Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Influence of interfacial carbide layer characteristics on thermal properties of copper–diamond composites

Copper–diamond composites are increasingly being considered for thermal management applications because of their attractive combination of properties, such as high thermal conductivity (λ) and low coefficient of thermal expansion (CTE). In this research, thermal properties of Cu–diamond composites with two different types of interfacial carbides (Cr₃C₂ and SiC) were studied. The interface thermal conductance (h _c) was calculated with Maxwell mean-field and differential effective medium schemes, wherein experimentally measured λ was entered as an input parameter. The λ and h _c of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites are higher than those reported in previous studies for Cu–diamond composites with no interfacial carbides. The value of h _c is intimately related to the morphology and thickness of the interface carbide layer, with the highest h _c being associated with a thin and continuous interface carbide layer. A lower h _c resulting from a thicker Cr₃C₂ layer can provide an alternate explanation for a previously reported trend in λ of Cu–Cr₃C₂–diamond composites with different Cr-contents. The experimentally measured CTE was compared with the Turner and Kerner model predictions. The CTE of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites is lower and better matches the model predictions than the previously reported CTE of Cu–diamond composite with no interfacial carbides. The CTE of Cu–Cr₃C₂–diamond composites agrees better with the Kerner model than the Turner model, which suggests that deformation during temperature excursions involves shear.     

Microstructure and microchemistry of the Al/SiC interface

The characteristics of the Al/SiC interface play a critical role in controlling the properties of SiC-reinforced aluminium composites and aluminium-brazed SiC ceramic joints. Recently, a detailed investigation on the wettability of SiC single crystals by aluminium and several of its alloys was conducted. In order to understand further the nature of the Al/SiC interface, high resolution and conventional transmission electron microscope techniques have now been used to investigate its microchemistry and microstructure. The results revealed the coexistence of two polytype structures, rhombohedral and hexagonal, in the SiC single crystal structure. Aluminium carbide (Al₄C₃) and silicon were the reaction products found at the Al/SiC interface. From diffraction patterns, epitaxial orientation relationships between the SiC substrate and Al₄C₃, Si were determined.     

高体积分数SiCp/A356复合材料的显微组织和电导率

铝基复合材料在电子封装领域存在着潜在的应用前景。为获得高体积分数的铝基复合材料,利用压力浸渗法制备了高体积分数SiC颗粒增强A356复合材料(SiC_p/A356),通过金相显微镜、XRD、SEM和EDS等分析手段对其物相、显微结构和电导率进行了表征。结果表明:用该方法制备的SiC_p/A356复合材料组织致密,颗粒分布均匀,界面结合性能较好;SiC增强颗粒与A356基体界面反应控制良好,仅有少量Al4C3脆性相生成。SiC粉体经颗粒表面氧化处理在其表面生成一层SiO_2薄膜,虽抑制了界面反应的发生,但也使复合材料的收缩减小,电阻率增大,导电性能变差。     

Microstructure, thermal behavior and mechanical properties of squeeze cast SiC, ZrO2 or C reinforced ZA27 composites

ZA27 alloy based composites were synthesized by stirring method, followed by squeeze casting. Stir casting was employed successfully to incorporate 5 vol.% of various reinforcement particulates, namely, SiC, ZrO₂ or C. The porosity in the composites was decreased by squeeze pressure. The presence of particles and/or application of squeeze pressure during solidification resulted in considerable refinement in the structure of the composites. The microstructures, X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDXA) results indicated that no significant reactions occurred at the interface between the SiC or C particles and ZA27 alloy. However, in case of ZrO₂ reinforced ZA27, the ZrO₂ reacted with Cu present in the molten ZA27 alloy, forming Cu₅Zr. Thermal analysis showed that both α and β nucleation and growth temperatures of the composites were lower than those of the ZA27 alloy. The presence of particles in the as-cast or squeezed composites led to not only an accelerated age hardening response, but also an increase in the peak hardness of the composites. The values of coefficient of thermal expansion (CTE) of the composites were drastically lower as compared to those of the ZA27 alloy. The tensile properties of the composites decreased as a result of the addition of the particles. Scanning electron microscope (SEM) pictures of the composites indicated that cracks mainly initiated at particle-matrix interface, propagated through the matrix and linked up with other cracks leading to failure of the composites.     

