Kinetics of carbonated decomposition of hydrogarnet with different silica saturation coefficients

Carbonated decomposition of hydrogarnet is one of the vital reactions of the calcification–carbonation method, which is designed to dispose of low-grade bauxite and Bayer red mud and is a novel eco-friendly method. In this study, the effect of the silica saturation coefficient(x) on the carbonation of hydrogarnet was investigated from the kinetic perspective. The results indicated that the carbonation of hydrogarnets with different x values(x = 0.27, 0.36, 0.70, and 0.73) underwent two stages with significantly different rates, and the kinetic mechanisms of the two stages can be described by the kinetic functions R3 and D3. The apparent activation energies at Stages 1 and 2 were 41.96–81.64 and14.80–34.84 k J/mol, respectively. Moreover, the corresponding limiting steps of the two stages were interfacial chemical reaction and diffusion.     

Thermoelectric properties of p-type Bi-Sb-Te compositionally graded thermoelectric materials with different barriers

In order to find more suitable materials as barriers and to improve the thermoelectric properties, p-type (Bi_l-xSb_x)₂ Te₃ (x=0.85, 0.9) two segments compositionally graded thermoelectric materials (CGTM) with different barriers were fabricated by conventional hot pressure method. Metals Fe, Co, Cu and Al were used as barriers between two segments. The effects of different barriers on thermoelectric properties of CGTM were investigated. The results show that metal Fe is more stable and suitable as the barrier.     

Oxidation pathway and kinetics of titania slag powders during cooling process in air

The oxidation pathway and kinetics of titania slag powders in air were analyzed using differential scanning calorimetry(DSC) and thermogravimetry(TG). The oxidation pathway of titania slag powder in air was divided into three stages according to their three exothermic peaks and three corresponding mass gain stages indicated by the respective non-isothermal DSC and TG curves. The isothermal oxidation kinetics of high titania slag powders of different sizes were analyzed using the ln-ln analysis method. The results revealed that the entire isothermal oxidation process comprises two stages. The kinetic mechanism of the first stage can be described as f(α) = 1.77(1-α) [-ln(1-α)]~((1.77-1)/1.77),f(α) = 1.97(1-α) [-ln(1-α)]~((1.97-1)/1.97), and f(α) = 1.18(1-α) [-ln(1-α)]~((1.18-1)/1.18). The kinetic mechanism of the second stage for all samples can be described as f(α) = 1.5(1-α)~(2/3)[1-(1-α)~(1/3)]~(-1). The activation energies of titania slag powders with different sizes(d_1 0.075 mm, 0.125 mm d_2 0.150 mm, and 0.425 mm d_3 0.600 mm) for different reaction degrees were calculated. For the given experimental conditions, the rate-controlling step in the first oxidation stage of all the samples is a chemical reaction. The rate-controlling steps of the second oxidation stage are a chemical reaction and internal diffusion(for powders d_1 0.075 mm) and internal diffusion(for powders 0.125 mm d_2 0.150 mm and 0.425 mm d_3 0.600 mm).     

B1500HS高强钢连续加热过程中奥氏体转变动力学分析

The dilatometric curves of B1500HS high-strength steel at different heating rates were measured by a Gleeble-3800 thermal simulator and analyzed to investigate the effect of heating rate on austenitization. Results show that the value of starting temperature and ending temperature of austenite transformation increase with the rise of heating rates, whereas the temperature interval of austenite formation decreases. The kinetic equation of austenite transformation was solved using the Johnson–Mehl–Avrami model, and the related parameters of the equation were analyzed by the Kissinger method. For those calculations, the activation energy of austenite transformation is 1.01 × 106 J/mol, and the values of kinetic parameters n and ln k₀ are 0.63 and 103.03, respectively. The relationship between the volume fraction of austenite and the heating time at different heating rates could be predicted using the kinetic equation. The predicted and experimental results were compared to verify the accuracy of the kinetic equation. The microstructure etched by different corrosive solutions was analyzed, and the reliability of kinetic equation was further verified from the microscopic perspective.     

