Patent Publication Number: US-6218648-B1

Title: Ceramic heater

Description:
FIELD OF THE INVENTION 
     The present invention relates to a ceramic heater comprising a resistance heating element embedded in ceramics. 
     BACKGROUND OF THE INVENTION 
     The ceramic heater comprising a resistance heating element of high-melting metal as embedded between a core and an insulation sheet covering the core is in widespread use as a heating means for the automotive oxygen sensor, glow system, etc. or as a heat source for devices for gassification of petroleum oil, such as a heater for use in semiconductor heating or an oil fan heater. 
     FIG.  5 ( a ) is a perspective view showing a typical ceramic heater of this type schematically and FIG.  5 ( b ) is a sectional view taken along the line A—A of FIG.  5 ( a ). 
     This ceramic heater comprises a cylindrical core  31 , an insulation sheet  32  wrapped around said core  31  with an adhesive layer  37  interposed, and a resistance heating element  33  embedded between said core and insulation sheet, with terminal portions of said resistance heating element  33  being connected to external terminals  34  disposed externally of said insulation sheet  32  and lead wires  36  being connected to said external terminals  34 , respectively. 
     As shown in FIG.  5 ( b ), each terminal portion of said resistance heating element  33  is connected to the corresponding external terminal  34  via a plated-through hole  35  provided under said external terminal  34  in the insulation sheet  32 . In this arrangement, as an electric current is applied between the external terminals  34  through the lead wires  36 , the resistance heating element  33  generates heat and thereby functions as a heater. 
     The insulation sheet  32  of said ceramic heater generally comprises Al 2 O 3  supplemented with, as sintering aids, SiO 2 , MgO, CaO, etc., and the SiO 2 , MgO, etc. are segregated as glass phases in the grain boundaries of alumina ceramics. 
     When a ceramic heater of this type is used as a heat source for the oxygen sensor of an automobile, for instance, a 12V direct current is applied between the terminals  34  of the ceramic heater, whereupon the resistance heating element  33  of the heater reaches to a high temperature of about 1000 to 1100° C. at the maximum. 
     Since the Mg and Ca in the insulation sheet  32  are present chiefly as glass phases in the grain boundaries, prolonged operation of the heater under such high-temperature DC conditions results in attraction of Mg 2+ and Ca 2+ in glass phases toward the negative pole so that the so-called migration, i.e. a shift of said metal ions toward the negative terminal, takes place. As this migration occurs, voids are formed in the grain boundaries of the alumina ceramics. 
     As the amount of voids in the alumina ceramics increases, the resistance heating element embedded beneath the insulation layer comes into contact with the air infiltrating into the voids, resulting in a progress in oxidation of the resistance heating element, with the result that not only is the resistance value of the heating element increased gradually but the resistance heating element as such expands due to oxidation. As a result, the heating temperature of the resistance heating element varies and the heating element becomes liable to be destroyed and, in extreme cases, develops a disconnection trouble. 
     SUMMARY OF THE INVENTION 
     In view of the above state of the art, the present invention has for its object to provide a ceramic heater wherein, even when a direct current is applied to the heater for many hours, the resistance heating element is not easily oxidized so that the resistance change of the resistance heating element due to such oxidation and heater degradation due to aging can be successfully prevented. 
     The present invention is directed to a ceramic heater which comprises 
     an insulation sheet comprising 88 to 95 weight % of Al 2 O 3  supplemented with, as sintering aids, 3 to 10 weight % of SiO 2 , 0.4 to 1.0 weight % of MgO and 1.0 to 2.5 weight % of CaO, 
     a core covered with said insulation sheet, 
     a resistance heating element of high-melting metal as interposed between said insulation sheet and said core, 
     an intermediate layer of an alumina ceramic body having a thickness of 5 to 50 μm, and 
     said alumina ceramic body containing 0.05 to 4 weight % of SiO 2 , 0.01 to 0.5 weight % of MgO and 0.01 to 1.2 weight % of CaO as interposed between at least a part of said resistance heating element and said core and/or between at least a part of said resistance heating element and said insulation sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a perspective view showing the construction of a ceramic heater of the invention and FIG.  1 ( b ) is a sectional view of the same; 
     FIG.  2 ( a ) is a schematic sectional view showing a stage in the fabrication of a ceramic heater according to the invention and FIG.  2 ( b ) is a plan view of the same; 
     FIG.  3 ( a ) is a schematic sectional view showing a further stage in the fabrication of a ceramic heater according to the invention and FIG.  3 ( b ) is a plan view of the same; 
     FIG.  4 ( a ) is a schematic sectional view showing a still further stage in the fabrication of a ceramic heater according to the invention and FIG.  4 ( b ) is a plan view of the same; 
     FIG.  5 ( a ) is a perspective view showing the construction of a conventional ceramic heater and FIG.  5 ( b ) is a sectional view of the same. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is now described in detail. 
