CA2154613C - Ceramic heater element with molybdenum disilicide and zirconia composition - Google Patents
Ceramic heater element with molybdenum disilicide and zirconia compositionInfo
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- CA2154613C CA2154613C CA 2154613 CA2154613A CA2154613C CA 2154613 C CA2154613 C CA 2154613C CA 2154613 CA2154613 CA 2154613 CA 2154613 A CA2154613 A CA 2154613A CA 2154613 C CA2154613 C CA 2154613C
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- zirconia
- heater element
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- sintered mixture
- molybdenum disilicide
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Classifications
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
- H05B2203/018—Heaters using heating elements comprising mosi2
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- Resistance Heating (AREA)
- Compositions Of Oxide Ceramics (AREA)
Abstract
A ceramic composition for the manufacture of an improved ceramic electric heater element is disclosed.
The composition contains 33-50 vol% molybdenum disilicide, the balance being partially stabilized zirconia or partially stabilized zirconia-ceria solid solution. The average grain size of the components in the sintered mixture made into the ceramic heater element of a desired shape, is less than 10µm. The ceramic heater element has a skin of zirconia or zirconia-ceria generated in-situ, having thickness in excess of 10µm. A portion of the zirconia or zirconia-ceria solid solution particles may be replaced by zirconia fibres. The sintered ceramic heater elements are suitable to be incorporated in an electrical resistance furnace or an induction furnace, and are capable of operating up to 2000°C under oxidizing conditions.
The composition contains 33-50 vol% molybdenum disilicide, the balance being partially stabilized zirconia or partially stabilized zirconia-ceria solid solution. The average grain size of the components in the sintered mixture made into the ceramic heater element of a desired shape, is less than 10µm. The ceramic heater element has a skin of zirconia or zirconia-ceria generated in-situ, having thickness in excess of 10µm. A portion of the zirconia or zirconia-ceria solid solution particles may be replaced by zirconia fibres. The sintered ceramic heater elements are suitable to be incorporated in an electrical resistance furnace or an induction furnace, and are capable of operating up to 2000°C under oxidizing conditions.
Description
21~613 Title: CERAMIC HEATER ~RRMF~T WITH MOLYBDENUM
DISILICIDE AND ZIRCONIA COMPOSITION
FIELD OF INVENTION
This invention relates to ceramic heating elements, more particularly zirconia based heating elements, which are capable of operating at medium and high temperatures, such as above 1500 C.
~K~ROUND OF THE INVENTION
The advanced materials utilized by modern industry often require prolonged heat treatment at temperatures well over 1300 C. There are electrical resistance furnaces commercially available which can provide temperatures above 1300 C having heater coils made of high melting point metals, such as tantalum, tungsten, molybdenum and their alloys. Other high temperature furnaces have graphite heating elements.
However, the above heater element materials cannot operate in oxidizing atmospheres and the furnace design must include special jackets for maintaining neutral or reducing atmospheres or vacuum. If oxygen or air is allowed to come into contact with the heater elements made of the above substances during operation, serious damage to the furnace may occur.
~eater elements and coils made of Nichrome or Kanthal wire can operate in air but cannot provide temperatures above 1200-1300 C. Platinum metal melts at 1660 C and although Pt-Rh alloy has a higher melting point, these are highly expensive heater materials for use in typical commercial applications. Usually the metallic heater component is either wound onto or is otherwise supported or enclosed by an electrically non-conductive ceramic body that is comprised within the heater element as a separate structural component.
Molybdenum disilicide is a ceramic substance which is oxidation and corrosion resistant at relatively 2 1 ~ 4 6 1 ~
high temperatures. Molybdenum disilicide differs from most ceramic substances by exhibiting electrical conductivity similar to metals. Molybdenum disilicide has been used in the manufacture of heater element contacts and heater elements, such that is described, for example in U.S. 2,622,304 issued to L.W.Coffer on Dec. 23 1952. Unsupported molybdenum disilicide heater elements however, are known to lose mechanical strength and shape retention above 1400 C. Another electrically conductive ceramic substance is silicon carbide.
Silicon carbide heaters may be able to operate above 1300 C in air, however such heaters become very sensitive to mechanical impact at high temperatures, and have therefore a short life span. Mixtures of silicon carbide and molybdenum disilicide are also known.
Another known substance, lanthanum chromate shows good electrical conductivity and stability at medium high temperatures, but is a relatively expensive ceramic material, and it also tends to have a volatile by-product which may contaminate the furnace atmosphere.
Zirconia (ZrO2) is stable at high temperature, melts around 2600 C and is a good electrical conductor at temperatures above 1100 C. Zirconia has monoclinic crystal structure below llO0 C. Monoclinic zirconia has high fracture toughness, but it is known to lose its mechanical strength above its transition temperature.
Monoclinic zirconia is also known to be a poor electrical conductor In its commercial applications zirconia is usually partially or fully stabilized to retain its desirable mechanical properties in a wider temperature range, by oxides of the alkaline earth metals, yttria or oxides of the rare earth metals. It is known that the addition of a lower valent metal oxide to zirconia for the stabilization of its tetragonal or cubic crystal structure, will increase the ionic conductivity of zirconia, however, stabilization will not substantially increase the electrical conductivity ' - ~lS4613 at temperatures below 800 C.
A heater element made of zirconia stabilized with 3-6% lime or magnesia, operating in cooperation with molybdenum metal rings and coil connectors, is described in U.S. 2,680.771 issued to S.S.Kistler on June 8 1954. In other heater element applications the low electrical conductivity of zirconia at low and medium high temperatures may be compensated by the heater element having specially designed configuration and/or preheaters. Such heater elements are usually made of partially or fully stabilized zirconia with no other admixed ceramic component. An electric furnace having a platinum alloy preheater operated as a separate heater element in conjunction with a zirconia or any other ceramic rod heater, is described in Japanese Patent 4-43588, issued to Mitsubishi Heavy Ind. Ltd. on Feb.13, 1992.
U.S. patents 5,073,689 and 5,154,785 issued to Tabata et al. on December 17, 1991 and October 13, 1992, respectively, describe zirconia heating elements made of 2 irconia reinforced with zirconia fibres. The zirconia powder and fibres utilized in the heater elements of Tabata et al. contain stabilizing oxides, various organic binders and other additives to improve the mechanical and electrical properties of zirconia heater elements produced. Moreover, the heater elements of Tabata et al. have special geometric design features, lead members, fabrication and impregnation steps so that the inherent high electrical resistance of zirconia at low temperatures may be diminished. The various additional process steps are likely to increase the production costs of the heater elements of Tabata et al.
Ceramic materials utilized in the manufacture of ceramic heaters and other high temperature resistant sintered ceramic articles, are known to contain molybdenum disilicide admixed with refractory oxides, usually for providing increased mechanical strength to 21~13 the article. British patent 725,577, published on March 9, 1955 and corresponding Canadian patent 612,139, issued on Jan.10, 1961, to Johnson Matthey and Mallory, describe ceramic articles made of molybdenum disilicide mixed with a refractory oxide, such as zirconia or alumina. The above British and Canadian patents teach in particular, ceramic compositions containing in various amounts MoSi2, Al2O3 and CaO as fluxing agent for the manufacture of engine chamber parts, and similar sintered articles required to have high wear and oxidation resistance. It is to be noted that alumina is a very good electrical insulator material. F.R.Charvat in U.S. 2,998,394 issued on Aug 29, 1961, describes electrical resistors made of molybdenum disilicide and clay. Lenie et al. in U.S. 3,108,887 issued on Oct. 29, 1963, describe erosion and corrosion resistant articles - having high electrical resistivity, made of mixtures of aluminum nitride and other refractory substances, such as molybdenum disilicide, zirconium oxide and the rare earth metal oxides. The oxide or silicide compound is added to improve the densification of the ceramic article made predominantly of aluminum nitride. Kaneko et al. in U.S. 4,907,015, issued on March 6, 1990, describe a heating element for a thermal printing head comprising an intermetallic compound, such as for example molybdenum disilicide, and an electrically insulating material, such as zirconia. Petrovic et al.
in U.S. 5,063,182, issued on Nov.5, 1991, describe a zirconia-molybdenum disilicide containing ceramic composition for the manufacture of articles required to have high fracture toughness. Japanese patent 63-257206 is directed to a slidPr material containing molybdenum disilicide and zirconia, to be utilized in a thin film magnetic head. It is of significance that in all of the above ceramic mixtures containing molybdenum disilicide and a refractory oxide, in particular zirconium dioxide, is added with the intention to increase the mechanical 21~13 strength and electrical resistance of the articles made of the mixture, and not with the object of improving the electrical conductivity of the mixture.
