Table of Content

28 February 2015, Volume 21 Issue 1

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Metallurgical Materials

Correlations between molar fusion entropy and the intrinsic undercoolability of liquid metals

LI Chen-hui, XU Ming-qin, HAN Xiu-jun, LI Jian-guo

2015, 21(1):  1-11.  doi:10.3969/j.issn.1007-2861.2014.04.014

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High undercoolings up to 340 K and 281 K for pure liquid iron are achieved using aerodynamic containerless levitation and a glass fluxing method, respectively. This implies that the nucleation of liquid iron under aerodynamic containerless solidification condition is closer to homogeneous nucleation. Based on the classical nucleation theory and Spaepen’s expression for solid/liquid interfacial energy, a correlation equation between intrinsic undercooling, namely, the undercooling corresponding to homogeneous nucleation and molar entropy is derived. From this equation, intrinsic undercoolings for a series of liquid metals can be predicted. Good agreement exists between the predicted and experimental results.

Microstructure evolution and hydrogen permeation performance of Nb₃₀Hf₃₅Ni_35–xCo_x alloys

LIANG Xiao, GUO Jing-jie, LI Xin-zhong, SU Yan-qing, FU Heng-zhi

2015, 21(1):  12-19.  doi:10.3969/j.issn.1007-2861.2014.04.013

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Nb₃₀Hf₃₅Ni_35–xCo_x (x = 0, 5, · · · , 35) alloys were prepared through tungsten arc-melting. The composition and structure of the alloys was analyzed with scanning electron microscope (SEM) and X-ray diffraction (XRD). Evolution of as-cast structures with the Co content was studied. Hydrogen permeation experiments were conducted at various temperatures and pressure differentials. Relationship between hydrogen permeability and composition of Nb-Hf-Ni (Co) alloys was built. It has been shown that, as the Co content increases, the content of impurities of intermetallic phase in the alloy and volume fraction of the primary bcc-Nb phase gradually reduce, while volume fraction of the bcc-Nb in the eutectic gradually increases. Eutectic morphology changes from rods to lamellas. Hydrogen permeability of the alloys increases with increasing Co content.

Influence of traveling magnetic field on flow in continuous casting of slab

TU Ting-sheng, DENG Kang, ZHANG Zhen-qiang, REN Zhong-ming

2015, 21(1):  20-27.  doi:10.3969/j.issn.1007-2861.2014.04.019

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This paper presents a mercury model experiment and coupling numerical simulation with Reynolds time-average model on molten metal flow in the mold of continuous casting of slab. The purpose is to study the influence of a traveling magnetic field (electromagnetic stirring) on the metal flow in the mold. The result shows that, in the mold, the stirring force from the traveling electromagnetic field makes the molten metal form a horizontal circumfluence, leading to destroy of the originally regular circumfluence. In the whole mold, the flow is three dimensional and non-symmetrical. Furthermore, the traveling magnetic field strengthens fluctuation of meniscus. The fluctuation is stronger on the side where the nozzle flow is opposite to the electromagnetic force.

Effect of sintering temperature and particle size on properties of porous silicon oxide ceramic core materials

XU Zi-ye, XUAN Wei-dong, ZHANG Jin-yao, REN Zhong-ming, WANG Huan,MA Chen-kai, YANG Fan, YU Jian-bo, LI Chuan-jun

2015, 21(1):  28-37.  doi:10.3969/j.issn.1007-2861.2014.04.016

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Samples of porous silicon oxide ceramic core were prepared using a heat-press molding method. The effects of different sintering temperatures and particle sizes on the structures and properties of the porous silicon oxide ceramic core were investigated. The result indicates that, as sintering temperature increases, the room temperature bending strength and high temperature bending strength increase and the porosity decreases. When the sintering temperature is 1 200°C, silicon oxide ceramic core shows the best overall performance for linear shrinkage rate of 2.75%, porosity of 24.69%, room temperature bending strength of 25.3 MPa and the high temperature bending strength of 44.23 MPa. Above 1 200°C, the room temperature bending strength and high temperature bending strength decrease obviously, while linear shrinkage rate and the porosity are stable. According to X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) fracture micrographs of the silicon oxide ceramic core, changes of the room temperature bending strength and high temperature bending strength are attributed to the differences of the compactness and cristobalite content at different sintering temperatures. In addition, the particle size of ceramic core has visible effects on porosity, linear shrinkage rate, and room temperature bending strength. The best overall property of the ceramic core occurrs when 25.33% of the particle sizes are less than 10 μm, 38.16% between 10 μm and 30 μm, 28.74% between 30 μm and 50 μm, and 7.77% greater than 50 μm but no more than 95 μm.

