RU2035396C1 - Method for producing powders of high-melting inorganic compounds - Google Patents

Abstract

FIELD: production of abrasives. SUBSTANCE: mixture of source reagents, e. g. SiO₂ and C for the production of SiC is placed at such a distance from electron source that the spot diameter would be equal to the width of the container working space. Then the electron source is switched on. The time of exposure is chosen from the relation submitted in the description. EFFECT: invention permits producing clean single-phase products, widen technological capabilities and increase efficiency of method.

Description

 The invention relates to inorganic chemistry, namely to high-temperature synthesis of powders of refractory inorganic compounds, for example carbides, borides, silicides, nitrides, etc.

 Refractory compound powders are used as abrasives, grinding pastes and suspensions for the production of high-temperature heaters, lubricants and refractories, catalysts, alloying additives, structural and other materials in various fields of technology.

 Closest to the invention is a method for producing powders of refractory inorganic compounds in the mode of self-propagating high-temperature synthesis (SHS) [1] ensuring the completeness of the reactions of formation of refractory compounds.

 The method consists in the formation of refractory compounds in the course of exothermic combustion reactions in the volume of a homogeneous charge mixture of reactants with local initiation of ignition of the mixture. SHS reactions are carried out in special sealed reactors at elevated pressure of the reaction or inert gas.

 The disadvantages of this method are limited technological capabilities due to the receipt of only those refractory compounds, the formation of which is accompanied by a significant exothermic effect; limited performance of the process due to its periodicity; the impossibility of shortening the period of the process, which is associated with the occurrence of post-processes following the heat release zone and affecting the completeness of the synthesis reaction, structure formation, phase composition formation, as a result of which the cooling time of the material after the passage of the heat wave should be no less than the characteristic duration of post-processes; insufficient quality of the final product due to chemical and structural heterogeneity of the product by volume; the high cost of refractory compounds obtained in a known manner, due to the use of pure materials as fuel (Al, Ti, Zr, Nb, Hf, Ta, etc.), and as oxidizing agents of pure non-metals B, C, N, S, P, Si and others. obtaining the final synthesis products in the form of very strong ingots or cakes, requiring subsequent grinding.

 The aim of the invention is to expand the technological capabilities of the method, increasing the productivity of the process and improving the quality of the target product.

(C_iΔT+ΔH

+ΔH

), (1), (1) где S площадь электрического луча на поверхности материала, м²,
Х высота насыпки материала, равная глубине проникновения электронов, м;
φ_i плотность i-го компонента смеси, кг/м³;
ν_i массовая доля i-го компонента смеси, отн.ед.The goal is achieved in that in the known method for producing refractory inorganic compounds, for example, carbides, nitrides, borides, sulfides, silicides, etc., including heat treatment of the mixture of the starting components and subsequent grinding of the obtained product, heat treatment of the mixture of the starting components is carried out under the influence of a beam of accelerated electrons during the time determined from the ratio:
t≥

(C _i ΔT + ΔH

+ ΔH

), (1), (1) where S is the area of the electric beam on the surface of the material, m ² ,
X the height of the filling material, equal to the depth of penetration of electrons, m;
φ _{i the} density of the i-th component of the mixture, kg / m ³ ;
ν _i mass fraction of the i-th component of the mixture, rel.

Heat capacity C _i i-th component of the mixture, J / ^kg. TO;
Δ T is the difference between the temperature of the reaction mixture and the ambient temperature, K;
Δ N _{f ni is the} heat of the phase transition, i-th component in the mixture, J / kg;
Δ N _{icn.i is the} heat of vaporization of the i-th impurity in the mixture, J / kg;
P power source, watts.

 The impact on the mixture of the starting components by a beam of accelerated electrons provides a local energy effect on the processed mixture, which allows you to concentrate energy on a given site. In this case, the following physicochemical transformations occur in the processed material; dissociation of molecules, ionization and excitation (formation of radicals) of molecules and atoms, bremsstrahlung ionizing radiation, which provide a high rate of chemical reactions. In addition, there is also a process of explosive evaporation of impurities contained in the starting components, which ensures high purity of the final product and the perfection of its crystal lattice.

