Structure and magnetic properties of hot pressed Co-based powder

Coarse-grained powders, prepared by short-time milling of Co_70.3Fe_4.7Si₁₀B₁₅ amorphous ribbons, were consolidated by hot pressing into disc-shaped specimens. The influence of the consolidation temperature on the structure and the magnetic properties of the bulk specimens was investigated. The lowest value of coercivity of 23 A/m was achieved for samples prepared by consolidation of the powder in the supercooled liquid region at a temperature of 813 K. In order to increase the saturation magnetic polarization crystalline iron powder was added to the amorphous Co_70.3Fe_4.7Si₁₀B₁₅ powder.     

The sound speed and attenuation in loose and consolidated granular formulations of high alumina cements

Clinkers of high alumina cements are separated into three granular formulations with particle sizes in the range 0.6-0.71 mm, 0.71-1.18 mm and greater than 1.18 mm. These are used to manufacture consolidated samples of porous concrete in an autoclave. The acoustic and microscopic properties of loose and consolidated porous samples of concrete are investigated using both experimental methods and mathematical modelling. Values of porosity, flow resistivity, tortuosity and parameters of the pore size distribution are determined and used to predict closely the sound speed, acoustic attenuation and normal incidence absorption coefficient of these materials. It is shown that high alumina cements do not require additional binders for consolidation and that the structural bonds in these cements are developed quickly between individual clinkers in the presence of water. The hydration product build-up during the consolidation process is insignificant which ensures good acoustic performance of the consolidated samples resulting from a sufficient proportion of the open pores. The value of porosity in the consolidated samples was found to be around 40%, which is close to that measured in some commercial acoustic absorbers. This work provides a foundation for the development of acoustically efficient and structurally robust materials, which can be integrated in environmentally sustainable concrete and masonry structures.     

Experimental Determination of the Response Envelope of fine limestone

Designing modern devices for bulk solids storage and handling requires an increasingly complex understanding of the handled bulk solids' mechanical behaviour. Models able to describe the behaviour of bulk solids can neither be formulated nor their parameters determined from common shear tests, since these do not contain sufficient information especially about the materials' stiffness characteristics. The paper describes the response envelope as a measure of the deformation resistance and its experimental determination from biaxial tests. It is shown that for fine limestone powder the stiffness in compression is only about 1/70 of the stiffness in expansion. The shape of the response envelope depends only on the ratio between the principal stresses at the stress state considered, whereas the size of the response envelope also depends on the stress magnitude.     

Transport and mechanical properties of Yb-filled skutterudites

A series of Yb-filled skutterudites were produced and powder samples consolidated using spark plasma sintering (SPS). The effect of different heating cycles on the resulting transport properties of the consolidated samples was explored. Specifically, the effect of sample uniformity on the electrical and thermal transport properties was explored. In addition to the optimal Yb-filling fraction, other factors, such as heating profiles and sintering conditions, were found to play a pivotal role in the performance of these materials. Large quantities of Yb-filled skutterudite material can be generated with high purity and uniformity. Resonant ultrasound spectroscopy was used to determine the elastic modulus and Poisson's ratio. Fracture strength was measured for 12 specimens and, taken with information obtained from resonant ultrasound spectroscopy and from thermal expansion and thermal transport characteristics, the thermal shock resistance parameter was evaluated. These parameters will be important for the engineering of thermoelectric modules based on skutterudite materials.     

在硅片上沉积厚二氧化硅的火焰水解法研究

用火焰水解和高温烧结的方法在单晶硅基片上制备了厚SiO2和B2O2-P2O2-SiO2光波导包层材料。并用扫描电镜(SEM)和X射线粉末衍射(XRD)方法对其微观形貌和物相结构进行了观察和检测。重点对硅基片上沉积厚SiO2时的龟裂和析晶问题进行了深入研究。从扫描电镜照片可以看出．火焰水解法形成的SiO2粉末呈多孔的蜂窝状结构。这种粉末具有很高的比表面积,因而很容易烧结成玻璃。X射线衍射图谱表明．这种粉末是完全非晶态的。经过烧结以后,从扫描电镜照片可以明显看出硅基片上的SiO2薄膜出现龟裂。同时,X射线衍射测试结果表明有少量SiO2析晶。而通过在SiO2中掺入B2O3、P2O5,上述龟裂和析晶完全消失。用这种工艺制备的SiO2波导包层材料厚度达到20μm以上,表面光滑、没有龟裂,而且是完全玻璃态的,可以用于制备性能优良的各种硅基二氧化硅波导器件。     

The influence of iron,fluorine and boron implantation on the magnetic properties of graphite

