Ferroelectric studies in solid-media high pressure devices

NRC publication: Ye

High-pressure studies of ferroelectric phases related to the earth and planetary interiors

To determine the effects of high pressure constraints on the overall behaviour of ferroelectrics (FE) of both the perovskite "soft-mode" and hydrogen-bonded types, the authors have conducted dielectric property measurements to 5.5 GPa have been conducted on single crystals and polycrystalline aggregates of several ferroelectrics (BaTiO\u2083, Pb(Zr, Ti)O\u2083, triglycine sulfate, Rochelle salt) for the purpose of determining the effects of high-pressure constraints on the overall behavior of ferroelectrics (FE) of both the perovskite 'soft-mode' and hydrogen-bonded types. It is found that dielectric hysteresis is possible in FE which are under high-pressure conditions of deviatoric stresses and strains in which the FE phase forms part of a composite system. The results indicate that the continued stability of these FE phases is not restricted to a particular polarization mechanism. The results also show that existing FE states occurring as elastic and dielectric inhomogeneities can persist to much higher pressures than expected on the basis of the pressure dependence of isolated and unconstrained FE crystals.NRC publication: Ye

Dielectric properties of ferroelectric perovskite Pb (ZrTi)03 ceramics in a solid matrix subjected to pressure

NRC publication: Ye

Hot creep of single crystals by bending: procedure, theory and pilot experiment on enstatite

Carrying out hot creep experiments by three-point bending of thin single crystals has three main advantages: (1) it permits plastic deformation in a soft machine under very small loads; (2) the forces and resolved stresses are well defined relative to the glide systems in oriented crystal bars; and (3) the deformation can be analysed without perturbing preparations by X-ray diffraction topography. Thus a complete creep experiment consists of strain and strain rate measurements during high temperature deformation followed by separate X-ray diffraction studies. The three-point bending rig is operated in high vacuum or controlled ambient atmospheres. Creep strains are determined with a displacement transducer from the central deflections of the crystal plate. Variable loads corresponding to resolved shear stresses, computed for chosen orientations of the glide systems, are applied electromagnetically to the central knife edge. Load or stress can be maintained constant by on-line feedback control. Elastic and plastic beam theory are used to obtain creep strains and strain rates from the maximum bending deflections. Calibration of the temperature dependence of the elastic deflections makes it possible to isolate elastic and plastic strains and to determine exact strain rates as functions of load, stress and time. The distributions of lattice strains, rotations, and dislocation concentrations identifiable with different creep stages are analysed by X-ray diffraction topography of the bent crystal plate in its entirety. Sets of images of both translation and section topographs as well as intensity scans are taken using different Bragg reflections. Particularly attractive features of the interpretation of creep experiments by bending, illustrated by the pilot experiment on enstatite, are: (a) the power n obtained for the strain dependence on the stress (in work-hardening) or the strain rate dependence on the stress (in steady flow) is identical to the power n describing the curve of the plastically deflected beam as a function of beam dimensions and load; (b) elastic and plastic strains are directly obtainable from the beam curvature; and (c) for single crystals of known orientations the plastic lattice curvature is related to the dislocation density

Dielectric properties of pyrophyllite as a function of water vapor pressure

The effects of water vapor pressure on the dielectric constant and dissipation factor of pyrophyllite have been measured in the frequency range 0.05-10 kHz. Both the dielectric constant and dissipation factor increase with increasing vapor pressure and decreasing measuring frequencies. This result has application in electrical measurements made in pyrophyllite-media high-pressure devices.NRC publication: Ye