B. A. Cook

Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material

The existence of a large thermoelectric figure of merit in (AgSbTe2)15(GeTe)85 has been known for many years. However, the nature of the crystallographic transformation in these materials from a high-temperature cubic to a low-temperature rhombohedral polymorph and its effect on electrical transport has not been clearly established. Transmission electron microscopy studies were performed that show extensive twinning in the low-temperature structure, resulting from lattice strain during the dilation along the (111) crystallographic direction. Analysis of differential scanning calorimetric studies indicates that the transformation is of second order, so that the high-temperature cubic phase is nonquenchable. High-temperature x-ray diffraction was performed to establish the transformation temperature, which was found to be complete upon heating at a temperature of 510K. Results of electrical conductivity measurements as a function of temperature on as-cast samples are discussed in terms of the observed twinning.

Si80Ge20 thermoelectric alloys prepared with GaP additions

Controlled amounts of GaP and P were added to a Si0.8Ge0.2 matrix by a powder‐metallurgical technique in order to evaluate the optimum composition for thermoelectric applications. Bulk determination of the gallium and phosphorus content in fully dense, hot pressed compacts was performed by inductively coupled plasma atomic emission spectroscopy. The transport properties of the compacts were characterized by Hall effect measurements at room temperature and by measurements of electrical resistivity, Seebeck coefficient, and thermal diffusivity to 1000 °C. Considerable variation in the electrical transport properties were found to accompany changes in the Ga/P ratio, in the total amount of dopant, and changes in other preparation conditions. Alloys with gallium phosphide additions exhibit carrier concentrations higher than those obtained in alloys doped only with phosphorus. Alloys with a nominal phosphorus content greater than 2.0 at. % were found to be overdoped and those containing less than 0.6 at. % pho...

Sn-Ag-Cu solders and solder joints: Alloy development, microstructure, and properties

Slow cooling of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens made by hand soldering simulated reflow in surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shearstrength testing, using Sn-3.5Ag (wt.%) as a baseline. Minor substitutions of either cobalt or iron for copper in Sn-3.7Ag-0.9Cu refined the joint matrix microstructure, modified the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and increased the shear strength. At elevated (150°C) temperature, no significant difference in shear strength was found in all of the alloys studied. Ambient temperature shear strength was reduced by largescale tin dendrites in the joint microstructure, especially by the coarse dendrites in solute poor Sn-Ag-Cu.

Electrical properties of Ga and ZnS doped ZnO prepared by mechanical alloying

A series of n-type ZnO alloys doped with Ga and ZnS were prepared by mechanical alloying. Densities of 95% to 98% of theoretical density were achieved by hot pressing the milled powders at 1000 and 1200 °C, respectively. The electrical resistivity and Seebeck coefficient of alloys containing 0.25–3.0 at. % Ga were characterized between 22 and 1000 °C. The magnitude of the resistivity and Seebeck coefficient at 22 °C ranged from 0.2 mΩ cm and −25 μV/°C for the most heavily doped specimen to 1.1 mΩ cm and −70 μV/°C for the lightly doped material. The alloys exhibit a positive temperature coefficient of resistivity and Seebeck coefficient with a nearly constant slope over the temperature range studied. Thermal diffusivity measurements on a specimen containing 1.0 at. % Ga were performed over the same temperature range. The thermal conductivity appears to follow a T−1 dependence, decreasing from 180 mW/cm °C at 22 °C to 82 mW/cm °C at 1000 °C. An estimate of the maximum dimensionless thermoelectric figure of ...

Extrusion and selected engineering properties of Mo–Si–B intermetallics

Abstract In this study, an extrusion process has been developed to produce defect free, high-density rods of Mo–Si–B material. An initial powder composition (53.5 vol.%, 91 wt.%) of 66 vol.% Mo 5 Si 3 B x (T1)–16 vol.% MoB–18 vol.% MoSi 2 was mixed with a paraffin-wax based binder (46.5 vol.%, 9 wt.%) and extruded using a twin-screw extruder. Following binder removal by a combination process of wicking and thermal degradation, the material was sintered at 1800°C. The bulk density of the sintered material was 90–92% of theoretical. Thorough binder removal was evidenced by low impurity levels: 258±6 ppm carbon and 772±10 ppm oxygen. The material demonstrated excellent high temperature oxidation resistance. The calculated parabolic rate constant is 1.1×10 −2 mg 2 /cm 4 /h at 1600°C. The extruded material was also successfully tested as a resistance heating element. These materials show promise for the development of heating elements with enhanced performance compared to current MoSi 2 -based heating elements.

