Trevor R. Finlayson

Characterization of domain structures from diffraction profiles in tetragonal ferroelastic ceramics

The underlying domain structures of ferroelastic ceramics have a large influence on their macroscopic electromechanical properties. The profile shape functions of certain pseudo-cubic peaks in diffraction patterns collected from these materials can provide a great deal of information about such domain structures. Using both constant-wavelength neutron and high-energy synchrotron x-ray diffraction, profile shape functions of the pseudo-cubic 002 reflection are evaluated in a soft, tetragonal PZT ceramic. Errors in the integrated intensity ratio, important for measuring the degree of domain boundary movement in these materials, are subject to further scrutiny. It is shown that an asymmetric Pearson VII type distribution, integrated numerically between reasonable limits, gives the most accurate value of relative domain populations in these materials. It is also shown that the diffuse scattering caused by ferroelastic domain walls may be used to estimate the amount of material that is affected by microstrains originating at these walls.

Time-resolved diffraction measurements of electric-field-induced strain in tetragonal lead zirconate titanate

The dynamic electric-field-induced strain in piezoelectric ceramics enables their use in a broad range of sensor, actuator, and electronic devices. In piezoelectric ceramics which are also ferroelectric, this macroscopic strain is comprised of both intrinsic (piezoelectric) and extrinsic (non-180° domain switching) strain components. Extrinsic contributions are accompanied by hysteresis, nonlinearity, and fatigue. Though technologically significant, direct measurement of these mechanisms and their relative contributions to the macroscopic response has not yet been achieved at driving frequencies of interest. Here we report measurements of these mechanisms in ceramic lead zirconate titanate during application of subcoercive cyclic driving electric fields using an in-situ stroboscopic neutron diffraction technique. Calculations are made from the diffraction measurements to determine the relative contributions of these different strain mechanisms. During applied electric field square waves of +0.5Ec unipolar and ±0.5Ec bipolar, at 1 Hz, non-180° domain switching is found to contribute 34% and 40% of the macroscopically measured strain, respectively.The dynamic electric-field-induced strain in piezoelectric ceramics enables their use in a broad range of sensor, actuator, and electronic devices. In piezoelectric ceramics which are also ferroelectric, this macroscopic strain is comprised of both intrinsic (piezoelectric) and extrinsic (non-180° domain switching) strain components. Extrinsic contributions are accompanied by hysteresis, nonlinearity, and fatigue. Though technologically significant, direct measurement of these mechanisms and their relative contributions to the macroscopic response has not yet been achieved at driving frequencies of interest. Here we report measurements of these mechanisms in ceramic lead zirconate titanate during application of subcoercive cyclic driving electric fields using an in-situ stroboscopic neutron diffraction technique. Calculations are made from the diffraction measurements to determine the relative contributions of these different strain mechanisms. During applied electric field square waves of +0.5Ec unipolar...

Chemical doping effect on the crystal structure and superconductivity of MgB2

Abstract Substituent alloys of Mg1−xMxB2 with M=Ti, Zr, Mo, Mn, Fe, Ca, Al, Ag, Cu, Y, and Ho and 0⩽x⩽10% have been synthesized by the solid-state reaction. The solid solubility for most of these dopants at Mg site is found to be very low except for M=Al. In most of these alloys, the Vegard relationship holds as the doping level is low. The lattice constants for the fictitious compounds such as AgB2, CuB2, FeB2 have been extrapolated. The superconductivity transition temperature, Tc, shows a systematic changes with the doping level in some of these substituent alloys. Relevant mechanisms of superconductivity suppression have been discussed.

Neutron diffraction study of the polarization reversal mechanism in [111]c-oriented Pb(Zn1∕3Nb2∕3)O3−xPbTiO3

The polarization reversal mechanism in [111]c-oriented Pb(Zn1∕3Nb2∕3)O3−xPbTiO3 has been investigated by in-situ neutron diffraction. Stepwise static-field measurements of the (222)c rocking curves confirm a two-stage polarization reversal mechanism via a sequence of non-180° domain reorientations. The time-resolved response has also been measured upon application of a bipolar square wave with a 30 s period to observe directly the relaxation times of diffracted neutron intensity during the reversal process. Upon application of a large antipolar field, the diffraction intensity increases quickly, before relaxing over a longer time period with an exponential decay constant, τ, of approximately 5.7 s. These large time constants correlate with a frequency dependence of the macroscopic strain-field response.

