US 2848587 A
Description (OCR text may contain errors)
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FIRE DETECTOR CABLE Filed Nov. 1'7, 1955 Elm 500 TEMPERATURE PF.)
INVENTOR Postal R01; art H 1.001 V COMPOSITION OF STARTING DXIDE 0F MANGANESE M 8 N B m. 8 8
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Patented Aug. 19, 1958 FE DETECTOR QABLE Robert H. Postal, Clifton, N. .l., assignor, by assignments, to McGraw-Edison (Company, Elgin, ill, a corporation of Delaware Application November 117, 1953, Serial No. 392,565 5 Claims. (fil. Elli-63) This invention relates to fire detector cables of the character described and claimed in the pending Kelly and Postal application Serial No. 241,992, filed August 15, 1951 (now Patent No. 2,740,874, dated April 3, i356) and having common ownership with the present application. The invention relates particularly to improved fire detector cables of this character, and to novel and improved methods of constructing such cables and of preparing the temperature-responsive material thereof.
These fire detector cables use temperature-responsive materials comprising the electronic non-stoichiometric oxidic semiconductorsi. e., compounds with a deviatien from a simple stoichiometric ratio. Such semiconductors have temperature-responsive characteristics depending on their oxygen contents but are notably instable. However, in the fire detector cables of the aforesaid application, stability is achieved by compacting the semiconductor in a sealed metal sheath of the cable under great pressure and by oxidizing the internal surfaces of the cable.
An object of. the present invention is to provide new and improved techniques in fabricating such cables, which enables cables having preselected temperature-resistance characteristics to be produced under controlled production methods.
Another object is to provide a new and improved fabricating technique by which cables can be produced economically under controlled conditions to have any preselected resistivity within a wide range at a given temperature.
Another object is to provide such improved fabricating technique which can be carried out accurately under mass production methods.
Another object is to provide a new and improved methed by which electronic non-stoichiometric oxidic semiconductors for the present type cable can be prepared to provide such cable with any given resistivity within a wide range of temperatures.
Another object is to provide fire detector cables having improved structural and operating characteristics, and especially greater uniformity of operating characteristics along each cable and greater stability of operation under repeated cyclings through wide temperature ranges.
It has been the prevailing thought that electronic nonstoichiometric oxides have very limited practical application. For example, in the article Controlled-valency semi-conductors by E. l. W. Verway, P. W. Haaijman, F. C. Romeijn and G. W. van Oosterhout, Philips Res. Rep. 5, 173-187, 1950, it is stated, page 176:
The application of non-stoechiometric compounds'as semi conductors in research and practice has rather severe limitations. One reason is a purely practical one, viz the circumstance that the preparation of a homogeneous and stable semi-conductor with a reproducible value of the specific resistance in this way is generally very difficult or even virtually impossible. Secondly the deviation from simple stoechiometry that can be realized is in most cases rather small and the possibility of variations of the specific resistance accordingly restricted,
i have found that electronic non-stoichiometric semiconductive metal oxides can be prepared under controlled reproducible conditions for use in the present type cable to provide cables with very stable operation having widely different specific resistances. This preparation is carried out by selecting, as initial ingredients, different oxides having substantially diiferent oxygen contents and by intimately mixing such oxides in predetermined proportions.
A further object of my invention is to provide a methad of producing fire detector cables under substantially invariable conditions which enables the operating characteristics of the cables to be predetermined on the basis of the metal oxide mix and the heat-treating operation.
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 cross sectional view of a fire detector cable incorporating my invention;
Figure 2 is a graph showing the resistancev variation of a cable at a given temperature with variation in the oxygen content of the temperature-responsive material using oxides of manganese; and
Figure 3 is a graph showing several resistance-temperature characteristics of cables using temperature-responsive manganese-oxide materials of different oxygen content. The present fire detector cable may be geometrically the same as described, in the. aforementioned application. For example, it comprises a central metal wire it) 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 element. The temperature-responsive material is packed into the cable under great pressure as by swaging the sheath to a reduced diameter after the material is loaded into the sheath. This temperature-responsive material preferably comprises the sole medium in the cable for holding the central wire centralized in relation to the sheath except for the use of ceramic beads 13 at the ends of the cable. The ends are closed air-tight by hermetic seals 14 each comprising a glass 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 by silver soldering at 18 and 19 respectively.
