Nord, Sven (Villagatan 18, Brom�lla, S-295 31, SE)
H�kansson, Arne (Per Albins gata 1, Brom�lla, S-295 32, SE)
Johansson, Thomas (Lars P�hls v�g 28, H�gan�s, S-263 52, SE)
Nord, Sven (Villagatan 18, Brom�lla, S-295 31, SE)
H�kansson, Arne (Per Albins gata 1, Brom�lla, S-295 32, SE)
|1.||Electric insulator of porcelain, in particular an electric highvoltage insulator, with at least one opaque, 0.1 1 mm thick glaze layer, c h a r act e r i z e d in that a film composed mainly of tin oxide, SnO2, is deposited on the said (at least one) opaque glaze layer, and that the film of mainly tin oxide deposited on the glaze layer has a thickness that is significantly less than that of the glaze layer.|
|2.||The porcelain insulator according to Claim 1, c h a r a c t e r i z e d in that the thickness ofthe tin oxide film is at most 1/10, preferably maximum 1/100, suitably at most 1/1000 ofthat ofthe glaze layer.|
|3.||The porcelain insulator according to Claim 1 or 2, c h a r a c t e r i z e d in that the opaque glaze layer has a thickness of 0.11 mm, preferably 0.20.5 mm, and that the tin oxide film has a thickness of 1 nm to 0.1 mm, preferably 10500 nm, suitably 10100 nm.|
|4.||The porcelain insulator according to Claims 13, c h a r a c t e r i z e d in that the glaze layer or layers are composed of electrically insulating material.|
|5.||The porcelain insulator according to Claims 13, c h a r a c t e r i z e d in that at least one glaze layer, coated with the said tin oxide film, consists of an electrically semiconductive material.|
|6.||The porcelain insulator according to Claims 14, c h a r a c t e r i z e d in that the deposited film of mainly tin oxide has an electric resistivity of 11000 MQ/sq, preferably 10200 MQ/sq.|
|7.||The porcelain insulator according to Claim 5, c h a r a c t e r i z e d in that the deposited film of mainly tin oxide in combination with the semiconducting glaze layer has an electrical resistivity of 11000 MQ/sq, preferably 10200 MQ/sq.|
|8.||The porcelain insulator according to Claims 17 c h a r a c t e r i z e d in that the deposited film of mainly tin oxide contains a functional amount of at least one substance which increases the electrical conductivity of the tinoxide film, preferably at least any compound belonging to the group including antimony oxides and fluorides, wherein, if the tin oxide film contains antimony oxide as a doping agent, the antimony oxide is present in an amount of 540 weight%, preferably 1030 weight%, and, if the tin oxide film contains fluoride, that the mentioned fluoride is present in an amount of 0.01 10 weight%, preferably in an amount of 0.15%, and that the total content of antimony oxide and fluoride together amount to maximum 40 weight%, preferably to max 30 weight%.|
|9.||Method for the manufacture of an electric insulator of porcelain with a surface coating on the porcelain body, comprising at least one glaze layer (3), c h a r a c t e r i z e d in that, on the unsintered body which shall form the porcelain body (2) of the insulator, a 0.11 mm thick layer of a composition is deposited, which is to form the glaze, that this layer is dried after which the body with said layer is fired at a temperature of between 1150 and 1450"C, preferably at a temperature between 1200 and 13500C, in a kiln such that a porcelain body is obtained with a 0.1 1 mm thick glaze layer and that thereafter, at a temperature below 1000"C and also below the softening point of the glaze, the glaze layer is coated with a film consisting mainly of tin oxide.|
|10.||Method according to Claim 9, c h a r a c t e r i z e d in that the tin oxide film is deposited on the glaze layer of the porcelain body during the cooling process in the firing kiln after firing of the porcelain body at the said firing temperature, as the fired body is caused to cool from the firing temperature to room temperature.|
|11.||Method according to Claim 10, c h a r act e r i z e d in that the tin oxide is deposited on the glaze layerof the porcelain body by injection of a tin compound into the firing kiln using a carrier in the firing kiln, by which the said tin compound through reaction with gas in the kiln and/or with the carrier and/or with other substances which are injected together with the tin compound, forms the said tin oxide which is deposited onto the glaze layer.|
|12.||Method according to Claims 911, c h a r a c t e r i z e d in that the tin compound is injected into the kiln when the glaze layer has a temperature of 300800°C, preferably 400700"C.|
