Enhanced negative thermal expansion of (KMg)3+-substituted Fe2Mo3O12 ceramics

Herein, we report the solid-state synthesis of (KMg)_xFe_2-xMo₃O₁₂ (0 = x ≤ 1.5) ceramics. Phase composition, crystal structure, morphology, phase transition and thermal expansion behavior of the (KMg)_xFe_2-xMo₃O₁₂ ceramics were investigated by XRD, Raman, XPS, HRTEM, EDX, SEM, TMA and high-temperature XRD. Results indicate that (KMg)³⁺ dual-cations have successfully replaced Fe³⁺ in Fe₂Mo₃O₁₂ ceramics and single-phase monoclinic (KMg)_xFe_2-xMo₃O₁₂ ceramics were prepared for 0.25 = x ≤ 1. (KMg)³⁺ introduction can increase the density of (KMg)_xFe_2-xMo₃O₁₂ ceramics and effectively improve their negative thermal expansion (NTE) performance. In addition, the phase transition temperature (Tc) of Fe₂Mo₃O₁₂ was reduced from 508.1 °C to room temperature with the increase of (KMg)³⁺-substitution. Monoclinic KMgFeMo₃O₁₂ ceramics was observed to show stronger NTE in a wider temperature range of 30–700 °C for the first time. Its corresponding coefficient of thermal expansion (CTE) is as high as ?17.21 × 10^?6 °C^?1. The distortion of [FeO₆/MgO₆] polyhedra in (KMg)_xFe_2-xMo₃O₁₂ caused by (KMg)³⁺-substitution contributed to the stronger NTE.     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Effect of Y doping on microstructure and thermophysical properties of yttria stabilized hafnia ceramics

A series of Y₂O₃-doped HfO₂ ceramics (Hf_1-xY_xO_2-0.5×, x?=?0, 0.04, 0.08, 0.12, 0.16 and 0.2) were synthesized by solid-state reaction at 1600?°C. The microstructure, thermophysical properties and phase stability were investigated. Hf_1-xY_xO_2–0.5x ceramics were comprised of monoclinic (M) phase and cubic (C) phase when Y³⁺ ion concentration ranged from 0.04 to 0.16. The thermal conductivity of Hf_1-xY_xO_2–0.5x ceramic decreased as Y³⁺ ion concentration increased and Hf_0.8Y_0.2O_1.9 ceramic revealed the lowest thermal conductivity of ~?1.8?W/m*K at 1200?°C. The average thermal expansion coefficient (TEC) of Hf_1-xY_xO_2–0.5x between 200?°C and 1300?°C increased with the Y³⁺ ion concentration. Hf_0.8Y_0.2O_1.9 yielded the highest TEC of ~?10.4?×?10^?6 K^?1 while keeping good phase stability between room temperature and 1600?°C.     

Electrical Properties of Triethylene Glycol Stabilized MnxCo1-xFe2O4 Nanoparticles

We reported on the synthesis and analysis of the composition, micro-structure, ac–dc conductivity performance and dielectric permittivity of triethylene glycol (TEG) stabilized Mn_xCo_1-xFe₂O₄ nanoparticles obtained by polyol method. Crystallite size from XRD and particle size from TEM micrographs are consistent with each other. Conductivity measurements were performed to investigate the influence of the coating with TEG on the conduction characteristics of Mn_xCo_1-xFe₂O₄ NP’s. The frequency-dependency of the ac conductivity shows electrode polarization effect. The dc conductivity is strongly temperature dependent and shows maximum conductivity of about 5 × 10^?5 S cm^?1 for x = 1.0 at 120 °C. Analysis of dielectric permittivity functions suggests that ionic and polymer segmental motions are strongly coupled.     