Some other heater related applications utilizing molybdenum disilicide are briefly reviewed below. In U.S. Patent 3,328,201 issued on June 27, 1967 to H.G. Scheible, a ceramic coating containing molybdenum disilicide and alumina, is provided to cover an alumina body enclosing a metallic heater wire. The device is utilized as a heater in an electron tube.
Japanese patent 2-227981 issued to Shinagawa Refract Co Ltd. on Nov. 9, 1990 describes a molybdenum disilicide heater rod protected from oxidation by a sheet of zirconia, which is laminated onto the molybdenum disilicide rod in a separate fabrication process step.
Kieffer et al. in U.S. 2,831,242 issued on Apr. 22, 1958, describe electric resistance heater elements made of molybdenum disilicide and molybdenum aluminide intermetallic compound, additionally containing refractory oxides, such as zirconia to increase the resistivity of the heater.
The heater elements having compositions as described in the above discussed publications, do not appear to have wide commercial success. There is a need for a relatively inexpensively produced ceramic heater element having a composition which combines the electrical conductivity of molybdenum disilicide at low and medium temperature range with the mechanical strength and electrical conductivity of partially stabilized zirconia, which is oxidation resistant at high temperatures, and which needs no preheater or special configuration in shape.
STAT~HENT OF INVENTION
An improved ceramic mixture for a sintered heater element made of particles of molybdenum disilicide and an oxide for reinforcing the mechanical strength of the molybdenum disilicide contained therein 2154~ 1 3 has been found. The improvement in the ceramic heater element comprises that the oxide in the sintered mixture is zirconia partially stabilized with an oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said mixture in 33-50 vol.~, the balance being zirconia, the average grain size of the molybdenum disilicide and the zirconia in said sintered mixture being less than 10~m, and that the ceramic heater element made of said sintered mixture has a continuous coating containing substantially zirconia having thickness in excess of 10~m generated in-situ, the improvement further comprises that the electrical conductivity of said heater element made of said mixture is greater than 0.1 (ohm.cm)-l and less than 10 (ohm.cm)~l in the temperature range 100-2000~C.
In another embodiment of the ceramic heater element the balance of the molybdenum disilicide containing sintered mixture is partially stabilized zirconia-ceria solid solution.
In yet another embodiment of the ceramic heater element a portion of the zirconia is present in the mixture in the form of zirconia fibres.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig.1 is a schematic representation of a typical heating element manufactured from the sintered mixture of the present invention.
Fig.2/a and 2/b are EDAX diffractograms taken at the core and at the surface, respectively, of the sintered ceramic heater element, and Fig.3 is a SEM photograph of the cross-section of the sintered ceramic heater element near the surface.
The preferred embodiments of the invention will be described hereinbelow and illustrated by the Figures in the examples.
215~ 13 DErATT.~.n DISCUSSION OF THE ~K~KKED EMBODIMENTS
As has been discussed above, the high temperature form of zirconia is known to exhibit electrical conductivity which renders it suitable for use as a heater element. The heat in the heater element may be generated either by resistance to an electric current passing through the zirconia element, or by induction. The high temperature cubic or tetragonal structure of zirconia may be retained at temperatures below the transition temperature by the addition of stabilizing oxides, which form solid solutions with the zirconia. It is known that zirconia forms solid solutions with di-valent alkaline earth metal oxides, such as magnesia, calcia and strontia, and with yttria and oxides of tri-valent rare earth metals. The rare earth metals are usually considered to be encompassing the lanthanum group metals including cerium and yttrium.
However, for the sake of emphasis, yttria is singled out here as a more commonly used stabilizing oxide, but oxides of other tri-valent rare earth metals may also be suitable in stabilizing the high temperature form of zirconia. Partially stabilized zirconia solid solutions may be formed when the stabilizing oxide concentration is above 3 mole%. Full stabilization of zirconia may be usually attained when the stabilizing oxide is present in the solid solution in excess of 7.5 mole%, depending on the stabilizing oxide. Fully stabilized zirconia exhibits predominantly ionic conductivity, the conductivity usually being greater than 10~1(ohm.cm)~l at temperatures higher than 1100 C. The conductivity of partially stabilized zirconia is expected to be one order of magnitude less. It is, however, known that the total conductivity of both partially and fully stabilized zirconia is less than 10~3(ohm.cm)~l at temperatures below 600 C, hence for obtaining a ceramic substance that may be utilized in manufacturing a ceramic heater, it is desirable that the conductivity be - 21~461~
increased in the lower temperature range by admixing with the zirconia a ceramic substance which is a good electronic conductor, such as molybdenum disilicide.
Molybdenum disilicide behaves more like a metallic conductor and has conductivity greater than lO(ohm.cm)~
in this temperature range.
There are other physical characteristics that need to be taken into consideration in selecting components for a mixture of ceramic substances suitable for producing ceramic heater elements, such as thermal conductivity, coefficient of thermal expansion, shape-retention, sinterability, mechanical strength and such like. As mentioned above, ceramic articles for use at high temperatures, made of molybdenum disilicide and a refractory oxide do not appear to enjoy great commercial success. It is suggested here, without in any way being bound by this argument, that mismatch of physical properties of the components, such as for example the coefficient of thermal expansion, may be responsible for the lack of commercial success. It is known that the thermal coefficient of expansion of partially stabilized zirconia is lower than that of fully stabilized zirconia. Furthermore, the fracture toughness and thermal shock resistance of partially stabilized zirconia are known to be higher than the fracture toughness and thermal shock resistance of fully stabilized zirconia. Thus although the electrical conductivity of partially stabilized zirconia is lower than the electrical conductivity of fully stabilized zirconia, for the purposes of manufacturing a ceramic heater element a mixture containing molybdenum disilicide and partially stabilized zirconia is preferred. It has now been observed that mismatch of the physical properties of the components may be substantially reduced if the components of the mixture are of similar fine particle size and are intimately mixed. In other words, it has been surprisingly found g that the particle size plays an important role in governing the electrical and mechanical properties of the ceramic heater element manufactured from such an intimate mixture. It has also been found that the S composition range allowing the combination of electrical and mechanical properties desirable in a ceramic heater element, lies between 33-50 vol.% molybdenum disilicide and 50-67 vol.% partially stabilized zirconia.
Furthermore, it has been found that for best results, the particle size of the particles making up the mixture should be less than 5~m. The even more preferred composition range of the intimate mixture was found to be 35-42 vol.% molybdenum disilicide and 58-65 vol.%
partially stabilized zirconia, having particles size less than 3~m.
The stabilizing oxide which, as discussed above, may be calcia, strontia, yttria or similar tri-valent rare earth metal oxide, is usually added in amounts between 3.5 to 7.0 mole% to partially stabilize the zirconia in its electrically conductive structure.
It is to be noted, that it is customary that the composition of commercially manufactured ceramic articles is expressed in vol.% or in weight%. It is however, acceptable and usual to refer to the amount of stabilizing oxide added to zirconia in mole%, since different oxides when added at the same mole% rate would be calculated to yield different vol.% values.
As mentioned above, good thermal conductivity is a property which is desirable in a ceramic component for manufacturing a ceramic heater element. Good thermal conductivity will lead to a more efficient heater element, and it may also prolong the expected useful life of the heater element. The thermal conductivity of partially stabilized zirconia is known to have values of about 8.5 10-3 cal.sec~l.cm~2/~C.cm~l at 100 C. Cubic ceria has been measured to exhibit thermal conductivity of about 21.6 10-3 cal.sec~l.cm~2/ C.cm~l at 21~4613 the same temperature. It is to be noted that ceria is normally found in its tetra-valent form and as such it has crystal structure similar to that of zirconia.
