Thermodynamic prediction of the nonstoichiometric phase Zr1–zCezO2–x in the ZrO2–CeO1.5–CeO2 system

A thermodynamic estimation of the ZrO₂–CeO₂ and ZrO₂–CeO_1.5 systems, as well as the cubic phase in the CeO_1.5–CeO₂ system has been developed and the complex relation between the nonstoichiometry, y, in Ce_zO_2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr_1–zCe_zO_2–x is described based on the thermodynamic estimation in the ZrO₂–CeO₂, CeO_1.5–CeO₂ and ZrO₂–CeO_1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO_1.5 fraction in CeO_1.5–CeO₂ in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO₂ and CeO_1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O₂ partial pressures on the mechanical properties of CeO₂-stabilised ZrO₂ ceramics     

The effect of Si–Bi2O3 on the ignition of the Al–CuO thermite

The ignition temperature of the Al–CuO thermite was measured using DTA at a scan rate of 50 °C min^?1 in a nitrogen atmosphere. Thermite reactions are difficult to start as they require very high temperatures for ignition, e.g. for Al–CuO thermite comprising micron particles it is ca. 940 °C. It was found that the ignition temperature is significantly reduced when the binary Si–Bi₂O₃ system is added as sensitizer. Further improvement is achieved when the reagents are nano-sized powders. For the composition Al + CuO + Si + Bi₂O₃ (65.3:14.7:16:4 wt.%), with all components nano-sized, the observed ignition temperature is ca. 613 °C and a thermal runaway reaction is observed in the DTA.     

Experimental study of the SnO2–ZrO2 phase diagram

The study of the SnO₂–ZrO₂ phase diagram in the 1230–1750 °C temperature range has shown the existence of an immiscibility gap, leading to two (Zr_1−xSn_x)O₂ and (Sn_1−yZr_y)O₂ limited solid solutions. Four compositions were synthesised for each solid solution, leading to pure phases, which were characterised by room-temperature and high-temperature X-ray diffraction. The unit-cell parameters of tetragonal (Sn_1−yZr_y)O₂, monoclinic (Zr_1−xSn_x)O₂ and tetragonal (Zr_1−xSn_x)O₂ were determined and correlated with the content of the substituted atom. The monoclinic to tetragonal and reverse reactions for the (Zr_1−xSn_x)O₂ series were also characterised (transition temperatures) when varying the tin mole fraction.     

On the Possibility of Forming Complexes with the Participation of O2– Ions in Melts and Glasses of the Na2O–SiO2–Cu2O System

The structural role of copper ions in melts (glasses) of the Na₂O–SiO₂–Cu₂O–CuO system is analyzed in the framework of the acid–base concept with due regard for the geometric (the radius ratio for Cu²⁽¹⁾⁺ and O^2– ions) and energy (the mean enthalpies of the Cu²⁽¹⁾⁺–O bonds) factors. It is demonstrated that copper ions in the structure fulfill the function of modifier cations. In these melts, the Cu¹⁺–Cu²⁺ redox equilibrium can be described without regard for the formation of [Cu²⁽¹⁾⁺O_4/2]^2(3)– ionic complexes (which could be incorporated into the structure of silicon–oxygen anions) and [Cu²⁺O _b/k ]^{2 – b/k} polyhedra providing the interaction between Cu²⁺ ions and anions. The influence of the formation of these polyhedra on the redox equilibrium is considered within the formalism of chemical thermodynamics. The composition dependence of the oxygen ion exponent pO is measured by an electromotive force (emf) technique. The ratio between the numbers of copper atoms with different valences is determined by chemical analysis. The experimental data obtained are in agreement with the theoretical inferences.     

Effect of Tungsten Oxide on the Crystallization Properties of Glasses of the B2O3–La2O3–Nb2O5 System

Glass Physics and Chemistry - The effect of tungsten oxide on the crystallization properties of glasses of the 37.5B2O3 22.5La2O3 (40 – x)Nb2O5 xWO3 (where x = 10, 15, 20, 30, 40 mol %)...     