Effects of Synthesis Temperature and Support Material on CO2 and CH4 Permeation through SAPO-34 Membranes

In this research, continuous SAPO-34 membranes were synthesized via secondary growth method onto both α-Al₂O₃ and mullite supports at three levels of synthesis temperature: 185, 195, and 220°C for 24 h. The synthesized membranes were characterized using XRD and SEM analysis and single gas permeation experiments. It was found out that support material and synthesis temperature both have significant effects on the membrane performance. At higher synthesis temperature, SAPO-34 crystals grown over the mullite support become more uniform and smaller in size but those grown on the α-Al₂O₃ support become larger. Effect of synthesis temperature on single gas permeation properties of the synthesized SAPO-34 membranes was also studied. For the mullite supported membranes, the CH₄ and CO₂ permeances decrease as synthesis temperature increases; but in the case of the alumina supported membranes, by increasing synthesis temperature, CH₄ and CO₂ permeances first decrease up to 195°C and then increase up to 220°C. Even in equal membrane thicknesses, the mullite supported membrane shows lower gas permenaces. Increasing synthesis temperature decreases CO₂/CH₄ ideal selectivity for the α-Al₂O₃ supported membranes, while increases for the mullite supported membranes. Under optimum synthesis conditions, at room temperature and 2 bar feed pressure, the CO₂ permeance through the α-Al₂O₃ and the mullite supported SAPO-34 membranes are 8.2 × 10^?7 and 8.5 × 10^?8 (mol/m² · s · Pa), respectively, and CO₂/CH₄ ideal selectivities are 51 and 61, respectively.     

Sc- and W-substitution and tuning of phase transition in the Al2Mo3O12

Al₂Mo₃O₁₂ is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al_{2. x}Sc_xMo₃O₁₂ (0 ≤ x ≤ 1) and Al₂Mo_3-xW_xO₁₂ (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al_1.6Sc_0.4Mo₃O₁₂ and Al₂Mo_0.5W_2.5O₁₂, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10^?6 °C^?1 in 100–550 °C and ?4.25 × 10^?6 °C^?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al₂Mo₃O₁₂, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.     

Double cation (KMg)3+ co-substitution to regulate negative thermal expansion property of ln2W3O12

To improve the negative thermal expansion (NTE) performance of ln₂W₃O₁₂, a novel series of NTE (KMg)_xln_2-xW₃O₁₂ ceramics were fabricated via the solid-state method. The effects of (KMg)³⁺ substitution on the phase composition, microstructure and thermal expansion property of the ln₂W₃O₁₂ ceramics were characterized using X-ray diffraction (XRD), Raman spectrometer (Raman), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), transmission electron microscope (TEM) and thermal mechanical analyzer (TMA). Results indicate that (KMg)³⁺ can partially replace In³⁺ in In₂W₃O₁₂ and form a new phase K_xMg_xln_2-xW₃O₁₂ with monoclinic symmetry. For x = 0.5, pure monoclinic (KMg)_0.5ln_1.5W₃O₁₂ ceramics is prepared and shows strong NTE. Its coefficient of thermal expansion is −7.89 × 10⁻⁶ °C⁻¹ in 30–700 °C, in addition, no phase transition was observed over the entire testing temperature range. These research results indicate that double cations co-substitution is an effective strategy to improve the NTE property of ln₂W₃O₁₂ through crystal structure modulation. This strategy could be extended to the performance modulation of other NTE materials.     

Structural and thermoelectric properties of hydrothermal-processed Ag-doped Fe2O3

Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) nanopowders with various Ag contents were synthesized at different hydrothermal reaction temperatures (150?°C and 180?°C). Their structural properties were fully investigated through an X-ray diffraction, a Fourier transform infrared spectroscopy, and an X-ray photoelectron spectroscopy. The hydrothermal reaction temperature, time, and Ag content remarkably affected the morphological characteristics and crystal structure of the synthesized powders. The Fe_2-xAg_xO₃ (0?≤?x?≤?0.04) powders synthesized at 150?°C for 6?h and the Fe_2-xAg_xO₃ (0.02?≤?x?≤?0.04) powders synthesized at 180?°C for 12?h formed the orthorhombic α-FeOOH phase with a rod-like morphology, whereas the Fe_2-xAg_xO₃ (0?≤?x?≤?0.01) powders synthesized at 180?°C for 12?h formed the rhombohedral α-Fe₂O₃ phase with a spherical-like morphology. The Fe_1.98Ag_0.02O₃ fabricated by utilizing Fe_1.98Ag_0.02O₃ powders synthesized at 180?°C showed the largest power factor (0.64?×10^?5 Wm^?1 K^?2) and dimensionless figure-of-merit (0.0036) at 800?°C.     