Ceria may also be stabilized in its cubic structure by the addition of alkaline earth metal oxides, yttria and oxides of tri-valent rare earth metals, and is known to be a good electrical conductor at medium and high temperatures. Moreover, stabilized ceria is known to form solid solutions with zirconia in a fairly wide range. In a another embodiment of the present invention a mixture of particles of molybdenum disilicide and particles of partially stabilized zirconia-ceria solid solution is used for the manufacture of ceramic heater elements. It was found that for obtaining a ceramic heater element having improved thermal conductivity, the ceria content of the zirconia-ceria solid solution in - the mixture of particles is between 3 to 10 vol.%. The ceramic heater element of this embodiment is made of 33-50 vol.% molybdenum disilicide and 50-67 vol.%
partially stabilized zirconia-ceria solid solution. The partic~e size in the mixture is usually less than 5~m.
The mixture is cast or compacted into a desired shape and sintered in argon or similar inert gas at high temperatures. The more preferred average size of the particles of compounds making up the molybdenum disilicide-stabilized zirconia-ceria ceramic mixture for the manufacture of ceramic heaters is less than 3~m.
It has been observed that partially stabilized solid solutions of zirconia-ceria may be obtained in the absence of other stabilizing oxides. It is believed, however, this argument is not considered to be binding, that in such instances a portion of the ceria is present in the crystal lattice in its tri-valent form, thereby creating vacancies and stabilizing the high temperature form of zirconia.
In yet another embodiment of the present invention a portion of zirconia making up the mixtures 2 1 ~
-as described above, is added in the form of zirconia fibres, preferably in a partially stabilized form.
The particles of molybdenum disilicide and zirconia or zirconia-ceria may be obtained by any conven~ional method, such as sol/gel coprecipitation or by other commercial processes for the production of fine particles. For best results it is preferable that the particles of partially stabilized zirconia or zirconia-ceria solid solution are obtained in a first step and are mixed with particles of molybdenum disilicide in a subsequent step. It is possible, however, that particles of molybdenum disilicide, zirconia or zirconia-ceria solid solution, or even zirconia and ceria in the appropriate ratio, and particles of the appropriate amount of stabilizing di- or tri-valent oxide are mixed in a single step, preferably in similar particle size of less than 5~m. The mixture of particles having the desired composition is subsequently wet or dry milled in conventional manner, for a prolonged period to obtain an intimate mixture, wherein the component particles have a smaller average size, preferably less than 3~m. If so desired, zirconia fibres may be added to the intimate mixture obtained by milling. No binders, either organic or inorganic, are normally required, but such may be added if so desired.
The mixture obtained may be subsequently cast, compacted or extruded into any required three dimensional shape to be utilized as a heater element in an electric furnace.
Any conventional method for obtaining a heater element of a desired shape or configuration may be applied.
There is no particular shape or configuration which is preferred, although it is usual practice that ceramic heater elements utilized in resistance heating have elongated shapes, such as for example, a bar having a circular cross-section. Such ceramic heater element is shown on Fig 1. It is common practice that a conventional heater element has length which is at least - _ 21~ 1613 3 times its diameter or width; that is, the aspect ratio of the elongated body is greater than 3.
Cylindrically shaped heaters may also be made to be used either in resistance heating or for induction heating.
The obtained "green" heater elements of the desired shape are allowed to dry in a conventional manner if produced by wet milling. The l'greenll ceramic heater elements may be presintered if so desired, and subsequently fired or sintered in an inert gas atmosphere, at temperatures above 1400 C. The length of the time period for sintering depends on the sintering temperature, but it is usual that the final sintering is conducted at above 1700 C for at least 2 hours in argon atmosphere. The sintered ceramic heater elements are subsequently annealed in air for about 30 min. at 9oo to 1500 C.
The components of the mixture in sintered ceramic heater element were found to have average grain size less than lO~m, and in the more preferred embodiment less than 6~m. It is believed that a grain is usually formed by the sintering of one to three particles of the same composition together. It is further believed that the desired mechanical strength and stability of the ceramic heater of the present invention is attained by having small particle sizes in the mixture prior to sintering, forming relatively small grains during the subsequent sintering step, which are likely to more readily accommodate the differences in the coefficients of expansion of the components. The small average grain size and the homogenous mixture of the components are likely to provide uniform and controllable electrical conductivity of the sintered ceramic heater element between room temperature and close to 2000~C.
It was found that the sintered ceramic heater elements having composition as stated above and having 215~613 been made as described above, exhibited desirable thermal conductance, mechanical strength and stability.
It is known that molybdenum disilicide may oxidize to volatile molybdenum oxides and silica at temperatures higher than 1500 C, leading to deterioration in the heater elements made substantially of molybdenum disilicide. It is thus desirable that a ceramic heater element containing molybdenum disilicide should have some means to retard the high temperature oxidation of the molybdenum disilicide therein. It has now been unexpectedly found that ceramic heater elements made of mixtures of molybdenum disilicide and partially stabilized zirconia or partially stabilized zirconia-ceria solid solutions in the composition ranges described above having average particle size which is less than 5~m, and sintering the obtained mixture in argon or similar inert gas atmosphere at temperatures above 1500 C and subsequently annealing the heater elements in air, will generate in-situ during the heat treatment a substantially zirconia containing, generally coherent layer or skin on the surface of the ceramic heater element. It is believed that the substantially coherent zirconia based layer or skin imparts protection from high temperature oxidation and further contributes to the mechanical strengh of the ceramic heater element made according to the present invention. The term in-situ is understood to mean that the layer or skin is formed on the surface of the ceramic heater element during sintering and subsequent annealing, in the absence of a separate coating step for the deposition of a layer, either prior or subse~uent to sintering. For the sake of clarity, the substantially zirconia or zirconia-ceria based layer or skin formed in-situ during the sintering and annealing treatment of the ceramic mixture will be referred to hereinbelow as a continuous zirconia or zirconia-ceria containing coating.
The ceramic heater made according to the 21~ 161~
present invention is provided with metallic leads for incorporating the heater element in an electrical circuit, that is adapted to having power applied between its end-portions for the passage of an electrical current. The metallic leads may be embedded in the opposing end-portions of the elongated ceramic heater structure before firing or may be affixed by other conventional methods, such as vapour deposition, electroless deposition, metallizing by plasma treatment and similar surface metallizing methods. If desired, heavier electrical leads, such as wires or sheets, may be additionally brazed or soldered to the metallized surface, or attached by applying contact pressure such as crimping.
A mixture was made of particles of zirconia partially stabilized with 5.1 mole% yttria, and molybdenum disilicide. The MoSi2 ratio to ZrO2 in the mixture was 40:60 by volume. The size of the particles in the mixture was about 5~m. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 18 hours. The particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting milled mixture was isostatically pressed at 20,000 psi, into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently sintered at 1800 C in argon for 2 hours and annealed at 1400 C in air for 20 min.
Each end of each bar was metallized by nickel coating deposited in 1.5 cm (5/8") wide metallic bands, by electroless deposition. A sheet of copper of similar size as the nickel deposit was tightly crimped over the metallized surface areas by conventional means, to ensure good mechanical and electrical contact. The heater element obtained is schematically shown on Fig.l, where 10 is the ceramic element and 12 represents the metallized and crimped end connections.
~ ~4~ ~ 3 The heater bars, cast and compacted by isostatic pressing, were observed to be of dark brown colour be~ore sintering, reflecting the dark colour of the molybdenum disilicide in the mixture. Subsequent to the high temperature sintering of the bars in an inert gas and annealing in air, the presence of a coherent whitish layer formed in-situ during the heat treatment was noted on the surface of the bars.
