Millimeter-Wave-HIP Combined Sintering to Fabricate AIN Ceramics with High Thermal Conductivity

Akira Kishimoto, Kouhei Yamashita, Syunsuke Ohura, and Hidetaka Hayashi

Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011
Submitted: 12 April 2011, Accepted: 24 May 2011

Topics Covered

Abstract

With the aim to improve the thermal conductivity of yttria dispersed AlN ceramics through a short time and low temperature treatment under nitrogen atmosphere, millimeter wave (MMW) heating was combined with post-HIP treatment. Relative density increased by post HIP treatment over 1600 ºC on previously MMW heated samples even with the same temperature. The thermal conductivity exceeded over 170W/mK when treated through MMW-HIP combined sintering with 1700 ºC +1800 ºC treatment by 0.5 h +0.5 h. This valued had been attained through the MMW treatment alone at 1800 ºC for 2 h under 3 wt% hydrogen mixed nitrogen atmosphere. The relative density has been saturated at the first MMW heating over 1700 ºC. On the other hand, thermal conductivity increased with the treatment temperature both in first MMW and post HIP. No significant pressure dependence can be seen in the thermal conductivity with this experimental condition.

Keywords

HIP, Millimeter-Wave, AlN, Thermal Conductivity

Introduction

With the recent development of highly integrated LSI, advanced substrate materials with higher thermal conductivity, higher electro insulation, and higher mechanical strength are needed. One of the most promising candidates for such substrate is aluminum nitride (AlN) because its theoretical thermal conductivity is as high as 320 W/mK and other electrical and thermal properties are excellent. Then it attracts much attention for heat sink material in light emitting diode, reaction container with the production of semiconductor, in addition to substrate for highly integrated electric circuit [1-4].

The chemical bond of AlN is covalent nature, then it is hard to sinter or it needs high temperature, long dowelling time, and particular atmospheric gas. To obtain a thermal conductivity over 200 W/mK, or the standard value for desirable heat sink substrate, it is needed to sinter at 1800 ºC for 2 h followed by annealing at 1850 ºC for 100 h [5]. Millimeter-wave irradiation heating has been estimated as a promising method for low temperature and rapid sintering of ceramics [6-9]. The millimeter-wave (30 GHz-300 GHz) including sub-millimeter-wave (20 GHz-30 GHz) is absorbed by a wide variety of ceramics leading to self-heating. By using such heating method, densification with low temperature and short holding time has been reported in several ceramics compared with conventional sintering [10,11]. We have already reported that AlN ceramics dispersed with 5 wt% of Y₂O₃ can be densified by millimeter-wave irradiation heating at 1800 ºC for 2 h in nitrogen atmosphere containing 3 vol.% of hydrogen and the resultant ceramics showed thermal conductivity as high as 170 W/mK [12]. Such processing condition, time and temperature, is still severe, and hydrogen introduction would be an obstruction for ordinary production.

On the other hand, Hot Isostatic Pressing (HIP) is another sintering method by which short time and low temperature densification is expected since it employs hydrostatic pressing during heat treatment. In order to densify a powder compacted body, capsule wrapping with materials possessing high melting point is needed which is also inconvenient to make complex shaped ceramics.

HIP treatment is reported to be effective for pre-sintered ceramics over 92 % to the theoretical density which is called capsule-free HIP treatment for self-capsuled materials [13,14]. In the present study, we tried to improve the properties of AlN ceramics by combining millimeter-wave heating following HIP treatment. Both millimeter-wave heating and capsule-free HIP treatment don’t employ solid state pressing media leading to facile fabrication of complex shaped ceramics.

Methods and Materials

As starting materials AlN powders (Toyo aluminum Co. Ltd., JC,median grain size ; 0.8 µm) and Y₂O₃ (Shin-etsu chemical Co. Ltd., UU-HP, median grain size; 25µm) were used.Both starting powders were weight to make Y₂O₃ content to the total weight as 3, 5, or 10 wt%. They were mixed in a planetary ball mill for 2.5 h by using acetone as media. After dried followed by crashing using alumina mortar, the resultant powder mixture of ca.1.6 g was uniaxially pressed under 25 MPa for 10 min. in 20f steel die followed by cold isostatic pressing under 130 MPa for 5 min.

