Advances in Preparation and Interface Regulation of High Thermal Conductivity Diamond and Copper Composite Materials

Excellent thermal conductivity mainly depends on new materials with excellent performance. The operating temperature of electronic products has a great influence on its efficiency and life. Relevant studies have shown that the life of electronic products is shortened exponentially when they work at higher temperatures. Therefore, new materials with excellent thermal conductivity have important practical significance and research value.

An ideal thermally conductive material must have high thermal conductivity, low thermal expansion coefficient, sufficient mechanical strength and low cost. Traditional heat-conducting materials can be divided into ceramic thermal conductive material, polymer thermal conductive material and metallic thermal conductive material according to their composition.

CTCM has high compactness, low coefficient of thermal expansion, and high mechanical strength. Common CTCMs mainly include Al2O3, SiC, BeO and AlN. Its thermal properties are shown in Figure 1. Although CTCM has a low coefficient of thermal expansion, its thermal conductivity is also low. Moreover, the processing and molding of ceramics is difficult, which limits the wide application of CTCM.

PTCM has good sealing, low density, good processability and low production cost. The most common PTCM is epoxy, whose thermal properties are shown in Figure 1. PTCM has low thermal conductivity, large thermal expansion coefficient and poor stability. Therefore, polymer thermally conductive materials cannot meet the requirements of high thermal conductivity, and generally can be applied to packaging materials that do not require high thermal conductivity.

The reason why the thermal conductivity of MTCM is generally higher than that of polymers and ceramics is that a large number of free electrons exist in metals, which can make heat transfer faster. MTCM is easy to process and low cost. Common MTCMs include copper, aluminum, silver, etc., and their thermal properties are shown in Figure 1. MTCMs have high thermal conductivity, but the mismatch between their thermal expansion coefficients and semiconductors limits their applications.

At present, traditional single-component thermally conductive materials can no longer meet the needs of electronic products for high thermal conductivity and low thermal expansion coefficient. The metal matrix composite thermal conductive material has the advantages of both the metal matrix and the reinforcing phase, and has high thermal conductivity, adjustable thermal expansion coefficient and good mechanical properties. Therefore, it has attracted more and more attention from researchers.

Diamond and copper composites combine the ultrahigh thermal conductivity of diamond with the low cost, ease of processing and high thermal conductivity of a copper matrix. It has great potential value in the application of high thermal conductivity materials, and has become a hot spot in the research of high thermal conductivity materials. However, the interfacial bonding between diamond and copper is generally poor. Even molten copper hardly wets diamond. The presence of voids at the Dia/Cu interface results in lower thermal conductivity than that of pure copper without applied high pressure (≥1 GPa). Therefore, the interface problem has become the focus of research on high thermal conductivity Dia/Cu.

1. Preparation Technology of Diamond/Copper Composites

The preparation technologies of diamond/copper composite materials mainly include High-temperature high-pressure sintering (HTHP), vacuum hot-pressing sintering (VHPS), spark plasma sintering (SPS) and melt infiltration , etc.

The high temperature and high pressure method (HTHP) is a method of filling the mixed powder into a mold and preparing a composite material in a short time under the action of high temperature and high pressure. Under the action of high temperature and high pressure, the powder is easier to flow, mass transfer and diffuse. Its sintering time is short and the prepared material has high density.

The thermal conductivity of the diamond/copper composite material prepared by the high temperature and high pressure method is as high as 920 W/(m·K), and the difficult wettability of diamond and copper under high temperature and high pressure is improved. This is due to the secondary nucleation and recrystallization of diamond to form a diamond-diamond skeleton.

The diamond/copper composite material prepared by high temperature and high pressure method has high density, and the formed diamond skeleton is helpful for heat conduction. But HTHP is very demanding on the mold. The small size and high cost of the prepared samples make it difficult to be widely used at present. Compared with the high temperature and high pressure method, the vacuum hot pressing sintering equipment is simple. Its mold requirements are low, and the size of the sintered product is larger.

