Structural,magnetic and magnetocaloric study of La0.7−xEuxSr0.3MnO3 (x=0.1, 0.2 and 0.3) manganites

This paper reports the structural, magnetic and magnetocaloric properties of La_0.7−xEu_xSr_0.3MnO₃ (x=0.1, 0.2 and 0.3) polycrystalline manganites elaborated using the solid-state reaction at high temperatures. The X-ray powder diffraction shows that all the prepared compounds are single phase. La_0.6Eu_0.1Sr_0.3MnO₃ is crystallized in the rhombohedral symmetry, whereas a structural transition towards orthorhombic system is observed for x≥0.2. Eu doping was found to induce a decrease of the Curie temperature T_C from 343 K (x=0.1) to 272 K (x=0.3). All compounds undergo a large magnetocaloric effect and have consequently potential applications in magnetic refrigeration domain around room temperature.     

Continuous hydrothermal synthesis of Ca1−xSrxTiO3 solid-solution nanoparticles using a T-type micromixer

In this work, a continuous hydrothermal synthesis method in supercritical water was applied to environmentally benign production of Ca_1−xSr_xTiO₃ (x = 0.0–1.0) solid-solution nanoparticles as key materials in conducting, electric, magnetic, and photocatalytic fields. A T-type micromixer (330 μm id) was introduced for rapid heating of stating solutions of Ca(NO₃)₂, Sr(NO₃)₂, and TiO₂ sol using turbulent flow mixing with preheated NaOH aqueous solution and also for exact control of reaction temperature. At 673 K and 30 MPa for 5.0 s mean residence time, Ca_1−xSr_xTiO₃ solid-solution nanoparticles having crystallite diameters of around 20 nm with monomodal diameter distributions were obtained without byproducts and production of CaTiO₃ and SrTiO₃ separately over the whole composition range. Effects of NaOH molality, Ca and Sr compositions in the starting solutions, and mean residence time on the reaction were examined. The results showed that TiO₂ sol dissolution and Ca_1−xSr_xTiO₃ precipitation were almost finished within 0.25 s mean residence time, and after that Ostwald ripening proceeded.     

Effect of site occupancies of rare earth ions on electrical properties in Ni-MLCC based on BaTiO3

The site occupancies of rare earth ions for Ba-site to Ti-site were quantitatively estimated in (Ba_1−x,R_x)(Ti_1−x,R_x)O₃ (R = Eu, Ho), (Ba_1−3xEu_2x)TiO₃, and Ba(Ti_1−2xEu_2x)O_3−x systems by applying the Rietveld refinement to the data obtained from the synchrotron radiation powder diffraction measurement. The occupational ratio of Ba-site to Ti-site for the larger rare earth ion (Eu) was 49/51 (x = 0.10), whereas for the smaller ion (Ho) was 9/91 (x = 0.01) in (Ba_1−x,R_x)(Ti_1−x,R_x)O₃ system. Furthermore, the occupational ratio was greatly dependent on the Ba/Ti ratio, in (Ba_1−3xEu_2x)TiO₃ system it was 92/8 (x = 0.03), whereas that in Ba(Ti_1−2xEu_2x)O_3−x system was 20/80 (x = 0.01). The Curie point shifted to lower temperature effectively with increasing in the occupational ratio of rare earth ion into Ba-site.     

Effect of La doping on tunable behavior of sol–gel-derived PST thin film

Pb_xSr_1−xTiO₃ and (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin films were prepared on ITO/glass substrate by sol–gel technique. The crystalline phase structures of thin films were investigated by XRD. The dielectric properties were measured by LCZ meter. Results show that the perovskite phase was stable in Pb_xSr_1−xTiO₃ thin film. Its lattice constant was found to increase with the increase of x. When Pb/Sr ratio was about 50/50, the transformation of the perovskite phase between cubic and tetragonal took place at room temperature. To ensure better tunability, Pb_0.4Sr_0.6TiO₃ thin film was selected in both paraelectric state (cubic structure) and near the phase transformation point between cubic and tetragonal. The tunability of more than 80% and figure of merit (FOM) of 14.17 were obtained. (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ was also prepared as La-doped thin film. Its dielectric constant was decreased below x = 0.4 and then increased above x = 0.4 with the increase in x. The inharmonic coefficient depicting ability of tunability of (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film kept constant due to the changeable shape of the crystal cell under electric field at a fixed temperature below x = 0.4, which is in good agreement with Johnson's theory. And the coefficient reveals a distinctive variable because of easily changeable shape under different bias above x = 0.4. The tunability of about 70% and FOM of about 7 were obtained in the (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film.     

