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Promoting the infrastructure - prestandardisation endeavours for ceramic powders and green bodies
(2002)
Tribology of Advanced Ceramics III: Principles of solid lubrication - materials and mechanisms
(2002)
The gelcasting process was used for manufacturing porous ceramic bodies from nano sized powders. 3 different powders with specific surface areas between 12 and 30 m²/g were used. Solid content of the corresponding slurries was dependent on the milling procedure and the use of a suitable dispersant. For bimodal GradeA powder solid content was as high as 58 vol%. For sintering temperatures between 1250 and 1600°C the porosity varied between 45 and 20%, whereas the pore radii varied between 50 and 91nm. The grain size of crystallites in the sintered microstructures was about 1um. The reflection ratio for visible light increased in comparison to a reference sample made from conventional powder from 0.8 to 0.95 in the wavelenght range around 400nm.
The interaction of nanosecond laser pulses in the ultraviolet wavelength range and femtosecond laser pulses in the near-infrared region with the direct semiconductor SiC and the composite compound SiC-TiC-TiB2 was investigated in respect to resulting physicochemical processes. Surface analytical techniques, such as XPS, µ-Raman, and EDX were used to identify the chemical changes between untreated and laser-treated areas. Single-pulse irradiation led to material
modifications in the condensed state in most instances. Multi-pulse results differed depending on the pulse duration. In the nanosecond case, melting of the surface and redeposited material (debris) were observed. With femtosecond pulses instead, only negligible melting and few debris could be detected. Additionally, periodic structures (ripples) appeared, with parallel orientation and in most cases two different periodicities for a particular material. Laser ablation of all the studied compound materials exhibited preferential ablation of the lighter component elements. Crystal structure changes of the irradiated surface were observed as a consequence of laser-induced melting and resolidification. An oxidation process interfered with all this processes so that unstoichiometric oxides were formed.
The isoelectric point is a material property for the characterization of acid-base behavior of the powder surface. The IEP of two alumina powders (AKP-30, Grade A) with different particle size characteristics in aqueous suspension has been determined by colloid vibration current (CVI) and particle charge detection (PCD) methods in moderately concentrated suspensions in order to qualify these methods for a future standardization. The solid loading of the suspensions were 1 and 5 vol%. The comparison of the IEP determined by PCD and CVI was in good agreement. However, the solid loading dependence of IEP and a large IEP hysteresis (at 1 vol%) were observed, which do not allow a proper determination of the IEP. The recording of the potential curve and therfore the determination of the IEP with CVI was difficult for the suspension of Grade A powder due to formation of agglomerates at pH values near the pHIEP.
Zeta potential as measured with the electrokinetic sonic amplitude (ESA) method has been compared to streaming potential as measured with the particle charge detection (PCD) method. The measurements have been carried out on identical suspensions of a-Al2O3, SiC and Si3N4 with a solids volume fraction of 1% and 10%. Zeta potential and streaming potential are related by a linear function. The correlation coefficients for this relationship are better than 0.99 for the suspensions with 1% solids loading. For the suspensions with 10% solids loading, correlation coefficients are slightly lower due to the influence of increasing viscosity. Regardless of lower correlation at the higher solids loading, the isoelectric pH is constant within the limits of accuracy of the pH measurements.