Consolidation processing

Solidification and deformation processes are very seldom used to fabricate bulk articles from ceramics and other materials with low ductility and malleability. These substances are brittle and suffer fracmre before the onset of plastic deformation. Additionally, ceramics normally have exceedingly high melting points, decompose, or react with most cm-cible materials at their melting temperatures. Many ceramics are worked with in powder form since the products of most solid-state chemical syntheses are powders. Fabricating a bulk part from a powder requires a consolidation process, usually compaction followed 

---

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Solid-state NMR spectroscopy was used for studying the formation of cubic mesoporous aluminophosphate thin films and powders. The analysis of the initial gel, the as-deposited materials and the thermally-treated materials elucidated the changes in the coordination of phosphorus and aluminium atoms and thus revealed how the framework formation and condensation proceeds. The consolidation process in thin films was different than the process in powders. Most probably this could be attributed to the effect of glass substrate. 

It is a matter of debate whether memories can be erased at all (Jacobs and Nadel 1985). However, there is some evidence from animal experiments that processes that lead to learning or LTP-induced changes in interneuronal communication can be reversed by specific opponents of the consolidation cascade, as for instance phosphatases (Genoux et al. 2002 Lin et al. 2003a,b). The decay of both LTP and memory seems to depend on NMDA receptors. However, there are conflicting data as to whether recurrent activation of NMDA receptors promotes decay or long-term consolidation processes (Shimizu et al. 2000 Villarreal et al. 2002). 

Most authors who have studied the consolidation process of solids in compression use the basic model of a porous medium having point contacts which yield a general equation of the mass-and-momentum balances. This must be supplemented by a model describing filtration and deformation properties. Probably the best model to date (ca 1996) uses two parameters to define characteristic behavior of suspensions (9). This model can be potentially applied to sedimentation, thickening, cake filtration, and expression. 

On-line consolidation techniques are commonly used in manufacturing processes, such as filament winding and tape laying. Despite differences in the machines used to implement these two manufacturing processes, similar procedures are required to ensure complete bonding between the composite layers. The basic components of on-line consolidation process are illustrated in Figure 7.2. 

Figure 7.2 Schematic diagram of the on-line consolidation process...

The process by which a thermoplastic matrix composite consolidates to form a laminated structure has been attributed to autohesive bond formation at the ply interfaces. Autohesive bond formation is controlled by two mechanisms (1) intimate contact at the ply interfaces, and (2) diffusion of the polymer chains across the interface (healing). The rate of autohesive bond formation and hence the speed of the composite consolidation process is directly related to the temperature-pressure-time processing cycle. 

Consolidation and development of interlaminar bond strength for thermoplastic matrix composites have been modeled by two mechanisms intimate contact and autohesion. Intimate contact describes the process by which two irregular ply surfaces become smooth (Fig. 13.10). In areas in which the ply surfaces are in contact, autohesion occurs, and the long thermoplastic polymer chains diffuse across the ply boundaries. Filament winding with thermoplastic matrix materials is considered an on-line consolidation process in that local... 

As is the case for Dic, complete autohesion and complete bonding correspond to Dau = 1 and Db = 1, respectively. To account for nonisothermal autohesion, which occurs in on-line consolidation processes such as filament winding, the degree of bonding must be calculated at discrete time steps and summed [21]. 

Stress-strain type equations have been developed for the compaction process, which help provide an understanding of the mechanisms involved in forming a tablet, as well as allowing for the prediction of compaction results. This predictive power of the compaction process is the basis for many scale-up approaches. However, there are compression and consolidation process aspects which are dependent on manufacturing scale, e.g., speed-sensitive materials, and this results in many problems encountered in transferring a technology to production scale. Unfortunately, these scale-sensitive processes have not been as extensively studied, and are less understood. 

This technique of post-trial manipulation of the modulation by DA of memory consolidation processes has now been extended to forms of memory mediated by other terminal domains. Packard and White (1991) showed that post-trial administration of d-amphetamine, the D2/3 agonist quinpirole, or the D1 receptor agonist SKF-38393 to the caudate (but not the hippocampus) all enhanced subsequent retention of an appetitive win-stay task carried out in a radial maze, whereas similar administrations to the hippocampus (but not the caudate) enhanced learning of a win-shift procedure in the same apparatus. These effects seem very difficult to explain simply in terms of general performance-altering effects of the drug. 

---