US2794943A - Selenium rectifier - Google Patents

Selenium rectifier Download PDF

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US2794943A
US2794943A US499271A US49927155A US2794943A US 2794943 A US2794943 A US 2794943A US 499271 A US499271 A US 499271A US 49927155 A US49927155 A US 49927155A US 2794943 A US2794943 A US 2794943A
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selenium
layer
barrier layer
base plate
rectifier
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Eannarino George
Parsons Robert
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Sarkes Tarzian Inc
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/06—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising selenium or tellurium in uncombined form other than as impurities in semiconductor bodies of other materials
    • H01L21/10—Preliminary treatment of the selenium or tellurium, its application to the foundation plate, or the subsequent treatment of the combination
    • H01L21/108—Provision of discrete insulating layers, i.e. non-genetic barrier layers

Definitions

  • a barrier layer between the layer of selenium and the counterelectrode the latter generally consisting of an alloy containing cadmium.
  • This barrier layer is strengthened during the forming of the rectifier, which comprises applying direct orpulsating current to the rectifier in the reverse or poorly conducting direction. It has been found that the effective rectifying action of selenium rectifiers can be improved by interposing an artificial barrier layer between the selenium and the counterelectrode. In the past, this artificial barrier layer has been made of shellac, nitrocellulose, nylon, or similar film-forming organic materials.
  • the object of the present invention to provide a selenium rectifier with a novel barrier layer having improved forward and reverse characteristics. It is a further object of this invention to provide a method for producing such barrier layers in Selenium rectifiers. Another object of the invention is to produce such artificial ie y s rom h combination of a carbohydrate and a basic organic nitrogen compound.
  • a barrier layer 4 is developed in accordance with the present disclosure, and then on top of the barrier layer 4 a counterelectrode 5 is produced.
  • the formation of the barrier layer is accomplished by applying to the selenium layer a thin coating of a dilute solution of a carbohydrate and an amine in a. volatile aqueous solvent, allowing the solvent to evap orate leaving a residue of carbohydrate, amine and other nonvolatile organic materials on the surface of the selenium, and finally applying to the selenium layer a counterelectrode such as cadmium or a cadmium alloy.
  • the finished rectifier cell is, then electroformed by passing a relatively high current through the rectifier cell in reverse direction.
  • the preferred method, of torming the rectifier is that disclosed in the application of George ICC Eannarino, Serial No. 475,705, filed December 16, 1954.
  • the forming of the cell may be conducted at low temperature or at high temperature, or both, depending upon the properties desired in the rectifier.
  • the solution of carbohydrate and amine in volatile solvent can be applied by any suitable method, such as by spraying, or roller coating, by brushing, by applying a small amount near the center of a spinning rectifier cell, or by dipping.
  • the combinations of carbohydrate and organic amine which are used in forming barrier layers in accordance with the present invention are produced by forming a solution of a soluble carbohydrate in an aqueous solution of a lower alcohol and adding sufiicient organic amine of the type hereinafter defined to adjust the pH to 6.5 to 7.5, and preferably 6.8 to 7.2.
  • Higher alcohols such as furfuryl alcohol and benzyl alcohol can be added in small amounts to improve the stability of the barrier layer.
  • Organic nitrogen compounds such as nitromethane, nitroethane, and other lower nitroparaffins may be added in small amounts, although neither these materials nor the higher alcohols are necessary to the successful operation of the invention.
  • sucrose dextrose
  • maltose lactose
  • arabinose and similar monoand disaecharides.
  • These sugars can be used in pure form, or they may be used in relatively crude form as they are obtained in nature, as for example, in the form of honey or molasses.
  • Carbohydrate derivatives such as glucuronic acid, mannitol, sorbitol, and other related materials are equally satisfactory in this invention.
  • Polysaccharides such as starch and glycogen are operative.
  • the solvents for the carbohydrate solution include aqueous solutions of lower alcohols, particularly those containing 5 to 10% of water in methanol, ethanol, npropanol, isopropanol, and the like. Mixtures of such lower alcohols may also be used. Certain other organic. solvents such as acetone or methyl ethyl ketone may be employed in small quantity to assist the solution of the carbohydrate and the organic amine.
