Controlled growth of silver nanoparticles in a hydrothermal process

A two-step synthesis was used to control the shape of silver nanoparticles prepared via reduction of Ag^＋ ions in aqueous Ag（NH3）2NO3 by poly（N-vinyl-2 First, a few spherical silver nanoparticles,-10 nm in size, were pyrrolidone） （PVP）. Then, in a subsequent hydrothermal treatment, the remaining Ag^＋ ions were reduced by PVP into polyhedral nanoparticles, or larger spherical nanoparticles formed from the small spherical seed silver nanoparticles in the first step. The morphology and size of the resultant particles depend on the hydrothermal temperature, PVP/Ag molar ratio and concentration of Ag^＋ ions. By using UV-visible spectroscopy （UV-vis）, transmission electron microscopy （TEM） and powder X-ray diffraction （XRD）, the possible growth mechanism of the silver nanoparticles was discussed. 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.     

DUAL-CHANNEL PARTICLE SIZE AND SHAPE ANALYZER

Fig. 1 shows a newly developed analyzer (Ankersmid CIS-100) that brings together two different measurement channels for accurate size and shape measurement of spherical and non-spherical particles. The size of spherical particles is measured by a HeNe Laser Beam; the size of non-spherical particles is analyzed by Dynamic Video Analysis of the particles‘ shape.     

The effect of far field flow on a spherical crystal growth in the undercooled melt

The effect of convective flow on a spherical crystal growth in the undercooled melt with a moderate far field flow is studied. The asymptotic solution of the evolution of the interface of the spherical crystal growth is obtained by the matched asymptotic expansion method. The analytic result shows that the convective flow in the undercooled melt has a strong effect on the evolution of spherical crystal growth. The convective flow induced by the far field flow makes the interface of the growing spherical crystal enhance its growth velocity in the upstream direction of the far field flow and inhibit growth in the downstream direction, and the interface of the decaying spherical crystal further decay in the upstream direction and inhibit decay in the downstream direction. The maximum growth velocity of the interface of the spherical crystal influenced by the far field flow is obtained.     

WET-GRANULATION RESEARCH WITH APPLICATION TO SCALE-UP

Summary Granulation is a unit operation by which larger granules are produced from fine, powdery particles to improve appearance, flow properties and mixedness, reduce dustiness and, in general, produce engineered particles with superior attributes. Agglomeration in wet granulation is achieved by introducing a “binder” fluid onto a shearing mass of fine powders. This paper gives a general overview of the process with emphasis on a simplified granulation model based on a dimensionless parameter containing inertia and viscous dissipation energies between colliding particles: the so-celled Stokes number. The model incorporates most common features of all granulation devices (mixers) used in the pharmaceutical industry.Also described in the paper is a computer simulation that captures the movement of flowing powder in an ideal mixer-granu/ator with constant shear rate. A fraction of the total number of particles is wet (covered by binder and there-fore “sticky”) while the rest of the particles are dry. The numerical simulation depicts two distinct regimes of agglomeration found in a typical granulator: granule growth and subsequent breakup. During granule growth-simulations, final granule size and shape distributions are obtained by analyzing the size and shape of formed granules using a pattern-recognition technique. A second kind of simulation, also using rapid granular flow modeling, follows the rotation and deformation of an “agglomerate” held together by a liquid binder. Results from these simulations yield critical values of the Stokes number. Below the cdticel value, the agglomerates are stable and only rotate in response to shear while above the critical value they break into two or more pieces. At the critical value, they attain a steady elongated shape. Using values of the critical Stokes number, the model predicts the size of formed granules.The existence of the critical state in which granules attain a characteristic elongated shape is used to measure shear forces in a granulator by employing calibrated “test” particles of known strength. This knowledge is employed in granulation scale-up to determine a kinematic rule that conserves stresses in the small and the large-scale machines. It was found that conserving the magnitude of internal stresses in the moving powder yields granules with similar attributes in granulators of different size.     

Pozzolanic reactivity of silico-aluminous fly ash

For some years it has been possible to control the particle size of fly ashes, by-products of thermal power stations. Incorporating these very fine particles （obtained by grinding and/or pneumatic selection） improves the physical-mechanical characteristics of mortars and concretes. In this study, we measured the lime consumption of the various fractions （granulometric and densimetric） and identified by X-ray diffraction the neoformed phases by the pozzolanic reaction, to show that it is not sufficient to simply define the pozzolanicity of products based on lime consumption since it does not take into account the nature of the phases formed. The size of the particles used in the test samples also has a determining effect on the quantity of lime consumed. Before comparing results, it is necessary to ensure that the size of the particles （of the global ash and its constituents） be the same. Two distinct neoformed ohases appear： CSH in the largest granular fractions （d〉 40 μm） and C3AH6 in the smaller fractions.     

