RU2525892C1 - Method of obtaining quartz ceramics - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of obtaining quartz ceramics includes production of slip from broken quartz glass, formation of a raw material blank by a method of casting in gypsum moulds, soaking the raw material blank with a liquid soaking solution, drying the soaked blank of quartz ceramics and following thermal processing. The soaking solution contains a mixture of Al(NO₃)₃, tetraethoxysilane, ethanol and water, the molar ratio of components provides in the soaking solution pH≥4, and thermal processing of the soaked quartz ceramics blank is realised at a temperature of 950-1200°C.

EFFECT: increase of strength and heat resistance of the products.

2 cl, 6 dwg, 2 tbl, 1 ex

Description

The invention relates to the ceramic industry, and more specifically to a technology for producing modified ceramic materials based on quartz glass with high-temperature strength for the manufacture of ceramic products for various purposes.

Quartz ceramics are widely used to create various refractory products for the glass industry and metallurgy (crucibles, ladles, mixers, nozzles, technological equipment), methods for its preparation are known from numerous scientific publications, including patent information.

In patent information US 20090206525, published on 08/20/2009 under the IPC index С04В 35/64, a method for producing quartz ceramic material is described, which includes forming a porous ceramic preform from a suspension of silica particles, followed by impregnation of the porous material with a liquid solution containing sintering additives (alkaline or alkaline earth metals, boron or phosphorus).

Patent publication US 20120098169, published April 26, 2012 according to the IPC index C04B 35/64, describes the preparation of high-density quartz ceramics using boron oxide as a sintering additive. Boron oxide was introduced into the composition of the material in the form of a powder at the stage of manufacturing the initial suspension of dispersed particles.

The use of boron compounds as a sintering aid for silica ceramics, introduced by impregnation of a porous ceramic preform with a liquid solution, is also described in patent publication US 20090206525.

Ceramics containing SiO ₂ and Al ₂ O ₃ are well known for their high strength properties and heat resistance and are described in the scientific and patent literature. Moreover, ceramic materials are described in the literature, characterized by different component ratios (Al ₂ O ₃ / SiO ₂ ) and obtained by various methods. So, in the patent US 4895814, published 01/23/1990 on the IPC index C04B 35/18, describes ceramics with high strength at high temperatures. The technological process described in this patent includes the use of amorphous powders of aluminum and silicon oxides, pressing their mixtures and sintering at temperatures of 1500-1750 ° C.

It should be noted that the resulting ceramic contains more than 50 wt.% Alumina.

In the patent of the Russian Federation 2436206, published December 10, 2011 according to the IPC index H01Q 1/42, hardening and sealing of the surface layer of quartz ceramics is carried out by modifying its surface layer to a depth of 1-3 mm by impregnation with an aqueous solution of chromium salt with a density of 1200-1400 kg / m ³ followed by drying and heat treatment of the material at a temperature of 450-750 ° C for 1-3 hours.

The method of impregnation with solutions containing chromium compounds of porous refractory products with the aim of hardening them was also described in US patent 3789096, published 01/29/1974 on indices IPC B23P 15/28, B24D 3/14? C04B 41/50. In this method, the technological process involves the complete impregnation of the ceramic preform with an aqueous or alcoholic solution of chromium compounds, followed by firing at the sintering temperature of the material. Impregnation is carried out throughout the volume by immersion of the workpiece in the solution. As in the previous analogues, chromium compounds are introduced before the firing of the material or product, and they are structural elements of the material, positively affecting one and negatively on other properties of the material and product. As described in RF patent 2436206, in relation to quartz ceramics, such a technical solution leads to increased crystallization of quartz glass, a decrease in heat resistance, strength and deterioration of the dielectric characteristics of the material.

In the patent US 5639412, published on 06/17/1997 on indices С04В 35/14, С04В 35/624, С04В 38/00, a sol consisting of silica particles, the surface of which is modified by metal ions, is described (as an example, the patent describes the modification of silica particles by ions aluminum). The sol, according to the description of the patent, has a pH <3, and the size of the colloidal particles is so small that it made it possible to form a porous body with pore size ≤20 Å from such particles.

