 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with a tin oxide-containing material, preferably an electronically conductive tin oxide-containing material.
Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery of choice for general purposes, such as starting an automotive vehicle, boat or airplane engine, emergency lighting,
electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multi-cell structure. Each cell comprises a set of vertical positive and negative plates formed of lead-acid alloy grids containing layers of electrochemically active pastes. The paste on the positive
plate when charged comprises lead dioxide, which is the positive active material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte, based on sulfuric acid, is interposed between the positive and
negative plates.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce light-weight lead-acid batteries, especially in the aircraft, electric car and automotive vehicle fields, have
placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density.
Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connected electrodes or cells. The bipolar plates must be impervious to electrolyte and be electrically conductive to
provide a serial connection between electrodes.
U.S. Pat. Nos. 4,275,130; 4,353,969; 4,405,697; 4,539,268; 4,507,372; 4,542,082; 4,510,219; and 4,547,443 relate to various aspects of lead-acid batteries. Certain of these patents discuss various aspects of bipolar plates.
Attempts have been made to improve the conductivity and strength of bipolar plates. Such attempts include the use of conductive carbon particles or filaments such as carbon, graphite or metal in a resin binder. However, such carbon-containing
materials are oxidized in the aggressive electrochemical environment of the positive plates in the lead-acid cell to acetic acid, which in turn reacts with the lead ion to form lead acetate, which is soluble in sulfuric acid. Thus, the active material
is gradually depleted from the paste and ties up the lead as a salt which does not contribute to the production or storage of electricity.
The metals fare no better; most metals are not capable of withstanding the high potential and strong acid environment present at the positive plates of a lead-acid battery. While some metals, such as platinum, are electrochemically stable, their
prohibitive cost prevents their use in high volume commercial applications of the lead-acid battery.
One approach that shows promise of providing benefits in lead acid batteries is a battery element, useful as at least a portion of the positive plates of the battery, which comprises an acid resistant substrate coated with a stable doped tin
oxide.
The combination of an acid resistant substrate coated with doped tin oxide has substantial electrical, chemical, physical and mechanical properties making it useful as a lead-acid battery element. For example, the element has substantial
stability in the presence of, and is impervious to, the sulfuric acid or the sulfuric acid-based electrolyte. The doped tin oxide coating on the acid resistant substrate provides for increased electrochemical stability and reduced corrosion in the
aggressive, oxidative-acidic conditions present on the positive side of lead-acid batteries.
Another application where substrates with coatings, e.g., electronically conductive coatings, find particular usefulness is in the promotion of chemical reactions, e.g., gas/liquid phase reactions, electro catalytic reactions, photo catalytic
reactions, redox reactions, etc. As an example of a type of reaction system, a catalytic, e.g., metallic, component is contacted with the material to be reacted, e.g., nitrogen oxides to be reduced, and a reducing gas is passed through or near to the
catalytic component to enhance the chemical reaction, e.g., the nitrogen oxide reduction to nitrogen. In addition, using a substrate for the catalytic component which is coated with an electronically conductive material is highly advantageous for
electro catalysis since an electronic field/current can be effectively and efficiently provided to or near the catalytic component for electron transfer reactions. Many types of chemical reactions can be advantageously promoted using coated substrates.
Tin-oxide containing coatings on substrates may promote electron transfer whether or not the chemical reaction is conducted in the presence of an electrical current or field. In addition, tin oxide coated substrates and sintered tin dioxides are useful
as gas sensors, as gas purifiers, as flocculants and as filter medium components. One or more other components, e.g., metal components, are often included in certain of these applications.
In many of the above-noted applications it would be advantageous to have an electronically conductive tin oxide which is substantially uniform, has high electronic conductivity, and has good chemical properties, e.g., morphology, stability, etc.
A number of techniques may be employed to provide conductive tin oxide coatings on acid resistant substrates. For example, the chemical vapor deposition (CVD) process may be employed. This process comprises contacting a substrate with a
vaporous composition comprising a tin component and a dopant-containing material and contacting the contacted substrate with an oxygen-containing vaporous medium at conditions effective to form the doped tin oxide coating on the substrate.
