In this study, Ni-W/carbon nanotube (CNT) composite platings are electrodeposited onto steel plate as a possible coating material for the polymer electrolyte membrane fuel cell (PEMFC) bipolar plate, which requires high corrosion resistance under acidic environment and low contact resistance. The dispersibility of CNT in the plating bath is improved by surface modification of CNT through hydrothermal treatment in mixed acid solution. The change in electrodeposition behavior of Ni-W plating by CNT addition is investigated by cathodic polarization measurement. The corrosion resistance under acidic environment is evaluated using 0.5M H2SO4 solution and contact resistance of the composite platings is measured and compared with Ni and Ni-W plating. The surface modification of CNT through hydrothermal treatment is found to increase the CNT content in the Ni-W/CNT composite plating up to 0.33 mass.% and sufficient incorporation of hydrothermally treated CNT in Ni-W plating results in low contact resistance and enhanced corrosion resistance than pristine Ni-W plating.

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Introduction Heat spreaders are used to ensure operation of electronic devices, such as smart phones, pads, LED and screen displays, by dissipating heat generated form integrated circuits. The ideal material working as heat spreaders should have high thermal conductivity. Cu and Cu alloys have been used as heat spreader materials from this point of view, but their heat dissipation performances are approaching their upper limit. Therefore, a heat dissipation material exhibiting even higher thermal conductivity is required for future electronics. Meanwhile, carbon materials are expected as next-generation heat spreader materials due to their extremely high thermal conductivities. Our group has previously reported on electroplating of Cu/carbon composites. In this study, we focused on electroplating of Cu/diamond composites because diamond is an interesting filler material that do not require orientation control due to its isotropic thermal conductivity. Typically, it has been suggested that the major issue for Cu/diamond composites is the adhesion at Cu/diamond interface because Cu and diamonds show low affinity. Surface modification with a silane coupling agent has been reported to be effective for improving the affinity of the interface, leading to improvement in thermal conductivity by reducing the thermal barrier at the interface [1]. Amino groups (-NH2) are expected to be attracted to the cathode side along with Cu2+ because it is positively charged in acidic solutions and to improve the Cu/diamond interface by forming complex with Cu. In addition, it has been reported that the -NH2 modification of BN improves the thermal conductivity of BN/epoxy resin composites [2]. In this study, we performed electroplating of Cu/diamond composites using -NH2 modified microdiamonds and investigated its effect on adhesion at Cu/diamond interface. Experimental The -NH2 modified diamonds were prepared beforehand by the following procedure. As a pretreatment, the microdiamonds (MDs: average diameter of 180 μm) were heated at 1350 ℃ for 2 hours in a vacuum (5 Pa or less), and subsequently heated at 425 ℃ in air. The pretreated MDs were added to 50 cm³ of toluene solution containing 5 wt% of 3-aminopropyltrimethoxysilane (APTMS) and stirred for 6 h at 65 ℃ for silanization. Then, 30 ml of methanol was added to prevent further reaction of unreacted APTMS molecules. -NH2 modified MDs were collected by filtration, sequentially washed with methanol, water and acetone and dried at 80 °C in a vacuum oven over night. Existence of -NH2 on diamond surface was confirmed by using ninhydrin reaction. Electroplating was performed at room temperature. The bath composition was 1.0 mol/dm3 CuSO4.5H2O aqueous solution adjusted to pH 2.0 using H2SO4. The -NH2 modified MDs were added to the bath. The cathode used was Cu and the anode used was Pt coil, the reference electrode was Ag/AgCl. Galvanostat was used to apply a constant current of 10 mA/cm2. The quantity of electricity was 250 C (theoretical film thickness of 92 μm). The adhesion at the Cu/diamond interface in the composite plating was evaluated by observing the surface and cross section using a scanning electron microscope (SEM). Results and Discussion Protrusions were formed on the surface of microdiamond after silanization. Silane oligomer aggregates formed by the self-condensation reaction of APTMS may have bonded to the diamond surface as reported elsewhere [3]. The ninhydrin solution turned from colorless to purple by ninhydrin reaction, implying the presence of -NH2 on modified MDs. From observation of Cu/diamond interface in the composite plating, well adhesion was obtained when using -NH2 modified MDs while an obvious void was observed when using pristine MDs. The result indicated that modification of diamond surface by -NH2 was effective to improve the adhesion at Cu/diamond interface. Conclusions MD surface was modified by -NH2 groups through silanization. Surface and cross-sectional observation of Cu/diamond composite platings using -NH2 modified MDs showed well adhesion at Cu/diamond interface. It was found that -NH2 modification of diamond surface was effective to improve the adhesion between Cu and diamond. Acknowledgement This work was partially supported by the JST-OPERA Program, Japan [grant number JPMJOP1843] and “Knowledge Hub Aichi”, Priority Research Project and Aichi Prefecture Research, Development Subsidy from Aichi Prefectural Government. Reference Y. Yamada, M. Miki, NIPPON GOMU KYOUKAISHI, 2013,86,5,133-139 K. C. Yung, H. Liem, J. Appl. Polym. Sci.,2007,106,3587-3591 Y. Nakamura, N. Karyu, M. Noda, S. Fujii, Journal of the Adhesion Society of Japan, 2016,52,1,9-15

Compositing diamonds with high thermally conductive metals, such as copper, have attracted much attention in recent years. In this study, we prepared copper/diamond composites by electroplating and investigated the effect of the surface functionalization of diamond on the interfacial adhesion of copper and diamond in the composite. Oxygen-containing groups were introduced on the diamond surface by a hydrothermal treatment and amino groups were further introduced using a silane coupling agent. Surface functionalization of diamond with both these groups led to drastically increased wettability against the copper sulfate solution and improved the interfacial adhesion of copper and diamond in the composite. Compared with that of the plating based on pristine diamond, the thermal conductivity of the platings based on diamond functionalized with oxygen-containing groups and amino groups increased by 42 and 31 W m[Formula: see text] K[Formula: see text], respectively. Thus, diamond surface functionalization effectively improved the adhesion at the copper/diamond interface in electroplated copper/diamond composites.