1,721,049 research outputs found
Hydrogen absorption processes in Mg(2)Ni-based systems: Thermal and mechanochemical kinetics
No full textMg2Ni/Ni and LaMg2Ni alloy powders were exposed to hydrogen under isothermal and
mechanical treatment conditions. In the former case, the amount of hydrogen absorbed
tends with time to a final asymptotic value. Once such a value has been reached, further
hydrogen absorption can be obtained only by submitting the powders to mechanical processing
in the presence of hydrogen. Hydrogen absorption processes under isothermal and
mechanical treatment conditions exhibited different kinetics and their rates have been
compared on a phenomenological basis starting from kinetic evidences. It appeared that
mechanical treatment enhances the rate of hydrogen absorption by four orders of
magnitude as a consequence of a mix of surface area enlargement, temperature rise and local structural excitation processes
Experimental and Theoretical Studies in Modern Mechanochemistry
No full textMechanochemistry is the branch of Chemistry focusing on the chemical
reactivity of solid substances submitted to the application of mechanical
forces under either elastic or plastic deformation conditions. A measure of
its ancestral roots is given by the many citations to mechanochemical
procedures in documents dating back to the Classical Roman Age. Quite
noticeably, even then such learning was referred to as remarkably ancient.
Other important tracks occasionally emerged during the Middle Age, when
the so-called via caloris of Alchemists dreaming the transmutation of matter
overwhelmed the slow comminution of powders in cold stone mortars. Yet,
Mechanochemistry only apparently was sunk in oblivion. The ancient
learning survived through the centuries, and was brought back to life by the
insatiable curiosity of little and great scientists.
After decades of continuous efforts, we are now aware that some of the
most important properties of materials in high-technology applications are
strongly influenced, or even controlled, by mechanochemical phenomena.
The interplay between mechanical stress and chemical reactivity lies for
example at the basis of the corrosion behavior as well as in metallic glasses
and ceramics where local atomic strain plays a critical role in structural
failure. At last, the entire field of Materials Science no longer ignores the
fascinating properties of mechanochemical reactions.
In contrast to their scientific and technological importance, our basic
understanding of even the simplest mechanochemical processes is at best
rudimentary. A deeper understanding and control of elementary processes in
Mechanochemistry is therefore needed. To call attention on the considerable
opportunities existing in this area of fundamental and applied research on
materials, we highlight in this book a number of past achievements and
promising recent developments in the field.
It is today widely accepted that only a highly interdisciplinary approach
holds the greatest promise to provide novel insight into the fundamental
features of physical, chemical and mechanical processes in the various
fields of Materials Science. The same is true for Mechanochemistry. As a
whole, the “mechanochemical” community is as diverse as virtually any
other in materials research, involving disciplines as disparate as
fundamental Physics and Chemistry on the one hand, and Engineering,
Physical Metallurgy and Physical Ceramics on the other. Yet, it is rare that
this diverse pool of expertise is ever brought together in a focused attack on
mechanochemical phenomena and properties. It is our hope that this volume
could give the reader a sense of the rich and fertile common ground which
already exists between the different disciplines, giving thus a contribution to
bring these very different communities closer together.
A central theme in much of the ongoing work on Mechanochemistry
concerns the interrelation between the chemical properties and the
underlying structure of materials. While for a number of scientific research
subjects many suitable experimental techniques for the characterization and
investigation of properties and processes have become available in recent
years, Mechanochemistry still poses a major challenge. This is mostly due
to the fact that in a general mechanochemical process only a small fraction
of atoms is directly affected. It is precisely this inhomogeneity that
undermines any effort to characterize in more detail the mechanochemical
process.
In organizing this book we soon realized that any attempt to cover a
wide field of research such as Mechanochemistry would necessarily
represent a highly selective endeavor. There are many ways of classifying
the contributions offered by the different authors that can be in principle
chosen. For example by distinguishing theoretical from experimental
studies, microscopic from macroscopic investigations, or fundamental from
applied research. The latter distinction appears to be particularly meaningful
for two reasons. First, because the methodologies required for the
investigations are fundamentally different. Second, because of the necessarily
different aims of the works. Nevertheless, a clean-cut separation between
fundamental and applied contributions is often difficult to find, thus the
chosen classification of contributions may not be the most satisfactory one.