Time-dependent magnetization behaviour for type II superconductors - V3Si

Abstract Time-dependent magnetization in the mixed state for high-Tc oxide superconducting materials has been widely reported and often attributed to features supposedly unique to these materials. Similar behaviour has been demonstrated for the A15-structure material, V3Si. Typical results for samples of V1−xSix alloys with the A15 structure, are presented and discussed in relation to the accepted theories for flux pinning in any type II superconductor.

Comparison of Neutron and Synchrotron Diffraction Measurements of Residual Stress in Bead-on-Plate Weldments

This paper explores the use of neutron and synchrotron diffractions for the evaluation of residual stresses in welded components. It has been shown that it is possible to achieve very good agreement between the two independent diffraction techniques. This study shows the significance of the weld start and end sites on the residual strain/stress distribution. Quantitative evaluation of the residual stress development process for multibead weldments has been presented. Some measurements were also taken before and after postweld stress relieving to establish the reduction and redistribution of the residual stress. The detailed measurements of residual stress around the weld achieved in this work significantly improve the knowledge and understanding of residual stress in welded components.

Heat capacity in magnetic and electric fields near the ferroelectric transition in triglycine sulfate

Specific-heat measurements are reported near the Curie temperature (TC=320K) on triglycine sulfate. Measurements were made on crystals whose surfaces were either nongrounded or short circuited, and were carried out in magnetic fields up to 9T and electric fields up to 220V∕cm. In nongrounded crystals the authors find that the shape of the specific-heat anomaly near TC is thermally broadened. However, the anomaly changes to the characteristic sharp λ shape expected for a continuous transition with the application of either a magnetic field or an electric field. In crystals whose surfaces were short circuited with gold, the characteristic λ shape appeared in the absence of an external field. This effect enabled a determination of the critical exponents above and below TC, and may be understood on the basis that the surface charge originating from the pyroelectric coefficient, dP∕dT, behaves as if shorted by external magnetic or electric fields.

The approach to saturation magnetization of nanocrystalline Fe90Zr7B3 alloy

The approach to the saturation magnetization of a nanocrystalline Fe90Zr7B3 alloy has been studied by means of differential magnetic susceptometry. The influence of the microstructure of the alloy on the approach to saturation magnetization has been studied. It has been found that for the optimum heat-treated sample (heated at 923 K for 0.25 h), which had an average grain size of 12 nm and a coercivity, Hc = 6.4 A m-1, the approach to saturation magnetization can be expressed using the law of approach to saturation magnetization, χ = aMsH-2 + 2bMsH-3 + c/2(H-1/2) (where χ is the differential magnetic susceptibility, Ms is the saturation magnetization, H is the magnetic field, and a, b and c are constants), without the H-3 term, while for the sample heated at 923 K for 240 h, which had an average grain size of 82 nm and a coercivity, Hc = 950 A m-1, the approach to saturation magnetization cannot be expressed in the same form. For both samples, the magnetic-anisotropy-related H-3 term cannot be found. As a result, the magnetic anisotropy in this nanocrystalline alloy cannot be determined by using the law of approach to saturation magnetization. The measurement with the differential susceptometer has been compared with the measurement made on the SQUID magnetometer. The result has shown that the measurement of differential susceptibility using a long ribbon sample on the differential susceptometer is more reliable than that using a small sample on the SQUID.

Martensite precursor observations using thermal expansion: NiAl

Abstract Measurements of thermal expansion, in the temperature range 2–300 K, were made along the three 〈100〉 directions of the cubic phase of a martensitically transforming Ni 0.625 Al 0.375 single crystal. Anisotropic and anomalous expansion for the three different directions were observed at temperatures about 115 K above the transformation temperature, of approximately 85 K. Optical observation showed that the onset temperature for the anomalous expansion is consistent with the temperature at which surface tilts appear and that the anisotropic expansion is due to the preferential selection of orientation of the martensite variants in the crystal. The amplitude of the observed anisotropy varies with applied stress which suggests that the relative fractions of the variants in the crystal with preferential orientation are stress dependent.

Anisotropic thermal expansion of polycrystalline V25.75at%Si

Abstract Thermal expansion measurements along three mutually orthogonal directions in polycrystalline V 3 Si display anisotropy up to about 90K, indicating that the onset of the cubic-tetragonal structural transformation in this compound can take place at temperatures well above those suggested by x-ray diffraction studies. In addition a small volume change accompanies the transformation. The results are discussed in relation to other anomalous properties of this compound.