基于弯曲测试技术的纳米梁厚度的精密测量

针对广泛应用的扫描电镜和台阶仪在测量纳米厚度时存在的破坏性和近似性等局限性,提出了一种基于弯曲测试技术实现纳米梁厚度精密测量的方法．该方法的核心思想是悬浮结构在载荷的作用下产生初始弯曲直至其下表面与衬底接触的过程中形成的载荷一位移曲线会出现斜率明显不同的两个直线段,其交点代表了悬浮结构下表面与衬底之间的初始接触,由此可测量出该悬浮结构下表面与衬底之间的间隙,从而间接得到结构的厚度值．分别采用原子力显微镜（AFM）和纳米压痕仪作为测试平台对单晶硅固支纳米梁进行了厚度测量,两种测量仪器得到了一致性较好的测量结果．讨论了测量随机误差、系统误差以及数据计算误差等对测试结果的影响和相应的误差降低方法．     

Electromagnetic confinement and shaping for plate-form p

The relationship between melt shape, electromagnetic pressure and magnetic field is studied for electromagnetic confinement and shaping of plate-form part. The results of experimental observation and theoretical inference can be summarized as follows. As the melt thickness a is large enough, causing the ratio of plate thickness to current theoretic skin depth a/delta larger than 2.2, the electromagnetic pressure acting on melt can be simply expressed as P-m = B-2/2 mu, and the melt shape would be known only by measuring the distribution of magnetic flux density As a is small and makes the ratio a/delta less than 2.2, the melt shape and electromagnetic pressure for confining and shaping are determined not only by magnetic flux density B but also by melt thickness a, electromagnetic parameter mu gamma and current frequency f. In this paper, an equation used to calculate electromagnetic pressure acting on "thin plate-form melt" is brought forward. The equation gives a precise relationship between electromagnetic pressure factor p and melt thickness a, electromagnetic parameter mu gamma and current frequency f.     

Distance determination by the gamma-ray time-of-flight method

The time-of-flight method is studied using coincident annihilation gamma rays with an energy of 511 keV to find the distance from a surface at which they are scattered back for detection. The gamma rays have a good penetration ability, which makes the method suitable for industrial circumstances in which the range is rendered opaque to light by flue gas and aerosols. An accuracy of 1 cm over a distance of 2 m can be obtained by increasing the averaging time by the square of the lengthening factor. The density of the surface can be studied by recording the width of the distribution of the surface measurements. This measurement needs a prolonged averaging time, but the thickness of the slag layer on top of a steel melt can be measured with a maximum error of 1.5 cm at short distances, for example     

First experiments to determine snow density from diffuse near-infrared transmittance

Snow density is one of the key properties to characterize a snow cover. We present diffuse near-infrared transmittance measurements with an integrating sphere setup in the laboratory. We analyze 8 snow samples taken from melt forms, decomposed, rounded, faceted and machine made snow. Reference measurements of specific surface area (optically equivalent grain size) and density are done by micro-computed tomography and used as input for transmittance calculations. A diffuse flux extinction model cannot be applied to simulate transmittance as our setup cannot be approximated by an infinite snow block thickness. Calculations with a more intricate radiative transfer model (DISORT) agree with our measurements within the estimated grain size and density variability for all probed natural snow types. Only our machine made snow shows a morphology which cannot be modeled by DISORT. Thus, our results show for the first time a direct experimental correlation between transmittance and snow specific surface area and density without the need for an empirical fitting parameter. We feel this to be an important step towards a possible high-resolution, quantitative optical measurement method to determine snow density in combination with an independent specific surface area measurement.     

