Method of obtaining nanostructured silicon-carbide ceramics

FIELD: chemistry.

SUBSTANCE: invention relates to inorganic chemistry, namely to obtaining silicon-carbide materials and products, and can be applied as thermal-protective, chemically and erosion resistant materials, used in creation of aviation and rocket technology, carriers with developed surface of heterogeneous catalysis catalysts, materials of chemical sensorics, filters for filtering flows of incandescent gases and melts, as well as in nuclear power industry technologies. To obtain nanostructures SiC ceramics solution of phenolformaldehyde resin with weight content of carbon from 5 to 40% with tetraethoxysilane with concentration from 1·10-3 to 2 mol/l and acidic catalyst of tetraethoxysilane hydrolysis id prepared in organic solvent; hydrolysis of tetraethoxysilane is carried out at temperature 0÷95°C with hydrolysing solutions, containing water and/or organic solvent, with formation of gel. Obtained gel is dried at temperature 0÷250°C and pressure 1·10-4÷1 atm until mass change stops, after which carbonisation is realised at temperature from 400 to 1000°C for 0.5÷12 hours in inert atmosphere or under reduced pressure with formation of highly-disperse initial mixture SiO2-C, from which ceramics is moulded by spark plasma sintering at temperature from 1300 to 2200°C and pressure 3.5÷6 kN for from 3 to 120 min under conditions of dynamic vacuum or in inert medium. Excessive carbon is burned in air at temperature 350÷800°C.

EFFECT: obtaining nanostructured silicon-carbide porous ceramics without accessory phases.

4 cl, 4 dwg, 3 ex

 

The invention relates to inorganic chemistry, particularly, to obtain the silicon carbide materials and products. The resulting SiC ceramics can be applied as thermal insulation, chemical and erosion-resistant materials used mainly for the development of aviation and rocket technology, media extended surface catalysts heterogeneous catalysis, materials, chemical sensors, silicon-carbide filter for filtering a flow of hot gases and melts, as well as in the technology of atomic energy, chemical and petrochemical industries.

Using the proposed method allows to obtain a silicon carbide materials necessary porosity in nanostructured state during spark plasma sintering (spark plasma sintering is, SPS) products.

A method of producing silicon carbide ceramics with the use of spark plasma sintering NANOKRISTALLIChESKOGO carborundum powder with addition as sintering additives mixture Y2O3and Al2O3or Al5Y3O12at a temperature of 1800°C (E. Ciudad, E. Sanchez-Gonzalez, O. Borrero-Lopez, F. Guiberteau, M. Nygren, A. L. Ortiz, Sliding-wear resistance of ultrafine-grained SiC densified by spark plasma sintering is with 3Y2O3+5Al2O3or Y3Al5O12additives // Scripta Materialia. 2013, 69(8), 598-601; M. Hotta, Microstructural contol for ultrafine-grained non-oxide structural ceramics // Journal of the Ceramic Society of Japan. 2012. 120(4). 123-130).

The main disadvantage of this method is the necessity of a preliminary synthesis or acquisition rather expensive NANOKRISTALLIChESKOGO powder of silicon carbide, which usually has an extremely high degree of aggregation, which determines the long introduction stage of a joint grinding and mixing of SiC with sintering additives, and the quality of the resulting product depends essentially on the parameters of this technological stage.

A method of producing ceramics complex composition ZrC-SiC as a result of a two-stage process (L. Kljajevic, S. Nenadovic, M. Nenadovic, D. Gautam, T. Volkov-Husovic, A. Devecerski, B. Matovic, Spark plasma sintering is of ZrC-SiC ceramics with LiYO2additive // Ceramics International(2013), 39(5), 5467-5476): 1) preliminary carbothermic reduction natural mineral zircon ZrSiO4activated charcoal, which was added to the natural raw material, at a temperature of 1600°C in vacuum for 1 hour; 2) spark plasma sintering obtained in the first stage product with a sintering additive LiYO2at a temperature of 1600°C for 3 min in a vacuum at a pressure of 35 MPa.

