Electricity: circuit makers and breakers – Special application – Change of inclination or of rate of motion responsive
Reexamination Certificate
2002-09-25
2004-03-16
Easthom, Karl D. (Department: 2832)
Electricity: circuit makers and breakers
Special application
Change of inclination or of rate of motion responsive
C200S220000, C200S233000
Reexamination Certificate
active
06706980
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of electrical switches and sensors, in particular, electrical switches and sensors relying on a liquid metal or metal alloy as an electrical conducting material for bridging an electrical gap between electrodes.
BACKGROUND OF THE INVENTION
Electrical switches and sensors that are usually referred to as “tilt” switches, such as thermostats, float controls, solenoids, relays, etc. are commonly used in a variety of electrical applications. The making or breaking of electrical contact in these switches, hence the electrical switching action, is generally accomplished by mechanical movement of the switch which causes a quantity of a bridging conducting material contained therein to flow from one location to another. Liquid mercury is used extensively in such electrical switches and sensors as the bridging conducting material.
In a typical switch application, liquid mercury is positioned inside a capsule or housing into which spaced apart electrodes or electrical contacts extend. Depending on the physical orientation of the housing, the liquid mercury can provide a conductive pathway between the electrodes, or be positioned such that there is an open circuit between the electrodes. The switch is closed and electrical contact made when the switch housing is moved in a manner such that the quantity of mercury flows toward a location in the housing at which the mercury bridges the spaced electrodes, thereby permitting the flow of electricity from one electrode to the other. Conversely, the switch is opened and electrical contact broken when the switch housing is moved in a manner such that the quantity of mercury flows towards and collects at a different position in the switch housing out of contact with at least one of the electrodes.
An important physical attribute of mercury for the purposes of electrical switch applications, aside from its ability to conduct electricity, is that it remains fluid throughout a wide temperature range thus enabling it to be used in many different environments, or in environments with constantly changing temperature parameters. Another important physical attribute of mercury is that it has significant surface tension and does not wet glass, metal or polymer surfaces.
A problem with mercury-based electrical switches is that mercury is toxic to humans and animals, and exposure to mercury is a significant concern in any application or process in which it is used. Utilization of mercury during manufacturing may present a health hazard to plant personnel, and the disposal of devices that contain mercury switches or the accidental breakage of mercury switches during use may present an indirect hazard to people within the immediate vicinity of the switch.
As a result of the toxicity of mercury, non-toxic replacements for mercury in electrical switch applications have been sought. A candidate for replacing mercury in electrical switches is liquid gallium metal or liquid gallium alloys. For example, U.S. Pat. No. 3,462,573 (Rabinowitz et al.) and Japanese Patent Application Sho 57-233016 to Inage et al. each disclose that gallium or gallium alloys may be useful as a replacement for mercury in electrical switches.
However, while gallium is non-toxic, it does not have all of the beneficial properties of mercury. For example, an important shortcoming of gallium and gallium alloys in electrical switch and sensor applications is the propensity of the gallium or gallium alloy to become oxidized. Even slight oxidation of gallium may be detrimental to the performance of the switch or sensor because oxidation of the metal reduces the surface tension of the metal in the liquid phase and may lead to wetting of the switch housing, unwanted bridging of the electrodes, sluggish movement of the metal, and poor contact between the metal and the electrodes. In U.S. Pat. No. 5,792,236 (Taylor et al.), it was recognized that gallium and gallium alloys are readily oxidized when exposed to ambient air, and that oxidized gallium alloys wet a number of materials, including glass. Taylor discusses that when the oxides in the metal components in the gallium alloy were removed and the formation of oxides during and after switch fabrication were prevented, the gallium alloy would not wet the switch housing material. To overcome this particular problem, Taylor discloses that the removal of the oxide from the metals may be accomplished by washing the metals in HCl, 30% NaOH, or other acids, bases, or reducing agents, and that after the oxides are removed, further oxidation of the metals should be avoided. Taylor discloses a multifaceted solution to the problem involving removal of the oxides from the metal constituents of the gallium alloy and maintaining the gallium alloy in an oxide free state prior to dispensing the alloy into a switch housing, and, because of the recognized rapid speed of oxidation of gallium or gallium alloys, the use of specialized dispensing apparatus designed to prevent oxide formation on the alloy during dispensing of the alloy into the switch housing. Taylor also discloses that coating the interior of the switch housing with a fluoroalkyl acrylate polymer prevents wetting of the housing by the gallium alloy, but it is noted that this coating of the polymer to the housing does not prevent stickiness to the electrodes. While these techniques may improve the functionality of gallium in an electrical switch or sensor, this would appear to require complex equipment and involve cumbersome processes for making electrical switches.
Further limitations of the prior art may be that washing the metal in acid, alkali or in a reducing agent to dissolve the oxide or chemically reduce it to gallium involves using strong chemical agents which attack the oxides on the gallium alloy. Traces of such chemical agents have to be removed from the gallium alloy prior to placement into a switch housing since the presence of a small quantity of these strong chemical agents will leave behind an oxide layer or in the long term will attack and deteriorate the gallium alloy surface thus compromising its surface tension and non wetting character. Taylor's patents describe separating the metallic parts from the aqueous acid or base reacting liquid and the gallium containing melt, but do not address removing traces of these strong chemical agents by repeated washings in deionized water. Water used for dissolving the acid, alkali or reducing agent generally has dissolved oxygen which may be a problem since this dissolved oxygen will re-oxidize the gallium alloy. Therefore, water free from dissolved oxygen and the reaction process, separation of the gallium alloy and the filling of the switch housing has to be done in inert, non-oxidizing conditions to prevent re-oxidation of gallium alloy.
In view of the drawbacks and problems with the prior art, a need exists for a novel approach to gallium-based electrical switch construction which will reduce or eliminate such drawbacks and problems.
SUMMARY OF THE INVENTION
The present invention provides gallium based switch or sensor devices, and a process for producing such devices using either in-situ or ex-situ separation of oxides from gallium or gallium alloys. The separation of oxides from gallium or gallium alloy in accordance with an embodiment of the present invention may be accomplished, and an oxide free state maintained, within the switch or sensor capsule or housing itself (in-situ), in which case the separation of oxides from the gallium or gallium alloy may be more efficiently carried out, and the switches or sensors may be less costly to produce, than the steps of the prior art. The separation in accordance with other embodiments of the present invention may also be carried out prior to the filling of a capsule or housing with gallium or gallium alloy (ex-situ).
Some embodiments of the present invention use oxide removing agents, either as a liquid or a gas, that are not strong enough to dissolve the oxide on the surface of the gallium or gallium alloy, but rather which physically sepa
Easthom Karl D.
Honeywell International , Inc.
Lee K.
LandOfFree
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