简介概要

Sintering behavior of aluminum nitride powder prepared by self-propagating high-temperature synthesis method

来源期刊：Rare Metals2018年第12期

论文作者：Liang Qiao Shu-Wen Chen Li-Qiang Jiang Kazuo Shinozaki Sheng-Lei Che

文章页码：1091 - 1095

摘要：Fully dense aluminum nitride（AIN） ceramics were synthesized by self-propagating high-temperature synthesis（SHS） method using AIN powder as raw material with Y₂ O₃ additive. The sintering behavior was studied at different sintering temperatures and additive contents. The change of phase compositions, secondary phase distributions and grain morphologies during sintering process were investigated. It is shown that fully dense ceramics using AIN powder prepared by SHS method can be obtained when the sintering temperature is above 1830 ℃. Both Y₂ O₃ content and sintering temperature have an important influence on the formation of Y-Al-O phase and grain shape. When Y₂ O₃ content is identified, the grain morphology converts from polyhedron into sphere-like shape with the rise of sintering temperature. At a certain sintering temperature,the grain size decreases with the increase in Y₂ O₃ content. The influencing mechanisms of different YAl-O secondary phases and sintering temperatures on the grain size and morphology were also discussed based on the experimental results.

稀有金属(英文版) 2018,37(12),1091-1095

Sintering behavior of aluminum nitride powder prepared by self-propagating high-temperature synthesis method

Liang Qiao Shu-Wen Chen Li-Qiang Jiang Kazuo Shinozaki Sheng-Lei Che

Research Center of Magnetic and Electronic Materials, College of Materials Science and Engineering, Zhejiang University of Technology

Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology

作者简介：*Sheng-Lei Che,e-mail: cheshenglei@zjut.edu.cn;

收稿日期：2 August 2014

基金：financially supported by the International Cooperation Project of Zhejiang Province (No. 2012C24007);

Sintering behavior of aluminum nitride powder prepared by self-propagating high-temperature synthesis method

Liang Qiao Shu-Wen Chen Li-Qiang Jiang Kazuo Shinozaki Sheng-Lei Che

Research Center of Magnetic and Electronic Materials, College of Materials Science and Engineering, Zhejiang University of Technology

Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology

Abstract：

Fully dense aluminum nitride(AIN) ceramics were synthesized by self-propagating high-temperature synthesis(SHS) method using AIN powder as raw material with Y₂ O₃ additive. The sintering behavior was studied at different sintering temperatures and additive contents. The change of phase compositions, secondary phase distributions and grain morphologies during sintering process were investigated. It is shown that fully dense ceramics using AIN powder prepared by SHS method can be obtained when the sintering temperature is above 1830 ℃. Both Y₂ O₃ content and sintering temperature have an important influence on the formation of Y-Al-O phase and grain shape. When Y₂ O₃ content is identified, the grain morphology converts from polyhedron into sphere-like shape with the rise of sintering temperature. At a certain sintering temperature,the grain size decreases with the increase in Y₂ O₃ content. The influencing mechanisms of different YAl-O secondary phases and sintering temperatures on the grain size and morphology were also discussed based on the experimental results.

Keyword：

Self-propagating high-temperature synthesis; AlN; Secondary phase; Liquid-phase sintering;

Received： 2 August 2014

1 Introduction

Owing to its high thermal conductivity,low dielectric loss,excellent electrical insulation and thermal expansion coefficient close to those of silicon,AlN ceramics are considered to be highly suitable as substrates and package materials for high-power integrated circuits and lightemitting diode (LED),structural parts for semiconductor processes and fillers for high thermal conductive resin [ 1, 2, 3] .

However,AIN is hard to achieve fully dense body as it belongs to the covalent compounds with small self-diffusion coefficient.Usually,rare-earth and/or alkaline earth oxides are added as sintering additives which will form the liquid phases to promote densification at lower temperatures [ 4, 5, 6] .Y₂O₃,as one of the most widely used additives [ 4, 7, 8, 9, 10] ,can produce the Y-Al-O eutectic liquid with Al₂O₃,which on the one hand facilitates densification process and on the other hand,eliminates the oxygen defects and purifies AlN lattice [ 11, 12] .Since Y₂O₃ forms eutectic liquid of Y₃Al₅O₁₂ with Al₂O₃ at the temperature of 1760℃,the liquid-phase sintering temperature using Y₂O₃ as the single sintering additive must be above1800℃ [ 13, 14] .

So far,the sintering study of commercial aluminum nitride powders prepared by the direct nitridation of aluminum or carbothermal reduction of alumina method has been commonly known.However,studies on the sintering characteristics of AlN powder synthesized by self-propagating high-temperature synthesis (SHS) are few as it was rarely produced on a large scale [ 15, 16] .In general,for their comparatively large particle size and mostly irregular particle shape,powders synthesized by SHS method are difficult to reach full densification in comparison with commercial aluminum nitride powders derived from carbo thermal reduction method.In this work,fully dense AlN ceramics were obtained by liquid sintering using AlN powder prepared by SHS method as starting material,and the study of sintering behavior was reported.

