P-type Alloys Research Articles

Grain orientation and transport properties of textured Bi2Te3 alloys

The new material developments in the field of thermoelectrics over the past two decades have resulted in new materials with unique transport properties as well as new synthetic approaches. Enhanced thermoelectric properties of these new materials through thermal conductivity reduction and electrical properties enhancement strategies such as nano-structuring and band engineering have also been reported. Nevertheless, the material system upon which all new thermoelectric material developments are compared with continues to be alloys of Bi2Te3. Approaches for thermoelectric property enhancements for Bi2Te3 alloys have also been undertaken, however, oriented polycrystalline materials continue to be of primary interest for thermoelectric device development. To this end we investigate n and p-type Bi2Te3 alloys provided by II-VI Inc. in order to elucidate the texturing in these materials and their transport properties with texturing. These materials are highly textured, as required in order to maximize the thermoelectric properties, with an 80% enhancement of the thermoelectric properties obtained with texturing.

Effect of defect interactions with interstitial Ag in the lattice of BixSb2−xTe3 alloys and their thermoelectric properties

In this study, the effect of Ag defects on the lattice and thermoelectric properties of Ag-doped BixSb2−xTe3 (BST) alloys are systematically investigated. Using density-functional calculations, we reveal that Ag impurity defects occupying interstitial sites in the BST affect the formation of intrinsic defects in the host materials. Ag interstitial defects, which are responsible for increasing the lattice parameter of the c axis, lower the formation energies of BiTe and SbTe acceptor defects more effectively in the Bi-rich BST than in the Bi-poor BST. In addition, these Ag interstitials induce the formation of AgBi and AgSb in BixSb2−xTe3. Therefore, Ag-induced defect interactions can be used to finely optimize the defects and doping density by varying the Bi content. Furthermore, the phonon thermal conductivity is reduced with the formation of nanoscale twin structures with various stacking faults. Finally, a high figure of merit of 1.4 at 423 K is achieved for p-type BST alloys.

Study of Radiation-Induced Defects in p-Type Si1-xGex Diodes before and after Annealing.

In this work, electrically active defects of pristine and 5.5 MeV electron irradiated p-type silicon–germanium (Si1−xGex)-based diodes were examined by combining regular capacitance deep-level transient spectroscopy (C-DLTS) and Laplace DLTS (L-DLTS) techniques. The p-type SiGe alloys with slightly different Ge contents were examined. It was deduced from C-DLTS and L-DLTS spectra that the carbon/oxygen-associated complexes prevailed in the pristine Si0.949Ge0.051 alloys. Irradiation with 5.5 MeV electrons led to a considerable change in the DLT spectrum containing up to seven spectral peaks due to the introduction of radiation defects. These defects were identified using activation energy values reported in the literature. The double interstitial and oxygen complexes and the vacancy, di-vacancy and tri-vacancy ascribed traps were revealed in the irradiated samples. The interstitial carbon and the metastable as well as stable forms of carbon–oxygen (CiOi* and CiOi) complexes were also identified for the electron-irradiated SiGe alloys. It was found that the unstable form of the carbon–oxygen complex became a stable complex in the irradiated and the subsequently annealed (at 125 °C) SiGe samples. The activation energy shifts in the radiation-induced deep traps to lower values were defined when increasing Ge content in the SiGe alloy.

Electronic quality factor for thermoelectrics.

Development of thermoelectrics usually involves trial-and-error investigations, including time-consuming synthesis and measurements. Here, we identify the electronic quality factor BE for determining the maximum thermoelectric power factor, which can be conveniently estimated by a single measurement of Seebeck coefficient and electrical conductivity of only one sample, not necessarily optimized, at an arbitrary temperature. We demonstrate that thousands of experimental measurements in dozens of materials can all be described by a universal curve and a single material parameter BE for each class of materials. Furthermore, any deviation in BE with temperature or doping indicated new effects such as band convergence or additional scattering. This makes BE a powerful tool for evaluating and guiding the development of thermoelectrics. We demonstrate the power of BE to show both p-type GeTe alloys and n-type Mg3SbBi alloys as highly competitive materials, at near room temperature, to state-of-the-art Bi2Te3 alloys used in nearly all commercial applications.

