The phonon, a quantum of atomic vibrations, is a core ingredient in the description of materials\u2019 behavior at both high and low temperatures. A harmonic theory of lattice dynamics treats phonons as independent, noninteracting normal modes with long lifetimes. The proper description of phenomena in solids requires the phonons to interact depending on temperature, or in other words, to act anharmonically. The phonon interaction in highly anharmonic crystals can result in intermodulation and an additional coherent scattering intensity at frequencies of the sums and differences of classical normal modes. At low temperatures, anharmonic interaction is triggered by nuclear quantum effects of zero-point motion, which can be observed\r\nas intermodulation and negative thermal expansion (NTE). In the thesis, I expand the general understanding of intermodulation phenomena using computational and experimental methods by adding missing parts expected in the theoretical intermodulation picture, such as phonon second harmonic generation and nuclear quantum intermodulation.
\r\n\r\nThe phenomenon of second harmonic generation (SHG) was found for phonons in anharmonic NaBr by inelastic neutron scattering. The temperature dependence of this phonon SHG was measured from 300 K to 650 K. At 300 K the second harmonic (SH) is seen as a high-energy branch around 33 meV, nearly independent of Q. The temperature effective potential (TDEP) method and classical molecular dynamics (MD) simulation with machine learning interatomic potential were able to reproduce the SH, and showed that SHG occurs with the flat transverse optical (TO) phonon branch. A classical model of a nonlinear medium explains the intensity and lifetime of the SH, compared to those of the TO modes. Also successful was a quantum model based on the Heisenberg-Langevin equation for interacting phonons coupled to a thermal bath, which also predicts a spectral distribution of the SH. The measured temperature dependence of the intensity of the second harmonic showed that it follows the Planck distribution of a one-phonon quasiparticle, and not two TO phonons.
\r\n\r\nThe anharmonic behavior of phonons and thermal expansion of hexagonal zinc were studied from 15 to 690 K by inelastic neutron scattering (INS) and ab initio simulations. Phonon spectra were measured for Q-points over the full Brillouin zone, giving the phonon density of states (DOS), and dispersions along high-symmetry directions. The dispersions were crisp at 15 K, but diffuse intensity was observed at energies above them. The dispersions broadened with temperature, T, and the diffuse intensity grew relatively stronger. This diffuse intensity appeared in all INS measurements and simulations, except for classical molecular dynamics at 15 K. The TDEP method was used to calculate the free energy and thermal expansion with the nuclear quantum effect from zero-point vibrational dynamics. For T < 100 K the nuclear quantum effect was essential for obtaining the negative thermal expansion, and path integral molecular dynamics (PIMD) was particularly effective for obtaining the negative thermal expansion in the basal plane. A Heisenberg-Langevin model for interacting phonons coupled to a thermal bath was able to reproduce the shape and intensity of the diffuse spectral features.
", "doi": "10.7907/sjpe-v132", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16450", "collection": "thesis", "collection_id": "16450", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302024-193052659", "primary_object_url": { "basename": "Pedro Guzman PhD Thesis 2024.pdf", "content": "final", "filesize": 8827433, "license": "other", "mime_type": "application/pdf", "url": "/16450/1/Pedro Guzman PhD Thesis 2024.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Developments in M\u00f6ssbauer Spectrometry: From Instrumentation to High Pressure Studies on Spins and Phonons", "author": [ { "family_name": "Guzman", "given_name": "Pedro", "orcid": "0000-0002-9726-8315", "clpid": "Guzman-Pedro" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Scott", "given_name": "Valerie", "orcid": "0000-0002-0267-9833", "clpid": "Scott-Valerie" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The well-established technique of 57Fe M\u00f6ssbauer spectrometry is used to investigate the local chemical environment in iron-containing materials. This technique relies on the recoil-free emission and absorption of \u03b3-rays by resonant nuclei within a solid. The key component of a M\u00f6ssbauer spectrometer is the velocity Doppler drive, which modulates the energy of the incident \u03b3-rays to detect the hyperfine structure of resonant nuclei. Since the 1970s, the conventional velocity Doppler drive has been constructed using a pair of electromagnetic coils, one for power and the second for feedback. An alternative M\u00f6ssbauer spectrometer was developed, utilizing an amplified piezoelectric actuator as the Doppler velocity drive under feedback control. The actuator, driven with a quadratic displacement waveform, produced a linear velocity profile and was optimized using measurements from a laser Doppler vibrometer (LDV). In transmission geometry, 57Fe M\u00f6ssbauer spectra of \u03b1-iron display minimal peak distortions, enabling M\u00f6ssbauer spectrometry in applications requiring compact size and low mass, such as geochemical studies on the Moon, Mars, or asteroids.
\r\n\r\nSynchrotron radiation is used for numerous experimental techniques, including X-ray diffraction (XRD), nuclear resonant inelastic X-ray scattering (NRIXS), and nuclear forward scattering (NFS), also known as synchrotron M\u00f6ssbauer spectrometry. Diamond-anvil cells, capable of reaching high pressures at various temperatures, combined with synchrotron experimental methods, provide the means to investigate the vibrational, magnetic, and thermophysical properties of materials. Measurements on 57Fe55Ni45 were conducted using synchrotron XRD, NRIXS, and NFS under various pressures and temperatures. XRD measurements at 298 K and 392 K under pressures up to 20 GPa confirmed a pressure-induced Invar effect between 7 GPa and 13 GPa, where the coefficient of thermal expansion is nearly zero. NFS measurements revealed a decrease in the magnetic moment of 57Fe under pressure, indicating an increase in magnetic entropy. The 57Fe phonon density of states (DOS) was measured with NRIXS from which a phonon entropy was extracted. Using thermodynamic Maxwell relations, magnetic and phonon contributions to thermal expansion were determined, demonstrating that the low thermal expansion in the pressure-induced Invar region stems from a competition between the thermal expansion from spins and from phonons.
