Thermal Properties Of Copper Matrix Composites Reinforced With Chromium Carbide-Coated Diamond Prepared Via Low-Temperature Molten Salts

As electronic packaging materials continue to integrate and miniaturize,heat flow density increases dramatically.Electronic devices require higher heat dissipation rates.High thermal conductivity and a suitable thermal expansion coefficient are desirable characteristics of electronic packaging materials.Diamond-reinforced metal matrix composites are a promising candidate due to the outstanding intrinsic thermal conductivity(700～2200 W·m^-1·K^-)and low thermal expansion coefficient(0.8×10^-6 K^-1)of diamond particles.However,due to diamonds’inherent characteristics,high-performance diamond/copper composites are challenging to prepare.To improve diamond/copper composite performance,regulating and optimizing the diamond-copper interface is critical.In this paper,dual coatings of chromium carbide using low-temperature molten salts and copper plating using electroless plating are examined.On this basis,diamond/copper composites were prepared through hot-press sintering,and their thermal properties were evaluated.A quaternary molten salt solution of Na Cl-KCl-Li Cl-Na F is utilized as the reaction medium to prepare chromium carbide on diamond surfaces at low temperatures.A reaction was conducted between diamonds and chromium powder at temperatures ranging from 600℃ to750℃ for 60 minutes.A chromium carbide coating was formed on the diamond surface as a result of the interaction of Cr²⁺chromium atoms with carbon atoms on the diamond surface,facilitated by the fluidity of the molten salt.At 600℃,no Cr₇C₃ coating was formed on the diamond surface,whereas at 630℃,the Cr₇C₃ coating was deposited preferentially on the（100）face of the diamond due to its higher reactivity.The diamond particles were completely and uniformly coated with Cr₇C₃ coating at 650℃,with a thickness of 160±10 nm,and no graphite phase was detected.Increasing the reaction temperature increased the thickness of the Cr₇C₃coating,indicating that the low-temperature molten salt reaction was advantageous in controlling the coating thickness.Upon reaching 750℃,the coating thickened and cracked,and peeled.It has been demonstrated that the addition of Na F to the molten salt system can reduce the viscosity and improve fluidity,thus promoting the reaction and permitting the preparation of thinner Cr₇C₃ coatings at lower temperatures.Furthermore,high-energy ball milling was used to activate Cr powder in a quaternary molten salt at 650℃,and it was found that determining the duration of high-energy ball milling affected the phase and structure of the chromium carbide coating.As the duration of ball milling increased,the peak intensities of Cr₇C₃ and Cr C gradually increased,as well as the thickness of the coating.After 36 hours of ball milling,however,the coating began to peel and crack,exposing the diamond surface once again.This may be due to the high-energy ball milling inducing numerous cracks in the interior of the Cr powder,resulting in grain refinement,increasing the specific surface area and activity of the Cr powder,and promoting the dissolution of chromium in the molten salt system and accelerating the reaction to generate chromium carbide.Copper electroless plating was applied to further modify the diamond particles with chromium carbide,resulting in the synthesis of double-layered diamond particles（Cu/Cr₇C₃@Diamond/Cu）.Within 20 minutes,a continuous copper coating was formed.The Cu/Cr₇C₃@Diamond particles were mixed with copper powder and sintered at 1000℃ and 50MPa,resulting in diamond/copper composites with a volume fraction of 55-65%.A composite containing less than 60 percent diamond volume fraction exhibited high relative density and good densification.Cu/Cr₇C₃@Diamond/Cu composites had fewer defects,such as voids,at the interface between Cu and the diamond,compared to diamond/copper composites without modification（Diamond/Cu）.The thermal conductivity of the composites prepared with modified diamond particles was significantly higher than that of Diamond/Cu and pure copper samples,indicating that Cu/Cr₇C₃@Diamond/Cu can effectively enhance thermal conductivity.The thermal conductivity of the composite material reaches a maximum value of 464.5 W·m^-1·K^-1 when the diamond volume fraction is 60 vol.%.As the diamond volume fraction increases,however,due to limitations in the hot-pressing process,the density of the composite material decreases and the thermal conductivity increases and then decreases as the diamond volume fraction increases.As the diamond volume fraction increases,the thermal expansion coefficient of Cu/Cr₇C₃@Diamond/Cu decreases.Composite materials with 60 vol.%diamond exhibit thermal expansion coefficients of 6.15 x 10^-6 K^-1 at 50℃ and 8.95 x 10^-6 K^-1 at 100℃.