Chemistry and physics of carbon—volume 1

BOOK REVIEW Chemistry

and

Ed. P. L. WALKER v-i-382 pp.

Physics JR.,Marcel

of

Carbon-Volume

Dekker,

1,

New York (1965);

Contents: Chapter

1. Dislocations

Graphite,

and Stacking Faults in S. Amelinckx, P. Delavignette

and M. Heerschap. Chapter

2. Gaseous Mass G. F. Hewitt.

Chapter

3. Microscopic Studies J. M. Thomas.

Chapter

4. Reactions

and

Transport

with

of Graphite

of Carbon Steam, Sabri

with

Graphite, Oxidation,

Carbon

Ergun

and

Dioxide Morris

Mentser. Chapter

of Carbon from Gases, 5. The Formation Howard B. Palmer and Charles F. Cullis.

Chapter

6. Oxjtgen Chemisorption

Thermoelectric L. G. Austin

Effects on Graphite

P. L. Walker and J. J. Tietjen.

Power,

Jr.,

IN THE entire periodic system, carbon has long held a unique place, through the complexity and diversity of its organic chemistry. As has only recently been recognised, elemental carbon presents a corresponding diversity of problems for the solid state. In recent years, scientific attack on these problems has been stimulated from various directions. Some of the technological potentialities of carbon have of course long been known, on a somewhat empirical basis. Amongst the most important of these is its comparative inertness to chemical attack even at quite high temperatures. Specialised demands of high temperature technology are growing rapidly. In the future this points to even more diversified applications of carbons as high temperature solid. Again, the electrical conductivity of certain varieties of carbon is sufficiently high to permit well tried and important uses as a conductor, in electric motors, in industrial electro-chemistry and in arc furnaces. But, in the future, more subtle uses of its electrical properties may be considered, which take into account the high anisotropy of well oriented graphitic carbons. Either alone or in combination with the electrical properties, new applications of the extremely high but anisotropic thermal conductivity of graphites are likewise emerging. Deepened understanding of carbon in its perfect crystallographic forms, as diamond or as graphite, is making new demands on crystal defect theory, even when such defects are low in total concentration. But new forms of carbon are also being identified. Many forms of carbon are, in fact, so grossly defective from a crystallographic standpoint that their reference to crystal norms at all may no longer be useful. Nevertheless, many of these forms of carbon should not be

described as amorphous, when these solids contain distinctive and quite specific local structural arrangements of carbon atoms. Some highly characteristic noncrystalline carbons have analogies with various high polymers, and this suggests new routes to their synthesis. Methods of removing heteroatoms without graphitisation may be found more subtle than mere coking. These new forms of carbons are being actively studied in relation to the organic compounds from which they are derived. Finally, the unique properties of carbon atoms as neutron targets have led to extensive applications in nuclear technology. As a consequence, many problems of radiation damage and annealing of such damage which are basic to solid-state physics have acquired large scale economic significance. This has stimulated scientific and technological research in novel directions. All these developments imply that foundation studies of the chemistry and physics of solid carbon demand an exceptionally wide range of specialised scientific disciplines. Without such studies, the newer technological applications will be seriously hampered. Various types of monograph are being produced to meet this demand. The present volume follows the scheme of enlisting distinguished workers in special sectors of the field, to write quite extensive chapters, dealing largely with their own specialities. This editorial approach leads to an exciting and interesting compilation, whose coverage is nevertheless and perhaps unavoidably erratic. A specialist writing about his own speciality is usually well worth reading, but may not give a very balanced critical account of cognate matters. This limitation must be recognised. With such a rapidly advancing subject, it does not seriously detract from the interest and stimulus of what is presented in this volume. The selection of articles in Volume 1 of this series (Volume 2 is already in preparation) may be considered in terms of two leading questions: (1) Does the article have some basic aspect of the physics or chemistry of carbon as its main objective, or does it have as its main aim to give an up-to-date account of a specific technological development? (2) Is the article mainly concerned with a detailed and up-to-date statement of the writer’s own interest in a special field, or is it mainly concerned to give a balanced critical account with a more omnibus coverage? Volume 1 sets out with an authoritative article on dislocations and stacking faults in graphite. The expertise of the authors in this domain is well known. This chapter presents much valuable material for the development of the physics of cooperative defects in solids. However, the treatment is somewhat formalistic and hence restricted. Carbon is a protean solid ; the range of defect

