Enhanced electrocaloric effect in Ba0.85Ca0.15Zr0.1Ti0.9–xSnxO3 ferroelectric ceramics

The present work deals with the study of effect of Sn addition (0%–6% by mole) on the electrocaloric (EC) performance of Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) ceramics. Maxwell's relations in conjunction with data from experimental observations were used for the indirect caloric measurements. Small percentage change with Sn addition is found to increase the EC performance of BCZT ceramics. A peak adiabatic temperature change of 0.84 K (353 K) was predicted for 0–32 kV/cm electric field, significantly higher than pristine BCZT ceramics. A simultaneous increase in specific entropy change was also observed.     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.

     

Concentration Dependence of the Electric Field Gradient in PbZr1−x Ti x O3 Ceramics

The concentration dependence of the electric field gradient (EFG) in PZT ceramics was studied by Time Differential Perturbed Angular Correlations (PAC) using the ¹⁸¹Ta radioactive probe in samples with x=0.3, 0.5 and 0.8 at room temperature. This phase presents two sites, probably related to defects close to probes, caused by oxygen and lead vacancies. We can also state that both the paraelectric and ferroelectric phases of these systems have two sites with different characteristics and that the high-frequency quadrupolar frequency site presents the highest fraction. The concentration dependence revealed by our results is discussed by comparing the latter with previous PAC studies. This revised version was published online in September 2006 with corrections to the Cover Date.     

Crystallography,magnetic properties and magnetocaloric effect in Gd4(BixSb1−x)3 alloys

A study of magnetic and thermal properties has been carried out on the alloys from the Gd₄(Bi_xSb_1−x)₃ series with x=0, 0.25, 0.5, 0.75, and 1. All of the alloys are ferromagnetic below their respective Curie temperatures which vary from 266 K for x=0 to 332 K for x=1.0. The magnetocaloric effect calculated from the temperature and magnetic field dependencies of the magnetization and heat capacity is moderate when compared to that of other materials, which order in the same temperature range. Both the magnetic ordering and the magnetocaloric effect peak temperatures increase nearly linearly with the increasing Bi content. Experimental magnetocaloric effect data obtained from two different measurement techniques are in excellent agreement.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.     

Mössbauer effect study of the pseudo-binary system (Ti1−x Nb x )Fe2

Mössbauer spectroscopy and X-ray diffraction measurements have been done on (Ti_1–x Nb _x )Fe₂ compounds in order to investigate the effect of Nb on the magnetic properties of TiFe₂. The experimental results show that Nb enters the lattice by filling Ti sites, thereby forming a continuous phase over the whole range of Nb concentrations. The Mössbauer spectra at 80 K fitted with a magnetic hyperfine field distribution show a continuous decrease of the average magnetic hyperfine field with increasing Nb concentration, as well as several different magnetic configurations forx0.3.     

The effect of LaPO4 in the LixLa2/3Ti1−xPxO3+x system

Lithium ion conductors of the overall composition Li_xLa_2/3Ti_1−xP_xO_3+x (hereafter referred to as LTP) based on La_2/3TiO₃ were prepared by solid state reaction at high temperature (1300 °C). AC impedance measurements indicate that the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ when x=0.28 − 0.35 at room temperature and have an activation energy of 18 kJ·mol⁻¹ in the temperature range from room temperature to 400 °C. X-ray powder diffraction patterns showed that the LTP system has a complex composition, which contains the solid solution perovskite Li_3xLa_2/3−xTiO₃ and LaPO₄.     

Antihysteresis of the electrical resistivity of graphene on a ferroelectric Pb(Zr x Ti1 − x )O3 substrate

A quantitative model is developed to explain the antihysteretic behavior of the electrical resistivity of graphene on a ferroelectric Pb(Zr _x Ti_{1 ? x} )O₃ substrate as a function of the gate voltage. The model takes into account the trapping of the electrons from the graphene layer by the states related to the graphene-ferroelectric interface. The finite energy gap of impurity states is also taken into account, which makes it possible to describe the well-known experimental dependences, including an increase and the subsequent saturation of a “memory window” with the switching gate voltage. The obtained estimates can be important for creating next-generation nonvolatile memory elements, which use the two stable values of electrical resistivity (one of them is attributed to logical “0” and the other, to “1”) that result from the antihysteresis effect.     

Quantum confinement mediated enhancement of optical kerr effect in CdS x Se1−x semiconductor microcrystallites

We first discuss theoretically the enhancement of the optical nonlinearity due to quantum confinement in semiconductor-doped glasses. This quantum confinement mediated enhancement also pertains to the figure of merit x ⁽³⁾/, in which case it is due to a decrease in the spectral width rather than in an increase of the matrix elements. We then report on frequency and size-dependent measurements which confirm the theoretically expected enhancement due to quantum confinement.     

