Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers
Introduction
Low temperature co-fired ceramic (LTCC) has been extensively used to make substrates for high density integrated devices packaging. The low processing temperature (<1000 °C) and multilayered structure allow to integrate passive components and circuits in the LTCC substrate.1, 2 However, higher speed, greater power and smaller size of contemporary integrated devices will produce more concentrated heat in the devices, which makes thermal management a great challenge in LTCC-based electronic packages.3 LTCC materials usually consist of glass and ceramic filler. Due to the low thermal conductivity of glass phase, the thermal conductivity of commercial LTCC materials is only about 2–4 W/m K.4 Consequently, there is an urgent need on high thermal conductivity LTCC materials to meet the demand of high performance electronic packaging.
One approach to improve the thermal conductivity of LTCC materials is to add high thermal conductive ceramic fillers into the glass matrix. AlN and Si3N4 are good candidate for high performance electronic devices packaging due to their excellent thermal, mechanical, and electrical properties.5, 6 Efforts have been made to prepare low temperature sintered high thermal conductivity ceramics by sintering AlN particles with glasses. Zhang et al. obtained a thermal conductivity of 10 W/m K by preparing glass–AlN composites using hot-press sintering at 850–1000 °C.7 Lee et al. reported a high thermal conductivity up to 11 W/m K by mixing AlN particles with microcrystal glass and sintering at 800 °C in vacuum.8 In these studies, the main approach for increasing thermal conductivity was only to optimize the proportion of AlN and glass, or the composition of the glass matrix. The improvement in the thermal conductivity of the glass–AlN ceramic composite is very limited, because the high thermal conductivity fillers are insulated by the glass phase and they cannot effectively contribute to the thermal conductivity of composites.
The formation of thermal conducting network in a composite may have a strong influence on the thermal conductivity.9, 10 One-dimensional materials with high thermal conductivity, such as whiskers and carbon nanotubes, could be the ideal candidates to construct thermal conducting paths on account of their large aspect ratio11. For example, Shi et al. increased the thermal conductivity of the polymer-based composite by 2.3 times through filling 47 vol% of the synthesized brushlike AlN nanowhiskers.12 Li et al. reported that the addition of 20 vol% β-SiC whiskers increases the thermal conductivity of vinylidene fluoride (PVDF)/barium titanate nanocomposites over 60%.13 Jakubinek et al. prepared single-walled carbon nanotube (SWCNT)/polystyrene composites and the thermal conductivity was increased by 50% for 1 mass% SWCNT loading.14 Although these studies were based on polymer composites, they have demonstrated the feasibility of constructing thermal conducting paths using one-dimensional materials.
In this work, we propose a strategy to build thermal conducting network in the borosilicate glass–AlN ceramic composites by connecting AlN particles using one-dimensional β-Si3N4 whiskers. A high thermal conductivity of 18.8 W/m K is obtained by loading 14 vol% β-Si3N4 whiskers. The results obtained confirmed the feasibility of this approach. In addition, the dielectric and mechanical properties of the ceramic composites are also investigated.
Section snippets
Experimental
The borosilicate glass used in this work is CaO–MgO–B2O3–SiO2 (CMBS) glass system. The composition of the CMBS glass is 25 wt% CaO, 10 wt% MgO, 30 wt% B2O3 and 35 wt% SiO2. The starting materials including CaCO3 (99.6%), MgO (99.7%), H3BO3 (99.5%), and SiO2 (99.0%) were mixed according to the given proportion and then melted at 1400 °C, holding at melting temperature for 2 h. The molten glass was quenched into de-ionized water and then ball-milled to 1 μm in mean particle size. High purity AlN powder
CMBS glass
Suitable glass matrix for glass–AlN composite is fairly important because it determines the densification behavior and final physical properties of the composite.15 The CMBS glass used in this work can obtain various crystallized phases by designing the glass composition and controlling heat treatment process.16, 17 Furthermore, CaO and MgO are also being considered as effective sintering aids to AlN ceramics,18 implying the good compatibility between the CMBS glass and AlN. Fig. 1 shows the
Conclusions
A strategy for fabricating high thermal conductivity low-temperature sintered ceramics is introduced. The enhancement of the thermal conductivity can be attributed to the formation of thermal percolation network in the matrix as a result of the loading of β-Si3N4 whiskers. Moreover, the crystalline phases in borosilicate glass can also promote the phonon conduction. The high thermal conductivity, low dielectric constant and loss, the enhanced mechanical properties make the low-temperature
Acknowledgements
The authors would like to thank Prof. Lidong Chen and Dr. Huili Liu at Energy Materials Research Center of Shanghai Institute of Ceramics, for the help in fabricating samples using SPS and measurement of the thermal diffusivity. This work was supported by The Science and Technology Commission of Shanghai Municipality (Nos. 10dz1140300, 10XD1404700) and the Innovation Fund of Shanghai Institute of Ceramics.
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