Composition dependent electrocaloric behavior of (SrxBa1-x)Nb2O6 ceramics

This article presents electrocaloric effect in (Sr_xBa_1-x)Nb₂O₆ ceramics (where x = 0.25, 0.50 and 0.75) using an indirect approach based on Maxwell's relations. Here, we have calculated various parameters of electrocaloric effect like temperature change (ΔT), entropy change (ΔS) and heat carrying capacity (ΔQ) of material due to the change in polarization under two different electric fields of 30 kV/cm and 20 kV/cm. (Sr_xBa_1-x)Nb₂O₆ ceramics is well known pyroelectric material, where by increasing Sr/Ba ratio the ferroelectric behavior turns towards relaxor behavior. While in terms of electrocaloric effect performance (temperature change ΔT) is increase as the Sr/Ba ratio increases. Additionally, maximum ΔT (0.30 K) was found for (Sr_xBa_1-x)Nb₂O₆ ceramic having x=0.75 molar concentration under the electric fields of 30 kV/cm.     

Glassy ferromagnetism in La0.75Sr0.25-xKxCoO3 polycrystalline perovskite cobaltites

In this paper, the structural and magnetic properties of La_0.75Sr_0.25-xK_xCoO₃ (0≤x≤0.15) powder samples are reported. X-ray diffraction analysis using Rietveld refinement show that all our samples crystallize in the rhombohedral structure with \( R\overline{3}c \) space group. At low temperatures, with increasing K amount, the samples change from ferromagnetic like behavior (x?=?0, 0.05 and 0.1) to spin glass one (x?=?0.15). The ferromagnetic-paramagnetic transition temperature decreases with increasing K amount from 235K (x?=?0) to 200K (x?=?0.1). In the paramagnetic phase, all our synthesized samples obey the Curie-Weiss law. In the vicinity of T_C, the magnetic entropy change |ΔS _M | for La_0.75Sr_0.2K_0.05CoO₃ sample reached maximum values of 0.47Jkg/K and 2.27Jkg/K under magnetic field variation in the range 1T and 7T, respectively. The magnetization hysteresis loops demonstrate a weakening the ferromagnetism with increasing K content.     

YBa2Cu3O7-x/SrTiO3//LaAlO3 heterostructures obtained by injection MOCVD

Abstract

YBa₂Cu₃O_7-x /Ba_xSr_1-xTiO₃ /LaAlO₃ heterostructures can be used as a basis for devices with voltage control in microwave circuits.

Ba_xSr_1-xTiO₃ (x=0–0.1) (BST) thin films have been epitaxially grown on LaAlO₃ substrates using injection MOCVD. The excellent crystalline quality of the obtained BST films can be proven by a FWHM of <0,2° for the rocking curve of the (002) BST reflection. An AFM study revealed flat surfaces, showing a surface roughness R_s as low as 1nm. YBa₂Cu₃O_7-x/Ba_xSr_1-x TiO₃//LaAlO₃ heterostructures were than optimised. The YBa₂Cu₃O_7-x (YBCO) layers obtained on Ba_xSr_1-xTiO₃ films are epitaxial with a FWHM of 0.45° for the (005) YBCO rocking curve and display very promising superconducting properties of T_c=92K.

Finally the microwave properties of the superconducting films were studied. For YBa₂Cu₃O_7-x layers directly deposited on LaAlO₃, surface resistance values of 0,32mΩ were obtained, while for YBa₂Cu₃O_7-x /SrTiO₃//LaAlO₃ heterostructures, higher values of 1mΩ at 8.5GHz were measured.     

