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Calorimetric Studies of the Energetics of Order-Disorder in the System Mg(1-x)Fe(x)Ca(CO(3))(2)

Description: Calorimetric studies by Chai and Navrotsky (1996) on dolomite-ankerite energetic have been extended by including two additional types of samples: a very disordered stoichiometric MgCa(CO{sub 3}){sub 2} prepared from low temperature aqueous solution and three largely ordered natural samples of intermediate iron content. Combining these data with previous work, three distinct trends of energetic can be seen: those for samples with nearly complete order, nearly complete disorder, and intermediate order. From these trends, the enthalpy of complete disordering is estimated to be 33 {+-} 6 kJ/mol for MgCa(CO{sub 3}){sub 2} and 18 {+-} 5 kJ/mol for FeCa(CO{sub 3}){sub 2}.
Date: February 10, 1999
Creator: Brady, P.; Dooley, D.; Navrotsky, A. & Reeder, R.
Partner: UNT Libraries Government Documents Department

Disorder-driven nonequilibrium melting studied by electron diffraction, brillouis scattering, and molecular dynamics

Description: In the present paper, a brief overview of the electron diffraction, Brillouin scattering and molecular dynamics studies of radiation-induced amorphization of ordered intermetallic compounds is presented. In these studies, measured changes in the velocity of surface acoustic phonons, lattice constant, and the Bragg-Williams long-range order parameter induced by irradiation were compared with the results of computer simulations of defect-induced amorphization. The results indicate that progressive chemical disordering of the superlattice structure during irradiation is accompanied by an expansion of the lattice and a large change in sound velocity corresponding to a {approximately} 50% decrease in the average shear modulus. The onset of amorphization occurs when the average shear modulus of the crystalline compound becomes equal to that of the amorphous phase. This elastic softening criterion for the onset of amorphization and the dependence of the average shear modulus on the long-range-order parameter are in excellent agreement with molecular dynamics simulations. Both the experimental observations and computer simulations confirm the predictions of the generalized Lindemann melting criterion which stipulates that thermodynamic melting of a defective crystal occurs when the sum of the dynamic and static mean-square atomic displacements reaches a critical value identical to that for melting of the defect-free crystal. In this broader view of melting, the crystal-to-glass transformation is a disorder-driven nonequilibrium melting process occurring at temperatures below the Kauzmann isentropic glass-transition temperature.
Date: December 21, 1999
Creator: Okamoto, P. R.; Lam, N. Q. & Grimsditch, M.
Partner: UNT Libraries Government Documents Department

Thermodynamics of paracrystalline silicon

Description: Fluctuation microscopy experiments have shown that the as-deposited structure of amorphous silicon thin films is paracrystalline. A paracrystal consists of small (< 3 nm in diameter) topologically crystalline grains separated by a disordered matrix. Here the authors consider the thermodynamics of paracrystalline silicon as a function of the grain size and the temperature. They offer a simple model that qualitatively explains the observed metastability of the ordered structure at low temperature (300 K), the relaxation towards a more disordered structure at intermediate temperatures (600 K), and the recrystallization at high temperatures (1,000 K).
Date: May 9, 2000
Creator: Voyles, P. M.; Treacy, M. M. J. & Gibson, J. M.
Partner: UNT Libraries Government Documents Department

Experimental methods and data analysis for fluctuation microscopy

Description: The authors have developed a new electron microscopy technique called fluctuation microscopy which is sensitive to medium-range order in disordered materials. The technique relies on quantitative statistical analysis of low-resolution dark-field electron micrographs. Extracting useful information from such micrographs involves correcting for the effects of the imaging system, incoherent image contrast caused by large scale structure in the sample, and the effects of the foil thickness.
Date: February 16, 2000
Creator: Voyles, P. M.; Treacy, M. M. J.; Gibson, J. M.; Jin, H.-C. & Abelson, J. R.
Partner: UNT Libraries Government Documents Department

Order-disorder behavior in KNbO{sub 3} and KNbO{sub 3}/KTaO{sub 3} solid solutions and superlattices by molecular-dynamics simulations.

