Intriguing experimental results raised the question about the fundamental mechanisms governing th... more Intriguing experimental results raised the question about the fundamental mechanisms governing the electron-hole coupling induced bipolar thermal conduction in semiconductors. Our combined theoretical analysis and experimental measurements show that in semiconductors bipolar thermal transport is in general a “conductivity-limiting” phenomenon and it is thus controlled by the carrier mobility ratio and by the minority carrier partial electrical conductivity for the intrinsic and extrinsic cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electronic band structure modulation and/or preferential minority carrier scatterings. We expect this study to be beneficial to the current interests in optimizing thermoelectric properties of narrow gap semiconductors.
The low weighted carrier mobility has long been considered to be the key challenge for improvemen... more The low weighted carrier mobility has long been considered to be the key challenge for improvement of thermoelectric (TE) performance in BiTeI. The Rashba-effect-induced two-dimensional density of states in this bulk semiconductor is beneficial for thermopower enhancement, which makes it a prospective compound for TE applications. In this report, we show that intercalation of minor Cu-dopants can substantially alter the equilibria of defect reactions, selectively mediate the donor-acceptor compensation, and tune the defect concentration in the carrier conductive network. Consequently, the potential fluctuations responsible for electron scattering are reduced and the carrier mobility in BiTeI can be enhanced by a factor of two to three between 10 K and 300 K. The carrier concentration can also be optimized by tuning the Te/I composition ratio, leading to higher thermopower in this Rashba system. Cu-intercalation in BiTeI gives rise to higher power factor, slightly lower lattice therm...
Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb) 2 Te 3 bulk materials... more Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb) 2 Te 3 bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi 0.48 Sb 1.52 Te 3 ) and specific nanostructures showed an increase of ∼50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.
Other uses, including reproduction and distribution, or selling or licensing copies, or posting t... more Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited.
In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark... more In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark plasma sintering (SPS) process was utilized to prepared n-type Bi 2 (Te 1Àx Se x ) 3 (x ¼ 0.0e1.0) solid solutions from high purity single elemental chunks. The substitution of tellurium by selenium has significant impacts on the electrical and thermal transport properties of the Bi 2 (Se x Te 1Àx ) 3 compounds in a manner which can be well understood using a valence bond rule and the corresponding change in band gap. Furthermore, the selenium substitution effectively adjusts the carrier density allowing an optimum value of w5 Â 10 À19 cm À3 . As a result, a maximum ZT of 1.05 at 420 K was achieved for the Bi 2 (Se 0.2 Te 0.8 ) 3 sample which also shows an improved average ZT of w0.97 in the entire measurement temperature range. By adopting the same p-type legs, the module fabricated by the MS-SPS Bi 2 (Se 0.2 Te 0.8 ) 3 material which acts as n-type legs shows w10% enhancement in thermoelectric conversion efficiency compared with the module fabricated by n-type zone melted ingots.
A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in... more A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in preparing high-performance (Bi,Sb) 2 Te 3 thermoelectric (TE) nanocomposites via creating and optimizing their resulting multiscale microstructures. (Bi,Sb) 2 Te 3 possesses a highly anisotropic crystal structure; therefore, it is important to investigate any potential correlation between the SPS conditions, the as-formed microstructures, and the resulting TE properties. In this work, we investigate the correlation between the SPS pressure, the microstructure texture, and the anisotropy of the total thermal conductivity in these melt-spun spark-plasma-sintered (Bi,Sb) 2 Te 3 compounds. The thermal conductivity has been measured in directions that are both perpendicular and parallel to the pressing (or force) direction by rearranging the sample geometry as described in the text. The results show that the anisotropy of thermal conductivity is ;0, 2-3, 6-7, and 13-15% for the samples sintered at pressures of 20, 30, 45, and 60 MPa, respectively. These results are consistent with an increasing degree of orientation observed by x-ray diffraction and electron microscopy.
