Supercritical Drying Preparation Process of Aerogel
The preparation of aerogels generally employs a two-step process, a first-step sol-gel process, and a second-stage drying process.
2 Sol-gel process
Using a compound containing a high chemically active component as a precursor, the precursor and the solvent are hydrolyzed or alcoholyzed to form a stable transparent sol system in the solution, and the sol is slowly polymerized between the aged colloidal particles to form a three-dimensional network structure of the coagulation Gels, gel networks are filled with fluids that lose fluidity. Therefore, more comprehensively, this method should be called the SSG method, namely the solution-sol-gel method. The most basic reactions are as follows:
(1) Solvation
Ionization reaction: ionizable precursor The metal cation Mz+ attracts a water molecule to form the solvent unit M(H2O)z+n (z is the valence of the M ion) and has a strong release of H+ in order to maintain its coordination number. the trend of.
M(H2O)nz+=M(H2O)n-1(OH)(z-1)+H+
Hydrolysis: Precursors of non-ionizable molecules, such as the metal alkoxide M(OR)n (n is the valency of metal M, R represents an alkyl group) reacts with water:
M(OR)n+xH2O=M(OH)x(OR)n-x+xROHM(OH)n
The reaction can continue until M(OH)n is generated.
(2) Polycondensation reaction
Dehydration polycondensation: -M-OH+HO-M- = -M-O-M-+H2O
Loss alcohol polycondensation: -M-OR+HO-M- = -M-O-M-+ROH
3 Drying process
The precursor obtained a gel after the Sol-gel process. The gel consists of an elastic solid network and a liquid solvent in the network. Nanostructured micropores exist in the gel. At the beginning of drying, enough liquids fill the pores of the gel. The decrease in gel volume is equivalent to the volume of liquid evaporation and there is no capillary force. When the gel volume reduction is less than the evaporation volume of the liquid, the liquid evaporates to expose the solid phase and the solid/liquid interface is replaced by a higher energy solid/air interface. To prevent the system energy from increasing, the liquid in the pores will flow outward. Solid/air interface. At this time, the liquid phase forms a meniscus in the gel pores, and the capillary pressure is generated due to the surface tension of the liquid. Capillary forces act on the pore walls of the gel micropores, which can lead to the collapse of a large number of gel network structures ] (see Figure 3). Therefore, in order to obtain an aerogel, it is necessary to eliminate the solvent in the network while maintaining the original gel network structure. In order to solve this problem, supercritical drying, freeze drying, normal temperature and pressure drying and conductive drying have been developed in recent years.
Supercritical drying
In aerogel preparation, supercritical drying is the most commonly used drying method. The so-called critical state refers to a state in which gas and liquid coexist. Each substance has its own critical temperature Tc and critical pressure Pc. Above the critical temperature Tc, no matter how much pressure is added, the gas cannot be liquefied; Pc refers to the pressure required for gas liquefaction at the critical temperature Tc.
As the liquid surface tension and temperature have the following relationship:
(γ is the surface tension of the liquid; γ0 is the liquid property constant related to intermolecular gravitation; T is the temperature of the system; TC is the critical temperature;) so according to the formula, under the critical condition (T=TC), the gas-liquid interface will Disappearing, the surface tension tends to zero, so that there is no additional capillary pressure in the gel pores. Therefore, supercritical drying can maintain the gel network structure and prevent the agglomeration of nanoparticles and the collapse of microporous structures.
The critical parameters of commonly used drying media are shown in Table 1. In practice, the three most commonly used drying media are methanol, ethanol, and carbon dioxide. However, methanol and ethanol are flammable, explosive, and critical conditions are more severe. Use carbon dioxide.
The key part of the device is the controller of temperature and pressure. According to the critical parameters of the drying medium, the temperature and pressure required for the supercritical drying are regulated by the temperature controller and the pressure reducing valve.