Diamond is the substance with the closest arrangement of atoms in nature, and it possesses outstanding thermal properties. In particular, the ultra-high thermal conductivity is one of the many extraordinary properties of diamond . The thermal conductivity of single crystal diamond at room temperature is as high as 2200W/(m·K), five times that of copper and ten times that of aluminum. In addition, diamond possesses an extremely high hardness, high elastic modulus, low thermal expansion coefficient, low density, outstanding radiation resistance, and stable physical and chemical properties. Furthermore, it exhibits low mass loss in a vacuum and produces no condensable volatiles. Thus, it is an ideal material for aerospace applications. Synthetic diamonds have attracted enthusiastic attention for applications in high-power lasers, high-power traveling-wave tubes, terahertz antennas, heat dissipation in high heat flux GaN chips, high-performance radiation detectors, and high-power microwave windows. Application research has been carried out in some of these areas . Over the past 30 years, the research on synthetic diamonds in China has achieved meaningful progress in some technical fields . There are mainly three methods for fabricating large size synthetic diamonds: hot filament chemical vapor deposition (CVD), direct current (DC) plasma jet CVD, and microwave plasma CVD. The hot filament CVD method dissociates H2 and CH4 at high temperatures by heating and deposits carbon elements. The DC plasma jet CVD method and the microwave plasma CVD method rely on the high temperature of the plasma or charged particles (electrons and ions) that collide with the molecules and atoms of H2 and CH4 to excite the precursor gases and deposit carbon elements to form a diamond structure. The hot filament CVD method can fabricate diamond discs with 180mm diameters and 2mm thicknesses, but it can hardly achieve materials thermal conductivities beyond 1000W/(m·K) under fast growth condition. The microwave plasma CVD method can fabricate high-quality single-crystal diamonds with thermal conductivities of more than 2000W/(m·K), but the diamonds are small in size (< Ø70mm) and too expensive. The DC plasma jet CVD method can fabricate high-quality polycrystalline diamonds with thermal conductivities of up to 2000W/(m·K) at a moderate price. Based on the requirements of the satellite in terms of product size, thermal conductivity, and batch production capacity, the high-thermal-conductivity diamond investigated in this article was developed by the DC plasma jet CVD method.
The temperatures of RF chips and electronic devices on a phased array antenna are closely related to the reliability and electrical performance. The failure rate of the microwave RF components in the T/R module increases exponentially with the increase in temperature, and the electrical performance of the microwave RF components deteriorates as the temperature rises. In addition, the phase of the transceiver component on the phased array antenna is affected by the temperature. To ensure phase control of the entire antenna array, the temperature consistency of microwave radio array is strictly required. To ensure that the antenna can function normally in orbit for a long time during its full lifespan, it is necessary to maintain the operating temperature of the antenna with a thermal control design. The temperature requirements of the satellite Ka phased array antenna are mainly the following: ① the temperature of the T/R module and other RF modules ranges from −10 to 50 °C, and ② the temperature gradient of all T/R modules is ≤10 °C.
High-thermal-conductivity diamond plates were embedded on the front and back sides of the frame structure of each transceiver component of the antenna in Figure 1. The diamond plate was installed on every antenna. The diamond film was fabricated by the DC arc plasma CVD method and cut by a high-power laser, simultaneously making several performance test samples from the original film. The surface roughness and flatness of the diamond films were controlled within an effective and reasonable range by polishing
The diamond plate was embedded in a aluminum frame of the transceiver component, with the interface filled with conductive epoxy adhesive that was later cured at a high temperature. The height difference between the secured diamond plate and the frame of the transceiver component was less than 100μm to satisfy the high-precision assembly design. A higher filling rate that ensured good heat transfer between the T/R module and the heat transfer interface of the diamond film was obtained by filling the installation interface between the T/R module and the diamond film with a conductive epoxy adhesive at a lower curing temperature. The abovementioned design enabled the accumulated heat of the T/R module to diffuse quickly through the diamond film to the entire transceiver frame, reducing the temperature gradients of the T/R modules. To improve the electrical conductivity between the T/R module and frame, gold was coated on the surface of diamond film by controlled sputtering. Thus, the surface of insulating diamond film could be electrically conductive while the binding force between the conductive adhesive and the diamond film was enhanced.
CSMH has a world-class diamond manufacturing capability, and currently produces diamond wafers, diamond heat sink, diamond-based gallium nitride and other products in mass production.
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