US20080265471A1 - Polycrystalline Sic Electrical Devices and Methods for Fabricating the Same - Google Patents

Polycrystalline Sic Electrical Devices and Methods for Fabricating the Same Download PDF

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US20080265471A1
US20080265471A1 US12/092,801 US9280106A US2008265471A1 US 20080265471 A1 US20080265471 A1 US 20080265471A1 US 9280106 A US9280106 A US 9280106A US 2008265471 A1 US2008265471 A1 US 2008265471A1
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ceramic body
sic
ceramic
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resin
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Curtis M. Colopy
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    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
    • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
    • C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
    • C04B35/571—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide obtained from Si-containing polymer precursors or organosilicon monomers
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    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/626—Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B
    • C04B35/62605—Treating the starting powders individually or as mixtures
    • C04B35/6269—Curing of mixtures
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/3217—Aluminum oxide or oxide forming salts thereof, e.g. bauxite, alpha-alumina
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/38—Non-oxide ceramic constituents or additives
    • C04B2235/3817—Carbides
    • C04B2235/3826—Silicon carbides
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    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/40—Metallic constituents or additives not added as binding phase
    • C04B2235/402—Aluminium
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/612—Machining
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/616—Liquid infiltration of green bodies or pre-forms
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
    • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/72—Products characterised by the absence or the low content of specific components, e.g. alkali metal free alumina ceramics
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    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/72—Products characterised by the absence or the low content of specific components, e.g. alkali metal free alumina ceramics
    • C04B2235/722—Nitrogen content
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/74—Physical characteristics
    • C04B2235/76—Crystal structural characteristics, e.g. symmetry
    • C04B2235/762—Cubic symmetry, e.g. beta-SiC
    • C—CHEMISTRY; METALLURGY
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/74—Physical characteristics
    • C04B2235/76—Crystal structural characteristics, e.g. symmetry
    • C04B2235/767—Hexagonal symmetry, e.g. beta-Si3N4, beta-Sialon, alpha-SiC or hexa-ferrites
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/74—Physical characteristics
    • C04B2235/77—Density
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N2227/00—Ignition or checking
    • F23N2227/42—Ceramic glow ignition

