Identification of the effect of grain size on levitation force of well-textured YBCO bulk superconductors

The levitation forces of samples with different grain size have been directly investigated and identified in well-textured YBCO bulks. A single-domain YBCO bulk of ∅30 mm was prepared by a top seeded melt growth process, then divided into two, three and four parts to acquire the levitation forces of samples with different grain size but with completely the same grain orientation. The maximum levitation force for the single-domain sample is about 1.68, 2.05 and 2.4 times higher than that of the samples with two, three, and four grain domains. It is directly confirmed that the levitation force of a single-domain YBCO bulk is much superior than that of samples with multiple domains. It is found that the levitation force is inversely proportional to the total length of grain boundaries of the corresponding sample.     

Dynamic analysis of laminated composite plates traversed by a moving mass based on a first-order theory

The dynamic response of angle-ply laminated composite plates traversed by a moving mass or a moving force is investigated. For this purpose, a finite element method based on the first-order shear deformation theory is used. Stationary and adaptive mesh techniques have been applied as two different meshing schemes. The adaptive mesh strategy is then used to avoid off-nodal position of moving mass. In this manner, the finite element mesh is continuously adapted to follow and comply with the path of moving mass. A Newmark direct integration method is employed to solve the equations of motion. Parametric study is directed to find out how different parameters like mass of the moving object as well as the type of the angle-ply laminated composite plates affect the dynamic response. Numerical results show the significant effects of the stacking order on the dynamic responses of the composite structures under a moving mass. It is found that although [30/−60/−60/30] lamination shows the highest maximum vertical deflection but [−45/45/45/−45] lamination has the highest value of the dynamic amplification factor. The dynamic amplification factor for different stacking orders and mass velocities is less than 1.25.     

分层式前驱坯体对YBCO单畴块材生长和磁通性能的影响

Y-Ba-Cu-O(YBCO)单畴块材通常是由统一分布的YBa₂Cu₃O_7-δ(Y123)和Y₂BaCuO₅(Y211)前驱坯体制备的。然而研究表明熔融生长的超导块材中Y211相粒子的浓度随着远离籽晶显著增大, 造成Y211相粒子的积聚以及限制单畴块材的超导性能。本文通过顶部籽晶熔融织构法(TSMTG)利用分层式前驱坯体制备了φ23 mm×9 mm的YBCO单畴块材, 并在生长形貌、微结构以及磁通俘获性能方面与常规方法制备的同尺寸YBCO单畴块材做了对比研究。研究结果显示, 使用分层式前驱坯体制备的YBCO单畴块材生长得更充分, Y211相粒子的平均粒径更小、分布更均匀, 并且在磁通俘获性能上有了显著的提高, 同时具备高对称性的磁通分布和强磁通俘获能力, 在77 K环境下最大磁通俘获场高达0.91 T。该结果对进一步提高YBCO超导块材的性能具有很好的参考价值。     

Analysis of the levitation force of pure and starch/polystyrene/MWCNT added bulk MgB2 superconductors using frozen image model under zero field cooling condition

The measurement of superconducting levitation force between permanent magnet and polycrystalline samples of pure and MgB₂ added with starch, polystyrene (PS) and multiwall carbon nanotube (MWCNT) have been performed under zero field cooling (ZFC) condition at 20 K in both descending and ascending modes. For this, the bulk pellets were synthesized by conventional solid state sintering technique. The XRD data indicate well developed MgB₂ phase. However, a decrease in lattice parameter ‘a = b’ have been observed for doped MgB₂ samples. Superconducting transition temperature of MgB₂ also decreases with starch/PS/MWCNT addition. Unlike MWCNT, the addition of starch/polystyrene is found to enhance the levitation force of MgB₂ superconductor. The levitation force between PM and investigated pellets in ZFC condition is explained well in terms of the updated version of modified frozen image model and the magnetic moment originated due to vertical motion of the superconductors have been estimated. It may be noted that except for MWCNT, addition of starch/PS in MgB₂ improves the magnetic moment generated by vertical movement of pure MgB₂. However, this improvement is more pronounced for 1 wt.% of PS added MgB₂, which indicates more flux trapping and hence better levitation properties in 1 wt.% of PS added MgB₂. The vertical stiffness estimated for pure and starch/PS/MWCNT doped MgB₂ samples indicate that the levitation force are more sensitive in the region close to the PM.     

