Papers by Keyword: High Pressure

Abstract: Using density function theory (DFT), evolutionary simulations for crystal structure prediction, the most stable compositions of Mg₂Si in the pressure range 0–30 GPa were obtained. The DFT results reproduce experimentally observed pressure-induced phase transitions and predict a new high-pressure structure Cmcm (space group 63).

Abstract: In the present study, we present a novel method to sinter Cr₃C₂ powders under high pressure without any addittives. The sintering Cr₃C₂ samples were charaterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), relative density measurements, Vicker’s hardness tests and Fracture toughness tests. The reasults show that Cr₃C₂ powders could be sintered to be bulk under the conditions of 3-5 GPa, 800-1200 °C and the heat preservation for 15 min. Moreover, the sintering body of Cr₃C₂ compound with the relative density of 99.84% by simultaneously tuning the pressure-temperature conditions exhibited excellent mechanical properties: a Vickers hardness of 20.3 GPa and a fracture toughness of ~8.9 MPam^1/2. These properties were much higher than that by using the previous methods. The temperature condition obtained good mechanical properties in the experiment was about 1/3 lower than that using any other methods owing to the high pressure.

Abstract: This paper aims to study the sintering process and mechanical properties of submicron polycrystalline diamond (SMPD) without any secondary phases and binder materials under pressure of 7-8 GPa and 1400 °C-1800 °C, using the bi-layer assembly and the conventional assembly methods. The as prepared samples were characterized by X-ray diffraction, scanning electron microscope, and Vickers indenter hardness tests. Well sintered specimen was obtained under the condition of 8 GPa and 1600 °C using the bi-layer assembly method, and an indentation test demonstrated a Vickers hardness of 52 GPa. The graphitization of diamond was found to be an important factor determining the hardness of samples sintered using the bi-layer assembly.

Abstract: With the depletion of low carbon dioxide (CO₂) content natural gas reserves, there is a pressing need to explore the vastly undeveloped high CO₂ content natural gas reserves and reduce the release of greenhouse gas CO₂ into environment. Our previous investigation on the absorption performance of CO₂ at high concentration level of 50% from mixture of CO₂-natural gas stream for 20wt% monoethanolamine (MEA) solution in countercurrent packed column indicated efficient removal at high pressure condition. In this present work, a combination mass transfer, chemical reaction of MEA as well as mass conservation equation was developed to model the removal behavior of the high pressure, high concentration CO₂ capture along the absorption column. The model developed in this study had satisfactorily represented the mass transfer behavior for high pressure and high CO₂ concentration gas removal along the absorption column.

Abstract: Super-high pressure (SHP) changes crystal structure and electronic distribution of metallic materials, which plays an important role in properties. Herein, a duplex Mg-7%wt.Li alloy was heat-treated under SHP (2 GPa) by cubic-anvil large-volume press with six rams for 2 h in the temperature range of 450~1350 °C. Microstructure, phase transformation behavior and mechanical properties were examined. Compared with the as-cast sample, the SHP samples after heat-treating from 450 °C to 750 °C under 2 GPa were composed of twinning in addition to duplex structure. Comparatively, the samples treated between 1050 °C and 1350 °C exhibit typical dendritic morphology. Phase transformation from Li₃Mg₇ phase or Li_0.92Mg_4.08 phase to Li₃Mg₁₇ phase occurred during the whole investigated temperature range, in which only the Li₃Mg₁₇ phase maintained when the temperature exceeds 1050 °C. The microhardness of the sample prepared at 750 °C under 2 GPa was 73.15HV, which is 1.5 times higher than that of the as-cast one. The improved microhardness is mainly attributed to the formation of nanosized twins during SHP treatment. These fine twins effectively prohibit the dislocation movement during deformation. It reveals the SHP is an effective approach to prepare high performation Mg alloys.