Description: We use molecular-dynamics simulation to examine the order-disorder behavior in pure ferroelectric KNbO{sub 3} and in KNbO{sub 3}-KTaO{sub 3} ferroelectric-paraelectric solid solutions and superlattices. We find that the order-disorder behavior is remarkably robust and plays an important role in both the polarization rotation associated with switching of the perfect crystal and in the dynamical behavior of the solid solutions and superlattices.
Date: February 18, 2002
Creator: Phillpot, S. R.; Sepliarsky, M.; Streiffer, S. K.; Stachiotti, M. G. & Migoni, R. L.
Partner: UNT Libraries Government Documents Department

Magnetic Compton scattering studies of the Invar alloy Fe{sub 3}Pt

Description: The magnetic Compton profiles (MCP) for both ordered and disordered Fe{sub 3}Pt samples have been measured above and below their Curie temperature. These measurements show that the average moment per atom at room temperature is 2.8{sub {mu}{sub B}} {plus_minus} 0.1{sub {mu}{sub B}} for disordered Fe{sub 3}Pt and 1.8{sub {mu}{sub B}} {plus_minus} 0.1{sub {mu}{sub B}} for ordered Fe{sub 3}Pt., At temperatures above {Tc}, we measured a substantial reduction in the moment (0.6{sub {mu}{sub B}} {plus_minus} 0.1{sub {mu}{sub B}} for disordered Fe{sub 3}Pt and 0.6{sub {mu}{sub B}} {plus_minus} 0.1{sub {mu}{sub B}} for ordered Fe{sub 3}Pt) and a change in the shape of the MCP. These results indicate a decrease of the d-like moment on the Fe atoms in the disordered phase. The MCP for the ordered phase shows a change in the total moment, yet the momentum distribution is substantially different and cannot be described by this analysis.
Date: February 1, 1995
Creator: Yahnke, C.J.; Srajer, G.; Haeffner, D.R.; Mills, D.M. & Assoufid, L.
Partner: UNT Libraries Government Documents Department

Spinodal Decomposition and Order-Disorder Transformation in a Water-Quenched U-6wt%Nb Alloy

Description: A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis has been employed to study phase stability and aging mechanisms of a water-quenched U-6wt%Nb (WQU6Nb) alloy subjected to different aging schedules that include artificial aging of WQ-U6Nb at 200 C, natural aging of WQ-U6Nb at ambient temperatures for 15 to18 years, and accelerative aging of the naturally aged (NA) alloy at 200 C. During the early stages of artificial aging at 200 C, the microhardness values continuously increase as a result of the development of a fine-scale compositional modulation (wavelength: 3 nm) caused by spinodal decomposition. Coarsening of the modulated structure occurs after prolonged aging of WQ-U6Nb at 200 C for 16 hours, which leads to a decrease of microhardness. Phase instability has also been found to occur in the NA alloy, in which the formation of partially ordered phase domains resulting from an atomic-scale spinodal modulation (wavelength: 0.5 nm) renders the appearance of antiphase domain boundaries (APBs) in TEM images. Although 18-year natural aging does not cause a significant change in hardness, it affects dramatically the aging mechanism of WQ-U6Nb subjected to the accelerative aging at 200 C. The result of microhardness measurement shows that the hardness values continuously increase until after aging for 239 hours, and the total hardness increment is twice in magnitude than that in the case of the artificial aging of water-quenched alloy at 200 C. The anomalous increment of hardness for the accelerative aging of NA alloy can be attributed to the precipitation of an ordered U{sub 3}Nb phase. It is accordingly concluded that the long-term natural aging at ambient temperatures can detour the transformation pathway of WQ U-6Nb alloy; it leads to the order-disorder transformation and precipitation of ordered phase in the alloy.
Date: September 12, 2006
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department

Halide electroadsorption on single crystal surfaces

Description: The structure and phase behavior of halides have been investigated on single crystals of Ag and Au using synchrotron x-ray scattering techniques. The adlayer coverages are potential dependent. For all halides studied the authors found that with increasing potential, at a critical potential, a disordered adlayer transforms into an ordered structure. Often these ordered phases are incommensurate and exhibit potential-dependent lateral separations (electrocompression). The authors have analyzed the electrocompression in terms of a model which includes lateral interactions and partial charge. A continuous compression is not observed for Br on Ag(100). Rather, they find that the adsorption is site-specific (lattice gas) in both the ordered and disordered phases. The coverage increases with increasing potential and at a critical potential the disordered phase transforms to a well-ordered commensurate structure.
Date: July 1, 1997
Creator: Ocko, B.M. & Wandlowski, T.
Partner: UNT Libraries Government Documents Department

