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排序方式: 共有1210条查询结果,搜索用时 281 毫秒
21.
SJ McPherson RN Gibson NA Collier TG Speer ND Sherson 《Canadian Metallurgical Quarterly》1998,206(3):665-672
Ancylostoma duodenale is still the dominant hookworm species in the Mediterranean area, India, China and Japan. In the present study, biopsied materials were taken from the small intestine of 30 patients infected only with A. duodenale and 12 cross matched controls. The results showed some pathological changes in severely infected cases. However, normal or insignificant changes were seen in the enzymatic activity of the intestinal mucosa. 相似文献
22.
G. Spanos H. S. Fang D. S. Sarma H. I. Aaronson 《Metallurgical and Materials Transactions A》1990,21(6):1391-1411
Isothermal transformation of austenite to bainite was studied by optical, replication, and thin foil transmission electron microscopy (TEM) in both hypo- and hypereutectoid Fe-C-2 wt pct Mn alloys (and a single 3 pct Mn alloy) containing from 0.1 to 1.37 wt pct C in order to characterize and explain the transitions which occur in bainite morphology as a function of carbon content and reaction temperature. A “morphology map” was constructed showing the temperaturecarbon composition(T-x) regions in which four different bainite morphologies predominate: upper bainite, lower bainite, nodular bainite, and inverse bainite. Calculations of the volume free energy changes associated with the nucleation of ferrite, ?G v α , and cementite, ?G v c , and the parabolic rate constant for growth of ferrite, αα, and cementite, αc, were performed in order to explain the observed morphological transitions. In hypoeutectoid alloys, where |?G v α | ? |ΔG v c | and αα ? α c , the Widmanstätten ferrite-dominated morphologies of upper and lower bainite predominate. The upper-to-lower bainite transition appears to be associated with the emergence of edge-to-face sympathetic nucleation at high values of |ΔG v α |. In hypereutectoid alloys, the ratios ΔG v α /ΔG v c and αα/αc are considerably smaller; hence, cementite can compete much more readily with ferrite during both nucleation and growth, resulting in the formation of nodular bainite. With decreasing temperature in the hypereutectoid regime,ΔG v α /ΔG v c , and especially αga/αc, increase, resulting in the replacement of nodular bainite by lower bainite at temperatures below about 250 °C to 275°C. 相似文献
23.
Zhao Z Adenwalla SD Ketterson JB Sarma BK 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1989,36(5):481-484
A technique is proposed for measuring the group velocity of sound by a frequency-modulated (FM) continuous wave (CW) technique. Detailed calculations are presented, showing that V(g)=(deltaomega(m)/deltaphi)d , where V(g), deltaomega(m), deltaphi, and d are, respectively, the group velocity, change of modulation frequency, phase change of the received signal detected by a biphase lock-in amplifier, and the distance traveled by the sound. The approach has the advantage of high resolution compared to the conventional time-of-flight measurement technique. 相似文献
24.
B. S. Shivaram M. W. Meisel Bimal K. Sarma W. P. Halperin J. B. Ketterson 《Journal of Low Temperature Physics》1986,63(1-2):57-113
A high-resolution, ultrasonic (12–89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid3He-B over a pressure range of 0–15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, theJ=2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close toT
c. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the sameJ
zsubstates of the differentJ modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in3He-B. A comparison of the observed Zeeman shifts with theoretical expressions has yielded information about particle-particle and particle-hole interaction effects in the superfluid. In the limitT 0 and above a threshold field, the real squashing mode has been found to possess additional structure. TheJ
z=0 substate has been observed to split into a doublet. The separation between the two components of the doublet is of the order of 100–200 kHz and remains independent of the magnetic field. The origin of the doublet may be understood in terms of a recent theory which postulates a texture-dependent collective mode frequency. Further, at extremely small fields the effects due to dispersion of the real squashing modes have been found to be important. The magnitude of the dispersion-induced mode splitting in zero field is found to be consistent with theoretical predictions. TheJ=2 squashing mode has also been studied in the presence of a magnetic field. TheJ
z=0 state of the squashing mode is observed to shift to lower temperatures in a magnetic field. An additional field dependence of the observed acoustic impedance is interpreted as the evolution of theJ
z=–1, –2 states, but appears to be inconsistent with theoretical predictions. 相似文献
25.
