Abstract:
A high-speed high temperature electrical resistance heater and method of making the same, said heater being in the form of a uniquely configured composite, non-homogeneous body having an esentially electrical-conducting material and an essentially electrically insulating material, wherein one of the materials is constituted as a porous matrix, and the other of the materials permeates the matrix of the one material. The electrically-conducting material has nodes and connective threads interconnecting the nodes, and electrical connections can be made to different places on the electrically-conducting material. The method involves forming a sintered matrix and impregnating the same with a ceramic substance, then sintering the impregnated matrix.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT. 
     Research and development of the present invention and application have not been Federally-sponsored, and no rights are given under any Federal program. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to electrical resistance heaters, and more particularly to heaters of the type incorporating PTC ceramic material which renders the heaters self-regulating at various preselected elevated temperatures. 
     2. DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR §§1.97-1.99 
     Various types of resistance heaters are disclosed in U.S. Pat. Nos. 3,700,857; 4,486,651; 4,541,898; 4,544,828; 4,613,455 and 4,633,064. 
     U.S. Pat. No. &#39;857 describes a heater consisting of a sintered mass of insulating refractory particles each of which has a thin film of electrically conductive material. U.S. Pat. No. &#39;651 shows a ceramic heater having an element formed by sintering a mixture of molybdenum disilicide and silicon nitride. 
     U.S. Pat. No. &#39;898 discloses a heating element composed of multiple finely divided particles of substance having a negative temperature coefficient of resistance, between which there are disposed areas of high resistance or areas of electrically non-conductive material. The resultant heater has different impedance characteristics according to the frequency of the A. C. wave that is applied to it. 
     U.S. Pat. No. &#39;455 involves mixtures of different types of ceramics, namely silicon nitride as an insulating ceramic and a mixture of titanium carbide and titanium nitride as a conductive ceramic. The ceramics are mixed in powder form, and thereafter sintered. U.S. Pat. No. &#39;064 illustrates a sintered heater element formed from an insulating ceramic powder such as silicon nitride, and a conductive ceramic powder such as MoSi 2 , WSi 2 , TiB 2  or TiC. 
     Finally U.S. Pat. No. &#39;828 relates to a heater formed by crushing PTC ceramic material, and mixing it with an insulating organic binder, to form the desired PTC resistor. 
     As presently understood, the devices disclosed in the above identified patents can be difficult to produce, since the ultimate resistance/temperature characteristics are largely determined by the relative proportions of the conductive material and insulating material, as well as the degree of sintering and the sintering temperature. It is believed that a reasonably close control of this resistance/temperature characteristic has, up to the present, been difficult to predict and achieve, as was uniformity of performance between different units of the same run. 
     In addition, where high temperatures on the order of 3000° F. are being generated during operation of the heaters, problems occur with establishing satisfactory electrical contact to the elements. The use of metal electrodes can be prohibitive, since the melting temperature of metal contacts is well below the 3000° F. figure noted above. As a result, most heaters of the prior art devices are intended to operate at temperatures well below this point. 
     SUMMARY OF THE INVENTION 
     The above disadvantages and drawbacks of prior PTC ceramic heaters are largely obviated by the present invention, which has for one object the provision of a novel PTC heater which is both low in manufacturing cost, and is characterized by improved control of the temperature/resistance characteristic. 
     A related object of the invention is to provide an improved PTC heater as above set forth, which is especially well adapted for use at temperatures approaching 3000° F, while still maintaining reliability over extended periods of use. 
     Still another object of the invention is to provide an improved PTC heater of the kind indicated, which is characterized by a physical configuration which leads to the development of one area that constitutes a hot spot or hot area; the configuration has other, spaced apart areas which, by virtue of their mass, do not become as hot as the hot spot or area, thereby facilitating making of electrical connections to the heater. 
