RU2259258C1 - Method of production of carbon-graphite articles - Google Patents

Abstract

FIELD: production of carbon-graphite articles; powder metallurgy; production of fuel compacts.

SUBSTANCE: proposed method includes mixing the graphite powder with binder, molding, polymerization and carbonization of binder followed by high-temperature treatment of blanks and extraction of them from mold. Used as binder is phenol formaldehyde resin. In the course of polymerization, molding pressure is reduced to zero and upon completion of polymerization process, temperature of blanks is risen to temperature below temperature of beginning of carbonization by 40-50°C; blanks are extracted from mold before carbonization.

EFFECT: reduction of rejects.

3 cl, 2 tbl, 4 ex

Description

The invention relates to the field of carbon-graphite materials and can be used in nuclear fuel technology and powder metallurgy.

Currently, carbon-graphite materials have found application in high-temperature gas-cooled reactors (VTGR), in which the fuel elements (fuel rods) are made in the form of a carbon-graphite matrix containing microfuel (MT). MT is a core of nuclear fuel with a diameter of 0.2-0.5 mm, coated with shells and pyrocarbon and silicon carbide.

After preliminary mixing of MT, graphite powder and a binder (coal tar pitch, phenol-formaldehyde resin, etc.), the fuel blanks are pressed and then subjected to heat treatment, during which the binder is polymerized, the binder is carbonized by pyrolysis with the release of solid coke residue and gaseous pyrolysis products ( phenol, carbon monoxide, hydrogen, etc.). The last stage of heat treatment is heating to 1800 ° C for the final removal of gaseous products.

In the process of obtaining carbon-graphite products at the stage of carbonization, the workpieces are compacted with a corresponding reduction in their size. So, to obtain cylindrical carbon-graphite products with a diameter of 12.5 mm and a height of 50 mm, molds with an inner diameter of 12.8-12.9 mm are used, and pressing along the height of the workpieces is carried out to a size of 51-52 mm. In this case, the extraction of blanks from the mold occurs with considerable effort, because when removing the blanks, the binder is in the solid state and the entire blank has an elastic aftereffect. The effort to extract the workpiece is 10-15 MPa, which can lead to the “storage" of these loads by the workpieces and the curvature of the workpieces at the stages of polymerization and carbonization.

The elastic aftereffect of the lateral surface of cylindrical products is explained by the fact that during the pressing process, the pressure acts from the upper and lower bases, and the height of the workpiece is halved. Due to the redistribution of compression stresses, tensile stresses arise on the side surface of the workpieces.

The curvature of the preforms can be compensated by applying an additional layer of coke grains to the resulting preform, followed by heat treatment up to 2000 ° C (German patent No. 3435863 dated 04.29.84, MKI G 21 C 3/22, 21/04). The disadvantage of this method is the complexity of the additional process steps.

The closest in technical essence to the problem being solved is the method of producing carbon-graphite products by carrying out all heat treatment processes (polymerization, carbonization, high-temperature processing) in injection molds (application for the FRG patent No. 19837969 from 08.21.98, MKM G 21 C 21/00). The disadvantage of this method is the complexity of the hardware design, which makes it impossible to use the method for mass production of products.

The basis of the present invention is the task of simplifying technological operations upon receipt of carbon-graphite products.

According to the invention, the problem is solved in that graphite powder is mixed with a binder, pressed, polymerization, binder carbonization, high-temperature processing and removal of preforms from the mold are carried out, while removing preforms from the mold is carried out before the carbonization operation, and during the polymerization, the pressing pressure reduce to zero, and then raise the temperature to a value of 40-50 ° C below the temperature of the onset of carbonization. In addition, the polymerization is carried out in the temperature range of 70-130 ° C, and the pressing pressure is removed during the end of the polymerization at a temperature of 130 ° C, and a further rise in temperature to 190-200 ° C. The proposed method differs from the known sequence of operations, as well as a new sign of the polymerization operation.

The inventors based on the studies found that when using phenol-formaldehyde resin as a binder, the process of its polymerization occurs in the temperature range of 70-130 ° C. When the pressing pressure is removed (usually 10 MPa) at the moment of polymerization completion (130 ° С) and the temperature rises further to 190-200 ° С (40-50 ° С lower than the carbonization onset temperature 240 ° С), the product diameter decreases by 0 , 07-0.09 mm compared to the inner diameter of the mold. Thus, the product is removed from the mold without any effort, which ensures the preservation of the cylindrical shape of the sample at subsequent stages of heat treatment.