Conventionally, the CVD process occurs simultaneously at high temperatures at very short contact times so that tin oxide is initially deposited on the substrate. However tin oxide can form off the substrate resulting in a low reagent capture rate. The
CVD process is well known in the art for coating a single flat surface which is maintained in a fixed position during the above-noted contacting steps. The conventional CVD process is an example of a "line-of-sight" process or a "two dimensional"
process in which the tin oxide is formed only on that portion of the substrate directly in the path of the tin source as tin oxide is formed on the substrate. Portions of the substrate which are shielded from the tin oxide being formed, e.g., such as
pores which extend inwardly from the external surface and substrate layers which are at least partially shielded from the depositing tin oxide by one or more other layers closer to the external substrate surface, do not get uniformly coated, if at all,
in a "line-of-sight" process. A particular problem with "line-of-sight" processes is the need to maintain a fixed distance between the tin source and the substrate. Otherwise, tin dioxide can be deposited or formed off the substrate and lost, with a
corresponding loss in process and reagent efficiency.
One of the preferred substrates for use with batteries, such as lead-acid batteries, are glass fibers, in particular a porous mat of glass fibers. Although the CVD process is useful for coating a single flat surface, for the reasons noted above
this process tends to produce non-uniform and/or discontinuous coatings on woven glass fiber mats. Such non uniformities and/or discontinuities are detrimental to the electrical and chemical properties of the coated substrate. A new process, e.g., a
"non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with tin oxide which
surfaces would not be directly exposed to vaporous tin oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step. In other words, a "three dimensional" process coats coatable substrate surfaces
which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate, e.g., the surfaces of the internal fibers of a porous mat of glass fibers.
In "Preparation of Thick Crystalline Films of Tin Oxide and Porous Glass Partially Filled with Tin Oxide", by R. G. Bartholomew et al, J. Electrochem, Soc. Vol 116, No. 9, p 1205(1969), a method is described for producing films of SnO.sub.2 on a
96% silica glass substrate by oxidation of stannous chloride. The plates of glass are pretreated to remove moisture, and the entire coating process appears to have been done under anhydrous conditions. Specific electrical resistivity values for
SnO.sub.2 -porous glass were surprisingly high. In addition, doping with SbCl.sub.3 was attempted, but substantially no improvement, i.e., reduction, in electrical resistivity was observed. Apparently, no effective amount of antimony was incorporated.
No other dopant materials were disclosed.
In "Physical Properties of Tin Oxide Films Deposited by Oxidation of SnCl.sub.2 ", by N. Srinivasa Murty et al, Thin Solid Films, 92(1982) 347-354, a method for depositing SnO.sub.2 films was disclosed which involved contacting a substrate with a
combined vapor of SnCl.sub.2 and oxygen. Although no dopants were used, dopant elements such as antimony and fluorine were postulated as being useful to reduce the electrical resistivity of the SnO.sub.2 films.
This last described method is somewhat similar to the conventional spray pyrolysis technique for coating substrates. In the spray pyrolysis approach, tin chloride dissolved in water at low pH is sprayed onto a hot, i.e., on the order of about
600.degree. C., surface in the presence of an oxidizing vapor, e.g., air. The tin chloride is immediately converted, e.g., by hydrolysis and/or oxidation, to SnO.sub.2, which forms a film on the surface. In order to get a sufficient SnO.sub.2 coating
on a glass fiber substrate to allow the coated substrate to be useful as a component of a lead-acid battery, on the order of about 20 spraying passes on each side have been required. In other words, it is frequently difficult, if not impossible, with
spray pyrolysis to achieve the requisite thickness and uniformity of the tin oxide coating on substrates, in particular three dimensional substrates.
Dislich, et al U.S. Pat. No. 4,229,491 discloses a process for producing cadmium stannate layers on a glass substrate. The process involves dipping the substrate into an alcoholic solution of a reaction product containing cadmium and tin;
withdrawing the substrate from the solution in a humid atmosphere; and gradually heating the coated substrate to 650.degree. C. whereby hydrolysis and pyrolysis remove residues from the coated substrate. Dislich, et al is not concerned with coating
substrates for lead-acid batteries, let alone the stability required, and is not concerned with maintaining a suitable concentration of a volatile dopant, such as fluoride, in the coating composition during production of the coated substrate.
Pytlewski U.S. Pat. No. 4,229,491 discloses changing the surface characteristics of a substrate surface, e.g., glass pane, by coating the surface with a tin-containing polymer. These polymers, which may contain a second metal such as iron,
cobalt, nickel, bismuth, lead, titanium, vanadium, chromium, copper, molybdenum, antimony and tungsten, are prepared in the form of a colloidal dispersion of the polymer in water. Pytlewski discloses that such polymers, when coated on glass surfaces,
retard soiling. Pytlewski is not concerned with the electrical properties of the polymers or of the coated substrate surfaces.
