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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

A novel approach to prepare dense AlN ceramics with high thermal conductivity was proposed. The aluminum isopropoxide (AIP) was introduced to prepare dense AlN ceramics possessing enhanced thermal conductivity via in situ reaction. The effect of AIP and sintering temperatures on the microstructure and properties of AlN ceramics were investigated. Results indicate that AIP is beneficial for the densification of AlN ceramics, and particularly, when the addition of AIP reaches 1 wt%, the density and thermal conductivity could reach up to 3.29 g/cm3, 182 W/mK, respectively. Finally, the mechanism of the in situ reaction was proposed.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

Aluminum nitride is considered to be a promising substrate and package material for high power integrated circuits on account of its high thermal conductivity, low dielectric constant, thermal expansion coefficient close to that of silicon and high electrical resistivity. However, AlN is difficult to sinter due to its highly covalent bond. For full densification, rare earth and/or alkaline earth oxides are often added as sintering aids in the fabrication of AlN ceramics.[1-5] These sintering aids play a dual role during the sintering process. One role is to form the liquid phase that promotes the densification by the process of liquid phase sintering. The other is to improve the thermal conductivity by decreasing the oxygen impurities in the AlN lattice. Y2O3 is an effective additive to achieve dense AlN ceramics most likely due to the liquid-phase formation of yttrium aluminates at temperatures around 1800°C since the eutectic temperature of Y2O3 and Al2O3 is around this region.[6-11] Therefore, the importance of Al2O3 in the formation of the liquid phase is evident. Meanwhile, it is known that the property of the sintered AlN ceramic containing Al2O3 would deteriorate, especially the thermal conductivity. The majority of Al2O3 in the AlN powders is due to the oxidation of the surface AlN.

In the present work, our focus is to direct investigate the effect of Aluminum oxide on the microstructure and properties of AlN ceramics, and the Aluminum oxide was introduced in the form of aluminum isopropoxide (C9H21AlO3, abbreviated as AIP). It is expected that in the sintering process the oxidization of AIP could absorb and exclude the oxygen element in the lattice of AlN powders via the in situ reaction. And the AlN ceramics possessing high thermal conductivity would be achieved.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

High purity and fine powders of AlN (Grade F, Tokuyama Soda, Tokyo, Japan) and aluminum isopropoxide (Sinopharm Chemical Reagent, Shanghai, China) were employed as raw materials. Y2O3 (Sinopharm Chemical Reagent) was used as sintering aid. Three kinds of powders were weighted according to the formula and milled together for 24 h in ethanol. The compositions of the specimens are shown in Table 1. Then the powders were dried and pressed into disks under a pressure of 60 MPa using polyvinyl butyral (PVB) as a binder. After PVB was burnt out at 650°C in air, the pellets were sintered at 1790°C and 1810°C for 4 h, respectively, in a graphite furnace with a flowing nitrogen atmosphere.

Table 1. The Compositions of Different Samples in the Experiments
SampleAlN (wt%)Y2O3 (wt%)AIP (wt%)
Sample 1955.00
Sample 2955.00.5
Sample 3955.01.0
Sample 4955.02.0

The relative densities and the shrinkages of the AlN-sintered body were measured using the Archimedes method and vernier caliper. X-ray diffractometer (XRD) (D/MAX-2500; Rigaku, Tokyo, Japan) with a CuKα1 radiation (λ = 0.15406 nm) was utilized to identify the crystal structures. Grain morphology of the samples was examined using scanning electron microscopy (SEM; LEO-1530, Oberkochen, Germany). The thermal conductivity at room temperature was measured using a laser flash technique.

Results and Discussions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

Figure 1 shows the shrinkages and densities of the samples with different amounts of AIP and different sintering temperatures. Compared with the ones containing AIP, the samples just with Y2O3 showed lower shrinkage and density at 1790°C. Even when the sintering temperature increased to 1810°C, pure AlN–Y2O3 ceramics still showed a low planar shrinkage ratio and density. However, the planar shrinkage ratio and density increased obviously with the addition of AIP to a higher value first and then decreased as the AIP content increased up to 2 wt% for two sintering temperature points. It also could be apparently observed in Fig. 1 that sample 3 possessed the largest shrinkages for two sintering temperature points, which indicated that 1 wt% AIP benefits achieving a maximum of shrinkage. Meanwhile, it means that dense AlN ceramics could be sintered at 1790°C with the AIP addition. The tendency of the density with the amount of AIP for the sample sintered at two temperature points is same. However, we consider that there exist two kinds of sintering mechanisms, one is solid-state sintering and another is the liquid sintering. Due to the different sintering mechanisms, the density of the samples for 1810°C is higher than that for 1790°C. The peak value of density (3.29 g/cm3) was attained for sample 3 sintered at 1810°C. The different shrinkages of the samples were most likely attributed to the effects of AIP on the liquid-phases under the different sintering temperatures.[12-14]

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Figure 1. Planar shrinkage ratios and density of AlN-xAIP ceramics.