     Yet another object of the invention is to provide an improved PTC heater in accordance with the foregoing, wherein relatively massive terminal portions are provided and connected by a relatively smaller bridge portion, the latter, by virtue of its smaller mass, becoming heated more rapidly than the terminal portions, thereby enabling electrical contacts to be made to the terminal portions as a result of their relatively lower temperature. 
     A still further object of the invention is to provide an improved PTC heater as outlined above, wherein relatively massive terminal portions are connected by a relatively smaller bridge portion, with the latter essentially assuming complete control over the current flow through the terminal portions when the bridge portion heats sufficiently. 
     A further object of the invention is to provide a PTC heater as above characterized, wherein one ceramic component is first formed into a matrix, and thereafter the second ceramic component is forced into the matrix so as to permeate the same. One of the ceramic components is electrically insulating, whereas the other ceramic component has a PTC resistance characteristic. 
     A still further object of the invention is to provide a composite, non-homogeneous PTC heater as above described, wherein there are formed, interspersed in a ceramic matrix, nodes and threads of a second ceramic. Either the first ceramic can be electrically insulating, with the other ceramic having a PTC characteristic, or vice-versa. 
     In accomplishing the above objects the invention provides a high-speed, high temperature electrical resistance heater comprising in combination a composite, non-homogeneous body constituted of an essentially electrically-conducting material and an essentially electrically insulating material, wherein one of the materials comprises a porous matrix, and the other material comprises a penetrant that permeates the porous matrix. The electrically-conducting material has nodes and connective threads interconnecting the nodes; means are provided for effecting electrical connections to different locations on the electrically-conducting material. 
     The invention further provides a high-speed high temperature electrical heater construction, comprising in combination means defining an elongate air-tunnel, and a plurality of PTC resistance members insulatedly mounted on the walls of the air tunnel and extending transversely thereof. The PTC resistance members have oppositely-disposed terminal portions for connection to an electrical supply, to effect energization of the same. 
     The invention also provides a novel method of making a high-speed, high-temperature electrical resistance heater constituted of a composite non-homogeneous body having an essentially electrically-conducting material and an essentially electrically-insulating material, which consists of the steps of forming one of the materials into a porous matrix, thereafter permeating the pores of the matrix with the other of the materials, and then firing the body to affix the materials in their assembled relation. 
     Other features and advantages will hereinafter appear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a strip heater of PTC material constructed in accordance with the principles of the present invention; electrical terminals are shown in section. 
     FIG. 2 is an edge elevation of the heater of FIG. 1. 
     FIG. 3 is a top plan view of a wafer like heater characterized by an annular bridge of thin cross section and an outer annulus of thickened cross section, for use as an ignitor, this construction constituting another embodiment of the invention. 
     FIG. 4 is a view, partly in side elevation and partly in section, of the heater of FIG. 3. 
     FIG. 5 is a top plan view of a cylindrical PTC heater constructed in accordance with the principles of the present invention, having a through bore of stepped dimension or diameter, constituting yet another embodiment of the invention. 
     FIG. 6 is an axial section of the heater of FIG. 5. 
     FIG. 7 is an axial section of the heater of FIGS. 5 and 6, having electrical connection means. 
     FIG. 8 is a section, greatly enlarged, taken on the line 8--8 of FIG. 1, showing a matrix of vitreous, electrically insulating material, and PTC material interspersed therein and taking the form of multiple nodes connected by multiple threads. 
     FIG. 9 is a top plan view of a modified heater, having relatively massive terminal portions connected by a smaller bridge portion, this constituting yet another embodiment of the invention. 
     FIG. 10 is a right end elevation of the heater of FIG. 9. 
     FIG. 11 is a top plan view of an air- or fluid-tunnel containing three heaters of the type illustrated in FIGS. 9 and 10. 
     FIG. 12 is a side elevation of the heater of FIG. 11. 
     FIG. 13 is a sectional view like that of FIG. 8, but differing therefrom in that the matrix is formed with nodes and threads of electrically insulating material. 