Dielectric and Magnetic properties of (1 − x)BiFeO3–xBa0.8Sr0.2TiO3 ceramics

The polycrystalline samples of (1 ? x)BiFeO₃–xBa_0.8Sr_0.2TiO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4 and x = 1) were prepared by the conventional solid state reaction method. The effect of substitution in BiFeO₃ by Ba_0.8Sr_0.2TiO₃ on the structural, dielectric and magnetic properties was investigated. X-ray diffraction study showed that these compounds crystallized at room temperature in the rhombohedral distorted perovskite structure for x ≤ 0.3 and in cubic one for x = 0.4. As Ba_0.8Sr_0.2TiO₃ content increases, the dielectric permittivity increases. This work suggests also that the Ba_0.8Sr_0.2TiO₃ substitution can enhance the magnetic response at room temperature. A remanent magnetization M_r and a coercive magnetic field H_C of about 0.971 emu/g and 2.616 kOe, respectively were obtained in specimen with composition x = 0.1 at room temperature.     

Dielectric spectroscopy and magnetometry investigation of Gd-doped strontium titanate ceramics

Structural, dielectric and magnetic properties of dense Gd-doped strontium titanate ceramics prepared by the conventional mixed oxide method are studied. Lattice parameter is found to decrease linearly in the Sr_1-1.5xGd _xTiO₃ system, confirming the incorporation of Gd onto the Sr site of the perovskite lattice of strontium titanate up to x = 0.05. Real and imaginary parts of the dielectric permittivity of Sr_1-1.5xGd _xTiO₃ ceramics exhibit a relaxation between 100 Hz and 1 MHz in the temperature range of 17–26 K, slightly shifting to higher temperatures with increasing Gd content. Such dielectric behaviour is attributed to the relaxation of individual dipoles formed by off-centre displacement of Gd³⁺ ions on Sr sites in a highly polarizable lattice of strontium titanate. Other dielectric relaxations observed in Sr_1-1.5xGd _xTiO₃ from 125 to 300 K are attributed to the oxygen vacancy related mechanisms. No magnetic anomaly but paramagnetic behaviour is observed for Sr_1-1.5xGd _xTiO₃ ceramics, indicating an absence of the magnetic order and hence magnetoelectric coupling.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Crystal structure and luminescence properties of novel Sr10−x(SiO4)3(SO4)3O:xEu2+ phosphor with apatite structure

In this paper, a series of novel luminescent Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors with apatite structure were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the PL thermal stability were investigated. Sr_9.92(SiO₄)₃(SO₄)₃O:0.08Eu²⁺ phosphor exhibits better thermal quenching resistance, retaining the luminance of 66.55% at 150 °C compared with that at 25 °C. The quenching concentration of Eu²⁺ in Sr₁₀(SiO₄)₃(SO₄)₃O was about 0.08 (mol) with the dipole–quadrupole interaction. The Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors exhibited a broad-band green emission at 538 nm upon excitation at 396 nm. The results indicate that Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors have potential applications as near UV-convertible phosphors for white-light UV LEDs.     

Improvement of dielectric loss of (Ba,Sr)(Ti,Zr)O3 ferroelectrics for tunable devices

(Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ (0.05 ≤ x ≤ 0.3) ferroelectric materials have cubic perovskite structure and show paraelectric properties at room temperature. Curie point shifted to a negative value as increasing Zr content in (Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ system. When Zr substituted 0.1 mol, the dielectric constant, dielectric loss, tunability, Curie point and FOM were 4500, 0.0005, 63%, −1.6 °C and 1260, respectively. This composition shows excellent microwave dielectric properties than those of (Ba_0.6Sr_0.4)TiO₃ ferroelectrics, which are limelight materials for tunable devices such as varactors, phase shifters and frequency agile filters, etc.     

Effect of Sr/Ca substitution on phase structure and photoluminescence properties of micro-SrxCa1−xAlSiN3: Eu2+ phosphor for high CRI white LEDs

Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors were prepared by using the high temperature solid state reaction in a 1.1 Mpa N₂ atmosphere. The phase structures, photoluminescence (PL) properties, and chromaticity properties of the phosphors affected by Sr/Ca Substitution have been investigated in detail. With increasing Sr content (x value), the crystal grain became bigger and the average grain size increased from 5 µm to 10 µm. PL emission bands of Sr_xCa_1−xAlSiN₃: Eu²⁺ showed a blue-shift from 660 (x=0) to 617 nm (x=0.8), the shoulder of the excitation spectra around 550 nm showed a slightly blue-shift and decay lifetime shortened from 776.96 (x=0.2) to 642.35 ns (x=0.8). Both the emission and excitation intensity of peak position increased with Sr content increased. The ideal white light with high CRI (Ra>88) can be obtained by mixing the Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors and commercial green phosphors with appropriate proportion of the components.     

