Nano Energy (2014) 7, 97–103
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High thermoelectric performance of MgAgSb-based materials Huaizhou Zhaoa,1,2, Jiehe Suia,b,1, Zhongjia Tangc, Yucheng Lana, Qing Jiea, Daniel Kraemerd, Kenneth McEnaneyd, Arnold Guloyc, Gang Chend, Zhifeng Rena,n a
Department of Physics and TcSUH, University of Houston, Houston, TX 77204, USA National Key Laboratory for Precision Hot Processing of Metals and School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China c Department of Chemistry, University of Houston, Houston, TX 77204, USA d Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b
Received 12 April 2014; accepted 15 April 2014 Available online 26 April 2014
KEYWORDS
Abstract
MgAgSb alloys; Ball milling; Nanostructure; Thermoelectric
Bismuth telluride has been the only thermoelectric material with thermoelectric figure-ofmerit (ZT) of 1–1.5 in the temperature range of 20 and 150 1C since its discovery in 1950s. It has been primarily used for cooling even though power generation has also been attempted for hot side of 250 1C and cold side of 20 1C. Here, we report our discovery of comparable ZT values in MgAgSb-based MgAg0.97Sb0.99 and MgAg0.965Ni0.005Sb0.99. The materials are made by ball milling powders of elements in a two-step process and hot pressing the powders into dense bulk samples with grains smaller than 20 nm. The small grain size together with point defects including vacancies and antisites in the structure are the main reasons for the very low thermal conductivity of 0.7 W m 1 K 1 at room temperature. The ZT values at room temperature are close to 1 and increase with temperature to a maximum of 1.4 at 475 K. Our discovery promises a new class of thermoelectric materials with high thermoelectric performance between 20 and 250 1C. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. E-mail address:
[email protected] (Z. Ren). 1 Equal contributors. 2 Current Address: The Intitute of Physics, Chinese Academy of Sciences P.O. Box 603, Beijing 100190, China. http://dx.doi.org/10.1016/j.nanoen.2014.04.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
Introduction Over the past decades, thermoelectric materials have been extensively studied for potentially broad applications in
98 refrigeration, waste heat recovery, solar energy conversion, etc. [1–8]. The efficiency of thermoelectric devices is governed by the materials' dimensionless figure of merit ZT ¼ ðS2 s=κÞT, where S, s, T; and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To date, in spite of the substantial improvements on the thermoelectric performance on many materials such as Bi2Te3 and its alloys [1,2,4], PbTe [9–11], PbSe [12], skutterudites [13,14], SiGe alloys [15,16], half-Heuslers [17,18], Bi2Te3 having ZT of 1 since its first discovery in 1950s remain to be the only material near room temperature for applications in cooling and low temperature waste heat recovery. The synthesis and crystal structure of MgAgSb was briefly studied before [19,20], but there had been no study on its thermoelectric properties till recently by Kirkham et al. reporting a ZT of 0.35 at room temperature [21]. Kirkham et al. reported that the crystal structure below 610 K is tetragonal, with a space group of I4c2 and the synthesis of pure MgAgSb phase is so challenging that they did not get phase pure samples using the conventional synthesis method. We found that phase pure materials can be achieved if the composition is a little bit Ag and Sb deficient, e.g., MgAg0.97Sb0.99, through a two-step ball milling and hot pressing method. We also found that the grains in the samples made by ball milling and hot pressing is smaller than 20 nm, which is much smaller than those in other thermoelectric materials made by the same method [4,15,18,22]. Because of the higher power factor from the higher phase purity, and lower thermal conductivity due to the small grain size and point defects of vacancies and antisites,we achieved ZT of 0.7 at 300 K and 1.2 at 450 K for MgAg0.97Sb0.99. By further alloying the Ag site using a very small amount of Ni, e.g., MgAg0.965Ni0.005Ag0.99, we obtained ZT of 1 at 325 K and 1.4 at 450 K.
