Transparent Polycrystalline Alumina Ceramic with Sub-Micrometre Microstructure by Means of Electrophoretic Deposition

Abstract

The optical quality attainable in coarse-grained polycrystalline alumina is severely limited by grain-boundary scattering, which is inherent to non-cubic materials. The optical properties of sub-micrometre polycrystalline alumina are of growing interest triggered by the fact that a decrease in the grain sizes of the final sintered material yields an improvement in the optical quality while the scattering mechanism changes as the grain size becomes comparable with the wavelength of light. To achieve transparent alumina ceramics with a fine-grained microstructure, however, porosity and other defects must be avoided. This necessitates the optimization of processing and sintering procedures. Electrophoretic deposition (EPD) is a colloidal process in which ceramic bodies are directly shaped from a stable suspension by application of an electric field. Electrophoretic deposition enables the formation of homogeneous, uniform green microstructures with high density, which can be sintered to transparency. It is a simple and precise technique to synthesize not only monoliths, but also composites with complex geometries [1]. Alumina green bodies were deposited from stabilized aqueous suspensions with and without doping. Green alumina compacts were evaluated based on their pore size distribution and density. Densification behaviour was characterized by dilatometric studies conducted at constant heating rate. Samples were sintered at different temperatures with subsequent post-densification by hot isostatic pressing. Transparency was evaluated by means of spectroscopic measurements. The measured in-line transmission of the samples at 645 nm was more than 50 % and that is 58 % of the value of sapphire. The influence of dopings on transparency was investigated. The mechanical properties of the samples were tested.

Preparation of High-Purity Glasses and Advanced Ceramics Via EPD of Nanopowders

“…6.28. These powders have to be perfectly dispersed to get an optimum green density and additives like MgO or ZrO 2 have to be added to control the grain growth during sintering [102]. The complexity of this optimizing process can be seen in Fig.…”

“…Therefore EPD seems to be a very promising shaping method especially for nanopowders or bimodal powder mixtures with nanopowders [10,11]. Furthermore it could be proofed that the highest green densities could be obtained with EPD [17][18][19]. This is of high importance for the preparation of optoceramics.…”

The effect of the particle size and the morphology of alumina powders on the processing of green bodies by electrophoretic deposition

“…Although water is environmentally and economically more acceptable, as well as having a high dielectric constant, there remains the problem of porosity in the deposits due to the formation of gas during the electrolysis of water (Anné et al, 2006;Uchikoshi and Sakka, 2008). In the case of simpleshaped components, however, the porosity of the deposits can be avoided by the use of a membrane in front of the deposition electrode (Clasen and Tabellion, 2003;Braun et al, 2006;Novak and König, 2009).…”

Freeze granulation: Powder processing for transparent alumina applications

“…3 In recent years, many advances have been made in production of transparent ceramics by various processing and sintering strategies. 1,[6][7][8][9][10][11][12][13][14] Mainstream ceramic processing techniques rely either on dry 11,14,15 or wet powder processing [1,6,9,13,16]-both allowing production of highly dense green bodies (GBs), one key factor for transparent ceramics-as well as on classic sintering followed by hot isostatic pressing (post-HIP) or novel fast sintering techniques. Among the latter, the most popular for transparent ceramics is pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS).…”