2 Experimental

Aluminum nitride powders (the average diameter of 50nanoparticles,D₅₀=3.01μm) synthesized by SHS method were used as starting material,and the morphology is shown in Fig.1.Commercial yttrium oxide was added as sintering additive with the amount of 3.0 wt%,5.0 wt%,7.5 wt%and 10.0 wt%,respectively.The mixtures were ball-milled by planetary milling for 2 h using ethanol as the mixing medium.After dried in vacuum oven,the mixed powders were granulated with polyvinyl butyral (PVB) and then pressed intoΦ23.0 mm×2.5 mm pellets.The green bodies were sintered in the graphite heater furnace at1800-1890℃for 2 h in a flowing nitrogen atmosphere after de-waxed at 600℃.

The shrinkages and the densities of the sintered pellets were measured by Vernier caliper and Archimedes displacement method,respectively.The phase compositions were identified by X-ray diffraction (XRD,PANalytical X'Pert PRO) with Cu Kαradiation.The fractural surfaces of sintered pellets were detected by scanning electron microscopy (SEM,Hitachi SU1510).

3 Results and discussion

3.1 Density and shrinkage

Figure 2 shows the relationship between the densities and sintering temperatures of AlN ceramics with different amounts of Y₂O₃ additives.It can be observed that the densities of sintered samples increase with the sintering temperature,and the same trend can be observed with the increase in Y₂O₃ content.The densities increase significantly when the sintering temperature is in the range of1800-1830℃.It begins to be stable when the temperature reaches 1830℃.Besides,at the same sintering temperature,the density is greatly influenced by Y₂O₃ content.There is an abnormal point in the density-temperature curve at 1800℃,the sample density gradually declines first with the increase in Y₂O₃ contents,whereas it rises again when Y₂O₃ content reaches a certain value.According to the principle of liquid-phase sintering,all above-mentioned densification changes are attributed to the state and compositions of the secondary phases.

Fig.1 SEM image of starting AlN powder

Fig.2 Densities of sintered samples with different Y₂O₃ additives at different sintering temperatures

Figure 3 presents the relationship between the shrinkages and sintering temperatures of AlN ceramics with different amounts of Y₂O₃ additive.As shown Fig.3,the shrinkages of sample increase gradually at 1830-1890℃with the rise in sintering temperatures,which is consistent with the density curve as shown in Fig.2.On the contrary,the shrinkages decrease with the increase in Y₂O₃ contents at 1800℃.Therefore,the reason for the increasing density of the sample with 10.0 wt%Y₂O₃ additive shown in Fig.2 can be associated with the excessive additive of Y₂0₃.

Fig.3 Shrinkages of sintered samples with different Y₂O₃ additives at different sintering temperatures

3.2 Phase compositions

The phase compositions of sample sintered at 1830℃for2 h with different Y₂O₃ additives are shown in Fig.4.In addition to the main phase of AlN,the generated yttrium aluminate phases transform with the sequence of YAG(Y₃Al₅O₁₂)→YAP (YAlO₃)→YAM (Y₄Al₂O₉) with the increase in the Y₂O₃ contents [ 11, 14] .According to Al₂O₃-Y₂O₃ binary phase diagrams,the rising content of Y₂O₃ enhances the formation of YAM.The reactions are as follows:

Besides the Y₂O₃ content,the sintering temperature also influences the phase compositions of AlN sintered samples.Figure 5 shows the phase compositions of the samples sintered at 1890℃for 2 h with 5.0 wt%Y₂O₃.Combining the XRD curve for 5.0 wt%Y₂O₃ additive in Fig.4 with Fig.5,for the sample containing 5.0 wt%Y₂O₃,it can be seen that the secondary phases are YAG and YAP at the sintering temperature of 1830℃.They convert to YAP and YAM when the temperature reaches 1890℃.This suggests that higher sintering temperature is helpful to the generation of yttrium-rich phase.The reason is that YAG and YAP phases can change to YAM in the graphite furnace with N₂ and C by the carbothermal reduction reaction [ 17] when the sintering temperature is high enough.Therefore,although there is little contribution to facilitating densification of sample by increasing sintering temperature above 1830℃,the formation of YAM can benefit the purification of AlN lattice and thus enhance the thermal conductivity.

Fig.4 XRD patterns of AlN samples sintered at 1830℃for 2 h with different Y₂O₃ additives

Fig.5 XRD pattern of AlN sample sintered at 1890℃for 2 h with5.0 wt%Y₂O₃ additives

Figure 6 shows the XRD patterns of AlN sample sintered at 1800℃for 2 h.It can be seen that there is a similar trend of the secondary phase with the increase in Y₂0₃ additives.YAG,YAP and YAM diffraction peaks can be obviously observed in the samples with the Y₂O₃contents of 3.0 wt%,5.0 wt%and 10.0 wt%,respectively.As confirmed in Al₂O₃-Y₂O₃ binary phase diagrams,YAG eutectic liquid may form near 1800℃.However,YAP and YAM remain solid state at this temperature.That is the reason why the shrinkage decreases with the increase in Y₂O₃ content from 3.0 wt%to 10.0 wt%at 1800℃as shown in Fig.3.