     FIG.  1 ( a ) is a perspective view showing the ceramic heater according to the invention schematically and FIG.  1 ( b ) is a sectional view taken along the line A—A of FIG.  1 ( a ). 
     As illustrated in FIGS.  1 ( a )-( b ), this ceramic heater, generally indicated at  10 , comprises a cylindrical core  11 , a resistance heating element  13  having terminals  14  disposed on its surface, an intermediate layer  17  covering said resistance heating element  13  and terminals  14 , and an insulation sheet  12  disposed so as to further cover the whole. 
     Each of said terminals  14  is exposed through a cutout  15  formed in said insulation sheet  12  and a lead wire  16  is connected and soldered to the exposed part of the terminal  14 . 
     The insulation sheet  12  has a thickness of 50 to 250 μm and comprises alumina ceramics composed of 88 to 95 weight % of Al 2 O 3  supplemented with, as sintering aids, 3 to 10 weight % of SiO 2 , 0.4 to 1.0 weight % of MgO and 1.0 to 2.5 weight % of CaO. 
     The core  11  also comprises substantially the same material. 
     Inclusion of SiO 2 , MgO, etc. as sintering aids at the above-mentioned amounts in the insulation sheet  12  is intended to insure the formation of a dense sintered compact without increasing the sintering temperature necessary for alumina ceramics to an excessive degree. 
     For mechanical protection of said resistance heating element  13  and protection thereof from oxidation, the thickness of the insulation sheet  12  is set at 50 to 250 μm. However, compared with the conventional ceramic heater not provided with the intermediate layer  17 , the thickness of the insulation sheet  12  can be remarkably reduced. 
     On the other hand, the intermediate layer  17  formed to directly cover the resistance heating element  13  has a thickness of 5 to 50 μm and is comprised of alumina ceramics containing 0.05 to 4 weight % of SiO 2 , 0.01 to 0.5 weight % of MgO and 0.01 to 1.2 weight % of CaO. 
     If the thickness of the intermediate layer  17  is less than 5 μm, the oxygen in the atmospheric air may have a chance to contact the resistance heating element  13  to oxidize it. On the other hand, the thickness of 50 μm is sufficient to preclude contact of the resistance heating element  13  with oxygen in the air. Thus, if the thickness exceeds 50 μm, the effect that can be realized will not be augmented any further but rather the conduction of heat will be sacrificed by the alumina ceramic layer. The more preferred thickness of the intermediate layer  17  is 10 to 15 μm. 
     Moreover, when the amount of SiO 2  in the intermediate layer  17  is less than 0.05 weight %, that of MgO is less than 0.01 weight %, or that of CaO is less than 0.01 weight %, the reduced total amount of the sintering aids detracts from the progress of sintering so that it will become difficult to obtain a dense layer necessary for preventing oxidation of the resistance heating element. On the other hand, if the amount of MgO exceeds 0.5 weight % or that of CaO exceeds 1.2 weight %, said migration will be ready to take place. It should also be understood that when the amounts of MgO and CaO are within the above ranges, the amount of SiO 2  need not be greater than 4 weight %. 
     The intermediate layer  17  may be disposed so as to cover the entire resistance heating element  13  or cover only a part of the resistance heating element  13 . When the intermediate layer  17  is disposed so as to cover a part of the resistance heating element  13 , it is preferably provided in the high-temperature part where the operating temperature of the resistance heating element  13  reaches 300° C. or higher. This is because in the low-temperature part, said migration does not readily proceed and the oxidation of the resistance heating element is also slow to progress. 
     While, in FIGS.  1 ( a )-( b ), the intermediate layer  17  is shown as interposed between the resistance heating element  13  and the insulation sheet  12 , it may be interposed only between the resistance heating element  13  and the core  11  or both between the resistance heating element  13  and the insulation sheet  12  and between the resistance heating element  13  and the core  11 , i.e. the resistance heating element  13  being sandwiched between the two intermediate layers. 
     The high-melting metal forming the resistance heating element  13  may for example be W, Ta, Nb or Ti. These metals may be used independently or in a combination of two or more species. Among these metals, W is most preferred. Any of those metals supplemented with Re is also useful. The high-melting metal may further contain ceramics such as Al 2 O 3  etc. in a minor proportion. 
     The process for fabricating the above ceramic heater according to the invention is now described. 
     FIGS.  2 ( a ) through  4 ( b ) are schematic views showing the flow of production of the ceramic heater  10 . In each figure, (a) represents a sectional view and (b) represents a plan view. 
