Rheology of ceramic systems

The rheological eqiration of a flitid state hnks the deformation y with the shear stress T. The rheogram generally used to represent this eqiration of state is  

Viscosity, dynamic or apparent, (t ) is the ratio of the shear stress (t) on the shear rate (y), its unit is Pa s. Viscosity represents the flow resistance of a fluid. 

The respective influences, on the rheological behavior, of the ceramic and suspensive phase (liquid, organic phase) are often determined using relative viscosity  

Normal or Newtonian fluids exhibit a linear relation between the shear stress and the shear rate ( = Fy). Viscosity does not depend on the shear stress. The rheogram of a Newtonian fluid is a straight line passing through the origin and with a slope equal to the viscosity. Most ceramic systems are non-linear or abnormal. 

The apparent viscosity of a shear-thimiing flnid decreases with the shear. This frequent behavior in ceramics is due to the orientation of the particles, particularly anisotropic particles, and of the polymeric chains in the direction of the flow, which reduces flow resistance. This behavior is described by a power law  

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Moore, F. Rheology of Ceramic Systems, MacLa-ren and Sons London, 1965. 

Measurement of Flow Properties For the precise scientific measurement of viscosity and thixotropy in absolute units, see Rheology of Ceramic Systems . Here we shall restrict ourselves to a consideration of the torsion viscometer. 

Rheology of Ceramic Systems, F. Moore, Institute of Ceramics Textbook Series... 

Moore F (1965) Rheology of Ceramic systems. Mac Laren and Sons Ltd. London... 

At this voliime fraction, the viscosity diverges because the shear stress is now given by the particle-particle contact in the tightly packed structure. As a result, we obtain a fluid with visco-elastic properties similar to polymeric solids. In ceramic processing, we extrude and press these pastes into green shapes. As a result, the rheology of ceramic pastes is of importance. The rheology of very concentrated suspensions is not particularly well developed, with the exception of model systems of monodisperse spheres. This section first discusses visco-elastic fluids and second the visco-elastic properties of ceramic pastes of monodisperse spheres. The material on visco-elastic fluids draws heavily from the book Colloidal Dispersions by Russel, Saville, and Schowalter [31]. 

Steady state rheometers with rotational cylinders or cone/plate (see Figure 5.12) are the most widely used to characterize the rheological behavior of ceramic systems with the determination of rheograms x = f(/). They consist of shearing the fluid between a surface at rest and a mobile surface. The advantage of a cone/plate rheometer with a low-angle cone is that it leads to constant x and / parameters at... 

This section on concentrated suspensions discusses the rheological behavior of sj tems which are colloidally stable and colloidally unstable suspensions. For stable sj tems, the rheology of sterically stabilized and electrostatically stabilized systems wiU be considered. For sterically stabilized suspensions, a hard sphere (or hard particle) model has been successfid. Concentrated suspensions in some cases behave rheologically like concentrated polymer solutions. For this reason, a discussion of the viscosity of concentrated polymer solutions is discussed next before a discussion of concentrated ceramic suspensions. 

Using the rheology of the ceramic system, we can write in terms of the velocity gradient dvjdx. Then, by integration, the velocity profile can be obtained. These velocity profiles for Newtonian, Bingham plastic, and Crossian rheology follow. 

Rheological properties of ceramic dispersions and green structures are related to the interaction forces between the particles and with the microstructure of the material. Unfortunately, detailed quantitative interpretation in terms of microstructure is only possible for model systems of monodisperse particles of which the interaction energies are precisely known [14]. 

Satava, V, and Tyle, P. (1994) Rheological behaviour of the system Portland clinker— Na COj—sulfonated polyphenolate— H20 Ceramics-Silikaty (Prague) 38,9-15. 

It has been noted that many polymeric systems have the capacity to induce what is called self lubrication. As a consequence it has not been a major focus of research activity, compared to metallic and ceramic systems, to study the lubrication of polymers per se, but more often to consider the infiuence of environmental sensitivity. For convenience the two are dealt with together here. We may also note that polymers are often included into polar and apolar solvents to modify the rheological behaviour as is conunon practice in the formulation of viscosity improvers and the development of drag reducing agents. These processes may have implications in rationalisation of environmental sensitivity. 

The last part of the book deals with Hquids crystals and ceramic microelectronic devices. Chapter 12, by Mendoza et al. reviews recent theoretical results on the rheology of systems consisting of a flow-aligning nematic contained in cells and capillaries under a variety of different flow conditions and under the action of applied electric fields. Finally, chapter 13, by Alias and Shapee, stresses the impact of silver paste rheology in the fabrication of ceramic microelectronic devices (low temperature co-fired ceramic LTCC devices). 

Ikkahashi, M., Kihira, H., Suzuki, S., and Ishigure, Y., Rheology of oxide/resin systems for injection molding, Ceram. Trans. (Ceram. Powder Sd. IV), 22, 387-392 (1991). 

Litman, A.M., Schott, N.R. and Tozlowski, S.W., Rheological properties of highly filled polyolefin/ceramic systems suited to injection moulding. Soc. Plant. Eng. Tech. 22 549-551 (1976). 

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