Melt-Spun SiGe Nano-Alloys: Microstructural Engineering Towards High Thermoelectric Efficiency

Silicon-germanium (SiGe) alloys are prominent high-temperature thermoelectric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spinning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The current methodology exhibited a TE figure-of-merit (ZT) ≈ 0.94 at 1123 K for a higher cooling rate of ∼3.0 × 107 K/s. This corresponds to ≈ 88% enhancement in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions, ≈ 45% higher than pressure-sintered p-type alloys, which results in a higher output power density, and TE conversion efficiency (η) ≈ 8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhancement is driven by selective scattering of phonons rather than of charge carriers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of synthesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials.

Power Factors of p-type Half-Heusler Alloys ScNiBi, YNiBi, and LuNiBi by <i>ab initio</i> Calculations

Power Factors of p-type Half-Heusler Alloys ScNiBi, YNiBi, and LuNiBi by <i>ab initio</i> Calculations

Near-room-temperature rhombohedral Ge1-xPbxTe thermoelectrics

Thermoelectric GeTe alloys have recently attracted renewed interests due to the extraordinary performance. It is particularly interesting in the low-symmetry structure, where the symmetry-breaking induced diversity in the arrangement in energy of split valence bands might enable an optimal band convergence for thermoelectric enhancements. This in principle leads to an extra degree of freedom for advancing GeTe thermoelectrics though a control of the degree of rhombohedral-distortion in the structure. Here we demonstrate this concept and realize its extraordinariness near room temperature, an extremely important temperature range for thermoelectric power generation/cooling applications in which very few options on materials other than conventional Bi2Te3 are currently available. This is enabled by a continuous manipulation of the rhombohedral-distortion in simple Ge1-xPbxTe solid solutions quenched from the high-temperature single phase region, a simultaneous optimization of carrier concentration and a reduction in lattice thermal conductivity due to PbTe-alloying. The resultant figure of merit, zT, at 200–350 K, is highly competitive to that of commercially available p-type Bi2Te3 alloys and higher than that of other known p-type thermoelectrics.

Origins of the enhanced thermoelectric performance for p-type Ge1-xPbxTe alloys

GeTe crystallizes with a strong deviation from the stoichiometric composition (in the direction of Te excess), which results in a growth of free carrier concentration (holes) up to p ~ 1021 cm−3. Such concentration is too high from the viewpoint of thermoelectric applications (maximum value of the figure of merit Z). The work demonstrates the possibility of increasing the thermoelectric efficiency of the GeTe-based material, used for the p-type leg of the thermoelectric couple over the temperature range of 600–825 K. The samples were prepared using Spark Plasma Sintering (SPS) technique. A systematic analysis of thermoelectric properties for Ge1-xPbxTe alloys (x up to 5 at.%) has been performed. An increase in the lead content in Ge1-xPbxTe alloys creates two parallel processes. The first process is a decrease of the lattice thermal conductivity κlat, which acts up to T ≈ 550 K, when the lead content in Ge1-xPbxTe alloys increases up to 5 at. %. However, the main process is to reduce deviation from the stoichiometric composition, which consequently leads to lower hole concentration p. Therefore, at a high temperature of T ≥ 550 K Fermi level EF is approaching the edge of the valence band Ev. As a result, the thermoelectric dimensionless figure of merit ZT increases up to 1.1 at the temperature of 825 K.

Response of p-type and n-type Half-Heusler thermoelectric materials to fast neutron irradiation

The thermoelectric materials working in radioisotope thermoelectric generators or space reactor power system will be exposed to neutron irradiation in space missions. Herein, the influences of neutron irradiation with doses of 1013n/cm2 and 1015n/cm2 on the transport properties of p- and n-type Half-Heusler thermoelectric alloys were investigated. It was found that, neutron irradiation showed negative effects on thermoelectric properties of p-type Hf0.5Zr0.5CoSb0.85Sn0.15 and p-type Hf0.5Ti0.5CoSb0.85Sn0.15. But neutron irradiation had no effects on thermoelectric properties of n-type Hf0.5Zr0.5NiSn0.985Sb0.015 and n-type ZrNiSn0.985Sb0.015. After 1015n/cm2 neutron irradiation, the electrical conductivity, power factor and ZT of p-type Half-Heusler alloys decreased by 7–22%, which was caused by the decreased carrier concentration. However, the radiation-induced changes in the thermoelectric properties proved to be essentially healed by annealing at 773 K ~ 873 K. The anti-annealing effects were also studied.

