US 3610888 A
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United States Patent Daniel D. Button Westborough, Mass.
Jan. 30, 1970 Oct. 5, 1971 Westinghouse Electric Corporation Pittsburgh, Pa.
Continuation-impart of application Ser. No. 640,529, May 23, 1967, now abandoned.
Inventor Appl. No. Filed Patented Assignee OXIDE RESISTOR HEATING ELEMENT 10 Claims, 9 Drawing Figs.
US. Cl 219/543, 117/221,117/224, 117/104 R, 117/106 R, 117/93.1 PF, 106/65, 252/519, 252/520, 252/521,
Int. Cl 1101b l/00, H011) 1/08 Field of Search 219/543; 338/308-9; 252/521, 518-521; 117/223, 224,106, 105, 93.1
 References Cited UNITED STATES PATENTS 2,976,505 3/1961 lchikawa 252/518 3,433,749 3/1969 Nishimoto et a1... 252/521 3,503,029 3/1970 Matsuoka.... 252/518 X 2,859,321 11/1958 Garaway 219/543 X 2,915,613 12/1959 Norton 219/543 X 3,019,198 l/196l Dumesmil 252/521 3,337,832 8/1967 l-lukee 338/309 Primary ExaminerWilliam L. Jarvis Attorneys-F. Shapoe and Lee P. Johns ABSTRACT: Mixed oxides composed of ions of alkaline earth, rare earth, and transition elements having perovskite crystal structures, the electrical conductivity of which make them useful as heating elements or as electrical conductors for use in high-temperature environments.
This invention results from work done under contract O C R 14-0 l0 00I-303 with the Qtffice of Saline Water of the United States Department of the Interior.
PATENIEnucI 5l97l (1610.888
smear-2 OXIDE RESISTOR HEATING ELEMENT CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to oxides having perovskite-like structures and having potentially good electrical conductivity.
2. Description of the Prior Art Mixed oxides having the perovskite structures are reasonably good electrical conductors. A perovskite structure has a cubic or pseudocubic unit cell and has a composition A80 The A ions are situated at the comers of the cell and the 13" ions are at the center with the (oxygen) ions centered in the faces of the cell. Each A-ion is twelvefold and B ion is sixfold coordinated by the oxygen neighbors and each oxygen ion islinked to four A and to two B cations. The perovskite structure is made up of large cations (A-site ions) surrounded by l2 oxygen ions, and of smaller cations (B-site ions) octahedrally surrounded by 6 oxygen ions.
For the purposes of this invention the perovskite materials include transition metal ions at the B-sites and rare earth ions occupy the A-sites. The electrical conductivity of these materials results from the interaction of the unfilled d orbitals of the transition metal ions. If the d orbitals overlap, a conduction band is formed. A wide band of many electrons will make a material having good electrical conduction properties.
The width of the conduction band is a function of the transition metal ion present and the separation of the transition metal ions in the perovskite-like lattice. The latter is controlled in part by the size of the A-site ions. The number of electrons available for conduction is a function of the temperature and the oxidation state of the transition metal ion. As the temperature is increases, electrons are raised to the conduction band and the number of electrons are raised to the conduction band and the number of electrons available for conduction is of the same order of magnitude and the transition metal ions present. In this case, the material has intrinsic conductivity with a positive temperature coefficient of resistivity. Below a certain temperature the material behaves as a nonnal oxide semiconductor.
Carriers can also be made available by doping the material and causing the transition metal ions to be present in more than one oxidation state. This is done, e.g., by substituting a two-plus ion for a three-plus ion in the A-site. In order for the material to preserve oxygen stoichiometry a proportional number of B-site (M) ions are raised to the four-plus oxidation state. The reverse of this process had the same effect, i.e., starting with a two-four (A B 0 material an substituting three-plus A-site ions to promote formation of three-plus transition metal ions. Thus either a P- or N-type semiconductor may be produced by this controlled valency method.
In the case of mixing two 3*, 3 [x+(A *B, O,)+(1-x) 3 B, O,)] materials, the most stable arrangement of oxidation states on the B-sites may be obtained by a fraction of one of the B ,ions dropping to a B,oxidation state and a proportionate number of the Bfi ions being raised to 8,, thus promoting mixed valence conductivity.
