Clarifying the mechanism of reverse structuring during electrodeposition in magnetic gradient fields

Deviating from the common expectation, magnetoelectrochemical structuring during deposition of diamagnetic ions was demonstrated, very recently. To achieve this, electrochemically inert paramagnetic ions have to be added to the electrolyte and the deposition has to be performed in a magnetic gradient field. A reverse structuring occurs, yielding thinner deposits near high gradient regions. In this paper we aim to clarify the mechanism of this reverse structuring. Potentiodynamic and potentiostatic investigations were performed, including measurements of the deposited mass with an electrochemical quartz crystal microbalance (EQCM). The convection of the electrolyte was studied in situ by astigmatism particle tracking velocimetry (APTV). It was revealed that during the reverse structuring a convection is induced in the electrolyte, which is directed away from the working electrode in regions of high magnetic gradients. Due to this additional convection, the overall deposition rate is increased, whereby it is locally reduced in regions of high magnetic gradients. The mechanism for reverse structuring is discussed in detail. Also, the influence of all relevant magnetic forces is addressed.     

Ultra-long periodically-twinned copper nanowires evolving from dendrites in interfacial electrodeposition

A simple and efficient template-free electrochemical route has been utilized to prepare ultra-long Cu nanowires at a steady water-oleic acid interface. The as-prepared nanowires have uniform diameters and aspect ratios of more than 1000, and periodically twinned structure. The investigation of the growth process has indicated that these nanowires actually evolve from the branches of dendrites. The morphology of the products can be tuned by controlling experimental parameters, such as electrodeposition time, metallic ions concentration, current density, the geometry of electric field and the property and state of the interfaces. The experimental results show that the steady water-oleic acid interface and a parallel electric field are crucial for the formation of dendritic Cu nanowires. It is believed that the higher Cu(2+) concentration near the water-oleic acid interface is favorable to induce interfacial electrodeposition. The electrochemical route is also suitable for the synthesis of other metal nanowires, such as Co nanowires.     

In situ observation of magnetic orientation process of feeble magnetic materials under high magnetic fields

Abstract

An in situ microscopic observation of the magnetic orientation process of feeble magnetic fibers was carried out under high magnetic fields of up to 10 T using a scanning laser microscope. In the experiment, carbon fibers and needle-like titania fibers with a length of 1 to 20 μm were used. The fibers were observed to gradually orient their axes parallel to the direction of the magnetic field. The orientation behavior of the sample fibers was evaluated on the basis of the measured duration required for a certain angular variation. As predicted from the theoretical consideration, it was confirmed that the duration required for a certain angular variation normalized by the viscosity of the fluid is described as a function of the fiber length. The results obtained here appear useful for the consideration of the magnetic orientation of materials suspended in a static fluid.     

In situ calibration for wide-angle, three-dimensional stereoscopic image analysis

Accurate measurement of three-dimensional object coordinates from stereoscopic images is an essential element in various applications that require three-dimensional position information. Conventionally, optical ray tracing has been the measurement method of choice. However, it requires accurate knowledge of geometrical and optical parameters, such as the image distance, camera locations relative to the object field, and size, shape, and refractive index of intervening elements, such as apparatus windows. On the other hand, all these parameters need not be known if an optical transformation method based on an in situ calibration experiment is used. Furthermore, the use of in situ calibration not only increases the effective accuracy of the measured three-dimensional object coordinates but also reduces significantly the computational time compared with conventional optical ray tracing. The computational efficiency of the technique used is essential, especially when the application requires multiple determinations of a large number of three-dimensional coordinates, such as is the case with three-dimensional particle-tracking velocimetry. The basic concept and formulation of an optical transformation method based on an in situ calibration experiment is introduced. The technique is first demonstrated with synthetic data, then case studies with actual in situ calibration data are discussed.     