保护气氛对Fe80Si9B11非晶带材表面特征及磁性能的影响

非晶合金带材的表面特征对磁性能有着重要的影响。为研究CO保护气氛对Fe₈₀Si₉B₁₁非晶带材表面鱼鳞纹的形成及非晶合金带材磁性能的影响规律,本文采用平面流铸法制备了Fe₈₀Si₉B₁₁非晶合金带材,利用金相显微镜、X射线衍射仪和磁性能测试仪研究了熔潭保护气体CO流量对非晶合金带材表面特征、微观结构及磁性能的影响。研究表明,不同CO流量条件下制备的合金带材均为非晶态,且CO流量对带材厚度几乎无影响;随CO流量增大,熔潭周围气体密度和气压降低,熔潭稳定性增强,裹入熔潭的气体变少,带材贴辊面气泡尺寸变小且数量减少,带材表面的鱼鳞纹间距增大。当CO流量为0时,带材表面比较粗糙,观察不到鱼鳞纹;当CO流量为0.25和0.5 m³/h时,带材表面鱼鳞纹间距分别为1.5、2.3 mm。随CO流量增大,由于气泡对畴壁的钉扎和鱼鳞纹细化磁畴的协同作用,使带材矫顽力和铁损降低,振幅磁导率增大;磁损耗分离结果表明,随CO流量增大,磁滞损耗降低,涡流损耗增大,且铁损降低主要由磁滞损耗降低引起的。     

Melt Spinning of Ni-12.5 wt. % Al Alloy

Melt spinner has been used for fabrication of Ni₃Al alloy (Ni-12.5 wt.% Al) ribbon. The optimal speed of wheel during melt spinning was 20.5 m s_-1. The thickness of the ribbon was between 14 and 60 μm and width 2 mm. During melt spinning some powder appeared as a by-product, dependent on the speed of copper wheel. Morphology of the ribbon and powder was observed. Ribbon tensile strength of up to 937 N mm^-2 and reduction of area up to 20% was measured.     

Production of metallic glass ribbons by the chill-block melt-spinning technique in stabilized laboratory conditions

The results of extensive studies on the production of metallic glass ribbons by a single jet chill-block melt-spinning technique in laboratory conditions are summarized with emphasis on the data of practical importance. A device that stabilizes quenching conditions by surrounding the melt puddle with an atmosphere of He gas is described. The conditions for high stability are defined. The dependence of ribbon width and thickness on the volumetric flow rate, injection angle and substrate velocity are experimentally determined in such stabilized conditions. The cross-sectional geometric uniformity of the ribbons, analysed by Talysurf, is shown to be comparable with those produced by commercial laboratories, and/or within specially constructed chambers.     

Digital photocalorimetric measurements of cooling rates in chill block melt spinning of MmNiCoMnAl5 hydride forming alloy

Abstract

A digital photocalorimetric technique has been developed and applied to obtain in situ temperature measurements from chill block melt spun ribbons of a MmNiCoMnAl5 hydride forming alloy. Compared with conventional colour transmission temperature measurements, this technique offers special advantages in terms of high resolutional and positional accuracy, which under the prevailing experimental conditions are found to be +-29 K and +-0.1 mm respectively. Moreover, it is shown that the cooling rate in the solid state is approximately 2.5 times higher than observed during solidification, indicating that the solid ribbon stays in intimate contact with the wheel surface down to very low metal temperatures before the bond is broken. During this contact period, the cooling regime shifts from near ideal in the melt puddle to near Newtonian towards the end, when heat transfer from the solid ribbon to the wheel becomes the rate controlling step.     

Magnetostriction Behavior of Pseudobulk CoFeBSiNb(Ga) Systems

Co-based ferromagnetic amorphous systems prepared by rapid quenching are parts of a very interesting group of materials from the perspective of new magnetic application possibilities. Practical applications of amorphous metallic glasses are in some cases limited by the thickness of the ribbon. In the present contribution as-cast and after annealing Co–Fe–B–Si–Nb ribbons (monolayer), pseudobulk (bilayer, trilayer) materials with increased thickness in comparison with ribbons (monolayer) and Co–Fe–B–Si–Nb-Ga ribbon were investigated by measurements of magnetostriction λ _s . These materials were prepared by planar flow casting method (PFC) and modified PFC method with a single crucible which has two or three nozzles for preparation of pseudobulk samples, respectively. The field dependence of the parallel [λ _par(H)] and perpendicular [λ _perp(H)] magnetostrictions in the sample plane was measured at room temperature. These dependencies were used to calculate the values of saturation magnetostriction λ _s . Microstructure of the as-cast samples as well as annealed samples was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM).     

