R. W. Whatmore

Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3

An applied electric field can reversibly change the temperature of an electrocaloric material under adiabatic conditions, and the effect is strongest near phase transitions. We demonstrate a giant electrocaloric effect (0.48 kelvin per volt) in 350-nanometer PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of 222°C. A large electrocaloric effect may find application in electrical refrigeration.

Pyroelectric devices and materials

The physics of pyroelectric detectors is reviewed, including a discussion of response and electronic noise and their dependence on device design and material parameters. Other sources of noise are described, particularly as generated by environmental effects such as microphony, together with techniques for their minimisation. The range of ferroelectric materials which have been assessed for use in pyroelectric detectors is reviewed and their properties compared, particularly from the aspect of application to different types of devices. Finally, an account is given of the wide range of applications for which pyroelectric detectors have been used, including a detailed description of both the pyroelectric vidicon and pyroelectric arrays and their application to thermal imagers.

Giant electrocaloric effect in the thin film relaxor ferroelectric 0.9 PbMg1/3Nb2/3O3-0.1 PbTiO3 near room temperature

The authors have recently observed a giant electrocaloric effect (12K in 25V) in 350nm sol-gel PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of 242°C. Here the authors demonstrate a giant electrocaloric effect (5K in 25V) in 260nm sol-gel films of the relaxor ferroelectric 0.9PbMg1∕3Nb2∕3O3–0.1PbTiO3 near the Curie temperature of 60°C. This reduction in operating temperature widens the potential for applications in cooling systems.

Ferroelectric materials for thermal IR detectors

The physics of pyroelectric infra red detectors as it applies to the selection of detector materials is briefly reviewed and the properties of a number of the more commonly used materials are compared. The use of ferroelectrics in the region of the Curie temperature as dielectric bolometers with an applied bias field is assessed and the properties of barium strontium titanate and lead magnesium niobate for use in this mode are reported and their potential compared with the conventional pyroelectrics.

Low temperature crystallization of lead zirconate titanate thin films by a sol-gel method

Pyroelectric lead zirconate titanate (PZT) thin films have been prepared by a sol-gel method and characterized by x-ray diffraction and transmission electron microscopy (TEM). A metastable Pt3Pb intermetallic phase has been identified. The formation of this metastable phase was found to depend on the drying temperature, the thickness of the as-deposited film, annealing temperature, and annealing time. Perovskite PZT was found to nucleate on top of the intermetallic phase, rather than directly on Pt. The improved lattice match between the intermetallic (a0=4.05 A) and perovskite PZT(a0=4.035 A) as compared to between Pt(a0=3.9231 A) and the perovskite is believed to substantially reduce the activation energy for the nucleation of perovskite on Pt. Using this effect, (111) perovskite PZT has been grown at a temperature as low as 440 °C. The formation of the intermetallic phase is believed to facilitate the (111) film orientation. The growth kinetics of the PZT were analyzed using the Avrami model, and from ...

Investigation of the electrocaloric effect in a PbMg2/3Nb1/3O3-PbTiO3 relaxor thin film

Permittivity measurements of a 0.93PMN-0.07PT thin film show a broad maximum near 35 °C, and an anomaly at the depolarizing temperature Td=18 °C on heating only, suggesting a dipolar glass-relaxor phase transition. No structural phase transition at 18 °C is apparent from ferroelectric hysteresis loops taken on field cooling and field heating. These loops show the thermal hysteresis expected for ferroelectric relaxors, which has not hitherto been experimentally verified in PbMg2/3Nb1/3O3-PbTiO3 thin films. Our data suggest the intriguing possibility of a giant electrocaloric effect (ΔT=9 K, ΔE=720 kV cm−1) at and near room temperature.

Pyroelectric ceramics and devices for thermal infra-red detection and imaging

Abstract Pyroelectric detectors offer major advantages in terms of cost and ease-of-operation over cooled photon detectors of long wavelength infra-red radiation. This paper reviews the requirements of a pyroelectric material to maximise its effectiveness in different types of detector and compares the properties of ferroelectric ceramics with those of different single crystal materials. The control of the electrical properties of modified lead zirconate by doping is discussed. The concept of using a ferroelectric close to Tc under an applied electric bias as a dielectric bolometer is introduced and the properties of three different ferroelectric ceramics, barium strontium titanate, lanthanum-doped lead magnesium niobate and lead scandium tantalate are compared. It is shown that figures-of-merit can be achieved in lead scandium tantalate under DC bias which are up to three times greater than more conventional pyroelectric materials. Finally, the use of pyroelectric ceramics is illustrated by reference to ...

Thin-film bulk acoustic resonators and filters using ZnO and lead-zirconium-titanate thin films

This paper presents the findings of a design, modeling, and fabrication study of ZnO and PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ thin-film bulk acoustic resonators and filters. Measurements of the high-frequency responses of ZnO resonators having different area are used to develop an acoustic model that accurately represents resonator impedance data. The models are also used to interpret S-parameter measurements on thin-film PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/-based resonators and a value for the effective coupling coefficient deduced. ZnO and PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ ladder filters were designed based on measured impedance data from single resonators. Ladder filters based on PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ have been fabricated for the first time. It is shown that the high coupling coefficient in PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ leads to bandwidths in the range 100/spl sim/120 MHz at a center frequency of 1.6 GHz, larger than the bandwidths of ZnO-based filters.

A TEM study of ordering in the perovskite, Pb(Sc1/2Ta1/2)O3

Ordering behaviour and the factors which influence the ordering have been studied in both single crystals and hot-pressed ceramics of Pb(Sc1/2Ta1/2)O3 using TEM techniques, including low-temperature microscopy. The characteristics of the structurally-ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases. The superlattice reflections are interpreted in terms of Pb2+ cation displacements. The non-interaction of ordered domains and ferroelectric domains is explained within this model.

Liquid-phase sintering of PZT ceramics

Abstract Lead zirconate titanate (abbreviated as PZT) ceramics are of considerable commercial importance for a host of piezoelectric and pyroelectric applications. Conventionally, many PZT ceramics are sintered at temperatures above 1250°C. Such extreme temperatures are undesirable due to the increased energy consumption, limitation of electrode material and evaporation of volatile components. A liquid-phase sintering aid incorporating Cu2O and PbO is presented which demonstrates a reduction in the required sintering temperature of these ceramics. This new aid is described with particular reference to a commercial PZT, termed Pz26, used industrially for its optimised piezoelectric properties. Pz26 has a composition near the morphotropic phase boundary and possesses a tetragonal crystalline structure. Typically this material is sintered between 1260 and 1300°C for 1 h to achieve the required densification. With the inclusion of sintering aid, sintered densities comparable to those obtained by conventional sintering are achieved at only 800°C. The optimum weight percentage of sintering aid varies for different ceramic materials, particle sizes, morphology and the desired sintering temperature. However, with standard “mixed-oxide” produced Pz26 powder and with a median particle size in the range 1.6–1.7 μm, a value of 5 wt.% allows sintering at 800°C, according to densification, dielectric and piezoelectric measurements (ϵ=873, tan δ=1.13 %, kp=43.1%). When finer grained powder is used (d0.5=1.1 μm), improved properties (ϵ=960, tan δ=1.04%, kp=51.7%) are obtained for an addition of 3 wt.% sintering aid and a sintering temperature of 850°C.