RU2736195C1 - Method of producing porous material based on silicon nitride - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to production of permeable composite materials based on silicon nitride intended for use as air and fluid filters, porous membranes, sorbents, neutralizer bases, composite elements of catalysts and catalyst supports. Method is implemented in a mode of self-sustained combustion of powders of ferroalloys of ferrosilicon or ferrosilicoaluminium, or wastes of their production. Alloys are preliminarily mixed with a solution of polyvinyl alcohol, the mixture is moulded from workpieces which are subjected to cryostructuring by freezing at temperature below 0° with further defrosting for 4–5 hours at room temperature and drying at 60 °C in vacuum at residual pressure of not more than 0.01 MPa. Components of mixture are taken in ratio, wt. %: ferroalloy powder - 50–70, polyvinyl alcohol - 3–10, water is the rest. Ferroalloy particles are smaller than 100 mcm. Combustion is carried out at pressure of 1–10 MPa. Freezing is performed at minus 0.5–25 °C for 15–20 hours.

EFFECT: method extends the field of application of articles from cheap, accessible ferroalloys without electricity consumption, fastness of the synthesis process, enables to recycle wastes from ferroalloy production, provides high operational characteristics of porous material (compressive strength 37_0–38_3 MPa, porosity 35_5–50_0 %, resistance to dynamic and static loads of articles), obtaining optimum equal porosity of material.

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Description

The invention relates to powder metallurgy, in particular to the production of permeable composite materials from silicon nitride, designed to work as air and liquid filters, porous membranes, sorbents, base neutralizers, catalyst components and catalyst carriers with wear resistance, high strength, corrosion and temperature resistance, inertness in relation to many aggressive media.

According to experts, silicon nitride is the most multifunctional among the known ceramic materials [L.N. Chukhlomina, Yu.M. Maximov, V.I. Vereshchagin. Self-propagating high-temperature synthesis of composite nitride-containing ceramic materials. - Novosibirsk: Nauka, 2013. - 260 p.].

In the review [Gilev V.G. Obtaining highly porous materials from silicon nitride. New refractories, 2017, No. 1, p. 40-47] presents the known methods of obtaining highly porous materials from silicon nitride. They are all energy intensive and expensive.

In contrast to traditional methods of obtaining composite ceramic materials (furnace, plasma-chemical, reaction sintering, hot pressing, etc.), the process of self-propagating high-temperature synthesis (SHS) makes it possible to quickly carry out using simple equipment, in the absence of energy consumption, under conditions of increased pressure and maximum temperatures. synthesis of nitride-containing composites. Economic costs per unit of production are reduced by 30-40%. The interaction of metals with nitrogen in the combustion mode refers to filtration combustion, the foundations of which were laid in the works of Academician A.G. Merzhanov, I.P. Borovinskaya, A.P. Aldushina. The traditional SHS technology for the synthesis of nitrides involves the use of powder materials.

SHS as a method for producing inorganic materials combines low energy consumption, wastelessness, and the possibility of dynamically varying the structural and other properties of products. These prerequisites make it possible to widely use an economical and simple technology for producing porous products [Advanced materials and technologies for self-propagating high-temperature synthesis: textbook. allowance / E.A. Levashov, A.S. Rogachev, V.V. Kurbatkina and others - M .: Ed. House MISIS, 2011. - p. 145-147]. The porous structure of SHS products depends on a number of factors, among which the composition and structure of the charge, the volumetric rate of release of impurity gases, and the presence of a liquid phase in the combustion wave are of primary importance. By varying these parameters, it is possible to regulate the porosity of the formed SHS-products in a very wide range: from practically zero porosity to very high (96%) [Amosov AP, Borovinskaya IP, Merzhanov A.G. Powder technology of self-propagating high-temperature synthesis of materials: Textbook. manual. / Under the scientific editorship of V.N. Antsiferova. - M .: Mechanical Engineering-1, 2007. - S. 379-386, 462-468].