Abstract: Aiming to characteristics of structure and transmitting medium of gas pipe with high temperature and high pressure, considering the coupling effect of static pressure and temperature, the thermal structure coupling model based on steady-state heat conduction is established according to the finite element theory. So method and process of numerical simulation on thermal structure coupling analysis are proposed. Simulation analysis is carried out for the gas pipe of a dry-gas compressor. The variation characteristic of inner-outer wall is revealed under the different temperature. The results indicate that the numerical modeling and simulation of thermal structure coupling of gas pipe with high temperature and high pressure can be realized. It can provide valuable reference opinions for site construction, real-time monitoring and safety analysis of pipe.

Abstract: Steam turbines are complex rotating machines working at high pressure and high temperature levels. Their high-pressure parts, which are subjected to the highest steam parameters, are most affected by these conditions and may suffer from creep deformation. Permanent changes in geometry become visible in high-pressure turbine casings when they are disassembled after certain time in operation.

Abstract: The general characteristic of current plate heat exchangers is first summarized in this paper. A new plate exchanger of multi-stream under high pressure is proposed, which is composed of the plates with opening the gyroidal flow-trough along the radius and the shell, it has some advantages, such as simple structure, high efficiency and available under high pressure. We introduce the structure of the plate, the shell and the slotting types. In addition, the process calculation method of heat-transfer is given and the strength calculation method for the key components of the novel exchanger is also presented. According to the stress analysis to the plate wall with ANSYS software, we have proved the reliability of the calculation method.

Abstract: Auger and SEM studies show that with increasing of MgB₂ manufacturing temperature from 600÷800 °C to 1050÷1100 °C the Mg-B-O nanolayers which are present in the MgB₂ matrix transform into distinct dispersed Mg-B-O inclusions. On the other hand the sizes of inclusions of higher magnesium borides (MgB_x, x=7 ÷ 25) which are also present in the MgB₂ matrix. The tendency is observed in a wide range of synthesis pressures (0.1 MPa-2 GPa). The described structural transformations are accompanied by an increase in critical current density, j_c, in low and medium magnetic fields and by transition from the grainboundary to the point pinning. The Ti addition results in a further increase in j_c due to: Ti promotes the formation of higher magnesium boride inclusions and localization (or segregation) of oxygen in MgB₂ matrix, and, hence, facilitates the formation of a homogeneous MgB₂ matrix with lower oxygen content, but with an increased number of Mg-B-O and MgB_x pinning centers. At low synthesis temperature Ti absorbs hydrogen forming titanium hydrides, thus preventing the formation of MgH₂ and provides the material densification. The positive effect of Ti addition is connected with the high ability of Ti to absorb hydrogen, oxygen, and magnesium. The results of the critical current and AC loss study by transformer method using rings from MgB₂ are discussed.

Abstract: The DTA and TG study in air of Ti₂Al (C_1-xN_x) and Ti₃AlC₂ synthesized under Ar 0.1 MPa pressure and densified in thermobaric conditions at 2 GPa, 1400 °C, for 1 h showed that the increase of the amount of TiC layers in Ti-Al-C MAX phases structures leads to the increase of their stability against oxidation: 321 MAX phase Ti₃AlC₂ are more stable than Ti₂AlC and Ti₂Al (C_1-xN_x) solid solutions both before and after thermobaric treatment. The oxide film formed on the surface of the highly dense (ρ=4.27 g/cm³, porosity 1 %) material based on nanolaminated MAX phase Ti₃AlC₂ (89 % Ti₃AlC2, 6 % TiC, 5 % Al₂O₃) manufactured by hot pressing (at 30 MPa) made the material highly resistant in air at high temperatures: after 1000 hours of exposition at 600 °C it demonstrated a higher resistance to oxidation than chromium ferrite steels (Crofer GPU and JDA types). Due to the surface oxidation self-healing of defects took place. Besides, the Ti₃AlC₂ material demonstrated resistance against high-temperature creep and after being kept in H₂ at 600 °C for 3h its bending strength reduced by 5 % only. At room temperature the Ti₃AlC₂ bulk exhibited microhardness Hμ = 4.6 GPa (at 5 N), hardness HV₅₀ = 630 (at 50 N ) and HRA = 70 (at 600 N), Young modulus was 140 ± 29 GPa, bending strength =500 MPa, compression strength 700 MPa, and fracture toughness K_1C=10.2 MPa·m^0.5.