The disadvantage of this method is coming from the nature of the raw material composition containing silicon carbide and zirconium carbide, the need for the introduction of sintering additives to reduce the temperature of manufacture of the material, an extra stage mesenyashin components (ZrSiO 4and carbon, the product of the first stage and LiYO2and conducting two separate technological operations - carbothermic synthesis of a mixture of carbides of zirconium and silicon and spark plasma sintering with the formation of ceramics.

There is a method of producing ceramic composition of ZrB2-SiC obtained with the use of spark plasma sintering, similar to the above-described method of obtaining ceramics ZrC-SiC (Hyeon-Cheol Oh, Sea-Hoon Lee, Sung-Churl Choi, a Two-step reduction process and spark plasma sintering is for the synthesis of ultra fine SiC and ZrB2powder mixtures // Int. Journal of Refractory Metals and Hard Materials. 2014. 42. 132-135). In the first stage are formed of complex powders ZrC-SiC as a result of synthesis on the basis of mixtures of ZrSiO4and carbon, obtained as a result of prolonged joint grinding directly in the fiery spark sintering at temperatures of 1400-1500°C for 1 hour in vacuum. Further, the products are subjected to joint grinding with boron carbide with a subsequent spark plasma sintering at 1400°C for 1 hour to form a composite ceramic composition of ZrB2-SiC.

The main disadvantage is the necessity of first obtaining mixtures of the oxide feedstock is ZrSiO4with carbon as a result of a joint grinding to create the best degree of homogenization and to optimize conditions for subsequent carbothermal synthesis. In addition�about, the aim of this work is to obtain a ceramic material composition of ZrB2-SiC rather than individual silicon-carbide material.

A method of obtaining ceramics based on silicon nitride Si3N4(Z. Taslicukur, F. Cinar Sahin. G. Goller, O. Yucel, N. Kuskonmaz, Reactive Spark Plasma sintering is of Si3N4Based Composites //Advances in Science and Technology. 2010. 62. 185-190) during the reaction sintering on the basis of powders of Si3N4, silicon dioxide, carbon, and sintering additives - Y2O3and AIN who are at the first stage are subjected to joint grinding for better distribution of the components is then conducted spark plasma sintering with simultaneous carbothermic obtaining silicon carbide component at a temperature of 1650°C at a pressure of na for 5 min.

The main negative side is the need stage of milling the starting powders for the homogenization of the components, the presence of sintering additives, chemical violate the purity, and the fact that the composition of the ceramics does not correspond to the silicon carbide.

There is a method of obtaining nanostructured silicon carbide with the use of the Sol-gel method (R. E. Simonenko, N. P. Simonenko, V. A. Derbenev, V. A. Nikolaev, D. V. Grashchenkov, V. G. Sevastyanov, E. N. Kablov, N. T. Kuznetsov, Synthesis of Nanocrystalline Silicon Carbide using the Sol-Gel Technique // Russian Journal of Inorganic Chemistry. 2013. 58(10). 1143-1151). Nanocrystalline powder of silicon carbide was obtained as a result �kislotnogo hydrolysis of tetraethoxysilane with the simultaneous presence of polymeric carbon source - phenol-formaldehyde resin with the formation of transparent gels were further drying, carbonization of the organic fragments at a temperature of 850°C in an inert atmosphere and carbothermic synthesis under dynamic vacuum at temperatures of 1100-1500°C.

The main disadvantage of this method is that it does not involve the production of silicon carbide products, but only the nanostructured powder, which can then be subjected to spark plasma sintering.

The closest in technical essence and achieved result is a method of manufacturing a ceramic composition B4C-SiC (F. C. Sahin, V. Arak, I. Akin, H. E. Kanbur, D. H. Genckan, A. Turan, G. Goller, O. Yucel, Spark plasma sintering is of B4C-SiC composites // Solid State Sciences. 2012. 14(11-12). 1660-1663) with the use of spark plasma sintering on the basis of the starting powders B4C-SiO2-C (from 5 to 20% of the resulting SiC), for most of homogenization which the first stage is the joint grinding of the powders, and then the synthesis of silicon carbide in the process of spark plasma sintering at a temperature 1700-1750°C and a pressure of 40 MPa for 5 min.