", "doi": "10.7907/hyry-q484", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16186", "collection": "thesis", "collection_id": "16186", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09222023-185858765", "primary_object_url": { "basename": "Bernal_Choban_Thesis.pdf", "content": "final", "filesize": 39913083, "license": "other", "mime_type": "application/pdf", "url": "/16186/1/Bernal_Choban_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Atomic Dynamics in Solids and Liquids from Inelastic Neutron Scattering", "author": [ { "family_name": "Bernal-Choban", "given_name": "Camille Marie", "orcid": "0000-0001-7550-3153", "clpid": "Bernal-Choban-Camille-Marie" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Granroth", "given_name": "Garrett", "orcid": "0000-0002-7583-8778", "clpid": "Granroth-Garrett" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "As temperature increases, atomic scale disorder, or entropy, drives the thermophysical properties of materials. One way it does this is by passing heat through materials in the form of vibrations. In solids, vibrational motions are called phonons, and their behaviors are used to predict macroscopic properties such as thermal expansion and thermal conductivity. Vibrational dynamics also exist in liquids but are traditionally less studied. Other forms of entropy include configurational and electronic entropy, which also evolve with temperature. Configurational changes in solids are often small, but in liquids, the prominence of diffusion makes this contribution significant. This dissertation addresses these atomistic components of entropy in two studies, one on bcc chromium and the other on the melting of monatomic systems.
\r\n\r\nIn the first study, phonon densities of states (DOS) of body-centered cubic chromium were measured by time-of-flight inelastic neutron scattering (INS) at temperatures up to 1493 K. Density functional theory calculations with both quasi-harmonic (QH) and anharmonic (AH) methods were performed at temperatures above the Neel temperature. Features in the phonon DOS decrease in energy (soften) substantially with temperature. A Born-von Karman analysis using fits to the experimental DOS reveals a softening of almost 17% of the high transverse phonon branch between 330 and 1493 K. The low transverse branch changes by approximately half this amount. The AH calculations capture the observed behavior of the two transverse phonon branches, but the QH calculations give some inverted trends. Vibrational entropies from phonons and electrons are obtained, and their sum is in excellent agreement with the entropy of chromium obtained by calorimetry, indicating that above 330 K, no explicit temperature-dependent magnetic contributions are necessary.
\r\n\r\nThe second investigation delves into the latent heat of melting, defined as TmΔSfus where Tm is the melting temperature and ΔSfus is the entropy of fusion. At the scale of atoms and electrons, ΔSfus has components from changes of atom configurations, atom vibrations, and thermal excitations of electrons. New data analyses were developed for inelastic neutron scattering to obtain changes in vibrational spectra upon melting. Combining these INS experiments with computational work using thermodynamic integration and molecular dynamics, components of ΔSfus were obtained for a total of six elements, Ge, Si, Bi, Sn, Pb, Li. Upon melting, there is always a positive change of configurational entropy, ΔSconfig. A baseline value of ΔSconfig=1.2kB/atom, approximately the value for Richard's rule, corresponds to zero change in the vibrational part of the entropy of fusion, ΔSvib. Elements having values of ΔSfus\r\nthat depart from this value of Richard's rule have both an additional ΔSvib and an additional ΔSconfig. Surprisingly, the extra ΔSconfig is close to 77% of ΔSvib, for both positive and negative deviations from Richard's rule. This implies a correlation between the change in the number of basins in a potential energy landscape and the change in the inverse of their curvature upon melting.
", "doi": "10.7907/3nv3-g144", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15259", "collection": "thesis", "collection_id": "15259", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012023-173549089", "type": "thesis", "title": "Experimental Study on the Thermodynamic Interactions of Phonons and Magnetism in Fe Systems", "author": [ { "family_name": "H\u00e4geli Lohaus", "given_name": "Stefan P.", "orcid": "0000-0002-4430-3834", "clpid": "H\u00e4geli-Lohaus-Stefan-P-H\u00e4geli" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Alp", "given_name": "Esen E.", "clpid": "Alp-Esen-E" }, { "family_name": "Scott", "given_name": "Valerie", "clpid": "Scott-Valerie" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The macroscopic thermophysical behavior of materials is governed by their atomic level excitations and how they store heat. Most of the thermal energy excites oscillations of the atoms, quantized as phonons, but in magnetic materials a considerable amount of heat is also absorbed by fluctuations of the electronic spins. This thesis explores the thermodynamics of phonons and magnetic spins in Fe-systems: we investigate the coupling between these excitations in Fe, Fe-Ni, and Fe-C, quantify their size dependency in nanocrystalline in Ni\u2083Fe, and assess their individual roles in the anomalous thermal expansion of Fe-Ni Invar.
\r\n \r\nMost materials expand when heated due to enhanced atomic oscillations. However, in 1895 C.E. Guillaume combined Fe and Ni to discover a material with near-zero thermal expansion, called Invar. This discovery was awarded the 1920 Physics Nobel Prize and sparked thousands of scientific investigations. Since the anomalous Invar effect is associated with magnetism, nearly all studies have focused on the electronic and spin structure of Fe-Ni. But phonons are needed to complete the picture, and to date, the anomalous Invar behavior is not fully understood. Here, we explore a method for measuring thermal expansion that is capable of isolating contributions from phonons and spins. Since the thermal energy of materials is related to entropy, the thermal expansion can be indirectly determined through individual entropic contributions by using a Maxwell relation. The phonon and magnetic entropies were measured by combining two nuclear resonant x-ray scattering techniques, with samples under pressure in diamond-anvil cells. We show that the Invar behavior stems from a competition between phonons and spins, that oppose each other for near-zero thermal expansion. A spin-phonon coupling improves the precision of this cancellation, extending the range of Invar behavior.
\r\n \r\nSuch a coupling of phonons and spin was also observed in pure Fe and Fe\u2083C cementite, as their phonon energies correlate to the change in magnetization. This motivated us to develop a magnetic quasi-harmonic model for Fe and Fe\u2083C, which accounts well for the deviation of phonon energies from pure volumetric effects of the conventional quasi-harmonic approximation.
\r\n\r\nThe thermodynamics of materials is also affected by the size of their crystallites. We determined the size effects on the heat absorption by phonons, electrons, and spins in nanocrystalline Ni\u2083Fe. All excitations become enhanced in the nanomaterial. In particular, the redistribution of spectral weights amplifies the phonon entropy. This helps stabilize the nanostructure against the enthalpy from its extra grain boundaries. However, the nanostructure is meta-stable, and the grains will grow into their bulk counterpart when diffusion is enabled at elevated temperatures.