Epoxy resin modified by novel polyphenylene ether

DUAN Jia-zhen1,2, YU Ruo-bing1,2, HU Lin1,2, JIAO Zheng3

2015, 21(1):  38-45.  doi:10.3969/j.issn.1007-2861.2014.01.028

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As a curing agent and modified agent of epoxy resin, modified polyphenylene ether (MPPE) containing hydroxyl group was synthesized, and characterized by Fourier transform infrared spectroscopy (FTIR). The properties of MPPE/E51 were investigated, and the following results were obtained. The curing temperature is gradually decreased with the increase of content of E51. Thermal stability of the epoxy resin thermosets is improved with the maximum T_d5% being 385 °C. The laminated sheet of MPPE/E51 has low dielectric constant (3.51, 1 GHz), dielectric loss (0.008 9, 1 GHz) and low water absorption. Impact strength of MPPE/E51 is up to 63.34 kJ/m².

Sensitive characteristics evaluation of electrochemical H₂S gas sensor

YU Wen-shuang1, LIU Ling2, GUO Yun1, YI Zhi-wei2, QIAN Yong-biao2

2015, 21(1):  46-53.  doi:10.3969/j.issn.1007-2861.2014.03.003

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The sensitive characteristics of electrochemical H₂S sensor are studied by applying six sigma statistical tools. The Minitab software is used to analyze the experimental data. The results show that the gas sensor has a quick response to H₂S gas, the average sensitivity is 0.82 μA/10⁻⁶H₂S, the gas sensor has a good linear relationship with concentration of H₂S in the measurement range of the sensor, the correlation index R_Sq = 99.9%, and the response time T₉₀ < 20 s. The influences of temperature on sensitivity and on the average response value of H₂S sensor are also discussed.

Template preparation of Ag nanoparticle arrays SERS-active substrate and highly sensitive detection of glucose

GUO He-shuai1, FU Qun1, LIN Wei1, ZHENG Xian-zheng1, LU Bo2, WU Ming-hong1, LEI Yong1

2015, 21(1):  54-63.  doi:10.3969/j.issn.1007-2861.2014.03.004

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A highly sensitive method for detecting glucose using Ag nanoparticle arrays surface-enhanced Raman scattering (SERS) active substrates based on the ultra-thin alumina mask (UTAM) was reported. The large-area highly ordered Ag nanoparticle arrays active substrates were fabricated using thermal evaporation with UTAM as a shadow mask in vacuum. The surface of active substrates can adsorb a layer of decanethiol/mercaptohexanol (DT/MH) self-assembled monolayers (SAM) through the surface pretreatment. Therefore the active substrates can be used for detecting glucose molecules. SERS measurement results of the active substrate with glucose molecules as probe molecules show strong SERS performance, and the enhanced signal is uniform and stable. The relative standard deviation (RSD) of the characteristic peak intensity is less than 10%. This preparation method has the advantages of simple operation and low cost.

Comparation of physicochemical characterization of PM_2.5 collected at different heights in the air above campus

ZHANG Rong-chi, LONG Ling, LU Sen-lin, LIU Ding-yu, HAO Xiao-jie, ZHANG Wen-chao, LIU Pin-wei

2015, 21(1):  64-71.  doi:10.3969/j.issn.1007-2861.2014.04.001

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PM_2.5 has become a major air pollutant. Air quality in campus has close relationship with the health of young people. This study focuses on mass concentration and physicochemical characterization of PM_2.5 collected at different heights above a campus. Ambient PM_2.5 particles were sampled synchronously at two heights (5 m and 40 m). Microscopic character and its chemical composition of the fine particles were characterized using scanning electron microscope-energy dispersive spectrometer (SEM-EDS). The following results are obtained. Mineral particles, soot aggregates and fly ash are ubiquitous in the campus PM_2.5, and gypsum particles, unknown particles can also be found. Mass and number concentrations at 5 m are higher than that of PM_2.5 collected at 40 m. The numbers of mineral particles in PM_2.5 collected at 5 m are more than that of fine particles sampled at 40 m. Soot aggregates in the PM_2.5 at 40 m are more abundant than that at 5 m. Both the particle types and the number of mineral particles collected during daytime are less than that sampled at night.