Crystallization kinetics of overlapping phases in Se70Te15Sb15 using isoconversional methods

The crystallization kinetics of Se70Te15Sb15 chalcogenide glass was studied by Differential Scanning Calorimetry(DSC) under non-isothermal conditions. This glass was found to have a double glass transition and double overlapped crystalline phases. The overlapped crystalline phases were successfully separated using the Gaussian fit model. The activation energy, Ec, and Avrami index, n, were determined by analyzing the data using the Matausita et. al. method. A strong heating rate depending on the activation energy for the two crystalline peaks was observed. The results indicated that the transformation from amorphous to crystalline phases is a complex process involving different mechanisms of nucleation and growth. The variation of activation energy with crystalline fraction was determined by Kissenger–Akahira–Sunose(KAS) method. The obtained results of directly fitting the experimental DSC data to the calculated DSC curves indicated that the crystallization process of Se70Te15Sb15 glass cannot be satisfactorily described by the Johnson–Mehl–Avrami(JMA) model. Simulation results indicated that the Sestak–Berggren(SB) model is more suitable to describe the crystallization process for the studied glass. The crystalline phases for the two events were identified by using x-ray diffraction(XRD) and scanning electron microscopy(SEM).     

Effect of initial microstructure on austenite formation kinetics in high-strength experimental microalloyed steels

Austenite formation kinetics in two high-strength experimental microalloyed steels with different initial microstructures compris-ing bainite–martensite and ferrite–martensite/austenite microconstituents was studied during continuous heating by dilatometric analysis. Austenite formation occurred in two steps:(1) carbide dissolution and precipitation and (2) transformation of residual ferrite to austenite. Di-latometric analysis was used to determine the critical temperatures of austenite formation and continuous heating transformation diagrams for heating rates ranging from 0.03°C×s?1 to 0.67°C×s?1. The austenite volume fraction was fitted using the Johnson–Mehl–Avrami–Kolmogorov equation to determine the kinetic parameters k and n as functions of the heating rate. Both n and k parameters increased with increasing heat-ing rate, which suggests an increase in the nucleation and growth rates of austenite. The activation energy of austenite formation was deter-mined by the Kissinger method. Two activation energies were associated with each of the two austenite formation steps. In the first step, the austenite growth rate was controlled by carbon diffusion from carbide dissolution and precipitation;in the second step, it was controlled by the dissolution of residual ferrite to austenite.     

Topography,structure,and formation kinetic mechanism of carbon deposited onto nickel in the temperature range from 400 to 850°C

The carbon deposition behavior on nickel particles was observed within the temperature range from 400 to 800°C in a pure methane atmosphere.The topography,properties,and molecular structure of the deposited carbon were investigated using field-emission scanning electron microscopy(FESEM),temperature-programmed oxidation(TPO) technology,X-ray diffraction(XRD),and Raman spectroscopy.The deposited carbon is present in the form of a film at 400–450°C,as fibers at 500–600°C,and as particles at 650–800°C.In addition,the structure of the deposited carbon becomes more ordered at higher temperatures because both the TPO peak temperature of deposited carbon and the Raman shift of the G band increase with the increase in experimental temperature,whereas the intensity ratio between the D bands and the G band decreases.An interesting observation is that the carbon deposition rate is suppressed in the medium-temperature range(M-T range) and the corresponding kinetic mechanism changes.Correspondingly,the FWHM of the G and D1 bands in the Raman spectrum reaches a maximum and the intensities of the D2,D3,and D4 bands decrease to low limits in the M-T range.These results indicate that carbon structure parameters exhibit two different tendencies with respect to varying temperature.Both of the two group parameters change dramatically as a peak function with increasing reaction temperature within the M-T range.     