Preparation and thermoelectric performance of nano rare-earth oxides dispersed p-type BiSbTe alloys by mechanical milling and spark plasma sintering

Simultaneously, the enhancement of power factor and reduction in thermal conductivity is a critical issue to improve the thermoelectric performance of a certain material. In the present work, the rare earth oxide, Pr6O11 was added to the Bi-Sb-Te material and the corresponding thermoelectric properties have been systematically investigated. The Seebeck coefficient was slightly improved due to the increase in effective mass by addition of rare earth oxides in BiSbTe matrix. The electrical conductivity was decreased with increasing rare earth oxide content due to the decreasing mobility owing to the carrier scattering at grain boundaries. The thermal conductivity was reduced of 8% for Pr6O11/BiSbTe composites in which the 4 wt% added Pr6O11 particles, act as a scattering center for middle to higher wavelength phonons and induce the phonon scattering. As a result, the maximum thermoelectric figure of merit, ZT of 0.71 was obtained for 2 wt% added BiSbTe/Pr6O11 composites at 350 K.

Optimizing the thermoelectric performance of p-type Mg3Sb2 by Sn doping

The effects of Sn doping on thermoelectric performance of Mg3Sb2-based materials were investigated. First principle calculation results show an increased density of state near the Fermi level and flattened band structure after Sn doping, predicting that the introduction of Sn will enhance the electric transport properties as an effective acceptor dopant. The maximum ZT of Mg3Sb2-xSnx (0 ≤ x ≤ 0.04) samples is 0.42 at 773 K with x = 0.02. Experiments demonstrate that Sn doping can promote the thermoelectric performance of p-type Mg3Sb2 alloys, however the doping effect is limited by the appearance of n-type Mg2Sn during the preparation processes. Consequently, for further enhancing the thermoelectric performance of p-type Mg3Sb2-based materials, co-doping or composite method can be introduced while select a reasonable Sn doping amount.

Tuning the nature of charge carriers by controlling dual substitution in single-filled thermoelectric skutterudite, Yb-CoSb3

Fe and Mn have been substituted for Co to study Yb0.3Fe0.7−xMnxCo3.3Sb12 alloys with x = 0.2 and 0.5. These alloys were made by simple vacuum alloying and hot pressing. Both the alloys were found to be essentially single phase with no second phase in agreement with earlier reports. The Seebeck coefficient of Fe-rich p-type alloy was found to increase with increasing temperature reaching a maximum value of 136 μVK−1 at 723 K while that of the Mn-rich n-type alloy increases to 249 μVK−1 at 520 K. The Seebeck coefficient in both alloys decreases after exhibiting a maximum, indicative of bipolar conduction with a small band gap. The electrical conductivity shows weak metallic behavior for the p-type alloy and degenerate semiconducting behavior for the n-type alloy. The charge carrier concentration in the p-type alloy (4.7 × 1019 cm−3) is lower by an order of magnitude compared with that in the n-type alloy (4.4 × 1020 cm−3) but with an order of magnitude higher mobility. The resultant maximum power factor for p-type is therefore ~ 0.83 mWm−1 K−2 compared with 1.3 mWm−1 K−2 for the n-type. The thermal conductivity of n-type, however, is found to be twice that of the p-type resulting in a maximum zT of 0.41 at 723 K for the n-type alloy compared with 0.42 at 723 K for the p-type alloy. These results show that compositional tuning can be used to make either p-type or n-type skutterudites with the same base composition and earth abundant elements.

