Novel Ceramic Processing Methods Using Responsive Polymers

Abstract

Optimal ceramic powder processing methods have to fulfil many requirements. Some of these requirements are obvious - like high reliability and good performance of the sintered material - but there is also a strong need of simple processing concepts which can lower the manufacturing cost, e.g. near net-shape, high flexibility, few process steps and water-based methods [1]. The high cost of manufactured parts is today probably the limiting factor of a large scale commercial implementat on of high performance ceramics. Many different approaches are being taken in this continuing effort of developing more robust and cost-effective ceramic processing methods. An important feature of optimal processing methods is versatility and high flexibility; it should be possible to exert a precise control of important suspension parameters like colloidal stability (degree of flocculation), rheology, phase separation and suspension microstructure and permeability. Depending on the forming method, the ideal conditions might vary drastically during the different processing steps [2]. For example, mixing of multi-component powder systems and removal of large, hard aggregates are best performed in well dispersed, low viscosity suspensions. Tape casting also requires stable suspensions while it may be beneficial to use weakly flocculated suspensions in slip casting and pressure filtration. Several alternative forming methods, e.g. freeze forming (Quickset) 131, get casting 141, direct coagulation casting (DCC) 151, and temperature induced flocculation (TIF) 161, require a well dispersed, low viscosity suspension of high concentration which should be transformed to a stiff green body in the mould. The details of these complex systems are starting to be unravelled with an improved knowledge of the fundamental forces and physical parameters acting in these systems, e.g. interparticle forces, dispersant interactions, solvent effects. This knowledge is being used in the design and development of novel processing methods, e.g. DCC 151 and vibraforming of salt induced flocculated suspensions [7]. Research is also being devoted to the development of ceramic processing systems with better suspension control using different additives. In this extended abstract, the characteristics of an important class of additives -responsive polymers- will be described and the current and future potential regarding their use in ceramic processing will be discussed. Responsive polymers - which are characterised by a reversible response to an external stimuli, such as temperature, pH and light -are the focus of considerable research efforts in many areas, e.g. controlled drug delivery, recycling, paints and coatings, and the potential in ceramic applications is high. Responsive polymers undergo conformational and phase changes in response to a change in e.g. temperature. Typically, the response is characterised by a transition from a polymer solution to some type of aggregated state or get structure. A polymer get is a fluid-containing, self-supporting system which implies some type of connectivity. This is also true for an aggregated or precipitated polymer solution but the concentration may be too low for a cell-spanning get to be formed. Hence, the responsive function can be identified with the breaking and forming ofjunctions between the polymer molecules. Now, there are many types of processes that can create junctions and thus gels, e.g.