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

Effect of Processing Parameters on Thermal Cycling Behavior of Al2O3-Al2O3 Brazed Joints

In the present study, alumina ceramics were active metal brazed at different temperatures ranging from 1163 K to 1183 K (890 A degrees C to 910 A degrees C) using TICUSIL (68.8Ag-26.7Cu-4.5Ti in wt pct) foil as filler alloy of different thicknesses. The brazed joints were subjected to thermal cycling for 100 cycles between 323 K and 873 K (50 A degrees C and 600 A degrees C). The microstructural and elemental composition analysis of the brazed joints were performed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) before and after thermal cycling. Helium (He) leak test and brazing strength measurement were also conducted after thermal cycling for 100 cycles. The joint could withstand up to 1 x 10(-9) Torr pressure and brazing strength was higher than 20 MPa. The experimental results demonstrated that joints brazed at the higher temperature with thinner filler alloy produced strong Al2O3-Al2O3 joints

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

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

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

Microscopic properties of MPCVD diamond coatings studied by micro-Raman and micro-photoluminescence spectroscopy

Diamond coatings were deposited on silicon (100) substrate using the microwave plasma chemical vapour deposition (MPCVD) technique at different process conditions. Process parameters such as CH4-H (2) gas mixture concentration, microwave power, chamber pressure and substrate temperature were varied. The diamond coatings were characterized by micro-Raman and micro-photoluminescence (PL) spectroscopy techniques. In this paper we report a comparison of the overall quality of MPCVD polycrystalline diamond coatings grown under different processing conditions in terms of stress distribution, thickness uniformity and surface roughness. Micro-Raman spectroscopy studies over various points on the deposited coating showed that the Raman line widths of diamond peak varied from 3.2 to 18.3 cm(-1) with the variation of CH4 and H-2 gas concentration. The micro-PL spectra suggested the presence of impurity concentration and defects within the diamond coating synthesized at different processing conditions. Transmission electron microscopy (TEM) images provide the direct evidence of the presence of crystal defects which corroborates the Raman and PL results. The coherence scanning interferometry (CSI) showed that surface roughness of diamond coating varied from 0.43 to 11 mu m with thickness at different positions of the three coating samples. It has been concluded that Raman line-width broadening and Raman-shift are due to the presence of crystal defects as well as non-uniform distribution of stresses present in the diamond crystals of the coating, due to the incorporation of Si as impurity element and non-uniform temperature distribution during growth. Defect density gets reduced at higher processing temperatures. It is also being proposed that better thickness uniformity and lower surface roughness can be achieved for coatings deposited at low methane concentration under optimized process conditions

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