Abstract
The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesised. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.
Highlights
The application of traditional thermoelectric metallic alloys is restricted by several challenges
Temperatures above 1673 K led to microcracks and unnecessary volatilisation of ZnO
In particular we propose that the following factors control the thermal conductivity response of Z9GO homologous compounds: (i) presence of twinned crystal structures; (ii) presence of inversion boundaries; (iii) alternating sequence of occupancy for Ga and Zn with different oxygen coordination in the TB boundaries along the b-direction; (iv) segregation of the Ga in the IBs having different oxygen coordination compared with oxygen coordinated Zn which occupy all the lattice sites between TB and IB boundaries
Summary
The application of traditional thermoelectric metallic alloys is restricted by several challenges. The first Ga2O3(ZnO)m compounds were synthesised by Nakamura et al [10] They proposed a “distorted” wurtzite structure for the solid solution range of (Ga2O3)x(ZnO)1-x (0≤x≤0.093) instead of a layered type structure. Michiue and Kimizuka [5] introduced a superspace formalism to describe the structure of the homologous series Ga2O3(ZnO)m, treating them as compositely-modulated structures, consisting of two subsystems. They suggested that the distribution of Ga and Zn in Ga2O3(ZnO)m resembles the arrangement of Zn cations in the Wurtzite structure of ZnO, and the arrangement of O anions resembles the framework made by O anions in the Wurtzite structure
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