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Nanoporous Materials Advanced Techniques for

Characterization, Modeling, and Processing

© 2011 by Taylor and Francis Group, LLC

Nanoporous Materials Advanced Techniques for

Characterization, Modeling, and Processing Edited by Nick Kanellopoulos

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

© 2011 by Taylor and Francis Group, LLC

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2011 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number: 978-1-4398-1104-7 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

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Contents Preface..................................................................................................................... vii Editor........................................................................................................................ix Contributors.............................................................................................................xi

Part Iâ•…Basic Principles, Recent Advances, and Expected Developments of Advanced Characterization Techniques 1 Scattering Techniques.................................................................................... 3 Gérald Lelong, David L. Price, and Marie-Louise Saboungi 2 Studying Diffusion and Mass Transfer at the Microscale.................... 53 Christian Chmelik, Douglas M. Ruthven, and Jörg Kärger 3 Nanoscale Microscopies............................................................................... 95 Anthony A.G. Tomlinson 4 Calorimetric Techniques............................................................................ 129 Philip L. Llewellyn 5 Combination of In Situ and Ex Situ Techniques for Monitoring and Controlling the Evolution of Nanostructure of Nanoporous Materials........................................................................................................ 165 G.N. Karanikolos, F.K. Katsaros, G.E. Romanos, K.L. Stefanopoulos, and N.K. Kanellopoulos

Part IIâ•…Fundamentals, Recent Advances, and Expected Developments of Simulation Methods 6 Mesoscopic Methods................................................................................... 223 P.M. Adler and J.-F. Thovert 7 Characterization of Macroscopically Nonhom*ogeneous Porous Media through Transient Gas Sorption or Permeation Measurements.............................................................................................. 253 J.H. Petropoulos and K.G. Papadokostaki v © 2011 by Taylor and Francis Group, LLC

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Contents

Part IIIâ•…Fundamentals, Recent Advances and Improvements, Membrane, Catalytic and Novel Processes Involving Nanoporous Materials 8 Synthesis Processes of Nanoporous Solids............................................ 283 P. Cool, V. Meynen, and E.F. Vansant 9 Sorption Processes...................................................................................... 307 F. Rodríguez-Reinoso, A. Sepúlveda-Escribano, and J. Silvestre-Albero 10 Fundamental Approach to Supercritical Hydrogen Adsorptivity of Nanoporous Carbons............................................................................. 353 Shigenori Utsumi and Katsumi Kaneko 11 Membrane Processes................................................................................... 387 A.F.P. Ferreira, M.C. Campo, A.M. Mendes, and F. Kapteijn 12 Diffusional Transport in Functional Materials: Zeolite, MOF, and Perovskite Gas Separation Membranes, Proton Exchange Membrane Fuel Cells, Dye-Sensitized Solar Cells.............................. 423 J. Caro 13 Zeolites and Mesoporous Aluminosilicates as Solid Acid Catalysts: Fundamentals and Future Challenges................................. 449 Ana Primo, Avelino Corma, and Hermenegildo García

Part IVâ•…C ase Studies of Applications of Advanced Techniques in Involving Nanoporous Materials 14 Recent Developments in Gas-to-Liquid Conversion and Opportunities for Advanced Nanoporous Materials........................... 481 Gabriele Centi and Siglinda Perathoner 15 Advanced Materials for Hydrogen Storage............................................ 513 Th.A. Steriotis, G.C. Charalambopoulou, and A.K. Stubos Index...................................................................................................................... 549

© 2011 by Taylor and Francis Group, LLC

Preface The advancement of materials science, nanotechnology, and biosciences depends on the effective use of advanced characterization and modeling techniques. During the last 30 years, there has been a tremendous improvement in the field of porous materials, with the development of increasing numbers of novel materials. During the same period, there have also been large numbers of significant breakthroughs in the development of advanced characterization and simulation techniques and their combinations. Since all the recent developments are largely scattered in a number of journals and conference proceedings, I believe that the concise information provided in this book covering diverse subjects will be a very useful reference for all scientists involved in the field of porous materials. This book aims to provide academic and industrial researchers of different disciplines and backgrounds with a concise yet comprehensive presentation of the state-ofthe-art, recent developments, and expected improvements of advanced characterization and simulation techniques and their applications to optimize processes involving sorbents, membranes, and catalysts. Nanoporous materials play an important role in chemical processing as, in many cases, they can successfully replace traditional, pollution-prone, and energy-consuming separation processes. These materials are widely used as sorbents, catalysts, catalyst supports, and membranes, and form the basis of innovative technologies, including high-temperature molecular sieve membrane separations and low-temperature reverse sorption membrane separations (hydrogen production, carbon dioxide capture and conversion, alkane/ alkene separation, methane conversion, hydrogen storage, FCC catalysis, etc.). This is mainly due to their unique structural or surface physicochemical properties, which can, to an extent, be tailored to meet specific processrelated requirements. Any equilibrium or dynamic process taking place within the nanopores of a solid is strongly influenced by the topology and the geometrical disorder of the pore matrix. The complete characterization of nanoporous materials still remains a difficult and frequently controversial problem, even if the equilibrium and transport mechanisms themselves are quite simple and well defined. This is mainly due to the great difficulty in accurately representing the complex morphology of the pore matrix. To this end, the application of combined techniques aided by advanced model analysis is of major importance as it is the most powerful method currently followed. On the other hand, no matter how thorough and complete the characterization, it is quite pointless if it is not related to the process under consideration, since one of the most important parameters in any application is the material’s ability to retain its properties over a certain period of time. The “changes” induced on materials during their utilization in specific vii © 2011 by Taylor and Francis Group, LLC

viii

Preface

applications are highly relevant and crucial for the economic viability of many applications (e.g., catalysis and separation processes). In this context, it is necessary to develop skills in establishing advanced combinations of “in situ” and “ex situ” techniques in order to expand our understanding of confinement phenomena in nanopores, to monitor and control the evolution of the properties of nanostructured materials, and to evaluate and optimize the performance of nanoporous sorbents, membranes, and catalysts involved in several important industrial processes. The book is organized as follows: Part I presents the basic principles and major applications of the most important characterization techniques, ranging from diffraction and spectroscopy to calorimetry, permeability, and other techniques. Part II presents computer simulation techniques, an indispensable complement to the combination of the aforementioned analytical Â�techniques. Part III covers the fundamentals and the recent advances in sorption, membrane, and catalyst processes, while Part IV presents two characteristic “case” studies of emerging areas of application of porous solids in the fields of gas-to-liquid conversion and hydrogen storage. This book is based on the experience gained from the workshops organized by the network of excellence INSIDE-PORES and is mainly the result of the workshop on NAnoPorous Materials for ENvironmental and ENergy Applications (NAPEN 2008), which was organized in Crete by three cooperating European networks of excellence, namely, the Networks of Excellence IDECAT on catalysis, the NANOMEMBRO on membranes, and the INSIDEPORES. I would like to thank Professors Gabriele Centi and Gilbert Rios, the coordinators of the cooperating networks of Excellence IDECAT and NANOMEMBRO. I would also thank my colleagues and students from Demokritos, A. Sapalides, G. Romanos, A. Labropoulos, S. Papageorgiou, V. Favvas, N. Kakizis, G. Pilatos, and E. Chatzidaki, for helping with the organization of NAPEN 2008. One of the major characteristics of this book is the impressive list of internationally well-known contributors. I would like to thank each one of them for their invaluable contributions. Special thanks are due to Jill Jurgensen and Allison Shatkin and their colleagues from Taylor & Francis Group for their help and patience. Last but not least, I would like to thank the European Commission for funding the three networks of excellence. I would also like to thank Dr. Soren Bowadt, who was in charge of these networks, for his valuable assistance and patience. Nick Kanellopoulos