Polycrystalline p-type Ca/Ce co-filled skutterudites Ca 0.5 Ce 0.5 Fe 4−x Ni x Sb 12 (0 ≤ 0 ≤ 0.7... more Polycrystalline p-type Ca/Ce co-filled skutterudites Ca 0.5 Ce 0.5 Fe 4−x Ni x Sb 12 (0 ≤ 0 ≤ 0.7) compounds have been successfully prepared by traditional melting-annealing-spark plasma sintering (SPS) method. The effective adjustment in carrier concentration dominates electronic transport behavior in a manner of significant decrease of electrical conductivity as well as the enhancement in Seebeck coefficient with increasing Ni-substitution, resulting in a slight degradation of power factor. Meanwhile, the thermal conductivity, dominated by electronic contribution, decreases monotonically with increasing Ni-substitution, and the negligible mass fluctuation between Ni and Fe gives rise to a slight change in phonon thermal conductivity. In particular, the lattice thermal conductivity of these co-filled skutterudites shows rather low value compared with other double filled analogs due to the large difference in localized frequencies between Ca and Ce elements. Due to the opposite tendency between electronic properties and thermal conductivities, ZT governed by carrier concentration increases firstly and then decreases. The maximum ZT value of 0.85 at 700 K is obtained in the sample Ca 0.5 Ce 0.5 Fe 3.5 Ni 0.5 Sb 12 with a carrier concentration of ∼2.6 × 10 20 cm −3 , and this peak value is comparable to some excellent p-type skutterudites reported so far. This study demonstrates that the effective control in carrier concentration can be easily realized through proper Fe-site substitutions without deterioration in carrier mobility and change in electronic structure, thus bringing about an optimization of thermoelectric figure of merit.
In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching proces... more In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching process combined with a subsequent spark plasma sintering process, and systematically investigated the doping effects of Ag on the thermoelectric properties. Ag substitution in the Pb site does not introduce resonant levels near the valence band edge or detectable change in the density of state in the vicinity of the Fermi level, but moves the Fermi level down and increases the carrier concentration to a maximum value of ∼4.7 × 10 19 cm −3 which is still insufficient for heavily doped PbSe compounds. Nonetheless, the non-monotonic variation in carrier concentration with increasing Ag content indicates that Ag doping reaches the solution limit at ∼1.0% and the excessive Ag presumably acts as donors in the materials. Moreover, the large energy gap of the PbSe-based material wipes off significant 'roll-over' in the Seebeck coefficient at elevated temperatures which gives rise to high power factors, being comparable to p-type Te analogues. Consequently, the maximum ZT reaches ∼1.0 for the 1.5% Ag-doped samples with optimized carrier density, which is ∼70% improvement in comparison with an undoped sample and also superior to the commercialized p-type PbTe materials.
Starting with elemental chunks of bismuth, antimony, tellurium and selenium, densified bulk mater... more Starting with elemental chunks of bismuth, antimony, tellurium and selenium, densified bulk materials (Bi 0.95 Sb 0.05 ) 2 (Te 1−x Se x ) 3 (x = 0.10, 0.13, 0.15 and 0.17) were prepared by melt spinning subsequently combined with a spark plasma sintering process. The prepared bulk materials display fine grain size and numerous layered structures with a size of 10-100 nm; moreover, details of the composition difference and phase difference cannot be observed. Measurements of electrical conductivity, Seebeck coefficient and thermal conductivity have been performed in the temperature range 300-500 K, and it is found that the thermoelectric properties are significantly affected by the content of selenium. All the prepared samples show higher ratios of electrical conductivity and total thermal conductivity compared with state-of-the-art commercial zone melted materials, mainly a large reduction in lattice thermal conductivity, which is more beneficial to the concept of 'electron crystal phonon glass'. Subsequently, the resulting thermoelectric figure of merit ZT value reaches a maximum of 1.0 at 460 K for the n-type (Bi 0.95 Sb 0.05 ) 2 (Te 0.85 Se 0.15 ) 3 bulk material. Compared with traditional zone melted materials, the peak ZTs move towards a higher temperature and this study demonstrates the possibility of preparing materials with high performance, which can be applied for low temperature power generation or multi-stage devices.
