Imperial College London

ProfessorRogerWhatmore

Faculty of EngineeringDepartment of Materials

Visiting Professor
 
 
 
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r.whatmore

 
 
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Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

343 results found

Whatmore RW, Ward SJ, 2023, Pyroelectric infrared detectors and materials—A critical perspective, Journal of Applied Physics, Vol: 133, Pages: 1-28, ISSN: 0021-8979

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 “figures-of-merit” (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 (0.1–100 Hz). 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[(Mg⅓Nb⅔)xTi1−x]O3 system and Na½Bi½TiO3–K½Bi½TiO3 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 perfo

Journal article

Roleder K, Majchrowski A, Lazar I, Whatmore RW, Glazer AM, Kajewski D, Koperski J, Soszynski Aet al., 2022, Monoclinic domain populations and enhancement of piezoelectric properties in a PZT single crystal at the morphotropic phase boundary, PHYSICAL REVIEW B, Vol: 105, ISSN: 2469-9950

Journal article

Cochard C, Granzow T, Fernandez-Posada CM, Carpenter MA, McQuaid RGP, Guy JGM, Whatmore RW, Gregg JMet al., 2021, Influence of charged walls and defects on DC resistivity and dielectric relaxations in Cu-Cl boracite, APPLIED PHYSICS LETTERS, Vol: 119, ISSN: 0003-6951

Journal article

Whatmore R, You Y-M, Xiong R-G, Eom C-Bet al., 2021, 100 years of ferroelectricity—A celebration, APL Materials, Vol: 9, ISSN: 2166-532X

Journal article

Whatmore RW, 2021, Pyroelectric crystals, ceramics, and thin films for IR sensors, Encyclopedia of Materials: Technical Ceramics and Glasses, Pages: 139-150, ISBN: 9780128185421

Pyroelectric infrared detectors are widely used in a host of applications, ranging from motion sensors used for people detection in intruder alarms, motion sensors, and light switches, through spectroscopic analysis equipment used in environmental monitoring to thermal imaging. There is a huge range of single crystal, polymer, ceramic, and thin-film materials that exhibit pyroelectricity. This article reviews the key properties needed from a pyroelectric material in order to get the best performance out of a device using it and compares and contrasts the different materials used and proposed for use in this application.

Book chapter

Guy JGM, Cochard C, Aguado-Puente P, Soergel E, Whatmore RW, Conroy M, Moore K, Courtney E, Harvey A, Bangert U, Kumar A, McQuaid RGP, Gregg JMet al., 2021, Anomalous motion of charged domain walls and associated negative capacitance in copper-chlorine boracite., Advanced Materials, Vol: 33, Pages: 1-10, ISSN: 0935-9648

During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with external electric fields. Favorably oriented dipolar regions (domains) grow at the expense of those in unfavorable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, the discovery that specific charged 90°domain walls in copper-chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarization is anti-aligned with the applied field, is reported. Polarization-field (P-E) hysteresis loops, inferred from optical imaging, show negative gradients and non-transient negative capacitance, throughout the P-E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this anomalous wall motion is directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency. This passes Jonscher's test for the misinterpretation of positive inductance and gives confidence that field-induced motion of these specific charged domain walls generates a measurable negative capacitance contribution to the overall dielectric response.

Journal article

Berenov A, Petrov P, Moffat B, Phair J, Allers L, Whatmore Ret al., 2021, Pyroelectric and photovoltaic properties of Nb doped PZT thin films, APL Materials, Vol: 9, ISSN: 2166-532X

Nb-doped lead zirconate titanate (PZT) films with up to 12 at. % of Nb were co-sputtered from oxide PZT and metallic Nb targets at a substrate temperature of 600 °C. Up to 4 at. % of Nb was doped into the perovskite structure with the formation of B-site cation vacancies for charge compensation. The preferential (111) PZT orientation decreased with Nb-doping within the solid solution region. The ferroelectric response of the films was affected by the large values of the internal field present in the samples (e.g., −84.3 kV cm−1 in 12 at. % Nd doped films). As-deposited unpoled films showed large values of the pyroelectric coefficient due to self-poling. The pyroelectric coefficient increased with Nb-doping and showed a complex dependence on the applied bias. The photovoltaic effect was observed in the films. The value of the photocurrent increased with the A/B ratio. The combined photovoltaic–pyroelectric effect increased the values of the measured current by up to 47% upon light illumination.

