Study On Magnetic Field Assisted Liquid-jet Cleaning Technology For The Polished KDP Crystal Surface

The functional performance of KDP crystal is affected by the residual impurities on the crystal surface after the magnetorheological finishing. It is difficult to remove the impurities from KDP crystal because of its low hardness and high brittleness, low fracture toughness and low deliquescence. The adsorption mechanism between the polishing fluid and crystal surface was studied in this thesis. The chemical removing method and the magnetic field assisted liquid-jet removing method were combined to clean the KDP crystal surface. The magnetic field assisted liquid-jet cleaning technology was proposed,and the magnetic field assisted liquid-jet cleaning device was designed and developed. Then, the removing process including soaking, liquid-jet cleaning and blow-dry in gas was proposed. Subsequently, the cleaning fluid was selected and prepared, and the process parameters of cleaning were optimized to remove effectively the residual impurities on the crystal surface.The adsorption behavior between the impurities and the polished KDP crystal surface was studied. It was noted that the polishing base fluid and solid particles were existed on the crystal surface. And the polishing base fluid was adsorbed on the crystal surface in the form of thin film. Most of the solid particles were adsorbed on the crystal surface by the polishing base fluid, and few particles were embedded inside the crystal. The compositions of the polishing fluid were tested, it was shown that the compositions of the solid particles and the polishing fluid were the iron particles and the butyldiglycol respectively. The adsorption mechanism between the crystal surface and the polishing base fluid was analyzed. It was noted that the polishing base fluid was adsorbed on the crystal surface due to the combination of induction force, dispersion force and hydrogen bonding interaction. The adsorption force between the crystal surface and the polishing fluid was obtained by testing. And the adsorption force was 5.7×10-5 N for the KDP crystal. The adsorption mechanism between the crystal surface and the iron powder was analyzed. The results showed that the iron powder particles were firmly adsorbed on the crystal surface because of the capillary adsorption force. And the range of capillary adsorption force was from 1.9×10-7 to 7.7×10-7N. The total force of the embedded particles was equal to the combination of capillary force and embedding force.According to the adsorption mechanism between the KDP crystal surface polished by the magnetorheological finishing and the polishing fluid, the magnetic field assisted liquid-jet cleaning method was proposed and the cleaning device was developed. The chemical removing method and the magnetic field assisted liquid-jet removing method were combined to clean the KDP crystal surface. The removing process including soak cleaning, liquid-jet cleaning and blow-dry in gas was built. The magnetic field assisted liquid-jet cleaning device was designed and developed. And the spindle system and the cleaning magnetic wheel were designed. The grooves at the bottom of the magnetic wheel changed the vertical impact force of the fluid into a horizontal shear force along the crystal surface.According to the two stages including the soaking cleaning and the liquid jet during the process of the magnetic field assisted liquid-jet cleaning, the soaking solution having the larger dissolving ability and osmotic force was prepared and the jet cleaning fluid that didn’t damage the crystal surface was selected. The compatibility between the soaking solution and the polishing base fluid was studied. And the soaking solution was consisted of the anhydrous ethanol having a higher compatibility and the ether having the low surface tension. The volume fraction of the ether in the soaking cleaning fluid was confirmed as 30%. The influence of environmental humidity, environmental temperature and the jet cleaning fluid on the deliquescence and the crack of the polished KDP crystal surface was studied. The results showed that the cyclohexane was the best jet cleaning fluid, and the environmental humidity should be under 50% and the temperature should be below 15℃.The fluid velocity field in the process of the magnetic field assisted liquid-jet, the stress field, strain field and the total deformation field of the crystal surface under the jet pressure were simulated and studied. The results showed that the maximum velocity was about 2 m/s in the center of the velocity field. The velocity of the cyclohexane fluid between the grooves was higher than that at the grooves, and the velocity field distribution was a sector. The impact kinetic energy of cyclohexane between the grooves was larger, and the cleaning ability was stronger. It was noted that the crystal surface stress value was less than the tensile strength, and the distribution of the strain field and the total deformation field was uniform when the standoff distance was 1 mm and the jet pressure was 0.3 MPa. In this case, the maximum strain on the crystal surface was 1.62×10-5, the maximum deformation was 1.56×10-4 mm. These research results provided the primary theoretical support for the optimization of process parameters during the magnetic field assisted liquid-jet cleaning. The relationship between the magnetic force and the standoff distance was studied. It was indicated that the magnetic force that the cleaning magnetic wheel attracted the single particle iron powder was larger than the capillary adsorption force, when the standoff distance was changed from 0 mm to 3 mm. And the removing effect of the cleaning magnetic wheel on the residual iron powder was verified by the experiments. The cleaning effect of the magnetic field assisted liquid-jet cleaning technology was studied by the experiment. The effectiveness of the magnetic field assisted liquid-jet cleaning device was verified. The results indicated that the residual impurities on the polished KDP crystals surface could be effectively removed by the magnetic field assisted liquid-jet cleaning technology.The relationship model between the number of the residual particles on the polished KDP crystal surface and the jet cleaning time, the standoff distance, the jet pressure and the velocity of the cleaning magnetic wheel was established. The results showed that the influence on the number of the residual particle was in the order of the standoff distance> the jet cleaning time> the jet pressure> the velocity of the cleaning magnetic wheel> the interaction effect between the standoff distance and the jet pressure> the interaction effect between the standoff distance and the velocity of the cleaning magnetic wheel. The optimum process parameters were obtained by the experiment. The results showed that the residual impurities on the polished KDP crystal surface could be effectively removed, when the optimal soaking time was 30 min. The optimum technological parameter combinations were that the jet cleaning time was 14 min, the standoff distance was 1.5 mm, the jet pressure was 0.27 MPa, the velocity of the cleaning magnetic wheel was 490 r/min.