Magnetocaloric study of monovalent-doped manganites Pr0.6Sr0.4−x Na x MnO3 (x = 0–0.2)

A systematic investigation of structural, magnetic, and magnetocaloric properties is reported for a series of monovalent sodium-doped manganites Pr_0.6Sr_0.4?x Na _x MnO₃ (x = 0, 0.05, 0.1, 0.15, and 0.2). Rietveld refinements of the X-ray diffraction patterns show that all powder samples are single-phased and crystallized in the orthorhombic structure with Pnma space group. Magnetic characterization and Arrott plot confirm the second-order phase transition at Curie temperature T _C decreasing from 310 K for x = 0 down to 272 K for x = 0.2. Magnetic entropy change is largest for x = 0 reaching 1.95 J kg^?1 K^?1 at 2 T field. This corresponds to a large relative cooling power of 102 J kg^?1 . Magnetic field sensitivity of magnetic entropy change and relative cooling power are analyzed and discussed.     

Structural, magnetic, infrared and Raman studies of La0.8Sr x Ca0.2-x MnO3 (0 ≤ x ≤ 0.2)

We report the results of magnetic, X-ray powder diffraction, infrared and temperature dependent Raman spectra of the La_0.8Sr _x Ca_0.2-x MnO₃ (0 ≤ x ≤ 0.2) manganite. The structure refinement using the Rietveld method indicates that the partial substitution of strontium for calcium (for x ≥ 0.15) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a rhombohedral phase. Magnetic measurement confirms the increase in the Curie temperature from 180 K for La_0.8Ca_0.2MnO₃ to 307 K for La_0.8Sr_0.2MnO₃, respectively. It is argued that the substitution with the larger Sr²⁺ ion strengthens the double-exchange interaction and gives rise to the observed increase of transition temperatures. All manganites show two IR active vibrational modes around 400 and 600 cm^?1. Moreover, when x ≤ 0.1, the absorption band around 400 cm^?1 splits into two peaks. In addition, we have analyzed the frequencies and widths of the observed Raman modes as a function of temperature for all samples with various Sr content. The mode splitting is attributed to both magnetic ordering and large orthorhombic distortion in doped rhombohedral manganites.     

Microstructure and microwave dielectric properties of xSm(Mg0.5Ti0.5)O3–(1 − x)Ca0.8Sr0.2TiO3 ceramics

xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ (x = 0.50–0.95) ceramics are prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties are investigated as a function of the x-value and sintering temperature. The single phase solid solutions were obtained throughout the studied compositional range. The variation of bulk density and dielectric properties are related with the x-value. Increasing sintering temperature can effectively promote the densification and dielectric properties of xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ ceramic system. With the content of Sm(Mg_0.5Ti_0.5)O₃ increasing, the temperature coefficient of resonant frequency τ _f value decreased, and a near-zero τ _f could be obtained for the samples with x = 0.80. The optimal microwave dielectric properties with a dielectric constant ε _r of 30.1, Q × f of 115,000 GHz (at 8.0 GHz), and τ _f of 8.9 ppm/°C were obtained for 0.80Sm(Mg_0.5Ti_0.5)O₃–0.20Ca_0.8Sr_0.2TiO₃ sintered at 1,550 °C for 3 h, which showed high density and well-developed grain growth.     

Microwave sintering effect on structural and dielectrical properties of Ba1−x(Sr/Pb)xTiO3 (x = 0.2 for Sr and Pb) ceramics

Microwave sintering has emerged in recent years as a new method for sintering a variety of materials that has shown significant advantages against conventional sintering procedures. Sr and Pb doped BaTiO₃ ceramics has been prepared by the high energy ball milling followed by conventional and microwave sintering. The phase formation was confirmed by X-ray diffractometer followed by Scanning electron microscopy, atomic force microscopy and Transmission electron microscopy. Dielectric constant was measured on both the samples and it is observed that, in Ba_0.8Pb_0.2TiO₃ (abbreviated as BPT), it increased more than one order of magnitude and in Ba_0.8Sr_0.2TiO₃ (abbreviated as BST), it increased two orders of magnitudes at room temperature and Curie transition temperature by microwave sintering. Interestingly the Curie transition temperature of BPT value decreased from 224 to 210 °C, where as in BST ferroelectric ceramics, no variation of transition temperature by conventional sintering and microwave sintering respectively. This promising technique has distinguished characteristics of energy saving, rapid processing and uniform temperature distribution throughout the samples.     

