Alumina airgel material preparation method and application
aerospace, energy, chemical and metallurgical low density, large surface area, high structure strength, low thermal conductivity, high temperature stability performance, and unique nano network structure, is widely used. Small series briefly described below alumina airgel material preparation method and application.
a process for preparing an alumina airgel material
airgel typically requires two steps: First, a sol-gel process a porous network structure, i.e. sol; second wet gel the drying process.
1. Preparation of the alumina sol prepared alumina aerogels
essentially as precursor species divided into: alkoxide method and two kinds of inorganic aluminum salt method. (1) an aluminum alkoxide method alkoxide method and the polymer is divided into particles Method methods. (A) Particle Methods particles alkoxide hydrolysis method in an excess of water and the resulting precipitate was larger particles, followed by addition of electrolyte such as peptizing acid, by peptization process the particles adsorbed on the surface of a given ion form an electric double layer, static role of electric power to form a stable alumina sol below. Alumina airgel particles prepared by Method disadvantages are: the network structure unstable and easily damaged, and a longer period of preparing. (B) a polymerization method is a rule polymer controlled hydrolysis of the metal alkoxide by adding a small amount of water, the reaction product was hydrolyzed directly in the polymerization, the formation of a network structure by chemical oxide in a solution, the sol directly into stable alumina sol . Polymerization alumina airgel advantages are: better performance, good finished shaped blocks, higher strength; disadvantages are: gel faster often need to add a chelating agent is stable and relatively high cost. (2) an inorganic aluminum salt an inorganic aluminum salt method is a method in an inorganic aluminum salt was added dropwise a solution of aqueous ammonia hydrolysis, until complete hydrolysis was evaporated after precipitation or centrifugation, the precipitate obtained was washed several times, then according to a certain proportion of added peptizer peptization, and controlling the pH of the sol to precipitate the generated re-dispersed uniformly to form a clear transparent alumina sol. Method inorganic aluminum salts are advantages: low cost starting material; Disadvantages are: larger sol particles prepared, the shorter the gel time, airgel powder easily formed, and a longer period.
2, square alumina airgel driedMethod
Currently, alumina airgel drying process mainly divided into: the supercritical drying method and a non-supercritical drying method. (1) supercritical drying method is the supercritical drying method more mature new drying technique, the main process are: in a supercritical state, a gas-liquid interface disappears, the surface tension no longer exists. In the process of supercritical fluid is discharged from alumina gel, does not cause contraction and collapse of the skeleton structure of its network, which finally can be maintained the original gel structure of bulk alumina airgel. Most at home and abroad using liquid CO2 and ethanol as the drying medium.
Supercritical drying advantages are: alumina airgel particulate porous network structure having excellent high temperature performance. The disadvantage is: The higher equipment requirements, the costs are more expensive.
(2) a non-supercritical drying method non-supercritical drying method is divided into two: one is alumina after aging the gel, replacing a large surface tension liquid gel using surface tension of the liquid, and at atmospheric pressure or sub-critical drying step to give an alumina airgel. Another alumina airgel after the aging treatment alkylated, while the organic solvent is replaced with water, and then dried under atmospheric pressure, alumina airgel was prepared. Pressure drying simpler equipment requirements; disadvantage is the relatively long period, and the structure and properties of aerogels by supercritical drying no good samples.
Second, the alumina-modified airgel
Currently, the main method for improving the high temperature performance of the alumina airgel modification is the introduction of additives. Additives are mainly rare earth oxides, alkaline earth oxides, silica and the like.
1, glycols alumina airgel
Modification of alumina airgel two yuan mainly in that the second phase is added to reduce surface hydroxyl groups results in alumina and inhibit sintering, improved alumina airgel high temperature performance. (1) alkali metal oxides and rare earth oxide added, alumina sol can reduce the surface energy, improve their high temperature performance. (2) silica was added to form Si-O-Si or Si-O-Al bridges during dehydroxylated, eliminating the anionic surface of the cavity alumina, alumina airgel improveHigh temperature stability.
2, alumina airgel polyol
Currently, improved sol-gel and preparative supercritical drying researchers use three yuan ultrafine NiO / La2O3 / Al2O3 catalyst airgel, not only retains the oxidation the main characteristics of aluminum airgel, but with the addition of nickel salts and lanthanum salts, airgel quality is improved, more easily crystallized alumina molded, high temperature performance and better adsorption ability.
Third, alumina airgel
1, super-insulation material having a nanoporous alumina airgel material structure, it has a lighter mass, smaller volume to an equivalent insulation effect, for use in insulation materials aviation engines, both play excellent insulating effect, but also reduce the weight of the engine, the other as a super thermal insulation material on outer space exploration vehicle, and the vehicle also has a good prospect.
2, high temperature catalysis
alumina having a high porosity aerogels , high specific surface area and the texture of open, having potential application in terms catalyst and catalyst support.
3, electrochemical aspects
alumina airgel can be used as high voltage insulation , high speed or speed of the integrated circuit substrate material, and a vacuum isolation dielectric electrode supercapacitors.