  • the basic organic nitrogen compounds which are suitable for use in developing the barrier layers of the present invention are those which have basic dissociation constants in the range of l l0 to 1X 10" and boiling points above 100 C.
  • Preferred amines include the polymethylenediamines such as hexamethylenediamine, tetramethylenediamine and N,N,N',N-tetrakis(2-hydroxypropyl)ethylenediamine.
  • the solutions of carbohydrates and amines contain small amounts of water in the order of 1 to 10% by volume in order to aid in the interaction of the chemicals of the barrier layer with each other and with the selenium and cadmium layers. In such solutions the amount of nonvolatile materials is in the range of 0.01% to 10% by weight, and preferably in the range of 0.01% to 1%, the remainder of the solution being volatile organic solvents and water.
  • the counterelectrode which forms a part of the rectifier may be composed of any of the metals used in selenium rectifier counterelectrodes. In practice cadmium or cadmium alloys are preferred.
  • the counterelectrode may be applied by evaporation under high. vacuum, by spraying or by any other suitable method.
  • alloys which may be employed as counterelectrodes are cadmium-bismuth, cadmium-tin, cadmium-bismuth-lead and similar alloys melting between and 300 C.
  • Rectifiers formed in accordance with the present invention have particularly advantageous properties. They have longer life in laboratory tests and in commercial operation than do corresponding selenium rectifiers formed by prior art. methods. For example, rectifiers produced with sucrose and hexamethylenediamine have exhibited are particularly stable against high humidity, high temperatures and high voltages.
  • Example 1 A solvent mixture is prepared from 780 volumes of methanol, 156 volumes of water,,31 volumes of furfuryl' alcohol and 31 volumes of benzyl. alcohol. To 100 milliliters of the foregoing solvent mixture are added 3 grams of crystalline sucrose. The mixture is warmed and stirred until the sucrose is completely dissolved, then cooled to room temperature. Suflicient hexamethylenediamine is added to adjust the pH to 7.0. The resulting solution is diluted with volumes of 90% aqueous isopropanol.
  • Selenium rectifiers are prepared as follows: aluminum base plates are plated with nickel and a thin amorphous selenium ,coating is pressed thereon. The selenium layer is converted to metallic selenium by heat treatment.
  • Example 3 2 grams of sucrose are dissolved in a solution of 20 milliliters of water in 80 milliliters of methanol. The pH of the solution is adjusted to 6.8 by the addition of N,N,N,N', tetrakis(2-hydroxypropyl)ethylenediamine. Theresulting solution is diluted with 9 volumes of 90% isopropanol. The solution thus produced forms a longlasting barrier layer, when applied to selenium.
  • Example 4 Selenium layers treated as above are then :coatedwith cadmium or cadmium alloys toa thickness of 0.001 inch to 0.01 inch. These cells are then electroformed by applying a current of relatively high current density in the range of l to 7 amperes, per square inch in the reverse direction, either in oil solution or in air. Rectifiers produced in accordance with this invention exhibit a relativelylong 4 life and high stability, particularly against high temperatures and humidity.
  • a unilateral conductor comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and an organic amine in an aqueous volatile solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
  • a unilateral conductor comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and an organic amine having a basic dissociation constant in the range of l l0 to l l0 and a boiling point above C. in an aqueous volatile solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
  • a rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and a polymethylenediamine in an aqueous volatile organic solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
  • a rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with, a dilute solution containing a monosaccharide and a.
  • a rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a disaccharide and a polymethylenediamine in a lower aqueous alcohol solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
  • a rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a disaccharide and a polymethylenediamine in a lower aqueous alcohol solvent at a pH in the range of 6.8 to 7.2, and a cadmium counterelectrode on said barrier layer.
  • a rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution of sucrose and a polymethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
  • a rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution of sucrose and hexamethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
  • a rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution ,of mannitol and a polymethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
  • a rectifier comprising an aluminum base plate, a