It should be noted that the use of sols described in US patent 5639412, as an impregnating solution for the modification and hardening of quartz ceramics is ineffective. In the described ash, the ratio of aluminum atoms to silicon is, in accordance with the description of the patent, [Al] / [Al] + [Si] = 0.01-0.05. When such a sol is impregnated with an initial ceramic material containing more than 99% SiO ₂ and having an open porosity of less than 30%, and subsequent heat treatment, the resulting modified material will contain no more than 0.03% Al ₂ O ₃ . A compositionally similar composite sol containing SiO ₂ and Al ₂ O ₃ and used for the manufacture of porous ceramics is described in US Pat. No. 5,610,109, published March 11, 1997 for IPC indices C04B 35/14, C04B 35/624, C04B 38/00.

By technical nature, the closest to the proposed material is a hardened ceramic material described in the patent of the Russian Federation 2458022, published on 08/10/2012 by the IPC indices C04B 35/14 and B82B 3/00. This patent describes nanomodified quartz ceramics comprising a porous ceramic base of silica glass grains and a modifying additive of alumina. As a basis, the described material contains fired quartz ceramics or products from it with an open porosity of 7-14%, obtained by water slip casting from a polydisperse suspension with grain sizes from 0.1 to 500 microns, with a particle content of 0.1-5.0 μm 20-30%, particles 60-500 μm 2-10%. As a modifying additive, the material contains α-Al ₂ O ₃ nanoparticles in an amount of 1.0-2.5 wt.%, Embedded in the junction zone of silica glass grains due to mass transfer. Α-Al ₂ O ₃ nanoparticles are obtained by impregnating a ceramic base with an aqueous solution of aluminum salt Al (NO ₃ ) ₃ · 9H ₂ O, drying and pyrolysis at a temperature of 400-600 ° C. The disadvantage of the prototype method is that the α-Al ₂ O ₃ nanoparticles introduced into the joint zones of the ceramic only fill part of the voids of the material and slightly increase its density, while at the same time practically not contributing to the additional binding of siliceous particles into a single framework.

The objective of the invention is to develop a method for producing high-strength heat-resistant quartz ceramics. This goal is achieved in that along with the modifying addition of aluminum salt, a material capable of forming colloidal silica particles is introduced into the composition of the impregnating solution, which in turn form chains and networks of colloidal silica particles, additionally connecting particles of siliceous particles of ceramic material.

A method for producing quartz ceramics includes the production of a slurry from a quartz glass battle, the formation of a crude preform by casting in gypsum molds, the impregnation of a crude preform with a liquid impregnating solution, drying of the impregnated quartz ceramic preform and subsequent heat treatment, in contrast to the prototype, the impregnating solution contains a mixture of Al (NO ₃ ) ₃ , tetraethoxysilane, ethanol and water, while the molar ratio of the components provides a pH≥4 in the impregnating solution, and heat treatment of the impregnated quartz ceramic blank carried out at a temperature of 950-1200 ° C.

The impregnating solution has a gelation time of 10 minutes to 6 hours.

Taking into account technical and economic requirements, the method for producing dense and mechanically strong quartz ceramics should ensure environmental safety of production, not require the use of complex and expensive equipment, and be based on the use of non-deficient and inexpensive raw materials. It is well known that during heat treatment at temperatures> 1200 ° C, crystallization of cristobalite occurs in quartz ceramics, accompanied by a significant decrease in the strength of the material (see [1]). Therefore, the temperature of the technological process of processing products made of quartz ceramics should not exceed 1200 ° C.

The use of organosilicon binders for the production of technical oxide ceramics is described in the scientific and technical literature. So, in [2], the results of developments on the use of tetraethoxysilane and other organosilicate compounds as a binder for powder materials in the manufacture of composite materials are presented.

The impregnation of preformed monolithic products from porous quartz ceramics with organosilicon polymer compounds is described in RF patent 2209494, published July 27, 2003 according to the IPC index H01Q 1/42. The ceramic products impregnated with these compounds were not subjected to heat treatment in subsequent technological processes and were polymer-ceramic composite in the structure of the material.

