Key Engineering Materials Vols. 368-372

Abstract: This paper investigated the influence of sintering temperature on the dielectric performance of alumina ceramic. Three different kinds of alumina ceramic samples were prepared using the same formula with three different sintering temperatures. The flashover voltage and surface charge distribution of the test samples were measured with a finger type electrode system under a positive pulse voltage application in a 1×10-4Pa vacuum circumstance. The experimental results showed that the samples with higher sintering temperature have worse dielectric performance. Based on the SEM observation, the change of dielectric performance could be attributed to the change of microstructure of alumina ceramics.

Abstract: Sol-gel method has been adopted to synthesize CuFeO2 powder. Precursor solutions and heat treatment conditions were studied using thermal analysis and XRD. Pure CuFeO2 was obtained using the prepared sol-gel precursor that heat treated at 950°C for 5 h under nitrogen atmosphere.

Abstract: A series of Cu1-xAlS2 (x = 0 ~ 0.08) bulk samples were synthesized by spark plasma sintering. The electrical and optical properties were investigated. P-type conductions for all samples were confirmed by both positive Seebeck coefficient and Hall coefficient. Bulk undoped CuAlS2 had a high conductivity of about 0.9 S/cm with a large band gap of 3.4 eV at room temperature. For vacancy-doped in Cu site, the carrier concentration was highly enhanced, reaching 1.7 × 1019 cm-3 for 8 mol% doped sample, and without decreasing the bang gap. The introduction of vacancies destroys the continuity of Cu-S network, which decreases the Hall mobility.

Abstract: We describe the production of complex shaped ceramic green bodies with high strength and reliability using a novel forming method: temperature-induced gelation. Gelation is performed by moderately decreasing the temperature of the suspension, which induces in situ gelation and forms a network to bridge the suspended particles, leading to a stiff green body. The gelation mechanism is based on the separation of dispersant KD1 from solvent or the collapse of adsorbed layer on particle surface, which depends on the stability of starting suspensions.

Abstract: Polycrystalline alumina was prepared at high temperature and high pressure from combustion reaction plus mechanical pressure. The defect structure in the sintered alumina was investigated by transmission electron microscopy and a regular hexagonal dislocation network was found. Diffraction contrast technique revealed that the regular hexagonal network on the (0001) plane was composed of three dislocations with Burgers vectors: 1/3[1210], 1/3[ 2110] and 1/3[1120]. The formation process of dislocation network was discussed.

Abstract: α-Al2O3 nanopowders were prepared by a novel synthesis process, using the nanosized α-Al2O3 obtained from pyrolyzing ammonium aluminum carbonate hydroxide as seeds and the self-dispersed nanosized AlOOH crystal powders as precursors. Based on their good self-dispersion in water, the α-Al2O3 seeds were dispersed evenly into the AlOOH sol by the new homodispersion mixing technique. This process enables the conversion of AlOOH to alumina at 190°C (hydrothermal temperature), in which the alumina is calcined to nanosized alpha-alumina having an average length to diameter ratio of 60nm:15nm at 930°C. In the synthesis reaction for transforming the AlOOH to alumina, the effect of superfine pulverization and self-dispersion of the precursors was studied.

Abstract: A modified starch was employed to be the pore former in the in-situ consolidation forming process of porous alumina ceramic. In order to prepare ceramic slurry with high dispersion, high stabilization and high solid volume loading, the effects of modified starch content, dispersant content, alumina solid volume loading, pH value of the slurry and ball milling time on the rheological behavior of alumina ceramic slurry were investigated. The results indicated that the viscosity of the slurry increased with the modified starch content, alumina solid volume loading and ball milling time. When the pH value of the slurry was 9.0, the modified starch-alumina slurry exhibited high fluidity. The modified starch-alumina slurry was a pseudoplastic fluid exhibiting shear thinning behavior. The slurry possessed a desirable plasticity to enable shape forming when adding 50 vol% modified starch and 1.0 wt% dispersant to the alumina slurry at pH 9.0 after 14 h ball milling treatment.

Abstract: Ammonium aluminium carbonate hydroxide (AACH), with a small quantity of γ-AlOOH, was synthesized through solid-state reaction at room temperature using AlCl3·6H2O and NH4HCO3 as raw materials and polyethylene glycol (PEG-10000) as the dispersant. After calcined at 1100°C for 1.5h, α-Al2O3 powders with primary particle sizes of 20~30nm were obtained. The crystal phase, particle size and morphology of the high-purity ultrafine α-Al2O3 were characterized. The results showed that a small quantity of γ-AlOOH in the AACH decomposed and formed crystal seeds. The presence of crystal seeds reduced the nucleation activation energy and therefore reduced the phase transformation temperature.

Abstract: The sintering activation energy of high-purity alumina powders with different particle sizes was evaluated under non-isothermal condition. It was found that, during sintering, the activation energy for the lower temperature stage is higher than that for higher temperature stage. The value of the activation energies for the powder compact with larger particle size was higher than that for the powder compact with smaller particle size. If the selected temperature interval for calculation was narrow enough, the evaluated activation energy values varied with the increasing temperature continuously.

Abstract: A new control method of the particle orientation structure with super high magnetic field is studied for Al2O3 ceramics. Al2O3 slurry was made with two types of Al2O3 particles, which one was spherical shape of, and the other one was elongated shape of. The Al2O3 ceramics of orientation structure was fabricated by drying the Al2O3 slurry under the super high magnetic field, cold isostatic pressing (CIP) and sintering. Effects of particle shape, particle size, the solid loading, slurry viscosity, dispersant content and the magnetic field strength on the particle orientation structure were examined in detail. The experimental results indicate that even the spherical Al2O3 particles can align under the magnetic field; the particle orientation degree changes with the particle shape and the solid loading under same magnetic field strength conditions, and the elongated particle system is easier to align than the spherical particle system; the particle orientation degree of the Al2O3 ceramics can be controlled by adjusting the particle shape, particle size, solid loading, slurry viscosity, dispersant content and magnetic field strength.