Powerful Mineral

The optimization of energy efficiency is one of the major challenges of the 21^st century. In many industrial applications, huge amounts of unused thermal energy are generated. Such waste heat is produced by melting furnaces, incineration plants, power plants and even motor vehicles – and could all be used for the generation of electrical energy. Along with having a positive environmental impact, this would also contribute significantly to increasing the overall efficiency and profitability of industrial plants. That’s where thermoelectrics come into play.

“Thermoelectric generators”, as they are known, are developed and can be employed in all areas where usable temperature differences are available. Such applications require the development of thermoelectric materials with high working temperatures and optimized efficiency.

For the development and optimization of thermoelectric materials, knowledge of the thermophysical and thermoelectric properties is essential. For assessment of the efficiency, the figure of merit (ZT value) is used. This thermoelectric figure describes how well or poorly suited a special material is for use in a thermoelectric generator. The ZT value thus yields information on the material’s efficiency.

With the SBA, the Seebeck coefficient, S, and electrical conductivity, σ, can be determined simultaneously. Using the LFA, the specific heat capacity, c_p, and thermal diffusivity, a, can be measured directly. Along with the density, ρ, the thermal conductivity, λ, can be calculated. 

With the SBA 458 Nemesis^® and the laser flash apparatuses LFA 427, LFA 457 and LFA 467, NETZSCH offers a complete solution for determination of the ZT value.

Currently, the enormous costs for development and the presently low efficiency of thermoelectric materials often prevent their application. To overcome this, the efficiency of thermoelectrics must be significantly increased via new developments and modifications.

The objective is to develop materials exhibiting low thermal conductivity, λ, with simultaneously high conductivity, σ, and a high Seebeck coefficient, S. The difficulty here is in the fact that these three properties can only be influenced independently of one another under certain conditions.

The following measurement examples show how the ZT value of a skutterudite system can be determined by means of a single sample.

For calculation of the dimensionless ZT value of skutterudite, the thermal diffusivity (figure 1, red curve) and the specific heat capacity (figure 1, black curve) were determined with the LFA 467 HyperFlash on a sample with a diameter of 12.7 mm. The measurements were carried out between room temperature and 400°C.

Calculation of the thermal conductivity is based on the results obtained by means of the following equation: λ = a·c_p·ρ (see figure 2).

With the SBA 458 Nemesis^®, the Seebeck coefficient and electrical conductivity of the sample already used for the LFA measurement was determined between RT and 350°C. The Seebeck coefficient increased from 100 µV/K to almost 160 µV/K while the electrical conductivity decreased from approx. 1300 S/cm to 1000 S/cm. The measurement results exhibit excellent reproducibility (± 2%) for both parameters (see figure 3).

The ZT value is calculated by means of the results obtained with the LFA und SBA on the same sample (see figure 4) using the following equation:

The plot in figure 5 represents the increase in ZT value between room temperature and 400°C with a maximum at 0.75.

It was demonstrated that the thermophysical properties – including thermal diffusivity and thermal conductivity, specific heat capacity, Seebeck coefficient and electrical conductivity – can be determined using only a single sample.