Translucent alumina ceramics was fabricated by a novel and simple gelling system using a water-soluble co-polymer of isobutylene and maleic anhydride (Isobam), acting as dispersant and gelling agent. Alumina slurry was prepared by mixing alumina powder, deionized water and 0.5wt% Isobam. Both rheological properties and gelling behaviors of the slurry were evaluated. Without an initiator or any other additives, gelation of the slurry occurred at room temperature in air atmosphere. After dried and presintered, the green body was sintered at 1850°C for 5h in vacuum furnace. In-line transmittance of the resultant alumina ceramics (1mm thick) was 29.5% at 600nm.
©2013 Optical Society of America
Since the appearance of translucent alumina in 1960s, it has been extensively fabricated as straight tube (arc tube) for high pressure sodium lamp. In 1990s, another new high-intensity discharge lamp called ceramic metal halide (CMH) lamp was developed. The shape of arc tube for CMH is complicated (middle part is a big hollow cave and two ends are capillaries), resulting in the difficulty of shaping. A processing option to produce tubes with such complicated shapes is gelcasting. In the past two decades, gelcasting has attracted much attention because it exhibits abilities to form green bodies with complicated shape, homogeneous microstructure, and high strength for handling and machining. The principle of gelcasting involves polymerization in slurries by means of free radical reaction , nucleophilic addition reaction [2,3], cross-linking reaction  between polyvinyl alcohol (PVA) and organotitanate, and thermogelation with natural agent such as agarose . However, less organic addition and simple operation are parts of the most important targets when researchers work on exploring new gelcasting system. Recently a novel gelling system  was developed at the authors’ lab for fabrication of traditional alumina ceramics using only a water soluble co-polymer with multi-functional groups (structural formula is shown as Fig. 1), which can be operated at room temperature in air. In this work, we investigate the gelcasting of translucent alumina ceramics by the novel gelling system.
4N high purity alumina powder (CR-10, Baikowski, Annecy, France, D50 = 0.45μm) doped with 600 ppm MgO was used as raw material. A water-soluble co-polymer powder of isobutylene and maleic anhydride commercially named Isobam 104 with an average molecule weight of 55000-65000 (Kuraray, Osaka, Japan) was used as both dispersant and gelling agent. The addition of Isobam 104 (simply noted as Isobam hereafter) for the slurry varied from 0.3 to 2 wt% referred to the weight of alumina powder. The solid loading changed from 32 to 42 vol%. After mixing, the slurry was degassed in a vacuum chamber to remove the trapped bubble and then casted into plastic molds for gelling. After demoulding and drying, the samples were presintered at 1000°C for 3h in air with heating speed 1°C/min. Final products were sintered at 1850°C for 5 hours in vacuum furnace. The rheological behavior of the slurry was characterized by a stress-controlled rheometer (Physica MCR301, Anton Paar, Graz, Austria) with a parallel plate (25 mm in diameter). The pore size of the calcined body was measured by mercury porosimeter (Auto Pore IV 9500, Micromeritics, USA). Microstructures of the green body and sintered body were observed by SEM (JSM, JEOL 6390). In-line transmittance of the translucent alumina with a thickness of 1 mm (double-surface polished) was measured by a UV-VIS-NIR spectrometer (Carry 5000 Spectrophotometer, Varian, USA).
3. Results and discussion
3.1. Effect of Isobam content on gelling of alumina slurry
Figure 2 shows the effect of Isobam on zeta potentials of alumina slurries. The isoelectric point of alumina powder is at pH = 9.2 without addition of Isobam. The addition of 0.5 wt% Isobam (referred to the weight of alumina powder) made this point move to pH = 4.2. The zeta potential was −50 mV at pH = 9.2 (Fig. 2), indicating the Isobam is an effective dispersant for CR 10 alumina powder, similar to the case with AES 11 alumina powder .
