Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

Crystal structure,microstructure and microwave dielectric properties of novel MgAl<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript> ceramic

In the present work, a novel MgAl₂Ti₃O₁₀ ceramic was obtained using a traditional solid-state reaction method. X-ray diffraction and energy dispersive spectrometer showed that the main MgAl₂Ti₃O₁₀ phase was formed after sintered at 1300–1450 °C. With rising the sintering temperature from 1300 to 1450 °C, the bulk density (ρ), relative permittivity (ε _r ) and Q?×?f value firstly increased, reached the maximum values (3.61 g/cm³, 14.9, and 26,450 GHz) and then decreased. The temperature coefficient of resonator frequency (τ _f ) showed a slight change at a negative range of ??94.6 to ??83.7 ppm/°C. When the sintering temperature was 1400 °C, MgAl₂Ti₃O₁₀ ceramics exhibited the best microwave dielectric properties with Q?×?f?=?26,450 GHz, ε _r ?=?14.9 and τ _f ?=???83.7 ppm/°C.     

Synthesis and high-temperature electrical conductivity of Ln<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and LnYTi<Subscript>2</Subscript>O<Subscript>7</Subscript> (Ln = Dy,Ho)

Ho₂Ti₂O₇ and LnYTi₂O₇ (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho₂Ti₂O₇ is 4 × 10^?4 S/cm, and that of HoYTi₂O₇ is 1 × 10^?3 S/cm, with activation energies E _a = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi₂O₇ (3 × 10^?4 S/cm at 740°C, E _a = 1.09 eV) is almost one order of magnitude lower than that of HoYTi₂O₇ (1 × 10^?3 S/cm at 740°C, E _a = 1.17 eV).     

Mechanochemical synthesis,crystal structure and ion conduction in the Gd<Subscript>2</Subscript>Hf<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7</Subscript> system

This work describes the mechanochemical synthesis, structural characterization and electrical properties of an interesting group of novel ionic conductors, with general formula Gd₂(Hf_2?x Ti _x )₂O₇. Different compositions in this system (x = 0, 0.4, 0.8, 1.2, 1.6 and 2) were obtained at room temperature, via a mechanochemical reaction between the corresponding elemental oxides, and characterized by using XRD, Raman spectroscopy and SEM. The XRD structural analysis by the Rietveld method revealed that all the Hf-containing compositions show a disordered fluorite-like structure instead of the expected pyrochlore-like atomic ordering, and the cation size mismatch criteria for pyrochlore stability. Increasing Ti content promotes a phase transformation to the pyrochlore structure with post-milling thermal treatments, which takes place in all samples on annealing at 1200 °C, except for Gd₂Hf₂O₇. These results were confirmed by Raman spectroscopy, which also suggests that the x = 0.4 sample has the highest degree of oxygen disorder in the system and that this disorder decreases with increasing Ti⁴⁺ content. Finally, all samples show the pyrochlore structure on firing at 1500 °C. Activation energies E _dc for oxygen migration were determined by using impedance spectroscopy and found to be within the ~0.9–1.2 eV range, whereas conductivity σ _dc values at 700 °C vary from 1.12 × 10^?6 to 2.75 × 10^?4 S cm^?1, with decreasing conductivity as Ti⁴⁺ content increases.     

Low temperature sintering and microwave dielectric properties of Li<Subscript>6</Subscript>Mg<Subscript>7</Subscript>Ti<Subscript>3</Subscript>O<Subscript>16</Subscript> ceramics with LiF additive for LTCC applications

Li₆Mg₇Ti₃O₁₆ ceramics were prepared by the conventional solid-state method with 1–5 wt% LiF as the sintering aid. Effects of LiF additions on the phase compositions, sintering characteristics, micro-structures and microwave dielectric properties of Li₆Mg₇Ti₃O₁₆ ceramics were investigated. The LiF addition could effectively lower the sintering temperature of Li₆Mg₇Ti₃O₁₆ ceramics from 1550 to 900 °C. For different LiF-doped compositions, the optimum dielectric permittivity (ε _r ) and quality factor (Q·f) values first increased and then decreased with the increase of LiF contents, whereas the temperature coefficient of resonant frequency (τ _f ) fluctuated between ??14.39 and ??8.21 ppm/°C. Typically, Li₆Mg₇Ti₃O₁₆ ceramics with 4 wt% LiF sintered at 900 °C exhibited excellent microwave dielectric properties of ε _r ?=?16.17, Q·f?=?80,921 GHz and τ _f ?=???8.21 ppm/°C, which are promising materials for the low temperature co-fired ceramics applications.     

Enhancing the microwave dielectric properties of Sr<Subscript>2</Subscript>Ca<Subscript>3</Subscript>Ta<Subscript>4</Subscript>TiO<Subscript>17</Subscript> ceramics by Zr Substitution

The compositions in Sr₂Ca₃Ta₄Ti_1?xZr_xO₁₇ (0?≤?x?≤?0.12) series were designed and fabricated by solid state sintering method. All the compositions formed single phases and crystallized in an orthorhombic crystal structure. Zr substitution led to the enhancing of the microwave dielectric properties by tuning the τ_f value through zero and increased the Q_uf_o value from 12,540 to 14,970 GHz with a slight decrease in ε_r. In the present study, a good combination of ε_r ~?51, Q_uf_o ~?145,43 GHz and τ_f ~ 3 ppm/°C were obtained for Sr₂Ca₃Ta₄Ti_0.90Zr_0.1O₁₇ ceramic sintered at 1575 °C for 4 h.     