Large relative cooling power of Bi-doped La0.8−xBixSr0.08(Ca0.55Ba0.45)0.12MnO3 (x=0.0, 0.1 and 0.3) perovskites: Magnetic and magnetocaloric properties

We report magnetic and magnetocaloric properties of La_0.8−xBi_xSr_0.08(Ca_0.55Ba_0.45)_0.12MnO₃ (x=0.0, 0.1 and 0.3) perovskite manganites. Polycrystalline samples have been synthesized in air by the sol gel method at a sintering temperature of 1150 °C. Powder X-ray diffraction data show that samples are phase pure and their cell parameters slightly decrease with increase in Bi content. Scanning electron micrographs show that the average particle size increases with increase in Bi content. The temperature dependent magnetization measurements show that the Curie temperatures decrease from 315 K (x=0.0) to 140 K (x=0.3) with increase in Bi content. The isothermal magnetization data is used to estimate the magnetic entropy changes (−$\Delta S_M$) and their calculated values are 1.12 J kg⁻¹ K⁻¹, 1.96 J kg⁻¹ K⁻¹ and 1.62 J kg⁻¹ K⁻¹ for x=0.0, 0.1 and 0.3 samples respectively under an applied magnetic field of 2.0 T. The corresponding values of relative cooling power (RCP) are 90 J kg⁻¹, 180 J kg⁻¹, 136 J kg⁻¹ for x=0.0, 0.1 and 0.3 samples respectively. These results of magnetocaloric effect in our samples suggest that they are promising materials for the magnetic refrigeration applications.     

Crystal structure and luminescence properties of green-emitting Sr1−xAl12O19:xEu2+ phosphors

In this paper, a series of novel luminescent Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the decay curves were investigated. The quenching concentration of Eu²⁺ in SrAl₁₂O₁₉ was about 0.15 (mol). Upon excitation at 378 nm, the composition-optimized Sr_0.85Al₁₂O₁₉:0.15Eu²⁺ exhibited strong broad-band green emission at 530 nm with the CIE chromaticity (0.2917, 0.5736). The results indicate that Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors have potential applications as green-emitting phosphors for UV-pumped white-light LEDs.     

The influence of Mo6+ doping on the luminescence properties of red-emitting phosphor Sr9Eu2W4−xMoxO24 (x = 0–4)

A series of red emitting phosphors Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) have been synthesized by solid-state reactions and their crystal structures, photoluminescence properties were studied. The excitation and emission spectra of Sr₉Eu₂W_4?xMo_xO₂₄ phosphors can be modified by Mo⁶⁺ doping. As the molybdate content increased, the Eu³⁺ emission intensity of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) under 395 nm excitation was found to increase and reached a maximum at x = 2. The excitation spectra, the emission intensities and the chromaticity coordinates of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) were compared to those of the conventional red phosphor Y₂O₂S: Eu³⁺. The intense red-emission under near-UV excitation suggests that Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) could be a potential candidate for white light generation by using near-UV LEDs. In this study, the effects of Mo⁶⁺ doping on the crystal structure and photoluminescence properties of Sr₉Eu₂W_4?xMo_xO₂₄ were discussed.     

Electronic conductivity,oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ is approximately 15%. The incorporation of Al³⁺ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr_0.7Ce_0.3Mn_0.9Al_0.1O_3−δ, measured in the oxygen partial pressure range from 10⁻¹⁸ to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O₂) below 10⁻⁴ to 10⁻² atm. The oxygen permeability of dense Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO₃-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ ceramics in air are (10.8–11.8) × 10⁻⁶ K⁻¹.     

White emitting Dy3+ activated perovskite titanates and energy transfer by Eu3+ codoping