In SHS technology, ferroalloys are used as starting reagents for obtaining nitride and oxynitride composite materials [Maksimov Yu.M., Chukhlomina LN, Braverman B.Sh., Smirnov L.A. Self-propagating high-temperature synthesis of nitrogen-containing alloys for metallurgy. - Novosibirsk: Nauka, 2014, 232 p .; L.N. Chukhlomina, Yu.M. Maximov, V.I. Vereshchagin Self-propagating high-temperature synthesis of composite nitride-containing ceramic materials. - Novosibirsk: Nauka, 20132. - 260 p.]. However, without external influence and compaction of bulk material, it is difficult to set the sample to the required shape. With an increase in the density of the charge, the amount of absorbed nitrogen in the samples decreases significantly, since it becomes difficult to filter nitrogen to the reaction zone. Therefore, samples with an initial charge density higher than 0.2 kg / m ³ fails azotirovat in CBC mode.

Known charge for obtaining a porous permeable material by the SHS method, containing in its composition as an additive of ferrosilicon in an amount of 1.0-5.0 wt. %, Ferrosilicon in the claimed amounts determines the completeness of combustion and the absence of a liquid phase, which entails an increase in the average pore size [RU 2154550, 2000; RU 2186657, 2002; RU 2507029, 2014; RU 2507030, 2014].

The disadvantages of the known technical solutions are the limitation of the scope of products made on the basis of the charge, their significant material consumption, insufficiently high compressive strength of the resulting porous material.

It is known to use organic compounds to form the porous structure of materials [J. Jue, B. Dong, H. Wang. Porous Si ₃ N ₄ fabricated by phase separation method using benzoic acid as pore-forming agent. - J. Amer Ceram. Soc. - 2011. - Vol. 94, no. 7. - P. 1989-1991].

To obtain porous materials by the SHS method, it is known to use liquid organic compounds from the group of hydroxyl-containing compounds in the form of a polyhydric alcohol as a gas-forming reductant [RU 2480310,2013].

The disadvantage of the named technical solution for obtaining an open-porous nanostructured metal is the use of expensive metals (Cu, Ag, Au).

It is also known to use a 10% solution of polyvinyl alcohol (PVA) as a temporary technological bond in the manufacture of complex-shaped products from a ceramic material based on silicon nitride by the sintering method [RU 2610744, 2015]. The disadvantage is low porosity (no more than 0.1%).

It is known to obtain porous materials [RU No. 2533510, 2014], including the preparation of a reaction mixture, obtaining a blank by pressing the reaction mixture in a forming mandrel and self-propagating high-temperature synthesis in air, while a liquid binder is additionally introduced into the reaction mixture, gasifying during thermal decomposition, drying is carried out billets in a forming mandrel with its subsequent extraction and hardening of the billet, and self-propagating high-temperature synthesis is carried out in mandrels made of refractory gas-permeable carbon-containing material. A 3-5% aqueous solution of polyvinyl alcohol is used as a liquid binder.

When implementing the invention, highly porous long-length ceramic blocks are obtained with an open porosity of 65-85%, an average pore section of 0.25 mm to 2 mm, a heat resistance of 1700 ° C, and a compressive strength of 0.5 to 50 MPa.

The disadvantages are the multicomponent composition of the charge and its high cost, the complexity of the technological equipment using refractory foamed carbon-containing material, the stepwise pre-pressing of each sample of the mixture, the limited range of products (only in the form of a pipe) with a specific field of application (only as a carrier for catalytic monoblocks for processing hydrocarbons, nozzles for organizing combustion).

A known method of producing porous ceramics from silicon nitride [CN 107353036, 2017].

The method consists in the fact that powder Si ₃ N ₄ , nanometer powder of silicon and surfactant - polyvinyl alcohol, is subjected to mixing and grinding in a ball mill to obtain surface modified silicon nitride. The mixture is milled in a ball mill to obtain a final solid slurry. Ceramic semi-finished product is fired, then sintering is carried out in gaseous nitrogen to obtain porous ceramic Si ₃ N ₄ . The method makes it possible to control the structure, ceramics have high porosity, the linear shrinkage rate decreases, mechanical properties such as flexural strength are improved.

The main disadvantage of this method is the high cost of silicon nitride powders and nanosized silicon powder, high energy consumption of the process.

The closest technical solution is an alloying material based on silicon nitride and a method for its production [RU 2218440, 2003]. In the method, as an initial alloy, I use ferrosilicon containing 40-95% silicon, which is ground into powder with a particle size of not more than 2.5 mm, with a particle content of less than 0.2 mm, not less than 45%, and placed in a nitrogen atmosphere at a pressure of 0.1016.6 MPa ... Then the alloy is heated to 1350-2100 ° C, initiating an exothermic reaction of the formation of silicon nitride, saturated with nitrogen in a self-sustained combustion mode, maintaining a given temperature until the end of combustion, while the material formed as a result of combustion is kept in a nitrogen atmosphere at an elevated pressure for a time sufficient to form the required content of silicon nitride in the final product. The final product is a strong, well-sintered material consisting of silicon nitride (50.0-77.8%)), iron (3.2-28.81%) and iron silicides (5.0-40.0%). There are traces of carbides and aluminides.