The choice of the processing method for products made of quartz ceramics is based on a consideration of the properties of the material being processed and the general technological scheme used for manufacturing products from it.

The general scheme used for manufacturing products from quartz ceramics is generally accepted (see, for example, [1]) and includes the following main technological operations:

1. The production of slip from the battle of quartz glass.

2. The formation of raw billets by casting in plaster molds.

3. Drying the raw product.

4. Heat treatment at temperatures of 1100-1200 ° C.

For technical and economic reasons (energy saving; no need to use additional sophisticated equipment), it is advisable to combine the process of hardening ceramic products with one or more stages of the process that are already in use.

In the proposed method, the stage of hardening of quartz ceramics is proposed to be combined with the stage of heat treatment of the material already used in the technological process at temperatures of 1100-1200 ° С. It is well known that during heat treatment at these temperatures, complete decomposition of aluminum nitrate occurs with the formation of oxide.

It is well known that in an aqueous solution tetraethoxysilane (TEOS) is hydrolyzed to form silicic acid:

$$Si{\left(O{C}_2{H}_5\right)}_{\mathrm{four}}+\mathrm{four}{H}_{2}O\to Si{\left(OH\right)}_{\mathrm{four}}+\mathrm{four}{C}_2{H}_{5}OH\left(\mathrm{one}\right)$$

The speed of the hydrolysis process depends on the chemical composition, pH and temperature of the solution. From reaction equation (1), it is clearly seen that for the TEOS hydrolysis to proceed completely, the molar ratio [H ₂ O] / [TEOS] in the solution should be ≥4.

The effect of the pH of the solution on the processes of TEOS hydrolysis and condensation of silica particles is described in detail in the literature (see, for example, [3]). In a neutral environment, the hydrolysis rate is low. To accelerate the hydrolysis of TEOS, the process is carried out in an alkaline or acidic environment.

In an alkaline medium at room temperature, the processes of TEOS hydrolysis and the growth of the formed SiO ₂ particles proceed very quickly. In such a solution, in a very short time, large particles of silica are formed, the solution quickly becomes cloudy and delaminates due to the deposition of these large particles. In addition, in an alkaline environment, precipitation of sparingly soluble metal hydroxides is possible. Therefore, for introducing modifying inorganic components into quartz ceramics, it is advisable to use impregnating solutions having a pH <7. Note that aqueous solutions of salts of strong acids and some metals (for example, aluminum) have a pH <7 due to partial hydrolysis.

The resulting silicic acid is able to form colloidal particles of silica, forming a colloidal solution (sol). The resulting colloidal particles, interacting with each other, form aggregates, the viscosity of the solution gradually increases and the liquid solution turns into a gel. The rate of transition of a solution from a liquid to a gel state also strongly depends on its chemical composition, pH, and temperature. With increasing viscosity of the solution, the speed of impregnation of ceramics with them sharply decreases. Therefore, the chemical composition and pH of the impregnating solution should be chosen so that during the entire process of impregnation of the initial quartz ceramic blank, the solution would remain in a liquid state and its viscosity would remain practically unchanged. This means that at room temperature, the gelation time of a freshly prepared solution (the time interval from mixing the initial components of a liquid solution to turning this solution into a gel) should exceed 10-15 minutes, i.e. the time during which at least the surface layers of a quartz ceramic product can be impregnated with a solution.

Upon contact of the considered freshly prepared impregnating solution with quartz ceramic, the silicic acid formed during the hydrolysis of TEOS can interact with its surface of silica particles that make up the ceramic structure. New chemical bonds formed as a result of this interaction will additionally bind silica particles and contribute to the hardening of ceramics. In addition, colloidal silica particles formed in the solution fill the pores and microcracks in the ceramic structure, contributing to the compaction and hardening of the ceramic material.

Specific example No. 1 for the manufacture of quartz ceramics.

Samples of quartz ceramics obtained by slip casting in gypsum molds and dried at room temperature were used as the starting material. The samples were in the form of stacks with dimensions of 65 × 8 × 8 mm and were characterized by porosity of ~ 25%.