Pulsed Laser Deposition of Zr1-xCexO2 and Ce1-xLaxO2-x/2 for Buffer Layers and Insulating Barrier in Oxide Heterostructures

We have studied the pulsed laser deposition (PLD) of Zr_1-xCexO₂ and Ce_1-xLa_xO₂-x/2 initially to grow buffer layers for perovskite films deposited on Si, LaAlO₃, SrTiO₃ and MgO and then to produce tunneling barriers for cuprate or manganite heterostructures. On (1 0 0) Si, the deposition of Zr_1-xCexO₂ (x = 0.12) produces a smooth epitaxial layer (R _RMS = 0.25/1m²), which allows the further deposition of high quality YBa₂Cu₃O₇ (Tc 88 K) and La_0.7Sr_0.3MnO₃ films. On the other hand, the use of Ce_1-xLa_xO_2-x/2 (0 x 0.4) makes it possible to match the YBa₂Cu₃O₇ and La_0.7Sr_0.3MnO₃ layers to various substrates. The buffer layers are epitaxially grown with a 45° rotation of the in-plane axes with respect to those of the substrate, and the smoothness is high (R _RMS = 0.25/1m²). In the case of an ultra-thin barrier (2.5 nm) of Ce_1-xLa_xO_2-x/2 sandwiched in a La_0.7Sr_0.3MnO₃, the out-of-plane mismatch of 2 induces distortions at the interface steps, which propagate into the topmost La_0.7Sr_0.3MnO₃ layer. This is in contrast to the case of SrTiO₃ barriers where an ideal crystal continuity in the growth direction is observed.     

Structural, Thermal and Electrical Properties of Ce-Doped SrMnO3

The Sr_1-xCe_xMnO_1-α system (0≤ x ≤ 0.5) was investigated with respect to its structural, thermal and electrical properties. Although un-doped SrMnO₃ has the perovskite structure above 1400°C, the structure is unstable at room temperature. However, partial substitution of Ce for Sr in SrMnO₃ stabilizes the perovskite structure down to room temperature. Single phase perovskite is obtained for 0.1≤ x ≤ 0.3 in Sr_1-xCe_xMnO_1-α, and it remains stable even following heat treatment at 800°C for 100 h. The dependence of the electrical conductivity on temperature was measured from room temperature to 1000°C in air. Ce doping dramatically enhanced the electrical conductivity of SrMnO₃. Sr_0.7Ce_0.3MnO_1-α exhibits a higher conductivity (290 S · cm^-1 at 1000°C) than that of La_0.8Sr_0.2MnO₃ (LSM, about 175 S · cm^-1) and remains n-type over the whole range of temperature examined. The thermal expansion coefficients in the system were nearly constant with values ranging between 1.24 × 10^-6 and 1.01 × 10^-6 cm/cm · K for temperatures of 50°C to 1000°C.     

Effect of Ni addition on PNZT tungsten bronze structure

Lead niobate zirconate titanate (PNZT) with general formula 0.5PbNb₂O₆–0.5Pb(Zr_xTi_1-x)O₃ (where x?=?0.15, 0.25, 0.35) and lead nickel niobate zirconate titanate (PNNZT) with general formula Pb(Ni_xNb_0.5-xZr_0.25Ti_0.25)O₃ (where x?=?0.167, 0.250, 0.333) have been prepared by conventional solid state reaction. The phase formation has been studied by X – ray diffraction analysis. Raman investigations have been carried out in the 200–1000 cm^?1 wave number range. The structural changes observed in the X – ray diffraction pattern has been further confirmed by Raman analysis. The major changes in the E(1TO), A₁(1TO) and E?+?B₁ phonon modes suggest the structural change from cubic to rhombohedral with increase in Ni concentration in PNNZT. The optical band gap has been calculated from UV – Vis absorption spectra.     

Microwave frequency conversion in coplanar waveguide on bilayered substrate with (Ba,Sr)TiO3 film

Abstract

Experimental and simulated spectra of microwave (30GHz) signal at the output of the coplanar waveguide (CPW) based on Ba_xSr_1-xTiO₃ film are presented and discussed. The modulation of Ba_xSr_1-xTiO₃ film dielectric constant by the application of high frequency (f ≈ 20MHz) control signal results in the generation of side frequency components in the output signal spectrum. The analysis of experimental data is performed on the basis of considering the CPW section with tunable effective dielectric constant as a phase modulator     

