This paper investigates sintering and phase transitions of La 0.7 Ca x CrO 3 (0.25 р x р 0.35), a material useful as electrical interconnections in solid oxide fuel cells. Heating of the quenched, metastable single-phase chromite resulted in exsolution of CaCrO 4 due to Ca solubility limitations below 1200°C. A transient liquid phase formed between 850°and 1000°C as the CaCrO 4 melted, causing partial densification in materials having 0.25 < x < 0.30. A slight increase in Ca content induced an additional liquid-phase sintering event, likely due to melting of Ca 3 (CrO 4 ) 2 , which facilitated nearcomplete densification by 1250°C. After enhancing sintering, the secondary phases redissolved into the chromite.
polyacrylamide was reduced to about half that of the polyacrylic acid in order to allow it to take up more of the water displaced from the polyacrylic acid. This is not yet the ideal muscle system. The current drives the shape change through a pH gradient which arises from electrolysis of water, which cannot be reversible in a closed system. The gas formed also causes instabilities in our system. Other workers have experimented with the release of calcium ions from polypyrrole as an alternative system for swelling and deswelling the gels without electrolysis of water. [10] The gels are also mechanically weak, but much stronger gels can be made by combining polyvinylalcohol with polyacrylic acid. [11] As outlined above, an ideal muscle should show an overall expansion or contraction along one axis in response to an applied field, with no overall volume change. This cannot be achieved with a single centrosymmetric material but is possible with an asymmetric series of layers. We have shown that stacked combinations of gels can be made to give linear contraction in response to applied electric field. In essence the asymmetry of gel stack balances the asymmetry of the applied field to allow a symmetric response. Other swollen polymer combinations should be capable of similar responses.
ExperimentalGels were made from solutions of acrylamide or acrylic acid at 200± 400 g/L of water. The cross-linking agent was methylenebisacrylamide at levels from 20±50 g/L. For the acrylamide, the initiator was potassium persulfate, 0.25 g/L and the accelerator was tetramethylethylenediamine (TEMED) 3.6 g/L. For the acrylic acid, the catalyst system was a potassium persulfate (1.14 g)-potassium metabisulfite (0.94 g) redox couple. Fumed silica (210 g/L) was added as a physical gelling agent and the pH adjusted to 3.8±4.0.Gel stacks were made by extrusion freeform fabrication. Acrylic monomer, water and catalyst is extruded from a syringe through a fine (0.2±1 mm diameter) nozzle which writes on a heated substrate. The composition of the gel can be changed by swapping syringes or by using a Y-junction between two syringe feeds. To prevent the monomer solution from flowing off the substrate before curing, 8 wt.-% of fumed silica is added to the solution, to give it a toothpaste-like rheology. The solution cures to cross-linked gel by thermally activated free radical polymerization within a few minutes of being deposited on the plate. 6-layer stacks were formed into bars 5 cḿ 0.5 cm (wide)´0.4 cm (thick), before swelling. A typical stack would have a polyacrylic acid face with 1±5 (x) layers and a polyacrylamide face of (6±x) layers with a combination of dense and open-mesh layers (written as a series of spaced lines). When used, wire electrodes were usually placed between layers 1 and 2, and between 5 and 6.This system allows interdiffusion and bonding between layers, which provides good stress transfer during the swelling processes. The early onset of gelation and the physical gelling agent prevent extensive mixing between layers. This...
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