Electroplating technology of conductive metal powd

With the rapid development of modern electronic industry and information industry, the number of electronic devices that produce electromagnetic interference has increased dramatically. Electromagnetic wave pollution has become one of the social hazards concerned by countries all over the world. Using conductive metal powder electroplating to develop electromagnetic shielding rubber filling material is an important technical means to deal with this pollution

conductive shielding technology

electromagnetic shielding materials are divided into surface conductive shielding materials and filled composite shielding materials. The filled composite shielding material is a new type of material introduced to the market after the surface conductive shielding material. It has a great potential to catch up and can be developed in a large number. As the core technology of filling composite shielding materials, the research of conductive fillers is also deepening

at present, many electromagnetic shielding rubber products with different material combinations have been developed abroad by using this technology, and their characteristic indexes are shown in Table 1

Table 1 characteristic indexes of electromagnetic shielding rubber products abroad

at present, pure silver powder is mainly used as conductive filler in China. Silver has excellent conductivity and oxidation resistance. Using silver powder or silver plated filler as electromagnetic shielding material has significant shielding effect. However, silver is expensive, dense, and not competitive in the market. It is only suitable for shielding materials in special occasions, and has poor shielding effect on low-frequency electromagnetic shielding, which is difficult to meet the needs of broadband electromagnetic shielding. Copper has good conductivity and moderate price, but its density is large and it is not easy to disperse in the polymer matrix, which affects the electromagnetic shielding effect of the composite. Moreover, copper powder is easy to be oxidized and deteriorated to reduce conductivity. Nickel powder is not as easy to oxidize as copper powder, but the conductivity of nickel is low. In order to improve the comprehensive performance of fillers, silver plating on copper or silver plating on nickel is often used. Graphite and carbon black have the characteristics of low cost and good dispersion, but they often have a certain electromagnetic shielding effect at a very high content, which will lead to a significant decline in the mechanical properties of the product. Moreover, the product itself is black, which affects the color diversity of the product, limits the scope of application, and is often used in the form of carbon powder nickel plating

generally, there are two ways of surface coating process of metal powder: electroplating and electroless plating. Northwest rubber and plastic research and Design Institute concluded through relevant research and analysis that electroplating is more stable, efficient and less polluting than electroless plating, and conducted corresponding research and development

electroplating process

taking silver/copper powder as an example, the electroplating process is: copper powder - degreasing - etching - activation - electroplating - washing - drying - Inspection - packaging for use. In the pretreatment before plating, the activation conditions must be strictly controlled. At the same time, ensuring that the plated surface is extremely bright and clean is also one of the most important conditions for obtaining a metal coating with tight connection and good appearance. After each step, water washing and neutralization treatment should be carefully carried out to ensure the normal progress of the next process and no pollution

after several months of testing conditions for electroplating, we have to choose the appropriate size of the pneumatic range of our company's products. Northwest rubber and plastic research and Design Institute has successfully produced electroplating products that meet the electromagnetic shielding performance of high conductive rubber (the volume resistivity of rubber reaches Ω cm). As shown in the figure

microstructure of electroplated products that meet the electromagnetic shielding performance of high conductive rubber

oil removal

according to chemical properties, grease can be divided into two categories: saponified and unsaponifiable. All vegetable and animal fats belong to the first category. They are esters of glycerol and high molecular organic acids (mostly stearic acid, oleic acid, palmitic acid, etc.). Mineral oils, such as gasoline, paraffin, Vaseline, and various lubricating oils, which are composed of mixtures of hydrocarbons with different compositions and thicknesses (from extremely light liquid to solid), belong to the second category of oils. No matter the first or second type of grease is actually insoluble in water, it must be removed from the metal surface by chemical treatment or electrochemical treatment in a solution of certain components. Generally, products with complex shapes should be degreased by chemical method

when removing oil by chemical method, alkali solution, alkaline salt solution and many special organic solvents should be used. The process of removing oil in alkaline solution is the saponification of animal and vegetable fats and the emulsification of other kinds of fats and oils. The emulsifiers used include water glass, fatty acids, soap, animal glue and various proteins. The concentration of these substances should be determined according to the emulsifying ability of each emulsifier and the type and quantity of oil dirt on the metal surface. In addition, the higher temperature of alkaline solution can strengthen the hydrolysis of alkaline salt, accelerate the saponification reaction of animal and vegetable oils, and accelerate the emulsification process of oil, which is one of the most important conditions to ensure that saponified and unsaponifiable oils are completely removed from the surface of products. Removing oil in organic solvents is a common dissolution process of saponified and unsaponifiable oils and fats. The solvents used are kerosene, gasoline, toluene, trichloroethylene, carbon tetrachloride, etc. The most effective oil solvents are dichloroethane, dichloroethylene, trichloroethylene, tetrachloroethylene and carbon tetrachloride. Unlike gasoline, kerosene, benzene and toluene, these substances do not burn and can be degreased at high temperatures. In addition, they dissolve grease well without corroding metals