Gonzalez-Oliver, C. J. R. and Kato, I. in "Sn (Sb)-Oxide Sol-Gel Coatings of Glass", Journal of Non-Crystalline Solids 82(1986) 400-410 North-Holland, Amsterdam, describe a process for applying an electrically conductive coating to glass
substrates with solutions containing tin and antimony. This coating is applied by repeatedly dipping the substrate into the solution or repeatedly spraying the solution onto the substrate. After each dipping or spraying, the coated substrate is
subjected to elevated temperatures on the order of 550.degree. C.-600.degree. C. to fully condense the most recently applied layer. Other workers, e.g., R. Pryane and I. Kato, have disclosed coating glass substrates, such as electrodes, with doped tin
oxide materials. The glass substrate is dipped into solution containing organo-metallic compounds of tin and antimony. Although multiple dippings are disclosed, after each dipping the coated substrate is treated at temperatures between 500.degree. C.
and 63020 C. to finish off the polycondensation reactions, particularly to remove deleterious carbon, as well as to increase the hardness and density of the coating.
Although a substantial amount of work has been done, there continues to be a need for a new method for coating substrates with doped tin oxide.
SUMMARY OF THE INVENTION
A new process for at least partially coating a substrate with a doped tin oxide-forming material has been discovered. In brief, the process comprises contacting the substrate with stannous chloride, in a vaporous form and/or in a liquid form, to
form a stannous chloride-containing coating on the substrate; contacting the substrate with a fluorine component, i.e., a component containing free fluorine and/or combined fluorine (as in a compound), to form a fluorine component-containing coating on
the substrate; and contacting the thus coated substrate with an oxidizing agent to form a fluorine doped tin oxide, preferably tin dioxide, coating on the substrate. The contacting of the substrate with the stannous chloride and with the fluorine
component can occur together, i.e., simultaneously, and/or in separate steps.
This process can provide coated substrates which have substantial electronic conductivity so as to be suitable for use as components in batteries, such as lead-acid storage batteries. Substantial coating uniformity, e.g., in the thickness of the
tin oxide-containing coating and in the distribution of dopant component in the coating, is obtained. Further, the present fluorine doped tin oxide coated substrates have outstanding stability, e.g., in terms of electrical properties and morphology, and
are thus useful in various applications. In addition, the process is efficient in utilizing the materials which are employed to form the coated substrate.
DETAILED DESCRIPTION OF THE INVENTION
In one broad aspect, the present coating process comprises contacting a substrate with a composition comprising tin chloride forming components, including stannic chloride, stannous chloride and mixtures thereof, preferably stannous chloride, at
conditions, preferably substantially non-deleterious oxidizing conditions, more preferably in a substantially inert environment or atmosphere, effective to form a stannous chloride-containing coating on at least a portion of the substrate. The substrate
is also contacted with at least one fluorine component at conditions, preferably substantially non-deleterious oxidizing conditions, more preferably in a substantially inert atmosphere, effective to form a fluorine component-containing coating on at
least a portion of the substrate. This substrate, including one or more coatings containing tin chloride, preferably stannous chloride, and fluorine component, is contacted with at least one oxidizing agent at conditions effective to convert tin
chloride to tin oxide and form a fluorine doped tin oxide, preferably tin dioxide, coating on at least a portion of the substrate. By "non-deleterious oxidation" is meant that the majority of the oxidation of stannous chloride coated onto the substrate
takes place in the oxidizing agent contacting step of the process, rather than in process step or steps conducted at non-deleterious oxidizing conditions. The process as set forth below will be described with reference to stannous chloride, which has
been found to provide particularly outstanding process and product properties.
The fluorine component-containing coating may be applied to the substrate before and/or after and/or during the time the substrate is coated with stannous chloride. In a particularly useful embodiment, the stannous chloride and the fluorine
component are both present in the same composition used to contact the substrate so that the stannous chloride-containing coating further contains the fluorine component. This embodiment provides processing efficiencies since the number of process steps
is reduced (relative to separately coating the substrate with stannous chloride and fluorine component). In addition, the relative amount of stannous chloride and fluorine component used to coat the substrate can be effectively controlled in this
"single coating composition" embodiment of the present invention.
In another useful embodiment, the substrate with the stannous chloride-containing coating and the dopant component-containing coating is maintained at conditions, preferably at substantially non-deleterious oxidizing conditions, for a period of
time effective to do at least one of the following: (1) coat a larger portion of the substrate with stannous chloride-containing coating; (2) distribute the stannous chloride coating over the substrate; (3) make the stannous chloride-containing coating
more uniform in thickness; and (4) distribute the dopant component more uniformly in the stannous chloride-containing coating. Such maintaining preferably occurs for a period of time in the range of about 1 minute to about 20 minutes. Such maintaining
is preferably conducted at the same or a higher temperature relative to the temperature at which the substrate/stannous chloride-containing composition contacting occurs. Such maintaining, in general, acts to make the coating more uniform and, thereby
provides for beneficial electrical conductivity properties. The thickness of the tin oxide-containing coating is preferably in the range of about 0.1 micron to about 10 microns, more preferably about 0.25 micron to about 1.25 microns.
