RU2637352C1 - Method of producing high density quartz ceramics and articles thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of producing high density quartz ceramics and articles thereof includes the preparation of an aqueous silica glass slurry with a density of 1.85-1.90 g/cm³, forming of ceramic billets of articles by the method of aqueous slip casting in gypsum moulds, drying and roasting in electric furnaces with air atmosphere. The aqueous slurry of quartz glass has a polydisperse grain composition in the range of 0.1-300 mcm, of which particles of up to 5 mcm are 25-35%, particles of up to 63 mcm are 94-98%. Moulding the billets of articles is carried out in plaster forms, the working surface that is saturated with a finely dispersed sludge of silica slurry or gating draining, and calcination is carried out at a maximum temperature of 1260-1280°C for 6-8 hours with intermediate exposures in 1100±50°C for 1±0.5 hours on the rise for equalizing the temperature of the article during cooling for calcining and removing thermal and shrinkage stresses. For producing water slurry, quartz glass and grinding media made of transparent quartz glass without a trace cristobalite are used, and grinding and stabilisation of surrey are carried out in mills lined with quartz glass or quartz ceramic.

EFFECT: method allows to produce large-sized articles, including thin-walled, with a high degree of uniformity, high strength and stable dielectric characteristics over a wide range of temperatures.

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Description

The invention relates to methods for producing high-density ceramic materials based on quartz glass - quartz ceramics with open porosity close to zero. Such material will find wide application for products operating in difficult thermal and climatic conditions.

Currently, methods for producing quartz ceramics and articles from it for the manufacture of steel pouring glasses and dispensers in metallurgy are well known (R.S. Churakova, E.P. Fedorova, Yu.A. Polonsky, E.I. Ermolaeva, A.M. Pozhivanov, ND Karpov, Glasses from quartz ceramics for casting steel at the UNRS // Ogneupory, 1972, No. 11, pp. 20-23), antenna radomes and radio-transparent windows of rockets (Yu.E. Pivinsky, E.I. Suzdaltsev, Quartz ceramics and refractories, 2008, vol. 1, chap. 16, 17, 18, pp. 569-640), heat-resistant model forms, pallets, shafts for molding, stamping, rolling ki of metals, alloys, glass (V.P. Markov, L.L. Chernina, A.M. Akhyan. Oversized products from quartz ceramics // Glass and Ceramics, 1981, No. 2, pp. 27-28). They include crushing, grinding of quartz glass and preparation of a water slurry of a given particle size distribution, casting of ceramic billets by the method of water slip casting in gypsum molds due to the capillary properties of gypsum, drying and firing products in electric furnaces.

The open porosity of such products and materials is in the range of 5-20%, the bending strength is 20-60 MPa. The disadvantage of materials and products from them is significant open porosity and the associated absorption of moisture in another liquid or air phase, which significantly limits the scope of the material.

There are known works on the development of a technology for producing quartz ceramics and products from it with porosity close to zero (P _open. ≥ 0.5%) due to the introduction of a small amount of boron-containing sintering activator boron oxide powders B ₂ O ₃ into the water slip (US patent No. 2973278, class CO3C 3/06, publ. 02/28/1961), boron nitride BN (AS USSR No. 501052, class CO4B 35/14, bull. No. 4, 1976).

In particular, the material OTM-107 TU1-598-249-87, quartz ceramic with the addition of 0.5-1.0% weight BN, which has an open porosity close to zero at a firing temperature of 1250 ° C with a shutter speed of 2-3 hours, and porous and sufficiently strong materials are obtained at a temperature of 1150-1200 ° C (F.Ya. Borodai, EI Suzdaltsev. Quartz ceramics doped with boron nitride - “niasite - BN” // Sat. New inorganic materials, 1976 , issue 3, p. 150-151).

The disadvantage of such materials and the method for their preparation is the introduction of alkali and alkaline-earth impurities into the ceramic structure together with the sintering activator, which accelerate the crystallization of quartz glass at temperatures above 1230 ° C, which is unacceptable to ensure high strength, heat resistance, and other valuable properties.

The second area of work on obtaining high-density quartz ceramics was the work related to the development of high-speed high-temperature sintering modes in order to eliminate or at least slow down the formation of cristobalite in the material during firing. The region of optimal sintering modes (FT Gorobets, RV Brailovskaya. Obtaining high-density quartz ceramics in gas furnaces "// Ogneupory, 1972, No. 5, p. 36-39) for obtaining high-density quartz ceramics in a gas furnace, is proposed working on natural gas. It is: at a temperature of 1350-1550 ° C, at a holding time of 6-60 minutes. The open porosity of ceramic samples did not exceed 1.5%.

The disadvantage of this method is the instability of the process and, as a result, significant fluctuations in the strength properties and an increase in the thermal expansion coefficient of the material to 10 × 10 ^-7 / ° C and higher, which is associated with the crystallization of the samples.

