Percolative clusters in KTaO3:Li

Abstract

Percolation of polarization is considered in solid solutions of incipient ferroelectric KTaO₃: Li. The Li impurities were surrounded by a sphere of the interaction radius. The latter was thought to be proportional to the correlation radius, whichcan be represented through the dielectric susceptibility, the temperature dependence of which is well known. This temperature dependence determines the dependence of the average size of the percolative clusters on temperature. It was shown that, in the Hyper-Rayleigh scattering case, the SHG intensity is just proportional to the average size that gives an opportunity to check the idea of existing the percolative clusters by straight comparison of the experimental and theoretical result.     

Ferroelectric switching and fatigue behavior for PZT/YBCO thin film heterostructures

Abstract

The dielectric and ferroelectric properties for Au/Pb(Zr,Ti)O₃/YBa₂Cu₃O_7?x heterostructures at low temperatures are reported. The fatigue behavior and the ferroelectric switching effect for the structures are also investigated. The PZT/YBCO thin film heterostructures were deposited on MgO(100) substrates by laser ablation. The ferroelectric and dielectric properties and optical response of the oriented PZT films with different thicknesses have been studied over the temperature range from 20 K to 300 K. The dielectric loss of the structure was found to decrease by an order of magnitude when the YBCO bottom electrode became superconducting. A very low fatigue rate of the structure has also been obtained below T _c of YBCO layer.     

Dielectric relaxation phenomenon of rare earth Nd3+ modified bismuth layer ferroelectric ceramics fabricated by plasma active sintering method

Abstract

The crystalline structure, dielectric relaxation and ferroelectric properties of the solid solution, Nd _x Bi_4-x Ti₃O₁₂ (NBIT) compound were measured. The Curie temperature of the NBIT ceramics was determined to be 490°C from dielectric measurements. The dielectric constant of the NBIT ceramics shows a small anisotropic property. Polarization switching was observed using a Sawyer-Tower circuit at 50 Hz. Remnant polarizations and coercive fields could not be confirmed since the hysteresis loops were not saturated. The large dielectric relaxation is observed in the frequency range between 100 kHz and 1 MHz.     

ELECTRICAL PROPERTIES OF LEAD FERROTANTALATE CERAMICS

ABSTRACT

Perovskite structures with high dielectric constants and magnetic properties play an important role in micro- and optoelectronics and have numerous practical applications. Relaxor type ferroelectric ceramics PbFe_1/2Ta_1/2O₃ with perovskite structure was subject of present studies. Pyrochlore free lead ferrotantalate ceramics were produced by solid state technology from oxides. Low- and infra-low frequency studies of dielectric properties of PbFe_1/2Ta_1/2O₃ ceramics have revealed considerable infra-low frequency dispersion at temperatures above the temperature of maximum dielectric permittivity. The observed dispersion described by lemniscates is due to a considerable Maxwell-Wagner relaxation.     

GHZ polarization dynamics in ferroelectric thin films

Abstract

We present results from experiments which measure the local dielectric response of ferroelectric thin films driven by microwave-frequency electric fields. The repetition rate of a mode-locked Ti:Sapphire laser is used to generate a microwave drive signal that is phase-locked to an optical probe pulse and applied to the ferroelectric thin film. The induced polarization change in the ferroelectric film is measured stroboscopically via the electro-optic effect. Polarization images are acquired by scanning the laser beam across the sample in a confocal geometry. Time resolution is achieved by changing the delay between the electrical pump and the optical probe. Initial results show large local phase shifts in the ferroelectric response of closely separated regions of a Ba_0.5Sr_0.5TiO₃ thin film. This new experimental technique may help to understand the physical mechanisms of dielectric loss in these materials.     

Surfaces structure of bulk and thin film ferroelectrics

Abstract

Based on the experimental results of thickness dependences of breakdown voltage and dielectric permittivity for BaTiO₃ family ceramics, (Pb, La)TiO₃ thin films and commercial multilayer capacitors, surface layer structures are discussed. Surface layers inside ferroelectric materials are consisting of non-ferroelectric Kanzig layer with low dielectric permittivity and with higher concentrations of impurities due to the inter diffusion between substrates and bulk, internal stresses induced by lattice mismatch, cubic-tetragonal phase transition, electric field induced anisotropies and internal bias field (space charge field). The Voltage - Current characteristics (V-I curve), D-E loops of ferroelectric materials show asymmetric behaviors. Saturation phenomena of V-I curves are observed only ferroelectric temperature region. The breakdown voltage almost depends on the non-ferroelectric Kanzig layer and the dielectric permittivity depends on the volume fraction of non-ferroelectric parts. Log-log plots of dielectric permittivity and breakdown voltage suggest that the thickness of non-ferroelectric Känzig layer should be at least more than 10～20 nm and in the grain boundary thickness of bulk ferroelectric materials should be more, especially in the liquid phase sintering, the thickness of grain boundary is a order of 0.1～0.2 μm.     

