Fabrication of hydroxyapatite ceramics with controlled pore characteristics by slip casting

Porous hydroxyapatite (HAp) ceramics with controlled pore characteristics were fabricated using slip casting method by mixing PMMA with HAp powder. The optimum conditions of HAp slip for slip casting was achieved by employing various experimental techniques, zeta potential and sedimentation, as a function of pH of the slips in the pH range of 4–12. HAp suspensions displayed an absolute maximum in zeta potential values and a minimum in sedimentation height at pH 11.5. The optimal amount of dispersant for the HAp suspensions was found at 1.0 wt% according to the viscosity of 25 vol% HAp slurry. The rheological behaviour of HAp slurry displays a shear-thinning behavior without thixotropy, which is needed in slip casting processing. The pore characteristics of sintered porous hydroxyapatite bioceramics can be controlled by added PMMA particle size and volume. The obtained ceramics exhibit higher strength than those obtained by dry pressing.     

Crystalline orientation of alumina ceramics prepared by electrophoretic deposition under a high magnetic field

Electrophoretic deposition (EPD) of alumina in a superconducting magnet was performed at various magnetic field strengths. A stable colloidal suspension of alumina appropriate for magnetic alignment was prepared in an ethanol medium by using a phosphate ester (PE) as a dispersant. The amount of PE appropriate for the stability of the alumina suspension was investigated by measuring the pH, zeta-potential and the relative density of the green compacts. The consolidation of alumina powder was performed by EPD under a magnetic field of 0–12 T. The degree of crystalline orientation of the sintered bodies was evaluated by X-ray diffraction (XRD) as a function of the applied magnetic field during deposition and the sintering temperature.     

Formation of feathery grains with the application of a static magnetic field during direct chill casting of Al-9.8wt%Zn alloy

Effect of a 0.2-T static magnetic field on the microstructure of a direct chill cast Al-9.8wt%Zn alloy slab was investigated. The static magnetic field transferred the microstructure from a mixture of equiaxed and columnar grains with the primary trunks growing in <100> directions to twinned lamellar feathery grains with the primary and secondary arms growing in <110> directions. The application of the static magnetic field results in the reduction of the heat discharge and solute mixing capacity through a damping effect on convection and thus a delay of the melt transformation to solid and a request to reduce the liquid/solid interface energy through reducing the interface area due to the loss of undercooling. The delay and the request account for the growth direction change and the formation of lamellas. The difference between the Al and Zn atomic radii and the related incoming flow facilitate the formation of the twins.     

外加磁场环境下45钢疲劳过程磁记忆特征及其损伤模型

为明确外加磁场环境下铁磁构件疲劳损伤过程磁记忆信号变化规律,以45钢为对象,开展了地磁场(40 A/m)及外加磁场(200、400、600 A/m)环境下的低周疲劳实验。获取了各疲劳损伤阶段的磁场强度法向分量H_p(y)及梯度值K,提取了H_p(y)极差值H_p(y)_Rv和K最大值K_max,结合经典疲劳损伤模型进行了磁特征损伤累积参数拟合,构建了疲劳损伤模型。结果表明：随着疲劳循环周次增加,各数据采集点的H_p(y)值、应力集中部位K值和H_p(y)_Rv值逐渐增大。相同循环周次下,随着外加磁场强度的增大,各数据采集点H_p(y)值和H_p(y)_Rv值逐渐增大,且对应于疲劳断裂阶段的K_max值逐渐增加。所构建的基于H_p(y)_Rv的疲劳损伤模型拟合度较高。本文可为铁磁构件疲劳损伤评估提供技术方法支持。     

Study of the direct current breakage process in an external nonuniform magnetic field

We study experimentally the influence of an external nonuniform magnetic field with transverse and longitudinal components on the electric characteristics, plasma configuration, and cathode spot arrangement of vaccum-arc discharge. It is revealed that for a cylindrical cathode, cathode spots are nonuniformly distributed on the cathode surface, the spot configuration has no axial symmetry, and the arrangement of spots changes in time depending on the induction value of the external pulsed magnetic field. With an increase in induction, spots (on average) are arranged closer to the cathode, i.e., displaced to the region of weaker transverse field. For two cathode geometries, the probabilities of direct current breakage depending on the induction of the external nonuniform magnetic field are experimentally determined. To determine the conditions of current breakage, a three-dimensional mathematical model of ion motion is suggested and the current breakage criterion is formulated. The trajectories of fast cathode ions in an electrode system with a ring cathode have been calculated using the model. Calculations were performed in a nonuniform magnetic field, the radial and axial components of which have been measured experimentally. It is shown that conditions of current breakage determined with this criterion agree with the results of experiment.     