Water-alcohol solutions based on tetraethoxysilane (TEOS, Si (C ₂ H ₅ O) ₄ ) with modifying components for impregnating samples of porous quartz ceramics were prepared. The chemical composition of the solutions is shown in Table 1. The ceramic samples were impregnated with solutions at room temperature for 10 minutes. After impregnation, the samples were dried at room temperature for 24 hours and heat treated in an electric muffle furnace. The maximum temperature for processing samples is shown in Table 2. When processing samples No. 1-25, when the specified maximum temperature is reached, heating immediately ceased and the samples cooled to room temperature. When processing samples No. 26-30, the samples were kept at a maximum temperature of 2 hours and then the samples were cooled to room temperature.

The apparent density and open porosity were determined by hydrostatic weighing in accordance with GOST 2409-80. Determination of tensile strength in bending was carried out by the method of three-point bending.

The results of comparative tests, shown in Table 2, show that the processing of products made of quartz ceramics leads to a significant increase in its bending strength. The observed increase in strength is manifested when using different durations of impregnation of ceramics with solutions and at different temperatures of its subsequent heat treatment (950-1200 ° C).

A comparison of the data on the density of ceramics shows that the density of the processed samples is higher. It should be noted that the density values of the processed samples are significantly lower than the density of dense amorphous silica (2.2 g / cm ³ ). The observed significant increase in the strength of ceramics with a relatively small increase in the density of samples suggests that treatment leads to the filling of the smallest pores and healing of microcracks that determine the strength of the material.

Table 1. The chemical composition of the impregnating solutions. No. p / p Chemical composition of solutions and characteristics of solutions The composition of the solid phase formed during drying and subsequent heat treatment of solutions ∗, mol.% Water g Ethanol, g TEOS g Al (NO ₃ ) ₃ 9H ₂ O, g Solution characteristics SiO ₂ Al ₂ O ₃ one 10 25 29th four The impregnating composition is transparent and uniform. 96.4 3.6 2 10 3 29th four The solution is heterogeneous, poor miscibility of the components is clearly visible 3 10 25 29th 40 The solution is heterogeneous, there are separate heterogeneous white flakes ∗ The table shows the calculated composition of the solid phase in molar percent of oxides.

Obviously, for a uniform and reproducible processing of ceramic samples, the impregnating solution must be homogeneous. The samples of solutions No. 2 and No. 3 we tested did not possess this property.

Tab. 2. Flexural strength of ceramic samples. No. p / p Duration of impregnation Processing temperature, ° С Impregnation Solution Number Strength,
kg / cm ² The average value of the strength P _cp .,
kg / cm ² Standard deviation, σ Relative standard deviation
σ / P _cf Apparent density
g / cm ³ Open porosity,% one - 950 - 7 eleven 9 0.82 1.72 21 2 - 950 - 10 3 - 950 - 7 four - 950 - 27 5 - 950 - 2 6 16 hours 950 one 189 145 56 0.39 1.81 17.1 7 16 hours 950 one 182 8 16 hours 950 one 155 9 16 hours 950 one 164 10 16 hours 950 one 35 eleven 20 minutes 950 one 131 140 24 0.17 12 20 minutes 950 one 121 13 20 minutes 950 one 136 fourteen 20 minutes 950 one 186 fifteen 20 minutes 950 one 124 16 90 min 950 one 123 135 10 0.08 17 90 min 950 one 135 eighteen 90 min 950 one 152 19 90 min 950 one 133 eleven - 1050 - eleven thirty 13 0.43 1.74 20,0 12 - 1050 - 39 13 - 1050 - 17 fourteen - 1050 - 43 fifteen - 1050 - 38

