Phase evolution and microwave dielectric properties of novel LiAl5−xZnxO8−0.5x-based (0 ≤ x ≤ 0.5) ceramics

Novel LiAl_5−xZn_xO_8−0.5x microwave dielectric ceramics were synthesized through a solid-state reaction route. Phase evolution of LiAl_5−xZn_xO_8−0.5x was determined by XRD analysis. The XRD results indicated that the phase compositions had a P4₃32 space group when 0 ≤ x ≤ 0.2 and a spinel structure when 0.3 ≤ x ≤ 0.5. The dielectric constant (ε_r) of this series’ solid solutions decreased with the increase in Zn doping content, which was in good agreement with the Clausius-Mossotti relation. Oxygen vacancy and the decreased degree of order degraded the quality factor (Q × f) of the two structures. The deterioration in quality factor was further verified by impedance spectroscopy. The temperature coefficient of the resonant frequency (τ_f) decreased with the increase in x and was correlated with the unit cell volume. Finally, CaTiO₃ was used as a compensation material to obtain a near-zero τ_f of the LiAl₅O₈ ceramic.     

Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Phase evolution and chemical durability of Zr1-xNd xO2-x/2 (0 ≤ x ≤ 1) ceramics

A series of Zr_1-xNd _xO_2-x/2 (0 ≤ x ≤ 1) ceramics was prepared by solid-state reaction method. The effects of Nd content on the phase evolution were investigated. The chemical durability of resulting waste forms was also examined. The results show that the ceramics with x < 0.1 show monoclinic and cubic zirconia phase, with 0.2 ≤ x < 0.4 exhibit a single cubic phase, with 0.4 ≤ x ≤ 0.6 exhibit a single pyrochlore phase, with 0.6 < x < 0.8 exhibit a single cubic phase and remain cubic phases and hexagonal Nd₂O₃ when 0.8 ≤ x ≤ 1. The unit cell parameters of the Nd-doped zirconia samples increase as the Nd content increases. Moreover, the normalized element release rates of Nd element in Nd-doped zirconia ceramics firstly decrease with leaching time and almost no change after 21 days (∼0⁻⁶ g m⁻² d⁻¹), demonstrating its good chemical durability.     

Structural,electrical and magnetic properties of Co0.5Zn0.5AlxFe2−xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) prepared via sol–gel route

Nanoparticles of Co_0.5Zn_0.5Al_xFe_2?xO₄ (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were synthesized by sol–gel method and the influence of Al³⁺ doping on the properties of Co_0.5Zn_0.5Fe₂O₄ was studied. X-ray diffraction studies revealed the formation of single phase spinel type cubical structure having space group Fd-3m. A decreasing trend of the lattice parameter was observed with increasing Al³⁺ concentration due to the smaller ionic radii of Al³⁺ ion as compared to Fe³⁺ ion. TEM was used to characterize the microstructure of the samples and particle size determination, which exhibited the formation of spherical nanoparticles. The particle size was found to be increases up to ～45 nm after annealing the sample at 1000 °C. Electrical resistivity was found to increase with Al³⁺ doping, attributed to the decrease in the number of Fe²⁺–Fe³⁺ hopping. The activation energy decreased with increasing Al³⁺ ion concentration, indicating the blocking of conduction mechanism between Fe³⁺–Fe²⁺ ions. The value of saturation magnetization decreased, when Fe³⁺ ions were doped with Al³⁺ ions in Co_0.5Zn_0.5Fe₂O₄; however, the coercivity values increased with increasing Al³⁺ ion content.     

Structure stability and microwave dielectric properties of double perovskite ceramics – Ba2Mg1−xCaxWO6 (0.0 ≤ x ≤ 0.15)

The structure stability of double perovskite ceramics – Ba₂Mg_1?xCa_xWO₆ (0.0 ≤ x ≤ 0.15) has been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small amount of Ca substitution for Mg increases the Mg/CaO bond strength, and hence the stability of the double perovskite. But it cannot completely suppress the decomposition of Ba₂Mg_1?xCa_xWO₆ at high temperature. Although space group Fm?3m is adopted for all compositions, nonrandom distribution of Ca²⁺ and Mg²⁺ on 4b-site within the short range scale is observed due to their large cation size difference. Small level doping of Ca (x ≤ 0.1) increases the dielectric permittivity monotonically, but does not affect the Q × f value greatly. As expected, the substitution of Ca tuned the temperature coefficient of resonant frequency (τ_f value) from negative to positive value. Excellent combined microwave dielectric properties with ?_r = 20.8, Q × f = 120,729 GHz, and τ_f = 0 ppm/°C could be obtained for x = 0.1 composition. However the Q × f value degrades considerably when the sample was stored under ambient conditions for a long time.     