The authors are not aware of the preparation of porous materials from industrial ferroalloys.

The present invention is directed to the development of a high-performance and low-energy-consuming method for producing porous nitride-containing composite materials with high strength, open microstructural porosity and wear resistance, from cheap, available raw materials (industrial ferroalloys or ferroalloy production waste).

The technical result of the proposed invention is to obtain a porous composite material based on silicon nitride with a microporous structure, high physical and mechanical properties.

The technical result is achieved by the fact that a method for producing a microstructural material is proposed, which consists in mixing a refractory component - ferrosilicon or ferrosilicoaluminum with a particle size of less than 100 microns with an aqueous solution of polyvinyl alcohol as a liquid binder, forming a preform of a given shape, which is frozen to a temperature below 0 ° C, sufficient for its structuring, followed by thawing for 4-5 hours at room temperature and drying at 60 ° C in vacuum with a residual pressure of not more than 0.01 MPa. The components of the mixture are taken in the following ratio, wt. %:

ferrosilicon or ferrosilicoaluminum - 50-70

polyvinyl alcohol - 3-10

water is the rest.

Self-sustaining combustion mode - self-propagating high-temperature synthesis (SHS) is carried out in a nitrogen atmosphere at a pressure of 1-10 MPa.

The ferrosilicon (FS) powder used is the PUD-75 grade with a silicon content of 82.0 wt. %., the rest is iron. According to the results of X-ray phase analysis (XRD), ferrosilicon is a two-phase material consisting of silicon and high-temperature leboite (FeSi ₂ ).

The grade FS65A15 is used as a ferrosilicoaluminum (FSA) powder, containing 60.1 wt. % silicon, 13.3 wt. % aluminum, the rest is iron. According to the results of X-ray phase analysis (XRD), the initial ferroalloy is a two-phase material consisting of silicon and high-temperature leboite. Using microspectral analysis of the initial FSA, it was found that aluminum is in the composition of a solid solution based on FeSi ₂ , forming two phases with different ratios of silicon and aluminum: FeSi _2.2 Al _0.4 and FeSi _1.6 Al _2.2 . In addition, aluminum is part of the impurity phases - Al _0.5 Fe _0.5 and Al ₂ O ₃ .

The results of chemical analysis show that the content of silicon in the alloy is 60%, aluminum - 13%, the rest is iron.

In the invention, the existing problem of obtaining articles of a certain shape from nitride-containing materials, which are difficult to process due to their high strength and low plasticity, is eliminated by one of the most effective methods for obtaining articles of a given shape by cryostructuring the initial finely dispersed reagents. The blockage of filtration channels of nitrogen molecules into the internal volume of a molded article can be eliminated by pre-forming matrices of cryogels based on polyvinyl alcohol (PVA), filled with finely dispersed ferroalloy powder.

The use of a viscous aqueous solution of polyvinyl alcohol in the composition of the charge is due to its good wettability (capillary impregnation) in relation to the surfaces of the ferroalloy powder particles. When the calculated volume of PVA solution is introduced into the powder mixture, its rapid and uniform distribution in the volume of the charge is observed. Within the claimed limits, the PVA solution provides an elastic cryogel matrix filled with ferroalloy powder. The formation of a cryogel before SHS nitriding makes it possible to avoid blocking the channels for the diffusion of nitrogen molecules into the inner volume of the molded material during SHS. In addition, PVA makes it possible to obtain the desired shape and size of the material during the formation of a ferroalloy powder. In the process of SHS in a nitrogen atmosphere under the influence of high temperatures, the decomposition of PVA molecules to carbon dioxide, water and ammonia occurs.

The ratio of ferroalloy, polyvinyl alcohol and water is selected based on the calculated content required to ensure a stable combustion process and further formation of material porosity.

Structuring modes - freezing at temperatures below 0 ° C, optimally minus 0.5-20 ° C, followed by slow defrosting for 4-5 hours at room temperature and drying at 60 ° C in vacuum with a residual pressure of no more than 0.01 MPa provide the required structure, form of the resulting material and the subsequent SHS process.

