Meltspun Ba 8 Ga 16−x Ge 30+x clathrates

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Ba8Ga16xGe30+x is the clathrate with the highest thermoelectric figure of merit known to date. However, no p-type material could be obtained by conventional synthesis from the melt. Here we show that the time and cost-effective melt spinning technique can produce Ba8Ga16xGe30+x in a metastable state, where x can be varied continuously from negative to positive values, resulting in both p- and n-type materials. The quenched phases were characterized by x-ray powder diffraction and transmission electron microscopy. It was surprising that they were perfectly crystalline, with large grain sizes of the order of a micrometer. Temperature dependent measurements of the electrical resistivity, Hall effect, thermopower, and thermal conductivity are presented and discussed in terms of a two-band model. I. INTRODUCTION

II. EXPERIMENTAL

Intermetallic clathrates are promising materials for thermoelectric applications. This is not only due to their low thermal and high electrical conductivities (phonon glass–electron crystal) but also due to the expectation that they are semiconductors and thus have large thermopower values. Band structure calculations of the stoichiometric compound Ba8Ga16Ge30 yield a semiconducting ground state.1 With conventional melting methods, however, such a state could not be realized. Metallike n-type samples are typically obtained instead.2 P-type polycrystalline samples could only be produced with ball milling and spark plasma sintering3 and p-type single crystals by growth from Ga flux.4 Here we use the melt spinning technique, which was discovered only recently to be applicable for the synthesis of cage compounds5,6 to obtain Ba8Ga16xGe30+x in a metastable state. This enables a smooth variation of x and hence a smooth crossover between n- and p-type conduction. It is interesting that the thermal conductivity differs drastically for these two kinds of samples. N-type samples show crystalline behavior, and p-type samples show characteristics of amorphous solids. Currently the melt spinning technique is being used for a number of other thermoelectric materials, for example, skutterudites, Bi2Te3, and PbTe.7–9 The interest for large scale thermoelectric applications is the much faster and thus less energy consuming process of melt spinning compared to conventional synthesis techniques. In the case of clathrates, typical annealing times of several weeks can be replaced by the melt spinning process of a few minutes.

High purity elements (Ba 99.95%, Ga 99.9999%, Ge 99.9999%) were used to prepare samples of the nominal compositions Ba8Ga16xGe30+x with x 5 0.1, 0, and 0.1, by high-frequency melting of the elements in a water cooled copper boat. The premelt was then put into a quartz nozzle to perform melt spinning with the same setup as in Refs. 5 and 6. In this process we inject the melt with a pressure of 300 mbar on a rotating copper wheel with a rotational speed of 1500 rpm. All steps were performed under a protective high purity Ar (99.999%) atmosphere. The resulting polycrystalline flakes have a thic