RU2845655C1 - Method of producing precursor for making ceramic articles based on boron carbide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to production of inorganic compounds and specifically to a carbothermic method of producing a mixture of polydisperse powder of boron, carbide and boron nitride as a precursor for further production of ceramic articles based thereon. Proposed method comprises the combined dissolution of boric acid and carbohydrates in hot water, drying, crushing in a ball mill, calcining the obtained mixture, cooling to room temperature, then second crushing in a ball mill, mixing with a nitriding additive and a nitride-formation catalyst, heat treatment in an induction furnace with continuous supply of argon. Precursor is obtained in two steps, with the following ratio of components, pts. wt.: 10 boric acid, 6.5-7.2 carbohydrates, 0.5 nitriding additive, 0.005 nitride-forming catalyst. After dissolving and drying, the obtained mixture is ground in a ball mill and calcined; thermal treatment of the mixture takes place in an induction furnace at temperature of 1100-1200 °C.

EFFECT: obtaining a precursor for further synthesis of higher boron carbides of high chemical purity at sufficiently low temperatures and without using toxic and explosive substances.

1 cl, 1 ex

Description

The invention relates to the production of inorganic compounds, specifically to a carbothermic method for obtaining a mixture of polydisperse powders of boron, carbide and boron nitride as a precursor for the further production of ceramic products based on boron carbide by powder metallurgy methods of sintering, hot pressing, etc., which have wear resistance, resistance to high-temperature oxidation and high chemical inertness when in contact with aggressive acidic and alkaline environments, as abrasive powders, etc.

Boron carbide (B ₄ C) is used in many modern industries. Various methods are known for synthesizing boron carbide powders: by synthesis from elements, plasma-chemical and self-propagating high-temperature synthesis (hereinafter referred to as SHS), spark/plasma sintering (SPS/PECS), metallothermic and carbothermic reduction. It is worth noting the morphological differences in the products obtained by different methods [Dimitar D. Radev, Edward Ampaw. Classical and contemporary synthesis methods of boron carbide powders. Comptes rendus de l'Acad´emie bulgare des Sciences, Volume 68, No 8, 2015, Р. 945-956].

Boron carbide is one of the hardest materials with diamond-like mechanical properties and is already used for a variety of applications including bulletproof glass, jet nozzles and end faces of mechanical seals, as well as for grinding and cutting tools. It is produced on an industrial scale by the classical carbothermic reduction of boron oxides at high temperatures, but obtaining pure boron carbide in processed forms such as films and fibers is difficult.

In addition, boron carbide (and boron) are used to synthesize lanthanum hexaboride, which, in turn, has found wide application in cathode compensators of stationary plasma engines of spacecraft. High emission characteristics of lanthanum hexaboride allow it to be used as a cathode material in electron microscopes and microanalyzers, as well as in charged particle accelerators (cyclotrons, synchrophasotrons, electron guns), in installations for welding refractory metals with an electron beam in a vacuum, in furnaces with electron heating, etc.

Organic compounds (sugars, alcohols), lamp black, petroleum coke, high-purity graphite, nanofibrous carbon, and methane are used as a carbon source for the synthesis of boron carbide.

There are various methods for producing boron carbide, for example, the SHS method by heating amorphous boron in a mixture of methane and argon [Kuketaev T.A., Kim L.M., Tulegulov A.D., Baltabekov A.S., Tagaeva B.S. Electron-beam boriding of structural steels with ultrafine powders. www.ntsr.info/upload/My/nauka/borstal.doc] . The SHS reaction began at a temperature of 400°C, the process was initiated by passing current through a tungsten spiral. The disadvantages of this method include the use of methane as a carbon source, which can lead to the formation of an explosive methane-air mixture.

A method for producing boron carbide nanopowder CN 102674356 A enriched in the isotope B10 is known, including mixing boric acid and glycerin in a molar ratio of (0.5-3):1 in a ball mill, heating and removing crystallization water, dehydration at a temperature of 450-650 °C, grinding and briquetting, and then heating to a temperature of 1300-1600 °C and cooling. The disadvantage of this method is enrichment in the isotope B10 (B11), which does not affect the features of the product production technology.

In patent RU 2576041 C2 The use of carbon black as a carbon source is proposed. The invention relates to the production of inorganic compounds, specifically to a carbothermic method for obtaining polydisperse boron carbide powders intended for obtaining abrasive powders for grinding and impact-resistant ceramics based on them. The method includes mixing boric acid and carbon black, compacting the batch, dehydrating it to obtain a sinter of boric anhydride with carbon, carbothermic reduction of boric anhydride to obtain boron carbide powder, and cooling. Before mixing, the carbon black is subjected to heat treatment, the charge is dehydrated according to the mode where it is initially heated to 140-160 ° C and held for 0.5-1 hour, then heated to 240-260 ° C and held for 0.5-1 hour, then heated to 380-430 ° C and held for 1-1.5 hours, after which carbothermic reduction of boric anhydride to 700 ° C is carried out in a vacuum, and further heating to 1800-1850 ° C is carried out at a rate of 4-6 deg / min in an argon stream and held for 2-3 hours, after which cooling is carried out in a vacuum. The result is a chemically pure polydisperse boron carbide powder of a given grain composition from 5 to 150 μm, which does not require intensive grinding. The disadvantage of this method is the high temperature of the final heat treatment, as well as the multi-stage dehydration scheme of the charge to obtain the sinter, which significantly complicates the technological process.

