Particle formation shape control

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Since the properties of these particulate materials are basically determined by their mean size, size distribution, external shape, internal structure, and chemical composition, the science in the mechanistic study of particle formation and the fundamental technology in their synthesis and characteristic control may constitute the background for the essential development of colloid science and pertinent industries. Scientists have now learned how to form monodispersed fine particles of different shapes of simple or mixed chemical compositions, and, as a result, it is now possible to design many powders of exact and reproducible characteristics for a variety of uses. These achievements are especially important in the manufacture of high-quality products requiring stringent specification of properties. 

The properties of filled materials are eritieally dependent on the interphase between the filler and the matrix polymer. The type of interphase depends on the character of the interaction which may be either a physical force or a chemical reaction. Both types of interaction contribute to the reinforcement of polymeric materials. Formation of chemical bonds in filled materials generates much of their physical properties. An interfacial bond improves interlaminar adhesion, delamination resistance, fatigue resistance, and corrosion resistance. These properties must be considered in the design of filled materials, composites, and in tailoring the properties of the final product. Other consequences of filler reactivity can be explained based on the properties of monodisperse inorganic materials having small particle sizes. The controlled shape, size and functional group distribution of these materials develop a controlled, ordered structure in the material. The filler surface acts as a template for interface formation which allows the reactivity of the filler surface to come into play. Here are examples ... 

Spray drying is by definition the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium [Masters, 1985]. This is an ideal process where the end-product must comply with precise quality standards regarding particle size distribution, residual moisture content, bulk density, and particle shape. It involves the atomization of a liquid feedstock into a spray of droplets and contacting the droplets with hot air in a drying chamber. The sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of solvent of matrix liquid from the droplets and formation of dry particles proceeds under controlled temperature and air flow conditions. 

One very special type of shape control results in hollow nanoparticles of different geometries. In principle, the process is rather simple, in that a particle of any structure may serve as template that is coated by a layer of the desired metal. Subsequent removal of the inner template, either by calcination or by selective dissolution, will result in the formation of hollow nanostructures. Whilst this general technique can be applied to many types of material, one special application that involves only metals is as follows. In this case, silver nanocrystals of different shapes are coated by gold layers, via a simple chemical process based on the reductive power of elementary silver towards Au. ... 

DNA is used as a stabilizer/template in the formation of CdS nanoparticles and its mesoscopic aggregates. Use of linear duplexes of DNA in solution help in this formation. In the beginning it was inferred that in DNA CdS nanoparticles split to form Cd and S . The role of DNA-nanoparticle interaction was unclear. When studied further it was realized that the DNA base sequence had a significant effect on the size of the CdS particles and their photophysical properties. DNA sequence in general and base adenine in particular had its influence on CdS particles. Coffer and co-workers 1996 developed a technique to synthesize well-defined mesoscale stractures in solution by binding a template DNA strand to a solid substrate. By controlling the particle composition, shape, length, and sequence of the DNA template via this approach a number of mesoscale structures were obtained. 

The drop generator established by Yim et al. for producing solder balls is shown in Fig. 26.5 as an example for a device suitable for use with melts [20]. This device combines a heatable reservoir for the metal melt with a solenoid-driven vibrator, which transmits oscillations by a disk mounted at the end of a shaft to the molten metal bath. The orifice piece is manufactured from ruby. The solidification of the solder drops is controlled by immersing them into a silicone oil bath. Particles produced with this device are nicely spherical, with typical diameters of 780 pm and a standard deviation of 26 pm. Apart from the spherical particles, irregular shapes are also observed, which are due to coalescence of the drops, either in the drop stream before entry into the oil bath, or in the oil bath prior to formation of a solid shell on their surface. 

Organic reactions can be understood in terms of a large toolbox with well-known reaction processes, better control in the particle formation by monitoring the reaction rates taking place along with their high-crystalline shape at moderate temperature with high chemical purity. 

Particle formation is very similar to that for carbon black, with very small, primary particles forming first and then fusing, while still mobile, into chain-like aggregates. Properties like particle size, surface area and shape are controlled by factors such as reactant ratio, temperature and turbulence. Because of the high temperature formation process, the surface is less hydroxylated than that of the precipitated silicas, and as a result adsorbs less water. The moisture loss at 105 °C is usually less than 1.5%. 

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