A third method of controlling valency is to control the atmosphere in which the material is fired. However, this method has the disadvantage that heating to moderate temperatures may cause changes in the material properties If one has a material of high conductivity with a suitable temperature coefficient of resistivity, a lower conductivity can be attained in at least two ways. The number of carriers present van be diluted by the addition of an inert ion on the B-sites in the perovskite lattice. Two ions which are suitable for this purpose are Al and Ga. Both of these ions will occupy B-sites and will not contribute to the conductivity. A second method is to prepare a mixture of the conducting material and an insulator.
Perovskite-like materials of most of the binary combinations of the rare earths, yttrium, and alkaline earths with the transition metals Ti through Ni have been prepared and reported in the literature. Electrical conductivity data has been reported for the systems LaMn0 CaMn0 LaFeO,-SrFeO,, and LaCoo srCoo Further work on perovskite-like materials is reported in Magnetic and Electrical Anomalies In LaCoO, by R. R. Heikes, R. C. Miller, and R. Mazelsky, Physica30, l600l608, I964).
U.S. Pat. No. 3,0l9,l98, issued to M. E. Dumesnil, discloses a thermistor composition which include mixed oxides of cerium, strontium, and manganese. Insomuch as these oxides have changes in resistance varying from -36 to 82 percent with a temperature increase of only from 25 to 125 C. and, as FIG. 3 indicates resistance changes by a factor of 25 or more occur in going up to several hundred degrees C., the oxides are not as useful as heating element materials as they would be if there were substantially no change in resistance.
SUMMARY OF THE INVENTION In accordance with this invention it has been found that the variation in resistivity and temperature coefiici nt of resistivity provide a wide latitude in selecting size and shape of a heating element. These oxides can be applied to a suitable substrate in order to provide the necessary mechanical strength for a variety of applications.
There are several limitations associated with the heating elements presently used in electric ranges and fry pans. Among these are: (1) poor thermal coupling, (2) large thermal inertia, (3) hazard of explosion of elements through vaporization of water associated with MgO insulation, (4) appearance.
Briefly, the present invention consists in providing a heating element, such as in an electric range or an electric frying pan, the heating element comprising a substrate composed of a dielectric material, a layer on the substrate and composed of mixed oxides of alkaline earth ions, rare earth ions, and transition metal ions, the transition metal ions having up to l0 electrons in the d shell, and which mixed oxides having a resistivity that is substantially independent of temperature changes.
DESCRIPTION OF THE DRAWINGS FIGS. 1, 2, and 3 are graphs illustrating the temperature behavior of resistivities for several materials indicated;
FIG. 4 is a perspective view of one embodiment of the invention;
FIGS. 5 and 6 are transverse sectional views showing other embodiments of that shown in FIG. 4;
FIG. 7 is a perspective view of the bottom surface of a spirally wound heating element for use as a burner on an electric range;
FIG. 8 is an enlarged vertical portion of the area A in FIG.
7; and FIG. 9 is a perspective view of a baking dish or electric fry- DESCRIPTION OF THE PREFERRED EMBODIMENTS The heating elements depicted in FIGS. 4 to 9 show various applications of the basic principle involved. In FIG. 4 a heating element 10 includes a dielectric member or substrate 12, a layer or coating 14 of oxide resistor material, and means including a pair of spaced terminals or contacts 16 and 18 for connecting the element 10 with a source of current. It is preferred that the substrate 12 have a relatively high coeffieient of thermal conductivity. For that purpose the substrate 12 may be composed of a borosilicate glass and. as shown in FIG. 5, may be of substantial thickness to completely support the layer 14 of mixed oxides. In the alternative, the substrate 12 is relatively thin in which case it is provided on the top surface, the surface opposite that of the layer 14, with a layer 20 of material having a coefficient of thermal expansion substantially equal to that of the conducting oxide layer 14 and having a relatively high coefficient of thermal conductivity. The sub strate 12 may or may not be provided with upturned side portions 22for retaining the layer 20 intact.