Magnetism-induced huge enhancement of the room-temperature thermoelectric and cooling performance of p-type BiSbTe alloys

Superparamagnetic Fe3O4 nanoparticles remarkably enhance the room-temperature thermoelectric and cooling performance of BiSbTe alloys.

Au and B co-doped p-type Si-Ge nanocomposites possessing ZT = 1.63 synthesized by ball milling and low-temperature sintering

The enhanced value of the dimensionless figure of merit was achieved in a P-type Si-Ge alloy containing a small amount of Au and B, Si0.62Ge0.31B0.03Au0.04. After the 6 h ball milling, the sample was sintered at 873 K under 400 MPa with a soaking time of 4 h. The precipitation of (Au) occurred in Si-Ge alloy matrix. The large Seebeck coefficient of 464 μVK−1 was achieved at 973 K, which might be due to the sharp peak, that consists mainly of 5d states of Au, near the top of the valence band in the electronic density of states. Assisted by the values of electrical resistivity ∼10 mΩcm and thermal conductivity 1.5 Wm−1 K−1, the large Seebeck coefficient led to a value of ZT exceeding 1.6, which is much higher than that of previously reported Si-Ge-based composites.

Influence of fast neutron and gamma irradiation on the thermoelectric properties of n-type and p-type SiGe alloy

The thermoelectric materials that work in radioisotope thermoelectric generators or space reactor power sources will be exposed to neutron irradiation and gamma irradiation for a long time. Herein, the influences of neutron irradiation doses between 1013 n/cm2 and 1015 n/cm2, and gamma irradiation of 104 Gy, on the transport properties of n- and p-type SiGe alloys were investigated. It was found that, neutron irradiation causes defects, and leads to a dramatic decrease in carrier mobility for both n- and p-type SiGe. As a result, the thermoelectric properties of both n- and p-type SiGe decline steeply after neutron irradiation. However, the thermoelectric properties of SiGe alloys can be restored by heat treatment at proper temperature. The n-type SiGe is more sensitive to neutron irradiation than p-type SiGe. After 1015 n/cm2 neutron irradiation, the electrical conductivity, power factor and ZT of n-type SiGe decrease to 0.014%, 5% and 3.8% that of an un-irradiated sample; while the electrical conductivity, power factor, and ZT of p-type SiGe decrease to 0.14%, 12% and 5.2% that of an un-irradiated sample. The healing temperature is 1273 K for n-type SiGe, while 623 K for p-type SiGe. Gamma irradiation has no obvious effect on the transport properties of SiGe alloys.

Impact of the scattering physics on the power factor of complex thermoelectric materials

We assess the impact of the scattering physics assumptions on the thermoelectric properties of five Co-based p-type half-Heusler alloys by considering full energy-dependent scattering times vs the commonly employed constant scattering time. For this, we employ density functional theory band structures and a full numerical scheme that uses Fermi's golden rule to extract the momentum relaxation times of each state at every energy, momentum, and band. We consider electron-phonon scattering (acoustic and optical), as well as ionized impurity scattering, and evaluate the qualitative and quantitative differences in the power factors of the materials compared to the case where the constant scattering time is employed. We show that the thermoelectric power factors extracted from the two different methods differ in terms of (i) their ranking between materials, (ii) the carrier density where the peak power factor appears, and (iii) their trends with temperature. We further show that the constant relaxation time approximation smoothens out the richness in the band structure features, thus limiting the possibilities of exploring this richness for material design and optimization. These details are more properly captured under full energy/momentum-dependent scattering time considerations. Finally, by mapping the conductivities extracted within the two schemes, we provide appropriate density-dependent constant relaxation times that could be employed as a fast first-order approximation for extracting charge transport properties in the half-Heuslers we consider.

