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Alumina green bodies were shaped by slip casting under a strong magnetic field. Alumina ceramics were obtained by hot isostatic pressing (HIP) at 1275°C and pressureless sintering at above 1700°C, respectively. HIP-sintered alumina, with in-line transmittance of 52% at 600nm and 82% at 2μm, showed a sub-micrometer grain microstructure and a slight grain orientation. However, XRD results demonstrated the grains of pressureless sintered alumina sintered at >1700°C oriented along c-axis, and texture microstructure was observed by SEM. It indicated that magnetic field assisted slip casting produced “seeds” of grain orientation, and orientation degree that could achieve depended on the sintering temperature.
© 2015 Optical Society of America
1. Introduction
As anisotropic ceramics, high purity transparent alumina ceramics have caught much attention due to its known and potential application as arc tube, optical device, cavity parts, armor and dome in the last half century. At equal relative density and grain size alumina exhibit higher hardness than other transparent ceramics [1–3], therefore preparation of alumina with sub-micrometer grains became significant through a fine powder and special sintering (HIP or SPS). Furthermore, it brought great improvement of in-line transmittance in alumina ceramics, especially the IR transmittance [2, 4–6]. On the other hand, transparent alumina ceramics as anisotropic ceramics just like FAP can be realized higher transmittance with a controlled microstructure by using a crystal orientation process based on magnetic field [7–10].Many group have been prepared the grain oriented alumina by strong magnetic field assisted colloidal process [11–13]. More recently, high purity alumina with high transmittance starting from UV was prepared by magnetic field assisted slip casting and sintering at hydrogen atmosphere [14].
In this report,alumina green bodies were formed with magnetic field assisted slip casting, oriented particles that can function as template were “seeded” by magnetic induced rotation effect. Analysis of HIP and pressureless sintered alumina were performed respectively in order to analyze the mainly attribution of grain orientation, which is grain growth or densification during sintering.
2. Experimental procedure
The raw material was high quality commercial α-alumina powders (TM-DAR, Taimei chemistry Co. Ltd., Tokyo, Japan.) with purity of 99.99%, a mean particle size about 250 nm and a BET specific surface area about 14.6 m2/g. 400 ppm MgO was added as sintering aid. Alumina suspension with about 20~25 vol% solids was prepared by mixing the raw powders with Polyacrylate ammonium (Ciba Specialty Chemicals Co., Basel, Switzerland) and deionized water, using ultrasonic dispersion for 30 minutes and planetary ball milling for 10 hours.
Green bodies were gained by slip casting the resultant alumina suspension into a porous mold which was placed in a Magnetic Field Instrument (12/14/98, Oxford Instruments plc. Oxon, UK). The magnetic field was 12T with direction parallel to the casting direction during shaping process, as indicated in Fig. 1.Then the samples were dried at 120°C for 24 hours and pre-sintered at 1250°C for 3 hours in air. The final densification of ceramics was carried out by hot isostatic pressing (HIP) sintering at 1275°C for 3h at a pressure of 160MPa in HIP equipment (RD 200/300-2000-200, Bairl Equipment Co., Ltd. Sichuan, China). Meanwhile, the shaped bodies were pressureless sintered at 1700-1880°C for 3h out in a vacuum tungsten furnace(ZW-30-20, Chenrong furnace Co., Ltd. Shanghai, China) with a vacuum level around 2 × 10−3 Pa.
In order to analyze the grain orientation, the ceramics were characterized by X-ray diffraction (XRD, D/MAX-2550V, Rigaku Industrial Co., Osaka, Japan) using a θ/2θ pattern on the testing slices [15–17]. The samples were cut horizontally (perpendicular to the magnetic field direction) and vertically (parallel to the magnetic field direction) from the alumina ceramics, noted as top and side slices, respectively.
Alumina ceramics were polished on both sides to a thickness of 1mm for the in-line transmittance measurement where an UV-VIS-NIR spectrophotometer (Cary 5000, Varian Inc., Palo Alto. US) and a Fourier Transform Infrared (FTIR) Spectrometer (EQUINOX 55, Bruker Optics Inc. German.) were employed. SEM (JSM-6300, JEOL, Tokyo, Japan) was used to observe the microstructure of the top and side slices.
3. Results and discussion
3.1 HIP sintering
Figure 2 illustrated microstructures of alumina ceramics shaped under 12T and 0T and HIP sintered at 1275°C/160MPa for 3h. The ruptured surface was parallel to the direction of magnetic field so as to observe the grain alignment if the grain orientation was intense enough and observable [11, 18, 19]. As can be seen in Fig. 2, there were not much differences between samples shaped with and without magnetic field, both two alumina ceramics showed equiaxed grain microstructure and average grain size of less than 1μm. It can be inferred that HIP sintering at a lower temperature was effective to restrain grain growth as compared with the particle size of raw powder (0.25μm), and the expected grain orientation of alumina shaped under 12T was not evidently observed in Fig. 2(a).