b-Zn 4 Sb 3 compounds doped with minute amounts of Cd were synthesized by the MS-SPS technique, w... more b-Zn 4 Sb 3 compounds doped with minute amounts of Cd were synthesized by the MS-SPS technique, which involves melt spinning (MS) followed by spark plasma sintering (SPS), and the microstructures, thermoelectric and thermodynamic properties were systematically characterized. The non-equilibrium MS-SPS technique generates multi-scale nanostructures in the MS-prepared ribbon-shape samples and the resulting compacted bulk materials. These unique multiple nanostructures result in substantial reductions in lattice thermal conductivities, particularly for samples with a large number of ZnSb nanodots with sizes of 10-30 nm. Meanwhile, Cd-doping remarkably improves the electrical properties of the (Zn 1Àx Cd x ) 4 Sb 3 compounds by a slight decrease in electrical conductivity and an apparent enhancement of the Seebeck coefficient. Therefore, the dimensionless figure of merits are significantly improved and the maximum value reaches $1.30 for the (Zn 0.99 Cd 0.01 ) 4 Sb 3 sample at 700 K, representing $13% and $23% improvements compared with the undoped MS-SPS sample and the 1% Cd-doped melting ingot, respectively. In particular, this value shows no degradation after 10 heat cycles from 300 to 700 K or 30 h annealing at 680 K in vacuum, whereas the ZT of neat sample decreases by $20% to a relatively low value of $1.0 after 30 h annealing. The enhanced thermal stability of ZT along with the suppressing effect on the low-temperature a-b phase transition clearly indicates a large improvement in thermodynamic stability as a result of minute Cd-doping. All the above-mentioned benefits make the minute Cd-doped b-Zn 4 Sb 3 compound prepared by the MS-SPS technique a promising candidate for mid-range temperature thermoelectric power generation applications.
Intriguing experimental results raised the question about the fundamental mechanisms governing th... more Intriguing experimental results raised the question about the fundamental mechanisms governing the electron-hole coupling induced bipolar thermal conduction in semiconductors. Our combined theoretical analysis and experimental measurements show that in semiconductors bipolar thermal transport is in general a “conductivity-limiting” phenomenon and it is thus controlled by the carrier mobility ratio and by the minority carrier partial electrical conductivity for the intrinsic and extrinsic cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electronic band structure modulation and/or preferential minority carrier scatterings. We expect this study to be beneficial to the current interests in optimizing thermoelectric properties of narrow gap semiconductors.
The low weighted carrier mobility has long been considered to be the key challenge for improvemen... more The low weighted carrier mobility has long been considered to be the key challenge for improvement of thermoelectric (TE) performance in BiTeI. The Rashba-effect-induced two-dimensional density of states in this bulk semiconductor is beneficial for thermopower enhancement, which makes it a prospective compound for TE applications. In this report, we show that intercalation of minor Cu-dopants can substantially alter the equilibria of defect reactions, selectively mediate the donor-acceptor compensation, and tune the defect concentration in the carrier conductive network. Consequently, the potential fluctuations responsible for electron scattering are reduced and the carrier mobility in BiTeI can be enhanced by a factor of two to three between 10 K and 300 K. The carrier concentration can also be optimized by tuning the Te/I composition ratio, leading to higher thermopower in this Rashba system. Cu-intercalation in BiTeI gives rise to higher power factor, slightly lower lattice therm...
Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb) 2 Te 3 bulk materials... more Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb) 2 Te 3 bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi 0.48 Sb 1.52 Te 3 ) and specific nanostructures showed an increase of ∼50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.
Other uses, including reproduction and distribution, or selling or licensing copies, or posting t... more Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited.