|13.||Method according to Claims 912, c h a r a c t e r i z e d in that the tin compound is injected into an oxidizing atmosphere in the firing kiln, when the temperature of the glaze layer has decreased to a temperature of 300800°C, preferably 400700"C, after completed firing in oxidizing or reducing atmosphere at a temperature between 1150 and 1450"C, preferably between 1200 and 13500C.|
DESCRIPTION OF THE BACKGROUND ART Electric high-voltage insulators of porcelain have conventionally a glaze layer, which has several functions. It is to increase the mechanical strength of the insulator, make the surface smooth and thereby prevent attachment of impurities onto the surface of the insulator, and give the surface a good wear resistance. However, the attachment of impurities onto the surface of the insulator cannot be completely avoided. Eventually the surface becomes worn, and in certain areas such as desert regions, this wear can be significant due to sandstorms etc. If as well the surface becomes moist, then the impurities deposited on the surface can become conductive, give rise to short-circuits and in this way cause significant power losses.
To prevent significant power losses due to moist insulators, special insulators with a glaze layer made of an electrically semiconductive material are used to a certain extent.
Such semiconductive glazes contain typically about 30% tin oxide, doped with a certain amount of antimony oxide to increase the electrical conductivity. The small electric currents passing over the surface of the insulator through the glaze layer cause heating of the surface and thereby counteract the deposition of moisture on the surface by condensation and/or assist in drying the surface. It is realized that the current leaks created by the semiconducting glaze layer are not desirable from a power-loss point of view; however the alternative--larger power losses due to moist, dirty surfaces--is considered worse. The technique has however another serious disadvantage and limitation, which is that the semiconductive glaze layer cannot be fired in a reducing furnace atmosphere, which is normally required in the production of electric porcelain insulators due to the chemical compositions of the clays used by most manufacturers.
DESCRIPTION OF THE INVENTION The present invention aims to attack the complex problem described above. In particular, the invention aims to offer an electric insulator of porcelain with a composition of the surface structure such that the surface deposition of moisture can be prevented in ways other than via a semiconductive glaze layer, or possibly in combination with such a layer. More particularly, the invention aims on the one hand to stimulate heating of the porcelain body through heat radiation inward by illumination since the porcelain body is coated with a coloured glaze layer, and on the other hand to reduce IR- emission from the insulator, especially during night-time or in the early morning hours, when the risk for condensation of moisture or frost is largest. According to one aspect of the invention, this is achieved in that the ceramic layer is coated with a thin transparent film mainly consisting of tin oxide. As well as decreasing the IR-emission, which counteracts condensation and eases drying in cold and dark climate conditions, the surface layer consisting mainly of tin oxide (hereafter called tin oxide layer) can give one, several or all of the following advantages: - improved surface wear resistance, which counteracts scratches, a rough finish or other surface flaws in which impurities can collect and become conductive when damp - a certain hydrophobic surface character - a certain heating of the surface through semiconductive properties of the tin oxide - a more even distribution of field strength over the surface of the insulator due to the semiconductive properties of the tin oxide film, by which local short-circuits and corona effects can be prevented - reduction of the total cost for manufacture of the insulator, provided that it is made smaller due to a more efficient insulating capacity - possibility for deposition of the tin oxide layer on the insulator in an oxidizing atmosphere even in such cases where the porcelain body of the insulator is made of a raw material which due to its composition must be fired in a reducing atmosphere.
