Crystallite imperfections

Within each crystallite, a varying number of imperfections may be found as shown in Figs. 3.4 and 3.5. These include  

Other crystalline imperfections likely caused by growth defects are screw dislocations and edge dislocations (Fig. 3.6). The presence of these imperfections may have a considerable influence on the properties of the bulk material. 

Thus in each graphitic material, the size, shape, and degree of imperfection of the basic crystallite, the general orientation of these crystallites, as well as the bulk characteristics such as porosity and amount of impurities, may vary considerably from one material to another. As a result, the properties of these various materials may show considerable differences. 

An important implication is that, whereas material differences in the various carbon materials were originally ascribed to the presence (or absence) of an amorphous component (as in lampblack for instance), a more realistic approach is to relate these differences to the size and orientation of the graphite crystallites. 

The specific structure and properties of the different graphitic materials will be reviewed in detail in subsequent chapters. 

---

Graphite is commonly produced by CVD and is often referred to as pyrolytic graphite. It is an aggregate of graphite crystallites, which have dimensions (L ) that may reach several hundred nm. It has a turbostratic structure, usually with many warped basal planes, lattice defects, and crystallite imperfections. Within the aggregate, the crystallites have various degrees of orientation. When they are essentially parallel to each other, the nature and the properties of the deposit closely match that of the ideal graphite crystal. 

Figure 2.18 Schematic of crystallite imperfections in graphite showing unfilled lattice, stacking fault and disinclination. Source Reprinted with permission from Pierson HO, Handbook of Carbon, Graphite, Diamond and Fullerenes, Noyes Publications, Park Ridge NJ, p. 49,1993. Copyright 1993, William Andrew Publishing.

Figure 3.5. Schematic of crystallite imperfections in graphite showing unfillecf lattice, stacking fault, and disinclination.

Pyrolytic graphite is an aggregate of graphite crystallites which have dimensions (LJ that may reach several hundred nm (see Ch. 3, Sec. 2). It has aturbostratic structure, usually with many warped basal planes, lattice defects, and crystallite imperfections. 

The 2500°C heat-treatment causes the reordering of the structure. The basal planes coalesce and become more parallel and closer together. The various crystallite imperfections such as vacancies, stacking faults, dislocations, and rotational disorders, tend to heal and disappear the crystallite size (Lg) increases the 002 line narrows considerably and becomes close to the position of the ideal graphite line as the interlayer spacing (d) decreases to approach that of the ideal graphite crystal (0.3354 nm). This observed reduction of the interlayer spacing is attributed in part to the removal of interstitial elements, mostly carbon.P l... 

The lower the temperature of crystallisation, the lower the melting point of the best crystal will be, but also the more imperfect crystallites are formed. The melting range of the sample with = 161 °C is, therefore, broader. 

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

Decreased mobility of adsorbed chains has been observed and proved in many cases both in the melt and in the solid state [52-54] and changes in composite properties are very often explained by it [52,54]. Overall properties of the interphase, however, are not completely clear. Based on model calculations the formation of a soft interphase is claimed [51], while in most cases the increased stiffness of the composite is explained by the presence of a rigid interphase [55,56]. The contradiction obviously stems from two opposing effects. Imperfection of the crystallites and decreased crystallinity of the interphase should lead to lower modulus and strength and larger deformability. Adhesion and hindered mobility of adsorbed polymer chains, on the other hand, decrease deformability and increase the strength of the interlayer. 

The determination of crystal structure in synthetic polymers is often made difficult by the lack of resolution in the diffraction data. The diffuseness of the reflections observed in most x-ray fiber patterns results from the small size and imperfect lattice nature of the polymer crystallites. Resolution of individual reflections is also made difficult from misorientation of the crystallites about the fiber axis. This lack of resolution leads to poor accuracy in measurement of peak positions. In particular, this lack of accuracy makes determination of layer line heights difficult with a corresponding loss of significant figures in evaluation of the repeat distance for the molecular conformation. In the case of helical conformations, the repeat distance may be of considerable length or, as we shall show, indeterminate and, in effect, nonperiodic. This evaluation requires high accuracy in measurements of layer line heights. 

The technique can be complemented by line-broadening analysis which gives valuable information on the size of individual crystallites. Variations of ratios between lines indicate cither order imperfections along... 

---