Thermophysical Properties of Ceramic Filler-Added BaO–ZnO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> Glass Composites for Barrier Ribs of Plasma Display Panels

The capability of BaO–ZnO–B₂O₃–P₂O₅ glass in hosting various ceramic fillers (up to 20 mass% of Al₂O₃, TiO₂, and ZnO) has been investigated. All the investigated filler-added glasses have demonstrated a reasonable densification at 550 °C to form stable ceramic filler-glass composites. Modifications of the thermophysical properties, such as coefficient of thermal expansion (CTE), glass transition temperature, and dilatometric softening temperature, by the addition of the fillers have been investigated and correlated to phase and microstructural evolution. The CTE of the fabricated composites with varying filler addition is well correlated with theoretical predictions based on the Turner equation considering the modification by phase evolution, which indicates the thermal property tuning potential of the BZBP-based glass composites for application to barrier ribs of plasma display panels.     

Casting particulate and fibrous metal-matrix composites by vacuum infiltration of a liquid metal under an inert gas pressure

A new method of making metal-matrix composites is reported. This method combines the essentials of three liquid-phase fabrication methods: (i) vacuum infiltration, (ii) infiltration under an inert gas pressure, and (iii) squeeze casting. In this method, the particulate or fibrous preform is placed in a mould and the matrix alloy is placed above the preform. The matrix alloy is heated to the liquidus temperature together with the mould and the preform under vacuum. Then an inert gas like argon is compressed on to the top surface of the matrix-alloy melt, forcing the melt to infiltrate the preform. The pressure is 1000 to 2500 psi. As the melt is just at liquidus temperature, it is much lower than that used in squeeze casting. Moreover, the pressure is an order of magnitude lower than that used in squeeze casting. The low temperature lessens the interfacial reaction between the matrix and the filler, while the low pressure essentially eliminates preform compression. This method has been successfully used to fabricate aluminium-matrix composites reinforced by short ceramic fibres, continuous ceramic fibres, SiC particles, Al₂O₃ particles, graphite flakes and SiC whiskers.     

A theoretical approach for the thermal expansion behavior of the particulate reinforced aluminum matrix composite Part I A thermal expansion model for composites with mono-dispersed spherical particles

Microstructural observation revealed that the increase in the volume fraction of SiC particles lowers the coefficient of thermal expansion (CTE) of the composite, and the CTE of the metal matrix composites is proportional to the size of the Si phase. To analyze the thermal expansion behavior of aluminum matrix composites, a new model for the CTE of the mono-dispersed binary composite on the basis of Ashelby's cutting and welding approach was proposed. In the theoretical model, it was considered that during cooling relaxation of residual stresses could create an elasto-plastic deformation zone around a SiC or Al₂O₃ particle in the matrix. The size of reinforced particles and other metallurgical factors of the matrix alloy and composite were also considered. In this model, the interacting effect between the reinforced hard particle and the soft matrix is considered by introducing the influence of the elasto-plastic deformation zone around a particle, which is distinguished from the previous models. It was revealed that the CTE of the composite are influenced by the particle volume fraction, the elastic modulus and Poisson's ratio as well as the elasto-plastic deformation zone size and the particle size.     