Microstructure and dielectric properties of Ca1–3/2xBixCu3Ti4O12 (x = 0, 0.05, 0.10, 0.15 and 0.20) ceramics

Influence of bismuth substitution on calcium site in CaCu₃Ti₄O₁₂ has been investigated. Compositions of Ca_1-3/2xBi_xCu₃Ti₄O₁₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were fabricated by solid-state sintering method. Crystal structure is remained cubic. X-ray diffraction indicates the presence of secondary phase of CuO in CCTO ceramics. Bismuth doping restrains the formation of CuO phase apparently. The grain size of CaCu₃Ti₄O₁₂ ceramics was greatly decreased by Bi³⁺ doping, resulting from the ability of bismuth to inhibit the grain growth. The dielectric and electric properties of CCTO ceramics were found to be influenced by bismuth doping. The fitting results of the complex impedance spectra showed an increase of the resistance of grain and grain boundary by bismuth substitution. Ca_0.70Bi_0.20Cu₃Ti₄O₁₂ showed the highest dielectric constant in the low frequency range. A modest composition such as Ca_0.85Bi_0.10Cu₃Ti₄O₁₂ expressed the optimized dielectric properties of higher dielectric constant (1.3 × 10⁴) and lower dielectric loss (0.06) than pure CCTO. The low and high temperature dielectric loss spectra demonstrate the interfacial polarization of the initial and secondary oxygen ionization, relating with the grain and grain boundary (the electrode contact for Ca_0.70Bi_0.20Cu₃Ti₄O₁₂) respectively.     

Synthesis of Ln0.6Sr0.4Co0.8Fe0.2O3 – δ (Ln = La, Pr, Nd, Sm) Perovskite-like Phases

Ln_0.6Sr_0.4Co_0.8Fe_0.2O_{3 –} (Ln = La, Pr, Nd, Sm) solid solutions are prepared by a procedure which makes it possible to markedly reduce the synthesis temperature and duration. X-ray diffraction, microstructural analysis, and density measurements are used to optimize conditions for the fabrication of dense substituted-cobaltite ceramics.     

High-Energy Storage Density and Efficiency of the xBi(Mg2/3Nb1/3)O3-(1 − x)(Ba0.8Sr0.2)TiO3 (0.08 ≤ x ≤ 0.14) Lead-Free Perovskite Structured Ceramics

The compounds xBi(Mg_2/3Nb_1/3)O₃-(1 − x)(Ba_0.8Sr_0.2)TiO₃ (xBMN-(1 − x)BST, 0.08 ≤ x ≤ 0.14) are prepared via the traditional solid-state reaction method and the ceramics are well densified in the sintering temperature range of 1280–1330 °C. X-ray diffraction analysis shows that all the ceramics crystallize into perovskite structure. Proper amounts of BMN additions can effectively reduce grain sizes of the xBMN-(1 − x)BST ceramics, resulting in more uniform microstructures. Accordingly, breakdown strength E_b is improved and a maximum value 250 kV cm⁻¹ is obtained in ceramic with x = 0.10. Meanwhile, recoverable energy storage density W_rec of the 0.1BMN-0.9BST ceramics reaches 2.03 J cm⁻³, and energy storage efficiency (η) is 96.8%. When the operating temperature increases to 150 °C, the W_rec and η values are about 1.02 J cm⁻³ under 150 kV cm⁻¹ and 89.8%, respectively.     

Structural and Magnetic Studies of Ca2?x Sm x MnO Compounds (x=0–0.4)

The Ca_2?x Sm _x MnO₄ (x=0?C0.4) compounds were synthesized by a solid?Csolid method and characterized by XRD at room temperature. The XRD profiles were indexed with a tetragonal structure (I/4mmm space group) for x??0.3 and orthorhombic one (Pnma space group) for x=0.4. The magnetic measurements were studied as a function of temperature (T=2?C300?K) and applied field (?? ₀ H=0?C10?T). When the temperature decreases, each compound has shown first a ferromagnetic?Cparamagnetic (FM?CPM) transition and then an antiferromagnetic?Cferromagnetic (AFM?CFM) one. The transition temperatures are found to be Sm-doping dependent. For all compounds, a spin-glass phenomenon is evidenced by FC/ZFC magnetization curves.     

