Nicholas P. Stadie, Maxwell Murialdo, Channing C. Ahn, and Brent Fultz
J. Amer. Chem. Soc., in press.
A thermodynamic study of the enthalpy of adsorption of methane on high-surface area carbonaceous materials was carried out from 238-526 K. The absolute quantity of adsorbed methane as a function of equilibrium pressure was determined by fitting isotherms to a generalized Langmuir-type equation. The adsorption of methane on zeolite-templated carbon (ZTC), an extremely high surface-area material with a periodic arrangement of narrow micropores, shows an increase in isosteric enthalpy with methane occupancy; that is, binding energies are greater as adsorption quantity increases. The heat of adsorption rises from 14 to 15 kJ mol-1 at ambient temperature, and then falls to lower values at very high loading (above a relative site occupancy of 0.6), indicating that methane-methane interactions within the adsorption layer become significant. The effect seems to be enhanced by a narrow pore-size distribution centered at 1.2 nm, corresponding to approximately the width of two monolayers of methane, and reversible methane delivery increases by up to 20% over MSC-30 at temperatures and pressures near ambient.
J. A. Munoz, M. S. Lucas, O. Delaire, M. L. Winterrose, L. Mauger, Chen W. Li, A. O. Sheets, M. B. Stone, D. L. Abernathy, Yuming Xiao, Paul Chow, and B. Fultz
Physical Review Letters 107, 115501 (2011).
Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by 0.22 +- 0.03 k_B/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.
Chen W. Li, Xiaoli Tang, J. A. Munoz, J. B. Keith, S. J. Tracy, D. L. Abernathy, and B. Fultz
Physical Review Letters, 107, 195504 (2011).
Cubic scandium trifluoride (ScF_3) has a large negative thermal expansion over a wide range of temperature. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF_3 from 7 to 750 K. The measured phonon densities of states (DOS) show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the DOS, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO_9 structure of ScF_3, and accounts for phonon stiffening with temperature and a signicant part of the negative thermal expansion.
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Progress in Materials Science, 55, 247-352 (2010).
The literature on vibrational thermodynamics of materials is reviewed. The emphasis is on metals and alloys, especially on the progress over the last decade in understanding differences in the vibrational entropy of different alloy phases and phase transformations. Some results on carbides, nitrides, oxides, hydrides and lithium-storage materials are also covered. Principles of harmonic phonons in alloys are organized into thermodynamic models for unmixing and ordering transformations on an Ising lattice, and extended for non-harmonic potentials. Owing to the high accuracy required for the phonon frequencies, quantitative predictions of vibrational entropy with analytical models prove elusive. Accurate tools for such calculations or measurements were challenging for many years, but are more accessible today. Ab-initio methods for calculating phonons in solids are summarized. The experimental techniques of calorimetry, inelastic neutron scattering, and inelastic x-ray scattering are explained with enough detail to show the issues of using these methods for investigations of vibrational thermodynamics. The explanations extend to methods of data analysis that affect the accuracy of thermodynamic information.
It is sometimes possible to identify the structural and chemical origins of the differences in vibrational entropy of materials, and the number of these assessments is growing. There has been considerable progress in our understanding of the vibrational entropy of mixing in solid solutions, compound formation from pure elements, chemical unmixing of alloys, order-disorder transformations, and martensitic transformations. Systematic trends are available for some of these phase transformations, although more examples are needed, and many results are less reliable at high temperatures. Nanostructures in materials can alter sufficiently the vibrational dynamics to affect thermodynamic stability. Internal stresses in polycrystals of anisotropic materials also contribute to the heat capacity. Lanthanides and actinides show a complex interplay of vibrational, electronic, and magnetic entropy, even at low temperatures.