The effect of phase transformation on the thermal expansion property in Al/ZrW2O8 composites

This article studied the effect of phase transformation on the thermal expansion property in Al/ZrW₂O₈ composites. The Al/ZrW₂O₈ composites of low-thermal expansion were fabricated by a squeeze casting method. The coefficient of thermal expansion (CTE) of as-made composites was discovered sharply increased at around 130 °C. The X-ray diffraction (XRD) spectra showed the existence of high-pressure γ-phase in the as-made composites. This high-pressure γ-phase was considered to be induced by the compressive residual stress originated from the thermal mismatch between Al matrix and ZrW₂O₈ particles. The in situ high-temperature XRD and the differential scanning calorimetry technique were used to study this thermally expanded abruption phenomenon. It was found that the phase transformation from high-pressure γ-phase to the low-pressure phases (α/β phase) in the composites should be responsible for fluctuation in the CTE of composites. Furthermore, using a proper heat treatment to eliminate the high-pressure phase in the composite, the Al/ZrW₂O₈ composites of low and uniform CTE (from 20 to 200 °C) could be achieved. And when temperature increased again, the thermal mismatch stresses between the metal matrix and ceramic particles in the composite were not large enough to re-induce the α-γ transformation.     

Thermal expansion behaviour of plasma sprayed Al–SiCp composites

Abstract

Al with 55 and 75 vol.-%SiC powders were free mechanically mixed or ball milled as feedstock. The powder feedstock was deposited onto a graphite substrate to form near net shape of Al/SiC composites by air plasma spraying. The pores and the gaps at the Al/SiC interface as well as at the boundary of Al grains exist extensively in the as sprayed composites. Coefficient of thermal expansion (CTE) of the sprayed composites was measured in the temperature range of 25–300°C. The composites plasma sprayed with Al–75SiC powder feedstock can reach a low CTE value of 8 × 10^?6 °C^?1. The effect of pore on the CTE of the composites has been discussed. The gap at Al/SiC interface has an influence on thermal expansion behaviour only at lower test temperatures. Reduction and elimination of the gap with temperature can offset the thermal expansion of the as sprayed composites, resulting in lower CTE at the beginning of the CTE test. Roughly quantitative consideration of the effect of the interfacial gaps between Al and SiC on CET was given. Linear rule of mixture (ROM), Turner and Kerner's models were used to estimate the CTE of the sprayed composites. It was found that ROM and Kerner's model give closer CTE prediction for the present composites.     

Fabrication,interface characterization and modeling of oriented graphite flakes/Si/Al composites for thermal management applications

Highly thermally conductive graphite flakes (G_f)/Si/Al composites have been fabricated using G_f, Si powder and an AlSi₇Mg_0.3 alloy by an optimized pressure infiltration process for thermal management applications. In the composites, the layers of G_f were spaced apart by Si particles and oriented perpendicular to the pressing direction, which offered the opportunity to tailor the thermal conductivity (TC) and coefficient of thermal expansion (CTE) of the composites. Microstructural characterization revealed that the formation of a clean and tightly-adhered interface at the nanoscale between the side surface of the G_f and Al matrix, devoid of a detrimental Al₄C₃ phase and a reacted amorphous Al–Si–O–C layer, contributed to excellent thermal performance along the alignment direction. With increasing volume fraction of G_f from 13.7 to 71.1 vol.%, the longitudinal (i.e. parallel to the graphite layers) TC of the composites increased from 179 to 526 W/m K, while the longitudinal CTE decreased from 12.1 to 7.3 ppm/K (matching the values of electronic components). Furthermore, the modified layers-in-parallel model better fitted the longitudinal TC data than the layers-in-parallel model and confirmed that the clean and tightly-adhered interface is favorable for the enhanced longitudinal TC.     

Synthesis and properties of dental zirconia–leucite composites

The dental zirconia–leucite composites were synthesized by high temperature solid-state method using potash feldspar, potassium carbonate and zirconia as raw materials. The mechanical properties and the coefficient of thermal expansion (CTE) of the prepared zirconia–leucite composites were tested. The results show that the bending strength, the fracture toughness and the metal–ceramic bonding strength of the prepared samples are about 110 MPa, 3·5 MPa/m^1/2 and 45 MPa, respectively. The CTE was about 13·73×10^–6 °C^–1 and close to that of Ni–Cr dental alloy (14·0×10^–6 °C^–1). The results indicate that the introduction of zirconia is beneficial to the improvement in the mechanical properties and CTE adjustment of porcelain material. The clinical application of the zirconia–leucite composites with good metal–ceramic bonding strength in the dental restoration could be envisioned.     