The layer 14 of mixed oxides has a perovskite-like structure the general fonnula of which is A130 in which the A-sites are filled with either three plus rare earth ions or two plus alkaline earth ions, and in which the B-sites are filled with transition metal ions. The rare earth ions include those elements of rare earth type 4] of Periodic Chart of the Atoms including lanthanum cerium, praseodymium, neodymium, promethium, simariu, europium, gadolinium, terbium, dystrosium, holmium, erbium, thulium, ytterbium, and lutetium, The alkaline earth ions include most of the Group 11 elements of the Periodic Chart of the Atoms including calcium, strontium, barium, and radium. A preferred listing of these elements would exclude beryllium and radium because of their toxic effect on personnel. The transition metal ions include the elements of the 4th, 5th, and 6th periods of the Periodic Chart of the Atoms including titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, technetium, euthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, and platinum.
The various elements are combined to form mixed or binary compounds which crystallize in the perovskite structure having resistivities which make them of potential interest as heating element materials. Accordingly, two oxides, a pair of bimetallic oxides, each of which includes metals of different valence are combined in the Table.
TABLE Solid Solutions of Binary Compounds with Perovskite-Like Structure Host Material Dopant Material Range of (X) (l-X) A'WPO, (X) I) ABO,
2 unt 3 ugno LaCrO, l) SrCrO, 0.00l-0.5 LaMnO, l) CaMnO, 0.00l-l .0 LaNlnO, l) SrMnO, 0.00l-l.0 LaMnO, I) BaMnO, 0.00I-l .0 Lat-e0, I SrFeO, 0.00l-I .0 EuFeO, l) BeFeO, SmFeO, l) BaFeO, LaCoO, I) SrCoO, 0.00I-l .0 GdCoO, l) CaCoO, 0.00l-I.0 LaNiO, l) SrNiO, 0.00l-0.l LaNiO, 3) ZrNiO, 0.00l-0.l LaVO, I) CaVO, 0.00l-I .0 LaVO, USIVO, 0.00l-l .0 LflCoO, 3) ThCoO, 0001-004 LaFeO, 3) ThFeO, 0.00l-0.5 LaMnO, 2) LaCrO, LaFeO, 2) LaCoO, 0.001-1 LaFeO, 2) LaCrO, NdFeO, 2) NdCrO, SmFeO, 2) SmCIO; YFeO, 2) YCrO, LaMnO, 2) LaGdO, 0.00l-I LaMnO, 2) LaNiO, 0.00l-I LaMnO, 2) LaCoO, 0.00l-I As shown in the Table the solid solutions of binary oxides includes a mixture of two oxides of different metallic content and in most cases different valences. For example, a l-X amount of oxide of two metals of one valence may be mixed with X amount of an oxide of a metal of a different valence or the same valence. As shown in the Table the range of the value X may vary from slightly above 0, such as 0.001 ,to as high as I.
For a given system of two binary compounds the elements in the A-sites are different and elements in the B-sites are the same but have different valence states. In the case of adding two binary compounds with common elements on the A-sites and different elements on the B-sites, both B-sites elements can assume different valance states. In these mixed binary compounds the electrons hop throughout the lattice which results in electrical conductivity.
Another embodiment of the invention is shown in FIG. 6 in which the layer 14 of ceramic material is disposed between a pair of dielectric members 24 and 26 having common end portions 28 and 30. As in FIGs. 4 and 5 the layer 14 in FIG. 6 extends between a pair of conductors l6 and 18.
Another embodiment of the invention is shown in FIG. 7 in which a pair of spiral-heating members 32 and 34 are disposed in a circular pattern in a manner similar to heating elements in an electric range. At the center of the spiral members, said members have a common or joined portion 36 to provide for continuity of an electric circuit which enters and leaves the spiral portions 32 and 34 through terminals or plugs having prongs 38 and 40. The periphery of the spiral-heating members 32 and 34 is enclosed within a protective dielectric member 42. As shown in FIG. 8 the spiral-heating members 32 and 34 include a layer or coating 44 of ceramic oxide material which is disposed on a dielectric substrate 46 and which in turn is provided with a layer 48 of material having a coefficient of thermal expansion substantially similar to that of the layer 44 and the substrate 46 to prevent separation of the members during heating and cooling. As shown in FIG. 8 the several spiral-heating members 32 and 34 are spaced from each other by a clearance 50 which clearance may be occupied by the surrounding atmosphere such as air or filled with a dielectric material such as glass.
