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The Process of Rapid Expansion of the Ultrafine Particles

Ultrafine particles have different definitions, often more stringent and more used is less than 3μm particle size is defined as ultrafine powder materials, they are generally divided into three levels: the micron powder sub-micron powder (particle size 0.1-1μm) and nano-powder. Ultrafine particles, in particular the preparation of nano-particles, high-tech in the current has become a hot area of the material, chemical industry, light industry, metallurgy, electronics, biomedical and other fields has been widely used. At present, preparation of ultrafine particles is: mechanical method, vapor and liquid method.

Supercritical fluid refers to the thermodynamic state, namely temperature and pressure at the critical point (liquid equilibrium endpoint) over the fluid. It is a gas, but is different from ordinary gas, is a dense high-pressure gas, has its own characteristics, so called the “supercritical fluid.”

Preparation of ultrafine particles active material, there is the traditional method has some limitations. For example, some degradation of biological products should not be easy to smash a way to change the use of their particle size and distribution of the state; and Gas Preparation of ultrafine particles, either physical or chemical vapor gas law, are accompanied by a large number of heat the occurrence of spray is operated under high temperature, biological active substances generally have heat-sensitive, easily decomposed at high temperatures, deactivation. Liquid phase method, there will sometimes harmful to the health of the residual organic solvents to remove the problem. Supercritical fluid technology, while equipment investment, but operated under mild conditions did not cause the inactivation of biologically active substances, degeneration, there is no adverse health solvent, and able to produce a small average particle size of fine particles, but also control the grain feet.

UK Hnnaya and Hgoarth describe the process of rapid expansion: “When a sudden decompression, solid like a gas in the snow or fog the glass out of the same deposition.” The last century and the early 80s, a formal application of this phenomenon prepared with the appropriate size and distribution of organic material powder. Rapid Expansion of use of the process of preparation of ultrafine particles in the solubility of the solute with the supercritical fluid density relationship, as from the rapid expansion of supercritical fluid state to low voltage, low-temperature gas state, the solute solubility dropped sharply. The flow of supercritical fluid ejected great (usually reach supersonic), the expansion time is short (10-8-10-5) s, this produced a strong mechanical disturbance and great super saturation ratio, super saturation ratio can be up to 106 or more. The former produces homogeneous nucleation conditions, which can form a very narrow particle size distribution; which produces fine particles, and thus obtain uniform particle size of ultrafine particles. Solute in the supercritical fluid has some solubility is a necessary condition for the process of rapid expansion.

Good raw material in the filling of high pressure extraction device (one or a few), make the solute extraction operation, the process of rapid expansion in the sedimentary units, supercritical fluid containing the solute through the rapid expansion of a special nozzle. Because in a very short period of time the solution becomes highly supersaturated solution, the formation of a large number of nuclei, which produce small, uniform size particles deposited in the expansion chamber. In order to prevent nozzle blocking, heat insulation on the nozzle to avoid premature crystallization.

Determined by the shape of solid materials, the characteristics of the material itself (crystalline or vitreous, mixtures or pure solute, etc.), also decided on the process parameters (temperature, pressure, nozzle shape and diameter size, fluid emission rate, expansion chamber size, etc.). For the pure solute is generally relatively narrow size distribution in order to obtain ultrafine particles. For preparation of the mixture within the framework in order to obtain material, micro-encapsulated active ingredients is in capsule or micro-spheres.

This process was applied, because they do not use organic solvents and the favorite. However, it requires a certain supercritical fluid solubility, so that application is limited; generally use it to prepare non-polar compounds. Sometimes, the appropriate co solvent can enhance solute solubility in supercritical fluids, but it needs to exclude the solute, thus increasing the process complexity and cost. But from the economic point of view, and the other supercritical preparation methods, the process of rapid expansion is preferred. Rapid expansion of the general process approach to prepare micron particles, the nozzle was in the application of appropriate circumstances, to prepare nana-particles.

In Preparation of the case, because the process of using a single nozzle Ershi relatively easy to implement. In extended to large-scale production, require multiple nozzles or porous plate O’clock, the process is not easy to control, Shide Kelifenbu wider range than a single nozzle.

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