The radiation of alumina melt in the middle IR spectral region at solidification in the course of free cooling

The results are presented of radiation intensity measurements in the middle IR spectral region from 3 to 11 μm at cooling and solidification in ambient air for both “thick” layers several millimeters thick and “thin” layers several tenths of a millimeter thick of a pure aluminum oxide melt. In the former case, the melt was held in a powder bed of the same material; in the latter case, a pool with the melt was created on a ceramic surface. The results on different wavelengths are presented as time dependences of the infrared spectrometer signals, proportional to the intensity of radiation emitted by the melt. In addition, the data on the time dependence of the emissivity for thick melt layers at a solidification plateau are presented. On the basis of the obtained results and the results of previous investigations and radiative-conductive heat transfer calculations at cooling and solidification, it is shown that the intensity of emitted radiation at crystallization is defined not only by optical properties of a melt and a crystal, but depends also on the thickness and the temperature field of the melt before the start of cooling. An almost horizontal section on the solidification plateau is caused by the presence of an isothermal two-phase zone; however, with a melt thickness of several millimeters, such a section of different length is observed only in the range of wavelengths from 5 to 11 μm.     

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

Abstract

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.     

Manufacture of amorphous alloy ribbons

Amorphous alloys are prepared in continuous ribbon form by rapid quenching directly from the melt. In particular, the process of chill block melt-spinning involves the continuous impingement of molten alloy against a rapidly moving substrate surface. Principles of chill block melt-spinning are presented with regard to the formation of continuous, rapidly-quenched amorphous alloy ribbon. The effects of numerous process variables on sample geometry and physical properties are examined through experimental results obtained by the author and by other researchers. Principles of narrow ribbon manufacture are extended to describe means of fabricating wide ribbon. Manufacturing problems unique to extended runs and potential solutions are cited. Effects of process parameters on magnetic and physical properties of as-cast samples are discussed.     

Coupled morphological and thin film instabilities generated by inclusions in crystal growth

The interaction of a foreign particle that is suspended in the melt with a planar solidifying interface may induce the onset of morphological instabilities provided that its distance from the interface falls below a critical value. This distance, which is of the order of the particle’s radius, depends on the governing processing and physical parameters. When the particle is in nearcontact with the solid-liquid interface, the disjoining pressure in the melt film that separates the particle from the interface influences the interaction. We derive an expression for the film thickness at which rupture occurs. The critical film thickness, which depicts the competition between the stabilizing influence of surface tension and thermal gradients and the destabilizing influence of the intermolecular forces, varies as (Sh)^1/4, where Sh is the Scheludko number that is modifed by the imposed thermal gradients. We note the existence of a critical value for the particle’s radius below which the stabilizing effects are primarily due to surface tension and above which they are due to the thermal gradients.     

Measurements of the relative momentum accommodation coefficient for different gases with a viscosity vacuum gauge

The viscosity vacuum gauges are based on the gas momentum transfer phenomena between a moving part of the gauge and a stationary surface. Thus, they may be used for the study of the momentum accommodation coefficient for various combinations of gas species and surfaces. The aim of the present work is to determine the momentum accommodation coefficient by means of the viscosity vacuum gauge with vibrating metal ribbon. The relative accommodation coefficient was computed from the measurements for Xe, Ar, He and H₂ on the bronze ribbon of the gauge in the molecular conditions. The values of the relative coefficients were 0.90 for Xe, 0.95 for Ar, 1 for He (values were normalised to data obtained for He) and 0.94 for H₂.     

Planar-flow spin-casting of molten metals: process behaviour

An experimental study of the planar-flow melt-spinning process was performed in order to gain a better understanding of the steady-state production of microcrystalline and amorphous ribbons. The dependence of the thickness of the ribbon product,T, on process parameters (wheel speed,U, nozzle/wheel gap,G, overpressure, ΔP, nozzle-slot breadth,R, and nozzle-slot width,W) was determined using an apparatus designed to deliver reproducible results. Thicknesses were reproducible to within 5%–8%. Guided by dimensional analysis, the non-dimensional thickness (T/G) was found to depend, within the experimental error, only on a non-dimensional pressure drop (ΔP/ρU ²) and slot breadth (R/G) for fixed thermal conditions. Data from the literature and our data, which considerably extend the range, correlate consistently on this basis. In contrast to the steady behaviour, the limits within which a uniform ribbon can be formed depend on a larger set of parameters; this dependence is sketched with the available data. Finally, a variety of observed ribbon surface textures (free meniscus side) is catalogued.     