Shubnikov-de Haas effect in tricrystals of the system of alloys Bi1−x Sbx (x=0.1; 0.13) with n-type conductivity

The Shubnikov-de Haas effect has been investigated in tricrystals of the alloy system Bi_1−x Sb_x (x=0.1; 0.13) with n-type conductivity in stationary (up to 14 T) and pulsed (up to 40 T) magnetic fields. Reconstruction of the internal boundary of the tricrystals was observed, along with a number of new component oscillations of ρ(B) indicating a rotation of the constant-energy surfaces of the L electrons on the intercrystallite boundary at an angle of ∼74° in the binary-trigonal plane. Fiz. Tverd. Tela (St. Petersburg) 41, 991–993 (June 1999)     

Evolution of structural,magnetic and magnetocaloric properties in Sn-doped manganites La0.57Nd0.1Sr0.33Mn1−x Sn x O3 (x = 0.05–0.3)

Structural and magnetic properties of manganites series La_0.57Nd_0.1Sr_0.33Mn_1?x Sn _x O₃ with (0.05 ≤ x ≤ 0.30) have been investigated, and the critical exponents and magnetocaloric effect are studied around the room temperature, to shed light on Sn substitution influence. A solid-state reaction method was used in the preparation. A structural study using Rietveld refinement of XRD patterns indicates rhombohedral structure with R \( \overline{3} \) c space group for (0.05 ≤ x ≤ 0.20) and shows the existence of a secondary phase attributed to the neodymium tin oxide (Nd₂Sn₂O₇) pyrochlore for x = 0.3. The variation of the magnetization (M) vs. temperature (T), under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition at the Curie temperature T _C. In addition, it was discovered that increasing the tin content leads to a reduction in magnetization and a lowering of T _C from 282 K (x = 0.05) to 158 K (x = 0.20) with increasing Sn substitution. The samples exhibit the characteristics of spin/cluster-glass state which is evident from (zero-field-cooled and field-cooled) magnetization vs. temperature curves. Indeed, the thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T ^3/2, in accordance with Bloch’s law. The spin-stiffness constant D obtained from the Bloch constant was determined. A large magnetocaloric effect has been observed in both samples (x = 0.05 and x = 0.10): the maximum entropy change, \( \left| {\varDelta S_{\text{M} }^{\text{peak}} } \right| \) , reaches the highest value of 3.22 J/kg K under a magnetic field change of 5 T with a RCP value of 56 J/kg for x = 0.10 composition. This opens an interesting opportunity to this compound to compete with materials which work as magnetic refrigerants near room temperature. Besides, we show that the samples follow the conventional behavior of a second-order ferromagnetic transition. This was possible by investigating the critical behavior at the transition region by adopting the modified Arrott plot method. The values of the critical exponents (β, γ, δ and n) are determined and they are between those predicted by the three-dimensional Heisenberg model.     

Neutron diffraction study of the oxide conducting δ1-phase of (Bi2O3)1−x(Y2O3)x (x=0.25)

The atomic parameters of the average cell of δ¹-Bi(Y)O_1.5 were determined by powder neutron diffraction. The evidence indicates that the disorder in the structure has many features in common with the structure of the oxygen deficient zirconia. Most oxygens (78%) are in sites displaced 0.335 Å along 〈100〉 directions from the normal fluorite positions, while a smaller proportion (22%) are displaced 0.80 Å along 〈111〉 directions and no oxygen remains in normal positions. In addition to the anion displacements a smaller displacement (0.25 Å) along the 〈111〉 direction was found for the cations. Comparison with the structure of β-Bi₂O₃ suggests that the displacements may be precursors to a δ→β phase transition.     

Dielectric, modulus and impedance analysis of lead-free ceramics Ba0.8La0.133Ti1−x Sn x O3 (x=0.15 and 0.2)

Titanate barium (BaTiO₃)-type oxide ceramics Ba_0.8La_0.133Ti_1?x Sn _x O₃ (BLTS) (here x=0.15 and 0.2) have been synthesized by the standard solid-state reaction method. Preliminary room temperature X-ray study confirms the formation of single-phase compounds in a rhombohedral crystal system. The electrical properties of BLTS were studied using the ac impedance spectroscopy technique over a wide range of temperature (120–320 K) in the frequency range of 40 Hz to 10 MHz. The presence of a single arc in the complex modulus spectrum at different temperatures confirms the single-phase character of the BLTS compounds.     

Surface nanostructure effects on optical properties of Pb(Zr x Ti1−x )O3 thin films

Optical scattering properties of nanostructured matter have crucial impact on performance efficiency of various photonic components, such as waveguides, display elements, and solar cells. In this paper, diffuse transmission properties of nanocrystalline Pb(Zr _x Ti_1?x )O₃ thin films with a high refractive index of ~2.5 and optical transmittance are presented. Thin films with a thicknesses ranging from 50 to 500 nm were studied using integrating sphere technique and results were compared to simulations performed by a scalar scattering theory. Thin films were deposited by pulsed laser deposition at room temperature on MgO(100) substrates and post-annealed at a temperature of 800 °C. Structural phase evolution-induced surface effects, which introduced periodicity on the film surface, cause the definite diffuse elements in transmission spectra of the films. Low and evenly distributed scattering amplitudes in k-space were seen for highly tetragonal- or trigonal-oriented films with non-textured surfaces, which led to low diffuse transmission values (T _D ≈ 5 %), while confined and increased scattering amplitudes in k-space were seen for tetragonal–trigonal-oriented films, with phase co-existence, which led to microstructure-induced textured surfaces and increased diffuse transmission values (T _D ≈ 50 %). For highly textured surfaces, scattering amplitudes distributed in tilted ellipsoid shape in k-space was observed. Difference between modeled and measured values was 3.8 % in maximum.     

Spin-reorientation, ferroelectricity, and magnetodielectric effect in YFe(1-x)Mn(x)O3(0.1 ≤ x ≤ 0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (～115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.