The Synthesis, Structrue And Physical Properties Of The Earth Metal Gd And Its Compouds

The rare earth elements and their compounds with nano-structure, which have characteristics of both rare earth and nano-materials, have received tremendous attention in recent years due to their unique structure and subsequent invariance in their excellent properties. The preparation for nano materials, however, is difficult due to their sensitive to oxidation involving the change in their composition, structure, and properties. As a result, research on the nano-structural rare earth elements and their compounds was impeded by half. In our study, a novel technique route is used to prepared nano-particles and full density bulk nano-crystalline of Gd metals. Investigation of these materials on their structure and physical properties, which could be served as data base in related field, was carried on.The nanoparticles of the pure rare-earth metals Gd have been prepared by the method of the inert-gas condensation. The production ratios and the mean nanoparticle sizes were effectively controlled by adjusting the important parameters of the fabrication technique. The structure of the particles was characterized using TEM, and XRD. Two kinds of crystal structures, hexagonal condensed packed (HCP) and face centered cubic (FCC), were observed simultaneously in the Gd nanoparticles, with increasing the mean particle size, the content of FCC structural Gd nanoparticles reduced gradually to zero.Spark Plasma Sintering (SPS) technology had been applied to synthesize the full density bulk nanocrystalline pure Gd under ultrahigh-pressure using the Gd nanoparticles with mean size of 15nm. With increasing the sintering temperature, the microstructure of bulk Gd sample sequentially went through four typical status transition from original particles - amorphous - amorphous and nanocrystalline - nanocrystalline. On the basis of special sintering mechanism of SPS and investigation on microstructure of the bulk Gd at different processing conditions, sintering dynamics of the bulk Gd samples by SPS under high pressure was proposed. The (00l) plane in Gd crystal structure with easy-axis anisotropy (C axis for hcp structure), which was explained as a pressure-induced anisotropy during SPS process, was observed. Moreover, when the mean grain size of the Gd crystal grains became larger than 5nm, a structural transition from FCC to HCP occurs as the only phase when the size is over 15nm . Research on the magnetism and electricity of bulk Gd samples showed that the existence of quite a few grain boundaries in the microstructure of the samples, in which the Gd-Gd atomic distance was different from that of in the Gd grains, led to series of changes such as decrease of Tc, saturate magnetization, atomic magnetic moment per Gd atom, magneto-caloric effect (MCE), and increase of electric resistance in the bulk nanocrystalline Gd samples.Another research is on the practical application of rare-earth Gd based magnetic refrigerant materials. At present, the room temperature magnetic refrigeration technique attracts more and more attention due to its good energy efficiency and environment amity. As a result, the Gd5(SixGe1-x)4 inter-metallic compounds had become the hotspot in the relative research field. From practical point of view, however, the working temperature span of the single phase alloy limited to application. Therefore, we firstly synthesized the Gd₅Si₂Ge₂ alloy and investigated the effects of the raw material purity, chemical composition and processing conditions on microstructure and MCE of the alloys. A new layer structural composite magnetic refrigerant material with composition of Gd₅(Si_xGe_1-x)₄ had been synthesized by SPS technique. The experimental result ofΔTad - T curve of the composite material has excellent result. The value ofΔTad of the composite exhibits a more constant tendency, which is suitable for practical application at room temperature refrigeration, when compared to those of the three components. Further observation showed that there were few obvious pores or cracks on the boundaries among layers, indicating the good sintering joint, and the atomic inter-diffusion region between different layers was less than～10μm. Consequently, it is expected that the MCE of the sintered composites would be rarely influenced by such small amounts of solid solution material.