On the structure of glass of the PbO-B2O3-SiO2 and CdO-SiO2-B2O3 systems

Using the method of X-ray scattering at large angles, the structure of single-phase glass of the PbO-B₂O₃-SiO₂ and CdO-B₂O₃-SiO₂ systems has been studied. The glass of the two-component PbO-B₂O₃ and PbO-SiO₂ systems of the three-component PbO-B₂O₃-SiO₂ system, containing from 10 to 60 mol % of SiO₂ at equal fractions of B₂O₃ and PbO, and the glass of a number of compositions of the CdO-B₂O₃-SiO₂ system containing from 30 to 60% of CdO are investigated. A change in the prevailing interatomic spacings in the dependence on the glass composition is determined. The boron-oxygen network provides the orderliness in the distribution of ions-modifiers observed at distances on the order of 6 Å. It is found that in contrast to boron-oxygen network, the silicon-oxygen network does not have the ability to order the distribution of the ions-modifiers.     

A coupled phase diagram experimental study and thermodynamic optimization of the Li2O-CaO-Al2O3 and Li2O-CaO-SiO2 systems,and prediction of the phase diagrams of the Li2O-CaO-Al2O3-SiO2 system

A coupled experimental phase diagram study and thermodynamic modeling of the Li₂O-CaO-Al₂O₃ and Li₂O-CaO-SiO₂ systems was conducted at 1 atm total pressure. Differential scanning calorimetry (DSC) measurements were performed in the Li₂O-CaO-Al₂O₃ and Li₂O-CaO-SiO₂ systems. In addition, the phase relations in the Li₂O-CaO-Al₂O₃ system were determined by equilibration/quenching experiments at 1643 and 1743 K, and the phases were characterized with X-ray diffraction (XRD) and Electron-probe micro analysis-wavelength dispersive spectroscopy (EPMA-WDS). The absence of ternary compounds or solid solutions was confirmed. Congruent melting of Li₂CaSiO₄ compound in the Li₂O-CaO-SiO₂ system was determined at 1350 ± 5 K. Thermodynamic optimization of the Li₂O-CaO-Al₂O₃ and Li₂O-CaO-SiO₂ systems was carried out based on new phase diagram experiments and critically evaluated literature data. The phase diagrams of the quaternary Li₂O-CaO-Al₂O₃-SiO₂ system were predicted using the thermodynamic models with optimized model parameters.     

Structural-chemical mechanism of formation of solid solutions in the La2SrAl2O2-Ho2SrAl2O7 system

The processes of phase formation in the La₂O₃-Ho₂O₃-SrO-Al₂O₃ system are investigated in the temperature range 1200–1500°C. The structural characteristics of the compounds described in the studied system are presented. It is established that the formation of (La_1?x Ho_x)₂SrAl₂O₇ solid solutions proceeds through the formation of LaAlO₃, LaSrAlO₄, SrAl₂O₄, and SrHo₂O₄ compounds. An increase in the holmium content and the temperature leads to a crossover from the mechanism in which the interaction of LaAlO₃ and LaSrAlO₄ is a limiting stage to the mechanism in which the decisive role is played by the interactions of SrAl₂O₄ with Ho₂O₃ and SrHo₂O₄ with Al₂O₃.     

The effects of the addition of tetragonal-ZrO2 on the mechanical properties of MgAl2O4 – ZrO2 composites

The effects of the addition of tetragonal 3mol% Y₂O₃ – ZrO₂ into a MgAl₂O₄ spinel matrix were investigated. MgAl₂O₄ spinel's lacking mechanical properties prevent further utilization in many structural and refractory applications even though it has excellent chemical and thermal stability. The addition of tetragonal-ZrO₂ was observed to improve the hardness, fracture toughness and biaxial flexural strength of MgAl₂O₄ materials. Moreover, the additions of 3mol% Y₂O₃ – ZrO₂ resulted in a reduction of the mean grain size.     