The stannous chloride which is contacted with the substrate is in a vaporous phase or state, or in a liquid phase or state at the time of the contacting. The composition which includes the stannous chloride preferably also includes the fluorine
component or components. This composition may also include one or more other materials, e.g., dopants, catalysts, grain growth inhibitors, solvents, etc., which do not substantially adversely promote the premature hydrolysis of the stannous chloride
and/or the fluorine component, and do not substantially adversely affect the properties of the final product, such as by leaving a detrimental residue in the final product prior to the formation of the tin oxide-containing coating. Thus, it has been
found to be important, e.g., to obtaining a tin oxide coating with good structural and/or electronic properties, that undue hydrolysis of the stannous chloride and fluorine component be avoided. This is contrary to certain of the prior art which
actively utilized the simultaneous hydrolysis reaction as an approach to form the final coating. Examples of useful other materials include organic components such as acetonitrile, ethyl acetate, dimethyl sulfoxide, propylene carbonate and mixtures
thereof; certain inorganic salts and mixtures thereof. These other materials, which are preferably substantially anhydrous, may often be considered as a carrier, e.g., solvent, for the stannous chloride and/or fluorine component to be contacted with the
substrate. It has also been found that the substrate can first be contacted with a stannous chloride powder, preferably with a film forming amount of stannous chloride powder, followed by increasing the temperature to the liquidus point of the stannous
chloride powder on the substrate to allow coating of, and preferably equilibration on, the substrate. The size distribution of the stannous chloride powder and the amount of such powder applied to the substrate are preferably chosen so as to distribute
the coating over substantially the entire substrate.
The stannous chloride and/or fluorine component to be contacted with the substrate may be present in a molten state. For example, a melt containing molten stannous chloride and/or stannous fluoride may be used. The molten composition may
include one or more other materials, having properties as noted above, to produce a mixture, e.g., a eutectic mixture, having a reduced melting point and/or boiling point. The use of molten stannous chloride and/or fluorine component provides
advantageous substrate coating while reducing the handling and disposal problems caused by a solvent. In addition, the substrate is very effectively and efficiently coated so that coating material losses are reduced.
The stannous chloride and/or fluorine component to be contacted with the substrate may be present in a vaporous state. As used in this context, the term "vaporous state" refers to both a substantially gaseous state and a state in which the
stannous chloride and/or fluorine component are present as drops or droplets in a carrier gas, i.e., an atomized state. Liquid state stannous chloride and/or fluorine component may be utilized to generate such vaporous state compositions.
In addition to the other materials, as noted above, the composition containing stannous chloride and/or the fluorine component may also include one or more grain growth inhibitor components. Such inhibitor component or components are present in
an amount effective to inhibit grain growth in the tin oxide-containing coating. Reducing grain growth leads to beneficial coating properties, e.g., higher electrical conductivity, more uniform morphology, and/or greater overall stability. Among useful
grain growth inhibitor components are components which include at least one metal, in particular potassium, calcium, magnesium, silicon and mixtures thereof. Of course, such grain growth inhibitor components should have no substantial detrimental effect
on the final product.
The fluorine component may be deposited on the substrate separately from the stannous chloride, e.g., before and/or during and/or after the stannous chloride/substrate contacting. If the fluorine component is deposited on the substrate
separately from the stannous chloride, it is preferred that the fluorine component be deposited after the stannous chloride.
Any suitable fluorine component may be employed in the present process. Such fluorine component should provide sufficient fluorine, e.g., fluoride, dopant so that the final fluorine doped tin oxide coating has the desired properties, e.g.,
electronic conductivity, stability, etc. Care should be exercised in choosing the fluorine component or components for use. For example, the fluorine component should be sufficiently compatible with the stannous chloride so that the desired fluorine
doped tin oxide coating can be formed. Fluorine components which have excessively high boiling points and/or are excessively volatile (relative to stannous chloride), at the conditions employed in the present process, are to be avoided since, for
example, the final coating may not be sufficiently doped and/or a relatively large amount of the fluorine component or components may be lost during processing. It may be useful to include one or more property altering components, e.g., boiling point
depressants, in the composition containing the fluorine component to be contacted with the substrate. Such property altering component or components are included in an amount effective to alter one or more properties, e.g., boiling point, of the
fluorine component, e.g., to improve the compatibility or reduce the incompatibility between the fluorine component and stannous chloride.
