Effect of chromium on the kinetics of the contact melting and the graphite-to-diamond transformation in the Co–Fe–C system

We have studied the effect of chromium concentration in alloys of the Co–Fe system on their interaction with graphite at p, T parameters of thermodynamic stability of diamond. It has been found that addition of chromium to the alloys stabilizes the Me₃C-type carbide, as a result, the structure of a layer of a contact melting at the alloy–graphite interface is identical to a horizontal section of the metastable phase diagram of the Fe–C system. It is shown that addition of chromium to the Co–Fe–C system lowers the eutectic melting temperature by 80–100 K. In this case, the coefficient of carbon diffusion in the melt increases by ∼20–30%. An increased surface activity of a Cr-containing melt with respect to graphite is noted, which is the reason of intensive intrusion of the melt deep into a graphite layer along the grain boundaries. As a result, the number of nucleating diamond crystals and the degree of the graphite→diamond transformation increases.     

Heterogeneous Equilibria in the Yb–Mn–O System

The heterogeneous equilibria attained during thermal dissociation and hydrogen reduction of YbMnO₃ and YbMn₂O₅ are investigated using the static method (vacuum circulation setup) and X-ray powder diffraction analysis of quenched solid phases. It is found that, at the boundary of the low-oxygen homogeneity region of YbMn₂O₅, this compound dissociates into YbMnO₃ and Mn₃O₄ with the liberation of oxygen. Upon the reduction, YbMnO₃ dissociates with the formation of Yb₂O₃ and MnO and the liberation of oxygen. Equations for the dissociation and reduction reactions of YbMn₂O₅ and YbMnO₃ are derived, and mass balance equations for these processes are written. A fragment of the isothermal section of the phase diagram of the Yb–Mn–O system at 900°C is constructed in the composition–oxygen pressure coordinates.     

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     

Effect of the graphite perfection on the HP–HT diamond synthesis in a Ni–Mn–C system

Changes in the graphite structure used as precursor for diamond synthesis is a phenomenon observed during high pressure and high temperature processes in the presence of a solvent–catalyst metals. In this work, experimental results have shown that the initial structure of the graphite material has a significant influence on the yield and the possibility of formation of diamond. An association between the degree of structural perfection of the graphite and the success in producing diamond crystals was found to exist even for conditions outside the field of diamond stability in the carbon P–T equilibrium diagram.     

Liquid–liquid equilibria for the ternary systems DMC–methanol–glycerol,DMC–glycerol carbonate–glycerol and the quaternary system DMC–methanol–glycerol carbonate–glycerol at catalytic reacting temperatures

Liquid–liquid equilibria of multicomponent systems involved in the synthesis of glycerol carbonate from dimethyl carbonate and glycerol were experimentally measured. Particularly, data for the ternary systems dimethyl carbonate + methanol + glycerol and dimethyl carbonate + glycerol carbonate + glycerol and the quaternary system dimethyl carbonate + methanol + glycerol carbonate + glycerol are provided at 333.2 K, 338.2 K and 343.2 K at atmospheric pressure since these temperatures prove relevant for the synthesis of carbonate glycerol from glycerol and dimethyl carbonate. The experimental data obtained were correlated with a good degree of agreement to the NRTL model in order to obtain the corresponding binary interaction parameters.     

Thermodynamic calculations on the chemical vapor deposition of Si–C–N from the SiCl4–NH3–C3H6–H2–Ar system

Based on the Gibbs free energy minimum principle and Factsage software, the thermodynamic phase diagram for the SiCl₄–NH₃–C₃H₆–H₂–Ar system was calculated. The effects of temperature, dilution ratio of H₂, total pressure on product types and distribution regions of reacted solid products were discussed. The results show that: (1) The area of SiC–Si₃N₄ increases at first, then decreases with the rising of temperature and reaches the maximum value at 1273.15 K. (2) The ratio of C/Si is the main factor for the deposition of SiC in the double phase of SiC–Si₃N₄. (3) The preferred deposition conditions of Si₃N₄ are: T=1173.15 K, H₂:SiCl₄=10:1, and P_Total=0.01 atm. Taking the deposition of SiC into consideration, the deposition of Si₃N₄ influences the formation of Si–C–N directly. (4) According to the influencing factors of depositing SiC and Si₃N₄, the suitable parameter for Si–C–N deposition can be determined. (5) Through the experimental verification, it can be demonstrated that Si–C–N can be obtained by low-pressure chemical vapor deposition (CVD), its product being amorphous and mainly constituted by Si–N and Si–C bonds. The obtained Si–C–N ceramics can transform to α-Si₃N₄ and SiC nano-crystal when heat-treated at 1773.15 K in N₂ for 2 h.     