Contribution to phase equilibria in the Ce2O3 rich part of the Ce2O3–SiO2–ZrO2 system

Four ternary compositions in the Ce₂O₃ rich part of the Ce₂O₃–SiO₂–ZrO₂ system were investigated. The corresponding mixtures of CeO₂, SiO₂ and ZrO₂ were fired at 1400 °C in an Ar + 7% H₂ atmosphere until the phase equilibria were attained. The samples were characterized by XRD, SEM and EDS analysis. The thermal effects and the mass changes of the fired compositions were monitored by DTA/TG analyses. The oxyapatite phase, Ce_4.67(SiO₄)₃O, and the fcc phases identified as the solid solutions (CeZr)O_2−y, (Ce_zZr_1−z)O_2−x and Ce₂Zr₂O_7+2δ were observed in this part of the Ce₂O₃–SiO₂–ZrO₂ system. The DTA/TGA analyses indicated the presence of the Ce⁺⁴ ions in the fired compositions, while the lowest melting temperature was noticed at 1627 °C.     

Effect of Li2O on the crystallization behavior of CaO-Al2O3-SiO2-Li2O-Ce2O3 mold slags

The effect of Li₂O on the crystallization properties of CaO-Al₂O₃-SiO₂-Li₂O-Ce₂O₃ slags was investigated. With increasing the Li₂O content, LiAlO₂ and CaCeAlO₄ were the main crystalline phases. LiAlO₂ formed for the charge compensating of Li⁺ ions to [AlO₄^5?]-tetrahedrons, and CaCeAlO₄ formed as a result of the charge balance of Ce³⁺ ions, Ca²⁺ ions, and [AlO₆^9?]-octahedrons. Increasing the content of Li₂O to 10%, the crystallization temperature was the highest, and the incubation time was the shortest. The crystallization ability was strong due to the three factors of strengthening the interaction between ions and ion groups, decreasing the polymerization degree, and increasing the melting temperature. Further increasing the content of Li₂O, the crystallization performance was obviously suppressed, because the melting temperature and the force between the cations and the anion groups decreased.     

Effect of SiO2, Fe2O3, and TiO2 on stabilization of cubic ZrO2 in the presence of Al2O3

Several compositions are investigated in order to determine the essence of the phase transformations occurring at temperatures up to 1600°C in the ZrO₂ — Ln₂O₃ (Ln is Nd, Y, Yb) — Al₂O₃ — SiO₂ (Fe₂O₃, TiO₂) systems. The efficiency of using Y₂O₃ and Yb₂O₃ to stabilize cubic ZrO₂ in the presence of a mixture of Al₂O₃ and SiO₂ (Fe₂O₃, TiO₂) is shown. Data show the possibility of fabricating high-quality zirconium-corundum articles with any proportion of Al₂O₃ and ZrO₂.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 17 – 20, June 1996.     

Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts: Effect of the presence of O2 and H2O and the addition of Pt

The complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O has been achieved on ultrafine powdered TiO₂ photocatalysts and the addition of H₂O was found to enhance the reaction. The photocatalytic reaction has been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C₂H₄ with O₂ into CO₂, CO, and H₂O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C₂H₄. The photoformed OH species as well as O ₂ ⁻ and O ₃ ⁻ anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O. Interestingly, small amount of Pt addition to the TiO₂ photocatalyst increased the amount of selective formation of CO₂ which was the oxidation product of C₂H₄ and O₂.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Effect of ZrO2 nanocrystals on the stabilization of the amorphous state of alumina and silica in the ZrO2-Al2O3 and ZrO2-SiO2 systems

Structural and chemical transformations in the ZrO₂-Al₂O₃ and ZrO₂-SiO₂ systems containing ZrO₂ nanocrystals are examined. It is shown that the presence of zirconia nanoparticles in the oxide mixture retards the processes of crystallization and phase formation. The revealed effect is explained by the space blocking of nucleation in the systems under consideration.     

Criterion for estimating the number of phases in phase-separated glass of the Na2O-K2O-B2O3-SiO2 system by dilatometric analysis

The data of dilatometry and electron microscopy of four series of xNa₂O-(8 ? x)K₂O-32B₂O₃-60SiO₂, xNa₂O-(8 ? x)K₂O-22B₂O₃-70SiO₂, xNa₂O-(6 ? x)K₂O-34B₂O₃-60SiO₂, and xNa₂O-xK₂O-(40 ? 2x)B₂O₃-60SiO₂ phase-separated glass heat-treated at 550°C for 144 h (for glass containing 70 mol % SiO₂) and 24 h (for glass containing 60 mol % SiO₂) for separation on phases are summarized. The comparison of dilatometric data and electron microscopy allow one to conclude that glass with a difference between the onset deformation temperature and a glass transition temperature of more than 100°C is phase-separated; and glass with a difference of less than 65°C is single-phase. Curves for the glass transition temperature as a function of the K₂O content reveal a mixed alkali effect, namely, minimums for glass containing 60% SiO₂, and maximums for glass containing 70% SiO₂.     