Figure 3 shows the effect of Isobam content on the rheological behaviors of slurries with a solid loading of 40 vol% at room temperature. It is clear that all slurries show pseudoplastic behaviors at low shear rates and shear-thinning characters with increase of shear rates. On the other hands, the viscosity of the slurry (40 wt% solid loading) with 0.3 wt% Isobam is about 2 Pa•s at a shear rate of 100 s−1 and that with 0.5 wt% Isobam is lower than 1 Pa•s. The viscosities increase slightly with further more Isobam from 0.8 wt% to 2 wt%. The higher viscosity of the slurry with 0.3 wt% Isobam should be ascribed to the insufficient dispersing of alumina powder in the slurry. Figure 4 shows the viscosities of slurries (0.5 wt% Isobam) with different solid loadings. It reveals that all slurries show a shear-thinning character. The viscosities increased with the increasing solid loadings from 32 vol% to 42 vol%. The viscosities of all the slurries were lower than 2 Pa·s at a shear rate of 100 s−1. The lower viscosities made the slurries suitable for casting.
Figure 5 shows storage and loss modulus of alumina slurry with a solid loading of 42vol% and 0.5wt% Isobam addition. With the time going, the storage modulus increased slowly, revealing the formation of microgel. The storage modulus increase quickly till 2600 seconds and then increase slowly after 6000 seconds, indicating that gelation is almost finished. This result demonstrated that the slurry mixed with CR 10 alumina powder, water and 0.5 wt % Isobam can gel spontaneously at room temperature in air, which is similar to the slurry with AES 11 alumina powder by Yang et al . Although microgel started at the beginning of the measurement or even during the mixing of the slurry, the loss modulus kept at lower level for a period of time, which is enough for the preparation of casting. Compared with most of the conventional gelcasting systems which need initiator  or moderate temperature [5,7] to induce polymerization, gelation of alumina slurry with Isobam is spontaneous at room temperature without additives in the present case. The mechanism of spontaneous gelling of alumina ceramics is under investigation.
3.2. Debinding and microstructure of alumina green body
The TG curve of the dried green body made from the slurry with 42vol% solid loading and 0.5wt% Isobam addition shows a mass loss of 0.7%, mainly occurring between 170 °C and 500 °C (Fig. 6). The lower mass loss comes from the lower Isobam addition, which makes the debinding process easier and more convenient. Meanwhile, lower exhaust was produced during debinding process so that Isobam is friendly to the environment.
The intrusion volume vs pore size of the presintered body is shown in Fig. 7. The pores were mainly distributed in 0.05~0.15μm, with a sharp peak at 0.12 μm. The pores existed come from the voids between alumina particles (D50 = 0.45μm). This result is in agreement to the homogeneous microstructure of the presintered body (Fig. 8).
3.3. Transmittance and microstructure of translucent alumina
Monolithic translucent alumina ceramics were obtained with good appearance in terms of defect-free surface (without visible pores or white and black spots), shown in Fig. 9. It demonstrated that the new gelling system with only one organic additive (Isobam 104) is suitable for the forming of translucent alumina. Further job is under going to prepare translucent alumina with complicated shape such as the arc tube for CMH. The samples were machined and polished to 1mm to test the transmittance. The in-line transmittance increased gradually from 20% to 60% in the range of 0.19~3μm (Fig. 10). In-line transmittance at 600nm was 29.5%, compared with that of translucent alumina ceramics gelled by epoxy resin and polyamine system using the same alumina powder CR10 .
Fracture microstructure of translucent alumina is shown in Fig. 11. It could be seen that the sample is dense with an average grain size of 20 μm. However a few pores appeared at the grain boundary which would decrease the in-line transmittance of the ceramics. Higher transmittance is expected when the sintering process is further optimized.
Translucent alumina ceramics was successfully fabricated by spontaneous gelcasting and vacuum sintering method. The advantages of this gelling system involve only one organic additive i.e. a copolymer with multi-functional groups and low addition (0.5 wt%) and gelling at room temperature in air. The presintered body and sintered body displayed homogenous microstructures. The grain size of sintered alumina is about 20 μm. The in-line transmittance of alumina (1mm thick) increased from 22% to 60% in the range of 0.19~3 μm.
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