Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

doi:10.1016/S0272-8842(03)00033-6

Ceramics International

Volume 29, Issue 8, 2003, Pages 893-896

https://doi.org/10.1016/S0272-8842(03)00033-6 Get rights and content

Abstract

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF₂ and YF₃ as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂. Liquid CaYAlO₄ mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F₂ at a relatively low temperature results in homogeneous YF₃ distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity.

Introduction

Aluminum nitride is a promising substrate and package material for high power integrated circuits because of its high thermal conductivity, low dielectric constant and thermal expansion coefficient close to that of silicon and high electrical resistivity [1], [2]. However, AlN is difficult to sinter due to its high covalent bonding. For full densification, rare-earth and/or alkaline earth oxides are often added as sintering aids in the fabrication of AlN ceramics [3], [4]. These sintering aids play a double role during the sintering. One is to form a liquid phase that promotes densification by liquid-phase sintering. The other is to improve the thermal conductivity by reducing the oxygen impurities in the AlN lattice. Hundere et al. [5] studied the chemical reactions when sintering AlN ceramics with YF₃ additions. Liu et al. [6] achieved AlN ceramics with a thermal conductivity above 170 W/m·K using CaF₂ and YF₃ as additives. However, the effect of YF₃ on the thermal conductivity was not clear from their study. In this paper, CaF₂, YF₃ and Y₂O₃ were used as sintering aids to explore the effect of YF₃ on the thermal conductivity of AlN ceramics.

Section snippets

Experimental procedure

The commercial AlN powder (H type, Tokuyama, Japan) used in this study has an oxygen impurity content of 0.9 wt.% and a specific surface area of 2.7 m²g⁻¹. AlN powder was mixed with CaF₂, YF₃ and Y₂O₃ (analytical reagents) and ball-milled by planetary milling for 2 h using ethanol as a mixing medium. The dried powder was mixed with polyvinyl butyral (PVB) binder before uniaxially pressing the powder into pellets 10 mm in diameter and 5 mm thick. The pellets were de-waxed at 550 °C and then

Results

Table 2 shows that the secondary phases in samples YFA and CYFA after sintering at different temperatures for 0.5 h. The chemical composition of the secondary phases in YFA samples changes in the order YOF → Y₃Al₅O₁₂ (YAG) → Y₄Al₂O₉ (YAM) → YN when the firing temperature increases from 1350 to 1650 °C. This is similar to the results achieved by Hundere et al. [5] However, it seems impossible that Y₂O₃ can be formed as an intermediate product from the reaction between YF₃ and Al₂O₃ as Hundere et

Discussion

In general, densification, grain boundary phases and oxygen content in the AlN lattice play an important role on the thermal conductivity of AlN ceramics. Since both sample CYFA and sample CYOA reach the density at 1750 °C, densification is not the main reason for the different thermal conductivities of the two samples.

Kurokawa et al. [9] published the relationship between thermal conductivity and oxygen concentration for a series of AlN ceramic samples, which were prepared with a sintering

Conclusions

Using CaF₂ and YF₃, dense AlN ceramics was achieved with a thermal conductivity of 180 W/m·K. The occurrence of liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂ is the main reason for the densification of AlN ceramics before 1650 °C. While, the densification is promoted by the action of liquid CaYAlO₄ at 1750 °C. Compared with the sample added with CaF₂ and Y₂O₃, grain boundary phases have no prominent effect on the thermal conductivity of AlN ceramics with CaF₂ and YF₃ addition. Oxygen concentration in the

Acknowledgements

This work was supported by Guangdong Research and Development Center for New Functional Ceramic Materials.

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Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Conclusions

Acknowledgements

J. Phys. Chem. Solids

J Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

Aluminum nitridea versatile but challenging material

Am. Ceram. Bull.

Role of Y2O3 and SiO2 additions in sintering of AlN

J. Am. Ceram. Soc.

Effect of CaCO3 addition on the sintering of AlN

J. Mater. Sci. Lett.

Thermal conductivity of AlN ceramics sintered with CaF₂ and YF₃

Role of Y₂O₃ and SiO₂ additions in sintering of AlN

Effect of CaCO₃ addition on the sintering of AlN