Lead Antimony Silver Tellurium Research Articles

Thermoelectric properties of p-type Ag1−x(Pb1−ySny)mSb1−zTem+2

The thermoelectric properties of Ag1−x(Pb1−ySny)mSb1−zTem+2 (4≤m≤16, −0.1≤x≤0.3, 1/3≤y≤2/3, 0.2≤z≤0.4; Lead Antimony Silver Tellurium Tin, LASTT-m) compositions were investigated in the temperature range of 300 to ~670K. All samples crystallize in the average NaCl-type structure without any noticeable second phase and exhibit very narrow bandgaps of <0.1eV. We studied a range of m values, silver concentrations (x), Pb/Sn ratios (y), and antimony concentrations (z) to determine their effects on the thermoelectric properties. The samples were investigated as melt grown polycrystalline ingots. Varying the Ag contents, the Pb/Sn ratios, and the Sb contents off-stoichiometry allowed us to control the electrical conductivity, the Seebeck coefficient, and the thermal conductivity. The electrical conductivity tends to decrease with decreasing m values. The highest ZT of ~1.1 was achieved at ~660K for Ag0.9Pb5Sn5Sb0.8Te12 mainly due to the very low lattice thermal conductivity of ~0.4W/(mK) around 660K. Also, samples with charge-balanced stoichiometries, Ag(Pb1−ySny)mSbTem+2, were studied and found to exhibit a lower power factor and higher lattice thermal conductivity than the Ag1−x(Pb1−ySny)mSb1−zTem+2 compositions.

Evolution of phase segregation and eutectic structures in AgPb18SbTe20

The evolution of phase segregation in stoichiometric quenched AgPbmSbTe2+m (m = 18, Lead–Antimony–Silver–Tellurium – LAST‐18) compounds was studied starting from the known pseudo‐binary diagrams among Ag2Te, PbTe, Sb2Te3 and AgSbTe2. The compositions of secondary phases indicate that liquid phase during cooling, even under quenching conditions, follows mainly the liquidus line on the 2PbTe–Ag0.45Sb0.55Te1.05 quasi‐binary section of the phase diagram until it reaches a critical point (18 mol.% of 2PbTe) and then turns to Ag2Te‐ and Sb2Te3‐rich sides of quasi‐ternary system. This has led to the formation of various secondary phases at various stages during the solidification, whose microstructural features and morphology strongly depend upon their chemical composition. Moreover, during solidification the local compositional fluctuations of liquid phase in combination with the shift of liquid composition towards Sb‐rich side of the phase diagram resulted in the development of eutectic microstructures in some regions of LAST‐18 matrix phase. This suggests there exists a miscibility gap and eutectic point below 600 °C on the 2PbTe–Ag0.45Sb0.55Te1.05 boundary line. These eutectic lamellar structures with a cumulative composition close to LAST‐3 are on the 200–500 nm length scales and possess thermal conductivity of 0.55–0.65 W/m K at room temperature. The low thermal conductivity of lamellar eutectic structures was later confirmed on bulk samples using laser flash analysis, where the samples were synthesized by quenching and annealing. The results clearly demonstrate that one can engineer the microstructures in LAST compounds by selecting the appropriate initial composition from quasi PbTe–Ag2Te–Sb2Te3 ternary phase diagram to lower the thermal conductivity further.

Thermal Fatigue of Cast and Hot-Pressed Lead-Antimony-Silver-Tellurium (LAST) Thermoelectric Materials

Lead-antimony-silver-tellurium (LAST) thermoelectric materials are candidates for waste-heat recovery applications. However, rapid heating and cooling (thermal shock) imposes thermomechanical stresses that can cause microcracking. Waste-heat recovery applications involve thermal fatigue, in which a series of hundreds or thousands of individual thermal shock events can lead to accumulation of microcrack damage in brittle thermoelectrics such as LAST. Microcracking in turn leads to a decrease in transport properties, such as electrical conductivity and thermal conductivity, and mechanical properties, including elastic modulus and strength. Thus, microcracking can affect both thermoelectric performance and mechanical integrity. In this study, LAST specimens were rapidly cooled (quenched) into a fluid (water or silicone oil) in order to compare the results with the vast majority of thermal shock studies of brittle materials that are quenched in a similar manner. Decreases in elastic modulus, E, with accumulating microcrack damage were measured using resonant ultrasound spectroscopy (RUS). The evolution of thermal fatigue damage observed in this study is also described well by an equation that successfully describes thermal fatigue damage in a variety of brittle materials.

