Search

Research into a gas sensing material with excellent performance to detect or remove toxic phosgene (COCl

Based on thermodynamic data for related pure substances, the relations of (nCl/nH)Eq and (nCl/nH)o have been plotted in the Si–Cl–H system. The results show that the difference of (nSi/nCl)o and (nSi/nCl)Eq is the driving force for polycrystalline silicon chemical vapor deposition (CVD). SiHCl3 is preferred for polycrystalline silicon deposition to SiCl4. SiH2Cl2 would be even better, but it is not stable as a gas and hence it is less frequently used. Then, thermodynamic simulation of polycrystalline silicon CVD in the Si–H–Cl system has been investigated. The pressure has a negative effect on polycrystalline silicon yield. The optimum temperature is 1400 K, at which the kinetic rate of rate-determining step for the main reaction is large enough. The excess hydrogen is necessary for polycrystalline silicon CVD in the Si–Cl–H system. However, the silicon deposition rate increases then decreases with increasing H2 molar fraction. The optimum H2 molar fraction should be determined by considering thermodynamics and transport phenomena simultaneously. Finally, the optimum conditions have been obtained as 1400 K, about 0.1 MPa, and H2 to SiHCl3 ratio of 15, which are close to the limited reported values in the open literature. Under the optimum conditions, the silicon yield ratio is 34.82% against 20% reported in the open literature.

In the present work, a Joule heating model by direct current (DC) has been developed for the rods arranged in a 48-rod Siemens reactor (SMS-48). The combined effect of heat dissipation (radiation, convection and reaction energy) and heat conduction produced by Joule effect has been examined by application of the developed model, taking into account variations of the rod radius and wall emissivity. The influence of the rod location, rod radius and wall emissivity on temperature profile and voltage-current curves during the electric heating process have been studied. Furthermore, the thermal and electrical behavior of silicon rod arranged in the SMS-48 and SMS-24 has been compared with each other. The dimensionless parameter ( p ) has also been introduced to investigate the effect of the number of rods and their arrangement on energy consumption. The interesting results show that the temperature profile of the rod arranged in the inner ring within SMS-48 is flatter than that in the SMS-24. The curve of current increases and becomes lower for the inner ring and keeps a linear relation for the outer ring in a manufacturing process. The required current and voltage increase when the wall emissivity increases. Enlarging the reactor capacity and meanwhile decreasing the dimensionless parameter ( p ) is an effective method to decrease energy consumption.

The modified Siemens process, which is the major process of producing polycrystalline silicon through current technologies, is a high temperature, slow, semi-batch process and the product is expensive primarily due to the large energy consumption. Therefore, the zinc reduction process, which can produce solar-grade silicon in a cost effective manner, should be redeveloped for these conditions. The SiCl 2 generation ratio, which stands for the degree of the side reactions, can be decomposed to SiCl 4 and ZnCl 2 in gas phase zinc atmosphere in the exit where the temperature is very low. Therefore, the lower SiCl 2 generation ratio is profitable with lower power consumption. Based on the thermodynamic data for the related pure substances, the relations of the SiCl 2 generation ratio and pressure, temperature and the feed molar ratio n Z n / n SiCl 4 are investigated and the graphs thereof are plotted. And the diagrams of K p Θ – T at standard atmosphere pressure have been plotted to account for the influence of temperature on the SiCl 2 generation ratio. Furthermore, the diagram of K p Θ – T at different pressures have also been plotted to give an interpretation of the influence of pressure on the SiCl 2 generation ratio. The results show that SiCl 2 generation ratio increases with increasing temperature, and the higher pressure and excess gas phase zinc can restrict SiCl 2 generation ratio. Finally, suitable operational conditions in the practical process of polycrystalline silicon manufacture by gas phase zinc reduction of SiCl 4 have been established with 1200 K, 0.2 MPa and the feed molar ratio n Z n / n SiCl 4 of 4 at the entrance. Under these conditions, SiCl 2 generation ratio is very low, which indicates that the side reactions can be restricted and the energy consumption is reasonable.