Synthesis,X-ray structure analysis,and Raman spectroscopy of R<Subscript>2</Subscript>TiO<Subscript>5</Subscript>-based (R = Sc,Y) solid solutions

Order–disorder transitions in xR₂O₃ · (1 ? x)TiO₂ (R = Sc, 0.4 ≤ x ≤ 0.5; R = Y, 0.5 ≤ x ≤ 0.6) solid solutions with highly imperfect fluorite-derived structures have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of a fluorite-like (Fm3m) disordered phase and a nanoscale (~10–100 nm) pyrochlore-like (Fd3m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The Raman spectra of Sc₂TiO₅ (Y₂TiO₅) contain broad lines in low- and high-frequency regions: at 190, 350, and 775 (134, 188, 365, 404, and 727) cm^?1. These lines are characteristic of a pyrochlore-like phase with a varying degree of order and a disordered fluorite-like phase, respectively. The pyrochlore-like phase Y₂Ti₂O₇ has two strong Raman peaks in the low-frequency region: at 312 and 527 cm^?1. The formation of nanodomains with different degrees of order is caused by the internal stress that arises from the high density of structural defects in the unit cells of the solid solutions.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Very high thermal stability with excellent dielectric,and non-ohmics properties of Mg-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4.2</Subscript>O<Subscript>12</Subscript> ceramics

In this work, the nominal CaCu_3?xMg_xTi_4.2O₁₂ (0.00, 0.05 and 0.10) ceramics were prepared by sintering pellets of their precursor powders obtained by a polymer pyrolysis solution method at 1100 °C for different sintering time of 8 and 12 h. Very low loss tangent (tanδ)?<?0.009–0.014 and giant dielectric constant (ε′) ～?1.1?×?10⁴–1.8?×?10⁴ with excellent temperature coefficient (Δε′) less than ±?15% in a temperature range of ??60 to 210 °C were achieved. These excellent performances suggested a potent application of the ceramics for high temperature X8R and X9R capacitors. It was found that tanδ values decreased with increasing Mg²⁺ dopants due to the increase of grain boundary resistance (R_gb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg²⁺ dopants in the structure. In addition, CaCu_3?xMg_xTi_4.2O₁₂ ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (E_b) due to Mg²⁺doping. The high values of ε′ (14012), α (13.64) and E_b (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu_2.90Mg_0.10Ti_4.2O₁₂ ceramic sintered at 1100 °C for 8 h.     

Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

Synthesis and photocatalytic activity of anatase-based aerogels

V_xTi_1–xO₂ (x = 5 and 10 mol %) solid solutions have been synthesized through supercritical drying in isopropanol at t = 250°C and p = 10 MPa. Their physicochemical properties and photocatalytic performance have been compared to those of an earlier synthesized Zn_xTi_1–xO₂ aerogel containing 10 mol % Zn [1]. It has been shown that increasing the vanadium content of V_xTi_1–xO₂ from 5 to 10 mol % leads to a decrease in hydrogen evolution rate in methanol/water splitting reaction under UV irradiation from 190 to 32 µL/(min g_catal), whereas in the case of the anatase-based aerogel solid solution containing 10 mol % Zn an opposite picture is observed: the hydrogen evolution rate in methanol/water splitting reaction under UV irradiation increases sharply to 700 µL/(min g_catal).     

Coordination of V<Superscript>4+</Superscript> in Vanadium Titanium Oxide Xerogels

V_{2 ? y}Ti _y O_{5 ? δ} · nH₂O (0 < y < 1.33, layered structure) and Ti_{1 ? y}V _y O_{2 + δ} · nH₂O (0 < y < 0.25, anatase structure) solid solutions are synthesized by a sol-gel process. ESR spectra are used to assess the state of tetravalent vanadium in the solid solutions before and after calcination at 350°C and after intercalation with K⁺. Six types of ESR signals arising from paramagnetic ions in different environments are identified. Computer simulation is used to evaluate the ESR parameters of the materials studied.     

Synthesis and High-Temperature Heat Capacity of Neodymium Titanate

Neodymium dititanate, Nd₂Ti2O₇ (monoclinic structure, sp. gr. P2₁), has been prepared by solid-state reaction in air at temperatures from 1673 to 1773 K using the Nd₂O₃ and TiO₂ oxides as starting materials. The high-temperature heat capacity of the resultant polycrystalline Nd₂Ti₂O₇ samples has been determined by differential scanning calorimetry. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of neodymium dititanate (enthalpy increment H°(T)–H°(320 K), entropy change S°(T)–S°(320 K), and reduced Gibbs energy Ф°(T)) in the temperature range 320–1053 K.     

