Inversion Boundaries and Phonon Scattering in Ga:ZnO Thermoelectric Compounds

Abstract

We investigated the high-temperature thermoelectric properties of Ga:ZnO bulk compounds, synthesized using a simple and scalable solid-state process. The effects of a low gallium content (x ≤ 0.04 in ZnGaO) on the structural features and electrical/thermal properties are reviewed. Transmission electron microscopy analyses showed that 2D, nonperiodic defects had formed from a doping content as low as x = 0.01 Ga. The structural description of these nanoscale interfaces is, for the first time, carefully investigated in such low-Ga-content samples by HAADF-STEM analyses combined with structural modeling. It was found that the formation of head-to-head inversion twin (h-IT) boundaries and tail-to tail inversion boundaries (t-IB) in the bulk compounds is responsible for strong phonon scattering, while maintaining relatively good electrical conductivity and thereby enhancing the thermoelectric properties. The absolute value of the Seebeck coefficient decreases abruptly from 475 μV/K for x = 0 down to 60 μV/K for x = 0.005 at 350 K. At the same time, the electrical resistivity drops from 1 ohm cm for x = 0 to 1.7 × 10 ohm cm for x = 0.005. For higher Ga additions (x > 0.01), the increase in electrical resistivity is likely linked to the formation of interface defects at a larger extent in the wurtzite structure. The thermal conductivity also drops sharply with the increase in the Ga content from ∼33 W/m K for x = 0 to ∼8 for x = 0.04 at 350 K. This study is progress toward the synthesis of other thermoelectric materials where nanoscale interfaces in bulk compounds provide tremendous opportunities for further enhancing both the phonon scattering and the overall figure of merit.

In2O3-Based Thermoelectric Materials: The State of the Art and the Role of Surface State in the Improvement of the Efficiency of Thermoelectric Conversion

“…The doping of sintered ZnO with gallium also did not noticeably improve its thermoelectric characteristics [109][110][111]. Despite an increase in conductivity by almost two orders of magnitude to 1 S/cm and a decrease in thermal conductivity to 3.5 W/m·K, the figure of merit for the ZnO:Ga ceramics did not exceed 0.1 at 1000 K. In the case of ZnO:Ga (1%) films deposited by atomic layer deposition (ALD) [112], the conductivity can reach~2 × 10 3 S/cm.…”

An alternative, faster and simpler method for the formation of hierarchically porous ZnO particles and their thermoelectric performance

“…Some notable examples of oxide-TE materials are Na x Co 2 O 4 and SrTiO 3 . 5,7,[9][10][11] We believe that these oxide-based materials could provide an inexpensive, environmentally-friendly viable high temperature thermoelectric material. The progress in developing n-type oxide TE materials with comparable ZT is still lacking, which calls for more research efforts in this area.…”

“…Using ZnO-based thermoelectric materials can be one of the good examples in this strategy. [5][6][7][12][13][14][15][16] ZnO is a promising n-type semiconductor due to its good Seebeck coefficient ($200 mV K…”

“…2,6,7 The temperature dependence of the electrical resistivity (1/s, Fig. 5a) and Seebeck coefficient (S, Fig.…”

“…5,7 The Zn-O bond has a covalent character because of the small electronegativity difference which leads to a relatively large carrier mobility. 7 However, the simple wurtzite crystal structure and light Zn and O elements result in thermal conductivity as high as $100 W m À1 K À1 (ref. [17][18][19] at RT (for example in ref.…”