 The high-temperature effect on the starting material causes a high concentration of internal stresses in the final product, as a result of which it is an easily crushed sponge structure. The implementation of this method allows the use of materials with a high content of impurities (which are then removed as a result of their explosive evaporation) as raw materials.

 The choice of the time of exposure to the starting components according to the proposed ratio allows you to adjust the process and, accordingly, the quality of the final product, depending on the physical and chemical properties of the starting materials, installation parameters.

 When the initial components are bombarded with a beam of accelerated electrons, energy is released in the absorption region by the volume of the electron beam, which increases the internal energy of the mixture and as a result, a temperature range is reached that meets the conditions for the formation of this refractory compound.

C_i+

H

+ Q′S, где ΔТ разность между температурой образования тугуплавкого соединения и температурой окружающей среды, К;
ν_i массовая доля i-го компонента в смеси, отн.ед.Given the energy costs of phase transitions and heat removal, the amount of heat required for the formation of a refractory compound is defined as:
q ΔT

C _i +

H

+ Q′S, where ΔТ is the difference between the temperature of formation of the refractory compound and the ambient temperature, K;
ν _i mass fraction of the i-th component in the mixture, rel.

C _{i is the} heat capacity of the i-th component, J / kg ^. TO;
Δ N _{f ni is the} heat of the phase transition of the i-th component of the mixture, J / kg;
Q ^I heat flux, W / m ² .

≥ ΔT

C_iρ_i+

H

+

H

или
t ≥

(C_iΔT+ΔH

+ΔH

)
Глубину проникновения электронов в обрабатываемый материал определяют из выражения
Х 2,35 ^. 10^-12 U²/ρ где U ускоряющее напряжение, В;
ρ плотность, кг/м³.Neglecting heat removal from the surface of the mixture of the starting components and assuming that the effective formation of refractory compounds occurs in the region of absorption of the beam by the volume of the mixture of components, and also taking into account the evaporation of impurities, we obtain the expression for calculating the exposure mode:

≥ ΔT

C _i ρ _i +

H

+

H

or
t ≥

(C _i ΔT + ΔH

+ ΔH

)
The penetration depth of electrons in the processed material is determined from the expression
X 2.35 ^. 10 ^-12 U ² / ρ where U is the accelerating voltage, V;
ρ density, kg / m ³ .

PRI me R 1. To obtain silicon carbide from silica and carbon, a mixture of reagents (SiO ₂ and C) is mixed in the ratio, wt. 0.71 (SiO ₂ ) 0.29 (C) and placed in a heat-resistant crucible, the material of which is chemically inert for this reaction mixture, for example graphite. The distance from the source to the surface to be treated is chosen so as to ensure that the diameter of the spot from the beam on the surface of the mixture of reagents is equal to the width of the working space of the crucible, that is, the width of the fill. The crucible with the reaction mixture placed in it is uniformly moved relative to the source (accelerated electrons of the ELV-4 installation). The exposure regimen is selected based on the formula (1). To prevent oxidation of the product isolated from the electron beam treatment zone, an inert atmosphere, for example argon, is created behind this zone.

2651 кг/м³; ρ_с 2220 кг/м³;

= 0,71; ν_с 0,29; С

= 741 Дж/кг ^. К; С_с 672 Дж/кг ^. К; ΔТ 2275-295 1980 К; Δ Н

1,42 ^. 10⁵ Дж/кг теплота плавления; ΔН_{ф n.c} 5,97^.10⁷ Дж/кг; S 2 ^. 10^-4 м²; Х 2,5^.10^-3 м; Р 3^.10⁴ Вт.The exposure time of the electron beam t is calculated by the formula (1). In this case, the parameters in the calculated expression (1) have the following numerical values:

2651 kg / m ³ ; ρ _s 2220 kg / m ³ ;

= 0.71; ν _s 0.29; FROM

= 741 J / kg ^. TO; C _with 672 J / kg ^. TO; ΔT 2275-295 1980 K; Δ N

1.42 ^. 10 ⁵ J / kg heat of fusion; ΔH _{f nc} 5.97 ^. 10 ⁷ J / kg; S 2 ^. 10 ^-4 m ² ; X 2.5 ^. 10 ^-3 m; P 3 ^. 10 ⁴ watts

At low concentrations of impurities, the values of Δ and _cn.i can be neglected.