Composition measurement of paraelectric SrTiO3 layer

Abstract

Since composition is an important parameter affecting the dielectric properties in paraelectric SrTiO₃ layers, composition is determined by Rutherford backscattering spectrometry (RBS) measurement. In this measurement, specifically for achieving precise composition measurement, the RBS spectra of Sr, Ti and O must be separated individually. This spectrum separation can only be attained when thin (800 A[ddot] thick) Sr_xTiO_y layers are deposited on graphite substrate. The measurement is performed for layers deposited at different O₂ partial pressure ratios and sputtering pressures. This measurement indicates that composition of O, y, in Sr_xTiO_y layer decreases from 3.7 to 2.7 with the decrease of O₂ partial pressure raito, R(=O₂/O₂ + Ar) from 1.0 to 0.83. Composition of Sr, x, also changes from 1.1 to 0.6 with this change. With the decrease of sputtering pressure from 10 to 5 mTorr, however, composition, y, is held at 2.7 and only the composition, x, increases from 0.6 to 1.1. This composition measurement is useful for the deposition of optimized dielectric layer employed in the charge storage capacitor.     

Conductivity and expansion at high temperature in Sr0.7La0.3TiO3−α prepared under reducing atmosphere

Sr_0.7La_0.3TiO_3−α specimens were prepared in reducing atmosphere, and the structural and electrical properties were studied. The lattice parameter of Sr_0.7La_0.3TiO_3−α at room temperature was larger than that expected from Vegard’s law between SrTiO₃ and LaTiO₃ due to the reductive expansion. The conductivity of this specimen was 100 S cm⁻¹ at 1000°C, p_O2 = 10⁻¹³ Pa. However, the conductivity was not preserved after an oxidation-reduction cycle. Over p_O2 = 10² Pa, the conductivity drastically dropped with increasing p_O2. The thermal expansion coefficient of Sr_0.7La_0.3TiO_3−α was 11.8 × 10⁻⁶ K⁻¹ in 9% H₂/N₂ (room temperature – 1000°C). In this Sr_0.7La_0.3TiO_3−α, the chemical expansion on oxidation reached Δl/l_o = 0.51%, when changing p_O2 from 10⁻¹¹ Pa to 2 × 10⁴ Pa (air) at 1000°C.     

Dielectric and piezoelectric properties of La2O3 doped (Bi0.5Na0.5)0.92(Ba0.8Sr0.2)0.08 TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.92(Ba_0.8Sr_0.2)_0.08 TiO₃+x mol% La₂O₃(x = 0, 0.1, 0.3, 0.5, 0.8) were synthesized by conventional solid state reaction. The crystal structure of all compositions is mono-perovskite ascertained by XRD. The grain size decreased and diffuse phase transition behavior was more evident with the increasing amount of La₂O₃. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p showed the maximum value of 165 pC/N and 0.322 at 0.3% and 0.1% La₂O₃ addition, respectively, and rapidly decreased when La₂O₃ addition over 0.5%. The loss tangent tanδ linearly increased and the mechanical quality factor Q_m linearly decreased with the increasing amount of La₂O₃.     

Advanced model of metal-organic chemical vapor Deposition of BaxSr1-xTiO3 oxides

Abstract

An advanced model of Ba_xSr_1-xTiO₃ Chemical Vapor Deposition is proposed. The model implies that metal-organic precursor decomposition occurs primarily at the reactor surfaces. Deposition of various parasitic solid phases (TiO₂, BaO, SrO, etc.) is taken into account. The model predicts deposition rate, solid phase composition and fraction of parasitic phases in the grown film as a function of growth parameters.     

Structure and microwave dielectric properties of La(2−x)/3NaxTiO3

The structure evolution, and microwave dielectric properties of La_(2?x)/3Na_xTiO₃ Ceramics( $ x = 0.02 \leqslant \times \leqslant 0.5 $ ) were investigated in this paper. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that all samples exhibit single phase. The concentration and ordering degree of A-site vacancies decrease with the increase of x value, and no A-site ordering exists when x?>?0.2. The dielectric constant decreases with the decrease of x value. The Q?×?f value increases up to x?=?0.1 and then decreases with the further increase of x value. A maximum Q?×?f value of 18,826 GHz is obtained when x?=?0.06. The temperature coefficient of resonant frequency exhibits positive value and decreases greatly with the decrease of x value.     

