Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-03-31
2002-12-10
Lovering, Richard D. (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C423S338000, C501S012000, C502S079000, C502S233000, C502S243000, C516S100000, C516S111000
Reexamination Certificate
active
06492014
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to composite gels and aerogels and more specifically to mesoporous composite gels and aerogels and their various uses.
2. Description of the Background Art
Xerogels and aerogels derived from the condensation and hydrolysis of metal alkoxide precursors have been studied for a variety of applications, including uses as optical, thermal, and electronic materials. Aerogels, because they are highly porous (80-99% by volume) and have a high surface area (up to 1000 m
2
/g), are especially well-suited to catalytic and sensing applications, where rapid transport of reactants (or detectable species) and large, accessible surface areas are critical to performance. In composite xerogels and aerogels, the gel structure can act as a host material for immobilized guest particles that perform catalytic, electrochemical or chemical sensing functions.
Typically, guest materials such as catalytic particles have been incorporated into xerogels and aerogels either by adding the guest material or a guest material precursor to a sol-gel precursor mixture before a sol-gel is formed or by impregnating materials into an already-formed xerogel or aerogel. A disadvantage to the method of adding a guest material to a sol-gel precursor mixture prior to forming a sol-gel is that the components may become so thoroughly mixed that the particles of the guest material become completely encapsulated by the sol-gel precursor material. Such encapsulation reduces the exposure of the particles of the guest material to the inner surface area of the subsequently formed gel and thus reduces the effectiveness of the composite for its intended use as a catalyst, sensor, fuel cell, etc. Further, thorough and prolonged mixing of a particulate guest material with a sol can lead to the loss of critical properties, particularly transport properties (which require intimate contact between guest particles) and chemical properties (which involve guest interaction with molecules in the mesopores). A disadvantage to the method of impregnating materials into an already formed aerogel is that the incorporated guest material may leach or wash out of the aerogel.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide new composite materials in which a guest solid particulate is fixed within a porous matrix.
It is an other object of the present invention to provide a new composite material in which a guest material fixed within a porous matrix can interact with an infiltrate within the matrix.
It is an other object of the present invention to provide a new composite material in which a guest material is incorporated into a porous matrix so that leaching or washing out of the guest material is minimized.
It is a further object of the present invention to provide new composite materials for use as catalysts; porous black composites (e.g., for blocking stray light); power source electrodes and electrode structures (where the term power source includes batteries, fuel cells, electrolytic capacitors, supercapacitors, photovoltaics, thermophotovoltaics, hybrid battery capacitors, etc.); thermoelectric materials; and chemical, optical, physical and biological sensors.
It is an additional object of the present invention to provide new, nanoscale porous composite materials that achieve transport paths for conductivity of ions, molecules, electrons, phonons, combinations thereof, etc., from guest-to-guest through the microstructure of the aerogel at low volume percentages of particulate guest.
These and additional objects of the invention are accomplished by commingling a particulate guest (such as a colloidal or dispersed (i.e., non-colloidal) solid or a powder) with a sol which is either about to gel or in which gelation has just started. After addition of the particulate, the mixture is then permitted to gel into a solid, gelled composite with open pores. This solid, gelled composite is then dried in a manner that prevents the collapse of open pores within the solid, gelled composite in which the gel acts as a “nanoglue” that holds the particles together. Introducing the guest particulate into a sol and forming a gel in this manner prevents encapsulation of the guest particles by the sol material while sufficiently incorporating the guest particles into the gel network so that the guest material does not leach or elute out during subsequent processing steps or during the subsequent use of the composite. The bulk and surface properties of both the guest material and sol material are retained on the nanoscale . The transport- and density-dependent properties of the composite gel can be tuned by varying the volume fraction of the guest material, thereby increasing the design flexibility of these nanoscale materials for optical, chemical, thermal, magnetic, and electronic applications. The chemical and physical properties of the composite material can be further engineered at multiple points during sol-gel processing by modifying the host solid, the guest solid, the composite gel, or the composite aerogel.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Throughout this application, all references cited are incorporated by reference in their entirety and for all purposes.
Typical precursors for gels or aerogels are metal alkoxides represented by the general formula (M(OR)
n
). For silica structures, the typical precursor is an oxysilane represented by the general formula (Si(L)
4-n
(OR)
n
, where R is organic (typically alkyl), where each —OR may be the same or different if more than one —OR is attached to the silicon, where n is an integer having a value of 1 to 4, and where L is any group other than —OR.
As used herein, the terms “sol”, “gel”, “xerogel” and “aerogel” are used in their commonly accepted meanings. In particular, the term “sol” refers to a colloidal suspension of precursor particles and “gel” refers to a wet three-dimensional porous network obtained by condensation of the precursor particles. Examples of sols include, but are not limited to silica sols, zirconia sols, vanadia sols, manganese oxide sols, magnesia sols, niobium oxide sols, alumina sols, tungsten oxide sols, yttria sols, tin oxide sols, cobalt oxide sols, nickel oxide sols, ceria sols, titania sols, calcia sols, aluminosilicate sols, or mixtures thereof. The sol could also be an non-oxidic or organic sol. As used herein, the term “network” is defined conventionally to mean a solid frame that sustains its shape and weight in the environment in which it is formed. That environment is the volume defined by the liquid phase precursors (solvent and any solutes) filling the vessel used for gelation. In the specification and the claims that follow, the onset of gelation is defined as the time at which the colloidal particles that comprise the sol (not to be confused with colloidal guest particles) begin to link together in the reaction volume. This point is accompanied by an increase in viscosity. In xerogels, the gel is dried under ambient conditions, leading to collapse of the pores, densification of the oxide structure and considerable shrinkage. In aerogels, the gel is dried under supercritical conditions to form a high surface area, high-porosity, ultra-low-density material. In supercritical drying, the pore-filling liquid is taken above its supercritical temperature and pressure before extraction, which prevents capillary forces from developing and then collapsing the pores of the gel. Other methods for preventing the collapsing of the pores and for forming aerogels are known, including evaporation of low surface tension liquids from the pores, freeze-dry extraction of the pore fluid, the addition of a low surface tension agent followed by evaporation, silanization of the wet gel followed by evaporation, etc.
In the present invention, a gel composite is formed by adding a guest particulate to a sol at or near the onset of gelation. The guest particulate may be in the form of a dispersed particulate, colloidal suspension or powder.
As used herein, the ter
Anderson Michelle L.
Cepak Veronica M.
Merzbacher Celia I.
Morris Catherine A.
Rolison Debra R.
Grunkemeyer Joseph T.
Lovering Richard D.
The United States of America as represented by the Secretary of
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