Tailoring particle shape for enhancing the homogeneity of powder mixtures:Experimental study and DEM modelling

The effect of particle shape modification on the segregation reduction of enzyme granules in laundry detergent powder mixtures was investigated,both experimentally and computationally using Deseret Element Method(DEM).The shape of modified enzyme particles was in such a way that the large and dense enzyme particles were layered by other fine particles in the detergent powder,by means of a process known in the literature as“seeded granulation”.It is found that the homogeneity of modified enzyme particles could be improved significantly comparing to the original spherical enzyme particles in powder mixtures.Overall,the results of this research demonstrated that the segregation-induced properties of the dense/spherical enzyme particles could be lowered by altering their shape,which could enable the enzyme particles to behave almost similar to other ingredients during the pile formation process.     

SURFACE MODIFICATION AND DISPERSION OF NANODIAMOND IN CLEAN OIL

The effect of different kinds of surfactants on the size distribution of nanodiamond particles in clean oil was studied.Results show that the dispersing stability of nanodiamond modified with surfactangts YS-1 and SB-18 simulta-neously is much better than those modified with either of them because of synergism of the surfactangts.And the particle size distribution in the system can be improved remarkably after the adoption of hyperdispersants such as SA-E and SA-F.Anchoring groups of those hyperdispersants can be bonded with the particle surface by chemical and/or hydrogen bonding and their soluble chains are well compatible with the dispersion media.As a result,the particles are uniformly distrbuted in the system owing to the steric stabilization.A very stable clean-oil based nanodiamond suspension with an average particle size of around 53.2 nm was prepared.     

MULTI-SCALE AGGREGATION OF PARTICLES IN GAS-SOLIDS FLUIDIZED BEDS

The multi-scale characteristics of clusters in a fast fluidized bed and of agglomerates in a fluidized bed of cohesive particles are discussed on the basis of large amounts of experiments.The cluster size and concentration are dominated by the local voidage of the bed.A cluster consists of many sub-clusters with different sizes and discrete par-ticles,and the sub-cluster size probability density distribution appears as a negative exponential function.The agglom-erates in a fluidized bed of cohesive particles also possess the multi-scale nature.The large agglomerates form a fixed bed at the bottom,the medium agglomerates are fluidized in the middle,and the small agglomerates and discrete parti-cles become the dilute-phase region in the upper part of the bed.The agglomerate size is mainly affected by cohesive forces and gas velocity.The present models for prediction the size of clusters and agglomerates can not tackle the in-trinsic mechanism of the multi-scale aggregation,and a challenging problem for establishing mechanistic model is put forward.     

EFFECTS OF SURFACTANT AND ACID TYPE ON PREPARATION OF CHITOSAN MICROCAPSULES

Chitosan microcapsules were prepared by a method involving emulsification and crosslinking. The effectsof surfactants and acid type for dissolving chitosan on the characteristics of chitosan microcapsules were investigated.The results showed that the mixed surfactant consisting of Span80 and Tween60 had an obvious effect on reducing the size of the microcapsules. The two-surfactant complex, formed on the basis of hydrogen bonding, strengthened the interfacial membrane in the emulsion, thus decreasing not only the size of the microcapsules but also the coalescence of dispersed chitosan liquid drops. In the case of monoacid such as hydrochloric acid or acetic acid for dissolving chitosan,the chitosan microcapsules obtained were spherical in shape with smooth surfaces. For diacids or triacid, the chitosanmicrocapsules obtained were also spherical, but their surfaces were covered by folds and crinkles. The number of carboxyl groups in the acids used influenced the chemical crosslinking between chitosan and the crosslinker (glutaraldehyde) as well as the morphology of the particles. For diacids or triacid, physical crosslinking occured due to electrostatic force, accompanied by substantial decrease of covalent crosslinking, leading to decreased strength of the microcapsules as shown by the collapse of microcapsule walls and the formation of multiple folds and cnnkles on their surfaces.     

TENSILE STRENGTH FOR SPLITTING FAILURE OF BRITTLE PARTICLES WITH CONSIDERATION OF POISSON＇S RATIO

The core mechanism of comminution could be reduced to the breakage of individual particles that occurs through contact with other particles or with the grinding media, or with the solid walls of the mill. When brittle particles are loaded in compression or by impact, substantial tensile stresses are induced within the particles. These tensile stresses are responsible for splitting failure of brittle particles. Since many engineering materials have Poisson‘s ratios very close to 0.3, the influence of Poisson‘s ratio on the tensile strength is neglected in many studies. In this paper, the state of stress in a spherical particle due to two diametrically opposed forces is analyzed theoretically. A simple equation for the tensile stress at the centre of the particle is obtained. It is found reasonable to propose this tensile stress at the instant of failure as the tensile strength of the particle. Moreover, this tensile strength is a function of the Poisson‘s ratio of the material. As the state of stress along the z-axis in an irregular specimen tends to be similar to that in a spherical particle compressed diametrically with the same force, this tensile strength has some validity for irregular particles as well. Therefore, it could be used as the tensile strength for brittle particles in general. The effect of Poisson‘s ratio on the tensile strength is discussed.     