More stable results on the sintering of quartz ceramics to open porosity close to zero were obtained in vacuum furnaces at temperatures of 1300–1350 ° C (N.V. Solomin, G.M. Tomilov. Moisture-proof ceramics "V-niasit" // Sat. New Inorganic Materials, 1976, issue 3, pp. 135-136). The absence of cristobalite in the composition of the material is ensured by the short duration of the sintering regime: the heating time of the vacuum furnace to t _max was 15-30 minutes, the exposure time τ = 10-30 minutes. According to this method, moisture-proof quartz ceramics "V-niasit" (material TCM - 991) was developed. It has an open porosity close to zero, and all properties, with the exception of optical ones, are close to the properties of quartz glass.

The disadvantage of this technical solution is the difficulty of obtaining specialized, large-sized products, the lack of the necessary vacuum furnace equipment.

The closest technical solution (prototype) is a method for producing quartz ceramics with zero open porosity, without sintering activators at relatively low firing temperatures in conventional electric furnaces with an air atmosphere due to the selection of the ratio of the fine-grained and coarse-grained fractions of the polydisperse slip, optimization of the slip preparation technology, molding and firing products (F.Ya. Borodai, EI Suzdaltsev. The influence of technological parameters on the properties of quartz ceramics // Refractories, 1975, No. 10, s . 50-54).

The disadvantage of the prototype is that the set technological parameters for the density of the slip, grain composition, sintering mode are optimal only when obtaining small-sized samples.

The objective of the invention is to find the technological parameters for producing high-density quartz ceramics by water slip casting in gypsum molds and subsequent firing in electric furnaces in the manufacture of large-sized products of complex profile with open porosity close to zero, while ensuring integrity and high quality of products.

The problem is solved in that the method of producing high-density quartz ceramics and products from it, including the preparation of an aqueous slip of silica glass with a density of 1.85-1.90 g / cm ³ , the molding of ceramic blanks, products by the method of aqueous slip casting in gypsum molds, drying and firing in electric furnaces with an air atmosphere, characterized in that the aqueous silica glass slurry has a polydisperse grain composition in the range of 0.1-300 μm, of which particles are up to 5 μm 25-35%, particles up to 63 μm 94-98%, forming blanks, wasp products It is available in gypsum forms, the working surface of which is impregnated with finely dispersed quartz slurry sludge or sprue drain, and firing is carried out at a maximum temperature of 1260-1280 ° C for 6-8 hours with intermediate exposure times at 1100 ± 50 ° C for 1 ± 0.5 h on the rise to equalize the temperature of the product and during cooling to anneal and relieve thermal and shrink stresses.

The method for producing high-density quartz ceramics and articles made of it is characterized in that quartz glass and grinding bodies made of transparent quartz glass without traces of cristobalite are used to prepare a water slurry, and grinding and stabilization of the slip are carried out in mills lined with quartz glass or quartz ceramics.

Our studies on obtaining products from quartz ceramics with a diameter of up to 450 mm, a height of up to 1000 mm and a wall thickness of 15-25 mm by water slip casting in gypsum molds and subsequent firing in electric furnaces with an air atmosphere showed that from polydisperse slips with a grain composition of 0.1 to 300 microns by selecting the ratio of fine (up to 5 microns) and coarse (more than 63 microns) fractions in the amount of 25-35% fine and 2-6% coarse, firing mode with isothermal sintering in the temperature range 1260-1280 ° C with an exposure of 6-8 hours, you can get high-quality products without cracks and warping at a porosity of 0.02-0.3% according to GOST 2409-2014.

To ensure the integrity, uniformity of the products and reduce heat and shrink stresses, intermediate shutter speeds are provided at temperatures of 1100 ± 50 ° C for 1 ± 0.5 hours in the areas of temperature rise and in the furnace. The ellipse of the products was 2–4 mm, and the difference in height and circumference was within ± 0.002 g / cm ³ , which is significantly lower than for similar products from porous quartz ceramics.

One of the main reasons complicating the production of high-density quartz ceramics is the cristobalitization of quartz glass, and hence the ceramic preform, when it is fired in the temperature range above 1250 ° C. This process is enhanced by impurities of the original silica glass, extraneous pollution during crushing, grinding of glass, molding ceramic blanks, etc. To reduce this phenomenon, it is necessary to use silica glass and grinding bodies made of transparent silica glass without traces of cristobalite, grinding and stabilization slip should be carried out in mills lined with quartz glass or quartz ceramics, and the molding of ceramic preforms, products should be carried out in gypsum forms, working the surface of which is impregnated with finely divided silica slurry or sludge draining sprue.

Table 1 shows the main properties of the high-density quartz ceramics developed by the proposed method in comparison with the well-known Niasit ceramics TU 1-596-195-2009.

Examples of the method

Crushed and washed with distilled water quartz tube made of transparent quartz glass TU 11-87 SCHLO.027.252, grinding bodies made of transparent quartz rods TU 21-RSFSR-587-82 together with distilled water in a ratio of 1: (0.5-0.7) : (0.20-0.23) was loaded into a TC A 115 ball mill lined with quartz ceramics, and an aqueous slip with a density of 1.85-1.90 g / cm ^{3 was} obtained by grinding. The slip was separated from the balls, poured into another mill with a quartz lining and it was stabilized in a rotating mill for 2-10 days to improve the casting properties and additional hydration of quartz glass grains.

