Effect of structure on the properties of fibrous carbon materials

Abstract

where area x is the strength of a pore-free material; P, porosity of the matrix; b = 0.405 I/d + 0.318 l/d + 1.22 [4] (l, d are the length and diameter of the pores, respectively). Tests of specimens in an air-plasma stream have shown that entrainment of the carbon-carbon material is also connected by an exponential relation with the porosity of the material [5]. The level of porosity of carbon-carbon materials is determined by the fabrication conditions and the type of fiber and binder. The effect of the starting material and processing parameters on altering porosity with the aim of improving the strength characteristics was examined in [ 1, 6, 7], but no quantitative estimate of the total effect of these factors on the porous structure of carbon-carbon composites was offered. The goal of the present work is to establish quantitative relations describing the dependence of parameters of the porous structure of the material on the structure of the carbon shell and the conditions under which the composite was fabricated. Four groups of pores may be distinguished in a carbon-carbon material with a fibrous filler. 1. Pores in Fiber Filaments. These are mainly closed pores, which comprise about 40% of the pores in graphitized fibers (processing temperature 2200-2500~ [8]. 2. Pores Associated with Textile Elements of the Filler - Filaments, Threads, Braids. Table 1 shows characteristics of the fibers used in the experiment. Assuming that these fiber elements are circular in cross section and considering the possibility of tetragonal or hexagonal lay-up in the material, we may calculate the dimensions of the pores in such element lays: req v ,x, 0.5-1.6 #m for filaments; reqv 'x, 8-10 #m for threads. 3. Pores Associated with the Lay-up of the Filler, i.e., with the Construction of the Shell in the Composite Material. These are mostly coarse pores, with r > 100 #m. 4. Binder Pores, Located in the Interfilament Spaces: These are the smallest pores. They are formed during hightemperature processing of the material (to 1000~ and higher) as a result of 10-60% weight toss involving removal of polymeric material (escape of volatile matter). Moreover, the difference in the coefficients of thermal expansion of the fibers and coke residues of the binder leads to additional cracking of the matrix and the formation of pores around the fibers during cooling after heat treatment.