     FIG. 14 is a top plan view of a modified air- or fluid-tunnel type of ignitor, illustrating connection and terminal devices, and 
     FIG. 15 is a side elevational view of the ignitor or heater of FIG. 14. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1, 2 and 8, and in accordance with the invention there is provided a novel and improved PTC (positive temperature coefficient of resistance) electric heater generally designated by the numeral 10. In the embodiment illustrated, the heater takes the form of a strip having end or terminal portions 12, 14, control portions 16, 18, and a single high-temperature bridge portion 20, the control portions 16, 18 being narrower than the terminal portions 12, 14, and the high-temperature bridge portion 20 being still narrower than the control portions 16, 18. When the terminal portions 12, 14 are connected to a source of voltage 22, current will flow through one terminal portion 12, one control portion 18, the bridge portion 20, the other control portion 16 and the other terminal portion 14, resulting in heating of all five portions. Since the relative masses of the terminal portions 12, 14 are greater than that of the bridge portion 20, the latter will heat more quickly and to a higher temperature, and ultimately limit the current to a predetermined, desired value. Such a value is below that which would heat the control or terminal portions to any significant degree, whereby these terminal portions 12, 14 remain sufficiently cool to enable metal contacts or terminals illustrated in section at 24 and 26, to be attached thereto. 
     As shown in FIG. 8, the heater 10 is constituted of a non-homogeneous substance, namely either two different ceramics or a PTC ceramic and a vitreous substance (such as a glass formulation). The material is fabricated by compressing particles or balls 28 of the vitreous substance in a suitable press (not shown), using heat if necessary, and thereafter sintering the mass so as to form a matrix generally designated by the numeral 27 of the vitreous substance. One definition of the word matrix, given in Webster&#39;s Third New International Dictionary, 1963, is as follows: &#34;3: a mass by which something is enclosed or in which something is embedded.&#34; As illustrated, there are interstices or passageways in the matrix 27. Following formation of the matrix, a PTC ceramic 30 is forced to permeate into the interstices of the matrix, and form nodes 32 therein, which nodes 32 are connected by thin threads 34. Electric current flow is from one node 32 to an adjacent node 32, through one or more of the connecting threads 34. 
     The PTC ceramic 30 can be introduced into the matrix 27 by a combination of heat and pressure, or heat alone. Alternately, a carrier solvent (not shown) containing finely divided particles of the PTC ceramic 30 can be introduced at the surface of the matrix 27, forced to flow into the passageways thereof and fill them, forming the nodes 32 and threads 34. 
     As presently understood, by adjusting the size of the passages in the initially formed matrix 27, the ultimate conductivity characteristics of the resistance heater can be established with considerable control, and the degree of control possible with the present invention is greater than that obtainable where mere mixing of PTC and insulating ceramic powders was employed to fabricate the resistance material, as in the prior art devices. 
     Heating of the threads 34 causes their resistance to increase; this occurs rapidly, and thus the current limiting which occurs similarly takes place very soon after the application of voltage to the terminal portions 12, 14 of the heater 10. 
     The knee of the temperature-resistance characteristic of the resulting PTC heater is intended to be, in all cases, at least 2500° F., preferably between 2500° F. and 3000° F., with self-limiting current flow occurring at a maximum temperature of 3000° F. 
     As an alternative, FIG. 13 illustrates a different embodiment involving the fabrication of a resistance heater 10a. In the disclosed figure, the matrix 27a is formed of PTC ceramic material 30a, and following this, an insulating vitreous substance 28a is introduced into the interstices of the matrix 27a, either by re-heating the matrix in the presence of finely divided particles of the vitreous substance, and/or the application of pressure and heat thereto. Alternately, a carrier solvent (not shown) containing the vitreous particles 28a in suspension could be employed, to force them into the passageways. 
     Several modifications of the heater construction shown in FIGS. 1, 2 and 8 are illustrated in FIGS. 3 and 4; FIGS. 5-7; FIGS. 9 and 10; FIGS. 11 and 12; and FIGS. 14 and 15. 