Particularly useful fluorine components for use in the present invention are selected from stannous fluoride, stannic fluoride, hydrogen fluoride and mixtures thereof. When hydrogen fluoride is used in the present invention, it has been found
that excellent dopant incorporation is achieved. It is believed that the hydrogen fluoride is able to react with the tin chloride compound, for example, stannous chloride, to form tin fluoride, for example, stannous fluoride, which provides for
substantial dopant incorporation. When stannous fluoride is used as a fluorine component, it is preferred to use one or more boiling point depressants to reduce the apparent boiling point of the stannous fluoride, in particular to at least about
850.degree. C. or less.
The use of a fluorine dopant is an important feature of certain aspects of the present invention. First, it has been found that fluorine dopants can be effectively and efficiently incorporated into the tin oxide-containing coating. In addition,
such fluorine dopants act to provide tin oxide-containing coatings with good electronic properties referred to above, morphology and stability. This is in contrast to certain of the prior art which found antimony dopants to be ineffective to improve the
electronic properties of tin oxide coatings.
The liquid, e.g., molten, composition which includes stannous chloride may, and preferably does, also include the fluorine component. In this embodiment, the fluorine component or components are preferably soluble in the composition. Vaporous
mixtures of stannous chloride and fluorine components may also be used. Such compositions are particularly effective since the amount of dopant in the final doped tin oxide coating can be controlled by controlling the make-up of the composition. In
addition, both the stannous chloride and fluorine component are deposited on the substrate in one step. Moreover, if stannous fluoride and/or stannic fluoride are used, such fluorine components provide the dopant and are converted to tin oxide during
the oxidizing agent/substrate contacting step. This enhances the overall utilization of the coating components in the present process. Particularly useful compositions comprise about 50% to about 98%, more preferably about 70% to about 95%, by weight
of stannous chloride and about 2% to about 50%, more preferably about 5% to about 30%, by weight of fluorine component, in particular stannous fluoride.
In one embodiment, a vaporous stannous chloride composition is utilized to contact the substrate, and the composition is at a higher temperature than is the substrate. The make-up of the vaporous stannous chloride-containing composition is such
that stannous chloride condensation occurs on the cooler substrate. If the fluorine component is present in the composition, it is preferred that such fluorine component also condense on the substrate. The amount of condensation can be controlled by
controlling the chemical make-up of the vaporous composition and the temperature differential between the composition and the substrate. This "condensation" approach very effectively coats the substrate to the desired coating thickness without requiring
that the substrate be subjected to numerous individual or separate contactings with the vaporous stannous chloride-containing composition. As noted above, previous vapor phase coating methods have often been handicapped in requiring that the substrate
be repeatedly recontacted in order to get the desired coating thickness. The present "condensation" embodiment reduces or eliminates this problem.
The substrate including the stannous chloride-containing coating and the fluorine component-containing coating is contacted with an oxidizing agent at conditions effective to convert stannous chloride to tin oxide, preferably substantially tin
dioxide, and form a fluorine doped tin oxide coating on at least a portion of the substrate. Water, e.g., in the form of a controlled amount of humidity, is preferably present during the coated substrate/oxidizing agent contacting. The vapor can be
added at elevated temperatures as saturated or super saturated steam in order to enhance overall conversion to tin oxide at reduced contacting times. This is in contrast with certain prior tin oxide coating methods which are conducted under anhydrous
conditions. The presence of water during this contacting has been found to provide a doped tin oxide coating having excellent electrical conductivity properties.
Any suitable oxidizing agent may be employed, provided that such agent functions as described herein. Preferably, the oxidizing agent (or mixtures of such agents) is substantially gaseous at the coated substrate/oxidizing agent contacting
conditions. The oxidizing agent preferably includes reducible oxygen, i.e., oxygen which is reduced in oxidation state as a result of the coated substrate/oxidizing agent contacting. More preferably, the oxidizing agent comprises molecular oxygen,
either alone or as a component of a gaseous mixture, e.g., air.
