Super- critical drying

Some of the particles were studied after supercritical point drying. The particles were dispersed in acetone by several centrifugation steps (45,000 x g). In a critical point dryer (Balzers Union, CPD 020), the carbon-coated grids and the dispersion in acetone were placed. Under pressure, the acetone was exchanged against liquid CO2 after increasing the temperature and passing the critical point, the particles adsorbed on the film were super-critically dried. 

Aerogel powders of binary aluminate and zirconate oxides were prepared by a sol-gel technique and super-critical drying. The synthesis conditions are described and initial properties. Changes in the surface area and phases present in these powders and several reference materials are followed as they are heated at various temperatures in air cmtaining 10 vol.% water vapor. In addition, a time series was done on the most sintering resistant material (LaAli lOig) in ordtf to estimate its sintering rate. 

Another approach is adopted from biological methods After BOE, the liquid is replaced with liquid C02 and dried at the super-critical PT state [18]. Systems are available commercially but suffer from low throughput. 

Other interesting polymerizations include the use of metal-activated hydrogen peroxide to deliver low molecular weight pol5uners (83,84), continuous polymerization of water-soluble monomers in extruders (85), dry polymerization of acrylic acid in super critical carbon dioxide (86,87) and on a powder bed (88), and the use of sodium nitrate mediated aqueous pol5unerization to allow high solids (89). 

VDF polymerization technology based upon super critical or liquid carbon dioxide as polymerization media has been reported (57-62). This technology offers an advantage in the polymer isolation step where a clean dry polymer is produced simply by depressimzation. The residual monomeifs) and CO2 can be recycled back to the reactor. PVDF is not soluble in CO2 (58-60) and as a result, additional pol5uneric stabilizers are required to produce stable particles. Adequate CO2 density for pol5unerization requires pressure significantly higher (typically >100 bar) than a conventional emulsion polymerization. 

For many purposes the commercial reagent-grade DMF, dried with molecular sieves, can be used DMF decomposes on distillation at atmospheric pressure. The most convenient way to purify DMF is probably to let it pass through a column [358] of alumina (e.g., ICN Alumina N-Super I). Other methods involve drying (CUSO4 [367], which also removes amines), azeotropic distillation with benzene, or percolation through molecular sieves [368] followed by fractional distillation at reduced pressure. DMF is difficult to obtain in an anhydrous form, but for most purposes a small water content is not critical. An impurity of A-methylformamide may act as a proton donor or otherwise interfere in the follow-up reactions [369]. DMF should be stored in the dark under nitrogen. The different purification methods have been compared and discussed [370]. 

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