© 2011 by Taylor and Francis Group, LLC

Editor Nick Kanellopoulos received his PhD from the Department of Chemical Engineering, University of Rochester, Rochester, New York, in 1975, and his diploma in chemical engineering from the National Technical University of Athens, Athens, Greece in 1970. He joined the Mass Transport Laboratory, Institute of Physical Chemistry, National Centre for Scientific Research Demokritos, Attiki, Greece, in 1976, and, since 1992, he has been the head of the “Membranes for Environmental Separations” Laboratory, NCSR Demokritos. His research interests include pore structure characterization, nanoporous membrane, and carbon nanotube systems, and the evaluation of their performance using a combination of in situ and ex situ techniques. Dr. Kanellopoulos is the author and coauthor of more than 140 papers; he is also the editor of Recent Advances in Gas Separation by Microporous Membranes (Elsevier Science) and the coeditor of Nanoporous Materials for Energy and Environment (Stanford Chong). He has received approximately 12 million euros in funding from over 50 European and national programs and has participated in three high technology companies in the field of nanoporous materials. He participated in the National Representation Committee of Greece for the FP6-NMP and FP7-NMP European programs in nanotechnology from 2001 to 2009. He is the coordinator of the European Network of Excellence in nanotechnology inside-pores.gr, a member of the Committee of the Kurchatov-Demokritos Project Center for Nanotechnology and Advanced Engineering, and the coordinator of the committee for the preparation and submission of the proposal for a Greek national nanotechnology program. He is also a Fulbright scholar and president of the Greek Fulbright Scholars Association.

ix © 2011 by Taylor and Francis Group, LLC

Contributors P.M. Adler Structure et Fonctionnement des Systèmes Hydriques Continentaux Université Pierre et Marie Curie Paris, France M.C. Campo Faculty of Engineering University of Porto Porto, Portugal J. Caro Institute of Physical Chemistry and Electrochemistry Leibniz University of Hannover Hannover, Germany Gabriele Centi Dipartimento di Chimica Industriale ed Ingegneria dei Materiali and Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materialis Laboratorio di Catalisi per una Produzione Sostenibile e L’energia Università di Messina Messina, Italy G.C. Charalambopoulou Institute of Nuclear Technology and Radiation Protection National Center for Scientific Research “Demokritos” Athens, Greece

Christian Chmelik Faculty of Physics and Geosciences University of Leipzig Leipzig, Germany P. Cool Laboratory of Adsorption and Catalysis Department of Chemistry University of Antwerpen Wilrijk, Belgium Avelino Corma Instituto Universitario de Tecnología Química Universidad Politécnica de Valencia Valencia, Spain A.F.P. Ferreira Faculty of Engineering University of Porto Porto, Portugal Hermenegildo García Instituto Universitario de Tecnología Química Universidad Politécnica de Valencia Valencia, Spain Katsumi Kaneko Department of Chemistry Graduate School of Science Chiba University Chiba, Japan xi

© 2011 by Taylor and Francis Group, LLC

xii

N.K. Kanellopoulos Institute of Physical Chemistry National Centre for Scientific Research “Demokritos” Athens, Greece F. Kapteijn Catalysis Engineering Delft University of Technology Delft, the Netherlands G.N. Karanikolos Institute of Physical Chemistry National Centre for Scientific Research “Demokritos” Athens, Greece Jörg Kärger Faculty of Physics and Geosciences University of Leipzig Leipzig, Germany F.K. Katsaros Institute of Physical Chemistry National Centre for Scientific Research “Demokritos” Athens, Greece Gérald Lelong Institute de Minéralogie et Physique des Milieux Condensés Université Paris 6 and Centre National de la Recherche Scientifique Université Paris 7 and Institut de Physique du Globe de Paris Institut de Recherche pour le Développement Paris, France © 2011 by Taylor and Francis Group, LLC

Contributors

Philip L. Llewellyn Laboratoire Chimie Provence CNRS-Université de Provence Marseille, France A.M. Mendes Faculty of Engineering University of Porto Porto, Portugal V. Meynen Laboratory of Adsorption and Catalysis Department of Chemistry University of Antwerpen Wilrijk, Belgium K.G. Papadokostaki Institute of Physical Chemistry National Center for Scientific Research “Demokritos” Athens, Greece Siglinda Perathoner Dipartimento di Chimica Industriale ed Ingegneria dei Materiali and Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materialis Laboratorio di Catalisi per una Produzione Sostenibile e L’energia Università di Messina Messina, Italy J.H. Petropoulos Institute of Physical Chemistry National Center for Scientific Research “Demokritos” Athens, Greece

xiii

Contributors

David L. Price Conditions Extrêmes et Matériaux: Haute Température et Irradiation Centre National de la Recherche Scientifique Université d’Orléans Orléans, France Ana Primo Instituto Universitario de Tecnología Química Universidad Politécnica de Valencia Valencia, Spain F. Rodríguez-Reinoso Laboratorio de Materiales Avanzados Departamento de Química Inorgánica Instituto Universitario de Materiales de Alicante Universidad de Alicante Alicante, Spain G.E. Romanos Institute of Physical Chemistry National Centre for Scientific Research “Demokritos” Athens, Greece Douglas M. Ruthven Department of Chemical Engineering University of Maine Orono, Maine Marie-Louise Saboungi Centre de Recherche sur la Matière Divisée Centre National de la Recherche Scientifique Université d’Orléans Orléans, France © 2011 by Taylor and Francis Group, LLC