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Description

. June 4, 1957 G. EANNARINOV ETAL' 2,794,943
SELENIUM RECTIFIER Filed April 5, 1955 INVENTORS Gsoaqs EANNAIZINO and ROBERT PARSONS BY m Attorneys.
United States Patent SELENIUM RECTIFIER George Eannarino and Robert Parsons, Bloomington, lntL, assignors to SarkesTarzian, Inc., Bloomington, inch, a summa i n f Indiana Application April 5, 1955, Serial No. 499,271
10 Claims. (Cl. 317-241) action of dry rectifiers such as selenium rectifiers, de-
pends largely upon the formation of a barrier layer between the layer of selenium and the counterelectrode, the latter generally consisting of an alloy containing cadmium. This barrier layer is strengthened during the forming of the rectifier, which comprises applying direct orpulsating current to the rectifier in the reverse or poorly conducting direction. It has been found that the effective rectifying action of selenium rectifiers can be improved by interposing an artificial barrier layer between the selenium and the counterelectrode. In the past, this artificial barrier layer has been made of shellac, nitrocellulose, nylon, or similar film-forming organic materials.
It is the object of the present invention to provide a selenium rectifier with a novel barrier layer having improved forward and reverse characteristics. It is a further object of this invention to provide a method for producing such barrier layers in Selenium rectifiers. Another object of the invention is to produce such artificial ie y s rom h combination of a carbohydrate and a basic organic nitrogen compound. These and other objects of the invention will be apparent from the ollo ing dis losur The invention is more readily understood by reference to the following description taken in conjunction with he accompanyin ng e-figure drawing which hows a cross-sectidn of a rectifier cell. The base plate 1, which may be formed of aluminum or iron, s coated wi h a layer of nickel 2 by electroplating or other suitable deposition method. A layerof selenium 3 is deposited on the nickel-plated base plate under heat and pressure,
and suitably heat-treated to form metallic selenium. On the layer of selenium, a barrier layer 4 is developed in accordance with the present disclosure, and then on top of the barrier layer 4 a counterelectrode 5 is produced.
The formation of the barrier layer is accomplished by applying to the selenium layer a thin coating of a dilute solution of a carbohydrate and an amine in a. volatile aqueous solvent, allowing the solvent to evap orate leaving a residue of carbohydrate, amine and other nonvolatile organic materials on the surface of the selenium, and finally applying to the selenium layer a counterelectrode such as cadmium or a cadmium alloy. The finished rectifier cell is, then electroformed by passing a relatively high current through the rectifier cell in reverse direction. The preferred method, of torming the rectifier is that disclosed in the application of George ICC Eannarino, Serial No. 475,705, filed December 16, 1954. The forming of the cell may be conducted at low temperature or at high temperature, or both, depending upon the properties desired in the rectifier. The solution of carbohydrate and amine in volatile solvent can be applied by any suitable method, such as by spraying, or roller coating, by brushing, by applying a small amount near the center of a spinning rectifier cell, or by dipping.
The combinations of carbohydrate and organic amine which are used in forming barrier layers in accordance with the present invention are produced by forming a solution of a soluble carbohydrate in an aqueous solution of a lower alcohol and adding sufiicient organic amine of the type hereinafter defined to adjust the pH to 6.5 to 7.5, and preferably 6.8 to 7.2. Higher alcohols such as furfuryl alcohol and benzyl alcohol can be added in small amounts to improve the stability of the barrier layer. Organic nitrogen compounds such as nitromethane, nitroethane, and other lower nitroparaffins may be added in small amounts, although neither these materials nor the higher alcohols are necessary to the successful operation of the invention.
Among the carbohydrates which can be employed successfully in developing the barrier layers of this invention are sucrose, dextrose, maltose, lactose, arabinose, and similar monoand disaecharides. These sugars can be used in pure form, or they may be used in relatively crude form as they are obtained in nature, as for example, in the form of honey or molasses. Carbohydrate derivatives such as glucuronic acid, mannitol, sorbitol, and other related materials are equally satisfactory in this invention. Polysaccharides such as starch and glycogen are operative.
The solvents for the carbohydrate solution include aqueous solutions of lower alcohols, particularly those containing 5 to 10% of water in methanol, ethanol, npropanol, isopropanol, and the like. Mixtures of such lower alcohols may also be used. Certain other organic. solvents such as acetone or methyl ethyl ketone may be employed in small quantity to assist the solution of the carbohydrate and the organic amine.
The basic organic nitrogen compounds which are suitable for use in developing the barrier layers of the present invention are those which have basic dissociation constants in the range of l l0 to 1X 10" and boiling points above 100 C. Preferred amines include the polymethylenediamines such as hexamethylenediamine, tetramethylenediamine and N,N,N',N-tetrakis(2-hydroxypropyl)ethylenediamine. The solutions of carbohydrates and amines contain small amounts of water in the order of 1 to 10% by volume in order to aid in the interaction of the chemicals of the barrier layer with each other and with the selenium and cadmium layers. In such solutions the amount of nonvolatile materials is in the range of 0.01% to 10% by weight, and preferably in the range of 0.01% to 1%, the remainder of the solution being volatile organic solvents and water.
The counterelectrode which forms a part of the rectifier may be composed of any of the metals used in selenium rectifier counterelectrodes. In practice cadmium or cadmium alloys are preferred. The counterelectrode may be applied by evaporation under high. vacuum, by spraying or by any other suitable method. Among the alloys which may be employed as counterelectrodes are cadmium-bismuth, cadmium-tin, cadmium-bismuth-lead and similar alloys melting between and 300 C.
Rectifiers formed in accordance with the present invention have particularly advantageous properties. They have longer life in laboratory tests and in commercial operation than do corresponding selenium rectifiers formed by prior art. methods. For example, rectifiers produced with sucrose and hexamethylenediamine have exhibited are particularly stable against high humidity, high temperatures and high voltages.
The following formulations are representative of solutions within the scope of this invention whichv produce desirable selenium retifiers.
Example 1 A solvent mixture is prepared from 780 volumes of methanol, 156 volumes of water,,31 volumes of furfuryl' alcohol and 31 volumes of benzyl. alcohol. To 100 milliliters of the foregoing solvent mixture are added 3 grams of crystalline sucrose. The mixture is warmed and stirred until the sucrose is completely dissolved, then cooled to room temperature. Suflicient hexamethylenediamine is added to adjust the pH to 7.0. The resulting solution is diluted with volumes of 90% aqueous isopropanol. Selenium rectifiers are prepared as follows: aluminum base plates are plated with nickel and a thin amorphous selenium ,coating is pressed thereon. The selenium layer is converted to metallic selenium by heat treatment. It is then sprayed with a thin coating of the above sucrosehexamethylenediamine solution. To the dry selenium layer a thin film of bismuthcadium alloy is applied by spraying. Rectifier cells so produced are electroformed at low temperature according to the method of the Eannarino application, Serial No. 475,705, filed December 16, 1954. The rectifiers made in this manner are uniform in performance and stable overperiods of time in excess of 5,000 hours.
Example .2
2 grams of mannitol are dissolved in a solvent mixture containing 80 milliliters of methanol, milliliters of water, 3 milliliters of furfuryl alcohol, and 3 milliliters of benzyl alcohol. Sufficient hexamethylenediamine is added to bring the pH to 7.2. The mixture is agitated and warmed nearly to its boiling point. The resulting solution is brought to room temperature and diluted 10:1 with 90% aqueous isopropyl alcohol. Rectifiers produced'with this coating as in Example 1 are uniform and stable over long periods of time.
Example 3 2 grams of sucrose are dissolved in a solution of 20 milliliters of water in 80 milliliters of methanol. The pH of the solution is adjusted to 6.8 by the addition of N,N,N,N', tetrakis(2-hydroxypropyl)ethylenediamine. Theresulting solution is diluted with 9 volumes of 90% isopropanol. The solution thus produced forms a longlasting barrier layer, when applied to selenium.
Example 4 Selenium layers treated as above are then :coatedwith cadmium or cadmium alloys toa thickness of 0.001 inch to 0.01 inch. These cells are then electroformed by applying a current of relatively high current density in the range of l to 7 amperes, per square inch in the reverse direction, either in oil solution or in air. Rectifiers produced in accordance with this invention exhibit a relativelylong 4 life and high stability, particularly against high temperatures and humidity.
While the invention has been described in connection with specific materials and conditions, it is to be understood that this descriptionis provided by way of illustration only, and is not a limitation of the scope of the invention.
What is claimed as new and desired to be secured by Letters Patent of the United States is:
l. A unilateral conductor comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and an organic amine in an aqueous volatile solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
2. A unilateral conductor comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and an organic amine having a basic dissociation constant in the range of l l0 to l l0 and a boiling point above C. in an aqueous volatile solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
3. A rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a carbohydrate and a polymethylenediamine in an aqueous volatile organic solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
4. A rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with, a dilute solution containing a monosaccharide and a.
polymethylenediamine in a lower aqueous alcohol solvent at a pH in the range of 6.5 to 7.5, and a counter electrode on said barrier layer.
5. A rectifier comprising a base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a disaccharide and a polymethylenediamine in a lower aqueous alcohol solvent at a pH in the range of 6.5 to 7.5, and a counterelectrode on said barrier layer.
6. A rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution containing a disaccharide and a polymethylenediamine in a lower aqueous alcohol solvent at a pH in the range of 6.8 to 7.2, and a cadmium counterelectrode on said barrier layer.
7. A rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution of sucrose and a polymethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
8. A rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution of sucrose and hexamethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
9. A rectifier comprising an aluminum base plate, a layer of selenium on said base plate, a barrier layer on said selenium layer formed by coating the selenium layer with a dilute solution ,of mannitol and a polymethylenediamine in aqueous lower alcohol solvent having a pH of 6.8 to 7.2, and a cadmium alloy counterelectrode layer on said barrier layer.
10. A rectifier comprising an aluminum base plate, a
layer of selenium on said base plate, a barrier layer on References Cited in the file of this patent said selenium layer formed by coating the selenium layer with a dilute solution of mannitol and hexamethylene- UNITED STATES PATENTS diamine in aqueous lower alcohol solvent having a pH of 2,193,598 Lotz Mar. 12, 1940 6.8 to 7.2, and a cadmium alloy counterelectrode layer 5 2,510,361 Addink June 6, 1950 on said barrier layer. 2,660,698 Black Nov. 24, 1953