A sintered ceramic heater bar made as described above was sectioned and subjected to EDAX
diffraction to analyze the composition of the bar. An EDAX diffractogram taken at the core or centre of the bar is shown on Fig.2/a, indicating the presence of zirconium, molybdenum and silicon. For the sake of clarity, the presence of oxygen is not recorded by EDAX
measurements, but it is taken that zirconium is present as zirconium dioxide and the molybdenum and silicon are in the form of molybdenum disilicide. The average particle size of the grains in the section were measured to be 5.8~m. Fig.2/b shows the EDAX diffractogram obtained on a section of the surface of the sintered ceramic heater bar. It can be seen that the surface consists predominantly of zirconium, with minor amounts of silicon and molybdenum also being present. It is likely but cannot be shown be EDAX analysis, that the silicon is present at least in part as silica and the molybdenum at least in part as molybdenum oxide. A SE~
picture taken of a section of the sintered ceramic heater bar close to its surface is shown on Fig.3. It can be seen that the coherent, predominantly zirconia layer formed on the surface of the ceramic heater during sintering and annealing is 25-50~m thick.
The heater bars made as described above, were subsequently connected to an electrical power source.
The voltage applied between the metallized ends was such that 35 amp. heating current was passing through the heater elements. The temperature of the bar, measured : ;j ~ ~ ~ 4 ~ 1 ~
_ - - 16 -pyrometrically, was steadily maintained at 1580 C for 3 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
A mixture was made of particles of zirconia partially stabilized with 5.1 mole% yttria, and molybdenum disilicide. The MoSi2 ratio to ZrO2 in the mixture was 35:65 by volume. The size of the particles in the mixture was around 5~m. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 18 hours. The particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting mixture was isostatically pressed at 20,000 psi into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently sintered at 1800 C in argon for 2 hours, and annealed in air at 1400 C for 20 min. The sintered heater bar had a white coating layer as observed in Example 1.
The ends of each bar obtained were metallized with a copper layer and had additional nickel sheets crimped over them, for incorporation in the electrical circuit of a furnace. The heater bars were similar to that shown on Fig.1.
The heater bars were subsequently connected to an electrical power source. The voltage applied between the metallized ends was such that 36 amp. heating current was passing through the heater elements. The temperature of the bar, measured pyrometrically, was steadily maintained at 1600 C for 3 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
~-;
~4~ 1 3 1 A mixture was made of particles of zirconia-ceria solid solution partially stabilized with 5.1 mole~6 yttria, and molybdenum disilicide. The zirconia-ceria 5 solid solution contained 6 vol.% ceria. The MoSi2 ratio to ZrO2-CeO2 in the mixture was 36:64 by volume. The size of the particles in the mixture was initially around 5,um. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 20 hours. The 10 particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting mixture was isostatically pressed at 20,000 psi into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently 15 sintered at 1780 C in argon for 2 hours and annealed in air at 1360~C for 20 min. The sintered bars were found to have a coherent white coating layer, simialar to the sintered bars of Example 1.
The ends of each bar obtained were metallized 20 with a copper layer and had additional nickel sheets crimped over them, for incorporation in the electrical circuit of a furnace. The heater bars were similar to that shown on Fig.1.
The heater bars obtained were subsequently 25 connected to an electrical power source. The voltage applied between the metallized ends was such that 35 amp. heating current was passing through the heater elements. The temperature of the bar, measured pyrometrically, was steadily maintained at 1620 C for 3 30 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
There was no noticeable difference in 35 performance between the heater bars of Example 2 and Example 3, it is, however, estimated that the heater bars made as described in Example 3, will have a longer " ., ~ ~ 5 ~ ~ ~ 3 _, useful life.
It has thus been shown that the heater element manufactured as described hereinabove, is capable of serving as a reliable fully ceramic heater element when fitted and connected in an electric furnace. The furnace may be of conventional design, having conventional insulation and conventional means for providing and controlling electric power for heating and a heater space therein. The ceramic heater elements of the present invention are usually arranged to enclose the heating space in the customary manner.
The ceramic heater element made according to this invention advantageously combines the electrical properties of molybdenum disilicide with that of partially stabilized zirconia. Moreover, the mechanical properties of partially stabilized zirconia advantageously reinforce the mechanical strength of molybdenum disilicide at higher temperatures. These features will allow an electrical furnace incorporating the ceramic heater element to operate reliably and without additional preheater requirements, between room temperature and temperatures as high as 2000 C.
The elements have no embedded metallic wire or any separate metallic component inside the ceramic body.
As mentioned above, metallic terminals for connection may be embedded at the extremities of the heater element. The ceramic heater elements obtained by the above process are capable of operating for a prolonged period in oxidizing atmosphere, that is in air.
It is to be noted that no specific design features are needed to accommodate high initial resistance characteristics, which would be the case when the heater element is made of substantially zirconia only. The heater elements described hereinabove are specially suited to be incorporated as resistance heater components, irrespective of shape.
The ceramic composition prepared in accordance f~
= . . ~
DISILICIDE AND ZIRCONIA COMPOSITION
FIELD OF INVENTION
This invention relates to ceramic heating elements, more particularly zirconia based heating elements, which are capable of operating at medium and high temperatures, such as above 1500 C.
~K~ROUND OF THE INVENTION
The advanced materials utilized by modern industry often require prolonged heat treatment at temperatures well over 1300 C. There are electrical resistance furnaces commercially available which can provide temperatures above 1300 C having heater coils made of high melting point metals, such as tantalum, tungsten, molybdenum and their alloys. Other high temperature furnaces have graphite heating elements.
However, the above heater element materials cannot operate in oxidizing atmospheres and the furnace design must include special jackets for maintaining neutral or reducing atmospheres or vacuum. If oxygen or air is allowed to come into contact with the heater elements made of the above substances during operation, serious damage to the furnace may occur.
~eater elements and coils made of Nichrome or Kanthal wire can operate in air but cannot provide temperatures above 1200-1300 C. Platinum metal melts at 1660 C and although Pt-Rh alloy has a higher melting point, these are highly expensive heater materials for use in typical commercial applications. Usually the metallic heater component is either wound onto or is otherwise supported or enclosed by an electrically non-conductive ceramic body that is comprised within the heater element as a separate structural component.
Molybdenum disilicide is a ceramic substance which is oxidation and corrosion resistant at relatively 2 1 ~ 4 6 1 ~
high temperatures. Molybdenum disilicide differs from most ceramic substances by exhibiting electrical conductivity similar to metals. Molybdenum disilicide has been used in the manufacture of heater element contacts and heater elements, such that is described, for example in U.S. 2,622,304 issued to L.W.Coffer on Dec. 23 1952. Unsupported molybdenum disilicide heater elements however, are known to lose mechanical strength and shape retention above 1400 C. Another electrically conductive ceramic substance is silicon carbide.
Silicon carbide heaters may be able to operate above 1300 C in air, however such heaters become very sensitive to mechanical impact at high temperatures, and have therefore a short life span. Mixtures of silicon carbide and molybdenum disilicide are also known.
Another known substance, lanthanum chromate shows good electrical conductivity and stability at medium high temperatures, but is a relatively expensive ceramic material, and it also tends to have a volatile by-product which may contaminate the furnace atmosphere.
Zirconia (ZrO2) is stable at high temperature, melts around 2600 C and is a good electrical conductor at temperatures above 1100 C. Zirconia has monoclinic crystal structure below llO0 C. Monoclinic zirconia has high fracture toughness, but it is known to lose its mechanical strength above its transition temperature.
Monoclinic zirconia is also known to be a poor electrical conductor In its commercial applications zirconia is usually partially or fully stabilized to retain its desirable mechanical properties in a wider temperature range, by oxides of the alkaline earth metals, yttria or oxides of the rare earth metals. It is known that the addition of a lower valent metal oxide to zirconia for the stabilization of its tetragonal or cubic crystal structure, will increase the ionic conductivity of zirconia, however, stabilization will not substantially increase the electrical conductivity ' - ~lS4613 at temperatures below 800 C.
A heater element made of zirconia stabilized with 3-6% lime or magnesia, operating in cooperation with molybdenum metal rings and coil connectors, is described in U.S. 2,680.771 issued to S.S.Kistler on June 8 1954. In other heater element applications the low electrical conductivity of zirconia at low and medium high temperatures may be compensated by the heater element having specially designed configuration and/or preheaters. Such heater elements are usually made of partially or fully stabilized zirconia with no other admixed ceramic component. An electric furnace having a platinum alloy preheater operated as a separate heater element in conjunction with a zirconia or any other ceramic rod heater, is described in Japanese Patent 4-43588, issued to Mitsubishi Heavy Ind. Ltd. on Feb.13, 1992.