A. Sepúlveda-Escribano Laboratorio de Materiales Avanzados Departamento de Química Inorgánica Instituto Universitario de Materiales de Alicante Universidad de Alicante Alicante, Spain J. Silvestre-Albero Laboratorio de Materiales Avanzados Departamento de Química Inorgánica Instituto Universitario de Materiales de Alicante Universidad de Alicante Alicante, Spain K.L. Stefanopoulos Institute of Physical Chemistry National Centre for Scientific Research “Demokritos” Athens, Greece Th.A. Steriotis Institute of Physical Chemistry National Center for Scientific Research “Demokritos” Athens, Greece A.K. Stubos Institute of Nuclear Technology and Radiation Protection National Center for Scientific Research “Demokritos” Athens, Greece J.-F. Thovert Institut PPRIME SP2MI Futuroscope, France

xiv

Anthony A.G. Tomlinson Consiglio Nazionale delle Ricerche Istituto per lo Studio dei Materiali Nanostrutturati Rome, Italy Shigenori Utsumi Department of Mechanical Systems Engineering Tokyo University of Science Chino, Japan

© 2011 by Taylor and Francis Group, LLC

Contributors

E.F. Vansant Laboratory of Adsorption and Catalysis Department of Chemistry University of Antwerpen Wilrijk, Belgium

Part I

Basic Principles, Recent Advances, and Expected Developments of Advanced Characterization Techniques

© 2011 by Taylor and Francis Group, LLC

1 Scattering Techniques Gérald Lelong, David L. Price, and Marie-Louise Saboungi Contents 1.1 Introduction.....................................................................................................4 1.2 Diffraction........................................................................................................5 1.2.1 Diffraction Formalism........................................................................ 6 1.2.2 Differences between Neutron and X-Ray Scattering................... 13 1.2.3 Selected Example.............................................................................. 14 1.3 Small-Angle Scattering................................................................................ 17 1.3.1 SAS Spectrometer............................................................................. 17 1.3.2 Small Angle Scattering vs. Diffraction.......................................... 19 1.3.3 What Does Q Probe?......................................................................... 19 1.3.4 Small-Angle Scattering Formalism................................................ 20 1.3.5 SAS from a Two-Phase System........................................................ 21 1.3.5.1 Guinier Approximation: Radius of Gyration................. 24 1.3.5.2 Porod Scattering................................................................. 25 1.3.5.3 Porod Invariant................................................................... 25 1.3.6 Form Factors...................................................................................... 26 1.3.7 Multicomponent Systems: Neutron Contrast Matching............. 28 1.3.8 Interacting Scatterers: Structure Factor.........................................30 1.3.9 SANS vs. SAXS.................................................................................. 33 1.3.10 Selected Examples............................................................................ 33 1.3.10.1 Porous Materials................................................................. 33 1.3.10.2 Soft Matter........................................................................... 35 1.3.10.3 New Developments............................................................ 36 1.4 X-Ray Absorption Spectroscopy................................................................. 37 1.5 Inelastic Scattering........................................................................................ 38 1.5.1 Selected Examples............................................................................44 References................................................................................................................ 48

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Nanoporous Materials

1.1╇ Introduction Neutrons produced by reactors and spallation sources, and x-rays produced by synchrotron sources, have wavelengths in the range of 0.1–1â•›n m, making the scattering experiments a powerful and versatile probe of nanoporous materials. In fact it is hard to find a comprehensive paper on some aspect of confinement on the nm scale that does not include references to scattering measurements. A case in point is the excellent topical review of Alcoutlabi and McKenna1 dealing with the effects of size and confinement on the melting temperature Tm (always depressed) and the glass transition temperature Tg (may increase, decrease, remain the same, or even disappear). Among other scattering results they refer to the work of Morineau et al., 2 who measured the density of confined liquid toluene through changes in the Bragg peak intensity in neutron diffraction measurements resulting from the change in contrast with the confined liquid. Whereas little or no changes were observed for confinement in mesoporous silicates with pore sizes of 3.5â•›n m and above, a decrease in density and an increase of 30â•›K in Tg were observed upon confinement in 2.4â•›n m pores. Alcoutlabi and McKenna also refer to the inelastic neutron scattering (INS) studies of Zorn et al.3 who observed a decrease in Tg on the confinement of salol in microporous silica glass together with a broadening of the relaxation spectra. These effects were discussed in terms of a cooperativity length scale that, since it cannot become larger than the confining dimensions, leads to an acceleration of the molecular dynamics compared with the bulk. In this chapter, we summarize the techniques of diffraction—essentially, the study of correlations in atomic arrangements on the scale of 0.1–1â•›nm, sometimes referred to as wide-angle neutron or x-ray scattering (WANS, WAXS), small-angle scattering (SAS) that measures density fluctuations on the length scale of 5–500â•›nm, and inelastic scattering that probes dynamical phenomena on the timescale of 0.1â•›ps to 0.1â•›ms. We include a brief description of x-ray absorption spectroscopy, often used in conjunction with scattering experiments: for example, extended x-ray absorption fine-structure spectroscopy (EXAFS) provides an element-specific structural probe through the scattering of the emitted photoelectron by neighboring atoms. Figure 1.1 compares the length and timescales probed by different scattering techniques with optical, dielectric, and nuclear magnetic resonance (NMR) spectroscopies. To flesh out these bare bones, we provide some examples of the application of these techniques from our own work, including selenium absorbed in zeolites, mesoporous silica nanopores, hydrogen adsorbed on carbon nanohorns, and glucose solutions confined in aqueous silica gels.

© 2011 by Taylor and Francis Group, LLC

5

Scattering Techniques

l (Å) 100

10

1 105

10–13 10

104

–12

103

TOF

10–11

102

10–10

Dielectric

NMR

10–7 10–6 10–5 10

–4

10

–3

Light scattering

τ(s)

10

–8

101

BS

10–9

100

NSE

10–1 10–2 10–3 10–4 10–5

SAS

Diffraction

10–6

10–2

Q~0

δE (µeV)

10

1000

–14

10–7 0.01

0.1

Q (Å–1)

1

10

Figure 1.1 Schematic representation of accessible length and time scales using light or neutron scattering techniques (time-of-flight (ToF), backscattering (BS), neutron spin echo (NSE), and SAS spectrometers) and spectroscopic methods (nuclear magnetic resonance [NMR] and dielectric spectroscopy).

1.2╇ Diffraction Diffraction is generally taken to mean the measurement of atomic or magnetic structure by scattering experiments. In principle, any particle can be used, but for most investigations of atomic structure neutrons, x-rays, and electrons are most common while neutrons and, in certain cases, x-rays can be also used to provide information about magnetic structure. The neutron is a subatomic particle with, as its name implies, zero charge, mass mnâ•›=â•›1.0087 atomic mass units, spin Iâ•›=â•›½ and magnetic moment μnâ•›=â•›−1.9132 nuclear magnetons. These properties combine to make the neutron a highly effective probe of condensed matter. The zero charge means that its interactions with a sample of a condensed material are confined to the short-ranged nuclear and normally weak magnetic interactions, so that the neutron can usually penetrate into the bulk of the sample.

