| The objective of this paper is to solve the problem of brittleness of ceramic materials by the way of preparing nano-composite powders of metal and ceramic. In our thought, firstly we should prepare nano-ceramic powders coated with metal, and then during the period of sintering, the film of metal could prevent the nano ceramics powder from dumpling and keep the powders to be nano-sized. It's well known that nano-materials have good ductility. On the other hand, because the metal phase was evenly distributed, the ductility of the ceramic composites could be promoted further. In this paper, we tried to get nano-composite powders of metal-coated ceramic by electroless cobalt plating of nano-A1203 powder.The nano-Al203 powder used in experiments were about 10 to 20 nanometer, and they were pretreated to produce catalytic activity before the electroless cobalt plating. The solution was alkaline, and hypophosphite (NaH2PO2H20) was used as a reducing agent. Plating was carried out at middle temperature/low temperature and ultrasonic radiation. Plating rate was determined by measuring the weight gain of powder after plating, and the ingredients of solution were determined by chemical analysis. The morphology of powders was observed using high-resolution transmission electron microscopy (HIRTEM); X-ray diffraction (XRD) pattern was used to analyze the phases of the powders; DSC analysis was carried out to confirm the phase change of the composites during the ascending process of temperature.At first, the feasibility of electroless cobalt plating of nano-M203 powder with ultrasonic radiation was studied. Ultrasonic radiation made the reaction happen at room temperature, furthermore, the reaction was accelerated, and more importantly, the powders were dispersed evenly so that every particle could be coated. The observation by HRTEM indicated that the size of particles coated with ultrasonic radiation at room temperature in alkaline hypophosphite bath was about 50-6Onm. The originally granular form of the nano-A1203 particles has changed into spheral-like shape. The X-ray analysis showed that the plate was the alloy of Co-PIn this paper, a number of deposition parameters, such as the pH, the temperature and the concentration of the bath, were studied.It was found that the plating was ended spontaneously after some time, named "plating time". The analysis of the solution after plating indicated that the exhaustion of hypophosphite caused the reaction to be stopped. The plating time was shortened when the pH and the temperature of the bath were increased. It was also shortened by increasing the concentration of cobalt sulfate and by decreasing the concentration of sodium citrate. When the bath load was increased, the plating time was shortened,Ivbecause the surface to be plated was increased. But the plating time became longer with increasing the concentration of hypophosphite, because the reduce agent that could be used was increased. Furthermore, the plating time can also be affected by the frequency and the power of the ultrasonic radiation and the condition of the preparation of the solution used for pretreating.All the factors studied bad an effect on plating rate, i.e., weight gain per unit of time. The results showed that the factors that could shorten the plating time increased the plating rate. But in the case of increasing the concentration of hypophosphite, the plating rate was increased although the plating time became longer when the concentration of hypophosphite was below 20g/l, because the weight gain of the powder was increased in the same time. Increasing the concentration of boracic acid can increase the plating rate, but the plating rate would not be increased when the concentration of boracic acid was reached to 25g/l or higher.The effects of bath composition and plating conditions on the composition were also investigated. The X-ray analysis showed that the plate was non-crystalline alloy of Co-P, when the concentration of hypophosphite was 45g/l or higher, which chang... |
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