Compounds having compositions of Mo3Sb7–xTex (where x = 1.5 or 1.6) have been investigated as candidate thermoelectric materials. These compounds are members of a class of semiconductors that includes previously known thermoelectric materials. All of these compounds have complex crystalline and electronic structures. Through selection of chemical compositions and processing conditions, it may be possible to alter the structures to enhance or optimize thermoelectric properties.
For the investigation, each specimen of Mo3Sb7–xTex was synthesized as follows:
- A mixture of specified proportions of Mo, Sb, and Te powders was heated for 7 days at a temperature of 750 °C in a covered boron nitride crucible in an evacuated, sealed fused silica ampule.
- The chemical reactions were quenched in cold water.
- In an argon atmosphere, the crucible was opened and the reacted powder mixture was ground in an agate mortar.
- The powder was annealed by again heating it as in step 1.
- The powder was formed into a dense cylindrical specimen by uniaxial pressing in a high-density graphite die for 1 hour at a pressure of about 20 kpsi (≈138 MPa) and temperature of 873 °C in an argon atmosphere.
The traditional thermoelectric figure of merit, Z, is defined by the equation Z = α2/ρκ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. Often, in current usage, the term “thermoelectric figure of merit” signifies the dimensionless product ZT, where T is the absolute temperature. The thermoelectric compatibility factor, s, is defined by the equation s = [(1 + ZT)1/2 – 1]/αT. For maximum efficiency, s should not change with temperature, both within a single material, and throughout a segmented thermoelectric-generator leg, the segments of which are made of different materials.
Values of ZT and s were calculated from measurements of the pertinent physical properties of Mo3Sb7–xTex specimens as functions at various temperatures, and the s values of Mo3Sb5.4Te1.6 were compared with those of other state-of-the-art thermoelectric materials (see figure). The ZT values of both Mo3Sb5.5Te1.5 and Mo3Sb5.4Te1.6 were found to increase with temperature up to 1,050 K, which is just below the decomposition temperature. The fact that ZT for x = 1.6 exceeds that for x = 1.5 might be taken as a hint that one could increase ZT by increasing x, except for the observation that attempts to synthesize Mo3Sb7–xTex having x > 1.6 resulted in specimens that appeared to be multiphase. Hence, other approaches to doping may be more promising.
The s value of Mo3Sb5.4Te1.6 was found to increase from about 1 V–1 at 300 K (room temperature) to about 2 V–1 at 1,000 K. This doubling of s indicates poor self-compatibility of Mo3Sb5.4Te1.6 over the affected temperature range. However, Mo3Sb5.4Te1.6 could be suitable for segmentation: for example, a Mo3Sb5.4Te1.6 segment (which can withstand a temperature >800 K) could be joined to a lower-temperature PbTe segment at an interface temperature at or slightly below 800 K, where their s values are equal.