© 2011 by Taylor and Francis Group, LLC

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Nanoporous Materials

Thermal neutrons for condensed matter research are usually obtained by slowing down energetic neutrons, produced by a nuclear reaction in either a fission reactor or an accelerator-driven spallation source, by means of inelastic collisions in a moderating material consisting of light atoms. Most of the slow neutrons thus produced will have kinetic energies on the order of kBT where T is the moderator temperature. Considering the wave nature of the neutron, its wavelength is given by

2 = k BT . 2mn λ 2

(1.1)

The neutron mass is such that for Tâ•›=â•›300â•›K, λâ•›∼â•›2 Å, a distance comparable to the mean atomic separation in a solid or liquid. Such neutrons are therefore ideally suited to studies of the atomic structure of condensed matter, discussed below. Furthermore, the kinetic energy of such a neutron is on the order of 25â•›meV, a typical energy for excitations in solids and liquids. Thus, both wavelength and energy are ideally suited to studies of the atomic dynamics of condensed matter in inelastic scattering experiments, discussed in Section 1.5. The magnetic moment of the neutron makes it a unique probe of magnetic structure and excitations: neutrons are scattered from the magnetic moments associated with unpaired electron spins in magnetic materials. Again, the wavelength and energy of a thermal neutron are such that both the magnetic structure and the dynamics of the spin system can be studied in the neutron scattering experiment. The x-ray is a photon with an energy conventionally taken in the range of keV. It has zero charge, zero magnetic moment, and spin Iâ•›=â•›1. An x-ray of energy Eâ•›=â•›hνâ•›=â•›hc/λâ•›=â•›12.398â•›keV has wavelength λâ•›=â•›1â•›Å, making it also, as is well known, a powerful probe of the structure of condensed matter. The electromagnetic field associated with a moving x-ray makes it, under appropriate circ*mstances, another probe of magnetic structure. To probe excitations in condensed matter, which typically have energies in the meV range, an energy resolution on the order of 10−7 is required in both incident and scattered beams, a formidable challenge that has recently been met in thirdgeneration synchrotron sources. 1.2.1╇ Diffraction Formalism We consider a simple scattering experiment shown schematically in Figure 1.2. We suppose that a beam of particles (neutrons, x-rays, or electrons) characterized by a wave vector k i⃗ falls on the sample. The magnitude of k i⃗ is 2π/λ and its direction corresponds to that of the beam. Usually the sample size is chosen such that most of the beam is transmitted: typically it is ∼mm with neutrons, and with x-rays it varies from ∼mm for light atoms © 2011 by Taylor and Francis Group, LLC

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Scattering Techniques

ki

2θ kf

dΩ ki

Scattering triangle: kf

Q = ki – kf

Figure 1.2 Geometry of a typical diffraction experiment.

to ∼μm for heavy atoms. Some particles are, however, scattered and can be measured with a detector placed, for example, in a direction k f⃗ â•›. If the incident beam is characterized by a flux Φ (particles crossing unit area per unit time), the sample has N identical atoms in the beam, and the detector subtends solid angle ΔΩ and has efficiency η, we may expect the count rate in the detector to be proportional (if ΔΩ is small enough) to all these quantities. In this case, the constant of proportionality is called the differential cross section and is derived as4

dσ C = . dΩ ηΦN(∆Ω)

(1.2)

The structural information obtained in a diffraction experiment is normally described by the variation of the intensity of the scattering with the scatÂ� tering vector Q⃗ :

Q = ki − k f

(1.3)

illustrated by the triangle in Figure 1.2. In the case of experiments on samples that are directionally isotropic—polycrystalline solids, glasses, and Â�liquids—the scattering depends only on the magnitude of the scattering Â�vector, the scalar quantity Qâ•›=â•›‖Q⃗ ‖. It is usual to fix the directions of k i⃗ and k f⃗ by means of appropriate collimators, detector placement, etc. and to fix the magnitude of one of these, generally ki, or sometimes a combination of ki and kfâ•›, for example one that corresponds to the total time-of-flight from sample to detector in the case of neutron scattering. The total intensity of the scattered particles measured in the detector is normally recorded, irrespective of any energy transfer that may take place, and Q⃗  is evaluated from Equation 1.3 under the assumption that the scattering is elastic, i.e., there is no energy exchange between the particle and the sample and so |k i⃗ |=|k f⃗ |. In the neutron case, significant © 2011 by Taylor and Francis Group, LLC

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Nanoporous Materials

inelastic scattering is always present and this can affect the structural interpretation. However, the experiments are usually designed to minimize the errors that result from these approximations, which can usually be taken care of by straightforward corrections. For elastic scattering, Q depends only on ki and the scattering angle 2θ, corresponding to the Bragg relation Qâ•›=â•›4π sin θ/λ. The nuclear interaction between a slow neutron and an atom can be expressed in a simple form. In the simplest case where the atoms in the sample are both noninteracting and identical, the differential cross section is just a constant: dσ = b2 , dΩ

(1.4)

where the scattering length b is normally a constant, depending on the atomic number Z and the atomic weight A of the nucleus, and its spin state relative to that of the neutron. Its magnitude depends on the details of the interaction between the neutron and the components of the nucleus. For this reason both sign and magnitude of b change in an irregular fashion with Z and A. In the x-ray case, the photon interacts with the electrons in the atom, and since these are distributed in space, the scattering factor is proportional to the total number of electrons and a form factor that represents the Fourier transform of their radial distribution. The differential cross section is then a function of the scattering vector Q: dσ = f 2 (Q). dΩ

(1.5)

In contrast with the neutron case, the x-ray scattering, which increases monotonically with Z, is independent of isotope and decreases with Q. The scattering length for neutrons and scattering factor for x-rays for two values of θ, and hence of Q, are shown in Figure 1.3. For convenience, comparison between experiment and theory is usually done in terms of a dimensionless quantity, the structure factor S(Q). In the case of neutron diffraction, this is related to the differential cross section by the relation:

dσ = dΩ

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2

n

∑ a =1

c a ba

n

(S(Q) − 1) + ∑ ca ba2 , a =1

(1.6)

9

Scattering Techniques

X-rays (sin θ)/λ = 0.5 Å–1

X-rays θ=0 Scattering amplitude 10–12 cm

5 4 3 2

–1

1H 7Li

40

20

Potential scattering contribution

Neutrons

Sc 56Fe

Cl

1 0

50Ni

V 60 Ti Mn 62Ni

80

100 Atomic weight

Figure 1.3 Scattering lengths for neutrons and scattering factor for x-rays for two values of scattering angle 2θ, as a function of atomic weight A.