Definitions

  • the present invention relates to novel electrical devices fabricated from polycrystalline silicon carbide (SiC) and methods for forming the same. More specifically, the present invention provides a method for fabricating polycrystalline silicon carbide (SiC) products infiltrated with SiC-containing preceramic precursor resins to substantially mask the deleterious effects of trace contaminants, typically nitrogen and aluminum, while reducing operative porosity and enhancing manufacturing ease.
  • SiC polycrystalline silicon carbide
  • Spark ignition systems are conventionally known to make use of an electronic control and an electrode located near a metal burner port that is electrically grounded.
  • a control opens a gas-supply valve and simultaneously provides a high voltage discharge (typically 20 kilovolts) to the electrode, which then “sparks” to the burner. This spark, in the presence of gas, ignites the burner.
  • the controls used with SiC-based hot surface ignition operate differently.
  • electrical power is applied to the igniter.
  • a gas-supply valve is opened.
  • hot surface SiC igniters may have various physical shapes, for example, the SiC hot surface igniter shown in U.S. Pat. No. 3,875,477 to Fredriksson, et al., the contents of which are incorporated herein by reference.
  • the igniter noted in Fredriksson shall be understood to represent a thin-profile igniter having a first wide cross-section parallel to the plane shown in FIG. 1 in U.S. Pat. No. 3,875,477 and a second narrow cross-section perpendicular to the first cross-section.
  • this type of hot surface igniter provides a serpentine-form of electrical connection from two pole ends.
  • a central region between the pole ends, having a reduced cross-section, is more resistive to electrical conduction, heats rapidly to allow hot surface ignition.
  • SiC hot surface igniters Yet despite the recognition provided to SiC hot surface igniters, certain detriments remain. It is now recognized that the density of the SiC hot surface igniter is critical in determining its electrical stability and mechanical strength. These factors (electrical stability and mechanical strength) are the two most important factors in the user-perceived quality and operating life of the igniter. Unfortunately, despite the above-recognition, in existing SiC igniter product examples, a typical 84% to 86% SiC density corresponds to a relatively high 16% to 14% interspermosing (meaning operable or flow-able interstecies between particles) porosity which is easily permeated by air and moisture, much like an open cell foam structure. Such detrimental porosity leads directly to excessive material oxidation at operational temperatures, mechanical stress, electrical property “aging” and resultant premature failure of the product.
  • interspermosing meaning operable or flow-able interstecies between particles
  • nitride nitride
  • SiC silicon carbide
  • a method for forming polymer-ceramic composite igniter shapes that are pyrolized in supporting fixtures at temperatures as low as 650° C. in nitrogen or other inert gas atmospheres converts the impregnated resin content to 70% to 80% silicon carbide by weight enabling a permanent SiC bonding phase at temperatures as low as 650° C., thereby creating a self supporting structure within an operational temperature ranges of 650° C. and approximately 2000° C., allowing substantially self-supporting/support-free firing orientations during downstream recrystallization and resultant production efficiencies.
  • the resulting material is a ceramic-bonded particulate ceramic composite that will later be recrystallized to attain desired electrical and mechanical properties.
  • the balance of the resin content (30% to 20% by weight) is volatilized as hydrogen, hydrocarbon and pyrophoric silane waste gases.
  • the supporting fixtures serve to limit dimensional distortion of the individual igniter shapes during the conversion reaction.
  • the bonding phase formed from the resin is amorphous (non-crystalline) SiC typically comprising up to 10% by weight of the composite material.
  • the original particulate matrix formed during slip casting comprises the balance, which is alpha phase SiC with a hexagonal crystal structure and/or beta phase SiC with a cubic crystal structure.
  • this invention provides an improved electrical SiC method and product, where the example product (the silicon carbide igniter above described) is of superior quality in terms of material properties (reflected in for example material density, consistency, electrical stability, mechanical strength, mechanical toughness and durability), improved operational life (in terms of durational time to failure), and other benefits.
  • the example product the silicon carbide igniter above described
  • the example product is of superior quality in terms of material properties (reflected in for example material density, consistency, electrical stability, mechanical strength, mechanical toughness and durability), improved operational life (in terms of durational time to failure), and other benefits.
  • the present invention will also result in substantial improvements to correspondingly processing time and manufacturing costs, firing energy, and product field replacement and quality control costs.
  • the present product and process invention is applicable to other SiC electrical components, for example, SiC heating elements and SiC infrared radiation devices.
  • the present invention relates to an improved method for production of silicon carbide igniter bodies exhibiting a permanent SiC bonding phase at temperatures as low as 650° C. resulting in an uncommon degree of structural strength for a ceramic bisque product.
  • FIG. 1 is a flow diagram of the proposed method without recycling.
  • FIG. 2 is a process flow diagram of the proposed method including a secondary densifying step.
  • FIG. 3 is a graphical representation of a net carrier concentration of nitrogen and aluminum approaching theoretical limits.