A "quasiharmonic model" is often used to extend the systematics of harmonic phonons to high temperatures by accounting for the effects of thermal expansion against a bulk modulus. Non-harmonic effects beyond the quasiharmonic approximation originate from the interactions of thermally-excited phonons with other phonons, or with the interactions of phonons with electronic excitations. In the classical high temperature limit, the adiabatic electron-phonon coupling can have a surprisingly large effect in metals when temperature causes significant changes in the electron density near the Fermi level. There are useful similarities in how temperature, pressure, and composition alter the conduction electron screening and the interatomic force constants. Phonon-phonon "anharmonic" interactions arise from those non-harmonic parts of the interatomic potential that cannot be accounted for by the quasiharmonic model. Anharmonic shifts in phonon frequency with temperature can be substantial, but trends are not well understood. Anharmonic phonon damping does show systematic trends, however, at least for fcc metals.
Trends of vibrational entropy are often justified with atomic properties such as atomic size, electronegativity, electron-to-atom ratio, and mass. Since vibrational entropy originates at the level of electrons in solids, such rules of thumb prove no better than similar rules devised for trends in bonding and structure, and tend to be worse. Fortunately, the required tools for accurate experimental investigations of vibrational entropy have improved dramatically over the past few years, and the required ab-initio methods have become more accessible. Steady progress is expected for understanding the phenomena reviewed here, as investigations are performed with the new tools of experiment and theory, sometimes in integrated ways.
Download draft of Feb. 6, 2011 (0.9 MB)
Book chapter in Characterization of Materials. Elton Kaufmann, Editor (John Wiley, New York, 2011).
Mossbauer spectrometry gives electronic, magnetic, and structural information from within materials. A Mossbauer spectrum is an intensity of gamma-ray absorption versus energy for a specific resonant nucleus such as 57Fe or 119Sn. For one nucleus to emit a gamma-ray and a second nucleus to absorb it with efficiency, both nuclei must be embedded in solids, a phenomenon known as the "Mossbauer effect." Mossbauer spectrometry looks at materials from the "inside out," where "inside" refers to the resonant nucleus.
Mossbauer spectra give quantitative information on "hyperfine interactions," which are small energies from the interaction between the nucleus and its neighboring electrons. The three hyperfine interactions originate from the electron density at the nucleus (the isomer shift), the gradient of the electric field (the nuclear quadrupole splitting), and the unpaired electron density at the nucleus (the hyperfine magnetic field). Over the years, methods have been refined for using these three hyperfine interactions to determine valence and spin at the resonant atom. Even when the hyperfine interactions are not easily interpreted, they can often be used reliably as "fingerprints" to identify the different local chemical environments of the resonant atom, usually with a good estimate of their fractional abundances. Mossbauer spectrometry is useful for quantitative phase analyses or determinations of the concentrations of resonant element in different phases, even when the phases are nanostructured or amorphous.
Most Mossbauer spectra are acquired with simple laboratory equipment and a radioisotope source, but the recent development of synchrotron instrumentation now allow for measurements on small 10 micron samples, which may be exposed to extreme environments of pressure and temperature. Other capabilities include measurements of the vibrational spectra of the resonant atoms, and coherent scattering and diffraction of nuclear radiation.
This article is not a review of the field, but an instructional reference that explains principles and practices, and gives the working materials scientist a basis for evaluating whether or not Mossbauer spectrometry may be useful for a research problem. A few representative materials studies are presented.
761 pages, 478 figures, 1,300 equations (Springer-Verlag, 2012).
Brent Fultz and James Howe
This book explains concepts of transmission electron microscopy (TEM) and x-ray diffractometry (XRD) that are important for the characterization of materials. The fourth edition has been updated to cover important technical developments, including electron tomography and new nanobeam methods.
This edition is not substantially longer than the third, but all chapters have been updated and revised for clarity. A new chapter on neutron scattering
follows the chapters on x-ray diffractometry and electron microscopy.
The book explains the fundamentals of how waves and wavefunctions interact with atoms in solids, and the similarities and differences of diffraction measurements with x-rays, electrons, or neutrons. Diffraction effects of crystalline order, defects, and disorder in materials are explained in detail. Both practical and theoretical issues are covered. This textbook can be used in an introductory-level or advanced-level course, since sections are identified by difficulty. Each chapter includes a set of problems to illustrate principles, and the extensive Appendix includes laboratory exercises.
Excerpts from the first edition of the book in Adobe Acrobat .pdf format can be found here.
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