Structure Stability and Microwave Dielectric Properties of (1 − x) Ba(Mg1/2W1/2) O3 + xBa(Y2/3W1/3)O3 Ceramics

The structure stabilities of double perovskite ceramics‐ (1 ? x) Ba(Mg_1/2W_1/2)O₃ + xBa(Y_2/3W_1/3)O₃ (0.01 ≤ x ≤ 0.4) have been studied by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectrometry in this study. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results showed that all the compounds exhibited face‐centered cubic perovskite structure. Part of Y³⁺ and W⁶⁺ cations occupied 4a‐site and the remaining Y³⁺ and Mg²⁺ distributed over 4b‐site, respectively, and kept the B‐site ratio 1:1 ordered. Local ordering of Y³⁺/Mg²⁺ on 4b‐site and Y³⁺/W⁶⁺ cations on 4a‐site within the short‐range scale could be observed with increasing Y‐doping content. The decomposition of the double perovskite compound at high temperature was successfully suppressed by doping with Y on B‐site. However, Ba₂Y_0.667WO₆ impurity phase appeared when x > 0.1. The optimized dielectric permittivity increased with the increase in Y doping. The optimized Q × f value was remarkably improved with small amount of Y doping (x ≤ 0.02) and reached a maximum value of about 160 000 GHz at x = 0.02 composition. Further increasing in Y doping led to the decrease in Q × f value. All compositions exhibited negative τ_f values. The absolute value of τ_f decreased with increasing Y‐doping content. Excellent combined microwave dielectric properties with ε_r = 20, Q × f = 160 000 GHz, and τ_f = ?21 ppm/°C could be obtained for x = 0.02 composition.     

Improving the Li-ion conductivity and air stability of cubic Li7La3Zr2O12 by the co-doping of Nb,Y on the Zr site

The elements Nb and Y were simultaneously substituted to the Zr sites of an Li₇La₃Zr₂O₁₂ (LLZO) electrolyte to improve its Li-ion conductivity and air stability. Samples of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were fabricated using a solid-state reaction method. The results show that the introduction of Nb and Y can stabilise cubic-phase LLZO. The total conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte can reach 8.29 × 10^?4 S cm^?1 at 30 °C when sintered at 1230 °C for only 15 h. Surprisingly, the conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ can be maintained at 6.91 × 10^?4 S cm^?1 after exposure to air for 1.5 months, indicating excellent air stability. Furthermore, a LiFePO₄/Li₇La₃ZrNb_0.5Y_0.5O₁₂/Li cell displayed stable charge/discharge and cycling performance at ambient temperature, suggesting there is potential to use Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte in Li-ion batteries. Additionally, the effects of varying the co-doping amount and dwelling time on the Li-ion conductivity of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were investigated.     

Effect of annealing temperature on the structural and magnetic properties of Ba-Pb-hexaferrite powders synthesized by sol-gel auto-combustion method

The annealing temperature (T_a) dependent development in phase purity, morphology and magnetic properties of Ba_xPb_1-xFe₁₂O₁₉ (x?=?0, 0.2, 0.4, 0.6, 0.8 and 1) powders synthesized via sol-gel auto-combustion route were studied. The hexagonal phase of PbFe₁₂O₁₉ forms directly via solid-state-reaction between α-Fe₂O₃ and PbO during annealing of the combustion product at T_a ~ 900?°C, but the BaFe₁₂O₁₉ phase forms at T_a ~ 1200?°C through the formation of BaFe₂O₄ and α-Fe₂O₃ as intermediate-phases at low T_a. The Ba_xPb_1-x Fe₁₂O₁₉-phase purity of the samples depend on T_a. For T_a >?1000?°C, PbO evaporates from Pb-containing samples. With increasing T_a, growth of hexagonal-shaped sharp-edged particles was observed for Barium-rich samples, however, the sharp-edges dissolute for Lead-rich samples. High saturation magnetization was observed for Ba-rich samples annealed at 1100?°C. For all the studied samples, magnetic coercivity increases with x but decreases with T_a. The magnetic properties were correlated with the structure, microstructure and grain-size of the samples.     