Mechanism of the Formation of Ba2Ti9O20-Based Phases in the Course of Solid-Phase Interaction in the BaO–TiO2(ZrO2) and Cs2O–BaO–TiO2(ZrO2) Systems

The mechanism of solid-phase interaction in the BaO–TiO₂(ZrO₂) and Cs₂O–BaO–TiO₂(ZrO₂) systems is investigated. It is established that the formation of the Ba₂Ti₉O₂₀ compound and Ba₂Ti₉O₂₀-based solid solutions is a multistage process proceeding through the formation of intermediate phases. The solid-phase interaction in the BaO–TiO₂(ZrO₂) system occurs through the formation of the BaTi₄O₉ intermediate compound. The Ba₂Ti₉O₂₀ single-phase product is formed only in the presence of ZrO₂ (0.82 mol %) upon heat treatment at a temperature of 1250°C for 5 h. In the Cs₂O–BaO–TiO₂(ZrO₂) system, the BaTi₅O₁₁ metastable intermediate phase is formed at the first stage of the solid-phase interaction. The Cs _x Ba_{2 – x/2}Ti_{9 – y} Zr _y O₂₀ single-phase solid solutions are prepared upon heat treatment at 1100°C for 1 h. It is demonstrated that, in the Ba₂Ti₉O₂₀ structure, cesium can isomorphously substitute for barium with the formation of Cs _x Ba_{2 – x/2}Ti_{9 – y} Zr _y O₂₀ solid solutions (0 x 0.8, y = 0 and 0.09).     

Crystallization of Glasses in the K2O–Nb2O5–SiO2System

The changes in the structure and phase composition of glasses in the K₂O–Nb₂O₅–SiO₂system upon their heat treatment in the temperature range 700–800°C are studied by the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction. It is demonstrated that the crystallization is the primary process giving rise to microinhomogeneities in glasses due to heat treatment. Nanocrystals of an unidentified niobium-containing phase precipitate in glasses with the formation of regions with a decreased content of potassium and niobium oxides. An increase in the duration of heat treatment at the studied temperatures results in an increase in the size of nanocrystals without change in their phase composition. This is accompanied by the disappearance of diffusion zones, which leads to a decrease in the SAXS intensity in the range of small scattering angles and, correspondingly, to a decrease in the light scattering intensity.     

Cobalt–silica interaction and the dehydrogenation of 2-propanol

The interaction between silica and cobalt was studied on supported catalysts with low silica loading. Below a threshold cobalt level of 0.41 wt%, the catalysts were inactive for dehydrogenation of 2-propanol at 450 K. Inactivity was attributed to irreducibility of cobalt ions. Samples that were impregnated at a level below the threshold, dried, calcined, then reimpregnated below the threshold level, redried and recalcined such that the total cobalt content exceeded the threshold, were inactive. These results are not consistent with a model in which a portion of the cobalt interacts with specific silica sites, forming an irreducible species. Rather, they suggest that strongly interacting cobalt ions are incorporated into the silica surface.     

Influence of mechanical activation of reactive mixtures on the microstructure and properties of SHS-ceramics MoSi2–HfB2–MoB

In this work, the influence of modifications of SHS-process on the microstructure and performance characteristics of composite ceramics MoSi₂-HfB₂-MoB with two-level structure was studied. Partial texturing of MoSi₂ grains in samples obtained by force SHS pressing technology was revealed. The effect of preliminary mechanical activation on the macrokinetic parameters of combustion and on the microstructure of the synthesized ceramics was studied. A significant grinding of the synthesized ceramics grain and an increasing of physical-mechanical properties are achieved by increasing the velocity and lowering the combustion temperature of the activated mixtures. The sample obtained by hot pressing of SHS powder from MA reaction mixture showed the most optimal combination of hardness (19.5 GPa), porosity (0.4%) and oxidation resistance (1.82∙10^-6 mg/(cm²∙s)).     

Preparation of PTCR ceramics in the BaO–Nb2O5–TiO2 system

The PTCR effect was investigated in the ferroelectric BaNb₂O₆ phase doped with TiO₂. Composite ceramics formed after sintering in a reducing atmosphere and subsequent reoxidation show the PTCR effect at around 70 and 300°C, respectively. Both PTCR anomalies are associated with the formation of high resistivity grain boundaries after controlled oxidation of reduced constituent phases.     