Properties of high reinforcement-content aluminum matrix composite for electronic packages

Aluminum matrix composites reinforced by a high volume fraction of ceramic particles provide a novel solution to electronic packaging technology, because of their high thermal conductivity, compatible and tailorable coefficient of thermal expansion (CTE) with chips or substrates, low weight, enhanced specific stiffness, and low cost. In this paper, SiC-particle-reinforced aluminum matrix composites are fabricated by the cost-effective squeeze-casting technology, and their microstructure characteristics, thermo-physical, and mechanical properties are investigated. The reinforcement volume fraction is as high as 70% and composites with linear CTE of 6.9–9.7×10^–6 °C^–1 and thermal conductivity of 120–170 W m^–1 °C^–1 are produced. The composites can be electric-discharge machined, ground, and electric-spark drilled. An electroless nickel layer is plated on the composite by the conventional procedures. Finally, their potential applications in electronic packaging and thermal management are illustrated via prototype examples.     

Thermophysical Properties of Ceramic Substrates with Modified Surfaces

Laser induced changes of thermophysical properties using a laser supported modification process have been studied. Metal–ceramic composites have been produced by a laser dispersing process. Two types of substrates have been included, namely, pure Al₂O₃ and Al₂O₃ reinforced with 10 mass% ZrO₂. As a modifying material during the laser process, hard metal powders like TiN and WC have been applied in order to produce a metal–ceramic composite with a metal concentration between 30 and 50%. Standard measurement techniques such as the laser-flash method and differential scanning calorimetry (DSC) have been used to measure the thermal diffusivity and the heat capacity of the ceramics before and after the laser processing. These properties have been evaluated within a temperature range from room temperature to 1400°C. The experimental results show that the effective thermal conductivity will be enhanced within the laser modified region. The increase of this heat transport property due to particle dispersion into the ceramic matrix depends on the thermal conductivity of the second-phase material. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Silicon carbide whisker copper-matrix composites fabricated by hot pressing copper coated whiskers

Copper-matrix SiC whisker composites containing 33–54 vol % SiC whiskers and with < 5 vol % porosity were fabricated by hot pressing SiC whiskers that had been coated with copper by electroless plating followed by electroplating. The highest Brinell hardness of 260 was attained at 50 vol % SiC whiskers. The lowest coefficient of thermal expansion (CTE) of 9.6 × 10^–6°C^–1 (at 25–150°C) was attained at 54 vol % SiC whiskers. The composites exhibited lower porosity, higher hardness, higher compressive yield strength, lower CTE, lower electrical resistivity and higher thermal conductivity than the corresponding composites made by hot pressing mixtures of copper powder and bare SiC whiskers.     

A comparative study of the coated filler method and the admixture method of powder metallurgy for making metal–matrix composites