Protonic Conduction in Perovskite-type Oxide Ceramics Based on LnScO3 (Ln=La, Nd, Sm or Gd) at High Temperature

Ionic transport properties of perovskite-type oxides based on LnScO₃, Ln_{1- x}Ca_xScO_3- (Ln=La, Nd, Sm and Gd) and LaSc_1-xMg_xO_3- were studied using an electrochemical method at elevated temperatures. Conductivity in these oxide systems increased by more than three orders of magnitude upon doping with divalent atoms such as Ca or Mg. However, when x0.1, conductivities are almost independent of x in La_{1- x}Ca_{xScO_3-} and LaSc_{1- x}Mg_xO_3-. Gd_0.9Ca_0.1ScO_3- showed lower conductivity than Ln_1-xCa_xScO_3- (Ln=La, Nd and Sm). Protonic conduction in these oxides under hydrogen containing atmospheres was confirmed by emf measurements of hydrogen concentration cells and by electrochemical hydrogen pumping using these oxides as a diaphragm. In LaSc_1-xMg_xO_3- the transport number of protons under hydrogen containing atmosphere was unity, suggesting that electronic conduction never became dominant, even under strong reducing conditions. Under condition of high oxygen partial pressure, the transport number of ions was less than 0.1, suggesting that the majority conductive carriers under such conditions were holes.     

Scaling for the refrigeration effects in lead-free barium titanate based ferroelectric ceramics

The refrigeration effect (ΔQ) or temperature change (ΔT) resulting from electro-caloric (EC) effect in BaTiO₃ multilayered structures and Ba_1-xSr_xTiO₃ ceramics are directly measured using differential scanning calorimetry (DSC). At high electric field and near the Curie temperature, the relation between ΔQ and the applied field E is found to follow a power-law relation ΔQ_max ~ E ^b , which is explained by the critical scaling near the Curie temperature. The exponent b is found to decrease with increasing disorder in the barium titanate based EC materials. Furthermore it is found that under the same electric field the maximum ΔQ of Ba_1-xSr_xTiO₃ ceramics which occurs around the Curie temperature first increases and then decreases with the increasing content of disorder x. A random-field ferroelectric transition model is used to investigate the effect of disorder on ΔQ and the scaling exponent b. The results from numerical solutions of this model are consistent with those from experiments. The scaling for the refrigeration effect in barium titanate based ferroelectrics is useful in the development of multilayered ferroelectrics with large cooling capacity for the practical application of lead-free barium titanate based ferroelectrics.     

Effect of Ba2+ – Sr2+ co-substitution on the structural and dielectric properties of Lead Titanate

Growing interest in developing new materials for device applications led to study of ferroelectric oxides in a wide range and variety of compositions. In the present work, polycrystalline samples of lead barium strontium titanate (Pb_1-xBa_0.5xSr_0.5xTiO₃) solid solution system have been synthesized. Phase formation studies and crystal structure analysis were carried out by X ray diffractometry at room temperature, which suggested formation of single phase compound with tetragonal structure up to x?=?0.8 and cubic structure for x?=?1.0. The XRD pattern has been analyzed by employing Rietveld method. The phase transition in the system was confirmed by Differential Scanning Calorimetry (DSC). Samples with 0.0????x????0.8 are in ferroelectric state whereas with x?=?1.0 is found to be in paraelectric state at room temperature. Co-substitution of Ba²⁺ and Sr²⁺ into lead titanate shows reduction in anisotropy as well as porosity. The dielectric studies of the system as a function of temperature and frequency were carried out in the range 323?K to 773?K and 100?Hz to 1?MHz respectively. Variation of dielectric constant and loss tangent with temperature shows peaking effect near Curie temperature. Frequency dependant dielectric studies clearly show that the dielectric constant and loss tangent decrease exponentially with increased frequency.     