Another embodiment of the invention is shown in FIG. 9 in which the heating element may be included as an integral part of an electric baking dish or frying pan 52 having sidewalls 54 and a bottom wall 56. A layer or coating 58 is provided on the top surface of the bottom wall 56 which coating is composed of the ceramic or mixed oxide which is heatable to an elevated temperature when current is applied to the spaced terminals 60 and 62 along opposite ends of the coating 58. Each terminal 60 and 62 includes a connecting prong 64 in a conventional manner. 0n the top side of the layer or coating 58 a dielectric material 66 such as Pyrex glam, is disposed to prevent food in the dish 52 from contacting the layer 58.
All of the heating elements shown in FIGS. 4 to 9 are dependent upon heat generated in the layers or coatings of ceramic or mixed oxide material when current is passed through said material from the terminals or conductors on opposite edges thereof. The heat so generated is transferred to the top surface of the heating element by conduction through the substrate and any other layer on the side of the substrate opposite that of the heated layer of oxide.
The layers or coatings may be applied to the substrates in a variety of methods such as flame spraying or plasma jet spraying, as illustrated by the examples. The preferred method depends on the coating thickness and resistivities desired. Thicknesses from 1 micron to fractions of an inch may be useful depending upon the application, even as much as up to 1 inch (25,400 microns).
Where the oxide has a relatively low resistivity (FIGS. 1 and 3), the layer or coating thickness may be smaller. C ontrarily, where the oxide resistivity is relatively high (FIG. 2). the thickness of the layer may be greater where the same power dissipation (or rating) is to be obtained. The power rating of a heating element is directly proportional to the thickness of the conducting layer and inversely proportional to its resistinty.
Systems which have been investigated in detail are LaFe0,- SrFeO LaCoO -SrCoO LaNiO -SrNiO and [AI-e0,- ThFeO These materials were prepared by reaction sintering of proper ratios of 111.0,, Th(NO SrCO and the oxides of the transition metals. Complete reaction was determined by X-ray difi'raction. The prepared powders were pressed into pellets and sintered in air at temperatures varying from l,200 to 1,400 C. Final density of the pellets was controlled by time and the temperature of sintering.
The electrical conductivity of the materials was measured in air as a function of temperature using the four-probe DC technique. The results of the electrical conductivity tests are shown for the various compositions in FIGS. 1, 2, and 3. The conductivity is observed to vary with the apparent density of the pellets, increasing with increasing density.
Coatings of the several perovskite materials have been applied to yttria stabilized zirconia discs. The parameter p/S has been measured on these coatings in order to evaluate their electrical properties. This parameter is measured directly and expressed in ohms. The value of p/8 can be predicted from a knowledge of the specific resistivity (p), expressed in ohmcentimeter, and the thickness of the coating (8), expressed in centimeters. The measured value of p/8 can be compared to the predicted value in order to determine whether the thin coating behaves like the bulk material.
For practical applications such as for use as heating element materials for instance in an electric range or an electric frying pan, the temperature of the material manifestly will vary considerably. For such purposes it is desirable that a mixed oxide having a relatively low resistivity at 25 C. and also a small change with temperature, such as shown in the curves of FIGS. 1 and 3, be employed. Thus, mixed oxides, such as (l- X) LaCoO,-(X) SrCoO (FIG. 1.) and l-X) LaNiO (X)SrNiO, (FIG. 3), are exemplary illustrations of the kinds of oxides that have coefficients of electrical resistance that change very little or only slightly when their temperature is varied between about 25 and 600 C. In other words, the resistivity temperature curve should be relatively flat over the range of use of the materials. A particularly useful material is obtained when the resistivity changes by a factor of less than 4 or 5 on the range of 25 to 600 C.
Coatings have been applied to zirconia substrate discs by three different techniques, namely, spray deposition sintering, an plasma spraying. These techniques are described in the following examples.
EXAMPLE I Spray deposition-The chlorides of La, Sr, and Co, in the ratio of 6:4:10 were dissolved in 0.1N HCl to form a two molar solution of the cations. This solution was sprayed into a tube which passed through two furnaces which can be maintained at different temperatures. Both furnaces were held at l,000 C., and the substrate discs were mounted 18 inches from the spray nozzle with their faces perpendicular to the tube axis. The solution was sprayed for one hour at the rate of 2.5 cc./min. with argon carrier gas flowing at 8,400 cc./min. This resulted in conducting coatings 10-12 1. thick. These coatings withstood three heating-cooling cycles between room temperature and l,000 C. with no indication of the coating separating from the disc. The value of p/8 measured at 0.54 ohms compared with a predicted value of 0.57 ohms. These results were achieved in spite of the large linear thermal expansion mismatch (approximately 8 x l0 in./in./ C.) between the zirconia disc and the La Sr CoO coating.
