Study on Fabrication and Properties of Graphite/Al Composites by Hot Isostatic Pressing-Rolling Process

Graphite/Al composites have attracted much attrition due to their excellent thermal properties. However, the improvement of thermal conductivity (TC) is limited by the difficulty in controlling the orientation of graphite and the poor wettability between graphite and aluminum. In this study, a novel process for fabricating the Graphite/Al composites is proposed, which involves fabricating graphite film and aluminum foil into laminate material. Then, taking a rolling method, the fractured and well oriented graphite film can help the composite achieve high TC while maintaining a certain strength. The result reveals that both single and total reduction have a significant influence on the diameter and orientation of the graphite, and by adjusting the process parameters, composites with high TC can be acquired at a relatively low reinforcement volume. This near-net-forming process can directly meet the thickness requirements for electronic packaging and avoids the exposure of graphite to the surface during secondary processing, which is promising to promote the application for high TC Graphite/Al composites in thermal management.

Vacuum-plasma processes at extreme field emission in diamond electron sources

Background and Objectives: The use of high-current field electron sources that satisfy various circuitry requirements as a part of electronic devices for various purposes suggests the possibility of matching their operation modes with the operating characteristics of the devices, as well as high reproducibility of emission parameters, stability and the necessary resource of reliability and durability. The stability and durability of field electron sources are extremely sensitive to the changes in the geometry of emission centers and to the state of their surface, which undergoes various destructive influences during operation. These changes are especially important in the case of high-current field-emission cathodes, which, as a rule, work under conditions of technical vacuum and high electric field intensities. The aim of the work was to study the possibility of creating field sources of electrons based on thin-film planar-end nanodiamond-graphite structures that satisfy various circuit requirements, as well as to study fundamental factors that lead to a change in their I–V characteristics and limit the maximum value of their field emission currents, stability and durability of high-current field emission. Materials and Methods: Emission structures were made of carbon films deposited in a microwave plasma of a low pressure gas discharge. The surface resistance of the films was 120 kOhm/□ and 1.2 mOhm/□. In the first type of emission structure, diamond-graphite films were mechanically separated into two parts. One part of the film was the cathode, the second served as the anode. Measurement of field emission characteristics in vacuum (2–4)·10-3 Pa. Between the cathode and the anode, voltage pulses with a duration of 10 μs and an amplitude of 0 to 3000 V were applied. In the second type of emission structure, field emission was carried out from the end face of a diamond-graphite film deposited on a polycor substrate. Field emission-voltage characteristics were measured in constant electric fields. Determination of the elemental composition of the surfaces of field emission structures after electrical tests was carried out using an energy dispersive microanalysis system. Results: It is shown that the steepness of the current-voltage characteristics, as well as the stability and durability in extreme operating conditions of high-current field electron sources based on film diamond-graphite nanocomposites, is determined by their surface resistance. Electron field sources based on low-resistance diamond-graphite structures, in comparison with high-resistance, have a high slope of the I–V characteristic, a lower threshold for the field intensity at the beginning of field emission, and the maximum field emission current is achieved at a lower electric field strength. The range of operating voltages providing the same maximum field emission current is many times higher for high-impedance electron sources than for low-impedance ones. The various nature of vacuum-plasma processes is established for extreme field emission in diamond-graphite electron sources with different surface resistance. In the case of lowresistance diamond-graphite composite film structures under extreme operating conditions of high-current planar-face autoemission structures, the main reasons for the instability of the emission and destruction parameters are the appearance of electrothermal breakdowns at the cathode of the “grid” characteristic of thin dielectric coatings during a sliding surface electric discharge. In the case of high-resistance diamond-graphite film structures, there is no branched network of electrothermal electrical breakdowns. In this case, as well as for high-current end field emission structures, the main nature of destruction under extreme operating conditions is erosion of the cathode part of the film. Erosion is caused by the processes of explosive electron emission, which is carried out from the nanodiamond emission centers of the composite carbon film structure with the appearance of a cathode plasma plume and the graphite component of the cathode material is sprayed onto the anode and into the interelectrode gap. Conclusion: The results can be used to predict the durability and stability of high-current field electron sources based on diamond-graphite film structures depending on their design, electrophysical characteristics, and vacuum operating conditions.

Preparation and Properties of C/SiC Composites Reinforced by High Thermal Conductivity Graphite Films

Ablation resistance as one important factor affecting the service life of SiC ceramic matrix composites that is highly valued in aerospace science and technology. In this study, high thermal conductivity (HTC) graphite films and carbon fibers reinforced C/SiC composites simultaneously, fabricating by precursor infiltration and pyrolysis (PIP) technology, to improve the ablation resistance of C/SiC composites. Three C/SiC composites were prepared from different quantity ratios of 2D fiber cloth to HTC graphite film with values of 1:0, 1:1, and 1:10. The microstructure, mechanical properties, thermal conductivity and ablation performance of C/SiC composites after plasma ablation test at 1500 °C for 600 s were investigated. The results showed that with the increase of graphite films’ contents, the thermal conductivity of composites was increased from 9.78 W/(m·K) to 333.34 W/(m·K). Additionally, the mass loss rate reduced from 1.18 to 0.74 mg/s and the linear ablation rate reduced from 0.64 to 0.18 mm/s, indicating that the addition of graphite films could effectively improve the ablation resistance of C/SiC composites.