[(2651·0,71·741·1980+1,42·10⁵) +
+ (2220·0,29·672·1980)] 6,02·10^-2 c
Время воздействия электронного пучка в данном случае 0,56 с, что превышает расчетное.Thus, putting the appropriate parameter values in the formula (1) for the mixture of reagents, calculated on the formation of SiC, we obtain:
t>

[(2651 · 0.71 · 741 · 1980 + 1.42 · 10 ⁵ ) +
+ (2220 · 0.29 · 672 · 1980)] 6.02 · 10 ^-2 s
The exposure time of the electron beam in this case is 0.56 s, which exceeds the calculated one.

Silicon carbide has the following characteristics: silicon content, wt. 69.95; carbon content, wt. 30.05; pycnometric density, g / cm ³ 3,212.

X-ray diffraction analysis: single-phase product: α-SiC with a hexagonal close-packed crystal lattice. Lattice parameter: a _about 3.075 A; with _about 15.12 A.

PRI me R 2. Obtaining In ₄ Si analogously to example 1, but the parameters in the formula (1) have the following numerical values: ρ _In 2340 kg / m ³ ; ρ _si 2330 kg / m ³ ; ν _at 0.61; ν _si 0.39; With _si 588 J / kg ^. TO; C _at 1286 J / kg ^. TO; ΔT 1495-295 1200 K.

The melting temperature of boron T _{pl. In} 2523 K, and the melting temperature of silicon T _pl.si 1724 K; therefore, the phase transformations of these substances in the synthesis temperature range can be neglected. The values for S, X and P will be the same as in the example of obtaining SiC.

[(2340·0,61·1286·1200) + (2330·0,39·558·1200)]
0,04 c
Реальное время воздействия электронного пучка составляет 0,26 с, что превышает расчетное.Substituting the corresponding parameter values in the formula (1), we obtain the following numerical value of the exposure time:
t>

[(2340 · 0.61 · 1286 · 1200) + (2330 · 0.39 · 558 · 1200)]
0.04 s
The actual exposure time of the electron beam is 0.26 s, which exceeds the calculated one.

 The resulting product is clean and uniform.

 The products obtained by the method according to the invention, with a correctly selected ratio of reagents from the point of view of the completeness of their reaction, are of high quality, which is a consequence of the higher chemical purity of this product. In addition to the process of explosive evaporation of impurities, the improvement of the product quality is affected by the fact that this product is an easily degradable spec, in contrast to the refractory compounds obtained by the SHS method and the basic method, which are very strong cakes and ingots that require crushing and grinding, during which powder contamination due to wear of the walls and grinding media of grinding units.

 The limitations of the SHS method selected as a prototype are related to the possibility of obtaining only those refractory compounds whose formation reactions are accompanied by a significant exothermic effect. The inventive method allows to obtain refractory compounds regardless of the thermal effect of the reaction of their formation. Thus, providing the possibility of obtaining refractory compounds obtained by the SHS method, as well as those that do not receive for the above reason, the SHS method is an extension of the technological capabilities of the method.

 The invention also allows to increase the productivity of the process by reducing the duration of the heat treatment and ensuring its continuity.

Claims (1)

METHOD FOR PRODUCING POWDERS OF REFINABLE INORGANIC COMPOUNDS, for example carbides, nitrides, borides, sulfides, silicides, including heat treatment of a mixture of the starting components and subsequent grinding of the obtained product, characterized in that, in order to expand the technological capabilities of the method, increase the productivity of the process and improve the quality of the target product, the heat treatment of the mixture of the starting components is carried out upon exposure to a beam of accelerated electrons, and the exposure time is selected from the expression

where S is the area of the electron beam on the surface of the processed material, m ² ;
x the height of the bulk material, equal to the depth of electron penetration, m;
P power source, W;
ρ _{i the} density of the i-th component of the mixture, kg / m ³ ;
ν _i mass fraction of the i-th component, relative ΔT is the difference between the temperature of the reaction mixture and the ambient temperature, K;
C _{i the} heat capacity of the i-th component of the mixture, J / kg · K;
ΔH _{fp, i is the} heat of the phase transition of the i-th component in the mixture of reagents, J / kg;
ΔH _{isp., I is the} heat of vaporization of the i-th impurity in the mixture, J / kg