Structure,Ferroelectric, Dielectric and Energy Storage Studies of Ba0.70Ca0.30TiO3, Ba(Zr0.20Ti0.80)O3 Ceramic Capacitors

Ba_0.70Ca_0.30TiO₃-(BCT),Ba(Zr_0.2Ti_0.8)O₃-(BZT) ceramics were fabricated by conventional mixed oxide route to develop inorganic dielectric materials suitable for use as an insulator with high dielectric constant and low energy loss for capacitor applications. The structural phase transition, ferroelectric, dielectric and energy storage properties of BCT, BZT ceramic capacitors were investigated. Room temperature X-ray diffraction (XRD) patterns revealed prominent peaks corresponding to tetragonal perovskite crystal structure for both BCT, BZT solid solutions. Slim ferroelectric hysteresis (P-E) loops were observed for BCT, BZT solid solutions. Temperature dependent dielectric property measurements of BCT, BZT solid solutions have shown a high dielectric constant and low dielectric loss. Room temperature (300K) breakdown field strength and energy densities were obtained from the integral area of P-E loops. For the BCT ceramics, the largest recoverable energy (unreleased energy) density is 1.41 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. For the BZT ceramics, the largest recoverable energy (unreleased energy) density is 0.71 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. Bulk BCT, BZT ceramics have shown interesting energy densities; these might be the strong candidate materials for capacitor applications.     

Ferroelectric lead iron-chromium-nickel niobate films

Abstract

The fabrication of ferroelectric films of modified lead iron niobate by a multiple magnetron sputtering technique with a subsequent rapid thermal annealing at 800°C for 5 seconds is reported. Since the magnetic properties of pure iron preclude its use in magnetron sputtering, a non-magnetic stainless steel was used as one of the target materials resulting in a ferroelectric of composition Pb[(Fe_0.7Cr_0.2Ni_0.1)_0.5Nb_0.5]O₃. The reaction sequence involved in the formation of the ferroelectric perovskite phase has been identified. The films exhibit unsaturated ferroelectric hysteresis loops with a remanent polarization of 15 μC/cm² and a coercive field of 100 kV/cm. The room temperature dielectric constant and dielectric loss at 1 kHz were 640 and 0.1, respectively. The dielectric constant showed a dielectric anomaly as a function of temperature in the form of a broad maximum around 90°C confirming the ferro-para electric phase transition. The films were highly insulating with a room temperature conductivity of ≈1 X^?12 Ω^?1 cm^?1, and an activation energy of 0.8 eV.     

Supression of size effects in ferroelectric films

Abstract

It is widely reported that the dielectric permittivity of ferroelectric films decreases with decreasing film thickness, and understanding and controlling these size effects are very important for charge storage application of these films. By combining phenomenological theory with careful experimental work, we have shown that the form of the boundary condition for the polarization plays a decisive role in the manifestation of size effects in ferroelectric films. We have taken two extreme boundary conditions to prove our point. For the case normal electrodes, it is assumed that the boundary condition for the component of polarization vector at the ferroelectric/electrode interface is P = 0. This case corresponds to the presence of a strong edge field, resulting in “freezing out” of the ferroelectric polarization at the interface and thus exhibiting severe size effects. However, if one utilizes conductive oxide electrodes that are ferroelectric in nature the polarization would not vanish at the ferroelectric/electrode interface and therefore the size effects are largely suppressed. To prove our point and to eliminate grain size, stress, and compositional effects, epitaxial SrTiO₃ thin films with stoichiometric composition on SrTiO₃ single crystal substrates were investigated. In fact, the experimental data also indicates that the use of ferroelectric electrodes indeed suppresses the size effects.     

Shift of Phase Transition Temperature in Strontium Titanate Thin Films

The results of structural and electrical characterizations of SrTiO₃ thin films deposited onto MgO and LaAlO₃ substrates by pulsed laser deposition technique are presented. The influence of substrate and annealing procedure on the crystalline structure and dielectric properties of these ferroelectric thin films are investigated. The obtained experimental data are analyzed in terms of the Landau theory taking into account the room-temperature lattice mismatch of the ferroelectric and substrates as well as the difference in their thermal expansion. It is shown that the behavior of the SrTiO₃ thin films could not be attributed to the effect of the film/substrate mechanical coupling. As a possible nature of the observed behavior one can consider the non-stoichiometry of the film composition caused by the chemical contact of the film with the substrate and by the annealing.     

