简介概要

Effects of heat treatment on the properties of powder injection molded AlN ceramics

来源期刊：Rare Metals2008年第1期

论文作者：Akhtar Farida

文章页码：70 - 73

摘要：The effects of two different heat-treatment atmospheres,nitrogen atmosphere and reducing nitrogen atmosphere with carbon,on the properties of Y2O3-doped aluminum nitride（AlN） ceramics were investigated.The AlN powder as a raw material was synthesized by self-propagating high-temperature synthesis（SHS） and compacts were fabricated by employing powder injection molding technique.The polymer-wax binder consisted of 60 wt.% paraffin wax（PW）,35 wt.% polypropylene（PP）,and 5 wt.% stearic acid（SA）.After the removal of binder,specimens were sintered at 1850°С in nitrogen atmosphere under atmospheric pressure.To improve the thermal conductivity,sintered samples were reheated.The result reveals that the heat-treatment atmosphere has significant effect on the properties and secondary phase of AlN ceramics.The thermal conductivity and density of AlN ceramics reheated in nitrogen gas are 180 W·m-1·K-1 and 3.28 g·cm-3 and the secondary phase is yttrium aluminate.For the sample reheated in reducing nitrogen atmosphere with carbon,the thermal conductivity and density are 173 W·m-1·K-1 and 3.23 g·cm-3,respectively,and the secondary phase is YN.

Rare Metals 2008,(01),70-73+2

Effects of heat treatment on the properties of powder injection molded AlN ceramics

Akhtar Farida

Institute of Powder Metallurgy University of Science and Technology Beijing,,Beijing 100083,China

作者简介：DU Xueli E-mail:d7288@126.com;

收稿日期：18 September 2006

Effects of heat treatment on the properties of powder injection molded AlN ceramics

Abstract：

The effects of two different heat-treatment atmospheres,nitrogen atmosphere and reducing nitrogen atmosphere with carbon,on the properties of Y2O3-doped aluminum nitride(AlN) ceramics were investigated.The AlN powder as a raw material was synthesized by self-propagating high-temperature synthesis(SHS) and compacts were fabricated by employing powder injection molding technique.The polymer-wax binder consisted of 60 wt.% paraffin wax(PW),35 wt.% polypropylene(PP),and 5 wt.% stearic acid(SA).After the removal of binder,specimens were sintered at 1850°С in nitrogen atmosphere under atmospheric pressure.To improve the thermal conductivity,sintered samples were reheated.The result reveals that the heat-treatment atmosphere has significant effect on the properties and secondary phase of AlN ceramics.The thermal conductivity and density of AlN ceramics reheated in nitrogen gas are 180 W·m-1·K-1 and 3.28 g·cm-3 and the secondary phase is yttrium aluminate.For the sample reheated in reducing nitrogen atmosphere with carbon,the thermal conductivity and density are 173 W·m-1·K-1 and 3.23 g·cm-3,respectively,and the secondary phase is YN.

Keyword：

powder injection molding;AlN ceramics;heat treatment;thermal conductivity;

Received： 18 September 2006

1. Introduction

Aluminium nitride(AlN)has attracted much attention,mainly because of its high theoretical thermal conductivity(319 W·m^-1·K^-1),low dielectric constant(8.0 at 1 MHz),and a thermal expansion coefficient(~4×10^-6 K^-1),which is close to that of silicon.These attributes make Al N an available material for fabricating heat sinks and packages for microelectronic applications[1].However,polycrystalline Al N ceramics have much lower thermal conductivities because of oxygen impurities and microstructural defects.Due to high covalent bonding Al N is difficult to sinter.For full densification,rare-earth oxides(such as yttria)are added as sintering aids[2].

Powder injection molding is a competitive manufacturing technology for the mass production of ceramic or metal components of high performance and complex shapes at low cost.Ceramic or metal powder is combined with a thermoplastic binder and the mixture is heated and injected into a mold cavity which contains the required shape.The molded compact is debound and then sintered to the required density[3-4].In this experiment,Al N ceramics are fabricated by powder injection molding technology.To improve the properties of Al N ceramics,the sintered Al N ceramics are reheated.The effects of heat-treatment atmosphere on the properties of AlN ceramics are investigated.

2. Experimental

2.1. Materials

Commercially available AlN powder synthesized from SHS,as described in Table 1,was used as a starting material.As a sintering aid,5 wt.%yttria(Y₂O₃,99.9%purity)was added by milling for 6 h in ethanol using polyethylene containers and Al₂O₃ milling media.During this step 1 wt.%stearic acid(SA)was added as a dispersant.A polymer-wax binder system composed of 35 wt.%polypropylene,60 wt.%paraffin wax and 5 wt.%stearic acid was chosen for injection molding.The feedstock was prepared with a solids loading of 62 vol.%.

Table 1.Properties of starting AlN powder 下载原图

Table 1.Properties of starting AlN powder

2.2. Mixing and molding

Mixing was performed in the torque rheometer.The binder components were first put in the mixer and allowed to melt completely.Then the powder was added to achieve the desired powder-loading and mixed at a rotation rate of80 r/min at 150°C for 40 min.A reciprocating-screw type injection molding machine(CJ50E)was used to prepare test pills with the dimensions of 12 mm in diameter and 5 mm thick.

