Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting Mg with Ni

The microwave dielectric properties of La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method at various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.71 g/cm³, dielectric constant (?_r) of 20.19, quality factor (Q × f) of 74,600 GHz, and temperature coefficient of resonant frequency (τ_f) of −85 ppm/°C were obtained for La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Resistivity, thermopower, and thermal conductivity of nickel doped compounds Cr1−xNixSb2 at low temperatures

Substitutional compounds Cr_1−xNi_xSb₂ (0 ≤ x ≤ 0.1) were synthesized, and the effect of Ni substitution on transport and thermoelectric properties of Cr_1−xNi_xSb₂ were investigated at the temperatures from 7 to 310 K. The results indicated that the magnitudes of the resistivity and thermopower of Cr_1−xNi_xSb₂ decreased greatly with increasing Ni content at low temperatures, owing to an increase in electron concentration caused by Ni substitution for Cr. Experiments also showed that the low-temperature lattice thermal conductivity of Cr_1−xNi_xSb₂ decreased substantially with increasing Ni content due to an enhancement of phonon scattering by the increased number of Ni atoms. As a result, the figure of merit, ZT, of lightly doped Cr_0.99Ni_0.01Sb₂ was improved at T > ∼230 K. Specifically, the ZT of Cr_0.99Ni_0.01Sb₂ at 310 K was approximately ∼29% larger than that of CrSb₂, indicating that thermoelectric properties of CrSb₂ can be improved by an appropriate substitution of Ni for Cr.     

Gaseous and electrochemical hydrogen storage kinetics of nanocrystalline Mg2Ni-type alloy prepared by rapid quenching

The nanocrystalline Mg₂Ni-type Mg₂Ni_1−xCu_x (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by direct melt quenching technique. The structures of the as-cast and quenched alloys were investigated by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the alloys was tested by using constant current to charge and discharge the electrode. The results indicate that the substitution of Cu notably rendered the grain refinement of the as-cast alloys without altering the major phase Mg₂Ni. All the as-quenched alloys exhibit a nanocrystalline structure without the presence of any amorphous phase. It is found that the substitution of Cu for Ni and rapid quenching significantly ameliorated the gaseous and electrochemical hydrogen storage kinetics of the nanocrystalline Mg₂Ni_1−xCu_x (x = 0-0.4) alloys. Furthermore, both the rapid quenching treatment and the Cu substitution results in a notable increase in the hydrogen diffusion coefficient (D) as well as the limiting current density (I_L) but an obvious decline in the electrochemical impedance.     

Structural characterization of layered LiNi0.85−xMnxCo0.15O2 with x = 0, 0.1, 0.2 and 0.4 oxide electrodes for Li batteries

We report the synthesis of LiNi_0.85−xCo_0.15Mn_xO₂ positive electrode materials from Ni_0.85−xCo_0.15Mn_x(OH)₂ and Li₂CO₃. XRD and XPS are used to study the effect of Mn-doping on the microstructures and oxidation states of the LiNi_0.85−xCo_0.15Mn_xO₂ materials. The analysis shows that Mn-doping promotes the formation of a single phase. With increasing substitution of Mn ions for Ni ions, the lattice parameter a decreases, while the lattice parameters c and c/a increase. XPS revealed that the oxidation states of Ni, Co and Mn in LiNi_0.85−xCo_0.15Mn_xO₂ compounds (where x = 0.1, 0.2 and 0.4) were +2/+3, +3 and +4. The substitution of Mn ions for Ni ions induces a decrease in the average oxidation state of Ni. Because the substitution of Mn for Ni ions is complex, the extent of the changes between the lattice parameter and L_M-O differ. The occupation of Ni in Li sites is affected by the ordering of Mn⁴⁺ with Ni²⁺ and Mn⁴⁺ with Li⁺.     

