Sintering behaviour and mechanical properties of porous alumina ceramics

Abstract

Alumina is a commercial ceramic material with high strength, good chemical durability and excellent electrical insulating properties. Porous alumina has been identied as the prime candidate as a porous bipolar membrane in an alternative lead acid battery design enabling higher power density. The rst generation porous alumina bipolar plates were fabricated by KeraNor AS by using silica as a sintering aid. These materials showed promising performance with respect to fracture strength, but the possibility of subcritical crack growth in acidic environments called for improvements. Particularly, the silica sintering aid used, causing the formation of a glass at the grain boundaries was identied as a potential problem. The primary goal of this thesis work was to develop alternative sintering aids to silica, which could result in a crystalline binding phase in the porous ceramics and possibly reduce the subcritical crack growth while maintaining the high fracture strength.

Alternative sintering aids were identied in the two ternary systems SrO-TiO2-Al2O3 and CaO-TiO2-Al2O3, where the two ternary oxide compounds SrTi3Al8O19 (STA) and CaTi3Al8O19 (CTA) were identied as prime candidates. The formation and stability of the STA and CTA compounds were investigated by thermal analysis, X-ray diraction and electron microscopy. It was found that both compounds were entropy stabilized and became stable above 1100° C and 1200° C respectively, which explained the diculty to fabricate ne grained powders of STA and CTA by the solid state method. An alternative fabrication process based on spray pyrolysis of an aqueous precursor solution was developed, and ne grained powders of STA and CTA were successfully produced by the spray pyrolysis technique. Thermal analysis and annealing experiments conrmed that STA decomposed by a peritectic reaction forming a liquid phase. CTA decomposed by a solid state reaction at 1400° C, while an invariant point leading to formation of a liquid phase was observed at 1446 ± 5° C. The solidus temperature in the STA-Al2O3 and CTA-Al2O3 systems revealed however that liquid phase sintering could take place down to 1398 ± 10° C and 1344 ± 15°C respectively. It was concluded that both CTA and STA would work well as sintering aids in the alumina ceramics.

Alumina ceramics using STA and CTA as sintering aids were fabricated in collaboration with KeraNor AS. The STA and CTA sintering aids were mixed with coarse grained alumina powder through an aqueous dispersion of the ne grained sintering aid. Several other mixing procedures were explored, but the use of the dispersion gave a relatively good distribution of the sintering aid in the nal alumina ceramics. This procedure was also shown to be easily implemented in the existing ceramic processing technology developed by KeraNor AS.

Porous alumina materials were fabricated with 1, 4 and 8 wt% of the STA sintering aid. Alumina ceramics with 3 wt% CTA sintering aid were produced. The evolution of the microstructure of the materials was studied as a function of the sintering time and temperature. The porosity and density of the materials were shown to correlate strongly with the sintering temperature and also the sintering time at low sintering temperatures. The density was not shown to be strongly in uenced by the amount of the sintering aid for the STA-alumina ceramics.

The fracture strength of the porous alumina based materials was measured by the Ball-on-Ring technique. The strength of the materials was largely dependent on the porosity and the type of sintering aid. The highest fracture strength of the alumina-STA materials was obtained for the material with 4 wt% STA sintered for 6 h at 1700° C with a density of 83.5 % and characteristic strength of 155 MPa. The highest strength of the alumina-CTA materials was obtained after sintering at 1650° C for 12 h with 197 MPa re ecting a density of 86.5 %. The fracture strength of the alumina-STA and the alumina-CTA materials was shown to be reduced after immersion in sulfuric acid (0.1 M) for one month. Fractography of the failed alumina-STA and alumina- CTA materials showed intergranular fracture mode. This was also demonstrated for the materials exposed to the acidic environment.

The fracture strength of porous alumina ceramics using 2.6, 4.0 and 4.8 wt% silica as sintering aid was measured. The alumina-silica materials possessed higher strength compared to the alumina-STA and the alumina-CTA materials at the same degree of porosity. The material with 2.6 wt% silica has the highest characteristic strength of 241 MPa. As the amount of silica was increased to 4.0 wt%, the strength decreased to 226 MPa, which was due to the formation of a secondary mullite phase at the grain boundaries. The fracture strength of the alumina-silica materials was less in uenced by exposure to acidic environments for one month when the silica content was increased. This was expected, since silica is a highly acidic oxide. Sub-critical crack growth in the alumina-silica materials were further characterized by the modied lifetime method. The subcritical crack growth rate was slightly higher for the material with lower silica content. The subcritical crack growth rate was established to be slightly higher in acid and water compared to in air.

The superior mechanical performance of the alumina-silica materials was discussed with respect to the transgranular fracture mode found by fractography, implying that the intergranular fracture mode found in the alumina-STA and alumina-CTA materials was the main reason for the lower mechanical performance of these materials.