Structural,microstructural, and magnetic studies of Y3Fe5-xNixO12 garnet nanoparticles

This paper reports the structural and magnetic properties of a series of Y₃Fe_5-xNi_xO₁₂ (x = 0, 0.05, 0.1, and 0.2) nanopowders synthesized by the citrate combustion method. We have discussed the change in different properties with the variation in calcination temperatures as well as the Ni ion substitution in yttrium iron garnet. X-ray diffraction study confirmed the desired garnet phase formation in all the calcined powders, and the crystallinity improved with an increase in calcination temperature. The crystallite sizes were observed in the range 47–52 nm and 84–94 nm for the samples calcined at 800 and 1000 °C, respectively. Scanning electron micrographs confirmed that the grains were in the nanometre range (132–170 nm) at 800 °C and increased (351–363 nm) at 1000 °C. Larger grains at high calcination temperature resulted in the enhanced saturation magnetization and a decrease in coercivity. Curie temperature (T_c) was observed in the range 558–560 K for all the calcined Y₃Fe_5-xNi_xO₁₂ samples. Nickel substitution for iron sites in Y₃Fe_5-xNi_xO₁₂ decreased the saturation magnetization and enhanced the coercivity. This could be related to the substitution of Ni ions for tetrahedral iron sites, which changed the magnetic exchange interactions of different lattice sites. The magnetic anisotropy constant (K) increases with the enhancement of calcination temperature, whereas it decreases with nickel ion substitution in Y₃Fe_5-xNi_xO₁₂. This study suggests that the structural and magnetic properties can be tuned by Ni substitution for the Fe ions in Y₃Fe₅O₁₂ garnets at different calcination temperatures, which make them promising candidates for various technological applications.     

Role of A-site (Sr), B-site (Y), and A,B sites (Sr,Y) substitution in lead-free BaTiO3 ceramic compounds: Structural,optical, microstructure,mechanical, and thermal conductivity properties

Strontium and Yttrium-doped and co-doped BaTiO₃ (BT) ceramics with the stoichiometric formulas BaTiO₃, B_1-xSr_xTiO₃, Ba_1-xY_xTiO₃, BaTi_1-xY_xO₃, Ba_1-xY_xTi_1-xY_xO₃, and Ba_1-xSr_xTi_1-xY_xO₃ (x = 0.075) noted as BT, BSrT, BYT, BTY, BYTY, and BSrTY have been synthesized through sol-gel method. X-ray diffraction (XRD) patterns of the prepared ceramics, calcined at a slightly low temperature (950 °C/3h), displayed that BT, BSrT, and BYT ceramics possess tetragonal structures and BTY, BYTY, and BSrTY have a cubic structure. The incorporation of the Ba and/or Ti sites by Sr²⁺ and Y³⁺ ions in the lattice of BaTiO₃ ceramic and the behaviors of the crystalline characteristics in terms of the Y and Sr dopant were described in detail. The scanning electron microscopy (SEM) images demonstrated that the densification and grain size were strongly related to Sr and Y elements. UV–visible spectroscopy was used to study the optical behavior of the as-prepared ceramic samples and revealed that Sr and Y dopants reduce the optical band gap energy to 2.74 eV for the BSrTY compound. The outcomes also demonstrated that the levels of Urbach energy are indicative of the created disorder following the inclusion of Yttrium. The measurements of the thermal conductivity indicated the influence of the doping mechanism on the thermal conductivity results of the synthesized samples. Indeed, the thermal conductivity of BaTiO₃ is decreased with Sr and Y dopants and found to be in the range of 085–2.23 W.m^-1. K^?1 at room temperature and decreases slightly with increasing temperature from 2.02 to 0.73-W.m^-1. K^?1. Moreover, the microstructure and grains distribution of the BT, BSrT, BYT, BTY, BYTY, and BSrTY samples impacted the compressive strength, hence; the compressive strength was minimized as the grain size decreased.     

