Abstract:
Electrical heating elements operable at high temperatures for long periods are produced by a method involving micropyretic synthesis. Compositions subjected to micropyretic synthesis comprises a powdery mass of electrically conductive and semiconductive material, a reactive system, a grain growth inhibitor and a plasticizer or extrusion agent.

Description:
This application is a continuation-in-part of application Ser. No. 8/242,362 filed May 13, 1994 now abandoned. 
    
    
     FIELD OF INVENTION 
     This invention relates to composite silicide or carbide heating element compositions which include improved combustion sources and refractory silicides such as tungsten silicide. This invention will enable combustion synthesized composite heating elements which include the combustion sources disclosed herein to be used at temperatures up to 900° C. for long periods of time in oxidizing atmospheres. 
     BACKGROUND OF THE INVENTION 
     This invention has been conceived to create heating elements which can work at high temperatures for long periods. Accordingly, this invention discloses new micropyretic sources and their combinations which allow for the first time, manufacture of heating elements displaying extended life at very high temperatures. A micropyretic synthesis method is used for the manufacture of heating elements, similar to that disclosed in our co-pending U.S. patent application Ser. No. 08/027,710, filed Mar. 8, 1993 (herein after referred to as &#34;the &#39;710 application&#34;, the contents of which are incorporated herein by way of reference). The synthesis method used in this invention differs from that disclosed in co-pending U.S. patent application Ser. No. 07/847,782, filed Mar. 5, 1992 (hereinafter referred to as &#34;the &#39;782 application&#34;, the contents of which are incorporated herein purely by way of reference), in an important aspect. As in the &#39;782 application, a combustible slurry is prepared, extruded and combusted. However, unlike the &#39;782 application, after combustion, these wire or other shapes are densified and homogenized by passing current, as described in the &#39;710 application. 
     The &#39;782 application discloses various combustion sources as part of the compositions, for the manufacture of heating elements. MoSi 2  composites are preferred in the &#39;782 application. MoSi 2  composites are very sensitive to phase composition and impurity at temperatures above 1600° C. The present compositions comprise new undisclosed combustion sources, grain growth inhibitors and other additives which substantially improve the applicability of heating elements manufactured using these compositions at temperatures up to 1900° C. Additionally, some of the combustion sources and their products described in the &#39;782 application may react with filler materials, and therefore may not be suitable as combustion sources for high temperature applications although they may be excellent for low and middle temperature applications (up to 1400° C.). The compositions of the present invention are designed so that the combustibles do not react with the silicides, which is believed to be one of reasons leading to the effective utilization of heating elements manufactured using the compositions of the present invention for extended periods at high temperatures. By utilizing combustibles which may react with the filler materials, the &#39;782 application teaches away from the present invention. 
     The high temperature superiority of heating elements manufactured using the compositions of the present invention, over the compositions of the &#39;782 application, has been observed by experimentation. These experiments have revealed that heating elements using the compositions of the present invention are stable at temperatures above 1400° C., for much longer periods than those manufactured using the compositions of the &#39;782 application. Without exception, a difference in time periods of at least two orders of magnitude has been observed. At temperatures above 1400° C. to 1600° C., after operation for 10-100 hours, heating elements using the &#39;782 compositions deteriorate substantially. Conversely, typical heating elements using the compositions of the present invention show similar deterioration at 1400° C. to 1600° C., only after 5000 hours. Additionally, above 1600° C., heating elements using the &#39;782 compositions last for less than 1 hour, whereas typical heating elements using the compositions of the present invention last for greater than 1000 hours. This tremendous increase in high temperature operability, is very beneficial and also quite unexpected. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide new combustion sources and compositions for manufacturing heating elements which are stable up to 1900° C. 
     It is a further object of the present invention to provide new combustion sources which will enhance the working temperature and life of heating elements and improve the oxidation resistance of heating elements at high temperatures. 
     It is another object of the present invention to provide new combustion sources which will prevent any possible reaction between the combustion sources, the combustion products, the silicides and the plasticizers. 
