Diamond/Copper composite materials, breaking through limitations! ZMSH is keeping up with the times.

With the continuous miniaturization, integration, and high performance of modern electronic devices—including computing, 5G/6G, batteries, and power electronics—the increasing power density has led to severe Joule heating and high temperatures within the devices. This results in performance degradation and device failure. Efficient thermal management has become a significant issue in electronic products. To alleviate this problem, integrating advanced thermal management materials onto electronic components can significantly enhance their heat dissipation capability.

Diamonds possess excellent thermal properties, exhibiting the highest isotropic thermal conductivity (k = 2300 W/mK) among all bulk materials, and have an extremely low coefficient of thermal expansion (CTE = 1 ppm/K) at room temperature. Diamond particle-reinforced copper matrix (diamond/copper) composites have attracted significant attention as a new generation of thermal management materials due to their potential high k values and adjustable CTE.

However, there are notable mismatches between diamond and copper in many performance aspects, including but not limited to CTE (with a significant difference in order of magnitude, as shown in Figure (a)) and chemical affinity (they are immiscible and do not undergo chemical reactions, as illustrated in Figure (b)).

These mismatches inevitably lead to the inherent low bonding strength of the diamond/copper composites during high-temperature manufacturing or integration processes, as well as high thermal stress at the diamond/copper interface. Consequently, the diamond/copper composites are prone to interface cracking, which significantly reduces thermal conductivity (when diamond and copper are directly bonded, their k value can be much lower than that of pure copper, even below 200 W/mK).

Currently, the main improvement method involves chemical modification of the diamond/diamond interface through metal alloying or surface metallization. The transition interlayer formed at the interface can enhance the interfacial bonding strength, and a relatively thicker interlayer is more beneficial in resisting interface cracking. As referenced in the literature, to achieve a bonding effect, the thickness of the interlayer needs to be on the order of hundreds of nanometers or even micrometers. However, the transition interlayers on the diamond/copper interface, such as carbides (e.g., TiC, ZrC, Cr3C2), exhibit lower intrinsic thermal conductivities (<25 W/mK), several orders of magnitude less than those of diamond or copper. From the perspective of improving interfacial heat transfer efficiency, it is essential to minimize the thickness of the transition interlayer because, according to the thermal resistance model, the interfacial thermal conductivity (G_cu-diamond) is inversely proportional to the interlayer thickness (d).

While a relatively thicker transition interlayer helps improve the interfacial bonding strength at the diamond/diamond interface, the excessive thermal resistance of the interlayer hinders heat transfer across the interface. Therefore, a significant challenge in integrating diamond and copper is to maintain a high interfacial bonding strength while not excessively introducing interfacial thermal resistance when employing interface modification methods.

The chemical state of the interface determines the interfacial bonding strength between heterogeneous materials. For example, chemical bonds are significantly stronger than van der Waals forces or hydrogen bonds. On the other hand, the thermal expansion mismatch on both sides of the interface (where T represents CTE and temperature) is another critical factor affecting the bonding strength of the diamond/copper composites. As shown in Figure (a), there is a significant difference in the order of magnitude of the thermal expansion coefficients between diamond and copper.

In general, thermal expansion mismatch has always been a key factor influencing the performance of many composites because the dislocation density around the filler significantly increases during cooling, especially in metal matrix composites reinforced with non-metallic fillers, such as AlN/Al composites, TiB2/Mg composites, SiC/Al composites, and the diamond/copper composites studied in this paper. Additionally, the preparation temperature of diamond/copper composites is relatively high, typically exceeding 900°C in conventional processes. The significant thermal expansion mismatch can easily generate thermal stress in a tensile state at the diamond/copper interface, leading to a sharp decline in interfacial adhesion and even interface failure.

In other words, the chemical state of the interface determines the theoretical potential for interfacial bonding strength, while thermal mismatch dictates the extent of reduction in interfacial bonding strength after high-temperature composite fabrication. Therefore, the final interfacial bonding strength is the result of the interplay between these two factors. However, most current studies focus on improving interfacial bonding strength by adjusting the chemical state of the interface, such as through the type, thickness, and morphology of the transition interlayer. The reduction in interfacial bonding strength due to severe thermal mismatch at the interface has not yet received sufficient attention.

