Parepistemes

A striking example of the importance of narrowing the focus in research, which is what the concept of the parepisteme really implies, is the episode (retailed in Chapter 3, Section 3.1.1) of Eilhard Mitscherlich s research, in 1818, on the crystal forms of potassium phosphate and potassium arsenate, which led him, quite unexpectedly, to the discovery of isomorphism in crystal species and that, in turn, provided heavyweight evidence in favour of the then disputed atomic hypothesis. As so often happens, the general insight comes from the highly specific observation. 

Some parepistemes are pursued by small worldwide groups whose members all know each other, others involve vast communities which, to preserve their sanity, need to sub-classify themselves into numerous subsets. They all seem to share the feature, however, that they are not disciplines in the sense that I have analysed these... 

A good way of demonstrating the importance of parepistemes, or in other terms, the virtues of subsidiarity, is to pick and analyse just a few examples, out of the many hundreds which could be chosen in the broad field of materials science and engineering. 

Stephen Keith, a historian of science, has examined the development of this parepisteme (Keith 1998), complete with the stops and starts caused by fierce competition between individuals and the discouragement of some of them, while a shorter account of the evolution of crystal-growing skill can be found in the first... 

The migration of one atomic species in another, in the solid state, is the archetype of a materials-science parepisteme. From small beginnings, just over a century ago, the topic has become central to many aspeets of solid-state science, with a huge dedicated literature of its own and specialised conferences attended by several hundred participants. 

Another subsidiary field of study was the effect of high concentrations of a diffusing solute, such as interstitial carbon in iron, in slowing diffusivity (in the case of carbon in fee austenite) because of mutual repulsion of neighbouring dissolved carbon atoms. By extension, high carbon concentrations can affect the mobility of substitutional solutes (Babu and Bhadeshia 1995). These last two phenomena, quenched-in vacancies and concentration effects, show how a parepisteme can carry smaller parepistemes on its back. 

The transport of charged ions in alkali halides and, later on, in (insulating) ceramics is a distinct parepisteme, because electric fields play a key role. This large field is discussed in Schmalzried s 1995 book, already mentioned, and also in a review by one of the pioneers (Nowick 1984). This kind of study in turn led on to the developments of superionic conductors, in which ions and not electrons carry substantial currents (touched on again in Chapter 11, Section 11.3.1.1). 

Diffusion now has its own specialised journal. Defect and Diffusion Forum, which published the successive comprehensive international conferences devoted to the parepisteme. 

In addition to the overarching role of the lUCr, there are numerous national crystallographic associations in various countries, some of them under the umbrella of bodies like the Institute of Physics in Britain. I doubt whether there is any other parepisteme so generously provided with professional assemblies all over the world. 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

This outline case-history is an excellent example of a parepisteme which began as a metallurgical curiosity and developed, at a leisurely pace, into a well-understood phenomenon, from which it became, at a much accelerated pace, an important industrial process. 

Parepistemes grow from an individual s curiosity, which in turn ignites curiosity in others if a piece of research is directly aimed at solving a specific practical problem, then it is part of mainline research and not a parepisteme at all. However, the improvement of a technique used for solving practical problems constitutes a parepisteme. 

A new parepisteme was under way its early stages were mapped in a classic text by McLean (1957), who worked in Rosenhain s old laboratory. Today, the atomic structure of interfaces, grain boundaries in particular, has become a virtual scientific industry a recent multiauthor book of 715 pages (Wolf and Yip 1992) surveys the present state, while an even more recent equally substantial book by two well-known authors provides a thorough account of all kinds of interfaces (Sutton and Balluffi 1995). In a paper published at about the same time, Balluffi... 

Many other parepistemes were stimulated by the new habits of precision in theory. Two important ones are the entropic theory of rubberlike elasticity in polymers, which again reached a degree of maturity in the middle of the century (Treloar 1951), and the calculation of phase diagrams (CALPHAD) on the basis of measurements of thermochemical quantities (heats of reaction, activity coefficients, etc.) here the first serious attempt, for the Ni-Cr Cu system, was done in the Netherlands by Meijering (1957). The early history of CALPHAD has recently been... 

The first detailed book to describe the practice and theory of stereology was assembled by two Americans, DeHoff and Rhines (1968) both these men were famous practitioners in their day. There has been a steady stream of books since then a fine, concise and very clear overview is that by Exner (1996). In the last few years, a specialised form of microstructural analysis, entirely dependent on computerised image analysis, has emerged - fractal analysis, a form of measurement of roughness in two or three dimensions. Most of the voluminous literature of fractals, initiated by a mathematician, Benoit Mandelbrot at IBM, is irrelevant to materials science, but there is a sub-parepisteme of fractal analysis which relates the fractal dimension to fracture toughness one example of this has been analysed, together with an explanation of the meaning of fractal dimension , by Cahn (1989). 

Recent texts have assembled impressive information about the production, characterisation and properties of semiconductor devices, including integrated circuits, using not only silicon but also the various compound semiconductors such as GaAs which there is no room to detail here. The reader is referred to excellent treatments by Bachmann (1995), Jackson (1996) and particularly by Mahajan and Sree Harsha (1999). In particular, the considerable complexities of epitaxial growth techniques - a major parepisteme in modern materials science - are set out in Chapter 6 of Bachmann s book and in Chapter 6 of that by Mahajan and Sree Harsha. 

The Lucalox story is a prime specimen of a valuable practical application of a parepistemic study begun for curiosity s sake. 

The comparatively new field of nanostructured materials has its own journals (though the first one has now been merged with another, broader journal) and frequent conferences it is a good example of a parepisteme which appears to be successful. The best single source of information about the many aspects of the field is a substantial multiauthor book edited by Edelstein and Cammarata (1996). 

As remarked above, surface science has come to be partitioned between chemists, physicists and materials scientists. Physicists have played a substantial role, and an excellent early overview of surface science from a physicist s perspective is by Tabor (1981). An example of a surface parepisteme that has been entirely driven by physicists is the study of the roughening transition. Above a critical temperature but... 

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