The main disadvantage is the need for pre-homogenization as a result of a joint grinding of the starting powders. Despite intensive grinding of the carbothermal synthesis required the use of a high t�of Imperator - 1700-1750°C. the Composition obtained by this method of a ceramic material different from silicon carbide due to the presence of boron carbide (B4C. in addition, this method does not allow to vary the porosity of the resulting material.

The invention aims at finding a method for producing nanostructured ceramics of silicon carbide without the inclusion of extraneous phases and sintering additives, allowing to obtain the values of porosity.

The technical result is achieved in that a method of producing nanostructured silicon carbide ceramics, namely, that prepare a solution of phenol formaldehyde resin with a mass carbon content of from 5 to 40% in an organic solvent or mixture of organic solvents with tetraethoxysilane with a concentration of 1·10-3to 2 mol/l and an acid catalyst hydrolysis of tetraethoxysilane, which carried out the hydrolysis of tetraethoxysilane at a temperature of 0÷95°C gidrolizuemye solutions, representing either water or an organic solvent or mixture of solvents containing water, with the formation of the gel, then drying the gel at a temperature of 0÷250°C and a pressure of 1·10-4÷1 ATM until the cessation of mass change, followed by carbonization of the xerogels heat treatment at a temperature of�e from 400 to 1000°C for 0.5÷12 hours or in an inert atmosphere, or under reduced pressure, with the formation of highly dispersed reactive starting mixture of SiO2-From which further molded ceramics by spark plasma sintering at a temperature of from 1300 to 2200°C and a pressure of 3.5÷6 kN for 3 to 120 minutes under dynamic vacuum or in an inert atmosphere, after which excess carbon is burned in air at temperatures of 350÷800°C.

It is advisable that phenol-formaldehyde resin is selected from the range: Bakulina resin, Novolac resin and resol resin.

The technical result is also achieved by the fact that as the organic solvents which dissolve the components, use of mono - and polynuclear alcohols, aliphatic and aromatic hydrocarbons, their halogen derivatives, ethers, aldehydes, ketones, organic acids and others, having a boiling point in the range 50÷50°C.

It is also advisable that as the acid catalyst hydrolysis of tetraethoxysilane using acids selected from the series: hydrochloric, nitric, formic, acetic, oxalic, maleic, citric.

The ratio of phenol-formaldehyde resin and tetraethoxysilane selected on the basis of the calculated content necessary for the further formation of excessive porosity of the carbon, which is burned out in air at temperatures of 350÷800°C.

The selected reage�you their concentration, solvent ratio and ensure the formation of homogeneous systems, which undergo hydrolysis and polycondensation, leading to the formation of a maximally homogeneous gels, where the metal atoms and the fragments of resin - phenol-formaldehyde resin uniformly distributed in the most that can synthesize most of disperse and reactive system SiO2-C, which are applicable for carrying out carbothermal synthesis of SiC already in the process of spark plasma sintering of nanostructured silicon carbide products.

The application of solutions of phenol-formaldehyde resin with a mass content resulting from pyrolysis of carbon less than 5% does not ensure high-quality silicon - carbon-containing gels, and above 40% does not allow the chemical nature of phenol-formaldehyde resin.

The use of concentrations of tetraethoxysilane less than 1·10-3mol/l prevents gelation, and more than 2 mol/l does not achieve a technical result, because it leads to the salting out phenol-formaldehyde resin from the solution.

The concentration and nature of the acid catalyst and gidrolizuemye agent is determined by the characteristics of specific syntheses is a necessary time of gelation and the structure of the gels. The time of hydrolysis caused by chemicals and fisiche�by their gelation.

The process of hydrolysis at a temperature of 0°C is very slow, and at temperatures over 95°C suitable for removal of water from the system and uneven in the amount of gelation.