Journal article

Crossley S, Whatmore RW, Mathur ND, Moya Xet al., 2021, Quasi-indirect measurement of electrocaloric temperature change in PbSc0.5Ta0.5O3 via comparison of adiabatic and isothermal electrical polarization data, APL Materials, Vol: 9, ISSN: 2166-532X

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 ∼2 K due to a maximum field change of 26 kV cm−1 for starting temperatures in the range of 300 K–315 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.

Journal article

Whatmore RW, 2021, Pyroelectric Crystals, Ceramics, and Thin Films for IR Sensors, Encyclopedia of Materials: Technical Ceramics and Glasses: Volume 1-3, ISBN: 9780128185421

Pyroelectric infrared detectors are widely used in a host of applications, ranging from motion sensors used for people detection in intruder alarms, motion sensors, and light switches, through spectroscopic analysis equipment used in environmental monitoring to thermal imaging. There is a huge range of single crystal, polymer, ceramic, and thin-film materials that exhibit pyroelectricity. This article reviews the key properties needed from a pyroelectric material in order to get the best performance out of a device using it and compares and contrasts the different materials used and proposed for use in this application.

Book chapter

Fernandez-Posada CM, Cochard C, Gregg JM, Whatmore RW, Carpenter MAet al., 2020, Order-disorder, ferroelasticity and mobility of domain walls in multiferroic Cu-Cl Boracite., Journal of Physics: Condensed Matter, Vol: 33, Pages: 1-11, ISSN: 0953-8984

Domain walls in Cu-Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F4 ̅3c → Pca21 to be modelled simply as being close to the order-disorder limit. High acoustic loss in the cubic phase, revealed by Resonant Ultrasound Spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ~10-5-10-6s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order-disorder process are immobile on this time scale. A Debye loss peak accompanied by ~1% elastic stiffening at ~40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ≥~0.01-0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

Journal article

Keeney L, Smith RJ, Palizdar M, Schmidt M, Bell AJ, Coleman JN, Whatmore RWet al., 2020, Ferroelectric behavior in exfoliated 2D aurivillius oxide flakes of sub-unit cell thickness, Advanced Electronic Materials, Vol: 6, Pages: 1-12, ISSN: 2199-160X

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 one‐half 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 field‐on and field‐off 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 electrode–oxide interface. The extracted values of spontaneous polarization, 0.04(±0.02) C m−2 and electrostrictive coefficient, 2(±0.1) × 10−2 m4 C−2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semi‐empirical 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.

Journal article

Crossley S, Nair B, Whatmore RW, Moya X, Mathur NDet al., 2019, Electrocaloric cooling cycles in lead scandium tantalate with true regeneration via field variation, Physical Review X, Vol: 9, ISSN: 2160-3308

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 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 are identified, unlike normal, from adiabatic measurements of polarization, at nearby measurement set temperatures, without assuming a constant heat capacity. The resulting high-resolution field-temperature-entropy maps of our material, and a small clamped companion sample, are 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.

Journal article

Whatmore R, 2019, Party poopers should look to their own helium waste, NEW SCIENTIST, Vol: 243, Pages: 26-26, ISSN: 0262-4079

Journal article

Peters JJP, Sanchez AM, Walker D, Whatmore R, Beanland Ret al., 2019, Quantitative high-dynamic-range electron diffraction of polar nanodomains in Pb2ScTaO6, Advanced Materials, Vol: 31, ISSN: 0935-9648

Highly B‐site ordered Pb2ScTaO6 crystals are studied as a function of temperature via dielectric spectroscopy and in situ high‐dynamic‐range electron diffraction. The degree of ordering is examined on the local and macroscopic scale and is determined to be 76%. Novel analysis of the electron diffraction patterns provides structural information with two types of antiferroelectric displacements determined to be present in the polar structure. It is then found that a low‐temperature transition occurs on cooling at ≈210 K that is not present on heating. This phenomenon is discussed in terms of the freezing of dynamic polar nanodomains where a high density of domain walls creates a metastable state.

