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  • Subject area(s): Engineering
  • Price: Free download
  • Published on: 7th September 2019
  • File format: Text
  • Number of pages: 2

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Abstract

Silica aerogels and xerogels are effective thermal insulation. Unfortunately the preparation of these materials is both time consuming as well as costly. The preparation of wet silica gels which can be effectively dried either super critically or under atmospheric pressure has both time consuming and expensive steps. For example, drawing the necessary quantity of water out of the gel to prepare for drying makes the procedure both times consuming and expensive. As a result of such problem, silica aerogels and xerogels have not yet reached full commercial effectiveness.

Chapter 1

Introduction

An aerogel is a former gel from which the liquid phase has dried out. To be an aerogel, as opposed to a xerogel, the solid structure in the gel needs to be preserved throughout the drying (Iler, 1979). The concept of aerogels was first investigated by Kistler (1931a) in the early thirties. Kistler's hypothesis was that the shrinkage was created by capillary forces during the process. To test the supposition Kistler heated his gel samples to the critical temperature of the liquid while the pressure was kept above the vapor pressure of the liquid. Above the critical temperature, the phase change form liquid to gas will not subject the gel structure to tension, which leaves a dry gel with the structure secure. The tests were successfully tested for a wide variety of gels (Kistler, 1931b).

Main objective of dissertation

The main objective of this dissertation is to use xerogel as an insulator in different applications to reduce heat transfer and to check its effectiveness compared to other insulators. These applications can be engineering applications or other. Xerogel can also be used to reduce power consumption that is also one of its applications and it is already being used in some applications.

Dissertation outline

The main goal of this dissertation to make xerogel in a laboratory by one of the convenient making processes and to use it as an aid or in a mechanism to reduce heat transfer in any engineering elements or   other mechanisms or structure and to substantiate its effectiveness in corresponding applications.

Chapter 2

Literature review

Micro-porous silica aerogel and xerogel structures are described which are suitable as insulation. These xerogels and aerogels are made by the process described herein which carefully controls the ratios and proportions of ingredients, catalyst, and solvent. Thermal insulation is an important and valuable product. Although many insulative compositions are already being applied, there is a continuing desire for energy conservation pushing a drive to achieve insulation with lower thermal conductivity. Micro-porous silica aerogel structures have excellent characteristics for insulation. M. Antonieta Maeip-Boulis and  Aheed G.Boulis has worked on microporous aerogel.

Ravindra Deshpande worked on manufacturing of wet silica gels.A wet silica gel can be prepared by a process comprising the steps of contacting a stable, aqueous, fluid, silica system with a pH greater than 7.5 with an ion exchange resin which removes metal ions and substitutes them with H+ ions and thereby reducing the pH to less than 5.0, adding an organic liquid to the silica system, however, the organic liquid added stays in one phase with the aqueous, silica, system, and stops the silica precipitation, and adding a base to the silica system so that the pH of the silica system is in the range of from 5.0 to 7.5, and allowing the wet silica gel to form. The wet silica gel produced by this process is characterized by its intense organic solvent content and the low concentration of basic metal ions.

Xerogel is a dried gel which is formed by drying the gel at atmospheric pressure, typically using elevated temperatures; shrinkage of the gel's network occurs during drying. But, in fact, the resulting xerogel is often reduced in volume 5 to 10 times compared to the original wet gel. Dierk Frank from Germany worked on dried xerogels. The undesired loss of volume has been noted in 'The Concise Chemical and Technical Dictionary' (Chemical Publishing Co. 1974) which tells that the removal of volatile fluid from the gel during the preparation of xerogel results in a harder and stronger mass. That shrinkage is not advantageous for properties such as porosity, density and thermal conductivity, which are very important for insulation. Less-dense xerogels are discussed in D. M. Smith, Better Ceramics through Chemistry V, Mat. Res. Soc. Symp. Proc. Volume 271. According to this reference, aging and surface derivatization can be used to manipulate the capillary pressure and gel structure strength of base-catalyzed alkoxide derived silica gels and subsequent formulation of xerogels under normal pressure conditions. Generally, the removal of pore fluids during drying in a non-super-critical drying process causes the gel network to break due to the high capillary pressure and further chemical reactions between surface sites.

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