Use when drying the gel temperature of 0°C much it slows down, and at >250°C possible release and uneven drying of the gel.

The pressure applied during drying of the gel, more than 1 ATM complicates the distillation of solvents and hydrolysis products, and less than 1·10-4ATM also may result in the release of the gel from the reactor.

At a temperature of carbonation of less than 400°C there is an incomplete pyrolysis of phenol-formaldehyde resin and other organic fragments of the gel, and at temperatures above 1000°C there is an integration of the formed silicon dioxide and carbon, reducing their reactivity in the process of carbothermal synthesis of silicon carbide.

The time of carbonization is less than 0.5 h does not allow to fully complete the processes of thermal degradation of organic gel fragments and the shutter speed is more than 12 hours did not increase the reaction yield.

The process of spark plasma sintering at temperatures below 1300°C does not allow the complete conversion of silica carbide, and carrying out the process at temperatures above 2200°C did not increase the reaction yield.

Use in manufacture�attachment ceramic pressure below 3.5 kN impossible due to the specific settings for spark plasma sintering and will not lead to the formation of compact samples the use of pressure above 6 kN does not affect the process for the production of nanostructured ceramics.

The use of exposure time on stage spark plasma sintering of samples less than 3 minutes (especially at low temperatures) does not allow for the complete conversion of silica carbide, and the use of time more than 120 minutes is not practical from the point of view of increasing the degree of conversion of SiO2in SiC.

Burning of excess carbon, which allows to vary the porosity of the resulting ceramics based on silicon carbide in air at a temperature below 350°C does not occur, and at temperatures above 800°C results in complete or partial oxidation of the synthesized nanocrystalline silicon carbide.

The invention consists in that for the production of nanostructured SiC-ceramics by the Sol-gel method for synthesizing highly dispersed starting mixture of SiO2-C, where the components are maximally homogeneous, almost at the molecular level are distributed in each other, which allows in situ, i.e. without isolation of the separate stages of pre-production of nanocrystalline SiC powder, conduct carbothermal synthesis of SiC directly during fabrication of ceramics by the method of spark plasma sintering. Thus by changing the initial ratio of tetraethoxysilane precursor of SiO 2and phenol-formaldehyde resin - carbon source, formed at the stage of carbonization by pyrolysis, due to the burning of a predetermined excess amount of carbon it is possible to vary the porosity of the resulting ceramic materials. In addition, the introduction in excess of the stoichiometric ratio, n(C):n(SiO2)≥3, the carbon, produced during the carbonization, prevents the aggregation of the components of the starting system SiO2-C and formed of SiC at elevated temperatures, i.e. contributes to the formation of nano-sized silicon carbide grains in ceramics.

Hydrolytic activity of the resulting solutions containing tetraethoxysilane and phenol-formaldehyde resin, may be varied by changing the type and concentration of acid catalyst, gidrolizuemye agent, solvent, temperature and time of thermal process.

Selected time-temperature regimes of drying, carbonization and carbothermal synthesis in the process, spark plasma sintering provide optimal conditions for the processes for the synthesis of highly starting mixtures of SiO2-C and silicon carbide ceramic materials based on them.

The essence of the claimed invention is illustrated by the following accompanying illustrations:

Fig. 1. Roentgenogram�and silicon carbide ceramics, example 1, the radiation ofCukα.

Fig. 2. Micrograph of silicon carbide ceramics, example 1 (according to scanning electron microscopy).

Fig. 3. Micrograph of silicon carbide ceramics, example 2 (according to scanning electron microscopy).

Fig. 4. Radiograph carborundum ceramics, example 3, the radiation ofCukα.

The achievement of the claimed technical result is confirmed by the following examples. The examples illustrate but do not limit the proposed technical solution.

