Microwave dielectric ceramics in the BaO–TiO2–ZnO system doped with Nb2O5

The microwave dielectric properties of BaO–TiO₂–ZnO (BTZ) system ceramics were studied as a function of the amount of Nb₂O₅ dopant. With the addition of 0–0.025 mol% Nb₂O₅, the substitution of Ti⁴⁺ ions with Nb⁵⁺ ions decreased the sintering temperature and increased the dielectric constant ε_r and quality factor Q of the system due to the similar atomic radius of Nb⁵⁺ and Ti⁴⁺ ions. When the amount of Nb⁵⁺ increased further (>0.025 mol%), Q was decreased by increasing the titanium vacancies. When the system doped with 0.025 mol% Nb₂O₅ was sintered at 1160 °C for 6 h, the ε_r, Qf₀, and the temperature coefficient of resonant frequency (τ_f) were 36.7, 41,000 GHz, and −5.0 ppm/°C, respectively, at 5 GHz.     

Silicates of leucite structure as environmentally safe forms of cesium and strontium immobilization

Silicates containing binary combinations of Cs cations with Mg, Sr, Cu, or Fe cations or a ternary combination of Cs, Cu, and Fe cations were prepared by the sol-gel procedure followed by heat treatment of the dried gels. Silicates of the composition Cs₂MgSi₅O₁₂, Cs₂SrSi₅O₁₂, Cs₂CuSi₅O₁₂, CsFeSi₂O₆, and Cs₂CuSi₅O₁₂·CsFeSi₂O₆ (Cs₃CuFeSi₇O₁₈), according to the results of X-ray phase analysis, belong to the leucite family, space group Ia3d. The temperature range of their formation is 370–450°C (data of differential scanning calorimetry and X-ray phase analysis). The silicates underwent no chemical or phase transformations on heating to 900–1000°C. No egress of Cs from the silicates to the gas phase was observed on heating for 4–6 h at 800°C. The leaching rates of Cs and Sr at 25°C on the seventh day reached 10⁻⁷ g cm⁻² day⁻¹ (powdered samples, steady-state mode). The X-ray diffraction patterns of the silicates before and after hydrolytic tests were essentially similar.     

Novel structural properties of the lead–vanadate–tellurate glass ceramics

In this paper, we have examined and analyzed the effects of systematic intercalation of the lead ions on vanadate–tellurate glass ceramics with interesting results. The structural properties of the lead–vanadate–tellurate glass ceramics of compositions xPbO·(100 − x)[6TeO₂·4V₂O₅], x = 0 − 100 mol%, are reported for the first time. It has been shown by X-ray diffraction that single-phase homogeneous glasses with a random network structure can be obtained in this system. Among these unconventional lead–vanadate–tellurate glass ceramics, we found that network formers are good host material for lead ions and are capable to intercalate a variety of species such as Te₂V₂ ⁵⁺O₉, Pb₃(V⁵⁺O₄)₂, Pb₂V₂ ⁵⁺O₇, and V₂O₅-rich amorphous phase. On the other hand, these glass ceramics contain V⁴⁺ and V⁵⁺ ions necessary for the electrical conduction. Based on these experimental results, we propose that the V⁴⁺=O bonds are created by two different mechanisms: the first of reduction of V⁵⁺ ions to V⁴⁺ ions and thus of creation of V⁴⁺=O bonds.     

Transport,magnetic and spectroscopic studies of nickel-gallium ferrites

X-ray, electrical conductivity, magnetic hysteresis and infrared spectroscopic studies for the system NiGa_2−2xFe_2xO₄ were carried out. All the compounds, 0 x⩽1 showed cubic symmetry. The activation energy, thermoelectric coefficient values decreased with increasing Fe³⁺ concentrations but with increasing number of Fe³⁺ ions, the values of lattice constant and magnetic moment increased. Compounds with x⩽0.5 are p-type while those with x⩾0.75 are n-type semiconductors. Magnetic hysteresis loops indicated that the compounds with x⩾0.25 are ferrimagnetic while the compound NiGa₂O₄ (x=0.0) is anti-ferromagnetic. Constant and low values of coercive force indicated that the compounds with x⩾0.50 exhibit multidomain behaviour. X-ray intensity calculations, electrical conductivity, magnetic hysteresis and infrared studies indicate the presence of Ga³⁺ ions at the tetrahedral site, Ni²⁺ ions at the octahedral site and Fe³⁺ ions are present in both tetrahedral and octahedral sites. The probable ionic configuration for the system NiGa_2−2xFe_2x O₄ is suggested to be Ga _1−x ³⁺ Fe _x ³⁺ [Ni²⁺ Fe³⁺ O ₄ ²⁻ .     