Influence of TiB2 content on the properties of TiC–SiCw composites

The impact of various volume percentages of TiB₂ additive (0, 10, 20, and 30) on the microstructure, relative density (RD), Vickers hardness, flexural strength, and thermal conductivity of as-sintered TiC-10 vol% SiC_w-based composite samples were scrutinized. All four samples were sintered using the SPS method under the following circumstances; sintering temperature of 1900 °C, dwell time of 7 min, and external pressure of 40 MPa. The best relative density of 98.73% was achieved for the sample with no TiB₂ additive, indicating the negative effect of TiB₂ additive on the RD and formation of porosity. The microstructural observations and XRD results confirmed the chemical interaction of TiO₂ and B₂O₃ oxide layers and SiC_w and in-situ formation of the TiSi brittle phase and TiC. The most significant values of flexural strength (511 MPa) and hardness (27.67 GPa) were related to TiC-10 vol% SiC_w and TiC-10 vol% SiC_w-30 vol% TiB₂ samples, respectively. On the contrary, the specimens with 30 vol% and 10 vol% TiB₂ as additive presented the poorest qualities of flexural strength (234 MPa) and Vickers hardness (22.12 GPa). Finally, the influence of the TiB₂ content on the thermal conductivity was evaluated, indicating the positive impact of this secondary phase on this characteristic, so with adding 30 vol% TiB₂ to TiC-10 vol% SiC_w, a thermal conductivity of 30.7 W/m.K was obtained.     

Aspects of the Role of Hydrogen in H2-Assisted HC–SCR Over Ag–Al2O3

The role of hydrogen in H₂-assisted HC–SCR of NO _x over Ag–Al₂O₃ is investigated by XPS and in situ DRIFT spectroscopy. Hydrogen does not reduce the surface silver species to metallic silver, however direct reduction of surface nitrates by hydrogen is observed. It is proposed that one important role of hydrogen is the removal of nitrates from the Ag–Al₂O₃ surface.     

Impact of the solidification path of FeOx–SiO2 slags on the resultant inorganic polymers

Aiming to reduce the carbon dioxide emissions associated with cement production, alternative binders such as inorganic polymers currently receive substantial attention and slags from the non-ferrous metallurgy are promising precursors. However, studies  that correlate their chemistry and crystallinity with the newly formed binder remain limited. In this work, the effect of three different solidification methods on glass formation and reactivity of FeO_x–SiO₂ slags, as well as on the molecular structure of the resultant Fe-rich inorganic polymers, was investigated. The inorganic polymers were synthesized by mixing the slags (approximate molar ratio FeO/SiO₂ = 1.6) with an alkali silicate solution (molar ratios SiO₂/Na₂O = 1.6 and H₂O/Na₂O = 25). Results demonstrated that higher cooling rates promoted higher glass formation and faster reaction kinetics when the slags were activated. ⁵⁷Fe Mössbauer spectroscopy indicated that all the slags consisted predominantly of Fe²⁺ ions with a minor amount of Fe³⁺ ions, regardless of the variability in glass content. The binder phase of all inorganic polymers consisted of iron in both Fe²⁺ and Fe³⁺ states, after 28 days of curing. After pulverizing the inorganic polymer pastes and exposing the powder to air for 28 additional days, the Fe²⁺ state in the binder transformed to Fe³⁺. The compressive strength evolution of the three slags showed that the 2-day strength was higher for the samples with a higher amorphous fraction, while after 28 days, this influence was less pronounced.     

Phase Equilibria in the Sections of the Ca(NO3)2–CO(NH2)2–H2O System and Deicing Properties of Nitrate–Urea Mixtures

Theoretical Foundations of Chemical Engineering - Phase equilibria in the sections of the Ca(NO3)2–CO(NH2)2–H2O system and the deicing properties of calcium nitrate and carbamide...     

