposite) bulk alloys represents a radical alternative to conventional homogeneous bulk materials. Nanocomposites are considered new materials different from their parent materials. Although they consist of similar atomic structures as the constituent elements, using size as a parameter effectively provides “new” materials in terms of their physical properties. The ability to tune the property of the multi-phase nanocrystalline material by controlling the material composition, crystallite sizes and distribution as well as manipulating the interface potential barrier height by alloying without deteriorating the nanocrystalline quality offers extra degrees of freedom for creating material structures with novel electrical properties.

Such new characteristics offer a landscape for discovering new material systems for engineering applications. For instance, maintaining the energy of hot electrons has been a long standing challenge in low band-gap photovoltaics (PV). A low band-gap PV material can harvest a larger part of the solar spectra; however, the electrons excited by high energy photons relax to the band edge giving up their extra energy to phonons. Therefore, the extra energy is wasted as heat.

A similar motivation exists for enhancing the power factor in thermoelectric materials. Instead of focusing on semiconducting materials with a highly asymmetric density of states close to the Fermi level to increase the average energy of charge carriers (hence the Seebeck coefficient), the multi-phase nanocrystalline approach can offer a new way via the non-equilibrium transport of charge carriers. This method alleviates the fundamental trade-off between the electrical conductivity and the Seebeck coefficient. The high electron density in the metal (>1022/cm-3) compensates for the negligible contribution to the conductivity from the majority of the electrons in the metal that have energies below the top of the interface potential barrier.

Over the last several years we have grown and tested over a thousand of nanostructured thermoelectric material samples. These materials often with different morphologies or alloy composition ratios have been characterized and provided us with valuable information. For examples, in order to enhance the TE properties of β-FeSi2, we developed nanocomposite of FeSi2 with SiGe in different composition ratios. Our materials have shown two fold enhancement in ZT of this material structure. For high temperature applications, we focused on several materials including higher manganese silicide (HMS), and SiGe. We also performed an extensive research on nanocomposite of Mg2Si-SiGe, which showed promising ZT enhancement. On another direction we worked on finding optimum electrical contact to HMS. We investigated several materials including Co, Ni, Cr, Ti, Mo, MnSi, MoSi2, and TiSi2 in search of the best contact material to HMS.

Our experimental data has shown that the nanocomposite of these metal silicides and SiGe can significantly enhance the thermoelectric power factor of SiGe by enhancing the Seebeck coefficient and/or reducing the thermal conductivity. Figure 1 plots the ZT of some of our recent samples showing the enhancement of figure-of-merit in both the p-type and n-type materials. Nanocomposite of (Bi,Sb)2Te3, α-FeSi2/SiGe, Mg2Si/SiGe, and nanocrystalline (NC) SiGe compared with that of RTG SiGe used in NASA space crafts are shown. The data shows that in all nanocomposite samples ZT is enhanced. We have also observed a significant improvement in ZT of p-type SiGe nanocomposite materials accounting for ~80% improvement in SiGe compared to its single crystalline bulk structures. For close to room temperature applications, we also worked on nanocomposite of BiTe-SbTe and achieved the highest reported ZT of this material structure. This research was funded by Air Force Office of Scientific Research and National Science Foundation.

 

