WO1995031416A1 - Sinter-homogenized heating products - Google Patents

Sinter-homogenized heating products Download PDF

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Publication number
WO1995031416A1
WO1995031416A1 PCT/US1995/005396 US9505396W WO9531416A1 WO 1995031416 A1 WO1995031416 A1 WO 1995031416A1 US 9505396 W US9505396 W US 9505396W WO 9531416 A1 WO9531416 A1 WO 9531416A1
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WO
WIPO (PCT)
Prior art keywords
composition
weight percent
temperatures
article
wsi
Prior art date
Application number
PCT/US1995/005396
Other languages
French (fr)
Inventor
Jainagesh A. Sekhar
Naiping Zhu
Original Assignee
Micropyretics Heaters International
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Micropyretics Heaters International filed Critical Micropyretics Heaters International
Priority to AU24645/95A priority Critical patent/AU2464595A/en
Priority to AU26068/95A priority patent/AU2606895A/en
Priority to PCT/US1995/006115 priority patent/WO1995031417A1/en
Publication of WO1995031416A1 publication Critical patent/WO1995031416A1/en

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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • 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
    • C04B35/58—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 borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
    • C04B35/58085—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 borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicides
    • C04B35/58092—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 borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicides based on refractory metal silicides
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • 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
    • C04B35/63—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 using additives specially adapted for forming the products, e.g.. binder binders
    • C04B35/632—Organic additives
    • C04B35/634—Polymers
    • C04B35/63448—Polymers obtained otherwise than by reactions only involving carbon-to-carbon unsaturated bonds
    • C04B35/63488—Polyethers, e.g. alkylphenol polyglycolether, polyethylene glycol [PEG], polyethylene oxide [PEO]
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • 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
    • C04B35/63—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 using additives specially adapted for forming the products, e.g.. binder binders
    • C04B35/632—Organic additives
    • C04B35/636—Polysaccharides or derivatives thereof
    • C04B35/6365—Cellulose or derivatives thereof
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • 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
    • C04B35/64—Burning or sintering processes
    • C04B35/65—Reaction sintering of free metal- or free silicon-containing compositions
    • C04B35/651—Thermite type sintering, e.g. combustion sintering
    • 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
    • H05B3/00—Ohmic-resistance heating
    • H05B3/10—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor
    • 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
    • H05B3/00—Ohmic-resistance heating
    • H05B3/10—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor
    • H05B3/12—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material
    • H05B3/14—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material the material being non-metallic
    • H05B3/148—Silicon, e.g. silicon carbide, magnesium silicide, heating transistors or diodes
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • 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

Definitions

  • This invention deals with composite suicide or carbide heating element compositions which comprise improved combustion sources and refractory suicides such as tungsten suicide. This invention will enable the combustion synthesized composite heating elements to be used at temperatures up to 1 900°C for long periods of time in oxidizing atmospheres.
  • This invention has been conceived to create heating elements which can work at high temperatures for long periods. Accordingly, this invention discloses new micropyretic sources and their combinations which allow for the first time, manufacture of heating elements displaying extended life at very high temperatures.
  • a micropyretic synthesis method is used for the manufacture of heating elements, similar to that disclosed in our co-pending United States patent application 08/027,710, filed March 8, 1 993 (herein after referred to as "the '710 application", the contents of which are incorporated herein by way of reference).
  • the synthesis method used in this invention differs from that disclosed in co-pending United States patent application 07/847,782, filed March 5, 1992 (hereinafter referred to as "the '782 application", the contents of which are incorporated herein purely by way of reference), in an important aspect.
  • the '782 application a combustible slurry is prepared, extruded and combusted.
  • these wire or other shapes are densified and homogenized by passing current, as described in the '710 application.
  • the '782 application discloses various combustion sources as part of the compositions, for the manufacture of heating elements. MoSi 2 composites are preferred in the '782 application.
  • MoSi 2 composites are very sensitive to phase composition and impurity at temperatures above 1600°C.