Ionic conduction and dielectric properties of yttrium doped LiZr2(PO4)3 obtained by a Pechini-type polymerizable complex route

We report on the ion transport properties of Li_1+xZr_2-xY_x(PO₄)₃ (0.05?≤ x?≤?0.2) NASICON type nanocrystalline compounds prepared through a Pechini-type polymerizable complex method. Structural properties were characterized by means of powder X-ray diffraction, Raman spectroscopy and electron microscopy with selected area electron diffraction. Impedance spectroscopy was utilised to investigate the lithium ion transport properties. Y³⁺ doped LiZr₂(PO₄)₃ compounds showed stabilized rhombohedral structure with enhanced total ionic conductivity at 30?°C from 2.87?×?10^?7 S?cm^?1 to 0.65?×?10^?5 S?cm^?1 for x=0.05 to 0.20 respectively. The activation energies of Li_1+xZr_2-xY_x(PO₄)₃ show a decreasing trend from 0.45?eV to 0.35?eV with increasing x from 0.05 to 0.20. The total conductivity of these compounds is thermally activated, with activation energies and pre-exponential factors following the Meyer-Neldel rule. The tanδ peak position shifts to the high-frequency side with increasing yttrium content. Scaling in AC conductivity spectra shows that the electrical relaxation mechanisms are independent of temperature.     

Effect of isovalent substitution on phase transition and negative thermal expansion of In2−xScxW3O12 ceramics

Solid solutions of In_2−xSc_xW₃O₁₂ (0≤x≤2) were successfully synthesized using the solid state reaction method. Effects of substituted scandium content on the phase composition, microstructure, phase transition temperatures and thermal expansion behaviors of the resulting In_2−xSc_xW₃O₁₂ (0≤x≤2) samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and thermal mechanical analyzer (TMA). Results indicate that the obtained In₂W₃O₁₂ ceramic undergoes a structure phase transition from monoclinic to orthorhombic at 248 °C. This phase transition temperature of In₂W₃O₁₂ can be easily shifted to a lower temperature by partly substituting the In³⁺ with Sc³⁺. When the x value increased from 0 to 1, the phase transition temperatures of In_2−xSc_xW₃O₁₂ (0≤x≤2) samples decreased from 248 to 47 °C. All the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics show fine negative thermal expansion below their corresponding phase transition temperatures. The negative thermal expansion coefficients of the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics change in the range from −1.08×10⁻⁶ °C⁻¹ to −7.13×10⁻⁶ °C⁻¹.     

Chemical zoning of calcium aluminoferrite formed during melt crystallization in CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₂(Al_xFe_1−x)₂O₅, with x<0.7 (ferrite), Ca₂SiO₄ (belite), Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇), were crystallized out of a complete melt during cooling at 8.3 °C/min. Upon cooling to 1370 °C, both the crystals of ferrite with x=0.41 and belite would start to nucleate from the melt. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.7. At ambient temperature, the ferrite crystals had a zonal structure, the x value of which successively increased from the cores toward the rims. The value of 0.45 was confirmed for the cores by EPMA. The chemical formula of the rims was determined to be Ca_2.03[Al_1.27Fe_0.68Si_0.02]_Σ1.97O₅ (x=0.65). As the crystallization of ferrite and belite proceeded, the coexisting melt would become progressively enriched in the aluminate components. After the termination of the ferrite crystallization, the C₃A and belite would immediately crystallize out of the melt, followed by the nucleation of C₁₂A₇. The C₁₂A₇ accommodated about 2.1 mass% Fe₂O₃ in the chemical formula Ca_12.03[Al_13.61Fe_0.37]_Σ13.98O₃₃, being free from the other foreign oxides (SiO₂ and P₂O₅).     

Structural,electrical, optical and dielectric properties of yttrium substituted cadmium ferrites prepared by Co-Precipitation method

