Thermoelectric Materials

Thermoelectricity refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. In modern technical usage, the term refers collectively to the Seebeck effect, Peltier effect, and the Thomson effect. Various metals and semiconductors are generally employed in these applications. One of the most commonly used materials in such applications is bismuth telluride(Bi2Te3).

Over recent decades, efforts have been made to improve the efficiency of thermal processes and the output energy of thermal engines. One approach is to generate electrical energy from heat which has been released to the environment. For such applications, thermoelectric materials with high working temperatures and optimized efficiency need to be developed.

Very often, accurate knowledge of certain thermal properties such as thermal stability, thermal diffusivity or thermal conductivity is of paramount importance in the application of these new materials. Measurements are necessary to resolve problems regarding a variety of issues, such as heat transfer within a given structure or the formation of thermally-induced stresses between two different materials which are in contact with each other.

Figure of Merit

Beneficial thermoelectric materials should possess high Seebeck coefficients, high electrical conductivity and low thermal conductivity. A high electrical conductivity is necessary to minimize Joule heating, while a low thermal conductivity helps to retain heat at the junctions and maintain a high temperature gradient. These three properties all factor into what is known as the “figure of merit”, Z. Since Z varies with temperature, a useful dimensionless figure of merit can be defined as ZT. The dimensionless figure of merit is calculated as follows:

where:
S = Seebeck coefficient or thermo power [μV/K]
σ = electrical conductivity [1/(Ωm)]
λ = total thermal conductivity [W/(m·K)]

ZT is a very convenient figure for comparing the potential efficiency of devices constructed of different materials. Values of ZT = 1 are considered good, but values in at least the 3-4 range would be considered essential in order to be competitive in terms of efficiency with regards to mechanical energy generation and refrigeration. To date, however, such values have not been achieved; the best reported ZT values have been in the 2-3 range.

Approximate figure of merit (ZT) for various p-type and n-type thermoelectric materials.
Source: G. Jeffrey Snyder, California Institute of Technology
http://thermoelectrics.caltech.edu. Reproduced with permission

Recommended literature:

Recommended literature:

Detailed Insight Into the World of Thermal Analysis

Detailed Insight Into the World of Thermal Analysis

To optimize thermoelectric devices, thermal properties must be known. The thermal conductivity has a direct impact on the efficiency of a thermoelectric material.

Some Application Examples

Some Application Examples

Pure Copper

Thermal Diffusivity Measurement

The thermal diffusivity of pure copper was measured for both the heating and cooling cycles. The large change in the thermal diffusivity at approximately 1080°C is due primarily to the change in the electronic component of the thermal conductivity upon melting or solidification. The fact that there is almost no difference in the thermal diffusivity values between the heating and cooling cycles indicates that no significant micorstructural changes occured. The measured values of the thermal diffusivity for both the solid and liquid regions deviated from those found in the literature by less than 2.5%. Note that the change in thermal diffusivity provides a good temperature calibration point for the LFA (melting point of pure copper = 1083°C). (measurement with LFA 427)

Pure Copper - Thermal Diffusivity

Ag1-xPb18MTe20

Thermal Conductivity Measurement

This plot shows the measurements carried out on AgPb18Te20 in the temperature range from 150°C to 370°C. The lattice conductivity can be calculated from the measured thermal conductivity. Here, the temperature dependence of the total thermal conductivity (λtot) and lattice thermal conductivity (λlatt) of AgPb18BiTe20 is demonstrated.
The inset depicts the temperature dependence of the lattice thermal conductivity of Ag1-xPb18BiTe20 (x = 0, 0.3), compared with the lattice thermal conductivity of AgPb18BiTe20 (presented in + symbol). (measurement with LFA 457 MicroFlash®)

Silver-Lead-Bismuth Telluride — Thermal ConductivityAg1-xPb18MTe20 (M = Bi, Sb); published by Kanatzidis et al., Northwestern University, IL, USA [1]. Measurements were carried out in the Netzsch LFA 457 MicroFlash®. The typical sample dimension with a diameter of 12.7 mm and thickness of 2 mm is used.

Lead Tellurides alloyed with Germanium and Silicium

Thermal Conductivity Measurement

In the lead telluride materials PbTe-Ge and PbTe-Ge1-xSix the thermal conductivity is easily tuned by alloying Ge with Si and reducing the Ge content [2] .

The below shown results are obtained in the temperature range between 25°C and 320°C. Plot A shows that Ge has a significant influence in the lattice conductivity of PbTe. With decreasing Ge content the lattice conductivity decreases over the entire temperature range. In addition, alloying Ge with Si of the PbTe-Ge (20%) composition a further reduction of the lattice conductivity can be observed (plot B). Similar behavior can be seen at constant mixing ratio of Ge and Si and decreasing Ge0.8Si0.2 content (plot C). Plot D shows that a ratio of 5% Ge-/Ge-Si achieves an optimum lattice thermal conductivity.    

[2] Sootsman, Joseph R.; He, Jiaqing; Dravid, Vinayak P., Li, Chang-Peng; Uher, Ctirad; Kanatzidis, Mercouri G. High Thermoelectric Figure of Merit and Improved Mechanical Properties in Melt Quenched PbTe – Ge and PbTe – Ge1-xSix Eutectic and Hyper-eutectic Composites J. Appl. Phys. (2009), 105, 083718. (measurement with LFA 457 MicroFlash®)

Lead Tellurides alloyed with Germanium and Silicium - Thermal Conductivity

Skutterudite

Cubic skutterudite materials of the form (Co,Ni,Fe)(P,Sb,As)3, have a potential for high ZT values due to their high electron mobility and high Seebeck coefficient. Unfilled CoSb3-based skutterudites are disadvantaged by their inherently large thermal conductivity, which lowers their ZT value. However, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted. These alter the thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity. This makes these materials behave like a PGEC (phonon-glass, electron crystal). It is proposed that in order to optimize ZT, phonons which are responsible for thermal conductivity must experience the material as they would in a glass (high degree of phonon scattering– lowering the thermal conductivity) while electrons must experience it as a crystal (very little scattering – maintaining the electrical conductivity).

Lattice Thermal Conductivity and Figure of Merit of La0.9CoFe3Sb12

The effect of introducing a nanoparticle layer in La0.9CoFe3Sb12 in order to reduce the thermal conductivity is investigated up to 550°C. The thermal conductivity (l)was calculated by using the heat capacity (cp) predetermined in the DSC 404 F1 Pegasus®. The lattice thermal conductivity was found by calculating the electrical thermal conductivity using the Wiedemann-Franz relationship and subtracting it from ltotal.

At 452°C, the ZT exhibits its maximum, and the 5 wt.-% nanocomposite shows the highest ZT with an improvement of nearly 15% over that of the control sample that contains no nanoparticles (orange dots). These results show that nanoparticles introduced in already optimized skutterudite systems can further reduce the thermal conductivity and therefore improve ZT within a broad temperature range.