RU2433108C1 - Method of producing aluminium nitride-based heat-conducting ceramic - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: invention relates to production of aluminium nitride-based heat-conducting ceramic which can be used in electronics and electrical engineering, particularly as substrate material for powerful microwave and power semiconductor devices, as well as other devices where there is need for good dielectric properties, strength and heat conductivity of the material. Powder of aluminium nitride with particle size less than 1 mcm, containing not less than 10% cubic phase of aluminium nitride with particle size less than 100 nm, and a technical additive selected from Y2O3, CaO, MgO in amount of not more than 6 wt % is used to prepare a mixture, with addition of not more than 65 vol. % organic component containing a solvent (mixture of methylethyl ketone with ethanol in ratio of 1:2), a dispersant (phosphate ether), binder (polyvinyl butyral) and a plasticiser (mixture of polyethylene glycol with dibutyl phthalate), followed by stirring with simultaneous deaeration for not less than 30 minutes rarefaction of air of not less than 0.15 atm. A belt is formed from the obtained slip, dried and cooled to room temperature. A plate is formed from the belt. The organic component is removed at temperature 150-500C. The obtained half-finished product is sintered at pressure 0.1-1.0 MPa in the atmosphere of a nitrogen-containing gas at temperature 1650-1820C for 1-3 hours and then cooled under given conditions. ^ EFFECT: increase in heat-conductivity to 230 W/mK, strength and output of the ceramic material. ^ 4 cl, 4 ex, 2 tbl

Description

The invention relates to the production of ceramic materials, in particular to the manufacture of ceramics based on aluminum nitride with high thermal conductivity, with a combination of slip casting and sintering of powders under pressure of a nitrogen-containing gas.

Ceramic based on aluminum nitride (AlN) with high thermal conductivity is intended for use in electronics and electrical engineering, primarily as a substrate material for powerful power and microwave semiconductor devices (diodes, transistors, thyristors), microcircuits, microassemblies and multi-chip modules, thermal modules and substrates of high-power LEDs, as well as other components and devices where high dielectric characteristics, strength and thermal conductivity of the material are required.

Ceramics based on aluminum nitride are intended to replace ceramics from beryllium oxide BeO, since the initial BeO powders are extremely toxic and have a harmful effect both on the human body (carcinogenic effect) and on the environment. Beryllium oxide powders have not been and are not produced in Russia. Currently, all ceramics with high thermal conductivity, both AlN and BeO, are supplied to Russia only by import. Therefore, the problem of obtaining high-conductivity ceramics based on aluminum nitride is relevant and timely.

In Russia, attempts were made to produce AlN ceramics by hot pressing and injection molding, but they all ended in failure for the following reasons:

- low quality AlN starting powders;

- low productivity of the hot pressing method and high cost of finished products;

- the lack of adequate demand for AlN ceramics, primarily due to the lack of TU standards for such ceramics intended for use in electronics, and the lack of experience at that time on its metallization.

There were no analogues in terms of the characteristics of ceramics described in the present invention in Russia.

A known method of producing a dense heat-conducting ceramic based on aluminum nitride, including sintering a powder of aluminum nitride obtained by the carbothermic method, in the presence of 2 wt.% Yttrium oxide or alkaline earth metal oxide at a temperature of 1850-1900 ° C in nitrogen, followed by annealing of sintered ceramics at a temperature of 1820-1880 ° C for 4-12 hours in a stream of a mixture of nitrogen and hydrogen (RU 2029752, С04В 35/58, 1995).

This method is economical and technological, but it allows you to get ceramics with insufficiently high thermal conductivity (not higher than 150 W / m · K). A known method of producing ceramic products, including based on aluminum nitride, which consists in preparing an exothermic powder mixture containing at least one component from the group: metal of group III-VIII of the periodic system of elements, boron, silicon, carbon as well as at least one inorganic compound from the group: boride, silicide, nitride of transition metals of IVB-VIB groups; carbide, nitride, silicon oxide; nitride, alumina; boron nitride, an oxide of an element of group II-IV, from the mixture, a product blank is formed, which is subjected to heat treatment in a nitrogen-containing gas combustion mode with a temperature of 2000 to 3000 ° C and a pressure of 0.1 to 1000 MPa (SU 1720258 A1, C04B 35/58, C04B 35/65, 1995). The porosity of the obtained products is 1-10%, the strength is 25-40 kg / mm ² , the microhardness is up to 91 HRA. In a known manner were obtained not only products but also plates, however, the thermal conductivity of such plates, as well as products, did not exceed 150 W / m · K.