Products were molded in plaster molds with a perforated metal frame in bulk. Before pouring the slip, the inner (working) surface of the mold was covered with a brush or spray gun with low-density sludge from a quartz slip or with a gate discharge. After drying, ceramic billets were sanded with an emery cloth to remove sagging, bumps and dirt.

Products were fired in domestic rotary hearth type CB-71 type rotary hearth furnaces or in Nabertherm wire-heated furnaces at t _max = 1260-1280 ° C with programmed firing mode.

Example 1. Products were molded from a slip with the following properties: density ρ = 1.90 g / cm ³ , the polydisperse grain composition as measured by the LA-950 Horiba installation was in the range 0.1-300 μm, of which up to 5 μm 35 %, particles over 63 microns - 4%. Dried and sanded on the outer surface of the product was fired in a CB-71 furnace at t _max = 1260 ° C with holding at a maximum temperature of τ = 7 h and holdings to equalize the temperature during heating and stress relief at 1100 ° C for 1 hour. Shrinkage during firing vertically 3.9-3.5%, horizontally 3.5-3.1%. The products had a density ρ = 2.14-2.15 g / cm ³ , open porosity P open _. = 0.04-0.30%, bending strength 71-75 MPa, dielectric constant ε at f = 10 ¹⁰ Hz 3.70 (at t = 20 ° C) and 3.80 (at t = 1200 ° C) , tgδ at / f = 10 ¹⁰ Hz, respectively (10-30) × 10 ^-4 and (70-90) × 10 ^-4 .

Example 2. Products were obtained by a similar technology for the preparation of slip and molding. An aqueous silica glass slip had a density ρ = 1.88 g / cm ³ , the fine fraction content (up to 5 μm) was 25%, and the large fraction (more than 63 μm) was 6%. After drying and sanding, the surface of the product was fired in furnaces with CB-71 silicon heaters at t _max = 1280 ° C with holding at a maximum temperature of τ = 6 hours and intermediate holdings at temperature rise and cooling at 1150 ° C for 0.5 hours. Shrinkage during firing was in the range: vertically 3.9-3.2%, horizontally - 3.6-3.4%. The products had a density ρ = 2.15-2.17 g / cm ³ , open porosity P open _. = 0.02-0.20%, bending strength 75-90 MPa, dielectric constant ε at f = 10 ¹⁰ Hz was 3.72 (at t = 20 ° C) and 3.85 (at t = 1200 ° C ), tgδ at f = 10 ¹⁰ Hz, respectively (20-30) × 10 ^-4 and (70-100) × 10 ^-4 .

Example 3. Products were molded from a polydisperse slip with a grain composition of 0.1-300 μm with a density ρ = 1.85 g / cm ³ , the content of the fine fraction (up to 5 μm) - 30%, coarse (more than 63 μm) - 2%. After drying and sanding, the surfaces of the product were fired in Nabertherm furnaces at t _max = 1270 ° C with holding at a maximum temperature of τ = 8 hours with intermediate holdings at temperature rise and cooling at 1050 ° C for 1.5 hours. Shrinkage during firing was within: vertical 3.50 ± 0.5%, horizontal - 3.0 ± 0.3%. Material properties in the shells: density ρ = 2.14-2.15 g / cm ³ , open porosity P open _. = 0.02-0.30%, bending strength 70-80 MPa, the average coefficient of linear thermal expansion in the temperature range 20-1200 ° C is in the range (4-7) × 10 ^-7 K ^-1 .

The advantage of the presented method is the possibility of obtaining a high-density ceramic material based on quartz glass with open porosity close to zero, having high strength and stable dielectric characteristics in a wide temperature range. The method allows to obtain large-sized products, including thin-walled, with a high degree of uniformity in density, physical and technical properties.

Claims (2)

1. A method of obtaining high-density quartz ceramics and articles made of it, including the preparation of an aqueous slurry of silica glass with a density of 1.85-1.90 g / cm ³ , the molding of ceramic workpieces by water slip casting in gypsum molds, drying and firing in electric furnaces with an air atmosphere, characterized in that the aqueous silica glass slurry has a polydisperse grain composition in the range of 0.1-300 μm, of which particles are up to 5 μm 25-35%, particles up to 63 μm 94-98%, the molding of product blanks is carried out in plaster molds, working the surface of which is impregnated with finely dispersed quartz slurry sludge or sprue drain, and firing is carried out at a maximum temperature of 1260-1280 ° C for 6-8 hours with intermediate shutter speeds at 1100 ± 50 ° C for 1 ± 0.5 h on rise to equalize the temperature on the product and during cooling for annealing and relieving thermal and shrink stresses. 2. The method of producing high-density quartz ceramics and articles thereof according to claim 1, characterized in that quartz glass and grinding bodies made of transparent quartz glass without traces of cristobalite are used for the preparation of an aqueous slurry, and grinding and stabilization of the slip are carried out in mills lined with quartz glass or quartz ceramics.