In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark... more In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark plasma sintering (SPS) process was utilized to prepared n-type Bi 2 (Te 1Àx Se x ) 3 (x ¼ 0.0e1.0) solid solutions from high purity single elemental chunks. The substitution of tellurium by selenium has significant impacts on the electrical and thermal transport properties of the Bi 2 (Se x Te 1Àx ) 3 compounds in a manner which can be well understood using a valence bond rule and the corresponding change in band gap. Furthermore, the selenium substitution effectively adjusts the carrier density allowing an optimum value of w5 Â 10 À19 cm À3 . As a result, a maximum ZT of 1.05 at 420 K was achieved for the Bi 2 (Se 0.2 Te 0.8 ) 3 sample which also shows an improved average ZT of w0.97 in the entire measurement temperature range. By adopting the same p-type legs, the module fabricated by the MS-SPS Bi 2 (Se 0.2 Te 0.8 ) 3 material which acts as n-type legs shows w10% enhancement in thermoelectric conversion efficiency compared with the module fabricated by n-type zone melted ingots.
A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in... more A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in preparing high-performance (Bi,Sb) 2 Te 3 thermoelectric (TE) nanocomposites via creating and optimizing their resulting multiscale microstructures. (Bi,Sb) 2 Te 3 possesses a highly anisotropic crystal structure; therefore, it is important to investigate any potential correlation between the SPS conditions, the as-formed microstructures, and the resulting TE properties. In this work, we investigate the correlation between the SPS pressure, the microstructure texture, and the anisotropy of the total thermal conductivity in these melt-spun spark-plasma-sintered (Bi,Sb) 2 Te 3 compounds. The thermal conductivity has been measured in directions that are both perpendicular and parallel to the pressing (or force) direction by rearranging the sample geometry as described in the text. The results show that the anisotropy of thermal conductivity is ;0, 2-3, 6-7, and 13-15% for the samples sintered at pressures of 20, 30, 45, and 60 MPa, respectively. These results are consistent with an increasing degree of orientation observed by x-ray diffraction and electron microscopy.
Polycrystalline p-type Ca/Ce co-filled skutterudites Ca 0.5 Ce 0.5 Fe 4−x Ni x Sb 12 (0 ≤ 0 ≤ 0.7... more Polycrystalline p-type Ca/Ce co-filled skutterudites Ca 0.5 Ce 0.5 Fe 4−x Ni x Sb 12 (0 ≤ 0 ≤ 0.7) compounds have been successfully prepared by traditional melting-annealing-spark plasma sintering (SPS) method. The effective adjustment in carrier concentration dominates electronic transport behavior in a manner of significant decrease of electrical conductivity as well as the enhancement in Seebeck coefficient with increasing Ni-substitution, resulting in a slight degradation of power factor. Meanwhile, the thermal conductivity, dominated by electronic contribution, decreases monotonically with increasing Ni-substitution, and the negligible mass fluctuation between Ni and Fe gives rise to a slight change in phonon thermal conductivity. In particular, the lattice thermal conductivity of these co-filled skutterudites shows rather low value compared with other double filled analogs due to the large difference in localized frequencies between Ca and Ce elements. Due to the opposite tendency between electronic properties and thermal conductivities, ZT governed by carrier concentration increases firstly and then decreases. The maximum ZT value of 0.85 at 700 K is obtained in the sample Ca 0.5 Ce 0.5 Fe 3.5 Ni 0.5 Sb 12 with a carrier concentration of ∼2.6 × 10 20 cm −3 , and this peak value is comparable to some excellent p-type skutterudites reported so far. This study demonstrates that the effective control in carrier concentration can be easily realized through proper Fe-site substitutions without deterioration in carrier mobility and change in electronic structure, thus bringing about an optimization of thermoelectric figure of merit.
