Abstract:
A ceramic heater which has an alumina rod, an alumina based ribbon sintered to the rod, and a platinum resistor element bonded to the ribbon. Additionally, a method of making a ceramic heater by preparing a ceramic slurry; combining the ceramic slurry with a binder component to form a slip; depositing the slip onto a carrier film at a controlled thickness such that a deposited slip is formed; heat curing the deposited slip to form a cured slip ribbon; applying a platinum paste onto the ribbon in a specific pattern, the paste forming a platinum resistor element on the ribbon; applying the ribbon with the platinum resistor element onto an alumina rod; and, heating the rod with the ribbon and the platinum resistor element thereon, whereby the ribbon is sintered to the rod and the platinum resistor element is sintered and bonded to the ribbon.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to ceramic heaters and more particularly to a ceramic heater having a platinum heating element which is resistent to oxidation. 
     BACKGROUND OF THE INVENTION 
     Ceramic heaters are generally known in the art. Normally, a ceramic heater will include an insulating portion, a heat generating portion, and electrical lead portions formed integrally with a ceramic body or substrate. The heater element and lead portions are normally formed of a single electrically conductive metal such as an inexpensive non-noble or base metal such as tungsten and molybdenum. However, the heater element and lead portions made of such metals are prone to oxidize during long periods of use at high operating temperatures in oxidizing atmospheres such as air. The oxidation may result in disconnection of the heat-generating portion of the ceramic heater and, thus, heater failure. 
     The art has sought to solve this problem by decreasing the amount of non-noble or base metal used in the ceramic heater. For example, U.S. Pat. No. 4,952,903 to Shibata et al., (hereinafter “Shibata”) teaches a ceramic heater including a ceramic body and a heater element formed of a cermet containing a ceramic material and a metal material which principally consists of at least one noble metal; and, including electrical lead portions formed of a metallic material consisting of at least one base metal or formed of a cermet containing ceramic material and metallic material. Shibata mentions the making of the heater element from a noble metal such as platinum or rhodium, but dismisses such use because of costs and the difficulty of bonding a noble metal to a ceramic substrate. The use of such noble metal would overcome the problems associated with oxidation of the metal. Thus, an economic and practical means of using such noble metals would be advantageous to the art of ceramic heaters. For these reasons, there remains room for improvement in the art. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide adequate binding of noble metals to a ceramic substrate. 
     It is also an object of this invention to provide a ceramic heater which does not require an outer sheath or cover and which is economical to manufacture. 
     It is another object of the present invention to provide a method of making a ceramic heater which provides for the screen printing of the heater element onto a ceramic sheet. 
     It is a further object of the present invention to provide a method of making a ceramic heater which does not require a cover layer to protect the heating element. 
     These and other objects of the invention are achieved by a ceramic heater comprising an alumina rod, an alumina based ribbon sintered to the rod, and a platinum resistor element bonded to the ribbon. These and other objects are also achieved by a method of making a ceramic heater comprising the steps of making a ceramic slurry; combining the ceramic slurry with a binder component to form a slip; depositing the slip onto a carrier film at a controlled thickness such that a deposited slip is formed; heat curing the deposited slip to form a cured slip ribbon; applying a platinum paste onto the ribbon in a specific pattern, the paste forming a platinum resistor element on the ribbon; applying the ribbon with the platinum resistor element onto an alumina rod; and, heating the rod with the ribbon and the platinum resistor element thereon, whereby the ribbon is sintered to the rod and the platinum resistor element is sintered and bonded to the ribbon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the ceramic heater of the present invention. 