  • the present compositions comprise new undisclosed combustion sources, grain growth inhibitors and other additives which substantially improve the applicability of heating elements manufactured using these compositions at temperatures up to 1900°C. Additionally, some of the combustion sources and their products described in the '782 application may react with filler materials, and therefore may not be suitable as combustion sources for high temperature applications although they may be excellent for low and middle temperature applications (up to 1400°C) .
  • the compositions of the present invention are designed so that the combustibles do not react with the suicides, which is believed to be one of reasons leading to the effective utilization of heating elements manufactured using the compositions of the present invention for extended periods at high temperatures. By utilizing combustibles which may react with the filler materials, the '782 application teaches away from the present invention.
  • heating elements using the '782 compositions deteriorate substantially.
  • typical heating elements using the compositions of the present invention show similar deterioration at 1400°C to 1 600°C, only after 5000 hours.
  • heating elements using the '782 compositions last for less than 1 hour, whereas typical heating elements using the compositions of the present invention last for greater than 1000 hours.
  • carbides such as tungsten carbide and metalloids such as carbon or silicon.
  • Such compounds are finely and homogeneously distributed in matrix of a major suicide phase e.g. in MoSi 2 or WSi 2 .
  • these finely distributed particles will also act as grain growth inhibitors and also influence the recrystallization behavior.
  • a pliable composition comprising by weight percent: (a) between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2 and mixtures thereof; (b) between about 5% and 50% of a combustible source which is selected from the group consisting of WO 3 + Al + Si, MoO 3 + Zr + Si, WO 3 + Zr + Si, WC + Si, Mo 2 C + Si and mixtures thereof, wherein the particle size of said Mo 2 C and WC, when present, is always submicronic ( ⁇ 1 ⁇ m); (c) between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB 2 , HfB 2 , SiC and mixtures thereof; (d) at least about 1 weight percent bentonite; and (e) at least about 3ml per 30g of the above listed components, of colloidal silica solution.
  • a method of manufacturing a composite article using the composition described above comprising the steps of: (a) premixing the powders comprising the combustible source in the composition; (b) blending said premixture with the other components of the composition; (c) forming a pliable slurry from said blend; (d) fashioning said slurry into a final desired article shape; (e) combusting said shape by ignition at a temperature between about 100°C and 1600°C; (f) initially applying sufficient current to said article so as to raise the temperature of said article to a minimum of 50% of the melting point in degrees Kelvin, of the lowest melting phase in the article, wherein the current applied is selected from the group consisting of a DC current, an AC current, a pulsed current and an induction current; and (g) greatly reducing the porosity of said article so as to make the repetitive distance between consecutive homogenous sections of said article to less than 0.002 m, by increasing said current applied so as to
  • a pliable composition comprising by weight percent: (a) between about 60 and 85% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2 and mixtures thereof; (b) about 1 5% of WO 3 + Al + Si as a combustible source; (c) about 2% of HfB 2 as a grain growth inhibitor; (d) about 1 weight percent bentonite; (e) about 3ml per 30g of the above listed components, of colloidal silica solution; and (f) about 0.5 weight percent C.
  • compositions which have been found to provide heating elements having longer working life up to 1900°C, are disclosed as follows:
  • composition 1 (all compositions herein are by weight percent of the total composition, unless otherwise indicated)
  • Colloidal Silica type 830 3ml/30g powder (this implies 3 ml
  • organic binders may be methyl cellulose, polyethylene glycol with an average molecular weight, of 200, polyethylene glycol with an average molecular weight of 300, glycerol, 99.5%, polyvinyl butyral, dioctyi adipate and their combinations)
  • compositions of the present invention comprise between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2 and mixtures thereof; between about 5% and 50% of a combustible source which is selected from the group consisting of WO 3 + Al + Si, MoO 3 + Zr + Si, W0 3 + Zr + Si, WC + Si, Mo 2 C -i- Si and mixtures thereof, wherein the particle size of said Mo 2 C and WC, when present, is always submicronic ( ⁇ 1 ⁇ m); between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB 2 , Hf B 2 , SiC and mixtures thereof; at least about 1 weight percent bentonite; and at least about 3ml/30g of colloidal silica solution.
