Water Calorimetry: A Correction to the Heat Defect Calculations

In a recent publication, we used a reaction model (model III) to calculate the heat defect for the irradiation of aqueous solutions with ionizing radiation at 21 °C. Subsequent work has revealed that the literature value used for one of the rate constants in the model was incorrect. A revised model (model IIIR) incorporates the correct rate constant for 21 °C. Versions of models III and IIIR were created for irradiations at 4 °C. For our current water calorimetry protocol, the values of the heat defect for H₂/O₂-water (water saturated with a flow of 43 % H₂ and 57 % O₂, by volume) at 21 °C predicted by model III and model IIIR are similar but the value for 4 °C predicted by III is 30 % smaller than the value predicted by IIIR. Model IIIR predicts that the values of the heat defect at 21 °C and 4 °C lie within the range −0.023±0.002, in agreement with the values obtained from our water calorimetry measurements done using pure water and H₂-saturated water at 21 °C and 4 °C. The yields of hydrogen peroxide in H₂/O₂-water at 21 °C and 4 °C were measured and agree with the predictions of model IIIR. Our water calorimetry measurements made with pure water and H₂-saturated water are now of sufficient quality that they can be used to determine the heat defect for H₂/O₂-water better than can be done by simulations. However, consistency between the three systems continues to be an excellent check on water purity which is crucial, especially for the pure water system.     

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O₂ and Ar–H₂–H₂O mixtures. It was found that in the low-pO₂ gases the oxide scales exhibited higher growth rates than in the high-pO₂ gases. The scales formed in the low-pO₂ gases showed substantially better adherence during cooling, than scales formed in the high-pO₂ gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O₂-H₂O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H₂O/O₂-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H₂O- or H₂-molecules or due to OH^? ions, cannot be derived with certainty from the present results.     

Experimental researches on characteristics of vapor–liquid equilibrium of NH3–H2O–LiBr system

An improved system of NH₃–H₂O–LiBr was proposed for overcoming the drawback of NH₃–H₂O absorption refrigeration system. The LiBr was added to NH₃–H₂O system anticipating a decrease in the content of water in the NH₃–H₂O–LiBr system. An equilibrium cell was used to measure thermal property of the ternary NH₃–H₂O–LiBr mixtures. The pressure–temperature data for their vapor–liquid equilibrium (VLE) data were measured at ten temperature points between 15–85 °C, and pressures up to 2 MPa. The LiBr concentration of the solution was chosen in the range of 5–60% of mass ratio of LiBr in pure water. The VLE for the NH₃–H₂O–LiBr ternary solution was measured statically. The experimental results show that the equilibrium pressures reduced by 30–50%, and the amount of component of water in the gas phase reduced greatly to 2.5% at T=70 °C. The experimental results predicted much better characteristics of the new ternary system than the NH₃–H₂O system for the applications.     

Determination of oxygen activities in highly diluted gas mixtures using a solid-state electrolyte cell

The applicability ofan oxygen sensor using stabilized zirconia as the solid-state electrolyte and catalytic Pt electrodes to measure oxygen activities in highly diluted gas mixtures was investigated. Various helium atmospheres containing low concentrations (in the µbar range) ofgaseous H₂,H₂O, CO, CO₂ and CH₄ impurities were used. For all gas mixtures considered an oxygen activity could be measured corresponding to the partial equilibrium ofH₂ and H₂O. The gas components CO and CO₂ did not affect the EMF ofthe cell, due to the low rates of their reactions at the Pt electrode relative to the H₂/H₂O equilibrium. Only CH₄ was seen to influence the measured oxygen partial pressure, P(O₂). It was found that the oxidation of CH₄ at Pt obeys a first-order rate law. Due to the extremely low kinetics of this reaction, only a slight reduction of p(O₂) from the H₂/H₂O equilibrium value of the bulk gas occurred.     