Combinatorial Tuning of Structural and Optoelectronic Properties in CuxZn1−xS

Summary P-type transparent conductors (TCs) are important enabling materials for optoelectronics and photovoltaics, but their performance still lags behind n-type counterparts. Recently, semiconductor CuxZn1−xS has demonstrated potential as a p-type TC, but it remains unclear how properties vary with composition. Here, we investigate CuxZn1−xS across the entire alloy space (0 ≤ x ≤ 1) using combinatorial sputtering and high-throughput characterization. First, we find a metastable wurtzite alloy at an intermediate composition between cubic endpoint compounds, contrasting with solid solutions or cubic composites (ZnS:CuyS) from the literature. Second, structural transformations correlate with shifts in hole conductivity and absorption; specifically, conductivity increases at the wurtzite phase transformation (x ≈ 0.19). Third, conductivity and optical transparency are optimized within a "TC regime" of 0.10

Critical evaluation of p-type doping effects in Bi-Sb alloys

We have investigated the doping effects of elements (Sn, Pb, Ge) with valence 4 in Bi-Sb alloys. The solubilities of these elements in Bi-Sb alloy are known to range from ∼3 to ∼0.5 to zero atomic percent, respectively. We employed a rapid solidification method to prepare samples that allowed us to achieve a high doping level of Ge and a high doping efficiency of Pb in Bi85Sb15. Using this method, unprecedented high Ge content (up to 13 atomic percent) could be solutionized in Bi-Sb. Microstructure analyses and thermoelectric transport property measurements have been performed on the doped samples which all showed p-type properties at cryogenic temperatures. The doping efficiency of the elements (Sn, Pb, Ge) was calculated based on the Hall measurements and discussed in light of donor and acceptor sites in Bi-Sb. A two-band effective mass model was used to simulate the thermoelectric transport properties within the Boltzmann transport theory. The present study revealed a gap-decreasing phenomenon in the undoped and doped samples. This gap-decreasing behavior is a heretofore unrealized obstacle to achieving a high thermoelectric figure of merit in p-type Bi-Sb alloys.

Microstructure to magnetic properties and nanoindentation mapping of mechanical properties relationship in amorphous and partially crystallized Fe–Si–B–P-type alloy

Microstructure, crystallization behavior, magnetic properties as well as nanoindentation mapping of mechanical properties were studied for amorphous and partially crystallized Fe72Co6Si4B9Mo1P8 alloy. X-ray diffraction accompanied by transmission electron microscopy studies confirm amorphicity of the as-quenched precursor, and complex nature of the material annealed at 873 K for 0.5 h. The primary at Tx1 = 766 K and secondary at Tx2 = 880 K crystallization temperatures were derived from DSC curve. Moreover, Curie points of 646 K and 1032 K for amorphous and crystalline phase created during heat treatment process, respectively, were also determined. Investigations of the magnetization versus temperature in zero field cooled mode allowed to determine Curie points for the amorphous and partially crystallized Fe72Co6Si4B9Mo1P8 alloy, as well as magnetic behavior of the investigated samples. Single phased and dual phased structures described by different mechanical properties for amorphous and annealed samples were confirmed by distribution maps of mechanical parameters. The as-quenched material is characterized by hardness of HVIT= (1050.31 ± 41.63) Vickers, elastic modulus of EIT= (107 ± 5) GPa and elastic deformation energy to total energy ratio of nIT= (46.68 ± 1.06) %. After annealing at 873 K bimodal distributions of mechanical parameters were observed. Evolution of hardness and elastic modulus as a function of penetration depth shows that with increasing of load both HVIT and EIT decrease. Moreover, elastic modulus for the amorphous and partially crystallized sample calculated for maximum load of 250 mN are almost the same.

High thermoelectric power factors of p-type half-Heusler alloys YNiSb, LuNiSb, YPdSb, and LuPdSb

Electronic structures and transport properties of YNiSb, LuNiSb, YPdSb, and LuPdSb were studied from first principles. Low effective masses of hole-like carriers (0.15–0.27) are a characteristic feature of these antimonides. A strong variation of a carrier relaxation time approximated for the materials within the deformation potential theory is revealed. The power factors of 4–5 mWK−2m−1 at room temperature were predicted for YNiSb and LuNiSb for carrier concentrations of about 1019 cm−3. They are comparable with power factors observed experimentally for the best known p-type thermoelectric materials, i.e., SnSe. The findings presented here indicate the Ni-based half-Heulser antimonides as very promising materials for thermoelectric applications.