where c_a is the atomic concentration ba is the average (over isotopes and spin states) of the neutron–nucleus scattering length ba2 is the mean square scattering length of element a present in the sample. This can be rewritten as follows:

dσ = dΩ

2

n

∑ a =1

c a ba

 n 2 S(Q) +  c a ba −   a =1

n

∑ a =1

  n 2 c a ba  +  ca ba2 − ba     a =1 2

(

) , 

(1.7)

where the leading term contains the structural information that is being sought here. The second term arises from random distributions of different elements (often referred to as Laue diffuse scattering) and the third term from random distributions of isotopes and spin states over the atoms belonging to a given element (generally called incoherent scattering). It is convenient to define the coherent and incoherent cross sections of element a:

( )

σ coh = 4π ba a

2

( )

2 .  2 and σ inc a = 4 π ba − ba  

(1.8)

It can be seen that the coherent cross sections enter into the first two terms of Equation 1.7 and the incoherent into the third. In the x-ray case, every atom of a given element scatters identically so the incoherent term does

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Nanoporous Materials

Table 1.1 Neutron Scattering Lengths b in Femtometers (1 fmâ•›=â•›10−15 m) and the Respective Coherent σcoh and Incoherent σinc Scattering Cross Sections in Barns (1 barnâ•›=â•›10−24 cm2) for Some Elements of the Periodic Table Element

b (fm)

σcoh (Barn)

σinc (Barn)

H H 6Li 7Li 12C 14N 16O 19F 23Na 27Al 28Si 31P 32S 70Ge

−3.742 6.674 2.000 −2.222 6.653 9.372 5.805 5.654 3.632 3.449 4.106 5.131 2.804 10.010

1.758 5.593 0.515 0.619 5.559 11.035 4.232 4.017 1.662 1.495 2.120 3.307 0.988 12.63

80.276 2.053 0.465 0.783 0 0.501 0 0.001 1.623 0.008 0 0.005 0 0

1 2

Source: Price, D.L. and Sköld, K., Introduction to neutron scattering, in Neutron Scattering, Methods of Experimental Physics, eds. D.L. Price and K. Sköld, Vol. 23 (Part A), p. 1, Academic Press, New York, 1986.

not appear. Table 1.1 gives the values of the neutron scattering lengths and the coherent and incoherent cross sections for a selection of elements and isotopes. By definition, S(Q) tends to unity at large Q, a property that is often used to normalize the intensities measured in a diffraction experiment. Its low-Q limit is related to the macroscopic compressibility χT: S(0) =

ρ0 V

 ∂V    k BT = ρ0 χT kBT , ∂P  T

(1.9)

where ρ0 is the number of atoms per unit volume. In between S(Q) exhibits a complex behavior that reflects the detailed atomic structure. In a crystalline sample—either single crystal or polycrystalline—there are sharp peaks called Bragg peaks that arise from diffraction from parallel crystallographic planes at Q values corresponding to 2πn/d, where n is an integer and d the plane spacing. There is also a continuous component, called diffuse scattering, arising from static and/or dynamic disorder. In © 2011 by Taylor and Francis Group, LLC

11

Scattering Techniques

Qmax ≈ 2π/σHS

S(Q)

3 2 S(Q ∞) = 1 1 S (Q = 0) = χρkT

1

2 3 Q (Å–1)

4

5

Figure 1.4 Typical form for S(Q) in a simple classical liquid or glass. (From Price, D.L., Experimental techniques, in High-Temperature Levitated Materials, Cambridge University Press, Cambridge, U.K., p. 45, 2010. With permission.)

fully disordered materials like liquids and glasses, the entire scattering is diffuse, with a generally oscillatory pattern that reflects the short- and intermediate-range order in the sample. A well-defined distance of closest approach between atoms that can be characterized by an equivalent hard-sphere diameter σHS will be reflected in oscillations in S(Q) with a period 2π/σHS. A typical form for S(Q) in a simple classical liquid is shown in Figure 1.4. In the case of x-ray diffraction, ba in the above equations and those that follow is replaced by fa(Q), the atomic form factor for species a. This results from the fact that the electrons in the atom from which the x-rays are scattered have a spatial distribution, while the nucleus from which the neutrons are scattered can be treated, for the present purposes, as a point object. Since the form factors are generally well tabulated this is not a major problem, but it can complicate the interpretation of the scattering from multicomponent systems. A pair correlation function g(r) that contains the structural information about the sample in real space is then calculated from S(Q) via the Fourier transform

1 g( r ) = 1 + 2 2π ρ0

Qmax

∫ Q(S(Q) − 1) 0

sin Qr M(Q) dQ, r

(1.10)

where M(Q) is a modification function that is often used to force the integrand to go smoothly to zero at Qmax and reduce the ripples that result from the finite limit of the integration. For systems with more than one type of atom—different elements, and sometimes different isotopes of the same element—S(Q) is a weighted sum of partial structure factors Sab(Q). Unfortunately, there are a number of alternative definitions of these in the literature: the S(Q) appearing in Equations 1.6 through 1.9 is called the Faber–Ziman definition after © 2011 by Taylor and Francis Group, LLC

12

Nanoporous Materials

its originators.5 Another definition of partial structure factors for binary �systems was �proposed by Bhatia and Thornton,6 where SNN(Q) describes the fluctuations in total particle density, SCC(Q) those in the relative concentrations, and SNC(Q) the cross-correlation of the two. For a two-component system Equation 1.9 applies to SNN(Q). The various definitions are linear combinations of each other and are given in the textbooks, for example, that of March and Tosi.7 For a multicomponent system, g(r) is correspondingly a weighted sum of partial pair correlation functions gab(r), which in the neutron case is given by g( r ) =

∑W g

ab

ab

(r ) =

a ,b

∑ a ,b

cacbbabb

c a ba

2

g ab (r ),

(1.11)

where a and b are the atom types Wab are weighting factors The partial pair distribution function gab(r) can be considered as the relative probability of finding a b atom at a distance r from an a atom at the origin. In a one-component system, the indices a, b disappear and only a single S(Q) and a single g(r) exist. In a system with n components, a full structural analysis requires n(n+1)/2 different measurements with different coefficients in Equation 1.11: in favorable cases, this may be accomplished with the use of isotope substitution in the case of neutron diffraction,8 by anomalous x-ray scattering (AXS) near an absorption edge, where the form factor has an additional component that varies rapidly with x-ray energy,9,10 or by a combination of the neutron and x-ray scattering.11 With a single measurement, only the average structure factor S(Q) can be determined; nevertheless, this may still contain useful information. For example, if a particular peak n in g(r) can be associated uniquely with a coordination shell for a pair of atom types a,b, the coordination number of b atoms about an average a atom for that shell is given by Can (b) =

cb Wab

∫ rT(r) dr,

(1.12)

n

where T(r)â•›=â•›4πρ0rg(r) and the integral is taken over the peak n, while the centroid of T(r) over the same peak gives the average coordination distance rabn . In a magnetic system, neutrons can also scatter from the magnetic moments associated with the unpaired electrons. In simple cases where the unpaired © 2011 by Taylor and Francis Group, LLC