", "doi": "10.7907/5sb5-fm96", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15181", "collection": "thesis", "collection_id": "15181", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05162023-225200255", "primary_object_url": { "basename": "Thesis_Ziyi_0522.pdf", "content": "final", "filesize": 12313820, "license": "other", "mime_type": "application/pdf", "url": "/15181/1/Thesis_Ziyi_0522.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Temperature Dependence of Gas Physisorption Energy: Experimental and Computational Studies of Krypton on Porous Carbon", "author": [ { "family_name": "Wang", "given_name": "Ziyi", "orcid": "0000-0003-0811-921X", "clpid": "Wang-Ziyi" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Goddard", "given_name": "William A., III", "orcid": "0000-0003-0097-5716", "clpid": "Goddard-W-A-III" }, { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Stadie", "given_name": "Nicholas", "orcid": "0000-0002-1139-7846", "clpid": "Stadie-P-Nicholas" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work comprehensively investigated the temperature dependence of physical adsorption energy, combining theoretical, computational, and experimental approaches. A thermodynamic analysis of the 2D ideal gas and the slit-pore models highlighted the role of van der Waal potentials in the adsorption energy and isotherm fitting methods, especially Henry's law. Experimental data of krypton adsorption on CNS-201 and MSC-30 porous carbon materials revealed a significant weakening in the isosteric adsorption energy with temperatures from 250 K to 330 K. Using the zero-coverage Henry's constants and Clausius\u2013Clapeyron equation, the adsorption energies weaken for 13% and 15% for CNS-201 and MSC-30. The corresponding changing rates are 4.35k_B for CNS-201 and 3.65k_B for MSC-30.
\r\n\r\nThe DFT-based computational study with the slit-pore model showed the van der Waal potentials of different-sized pores. Then it showed how the structures of the pores significantly influence the surface dynamics and the internal energies of the adsorbates at different temperatures. Gas molecules adsorbed in pores of different sizes have different heat capacities larger than the gas phase, leading to a temperature dependence of adsorption energy. Monte Carlo calculation indicated that displacements of adsorbent atoms caused by thermal vibration slightly weaken the van der Waal potentials but have a negligible effect on the temperature dependence of the adsorption energy.
\r\n\r\nThe distribution of pore sizes plays a crucial role in the temperature dependence of the overall adsorption energy. With increasing temperature, the pores with higher energy states become more accessible due to the Boltzmann distribution, weakening the statistically averaged internal energy. Adsorption energy weakening of 5% and 15% for CNS-201 and MSC-30 are given by combining the computational van der Waal potentials and experimentally measured pore sizes. The changing rates are 0.62k_B and 2.03k_B.
", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15222", "collection": "thesis", "collection_id": "15222", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292023-054311609", "primary_object_url": { "basename": "QuineThesis_Caltech.pdf", "content": "final", "filesize": 61426395, "license": "other", "mime_type": "application/pdf", "url": "/15222/1/QuineThesis_Caltech.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Tunability of Gas Adsorption Enthalpies in Carbonaceous Materials for Energy-Related Applications", "author": [ { "family_name": "Quine", "given_name": "Cullen Mackenzie", "orcid": "0000-0002-7301-0969", "clpid": "Quine-Cullen-Mackenzie" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Ahn", "given_name": "Channing C.", "clpid": "Ahn-C-C" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Schwab", "given_name": "Keith C.", "orcid": "0000-0001-8216-4815", "clpid": "Schwab-K-C" }, { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Stadie", "given_name": "Nicholas", "orcid": "0000-0002-1139-7846", "clpid": "Stadie-P-Nicholas" }, { "family_name": "Ahn", "given_name": "Channing C.", "clpid": "Ahn-C-C" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "div_eng" } ], "abstract": "Carbonaceous materials provide a porous, high surface area framework for the adsorption of gases through physisorption. Physisorption operates through van der Waals forces, resulting in highly reversible, densified gas storage. The density of adsorbed gas species approaches the bulk liquid density, providing a method to increase the volumetric energy density of hydrogen and natural gas at conditions where the adsorbate is a non-liquid in the bulk phase. This dissertation explores the tunability of the strength of gas adsorption to surfaces of carbon adsorbents, known as the enthalpy of adsorption. Two methods are studied: modification of the surface atomic composition and microstructural changes to the carbon porosity. Applications are considered for both energy storage and carbon capture applications.
\r\n\r\nThe first chapter presents a brief overview of the energy storage field, with emphasis on non-conventional methods to store gases efficiently. Chapter 2 provides the thermodynamic and statistical mechanical derivations used throughout this work, and the assumptions that go into the models used to analyze adsorption data. Chapter 3 reports work on a copper-modified commercial carbon MSC-30 for hydrogen storage, which exhibits an activated dissociative chemisorption desorption feature around ambient temperature. Chapter 4 presents the densification of a novel architected carbon structure, zeolite-templated carbon, for adsorbed natural gas storage. Through the pelletization process, the pore morphology of the underlying adsorbent framework is compressed, resulting in increased adsorption enthalpies with applied pelletization pressure. Chapter 5 focuses on the tunability of pore structure through potassium hydroxide activation, and the resulting adsorption properties pertinent to carbon dioxide capture from a simulated flue-gas stream. The last chapter provides insight into the work as a whole and identifies areas of future work that would improve the fundamental understanding and broader impact of adsorbent materials.
", "doi": "10.7907/r5ad-1j85", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14625", "collection": "thesis", "collection_id": "14625", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262022-015714738", "type": "thesis", "title": "Thermal Behavior of Cuprous Oxide: a Comprehensive Study of Three-Body Phonon Effects and Beyond", "author": [ { "family_name": "Saunders", "given_name": "Claire Nicole", "orcid": "0000-0001-7973-3722", "clpid": "Saunders-Claire-Nicole" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Schwab", "given_name": "Keith C.", "orcid": "0000-0001-8216-4815", "clpid": "Schwab-K-C" }, { "family_name": "Minnich", "given_name": "Austin J.", "orcid": "0000-0002-9671-9540", "clpid": "Minnich-A-J" }, { "family_name": "Goddard", "given_name": "William A., III", "orcid": "0000-0003-0097-5716", "clpid": "Goddard-W-A-III" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Granroth", "given_name": "Garrett", "orcid": "0000-0002-7583-8778", "clpid": "Granroth-Garrett" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Phonons, or quantized normal modes of crystal vibrations, are responsible for much of the thermophysical behavior in solid-state systems. This behavior includes properties like thermal expansion, defined as the change in material volume in response to temperature. Typically, materials expand upon heating and contract upon cooling; however, some undergo anomalous or negative thermal expansion (NTE). This study focuses on a material with NTE, cuprous oxide (Cu2O), commonly known as cuprite. Using computational and experimental methods, we identify the underlying mechanisms of the NTE and how these mechanisms relate to temperature-dependent phonon behavior with temperature, using both computational and experimental methods.