H. Zhao et al. milling Mg with Ag first and then adding Sb for further ball milling and hot pressing, and (2) make composition deficient in both Ag and Sb as MgAg0.97Sb0.99. Based on the phase pure composition MgAg0.97Sb0.99, we further studied substitution of Ag by Ni to improve the thermoelectric properties by reducing the thermal conductivity, and found that Ni substituted composition MgAg0.965Ni0.005Ag0.99 yields the better thermoelectric properties. The densities of disc samples measured by the Archimedes method are 6.21, 6.17, and 6.20 g cm 3 for MgAgSb, MgAg0.97Sb0.99, and MgAg0.965Ni0.005Ag0.99, respectively, 99% of the theoretical density.
Characterizations X-ray diffraction spectra analysis was collected on a PANalytical multipurpose diffractometer with an X'celerator detector (PANalytical X'Pert Pro). Scanning electron microscopy (SEM, JEOL 6340F) was used to study the grain size of the freshly broken pieces of the MgAg0.97Sb0.99 sample. Transmission electron microscopy (TEM, JEOL 2010F) was used to study the structural details of grains observed by the SEM. The TEM samples were prepared as follows: a small piece of MgAg0.97Sb0.99 disk sample was gently hand ground, and the obtained suspension was dipped onto a typical carbon-coated Cu grid, which can be used for TEM observation after drying. The grain was selected for TEM observation. Thermal properties including specific heat (Cp), diffusivity (D), and electrical transport properties were performed on all samples. Temperature dependent Cp is measured by DSC 404 C (NETZSCH). Thermal diffusivities (D) were measured by LFA 457 (NETZSCH), the diameter of disc samples were 12 mm, with thickness of 2 mm, and coated by 10 μm amorphous carbon. ZEM-3 (ULVAC-RIKO) was used to measure the electrical transport properties for all samples with the sample dimensions of 2.5 2.5 11 mm3.
Material and methods Results and discussion Synthesis process We used a two-step process combining ball milling with hot pressing to synthesize the material. We first loaded 0.8102 g magnesium (Mg, Sigma Aldrich, 99.8% metal basis) and 3.5956 g silver (Ag, Sigma Aldrich, 99.9% metal basis) metal pieces (Mg:Ag = 1:1, atomic ratio) into a stainless steel jar with balls inside the argon-filled glove box, followed by ball milling for 8 h leading to formation of MgAg phase. Following this step, we added 4.0587 g of antimony (Sb, Sigma Aldrich, 99.8% metal basis) chunks into the jar inside the glove box corresponding to atomic ratio of Mg:Ag:Sb= 1:1:1, with another ball milling of 5 h. The final powders were hot pressed under DC current at 575 K for 8 min. The as-pressed disc was then annealed at 575 K in air for 30 min prior to structure characterizations and property measurements. We first started with Mg:Ag:Sb = 1:1:1 sample and found we always got some impurity phases related to Ag and Sb, which is in the range of 1–2% estimated by the XRD analysis. The content of impurities in our MgAgSb sample is significantly less than that of samples reported by Kirkham [21]. To completely eliminate the impurities, we found two things are necessary: (1) use a two-step synthesis process by ball
Figure 1A shows the XRD patterns of the three compositions we studied: MgAgSb, MgAg0.97Sb0.99, and MgAg0.965Ni0.005 Sb0.99. Although all compositions are nominal, the actual compositions examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) are basically the same as the nominal compositions. During the XRD pattern refinement for MgAg0.97Sb0.99, we noticed that antisite between Ag and Mg might have happened in the structure. The antisite occupation between Mg and Ag in multiple alloy phases had been reported [23,24], we believe this is also possible in the MgAgSb system. The successful demonstration of the synthesis of single phase from composition MgAg0.97Sb0.99 by a two-step ball milling and hot pressing process would pave the way for synthesis of a broad range of similar alloys in the future. SEM and TEM observations were carried out for sample MgAg0.97Sb0.99. Figure 1B shows the SEM image of a freshly broken surface. It seems that the grains are in the range of 100–200 nm with a fairly good uniformity and appear densely packed, consistent with its high relative density. It is generally understood that SEM image is not conclusive on determining the grain size. So TEM images were taken on
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Figure 1 Structure characterizations: (A) the observed XRD (X-ray diffraction) patterns for MgAgSb and MgAg0.97Sb0.99, and Ni substituted MgAg0.965Ni0.005Sb0.99 samples, impurity peaks were indicated by arrows, the calculated XRD pattern was shown at the bottom for comparison; (B) SEM image of a freshly broken surface of disc sample MgAg0.97Sb0.99; (C) low magnification TEM image with selected area electron diffraction pattern as the inset of sample MgAg0.97Sb0.99; (D) high resolution TEM image of MgAg0.97Sb0.99.
