Guangqiang Li

Effect of Inclusions’ Behavior on the Microstructure in Al-Ti Deoxidized and Magnesium-Treated Steel with Different Aluminum Contents

To clarify the precipitation behavior of beneficial inclusions and mechanism of their effects on microstructure, the effect of aluminum content on inclusion’s characteristics and their influence on the refinement of microstructure in Al-Ti complex deoxidized magnesium-treated steels were systematically investigated based on experiment and calculation. The results showed that due to the dual effects of Ti and Mg deoxidation, a large amount of finely dispersed Al2O3-TiOx-MgO inclusions in low aluminum steel with a complex multilayer or mosaic structure were formed, whereas a relatively smaller amount of Al2O3-MgO inclusions with the simple bundle structure were observed in high aluminum steel. The Al2O3-TiOx-MgO core oxides are more conducive to the precipitation of multiple manganese sulfides with thinner thickness on their local surfaces. Thus, the inclusion deformation, which mainly depends on the surface manganese sulfides layer, is smaller in low aluminum steel than that in high aluminum steel. Complex inclusions in low aluminum steel can pin austenite grain boundaries and induce interlocking acicular ferrite effectively. In addition to the small size and chemical composition of inclusions, the complex structure of oxides and the precipitation of multiple MnS on their surface are important to the nucleation of interlocking AFs on inclusions in Ti-deoxidized Mg-treated steel. The AFs quantity is much more, and the grain size is more uniform in low aluminum steel than that in high aluminum steel.

The effect of methane decomposition on the formation and magnetic properties of iron carbide prepared from oolitic hematite

A chemical metallurgical method was used to prepare iron carbide from high phosphorus oolitic hematite in an atmosphere of H2/CH4. The relationship between the rate of methane decomposition and the iron carbide formation was discussed by using the Mossbauer spectrum to accurately determine the content change of iron carbide with time in the carburized product. Moreover, the carburized samples were characterized by transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD), and the magnetic properties were examined by a vibrating sample magnetometer (VSM). The results show that 1023 K is the optimum temperature for iron carbide preparation from high phosphorus oolitic hematite. After deep reduction in the initial period of contact with CH4, the conversion of deposited carbon to the iron carbide rises rapidly, and then declines quickly. A polycrystalline electron diffraction analysis reveals that iron carbide, graphite and quartz are in the carburized sample. The iron carbide prepared from high phosphorus oolitic hematite in an H2/CH4 atmosphere is a soft magnetic material with relatively high magnetic properties.

Colossal Volume Contraction in Strong Polar Perovskites of Pb(Ti,V)O3

The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb(Ti1-xVx)O3 by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (c/a) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti-O and Pb-O2 bonds from first-principles calculations. As a consequence, Pb(Ti0.9V0.1)O3 exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion αV = -3.76 × 10-5/°C (25-550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb(Ti0.7V0.3)O3 during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferroelectrostriction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics.

Elemental Diffusion and Service Performance of Bi2Te3-Based Thermoelectric Generation Modules with Flexible Connection Electrodes

In this work, the elemental diffusion and service performance of Bi2Te3-based thermoelectric generation (TEG) modules with flexible Al electrodes were evaluated at a temperature difference of 240°C and a cold junction temperature of 50°C. The results indicated that while the maximum output power (Pmax) and open circuit voltage (U0) first increased rapidly and then decreased gradually with service time, the dynamic inner-resistance (Ri) showed the opposite trend. Obvious defects and elemental diffusion across the interfaces were observed and resulted in the performance degradation of the TEG modules. The Ni barrier layer with a thickness of 8–10xa0μm could not effectively restrain the elemental diffusion for the TEG applications at the high operating temperatures. Al was not suitable as the electrode material for the Bi2Te3-based TEG modules due to its ready absorption of Se from the n-type thermoelectric legs. Encouragingly, we found that the Al electrode could restrain the diffusion of the other elements such as Bi, Te, Sb, Cu, Ni, and I. These results provided insight into the improvement of the service performance of the TEG modules.