The technique of cryotropic formation of objects from ferroalloy powders can be used to develop an optimal method for obtaining nitride-containing products of a given shape, which have high strength, porosity, and commercial value. In addition to nitride-containing powders, this technique can be used on any powders and dispersed mixtures to obtain green compacts for subsequent sintering or SHS. The challenge is to develop organic binder formulations that combine good powder surface wettability, optimum viscosity, sufficient workpiece strength after solvent evaporation, and minimal carbon contamination after thermal decomposition of the binder.

Ferroalloys nitriding is carried out in the filtration combustion mode, which is characterized by a strong dependence of the regularities of combustion and the degree of nitriding on the pressure and dispersion of the initial powder. It has been established that the optimal particle size of the initial ferroalloys should be less than 100 microns. The combustion rate of ferrosilicon and ferrosilicoaluminum rapidly decreases with an increase in the particle size of more than 100 μm, which, in turn, leads to a decrease in the nitrogen content in the combustion products (and, accordingly, to a decrease in the degree of conversion of silicon into nitride) due to the large underburning of the used ferroalloys.

Products with the maximum degree of nitriding can be obtained at a pressure of 1-10 MPa, when the necessary pressure drop is created, providing a sufficient supply of nitrogen to the reaction zone. A further increase in the initial pressure is impractical, since it leads to a decrease in the nitrogen content in the nitride due to an increase in temperature and sintering of the reaction products. At a pressure of less than 1 MPa, the process is not possible, since ferrosilicon and ferrosilicon do not burn at this pressure.

The initiation of the combustion process is carried out from a powdery igniting mixture using a tungsten coil, through which an electric current is passed. Raising the pressure above 10 MPa is impractical for economic reasons and from the point of view of safe use of equipment.

After passing the combustion front, the sample is kept in a nitrogen atmosphere until it cools completely. The pressure is then released and the SHS product is recovered from the plant. The method is characterized by low energy and labor costs, since it proceeds quickly enough (0.1-0.5 hours).

At present, to obtain an article of a given configuration, ferroalloy powders are pressed under high pressure, which leads to a deterioration in nitrogen filtration, a decrease in the conversion rate, or the impossibility of carrying out the reaction.

The combustion rates of ferroalloys in nitrogen are much lower than that of elemental powders, which provides a higher degree of nitriding. Silicon nitride obtained by the interaction of an alloy (ferrosilicon, ferrosilicoaluminum) with nitrogen in the filtration combustion mode, along with the known a- and p- modifications having a hexagonal crystal lattice, form polymorphic modifications with tetragonal and orthorhombic crystal lattices, which indicates a fundamentally different mechanism of nitride formation when burning the alloy.

The figure 1 shows a diagram of the synthesis of a porous composite material based on silicon nitride.

Figure 2 shows a diagram of the combustion process.

Figure 3 shows an electron micrograph of a sample of the finished material obtained under the conditions of the proposed method. The picture shows that the finished material has a homogeneous microstructure without the inclusion of unreacted reagents due to evaporation during the synthesis.

The strength properties of nitride-containing porous materials are assessed by the value of the ultimate compressive strength and determined on a Press PGM-100MG4 device according to GOST 10180. The porosity is assessed on a Quantachrome PoreMaster 33 porosimeter.

Below are examples of the implementation of the method.

Example 1.

Take 100 g of powder of ferrosilicon (FS) grade PUD-75 with a silicon content of 82.0 wt. %. According to the results of X-ray phase analysis (XRD), ferrosilicon is a two-phase material consisting of silicon and high-temperature leboite (FeSi ₂ ). Take 40 g of an aqueous solution of PVA, the mass content of the polymer in which is 2% wt., Is placed in a plastic form with a volume of 90 ml and while stirring the polymer solution, add 60 g of PS (particle size less than 100 μm) with a bulk density of 1.5 g / cm ³ . The viscous suspension is thoroughly mixed, briquettes with a diameter of 40 mm, a height of 50 mm are formed and placed in a refrigerator at a temperature of -20 ° C for 20 hours. Then defrost at room temperature for 4 hours. After thawing, a cryogel filled with PS is formed. Then the sample is dried at a temperature of 60 ° C by evacuation (until the sample is completely dehydrated). After drying, a strong briquette containing ferrosilicon is obtained. Next, the resulting briquettes are placed in a constant pressure unit and burned in a nitrogen atmosphere at a pressure of 4 MPa. Ignition is carried out from the powdery igniting mixture using a tungsten coil through which an electric current is passed. After passing the combustion front, the sample is kept in a nitrogen atmosphere until it cools completely, then the pressure is released, and the SHS product is removed from the installation for further research. Other examples, as well as the phase composition of the obtained porous materials, the results of measuring the compressive strength and porosity are given in the table.