Iron, fluorine and boron ions were implanted into highly oriented pyrolytic graphite (HOPG). The samples were characterized before and after ion implantation as well as after heat treatments in vacuum by measurements of the magnetic moment and element analysis. Whereas the main magnetic contribution remains diamagnetic the paramagnetic one clearly increases with implantations and correlates with the amount of implanted ions. It is shown that a large part of the paramagnetic contribution is caused by the structural disorder created by particle bombardment using iron, fluorine or boron. All implanted HOPG samples show practically no change of the small ferromagnetic signal observed in their virgin state. No particular influence of iron on the ferromagnetic properties of HOPG is observed, up to ∼4000 μg/g Fe-concentration in the implanted region. For comparison, ferrous sulphates were added to ultra-clean graphite powder. This iron addition increases the number of paramagnetic spins proportional to the iron content in the untreated samples. In heat-treated samples however, a clear ferromagnetic behaviour is observed due to the formation of a ferromagnetic iron compound.     

Dynamic compaction of copper powder: 2D numerical simulation and experimental results

Abstract

A method for plate-impact dynamic compaction of copper powder has been developped. The optimization of the experimental set-up (impedance adjustments, tensile wave traps, relative thickness of impactor and target,…) is presented.

2D axisymetrical numerical simulations have been performed with a Lagrangian finite element code. Geometrical characteristics of the experimental set-up as well as the dynamic response of the powder (Reaugh equation of state) and of the material of the set-up have been taken into account. These simulations show that, due to the difference in shock velocities in the container and in the powder, the powder is submitted to 2D loading waves. As a matter of fact the powder may be loaded by a non-planar shock wave propagating in the as-expected direction, as well as by a sweeping wave initiated at the bottom of the powder container, and propagating obliquely from the bottom-up. This second wave loads the bottom of the powder first. The influence of the impactor thickness as well as its material on the shock front shape and on the shock density-pressure history of the material has been studied. 1D simulations are shown not to evaluate properly the stress history and the energy deposition in the powder sample.

Metallographic observations as well as X-ray tomography experiments have been performed on consolidated samples. A very good agreement has been found between results of 2D numerical simulations and the observed final shape and density maps of the samples. The shape of deformed powder particles are also in agreement with the expected shock history.     

Interpretation of the stress dependence of creep by a mixed climb mechanism in TiAl

The behaviour of ordinary dislocations in TiAl alloy creep-deformed at 750°C has been investigated. Two alloys processed by the cast and powder metallurgy routes were crept under tensile stresses of 150 and 80?MPa, respectively. Transmission electron microscopy was performed on the crept samples to determine the characteristics of the dislocations. The stress dependence of the dislocation mechanisms was determined from stress jumps performed during creep deformation. Complementary in-situ heating experiments performed on previously crept samples are also presented. From these experimental investigations, it is shown that some ordinary dislocations move by a mixed climb mechanism for which the elementary process is the nucleation and the lateral propagation of a jog pair. The coherency between this mechanism and the determined activation parameters, as well as the driving force at its origin, are discussed.     

Stress Inhomogeneity in Powder Specimens Tested in the Jenike Shear Cell: Myth or Fact?

In the mechanical characterization of powders using the direct shear testers such as the Jenike shear cell, the existence of a uniform or well‐defined stress field in a powder specimen is assumed. This assumption has not been subjected to any serious scrutiny in the literature. In this study, the normal stress variation in a silica powder was locally determined by locating a pressure‐sensitive TekScan pad at the bottom section of a Jenike shear cell. A computer simulation of the consolidation and pre‐shearing stages of the Jenike test procedure was performed using the Discrete Element Method (DEM). The paper presents both experimental and computational evidence for the existence of a complex stress field in the powder specimen, thus clearly invalidating the long‐standing stress homogeneity assumption in the direct shear testing of powders. The implications of the stress inhomogeneity in terms of the accuracy of the material properties extracted from the Jenike test are also presented.     

The Crystalline Structure of Nascent Ultra High Molecular Weight Single Particles and Its Change on Heating,as Revealed by in-situ Synchrotron Studies

Abstract

A solvent-free route to high performance ultra-high molecular weight polyethylene (UHMWPE) film threads is currently under intensive development. It involves compaction/sintering of UHMWPE reactor powder at T?<?T_m followed by orientation hardening of the sintered film cut into narrow strips. However, not any kind of reactor powder can be transformed into the desired high-performance material. The presence of a monoclinic crystalline phase (MP) in the powder is considered as one of the key parameters indicating its applicability for solvent-free processing. Since the MP is stable only under stress, the assumption has been made that the observed MP is generated during tableting for X-ray analyses rather than during synthesis of the nascent powder. We show that comparative X-ray analysis of a tablet and a single “virgin” particle using synchrotron radiation indicates that the MP content in the virgin particle was far less than that in the compressed tablet. Only the (001) MP peak was resolved while the others were severely overlapped with the normal, orthorhombic reflections. Thus, it supports our idea that the widely observed MP phase is, for the most part, generated during the sample preparation for the X-ray analyses.     