The Formation of CrO2+2 in the Reaction of Cr2+ + O2 in Aqueous Acid Solutions

The reduction rate of Cr³⁺ by e_aq was determined, k₈ = 1.7 × 10¹⁰ M^?1 s^?1. The reaction of Cr²⁺ with O₂ was studied, k₂ = 1.6 × 10⁸ M^?1 s^?1. The spectrum of CrO²⁺₂ was obtained both with the pulse radiolysis method and by mixing Cr²⁺ with excess of O₂. It was shown that CrO²⁺₂ decays slowly to yield HCrO^?₄. The results suggest that the reaction of Cr²⁺ with O₂ is a two electron transfer process.     

Influence of preparation method on the catalytic performances of Re2O7/SiO2-Al2O3 catalysts in the metathesis of ethylene and 2-pentene

The metathesis of ethylene and 2-pentene for propylene production was investigated over Re₂O₇/SiO₂-Al₂O₃ catalysts. The SiO₂-Al₂O₃ (50 wt% Al₂O₃) were obtained by co-precipitation, physical mixing, and sol–gel coating. The presence of isolated layers/grains γ-Al₂O₃ either in bulk (physical mixing) or on the surface (sol–gel coating) resulted in isomerization-free metathesis. On the contrary, the Re₂O₇ catalysts supported on co-precipitated SiO₂-Al₂O₃ or the direct synthesis showed both metathesis and double-bond shift isomerization activities and as a consequence, higher propylene yield was obtained via the secondary metathesis reaction. The catalytic properties were also correlated with the Al-atom rearrangements and the acidic characters/strengths.     

The issue of selectivity in the direct synthesis of H2O2 from H2 and O2: the role of the catalyst in relation to the kinetics of reaction

The kinetic behavior in the direct synthesis of H₂O₂ with Pd–Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation (DP) methods on α-Al₂O₃ asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br⁻, pH) determine the selectivity in H₂O₂ formation, influencing the rate of the consecutive reduction of H₂O₂ (which is faster with respect to H₂O₂ decomposition on the metal surface) and/or of direct H₂ + O₂ conversion to H₂O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel and consecutive to H₂O₂ formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics. Increasing the noble metal loading on the membrane not only increases the productivity to H₂O₂, but also the selectivity, due to the formation of larger, less defective, Pd particles.     

Comparative study of the electrical characteristics of ALD-ZnO thin films using H2O and H2O2 as the oxidants

ZnO thin films were deposited via atomic layer deposition (ALD) using H₂O and H₂O₂ as oxidants with substrate temperatures from 100°C to 200°C. The ZnO films deposited using H₂O₂ (H₂O₂-ZnO) showed lower growth rates than those deposited with H₂O (H₂O-ZnO) at these temperature range due to the lower vapor pressure of H₂O₂, which produces fewer OH⁻ functional groups; the H₂O₂-ZnO films exhibited higher electrical resistivities than the H₂O-ZnO films. The selection of H₂O₂ or H₂O as oxidants was revealed to be very important for controlling the electrical properties of ALD-ZnO thin films, as it affected the film crystallinity and number of defects. Compared to H₂O-ZnO, H₂O₂-ZnO exhibited poor crystallinity within a growth temperature range of 100-200°C, while H₂O₂-ZnO showed a strong (002) peak intensity. Photoluminescence showed that H₂O₂-ZnO had more interstitial oxygen and fewer oxygen vacancies than H₂O-ZnO. Finally, both kinds of ZnO thin films were prepared as transparent resistive oxide layers for CIGS solar cells and were evaluated.     