Journal article

McQuaid R, Campbell M, Whatmore R, Kumar A, Gregg JMet al., 2017, Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite, Nature Communications, Vol: 8, ISSN: 2041-1723

Ferroelectric domain walls constitute a completely new class of sheet-like functional material. Moreover, since domain walls are generally writable, erasable, and mobile, they could be useful in functionally agile devices: for example, creating and moving conducting walls could make or break electrical connections in new forms of reconfigurable nanocircuitry. However, significant challenges exist: site-specific injection and annihilation of planar walls, which show robust conductivity, has not been easy to achieve. Here, we report the observation, mechanical writing and controlled movement of charged conducting domain walls in the improper ferroelectric Cu3B7O13Cl. Walls are straight, tens of microns long, and exist as a consequence of elastic compatibility conditions between specific domain pairs. We show that site-specific injection of conducting walls of up to hundreds of microns in length can be achieved through locally applied point-stress and, once created, that they can be moved and repositioned using applied electric fields.

Journal article

Keeney L, Downing C, Schmidt M, Pemble ME, Nicolosi V, Whatmore Ret al., 2017, Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material, Scientific Reports, Vol: 7, ISSN: 2045-2322

The five-layer Aurivillius phase Bi6TixFeyMnzO18 system is a rare example of a single-phase room temperature multiferroic material. To optimise its properties and exploit it for future memory storage applications, it is necessary to understand the origin of the room temperature magnetisation. In this work we use high resolution scanning transmission electron microscopy, EDX and EELS to discover how closely-packed Ti/Mn/Fe cations of similar atomic number are arranged, both within the perfect structure and within defect regions. Direct evidence for partitioning of the magnetic cations (Mn and Fe) to the central three of the five perovskite (PK) layers is presented, which reveals a marked preference for Mn to partition to the central layer. We infer this is most probably due to elastic strain energy considerations. The observed increase (>8%) in magnetic cation content at the central PK layers engenders up to a 90% increase in potential ferromagnetic spin alignments in the central layer and this could be significant in terms of creating pathways to the long-range room temperature magnetic order observed in this distinct and intriguing material system.

Journal article

Whatmore R, 2017, Ferroelectric materials, Springer Handbooks

Ferroelectric materials offer a wide range of useful properties. These include ferroelectric hysteresis (used in nonvolatile memories), high permittivities (used in capacitors), high piezoelectric effects (used in sensors, actuators and resonant wave devices such as radio-frequency filters), high pyroelectric coefficients (used in infra-red detectors), strong electro-optic effects (used in optical switches) and anomalous temperature coefficients of resistivity (used in electric-motor overload-protection circuits). In addition, ferroelectrics can be made in a wide variety of forms, including ceramics, single crystals, polymers and thin films – increasing their exploitability. This chapter gives an account of the basic theories behind the ferroelectric effect and the main ferroelectric material classes, discussing how their properties are related to their composition and the different ways they are made. Finally, it reviews the major applications for this class of materials, relating the ways in which their key functional properties affect those of the devices in which they are exploited.

Book chapter

Whatmore R, 2017, Ferroelectric Materials, SPRINGER HANDBOOK OF ELECTRONIC AND PHOTONIC MATERIALS, 2ND EDITION, Editors: Kasap, Capper, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 589-614, ISBN: 978-3-319-48931-5

Book chapter

Faraz A, Maity T, Schmidt M, Deepak N, Roy S, Pemble ME, Whatmore RW, Keeney Let al., 2016, Direct visualization of magnetic-field-induced magnetoelectric switching in multiferroic aurivillius phase thin films, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 100, Pages: 975-987, ISSN: 0002-7820

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.

