Exploration and Optimization of Tellurium‐Based Thermoelectrics

Chapter 5. Physical Property Measurements
Physical property measurements are a critical component to understanding and analyzing
thermoelectric materials. Since thermoelectric materials are governed by the dimensionless figure of
merit (ZT) discussed in Section I, the instruments in this chapter generally measure one or more of
those pertaining variables. Physical property measurements not only determine whether a material will
make a good potential application‐grade thermoelectric, but how a material’s behaviour changes with
experimental variations. These property trends are quite valuable as they give an idea of how the
charge carriers within are behaving – something that is typically not directly measured through
instruments. For example, an increase in the electrical conductivity (σ) magnitude can suggest that
there are more charge carriers available to travel through the crystal lattice whilst the shape of the
electrical conductivity curve can imply changing behaviours such as intrinsic semiconductor, extrinsic
semiconductor, semimetal, etc. These concepts (with some examples) were discussed in the previous
Section I.
The vast majority of samples were measured on‐site with instruments belonging to the Kleinke
group utilizing cold‐pressed pellets annealed at temperatures ~100 – 150 K below the material’s melting
point. As a result, pellet densities were between 80 and 90 % of ; this was determined by
weighing the pellet on an analytical balance and measuring the dimensions with a Vernier calliper,
accurate to a 100 th of a millimetre. In order to make proper pellets, powder on the order of 200 – 400
mg was typically used. Selected samples were sent to Professor Donald Morelli of Michigan State
University for hot‐pressing, which entails simultaneously heating the sample and pressing it into pellet
form with an 11.7 mm diameter. This technique generally yields ’s between 95 and 98 % of ,
further improving the outcome of physical property results. Due to the presence of air pockets, grain
boundaries, or potential internal cracks, it is inaccurate to perform property measurements on purely
cold‐pressed samples, as the aforementioned deformities can lower by almost three orders of
magnitude and sometimes to ½ of its realistic value; while the S magnitude is generally unaffected,
strange curve shapes, or behaviours, and even bogus p‐n transitions have empirically been observed on
cold‐pressed samples.
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