Elastic modulus, biaxial fracture strength, electrical and thermal transport properties of thermally fatigued hot pressed LAST and LASTT thermoelectric materials

Harvesting of waste heat may lead to macrocrack and/or microcrack damage accumulation in thermoelectrics. No studies in the open literature address the thermal fatigue of any thermoelectric material. This study characterizes the thermal fatigue behavior for two PbTe-based thermoelectric materials, n-type LAST (lead–antimony–silver–tellurium) and p-type LASTT (lead–antimony–silver–tellurium–tin). The mechanical properties (fracture strength, elastic moduli) were evaluated for up to 200 thermal fatigue cycles. In addition, the electrical and thermal transport properties were evaluated for n- and p-type specimens for thermal cycling. The elastic moduli were relatively insensitive to thermal fatigue treatment. The fracture strength, σf, of the thermally fatigued LASTT specimens was in a band of from 25 to 40 MPa while σf of the thermally fatigued LAST ranged from 15 to 38 MPa. The thermopower and electrical conductivity of LASTT samples showed small deviations from the low temperature trend near 600 K and the data repeated well after the first temperature cycle for all samples. For the n-type LAST samples, the electrical conductivity and thermopower showed larger deviations from the low temperature trend near 500 K with some samples requiring several temperature cycles before showing repeatability in the data, suggesting a possible secondary phase in the samples.

Microstructure analyses and thermoelectric properties of [formula omitted

This study reports microstructural investigations of long-term annealed Ag1−xPbmSb1+yTe2+m (m=18, x=y=0, hereinafter referred to as AgPb18SbTe20) (Lead–Antimony–Silver–Tellurium, LAST-18) as well as of Ag1−xPb18Sb1+yTe20, i.e. Ag-deficient and Sb-excess LAST-18 (x≠0,y≠0), respectively. Two different length scales are explored. The micrometer scale was evaluated by SEM to analyze the volume fraction and the number of secondary phases as well as the impact of processing parameters on the homogeneity of bulk samples. For AgPb18SbTe20, site-specific FIB liftout of TEM lamellae from thermoelectrically characterized samples was accomplished to investigate the structure on the nanometer scale. High-resolution TEM and energy-filtered TEM were performed to reveal shape and size distribution of nanoprecipitates, respectively. A hypothesis concerning the structure–property relationship is set out within the frame of a gradient annealing experiment. This study is completed by results dealing with inhomogeneities on the micrometer scale of Ag1−xPb18Sb1+yTe20 and its electronic properties.

Synthesis and Thermoelectric Properties of LAST System Bulk Materials: Substitution of Sulfur for Tellurium

Nonstoichiometric lead-antimony-silver-tellurium (LAST) system thermoelectric bulk materials Ag0.8Pb22.5SbTe20−xSx (x = 0 to 8.0) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The electrical and thermal transport properties were investigated in the temperature range of 300 K to 700 K. The x-ray diffraction (XRD) results indicated that sulfur entered the PbTe during MA process, but gradually precipitated in the sintering process in the form of PbS from the PbTe matrix when the sulfur content was changed from x = 2.0 to x = 8.0. It was confirmed that the addition of sulfur effectively reduced the lattice thermal conductivity. A low thermal conductivity of 0.83 W m−1 K−1 at 673 K was obtained for the Ag0.8Pb22.5SbTe12S8 sample. Benefiting from the high electrical conductivity, the Ag0.8Pb22.5SbTe18S2 sample reached a maximum ZT value of ∼0.97 at 673 K, which is 32% higher than its counterpart without sulfur substitution.