The sheath and center wire must be capable of withstanding extremely high temperatures running beyond 2000 F. when engine oil fires on aircraft are to be detected. The sheath should also be ductile to enable the swaging aforementioned and to enable sharp bending of the finished cable in meeting particular installation requirements. Nickel-iron alloy of about 42% nickel and the remainder iron admirably fulfils these requirements. As for the center wire, there is required a metal which will also acquire a low-resistance contact with the semiconductive metal oxide under the conditions of fabrication of the cable. Except for the noble metals--which are hardly acceptable in most practical applications because of their high costthe run of such metals will form an oxidized surface layer in an oxygen atmosphere at flame temperatures. A requirement of such oxidizable. metals is therefore not only that they withstand flame temperatures without melting but also that the oxides thereof have low electrical resistivity and the capability of bonding physically with the metal oxide of the temperature-responsive material by mutual interlacing of the 3 oxides at flame temperatures. It has been found that substantially pure iron wire fulfils admirably all of these requirements.
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 .012" thickness. Wide variations in these particular dimensions may of course be made.
As aforementioned and as will be understood from the Kelly et al. Patent No. 2,740,874, by using an airtight metal-sheathed cable having oxidized internal surfaces and by densely compacting or otherwise consolidating the temperature-responsive material in the cable, it becomes possible to obtain stable reproducible temperature-responsive characteristics with the use of electronic semiconductive metal oxides that are generally recognized as being very instable. It is not fully understood why stable operation is achieved from such cable construction, but it is believed that the heavy compacting of the semiconductor precludes gaseous diffusion therethrough and that this is an important factor. Each of the semiconductors usable for the present purpose has a temperature-resistance characteristic dependent upon its oxygen content, such that the resistance at a given temperature increases as the oxygen content is reduced. Also, each of the semiconductors has the property that it tends to release oxygen as gas when heated under given pressure conditions to certain temperature regions, the temperature region being higher as the oxygen content of the semiconductor is smaller, and vice versa. Although this temperature region may not fall for certain semiconductors in the operate temperature range of the cable at which an alarm is to be sounded or a firecombative apparatus is to be put into operation-thc operate temperature range being typically between 200 F. and 1000 F.it is to be recognized that in practice the cables are subjected to much higher temperaturesas high as 2200 F. and more when detecting oil fires on aircraftbefore the fire can be coped with. These higher temperatures exceed the abovementioned gas-releasing temperature regions of the usable electronic oxidic semiconductors with the result that oxygen gas is released during each fire-detecting operation. When gaseous diffusion through the semiconductor is precluded, the oxygen released from each elemental portion as the cable is heated is retained in intimate association with that portion for total recombination therewith as the cable is cooled to assure that the semiconductor will return to its original state and have again the same resistivity after each heating-cooling cycle. The oxides which are usable for the present type of cable are those of the metals: cobalt, manganese, chrome, nickel and copper.
The present invention resides particularly in a new and improved fabricating technique which enables cables to he produced under controlled conditions having a given resistivity within a wide temperature range of operate temperatures. Also the cables so produced have improved construction and greater operating stability and greater uniformity.
It has been found that electronic non-stoichiometric oxidic semiconductors can be produced to have different prescribed oxygen contents and that these oxides can be intimately mixed in preselected proportions to provide improved temperature-responsive materials for the present type cable, to enable cables to be produced having an operate resistance, say 100 ohms for a 50 length of cable of the dimensions aforementioned, at different operate temperatures in a range approximating between 200 F. and 1000 F. Accordingly I am able to produce a family of cables using the same basic metal oxide to meet specific requirements of different fire-detection applications. By way of preferred example, I herein next illustrate my in vention in connection with the use of oxides of manganese, it being understood that the same technique applies similarly to the other oxides abovementioned.