  • This invention relates to novel electrical devices, including the hot surface igniters discussed above, fabricated from polycrystalline silicon carbide (SiC) and methods for forming the same.
  • a hot surface igniter is merely a representative example wherein the heating element portion is composed of silicon carbide, that the present invention is not limited thereto and has broader applications throughout the industry.
  • the product igniter formulation decreases deterioration substantially by further reducing the porosity to 12% to 10% (a substantive and material improvement of 25%-40%) where the individual pores become more isolated and substantially closed from a micro-structural viewpoint.
  • This at least 25% decrease in porosity (density improvement) and the resultant substantial closing of all pore reduces atmospheric infiltration and extends service life up to three-fold (300% improvement) over conventional igniters (See Table 1).
  • a substantial increase in mechanical strength is also achieved by virtue of both smaller pore sizes and higher density. To the product consumer these improvements mean reduced breakage during appliance transportation and use, and a significant decrease in the total cost of appliance ownership.
  • the proposed invention for creating a ceramic body creates a higher density igniter material by starting with a conventional slip casting process step 100 based on precise selection of the SiC particle sizes, fluid carrier and deflocculants as is known in the art to form a green body (also alternatively referred to herein as a loaf, green loaf, or precursor loaf).
  • the SiC loaves are vacuum-pressure impregnated (or optionally infiltrated) in a step 102 with an organo-metallic resin, principally a SiC-precursor-resin.
  • This SiC resin is preferably allylhydrido-polycarbosilane (AHPCS) and more preferably allylhydrido-polycarbosilane (AHPCS) supplied by Starfire Systems, Malta, N.Y., under the trade name SMP-10 Matrix Polymer.
  • Optional or alternative resins include polyborosiloxane and polysilazane, although these are non-preferred.
  • SiC resins are represented, for example by the following chemical formula (I):
  • the bodies are heated in a curing step 103 to a peak temperature of between 200° C. to 450° C., and more preferably 250° C. to 400° C. in a nitrogen or another inert gas atmosphere to complete curing and cross-linking in a single step.
  • the green loaves are machined in a step 104 to a desired geometry.
  • the loaves are sliced and slotted into a manufacturer-selected individual igniter shape, for example but not limited to the igniter shape noted in U.S. Pat. No. 3,875,477.
  • the polymer-ceramic composite igniter shapes are then pyrolized in a pyrolyzing step 105 in supporting fixtures at temperatures as low as 650° C. in nitrogen or another inert gas (Argon for example) atmosphere.
  • This pyrolizing step 105 converts the impregnated resin content to 70% to 80% silicon carbide by weight.
  • the resulting material is therefore a ceramic-bonded particulate ceramic composite that will later be recrystallized to attain desired electrical and mechanical properties.
  • the balance of the resin content (30% to 20% by weight) is volatilized as hydrogen, hydrocarbon and pyrophoric silane waste gases.
  • the supporting fixtures serve at this stage only to limit dimensional distortion of the individual igniter shapes during the conversion reaction in step 105 .
  • the bonding phase formed in step 105 from the resin is amorphous (non-crystalline) SiC typically comprising up to 10% by weight of the composite material.
  • the original particulate matrix formed during slip casting comprises the balance which is alpha phase SiC with a hexagonal crystal structure and/or beta phase SiC with a cubic crystal structure.
  • the use of a Si-resin, the preferred suggested AHPCS resin, or the particularly preferred composition noted above is recognized as now forming a permanent SiC bonding structure at a relatively low processing temperature in contrast to the conventional art that employs a temporary, organic adhesive bond that exists only while the green loaf is sliced and slotted to form the individual igniters and converts only at very high temperatures prohibiting the generation of a self-supporting (actually bonded) structure prior to very-high temperature recrystallization in a step 107 .
  • the present invention provides for a method that enables a permanently bonded SiC structure that self-supports between the pyrolysis temperature and recrystallization temperature.
  • an additional cycle step 108 may be provided on a repeated basis to further augment density.
  • each succeeding cycle step 108 will result in increasing density until all the pores are closed or rendered unavailable by narrowing to such an extent that the surface tension of the resin may not be over come by hydraulic assistance and pore entry is prohibited.
  • the proposed method is also in clear contrast to the conventional suggestion to impregnate a green body with slurry composed of very fine silicon carbide particles ( ⁇ 5 microns).
  • the proposed method results in a permanent bond structure that correspondingly facilitates several novel and unexpected derivative results, including strength within a temperature range from 650° C.-2000° C. that are very useful in the further processing of a pre-recrystallized body (here represented by the example igniter product).
  • the use of AHPCS resin impregnation creates a supplemental density providing more flexibility in the slip casting formation of the original SiC particulate loaf resulting in substantially improved handling and processing capacity.
  • casting parameters such as the SiC slip inter-particle spacing, packing modulus, surface area and colloidal content may be manipulated in manners known in the art, but now with reduced constraint allowing a system to achieve lower slip viscosities, reduced air bubble entrapment and greatly reduced shrinkage (a problem particularly noted in U.S. Pat. No. 6,692,597).
  • this additional flexibility facilitates a better ordered deposition of the slip cast particles resulting in a corresponding reduction in micro-structural defects in the loaf.
