This report presents an Executive Summary of the various elements of the Materials Sciences and Engineering Program which is funded by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Sandia National Laboratories, New Mexico. A general programmatic overview is also presented.
Detailed studies of the properties of ceramic CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO) have clarified the physics of this interesting material and revealed several features not reported before. The dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor, consisting of semiconducting grains and insulating grain boundaries as also concluded by others. The kinetics of the main [low-temperature (T)] relaxation reveal that two different thermally activated processes in CCTO grains control the dynamics. A likely candidate defect responsible for the two processes is the oxygen vacancy which is a double donor. A higher-T relaxation is determined by grain boundary conduction. Both Nb and Fe doping lowered both the apparent dielectric constant {var_epsilon}{prime} and the dielectric loss, but increased Fe doping led to more dramatic effects. At 3 at.% Fe doping, the anomalous {var_epsilon}{prime}(T) response was removed, making the CCTO an intrinsic, very-low-loss dielectric. The intrinsic {var_epsilon}{prime}({approx}75) and its T dependence are measured and shown to be largely determined by a low-lying soft TO phonon. At low T, cubic CCTO transforms into an antiferromagnetic phase at T{sub N} = 25 K. T{sub N} is essentially independent of Nb doping (up to 4 at.%) and of hydrostatic pressure (up to {approx}7 kbar), but decreases significantly with Fe doping. Analysis of the high-T dependence of the magnetic susceptibility provided insight into the role of Fe as a dopant. Finally, an {var_epsilon}{prime}(T) anomaly associated with the onset of antiferromagnetic order has been discovered, providing evidence for coupling between the polarization and sublattice magnetization. The possible origin of this coupling is discussed.
The influences of hydrostatic pressure and biasing electric field on the dielectric properties and phase behavior of a single crystal of the perovskite compound Pb(Sc{sub 0.5}Nb{sub 0.5})O{sub 3}, (PSN) have been investigated. On cooling from high temperatures, the crystal first enters a relaxor (R) state and then spontaneously transforms to a ferroelectric (FE) phase at a temperature, T{sub c}, substantially below the peak temperature, T{sub m}, in the dielectric susceptibility. Based on earlier work on ceramic samples, this behavior suggests substantial chemical (Sc and Nb) disorder at the B sites. Pressure enhances the R state with strong indications that the FE phase should vanish at a pressure somewhat higher than the highest pressure reached in the experiments, making the R state the ground state of the crystal at reduced volume. A significant feature of the temperature (T)-pressure (P) phase diagram is the finding that the T{sub c}(P) phase line should terminate at a pressure between 10 and 15 kbar in a manner akin to a critical point; however, in the case of PSN this feature represents a FE-to-R crossover. Such behavior suggests that a path can be defined that takes the crystal from the FE phase to the R state without crossing a phase boundary. A biasing electric field favors the FE phase over the R state, and the results indicate that the R state vanishes at 5 kV/cm. The magnitudes of both the high T Curie-Weiss constant, C, and the change in entropy (or latent heat) at T{sub c} are found to be comparable to those of simple displacive perovskite oxides such as BaTiO{sub 3} and PbTiO{sub 3}.
Dielectric spectroscopy, lattice structure, and thermal properties have revealed the relaxor dielectric response of Ba-substituted lead zirconate/titanate (PZT) having the composition (Pb0.71Ba0.29) (Zr0.71Ti0.29)O3 and containing 2 at. % Bi as an additive. The relaxor behavior is attributed to the compositional disorder introduced by the substitution of Ba2+ at the A site and Bi3+/5+ at the B site (and possibly A site) of the ABO3 PZT host lattice. Analysis of the results gives clear evidence for the nucleation of polar nanodomains at a temperature much higher than the peak (Tm) in the dielectric susceptibility. These nanodomains grow in size as their correlation length increases with decreasing temperature, and ultimately their dipolar fluctuations slow down below Tm leading to the formation of the relaxor state. The influences of hydrostatic pressure on the dielectric susceptibility and the dynamics of the relaxation of the polar nanodomains were investigated and can be understood in terms of the decrease in the size of the nanodomains with pressure. The influence of dc electrical bias on the susceptibility was also investigated. Physical models of the relaxor response of this material are discussed.
