The mechanism of type II pyroelectric detectors

A theoretical model of type II pyroelectric detectors, which considers primary, secondary and tertiary effects, is given. The responsivities vs the target width and frequency are derived and calculated for LiTaO₃ type II pyroelectric detectors. The results are in agreement with those of other experiments.     

The detection mechanism of LiTaO3 type II pyroelectric detectors.: III. The total pyroelectric effects

The contributions of the primary and secondary pyroelectric effects and that of the tertiary pyroelectric effect are both considered in the discussion of the detection mechanism of type II pyroelectric detectors, and the total signal voltage and the voltage responsivity are given and analysed.     

The detection mechanism of LiTaO3 type II pyroelectric detectors.: I. The primary and secondary pyroelectric effects

The contributions of the primary and secondary pyroelectric effects in LiTaO₃ type II pyroelectric detectors are discussed in terms of theories of thermoconductivity and electromagnetic field, and formulas for signal voltage and responsivity are given.     

The off-axis pyroelectric effect observed for lithium tetraborate

We find a pyroelectric current along the 〈110〉 direction of stoichiometric Li₂B₄O₇ so that the pyroelectric coefficient is nonzero but roughly 10⁻³ smaller than along the 〈001〉 direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the 〈110〉 direction can be correlated with anomalies in the elastic stiffness contributing to concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that an inverse piezoelectric effect occurs at the (110) surface but not the (100) surface. Both effects along the 〈110〉 direction or at the (110) surface are distinct the conventional as a bulk pyroelectric effect.     

Induced pyroelectric effect in a planar field

The effect of an induced deformation and the influence of a planar electric field on the ground state of the ferroelectric Ba_0.7Sr_0.3O₃ thin films deposited on (001)-oriented cubic substrates have been investigated. The dependence of the pyroelectric coefficient on the electric field strength E _x in the film plane has been constructed for the values of the induced strain that correspond to different ground states of the film. In the regions where the film can operate in the mode of a dielectric bolometer, there is an electric field in which the pyroelectric coefficient reaches the extreme value. There is also a value of the induced strain at which the pyroelectric effect is most pronounced.     

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe²⁺-doped) lithium niobate crystals (LiNbO₃) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP _s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO₃ and LiNbO₃:Fe (0.1 ppm Fe²⁺) crystals with c axis along the direction of laser propagation.     

The physics of pyroelectric infrared devices

The absorption of radiation with pyroelectric detectors and the thermal properties of these devices are discussed using a simple physical picture — the physics of waves. Considered are the reflection, transmission and interference of electromagnetic and of thermal waves within the pyroelectric sensor arrangement. In particular, thin metal films, quarter wavelength structures, and anti-reflection coatings on metal films as absorber structures are discussed. The effect of the substrate on the pyroelectric response is treated and new figures of merit are introduced for the comparison of sensor materials which are mounted on a heat sink.     

Production of high-quality metal-coated pyroelectric SrTiO3 films

A procedure for the production of metal-coated quasi-amorphous pyroelectric thin SrTiO₃ films is described. The films are grown in a modified 306 Edwards magnetron sputtering setup under controlled thermal conditions and stabilized high-accuracy vacuum and gas pressure conditions. Three-layer 200 nm metal-100 nm SrTiO₃-100 nm metal films are studied. The pyroelectric nature of the electric response of these films to heating is directly established, since metallic contacts made from the same material do not distort the measurement results, which excludes the effect of a contact potential difference on the measurement results.     

Preparation and assessment of piezo- and pyroelectric poly (vinylidene fluoride-hexafluoropropylene) copolymer films

Films of poly(vinylidene fluoride-hexafluoropropylene) copolymer [P(VDF-HFP)] were cast from a dimethylsulfoxide (DMSO)/acetone solution of Solef?85-15 P(VDF-HFP) copolymer powder grade 21508. Undrawn and uniaxially drawn cast copolymer films were investigated with respect to their piezo- and pyroelectric properties. Quasistatic charge integration was employed for the determination of the poling-field dependence of the piezoelectric d₃₁ and pyroelectric p₃ coefficients. In addition, the thermal stability of the pyroelectric effect was studied with a combination of thermally stimulated discharge (TSD) and temperature-modulation techniques. Cast copolymer films could withstand electrical poling fields of up to 400 MV/m. The maximum values of d₃₁=30 pC/N and p₃=49 μC/(m²K) for uniaxially drawn samples are similar to those found on commercial PVDF films and much higher than those on pure PVDF films cast from solution. Samples kept for 5 min at 150 °C still exhibit 30–40% of the initial pyroelectric effect [up to around 20 μC/(m²K)]. After this annealing step, no further decay of the pyroelectric coefficient could be observed during storage at 120 °C for several hours. Received: 5 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