Crystal structure and mixed ionic–electronic conductivity of La1−xCaxMnO3 (0 ≤ x ≤ 0.8) produced by mechanosynthesis

The aim of this work was to study the relationship between the crystalline structure, the mixed ionic–electronic conductivity and the calcium content in calcium-doped lanthanum manganites (CLM, La_1?xCa_xMnO₃) synthesized by reactive ball milling. Mechanosynthesis was employed to produce nanocrystalline CLM with varying calcium content (x = 0–0.8 in increments of 0.1). Powders of Mn₂O₃, La₂O₃ and CaO mixed in the stoichiometric ratio were used as raw materials. The mechanosynthesis was carried out using a high-energy shaker mixer/mill. X-ray powder diffraction and Rietveld refinement were used to determine the crystalline structure as a function of calcium content. The four-point probe resistivity test was used to measure the electrical resistivity of the compacted and sintered powders using a DC milli-ohm meter. The results showed that the substitution of the La³⁺ ion by the Ca²⁺ ion during mechanosynthesis only changed the lattice parameters but not the orthorhombic Pnma structure. The mixed ionic–electronic conductivity increased with the Ca²⁺ content. The best conductivity was observed for the composition of La_0.2Ca_0.8MnO₃.     

Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2 and 0.8)

Microstructural changes due to kinetic demixing within sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1?xO_3?δ, x = 0.2 and 0.8: BSCF5528 and BSCF5582, respectively) have been investigated. When the specimens were subjected to 2 A/cm² at 1000 °C and pO₂ = 10^?5 atm, there was a significant enhancement of grain growth as well as 2nd phase formation observed in BSCF5528. At the anode, cobalt deficient aggregates within the grains; and, at the cathode, cobalt rich 2nd phase particles were observed on the grain surfaces of the microstructure. Such phenomena were not observed in BSCF5582, even under higher current density (7 A/cm²) and longer delay time. These results were explained by the kinetic demixing/decomposition.     

Structure and microwave dielectric characteristics of Sr2[Ti1−x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x ≤ 0.50, and only minor amount (1-2 wt%) of Sr₃Ti₂O₇ secondary phase was detected for the compositions x ≥ 0.25. The temperature coefficient of resonant frequency τ_f of Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ ceramics was significantly improved from 132 to 14 ppm/°C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant ɛ_r decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: ɛ_r = 25.1, Qf = 77 580 GHz, τ_f = 14 ppm/°C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.     

Impact of Mg-Zr substitution on the magnetic and microwave absorption properties of BaFe12-2xMgxZrxO19 (0.25 ≤ x ≤ 1.5)

In this study, magnesium-zirconium–substituted M-type barium hexaferrites BaFe_12-2xMg+_xZr_xO₁₉ (BFMZO, 0.25 ≤ x ≤ 1.5) nanoparticles were successfully synthesized by sol-gel autocombustion technique. On one hand, the effects of Mg-Zr substitution concentration on the magnetic features of doped magnetic nanoparticles were investigated which showed that increasing the doping concentration causes the saturation magnetization to decrease. On the other hand, the influence of the different layer thicknesses (2, 3, and 4 mm) of BFMZO on the microwave absorption was investigated in X-band frequencies (8-12 GHz). Absorption results showed that increasing the film thickness from 2 to 3 mm causes microwave absorption to increase. Moreover, the morphological study reveals that aggregation percentage decreased when the substitution concentration increased. Therefore, size, magnetic, and absorption properties are tunable by substitution concentration.     