Impact of spatial data availability on the temperature and precipitation prediction characteristics of Weyib River basin in Ethiopia has been investigated using CMIP5-CanESM2 model for the RCP8.5, RCP4.5 and RCP2.6 scenarios. The objective of the present study is to characterize how future temperatures and precipitation prediction under CMIP5-CanESM2 model output varies against diverse averaged arbitrary spatial weather stations found in the basin. The statistical downscaling model tested and verified using the observed daily data for twelve, six and three averaged arbitrary spatial weather stations as well as for a single weather station was used to predict the future climate scenarios. The results revealed that the mean annual daily maximum and minimum temperature and precipitation for twelve, six and three arbitrary spatial stations have revealed an increasing trend in the upcoming periods until the end of the century. In single station analysis, the trend itself has changed from increasing trend to decreasing trend in case of maximum and minimum temperature. In case of precipitation, no visible trend has been observed in case of single station analysis. Therefore, the variation in amount and distribution of precipitation and temperature among the four averaged spatial stations in the same study area might affect the water resources and agriculture of the basin and also instead of using a single weather station to predict future climate variables for a particular study basin, it is more reliable using averages of numerous spatial weather stations data. 相似文献
26.
正事实上,绝大多数攻击者是经由用户及其设备进入企业的。现实情况往往是黑客跟踪一个用户,然后此用户成为帮助黑客到达数据库或用户凭据的一个管道。因此,企业不仅需要端点安全和网络安全,而且还要将不同的安全方案集成起来。安全厂商也应当听从时代的召唤,建设针对新威胁的安全方案,迎接深度安全的新时代。 相似文献
27.
The deposition of dense solid particles in a downward, fully developed turbulent square duct flow at Reτ = 360, based on the mean friction velocity and the duct width, is studied using large eddy simulations of the fluid flow. The fluid and the particulate phases are treated using Eulerian and Lagrangian approaches, respectively. A finite-volume based, second-order accurate fractional step scheme is used to integrate the incompressible form of the unsteady, three-dimensional, filtered Navier-Stokes equations on an 80 × 80 × 128 grid. A dynamic subgrid kinetic energy model is used to account for the unresolved scales. The Lagrangian particle equation of motion includes the drag, lift, and gravity forces and is integrated using the fourth-order accurate Runge-Kutta scheme. Two values of particle to fluid density ratio (ρp/ρf = 1000 and 8900) and five values of dimensionless particle diameter (dp/δ × 106 = 100, 250, 500, 1000 and 2000, δ is the duct width) are studied. Two particle number densities, consisting of 105 and 1.5 × 106 particles initially in the domain, are examined.Variations in the probability distribution function (PDF) of the particle deposition location with dimensionless particle response time, i.e. Stokes number, are presented. The deposition is seen to occur with greater probability near the center of the duct walls, than at the corners. The average streamwise and wall-normal deposition velocities of the particles increase with Stokes number, with their maxima occurring near the center of the duct wall. The computed deposition rates are compared to previously reported results for a circular pipe flow. It is observed that the deposition rates in a square duct are greater than those in a pipe flow, especially for the low Stokes number particles. Also, wall-deposition of the low Stokes number particles increases significantly by including the subgrid velocity fluctuations in computing the fluid forces on the particles. Two-way coupling and, to a greater extent, four-way coupling are seen to increase the deposition rates. 相似文献
28.
29.
Poly(vinyl alcohol) (PVA) can be dissolved in a nonaqueous medium in the presence of catalytic concentration of ethyl nitrate dimethyl sulfoxide, C2H5ONO2·DMSO. From the PVA solution, poly(vinyl propionate), PVPR was prepared by the homogeneous esterification of PVA with propionic acid. The ester thus formed contained some unconverted hydroxyl group. The formation of the ester was confirmed by the IR and 1H‐NMR spectra. The molecular weight of the ester was determined by GPC and intrinsic viscosity (η) was determined by viscometric method. Glass transition temperature, Tg, was obtained from differential scanning calorimetric (DSC) analysis. Thermal stabilities of the ester were checked by thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) analysis. The efficiency of the ester as a flow improver of crude oil was also examined. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5675–5679, 2006 相似文献
30.
Ritesh Rawal Antonio Feteira Alberto Arenas Flores Neil C. Hyatt Anthony R. West Derek C. Sinclair Kumaravinothan Sarma Neil McN. Alford 《Journal of the American Ceramic Society》2006,89(1):336-339
The dielectric properties of dense ceramics of the "twinned" 8H-hexagonal perovskite Ba8 Nb4 Ti3 O24 are reported. Single-phase powders were obtained from the mixed-oxide route at 1325°C and ceramics (>92% of the theoretical X-ray density) by sintering in air or flowing O2 at 1400°–1450°C. The ceramics are dc insulators with a band gap >3.4 eV that resonate at microwave frequencies with relative permittivity, ɛr ∼44–48, quality factor, Q × f r ∼21 000–23 500 GHz (at f r ∼5.5 GHz) and temperature coefficient of resonant frequency, TC f ,∼+115 ppm/K. 相似文献