WO2014120044A1

WO2014120044A1 - Method for doping an aluminium oxide ceramic

Info

Publication number: WO2014120044A1
Application number: PCT/RU2014/000016
Authority: WO
Inventors: Пётр Яковлевич ДЕТКОВ; Валентин Кириллович МЯКИН; Игорь Леонидович ПЕТРОВ
Original assignee: Detkov Pyotr Yakovlevich; Myakin Valentin Kirillovich; Petrov Igor Leonidovich
Priority date: 2013-01-30
Filing date: 2014-01-15
Publication date: 2014-08-07
Also published as: CN105189407A; RU2525889C1; CN105189407B; HK1219942A1; RU2013106318A

Abstract

A method for doping an aluminium oxide ceramic involves producing a blank from slurry, removing the technical binder, and firing, wherein, according to the invention, upon removal of the technical binder, the blank is saturated with an aqueous solution of zirconyl nitrate ZrO(NO₃)₂×2H₂O, and is then heated, the temperature being increased to 400ºC.

Description

METHOD FOR ALLOYING ALUMINUM OXIDE CERAMICS

The invention relates to technologies for producing ceramic materials, in particular to methods of doping ceramics, and can be used in the field of electrical engineering and mechanical engineering for the manufacture of high-strength ceramic products.

A known method of manufacturing ceramic products based on zirconium and aluminum oxides, including the preparation of a workpiece from ultrafine powders of the aforementioned oxides, its preliminary sintering and subsequent high-temperature deformation at a temperature of 1400-1600 ° C, according to the invention, high-temperature deformation of the workpiece is carried out at a pressure 3-10 MPa and then subjected to recrystallization calcined under vacuum at a residual a pressure of not lower than 5 to 10 ^"5 torr and a temperature of 1500-1700 ° C, the content of aluminum hydroxy yes a mixture of ultrafine powders is 10 to 80 wt.% (the invention according to the patent RU 2304566).

A known method of producing alumina ceramics by mixing alumina and an organotitanium additive, drying, molding and firing, according to the invention, in the process of mixing an additional zirconium-organic compound and water are introduced in the following ratio of components, wt.%: Alumina - 96.0-97 , 8, organo-titanium compound - 0.6-1.5, organo zirconium compound - 0.6-1.5, water - the rest (invention according to the copyright certificate SU 1747424). At the same time, the high strength properties provided by the introduction of a zirconium-containing additive in the form of an organ zirconium compound can be achieved at a temperature of 1400-1500 ° C.

A known method for producing and composition of ceramic material described in patent for invention RU 1793576, selected as a prototype. The powder of corundum ceramics containing magnesium oxide, sodium oxide, silicon oxide, micro impurities and alumina is mixed with partially stabilized zirconia and calcium fluoride in the following ratio of components, May: stabilized zirconia - 5.0-15, 0, calcium fluoride - 0.5-6.0, magnesium oxide - 0.2-0.4, sodium oxide - 0.1-0.2, silicon oxide - 0.1-0.2, trace elements - 0, 02-0.04, alumina 78.16-94.08. Ceramics are prepared as follows: a casting slurry is prepared from a fine powder, the composition of the slurry, May: a mixture of powders - 85.5, a binder - 14.5 (paraffin - 14, wax - 0.5), the products are molded by hot casting under a pressure of 1-4 atm. at a slurry temperature of 60-65 ° C, preliminary burning of the binder in a bed of aluminum oxide to 90 ° C with a temperature rise of 50 ° C per hour, final firing in air.

The disadvantages of all the above methods of using zirconia to increase the strength of alumina ceramics will be described below.

In modern industrial technologies, finely dispersed zirconia is introduced into alumina powder at the technological stage of preparation of the powder mixture. The heterogeneity of the mixing of the components increases when a technological bond is introduced into the mixture and creates the risk of critical defects, which, when sintering, lead to various shrinkage in the zones of non-homogeneity, the occurrence of stresses and other defects. Defects that appear during the preparation of the ceramic mass (slip), then appear during the molding (molding) of the products as a result of the deformation of the mass and the emergence of sliding surfaces. The appearance of defects during mass preparation and molding essentially leads to uncontrollability of the processes of reproducing good strength of ceramic products in various zones, especially of a complex configuration.

