Novel Ceramics and Glass-Ceramics by Microwave and Conventional Processing: A Review

Microwave heating technique has attracted considerable attention for the processing of various materials such as ceramics, glasses, polymers, composites and even metals. Researchers are trying to apply this technology to new areas. The present review presents a short overview of some recent applications of conventional and/ or microwave processing for the synthesis of novel ceramics and glass-ceramics

Glass-ceramic glazes for future generation floor tiles

Glaze in the CaO-MgO-Al2O3-SiO2 system was heated at 950-1190 degrees C for 2 h and characterized. X-ray diffraction showed that only -brace amount of mullite was formed in the glass-ceramic glaze heated at 950 degrees C. Both mullite and alpha-cordierite were formed in the glass-ceramic glaze heated at 1050 degrees C as primary and secondary phases. Glass-ceramic glazes heated at 1120 degrees C and 1190 degrees C contained alpha-cordierite and mullite as major and minor phases. Rietveld analysis revealed that the amount of alpha-cordierite increased and mullite decreased with increasing heating temperature. Field emission scanning electron microscopy showed presence of mullite crystals dispersed within residual glassy phase in the glass-ceramic glazes heated at 950 degrees C and 1050 degrees C. In the microstructures of glass-ceramic glazes heated at 1120 degrees C and 1190 degrees C alpha-cordierite crystals were mainly appeared. Energy Dispersive X-ray analysis corroborated X-ray diffraction results. Vickers rnicrohardness measurement demonstrated highest hardness (8.38 1 0.07 GPa) of the glass-ceramic glaze heated at 1190 degrees C. (C) 2012 Elsevier Ltd. All rights reserved

Interfacial properties of metallized alumina ceramics

An alumina ceramic material (purity-96%) was metallized by the conventional molybdenum-manganese (Mo-Mn) process in which an alumina substrate was coated with Mo-Mn paste and subsequently heat treated at 1400 A degrees C for 10 min. During the entire process a moist H-2 and N-2 gas mixture (dew point-20 A degrees C) with 3:1 ratio was passed continuously through the furnace. X-ray diffraction analysis of the metallized alumina substrate identified only molybdenum phase at the surface of the metallizing layer. The microstructural observations of the metallized alumina substrate were made by scanning electron microscopy. Energy dispersive X-ray analysis showed the elemental compositions along the cross-sectional region of the metallized alumina substrate. The adhesion of the metallic coatings on the alumina substrates was evaluated qualitatively by a scratch testing technique and quantitatively by an adhesion tester. Nanohardness measurements showed gradual change in the nanohardness values across the metallized alumina substrate

Thermal cycling behavior of alumina-graphite brazed joints in electron tube applications

Alumina was joined with graphite by active metal brazing technique at 895, 900, 905, and 910 degrees C for 10 min in vacuum of 0.67 mPa using Ti-Cu-Ag (68.8Ag-26.7Cu-4.5Ti; mass fraction, %) as filler material. The brazed samples were thermal cycled between 30 and 600 degrees C and characterized. X-ray diffraction results show strong reaction between titanium and carbon as well as titanium and alumina. Scanning electron microscopy and helium leak tests show that the initial and thermal cycled brazed samples are devoid of cracks or any other defects and hermeticity in nature. Brazing strength of the joints is found to be satisfactory

Cordierite based glass-ceramic glazed floor tiles by microwave processing

Cordierite based glass-ceramics were used to glaze floor tiles by microwave and conventional processing techniques. The surface roughness (R-a) values of the microwave and conventionally processed glazes were similar to 3.73 mu m and similar to 11.96 mu m, respectively. The Vickers hardness value for the conventionally processed glaze was similar to 1.63 GPa while it was similar to 5.04 GPa for the microwave processed glaze. Gradual transition of nanohardness and Young's modulus values was observed from the substrate to glaze for both conventional and microwave processed glazed tiles. Scratch and impact resistance of the microwave processed glass-ceramic glaze was superior to those of the conventionally processed one. (C) 2014 Elsevier Inc. All rights reserved

Influence of the microwave plasma CVD reactor parameters on substrate thermal management for growing large area diamond coatings inside a 915 MHz and moderately low power unit