Nonlinear calculation of three-dimensional static magnetic fields

We present here the principle and structure of a method to calculate the three-dimensional (3-D) static magnetic fields which have already permitted us to study hybrid magnets for magnetic resonance imaging and ion confinement. Field sources can be issued from resistive or superconducting coils, permanent magnets, and other magnetic bodies such as soft iron. It can be extended to very low-frequency fields calculation as long as eddy current effects do not intervene. We call this method CALMAG3D     

In situ nanocompression testing of irradiated copper

Increasing demand for energy and reduction of carbon dioxide emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates a fundamental understanding of how radiation-induced defects alter mechanical properties. Ion beams create radiation damage efficiently without material activation, but their limited penetration depth requires small-scale testing. However, strength measurements of nanoscale irradiated specimens have not been previously performed. Here we show that yield strengths approaching macroscopic values are measured from irradiated ~400 nm-diameter copper specimens. Quantitative in situ nanocompression testing in a transmission electron microscope reveals that the strength of larger samples is controlled by dislocation-irradiation defect interactions, yielding size-independent strengths. Below ~400 nm, size-dependent strength results from dislocation source limitation. This transition length-scale should be universal, but depends on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nanoscale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms.     

In situ light-scattering measurements of morphologically evolving flame-synthesized oxide nanoaggregates

Nonspherical Al(2)O(3) aggregates produced in a laminar counterflow nonpremixed methane flame were investigated with an in situ laser light-scattering (LLS) technique in combination with a thermophoretic sampling-transmission electron microscope (TS-TEM) method. These flame-synthesized nanoparticles clearly underwent morphological changes following their formation (from precursor trimethylaluminum hydrolysis), mainly as a result of aggregation and sintering processes in the ~3.3 x 10(4) K/s heating environment. To characterize this particulate morphological evolution conveniently we made multiangular absolute LLS measurements and interpreted them based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Optically determined fractal dimension D(f), mean radius of gyration, aggregate size distribution, and local particle volume fraction phi(p) were found to be consistent with our independent ex situ TS-TEM experiments. D(f) (optically inferred) increased from 1.60 to 1.84 with axial position, confirming the morphological evolution of alumina aggregates owing to finite-rate, spatially resolved high-temperature sintering. An extension of our TS-TEM method was successfully applied, for the first time to our knowledge, to inorganic particles. phi(p) inferred by means of this ex situ technique generally agreed with that from the in situ LLS technique, supporting our interpretation of both measurements. Moreover, an optically inferred net sintering rate of alumina aggregates approaching the flame was estimated to be consistent with the available TEM data. The LLS methods and results presented here are expected to permit more comprehensive mechanistic analyses of nanoaggregate sintering and coagulation kinetics in such flame environments, ultimately improving the modeling of more-complex (e.g., turbulent, high-pressure) combustion systems involving nanoparticle formation and evolution.     

Undercooling behavior of glass-fluxed Sb melts under gradient magnetic fields

The influence of gradient magnetic field on the undercooling behavior of glass-fluxed pure Sb melts was investigated using a superconducting magnet. It was found that under a positive gradient magnetic field, the mean undercooling of pure Sb melts increased with increasing magnetic field intensity. However, under a negative gradient magnetic field, the mean undercooling showed a decreasing tendency following an initial increase with increasing magnetic field intensity. The results were discussed by considering the Lorentz force and the magnetization force imposed by the gradient magnetic field.     

In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

Nuclear magnetic resonance techniques can be used to monitor in situ the dynamical behaviour of point and line defects in materials during deformation. These techniques are non-destructive and non-invasive. We report here the atomic transport, in particular the enhanced diffusion during deformation by evaluating the spin lattice relaxation time in the rotating frame, T ₁,in pure NaCl single crystals as a function of temperature (from ambient to about 900K) and strain-rate (to 1.0s^–1) in situ during deformation. The strain-induced excess vacancy concentration increased with the strain-rate while in situ annealing of these excess defects is noted at high temperatures. Contributions due to phonons or paramagnetic impurities dominated at lower temperatures in the undeformed material. During deformation, however, the dislocation contribution became predominant at these low temperatures. The dislocation jump distances were noted to decrease with increase in temperature leading to a reduced contribution to the overall spin relaxation as temperature is increased. Similar tests with an improved pulse sequence (CUT-sequence), performed on ultra-pure NaCl and NaF single crystals revealed slightly different results; however, strain-enhanced vacancy concentrations were observed. The applicability of these techniques to metallic systems will be outlined taking thin aluminium foils as an example.     

