Compositions – Electrolytes for electrical devices
Patent
1997-04-17
1999-09-07
Lovering, Richard D.
Compositions
Electrolytes for electrical devices
10628711, 10628716, 106490, 428405, 428404, 516100, E04B 174, B01J 1300, C09K 300
Patent
active
059483148
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a composition comprising from 30 to 95% by volume of aerogel particles and at least one aqueous binder, a process for the preparation thereof, and the use thereof.
Because of their very low density and high porosity, aerogels, especially those having porosities greater than 60% and densities less than 0.6 g/cm.sup.3, have extremely low thermal conductivity and therefore find application as heat-insulating materials, as described in EP-A-0 171 722.
However, the high porosity also results in low mechanical stability, both of the gel from which the aerogel is dried and also of the dried aerogel itself.
It is also known that aerogels possess exceptionally low dielectric constants having values between 1 and 2, depending on the density of the aerogel. Aerogels are therefore also predestined for electronic applications, for example for high-frequency applications (S. C. W. Hrubesh et al., J. Mater, Res. Vol. 8, No. 7, 1736-1741).
In addition to the abovementioned mechanical disadvantages of aerogels, it is very disadvantageous for electronic applications if the dissipation factor is high. It is known that the dissipation factor on the internal surface is increased by hydrophilic and polar surface groups or adsorbed molecules.
Aerogels are prepared by drying a suitable gel. A dried gel is referred to as aerogel if the gel liquid is removed at temperatures above the critical temperature and starting from pressures above the critical pressure.
If, by contrast, the gel liquid is removed with formation of a liquid-vapor boundary phase, the resulting gel is often also referred to as a xerogel. Unless otherwise stated, the term "aerogel" in the present application also includes xerogels as well as mixtures of these.
The process of shaping of the aerogel is completed during the sol/gel transition. Once the solid gel structure has formed, the external shape can be changed only by comminution, for example by grinding.
For many applications, however, it is necessary to use the aerogels in particular shapes. For this it is necessary, following the preparation of the aerogel, i.e. following the drying, to carry out a shaping step without a significant change in the internal structure of the aerogel with respect to its application.
EP-A-0 377 301 discloses a stable, pumpable, aqueous suspension whose content of amorphous silica is, however, restricted to from 5 to 15% by weight. The low silica content makes possible the formation of a stable, pumpable, aqueous suspension, but excludes any application as a heat-insulating material.
EP-A-0 340 707 discloses an insulating material having a density of from 0.1 to 0.4 g/cm.sup.3 comprising at least 50% by volume of silica aerogel particles having a diameter between 0.5 and 5 mm, bound together by at least one organic and/or inorganic binder. The comparatively coarse particle size results in the insulating materials prepared having a non-uniform appearance, which is disadvantageous for many applications. Particularly thin layers (<<0.5 mm) are necessary especially for electronic applications, and these cannot be prepared using the abovementioned aerogel particles.
Furthermore, the coarse particle size of the aerogel makes it impossible to produce film-like heat-insulation articles having a thickness less than 0.5 mm. Film layers of greater thickness are also adversely affected by aerogel particles which are comparatively large in relation to the film thickness, because, especially at the perimeters, an increased binder proportion is necessary, and this has an adverse effect on the thermal conductivity and the electronic properties of the dried film or the dried shaped article.
EP-A-0 340 707 goes on to describe a process in which the aerogel particles are coated and/or mixed with a binder, and the paste thus obtained is then hardened in a mold. Because of the large density difference between the aerogel and the inorganic and/or organic binder, and the size of the aerogel particles, the mixed paste tends to separate, i.e. it is unstable over the relati
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Geiss Gerhard
Muller Hans-Karl
Prass Werner
Scheunemann Ude
Zimmermann Andreas
Hoechst Aktiengesellschaft
Lovering Richard D.
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