干胶制备过程.docx

1、干胶制备过程Aerogel ProcessingINTRODUCTIONA gel results from a condensation of molecules or particles in a solvent. It is constituted by tenuous and entangled chains of solid wetted by a liquid which occupies the whole volume located between solid chains. The liquid is a mixture of solvent, unreacted mole

2、cules inducing gelation and by-products of chemical reactions. It is obvious that only the network is of interest for material applications. There are many ways to remove the liquid located within the pores of the gel. A dried gel is named “xerogel” (from the Greek work that means dried).Drying is o

3、ften performed by a gentle solvent evaporation at temperatures close to room temperature. In the course of solvent evaporation, the shape of the liquidvapor interface changes with time. The curvature radius of the meniscus decreases (Fig. 25-1) and, associated to this curvature, capillary forces tak

4、e place. The pressure difference, P, between vapor and liquid is given by Laplaces relation: (25-1)where LV is the liquidvapor surface energy and R is the curvature radius of the meniscus (here assumed spherical). The liquid is consequently under a tension stress and conversely the solid network is

5、submitted to a compression stress. Because of the weak mechanical properties of the gel network a shrinkage occurs. The pore volume of the xerogel is well lower than that of the starting gel.Hence pronounced textural modifications happen. This is the first serious drawback that we must avoid to pres

6、erve the expanded texture of the solid network.The volume shrinkage of the gel during drying induces an increase of its stiffness. At a given time, the solid network is no more compliant and the meniscus recedes in the pores. At this moment, the stress is maximum since the curvature radius correspon

7、ds to that of the shrunk pore (assumed cylindrical). Associated to evaporation, the liquid flows from the core of the gel to the surface. This flow is hindered by the solid arms of the gel. A gel is badly permeable because the size of the pores lies mainly within the range 0.210 nm indicating that a

8、 gel is a mesoporous material. According to the Darcys law, the liquid flow, J, is related to permeability, D, by the relation: (25-2)where P is the pressure gradient and is the liquid viscosity.Because of the stress gradient, the solid network may crack. This kinetic approach explains why cracking

9、is related to the evaporation rate. Indeed the evaporation rate controls the liquid flow. The drying of the gel has been very precisely studied by G.W. Scherer in a series of papers listed in Chapter 8 (Brinker, 1990). A gel dried very slowly will produce a free crack xerogel. Many authors report dr

10、ying treatments the duration of which is of several months. That is usually done by covering the gels with a plastic film in which many holes are punctured.Figure 25-1. Evolution of the curvature of liquidvapor meniscus at the surface of a pore as a function of drying time, t.Cracking of the solid p

11、art of the gel is the second drawback usually encountered during drying. Freeze drying and supercritical drying are two processes which have been investigated to circumvent these difficulties.Freeze drying consists of lowering the temperature of liquid to induce a crystallization phenomenon. The sol

12、vent is then removed from its vapor state by decreasing the pressure (sublimation). This process applies well to solvents showing an appreciable vapor pressure at temperatures lower than crystallization temperature. Low molecular weight alcohols have very low crystallization temperatures (methanol:

13、94C, ethanol: 117C). Water which transforms into ice crystal shows an important volume change associated to this transformation. The solid part of the gel is highly stressed and usually breaks into small pieces (Pajonk, 1989). Moreover the sublimation rate is quite slow. It is of about 140 kg/m2 h a

14、t 15C. A solution which may, in imagination, avoid the large volume change produced by crystallization, is to transform liquid into glass. Unfortunately glass formation domain often occurs near eutectic point composition. As exemplified the glass temperature of mixture H2O-CH3OH is too low (157C) (V

15、uillard, 1961) to perform then sublimation at appreciable rate. Finally, one among the best liquids seems to be terbutanol whose the melting temperature is 25C and which has a sublimation rate of 2800 kg/m2 h at 0C. This solvent is not usual and a previous solvent exchange is often required. The tex

16、tural properties of the gel such as the pore volume and the pore size distribution are approximately preserved. Nevertheless it seems difficult to obtain monolithic samples having significant thickness (higher than 10 mm) (Degn Egeberg, 1989). A detailed analysis of the nucleation and crystallizatio