Selenium doping induced two antiferromagnetic transitions in thiospinel compounds CuCo2S4-xSex (0 ≤ x ≤ 0.8)

A series of copper thiospinel compounds, CuCo₂S_4-xSe_x (x = 0, 0.2, 0.4, 0.6, 0.8), have been successfully synthesized by solid-state reaction and their structure and magnetic properties have been studied. The Rietveld refinements of X-ray diffractions indicate that both the lattice constants and the nearest-neighbor Cu-Cu distances increase with increasing selenium doping. A weakly antiferromagnetic transition occurring at about 4 K is observed in CuCo₂S₄. Two antiferromagnetic transitions at about 3.5 K and 6 K are observed in selenium-doped samples, which suggest that the exchange couplings associated with Cu-S(Se)-Cu and Cu-Se(S)-Cu, respectively, are responsible for the two antiferromagnetic transitions. Detailed analysis of the experimental results further indicates that the nearest-neighbor molecular field coefficient is comparable to the next-neighbor molecular field coefficient. We propose a reasonable model to explain this phenomenon.     

Novel scheelite-type [Ca0.55(Nd1-xBix)0.3]MoO4 (0.2 ≤ x ≤ 0.95) microwave dielectric ceramics with low sintering temperature

Novel scheelite-type [Ca_0.55(Nd_1-xBi_x)_0.3]MoO₄ (0.2 ≤ x ≤ 0.95) ceramics were prepared using the solid-state reaction method. According to the X-ray diffraction data, a solid solution was formed in 0.2 ≤ x ≤ 0.95 and all the samples belong to pure scheelite phase with the tetragonal structure. As revealed by Raman spectroscopy, the number of vibrational modes decreased with the increase in x value, which further indicated that Bi³⁺ ions occupied A-site of scheelite structure. As the x value increased, the sintering temperature decreased from 740°C to 660°C; the permittivity increased from 12.6 to 20.3; the Qf value first decreased slightly and gradually remained stable. Based on the infrared reflectivity spectrum analysis, the calculated permittivity derived from the fitted data shared the same trend with the measured value. The [Ca_0.55(Nd_0.05Bi_0.95)_0.3]MoO₄ ceramic sintered at 660 °C attained a near-zero value temperature coefficient ~τ_f (−7.1 ppm/°C) and showed excellent microwave dielectric properties with a ɛ_r ~ 20.3 and a Qf ~ 33 860 GHz, making this system a promising candidate in the ultralow temperature cofired ceramic (ULTCC) technology.     

New high Q microwave dielectric ceramics with rock salt structures: (1 − x)Li2TiO3 + xMgO system (0 ≤ x ≤ 0.5)

The structural evolution and microwave dielectric properties of (1 ? x)Li₂TiO₃ + xMgO system (0 ≤ x ≤ 0.5) have been investigated in this paper. The ordering degree decreased with the increase of MgO content. The microcracks and cleavage on (0 0 1) due to the weak Li–O bonds disappeared with the increase of MgO content. The dielectric constant and temperature coefficient of resonant frequency decreased with the increase of MgO content. The Q × f value increased with x up to x = 0.2 and then decreases with the further increase of x. An excellent combined microwave dielectric properties could be obtained when x = 0.24, ?_r = 19.2, Q × f = 106,226 GHz and τ_f = 3.56 ppm/°C.     

Enhanced “overall” ionic conductivity in Li1+xAlxTi2−x(PO4)3 (0.3 ≤ x ≤ 0.7) ceramics prepared from sol-gel powders by spark plasma sintering

The Li_1+xAl_xTi_2?x(PO₄)₃ (LATPx) series displays the highest “bulk” reported conductivity, but a much lower “overall” contribution, that changes with the powder preparation and sintering conditions. In this work, the preparation of LATPx ceramics is discussed, by using the sol-gel technique for powders synthesis and mild spark plasma (SPS) for ceramics sintering at 800 °C. An “overall” conductivity ~ 2.10^?3 Ω^?1 cm^?1 was obtained for the x = 0.4 composition, that was the result of a high “bulk” conductivity, an optimized microstructure and almost full density, in absence of micro-cracks, with a small content of secondary phases and clean grain boundaries. Fast-ion ceramics prepared by SPS are good candidates for solid electrolytes in all solid state batteries (ASSB).     