13

Scattering Techniques

electrons can be associated with a particular atom, the magnetic scattering can be described by making the substitution 1 ba → g n r0 f ma (Q) g aSa 2

(1.13)

in the formalism given above, where gnâ•›=â•›−1.9132 is the g-factor for the neutron, r0â•›=â•›2.8179 fm is the classical radius of the electron, and fma(Q), ga, and Sa are the magnetic form factor, g-factor, and spin operator of the unpaired electrons on the atoms of element a. It is clear that if there are correlations between the orientations of the magnetic moments in the system with their positions, i.e., some kind of magnetic ordering, there will be a structuredependent term in the magnetic scattering analogous to the first term in the expression for the nuclear scattering, Equation 1.6. If, on the other hand, the orientations of the magnetic moments are completely random, as in a paramagnetic system, the magnetic scattering is independent of the structure and can be described by a term 2 2 g n r0 f ma (Q) µ 2a 3

(1.14)

analogous to the second term in Equation 1.6, μ⃗aâ•›=â•›1/2gaSa⃗ being the magnetic moment of the ath atom. It is clear from the Fourier transform in Equation 1.10 that long-range structural information will tend to dominate the scattering at low Q and short-range at high Q. Thus, the need to get accurate information about nearest-neighbor correlations, such as bond distances and coordination numbers, has driven the development of diffractometers with a large Q range, exploiting epithermal neutrons from pulsed spallation sources and high-energy x-rays from third-generation synchrotron sources. 1.2.2╇ Differences between Neutron and X-Ray Scattering Experimentally, a significant difference is that x-rays with energies available in a laboratory source or in a typical synchrotron beam are more highly absorbing than neutrons, which must be taken into account when designing sample containers and environmental equipment. With the x-ray energies on the order of 100â•›keV available from third-generation synchrotron sources, the absorption is much less significant. Other differences include the following:

1. Since the form factors depend on Q and fall off as Q increases, measurements at high Q values (e.g., beyond 5â•›Å−1) become more difficult. Also, the Q dependence has to be taken into account when calculating structure factors as in Equation 1.6. This is a significant problem when the sample has more than one atom.

© 2011 by Taylor and Francis Group, LLC

14

Nanoporous Materials

2. Since the form factors are not significantly isotope dependent, the scattering is always coherent in the sense used in neutron scattering. 3. Form factors are not generally energy dependent but have a strong energy variation near an absorption edge, as well as an imaginary component. This behavior is called anomalous scattering and can be exploited to distinguish scattering from a specific element, somewhat like isotope substitution in the neutron case. 4. Since x-ray energies of 10â•›keV and higher must generally be used to get an adequate Q range for studies of atomic structure and dynamics, it is difficult to get energy resolution in inelastic x-ray scattering (IXS) comparable to that obtainable in INS. At present the limit is about 1â•›meV. A compensating advantage is that the velocity of x-rays is orders of magnitude higher than any sound velocity in condensed matter, so that the kinematic restrictions that make it hard to make dynamical measurements at low Q and high E with INS do not apply.4 5. X-rays do not have a magnetic moment and so the interaction with magnetic moments in condensed matter is much weaker. On the other hand, polarization of an x-ray beam can be exploited for magnetic studies, as in magnetic circular dichroism, for example.

These examples show that neutrons and x-rays have many complementary features, and it is often important to use both techniques, as well as others described below, to investigate a complex material or phenomenon. 1.2.3╇ Selected Example An example of a diffraction measurement on a material confined in a porous host is the study of highly loaded Se in a Cu2+ ion-exchanged Y zeolite12 by AXS,10 which was complemented by diffuse reflectance and Raman spectroscopy measurements. The diffraction measurements were made at two energies, 20 and 300╛eV below the K absorption edge of Se at 12,658╛eV. Near an absorption edge of an element a, the scattering factors

f a (Q, E) = f a 0 (Q) + f a′(Q, E) + i f a′′(Q, E)

(1.15)

are complex with anomalous terms that vary strongly with energy. Accordingly, the weighting factors Waj involving the element a in the x-ray analogue of Equation 1.11 can be altered by tuning the x-ray energy near the absorption edge. From measurements at two energies below that edge, the difference structure factor Sa(Q) associated with the element can be derived from the relation

I (Q , E1 ) − I (Q , E2 ) = 2ca ∆f a′ f (Q) [Sa (Q) − 1] + 2ca ∆f a′ f a (Q).

© 2011 by Taylor and Francis Group, LLC

(1.16)

15

Scattering Techniques

4

SSe (Q)

2 0 –2 –4 –6

2

4 Q (Å–1)

6

8

Figure 1.5 Difference structure factor S Se(Q) of Cu–Y zeolite loaded with 12.5 Se per supercage derived from two diffraction experiments at 20 and 300â•›eV below the K absorption edge of selenium. (With permission from Goldbach, A., Saboungi, M.-L., Iton, L.E., and Price, D.L., Approach to band-gap alignment in confined semiconductors, J. Chem. Phys., 115, 11254, 2001.)

The difference pair correlation function ga(r) can be obtained by Fourier transformation of Sa(Q), through a relation analogous to Equation 1.10. Figure 1.5 shows the difference structure factor SSe(Q) determined from the diffraction experiments at the two energies, corrected for resonant Raman scattering, dead-time effects, Compton scattering, multiple scattering, and absorption in the sample. It can be seen to consist of a smoothly varying diffuse component with sharp positive and negative spikes superimposed on it. The diffuse component arises from the encapsulated Se, estimated from weight balance to amount to about 12.5 Se atoms per zeolite supercage. The sharp spikes result from a slightly imperfect cancellation of the large Bragg scattering from the zeolite host in the difference of the two measurements. The fact that the Se scattering is diffuse instead of following the Bragg peaks of the zeolite host shows that the Se atoms have a disordered structure, out of registry with the crystalline lattice of the zeolite host. Figure 1.6 shows the corresponding pair correlation function in real space. For technical reasons, the function 4πρ0ga(r) was used in the analysis rather than ga(r) itself. The spikes that appeared in SSe(Q) due to the imperfect cancellation of the zeolite Bragg peaks do not lead to any observable peaks in TSe(r) because of their very low weight. The oscillations below 2╛Šare due to truncation effects caused by the limited Q range of the SSe(Q) measurement. The distinguishable peaks in the region above 2╛Šwere fitted with Gaussian functions. The first peak centered at R SeSe(1)â•›=â•›2.39 ± 0.02╛Šreflects the intramolecular Se correlation. This distance is significantly longer than the © 2011 by Taylor and Francis Group, LLC

16

TSe (r)

Nanoporous Materials

0 0

1

2

3 r (Å)