Perovskite type titanate phosphors Sr_0.97−xDy_0.03Li_xTi_1−xNb_xO₃, Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ and Sr_0.87−yDy_0.03Eu_yLi_0.1Ti_0.9Nb_0.1O₃ were prepared by conventional solid state method. Herein, white light emission from Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors and the lowering of its color temperature through codoping with Eu³⁺ ions are reported. Raman measurements have shown that the incorporation of dopants alters the vibrational properties of these phosphors significantly, indicating the reduction of the local symmetry in the crystal lattice. The addition of LiNbO₃ in SrTiO₃:Dy³⁺ phosphor enhances the luminescence intensity and the yellow to blue ratio resulting in emission of high quality white light with color coordinates corresponding to that of standard white. Life time measurements and data fits of Sr_0.9−xDy_xLi_0.1Ti_0.9Nb_0.1O₃ phosphors revealed the biexponential behaviour of luminescence decay profiles. From Judd-Ofelt analysis it is found that the intensity parameter Ω₂ increases with Dy³⁺ concentration and a quantum efficiency of 90.4% was obtained for optimum concentration. In the case of Dy³⁺ and Eu³⁺ codoped phosphors, the color coordinates are found to be sensitive to the Eu³⁺ concentration and the highest energy transfer efficiency of 92% was obtained for the phosphor doped with 10 mol% Eu³⁺. The emission color changes from cold white to reddish orange when the wavelength of excitation alters from 452 to 388 nm, since the energy transfer mechanism alone take place under 452 nm excitation and both direct absorption and the energy transfer mechanism occurs under 388 nm excitation.     

Catalytic activity of nanometer La1−xSrxCoO3 (x = 0, 0.2) perovskites towards VOCs combustion

Combustive oxidation of volatile organic compounds (VOCs), such as propyl alcohol, toluene and cyclohexane, were studied. The combustion was catalyzed by nanoparticles of La_1−xSr_xCoO₃ (x = 0, 0.2) perovskites prepared by a co-precipitation method. The results showed high activities of the perovskite catalysts. Compared to LaCoO₃, in particular, La_0.8Sr_0.2CoO₃ was much higher in catalytic ability. The total oxidation of VOCs followed the increasing order: cyclohexane < toluene < propyl alcohol. The T_99% of cyclohexane was 40 °C lower than that of toluene, which appeared to be determined by the bond strengths of the weakest C–H and C–C bonds. The 100-h stability experiments showed that La_1−xSr_xCoO₃ (x = 0, 0.2) perovskite was highly stable.     

Investigation of Ba1−xSrxTiO3 ceramics prepared from powders synthesized by the modified Pechini route

Ba_1−xSr_xTiO₃ (x = 0, 0.20, 0.25, 0.30 and 0.35) nanopowders were prepared by Pechini method from titanium isopropoxide, barium and strontium carbonates using citric acid as a chelating agent and ethylene glycol as an esterification agent. X-ray diffraction data show the formation of (Ba,Sr)TiO₃ solid solutions, free from secondary phases as BaCO₃ or Ti-rich oxides, when the polymeric precursors were calcined in air at 850 °C for 2 h. Ceramic pellets with relative density of 85–93% were obtained after sintering at 1350 °C for 3 h. High values of the dielectric constants (of ∼1500–12,000), low losses at the room temperature and a shift of the ferro-para phase transition temperature in the range of 7–127 °C with x decreasing were found. Lower values of the Curie constant for higher x indicate the increase of the chemical and electrical local heterogeneity degree.     

Precise measurement of the dielectric properties of BaxSr1−xTiO3 thin films by on-wafer through-reflect-line (TRL) calibration method

Polycrystalline Ba_xSr_1−xTiO₃ (x = 0.3, 0.4, 0.5) (BST) thin films with a thickness of 200 nm were deposited on r-cut sapphire substrates by rf sputtering method. The permittivity and loss tangent of the films were successfully observed in the range of 1–3 GHz, by utilizing the on-wafer through-reflect-line (TRL) calibration method although the estimated relative permittivity depended on an applied power to waveguides and the loss tangent had the dispersion around 1 GHz even in the case of 2 μm-thick aluminum. Finally, we concluded that the BST thin film with x = 0.4 is the most suitable for microwave tunable devices because it had the lowest loss tangent and relatively high permittivity.     

Defect chemistry of a high-k ‘Y5V’ (Ba0.95Eu0.05)TiO3 ceramic

The influence of the sintering temperature (T_s) on the structure, dielectric and valence-state properties of (Ba_1−xEu_x)TiO₃ (x=0.05) ceramics was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), Raman spectroscopy, and dielectric temperature measurements. An increase in T_s can increase the solubility limit of Eu in BaTiO₃. When the T_s was increased to 1450 °C, a high-k ‘Y5V’ (ε′_RT=8500) ceramic (C-BE5T) with a single-phase cubic structure was obtained. The dielectric peak shifted rapidly toward lower temperatures with increasing T_s at a rate of −0.46 °C/K. A symmetric (200) XRD peak, exaggerated grain growth (5.6 μm), a mixed valence of Eu²⁺/Eu³⁺, an asymmetric main Raman band at 2494 cm⁻¹ and a weak sharp band at 1516 cm⁻¹ in the high-wavenumber region are characteristics of cubic symmetry of C-BE5T. The formation of a solid solution of C-BE5T and defect chemistry are discussed.