Pressure as a probe of the glassy state of ferroelectrics with random site disorder

Description: Results on lanthanum-modified lead zirconate-titanates (PLZTs) have revealed a pressure-induced crossover from a normal ferroelectric to a relaxor state and the continuous evolution of the dynamics and energetics of the relaxation process. This crossover appears to be a general feature of soft mode ferroelectrics with random site dipolar impurities or polar nanodomains and results from a large decrease in the correlation radius among polar nanodomains -- a unique property of soft mode ferroelectrics.
Date: March 10, 1998
Creator: Samara, G.A. & Hansen, L.V.
Partner: UNT Libraries Government Documents Department

Particle-induced amorphization of complex ceramics. Final report

Description: The crystalline-to-amorphous (c-a) phase transition is of fundamental importance. Particle irradiations provide an important, highly controlled means of investigating this phase transformation and the structure of the amorphous state. The interaction of heavy-particles with ceramics is complex because these materials have a wide range of structure types, complex compositions, and because chemical bonding is variable. Radiation damage and annealing can produce diverse results, but most commonly, single crystals become aperiodic or break down into a polycrystalline aggregate. The authors continued the studies of the transition from the periodic-to-aperiodic state in natural materials that have been damaged by {alpha}-recoil nuclei in the uranium and thorium decay series and in synthetic, analogous structures. The transition from the periodic to aperiodic state was followed by detailed x-ray diffraction analysis, in-situ irradiation/transmission electron microscopy, high resolution transmission electron microscopy, extended x-ray absorption fine structure spectroscopy/x-ray absorption near edge spectroscopy and other spectroscopic techniques. These studies were completed in conjunction with bulk irradiations that can be completed at Los Alamos National Laboratory or Sandia National Laboratories. Principal questions addressed in this research program included: (1) What is the process at the atomic level by which a ceramic material is transformed into a disordered or aperiodic state? (2) What are the controlling effects of structural topology, bond-type, dose rate, and irradiation temperature on the final state of the irradiated material? (3) What is the structure of the damaged material? (4) What are the mechanisms and kinetics for the annealing of interstitial and aggregate defects in these irradiated ceramic materials? (5) What general criteria may be applied to the prediction of amorphization in complex ceramics?
Date: August 1, 1998
Creator: Ewing, R. C. & Wang, L. M.
Partner: UNT Libraries Government Documents Department

Silicon carbide amorphization by electron irradiation

Description: Observations made more than ten years ago showed that SiC could be made amorphous at cryogenic temperatures by in-situ 300kV electron irradiation. However, high voltage electron microscope (HVEM) results indicate a threshold voltage of 725 kV for amorphization of SiC at 140 K. In addition, a recent review exposes the considerable uncertainty in the literature regarding displacement energies for SiC. Therefore, further experiments have been performed in a Philips CM30 (LaB{sub 6} cathode) with a Gatan double-tilt cooling holder in an attempt to determine the threshold voltage for amorphization at {approximately} 140 K. Sintered {alpha}-SiC (defected 6H polytype), beam direction B = <11{bar 2}0>, and probes containing {approximately} 75 nA in {approximately} 0.5 {micro}m, were used. Amorphization occurred in <10 min at 300 kV and after {approximately} 60 min at 180 kV; visible darkening occurred at lower voltages and doses. Similar behavior occurred for B = [0001]. The critical dose for amorphization was measured as a function of accelerating voltage. Probe current profiles were measured by post-specimen scanning (CM30 SCIM mode with 100 {micro}m diameter Gatan STEM detector) images of the focused probes positioned in a hole, and probe currents were measured from the exposure time, which had previously been calibrated with a Faraday cup.
Date: February 1, 1998
Creator: Bentley, J.
Partner: UNT Libraries Government Documents Department

The effect of disorder on the critical points in the vortex phase diagram of YBCO

Description: The effect of line disorder induced by heavy ion irradiation and of point disorder induced by proton and electron irradiation on the upper and lower critical points in the vortex phase diagram of YBCO is presented. The authors find that dilute line disorder induces a Bose glass transition at low fields which is replaced at the lower critical point by first order melting at higher fields. Strong pinning point defects raise the lower critical point, while weak pinning point defects have little or no effect on the lower critical point. The upper critical point is lowered by point disorder, but raised by line disorder. First order melting is suppressed by point disorder in two ways, by lowering of the upper critical point only for weak point pins, or by merging of the upper and lower critical points for strong point pins. The differing responses of the upper and lower critical points to line and point disorder can be understood in a picture of transverse and longitudinal spatial fluctuations.
Date: January 19, 2000
Creator: Crabtree, G. W.; Kwok, W. K.; Paulius, L. M.; Petrean, A. M.; Olsson, R. J.; Karapetrov, G. et al.
Partner: UNT Libraries Government Documents Department