     It is yet another object of this invention to provide combustion synthesis product(s) with increased creep resistance and strength, by forming in situ compounds between carbides such as tungsten carbide and metalloids such as carbon or silicon. Such compounds are finely and homogeneously distributed in matrix of a major silicide phase e.g. in MoSi 2  or WSi 2 . In addition, these finely distributed particles will also act as grain growth inhibitors and also influence the recrystallization behavior. 
     In accordance with the present invention, there is provided a pliable composition comprising by weight percent: (a) between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2  and mixtures thereof; (b) between about 5% and 50% of a combustible source which is selected from the group consisting of WO 3  +Al+Si, MoO 3  +Zr+Si, WO 3  +Zr+Si, WC+Si, Mo 2  C+Si and mixtures thereof; (c) between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB 2 , HfB 2 , SiC and mixtures thereof; (d) at least about 1 weight percent bentonite; and (e) at least about 3 ml per 30 g of the above listed components, of colloidal silica solution. 
     In accordance with another aspect of the present invention there is provided a method of manufacturing a composite article using the composition described above, comprising the steps of: (a) premixing the powders comprising the combustible source in the composition; (b) blending said premixture with the other components of the composition; (c) forming a pliable slurry from said blend; (d) fashioning said slurry into a final desired article shape; (e) combusting said shape by ignition at a temperature between about 100° C. and 1600° C.; (f) initially applying sufficient current to said article so as to raise the temperature of said article to a minimum of 50% of the melting point in degrees Kelvin, of the lowest melting phase in the article, wherein the current applied is selected from the group consisting of a DC current, an AC current, a pulsed current and an induction current; and (g) greatly reducing the porosity of said article so as to make the repetitive distance between consecutive homogenous sections of said article to less than 0.002 m, by increasing said current applied so as to cause the elimination of thermal and mass gradients. 
     In accordance with a further aspect of the present invention, there is provided a pliable composition comprising by weight percent: (a) between about 60 and 85% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2  and mixtures thereof; (b) about 15% of WO3+Al+Si as a combustible source; (c) about 2% of HfB 2  as a grain growth inhibitor; (d) about 1 weight percent bentonite; (e) about 3 ml per 30 g of the above listed components, of colloidal silica solution; and (f) about 0.5 weight percent C. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Examples of specific compositions which have been found to provide heating elements having longer working life up to 1900° C., are disclosed as follows: 
     
         ______________________________________(all compositions herein are by weight percent of the totalcomposition, unless otherwise indicated)______________________________________Composition 1WSi.sub.2           10MoSi.sub.2          71.5WO.sub.3 + 2Al + 2Si.sub.2               15HfB.sub.2           2C (present in the form of               0.5graphite in all compositions herein)Bentonite           1Colloidal Silica (type 830               3 ml/30 g powderNycol corporation)  (this implies 3 ml               per 30 g of the               remaining components)Composition 2WSi.sub.2           20MoSi.sub.2          61.5WO.sub.3 + 2Al + 2Si.sub.2               15HfB.sub.2           2C                   0.5Bentonite           1Colloidal Silica    3 ml/30 g powderComposition 3WSi.sub.2           30MoSi.sub.2          51.5WO.sub.3 + 2Al + 2Si.sub.2               15HfB.sub.2           2C                   0.5Bentonite           1Colloidal Silica    3 ml/30 g powderComposition 4WSi.sub.2           40MoSi.sub.2          41.5WO.sub.3 + 2Al + 2Si.sub.2               15HfB.sub.2           2C                   0.5Bentonite           1Colloidal Silica    3 ml/30 g powderComposition 5WSi.sub.2           50MoSi.sub.2          31.5WO.sub.3 + 2Al + 