Studies of the dielectric properties and phase behavior of an {sup 18}O-substituted SrTiO{sub 3} (>97% {sup 18}O), or STO-18, crystal at 1 bar and as functions of hydrostatic pressure and applied dc biasing electric field have shed much light on the mechanism of the {sup 18}O-induced ferroelectric transition in this material. Dielectric measurements reveal an equilibrium phase transition (T{sub c} {approx_equal} 24K at 1 bar) and an enhancement of the static dielectric constant {var_epsilon} over that of normal (i.e., {sup 16}O) SrTiO{sub 3}, or STO-16, over a large temperature range above T{sub c}. This enhancement is quantitatively shown to be attributed to additional softening of the ferroelectric soft-mode frequency ({omega}{sub s}) of STO-16, in agreement with lattice dynamic calculations. Thus, in STO-18, two effects due to the heavier mass of {sup 18}O conspire to induce the transition: (i) this additional softening of {omega}{sub s} and (ii) damping of quantum fluctuations. Pressure lowers T{sub c} at the large initial rate of 20 K/kbar and completely suppresses the ferroelectric state leading to a quantum paraelectric state at 0.7 kbar, confirming earlier results. Very large effects of a biasing dc electric fields on the peak temperature and {var_epsilon} are also observed in the quantum regime reflecting the small characteristic energies of the system. The results also reveal a dielectric relaxation process near 10 K with interesting properties. The implications of all the results on our understanding of the physics of STO-18 are discussed.
The ABO3 perovskite oxides constitute an important family of technologically important ferroelectrics whose relatively simple chemical and crystallographic structures have contributed significantly to our understanding of ferroelectricity. They readily undergo structural phase transitions involving both polar and non-polar distortions from the ideal cubic lattice. This paper focuses on the mixed perovskite system KTa1-xNbxO3, or KTN, which has turned out to be a model system. While the end members KTaO3 and KNbO3 might be expected to be similar, in reality they exhibit very different properties. Their mixed crystals, which can be grown over the whole composition range, exhibit a rich set of phenomena whose study has added greatly to our current understanding of the phase trsitions and dielectric properties of these materials. Included among these phenomena are soft mode response, ferroelectric (FE)-to-relaxor (R) crossover, quantum mechanical suppression of the transition, the appearance of a quantum paraelectric state and relaxational effects associated with dipolar impurities. Each of these phenomena is discussed briefly and illustrated. Some emphasis is on the unique role of pressure in elucidating the physics involved.
Studies of the influences of temperature, hydrostatic pressure, dc biasing field and frequency on the dielectric constant ({epsilon}{prime}) and loss (tan {delta}) of single crystal [pb (Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905} (PbTiO{sub 3}){sub 0.095}, or PZN-9.5PT for short, have provided a detailed view of the ferroelectric (FE) response and phase transitions of this technologically important material. While at 1 bar, the crystal exhibits on cooling a cubic-to-tetragonal FE transition followed by a second transition to a rhombohedral phase, pressure induces a FE-to-relaxer crossover, the relaxer phase becoming the ground state at pressures {ge}5 kbar. Analogy with earlier results suggests that this crossover is a common feature of compositionally-disordered soft mode ferroelectrics and can be understood in terms of a decrease in the correlation length among polar domains with increasing pressure. Application of a dc biasing electric field at 1 bar strengthens FE correlations, and can at high pressure re-stabilize the FE response. The pressure-temperature-electric field phase diagram was established. In the absence of dc bias the tetragonal phase vanishes at high pressure, the crystal exhibiting classic relaxor behavior. The dynamics of dipolar motion and the strong deviation from Curie-Weiss behavior of the susceptibility in the high temperature cubic phase are discussed.