The substrate may be composed of any suitable material and may be in any suitable form. Preferably, the substrate is such so as to minimize or substantially eliminate the migration of ions and other species, if any, from the substrate to the tin
oxide-containing coating which are deleterious to the functioning or performance of the coated substrate in a particular application. In addition, it can be precoated to minimize migration, for example a silica precoat and/or to improve wetability and
uniform distribution of the coating materials on the substrate. In order to provide for controlled electronic conductivity in the fluorine doped tin oxide coating, it is preferred that the substrate be substantially non-electronically conductive when
the coated substrate is to be used as a component of an electric energy storage battery. In one embodiment, the substrate is inorganic, for example glass and/or ceramic. Although the present process may be employed to coat two dimensional substrates,
such as substantially flat surfaces, it has particular applicability in coating three dimensional substrates. Thus, the present process is a three dimensional process. Examples of three dimensional substrates which can be coated using the present
process include spheres, extrudates, flakes, single fibers, fiber rovings, chopped fibers, fiber mats, porous substrates, irregularly shaped particles, e.g., catalyst supports, multi-channel monoliths and the like. Acid resistant glass fibers,
especially woven and non-woven mats of acid resistant glass fibers, are particularly useful substrates when the fluorine doped tin oxide coated substrate is to be used as a component of a battery, such as a lead-acid electrical energy storage battery.
More particularly, the substrate for use in a battery is in the form of a body of woven or non-woven fibers, still more particularly, a body of fibers having a porosity in the range of about 60% to about 95%. Porosity is defined as the percent or
fraction of void space within a body of fibers. The above-noted porosities are calculated based on the fibers including the desired fluorine doped tin oxide coating.
The substrate for use in lead-acid batteries, because of availability, cost and performance considerations, preferably comprises acid resistant glass, more preferably in the form of fibers, as noted above.
The substrate for use in lead-acid batteries is acid resistant. That is, the substrate exhibits some resistance to corrosion, erosion and/or other forms of deterioration at the conditions present, e.g., at or near the positive plate, or positive
side of the bipolar plates, in a lead-acid battery. Although the fluorine doped tin oxide coating does provide a degree of protection for the substrate against these conditions, the substrate should itself have an inherent degree of acid resistance. If
the substrate is acid resistant, the physical integrity and electrical effectiveness of the doped tin oxide coating and of the whole present battery element, is better maintained with time relative to a substrate having reduced acid resistance. If glass
is used as the substrate, it is preferred that the glass have an increased acid resistance relative to E-glass. Preferably, the acid resistant glass substrate is at least as resistant as is C-or T- glass to the conditions present in a lead-acid battery.
Typical compositions of E-glass and C-glass are as follows:
______________________________________ Weight Percent E-glass C-glass T-glass ______________________________________ Silica 54 65 Alumina 14 4 6 Calcia 18 14 10* Magnesia 5 3 -- Soda + Potassium Oxide 0.5 9 13 Boria 8 5 6 Titania + Iron
Oxide 0.5 -- -- ______________________________________ *including MgO
Preferably the glass contains more than about 60% by weight of silica and less than about 35% by weight of alumina, and alkali and alkaline earth metal oxides.
The conditions at which each of the steps of the present process occur are effective to obtain the desired result from each such step and to provide a fluorine doped tin oxide coated substrate. The substrate/stannous chloride contacting and the
substrate/fluorine component contacting preferably occur at a temperature in the range of about 250.degree. C. to about 375.degree. C., more preferably about 275.degree. C. to about 350.degree. C. The amount of time during which stannous chloride
and/or fluorine component is being deposited on the substrate depends on a number of factors, for example, the desired thickness of the doped tin oxide coating, the amounts of stannous chloride and fluorine component available for substrate contacting,
the method by which the stannous chloride and fluorine component are contacted with the substrate and the like. Such amount of time is preferably in the range of about 0.5 minutes to about 20 minutes, more preferably about 1 minute to about 10 minutes.
If the coated substrate is maintained in a substantially non-deleterious oxidizing environment, it is preferred that such maintaining occur at a temperature in the range of about 275.degree. C. to about 375.degree. C., more preferably about
300.degree. C. to about 350.degree. C. for a period of time in the range of about 0.1 minutes to about 20 minutes, more preferably about 1 minute to about 10 minutes. The coated substrate/oxidizing agent contacting preferably occurs at a temperature
in the range of about 350.degree. C. to about 600.degree. C., more preferably about 400.degree. C. to about 550.degree. C., for a period of time in the range of about 0.1 minutes to about 10 minutes. A particular advantage of the process of this
invention is the temperatures used for oxidation have been found to be lower, in certain cases, significantly lower, i.e., 50.degree. to 100.degree. C. than the temperatures required for spray hydrolysis. This is very significant and unexpected,
provides for process efficiencies and reduces, and in some cases substantially eliminates, migration of deleterious elements from the substrate to the tin oxide layer. Excessive sodium migration, e.g., from the substrate, can reduce electronic
conductivity.