     FIG. 2 is a schematic representation of the method of making the ceramic heater of the present invention. 
     FIG. 3 is a schematic representation of the method of making the slip of the present invention. 
     FIG. 4 is a schematic representation of the method of making the ribbon of the present invention. 
     FIG. 5 is a schematic representation of the method of manufacturing the resistor element of the present invention. 
     FIG. 6 is a schematic representation of the method of manufacturing the ceramic heater with the resistor element as taught in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective view of the ceramic heater of the present invention. As shown, the ceramic heater comprises a rod portion  3  which is preferably an alumina rod but can comprise any suitable insulating material. Alumina is preferable in this instance because of its physical and thermal robustness. Also, as shown, the resistor element  9  is printed onto a cured slip ribbon  5  which is in communication with the alumina rod  3 ; however, the resistor element  9  may be screen printed directly onto the rod  3  (not shown). In a preferred embodiment, the resistor element  9  is made from a platinum paste but may comprise some other noble metal or combination comprising a noble metal. The resistor element  9  is sintered and bonded onto the ribbon  5  which is further sintered onto the alumina rod  3  to form the ceramic heater  1 . In certain instances, however, it may be preferable to sinter or bond the resistor element  9  directly onto the rod  3  using the method of this invention without involving the ribbon element  5 . 
     FIG. 2 is a schematic representation of the method by which the ceramic heater  1  of the present invention is made. The first step  11  in making the ceramic heater  1  is to make the ceramic tape or the cured slip ribbon  5 . The second step  31  includes screen printing the resistor element  9  onto the ribbon  5 . The third step  41  includes manufacturing the heater  1 . And the final step  49  involves bonding and sintering the heater elements and the ceramic particles together. These steps will be described in more detail below. 
     The first step  11  is more fully detailed with reference to FIG.  3 . FIG. 3 is a schematic representation of the method of making the cured slip ribbon  5  of the present invention. In making the ceramic slurry  13  used in the present invention, dried ceramic powders, such as Al 2 O 3,  MgO, SiO 2,  ZrO 2  and CaCO 3,  are weighed, blended and then wet out by conventional means to form the slurry  13 . The ceramic components to the slurry  13  are mixed by conventional means, for example in mixing tanks, for approximately one hour to ensure consistency in the mixture. Thereafter, the slurry  13  is transferred into the vibratory mill where the ceramic particles are broken down to create more surface area. The process of breaking down the ceramic particles makes the alumina in the slurry  13  more reactive and, thus, allows for a lower sintering temperature. Second, the breaking down process allows the forming of a ceramic tape comprising more densely packed particles which reduces variability throughout the ceramic tape or ribbon  11 . 
     Once the milling process is completed, the slurry  13  is removed from the vibratory mill and returned into the mixing tanks where the weight is recorded and used to calculate the proper binder addition. Once the proper binder addition is calculated, the slurry  13  is combined with a binder compound  15  to produce the ceramic slip  17 . In a preferred embodiment of the invention, the binder  15  is a cellulose binder compound. The method of the present invention is to manufacture the binder compound  15  by combining the necessary raw materials and “cooking” the solution in a crock-pot type apparatus. By cooking the solution, materials such as polyethyleneglycol and polyvinylalcohol melt down into a viscous fluid which is then added to the slurry  13  to form the ceramic slip  17 . 
     In a preferred embodiment of the present invention, the ceramic slip  17  formulation (by weight) will be as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Al 2 O 3   
                 51% 1   
                 PEG 3350 
                 0.5% 
               