An investigation into the structure of aqueous NaCl electrolyte solutions under magnetic fields

Molecular dynamics simulations are performed to investigate the effects of magnetic fields with intensities of 1–10 T on aqueous NaCl electrolyte solutions at 298 K. The simulations employ the F3C (flexible three centered) water model and investigate electrolyte solutions with both low (1 M) and high (5 M) NaCl concentrations. The results show that the self-diffusion coefficient of the water molecules decreases in a low-concentration solution as the magnetic field intensity is increased, but increases in a high-concentration solution. The magnetic field enhances the mobility of the Na⁺ and Cl⁻ ions in both low- and high-concentration solutions. The average number of hydrogen bonds increases when the magnetic field is applied to pure water or to a solution with a low NaCl concentration, but decreases in a solution with a high-concentration. The results show that the enhanced mobility of the ions under a magnetic field causes serious damage to the hydrogen bond network in the high-concentration solution. Conversely, in the low-concentration solution, the structural behavior is dominated by the properties of the water molecules, and hence the hydrogen bonding ability is enhanced as the magnetic field is increased.     

Low-temperature magnetic and thermal properties of CePd2In in magnetic fields

A detailed study of the low-temperature magnetization M and specific heat C of CePd₂In is reported. For B 0, M and C show maxima at T_N 1.25 K indicating an antiferromagnetic phase transition as observed previously. In a magnetic field B the transition seen in M is shifted to lower T while T_N as determined from the specific-heat maximum is essentially unaltered up to fields of 6 T. This different behavior is attributed to magnetic anisotropy of the polycrystalline samples. The hyperfine contribution to C is discussed in terms of nuclear quadrupole splitting as well as Zeeman splitting by applied and transferred hyperfine fields. A rough estimate of the Kondo temperature from C sufficiently above T_N yields T_K 1.5 K.     

In situ fabrication of titanium carbide reinforced copper MMC

Abstract

The in situ fabrication of titanium carbide reinforced copper and aluminium bronze (AB2) composites by carbothermal reduction of titanium in an induction furnace has been investigated. An inert atmosphere was maintained with carbon monoxide created as a byproduct from the heat of reaction between the induction field, graphite crucible and graphite lid. Titanium carbide particles of the order 1–3 μm were formed in aluminium bronze at approximately 1250°C and, in copper, particles of order 1–6 μm were produced at approximately 1330°C. Dispersion concentrations of titanium carbide of 20% and 6.5% were obtained for copper and aluminium bronze respectively. In addition, evidence is presented indicating that iron could be used as a dispersion medium for titanium carbide particulates in aluminium bronze alloys.     

Observation of the static nuclear magnetism of pure metallic copper in low magnetic fields

Using a specially designed mixing chamber for a dilution refrigerator and a magnetometer employing a superconducting device we have compared, over the temperature range 12 to 100 m°K, the static temperature-dependent moment of a sample of pure copper (resistance ratio 31,500) in a field of 5.8 G with the 17 Hz susceptibility of powdered cerium magnesium nitrate (CMN). Using cross-calibrational information from experiments on Cu(Mn) alloys we deduce that within the accuracy of measurement the observed temperature-dependent magnetic moment of the pure copper can be ascribed to nuclear magnetism only, with a calculated Curie constant of 4.5×10^–8°K. The experiments are not sufficiently precise to shed new light on the CMN temperature scale, but are not in disagreement with our earlier conclusions. A remarkable but unexplained electronic Curie-law background magnetization is observed in the empty magnetometer.Supported by the U.S. Atomic Energy Commission under Contract AT(04-3)-34, P.A. 143.     