The pressure existing or maintained during each of these steps may be independently selected from elevated pressures (relative to atmospheric pressure), atmospheric pressure, and reduced pressures (relative to atmospheric pressure). Slightly
reduced pressures, e.g., less than atmospheric pressure and greater than about 8 psia and especially greater than about 11 psia, are preferred.
The fluorine doped tin oxide coated substrate of the present invention may be, for example, a catalyst itself or a component of a composite together with one or more matrix materials. The composites may be such that the matrix material or
materials substantially totally encapsulate or surround the coated substrate, or a portion of the coated substrate may extend away from the matrix material or materials.
Any suitable matrix material or materials may be used in a composite with the fluorine doped tin oxide coated substrate. Preferably, the matrix material comprises a polymeric material, e.g., one or more synthetic polymers, more preferably an
organic polymeric material. The polymeric material may be either a thermoplastic material or a thermoset material. Among the thermoplastics useful in the present invention are the polyolefins, such as polyethylene, polypropylene, polymethylpentene and
mixtures thereof; and poly vinyl polymers, such as polystyrene, polyvinylidene difluoride, combinations of polyphenylene oxide and polystyrene, and mixtures thereof. Among the thermoset polymers useful in the present invention are epoxies,
phenol-formaldehyde polymers, polyesters, polyvinyl esters, polyurethanes, melamineformaldehyde polymers, and urea-formaldehyde polymers. Also included among the useful polymeric materials are natural and synthetic rubber materials, such as styrene
butadiene rubbers; acrylonitrile rubbers, such as acrylonitrile butadiene styrene rubbers; ethylene propylene rubbers; chlorinated derivatives thereof; and mixtures thereof.
When used in battery applications, the present doped tin oxide coated substrate is preferably at least partially embedded in a matrix material. The matrix material should be at least initially fluid impervious to be useful in batteries. If the
fluorine doped tin oxide coated substrate is to be used as a component in a battery, e.g., a lead-acid electrical energy storage battery, it is situated so that at least a portion of it contacts the positive active electrode material. Any suitable
positive active electrode material or combination of materials useful in lead-acid batteries may be employed in the present invention. One particularly useful positive active electrode material comprises electrochemically active lead oxide, e.g., lead
dioxide, material. A paste of this material is often used. If a paste is used in the present invention, it is applied so that there is appropriate contacting between the fluorine doped tin oxide coated substrate and the paste.
In order to provide enhanced bonding between the fluorine doped tin oxide coated substrate and the matrix material, it has been found that the preferred matrix materials have an increased polarity, as indicated by an increased dipole moment,
relative to the polarity of polypropylene. Because of weight and strength considerations, if the matrix material is to be a thermoplastic polymer, it is preferred that the matrix be a polypropylene-based polymer which includes one or more groups
effective to increase the polarity of the polymer relative to polypropylene. Additive or additional monomers, such as maleic anhydride, vinyl acetate, acrylic acid, and the like and mixtures thereof, may be included prior to propylene polymerization to
give the product propylene-based polymer increased polarity. Hydroxyl groups may also be included in a limited amount, using conventional techniques, to increase the polarity of the final propylene-based polymer.
Thermoset polymers which have increased polarity relative to polypropylene are more preferred for use as the present matrix material. Particularly preferred thermoset polymers include epoxies, phenol-formaldehyde polymers, polyesters, and
polyvinyl esters.
A more complete discussion of the presently useful matrix materials is presented in Fitzgerald, et al U.S. Pat. No. 4,708,918, the entire disclosure of which is hereby incorporated by reference herein.
Various techniques, such as casting, molding and the like, may be used to at least partially encapsulate or embed the fluorine doped tin oxide coated substrate into the matrix material or materials and form composites. The choice of technique
may depend, for example, on the type of matrix material used, the type and form of the substrate used and the specific application involved. Certain of these techniques are presented in U.S. Pat. No. 4,547,443, the entire disclosure of which is hereby
incorporated by reference herein. One particular embodiment involves pre-impregnating (or combining) that portion of the doped tin oxide coated substrate to be embedded in the matrix material with a relatively polar (increased polarity relative to
polypropylene) thermoplastic polymer, such as polyvinylidene difluoride, prior to the coated substrate being embedded in the matrix material. This embodiment is particularly useful when the matrix material is itself a thermoplastic polymer, such as
modified polypropylene, and has been found to provide improved bonding between the fluorine doped tin oxide coated substrate and the matrix material.