4

5

6

Figure 1.6 Difference pair correlation function TSe(r) calculated from the difference structure factor displayed in Figure 1.5. (With permission from Goldbach, A., Saboungi, M.-L., Iton, L.E., and Price, D.L., Approach to band-gap alignment in confined semiconductors, J. Chem. Phys., 115, 11254, 2001.)

corresponding nearest-neighbor distances of Se encapsulates in Nd-Y, La-Y, and Ca-Y zeolites which have values ranging from 2.32 to 2.34╛Šderived by a procedure identical to that used for the Cu-Y zeolite,13,14 pointing to a weakening of the intrachain bonding in comparison to other zeolites. Except for trigonal Se, it also exceeds the values found in bulk Se forms, 2.336╛Šin monoclinic Se and 2.356╛Šin amorphous Se. The second peak extends between 3.0 and 4.5╛Šand contains three types of correlations: secondary Se–Se correlations, Se–O encapsulate framework interactions, and Se–Cu2+ pairs. Since the three Gaussian functions fitted to this peak are not completely resolved, the corresponding distances were assigned with significant error bars: RSeSe(2)â•›=â•›3.65â•›±â•›0.10â•›Å, R SeOâ•›=â•›3.95â•›±â•›0.05â•›Å, and, for the small first component, R SeCuâ•›=â•›3.30â•›±â•›0.05â•›Å. This last component was not observed in the other zeolite studies just mentioned. While it was not possible to obtain absolute coordination numbers from these data, the areas of the peaks at RSeSe(1) and R SeSe(2) were similar, as expected for isolated rings or extended chains, suggesting that intermolecular Se–Se interactions play a minor role in this material as in the other zeolites. The results presented here, together with the complementary Raman scattering measurements, indicated significant interactions between the incorporated Se and the Cu–Y matrix that modify the semiconductor’s electronic structure. The absence of Raman bands characteristic of Se8 rings suggested the formation of long Se chains inside the voids of the zeolite. The similarity of the values of the peak areas around RSeSe(1) and R SeSe(2) in the AXS measurement showed that these chains are mostly isolated. At RSeSe(1)â•›=â•›2.39╛Šthe first Se–Se distance is extraordinarily large for a covalent Se bond which points to a weakening of the intrachain bonding in comparison to © 2011 by Taylor and Francis Group, LLC

Scattering Techniques

17

the structure of Se chains in the other zeolites. This conclusion is corroborated by the large red shift 2+ of the encapsulate Raman band, while the width Cu of this feature implies strong irregularities within Se the Se chains. Altogether, these features point to a new type of interaction between the encapsulated Figure 1.7 Se and the Cu2+ ion. This interaction was identified Schematic illustration of bonding situations between with the short-range correlation at 3.30â•›Å, which did not appear in the pair distribution functions the encapsulated Se and the metal cations in Cu–Y obtained for other Se zeolite encapsulates. The zeolite. authors concluded that Cu2+ ions could be coordinated to one, two, or even more Se atoms of chain fragments of various length and that these distinct bonding situations could randomly alternate along the chain, as shown schematically in Figure 1.7. This study demonstrated the possibility of cation-directed band-gap alignments in zeolite-encapsulated semiconductors and established a convenient method for adjusting the electronic levels of clusters and molecules hosted in molecular sieves, which may be expedient for potential technical devices such as lasers or sensors.

1.3╇ Small-Angle Scattering SAS is a nondestructive technique and a very effective probe to study geometry and texture of inhom*ogeneities in the mesoscopic and macroscopic range, i.e., between 5 and 500â•›nm* according to the IUPAC definition.15–18 Because of the size range explored, SAS is a perfect complement of scanning and transmission electron microscopy (SEM, TEM) as well as diffraction (Figure 1.8). Small-angle neutron scattering (SANS) and its x-ray analog (SAXS) can be extremely useful in biology, polymer science, materials science, and chemistry. In this case the weaker scattering power gives the neutron measurements an advantage, since the samples are usually of manageable size—1–2â•›mm thick. Another advantage is that, as we will see later, contrast matching is much easier with neutrons, especially in systems containing light elements and in particular hydrogen atoms. 1.3.1╇ SAS Spectrometer A SANS spectrometer is composed first of a monochromator capable of selecting wavelengths λ 0 in the range 5–20â•›Å, followed by several diaphragms (collimators) used to produce a parallel beam (Figure 1.9). The scattered neutrons * The corresponding scattering vector range is approximately 10 −3â•› 0, LaE < 0 reported for the barriers (A, C, D, G, H, K) constructed by unsymmetrical compaction (keeping the piston at Xâ•›=â•›l fixed, as illustrated in Figure 7.4a) indicate an overall tendency for ψ(X) (and hence for ε(X)) to attain higher values on the upstream side of the barrier; a prediction that is again verified by the pertinent X-ray imaging plots of Figure 7.4b. X-ray imaging also shows how the aforesaid variation in bulk density arises, given that, in all cases listed in Table 7.1, the barrier surface facing the mobile piston during unsymmetrical compaction, became the upstream surface (Xâ•›=â•›0) © 2011 by Taylor and Francis Group, LLC

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Nanoporous Materials

Free Pressurized

Local bulk density (g/cm3)

2.0

Low

εL

1.8

High

1.6

εL

0.0

Mobile piston X=0

0.4 Vpi(X)

0.8 (cm3)

Fixed piston X=l

Figure 7.6 Typical example of variation of densification along X in an unsymmetrically compacted (alumina) barrier: (– – – –) in the pressurized state (local densification higher, hence local porosity εL lower, on the mobile piston side); (——) after the barrier has “sprung back” elastically, upon release of the compacting pressure (local densification lower, hence local porosity εL higher, on the mobile piston side).

in subsequent permeation runs. As illustrated in Figure 7.6 for an unsymmetrically compacted alumina barrier, the densification produced under compression is highest (i.e., εL is lowest) at Xâ•›=â•›0 and diminishes (i.e., εL rises) progressively up to Xâ•›=â•›l (as might reasonably be expected on the grounds of increasing frictional compressive pressure losses); this pattern tends, in fact, to remain imprinted in compacts of ductile materials (e.g., in powder metallurgy; Goetzel 1949). However, compacts of hard materials, as is the case here, tend to recover or “spring back” elastically, upon decompression, and it is the extent to which such an expansion can occur (against frictional resistance), at various locations X, that determines the pattern of macroscopic structural inhom*ogeneity, which finally characterizes the finished barrier (see Savvakis and Petropoulos 1982 and Galiatsatou et al. 2006a for further details). This is also the reason why the turning point in the plot of local bulk density for E in Figure 7.4b is a maximum [(i.e., a minimum for εL(X)]. Furthermore, the possibility that the magnitude of the reduced parameter ∆Lah /∆Las might serve as a useful indication of the overall degree of inhom*ogeneity of a barrier, for a given unsymmetrical functional form of ψ(X), is suggested by the fact that the highest values of this parameter recorded in Table 7.1 pertain to barriers D and H, which were constructed under conditions Â� known to intensify nonuniformity of densification, namely © 2011 by Taylor and Francis Group, LLC

