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Discontinuous Thermal Expansions and Phase Transformations in Crystals at Higher Temperatures

Description: The purpose of this investigation is to make more detailed studies of transformations. Fourteen compounds have been examined by high temperature X-ray diffraction for this purpose. The investigations have been carried out in such a way as to reveal: 1. the existence of transformations, 2. the influence of polarizability on thermal expansion, 3. the anisotropy of expansion, and 4. the discontinuity of thermal expansion.
Date: 1967
Creator: Hsu, Yuan Tsun
Partner: UNT Libraries

Phase transformation of poled "chem-prep" PZT 95/5-2Nb ceramic under quasi-static loading conditions.

Description: Specimens of poled 'chem-prep' PNZT ceramic from batch HF803 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions at three temperatures of -55, 25, and 75 C and pressures up to 500 MPa. The objective of this experimental study was to obtain the electro-mechanical properties of the ceramic and the criteria of FE (Ferroelectric) to AFE (Antiferroelectric) phase transformations so that grain-scale modeling efforts can develop and test models and codes using realistic parameters. The poled ceramic undergoes anisotropic deformation during the transition from a FE to an AFE structure. The lateral strain measured parallel to the poling direction was typically 35 % greater than the strain measured perpendicular to the poling direction. The rates of increase in the phase transformation pressures per temperature changes were practically identical for both unpoled and poled PNZT HF803 specimens. We observed that the retarding effect of temperature on the kinetics of phase transformation appears to be analogous to the effect of shear stress. We also observed that the FE-to-AFE phase transformation occurs in poled ceramic when the normal compressive stress, acting perpendicular to a crystallographic plane about the polar axis, equals the hydrostatic pressure at which the transformation otherwise takes place.
Date: October 1, 2004
Creator: Lee, Moo Yul; Montgomery, Stephen Tedford & Hofer, John H.
Partner: UNT Libraries Government Documents Department

Development of a carburizing and quenching simulation tool: A material model for low carbon steels undergoing phase transformations

Description: An internal state variable formulation for phase transforming alloy steels is presented. We have illustrated how local transformation plasticity can be accommodated by an appropriate choice for the corresponding internal stress field acting between the phases. The state variable framework compares well with a numerical micromechanical calculation providing a discrete dependence of microscopic plasticity on volume fraction and the stress dependence attributable to a softer parent phase. The multiphase model is used to simulate the stress state of a quenched bar and show qualitative trends in the response when the transformation phenomenon is incorporated on the length scale of a global boundary value problem.
Date: June 24, 1996
Creator: Bammann, D.; Prantil, V. & Kumar, A.
Partner: UNT Libraries Government Documents Department

Phase transitions: An overview with a view

Description: The dynamics of phase transitions plays a crucial role in the so- called interface between high energy particle physics and cosmology. Many of the interesting results generated during the last fifteen years or so rely on simplified assumptions concerning the complex mechanisms typical of nonequilibrium field theories. After reviewing well-known results concerning the dynamics of first and second order phase transitions, I argue that much is yet to be understood, in particular in situations where homogeneous nucleation theory does not apply. I present a method to deal with departures from homogeneous nucleation, and compare its efficacy with numerical simulations. Finally, I discuss the interesting problem of matching numerical simulations of stochastic field theories with continuum models.
Date: October 1, 1997
Creator: Gleiser, M.
Partner: UNT Libraries Government Documents Department

Nanostructured block copolymers.

Description: Block copolymers are well known to undergo micro-phase separation. Typical domain sizes range from a few to about 100 nanometers. With use of OPV{sub n-}PEG{sub m} diblock copolymers, long nanofiber, short nanorod, and lamella morphologies have been observed. OPV is oligo(phenylenevinylene) and PEG is poly(ethyleneglycol). The OPV segment is photoluminescent as well as electroluminescent. The micrometer long OPV{sub 13-}PEG{sub 45} nanofiber consists of OPV{sub 13} inner core with radius of 5.2 nm, and PEG{sub 45} outer shell with overall fiber radius of 7.9 nm. The nanorod has similar core-shell composition but with much shorter length of {approx}100 nm. Lamella morphology consists of alternating OPV and PEG layers. The structural characterization and physical properties of these nanostructured materials and their implication are summarized.
Date: July 9, 2002
Creator: Wang, H. H.; Wang, H.; Yu, L.; Han, C. Y.; Csencsits, R.; Willing, G. A. et al.
Partner: UNT Libraries Government Documents Department

