Systematic study on densification of alumina fine powder during milliwave sintering at 28 GHz

Abstract

Pure ultrafine alumina powder (0.2 m average grain size (GS)) was sintered with a 28 GHz millimeterwave radiation source by systematically varying sintering temperature (ST), holding time (HT), and heating rate (HR). Densification, microstructure and grain growth effects were analyzed after millimeterwave sintering (MMWS). Alumina compacts were densified under rapid heating at 1100 • C without any HT up to 96% TD by retaining sub-micrometric GS (∼0.4 m) and homogeneous microstructure. The ST was increased to 1200 • C (without HT, 50 • C/min HR) to achieve densification up to 98.6% TD, however, at the expense of an increased GS of ∼2 m. Microstructure was maintained uniform throughout the sample even under extremely fast firing conditions of 200 • C/min HR except for the case in which HT was changed. Remarkable grain growth was observed when either ST or HT increased above the reference values.

Refitting of Zirconia Toughening into Open-Cellular Alumina Foams by Infiltration with Zirconyl Nitrate

“…Eventually, an average coating thickness of 33 µm is calculated from the difference m inf − m th equivalent to a thickening of the strut diameter by 66 µm. A volumetric packing density of the alumina particles in the coating of 31% of the theoretical density of alumina has been assumed based on the alumina volume fraction of the initial infiltration dispersion [55,56]. However, from SEM micrographs of alumina-infiltrated foams an inhomogeneous coating with sub-micrometer Al 2 O 3 particles with a thickness in the range of 1 to 20 µm was observed.…”

Manufacturing of Large and Polished Ceramic Pistons by Cold Isostatic Pressing

“…In an automated atomizer unit (Niro Atomizer D200), the slurry was fed at 21.4 bars and dried at 280 °C. The density of the sludge was 1.95 g/cm 3 during the drying process and the morphology of the granules after spray drying is shown in Figure 3. After the spray-drying process, a polyoxyethylenebased additive (ZUSOPLAST O 59, Zschimmer & Schwarz Chemie GmbH.)…”

“…In the case of alumina-based advanced ceramics, especially where a particular spare part must fit precisely, the purity of the raw materials and the surface roughness of the finished product are particularly important. The surface finish of alumina ceramics can significantly affect their performance, especially in applications that require a low level of friction, tight tolerances and exceptional aesthetics [3]- [4]. Therefore, the development of effective polishing techniques for alumina ceramics has been extensively studied over the years.…”