543

BOOK problems which can be discussed with respect to crystalline graphite as the only reference norm is much too limited to provide an adequate scientific background for various properties that depend on defect structure. Even within the confines of Chapter 1 as chosen by the authors, the extreme anisotropy of graphite appears to lead to over-emphasis of those types of defect that are most readily observed with existing techniques of study. Other methods of revealing cooperative defects may be expected to be developed in due course; one example is oxidative etching, which is brilliantly described in another chapter in Volume 1. Ultimately, a more comprehensive presentation of defect structures to be found in near-ideal graphites may emerge. We need to have more information too, about the role played by heteroatoms incorporated in solid carbon, associated with the various dislocations and cooperative defects. The chemical nature of the carbon atom is unlikely to permit free vale&es to any extreme degree. How the valency of carbon is satisfied in defect solids is a very interesting question; no general answer seems yet available. This introductory chapter rightly deals with near-ideal graphite. It is to be hoped that subsequent volumes will contain complementary studies of comparable quality on more grossly defective carbons. Chapter 2 deals with technological problems. In the main, these are of the greatest importance for gas-cooled graphite reactors. Conventional applied physics of the kinetic theory of gases is well worked out in detail. This chapter provides an excellent instance of the stimulus given to the development of basic principles by technological needs. There are interesting boundary effects at the carbon-gas interfaces in pores, which must be taken as empirical data in present theory. Future developments of transport studies, in pores or in the inverse case of fluidised beds, may well repay a less ad hoc approach to boundary slip, and may attempt to relate it to the surface structure of the carbons used. Chapter 3 gives numerous illustrations of optical microscopy, as applied to the study of the oxidation of graphite. As a field of inquiry, the oxidation of graphite ‘has already elicited a prodigious literature’. The present article deals in a very personal manner and with great virtuosity, with a deliberately limited sector of this field. Optical microscopy brings out in a clear and suggestive way important topological features of the oxidation of graphite. These are of special importance because of its highly anisotropic structure and in view of the unusual chemical bonding of the carbon atoms. The author stresses the role of non basal dislocations, which some still regard as controversial, in the oxidation of the solid. A stimulating account of striking catalytic effects of the

REVIEW oxidation of carbon due to metal impurities. Rival theories are based on local electronic influences, and on intermediate chemical compound formation. Stimulated by these opposing theories, much novel experimental work is emerging. In due course, this should throw much light on the chemical reactivity of carbons, and should also react on theories of catalysis generally. It is important to remember, however, that most of the studies described in the article refer to near-ideal graphite. With types of carbons still describable as grossly defective graphites, or with carbons involving other non-graphitic localised structures, quite different mechanisms of chemical attack may predominate, whether by oxygen or by other reagents. Chapter 4 is technological. It deals with the basic reactions in the gasification of coke, using carbon dioxide or steam. As is appropriate for such a long established field of carbon technology, the presentation is systematic and thorough, though without any startling novelties. Chapter 5 by contrast gives a stimulating account of an extremely incomplete and very uneven field of knowledge, in dealing with the formation of carbons from gases. This remains one of the perennial basic themes in the physics and chemistry of carbon. A whole monograph is needed to assess the very differem types of carbon-forming mechanisms indicated by the authors in this article, which is too short to do justice to the themes involved. However, at the end of their article, the authors give a useful and provocative summary of their views about unsolved problems and their suggestions for future work. Whether readers will agree or disagree with their summary, this kind of treatment certainly maxir&es the incentive to further research. Finally, the closing chapter deals with effects which may have very interesting implications but which are physically quite intricate. The thermoselectric power even of fairly pure carbon is an involved subject; complex roles must be attributed to any heteroatoms incorporated and to various kinds of structural defects. The special effect considered in this chapter is due to adsorbed oxygen. Suggestive modifications of T.E.P. are found, and the adsorbed oxygen is considered to act as electron acceptor, removing electrons from the conduction system in forming a surface complex. In all, the six chapters of the first volume comprise a judicious blend of technological expertise, and exploratory physics and chemistry on frontiers of knowledge that are highly stimulating for new research. In both directions this publication should advance the subject very substantially and can be strongly recommended. A. R. UBBELOHDE