Thus prepared powder compacted bodies were heat treated for 0.5 h or 2 h at 1600, 1700 or 1800ºC in nitrogen atmosphere using a millimeter-wave irradiation equipment (24 GHz gyrotron generator system, MSP Ltd., Osaka, Japan). In the millimeter-wave irradiation heating, the thermal insulation setup was followed that reported previously and the thermocouple was inserted between the first and second samples piled of fourfold to reduce the temperature monitoring error. The temperature was elevated at a rate of 20 ºC/min up to 1500 ºC then 10 ºC to the predetermined temperature. After holding, the temperature was decreased at a rate of 25 ºC down to 500 ºC then furnace cooled.

Post-HIP treatment was conducted using Dr. HIP (Kobe Steel Ltd., Kobe, Japan). The treatment temperatures, pressure and holding time were varied as 1600-1800 ºC, 190 MPa, and 0.5 h, respectively. Treatment temperature was elevated at 10 ºC /min. and gas pressure was applied from the temperature reached 500 ºC.

Apparent density was measured by the Archimedes method. Thermal conductivity was measured by the laser flash method (LFA 447 Nanoflash, Netzsch, Selb, Germany) on samples cut to f6 disk by a ultrasonic cutting machine. To confirm the crystalline phase formed, X-ray powder diffraction was conducted by RINT2000 (Rigaku Co. Ltd., Tokyo, Japan) using CuKa radiation. Microstructural observation was conducted on fractured surface through SEM (S-4300, Hitachi Co. Ltd., Tokyo, Japan).

Figure 1. Relative densities (top) and Thermal conductivity (bottom) of yttria added AlN ceramics with millimeter wave heating. Yttria addition; 3, 5 and 10 wt%. Heating temperature 1600, 1700 and 1800 ºC for 0.5 h [21].

Figure 2. Relative densities (top) and Thermal conductivity (bottom) yttria added AlN ceramics with combination of millimeter wave heating and post-HIP at 1800 ºC for 0.5 h under nitrogen pressure of 190 MPa.

Figure 3. SEM photos of fracture surfaces for variously Y₂O₃ added AlN ceramics with combination of millimeter wave heating at 1600 ºC for 0.5 h. and post-HIP at 1800 ºC for 0.5 h under nitrogen pressure of 190 MPa. Yttria addition; 3 (top), 5 (middle) and 10 (bottom) wt %.

Results and Discussions

After Millimeter-Wave Irradiation Heating

In Figure 1 (top) relative density to the theoretical one is illustrated on AlN dispersed with 3, 5, or 10 wt% of Y₂O₃ heat-treated at 1600, 1700, or 1800 ºC by the millimeter-wave irradiation. By using the millimeter-wave heating relative density over 85 % was attained in any sample irrespective of Y₂O₃ content. These results accord with that reported previously. In samples heat-treated at 1600 ºC, the relative density increases with an increase in Y₂O₃ addition amount, indicating a decrease in sintering temperature with addition. This is probably due to the liquid phase formed from the added Y₂O₃ facilitating the rearrangement of grains [15-18].

As references, samples dispersed with 3 wt% of Y₂O₃ were heated at 1600 ºC for 0.5 h using conventional furnace with nitrogen atmosphere or capsule-HIP with nitrogen gas pressure of 190 MPa. The relative densities are both around 70 %, i.e., 70.2 % and 70.7 %, which are inferior to that sintered by the millimeter-wave irradiation. These results demonstrated the advantage of millimeter-wave irradiation heating for short time and low temperature sintering of AlN ceramics.

In Figure 1 (bottom) thermal conductivities are illustrated on samples dispersed with various Y₂O₃ after the millimeter-wave irradiation heating at 1600 to 1800 ºC. The thermal conductivity increases with increasing the heating temperature. For example, heat-treatment at 1800 ºC gave favorable high thermal conductive materials with thermal conductivity over 160 W/ mK irrespective of Y₂O₃ dispersion amount. Such temperature dependence of thermal conductivity would be ascribed to the densification improvement with higher treatment temperature.

