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Organisationseinheit der BAM
Topical issue: Geopolymers
(2017)
Geopolymers have gained very much momentum in recent years, in science and in industry. These highly promising and versatile materials have been researched with a view to applications as diverse as fire-resistant/refractory materials, as precursors for ceramics, in waste stabilization/immobilization, and as novel building materials. The great interest in these materials is reflected, inter alia, by the installment of a session on geopolymers at the 92nd Annual Meeting of the Deutsche Keramische Gesellschaft (DKG; German Ceramic Society) in March 2017. Therefore this Special Issue is dedicated to disseminating some of the recent findings on these materials. In addition, it contains reviews by highly experienced researchers in the field, highlighting very different aspects such as fiber reinforcement and synchrotron-based characterization methods.
Partly due to the interest from very different fields, the term „geopolymer“ is used with two different meanings. Sometimes it is used in the sense to denote the ceramic-like materials produced by activation of metakaolin with alkali silicate solutions, and closely related materials. On the other hand, sometimes it is used in a much broader sense to refer to any material that is produced by alkali-activation of many different aluminosilicate precursors, such as fly ash and granulated blast furnace slag waste materials, often serving as a cementitious binder. The present issue focusses on applications and materials that may be classified as ceramic, but also contains a review on the use of alkali-activated materials as construction materials to make the reader familiar with this approach too. Additionally, some of the materials described in this issue may be regarded as being „on the borderline“ between ceramics and cements, which is something that can be said about many studies in the geopolymer field.
The editors are very grateful to each of the authors of the articles in the present issue, as all of them have contributed high quality research based on their extensive experience. In particular, the historial perpectives on the origins and developments of geopolymers and of alkali activated materials have been authoritatively documented in contributions, by Joseph Davidovits and Pavel Krivenko. Dr. Krivenko was the Ph.D. student of Viktor D. Glukhovsky who first introduced alkali activated cements in 1957. It is hoped that the readers of this issue will enjoy reading it as much as the editors enjoyed putting it together, and that the issue will stimulate more high quality studies of geopolymers, perhaps even from research groups that have not yet been active in this area.
The purpose of "The Metallographist":
The results of the important researches regarding metallography are disseminated throughout the scientific papers and transactions of scientific societies chiefly of France, England, Germany and the United States, and even if the busy engineer or metallurgist, the metal producer or consumer in general, had the necessary knowledge of the three languages, and easy access to these publications, he could not possibly find time to go over such an amount of literature in order to extract from it what is of interest and importance to him. This is precisely the work which The Metallographist proposes to do for its readers by presenting to them every three months a clear, exhaustive and comprehensive review of what has been accomplished in metallography during the previous quarter.
The most important articles and memoirs will be reproduced in full, while those of minor importance will be condensed, accompanied in every case by such explanatory notes as will make them comprehensible and fruitful for others besides specialists. The Metallographist will publish moreover, besides the results of the investigations carried on in the Boston Testing Laboratories, original articles by eminent authorities and active workers who have kindly promised their collaboration. The editor will never lose sight of the fact that the importance of these investigations, to the world at large, lies chiefly in their ability to lead to deductions of industrial value, and his presentation of the subject will be conducted accordingly.
What is meant by ‘Micro Non-Destructive Testing and Evaluation’? This was the central subject of debate in this Special Issue.
At present, sub-millimeter-size components or even assemblies are pervading the industrial and scientific world. Classic examples are electronic devices and watches (as well as parts thereof), but recent examples encompass additively manufactured lattice structures, stents, or other microparts. Moreover, most assemblies contain micro-components. Testing such components or their miniaturized parts would fit well within the topic of micro non-destructive testing and evaluation.
In all cases, performance and integrity testing, quality control, and dimensional tolerances need to be measured at the sub-millimeter level (ideally with a spatial resolution of about a micron); most of the time, such features and components are embedded in much larger assemblies, which also need to be taken into account. The solution to this dilemma (i.e. measuring large parts with high resolution) depends on the part and on the problem under consideration.
Another possible definition of micro non-destructive testing and evaluation can relate to the characterization of micro-features (e.g., the microstructure) in much larger specimens, such as damage in concrete cores or porosity in additively manufactured components. A further aspect is the use of microscopic probes to evaluate macroscopic properties. This is the case, for instance but not at all exclusively, in the use of diffraction techniques to determine macroscopic stress.
The splits between testing and characterization at the micro-level (or of micro parts) from one side and handling of macroscopic assemblies on the other represent a great challenge for many fields of materials characterization. On top of that, including the use of microscopic methods to test integrity would add a further level of complexity.
Imaging, mechanical testing, non-destructive testing, measurement of properties, structural health monitoring, and dimensional metrology all need to be re-defined if we want to cope with the multi-faceted topic of micro non-destructive testing and evaluation.
The challenge has already been accepted by the scientific and engineering communities for a while but is still far from being universally tackled. This Special Issue yields an interesting answer to the questions posed above. It presents the progress made and the different aspects of the challenge as well as at indicates the paths for the future of NDT&E.