ES2326069B1 - MANUFACTURING PROCEDURE OF A CERAMIC RESISTIVE DEVICE FROM CELLULOSIC PRECURSORS AND PRODUCT SO OBTAINED. - Google Patents

Abstract

Device manufacturing procedure ceramic resistive from cellulosic precursors and product so obtained.

Device manufacturing procedure ceramic resistive of the type manufactured with multiphase materials of silicon carbide and silicon from cellulosic precursors characterized in that it comprises at least a first stage configured for drying said cellulosic precursor; a second stage of pyrolysis; a third machining stage; a fourth stage of union; a fifth stage of infiltration; a sixth stage of selective removal of silicon; a seventh stage of metallization; an eighth stage of electrical connection configured to obtain a high quality electrical connection; and a novena thermal and electrical insulation stage.

Description

Device manufacturing procedure ceramic resistive from cellulosic precursors and product so obtained.

The main object of the present invention it refers to a device manufacturing procedure resistive made of multiphase carbide materials silicon and silicon, specifically from the infiltration of metal alloys in carbon preforms obtained by pyrolysis of cellulosic precursors, as well as the product obtained by this procedure.

The present invention is applicable, fundamentally, to heating and dissipation processes of Energy in various industrial sectors. Thus, it is possible to obtain components for applications based on obtaining an area hot, maintaining optimal hardness properties, friction resistance, thermal conductivity, conductivity electrical, mechanical resistance, porosity, resistance to corrosion, resistance to thermal shock and low density.

Prior art

The start of research in new ceramic materials have been due to the limitations of the metal alloys for use in structural applications and / or aggressive environments at high temperatures [1]. The materials ceramics have a higher melting point than metals, what which allows them to endure for extended periods of time the effect of temperature and mechanical stress. The development of materials that can be used at temperatures higher than  Metals (> 1000ºC) have many advantages and new applications. For example, the increase in working temperature of engines and turbines increase efficiency and reduce gas emissions pollutants [2-5].

One of these ceramics is silicon carbide (Sic). This material was first synthesized in 1980 by E.G. Acheson in an attempt to make rhinestones. This material does not exist naturally, although its meteorite training This ceramic has the following properties that make it together the best candidate for high temperature structural applications, superior to others ceramic materials such as alumina (Al 2 O 3), nitride of silicon (Si_ {N} {4}) or zirconia (ZrO2) [5]:

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Low Density: Very important for aerospace applications.

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high mechanical resistance at high temperatures: Does not decrease significantly with the temperature up to 1500 ° C.

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Low coefficient of thermal expansion: Does not create tensions in the zones of contact with other components during the cycles of heating.

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high thermal conductivity: Prevents overheating in joints ceramic metal

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high chemical stability: The decomposition temperature is 2400 ° C.

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Excellent corrosion resistance and oxidation.

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high resistance to thermal shock.

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high abrasion resistance in temperature.

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high Hardness: very close to that of the diamond.

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Fissure propagation resistance. It is inferior to that of other ceramics. The improvement of this property in SiC is one of the current challenges in materials science [6].

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The main limitation for the use of this material has been the manufacture of macroscopic sized parts. Silicon carbide powder has been used as an abrasive material since its discovery. During World War II it tried to use as a heating element, however it was Impossible to obtain high density SiC parts. In 1974, S. Prochaza [7, 8] discovered that it was possible to manufacture parts SiC from compacted powder at high temperature (sintered) with small additions of boron and carbon. From this moment the SiC It has received great attention for use in the following fields of application [2-5, 9, 10]:

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Abrasion resistant components and Corrosion - Mechanical seals, valves, surface reduction, cutting parts in the paper industry, etc.

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Temperature resistant components - Heat exchangers, ceramic fans, elements of heating, protective tubes, etc.

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Engine component and turbines

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Components for the steel industry and other metals (refinement and manufacturing).

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Holder catalyst.

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Cooling walls in reactors nuclear fusion.