Vacuum hot press sintering (VHPS) is one of the powder metallurgy methods. The composite material is prepared by putting the mixed powder into the mold, and going through the process of heating, pressurizing, keeping the pressure, cooling, demoulding and so on in the vacuum hot pressing furnace. The vacuum hot pressing sintering equipment consists of three parts: vacuum system, pressurization system and heating system. The schematic diagram of the equipment is shown in Figure 2.

Vacuum hot pressing sintering has the advantage of generating thermal stress during sintering. And the composition of the composite material is easier to control. However, VHPS is limited by the mold, and its pressure is generally below 100 MPa. The improvement of the bonding degree of copper and diamond interface is limited, which requires high control of sintering parameters and selection and addition of active elements. The preparation efficiency of VHPS is also low, and it is challenging to prepare Dia/Cu with excellent thermal properties. Compared with the vacuum hot pressing sintering method, spark plasma sintering is a new, fast and efficient composite material preparation method.

Spark plasma sintering (SPS) is a method of sintering powder under the combined action of pulse current and axial pressure through the plasma generated by instantaneous spark discharge. Its equipment is shown in Figure 3. The uniform distribution of spark discharge points during SPS sintering makes the sample evenly heated and rapidly diffused. The prepared material is uniform and dense, and is suitable for sintering of composite materials that are difficult to densify.

Spark plasma sintering heats up and cools down quickly. The sintering temperature is relatively low and the efficiency is high. Usually the sintering temperature of Dia/Cu is 800~970℃, which will not exceed the melting point of copper. Sintered molds in this temperature range are generally graphite molds. The fracture strength of the graphite mold is less than 100 MPa. Therefore, the sintering pressure is generally 50-80 MPa. In this sintering pressure range, it is difficult for the composite to become completely dense. Voids within the material increase the thermal resistance and reduce the thermal conductivity of Dia/Cu. Therefore, the future research direction of diamond/copper composite materials prepared by SPS should include the development and selection of high temperature resistant and high strength abrasive tools. Control the interface composition and interface thickness during sintering and study the thermal deformation behavior of diamond/copper composites, so as to improve the compactness of composites.

The melt infiltration method (Infiltration) is a method in which the matrix heated to a molten state is infiltrated into the gap of the reinforcement with a higher melting point, and then cooled and solidified to prepare a composite material. The interstitial space of the reinforcement is the volume fraction of the matrix. Infiltration can be divided into pressureless infiltration (Pressureless infiltration, PLI) and pressure melt infiltration (Pressure infiltration, PI).

Pressureless melt infiltration (PLI) refers to the method of preparing composite materials by infiltrating the molten matrix into the pores of the reinforcement preform mainly relying on capillary force without external force. This method generally uses a binder to make a preform from diamond and then places copper or copper alloy on top of the preform. Raise the temperature above the liquidus line of copper or copper alloy (about 1200°C) in a gas atmosphere. The copper or copper alloy melt spontaneously infiltrates the preform to form a diamond/copper composite.

The pressureless infiltration condition is simple. The operation is convenient, and it is the easiest to realize. However, the requirement for wettability between the matrix and the reinforcing phase is high, and the binder added during the preparation of the preform cannot be completely removed, which reduces the thermal conductivity of the matrix and increases the interface thermal resistance. When the volume fraction of diamond is high, the molten copper cannot completely fill the gaps of diamond spontaneously, while the pressure melt infiltration method can promote the filling of the gaps by the melt through external pressure.

Pressure melt infiltration (PI) refers to the method of adding external force to promote infiltration and solidifying under pressure to prepare composite materials during the infiltration process. Compared with pressureless infiltration, the preparation of Dia/Cu by pressure infiltration requires a shorter time and higher efficiency, and the prepared Dia/Cu has a higher density.