Conductivity of Sm<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Sm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The oxygen-ion conductivity of porous materials, the coarse-grained pyrochlore-like Sm₂Ti₂O₇ and fine-grained Sm₂TiO₅ compounds, produced by mechanical activation of initial oxides is studied at 400–1000 °C. The Sm₂TiO₅ samples contain ～15 wt % of the nanosized pyrochlore-like Sm₂TiO₅ phase in addition to the rhombic phase. As determined by impedance spectroscopy, the ionic conductivities of Sm₂TiO₅ and Sm₂Ti₂O₇ at 1000°C are 1.3 × 10^?3 and 1.8 × 10^?4 S cm^?1, and the activation energies of the bulk and grainboundary conductivities of the materials are 1.04 and 1.24 eV for Sm₂TiO₅ and 1.69 and 1.80 eV for Sm₂Ti₂O₇.     

Preparation,structure and microwave dielectric properties of 3MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–3TiO<Subscript>2</Subscript> ceramics

3MgO–Al₂O₃–3TiO₂ (MAT) ceramics were prepared by a conventional solid-state reaction method. The crystal structure, sintering behavior and microwave dielectric properties of ceramics were investigated using X-ray diffraction, scanning electron microscopy and network analyzer. MAT ceramics contained the coexistence of three phases, including MgAl₂O₄, MgTiO₃ and MgTi₂O₅. The ceramics sintered at 1350 °C for 4 h presented excellent comprehensive performances with relative permittivity (ε _r ) of 15.4, quality factor (Q × f) of 91,000 GHz and temperature coefficient of resonant frequency (τ _f ) about ?55.1 ppm/°C.     

Homogeneity Range of Samarium Manganite in Air

The solubility limits of samarium and manganese oxides in SmMnO₃ are determined using x-ray diffraction analysis of Sm_{2 ? x}MnxO₃ samples (0.90 ≤ x ≤ 1.20, Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 800 and 1400°C. The composition dependences of lattice parameters are presented for samarium manganite synthesized at 1400°C. The solubility of samarium oxide in SmMnO₃ is tentatively attributed to structural defects, and that of manganese oxides is interpreted in terms of structural defects, oxygen nonstoichiometry of samarium manganite, the disproportionation reaction 2Mn³⁺ =Mn²⁺ + Mn⁴⁺, and partial substitution of the resulting Mn₂₊ for Sm₃₊ on the cuboctahedral site of the perovskite structure.     

High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.     

Effect of high pressures and temperatures on the structure and properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

We have prepared ceramic CaCu₃Ti₄O₁₂ samples by solid-state reaction and investigated the effect of high-pressure/high-temperature processing (p = 8.0 GPa, t = 1100°C) on the structure and electrical properties of CaCu₃Ti₄O₁₂.     

Stability region of Gd<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 ± δ</Subscript> solid solutions in air

The solid-solution range of Gd_{2 ? x} Mn _x O_{3 ± δ} has been determined using X-ray diffraction analysis of samples with 0.90 ≤ x ≤ 1.20 (Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Gd-Mn-O system in air. It is shown that gadolinium manganite with the perfect metal stoichiometry, GdMnO_{3 ± δ}, does not exist. The material of this composition in air consists of two phases: a Gd_{2 ? x} Mn _x O_{3 ± δ} solid solution with an orthorhombic perovskite-like structure and the binary oxide Gd₂O₃. The solid solution extends to the composition Gd_0.96Mn_1.04O_{3 ± δ}. Over the entire temperature range studied, gadolinium oxide does not dissolve in Gd_0.96Mn_1.04O_{3 ± δ}. Mn₃O₄ exhibits significant solubility in Gd_{2 ? x} Mn _x O_{3 ± δ}. In particular, Gd_0.86Mn_1.14 O_{3 ± δ} is single-phase by X-ray diffraction in the temperature range 1185–1400°C. Below 1185°C, Gd_{2 ? x} Mn _x O_{3 ± δ} is in equilibrium with another gadolinium manganite, GdMn₂O₅. With decreasing synthesis temperature, the GdMn₂O₅ solubility in Gd_{2 ? x} Mn _x O_{3 ± δ} drops precipitously. At 900°C, the only single-phase sample was Gd_0.96Mn_1.04O_{3 ± δ}.     

A new niobate-based CaO–2CuO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> microwave dielectric ceramic composite for LTCC applications

A novel CaO–2CuO–Nb₂O₅ (CCN) ceramic composite was prepared by the solid-state reaction method in the temperature range of 810–890 °C. Typically, the CCN sintered at 870 °C exhibited the excellent microwave properties of ε _r ?=?15.7, Q?×?f?=?28,700 GHz, τ _f = ? 38.4 ppm/°C. The τ _f of CCN was turned to be near zero by adding TiO₂, while the ε _r increased slightly and the Q?×?f decreased. The 0.91CCN–0.09TiO₂ ceramic sintered at 920 °C showed modified properties of ε _r ?=?16.9, Q?×?f?=?21,500 GHz, τ _f = ? 1.6 ppm/°C, which shows potential in LTCC applications.