U.S. patents 5,073,689 and 5,154,785 issued to Tabata et al. on December 17, 1991 and October 13, 1992, respectively, describe zirconia heating elements made of 2 irconia reinforced with zirconia fibres. The zirconia powder and fibres utilized in the heater elements of Tabata et al. contain stabilizing oxides, various organic binders and other additives to improve the mechanical and electrical properties of zirconia heater elements produced. Moreover, the heater elements of Tabata et al. have special geometric design features, lead members, fabrication and impregnation steps so that the inherent high electrical resistance of zirconia at low temperatures may be diminished. The various additional process steps are likely to increase the production costs of the heater elements of Tabata et al.
Ceramic materials utilized in the manufacture of ceramic heaters and other high temperature resistant sintered ceramic articles, are known to contain molybdenum disilicide admixed with refractory oxides, usually for providing increased mechanical strength to 21~13 the article. British patent 725,577, published on March 9, 1955 and corresponding Canadian patent 612,139, issued on Jan.10, 1961, to Johnson Matthey and Mallory, describe ceramic articles made of molybdenum disilicide mixed with a refractory oxide, such as zirconia or alumina. The above British and Canadian patents teach in particular, ceramic compositions containing in various amounts MoSi2, Al2O3 and CaO as fluxing agent for the manufacture of engine chamber parts, and similar sintered articles required to have high wear and oxidation resistance. It is to be noted that alumina is a very good electrical insulator material. F.R.Charvat in U.S. 2,998,394 issued on Aug 29, 1961, describes electrical resistors made of molybdenum disilicide and clay. Lenie et al. in U.S. 3,108,887 issued on Oct. 29, 1963, describe erosion and corrosion resistant articles - having high electrical resistivity, made of mixtures of aluminum nitride and other refractory substances, such as molybdenum disilicide, zirconium oxide and the rare earth metal oxides. The oxide or silicide compound is added to improve the densification of the ceramic article made predominantly of aluminum nitride. Kaneko et al. in U.S. 4,907,015, issued on March 6, 1990, describe a heating element for a thermal printing head comprising an intermetallic compound, such as for example molybdenum disilicide, and an electrically insulating material, such as zirconia. Petrovic et al.
in U.S. 5,063,182, issued on Nov.5, 1991, describe a zirconia-molybdenum disilicide containing ceramic composition for the manufacture of articles required to have high fracture toughness. Japanese patent 63-257206 is directed to a slidPr material containing molybdenum disilicide and zirconia, to be utilized in a thin film magnetic head. It is of significance that in all of the above ceramic mixtures containing molybdenum disilicide and a refractory oxide, in particular zirconium dioxide, is added with the intention to increase the mechanical 21~13 strength and electrical resistance of the articles made of the mixture, and not with the object of improving the electrical conductivity of the mixture.
Some other heater related applications utilizing molybdenum disilicide are briefly reviewed below. In U.S. Patent 3,328,201 issued on June 27, 1967 to H.G. Scheible, a ceramic coating containing molybdenum disilicide and alumina, is provided to cover an alumina body enclosing a metallic heater wire. The device is utilized as a heater in an electron tube.
Japanese patent 2-227981 issued to Shinagawa Refract Co Ltd. on Nov. 9, 1990 describes a molybdenum disilicide heater rod protected from oxidation by a sheet of zirconia, which is laminated onto the molybdenum disilicide rod in a separate fabrication process step.
Kieffer et al. in U.S. 2,831,242 issued on Apr. 22, 1958, describe electric resistance heater elements made of molybdenum disilicide and molybdenum aluminide intermetallic compound, additionally containing refractory oxides, such as zirconia to increase the resistivity of the heater.
The heater elements having compositions as described in the above discussed publications, do not appear to have wide commercial success. There is a need for a relatively inexpensively produced ceramic heater element having a composition which combines the electrical conductivity of molybdenum disilicide at low and medium temperature range with the mechanical strength and electrical conductivity of partially stabilized zirconia, which is oxidation resistant at high temperatures, and which needs no preheater or special configuration in shape.
STAT~HENT OF INVENTION
An improved ceramic mixture for a sintered heater element made of particles of molybdenum disilicide and an oxide for reinforcing the mechanical strength of the molybdenum disilicide contained therein 2154~ 1 3 has been found. The improvement in the ceramic heater element comprises that the oxide in the sintered mixture is zirconia partially stabilized with an oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said mixture in 33-50 vol.~, the balance being zirconia, the average grain size of the molybdenum disilicide and the zirconia in said sintered mixture being less than 10~m, and that the ceramic heater element made of said sintered mixture has a continuous coating containing substantially zirconia having thickness in excess of 10~m generated in-situ, the improvement further comprises that the electrical conductivity of said heater element made of said mixture is greater than 0.1 (ohm.cm)-l and less than 10 (ohm.cm)~l in the temperature range 100-2000~C.
In another embodiment of the ceramic heater element the balance of the molybdenum disilicide containing sintered mixture is partially stabilized zirconia-ceria solid solution.
In yet another embodiment of the ceramic heater element a portion of the zirconia is present in the mixture in the form of zirconia fibres.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig.1 is a schematic representation of a typical heating element manufactured from the sintered mixture of the present invention.
Fig.2/a and 2/b are EDAX diffractograms taken at the core and at the surface, respectively, of the sintered ceramic heater element, and Fig.3 is a SEM photograph of the cross-section of the sintered ceramic heater element near the surface.
The preferred embodiments of the invention will be described hereinbelow and illustrated by the Figures in the examples.
215~ 13 DErATT.~.n DISCUSSION OF THE ~K~KKED EMBODIMENTS
As has been discussed above, the high temperature form of zirconia is known to exhibit electrical conductivity which renders it suitable for use as a heater element. The heat in the heater element may be generated either by resistance to an electric current passing through the zirconia element, or by induction. The high temperature cubic or tetragonal structure of zirconia may be retained at temperatures below the transition temperature by the addition of stabilizing oxides, which form solid solutions with the zirconia. It is known that zirconia forms solid solutions with di-valent alkaline earth metal oxides, such as magnesia, calcia and strontia, and with yttria and oxides of tri-valent rare earth metals. The rare earth metals are usually considered to be encompassing the lanthanum group metals including cerium and yttrium.
However, for the sake of emphasis, yttria is singled out here as a more commonly used stabilizing oxide, but oxides of other tri-valent rare earth metals may also be suitable in stabilizing the high temperature form of zirconia. Partially stabilized zirconia solid solutions may be formed when the stabilizing oxide concentration is above 3 mole%. Full stabilization of zirconia may be usually attained when the stabilizing oxide is present in the solid solution in excess of 7.5 mole%, depending on the stabilizing oxide. Fully stabilized zirconia exhibits predominantly ionic conductivity, the conductivity usually being greater than 10~1(ohm.cm)~l at temperatures higher than 1100 C. The conductivity of partially stabilized zirconia is expected to be one order of magnitude less. It is, however, known that the total conductivity of both partially and fully stabilized zirconia is less than 10~3(ohm.cm)~l at temperatures below 600 C, hence for obtaining a ceramic substance that may be utilized in manufacturing a ceramic heater, it is desirable that the conductivity be - 21~461~
increased in the lower temperature range by admixing with the zirconia a ceramic substance which is a good electronic conductor, such as molybdenum disilicide.
Molybdenum disilicide behaves more like a metallic conductor and has conductivity greater than lO(ohm.cm)~
in this temperature range.