Synthesis and electrical properties of Ce1−xGdxO2−x/2 (x = 0.05–0.3) solid solutions prepared by a citrate–nitrate combustion method

Ce_1?xGd_xO_2?x/2 (GDC) powders with different Gd³⁺ contents (x = 0.05–0.3) were prepared by a simple citrate–nitrate combustion method. The influence of the Gd³⁺ doping content on the crystal structure and the electrical properties of GDC were examined. Many analysis techniques such as thermal analysis, X-ray diffraction, nitrogen adsorption analysis, scanning electron microscopy and AC impedance analysis were employed to characterize the GDC powders. The crystallization of the GDC solid solution occurred below 350 °C. The GDC powders calcined at 800 °C showed a typical cubic fluorite structure. The lattice parameter of GDC exhibited a linear relationship with the Gd³⁺ content. As compared with that sintered at other temperatures, the GDC pellet that sintered at 1300 °C had a high relative density of 97%, and showed finer microstructure. The conductivity of GDC was firstly increased and then decreased with the increase of the Gd³⁺ content. The sintered GDC sample with the Gd³⁺ content of 0.25 exhibited the highest conductivity of 1.27 × 10^?2 S cm^?1 at 600 °C.     

Crystal structure,densification, and microwave dielectric properties of Li3Mg2(Nb(1−x)Mox)O6+x/2 (0 ≤ x ≤ 0.08) ceramics

A novel system Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 (0 ≤ x ≤ 0.08) microwave dielectric ceramics were fabricated by the solid-state method. The charge compensation of Mo⁶⁺ ions substitution for Nb⁵⁺ ions was performed by introducing oxygen ions. The X-ray diffraction patterns and Rietveld refinements indicated Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with single phase and orthorhombic structure. Micro-structure and density confirmed that the grain of Li₃Mg₂(Nb_(1-x)Mo_x)O_6+x/2 ceramics grew well. In addition, the permittivity of Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with the same trend as density decreased slightly with increasing Mo⁶⁺ ions content. However, the Q*f and τ_f were obviously improved with an appropriate amount of Mo⁶⁺ ions. When x ≤ 0.04, the Q*f was closely related to the bond valence of samples, while when x ≥ 0.06, the Q*f was closely related to the density of samples. The variations of τ_f and oxygen octahedral distortion were the opposite. In conclusions, the Li₃Mg₂(Nb_0.98Mo_0.02)O_6.01 ceramic sintered at 1200°C for 6 hours exhibited outstanding properties: ε_r ~ 15.18, Q*f ~ 116 266 GHz, τ_f ~ −15.71 ppm/^oC.     

Structural and optical properties of Mg1−xMnxP2O6 (x = 0–1.0) magnesium metaphosphate

Structural and optical properties of Mg_1−xMn_xP₂O₆ (x = 0–1.0) magnesium metaphosphate were investigated in detail. The complete solid solution of MgP₂O₆–MnP₂O₆ is confirmed as monoclinic space group C2/c. The dynamic luminescence was studied by changing the Mn²⁺ content (0–100 mol%) and temperature (10–300 K). There is a good chemical homogeneity in Mg_1−xMn_xP₂O₆ (x = 0–1.0), which can be supported by the linearly varying cell size and the gradually changing vibration spectrum. However, the optical properties of the solid solution do not show a continuous change trend, that is, an obvious inflection point appeared when x = 0.5. Mg_1−xMn_xP₂O₆ (x = 0.1–0.5) shows a dominant O²⁻ → Mn²⁺ charge transfer (CT) absorption in the near UV region and feeble d–d transitions of Mn²⁺ in visible wavelength region. However, Mg_1−xMn_xP₂O₆ (x = 0.6–1.0) presents a strong d–d absorption transition and nearly disappeared CT band. The changing trend of optical absorption is also maintained in the excitation and emission of the solid solutions. In Mg_1−xMn_xP₂O₆ (x = 0.1–0.5), (Mn, Mg)O₆ octahedron has slight distortion, and the effective luminescence only occurs when CT band excitation is used. In contrast, in Mg_1−xMn_xP₂O₆ (x = 0.6–1.0), (Mn, Mg)O₆ octahedron is highly distorted, and only excitation at d–d transition produces effective luminescence. This research highlights the critical role of MnO₆ octahedral distortions in the luminescence properties of Mn²⁺ activators. The research provides a reference for developing optical materials.     