尾矿颗粒沉降和分选规律研究1)

开展了尾矿颗粒在流体中沉降和分选规律研究。首先,对尾矿颗粒在流体中受力特征和各种力作用效果进行分析;其次,对牛顿流体中尾矿颗粒沉降和分选进行研究;再次,对宾汉浆液中尾矿颗粒沉降和分选进行研究;最后,对理论公式进行了讨论和结果验证。论文确定了尾矿颗粒在不同流体中的沉降特点,提出在浆体流动中不沉的最大粒径,推导出球形颗粒的沉降微分方程,推导出颗粒最终沉速和沉降距离,求得了颗粒水平运移距离,结果讨论验证了理论公式的正确性。     

Numerical simulation of oxide nanoparticle growth characteristics under the gas detonation chemical reaction by space-time conservation element–solution element method

Under harsh conditions (such as high temperature, high pressure, and millisecond lifetime chemical reaction), a long-standing challenge remains to accurately predict the growth characteristics of nanosize spherical particles and to determine the rapid chemical reaction flow field characteristics. The growth characteristics of similar spherical oxide nanoparticles are further studied by successfully introducing the space-time conservation element–solution element (CE/SE) algorithm with the monodisperse Kruis model. This approach overcomes the nanosize particle rapid growth limit set and successfully captures the characteristics of the rapid gaseous chemical reaction process. The results show that this approach quantitatively captures the characteristics of the rapid chemical reaction, nanosize particle growth and size distribution. To reveal the growth mechanism for numerous types of oxide nanoparticles, it is very important to choose a rational numerical method and particle physics model.     

Growth characteristics of spherical titanium oxide nanoparticles during the rapid gaseous detonation reaction

The nanosize grain growth characteristics of spherical single-crystal titanium oxide (TiO₂) during the rapid gaseous detonation reaction are discussed. Based on the experimental conditions and the Chapman–Jouguet theory, the Kruis model was introduced to simulate the growth characteristics of spherical TiO₂ nanoparticles obtained under high pressure, high temperature and by rapid reaction. The results show that the numerical analysis can satisfactorily predict the growth characteristics of spherical TiO₂ nanoparticles with diameters of 15–300 nm at different affecting factors, such as concentration of particles, reaction temperature and time, which are in agreement with the obtained experimental results. We found that the increase of the gas-phase reaction temperature, time, and particle concentration affects the growth tendency of spherical nanocrystal TiO₂, which provides effective theoretical support for the controllable synthesis of multi-scale nanoparticles.     

SiC/环氧树脂复合材料冲蚀磨损性能的研究

采用环氧树脂为粘接剂制备了SiC/环氧树脂复合材料,在自制的射流式冲蚀磨损试验机上研究了SiC/环氧树脂的冲蚀磨损性能.结果表明:大尺寸SiC颗粒制备的复合材料较小尺寸SiC颗粒制备的复合材料具有更好的冲蚀磨损性能,且大尺寸SiC颗粒复合材料的冲蚀磨损性能优于Q235钢,而小尺寸SiC颗粒复合材料则低于Q235钢.随着冲蚀角度的变化,其平行材料表面的切削分量和垂直材料表面的冲击分量将会发生变化,低角度冲蚀磨损机理以显微切削和碾压造成环氧树脂及SiC颗粒的层片状脱落为主转变为高角度冲蚀磨损以SiC颗粒碎裂造成环氧树脂疲劳脱落为主.     

A NOVEL HYDROTHERMAL SYNTHESIS METHOD FOR BARIUM FERRITE

In the present work, fine barium ferrite powder has been synthesized through a one-step hydrothermal process in an autoclave at [OH^-]/[Cl^-] ratio of 2:1 in the temperature range from 180 to 260℃ using barium chloride(BaCl2), ferrous chloride (FeCl2) and potassium nitrate (KNO3) as the starting materials. Both particle size and saturation magnetization (Ms) increase with increasing hydrothermal reaction temperature, while the intrinsic coercivity (iHc) peaks at 685 Oe at 230℃. Morphology progress from the barium ferrite precursor particles to the barium hexaferrite particles has been monitored with increasing hydrothermal reaction time at 230℃ in the autoclave.     

