A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

Microwave dielectric properties and sintering behavior of nano-scaled (α + θ)-Al2O3 ceramics

The dielectric properties at microwave frequencies and the microstructures of nano (α + θ)-Al₂O₃ ceramics were investigated. Using the high-purity nano (α + θ)-Al₂O₃ powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with θ-phase Al₂O₃ addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure α-Al₂O₃. Relative density of sintered samples can be as high as 99.49% at 1400 °C for 8 h. The unloaded quality factors Q × f are strongly dependent on the sintering time. Further improvement of the Q × f value can be achieved by extending the sintering time to 8 h. A dielectric constant (_r) of 10, a high Q × f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (τ_f) of −39.88 ppm/°C were obtained for specimen sintered at 1400 °C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

The effect of Sb2O5 additions on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The sintering behavior and dielectric properties for perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramic with Sb₂O₅ doping was explored. A small amount of Sb₂O₅ (2.5 wt.%) led to high densification at temperatures < 1060 °C. The dielectric constant increased and the temperature coefficient decreased with increasing concentration of Sb₂O₅, and the dielectric constant reached 673, combined with a low temperature coefficient of 147 ppm/°C, and dielectric loss of 0.0044 (at 1 MHz) for the sample with 3.5 wt.% Sb₂O₅ sintered at 1080 °C.     

Composition dependence of optical constants of Ge1 − xSe2Pbx thin films

Optical constants of vacuum-evaporated thin films in the Ge_1 − xSe₂Pb_x (x = 0, 0.2, 0.4, 0.6) system were calculated from reflectance and transmittance spectra. It is found that the films exhibit a non-direct gap, which decreases with increasing Pb content. The variation in the refractive index and the imaginary part of the dielectric constant with photon energy is reported. The relationship between the optical gap and chemical composition in chalcogenide glasses is discussed in terms of the average heat of atomization.     

Fabrication and optical properties of NUV-emitting SiO2–SrB4O7:Eu glass–ceramic thin films

SiO₂–SrB₄O₇:Eu²⁺ glass–ceramic thin films were fabricated for possible application in near ultraviolet (NUV) emitting devices. Nano-sized SrB₄O₇:Eu²⁺ powders were prepared by a Pechini-type sol–gel method and a subsequent ball-milling treatment. The powders showed NUV emissions centered at 367 nm, upon irradiation with UV of shorter wavelengths, due to an allowed 4f⁶5d¹ → 4f⁷ electronic transition of Eu²⁺ ions. The glass–ceramic thin films were prepared by dip-coating of tetraethylorthosilicate (TEOS) solutions dispersed with the nano-sized SrB₄O₇:Eu²⁺ powders and a subsequent heat-treatment. It was found that the glass–ceramic thin films had relatively high thermal stability up to 800 °C in terms of the Eu²⁺ emissions. SiO₂ layers surrounding SrB₄O₇:Eu²⁺ appeared to be effective for the surface passivation of the phosphor particles.     

Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li⁺ content on electrochemistry of Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO₂ structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li⁺ is possibly incorporated in structure of layered Li_1.2Ni_0.2Mn_0.6O₂ (x = 0), accompanying oxidization of Ni²⁺ to Ni³⁺ to form Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni³⁺ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) in this work delivered the highest discharge capacity of 219 mAh g⁻¹ between 4.8 and 2.0 V. Increasing Li content (x value in Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂) reduced charge–discharge capacities, but significantly enhancing cycleability.     

Silver cofirable Bi1.5Zn0.92Nb1.5O6.92 microwave ceramics containing CuO-based dopants

Bi₂O₃–ZnO–Nb₂O₅ system has emerged as a good low sintering (1050 °C) microwave material because it exhibits high dielectric constant and low temperature coefficient of resonance frequency (τ_f). We have lowered the sintering temperature of Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) below 900 °C by using 3 wt.% of CuO-based dopants, such as 0.21BaCO₃–0.79CuO (BC) and 0.81MoO₃–0.19CuO (MC). The doped BZN exhibits high microwave dielectric constant at 2.3 GHz (k  120). The interfacial behavior between BZN and silver was investigated by using X-ray diffractometer, scanning electronic microscope, and electronic probe microanalyzer. The extent of silver migration of MC and BC dopants is reduced at least by one order of magnitude as compared with V₂O₅ dopant when the samples was prepared at 900 °C for 4 h. Thus, CuO-based dopants can replace V₂O₅ to lower the sintering temperature of BZN and to be cofired with silver.     

Electrochemical characteristics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ–Sm0.2Ce0.8O1.9 composite materials for low-temperature solid oxide fuel cell cathodes

In this paper, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ–xSm_0.2Ce_0.8O_1.9 (BSCF–xSDC, x = 0–60 wt.%) composite cathodes were prepared by soft chemical methods, and then examined for potential applications in lower temperature solid oxide fuel cells. Both DC polarization and AC impedance spectroscopy measurements indicated that the addition of SDC electrolyte into BSCF remarkably improved the electrochemical properties. The optimum composition was found to be BSCF–30SDC, which exhibited 5.5 times higher polarization current density and 15.1% polarization resistance, compared with the pure-phase BSCF cathode at 550 °C.     

