US2444430A - Metal rectifier element - Google Patents

Description

Patented July 6, 1948 METAL RECTIFIER ELEMENT Hyman Samuel Denmark, New York, N. Y., as-

signor to Federal Telep hone & Radio Corporation, New York, N. Y., a corporation of Dela- Application July 13, 1943, Serial No. 494,457

(Cl. F75-366) 4 Claims.

1 This invention relates to metal rectiflers of the selenium plate type designed for use in series or stack assemblies wherein the electric circuits are made through contact washers having pressure contact engagement with the electrodes of the respective plates. An important object of the present improvements is to produce, by a new method `of manufacture, an improved selenium rectifier plate' structure which in use has important functional advantages materially contributing to greater durability and stability over long periods of use and in particular to more successfully overcome certain objectionable factors present in the constructions of selenium rectifier plates as heretofore made.

In present practice the selenium rectifier plates are composed of a backing plate, commonly of disc formation providing one electrode having one face thereof coated with alayer of selenium forming an` imperfect conductor and over which is applied an alloy coating of good conducting material forming the counter-electrode, the barrier layer for effective rectification being formed between the selenium and the alloy coating in the known manner. In the series assembling of the plates as now made spring contact washers are interposed between the plates and arranged to engage the opposite electrodes to establish the electrical connection therebetween.

In the assembled units as referred to there is present an objectionable condition incident to the peculiar characteristic of the selenium rectifier plate in that it is electrically affected by or susceptible to pressure such that when pressure is exerted on its active area there is a tendency for an increase to take place in the reverse current thereby reducing its rectifying values. Limiting of the pressure contact to reduce this eniciency loss reduces the efliciency of the electrical contact and is further objectionable when it is desired to moisture proof the rectifying unit by coating with a protective varnish which with limited pressure tends to penetrate between the contact washers and electrodes.

While attempts have been made to remedy the objections aforesaid, for inst-anceas disclosed in Richards Patent 2,314,014 of March 16, 1943, by the elimination of the active or barrier layer from the pressure area of the plates such attempts have been only partially successful in certain structures under the condition of substantial clamping pressures to obtain good electrical contact, particularly from the tendency to fracture of the insulating layer'and of the counter-electrode alloy at the region of the junction between the selenium and the insulation as the result of varied compressivel strength of the diierent materials, non-uniformity in thickness resulting from manufacture and from non-uniformity of expansion and contraction from heating in use.

The present improvements comprise a structural arrangement employing rectifier plates of a further improved form having selenium and counter-electrode materials' of uniform thickness and of homogeneous materials substantially coextensive with both the active, or barrier, area and with .the pressure protection areas thereof thereby providing greater stability over long periods of use. This is accomplished by the employment of an electrode base plate upon one side of which is applied the usual selenium powder layer while masking or covering a pressure contact area and then, with masking of the first `applied layer, completing the layer over the pressure protection area with a special selenium powder containing about .1% of thallium and pres'- sure and heat treating to unite with the plate.

The employment of this thallium content selenium without any halogen makes this special selenium covered area an insulator so that there is no barrier or pressure responsive blocking area at the protected portion. There is thereby provided a selenium layer of uniform thickness and of homogeneousv material extending over both the active and protected or insulated areas vand upon which the counter-electrode alloy is then sprayed as a covering of uniform thickness to be co-extensive with both the active and protectively insulated areas thereof, thus permitting pressure contact engagement with the electrodes without danger Yof fracture and short circuiting and with elimination of pressure effect' on the barrier layer. The homogeneous material layers of uniform thickness as thus provided fully eliminate the objections aforesaid such as danger of cracking result-ing from uneven thickness, non-uniform expansion -and unlike compressive strength of the insulating materials as heretofore proposed.

The improved rectifier plate and method of manufacture will be more fully understood by reference to the accompanying drawing wherein like reference characters are applied to the corresponding parts in the several views.

In the drawing:

Fig. 1 is a central cross-sectional view through the base electrode disc having a masking disc temporarily positioned thereon as a rst step in manufacture;

therewith; l

r form thickness Fig. 4 is a similar view of the completed rectier plate having the counter-electrode alloy applied over the selenium layer;

Fig.l 5 is a. face view of the completed rectifier plate of Fig. 4; p

Fig. 6 is a view corresponding to Fig. 4 showing a desirable modification wherein the insulating selenium layer is thicker than the active layer thereof and has overlapping relation thereto, and

Fig. 7 is a side elevation of rectifier assembly.

