US2764659A - Resistance type fire detector cable - Google Patents

Description

R. H. POSTAL RESISTANCE TYPE FIRE DETECTOR CABLE Sept. 25, 1956 Filed June 27, .1955

0 0 00 M 0 0 wwnw s a 3 z 01m no *mmwom Lam m @Smom m fiWumQ Cable Oxide 0.05 0.10 I Effechve Aromlc INVENTOR Robe?! H. P052221 BY United States Patent RESISTANCE TYPE FIRE DETECTOR CABLE Robert H. Postal, Clifton, N. J., assignor to Thomas A.

Edison, Incorporated, West Orange, N. J., a corporation of New Jersey Application June 27, 1955', Serial No. 518,033 8 Claims. (Cl. 201-63) This invention relates to temperature-responsive resistance devices and to novel temperature-responsive resistance material and methods of preparing the same. The invention relates particularly to improved fire detection apparatus in the form of a flexible cable of indefinite length, and is described in terms of such cable but without intending any unnecessary limitation thereto.

A fundamental feature of my invention resides in the provision of a novel electronic oxidic semiconductor which can be produced under controlled conditions to have a preset electrical conductivity and which is characterized as having an inherent stability unlike the usual semiconductors, enabling it while in the cable to be cycled through extreme temperature ranges without undergoing any substantial change in its physical characteristics and operationalbehavior.

The novel semiconductive material of my invention comprises manganese oxide having lithium in its crystal lattice and having no appreciable excess oxygen over the crystal requirements, such material being hereinafter referred to as lithium-doped manganese oxide. It has been known that different electronic oxidic semiconductors can be rendered more electrically conductive by causing the crystal lattices thereof to contain excess oxygen over and above the simple stoichiometric ratio, and that by controlling the amount of excess oxygen so contained the electrical conductivity can be varied. In the pending Kelly-Postal application Serial No. 241,992, filed August 15, 1951, and the Postal application Serial No. 392,565, filed November 17, 1953, both of which applications have common ownership with the present application, apparatus and methods are taught by which electronic semiconductive material with excess oxygen can be utilized commercially for fire detection purposes while maintaining the material in a stable, reproducible state although subjected to wide temperature cycling, but such stability is achieved by a special processing of the material and by maintaining it under exacting environmental conditions such that the oxygen which is driven off during the heating half-cycle is caused to recombine totally with the semiconductive material during the cooling half-cycle. While such semiconductive oxides with excess oxygen can be provided in cables for direct operation of a receiving instrument at any operate temperature within a wide range without need for amplification-the term operate temperature being herein employed to mean that temperature at which a 50' length of cable has a resistance between center wire and sheath of 100 ohms-such material is suited particularly for higher operate temperatures above 500 F.

The lithium-doped manganese oxide of my invention has a greater inherent stability than the electronic semiconductive oxides with excess oxygen, which enables it to be used under less exacting conditions at a resultant lower cost, and is particularly adapted for fire detection cables of the character abovedescribed but which are to have operate points below 500 F. Moreover, I find that this lithiumdoped manganese oxide can be readily processed so that its conductivity can be accurately controlled solely by the amount of lithium introduced into the manganese oxide crystal.

Objects of my invention are therefore to provide electronic semiconductive oxide materials having more readily controlled electrical conductivity and greater inherent stability than the other semiconductive materials heretofore known, to provide fire detection apparatus using such improved semiconductive materials which are more dependable and more economical to produce, to provide such fire detection apparatus capable of operating directly receiving devices without need for'amplification and at operate temperatures below 500 F and to provide new and improved methods of processing such improved electronic oxidic semiconducting materials and of fabricating heat-detecting and measuring apparatus using the same.

These and other objects and features of my invention will be apparent from the following description and the appended claims.

In the description of my invention reference is had to the accompanying drawings, of which:

Figure l is a fractional view, partly in axial section, of a fire detection cable according to my invention, and showing also an operating circuit for this cable;

Figure 2 is a perspective view of a second form of fire detection cable according to my invention showing an end of the cable in cross section; and

Figure 3 is a graph showing a variation of the operate temperature of my first form of cable with change in the lithium content of the semiconductive manganese oxide in the cable.

The cable shown in Figure 1 comprises a central metal wire 10 constituting one electrode of the cable, a spaced surrounding metal sheath 11 constituting a second electrode and an intervening temperature-responsive material 12 which comprises electronic semiconductive metal oxide in its entirety or as its principal ingredient. The temperature-responsive material is packed into the cable under pressure as by swaging the sheath to a reduced diameter after the material is loaded into the sheath, the packing of the material under pressure being principally to assure a positive electrical contact between the ma terial and the sheath and wire. The temperature-responsive material preferably comprises the sole medium in the cable for holding the central wire in axial relation to the sheath except for the possible use of ceramic beads 13 at the ends of the cable. The ends of the cable are preferably closed airtight by hermetic seals 14 each comprising a glass or ceramic bead 15 fuzed to outer and inner tubing sections 16 and 17. These tubing sections are telescoped onto the sheath and wire respectively and the ends thereof are secured airtight to the sheath and wire as by silver soldering.