The preparation process, as shown in Figure (a), includes three main stages. First, a nominal thickness of 70 nm of a thin titanium (Ti) coating is deposited on the surface of diamond particles (model: HHD90, mesh size: 60/70, Huanghe Whirlwind Co., Ltd., Henan, China) using radio frequency magnetron sputtering at 500°C. High-purity titanium targets (purity: 99.99%) are used as the source material, and argon gas (purity: 99.995%) serves as the sputtering gas. The thickness of the Ti coating is controlled by adjusting the deposition time. During the deposition process, a substrate rotation technique is employed, allowing all surfaces of the diamond particles to be exposed to the sputtering atmosphere, ensuring that the Ti element is uniformly deposited on all surface planes of the diamond particles (mainly including two types of facets: (001) and (111)).

Secondly, during the wet mixing process, 10 wt% of alcohol is added to ensure a uniform distribution of the diamond particles within the copper matrix. Pure copper powder (purity: 99.85 wt%, particle size: 5–20 μm, Zhongnuo Advanced Materials Technology Co., Ltd., China) and high-quality single-crystal diamond particles are used as the matrix (55 vol%) and the reinforcing phase (45 vol%), respectively.

Finally, alcohol is removed from the pre-pressed composite material in a high vacuum of 10^-4 Pa, and the copper-diamond composite material is densified using powder metallurgy methods (spark plasma sintering, SPS).

In the SPS preparation process, we innovatively proposed a Low-Temperature High-Pressure (LTHP) sintering technique, combining it with thin interface modification (70 nm). To reduce the thermal resistance introduced by the coating itself, a thin interface modification layer (70 nm) was employed. For comparison, we also prepared the composite materials using the traditional High-Temperature Low-Pressure (HTLP) sintering process. The HTLP sintering technique is a conventional method widely used in previous works to integrate diamond and copper into dense composites. This HTLP process typically uses a high sintering temperature of over 900°C (close to the melting point of copper) and low sintering pressure of about 50 MPa. However, in our proposed LTHP process, the sintering temperature is set at 600°C, significantly lower than the melting point of copper. At the same time, by replacing traditional graphite molds with hard alloy molds, the sintering pressure can be substantially increased to 300 MPa. The sintering time for both processes is 10 minutes. Additional details on optimizing the LTHP process parameters are provided in the supplementary materials. The experimental parameters for the different processes (LTHP and HTLP) are shown in Figure (b).

The conclusions of the above research aim to overcome these challenges and elucidate the mechanisms for improving the thermal transport properties of diamond/copper composites:

1. A new integration strategy was developed that combines ultra-thin interface modification with the LTHP sintering process. The resulting diamond/copper composite achieved a high thermal conductivity value (k) of 763 W/mK, with a coefficient of thermal expansion (CTE) value of less than 10 ppm/K. Additionally, a high k value was obtained even at a lower diamond volume fraction (45% compared to the 50%-70% typical in conventional powder metallurgy processes), indicating that costs can be significantly reduced by decreasing the amount of diamond filler.

2. Through the proposed strategy, the refined interface structure was characterized as a layered structure of diamond/TiC/CuTi2/Cu, which greatly reduced the thickness of the transition layer to approximately 100 nm, much less than the several hundred nanometers or even micrometers previously used. However, due to reduced thermal stress damage during the preparation process, the interface bonding strength was still enhanced to covalent bond levels, with an interface bonding energy of 3.661 J/m².

3. Due to its ultra-thin nature, the carefully crafted diamond/copper interface transition layer exhibits low thermal resistance. Meanwhile, molecular dynamics (MD) and ab initio simulation results indicate that the diamond/titanium carbide interface has excellent phonon property matching and outstanding thermal transfer capability (G > 800 MW/m²K). Thus, the two potential thermal transfer bottlenecks are no longer limiting factors for the diamond/copper interface.

The interface bonding strength effectively increased to covalent bond levels. However, the interface thermal transfer capability (G = 93.5 MW/m²K) remained unaffected, achieving an excellent balance between these two critical factors. Analyses suggest that the simultaneous improvement of these two key factors is the reason for the superior thermal conductivity of the diamond/copper composites.

 

ZMSH'S Solution

Copper substrate single crystal Cu wafer 5x5x0.5/lmm 10x10x0.5/1mm 20x20x0.5/1mm a=3.607A

Al substrate single crystal Aluminum substrate purity 99/99% 5×5×1/0.5 mm 10×10×1/0.5 20x20x0.5/1mm