Influence of the preparation method on the characteristics of TiO2-CeO2 supports

The preparation by a sol-gel process of TiO₂-CeO₂ supports (Ce/Ti atomic ratio=1/1) was studied by varying the sol-gel processing conditions. Starting materials were Ti[OCH(CH₃)₂]₄ and CeCl₃-7H₂O. The samples were characterised before and after calcination at 400 °C by BET, porosity measurements, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Working in acidic conditions leads to the formation of a compound that induces the growth of CeO₂ on amorphous TiO₂ during calcination. In a basic medium, a well defined CeTiO₃ compound is obtained that allows the formation of a real mixed TiO₂-CeO₂ compound after calcination. It presents a specific area of 70 m² g^–1, a narrow distribution of mesopores and easily reducible cerium oxide.     

Synthesis and structure of the n = 4 member of the framework aluminium alkylenediphosphonate series Al2[O3PC n H2n PO3](H2O)2F2

We report here the solvothermal synthesis and crystal structure of the hybrid inorganic-organic framework material Al₂[O₃PC₄H₈PO₃](H₂O)₂F₂?2H₂O (monoclinic, 1P2₁ /m, a = 4.961(2) Å, b = 11.930(5) Å, c = 10.727(5) Å, β = 93.972(6)^°, Z = 2, R(F, F ² > 2σ) = 0.094, R _w(F ², all data) = 0.262), the third member of the Al₂[O₃PC _n H_{2 n} PO₃](H₂O)₂F₂ framework series. The structure is formed from corrugated chains of corner-sharing AlO₄F₂ octahedra in which alternating AlO₄F₂ octahedra contain two fluorine atoms in a trans or a cis configuration. The diphosphonate groups link the chains together through Al-O-P-O-Al bridges and through the butyl groups to form a three-dimensional framework structure containing a one-dimensional channel system consisting of one type of channel only. The channels contain four extra-framework water molecules per unit cell. The formation of this member of the series shows that the form of the alkyl chain can successfully define the number of channel types and the channel length in this hybrid framework system.     

Avoiding the intermediate phase Zr2WP2O12 to develop a larger-negative-thermal-expansion-coefficient material Zr2W2P2O15

Zr₂W₂P₂O₁₅ with larger-negative-thermal-expansion-coefficient (NTEC, −4.01×10⁻⁶ K⁻¹, 143–673 K) is developed by controlling reaction process to avoid the intermediate phase of Zr₂WP₂O₁₂, whose smaller NTEC limits its application. From 1473 K to1573 K, Zr₂WP₂O₁₂ forms easily but the reaction between Zr₂WP₂O₁₂ and WO₃ to generate Zr₂W₂P₂O₁₅ is difficult even followed by heating up to 1673 K. However, putting raw materials of ZrO₂, WO₃ and NH₄H₂PO₄ into a pipe furnace at sintering temperature 1673 K directly, the Zr₂W₂P₂O₁₅ is formed avoiding the intermediate phase of Zr₂WP₂O₁₂.     

Mass spectrometric investigation of the thermodynamic properties of glass melts in the Cs2O-B2O3-SiO2 system at high temperatures

The composition of the vapor and the activities of cesium metaborate CsBO₂ in glass melts of the Cs₂O-B₂O₃-SiO₂ system at a temperature of 1020 K are determined using high-temperature mass spectrometry. The activities of the individual oxides Cs₂O, B₂O₃, and SiO₂ in glass melts of the Cs₂O-B₂O₃-SiO₂ system at a temperature of 1020 K are calculated by the Wagner method from the obtained values of the activities of CsBO₂. The integral and excess Gibbs energies are determined.     

Catalytic performances of NiO–CeO2 for the reforming of methane with CO2 and O2

Ce_1−xNi_xO₂ oxides with x varying from 0.1 to 0.5 were prepared by precipitation and characterized by XRD, TPR and XPS techniques. Three kinds of Ni phases co-exist in Ce_1−xNi_xO₂ catalysts: (1) aggregated NiO on the support CeO₂, (2) highly dispersed NiO with a strong interaction with CeO₂ and (3) Ni atoms incorporated into the CeO₂ lattice that forms the solid solution. For the reforming of methane with CO₂ and O₂, the Ce_1−xNi_xO₂ oxides prepared by precipitation are superior catalysts, which had a high catalytic activity and thermal stability, especially Ce_0.8Ni_0.2O₂. The catalytic activity is affected by the Ni dispersion on the surface of the catalyst. The Ni–CeO₂ solid solution formed by the Ni species incorporated into CeO₂ promotes the capability of coking resistance.