Deformation of copper under the action of strong magnetic fields and cryogenic temperatures

Results are presented for an experimental study of the effect of a strong constant and pulsed magnetic field on the mechanical characteristics and deformation features of copper M1 at 293–4.2 K. Dependences have been obtained for the increase in specimen strain on the intensity of a pulsed magnetic field and flow stress, and possibilities have been studied for increasing the effectiveness of pulsedmagnetic deformation of metals. A calculation relationship is suggested for determining the critical stress level under strong skin-effect conditions of a magnetic field.Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 12, pp. 32–36, December, 1989.     

Integrated gray-level gradient method applied for the extraction of three-dimensional velocity fields of sprays in in-line digital holography

In this study, an integrated gray-level gradient method is applied to extract the three-dimensional (3D) velocity fields of sprays. This method consists of a conventional edge-sharpness method and a new method, namely, the overall-sharpness method, which is an efficient supplement of the former. And then the synchronization system is designed and assembled to record double-exposure spray holograms in a short time interval. Finally, using the integrated gray-level gradient method and some image processing techniques, the 3D coordinates of droplets are easily obtained, which can be used to evaluate the 3D velocity fields and the size features of spray droplets in different spray injection pressures. It proves that the integrated gray-level gradient method is well applied to measure the characteristics of sprays in in-line digital holography.     

In situ TEM observations of high-strain-rate deformation and fracture in pure copper

The plastic deformation of polycrystalline metals at high strain rates is controlled by the way defects (dislocations and twins) nucleate, propagate, and interact in the microstructure. To-date, the role of these defects has been estimated based on dynamic mechanical measurements coupled with ex situ investigations of the deformed microstructure. However, such investigations are fundamentally limited in their ability to characterize transient mechanisms. Here, we present for the first time direct, experimental observations of the nucleation, motion, and interaction of defects and cracks during deformation of pure copper at strain rates between 10³ and 10⁴ s⁻¹. These observations are enabled by coupling a custom-built in situ high-rate straining stage with nanosecond-resolution dynamic transmission electron microscopy. The results show that while twins play only a minor role in the deformation of copper at quasi-static strain rates, the twin nucleation rate increases markedly at high strain rates. The preferred nucleation sites for twins also change, and the new twin interfaces become preferential paths for crack propagation, facilitating fracture through the original grains.     

Effect of three-dimensional melt pool convection on process characteristics during laser cladding

A three-dimensional heat transfer model is developed to simulate the cladding process that include the different physical phenomena such as heat transfer, phase changes, addition of powder particles and fluid flow due to Marangoni–Rayleigh–Benard convection. It is found that the Rayleigh–Benard convection is insignificant and Marangoni–Benard convection is dominant for the studied cases. By varying the scanning speed and Marangoni number the melt pool size and strength of convection are changed and its influence on clad built-up geometry, dilution level, maximum and average melt pool temperatures and the form and scale of the microstructure of the solidified clad track has been studied.     

In situ monitoring of the microstructure of detergent drops during drying using a rapid nuclear magnetic resonance diffusion measurement

The application of a nuclear magnetic resonance (NMR) diffusion measurement suitable for monitoring, in situ, the evolution of microstructure in commercial surfactant-based detergent drops as a response to drying is demonstrated for the first time. We utilise pulsed field gradient (PFG) NMR diffusion techniques to observe variations in self-diffusion coefficient, surface-to-volume ratio and characteristic pore size by probing the water content of the drops. Previously, we quantified the evolution of microstructure in vials of detergent paste using conventional PFG techniques. Now we apply a rapid PFG technique to enable this measurement during the relatively fast in situ drying of a drop with spherical geometry, relevant to spray drying processes. A finer structure is seen to form in the drops compared to the paste in the vials.     

A numerical analysis of time-dependent two-dimensional magnetic fields

The nonlinear diffusion equation as applied to two-dimensional time-dependent magnetic fields is solved with a finite element algorithm. This algorithm permits the analysis of problems possessing complex geometries, induced eddy currents, permanent magnets, and nonperiodic excitation currents. The numerical procedure utilizes implicit time stepping with an iterative scheme to solve the resulting set of equations. Two examples of applications of this program are presented.