Effect of particle size on the microstructure and thermal conductivity of Al/diamond composites prepared by spark plasma sintering

Spark plasma sintering (SPS) was used to fabricate Al/diamond composites. The influence of diamond particle size on the microstructure and thermal conductivity (TC) of composites was investigated by combining experimental results with model prediction. The results show that both composites with 40 μm particles and 70 μm particles exhibit high density and good TC, and the composite with 70 μm particles indicates an excellent TC of 325 W·m⁻¹·K⁻¹. Their TCs lay between the theoretical estimated bounds. In contrast, the composite with 100 μm particles demonstrates low density as well as poor TC due to its high porosity and weak interfacial bonding. Its TC is even considerably less than the lower bound of the predicted value. Using larger diamond particles can further enhance thermal conductive performance only based on the premise that highly dense composites of strong interfacial bonding can be obtained.     

Microstructure and thermal expansion of Ti coated diamond/A1 composites

A titanium coating fabricated via vacuum vapor deposition for diamond/Al composites was used to improve the interfacial bonding strength between diamond particles and Al matrix, and the Ti coated diamond particles reinforced Al matrix composites were prepared by gas pressure infiltration for electronic packaging. The surface structure of the Ti coated diamond particles was investigated by XRD and SEM. The interfacial characteristics and fracture surfaces were observed by SEM and EDS. The coefficient of thermal expansion(CTE) of 50% (volume fraction) Ti coated diamond particles reinforced Al matrix composites was measured. The Ti coating on diamond before infiltration consists of inner TiC layer and outer TiO₂ layer, and the inner TiC layer is very stable and cannot be removed during infiltration process. Fractographs of the composites illustrate that aluminum matrix fracture is the dominant fracture mechanism, and the stepped breakage of a diamond particle indicates strong interfacial bonding between the Ti coated diamond particles and the Al matrix. The measured low CTEs (5.07×10⁻⁶−9.27×10⁻⁶K⁻¹) of the composites also show the strong interfacial bonding between the Ti coated diamond particles and the Al matrix.     

Pressure infiltrated Cu/diamond composites for LED applications

Diamond reinforced copper (Cu/diamond) composites were prepared by a pressure infiltration technique.The composites show a super high conductivity of 713 W m-1 K-1 in combination with an extremely low coefficient of thermal expansion (CTE) of 7.72 × 10-6 K-1 (25-100 ℃),which are achieved by modifying the copper matrix with adding 0.3 wt.% of boron to get a good thermal contact between the matrix and the diamond particles.By adopting a series of postmachining techniques the composites were made into near-net-shape parts,and an electroless silver coating was also successfully plated on the composites.Finally,their potential applications in the thermal management of light emitting diodes (LED) were illustrated via prototype examples.     

高分数GNFs/6061Al基复合材料的放电等离子体烧结制备及其显微组织与性能

采用放电等离子体烧结(SPS)工艺在610℃制备30％～50％(质量分数)纳米石墨片(GNF)/6061Al基复合材料,研究烧结压力及GNF含量对复合材料显微组织和力学、热学性能的影响.结果表明,SPS有效抑制GNFs/6061Al基复合材料中Al4C3等界面反应产物的生成.随着GNF含量的增加,GNFs团聚程度增加,...     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

Predicted interfacial thermal conductance and thermal conductivity of diamond/Al composites with various interfacial coatings

The interfacial thermal conductance (ITC) and thermal conductivity (TC) of diamond/Al composites with various coatings were theoretically studied and discussed. A series of predictions and numerical analyses were performed to investigate the effect of thickness, sound velocity, and other parameters of coating layers on the ITC and TC. It is found that both the ITC and TC decline with increasing coating thickness, especially for the coatings with relatively low thermal conductivity. Nevertheless, if the coating thickness is close to zero, or quite a small value, the ITC and TC are mainly determined by the constants of the coating material. Under this condition, coatings such as Ni, TiC, Mo 2 C, SiC, and Si can significantly improve the ITC and TC of diamond/Al composites. By contrast, coatings like Ag will exert the negative effect. Taking the optimization of interfacial bonding into account, conductive carbides such as TiC or Mo 2 C with low thickness can be the most suitable coatings for diamond/Al composites.     

