US2452577A - Dry rectifier element and method of manufacture - Google Patents

Description

.; 1948. c. A. KOTTERMAN ,452,77

DRY RECTIFIER ELEMENT AND METHOD OF MANUFACTURE Filed Oct. 7, 1945 T, WWW I IlllH 2 V77 J Wfl/ ////J INVENTOR. 67/557257? 4. AOTIf/F'AM Patented Nov. 2, 1948 DRY RECTIFIER ELEMENT AND METHOD OF MANUFACTURE Chester A. Kotterman, Livingston. N. J assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application October 7, 1943, Serial No. 505,330

4 Claims.

er is applied by sprinkling a suitable material in powder form on the base plate and compressing it thereon at an elevated temperature whereby the powder softens sufllciently to form a uniform layer and adhere to the plate. When the semi-conducting material is selenium, pressures of three to five thousand pounds per square inch and temperatures of 110 C. to 135 C. are satisfactory. The selenium coated base plate may then be further heat treated to obtain the required crystal form of the selenium and is further processedto adapt it for use as a rectifier. Specifically, a, counter-electrode is applied over the surface, of the selenium, as by spraying with a suitable conducting substance such as Woods metal, and the element may then be given an electro-forming treatment by the application of voltage between the base plate and the counterelectrode.

It has been found in using such rectifier elements that the optimum thickness for the semiconducting layer is within a definite range, usually of the order of .003 to .006 inch. When thinner layers are used the rectifyi g action b'eaks down while when thicker layers are used the rectifiers offer too much resistance. to the passage of current, resulting decreased efficiency and heating of the rectifiers to a point too near their maximum safe operating temperature, which is about 75 C. In prior practice, where ordinary fiat discs are used for the base plates, the value of the pressure applied has been critical in determining the thickness of the layers produced, since, assuming an excess of powder is used, too little pressure results in insufficient adhesion and too much pressure tends to extrude too much of the softened material laterally. It therefore has been difiicult to calculate the exact pressure required to produce the optimum thickness.

The present invention produces in a simple manner dry rectifier elements with semi-conducting layers of specific, predetermined thickness by using base plates having upstanding circumferential rims. The height of the rim determines the thickness of the layer, since the upper 2 press platen used in the pressing Operation is substantially larger than the element and is arrested by the rim; so that regardless of the pressure applied, there will be no thinning out of the selenium layer beyond the desired limit. By making use of a suitable punch press for punching "out the base plates, plates with rims of the desired height can be obtained in one simple operation from which in turn rectifiers with semi conducting layers of a thickness corresponding to the height of said rims can be produced by standard methods. A corollary object of theinvene tion is to eliminate the necessity for highly accurate measurement of the quantity of powder used, and at the same time to obtain a semiconducting layer of specific thickness. A further object of the invention is to reduce-the occurrence of chipped edges in said layer.

Other objects and advantages will appear in the following detailed description in connection with the attached drawings in which:

Fig. 1 is a face view and Fig. 2 a. cross section of a base plate with a raised circumferential rim, the height of the rim being exaggerated;

Fig. 3 is an expanded view of the press platens and an interposed base plate shown in section and carrying the selenium powder; and

Fig. 4 is a cross section of a completed rectifier made according to the invention.

Figs. 1 and 2 show a base plate of a type readily punched out by a suitable punch press, having a raised rim l around the edge, with respect to which the surface 2 of the plate 3 is depressed. The elevation of the rim is preferably about .004

- inch, representing the desired thickness of the selenium layer, but may be more or less than this, and of course is exaggerated in the drawings to make the construction clear. The width of the rim may vary, one embodiment having a rim Width of about one sixteenth of an inch. The plate will ordinarily have a central opening a as shown so that a number of rectifiers may be mounted in known manner with suitable spacing members interposed on an insulated spindle running through the openings to form a stack,

Figure 3 illustrates the method of forming the selenium layer. A layer of selenium powder 5 is applied to the surface 2 of the base plate 3 as by sifting it in place; and an excess of powder is advantageously 'used to assure the continuity and uniform thickness of the selenium layer throughout said surface. Platens 6 are heated to a temperature at which the powder 5 will soften and fuse; and when said platens are brought together with the base plate 3 carrying 3 the powder 5 located between them, the application of heat and pressure will change the powder 5 to a fluent mass which will be forced into efi'ective engagement with surface 2, the surplus material flowing out over the rim I and into the aperture 4 where it is readily removed. The pressures and temperatures employed are known to those skilled in the art and are the same to those which are used in the ordinary pressed powder method of applying the selenium layer, except that substantially higher pressures can be employed without damage because of the use of the rim I. Such higher pressures are advantageous because they produce substantially better adhesion to the surface 2, as well as greater assurance of uniformity in thickness of the selenium layer.

The unit is then treated in the usual manner to form the finished disk. This treatment normally includes a second heat treatment, ordinarily at a temperature of about 200 C., to transform the selenium into the desired crystalline form. The counter electrode is thenapplied to thickness and a-flat surface extending inwardly from the rim; disposing on said surface a quantity of a thermoplastic semi-conducting material,

and forming said material into a flat layer flush with the upper margin of the rim by pressing a flat hot surface against the material and forcing said hot surface toward the base plate until arrested by the rim, the heat and pressure being sufllcient to fuse the material and form a continuous uniform layer strongly adherent to the base plate surface.

2. A process as set forth in claim 1 in which an excess of the semi-conducting material is disposed on said surface, the excess material being laterally extruded upon the application of heat and pressure.

3. A process asset forth in claim 1 in which the semi-conducting material is in the form of a the selenium as by spraying, and the disk is ment having a semi-conducting layer of specific thickness, comprising forming a base plate with an upstanding rim having a height equal to said powder.

4. A process as set forth in claim 1 in which the semi-conducting material is selenium.

CHESTER A. KO'ITERMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,611,653 Lilienfeld Dec. 21, 1926 1,826,955 Ruben Oct. 13, 1931 1,954,950 Russell Apr. 17, 1934 1,989,463 Ruben Jan. 29, 1935 2,032,439 Ruben Mar. 3, 1936 2,189,580 Hewlett Feb. 6, 1940 FOREIGN PATENTS Number Country Date 40 68,188 Sweden Oct. 8, 1929

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