What is Different in Mixtures? From Critical Point to High Pressures

It is well known that the properties of mixtures may be very different from those of the pure constituent components, due to the unlike forces. Even for mixtures composed of simple molecules, this behavior can be rather complicated at high pressures, where the system may exhibit gas–gas equilibrium, a critical double point and density inversions. A few mixtures such as He–H₂ and He–N₂ are discussed. It is demonstrated that Raman spectroscopy is an important tool not only for obtaining knowledge about the dynamical behavior of mixtures but also for determining phase equilibria at high pressures. The frequency shift, the linewidth, and the line shape provide information about the solubility, the state of aggregation, and other conditions of the system. Recently, a new type of compound has been found, the so-called van der Waals compound. The results obtained in simple mixtures, such as Ne–N₂ and Xe–N₂, are compared with the results of molecular dynamics (MD) simulations of hard sphere mixtures. It is pointed out that energy, and not a geometrical effect (or entropy), is the driving force for compound formation.     

Formation of hydrogen peroxide by galena and its influence on flotation

The formation of hydrogen peroxide (H₂O₂), an oxidising agent stronger than oxygen, during the grinding of galena (PbS) was examined. It was observed that galena generated H₂O₂ in the pulp liquid during wet grinding and also when the freshly ground solids were placed in water immediately after dry grinding. The generation of H₂O₂ during either wet or dry grinding was thought to be due to a reaction between galena and water, when the mineral surface is catalytically active, to produce OH free radicals by breaking down the water molecule. It was also shown that galena could generate H₂O₂ in the presence or absence of dissolved oxygen in water.The concentration of H₂O₂ formed increased with decreasing pH. The effects of using mixtures of pyrite or chalcopyrite with galena were also investigated. In pyrite–galena mixture, the formation of H₂O₂ increased with an increase in the proportion of pyrite. This was also the case with an increase in the fraction of chalcopyrite in chalcopyrite–galena mixtures. The oxidation or dissolution of one specific mineral rather than the other in a mixture can be explained better by considering the extent of H₂O₂ formation rather than galvanic interactions. It appears that H₂O₂ plays a greater role in the oxidation of sulphides or in aiding the extensively reported galvanic interactions. This study highlights the necessity of further study of electrochemical and/or galvanic interaction mechanisms between pyrite and galena or chalcopyrite and galena in terms of their flotation behaviour.     

Evaluation of ion conductivity of ZrO2 thin films prepared by reactive sputtering in O2, H2O, and H2O + H2O2 mixed gas

Hydrated ZrO₂ thin films were prepared by reactive sputtering in O₂, H₂O, and H₂O + H₂O₂ mixed gas, and the effect of the sputtering atmosphere on ion conductivity of the films was investigated. The results showed that the films deposited in O₂ gas exhibited poor ion conductivity; however, the ion conductivities of the films deposited in the other two kinds of atmosphere were similar and 300-500 times higher than that of the films deposited in O₂ gas. It was indicated that the higher ion conductivity of the films was caused by lower film density and higher water content.     

Chemical Vapor Deposition of Niobium in the NbCl5–H2–O2 System

Chemical vapor deposition of niobium from NbCl₅–H₂–O₂ mixtures onto heated substrates was studied. Data are presented on the microstructure, phase composition, and microhardness of metastable oxygen-containing Nb.     

Nucleation and growth of aluminium oxide on silicon in the CVD process

Films of aluminium oxide have been formed on single crystal silicon substrates using AlCl₃-CO₂-H₂ gas mixtures in a cold-walled chemical vapour deposition (CVD) reactor. The nucleation and subsequent growth of the deposit have been observed under the varying process parameters. It is found that the nucleation and growth of the Al₂O₃ are dependent on the H₂O flux and H₂O supersaturation. An activation energy of 34.8 Kcal mol^–1 is obtained for the growth rate indicating that the CVD of Al₂O₃ on silicon is a thermally activated process and limited by surface reaction. Scanning electron micrographs (SEM) show that the deposited films are amorphous at low temperature, 850° C, but change to fine grained polycrystalline structure at high temperature, 1000° C.     

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide (Small 33/2023)

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H₂O₂ production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H₂O₂ to O₂. Here, ZnO passivator-coated BiVO₄ photoanode is reported for selective PEC H₂O₂ production. Both the H₂O₂ selectivity and production rate increase in the range of 1.0–2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO₄ upon ZnO coating, facilitating H₂O₂ generation and suppressing the competitive reaction of O₂ evolution. The ZnO overlayer also inhibits H₂O₂ decomposition, accelerates charge extraction from BiVO₄, and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H₂O₂ synthesis from PEC water oxidation.     