Preparation and radar-absorbing properties of Al_2O_3/TiO_2/Fe_2O_3/Yb_2O_3 composite powder

Al_2O_3/Ti O_2/Fe_2O_3/Yb_2O_3 composite powder was synthesized via the sol–gel method. The structure,morphology,and radar-absorption properties of the composite powder were characterized by transmission electron microscopy,X-ray diffraction analysis and RF impedance analysis. The results show that two types of particles exist in the composite powder. One is irregular flakes(100–200 nm) and the other is spherical Al_2O_3 particles(smaller than 80 nm). Electromagnetic wave attenuation is mostly achieved by dielectric loss. The maximum value of the dissipation factor reaches 0.76(at 15.68 GHz) in the frequency range of 2–18 GHz. The electromagnetic absorption of waves covers 2–18 GHz with the matching thicknesses of 1.5–4.5 mm. The absorption peak shifts to the lower-frequency area with increasing matching thickness. The effective absorption band covers the frequency range of 2.16–9.76 GHz,and the maximum absorption peak reaches-20.18 d B with a matching thickness of 3.5 mm at a frequency of 3.52 GHz.     

Ultrafast dynamics and dissociative ionization of CS<Subscript>2</Subscript> molecules studied via the femtosecond pump-probe method

The ultrafast dynamics and dissociative ionization of CS2 were studied using the pump-probe method with time-of-flight mass spectroscopy. The transient behavior of both parent ion (CS2+) and fragment ions (S+ and CS+) was observed. It was found that all the ionic signals decay exponentially with lifetimes that were different for delay times, t>0 and t<0, which can be attributed to the evolution of different Rydberg states pumped by 267-nm and 400-nm laser pulses. The lifetimes of two Rydberg states were obtained simultaneously from one fitting of the transients. The fragment ions were produced by the dissociation of CS2+, and it is suggested that the final ionic state is the C2Σg+ state of CS2+ based on the measured S+/CS+ branching ratio. The S+/CS+ ratio is dependent on the delay time of the two lasers, indicating that the dissociation process of CS2+ is related to the evolution of the intermediate Rydberg state.     

Staged reaction kinetics and characteristics of iron oxide direct reduction by carbon

Staged reduction kinetics and characteristics of iron oxide direct reduction by carbon were studied in this work. The characteristics were investigated by simultaneous thermogravimetric analysis, X-ray diffraction (XRD), and quadrupole mass spectrometry. The kinetics parameters of the reduction stages were obtained by isoconversional (model-free) methods. Three stages in the reduction are Fe₂O₃→Fe₃O₄, Fe₃O₄→FeO, and FeO→Fe, which start at 912 K, 1255 K, and 1397 K, respectively. The CO content in the evolved gas is lower than the CO₂ content in the Fe₂O₃→Fe₃O₄ stage but is substantially greater than the CO₂ contents in the Fe₃O₄→FeO and FeO→Fe stages, where gasification starts at approximately 1205 K. The activation energy (E) of the three stages are 126–309 kJ/mol, 628 kJ/mol, and 648 kJ/mol, respectively. The restrictive step of the total reduction is FeO→Fe. If the rate of the total reduction is to be improved, the rate of the FeO→Fe reduction should be improved first. The activation energy of the first stage is much lower than those of the latter two stages because of carbon gasification. Carbon gasification and Fe_xO_y reduction by CO, which are the restrictive step in the last two stages, require further study.     

Limitation of the Johnson-Mehl-Avrami equation for the kinetic analysis of crystallization in a Ti-based amorphous alloy

The primary crystallization of the Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ amorphous alloy was studied by isochronal differential scanning calorimetry (DSC). The activation energy was determined by the Kissinger-Akahira-Sunose method. Trying to analyze the crystallization kinetics of the Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ amorphous alloy by two different methods, it was found that the crystallization kinetics did not obey the Johnson-Mehl-Avrami equation. A modified method in consideration of the impingement effect was proposed to perform kinetic analysis of the isochronal crystallization of this alloy. The kinetic parameters were then obtained by the linear fitting method based on the modified kinetic equation. The results show that the isochronal crystallization kinetics of the amorphous Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ alloy is heating rate dependent, and the discrepancy between the Johnson-Mehl-Avrami method and the modified method increases with the increase of heating rate.     

