Sidney B. Lang

Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool

Changes in the net dipole moment of certain materials form the basis for a broad range of IR detectors.

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.

Laser intensity modulation method (LIMM): Experimental techniques, theory and solution of the integral equation

Abstract The Laser Intensity Modulation Method (LIMM) was developed for the determination of the spatial distribution of polarization or space charge through the thickness of a dielectric sample. It has been used in measurements on a number of ferroelectric ceramics and polymers, single crystals and pyroelectric glass-ceramics. Its applications have included poling studies, determination of crystallite orientation in glass-ceramics and influence of surface treatments on polarization of ceramics. The experimental technique and the theory are reviewed, and modifications for increase of accuracy are discussed. A major difficulty in the analysis of LIMM measurements is the necessity of solving a Fredholm integral equation of the 1st kind using experimental data. This is an ill-posed problem which can inherently yield a number of solutions, each of which satisfies the measured data to within experimental accuracy. A technique was developed for solving this class of equations by a modification of the constraine...

Conductivity and space charge in LDPE containing nano- and micro-sized ZnO particles

DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm.

Ferroelectric Polarization in Nanocrystalline Hydroxyapatite Thin Films on Silicon

Hydroxyapatite nanocrystals in natural form are a major component of bone- a known piezoelectric material. Synthetic hydroxyapatite is widely used in bone grafts and prosthetic pyroelectric coatings as it binds strongly with natural bone. Nanocrystalline synthetic hydroxyapatite films have recently been found to exhibit strong piezoelectricity and pyroelectricity. While a spontaneous polarization in hydroxyapatite has been predicted since 2005, the reversibility of this polarization (i.e. ferroelectricity) requires experimental evidence. Here we use piezoresponse force microscopy to demonstrate that nanocrystalline hydroxyapatite indeed exhibits ferroelectricity: a reversal of polarization under an electrical field. This finding will strengthen investigations on the role of electrical polarization in biomineralization and bone-density related diseases. As hydroxyapatite is one of the most common biocompatible materials, our findings will also stimulate systematic exploration of lead and rare-metal free ferroelectric devices for potential applications in areas as diverse as in vivo and ex vivo energy harvesting, biosensing and electronics.

Pyroelectric polymer electrets

Pyroelectricity is the electrical response of a material to a change in temperature. It exists in polymers which contain spontaneous or frozen polarization resulting from oriented dipoles. This review describes the physical basis for the existence of the pyroelectric effect and discusses its behavior in a number of amorphous, semicrystalline, crystalline and liquid crystalline polymers. Some of the techniques for measurement of the pyroelectric effect are presented and its use in pyroelectric relaxation spectroscopy and thermal analysis is described. The spatial distribution of polarization and space charge through the thickness of polymer films can be determined by several thermal pyroelectric techniques. Recent developments in the integration of pyroelectric polymer infrared detectors with solid-state silicon devices are described. One of the most important recent applications of polymers is in photopyroelectric spectroscopy which can be used for the determination of a number of thermal and optical properties of materials. Finally, some miscellaneous applications in physics, chemistry and biology are presented.

Pyroelectric, piezoelectric, and photoeffects in hydroxyapatite thin films on silicon

Tofail et al.3 have found that the dipole of HA is the hydroxyl (OH) ion, which lies along the crystallographic c-axis within the tunnel formed by phosphate (PO4) tetrahedra. In adjacent tunnels, the OH ions could point in a parallel direction or in an anti-parallel one. We believe that the high-temperature calcination crystallized the HA film on silicon and converted most of the OH pairs to a parallel configuration. Thus the HA developed a domain structure with randomly oriented dipoles. Upon cooling, sufficient domains reoriented in polarity so as to result in a net polar structure. This is due to the crystalline character of the silicon substrate and texturing. The photocurrent is caused by the silicon alone.

Guide to the Literature of Piezoelectricity and Pyroelectricity. 23

A bibliography is given containing 2067 references published during 2002 and 2003 on piezoelectric and pyroelectric properties of materials and their applications. It contains listings of journal articles with complete bibliographic citations. This bibliography is the continuation of a series published semi-annually.

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.

Technique for the measurement of thermal diffusivity based on the laser intensity modulation method (LIMM)

Abstract A new technique has been developed for measurement of the thermal diffusivity of samples with thicknesses in the range from several micrometers to about a millimeter. The test sample is attached to a thin sheet of electroded and poled polyvinylidene fluoride (PVDF) which acts as a pyroelectric detector. The other surface of the sample is illuminated with a sinusoidally-modulated laser beam which causes the propagation of temperature waves through the sample into the PVDF. The phase retardation of the waves, which is a unique function of the sample thickness and thermal diffusivity, is determined from the pyroelectric current generated by the PVDF. The thermal diffusivity of the sample is then found by matching the experimental phase angle measurements to the analytical solution using a minimization algorithm. Computer simulation calculations are used to make an error analysis. Experimental results for five polymeric and two ceramic materials are presented.