17、n phenomena occurring in the liquid wetting the solid part of the gel has been done by Scherer (Scherer, 1993). Crystallization starts from the liquid located at the external gel surface and the crystalliquid interface moves from the surface to the core. Thus stresses appear as a consequence of the

18、solid crust which forms at the surface and the volume change associated to the liquidcrystal transformation.Since the main consequences of drying are the shrinkage and the breakage, several experiments have been performed to overcome these drawbacks. We must underline that cracking has been chemical

19、ly avoided by adding to the starting solution some compounds which give rise to gel having a narrow pore size distribution (formamide, glycerol, oxalic acid). Chemical additives controlling the drying step work well both with aqueous gels (Shoup, 1988) and those prepared from organometallic compound

20、s (Hench, 1986).The increase of the stiffness of the solid part of the gel by a dissolution-redeposition effect allows to preserve the monolithicity of the gel while reducing the shrinkage (Mizuno, 1988). It is worth noticing that ageing the wet gel in a solution containing monomers gives analogous

21、results (Einarsrud, 1998). An alternative way to produce crack free samples is to synthesize gels having very small pore sizes. During drying, nucleation and growth of bubbles occur within the liquid. This cavitation phenomenon induces the segmentation of the liquid which becomes under a lower tensi

22、le stress (Sarkar, 1994). On the other hand we must underline that sometimes cracking can be regarded as an advantage. As an example, an extensive cracking is beneficial in the synthesis of abrasive powders issued from solgel process.THE SUPERCRITICAL DRYING-PROCESSThe supercritical drying process h

23、as been proposed by Kistler. (Kistler, 1932) to dry, without textural modification, very tenuous solids wetted with a solvent.The main idea is to avoid capillary forces, which occur during drying, by a very peculiar pressure and temperature schedule applied to the liquid. Regarding only the liquid p

24、hase of the gel, it is obvious that one can modify its state by changing thermodynamic parameters such as the pressure and the temperature.Figure 25-2 shows a typical phase diagram for a pure compound. The parameters, P, T, v (usually the specific volume) are the variables which determine the state

25、equation.Figures 25-3 and 25-4 correspond to some projections of the previous three-dimensional diagram.Figure 25-2. Typical P, T, v diagram of a chemical compound.Figure 25-3. Pressure-specific volume diagram issued from diagram Figure 25-2.Figure 25-4. Pressure-temperature diagram showing the diff

26、erent domains solid, liquid and vapor and supercritical fluid (SF).The principle of supercritical drying is easily understood owing to Figure 25-4. The point, a, defines the couple pressure-temperature at which the three states of the compound are in equilibrium. Under atmospheric pressure, Pat, the

27、 liquid transforms into vapor at boiling temperature (TB). The point, c, is the boundary of the vaporization curve corresponding to liquidvapor separation. The point, c, is named the critical point. For a given compound the critical point is determined by associated critical pressure and temperature

28、 values. Above this point there is a continuum between the liquid and the vapor which can no more be distinguished. In this domain, there is an unique state named supercritical fluid (SF). This domain is not well defined. However a crude approximate consists in locating the supercritical fluid domai

29、n by a P, T area as indicated in Figure 25-4.At room temperature (TR) starting with a liquid (N) and increasing both the temperature and the pressure, the compound follows the path N Q (Fig. 25-5). At Q, the compound is supercritical under its fluid state. It can be observed that starting with the v

30、apor state at low pressure (M) and increasing again the temperature and the pressure, the compound reaches the point Q where it is in the same state than that previously mentioned. Thus we have obtained the same homogeneous and unique state using different paths. For a given compound, its properties

31、 depend obviously on the pressure and temperature values and can be easily varied accordingly.Figure 25-5. Different paths to reach the supercritical fluid domain.It is evidenced that starting from the liquid state (point N) and increasing the temperature and pressure up to the supercritical fluid s

32、tate (point Q), an adequate decrease in the temperature and pressure (see full arrow) will lead to the vapor state (point M). The net effect of these successive steps results in the transformation of liquid into vapor. A drying step has been carried out. The change from the liquid to the vapor follows a path that avoids the vaporization curve (ac). During heating, the surface energy associated to the interface liquidgas progressively decreases and vanishes when the superfluid state is attained. Consequently capillary forces (see equation (1) are no more acting and the solid part of the g