Nano replacement supercritical drying drying
     Drying method is the so-called displacement liquid phase resulting precipitate was dried in a solvent before use water displacer ( including organic solvents, liquid carbon dioxide , supercritical carbon dioxide, etc. ) for the replacement process , and then using an appropriate drying method to obtain nano-materials product method of operation .
In the wet chemical method nanopowders , you need pretreatment before drying filtering process, washed several times with deionized water would be liquid residue removed as much as possible of the various impurity ions , but in the wash after the drying process, will inevitably lead to agglomeration of the powder particles . A surface tension lower than water, an organic solvent instead of water remaining in the particles , less agglomeration of the powder can be obtained . The amount of the solvent for the gel , is often required for subsequent drying process .
 ( 1 ) an organic solvent, supercritical drying the hydrogel is not suitable for supercritical drying , so the use of inorganic salts ( such as water glass, etc. ) made ​​of a hydrogel , need alcohols ( such as methanol ) replacement of the water therein to obtain methanol gel . Gel in a desiccator and then methanol , adding a certain amount of methanol , to a certain temperature and pressure rate of methanol to achieve a supercritical state ( critical point of a yeast 240 ℃, 7.9MPa), so that the inner holes the methanol solvent into a supercritical fluid gradually to maintain the system in a supercritical state for a time constant slow release of methanol solvent under reduced pressure , which is 1931 using supercritical methanol were prepared by drying a first airgel - silica airgel . Subsequently, ethanol , acetone and for replacement of the supercritical drying method, an organic solvent to prepare a A1z [) 2, T are 2, Zr02, organic aerogels , carbon aerogels , etc. ; addition, the supercritical drying of ethanol Preparation of nickel hydroxide , zinc oxide, chromium dioxide , nickel oxide nano-powder .
    ( 2 ) supercritical carbon dioxide drying in general . With an alcohol ( such as methanol ) as the solvent at high temperature of a surface of the gel network can acetification , the resulting surface has a hydrophobic airgel , therefore, easy to absorb moisture in the air and the destruction of their nano-porous structure , very stable , long-term storage without changes . Only a relatively high critical temperature of alcohols , and because flammable. Methanol is also toxic , therefore , with an alcohol as a solvent having a certain degree of risk . In addition , with an alcohol as a solvent in a supercritical drying process the gel network will have a certain impact, especially in the presence of water and in the presence of an alkaline catalyst . Impact will be even greater. The mechanism of this effect with the aging process as the gel can also be considered to be more critical drying process of high temperature and pressure exacerbated by the aging of the gel network , the gel network variable rent, stiffness becomes larger than the surface area decreased, this is disadvantageous for some Aerogels . 1985 TewaT5 such as the use of supercritical carbon dioxide drying alcohol , not only the supercritical drying process greatly reduced operating temperatures , but also the safety and reliability of the device is improved . Under the operating conditions , the solid carbon dioxide basic skeleton of the gel is chemically inert , is a purely physical process . Because the solubility of carbon dioxide in water is very small , so during supercritical carbon dioxide prior to drying the gel with an organic solvent substitution of water , liquid carbon dioxide and then the organic solvent substitution . Supercritical carbon dioxide airgel dried , the surface has a strong hydrophilic , it is easily absorbed moisture from the air , so put a long time because of water gradually becomes milky , and in severe cracking . Adsorbed water - generally by heating (100-250 ℃) to remove , and does not affect the nano- porous structure . Currently, the preparation of airgel or nano powders are mostly based on supercritical carbon dioxide drying method .
    ( 3 ) supercritical carbon dioxide extraction of supercritical carbon dioxide drying if the drying process used in solvent substitution liquid carbon dioxide becomes supercritical carbon dioxide , when the operation is to supercritical carbon dioxide extraction dry. Drying with supercritical carbon dioxide compared with supercritical carbon dioxide extraction drying of the entire drying time is greatly reduced , significantly reducing operating costs . However, during operation , to ensure the operating temperature and pressure in excess of binary mixtures ( such as ethanol and carbon dioxide ) the critical temperature and pressure , otherwise would have been the porous structure of airgel