               
                   
                 H 2 O 
                 40% 
                 PEG 8000 
                 0.5% 
               
               
                   
                 PVA 
                  3% 
                 Darvan 821A 
                 0.4% 
               
               
                   
                 Glycerol 
                  1.9% 
                 MgO 
                 0.4% 
               
               
                   
                 SiO 2   
                  1.6% 
                 ZrO 2   
                 0.1% 
               
               
                   
                 CaCO 3   
                  0.6% 
               
               
                   
                   
               
               
                   
                   1 The weights identified in this formulation are approximate weights.  
               
             
          
         
       
     
     After the addition of the binder  15 , the ceramic slip  17  is mixed for approximately one hour. The slip  17  is then pumped through a series of filters  18 , for example fiber woven filters, and into at least one slip casting tank. The filtration process removes excessively large particles or conglomerates to ensure consistency. The slip  17  is then de-aired  19  in the casting tank for approximately twelve (12) hours. During this time, the slip  17  is kept in suspension by slow rotation of the mixing blade. This allows any entrapped air to escape from the slip  17  so that pinholes will not form when casting the ribbon  27 . 
     FIG. 4 is a schematic representation of the method of making the ribbon  27  of the present invention. Once the slip  17  is sufficiently processed, it is pumped from a tank, such as a casting tank, into a reservoir. A carrier film  21  is passed through the reservoir, entering one end and exiting the opposite, such that the slip composition  17  is deposited onto the carrier film  21  to make a deposited slip. The deposited slip is then brought into contact with a blade, such as a “doctor-blade”  23 . In a preferred embodiment, the deposited slip travels underneath the doctor blade  23  which is positioned at a predetermined distance above the carrier film. This process controls the thickness of the slip  17  which is allowed to remain deposited on the carrier film  21  and thus controls the resulting thickness of the ceramic tape or ribbon  27 . 
     After passing under the “doctor-blade”  23 , the deposited slip is cured. In a preferred embodiment the deposited slip is cured  25  by causing the deposited slip to travel through a heated chamber where the deposited slip is dehydrated. After exiting the chamber, the cured slip or tape may be stored  27  for later use by winding up on a reel, or by any other conventional means of storage. 
     FIG. 5 is a schematic representation of the method of manufacturing the resistor element of the present invention. When the user is ready to prepare  31  the heater resistor element  9 , the stored slip, otherwise called the ceramic tape or ribbon  27 , is prepared into sheets of predetermined size  33 . In the preferred embodiment, the ceramic tape or ribbon  27  will be cut into rough squares approximately 4 inches by 4 inches. The individual sheets of ceramic tape or ribbon  27  provide a printing surface. In order to ensure consistency and accuracy, the printing surface is secured in place by a holding means  35 . In the preferred embodiment, the holding means is a vacuum chuck which holds the printing surface in place during the printing process. The printing is accomplished by using a screen which is shaped having a specific patten. The pattern of the screen corresponds directly to the intended or desired shape of the resistor element  9 . The screen may be held in place by a frame, such as a metal frame or by any conventional method. 
     A platinum paste is then made and applied to a surface of the screen  37 . A device, such as a squeegee, is then used to force the paste through the screen  38  and onto the printing surface of the ribbon  27 . The printing surface is then removed from the holding means and allowed to dry  39 , such as in a drying box, a table, or some other flat surface, to form the resistor element. 
     FIG. 6 is a schematic representation of the method of manufacturing the ceramic heater  1  with the resistor element  9  as taught in the present invention. To complete the manufacture  41  of the heater  1 , the individual resistor patterns are cut out of the ribbon  27  and removed from the carrier film  43 . The resistor element  9  is inverted and a binder solution is applied to a backside of the resistor element  9  opposite the platinum paste  45 . The binder solution used is preferably the same alumina binder composition previously mixed with the ceramic solution to form the slip  17 , but may be any equivalent binder solution. The resistor element is then applied to a pre-fired alumina rod. In a preferred embodiment, the resistor element  9  is applied by rolling  47  the rod  3  over the side of the resistor element  9  containing the binder solution, causing the resistor element  9  to wrap itself round the rod  3  to form the “green” heater. 
     The “green” heater is inspected to ensure a smooth and uniform wrap of the resistor element  9  to the rod  3 . Once inspected, the “green” heater is “baked-out” to remove any organic materials from the heater components  49 A and to center the ceramic particles. The heater  1  is heated through a controlled heating profile which is completed at approximately 625° Celsius. After the heater completes the “bake-out” phase  49 A, it is then “fired” by going through a second controlled heating profile  49 B which is completed at approximately 1550° Celsius. 
     The heater  1  that is produced in accordance with this invention having the platinum resistor element  9  (or heating element) overcomes the problems of the prior art because it is economical to produce and will not oxidize when exposed to air; thus, there is no need for an outer sheath or cover element. The method of the present invention allows for dense packing of particles while forming the ceramic tape or ribbon, reducing variability throughout the ceramic tape. The method further provides for the screen printing of the heater element onto a ceramic tape in a desired pattern. 
     It will be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those described, as well as many variations, modifications and equivalent arrangements will be apparent from or reasonably suggested by the present invention and foregoing description thereof, without departing from the substance or scope of the present invention as defined by the following appended claims.