SHS纳米/微米块体复相陶瓷微观结构与断裂

通过在(CrO3 Al)燃烧体系中添加一定量的ZrO2(2Y)粉末,利用SHS冶金技术直接制备出Al2O3-35vol%ZrO2纳米/微米结构块体复相陶瓷,研究该复相陶瓷的微观结构与断裂行为.研究发现:该复相陶瓷基体主要由纳米/微米相晶内型结构共晶体组织构成;Vickers压痕试验显示引发陶瓷裂纹扩展的压痕压制临界载荷为30 kg;ZrO2相所具有的应力诱发相变增韧机制和微裂纹增韧机制均很微弱;裂纹扩展主要受纳米/微米相晶内型结构共晶体控制,使该复相陶瓷在断裂过程中呈现出强烈的裂纹偏转绕过机制.     

The effect of alternating magnetic field on the process of semi-continuous casting for AZ91 billets

Billets of AZ91 alloy under different alternating magnetic field conditions were examined. Effects of alternating magnetic field on microstructures, hardness, hot-tearing tendency and casting speed are studied. The results show that with a relatively low frequency of alternating magnetic field employed during the casting, the microstructures in the entire cross section of the billet are changed from columnar dendrites to equiaxed dendritic shapes; the hardness of billet is increased and the hot-tearing tendency is reduced. And it is possible to produce AZ91 billets at a relatively high casting speed with an alternating magnetic field.     

Determination of the magnetic moment by measuring the external magnetic field close to electrical equipment

Translated from Izmeritel'naya Tekhnika, No. 10, pp. 36–37, October, 1992.     

Effect of a magnetic field on the hysteresis transitions during floating-zone crystal growth

The effect of a constant axial magnetic field on the thermosolutocapillary (TSC) convection in the floating liquid zone with an undeformed free surface under microgravity (MG) conditions has been studied. It is established that several stable flow regimes may coexist under such conditions in a certain range of parameters. The boundaries of transitions between various convection regimes under MG conditions with various Hartmann numbers have been determined. It is shown that the constant axial magnetic field suppresses TSC convection, so that the region of uncertainty decreases and shifts toward higher Marangoni numbers. The dependence of the structure and intensity of flow on the floating-zone length to radius ratio has been analyzed.     

The characteristics of single crystal bismuth wires produced by the Ohno continuous casting process

Bismuth wires approximately 2 mm in diameter were produced by the heated mould Ohno Continuous Casting process and their mechanical characteristics evaluated. It was found that cast wires produced under all speed conditions from 30 to 170 mm min^–1 were single crystals. Cast wires with an angle between the cleavage plane and wire axis of less than about 40° were significantly more ductile than those with an angle of over 57°. The former could be bent repeatedly before fracture occurred whereas the latter fractured after only a single bend. It was also found that the bismuth wires with a cleavage angle of less than 45°, exhibited tensile elongations up to about 130%.     

On the dissipation process in Bi2Sr2Ca2Cu3O10 tapes and bulk samples with preferential grain orientation in zero external magnetic field

Current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O₁₀ tapes and bulk samples with preferentially oriented crystallites have been measured in zero applied magnetic field and for temperatures close to the mean-field critical-temperature,T _c0. It was shown that the power-law dependence,V=AI ^a, valid for two-dimensional systems, does not correctly describe the data, which clearly indicate the existence of a finite critical-current density,J _c. The experimentalI-V curves are fitted quite well with the model which attributes the finite critical-current density to the coupling between the CuO₂ double layers. It was found thatJ _c vanishes at a temperature value belowT _c0. This behavior can result from the occurrence of vortex fluctuation-induced layer decoupling and/or from the contribution of entropy to the vortex unbinding process.     

Expansion of a conducting shell by the magnetic field of an external inductor

The problem on magnetic field diffusion into the internal cavity of a hollow cylindrical conducting shell is solved analytically. In the case of an external field pulse representing a half-wave B _e = B _m sin(πt/T), a region of the geometric parameters (thickness, diameter) is determined, for which the shell can be expanded under the action of the residual internal magnetic field.     

Effect of casting parameters on the freezing point position of the 304 stainless steel during twin-roll strip casting process by numerical simulation

This study focuses on the numerical simulation of the temperature fields of the 304 stainless steel during twin-roll strip casting process. The cracks and fratographs of the 304 stainless steel strip with the practical casting parameters were observed by Scanning Electron Microscopy. The exit temperature of the 304 stainless steel during twin-roll strip casting process was measured. Meanwhile, it was simulated by means of finite element model. The observed results show that the strip from outside to inside are composed of refined crystal zone, columnar crystal one, and equated axis crystal one. The cracks occur on the strip surface, extended to interior along the columnar crystal boundary, and then, ended at the juncture between the columnar crystal zone and equated axis crystal one. The simulated result was compared to the measured temperature, which showed a good agreement, thus indicating that this model is valid and accurate. Then, by employing this model, the effects of the casting parameters, such as casting speed and casting temperature, on the freezing point position of the 304 stainless steel during twin-roll strip casting process were simulated too. The simulated results show that, the freezing point position is decreased with the increasing casting speed and casting temperature. For producing high-quality strip of 304 stainless steel during twin-roll strip casting, the limits of casting speed and casting temperature are 1.1 m·s⁻¹ and 1,570 °C, respectively.     