  • a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi 2 , MoSi 2 and mixtures thereof; between
  • the combustion sources have to be premixed before mixing with other powders, in order to produce sufficient heat during combustion.
  • the weight percentage of combustibles in the compositions of the present invention will not be less than 5 wt%, but will not be more than 90%.
  • a combustible content of more than 50% will lead to too strong combustion, and therefore, to samples (heating elements) with large pores and deformed shapes.
  • Compositions with combustibles less then 5% will not be able to produce enough heat to sinter the wire and therefore the wire may not be conductive enough to be finally sintered, densified and homogenized by passing current.
  • the addition of WSi 2 or MoSi 2 increases the working temperature, life of heating elements at high temperature and creep resistance, but WSi 2 or MoSi 2 content of more than 90 weight percent will drastically decrease thermal shock resistance and oxidation resistance of heating element.
  • compositions of the present invention comprise from about 10 to 30 percent WSi 2 , up to 4% SiC, up to 3% TiB 2 , up to 3% HfB 2 (said SiC, TiB 2 and HfB 2 being at least 0.5 percent), close to 2% bentonite and close to 1 5ml/100g of powder of colloidal silica.
  • the most preferred composition for manufacturing high temperature heating elements is as follows: 1 5-30 percent of WSi 2 , 1 5-20 percent combustibles, 1 -2 percent TiB 2 and HfB 2 and 0.5-1 percent C. Heating elements made from compositions 1 , 2 and 3 above, lasted for over 1000 hours at temperatures from 1700 to 1750°C.
  • Yet another preferred and unique composition discloses the use of a combination of WC + Si or Mo 2 C + Si with WO 3 + Al + Si or
  • WO 3 + Zr + Si The content of carbide combustion source (WC + Si or Mo 2 C + Si) will be from 1 to 50%; oxide combustion source (WO 3 + Al + Si or WO 3 + Zr + Si) will be 5-30%. The total weight percent of the combustion source is at least 5%. In its more preferred aspects, the carbide combustion source ranges from 1 5 to 20%, oxide combustion source will be 5-10 wt%. By using carbide combustion sources, alumina content in the final products will be tremendously reduced. It was found that heating elements manufactured using a combination of carbide and oxide combustion sources (for instance, composition 9), show excellent temperature capacity and improved life up to 1 850°C.
  • compositions discloses WO 3 + Zr + Si and Mo0 3 + Zr + Si as combustion sources or combinations thereof. These compositions show strong and stable combustion when mixed with MoSi 2 and WSi 2 powders. The final products of these combustion sources are WSi 2 (or MoSi 2 ) and ZrO 2 . Part of ZrO 2 will react further with SiO 2 during sintering to form zircon which has high melting point and phase stability with MoSi 2 and WSi 2 . The total weight percent of the combustion source in these compositions is at least 5%. In its more preferred aspects, the WO 3 + Zr + Si combustion source will be from 10 to 25 weight percent. It was found that the compositions using this combustion source show excellent sintering properties and high temperature capability up to 1850°C.
  • compositions suited for preparing heating elements which may be operated at temperatures up to 1 900°C or for very long durations at lower temperatures such as 1750°C, which fully satisfy the objects, aspects and advantages set forth above.
  • the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.
  • a particular composition may comprise other combustion sources. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.

Abstract

Electrical heating elements operable at high temperatures for long periods are produced by a method involving micropyretic synthesis. Compositions subjected to micropyretic synthesis comprises a powdery mass of electrically conductive and semiconductive material, a reactive system including a sub-micron particle size reactant capable of undergoing micropyretic synthesis, a grain growth inhibitor and a plasticizer or extrusion agent.