The bonding between the matrix material and the fluorine doped tin oxide coated, acid-resistant substrate is important to provide effective battery operation. In order to provide for improved bonding of the fluorine doped tin oxide coating (on
the substrate) with the matrix material, it is preferred to at least partially, more preferably substantially totally, coat the fluorine doped tin oxide coated substrate with a coupling agent which acts to improve the bonding of the fluorine doped tin
oxide coating with the matrix. This is particularly useful when the substrate comprises acid resistant glass fibers. Any suitable coupling agent may be employed. Such agents preferably comprise molecules which have both a polar portion and a non-polar
portion. Certain materials generally in use as sizing for glass fibers may be used here as a "size" for the doped tin oxide coated glass fibers. The amount of coupling agent used to coat the fluorine doped tin oxide coated glass fibers should be
effective to provide the improved bonding noted above and, preferably, is substantially the same as is used to size bare glass fibers. Preferably, the coupling agent is selected from the group consisting of silanes, silane derivatives, stannates,
stannate derivatives, titanates, titanate derivatives and mixtures thereof. U.S. Pat. No. 4,154,638 discloses one silane-based coupling agent adapted for use with tin oxide surfaces. The entire disclosure of this patent is hereby expressly
incorporated by reference herein.
In the embodiment in which the fluorine doped tin oxide coated substrate is used as a component of a bipolar plate in a lead-acid battery, it is preferred to include a fluid-impervious conductive layer that is resistant to reduction adjacent to,
and preferably in electrical communication with, the second surface of the matrix material. The conductive layer is preferably selected from metal, more preferably lead, and substantially non-conductive polymers, more preferably synthetic polymers,
containing conductive material. The non-conductive polymers may be chosen from the polymers discussed previously as matrix materials. One particular embodiment involves using the same polymer in the matrix material and in the conductive layer. The
electrically conductive material contained in the non-conductive layer preferably is selected from the group consisting of graphite, lead and mixtures thereof.
In the bipolar plate configuration, a negative active electrode layer located adjacent, and preferably in electric communication with, the fluid impervious conductive layer is included. Any suitable negative active electrode material useful in
lead-acid batteries may be employed. One particularly useful negative active electrode material comprises lead, e.g., sponge lead. Lead paste is often used.
In yet another embodiment, a coated substrate including tin oxide, preferably electronically conductive tin oxide, and at least one additional catalyst component in an amount effective to promote a chemical reaction is formed. Preferably, the
additional catalyst component is a metal and/or a component of a metal effective to promote the chemical reaction. The promoting effect of the catalyst component may be enhanced by the presence of an electrical field or electrical current in proximity
to the component. Thus, the tin oxide, preferably on a substantially non-electronically conductive substrate, e.g., a catalyst support, can provide an effective and efficient catalyst for chemical reactions, including those which occur or are enhanced
when an electric field or current is applied in proximity to the catalyst component. Thus, it has been found that the present coated substrates are useful as active catalysts and supports for additional catalytic components. Without wishing to limit
the invention to any particular theory of operation, it is believed that the outstanding stability, e.g., with respect to electronic properties and/or morphology and/or stability, of the present tin oxides plays an important role in making useful and
effective catalyst materials. Any chemical reaction, including a chemical reaction the rate of which is enhanced by the presence of an electrical field or electrical current as described herein, may be promoted using the present catalyst component tin
oxide-containing coated substrates. A particularly useful class of chemical reactions are those involving chemical oxidation or reduction. For example, an especially useful and novel chemical reduction includes the chemical reduction of nitrogen
oxides, to minimize air pollution, with a reducing gas such as carbon monoxide, hydrogen and mixtures thereof and/or an electron transferring electrical field. O course, other chemical reactions, e.g., hydrocarbon reforming, dehydrogenation, such as
alkylaromatics to olefins and olefins to dienes, hydrodecyclization, isomerization, ammoxidation, such as with olefins, aldol condensations using aldehydes and carboxylic acids and the like, may be promoted using the present catalyst component, tin
oxide-containing coated substrates. As noted above, it is preferred that the tin oxide in the catalyst component, tin oxide-containing substrates be electronically conductive. Although fluorine doped tin oxide is particularly useful, other dopants may
be incorporated in the present catalyst materials to provide the tin oxide with the desired electronic properties. For example, antimony may be employed as a tin oxide dopant. Such other dopants may be incorporated into the final catalyst component,
tin oxide-containing coated substrates using one or more processing techniques substantially analogous to procedures useful to incorporate fluorine dopant, e.g., as described herein.
Particularly useful chemical reactions as set forth above include the oxidative dehydrogenation of ethylbenzene to styrene and 1-butene to 1,3-butadiene; the ammoxidation of propylene to acrylonitrile; aldol condensation reactions for the
production of unsaturated acids, i.e., formaldehyde and propionic acid to form methacrylic acid and formaldehyde and acetic acid to form acrylic acid; the isom | | |
