Compositions – Electrically conductive or emissive compositions – Metal compound containing
Patent
1996-06-26
1998-02-24
Bos, Steven
Compositions
Electrically conductive or emissive compositions
Metal compound containing
H01B 106, C01G 1902
Patent
active
057209049
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the preparation and use of electroconductive doped tin oxide, and particularly to the preparation and use of electroconductive doped tin oxide in bulk form.
BACKGROUND OF THE INVENTION
Doped tin oxide is an example of a semiconductor. The lattice of the latter can accommodate ions of different valencies. Such ions are not necessarily of similar size to tin (IV) or oxygen (II). The relative concentration of the differing ions can be used to control the electrical conductivity.
It is well known to dope tin oxide to produce and electrically conductive material. Thus for example antimony doped films have been commonly used as transparent conductive films for light modulating devices and displays, and there is also a considerable body of literature relating to the formation of fluoride doped films. However, references to the production of electroconductive doped tin oxide in bulk, for example as a powder, as opposed to thin films, are rare.
While at first this may appear strange, it should be remembered that simultaneous co-precipitation of anions is rarely, if ever, possible. When forming a film, conditions may be such that the simultaneous thermal decomposition of dopant and tin containing chemicals, upon contact with a heated substrate, increases the chances of dopant being incorporated into the tin oxide lattice. Examples of the reactions employed for film formation are: BuSnCl.sub.3 +Sn(BF.sub.4).sub.2 ; Bu.sub.2 Sn(OBu).sub.2 +CF.sub.3 CO.sub.2 H; SnCl.sub.4 +NH.sub.4 F; and MeSnCl.sub.3 +HF.
DESCRIPTION OF THE PRIOR ART
A method of producing fluoride doped tin oxide is disclosed in EP 0 448 946 in which a tin (II) oxide or oxide precursor is intimately mixed with tin (II) fluoride and heated in an oxidising atmosphere to produce fluoride doped tin (IV) oxide.
Common dopants for tin oxide are antimony and fluoride. These suffer from a number of disadvantages, not least that the starting materials are toxic and unpleasant. In addition, fluoride doped tin oxides are temperature sensitive and lose fluoride and electrical conductivity when exposed to high temperature. One alternative dopant which shows promise is phosphorus. However, to date the only technique reported for producing phosphorus doped tin (IV) oxide, as disclosed in U.S. Pat. No. 4,514,322, involves milling metastannic acid with elemental phosphorus. This is clearly not a desirable operation.
One method for producing a fluorine doped tin oxide material in bulk form is disclosed in Japanese Patent Application HEISEI 2-197014, published 3 Aug., 1990 (filing no. HEISEI 1-17196). This describes the treatment of stannic oxide with fluorine gas in an inert gas diluent, preferably at elevated temperatures (300.degree. to 600.degree. C.). Clearly a process involving fluorine gas, particularly at elevated temperatures, has attendant difficulties, and one of our aims was to develop a simpler process than this.
Another method is described in EP 0 441 427 in which a non-aqueous solution of a tin (IV) salt is combined with an aqueous fluoride, the resulting co-precipitated hydroxide materials being dried and heated to at least 600.degree. C., preferably under nitrogen. Whilst the fluoride could be tin (II) fluoride, there is no teaching that air oxidation of a stannous component could be beneficial.
European Patent Application 0 235 968 A discloses a process in which an aqueous medium containing a tin (II) carboxylate and a dopant material is treated with hydrogen peroxide to provide a transparent solution. After shaping (for example, formation of a membrane on a substrate) or conversion to a powder, e.g. by spray drying, this material is calcined to produce conductive tin oxide. Clearly in such a case oxidation by hydrogen peroxide is a "wet" process at relatively low temperatures, as compared to the relatively high temperature oxidation of a solid material in embodiments of the present invention.
SUMMARY OF THE INVENTION
The present invention provides a method of producing an electroconductive dop
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Alcan International Limited
Bos Steven
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