Dynamic X-ray Diffraction Measurements Research Articles

The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride).

Piezoelectricity describes interconversion between electrical charge and mechanical strain. As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exceptions are poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)), which exhibit a negative longitudinal piezoelectric coefficient. Reported explanations exclusively consider contraction with applied electric field of either the crystalline or the amorphous part of these semi-crystalline polymers. To distinguish between these conflicting interpretations, we have performed in situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure.

Laser-Based Dynamic Compression of Solids to Ultrahigh Pressures

Laser-based dynamic compression provides new opportunities to study the structures and properties of materials to ultrahigh pressure conditions. In this technique, high-powered laser beams are used to ablate a sample surface and by reaction a compression wave is generated and propagate through the sample. By controlling the shape and duration of the laser pulse, either shock or ramp (shockless) compression can be produced. Diagnostics include velocity interferometry (from which the stress-density response of the material can be determined) and x-ray diffraction from which structural information is obtained. Magnesium oxide is a fundamental ionic solid which has been extensively examined at high pressures. Theoretical studies predict a change in MgO from a rocksalt (B1) crystal structure to a cesium chloride (B2) structure at pressures of about 400–600 GPa but diamond anvil cell experiments have not been able to reach these pressures. Here we present dynamic X-ray diffraction measurements of ramp-compressed magnesium oxide. We show that a solid–solid phase transition, consistent with a transformation to the B2 structure occurs near 600 GPa. On further compression, this structure remains stable to 900 GPa. Our results provide an experimental benchmark to the equations of state and transition pressure of magnesium oxide, and may help constrain interior properties of super-Earth extrasolar planets. We have also examined the high-pressure behavior of molybdenum under both shock and ramp loading. The melting curves and high-pressure phase diagrams of transition metals have been controversial, and Mo is an excellent test case for resolving these discrepancies. We have conducted shock compression experiments on Mo with an X-ray diffraction diagnostic to address previous claims of high-pressure phase transitions and to determine the location of the Hugoniot melting point. We have also carried out ramp compression experiments to test predictions of phase transitions in Mo at ultrahigh pressures and low temperatures.

Experimental evidence for a phase transition in magnesium oxide at exoplanet pressures

Little is known about the structure of possible mantle materials of extra-solar super-Earths with interior pressures of up to 1,000 GPa. Dynamic X-ray diffraction measurements of ramp-compressed magnesium oxide, an important component of Earth’s mantle, show a solid–solid state transition at about 600 GPa, with a high-pressure structure that is stable up to 900 GPa.

Determination of equilibrium humidities using temperature and humidity controlled X-ray diffraction (RH-XRD)

Confined growth of crystals in porous building materials is generally considered to be a major cause of damage. We report on the use of X-ray diffraction under controlled conditions of temperature and relative humidity (RH-XRD) for the investigation of potentially deleterious phase transition reactions. An improved procedure based on rate measurements is used for the accurate and reproducible determination of equilibrium humidities of deliquescence and hydration reactions. The deliquescence humidities of NaCl (75.4 ± 0.5% RH) and Ca(NO 3) 2·4H 2O (50.8 ± 0.7% RH) at 25 °C determined with this improved RH-XRD technique are in excellent agreement with available literature data. Measurement of the hydration of anhydrous Ca(NO 3) 2 to form Ca(NO 3) 2·2H 2O revealed an equilibrium humidity of 10.2 ± 0.3%, which is also in reasonable agreement with available data. In conclusion, dynamic X-ray diffraction measurements are an appropriate method for the accurate and precise determination of equilibrium humidities with a number of interesting future applications.

Phase Transitions in LaNi4Co during Electrochemical Cycling An In Situ X-Ray Diffraction Study

Phase and crystal structure changes during the electrochemical hydriding and dehydriding of were investigated using in situ X-ray diffraction (XRD). A specially designed cell allowed dynamic XRD measurements during five charge-discharge cycles. This enabled the direct observation of the activation process. In addition, crystal lattice information derived from these measurements help to explain the long cycle life of cobalt containing -type battery alloys. The formation of an intermediate γ-phase hydride between the hydrogen solid-solution α-phase and the fully hydrided β-phase was clearly observed during absorption and desorption. The volume expansion in the formation of the γ and β hydride phases is highly anisotropic. Lattice expansion in the α-phase to γ-phase transformation occurs mainly in the basal plane, whereas the transition from the γ-phase to the β-phase causes a lattice expansion in the c axis direction. It is believed that the two-step phase transition in this Co-substituted alloy generates less internal stress than the single-step volume expansion of the archetype compound. This reduces the stress-induced pulverization that occurs during electrochemical cycling. Consequently, the metal hydride electrode maintains larger particle sizes and, thus, smaller surface areas subject to corrosion by the electrolyte, which is the principle cause of capacity loss. © 2001 The Electrochemical Society. All rights reserved.