There are other physical characteristics that need to be taken into consideration in selecting components for a mixture of ceramic substances suitable for producing ceramic heater elements, such as thermal conductivity, coefficient of thermal expansion, shape-retention, sinterability, mechanical strength and such like. As mentioned above, ceramic articles for use at high temperatures, made of molybdenum disilicide and a refractory oxide do not appear to enjoy great commercial success. It is suggested here, without in any way being bound by this argument, that mismatch of physical properties of the components, such as for example the coefficient of thermal expansion, may be responsible for the lack of commercial success. It is known that the thermal coefficient of expansion of partially stabilized zirconia is lower than that of fully stabilized zirconia. Furthermore, the fracture toughness and thermal shock resistance of partially stabilized zirconia are known to be higher than the fracture toughness and thermal shock resistance of fully stabilized zirconia. Thus although the electrical conductivity of partially stabilized zirconia is lower than the electrical conductivity of fully stabilized zirconia, for the purposes of manufacturing a ceramic heater element a mixture containing molybdenum disilicide and partially stabilized zirconia is preferred. It has now been observed that mismatch of the physical properties of the components may be substantially reduced if the components of the mixture are of similar fine particle size and are intimately mixed. In other words, it has been surprisingly found g that the particle size plays an important role in governing the electrical and mechanical properties of the ceramic heater element manufactured from such an intimate mixture. It has also been found that the S composition range allowing the combination of electrical and mechanical properties desirable in a ceramic heater element, lies between 33-50 vol.% molybdenum disilicide and 50-67 vol.% partially stabilized zirconia.
Furthermore, it has been found that for best results, the particle size of the particles making up the mixture should be less than 5~m. The even more preferred composition range of the intimate mixture was found to be 35-42 vol.% molybdenum disilicide and 58-65 vol.%
partially stabilized zirconia, having particles size less than 3~m.
The stabilizing oxide which, as discussed above, may be calcia, strontia, yttria or similar tri-valent rare earth metal oxide, is usually added in amounts between 3.5 to 7.0 mole% to partially stabilize the zirconia in its electrically conductive structure.
It is to be noted, that it is customary that the composition of commercially manufactured ceramic articles is expressed in vol.% or in weight%. It is however, acceptable and usual to refer to the amount of stabilizing oxide added to zirconia in mole%, since different oxides when added at the same mole% rate would be calculated to yield different vol.% values.
As mentioned above, good thermal conductivity is a property which is desirable in a ceramic component for manufacturing a ceramic heater element. Good thermal conductivity will lead to a more efficient heater element, and it may also prolong the expected useful life of the heater element. The thermal conductivity of partially stabilized zirconia is known to have values of about 8.5 10-3 cal.sec~l.cm~2/~C.cm~l at 100 C. Cubic ceria has been measured to exhibit thermal conductivity of about 21.6 10-3 cal.sec~l.cm~2/ C.cm~l at 21~4613 the same temperature. It is to be noted that ceria is normally found in its tetra-valent form and as such it has crystal structure similar to that of zirconia.
Ceria may also be stabilized in its cubic structure by the addition of alkaline earth metal oxides, yttria and oxides of tri-valent rare earth metals, and is known to be a good electrical conductor at medium and high temperatures. Moreover, stabilized ceria is known to form solid solutions with zirconia in a fairly wide range. In a another embodiment of the present invention a mixture of particles of molybdenum disilicide and particles of partially stabilized zirconia-ceria solid solution is used for the manufacture of ceramic heater elements. It was found that for obtaining a ceramic heater element having improved thermal conductivity, the ceria content of the zirconia-ceria solid solution in - the mixture of particles is between 3 to 10 vol.%. The ceramic heater element of this embodiment is made of 33-50 vol.% molybdenum disilicide and 50-67 vol.%
partially stabilized zirconia-ceria solid solution. The partic~e size in the mixture is usually less than 5~m.
The mixture is cast or compacted into a desired shape and sintered in argon or similar inert gas at high temperatures. The more preferred average size of the particles of compounds making up the molybdenum disilicide-stabilized zirconia-ceria ceramic mixture for the manufacture of ceramic heaters is less than 3~m.
It has been observed that partially stabilized solid solutions of zirconia-ceria may be obtained in the absence of other stabilizing oxides. It is believed, however, this argument is not considered to be binding, that in such instances a portion of the ceria is present in the crystal lattice in its tri-valent form, thereby creating vacancies and stabilizing the high temperature form of zirconia.
In yet another embodiment of the present invention a portion of zirconia making up the mixtures 2 1 ~
-as described above, is added in the form of zirconia fibres, preferably in a partially stabilized form.
The particles of molybdenum disilicide and zirconia or zirconia-ceria may be obtained by any conven~ional method, such as sol/gel coprecipitation or by other commercial processes for the production of fine particles. For best results it is preferable that the particles of partially stabilized zirconia or zirconia-ceria solid solution are obtained in a first step and are mixed with particles of molybdenum disilicide in a subsequent step. It is possible, however, that particles of molybdenum disilicide, zirconia or zirconia-ceria solid solution, or even zirconia and ceria in the appropriate ratio, and particles of the appropriate amount of stabilizing di- or tri-valent oxide are mixed in a single step, preferably in similar particle size of less than 5~m. The mixture of particles having the desired composition is subsequently wet or dry milled in conventional manner, for a prolonged period to obtain an intimate mixture, wherein the component particles have a smaller average size, preferably less than 3~m. If so desired, zirconia fibres may be added to the intimate mixture obtained by milling. No binders, either organic or inorganic, are normally required, but such may be added if so desired.
The mixture obtained may be subsequently cast, compacted or extruded into any required three dimensional shape to be utilized as a heater element in an electric furnace.
Any conventional method for obtaining a heater element of a desired shape or configuration may be applied.
There is no particular shape or configuration which is preferred, although it is usual practice that ceramic heater elements utilized in resistance heating have elongated shapes, such as for example, a bar having a circular cross-section. Such ceramic heater element is shown on Fig 1. It is common practice that a conventional heater element has length which is at least - _ 21~ 1613 3 times its diameter or width; that is, the aspect ratio of the elongated body is greater than 3.
Cylindrically shaped heaters may also be made to be used either in resistance heating or for induction heating.
The obtained "green" heater elements of the desired shape are allowed to dry in a conventional manner if produced by wet milling. The l'greenll ceramic heater elements may be presintered if so desired, and subsequently fired or sintered in an inert gas atmosphere, at temperatures above 1400 C. The length of the time period for sintering depends on the sintering temperature, but it is usual that the final sintering is conducted at above 1700 C for at least 2 hours in argon atmosphere. The sintered ceramic heater elements are subsequently annealed in air for about 30 min. at 9oo to 1500 C.
The components of the mixture in sintered ceramic heater element were found to have average grain size less than lO~m, and in the more preferred embodiment less than 6~m. It is believed that a grain is usually formed by the sintering of one to three particles of the same composition together. It is further believed that the desired mechanical strength and stability of the ceramic heater of the present invention is attained by having small particle sizes in the mixture prior to sintering, forming relatively small grains during the subsequent sintering step, which are likely to more readily accommodate the differences in the coefficients of expansion of the components. The small average grain size and the homogenous mixture of the components are likely to provide uniform and controllable electrical conductivity of the sintered ceramic heater element between room temperature and close to 2000~C.
It was found that the sintered ceramic heater elements having composition as stated above and having 215~613 been made as described above, exhibited desirable thermal conductance, mechanical strength and stability.
It is known that molybdenum disilicide may oxidize to volatile molybdenum oxides and silica at temperatures higher than 1500 C, leading to deterioration in the heater elements made substantially of molybdenum disilicide. It is thus desirable that a ceramic heater element containing molybdenum disilicide should have some means to retard the high temperature oxidation of the molybdenum disilicide therein. It has now been unexpectedly found that ceramic heater elements made of mixtures of molybdenum disilicide and partially stabilized zirconia or partially stabilized zirconia-ceria solid solutions in the composition ranges described above having average particle size which is less than 5~m, and sintering the obtained mixture in argon or similar inert gas atmosphere at temperatures above 1500 C and subsequently annealing the heater elements in air, will generate in-situ during the heat treatment a substantially zirconia containing, generally coherent layer or skin on the surface of the ceramic heater element. It is believed that the substantially coherent zirconia based layer or skin imparts protection from high temperature oxidation and further contributes to the mechanical strengh of the ceramic heater element made according to the present invention. The term in-situ is understood to mean that the layer or skin is formed on the surface of the ceramic heater element during sintering and subsequent annealing, in the absence of a separate coating step for the deposition of a layer, either prior or subse~uent to sintering. For the sake of clarity, the substantially zirconia or zirconia-ceria based layer or skin formed in-situ during the sintering and annealing treatment of the ceramic mixture will be referred to hereinbelow as a continuous zirconia or zirconia-ceria containing coating.
