RU2191759C2 - Method of production of porous material - Google Patents

Abstract

FIELD: ceramics technologies; production of membrane and microporous materials from sialons used as filtering and insulating materials. SUBSTANCE: method includes mixing clay and carbon components, molding material or article from mixture thus obtained followed by burning which is performed in nitrogen atmosphere at temperature of 1400 to 1450 C continued during 3 to 4 hours at the following relationship of components contained in mixture, mass-%: clay component, 76 to 82 and carbon component, 18 to 24; pores of material thus produced is no less than 1.1 mcm. Used as clay component is kaolin and graphite is used as carbon component. Molding is performed at pressure of 25 to 100 MPa. EFFECT: enhanced permeability of material. 2 cl, 3 tbl

Description

 The invention relates to the field of ceramic technologies, namely to the production of membrane and ultra-porous materials from sialons intended to work as filtering and heat-insulating materials.

 The most common way to obtain porous materials is the method of burnable additives. The method includes mixing powders of the base material and a burnable, most often carbon additive, molding the product from a mixture, burning the burnable additive (usually in air) and sintering. The method is intended to obtain porous materials from oxides. It is also known the application of this method to obtain membrane materials from reaction-sintered silicon nitride (RSNA) [1].

 Materials from RSNA can have small pore sizes, but in this case they have permeability limitations, and the use of burnable starch additives to increase the porosity and permeability of RSNA leads to a significant increase in the pore size of the resulting materials to 5-10 μm.

During the reaction sintering of silicon nitride, a reaction proceeds
3Si + 2N ₂ = Si ₃ N ₄ , (1)
which is accompanied by an increase in the volume and mass of the solid phase, which leads to a decrease in porosity, pore size and permeability.

 However, this method does not make it possible to obtain a sialon.

It is known that porous materials from Sialon with a porosity of 60-80% are obtained by nitrogen annealing of porous preforms from a mixture of silicon with Al ₂ O ₃ and AlN additives, first at temperatures 1300-1400 ^o С, and then at temperatures 1500-1600 ^o С [2 ].

 The disadvantage of this method is that the pore sizes of the resulting material are large - about 50-100 microns.

Closest to the technical nature of the claimed is a method of obtaining a porous material, which includes mixing clay and carbon components, molding from a mixture of material or product and subsequent annealing in a nitrogen atmosphere at a temperature of 1400-1450 ^o C for 9-25 hours [3].

By this method, a porous intermediate product is obtained, which can easily be pulverized by crushing. The pore size of the intermediate product is at least 50 μm (up to 1-5 mm, depending on the size of the sawdust used), which ensures ease of removal of the gases formed and saturation with nitrogen. According to the known method for producing porous cylinders from sialon, molding is carried out both manually (in particular, in example 1, a mixture of clay, carbon and sawdust is molded into cylinders by adding water manually), and at a pressing pressure of 800 kg / cm ² (example 14).

 Sialon material obtained by the prototype method at the intermediate stage has such disadvantages as low strength and large pore size, therefore, it is not suitable for use as a membrane material.

 The inventive method provides a durable porous sialon membrane material with a pore size of the order of 1 μm with increased permeability compared to similar materials with similar pore sizes. This is achieved by choosing the composition of the mixtures, the grinding conditions, mixing and molding of the material, the firing time in a nitrogen atmosphere.

The inventive method for producing a porous material, comprising mixing clay and carbon components, molding from a mixture of material or product and subsequent annealing in a nitrogen atmosphere at a temperature of 1400-1450 ^o C, characterized in that the molding is carried out by pressing at a pressure of 25-100 MPa, annealing is carried out in for at least 3-4 hours in the following ratio of components in the mixture, wt. %:
Clay component - 76-82
Carbon component - 18-24
with obtaining material with a pore size of less than 1.1 microns.

 In this case, kaolin is taken as the clay component, and graphite as the carbon component.

 The synthesis of sialon is accompanied by reactions with negative mass and volume effects.

Последняя реакция является наиболее оптимальной в смысле получения максимальных отрицательных объемных эффектов при минимальном введении углерода по сравнению с простым выжиганием углеродных добавок в способе-прототипе (табл. 1).High strength and permeability of the material in the proposed method are achieved because a reaction occurs in this reaction system

The latter reaction is the most optimal in the sense of obtaining maximum negative volumetric effects with minimal introduction of carbon compared to the simple burning of carbon additives in the prototype method (table. 1).

In the table. 1 also introduced data carbothermal reduction - nitriding of silicon oxide according to the reaction
3SiO ₂ + 6C + 2N ₂ = Si ₃ N ₄ + 6CO (3)
which leads to a decrease in mass by 44.4% and volume by 60.8%, but requires the introduction of a very large amount (37.4 wt.%) of carbon into the initial mixture, which prevents the production of durable material.

 The membrane material obtained by the new method is made of sialon and has high chemical resistance, as well as low thermal conductivity and high porosity and thermal strength, which makes it possible to use it as an effective heat insulator.

 The method is illustrated by examples of specific performance.

