**3.2 Determining filler activity by sintering ability**

As an example, let us consider the curve for the change in strength of pitch composites with anthracite (**Figure 4**). The strength of the pitch composite increases (the left branch of the curve in **Figure 4**), because the high modulus filler reduces the ability of the entire composition to deform. With an increase in the degree of filling, the individual particles of the filler approach and their boundary (interphase) layers begin to interact with one another, forming a film structure of the matrix between the particles (**Figure 5a**).

We do not take into account here that individual particles in composites are rarely observed [22]. Since binders that solidify are typical nonequilibrium systems, the loss of stability leads to the spontaneous formation of primary clusters—a decrease in surface energy occurs due to a reduction in the interfacial surface and, as a consequence, aggregation of the filler particles. The filler particles on their surface have a layer of a pitch matrix, the so-called interfacial layer (IFL) [23]. There is no clear boundary between IFL and the pitch matrix, but it is established that IFL reduces the concentration of stresses on the surface of the matrix and filler, which affects deformation and fracture in composites [24]. The thickness of the interphase layer

**25**

**Figure 4.**

*Determining the Filler Activity in the Sintering of Pitch Composites*

responds to the maximum on the curve (**Figure 4**).

due to capillary phenomena.

were previously expressed [26].

*composite on the amount of filler (anthracite) in it.*

depends on the radius of the filler particles and the fractal dimension of its surface [25]. At low filler concentrations in the matrix, the interphase layers are not an independent phase in the volume of the composite, which does not affect its properties. Considering the fact that the interfacial layer is an oriented ordered structure, the film has enhanced strength compared to the structure of the remaining volume of the pitch matrix. At the critical point, the complete transition of the pitch matrix to the film structure of the IFL is the main factor of strength increasing of the pitch composites. In addition, the filler-binder ratio at this point is optimal, which cor-

In the case of a critical binder-filler ratio, the pitch matrix is completely on the surface of the filler particles in a structured (interfacial) layer (**Figure 5a**). The activity of the filler is maximum. With a further increase in the content of the filler in the composite (the right branch of the curve in **Figure 4**), the activity of the filler does not decrease, but the pitch matrix cannot cover the entire surface of the filler grains. The film is not enough to cover the entire surface of the filler grains. The binder film is divided into separate fragments (**Figure 5b**), which is accompanied by a sharp decrease in the strength of the final composite. In this consideration, we do not take into account the effect of agglomeration of filler particles. Although the amount of the matrix and probably its cohesive strength does not change, the strength of the composite decreases. The strength of the matrix on the surface of the filler particles can be another indicator of the activity of the filler. In the actual conditions of formation of the contact surface of the filler with a liquid binder, the processes of diffusion of the adhesive into the filler play an important role. A part of the pitch can penetrate into the surface layer of the filler

The removal of volatile substances from the pitch when carbonized leads to the appearance of shrinkage stresses and defects and, as a result, to rupture the film. That is, a pitch that is carbonized to the semicoke state may not completely cover the filler surface and not create a continuous "carbon skeleton," the ideas of which

*Experimental points and approximation by the parabola equation for the dependence of the strength of a pitch* 

*DOI: http://dx.doi.org/10.5772/intechopen.82012*