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On the other hand, the main methods of SiC parts manufacturing are listed below:

(a) Hot sintered without pressure [11-13]: SiC powder is mixed with small amounts of B and C and heated at temperatures between 2000-2300 ° C. The main drawbacks of this technique are that high temperatures make it necessary to use more expensive technologies; Temperature control is very critical and can only be done with optical pyrometers, which present precision problems; transformations between the different SiC polymorphisms can occur leading to anomalous grain size growth; It needs a final finish, which considerably increases costs due to the wear resistance of the material; the resistance of the final product decreases considerably at high temperatures; and the additives used for sintering can alter other properties such as corrosion resistance.

(b) Hot sintering with pressure [14]: Produces materials with greater resistance than those produced by sintering without pressure. The main drawbacks of this technique is that it is only applicable for simple geometries and the cost is even higher than that of sintering without pressure.

(c) Chemical deposition in vapor phase [15, 16]: It is produced from the reaction in a gas containing Si and C, subsequently deposited on a substrate. Produces pure SiC without additives. The main drawbacks are that only thin sheets can be produced; the growth rate is very slow; and the size of the grains varies systematically during the deposition process.

(d) Compacted by reaction [17,18]: This technique is a mixture of SiC powder and C is reacted with gaseous or liquid SiC. The manufacturing temperature is lower (1410 ° C). The main drawback is that the contact areas between the SiC grains are small and the material has very low resistance to high temperature since it is controlled by the flow of silicon [19-21].

(e) Reactive infiltration [22-25]: Manufacture of silicon carbide from the infiltration of liquid silicon into artificial carbon preforms. The main drawback is that the structure needs optimization in its interconnectivity and does not present directionality. The pores are of uniform size without nesting, which limits certain applications in which the specific surface is important.

(f) Manufacturing by pyrolysis and infiltration with silicon of plant precursors : There are patent documents related to the manufacture of silicon carbide from the infiltration of liquid silicon into natural carbon preforms [26], as well as related to the manufacture of Porous ceramics and multiphasic materials from cellulosic precursors in which they claim the processes of reactive and non-reactive infiltration of metal alloys in silicon carbide manufactured according to the first patent. The microstructure and properties of these SiC ceramics manufactured by infiltration of liquid silicon into charcoal are described in various publications [27-37], as well as the modeling of the manufacturing process [38] and its mechanical behavior [39-41] . The results obtained indicate that these ceramics have multiple advantages over those obtained by other manufacturing processes, which make them susceptible to being used in a wide range of applications [42-46].

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In no case, the current state of the art explains a novel procedure in the manufacture of devices consistent resistors, essentially in a bonding procedure of these materials, a procedure for the elimination of silicon, a procedure for its electrical connection, and a procedure for thermal and electrical insulation, as well as resistive devices manufactured under that procedure.

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References

[1] WD Kingery , "Social needs and ceramic technology", Am. Ceram. Soc. Bull ., 59 (6), 598-600 ( 1980 ).

[2] HB Strock , " Spectrum Materials Manufacturing ", 35, 1-11 ( 1992 ).

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[4] SJ Dapkunas , "Ceramics Heat Exchangers", Am. Ceram. Soc. Bull ., 67 \ Box 2 \ Box 388-91 ( 1988 ).

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[5] K. Yamada and M. Mori , "Properties and applications of SiC ceramics", in "Silicon carbide ceramics", pp. 13-44, Elsevier Applied Science , ISBN 1-85166-560-9, 1991 .

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[7] S. Prochazka , GE report , SRD-72-035, 1972 .

[8] S. Prochazca , "The role of boron and carbon in the sintering of silicon carbide", Special Ceramics Vol. 6, British Ceramics Research Association , 171-81, 1975 .

[9] G. Wei , and V. Tennery , "Evaluation of tubular ceramics heat exchanger materials in residual oil combustion environments", ORNL / TM-757, 1981 .

[10] G. Trantina , "Design techniques for ceramics in fusion reactors", Nucl. Eng. Des . 54 (1), 676-677, 1979 .

