Papers by Keyword: Constrained Sintering

Abstract: The sintering of thin films is widely used for surface coatings and because of its technological importance has generated extensive research interest. During the sintering process, the thin film is constrained by the substrate, which generates considerably high levels of stresses. Crackings are often observed and are considered as one of the major problems of the surface coating technique. This paper has proposed a new numerical method in order to tackle the traditional difficulties in simulating multi-crackings during constrained sintering. Main features of the present method include: (i) the material data is represented by an anisotropic constitutive law, (ii) a new numerical scheme is developed for the crack initialization and growth based on the material point method, (iii) the 3D viscous film shrinkage model is solved by using a dynamic FE scheme, and (iv) the random nature of the initial green body density is represented by statistical variabilities. It is shown that the model proposed by the present paper is capable for the nucleation and propagation of multi-cracks in a straightforward manner. Cracking patterns are shown to be consistent with experimental understandings. The focus of the paper is on the numerical issues and demonstrating the capacity of the model.

Abstract: The constrained sintering behavior of yttria stabilized zirconia coatings containing 1 wt. % alumina produced using electrophoretic deposition has been studied. Coatings have been characterized by scanning electron microscopy, micro-hardness and fluorescence spectroscopy after sintering in air at 1100°C and 1200°C. These coatings were compared with those that additionally contained 1 wt. % soda-lime glass. Micro-hardness and fluorescence spectroscopy are shown to accurately monitor the density of the coatings. Fluorescence spectroscopy showed that the density across the coating is even, as expected from modelling literature. The addition of 1 wt. % of soda-lime glass is shown to improve the sintered density of coatings.

Abstract: The synthesis of multilayer membranes for solid oxide fuel cells by means of co-firing requires choice of compatible materials as well as a precise balance between densification rates of the various layers throughout the densification process. One attractive practical objective is the formation of dense electrolyte films on invariant, preformed electrode substrates, such as porous electrodes of either ceramic or metal conductors. The formation of dense 10-20 micrometer thick Ceria-based membranes on invariant substrates has been achieved by adjusting of the film sintering properties trough selection of particle size, initial density, and novel sintering additives. The accompanying theory pertaining to constrained sintering is examined, to determine the conditions under which fully dense electrolyte layers can be produced on invariant or minimally variant substrates.

Abstract: When structures that consist of powders of two or more materials, such as low temperature co-fired ceramic packages or a planar solid oxide fuel cells, are sintered, the mismatch in the sintering shrinkage rates between the different materials produces stress, since the faster shrinking materials are constrained by those that shrink at a slower rate. These stresses can lead to the formation of defects such as cracks or shape distortion. Results of recent model experiments to study the constrained sintering in multi-material systems will be discussed for two model geometries. The first geometry is a simple bi-layer consisting of single layers of two different materials bonded together. Results of in situ observation of sintering bi-layers will be presented and discussed in relation to the properties of the individual layers such as their free sintering rates, uniaxial viscosities and viscous Poisson’s ratio. The second geometry studied was that of a ring of one material filled with a cylinder of a second, slower shrinking material. In this case, the results of several variations of this geometry including filling the ring either fully or partly with a rigid, nonshrinking cylinder or with rigid, non-cylindrical shapes will be presented and discussed.

Abstract: Co-firing of multiple materials results in the formation of internal stress due to the difference in shrinkage behavior. This internal stress causes retardation of sintering, crack formation and/or de-lamination at the interface. To reduce internal stress in layered structures, homogeneous pressing via centrifuge has been attempted. The effect of centrifugal pressing was demonstrated by film sintering, in which crack suppression and void removal by the present process were revealed. Owing to the homogeneous microstructure, the thermal stability of the film was significantly improved.