镀TiC金刚石/铝复合材料的界面及热膨胀性能

采用气压浸渗法制备高体积分数的镀TiC金刚石/铝复合材料,通过SEM和EDS等手段对复合材料的断口形貌进行分析,并研究TiC镀层对复合材料界面和热膨胀性能的影响。结果表明:TiC镀层改善金刚石颗粒与铝合金基体之间的选择性粘结现象,断裂方式以基体断裂为主。部分TiC会被氧化成TiO2并与铝合金基体反应生成Al2O3,从而实现金刚石颗粒与铝合金基体之间良好的界面结合;TiC镀层有效地降低复合材料的热膨胀系数(CTE),增强复合材料热膨胀性能的稳定性。在体积分数相同的情况下,CTE随金刚石颗粒尺寸的减小而减小。     

Enhanced mechanical properties in Al/diamond composites by Si addition

Diamond particles reinforced aluminum–silicon matrix composites,abbreviated as Al(Si)/diamond composites,were fabricated by squeeze casting.The effect of Si content on the microstructure and mechanical properties of the composites were investigated.The mechanical properties are found to increase monotonically with Si content increasing up to 7.0 wt%.The Al-7.0 wt% Si/diamond composite exhibits tensile strength of 78 MPa,bending strength of 230 MPa,and compressive strength of426 MPa.Al–Si eutectic phases are shown to connect with Al matrix and diamond particles tightly,which is responsible for the enhancement of mechanical properties in the Al(Si)/diamond composites.     

Near-net shaped Al/SiCP composites via vacuum-pressure infiltration combined with gelcasting

To solve the problem of difficult machining, the near-net shaped Al/SiC_P composites with high volume fraction of SiC particles were fabricated by vacuum-pressure infiltration. The SiC_P preform with a complex shape was prepared by gelcasting. Pure Al, Al4Mg, and Al4Mg2Si were used as the matrices, respectively. The results indicate that the optimal parameters of SiC_P suspension in gelcasting process are pH value of 10, TMAH content of 0.5 wt.%, and solid loading of 52 vol.%. The Al matrix alloyed with Mg contributes to improving the interfacial wettability of the matrix and SiC particles, which increases the relative density of the composite. The Al matrix alloyed with Si is beneficial to inhibiting the formation of the detrimental Al₄C₃ phases. The Al4Mg2Si/SiC_P composite exhibits high relative density of 99.2%, good thermal conductivity of 150 W·m⁻¹·K⁻¹, low coefficient of thermal expansion of 10.1×10⁻⁶ K⁻¹, and excellent bending strength of 489 MPa.     

Interface and thermal expansion of carbon fiber reinforced aluminum matrix composites

Two kinds of unidirectional PAN M40 carbon fiber(55%,volume fraction) reinforced 6061Al and 5A06Al composites were fabricated by the squeeze-casting technology and their interface structure and thermal expansion properties were investigated.Results showed that the combination between aluminum alloy and fibers was well in two composites and interface reaction in M40/5A06Al composite was weaker than that in M40/6061Al composite.Coefficients of thermal expansion(CTE) of M40/Al composites varied approximately from(1.45-2.68)×10-6 K-1 to(0.35-1.44)×10-6 K-1 between 20 °C and 450 °C,and decreased slowly with the increase of temperature.In addition,the CTE of M40/6061Al composite was lower than that of M40/5A06Al composite.It was observed that fibers were protruded significantly from the matrix after thermal expansion,which demonstrated the existence of interface sliding between fiber and matrix during the thermal expansion.It was believed that weak interfacial reaction resulted in a higher CTE.It was found that the experimental CTEs were closer to the predicted values by Schapery model.     