Microstructures and nanostructures in long-term annealed AgPb18SbTe20 (LAST-18) compounds and their influence on the thermoelectric properties

Abstract

An in situ SEM experimental study of the thermal stability of a LAST thermoelectric material

Thermal stability is a requirement for the deployment of thermoelectric (TE) materials in the application of power generation. Lead–antimony–silver–tellurium (LAST) is an emerging material with promising TE properties. This study focuses on the thermal stability of a LAST sample with composition Ag0.86Pb19SbTe20 fabricated from a cast ingot. Using a customized heating stage, the morphology of the LAST particles was studied via scanning electron microscopy (SEM) in situ, between room temperature and 848 K. The material was found to be stable below 823 K. The inclusion phase, which was antimony-rich, had a lower thermal stability than the PbTe-rich matrix. SEM findings were consistent with the results of a thermogravimetric analysis. The results from an in situ heating study suggest that protective environments should be used during the processing and application of LAST materials.

Electrical behaviors for growth of LAST alloys using vapor phase deposition

The well-dispersed crystal structures of LAST (Lead Antimony Silver Tellurium) alloys grown by a vapor phase deposition method are demonstrated to have excellent electrical characteristics in this study. The atomic ratio of Pb to Te in the alloys are kept at 0.85–0.88. The distribution of component concentrations depends on the deposition temperature, and the Ag- and Sb-rich segments are fabricated at high deposition temperature. Experimental data suggest that the higher Sb content in the crystal increases carrier concentration up to ∼10 19 (per cm). The electrical behavior of the crystal is n-type under Sb doping, and it was transferred into p-type once the samples are dominated by Ag doping, which also raises the mobility to a level as high as ∼10 3 cm 2/V. Test results of this study have indicated that both Ag and Sb elements contribute to the electrical conductivity enhancement, however, an excess amount of Ag may significantly deteriorate the electrical performance instead.

Porosity dependence of elastic moduli in LAST (Lead–antimony–silver–tellurium) thermoelectric materials

Porosity in thermoelectric materials affects the thermal, electrical and mechanical properties of the materials. In this study, the resonant ultrasound spectroscopy technique was used to determine the Young's modulus, E, shear modulus, G, and Poisson's ratio, ν, of 12 hot pressed and five cast polycrystalline specimens of the thermoelectric material LAST (lead–antimony–silver–tellurium) as a function of volume fraction porosity, P, for P ranging from 0.01 to 0.14. A least-squares fit of the Young's and shear moduli data to the relationships E = E D (1 − b PE P) and G = G D (1 − b PG P), respectively yielded E D = 58.4 ± 0.6 GPa and G D = 23.0 ± 0.2 GPa, where E D and G D are the estimated Young's modulus and shear modulus at room temperature for theoretically dense specimens, respectively. The unitless, material-dependent constants b PE and b PG were b PE = 3.5 ± 0.2 and b PG = 3.5 ± 0.2.

Agglomeration during wet milling of LAST (lead–antimony–silver–tellurium) powders

LAST (lead–antimony–silver–tellurium) compounds comprise a family of semiconducting materials with good thermoelectric properties. However, the as-cast form of LAST exhibits large grain size and hence low mechanical strength. Powder processing can produce a fine powder particle size that enhances fracture strength, however the powders tend to agglomerate if the individual powder diameters are less than a few microns across. Dry milling or wet milling (hexane additions of 0cm3 and 10cm3) produced hard agglomerates roughly 40μm in diameter while wet milling with hexane additions of 25cm3, 30cm3 or 50cm3 resulted in small, porous agglomerates roughly 20μm in diameter. Thus, by adjusting the amount of milling liquid used while milling LAST powders, one can shift from hard to soft agglomerates, where the literature shows that soft agglomerates are less harmful to the final, sintered product. Also, in agreement with the results from the literature on other materials, wet milling of LAST powders produced smaller particle sizes but required longer times to reach the grindability limit.