  • SiC-precursor resin impregnation provides a convenient route for homogeneous dispersion of aluminum dopant compounds (operating as a defect originator causing electrical conductivity variation when uncontrolled) throughout the loaf material.
  • At least three variants of the allylhydrido-polycarbosilane (AHPCS) polymer resin may be customized to accomplish and tailor these results, including: 1) a variant suspension formed by adding very fine alumina or aluminum particles in the liquid resin prior to impregnation (Variant No.
  • the low conversion temperature of the SiC-precursor resin provides an effective means of capturing (prohibiting loss as a gas) elemental aluminum (Al) (given its low melting temperature of 660° C.) in the solid SiC matrix before it can be substantially vaporized. It is notable that these methods may be combined in various ways to achieve and improve the desired electrical and mechanical attributes of the SiC product.
  • the method of the present invention provides for the formation of a permanent SiC bonding phase at temperatures as low as 650° C., and hence provides a superior and uncommon degree of structural strength for a bisque product (having for example a strength sufficient to enable high-precision machining to achieve 3-dimensional structures).
  • the substantially improved strength of the bisque product provides greatly improved versatility in fixturing the product for high temperature (>1800° C.) recrystallization firing.
  • the initial bond strength provided by the proposed method enables the amorphous SiC phase (pre-fired) of the individual igniters to be fixtured in a non-horizontal support or non-long-face (free-standing or hanging vertically) position without additional support in a manner that places a first wide cross-section face of an exemplary igniter (as orientationally defined as noted above) substantially along a plane parallel to the direction of gravity without additional support.
  • the present invention also facilitates an unobstructed environment for more homogeneous gaseous diffusion of aluminum, nitrogen, and other desired semiconductor dopants into the SiC lattice structure. Additionally, the kiln load density, or number of igniter shapes fired together in a given volume, may be substantially increased due to the more efficient vertical setting method. This increases kiln productivity while reducing the energy requirement per unit of product. Further unit energy savings may be obtained using a smaller kiln with proportionately lower heat loss expressly designed around the more efficient vertical setting method.
  • the utilization of the SMP-10 Matrix Polymer as an impregnate unexpectedly promotes lower temperature and/or shorter time recrystallization of the SiC structure during high temperature firing (>1800° C.) allowing a faster manufacturing process.
  • the resin-derived amorphous SiC phase and the continued evolution of remnant hydrogen from the same amorphous SiC, both play a role in accelerating the SiC sublimation-condensation reactions which result in epitaxial growth onto larger crystals present in the ceramic-bonded particulate ceramic composite.
  • the amorphous SiC is believed to “jump start” or act as a seed for overall nucleation and recrystallization as initial nano-sized crystals are first formed from this phase.
  • the evolving hydrogen gas is believed to chemically etch the SiC particulates removing the potentially blocking surface oxidation thus facilitating (speeding) earlier sublimation-condensation of these micro-sized crystals as well.
  • this approach maximizes nitrogen content in the SiC lattice, which substantially decreases the device's response time from room to operating temperature (See Table II). While not wishing to be held to a theory, it is believed that the nitrogen donor carrier creates a positive conductivity temperature coefficient by virtue of its lower ionization energy and higher electron mobility as compared to the aluminum acceptor carrier.
  • FIG. 3 illustrating the data in Table III representing the sum or net solubility of nitrogen at a carbon site in a SiC lattice (N[C-site]), the sum or net solubility of aluminum at a silicon site in the SiC lattice (Al[Si-site]), with the delta of these two dopant solubilitys represented by (Al[Si]—N[C])).
  • This graphical representation is noted against a rising temperature profile to achieve a theoretical optimum solubility limit.
  • means- or step-plus-function clauses are intended to cover the structures described or suggested herein as performing the recited function and not only structural equivalents but also equivalent structures.
  • a nail, a screw, and a bolt may not be structural equivalents in that a nail relies on friction between a wooden part and a cylindrical surface, a screw's helical surface positively engages the wooden part, and a bolt's head and nut compress opposite sides of a wooden part, in the environment of fastening wooden parts, a nail, a screw, and a bolt may be readily understood by those skilled in the art as equivalent structures.

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  • Organic Chemistry (AREA)
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US12/092,801 US20080265471A1 (en) 2005-11-07 2006-11-07 Polycrystalline Sic Electrical Devices and Methods for Fabricating the Same
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Cited By (3)

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Publication number Priority date Publication date Assignee Title
WO2015065764A1 (fr) * 2013-10-30 2015-05-07 United Technologies Corporation Article en céramique et procédé associé faisant appel à l'infiltration de particules et à l'infiltration d'un polymère pré-céramique
US9951952B2 (en) 2014-10-15 2018-04-24 Specialized Component Parts Limited, Inc. Hot surface igniters and methods of making same
US20200392046A1 (en) * 2017-11-29 2020-12-17 Commissariat A L'energie Atomique Et Aux Energies Alternatives Particulate composite ceramic material, part comprising said material, and method for the preparation of said part

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