Highly crystalline germanium (Ge) nanocrystals in the size range 2--10 nm were grown in inverse micelles and purified and size-separated by high pressure liquid chromatography with on-line optical and electrical diagnostics. The nanocrystals retain the diamond structure of bulk Ge down to at least 2.0 nm (containing about 150 Ge atoms). The background- and impurity-free extinction and photoluminescence (PL) spectra of these nanocrystals revealed rich structure which was interpreted in terms of the bandstructure of Ge shifted to higher energies by quantum confinement. The shifts ranged from {minus}0.1 eV to over 1 eV for the various transitions. PL in the range 350--700 nm was observed from nanocrystals 2--5 nm in size. The 2.0 nm nanocrystals yielded the most intense PL (at 420 nm) which is believed to be intrinsic and attributed to direct recombination at {Gamma}. Excitation at high energy (250 nm) populates most of the conduction bands resulting in competing recombination channels and the observed broad PL spectra.
Results on a variety of mixed ABO{sub 3} oxides have revealed a pressure-induced ferroelectric-to-relaxor crossover and the continuous evolution of the energetics and dynamics of the relaxation process with increasing pressure. These common features have suggested a mechanism for the crossover phenomenon in terms of a large decrease in the correlation length for dipolar interactions with pressure--a unique property of soft mode or highly polarizable host lattices. The pressure effects as well as the interplay between pressure and dc biasing fields are illustrated for some recent results on PZN-9.5 PT,PMN and PLZT 6/65/35.
Pressure studies have provided new insights into the physics of compositionally-disordered ABO{sub 3} oxide relaxors. Specifically, results will be presented and discussed on a pressure-induced ferroelectric-to-relaxer crossover phenomenon, the continuous evolution of the energetic and dynamics of the relaxation process, and the interplay between pressure and electric field in determining the dielectric response.
A pressure-induced crossover from normal Ferroelectric-to-Relaxer behavior has been observed in single crystal [Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905}(PbTiO{sub 3}){sub 0.0095}, or PZN - 9.5% PT. Analogy with similar observations for other perovskites indicates that this crossover is a general feature of compositionally-disordered soft mode ferroelectrics. The Pressure-Temperature phase diagram has been also determined.
Pressure studies have provided new insights into the physics of compositionally-disordered ABO{sub 3} oxide relaxors. Specifically results are presented and discussed on a pressure-induced ferroelectric-to-relaxor crossover phenomenon, the continuous evolution of the energetics and dynamics of the relaxation process, and the interplay between pressure and electric field in determining the dielectric response.
It is shown that lattice disorder induced by Nb and Ca substitution has a strong influence on the dielectric and relaxational properties of KTaO{sub 3}. Both substituents are believed to occupy off-center positions at the Ta site, and the difference in valence between the Ca{sup 2+} and Ta{sup 5+} ions leads to the formation of oxygen vacancies (V{sub 0}). Specifically, for a KTa{sub 1{minus}x}Nb{sub x}O{sub 3}:Ca crystal with x = 0.023 and with a 0.055 at.% Ca doping they observe: (1) a ferroelectric transition at atmospheric pressure (1 bar); (2) a large enhancement of the transition temperature by Ca doping; (3) a pressure-induced crossover from ferroelectric-to-relaxor behavior; (4) the impending vanishing of the relaxor phase at high pressure; (5) the reorientation of the Ca-oxygen vacancy (Ca:V{sub 0}) pair defect; and (6) the variation of the energetics and dynamics of this reorientation with pressure. Most of these effects are associated with Nb- and Ca-induced dipolar entities and appear to be general features of soft mode ferroelectrics with random-site polar nanodomains. The ferroelectric-to-relaxor crossover can be understood in terms of a large decrease with pressure in the correlation length among polar nanodomains--a unique property of soft ferroelectric mode systems.
