This paper presents a reasonable approach to this issue, i.e., computer controlled magnetorheological finishing (MRF). In MRF, magnetically stiffened magnetorheological (MR) abrasive fluid flows through a preset converging gap that is formed by a workpiece surface and a moving rigid wall, to create precise material removal and polishing. Tsinghua University recently completed a project with MRF technology, in which a 66 mm diameter, f/5 parabolic mirror was polished to the shape accuracy of λ/17 RMS (λ=632.8nm) and the surface roughness of 1.22 nm Ra. This was done on a home made novel aspheric computer controlled manufacturing system. It is a three-axis, self-rotating wheel machine, the polishing tool is driven with one motor through a belt. This paper presents the manufacturing and testing processes, including establish the mathematics model of MRF optics on the basis of Preston equation, profiler test and relative coefficients, i.e., pressure between workpiece and tool, velocity of MR fluid in polishing spot, tolerance control of geometrical parameters such as radius of curvature and conic constant also been analyzed in the paper. Experiments were carried out on the features of MRF. The results indicated that the required convergent speed, surface roughness could be achieved with high efficiency.
In this work, excellent property infrared (IR) window composite materials was obtained by inorganic-organic compound method. Scanning Electron Microscopy (SEM) observation found that inorganic particles were distributed uniformly in the organic matrix, and good combination between inorganic and organic phase were obtained. The measurement of vitrification point showed that glass temperature of composite materials were higher than that of pure plastic matrix, so the temperature range of application of this kind of composite materials was expanded and its transmittivity was about 80% when wave length was before 30μm.
In this paper, Cz growth of silicon single crystal in a magnetic field was studied with numerical simulation method. The results of numerical simulation showed that convection of melt could be controlled when the magnetic field was added, moreover, forced convection was controlled much larger than natural convection. The oxygen concentration of the interface would decrease with the strengthening of the magnetic field intensity, but the temperature field of the melt did not change with the strenghtening of the magnetic field.
The PWO crystal was grown by Cz method. We found the cause of crystal cracking was as follows: lateral crack; longitudinal length crack; along the cleaveage place crack; random fracture. Structure analysis shows that structure stress which cause the random fracture can be decreased through select high pure raw material and perfect sub-crystal, and main factor of crystal cracking is caused by thermal stress. Through a lot of experiments we have known that crystal cracking of thermal stress included not only along cleavage place crack but also lateral stratum crack and longitudinal length crack which was caused by dissimilar coefficient of expansion in all directions. During designing stable temperature field and selecting rational technology parameters and annealing process, perfect crystal was grown.