Microstructure development in (Co,Ta)-doped SnO2-based ceramics with promising varistor and dielectric properties

Following the $\mathit{3}Sn{\mathit{(}IV\mathit{)}}_{Sn\mathit{(}IV\mathit{)}}^{\times }\leftrightharpoons Co{\mathit{(}II\mathit{)}}_{Sn\mathit{(}IV\mathit{)}}^{\mathrm{″}}+2Ta{\mathit{\left(}V\mathit{\right)}}_{Sn\mathit{(}IV\mathit{)}}^{\cdot }$ charge compensation mechanism we optimized densification and electrical properties of Ta₂O₅-doped SnO₂–CoO ceramics. We show that incorporation of acceptor dopant Co²⁺ in SnO₂ is promoted after the addition of donor dopant like Ta⁵⁺, whereas any surplus of Co would form secondary Co₂SnO₄ phase. A balanced addition of both dopants is needed to promote densification, and any surplus of donor dopants that remain present at the grain boundaries retard the grain growth and deteriorate electrical properties. Varistor and dielectric properties are then strongly influenced by donor doping. Optimum varistor properties (α = 40, U_T = 272 V/mm, I_L = 1.2 μA) were measured for the sample with 1 mol% Ta₂O₅ and the best dielectric properties (ε = 6525; tan(δ) = 0.057@1kHz) were measured for the sample with 0.10 mol% Ta₂O₅ with the largest SnO₂ grain sizes.     

Small polaron hopping conduction in NiMnO3/NiMn2O4 nano-cotton and its emerging energy application with MWCNT

Self-assembled ilmenite/spinel NiMnO₃/NiMn₂O₄ nano-cotton and it's MWCNT blended composites were synthesized via one-step sol-gel auto combustion and ultra-sonication techniques with a perspective to investigate its dielectric, conduction mechanism and enhanced electrochemical performance to explore its applications as supercapacitor materials and high-temperature energy storage devices. The as-prepared double phase oxides were characterized initially by X-ray diffractometry, FTIR and Raman techniques to ascertain its phase formation and structural information. The FESEM and TEM micrographs reveal the growth of nanoparticles homogeneously and cotton-like morphology over observed space. The EDS elemental analysis and mappings exposed the true stoichiometrical ratios of elemental particles Ni, Mn, O and C and their uniform distribution. The coercivity (264 Oe) and saturation magnetization (1.2 emu/gm.) of MWCNT blended NiMnO₃/NiMn₂O₄ oxides curtailed significantly than NiMnO₃/NiMn₂O₄ which recommends its approach towards the superparamagnetic region. The fitted Nyquist plot ($Z^{\text{'}}vs.Z\text{'}\text{'}$) distinguished the individual contribution of grain and grain boundary and activation energy for grain (0.87 eV) and grain boundary (1.2 eV) was estimated from liner plot of extracted data. The frequency dispersing conductivity spectra was fitted with a modified equation of Jonscher's Power Law proposed by us, ${{\sigma }_{ac}\left(\omega \right)={\sigma }_0\left(dc\right)+A(\omega -b)}^n$ and temperature dependent frequency exponent (n) and Ln (A) suggests non-overlapping small polaron tunneling (NSPT) model for its conduction mechanism. Displacement of both Ni²⁺ and Ni³⁺ ions through the channel like cavities of ilmenite/spinel structure along a, b and c axis provides strong support of the NSPT model. The electrochemical supercapacitor properties of NiMnO₃/NiMn₂O₄ and MWCNT blended NiMnO₃/NiMn₂O₄ were examined in 2 M aqueous KOH solution and excellent capacitive properties were observed for both compounds. Higher capacitance value (1347 F-g⁻¹) and energy density 140 W h kg⁻¹ were obtained at a current density of 5 A g⁻¹ for MWCNT blended NiMnO₃/NiMn₂O₄ with compared to its parent counterparts.     

