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Graded intermetallic-ceramic multilayer composites by tape casting
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kodentsov, Alexander A. Snel, Marcel |
| Copyright Year | 2003 |
| Abstract | Regulations and safety concerns will force more and more productions plants to move away from organic solvents based processing to water based processing. This move is less obvious than it appears at first glance: organic solvents have distinctive advantages over water, in particular the evaporation rate of organic solvents is higher than of water and evaporation usually happens at lower temperatures, yielding a potentially higher production rate. Other important properties for tape casting are the usually lower contact angle of organic solvents and the absence of reactions between solvents and metal powders. Two methods are normally used to protect the metal powders from oxidation by water: the surface of the powder is chemically modified by a hydrophobic treatment, usually by long-chained organic molecules, or low concentrations of some weak acids is used, like H3PO4. In this work, the first method was pursued. To stabilize the powder a coating of stearic acid is applied to the powder surface to yield it hydrophobic and hence protect it from oxidation. Unfortunately, to apply the coating it is still necessary to process the powders in a organic solvent to be able to apply the hydrophobic coating before the water-based tape casting process can be used. Secondly, the hydrophobization process makes it harder to process the powder in water. In chapter 4 it was shown that it is possible to use this coating technique for mixtures of aluminium and alumina to produce, after sintering in air, alumina monoliths. It was shown by TGA/DSC that aluminium is not oxidized during the processing prior to the thermal treatments. The properties of the green tapes were good and allowed for successful lamination. Typically densities that were achieved were near 97 %, although an anomaly was found at 1550 ° C at which their is a decrease in density compared to 1500 ° C and 1600 ° C. The three point bending strength of the materials did increase with higher sintering temperatures. Anomalous grain growth due to the absence of enough magnesium accounts for this behaviour. The processing of nickel oxide-titanium-titania powders were based upon the results obtained for the aluminium-alumina mixtures for which the results were shown in chapter 5. It shows that the technique is versatile and can be applied to different systems. It also became clear that the specific amount of stearic acid is different for each system and has to be optimized; an excess of stearic acid will lead to the formation of agglomerates in the green tape. In this chapter it was also shown that tensile testing can be used to optimize the powder content. With increasing amount of powder, the maximum strain of the green tapes decrease as expected, however, the Young's modulus and the tensile strength of the green tape showed a maximum, where a continuous increase was expected. This maximum is the result of the formation of agglomerates at higher volume loadings which effectively act as voids in the green tape, resulting in a decrease in strength. Sintering was conducted at 1350 ° C and yielded materials a composite of NiTi2-Ni3Ti-TiO with ~5 % porosity. The microstructure, however, showed, that instead of the expected interpenetrating network of both ceramic and intermetallic phase, the ceramic phase forms a loosely connected network of ~ 25 µm grains, despite making up ~ 60 % of the microstructure. A mechanism for this resulting network was proposed: the initial TiO2 grains of about ??20 µm grains will form the final grains; after reaction at 750 ° C forming NiTi2, Ni3Ti and TiO, the NiTi2 will wet the TiO particles; at the sintering temperature of 1350 ° C the Ni3Ti will fill the voids between the TiO particles; at locations where the TiO is not wetted and in contact with each other, necking will occurs, in other locations the NiTi2 prevents the necking. Chapter 6 focuses on solving this problem by adding a dopant to the powder mixture. The dopants could have various results, of which the introduction of a mixed oxide phase with similar lattice as NiTi2 was the most common, followed by the introduction of a fourth phase, which did not resemble NiTi2. Only barium was found to influence the microstructure to a great extent, however, the resulting samples were mechanically weak and powder-like. Only aluminium was found to increase the interconnectivity by a certain degree, although it did not result in a truly (random) interpenetrating ceramic-intermetallic network. Further possibilities to promote the formation of a ceramic network by altering the processing is investigated in chapter 7. It is found that increasing the aluminium content increases the formation of a network, but also increases the porosity of the sample, possibly due to the higher sintering temperatures needed for the aluminium phases. Varying the sintering profile using slower heating rates around the reaction temperature did not influence the microstructure; increasing the holding time resulted in larger grains of TiO but did not promote the formation of a network. Decreasing the initial particle size of titanium oxide, by either using more milling cycles, or by using smaller starting powders, did not increase the interconnectivity, but resulted in higher porosity, due to more difficult processing of smaller powders, and the resulting increase in the viscosity of the slurry. Chapter 8 focused on the mechanical properties of (Al-)Ni-Ti-O composites. There is no significant difference between the Ni-Ti-O composites and the 5% Al-Ni-Ti-O composites; both have a 3pb strength of 120 MPa. 20% Al-Ni-Ti-O composites have a significantly lower 3pb strength, due to a large reduction in the amount of intermetallic phases, resulting in a porous ceramic network. For the undoped composites, multicompositions revealed no increase in the 3pb strength due to difference in thermal expansion coefficient. For 5% Al-Ni-Ti-O composites an effect was observed for composition richer in ceramic content; if the compositions were richer in intermetallics no effect of the layering was observed. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://pure.tue.nl/ws/files/3307013/200911481.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