     In particular, FIGS. 3 and 4 illustrate a wafer-like heater 36 having a stem 38, and characterized by a thickened outer annular rim 40, and a relatively thinner, inner annulus 42, the latter constituting the hot spot or bridge portion of the heater. When a source of voltage is applied between the stem 38 and the outer annular rim 40, current flows through this outer annular rim 40, through the thin annular bridge portion 42 and to the stem 38. The application of either a. c. or d. c. would produce desired heating effects. The inner annular or bridge portion 42, being relatively thinner than the outer annular portion 40, heats more rapidly than the latter and the stem 38, and to a higher temperature; this permits the use of metal terminals (not shown) with the heater, without the danger of melting. The bridge portion 42 in heating more quickly, rapidly limits the maximum current flow through the outer rim 40 and stem 38, and results in a self-regulating heater that is hottest at the inner annulus 42. Such a construction would have application for ignitors of various types, such as cigar lighters. The material of the heater is similar to that diagrammatically shown in FIGS. 8 or 13. 
     FIGS. 5-7 illustrate another embodiment of the invention. A heater 44 has a generally cylindrical outer surface 46, with a through bore 48 characterized by stepped dimensions or diameters. There is thus formed a first annular terminal portion 50, a first annular control portion 52, an annular bridge portion 54 which exhibits maximum heating and reaches the highest temperature, another annular control portion 56, and another annular terminal portion 58 having a larger bore than the portion 50. Electrical terminals 60, 62 contact the end surfaces of the terminal portions 50, 58, as shown. The one terminal 60 comprises an end cap 61, and a stem 64 which passes through the heater 44 and is spaced from the terminal portion 58 by virtue of the larger bore of the latter. An electrically insulating bushing 66 is interposed between the stem 64 and the terminal 62. 
     When voltage is applied between the terminals 60, 62, the bridge portion 54, being less massive than the control portions 52, 56, or terminal portions 50, 58, heats more rapidly, quickly limiting the current flow through the entire assemblage. Again, the high temperature is restricted to the bridge portion 54, and the greater masses of the control and terminal portions limit their increase in temperature to much lower values. The advantage noted above, namely reducing the likelihood of the metal terminals 60, 62 melting, is retained without sacrifice of the high-temperature ignition point provided by the bridge portion 54. 
     FIGS. 9 and 10 illustrate yet another embodiment of the invention. The heater 68 has the form of a modified prism of triangular cross-sectional configuration. The heater 68 has terminal portions 70, 72, control portions 74, 76, and a bridge portion 78. FIGS. 11 and 12 show three such heaters 68 installed in a ceramic air-tunnel 80. The tunnel has pairs of diametrically opposed triangular openings of slightly greater size than that of the terminal portions 70, 72 of the respective heaters 68. Suitable ceramic cement 82 can be employed to mount the heaters 68 in the openings in the tunnel 80. The heaters are electrically insulated, since the tunnel is of insulation and does not short circuit the terminal portions 70, 72. Electrical connections (not shown) can be made to those parts of the terminal portions 70, 72 which protrude through the openings in the tunnel 80. Air or other fluid-like substances, such as fuel vapor, passing through the tunnel 80 will be rapidly heated by the bridge portions 78 of the three heaters 68. The high temperatures achievable are sufficient to cause very rapid and positive ignition of the vapors. This construction is believed to have particular application in the case of a re-light, in flight, of a jet or rocket engine, a problem which has proved to be troublesome in the past due to unfavorable environmental conditions such as high air velocity which tends to cool off heaters, cold temperatures as experienced at high altitudes, and the presence of moisture in the form of fog, rain, or ice in the fuel stream that is being re-ignited. 
     Still another embodiment of the invention is illustrated in FIGS. 14 and 15, showing a composite air-tunnel generally designated 80a. Three heaters are provided, which can be identical to that designated 68 in FIGS. 9 and 10. In accordance with the invention, a cylindrical ceramic housing 86 (electrically non-conductive) encircles two semi-cylindrical spaced-apart electrically conductive sleeve-like members or electrodes 88, 90 each of which has multiple openings. The respective terminal portions of the heaters 68 are electrically in contact with the walls of the openings, and preferably held in place by conductive cement. 