Proximity and interaction effects in arrays of I-bars

Calculations have been made of interactions and fields in two-dimensional periodic arrays of I-bars. The results of the study shed light on several questions of interactions among bubble propagation elements. We have calculated interaction effects in arrays of I-bars as a function of both longitudinal and transverse spacing between bars. Three different geometries of I-bar show similar effects; the transverse and longitudinal interactions have opposing signs and are stronger in the transverse direction. We also have calculated local and average fields for an array of I-bars of geometries chosen to facilitate comparison with recent measurements. The local field in the gap between elements is found to be highly nonuniform but with an average value that obeys a simple relationship as the gap length is varied. The field profile is also nonuniform in the transverse direction, and exhibits a proximity enhancement that depends upon gap dimension. These results support the interpretation that bar-bar interactions can influence the magnetization in propagation arrays.     

Influence of rapid solidification route on microstructure and properties of a thermally stable Al-Cr-Zr alloy

An Al-5% Cr-2% Zr (wt%) alloy for elevated temperature service has been rapidly solidified (RS) by gas atomization and planar flow casting (PFC). A comparative study of the atomized powder and the planar flow cast ribbon has been performed during consolidation by extrusion. Particular attention has been given to the microstructural changes during fabrication and the influence of RS particulate type on extrusion performance, microstructure and mechanical properties. Microstructurally it has been shown that the more homogeneous PFC ribbon has advantages over the atomized powder and can improve mechanical properties. However, optimization of the thermomechanical processing of the PFC ribbon is necessary to achieve significant performance benefits in comparison to the atomized powder.     

Wetting pattern characterization of rapidly solidified centrifuge melt-spun metallic ribbons

The wetting pattern characterization of rapidly solidified centrifuge melt-spun metallic Al-Ge ribbons has been enabled by measuring the roughness of the rim-contact side of the ribbons. Roughness parameter values were obtained using the SEM ability of amplitude modulation and line scanning of the smooth ribbon surfaces, and micro-computerized digitizing of the line profiles. In such a manner, quantification of the wetting patterns is possible. As a result, the respective contribution of the ejection pressure and the extraction velocity on the wetting patterns of the solidifying ribbons is evaluated. In centrifuge melt spinning, the roughness of the rim-contact surface of the ribbons increases when extraction velocities are increased. Despite the additional roughness and the bad thermal contact conditions for effective heat transfer, higher cooling rates (as measured by secondary dendritic arm spacings), are achieved. It appears that a kind of dragging occurs, reducing the ribbon thickness by a shear drawing mechanism. This behaviour is characteristic of centrifuge melt spinning because of the specific hydraulic and heat transfer features involved with the counter-rotating of the crucible and the substrate.     

Model based gap and melt rate control for VAR of Ti-6Al-4V

A new controller has been designed for vacuum arc remelting titanium alloys based on an accurate, low order, nonlinear, melting model. The controller adjusts melting current and electrode drive speed to match estimated gap and melt rate with operator supplied reference values. Estimates of gap and melt rate are obtained by optimally combining predictions from the model with measurements of voltage, current, and electrode position. Controller tests were carried out at Timet Corporation’s Henderson Technical Laboratory in Henderson, Nevada. Previous test results were used to correlate measured gap to voltage and current. A controller test melt was performed wherein a 0.279 m diameter Ti-6Al-4V electrode was melted into 0.356 m diameter ingot. Commanded melt rate was varied from 20 to 90 g/s and commanded gap was held at 1.5 cm. Because no measure of electrode weight was available on the test furnace, electrode position data were analyzed and the results used to determine the actual melt rate. A gap-voltage-current factor space model was used to check estimated gap. The controller performed well, and both melt rate and electrode gap control were successfully demonstrated.     

Photonic-Band Structures in Macroporous Silicon

Macroporous Si was prepared by electrochemical etching at a linearly varying applied voltage. A relationship between the process parameters (illumination intensity, current density, and applied voltage) was derived in a diffusion–drift model. The experimental dependences of the ratio of the photohole concentration to the current density on the distance between the illuminated surface and pore bottom were shown to agree with the diffusion–drift model for nonequilibrium-hole transport provided that the anode thickness exceeds the diffusion length of holes and the pore radius is comparatively large. The photonic band gap of the two-dimensional macroporous Si structure was calculated by the plane-wave method. The transmittance of macroporous Si was measured. The effect of surface recombination on the lifetime of nonequilibrium charge carriers was assessed.