please I need this answer today it is urgent Below is a schematic of glassy phas
ID: 1082185 • Letter: P
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please I need this answer today it is urgent
Below is a schematic of glassy phase wetting the grain boundaries of a ceramic in various ways (o is the wetting angle) at triple junctions or quadruple junctions where 3 or 4 grains come together. In one extreme case, a continuous glassy intergranular film (IGF) forms along the boundaries, while in the other extreme case, the glassy phase is confined to the junctions. Explain whether you expect the properties of these ceramics (2 extremes) to.be different, and if so why and how. (5) 7. -13* dishedExplanation / Answer
In one case, a continuous intergranular glass film(IGF) forms along the bondary, while in the other, glassy phase is confined to the junctions. This difference in the structure, location and magnitude of IGF will cause the two ceramics to have different properties. We will first analyse the IGF and find out the parameters which will vary the structure, wetting angle and other variables regarding the ceramic structure. After that, we can compare the two extremes.
Consider the force balance on an intergranular liquid glass film that is stable at high temperature. The force acting to draw the two adjacent grains closer together and thin the film is the capillarity force due to the concave curvature at the three and four-grain junctions. At equilibrium this force is balanced by an equal force, which may be termed the disjoining pressure.
This force, which is everywhere normal to the glass phase, is in reality a combiqation of forces given by summation of the following forces :
As the material of the two grains is assumed to be the same the dispersion force contribution to the disjoining pressure will always be negative (meaning an attractive force). In the absence of any information the contribution from any electrical double layer overlap is assumed to be zero leaving only the adsorption and structural components to contribute to a positive disjoining pressure.
Large distances between the grains the degree of conformation will be small, but as the distance is decreased it will necessarily become greater. When the distance is reduced to values only a few times the size of the basic tetrahedhral units the conformation will become very severe, and distortion of the ceramic units will occur in trying to match the ordered monolayers on both grains simultaneously. This distortional energy will be manifest as a positive structural contribution to the disjoining pressure, and is responsible for the observed stability and thickness constancy of the intergranular phase in hot-pressed ceramics.
The structural contribution to the disjoining pressure also allows a number of further predictions to be made:
Firstly, there should exist a preferred orientation to the ceramic tetrahedra on the surfaces of the crystalline grains. Secondly, the glass phase should exhibit an orientational anisotropy that becomes more marked the thinner is the intergranular film. The deviation from bulk viscosity is expected to also become more marked in the same way. Thirdly, if the structural contribution to the disjoining pressure is dominant then intergranular films of constant thickness will only exist in ceramics in which there is a similarity in structure between the major phase and the intergranular phase.
Low angle grain boundaries, with angles up to C, will be free of glass whereas all high angle boundaries with the exception of deep "cusp" orientations, will contain an intergranular glass phase.
Grain boundaries are known to have a significant effect on the transport properties of ceramics. Grain boundaries with a reduced concentration of free electrons raise the total resistivity of the material. In nanocrystalline ceramics, where the grain size does not exceed the Debye screening length, the size effect may increase the ionic conductivity. The presence of continuous liquid channels along grain boundaries ensures high diffusional permeability of the material. In some cases, a solid phase may spontaneously disperse in a liquid to form a diphasic, thermodynamically stable system.
Let us consider the effect of various stimuli on the IGF:
These results will manifest in the two extremes in different ways. All the effects considered above will be more pronounced in the continuous IGF, while the isolated glass phase IGF ceramic will exhibit the mentioned properties but their extent will be limited due to lack of a continuous boundary.
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