Abstract:
A ceramic heater for a gas sensor has a heater substrate, a laminating substrate made of the same material as the heater substrate, and an electrode made of platinum and at least one lanthanide oxide disposed between the heater substrate and the laminating substrates. The ceramic heater exhibits improved durability without migration patterns, is free of cracks on the substrate, is free of short circuits in the heat electrode and is low in production cost.

Description:
This application is a continuation of application Ser. No. 08/549,313, filed Oct. 27, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates, in general, to a heater for heating an automobile sensor and, more particularly, to an improvement in durability along with the heater and the ceramic heater. 
     2. Description of the Prior Art 
     Owing to superior endurance at high temperature, ceramic heaters are useful for a plurality of purposes, especially, for automobile sensors. An automobile has a solid electrolyte air/fuel (A/F) sensor for detecting the oxygen content contained in an exhaust gas from the engine, in order to purify the exhaust gas and to enhance the fuel consumption ratio. Such sensor can respond only when it is heated to at least about 800 degrees centigrade. 
     For the respondence, a heater is adopted to elevate the temperature. The heater is generally made by constructing an electrode on a substrate with high thermal conductivity. The electrode consists typically of platinum in combination with additives. Recently, there has been a strong demand for low-resistant heaters which are capable of elevating temperature more rapidly. The low resistance can be accomplished by incorporating a variety of ceramic powders in the heater. 
     When Al 2  O 3 , a typical material for the substrate, is sintered, sinter-aiding agents and/or grain-growth inhibitors are usually added because of its high sintering temperature Oxides, such as SiO 2 , MgO, CaO and the like, are used as these additives because it is easy for them to make Al 2  O 3  glassy upon sintering. Other additives exist for these purposes. For example, oxides of Periodic Table Group IIIa metal, for example, Y 2  O 3 , are added for reducing the sintering temperature and ZrO 2  for inhibiting grain growth of Al 2  O 3 . However, heaters employing such Al 2  O 3  substrates, when used at high temperature under direct current high voltage (e.g. automobile power 12V), allow the alkaline metal ions contained in the substrates, that is, Mg 2+ , Ca 2+ , to migrate into the negative terminal and thereby to give rise to segregation. As a result, compounds with low melting points are produced, which cause cracks on the surface of the heater and thus, short circuits in the heater electrode. 
     In order to prevent the migration, a variety of techniques have been undertaken, e.g. high purity of Al 2  O 3  or small amount of Al 2  O 3  in Pt. However, these techniques can relieve the migration only to some degree and the purer Al 2  O 3  requires a higher sintering temperature. 
     Referring to FIG. 1, there is shown a heater which is presently commercially available. As shown in this figure, it consists of two laminating substrates 1, 5 between which a migration pattern 2, a heater substrate 3 and a heater electrode 4 are, in sequence, formed. The migration pattern 2 is interposed between the laminating substrate 1 and the heater substrate 3, with the aim of preventing the generation of cracks on the heater substrate 3 and short circuits in the heater electrode 4. This heater, however, is disadvantageous in that its construction is done by complicated processes, which results in high production cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a heater for heating an automobile sensor, preventive of migration of alkaline metal ions into negative terminal in the heater. 
     It is another object of the present invention to provide a heater for heating an automobile sensor, in which durability can be improved without migration pattern. 
     It is a further object of the present invention to provide a heater for heating an automobile sensor, free of cracks on the substrate and short circuits in the heat electrode. 
     It is still another object of the present invention to provide a heater for heating an automobile sensor, having a simple duplicate structure which is low in production cost. 
     In accordance with an aspect of the present invention, there is provided a heater comprising a heating electrode comprised of platinum and at least one of lanthanide oxides. 
     In accordance with another aspect of the present invention, there is provided a heater comprising a heating electrode comprised of platinum and cerium oxide. 