Dynamics of the Si(111) surface phase transition

Description: The authors have used low-energy electron microscopy to investigate the dynamics of the Si(111) 7 x 7 {r_arrow} 1 x 1 phase transition. Because the densities of the two phases differ, the phase transformation is analogous to precipitation in bulk systems: additional material must diffuse to the phase boundaries in order for the transformation to occur. By measuring the size evolution of an ensemble of domains, and comparing the results to simulations, they have identified a new mechanism of precipitate growth. The source of material necessary for the transformation is the random creation of atom/vacancy pairs at the surface. This mechanism contrasts sharply with classical theories of precipitation, in which mass transport kinetics determine the rate of transformation.
Date: January 27, 1999
Partner: UNT Libraries Government Documents Department

Do phase transitions survive binomial reducibility and thermal scaling?

Description: First order phase transitions are described in terms of the microcanonical and canonical ensemble, with special attention to finite size effects. Difficulties in interpreting a `caloric curve` are discussed. A robust parameter indicating phase coexistence (univariance) or single phase (bivariance) is extracted for charge distributions. 9 refs., 4 figs.
Date: May 1, 1996
Creator: Moretto, L.G.; Phair, L. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

Synthesis and Physical Properties of Environmentally Responsive Polymer Gels

Description: Polymer gels undergo the volume phase transition in response to an infinitesimal environmental change. This remarkable phenomenon results in many potential applications of polymer gels. This dissertation systematically investigates the chemical and physical properties of polymer gels. It is found that infrared radiation laser not only induces a volume phase transition in N-isopropylacrylamide (NIPA) gel, but also causes the gel to bend toward the laser beam. The transmission of visible laser light through a NIPA gel can also be controlled by adjusting the infrared laser power. A new class of environmentally responsive materials based on spatial modulation of the chemical nature of gels has been proposed and demonstrated. Three simple applications based on the modulated gels are presented: a bi-gel strip, a shape memory gel, and a gel hand. The bending of bi-gels has been studied as a function of temperature, acetone aqueous solution, and salt solution. As the polymer network concentration increases, the behavior of shear modulus of acrylamide (PAAM) gels deviates significantlyfromthe classical theory. The ionic NIPA gels undergo two sequential volume phase transitions: one occurs in dilute NaCl solution, the other occurs in concentrated NaCl solution. An interpenetrating polymer network (IPN) of PAAM--NIPA has also been synthesized using free radical polymerization. It is found that the IPN gels preserve the essential properties of individual components. The volume phase transition of the IPN gels can be triggered by multiple external stimuli including temperature, acetone concentration, and salt concentration.
Date: May 1996
Creator: Zhang, Xiaomin
Partner: UNT Libraries

Random organization and plastic depinning

Description: We provide evidence that the general phenomenon of plastic depinning can be described as an absorbing phase transition, and shows the same features as the random organization which was recently studied in periodically driven particle systems [L. Corte, Nature Phys. 4, 420 (2008)]. In the plastic flow system, the pinned regime corresponds to the absorbing state and the moving state corresponds to the fluctuating state. When an external force is suddenly applied, the system eventually organizes into one of these two states with a time scale that diverges as a power law at a nonequilibrium transition. We propose a simple experiment to test for this transition in systems with random disorder.
Date: January 1, 2008
Creator: Reichhardt, Charles & Reichhardt, Cynthia
Partner: UNT Libraries Government Documents Department

Progress report on DOE research project [Thermodynamic and kinetic behavior of systems with intermetallic and intermediate phases]

Description: A theoretical investigation was made of the coherent displacive phase transformation between two equilibrium single-phase states producing several orientation variants of the product phase. The research was focused on a behavior of coherent systems (martensitic systems, metal and ceramic, and ferroelectric systems) with defects. The computer simulation demonstrated that randomly distributed static defects may drastically affect the thermodynamics, kinetics, and morphology of the transformation. In particular, the interaction of the transformation mode with the defects may be responsible for appearance of two new fields in the phase diagram: (i) the two-phase field describing the tweed microstructure, which consists of the retain parent phase and the variants of the product phase and (ii) the single-phase field describing the tweed microstructure, which consists of the variants of the product phase. These new fields can be attributed to the pre-transitional states observed in some of th e displacive transformations. The microstructure evolution resulting in formation of the thermoelastic equilibrium is path dependent. This unusual behavior is expected in systems with a sharp dependence of the transition temperature on the defect concentration.
Date: December 13, 2000
Creator: Tsakalakos, T.; Semenovskaya-Khachaturyan, S. & Khachaturyan, A.G.
Partner: UNT Libraries Government Documents Department