About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La₂O₃ crystal structure. In order to explain the two oxygen sites in La₂O₃, it has been supposed that O _II ¹⁻ ions occupy the tetrahedral site while O _II ²⁻ occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]¹⁺. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]¹⁺ and [BiO]¹⁺ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are O _II ¹⁻ ions. The bivalent character of copper is then obtained through covalent bonds. For La_2−x Sr _x O₄ compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF₂.     

Preparation of nano-sized ABi2Nb2O9 (A?=?Ca2+, Sr2+, Ba2+) ferroelectric ceramics by soluble Nb(V) tartarate precursor and their dielectric characteristics after sintering

ABi₂Nb₂O₉ (A = Ca²⁺, Sr²⁺, Ba²⁺) were prepared by using an aqueous solution method from the mixture of water-soluble M-EDTA (M = Ca²⁺, Sr²⁺, Ba²⁺), Bi-EDTA, Nb-tartarate and TEA as the starting materials. A black fluffy mass resulted on heating the solution mixture at ∼200 °C. X-ray powder diffraction (XRD) showed that a single phase with the layered perovskite structure was formed after calcining the black fluffy mass at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The particle size obtained from TEM lay between 20 and 30 nm. Structural electron microscopy (SEM) revealed that the average grain size after sintering at 900 °C for 4 h ranges from 1.2 to 1.67 μm with a relative compact density of ≥90%. A comparative study on dielectric constant and corresponding tangent loss were carried out at 1 kHz, 100 kHz, 1 MHz, and 5 MHz from which the Curie temperature (T _c) was found to be at 935, 450, and 170 °C with peak dielectric constants at 930, 595, and 545, respectively, for CBN (CaBi₂Nb₂O₉), SBN (SrBi₂Nb₂O₉), and BBN (BaBi₂Nb₂O₉) measured at frequency 100 kHz with very low tangent loss.     

Use of Purbe diagrams in prediction of optimum conditions of separation and determination of halogens in natural objects using ionometry

Directional oxidation in the solution of X ⁻ ion being determined down to elementary halogen X ₂ and separation of it using gas extraction is promising for the separation of halogenide ions (Cl⁻, Br⁻, I⁻). The coinciding Purbe diagrams for redox systems of halogens X ⁻/X ₂ and most widely used oxidizers, namely, MnO₄⁻/MnO₂, MnO₄⁻/Mn²⁺, PbO₂/Pb²⁺, Cr₂O₇⁻/Cr³⁺, HNO₂/NO, were fragmentarily plotted. The predicted working pH intervals of the medium for directional quantitative oxidation of the given halogenide ion by a particular oxidizer were ascertained graphically using the diagrams. The possible schemes for the independent ionometric determination of Cl⁻, Br⁻ and I⁻ ions were proposed.     

Catalytic wet oxidations of aromatic compounds over supported copper oxides

Catalytic wet oxidations of naphthalene as a model compound of persistent aromatic compounds were carried out with hydrogen peroxide in a closed autoclave lined with Teflon. CuO/Al₂O₃ and CuO/AC catalyst showed the high activity for the naphthalene oxidation with hydrogen peroxide of 1.0 mol L⁻¹ at 100 °C. Naphthalene, whose initial concentration was 1.0 g L⁻¹, was converted completely and the concentration of water-soluble organic compounds in the resultant aqueous solution was less than 25 ppm-C. In contrast, platinum, and manganese oxide, silver oxide, and ruthenium oxide catalysts consumed hydrogen peroxide preferentially. Iron and nickel oxides catalysts showed lower activity than the copper oxide catalyst. During the reaction, the intermediate organic acids were formed and then were oxidized. Simultaneously, copper species of CuO catalysts were dissolved and then were precipitated. The precipitated copper species on the catalyst support showed the catalytic activity. CuO/Al₂O₃ catalysts showed high activity for the six successive batch reactions with the treatment of sodium carbonate after the reaction to precipitate copper ions.     