Example 1. In 6 ml of acetone was dissolved 15,6 ml of tetraethoxysilane, 14 g of an ethanol solution of phenol formaldehyde resin with a mass content of carbon 19% and 12 ml of formic acid. Then, the solution was heated to a temperature of 35°C and was added to 4 ml of water. After hydrolysis and gelation of the gel dried at atmospheric pressure and a temperature of 120°C until the cessation of the mass change. The resulting xerogel subjected to heat treatment at 800°C under reduced pressure to achieve pyrolysis of organic components and production of highly dispersed maximum�about homogeneous starting mixture of SiO 2-C, which was placed in a graphite mold, to summarize, was evacuated and exposed electrical impulses at a temperature less than 1800°C under a pressure of 4.5 kN aged for 35 min. Excess carbon is burned in air at 550°C. According to x-ray analysis (Fig. 1) is formed exclusively rhombohedral silicon carbide without the inclusion of extraneous phases, with an average crystallite size of 50 nm. Scanning electron microscopy showed (Fig. 2) that is formed nanostructured porous ceramics with an average particle size of 90 nm, are combined into aggregates.

Example 2. In 40 ml of ethanol was dissolved 5 ml of tetraethoxysilane, 28 g of a solution of phenol formaldehyde resin with a mass carbon content of 40% and 3 ml of hydrochloric acid. Then, the solution was cooled to 0°C and was added to 25 ml of water. After hydrolysis and gelation, the gel was dried at a pressure of 1·10-3-1·10-4ATM and a temperature of 250°C until the cessation of the mass change. The resulting xerogel subjected to heat treatment at 400°C under reduced pressure to achieve pyrolysis of the organic components and the maximum production of highly dispersed homogeneous starting mixture of SiO2-C, which was placed in a graphite mold, to summarize, was evacuated and exposed e�actrices pulses at a temperature less than 1300°C under a pressure of 6 kN aged for 120 min. Excess carbon is burned in air at 350°C. According to x-ray analysis formed cubic silicon carbide without the inclusion of extraneous phases, with an average crystallite size of 35 nm. Scanning electron microscopy showed (Fig. 3) that is formed nanostructured porous ceramic with a particle size of 40-60 nm, are combined into aggregates.

Example 3. In 5 ml of ethanol was added 10 ml of tetraethoxysilane, 110 g of a solution of phenol formaldehyde resin with a mass content of carbon 15% and 14 g of citric acid. Then, the solution was heated to a temperature of ~95°C and add 10 ml of water. After hydrolysis and gelation, the gel was dried at a pressure of 0.01-0.1 ATM and a temperature of 0°C until the cessation of the mass change. The resulting xerogel was subjected to thermal treatment at a temperature of 1000°C for 0.5 h in an inert atmosphere to achieve pyrolysis of the organic components and the maximum production of highly dispersed homogeneous starting mixture of SiO2-C, which was placed in a graphite mold, to summarize and exposed electrical impulses at a temperature of ~2200°C under a pressure of 3.8 kN aged for 3 min in an inert environment. Excess carbon is burned in air at a temperature of 950°C. According to x-ray analysis (Fig. 4) is formed of hexagonal CT�ID silicon without the inclusion of extraneous phases, with an average crystallite size of 90 nm. Scanning electron microscopy showed that the formed nanostructured porous ceramics with particle size ~100 nm, are combined into aggregates.

Thus, the inventive method has the following advantages:

- allows you to obtain silicon carbide ceramic of the desired porosity without introducing contaminants sintering additives or unauthorized use of ceramic powders;

- gives the possibility of forming a ceramic silicon nanostructured materials in the state;

- allows you to exclude specific stage of preliminary joint grinding of the starting powders for mixing the components (silica and carbon) due to the synthesis of the starting mixture of SiO2-C, where the components are maximally homogeneous, almost at the molecular level are distributed in each other, which allows to reduce the temperature stage carbothermic synthesis in situ at the time of manufacture ceramics by spark plasma sintering;

- without adding additional stabilizing additives to form gels that do not undergo delamination or deposition;

- application of the process of spark plasma sintering due to the fact the method allows to keep low the size of individual particles with simultaneous f�armirovanie durable ceramic.