\r\n\r\nComputationally, we interpret temperature-dependent changes in phonon energies with perturbation theory. Assuming that the bonds between atoms behave like simple harmonic oscillators, we model the observed random motion of the atoms around their equilibrium positions with quasi-harmonic (QH) and anharmonic (AH) approximations. Furthermore, the perturbations in the atom position allow us to model phonon energy changes in response to temperatures.
\r\n\r\nWhile these models, particularly AH models, have proven accurate in predicting the phonon behavior, experimental methods, like inelastic neutron scattering (INS), remain the gold standard for validation. This study presents INS data from single-crystal cuprite measured on the Wide-Angular Range Chopper Spectrometer (ARCS) at the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS). We present INS data collected at 10 K, 300 K, 700 K, and 900 K. The post-processing workflow included: (1) binning with the software package Mantid, (2) reducing with a multiphonon background correction for polyatomic crystals, and (3) condensing into a single irreducible wedge in the first Brillouin zone (BZ). From this, we obtain a four-dimensional scattering function S(Q, E). Our AH calculations use the stochastic-Temperature Dependent Effective Potential (sTDEP) and the Machine Learning Interatomic Potential (MLIP) methods. The former method uses perturbation theory to include cubic and quartic AH contributions. The latter uses machine learning (ML), which in principle, includes all orders of AH terms.
\r\n\r\nThis investigation of the NTE of cuprite demonstrates that QH and AH models successfully predict anomalous NTE behavior. However, only AH calculations show the temperature-dependent phonon behavior seen in INS results. This discrepancy likely stems from a fortuitous cancellation of cubic and quartic AH terms giving an apparent success of QH models for the NTE. Ultimately, a correct prediction of thermal expansion with incorrect phonons reinforces the need to look at the role of higher-order terms in the temperature-dependent behavior of this material.
\r\n\r\nDespite the success of sTDEP at predicting phonon frequency shifts, it could not account for the newly observed diffuse inelastic intensity (DII) in the INS phonon spectra. For this, MLIP was more effective.
\r\n\r\nThis work provides complementary models to explain the origins of the DII, which is likely an emerging category of AH feature best described as a local nonlinear many-body process. We investigate phonon dissipation, the dynamics of systems coupled to their environments, Brownian motion, and discontinuities due to impulse transfer effects. We conclude by addressing the potential applications of the results and their role in future work on thermal lattice dynamics.
", "doi": "10.7907/mate-2v65", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14438", "collection": "thesis", "collection_id": "14438", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12012021-011312125", "primary_object_url": { "basename": "Mudide_Thesis_2021.pdf", "content": "final", "filesize": 5179437, "license": "other", "mime_type": "application/pdf", "url": "/14438/1/Mudide_Thesis_2021.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "The Limits of The Quasi-Harmonic Approximation: Anharmonicity in Germanium and the Entropy of Melting", "author": [ { "family_name": "Mudide", "given_name": "Shiva", "clpid": "Mudide-Shiva" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Libbrecht", "given_name": "Kenneth George", "orcid": "0000-0002-8744-3298", "clpid": "Libbrecht-K-G" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Politzer", "given_name": "Hugh David", "orcid": "0000-0002-4983-6621", "clpid": "Politzer-H-D" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Inelastic Neutron Scattering (INS) measurements were made at the Wide Angular-Range Chopper Spectrometer (ARCS) on Germanium at temperatures higher than what has been done before, from 296 K to 1203 K. Raw data was used to calculate the dynamic structure factor. Multi-phonon and multiple scattering events were accounted for and subtracted. These dynamic structure factors were then used to calculate single phonon density of states (DOS) for temperatures throughout the said temperature range. Thermal softening of the phonon modes was observed. The softening was quantitatively characterized with several Gruneisen parameters to better understand the effects phonon anharmonicity in Germanium. We find the quasi-harmonic approximation alone cannot explain the large phonon softening. The vibrational entropy contribution to the total entropy was also determined. We find that the vibrational entropy makes up almost all of the total entropy in Germanium, even at elevated temperatures.
\r\n\r\nWe also conduct melting experiments to ensure containment of Si, Bi, and Pb in quartz ampules. These metals will be heated through their melting points at ARCS in the near future in order to determine the vibrational entropy contribution to the latent heat of melting. Furthermore, we write an algorithm based on the work of Sivia to determine the number of phonon modes there is the maximum evidence for in any given phonon DOS.
", "doi": "10.7907/9mn0-y471", "publication_date": "2021", "thesis_type": "senior_major", "thesis_year": "2021" }, { "id": "thesis:13738", "collection": "thesis", "collection_id": "13738", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292020-150856962", "primary_object_url": { "basename": "CaltechThesis_YangShen.pdf", "content": "final", "filesize": 69548128, "license": "other", "mime_type": "application/pdf", "url": "/13738/1/CaltechThesis_YangShen.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Phonon Anharmonicity at the Limits of Perturbation Theory", "author": [ { "family_name": "Shen", "given_name": "Yang", "orcid": "0000-0001-6838-0925", "clpid": "Shen-Yang" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Minnich", "given_name": "Austin J.", "orcid": "0000-0002-9671-9540", "clpid": "Minnich-A-J" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" }, { "family_name": "Hellman", "given_name": "Bj\u00f6rn O.", "orcid": "0000-0002-3429-7856", "clpid": "Hellman-B-O" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Phonons, as the building blocks of solid-state physics, have been studied for almost one hundred years. The harmonic model is helpful when introducing the concepts and offering a basic physics picture of atomic vibrations. However, there are many properties that cannot be explained by the harmonic model or its extension to the quasiharmonic approximation (QHA), which ignores the pure temperature dependence of phonon frequencies. The rapid development of materials science requests a deep understanding of the phonon behaviors at elevated temperatures, where phonon-phonon interactions, as a main source of phonon anharmonicity, account for a number of abnormal phonon behaviors and the thermodynamical properties of many materials. In this thesis, I present the phonon anharmonicity in metals of Pd and Pt, the metallic compound FeGe2, and the polar material NaBr to show the limitation of the harmonic or QH model and the importance of taking anharmonic effects into consideration.