Microstructure and kinetics of the plutonium $beta$ $Yields$ $alpha$ and $gamma$ $Yields$ $alpha$ transformations

Description: Supported-discharge cathodic etching and SEM examination techniques were developed for $alpha$-Pu and used to investigate the $alpha$ microstructures formed by the $beta$/sub $alpha$/ $Yields$ $alpha$ and $beta$/sub $gamma$/ $Yields$ $alpha$ transformations in high-purity extruded Pu at low and high $alpha$-phase temperatures ($beta$/sub $alpha$/ and $beta$/sub $gamma$/ are $beta$ formed from $alpha$ and $gamma$, respectively). Low (200$sup 0$K) $beta$/ sub $alpha$/ $Yields$ $alpha$ transformation temperatures produced $alpha$ with a grain size that was finer than that of the original extruded $alpha$ and substantially smaller than the grain size of the $alpha$ formed from $beta$/sub $gamma$/. At high $alpha$-phase temperatures (343 to 370$sup 0$K), $beta$/sub $alpha$/ transformed to $alpha$ by the surface nucleation and inward growth of colonies of columnar $alpha$ grains oriented radially with respect to the original nucleation site. The transformation kinetics could be correlated with the microstructural observations based on a correspondence between the long axes of the grains and the (010) direction of the monoclinic $alpha$-Pu structure. Significant changes in the size, shape, and orientation of the $alpha$ grains were noted with increasing transformation temperature. Conversely, the grain size of the $alpha$ formed from $beta$/sub $gamma$/ was essentially constant at all transformation temperatures. Metallographic evidence of the $gamma$ $Yields$ $alpha$ skip transformation or a significant change in the mode of the $gamma$ $Yields$ $alpha$ transformation was obtained by a comparison of the $alpha$ microstructures produced by quenching $gamma$ to high (362 to 368$sup 0$K) and low (200$sup 0$K) $alpha$-phase temperatures. (auth)
Date: January 1, 1975
Creator: Allen, R.P. & Arrow Smith, H.W.
Partner: UNT Libraries Government Documents Department

The effects of composition, temperature and sample size on the sintering of chem-prep high field varistors.

Description: The sintering behavior of Sandia chem-prep high field varistor materials was studied using techniques including in situ shrinkage measurements, optical and scanning electron microscopy and x-ray diffraction. A thorough literature review of phase behavior, sintering and microstructure in Bi{sub 2}O{sub 3}-ZnO varistor systems is included. The effects of Bi{sub 2}O{sub 3} content (from 0.25 to 0.56 mol%) and of sodium doping level (0 to 600 ppm) on the isothermal densification kinetics was determined between 650 and 825 C. At {ge} 750 C samples with {ge}0.41 mol% Bi{sub 2}O{sub 3} have very similar densification kinetics, whereas samples with {le}0.33 mol% begin to densify only after a period of hours at low temperatures. The effect of the sodium content was greatest at {approx}700 C for standard 0.56 mol% Bi{sub 2}O{sub 3} and was greater in samples with 0.30 mol% Bi{sub 2}O{sub 3} than for those with 0.56 mol%. Sintering experiments on samples of differing size and shape found that densification decreases and mass loss increases with increasing surface area to volume ratio. However, these two effects have different causes: the enhancement in densification as samples increase in size appears to be caused by a low oxygen internal atmosphere that develops whereas the mass loss is due to the evaporation of bismuth oxide. In situ XRD experiments showed that the bismuth is initially present as an oxycarbonate that transforms to metastable {beta}-Bi{sub 2}O{sub 3} by 400 C. At {approx}650 C, coincident with the onset of densification, the cubic binary phase, Bi{sub 38}ZnO{sub 58} forms and remains stable to >800 C, indicating that a eutectic liquid does not form during normal varistor sintering ({approx}730 C). Finally, the formation and morphology of bismuth oxide phase regions that form on the varistors surfaces during slow cooling were studied.
Date: September 1, 2007
Creator: Garino, Terry J.
Partner: UNT Libraries Government Documents Department

Phase transformation of "chem-prep" PZT 95/5-2Nb HF1035 ceramic under quasi-static loading conditions.