Impacts on geochemical phases of nitrogen and phosphorus in sediments of Xinanjiang River by river impounding

QIAN Hui-jun, GAO Yang, CAO Man, FU Jia-nan, MA Jing, WANG Fu-shun

2015, 21(1):  72-80.  doi:10.3969/j.issn.1007-2861.2014.01.031

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The Xinanjiang Reservoir and the related river reaches were investigated. Sediments were collected along the upstream, reservoir backwater area, central reservoir and downstream the dam. The geochemical phases of phosphorus and nitrogen were determined. The results show that the total phosphorus (TP) and total nitrogen (TN) concentrations in backwater area sediments increase by 37.3% and 34.2% respectively as compared to that in upstream sediments, for the reason of sedimentation and aquaculture input. Due to the impacts of impounding, phosphorus and nitrogen concentrations in downstream sediments decrease by 39.5% and 74.1% respectively as compared to that in reservoir sediments. Among the various forms of phosphorus, the ratio of Ca-bound phosphorus (Ca-P) (13.8%∽31.9%) and Fe-bound phosphorus (Fe-P) (11.5%∽26.2%) in total phosphorus were next only to organic phosphorus (OP) (48%∽69%). The organic phosphorus deposited in the backwater area and central reservoir sediments is gradually decomposed and transformed to Fe-P, leading to the variation in phosphorus forms along the river course. Meanwhile, organic nitrogen (ON) is the main form of TN. Its content is low in the upper reaches sediments, and increases rapidly downward toward the central reservoir, but decreases significantly in the downstream sediments. The percentage of the content of ammoniac nitrogen (NH⁺₄ -N) in TN has less changed along the river channel. The results reveal that the presence of dam has an obvious retention effect on phosphorus and nitrogen, which also changes the stoichiometric ratio of nutrients in sediments.

Variations of nitrate and nitrite contents in vegetables during growth and storage

YAN Xiao-juan1, YAN Li-jun1,2, HU Xue-feng1,3, WANG Jian4

2015, 21(1):  81-87.  doi:10.3969/j.issn.1007-2861.2014.01.034

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Field experiments indicate that nitrate content in Chinese cabbage (brassica chinensis) is significantly raised when applying chemical fertilizers. Moreover, the nitrate content in its petioles is significantly higher than that in leaves. The nitrite content in fresh vegetables is usually less than 1 mg/kg FW (fresh weight), but may be significantly raised when it is stored. Especially, the nitrite content in vegetables sharply increases to several hundred mg/kg FW as soon as the leaves are decayed when it is stored at room temperature, posing a risk to human health. Pretreatments of vegetables before storage also significantly influence of nitrite accumulation. It should be noted that the nitrite content in vegetables stored as homogenate often increases sharply in a short term.

A novel coupon-type electrical resistance probe for environmental corrosion monitoring

FENG Bei1,2, WENG Yong-ji1,3, LI Xiang-yi1,3, LI Jun-hao1, JIAO Zheng2

2015, 21(1):  88-96.  doi:10.3969/j.issn.1007-2861.2104.01.030

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A novel electrical resistance (ER) probe is designed. It includes 4 coupons, 2 of them exposed in a corrosion environment and 2 sealed in epoxy for monitoring the environmental corrosion rates and deviations. A corrosion index P is introduced to eliminate the temperature influence. The mean corrosion rates within individual periods of time are calculated based on the P-t curves. The probe’s sensitivity in thickness is determined based on the P-R_x (corroded coupon’s resistance) curves. In addition, the influences of output line resistance and the pitting factors on the measure errors are investigated. Ratios of two contrast resistances are analyzed with a statistical method to determine the unusual fluctuation, and used as an index for sealing damage alarm. Comparison with weightlessness experiments shows that this kind of ER probes can offer approximately consistent results for quick estimation of the degree of environmental corrosion.

