Electrocaloric properties of (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 ferroelectric ceramics near room temperature

Electrocaloric effects of (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [abbreviated as (1 − x)PMN–xPT] ferroelectric ceramics with x being, 0.08, 0.10, and 0.25, respectively, were measured near room temperature, and the origins of the electrocaloric effects of these ceramics were discussed. It was found that these ceramics possess large electrocaloric effect with ΔT being, more than 1 K under an electric field of 1.5 kV mm⁻¹ in a wide temperature range (more than 10 K) near room temperature, and this effect is due to both of the electrocaloric effect resulting from the electric field induced first-order phase transition and the linear electrocaloric effect. It is expected that these ceramics could be used for multi-stage cascade ferroelectric refrigeration near room temperature.     

Processing and electrical properties of (1 − x)[(1 − y)(Pb(Mg1/3Nb2/3)O3)–y(Pb(Yb1/2Nb1/2)O3)]–xPbTiO3 ceramics

Ferroelectric ceramics in the vicinity of morphotropic phase boundary (MPB) with compositions represented as (1 ? x)[(1 ? y)(Pb(Mg_1/3Nb_2/3)O₃)–y(Pb(Yb_1/2Nb_1/2)O₃)]–xPbTiO₃ were prepared by solid state reaction. The addition of PYbN to PMN–PT decreased the sintering temperature from 1200 °C (y = 0.25) to 1000 °C (y = 0.75). The PT content, where the MPB was observed, increased with the PYbN addition. A remanent polarization value of 28.5 µC/cm² and a coercive field value of 11 kV/cm were measured from 0.62[0.25PMN–0.75PYbN]–0.38PT ceramics, which were close to the ones measured from PMN–0.32PT ceramics. In addition, the Curie temperature was found to increase with PYbN additions.     

Influence of Al2O3 on low-field spin-polarized tunneling magnetoresistance of (1 − x) La0.7Ca0.3MnO3 + x Al2O3 composites

In this study we report the effect of Al₂O₃ on the low field magnetoresistance (LFMR) of (1 ? x) La_0.7Ca_0.3MnO₃ + x Al₂O₃ composite synthesized through a solid-state reaction method combined with an energy milling method. Based upon a spin-polarized tunneling of conduction electrons at the grain boundaries, we have proposed a phenomenological model to explain the observed electrical transport behavior over the whole temperature range (5  300 K), especially the gradual drop of metal-insulator transition temperature (T_p = T_max) as a function of increasing Al₂O₃ content, while the ferromagnetic–paramagnetic transition temperature (T_C) remains almost constant (T_C = 250 K).     

Effect of bond valence on microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics

The origins of microwave dielectric properties (1 ? x)CaTiO₃–x(Li_0.5La_0.5)TiO₃ (0.2 ≤ x ≤ 0.8) ceramics, prepared by a conventional solid-state reaction method, were investigated based on the theory of bond valence. The XRD and SEM results showed that complete solid solutions with orthorhombic perovskite structure were formed in the whole investigated compositional range. The dielectric constant (?_r), quality factor (Q × f) and temperature coefficient of resonant frequency (τ_f) were closely related to B-site, A-site and the difference between A-site and B-site bond valences of ABO₃ perovskite compounds, respectively. As x value increased from 0.2 to 0.8, the dielectric constant increased from 198.3 to 276.8, the Q × f value decreased from 4340 to 1880 GHz, and the τ_f value varied from +489.7 to ?178 ppm/°C. For practical applications, excellent microwave dielectric properties of ?_r = 245, Q × f = 2750 GHz and τ_f = +0.75 ppm/°C were obtained for 0.4CaTiO₃–0.6(Li_0.5La_0.5)TiO₃ ceramics.     