A known method of producing a heat-conducting ceramic based on aluminum nitride with high thermal conductivity, which includes the preparation of a mixture of aluminum nitride powder in the form of microcrystals with a needle-shaped particles, obtained in the mode of self-propagating high-temperature synthesis, or a mixture obtained by any of the known methods of aluminum nitride with at least 5 wt. .% aluminum nitride obtained in the mode of self-propagating high-temperature synthesis (in the combustion mode) and containing particles in the form of single crystals fibers, in the presence of not more than 6 wt.% yttrium oxide, preparing the mixture by granulating the mixture on a bundle of synthetic rubber in gasoline, pressing it in a steel mold at a pressure of 0.1 MPa, then compacts with a density of 2.2-2 , 4 g / cm ³ are placed in a graphite glass filled with coarse powder of aluminum nitride, subsequent sintering of the compacts is carried out at a temperature of 1850-1900 ° C for 1 hour in a medium of chemically pure nitrogen at a pressure of at least 0.12 MPa with annealing of sintered ceramics in the same gaseous medium at a temperature of 1750-1800 ° C for 8-10 hours (RU 2144010 C1, C04B 35/581, 2000). The well-known technological and economical method for producing heat-conducting ceramics based on aluminum nitride provides thermal conductivity of 206-217 W / m · K when using powders with a normal level of impurities. The disadvantages of the known invention are the low bending strength of the material (not more than 150 MPa), a high degree of shrinkage during preparation of the mixture before sintering (more than 30%) and a low yield of usable material (not more than 60%).

The closest analogue of the claimed invention in terms of essential features is a method for producing heat-conducting ceramic from aluminum nitride, disclosed in patent US 4814302, С04В 35/58, 03/21/1989. The known method includes the preparation of a mixture of aluminum nitride powders with a fineness of less than 2 μm and a specific surface area of up to 10 m ² / g and a sintering agent selected from the series Y ₂ O ₃ , CaO, MgO in an amount of not more than 6 wt.%, Preparing a cast slip of these powders - an inorganic component and an organic component containing a solvent, dispersant, a binder and a plasticizer, forming a slip in the form of a tape, drying, molding from a ribbon of a semi-finished product in the form of plates and sintering of the semi-finished product at a temperature of more than 1700 ° C od pressure in the nitrogen-containing gas atmosphere.

The main disadvantages of this method are the low thermal conductivity and low strength of the target product in bending due to the high content of impurities in the initial aluminum nitride (more than 3 wt.%), A high degree of shrinkage (more than 30%).

The technical result of the invention is to increase thermal conductivity, strength and yield of heat-conducting ceramics while reducing the degree of shrinkage.

The technical result is achieved by combining slip casting and high-temperature sintering of the intermediate product (semi-finished product) in a nitrogen-containing atmosphere.

The technical result is achieved in that a method for producing a heat-conducting ceramic based on aluminum nitride, including the preparation of a mixture of aluminum nitride powders and a sintering additive selected from the series Y ₂ O ₃ , CaO, MgO in an amount of not more than 6 wt.%, Preparation of a cast slip from of said powders — inorganic component and organic component, casting of blanks, drying and high-temperature sintering of the obtained semi-finished product, according to the invention, a powder containing at least 10% of aluminum nitride is used of the ubic phase of aluminum nitride with nanosized particles less than 100 nm, the preparation of a casting slip involves sequentially: mixing the components of the charge, to which not more than 65 vol.% organic component is added, containing as a solvent a 1: 2 mixture of methyl ethyl ketone and ethanol as a dispersant phosphate ester, mainly BEYCOSTAT C213, as a binder polyvinyl butyral PVB and as a plasticizer, a mixture of polyethylene glycol with dibutyl phthalate, mainly in a 1: 1 ratio; mixing these components with simultaneous deaeration for at least 30 minutes with a vacuum of at least 0.015 MPa; feeding the obtained slurry to an injection machine for forming a tape with a thickness of 0.1 to 1.5 mm, with a density of 1.90-2.0 g / cm ³ , with a conveyor speed of 10 mm / min, drying the tape at a temperature of 30-40 ° C for no more than 180 minutes; subsequent cooling of the tape to room temperature at the outlet of the injection plant in a natural way; forming plates from tape by stamping; followed by heating of the plates at a temperature of 400 to 500 ° C, at which the removal of the components of the organic component occurs, after which the resulting semi-finished product in the form of a preform is sintered at a pressure of 0.1-1.0 MPa in an atmosphere of nitrogen-containing gas at a temperature of 1650-1820 ° C within 1-3 hours, followed by cooling the resulting ceramics to 1200 ° C at a rate of 100 ° C / h, then at a speed of 300 ° C / h to room temperature. Before forming the plates in the tape, perforation of holes with a diameter of 0.2 mm or more is performed. As a nitrogen-containing gas, nitrogen or a mixture of nitrogen with hydrogen or argon is used, in an amount of not more than 10 vol.%, And these sintering additives are introduced into the mixture separately or in a mixture.