\r\n\r\nInelastic neutron scattering (INS) was used to measure the phonon density of states (DOS) in fcc Pd and Pt metal at temperatures from 7 K to 1576 K. Both phonon-phonon interactions and electron-phonon interactions were calculated by methods based on density functional theory (DFT) and were consistent with the measured shifts and broadenings of phonons with temperature. Contributions to the entropy from phonons and electrons were assessed and summed to obtain excellent agreement with prior calorimetric data. The QH entropy is positive for both phonons and electrons but larger for phonons. The anharmonic phonon entropy is negative in Pt, but in Pd it changes from positive to negative with increasing temperature.
\r\n \r\nPhonon dispersions in a single crystal of FeGe2 with the C16 structure at 300, 500, and 635 K were measured by INS. Phonon DOS were also measured on polycrystalline FeGe2 from 325 to 1050 K, and the Fe partial DOS was obtained from polycrystalline 57FeGe2 at 300 K using nuclear resonant inelastic X-ray scattering (NRIXS). The dominant feature in the temperature dependence of the phonon spectrum is thermal broadening of high-energy modes. The energy shifts of the low- and high-energy parts of the spectrum were almost the same. DFT calculations performed with the QHA gave results in moderate agreement with the experimental thermal energy shifts, although the isobaric Gr\u00fcneisen parameter calculated from the quasiharmonic model was smaller than that from measurements. The thermal broadening of the phonon spectrum and dispersions, especially at high energies, indicates a cubic anharmonicity to second order that should also induce phonon shifts. There are cancellations of different anharmonic contributions to energy shifts, giving average phonon shifts in moderate agreement to calculations with the QHA. The different parts of the large phonon contribution to the entropy are separated for FeGe2, showing modest but interpretable anharmonic contributions.
\r\n\r\nAll phonons in a single crystal of NaBr were measured by INS at temperatures of 10, 300 and 700 K. Even at 300 K the phonons, especially the longitudinal optical (LO) phonons, showed large shifts in frequencies, and showed large broadenings in energy owing to anharmonicity. The QHA was an unqualified failure for predicting the temperature dependence of phonon frequencies, even at 300K, and it predicted a thermal expansion that was in error by a factor of four. Ab initio computations that included both anharmonicity and quasiharmonicity successfully predicted both the temperature dependence of phonons and the large thermal expansion of NaBr. The frequencies of LO phonon modes decrease significantly with temperature owing to the real part of the phonon self-energy from explicit anharmonicity. The origin of the large cubic anharmonicity was identified with nearest-neighbor Na-Br bonds. Anharmonicity is not a small correction to the QHA predictions of thermal expansion and thermal phonon shifts, but anharmonicity dominates the behavior.
\r\n\r\nNew spectral features were found in phonon dispersions of NaBr at 300 K. Ab initio calculations based on anharmonic perturbation theory also showed these spectral features as \"many-body effects\". Their physical origin is better elucidated with a Langevin model, similar that in recent work in optomechanics. The transverse optic (TO) part of the new features originates from phonon intermodulation between the transverse acoustic (TA) and TO phonons. The LO spectral features originate from three-phonon coupling between the TA modes and the TO lattice modes.
", "doi": "10.7907/e48r-2j94", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11433", "collection": "thesis", "collection_id": "11433", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03272019-174351662", "type": "thesis", "title": "High Temperature Electron-Phonon and Magnon-Phonon Interactions", "author": [ { "family_name": "Yang", "given_name": "Fred Chae-Reem", "orcid": "0000-0002-5615-5170", "clpid": "Yang-Fred-Chae-Reem" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Minnich", "given_name": "Austin J.", "orcid": "0000-0002-9671-9540", "clpid": "Minnich-A-J" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" }, { "family_name": "Schwab", "given_name": "Keith C.", "orcid": "0000-0001-8216-4815", "clpid": "Schwab-K-C" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Computational materials discovery and design has emerged in order to meet the surge in demand for new materials for applications ranging from clean alternative energy to human welfare. This acceleration of materials discovery is exhilarating, but the applications of new advanced materials can be limited by their thermodynamic stability. Accurate calculations of the Gibbs free energy, a measure of thermodynamic stability, require a deep understanding of atomic vibrations, a main source of entropy in materials. This deep understanding of atomic vibrations requires us to treat phonons (quantized lattice vibrations) beyond the harmonic model by considering their interactions with various excitations. In this thesis, I present the effects of high temperature interactions of phonons with electrons and magnetic excitations on the thermodynamics of FeTi, vanadium, and Pd3Fe.
\r\n\r\nA combination of ab initio calculations, inelastic neutron scattering (INS), and nuclear resonant inelastic x-ray scattering (NRIXS) showed an anomalous thermal softening of the M5\u2212 phonon mode in B2-ordered FeTi and a thermal stiffening of the longitudinal acoustic N phonon mode in body-centered-cubic vanadium. Computational investigations involving electronic band unfolding were performed to identify the nesting features on Fermi surfaces crucial to high temperature electron-phonon interactions in FeTi and vanadium. These investigations showed that the Fermi surface of FeTi undergoes a novel thermally driven electronic topological transition (ETT), in which new features of the Fermi surface arise at elevated temperatures. This ETT was also observed in vanadium, but the effects were overtaken by the thermal smearing of the Fermi surface that decreased the rate of electron-phonon scattering.
\r\n\r\nIron phonon partial densities of states of Pd3Fe were measured with NRIXS from room temperature through the Curie transition at 500 K. The experimental results were compared to ab initio spin-polarized calculations that modeled the finite-temperature thermodynamic properties of Pd3Fe with magnetic special quasirandom structures (SQSs) of magnetic moments. The scattering measurements and first-principles calculations showed that the iron partial vibrational entropy is close to what is predicted by the quasiharmonic approximation owing to a cancellation of effects: phonon-phonon and magnon-phonon interactions approximately cancel a ferromagnetic optical phonon stiffening.