Doping effects of Li–Sb content on the structure and electrical properties of [(Na0.5K0.5)1 − x(Li)x(Sb)x(Nb)1 − xO3] lead-free piezoelectric ceramics

Ceramic samples of [Na_0.5K_0.5]_1 ? x(Li)_x(Sb)_x(Nb)_1 ? xO₃ (NKNLS) (x = 0.04–0.06) were prepared by high temperature solid-state reaction method. X-ray diffraction analysis of the powder samples suggests the formation of a single-phase material with transformation from orthorhombic to tetragonal crystal structure with increase in Sb content. Dielectric studies show a diffuse phase transition about 100 °C and another phase ferroelectric–paraelectric transition at 330 °C. Polarization vs. electric field (P–E) hysteresis studies show maximum remanent polarization (Pr ～ 0.66 C m^?2) for composition x = 0.05. AC conductivity in the compound increases with increase in temperature which may be attributed due to oxygen vacancies and show negative temperature coefficient of resistance (NTCR) effect.     

High dielectric constant observed in (1 − x)Ba(Zr0.07Ti0.93)O3–xBa(Fe0.5Nb0.5)O3 binary solid-solution

Binary solid-solutions of the (1 ? x)Ba(Zr_0.07Ti_0.93)O₃–xBa(Fe_0.5Nb_0.5O₃) system, with 0.1 ≤ x ≤ 0.9,were fabricated via a solid-state processing technique. X-ray diffraction analysis revealed that all samples exhibited a single perovskite phase. The BaFe_0.5Nb_0.5O₃ also promoted densification and grain growth of the system. Dielectric measurements showed that all samples displayed a relaxor like behavior. The x = 0.1 sample presented a dielectric-frequency and temperature with low loss tangent (<0.07 at 10 kHz). For x > 0.2 samples, the dielectric data showed a broad dielectric constant–temperature curve with a giant dielectric characteristic. In addition, a high dielectric constant > 50,000 (at 10 kHz and temperature > 150 °C) was observed for the x = 0.9 sample.     

Phase transition in (1 − x)Bi(Mg1/2Ti1/2)O3-xPbTiO3 ceramics

The phase structure and phase transition of (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ (BMT-PT) ceramics with x = 0.0–0.42 were investigated. It was found that pure perovskite phases were achieved for x ≥ 0.28, while Bi₄Ti₃O₁₂ or Bi₁₂TiO₂₀ phase existed for x ≤ 0.15. The anomaly dielectric peaks were observed around 620 °C for BMT-(0.28–0.38)PT samples, thus phase transition in (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ ceramics was studied using thermal expansion. It was found that dielectric anomalies at ~ 620 °C were resulted from the phase transition of the second phase and defects inside samples.     

Synthesis and characterization of xTiO2(1 − x)α-Fe2O3 magnetic ceramic nanostructure system

Rutile-doped hematite xTiO₂(1 ? x)α-Fe₂O₃ (x = 0.0–1.0) nanostructures were synthesized using mechanochemical activation by ball milling. Their complex structural, magnetic and thermal properties were characterized by X-ray diffraction, Mössbauer spectroscopy and simultaneous DSC–TGA. XRD patterns yielded the dependence of lattice parameters and grain size as a function of ball milling time. For the molar concentrations x = 0.1 and 0.3, the Mössbauer spectra were fitted with one, two, three or four sextets, corresponding to the degree of Ti ion substitution of Fe ions in hematite lattice. After 12 h of ball milling, the completion of Ti ion substitution of Fe ions in hematite lattice occurs for x = 0.1 and 0.3. For x = 0.5 and 0.7, Mössbauer spectra fitting required sextets and a quadrupole-split doublet, representing Fe ions substituting Ti ions in the rutile lattice. The completion of Fe ion substitution of Ti ions in rutile lattice was not observed, as indicated by XRD patterns and Mössbauer spectra for these two molar concentrations. Simultaneous DSC–TGA measurements revealed that the mechanochemical activation by ball milling has a strong effect on the thermal behavior of this nanostructure system. The enthalpy dropped dramatically after 2 h of milling time, indicating the strong solid–solid interactions between TiO₂ and α-Fe₂O₃ after ball milling. The change in weight loss of hematite was caused by the decrease of grain size and ion substitutions between Fe and Ti after mechanochemical activation.     