Vapor–liquid equilibrium in the sodium carbonate–sodium bicarbonate–water–CO2-system

Vapor–liquid equilibria of the carbon dioxide loaded sodium carbonate–water system were measured in the temperature range 40–80 °C and for sodium carbonate concentrations 8–12 wt%. In addition the vapor pressure of water over 10–30 wt% sodium carbonate solutions for the temperature range 27–100 °C was measured in an ebulliometer. The system was modeled using the electrolyte-NRTL model. Experimental vapor–liquid data from this study as well as data available in the literature from 25 to 195 °C and for sodium carbonate concentrations from 0.5 to 12 wt% were used for parameter fitting. The average deviation of the model predictions compared to all experimental data found is 9.8% for the partial pressure of CO₂. For vapor pressure of water the standard deviation is 0.6% up to 100 °C and 30 wt% sodium carbonate solutions.     

Morphotropic phase boundary in the BNT–BT–BKT system

Lead-free x Bi_0.5Na_0.5TiO₃–y BaTiO₃–z Bi_0.5K_0.5TiO₃ piezoelectric ceramics were synthesized by a conventional solid state reaction method. The microstructure, ferroelectric and piezoelectric properties of the ceramics were investigated. Structure measurements by X-ray diffraction with Rietveld refinement have allowed us to specify more precisely the morphotropic phase boundary (MPB) in this system. For (1 ? x) BNT–x BT solid solution ceramics, the 0.94 BNT–0.06 BT morphotropic composition shows the higher values with d₃₃ = 170 pC/N, k_p = 0.35 and k_t = 0.53. In the case of (1 ? x) BNT–x BKT compositions, the d₃₃, k_p and k_t are, respectively, 137 pC/N, 0.39 and 0.54 for the 0.80 BNT–0.20 BKT ceramic. On the other hand, the ternary 0.865 BNT–0.035 BT–0.100 BKT morphotropic composition shows high piezoelectric constant and electromechanical coupling factors (d₃₃ = 133 pC/N, k_p = 0.26 and k_t = 0.57).     

Gas-solid displacement reactions in the Ti–W–C system

Displacement reactions between binary and ternary ceramics in the Ti–W–C system and reactive gaseous atmospheres are investigated in this work. Specifically, WC and 50:50 wt% TiC:WC solid solution powders were exposed to flowing hydrogen gas, or equilibrated against an excess of titanium in the presence of iodine, to form metallic tungsten and TiC solid products. In the case of pure WC reacting with hydrogen, transformation to metallic tungsten occurred as a result of removal of chemically bound carbon as gaseous hydrocarbons. In the case of pure WC reacting with titanium iodide vapors, transformation was accompanied by the appearance of TiC as a solid product formed at the gas-solid interface. In the case of 50:50 wt% TiC:WC solid solution powders, hydrogen was generally found to be an ineffective displacing reagent, whereas reaction with titanium iodide vapors was observed to proceed virtually to completion, resulting in a two phase product mixture comprising metallic tungsten and TiC. For the latter case, a variety of microstructures could be observed within a given batch, including tungsten platelets and/or lamellae in a TiC matrix, or coarse tungsten grains interspersed with TiC grains. These morphological variations are speculated to arise from compositional variation in the starting material and the occurrence of local rapid coarsening along fast diffusion pathways within reacting agglomerates and polycrystalline primary particles. The observed reaction products and relative efficacy of gaseous reagents to promote displacement reactions in the Ti–W–C system are rationalized on the basis of thermodynamic predictions. The reaction between 50:50 wt% TiC:WC solid solution powders and titanium iodide vapors constitutes the first known report of an internal displacement reaction proceeding via gaseous intermediates in a nonoxide ceramic system.     

Coagulation–bubbling–ultrafiltration: Effect of floc properties on the performance of the hybrid process

A novel hybrid process of coagulation–bubbling–ultrafiltration was proposed to study membrane fouling phenomena by surface water. Relationship of bubbles, flocs and the hollow fibers was explored. When applying less than 20 mL/min gas flow rate, membrane fouling was accelerated with air bubbles introduced. When gas flow rate increased further to 40 mL/min and 60 mL/min, TMP showed a two-stage development trend, which was a fast development in the first few hours followed with a relatively slow development after about 4 h. Unified membrane fouling index (UMFI) increased from 0.00216 (without bubbles) to 0.00274 m²/L (40 mL/min gas flow rate) and 0.00219 m²/L (60 mL/min gas flow rate). As gas flow rate increased, bubble size became bigger, and its distribution range became wider, resulting in higher shear rate in the ultrafiltration column, which led to severe floc breakage. Flocs of small size and compact structure accelerated membrane fouling, resulting in highest UMFI value under 40 mL/min gas flow rate. However, under 60 mL/min gas flow rate, with largest bubbles and highest shear rate examined in this study, concentration polarization was effectively limited. As a result, TMP development slowed down when pore blockage reached equilibrium.     

