Microsphere sintering, porous

In microsphere sintering, pre-synthesized polymer microspheres or polymer/ceramic/bioactive addiction composites are sintered to produce a 3-D porous scaffold (56). Bioactive scaffolds can be fabricated through this technique, and they are demonstrated to be supportive to human osteoblast-like cells adhesion, growth, and mineralization (57). Scaffolds fabricated through this technique can have graded porosity structures. Mechanical properties close to cancellous bone also become possible when the microspheres are sintered into... 

A newer and less expensive alternative to the microchannel plate is the microsphere plate (MSP). As illustrated in Figure 3.6, this electron multiplier consists of glass beads with diameters from 20 to 100 pm that are sintered to form a thin plate with a thickness of 0.7 mm. This plate is porous with irregularly shaped channels between the planar faces. The surfaces of the beads are covered with an electron emissive material and the two sides of the plate are coated to make them conductive. The operating principle of this electron multiplier is similar to that of the microchannel plate. A potential difference of between 1.5 and 3.5 kV is applied across the plate, with the output side of the plate at the more positive potential. When particles hit the input side of the microsphere plate, they produce secondary electrons. These electrons are then accelerated by the electric field through the porous plate and collide with other beads. Secondary electron multiplication in the gaps occurs and finally a large number of secondary electrons are emitted from the output side of the plate. 

The small particle size of the silica is important not only in enabling the silica to flow to the peripheral region of the porous microsphere but also in forming the hard peripheral oxide-rich shell. Particles of silica 2-3 nm in diameter sinter together to some extent even under the temperature conditions encountered in a conventional spray drying process, whereas particles 10-100 nm do not sinter below 700-1000°C. As a result, attrition resistance of the catalyst, catalyst precursor or catalyst support particle is a function of the particle size and degree of aggreggation of the silica formed by dehydration. 

FIGURE 5.4 Different types of porous chitosan scaffolds fabricated by (a) thermally induced phase separation [129], (b) dense CO gas foaming [132], (c) sintering chitosan microspheres, and (d) electrospinning [157]. Reprinted with permission. Copyright 1999, 2005, 2011 Elsevier. 

J.L. Brown, L.S. Nair, C.T. Laurencin, Solvent/non-solvent sintering a novel route to create porous microsphere scaffolds for tissue regeneration, J. Biomed. Mater. Res. B Appl. Biomater. 86 (2008) 396-406. 

The sintering behavior of alumina-zirconia powders prepared from inorganic precursors by a simple sol-gel process (Montanaro and Guilhot, 1989) with gel precipitation followed by peptization and consequent spraying of the sol through a nozzle to produce microspheres was studied by Negro and Montanaro (1996). Powders collected from such a process on calcination at 1200°C for 0.5 h produced porous microspheres. Hot pressing... 

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