", "doi": "10.7907/1KP5-CJ98", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11390", "collection": "thesis", "collection_id": "11390", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02112019-143327096", "primary_object_url": { "basename": "Weadock_Nicholas_2019_Final.pdf", "content": "final", "filesize": 25632235, "license": "other", "mime_type": "application/pdf", "url": "/11390/1/Weadock_Nicholas_2019_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Addressing Thermodynamic Inefficiencies of Hydrogen Storage in Transition Metal Hydrides", "author": [ { "family_name": "Weadock", "given_name": "Nicholas Joseph", "orcid": "0000-0002-1178-7641", "clpid": "Weadock-Nicholas-Joseph" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "See", "given_name": "Kimberly", "orcid": "0000-0002-0133-9693", "clpid": "See-Kimberly" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Transition metal hydrides (MH) are an attractive class of materials for several energy technologies. Primary benefits include their large volumetric storage capacity (often exceeding that of liquid hydrogen) and capability to absorb and desorb hydrogen for hundreds of cycles. In this thesis, we set out to understand two of the thermodynamic inefficiencies of MH: the pressure hysteresis associated with hydrogen absorption and desorption and the corrosion and dissolution of high capacity MH alloys in high pH electrolyte environments.
\r\n\r\nThe volume change associated with hydriding transitions can exceed 10%, and a macroscopic nucleation barrier resulting from coherency strains has been proposed as the origin of the pressure hysteresis. We investigated this hypothesis for the palladium-hydrogen system. The hysteresis and phase transformation characteristics of bulk and nanocrystalline PdH were characterized with coupled in situ X-ray diffraction and pressure composition isotherm measurements. Size effects are observed in the total hydrogen uptake and hydrogen solubility in the hydride phases. Experimentally determined hysteresis energies were found to be comparable to the misfit strain between the Pd and PdH phases and much larger than the energy for dislocation formation. Theoretical predictions of pressure hysteresis overestimate the experimentally measured hysteresis, and we suggest methods of accommodation which could explain the discrepancy. Finally, we propose that an effect of the nucleation barrier is to split the coherent spinodal phase diagram and introduce directionally dependent phase boundaries.
\r\n\r\nWe report a successful development of Ti29V62-xNi9Crx (x = 0, 6, 12) body-centered cubic (BCC) MH electrodes for MH batteries by addressing vanadium corrosion and dissolution in potassium hydroxide electrolytes. The effectiveness of a limited oxygen environment and vanadate ion addition against corrosion are compared to the effects of Cr substitution. By identifying oxygen as the primary source of corrosion and eliminating oxygen with an Ar-purged cell, the Cr-free alloy electrode achieved a maximum capacity of 594 mAh/g, double the capacity of commercial AB5 MH electrodes. With modified coin cells suppressing oxygen evolution, the cycle stability of the Ti29V62Ni9 alloy electrode was greatly improved with either vanadate ion additions to the electrolyte or Cr-substitution in the alloy. Both approaches lead to reversible capacity of 500 mAh/g for 200 cycles.
", "doi": "10.7907/ANY4-VA70", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:9907", "collection": "thesis", "collection_id": "9907", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08292016-233907648", "primary_object_url": { "basename": "Murialdo_Maxwell_FullThesis (Final Complete).pdf", "content": "final", "filesize": 25505593, "license": "other", "mime_type": "application/pdf", "url": "/9907/85/Murialdo_Maxwell_FullThesis (Final Complete).pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Anomalous Thermodynamics of Nonideal Gas Physisorption on Nanostructured Carbons", "author": [ { "family_name": "Murialdo", "given_name": "Maxwell Robert", "clpid": "Murialdo-Maxwell-Robert" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" }, { "family_name": "Faber", "given_name": "Katherine T.", "orcid": "0000-0001-6585-2536", "clpid": "Faber-K-T" }, { "family_name": "Goddard", "given_name": "William A., III", "orcid": "0000-0003-0097-5716", "clpid": "Goddard-W-A-III" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Mesoporous and microporous adsorbents play critical roles in gas storage and separation applications. This thesis describes previously unexplored anomalous thermodynamics in the field of gas physisorption and their impact on energy relevant gases including methane, ethane, krypton and carbon dioxide. Physisorption occurs when an adsorbent induces gas molecules to form a locally densified layer at its surface due to physical interactions. This increases gas storage capacity over pure compression and its efficacy is dependent on the surface area of the adsorbent and the isosteric heat of adsorption. The isosteric heat of adsorption is the molar change in the enthalpy of the adsorptive species upon adsorption and serves as a measure of adsorbent-adsorbate binding strength.
\r\n\r\nUnlike conventional adsorbate-adsorbent systems, which have isosteric heats of adsorption that decrease with surface loading, zeolite-templated carbon is shown to have isosteric heats of methane, ethane and krypton adsorption that increase with surface loading. This is a largely beneficial effect that can enhance gas storage and separation. The unique nanostructure and uniform pore periodicity of the zeolite-templated carbon promote lateral interactions among the adsorbed molecules that cause the isosteric heats of adsorption to increase with loading. These results have been tested and corroborated by developing robust fitting techniques and thermodynamics analyses. The anomalous thermodynamics are shown to result from cooperative adsorbate-adsorbate interactions among the nonideal species and are modeled with an Ising-type model.
\r\n\r\nAs a second theme of this thesis, the study of nonideal gas adsorption has enabled the development of a Generalized Law of Corresponding States for Physisorption. A predictive understanding of high-pressure physisorption on a variety of adsorbents would facilitate the further development of tailored adsorbents and adsorption analysis. Prior attempts at developing a predictive understanding, however, have been hindered by nonideal gas effects.
\r\n\r\nBy approaching physisorption from both empirical and fundamental perspectives, a Generalized Law of Corresponding States for Physisorption was established that accounts for a number of nonideal effects. This new Law of Corresponding States allows one to predict adsorption isotherms for a variety of classical gases from data measured with a single gas. In brief: \"At corresponding conditions on the same adsorbent, classical gases physisorb to the same fractional occupancy.\"
\r\n\r\nCorresponding conditions are met when the reduced variables of each nonideal gas are equivalent, and fractional occupancy gives the fraction of occupied adsorption sites. This Law of Corresponding States for Physisorption is determined using monolayer, BET and Dubinin-Polanyi adsorption theories along with measured adsorption isotherms across a number of conditions and adsorbents. Furthermore, the anomalous cooperative adsorbate-adsorbate interactions discussed in this thesis are shown to be consistent with the Generalized Law of Corresponding States for Physisorption.