Journal article

Keeney L, Schmidt M, Amann A, Maity T, Deepak N, Ahmad F, Petkov N, Roy S, Pemble ME, Whatmore RWet al., 2016, 25. Novel approaches for genuine single phase room temperaturemagnetoelectric multiferroics, Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects, Volume I & II, Publisher: Wiley, ISBN: 978-1-118-93575-0

This chapter reviews approaches currently under investigation for the fabrication of single-phase magnetoelectric multiferroics, from bulk ceramics to those in thin-film form. It presents an approach of inserting magnetic ions into the Aurivillius phase, layer-structured ferroelectric materials, whereby thin films of average composition Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) demonstrate room temperature ferroelectricity, ferromagnetism, and magnetoelectric coupling. The chapter also discusses the importance of careful microstructural analysis of the materials and the application of a statistical model to determine a confidence level that the observed effects are from genuine single-phase magnetoelectric multiferroics. It reviews how careful phase analysis and statistical treatment of the data confirmed that the B6TFMO phase is a single-phase multiferroic to a confidence level of 99.5%. Finally, it summarizes how direct evidence of magnetoelectric coupling in the B6TFMO thin films was obtained. This review demonstrates that with materials development and design, the development of room temperature multiferroic materials can be achieved.

Book chapter

Skiadopoulou S, Kamba S, Drahokoupil J, Kroupa J, Deepak N, Pemble ME, Whatmore RWet al., 2015, Comment on "Interesting Evidence for Template-Induced Ferroelectric Behavior in Ultra-Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates", ADVANCED FUNCTIONAL MATERIALS, Vol: 26, Pages: 642-646, ISSN: 1616-301X

X-ray diffraction, second-harmonic generation and infrared reflectance investigations reveal no evidence for a polar phase or ferroelectric phase transition in 1.6% tensile strained anatase TiO2 thin films. This indicates that the previously-reported potential ferroelectric behaviour, observed using piezoelectric force microscopy, may have been defect related, or the polar distortion is too small to detect using these methods.

Journal article

Douglas AM, Kumar A, Whatmore RW, Gregg JMet al., 2015, Local conductance: A means to extract polarization and depolarizing fields near domain walls in ferroelectrics, Applied Physics Letters, Vol: 107, ISSN: 0003-6951

Conducting atomic force microscopy images of bulk semiconducting BaTiO3 surfaces show clearstripe domain contrast. High local conductance correlates with strong out-of-plane polarization(mapped independently using piezoresponse force microscopy), and current-voltage characteristicsare consistent with dipole-induced alterations in Schottky barriers at the metallic tip-ferroelectricinterface. Indeed, analyzing current-voltage data in terms of established Schottky barrier modelsallows relative variations in the surface polarization, and hence the local domain structure, to bedetermined. Fitting also reveals the signature of surface-related depolarizing fields concentratednear domain walls. Domain information obtained from mapping local conductance appears to bemore surface-sensitive than that from piezoresponse force microscopy. In the right materials systems,local current mapping could therefore represent a useful complementary technique for evaluatingpolarization and local electric fields with nanoscale resolution

Journal article

Deepak N, Carolan P, Keeney L, Zhang PF, Pemble ME, Whatmore RWet al., 2015, Bismuth Self-Limiting Growth of Ultrathin BiFeO3 Films, CHEMISTRY OF MATERIALS, Vol: 27, Pages: 6508-6515, ISSN: 0897-4756

Bismuth ferrite (BiFeO3) is a widely studied material, because of its interesting multiferroic properties. Bismuth self-limiting growth of single-phase BiFeO3 (BFO) has previously been achieved using molecular beam epitaxy (MBE), but the growth of BFO by chemical vapor deposition (CVD) has proved to be very challenging, because of the volatile nature of bismuth. The growth window regarding temperature, pressure, and precursor flow rates that will give a pure perovskite BFO phase is normally very small. In this work, we have studied the metal–organic CVD (MOCVD) growth of epitaxial BFO thin films on SrTiO3 substrates and found that by carefully controlling the amount of the iron precursor, Fe(thd)3 (where thd = 2,2,6,6 tetra-methyl-3,5-heptanedionate), we were able to achieve bismuth self-liming growth, for the first time. The effect of the volume of the bismuth and iron precursors injected on the growth of BFO thin films is reported, and it has been found that the phase-pure films can be prepared when the Bi/Fe ratios are between 1.33 and 1.81 under temperature and pressure conditions of 650 °C and 10 mbar, respectively, and where the O2 gas flow was kept constant to 1000 sccm out of a total gas flow of 3000 sccm. Piezoresponse force microscopy (PFM) studies demonstrate the presence of bipolar switching in ultrathin BFO films.