     The adjacent longitudinal edges of the electrodes 88, 90 are spaced from one another and separated by ceramic insulator strips 91, so as not to be short-circuited. The ceramic housing 86 has three pairs of internal grooves 92 provided with ledges or steps on which the heaters 68 rest. The electrodes 88, 90 are secured in the ceramic housing 86 by suitable cement. One electrode 88 can be electrically positive, while the other electrode 90 can be electrically negative. (In the case of A. C., the electrodes 88, 90 can constitute different sides of the A. C. circuit). Electrical connections can be made to the upper ends of the electrodes, and hence the terminal portions of the heaters 68, via suitable terminals. Alternatively, or in conjunction with the above, six metal leaf springs 95 can occupy the six grooves 92 respectively and press against the terminal end portions of the heaters 68 to effect electrical connection thereto. The leaf springs can have paired terminal portions 93 and 94, as shown. This construction has the advantage that the electrodes 88, 90 being long and of relatively high mass with respect to that of the heaters, can effectively dissipate much of the heat conducted thereto by virtue of their surface contact with the terminal portions of the heaters, virtually eliminating problems with melting of the metal of which the electrodes is constituted. 
     The examples shown are intended to be illustrative only, and it is believed that many other applications of the principles set forth above can be developed in the electric heater art. 
     As presently understood, the areas of the heaters that have been referred to as &#34;control portions&#34; have the effect of dissipating heat generated in the &#34;bridge portions&#34;; making the control portions more massive would reduce the steady state temperature of the bridge portions somewhat, as well as slowing the heating effect somewhat, whereas making the control portions less massive would have the opposite effects. 
     The invention as set forth above therefore provides an improved method of making high temperature PTC electrical heaters of the kind described above. Referring to FIGS. 8 and 13, for example, this method comprises forming a sintered porous matrix of one ceramic material, and thereafter impregnating the matrix with another ceramic material, one material being electrically insulating and other material being electrically conducting, but resistive. 
     Referring to FIG. 8, the insulating material comprises the granules or &#34;balls&#34; 28 which are compressed in a mold (not shown) and then fired or sintered to form a matrix. The matrix is then forcefully impregnated with the electrically conducting PTC material 30 consisting of particles, to form the nodes 32 and threads 34 which form the electrically conducting PTC grid of the heater 10. 
     In the case of FIG. 13, the PTC conducting or resistive material comprises the ceramic granules or &#34;balls&#34; 30a which are compressed in a mold (not shown) and then fired or sintered to form a matrix having conducting nodes and threads at and adjacent the points of contact of the particles. This matrix is then forcefully impregnated with the electrically insulating ceramic material 28a. 
     From the above it can be seen that I have provided novel and improved heater constructions and methods for making the same. The constructions are rugged and reliable in use, and lend themselves to connection to suitable sources of electricity without the need for sophisticated clips or terminals that would otherwise have to withstand extremely high temperatures. The illustrated structures provide for rapid heating of a fluid or solid to extremely high temperatures, approaching 3000° F. Where the PTC material employed is molybdenum disilicide, the knee of the temperature/resistance curve is in this temperature range. Stated differently, the increase in resistance occurs very rapidly as 3000° F is approached. This fact makes the heaters well adapted for use as ignitors, whether for fuel in an aircraft, fuel in a diesel engine (glow plug), or as ignitors for electric cigar lighters. 
     The disclosed devices and methods are thus seen to represent distinct advances and improvements in the field of electric heaters. 
     Variations and modifications are possible without departing from the spirit of the invention. 
     Each and every one of the appended claims defines an aspect of the invention which is separate and distinct from all others, and accordingly it is intended that each claim be treated as such when examined in the light of the prior art devices in any determination of novelty or validity.