[11] C. Greskovic and JH Rosolowski , "Sintering of covalent solids", J. Am. Ceram. Soc , 59 (7-8), p. 336-43, 1976 .

[12] W. Bocker and H. Hausner , "Observations on the sintering characteristic of submicron silicon carbide powders", Science of Ceramics 9, pp. 168-75, 1977 .

[13] H. Tanaka , "Sintering of SiC", in "Properties and applications of SiC ceramics", "Silicon carbide ceramics", pp. 213-238, Elsevier Applied Science , ISBN 1-85166-560-9, 1991 .

[14] RA Alliegro , LB Coffin and JR Tinklepaught , J. Am. Ceram. Soc . 39, p. 386 ( 1956 ).

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[16] T. Hirai , H. Asakura , and M. Sasaki , Bull. Japan Inst. Met , 26, p. 809 ( 1987 ).

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[19] B. John , and J. Wachtman , "Structural Ceramics" Vol. 29, Academic Press , 91-163, 1989 .

[20] BJ Hockey , and SM Wiederhorn , "Effect of microstructure on the creep of siliconized silicon carbide", J. Am. Ceram. Soc . 75 [7] 1822-30 ( 1992 ).

[21] SM Wiederhorn , BJ Hockey and JD French , "Mechanisms of deformation of silicon nitride and silicon carbide at high temperatures", J. European Ceramic Society , Volume 19, Issues 13-14, 2273-2284, 1999 .

[22] M. Singh , DR Behrendt , "Reactive melt infiltration of silicon-niobium alloys in microporous carbon", J. Mater. Res ., 1994 ; 9: 1701.

[23] M. Singh , DR Behrendt , "Reactive melt infiltration of silicon-molybdenum alloys in microporous carbon", Mater. Sci. And Eng ., 1995 ; A194: 193.

[24] M. Singh , DR Behrendt , "Microstructure and mechanical properties of reaction-formed silicon carbide (RFSC) ceramics", Mater. Sci. And Eng ., 1994 ; A187: 183.

[25] A. Muñoz , J. Martínez Fernández , A. Domínguez Rodríguez , M. Singh , "High temperature compressive strength of reaction formed silicon carbide (RFSC) ceramics", J. Europ. Ceram Soc ., 1998 ; 18:65.

[26] "Procedure to fabricate silicon carbide ceramics from natural precursors ". International patent PCT / ES 02/00483 (11/4/2002).

[27] J. Martínez-Fernández , FM Valera-Feria and M. Singh , "Microstructure and thermomechanical characterization of bimorphic silicon carbide-based ceramics", Scripta Materialia 43 ( 2000 ) 813-818.

[28] J. Martínez-Fernández , FM Valera-Feria FM, A. Domínguez Rodríguez , M. Singh , "Microstructure and thermomechanical characterization of bimorphic silicon carbide-based ceramics, Environment Conscious Materials"; Ecomaterials ISBN: 1-894475-04-6. Canadian Institute of Mining, Metallurgy, and Petroleum., Pp. 733-740 ( 2000 ).

[29] FM Varela – Feria , J. Martínez – Fernández , AR de Arellano – López , and M. Singh , "Low Density Biomorphic Silicon Carbide: Microstructure and Mechanical Properties", J. Europ. Ceram Soc ., Vol. 22 [14-15] pp. 2719-2725 ( 2002 ).

[30] M. Singh , J. Martínez – Fernández , AR of Arellano – López , "Environmentally Conscious Ceramics (Ecoceramics) from Natural Wood Precursors", Current Opinions on Solid State & Material Science Vol. 7, pp. 247-254 ( 2003 ).

[31] Y. Kardashev , I. Burenkov , B. Smirnov , AR of Arellano-López , J. Martínez-Fernández , FM Varela-Feria , "Elasticity and Inelasticity of Biomorphic Silicon Carbide Ceramics", Physics of the Solid State , V .46, N10 ( 2004 ) 1873-1877.