Journal article

Deepak N, Carolan P, Keeney L, Pemble ME, Whatmore RWet al., 2015, Tunable nanoscale structural disorder in Aurivillius phase, n=3 Bi4Ti3O12 thin films and their role in the transformation to n=4, Bi5Ti3FeO15 phase, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 5727-5732, ISSN: 2050-7526

Naturally super-latticed Aurivillius phase ferroelectrics can accommodate various magnetic ions, opening up the possibility of making new room temperature multiferroics. Here, we studied the growth of single-phase Aurivillius phase Bi5Ti3FeO15 (BTFO) thin films, grown onto single crystalline SrTiO3 (STO) substrates, by doping Bi4Ti3O12 (BTO) with iron by liquid injection metal–organic chemical vapour deposition. The crystalline properties of the resulting films were characterized by X-ray diffraction and transmission electron microscopy. It has been found that the structural properties of the films depend strongly on the relative iron and titanium precursor injection volumes. Nanoscale structural disorder starts to occur in BTO films on the onset of iron precursor flow. A small iron precursor flow causes the formation of half-unit cells of BTFO inside BTO lattice, which in turns causes disorder in BTO films. This disorder can be tuned by varying iron content in the film. Atomic force microscopy shows how the growth mode switches from island growth to layer-by-layer growth mode as the composition changes from BTO to BTFO.

Journal article

Alpay SP, Mantese J, Trolier-McKinstry S, Zhang Q, Whatmore RWet al., 2014, Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion, MRS Bulletin, Vol: 39, Pages: 1099-1111, ISSN: 0883-7694

Thin-film electrocaloric and pyroelectric sources for electrothermal energy interconversion have recently emerged as viable means for primary and auxiliary solid-state cooling and power generation. Two significant advances have facilitated this development: (1) the formation of high-quality polymeric and ceramic thin films with figures of merit that project system-level performance as a large percentage of Carnot efficiency and (2) the ability of these newer materials to support larger electric fields, thereby permitting operation at higher voltages. This makes the power electronic architectures more favorable for thermal to electric energy interconversion. Current research targets to adequately address commercial device needs including reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly freestanding elements with thicknesses in the range of 1–10 μm. This article describes the current state-of-the-art materials, thermodynamic cycles, and device losses and points toward potential lines of research that would lead to substantially better figures of merit for electrothermal energy interconversion.

Journal article

Dul'kin E, Salje EKH, Aktas O, Whatmore RW, Roth Met al., 2014, Ferroelectric precursor behavior of highly cation-ordered PbSc0.5Ta0.5O3 detected by acoustic emission: Tweed and polar nanoregions, Applied Physics Letters, Vol: 105, ISSN: 1077-3118

Highly ordered ferroelectric PbSc0.5Ta0.5O3 crystals were studied by acoustic emission over a widetemperature range. Acoustic emission was found at three characteristic temperatures: 330, 409, and600 K, which are close to those, known from the same disordered crystals, containing polarnanoregions. The microstructure in our crystals contains structural “tweed” rather thannanoregions. The coincidence of acoustic emission temperatures points towards a close structuralrelationship between nanoregions and “tweed.” Under electric field, these temperatures shiftsimilar to “critical end point” behavior. The obtained data prove that acoustic emission detectssignals in a wider parameter space than previously expected.

Journal article

Schmidt M, Amann A, Keeney L, Pemble ME, Holmes JD, Petkov N, Whatmore RWet al., 2014, Absence of Evidence ≠ Evidence of Absence: Statistical Analysis of Inclusions in Multiferroic Thin Films, SCIENTIFIC REPORTS, Vol: 4, ISSN: 2045-2322

Journal article

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