Thermoelectric properties of p-type (Bi0.15Sb0.85)2Te3-PbTe graded thermoelectric materials with different barriers

The p-type (Bi_0.15Sb_0.85)₂Te₃ and PbTe are typical thermoelectric materials used for low and middle temperature range and functional graded materials (FGM) is an inevitable way to widen the working temperature range. Here two segments graded thermoelectric materials (GTM) consisting of (Bi_0.15Sb_0.85)₂Te₃, PbTe and different barriers were fabricated by the common hot pressure method. Metals Fe, Mg and Ni were used as barriers between the two segments. The diffusion of different barriers between the barriers and bases were analyzed by electron microprobe analysis (EMA). The phase and crystal structures were determined by X-ray diffraction analysis (XRD). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The results show that the compositional diffusion occurs when there is no barrier at the interface of the two segments, and the diffusion of Pb is most obvious; as the barrier material, the diffusion of metals Fe, Mg and Ni between different bases is not very obvious, and the thermoelectric properties of GTM is much better than that of the original segment.     

Thermoelectric Properties of N-typer Bi2Te3-PbTe Graded Thermoelectric Materials with Different Barriers

In order to investigate the adaptability of thermoelectric materials system with different barriers to functional graded thermoelectric materials, n-type Bi₂Te₃ and PbTe two segments graded thermoelectric materials (GTM) with different barriers were fabricated by conventional hot pressing method. Metals Cu, Al, Fe, Co and Ni were used as barriers between two segments. The effects of different barriers on thermoelectric properties of GTM were investigated. The phase and crystal structures were determined by x-ray diffraction analysis (XRD). The distributions of different compositions were analyzed by electron microprobe analysis (EMA). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The electric conductivity of samples was measured at 303 K by the four-probe technique. To measure the Seebeck coefficient, heat was applied to the samples, which were placed between two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature differences (△T<275 K) between the both ends of the samples. The Seebeck coefficient of the samples was determined from the E/△T.     

Effect of Nb on the transformation kinetics of low carbon (manganese) steel during deformation of undercooled austenite

The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 (80% reduction) in Nb-free and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel,the transformation incubation period of Nb-bearing steel was pro-longed and the transformation kinetics curves parallelly moved to higher strain because of the solute Nb drag effect. Studies on kinetics also showed that the deformation-enhanced ferrite transformation (DEFT) of the two steels were composed of three stages,which can be expressed by the J-M-A equations individually. However,the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels,and the same in the third stage for both the steels corresponding to the nuclea-tion of retained austenite.     

Slag formation path during dephosphorization process in a converter

The slag formation path is important for efficient dephosphorization in steelmaking processes. The phosphorus capacity and the melting properties of the slag are critical parameters for optimizing the slag formation path. Regarding these two factors, the phosphorus par-tition ratio was calculated using the regular solution model (RSM), whereas the liquidus diagrams of the slag systems were estimated using the FactSage thermodynamic package. A slag formation path that satisfies the different requirements of dephosphorization at different stages of dephosphorization in a converter was thus established through a combination of these two aspects. The composition of the initial slag was considered to be approximately 15wt%CaO–44wt%SiO2–41wt%FeO. During the dephosphorization process, a slag formation path that fol-lows a high-iron route would facilitate efficient dephosphorization. The composition of the final dephosphorization slag should be approxi-mately 53wt%CaO–25.5wt%SiO2–21.5wt%FeO. The composition of the final solid slag after dephosphorization is approximately 63.6wt%CaO–30.3wt%SiO2–6.1wt%FeO.     

Metal deformation in skew rolling step-shaft

The process of rolling step-shaft was simulated with the help of the FEM (Finite-Element Model) software-DEFORMTM 3D. The characteristics of metal flow in different periods were analyzed, and the curve of variety in non-round area at different crosssections of the rolled piece, the curve of variety in bulge volume and the curve of variety in non-column volume were obtained. On this base the method of tool design is worked out and validated by experiment.     