the same sample and are shown in Figure 1C–E. Figure 1C and D shows the low and medium magnification TEM images, respectively. They clearly show that the grains are in the range of 10–20 nm, and confirmed by the selected area electron diffraction as shown in the inset of Figure 1, much smaller than what was shown by the SEM image in Figure 1B. A high resolution TEM image as shown in Figure 1E further demonstrate that the large grains in Figure 1B are composed of much smaller grains of around 5–10 nm. The very small grain size may have played significantly important roles in the low thermal conductivity shown in Figure 2. The specific heat (Cp) dependence of temperature is shown in Figure 2A, two phase transitions at 610 K and 682 K for MgAgSb samples were observed, corresponding to the α phase below 610 K, intermediate β phase between 610 and 682 K, and γ phase above 682 K. It is interesting to note that MgAg0.97Sb0.99 and the Ni substituted MgAg0.965 Ni0.005Ag0.99 samples have lower second phase transition temperature for which we do not understand it at the moment. The inset in Figure 2A shows the enlarged Cp curve of the low temperature range. It is noticed that MgAgSb sample has slightly higher Cp values ( 0.34 J g 1 K 1 at 550 K) than those of MgAg0.97Sb0.99 and MgAg0.965Ni0.005 Ag0.99 in the range of 0.3–0.32 J g 1 K 1. The Cp curve of MgAg0.965Ni0.005Ag0.99 is slightly higher than that of MgAg0.97Sb0.99 above 450 K. The Cp value (dished line) calculated from Dulong–Petit law was put into the inset
for comparison [25]. Thermal conductivity was then calculated based on κ ¼ DρCp , where ρ is the measured density of the samples. The thermal diffusivities and thermal conductivities of MgAgSb and MgAg0.97Sb0.99 are shown in Figure 2B and C, respectively. The carrier and lattice contributions to the thermal conductivity are calculated based on the Wiedemann–Franz law κtotal = κcarrier + κlattice, and κcarrier = LsT, where s is the electrical conductivity and T the absolute temperature, and L the Lorenz number. Figure 2D shows the temperature dependence of κcarrier and κlattice. For a given material, the value of Lorenz number depends on the detailed band structure, position of the Fermi level and the temperature [26]. Sample MgAg0.97Sb0.99 has lower thermal conductivity over the whole temperature range. This can be ascribed to the strong phonon scattering due to the deficiencies at both the Ag and Sb sites and the very small grain size. The lattice thermal conductivity κL of MgAg0.97Sb0.99 is 0.6 W m 1 K 1 at 300 K, and 0.5 W m 1 K 1 at 570 K (Figure 2D). With the impurity phases in the MgAgSb sample, we observed a higher thermal conductivity of 0.8 W m 1 K 1 at 300 K (Figure 2D), which is understandable since the impurity has so much higher thermal conductivity than the matrix phase. The electrical transport properties of samples MgAgSb and MgAg0.97Sb0.99 are shown in Figure 2E–H. Figure 2E
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Figure 2 Thermoelectric properties of MgAgSb and MgAg0.97Sb0.99 samples: (A) temperature dependent heat capacities, inset shows heat capacity from 300 to 600 K; (B) temperature dependent thermal diffusivities; (C) temperature dependent total thermal conductivity; (D) temperature dependent carrier and lattice thermal conductivity; (E) temperature dependent electrical resistivity; (F) temperature dependent Seebeck coefficient; (G) temperature dependent power factor (PF); (H) temperature dependent figure of merit ZT.