[32] TS Orlova , BI Smirnov , AR de Arellano López , J. Martínez Fernández , R. Sepúlveda , "Anisotropy of electric resistivity of Sapele-based biomorphic SiC / Si composites", Physics of the Solid State , V.47, N2 ( 2005 ), 220-223.

[33] LS Parfenieva , BI Smirnov , IA Smirnov , H. Misiorek , J. Mucha , A. Jezowski , AR de Arellano-López , J. Martínez-Fernández , R. Sepúlveda , "Thermal Conductivity of Bio-SiC and the Si Embedded in Cellular Pores of the SiC / Si Biomorphic Composite ", Physics of the Solid State , Vol. 49, No. 2, pp. 211-214 ( 2007 ).

[34] TS Orlova , DV Il'in , BI Smirnov , IA Smirnov , R. Sepúlveda , J. Martínez-Fernández , and AR de Arellano-López , "Electrical Properties of Bio-SiC and Si Components of the SiC / Si Biomorphic Composite ", Physics of the Solid State , Vol. 49, No. 2, pp. 205–210 ( 2007 ).

[35] IA Smirnov , BI Smirnov , H. Misiorek , A. Jezowski , AR de Arellano-López , J. Martínez-Fernández , FM Varela-Feria , AI Krivchikov , GA Zviagina , and KR Zhekov , "Heat Capacity and Velocity of Sound in the SiC / Si Biomorphic Composite ", Physics of the Solid State , Vol. 49, N 10 1839-1844 ( 2007 ).

[36] IA Smirnov , BI Smirnov , AI Krivchikov , H. Misiorek , A. Jezowski , AR de Arellano-López , J. Martínez-Fernández , R. Sepulveda . "Heat Capacity of Silicon Carbide at Low Temperatures", Physics of the Solid State , V49, N2 1835-1838 ( 2007 ).

[37] AR by Arellano López , J. Martínez Fernández , FM Varela Feria , RE Sepúlveda , MJ López Robledo , J. Llorca , JY Pastor , M. Presas , KT Faber , VS Kaul , KE Pappacena , TE Wilkes , "Processing, microstructure and mechanical properties of SiC-based ceramics via naturally derived scaffolds ", Mechanical Properties and Performance of Engineering Ceramics and Composites Vol. 2, pp. 635-650, Wiley and Sons ( 2007 ).

[38] FM Varela-Feria , J. Ramírez-Rico , AR of Arellano-López , J. Martínez-Fernández , "Reaction-formation Mechanisms and Microstructure Evolution of Biomorphic SiC", Journal of Materials Science , DOI 10.1007 /
s10853-007-2207-4, in press ( 2008 ).

[39] J. Martínez Fernández , A. Muñoz , AR de Arellano López , FM Valera Feria , A. Dominguez-Rodriguez , M. Singh , "Microstructure-mechanical property correlation in siliconized silicon carbide ceramics", Acta Materialia , 51 [11 ] pp. 3259-3275 ( 2003 ).

[40] BK Kardashev , BI Smirnov , AR de Arellano-López , J. Martínez-Fernández , FM Varela-Feria , "Elastic and anelastic properties of SiC / Si ecoceramics", Materials Science and Engineering A, A 442 pp. 444-448 ( 2006 ).

[41] AI Shelykh , BI Smirnov , IA Smirnov , AR de Arellano-López , J. Martínez-Fernández , FM Varela-Feria , "Linear expansion coefficient of biomorphic composite SiC / Si", Physics of the Solid State , 48 [2 ] ( 2006 ), 202-203.

[42] AR de Arellano-López , J. Martínez-Fernández , P. González , C. Domínguez , V. Fernández-Quero , M. Singh , "Biomorphic sic: a new engineered ceramic material" (first issue), Int. Journal of Applied Ceramic Technology Vol. 1 pp. 95-100 ( 2004 ).