Used kaolin Kyshtym deposits, which include SiO ₂ 45.39%; Al ₂ O ₃ 36.39% and impurities Fe ₂ O ₃ 1.03%; TiO ₂ 0.49%; CaO 0.89%; MgO 0.53%; K ₂ O 1.30%; Na ₂ O 0.1%. The composition of kaolin is close to the molecular formula (2SiO ₂ • Al ₂ O ₃ ), upon annealing it decomposes into mullite and silicon oxide, the final product during carbonitriding of this composition should be mainly Si ₃ Al ₃ O ₃ N ₅ according to the reaction equation (2 )

Kaolin was preliminarily annealed at a temperature of 800 ^o C, since this temperature corresponds to the almost complete removal of chemically bound water from kaolinite (without diffusion "coarsening" of its submicron particles) and the appearance of highly active metakaolinite in the grain volume.

 Colloidal graphite C-1 was used as a reducing agent. Mixtures of kaolin and graphite in an amount of 12 to 28 wt.% Were prepared by dry mixing in a Sand planetary mill at a rotation speed of 180 and 340 rpm for 30-60 minutes.

 In the experiments, compacts with a diameter of 50 mm and a height of 4-5 mm were used; annealing was carried out in an atmosphere of nitrogen and dissociated ammonia. Extruded in a steel mold at a pressure of 100, 50 and 25 MPa with a technological bundle of 5% aqueous solution of polyvinyl alcohol (PVA) in an amount of 1-1.5 ml per 10 g of the mixture.

For annealing, a continuous furnace with a graphite heater with a gas filling volume of 100 l was used. The furnace was pre-evacuated to a pressure of 10 ^-1 mmHg and filled with technically pure nitrogen. The temperature was measured with a pyrometer with an accuracy of +20 ^o C.

Heating was carried out with speeds in the range of 20-1000 ^o С - 16-17 deg / min, in the range of 1000-1400 ^o С - 3-7 deg / min. Controlled changes in mass, size of samples, phase composition. Gas permeability was determined according to GOST 25283-82, the maximum and average pore size was determined by the pressure of liquid displacement from the pores by gas according to GOST 26849-86. Ethyl alcohol was used as a liquid, and nitrogen was used as a displacing gas.

 In the table. Table 2 summarizes the results of studies of membrane materials from sialons; for comparison, some of the samples were sintered in air (Table 3).

During the heat treatment of tablets, shrinkage is observed, and with its increase, permeability decreases. The porosity, permeability and pore size of the materials obtained by this method can be controlled to a small extent by pressing pressure, heating rate in the range of 1000-1400 ^o C and the carbon content in the initial mixture. The resulting materials have the following properties: composition (main phases) - β-sialon (Si ₃ Al ₃ O ₃ N ₅ ), α-Al ₂ O ₃ ; gas permeability - 0.2-0.8 microns ² pore size: according to the method of bubbles GOST 26849-86 - about 1 micron; according to mercury porosimetry, about 0.8 microns.

Insufficient exposure (less than 3 hours) at the synthesis temperature leads to incomplete reactions, with low permeability values, and a decrease in mass. Annealing over 4 hours leads to an increase in D _m , permeability and pore size (examples 4 'and 11').

 The optimum carbon content of 18-24 wt. % With a lower carbon content in the initial charge, the material consists mainly of the X phase and is characterized by low porosity and permeability. At a higher content, the material consists of 15R sialon and has too much porosity and low strength (Example 12).

 Mixture preparation (grinding) ensures the achievement of shrinkage during reaction sintering. Inadequate grinding leads to insufficient shrinkage and an increase in pore size, an increase in porosity, and a decrease in strength (see Table 2).

 During molding, the mixture is pressed under pressure. With manual pressing (see example 7, table. 2), a material with an average pore diameter of 1.8 μm is obtained.

 The use of dissociated ammonia as a sintering medium leads to the formation of a liquid film on the surface of the samples at firing temperatures, which impedes saturation of the material with nitrogen and the production of the required material. Sintering in air does not produce sialons, but oxide ceramics. As can be seen from the comparison table. 2 and 3, the materials obtained by the proposed method, with the same pore size (1 μm) have an advantage in permeability.

The advantages of sialon material according to the proposed method:
- small pore size of 0.8-1.2 microns;
- relatively high gas permeability (K) 0.04-0.08 μm ² ;
- high porosity (P) 50-65%;
- low coefficient of thermal expansion and high resistance to thermal shock;
- low thermal conductivity (good heat-insulating properties).

 These sialon materials can be used as membrane and heat insulating materials and have advantages over similar oxide materials under conditions of sharp heat exchange.

Sources of information taken into account
1. Antsiferov V.N., Gilev V.G. Ceramic membranes from reaction-sintered silicon nitride on nitride and oxide substrates // Refractories and technical ceramics. 1998.12.

 2. Antsiferov V.N., Gilev V.G., Lanin A.G., Yakushev P.M. Features of obtaining highly porous materials from silicon nitride, silicon oxynitride and sialons // Refractories and technical ceramics, 1996. 11. P.8-13.

 3. European patent 0023869, cl. C 04 B 35/58, 02/11/1981.

Claims (2)

1. A method of obtaining a porous material, including mixing clay and carbon components, molding from a mixture of material or product and subsequent annealing in a nitrogen atmosphere at 1400-1450 ^o C, characterized in that the molding is carried out by pressing at a pressure of 25-100 MPa, annealing is carried out in within 3-4 hours, with the ratio of the components of the mixture, wt.%:
Clay component - 76-82
Carbon component - 18-24
with obtaining material with a pore size of less than 1.1 microns.  2. The method according to claim 1, characterized in that kaolin is taken as the clay component, and graphite is taken as the carbon component.

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