Influence of poling methods on the orientational dynamics of 2-methyl-4-nitro-aniline in poly(methyl methacrylate) studied by second harmonic generation

The magnitude and stability of the induced dipolar orientation of 2-methyl-4-nitroaniline (MNA)/poly(methyl methacrylate) (PMMA) guest/host system is investigated. The chromophores are aligned using both the corona discharge and contact electrode poling techniques. The magnitude of order parameter (also an indicator for the second order nonlinear susceptibility) is measured by recording absorbances of the poled (by the two different techniques) and unpoled PMMA films at different concentrations of MNA. Under the same conditions the corona poling technique creates a higher alignment of molecules along the field direction. The time dependence of the second harmonic intensity of the MNA/PMMA film prepared by the two techniques can be described by a Kohlrausch-Williams-Watts stretched exponential. The temperature dependence of the decay time constant is found to generally follow a modified Williams-Landel-Ferry (WLF) or Vogel-Tamann-Fulcher (VTF) equation. The glass transition temperature seems to be the single most important parameter for determining the relaxation time tau(T).

Analogous stress and electric field driven structural transformation and decrease in polarization coherence on poling around the morphotropic phase boundary in BiScO3-PbTiO3

The nature of the stress and electric field driven structural and microstructural transformations in the morphotropic phase boundary (MPB) compositions of the high Curie point piezoelectric system BiScO3-PbTiO3 has been examined by ex situ based techniques. Using a powder poling technique, which is based on the concept of exploiting the irreversible structural change that occurs after the application of a strong electric field and stress independently, it was possible to ascertain that both moderate stress and electric field induce identical structural transformation-a fraction of the monoclinic phase transforms irreversibly to the tetragonal phase. Moreover, analysis of the dielectric response before and after poling revealed a counterintuitive phenomenon of poling induced decrease in the spatial coherence of polarization for compositions around the MPB and not so for compositions far away from the MPB range. Exploiting the greater sensitivity of this technique, we demonstrate that the criticality associated with the interferroelectric transition spans a wider composition range than what is conventionally reported in the literature based on bulk x-ray/neutron powder diffraction techniques.

Unraveling the nature of electric field- and stress- induced structural transformations in soft PZT by a new powder poling technique

A `powder-poling' technique was developed to study electric field induced structural transformations in ferroelectrics exhibiting a morphotropic phase boundary (MPB). The technique was employed on soft PZT exhibiting a large longitudinal piezoelectric response (d(33) similar to 650 pCN(-1)). It was found that electric poling brings about a considerable degree of irreversible tetragonal to monoclinic transformation. The same transformation was achieved after subjecting the specimen to mechanical stress, which suggests an equivalence of stress and electric field with regard to the structural mechanism in MPB compositions. The electric field induced structural transformation was also found to be accompanied by a decrease in the spatial coherence of polarization.

A field model interpretation of crack initiation and growth behavior in ferroelectric ceramics: change of poling direction and boundary condition

Strain energy density expressions are obtained from a field model that can qualitatively exhibit how the electrical and mechanical disturbances would affect the crack growth behavior in ferroelectric ceramics. Simplification is achieved by considering only three material constants to account for elastic, piezoelectric and dielectric effects. Cross interaction of electric field (or displacement) with mechanical stress (or strain) is identified with the piezoelectric effect; it occurs only when the pole is aligned normal to the crack. Switching of the pole axis by 90degrees and 180degrees is examined for possible connection with domain switching. Opposing crack growth behavior can be obtained when the specification of mechanical stress sigma(infinity) and electric field E-infinity or (sigma(infinity), E-infinity) is replaced by strain e and electric displacement D-infinity or (epsilon(infinity), D-infinity). Mixed conditions (sigma(infinity),D-infinity) and (epsilon(infinity),E-infinity) are also considered. In general, crack growth is found to be larger when compared to that without the application of electric disturbances. This includes both the electric field and displacement. For the eight possible boundary conditions, crack growth retardation is identified only with (E-y(infinity),sigma(y)(infinity)) for negative E-y(infinity) and (D-y(infinity), epsilon(y)(infinity)) for positive D-y(infinity) while the mechanical conditions sigma(y)(infinity) or epsilon(y)infinity are not changed. Suitable combinations of the elastic, piezoelectric and dielectric material constants could also be made to suppress crack growth. (C) 2002 Published by Elsevier Science Ltd.

Corona poling of liquid crystalline polymers with nitroazobenzene side chains

The orientational behavior of liquid crystalline polymers with para-nitro azobenzene as side chains under electric field was studied by UV-visible spectroscopy. The results showed that lambda(max) of the poled polymer films was around 394nm, compared to that of the unpoled films, the absorption decreased due to poling. The orientational parameters increased linearly with the increase of the electric field. The temporal stability of the poled polymer film is good at room temperature. This kind of materials showed promise application as nonlinear optical component in photorefractive polymers.