Effects of Precursors with Fine BaZrO3 Inclusions on the Growth and Microstructure of Textured YBCO

We prepared precursors with fine BaZrO₃ inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y_1.5Ba₂Cu₃O _y + x BaZrO₃ (x = 0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa₂Cu₃O_7-/Y₂BaCuO₅ (YBCO). The BaZrO₃ particles were about 100 nm in size in the textured YBa₂Cu₃O_7- (Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO₃ particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO₃ fine particles exclude themselves as effective flux pinning centers in the textured YBCO.     

The Holographic study for the high T_c superconductor YBCO and recent development of studies of a class of inverse problems

A dream or interesting problem is to obtain more information based on one measurement,for example to obtain all the thermodynamic functions based on specific heat only. We call it“holographic“study.After more than 10 year‘s efforts,we suggest one possibility(or only one)is to solve an inverse problem(specific heat-phonon spectrum inversion,SPI).In this talk we will review recent development of studies of a class of inverse problems,including Dai‘s exact solution and Chen‘s formula and their unification.T...     

前驱液中添加聚乙烯吡咯烷酮对YBa2Cu3O7-δ膜形貌和性能的影响

用三氟乙酸金属有机物沉积(TFA-MOD)方法在LaAlO3(100)基底上生长YBa2Cu3O7-δ(YBCO)超导薄膜,研究了向前驱液中添加聚乙烯吡咯烷酮(PVP)对YBCO薄膜微结构和超导性能的影响.涂膜在氧气环境中进行200～250℃热解,再经775℃氩气环境下结晶后获得YBCO超导薄膜.在相同热处理条件下,未添加PVP的前驱液制得的YBCO薄膜临界电流密度为4050A/cm2,添加PVP的前驱液制得的YBCO薄膜临界电流密度为5800A/cm2.后者表现出较少的孔洞,较强的c轴取向,较纯的双轴织构和较高的临界电流密度.因此,向前驱液中添加PVP的化学方法可以改进YBCO涂层导体的MOD制备过程.总压,氧分压和热处理温度等工艺条件将进一步优化,以提高临界电流密度.     

Effects of operating speed on production quality and throughput

A key determinant of a manufacturing system's performance is its operating speed. While it is generally assumed that overall production throughput increases with operating speed, this is not necessarily the case where quality deteriorates as a result of the higher speed. In this paper we derive a representative quality–speed relationship and examine the productivity implications. We develop models to capture the effects of rework, repair, and scrap on system throughput and illustrate the resulting trade-off between higher productivity and the need for additional processing. Empirical evidence is also presented to motivate the work.     

Effects of average speed enforcement on speed compliance and crashes: A review of the literature

Average speed enforcement is a relatively new approach gaining popularity throughout Europe and Australia. This paper reviews the evidence regarding the impact of this approach on vehicle speeds, crash rates and a number of additional road safety and public health outcomes. The economic and practical viability of the approach as a road safety countermeasure is also explored. A literature review, with an international scope, of both published and grey literature was conducted. There is a growing body of evidence to suggest a number of road safety benefits associated with average speed enforcement, including high rates of compliance with speed limits, reductions in average and 85th percentile speeds and reduced speed variability between vehicles. Moreover, the approach has been demonstrated to be particularly effective in reducing excessive speeding behaviour. Reductions in crash rates have also been reported in association with average speed enforcement, particularly in relation to fatal and serious injury crashes. In addition, the approach has been shown to improve traffic flow, reduce vehicle emissions and has also been associated with high levels of public acceptance. Average speed enforcement offers a greater network-wide approach to managing speeds that reduces the impact of time and distance halo effects associated with other automated speed enforcement approaches. Although comparatively expensive it represents a highly reliable approach to speed enforcement that produces considerable returns on investment through reduced social and economic costs associated with crashes.     

Thermomechanical correction to the drag force on a sphere and a cylinder moving in HeII

The drag force on a sphere and a cylinder moving in HeII is considered on the basis of the full system of the two-fluid dissipative hydro-dynamical equations. It is shown, that due to the thermomechanical effect in superfluid there exists a specific correction to the drag force on a body. The value of this thermomechanical correction F has the order of the ratio1 _T/R or1 _T ² /R² (R is the size of the body, and1 _T is the length of the formation of the convective heat flow). The value of F grows at low temperatures, where1 _T is proportional to the characteristic time of the phonon-roton scattering process, and in the region near the -point, where1 _T has the same temperature dependence as the order parameter correlation length 1_T (T–T)^–2/3. The effects under discussion can be investigated experimentally by measurements of the mobility of small objects. A comparison is given with the data of the mobility of positive and negative ions in HeII near the -point.     