Strain tensor effects on SrTiO3 incipient ferroelectric phase transition

Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO₃ thin films epitaxially grown on (110) DyScO₃ substrates using molecular beam epitaxy (MBE) are extremely strained (i.e., ～1% in-plane tensional strain) from 3.905 Å of bulk SrTiO₃. The room temperature dielectric constant and its tuning of the films are observed to be 6000 and 75% with an electric field of 1 V/μm, respectively. The control of strain in SrTiO₃ provides a basis for room temperature tunable microwave applications by elevating its phase transition peak to room temperature. Also, a significant in-plane anisotropy in dielectric constant and tuning was observed in these SrTiO₃ films. The observed in-plane anisotropic dielectric properties have been interpreted based on the phenomenological thermodynamics of film strain.     

Diffuse Dielectric Anomaly in Ferroelectric Materials

It has not been clear whether the diffuse dielectric anomaly by Debye-type dielectric relaxation is extrinsic or intrinsic in origin although it has been frequently found in ferroelectric materials regardless of their structures and ferroelectric properties. We experimentally investigated the extrinsic nature of the diffuse dielectric anomaly in ferroelectric oxides and sulfide such as BaTiO₃, Pb_0.9La_0.1TiO₃, and SnP₂S₆. The advanced fitting method using the modified Debye relaxation equation was introduced in order to explain the temperature dependent behavior of the diffuse dielectric anomaly. It was confirmed that the diffuse dielectric anomaly was a competitive phenomenon between the dielectric relaxation and the electrical conduction of the relaxing species. It was also proved that the activation energy of the dielectric relaxation should be always higher than the conductivity activation energy of the relaxing species in the diffuse dielectric anomaly.     

Dielectric anomalies of Pb0.7La0.2TiO3-based perovskite

Dielectric transitions from normal ferroelectric to relaxor-like and then to quantum paraelectric-like behaviour were observed by substituting 10 to 60% of Ca²⁺ for Pb²⁺ into A-site of Pb_0.7La_0.2TiO₃ with the required stoichiometry of Pb_0.7(1−x)Ca_0.7x La_0.2TiO₃ (PCLT). The quantum ferroelectric relation that commonly applies to SrTiO₃-based perovskites, T _maxα (X − X _c )^1/2, fails to describe the observed dielectric anomalies in PCLT, whereby the transition temperature vanishes with a finite slope. Quantum ferroelectric behaviour that exhibits a sharp dielectric peak but violates the Curie-Weiss law was not observed throughout the composition range studied. Unlike the typical quantum paraelectric SrTiO₃, the quantum paraelectric-like behaviour observed for PCLT with x = 0.55 and 0.60 exhibit negative transition temperatures, as shown by the fittings to both the Curie-Weiss and Barrett’s relations. Thermal hysteresis was surprisingly observed in substituted PCLT with x = 0.30 and 0.40 that exhibit frequency dispersive relaxation. To establish a correlation between the dielectric anomalies and structural parameters, analyses on global and local perovskite lattices were carried out using X-ray diffraction technique and micro-Raman spectroscopy.     

Structural and electrical properties of SrBi2VxNb2?xO9 ferroelectric ceramics: Effect of temperature and frequency

Aurivillius type bismuth layered materials have received a lot of attention because of their application in ferroelectric non-volatile random access memories. Among bismuth layer structured ferroelectric ceramics SrBi₂Ta₂O₉ (SBT)/SrBi₂Nb₂O₉ (SBN) are of great interest for researchers because of their fatigue resistance and less distorted structure. Recently vanadium substitution in SBN/SBT has shown interesting electric and dielectric properties. In the present work, processing conditions, microstructure and electrical studies of vanadium doped SBN ferroelectric ceramics have been performed. Samples of compositions SrBi₂V _x Nb_2-x O₉, x = 0.0, 0.1, 0.3, 0.5 were prepared by solid-state reaction technique using high purity oxides / carbonates. The samples were calcined at 700 °C and sintered at 800 °C. X-ray diffractograms show that a single phase layered perovskite structure is formed in all the samples. Effect of partial substitution of pentavalent niobium ion (0.68 ?) by smaller pentavalent vanadium ion (0.59 ?) at B site on the microstructure, Curie temperature, Dielectric constant, Dielectric loss and electrical conductivity have been investigated. Dielectric properties of SBVN have been investigated from room temperature to 500 °C and frequency of 100 Hz to 1 MHz. Dielectric constant values at their respective Curie points are observed to increase with increasing vanadium concentration. Curie temperature is observed to be maximum in x = 0.1 vanadium doped sample. Strong relaxor like dielectric relaxation at the transition temperatures have been observed. With increasing vanadium concentration the dielectric loss is observed to increase significantly. It is also observed that dielectric loss increases with increase in temperature. The variation of conductivities in these samples is also reported.     