On the other hand, thermal conductivity tends to decrease with increasing the Y₂O₃ addition amount while sample density increases with the addition amount. In general thermal conductivity is affected by lattice defects such as vacancy, substitution atoms, dislocation, and pores [15,17]. In the present case, dispersed Y₂O₃ is thought to play a role to form lattice defects. It has already been reported that dispersed oxide is dissolved into AlN to make aluminum vacancy and substitution of oxide for nitrogen. These defects associate to form layered defects which act as phonon scattering site reducing the thermal conductivity.

Effect of Post-HIP Treatment on Density and Thermal Conductivity

The millimeter-wave irradiation sintered samples were subjected to post-HIP treatment at temperature of 1600 - 1800 ºC, under nitrogen gas pressure of 50 – 190 MPa, and with holding time of 0.5 h. In Figures 2 (top) and (bottom), relative densities and thermal conductivities on samples firstly millimeter-wave sintered followed by post-HIP treatment at 1800 ºC for 0.5 h under nitrogen pressure of 190 MPa are illustrated as examples.

After post-HIP treatment, the density improved tremendously even with the sample firstly sintered by millimeter-wave heating at 1600 ºC. Since the density further increases by the post-HIP treatment even with the same sintering temperature as that of millimeter-wave sintering, gas pressure as high as 190 MPa effectively act to improve the density. This is due to the exclusion of pores and modification of crack-like defects by such high pressure in addition of facilitation of sintering. The relative density increases with the Y₂O₃ addition amount in samples millimeter-wave sintered at 1600 ºC followed by post-HIP treatment at 1800 ºC under 190 MPa. This can also be ascribed to the liquid phase formed by the reaction between added Y₂O₃ and Al₂O₃ existing on the AlN grain [18].

When the thermal conductivity is compared before and after HIP treatment, it improves accompanied by density improvement. The addition amount dependency, however, are different between density and thermal conductivity. In the samples millimeter-wave sintered at 1600 ºC for 0.5 h, thermal conductivity decreases with an increase in addition amount while the density increase with addition amount.

In AlN ceramics, high temperature sintering is reported to scavenge the impurity oxygen from the grain to segregate at grain boundary, enhancing the thermal conductivity [19,20]. Such scavenging effect is insufficient to remain considerable amount of oxygen in grain reducing the thermal conductivity in the case of low-temperature sintered sample. Figure 3 shows the SEM photos of fracture surfaces. Grain boundary becomes smooth with increasing the addition amount, indicating the formation of liquid phase [21].

As a reference, the same post-HIP treatment at 1700 ºC for 0.5 h under gas pressure of 190 MPa was conducted on 3 wt% of Y₂O₃ added AlN sintered at 1600 ºC for 0.5 h in nitrogen atmosphere with conventional furnace. After post-HIP treatment, the thermal conductivity remained at 93.1 W/mK while the relative density increased from 70.2 % to 94.3 % [21]. The thermal conductivity of MMW-postHIP treated sample with the same composition and processing parameter (temperature and time) was 138 W/mK, demonstrating a superiority of millimeter-wave irradiation heating in the improvement of properties including thermal conductivity.

Conclusions

With the aim to improve the thermal conductivity of yttria dispersed AlN ceramics through a short time and low temperature treatment under nitrogen atmosphere, millimeter wave (MMW) heating was combined with post-HIP treatment. Relative density increased by post HIP treatment over 1600 ºC on previously MMW heated samples even with the same temperature. The thermal conductivity exceeded over 170 W/mK when treated through MMW-HIP combined sintering with 1700 ºC +1800 ºC treatment by 0.5 h +0.5 h. This valued had been attained through the MMW treatment alone at 1800 ºC for 2 h under 3 wt% hydrogen mixed nitrogen atmosphere.

The relative density has been saturated at the first MMW heating over 1700 ºC. On the other hand, thermal conductivity increased with the treatment temperature both in first MMW and post HIP.

Acknowledgements

This work was supported in part by Grants-in-Aid Grant-in-Aid for Scientific Research on Priority Areas (21017005) from Japan Society for the Promotion of Science (JSPS).

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Contact Details

Akira Kishimoto, Kouhei Yamashita, Syunsuke Ohura, and Hidetaka Hayashi
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan

Email: [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[1] (2011) 14-18.