Non-ferroelectric Materials Research Articles

Enhanced flexoelectric effect in a non-ferroelectric composite

Direct flexoelectric effect was investigated in a non-ferroelectric composite (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7/Ag (BZN/Ag) where the structure symmetry permits no macro-piezoelectricity. The flexoelectric coefficient of the BZN/Ag composite approaches 0.17 μC/m at room temperature. This value is 3–4 orders of magnitude higher than those of common dielectrics. Our research confirms the existence of flexoelectric effect in insulated non-ferroelectric materials except for ferroelectric relaxors.

Anomalous dielectric behaviour in centrosymmetric organic–inorganic hybrid chlorobismuthate(III) containing functional N,N-dimethylethylammonium ligand. Crystal structure and properties

The structure of [C2H5NH(CH3)2+]3[BiCl6−] (abbreviated as DCB) was determined by a single-crystal X-ray diffraction at 115K. The compound adopts tetragonal symmetry with the space group I41/acd; a=23.35Å, c=17.60Å, V=9598Å3 and Z=16. The crystal structure of DCB is built up of isolated [BiCl6]3− units and N,N-dimethylethylammonium counterions that are accommodated in the large voids. At ambient temperature two-thirds of the counterions appear to be dynamically disordered. Dynamics of this type of cations contributes to the enhanced dielectric permittivity of DCB. A low frequency dielectric relaxation process that takes place between 200 and 300K is characterized by an exceptionally large dielectric increment, Δϵ>100, which is unprecedented in nonferroelectric materials. The molecular motions of the N,N-dimethylethylammonium cations were studied by means of H1 NMR spin-lattice relaxation time measurements.

Piezoelectric force microscopy of crystalline oxide-semiconductor heterostructures

Thin films of epitaxial SrTiO3 grown on silicon exhibit compressive in-plane strain that may stabilize ferroelectricity in this normally non-ferroelectric material. We investigate this possibility by using an ultra-high vacuum atomic force microscope to measure the local force response of coherently strained SrTiO3 films on silicon to an applied ac electric field. The observed cantilever response is different in regions that were previously written with positive and negative voltages, but the frequency dependence of this response indicates that the dominant forces are related to electrostatic charging rather than ferroelectricity.

Fabrication of aluminium doped zinc oxide piezoelectric thin film on a silicon substrate for piezoelectric MEMS energy harvesters

Thin film piezoelectric materials play an essential role in micro electro mechanical system (MEMS) energy harvesting due to its low power requirement and high available energy densities. Non-ferroelectric piezoelectric materials such as ZnO and AlN are highly silicon compatible making it suitable for MEMS energy harvesters in self-powered microsystems. This work primarily describe the design, simulation and fabrication of aluminium doped zinc oxide (AZO) cantilever beam deposited on silicon substrate. AZO was chosen due its high piezoelectric coupling coefficient, ease of deposition and excellent bonding with silicon substrate. Doping of ZnO with Al has improved the electrical properties, conductivity and thermal stability. The proposed design operates in transversal mode (d31 mode) which was structured as a parallel plated capacitor using Si/Al/AZO/Al layers. The highlight of this work is the successful design and fabrication of Al/AZO/Al on silicon as the substrate to make the device CMOS compatible for electronic functionality integration. Design and finite element modeling was conducted using COMSOL™ software to estimate the resonance frequency. RF Magnetron sputtering was chosen as the deposition method for aluminium and AZO. Material characterization was performed using X-ray diffraction and field emission scanning electron microscopy to evaluate the piezoelectric qualities, surface morphology and the cross section. The fabricated energy harvester generated 1.61 V open circuit output voltage at 7.77 MHz resonance frequency. The experimental results agreed with the simulation results. The measured output voltage is sufficient for low power wireless sensor nodes as an alternative power sources to traditional chemical batteries.