Getting spark plasma sintering with the use of the Sol-gel method, silicon carbide ceramics, characterized by a nanocrystalline structure, a predetermined porosity and lack of inclusions of foreign phases, can be used to create heat, chemically and erosion-resistant materials, used mainly for the development of aviation and rocket technology, media extended surface catalysts heterogeneous catalysis, materials, chemical sensors, silicon-carbide filter for filtering a flow of hot gases and melts in the technology of atomic energy, chemical and petrochemical industries.

1. A method of producing nanostructured silicon carbide ceramics, namely, that prepare a solution of phenol formaldehyde resin with a mass carbon content of from 5 to 40% in an organic solvent or mixture of organic solvents with tetraethoxysilane with a concentration of 1·10-3to 2 mol/l and an acid catalyst hydrolysis of tetraethoxysilane, which carried out the hydrolysis of tetraethoxysilane at a temperature of 0÷95°C gidrolizuemye solutions, representing either water or an organic solvent or mixture of solvents containing water, with the formation of the gel, then drying the gel when the pace�the atur 0÷250°C and a pressure of 1·10 -4÷1 ATM until the cessation of mass change, followed by carbonization of the xerogels heat treatment at a temperature of from 400 to 1000°C for 0.5÷12 hours or in an inert atmosphere or under reduced pressure, with the formation of highly dispersed reactive starting mixture of SiO2-C, from which further molded ceramics by spark plasma sintering at a temperature of from 1300 to 2200°C and a pressure of 3.5÷6 kN for 3 to 120 minutes under dynamic vacuum or in an inert atmosphere, after which excess carbon is burned in air at temperatures of 350÷800°C.

2. A method according to claim 1, characterized in that the phenol-formaldehyde resin is selected from the range: Bakulina resin, Novolac resin and resol resin.

3. A method according to claim 1, characterized in that as the organic solvents which dissolve the components, use of mono - and polynuclear alcohols, aliphatic and aromatic hydrocarbons, their halogen derivatives, ethers, aldehydes, ketones, organic acids and others, having a boiling point in the range 50÷150°C.

4. A method according to claim 1, characterized in that as the acid catalyst hydrolysis of tetraethoxysilane using acids selected from the series: hydrochloric, nitric, formic, acetic, oxalic, maleic, citric.



 

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34 cl, 8 ex, 5 tbl, 9 dwg

FIELD: electricity.

SUBSTANCE: invention is related to electrochemical installation intended to shape nanosized coating and may be used in semiconductor and electronics industry. The installation contains a computer, a controller and manipulator 1 mounted at the rack 2 rotatable around vertical axis and equipped with holder 3 for a processed sample 4. Around the manipulator 1 rack there are electrochemical cells 5 with electrodes connected to one pole of current source. The sample 4 submerged to electrochemical cells is connected to the other pole of current source. Holder 3 is installed so that it can be moved in regard to manipulator 1, and at that sample 4 in downwardmost position of holder 3 is placed in one of electrochemical cells. One of electrochemical cells is made as measuring cell 7 to control parameters of the processed sample 4. The installation is equipped with tube-type furnace 8 intended for thermal processing of the sample.

EFFECT: potential determining and setting of the required parameters for obtained nanomaterial against absolute value and conditions of their change.

4 dwg

FIELD: physics.

SUBSTANCE: proposed shutter comprises locally smelting or evaporating mirror metal film located in focal area of the lens and secured by translucent substrate. On radiation side said substrate includes also the ply of translucent liquid of solid sol with nanoparticles in size smaller than radiation wavelength. Mirror film is arranged on said substrate on radiation side or opposite side.

EFFECT: lower threshold of shutter operation.

4 dwg

FIELD: chemistry.

SUBSTANCE: method includes treating the surface of crystalline silicon by electrochemical etching in hydrofluoric acid solution with concentration of 20-30% while supplying current with surface density of 750-1000 mA/cm2 for 5-30 s to obtain hydrophobic silicon or supplying current with surface density of not more than 650 mA/cm2 for 5-30 s to obtain hydrophilic silicon.

EFFECT: method enables to obtain a surface with multimodal nano- or microporosity in a single step.

4 dwg

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