Industrial Carbon and Graphite Materials. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9783527674053
Скачать книгу
principal all carbon‐containing substances are suited as raw materials for the production of non‐graphitic or graphitic solid carbon materials as long as sufficient carbon remains after the first thermal degradation, the so‐called pyrolysis. Other technical synonyms for heat treatment below graphitization temperatures (>2500 K) under the exclusion of oxygen are coking, calcining, and baking. Whether the product of pyrolysis is a non‐graphitizable or graphitizable carbon depends in general on the mobility of the molecules during pyrolysis. This means the capability to arrange the necessary microstructural pre‐order for the subsequent solid‐state healing process under graphitization conditions (>2500 K). The principle is illustrated in Figure 5.7.

      Carbon‐containing materials passing through a liquid or gaseous form during pyrolysis give graphitizable carbons. Materials that remain solid under pyrolysis conditions maintain their microstructural arrangement also under graphitization treatment conditions. Transitions between solid‐, liquid‐, and gas‐phase pyrolysis do exist and can result in partially graphitic regions after graphitization treatment.

      Examples for a solid‐phase pyrolysis are chars derived from natural matter and glass‐like carbon derived from thermosetting resins. Due to the original structural disorder and impossibility of molecular rearrangement during pyrolysis, these materials do not graphitize.

      Another industrially important example is polyacrylonitrile (PAN) based carbon fibers. PAN precursor fibers are stabilized by a thermal treatment in air. The once stabilized fibers do not melt during subsequent pyrolysis. Further treatment at graphitization temperatures (high‐modulus carbon fibers) does not result in a graphitic carbon fiber.

Schematic illustration of the gaseous, liquid, and solid pyrolysis and their products. Photo depicts an optical micrograph of carbonaceous mesophase from heated anthracene oil. Schematic illustration of the carbonaceous mesophase structure and mechanism of growth by coalescence of spherulites.

      After the formation of a sufficient number of spherulites, so‐called secondary quinoline insoluble (QIs), they touch each other and coalesce under the formation of bigger spherules. With ongoing growth, the spherulitic mesophase is transferred into the bulk mesophase, viscosity increases, and the carbonaceous material becomes solid (Figure 5.9). Formation and structure of the mesophase are essentially governed by three limiting conditions:

      1 1. The shape of the polyaromatic molecules must favor the formation of liquid crystals (i.e. highly aromatic, few heteroatoms, and few aliphatic side chains).

      2 2. The fluidity of the system.

      3 3. The reaction or condensation rate must be smaller than the ordering rate.

      The chemistry of the pyrolytic conversion of hydrocarbons to solid carbon involves numerous chemical reactions. Generalized schemes have been developed by studying simpler models [25, 26]. The major reactions involved are:

       Bond cleavage and formation of free radicals.

       Molecular rearrangement.

       Thermal polymerization/polycondensation.

       Aromatization.

       Elimination of aliphatic side chains and dehydrogenation.

      After pyrolysis further heat treatment to above 2500 K is necessary to obtain synthetic graphite. Industrially most important is the heat treatment of graphitizable carbon compounds above 2500 K by means of electrically heated furnaces known as Acheson [29] and Castner furnaces [30] (see → Carbon, 4. Industrial Carbons). Today the commonly used technical terminus is lengthwise graphitization (LWG). For granular forms of carbon or powders, electrically heated shaft furnaces or fluidized‐bed reactors are used [27, 28, 31].

Schematic illustration of the reaction scheme for carbonization and graphitization.