Application of High Permittivity Bismuth Copper Titanate in Multilayer Capacitors

A nonferroelectric high permittivity compound with the perovskite structure, Bi2/3Cu3Ti4O12, was synthesized at 900 ◦C. The ceramic powder and appropriate organic additives were used for preparation of a slurry for tape casting. The obtained green tapes were smooth, flexible, and with a uniform thickness of 25 μm after drying at 50 ◦C. Ag electrodes were screen printed on green rectangular sheets cut by a laser. Subsequent operations were screen printing of Ag internal electrodes, stacking of green sheets, isostatic lamination, cutting, deposition of external terminations and co-firing of dielectric and conductive layers at 850 ◦C. Scanning electron microscopy observations showed well sintered, dense, fine-grained microstructure of ceramic layers and good cooperation with the electrodes made of commercial Ag paste. Capacitance and dissipation factor of multilayer capacitors were examined in the temperature range from −55 to 330 ◦C at frequencies 10 Hz–2 MHz. The fabricated multilayer capacitors exhibit high capacitance and relatively low temperature coefficient of capacitance in the temperature range from −55 to 110 ◦C. The obtained lead-free high permittivity nonferroelectric material Bi2/3Cu3Ti4O12 is a spontaneously formed internal barrier capacitor. This material seems to be a promising alternative for conventional lead-based relaxor dielectrics in multilayer ceramic capacitors.

Ferroelectricity in Strain-FreeSrTiO3Thin Films

Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO3. By a direct comparison of the strained and strain-free SrTiO3 films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all SrTiO3 films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.

Ferroelectric phase transitions in three-component short-period superlattices studied by ultraviolet Raman spectroscopy

Vibrational spectra of three-component BaTiO3/SrTiO3/CaTiO3 short-period superlattices grown by pulsed laser deposition with atomic-layer control have been investigated by ultraviolet Raman spectroscopy. Monitoring the intensity of the first-order phonon peaks in Raman spectra as a function of temperature allowed the determination of the ferroelectric phase transition temperature Tc. Raman spectra indicate that all superlattices remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below Tc. The dependence of Tc on the relative thicknesses of ferroelectric (BaTiO3) to nonferroelectric materials (SrTiO3 and CaTiO3) has been studied. The highest Tc was found in superlattices having the largest relative amount of BaTiO3, provided that the superlattice maintains its coherency with the substrate. Strain relaxation leads to a significant decrease in the ferroelectric phase transition temperature.

Ferroelectric and magnetic properties of multiferroic BiFeO3-based composite films

BiFeO(3)-based composite films were fabricated onto the Pt/Ti/SiO(2)/Si(100) substrates by a chemical solution deposition (CSD) method using the precursor solutions with various excess iron composition followed by annealing at 923 K for 30 minutes under oxygen gas flow. Coexistence of spontaneous magnetization and remanent polarization could be obtained in the BiFeO(3)-based composite films with high excess iron composition. The remanent magnetization of almost 20 emu/cm(3) and the magnetic coercive field of 1.5 kOe were obtained at the iron composition ratio of Fe/Bi = 1.25. In this specimen, the remanent polarization at 90 K was approximately 10 microC/cm(2) at the electric field of 1500 kV/cm. Structural analysis suggested that the remanent polarization has a possibility to increase by suppressing the formation of the secondary phases of Bi(2)Fe(4)O(9) and alpha-Fe(2)O(3), these are the nonferroelectric material as well as antiferromagnetic phase.

Microstructure and dielectric properties of NaxTiyNi1−x−yO (x = 0.05–0.30, y = 0.02)

The microstructure of NaxTiyNi1−x−yO (abbreviated as NaTNO), a high dielectric material, has been investigated by scanning electron microscopy. Dielectric dispersion and complex impedance of NaTNO ceramics (in the frequency range 10–107 Hz) are discussed. The relatively lower value of the dielectric constant (ε′) of NaTNO compared with those of LixTiyNi1−x−yO (LTNO) and KxTiyNi1−x−yO (KTNO) is attributed to the typical distinguishing behaviour of its microstructure. Ac impedance spectroscopic studies indicate that the internal barrier layer capacitance effect, arising from differing electrical properties of grains and grain boundaries, is responsible for high dielectric permittivity of this non-ferroelectric material. The ratios of dielectric dispersion strengths, in the low (<100 kHz) and high (>100 kHz) frequency ranges, increase with an increase in the Na content. Deviation of the measured ε′ value of NaTNO from that predicted by the boundary layer capacitance mechanism is attributed to its typical microstructure.