The sheath and center wire must be capable of with standing extremely high temperatures running beyond 2000 F. when engine oil fires on aircraft are to be de tected. The sheath should also be ductile so that it can be swaged as aforementioned and so as to enable sharp bending of the finished cable to meet particular installation requirements. Nickel, iron, copper or alloys thereof are suitable metals for the sheath. Likewise, the same materials are suitable for the center Wire. By way of typical example, the sheath may have an outside diameter of .070" and a Wall thickness of .011", and the center Wire may have a diameter of .020", leaving an intervening layer of temperature-responsive material of .014" thickness. Wide variations in these particular dimensions are of course permissible.

Fire detection cables using electronic semiconductive oxidic materials have a relatively steep resistance-temperature characteristic enabling them to be used to op- "ice fix all? crate a receiving instrument directly without need for amplification. Thus, the present cable may be connected in an electrical circuit serially including a battery 21 and a receiving instrument Z2 diagrammatically shown. This receiving instrument may be a relay for controlling other apparatus or it may be an alarm device itself.

In preparing the lithium-doped manganese oxide of my invention, I preferably mix an oxide of manganese, suitably nianganous oxide (MnO) with an oxide of lithium (LizO), the lithium being added in an amount proportional to the conductivity desired for the end material. Preferably,- the mixture is produced by thoroughly mixing manganous oxide with a water solution of lithium acetate, and then mixing and concurrently heating the same until suflicient water is driven off to provide a thick sludge. This sludge is then heated in air to decompose the lithium acetate and convertit to the oxide, a suitable temperature for this purpose being 600 F. After this heating step the material is a dry, intimate mixture of manganous oxide and lithium oxide.

Next the foregoing mixture is heated in air to a sintering temperature of the order of 1200 C. for about an hour. It is believed that in this heating step lithium ions enter the crystal lattice of the manganese oxide to provide a chemical combination of manganese oxide and lithium which is herein otherwise referred to as lithiumdoped manganese oxide. As this product is subsequently cooled, oxygen from the air also enters the lattice to provide it with an excess of oxygen over the crystal requirements. For each excess oxygen atom held in the lattice two manganese ions are made trivalent. Also, for each lithium ion taken into the lattice a neighboring manganese ion is made trivalent. The composition thus becomes enriched with trivalent manganese ions.

Trivalent manganese ions maintained in the lattice by excess oxygen are unstable in that loosely held oxygen is driven oil with consequent reduction of manganese ions from the conductive trivalent to the non-conductive divalent state and with resultant increase in the resistivity of the composition, as the composition is heated to a firing temperature. In accordance with my invention, the excess oxygen is purposely driven off by heating the lithiurn-dcped manganese oxide in an inert or reducing atmosphere, such as nitrogen or hydrogen, at a temperature of the order of i006 C. By so driving off the excess oxygen, the conductivity becomes solely dependent on the lithium present. Thus, by starting with different proportions of lithium acetate in the initial mix, the conductivity of the final product can be varied.

The electrical conductivity of the final product increases as the number of trivalent manganese ions is increased, but it appears that the electrical conductivity is not dependent solely on the trivalent manganese ions since many tests indicate that lithium-doped manganese oxide has a greater conductivity than can be accounted for by the presence of trivalent manganese ions. However, it will be understood that I intend no unnecessary limitation of my invention in regard to the manner in which the lithium affects the electrical conductivity of the manganese oxide, whether it be solely by chemical combination of the lithium with the manganese oxide or not, or whether it be by conversion of manganese ions from lower to higher states of valency or by other means.

Alternatively, take-up by the material of excess oxygen at any stage in the processing thereof may be avoided by allowing the material, following the heating step in air, to cool in air firstly to about 1000 C. and thereupon to cool to room temperature in an inert or reducing atmos phere. This may be accomplished in any suitable Way by shifting the material from an air atmosphere to an inert or reducing atmosphere when in the cooling thereof the temperature has fallen to about 1000 C.

Following the processing step abovedes'cribcd, there tends to be a small amount of free lithium oxide in the material. This residue of lithium oxide has the property of coating the lithium-doped manganese oxide not only to increase the resistance of the material but also to render it less controllable. However, by thoroughly Washing the material following the heat-reducing step, the free lithium oxide is removed with the result that the con ductivity of the material can be controlled accurately according to the amount of lithium. present; moreover, the conductivity will then be uniform from batch to batch.