The ceramic heater made according to the 21~ 161~
present invention is provided with metallic leads for incorporating the heater element in an electrical circuit, that is adapted to having power applied between its end-portions for the passage of an electrical current. The metallic leads may be embedded in the opposing end-portions of the elongated ceramic heater structure before firing or may be affixed by other conventional methods, such as vapour deposition, electroless deposition, metallizing by plasma treatment and similar surface metallizing methods. If desired, heavier electrical leads, such as wires or sheets, may be additionally brazed or soldered to the metallized surface, or attached by applying contact pressure such as crimping.
A mixture was made of particles of zirconia partially stabilized with 5.1 mole% yttria, and molybdenum disilicide. The MoSi2 ratio to ZrO2 in the mixture was 40:60 by volume. The size of the particles in the mixture was about 5~m. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 18 hours. The particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting milled mixture was isostatically pressed at 20,000 psi, into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently sintered at 1800 C in argon for 2 hours and annealed at 1400 C in air for 20 min.
Each end of each bar was metallized by nickel coating deposited in 1.5 cm (5/8") wide metallic bands, by electroless deposition. A sheet of copper of similar size as the nickel deposit was tightly crimped over the metallized surface areas by conventional means, to ensure good mechanical and electrical contact. The heater element obtained is schematically shown on Fig.l, where 10 is the ceramic element and 12 represents the metallized and crimped end connections.
~ ~4~ ~ 3 The heater bars, cast and compacted by isostatic pressing, were observed to be of dark brown colour be~ore sintering, reflecting the dark colour of the molybdenum disilicide in the mixture. Subsequent to the high temperature sintering of the bars in an inert gas and annealing in air, the presence of a coherent whitish layer formed in-situ during the heat treatment was noted on the surface of the bars.
A sintered ceramic heater bar made as described above was sectioned and subjected to EDAX
diffraction to analyze the composition of the bar. An EDAX diffractogram taken at the core or centre of the bar is shown on Fig.2/a, indicating the presence of zirconium, molybdenum and silicon. For the sake of clarity, the presence of oxygen is not recorded by EDAX
measurements, but it is taken that zirconium is present as zirconium dioxide and the molybdenum and silicon are in the form of molybdenum disilicide. The average particle size of the grains in the section were measured to be 5.8~m. Fig.2/b shows the EDAX diffractogram obtained on a section of the surface of the sintered ceramic heater bar. It can be seen that the surface consists predominantly of zirconium, with minor amounts of silicon and molybdenum also being present. It is likely but cannot be shown be EDAX analysis, that the silicon is present at least in part as silica and the molybdenum at least in part as molybdenum oxide. A SE~
picture taken of a section of the sintered ceramic heater bar close to its surface is shown on Fig.3. It can be seen that the coherent, predominantly zirconia layer formed on the surface of the ceramic heater during sintering and annealing is 25-50~m thick.
The heater bars made as described above, were subsequently connected to an electrical power source.
The voltage applied between the metallized ends was such that 35 amp. heating current was passing through the heater elements. The temperature of the bar, measured : ;j ~ ~ ~ 4 ~ 1 ~
_ - - 16 -pyrometrically, was steadily maintained at 1580 C for 3 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
A mixture was made of particles of zirconia partially stabilized with 5.1 mole% yttria, and molybdenum disilicide. The MoSi2 ratio to ZrO2 in the mixture was 35:65 by volume. The size of the particles in the mixture was around 5~m. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 18 hours. The particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting mixture was isostatically pressed at 20,000 psi into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently sintered at 1800 C in argon for 2 hours, and annealed in air at 1400 C for 20 min. The sintered heater bar had a white coating layer as observed in Example 1.
The ends of each bar obtained were metallized with a copper layer and had additional nickel sheets crimped over them, for incorporation in the electrical circuit of a furnace. The heater bars were similar to that shown on Fig.1.
The heater bars were subsequently connected to an electrical power source. The voltage applied between the metallized ends was such that 36 amp. heating current was passing through the heater elements. The temperature of the bar, measured pyrometrically, was steadily maintained at 1600 C for 3 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
~-;
~4~ 1 3 1 A mixture was made of particles of zirconia-ceria solid solution partially stabilized with 5.1 mole~6 yttria, and molybdenum disilicide. The zirconia-ceria 5 solid solution contained 6 vol.% ceria. The MoSi2 ratio to ZrO2-CeO2 in the mixture was 36:64 by volume. The size of the particles in the mixture was initially around 5,um. The mixture was milled in a vibromill in the presence of iso-propyl alcohol for 20 hours. The 10 particle size of the particles in the mixture subsequent to milling was found to be less than 3~m. The resulting mixture was isostatically pressed at 20,000 psi into 25 cm (10 in.) long bars, having 1.8 cm (3/4 in) diameter cross-section. The bars were subsequently 15 sintered at 1780 C in argon for 2 hours and annealed in air at 1360~C for 20 min. The sintered bars were found to have a coherent white coating layer, simialar to the sintered bars of Example 1.
The ends of each bar obtained were metallized 20 with a copper layer and had additional nickel sheets crimped over them, for incorporation in the electrical circuit of a furnace. The heater bars were similar to that shown on Fig.1.
The heater bars obtained were subsequently 25 connected to an electrical power source. The voltage applied between the metallized ends was such that 35 amp. heating current was passing through the heater elements. The temperature of the bar, measured pyrometrically, was steadily maintained at 1620 C for 3 30 hours. When the power was switched off the heater was allowed to cool to room temperature. The heater element was subjected to several heating and cooling cycles subsequently, without drop in performance.
There was no noticeable difference in 35 performance between the heater bars of Example 2 and Example 3, it is, however, estimated that the heater bars made as described in Example 3, will have a longer " ., ~ ~ 5 ~ ~ ~ 3 _, useful life.
It has thus been shown that the heater element manufactured as described hereinabove, is capable of serving as a reliable fully ceramic heater element when fitted and connected in an electric furnace. The furnace may be of conventional design, having conventional insulation and conventional means for providing and controlling electric power for heating and a heater space therein. The ceramic heater elements of the present invention are usually arranged to enclose the heating space in the customary manner.
The ceramic heater element made according to this invention advantageously combines the electrical properties of molybdenum disilicide with that of partially stabilized zirconia. Moreover, the mechanical properties of partially stabilized zirconia advantageously reinforce the mechanical strength of molybdenum disilicide at higher temperatures. These features will allow an electrical furnace incorporating the ceramic heater element to operate reliably and without additional preheater requirements, between room temperature and temperatures as high as 2000 C.
The elements have no embedded metallic wire or any separate metallic component inside the ceramic body.
As mentioned above, metallic terminals for connection may be embedded at the extremities of the heater element. The ceramic heater elements obtained by the above process are capable of operating for a prolonged period in oxidizing atmosphere, that is in air.
It is to be noted that no specific design features are needed to accommodate high initial resistance characteristics, which would be the case when the heater element is made of substantially zirconia only. The heater elements described hereinabove are specially suited to be incorporated as resistance heater components, irrespective of shape.
The ceramic composition prepared in accordance f~
= . . ~
4 ~ ~ 3 'J
with the process described hereinabove, may be utilized in manufacturing heater elements adapted to heating by current induced by a changing electromagnetic field;
that is the heater element is equally useful for induction heating. In this case, as a skilled person will be aware, there is no need for lead elements to be attached to the heater for incorporating the heater in the electric circuit. The heater elements utilized in induction heating are usually, but not necessarily, cylindrically shaped. However, some means have to be provided for supporting the heater element in a position wherein current and heat may be generated, as defined by the electromagnetic field.
The composition of the present invention may also be used to provide a continuous ceramic heating surface, such as is utilized in a high temperature isostatic press. The ceramic heating surface in such commercial application would replace the conventional embedded metallic heating elements and the heat could be directly generated.