Pressure infiltration is a relatively complex process. The preparation of the reinforcement preform, the melting of the matrix, the flow of gas during the infiltration process, and the solidification of the matrix all have a great influence on the properties of the sample. This method has higher requirements on the design of graphite mold, control of sintering parameters and selection of sintering equipment. At the same time, diamond is a metastable state of carbon at room temperature. In high temperature environment (>900°C), graphitization transformation is easy to occur. Therefore, while ensuring interfacial bonding, effectively reducing the reaction temperature is the key to preparing Dia/Cu with excellent comprehensive properties.

The thermal conductivity of the composites obtained by the different preparation methods described above is shown in Fig. 4. It can be seen that the composite materials prepared by high temperature and high pressure method and pressure melt infiltration method have high thermal conductivity. This shows that no matter which method is used to prepare high thermal conductivity composite materials, it is inseparable from the corresponding pressure. However, vacuum hot pressing sintering and spark plasma sintering are limited by the compressive strength of the mold in the preparation of composite materials, which makes their thermal conductivity relatively low. The development and selection of high-temperature-resistant and high-strength sintered abrasives will be one of the future research directions for vacuum hot pressing sintering and spark plasma sintering. The various techniques for preparing high thermal conductivity Dia/Cu described above have their own advantages and disadvantages.

2. Interface Control of Diamond/Copper Composite Materials

The problem of high interfacial energy and poor wettability between diamond and copper seriously degrades the thermal conductivity of Dia/Cu while reducing its mechanical properties. The key to improving the performance of Dia/Cu lies in optimizing the interfacial bonding, reducing interfacial voids, and reducing interfacial thermal resistance. At present, in addition to the various sintering methods mentioned above, it is to introduce a transition layer at the Dia/Cu interface that has a good ability to combine with both diamond and copper. The commonly used methods are alloy the copper matrix and metallize the diamond surface.

Copper matrix alloying is to dope copper with a small amount of active elements (such as Ti, B, Cr, Zr, etc.) to improve the wettability of the Dia/Cu interface and optimize the interface bonding. The main methods of alloying copper substrates include alloy smelting (AS), gas atomization (GA) and so on. Alloy smelting (AS) is a process in which metals and additives are melted in a heating furnace to undergo physical and chemical changes and form alloys.

The alloying elements introduced by the alloying of copper matrix can form a carbide transition layer on the diamond surface, improve the wettability of Dia/Cu, fill the interfacial gap, optimize interfacial bonding, and improve thermal performance. The thickness of the carbide layer can be controlled by the doping amount of active elements. However, if the various carbide-forming elements added remain in the matrix, they will increase phonon scattering during heat transfer and reduce the thermal conductivity of the copper matrix, thereby reducing the thermal conductivity of Dia/Cu.

Therefore, when choosing doping alloy elements to improve thermal conductivity, elements that are easy to be carbonized by diamond and have good wettability with copper should be selected. Take care to avoid poor thermal conductivity. At the same time that the serious elements are diffused in the matrix, attention should be paid to controlling the amount of alloying elements. This makes the carbide layer thin and uniform to reduce the interface thermal resistance, avoiding the discontinuity of the carbide layer caused by too little addition of alloying elements, too thick transition layer caused by too much addition, or too much residue in the copper matrix, etc. question. Compared with ACM, diamond surface metallization is the pretreatment of diamond before sintering, which can effectively prevent the problem of the reduction of Dia/Cu thermal conductivity caused by insufficient alloying elements or residues in the copper matrix.

Diamond surface metallization (MDS) is the process of pre-treating diamond to make the diamond surface react with elements that are easy to react with carbon (such as Ti, W, Cr, Mo, etc.) to form a continuous dense carbide and active element coating. MDS methods include electroless plating ( EP), ion beam sputtering (IBS), magnetron sputtering (MS), vacuum micro evaporation plating (VMEP ), powder covered sintering (PCS), salt bath coating (SBC) and sol-gel coating (SGC), etc.

Electroless plating (EP) is a process of controlling metal deposition on the surface to be plated by using chemical reduction reaction in the absence of an external power source and the action of a strong reducing catalyst (Ni, Co, etc.). Before EP, the diamond surface is generally pretreated by cleaning, catalysis, etching, sensitization, and activation.