As an aluminum nitride powder, a powder with a specific surface area of 10-12 m ² / g, with an average particle size of less than 1 μm and containing at least 10% of the cubic phase of aluminum nitride with nanosized particles of less than 100 nm, with an impurity content of: O is not more than 0 7% by weight; C - 130-270 ppm; Ca - 200-240 ppm; Si - 39-48 ppm; Fe - 10-14 ppm. The use of such a powder of aluminum nitride promotes the formation of crystallization centers and increases the density of the final material to 96-98% of theoretical.

The addition of sintering technological additives Y ₂ O ₃ , CaO, MgO in an amount of not more than 6 wt.% To the composition of the charge ensures the formation of a dense structure of aluminum nitride during hot pressing, while the introduction of magnesium and calcium oxides allows the sintering stage to be carried out under milder conditions ( T = 1650-1750 ° C and P = 0.1-0.2 MPa), compared with the sintering mode of the composition of aluminum nitride - yttrium oxide (T = 1750-1820 ° C and P = 0.1-1.0 MPa ) Yttrium oxide also forms a liquid phase during sintering, providing ceramic compaction, and purifies it of oxygen.

The use of slip casting from inorganic (aluminum nitride + processing aids) and organic components reduces shrinkage during sintering of the semi-finished product, as well as increasing the density or porosity of the final material. Each component of the organic component of the slip, as well as the features of the slip casting and sintering method, play an indispensable role and contribute, together with the totality of the features of the invention, to the achievement of a technical result.

So, a dispersant, which is used as phosphate ester, mainly of the brand BEYCOSTAT C213, prevents the adhesion of particles of powder powders.

A binder, which is used as PVB polyvinyl butyral, provides flexibility of the ceramic tape after casting, the possibility of its storage and processing (cutting, punching, hole punching, etc.). A plasticizer, which is used as a mixture of polyethylene glycol with dibutyl phthalate, mainly in a 1: 1 ratio, provides softening of the binder and increasing the flexibility of the ceramic tape after casting.

In addition, the binder and plasticizer prevent adhesion of the slip to the conveyor during the casting process.

These components of the organic component do not limit the use of other known components in the preparation of the charge. In this case, the experiments were tested on known components presented in the formula.

The essence of the method is confirmed by examples.

Example 1

The mixture is prepared by mixing powders of aluminum nitride (95 wt.%), Obtained by the SHS method, with a specific surface area of 10 m ² / g, with particle sizes less than 1 μm and yttrium oxide (5 wt.%) With particle sizes less than 1 μm, while aluminum nitride contains 10 wt.% cubic phase with particle sizes ≤60 nm. Further, this mixture will be designated as the inorganic component of the K ₁ slip. Then a casting slip is prepared, for this purpose an organic component K ₂ is added to the inorganic component (35 wt.%) Containing aluminum nitride and Y ₂ O ₃ , containing: solvent 53 wt.% In the form of an azeotropic mixture of methyl ethyl ketone with ethanol (MEK / EtOH), a dispersant that uses phosphate ester, mainly BEYCOSTAT C213 brand in an amount of 0.6 wt.%, A binder of 5.4 wt.%, Which uses polyvinyl butyral PVB, a plasticizer, which is used as a mixture of polyethylene glycol 3 wt.% with dibutyl phthalato m 3 wt.%, in a ratio of 1: 1.

Slip preparation is carried out in a ball mill with grinding media. A ball mill must simultaneously provide deaeration (removal of air bubbles) from the mixture of components.