The invention proposes to prevent the possibility of the appearance of free carbon impurities and reduce energy costs. RU 2550848 C2. Powders of amorphous boron and highly dispersed carbon material (nanofibrous carbon) are weighed so that the content of highly dispersed carbon material in the sample is 17.6-29.5% by weight (the sample weight is within 30-50 grams), after which the mixture is sifted through a sieve with a cell size of 100 μm. During sifting, boron is well mixed with highly dispersed carbon material. Then the mixture is loaded into a glassy carbon crucible with an internal diameter of 15 mm and an internal space height of 60 mm (its volume is 10.603 cm³). With a charge density of 1.8 g/cm³The mass is approximately 19 grams. The glassy carbon crucible is closed with a graphite lid and placed in a quartz reactor, which in turn is inserted into the inductor of an induction furnace. To prevent boron nitriding (boron nitride will form along with boron carbide), the quartz reactor is flushed with argon. One synthesis cycle consists of the following operations:

- heating the reactor to a given temperature (1700-1800°C) - 5 minutes;

- synthesis at a temperature of 1700-1800°C - 15-20 minutes;

- reactor cooling - 15 minutes.

The disadvantages of this method include the use of expensive nanofibrous carbon.

As an alternative to high-temperature powder technologies, there has been a great deal of interest in recent years in the development of polymer precursors for the production of ceramic materials.

Patent US 7635458 B1 discusses the production of ultrafine boron carbide powders from liquid boron-containing precursors and/or carbon-containing precursors. Liquid starting materials are fed together or separately into a plasma system, where the starting materials react to form boron carbide in the form of ultrafine particles. However, the described method requires significant costs, due to the need to use an expensive boron-containing precursor (trimethylboroxine, trimethylborate, triethylborate, or a combination thereof).

Methods for producing boron carbide using various organic compounds as a carbon source make it possible in some cases to reduce the firing temperature, but often contaminate the product with carbon, which is difficult to remove.

Low-temperature synthesis of boron carbide is proposed in [Materials Letters (2013) pp. 1839-1841. Low-temperature synthesis of boron carbide powder from condensed boric acid-glycerin product. Masaki Kakiage, Naoki Tahara, Ikuo Yanase, Hidehiko Kobayashi. Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan] . Boric acid (H ₃ BO _3, 99.5%) and glycerol (C ₃ H ₈ O ₃ , 99.0%) were used as starting materials. Crystalline B ₄ C powder was synthesized from the condensed product of H ₃ BO ₃ -C ₃ H ₈ O ₃ composition by carbothermic reduction. Borate-ether bonds (BOC) were formed in the condensed product. The condensed product was obtained by dehydration at 150°C of a mixture of equimolar amounts of boric acid and glycerol. This condensed product was placed in an alumina crucible and heated in air at 250°C for 2 h, then at 350°C for 2 h, after which pyrolysis was carried out in air at 450-650°C for 2 h to obtain a precursor powder from which excess carbon was removed. The formation of crystalline B ₄ C began at 1150°C; finally, crystalline B ₄ C powder without residual carbon was synthesized by heating at 1250°C for 5 h in an Ar flow. The disadvantage of the method is the multi-stage dehydration scheme of the batch to obtain a sinter, which significantly complicates the technological process.

The closest solution selected as a prototype is patent US 3885022 A. In this case, a method for obtaining boron carbide from an aqueous-alcoholic solution of a carbon source is described. The method is carried out by mixing a boron source, a hydrocarbon source and a polyhydric alcohol together to form a solution, which is heated to form boron carbide. Then, the said solution is dried at a temperature relatively close to the boiling point of the alcohol. Next, the dried material is heated in a hydrogen atmosphere at a temperature of 690 ° C to 710 ° C to obtain a granulated product. After that, the said granulated product is fired at a temperature of at least 1700 ° C in an inert gas atmosphere to obtain loose boron carbide powder. The disadvantage of this method is the use of explosive hydrogen gas, as well as toxic ethylene glycol, which can also spontaneously ignite when heated to + 380 ° C.

The objective of the claimed invention is to obtain a precursor for the manufacture of ceramic products based on boron carbide of increased chemical purity at fairly low temperatures and without the use of toxic and explosive substances .