Our studies of the shock compression response of PVDF polymer are continuing in order to understand the physical properties under shock loading and to develop high fidelity, reproducible, time-resolved dynamic stress gauges. New PVDF technology, new electrode configurations and piezoelectric analysis have resulted in enhanced precision gauges. Our new standard gauges have a precision of better than 1% in electric charge release under shock up to 15 GPa. The piezoelectric response of shock compressed PVDF gauges 1 mm{sup 2} in active area has been studied and yielded well-behaved reproducible data up to 20 GPa. Analysis of the response of these gauges in the {open_quotes}thin mode regime{close_quotes} using a Lagrangian hydrocode will be presented. P(VDF-TrFE) copolymers exhibit unique piezoelectric properties over a wide range of temperature depending on the composition. Their properties and phase transitions are being investigated. Emphasis of the presentation will be on key results and implications.
The BES Materials Sciences Program has the central theme of Scientifically Tailored Materials. The major objective of this program is to combine Sandia`s expertise and capabilities in the areas of solid state sciences, advanced atomic-level diagnostics and materials synthesis and processing science to produce new classes of tailored materials as well as to enhance the properties of existing materials for US energy applications and for critical defense needs. Current core research in this program includes the physics and chemistry of ceramics synthesis and processing, the use of energetic particles for the synthesis and study of materials, tailored surfaces and interfaces for materials applications, chemical vapor deposition sciences, artificially-structured semiconductor materials science, advanced growth techniques for improved semiconductor structures, transport in unconventional solids, atomic-level science of interfacial adhesion, high-temperature superconductors, and the synthesis and processing of nano-size clusters for energy applications. In addition, the program includes the following three smaller efforts initiated in the past two years: (1) Wetting and Flow of Liquid Metals and Amorphous Ceramics at Solid Interfaces, (2) Field-Structured Anisotropic Composites, and (3) Composition-Modulated Semiconductor Structures for Photovoltaic and Optical Technologies. The latter is a joint effort with the National Renewable Energy Laboratory. Separate summaries are given of individual research areas.
This report provides an Executive Summary of the various elements of the Materials Sciences Program which is funded by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy at Sandia National Laboratories, New Mexico.
This publication is designed to inform present and potential customers and partners of the DOE Center of Excellence for the Synthesis and Processing of Advanced Materials about significant advances resulting from Center-coordinated research. The format is an easy-to-read, not highly technical, concise presentation of the accomplishments. Selected accomplishments from each of the Center`s seven initial focused projects are presented. The seven projects are: (1) conventional and superplastic forming; (2) materials joining; (3) nanoscale materials for energy applications; (4) microstructural engineering with polymers; (5) tailored microstructures in hard magnets; (6) processing for surface hardness; and (7) mechanically reliable surface oxides for high-temperature corrosion resistance.
This document is a compilation of the Field Work Proposals (FWP) for the DOE BES Materials Sciences Program. The program is directed toward Scientifically Tailored Materials, specifically for energy applications.
Effects of frequency, temperature and hydrostatic pressure on dielectric properties, molecular relaxations, and phase transitions of PVDF and a copolymer with 30 mol % trifluoroethylene are discussed. Pressure causes large slowing down of the {beta} molecular relaxations as well as large increases in the, ferroelectric transition temperatures and melting points, but the magnitudes of the effects are different for the different transitions. These effects can be understood in terms of pressure-induced hindrance of the molecular motions and/or reorientations. A unique application of these polymers as time-resolved dynamic stress gauges based on PVDF studies under very high pressure shock compression is discussed.
This report is divided into: budget, capital equipment requests, general programmatic overview and institutional issues, DOE center of excellence for synthesis and processing of advanced materials, industrial interactions and technology transfer, and research program summaries (new proposals, existing programs). Ceramics, semiconductors, superconductors, interfaces, CVD, tailored surfaces, adhesion, growth and epitaxy, boron-rich solids, nanoclusters, etc. are covered.
