Electromagnetic shielding properties of electroless Ni-P and Ni-B coated carbonfiber and effects of addition W

As a result of the development of technology, the rapid increase in the use of electronic devices has led to an increase in the spread of electromagnetic waves. Measures against electromagnetic waves have begun to be taken because of the negative effects on both the living and other electronic devices. Electromagnetic shielding methods, which are produced as a solution to the problem of electromagnetic interaction, change with the development of the technology. Electromagnetic shielding is defined as the damping or diminishing of electromagnetic waves by the conductive shielding material placed between the source and the victim. These shielding materials are mostly metals. However, the new generation of materials is being studied because of the high weight and cost of mass-used metal materials, as well as processing and corrosion problems. Because of the advantages of lightness, the production of conductive polymers or polymer matrix composites by various conductive dopants or conductive coatings is a current research topic. Carbon fiber materials are preferred as dopants because they are conductive, have high strength, and have good corrosion resistance. In order to increase the conductivity of carbon fibers, metallic coating is applied by electroless coating method. Thus, with higher conductivity, high strength and lightweight material is obtained. Many metals are used as the metallic coating material, but due to the properties such adhesion between carbon fiber and high oxidation resistance, nickel has been found suitable for coating and many studies have been made. Electroless coating method is preferred in many applications such as aerospace, automotive, polymer, electronics, chemistry and machinery. The basic reasons are that it can be applicable to all surfaces including insulating surfaces; it can cover even complex geometric bodies homogeneously; it has high corrosion resistance and hardness. By using electroless coating pure nickel or nickel alloys with the addition of phosphorus, boron or a third element (Sn, W, Mo, etc.) can be coated. Electroless nickel plating is achieved by the reduction of the nickel ions to metallic nickel on the surface which has the catalytic effect. The nickel metal also has a catalytic property so reaction proceeds on the deposited nickel surface. Depending on the content of the bath, phosphorus or boron are included in the structure in the form of an alloy coating during the reaction. In this study, nickel-phosphorus, nickel-boron, nickel-tungsten-phosphorus and nickel-tungsten-boron coatings have been obtained. The obtained coatings were produced within the parameters of pH, temperature and time. Before coating, the carbon fiber surface activation and sensitization application was carried out by applying five different pretreatment baths, which are: etching (20 min 65 °C), neutralization (2 min, room temperature), pre-immersion (1 min, room temperature), activation (4 min, room temperature). After each pretreatment treatment, the sample was cleaned by submerging it in distilled water. After the surface activation processes, carbon fiber samples were immersed in bath which was placed on a magnetic stirrer and heated by the heating bath circulator to the desired temperature. Coating baths NiKlad ELV 811 for Ni-P baths NiKlad 752 baths for Ni-B baths were supplied from MacDermid. In order to create Ni-W-P and Ni-W-B baths, 5 g/L sodium tungstate (Na2WO4.2H2O) salt was added to the baths. The Ni-P coating process was carried out at 75 °C, 85 °C and 95 °C for 10 min and 20 min at pH 4.8 indicated in the bath usage prescription. Besides, Ni-B coating was carried out at 65 °C, 75 °C, 85 °C for 10 min and 20 min at pH 6.5 indicated in the bath usage prescription. The Ni-W-P coating process was performed at pH 4.8, pH 6, pH 7 at 75 ° C for 20 min and Ni-W-B coating at pH 6, pH 6.5, pH 7 at 65 ° C for 20 min to measure the effect of the W content. Heat treatment was applied atmospheric conditions at 200 °C, 300 °C and 400 °C; moreover heat treatment also applied under vacuum conditions at 300 °C and 400 °C temperatures. For Ni-W-P and Ni-W-B coatings, heat treatment was applied under atmospheric and vacuum conditions at 300 °C and 400 °C temperatures. Measurements were made at 1 MHz and 8 GHz using the coaxial