Formation and thermal properties of fluorite-pyrochlore composite structure in La2(ZrxCe1 − x)2O7 oxide system

Rare-earth oxides of La₂(Zr_xCe_1 ? x)₂O₇ for thermal barrier coatings (TBCs) are fabricated via a solid-state reaction at 1600 °C. As the phase formation, microstructure, and thermal properties of these oxides are examined, a fluorite–pyrochlore composite structure is found in the La₂(Zr_xCe_1 ? x)₂O₇ system. This composite structure is composed of coarse Ce-rich fluorite and fine Zr-rich pyrochlore grains. From XRD and microstructural analysis, the lattice parameter and volume fraction of each phase are evaluated in order to obtain the intrinsic thermal conductivity value of composite-structured oxide with porosity calibration. The thermal conductivity of the composite structure is similar to that of pyrochlore La₂Zr₂O₇, which is attributed to phonon scattering by phase boundaries.     

Thermo-optical properties of LaMg1 − xNixAl11O19 (0 ≤ x ≤ 1) hexaaluminates for metallic thermal protection system

LaMg_1 ? xNi_xAl₁₁O₁₉ (x = 0, 0.25, 0.5, 0.75, 1) ceramics are fabricated by pressureless-sintering method at 1700 °C for 10 h in air. The microstructure and thermo-optical properties of LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics are investigated by the X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The influence of NiO doping on structure and thermo-optical properties of LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics is investigated. The partial substitution of Ni²⁺ for Mg²⁺ results in a significant increase in emissivity at low wavelengths as compared with unmodified LaMgAl₁₁O₁₉. When the Ni²⁺ content increases to x = 0.75 or above, LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics have a high emissivity value above 0.70 at low wavelengths at 500 °C. The measured emissivity of all LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics shows a similar trend in the wavelength range of 6 to 14 μm.     

Novel SrCo1 − 2x(Fe,Nb)xO3 − δ (x = 0.05, 0.10) oxides targeting CO2 capture and O2 enrichment: Structural stability and oxygen sorption properties

The novel Fe/Nb co-doped SrCo_1 ? 2x(Fe,Nb)_xO_3 ? δ (x = 0.05, 0.10) perovskite oxides were synthesized by the solid-state method. Structural and chemical stability of the SrCo_1 ? 2x(Fe,Nb)_xO_3 ? δ (x = 0.05, 0.10) oxides were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and X-ray diffraction (XRD). The results demonstrated that the structural and chemical stability of the Fe/Nb co-doped SrCo_1 ? 2x(Fe,Nb)_xO_3 ? δ (x = 0.05, 0.10) is improved significantly. The oxygen sorption properties of the SrCo_1 ? 2x(Fe,Nb)_xO_3 ? δ (x = 0.05, 0.10) oxides were investigated between 300–900 °C in air, and the high oxygen sorption capacity of 11.5 and 10.3 mL O₂ (STP)/g oxide, respectively, are obtained.     

Microwave dielectric properties of Ba(Sn x Zn(1−x)/3Nb(1−x)2/3)O3 (0 ⩽ x ⩽ 0.32) ceramics

The dielectric properties of the (1–x)Ba(Zn_1/3Nb_2/3)O₃–xBaSnO₃ (0 x 0.32) composition at microwave frequencies were investigated in this study. With the addition of BaSnO₃, the dielectric Q(Q _d) value of Ba(Zn_1/3Nb_2/3)O₃ (BZN) can be improved and a small temperature coefficient of resonant frequency (_f) can be achieved. When 22.6 mol % of Sn is added to BZN, the characteristics of the Ba(Sn_0.226Zn_0.258Nb_0.516)O₃ ceramics sintered at 1500°C are as follows: dielectric constant _r = 32, _f = + 12 p.p.m.°C¹ and high Q _d value of 9700 at 10 GHz. Based on the classical dispersion theory and the logarithmic mixing rule, the effects with additions of substitutional element of BaSnO₃ on the microwave dielectric properties of Ba(Zn_1/3Nb_2/3)O₃ can be mostly explained.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.     