Description

SINTER-HOMOGENIZED HEATING PRODUCTS
Field of Invention
This invention deals with composite suicide or carbide heating element compositions which comprise improved combustion sources and refractory suicides such as tungsten suicide. This invention will enable the combustion synthesized composite heating elements to be used at temperatures up to 1 900°C for long periods of time in oxidizing atmospheres.
Background of the Invention
This invention has been conceived to create heating elements which can work at high temperatures for long periods. Accordingly, this invention discloses new micropyretic sources and their combinations which allow for the first time, manufacture of heating elements displaying extended life at very high temperatures. A micropyretic synthesis method is used for the manufacture of heating elements, similar to that disclosed in our co-pending United States patent application 08/027,710, filed March 8, 1 993 (herein after referred to as "the '710 application", the contents of which are incorporated herein by way of reference). The synthesis method used in this invention differs from that disclosed in co-pending United States patent application 07/847,782, filed March 5, 1992 (hereinafter referred to as "the '782 application", the contents of which are incorporated herein purely by way of reference), in an important aspect. As in the '782 application, a combustible slurry is prepared, extruded and combusted. However, unlike the '782 application, after combustion, these wire or other shapes are densified and homogenized by passing current, as described in the '710 application. The '782 application discloses various combustion sources as part of the compositions, for the manufacture of heating elements. MoSi2 composites are preferred in the '782 application. MoSi2 composites are very sensitive to phase composition and impurity at temperatures above 1600°C. The present compositions comprise new undisclosed combustion sources, grain growth inhibitors and other additives which substantially improve the applicability of heating elements manufactured using these compositions at temperatures up to 1900°C. Additionally, some of the combustion sources and their products described in the '782 application may react with filler materials, and therefore may not be suitable as combustion sources for high temperature applications although they may be excellent for low and middle temperature applications (up to 1400°C) . The compositions of the present invention are designed so that the combustibles do not react with the suicides, which is believed to be one of reasons leading to the effective utilization of heating elements manufactured using the compositions of the present invention for extended periods at high temperatures. By utilizing combustibles which may react with the filler materials, the '782 application teaches away from the present invention.
The high temperature superiority of heating elements manufactured using the compositions of the present invention, over the compositions of the '782 application, has been observed by experimentation. These experiments have revealed that heating elements using the compositions of the present invention are stable at temperatures above 1400°C, for much longer periods than those manufactured using the compositions of the '782 application. Without exception, a difference in time periods of at least two orders of magnitude has been observed. At temperatures above 1400°C to
1600°C, after operation for 10-100 hours, heating elements using the '782 compositions deteriorate substantially. Conversely, typical heating elements using the compositions of the present invention show similar deterioration at 1400°C to 1 600°C, only after 5000 hours. Additionally, above 1600°C, heating elements using the '782 compositions last for less than 1 hour, whereas typical heating elements using the compositions of the present invention last for greater than 1000 hours.
Clearly this tremendous increase in high temperature operability, is very beneficial and also quite unexpected.
Summary of the Invention
It is a primary object of the present invention to provide new combustion sources and compositions for manufacturing heating elements which are stable up to 1 900°C.
It is a further object of the present invention to provide new combustion sources which will enhance the working temperature and life of heating elements and improve the oxidation resistance of heating elements at high temperatures.
It is another object of the present invention to provide new combustion sources which will prevent any possible reaction between the combustion sources, the combustion products, the suicides and the plasticizers.
It is yet another object of this invention to provide combustion synthesis product(s) with increased creep resistance and strength, by forming in situ compounds between carbides such as tungsten carbide and metalloids such as carbon or silicon. Such compounds are finely and homogeneously distributed in matrix of a major suicide phase e.g. in MoSi2 or WSi2. In addition, these finely distributed particles will also act as grain growth inhibitors and also influence the recrystallization behavior. In accordance with the present invention, there is provided a pliable composition comprising by weight percent: (a) between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi2, MoSi2 and mixtures thereof; (b) between about 5% and 50% of a combustible source which is selected from the group consisting of WO3 + Al + Si, MoO3 + Zr + Si, WO3 + Zr + Si, WC + Si, Mo2C + Si and mixtures thereof, wherein the particle size of said Mo2C and WC, when present, is always submicronic ( < 1μm); (c) between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB2, HfB2, SiC and mixtures thereof; (d) at least about 1 weight percent bentonite; and (e) at least about 3ml per 30g of the above listed components, of colloidal silica solution.