The studies of phase relation, microstructure, magnetic transition, magnetocaloric effect in (Gd1−xErx)5Si1.7Ge2.3 compounds

The phase relation, microstructure, Curie temperatures (T_C), magnetic transition, and magnetocaloric effect of (Gd_1−xEr_x)₅Si_1.7Ge_2.3 (x = 0, 0.05, 0.1, 0.15, and 0.2) compounds prepared by arc-melting and then annealing at 1523 K (3 h) using purity Gd (99.9 wt.%) are investigated. The results of XRD patterns and SEM show that the main phases in those samples are mono-clinic Gd₅Si₂Ge₂ type structure. With increase of Er content from x = 0 to 0.2, the values of magnetic transition temperatures (T_C) decrease linearly from 228.7 K to 135.3 K. But the (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds display large magnetic entropy near their transition temperatures in a magnetic field of 0-2 T. The maximum magnetic entropy change in (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds are 24.56, 14.56, 16.84, 14.20, and 13.22 J/kg K⁻¹ with x = 0, 0.05, 0.1, 0.15, and 0.2, respectively.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

An open aperture z-scan study of Sr2CeO4 blue phosphor

Sr₂CeO₄ blue phosphor has been prepared by the solid-state reaction method. The X-ray diffraction (XRD) study confirms the structure of the system to be orthorhombic. High resolution electron transmission microscopy reveals that Sr₂CeO₄ prepared by the solid state reaction method is composed of elongated spherical structures of length ∼0.2-0.6 μm and width ∼90-150 nm. The excitation spectrum shows a broad band which peaks at 275 nm. The emission spectrum shows a broad band which peaks at 467 nm when excited at 275 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer (CT) state of the Ce⁴⁺ ion. The Commission International de l’Eclairage (CIE) co-ordinates are x = 0.15, and y = 0.23. The nonlinear absorption behavior of Sr₂CeO₄ has been investigated using the open aperture z-scan technique. The calculated effective two-photon absorption coefficient shows that the Sr₂CeO₄ blue phosphor is a promising optical limiting material.     

Effect of the impurity LixNi1−xO on the electrochemical properties of 5 V cathode material LiNi0.5Mn1.5O4

The formation of impurity Li_xNi_1−xO when synthesizing spinel LiNi_0.5Mn_1.5O₄ using solid state reaction method, and its influence on the electrochemical properties of product LiNi_0.5Mn_1.5O₄ were studied. The secondary phase Li_xNi_1−xO emerges at high temperature due to oxygen deficiency for LiNi_0.5Mn_1.5O₄ and partial reduction of Mn⁴⁺ to Mn³⁺ in LiNi_0.5Mn_1.5O₄. Annealing process can diminish oxygen deficiency and inhibit impurity Li_xNi_1−xO. The impurity reduces the specific capacity of product, but it does not have obvious negative effect on cycle performance of product. The capacity of LiNi_0.5Mn_1.5O₄ that contains Li_xNi_1−xO can deliver about 120 mAh g⁻¹.     

Microwave dielectric properties of (1 − x)ZnMoO4-xTiO2 composite ceramics

(1 − x)ZnMoO₄-xTiO₂ (x = 0.0, 0.05, 0.158, 0.25, and 0.35) composite ceramics were synthesized by the conventional solid state reaction process. The sintering behavior, phase composition, chemical compatibility with silver, and microwave dielectric properties were investigated. All the specimens can be well densified below 950 °C. From the X-ray diffraction analysis, it indicates that the triclinic wolframite ZnMoO₄ phase coexists with the tetragonal rutile TiO₂ phase, and it is easy for silver to react with ZnMoO₄ to form Ag₂Zn₂(MoO₄)₃ phase and hard to react with TiO₂. When the volume fraction of TiO₂ (x value) increasing from 0 to 0.35, the microwave dielectric permittivity of the (1 − x)ZnMoO₄-xTiO₂ composite ceramics increases from 8.0 to 25.2, the Q_f value changes in the range of 32,300-43,300 GHz, and the temperature coefficient τ_f value varies from −128.9 to 157.4 ppm/°C. At x = 0.158, the mixture exhibits good microwave dielectric properties with a ?_r = 13.9, a Q_f = 40,400 GHz, and a τ_f = +2.0 ppm/°C.     

