S. Bhattacharyya

Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5xa0mol% Y2O3) containing 2, 5, 7.5 and 10xa0wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100xa0nm) the TZP particles were only of spherical nature (∼50xa0nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO2. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5xa0wt% TZP, while it was 92% for 7.5xa0wt% and 90% for 10xa0wt% TZP compositions. X-ray analysis of sintered samples shows that with 2xa0wt% TZP, the retained phases were only HA and t-ZrO2. However, for 5, 7.5 and 10xa0wt% TZP addition both TCP and CaZrO3 were also observed along with HA and t-ZrO2. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72xa0MPa, it was 35.5xa0MPa for diametral compression. The strength shows a decreasing trend at higher ZrO2 content. SEM pictures show near uniform distribution of ZrO2 in HA matrix. The reduction in sintered density at higher ZrO2 content could be related to difference in the sintering behaviour of HA and ZrO2.

Preparation of alumina–high zirconia microcomposite by combined gel-precipitation

Abstract Microcomposites of alumina and zirconia containing equal volume fractions of the two are prepared by a combined gel-precipitation route using aluminium chloride and zirconium oxychloride as precursors. Dried gel powders calcined between 375 and 800 °C show cubic phase as the only crystalline phase. Zirconia gets converted to tetragonal phase above 900 °C and is retained mostly in this form upto 1000 °C. At the higher temperatures, it converts to the monoclinic phase. Alumina starts to crystallize into δ-form at 900 °C and is converted to α-form at 1250 °C. Pressed pellets sintered at 1400 °C achieve densities close to theoretical in 4 h. Microstructures show evidence of segregation and coarsening in a composite matrix of alumina and zirconia mixed at a submicron level.

Nano Alumina: A Review of the Powder Synthesis Method

Nanocrystalline ceramic materials are those with size smaller than 100 nm which have great importance in the field of nanotechnology. Nano-sized materials have properties superior to bulk materials, including improved surface area-to-volume ratio and high strength and toughness. This review paper presents an outline of the preparation of nano alumina by different methods such as sol-gel, combustion, precipitation, hydrothermal and leaching from kaolin. Nano alumina has a broad range of applications, so development of cost-effective processing routes for synthesis of nano alumina is the most prominent industrial challenge. These methods highlight the benefit of controlled particle size distribution with less agglomeration, improved morphology and controlled generation of pure nano alumina phase during the synthesis process. More research should be carried out to discover processing conditions that advance these goals.