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Economic industrial spray drying of ceramic slurries aims for as high as possible solids content. Investigated slurries of up to 80 wt% solids content were analyzed regarding stability while staying processable for granule production via spray drying.
Preliminary stability examinations were carried out on the one hand via zeta potential measurements and on the other hand by optical centrifuge analysis for determination of suitable additive type, quantity and composition while even allowing the detection of potential side effects. The processability of the slurry for spraying has primarily been quantified by viscosity measurements.
Early spray dried granules turned out to have internal voids and/or hard shells leading to defective sinter bodies and low density. Focusing on the root of these voids, the “hollow hard granules”, a controlled destabilization and flocculation was initiated by weakening electrostatic repulsion and approaching the isoelectric point. Destabilization, quantifiable by optical centrifugation, leaded to a change in speed of clarification as well as packing density, influencing movement speed of the phase boundary and the final height of the sediment, respectively. For sufficient destabilization, the solids content needed to be reduced in order to keep the viscosity suitable for the following spray drying procedure.
The versatile controlled destabilization of the ceramic slurry finally leaded to a significantly reduced fraction of hollow granules featuring a sinter body of higher density with smaller pores and a narrower pore size distribution, additionally this destabilization approach has shown to be transferrable with excellent results to zirconia and even ZTA (zirconia toughened alumina) composite materials.
Generation of ceramic green bodies in the additive manufacturing by Laser Induced Slip-casting (LIS)
(2016)
For the additive manufacture of large components usually powder-based methods are used. A powder is deposited layer wise by a recoater, then, the component structure is printed to the powder bed or sintered by a laser. In slurry based methods, the slurry is deposited by a doctor blade and dried before the printing of binder or the laser treatment. The new method of laser-induced slip casting is also a slurry-based method and the layers deposited sequential. However the slip is not dried and the structure is written directly by a laser into the suspension. The wall thickness of the ceramic material can be adjusted by the laser spot size and treatment time. The water is evaporated by the laser and a green body is formed locally. Because of its porosity, water is taken from the surrounding suspension and the wall thickness of the green body increases with treatment time. Due to the use of highly filled suspensions, the green body is stable in the ceramic slurry. Large green bodies can be built which have no visible layers in the microstructure.
Generation of ceramic green bodies in the additive manufacturing by Laser Induced Slip-casting (LIS)
(2016)
For the additive manufacture of large components usually powder-based methods are used. A powder is deposited layer wise by a recoater, then, the component structure is printed to the powder bed or sintered by a laser. In slurry based methods, the slurry is deposited by a doctor blade and dried before the printing of binder or the laser treatment. The new method of laser-induced slip casting is also a slurry-based method and the layers deposited sequential. However the slip is not dried and the structure is written directly by a laser into the suspension. The wall thickness of the ceramic material can be adjusted by the laser spot size and treatment time. The water is evaporated by the laser and a green body is formed locally. Because of its porosity, water is taken from the surrounding suspension and the wall thickness of the green body increases with treatment time. Due to the use of highly filled suspensions, the green body is stable in the ceramic slurry. Large green bodies can be built which have no visible layers in the microstructure.
The ProboStat is a multi-purpose measurement cell suitable for various electrical and physical measurements under different atmospheres and at high temperatures. Disc and bar shaped samples are sandwiched between platinum electrodes at the top of the tubular cell. The gas tight assembly can be inserted into a furnace. Different gases can be flushed through the tube. For this study, a ProboStat was adapted to measure volume resistivity of ceramic insulators at high temperatures according to standards.
The standardized measurement of volume resistivity of ceramic insulators requires the consideration of many specifications including sample diameter, thickness, electrode design, and the proportion of these characteristics. Measurements are ideally performed in a state of dielectric equilibrium. The time-related slope of resistivity of a specific sample follows a power function. Thus, care must be taken when choosing a charge time or defining the duration of a measurement. As fringing of the guarded electrode occurs under high voltage, the effective electrode area for evaluation of the results should be corrected with respect to sample thickness and electrode design. The demands of effective standards on sample geometry and electrode design are stricter for room temperature measurements than for high temperature measurements.
To perform high temperature measurements on ceramic samples that also fulfill the demands on room temperature measurements, a ProboStat was equipped with a dedicated large sample setup for discs with diameters of up to 60 mm. The volume resistivity of different alumina samples was first measured at room temperature in a standard test fixture and then compared to results obtained with the ProboStat. All measurements were performed for at least 100 min using a 26 mm guarded electrode. High temperature measurements at 500 °C were performed using the same samples. Room temperature values obtained with the standard test fixture are in the order of 10^17 Ohm·cm. The quantitative effect of electrode area correction is presented. Practical issues related to the use of the multi-purpose cell are addressed. These include electrode material selection, application of electrodes, and compensation of leakage currents. High temperature results of volume resistivity of the different alumina samples are presented. The validity is discussed with respect to the suitability of the multi-purpose cell for such measurements.