The tuning (optimization) of the reactor parameters is very important in order to achieve spatial temperature uniformity across the large area substrate for growing high quality and uniformly thick diamond coatings. It is also important to minimize the thermal stress, arising from temperature non-uniformity, for producing crack free large area diamond coatings. So by varying different process parameters, such as chamber pressure, microwave power, gas flow rate and cavity lengths (by changing the probe, short and stage positions), the resultant variation of substrate surface temperature can be measured. By tuning these parameters the geometry as well as the density of the plasma discharge which directly influences substrate surface temperature may be controlled. It has been shown that by suitable manipulation of the reactor parameters one could achieve at best 80 degrees C variation in temperature uniformity over 100 mm diameter, which indeed has resulted in a very uniformly thick (+/- 12.8%) high quality polycrystalline diamond (PCD) coating. (c) 2012 Elsevier B.V. All rights reserved

Influence of growth conditions on microstructure and defects in diamond coatings grown by microwave plasma enhanced CVD

Diamond coatings were grown on SiO2/Si substrate under various process conditions by microwave plasma chemical vapour deposition (MPCVD) using CH4/H-2 gas mixture. In this paper, we present a microstructural study to elucidate on the growth mechanism and evolution of defects, viz., strain, dislocations, stacking faults, twins and non-diamond impurities in diamond coatings grown under different process conditions. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the diamond coatings. It has been shown that our new approach of prolonged substrate pre-treatment under hydrogen plasma yielded a new growth sequence that the SiO2 layer on the Si substrate was first reduced to yield Si layer of similar to 150 nm thickness before diamond was allowed to grow under CH4-H-2 plasma, created subsequently. It has also been shown that Si and O as impurity from the substrate hinders the initial diamond growth to yield non-diamond phases. It is being suggested that the crystal defects like twins, stacking faults, dislocations in the diamond grains and dislocations in the intermediate Si layer are generated due to the development of non-uniform stresses during diamond growth at high temperature

Large area deposition of polycrystalline diamond coatings by microwave plasma CVD

Polycrystalline diamond (PCD) films have been grown over 100 mm diameter silicon (100) substrate, using microwave plasma chemical vapour deposition (MPCVD) technique. The deposition was carried out inside a 15 cm diameter quartz chamber with microwave power of 15 kW at 915 MHz frequency. Uniform substrate surface temperature of 1050 degrees C with plasma heating was maintained with simultaneous cooling arrangement. The pressure was 110 Torr and the microwave incident power was 8.5 kW. Temperature uniformity and plasma geometry over the substrate are the key parameters for producing uniformly thick MPCVD diamond films of high quality. Thickness uniformity of as-deposited films is +/- 10% across 100 mm diameters with a growth rate of 1 mu m.h(-1). The grown PCD was characterized by X-ray diffractometry (XRD), Raman spectrometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and bright field imaging technique. Experimental results indicate columnar growth of a very densely crystalline PCD with (111) facets of high quality morphology

Synthesis and characterisation of freestanding diamond coatings

Freestanding polycrystalline diamond (PCD) coatings are of immense technological importance. PCD has been grown over silicon substrates by microwave plasma assisted chemical vapor deposition (MWPACVD) process. The coatings are grown by suitable optimisation of the growth parameters of a 915 MHz microwave reactor. Thereafter, 1:1:1 solution of hydrofluoric acid (HF); nitric acid (HNO3) and acetic acid (CH3COOH) is used to etch out the silicon wafer from the backside of the coating. Hereby, freshly generated nucleation surface, could be characterised by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and stylus profilometer and could be compared with the growth side. It is found that both the nucleation side and growth side are of very high quality (full width at half maxima, i.e., FWHM < 8 cm(-1)). The growth side is (111) textured, whereas, the nucleation side is very smooth with embedded detonation-nano-diamond (DND) agglomerates. These freestanding coatings are successfully laser cut into different geometrical shapes. They are found to be optically translucent having high refractive index. Cross-sectional microscopy of the laser cut edge reveals novel melting features of the CVD grown diamond columns