The methods of hot pressing and hot isostatic pressing make it possible to provide the highest strength of ceramic materials based on aluminum oxide, however, these methods are expensive and require the use of sophisticated equipment and accessories, and are also limited by simple forms of manufactured products.

The process of high-temperature firing (sintering) mainly shows, but does not determine, as is commonly believed, the level of homogeneity and homogeneity of the material incorporated in the manufacture of slip and workpieces.

The technical task of the alleged invention is the creation of a simpler and more economical way to obtain durable ceramics, providing products of any shape.

The technical result of the invention is to increase the strength and reduce the dispersion of the strength of alumina ceramics.

To achieve the task in a method of doping alumina ceramics, including obtaining a workpiece from a slip, removing technological ligaments and firing, according to the invention, after removing the technological ligaments, the preparation is impregnated with an aqueous solution of zirconyl nitrate ΖΓΟ (Ν0 ₃ ) ₂ * 2 Н ₂ 0, then it is heated with increasing temperature to 400 ° С.

Impregnation of zirconyl nitrate with an aqueous solution ensures saturation of the internal capillary-porous structure of the preform with the solution, which contributes to the uniform coating of each A1 ₂ 0 ₃ particle with the micro-layer of the solution.

The water content in the ZrO (N0 ₃ ) ₂ ^x 2 Н ₂ 0 salt ensures the maximum possible density of the solution, which allows to achieve a high concentration of zirconium dioxide in the product and to provide the required strength.

Subsequent heating of the preform with an increase in temperature to 400 ° C ensures moisture removal, decomposition of zirconyl nitrate to zirconium dioxide with the formation of a uniformly distributed layer of Zr0 ₂ particles in the monoclinic phase on the surface of A1 ₂ 0 ₃ particles, and removal of volatile decomposition products. Upon further high-temperature firing at a temperature of 1600–1650 ° С, zirconia particles pass into the tetragonal phase, which is preserved upon cooling to room temperature, which ensures an increase in ceramic strength and a decrease in strength dispersion.

In our works, zirconium dioxide was introduced in the form of aqueous solutions of salts into a ceramic semi-finished product, which represents a ceramic preform after forming and removing a technological binder from it. After ligament removal, the ceramic semi-finished product having a capillary-porous structure was saturated with an aqueous solution of zirconyl nitrate ZrO (N0 ₃ ) ₂ ^χ 2 H ₂ 0. Subsequent stepwise heating of the ceramic billet removes moisture, decomposes zirconyl nitrate to zirconium dioxide, and removes volatile products.

In an aqueous solution, zirconyl nitrate is in a state of molecular dis-

(particle size less than 10 ^" ° m) and fills all defects and microdefects of the alumina preform. After thermal decomposition of zirconyl nitrate, removal of moisture and volatile products, zirconia obtained in this way in an ultrafine state is distributed throughout the ceramic the surface of each alumina particle.

The arrangement of zirconia introduced in the aqueous solution mirrors the porous structure of the preform. Capillaries and pores represent is the free volume previously occupied by the technological bundle covering the surface of aluminum oxide particles at the stage of preparing the slip.

At the stage of the study on saturation of zirconyl nitrate with an aqueous solution, experiments were performed to identify the nature of saturation, the nature of the interaction of the preform with the solution, and the uniformity (unevenness) of the distribution of the solution in the volume of the preform. This knowledge is necessary for assessing the possibilities of saturating ceramic billets with complex configurations with an aqueous solution, and ultimately for assessing the possibilities of manufacturing ceramic products of simple and complex shapes with increased strength.

It has been established experimentally and by calculation that the capillary-porous structure of the preform is completely filled with zirconyl nitrate regardless of the location (horizontal, vertical, inclined) of the preform in the solution. The full filling gives reason to attribute such capillaries and pores to a system in which the surface of a liquid (an aqueous solution of zirconyl nitrate) takes the form due to surface tension forces, slightly distorted by gravity, i.e. capillary potential is much greater than gravity (φ _κ > fpt.) - This means the possibility of simultaneous saturation of ceramic billets of complex shape with zirconyl nitrate, and in the end, with zirconia nanoparticles and the possibility of obtaining a variety of ceramic products with increased strength. Zirconia nanoparticles formed as a result of thermal decomposition of zirconyl nitrate are tetragonal crystals with a size of tens of nanometers, which provide an increase in the strength of ceramics.