Selected Representative Publications

  1. Enhancement of thermoelectric power factor of silicon germanium films grown by electrophoresis deposition, Amin Nozariasbmarz, Armin Tahmasbi Rad, Zahra Zamanipourb, Jerzy S. Krasinskib, Lobat Tayebia, Daryoosh Vashaee, Scripta Materialia, ISSN 1359-6462, http://dx.doi.org/10.1016/j.scriptamat.2013.06.025 (2013).
  2. The effect of phase heterogeneity on thermoelectric properties of nanostructured silicon germanium alloy, Zahra Zamanipour, Payam Norouzzadeh, Jerzy S. Krasinski, Lobat Tayebi, Daryoosh Vashaee, J. Appl. Phys. 114, 023705 (2013); http://dx.doi.org/10.1063/1.4813474.
  3. Comparison of boron precipitation in p-type bulk nanostructured and polycrystalline silicon germanium alloy, Zahra Zamanipour, Jerzy S. Krasinski, and Daryoosh Vashaee, J. Appl. Phys. 113, 143715 (2013); doi: 10.1063/1.4801388
  4. The effect of synthesis parameters on transport properties of nanostructured bulk thermoelectric p-type silicon germanium alloy, Zahra Zamanipour, Xinghua Shi, Arash M. Dehkordi, Jerzy S. Krasinski, Daryoosh Vashaee, Physica Status Solidi (b), DOI: 10.1002/pssa.201228102 (2012)
  5. An investigation of electrical contacts for higher manganese silicide, Xinghua She, Zahra Zamanipour, Jerzy S. Krasinski, Alan Tree and Daryoosh Vashaee, Journal of Electronic Materials, Volume 41, Number 9 (2012), 2331-2337
  6. Enhancement in thermoelectric power factor of polycrystalline Bi0.5Sb1.5Te3 by crystallite alignment, Arash Mehdizadeh Dehkordi, Daryoosh Vashaee, Physica Status Solidi (a), DOI: 10.1002/pssa.201228147 (2012)
  7. Comparison of thermoelectric properties of p-type nanostructured bulk Si0.8Ge0.2 alloy with Si0.8Ge0.2composites embedded with CrSi2 nano-inclusisons, Zahra Zamanipour and Daryoosh Vashaee, J. Appl. Phys. 112, 093714 (2012) http://dx.doi.org/10.1063/1.4764919
  8. Synthesis, characterization, and thermoelectric properties of nanostructured bulk p-type MnSi1.73, MnSi1.75, and MnSi1.77,       Z. Zamanipour, X. Shi, M. Mozafari , J. S. Krasinski, L. Tayebi, D. Vashaee, Ceramics International, (In press-2012 Available online 5 September 2012, ISSN 0272-8842, 10.1016/j.ceramint.2012.08.086. )
  9. The effect of nanostructuring on thermoelectric transport properties of p-type higher manganese silicide MnSi1.73, Payam Norouzzadeh, Zahra Zamanipour, Jerzy Krasinski, Daryoosh Vashaee, J. of Applied Physics, 112, 124308 (2012); doi: 10.1063/1.4769884
  10. Increased Phonon Scattering by Nanograins and Point Defects in Nanostructured Silicon with a Low Concentration of Germanium, Chen, G. , Zhu, G.H. , Lee, H. , Lan, Y.C. , Wang, X.W. , Joshi, G. , Wang, D.Z., Yang, J., Vashaee, D., Guilbert, H., Pillitteri, A., Dresselhaus, M.S., Ren, Z.F., Physical Review Letters, Volume 102, Issue 19, 14 May 2009, Article number 196803
  11. High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Bed Poudel, Qing Hao, Yi Ma, Yucheng Lan, Xiao Yan, Dezhi Wang, D. Vashaee, M. Dresselhaus, Gang Chen, and Z. F. Ren, , Science Express Research Articles, 20 March 2008, 10.1126/science.1156446.
  12. Thermionic Power Generation at High Temperatures Using SiGe/Si Superlattices, Daryoosh Vashaee, Ali Shakouri, J. Appl. Phys. 101, 053719 (2007).
  13. Enhancement of Thermoelectric Power Factor in SiGe-CrSi2 Composite Alloy, Zahra Zamanipour, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  14. Economical FeSi2-SiGe Composites for Thermoelectric Power Generation, Yin Liu, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  15. Thermal and Thermoelectric Properties of Nanostructured Versus Crystalline SiGe, Lobat Tayebi, Zahra Zamanipour, Masoud Mozafari, Payam Norouzzadeh, Jerzy S. Krasinski, Kenneth F. Ede, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  16. Thermoelectric Properties of Mg2Si Doped with Bi and Al with Conductive Glass Inclusion, Nikhil Satyala, Jerzy S. Krasinski, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  17. A Method to Measure the Thermal Conductivity of Thermoelectric Nanowires, Nahida Akhter, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  18. Enhancement of Thermoelectric Efficiency of MnSi1.75 with the Addition of Externally Processed Nanostructured MnSi, Xinghua Shi, Zahra Zamanipour, Kenneth F. Ede, Jerzy S. Krasinski, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  19. Cost Effective Synthesis of Bulk Thermoelectric Higher Manganese Silicide for Waste Heat Recovery and Environmental Protection, Xinghua Shi, Zahra Zamanipour, Kenneth F. Ede, Jerzy S. Krasinski, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  20. Anomalous Thermoelectric Behavior of BiSeTe Doped with SiGe:As, Oonnittan Jacob Panachaveettil, Ranji Vaidyanathan, Jerzy S. Krasinski, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  21. Differential Thermal Analysis of Nanostructured Si0.8Ge0.2 Thermoelectric Material, Lobat Tayebi, Zahra Zamanipour, Masoud Mozafari, Payam Norouzzadeh, Kenneth F. Ede, Jerzy S. Krasinski, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  22. Economical Preparation of Nanostructured Bulk (BixSb1-x)2Te3 Thermoelectrics, Arash Mehdizadeh Dehkordi, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  23. Increase of Boron Precipitation in Nanostructured P-type Silicon Germanium Thermoelectric Alloys, Zhaihui Gao, Zahra Zamanipour, Daryoosh Vashaee, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  24. Enhancement of Doping Concentration in P-Type SiGe Thermoelectric Alloy with the Addition of CrSi2,Parvaneh Rouhani, Zahra Zamanipour, Jerzy S. Krasinski, Daryoosh Vashaee, Lobat Tayebi, IEEE Green Technologies Conference 2012 – Energy Generation & Storage Technologies, Tulsa, Oklahoma, April 19-20, 2012
  25. Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach, Zhifeng Ren, Bed Poudel, Yi Ma, Yucheng Lan, Xiaowei Wang, Giri Joshi, Gaohua Zhu, Jian Yang, Bo Yu, Xiao Yan, Hui Wang, Dezhi Wang, Qing Hao, Hohyun Lee, Austin Minnich, Andrew Muto, Daryoosh Vashaee, Mildred Dresselhaus, Gang Chen, Annual American Physical Society Meeting, March 16-20, 2009, Pittsburgh, PA.
  26. Mechanism of Enhancement of Thermoelectric Figure-of-Merit in Nanocrystalline Materials, Daryoosh Vashaee, Austin Minnich, Qiang Hao, Andrew Muto, Xiaoyuan Chen, Bed Poudel, Yi Ma, Yucheng Lan, Bo Yu, Xiao Yan, Dezhi Wang, Z. F. Ren, Junming Liu, Mildred S. Dresselhaus, Gang Chen, Japan-U.S. nanotechnology symposium, July 2008, Boston, Massachusetts.
  27. Thermoelectric Transport In Silicon Germanium Nanocomposite, Hohyun Lee, Daryoosh Vashaee, Xiaowei Wang, Giri Joshi, Gaohua Zhu, Dezhi Wang, Zhifeng Ren, Sabah Bux, Richard Blair, Pawan Gogna, Jean-Pierre Fleurial, Ming Y. Tang, Mildred S. Dresselhaus, Gang Chen, ASME International Mechanical Engineering Congress and Exposition IMECE October 31 – November 6, 2008, Boston, Massachusetts.