Stirring is carried out for one hour, with a deaeration time of 30 minutes, with a dilution of air of 0.15 atm (0.015 MPa). The finished mixture (slip) is fed into the injection machine with a container movement speed of 10 mm / min. The drying of the tape is carried out at a temperature of 40 ° C for 150 minutes, then the tape is cooled when it leaves the plant naturally. From the obtained tape with a thickness of 0.1 mm, plates with a density of 2.0 g / cm ^{3 are formed} . The plates are perforated in order to obtain holes with a diameter of 0.20 mm or more. After that, the formed perforated plates are heated at a temperature of from 400 to 500 ° C until the components of the organic component are completely removed. Next, the obtained semi-finished product (billet) is sintered under a nitrogen pressure of 1.0 MPa (10 atm) at a temperature of 1820 ° C for 1 hour, followed by cooling of the resulting ceramics to a temperature of 1200 ° C at a speed of 100 ° C / hour, then at a speed of 300 ° C / hour to room temperature.

The resulting ceramic is a product in the form of a plate of aluminum nitride with a density of 98% of theoretical, bending strength of 450 MPa, thermal conductivity of 215 W / m · K with a yield of 85% and a shrink of 10%.

Example 2

The mixture is prepared by mixing powders of aluminum nitride 94 wt.%, With a specific surface area of 12 m ² / g, with particle sizes less than 1 μm and a mixture of calcium oxide and magnesium in equal proportions (3 + 3 = 6 wt.%) With particle sizes less than 1 μm, while aluminum nitride contains 15 wt.% cubic phase with particle sizes ≤100 nm. Then a casting slip is prepared, for this purpose, an organic component K ₂ is added to the inorganic component K ₁ (36.0 wt.%) Containing aluminum nitride and CaO + MgO, including: solvent 51.0 wt.% In the form of an azeotropic mixture of methyl ethyl ketone with ethanol (MEK / EtOH), a dispersant, which is used as phosphate ester, mainly brand BEYCOSTAT C213 in an amount of 0.4 wt. %, a binder of 5.6 wt.%, which is used polyvinyl butyral PVB, a plasticizer, which is used as a mixture of polyethylene glycol 3.5 wt.% with dibutyl phthalate 3.5 wt.%, in a ratio of 1: 1.

Slip preparation is carried out in a ball mill with grinding media. A ball mill must simultaneously provide deaeration (removal of air bubbles) from the charge.

Stirring is carried out for one hour, with a deaeration time of 45 minutes with a dilution of air of 0.1 atm (0.01 MPa). The finished slip is fed to the injection plant with a container speed of 10 mm / min. The drying of the tape is carried out at a temperature of 30 ° C for 180 minutes, then the cooling of the tape is carried out upon exiting the unit in a natural way. From the obtained tape with a thickness of 1.0 mm, plates with a density of 1.90 g / cm ^{3 are formed} , after which the resulting plates are heated at a temperature of 400 to 500 ° C until the components of the organic component K _{2 are} completely removed.

Next, the obtained semi-finished product (billet) in the form of a plate is sintered under a pressure of 1 MPa (10 atm) of a mixture of nitrogen and 10 vol.% Hydrogen at a temperature of 1650 ° C for 3 hours, followed by cooling of the resulting ceramics to a temperature of 1200 ° C at a speed of 100 ° C / hour, then at a rate of 300 ° C / hour to room temperature.

The resulting ceramic is a plate with a density of 97% of theoretical with a bending strength of 350 MPa and a thermal conductivity of 220 W / m · K, with a yield of usable plates of 80% and a shrinkage of 15%.

Example 3

The mixture is prepared by mixing powders of aluminum nitride 97 wt.%, Obtained by the SHS method, with particle sizes less than 1 μm and a specific surface area of 11 m ² / g and Y ₂ O ₃ (3 wt.%) With particle sizes less than 1 μm, with nitride aluminum contains 25 wt.% cubic phase with particle sizes ≤90 nm. Then a casting slip is prepared, for this purpose, an organic component K ₂ is added to the inorganic component K ₁ (35.0 wt.%) Containing aluminum nitride and Y ₂ O ₃ , including: a solvent of 52.4 wt.% In the form of an azeotropic mixture methyl ethyl ketone with ethanol (MEK / EtOH), a dispersant, which is used phosphate ester, mainly BEYCOSTAT C213 in the amount of 0.5 wt.%, a binder of 5.4 wt.%, which is used polyvinyl butyral PVB, plasticizer, the quality of which is used a mixture of polyethylene glycol 3.35 wt.% with dibutyl phthalate 3.35 wt.%, in a ratio of 1: 1.