First-principles approaches to materials stability

Description: Parameter-free electronic structure approaches are now being used to predict chemical order, and to a lesser extent, structural transformations in multi-component alloys, as a function of temperature, concentration and pressure. The underlying state-of-the-art framework will be briefly reviewed, and applications to specific aspects of the statics and kinetics of alloy transformations will be discussed. Finally special emphasis will be put on the relations between stability and mechanical properties in substitutional alloys with examples pertaining to the energetics of antiphase boundaries and interfaces.
Date: December 1, 1994
Creator: Turchi, P.E.A.
Partner: UNT Libraries Government Documents Department

Pressure-induced amorphizations of silica analogues: A probe of the relationship between order and disorder. Final report, August 1, 1993--July 31, 1996

Description: Efforts of this research were to determine the relationship between order and disorder in amorphous materials using high pressure techniques. Specifically, the experiments utilized high pressure x-ray diffraction, electron transmission microscopy, and Raman scattering.
Date: August 1, 1997
Creator: Hammack, W.S.
Partner: UNT Libraries Government Documents Department

Structural ground-state of La{sub 2}CuO{sub 4} in the LTO phase: Evidence of local disorder

Description: We present neutron scattering data which indicate that the local structure of La{sub 2}CuO{sub 4} in the LTO phase is disordered. Data from two different diffractometers are compared and systematic differences between the pair distribution functions (PDF) of the LTO model and the data are reproduced in both cases.
Date: June 1, 1994
Creator: Billinge, S. J. L.; Kwei, G. H. & Takagi, H.
Partner: UNT Libraries Government Documents Department

Microstructure and Mechanical Instability of Water-Quenched U-6wt% Nb Alloy Affected by Long-Term Aging

Description: A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased and no age softening was found after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation pathway of WQ-U6Nb, and it leads to order-disorder transformation, precipitation hardening, and ductility embrittlement of WQ-U6Nb alloy.
Date: December 6, 2005
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department

Spinodal Ordering and Precipitation in U-6 wt% Nb

Description: A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQU6Nb) alloy subjected to different aging schedules including artificial aging at 200 C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200 C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200 C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200 C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200 C. Microhardness values of the NA alloy continuously increased after isothermal aging at 200 C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted {alpha} phase and Nb-enriched U{sub 3}Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation pathway of WQ-U6Nb, and it leads to order-disorder transformation and precipitation hardening of WQ-U6Nb alloy.
Date: December 19, 2005
Creator: Hsiung, L & Zhou, J
Partner: UNT Libraries Government Documents Department

Report on 1992/1993 activities

Description: Over the past several years, at least four different techniques have been developed for probing the behavior of homopolymers and block copolymers at surfaces and interfaces. These include neutron reflectivity, grazing incidence x-ray scattering, dynamic secondan, ion mass spectrometry and electron microscopy. In the upcoming year It is proposed to make key uses of these developments to gain a quantitative understanding of the Interfacial behavior of homopolymers and block copolymers. In addition, the newly discovered lower disorder to order phase transition will be investigated further, is the role a surface or an interface plays in influencing the structure and segmental distribution of polymers in the vicinity of an interface. These areas are outlined below.
Date: July 1, 1996
Creator: Russell, T.P.
Partner: UNT Libraries Government Documents Department

