Thermoelectric Materials:

Utilizing Geothermal Heat to Solve the Energy Problem

The amount of electricity which can be generated by solar and wind facilities, currently the most common sources for renewable energy, vary greatly depending on the time of day or the weather, making them inefficient to use. Geothermal and industrial exhaust heat are being evaluated as stable, renewable sources of energy. Thermoelectric conversion, which can convert heat directly into electrical power, is particularly well-regarded as an environmentally friendly electricity generation technology, with no moving parts (like turbines) and no carbon dioxide emission.

Thermoelectric devices generate electricity based on differences in temperature between each side of their materials. As depicted in the following diagram, thermoelectric devices are created by connecting p-type and n-type thermoelectric materials via electrodes in a zigzagging pattern called a π-type structure. The most well-known thermoelectric materials are bismuth telluride (Bi2Te3) and lead telluride (PbTe).

<Thermoelectric Device>

Arranged from top to bottom are a hot ceramic plate, a metal electrode, and a plated solder layer. Electricity is produced through the heat generated by the temperature difference between the p-type and n-type thermoelectric materials connected with a conducting wire to the electrode with lower temperature. Figure: Structure of a thermoelectric device P-type and n-type thermoelectric materials are arranged in a zigzag pattern called a pi-structure between high-temperature and low-temperature layers. Each layer has a plated solder layer, a metal electrode, and a ceramic plate. Electricity is produced by connecting the set of p-type and n-type materials to the plates using a conducting wire on both ends.

In order to promote the wide adoption of waste-heat renewable energy, Panasonic is actively engaged in the research and development of low-cost and high-performance thermoelectric materials which don’t require rare and expensive elements, such as tellurium.

Developing High-performance Thermoelectric Materials

The efficiency of a material’s thermoelectric energy conversion is determined by its figure of merit: ZT = S2 σT/κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and the temperature of the material, respectively. High-performance thermoelectric materials must be thermally insulating (a low κ value) but also electrically conductive (a high σ value). However, because these two characteristics usually exist in a contrary relationship, it can be difficult to create materials with enough of both.

To solve this problem, Panasonic has developed new thermoelectric materials - magnesium antimonide bismuthide (Mg3(Sb,Bi)2) and zirconium nickel antimonide (Zr3Ni3Sb4) - by using computational material engineering technology1,2. Magnesium antimonide bismuthide in particular could be synthesized from low-cost and low-toxicity materials by focusing on their intrinsic defect formation mechanism, leading to a high thermoelectric figure of merit: ZT = 1.51 at 440°C.

<High-performance Thermoelectric Material>

Diagram: Crystal structure of Mg3Sb2
Graph: Temperature dependence of thermoelectric figure of merit (ZT). High thermoelectric conversion efficiency, ZT = 1.51, is achieved at a temperature of 440 degrees Celsius.

(Upper) Crystal structure of Mg3Sb2
(Lower) Temperature dependence of thermoelectric figure of merit (ZT), showing comparisons between known thermoelectric materials3)

Recent Updates

Panasonic is using thermoelectric materials to develop devices for generating electricity. Soon, it will be possible to efficiently convert even relatively cool heat sources of less than 200°C to electricity, allowing the world to take advantage of unused heat sources like geothermal heat and hot waste water from factories.

1) Adv. Mater. 28, 10182 (2016).
2) Appl. Phys. Lett. 104, 122103 (2014).
3) Nat. Mater. 7, 105 (2008).