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Figure 3 Comparison of the thermoelectric properties of MgAg0.965Ni0.005Sb0.99 with those of MgAg0.97Sb0.99 and Bi0.4Sb1.6 Te3nanocomposite: (A) temperature dependent total thermal conductivity; (B) temperature dependent electrical resistivity; (C) temperature dependent Seebeck coefficient; and (D) temperature dependent figure of merit ZT.
shows the temperature dependent electrical resistivity, MgAg0.97Sb0.99 has higher electrical resistivity than that of MgAgSb below 425 K, but comparable with that of MgAgSb above 425 K. It is about 2.3 10 5 Ω m at room temperature, increases to 3 10 5 Ω m at 350 K before it decreases to 1.6 10 5 Ω m at 575 K. Compared to Sb substituted Bi0.4Sb1.6Te3(4), the electrical resistivity of both MgAgSb and MgAg0.97Sb0.99 is much higher, leaving more room for further optimization. Figure 2F shows the temperature dependent Seebeck coefficient for both samples. For MgAg0.97Sb0.99, the Seebeckcoefficient starts at 210 mV K 1 at 300 K and then increases to 240 mV K 1 before decreasing to 180 mV K 1 at 575 K. These Seebeck values are comparable to those of Bi0.4Sb1.6Te3 bulk alloy [4]. Based on the simple formula on energy gap E G ¼ 2eSmax T developed by Goldsmid [27], where Smax and T refer to the peak Seebeck coefficient and corresponding temperature, respectively, we estimated the band gap E G of MgAg0.97Sb0.99 to be of 0.16 eV at 370 K, which is similar to that of Bi0.4Sb1.6Te3. Figure 2G shows the power factor dependence of temperature. The power factors of the two samples are similar below 375 K, and increases with temperature but much lower than that of Bi0.4Sb1.6Te3 [4]. Considering the high electrical resistivity of MgAg0.97Sb0.99, optimization of carrier concentration by doping will potentially improve the power factors. The figure of merit ZT is shown in Figure 2H, with the reported literature value shown for comparison [21]. A ZT
value of 0.7 at room temperature is the first material to exhibit such a high ZT after Bi2Te3 was discovered in 1950s. The ZT values keep increasing with temperature with a maximum of 1.2 obtained at 450 K. For the stoichiometric composition MgAgSb with impurity phases, we achieved ZT of 0.5 at room temperature, higher than the reported value of 0.3 [21] due to the less content of Ag and Sb related impurity phases in our sample. Considering the high electrical resistivity and lattice thermal conductivity, we see potentials of further improvement of the ZT values by optimizing the sample preparation process and composition. To optimize the composition, we have many options since there are three atoms in the material. We choose to start with the Ag site since a small deficiency on Ag increased the power factor (Figure 2G) and also reduced its lattice thermal conductivity (Figure 2C). We found Ni can further reduce the thermal conductivity without degrading the power factor. Figure 3A–D shows the thermoelectric properties of the Ni substituted samples: MgAg0.965Ni0.005Ag0.99. Ni alloying into Ag site clearly increased the electrical resistivity (Figure 3A) and accordingly the Seebeck coefficient (Figure 3B), but decreased the thermal conductivity to 0.7 W m 1 K 1 at 300 K (Figure 3C). Because of the lower thermal conductivity, we achieved ZT of 0.9 at 300 K and 1 at 325 K in sample MgAg0.965Ni0.005Ag0.99 (Figure 3D). The peak ZT of 1.4 was obtained at 450 K (Figure 3D), comparable to the highest reproducible ZT of Bi0.4Sb1.6Te3(4). For comparison, we also plotted the ZT of nanostructured Bi0.4Sb1.6Te3 in
102 Figure 3D. It is very clear that MgAg0.965Ni0.005Ag0.99 has the full potential to be even better than Bi0.4Sb1.6Te3 after further optimization.
Conclusions In conclusion, we have achieved ZTs of 1 at near room temperature in MgAgSb-based materials: MgAg0.965Ni0.005 Ag0.99, through improving the phase purity and Ni alloying to Ag site. Such a ZT at near room temperature is comparable with that of the best p-type Bi0.4Sb1.6Te3. The key to achieve the high ZT at near room temperature is to obtain phase pure samples by synthesis process and composition optimization. The point defects of Ag and Sb deficiency and antisites together with the nano grain size are the reasons for the low thermal conductivity of 0.7 W m 1 K 1 at near room temperature. Considering the fact that Bi0.4Sb1.6Te3 has been the only thermoelectric material with ZT 1 since 1950s, achieving similar or higher ZT at room temperature in a new material could be extremely important for finding more materials with higher ZT values. Further optimization of alloying in each site and carrier concentration could lead to even higher power factor and ultimately higher ZT.
