Infoscience

Thesis

High-temperature wettability and thermal conductivity of aluminium nitride reinforced metal matrix composites

Aluminum nitride (AlN) has been applied to various applications, including electronic substrates and packaging, semiconductor production components, wide band gap semiconductor, light-emitting diodes (LED) and acoustic wave devices; at the same time, it is also attractive as a reinforcement in Metal Matrix Composites (MMCs) for thermal management applications with tailored thermal properties. In this thesis, two main subjects are investigated in terms of both the composite processing and its thermal property, namely (i) the high-temperature wettability between AlN and liquid metals and (ii) the thermal conductivity behavior of AlN reinforced metal matrix composites and the associated question of the interface thermal conductance at the AlN/metal interface. High-temperature wettability of AlN particle preforms with pure Al, Cu, Sn and Pb, was investigated by means of drainage curve measurements during pressure infiltration. The measured drainage curves show good agreement with the semi-empirical model in soil science proposed by Brooks and Corey. The total work of immersion, Wi, can be directly calculated by integration of the drainage curves, in turn giving the apparent contact angle during infiltration at high temperature. For Cu, Sn and Pb, the apparent contact angles deduced by integration of experimental drainage curves are in the range 120-140° and agree well with corresponding sessile drop data from the literature. Data for AlN and α-Al2O3 are furthermore similar for both methods of measurement, which results from the presence of a native oxide layer on the AlN particles, which is not eliminated from the particle surface in these systems. For the wettability between AlN/Pb and α-Al2O3/Pb systems, a slight difference is observed and this behavior is attributed to the transformation of the native oxide layer with temperature and the presence of dissolved oxygen in the melt. On the contrary, the wettability of AlN/Al system gives higher value than the sessile drop data measured in vacuum, corresponding rather to the value measured under He-H2 gas at 1373K. This indicates that in the present study there is no elimination of the native oxide layer on the AlN by the formation of Al2O gas, which normally occurs in high vacuum. Moreover, the results of the present technique gradually deviate to the sessile drop data as the initial contact angle gets close or below 90° especially in the α-Al2O3/Al system, suggesting that there is a minimum value, around 105-110°, below which the present infiltration-based contact-angle measurement technique may lose validity. Thermal conductivity behavior of AlN reinforced metal matrix composites produced by pressure infiltration technique has been characterized at both room and elevated temperature in two different microstructural systems, namely (i) particle reinforced system (PRCs) and (ii) partially sintered particle system, i.e., an interpenetrating composite in which the AlN is also a continuous phase (IPCs). The phase contrast of thermal conductivity between AlN (Κd) and matrix metals (Κm) is varied within a range of 0.4<Κd/Κm<4.5 by changing matrix metals (i.e. Cu, Al, Sn and Pb). For the PRCs, the interface thermal resistance manifests itself by a decrease in the thermal conductivity of composites as the particle size decreases. For the case that the matrix is less conducting than the AlN inclusion, a critical particle diameter can be defined below which the thermal conductivity of composites falls below the matrix thermal conductivity. This diameter is around 10 µm for both the AlN/Sn and AlN/Pb PRCs. At high phase contrast, particularly Κd/Κm>4, the differential effective medium (DEM) scheme still yields consistent results for the thermal conductivity of composites, whereas the mean field scheme deviates as the phase contrast increases. The effect of partially sintering of AlN particles for the IPCs is evident to increase the thermal conductivity of composites with fine particle size, where the PRCs have a difficulty to increase their thermal conductivity due to the interface thermal resistance. With increasing the interconnection between the particles, there is a transition from the thermal conductivity of PRCs to that of the inverted phase arrangement, i.e. where the highly conducting AlN phase acts as a matrix. Interface thermal conductance at the AlN/metal interface has been evaluated at R.T. by means of the size effect of reinforcement on the thermal conductivity of composites and by the direct TTR (Transient Thermal Reflectance) method on idealized AlN/metal interfaces. Data show the increase of interface thermal conductance in the series of Pb<Sn<Cu<Al as the Debye temperature ratio between metal and AlN increases. The results are in good agreement with data from the literature both measured by the TTR method and in composites. It is further confirmed by measurements on the present composites that the interface between solids with large difference in Debye temperature, in the present case between AlN and Pb, has higher interface thermal conductance than that would be expected based on theory, namely the Acoustic-Mismatch-Model (AMM) and Diffuse-Mismatch-Model (DMM). This behavior is consistent to the literature data with highly dissimilar interfaces supposedly due to inelastic phonon effect. However, a quantitative theoretical explanation for this observation is still required.

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