Origin of giant dielectric response in ferroelectric thin film multilayers

Abstract

A thermodynamic theory of dielectric response in ferroelectric thin film multilayers is developed. The solution of Lame equation for static dielectric susceptibility has shown that susceptibility diverges at the transition temperature of the thickness induced ferroelectric phase. This divergence is shown to be the origin of the giant dielectric response observed in some multilayers. The theory gives an excellent fit to the temperature dependence of the giant susceptibility observed recently in multilayers of PbTiO₃-Pb_1-xLa_xTiO₃ (x = 0.28).     

Separation of the bulk and grain boundary contributions to the total conductivity of solid lithium-ion conducting electrolytes

The transport properties of lithium-ion conducting Li_3xLa_2/3-xTiO₃ are studied for bulk and grain-boundary effects. This paper introduces a procedure for investigating bulk and grain-boundary polarization contributions using electrochemical impedance spectroscopy (EIS) and subsequent analysis via the distribution function of relaxation times (DRT) [1]. The frequency range of impedance spectroscopy is extended up to 120 MHz to resolve all conductivity contributions occurring in a polycrystalline solid electrolyte. Intra grain (bulk) and inter grain (grain boundary) conductivity contributions are separated using (i) a systematic variation of solid electrolyte contacting, (ii) two different solid electrolyte microstructures and activation energies were determined using adequate equivalent circuit models. Finally, these results are supported by SEM analysis, revealing different grain size distributions and different contents of inhomogeneities in Li_3xLa_2/3-xTiO₃ solid electrolytes sintered at 1400°C and at 1450°C.     

Equivalent circuit models using CPE for impedance spectroscopy of electronic ceramics

Complex immittance (Impedance Z, Modulus M, Admittance Y, Permittivity ?) spectra for some equivalent circuit models involving resistances, capacitances and constant phase angle elements (CPE) are calculated for different ratios of the parameters. A comparison of experimentally obtained complex immittance plots with these diagrams greatly facilitates the search for the most appropriate equivalent circuit representing the electrical properties of electronic ceramics. An equivalent circuit for Ba_1-xSr_xTiO₃ (x = 0.35) ceramic system is developed by using these simulated plots.     

Structural, Thermal and Electrical Properties of Ce-Doped SrMnO3

The Sr_1-xCe_xMnO_1- system (0 x 0.5) was investigated with respect to its structural, thermal and electrical properties. Although un-doped SrMnO₃ has the perovskite structure above 1400°C, the structure is unstable at room temperature. However, partial substitution of Ce for Sr in SrMnO₃ stabilizes the perovskite structure down to room temperature. Single phase perovskite is obtained for 0.1 x 0.3 in Sr_1-xCe_xMnO_1-, and it remains stable even following heat treatment at 800°C for 100 h. The dependence of the electrical conductivity on temperature was measured from room temperature to 1000°C in air. Ce doping dramatically enhanced the electrical conductivity of SrMnO₃. Sr_0.7Ce_0.3MnO_1- exhibits a higher conductivity (290 S · cm^-1 at 1000°C) than that of La_0.8Sr_0.2MnO₃ (LSM, about 175 S · cm^-1) and remains n-type over the whole range of temperature examined. The thermal expansion coefficients in the system were nearly constant with values ranging between 1.24 × 10^-6 and 1.01 × 10^-6 cm/cm · K for temperatures of 50°C to 1000°C.     

Dielectric properties of (Ba1-2x Sr x Ca x )TiO3 ferroelectric ceramics

Ferroelectric ceramics of (Ba_1?2x Sr _x Ca _x )TiO₃ (0?≤?x?≤?0.30) were prepared by a routine solid-state reaction technique. Co-substitution of Sr²⁺ and Ca²⁺ for Ba²⁺ with equal mole in BaTiO₃ restrain the maximal dielectric constant K _m strongly when 0.2?≥?x?>?0. However, composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ shows the higher maximal dielectric constant. With increasing Sr and Ca content x, temperature T _m of the maximal dielectric constant K _m shifts to low temperature and all selected compositions for study exhibit a broad phase transition temperature range. The composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ presents characteristics of ferroelectric relaxor, value of K _m decreases and temperature T _m increases with increasing frequency.