Acknowledgments The work performed at University of Houston is funded by Air Force Office of Scientific Research MURI program under contract FA9550-10-1–0533 (synthesis of MgAgSb), the Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award number: DE-SC0001299/DE-FG0209ER46577 (characterization of MgAgSb), and DOE under contract number DE-FG02-13ER46917/DE-SC0010831 (Ni substitution and characterization), and the work supported at MIT is funded by the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award number: DE-SC0001299/DE-FG02-09ER46577 (efficiency calculation and measurement).
References [1] H.J. Goldsmid, R.W. Douglas, J. Appl. Phys. 5 (1954) 386. [2] H.J. Goldsmid, Thermoelectric Refrigeration, Plenum, New York, 1964. [3] B.C. Sales, Science 295 (2002) 1248. [4] B. Poudel, et al., Science 320 (2008) 634. [5] R.D. Abelson, in: D.W. Rowe (Ed.), Thermoelectrics Handbook, CRC press, Boca Raton, 2006, ch.56:1-26. [6] L.E. Bell, Science 321 (2008) 1457. [7] D. Kraemer, et al., Nat. Mater. 10 (2011) 532. [8] G. Mahan, B.C. Sales, J. Sharp, Phys. Today 50 (3) (1997) 42. [9] J.P. Heremans, et al., Science 321 (2008) 554. [10] K. Biswas, et al., Nature 489 (2012) 414. [11] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2008) 105. [12] Y. Pei, H. Wang, G.J. Snyder, Adv. Mater. 24 (2012) 6125. [13] X. Shi, et al., J. Am. Chem. Soc. 133 (2011) 7837. [14] G.S. Nolas, D.T. Morelli, T.M. Tritt, Annu. Rev. Mater. Sci. 29 (1999) 89.
H. Zhao et al. [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
G.H. Zhu, et al., Phys. Rev. Lett. 102 (2009) 196803. B. Yu, et al., Nano Lett. 12 (2012) 2077. X. Yan, et al., Nano Lett. 11 (2011) 556. G. Joshi, et al., Adv. Energy Mater. 1 (2011) 643. H. Nowotny, W. Sibert, Z. Metallkd. 33 (1941) 391. B.R.T. Frost, G.V. Raynor, Proc. R. Soc. Lond. A 203 (1950) 132. M.J. Kirkham, et al., Phys. Rev. B 85 (2012) 144120. Q. Zhang, et al., J. Am. Chem. Soc. 134 (2012) 17731. R.S. Busk, Metall. Pet. Eng. 188 (1950) 1460. R.B. Hill, M.H.J. Axon, J. Inst. Met. 85 (1957) 109. L.D. Landau, E.M. Lifshitz, Statistical Physics Pt. 1. Course in Theoretical Physics5, Pergamon Press, Oxford 193–196. [26] K.C. Lukas, et al., Phys. Rev. B85 (2012) 205410. [27] H.J. Goldsmid, Introduction to Thermoelectricity, SpringerVerlag, Berlin Heidelberg, 2010.
Dr. Huaizhou Zhao was a research associate in the Department of Physics and TCSUH at the University of Houston and currently he serves as an Associate Professor in the Institute of Physics at Chinese Academy of Sciences. He obtained his Ph.D. degree in Condensed Matter Physics from the Institute of Physics, Chinese Academy of Sciences in 2006, and Master degree in Solid State Chemistry from Beijing Normal University in 2003. He specializes in X-ray powder diffraction structural refinement, Solid State Chemistry, Nano-technology. He has published extensively in the area of metal nitrides, nanosensors, and thermoelectric materials. Dr. Jiehe Sui is currently a visiting scholar in the Department of Physics and TcSUH at the University of Houston, USA. He received his Ph.D. degree in Materials Science and Engineering from Harbin Institute of Technology, China. His current research is mainly on synthesis and characterization of nanostructured thermoelectric materials.