The glaze layer, in particular each and every glaze layer if several glaze layers are laid upon each other, is 0.1-1 mm thick, preferably 0.2-0.5 mm thick while the tin oxide layer according to the invention is much thinner, normally a maximum of one-tenth the thickness of the glaze layer, preferably one-hundredth at the most, and in certain applications one-thousandth the thickness of the glaze layer or in absolute values 10 A (Angstrom units) to 0.1 mm, preferably 100 A to 500 nm, suitably 100-1000 A thick.
As the thin tin oxide layer is transparent, the opaque, normally coloured or more or less
dark glazes can on illumination absorb the falling light and contribute to heating up and accumulating heat in the porcelain body.
Tin oxide has, as mentioned, a certain electrical semiconductive capacity. As is known, this can be strengthened by doping of the tin oxide, SnO2 with antimony oxide (antimony pentoxide) Sb2O5, so that the tin oxide layer contains 5-40 weight-%, preferably 10-30 weight-% antimony oxide. It is possible to partly or completely replace the antimony oxide with 0.01-10% fluoride of a type which can, as known, increase the conductivity of tin oxide. According to one aspect of the present invention, this can be utilized to conduct electrical current through the tin oxide film on top of the glaze layer to heat the surface of the insulator and thereby prevent condensation of moisture onto the surface and/or dry the surface. The surface resistivity of the tin oxide film should in this case amount to between 1 and 1000 MQ per square (MQ/sq), preferably 10-200 MQ/sq.
This can be achieved by optimal addition of antimony oxide and/or of a fluoride within the mentioned range as well as by giving the tin oxide film an adequate thickness. The tin oxide film can be applied in several layers in order to achieve the level of surface resistivity mentioned.
According to the invention, a characteristic feature of the composite surface coating is also that there is a distinct transition between the glaze layer and the tin oxide film, i.e. in the boundary layer between the glaze layer and the tin oxide film, there is, if any, only an extremely thin stratum of transitional phases or mixtures of glaze and tin oxide (deposited on the exterior of the glaze layer). Characteristic as well is that the exterior of the glaze layer (i.e. the surface upon which the tin oxide film is deposited) has a high surface fineness, which should be of significant importance for decreasing IR- emission.
The surface of the porcelain body can however be comparatively rough; this roughness is evened out by the glaze layer coating the porcelain body.
It is also possible to achieve the aimed resistivity for the surface coating of the insulator in part or in principle (i.e. for the integrated surface structure comprised of at least one glaze layer and at least one tin oxide layer upon the glaze layer) by incorporating tin oxide (SnO2), preferably doped with antimony oxide, Sb2O5, to a level of 2-15 weight-%, preferably 4-10 weight-%, in the glaze layer. The composite surface coating consisting of glaze layer and tin oxide film should in this case have a resistivity of 1-1000, preferably 10-200 MQ/sq. According to one possible embodiment, as much tin oxide (preferably doped with antimony oxide) can be incorporated into the glaze layer, e.g. a total content of 20-40 weight-% SnO2 + Sb205 in the glaze layer, that the glaze layer
alone obtains a resistivity of 1-1000, preferably 10-200 MQ/sq, while the tin oxide film is made so thin and/or is not doped with antimony oxide so that the conductivity of the tin oxide film becomes insignificant in comparison with that of the semiconducting glaze layer. In this case the tin oxide film functions as a thermal insulator by reducing emission of the heat generated by the electric current in the glaze layer. The efficiency of the semiconductive glaze layer in keeping the surface of the insulatos dry is thereby increased. To achieve this effect, it is sufficient that the tin oxide film be given a thickness of 10-1000 A, preferably 100-1000 A.
The method for obtaining the composite surface coating according to the invention is characterized by the laying of a 0.1-2 mm thick, preferably 0.2-1 mm thick layer of a composition which forms the glaze onto the form of porcelain paste which is to become the porcelain body of the insulator, that this layer is dried and thereafter the body with the mentioned layer is fired at a temperature of between 1150 and 1450"C, preferably at a temperature of between 1200 and 13 50"C in a firing kiln so that a porcelain body is obtained with a 0.1-1 mm thick, preferably 0.2-0.5 mm thick glaze layer and that subsequently, at a temperature below 1000"C, preferably at a temperature between 300 and 800"C (which is below the softening point of the glaze layer) the glaze layer is coated with a film consisting mainly of tin oxide. The tin oxide film can be deposited onto the glaze layer in a separate operation in a separate furnace, however it is suitably performed in a firing kiln during cooling of the insulator from the high firing temperature.