2Si.sub.2               15HfB.sub.2           2C                   0.5Bentonite           1Colloidal Silica    3 ml/30 g powderComposition 6WSi.sub.2           20MoSi.sub.2          61.5WO.sub.3 + 2Al + 2Si.sub.2               72WO.sub.3 + 3Zr + 4Si               8HfB.sub.2           2C                   0.5Bentonite           1Colloidal Silica    3 ml/30 g powderComposition 7WSi.sub.2           20MoSi.sub.2          57WO.sub.3 + 2Al + 2Si.sub.2               5WC + 3Si            15TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 8WSi.sub.2           20MoSi.sub.2          54WO.sub.3 + 2Al + 2Si.sub.2               8WC + 3Si            15TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 9WSi.sub.2           20MoSi.sub.2          52WO.sub.3 + 2Al + 2Si.sub.2               10WC + 3Si            15TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 10WSi.sub.2           20MoSi.sub.2          47WC + 3Si            15WO.sub.3 + 2Al + 2Si.sub.2               15TiB.sub.2           2Bentonite           1Water with organic binders                5 ml/30 g powder.(organic binders may be methyl cellulose, polyethyleneglycol with an average molecular weight. of 200, polyethyleneglycol with an average molecular weight of 300, glycerol,99.5%, polyvinyl butyral, dioctyl adipate and theircombinations)Composition 11WSi.sub.2           20MoSi.sub.2          57MO.sub.2 C + 5Si    15WO.sub.3 + 2Al + 2Si.sub.2               5TiB.sub.2           2Bentonite           1Colloidal Silica    (5 ml/30 g powder)Composition 12WSi.sub.2           20MoSi.sub.2          54MO.sub.2 C + 5Si    15MoO.sub.3 + 2Al + 2Si               8TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 13WSi.sub.2           20MoSi.sub.2          57Mo.sub.2 C + 5Si    15WO.sub.3 + 2Al + 2Si.sub.2               5HfB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 14WSi.sub.2           20MoSi.sub.2          47WC + 3Si            25WO.sub.3 + 2Al + 2Si.sub.2               5TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 15WSi.sub.2           20MoSi.sub.2          47Mo.sub.2 C + 5Si    25WO.sub.3 + 2Al + 2Si               5TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 16WSi.sub.2           20MoSi.sub.2          47Mo.sub.2 C + 5Si    25WO.sub.3 + 2Al + 2Si.sub.2               5TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g powderComposition 17MoSi.sub.2          872WO.sub.3 + 3Zr + 4Si               10TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 18MoSi.sub.2          822WO.sub.3 + 3Zr + 4Si               5WO.sub.3 + 2Al + 2Si               10TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 19MoSi.sub.2          772WO.sub.3 + 3Zr + 4Si               10WO.sub.3 + 2Al + 2Si               10TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 20WSi.sub.2           20MoSi.sub.2          622WO.sub.3 + 3Zr + 4Si               5WC + 3Si            10TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 21MoSi.sub.2          872WO.sub.3 + 3Zr + 4Si               10Si.sub.3 N.sub.4    2Bentonite           1Colloidal Silica    5 ml/30 gComposition 22MoSi.sub.2          822WO.sub.3 + 3Zr + 4Si               15SiC                 2Bentonite           1Colloidal Silica    5 ml/30 gComposition 23MoSi.sub.2          772WO.sub.3 + 3Zr + 4Si               20TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 24MoSi.sub.2          772WO.sub.3 + 3Zr + 4Si               102MO.sub.3 + 3Zr + 4Si               10TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 gComposition 25WSi.sub.2           10MoSi.sub.2          722MoO.sub.3 + 3Zr + 4Si               10WC + 3Si            5TiB.sub.2           2Bentonite           1Colloidal Silica    5 ml/30 g______________________________________ 
    
     Broadly stated, the compositions of the present invention comprise between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2  and mixtures thereof; between about 5 % and 50% of a combustible source which is selected from the group consisting of WO 3  +Al+Si, MoO 3  +Zr+Si, WO 3  +Zr+Si, WC+Si, Mo 2  C+Si and mixtures thereof; between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB 2 , HfB 2 , SiC and mixtures thereof; at least about 1 weight percent bentonite; and at least about 3 ml/30 g of colloidal silica solution. 