Phase Composition of the Products of Fe Oxidation in Fe + C + NaCl + H2O + O2 Exothermic Mixtures

Using thermodynamic analysis of the Fe–C–NaCl–H₂O–O₂ system and experimental studies (x-ray diffraction and Mössbauer spectroscopy) of exothermic mixtures containing Fe metal, activated carbon, water, and NaCl, we identified the state of Fe and determined the phase composition of the reaction products at different stages of oxidation with atmospheric oxygen. The calculation and experimental results are in reasonable agreement. Under the conditions of restricted access for air, the main oxidation product is magnetite, Fe₃O₄. Free access for air leads to the formation of hydrous ferric oxide, Fe₂O₃ · nH₂O. The most stable phase under the conditions of interest is goethite, Fe₂O₃ · H₂O (-FeOOH). Storage of incompletely oxidized samples away from air for 7–14 days leads to partial reduction of iron(III) oxide phases to Fe₃O₄ and -Fe.     

Gaseous reduction of Fe2O3 compacts at 600 to 1050° C

Dense Fe₂O₃ briquettes were isothermally reduced with hydrogen, carbon monoxide and H₂-CO mixtures at 600 to 1050‡ C. The course of reduction was followed by measuring the oxygen weight loss, as a function of time, using a thermogravimetric technique. Microscopic examination, X-ray and carbon analyses were also used to elucidate the kinetics and mechanisms of reduction of Fe₂O₃ briquettes. In the initial stages of reduction, the highest reduction rate was obtained in hydrogen while the slowest was in carbon monoxide. In CO-H₂ gas mixtures, the rate decreased with increasing amounts of carbon monoxide. In the later stages of reduction, a minimum reduction rate was only observed during the reduction with hydrogen and hydrogen-rich gas mixtures due to the formation of denseγ-iron at 900 to 950‡ C. This was eliminated in carbon monoxide and carbon-monoxide-rich gas mixtures due to the carbon deposition in the samples and secondary reactions.     

Solubilities of ethylene,ethane, and carbon dioxide in mixed solvents consisting of methanol,acetone, and water

The amount of gas absorbed in a liquid solvent or solvent mixture is measured with a high-precision gas burette. Solubility coefficients (Henry and Ostwald coefficients) of C₂H₄ and C₂H₆ are determined at ambient temperature and atmospheric pressure (1) in the pure solvents C₃H₆O, CH₃OH, and H₂O; (2) in the three binary mixtures of the solvents as a function of the composition of the solvent mixture; and (3) in several ternary mixtures of arbitrary composition. The experimental data are presented in diagrams and tables.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Chemical Etching of InAs, InSb, and GaAs in H2O2–HBr Solutions

The kinetics and mechanisms of InSb, GaAs, InAs, and InAs dissolution in H₂O₂–HBr mixtures were studied. The dissolution rate was determined as a function of solution composition, and the rate-limiting steps were identified. The dissolution process was shown to be diffusion-limited for all the materials studied, independent of solution composition. The composition range was determined in which H₂O₂–HBr solutions can be used for chemical polishing of InAs. Doping was shown to have a strong effect on the dissolution rate.     

Isochoric Heat Capacity Measurements for 0.5 H2O+0.5 D2O Mixture in the Critical Region

The isochoric heat capacity C _V of an equimolar H₂O+D₂O mixture was measured in the temperature range from 391 to 655 K, at near-critical liquid and vapor densities between 274.05 and 385.36 kgm^–3. A high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter was used. The measurements were performed in the one- and two-phase regions including the coexistence curve. The uncertainty of the heat-capacity measurement is estimated to be ±2%. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and densities at saturation were extracted from the experimental data for each measured isochore. The critical temperature and the critical density for the equimolar H₂O+D₂O mixture were obtained from isochoric heat capacity measurements using the method of quasi-static thermograms. The measurements were compared with a crossover equation of state for H₂O+D₂O mixtures. The near-critical isochoric heat capacity behavior for the 0.5 H₂O+0.5 D₂O mixture was studied using the principle of isomorphism of critical phenomena. The experimental isochoric heat capacity data for the 0.5 H₂O+0.5 D₂O mixture exhibit a weak singularity, like that of both pure components. The reliability of the experimental method was confirmed with measurements on pure light water, for which the isochoric heat capacity was measured on the critical isochore (321.96 kgm^–3) in both the one- and two-phase regions. The result for the phase-transition temperature (the critical temperature, T _{C, this work}=647.104±0.003 K) agreed, within experimental uncertainty, with the critical temperature (T _{C, IAPWS}=647.096 K) adopted by IAPWS.     