Structural dependence of microwave dielectric properties of spinel structured Mg2(Ti1-xSnx)O4 solid solutions: Crystal structure refinement,Raman spectra study and complex chemical bond theory

Mg₂(Ti_1-xSn_x)O₄ (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg₂(Ti_1-xSn_x)O₄ average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg₂(Ti_1-xSn_x)O₄ compounds were obtained (${\epsilon }_r$= 12.18, $Q\times f$?=?170,130?GHz, ${\tau }_f$?=??53.1?ppm/°C, x?=?0.2).     

Effects of Zn substitution on high-frequency properties of Ba1.5Sr1.5Co2-xZnxFe22O41 hexaferrites

Ba_1.5Sr_1.5Co_2-xZn_xFe₂₂O₄₁(x = 0,0.4,0.8,1.2,1.6,2.0) hexaferrites substituted by Zn were prepared via conventional solid-state reaction. The influences of Zn substitution content on phase formation, microstructure, and magnetic properties, especially high-frequency magnetic properties, were systematically investigated. Results indicate that, when sintering temperature is 1200 °C, pure ferrite phase of Co₂Z is formed, and Zn²⁺ is successfully incorporated into the lattice. Appropriate amount of Zn substitution can improve the densification and saturation magnetization ($M_s$) of Z-type ferrite, thereby effectively promoting permeability. However, due to limitations of Snoek's law, cut-off frequency is found to be reduced with Zn substitution. For x = 0.4, the sample exhibits both high permeability (up to 22.49) and high cut-off frequency (~900 MHz), with Snoek constant $({\mu }_i?1)\times f_r$ reaching 19.337 GHz. Furthermore, magnetic loss for the sample with x = 0.4 is found to be low. It is anticipated that this material will have extensive application prospects for miniaturization and high frequency antenna applications.     

Enhanced resistance in Bi(Fe1-xScx)O3-0.3BaTiO3 lead-free piezoelectric ceramics: Facile analysis and reduction of oxygen vacancy

We reported a facile analysis and reduction of oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}^{??}$) in 0.7Bi(Fe_1-xSc_x)O₃-0.3BaTiO₃ (0≤x≤0.08) ceramics. The leakage current mechanism was investigated intensively. Our results indicated that oxygen vacancies are the main cause for the high conductivity in BF-BT ceramics, and their concentration was quantitatively estimated from the Bi³⁺ content and the average oxidation state of iron. The ${\mathrm{V}}_{\mathrm{O}}^{??}$ concentration was effectively suppressed and the insulation resistance was enhanced by almost two orders of magnitude after doping 2%mol Sc³⁺. The enhanced insulation resistance contributed to excellent piezoelectric properties with d₃₃ = 165 pC/N, T_C = 505 °C, and k_p = 26%. The proposed analysis method used to quantify the ${\mathrm{V}}_{\mathrm{O}}^{??}$ concentration provides valuable indications to reduce the leakage current density and improve the piezoelectric properties of BF-BT based ceramic.     

Correlation of the crystal features,magnetic parameters,and electronic structure of Bi-substituted BaFe12-xBixO19 hexaferrites: Theoretical background

Herein, using first-principles density functional theory (DFT) calculations, we have investigated the effects of Bi substitution on the structural, electronic, and magnetic properties of barium hexaferrite (BaFe_12-xBi_xO₁₉, x = 0; 0.5; 1.5 and 2). As a result of the calculation, it was determined that the most stable structure exists if the spin of the Fe atom on the 2a, 2b, and 12k positions of the barium hexaferrite compound is taken in the upward direction. The calculated lattice constant $\left(\raisebox{1ex}{$\mathrm{c}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{a}$}\right.=3.9\right)$ and magnetic moment $\left(4.24{\mu }_{\mathrm{B}}\right)$ of iron ions are in reasonable agreement with other experimental works. Moreover, the presence of bismuth reduces the electronic band gap. Energy gain and magnetic anisotropy energy calculations for FIM, FM, and NM states were performed for the most stable states. It has been established that the most stable structural state is characteristic of х = 0.5. It has been calculated that substitution by the large Bi³⁺ ion dramatically changed the electronic structure and sharply reduced the band gap. This paper is the first step towards establishing the nature of the distribution of ions in M-type hexaferrites under conditions of substitution by ions with a large ionic radius.     