Oxides of manganese can be produced in various forms having widely-different oxygen content ranging from manganous oxide (MnO) to the higher forms such as MnO including the non-stoichiometric intermediate forms. By experiment it has been found that a 50' length of cable of the dimensions aforementioned, using a substantially pure iron center wire, has a resistance of ohms and an operate temperature of approximately 535 F. when the starting oxide of manganese which is loaded into the cable has the composition MnO To obtain this particular composition I mix two readily-preparable forms of oxide of manganese having respectively lesser and greater percentage oxygen content-i. e., ratio of oxygen to manganese-than that of MnO Preferably, I use a near form of manganous oxide prepared by firing manganese dioxide at 1000 C. in a pure hydrogen atmosphere for one hour, and a second oxide of manganese obtained by firing such manganous oxide in air at 1000 C. for about one hour. The first oxide so obtained measuser typically by chemical analysis to be MnO and the air-fired oxide measures to be MnO Since both of these oxides are prepared in definite atmospheres-the one being prepared in air for convenience-repeated production runs will result in uniform materials. It may be parenthetically noted that on firing the manganese dioxide in hydrogen the color thereof changes from black to light green and upon subsequently firing the green oxide in air the color changes to brown.
These green and brown oxides are micropulverized and run through a 325-mesh screen, and are then mixed in such proportions as to give a composition having an oxygen content represented by the formula MHOLUDQ. The formula for compacting the specific proportions of the two oxides stems from the equation that the content of oxygen by weight in the green material plus the content of oxygen by weight in the brown material must equal the content of oxygen by weight in the resulting mix. For example, if X represents the parts by weight of the green material then (100-X) represents the parts by weight of the brown material. The oxygen content of X parts of the green material (Mno will be inoz of (IOU-X) parts of the brown material (MHOL25I1 will be and of 100 parts of the final mix will be 1.009 umc On equating and substituting the atomic weights for oxygen and manganese and solving the equation, it will be found, for example, that 96.975 parts of the green oxide 3.025 parts of the brown oxide by weight are required to give the desired mix.
Having prepared the desired oxide mix, the same is mixed with an extruding lubricant such as Veegum, described in the Kelly et al. Patent No. 2,740,874, and with 10% by Weight of water, and the mix is then extruded under great pressure on the iron center wire with a diameter of .100", the wire being first roughened to prevent oxide slippage. This extrusion is dried in air for about twenty-four hours and is then passed through an extrusion furnace for about fifteen minutes at 1500 F. in an inert atmosphere which may be commercially pure nitrogen. Since the particular oxide of manganese here considered has a low oxygen content it does not release oxygen in such nitrogen atmosphere at this temperature. The treatment does, however, render the mix firmly adherent to the center wire. Directly after this treatment the extrusion is threaded into a clean metal sheath such as above described, having initially such inside diameter as to clear the extrusion by about .005. The sheath is drawn next through dies to establish a firm contact between it and the extrusion, and then the sheath is swaged progressively to a diameter of about .070" to compact the manganese-oxide mix under great pressure.
The swaged cable is then heat-treated at about 1000 C. for fifteen to twenty minutes in a reducing atmosphere of pure carbon monoxide and is sealed at its ends to complete its construction. This final heat treatment is the same for all cables, the purposes thereof being: (1) to provide the inside wall of the sheath and the surface of the center wire with oxide films of the respective metals formed by oxygen from the metal oxide extrusion to thicknesses representing a substantially stable state of oxi dation whereat substantially no further oxidation will take place in the practical use of the cables, and (2) to anneal the sheath from the brittle condition thereof resulting from the cold rolled swaging operations. In this final heat treatment, oxygen will diffuse through the semiconductor to the metal sheath and center wire because of their greater afiinity for oxygen at the heat-treating temperature. Experimental analysis shows that whereas the composition of the semiconductor was originally MnO it is MnO after the final heat treatment. Since the final heat treatment is substantially the same for all cables, the electrical resistivity of each cable may be determined substantially on the basis of the oxygen content of the oxide mixture loaded into the cable.
In the above example, the metal oxide mixture which is loaded into the cable is purposely prepared to have a greater amount of oxygen concentration than is required in the end product by the amount of oxygen lost to the sheath and center wire in the final heat-treating operation. If the sheath and center wire were made of substantially non-oxidizable metals such as the noble metals, the final heat treatment would not be necessary and no appreciable allowance for oxygen loss would have to be made.