[43] LS Parfeneva , BI Smirnov , IA Smirnov , D. Wlosewicz , H. Misiorek , A. Jezowski , J. Mucha , AR of Arellano-López , J. Martínez-Fernández , FM Varela-Feria , and AI Krivchikov , " Heat Capacity of a White-Eucalyptus Biocarbon Template for SiC / Si Ecoceramics ", Physics of the Solid State , Vol. 48, No. 11, pp. 2056–2059 ( 2006 ).

[44] LS Parfenieva , TS Orlova , BI Smirnov , IA Smirnov , H. Misiorek , J. Mucha , A. Jezowski , AR de Arellano-López , J. Martínez-Fernández , FM Varela-Feria , "Anisotropy of the Thermal Conductivity and Electrical Resistivity of the SiC / Si Biomorphic Composite Based on a White-Eucalyptus Biocarbon Template ", Physics of the Solid State , 48 [11] ( 2006 ) 2281-2288.

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[45] LS Parfenieva , TS Orlova , NF Kartenko , NV Sharenkova , BI Smirnov , IA Smirnov , H. Misiorek , A. Jezowski , J. Mucha , AR de Arellano-López , J. Martínez-Fernández , FM Varela-Feria , "Thermal and electrical conductivity of a white eucalyptus bio-carbon matrix for ecoceramic SiC / Si", Physics of the Solid State , 48 [3] ( 2006 ) 441-446.

[46] VV Popov , TS Orlova , J. Ramirez-Rico , AR de Arellano-López , and J. Martínez-Fernández , "Electrical Properties of the SiC / Si Composite and the Biomorphic SiC Ceramic Fabricated from Spanish Beech Wood", Physics of the Solid State , Vol. 50, No. 10, pp. 1819–1825 ( 2008 ).

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Description of the invention

As indicated, it is the subject of present invention the development of a manufacturing process of a ceramic resistive device based on multiphase materials of silicon carbide and silicon from the infiltration of metal alloys in carbon preforms obtained by pyrolysis of cellulosic precursors.

The selection of said cellulosic precursors It is a step of great importance, where you can use both natural wood of different densities (pine, sapelli, beech, etc.), as processed wood (pressed wood, fiberboard medium density wood, etc.). It is an indispensable condition that selected precursors have a pore distribution that produce an optimal microstructure for infiltration. The The need to obtain complex final forms makes use essential of a versatile and robust system of joining these materials, which It is also included in the present invention. Similarly, it has developed a method for the selective removal of silicon, a method for the electrical connection of the device, as well as its thermal and electrical insulation.

The manufacturing process of a ceramic resistive device of the type manufactured with materials multiphase silicon carbide and silicon from cellulosic precursors, object of the present invention comprises, essentially and at least the following stages:

(i) a first stage configured for drying of said cellulosic precursor;

(ii) a second stage of configured pyrolysis to subject the cellulosic precursor to a pyrolysis process consisting of decomposition by heating of matter organic and where this process is carried out in the absence of oxygen, so that volatile substances and water they disappear as non-polluting gases, leaving the coal as process waste;

(iii) a third stage of machining configured to obtain optimal dimensions such that allow to control the resistance in the hot zone keeping power dissipation per area below limits bearable by the material itself;

(iv) a fourth stage of union where they put in contact the preforms of coal, already mechanized, covering previously the joint surface with a paste rich in carbon;

(v) a fifth stage of infiltration with at minus a silicon-rich metal alloy of the element obtained in the previous stages;

(vi) a sixth stage of selective elimination of silicon / metals / phases produced by reaction in the fifth infiltration stage, where, said sixth stage is configured to increase the resistivity in the hot zone of the device heater;

(vii) a seventh stage of metallization configured to facilitate the electrical connection in the cold zone of the heating element in such a way that the multiphase material rich in silicon carbide and silicon itself is metallized by evaporation of noble metals.

(viii) an eighth stage of electrical connection configured to obtain a high quality electrical connection; Y

(ix) a ninth stage of thermal insulation and electric.

With the described procedure it is achieved solve the technical problem posed by a procedure of manufacture of ceramic resistive devices such that components for applications based on the obtaining a hot zone, maintaining some properties optimum hardness, friction resistance, conductivity thermal, electrical conductivity, mechanical resistance, porosity, corrosion resistance, thermal shock resistance and low density.