Analysis of contact force distribution in a moving granule bed subjected to shear deformation by a set of rollers

Large contact forces on granules could give rise to undesirable attrition. In a new device, referred to as the Particle Shear and Impact tester, granules are subjected to repeated and sequential impact and shearing, the latter effected by two counter-rotating rollers with differential angular speeds and an adjustable gap between the rollers. This enables the contact force distribution to be varied in order to apply representative contact forces, as experienced in manufacturing plants. Granule flow in the rollers is simulated by Distinct Element Method for several roller gap sizes. The resulting contact force distribution is compared to that from shear cell simulations for representative plant normal stresses (8 to 15 kPa), in order to calibrate the appropriate gap size. The 90th percentile of the contact forces distributions from the two simulations are matched to set the gap size. A roller gap size approximately 3.5 times the 90th percentile of the particle size (based on number distribution) gives a good match of the interparticle contact forces between the rollers and the shear cell. This enables replicating the stresses that granules experience in plants, whether during handling and transport, or during more severe stressing conditions, e.g. compaction or even grinding, thereby assessing attrition or fragmentation propensity of granules.     

Second and third interaction virial coefficients of the (methane+propane) system determined from the speed of sound

The speed of sound has been measured in the binary gaseous mixture (0.85CH₄+0.15C₃H₈) along seven isotherms at temperatures between 225 and 375 K and at pressures up to 1.4 M Pa. From the measurements, second and third acoustic virial coefficients of the mixture were obtained. These results were analyzed together with values of the second and third acoustic virial coefficients of the two pure components to obtain a set of model intermolecular potential-energy functions for the methane-propane system. Nonpairwise additivity of the intermolecular forces was included in this analysis. Ordinary second and third interaction virial coefficients calculated from the model are reported, as are the second and third virial coefficients of the pure components. Gas densities calculated by means of these virial coefficients for the mixture (0.9298CH₄+0.0702C₃H₈) were found to agree with experimental values at temperatures between 280 and 330 K to within 0.02% at pressures up to 3 MPa and to within 0.08% at 4MPa.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

激励力作用位置对圆柱壳辐射声功率影响研究

潜艇上的机械设备不平衡运转会产生激励力,不平衡运转设备引起的艇体噪声是辐射噪声的主要成分。因此研究激励力的位置与声功率的关系是非常具有意义。文章建立了环肋圆柱壳结构声振动分析模型,基于力辐射模态从激励力作用位置变化方面对环肋圆柱壳进行数值计算和分析,研究激励力作用在不同位置处时,辐射声功率的变化规律。为了给出激励力作用位置与壳体辐射声功率的关系,运用力辐射模态概念,依据力辐射模态形态特征,分析判断激励力作用位置对辐射声功率的影响,为有效地减少圆柱壳辐射声功率提供了更为方便的技术手段。     

Free and forced vibration of a functionally graded beam subjected to a concentrated moving harmonic load

In this paper, free vibration characteristics and the dynamic behavior of a functionally graded simply-supported beam under a concentrated moving harmonic load are investigated. The system of equations of motion is derived by using Lagrange’s equations under the assumptions of the Euler–Bernoulli beam theory. Trial functions denoting the transverse and the axial deflections of the beam are expressed in polynomial forms. The constraint conditions of supports are taken into account by using Lagrange multipliers. It is assumed that material properties of the beam vary continuously in the thickness direction according to the exponential law and the power-law form. In this study, the effects of the different material distribution, velocity of the moving harmonic load, the excitation frequency on the dynamic responses of the beam are discussed. Numerical results show that the above-mentioned effects play very important role on the dynamic deflections of the beam.     

Dynamic response of a beam to the train of moving forces driven by an Erlang renewal process

Dynamic response of a beam to a random train of moving forces moving with the same velocity is considered. Unlike a widely used Poisson process model, a more adequate Erlang renewal process is used as a process driving the train of forces. Normal-mode approach is used to convert the problem into that of a renewal driven train of general pulses. Consequently the modal responses are the filtered renewal processes and are expressed as integrals with respect to the response to a single pulse (passage of a force) and to the increments of the counting renewal process. The expressions for the mean values and cross-correlations of modal responses are obtained as single and double integrals, respectively. The results are presented in terms of the renewal density of the underlying Erlang counting process. Mean value and variance of the mid-span deflection of the beam are determined by numerical evaluation of the pertinent integrals. Numerical analysis is carried out for the Erlang processes with integer parameter $k=2$, $k=3$ and 4 and, for comparison, also for a Poisson process. Different traffic conditions such as the velocity and the mean arrival rate of vehicles are taken into account.     

