Monodisperse particles sintering

Ordered porous carbons were synthesized by replication of colloidal templates made from 30 - 100 nm diameter silica spheres and removal of the silica templates using aqueous HP. To create the templates, the monodisperse particles were pressed into pellets and then sintered slightly at their points of contact. The silica template were filled with carbon precursor solution of divinylbenzene (DVB) and a free radical initiator, azobisisobutyronitrile (AIBN). Polymerization and carbonization of the precursor solution and subsequent dissolution of the silica templates leave a polycrystalline network of carbon with interconnected uniform pores. The degree of order of the silica template is faithfully reproduced in the carbon replicas. 

In this section we examine many of the methods that have been developed to make monodisperse materials. Whether or not they are ideal for ceramics fabrication, monodisperse particles are certainly important as model systems for testing theories of colloidal stability, light scattering, drying, and sintering. 

The need for ceramics with improved mechanical properties has led to tremendous interest in methods for preparation of superior powders. In particular, many studies have examined methods for making monodisperse particles from solution. This is based on the argument that the ideal powder should be pure, stoichiometric, dense, equiaxed (i.e., spheroidal), and nearly monodisperse. Uniform particle size is said to facilitate preparation of stable dispersions, as well as dense, uniform powder compacts (76]. However, if the particles are well dispersed, a denser body can be made with a range of particle sizes, as the smaller ones fit into the spaces between the larger ones. It is sometimes argued that the best sintering... 

Colloidosomes are a recent class of microcapsules and thus far have only been applied to catalysis and drug delivery in a few cases. The term colloidosomes was coined by Anthony Dinsmore and colleagues in 2002 to refer to capsules where the shells are composed of close-packed layers of monodisperse colloidal particles (usually micron-sized polymer beads) that have been linked together by sintering. 

The simplest photonic crystal is nature s opal. The artificial opal is composed of monodispersed spheres of a dielectric, usually silica. Considerable work has been done using latex or polystyrene spheres, but we largely will restrict ourselves here to ceramics. In producing high-quality photonic crystals, care must be taken in each of the three main steps particle synthesis, sedimentation, and sintering. 

The other simplifying assumptions of the models must also be remembered. The extension of the two-sphere geometry to real powder compacts is valid only if the particles are spheres of the same size arranged in a uniform pattern. In practice, this system is, at best, approached only by the uniform consolidation of monodisperse powders by colloidal methods (see, for example, the work of Barringer and Bowen discussed in Chapter 1). Coble (17) considered the effect of a particle size distribution on the initial stage of sintering by considering a linear array of spheres. 

Studies by Ciftcioglu et al. and Milne et al. also demonstrated another important fact about sintering. Monodispersed powders will form ordered areas of close packed particles, which are characterized by uniform and small pore sizes. This condition will favor densification of the material at a lower temperature and in shorter times. It will also lead to smaller and uniform grain sizes within those areas. How-... 

T. Shimohira, A. Makishima, K. Kotani, and M. Wakakuwa Sintering of monodispersed amorphous silica particles, in Proceedings of International Symposium of Factors in Densification and Sintering of Oxide and Non-Oxide Ceramics (S. Somiya S. Saito, eds.) Tokyo Institute of Technology, Tokyo (1978) 119 127... 

Ordered stmctures can also play a key role in the manufacture of membranes with controlled pore size distributions. Ramakrishnan et al. (2003) synthesized membranes by depositing monodisperse latex particles onto a base substrate to form an ordered stmcture. The voids between the particles serve as narrowly distributed pores and the size of the pores is controlled by the size of the particles used in deposition. Monodispersity plays a key role in achieving pores of uniform size and in the processing step, which involves the sintering of the formed stmcture to achieve stability. As with the processing of ceramic suspensions. 

Shimohira T., Makishima A., Kotani K., Wakakuwa M. Sintering of monodispersed amorphous silica particles. Proceedings of the International Symposium on Factors in Densification and Sintering of Oxide and Non-oxide Ceramics, October 3-5, 1978, Hakone, Japan Smith J.P., Messing G.L. Sintering of bimodaUy distributed alumina powders. J. Am. Ceram. Soc. 1984 67 238-242... 

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