HUANG Junjie ^{1, 4}, ZHANG Hao ^{1, 4}, ZHAO Jianwei ¹, HE Bin ¹,

WANG Ximeng ², XIAO Zhuohao ³, KONG Lingbing ¹

(1. College of New Materials and New Energies, Shenzhen Technology University, Shenzhen 518118, Guangdong, China;

2. Zhongshan Institute of Changchun University of Science and Technology, Zhongshan 528437, Guangdong, China;

3. School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi, China;

4. College of Applied Technology, Shenzhen University, Shenzhen 518000, Guangdong, China)

Extended abstract:[Background and purposes] Transparent polycrystalline alumina ceramics (PCA) have attracted extensive attention as a viable alternative to traditional glass and single crystal materials, due to their excellent mechanical properties (high hardness, fracture toughness and flexural strength), making them widely studied materials in industrial and defense applications. Although progress has been made in sintering technology, it is challenging to achieve high optical transmittance for PCA, due to the birefringence effect related to its non-cubic crystal structure, thus requiring simultaneous reduction in porosity (<0.05%) and grain size (<1 μm) during the preparation process. In addition, in the SPS process, the development of inhomogeneous microstructure induced by temperature gradient and pressure gradient is widely considered to be the main factor hindering the further improvement of mechanical and optical properties. In this work, transparent alumina was prepared by using a two-step SPS process using untreated commercial powders at 1150–1350 ℃. Effects of SPS parameters on sintering behavior and optical properties of the prepared samples were systematically studied.[Methods] Commercial α-Al₂O₃ powder with an average particle size of 0.17 μm (99.99% purity, TM-DAR, Taimei Chemicals Co., Tokyo, Japan) was used in this study. The as-received powder was directly poured into a 16 mm diameter graphite die without any special treatment or additives. The sintering experiments were conducted using an SPS equipment (SPS-20T-10-IV, Shanghai Chenhua Science and Technology Co., Ltd., China). The temperature was measured by using an optical pyrometer with a non-penetrating hole positioned at the top of the graphite mold. Sintering was conducted at temperatures of 1150–1350 ℃ at an applied pressure of 50 MPa. The samples were rapidly heated to 600 ℃ in 1 min, then a heating rate of 50 ℃·min⁻¹ was used up to 800 ℃, where it was held for 3 min. During this holding stage, the pressure was increased from 40 MPa at room temperature to 50 MPa. Subsequently, the temperature was increased at a same heating rate of 50 ℃·min⁻¹ up to the 1150℃, with dwell times of 30–100 min. Then, the samples were cooled at a rate of 50 ℃·min⁻¹ to 600 ℃, while the pressure was reduced to 40 MPa, thus completing the entire process. The obtained disc-shaped samples were ground and polished to an optical level and had final dimensions of 16 mm in diameter and 0.8 mm in thickness. TFT measurements in the wavelength range of 200–2500 nm were conducted using a double-beam spectrophotometer (Lambda1050+, PerkinElmer, Inc., USA), with the samples directly placed in contact with a 5×5 mm square aperture in front of the integrating sphere. RIT was measured in the wavelength range of 200–800 nm using the same spectrophotometer by inserting a pinhole (diameter of 2 mm) in front of the detector to allow the measurement of only the specularly transmitted portion of the incident light beam. The distance between the sample and the detector was sufficiently far to exclude scattered light >0.5°. The RIT data were normalized at similar thickness of d2=0.80 mm using the equation RIT(d₂)=(1−R_S) [(RIT(d₁)/(1−R_S)]^d2/d1, where R_S denote the total normal surface reflectance (0.14 for transparent alumina), RIT (di) is the RIT for a sample with given thickness. For microstructure observation, the samples were polished and thermally etched in air at a temperature of 200 ℃ below the SPS sintering temperature for 5 h, by using a scanning electron microscope (SEM, SU8100, Hitachi High-Tech Corporation, Japan).[Results] According SEM results, the non-uniformity of the sample is greatly dependent on the sintering temperature, while the samples SPS sintered at 1150 ℃ is more uniform. When the temperature is increased to 1250 ℃ and 1350 ℃, grain coarsening phenomenon expands from the central region of the sample, so that the central region of the sample gradually becomes opaque. The two-step SPS process at 1150 ℃ for 60 min can fully densify and improve uniformity of the samples, while the grain size is 0.25 μm. Infrared transmittance of the sample sintered at 1250 ℃ for 60 min reached 80%. The sample with 150 ℃ heating and cooling rate at 1250 ℃ for 60 min and additional SPS annealing step (1000 ℃/20 min) shows a higher total forward transmittance, with a maximum value of 83%, which is close to the theoretical value. The RIT spectrum has a great correlation with the microstructure of the sample. The sample sintered at 1150 ℃ for 60 min has the highest RIT, which is 46.5%. After heat treatment at 950 ℃ for 5 h in air, this value is increased to 49%. Moreover, the additional SPS annealing step can effectively improve the TFT and infrared transmittance, but it may also deteriorate the RIT.[Conclusions] Transparent sub-micron PCA ceramics were prepared by using two-step SPS process with untreated commercial alumina powder. The relative density of all samples is >99%, while the uniformity of microstructure and the number of defects are dependent on the SPS parameters. The combination of two-step SPS sintering and optimized SPS parameters can be used significantly control the grain size and improve uniformity of the submicron PCA ceramics. The fine grains and pores make the transmittance curves of the samples in the near-infrared and infrared bands show similar characteristics. Among them, the sample SPS sintered at 1250 ℃ for 60 min at a heating and cooling rate of 50 ℃·min⁻¹ has the highest infrared transmittance, which is close to the level of sapphire. TFT of the sample sintered at heating/cooling rate of 150 ℃·min⁻¹ at 1250 ℃ for 60 min, followed by additional SPS annealing at 1000 ℃ for 20 min, reached 80% at 1150 nm and 83% at 2000 nm, which is very close to the theoretical value of 86%. The grain size (0.25 μm) of the sample sintered at 1150 ℃ for 60 min with a heating and cooling rate of 50 ℃·min⁻¹ is finer and more uniform. The sample with a thickness of 0.8 mm achieves 46.5% RIT at 640 nm, which is increased to 49% after thermal annealing. The use of annealing steps in SPS can improve TFT performance, but prolonged sintering time will lead to grain coarsening and adversely affect RIT. Therefore, the introduction of annealing process in SPS requires careful design and consideration of sintering kinetics and microstructure stability.

Key words: alumina (Al₂O₃); spark plasma sintering (SPS); total forward transmission/transmittance (TFT); real in-line transmission/transmittance (RIT)