Powder supercritical fluid techniques

Supersaturation of solutions to precipitate ceramic powders is attained by means of supercritical fluids. This technique, termed rapid expansion of supercritical solutions (RESS), is reviewed elsewhere. ... 

The importance of particle sizing is that only small particles will reach the lung, since the nose and mouth will remove any larger particles. There is continuing debate as to what maximum size will reach the lung, but consensus is for around 5 to 7 um. For alveoli penetration, a size of 3 lam is required, but very fine particles, below 1 jun, may be exhaled. It is the size of the droplets in the aerosol cloud that is important for dry powders and fully evaporated liquid suspensions, this maybe the original powder particle. Hence, it is essential to have powders in the micron range, and normal micronisation will usually produce a size of 1 to 3 jun. Newer techniques, such as supercritical fluid extraction, can produce smaller particles. Naturally, if the product is a solution, the particle size formed on evaporation of the solvent will depend on solution concentration. 

Drying techniques have been explored for pharmaceutical biopolymer formulations drying with the aid of a supercritical fluid is especially attractive for reasons of mild process conditions, cost-effectiveness, possible sterilizing properties of supercritical carbon dioxide, capability of producing microparticulate protein powders, and feasibility of scaling up. 

See also-. Extraction Supercritical Fluid Extraction. Forensic Sciences Gunshot Residues Thin-Layer Chromatography. Fourier Transform Techniques. Gas Chromatography Mass Spectrometry Forensic Applications. Microscopy Appiications Forensic. X-Ray Absorption and Diffraction X-Ray Absorption X-Ray Diffraction - Powder. 

Supercritical drying of porous silicon films on wafers has been particularly successful (see references in Table 1) and should be applicable to other physical forms such as membranes, microparticles, nanowire arrays, and nanoparticles. Of particular relevance in this regard is that the silicon MEMS community also utilizes this technique to avoid micromachined structure stiction, to clean silicon surfaces, and to improve device yield (Kim et al. 1998 Jafri et al. 1999 Namatsu et al. 1999). There are now commercially available systems for both silicon powder and wafer batch processing (Fig. 4). Supercritical fluids could also be utilized in the purification of porous silicon (Koynov et al. 2011). 

Sub- and supercritical fluid extraction with CO2 has several advantages. One is that CO2 is nontoxic, nonflammable, and noncorrosive. Also, the end product is obtained as a powder without the need for drying [197]. However, the relatively high equipment purchase and maintenance pose a major constraint. This technique has been successfully used to extract polyphenols from grape pomace, generally using CO2 modified with methanol [198-201] or ethanol as cosolvent [202]. 

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