Measurement of Gibbs energies of formation of CoF2 and MnF2 using a new composite dispersed solid electrolyte

Gibbs energies of formation of CoF₂ and MnF₂ have been measured in the temperature range from 700 to 1100 K using Al₂O₃-dispersed CaF₂ solid electrolyte and Ni+NiF₂ as the reference electrode. The dispersed solid electrolyte has higher conductivity than pure CaF₂ thus permitting accurate measurements at lower temperatures. However, to prevent reaction between Al₂O₃ in the solid electrolyte and NiF₂ (or CoF₂) at the electrode, the dispersed solid electrolyte was coated with pure CaF₂, thus creating a composite structure. The free energies of formation of CoF₂ and MnF₂ are (± 1700) J mol⁻¹; {fx37-1} The third law analysis gives the enthalpy of formation of solid CoF₂ as ΔH° (298·15 K) = −672·69 (± 0·1) kJ mol⁻¹, which compares with a value of −671·5 (± 4) kJ mol⁻¹ given in Janaf tables. For solid MnF₂, ΔH°(298·15 K) = − 854·97 (± 0·13) kJ mol⁻¹, which is significantly different from a value of −803·3 kJ mol⁻¹ given in the compilation by Barinet al.     

Low temperature synthesis of glassy solids of the system Al2O3-P2O5-SiO2

Glassy solids of the system Al₂O₃-P₂O₅-SiO₂, which have not been obtainable because of the devitrification of glass during the cooling process after melting, were obtained by the gel method. After the gels of the system Al₂O₃-P₂O₅-SiO₂ were prepared from AlCl₃·6H₂O, H₃PO₄ and Si(OC₂H₅)₄, they were heat treated up to 800° C to obtain glassy solids. In SiO₂ concentrations ranging from 75 to 82 mol % in batch compositions, non-porous transparent solids were obtained, while in SiO₂ concentrations above 87 mol %, porous transparent solids were obtained. Tridymite was precipitated because of the crystallization of glassy solids during heat treatment above 800° C. It depended upon the microstructure of the gels whether non-porous glassy solids were obtained or not. The thermal expansion coefficients of the glassy solids were greatly dependent upon the concentration of P₂O₅, ranging from 1.7×10⁻⁶ to 4.2×10⁻⁶ (1/° C).     

Rubidium ion conducting Rb2 ? 2xAl2 ? xAxO4 (A = Nb, Ta) solid electrolytes

Rb_{2 − 2x} Al_{2 − x} A _x O₄ (A = Nb, Ta) solid solutions have been synthesized, and their conductivity has been measured as a function of temperature and composition. The highest rubidium ion conductivity in the Rb_{2 − 2x} Al_{2 − x} A _x O₄ solid solutions is 3.16 × 10⁻³ S/cm at 300°C and ∼ 2 × 10⁻² S/cm at 700°C. The high rubidium ion conductivity of the synthesized solid electrolytes is mainly due to the formation of rubidium vacancies when Nb⁵⁺ or Ta⁵⁺ substitutes for Al³⁺ and to the specific features of the crystal structure of RbAlO₂.     

Microstructure, mechanical, and in vitro properties of mica glass-ceramics with varying fluorine content

The design and development of glass ceramic materials provide us the unique opportunity to study the microstructure development with changes in either base glass composition or heat treatment conditions as well as to understand processing-microstructure-property (mechanical/biological) relationship. In the present work, it is demonstrated how various crystal morphology can develop when F⁻ content in base glass (K₂O–B₂O₃–Al₂O₃–SiO₂–MgO–F) is varied in the range of 1.08–3.85% and when all are heat treated at varying temperatures of 1000–1120°C. For some selected heat treatment temperature, the heat treatment time is also varied over 4–24 h. It was established that with increase in fluoride content in the glass composition, the crystal volume fraction of the glass-ceramic decreases. Using 1.08% fluoride, more than 80% crystal volume fraction could be achieved in the K₂O–B₂O₃–Al₂O₃–SiO₂–MgO–F system. It was observed that with lower fluoride content glass-ceramic, if heated at 1040°C for 12 h, an oriented microstructure with ‘envelop like’ crystals can develop. For glass ceramics with higher fluorine content (2.83% or 3.85%), hexagonal-shaped crystals are formed. Importantly, high hardness of around 8 GPa has been measured in glass ceramics with maximum amount of crystals. The three-point flexural strength and elastic modulus of the glass-ceramic (heat treated at 1040°C for 24 h) was 80 MPa and 69 GPa of the sample containing 3.85% fluorine, whereas, similar properties obtained for the sample containing 1.08% F⁻ was 94 MPa and 57 GPa, respectively. Further, in vitro dissolution study of the all three glass-ceramic composition in artificial saliva (AS) revealed that leached fluoride ion concentration was 0.44 ppm, when the samples were immersed in AS for 8 weeks. This was much lower than the WHO recommended safety limits of 1.5 ppm. Among all the investigated glass-ceramic samples, the glass ceramic with 3.85% F⁻ content in base glass (heat treated at 1040°C for 12 h), exhibits the adherence of Ca–P layer, which consists of spherical particles of 2–3 μm. Other ions, such as Mg⁺² and K⁺¹ ion concentrations in the solution were found to be 8 and 315 ppm after 8 weeks of leaching, respectively. The leaching of all metal ions is recorded to decrease with time, probably due to time-dependent kinetic modification of sample surface. Summarizing, the present study illustrates that it is possible to obtain a good combination of crystallization, mechanical and in vitro dissolution properties with the careful selection of base glass composition and heat treatment conditions.     