Timoshenko composite beam model analysis of steel beam and brick masonry for foundation underpinning of masonry wall

XIANG Xiao-xiao, WU Li-wei, YANG Xiao

2015, 21(1):  97-105.  doi:10.3969/j.issn.1007-2861.2014.01.011

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Regarding the steel clamping beams and brick masonry between them as a composite beam and based on the Timoshenko elastic beam model, the governing equation for bending deformation of composite beam is established. The analytical solution for bending deformation of the composite beam of steel and brick masonry is presented. Considering the arch effect of the brick masonry, the foundation underpinning of the brick wall of masonry structure is investigated. The maximum deflection and maximum stresses of the steel-brick masonry composite beam for different models of H-type steel clamping beam are obtained, and the maximum length of the foundation underpinning for single stage is given. It is shown that deflection and stresses of the steel-brick masonry composite beam decrease with the model number of the H-type steel beam increasing, but the load held by the steel beam and the confining pressure of the steel-brick masonry composite beam are unchanged. Furthermore, the deflection of the Timoshenko composite beam is larger than that of the Euler composite beam, but the stresses and confining pressure are the same. Therefore, in the foundation underpinning design, the model of an Euler composite beam can be used for strength analysis, and the Timoshenko composite beam model can be used for stiffness analysis.

Simplified calculation of shield tunnel’s equivalent bending rigidity

HUANG Yan-xiang, ZHANG Meng-xi, LI Lei

2015, 21(1):  106-116.  doi:10.3969/j.issn.1007-2861.2014.01.036

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Based on the studies of conventional longitudinal equivalent rigidity ratio, the tunnel’s displacement and deformation are calculated using a variable rigidity beam elasticity calculation method. A simplified expression of the tunnel’s longitudinal equivalent rigidity ratio is obtained. The analysis shows that the longitudinal equivalent rigidity ratio is linearly proportional to the number of bolts. At the same time, the longitudinal equivalent rigidity ratio nearly parabolically increases with the increasing of the bolt radius. However, with the increase of segment ring thickness and the segment modulus of elasticity, the longitudinal equivalent rigidity ratio is reducing. Because of the stress concentration caused by the tunnel’s rigidity change, the tunnel longitudinal equivalent bending rigidity and effectiveness of the rigidity reduce with the increasing of depth. The proposed model is more widely applicable and is of reference value for longitudinal stability design of shield tunnels.

Dynamic deformation characteristics of unsaturated silt under suction control

JIA Lei, SUN De-an, HAO Fei

2015, 21(1):  117-127.  doi:10.3969/j.issn.1007-2861.2014.01.013

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By using a suction-controllable dynamic triaxial testing system USTX-2000 for the unsaturated and saturated soils developed by GCTS Company, the dynamic deformation characteristics of unsaturated silt have been studied with suctions being controlled. Test results of dynamic deformation characteristics are obtained including the skeleton curve of dynamic stress-strain relation, dynamic modulus, and damping ratio. The following results are obtained. Skeleton curves of unsaturated silt under suction control are hyperbolic under dynamic loading, and the larger the suction, the higher the skeleton curves. The dynamic modulus of unsaturated silt increases with the increasing suction, and the effect of suction on dynamic modulus is less than that of the confining pressure. The damping ratio of unsaturated silt decreases with the increasing suction, and the effect of suction on the damping ratio is less than that of the confining pressure. Changes in the skeleton curves of dynamic stress-strain relation, dynamic modulus and damping ratio with the suction can be explained by the change in the average skeleton stress.

High performance close-loop control of optimal PWM based on stator flux trajectory tracking

SONG Wen-xiang, JIANG Shu-hao, RUAN Zhi-yong, YANG Ying, RUAN Yi

2015, 21(1):  128-140.  doi:10.3969/j.issn.1007-2861.2014.03.021

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Lower harmonic distortion at low switching frequency requires optimal pulse patterns. But it cannot be directly used in high performance systems. A stator flux trajectory tracking control (FTTC) system is studied, and a pulse width modulation (PWM) correction method is proposed. Based on this method, dynamic modulation errors and high overcurrents as the operating conditions change are discussed. Furthermore, high performance closed-loop control of optimal PWM based on FTTC with a self-controlled machine model can be realized. Simulation results show that both fast dynamic response and low harmonics characteristic can be achieved at a low switching frequency of 200∽300 Hz. Its harmonic characteristic is more outstanding than that of model predictive direct control methods.