D. K. Das-Gupta

Laser‐intensity‐modulation method: A technique for determination of spatial distributions of polarization and space charge in polymer electrets

A new method for measuring the spatial distribution of combined polarization and space charge in poled polymer electrets has been developed. The technique, called the Laser Intensity Modulation Method (LIMM), is based upon measurement o-f the pyroelectric current generated by the electret when it is heated with a laser beam which is intensity-modulated at frequencies varying from 200 to 2500 Hz. A special analytical technique is used to trans-form the current-frequency measurements to spatial distributions. The advantages and limitations of LIMM are illustrated by means of calculations based upon simulated data. Experimental results on laboratory-poled samples are presented.

A technique for determining the polarization distribution in thin polymer electrets using periodic heating

Abstract The variation in the polarization distribution through the thickness of a polymer electret is determined by heating each electrode of the sample with a laser beam which is modulated at various frequencies between 0.1 and 100 kHz. The periodic heating induces temperature waves which are attenuated exponentially as a function of both depth in the sample and frequency of modulation. The resulting “thermal probe” heats regions having different polarizations and produces an AC pyroelectric current. A linear least-squares analysis of the experimental current-frequency data gives the polarization distribution.

Analysis of the thermally stimulated discharge current around glass-rubber transition temperature in polyethylene terephthalate

The nature of the thermally stimulated discharge current (TSDC) for polyethylene terephthalate samples in the temperature range from room temperature to above glass-rubber transition temperature of the amorphous phase is analyzed. The well conditioning of the sample is strictly necessary in order to have a good reproducibility and accuracy of results. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows through the sample during polarization. The peak position changes as well, if the sample is polarized in air or in oxygen and the nature of change is more important in the case of oxygen. The shape of the peak is complex and at least four shoulders have been identified around 85, 90, 105, and 125 °C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the glass-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For...

Pyroelectricity in polymers

Abstract The piezo- and the pyroelectric properties of poled polyvinylidene fluoride (PVDF) and its copolymers arise primarily from the ferroelectric orientation of the C-F dipoles rather than trapped space charges. Composite films, consisting of ceramic powder, introduced in PVDF matrix, combine the mechanical strength of polymeric films with high piezo- and pyroelectric responses of the ceramics. Present paper provides a brief review of the nature and origin of pyroelectricity in (i) PVDF, (ii) its copolymer vinylidene fluroide-trifluoroethylene (BDF-TrFE) and (iii) their composites (polymer-ceramic).

A new technique for determination of the spatial distribution of polarization in polymer electrets

Abstract A new method for measuring the spatial distribution of polarization in poled polymer electrets has been developed. The technique, called the Laser Intensity Modulation Method (LIMM), is based upon measurement of the pyroelectric current generated by the electret when it is heated with a laser beam which is intensity-modulated at frequencies varying from 100 Hz to 100 kHz. A special analytical technique is used to transform the current-frequency measurements to spatial distributions. The method gives high resolution and is very stable with respect to measurement errors. The technique can be easily implemented in a laboratory with conventional equipment. Experimental results on a number of samples are presented.

Pyroelectricity: Fundamentals and applications

Publisher Summary “Pyroelectricity” is defined as the manifestation of the temperature dependence of the spontaneous polarization of certain anisotropic solids. The chapter contains a brief presentation of the two-and-a-half-millennium history of pyroelectricity, a detailed discussion of the fundamentals of the phenomenon, and a description of many of its applications. The pyroelectric effect is closely related to piezoelectricity and is the basis for ferroelectricity. Despite many years of research, there are undoubtedly more pyroelectric phenomena and more pyroelectric materials to be discovered. Biological pyroelectricity may be a particularly fruitful area for future research. Up to the present time, the applications of pyroelectricity have been principally in the area of infrared detection with a strong emphasis on security uses. The current emphasis on infrared imaging is largely directed toward the consumer market, and this may have a very strong impact on the daily lives of people. It can be safely predicted that the future holds many exciting developments in the fundamentals and applications of pyroelectricity.

Electroactive Properties of Flexible Piezoelectric Composites

A flexible piezoelectric composite with 0-3 connectivity, made from Lead Zirconate Titanate (PZT) powder and vegetable-based polyurethane (PU), was doped with small amount of semiconductor powder. As a result a composite with 0-0-3 connectivity was obtained. The nature of absorption and steady state electrical conduction and the dielectric behaviour have been studied for this ceramic/polymer composite. The dielectric loss processes of the composite were observed to be dominated by those the polymer. Adding a semiconductor phase in the composite the electrical conductivity can be controlled and a continuous electric flux path could be created between the PZT grains. This composite may be poled at low voltage and in shorter time compared with composites without a conductive phase.

Nonisothermal and isothermal discharging currents in polyethylene terephthalate at elevated temperatures

The thermally stimulated discharge current and the isothermally final discharging current have been measured, in vacuum and in different ambient gases for “as-received” polyethylene terephthalate specimens, in order to understand the nature of the origin of the released current in the temperature range from glass–rubber transition temperature up to 220 °C. The behavior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized states in the material. The current spectrum is determined by the space-charge existing in the as-received sample, and by the adsorbed and/or absorbed gases and water vapors. The movement of the ions, resulting from the interaction of the adsorbed and absorbed gases with the parasitic space charge, in the field produced by the space charge, is responsible for observed change in polarity of the current during nonisothermal and/or isothermal measurements and for the appearance of the ρ or space-charge p...

PTCa/PEKK piezo-composites for acoustic emission detection

Abstract Piezoelectric ceramic/polymer composites with mixed connectivity combine the high values of piezoelectric coefficients of the ferroelectric ceramics and the formability and flexibility of the polymer, making them suitable for in situ acoustic emission (AE) sensors. The present paper reports a study of the piezoelectric and dielectric properties of the ceramic/polymer composite films composed of the calcium modified lead titanate (PTCa) ceramic and the thermoplastic polymer, polyetherketoneketone (PEKK). The mixed connectivity model is used to calculate the percentage of 1–3 connectivity in each composite. Additionally, the sensors were embedded in a carbon fibre reinforced composite (CFRC) and were able to detect simulated AE.

Dielectric relaxation of vegetable-based polyurethane

Vegetable-based polyurethane (PU) was prepared in the thin film form by spin coating. This polymer is synthesised from castor oil, which can be extracted from the seeds of a native plant in Brazil called mamona. This polymer is biocompatible and is being used as material for artificial bone. The PU was characterised by dielectric spectroscopy in a wide range of frequency (10−5 Hz to 105 Hz) and by thermally stimulated discharge current (TSDC) measurements. The glass transition temperature (Tg = 39°C) was determined and using the initial rise method the activation energy was found to be 1.58 eV.