     In accordance with another aspect of the present invention, there is provided a heater for heating an automobile sensor, comprising a heater substrate; an electrode comprised of platinum and at least one of lanthanide oxides; and a laminating substrate comprised of a same material as said heater substrate; and wherein said electrode is disposed between said heater substrate and said laminating substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and other features and advantages of the present invention will be more apparent from the following detailed description taken with reference to the accompanying drawings, in which: 
     FIG. 1 is an exploded perspective view showing a conventional ceramic heater for automobile air/fuel sensor; 
     FIG. 2 is an exploded perspective view showing a ceramic heater for automobile air/fuel sensor, according to the present invention; 
     FIG. 3 is a graph showing the durability  vs. additive amount of a ceramic heater for automobile air/fuel sensor of the present invention and a conventional one with 3.5 ohm of resistance respectively;! of a heater of the present invention as a function of the amount of a lanthanide oxide added to the heater and the durability of a conventional heater that does not contain any lanthanide oxide, both heaters having a resistance of 3.5 ohms; and 
     FIG. 4 is a graph showing durability of a ceramic heater for automobile air/fuel sensor of the present invention and a conventional one. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, an oxide of lanthanides is added in a platinum electrode formed on an Al 2  O 3  substrate and, optionally, in the Al 2  O 3  substrate. The resulting ceramic heater can be preventive of migration of the alkaline metal ions into negative terminal therein, without employing a migration pattern. 
     This application of the preferred embodiments of the present invention will be best understood by referring to the accompanying drawings 
     First, with reference to FIG. 2, there is shown a ceramic heater, according to the present invention. As shown in this figure, it has a duplicate structure consisting of a heater substrate 6 and a laminating substrate 8 between which a heater electrode 7 is formed. 
     A green sheet of the heater substrate is prepared from a formulation comprising particles of a mixture of Al 2  O 3 , CaO, Sio 2  and MgO (average diameter 0.5 μm), a sintering agent of YSZ and a PVB based binder, using a doctor blade process. 
     For heater electrode 7, a paste which is prepared by adding CeO 2  and a PVB-based binder to platinum is screen printed on the green sheet of heater substrate 6. The lanthanides oxide can play a key role in preventing the positive ions, Ca 2  +, Mg 2  +from migrating into the negative terminal and have little adverse influence on the conductivity of platinum by virtue of its superior conductivity. In addition, they can increase the adhesive strength of heater electrode 7 to heater substrate 6. CeO 2 , an oxide capable of storing oxygen, can provide oxygen continuously so as to prevent the generation of positive ions, Mg 2+ , Ca 2+   in the heater electrode. 
     In accordance with the present invention, the lanthanides oxide is added in an amount of about 3 to about 20% by weight, based on the weight amount of platinum and preferably about 8 to about 12% by weight. For example, as shown in FIG. 3, if too much lanthanides oxide is used, the resulting platinum electrode is difficult to sinter, as will be described shortly. On the other hand, if too little lanthanides oxide is used, there is little effect of preventing the migration. In the paste, the PVB-based binder ranges in quantity from about 5 to about 50% by weight of the paste. 
     Another green sheet is prepared in the same manner as in the heater substrate and used as a laminating substrate which overlays the heater substrate. The duplicate structure thus obtained is sintered at a temperature of about 1,500° C. for about an hour, to give a ceramic heater. 
     Optionally, CeO 2  may be added to each of the heater substrate and the laminating substrate in up to 20% by weight of platinum in the heater electrode, with the aim of strengthening the above-mentioned effects. 
     As explained above, the ceramic heater of the present invention adopts a duplicate structure, which is much simpler than the conventional multiplicate structure, e.g. triplicate structure. Use of an appropriate amount of CeO 2  as an additive for the heater electrode restrains the migration, even though the ceramic heater is used at high temperatures under the application of high direct current voltages (e.g. 10 to 13 V), because the ability of CeO 2  to store oxygen allows oxygen to be provided so constantly as to prevent generation of the alkaline metal ions, Ca 2+ , Mg 2+ , in the Al 2  O 3  substrate. Accordingly, there is obtained an effect of preventing short circuits in the electrode and cracks on the substrate, in accordance with the present inventions. In addition, since CeO 2  readily forms a solid solution with Al 2  O 3 , CaO, Sio 2 , MgO and the like, the heater electrode becomes anchored to the heater substrate, reinforcing the adhesive strength therebetween. 
