US3857175A

US3857175A - Method of manufacturing reed switches with oxidized rhodium contacts

Info

Publication number: US3857175A
Application number: US00421788A
Authority: US
Inventors: T Yokokawa; C Kawakita
Original assignee: Oki Electric Industry Co Ltd
Current assignee: Oki Electric Industry Co Ltd
Priority date: 1973-01-25
Filing date: 1973-12-05
Publication date: 1974-12-31
Anticipated expiration: 1991-12-31

Abstract

The cooperating contacts of a reed switch are made of rhodium oxide. Such contacts are prepared by plating rhodium on the reeds of reed switches and then oxidizing the surface layers of the plated rhodium contacts by heating them in an oxygen containing atmosphere at a temperature of from 380* to 580*C for 3 to 15 minutes.

Description

United States Patent [191 Yokokawa et al.

[ Dec. 31, 1974 [5 METHOD OF MANUFACTURING REED 3,606,498 9/1971 Ohno 200/144 B SWITCHES WITH OXIDIZED RHODIUM 3,663,777 5/1972 Steinmetz et a1. 200/ 166 C CONTACTS [75] Inventors: ToshikiYokokawa; Qhihiro Primary Examiner c Lanham Kawak'ta both of Tokyo Japan Assistant Examiner-James R. Duzan 731 Assignee: 0K1 Electric Industry 00., Ltd., Attorney, Agent, or Firm-Dike, Bronstein, Roberts.

Tokyo Japan Cushman & PfLll'ld [22] Filed: Dec. 5, 1973 [21] Appl. No; 421,788

Related US. Application Data [57] ABSTRACT [62] Division of Ser. No. 327,098, Jan. 26, 1973, Pat. No.

33135051 The cooperating contacts of a reed switch are made of v rhodium oxide. Such contacts are prepared by plating 52 us. Cl. 29/622, 29/630 c rhodium on the reeds of reed Switches and 51 Int. Cl. HOlh 11/00 dizing the Surface layers of the Plated rhodium 58 Field of Search... 29/622, 630 c, 195 T, 527.2, contacts by heating them m an Oxygen Containing 29/5274; 200/1 C 144 14 3 635 mosphere at a temperature of from 380 to 580C f0! 3 to 15 minutes.

[56] References Cited UNITED STATES PATENTS 3 Claims, 5 Drawing Figures 3,222,489 12/1965 Chaikin 200/166 C 1 I fr i \N I PATENTEB 1974 3.857. 175

sum 1 0F 2 OONTAOT RESTSTANOE rm (21 START m 1 0 10 NUMBER OE OPERATION OONTAOT NESISTANOE (m 0) START 10 m 10 NUMBER OF OPERATION PATENTEDBECW 3,857,175

5 5 E g 10 a E E a E :5 E 5 z am) 350 Min 450 500 550 600 650 BAKING TEMPERATURE "B METHOD OF MANUFACTURING REED SWITCHES WITH OXIDIZED RHODIUM CONTACTS This is a division, of application Ser. No. 327,098 filed Jan. 26, 1973 now U.S. Pat. No. 3,813,508.

BACKGROUND OF THE INVENTION This invention relates to a reed switch having contacts whose contact resistance does not increase after a number of operations and to a method of preparing such contacts.

A conventional reed switch comprises a sealed glass envelope 1, a pair of reeds 2 usually made of Fe-Ni Permalloy, and contacts 3 on the inner ends of reeds 2 said contacts being usually formed by plating a nobel metal.

Although rhodium has generally been used as the nobel metal because it is suitable for use as the contact material for reed switches, has a disadvantage in that the contact resistance between contacts of rhodium increases abnormally after a relatively small number of operations. Although a number of reasons for this may be considered, surface adsorption and catalytic action inherent to rhodium are considered to be the major reasons. When contacts of rhodium whose surfaces have adsorbed various organic substances or other impurities are opened and closed many times polymers are formed by the energy of collision of the contacts any by the catalytic action of rhodium and such polymers cause an increase in the contact resistance.

Accordingly, it is desirable to develop a novel method of surface treatment of the contact which can eliminate the catalytic action of rhodium thereby providing reliable contacts free from any abnormal increase in the contact resistance.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved reed switch provided with rhodium contacts which do not increase their contact resistance during use.

Another object of this invention is to provide a novel method of surface treatment of the rhodium contacts of reed switches.

According to this invention, these and other objects can be accomplished by oxidizing the rhodium contacts of a reed switch so as to form extremely thin films of rhodium oxide. To this end, the contacts plated with thin films of rhodium are heated to a temperature of 480 1': 100C for 3 to minutes in an oxygen containing atmosphere. The resulting thin films of rhodium oxide greatly reduce the adsorption of impurities and catalytic action of metallic rhodium.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a conventional reed switch;

FIG. 2 is a graph showing the variation in the contact resistance of a prior art reed switch having rhodium contacts;

FIG. 3 is a graph showing the variation in the contact resistance of the rhodium contacts prepared in accordance with this invention;

the deterioration of the contact resistance and baking temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT As above described, according to this invention, contacts of reed switches are prepared by plating rhodium on the reeds of reed switches and then oxidizing the plated rhodium in an oxygen containing atmosphere under conditions described above. To compare the variation in the contact resistance of the prior art rhodium contacts and rhodium contacts having thin layers of rhodium oxide which are prepared by the method of this invention, the following experiment was made. More particularly, as shown in FIG. 4, a dish shaped container 9 containing leads 7 for reed switches having contacts prepared by the method of this invention and another container 9' containing reeds having prior art rhodium plated contacts were placed on a perforated shelf 6 of a sealed vessel 4. A quantity of benzol 5 was placed in the bottom of vessel 4 which was exhausted by means of a vacuum pump (not shown) connected to an exhaust pipe 8. After 24 hours, the

reeds

7 and 7 were taken out from the vessel 4 and were respectively used to fabricate reed switches and life tests were carried out for these reed switches. As shown in FIG. 2, the contact resistance of conventional contacts increased greatly after a relatively small number of operations, whereas the contact resistance of the contacts treated in accordance with this invention varied only a little after many operations (that is 10 as shown in FIG. 3.

When inspected with a microscope, the prior art contacts were observed to have formed polymers because they have adsorbed benzol which was converted into polymers by the energy of the collision of contacts and the catalytic action of rhodium. On the other hand no polymer was noted on the contacts of reeds of this invention.

FIG. 5 shows a preferred range of oxidation temperature, wherein the ordinate shows the percentage of rejected contacts due to excessive increase in the contact resistance after 10 operations (which is a typical number of cycles for the life test) and the abscissa represents the baking or oxidation temperature of plated rhodium contacts. As can be noted from FIG. 5 with a baking temperature of from about 380C to about 580C, the percentage of the rejected contacts is approximately zero but increases rapidly at temperatures below 380C and above 580C. This is because at temperatures below 380C the reaction is not sufficient whereas at temperatures above 580C the surface layer of rhodium undergoes recrystallization due to heat or the diffusion of the plated metal occurs.

What is claimed is:

l. A method of manufacturing a reed switch comprising the steps of forming rhodium contacts on leads, oxidizing said rhodium contact and sealing said leads with said oxidized rhodium contacts in a sealed envelope.

2. The method according to claim 1 wherein said oxidation is carried out by heating said rhodium contact in an oxygen containing atmosphere at a temperature of from 380C to 580C for 3 to l5 minutes.

3. The method according to claim 1 wherein said rhodium contacts are formed by plating rhodium on said leads.

Claims

1. A METHOD OF MANUFACTURING A REED SWITCH COMPRISING THE STEPS OF FORMING RHODIUM CONTACTS ON LEADS, OXIDIZING SAID RHODIUM CONTACT AND SEALING SAID LEAD WITH SAID OXIDIZED RHODIUM CONTACTS IN A SEALED ENVELOPE.

2. The method according to claim 1 wherein said oxidation is carried out by heating said rhodium contact in an oxygen containing atmosphere at a temperature of from 380*C to 580*C for 3 to 15 minutes.