Dielectric properties of the system Ba1−xLaxTi1−xCo x O3

The dielectric behaviour of compositions withx=0.01, 0.05, 0.10 and 0.20 in the system Ba_1–x La _x Ti-_1-x Co _x O₃ was studied in the temperature range 300–473 K. The compositions withx = 0.01 and 0.05 show a diffuse ferroelectric-paraelectric phase transition, while other compositions do not show this transition in this temperature range. The frequency dependence of dielectric constant and dielectric loss in the samples withx0.05 indicates that spacecharge polarization contributes significantly to their observed dielectric parameters.     

Density functional theories study on the properties of Ga1 − xCrxAs

The structural, electronic, magnetic and optical properties of Ga_1 − xCr_xAs (x = 0, 6.25%, 12.5%) have been studied by first-principles calculations based on the HSE hybrid density functional theories. The optical properties, including the complex dielectric function, optical refractive index, extinction coefficient and absorption coefficient are discussed for radiation up to 15 eV. The results predicate that the system of Ga_1 − xCr_xAs exhibits typical half-metallic properties, in which Cr forms deep levels in the forbidden band and reduces the energy gap, increases static dielectric constant and obviously red-shifts the absorption edge. With the increase of the fraction of Cr, the material gradually exhibits noticeable anisotropy in the photon energy range of 0–5 eV. In addition, the p-d hybridization reduces the magnetic moment of Cr from its free space charge value of 3 μ_B and a smaller atomic magnetic moments of As and Ga atoms are generated.     

Density variation and piezoelectric properties of Ba(Ti1 − x Sn x )O3 ceramics prepared from nanocrystalline powders

Nanocrystalline powders of tin-doped barium titanate with different concentrations of tin have been synthesized by a combination of solid state reaction and high-energy ball milling. The average particle size of the milled powders as determined from TEM analysis was about 5·96 nm. Analysis of all the milled powders using X-ray diffraction method showed single phase perovskite structure. The density variation of the ceramics with sintering temperature has been studied by sintering the samples at different temperatures. Density variation results show that 1350°C is the optimum sintering temperature for tin-doped barium titanate ceramics. SEM micrographs show high density and increasing trend of grain size with increasing content of Sn. The ferroelectricity decreases with increasing concentration of Sn. The electromechanical coupling coefficient also decreases with increasing Sn content corroborating decreasing trend of ferroelectricity. The bipolar strain curves show piezoelectric properties of the prepared ceramics.     

Multiferroic Properties of (Bi0.9Gd0.1FeO)1−x (BaTiO3) x Ceramics

Conventional solid-state reaction method has been employed for the synthesis of polycrystalline (Bi_0.9Gd_0.1FeO)_1?x (BaTiO₃) _x for x=0.1, 0.2 and 0.3, ceramics samples. The effect of BaTiO₃ content on the multiferroic properties of Gd-doped BiFeO₃ ceramics has been presented. Pure perovskite phase with high density has been obtained by optimizing the synthesis approach, calcination and sintering strategies. Structural analysis carried out using X-ray diffraction confirms the formation of desired morphotropic phase. The dielectric properties have been investigated at different concentration of BaTiO₃ as function of temperature, revealing that by increasing the BaTiO₃ content dielectric constant increases while dielectric losses decrease. Magnetic study shows that initially saturation magnetization increases with increase in BaTiO₃ content up to x=0.1; however, afterwards it decreases for higher concentration of BaTiO₃. According to ferroelectric measurements, PE loops (with low coercive field) are observed at room temperature. The remnant polarization (P _r ) has been found to be 0.169, 0.619 and 0.760 μC/cm², respectively, for samples with x=0.1, 0.2 and 0.3. Magnetoelectric coupling in as-synthesized samples has been indirectly deduced by an anomaly observed at magnetic transition temperature.