EXAMPLE ll Sintering-An aqueous suspension of the finely divided powder of La Sn CoO was prepared using goulac as the suspending agent. The zirconia substrates were hung in a basket in this suspension and the particles La,,, ,Sr,, CoO were allowed to settle on it. The coatings, thus obtained, were air dried and then sintered at temperatures between 1,200"- l,400 C. This resulted in well sintered coatings which adhered poorly to the zirconia substrates. However, the value of for these coatings was the same as that predicted from the thickness and resistivity.
EXAMPLE in Plasma spraying-Finely divided powder of La Sr CoO was sprayed on yttria stabilized zirconia discs preheated to 900 C. using argon and nitrogen as the plasama gas. Coatings approximately 10p. thick were obtained by this process. The coatings were more adherent than sintered coatings but less adherent than vapor-deposited coatings. The measured value of p/& was 4-10 times greater than calculated from the re sistivity and thickness. This difference was a result of a loss of cobalt oxide during the spraying process.
The results obtained with the above three methods indicate that it is possible to obtain thin coatings of perovskite-like materials with electrical properties similar to the bulk material. ln addition, by vapor deposition, it has been possible to get the coatings to adhere to materials with linear thermal expansion coefficients greatly different from the coating material.
it is understood that the above specification and drawings are merely exemplary and not in limitation of the invention.
What is claimed is:
1. An electrically conductive element comprising a sub strate of dielectric material, a layer of oxide resistor material disposed on the substrate, the layer being composed of a solid solution of two mixed binary oxides of alkaline earth, rare earth, and transition metals, the transition metals having not more than 10 electrons in the d shell, the mixed oxides having a Perkovskite crystal structure characterized by a general compositional unit cell A where A represents a rare earth ion in one oxide and an alkaline earth ion in the other oxide, B represents a transition metal ion at the center of each oxide unit cell, 0 represents an oxygen ion at the center of the faces of the cell of each oxide; and the mixed oxides having a relatively flat resistivity temperature curve over the range of use of the material.
2. The element of claim 1 in which the layer has a thickness ranging from 1 p. to 25,400
3. The element of claim 2 in which the layer thickness is about 2 4. The element of claim 1 in which the layer is a plasmasprayed deposit.
5. The element of claim 1 in which the layer is a sintered body.
6. The element of claim 1 in which the layer is a deposit derived from sprayed solutions or suspensions of compounds of the constituents.
7. The element of claim 1 in which the binary compound is composed of mixed oxides having a perovskite crystal structure and in which the resistivity of the oxide is controlled by the addition of an inert ion selected from the group consisting of aluminum and gallium in the B-sites of the perovskite crystal.
8. An electrically conductive element comprising a substrate of dielectric material, a layer on the substrate which layer is composed of (l-X) LaCoO,(X)SrCoO;, wherein X varies from 0.3 to 0.6 and the mixed oxides having a relatively flat resistivity temperature curve over the range of from about 25 to 600 C.
9. An electrically conductive element comprising a substrate of dielectric material, a layer on the substrate which layer is composed of (l-X)LaNio (X)SrNi0 wherein X varies form 0.9 to 1.0, and the mixed oxides having a relatively flat resistivity temperature curve over the range of from about 25" to 600 C.
10. An electrical appliance comprising a heating element, the heating element comprising an electrically insulating substrate, an electrically conductive layer disposed thereon and means for supplying electric current to the layer to provide for heat being developed in the layer, the layer being composed of a solid solution of two mixed binary oxides of alkaline earth,
rare earth, and transition metals, the transition metals havingnot more than 10 electrons in the d shell, the mixed oxides having a perovskite crystal structure characterized by a general compositional unit cell A where A represents a rare earth ion in one oxide and an alkaline earth ion in the other oxide, B represents a transition metal ion at the center of each oxide unit cell, represents an oxygen ion at the center of the faces of the cell of each oxide, and the mixed oxide hav-