Formation kinetics and transition mechanism of CaO·SiO2 in low-calcium system during high-temperature sintering

The crystal structure,formation kinetics and micro-morphology of CaO·SiO2 during high-temperature sintering process were studied in low-calcium system by XRD,FT-IR,Raman and SEM-EDS methods.When the molar ratio of CaCO3 to SiO2 is 1.0,β-2CaO·SiO2 forms firstly during the heating process,and then CaO·SiO2 is generated by the transformation reaction of pre-formed 2CaO·SiO2 with SiO2.3CaO·SiO2 and 3CaO·2SiO2 do not form either in the heating or sintering process.Rising the sintering temperature and prolonging the holding time promote the phase transition of 2CaO·SiO2 to CaO·SiO2,resulting in the sintered products a small blue shift and broadening in Raman spectra.The content of CS can reach 97.4%when sintered at 1400℃ for 1 h.The formation kinetics of CaO·SiO2 follows the second-order chemical reaction model,and the corresponding apparent activation energy and pre-exponential factor are 505.82 kJ/mol and 2.16×10^14 s^−1 respectively.

The formation kinetics and control of biofilms by three dominant fungi species isolated from groundwater

Filamentous fungi can enter drinking water supply systems in various ways,and exist in suspended or sessile states which threatens the health of individuals by posing a high risk of invasive infections.In this study,the biofilms formation kinetics of the three genera of fungal spores,Aspergillus niger(A.niger),Penicillium polonicum(P.polonicum)and Trichoderma harzianum(T.harzianum)isolated fromthe groundwater were reported,as well as the effects of water quality parameters were evaluated.In addition,the efficiency of low-concentrations of chlorine-based disinfectants(chlorine,chlorine dioxide and chloramine)on controlling the formation of fungal biofilms was assessed.The results showed that the biofilms formation of the three genera of fungi could be divided into the following four phases:induction,exponential,stationary and sloughing off.The optimum conditions for fungal biofilms formation were found to be neutral or weakly acidic at 28°C with rich nutrition.In fact,A.niger,P.polonicum,and T.harzianum were not observed to form mature biofilms in actual groundwater within 120 hr.Carbon was found to have the maximum effect on the fungal biofilms formation in actual groundwater,followed by nitrogen and phosphorus.The resistance of fungal species to disinfectants during the formation of biofilms decreased in the order:A.niger>T.harzianum>P.polonicum.Chlorine dioxide was observed to control the biofilms formation with maximum efficiency,followed by chlorine and chloramine.Consequently,the results of this study will provide a beneficial understanding for the formation and control of fungal biofilms.

The dual action of N_(2)on morphology regulation and mass-transfer acceleration of CO_(2)hydrate film

The morphology characteristics of CH_(4),CO_(2),and CO_(2)+N_(2)hydrate film forming on the suspending gas bubbles are studied using microscopic visual method at supercooling conditions from 1.0 to 3.0 K.The hydrate film vertical growth rate and thickness along the planar gas-water interface are measured to study the hydrate formation kinetics and mass transfer process.Adding N_(2)in the gas mixture plays the same role as lowering the supercooling conditions,both retarding the crystal nucleation and growth rates,which results in larger single crystal size and rough hydrate morphology.N_(2)in the gas mixture helps to delay the secondary nucleation on the hydrate film,which is beneficial to maintain the porethroat structure and enhance the mass transfer.The vertical growth rate of hydrate film mainly depends on the supercooling conditions and gas compositions but has weak dependence on the experimental temperature and pressure.Under the same gas composition condition,the final film thickness shows a linear relationship with the supercooling conditions.The mass transfer coefficient of CH_(4)molecules in hydrates ranges from 4.54×10^(-8)to 7.54×10^(-8)mol·cm^(-2)·s^(-1)·MPa^(-1).The maximum mass transfer coefficient for CO_(2)t N_(2)hydrate occurs at the composition of 60%CO_(2)t 40%N_(2),which is 3.98×10^(-8)mol·cm^(-2)·s^(-1)·MPa^(-1).

