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Silica Sol Casting The Permeability Is Determined By The Capillary Microstructure Of The Gel

Silica sol is a high quality water-based binder commonly used in investment casting. Because of its shell does not require chemical hardening, high temperature strength, high temperature resistance to deformation ability, easy to dubbed high-quality liquid than the high-quality paint, since the sixties of last century was introduced after the investment casting, shelling technology But the published literature [1] is mostly on the shell process and process parameters control and the choice of refractory materials and other aspects of the discussion, while the investment casting process shell micro Structural analysis has not yet seen the relevant reports.

In this paper, we discuss the change of the microstructure of the shell during the process of Silica Sol Casting from the perspective of colloidal chemistry, and hope to provide some effective theoretical guidance for the optimization of the shell technology.

2. Introduction to professional terms

Some technical terms in the field of chemistry and casting the understanding of the field is not the same, for the convenience of discussion, we unified the use of colloidal chemistry to understand the terminology, and the use of narrow understanding. The briefing is as follows:

2.1 colloid

First colloid is not a special substance, nor is the nature of matter, but the existence of a special state of material. The colloidal system is characterized by a certain size of the dispersed phase particles (the diameter of the dispersed phase particles is 1 to 100 nm), and there is a significant physical interface between the colloidal particles and the dispersion medium. Therefore, the colloidal system is a two-phase or multi- Uneven dispersion system.

2.2 sol

From the definition of colloids, colloidal systems can be formed in dispersed media in different states as long as the particle sizes of the dispersed phases of different aggregates are between 1 and 100 nm. In addition to the dispersed phase and dispersion medium are gas and can not form colloidal system, the remaining eight kinds of dispersion system can form a colloid system. When the dispersion medium is solid, it is called solid sol; when the dispersion medium is gas, it is called aerosol, and the colloidal dispersion system with liquid dispersion is called liquid sol or sol.

2.3 Gel

Also known as jelly. The colloidal particles in the sol can be connected to each other under certain conditions to form a space network structure, the structure of the gap filled with liquid as a dispersion medium, regardless of the amount of liquid, this will lose the liquidity of the dispersion system called gel. Fresh gel called wet gel, when the liquid in the gel all lost also known as the gel, is the dry gel, the structure of the dry gel inside the filling is full of gas.

Gels are filled with water when the liquid is called water.

Gel has a certain geometric shape, with a solid mechanical properties, such as strength, elasticity and yield value. But from the internal structure, it is different from the usual solid, which consists of solid-liquid two-phase composition, but also has some properties of the liquid, such as ions in the fresh hydrogel diffusion rate close to the diffusion rate in aqueous solution The This shows that the fresh hydrogel, the dispersed phase and dispersion medium are continuous phase, which is the structural characteristics of the gel.

2.4 capillary phenomenon

The continuous dispersed phase forms the solid skeleton of the gel, and the continuous dispersion medium forms the fluid portion of the gel, forming the particle size of the colloid so that the gel has a microstructure of the capillary. Capillary phenomenon refers to the liquid inside the fine tubular object, due to the adhesion between the liquid and the wall and the liquid itself, the difference between the cohesion in the vertical tube within the rise or fall phenomenon, and the role of these two forces is the capillary force The role.

The gel structure of fresh gel is filled with liquid dispersion medium. With the progress of gel drying, the gel will change from liquid-solid two-phase to liquid-solid three-phase, and the final liquid phase will be replaced by gas phase and become dry gel The The permeability of the visible shell is determined by the microstructure of the capillaries of the gel. It is also seen that the drying process is a very critical and important link in the gel formation process and that factors related to the gel drying rate will ultimately affect the gel capillary Of the microstructure.

2.5 gelling

A certain concentration of sol in the appropriate conditions to form a gel process known as gel.

During the gelation process of silica sol, the capillary structure of the colloidal silica gel was formed by hydrogen bonding. The hydroxylation of the hydroxyl group was formed by the hydrogen bond in the late stage. After drying, the hydrogen bond was continuously formed into the covalent bond of the silica and the three-dimensional growth of the micelles, but the crosslinking speed in each direction was different , The cross-linking density will be different, and ultimately the formation of heterogeneous, with complex micro-area, dendritic polymer structure of the gel.

3. The formation process of the microstructure of the investment casting shell

Silica sol coating is a mixture of binder silica sol, refractory powder and a small amount of functional auxiliaries. The process of Silica Sol Casting is essentially a gelation process of a silica sol coating, and then the gel changes from a wet gel to a dry gel. Drying is a critical process, and the capillary force during drying causes the type The internal microstructure of the shell undergoes a series of physical and chemical changes that can be broadly divided into four stages, but there is no strict boundary, or gradation or overlap between each stage.

3.1 Sol-gel transition phase

Silica sol coating infiltration mold, due to the surface of the coating evaporation of water, the surface and the internal will produce a poor humidity, the internal moisture in the humidity difference driven by liquid water in the form of continuous diffusion to the surface. At the same time, the silica sol particles continue to be precipitated in colloidal dispersion. The precipitated silica particles are bound by hydrogen bonds to form a skeleton, forming a connected polymer capillary structure, and finally losing the flowability into gel , Where the gel is a wet gel.

Since the evaporation process is carried out on the surface of the coating, the drying process is at a constant speed drying stage and the evaporation rate of water is approximately equal to the evaporation rate of water in the open vessel at this temperature since the evaporation rate is proportional to ΔP = PV-PA, where PA is Environmental vapor pressure; PV is the system liquid vapor pressure. So the ambient vapor pressure, that is, the humidity between the shell determines the evaporation rate of water at this stage. At the same time, with the evaporation of water, the volume shrinkage of the coating, the rate of volume shrinkage and the rate of evaporation of water can be approximated.

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