Claims (1)

1. A UNILATERAL CONDUCTOR COMPRISING A BASE PLATE, A LAYER OF SELENIUM ON SAID BASE PLATE, A BARRIER LAYER ON SAID SELENIUM LAYER FORMED BY COATING THE SELECIUM LAYER WITH A DILUTE SOLUTION CONTAINING A CARBOHYDRATE AND AN ORGANIC AMINE IN AN AQUEOUS VOLATILE SOLVENT AT A PH IN THE RANGE OF 6.5 TO 7.5, AND A COUNTERELECTRODE ON SAID BARRIER LAYER.
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2828453A (en) * 1955-06-15 1958-03-25 Westinghouse Brake & Signal Selenium rectifiers
US2841749A (en) * 1955-04-11 1958-07-01 Sarkes Tarzian Selenium rectifier
US20060231349A1 (en) * 2002-05-21 2006-10-19 Mitsubishi Denki Kabushiki Kaisha Buffer device for elevator

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2193598A (en) * 1936-09-05 1940-03-12 Westinghouse Electric & Mfg Co Method of producing selenium rectifiers
US2510361A (en) * 1944-04-06 1950-06-06 Hartford Nat Bank & Trust Co Method of producing selenium rectifiers
US2660698A (en) * 1951-10-29 1953-11-24 Standard Telephones Cables Ltd Selenium rectifier

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2193598A (en) * 1936-09-05 1940-03-12 Westinghouse Electric & Mfg Co Method of producing selenium rectifiers
US2510361A (en) * 1944-04-06 1950-06-06 Hartford Nat Bank & Trust Co Method of producing selenium rectifiers
US2660698A (en) * 1951-10-29 1953-11-24 Standard Telephones Cables Ltd Selenium rectifier

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2841749A (en) * 1955-04-11 1958-07-01 Sarkes Tarzian Selenium rectifier
US2828453A (en) * 1955-06-15 1958-03-25 Westinghouse Brake & Signal Selenium rectifiers
US20060231349A1 (en) * 2002-05-21 2006-10-19 Mitsubishi Denki Kabushiki Kaisha Buffer device for elevator
US20060249334A1 (en) * 2002-05-21 2006-11-09 Mitsubishi Denki Kabushiki Kaisha Buffer device for elevator

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