Wettability and interface microstructure were studied for Nb-containing melts in the contact with ceramic oxide materials of Al2O3-SiO2 system. Ni-40.5 at.%Nb and Au-(0-20) at.%Nb alloys were investigated as prospective alloys for high temperature brazes. Ceramic samples ranged in composition from pure Al2O3 to pure SiO2. The wettability was measured by the sessile drop method. Microstructure of the oxide/metal interface was investigated by scanning electron microscopy and X-ray diffraction methods. The results showed that contact angles decreased with the increase of SiO2 content in the solid substrate for all the melts. Al2O3 was dissolved insignificantly in the melts under study. SiO2 formed intermediate chemical compounds at the interface with the melts: NbO, Nb6Ni6O and Nb5Si3 with the Ni-based melt and NbO2, Nb2O5 with the Au-based melt. Obtained results allow consideration of Nb alloys under study as high-temperature brazes for the Al2O3-SiO2 materials.

The effects of structural properties on the d.c. and a.c. electrical properties of different weight gain reaction bonded silicon nitride (RBSN) have been studied in this work. The degree of nitridation is assessed by the ‘weight gain’ of the ceramic, the percentage by which the weight is increased in the nitriding reaction. From X-ray diffraction (XRD) patterns, it is observed that a higher degree of nitriadation sample has strong α-silicon nitride peaks. Intensity of α-silicon nitride peaks decreases with decreasing weight gain. The higher degrees of nitridation, the samples have less significant Si peak. XRD patterns were recorded to calculate the lattice parameters of RBSN. The lattice parameters for three weight gain RBSN samples are found to be a =b = 7.7727 Å, c= 5.6565 Å (26% weight gain), a=b= 7.6272 Å, c= 5.6374 Å (42% weight gain) and a=b=7.6158 Å, c= 5.7732 Å (58.27% weight gain) and are in good agreement with the reported values from XRD patterns. Porosity (%) and surface morphology was observed by SEM.

Positron annihilation lifetime spectroscopy (PALS) is a direct probe of size and concentration of nano-scale defects in materials, because it is very sensitive to electron density. The lifetime of positron (τ) and its intensity (I) can be used to characterize the defects concentration. In the present work, PALS has been applied to measure the variation of positron lifetime parameters for polycrystalline samples with chemical formula Zn0.5Cu0.5Fe1.98R0.02O4 (R = Gd, Sm, Nd and La). The variation of positron annihilation lifetime parameters (I1, I2, τav and k) with the ionic radius of rare earth ions, homogeneity, grain size and electrical resistivity have been studied. The inter-granular pores and grain boundaries defects are increased with the increasing of the ionic radius of the rare earth ions and decreased for Gd-sample. The defects inside and outside the grains are distributed homogeneously for Sm-sample only. A positive correlation has been found between positron lifetime parameters and ionic radius of rare earth ions as well as the electrical resistivity.