     The combustion sources have to be premixed before mixing with other powders, in order to produce sufficient heat during combustion. The weight percentage of combustibles in the compositions of the present invention will not be less than 5 wt %, but will not be more than 90%. A combustible content of more than 50% will lead to too strong combustion, and therefore, to samples (heating elements) with large pores and deformed shapes. Compositions with combustibles less then 5% will not be able to produce enough heat to sinter the wire and therefore the wire may not be conductive enough to be finally sintered, densified and homogenized by passing current. The addition of WSi 2  or MoSi 2  increases the working temperature, life of heating elements at high temperature and creep resistance, but WSi 2  or MoSi 2  content of more than 90 weight percent will drastically decrease thermal shock resistance and oxidation resistance of heating element. 
     The preferred compositions of the present invention comprise from about 10 to 30 percent WSi 2 , up to 4% SiC, up to 3% TiB 2 , up to 3% HfB 2  (said SiC, TiB 2  and HfB 2  being at least 0.5 percent), close to 2% bentonite and close to 15 ml/100 g of powder of colloidal silica. The most preferred composition for manufacturing high temperature heating elements is as follows: 15-30 percent of WSi 2 , 15-20 percent combustibles, 1-2 percent TiB 2  and HfB 2  and 0.5-1 percent C. Heating elements made from compositions 1, 2 and 3 above, lasted for over 1000 hours at temperatures from 1700° to 1750° C. 
     Yet another preferred and unique composition discloses the use of a combination of WC+Si or Mo 2  C+Si with WO 3  +Al+Si or WO 3  +Zr+Si. The content of carbide combustion source (WC+Si or Mo 2  C+Si) will be from 1 to 50%; oxide combustion source (WO 3  +Al+Si or WO 3  +Zr+Si) will be 5-30%. The total weight percent of the combustion source is at least 5%. In its more preferred aspects, the carbide combustion source ranges from 15 to 20%, oxide combustion source will be 5-10 wt %. By using carbide combustion sources, alumina content in the final products will be tremendously reduced. It was found that heating elements manufactured using a combination of carbide and oxide combustion sources (for instance, composition 9), show excellent temperature capacity and improved life up to 1850° C. 
     Yet another preferred and unique composition discloses WO 3  +Zr+Si and MOO 3  +Zr+Si as combustion sources or combinations thereof. These compositions show strong and stable combustion when mixed with MoSi 2  and WSi 2  powders. The final products of these combustion sources are WSi 2  (or MoSi 2 ) and ZrO 2 . Part of ZrO 2  will react further with SiO 2  during sintering to form zircon which has high melting point and phase stability with MoSi 2  and WSi 2 . The total weight percent of the combustion source in these compositions is at least 5%. In its more preferred aspects, the WO3+Zr+Si combustion source will be from 10 to 25 weight percent. It was found that the compositions using this combustion source show excellent sintering properties and high temperature capability up to 1850° C. 
     Optimal (these are merely preferred particle sizes) particle sizes are disclosed for various the components listed above as follows: 
     
         ______________________________________SiC                1 μmWSi.sub.2          4 μmZr                 3.5-4 μmMoSi.sub.2         4 μmC                  300 mesh (15 μm)WO.sub.3           8 μmMo.sub.2 C         325 mesh (44 μm)Al                 325 mesh (44 μm)Si                 325 mesh (44 μm)HfB.sub.2          325 mesh (44 μm)WC                 1 μmTiB.sub.2          1 μmMoO.sub.3          8 μmSi.sub.3 N.sub.4   325 mesh (44 μm)Bentonite          5 μmColloidal silica   nanosize colloid______________________________________ 
    
     Thus it is apparent that there have been provided, in accordance with the invention, compositions suited for preparing heating elements which may be operated at temperatures up to 1900° C. or for very long durations at lower temperatures such as 1750° C., which fully satisfy the objects, aspects and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. For example, it is contemplated that in addition to the combustion sources described and claimed herein, a particular composition may comprise other combustion sources. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.