Thermal properties of diamond/Cu composites enhanced by TiC plating with molten salts containing fluoride and electroless-plated Cu on diamond particles

Highlights

• Using a ternary molten salt containing F, coat diamond particles with TiC.

• The temperature as low as 700 °C formed continuous thin TiC coatings.

• Cu-TiC coating is readily achieved by electroless plating and molten salt treatment.

• Cu/diamond Composites with dual-coating diamond exhibit favorable thermal properties.

Abstract

To improve the thermal properties of diamond/Cu composites for thermal management applications, we designed dual-coated diamond particles for preparing composites. A thin TiC coating is formed on diamonds using molten salt with NaCl-KCl-NaF, then electroless plating of Cu is applied. The effects of salt bath temperature on the formation of TiC coating and the percentage of modified diamonds on the thermal performance of the composites were investigated. At temperature as low as 700 °C, Ti and diamond particles forms a TiC coating with a thickness of 1 μm in molten salt containing fluoride. In response to an increase in temperature, the thickness of the formed TiC coating increases and is prone to cracking and flaking. Cu coating well coats the TiC/diamond by electroless plating. The hot-pressing diamond/Cu composite with 60 vol% modified diamonds exhibits the best thermal conductivity of 495.5 W·m⁻¹·K⁻¹, slightly below the values predicted by the model H-J and DEM, and lower thermal expansion coefficient of 5.86 × 10⁻⁶ K⁻¹, corresponding to the Kerner model.

Introduction

The microelectronic industry has experienced tremendous growth in the last few decades and this has dramatically increased the power density of electronic equipment. However, high-performance heat dissipation material is still the most challenging aspect of developing next-generation high-power electronic equipment [1], [2], [3], [4], [5]. Diamond has an extraordinary thermal conductivity (TC) of about 2200 W·m⁻¹·K⁻¹ and a low coefficient of thermal expansion (CTE) of 1.2 × 10⁻⁶ K⁻¹, which makes it an ideal reinforcement material for thermal management materials. The TC of pure Cu can reach up to 396 W·m⁻¹·K⁻¹, the second highest among metals. In addition, it is inexpensive and easy to process. Unfortunately, the CTE of pure Cu is approximately 17 × 10⁻⁶ K⁻¹, due to its high thermal expansion coefficient, it cannot meet the requirements of electronic packaging materials. Therefore, diamond reinforced Cu matrix (diamond/Cu) composites are considered an attractive candidate material by increasing TC and limiting the CTE to a reasonable value.

For diamond reinforced Cu matrix composites, the interface bonding and structure between the reinforced phase and the matrix phase are critical to achieving the ideal thermal performance of the composites [6], [7], [8]. However, diamond and Cu are non-wetting and do not react, resulting in a large number of air gaps at the interface, thereby greatly reducing the theoretical TC of the composite. A variety of methods have been explored for addressing the interface defects, such as electroforming, Cu alloying, and surface modification of diamonds and so on.

In alloying, Cu alloys with a small amount of strong carbide-forming elements, such as Ti, Zr, Cr, and B [9], [10], [11], [12], [13], [14]_. During the process of infiltration, the diamond and carbide forming elements will react at the interface to form a carbide layer. It is inevitable that alloying elements will dissolve into the Cu matrix, which will reduce its TC. It is also hard to accurate regular the amount of alloying elements. Furthermore, alloying generally utilizes liquid phase sintering to fabricate composites. As sintering temperatures are higher than the melting point of Cu, unmodified diamond surfaces are readily graphitized. Unlike alloying, electroforming is simpler and more feasible because it does not require high temperature or high pressure. Although it is possible to prepare diamond/Cu composites with high TC and good interface without voids, it requires careful control of preparation parameters, such as electrolyte concentration, current density, PH, and rate of deposition [15], [16], [17], [18], [19]. It can take up to tens of hours to prepare a Cu sheet in millimeter thickness on diamond hybridized to it. It is unclear whether thermal fatigue between the unmodified diamond and electrode position reduces its TC.

When a diamond particle is coated with strong carbide-forming elements, such as Ti, Cr, B, Mo and W [20], [21], [22], [23], [24], the carbide coating effective improves interfacial bonding between diamond and Cu matrix. In addition, it prevents the diamond surface experience graphitization at relatively high temperatures. Carbide coatings can be formed on diamond particles by the molten salt method [25], [26], magnetron sputtering [27], [28] and vacuum evaporation [29], [30]. However, the diamond and coating are a physical combination by magnetron sputtering and vacuum evaporation. The binding force is not strong. Molten salt method is arguably one of the simplest methods to form a coating on diamond surface. In the process of coating reaction, molten salt environment is used as the reaction medium, and the diamond surface reacts with strong carbide elements to form carbide layer. In many cases, NaCl-KCl binary molten salts is used as the reaction medium, however, ternary molten salts have a lower melting point and higher melting heat, and can react at lower temperatures to form coating. Kang et al. coated diamond particles with TiC by heating diamond and Ti powder in a molten mixture of NaCl and KCl at 900 °C–1050 °C [31]. Kim et al. fabricated uniform and complete Cr₃C₇ on diamond surface in molten mixtures with LiCl-KCl-NaCl over the temperature range of 800 °C [32].

In this study, diamond particles were coated with titanium carbide using molten salt with NaCl, KCl, and NaF. In the preparation of composites, the Cu coating deposited on the diamond particles surface by short-time electroless plating further to enhance the interfacial bonding between diamond and Cu matrix. The TiC coating enhances the bond between diamond and copper by preventing their disconnection. Sintering of composites results in TiC–Cu transiting to Cu–Cu due to the introduction of Cu coatings [33]. To investigate the effect of dual-coated diamond particles on the thermal properties of diamond/Cu composites, hot-press sintering composites reinforced with different volume fractions were prepared.