Effect of “Ph–NH–CH2–” group on the luminescence properties of sol–gel silica xerogel

The effect of Ph–NH–CH₂– group (Ph: phenyl) on the luminescence properties of silica xerogel by sol–gel method has been investigated. The absorption spectra, excitation spectra and emission spectra of Ph–NH–CH₂– group embedded in the network of silica xerogel have a red shift with increasing the ratios of Ph–NH–CH₂– group in the samples. The relative photoluminescence (PL) intensity of the hybrid silica xerogel sample decreases with increasing the ratios of Ph–NH–CH₂– group. From the complex luminescent centers, the novel luminescence properties have been observed from the hybrid silica xerogel sample containing Ph–NH–CH₂– group.     

A comprehensive study on the effect of TeO2 on the radiation shielding properties of TeO2–B2O3–Bi2O3–LiF–SrCl2 glass system using Phy-X / PSD software

The present work deals with the detailed investigations on the radiation shielding features of TeO₂–B₂O₃–Bi₂O₃–LiF–SrCl₂ glass system. The recent software namely Phy-X/PSD has been used to report the change in the photons attenuation features of this glass system with increasing the content of TeO₂ from 25 to 85 mol% between 347 keV and 2506 keV. The results show that replacing B₂O₃ with TeO₂ causes the increase in the density of the glass from 3.66 to 5.91 g cm⁻³ which lead to increase the linear attenuation factor (μ). The μ has a direct relationship with energy and has a highly large value at 0.347 MeV (between 0.550 and 0.896 cm⁻¹). The maximum effective atomic number occurs at 0.347 MeV and equals to 17.75, 19.78, 21.94, 24.24, 26.68, 29.29 and 32.07 for the TeBBiLS1- TeBBiLS7 samples. Thereafter, Z_eff decreases quickly down to 0.826 MeV, and becomes almost constant between 1.173 and 1.333 MeV and very slightly increases at 2.506 MeV. The tenth value layer results imply that increasing the weight percentage of TeO₂ in the glasses from 25 to 85 mol% can reduce the thickness needed to attenuate or block the photons. TeBBiLS1 has the maximum half value layer (HVL) and was found to be in the range of 3.034–4.823 cm and TeBBiLS7 sample has the lowest HVL. Also, the results revealed that as the density increased, the mean free path (MFP) gradually decreased, and the attenuation ability improves.     

Determination of surface area and porosity of sol–gel derived ceramic powders in the system TiO2–SiO2–Al2O3

The surface characteristics of ternary ceramic powders in the system TiO₂–SiO₂–Al₂O₃ prepared by a sol–gel technique were studied through nitrogen gas adsorption at liquid nitrogen temperature and application of the Brunauer–Emmett–Teller (BET) equation. The total surface area, total pore volume and pore radius of the powders were calculated through the construction of plots relating the amount of nitrogen gas adsorbed V₁ and the thickness of the adsorbed layer t (V₁−t plots). These results permitted the detection of the different types of pores present in the samples. In a first approximation, the V₁−t plots seem to demonstrate the correctness of the results obtained for the total specific area of the investigated samples. The influence of thermal treatment (drying, calcination) on the surface characteristics of the investigated powders was assessed.     

Liquid Immiscibility Phenomena in Melts of the ZrO2–FeO–Fe2O3 System

The ZrO₂–FeO _x melts prepared by induction melting in a cold crucible in air are investigated experimentally. It is found that the immiscibility region exists in the liquid phase. The theoretical fusibility curves (calculated according to the authors' DIATRIS 1.2 program) for the ZrO₂–FeO–Fe₂O₃ system are compared with the experimental temperatures of the formation of the secondary phase, liquidus and solidus lines, and X-ray microprobe spectroscopic and X-ray powder diffraction data for quenched and slowly cooled samples. It is demonstrated that the immiscibility region in the liquid phase in an air atmosphere is observed in the temperature range 1870–2230°C and the concentration range 34–82 wt % ZrO₂.     

Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al₂O₃·5SiO₂ to 0·5MgO·1·4Al₂O₃·5SiO₂ due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg₂Al₄Si₅O₁₈), spinel (Al₆Si₂O₁₃) and sapphirine (Mg₄Al₈Si₂O₂₀) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al₂O₃·5SiO₂ the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al₂O₃·5SiO₂ nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©