In the preferred embodiment of -the invention as here shown the base electrode I is in the form a portion of a typical of an annular flat iron 'disc having a central circular aperture 24 to receive the customary clamping bolt 0i the assembled unit having multiple plates with interposed spring contact washers for series electrical connection in the known manner. As made in accordance with the present improved method, the base electrode, so formed, has temporarily applied to its inner annular portion a masking disc 3 of circular form and of a diameter desirably slightly greater than that of the electrical contact washers to be employed therewith. With the masking disc in position thereon as shown in Fig. 1 the active selenium layer is applied in powder form to the surface shown at 4 in Fig. 2 covering an annular area outwardly of the inn-er annular area protected by the masking disc 3. This inner area is-hereinafter referred to as the pressure area in that it is subjected to the pressure of the electrical contact washers in the final assembly. The active selenium layer 4 referred to consists of the ordinary selenium customarily applied in powdered form. Thereafter an inwardly continued layer of a special selenium powder is applied to the base electrode by masking of the active selenium layer as shown in dotted lines in Fig. 2. The continuous, combinedy selenium and special selenium layer is then subjected to pressure and heat treating to unite with the plate I in the customary manner. shown in Fig. 3 extends the selenium layer over the inner pressure area at 6 of uniform thickness with the active layer and is of a special selenium containing .1% of thallium as referred to. This pressure area layer having thallium content without any halogen makes the selenium layer of this inner area an insulator and accordingly an inactive area as related to blocking effect and correspondingly non-pressure sensitive in influence on the rectification thereby to permit pressure contact at this portion without affecting proper operation and avoiding possible damage to the barrier area. As shown in Fig, 4, the'counter electrode alloy layer 8 is in the usual manner lapplied by spraying to cover substantially the full active selenium area and to be of uniform thickness, and the disc is then processed for the electro-forming of the barrier layer. There is thus provided an improved rectifier plate having uniselenium and counter-electrode layers and wherein the'selenium layer includes a homogeneous portion, or continuation of the active area, of insulating character positioned to receive and resist the pressure of the electrical contact washers in an area removed from the barrier formation with the substantial benefits as described.

In Fig. 6 there is shown a desirable modification wherein the pressure area layer of selenium 6 of insulating properties is thickened relative to that of the outward active selenium of the plate as` ing a base element layer and is applied to the plate to be overlapping in relation to the active layer at the outer marginal portion of the insulation. In connection therewith the electrical contact washer is shown at I0 to have a contact area of slightly smaller diameter than that of the insulating selenium area, of major thickness thereby avoiding compressive stresses at the selenium junction or area of overlapping.

What is claimed is:`

1. In a selenium rectifier of the type comprisbearing a selenium layer` carrying a counter-electrode and adapted to being supported in operating'condition by application of pressure against a portion of the surface of said counter-electrode, the improvements that comprise interposing between the base element and the counter-electrode in the zone underlying the portion of the counter-electrode against which pressure is applied, an electrically insulative body comprising selenium having thallium f admixed therewith, .the thallium content of said This continued layer as Icomprising a metallic disc body being in the order weight.

2. In a selenium rectifier loi' 0.1% based upon element of the type carrying a layer of selenium with a superposed counter-electrode and adapted to being mounted upon a spindle extending through a centrally located opening, said element when in use being subjected to pressure applied to the counter-electrode in the region near said opening, the improvements that coinprise interposing between the counter-electrode and the disc in said Yregion an electrically insulative body comprising selenium containing thallium in an amount of the order of 0.1% based upon Weight.

3. In a selenium rectifier element of the type vcomprising a, metallic disc bearing a selenium layer with an overlying counter-electrode, adapted to being supported under pressure applied in a region adjoining a centrally located opening formed in the element, the improvement that comprises interposing'between the disc and the counter-electrode, in an annular zone adjacent the centrally located opening, an electrically insulative mass of selenium containing about 0.1% by weight of thallium.

4. In a process for the manufacture of selenium rectiers wherein a selenium layer is applied to a metallic base plate and a `counter-electrode is thereafter applied to the selenium layer and wherein an electrically insulative body is interposed in a localized portion of the selenium layer between the counter-electrode and the base plate, the improvement that comprises forming said electrically insulative body of selenium containing about 0.1% of thallium based upon weight.

HYMAN S. DENMARK.

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

UNITED STATES PATENTS Number Name Date 2,157,895 Hein May 9, 1939 2,182,377 Guanella Dec. 5, 1939 2,215,999 Brunke Sept. 24, 1940 2,235,051 Thompson Mar. 18, 1941 2,244,664 Addink June 10, 1941 2,279,746 Thompson Apr- 14, 1946 2,314,104 Richards et al Mar. 16, 1943 2,343,379 Kotterman Mar. 7, 1944

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