Similarly, the resistive device ceramic obtained by the described procedure allows to be used in any type of application where they are indispensable its properties in terms of hardness, resistance to friction, thermal conductivity, electrical conductivity, resistance Mechanical, porosity, corrosion resistance, resistance to thermal shock and density as, by way of example, elements of heating, ignition systems, high heatsinks temperature and surge protectors.

Throughout the description and the claims the word "comprises" and its variants not they intend to exclude other technical characteristics, additives, components or steps. For experts in the field, others objects, advantages and features of the invention will come off partly from the description and partly from the practice of invention. The following examples and drawings are provided by way of of illustration, and are not intended to be limiting of the present invention In addition, the present invention covers all possible combinations of particular and preferred embodiments here indicated.

Preferred Embodiment of the Invention

In a preferred embodiment, the process of manufacturing a ceramic resistive device from cellulosic precursors, object of the present invention comprises, at least:

(i) a first stage of drying the precursor configured so that the precursor is subjected to a first phase drying, performed for a time between 12 and 36 hours in an oven at a temperature between 50ºC and 150 ° C if the precursor has been previously treated for use industrial or for 36 to 150 hours otherwise;

(ii) a second stage of pyrolysis, configured to subject the cellulosic precursor to a pyrolysis process consisting of decomposition by heating of matter organic and where this process is carried out in the absence of oxygen, so that volatile substances and water they disappear as non-polluting gases, leaving the coal as process waste More specifically, pyrolysis is performed with partial oxygen pressures of the order of 10-1 Torr or lower, at heating rates below 2ºC per minute up to temperatures higher than 600ºC and later cooling at a speed between 1 and 15 ° C per minute from the maximum temperature reached to room temperature;

(iii) a third stage of machining, where after the process of carbonization of the cellulosic precursor the Preforms are easily machinable. Media are used robotized by numerical control to obtain the definitive form of the heating device to be manufactured. It's special importance the design of the optimal dimensions that increase the resistance in the dissipative zone, maintaining the dissipation of power per area below the limits supported by the material.

(iv) a fourth stage of union where they put in contact the preforms of coal, already mechanized, covering previously the bonding surface with a carbon rich paste, in such a way that once the piece joined in the next procedure stage, the joints have a similar resistance to the materials developed through this invention, so that do not produce a decrease in their properties during use in specific applications. It is essential that the union be of extraordinary quality This step allows to obtain complex shapes in 3D

(v) a fifth infiltration stage where This process can be performed with various metal alloys rich in silicon, applying partial oxygen pressures below 10-1 torr and temperatures below 1800 ° C. He heating process is carried out at speeds below 20ºC per minute from room temperature to reach temperature final infiltration, which will depend on the cellulosic precursor used. The cooling process is similarly performed. up to room temperature The silicon alloys used in the infiltration must be sprayed for the process to be effective, distributing the powder evenly over the carbon preform, so that by capillarity complete infiltration is achieved. The whole process is carried out in a non-reactive crucible, for example of boron nitride

The amount of silicon will be stoichiometric, according to the atomic reaction (1: 1) of the compound SiC. It will weigh first the carbon preform, then the number of moles of carbon and subsequently the amount of the alloy of silicon to use. Usually this amount is increased by approximately 15-25% to ensure a reaction full carbon. Still the unreacted carbon burns in the use of the piece at high temperature, affecting its microstructure During the metal infiltration process, the interaction between molten metal and porous carbon structure It plays a fundamental and essential role. If the pressure, the molten metal must wet the porous structure of Coal. Wettability describes whether a liquid will separate from a solid substrate or even adheres to wet it. When a liquid and a solid are in contact, the balance of the surface energies between steam and solid, solid and liquid, and Vapor and liquid determines the wettability of the liquid in solid.