Thermal expansion properties of Ln2−xCrxMo3O12 (Ln = Er and Y)

Structures and thermal expansion properties of Ln_2−xCr_xMo₃O₁₂ (Ln = Er and Y) have been investigated by X-ray powder diffraction. Rietveld analysis results of Ln_2−xCr_xMo₃O₁₂ indicate that compounds Er_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.3) and Y_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.2) crystallize in orthorhombic structure and exhibit negative thermal expansion, while both monoclinic and orthorhombic compounds Er_2−xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) and Y_2−xCr_xMo₃O₁₂ (1.8 ≤ x ≤ 2.0) possess positive coefficient of thermal expansion. The coefficients of linear thermal expansion of orthorhombic Ln_2−xCr_xMo₃O₁₂ change from negative to positive with increasing chromium content. Thermogravimetric and differential scanning calorimetry have been used to study the hygroscopicity and the phase transition temperature.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

Dielectric, magnetic and spectroscopic properties of Li2O–WO3–P2O5 glass system with Ag2O as additive

Li₂O–WO₃–P₂O₅ glasses containing small concentrations of Ag₂O from 0 to 1 mol% were prepared. A number of studies viz., chemical durability, dielectric studies (constant ′, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature), spectroscopic (infrared, optical absorption ESR spectra) and magnetic susceptibility studies of these glasses, have been carried out. The interesting variations observed in all these properties with the concentration of Ag⁺ ions have been analyzed in the light of different oxidation states and environment of tungsten ions in the glass network.     

High-temperature oxygen non-stoichiometry, conductivity and structure in strontium-rich nickelates La2−xSrxNiO4−δ (x = 1 and 1.4)

Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2−xSr_xNiO_4−δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating.     

Structural, dielectric, impedance and optical properties of CaBi2B2O7 glasses and glass-nanocrystal composites

Optically clear glasses were fabricated by quenching the melt of CaCO₃–Bi₂O₃–B₂O₃ (in equimolecular ratio). The amorphous and glassy characteristics of the as-quenched samples were confirmed via the X-ray powder diffraction (XRD) and differential scanning calorimetric (DSC) studies. These glasses were found to have high thermal stability parameter (S). The optical transmission studies carried out in the 200–2500 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent between 400 nm and 2500 nm. The glass-plates that were heat-treated just above the glass transition temperature (723 K) for 6 h retained ≈60% transparency despite having nano-crystallites (≈50–100 nm) of CaBi₂B₂O₇ (CBBO) as confirmed by both the XRD and transmission electron microscopy (TEM) studies. The dielectric properties and impedance characteristics of the as-quenched and heat-treated (723 K/6 h) samples were studied as a function of frequency at different temperatures. Cole–Cole equation was employed to rationalize the impedance data.     

Piezoelectric, ferroelectric and dielectric properties of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

La₂O₃ (0–0.8 wt.%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La₂O₃ has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La₂O₃. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La₂O₃. At room temperature, the ceramics doped with 0.6 wt.% La₂O₃ show superior performance with high piezoelectric constant (d₃₃ = 167 pC/N), high planar electromechanical coupling factor (k_p = 0.30), high mechanical quality factor (Q_m = 118), high relative dielectric constant (ε_r = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.     

Crystallization, microstructure development and dielectric behaviour of glass ceramics in the system [SrO · TiO2]-[2SiO2 · B2O3]-La2O3

Various glasses in the system (65 – x)[SrO · TiO₂]-(35)[2SiO₂ · B₂2O₃]-(x)La₂O₃, where x = 1,5,10 (wt%) were prepared by melting in alumina crucible (1375–1575 K). Heat treatment schedules were selected from DTA plots of respective glasses. X-ray diffraction studies of glass ceramic samples containing different concentrations of La₂O₃ revealed the formation of Sr₂B₂O₅, Sr₃Ti₂O₇ and TiO₂ (rutile) phases. The addition of La₂O₃ results in the development of well formed, elongated crystallites of different phases. Results of the dielectric behaviour demonstrate higher values of dielectric constant for some of the glass ceramic samples. This can be ascribed to the relaxation polarization at the crystal-glass interface due to conductivity differences between crystalline and glassy phases.     

Study of dielectric, magnetic and ferroelectric properties in Bi1 − xGdxFeO3

Gd-doped BiFeO₃ polycrystalline ceramics were synthesized by solid-state reaction method and their dielectric and magnetic properties were investigated. X-ray diffraction pattern showed that Bi_1 − xGd_xFeO₃ (x = 0, 0.05 and 0.1) ceramics were rhombohedral. The Gd substitution has suppressed the usual impurity peaks present in the parent compound and we obtained single phase Bi_0.9Gd_0.1FeO₃ ceramic. Gd substitution reduced the antiferromagnetic Néel temperature (T_N) in Bi_1 − xGd_x FeO_3. An anomaly in the dielectric constant(ε) and dielectric loss(tan (δ)) in the vicinity of the antiferromagnetic Néel temperature (T_N) was observed. Ferroelectric and magnetic hysteresis loops measured at room temperature indicated the coexistence of ferroelectricity and magnetism. The room temperature magnetic hysteresis loops were not saturated, but the magnetic moment was found to increase with increase in Gd concentration.     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 ? x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 – x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 – x} .     

Effects of Ho2O3 addition on defects of BaTiO3

Effects of Ho₂O₃ addition on defects of BaTiO₃ ceramic have been studied in terms of electrical conductivity at 1200 °C as a function of oxygen partial pressure (P_O2°) and oxygen vacancy concentration. The substitution of Ho³⁺ for the Ti site in Ba(Ti_1−xHo_x)O_3−0.5x resulted in a significant shift of conductivity minimum toward lower oxygen pressures and showed an acceptor-doped behavior. The solubility limit of Ho on Ti sites was confirmed less than 3.0 mol% by measuring the electrical conductivity and the lattice constant. Oxygen vacancy concentrations were calculated from the positions of P_O2° in the conductivity minima and were in good agreement with theoretically estimated values within the solubility limit. The Curie point moved to lower temperatures with increasing the oxygen vacancy concentration and Ho contents.