Structural, dielectric and conductivity studies of Na2Pb2La2W2Ti4Nb4O30 ferroelectric ceramic

A new single phase orthorhombic ferroelectric ceramic Na₂Pb₂La₂W₂Ti₄Nb₄O₃₀ (NPLWTN) was prepared via high-temperature solid-state reaction method. The grain morphology of the compound was analyzed by scanning electron microscopy (SEM). Studies of dielectric properties (??_r and tan??) of the compound at different frequencies (10²?C10⁶?Hz) in a wide temperature range (300?C700?K) showed multiple phase transitions in it. First phase transition observed at 335?K related to structural type (ferroelectric-ferroelectric) and the second one observed at 536?K is related to the ferroelectric to paraelectric. The ferroelectric property of the compound at room temperature was confirmed by polarization (hysteresis) study. Broadened dielectric peaks at low frequencies were observed above ferroelectric to paraelectric phase transition temperature (Tc). The values of exponent n(T) and pre-factor A(T) at and around Tc were obtained by the fitting ac conductivity data with Jonscher??s universal power law. From the variation of n(T) and A(T) with temperature, the strength of interaction among the charge carriers with the crystal lattice and the strength of polarisability at phase transition are observed. The activation energy of the compound in low and high temperature range suggests the conduction mechanism in the material.     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.     

Dielectric, ferroelectric and ferromagnetic properties of 0.8PbFe0.5Ta0.5O3-0.2PbTiO3 ceramics

Lead based complex compounds, 0.8PbFe_0.5− Ta_0.5O₃-0.2PbTiO₃ (0.8PFT-0.2PT) ceramics, were prepared by the solid state reaction method, and the corresponding dielectric, ferroelectric and ferromagnetic properties were investigated. As the PT phase was added to the PFT phase, the Curie temperature of 0.8PFT-0.2PT ceramics increased. The ferroelectric P-E and ferromagnetic M-H hysteresis loops were observed at the same time. The ferroelectric properties depend on the formation of the perovskite 0.8PFT-0.2PT phase; however, the ferromagnetic properties depend on the formation of the pyrochlore Pb₃FeTaO₇ phase.     

Phase transition and dielectric properties of PbLa(Zr,Sn,Ti)O3 antiferroelectric ceramics under hydrostatic pressure

The influences of temperature and hydrostatic pressure on the dielectric constant (? _r) and loss (tanδ) of PbLa(Zr,Sn,Ti)O₃ antiferroelectric ceramics have been studied. Temperature-induced transition from tetragonal antiferroelectric phase to cubic paraelectric phase and low-temperature frequency dispersion are observed. The dielectric behavior as a function of temperature was determined by XRD measurement. It has been found from experimental results that the ferroelectric–antiferroelectric phase transition temperature decreased and antiferroelectric–paraelectric phase transition temperature increased with increasing pressure.     

Ferroelectric properties of heterolayered lead zirconate titanate thin films

Heterolayered Pb(Zr_{1 − x} Ti _x )O₃ thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers were successfully deposited via a multistep sol-gel route assisted by spin-coating. These heterolayered PZT films, when annealed at a temperature in the range of 600–700^∘C show (001)/(100) preferred orientation, demonstrate desired ferroelectric and dielectric properties. The most interesting ferroelectric and dielectric properties were obtained from the six-layered PZT thin film annealed at 650^∘C, which exhibits a remanent polarization of 47.7 μC/cm² and a dielectric permittivity of 1002 at 100 Hz. Reversible polarization constituents a considerably high contribution towards the ferroelectric hysteresis of the heterolayered PZT films, as shown by studies obtained from C-V and AC measurement.     

Preparation of Pb(Ti,Zr)O3/RuO2 multilayers in situ by pulsed laser ablation deposition

Abstract

RuO₂/PbZr_0.52Ti_0.48O₃ multilayers have been prepared by pulsed laser deposition and characterised by Grazing Incident Angle Bragg Diffraction and scanning electron microscopy. The results are correlated with the different deposition conditions and post-growth thermal annealing. The structure of the paraelectric pyrochlore phase (Pb₂(Zr,Ti)₂O₆) and its stability relative to the ferroelectric perovskite is discussed.