To improve wettability, infiltration It can be made with alloys, which allows you to modify the angle of wet. The use of alloys also modifies the melting point, so that the desired reactions can occur at a lower temperature.

(vi) a sixth stage of selective elimination of silicon / metals / phases produced by reaction, where, due to the silicon-rich alloys used in the process of infiltration are usually electrical conductors, to increase the resistivity in the hot zone of the heating device, it They usually selectively remove. To carry out this process a chemical attack can be carried out with solutions containing one to several of the following acidic compounds: HF, HNO 3, HCl, H 2 SO 4. Or you can also perform a procedure electrochemical with salt water.

In the first case, to eliminate any excess silicon in the pores a mixture of the acid is used nitric (HNO3) and hydrofluoric acid (HF). Nitric acid first oxidizes silicon to form silicon dioxide (SiO_ {2}) that is removed from the pores by the HF:

one

In the second case, to remove silicon residual is used a bath with salt water, with a concentration between 2 and 30 g of salt per liter of water. Using two electrodes, a DC electric current that causes a electrochemical reaction of salt oxidation, favoring residual silicon removal.

After removing excess silicon, it will Obtains a porous silicon carbide structure. The areas that not interested in being chemically attacked will be covered with enamel, to prevent acid solutions from attacking the material multiphase rich in silicon carbide and silicon.

(vii) a seventh stage of metallization, in where to facilitate the electrical connection in the cold zone of the heating element, the carbide-rich multiphase material of silicon and silicon itself is usually metallized by evaporation of noble metals, such as gold or silver. The process itself is carried out under partial oxygen pressures lower than 10 ^ - 3 Torr, getting to deposit a film of several micrometers thick.

(viii) an eighth stage of electrical connection in where cold plates are already placed on metallic areas Pure silver that are connected to high temperature cables. This form ensures a high quality electrical connection.

(ix) a ninth stage of thermal insulation and electrical, where depending on the application for which develop the heating device, the cold parts and the Connection zone are thermally and electrically isolated. To do this they mechanize ceramic bodies, generally of silicates, which are electrical insulators, inside which the elements will be sealed heaters with high temperature ceramic adhesives too electrically insulating.

Among the advantages of the procedure as well described, after the relevant tests and tests, we can highlight:

- Low cost, due to the low temperatures of processed, between 600 and 900ºC lower than the processed by sintered, since it is not necessary to start from carbide powder silicon, and because the pieces do not need final finishing, well the mechanization of the carbon preform avoids the latter He passed.

- Use of regenerable materials, with the consequent non-production of environmental pollution, being possible the manufacture of complex shapes with the simple pre-molding of the wood of origin, and the use of additives is not necessary. further This procedure is performed at a faster manufacturing speed and at lower temperature than manufacturing procedures by reaction with gases.

- The ceramics obtained have the structure fibrous wood used in manufacturing, ideal structure for optimal mechanical properties as it is the result of improvement of the evolutionary process. The resulting products of infiltration with silicon, reach 50% densities lower, resistance similar to silicon carbide sintered and far superior to those of silicon carbide compacted by reaction.

- A structure is obtained naturally similar to that of continuous fiber composite materials, materials designed to improve the intrinsic low toughness of Ceramics

- You can get a wide range of microstructures and properties for specific applications simply using the right vegetable precursor.

- The generation of mixed materials ceramic-metal, opens a wide field of applications associated with the generation of materials multifunctional, which combine high mechanical strength with electrical, magnetic, thermal and optical properties designed to measure. In particular, with a tight selection of alloys in the infiltration process, and subsequent elimination by attack chemical, we can control the resistivities of the different zones of the heating device.

- The reaction junctions described in this invention allow the manufacture of these materials with shapes complex and robust, which opens the range of applications since these are hardly attainable with the conventional methods of manufacturing silicon carbide commercial.