The task is solved by the following method of obtaining a precursor for the manufacture of ceramic products based on boron carbides. To do this, boric acid and carbohydrates are dissolved together in hot water, dried, crushed in a ball mill, the mixture is calcined, cooled to room temperature, then re-crushed in a ball mill, mixed with a nitriding additive and a nitride formation catalyst.

The essential distinguishing features of the claimed technical solution are:

1. The active mixture is obtained using the following ratio of components, mass parts:

boric acid 10 carbohydrates 6.5-7.2 nitriding additive 0.5 nitride catalyst 0.005;

2. sugar is used as carbohydrate;

3. joint dissolution of boric acid and carbohydrates is carried out in hot water at 80-90°C, further drying - at 150°C;

4. the resulting mixture, after dissolution and drying, is ground in a ball mill and calcined at a temperature of 450°C;

5. Urea is used as a nitrogenating additive;

6. lithium hydroxide or carbonate, calculated as lithium oxide, is used as a catalyst for nitride formation;

7. The final heat treatment of the mixture is carried out in an induction furnace at a temperature of 1100-1200°C.

An example of the implementation of the method for obtaining a precursor for the manufacture of ceramic products based on boron carbide.

The process is carried out in two stages:

1) obtaining a mixture (melt) of boron oxide with active carbon;

2) synthesis of amorphous particles of boron, carbides and nitrides of boron.

At stage 1, boric acid (10 parts by weight) and carbohydrates (6.5-7.2 parts by weight) are dissolved together in hot water (80-90°C). The mixture is then dried at 150°C for 3 hours, then crushed in a ball mill for 30 minutes. After that, the mixture is calcined at 450°C, and after cooling to room temperature, it is crushed again in a ball mill for 30 minutes, then mixed with a nitriding additive (urea 0.5 parts by weight).

At stage 2, the resulting mixture is placed in a graphite crucible of an induction furnace (filled without tamping to two-thirds of the crucible volume), then a protective layer of activated charcoal is laid. The crucible is tightly closed with a graphite lid with two drilled holes for a quartz (ceramic) tube through which argon will be supplied and exhaust gases will be removed. The graphite crucible with the lid is covered with a blanket of high-temperature material for additional protection from the effects of oxygen.

Carbonization (reduction of boron and carbide formation) is carried out at a temperature of 1100-1200°C with continuous supply of argon for 30 minutes. Argon is supplied until the furnace cools down to 200°C. After complete cooling, the powders are sifted through a sieve to remove activated charcoal and washed with warm distilled water to remove boron oxide.

A decrease in the amount of carbohydrates (sugar) below 6.5 parts by weight leads to an incomplete reaction and residual content of glassy boron oxide, an increase above 7.2 parts by weight leads to the appearance of carbon (soot) in the mixture, which is almost impossible to remove. A decrease in the amount of nitriding additive (urea) below 0.5 parts by weight leads to the formation of lower carbides, an increase above 0.5 parts by weight is inappropriate. The nitriding additive in the reaction mixture acts as a mineralizer, i.e. a substance that facilitates the production of boron carbides of higher degrees of carbonization (B ₄ C and B ₃ C).

Increasing the amount of lithium carbonate or hydroxide in terms of lithium oxide above 0.005 parts by weight prevents the appearance of _B4C and promotes the formation of excess BN; decreasing it below 0.005 parts by weight helps to reduce the required amount of BN. It is known that in the presence of lithium compounds, highly dispersed boron nitride with a graphite-like structure is formed during carbothermal reduction [T.S. Bartnitskaya, V.I. Lyashenko, A.V. Kurdyumov et al. Formation of graphite-like BN in the presence of lithium compounds. Powder Metallurgy, 1994, No. 7/8 (374), pp. 1-8. 12. Bartnitskaya T.S., Vlasova M.V., Lyashenko, A.V. et al. Formation of highly dispersed boron nitride during carbothermal reduction in the presence of lithium compounds. Powder Metallurgy, 1993, No. 1, pp. 64-73.] A small content of boron nitride plays the role of a sliding additive and promotes better forming of blanks in the manufacture of ceramic products by various methods (pressing, vibration forming).

Claims (1)

A method for producing a precursor for manufacturing ceramic products based on boron carbide, comprising jointly dissolving boric acid and carbohydrates in hot water at a temperature of 80-90°C, drying at 150°C, crushing in a ball mill, calcining the resulting mixture at a temperature of 450°C, cooling to room temperature, then a second crushing in a ball mill, mixing with a nitriding additive and a nitride formation catalyst, heat treatment in an induction furnace at a temperature of 1100-1200°C with a continuous supply of argon, characterized in that the initial components are used in the following ratio, parts by weight: boric acid 10, carbohydrates 6.5-7.2, nitriding additive - urea 0.5, nitride formation catalyst - lithium hydroxide or carbonate in terms of lithium oxide 0.005.