     As illustrated in FIGS.  2 ( a )-( b ), an adhesive layer  22  is first formed on a releasable plastic film  21  and, then, a conductive paste layer  23   a  forming said resistance heating element  13  and a conductive paste layer  23   b  forming said terminals  14  are formed. 
     The adhesive layer  22  is formed in order that, in assembling the heater, the parts of terminals  14  which are to be exposed through the cutouts  15  may be firmly secured to the core  11 . Moreover, the conductive paste layer  23   a  and conductive paste layer  23   b  are disposed one adjoining the other so that they may be firmly secured to each other. 
     Then, as shown in FIGS.  3 ( a )-( b ), a green sheet layer  24  for said intermediate layer  17  is formed so as to cover most of the conductor paste layer  23   a  and conductor paste layer  23   b . On top of the green sheet layer  24  so as to cover the whole assembly, a green sheet  25  layer to serve as an insulation sheet  12  is formed. 
     However, the parts of conductor paste layer  23   b  corresponding to the cutouts to form after firing are not covered with the green sheet layer  25  but kept exposed. 
     The green sheet  24  to serve as said intermediate layer  17  may be disposed in such a manner that it covers only the conductive paste layer  23   a  or only the part where the temperature reaches 300° C. or higher in use of the heater. 
     Then, as illustrated in FIGS.  4 ( a )-( b ), the laminate  20  shown in FIGS.  3 ( a )-( b ) is turned back so that the insulation sheet  25  will become the underside and set rigidly on a platform  26  by means of, for instance, air suction. Then, a plastic film  21  is peeled off. Thereafter, although not illustrated in FIGS.  4 ( a )-( b ), the core  11  is set in position on the laminate  20  which is then wrapped around said core  11  to construct a green molding for firing. This green molding is sintered at a predetermined temperature to provide the ceramic heater  10 . 
     In the ceramic heater thus fabricated, the resistance heating element is surrounded by an intermediate layer which is lean in SiO 2 , MgO, etc. and, hence, hardly allows migration of MgO etc. Therefore, the resistance heating element will hardly be oxidized even if a direct current flows through the ceramic heater for many consecutive hours, with the result that the change in resistance of the resistance heating element due to such oxidation and the degradation of the heater by aging can be successfully prevented. 
     EXAMPLES 
     The following examples are further illustrative of the present invention but by no means limitative of the scope of the invention. 
     Example 1 
     In accordance with the process described in detail above, the ceramic heater  10  shown in FIGS.  1 ( a )-( b ) was fabricated. The sintering temperature used was 1600° C. The resistance heating element  13  of the ceramic heater  10  thus fabricated was composed of 80 weight % of W, 17 weight % of Re and 3 weight % of Al 2 O 3 ; the intermediate layer  17  was comprised of alumina ceramics having a thickness of 15 μm and containing 0.1 weight % of SiO 2 , 0.05 weight % of MgO and 0.05 weight % of CaO; and the insulation sheet  12  was composed of 92.5 weight % of Al 2 O 3  supplemented with, as sintering aids, 5.8 weight % of SiO 2 , 0.5 weight % of MgO and 1.2 weight % of CaO and had a thickness of 200 μm. 
     The ceramic heater  10  thus fabricated was connected to a DC source, whereupon the heater temperature rose to 1000° C. The time to a 10% change in resistance was measured. The time was 10000 hours. 
     The percent change in resistance can be expressed by the following expression (1). 
     
       
         Change in resistance (%)=[(resistance value after test−resistance value before test)×100]/(resistance value before test)  (1) 
       
     
     Comparative Example 1 
     A ceramic heater was fabricated according to the conventional design illustrated in FIGS.  5 ( a )-( b ). The sintering temperature was 1600° C. The material formulation for the resistance heating element  13  of the ceramic heater  30  was the same as that used in Example 1, and the insulation sheet  12  was composed of 85 weight % of Al 2 O 3  supplemented with, as sintering aids, 12 weight % of SiO 2 , 1.0 weight % of MgO and 2.0 weight % of CaO and had a thickness of 250 μm. No intermediate layer was provided. 
     The ceramic heater  30  thus obtained was connected to a DC source. The time to a 10% change in resistance was measured in the same manner as in Example 1. The time was found to be 6000 hours. 
     It will be apparent from the foregoing resistance change data generated in Example 1 and Comparative Example 1 that the resistance change of the resistance heating element due to the migration of Mg 2+ and other ions could be effectively inhibited by providing an intermediate layer in the ceramic heater. 
     In the ceramic heater of the present invention wherein an intermediate layer which is lean in SiO 2 , MgO, etc. and, hence, hardly allows migration is disposed adjacent to the resistance heating element, even when the a direct current flows to this ceramic heater over a long time, the resistance heating element is not easily oxidized so that the resistance change of the resistance heating element due to such oxidation and heater degradation due to aging can be effectively inhibited.