As integrated circuits in electronic devices become increasingly complex and chip sizes continue to shrink, excessive heat accumulation during operation degrades device performance and can even cause short circuits and other failures. To ensure stable, safe, and efficient operation of electronic equipment, the development of packaging materials with high thermal conductivity, low coefficient of thermal expansion, low density, and excellent flexural strength has become a research focus.
01 Diamond/Silicon Carbide: A Powerful Combination for Thermal Management
Diamond has the highest thermal conductivity of any naturally occurring material known to man, combined with a low thermal expansion coefficient, high strength and hardness, and excellent chemical stability, making it an ideal thermal management material. Silicon carbide also possesses a low thermal expansion coefficient and superior thermal conductivity. By incorporating diamond as a reinforcing phase into silicon carbide, diamond/silicon carbide composites with tunable thermal expansion coefficients and high thermal conductivity can be achieved. Compared with diamond-reinforced metal matrix composites, diamond and silicon carbide have similar structures, and their wettability and thermal expansion matching are much better than those between metals and diamond.
02 Fabrication Processes for Diamond/Silicon Carbide Composites
Diamond is prone to graphitization at high temperatures; the resulting graphite has significantly lower thermal conductivity and flexural strength than diamond. To successfully fabricate diamond/silicon carbide composites, effective control of diamond graphitization is essential. Since silicon carbide matrix cannot be directly infiltrated into a diamond preform, the silicon carbide matrix is typically obtained through silicon‑carbon reactions. The main fabrication processes for diamond/silicon carbide composites are roughly as follows.
(1) High‑Pressure High‑Temperature (HPHT) Sintering
In the HPHT method, diamond micropowder and pure silicon powder are thoroughly mixed and then subjected to high temperature and high pressure to undergo an in‑situ reaction forming silicon carbide, ultimately yielding diamond/silicon carbide composites. The advantage of this method is that under high‑pressure conditions, diamond does not undergo significant graphitization, and the gaps between diamond particles can be effectively reduced, producing composites with high diamond volume fraction and high density. However, the high‑pressure process demands expensive equipment and imposes significant limitations on the shape of the fabricated composites.
(2) Spark Plasma Sintering (SPS)
SPS is similar to HPHT in that it sinters and densifies the preform under high temperature and pressure. The difference lies in that SPS uses high‑energy electric sparks to generate instantaneous discharge between powder particles, producing high‑temperature plasma that releases a large amount of heat. Therefore, SPS features rapid heating rates and short sintering times. In addition, the sintering pressure in SPS is lower than that in HPHT.
(3) Hot Isostatic Pressing (HIP)
In the HIP process, diamond and silicon powders are placed in a sealed container and subjected to equal pressure from all directions, while sintering and densification are completed at elevated temperatures. HIP typically operates at pressures in the range of several hundred megapascals. Its advantages include lower sintering pressure and the ability to produce larger volumes and complex‑shaped samples. However, the process is complicated and has low production efficiency.
(4) Precursor Infiltration and Pyrolysis (PIP)
The PIP method uses a silicon carbide precursor (polycarbosilane) that is heated and pyrolyzed to form silicon carbide, which then serves as the matrix binding the diamond particles together. The advantages of this process are low sintering temperature, capability for large‑scale or complex‑shaped samples, and no residual silicon in the composite. However, due to the low conversion yield of the precursor, multiple cycles are often required to improve density.
(5) Chemical Vapor Infiltration (CVI)
CVI is a relatively mild process; at the low infiltration temperatures, diamond particles remain stable and graphitization is avoided. Compared with other techniques, the silicon carbide phase is derived from chemical vapor deposition on the material surface, so the silicon phase can be completely eliminated during the CVI process. Composites prepared by this method have relatively low density and are typically used for producing thin‑sheet samples.
(6) Reactive Infiltration (RI)
RI is a densification process in which a solid is melted by heating and infiltrated into a prepared preform. Depending on the wettability between the reinforcement and the matrix, either pressure‑assisted or pressureless infiltration can be employed.
In this process, diamond particles and graphite powder are uniformly mixed, with resin used as a binder, and then cold‑pressed or hot‑pressed to form a preform. The preform is then subjected to debinding and pyrolysis to completely carbonize the binder and increase the preform porosity. Finally, silicon infiltration is carried out by heating in vacuum or an inert atmosphere. During infiltration, liquid silicon reacts with the pyrolytic carbon and the added graphite powder to form silicon carbide. After the carbon reaction is complete, the remaining pores in the preform are filled with liquid silicon, resulting in a dense diamond/silicon carbide composite. Compared with other processes, RI does not require complex procedures or expensive equipment; the process is simple yet enables near‑net‑shape forming of the composite.