Crystal growth of tetragonal ZrO2 in the glass system ZrO2-SiO2 prepared by the sol-gel process from metal alkoxides

Glasses in the system ZrO₂-SiO₂ containing 40 to 60 mol % ZrO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by heat treatment at 800 and 1200° C, and its crystal growth was measured by differential thermal and X-ray diffraction analyses. At 800 to 900° C, tetragonal ZrO₂ crystals grew three-dimensionally and the activation energy for growth was calculated as about 680 kJ mol⁻¹. On the other hand, the secondary growth of tetragonal ZrO₂ at 1000 to 1200° C followed the cube-root-of-time law. The activation energy for secondary growth was about 380 kJ mol⁻¹. It is suggested that the diffusion of Zr⁴⁺ ions is the rate-limiting process for the secondary crystal growth of tetragonal ZrO₂.     

Thermal and dielectric properties of the LTCC composites based on the eutectic system BaO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The low-temperature co-fired ceramic (LTCC) composites containing quartz based on the eutectic system BaO–Al₂O₃–SiO₂–B₂O₃ are fabricated at the sintering temperature below 980 °C. Preparation process and sintering mechanism were described and discussed, respectively. The results indicated that the addition of quartz to the eutectic system can availably improve dielectric properties of the LTCC composites. In addition, The LTCC composites with optimum compositions, which were obtained by the regulation of an Al₂O₃ content in the composite, can express excellent dielectric properties (permittivity: 5.94, 5.48; loss: 7 × 10⁻⁴, 5 × 10⁻⁴), considerable CTE values (11.7 ppm. °C⁻¹, 10.6 ppm. °C⁻¹) and good mechanical properties (128 MPa,133 MPa).     

Synthesis of hexagonal Al2−x Cr x O3 nanodisks by a thermal decomposition of mesoporous Cr4+:Al2O3 powders

Monodispersed hexagonal Al_2−x Cr _x O₃ nanodisks are synthesized through a reactive doping of Cr⁶⁺ cations in a hydrogenated mesoporous AlO(OH)·αH₂O powder followed by annealing at 1,200 °C in air. The reaction was carried out by a drop wise addition of an aqueous Cr⁶⁺ solution (0.5–1.0 M) to AlO(OH)·αH₂O, at room temperature. Al_2−x Cr _x O₃ nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr⁴⁺:Al₂O₃ composites in the temperature range 400–1,000 °C. The nanodisks of ∼50 nm diameter and thickness of ∼16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 °C). The Cr³⁺ ↔ Al³⁺ substitution, x ≤ 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.     

Effect of secondary heat treatment on the spectroscopic properties of Na<Subscript>2</Subscript>O-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

We have studied the optical absorption and luminescence spectra of 45Na₂O · xNb₂O₅ · (55 − x)P₂O₅ glasses containing 5, 10, 20, 25, 30, and 35 mol % Nb₂O₅. The results indicate that the absorption band around 26000 cm⁻¹, responsible for the yellow color of the glasses, is due to the [Nb^(5+)--O⁻] center and disappears upon secondary heat treatment. Heat treatment of europium-doped glasses increases the concentration of Eu³⁺ centers in an asymmetric environment, which is accompanied by an increase in luminescence efficiency. The reason for this is that the Eu³⁺ ions are located outside the niobate subsystem of the glass matrix. The europium in the glasses studied acts as a protector ion.     