The yttrium substituted cadmium ferrites having composition Cd_1-xY_xFe₂O₄ (X = 0.00, 0.125, 0.250, 0.375, 0.500) were synthesized by the co-precipitation method and sintered at 1100 °C for 6 h. Structural, morphological, electrical, optical and dielectric characteristics were explored by XRD, SEM, EDS, FTIR, I–V two probes, UV–Vis and LCR techniques.XRD results confirmed the cubic structure of spinel ferrites. A decrease in lattice constants of the prepared samples was observed with the substitution of Y ions and was attributed to the difference in ionic radii of Y³⁺ (0.95 Å) and Cd²⁺ (0.97 Å) ions. Cationic distributions, ionic radii of both tetrahedral and octahedral sites, tolerance factor, oxygen positional parameters, bond lengths, interatomic distances, positional parameters and bond length angles were calculated from XRD data. The morphology of the prepared ferrites was studied using SEM and results ratified the XRD results. EDS confirmed the presence of all inserted elements in Cd_1-xY_xFe₂O₄ composition. DC resistivity and drift mobility of soft-ferrites were found to be increased from 1.047 × 10⁸–4.822 × 10¹⁰ Ω-cm and 5.87 × 10⁻¹² – 1.045 × 10⁻¹⁴ cm²V⁻¹s⁻¹, respectively, at 523 K with yttrium content confirming the behavior of semiconductor materials. The optical band gap energy calculated from the UV–Vis pattern of the Cd_1-xY_xFe₂O₄ system was decreased from 3.6011 to 2.8153 eV. DC resistivity and optical band gaps exposed inverse relation. FTIR results revealed lower and upper-frequency absorption bands in the ranges of 419.31–417.01 cm⁻¹ and 540.95–565.70 cm⁻¹, respectively. Dielectric constant and dielectric losses were in decreasing order, while ac conductivity revealed rising behavior with increasing frequency. Results showed the potential of yttrium doped Cd nanoferrites for applications in high-frequency microwave absorbing devices.     

Tailoring structure and piezoelectric properties of CaBi2Nb2O9 ceramics by W6+-Doping

Calcium bismuth niobate (CaBi₂Nb₂O₉) is a typical bismuth-layer structured piezoelectrics (BLSPs) with a high Curie temperature (T_C) of ～943 °C, but it has low piezoelectric coefficient and high-temperature resistivity which severely limits signal acquisition in the high-temperature piezoelectric vibration sensors. Ion-doping modification is regarded as an effective way to enhance electrical properties. In this work, W⁶⁺ donor-doping at Nb⁵⁺ site in the CaBi₂Nb_2-xW_xO₉ (x = 0, 0.020, 0.025, 0.030, 0.035 and 0.040) piezoelectric ceramics with T_C of 931 ± 2 °C were fabricated by the conventional solid-state reaction method. The effects of W⁶⁺-doping on crystal structure of CaBi₂Nb_2-xW_xO₉ as well as microscopic morphology and electrical properties of ceramics were investigated systematically. The tetragonality, isotropy and electrical properties of the ceramics were enhanced with the introduction of W⁶⁺ dopant. It was found that x = 0.025 was the optimal W⁶⁺-doping ratio that yielded remnant polarization of 8.0 μC/cm², electrical resistivity of 3.0 × 10⁶ Ω cm at 600 °C, piezoelectric coefficient (d₃₃) of 14.4 pC/N, and good thermal depoling property. Our work has established a feasible approach to tune the structure of CaBi₂Nb₂O₉ to improve piezoelectric properties for potential applications in high-temperature piezoelectric vibration sensors.     

Effects of W6+ substitution on crystal structure and microwave dielectric properties of Li3Mg2NbO6 ceramics

Li₃Mg₂(Nb_1-xW_x)O_6+x/2 (0 ≤ x ≤ 0.08) ceramics were synthesized by the solid-state reaction route. The effects of W⁶⁺ substitution on the phase composition, microstructure and microwave dielectric properties of Li₃Mg₂NbO₆ ceramics were investigated systematically. The XRD results showed that all the samples formed a pure solid solution in the whole doping range. The SEM iamges and relative density revealed the dense structure of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The relationship between the crystal structure and dielectric properties of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics was researched through polarizability, average bond valence, and bond energy. The substitution of W⁶⁺ for Nb⁵⁺ in Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics significantly promoted the Q × f values. In addition, the increase of W⁶⁺ content improved the thermal stability of the Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The Li₃Mg₂(Nb_0.94W_0.06)O_6.03 ceramics sintered at 1175 °C for 6h possessed excellent properties: ε_r ~ 15.82, Q × f ~ 124,187 GHz, τ_f ~ −18.28 ppm/°C.