In accordance with another aspect of the present invention there is provided a method of manufacturing a composite article using the composition described above, comprising the steps of: (a) premixing the powders comprising the combustible source in the composition; (b) blending said premixture with the other components of the composition; (c) forming a pliable slurry from said blend; (d) fashioning said slurry into a final desired article shape; (e) combusting said shape by ignition at a temperature between about 100°C and 1600°C; (f) initially applying sufficient current to said article so as to raise the temperature of said article to a minimum of 50% of the melting point in degrees Kelvin, of the lowest melting phase in the article, wherein the current applied is selected from the group consisting of a DC current, an AC current, a pulsed current and an induction current; and (g) greatly reducing the porosity of said article so as to make the repetitive distance between consecutive homogenous sections of said article to less than 0.002 m, by increasing said current applied so as to cause the elimination of thermal and mass gradients. In accordance with a further aspect of the present invention, there is provided a pliable composition comprising by weight percent: (a) between about 60 and 85% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi2, MoSi2 and mixtures thereof; (b) about 1 5% of WO3 + Al + Si as a combustible source; (c) about 2% of HfB2 as a grain growth inhibitor; (d) about 1 weight percent bentonite; (e) about 3ml per 30g of the above listed components, of colloidal silica solution; and (f) about 0.5 weight percent C.
Detailed Description of the Preferred Embodiment
Examples of specific compositions which have been found to provide heating elements having longer working life up to 1900°C, are disclosed as follows:
Composition 1 (all compositions herein are by weight percent of the total composition, unless otherwise indicated)
WSi2 10
MoSi2 71 .5
WO3 + 2AI + 2Si2 1 5
HfB2 2
C (present in the form of 0.5 graphite in all compositions herein)
Bentonite 1
Colloidal Silica (type 830 3ml/30g powder (this implies 3 ml
Nycol corporation) per 30g of the remaining components)
Composition 2 WSi2 20 MoSi2 61 .5 W03+ 2AI+ 2Si2 15
HfB2 2
C 0.5
Bentonite 1
Colloidal Silica 3ml/30g powder
Comoosition 3
WSi2 30
MoSi2 51.5
WO3+ 2AI + 2Si2 15
HfB2 2
C 0.5
Bentonite 1
Colloidal Silica 3ml/30g powder
ComDosition 4
WSi2 40
MoSi2 41.5
WO3+ 2AI + 2Si2 15
HfB2 2
C 0.5
Bentonite 1
Colloidal Silica 3ml/30g powder
ComDosition 5
WSi2 50
MoSi2 31.5
WO3+ 2AI + 2Si2 15
HfB2 2
C 0.5
Bentonite 1
Colloidal Silica 3ml/30g powder Composition 6
WSi2 20
MoSi2 61.5
WO3 + 2AI + 2Si2 7
2WO3 + 3Zr + 4Si 8
HfB2 2
C 0.5
Bentonite 1
Colloidal Silica 3ml/30g powder
Comoosition 7
WSi2 20
MoSi2 57
WO3+ 2AI + 2Si2 5
WC + 3Si 15
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
Comoosition 8
WSi2 20
MoSi2 54
WO3 + 2AI + 2Si2 8
WC + 3Si 15
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
ComDosition 9
WSi2 20
MoSi2 52 W03 + 2AI + 2Si2 10
WC + 3Si 15
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
Comoosition 10
WSi2 20
MoSi2 47
WC + 3Si 15
W03 + 2AI + 2Si 15
TiB2 2
Bentonite 1
Water with organic binders 5ml/30g powder. (organic binders may be methyl cellulose, polyethylene glycol with an average molecular weight, of 200, polyethylene glycol with an average molecular weight of 300, glycerol, 99.5%, polyvinyl butyral, dioctyi adipate and their combinations)