The improvement of pyroelectric properties of PZT thick films on Si substrate by TiOx barrier layer

The effects of TiO_x diffusion barrier layer thickness on the microstructure and pyroelectric characteristics of PZT thick films were studied in this paper. The TiO_x layer was prepared by thermal oxidation of Ti thin film in air and the PZT thick films were fabricated by electrophoresis deposition method (EPD). To demonstrate the barrier effect of TiO_x layer, the electrode/substrate interface and Si content in PZT thick films were characterized by scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS), respectively. The TiO_x barrier thickness shows significant influence on the bottom electrode and the pyroelectric performance of the PZT thick films. The average pyroelectric coefficient of PZT films deposited on 400 nm TiO_x layer was about 8.94 × 10⁻⁹ C/(cm² K), which was improved by 70% than those without diffusion barrier layer. The results showed in this study indicate that TiO_x barrier layer has great potential in fabrication of PZT pyroelectric device.     

The pyroelectric signal from triglycine sulphate

The pyroelectric signal from triglycine sulphate has been studied by a focussed He-Ne laser beam and a phase sensitive detection method. It is observed that anti-parallel domains give rise to pyroelectric signals of opposite polarity. The pyroelectric signal is zero when the laser spot is in between anti-parallel domains. The intensity of the pyroelectric signal is recorded and the domain structure is determined.     

Experimental and theoretical investigation of the pyroelectric effect of the p-n junction in a paraelectric non-polar semiconductor

We describe here the first comprehensive investigation of a pyroelectric response of a p-n junction in a non-polar paraelectric semiconductor. The pyroelectric effect is generated by the, temperature dependent, built-in electrical dipole moment. High quality PbTe p-n junctions have been prepared specifically for this experiment. The pyroelectric effect was excited by a continuous CO₂ laser beam, modulated by a mechanical chopper. The shape and amplitude of the periodic and single-pulse pyroelectric signals were studied as a function of temperature (10-130 K), reverse bias voltage (up to −500 mV) and chopping frequency (4-2000 Hz). The pyroelectric coefficient is ≈10⁻³ μC/cm²K in the temperature region 40-80 K. The developed theoretical model quantitatively describes all the experimental features of the observed pyroelectric effect. The time evolution of the temperature within the p-n junction was reconstructed.     

Potentiality of SBN textured ceramics for pyroelectric applications

Textured Sr_xBa_1−xNb₂O₆ (SBN) ferroelectric ceramics with x = 0.53 and 0.63 were fabricated by hot forging process. The objective was to obtain, in the ceramic form, the strong anisotropy of the electric properties that these materials possess in the single crystal form. Properties such as electric permittivity, pyroelectric coefficient and dielectric loss showed an anisotropy between the perpendicular and parallel direction with respect to the pressure axis (applied pressure during the forging of the ceramics). A high pyroelectric coefficient, comparable with these published for SBN single crystal with the same composition, was obtained for the SBN53/47 ceramic, when measured in the perpendicular direction to the pressing axis. From the calculus of the pyroelectric figures of merit, it was possible to conclude that the textured SBN53/47 ceramic has a high potential to be used as pyroelectric elements. This ceramic, cut in the perpendicular direction to the pressing axis, possess high potential as fast pulse detector but the same ceramic, cut in the parallel direction to the pressing axis, has better properties to be used as large area and point detectors.     

High-performance pyroelectric single crystals for uncooled infrared detection applications

Uncooled pyroelectric infrared detectors based on ferroelectric single crystals 0.74Pb(Mg_1/3Nb_2/3)O₃–0.26PbTiO₃ (PMN–0.26PT) were fabricated. The performances of pyroelectric detectors dependence on detector fabrication temperature, absorption layer, and element thickness were compared. The room-temperature voltage responsivity (R_v) of 200 V/W and specific detectivity (D^*) of 10⁸ cm Hz^1/2/W at 12.5 Hz have been achieved. The results reveal that the better pyroelectric response can be expected by controlling temperature below 70 °C during the fabrication of the pyroelectric detectors, selecting absorption layer with high absorption coefficient, and decreasing the thickness of the elements.     