electrochemical oil removal is carried out on the cathode or anode in alkaline solution. The more commonly used method is the cathodic degreasing method or combined treatment method - degreasing on the cathode first, and then on the anode. In some cases, the efficiency of electrochemical oil removal method is several times higher than that of ordinary chemical oil removal method. The mechanism of the electrochemical oil removal process is that the oil is emulsified by the hydrogen bubbles separated from the cathode and the oxygen bubbles separated from the anode. The stirring effect of the separated gas on the electrolyte can accelerate the oil removal process. The electrolyte prepared with the following reagents can be used for oil removal: caustic soda, sodium carbonate, potassium carbonate, sodium phosphate, sodium cyanide or potassium cyanide, etc. Sometimes, a small amount of some emulsifier can be added to these solutions: soap or water glass. The current density during oil removal shall ensure that enough bubbles can be separated out, which can not only mechanically separate the oil droplets, but also stir the solution. Therefore, increasing the current density can have a great impact on the oil removal speed. Generally, the current density should be maintained in the range of 3~10 AMPS/decimeter 2. Increasing the temperature of the solution can improve the conductivity of the solution. When the temperature of the solution reaches 60~70 ℃, the effect of oil removal is roughly the same as that of chemical oil removal. Generally, the temperature of the electrolyte is maintained in the range of 60~80 ℃

etching

etching is the process of treating products in the solution of acid and acid salt or alkali to remove oxides and oxidized sub substances on the metal surface. Etching can be carried out by chemical etching and electrochemical etching. The etching method should be selected according to the properties of the metal, the characteristics and thickness of the surface oxide layer, and the characteristics of pre processing and post processing. In addition, the effect of etching is only effective when the oil stain is removed from the surface of the product in advance

chemical etching method is to immerse the product in a solution of appropriate concentration of acid or base that can react with the metal oxide. In fact, acid solution is used for etching. The mixture of nitric acid, sulfuric acid and hydrochloric acid is usually used when etching copper and copper alloys. See Table 2 for specific chemical etching conditions until normal

Table 2 chemical etching conditions when etching copper and copper alloys

electrochemical etching can be carried out on the anode or cathode. Anodic etching is based on the electrolytic dissolution of metals and the mechanical shedding of oxygen from oxides. At this time, hydrogen is violently separated out on the cathode. The electrolyte used can be acid solution or the solution of corresponding metal salt. The electrode uses the products to be etched as the anode, and copper, iron, lead, etc. as the cathode. The current density used is usually high, generally 5-10 AMPS/decimeter 2 or more. Cathodic etching is achieved by the reduction of hydrogen released violently and the mechanical removal of metal oxides. During cathodic etching, the best effect can be obtained by using an electrolyte containing a mixture of sulfuric acid and hydrochloric acid. Lead, lead antimony alloy or silicon pig iron can be used as the anode, and the current density used is the same as that of anodic etching

etching products by electrochemical method is much faster than single chemical treatment in most cases, and in some cases where chemical etching is difficult to use or cannot be used at all, electrochemical method can often obtain good results

activation

activation is the last and necessary process before the product is immersed in the electroplating bath. The activation energy can quickly remove the thin oxide layer often formed on the surface of the cleaned products during transportation or storage. In addition, it can also slightly etch the outer layer of the metal and present the crystalline structure of the metal. In this way, good adhesion between the surface of the base metal and the coating can be ensured. In order to avoid damage to the metal surface, the activation operation time should be very short. Copper and copper alloy products are activated in 5~10% sulfuric acid solution or diluted mixed solution of sulfuric acid and nitric acid, or electrochemically activated in 3~4% potassium cyanide solution added with 2~3% sodium carbonate or potassium carbonate. At this time, the temperature of the solution is room temperature and the current density is 3~5 A/min M2

electroplating

the precipitation of metal on the cathode can be regarded as a crystallization process. This crystallization process is divided into two steps: the formation of crystal core (crystal core) and the growth after the formation of crystal core. The higher the rate of crystal core formation, that is, the more crystals formed on the covered surface per unit time, the finer the crystals of the deposited layer. The growth of crystal is not uniformly carried out on all surfaces of the crystal growth surface, but only on the activated part of the growth surface. The practice of electrolytic deposition of metals for many times shows that all the factors that affect the cathodic polarization change the structure of the deposited layer accordingly. These factors mainly include: the ion concentration of the metal precipitated, the current density, the stirring speed, and other metal salts, acids, and organic substances added to the electrolyte

if other conditions (temperature, current density, etc.) remain unchanged, when the electrolyte concentration is high, many crystals are formed on the cathode when electrolysis begins, but under the same current intensity, the number of growing crystals gradually decreases after a period of time. This is because: as the growing crystal surface increases, the real current density decreases, and when it decreases below a certain value, some crystals begin to passivate and stop growing, and only some crystals continue to grow. The greater the electrolyte concentration, the more passivators in the electrolyte to reduce the effective area of the cathode

in the process of electrolysis, salts of alkali metals or alkaline earth metals are often added to the salt solution of the metal to be precipitated, and in some cases, equivalent acids are also added. Some additives can not only increase the conductivity of the solution, but also increase the cathodic polarization, thus promoting the formation of fine crystalline deposition layer. For example, adding aluminum sulfate, boric acid, oxalic acid or sodium oxalate to zinc sulfate or nickel sulfate solutions can make these solutions have buffering properties. Add special organic substances to the metal salt solution. In a meeting organized by the American Association of automotive and equipment manufacturers (Mema), organic substances have a significant impact on the organization of the electrodeposited layer. These organic substances can form colloidal solution or molecular solution