Measurements of the effects of pressure on the thermal electron emission rate and capture cross section for a variety of deep electronic levels in GaAs, GaP and their alloys have yielded the pressure dependences of the energies of these levels in the bandgaps, allowed evaluation of the breathing mode lattice relaxations accompanying carrier emission or capture by these levels and revealed trends which lead to new insights into the nature of the responsible defects. Emphasis is on deep levels believed to be associated with simple defects. Specifically, results will be summarized for the donor levels of the dominant native defect known as EL2 in CAM, which is believed to be associated with the arsenic antisite, and on the radiation-induced El and E2 levels in GaAs, GaP and their alloys, which are believed to be due to arsenic (or phosphorous) vacancies. The results are discussed in terms of models for the defects responsible for these deep levels.
Research programs from Sandia Laboratory in Materials Science are briefly presented. Significant accomplishments include: preparation of Tl superconductors under equilibrium conditions, development of force-feedback sensor for interfacial force microscope, predictive model of hydrogen interactions in silicon dioxide on silicon, layer-by-layer sputtering of Si (001), oscillatory As{sub 4} surface reaction rates during molecular beam epitaxy of AlAs, GaAs and InAs, the effects of interfacial strain on the band offsets of lattice matched III-V semiconductor, a new mechanism for surface diffusion, solid solution effects in Tl-containing superconductors, record high superconducting transitions for organic materials, atomic vibrations in boron carbides and a method for studying radical/surface reactions in chemical vapor deposition (CVD).
The BES Materials Science program at Sandia Albuquerque has the central theme of Scientifically Tailored Materials. The major objective of this program is to combine Sandia's expertise and capabilities in the areas of solid state sciences, advanced atomic-level diagnostics and materials-processing science to produce new classes of tailorable materials for the US energy industry, the electronics industry and for defense needs. Current research in this program includes the physics and chemistry of ceramics, the use of energetic particles for the synthesis and study of materials, high-temperature and organic superconductors, tailored surfaces for materials applications, chemical vapor deposition sciences, strained-layer semiconductors, advanced growth techniques for improved semiconductor structures and boron-rich very high temperature semiconductors. A new start just getting underway deals with the atomic level science of interfacial adhesion. Our interdisciplinary program utilizes a broad array of sophisticated, state-of-the-art experimental capabilities provided by other programs. The major capabilities include several molecular-beam epitaxy and chemical-vapor-deposition facilities, electron- and ion-beam accelerators, laser-based diagnostics, advanced surface spectroscopies, unique combined high-pressure/low-temperature/high-magnetic-field facilities, and the soon to be added scanning tunneling and atomic force microscopies.
The goals of our Basic Energy Sciences (BES) Materials Science Program at Sandia are: (1) Perform basic, forefront interdisciplinary research using the capabilities of several organizations. (2) Choose programs broadly complementary to Sandia's weapons laboratory mission, but separably identifiable. (3) Perform research in a setting which enhances technological impact because of Sandia's spectrum of basic research, applied research and development engineering. (4) Use large, capital-intensive research facilities not usually found at universities. The BES Materials Science program at Sandia Albuquerque has the central theme of Scientifically Tailored Materials. The major objective of this program is to combine Sandia's expertise and capabilities in the areas of solid state sciences, advanced atomic-level diagnostics, and materials-processing science to produce new classes of tailorable materials for the US energy industry, the electronics industry and for defense needs. Current research in this program includes ion-implantation-modified materials, physics and chemistry of ceramics, tailored surfaces for materials applications, strained-layer semiconductors, chemical vapor deposition, surface photo kinetics, organic and high-temperature superconductors, advanced growth techniques for improved semiconductor structures and boron-rich very high temperature semiconductors.
The hydrostatic pressure dependence of the /beta/ molecular relaxation process of polyvinylidene fluoride (PVDF) has been investigated to 20 kbar. This relaxation is known to have a strong influence on the electrical and mechanical properties of PVDF. The observed large slowing down of the relaxation process is discussed in terms of the Vogel/endash/Fulcher equation. There is an increase in both the energy barrier to dipolar motion and the reference temperature (T/sub 0/) for the kinetic relaxation process which represents the ''static'' dipolar freezing temperature for the process.
