Ceramic Paste Rheology

In Chapter 12 of this book, the mechanical properties of ceramic suspensions, pastes, and diy ceramic powders are discussed. Ceramic suspension rheology is dependent on the viscosity of the solvent with polymeric additives, particle volume fraction, particle size distribution, particle morphology, and interparticle interaction energy. The interparticle forces play a veiy important role in determining the colloidal stability of the suspension. If a suspension... 

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]. 

In some cases, an extrudable and injectable paste may consist of 65% vol. ceramic powder and 35% vol. polymeric binder. In others, an extrudable paste may consist of a highly loaded aqueous suspension of clay particles such that its rheology is plastic. Hie low shear (i.e., <100 sec ) viscosity of such a paste is between 2000 and 5000 poise at ambient temperature. Highly nonlinear stress strain curves are typical of ceramic pastes, as well as time dependent thixotropy. In many cases, pastes behave like visco-elastic fluids. This complex rheological behavior of ceramic pastes has made theoretical approadies to these problems difficult. For this reason, the discussion in this chapter is limited to Newtonian fluids where analytical solutions are possible, with obvious consequences as to accuracy of these equations for non-Newtonian ceramic pastes. 

Mazeo FA (2001) Extrusion and rheology of fine particulated ceramic pastes. Dissertation, Rutgers University New Jersey... 

Graczyk J, GleiBle W (1992) The rheometric characterization of ceramic pastes for catalysts. In Theoretical and Applied Rheology, Proc Xith Int Congress on Rheology, Bruessels, pp 601-603... 

The most important aspects of ceramic pastes will be addressed in this chapter the rheological properties, the handling and delivery, the hardening (for cements), the mechanical properties, the biological properties, and finally some... 

Morissette, S.L., Lewis, J.A., Clem, P.G., Cesarano III, J. Dimos, D.B. (2001). Direct-Write Fabrication of Pb(Nb,Zr,Ti)03 Devices Influence of Paste Rheology on Print Morphology and Component Properties. Journal of the American Ceramic Society, Vol. 84, No. 11, pp. 2462-2468. 

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). 

These relate mainly to the degree of purity of the powders and the nature of the particles surface. Purity depends on the source of the raw materials and the transformation processes that give rise to impurities (iron, heavy metals, salts, carbon, etc.). It will determine to a large extent the sintering reactivity, with the possible formation of a second intergranular phase and the final properties of the piece (mechanical, chemical, electric, etc.). The surface properties of the particles determine the mechanisms of species adsorption and dissolution. They will control the dispersion properties, homogenity and the rheological behavior of the suspensions and ceramic pastes (section 5.3). 

In ceramics, the term plastic mass or paste denotes an easily workable mixture which keeps its shape after forming. Up to a certain stress, such a mixture behaves as a solid and exhibits approximately elastic behaviour. It is irreversibly deformed only beyond a critical stress called the yield point. Beyond the yield point its behaviour can be characterized by the respective deformation rate from which the so-called plastic viscosity can be determined. The rheological behaviour involved is essentially of the... 

Macromolecules are very important in the processing of powders into porous support layers. They are used to ensure the colloidal stability of ceramic suspensions, to control the rheology of suspensions and pastes, to avoid cracking... 

Chandler HW, George SD, Liddle J (2002) Deformation and flow of stiff pastes review of rheology of some soft solids. J Br Ceram Trans vol 101 No 2 pp 47-57... 

The second rheological property that should be carefully considered while designing a ceramic bone substitute is paste cohesion (= cohesiveness, nondecay ). Specifically, it is the ability of the paste to keep its geometrical integrity in an aqueous solution. For a cement, poor cohesion may prevent setting and may lead to negative in vivo reactions due to the release of microparticles. ... 

The organic medium serves as a vehicle for dispersion of the finely divided solids in such form that the composition can readily be applied to a ceramic or other substrate base. Thus, the organic medium must primarily be one in which the solids are dispersible with an adequate degree of wettability and the organic materials are soluble. Secondly, the rheological paste properties must be such that they lend good application properties to the dispersion. 

Phair, J.W. Kaiser, F.-J. (2009). Determination and Assessment of the Rheological Properties of Pastes for Screen Printing Ceramics, Annual Transactions of the Nordic Rheology Society, Vol. 17, Iceland, August 2009. 

Each mineral raw material has a specific influence on the rheology of the paste, the development of the microstmctme, the phases formation during the heat treatment and the properties of the finished product. The manufacture of all silicate ceramics requires such a large number of raw materials, which cannot be discussed here. Only those most commonly used, i.e. clays, feldspars and silica, will therefore be described. 

---