In this article and the followings, the focus will be on how large numbers of these crystallites are combined to form synthetic carbon and graphite products. The various types of synthetic materials will be reviewed including their production processes, their properties and characteristics, and their present and potential applications.
In terms of size, the review proceeds from the size of a single carbon atom, to that of a graphite crystal, composed of thousands of atoms, to that of a graphite product such as an electrode, which may weight hundreds of kilograms.
Natural graphite, which is found in found in abundance in many areas of the world, has been used since historical times, but its applications always were limited. In the last century, the advent of synthetic graphite and carbon has considerably increased the scope of applications, although natural graphite still remains the material of choice in a few cases. A large majority of graphite and carbon products are now synthetic and these products are continuously being improved and upgraded.
The carbon terminology was reviewed in ch.3. sec.1.1, and its proper use is important as confusion can sassily prevail because of the many variations of graphite and carbon products. The synthetic graphite and carbon products can be divided into six major categories:
- Molded graphite and carbon.
- Vitreous / glassy carbon.
- Pyrolytic graphite and carbon
- Carbon fibers
- Carbon composites and carbon-carbon
- Carbon and graphite powders and particles
These divisions may appear arbitrary and overlapping in some cases; for instance, fibers and carbon-carbon are generally polymeric carbons, although pyrolytic graphite is often used in their processing. These divisions however correspond to specific and unique processes, with resulting products that may have different properties. These variations in properties, as stated in Ch.3, stem from the nature of the polycrystalline aggregates, their different crystallite sizes and orientation, various degrees of porosity and purity, and other characteristics.
General characteristics of synthetic graphite and carbon: Many new graphite- and carbon-materials with improved characteristics have been developed in the last two decades. Some of these materials have a strongly anisotropic structure and properties that approach those of the perfect graphite crystal. Others have a lesser degree of anisotropy which is not always a disadvantage as, in many cases, isotropic properties are a desirable feature, as will be seen in later chapters.
A common characteristic of graphite and carbon materials, whatever their origin or processing, is that they are all derived from organic precursors: molded graphite from petroleum coke and coal-tar, pyrolytic graphite from methane and other gaseous hydrocarbons, vitreous carbon and fibers from polymers, carbon black from natural gas, charcoal from wood, coal from plants, etc.
These organic precursors must be carbonized and, more often than not, graphitized, in order to form carbon and graphite materials. The critical and complex phenomena of carbonization and graphitization are the two common features of the production of all these synthetic materials with the notable exception of pyrolytic graphite, which is produced by the entirely different process of vapor deposition. These two phenomena, carbonization and graphitization, are the topics of this chapter.