Fig. 2 Microstructures of alumina ceramics shaped under (a)12T and (b) 0T magnetic field, and HIP sintered at 1275°C/160MPa for 3h.
Figure 3Figure 3 showed photos of alumina ceramics obtained by HIP sintering. By comparison of samples shaped under 12T and 0T we concluded that both two alumina ceramics were transparent (not like traditional translucent alumina).Their in-line transmittance at UV-visible region were measured and shown in Fig. 4(a).Both curves of alumina ceramics exhibited typical shape of sub-μm transparent alumina [6, 19]. The in-line transmittance of HIP sintered alumina shaped under 12T (Fig. 4(a)) was higher than that of alumina shaped without magnetic filed, which were ~52% and 48% at the wavelength of 600nm, respectively.
Fig. 4 In-line transmittance of HIP sintered alumina shaped under 12T and 0T (a) UV-visible region and (b) infrared region.
XRD results of alumina ceramics shaped under 12T and 0T were shown in Fig. 5.There were two diffractive peaks (006) and (1010) in XRD pattern of HIP sintered alumina shaped under 12T (Fig. 5(a)) which relative intensity were increased as comparison with the alumina shaped under 0T (Fig. 5(b)). (006) and (1010) planes have low interplanar angles between c-planes, 0° and 17.5°, respectively.
Fig. 5 XRD patterns of HIP sintered alumina at 1275°C/160MPa for 3h shaped under (a) 12T, and (b) 0T.
Contrarily, there were several peaks, like (110), (012), (113), (124), and (030) with weakened relative intensity in Fig. 5(a), which their corresponding crystal planes have higher interplanar angles between c-planes. From the XRD results, a conclusion can be drawn that HIP sintered alumina shaped under 12T showed slight grain orientation along c-axis, resulting in a higher transmittance than that of samples shaped under 0T (Fig. 4).
The bending strength of HIP-sintered alumina were as high as about 610MPa and the in-line transmittance was 82% at 2μm as shown in Fig. 4(b). High mechanical properties and excellent IR transmittance were predominant characteristics of sub-μm transparent alumina [2, 6].
3.2 Pressureless sintering
Comparison was made by sintering the same green bodies shaped under 12T pressureless in vacuum at 1700-1880°C for 3h. XRD patterns tested on top of alumina were illustrated in Fig. 6. All the peaks which their corresponding crystal planes have higher angles (>45°) with c-planes were absent on the patterns, which means the grain orientation were greatly increased.
Fig. 6 XRD tested on top of alumina shaped under 12T and pressureless sintered at 1700~1880°C for 3h.
The diffraction relative intensity of peaks (006) was enhanced when sintering temperature increased, and the relative intensity of peaks with small interplanar angles to c-planes (00l), such as (104), (018) and (1010), were decreased because the samples tended to gain almost total orientation. This demonstrated that grain orientation degree increased with the increase of sintering temperature. By use of XRD pattern from alumina without magnetic field as a reference, the orientation facotrs of alumina shaped under 12T and sintered at 1700°C - 1880°C in Fig. 6 are calculated to be 0.13, 0.29, 0.35 and 0.45, respectively [8]. Alumina shaped under 12T and pressureless sintered at 1700-1880°C exhibited grains oriented paralleled to c axis and aligned along the direction of assisted magnetic field.
Figure 7 illustrated the fracture surfaces of alumina ceramics sintered at 1700-1880°C for 3h, grains grew from a grain size of 12μm to 45μm with the increase of sintering temperature. Moreover, there were pores inside ceramic bodies, which resulted in the opacity of the alumina samples. The raw alumina powders were fine (~250nm), the high sintering activity provided strong driving force for grain growth, but fine particles promoted agglomeration and resulted in inhomogeneous mutual coordination of particles, the pore distribution considerably retarded densification [20]. Well dispersion of the particles in suspensions and optimized subsequent sintering process may ameliorate this inferiority of densification situation.
Fig. 7 Fracture surfaces (parallel to top) of alumina ceramics shaped under 12T and pressureless sintered at (a) 1700°C, (b) 1800°C, (c) 1850°C, and (d)1880°C for 3h, respectively.
Figure 8 showed fracture of alumina pressureless sintered at 1800°C for 3h. The grains were obviously textured, flat disk and anisotropic growth, which were aligned parallel to top surface, i.e. perpendicular to the magnetic direction or c-axis.
4. Conclusions
HIP sintered (1275°C/160MPa/3h) alumina shaped under the assistance of magnetic field showed sub-micrometer grain. The in-line transmission was 52% at 600nm and 82% at 2μm. XRD results revealed that the grains were slightly oriented.
When pressureless sintered at 1700°C-1880°C for 3h, alumina ceramics were opaque, grain orientation happened as illustrated by XRD and SEM. The alumina oriented grains were flat disk shaped, anisotropic growth, textured and aligned perpendicular to magnetic field.It seems that grain orientation was attributed to the degree of grain growth, not that of bulk densification.
Acknowledgments
This work was financially supported by China National 863 Project (2009AA03Z440).
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