Comoosition 11
WSi2 20
MoSi2 57
Mo2C + 5Si 15
WO3+ 2AI + , 2Si 5
TiB2 2
Bentonite 1
Colloidal Silica (5ml/30g powder)
Comoosition 12
WSi2 20
MoSi2 54
Mo2C + 5Si 15
M0O3+ 2AI + 2Si 8 TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
> Comoosition 13
WSi2 20
MoSi2 57
Mo2C + 5Si 1 5
WO3 + 2AI + : ZSi 5 ι HfB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
Comoosition 14
WSi2 20
MoSi2 47
WC + 3Si 25
WO3 + 2AI + 2Si 5
TiB2 2
» Bentonite 1
Colloidal Silica 5ml/30g powder
ComDosition 15
WSi2 20 i MoSi2 47
Mo2C + 5Si 25
WO3 + 2AI + 2Si 5
TiB2 2
Bentonite 1 l Colloidal Silica 5ml/30g powder ComDosition 1 6
WSi2 20
MoSi2 47
Mo2C + 5Si 25
WO3 + 2AI + 2Si 5
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g powder
Comoosition 17
MoSi, 87
2W03 + 3Zr + 4Si 10 TiB, 2
Bentonite 1
Colloidal Silica 5ml/30g
Comoosition 18
MoSi2 82
2WO3 + 3Zr + 4Si 5
WO3 + 2AI + 2Si 10
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g
ComDosition 1 9
MoSi2 77
2WO3 + 3Zr + 4Si 10
WO3 + 2AI + 2Si 10
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g Composition 20 WSi, 20
MoSi2 62
2WO3 + 3Zr + 4Si 5
WC + 3Si 10
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g
Comoosition 21
MoSi2 87
2WO3 + 3Zr + 4Si 10
Si3N4 2
Bentonite 1
Colloidal Silica 5ml/30g
ComDosition 22
MoSi2 82
2WO3 + 3Zr + 4Si 1 5
SiC 2
Bentonite 1
Colloidal Silica 5ml/30g
Composition 23
MoSi2 77
2WO3 + 3Zr +4Si 20
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g Composition 24
MoSi2 77
2W03 + 3Zr + 4Si 10
2M03 + 3Zr + 4Si 10
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g
Comoosition 25
WSi2 10
MoSi2 72
2Mo03 + 3Zr + 4Si 10
WC + 3Si 5
TiB2 2
Bentonite 1
Colloidal Silica 5ml/30g
Broadly stated, the compositions of the present invention comprise between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi2, MoSi2 and mixtures thereof; between about 5% and 50% of a combustible source which is selected from the group consisting of WO3 + Al + Si, MoO3 + Zr + Si, W03 + Zr + Si, WC + Si, Mo2C -i- Si and mixtures thereof, wherein the particle size of said Mo2C and WC, when present, is always submicronic ( < 1μm); between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB2, Hf B2, SiC and mixtures thereof; at least about 1 weight percent bentonite; and at least about 3ml/30g of colloidal silica solution.
The combustion sources have to be premixed before mixing with other powders, in order to produce sufficient heat during combustion. The weight percentage of combustibles in the compositions of the present invention will not be less than 5 wt%, but will not be more than 90%. A combustible content of more than 50% will lead to too strong combustion, and therefore, to samples (heating elements) with large pores and deformed shapes. Compositions with combustibles less then 5% will not be able to produce enough heat to sinter the wire and therefore the wire may not be conductive enough to be finally sintered, densified and homogenized by passing current. The addition of WSi2 or MoSi2 increases the working temperature, life of heating elements at high temperature and creep resistance, but WSi2 or MoSi2 content of more than 90 weight percent will drastically decrease thermal shock resistance and oxidation resistance of heating element.
