US2429089A - Production of electrical condenser plates - Google Patents

Description

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PRODUCTION oF ELECTRICAL CONDENSER PLATES Ernest Robert Box, London, England, assignor to.

Johnson, Matthey & Company Limited, London, England, a British company No Drawing. Application September 14, 1944, Serial No. 554,143. In Great Britain October 2 Claims. (01. 117-46) In my prior application Serial No. 487,651, filed May 19, 1943, I have described a process for pro-' ducing electrical condenser plates which consists in applying to a suitable dielectric by means of a meshed printing screen a paste comprising a mixture of an organic liquid screen-printing medium with a metal compound decomposable by heat to leave a residual metal film and thereafter firing at such a temperature and for such a period of time that the organic matter is burned away and the metal compound is decomposed to yield a layer of metal which adheresfirmly to the dielectric. 7

I have now found that in the said process I can use, instead of a metal decomposable by heat, metallic silver provided the silver is in a finely divided form. i i

The process according to the present invention therefore consists in applying to a suitable dielectric by forcing through a stencil or the like affixed toa meshed-printingscreen by means of a roller or squeegee a paste comprising a mixture of an organic liquid screen-printing medium with finely divided metallic silver and thereafter firing at such a temperature and for such a period of time that the organic matter is burned away and the silver is left as a layer adhering firmly to the dielectric.

The silver is preferably brought into a finely divided form for use according to the present invention by a precipitation method.

For example the silver may be directly precipitated from a solution of a silver salt, such as silver nitrate, by the addition of an organic reducing agent, such as formic acid, or by the addition of an inorganic reducing agent, such as sodium hypophosphite.

The finely divided silver may also be obtained from a solution of a silver salt by displacement of the silver therefrom by adding to the solution an excess of another metal, such as zinc, which is preferably in finely divided form. The resulting powder, which consists of the whole of the silver from the silver compound together with the excess of the other metal, may then be freed from said other metal by treatment with an acid, such as dilute sulphuric acid, which will dissolve the other metal but which will have no effect on the silver.

Another method of preparing the finely divided silver is by the reduction of precipitated silver chloride by means of zinc powder and sulphuric acid.

Apart from the substitution of metallic silver for the metal compounds described in my said prior specification Serial No. 487,651, the process is identical with the process described in the said application. V

In the process, as now practised, the screen consists of a piece of, for example, silk or other woven material stretched over a frame and the meshes of the screen are obscured by a stencil or by a suitable filler except at the parts where it is desired that the paste comprising the metallic silver and the liquid medium should pass through onto the dielectric. While the paste is usually applied to the dielectric in the shape of a rectangle or square, any other shape may be applied if desired. Thepaste is forced through the screen for example by means of a roller or squeegee after the dielectric has been located in the appropriate position under the screen.

"It is preferred to use mica as the dielectric, but any other dielectric which is suitable for condenser plates and which is capable of withstanding the firingoperation may be used. Examples of such other dielectrics are glass and porcelain.

The liquid screen-printing medium which is used for making the powdered silver into a paste suitable for application to the dielectric by means of a screen should be relatively viscous so as to form a paste which can be squeezed cleanly through the mesh of the printing screen to give a continuous uniform layer of paste which when subsequently heated leaves a residual metal film free from bubbles and cracks. Examples of such media are solutions of a cellulose ester or ether in a volatile organic ester solvent, or solutions of drying oils, gums, and synthetic and natural resins in appropriate volatile solvents. I have found that very satisfactory results may be obtained with a solution of cellulose nitrate or ethyl cellulose in ethyl, amyl or butyl acetate, lactate or phthalate. In some cases it is advantageous to add to the liquid medium a plasticiser, such as diethyl or dibutyl phthalate, medicinal paraffin, castor oil or methyl abietate.

Adherence of the metal film to the dielectric may be improved where desired by the'addition to the paste of a suitable flux, such for example as lead silicate, lead borate or lead boro-silicate, or the corresponding bismuth compounds or mixtures of two or more of the aforesaid.

After the paste comprising the metallic silver, with or without the addition of a flux,.has been applied to the dielectric by means of a screen, the firing operation is carried out by heating at such a temperature, generally from 400 to 600 0., and for such a period, generally from 2 hours to 5 minutes (depending on the temperature), that 3 the organic matter is burned away, the metallic silver is converted into a metal film and this film adheres firmly to the dielectric.

The film of metal thus produced on the dielectric is of uniform thickness and texture throughout and the condenser plates obtained have excellent electrical properties.

The following example will further illustrate how the said invention may be carried out in practice, but the invention is not restricted to this eXample, ,7 V

Example A paste of a consistency suitable for application through a screen is made from finely pow dered silver and an 8% solution of cellulose'nitrate in butyl lactate. If desired, 3% of lead boro-' silicate, calculated on the Weight of Silver, may also be added as a flux to improve the adherence of the silver film later to be formed.

A strip of mica, cut to the appropriate size, is then located beneath a screen to which has beeh'applied a stencil having an aperture of the exact area and shape of the film of silver which it is desired to apply to the mica. The'paste'prepared as above is then forced through the screen on to the 'mica by drawing asqueegee across'th'e upper surface of th'e's'cr'een. Thela'yer of paste is allowed to dry and thenanother plied to the reverse side "of'the mica.

The mica bearing the two'layers of paste is then fired at a temperature'off500" Cffo'r 40 minutes.

This method of application, followed by the fir ing process as described, provides on each"sid'e of the plate a continuous compact film of silver of uniformthickns'sand texture; This film adheres firmly to the plate and 311 air between the mica and the silver isfthe'reby excluded. Con

denser plates" produced as described in this example are characterised by' highi'electrical capacity, non-variability of capacity under varying working conditions and minimumdevia'tionof the layer is ap-' plates which consists in applying to mica through a meshed printing screen a paste made from finely V divided'metallicsilver, a lead borosilicate flux and a solution of cellulose nitrate in a solvent selected from the group consisting of amyl lactate and butyl lactate, and thereafter firing at a temperature from about 400 C. to 600 C. for from about two hours to about five minutes, depending on the temperature, until the organic matter is burned away and the finely divided metallic silver is converted into a layer of silver free of bubbles and which adheres firmly to the mica."

cracks and of uniformv thickness, and texture ERNEST onner ox.

REFERENCES CITED The following references areof record in the.

. file of this patent:

UNITED STATES PATENTS Number Name Date 1,300,394 Hoffman Apr. 15, 1919 1,922,387 M11118? Aug. 15, 1933, 1,977,625 Ernst Oct 23, 1934. 2,344,081 Claude Mal, 14,1945.

FOREIGN PATENTS Number Country I Date 407,521 Great Britain Mar. 22, 193i 475,935 Great Britain Nov. 2 1937