A preservation study of carbon nanotubes in alumina-based nanocomposites via Raman spectroscopy and nuclear magnetic resonance

Raman spectroscopy was used to study the preservation of the carbon nanotube structure in nanotube-reinforced alumina nanocomposites consolidated via spark plasma sintering (SPS). A series of Raman spectroscopy experiments was used to identify the thermal breakdown temperature of single-walled carbon nanotubes (SWCNTs) embedded in nanocrystalline alumina. It was found that the carbon nanotube structure remains intact after sintering at 1150 °C, but almost completely breaks down by 1350 °C after only 5 min. Also, ²⁷Al nuclear magnetic resonance (NMR) was used to study the chemical and structural effects of high-energy ball milling (HEBM) and SPS consolidation on pure alumina and SWCNT-alumina nanocomposites. HEBM does not change the mixed coordination number of the as-received alumina, but slight peak shifts indicate residual stresses. No Al₄C₃ was detected in any of the consolidated samples – even up to 1550 °C for 10 min. Thus, it is concluded that consolidation of carbon nanotube-reinforced composites should be completed at temperatures below ∼1250 °C in order to preserve the carbon nanotube structure. PACS 61.18.Fs; 61.46.Fg; 61.82.Rx; 62.25.+g; 76.60.-k     

Three-dimensional strain localization of water-saturated clay and numerical simulation using an elasto-viscoplastic model

Since strain localization is a precursor of failure, it is an important subject to address in the field of geomechanics. Strain localization has been analysed for geomaterials by several researchers. Many of the studies, however, treated the problems brought about by strain localization as two-dimensional problems, although the phenomena are generally three-dimensional. In the present study, undrained triaxial compression tests using rectangular specimens and their numerical simulation are conducted in order to investigate the strain localization behaviour of geomaterials under three-dimensional conditions. In the experiments, both normally consolidated and over-consolidated clay samples are tested with different strain rates. Using the distribution of shear strain obtained by an image analysis of digital photographs taken during deformation, the effects of the strain rates, the dilation, and the over-consolidation on strain localization are studied in detail. The analysis method used in the numerical simulation is a coupled fluid-structure finite element method. The method is based on the finite deformation theory, in which an elasto-viscoplastic model for water-saturated clay, which can consider structural changes, is adopted. The results of the simulation include not only the distribution of shear strain on the surfaces of the specimens, but also the distributions of strain, stress, and pore water pressure inside the specimens. Through a comparison of the experimental results and the simulation results, the mechanisms of strain localization are studied under three-dimensional conditions.     

Coercivity enhancement of HDDR-processed Nd-Fe-B permanent magnet with the rapid hot-press consolidation process

High coercivity, fully dense anisotropic permanent magnets of submicron grain sizes were produced by rapid hot-press consolidation of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-Co-B powders. In the hot-press process, the coercivity of the consolidated material showed a sharp minimum prior to full densification. Thereafter, it reached a value 25% higher than that of the initial powder. Scanning electron microscopy and transmission electron microscopy observations revealed that the variation in H_cJ was caused by a redistribution of Nd along the grain boundaries during hot pressing and that the high coercivity was attributable to the formation of thin, continuous Nd-rich phase along the grain boundaries.     

Neutron depolarization study on the magnetic correlation length of nickel ferrite with different packing densities

The magnetic correlation length of a mixed nickel ferrite powder was studied by a newly commissioned depolarized neutron beamline at the W3 port of Tsing Hua Open Pool Reactor (THOR). In this work, Ni ferrite powder samples with different packing densities were studied. The magnetic correlation lengths of the sample were observed to be 2 μm at virgin state and about 3.1 μm at remanent state from the packing density of 20–60%. This magnetic domain size is smaller than particle size. No significant change of domain size at this packing density implies the domain wall motion is hindered by the porosity effectively up to at least 60% of packing density.     

Experiment study of powder flow feed behavior of laser solid forming

A photographic system for the powder feed process of laser solid forming (LSF) was developed using a high speed camera, and the powder feed behaviors (the particle speed and the powder flow concentration) were described based on the powder flow images. The influences of the powder feed parameters and the distance below the nozzle exit plane on the powder feed behaviors were discussed, and the influences of the powder feed behaviors on the deposited layer quality were also investigated. It can be seen that the smooth finish of the deposited layer surface was improved remarkably by increasing the particle speed, and the deposited layer height decreases with the increase of the particle speed. It can also be found that the variation of the deposited layer height with the increase of the distance between the deposited surface and the nozzle exit plane is similar to that of the powder mass concentrations on the vertical symmetry axis.     

