| With the development of electronic device towards miniaturization, lightweight and high performance, the heat in a unit of electronic device increases ceaselessly. The consequent temperature rise as well as the increase of the thermal stresses between packaging materials and chips can significantly affect the performance, reliability and life. Heat dissipation of electronic devicesis has become the bottleneck of development and application. In order to effectively dissipate heat, thermal management materials should have a thermal conductivity(TC) as high as possible and a low coefficient of thermal expansion(CTE) which matches to the semiconductors, thus avoiding the failure caused by the thermal stresses due to the CTE mismatch. Traditional thermal management materials suffer of insufficient TCs and are unable to combine high TC and low CTE, thus can not meet the thermal dissipation requirements for high power electronic devices.Aluminum matrix composite may be a promising high-performance thermal management material due to relatively high TC and adjustable CTE, in addition, low density and ease of machining. However, the TC and CTE always can not be satisfied simultaneously, besides it should meet the requirements of the machinability, mechanical properties, density and cost, which restrained its development.In this paper, emphasis is put on how to enhance the TC and control the CTE of aluminum matrix composite. By optimizing the composite configuration of the graphite-aluminum composites and controlling the interfacial structure, high TC of the composites as well as low thermal expansion was achieved simultaneously. Meanwhile, the machinability and mechanical properties can meet the requirements.Firstly, the condition of realizing the high TC was analyzed theoretically. The theoretical models were derived to predict the TC of the graphite-aluminum composites within the framework of the effective medium theory. The quantitative relations between the TC of the composites and the microstructures, including composite configuration and interfacial structure, were also established to provide theoretical guidance for the design of graphite-aluminum composites. Secondly, we optimized the composite configuration. Several different graphite-aluminum composites with different composite configurations including in-plane randomly distributed chopped fibers, oriented graphite flakes alignment and interpenetrating networks were fabricated. These composite configurations were also compared with the conventional dispersed graphite particles configuration. By comparison of the thermal enhancement capacities of these different composite configurations, the optimal configuration was confirmed. Meanwhile, interfacial structure with a low interfacial thermal resistance(ITR) was achieved by suppressing the harmful interfacial reactions, and thus realized the high thermal properties of composites. On this basis, the graphite fillers, matrix materials and interfacial structures were further optimized. Thus, the TC of graphite-aluminum composites was further improved. Finally, the thermal expansion and mechanical properties of the composites were studied. The graphite-aluminum composites with high TC and low CTE were eventually achieved.The study on the thermal enhancement capacities indicates that the oriented graphite flakes alignment has the highest capacity among these composite configurations. Based on the effective medium theory, the corresponding theoretical models for predicition of the composite TC were derived in this work. The experimental TCs showed good agreement with the predictions, which confirmed the validity. The influence rule of the graphite flakes size on the TC of composites was also investigated. The results indicate that the TC of composites increases with the increase of the diameter of graphite flakes, but the increase slows down when the diameter reaches to a certain value. Besides, the TC of composites increases with the increase of the reciprocal of the aspect ratio of graphite flakes. There was significant difference in thermal enhancement behavior for oriented graphite flakes alignment in different matrices. The theoretical model reasonably elucidated this phenomenon. It can be attributed to the influence of effective phase contrast(defined as the ratio between effective TC of the inclusions and TC of the matrix). With increasing effective phase contrast, the thermal enhancement behavior of oriented graphite flakes alignment switches from linearity into non-linearity.The characterization of interfacial structure revealed that an amorphous layer formed at the interface between graphite flake and aluminum matrix. The interface at the top surface of graphite is reactive Al-Si-O-C amorphous layer with a thickness of 40-50 nm. While the interface at the side surface has a thickness of only 5-8 nm and no reaction products were detected. Fast Fourier transform confirmed that the interface is amorphous carbon layer. The experimental TC values are very close to the theoretical values in the case of zero thickness interface(only consider Kapitza thermal resistance), which suggests that nanoscale amorphous carbon layer does not cause an obvious adverse effect on interfacial heat transfer. The ITR of 1.3×107 W/m2 K was derived, which the same order of magnitude as the value in the case of zero thickness interface(4.5×107 W/m2 K). This study also found that heat treatment can significantly reduce the TC of the composites. It can be attributed to the enrichment of interfacial oxygen atoms that enhanced the phonon scattering and harmful interfacial reaction products leading to degradation of the ITR. Constructing a suitable inter layer and limiting its thickness under the critical thickness can achieve a higher interfical thermal conducitivity. Theoretical calculation indicates that highly thermal conductive diamond, silver and copper are unsuitable as interlayer for graphite/aluminum interface, while silicon is the most suitable material.The investigation of thermal expansion behavior of graphite flakes-Si-Al composites indicates that the CTE along the xy direction decreases from 11.4 ppm/K to 7.7 ppm/K with increasing volume fraction of graphite flakes from 13.7 to 71.1 vol. %. This is consistent with Kerner model. However, the CTE along the z direction shows an anomalous thermal expansion behavior, i.e. the CTE along the z direction is even lower than that along the xy direction and decreases with the increase of the volume fraction of graphite flake.By optimizing the composite configuration and controlling the interfacial structure, the graphite flakes-Si-Al composites with high TC and low CTE was eventually achieved. Along the xy direction, the TC of composites is 526 W/m K, which is 400% higher than that of matrix. The CTE is 7.7 ppm/K, which matches to the semiconductors. The density of the composite is 2.32 g/cm3. The mechanical properties meet the use requirements of heat sink in semiconductor industry. By replacing the silicon particles with diamond, the TC of composite reaches to 630W/m K. |
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