复合材料翼盒壁板低速冲击接触力、凹坑特性及其模拟

为预测复合材料层合板的低速冲击特性,提出一种基于弹簧-质量模型的冲击分层响应模拟方法。首先根据假定的分层数量和分层面积计算分层前后的弯曲刚度与挠度变化,进而模拟出含分层过程的冲击头运动时间历程。然后,由冲击头的加速度计算接触力,并基于材料的弹-塑性接触定律计算出永久凹坑深度。随后,对共固化复合材料翼盒的壁板进行了低速冲击实验,冲击点分为3种类型:2个梁间蒙皮的中点位置、蒙皮与翼梁T型连接的胶接区边界和梁的正上方。接着,分析对比了不同冲击点的接触力、分层阈值力和凹坑深度的特点。最后,使用所提出的模型计算第1种冲击点的接触力、永久凹坑深度和吸收能量,并分析了矩形板长宽比对薄膜刚度及接触力的影响。结果表明:模拟的接触力与实测结果吻合良好,对永久凹坑深度和吸收能量的模拟精度也较高,所提模拟方法有效;随着长宽比的减小,薄膜刚度迅速增大,从而使接触力显著增大。      

Effects on speed and safety of point-to-point speed enforcement systems: Evaluation on the urban motorway A56 Tangenziale di Napoli

In this paper, we evaluated the effects on speed and safety of the point-to-point (P2P) speed enforcement system activated on the urban motorway A56 in Italy. The P2P speed enforcement is a relatively new approach to traffic law enforcement that involves the calculation of the average speed over a section. To evaluate the speed effects, we performed a before–after analysis of speed data investigating also effects on non-compliance to speed limits. To evaluate the safety effects, we carried out an empirical Bayes observational before-and-after study.     

Driving speed and the risk of road crashes: a review

Driving speed is an important factor in road safety. Speed not only affects the severity of a crash, but is also related to the risk of being involved in a crash. This paper discusses the most important empirical studies into speed and crash rate with an emphasis on the more recent studies. The majority of these studies looked at absolute speed, either at individual vehicle level or at road section level. Respectively, they found evidence for an exponential function and a power function between speed and crash rate. Both types of studies found evidence that crash rate increases faster with an increase in speed on minor roads than on major roads. At a more detailed level, lane width, junction density, and traffic flow were found to interact with the speed-crash rate relation. Other studies looked at speed dispersion and found evidence that this is also an important factor in determining crash rate. Larger differences in speed between vehicles are related to a higher crash rate. Without exception, a vehicle that moved (much) faster than other traffic around it, had a higher crash rate. With regard to the rate of a (much) slower moving vehicle, the evidence is inconclusive.     

Effects of nanoparticle migration on force convection of alumina/water nanofluid in a cooled parallel-plate channel

Force convective heat transfer of alumina/water nanofluid inside a cooled parallel-plate channel in the creeping flow regime and the presence of heat generation is investigated theoretically. A modified two-component four-equation non-homogeneous equilibrium model is employed for the alumina/water nanofluid that fully accounts for the effects of nanoparticles volume fraction distribution. To impose the temperature gradients across the channel, the upper wall is subjected to a prescribed wall heat flux while the bottom wall is kept adiabatic. Moreover, due to the nanoparticle migration in the fluid, the no-slip condition of the fluid–solid interface at the walls is abandoned in favor of a slip condition that appropriately represents the non-equilibrium region near the interface. The results indicated that nanoparticles move from the adiabatic wall (nanoparticles depletion) toward the cold wall (nanoparticles accumulation) and construct a non-uniform nanoparticle distribution. Moreover, the anomalous heat transfer rate occurs when the Brownian motion takes control of the nanoparticle migration (smaller nanoparticles).     

The stress response of a semi-infinite micropolar granular material subject to a concentrated force normal to the boundary

We consider the problem of a two dimensional semi-infinite granular material subject to a concentrated or point force normal to the boundary. This boundary value problem was originally solved for a classical elastic material by Flamant in 1892 and, hence, is also known as the Flamant problem (Johnson [8]). In this paper, the granular material is considered as an elastic micropolar or Cosserat continuum and is represented by a particular form of the general constitutive law derived in Walsh and Tordesillas [29]. The stress distribution predicted by the model is in good agreement with experimental data for small strains. In particular, two important features that are captured by the proposed model are: (i) the presence of tensile stress response regions, and (ii) the dependence of the stresses on the microstructural properties, i.e. the particles normal, tangential and rotational stiffness constants. The proposed analysis utilizes two new stress functions, similar to Airys stress functions in classical elastic theory.The support of the US Army Research Office through a grant to AT (Grant No. DAAD19-02-1-0216) and the Melbourne Research and Development Grant scheme is gratefully acknowledged. We thank our reviewers for their useful suggestions and insightful comments.