Example 2.

Take 100 g of powder of ferrosilicoaluminum (FSA) grade FS65F15 with a silicon content of 60.1 wt. %, aluminum 13.3 wt. % According to the results of X-ray phase analysis (XRD), ferrosilicon is a two-phase material consisting of silicon and high-temperature leboite (FeSi ₂ ). Fe _0.5 Al _0.5 and Al ₂ O ₃ are present as impurity phases. Take 50 g of an aqueous solution of PVA, the mass content of the polymer in which is 2.5% wt, placed in a plastic mold with a volume of 90 ml and while stirring the polymer solution add 50 g of PSA with a bulk density of 1.5 g / cm ³ . A viscous suspension containing 2.5% wt. PVA is thoroughly mixed, briquettes with a diameter of 40 mm, a height of 50 mm are formed and placed for 15 hours in a refrigerator at a temperature of -20 °. Then defrost at room temperature for 4 hours. After thawing, a cryogel filled with PS is formed. Then the sample is dried at a temperature of 60 ° by evacuation (until the sample is completely dehydrated). After drying, a strong briquette is obtained containing ferrosilicoaluminum. Next, the resulting briquettes are placed in a constant pressure unit and burned in a nitrogen atmosphere at a pressure of 4 MPa. Ignition is carried out from the powdery igniting mixture using a tungsten coil through which an electric current is passed. After passing the combustion front, the sample is kept in a nitrogen atmosphere until it cools completely, then the pressure is released, and the SHS product is removed from the installation for further research.

In example 3, the method is carried out analogously to example 1, only the waste of ferrosilicon production of the Novokuznetsk ferroalloy plant is used as a ferroalloy.

Examples, as well as the phase composition of the obtained porous materials, the results of measuring the compressive strength and porosity are given in the table.

The method significantly expands the scope of application of products made of cheap, affordable, unrefined ferroalloys in the absence of electricity consumption, rapidity of the synthesis process, on simple technological equipment with a long operating period, in the absence of raw material losses during nitriding and the elimination of harmful emissions into the environment, the unobvious use of cryostructuring. In addition, a very fine fraction (dust), which is formed during crushing of ferroalloys, can serve as a raw material. Utilization of this dust - a rather difficult task for ferroalloy plants - can be successfully solved by the proposed method.

The method provides for the production of advanced high-quality porous ceramics based on silicon nitride with improved performance properties (compressive strength 37-39 MPa, porosity 35.5-50.0%, resistance to dynamic and static loads of products made on the basis of the obtained porous material) due to the formed homogeneous microstructure without the inclusion of unreacted reagents, obtaining optimal uniform porosity of the material, as well as the possibility of forming complex-shaped products.

Claims (9)

1. A method for producing a porous material, including the initiation of a self-sustaining combustion mode of a ferroalloy powder in a nitrogen atmosphere at an elevated pressure, followed by cooling the resulting product, characterized in that the ferroalloy powder is pre-mixed with an aqueous solution of polyvinyl alcohol, from the mixture blanks are formed, which are subjected to cryostructuring by freezing at temperature below 0 ° C, followed by defrosting for 4-5 hours at room temperature and drying at 60 ° C in vacuum with a residual pressure of not more than 0.01 MPa. 2. A method according to claim 1, characterized in that the ferrosilicon or ferrosilicoaluminum powder or their production waste is used as the ferroalloy powder. 3. A method according to any one of claims. 1 and 2, characterized in that the components of the mixture are taken in the following ratio, wt%: ferroalloy powder - 50-70 polyvinyl alcohol - 3-10 water is the rest. 4. A method according to any one of claims. 1-3, characterized in that the ferroalloy powder has a particle size of less than 100 microns. 5. The method according to any one of claims. 1-4, characterized in that the combustion is carried out at a pressure of 1-10 MPa. 6. The method according to any one of claims. 1-5, characterized in that freezing is carried out at minus 0.5-25 ° C for 15-20 hours.

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