Following the aforementioned distinction between fundamental and
applied research, the first part of the book is devoted to contributions
dealing with the fundamental features of Mechanochemistry, including
thermodynamic as well as kinetic and atomistic studies. By contrast with the
emphasis on fundamentals, the second part is instead dedicated to a variety
of applications of mechanochemical methods devoted to the preparation of
inorganic and organic materials and to the processing of minerals and
wastes. The two parts of the book are opened by two introductory chapters.
The former focuses on materials properties and chemical reactions under
well-controlled and well-defined loading conditions, so introducing most of
the fundamental concepts. The latter offers instead an overview of the
reactivity enhancement promoted by the mechanical treatment in various
apparatuses.
We have greatly benefited from discussions with our colleagues at the
University of Sassari and have received many useful suggestions from them
as well as from all the contributors to this book. We would like to
acknowledge the long-standing support we have received from all of our
collaborators and students participating in formation programs. A special
thank goes to Dr. Giuseppe Manai, for his continuous assistance and
suggestions. We are indebted to Dr. Pietro De Martini, Banco di Sardegna,
for his fundamental guide for fund raising. We are grateful to Prof. S.
Rubino for his kind support and to Dr. Giovanna Tuveri, who followed our
case at the Fondazione Banco di Sardegna.
This work has been made possible by the funds kindly placed at our
disposal by the Fondazione Banco di Sardegna. We wish to express here all
of our deep gratitude to the Board of Directors. The University of Cagliari
and the University of Sassari are as well acknowledged for the opportunity
we had to use laboratories and facilities necessary to our study
The mechanochemical conversion of acetone to methyl isobutyl ketone over Cu-Mg based substrates
No full textSelected results concerning the acetone conversion to methyl isobutyl ketone promoted by mechanical energy are presented. Cu-MgO composites at different Cu contents and amorphous Cu40Mg60, prepared by different ball milling techniques, were employed as solid substrates for the mechanochemical process. The conversion and selectivity values of the acetone transformation were followed under Ar and H-2 reaction atmosphere and as a function of the impact energy. The mechanical treatment under H-2 resulted mainly in acetone hydrogenation leading to 2-propanol, while under Ar, the formation of diacetone alcohol, mesityl oxide and methyl isobutyl ketone (MIBK) was observed. Conversion changes and selectivity shifts were observed depending on the substrates employed, and the largest amount of MIBK; was obtained when amorphous Cu40Mg60 was used. An important structural transformation occurred during the mechanochemical runs. (C) 1999 Elsevier Science S.A. All rights reserved
Estimation of Mechanochemical Effects in Heterogeneous Processes
No full textThe present work focuses on the comparison of mechanochemical and thermal processes on a kinetic basis, with the aim of pointing out the higher efficiency of mechanically activated reactions in solid-solid, solid-liquid and solid-gas systems. The comparison relies upon the correlation between processing parameters, reaction kinetics and local deformation events. The obtained results are ascribed to the coupling of mechanical deformation with thermodynamic driving force
MECHANICAL ALLOYING PROCESSES AND REACTIVE MILLING
No full textThe great flexibility of mechanically driven processes in designing functional materials has stimulated wide attention in the different side branches and research fields. Our current interest concerns both the inherent chemical properties of amorphous metal powders-in view of a possible exploitation in heterogeneous catalysis-as well as their chemical behaviors under reactive milling. Chemical effects were found to be responsible for changing the course of Mechanical Alloying processes (MA). Furthermore, deep structural characterizations of the powders under milling have revealed the subtle and sometimes unavoidable influence of gaseous contaminants from the reaction environment. These results were obtained from the use of complementary diffraction techniques and from the neutron diffraction analysis in particular. It is only natural to refer to these chemical aspects in terms of reactive milling and Mechanochemistry. Within this framework we focused on the active role of hydrogen, as a gaseous reagent or released in activated form from metal hydride lattices, to steer the end-products of MA and Mechanical Milling (MM) processes. On this basis, the possibility to run gaseous hydrogenation reactions under milling was also proven. For a better understanding of the underlying mechanism, MA and Mechanochemistry both require the proper definition of the main milling parameters. To this extent, an experimental method is briefly outlined for the direct evaluation of the collision frequency and energetical factors in ball milling processes
Composto di ossido ternario a base di Magnesio e Niobio, mesoporoso, altamente ordinato, procedimento per la sua preparazione e suoi usi.
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