Following the preparation of the material as abovedescribed, the same is extruded onto the center wire and threaded through the sheath, and then the sheath is reduced to the desired diameter by successive swaging operations. Since the swaging operations leave the sheath and center wire in a hardened state, the cable is next annealed by heating it in a reducing atmosphere, say carbon monoxide, to a temperature of about 1000" C. for fifteen minutes. After the cable is cooled from this annealing operation, the ends thereof are hermetically scaled as beforedescribed.

Since the trivalent manganese ions held by a lithium bond are highly stable, I obtain a cable which can be fired through a temperature range up to 1000 C., and more, without altering its conductivity characteristic. Because the present serniconductive material, unlike the electronic semiconductive oxides with oxygen, does not tend to release any gas or undergo any decomposition when so heated, it can be used under less exacting environmental conditions than can the oxygen-enriched semi.- conductive material. For instance, the extreme compacting of the oxygen-enriched materials in the cable sheath, which is so essential to achieve stability with those materials as is disclosed in the aforementioned pending applications, is no longer essential with the present material. However, compacting of the material is still desirable for the purpose of providing positive electrical contact between it and the sheath and center wire, as aforementioned.

In Figure 3 there is shown a graph of the operate temperatures versus percent lithium content (on an atomic basis) of the semiconducth e manganese oxide for a cable having the typical dimensions hereinbefore described. A lithium-dopcd manganese oxide having a 1 atomic percent lithium content is obtained, for example, by the process hereinbefore described, when that process is C?!- ried out with initial ingredients consisting of 1 part of manganese oxide and .0292 part of lithium acetate.

An alternative form of cable embodying my invention is shown in. Figure 2. This embodiment comprises a sheath 23, two spaced internal wires 24 and 25 running the length of the sheath, and a spacing scmiconductive oxide 265 between the wires and sheath. Each end of the sheath is sealed by a hermetic seal 27 of the type hereinbefore described. In this cable construction the sheath serves only as a protective covering, in view of which a firm electrical contact of the spacing material with the sheath is no longer necessary. The external electrical circuit i8 is in this case connected to the two internal wires. Each element of this cable may be made of the same material as is that of the corresponding element in my first embodiment abovcdescribed.

The particular embodiments of my invention herein de scribed are intended to be illustrative and not necessarily limitativc of my invention since the same are subject to changes and modifications without departure from the scope of my invention, which i endeavor to express according to the following claims.

I claim:

1. A resistance-type temperature-responsive device of indefinite continuous length comprising two spaced rnctal conductors having the space therebetween sealed from the atmosphere, and a temperatureresponsive material filling said space and in intimate electrical contact with said conductors, said material comprising an oxide of manganese containing lithium ions in its crystal lattice.

2. A resistance-type temperature-responsive cable of indefinite continuous length comprising a metal sheath and a spaced center wire, and a temperature-responsive resistance material filling the space between said center wire and sheath, said material being compacted in said sheath to have a positive electrical contact with said center wire and sheath, and said material comprising an oxide of manganese having lithium ions in its crystal lattice.

3. The device set forth in claim 1 in which said temperature-responsive material is substantially free of excess oxygen over the stoichiometric requirements of the manganese and lithium ions in the crystal lattice.

4. Thetemperature-responsive device set forth in claim 1 in which said temperature-responsive material contains trivalent manganese ions in proportion to the lithium ions present and contains substantially no free lithium oxide.

5. A temperature-responsive resistance material stable at high temperatures in excess of 1000 C. in a reducing atmosphere, comprising a predominant proportion of manganese oxide and a minor proportion of lithium, said material comprising said oxide and lithium in chemical combination with lithium ions in the crystal lattice of the manganese oxide, and said material being substantially free of oxygen in excess of the stoichiometric requirements of the manganese and lithium ions in the crystal lattice.

6. A temperature-responsive resistance material stable at a firing temperature of the order of 1000 C. in a reducing or neutral atmosphere, comprising manganese oxide containing lithium ions in the vacant manganese ion sites of the crystal lattice, said oxide being rich in trivalent manganese ions and containing substantially no free lithium oxide.

'7. A temperature-responsive resistance material stable at high temperatures in excess of 1000 C. in a reducing atmosphere, comprising an intimate mixture of a predominant proportion of manganese oxide and a minor proportion of lithium oxide fired in air at about 1200 C. and thereupon cooled, the cooling from about 1000 C. being in an inert or reducing atmosphere to prevent take-up of excess oxygen during the cooling step.

8. The method of preparing a temperature-responsive resistance material stable at a firing temperature of the order of 1000 C. in a reducing or neutral atmosphere which comprises providing an intimate mixture of an oxide of manganese and an oxide of lithium, heating said mixture in air to a sintering temperature of the order of 1200 C. for about one hour to introduce lithium into the crystal lattice, and causing said material to be cooled, the final cooling of said material from about 1000 C. to room temperature being in a reducing atmosphere whereby to prevent oxygen from being taken up by the material during the cooling thereof.

No references cited.

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