Phase formation and kinetics in melt-spun and sputtered alloys along the line Cu40Ni60Cu30Ni45Sn25

The ternary system CuNiSn includes intermetallic phases of DO19, 2H and DO3 structure. These have some interesting properties for future mechanical investigations. Recently, CuNiSn alloys were investigated to understand the process of phase formation and to learn how to produce single-phase and well defined multi-phase material. The results show complex phase formation and leave some open questions, especially concerning the low-temperature range. The present investigation uses sputtered and melt-spun CuNiSn alloys to get information about the influence of different preparation methods. Isothermal and dynamic X-ray diffraction measurements together with TEM investigations reveal information about the phase diagram and kinetics of phase formation down to 500 °C. Proposed equilibrium diagrams are presented: a vertical section along Cu40Ni60Cu30Ni45Sn25 and isothermal cuts across the Sn-poor side of the ternary CuNiSn diagram. The precipitation of intermetallic phases in supersaturated ternary solid solutions is shown to be a two-step process.

Dynamic X-ray diffraction measurements following photolytic relaxation and activation of skinned rabbit psoas fibres

1) The ATP binding and crossbridge dissociation in muscle fibres is as fast as in solution, has a Q10 ca. 2-3, and is not measurably strain sensitive. 2) The final ADP release from the AM.ADP state achieved by adding ADP to rigor fibres must be greater than or equal to 69 sec-1 at 10 degrees C, and the combination of this rate and the ADP rebinding rate at 1 mM ADP limits the ATP induced crossbridge dissociation rate at greater than 2 mM ATP, but these kinetics were not strain sensitive. The strain sensitive steps must occur earlier on the attached pathway. 3) On activation, the equatorial changes thought to reflect crossbridge attachment are faster than tension production. The 10 intensity may change slightly ahead of the 11. This rate was not very temperature sensitive unlike the tension producing step in the mechanism. 4) The re-equilibration of equatorial intensity levels was much faster on activation from the rigor state than from the relaxed state. We conclude that crossbridges do not necessarily move far from the thin filaments when they detach in a fully activated thin filament system. 5) The 14.3 nm meridional intensity increases greater than 200% on fibre activation at 24 degrees C. The structural reorganisation of the heads responsible for this increase is associated with the tension generating step in the ATPase mechanism rather than the initial binding of bridges.

Rheo-Optical Studies on the Deformation Mechanism of Semicrystalline Polymers. XII. On the Nature of Alpha and Beta Mechanical Dispersions of High-Density Polyethylene in Relation to the Mechanism of Spherulite Deformation

Dynamic birefringence and dynamic X-ray diffraction measurements were carried out from 0.008 to 4.3 Hz at various temperatures from −40 to 100°C and also from 0.05 to 3.2 Hz at various temperatures from −30 to 100°C, respectively, in order to study the nature of α and β mechanical dispersions in spherulitic high-density polyethylene. Three mechanical dispersions analogous to the β, α1, and α2 processes were obtained and resolved quantitatively into each component. Two dispersions corresponding to α and β processes were observed in the dynamic birefringence and dynamic crystal orientation studies. These α and β dispersions were interpreted in terms of relative contributions of intralamellar crystal-grain reorientation and lamellar orientation, both of which are associated with the dynamic-tensile deformation of spherulitic crystalline texture. Besides the crystalline contributions, substantial noncrystalline contribution to the α process was seen and is believed to have arisen from the orientation of noncrystalline chains in the crystal-grain boundaries. The noncrystalline contribution to the β process was observed to have a large negative in-phase component and a slightly positive out-of-phase component of the dynamic stress-optical coefficient function. This anomalous behavior is explained in terms of the combined contributions from the orientational and distortional birefringence of interlamellar noncrystalline chains as well as on the basis of negative form birefringence. In the dynamic crystal-lattice deformation studies, two dispersions corresponding to the α1 and α2 mechanical dispersions were detected. The former α1 lattice dispersion is considered to be associated with the α crystal-orientation dispersion, whereas the latter α2 lattice dispersion is believed to be inherent, arising from incoherent lattice vibrations within crystals in which the intermolecular potential suffers a smearing-out effect.

Rheo-Optical Studies on the Deformation Mechanism of Semicrystalline Polymers. III. An Improved Technique for Oscillatory Measurements in Rheo-Optics and Its Application to Dynamic X-Ray Diffraction Measurements

An improved technique, “Multi-Channel Narrow Sector Technique,” was presented for measuring rheo-optical responses of polymeric materials against oscillatory mechanical excitation. The technique is based on the Fourier expansion of the response accumulated at equally divided narrow phase intervals of the oscillatory excitation over many cycles of the oscillation, and is available for determination of the rheo-optical properties of the materials even under one or both of the following conditions: (i) the excitation being distorted from a genuine sinusoidal one, or/and (ii) the response being of nonlinear characteristics. The multi-channel narrow sector technique is applied to the dynamic X-ray diffraction measurements in association with the use of a microcomputer. Some results for two kinds of semicrystalline polymer films, a tubular-extruded film of a poly(butene-1) and a melt-crystallized isotropic film of a low-density polyethylene are demonstrated. Determination of the dynamic shift and the intensity variation of overlapped peaks of X-ray diffraction from crystal planes, which is necessary for evaluating the dynamic lattice deformation and orientation of the polymer crystals, is made by means of the least square method. Some typical results are further demonstrated for the dynamic behavior of the (110) and (200) crystal planes of the low-density polyethylene film.