3.3 Microstructure

Figures 7,8 and 9 show the micros truv tures of the fractural surfaces of AlN samples sintered at 1800,1830 and1890℃for 2 h with different Y₂O₃ additives,respectively.In the case of introducing Y₂O₃ as sintering aid,the formation of liquid Y-Al-O above 1800℃has important effects on improving densification,transition of phase compositions and conversion of grain morphology and size for the sintered samples.

Fig.6 XRD patterns of AlN samples sintered at 1800℃for 2 h with different Y₂O₃ additives

Fig.7 SEM images of A1N samples sintered at 1800℃for 2 h with different Y₂O₃ additives:a 3.0 wt%,b 5.0 wt%,c 7.5 wt%and d 10.0 wt%

A large number of pores and unobvious secondary phases between grain boundaries can be observed at1800℃in Fig.7.Combined with the result of XRD curve for 5.0 wt%Y₂O₃ additive shown in Fig.6,it can be concluded that at 1800℃,dispersive Y₂O₃ reacts with aluminum oxide generated on the surface of the aluminum nitride to form isolated solid-state YAP phase,which has no obvious liquid sintering effect.The grains in the four samples show the similar polyhedron.

When the temperature increases to 1830℃,the porosity in the four samples decreases significantly and becomes unobvious with the grains packing closely,and this agrees with the increasing shrinkage as shown in Fig.3.The densification process becomes slow at 1830-1890℃.Besides,more liquid phases generate and migrate at this temperature,making some YAP react with the residual Al₂O₃ to form YAG (XRD curves for 5.0 wt%Y₂O₃ additive in Figs.4,6).It can be found that the secondary phases well distribute in intergranular and triangle grain boundaries as shown in Fig.8c,d.With the further increase in the sintering temperature,the liquid layers around grains gradually become thicker and even completely encapsulate the grains as shown in Fig.9.The grain size decreases and the grain morphology transforms from polyhedron into sphere-like shape with the increase in Y₂O₃ contents as revealed in Figs.8 and 9.In addition,the two changes occurring on the grains are more significant when the sintering temperature is higher.The high sintering temperature on the one hand decreases the viscosity of Y-Al-O liquid phase and promotes the liquid distribution and on the other hand,makes the grain edges easily dissolve into the liquid phase,which causes the increase in the thickness of the grain boundary phase and the sphere-like morphology of the grains.Therefore,although there are lots of edges and corners in starting AlN powder after grinding,the grain morphology still develops from polyhedron into sphere-like shape by the liquid-phase sintering.Furthermore,when the oxygen content in AlN powder is the same,the more the Y₂O₃ is added,the more the Y-AlO liquid phases are created.The enough liquid layers between the grains will prevent the grains from bonding to each other in the later stage of liquid-phase sintering,thereby inhibiting grain growth further.

Fig.8 Backscattered electron (BSE) images of AlN sample sintered at 1830℃for 2 h with different Y₂O₃ additives:a 3.0 wt%,b 5.0 wt%,c 7.5 wt%and d 10.0 wt%

Fig.9 BSE images of AlN samples sintered at 1890℃for 2 h with different Y₂O₃ additives:a 3.0 wt%,b 5.0 wt%,c 7.5 wt%and d 10.0 wt%

The fractural surface also displays different morphologies with varied secondary phases.The transgranular and intergranular fractures can be found when the amount of secondary phases between the grains is small as shown in Fig.8 a.However,it exhibits the intergranular fracture when the amount of secondary phase increases as shown in Figs.8d and9a.This implies that fracture is more likely to happen in the Y-Al-O grain boundary phases.Accordingly,the existence of the grain boundary phases can not only decrease the thermal conductivity,but also go against improving the mechanical properties of the sintered AlN ceramics.

4 Conclusion

Fully dense AlN ceramics are obtained by liquid sintering when the sintering temperature is above 1830℃using AlN powder prepared by SHS as starting material and Y₂O₃ as sintering additive.In the experiments,the density of the sintered sample reaches 3.33 g·cm^-3 with 5.0 wt%Y₂o₃ additive at the temperature of 1850℃for 2 h.With the increase in addition of Y₂O₃ and the sintering temperature,the secondary phase has a successive phase transition:YAG→YAP→YAM.

Both the additive contents and sintering temperatures have an important impact on the grain morphology and size of sintered AlN ceramics.When Y₂O₃ content is identified,the grain size increases and the grain morphology transforms from polyhedron to sphere-like shape,along with the rise in sintering temperature.At a certain sintering temperature,the grain size decreases and the grain morphology still converts from polyhedron to sphere-like shape along with the increase in secondary phase content.Therefore,to obtain polyhedral grains with uniform size and close packing,the sintering temperature and the Y₂O₃ content need to be controlled strictly.

Acknowledgments This study was financially supported by the International Cooperation Project of Zhejiang Province (No.2012C24007).