High-temperature treatment of carbon-carbon composite materials (1)

High-temperature treatment (HTT) of blanks after carbonization is an important stage of production process in the manufacture of carbon composites and structural graphite. High-temperature treatment treatment controls the true density of carbon materials, its thermal and electrical conductivity, and also oxidation rate and other chemical processes. At the same time, analysis of structural parameter formation for carbon-carbon composite materials, establishment of temperature limits and instructions for HTT have received less attention compared with two other main production processes, i.e., carbonization of blanks and carbon matrix compaction.

The aim of this work is to study formation of C/C structural parameters during high-temperature treatment, substantiation on the basis of this of specifications for production processes, and establishment of the completion limits for individual HTT stages. Currently domestic industry is based on C/C series production catering primarily for electrical engineering and chemical enterprises with high-strength and heat-resistant carbon materials and large thin-walled shapes. Clarification of temperature limits for improvement of the structure is of considerable economic importance on a background of a steady increase in cost of power generation resources.

The composites studied were based on carbon fiber from polyacrylonitrile filament and combined carbon matrix of phenol formaldehyde resin coke and pyrolytic deposition of carbon within pores. The fundamental production scheme for manufacturing CCM products has been described previously. Reinforcement of the structure was created by winding or laying on a mandrel. An autoclave or compression method was used in order to harden the phenolformaldehyde of the original binder. Carbonization was carried out within furnaces with a controlled reducing gas atmosphere. The raw material for preparing pyrolytic carbon supply-line hydrocarbon natural gas, containing 90-95% carbon, and the rest was mainly hydrogen.

Study procedure:

Measurements and testing during a study of composite materials (CM) and CCCM properties, monitoring of production operations for CM preparation, their carbonization and compaction, and also HTT, were carried out by means of metrologically certified measurement facilities by procedures of the OAO NIIgrapit Test center carbon materials.

The relative change in specimen dimensions and component dimensions in three mutually perpendicular directions were measured by a standard instrument. Density and open porosity were determined by a hydrostatic method. Open porosity from 5 to 35% was certified within these limits by procedure MI 00200851-162-2009.

The ultimate strength in bending was determined according to procedure MI 0020085-335-2010 in specimens with a size of 100*15* δ with distance between the supports of 80mm. Specimen thickness δ did not exceed 6 mm and was equal to the thickness of test plate or shell material. Ultimate strength in bending was determined as the quotient of maximum bending moment by the moment of transverse bending resistance. The determination error for the breaking load did not exceed 1%.

Measurement of linear thermal expansion coefficient was carried out by determining specimen elongation directly by procedure MI 00200851.163-2007. Direct measurement of displacement was carried out by means of a horizontal microscope. According to the curve for the dependence of relative elongation on temperature measurements the average LTEC was calculated in prescribed temperature ranges. Tst temperature was measured by an optical pyrometer. Measurements were performed in a laboratory electric vacuum furnace in a pure argon atmosphere. In order to reduce the error of optical temperature measurement due incomplete grey body radiation as a result of radiation of the boundaries a special form of specimen edge was developed. The absolute error of the procedure was 0.1*10-6/K.

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