Numerical Study on the Effect of Electrode Polarity on Desulfurization in Direct Current Electroslag Remelting Process

In order to clarify the influence of electrode polarity on desulfurization in direct current (DC) electroslag remelting process, a transient three-dimensional coupled mathematical model has been established. The finite volume method was invoked to simultaneously solve the mass, momentum, energy, and species conservation equations. The Joule heating and Lorentz force were fully coupled through calculating Maxwell’s equations with the assistance of the magnetic potential vector. The motion of the metal-slag interface was described by using the volume of fluid approach. An auxiliary metallurgical kinetics module was introduced to determine the thermochemical and the electrochemical reaction rates. A reasonable agreement between the measured data and the simulated results are observed. A longer time and a larger area for the desulfurization can be provided by the metal pool-slag interface when compared with the metal droplet-slag interface. The electrochemical transfer rate at the metal pool-slag interface is positive in the DC reverse polarity (DCRP) remelting, while in the DC straight polarity (DCSP) remelting, the electrochemical transfer rate is negative at this interface. The desulfurization progress in the DCSP remelting thus is fall behind that in the DCRP remelting. The desulfurization rate of the DCRP remelting is around 70 pct and the rate of the DCSP remelting is about 40 pct.

A coupled mathematical model and experimental validation of oxygen transport behavior in the electro-slag refining process

AbstractElectro-slag refining process is widely employed in steel industry for the production of special alloys used in ocean, aeronautics, and nuclear industries. Because of the adverse effect on the ductility of metal, it is critical to remove oxygen in the process. This study established a transient three-dimensional (3D) coupled mathematical model for understanding oxygen transport behavior in the electro-slag refining process. The finite volume method was invoked to simultaneously solve mass, momentum, energy, and species conservation equations. Using the magnetic potential vector, Maxwell’s equations were solved, during which the obtained Joule heating and Lorentz force were coupled with the energy and momentum equations, respectively. The movement of metal–slag interface was described through the application of the volume of fluid (VOF) technique. Additionally, an auxiliary metallurgical kinetic module was introduced to determine the electrochemical reaction rate. An experiment was then conducted to validate the model, where the predicted oxygen contents agreed with the measured data within an acceptable accuracy range. Oxygen redistribution in both fluids is clarified: its transport rate at the metal droplet–slag interface is approximately one order of magnitude larger than that at the metal pool–slag interface. Further, the oxygen content in the metal pool is shown to increase with time, while the content in the slag layer is decreased. In order to effectively remove the oxygen in the metal, one more positive electrode, which is more likely to react with the oxygen, is proposed to be added in the unit.Graphical abstractDistributions of the electric streamlines and the phase distribution at 151.25 s with a current of 1800 An

The Behavior of Phosphorus During Reduction and Carburization of High-Phosphorus Oolitic Hematite with H2 and CH4

High-phosphorus oolitic hematite has not been widely utilized due to high content of phosphorus. Ca3(PO4)2 is the main component containing phosphorus in high-phosphorus oolitic hematite. In the present work, the thermodynamics was studied for Ca3(PO4)2 reduction by H2 gas and then carburization by CH4 gas. The results show that phosphorous in Ca3(PO4)2 cannot be reduced from gangue during the reduction of hematite and the formation of iron carbide at the temperature from 923xa0K to 1073xa0K (650xa0°C to 800xa0°C), in H2 and CH4 atmosphere. Reduction and carburization experiments were carried out. And phosphorus in reduced and carburized specimens was analyzed by EDS and wet chemical method. The results confirmed that phosphorous cannot be reduced during the preparation of iron carbide from this iron ore. So the metallic iron or iron carbide can be prepared without the reduction of phosphorous at relatively low temperature, which can be a new route of utilizing high-phosphorus oolitic hematite. After fine milling–magnetic separation, the 99.47xa0pct of Fe3C-containing material was recovered, but the dephosphorization rate reached to 19.37xa0pct only.