The preferred compositions of the present invention comprise from about 10 to 30 percent WSi2, up to 4% SiC, up to 3% TiB2, up to 3% HfB2 (said SiC, TiB2 and HfB2 being at least 0.5 percent), close to 2% bentonite and close to 1 5ml/100g of powder of colloidal silica. The most preferred composition for manufacturing high temperature heating elements is as follows: 1 5-30 percent of WSi2, 1 5-20 percent combustibles, 1 -2 percent TiB2 and HfB2 and 0.5-1 percent C. Heating elements made from compositions 1 , 2 and 3 above, lasted for over 1000 hours at temperatures from 1700 to 1750°C.
Yet another preferred and unique composition discloses the use of a combination of WC + Si or Mo2C + Si with WO3 + Al + Si or
WO3 + Zr + Si. The content of carbide combustion source (WC + Si or Mo2C + Si) will be from 1 to 50%; oxide combustion source (WO3 + Al + Si or WO3 + Zr + Si) will be 5-30%. The total weight percent of the combustion source is at least 5%. In its more preferred aspects, the carbide combustion source ranges from 1 5 to 20%, oxide combustion source will be 5-10 wt%. By using carbide combustion sources, alumina content in the final products will be tremendously reduced. It was found that heating elements manufactured using a combination of carbide and oxide combustion sources (for instance, composition 9), show excellent temperature capacity and improved life up to 1 850°C.
Yet another preferred and unique composition discloses WO3 + Zr + Si and Mo03 + Zr + Si as combustion sources or combinations thereof. These compositions show strong and stable combustion when mixed with MoSi2 and WSi2 powders. The final products of these combustion sources are WSi2 (or MoSi2) and ZrO2. Part of ZrO2 will react further with SiO2 during sintering to form zircon which has high melting point and phase stability with MoSi2 and WSi2. The total weight percent of the combustion source in these compositions is at least 5%. In its more preferred aspects, the WO3 + Zr + Si combustion source will be from 10 to 25 weight percent. It was found that the compositions using this combustion source show excellent sintering properties and high temperature capability up to 1850°C.
Optimal particle sizes are i disclosed for various the components listed above as follows:
Figure imgf000016_0001
WSi2 - 4μm
Figure imgf000016_0002
MoSi2 - 4μm
Figure imgf000016_0003
Mo2C - -325 mesh (-44 /m)
Figure imgf000016_0004
HfB2 - -325 mesh (-44//m)
Figure imgf000016_0005
TiB2 - 1 m
Figure imgf000017_0001
Si3N4 - -325 mesh (-44//m)
Bentonite - 5μm
Colloidal silica nanosize colloid
Thus it is apparent that there have been provided, in accordance with the invention, compositions suited for preparing heating elements which may be operated at temperatures up to 1 900°C or for very long durations at lower temperatures such as 1750°C, which fully satisfy the objects, aspects and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. For example, it is contemplated that in addition to the combustion sources described and claimed herein, a particular composition may comprise other combustion sources. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.

Claims

1 . A pliable composition comprising by weight percent:
(a) between about 10 and 90% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi2, MoSi2 and mixtures thereof;
(b) between about 5% and 50% of a combustible source which is selected from the group consisting of W03 + Al + Si, MoO3 + Zr + Si, WO3 + Zr + Si, WC + Si, Mo2C + S1 and mixtures thereof, wherein the particle size of said Mo2C and WC, when present, is always submicronic ( < 1μm);
(c) between about 0.5 to 10% of grain growth inhibitors selected from the group consisting of TiB2, HfB2, SiC and mixtures thereof; (d) at least about 1 weight percent bentonite; and
(e) at least about 3ml per 30g of the above listed components, of colloidal silica solution.
2. The composition of claim 1 further comprising up to 1 weight percent carbon; up to 2 weight percent bentonite; and up to 1 5ml per 30g of the above listed components, of colloidal silica.