Patent studies did not reveal methods characterized by the claimed combination of features, therefore, it can be assumed that this method meets the criterion of "novelty."

The proposed method can be implemented on an industrial scale and will find application in the field of electrical engineering and mechanical engineering for the manufacture of high-strength ceramic products, i.e. characterized by the criterion of “industrial applicability”. The essence of the claimed technical solution is illustrated on the graph of the dependence of the temperature of the furnace on time when removing a temporary binder from ceramic billets.

The method is as follows.

First, according to the basic technological requirements of the slip casting method, a preform is obtained. After removal of the temporary technological binder as a result of partial sintering, a ceramic billet having a capillary-porous internal structure is obtained. The shape, volume, spatial structure of the pores repeat the remote temporal ligament and consists mainly of pores and capillaries, the radius of which is ~ 10 ^"7 m.

After removal of the temporary ligament, the preform is saturated with an aqueous solution of zirconyl nitrate ZrO (N0 ₃ ) ₂ ^χ 2 H ₂ 0. When saturated, the internal capillary-porous structure of the preform is filled with an aqueous solution that is firmly held in the capillaries and pores. An aqueous solution of zirconyl nitrate fills the capillary-porous interior of the ceramic billet, covering the surface of each particle A1 ₂ 0 _{3 with a} micro layer. Subsequent removal of water, decomposition of zirconyl nitrate to zirconium dioxide and removal of volatile products is carried out in the process of heating the workpiece with increasing temperature to 400 ° C. After decomposition of zirconyl nitrate on the surface of A1 ₂ 0 ₃ particles, ultrafine particles of Zr0 _{2 are formed} in the monoclinic phase. Upon subsequent heating> 1200 ° C, the particles of zirconium dioxide transfer to the tetragonal phase, which is maintained upon cooling to room temperature. The firing of the ceramic sample is carried out at a temperature of ~ 1600 - 1650 ° C.

A feature of the present method of doping alumina ceramics is that doping occurs with zirconia released during the thermal dissociation of aqueous solutions of salts. Zirconium dioxide is formed in a highly dispersed state and is evenly distributed in the volume of the ceramic.

The use of aqueous solutions of zirconium salts allows us to solve a number of problems:

- zirconium dioxide is introduced and uniformly distributed directly in the matrix of aluminum oxide;

- particles of zirconium dioxide are formed in an ultrafine state;

- provides homogenization of the resulting ceramic structure and lower burning (elimination) of defects in it, especially pores;

- it is possible to vary the amount of introduced zirconium dioxide;

- the tetragonal phase of zirconia is stored in the ceramic material to room temperature;

- decrease in the level of internal stresses;

- reduction of strength dispersion;

- increase the strength and reliability of ceramic products.

The proposed method for doping alumina ceramics with zirconia was tested on the example of VK-94-1 ceramics, industrially produced and widely used in Russia.

A sample was prepared from a slip of the following composition:

- the content of A1 ₂ 0 ₃ - 95% of the mass

- density - 2.6 g / cm

- specific surface of ceramic powder - 5500 cm / g

- paraffin content - 12.5% of the mass

- paraffin density - 0.92 g / cm

As a result of introducing ~ 9% (by weight) of zirconium dioxide by the proposed method, the value of bending strength changed from 250-300 MPa to 500-600 MPa.

From the point of view of the smallest investment with a noticeable (1.5-fold) increase in strength properties and the possibility of incorporation into existing technologies, we propose the development of new methods for alloying alumina ceramics obtained by hot slip casting.

The applicant conducted experiments on hardening alumina ceramics.

The studies were carried out on cylindrical samples with a diameter of 7 ± 1 mm and a length of 60 ± 5 mm. The samples were made from a slip industrial production brand VK94-1. The strength of the samples was evaluated according to the results of tests for three-point bending according to the Russian standard.

In the process of conducting preliminary studies, the following were selected:

- temperature regimes of preliminary firing of samples to remove time ligaments;

- the required amount of zirconium salts (tentatively);

- high-temperature firing of samples in air.