Copper–matrix composites were made by powder metallurgy (PM). The reinforcements were molybdenum particles, silicon carbide whiskers and titanium diboride platelets. The coated filler method, which involves a reinforcement coated with the matrix metal, was used. In contrast, conventional PM uses the admixture method, which involves a mixture of matrix powder and reinforcement. For all the composite systems, the coated filler method was found to be superior to the admixture method in providing composites with lower porosity, greater hardness, higher compressive yield strength, lower coefficient of thermal expansion (CTE), higher thermal conductivity and lower electrical resistivity, though the degree of superiority was greater for high than low reinforcement contents. In the coated filler method, the coating on the reinforcement separated reinforcement units from one another and provided a cleaner interface and stronger bond between reinforcement and matrix than the admixture method could provide. The highest reinforcement content attained in dense composites (<5% porosity) made by the coated filler method was 70 vol% Mo, 60 vol% TiB2 and 54 vol% SiC. The critical reinforcement volume fraction above which the porosity of composites made by the admixture method increases abruptly is 60% Mo, 42% TiB2 and 33% SiC. This fraction increases with decreasing aspect ratio of the reinforcement. Among Cu/Mo, Cu/TiB2 and Cu/SiC at the same reinforcement volume fraction (50%), Cu/Mo gave the lowest CTE, highest thermal conductivity and lowest electrical resistivity, while Cu/SiC gave the greatest hardness and Cu/TiB2 and Cu/SiC gave the highest compressive yield strength. Compared to Cu/SiC, Cu/TiB2 exhibited much higher thermal conductivity and much lower electrical resistivity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites

A new type of hybrid SiC foam–SiC particles–Al composites (V_SiC = 53, 56.2 and 59.9%) to be used as an electronic packaging substrate material were fabricated by squeeze casting technique, and their thermal expansion behavior was evaluated. The coefficients of thermal expansion (CTEs) of the hybrid composites in the range of 20–100 °C were found to be between 6.6 and 7.7 ppm/°C. The measured CTEs are much lower than those of SiC particle-reinforced aluminum (SiC_p–Al) composites with the same content of SiC because of the characteristic interpenetrating structure of the hybrid composites. A material of such a low CTE is ideal for electronic packaging because of the low thermal mismatch (and therefore, low thermal stresses) between the electronic component and the substrate. To achieve similar CTEs in SiC_p–Al composites, the volume fraction of SiC would be much higher than that in the hybrid composites.     

Microstructure and mechanical properties of cast (Al–Si)/SiCp composites produced by liquid and semisolid double stirring process

Abstract

A stirring process containing two steps, i.e. liquid and then semisolid stirring, was used to produce SiC particle reinforced aluminium matrix composites. The major advantages of this process are that full wetting of SiC particles by molten aluminium can be readily achieved at relatively low stirring rates, and undesirable Al₄ C₃ is not formed at the Al/SiC interface due to lower processing temperatures. Cast Al–Si matrix composites reinforced with 15 and 20 vol.-%SiC particles were produced in the present work. The mechanical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained without employing semisolid stirring, the aggregation of SiC particles observed in the microstructure of composites resulted in quite poor mechanical properties. Observations and analyses indicated that some Al/SiC interfaces were very clean, and a reaction product of spinel MgAl₂O₄ was also found at some Al/SiC interfaces. Silicon dioxide (SiO₂ ) was found to exist on the surface of as purchased and 250°C dried SiC powders. This SiO₂ is involved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.     

Synthesis and characterization of borosilicate glass/β-spodumene/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites with low CTE value for LTCC applications

Glass?+?ceramic composites based on low-softening-point borosilicate (BS) glass, β-spodumene and Al₂O₃ were produced in this work. The influence of ceramic filler composition on the microstructure, sintering quality, mechanical properties, thermal properties and dielectric properties of composites were studied. XRD and DSC indicated that both kinds of ceramic filler as well as the BS glass maintained their characteristics after sintering. The addition of β-spodumene would decrease the coefficient of thermal expansion (CTE) value of composites to match with silicon well. The better wetting behavior between β-spodumene and BS glass would lead to better sintering quality, microstructure and dielectric properties for composites containing more β-spodumene. With appropriate Al₂O₃ content, the flexural strength of composites could be enhanced. Composite with 45 wt% BS glass, 30 wt% β-spodumene and 25 wt% Al₂O₃ sintered at 875 °C showed good properties which meet the requirements of low temperature co-fired ceramic applications: dense microstructure with high relative density of 96.27%, proper CTE value of 3.57 ppm/°C, high flexural strength of 156 MPa, low dielectric constant of 6.20 and low dielectric loss of 1.9?×?10^?3.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.