Description: Specimens of poled and unpoled ''chem-prep'' PNZT ceramic from batch HF1035 were tested under hydrostatic, uniaxial, and constant stress difference loading conditions at -55, 25, and 75 C. The objective of this experimental study was to characterize the mechanical properties and conditions for the ferroelectric (FE) to antiferroelectric (AFE) phase transformations of this ''chem-prep'' PNZT ceramic to aid grain-scale modeling efforts in developing and testing realistic response models for use in simulation codes. As seen from a previously characterized material (batch HF803), poled ceramic from HF1035 was seen to undergo anisotropic deformation during the transition from a FE to an AFE phase. Also, the phase transformation was found to be permanent for the two low temperature conditions, whereas the transformation can be completely reversed at the highest temperature. The rates of increase in the phase transformation pressures with temperature were practically identical for both unpoled and poled PNZT HF1035 specimens. We observed that temperature spread the phase transformation over mean stress analogous to the observed spread over mean stress due to shear stress. Additionally, for poled ceramic samples, the FE to AFE phase transformation was seen to occur when the normal compressive stress, acting perpendicular to a crystallographic plane about the polar axis, equals the hydrostatic pressure at which the transformation otherwise takes place.
Date: July 1, 2006
Creator: Montgomery, Stephen Tedford; Lee, Moo Yul; Meier, Diane A. & Hofer, John H.
Partner: UNT Libraries Government Documents Department

Isentropic compression of argon

Description: The compression was done in an MC-1 flux compression (explosive) generator, in order to study the transition from an insulator to a conductor. Since conductivity signals were observed in all the experiments (except when the probe is removed), both the Teflon and the argon are becoming conductive. The conductivity could not be determined (Teflon insulation properties unknown), but it could be bounded as being {sigma}=1/{rho}{le}8({Omega}cm){sub -1}, because when the Teflon breaks down, the dielectric constant is reduced. The Teflon insulator problem remains, and other ways to better insulate the probe or to measure the conductivity without a probe is being sought.
Date: June 1, 1997
Creator: Veeser, L.R.; Ekdahl, C.A. & Oona, H.
Partner: UNT Libraries Government Documents Department

Phase change in uranium: Discrepancy between experiment and theory

Description: Using a diamond-anvil cell (DAC) phase transformation and room temperature Equation of State (EOS) for some actinides and lanthanides were studied to multimegabar (megabar = 100 GPa) pressures. Experimental data are compared with the theoretically predicted crystal structural changes and the pressure-volume relationships. There is a general agreement between theory and experiment for the structural changes in the lighter actinides, however in detail there are some discrepancies still. A generalized trend for the phase transformations in the lanthanides can be seen, which again has broad agreement with theory. We conclude that an accurate and robust theoretical base for predicting the phase transformations in the f-electron metals can be developed by incorporating the DAC data.
Date: July 22, 1996
Creator: Akella, J.
Partner: UNT Libraries Government Documents Department

Dislocation accumulation at large plastic strains -- An approach to the theoretical strength of materials

Description: The usual method of introducing engineers to the concept of dislocations and their role in plastic flow is to compare an estimate of the theoretical strength of solid (of order {micro}/30 where {micro} is the shear modulus) and the observed strength of either single crystals ({mu}/10{sup 4}) or practical engineering material such as structural steels where the yield stress in shear is of order {mu}/10{sup 3}. However, if one considers the problem in reverse, one can consider the accumulation of dislocations as an important mechanism by which one can produce engineering materials in which the strength level approaches the theoretical strength. If one assumes that the flow stress can be expressed in terms of te mean free path between stored dislocations or as the square root of the global dislocation density, then one can see the influence of dislocation density in a diagrammatic form. It is clear that the strengthening by dislocation accumulation due to large imposed plastic strains represents an important approach both to the development of new, potentially valuable, engineering materials and an important area of basic understanding in terms of the mechanical response of materials close to their theoretical strength. Thus, this article will survey some of the factors which influence dislocation accumulation at large strains and the consequences of such accumulation processes.
Date: April 1, 1999
Creator: Embury, J.D. & Han, K.
Partner: UNT Libraries Government Documents Department

A model for the shock-induced phase transition in iron

Description: It has long been known that the shock induced {alpha} to {epsilon} phase transition in iron exhibits significant metastability in the two phase region above 13 GPa. The authors have developed a simple, physically motivated model that accurately describes the metastable phase transition surface for shock loaded iron during numerical simulations. It is demonstrated here that the metastability of Fe can have a substantial impact on numerical simulations of time-resolved experiments.
Date: October 1, 1997
Creator: Boettger, J.C. & Wallace, D.C.
Partner: UNT Libraries Government Documents Department

First-principles simulations and Z-contrast imaging of impurities at <001> tilt grain boundaries in MgO

Description: First-principles density-functional calculations were used to study the effects of CA impurities on the {Sigma} = 5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
Date: December 1, 1997
Creator: Yan, Y.; Chisholm, M.F.; Pennycook, S.J.; Duscher, G.; Maiti, A. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department