95 research outputs found
Pyroelectric infrared detectors and materialsāA critical perspective
Pyroelectric infrared detectors (PIRDs) have a number of advantages over other IR sensors, including room-temperature operation, wide wavelength sensitivity, and low cost, leading to their use in many applications and a market expected to reach U.S.$68 million by 2025. Physical models that can be used to accurately predict the performances of PIRDs of different types are reviewed in detail. All polar dielectrics exhibit the pyroelectric effect, so there are many materials potentially available for use in PIRDs. Traditionally, a range of āfigures-of-meritā (FoMs) are employed to aid the selection of the best material to use in a given application. These FoMs, and their utility in determining how a given pyroelectric material will behave in a PIRD, are reviewed in the light of the physical models and the availability of dielectric data, which cover the frequency ranges of greatest interest for PIRDs (0.1ā100āHz). The properties of several pyroelectric materials are reviewed, and models are derived for their dielectric properties as functions of frequency. It is concluded, first, that the availability of full-frequency dielectric data is highly desirable if accurate predictions of device performance are to be obtained from the models and that second, the FoMs have practical utility in only very limited circumstances. Thus, they must be used with considerable care and circumspection. The circumstances under which each FoM is likely to give a good prediction for utility are discussed. The properties of some recently researched pyroelectric materials, including lead-containing single crystals in the Pb[(Mgā
Nbā
)xTi1āx]O3 system and NaĀ½BiĀ½TiO3āKĀ½BiĀ½TiO3 based lead-free crystals and ceramics, are reviewed in the light of this, and their properties and potential for device applications compared with the industry-standard material, LiTaO3. It is concluded that while there is potential for significant device performance improvements by using improved materials, especially with the PMN-PT-based materials, factors such as temperature stability, uniformity, and ease-of-processing are at least as important as device performance in determining material utility. The properties reported for the new lead-free materials do not, as yet, promise a performance likely to compete with LiTaO3 for mm-scale detectors, a material that is both readily available and lead-free
Electrocaloric Cooling Cycles in Lead Scandium Tantalate with True Regeneration via Field Variation
There is growing interest in heat pumps based on materials that show thermal
changes when phase transitions are driven by changes of electric, magnetic or
stress field. Importantly, regeneration permits sinks and loads to be thermally
separated by many times the changes of temperature that can arise in the
materials themselves. However, performance and parameterization are compromised
by net heat transfer between caloric working bodies and heat transfer fluids.
Here we show that this net transfer can be avoided-resulting in true, balanced
regeneration-if one varies the applied electric field while an electrocaloric
(EC) working body dumps heat on traversing a passive fluid regenerator. Our EC
working body is represented by bulk PbSc0.5Ta0.5O3 (PST) near its first-order
ferroelectric phase transition, where we record directly measured adiabatic
temperature changes of up to 2.2 K. Indirectly measured adiabatic temperature
changes of similar magnitude were identified, unlike normal, from adiabatic
measurements of polarization, at nearby starting temperatures, without assuming
a constant heat capacity. The resulting high-resolution
field-temperature-entropy maps of our material, and a small clamped companion
sample, were used to construct cooling cycles that assume the use of an ideal
passive regenerator in order to span 20 K. These cooling cycles possess
well defined coefficients of performance that are bounded by well defined
Carnot limits, resulting in large (50%) well defined efficiencies that are
not unduly compromised by a small field hysteresis. Our approach permits the
limiting performance of any caloric material in a passive regenerator to be
established, optimized and compared; provides a recipe for true regeneration in
prototype cooling devices; and could be extended to balance active
regeneration.Gates Cambridge, the Winton Programme for the Physics of Sustainabilit
Ultra high resolution of PZT 30/70 domains as imaged by PFM
iezoforce microscopy (PFM) has been used to determine the domain structure of lead zirconate titanate (PZT) (30/70) on an indium tin oxide (ITO)/glass substrate with a TiO2 boundary layer. The PZT nucleates into the perovskite form in a random crystallographic manner, which leads to a random domain structure in the final film. Using PFM it has been possible to visualize the domain structure of the PZT and determine that the domain structure has features as fine as 8 nm herringbone patterns. The possible impact of these structures for future devices utilizing nanoscale features of PZT and especially FeRAM developments is highlighted
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Ferroelectric precursor behavior in PbSc<inf>0.5</inf>Ta <inf>0.5</inf>O<inf>3</inf> detected by field-induced resonant piezoelectric spectroscopy