\r\n", "doi": "10.7907/Z9GH9FXM", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:6738", "collection": "thesis", "collection_id": "6738", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11232011-221537072", "primary_object_url": { "basename": "Thesis_combo.pdf", "content": "final", "filesize": 4382483, "license": "other", "mime_type": "application/pdf", "url": "/6738/1/Thesis_combo.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Study of the Thermodynamics and Kinetics of Li\u2093FePO\u2084 as a Cathode Material for Li Batteries", "author": [ { "family_name": "Tan", "given_name": "Hongjin", "clpid": "Tan-Hongjin" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" }, { "family_name": "Rossman", "given_name": "George Robert", "orcid": "0000-0002-4571-6884", "clpid": "Rossman-G-R" }, { "family_name": "Jackson", "given_name": "Jennifer M.", "orcid": "0000-0002-8256-6336", "clpid": "Jackson-J-M" }, { "family_name": "Jones", "given_name": "Simon C.", "clpid": "Jones-Simon-C" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Olivine-type LiFePO4 has been recognized as one of the most promising cathode materials for rechargeable Li batteries. Its advantages include high capacity, high stability, nontoxicity, and low cost. Our methods for synthesizing nanocrystalline LixFePO4 with the olivine structure are described. Solid-state reactions and precipitation reactions were both successful, and ball milling was especially effective at reducing crystallite sizes. Diffractometry and microscopy were used to characterize these materials, and results of impurity phases, excess Fe3+, and internal stresses are reported for the different types of synthesis.
\r\n\r\nApplications of lithium-ion batteries, including automotive applications, require fast kinetics and high conductivity of ions and electrons. Unfortunately, LixFePO4 has the electronic structure of an insulator, an entirely unsatisfactory situation if it is to be used as a battery electrode. Electrical conductivity in LixFePO4 occurs by the motion of small polarons, which are valence electrons at Fe atoms plus their distorted local environments. Electrical conductivity of LixFePO4 is interpreted in terms of small polaron hopping. There are other factors of importance in these measurements, such as impurities or defects that block the one-dimensional conduction channels of the olivine structure of LixFePO4.
\r\n\r\nWe studied the polaron hopping directly, which allows us to understand the intrinsic electrical conductivity, and how it depends on microstructure and composition of LixFePO4. The experimental technique was Mossbauer spectrometry, which has been used for many years as a means for determining the fractions of Fe2+ and Fe3+ in a material. Usually the spectral signatures of Fe2+ and Fe3+ are distinct. When valence electrons hop between Fe2+ and Fe3+ at a frequency of 108 Hz or higher, however, the valence changes during the timescale of the Mossbauer measurement and the spectrum is blurred. By measuring Mossbauer spectra at elevated temperatures, we can determine the fractions of Fe atoms participating in polaron hopping, and determine the activation energy of the process. From this we estimate intrinsic electrical conductivities of 10-7S/cm at room temperature for nanocrystalline Li0.5FePO4, for example. We find a comparable conductivity for LixFePO4 prepared as a solid solution, but the conductivity of conventional LixFePO4 is much lower.
\r\n\r\nThere has been much discussion about how surface area might thermodynamically stabilize the solid solution phase of nanocrystalline LixFePO4. In a series of X-ray diffraction measurements, some at elevated temperatures, we found the solid solution phase of LixFePO4 to be especially robust at room temperature when the material was prepared in nanocrystalline form. Moreover, the consistent phase transition temperature around 200\u00b0C was observed, as evidence for the unchanged equilibrium phase diagram by crystallite size. This is consistent with our evaluation on the boundaries of the two-phase mixture of triphylite and heterosite during Li insertion and extraction. Profiles of entropy and enthalpy changes were evaluated by open-circuit voltage measurements. The boundaries were found at x=0.05 and 0.95 in the LixFePO4 with crystal size of 70 nm, similar to the reported values on bulk-LixFePO4. These are important in practice, because electrochemical lithiation and delithiation at room temperature should remain as a two-phase transformation, even if a solid solution of lithium is present in the initial electrode material.
", "doi": "10.7907/NQC1-J605", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:5145", "collection": "thesis", "collection_id": "5145", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12272005-154952", "primary_object_url": { "basename": "Bogdanoff_pd_2002.pdf", "content": "final", "filesize": 4489294, "license": "other", "mime_type": "application/pdf", "url": "/5145/1/Bogdanoff_pd_2002.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Phonon Entropy of Metals and Alloys: The Effects of Thermal and Chemical Disorder", "author": [ { "family_name": "Bogdanoff", "given_name": "Peter David", "clpid": "Bogdanoff-Peter-David" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Goddard", "given_name": "William A., III", "orcid": "0000-0003-0097-5716", "clpid": "Goddard-W-A-III" }, { "family_name": "Haile", "given_name": "Sossina M.", "orcid": "0000-0002-5293-6252", "clpid": "Haile-S-M" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Vibrational entropy is important for the thermodynamics of alloying, alloy formation, phase transitions and phase stability at high temperature. Vibrational entropies of alloying and alloy formation were calculated for 32 binary alloys and intermetallic compounds using phonon DOS curves taken from the literature. The vibrational entropies of formation span a wide range from -0.64 to +0.55 kB/atom, and the vibrational entropies of alloying ranged from -0.39 to +1.0 kB/atom. This range exceeds the range of configurational entropy of a binary alloy, which reaches a maximum value of +0.69 kB/atom and a minimum value of 0 kB/atom.
\r\n\r\nThe vibrational entropy of the NiTi martensitic transition was measured using low-temperature calorimetry and inelastic neutron-scattering. The high-temperature B2 phase of NiTi has a vibrational entropy 0.5 kB/atom larger than the low-temperature martensitic phase. The difference in vibrational entropy accounts for the total entropy of the austenitic-martensitic phase transition.
\r\n\r\nInelastic neutron scattering was used to show that the phonon DOS of V is unchanged between 20 and 1000\u00b0C, inconsistent with the phonon softening expected from thermal expansion. It is found that the effects of volume expansion and rising temperature exert equal and opposite shifts on the phonon DOS. The pure temperature dependence of the phonon DOS is due to strong phonon-phonon scattering, which in turn leads to a large anharmonic vibrational entropy contribution at high temperature.
\r\n\r\nThe vibrational entropy of eight chemically disordered Cu-Au alloys was measured using inelastic neutron scattering. The analysis of the phonon entropy of a disordered alloy was performed in a novel way by modeling the partial vibrational entropies of Cu and Au. The partial vibrational entropies of Cu and Au were shown to be slowly varying and smooth functions of composition. The vibrational entropy of disordering in Cu3Au is calculated as 0.24 \u00b1 0.02 kB/atom, substantially larger than results predicted from recent theoretical work.