Although the present invention has been described with reference to the preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.
with the process described hereinabove, may be utilized in manufacturing heater elements adapted to heating by current induced by a changing electromagnetic field;
that is the heater element is equally useful for induction heating. In this case, as a skilled person will be aware, there is no need for lead elements to be attached to the heater for incorporating the heater in the electric circuit. The heater elements utilized in induction heating are usually, but not necessarily, cylindrically shaped. However, some means have to be provided for supporting the heater element in a position wherein current and heat may be generated, as defined by the electromagnetic field.
The composition of the present invention may also be used to provide a continuous ceramic heating surface, such as is utilized in a high temperature isostatic press. The ceramic heating surface in such commercial application would replace the conventional embedded metallic heating elements and the heat could be directly generated.
Although the present invention has been described with reference to the preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.
Claims (15)
1. In a ceramic heater element made of a sintered mixture essentially composed of particles of molybdenum disilicide and particles of a first oxide added for reinforcing the mechanical strength of the molybdenum disilicide, the improvement comprising that said first oxide in the sintered mixture is zirconia, partially stabilized with a second oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said sintered mixture in 33-50 vol.%, the balance being zirconia, the average grain size of said molybdenum disilicide and said zirconia in said sintered mixture being less than 10µm, and that the ceramic heater element made of said sintered mixture has a continuous zirconia coating having thickness in excess of 10µm generated in-situ, the improvement further comprising that the electrical conductivity of said ceramic heater element made of said sintered mixture of particles is greater than 0.1 (ohm.cm)-1 and less than 10 (ohm.cm)-1 in the temperature range 100-1900°C.
2. A ceramic heater element made of a sintered mixture of particles as claimed in claim 1, wherein the vol.% of said molybdenum disilicide in said sintered mixture ranges between 35 and 42, and the average grain size of said molybdenum disilicide and said zirconia in said sintered mixture is less than 6µm.
3. A ceramic heater element made of a sintered mixture of particles as claimed in claim 1, wherein said balance of zirconia in said mixture further comprises zirconia fibres in 2 to 10 vol.%, and said zirconia fibres are partially stabilized.
4. A ceramic heater element made of a sintered mixture of particles as claimed in claim 1, wherein said zirconia particles in said mixture are partially stabilized with 3.5 to 7 mole% of said second oxide.
5. In a ceramic heater element made of a sintered mixture essentially composed of particles of molybdenum disilicide and particles of a first oxide added for reinforcing the mechanical strength of the molybdenum disilicide, the improvement comprising that said first oxide in the sintered mixture is zirconia-ceria solid solution, partially stabilized with a second oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said sintered mixture in 33-50 vol.%, the balance being zirconia-ceria solid solution, the average grain size of said molybdenum disilicide and said zirconia-ceria solid solution in said sintered mixture being less than 10µm, and that the ceramic heater element made of said sintered mixture has a continuous zirconia-ceria solid solution coating having thickness in excess of 10µm generated in-situ, the improvement further comprising that the electrical conductivity of said ceramic heater element made of said sintered mixture of particles is greater than 0.1 (ohm.cm)-1 and less than 10 (ohm.cm)-1 in the temperature range of 100-1900 C.
6. A ceramic heater element made of a sintered mixture of particles as claimed in claim 5, wherein the vol.% of said molybdenum disilicide in said sintered mixture ranges between 35 and 42, and the average grain size of said molybdenum disilicide and said zirconia-ceria solid solution in said sintered mixture is less than 6µm.
7. A ceramic heater element made of a sintered mixture of particles as claimed in claim 5, wherein said particles of zirconia-ceria solid solution in said mixture contain 3 to 10 vol.% ceria.
8. A ceramic heater element made of a sintered mixture of particles as claimed in claim 5, wherein said particles of zirconia-ceria solid solution in said mixture are partially stabilized with 3.5 to 7 mole% of said second oxide.
9. In a ceramic heater element made of a sintered mixture essentially composed of particles of molybdenum disilicide and particles of a first oxide added for reinforcing the mechanical strength of the molybdenum disilicide, the improvement comprising that said first oxide is zirconia-ceria solid solution partially stabilized with a second oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said mixture in 33-50 vol.%, the balance being fibres of zirconia and zirconia-ceria solid solution, the average grain size of said molybdenum disilicide and said zirconia-ceria solid solution being less than 10µm, and that the ceramic heater element made of said sintered mixture has a continuous zirconia-ceria coating having thickness in excess of 10µm generated in-situ, the improvement further comprising that the electrical conductivity of said ceramic heater element made of said sintered mixture is greater than 0.1 (ohm.cm)-1 and less than 10 (ohm.cm)-1 in the temperature range of 100-1900 C.
10. A ceramic heater element made of a sintered mixture as claimed in claim 9, wherein said zirconia fibres are present in said mixture in 2 to 10 vol.%, and said fibres are partially stabilized.
11. A ceramic heater element made of a sintered mixture as claimed in claim 9, wherein said particles of zirconia-ceria solid solution in said mixture contain 3 to 10 vol.% ceria.
12. A ceramic heater element made of a sintered mixture as claimed in claim 9, wherein said particles of zirconia-ceria solid solution are partially stabilized with 3.5 to 7 mole% of said second oxide.
13. In an electric furnace comprised of a furnace housing, means within said housing to support a heater element and means to connect said heater element to an electrical power source, and means to control said electrical power applied to said heater element, insulating means within said furnace housing, said insulating means defining a heater space within said furnace, means for having access to said heater space, and at least one ceramic heater element made of a sintered mixture essentially composed of particles of molybdenum disilicide and particles of a first oxide added for reinforcing the strength of the molybdenum disilicide, the improvement comprising that said first oxide in the sintered mixture is zirconia, partially stabilized with a second oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said sintered mixture in 33-50 vol.%, the balance being zirconia, the average grain size of said molybdenum disilicide and said zirconia in said sintered mixture being less than 10µm, and that the ceramic heater made of said sintered mixture has a continuous zirconia coating having thickness in excess of 10µm generated in-situ, the improvement further comprising that the electrical conductivity of said ceramic heater element made of said sintered mixture of particles is greater than 0.1 (ohm.cm)-1 and less than 10 (ohm.cm)-1 in the temperature range of 100-1900°C.
14. In an electric furnace comprised of a furnace housing, means within said housing to support a heater element and means to connect said heater element to an electrical power source, and means to control said electrical power applied to said heater element, insulating means within said furnace housing, said insulating means defining a heater space within said furnace, means for having access to said heater space, and at least one ceramic heater element made of a sintered mixture essentially composed of particles of molybdenum disilicide and particles of a first oxide added for reinforcing the mechanical strength of the molybdenum disilicide, the improvement comprising that said first oxide is zirconia-ceria solid solution partially stabilized with a second oxide of a metal selected from the group consisting of an alkaline earth metal, yttrium and a tri-valent rare earth metal, and that molybdenum disilicide is contained in said mixture in 33-50 vol.%, the balance being zirconia-ceria solid solution, the average grain size of said molybdenum disilicide and said zirconia-ceria solid solution being less than 10µm, and that the ceramic heater element made of said sintered mixture has a continuous zirconia-ceria coating having thickness in excess of 10µm generated in-situ, the improvement further comprising that the electrical conductivity of said ceramic heater element made of said sintered mixture of particles is greater than 0.1 (ohm.cm)-1 and less than 10 (ohm.cm)-1 in the temperature range of 100-1900°C.
15. An electric furnace as claimed in claim 14, wherein said sintered mixture of which said ceramic heater element is made of, further comprises partially stabilized zirconia fibres in 2 to 10 vol%.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US28236594A | 1994-07-29 | 1994-07-29 | |
| US08/282,365 | 1994-07-29 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2154613A1 CA2154613A1 (en) | 1996-01-30 |
| CA2154613C true CA2154613C (en) | 1998-10-27 |
Family
ID=23081186
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2154613 Expired - Fee Related CA2154613C (en) | 1994-07-29 | 1995-07-25 | Ceramic heater element with molybdenum disilicide and zirconia composition |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2154613C (en) |
-
1995
- 1995-07-25 CA CA 2154613 patent/CA2154613C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2154613A1 (en) | 1996-01-30 |
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