Characterization of Inhom*ogeneity via Transport Properties

267

single-step compaction for D (Barrer and Gabor 1959, 1960; Savvakis and Petropoulos 1982) and lower overall densification (higher ε, due to lower applied compressive pressure) for H (Galiatsatou et al. 2006b), as compared with Â�multistep compaction to high overall densification (low ε) for barriers A, C, G, H, K. On the other hand, the technique of X-ray imaging with interposed X-rayopaque discs used by Savvakis and Petropoulos (1982) (see legend of Figure 7.4b) reveals that the time-lag behavior of barriers constructed by one-step compaction is subject to an additional complication, namely the presence of a radial component of macroscopic inhom*ogeneity (also known in powder metallurgy; see, e.g., Goetzel 1949), resulting in two-dimensional macroscopic variability of local densification (cf. plots for barriers of type D and E in Figure 7.4c). Nevertheless, there is a clear pattern of variation along X of the mean densification within each segment delimited by the broken lines in Figure 7.4c (cf. the resulting plots for barriers of type D and E in Figure 7.4b), which parallels the intersegmental variation of densification along X displayed by barriers of similar εL(X) functional form prepared by multistep compaction (exemplified by the plots for barriers of type D and C in Figure 7.4b). This gross axial structural similarity is duly reflected in the observed qualitative similarity of time-lag behavior shown in Table 7.1. Note that radial intrasegmental variability of local densification is also found in multistep compacts. It is not detectable by the above X-ray imaging technique, but may be estimated indirectly via suitable X-ray diffraction measurement (Savvakis and Petropoulos 1982) of the degree of orientation (alignment) of graphite particles at various locations in the barrier. (Due to the fact that the said particles are platelets, which pack most efficiently when parallel to one another, their degree of orientation is closely correlated with the extent of densification.) The results obtained (see Savvakis and Petropoulos 1982 for details) reveal a radial pattern of mild rise in densification from the center to the periphery of the barrier, which changes little with X, and should, therefore, not give rise to any appreciable time-lag discrepancies (in view of the fact that pure radial space dependence has no effect on ideal time-lag behavior; e.g., Petropoulos 1985). This is in marked contrast to the two-dimensional pattern exhibited by the corresponding single-step unsymmetrical barrier (D in Figure 7.4c). Here, the radial rise of densification from the center to the periphery of the barrier is very marked near the mobile piston (Xâ•›=â•›0), becomes gradually attenuated with increasing X, and is finally reversed near the fixed piston (Xâ•›=â•›l). For completeness, it is worth noting that the aforementioned X-ray diffraction results also yield the direction of the local orientation of the graphite platelets, which should affect the effective local value of DT (via a microscopic “pore orientation factor” included therein; Savvakis et al. 1982; Galiatsatou et al. 2006b). The said particle orientation was found (Savvakis and Petropoulos 1982) to vary radially (in both single- and multistep compacts) from effectively normal to X (minimum local DT), in the central region, to effectively parallel to X (maximum local © 2011 by Taylor and Francis Group, LLC

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Nanoporous Materials

DT) at the wall of the die. This radial variation of D T is much the same at all X and should not, therefore, contribute significantly to deviation from the ideal time-lag behavior. Accordingly, the observed non-Fickian time-lag behavior of multistep compacts should be representative primarily of intersegmental X-dependence of S and/or DT (see also Petropoulos and Roussis 1969a in this respect), while that of single-step compacts would be expected to include an effect stemming from the X-dependence of the radial variability of these transport parameters (to an extent that cannot be estimated, at present, due to lack of appropriate theoretical background). It follows that assessment of the potential diagnostic value of time-lag analysis for elucidation of such complex inhom*ogeneity effects requires further detailed study, complemented with parallel investigation of transient diffusion kinetics, which are sensitive to radial inhom*ogeneity. Furthermore, the above discussion illustrates the need to take due account also of the space variability of other relevant structural parameters, such as the orientation factor in graphite barriers considered above (see Galiatsatou et al. 2006b in particular) and the general pore structure factors κg, κs (cf. Barrer and Gabor 1959; Nicholson and Petropoulos 1982; Savvakis et al. 1982; Kanellopoulos et al. 1985; Petropoulos and Papadokostaki 2008), derived from appropriate sorption and permeability or relative permeability data. For examples of the fruitful combination of information derived from the said pore structure factors with time-lag analysis, see Petropoulos and Petrou (1992) and Petropoulos and Papadokostaki (2008). For examples of other discussions of non-Fickian time lag analysis or uses thereof, see Crank (1975a), Ash et al. (1968, 1979), Ash (2006), Rutherford and Do (1997).

7.4╇Transient Diffusion Kinetics (ConcentrationIndependent Systems) An analogous method of analysis of transient state diffusion kinetics has been proposed (Tsimillis and Petropoulos 1977; Amarantos et al. 1983; Grzywna and Petropoulos 1983a,b; Petropoulos 1985), based on the consideration that, in any experiment, the kinetic behavior of the system represented by S(X), DT(X) will generally deviate from that of the corresponding ideal system represented by Sâ•›=â•›Se, Dâ•›=â•›De, in either of two ways: (1) ideal kinetics is obeyed, but with a different effective diffusion coefficient Dn, where nâ•›=â•›l, 2,… denotes a particular kinetic regime (Dn is usually deduced from a suitable linear kinetic plot) or (2) ideal kinetics is departed from, in which case one is reduced to comparison between the (nonlinear) experimental plot and the corresponding calculated ideal (linear) one. © 2011 by Taylor and Francis Group, LLC

Characterization of Inhom*ogeneity via Transport Properties

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The following effective diffusion coefficients Dn may be defined on the basis of standard sorption and permeation experiments (Grzywna and Petropoulos 1983a,b; Petropoulos 1985) (when absorption or desorption conditions Â� need to be specified, superscripts a or d, respectively, are again used): 1. Sorption The barrier is initially equilibrated at aâ•›=â•›a1. a. In “unsymmetrical” sorption experiments (Tsimillis and Petropoulos 1977; Grzywna and Petropoulos 1983a), the barrier is exposed to penetrant at Xâ•›=â•›0 and is blocked at Xâ•›=â•›l, namely

∂a(X = l , t) =0 ∂X

a(X , t = 0) = a1 ; a(X = 0 , t) = a0 ;

Denoting by Qt, Q∞ the amounts of penetrant sorbed (absorbed if a0â•›>â•›a1 or desorbed if a0â•›

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