Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
1999-12-01
2001-09-18
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S521300, C423S022000
Reexamination Certificate
active
06290880
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to aerogel composite materials and methods of making them. In particular, the invention relates to an aerogel structure having an electrically connected network of ruthenium dioxide deposited throughout the structure and to methods of making the composite.
2. Background of the Related Art
Ruthenium dioxide (RuO
2
), one of the platinum group metal oxides, is an important industrial material due to its metallic electrical conductivity (RuO
2
single crystal conductivity approaches 10
5
S/cm at 25° C.) along with its excellent chemical and thermal stability and diffusion barrier properties. These characteristics have led to the use of ruthenium dioxide in electrodes for catalysis, electrolysis, photovoltaic devices, capacitors, thick and thin film resistors, etc.
Many techniques based on chemical vapor deposition (CVD) have been developed for depositing dense RuO
2
films on flat substrates, including: sputtering or evaporating ruthenium metal in the presence of oxygen; plasma decomposition of Ru-bearing gases by glow discharge; thermal or photolytic decomposition of one of several organometallic precursors. Deposition by reacting oxygen with evaporated metal vapor can be activated by applying a dc current or r.f. radiation, as described in U.S. Pat. No. 5,055,319 to Bunshah et al. In Yuan et al. “Low-Temperature Chemical Vapor Deposition of Ruthenium Dioxide from Ruthenium Tetroxide: A Simple Approach to High-Purity RuO2 Films” Chem. Mater. 5 (1993) pp 908-910, incorporated herein by reference, the deposition of RuO
4
, which spontaneously reduces to RuO
2
, by CVD is described. The precursor was either RuO
4
in a solution of water, pentane or carbon tetrachloride or pure RuO
4
solid. Using this approach, RuO
2
films 1-micron thick with resistivities of about 10
−2
ohm-cm were prepared.
For many RuO
2
applications such as catalytic and sensing applications, it is desirable that the RuO
2
material have the highest possible surface area in order to maximize the number of reaction sites. Conventionally, porous RuO
2
electrodes are prepared by dip-coating a substrate in RuCl
3
solution and heating in air to decompose the salt to RuO
2
. A technique for increasing the porosity of RuO
2
by doping the ruthenium chloride solution with lanthanum chloride and, after firing, removing the lanthanum oxide by dissolving in sulfuric acid is described in Takasu et al., J. Alloys Comp. 261 (1997) p. 172, incorporated herein by reference. The RuO
2
is stable and is five times “rougher” than the sample prepared without La doping. These materials have good electrical conductivity, but the surface area is still fairly low.
Aerogels are a class of materials typified by extremely high surface area (up to 1000 m
2
/g) and porosity (up to greater than 99%). These properties are generally achieved by extracting the solvent from the pores of a wet porous gel under supercritical conditions, thereby avoiding shrinkage caused by capillary forces that develop during ambient drying. Although a wide range of aerogel compositions are possible, silica is the most widely studied. When formed by catalyzed hydration and polycondensation of a metal alkoxide solution, followed by exchange of pore-filling solvent with, and then removal of, supercritical carbon dioxide, silica forms a relatively robust monolith with extremely low electrical and thermal conductivity.
Efforts have been made previously to develop techniques to deposit Ru oxide on porous substrates. U.S. Pat.No. 4,298,439 to Gafney, incorporated herein by reference, claims a process for adsorbing RuCl
3
in aqueous solution in/on a porous glass and then oxidizing in air at 120° C. for one week to obtain the oxide. There is no indication whether this process resulted in a conductive film. Miller et al, J. Electrochem Soc. 144 (1997) L309, incorporated herein by reference, discloses a method of depositing Ru oxide by heating a volatile organometallic Ru compound in the presence of carbon aerogel in a sealed reactor. Decomposing the deposited organometallic by heating in flowing argon resulted in 2-nm Ru particles dispersed throughout the aerogel pores. The Ru/carbon aerogel composite had significantly higher specific capacitance than the untreated aerogel, but the Ru phase did not form its own electrically conductive network.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrically conducting structure having a high surface area.
It is a further object of the present invention to provide a method of forming an electrically connected deposit of RuO
2
throughout an aerogel.
It is a fierier object of the present invention to provide a method of forming an electrically connected deposit of RuO
2
, wherein the method does not require high temperatures.
These and other objects are achieved by an electrically conducting composite made by a method comprising the steps of providing an aerogel structure, exposing the aerogel structure to a mixture of RuO
4
and a nonpolar solvent in an inert atmosphere, wherein the mixture is held initially at a first temperature that is below the ambient temperature and below the temperature at which RuO
4
decomposes into RuO
2
in the nonpolar solvent and in the presence of the aerogel, and allowing the mixture to warm to a second temperature that is above the temperature at which RuO
4
decomposes to RuO
2
in the nonpolar solvent and in the presence of the aerogel, wherein the rate of warming is controlled so that as the mixture warms and the RuO
4
begins to decompose into RuO
2
, the newly formed RuO
2
is deposited throughout the aerogel structure as an electrically connected conductive deposit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The aerogel structure of the present invention can be any conventionally known aerogel material. Preferably, the aerogel structure is made of a nonconducting material, such as silica. Typically, the aerogel structure is a silica aerogel prepared by acid- or base-catalyzed hydration and condensation of a metal alkoxide, tetramethoxysilane (TMOS), followed by washing to replace the pore liquid with acetone and then drying under supercritical CO
2
. The resulting monolithic aerogel consists of microporous (less than 2 nm pores) clusters that are about 10 nm in diameter, connected in a three-dimensional mesoporous (2-50 mn pores) network. The as-dried material has a surface area of about 800 m
2
/g. In order to strengthen the aerogel to allow refilling of the pores by a pentane solution, the aerogel is partially densified by sintering, typically at 900° C. After sintering, the mocropores are gone, and the partially densified aerogel is about 80% porous with a surface area of about 400-500 m
2
/g. Collapsing the micropores within the silica domains provides a material that still has an ultra-high surface area, but does not have an extensive microporous area that would trap and isolate a deposited material.
To create an electrically connected deposit of RuO
2
throughout the aerogel, the aerogel is exposed to a mixture of RuO
4
and a nonpolar solvent. A nonpolar solvent such as pentane is preferred over an aqueous or nonpolar solvent because it has a lower surface tension, which minimizes capillary forces during re-wetting and re-drying of the aerogel at subcritical conditions. The mixture is initially kept at a temperature below the ambient temperature and below the temperature at which RuO
4
decomposes into RuO
2
(the temperature varies according to the solvent). Then the mixture is allowed to warm above the temperature at which RuO
4
decomposes into RuO
2
in the particular solvent and in the presence of the aerogel structure (In the presence of a substrate such as an aerogel, RuO
4
decomposes at a lower temperature than it does in the absence of the substrate.). The rate of warming of the mixture is controlled so that the mixture has time to completely infiltrate the aerogel before the RuO
4
decomposes. In this way, when the RuO
4
decomposes,
Berry Alan D.
Long Jeffery W.
Merzbacher Celia I.
Rolison Debra R.
Ryan Joseph V.
Karasek John J.
Ketner Philip E.
Kopec Mark
The United States of America as represented by the Secretary of
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