Magnetoelectric coupling studies in lead-free multiferroic (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3−(Ni0.7Zn0.3)Fe2O4 ceramic composites

Lead-free multiferroic 3–0 type particulate composites with a composition (1?x)(Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃) – x(Ni_0.7Zn_0.3Fe₂O₄) [(1?x)BCZT – xNZFO with 0 ≤ x ≤ 100 at%] were prepared using solid state reaction method. Structural and microstructural analysis using XRD, FESEM and Raman techniques confirmed the phase formation of the ferroelectric (BCZT) and magnetostrictive (NZFO) phases without any detectable presence of impurity phases. Rietveld refinement of the XRD data revealed a tetragonal (P4mm) and a cubic structure (Fd$\stackrel{￣}{3}$m) for the BCZT and NZFO phases, respectively. Elemental compositions of the constituent phases were assessed by EDS and XPS analyses. Electrical, magnetic, and magnetoelectric (ME) measurements were performed. The composites exhibit typical well-saturated magnetic hysteresis (M?H) loops at room temperature, having very low coercive field ($H_C$) values, indicating their soft ferromagnetic behavior. Various parameters extracted from the M?H curves including $H_C$, magneto-crystalline anisotropy, squareness, and magnetization were found to depend on x. Frequency dependence of capacitance and admittance exhibited a resonance behavior corresponding to the radial mode of the electromechanical resonance (EMR). ME coefficients were studied in both longitudinal (${\alpha }_{\text{E}33}$) and transverse (${\alpha }_{\text{E}31}$) modes. The highest coupling coefficients, ${\alpha }_{\text{E}31}$ ～14.5 mV/Oe.cm and ${\alpha }_{\text{E}33}$ ～13 mV/Oe.cm were obtained for composite with 50 at% NZF at off-resonance frequency of 1 kHz. At the EMR frequency of 314 kHz, the ${\alpha }_{\text{E}31}$ value in 0.5BCZT-0.5NZFO composite enhanced enormously to ～5.5 V/Oe.cm. The studies conclude that x = 0.5 is an optimum atomic fraction of NZFO in the particulate composite for maximum ME coupling.     

La0.7−xLnxCa0.3MnO3(Ln=ProrSm) perovskites as electrode materials for SOFCs

In this work, the Pechini method was used to synthesize $L{a}_{0.7?x}L{n}_{x}C{a}_{0.3}Mn{O}_{3,}(Ln=ProrSm)$-${\text{La}}_{1?\text{x}}{\text{Ln}}_{\text{x}}{\text{Ca}}_{0.3}{\text{MnO}}_3$ type perovskites, evaluating the effect of the type of cation and composition on the structural, morphological and textural properties. The use of similar rare earth cations was studied to promote weak bonds between the reactive surface and adsorbed oxygen species that could facilitate the oxygen reduction reaction and thus improve electrochemical performance of SOFC devices. To achieve this goal, different compositions of Pr or Sm ions (x = 0.1, 0.3, 0.5 or 0.6) were used for the partial substitution of lanthanum at the A site. The results indicated that the substitution of La by Pr or Sm did not modify the original orthorhombic perovskite structure of Ca-doped lanthanum manganites. However, a shift in the main reflections can be obtained as the content of both cations increases due to cell distortion. Rietveld refinement confirms that the crystal structure belongs to the Pnma space group with a large distortion along the b-axis but no secondary phase formation. The compensated charge neutrality of the Mn³⁺/Mn⁴⁺ ratio influences the octahedral sites of MnO₆ and cell volume. Orthorhombic distortion ($\frac{c}{\sqrt{2}}<a<b$) occurs through Jahn-Teller mechanism promoting a hole-doped system for electrical conductivity. The adsorption-desorption isotherms reveal that in any composition of Pr, a mesoporous isotherm (type IV) is obtained. In contrast, the isotherms changed from micro- to meso-porous features depending on the amount of Sm substitution at the A-site. All the prepared samples showed a soft granular structure with agglomerates ranging between 200 and 300 nm with well-interconnected pores. Comparing the La substitution by Pr or Sm, it was found that Sm can form perovskites that can better promote oxygen vacancies and triple boundary phase formation, which is essential in SOFC devices.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.