Ion sputtering (IBS) is to introduce a small amount of inert gas or air molecules into a vacuum container to be ionized under the action of an electric field. The plasma generated by it bombards the surface of the metal target, sputters out the target atoms and deposits them on the diamond surface. The film layer prepared by ion sputtering method is easy to adhere to the diamond surface. However, it is also particularly easy to bombard ions into the film layer to affect its performance. There are few studies on the use of ion sputtering to coat the diamond surface.

The principle of magnetron sputtering (MS) is basically the same as ion sputtering. But the magnetic field introduced by magnetron sputtering can control the movement of electrons near the cathode target. This ionizes more gas ions to bombard the target, increasing efficiency while keeping the ions from bombarding the diamond surface.

Metallization of the diamond surface by sputtering allows precise control of the thickness of the resulting coating. However, the distribution of film layers on each surface of the obtained diamond particles is not uniform. In order to ensure the combination of the coating and the diamond, it is generally necessary to treat the sputtered diamond in a vacuum (atmosphere) furnace. The diamond reacts with the coating to form carbides. The reaction temperature and time must be accurately controlled, which increases the difficulty of accurately controlling the composition and thickness of the surface transition layer.

Vacuum micro-evaporation (VMEP) is a process in which the vaporized and escaped atoms of the metal are heated in a vacuum container to react with the diamond surface and condense to form a film.

VMEP has the advantages of simple process, easy control of conditions, high film formation purity, good uniformity, relatively low coating temperature, small damage, and low cost. This also applies to the plating of carbide formers such as W, Ti, Cr, Mo, etc. But VMEP devices are complex. Interfacial defects are formed when combined with copper, affecting the thermal properties of the material.

Powder covered sintering (PCS) is a process of directly mixing metal or metal compounds with diamond particles, and causing them to undergo a diffusion reaction in a vacuum or inert atmosphere high-temperature furnace to form a carbide layer, also known as diffusion coating.

The thermal conductivity of diamond/copper composites prepared using different interface regulation methods described in this paper is shown in Figure 5. The thermal conductivity of Dia/Cu is closely related to the method of interface regulation and the type of coating elements. Regardless of the interface regulation process, carbide-forming elements (Ti, B, Cr, Zr, W, B, Mo, etc.) have the potential to improve the thermal conductivity of Dia/Cu. The transition layer formed by the reaction of these elements with the diamond surface can improve the wettability and bonding of the Dia/Cu interface and increase the thermal conductivity.

However, the actual thermal conductivity of diamond/copper composites is generally smaller than the theoretical value. This is mainly because the interface bonding of Dia/Cu in actual production has not reached the ideal state. The composition, continuity, and thickness of carbides have not been precisely controlled.

3. Conclusion and Outlook

The diamond/copper composite material has a high thermal conductivity and a thermal expansion coefficient matching that of semiconductor materials. It has broad application prospects in the fields of military industry, integrated circuits, 5G communications and new energy vehicles.

Future research on high thermal conductivity diamond/copper materials should focus on the following aspects.

(1) Research on the diamond skeleton structure under high temperature and high pressure conditions. The adjustment process ensures that the diamonds are not graphitized, and at the same time, the diamonds can be reunited to form bonds, forming more efficient diamond heat conduction channels to improve the thermal conductivity of the composite material.

(2) Focus on research on overplating. Regardless of the preparation process, the overcoating layer is very important to improve the thermal conductivity of the composite material. The overplating layer between diamond and copper should be continuous, dense, thin and uniform, with low thermal resistance.

(3) Design-optimize composite materials from the micro-nano scale. To reveal the action mechanism and influence law of various factors (especially interfacial bonding) on the thermal conductivity of composite materials at the nanometer scale.

(4) Production costs are equally important. The thermal conductivity of the reported diamond/copper composites has been far ahead of the application. The main reason is the cost issue. In the future, attention should be paid to how to use industrial-grade raw materials and equipment to prepare high-performance thermal conductive materials.