Enhanced second order non-linear optical properties of silica sol-gel thin films through photo-induced poling

Sol-gel derived inorganic materials are of interest as hosts for non-linear optically active guest molecules and they offer particular advantages in the field of non-linear optics. Orientationally ordered glasses have been prepared using a sol-gel system based on tetramethoxysilane, methyltrimethoxysilane and a non-linear optical chromophore Disperse Red 1. The novel technique of photo-induced poling was used to generate enhanced levels of polar order. The level of enhancement is strongly dependent on the extent of gelation and an optimum preparation time of ∼100 h led to an enhancement factor of ∼5. Films prepared in this manner exhibited a high stability of the polar order.

A comparison of photoinduced poling and thermal poling of azo‐dye‐doped polymer films for second order nonlinear optical applications

Photoinduced poling (PIP) is a new technique which allows the room‐temperature preparation of guest/host polymer films exhibiting significant polar order for nonlinear optical applications. We report a comparison of this novel technique with the conventional electrode poling procedure performed at the glass transition temperature of the polymer using disperse red 1/poly(methylmethacrylate) films. In particular, in situ second harmonic generation measurements show that levels of polar order achieved using these two techniques are similar. In contrast, the stability of the polar order is reduced by up to 20 times in terms of the decay time constant in films prepared using PIP although the stability is very dependent upon the temperature at which the poling was performed.

STUDY OF POLING BEHAVIOR OF BIAXIALLY STRETCHED POLY(VINYLIDENE FLUORIDE) FILMS USING THE CONSTANT-CURRENT CORONA TRIODE

This article reports systematic results of corona poling measurements obtained on biaxially stretched polyvinylidene fluoride films charged in dry air. Charging was performed using the constant current corona triode. The dependence of the poling process on the sample thickness, charging current, and successive charging processes was investigated. Phase transitions from alpha to delta and to beta phases were observed when virgin samples were corona charged. The thermal pulse technique showed that the polarization profiles during charging can be made consistently almost uniform and that the ferroelectric reorientation can be associated with the rising plateau region displayed on potential buildup curves. (C) 1995 American Institute of Physics.

Electrode poling of cellular polypropylene films with short high-voltage pulses

Recently, piezoelectric cellular polypropylene (PP) was proposed as a new type of quasi-ferroelectric. The observed hysteresis of the charge density as a function of the electric field could be explained as field-dependent charging inside the gas-filled voids. Interestingly enough, the measurable poling behavior of the macroscopic dipoles formed by charges that are trapped at the internal void surfaces is phenomenologically completely identical to the cooperative poling behavior of microscopic molecular dipoles in ferroelectric polymers. Therefore, it can be assumed that charge separation (or charge redistribution) and subsequent trapping in cellular PP is a rather fast switching process. In order to examine the poling dynamics, we developed an experimental setup for pulsed poling. High-voltage pulses with a duration of 45 μs (FWHM) were applied in direct contact to two-side metallized cellular PP films. The pulsed poling yields piezoelectricity in the cellular PP. We study and discuss the dependence of the resulting piezoelectricity on the poling field. We also characterize the charge separation during application of higher electric poling fields of up to -10 kV in direct contact to the two-side metallized films for longer times.

Influence of the processing conditions and corona poling on the morphology of β-PVDF

The α → β phase transformation of PVDF through the stretching process at different temperatures was investigated. The optimum stretching conditions were studied and characterised by infrared spectroscopy and differential scanning calorimetry. The maximum β-phase content was achieved at 80°C and a stretch ratio of 5. These samples were poled at several electric fields by the corona charge method. The effect of the electric field on the phase transformation was studied by infrared spectroscopy.

Bragg Gratings, Photosensitivity and Poling in Glass Waveguides (BGPP

Polymer optical fibre Bragg gratings are useful for strain sensor applications for large dynamic range. We report recent progress in developing polymer optical fibres with higher photosensitivity and fabricating POF gratings at alternative wavelength.

Bragg Gratings, Photosensitivity and Poling in Glass Waveguides (BGPP

Polymer optical fibre Bragg gratings are useful for strain sensor applications for large dynamic range. We report recent progress in developing polymer optical fibres with higher photosensitivity and fabricating POF gratings at alternative wavelength.

Fractional order of poling period for broadly tunable second harmonic generation

We demonstrate the possibility to use a fractional order of poling period of nonlinear crystal waveguides for tunable second harmonic generation. This approach allows one to extend wavelength coverage in the visible spectral range by frequency doubling in a single crystal waveguide.