Claims (11)

1. Method of manufacturing a ceramic resistive device of the type manufactured with multiphase materials of silicon carbide and silicon from cellulosic precursors characterized in that it comprises at least the following steps: (i) a first stage configured for drying of said cellulosic precursor; (ii) a second stage of configured pyrolysis to subject the cellulosic precursor to a pyrolysis process consisting of decomposition by heating of matter organic and where this process is carried out in the absence of oxygen, so that volatile substances and water they disappear as non-polluting gases, leaving the coal as process waste; (iii) a third stage of machining configured to obtain optimal dimensions such that allow to control the resistance in the hot zone keeping power dissipation per area below limits bearable by the material itself; (iv) a fourth stage of union where they put in contact the preforms of coal, already mechanized, covering previously the joint surface with a paste rich in carbon; (v) a fifth stage of infiltration with at minus a silicon-rich metal alloy of the element obtained in the previous stages; (vi) a sixth stage of selective elimination of silicon / metals / phases produced by reaction in the fifth infiltration stage, where, said sixth stage is configured to increase the resistivity in the hot zone of the device heater; (vii) a seventh stage of metallization configured to facilitate the electrical connection in the cold zone of the heating element in such a way that the multiphase material rich in silicon carbide and silicon itself is metallized by evaporation of noble metals; (viii) an eighth stage of electrical connection configured to obtain a high quality electrical connection; Y (ix) a ninth stage of thermal insulation and electric.

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2. Procedure according to claim 1 where the first stage of drying is carried out in a first phase and with a form selected from: (a) for a time between 12 and 36 hours in an oven at a temperature between 50ºC and 150 ° C if the precursor has been previously treated for use industrial; or (b) for 36 to 150 hours in a stove at one temperature between 50ºC and 150ºC if the precursor has not previously treated for industrial use.

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3. Procedure in accordance with the claims 1 and 2 wherein the second pyrolysis stage is performs with partial oxygen pressures of the order of 10-1 Torr or lower, at heating rates below 2ºC per minute up to temperatures above 600ºC and later cooling at a speed between 1 and 15 ° C per minute from the maximum temperature reached to room temperature. 4. Procedure in accordance with previous claims wherein in the third stage of machining means roboticized by numerical control are used to mechanize the carbon preform and obtain the definitive form of heating device to be manufactured. 5. Procedure in accordance with previous claims wherein the sixth stage of elimination Selective silicon / metal / phases is implemented by at least one of The following solutions: (a) a chemical attack with solutions that contain one to several of the following acidic compounds: HF, HNO 3, HCl, H 2 SO 4; (b) an electrochemical procedure with water salty, and where the areas that do not interest be chemically attacked will be covered with enamel, so that prevent acid solutions from attacking multiphase material Rich in silicon carbide and silicon.

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6. Procedure in accordance with the claim 5 wherein the chemical attack to eliminate any excess silicon in the pores a mixture of the acid is used nitric (HNO3) and hydrofluoric acid (HF), so that nitric acid first oxidizes silicon to form the dioxide of silicon (SiO2) that is removed from the pores by the HF: 2 7. Procedure in accordance with the claim 5 wherein the electrochemical process with water Salty is implemented to remove residual silicon with a bath with salt water, with a concentration between 2 and 30 g of salt per each liter of water, so that using two electrodes, it is made pass an electric current through direct current that causes an electrochemical reaction of salt oxidation such that the residual silicon is removed, and where, in addition, a Porous silicon carbide structure. 8. Procedure in accordance with previous claims wherein the seventh stage of metallization is carried out under partial oxygen pressures less than 10 - 3 Torr, getting to deposit a film of several micrometers thick noble metal. 9. Procedure in accordance with the previous claims wherein in the eighth stage of connection electric in cold areas already metallized plates are placed Pure silver that are connected to high temperature cables. 10. Procedure in accordance with previous claims wherein in the isolation stage electrical and thermal the product resulting from the previous stages it is assembled in an electrical insulating ceramic body, in whose inside the heating elements are sealed with adhesives High temperature ceramics also electrically insulating. 11. Ceramic resistive device of the type manufactured with multiphase materials of silicon carbide and silicon from cellulosic precursors manufactured according to claims 1 to 10.