Formation and characterization of cerium conversion coatings on magnesium alloy

Chemical conversion treatment by rare earth metal salt solution was considered as an alternative to chromium chemical conversion treatment to improve the corrosion resistance of magnesium alloys. In this study, cerium conversion coatings formed on AZ31 magnesium alloy were characterized and the formation mechanism was discussed. X-ray photoelectron spectroscopy （XPS） analysis showed that cerium conversion coating consisted of cerium hydroxides/oxides, in which both tetravalent cerium Ce（Ⅳ） and trivalent cerium Ce（Ⅲ） species co-existed. Cerium conversion coating was a two-layer structure. Atomic force microscopy （AFM） images revealed that the morphology of the inside layer was different from that of the outside one, which was responsible for the inherent adhesive weakness of the coating. Corrosion potential （Ecorr） measurements indicated that poor adhesion limited the improvement of the corrosion resistance of the coating. During the treating process, the increased pH value of the cerium salt solution led to the precipitation of cerium hydroxides/oxides. The formation kinetics of the coating followed a parabolic curve.

Mineral transition and formation mechanism of calcium aluminate compounds in CaO-Al2O3-Na2O system during high-temperature sintering

The mineral transition and formation mechanism of calcium aluminate compounds in CaO-Al2O3-Na2O system during the hightemperature sintering process were systematically investigated using DSC-TG,XRD,SEM-EDS,FTIR,and Raman spectra,and the crystal structure of Na4Ca3(AlO2)10 was also simulated by Material Studio software.The results indicated that the minerals formed during the sintering process included Na4Ca3(AlO2)10,CaO·Al2O3,and 12 CaO·7 Al2O3,and the content of Na4Ca3(AlO2)10 could reach 92 wt%when sintered at 1200°C for 30 min.The main formation stage of Na4Ca3(AlO2)10 occurred at temperatures from 970 to 1100°C,and the content could reach82 wt%when the reaction temperature increased to 1100°C.The crystal system of Na4Ca3(AlO2)10 was tetragonal,and the cells preferred to grow along crystal planes(110)and(210).The formation of Na4Ca3(AlO2)10 was an exothermic reaction that followed a secondary reaction model,and its activation energy was 223.97 kJ/mol.

Rapid hydrate formation and efficient gas separation based on aqueous solid carrier

The long hydrate induction time and limited gas-liquid contact area leads to slow hydrate formation rate and low water-hydrate conversion rate.Porous media are often used to promote hydrate formation because of their large specific surface area.Consequently,we used 3A molecular sieve as a water-carrying solid in this work,and investigated the dynamic renewal of the gas-liquid interface and its effect on hydrate formation.The formation kinetics of ethane hydrate was first measured in an aqueous molecular sieve system.Then the separation of(H_(2)+CH_(4)+C_(2)H_(6)+C_(3)H_(8))gas mixture was conducted via hydrate formation.The results show that the formation rate and gas storage capacity of ethane hydrate can be greatly improved by using aqueous molecular sieve.Compared with a pure water system under the same temperature and pressure,aqueous molecular sieve has obvious advantages in separation effect and energy consumption for separating gas mixtures.

Accelerated methane storage in clathrate hydrates using surfactantstabilized suspension with graphite nanoparticles

In this study,enhanced kinetics of methane hydrate formation in the sodium dodecyl sulfate(SDS)solution with different concentrations of suspended graphite nanoparticles(GNPs)were investigated at 6.1-9.0 MPa and 274.15 K.The GNPs with rough surfaces and excellent thermal conductivity not only provided a considerable number of microsites for hydrate nucleation but also facilitated the fast hydrate heat transfer in the suspension system.At a relatively low pressure of 6.1 MPa,the suspension with 0.4 wt%of GNPs exhibited the minimum induction time of 22 min and maximum methane uptake of 126.1 cm3·cm-3.However,the methane storage performances of the suspensions with higher and lower concentrations of GNPs were not satisfactory.At the applied pressure,the temperature increase arising from the hydrate heat in the suspension system with the optimized concentration(0.4 wt%)of GNPs was more significant than that in the traditional SDS solution.Furthermore,compared with those of the system without GNPs,enhanced hydration rate and storage capacity were achieved in the suspensions with GNPs,and the storage capacities were increased by 3.9%-17.0%.The promotion effect of GNPs on gas hydrate formation at low pressure is much more obvious than that at high pressure.