The preparation of the charge is carried out in a ball mill with grinding media with simultaneous deaeration.

Stirring is carried out for one hour, with a deaeration time of 30 minutes and a vacuum of 0.15 atm (0.015 MPa). The finished slip is fed to the injection plant with a container speed of 10 mm / min. The drying of the tape is carried out at a temperature of 30 ° C for 180 minutes, then the cooling of the tape is carried out when leaving the installation in a natural way. From the obtained tape with a thickness of 1.0 mm, plates with a density of 2.0 g / cm ^{3 are formed} , after which the plates are heated at a temperature of from 400 to 500 ° C to completely remove the organic component.

Next, the obtained semi-finished product in the form of a plate without hole punching is sintered under a pressure of 1.0 MPa (10 atm) of a mixture of nitrogen and 10 vol.% Argon at a temperature of 1750 ° C for 2 hours, followed by cooling of the resulting ceramics to a temperature of 1200 ° C at a speed 100 ° C / hour, then at a rate of 300 ° C / hour to room temperature.

The resulting ceramic is a plate with a density of 98% of theoretical, with a shrinkage of 8.0%, a bending strength of 300 MPa and a thermal conductivity of 230 W / m · K. The yield of usable plates is 90%.

Example 4

Everything, as in example 2, only the sintering additive is a mixture: 2 wt.% Y ₂ O ₃ +4 wt.% CaO, and sintering is carried out under a nitrogen pressure of 0.1 MPa.

The resulting ceramic is a plate with a density of 98% of theoretical, bending strength of 450 MPa, thermal conductivity of 215 W / m · K with a yield of 85% and a shrink of 16%.

After sintering, the ceramics are ground and, if necessary, polished. These operations are performed on special installations that provide automatic control of the thickness and quality of the surface. As abrasive materials, materials based on artificial diamond are used, which ensure the greatest productivity of the plants. Then measure the performance by known methods.

Additionally, you can specify that in order to improve the performance and quality of AlN tapes, the injection plant can have two slip containers, which are filled one at a time, and from which the slip is supplied alternately for the smooth operation of the injection machine. The length of its conveyor can be from 12 to 18 m, depending on the width and thickness of the manufactured AlN tape.

The blade of the injection machine provides the formation of AlN tape of a certain thickness - from 0.1 to 1.5 mm. Typically, such installations provide a tape width of 300 or 600 mm. Moreover, a narrower tape will be distinguished by better quality, namely uniformity in thickness. The resulting tape is wound into a roll. Usually before winding it is cut longitudinally into strips 100–200 mm wide. The obtained rolls are the feedstock for sintering AlN ceramics. They can be stored for several weeks before sintering.

Immediately before sintering, the plates are formed from the tape by means of stamping. Perforations of holes with a diameter of 0.25 mm or more can be made in the plates to prevent their walls from closing during sintering. The dimensions of the plates, as a rule, should be no more than 150 × 150 mm. This is due to the effect of their expansion during sintering, since the edges of the plates in the stack warm up faster than the middle. To prevent swelling, the plates are loaded into the furnace in cuvettes, ensuring uniform heating of the stack of plates along the edges and in the middle. The cuvettes are “boxes” made of boron nitride, into which the plates are loaded, placing them one on one and pouring layers of boron nitride powder.

For clarity, the table summarizes the main parameters of the method and the properties of the obtained ceramics according to the proposed method.

As can be seen from tables 1 and 2, the proposed method for the manufacture of ceramics based on aluminum nitride by a combination of slip casting and high-temperature sintering at an excess pressure of nitrogen-containing gas makes it possible to obtain ceramics with a maximum thermal conductivity of 230 W / m · K, bending strength of 450 MPa, at the exit usable plates up to 90%.