放电等离子烧结法和热压法制备的镀钛金刚石/铜多层复合材料的结构和热性能

采用放电等离子烧结技术(SPS)和热压法(HP)分别制备用于电子封装领域的多层镀钛金刚石/铜复合材料获得。借助扫描电子显微镜(SEM)分析了复合材料的显微组织,同时对热导率(TC)和热膨胀系数(CTE)等热性能参数进行了分析。层状复合材料的热导率理论值参考改良的哈塞尔曼-约翰逊(HJ)模型,同时考虑TiC界面的影响计算,结果为446.66 W·(m·K)~(-1),而热膨胀系数则通过热膨胀仪测试确定。结果显示,经放电等离子烧结的试样与经热压制备的试样相比,缺陷相对较少,界面的结合对于复合材料热导率的影响十分明显。提出了一个界面影响的模型示意图,热导率随着碳化物层厚度的增加和气孔的出现而减小。由此可见,实现高热导率的条件是复合材料中的碳化物层较薄、同时没有气孔的出现。     

Reinforcement architectures and thermal fatigue in diamond particle-reinforced aluminum

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK^?1) and low coefficient thermal expansion (CTE ≈ 1 ppm K^?1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond–Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure.     

Microstructure and Oxidation Behavior of Al and Al/NiCrAlY Coatings on Pure Titanium Alloy

To improve the oxidation resistance of Ti alloys, a NiCrAlY coating was deposited as diffusion barrier between aluminum overlay coating and pure Ti substrate by air plasma spraying method. The microstructure and oxidation behavior of Al coatings with and without NiCrAlY diffusion barrier were investigated in isothermal oxidation tests at 800 °C for 100 h. The results indicate that the weight gain of the Al/NiCrAlY coating was 4.16 × 10^?5 mg² cm^?4 s^?1, whereas that of the single Al coating was 9.52 × 10^?5 mg² cm^?4 s^?1 after 100 h oxidation. As compared with single Al coating, the Al/NiCrAlY coating revealed lower oxidation rate and excellent oxidation resistance by forming thin Al₂O₃ + NiO scales at overlaying coating/diffusion barrier and diffusion barrier/substrate interfaces. Meanwhile, the inward diffusion of Al and the outward diffusion of Ti were inhibited effectively by the NiCrAlY diffusion barrier.     

镀TiC金刚石/铝复合材料的热膨胀性能

采用气压浸渗法制备了金刚石体积分数为65%的铝基复合材料,分析了复合材料的显微组织并对热膨胀系数(CTE)进行了测试,研究了镀TiC金刚石/铝复合材料的热膨胀性能。结果表明,金刚石颗粒在铝合金基体中分布均匀,组织致密;TiC镀层有效地改善了金刚石颗粒与铝合金基体间选择性粘结现象,增强了金刚石与基体间的界面结合;镀TiC使复合材料热膨胀系数明显降低,Turner模型和Kerner模型的均值可以预测其热膨胀系数,而对于未镀层的复合材料则可以用Kerner模型进行预测。     

金刚石/碳化硅/铝复合材料的热膨胀性能

采用气压浸渗法制备金刚石/碳化硅/铝复合材料,研究复合材料的断口形貌以及界面反应,测试复合材料的热膨胀性能。结果表明:金刚石表面Ti镀层使得其选择性粘附不同于未镀钛金刚石的,而在各个面上均粘附有Al,金刚石与基体间有着良好的界面结合,断裂方式以基体断裂为主,其界面反应后,Ti以Al3Ti和Ti-Al-Si等金属间化合物的形式析出,提高金刚石/铝界面的结合强度,降低复合材料的热膨胀系数;随着金刚石颗粒粒径的增大,金刚石和碳化硅颗粒间粒径比的增大增加了整个复合材料的体积分数,从而降低了其热膨胀系数;金刚石颗粒粒径增大导致热膨胀系数升高。这两方面共同影响复合材料的热膨胀系数,但前者起主导作用;金刚石和碳化硅在不同配比下的热膨胀系数随着复合材料中碳化硅含量的增加逐渐增大,Terner模型与Kerner模型的计算平均值能较好地预测实验结果。     

