Silver Plating Of Ceramic Powders

The Ag-coated composite powder is a kind of addition agent, which is of great value in the application in the fields of conducting resin, wave-absorbing materials and optical apparatus. The importance of the composites could be attributed to the good associativity between Ag and the powder and the equably coating of silver on the substrate. In this paper, we tried to get metal-coated composite powders with different particle sizes by electroless silver plating on different ceramic powders such as nano-Al2O3, micro-cenospheres and submirco-SiO2.The nano-Al2O3 powder with average size about 10 to 20 nanometers was pretreated to produce catalytic activity before the electroless silver plating. The formaldehyde (HCHO) was used as the reducing agent and plating was carried out at normal temperature and ultrasonic radiation. The ingredients of solution were determined by chemical analysis. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were used in the investigation.The feasibility of electroless silver plating of nano-Al2O3 powder with ultrasonic radiation was studied. Ultrasonic radiation improved the dispersibility of powders and the uniformity of coating. The TEM micrographs of composite powders indicated that the size of particles after electroless plating increased from 10~20nm to 50~60nm and the originally granular form changed into spheral-like shape. The result of X-ray analysis indicated that the plate was made of metal silver.The influences of the composition of solutions and plating conditions on electroless silver plating was also investigated. The reaction ended when pH value reduced below 10 which resulted in the lost of reducing ability of formaldehyde and the reaction time was 1.5~2h. The length of the gestating time (the time before reaction) reflected the stability of the bath. Increasing the formaldehyde dosage and pH value or decreasing NH4OH dosage can shorten the gestating time, but the stability of the bath was declined. On the other hand, when pH value and coating temperature decreased and the NH4OH dosage increased, the stability of the bath was increased. The weight gain of powders could be slightly increased with the increasing pH value, formaldehyde dosage and powder loadage, but the most utilization ratio of silver was only 35.2 percent. The composite powders with the silver content between 13%~70% were produced.Two kinds of micro-cenospheres with the density of 2.5g/cm3 and 0.6054g/cm3 were respectively electroless silver plated with NaOH introduced into the bath. The plating was carried out at agitation machine without preteatment and ultrasonic radiation. The morphology of powders was observed using HRTENM, scanning electron microscope (SEM) and metallographic microscope.The weight gain greatly increased and approached the theoretical value by the addition of NaOH that increased the pH value of solution. The composite powders with compact and continuous silver coating layer were produced. The size of silver particles was between 50nm and 400nm.All the factors in the composition of the bath and plating conditions had influences on the evenness of the electroless silver plating on cenospheres. When pHvalue was increased, the bath tended to be self-decomposed and the compactness of the coating layer was improved. The increasing loadage led to the decreasing of self-decomposition and the size of silver particles could be adjusted. To decrease the reacting velocity of the bath and self-decomposition, the method of continuously dripping the silver solution into the reducing solution was introduced. As a result, the surface pattern of the Ag-cenosphere composite powders was improved. The great amount of stabilizing agent led to the non-uniform of the plating layer. The addition of the high polymer could decrease the size of silver particles and help to increase the dispersibility of powders.After the pretreatment, the submicron-SiO2 powder with 300nm in size was firstly coated with copper and then the copper-coated powder was...