Characterization of dry milled powders of LAST (lead–antimony–silver–tellurium) thermoelectric material

Although thermoelectric (TE) materials are often fabricated as cast ingots, there has been recent interest in the powder processing of these materials. Cast TE materials typically have grain sizes as large as several hundred microns, but powder processing (grinding, milling and then sintering) can produce dense specimens with a reduced grain size, an improved mechanical integrity and enhanced TE properties. In the TE literature when powder processing is employed, little or no description is provided of the powder processing techniques and the powder processing parameters are either not characterized or only a mean particle size is given. In this study, powders milled from solid cast ingots of the TE compound LAST (lead–antimony–silver–tellurium) were characterized via Coulter Counter, to measure the mean powder particle size and the size distribution, scanning electron microscopy (SEM), to examine the powder particle morphology, and X-ray diffraction (XRD), to detect possible phase changes or amorphization. The impurity levels in the milled powders were examined by wear rate measurements on the milling media, as well as by energy dispersive X-ray analysis and inductively coupled plasma measurements.

Nanostructured Thermoelectric Materials and High-Efficiency Power-Generation Modules

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.

Hardness as a function of composition for n-type LAST thermoelectric material

Compositional changes (alloying) in metals, ceramics and semiconductors lead to changes in microstructure and mechanical properties that range from subtle to dramatic. In particular, hardness measurements have been widely used to track changes in mechanical properties as a function of composition. In this study, composition dependence of the Vickers indentation hardness, H, is examined for 17 specimens that include 15 different n-type compositions within the quaternary chalcogenide family of LAST (lead-antimony-silver-tellurium) high-temperature thermoelectric materials. In addition, nanoindentation measurements were made on six of the 17 specimens that were indented via Vickers indentation. The H values obtained independently by the microindentation (Vickers indentation) and nanoindentation techniques agreed very well. In this study, relationships from the literature for the parabolic dependence of H on the single compositional variable x for pseudo binaries AB 1− x C x is successfully extended to describe the composition dependence of H for a true quaternary system. Possible implications of hardness change on fabrication and in-service reliability are also discussed.

Weibull analysis of the biaxial fracture strength of a cast p-type LAST-T thermoelectric material

This is the first study applying Weibull statistics to the strength distribution of a thermoelectric (TE) material and to determine the fracture strength of a member of the LAST-T (lead–antimony–silver–tellurium–tin) family of high-temperature TE compounds. The p-type TE material Ag0.9Pb9Sn9Sb0.6Te20 was cast from the melt and specimens cut from the resulting ingot were fractured in a ball-on-ring biaxial fracture test. The Weibull parameters (characteristic strength and Weibull modulus) were obtained from the fracture data. Implications of the Weibull analysis on the fabrication of TE devices are discussed.

Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m. The Myth of Solid Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m. The Myth of Solid Solutions

The nature of the thermoelectric materials Ag(1-x)Pb(m)SbTe(m+2) or LAST-m materials (LAST for Lead Antimony Silver Tellurium) with different m values at the atomic as well as nanoscale was studied with powder/single-crystal X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy. Powder diffraction patterns of different members (m = 0, 6, 12, 18, infinity) are consistent with pure phases crystallizing in the NaCl-structure-type (Fmm) and the proposition that the LAST family behaved as solid solutions between the PbTe and AgSbTe2 compounds. However, electron diffraction and high resolution transmission electron microscopy studies suggest the LAST phases are inhomogeneous at the nanoscale with at least two coexisting sets of well-defined phases. The minority phase which is richer in Ag and Sb is on the nanosized length scale, and it is endotaxially embedded in the majority phase which is poorer in Ag and Sb. Moreover, within each nanodomain we observe extensive long range ordering of Ag, Pb, and Sb atoms. The long range ordering can be confirmed by single crystal X-ray diffraction studies. Indeed, data collections of five different single crystals were successfully refined in space groups of lower symmetry than Fmm including P4/mmm and Rm. The results reported here provide experimental evidence for a conceptual basis that could be employed when designing high performance thermoelectric materials and dispel the decades long belief that the systems (AgSbTe2)(1-x)(PbTe)x are solid solutions.