Hydrogen storage characteristics of nanocrystalline and amorphous Mg2Ni-type alloys prepared by melt spinning

In order to improve the hydrogen storage characteristics of the Mg₂Ni-type alloys, Ni in the alloy is partially substituted by element Mn, and melt-spinning technology is used for the preparation of the Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The microstructures of the as-cast and spun alloys are characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys are measured by an automatically controlled Sieverts apparatus. The electrochemical performances are tested by an automatic galvanostatic system. The results show that the as-spun Mn-free alloy holds typical nanocrystalline structure, whereas the as-spun alloys containing Mn displays a nanocrystalline and amorphous structure. The hydrogen absorption and desorption capacities and kinetics of the alloys increase with rising spinning rate. Additionally, melt spinning markedly improves the electrochemical hydrogen storage capacity and cycle stability of the alloys containing Mn. With an increase in the spinning rate from 0 (As-casts is defined as spinning rate of 0 m/s) to 30 m/s, the discharge capacity of the (x = 0.3) alloy mounts up from 92.3 to 211.1 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle grows from 36.21% to 76.02%.     

An investigation on hydrogen storage kinetics of nanocrystalline and amorphous Mg2Ni1−xCox (x = 0-0.4) alloy prepared by melt spinning

In order to improve the hydrogen storage kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂Ni_1−xCo_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and spun alloys are characterized by XRD, SEM and TEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-spun alloys is tested by an automatic galvanostatic system. The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The electrochemical impedance spectrums (EIS) and the Tafel polarization curves are plotted by an electrochemical workstation. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg₂Ni-type alloy. Furthermore, the substitution of Co for Ni, instead of changing major phase Mg₂Ni, leads to forming secondary phases MgCo₂ and Mg. Both the melt spinning treatment and Co substitution significantly improve the hydrogen absorption and desorption kinetics. The high rate discharge ability, the hydrogen diffusion coefficient and the limiting current density of the alloys significantly increase with raising both the spinning rate and the amount of Co substitution.     

Comparison of dehydriding kinetics between pure LaNi5 and its substituted systems

The kinetic mechanisms of dehydriding reaction in La_0.8M_0.2Ni₅ and LaNi_4.5T_0.5 were comparatively investigated by the Chou model and the first order model, and the former was superior to the latter from the comparison of theoretical calculated results and experimental data. According to the characteristic time (t_c) in the Chou model, substituting La (t_c = 71.36 s) with Ce (t_c = 35.64 s), Nd (t_c = 30.82 s) and Pr (t_c = 28.86 s) increased the dehydriding reaction rate, but partial substitution of Ni (t_c = 71.36 s) with Co (t_c = 102.29 s), Fe (t_c = 116.70 s) and Cu (t_c = 120.62 s) decreased the hydrogen desorption reaction rate. The Chou model was also used to discuss the effect of different amount of Ni substituted with Fe and Co. The dehydriding reaction rates of La(Ni_1−xFe_x)₅ increased with x leveling from 0 to 0.20 but decreased with x increasing from 0.20 to 0.30, while that of LaNi_5−yCo_y decreased with increasing the amount of Co from 0 to 3.00. In addition, the Chou model was successfully applied to predicting the kinetic properties of LaNi_4.8Sn_0.2 at 40 °C and 60 °C, which exhibited an excellent agreement with the experimental data.     