The meaning of the invention lies in the fact that during the polymerization of the resin, the workpiece is compressed, to a certain extent, similar to compression in the carbonization process.

The proposed method was carried out in two versions: mixed graphite powder with a binder for further production of carbon-graphite products and mixed graphite powder with a binder and microfuel (MT) for further production of fuel compacts (by analogy of fuel elements).

Used graphite grade MPG-KS with particle sizes of 5-100 microns, as a binder - phenol-formaldehyde resin. MTs were spheres with a diameter of 0.7 mm, containing a fuel core with a diameter of 0.2 mm and four shells: pyrocarbon with a density of 1.0 g / cm ³ , pyrocarbon with a density of 1.8 g / cm ³ , silicon carbide with a density of 3.2 g / cm ³ , pyrocarbon with a density of 1.8 g / cm ³ .

In the table. 1, 2 give examples of the production of carbon-graphite products and fuel compacts according to the proposed method (d _o is the inner diameter of the mold, d ₂ is the diameter of the workpiece extracted from the mold before the carbonization process according to the proposed method, t _o , p _o are the temperature, respectively and pressing pressure, p ₁ , t ₁ , t ₂ , p ₂ , t ₃ , p ₃ - respectively, the temperature and pressure in the polymerization process, t ₄ - temperature of the end of the process according to the proposed method.

Table 1.
Modes of obtaining blanks carbon products No. d _o
mm t _o
° C p _about
MPa t ₁
° C p ₁
MPa t ₂
° C p ₂
MPa t ₃ ,
° C p ₃ , MPa t ₄
° C d ₂
mm 1 12.80 100 10 110 7 120 4 190 0 190 12.73 2 12.60 100 10 110 7 120 4 200 0 200 12.52 3 12.60 100 10 110 0 120 0 220 0 220 12.59 4 12.60 100 10 110 7 120 0 180 0 180 12.58 table 2
Modes of obtaining blanks of fuel compacts No. d _o
mm t _o
° C p _o
MPa t ₁
° C p ₁
MPa t ₂
° C p ₂
MPa t ₃ ,
° C p ₃
MPa t ₄
° C d ₂
mm 1 12.80 100 10 110 7 120 4 130 0 190 12.72 2 12.60 100 10 110 7 120 4 130 0 200 12.51 3 12.60 100 10 110 0 120 0 130 0 220 12.59 4 12.60 100 10 110 7 120 7 130 0 130 12.59

From the data table. 1, 2 it follows that a significant reduction in the diameter of the workpieces was achieved when obtaining carbon-graphite products and compacts in examples 1, 2, when the heat treatment of the workpieces after a temperature of 130 ° C takes place without the application of pressure in a “restrained” state.

Examples No. 3.4 (Tables 1, 2) show that increasing the temperature above 200 ° C or decreasing below 190 ° C, as well as relieving pressure at 100 ° C or maintaining pressure at 130 ° C, do not significantly reduce the diameter of the workpieces.

The batch of preforms obtained by the proposed method, was subsequently subjected to carbonization and high temperature processing. High temperature treatment was carried out at 1800 ° C. For the criterion of effectiveness of the proposed method with a simplified hardware design selected curvature of the products. The yield of the proposed method for carbon-graphite products and fuel compacts was 100%. For the criterion of curvature, the height difference of cylindrical compacts measured at different points of the upper and lower bases of the cylinders, exceeding 0.2 mm (at a nominal height of 50 mm), was taken.

Claims (3)

1. A method for producing carbon-graphite products, comprising mixing graphite powder with a binder, pressing, polymerizing and carbonizing the binder, further processing the billets and removing the billets from the mold, characterized in that phenol-formaldehyde resin is used as the binder, and the pressing pressure is reduced during polymerization to zero, and after the end of the polymerization process, the temperature of the preforms is raised to a value of 40-50 ° C below the temperature of the onset of carbonization, while blanks from the mold are carried out before the carbonization operation. 2. The method according to claim 1, characterized in that the polymerization is carried out in a temperature range of 70-130 ° C. 3. The method according to claim 1, characterized in that the pressing pressure is removed during the end of the polymerization at a temperature of 130 ° C and a further rise in temperature to 190-200 ° C.