In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching proces... more In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching process combined with a subsequent spark plasma sintering process, and systematically investigated the doping effects of Ag on the thermoelectric properties. Ag substitution in the Pb site does not introduce resonant levels near the valence band edge or detectable change in the density of state in the vicinity of the Fermi level, but moves the Fermi level down and increases the carrier concentration to a maximum value of ∼4.7 × 10 19 cm −3 which is still insufficient for heavily doped PbSe compounds. Nonetheless, the non-monotonic variation in carrier concentration with increasing Ag content indicates that Ag doping reaches the solution limit at ∼1.0% and the excessive Ag presumably acts as donors in the materials. Moreover, the large energy gap of the PbSe-based material wipes off significant 'roll-over' in the Seebeck coefficient at elevated temperatures which gives rise to high power factors, being comparable to p-type Te analogues. Consequently, the maximum ZT reaches ∼1.0 for the 1.5% Ag-doped samples with optimized carrier density, which is ∼70% improvement in comparison with an undoped sample and also superior to the commercialized p-type PbTe materials.
Starting with elemental chunks of bismuth, antimony, tellurium and selenium, densified bulk mater... more Starting with elemental chunks of bismuth, antimony, tellurium and selenium, densified bulk materials (Bi 0.95 Sb 0.05 ) 2 (Te 1−x Se x ) 3 (x = 0.10, 0.13, 0.15 and 0.17) were prepared by melt spinning subsequently combined with a spark plasma sintering process. The prepared bulk materials display fine grain size and numerous layered structures with a size of 10-100 nm; moreover, details of the composition difference and phase difference cannot be observed. Measurements of electrical conductivity, Seebeck coefficient and thermal conductivity have been performed in the temperature range 300-500 K, and it is found that the thermoelectric properties are significantly affected by the content of selenium. All the prepared samples show higher ratios of electrical conductivity and total thermal conductivity compared with state-of-the-art commercial zone melted materials, mainly a large reduction in lattice thermal conductivity, which is more beneficial to the concept of 'electron crystal phonon glass'. Subsequently, the resulting thermoelectric figure of merit ZT value reaches a maximum of 1.0 at 460 K for the n-type (Bi 0.95 Sb 0.05 ) 2 (Te 0.85 Se 0.15 ) 3 bulk material. Compared with traditional zone melted materials, the peak ZTs move towards a higher temperature and this study demonstrates the possibility of preparing materials with high performance, which can be applied for low temperature power generation or multi-stage devices.
b-Zn 4 Sb 3 compounds doped with minute amounts of Cd were synthesized by the MS-SPS technique, w... more b-Zn 4 Sb 3 compounds doped with minute amounts of Cd were synthesized by the MS-SPS technique, which involves melt spinning (MS) followed by spark plasma sintering (SPS), and the microstructures, thermoelectric and thermodynamic properties were systematically characterized. The non-equilibrium MS-SPS technique generates multi-scale nanostructures in the MS-prepared ribbon-shape samples and the resulting compacted bulk materials. These unique multiple nanostructures result in substantial reductions in lattice thermal conductivities, particularly for samples with a large number of ZnSb nanodots with sizes of 10-30 nm. Meanwhile, Cd-doping remarkably improves the electrical properties of the (Zn 1Àx Cd x ) 4 Sb 3 compounds by a slight decrease in electrical conductivity and an apparent enhancement of the Seebeck coefficient. Therefore, the dimensionless figure of merits are significantly improved and the maximum value reaches $1.30 for the (Zn 0.99 Cd 0.01 ) 4 Sb 3 sample at 700 K, representing $13% and $23% improvements compared with the undoped MS-SPS sample and the 1% Cd-doped melting ingot, respectively. In particular, this value shows no degradation after 10 heat cycles from 300 to 700 K or 30 h annealing at 680 K in vacuum, whereas the ZT of neat sample decreases by $20% to a relatively low value of $1.0 after 30 h annealing. The enhanced thermal stability of ZT along with the suppressing effect on the low-temperature a-b phase transition clearly indicates a large improvement in thermodynamic stability as a result of minute Cd-doping. All the above-mentioned benefits make the minute Cd-doped b-Zn 4 Sb 3 compound prepared by the MS-SPS technique a promising candidate for mid-range temperature thermoelectric power generation applications.
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Papers by Shanyu Wang