Stability and growth of gas hydrates below the ice–hydrate–gas equilibrium line on the P–T phase diagram

Using a previously developed experimental technique, the behavior of small methane and propane hydrate samples formed from water droplets between 0.25 and 2.5 mm in size has been studied in the pressure–temperature area between the ice–hydrate–gas equilibrium line and the supercooled water–hydrate–gas metastable equilibrium line, where ice is a stable phase. The unusual persistence of the hydrates within the area bounded by these lines and the isotherms at T=253 K for methane hydrate or at T=263 K for propane hydrates was observed. This behavior has not previously been reported. For example, in the experiment carried out at 1.9 MPa and 268 K, the methane hydrates existed in a metastable state (the equilibrium pressure at 268 K is 2.17 MPa) for 2 weeks, then immediately dissociated into liquid supercooled water and gas after the pressure was isothermally decreased slightly below the supercooled water–hydrate–gas metastable equilibrium pressure. It was found that dissociation of metastable hydrate into supercooled water and gas was reversible. The lateral hydrate film growth rates of metastable methane and propane hydrates on the surface of supercooled water at a pressure below the ice–hydrate–gas equilibrium pressure were measured. The temperature range within which supercooled water formed during hydrate dissociation can exist and a role of supercooled water in hydrate self-preservation is discussed.     

Structure and phase formation in the Ti–Al–Nb system in the thermal explosion mode

This paper presents the results of the structure and phase formation in the Ti/Nb/2Al, Ti/Nb/2.5Al and Ti/Nb/3Al systems in the thermal explosion mode of self-propagating hightemperature synthesis. The morphology, phase composition, microstructure, and physical properties have been studied. It has been found that compounds with the highest content of aluminum have the most homogeneous composition and the lowest porosity. The main phase of the synthesis product is a phase based a solid solution of Nb in γ-TiAl.     

Phase equilibria in the calcium nitrate–water–isopropyl alcohol system

Results of an investigation of the solubility of the components of the water–isopropyl alcohol system in the temperature range of 253–268 K are given. Using fractional fusion, the concentrations of twocomponent water–isopropyl alcohol system have been determined, where fusion occurs more homogeneously. Phase diagrams of the calcium nitrate–water–isopropyl alcohol systems have been plotted at temperatures of 253, 263, and 268 K. The working area where compositions can be chosen for preparing the process liquid with a low freezing point has been determined.     

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.     

Decomposition reactions in the SiC–Al–Y–O system during gas pressure sintering

The substantial densification, that occurred in the SiC–Al–Y–O system was explained in the present work by analysing possible chemical reactions and their dependence on initial particle associations, i.e. homogeneity of mixing, the physical and chemical state of additives, pressurised sintering environment over the reactants and temperature of sintering. Hydroxyhydrogel powder precursors were found to be better than mechanically mixed SiC–YAG powder and pre-forming of YAG by holding the specimens at the temperature of 1400°C for 2 h were found to be the best. Decomposition reactions within the system could be controlled by using finer SiC and applying gas pressure over the reactants.     

Using the Sol–Gel Technology for the Treatment of Barley Seeds

Silica sols obtained by acidic or alkaline hydrolysis of tetraethoxysilane in an excess of water or ethanol in the presence of a number of salts and/or acids are used to treat the surface of barley seeds. The surface state and the elemental composition of the seeds are studied before and after their treatment in silica sols. The conditions of the sol–gel synthesis of silica sols, their effect on the state and chemical composition of the seed surface, and the sowing characteristics are analyzed. The plant’s resistance to phytopathogen, the causative agent of root rot is studied.     

Effect of the processing on the production of cordierite–mullite composite

In this study, effect of process on the production of cordierite–mullite composite was studied. For this reason two different processing methods were used in the production of cordierite–mullite composites. In first process, in situ cordierite–mullite composites were produced from cordierite and mullite layers which were formed by using aqueous tape casting method. In second one, composite was produced by addition of pre-produced mullite powders (in different weight percents, 0–30) into cordierite starting powders. The results show that the addition of pre-sintered mullite powders to the cordierite slip has more effect on densification behavior and mechanical properties of composites than layered production method.