transmission line method to determine electromagnetic interference shielding values of coated and uncoated fiber. As a result, Ni-B and Ni-P coating thicknesses increase with increasing temperature and time. Ni-W-P and Ni-W-B coatings have been obtained by applying different pH at constant temperature and time, and also that the coating thickness is increasing in Ni-W-P coatings with increasing pH. Ni-P and Ni-B coatings were applied on both carbon fibers and mat carbon fibers and EMI shielding properties of mat carbon fibers are better than carbon fibers. When the EMI results from the coatings applied to the fiber are examined, the best EMI shielding effectiveness is achieved with Ni-B 75 °C 10 min pH 6.5 coating which is about 30 dB in 1-5 GHz frequency range. EMI shielding effectiveness of Ni-P coatings applied on mat carbon fibers 35±2 dB in 1-5 frequency range achieved by 75 °C 20 min. pH 4.8 coating and Ni-B coatings has 40±2 dB EMI shielding effectiveness achieved by 65 °C 20 min pH 6.5 coating. W addition in Ni-B coatings affects EMI shielding effectiveness adversely. The best EMI shielding is achieved by pH 6 coating in 0-1 GHz frequency range with 30 dB. The shielding effectiveness was measured as 20 dB at higher frequencies. Furthermore, when W effect on Ni-P coatings are examined, EMI shielding effectiveness is decreased for pH 4.8 coating, but for pH 6 and pH 7 coatings EMI shielding effectiveness values are increased. Especially, at 0-1 GHz frequency range, for pH 7 coatings EMI shielding effectiveness value is about 50 dB and at higher frequencies the best shielding is achieved by pH 6 coating with 40 dB. The reflection loss values are calculated according to ASTM D4935-10 standard. As a result of calculations, the max absorption loss of Ni-B coatings is 20%, and for Ni-P coatings is 40%. Depending on the amount of increasing W, the absorption loss in Ni-W-P coatings becomes more active and reaching about 50%. After different coating types are applied to the carbon fibers, to investigate crystallization effect on EMI shielding effectiveness heat treatments at different temperatures are applied to the obtained coatings. As a result, heat treatment applied at atmospheric conditions affect Ni-P and Ni-B coating adversely and cause significant decrease in EMI shielding effectiveness. In Ni-W-B and Ni-W-P coatings, the change in the EMI shielding effectiveness is observed depending on the pH value, but when evaluated in general it causes a decrease in the shielding effectiveness value. EMI shielding effectiveness value of Ni-B coating is reduced to 10 dB especially in 1-5 GHz frequency range with increasing temperature and EMI shielding effectiveness of Ni-P coating is reduced to 20 dB. It has been decided to apply heat treatment under vacuum conditions considering that this decrease in EMI shielding efficiency is due to the fact that oxide formation during atmospheric condition heat treatment negatively affects surface properties and reduces the nickel content in the coating. The vacuum heat treatment generally affects the EMI shielding properties positively. EMI shielding efficiency reaches in 0-1 GHz frequency 50±5 dB for Ni-B coating and 40±3 dB for Ni-P coating with vacuum heat treatment applied at 400 °C temperatures. At higher frequencies, for Ni-B coating 40 dB; for Ni-P coating 35 dB EMI shielding effectiveness is obtained. Ni-W-B pH 6.5 coating, EMI SE was reached 60±2 dB in 0-5 GHz frequency range by vacuum heat treatment applied at 400 °C temperature. Moreover, Ni-W-P pH 7 coating, 55 dB EMI SE in 0-1 GHz frequency range and a 45 dB EMI SE in 1-5 GHz range were obtained with 400 °C vacuum heat treatment. Heat treatment under vacuum conditions also affects mechanisms of shielding. The increasing temperature cause increase in reflection loss and decrease in absorption loss in shielding mechanisms of Ni-B and Ni-P coatings. Besides, with increasing temperature for Ni-W-B coatings absorption loss is firstly decrease and then increase to approximately 30%. Moreover, absorption loss in Ni-W-P coatings increases with increasing temperature and reach approximately 50%. When all coatings and applied heat treatments are evaluated, the best EMI SE value (60±2 dB) in 0-5 GHz frequency range is achieved with 400 °C vacuum heat treatment applied on Ni-W-B coating. Furthermore, absorption loss is approximately 30% and reflection loss is 70%.