Table 1 Components of Inorganic K ₁ and Organic Components K ₂ The composition of the components K ₁ + K ₂ for the slip according to the examples, vol.% one 2 3 four K ₁ 35.0 - - - 95 wt.% AlN, soda. 10 wt.% Cubic phase + 5 wt.% Y ₂ O ₃ 94 wt.% AlN, soda. 15 wt.% Cubic phase + 6 wt.% (3CaO + 3MgO) - 36.0 - 36.0 97 wt.% AlN, soda. 25 wt.% Cubic phase + 3 wt.% Y ₂ O ₃ - - 35.0 - 95 wt.% AlN, soda. 25 wt.% Cubic phase + 2 wt.% Y ₂ O ₃ +4 CaO - K ₂ 53.0 51.0 52,4 51.0 Solvent, azeotropic mixture of methyl ethyl ketone with ethanol, MEK / EtOH Dispersant, phosphate ester, Beycostat C213 0.6 0.4 0.5 0.4 Binder, Polyvinyl Butyral (PVB) 5,4 5,6 5,4 5,6 Plasticizer, Polyethylene glycol: Dibutyl phthalate = 1: 1 3.0 + 3.0 3.5 + 3.5 3.35 + 3.35 3.5 + 3.5 Parameters of the synthesis of ceramics by examples R, MPa 1,0 1,0 1,0 0.1 T-ra, ° C 1820 1650 1750 1750 Gas medium nitrogen nitrogen + 10 vol.% hydrogen nitrogen + 10 vol.% argon Nitrogen

table 2 Properties of ceramics by examples Example Number one 2 3 four Thermal conductivity, W / m · K 215 220 230 215 Bending Strength, MPa 450 350 300 400 Shrink% 10 fifteen 8 16 Dielectric Strength, kV / mm twenty twenty twenty 19.5 KTR, 1 · 10 ^-6 / ° C 4.6 4.6 4.6 4,5 Volume resistance, Ohm · cm ≥1 · 10 ¹³ ≥1 · 10 ¹³ ≥1 · 10 ¹³ 0.910 ¹³ Dielectric constant (at f = 1 MHz) 9.0 9.0 9.0 8.0 The yield of material,% 85 80 90 85

Claims (4)

1. A method of obtaining a heat-conducting ceramic based on aluminum nitride, including the preparation of a mixture of powders of aluminum nitride and a sintering agent selected from the series Y ₂ O ₃ , CaO, MgO, in an amount of not more than 6 wt.%, The preparation of a cast slip from these powders - inorganic component and organic component, casting of preforms, drying and high-temperature sintering of the obtained semi-finished product, characterized in that a powder containing at least 10% cubic phase of aluminum nitride from nanor is used as aluminum nitride particles of less than 100 nm, the preparation of the casting slip includes the following: mixing the components of the mixture, to which no more than 65 vol.% organic component is added, containing as a solvent a mixture of methyl ethyl ketone with ethanol in a ratio of 1: 2; as a dispersant, phosphoric acid ester, mainly BEYCOSTAT C213; as a binder - polyvinyl butyral
PVB and, as a plasticizer, a mixture of polyethylene glycol with dibutyl phthalate, preferably in a ratio of 1: 1; mixing these components with simultaneous deaeration for at least 30 minutes with a vacuum of at least 0.015 MPa; feeding the obtained slurry to an injection machine for forming a tape with a thickness of 0.1 to 1.5 mm, with a density of 1.90-2.0 g / cm ³ , with a conveyor speed of 10 mm / min, drying the tape at a temperature of 30-40 ° C for no more than 180 minutes; subsequent cooling of the tape to room temperature at the outlet of the injection plant in a natural way; forming the plates from the tape by stamping, followed by heating the plates at a temperature of from 400 to 500 ° C, at which the components of the organic component are removed, after which the resulting semi-finished product in the form of a preform is sintered under a pressure of 0.1-1.0 MPa in an atmosphere of nitrogen-containing gas at a temperature of 1650-1820 ° C for 1-3 hours, followed by cooling of the obtained ceramics to 1200 ° C at a speed of 100 ° C / h, then at a speed of 300 ° C / h to room temperature. 2. The method according to claim 1, characterized in that before forming the plates in the tape, perforation of holes with a diameter of 0.2 mm or more is performed. 3. The method according to claim 1, characterized in that nitrogen or a mixture of nitrogen with hydrogen or argon in an amount of not more than 10 vol.% Is used as a nitrogen-containing gas. 4. The method according to claim 1, characterized in that the said sintering additives are introduced into the mixture separately or in a mixture.