热处理对金刚石/2024Al复合材料微观组织和热物理性能的影响

采用挤压铸造法制备粒径为5μm、体积分数为50%的金刚石/2024Al 复合材料。退火处理后对其金相组织界面反应、界面结合情况以及金刚石颗粒的内部缺陷进行观察与分析,并对其热物理性能进行测试与研究。结果表明,金刚石/2024Al 复合材料的组织致密,无明显的气孔、夹杂等缺陷;颗粒为不规则多边形,有棱角,分布比较均匀。透射电镜观察表明,部分金刚石颗粒内部有位错和层错存在,而2024Al 基体中的位错密度较大,金刚石/2024Al界面处有较多的界面反应物生成,可能为Al2Cu。复合材料在20~100°C温度区间内的平均热膨胀系数为8.5×10-6°C-1,退火处理的复合材料其热膨胀系数有一定程度的降低;随着温度的升高,复合材料的平均热膨胀系数也呈现增加的趋势。复合材料的热导率约为100 W/（m·K）,退火处理能够提高复合材料的热导率。     

Improved Thermal Performance of Diamond-Copper Composites with Boron Carbide Coating

B₄C-coated diamond (diamond@B₄C) particles are used to improve the interfacial bonding and thermal properties of diamond/Cu composites. Scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were applied to characterize the formed B₄C coating on diamond particles. It is found that the B₄C coating strongly improves the interfacial bonding between the Cu matrix and diamond particles. The resulting diamond@B₄C/Cu composites show high thermal conductivity of 665 W/mK and low coefficient of thermal expansion of 7.5 × 10^?6/K at 60% diamond volume fraction, which are significantly superior to those of the composites with uncoated diamond particles. The experimental thermal conductivity is also theoretically analyzed to account for the thermal resistance at the diamond@B₄C-Cu interface boundary.     

Thermal conductivity behavior of SPS consolidated AlN/Al composites for thermal management applications

AlN/Al composites are a potentially new kind of thermal management material for electronic packaging and heat sink applications.The spark plasma sintering (SPS) technique was used for the first time to prepare the AlN/Al composites,and attention was focused on the effects of sintering parameters on the relative density,microstructure and,in particular,thermal conductivity behavior of the composites.The results showed that the relative density and thermal conductivity of the composites increased with increasing sintering temperature and pressure.The composites sintered at 1550 ℃ for 5 min under 70 MPa showed the maximum relative density and thermal conductivity,corresponding to 99% and 97.5 W m-1 K-1,respectively.However,the thermal conductivity of present AlN/Al composites is still far below the theoretical value.Possible reasons for this deviation were discussed.     

Microstructure and thermal conductivity of copper matrix composites reinforced with mixtures of diamond and SiC particles

The thermal conductivity of diamond hybrid SiC/Cu,diamond/Cu and SiC/Cu composite were calculated by using the extended differential effective medium (DEM) theoretical model in this paper.The effects of the particle volume fraction,the particle size and the volume ratio of the diamond particles to the total particles on the thermal conductivity of the composite were studied.The DEM theoretical calculation results show that,for the diamond hybrid SiC/Cu composite,when the particle volume fraction is above 46% and the volume ratio of the diamond particles to the SiC particles is above 13:12,the thermal conductivity of the composite can reach 500 W·m-1·K-1.The thermal conduc-tivity of the composite has little change when the particle size is above 200μm.The experimental results show that Ti can improve the wettability of the SiC and Cu.The thermal conductivity of the diamond hybrid SiCTi/Cu is almost two times better than that of the diamond hybrid SiC/Cu.It is feasible to predict the thermal conductivity of the composite by DEM theoretical model.     

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m^?1 K^?1 for EB-PVD YSZ coatings to about 0.7 W m^?1 K^?1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni₃Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.