Phonon effects in the two-magnon Raman scattering in spin-Peierls systems

Description: Recently discovered spin-Peierls (SP) transition in the compound CuGeO{sub 3} revived the interest in theoretical studies of low dimensional spin systems and lattice spin coupling effects. Special attention has been paid to the frustration effects and their consequence on the dynamical properties. In fact, it was pointed out earlier by some of the preset authors that there are frustrated SP fluctuations in the MX materials near the crossover region. In the study of the excitations in the CuGeO{sub 3} compound, several groups have conducted two-magnon Raman scattering experiments. In addition, several theoretical studies were carried out which, however, neglected the phonon effects totally. In the present study, the authors show that the phonon effects are important in the two-magnon Raman scattering. The phonon effects enter the Raman scattering process in two ways. First, there is a strong spin phonon coupling; naturally, the photon-magnon coupling has a phonon term, which has strength {partial_derivative}J/{partial_derivative}u, where u is lattice distortion. Second, without frustrated spin-spin coupling J{sub 2}, there is no two-magnon Raman scattering in the high temperature uniform phase, as the scattering term commutes with the Hamiltonian. However, since the lattice fluctuations (quantum or thermal) will introduce disorder in the spin-spin coupling, there is phonon induced two-magnon Raman scattering even without J{sub 2}.
Date: October 1, 1996
Creator: Zang, J.; Roeder, H.; Gammel, J.T.; Saxena, A. & Bishop, A.R.
Partner: UNT Libraries Government Documents Department

Glassy dynamics of driven elastic manifolds

Description: We study the low-temperature dynamics of an elastic manifold driven through a random medium. For driving forces well below the zero- temperature depinning force, the manifold advances via thermally activated hops over the energy barriers separating favorable metastable states. We develop a scaling theory of the thermally activated dynamics (creep) and find a nonlinear glassy response for the driven manifold, {upsilon}{approximately}exp(-const{times}F{sup - {mu}}). We consider an exactly solvable 1-D model for random driven dynamics which exhibits a creep-like velocity-force characteristic. We discuss a microscopic mechanism for the creep motion and show that the distribution of waiting times for the hopping processes scales as a power law. This power-law distribution naturally yields an exponential response for the creep of the manifold.
Date: December 31, 1996
Creator: Vinokur, V.M.
Partner: UNT Libraries Government Documents Department

Electronic structure and phase equilibria in ternary substitutional alloys

Description: A reliable, consistent scheme to study phase equilibria in ternary substitutional alloys based on the tight-binding approximation is presented. With electronic parameters from linear muffin-tin orbital calculations, the computed density of states and band structures compare well with those from more accurate {ital ab}{ital initio} calculations. Disordered alloys are studied within the tight-binding coherent-potential approximation extended to alloys; energetics of ordered systems are obtained through effective pair interactions computed with the general perturbation method; and partially ordered alloys are studied with a novel simplification of the molecular coherent-potential approximation combined with the general perturbation method. The formalism is applied to bcc-based Zr-Ru-Pd alloys which are promising candidates for medical implant devices. Using energetics obtained from the above scheme, we apply the cluster- variation method to study phase equilibria for particular pseudo- binary alloys and show that results are consistent with observed behavior of electronic specific heat coefficient with composition for Zr{sub 0.5}(Ru, Pd){sub 0.5}.
Date: April 26, 1996
Creator: Traiber, A.J.S.; Allen, S.M.; Turchi, P.E.A. & Waterstrat, R.M.
Partner: UNT Libraries Government Documents Department

J{sub c} and vortex pinning enhancements in Bi-, Tl-, and Hg-based cuprate superconductors via GeV proton irradiation

Description: Randomly oriented columnar defects provide an efficient mechanism for stabilizing the magnetic flux and current density in a variety of high-{Tc} superconductors. Protons with 0.8 GeV energy cause heavy constituent nuclei (Hg, Tl, Pb, Bi,...) to fission and the resultant fragments generate isotropically distributed columnar tracks. The main results are a significant enhancement of the persistent current density J, a shift of the irreversibility line towards higher fields and temperatures, and a marked reduction in the rate of current decay with time. With increasing crystalline disorder, {Tc} decreases at the rate of {approximately} 0.1--1 K per 10{sup 16} proton/cm{sup 2}. The optimal proton fluence lies in the range of (5--20) x 10{sup 16} p/cm{sup 2}. At these levels, J is enhanced by one or more orders of magnitude compared with unirradiated virgin materials and can be significantly large at T {ge} 100 K in tesla fields. In addition, the logarithmic decay rate dln(J/dln(t)) is diminished. By analyzing the decay rate of J with time in a Maley framework, the authors obtain the effective pinning energy U(J) of irradiated materials for comparison with the virgin superconductors.
Date: December 1, 1997
Creator: Thompson, J.R.; Ossandon, J.G.; Krusin-Elbaum, L.; Song, K.J.; Christen, D.K.; Paranthaman, M. et al.
Partner: UNT Libraries Government Documents Department