Journal of University of Science and Technology Beijing, Mineral, Metallurgy, Material最新文献

The effects of Cu addition and annealing treatment on the magnetic properties and microstructure of Nd_12.3Fe_81.7–xCu_xB₆ (x=0–1.2) ribbons melt-spun and annealed were systematically investigated by the methods of vibrating sample magnetometer (VSM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Optimum magnetic properties were achieved by annealing melt-spun Nd_12.3Fe_81.5Cu_0.2B₆ ribbons at 550°C for 15 min, which only contained Nd₂Fe₁₄B phase. The remanence, coercive force, and maximum energy product increase by 18.4%, 36.2%, and 49% respectively compared with those of Cu-free samples. The significant improvement in magnetic properties originates from the finer grains of the samples by introducing Cu, which leads to the stronger exchange-coupling between neighboring grains.

In-situ stress measurement using the hydraulic fracturing technique was made at Wanfu Coal Mine in Shandong Province, China. To solve problems caused by great measuring depth and extra thick overburden soil layers in the mine, a series of improved techniques were developed for the traditional hydraulic fracturing technique and equipment to increase their pressure-enduring ability and to ensure safe and flexible removal of the sealing packers with other experimental apparatus. Successful in-situ stress measurement at 37 points within 7 boreholes, which were mostly over 1000 m deep, was completed. Through the measurement, detailed information of in-situ stress state has been provided for mining design of the mine. The improved hydraulic fracturing technique and equipment also provide reliable tools for in-situ stress measurement at great depth of other mines.

Olivine LiFePO₄/C composite cathode materials were synthesized by a solid state method in N₂ + 5vol% H₂ atmosphere. The effects of different iron sources, including Fe(OH)₃ and FeC₂O₄·2H₂O, on the performance of as-synthesized cathode materials were investigated and the causes were also analyzed. The crystal structure, the morphology, and the electrochemical performance of the prepared samples were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), laser particle-size distribution measurement, and other electrochemical techniques. The results demonstrate that the LiFePO₄/C materials obtained from Fe(OH)₃ at 800°C and FeC₂O₄·2H₂O at 700°C have the similar electrochemical performances. The initial discharge capacities of LiFePO₄/C synthesized from Fe(OH)₃ and FeC₂O₄·2H₂O are 134.5 mAh·g⁻¹ and 137.4 mAh·g⁻¹ at the C/5 rate, respectively. However, the tap density of the LiFePO₄/C materials obtained from Fe(OH)₃ are higher, which is significant for the improvement of the capacity of the battery.

The experimental results of the thermal conductivities of xonotlite-type calcium silicate insulation materials were presented at different temperatures and pressures. Two appropriative surroundings, i.e. an elevated temperature surrounding from ambient temperature to 1450 K and a vacuum surrounding from atmosphere pressure to 10⁻³ Pa, were designed for the transient hot-strip (THS) method. The thermal conductivities of xonotlite-type calcium silicate with four densities from ambient temperature to 1000 K and 0.045 Pa to atmospheric pressure were measured. The results show that the thermal conductivity of xonotlite-type calcium silicate decreases apparently with the fall of density, and decreases apparently with the drop of pressure, and reaches the least value at about 100 Pa. The thermal conductivity of xonotlite-type calcium silicate increases almost linearly with T³, and increases more abundantly with low density than with high density. The thermal conductivity measurement uncertainty is estimated to be approximately 3% at ambient temperature, and 6% at 800 K.

LiAlH₄ doped with Ni and Ce(SO₄)₂ additives and the effect of doping on temperature and hydrogen release were studied by pressure-content-temperature (PCT) experiment and X-ray diffraction (XRD) analysis. It is indicated that doping with Ni induces a significant decrease in temperature in the first step and LiAlH₄ doped with 1mol% Ni presents the most absorption of hydrogen. Doping with Ce(SO₄)₂ also causes a marked decrease, while the amount of hydrogen release changes only slightly. The results from X-ray diffraction analysis show that doping does not cause any structural change; Ni and Ce-containing phases are not observed at room temperature or even at 250°C.

In order to investigate stress corrosion cracking (SCC) of X70 pipeline steel and its weld joint area in acidic soil environment in China, two simulating methods were used: one was to obtain bad microstructures in heat affected zone by annealing at 1300°C for 10 min and then, quenching in water; the other was to get different simulating solutions of acidic soil in Yingtan in southeast China. The SCC susceptibilities of X70 pipeline steel before and after quenching in the simulating solutions were analyzed using slow stain rate test (SSRT) and potentiodynamic polarization technique to investigate the SCC electrochemical mechanism of different microstructures further. The results show that SCC appears in the original microstructure and the quenched microstructure as the polarization potential decreases. Hydrogen revolution accelerates SCC of the two tested materials within the range of −850 mV to −1200 mV vs. SCE. Microstructural hardening and grain coarsening also increase SCC. The SCC mechanisms are different, anodic dissolution is the key of causing SCC as the polarization potential is higher than the null current potential, and hydrogen embrittlement will play a more important role to SCC as the polarization potential lower than the null current potential.

In order to prevent carbon/carbon composites from oxidation at 1873 K, an efficient oxidation protective SiC/Si-W-Mo coating was prepared by a two-step pack cementation technique. The microstructures and the phase composition of the as-received multi-coating were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is seen that the compact multi-coating is composed of α-SiC, Si, and (W_xMo_1–x)Si₂. Oxidation test shows that, after oxidation at 1873 K in air for 102 h and thermal cycling between 1873 K and room temperature for 10 times, the weight loss of the SiC/Si-W-Mo coated C/C composites is only 0.26%. The invalidation of the multi-coating is attributed to the formation of penetrable cracks in the coating.

The ultra-fine bainitic microstructure of a 900 MPa low carbon bainitic Cu-Ni-Mo-B steel was obtained by a newly developed relaxation precipitation control (RPC) phase transformation processing. In a pan-cake like prior-austenite grain, the microstructure consisted of lath bainite, a little of abnormal granular bainite, and acicular ferrite. The effect of zirconium carbonitrides on the austenite grain coarsening behavior was studied by transmission electron microscopy (TEM). The results show that, the lath is narrower with increasing cooling rate. The ratio of all kinds of bainitic microstructure is proper with the intermediate cooling rate; and Zr-containing precipitates distribute uniformly, which restrains austenite grain growing in heat-affected welding zone.