Powerful Mineral

Skutterudite – The Ideal Candidate for Thermoelectric Applications

1) The Importance of Thermoelectric Materials and Their Application Possibilities

The optimization of energy efficiency is one of the major challenges of the 21st 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.

2) Determination of Thermophysical and Thermoelectric Properties of Materials

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, cp, 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.

3) Skutterudite as an Appropriate Material for Thermoelectric Applications

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.

Skutterudite in particular has the potential for excellent electrical properties. Skutterudite is a mineral consisting of cobalt and arsenic, often contaminated by rare earths. It belongs to the mineral class of sulfides. It owes its name to the city of Skutterud in Norway, which is where this naturally occurring mineral, CoAs3, was first discovered in 1928. It was only in the mid-50s that its excellent electrical properties were recognized. Skutterudite features a very high charge carrier mobility and a medium-sized Seebeck coefficient. Its thermal conductivity, on the other hand, is far too high to have made its efficient use in thermoelectrical applications possible at that point in time. In the 70s, the crystal structure typical for skutterudite was discovered, which can be modified optimally. Two voids in the elementary cell can be filled by the insertion of foreign atoms. This way, the thermal conductivity of skutterudite can be reduced. Since then, skutterudites have been potential candidates for more efficient thermoelectric converters with which, for example, waste heat from the exhaust systems of automobiles could be directly converted into electricity.

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

LFA Measurement

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·cp·ρ (see figure 2).

Fig 1: Measurement of the thermal diffusivity (red curve) and the specific heat capacity (black curve) with the LFA method
Fig. 2: Determination of the thermal conductivity

SBA Measurement

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).

Fig. 3: Determination of the Seebeck coefficient and electrical conductivity between RT and 350°C with the SBA 458 Nemesis®
LFA 467 HyperFlash® – Light Flash Apparatus

The new LFA 467 HyperFlash® features a wide temperature range, very high efficiency (with its sample changer for 16 samples), extremely fast data acquisition rates and an intelligent lens system (ZoomOptics) between the sample and detector.

SBA 458 Nemesis®

Thermoelectric materials should possess high working temperatures and optimized efficiency. The relative performance is described by the figure of merit (ZT). It highlights the importance of the Seebeck coefficient with respect to the performance. The SBA 458 Nemesis® allows for simultaneous measurement of the Seebeck coefficient and electrical conductivity under indentical conditions.

ZT Value

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:

Fig. 4: Only one sample needs to be used for both the LFA and SBA measurements. There is no need for any additional sample preparation to adjust the sample geometry.
Fig. 5: Increase in ZT value between room temperature and 400°C. The maximum is at 0.75.

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

Summary

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.