Thermoelectric Ba8Ga16Ge30 clathrates for waste heat recovery

Licentiatavhandling, 2010

The modern, highly industrialized, society of today requires huge amounts of energy to function. This has resulted in a heavy dependence on the very energy efficient fossil fuels, a source of energy which of course is not infinite and we are slowly approaching the day when oil runs out. As a result of this dependence on fossil fuels we see steadily increasing energy prizes and environmental problems related to the combustion of these fuels. One way of improving fuel economy is to recycle the waste heat produced in all combustion processes. In a normal combustion engine only about one third of the energy released is transformed into usable energy while two thirds are lost as waste heat. To reduce these losses thermoelectric materials can be used to transform some of this waste heat to electrical energy, which can be stored in batteries or used directly. In this thesis the thermoelectric material Ba8Ga16Ge30 is studied with the aim of using it for waste heat recovery in heavy trucks. The materials has been synthesised with direct solid state reaction of pure elements and the phase purity has been investigated with powder XRD analysis. The thermoelectric properties which determine the efficiency of the material have been carefully studied using several methods. Seebeck coefficient and electrical resistivity have been measured with both commercial equipment and laboratory setups, the former utilizing a small ΔT differential method and the four-point-probe while the latter using a light-pipe system and the van der Pauw technique. The thermal conductivity was investigated with two different techniques, the laser flash method and the transient plane source technique. With the help of these techniques the thermal stability of the p-doped version of Ba8Ga16Ge30 has been mapped out, revealing irreversible material property transformations at temperatures above 400°C. At 600°C a transformation from p- to n-type was observed while treatment at 800°C resulted in a close to intrinsic material. Also the effects of partially substituting Ga or Ge in the n-type Ba8Ga16Ge30 with Sb or Zn have been studied. Here it was seen that partially substituting Ge with Zn results in enhanced thermopower and reduced thermal conduction, improving the materials thermoelectric potential up to around 400°C. Partially substituting Ga with either Sb or Zn results, however, in an unexpected deterioration of all thermoelectric properties, including a curious simultaneous increase in both thermal conduction and electrical resistance. Keywords: Clathrates, Ba8Ga16Ge30, waste heat recovery, thermal stability, semiconductors, substitution, Seebeck coefficient, electrical resistance, thermal conductivity