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Figure 2 shows the schematic of the process of preparation of AlN ceramics. In the ball milling process, a part of AIP could dissolve into alcohol and AIP could mix through with AlN and Y2O3 powders. After the alcohol was dried out, the AIP wrapped up the AlN powders and located on the surface of AlN.

image

Figure 2. The schematic of the process of preparation of AlN ceramics via in situ reaction.

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Figure 3 shows a typical X-ray diffraction pattern for the as-sintered samples at 1810°C. As expected, the major crystalline phase was identified as AlN. However, additional peaks were obtained, which could be consistently indexed as yttrium aluminum garnet Y3Al5O12 (YAG). The second phase YAG was identified as the product of the reaction of Al2O3 and Y2O3. With the AIP increasing, the relative intensity of YAG's peaks increased. This indicated that the amount of YAG increased with the content of AIP. In the sintering process, during oxidation AIP would absorb the oxygen in the lattice as well as in air to form aluminum oxide. However, the oxygen content of sintering surroundings was very low under N2 atmosphere. Thus, the fixation of oxygen from the lattice was quite obvious. Then, the aluminum oxide would react with Y2O3 at the eutectic temperature to form a liquid phase which would promote the densification of the ceramics. Hence, it is considered that the thermal conductivity of AlN would be enhanced by further exclusion of oxygen in the lattice of AlN and dense AlN ceramics would be obtained.

image

Figure 3. The XRD patterns of the samples sintered at 1810°C.

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Figure 4 shows the microstructure of the fracture surfaces of samples sintered at 1790°C and 1810°C. For AlN's highly covalent bond, it is difficult for AlN to achieve full density. However, in Fig. 4 seldom pores could be observed. At the same time, it is also observed that in sample 1 and 2 the grains have an inhomogeneous size distribution. However, they are more homogeneous in sample 3 and 4, and AlN crystallizes well, which implies that AIP improves the growth of the AlN grains. Hence, it could be considered that the uniform and dense microstructure of the AlN ceramics was caused by the appearance of liquid phase.

image

Figure 4. SEM micrographs of the fractures of the samples sintered at 1790°C and 1810°C.

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The compositional analysis using EDS in Fig. 5, further confirms that the secondary phase at grain boundary was indeed YAG. The origin of YAG secondary phase in the sintered AlN samples is postulated to be as follows. It is known that the oxygen is present in two forms. One is a thin layer of aluminum oxide on the AlN powder surface and the other is in the form of dissolved oxygen in AlN lattices. Under an appropriate temperature, Y2O3 reacts with the Al2O3 layer to form a low melting-point eutectic phase. This phase is assumed to be liquid at the sintering temperature, and promotes the sintering of AlN on account of the high mobility of atoms in liquid-phase. In our work, Al2O3 mainly stemmed from the oxidization of AIP. With temperature going up, oxygen in AlN lattices would diffuse toward the AlN surface under the gradient of concentration and would be absorbed by AIP on the surface of AlN powders. In conclusion, the existence of AIP not only promotes the dense sintering of AlN, but also plays an important role in excluding oxygen in the lattice.

image

Figure 5. The SEM and EDS of sample 2 sintered at 1810°C

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Figure 6 presents the thermal conductivity of samples with different contents of AIP under two sintering temperature points. The trends of thermal conductivity of samples sintered at 1790°C and 1810°C, are pretty similar, first increase and then decrease. The maximum value 182 W/mK was obtained in sample 3 for 1810°C. This performance is better than that of pure AlN (Y2O3) ceramics.[12] The sintering temperature and dwell time were reduced compared with the conventional sintering of AlN (Y2O3) ceramics, which is meaningful for large-scale industry production of AlN ceramics. The increase in the thermal conductivity could be ascribed to the densification of AlN. And the excessive amount of secondary phase may be the cause for the decrement of thermal conductivity. The presence of the secondary phases along the AlN grain boundaries disrupts the connections between high thermal conductivity AlN grains. This is due to the fact that the thermal conductivity of the secondary phases is low, that is, YAG only 11 W/mK.[15]

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Figure 6. Thermal conductivity of samples with a different content of AIP.

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In summary, a novel approach has been proposed to prepare dense AlN ceramics possessing enhanced properties.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

Dense AlN ceramics was fabricated via in situ reaction. The AIP was introduced to further exclude the O in the lattice of AlN. All the sintered samples contained YAG as a second phase, which was believed to have been formed by reaction of sintering additive Y2O3 with the aluminum oxide mainly produced by the oxidization of AIP. As the content of AIP increased, the distribution of grains became more uniform. At the same time, the densities and thermal conductivities improved with the amount of AIP and (or) sintering temperature increasing. The peak values 182 W/mK were achieved in sample 3 under 1810°C.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References

This research grant is provided by Korea Association of Small Business Innovation Research.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussions
  6. Conclusions
  7. Acknowledgment
  8. References
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