Structural and Electrical Properties of Monovalent Doped Manganites Pr0.6Sr0.4−x K x MnO3 (x=0, 0.05 and 0.1)

Structural and electrical properties of the mixed-valence monovalent doped manganites Pr_0.6Sr_0.4?x K _x MnO₃ (x=0, 0.05 and 0.1), prepared using the conventional solid-state synthesis method, have been investigated. Rietveld refinement of the X-ray diffraction patterns at room temperature shows a slight increase in unit cell volume with K content and confirms that all powder samples are single phase and crystallize in the orthorhombic structure with Pnma space group. Electrical resistivity measurements under and without magnetic field show a transition from the metallic to insulating behavior. The electrical conductivity is improved with increasing K content. The resistivity data in metallic region were fitted according to the electron–electron scattering process while in insulating region they were fitted using the small polaron hopping SPH model and Mott’s variable range hopping VRH model.     

Dielectric and Piezoelectric Properties of (Na0.5Bi0.5)(Ti1–xMnx)O3 (x = 0–0.1) Modified Ceramics

Inorganic Materials - We have studied the crystal structure and dielectric and local piezoelectric properties of (Na0.5Bi0.5)(Ti1–xMnx)O3 (x = 0–0.1) modified sodium bismuth...     

Structure,morphology, dielectric,and impedance properties of (1-x) (Al0.2La0.8TiO3) + (x) (CuTiO3) (x = 0.2–0.8) nanocomposites

Journal of Materials Science: Materials in Electronics - (1?x) (Al0.2La0.8TiO3) + (x) (CuTiO3) (x?=?0.2–0.8) [ALTCT] nanocomposites were prepared through hydrothermal...     

Magnetocaloric Effects in Pr0.6−x Er x Sr0.4MnO3 (0.0≤x≤0.2) Manganese Oxides

The effects of partial substitution of praseodymium by erbium on the structural, magnetic, and magnetocaloric properties of Pr_0.6?x Er _x Sr_0.4MnO₃ (0.0≤x≤0.2) powder samples have been studied. Our polycrystalline compounds were synthesized by the conventional solid state reaction at high temperature. Rietveld refinement of the X-ray diffraction patterns using Fullprof program shows that all our samples are single phase and crystallize in the orthorhombic structure with the Pnma space group. The unit cell volume decreased with increasing the Er amount. Magnetic measurements show that all our samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. The Curie temperature T _C shifts to lower values with increasing Er content. From the magnetization isotherms at different temperatures, magnetic entropy changes ΔS _M and relative cooling power RCP have been evaluated. The maximum of the magnetic entropy changes for the Pr_0.45Er_0.15Sr_0.4MnO₃ sample is found to be $| \Delta S_{M}^{\max } | = 2.66~\mathrm{J}\,\mathrm{kg}^{-1}\,\mathrm{K}^{-1}$ under a magnetic applied field change of 2 T.     

Microwave dielectric properties of (Ca0.8Sr0.2)(SnxTi1−x)O3 ceramics

In this paper, we study the behavior of the B-site behavior with the incorporation of Sn⁴⁺ ion in (Ca_0.8Sr_0.2)TiO₃ ceramics. An excess of Sn⁴⁺ resulted in the formation of a secondary phase of CaSnO₃ and SrSnO₃ affecting the microwave dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics. The dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics were improved because of the solid solution of Sn⁴⁺ substitution in the B-site. The temperature coefficient of resonant frequency (τ_f) of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics also improved with increasing Sn content.     

Effect of Ti on glass-forming ability and magnetic properties of Fe81Si4B12−x P2Cu1Ti x (x = 0–3) soft magnetic alloys

The effect of Ti substitution for B on the glass-forming ability, microstructure and soft magnetic properties of Fe₈₁Si₄B_12?x P₂Cu₁Ti _x (x = 0–3) alloy ribbons made by single roller melt spinning method were investigated. The experimental results demonstrate that proper doping of Ti effectively improves the glass-forming ability, widening the optimum annealing temperature range, decreasing the sensitivity of coercivity on annealing temperature, increasing the volume fraction of nanocrystalline α-Fe phase and suppressing the grain growth in this alloy system. For alloy ribbons Fe₈₁Si₄B_12?x P₂Cu₁Ti _x (x = 0.5–2.5) annealed at 475 °C for 5 min, the saturation magnetostriction λ _s and the magnetic flux density B ₈₀₀₀ gradually decrease with increasing x, while the coercivity H _c decreases with increasing x until x = 2.0 and then increases. Nanocrystalline alloy Fe₈₁Si₄B₁₀P₂Cu₁Ti₂ shows excellent magnetic properties with low λ _s of 7 ppm, low H _c of 7.5 A/m and high B ₈₀₀₀ of 1.73 T which is promising for the future industrial applications.