Fabrication and property characterization of MNb2O6 (M=Cu, Zn)

Abstract

Binary niobate ceramics are well-known in functional materials. For example, ZnNb2O6 is a microwave dielectric material that can be an alternative compromise to expensive tantalum-based complex perovskites. And CuNb2O6 is a narrow bandgap semiconductor with great potential for photovoltaic devices. This thesis is covered the structure and properties analysis of solid-state reaction synthesised ZnNb2O6 and CuNb2O6. ZnNb2O6 ceramic was determined to be a single orthorhombic phase with a space group symmetry of Pbcn by X-ray powder diffraction (XRPD) and electron diffraction techniques. Results of energy dispersive spectroscopy (EDS) element mapping along with the X-ray photoelectron spectroscopy (XPS) indicate the materials are chemically homogeneous with a stoichiometric composition. The result of current-voltage (I-V) measurement showed an interesting resistive switching behaviour under either a DC (direct current) or low-frequency AC (alternating current) electrical field. Through systematic investigation of electronic and dielectric behaviours by varying electrode materials (gold, silver, and platinum), such a resistive switching behaviour results from a combination of electron transport within the material, electrode/interface effect and intrinsic ion displacement. It is the first time to observe such a resistive switching effect in bulk ceramics rather than previously reported in nano-scaled thin films. This work lightens the promising application of bulk resistive switching devices for future resistive random-access memory and neural network applications. The synthesis of CuNb2O6 is more challenging as most ceramics obtained are either monoclinic single-phase or a mixture of monoclinic and orthorhombic phases. The XRPD pattern of the polished or ground same sample indicates that the stress processing can lead to phase transformation from monoclinic to orthorhombic. In optical absorption spectra, the higher orthorhombic phase ratio ceramic had higher visible light and lower ultra-violet light absorption. Because the absorbability differences and stress stimuli in CuNb2O6 ceramic experienced a one-way stress treatment by plastic, its photocurrent would increase under a 400nm light illumination. This work brings a new perspective to enhance photoresponse in this type of material. Two types of orthorhombic CuNb2O6 powder were obtained by grounding the 1000 celsius sintered mix-phase CuNb2O6 ceramics (earth-yellow powder) or heating the CuNb2O6 ceramics to 1100 celsius (black powder). EDS element mapping indicates both samples' Cu/Nb ratio is 1:2, but XPS spectra indicate the black CuNb2O6 powder contains higher Cu+ ions than the earth-yellow CuNb2O6 powder. XRPD and Electron paramagnetic resonance (EPR) spectroscopy results indicate that the crystal structure in the black CuNb2O6 powder is elongated. Both CuNb2O6 powders have frequency-independent antiferromagnetic performance. This work differentiates two types of orthorhombic CuNb2O6 powder that used to claim as the same in structure, composition, and magnetic behaviours. Dielectric characterization shows the 1:1 CuNb2O6-ZnNb2O6 composite presents better in a lower dielectric loss. An asymmetric resistive switch was presented in the DC I-V curve as well. This combination of p-type CuNb2O6 and n-type ZnNb2O6 is potentially valuable from the rich p-n junction structure that can apply to electro-photocatalysis fields. Show more