2.3. Debinding and sintering

The binder was removed through debinding process in two steps:solvent debinding followed by thermal debinding.For solvent debinding,the compacts were completely immersed in trichloroethylene for 6 h at room temperature.After solvent debinding,the dried specimens were then put in a thermal debinding furnace in air atmosphere.For sintering,the specimens were placed in an AlN crucible and sintered at1850°C for 6 h in flowing nitrogen gas under atmospheric pressure,and thus sample A was obtained.Successive heat-treatment cycles of the sintered samples were performed using the same equipment at 1800°C for 4 h in nitrogen gas(sample B)and reducing nitrogen atmosphere with carbon(sample C),respectively.

2.4. Sample characterization

X-ray diffraction(XRD)with Cu K_αwas used to identify the produced phases.The fracture surfaces of the sintered samples were observed by scanning electron microscopy(SEM).The thermal diffusivity at room temperature was measured using a laser flash technique.The density of the sintered specimen was measured using the Archimedes displacement method.

3. Results and discussion

Table 2 shows the density and thermal conductivity of the samples.The thermal conductivity at room temperature was calculated from the following equation[5]:

whereαis the thermal diffusivity,ρis the density,and C_p isthe specific heat capacity of AlN.The specific heat capacity of pure AlN at room temperature(0.734 J·g^-1·K^-1)was utilized in this experiment.

From Table 2,it is seen that heat treatment can effectively improve the thermal conductivity and density of sintered samples.Especially for sample B reheated in nitrogen atmosphere,its thermal conductivity increases from 131 to180 W·m^-1·K^-1,and its density increases from 3.19 to 3.28g·cm^-3.The density and thermal conductivity of sample C reheated in reducing nitrogen atmosphere with carbon are3.23 g·cm^-3 and 173 W·m^-1·K^-1,respectivley.Of all the factors influencing Al N ceramics’thermal conductivity,the oxygen content dissolved in the AlN lattice and microstructure defects are two very important reasons.The microstructure of practical polycrystalline Al N is composed of Al N grains,grain boundary phases,and remaining pores.The thermal conductivity of Al N ceramics depends on not only the inpidual thermal conductivity values of the various phases but also the distribution of the phases.The overall thermal conductivity of AlN ceramics may be increased by segregating the secondary phase to locate at the triple-grain junctions instead of along the grain boundaries.With the decrease of the secondary phases and remaining pores,the thermal conductivity of AlN ceramics will be improved[6-7].

Table 2.Thermal conductivity and density of sintered samples 下载原图

Table 2.Thermal conductivity and density of sintered samples

Fig.1 shows the microstructure photographs of fracture surfaces of sintered samples A,B,and C.It indicates that samples A and C are not fully sintered with some visible remaining pores.Due to no visible pores in sample B,its density is up to 3.28 g·cm^-3.For Y₂O₃-doped AlN ceramics,the inter-granular phase of yttrium aluminates moves to triple-grain junctions with increasing the holding time at sintered temperature[8].Low porosity and migration of inter-granular phase resulted in the increasing of thermal conductivity of sample B.Although there are some visible remaining pores in sample C,fewer secondary phases than other samples increase its thermal conductivity from 131 to173 W·m^-1·K^-1.

Fig.2 illustrates the XRD patterns of the sintered samples Al N is identified as the main phase for all the three samples.The secondary phases of samples A and B are identified as yttrium aluminates,but that of sample C is YN.During sintering of debound AlN green bodies in nitrogen atmosphere,Y₂O₃ reacted with Al₂O₃ covers the AlN particles to form yttrium aluminates(Y₄Al₂O₉,Y₃Al₅O₁₂,or YAlO₃)[9].In the case of reheating the sintered sample in a reducing nitrogen atmosphere with carbon,the following reactions occurred,which resulted in a decrease in oxygen content and purified the grain boundary[10]:

The decreasing of oxygen content and inter-granular phases increased the thermal conductivity of sample C;however,possibly the trapping of some CO gas in the sintered body restricted the density and thermal conductivity from further increasing.

Fig.1.SEM microstructures of sintered samples A to C.

Fig.2.XRD patterns of sintered samples A to C(PDF#:AlN25-1133,YN 35-0779,Y4Al2O9 14-0475,and Y3Al5O12 33-0040).

4. Conclusions

Heat treatment effectively increased the thermal conductivity of injection molded Al N ceramics.For injection molded Al N ceramics reheated in nitrogen atmosphere,its thermal conductivity increased when the porosity decreased and inter-granular phase migrated to triple-grain junctions.The component of the secondary phase was still yttrium aluminates.For AlN ceramics reheated in a reducing nitrogen atmosphere with carbon,the reducing reaction between carbon and yttrium aluminates decreased the oxygen content and purified the grain boundary,resulted in the improving of the thermal conductivity of Al N ceramics.The secondary phase of yttrium aluminates was reduced into YN.