Characterizing the Rheology of Powders by studying dynamic avalanching of the powder

Avalanching of solids is an important multivariable dependent phenomenon that can be a valuable tool for characterization of solids flowability. Although avalanching by itself is primarily chaotic it appears that the fractal analysis of its avalanching mass data can yield very interesting and significant information that is of importance in determining suitability of a material in solids handling and pneumatic transport.     

Studying the Avalanching Behaviour of a Powder in a Rotating Disc

The use of a rotating disc to study the avalanching behaviour of a powder is discussed. It is shown that the strange attractor plotted in discreet time maps summarizes useful information on the rheological behaviour of powders and powder mixtures. In particular it is shown that the avalanching behaviour is related to the particle size distribution of the powder and that one can study the changes in rheological behaviour as another powder is mixed with it. The strange attractor patterns generated are dependent upon the environmental conditions under which the experiments are carried out. For this reason the measurements are referred to as an assessment of the holistic powder rheology. The potential use of the disc to study the holistic rheology of powder systems is outlined.     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Avalanching patterns of irregular sand particles in continual discrete flow

We investigate the flow patterns of irregular sand particles under avalanching mode in a rotating drum by using the spatial filtering velocimetry technique.By exploring the variations of velocity distribution of granular flow,we find a type of avalanching pattern of irregular sand particles which is similar to that of spherical particles flow.Due to the fact that the initial position of avalanche in this pattern locates at the middle of the drum and the avalanche propagates toward the edge area gradually,we named it as mid-to-edge avalanching pattern.Furthermore,we find another avalanching pattern which slumpS from the edge and propagates toward the opposite edge of the flow surface,named as edge-to-edge pattern.By analyzing the temporal and spatial characteristics of these two types of avalanching patterns,we discover that these two types of avalanche patterns are caused by that the avalanching particles constantly perturb the axial adjacent particles.Thus,the particles on the flow surface are involved in avalanching sequentially in order of the axial distance from the initial position.     

Step excitation source for ultrasonic pulse transducers

A circuit is described for exciting ultrasonic transducers by step discharge through avalanching transistors. While a free-running mode is possible, the circuit is designed for synchronous operation at 1 and 10 kHz repetition rates. During the receive mode the transducer is decoupled from the power source. The output is obtained from a buffer amplifier to match a 50 Ω coaxial cable. The circuit described produces a 250 V step discharge into a 24 pf load in 10 ns.     

Exactly solvable sandpile with fractal avalanching

A simple one-dimensional sandpile model is constructed which possesses exact analytical solvability while displaying both scale-free behavior and fractal properties. The sandpile grows by avalanching on all scales, yet its shape and energy content are described by a simple, continuous (but nowhere differentiable) analytical formula. The avalanche energy distribution and the avalanche time series are both power laws with index -1 ("1/f spectra").     

A new TEM method for the characterization of the tertiary γ′ nano-precipitates in a PM disk superalloy: influence of ageing

An original method for characterizing the γ′-phase tertiary precipitates in a Ni-based superalloy manufactured by powder metallurgy is described. This investigation is made using post mortem transmission electron microscopy (TEM). It is based on the analysis of sheared areas within crept specimens, which allows the precipitates revealed by the dislocations in their glide plane to be observed. The characteristics of these nano-precipitates, i.e. their size, their volume fraction and the channel width between them, are determined for two different heat treatments (HTs). The results show a wide distribution of the microstructural parameters for a given HT, but only slight differences between the microstructures produced by the two different HTs. This microstructural information allows a better understanding of the wide variety of the deformation micromechanisms observed during creep at high temperature.     

The Use of an Antistatic Agent to Improve Powder Flowability

Small quantities of Larostat 519, an anti-static agent, were added to four samples of a commercial organic powder which was giving flowability problems believed to be caused by electrostatics. The effects on standardised bulk densities, on the Hausner Ratio, and on the behaviour of the samples when exposed to various controlled relative humidities in an experimental rotating inclined pan, were significant, and are reported and interpreted.     

双硫腙包覆钛酸锶钡粉体对水中镉的吸附性能

用草酸盐化学共沉淀法合成钛酸锶钡粉体,用双硫腙对该粉体表面进行包覆修饰,制成新型有机包覆吸附剂。用傅里叶变换红外光谱对包覆粉体进行了表征;以火焰原子吸收为检测手段,研究了该吸附剂对水中Cd2+ 的吸附性能,考察了洗脱条件;通过红外光谱,初步探讨了包覆和吸附作用机理。结果表明,双硫腙以氢键作用牢固地包覆于钛酸锶钡粉体上;该包覆粉体对水中的Cd2+具有较高的选择性吸附能力,其吸附量受介质的pH值影响,当pH值小于3时,该吸附剂对水中的Cd2+几乎不吸附,当pH值大于6时,吸附量达到最大;常温下,20 min内吸附达到平衡;吸附在双硫腙包覆钛酸锶钡粉体上的Cd2+,可用5% EDTA溶液完全洗脱。建立了吸附富集,火焰原子吸收法测定水中Cd2+的新方法,方法检出限为0.2 μg·L-1,相对标准偏差为2.9%。应用于自来水和地表水中Cd2+的测定,结果满意。     

利用MC模拟研究MRPC性能

利用Monte Carlo方法模拟了多气隙电阻板室(MRPC)的气体雪崩和信号收集过程,通过模拟与实验数据的比较分析,研究了MRPC粒子探测效率和信号幅度等性能,并讨论了一些外部条件的影响.     