Active anion manipulation for emergence of active functions in the nanoporous crystal 12CaO·7Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: a case study of abundant element strategy

This article reviews our approach to render 12CaO · 7Al₂O₃ (C12A7) electronically active using a new concept of ‘active anion manipulation’, where nanostructures embedded within the C12A7 crystal lattice are intentionally utilized to generate chemically unstable (‘water-free active’) anions. Anionic active oxygen radicals, O⁻ and O₂⁻, are formed efficiently in C12A7 cages under high oxygen activity conditions. The configuration and dynamics of O₂⁻ in cages are revealed by a combination of continuous-wave and pulsed electron paramagnetic resonance (EPR). It is demonstrated that metal-loaded C12A7 is a promising oxidation catalyst for syngas (CO + H₂) formation from methane. Furthermore, the O⁻ ion, the strongest oxidant among active oxygen species, can be extracted from the cage into an external vacuum by applying an electric field with thermal assistance, generating a high-density O⁻ beam in the order of μA cm⁻². In contrast, heat treatment of C12A7 in a hydrogen atmosphere forms H⁻ ions in the cages. The resultant C12A7:H⁻ exhibits a persistent insulator-conductor conversion upon ultraviolet-light or electron-beam irradiation. The irradiation-induced conversion mechanism is examined by first-principle theoretical calculations. Furthermore, the presence of a severely reducing environment causes the complete substitution of electrons for anions in the cages. The resulting C12A7:e⁻, which exhibits excellent stability and an electrical conductivity greater than 100 S cm⁻¹, is regarded as an ‘electride’, an ionic compound in which electrons serve as anions. The C12A7 electride exhibits a high potential for applications involving cold cathode and thermal field electron emissions due to its small work function. Electride fabrication methods suitable for large-scale production via melt processing are described. It is also demonstrated that proton or inert gas ion implantations into C12A7 thin films at elevated temperatures are effective for both H⁻ and electron doping.     

NiaSi2O5（OH）4 Multi-Walled Nanotubes with Tunable Magnetic Properties and Their Application as Anode Materials for Lithium Batteries

Highly crystalline and thermally stable pure multi-walled Ni₃Si₂O₅(OH)₄ nanotubes with a layered structure have been synthesized in water at a relatively low temperature of 200–210 °C using a facile and simple method. The nickel ions between the layers could be reduced in situ to form size-tunable Ni nanocrystals, which endowed these nanotubes with tunable magnetic properties. Additionally, when used as the anode material in a lithium ion battery, the layered structure of the Ni₃Si₂O₅(OH)₄ nanotubes provided favorable transport kinetics for lithium ions and the discharge capacity reached 226.7 mA·h·g⁻¹ after 21 cycles at a rate of 20 mA·g⁻¹. Furthermore, after the nanotubes were calcined (600 °C, 4 h) or reduced (180 °C, 10 h), the corresponding discharge capacities increased to 277.2 mA·h·g⁻¹ and 308.5 mA·h·g⁻¹, respectively.      

Structural studies of some V2O5-P2O5-B2O3-Fe2O3 glass systems

X-ray diffraction and infrared measurements were performed on vanadium borophosphate glass containing different amounts of iron ranging from 0–7.5 mol % and heat treated at 300 °C for various times. The structure and phase separation could be determined for each glass composition. V₂O₅ was the main precipitated phase in all heat-treated samples, and its amount was dependent on the heat-treatment time and Fe₂O₃ content. Also FeP was detected in samples heat treated for 24 h. The infrared measurements showed the presence of both V⁴⁺ and V⁵⁺. The symmetry of V₂O ₇ ⁴⁻ and VO ₄ ³⁻ groups was found to increase with increasing Fe₂O₃ content. It was also found that some PO₄ changed to BO₃, forming a non-bridging oxygen.     

Dynamic formation of zircon during high temperature deformation of zirconia–silica composites with alumina additions

A three phase ceramic composite of 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ), SiO₂, and Al₂O₃ was evaluated for potential high temperature superplasticity. The amorphous SiO₂ content was 5 wt.%, and increasing additions of Al₂O₃ were made. The effect of varying the Y₂O₃ stabilizer concentration in ZrO₂ was also studied. Samples sintered at 1200 °C contained only YSZ, Al₂O₃, and amorphous SiO₂, but ZrSiO₄ formed in the samples above 1300 °C. Mullite (3Al₂O₃ · 2SiO₂) was not detected in any samples. Specimens of 1 wt.% Al₂O₃–YSZ/SiO₂ had an anomalously high deformation rate of ∼2 × 10⁻⁴ s⁻¹ at 1200 °C when compared to YSZ/SiO₂ without Al₂O₃ (∼4 × 10^-5 s⁻¹). Higher amounts of Al₂O₃ additions decreased the strain rate. Extensive deformation of Al₂O₃ doped YSZ/SiO₂ at 1200 °C induced the formation of ZrSiO₄ due to enhanced reaction rates. This distributed, yet locally interconnected, zircon phase rapidly eroded the strain rate after ∼60% deformation.