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Thermal Conductivity Testing of Thermoelectric Materials

Thermoelectricity is a phenomenon where a temperature gradient between two junctions of a system can create electric potential, or vice-versa. The creation of electric potential (and thus a current) from a temperature gradient is known as the Seebeck effect, and was first observed and reported by Estonian physicist Thomas Johann Seebeck in 1821. The Peltier effect is responsible for the opposite effect, or the generation of a heating or cooling effect from the application of an electric current. The direction of the applied current determines whether a heating or cooling effect is produced. 

A third related effect was predicted in 1854 by William Thomson (Lord Kelvin) which stated that heat would be absorbed or produced when applying a current to a system with a temperature gradient. This would come to be known as the Thomson effect.  

The Seebeck, Peltier and Thomson effects are related by thermodynamics. Other important properties when discussing thermoelectricity are thermopower (also known as the Seebeck coefficient, S) which is equal to the difference in thermoelectric voltage divided by the difference in temperature (S=ΔV/ΔT). 

Thermal Conductivity Applications Thermoelectric Generator Diagram Mike

Figure 1. Setup for a thermoelectric device, composed of two faces that are separated by the thermoelectric material, made up of p and n type semiconductors.

Through the use of thermoelectric materials, heating or cooling effects can be produced by the application of a current, or vice-versa. In this way, electricity can be generated in hot or cold environments. Unfortunately, this technology has not yet become widespread due to the poor conversion efficiency experienced by these systems. Some of the more efficient systems recently developed may experience upwards of 5% conversion efficiency, but this is near the upper limit.  

Researchers have spent decades trying to improve the efficiency of thermoelectric materials and are slowly making progress towards better materials. There are three fundamentally important properties of a thermoelectric system that must be optimized in order to produce a near-ideal system. Unfortunately these three properties are closely related and are usually inherent properties of a material. An ideal thermoelectric material will have a high electrical conductivity to minimize Joule heating, a high thermopower to maximize voltage generated per degree of temperature gradient, and a low thermal conductivity in order to maintain a high temperature gradient between the sides of different temperature. Thermal conductivity measurements on thermoelectric materials can easily be made using Hot Disk TPS instruments.  

These three properties can be used to calculate figure of merit (ZT) which is an overall indicator of the quality of a thermoelectric material. ZT = S2σT/λ, where S is thermopower/Seebeck coefficient, σ is electrical conductivity, T is absolute temperature and λ is thermal conductivity.  

There are a variety of systems that are being investigated for their use as thermoelectric materials, and some of the more common ones include: Bi2Te3, Sb2Te3, and CaMnO3. These systems have been investigated in some cases for many decades now. The most effective thermoelectric materials being produced today involve metal alloys or doped systems in which the electrical and thermal properties are altered through the dopant.  

There are many applications for thermoelectric materials in both everyday life and the scientific and engineering disciplines. Primarily, thermoelectric materials are used in niche heating/cooling areas, where conversion efficiency isn’t an important factor. Some thermoelectric materials are currently being used in vehicle seat heating/cooling systems, optoelectronics and small refrigerators. In addition to these applications, thermocouples also function via thermoelectrics and are a common device in many laboratories.  

Another prominent use of this technology is in radioisotope thermoelectric generators (RTGs) which are used to provide electricity to space vessels. Through the use of radioactive plutonium-238, heat is provided to a thermoelectric material, and thus electricity can be generated from this heat. RTGs have been used by NASA in many of their missions, such as Apollo, Viking, Voyager, Galileo and Cassini, in addition to the Mars rover Curiosity being the first rover powered with a special Multi-Mission RTG (MMRTG).  

Since thermal conductivity is important for the design of thermoelectric materials, the analysis of thermoelectric materials is part of what we do here at Thermtest.  

If the energy conversion efficiency of thermoelectric materials can be significantly improved, then the applications for these materials will become virtually limitless. Due to this, a significant amount of research is done each year to develop new thermoelectric materials, or improve the currently used systems. Research performed by Chen et al. looks at using vertically aligned silicon nanowires with large surface areas in thermoelectric materials. The thermal conductivities of the prepared silicon nanowires were measured using the Hot Disk TPS 2500 Thermal Conductivity System. Due to the size and shape of the composites to be measured, the special TPS Slab Module was used for the determination of thermal conductivity. The authors method of preparing the nanowires has resulted in up to a 43% reduction in thermal conductivity, as compared to bulk silicon. Shown below are the thermal conductivity results. The length of the nanowire was negatively correlated with thermal conductivity, and the longest nanowires resulted in the lowest thermal conductivity.

Table 1. Measured Thermal Conductivities of Silicon Nanowire Composites

Length of SiNWs (μm) bulk 35 60 130 143 215 320
Effective thermal conductivity (W/m·K) 138 120 115 111 107 98 79

To read more about this research, follow this link: 

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-18-S3-A467&id=205649 

Learn More About Hot Disk Transient Plane Source (TPS)

The Hot Disk Transient Plane Source (TPS) technique allows for precise thermal conductivity measurement of a huge array of materials ranging in thermal conductivity from 0.005 to 1800 W/m∙K . TPS is capable of measuring bulk and directional thermal properties of solids, liquids, pastes and powders.

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