Vacuum ultraviolet spectroscopic properties of KSrPO4:Tb

KSrPO₄:Tb³⁺ phosphors were prepared by a solid-state method and their photoluminescence properties were investigated under vacuum ultraviolet excitation. In the excitation spectrum monitoring at 544 nm, the band in the region of 120-162 nm can be attributed to be the overlap of host absorption and charge transfer transition of O²⁻ → Tb³⁺, and the band ranging from 162 to 300 nm was assigned to the f-d transition of Tb³⁺. The photoluminescence spectrum shows that the phosphors exhibited a strong green emission around 544 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ under the excitation of 147 nm. Optimal emission intensity was obtained when x = 7% in KSr_1-xPO₄:xTb³⁺ and the luminescent chromaticity coordinates were calculated to be (x = 0.317, y = 0.522) for KSr_0.93PO₄:7%Tb³⁺.     

Studies on the power factor of (Ba,Sr)Co2+xRu4−xO11 compounds

We have prepared polycrystalline single-phase ACo_2+xRu_4−xO₁₁ (A = Sr, Ba; 0 ≤ x ≤ 0.5) using the ceramic method and we have studied their structure, electrical resistivity and Seebeck coefficient, in order to estimate their power factor (P.F.). These layered compounds show values of electrical resistivity of the order of 10⁻⁵ Ωm and their Seebeck coefficients are positive and range from 1 μV K⁻¹ (T = 100 K) to 20 μV K⁻¹ (T = 450 K). The maximum power factor at room temperature is displayed by BaCo₂Ru₄O₁₁ (P.F.: 0.20 μW K⁻² cm⁻¹), value that is comparable to that shown by compounds such as SrRuO₃ and Sr₆Co₅O₁₅.     

Synthesis and photoluminescence properties of color-tunable BaLa2WO7:Eu phosphor

Color-tunable phosphors BaLa_2−xEu_xWO₇ were synthesized via a solid-state reaction. The absorption, excitation, emission and decay curves were obtained to study the luminescence properties. The experimental results indicate that BaLa_2−xEu_xWO₇ phosphors have two regions in the excitation spectra: one is assigned to the charge-transfer state (CTS) band at about 338 nm, and the other is assigned to the intra-4f transitions at 360-600 nm. The emission spectra of BaLa_2−xEu_xWO₇ phosphors excited at 395 nm exhibit a series of sharp peaks, which are attributed to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions. Luminescence from higher excited states, such as ⁵D₁, ⁵D₂, and ⁵D₃, were also observed at low Eu³⁺ concentration. The optimal emission intensity of ⁵D₀ → ⁷F₂ red emission is at x = 0.4 (BaLa_1.6Eu_0.4WO₇). The chromaticity coordinates of BaLa_2−xEu_xWO₇ phosphors vary with Eu³⁺ content from white, orange-red, to red, making it a candidate for a white-light-emitting phosphor in UV-LEDs.     

Effect of Th doping on superconductivity in CePt3Si

Alloys from the solid solution Ce_1−xTh_xPt₃Si (x = 0.0, 0.02, 0.04, 0.08, 0.1, 0.2 and 1.0) were prepared by arc-melting. X-ray Rietveld powder analyses revealed that alloys in the compositional range 0 ≤ x ≤ 0.2 crystallize with the CePt₃B-type with a random distribution of Ce and Th atoms in positions 1(b) (1/2, 1/2, z) of the noncentrosymmetric space group P4mm. Th-doping results in a rapid suppression of the superconductivity. The alloy with x = 0.02 shows the onset of superconducting state at T_c = 400 mK, while that with x = 0.04 remains in normal metallic state at least down to 70 mK.     

Phase, structural, and magnetocaloric properties of high temperature annealed LaFe11.6Si1.4BX