Process mineralogy as a key factor affecting the flotation kinetics of copper sulfide minerals

The aim of this study is to apply process mineralogy as a practical tool for further understanding and predicting the flotation kinetics of the copper sulfide minerals. The minerals' composition and association, grain distribution, and liberation within the ore samples were analyzed in the feed, concentrate, and the tailings of the flotation processes with two pulp densities of 25 wt% and 30 wt%. The major copper-bearing minerals identified by microscopic analysis of the concentrate samples included chalcopyrite(56.2 wt%), chalcocite(29.1 wt%),covellite(6.4 wt%), and bornite(4.7 wt%). Pyrite was the main sulfide gangue mineral(3.6 wt%) in the concentrates. A 95% degree of liberation with d_(80) 80 μm was obtained for chalcopyrite as the main copper mineral in the ore sample. The recovery rate and the grade in the concentrates were enhanced with increasing chalcopyrite particle size. Chalcopyrite particles with a d_(80) of approximately 100 μm were recovered at the early stages of the flotation process. The kinetic studies showed that the kinetic second-order rectangular distribution model perfectly fit the flotation test data. Characterization of the kinetic parameters indicated that the optimum granulation distribution range for achieving a maximum flotation rate for chalcopyrite particles was between the sizes 50 and 55 μm.     

STUDY ON SLOW-RELEASE CHARACTERISTICS OF MICROCAPSULES WITH EMULSION NON-SOLVENT ADDITION METHOD

An O / W emulsion non-solvent addition method was used to prepare ethyleellulose (EC) microcapsules of water soluble pharmaceutical (theophylline). The solvent and non-solvent reagents used in this work were toluene and cyclohexane. The effects of polymer concentration, core wall ratio and particle size on the kinetics as well as the dissolution rate of the drug were investigated. The results show that the dissolution rate increases with the increasing of polymer concentration and core wall ratio. The release of microcapsules prepared with different procedures can be selectively profiled with first order and Higuchi matrix kinetic models.     

Phase-field simulation on microstructure evolution of D0_(19) phase in γ/γ′structure of Co–Al–W superalloys

The morphological evolution and precipitation kinetics of γ′ and D0_(19)(Co_3 W) phase in Co–Al–W alloys at 900 °C have been studied by applying Phase-field method and experiment in order to understand the transformation process of γ′ phase and D0_(19) phase. The growth processes of D0_(19) phase and precipitation of γ′ phase under elastic fields were simulated through coupling with thermodynamics and dynamics databases. The simulation results indicate that the misfit δ≥ 0.53% has a greater impact on γ′ particle morphology in γ/γ′ structure.Co–Al–W alloy with low Al and high W is one of the factors to promote the precipitation of D0_(19) phase. Three stages during aging, namely the γ′ phase incubation stage, the γ′ phase fast nucleation and growth stage, and the transformation from γ′ phase to D0_(19) phase stage can be observed with the non-constant coarsening rate that varying with the decrease of γ′ phase. The particle size distribution(PSD) during the precipitation of D0_(19) phase is more in line with MLSW theory than LSW theory. This simulation results are in good agreement with the experiment results to help analyze microstructure evolution of Co–Al–W alloy.     

Structural characteristics and mechanism of hydrogen-induced disproportionation of the ZrCo alloy

The intermetallic compound ZrCo was prepared, and its hydrogen-induced disproportionation in hydrogen desorption processes was investigated. The hydrogenation-dehydrogenation thermodynamics of the ZrCo alloy was evaluated by pressure-composition isotherm measurements at different temperatures. The kinetic processes of hydrogen-induced disproportionation at different temperatures under certain pressures were detailedly studied. The disproportionation rate of the ZrCo alloy increased with the increases of temperature and initial hydrogen pressure under experimental conditions. However, the maximum attainable extent of disproportionation did not change much with an increase in temperature or initial hydrogen pressure. The crystallographic structure analysis of the ZrCo alloy combining with its corresponding dehydrogenation kinetic curves under the conditions of an initial hydrogen pressure of 0.2 MPa and a temperature of 723 K indicated that the basic process of disproportionation reactions was composed of four stages:rapid dehydrogenation of ZrCoH₃, equilibrium of dehydrogenation, simultaneity of dehydrogenation and disproportionation, and completion of disproportionation.