Preferred Orientation in Pyrolytic Graphite

The preferred orientation in several pyrolytic graphites prepared under various conditions was investigated by means of an X-ray technique. Two kinds of the preferred orientation parameters were measured. A simple relationship exists between them as proposed by Fischbach. Relationships among several measures of preferred orientation in common use are discussed. The preferred orientation of pyrolytic graphite is affected by the preparation conditions such as deposition temperature and gas pressure. The preferred orientation deteriorates at temperatures below about 2000℃ and the deterioration becomes marked at low temperatures under higher gas pressure. The method employed in the present work is in principle applicable to measurements of the preferred orientation of other vapor deposited materials

Siliconated Pyrolytic Graphite : Part 4. Electrical Resistivity

Investigations have been made on the electrical resistivity ρ of siliconated pyrolytic graphite (PG(Si), 0.02 to 4 wt % silicon) prepared by pyrolysis of a mixture of propane gas and silicon tetrachloride vapour at various deposition temperatures, total gas pressures, and partial pressures of silicon tetrachloride vapour. With increase in the partial pressure of silicon tetrachloride, ρ_a decreases and ρ_c increases. The electrical anisotropy (ρ_c/ρ_a) of PG(Si) is two orders of magnitude higher than that of PG, at deposition temperatures between 1600 and 1700℃ and a total gas pressure of 50 torr. Effects of the silicon content, density and structural features on the resistivities and the anisotropy have been discussed. The anisotropy is closely related to the preferred orientation, and high values of ρ_c/ρ_a induced by discontinuity in the stacking of crystallites are lowered in silicon-rich PG(Si) by the presence of SiC between the crystallites

Siliconated Pyrolytic Graphite : Part 2. The State of Silicon Present in Siliconated Pyrolytic Graphite

By X-ray diffraction, surface oxidation, X-ray microanalysis, electron diffraction, and electron microscopy, the state of silicon present in siliconated pyrolytic graphite has been examined on several samples prepared under a variety of conditions. In the siliconated pyrolytic graphite prepared at the deposition temperatures below 1730℃, the greater part of the silicon occurs as β-SiC. It does not segregate in the cone boundaries but disperses uniformly. It exists as flake-1ike single crystals, whose size increases with decreasing temperature. The (111) plane of β-SiC is parallel to the (001) planes of graphite

The Electrical Anisotropies of Pyrolytic Graphite and Its Compounds

The electrical resistivities and the electrical anisotropy of the pyrolytic graphite compounds containing silicon (0.02-4wt% Si ; PG(Si) ) or bromine (0.1-12 wt% Br ; PG(Br) ) have been examined at room temperature. The anisotropy of PG(Si) was closely related to its preferred orientation which depends on the deposition temperature. The anisotropies of PG and PG(Si) are attributed to discontinuity in the stacking of the crystallites, as proposed by Guentert and Klein ; the discontinuity increases with preferred orientation. Low values of the anisotropy for PG(Si), containing large amounts of silicon and having the considerably high preferred orientation, result from the disappearance in the discontinuity because of the presence of SiC between the crystallites. The anisotropy of PG(Br) increases with the amount of bromine. Almost all the bromine atoms may be ionized in PG(Br) according to Blackman et al. The effect of ionization on the anisotropy is not clear

Chemical Vapor-Deposited Amorphous Silicon Nitride

Chemical vapor-deposited amorphous Si_3N_4 (CVD-amorphous Si_3N_4) up to 4.2mm in thickness has been prepared from a gaseous mixture of NH_3 and H_2-carried SiCl_4 under various deposition conditions. The formation of the CVD-amorphous Si_3N_4 depended strongly on the deposition temperature, total gas pressure and gas flow rate. The CVD-amorphous Si_3N_4 prepared at 1100-1300℃ does not crystallize by heating at each deposition temperature. Their density and deposition rate are markedly dependent on deposition conditions and have maximum values of 3.00g/cm^3 (94% of the theoretical density of α-Si_3N_4) and 0.36mm/hr, respectively. The Vickers microhardness of the CVD-amorphous Si_3N_4 at room temperature varies between 2200 and 3200kg/mm^2 according to its deposition conditions. The hardness at 1300℃ is 1200～1300 kg/mm^2. The thermal conductivity was 0.010cal/cm/sec/℃ at 20℃ and 0.012cal/cm/sec/℃ at 1300℃. The thermal expansion coefficient at 20～1200℃ is 2.99±0.05/℃. The formation mechanism and the effect of gas flow patterns on the deposition rate of the CVD-amorphous Si_3N_4 are also discussed

Chemical Vapour-Deposited Silicon Nitride : Part 1. Preparation and Some Properties

Pyrolytic Si_3N_4 has been deposited on a graphite substrate, using a mixture of SiCl_4, NH_3 and H_2. The pyrolysis is performed with deposition temperatures of 1100 to 1500℃, total gas pressures of 5 to 300 Torr, and flow rates of H_2=700, NH_3=60 and SiCl_4 (1iq.)=0.8 cm^3 min^. Massive amorphous and crystalline pyrolytic forms of Si_3N_4 are prepared at a maximum thickness of 4.6 mm. The effects of deposition conditions on some properties of the deposited products and the dependence of formation of amorphous or crystalline deposits on deposition temperature and total pressure were investigated. The surface and cross-sectional structures show growth cones and oriented crystals which are strongly dependent on the deposition conditions. The thin deposits are translucent ; the thick deposits vary in colour from white to black. The silicon content is close to the theoretical composition and independent of the deposition conditions, while the oxygen content increases with decreasing deposition temperature and total pressure. No segregation of silicon and nitrogen at cone boundaries was found

Th-ThC Phase Diagram

The partial phase diagram in the Th-C system between pure Th and ThC was studied by metallographic and X-ray techniques and by measurements of electrical resistivity in quenched and slow cooled Th-C alloys. The peritectic point of the reaction, liq.+ThC⇄α-Th, was found at 16 at%C and about 1875℃ and the peritectic composition of ThC was 33 at%C. The eutectic reaction occurred at 1650℃ and its composition of α-Th and β-Th was 6.5 and <0.5 at%C, respectively. The α⇄β transformation temperature rose gradually with the carbon content up to 2.3 at%C and rose rapidly between the compositions of 2.3 and 3.7 at%C. The phase boundaries of α/ThC+α, ThC+a/ThC. ThC/ThC+ThC_2 and liq.+ThC/ThC were also established