     A better understanding of the present ceramic heater may be obtained in light of following examples which are set forth to illustrate, and are not to be construed to limit, the present invention. 
     EXAMPLE I 
     Particles (average diameter 0.5 μm) of a mixture consisting of 96 weight percent Al 2  O 3 , 0.6 weight percent CaO, 2.4 weight percent SiO 2  and 1 weight percent of MgO were added with a sintering agent of 6 mole percent YSZ in an amount of 1 weight percent of the particles and then, mixed with a PVB based binder such as that sold by Sekisui Co. Ltd., Japan under the trademark designation &#34;BMS&#34;, in an amount of 10 weight percent of the particles. The resulting mixture was formed into a green sheet with a dimension of 5mm×50mm×0.8mm by a doctor blade process. 
     Platinum was formulated with 3 weight percent CeO 2  to 20 weight percent PVB based binder, to give a paste which was then screen printed on the green sheet. 
     Another green sheet with a dimension of 5mm×50mm×0.4mm which was prepared in the same manner as that for the above green sheet was laminated thereon. The resulting laminate was subjected to sintering at about 1,500° C. for about an hour in the air, to give a ceramic heater. 
     EXAMPLE II 
     A ceramic heater was prepared in the similar manner to that of Example I, except that the amount of CeO 2  was 20 weight percent based on the weight of platinum. 
     EXAMPLE III 
     Particles (average diameter 0.5 μm) of a mixture consisting of 96 weight percent Al 2  O 3 , 0.6 weight percent CaO, 2.4 weight percent SiO 2  and 1 weight percent of MgO were added with a sintering agent of 6 mole percent YSZ in an amount of 1 weight percent of the particles and then, mixed with a PVB based binder such as that sold by Sekisui Co. Ltd., Japan under the trademark designation &#34;BMS&#34;, in an amount of 6 weight percent of the particles. The resulting mixture was formed into a green sheet with a dimension of 5mm×50mm×0.8mm by a doctor blade process. 
     Platinum was formulated with 6 weight percent CeO 2  and 10 weight percent PVB based binder, to give a paste which was then screen printed on the green sheet. 
     Another green sheet with a dimension of 5mm×50mm×0.4mm which was prepared in the same manner as that for the above green sheet was laminated thereon. The resulting laminate was subjected to sintering at about 1,500° C. for about an hour in the air, to give a ceramic heater. 
     Comparative Example I 
     A ceramic heater was prepared in the similar manner to that of Example, except that CeO 2  was not added. 
     The ceramic heaters obtained in Example I and Comparative Example I were tested for durability. For this test, heater resistance was confined to a range of 3 to 4 ohms and a severe condition of 1,000° C. was taken by applying a voltage of 12.5 to 13.5 V. Results are given as shown in FIGS. 3 and 4 in which duration time is plotted with respect to additive amounts and heater resistance, respectively. As apparent from this plot, the ceramic heater containing CeO 2  (represented by solid lines) shows much longer duration time than that devoid of CeO 2  (represented by dotted lines), averaging 270 hours. This is owing to reduction in the migration. Particularly, the quite narrow distribution of the measured times demonstrates superiority in reproductivity and reliability. Further, the higher resistance, the longer duration time. From these results, it is proved that addition of CeO 2  in the electrode brings about an effect of prolonging duration time. 
     As described hereinbefore, CeO 2  can readily form a solid solution with the MgO, CaO, SiO 2  and Al 2  O 3 , resulting in an excellent adhesion of the platinum electrode to the Al 2  O 3  substrate. In addition, the lanthanides oxide restrains the migration of the alkaline metal ions, Mg 2+ , Ca 2+   owing to its superior protector and oxygen-storing ability, thereby preventing short circuit of the heat electrode as well as cracks on the Al 2  O 3  substrate. Further, the present ceramic heater has a duplicate structure free of the conventional migration pattern, which greatly contributes to simplification of production process and to reduction of production cost. 
     Other features, advantages and embodiments of the present invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosures. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed.