Ferroelectric properties of anisotropically strained epitaxial NaNbO3 films grown on NdGaO3

Due to the lattice mismatch between the film and substrate, an anisotropically in-plain strain can be imposed to an epitaxially grown film, which (i) depends on a number of parameters and (ii) can strongly affect the electronic properties of this thin layer. NaNbO3 films are epitaxially deposited on (110) NdGaO3 via Metal Organic Chemical Vapor Deposition (MOCVD). X-ray analysis shows that up to a thickness of approximately 15nm, the film is completely strained, while above this thickness, relaxation of the compressive strain takes place. The analysis of the complex permittivity of three films of different thickness (27 – 80nm) as a function of temperature, electric field direction, AC and DC electric field reveals that (i) the compressive strain shifts the temperature of maximum (Tmax) from about 628K of a bulk NaNbO3 to close to room temperature of thin films, (ii) the room temperature permittivity of these strained films is enhanced by a factor of 1.06 – 2.74 compared to that of bulk material, (iii) there is a strong anisotropy in all ferroelectric characteristics for electric field orientation along the [110] and [001] orientations of NdGaO3, and (iv) a strong dependence of the permittivity on the ac amplitude of the electric field as well as the dc component of the electric field. The experimental results are discussed in terms of theories on domain wall mobility and pinning as well as effect of relaxor ferroelectrics. These strained films represent ideal candidates for the applications of sensors, such as surface acoustic wave sensor

Combined impact of strain and stoichiometry on the structural and ferroelectric properties of epitaxially grown Na1+x_{1 + x} Nb O3+δ_{3 + δ} films on (110) NdGa O3_{3}

We demonstrate that the strain of an epitaxially grown film, which is induced by the lattice mismatch between the crystalline substrate and film and relaxes with increasing film thickness, can be conserved beyond the critical thickness of plastic relaxation of the respective film/substrate heterostructure system by adding epitaxially embedded nanoprecipitates and/or nanopillars of a secondary phase. By doing so we modify the ferroelectric properties of the film in a very controlled way. For this purpose, strained Na1+xNbO3+δ films are epitaxially grown on (110)NdGaO3 and their structural and electronic properties are investigated. X-ray diffraction and transmission electron microscopy analysis indicate that in addition to the epitaxially grown majority phase NaNbO3, a second phase NayNbO3+δ is present in the films and forms crystalline precipitates and vertically aligned pillars a few nanometers in diameter. For large enough concentrations, this secondary phase appears to be able to suppress the plastic relaxation of the NaNbO3 matrix. In contrast to stoichiometric films and films with small Na excess, which demonstrate strain relaxation for film thickness exceeding a few nanometers and relaxor-type ferroelectric behavior, the Na1+xNbO3+δ film with the largest off-stoichiometry (grown from a target with x=17%) exhibits the “classic” ferroelectric behavior of unstrained NaNbO3 with a hysteretic structural and ferroelectric transition. However, the temperature of this hysteretic transition is shifted from 616 K to 655 K for unstrained material to room temperature for the strained Na1+xNbO3+δ film grown from the off-stoichiometric target

Electronic characterization of polar nanoregions and their impact on the ferroelectric property of strain-engineered relaxor-type NaNbO3 films

Due to the lattice mismatch between the film and substrate, an anisotropically in-plain strain can be imposed to an epitaxially grown film, which (i) depends on the lattice parameters, thermal expansion coefficient and the thickness of the film and (ii) can strongly affect the electronic properties of this thin layer. NaNbO3 films of different thickness are epitaxially deposited on (110) NdGaO3 via Metal Organic Chemical Vapor Deposition (MOCVD). X-ray analysis shows that up to a thickness of approximately 15nm, the film is completely strained, while above this thickness, relaxation of the compressive strain takes place. The analysis of the complex permittivity of films of different thickness (27 – 80nm) as a function of temperature, frequency, AC and DC electric field reveals that the strain not only leads to a modification of the permittivity and the ferroelectric transition temperature, it also results in a relaxor-type behavior, such as:(i) The compressive strain shifts the temperature of maximum (Tmax) from about 628K of a bulk NaNbO3 down to 125K of thin films.(ii) It also enhances the corresponding permittivity from ε’ ≈ 500 up to ε’ = 1500.(iii) Both effects, reduction of phase transition temperature and enhancement of ε´, depend on the thickness of the film since the strain relaxes with increasing film thickness.(iv) Finally, the films show a characteristic frequency and electric field dependence of ε´, which is discussed in terms of Vogel-Fulcher equation and Rayleigh law, respectively. The electric field dependence of the resulting freezing temperature TVF allows a direct estimation of the maximum size of the polar nano-regions (PNRs). For our samples we obtain VPRN (TVF) ≈ 93nm3 which results in a diameter of 5.6nm if we assume a spherical shape of the PNRs. This value agrees with literature predictions that however are obtained by indirect estimation, e.g. microscopic observation of defect arrangements and simulations. Furthermore, the irreversible contribution to the polarization displays a characteristic peak around TVF, it vanishes above the Burns temperature and is frequency dependent and, thus seem to be caused by the presence and mobility of PNRs. All these extraordinary properties of this strain-engineered relaxor-type ferroelectric system are discussed in terms of existing theories, its potential for sensor applications will be sketched

Impact of Anisotropic Biaxial Compressive and Tensile Strain on the Properties of Epitaxial Ferroelectric Films

The impact of anisotropic biaxial strain on the ferroelectric properties of thin oxide films (20-100nm) are examined using the example of epitaxial NaNbO3 and SrTiO3 films that are grown on single-crystalline oxide substrates with different lattice mismatch, leading to compressive and tensile in-plane strain, respectively. Generally, tensile in-plane strain leads to an increase of the ferroelectric in-plane transition temperature whereas compressive strain tents to decrease the transition temperature. Shifts of the transition temperature by several 100K can easily be obtained via this method leading to room-temperature permittivity of several 1000. Our investigations have shown that the phase transition itself and the ferroelectric states of the anisotropically strained films turn out to be highly complex. First, the transition temperature depends on the direction of the applied electric field which contradicts the concept of a uniform phase transition for a given system. Second, all systems, that we examined, showed relaxor properties which are usually expected for systems consisting of a mixture of phases. Third, most ferroelectric properties strongly depend on the applied electric field. This can partially be explained by Rayleigh law, however especially for the tensile strained SrTiO3 terms of higher order in the field dependence of the permittivity indicate the strong impact of pinning of domain walls and polar regions (e.g. polar nano regions). Finally at elevated temperature an anisotropic conductivity is observed. The latter might attributed to domain wall conductance. The different observations are discussed in terms of existing models, potential application of the different properties will be sketched.Keywords: anisotropic strain, thin films, ferroelectrics, high-k materia

Co-morbidity Factors Associated with Influenza in Nigeria

In Nigeria, anecdotal reports of poultry disease with high mortality began in December 2005. H5N1 was first confirmed on February 8, 2006 among poultry from a commercial farm in Kaduna state in northern Nigeria. This was the first confirmation of the presence of H5N1 virus in Africa. Shortly after H5N1 infection in birds was confirmed in Nigeria, national and local authorities initiated culling of birds at farms with laboratory confirmed and probable out breaks of H5N. On January 17, 2007, Nigeria recorded the first human fatal case of avian influenza in a 22year old lady

Electronic characterization of polar nanoregions in relaxor-type ferroelectric NaNbO3 films

Strained NaNbO3 films of different thicknesses are epitaxially grown on (110) NdGaO3 substrates. A detailedanalysis of the permittivity of these films demonstrates that strain not only leads to a modification of thepermittivity and the ferroelectric transition temperature, it also results in a pronounced relaxor-type behavior andallows a direct estimation of the size and mobility of the polar nanoregions (PNRs). The compressive strain reducesthe transition temperature to 125 K and enhances the corresponding permittivity up to ε ≈ 1500 for the thinnestfilm. Since the strain relaxes with increasing film thickness, both effects, reduction of phase transition temperatureand enhancement of ε, depend on the thickness of the film. The films show a characteristic frequency and electricfield dependence of ε, which is discussed in terms of the Vogel-Fulcher equation and Rayleigh law, respectively.Using the electric field dependence of the resulting freezing temperature TVF, allows a direct estimation of thevolume of the PNRs at the freezing temperature, i.e. from 70 to 270 nm3. Assuming an idealized spherical shapeof the PNRs, diameters of a few nanometers (5.2–8 nm) are determined that depend on the applied ac electricfield. The irreversible part of the polarization seems to be dominated by the presence and mobility of the PNRs. Itshows a characteristic peak at low temperature around TVF, vanishes at a temperature where the activation energyof the PRNs extrapolates to zero, and shows a frequency dispersion that is characteristic for relaxor-type behavior

Tailoring the Properties of Ferroelectric Films via Compressive and Tensile Strain

The impact of anisotropic biaxial strain on the ferroelectric properties of thin oxide films (20-100nm) are examined using the example of epitaxial NaNbO3 and SrTiO3 films that are grown on single-crystalline oxide substrates with different lattice mismatch, leading to compressive and tensile in-plane strain, respectively. Generally, tensile in-plane strain leads to an increase of the ferroelectric in-plane transition temperature whereas compressive strain tends to decrease the transition temperature. Shifts of the transition temperature by several 100K can easily be obtained via this method leading to room-temperature permittivity of several 1000. Our investigations have shown that the phase transition itself and the ferroelectric states of the anisotropically strained films turn out to be highly complex. First, the transition temperature depends on the direction of the applied electric field which contradicts the concept of an uniform phase transition for a given system. Second, all systems, that we examined, showed relaxor properties which are usually expected for systems consisting of a mixture of phases. Third, most ferroelectric properties strongly depend on the applied electric field. This can partially be explained by Rayleigh law, however especially for the tensile strained SrTiO3 terms of higher order in the field dependence of the permittivity indicate the strong impact of pinning of domain walls and polar regions (e.g. polar nano regions). Finally at elevated temperature an anisotropic conductivity is observed. The latter might attributed to domain wall conductance. The different observations are discussed in terms of existing models, potential application of the different properties will be sketched

Impact of Compressional and Tensile Biaxially-Anisotropic Strain on the Ferroelectric Properties of Epitaxial NaNbO3 and SrTiO3 Films

Impact of Compressional and Tensile Biaxially-Anisotropic Strain on the Ferroelectric Properties of Epitaxial NaNbO3 and SrTiO3 Films R. Wördenweber1, J. Schwarzkopf2, Biya Cai1, Yang Dai1, D. Braun2, J. Schubert1, E. Hollmann11Peter Grünberg Institute (PGI) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany2Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, GermanyThe impact of anisotropic biaxial strain on the ferroelectric properties of thin oxide films (20-100nm) are examined for the example of epitaxial NaNbO3 and SrTiO3 films that are grown on different single-crystalline oxide substrates with varying lattice mismatch. Generally, tensile in-plane strain leads to an increase of the ferroelectric in-plane transition temperature whereas compressive strain tents to decrease the transition temperature. Shifts of the transition temperature by several 100K can easily be obtained via this method. Our investigations have shown that the phase transition itself and the ferroelectric states of the anisotropically strained films turn out to be highly complex. First, the transition temperature depends on the direction of the applied electric field which contradicts the concept of a uniform phase transition for a given system. Second, all systems, that we examined, showed relaxor properties which are usually expected for systems consisting of a mixture of phases. Third, ferro- to antiferroelectric transitions are observed. These transitions seem to be connected to the presence of polar nanoregions. Finally, all systems show a distinct frequency dependence of the complex permittivity at low frequencies (typically <1kHz) and intermediate temperatures around room temperature. Again this behavior seems to be connected to the presence of polar nanoregions and points to an additional Maxwell-Wagner like mechanism that seems to be present in the temperature regime where the polar nanoregions are present and mobile

Forward Kinematic Modeling of Conical-Shaped Continuum Manipulators

International audienceSUMMARY Forward kinematics is essential in robot control. Its resolution remains a challenge for continuum manipulators because of their inherent flexibility. Learning-based approaches allow obtaining accurate models. However, they suffer from the explosion of the learning database that wears down the manipulator during data collection. This paper proposes an approach that combines the model and learning-based approaches. The learning database is derived from analytical equations to prevent the robot from operating for long periods. The database obtained is handled using Deep Neural Networks (DNNs). The Compact Bionic Handling robot serves as an experimental platform. The comparison with existing approaches gives satisfaction

Synthesis, structural and vibrational studies of lacunar apatite type NaPb2CaSr(PO4)3 and application to phosphoric acid demetallization

NaPb2CaSr(PO4)3 lacunar apatite was synthesized by the solid‐state reaction method. The compound was characterized by X‐ray diffraction, Raman and infrared spectroscopies. The crystal structure of this new compound has been determined by Rietveld method, adopting the space group P63/m (No. 176), with a = b = 9.6684(3) Å and c = 7.1104(3) Å. The analysis shows that the M(2) sites are shared by four cations Ca, Sr, Pb and Na. While the M(1) sites are occupied by the Ca, Sr and Pb cations. The structure was described as built up from [PO4]3‐ tetrahedra and Pb2+/Ca2+/Sr2+ of six fold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. The tunnels are connected by cations of mixed sites (4f) half occupied by Pb2+/Ca2+/Sr2+ and half by Na+ mixed alkali cation. The results of vibrational spectroscopy are in good agreements with the XRD measurements, the assignments of internal modes of (PO4)3‐ tetrahedra have been made and corroborate well with factor group analysis for the symmetry P63/m. An attempt to purify phosphoric acid is done using our apatite and the results are promising