Dr. Yucheng Lan is currently a research assistant professor of physics at University of Houston. He obtained his Ph.D. degree from the Institute of Physics, Chinese Academy of Sciences, master and bachelor degrees from Jilin University, China. He specializes in X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy and Raman scattering. He has published extensively in the area of superconductors, III-V nitrides, thermoelectric materials, photocatalytic materials, and nanosensors. Dr. Qing Jie is currently a research associate in the Department of Physics and TCSUH at the University of Houston, USA. He got his Ph.D. degree in Materials Science and Engineering from the Stony Brook University, NY, USA, in 2010, a Master degree in Material Science from Shanghai Institute of Ceramic, Shanghai, China in 2003, and a Bachelor degree in Materials Science and Engineering from Tsinghua University, Beijing, China in 2000. His current interests cover the fabrication of thermoelectric devices for different temperature range, thermal
High thermoelectric performance and electrical properties measurement, as well as exploring novel materials for energy conversion Daniel Kraemer iis currently a Ph.D. candidate in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, USA. He received his Master’s and Bachelor’s degree in Mechanical Engineering from the Swiss Federal Institute of Technology in Zurich, Switzerland and a Diploma in Process Engineering from the University of Applied Sciences in Frankfurt, Germany. His current research ranges from thermoelectric system/device design and power conversion efficiency measurements to electrical and thermal material properties measurements as well as radiative heat transfer and optical properties measurements. Kenneth McEnaney is a Ph.D. candidate in the Department of Mechanical Engineering at the Massachusetts Institute of Technology. He received his bachelor's degree in Mechanical Engineering from Cornell University. His research focuses on thermoelectric devices and radiative heat transfer.
Dr. Arnold M. Guloy is currently a Professor of Chemistry at the University of Houston and TCSUH. He received his Ph.D. in Inorganic Chemistry from Iowa State University, under the supervision of the late Prof. John D. Corbett. He was an IBM Postdoctoral Fellow at the IBM Research Center at Yorktown Heights prior to joining the faculty at the University of Houston. His research in solid state chemistry focuses on the syntheses and characterization of polar intermetallics and Zintl phases with novel structures and unique properties. He has received the NSF CAREER Award and the University of Houston Award for Excellence in Research and Scholarship. He was a Visiting Professor the Max Planck Institute for Chemical Physics of Solids in Dresden.
103 Dr. Gang Chen is currently the Carl Richard Soderberg Professor of Power Engineering at Massachusetts Institute of Technology. He obtained his Ph.D. degree from UC Berkeley in 1993 working under the Chancellor Chang-Lin Tien, master and bachelor degrees from Huazhong University of Science and Technology, China. He was a faculty member at Duke University (1993– 1997), University of California at Los Angeles (1997–2001), before joining MIT in 2001. He is a member of National Academy of Engineering. He is a recipient of the NSF Young Investigator Award, the ASME Heat Transfer Memorial Award, and the R&D 100 Award. He is a Guggenheim Fellow, an AAAS Fellow, an ASME Fellow, and an APS Fellow. He has published extensively in the area of nanoscale energy transport and conversion and nanoscale heat transfer. He is the director of Solid-State Solar–Thermal Energy Conversion Center funded by the US DOE’s Energy Frontier Research Centers program. Dr. Zhifeng Ren is currently an M.D. Anderson Chair Professor in the Department of Physics and TcSUH at the University of Houston. He obtained his Ph.D. degree from the Institute of Physics Chinese Academy of Sciences in 1990, master degree from Huazhong University of Science and Technology in 1987, and bachelor degree from Sichuan Institute of Technology in 1984. He was a postdoc and research faculty at SUNY Buffalo (1990–1999) before joining BC as an Associate Professor in 1999. He specializes in thermoelectric materials, solar thermoelectric devices and systems, photovoltaic materials and systems, carbon nanotubes and semiconducting nanostructures, nanocomposites, bio agent delivery and bio sensors, flexible transparent conductors, superconductors, etc. He is a fellow of the APS and AAAS. Recently he was elected as a fellow of the National Academy of Inventors (NAI). He was recipient of the R&D 100 award in 2008 and the 2014 Edith & Peter O’Donnell Award in Science of The Academy of Medicine, Engineering, and Science of Texas.