Relationships Between Particle Properties and Bulk Behaviour for some industrial powders

A series of industrial powders, most of them organic, were characterized in terms of volumetric mean size, size distribution, density, particle shape, water rentention characteristics (under a series of controlled ambient relative humidities) and loosely packed and densely packed voidages (giving the compressibility). The behaviour of the powders in powder handling apparatus (a precision powder filler) was also determined. The minimum and maximum bulk densities of the powders were related to the properties of the individual particles by means of a semi-empirical equation. The filling capacity of the powder handling apparatus was found to be related to the maximum bulk density of the powder while the filling accuracy turned out to be related to the compressibility of the powder. Knowing these interrelations made it possible to predict the powder behaviour in the handling apparatus directly on basis of the particle properties.     

Fourier‐transform Raman analysis of milk powder: a potential method for rapid quality screening

In this work, FT‐Raman spectroscopy was explored as a fast and reliable screening method for the assessment of milk powder quality and the identification of samples adulterated with whey (1–40% w/w). Raman measurements can easily differentiate milk powders without the need of sample preparation, whereas the traditional methods of quality control, including high‐performance liquid chromatography, are laborious and slow. The FT‐Raman spectra of whole, low‐fat, and skimmed milk powder samples were obtained and distinguished from commercial milk powder samples. In addition, the exploratory analysis employing data from Raman spectroscopy and principal component analysis (PCA)allowed the separation of milk powder samples according to type,identifying differences between samples in the same group. Multivariate analysis was also developed to classify the adulterated milk powder samples using PCA and partial least squares discriminate analysis (PLS‐DA). The resulting PLS‐DA model correctly classified 100% of the adulterated samples. These results clearly demonstrate the utility of FT‐Raman spectroscopy combined with chemometrics as a rapid method for screening milk powder. Copyright © 2011 John Wiley & Sons, Ltd.     

Acoustic emission monitoring of surface hardening by laser

The acoustic emission signals from the laser surface heating of En8 steel, a stainless steel, 0.25% C mild steel, an /β brass, and Al-4% Cu were analysed. The stainless steel was acoustically inactive but it was found that transformation processes in En8 and mild steel emitted a signal which could be used as part of a feedback loop to control the extent of the process. Similarly the process could be controlled in the case of the non-ferrous alloys by the monitoring of the emissions associated with one or more of the following: surface melting, cracking and an order-disorder transformation.

The acoustic emission signals measured during laser cladding, in which powder is blown into the laser generated weld pool, were totally dominated by the noise from the powder striking the surface. These signals (in particular the RMS) appeared capable of being used to monitor the powder feed rate and the behaviour of any auxillary equipment such as an ultrasonic vibrator.

The acoustic signals from a cooling clad trace of Stellite 6 on En3 steel plate varied with the size of the deposit and the intensity of the residual stresses allowing a method of comparing the effect of vibration on residual stress in vibrated or non-vibrated specimens.     

Relaxation and Creep of Dry Bulk Solids

In former investigations it has been shown that creep (constant stress, altering strain) and relaxation (constant strain, decreasing stress) can be observed with dry bulk solids. Both effects are covered when investigating the time dependent behaviour of bulk solids where time dependence can also mean an increase of the deformation resistance with increasing deformation rate. In this paper the investigated time dependent effects do not include time consolidation. The effects of creep and relaxation are often neglected for bulk solids because in many applications the influence of these time dependent behaviours is of minor importance. A deeper insight into the bulk solids flow characteristics and mechanisms can only be obtained when time dependence is taken into consideration.     

Stress insensitive NiCuZn ferrite compositions for microinductor applications

Two series of stoichiometric and iron deficient NiCuZn ferrites were prepared by the conventional ceramic method. The formation of single phase was confirmed by X-ray diffraction in pure ferrites and a second phase was noticed in SiO₂ added ferrite samples. Initial permeability measurements on these samples were carried out in the temperature range of 30–300 °C. The effect of external applied stress on the open magnetic circuit type coil with these ferrite cores was studied by applying uniaxial compressive stress parallel to the magnetizing direction and the change in the inductance was measured. These studies show that stress sensitivity is more in stoichiometric ferrite samples than in iron deficient ferrite samples. In order to make them stress insensitive various amounts of SiO₂ were added and the stress insensitivity was examined. SiO₂ addition was found to be effective in reducing the stress sensitivity. This was confirmed by elastic behaviour studies at room temperature on these ferrite samples by employing composite oscillator technique. These ferrite compositions have been developed for their use as core materials for microinductor applications.