A novel experimental technique, resonant piezoelectric spectroscopy (RPS), has been applied to investigate polar precursor effects in highly (65%) B-site ordered PbSc0.5Ta0.5O3 (PST), which undergoes a ferroelectric phase transition near 300 K. The cubic-rhombohedral transition is weakly first order, with a coexistence interval of ā¼4 K, and is accompanied by a significant elastic anomaly over a wide temperature interval. Precursor polarity in the cubic phase was detected as elastic vibrations generated by local piezoelectric excitations in the frequency range 250ā710 kHz. The RPS resonance frequencies follow exactly the frequencies of elastic resonances generated by conventional resonant ultrasound spectroscopy (RUS) but RPS signals disappear on heating beyond an onset temperature, Tonset, of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between Tonset and the transition point, Ttrans=300 K, has locally polar symmetry even while it remains macroscopically cubic. It is proposed that this precursor behavior could involve the development of a tweed microstructure arising by coupling between strain and multiple order parameters, which can be understood from the perspective of Landau theory. As a function of temperature the transition is driven by the polar displacement P and the order parameter for cation ordering on the crystallographic B site Qod. Results in the literature show that, as a function of pressure, there is a separate instability driven by octahedral tilting for which the assigned order parameter is Q. The two mainly displacive order parameters, P and Q, are unfavorably coupled via a biquadratic term Q2P2, and complex tweedlike fluctuations in the precursor regime would be expected to combine aspects of all the order parameters. This would be different from the development of polar nanoregions, which are more usually evoked to explain relaxor ferroelectric behavior, such as occurs in PST with a lower degree of B-site order.RUS facilities in Cambridge were established through support from the NERC NE/B505738/1) to MAC. EKHS thanks the Leverhulme foundation (RG66640) and EPSRC (RG66344) for financial support.This is the accepted version of an original article published in Physical Review B and available online at http://link.aps.org/doi/10.1103/PhysRevB.88.174112
Quasi-indirect measurement of electrocaloric temperature change in PbSc0.5Ta0.5O3 via comparison of adiabatic and isothermal electrical polarization data
Electrically driven adiabatic changes of temperature are identified in the archetypal electrocaloric material PbSc0.5Ta0.5O3 by comparing isothermal changes of electrical polarization due to the slow variation of electric field and adiabatic changes of electrical polarization due to the fast variation of electric field. By obtaining isothermal (adiabatic) electrical polarization data at measurement (starting) temperatures separated by <0.4 K, we identify a maximum temperature change of ā¼2 K due to a maximum field change of 26 kV cmā1 for starting temperatures in the range of 300 Kā315 K. These quasi-indirect measurements combine with their direct, indirect, and quasi-direct counterparts to complete the set and could find routine use in the future
Ferroelectric Behavior in Exfoliated 2D Aurivillius Oxide Flakes of SubāUnit Cell Thickness
Ferroelectricity in ultrasonically exfoliated flakes of the layered Aurivillius oxide Bi5Ti3Fe0.5Co0.5O15 with a range of thicknesses is studied. These flakes have relatively large areas (linear dimensions many times the film thickness), thus classifying them as 2D materials. It is shown that ferroelectricity can exist in flakes with thicknesses of only 2.4 nm, which equals oneāhalf of the normal crystal unit cell. Piezoresponse force microscopy (PFM) demonstrates that these very thin flakes exhibit both piezoelectric effects and that the ferroelectric polarization can be reversibly switched. A new model is presented that permits the accurate modeling of the fieldāon and fieldāoff PFM time domain and hysteresis loop responses from a ferroelectric during switching in the presence of charge injection, storage, and decay through a Schottky barrier at the electrodeāoxide interface. The extracted values of spontaneous polarization, 0.04(Ā±0.02) C mā2 and electrostrictive coefficient, 2(Ā±0.1) Ć 10ā2 m4 Cā2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semiāempirical scaling law expected for ferroelectric materials. This constitutes the first evidence for ferroelectricity in a 2D oxide material, and it offers the prospect of new devices that might use the useful properties associated with the switchable ferroelectric spontaneous polarization in a 2D materials format
Direct visualization of magnetic-field-induced magnetoelectric switching in multiferroic aurivillius phase thin films
Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi6Ti2.8Fe1.52Mn0.68O18, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi6Ti2.99Fe1.46Mn0.55O18, via the liquid injection chemical vapor deposition technique. High-resolution magnetic measurements reveal a considerably higher in-plane ferromagnetic signature than CSD grown films (MS=24.25 emu/g (215 emu/cm3), MR=9.916 emu/g (81.5 emu/cm3), HC=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in-plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD-grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching
Active layers of high-performance lead zirconate titanate at temperatures compatible with silicon nano- and microelecronic devices
Applications of ferroelectric materials in modern microelectronics will be greatly encouraged if the
thermal incompatibility between inorganic ferroelectrics and semiconductor devices is overcome.
Here, solution-processable layers of the most commercial ferroelectric compound ā morphotrophic
phase boundary lead zirconate titanate, namely Pb(Zr0.52Ti0.48)O3 (PZT) ā are grown on silicon
substrates at temperatures well below the standard CMOS process of semiconductor technology.
The method, potentially transferable to a broader range of Zr:Ti ratios, is based on the addition of
crystalline nanoseeds to photosensitive solutions of PZT resulting in perovskite crystallization from
only 350 Ā°C after the enhanced decomposition of metal precursors in the films by UV irradiation. A
remanent polarization of 10.0 Ī¼C cmā2 is obtained for these films that is in the order of the switching
charge densities demanded for FeRAM devices. Also, a dielectric constant of ~90 is measured at
zero voltage which exceeds that of current single-oxide candidates for capacitance applications. The
multifunctionality of the films is additionally demonstrated by their pyroelectric and piezoelectric
performance. The potential integration of PZT layers at such low fabrication temperatures may redefine
the concept design of classical microelectronic devices, besides allowing inorganic ferroelectrics to
enter the scene of the emerging large-area, flexible electronics
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