In Figure 2 there is shown a graph, determined empirically, which shows the resistances of 50' lengths of cable of the type and dimensions abovementioned, at an operate temperature of 500 F., for starting oxides of manganese having different oxygen content ranging from MnO to MnO For example, this graph shows that a cable using a starting oxide mixture whose composition is MnOLogg will have approximately 200 ohms resistance at 500 F. As shown by the graph of Figure 3, with particular reference to curve A, the temperature at which this cable will have 100 ohms is 535 F. Curve A of Figure 3 is determined experimentally. Once such curve is known, there may be readily drawn a family of such characteristics for cables having different oxide mixtures, since these characteristics run along each other with approximately equal spacing. For example, the graph of Figure 2 shows that a 50' cable of the aforesaid dimensions will have approximately 45 ohms at 500 F., When a starting oxide mix of the formula MnO is used. This establishes point P on graph 3. The resistance vs. temperature curve for a cable with this particular oxide mixture will then be approximately as shown by curve B. This latter cable will thus have approximately 100 ohms resistance at a temperature of about 460 F.
It will be understood that the examples herein specifically described are intended to be illustrative and not limitative of my invention since variations in the temperature and other conditions and in the materials may be made without departure from the scope of my invention, which I endeavor to express according to the fol' lowing claims,
1. A resistance-type temperature-responsive cable of indefinite continuous length comprising an impervious metal sheath and a spaced metal center wire having oxidized surfaces; means sealing said cable at the ends; a temperature responsive material filling the space between said sheath and center wire, said material comprising predominantly an electronic non-stoichiometric oxidic semiconductor tending to release oxygen when heated and to recombine with available oxygen when cooled and having temperature-responsive characteristic depending on the percentage oxygen content thereof, said semi-conductor being selected from the group consisting of the oxides of Co, Mn, Cr, Ni and Cr, said semiconductor comprising a plurality of semiconductive metal oxides of the same metal, said oxides having different percentage oxycontents and being in a finely mixed pulverulent state compacted in said sheath into an essentially solid mass substantially impervious to diffusion of released oxygen gas therethrough when the cable is heated.
2. The temperature-responsive cable set forth in claim 1 wherein said oxides have preselected lesser and greater percentage oxygen contents and are predeterminately proportioned by weight to provide said material with a predetermined intermediate percentage oxygen content adapted to provide said cable With a preselected resistivity at a preset temperature.
3. The temperature-responsive device set forth in claim 1 wherein said semiconductor comprises a mixture of substantially manganous oxide (MnO) and an oxide of manganese having a higher oxygen content.
4. A resistance-type fire-detection cable comprising a metal sheath having an inside wall of a metal oxidizable to form a semiconductive oxide, a center wire of iron, an intervening temperature-responsive material comprising an electronic semiconductor of a type which has a resistivity at a given temperature depending on the oxygen content thereof, said semiconductor being selected from the group consisting of the oxides of Co, Mn, Cr, Ni and Cu, said semiconductor being compacted in said sheath into a solid homogeneous mass substantially free of air spaces, the inside wall of said sheath and said center wire being oxidized by oxygen from said semiconductor, said center wire being bonded mechanically to said semiconductor by interlacing of oxides of the semiconductor and of the iron oxide on the center wire.
5. A temperature-responsive fire-detection cable comprising a metal sheath and a center wire having its surfaces in a stabilized state of oxidation, an intervening temperature-responsive material comprising an electronic non-stoichiometric oxidic semiconductor selected from the group consisting of the oxides of Co, Mn, Cr, Ni and Cu, said semiconductor releasing oxygen when heated and tending to recombine with available oxygen when cooled and having a resistivity which at a given temperature varies according to the oxygen content thereof, said semiconductor being highly compacted in said sheath and said semiconductor comprising an intimate mixture of oxides having different oxygen contents, said different oxides being proportioned to give said cable a preset resistivity at a given temperature.
References Cited in the file of this patent UNITED STATES PATENTS