For the control group of samples were carried out:

- injection molding of a slip under pressure;

- preliminary firing (to remove a temporary bundle, T _max = 1050 - 1 100

° C);

- saturation with solutions of zirconium salts and drying of samples (T _max = 400 ° C);

- high-temperature firing in air (T _max = 1600 - 1650 ° C);

- Strength tests in three-point bending.

Billets of cylindrical samples (diameter 7 ± 1 mm, length 60 ± 5 mm) were made from a slip of the VK94-1 grade, consisting of 87.5% (May.) Ceramic powder and 12.5% paraffin.

Slip components had the following characteristics:

Ceramic charge, May. :

- content of A1 ₂ 0 ₃ 95%;

- the total content of Si0 ₂ and MnO 5%;

- density 2.6 g / cm;

- specific surface area 5500 cm / g;

Paraffin:

- density 0.92 g / cm.

The slip slip injection method was used, subject to the following process conditions:

- casting at room temperature;

- slip temperature 90 ± 2 ° C;

- the pressure of the slip in the form of 5.2 - 6.4 kgf / cm;

- slip feed time in the form 30 ± 5 sec.

After filling the mold with a slip, the pressure was reduced to atmospheric. Then the mold was disassembled and the blank was taken out. Conducted visual inspection for visible defects.

The obtained ceramic preforms were placed in a ceramic cell and filled with alumina so that the alumina layer between the preforms was not less than 5 mm, between the blanks and the walls of the cell - at least 10 mm. Used alumina with a mass fraction of a - A1 ₂ 0 ₃ - not less than 35%, specific surface area - 10 m ² / g, mass fraction of moisture - not more than 2.5%.

The cuvette with the blanks was placed in a high-temperature laboratory chamber electric resistance furnace. The used electric furnace allows heat treatment at temperatures up to 1,100 ° C.

The graph of the furnace temperature change over time (Fig. 1) consisted of several stages:

- stepwise heating to a temperature of 1050 - 1100 ° C - 16 hours;

- exposure at a temperature of 1050 - 1 100 ° C - 100 min;

- cooling to room temperature - 8 hours.

After cooling, the cuvette with blanks was removed from the furnace, the blanks were removed from alumina.

After removal of the temporal ligament, the preforms were impregnated with an aqueous solution of ZrO (N0 ₃ ) ₂ × 2Н ₂ 0. salt. Samples were placed horizontally in the aqueous solution of salt and kept for some time, periodically turning them around its axis.

After completion of the impregnation, the samples were removed from the solution and dried to remove moisture, thermal destruction of the zirconium salt and removal of the products of thermal destruction.

Drying was carried out according to the following scheme:

at a temperature of +80 - 90 ° С - 1 hour;

at a temperature of +400 ° С - 1 hour.

After drying the blanks, they were fired in air at high temperatures in a furnace with fibrous insulation, a stainless steel casing, designed for universal use at temperatures up to 1800 ° C. The preforms were placed on corundum substrates.

The temperature regime of firing consisted of several stages:

- uniform heating to a temperature of 1600 - 1650 ° C - 12 hours;

- exposure at a temperature of 1600 ° C - 1 hour;

- cooling to room temperature - 3 to 4 hours.

For 15 samples obtained by the described method, tests were conducted strength according to the federal test standard adopted in Russia.

The tests were carried out using the three-point bending method for cylindrical specimens with a diameter of 7 ± 1 mm and a length of 60 ± 5 mm with a distance between supports of 50 mm according to the scheme.

The table shows the strength test results of ceramic samples at three-point bending.

The conducted experimental studies allow us to draw the following conclusion: the proposed method of doping alumina ceramics using the impregnation of ceramic billets with solutions of zirconium salts with their subsequent firing, increases the strength of cylindrical samples (7 mm in diameter, 60 mm long) by 30 - 40%.

Claims

FORMULA

A method of doping alumina ceramics, including obtaining a blank from a slip, removing the technological binder and firing, characterized in that after removing the technological binder, the preform is impregnated with an aqueous solution of zirconyl nitrate ZrO (N0 ₃ ) ₂ x 2 H ₂ 0, then it is carried out heating with increasing temperature up to 400 ° С.