Sintering and dielectric properties of Cu 2Ta 4O 12 ceramics

In this work, Cu 2Ta 4O 12 ceramic was investigated as a promising, lead-free, nonferroelectric material with high dielectric permittivity. The results of impedance spectroscopy studies carried out at frequencies 10 Hz to 2 MHz over a wide temperature range from −55 to 700 °C were analyzed in the impedance, dielectric permittivity and electric modulus formalisms. In complex impedance plots two distinct arcs were distinguished, ascribed to the semiconducting grains and to the insulating grain boundaries. Cu 2Ta 4O 12 ceramic was found to exhibit a high dielectric permittivity exceeding 10,000 at low frequencies in the temperature range 150–740 °C. High permittivity of this material was attributed to the formation of internal (grain boundary) barrier layer capacitors. The influence of sintering conditions on microstructure, composition and dielectric properties of Cu 2Ta 4O 12 ceramics was also studied.

Towards local electromechanical probing of cellular and biomolecular systems in a liquidenvironment

Electromechanical coupling is ubiquitous in biological systems, with examples ranging fromsimple piezoelectricity in calcified and connective tissues to voltage-gated ion channels,energy storage in mitochondria, and electromechanical activity in cardiac myocytes andouter hair cell stereocilia. Piezoresponse force microscopy (PFM) originally emerged as atechnique to study electromechanical phenomena in ferroelectric materials, and in recentyears has been employed to study a broad range of non-ferroelectric polar materials,including piezoelectric biomaterials. At the same time, the technique has been extendedfrom ambient to liquid imaging on model ferroelectric systems. Here, we present resultson local electromechanical probing of several model cellular and biomolecularsystems, including insulin and lysozyme amyloid fibrils, breast adenocarcinomacells, and bacteriorhodopsin in a liquid environment. The specific features ofPFM operation in liquid are delineated and bottlenecks on the route towardsnanometre-resolution electromechanical imaging of biological systems are identified.

Whispering-gallery-mode analysis of phase-matched doubly resonant second-harmonic generation

We propose a coupled modes analysis of second-harmonic generation in microdisk resonators. We demonstrate that whispering gallery modes can be used to obtain a combination of modal and geometrical quasi-phase-matching (without domain inversion) to obtain efficient conversion in isotropic and nonferroelectric materials such as III-V semiconductor compounds. Finally we use an analytical model to describe the coupling between a bus waveguide and the nonlinear microdisk to achieve an optimization scheme for practical configuration.

Internal Barrier Layer Capacitance Effect in Hexagonal Perovskite Ba4YMn3O11.5 Ceramics

Ba4YMn3O11.5, a 12R hexagonal perovskite that consists of face-sharing Mn3O12 trimers isolated by single YO6 octahedral layers, is presented. The presence of locally 5- and 6-coordinated Y3+ and Mn4+ cations is observed in this structure. A high dielectric permittivity (er ≈ 1 × 104), which is almost frequency independent over the ∼1 × 102 to 1 × 105Hz range from room temperature to 120 °C, has been observed in ceramics of this material. AC impedance spectroscopy indicates that the internal barrier layer capacitance effect arising from the differing electrical properties of the grains and grain boundaries is responsible for the high dielectric permittivity of this non-ferroelectric material.

Low temperature sputter deposition of Ba0.96Ca0.04Ti0.84Zr0.16O3 thin films on Ni electrodes

ABSTRACTThe integration of high-K materials, such as ferroelectrics in the paraelectric state, in integrated circuits presents several challenges. If high-K materials are deposited on-chip after or between Al metalization steps, then these challenges include limits on processing gas composition, deposition temperature and electrode material. Specifically, the atmosphere present during deposition and annealing must be oxygen-free; the deposition and annealing temperatures must not exceed 450°C; and the electrode material must be etchable with chemical techniques. We studied rf magnetron sputtered Ba0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) with Ni electrodes because this system meets all the above requirements. The BCTZ deposition process uses pure Ar as the sputter gas and a substrate temperature of 450°C. Subsequent anneals may be performed in a reducing (forming gas) atmosphere with little effect on either the dielectric constant or leakage current. The Ni electrodes provide a good substrate for BCTZ films and are much easier to integrate than Pt films. Observed values for the relative dielectric constant K, exceeding 100, were not as high as for BCTZ films on Pt electrodes, however these values are sufficient to provide a clear advantage over other, non-ferroelectric materials. Overall, the device characteristics observed prove that the Ni/BCTZ/Ni capacitor is a valuable technology for on-chip capacitor applications.

