New oxidation protective coatings for thermoelectric materials

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Oxidation protective glass coating for magnesium silicide based thermoelectrics

A Mg2Si-Mg2Sn based thermoelectric material (TE), with composition of Mg2Si0.487Sn0.5Sb0.013, produced by ball-milling combined with spark plasma sintering, is successfully coated with a new silica-based glass, which is specifically designed, characterised and tested as an oxidation protective coating for mid-temperature range (up to 500 °C) applications. Despite the relatively high coefficient of thermal expansion (CTE) of Mg2(Si,Sn) based materials, very good thermo-mechanical compatibility between the substrate and the coating material is obtained. Oxidation tests, performed at 500 °C for 120 hrs in air, demonstrate the effectiveness of the glass coating for the protection of Sb doped Mg2(Si,Sn) thermoelectric materials

Glass-ceramic joining of Fe22Cr porous alloy to Crofer22APU: interfacial issues and mechanical properties

This work deals with the joining of porous Fe22Cr ferritic stainless steel to a dense Crofer22APU plate by using a silica-based, Ba-containing glass-ceramic. The chemical and interfacial stability and the mechanical properties of the joints were evaluated before and after thermal ageing at 700 ◦ C for 500hrs. The sintering behaviour of the glass was assessed by using heating stage microscopy (HSM) to study the influence of a porous metal substrate on the shrinkage of the joining material. Scanning electron microscopy revealed that there were no defects or cracks at the porous alloy/glass-ceramic interface for both the as-joined samples and the samples after thermal ageing at 700 ◦C for 500 h. However, at this exposure temperature, the porous alloy started to form an oxide scale at the interface with the glass-ceramic and the internal surface of the porous alloy. Finally, the evaluation of the mechanical properties by tensile testing showed that the properties were not affected by thermal ageing at 700°

Orthoenstatite to forsterite phase transformation in magnesium germanate ceramics

Magnesium germanate (MgGeO3) ceramics represent a model system for geological study of the upper earth's mantle and are recently being investigated for applications in which red luminescence properties are required. The mechanical and optical properties of MgGeO3 are greatly influenced by the its polymorphic nature and by the complex series of phase transformations taking place in response to temperature variations and the application of mechanical pressure, as well-documented in the literature. In the present study, an additional phase transformation from orthoenstatite MgGeO3 to forsterite Mg2GeO4 with an olivine-type structure is identified in ceramics sintered in range 1350–1500 °C and ascertained by the refinement of the relative X-ray diffraction patterns. The phase transformation is believed to be driven by the volatilization of GeO2 during high-temperature sintering and it should be taken into consideration when targeting single phase and dense MgGeO3 ceramics

The effect of processing conditions on phase and microstructure of CaGeO3 ceramics

The present study illustrates the effects of processing conditions on phase and microstructure of CaGeO3 ceramics. It is shown that the fabrication of single phase, high density and crack-free CaGeO3 ceramics is particularly challenging using conventional sintering or spark plasma sintering. The main difficulties in achieving single phase are predominantly related to the segregation of an additional phase identified as Ca5Ge3O11, most likely driven by the volatilization of GeO2 during processing at high temperature. This can be mainly suppressed by adding an appropriate excess amount of GeO2. High density can be achieved by conventional sintering and spark plasma sintering (SPS), but in the temperature ranges that lead to the formation of Ca5Ge3O11. The presence of cracks observed in the ceramics is attributed to the development of local residual stresses caused by the anisotropy of the thermal expansion and elastic properties, and it can be avoided under certain SPS conditions