Sol-gel-derived pyroelectric barium strontium titanate thin films for infrared detector applications

Quality pyroelectric Ba_0.8Sr_0.2TiO₃ films have been successfully fabricated by a sol-gel processing using a highly diluted precursor solution. The remnant polarization of the films decreases with the temperature increasing, which results in a large pyroelectric coefficient at room temperature. Infrared response measured using a 500-K chopped blackbody at room temperature exhibits a typical pyroelectric response waveform. Frequency dependence of the infrared response measurement for a pixel with area of 2.5×10^-3 cm² showed that the maximum response output voltage of 3.2 mV was obtained at 6 Hz. Better infrared response can be expected by the improvement in thermal isolation of the pixels and electrode materials. Received: 26 April 2000 / Accepted: 9 May 2000 / Published online: 9 August 2000     

A discussion on type II pyroelectric detectors

A theoretical model of type II pyroelectric detectors is given. The responsivity vs geometrical and physical parameters of the sensors are derived and calculated. The results are mainly in accord with the experiments.     

Effect of electrode on dielectric susceptibility and pyroelectric properties of a ferroelectric thin film capacitor using landau-khalatnikov theory

The influence of electrodes on the dielectric susceptibility and pyroelectric properties of a ferroelectric thin film with surface transition layers has been investigated within the framework of Landau-Khalatnikov dynamic theory. The contribution of the electrodes is reflected by the depolarization field in the free-energy function. The large electrode effect implies the strong depolarization field in ferroelectric thin films. The results show that the electrode materials can greatly impact the dynamic dielectric and pyroelectric properties of a ferroelectric thin film.     

The influence of structural and electrical parameters of the pyroelectric phase on the pyroelectric properties of a polymer-pyroelectric ceramic composite

The influence of structural and electrical parameters of the pyroelectric phase on the pyroelectric properties of composites based on poly(vinylidene fluoride), vinylidene fluoride-tetrafluoroethylene copolymer (F-42), and pyroelectric ceramics of the lead zirconate titanate family with different structures is investigated. It is revealed that the pyroelectric coefficient of the composite is determined by the reorientation polarization and the mobility of domains in the pyroelectric ceramic phase, which depend on the homogeneous parameter of the spontaneous strain of the perovskite cell.     

对热释电摄像机调制方法的研究

本文主要分析了用于红外干涉测试中的热释电摄像机的工作原理及其调制特性，比较了几种调制方式的优缺点，最后提出了热释电成像的移相调制法，考虑了优选调制频率对测试结果的影响，并给出实测结果。     

The evaluation of the performance of two pyroelectric detectors with vertically aligned multi-walled carbon nanotube coatings

Two LiTaO₃ pyroelectric detectors coated with vertically aligned multi walled carbon nanotube (MWCNT) black coatings were assembled and evaluated using NPL’s detector characterisation facilities. The vertically-aligned nanotube array (VANTA) black coatings were grown on a silicon substrate and subsequently lifted off the silicon and bonded on the pyroelectric crystal substrates. Despite some drawbacks, this method was shown to provide a reliable way of coating delicate substrates such as pyroelectric crystals with VANTA coatings. The performance of the coated and uncoated detectors was evaluated and compared by coating only half of the active area of the test detectors, leaving the other half uncoated. The relative spectral responsivity of the VANTA-coated pyroelectric detectors was shown to be spectrally flat in the 0.8–14 μm wavelength range within the uncertainty of the measurements. The spatial uniformity of response of both detectors exhibited fine structure which was assigned either to the thickness of the VANTA coatings or to their bonding to the pyroelectric crystal. Both coated and uncoated detectors exhibited a super-linear response. This observation was expected in the case of the uncoated detectors, but was surprising in the case of the coated detectors and indicates that the thermal conductivity of vertically aligned multi-walled carbon nanotubes is high along their long axis. The spatial variations of the phase delay experienced by the signal propagating through the VANTA coatings indicate that the thermal diffusivity of the coatings is not spatially uniform.