3. The composition of claim 1 further comprising up to 20 weight percent bentonite.
4. A method for the preparation of integral articles having improved mechanical stability, room temperature fracture toughness, and oxidation resistance at temperatures up to 1900°C, and stable electrical conductivity, comprising the steps of: (a) premixing the powders comprising the combustible source in the composition of claim 1 ; (b) blending said premixture with the other components of the composition of claim 1 ;
(c) forming a pliable slurry from said blend; (d) fashioning said slurry into a final desired article shape;
(e) combusting said shape by ignition at a temperature between about 100°C and 1600°C;
(f) initially applying sufficient current to said article so as to raise the temperature of said article to a minimum of 50% of the melting point in degrees Kelvin, of the lowest melting phase in the article, wherein the current applied is selected from the group consisting of a DC current, an AC current, a pulsed current and an induction current; and
(g) greatly reducing the porosity of said article so as to make the repetitive distance between consecutive homogenous sections of said article to less than 0.002 m, by increasing said current applied so as to cause the elimination of thermal and mass gradients.
5. An integral article having improved mechanical stability, room temperature fracture toughness, and oxidation resistance at temperatures up to 1900° C, and stable electrical conductivity, produced in accordance with the process of claim 4.
6. An electrical heating element capable of being used at temperatures up to 1 900°C formed by micropyretic synthesis of the composition of claim 1 .
7. An integral article having improved mechanical stability, room temperature fracture toughness, and oxidation resistance at temperatures up to 1 900°C, and stable electrical conductivity, comprising an integral article formed by micropyretic synthesis of the composition of claim 1 .
8. An electrical heating element suitable for use as a high temperature indicator comprising an integral article formed by micropyretic synthesis of the composition of claim 3.
9. An electrical heating element capable of being used at temperatures up to 1 900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 2.
10. The composition of claim 2, wherein said combustible source is selected from the group consisting of Mo03 + Zr + Si, WO3 + Zr + Si and mixtures thereof.
1 1 . An electrical heating element capable of being used at temperatures up to 1900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 10.
12. The composition of claim 10 wherein said combustible source is between about 10 to 25 weight percent WO3 + Zr + Si.
13. An electrical heating element capable of being used at temperatures up to 1 900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 12.
14. The composition of claim 1 comprising about 10 to 30 weight percent WSi2, from about 0.5 weight percent to 4 weight percent SiC, from about 0.5 weight percent to 3 weight percent TiB2, from about 0.5 weight percent to 3 weight percent HfB2, up to 2 j weight percent bentonite and up to 1 5ml per 30g of the above listed components, of colloidal silica.
1 5. An electrical heating element capable of being used at temperatures up to 1 900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 14.
16. The composition of claim 14 comprising 1 5-30 weight percent of WSi2, 1 5-20 weight percent combustibles, 1 -2 weight percent TiB2 and HfB2 and further comprising 0.5-1 weight percent C.
17. An electrical heating element capable of being used at temperatures up to 1 900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 1 6.
18. A pliable composition comprising by weight percent:
(a) between about 60 and 85% of a powdery mass of electrically conductive and/or semiconductive material selected from the group consisting of WSi2, MoSi2 and mixtures thereof;
(b) about 15% of WO3 + Al + Si as a combustible source;
(c) about 2% of HfB2 as a grain growth inhibitor;
(d) about 1 weight percent bentonite; (e) about 3ml per 30g of the above listed components, of colloidal silica solution; and (f) about 0.5 weight percent C.
1 9. An electrical heating element capable of being used at temperatures up to 1900°C comprising an integral article formed by micropyretic synthesis of the composition of claim 18.
20. A integral article formed by micropyretic synthesis of the composition of claim 1 for use as a gas ignitor.
21 . An electrical heating element capable of being used at temperatures up to 1 900°C formed by micropyretic synthesis of the composition of claim 1 , said article comprising WSi2, MoSi or a mixture thereof.
22. The composition of claim 1 , further comprising additional combustion sources.
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