Relaxor-Like Dielectric Relaxation: Artifacts and Intrinsic Properties

The existing results on the relaxor-like dielectric behavior which is characterized by a broad maximum in a dielectric permittivity as a function of temperature and the low frequency dielectric dispersion are reviewed for different ferroelectric and non-ferroelectric materials. The general and distinctive features of the relaxor and relaxor-like dielectric anomalies are discussed. Brief discussion of the existing treatments of dielectric data, which take into account not only, the bulk properties of the materials but also the interfacial effects is given. New model explaining the relaxor-like response on the base of the association-dissociation theory of impurity dipole complexes is proposed.

Giant dielectric permittivity observed in Li and Ti doped NiO.

A giant low-frequency dielectric constant ( epsilon 0 approximately 10(5)) near room temperature was observed in Li,Ti co-doped NiO ceramics. Unlike currently best-known high epsilon 0 ferroelectric-related materials, the doped oxide is a nonperovskite, lead-free, and nonferroelectric material. It is suggested that the giant dielectric constant response of the doped NiO could be enhanced by a grain boundary-layer mechanism as found in boundary-layer capacitors. In addition, there is about a one-hundred-fold drop in the dielectric constant at low temperature. This anomaly is attributed to a thermally excited relaxation process rather than a thermally driven phase transition, as for that yielding ferroelectrics.

Origin of apparent colossal dielectric constants

Experimental evidence is provided that colossal dielectric constants ${\ensuremath{\varepsilon}}^{\ensuremath{'}}&gt;~1000,$ sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner-type contributions of depletion layers at the interface between sample and contacts or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in nonferroelectric materials is of order ${10}^{2}.$

Transient Behavior of the Polarization in Ferroelectric Thin Film Capacitors

AbstractThe understanding of the polarization switching process of ferroelectric capacitors is highly relevant for the development and optimization of FeRAM devices. We report on the characterization of Pb(Zr,Ti)O3 thin films which have been studied by means of dedicated rectangle pulse measurements. Decreasing the voltage level of the excitation pulses decelerates the polarization switching significantly to the range of milliseconds and reduces the switchable polarization. In this work the influence of niobium (Nb) doping on the switching properties of PZT thin films prepared by CSD are investigated to reach the aspired conditions of low voltage operation, read and write access pulses in the range of nanoseconds. For the implementation of the transient behavior of ferroelectric capacitors in circuit design and simulation tools it is necessary to develop a model which precisely describes the polarization hysteresis, the pulse switching behavior as well as the small signal capacitance. The fundamental considerations for this model are presented, based on an ideal ferroelectric capacitor, taking into account the Curie-von Schweidler behavior. The latter is observed in non-ferroelectric high-K materials as well as in ferroelectric thin films.

Systematic prediction of new inorganic ferroelectrics in point group 4.

The latest release of the Inorganic Crystal Structure Database contains a total of 87 entries corresponding to 70 different materials in point group 4. The structures reported for 11 materials in space group P4 satisfy the criteria for ferroelectricity, as do four in P4(1), one each in P4(2) and P4(3), 12 in I4, including seven that form three families, and another three in I4(1). Three previously known ferroelectrics were also listed in I4 and one in I4(1). In addition, the listing for point group 4 contains 22 entries for nonferroelectric materials and three with misassigned space groups. Among the newly predicted ferroelectrics in point group 4, assuming the validity of the underlying structural reports, are Ce(5)B(2)C(6), modulated NbTe(4), Na(3)Nb(12)O(31)F, Ca(2)FeO(3)Cl, K(4)CuV(5)O(15)Cl, TlBO(2), CrOF(3), PbTeO(3), VO(HPO(3))(H(2)O).3H(2)O, MgB(2)O(OH)(6), beta-tetragonal boron, CuBi(2)O(4), WOBr(4), Na(8)PtO(6), SbF(2)Cl(3), Ba(1.2)Ti(8)O(16), Ni[SC(NH(2))(2)](4)Cl(2), Ca(2)SiO(3)Cl(2), the mineral caratiite, NbAs, beta-NbO(2) and Ag(3)BiO(3).

Design of a Michelson interferometer for the measurement of electrostrictive strains

In the field of quadratic electrostriction of non-ferroelectric materials, efforts to optimize interferometers for the accurate measurement of small strains are being undertaken because the displacements that have to be measured are very small (10 −13m). Here, a design for a Michelson interferometer is proposed that combines a double-face detection scheme with a null method.