Nondestructive effect of the cusp magnetic field on the dendritic microstructure during the directional solidification of Nickel-based single crystal superalloy

The mechanical-property improvement of directionally-solidified Nickel-based single crystal(SC)superalloy with the single-direction magnetic fields is limited by their destructiveness on the dendritic microstructure.Here,the work present breaks through the bottleneck.It shows that the application of the cusp magnetic field(CMF)ensures that the dendrites are not destroyed.This feature embodies that the primary dendrite trunks arrange regularly and orderly,as well the secondary dendrite arms grow symmetrically.By contrast,both the unidirectional transverse and longitudinal magnetic field destroy the dendrite morphology,and there are a number of stray grains near the totally-re melted interface.The nondestructive effect is achieved mainly by the combined action of the thermoelectromagnetic force on the dendrites and thermoelectromagnetic convection in the melt during directional solidification.The investigation should contribute a new route for dramatically and effectively improving the crystal quality and mechanical properties of the directionally-solidified alloys.     

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magne-sium alloys.However,the roles of slips and twins in the room-temperature deformation of Mg-rare earth(Mg-RE) alloys with high contents of rare earth elements is rarely investigated.Here,the microstruc-tural evolution and deformation mechanism of an aged Mg-SY-2Nd-3Sm-0.SZr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy.The results indicated that in the early stage of defor-mation,the Mg-RE alloy was mainly controlled by the slip of dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin.With an increase in the strain,the grain orientation became more suitable for the initiation of pyramidal Ⅱ dislocations in the later stage of deforma-tion;these dominated the deformation mechanism.In the twin evolution of the Mg-RE alloy,there were three types of twin-twin interaction behaviors:(i) single twin variant 'parallel'structure,(ii) single twin variant 'cross' structure,and (iii) multi twin variant 'cross' structure.In addition,three types of twin-grain boundary interaction behaviors were summarized:(i) twin 'refracting through'grain boundary,(ii) twin'parallel through'grain boundary,and (iii) twin 'fusing through'grain boundary,which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.     

外加磁场对应变RTD电流负阻“平台”的抑制作用

本文在实验上观察到GaAs/A1As/InGaAs应变共振隧穿二极管(RTD)在电流负阻区出现一段"平台"现象,该现象来源于发射极子能级和量子阱子能级之间的相互耦合.在外加磁场的作用下,它们之间的耦合得到抑制,在一定的磁场下,负阻"平台"现象消失.此外,峰值电流随磁场增加而减小,导致峰谷电流比下降,同时峰值电压也发生相应的变化.     

Evolution of the polarization state in material media with uniform linear birefringence and an applied nonhomogeneous external magnetic field: application to bulk-type Faraday current sensors

In this work we find the exact solution for the evolution of the polarization state of a light wave that propagates in a material medium with uniform linear birefringence and a nonuniform external magnetic field. The obtained results could be used to improve the precision of existing Faraday current sensors.     

On the dissipation process in Bi2Sr2Ca2Cu3O10 tapes and bulk samples with preferential grain orientation in zero external magnetic field

Current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O₁₀ tapes and bulk samples with preferentially oriented crystallites have been measured in zero applied magnetic field and for temperatures close to the mean-field critical-temperature,T _c0. It was shown that the power-law dependence,V=AI ^a, valid for two-dimensional systems, does not correctly describe the data, which clearly indicate the existence of a finite critical-current density,J _c. The experimentalI-V curves are fitted quite well with the model which attributes the finite critical-current density to the coupling between the CuO₂ double layers. It was found thatJ _c vanishes at a temperature value belowT _c0. This behavior can result from the occurrence of vortex fluctuation-induced layer decoupling and/or from the contribution of entropy to the vortex unbinding process.     

Double refraction and reflection of sequential crystal interfaces with arbitrary orientation of the optic axis and application to optimum design

The general formulation of double refraction or internal double reflection for any directions of incidence and arbitrary orientation of the optic axis in a uniaxial crystal is analysed in terms of Huygens’ principle. Then double refraction and double reflection along the sequential interfaces in a crystal are discussed. On this basis, if the parameters of the interface are chosen appropriately, the range of angular separation between the ordinary ray and extraordinary ray can be much greater. It is useful for crystal element design. Finally, as an example, an optimum design of the output end interface for a 2?×?2 electro-optic switch is given.