Microwave dielectric properties of SrLa[Ga1−x(Mg0.5Ti0.5)x]O4 and SrLa[Ga1−x(Zn0.5Ti0.5)x]O4 (x = 0.2-0.8) ceramics

SrLa[Ga_1−x(R_0.5Ti_0.5)_x]O₄ (R = Mg, Zn) ceramics were prepared by a standard solid state sintering method. The single-phase ceramics with K₂NiF₄-type layered perovskite structure and I4/mmm space group were obtained, indicating that SrLa(R_0.5Ti_0.5) and SrLaGaO₄ can form the unlimited solid solutions. With increasing x for R = Mg and Zn, ε_r increases monotonously, the Qf value first increases and then decreases, while τ_f increases from a negative to a positive value. The optimized microwave dielectric properties were obtained as following: ε_r = 23.3, Qf = 89 400 GHz, τ_f = −0.8 ppm/°C for SrLa[Ga_0.6(Mg_0.5Ti_0.5)_0.4]O₄ and ε_r = 23.3, Qf = 76 200 GHz, τ_f = 0.2 ppm/°C for SrLa[Ga_0.7(Zn_0.5Ti_0.5)_0.3]O₄, indicating that the present solid solution ceramics are the promising candidates as microwave resonator materials for the telecommunication applications.     

A novel series of Ba0.5Sr0.5Al0.2−xMgxFe0.8O3−ξ (x ≤ 0.2) membranes for oxygen permeation application

The pervoskite-type oxides have received attention due to their potential applications in catalysis, fuel cells, sensors, gas separable membranes, and electrolytes. In view of the importance of oxygen separation from air, stable Ba_0.5Sr_0.5Al_0.2−xMg_xFe_0.8O_3−ξ (x = 0–0.2) powders have been synthesized by decomposition of sol–gel derived oxalate at 950 °C for 5 h and characterized with regard to formation, nature of iron species, oxygen permeation, and electrical conductivity. It is shown that magnesium substitution leads to (i) a stable perovskite-type cubic phase with ‘a’ = 3.953–3.978 Å, (ii) weakening of metal–oxygen bond, (iii) reduction of Fe⁴⁺ ions, and (iv) enhancement of oxygen deficiency and electrical conductivity. Their compact discs act as stable oxygen permeable filters with flux density of ∼3.013–3.355 μmol cm⁻² s⁻¹ at 1000 °C. The maximum value corresponds to composition x = 0.2 and hence can be a potential membrane for oxygen separation technology.     

Microstructure,magnetism, and high-frequency performance of polycrystalline Ni0.5Zn0.5Sm0.025HoxFe1.975−xO4 ferrites

Novel polycrystalline Ni_0.5Zn_0.5Sm_0.025Ho_xFe_1.975−xO₄ (x = 0-0.06) ferrites were fabricated by a traditional solid-state reaction sintering method. The codoping effects of Sm and Ho on the microstructure, magnetism, and high-frequency performance of Ni–Zn ferrites were investigated. The substitution of Sm³⁺ and Ho³⁺ ions led to an apparent increase in the lattice constants. However, further increasing the addition of both dopants introduced SmFeO₃ or HoFeO₃ foreign phases at the boundaries of the polycrystalline grains. As the content of Ho³⁺ ions increased, the relative density and average grain size of the specimens decreased accordingly. Moreover, the substitution of Sm³⁺ clearly decreased the saturation magnetization and complex permeability, which further decreased with the doping of Ho³⁺. The evolution of the Curie temperature showed an opposite trend, reaching the highest temperature of 278°C when x = 0.03. Similarly, the coercivity and resonance frequencies also displayed opposite trends compared to those of the saturation magnetization and complex permeability. The codoping of Sm³⁺ and Ho³⁺ more effectively lowered the magnetic and dielectric loss tangent of the specimens compared with the undoped or single dopant modified ferrites.