", "doi": "10.7907/045G-6426", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:6129", "collection": "thesis", "collection_id": "6129", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10122010-083542958", "type": "thesis", "title": "Interaction of Hydrogen with Novel Carbon Materials", "author": [ { "family_name": "Ye", "given_name": "Yun", "clpid": "Ye-Yun" } ], "thesis_advisor": [ { "family_name": "Cass", "given_name": "Glen Rowan", "clpid": "Cass-G-R" }, { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Ahn", "given_name": "Channing C.", "clpid": "Ahn-C-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Johnson", "given_name": "William Lewis", "clpid": "Johnson-W-L" }, { "family_name": "Bowman", "given_name": "Robert Clark, Jr.", "clpid": "Bowman-Robert-Clark-Jr" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The hydrogen storage properties of sonic carbon materials were studied. Graphite nanofibers (GNF) were synthesized by catalytic decomposion of ethylene and hydrogen. Catalyst supported carbon materials were prepared by impregnation process. Hydrogen desorption and adsorption properties of graphite nanofibers, single-walled carbon nanotubes (SWNT), fullerene materials and catalysts supported carbon materials were measured volumetrically using a Sievert's apparatus. The hydrogen desorption capacity of GNF was typically less than 0.2 wt.%. A phase transition between crystal SWNT and a new hydride phase was found at high pressures at 80K. The phase transition was of first order, and involved the separation of the individual tubes within a rope, exposing a high surface area for hydrogen adsorption. From the change in chemical potential of the hydrogen gas upon adsorption, we were able to calculate the cohesive van der Waals energy between the tubes as 5 mcV/C atom. This is much smaller than expected from previous theoretical work, and shows that defects in the crystal structure cause large suppressions of the cohesive energy. We were able to alter this cohesive energy by changing the state of the material. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt% for the first cycle to a capacity of 4.2 wt% for the fourth cycle. Correspondingly, the surface area increased from 0.9 m^2/gm to 11 m^2/gm and showed a phase transformation, characterized by X-ray powder diffraction. By adding Ni particles onto the sample, the hydrogen storage capacity of fullerite and activated carbon sample was increased. The adsorption of hydrogen on Ni particle can not account for the total increased capacity even by assuming complete coverage of hydrogen molecules on the Ni particle surface.", "doi": "10.7907/z8wn-gs34", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6145", "collection": "thesis", "collection_id": "6145", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10152010-111044693", "primary_object_url": { "basename": "Good_NR_2001.pdf", "content": "final", "filesize": 3000007, "license": "other", "mime_type": "application/pdf", "url": "/6145/1/Good_NR_2001.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Influence of Texture on the Magnetoelastic Properties of Polycrystalline TbDy Alloys", "author": [ { "family_name": "Good", "given_name": "Nathan Ray", "clpid": "Good-Nathan-Ray" } ], "thesis_advisor": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" } ], "thesis_committee": [ { "family_name": "Fultz", "given_name": "Brent T.", "orcid": "0000-0002-6364-8782", "clpid": "Fultz-B-T" }, { "family_name": "Crocker", "given_name": "John C.", "clpid": "Crocker-J-C" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "An investigation into the influence of texture on the magneto elastic properties of cold-rolled polycrystalline terbium-dysprosium alloys has been performed. Significant influence of grain orientations on the thermal expansion, magnetostrictive and magnetomechanical damping properties of TbDy were observed.
\r\n\r\nDrop cast ingots of TbDy alloys were deformed by cold-rolling and annealing with the aim of reorienting grains for large magnetostrictive strains and damping capacities. Thermal expansion coefficients of single crystal and polycrystalline samples of TbDy were measured from 77 - 570 K. These measurements confirmed the expected strong reorientation of the c-axes of grains toward the direction of applied force during deformation. The anisotropy of thermal expansion between the rolling and applied stress directions provided a measure of the effectiveness of various sample preparations for maximizing magnetocrystalline anisotropy. Furthermore, the effects of magnetic phase transitions on thermal expansion through the Curie and N eel points of TbDy revealed thermal expansion anisotropy between grain orientations in the rolling and transverse directions of deformation.\r\nMagnetostrictive strains along the rolling direction of polycrystalline TbDy alloys were measured at 77 K. Saturation magnetostriction of up to 55% of previous single crystal results were observed. Minimal applied stress was required to obtain near maximum magnetostrictions for all of the samples tested. This suggests a preloading mechanism within the grain structure of polycrystalline TbDy not present in single crystals. Also in contrast with single crystal measurements, the performance of more economical commercial purity (99.7%) samples was seen to be somewhat higher than similarly prepared high purity (99.94%) samples. Resistance to deterioration of performance over multiple cycles was observed, as changes in magnetostriction over 150 cycles at 0.1 Hz were within measurement errors. By comparing thermal expansion anisotropy of TbDy samples with peak magnetostrictive strain, a clear proportionality between texture and magnetostrictive performance was established. Deviations from this pattern by samples with more deformation and annealing suggest microstructural mechanisms beyond average grain orientation impacting magnetostriction.
\r\n\r\nMagnetomechanical damping effects were observed for polycrystalline TbDy samples through compression stress-strain curves. Elastic moduli at 77 K was measured to be up to 80% less than at 300 K, with a large hysteresis present in the stress-strain curves of all samples tested below the Curie point. Damping capacity was measured as the stress-strain hysteresis loop divided by the total area under the stress-strain curve. Damping capacities up to 23% were measured for polycrystalline TbDy alloys. Larger magnetomechanical damping was observed at lower strains, with higher strains corresponded to saturation of magnetic domain realignment, smaller damping capacities and larger elastic moduli. Samples with larger magnetostrictions displayed larger damping capacities over a wide range of applied stresses, and also had consistently lower elastic moduli at 77K.
\r\n\r\nMechanisms of damping were investigated by fitting magnetostriction, elastic modulus and estimated strain of damping saturation of TbDy alloys to a model of magnetomechanical energy dissipation. This model relates magnetomechanical damping to magnetic hysteresis, neglecting microstructural influences not present in magnetostriction data. The damping capacities predicted by this model were approximately an order of magnitude higher than experimental results. This result suggests a prominent role of microstructural interactions in the domain realignments responsible for magnetoelastic damping.
", "doi": "10.7907/nh91-2f05", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" } ]