Synthesis of spherical Y2O3:Er emitting particles with variable radial composition by controlled double-jet precipitation of layered precursors

Spherical (Y_0.98Er_0.02)₂(OH)₅(NO₃)·xH₂O particles were synthesized by controlled double-jet precipitation, with a “core” of pure layered yttrium hydroxide nitrate, and a “shell” of co-precipitated yttrium-erbium layered hydroxide nitrates. With an increase in precipitation pH from 7 to 9.5, the size of layered “building units” decreases and the architecture of their assemblies changes from flower-like through network-like spherical to irregular agglomerates. From there, spherical particles gradually increase their diameter due to the continuous uniform growth of curved layered sheets on their surface. It was established that such growth behavior and network-like architecture of spherical particles was retained even when yttrium was replaced by erbium ions in the layered host lattice during the formation of an Er-enriched “shell”. Analysis of SEM, EDS, XPS, photoluminescence spectra and concentration quenching effects of heat-treated Y₂O₃:Er (2 at.%) particles indicate that the radial distribution of erbium in particles is most controllable in a narrow pH range of co-precipitation of layered precursors (pH 8). Вy widely varying the elemental composition of “building units” during co-precipitation, one can simultaneously finely control the composition of layered hydroxides in the radial direction of the spherical particles and grow multicomponent “multi-shell” powders with desired properties.     

Relationship Between the Microstructure/Pore Structure of Oil-Well Cement and Hydrostatic Pressure

In cementing operations, hydrostatic pressure reduction in cement slurries is a serious threat to operation safety and cementing quality. This study combines X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, low-field nuclear magnetic resonance, and nano-computed tomography to investigate the mechanism of hydrostatic pressure reduction in cement slurries. The experimental results reveal that hydrostatic pressure transmission in a fresh cement slurry follows Pascal’s law. However, in the slurry, some cement particles undergo sedimentation with an increase in the hydration time, which reduces a part of the hydrostatic pressure. Further, another part of the hydrostatic pressure, which is maintained by the free water in the slurry pores, is reduced during the hardening stage. When the hydration reaction of the slurry is accelerated, the pore water in the slurry is supersaturated and hydration products start to rapidly nucleate and grow between the cement particles. These hydration products are porous gel structures and can change the pore structure of the cement slurry; a macro-pore is divided into many micropores, such as capillary pores and gel pores. Because these pores are filled by water in the slurry, during this process, free water in the macro-pores is changed to capillary water and gel water. However, gel water and capillary water cannot transmit hydrostatic pressure in the cement slurry. Meanwhile, in the fresh cement slurry, many pores containing free water are connected and some hydration products rapidly grow in the macro-pores and fill them, which may reduce the column height of the free water in the pores and lead to hydrostatic pressure reduction in the slurry.     

纯铅化学机械抛光中工艺参数对抛光性能的影响

为获得高质量纯铅表面,采用化学机械抛光(CMP)的方法并辅以自制抛光液,研究了胶体二氧化硅抛光颗粒的形状、粒径和浓度、加载压力、抛光头与抛光盘转向和转速、抛光液流量等工艺参数对铅片表面材料去除率和粗糙度的影响.研究表明:小粒径异形(眉形)胶体二氧化硅抛光颗粒相较于大粒径球形颗粒更有利于铅片抛光,抛光颗粒的粒径和浓度对纯铅抛光性能的影响主要取决于铅片表面与胶体二氧化硅颗粒以及抛光垫表面丝绒的耦合作用关系.随着加载压力、抛光头与抛光盘转向和转速、抛光液流量的改变,铅片表面和抛光垫之间驻留的层间抛光液的厚度以及状态发生改变,从而直接影响抛光液的流动性、润滑性和分散性,以及影响抛光颗粒和化学试剂与铅片表面的机械化学作用,进而影响抛光质量和材料去除率.通过对工艺参数影响的研究和对工艺参数的优化,最终获得了表面粗糙度R_a为1.5 nm的较为理想的超光滑纯铅表面,同时材料去除率能够达到适中的380?/min.     

Effects of magnetic fields on improving mass transfer in flue gas desulfurization using a fluidized bed

The effects of magnetic fields on improving the mass transfer in flue gas desulfurization using a fluidized bed are investigated in the paper. In this research, the magnetically fluidized bed (MFB) is used as the reactor in which ferromagnetic particles are fluidized with simulated flue gas under the influence of an external magnetic field. Lime slurry is continuously sprayed into the reactor. As a consequence, the desulfurization reaction and the slurry drying process take place simultaneously in the MFB. In this paper, the effects of ferromagnetic particles and external magnetic fields on the desulphurization efficiency are studied and compared with that of quartz particles as the fluidized particles. Experimental results show that the ferromagnetic particles not only act as a platform for lime slurry to precipitate on like quartz particles, but also take part in the desulfurization reaction. The results also show that the specific surface area of ferromagnetic particles after reaction is enlarged as the magnetic intensity increases, and the external magnetic field promotes the oxidation of S(IV), improving the mass transfer between sulphur and its sorbent. Hence, the efficiency of desulphurization under the effects of external magnetic fields is higher than that in general fluidized beds.