This is favourable for several reasons. Firstly, the procedure does not require a separate kiln for depositing the tin oxide film onto the glaze layer; secondly, the glazed porcelain body does not need to be heated up again to a temperature optimal for the tin oxide coating; thirdly, the porcelain body cools from the firing temperature so slowly that the glazed porcelain body has a temperature within the optimal interval for the tin oxide coating during a relatively long period so that the tin oxide coating can be performed without any problem; fourthly, that for the tin oxide coating phase a suitable oxidizing atmosphere can be achieved before execution of the tin oxide coating, as well for the case where a previous high-temperature firing was performed in a reducing atmosphere as for the case where an oxidizing atmosphere was used.
Several non-organic or organic tin compounds are useable as precurors for the formation of a tin oxide film on the glaze layer. These precurors can be composed of gases, liquids, be present in solutions or in the form of a powder. Common for suitable precurors is that they can be easily decomposed and/or react at the chosen coating temperature to form tin oxide, and in anticipated cases even antimony oxide or another desirable oxide
or fluoride, which is deposited onto the glaze layer on the porcelain body. The optimal temperature depends on the choice of precursor but lies within the temperature range 300-800"C, and normally 400-700"C for most considered precursors. Among the multitude of precurors considered for the formation of tin oxide for deposition onto the glaze layer, the following can be mentioned: Powder-form precurors The following organic tin compounds can be mentioned from this group: dibutyltin oxide (C4Hg)2SnO, dipropyltin di-trifluoroacetate, (C3H7)2Sn(CF3COO)2, and dibutyltin di- trifluoroacetate, (CJi9)2Sn(CF3COO)2, with a particle size of 10-60mm. These compounds can be injected into an oxidizing carrier gas, e.g. air, which can even contain hydrofluoric acid to, if desired, increase the electrical conductivity of the deposited tin oxide film. An optimal temperature of the substrate, the glaze layer, lies within the temperature range of 400-700°C for these compounds.
Liquid precurors and/or precurors in solution This group contains a number of non-organic compounds such as tin tetrachioride, SnCl4 and tin acid, H2SnO3. In this context, antimony trichloride, SbC13 and antimony pentachloride, SbCI5 should also be mentioned as possible precurors for the doping of tin oxide with antimony oxide. Among organic tin compounds, there is a larger number, such as monobutyltin trichloride, C4H9SnC13, butyltin methoxide, C4H9Sn(OCH3)3 and butyltin butoxide, C4H9Sn(OC4H9)3, which are dissolved in suitable organic solvents such as methanol or butanol and sprayed into the kiln or injected in the form of vapour with carrier gas. Further, dimethyltin chloride, monophenyltin trichloride, monobutyltin chloride and reactive organometallic salts in organic solvents, such as methylene chloride and trichloroethylene can be mentioned. Monophenyltin trichloride and monobutyltin chloride are injected preferably in the form of vapour. The optimal temperature for deposition of tin oxide film using the mentioned compounds lies generally within the temperature range 400-700"C and optimally at approx. 550-600"C.
This aspect and other aspects of the invention will be evident from the following description of a preferred embodiment and from the following claims.
BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described with reference to the accompanying drawing, in which
Figure 1 shows partly schematically a high-voltage insulator of porcelain with typical appearance, to which the invention can be applied; and Figure 2 illustrates schematically a section through a surface portion under very strong magnification.
