Patent Application
20080055038
The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
By way of overview, a striker pin in a thermal switch configured as a mechanical link between a bimetallic disk and an armature spring is provided. The striker pin includes a pin of molded ceramic material. The pin has a generally cylindrical shape, a first axial end, and a second axial end. The first axial end is fastenable in fixed relation to an armature spring. A metallizing film is fused to the second axial end. A metallic/synthetic deposit is fused to the metallizing film such that the metallic deposit substantially covers the second axial end.
The thermal switch is designed for use in high reliability applications such as Space Science Satellites, Defense Satellites, Commercial Satellites, Manned Space Flight Programs and High-Value Terrestrial Applications. Because of the operating environment and the extremely high cost of repair (requiring a separate space flight for replacement) the switches are developed and fabricated to have long life (20+ years) and high reliability while operating under extreme conditions. The switches are bimetallic snap action type relying upon the designed thermostatic characteristics of a bimetallic disk.
A hermetic glass seal 32 holds the external terminal post 26 fixed in one of two perforations to the header plate 44, while a hermetic glass seal 34 holds the external terminal past 28 fixedly in the other perforation. An armature spring is riveted to the top of the terminal post 23. A stationary contact is riveted to the top of the terminal post 24. A striker pin 13 is affixed to the armature spring 21 and bearing against the bimetallic disk 18 during one operating state (contacts open). A metallic/synthetic deposit 16 is affixed at an end of the striker pin 13.
The bimetallic disk 18 actuates by detecting temperature change above or below its operational set points. It actuates by deforming convexly. In doing so the bimetallic disk 18 presses against the striker pin forcing the armature spring 21 to open or to close a pair of electrical contacts (29 and 27) depending upon the designed cycle of the switch 10.
The striker pin 13 includes a ceramic material with a bonded lubricious metallic/synthetic deposit 16, such as an autocatalytic nickel matrix that includes second phase particles that impart additional advantageous properties. An example of the second phase particles include Polytetrafluoroethylene (PTFE or Teflon®), such as that produced by Coating Technologies Inc. of Phoenix, Ariz. under the brand name NP3. Fusion of the ceramic material and the metallic/synthetic deposit 16 is done by a process of metallizing a film layer, such as a nickel film or other metals as described below, and then plating the surface to achieve strong mechanical bonding.
At a block 48, a refractory metal paint, preferable including molybdenum or a similar substance, is applied at the intended site of the metallic/synthetic deposit 16 on the green ceramic pin. In one embodiment, the refractory metal paint includes a small amount of manganese (around 10% is generally suitable). The refractory metal paint is generally applied by either brushing or screen printing onto the ceramic surface to be metallized to form a metallic layer.
At a block 51, the pin with the refractory metal paint is fired (heated). Firing serves two purposes. First, firing cures the ceramic pin bringing it to its vitreous state. Firing also sinters a boundary between the green ceramic and the refractory metal paint causing the metal paint to bond to the ceramic pin. As the ceramic enters the glass phase of firing, the ceramic is drawn into the interstices of the refractory metal paint, i.e., a molybdenum layer of the paint. The added manganese then has two effects. First, upon heating during the sintering, the manganese is oxidized to form manganese oxide, which, at temperature, enhances the permeation of the ceramic in the glass phase into the molybdenum layer. Second, the manganese penetrates down ceramic grain boundaries of the pin and changes the properties of the ceramic in the glass phase. These two changes decrease both the thermal expansion mismatch between the molybdenum layer and the ceramic, and alter the glass transition temperature of the ceramic pin. The results may be enhanced where firing occurs under a greater atmospheric pressure resulting in what is known as “densification,” i.e. the further migration of metals in to the boundary region. As a result, there is less residual stress at the metallized interface, which leads to a stronger bond than had previously been achieved with the refractory metals alone.
Once a defect free molybdenum-manganese layer has been successfully applied and fired, the resulting pin is plated with a thin layer of a suitable metallic and synthetic combination. The immediate plating with metallic/synthetic material prevents oxidation of the Mo—Mn layer. Usually, the metallic/synthetic material is deposited either by electroplating, electroless plating, by the reduction in hydrogen of nickel oxide paint or by some other fusing process. Upon plating, the pin is suitable for use as the striker pin 13.
In another embodiment the refractory metal paint or film includes Nickel.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment.