16 12 min 1050 one 220 261 31 0.12 1.78 18.8 17 12 min 1050 one 277 eighteen 12 min 1050 one 240 19 12 min 1050 one 311 twenty 12 min 1050 one 259 21 12 min 1200 one 288 325 55 0.17 1.79 17.8 22 12 min 1200 one 312 23 12 min 1200 one 362 24 12 min 1200 one 326 25 12 min 1200 one 337 26 - 1200 ∗∗ - 98 116 27 0.23 - - 27 - 1200 ∗∗ - 75 28 - 1200 ∗∗ - 120 29th - 1200 ∗∗ - 148 thirty - 1200 ∗∗ - 142 31 12 min 900 one 183 226 29th 0.13 32 12 min 900 one 233 33 12 min 900 one 262 34 12 min 900 one 202 35 12 min 900 one 249 36 12 min twenty one 152 137 23 0.17 37 12 min twenty one 159 38 12 min twenty one 146

39 12 min twenty one 132 40 12 min twenty one 95 ∗ Samples No. B-11 were impregnated for 16 hours at room temperature.
∗∗ When processing samples No. 26-30, the samples were kept at a maximum temperature of 2 hours and then the samples were cooled to room temperature.

Figure 1 shows the dependence of the strength (kg / cm ² ) of the initial (curve 1) and impregnated with a solution (curve 2) ceramic samples from the heat treatment temperature. During the heat treatment of these samples, heating ceased immediately upon reaching the set temperature and the samples were removed from the furnace after it was cooled to room temperature. The graphs show that hardening of the raw starting samples is observed only at heat treatment temperatures of more than 1000-1100 ° C. At a temperature of 900 ° C, the strength of the initial and processed samples is the same. A significant increase in the strength of samples impregnated with solution No. 1 (see Table 1) is observed at temperatures> 900 ° C. That is why the claimed temperature range of material processing is 950-1200 ° C.

Figure 2 shows the dependence of the relative deviation of the strength values of the samples of untreated (curve 1) and solution-treated (curve 2) quartz ceramics from the heat treatment temperature. It can be seen that with an increase in the heat treatment temperature, the values of the relative deviation of the strength values decrease, i.e. the spread in strength values of samples treated under the same conditions becomes smaller. It should be noted that the values of the relative deviation of the strength values of the samples treated with solution 1 are significantly less than for untreated ceramics in the entire range of heat treatment temperatures used. This indicates that the proposed method for processing quartz ceramics provides not only a significant increase in strength, but also a significant reduction in the spread of its values, i.e. the stability of the strength characteristics of the material increases.

Figure 3 and 4 shows electron microscopic images of the surface of the samples, the original quartz ceramics, not subjected to liquid processing (Figure 3), and a sample of ceramics subjected to processing in accordance with the proposed method (Figure 4). Both ceramic samples were subjected to heat treatment at 1050 ° C. A comparison of the above images shows that there are no fundamental changes in the structure of the material. This fact is not surprising, given the overall low (about 25%) porosity of the initial ceramic samples and the low solids content in the used solution. However, attention is drawn to the visually clearly manifested somewhat greater coherence of the material structure.

A substantially different picture is observed for electron microscopic images of fracture areas of samples (Figs. 5 and 6). If for a ceramic sample not subjected to hardening treatment, complete destruction of the material structure is observed (Figure 5), then the sample subjected to hardening treatment (Figure 6) retains the silica particles bound together.

Literature

1. Pivinsky Yu.E., Romashin A.G. Quartz ceramics. - M.: Metallurgy, 1974, 264 p.

2. Yao I.M. Composite ceramic materials based on organosilicon binders and refractory oxygen-free fillers. - The dissertation for the degree of candidate of technical sciences. Kazan State Technological University, Kazan, 2000.

3. Ayler R. Chemistry of silica, part 1. M.: Mir, 1982, 416 p.

Claims (2)

1. A method of producing quartz ceramics, including the production of a slurry from a battle of quartz glass, the formation of a crude billet by casting into gypsum molds, the impregnation of a raw billet with a liquid impregnating solution, drying of the impregnated quartz ceramic billet and subsequent heat treatment, characterized in that the impregnating solution contains an Al mixture ( NO _{3) 3,} tetraethoxysilane, ethanol and water, wherein the molar ratio of the components provides a rN≥4 impregnating solution, and heat treatment prepreg quartz Ceram ki is carried out at a temperature of 950-1200 ° C. 2. The method according to claim 1, in which the impregnating solution has a gelation time of from 10 minutes to 6 hours.

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