DESCRIPTION OF A PREFERRED EMBODIMENT A high-voltage insulator is identified generally in Fig. 1 with the reference numeral 1. In Fig. 2 the porcelain body of the insulatos has been designated 2, a glaze layer designated 3 and a film which consists of principally tin oxide SnO2, possibly doped with antimony oxide, Sb205, or another compound which can promote the semiconductive properties of the film, has been designated 4. The porcelain body 2 has a comparatively rough surface 5, which is smoothened by the glaze layer 3 which has a surface 6 that forms a substrate for the tin oxide film 4, which has a very high surface fineness, by which the tin oxide film 4 obtains good reflection factor for IR-radiation from the porcelain body 2 and from the glaze layer 3.
EXAMPLE 1 A layer of a slurry containing a mixture with the following composition in weight-%: 71.3 SiO2, 13.7 Awl203, 9.4 CaO, 1.7 MgO, 3.2 K2O, 0.7 Na2O, was deposited on a body of a material of the type used for porcelain insulators, with the form shown in Fig. 1, and which can be fired in an oxidizing atmosphere without problem.
The layer was dried, after which the body was introduced into a firing kiln and fired in a conventional way in an oxidizing atmosphere at a temperature between 1230 and 12700C for four hours, wherein the porcelain body sintered and formed porcelain 2, Fig. 2. At the firing temperature the applied surface coating melted and formed on cooling of the porcelain body in the firing kiln an approx. 0.3-0.4 mm thick opaque, darkly coloured glaze layer 3, Fig. 2. The whole cooling process, i.e. from the firing temperature to room temperature, took about 20 hours. When the glaze layer 3, Fig. 2 had a temperature of about 580"C, a thin tin oxide film 4, Fig. 2, was deposited on the stabilized, hard glaze layer 3. The precursor used for tin oxide was liquid monobutyltin trichloride, vapourized in a jet of air as carrier gas, which was heated to about 1600C.
The gas jet was injected into the firing kiln through nozzles directed towards the surface of the insulator. Tin oxide, SnO2, was formed here by certain reactions (which have not been studied in detail), and deposited as an approx. 500-700 A thin transparent film on the dark glaze layer.
EXAMPLE 2 Pretreatment of the insulator body, the application of the glaze layer and the firing of the porcelain body were the same as for Example 1. Tin tetrachloride, SnC13, was used however as a precursor for the tin oxide and antimony pentachloride, SbC15, was used as a precursor for antimony oxide, Sb205 intended as doping substance for the tin oxide in the tin oxide film. The mentioned compounds of tin and antimony were injected into the kiln together with H202 towards the insulator, as its surface had cooled to a temperature of approx. 590"C. By reaction with water, SnO2 and Sb205 were formed which were deposited as a thin transparent film onto the ceramic layer.
EXAMPLE 3 This experiment was carried out in the same way as in Example 1 with the difference being that the ceramic layer 3 contained as well 25% antimony-doped tin oxide, more exactly approx. 20% SnO2 and approx. 5% of substantially antimony pentoxide Sb2O5.
EXAMPLE 4 This experiment was carried out in the same way as in Example 1 with the exception that the firing of the porcelain body with the glaze layer was performed in a reducing atmosphere. When the reducing firing was finished and the body cooled so that the glaze layer 3, Fig. 2, had a temperature of approx. 580"C, the thin tin oxide film was deposited as before under an oxidizing atmosphere in the firing kiln.
EXAMPLE 5 A porcelain plate was coated with a glaze layer of the same type as was produced in Example 1. The glazed plate was heated to 6000C in an experimental oven, whereafter a fluorine-doped organo tin formulation was spray atomized in the oven, the formulation having a tin content of approx. 21-40% and a chlorine content of approx. 18-35% to provide a tin oxide coating on the glaze layer. Also CVD (chemical vapour deposition) was tested. Irrespective of the deposition technique, but depending on the coating thickness, a surface resistivity ranging from approx. 10 Q/sq, when the tin oxide coating was in the order of 200-300 nm, to about 50 MQ/sq. when the coating thickness was in the order of 10-30 nm, was achieved. Further, an infra-red reflection of 80-58% (integral reflection at wavelengths between 2 m-15 clam) was achieved. Moreover, the coating layer applied with the above chemical formulation had a high degree of chemical resistance against environmental attack, compared to the standard, uncoated glaze surface.