Investigation of the Behaviour of Powders under and after Consolidation

A biaxial tester was utilized in order to characterize mechanical powder behaviour. Tests were aimed at characterizing the virgin consolidation behaviour of CRM 116 limestone powder, standardized by the Community Bureau of Reference (BCR). The tests indicated that the virgin consolidation behaviour of the powder can be represented by means of topographic maps and surface plots in the principal stress space irrespective of the consolidation stress paths. Resulting strains and bulk densities can also be represented in the same manner. Reloading tests were also carried out on samples consolidated by following different stress paths (consolidation procedures). By applying stresses to the previously consolidated samples and determination of points of initial plastic deformations, the yield surface for each specific type of consolidation was drawn. These tests revealed anisotropic behaviour of the powder, depending on the type of consolidation.     

表面修饰的钛酸钡纳米粉体的制备

采用水热法制备出表面包裹有硬脂酸的钛酸钡钠米粉体,并运用一系列手段对其微结构进行了表征。结果表明：产物粒径较小,粒度分布较窄,单分散性较好,其表面为非极性,同时表现出良好的流动性能。认为钛酸钡纳米粉体表面极性的改变是由于其表面包裹了一层硬脂酸,并且包裹层降低了粉体间的相互作用力,从而提高了粉体的流动性。     

Distribution of anomalous exponents of natural images

Recent studies of correlations of intensity in databases of natural images revealed a remarkable property. The two point correlations are described in terms of power law behavior, with an exponent which seems to be robust. In the present Letter we consider the statistical meaning of that result. We study many individual images of one of the databases considered. We find that the same law characterizing the correlations in the whole database governs also images randomly chosen from that database, with one essential difference. The exponent characterizing each image is specific and differs from the exponent characterizing the whole database. The distribution of single image exponents has been measured and found to exhibit a rather heavy tail. The database exponent cannot, thus, be considered as a statistical representative of a single image exponent. Possible reasons for the diversity in image exponents are discussed.     

Influence of Tester Geometry,Speed and Procedure on the Results from a Ring Shear Tester

Automated ring shear testers have gained widespread acceptance in the chemical industry. In this paper, comparative measurements with different shear testers are described and possible factors influencing the measurement of flowability, are discussed. The measurement results presented show a dependency on the bulk solid investigated. No significant influence of the shear tester was measured for easy flowing bulk solids. An influence of the shear tester is observed for poor flowing, compressible bulk solids. Experiments have shown that test procedure and shear cell design (tester geometry) affect the measurements considerably.     

密集斜槽流颗粒靶的流动性

研究了中子辐照装置密集斜槽流颗粒靶设计的流动性,该靶设计能够提供适用于聚变反应堆材料研究的高能中子。本文通过数值模拟和理论分析的方法描述了流动的稳定性和稳固性,这在其他文献中很少被提及。结果发现,25°对于稳定的流动是可以接受的,这种颗粒流有潜力成为热介质的选择。     

Enhancement of second harmonic generation efficiency: Growth and characterization of magnesium(II)-doped tetrakis(thiourea)nickel(II) chloride crystals

The influence of magnesium doping on the properties of tetrakis(thiourea)nickel(II) chloride crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy. The incorporation of Mg(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The crystal is further characterized by Kurtz powder technique and dielectric studies.     

Scales and kinetics of granular flows

When a granular material experiences strong forcing, as may be the case, e.g., for coal or gravel flowing down a chute or snow (or rocks) avalanching down a mountain slope, the individual grains interact by nearly instantaneous collisions, much like in the classical model of a gas. The dissipative nature of the particle collisions renders this analogy incomplete and is the source of a number of phenomena which are peculiar to "granular gases," such as clustering and collapse. In addition, the inelasticity of the collisions is the reason that granular gases, unlike atomic ones, lack temporal and spatial scale separation, a fact manifested by macroscopic mean free paths, scale dependent stresses, "macroscopic measurability" of "microscopic fluctuations" and observability of the effects of the Burnett and super-Burnett "corrections." The latter features may also exist in atomic fluids but they are observable there only under extreme conditions. Clustering, collapse and a kinetic theory for rapid flows of dilute granular systems, including a derivation of boundary conditions, are described alongside the mesoscopic properties of these systems with emphasis on the effects, theoretical conclusions and restrictions imposed by the lack of scale separation. (c) 1999 American Institute of Physics.