Coefficient of thermal expansion of AlMgB14

Abstract The coefficient of thermal expansion (COTE) of AlMgB14 was measured by dilatometry and by high temperature X-ray diffraction using synchroton radiation. The COTE over the temperature range 298 K to 1373 K was determined to be 9×10−6 K−1, which is relatively close to the value of 8.3×10−6 K−1 for pure B. The anisotropy of the COTE appears to be relatively small.

Electrical properties of some (1, 1, 1) intermetallic compounds

Many intermetallic compounds form with a gap in their density of states at the Fermi level, giving rise to relatively large Seebeck coefficients, on the order of −150 to −300 μV/deg. at room temperature. Consequently, when combined with reasonable carrier mobilities in the range of 30 to 50 cm2/V-s, ‘half-Heusler’ compounds, such as MNiSn where M = (Ti, Zr, Hf), become attractive candidates for intermediate temperature (300°C to 600°C) thermoelectric applications. Samples of TiNiSn were prepared by arc melting and homogenized by various heat treatments. The temperature dependence of the electrical resistivity, Seebeck coefficient, and thermal diffusivity of these samples was characterized between 22°C and 900°C. The electrical resistivity and thermopower both decrease with increasing temperature, consistent with semiconducting behavior. The electrical power factor, defined as S2/ρ where S is the Seebeck coefficient and ρ is the resistivity, appears quite sensitive to the degree of homogenization in the microstructure and values in excess of 25 μW/cm-°C2 were observed in nearly single phase alloys within the 300 to 600°C temperature range. A brief survey of other selected ternary intermetallic compounds is also presented.

Characterization of thermoelectric elements and devices by impedance spectroscopy.

This article describes a new measurement technique that utilizes impedance spectroscopy for the characterization of thermoelectric materials and devices. Two circuit models were developed and used to help explain the impedance spectroscopy data using transmission line theory and a coupled electrothermal model. Two testing configurations have been investigated including one based on a sinusoidal source (ac lock-in technique) and one based on a pulsed wave source. Methods for reducing the measurement times for this technique are discussed. In addition, the influence of radiation losses on this measurement technique has also been analyzed to further understand the limitations of this technique at higher temperatures.

Al2MgO4, Fe3O4, and FeB impurities in AlMgB14

Abstract AlMgB 14 materials with Si and Ti additions produced by mechanical alloying/hot uniaxial pressing have recently been found to display hardness greater than 40 GPa. Al 2 MgO 4 and Fe 3 O 4 , and FeB are common impurities in AlMgB 14 produced in this manner. The characterization of these impurities and their effects on the hardness and electrical properties of AlMgB 14 without Si and Ti additions are presented in this study.

Parasitic effects of oxygen on the thermoelectric properties of Si80Ge20 doped with GaP and P

The presence of oxygen in silicon‐germanium can have a significant effect on the sintering, microstructural, and transport properties of these alloys. A neutron activation study was performed to follow the total oxygen content during the preparation of mechanically alloyed n‐type Si‐20 at. % Ge alloys using either fine powders, polycrystalline Czochralski chunk, or single‐crystal float zoned starting materials. The alloys were prepared with a nominal composition of Si0.747Ge0.187(GaP)0.016P0.034. The total oxygen concentration was measured in the starting materials, after 6 h of mechanical alloying in a helium environment, after hot pressing, and after a 3 h, 1100 °C heat treatment in fused silica ampoules. Alloys that contained oxygen levels in excess of 2.0 at. % exhibited low carrier mobility and low thermal conductivity, whereas those that contained less than 0.6 at. % oxygen possessed high mobility and thermal conductivity. The microstructure, as observed by optical metallography and scanning electro...