Structural-Energy Characteristics of Tribotechnical Contact in Unsteady Operational Modes

The current tribological research is intended to achieve maximum wear resistance under the structural adaptability of tribocoupling elements, which requires application of means to reduce the activation of the metal surface layers, decrease in frictional work, regulation of passivation and temperature control. The aim of this study is to identify the patterns that influence the kinetics formation of the boundary layers of lubricating mineral gear oil on activated friction contact surfaces, and the increment of the friction specific work on wear-resistant steel 42Cr4 and 100Cr6 in frequent start and stop operation mode. Due to the activation of surface layers of metal in the non-stationary operating conditions of the contact surfaces, the gradual forming of the boundary lubricant adsorption layers with increased effective viscosity in contact occurs, exhibiting high adaptation ability, and the boundary layer thickness is from 0.2 to 4 microns. This research analyzed the lubricating ability of oil at the starting maximum torque of friction, showing that the thickness of the oil layer formed in contact had a dual nature—boundary and hydrodynamic. The pure rolling conditions promote localization of shear in the lubricating layers, and the high frictional properties of the transmission oil have been identified regardless of the hardness of the investigated surfaces.

Effects of Vanadium on Structure and Tensile Properties of Tempcore Steel Bars

Thermomechanical processing is a metallurgical operation to produce high-strength steel bars (rebars), through combining plastic deformation with thermal processes like heat treatment, water quenching, heating, and cooling at various rates into a single process. Ribbed reinforcing steel bars (rebars) are used for the reinforcement of concrete structures. Tempcore is a unique process to produce high-yield-strength rebars from mild steel without addition of a high weight percentage of costly alloying elements. The strength of rebar originates from the formation of a surface layer consisting of quenched and tempered martensite that surrounds a core composed of ferrite and pearlite. The economic advantages of this process are significant in comparison to those processes requiring alloying elements or further metal working to improve the mechanical properties. However, when there is a limitation in the water-cooling capacity, the required volume fraction of the martensite layer can’t be accomplished particularly when rolling bigger diameters of 32 mm - 40 mm at a higher rolling speed to maintain high productivity. Accordingly, a small addition of microalloying elements vanadium or niobium could be used in combination with Tempcore process to obtain high-strength steel rebars. In this contribution, 0.06 weight percentage of vanadium is added to the Tempcore treated rebars to satisfy ASTM A 706 Standard of Rebar Grade 80 PSI [550 MPa]. In order to decrease the trials in the steel plant floor, thermodynamics equilibrium calculations are predicted by Thermo-Calc, CCT, TTT diagrams are calculated by JMat Pro and the kinetics evolution of the vanadium carbonitrides precipitates are predicted by the computational database Mat Calc. High yield strength and tensile strength are obtained due to the effect of fine dispersions of nanometer-scale vanadium carbonitrides precipitates inspected by transmission electron microscope.

Linear free energy relationships between reaction rate constants and equilibrium constants of complex compounds——III. Kinetics and mechanisms of ternary complex formation between (5-X-1, 10-phenanthroline)copper(II) and threonine

The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO_2, Cl, H, CH_3) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, k_f(M^(-1).s^(-1)), for the complexation reaction, CuA + LCuAL, are as follows; X=NO_2, 8.68×10~8; X=Cl, 7.13×10~8; X=H, 6.12×10~8; X=CH_3, 5.42×10~8. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability(logK_(CuAL)^(CuA) of the complexes CuAL and log kf as follows: IogK_(CuAL)^(CuA)=0.13 + 0.83 logk_f, r=0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(k_f^R/k_F^O) = 0.097σ, r=0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.

Gas hydrate formation in fine sand

Gas hydrate formation from two types of dissolved gas (methane and mixed gas) was studied under varying thermodynamic conditions in a novel apparatus containing two different natural media from the South China Sea. The testing media consisted of silica sand particles with diameters of 150-250 μm and 250-380 μm. Hydrate was formed (as in nature) in salt water that occupies the interstitial space of the partially water-saturated silica sand bed. The experiments demonstrate that the rate of hydrate formation is a function of particle diameter, gas source, water salinity, and thermodynamic conditions. The initiation time of hydrate formation was very short and pressure decreased rapidly in the initial stage. The process of mixed gas hydrate formation can be divided into three stages for each type of sediment. Sand particle diameter and water salinity also can influence the formation process of hydrate. The conversion rate of water to hydrate was different under varying thermodynamic conditions, although the formation processes were similar. The conversion rate of methane hydrate in the 250-380 μm sediment was greater than that in the 150-250μm sediment. However, the sediment grain size has no significant influence on the conversion rate of mixed gas hydrate.