Effect of water on monetite synthesis in the solid state

Synthesis of calcium hydrogen phosphate CaHPO₄ can be carried out with a milling apparatus into which solid calcium dihydrogen phosphate Ca(H₂PO₄)₂.H₂O and tricalcium phosphate Ca₃(PO₄)₂.1/6H₂O mixtures are introduced. The effect of water vapor pressure P_H2O on this reaction has been investigated. At a given temperature (60°C) the curves of degree of conversion as a function of milling time depend on P_H2O. Formation of brushite CaHPO₄.2H₂O during the course of reaction can be noted for a particular range of water vapor pressure.If some amounts of brushite are added to the initial mixture, the kinetics of the reaction developed under water vapor are increased. Experiments on the evolution of products after a milling treatment give evidence of the important dependence of the reaction rate on water vapor pressure.     

Heat release in combustion of burning composite thermite mixtures on the base of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al system

The heat release of composite thermite mixtures of Fe₂O₃/TiO₂/Al system is investigated using a BKS-3 calorimeter. The region of compositions is identified, in which one can smoothly adjust the level and rate of heat release.     

Enthalpie de melange du systeme vitreux albite-orthose (Na,K) AlSi3O8

Using a high temperature Calvet calorimeter the authors have measured the enthalpy of dissolution of vitreous albite, orthose and their mixtures into a 2PbO+B₂O₃ liquid bath at 970 K. From these values, the enthalpy of mixing the vitreous system albite-orthose (Na, K) AlSi₃O₈ at this temperature has been deduced.     

Breakdown of the protective oxide on 11 % Cr steel at high temperature in the presence of water vapor and oxygen,the influence of chromium vaporization

Abstract

We report on the effect of water vapor and oxygen on the oxidation of a ferritic/martensitic 11 % Cr steel (CrMoV11 1). The influence of pH₂O, exposure time, gas velocity and temperature was investigated. The samples were exposed to dry O₂, O₂+10 or 40 % H₂O for up to 336 hours. Total pressure was 1 atm (1.02 × 10⁵ Pa). The gas velocity was between 0.05 and 10 cm/s while temperature was in the range 450–700°C. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, GDOES, FIB and TEM/EDX. Oxidation is strongly affected by the vaporization of CrO₂(OH)₂ in H₂O/O₂ environment. The mechanism of vaporization of CrO₂(OH)₂ from a Cr₂O₃ surface is modelled by DFT calculations. In the absence of chromium vaporization the alloy forms a protective oxide consisting of a corundum-type solid solution (Fe_1–xCr_x)₂O₃. The vaporization of chromium tends to deplete the oxide in chromium. In some cases the oxide remains protective in spite of chromium depletion while in other cases there is a transition to breakaway oxidation. In the latter case a thick layered scale forms, consisting of an outer hematite part and an inner iron-chromium spinel. Oxidation behavior in an O₂+H₂O environment is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium by diffusion in order to compensate for the losses by vaporization. The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.     

Preparation of spinel lithium ferrite by thermal treatment of spray-dried formates

The thermal decomposition of mixtures of complex ferric formate, /Fe₃(HCOO)₆(OH)₂/HCOO.4H₂O, and LiHCOO.H₂O prepared by spray-drying of their water solutions has been investigated. It was shown that the mixtures decompose at lower temperatures than the individual compounds in them. It was established that stoichiometric Li_O, ₅Fe₂, ₅O₄ can be prepared by slow and careful heating of formate mixture with lithium in excess to the stoichiometric amount (Li:Fe = 1:4, 4). The final product obtained is powdery Li_O, ₅Fe₂, ₅O4 with a large surface area.