The phase relation, microstructural, hysteresis, Curie temperature, and magnetocaloric effects of LaFe_11.6Si_1.4B_x (x = 0.1, 0.2, 0.3, 0.4, and 0.5) prepared by arc-melting and then annealed at 1373 K (1.5 h) + 1523 K (5 h) were investigated. It was found that the main phase is NaZn₁₃-type phase, the impurity phases include α-Fe, Fe₂B, and small amount of La₅Si₃. The boron atom can dissolve into the crystal lattice of LaFe_11.6Si_1.4B_x to form interstitial solid solution, but the content of solid solution is not up to x = 0.5. For LaFe_11.6Si_1.4B_x (x = 0.1, 0.3, and 0.5) compounds, the Curie temperature T_C increases from 190.6 to 198.3 K with the increasing of B content from x = 0.1 to 0.5. The first order magnetic transition behavior becomes weaker and magnetic entropy change ΔS_M (T, H) drops with the increasing of B content, respectively. However, ΔS_M (T, H) still remains a large value, 11.18 J/kg K, when x reaches to 0.5 at 0-2 T. An attractive feature is that both thermal and magnetic hysteresis can be reduced remarkably by introducing B. The maximum magnetic hysteresis loss near T_C drops from 22.52 to 4.95 J/kg when the content of B increases from x = 0.1 to 0.5.     

Effect of Ni substitution on the structural and transport properties of NixMn0.8−xMg0.2Fe2O4; 0.0 ≤ x ≤ 0.40 ferrite

Ni_xMn_0.8−xMg_0.2Fe₂O₄; 0.0≤ x ≤0.40 was prepared by standard ceramic technique, presintering was carried out at 900 °C and final sintering at 1200 °C with heating/cooling rate 4 °C/min. X-ray diffraction analyses assured the formation of the samples in a single phase spinel cubic structure. The calculated crystal size was obtained in the range of 75-130 nm. A slight increase in the theoretical density and decrease in the porosity was obtained with increasing the nickel content. This result was discussed based on the difference in the atomic masses between Ni (58.71) and Mn (54.938). IR spectral analyses show four bands of the spinel ferrite for all the samples. The conductivity and dielectric loss factor give nearly continuous decrease with increasing Ni-content. This was discussed as the result of the significant role of the multivalent cations, such as iron, nickel, manganese, in the conduction mechanism. Anomalous behavior was obtained for the sample with x = 0.20 as highest dielectric constant, highest dielectric loss and highest conductivity. This anomalous behavior was explained due to the existence of two divalent cations on B-sites with the same ratio, namely, Mg²⁺ and Ni²⁺.     

Effect of lanthanum doping on linear and nonlinear-optical properties of Bi3TiNbO9 thin films

Lanthanum doped Bi₃TiNbO₉ thin films (LBTN-x, La³⁺ contents x = 5%, 15%, 25% and 35 mol.%) with layered perovskite structure were fabricated on fused silica by pulsed laser deposition method. Their linear and nonlinear optical properties were studied by transmittance measurement and Z-Scan method. All films exhibit good transmittance (>55%) in visible region. For lanthanum doping content are x = 5%, 15% and 25 mol.%, the nonlinear absorption coefficient of LBTN-x thin films increases with the La³⁺ content, then it drops down at x = 35 mol.% when the content of La³⁺ in (Bi₂O₂)²⁺ layers is high enough to aggravate the orthorhombic distortion of the octahedra. We found that, 25 mol.% is the optimal La³⁺ content for LBTN-x thin films to have the largest nonlinear absorption coefficient making the LBTN-x film a promising candidate for absorbing-type optical device applications.     

Effects of Tl-filling into the voids and Rh substitution for Co on the thermoelectric properties of CoSb3

In order to enhance the thermoelectric (TE) properties of CoSb₃, we tried to reduce the lattice thermal conductivity (κ_lat) by filling Tl into the voids and substitution of Rh for Co. We prepared polycrystalline samples of Tl_x(Co_1−yRh_y)₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20 and y = 0.1, 0.2) and examined their TE properties from room temperature to 750 K. All the samples indicated negative values of the Seebeck coefficient (S). Both the electrical resistivity and the absolute values of the S decreased with increasing the Tl-filling ratio. The Tl-filling and Rh substitution reduced the κ_lat, due to the rattling and the alloy scattering effects. The minimum value of the κ_lat was 1.54 W m⁻¹ K⁻¹ at 550 K obtained for Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂. Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂ exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.58 at around 600 K.