Effect of Nanopowders Addition on the Thermoelectric Properties of n-Type Bi2Te3 Nanocomposites

Bi2Te3 nanopowders with an average grain size of about 40 nm were synthesized by vacuum arc plasma evaporation. The n-type bulk Bi2Te3 nanocomposites were prepared by consolidating the mixtures of these nanopowders and mechanically alloyed powders using plasma activated sintering, and the effect of the addition of nanopowders on the thermoelectric properties of bulk Bi2Te3 nanocomposites was investigated. With increasing the content of the added nanopowders, the electrical resistivity and Seebeck coefficient of the nanocomposites increased and the thermal conductivity decreased. When the content of the added nanopowders was 20 wt%, the thermal conductivity decreased by 21.4% compared with that of the nanopowders-free compounds. A preliminary investigation showed that the addition of nanopowders was an effective way to decrease the thermal conductivity and increase the thermoelectric efficiency.

Thermal Stability of Zone Melting p -Type (Bi, Sb) 2 Te 3 Ingots and Comparison with the Corresponding Powder Metallurgy Samples

During the past few decades, Bi2Te3-based alloys have been investigated extensively because of their promising application in the area of low temperature waste heat thermoelectric power generation. However, their thermal stability must be evaluated to explore the appropriate service temperature. In this work, the thermal stability of zone melting p-type (Bi, Sb)2Te3-based ingots was investigated under different annealing treatment conditions. The effect of service temperature on the thermoelectric properties and hardness of the samples was also discussed in detail. The results showed that the grain size, density, dimension size and mass remained nearly unchanged when the service temperature was below 523xa0K, which suggested that the geometry size of zone melting p-type (Bi, Sb)2Te3-based materials was stable below 523xa0K. The power factor and Vickers hardness of the ingots also changed little and maintained good thermal stability. Unfortunately, the thermal conductivity increased with increasing annealing temperature, which resulted in an obvious decrease of the zT value. In addition, the thermal stabilities of the zone melting p-type (Bi, Sb)2Te3-based materials and the corresponding powder metallurgy samples were also compared. All evidence implied that the thermal stabilities of the zone-melted (ZMed) p-type (Bi, Sb)2Te3 ingots in terms of crystal structure, geometry size, power factor (PF) and hardness were better than those of the corresponding powder metallurgy samples. However, their thermal stabilities in terms of zT values were similar under different annealing temperatures.

Large spontaneous polarization in polar perovskites of PbTiO3–Bi(Zn1/2Ti1/2)O3

PbTiO3–Bi(Zn1/2Ti1/2)O3 is considered to be a promising high-ferroelectric performance material in the Pb/Bi-based perovskite family. In the present study, a whole set of (1 − x)PbTiO3–xBi(Zn1/2Ti1/2)O3 (0 ≤ x ≤ 1) solid solutions have been prepared by the conventional solid-state and high-pressure vs. high-temperature methods. The effect of Bi(Zn1/2Ti1/2)O3 on the crystal structure has been investigated by synchrotron X-ray powder diffraction. Unlike most PbTiO3-based perovskites, the present system exhibits a continuously enhanced tetragonality (c/a) and large spontaneous polarization (PS) properties. The enhanced c/a is ascribed to the large spontaneous polarization displacements induced by the strong Pb/Bi–O hybridization and coupling interactions between Ti/Zn and Pb/Bi cations, which can be evidenced from the lattice dynamics study and first-principles calculations. Accordingly, the TC (i.e., x = 0.5) is expected to be approximately 1000 °C if its perovskite structure can be stabilized. The present study provides a route to obtain large-PS in PbTiO3-based ferroelectric materials by introducing isostructural perovskites with strong polarity.