Short-time plasma surface modification of HDPE powder in a Plasma Downer Reactor - process, wettability improvement and ageing effects

The effectiveness of improving the wettability of HDPE powders within less than 0.1 s by plasma surface modification in a Plasma Downer Reactor is investigated. A correlation is revealed between the XPS results (O/C-ratio) and the wettability (contact angle, polar surface tension by capillary rise method). The O₂-content in the plasma feed gas has been adjusted for best wettability properties. XPS results indicate the formation of CO and COOH functional groups on the powder surface. The O/C-ratio increased from 0.0 (no oxygen on the non-treated powder) up to 0.15 for the plasma treated HDPE powder surface. With pure O₂-plasma treatment, a water contact angle reduction from >90° (no water penetration into the untreated PE powder) down to 65° was achieved. The total surface free energy increased from 31.2 to 45 mN/m. Ageing of treated powders occurs and proceeds mostly within the first 7 days of storage. Contact angle measurements and O1s/O2s intensity ratio data support that ageing is mainly a diffusion-controlled process. Nevertheless, XPS results show the presence of oxygen functional groups even after 40 days, which explains why the powder is still dispersible in water without any addition of surfactants.     

Experimental Study on Powder Flowability Using Vibration Shear Tube Method

The flowability of powders with different mass median diameters ranging from micrometers to nanometers was measured using the vibration shear tube method. In the measurement system used in this study, the powder was discharged through a narrow gap between a vibrating tube edge and a flat bottom surface, where each particle could experience high shear forces to overcome the adhesion and friction forces. The vibration amplitude was increased during the measurement, and the mass of particles discharged was measured at constant time intervals. From the relationship between the mass flow rate and the vibration acceleration, static and dynamic properties of the powders were evaluated using the critical vibration acceleration, characteristic mass flow rate, and gradient of mass flow rate. The correlation between the static and dynamic properties was studied in detail.     

Close-coupled nozzle atomization integral simulation and powder preparation using vacuum induction gas atomization technology

We simulate the gas-atomization process of a close-coupled annular nozzle for vacuum induction gas atomization at a three-dimensional scale.Moreover,the relationship between the simulated droplet type and experimentally metallic powder is established by comparing the morphology of droplets with powders.Herein,the primary atomization process is described by the volume-of-fluid(VOF)approach,whereas the prediction of powder diameter after secondary atomization is realized by the VOF-Lagrangian method.In addition,to completely reflect the breaking and deformation process of the metallic flow,we employ the VOF model to simulate the secondary atomization process of a single ellipsoidal droplet.The results show that the primary atomization process includes the formation of surface liquid film,appearance of serrated ligaments,and shredding of ligaments.Further,gas recirculation zone plays an important role in formation of the umbrella-shaped liquid film.The secondary atomization process is divided into droplet convergence and dispersion stages,and the predicted powder diameter is basically consistent with the experiment.In general,the four main powder shapes are formed by the interaction of five different typical droplets.     

Physics of Radio-Frequency Plasmas,by Pascal Chabert and Nicholas Braithwaite

The mechanism whereby powders may be compressed to form solid objects is not properly understood, largely because, until recently, the process has not been studied by sufficiently powerful techniques. This paper describes a number of techniques developed for a detailed study of powder compaction. Methods for measuring specific surface areas of powders and powder compacts, and hence the areas of contact between the particles, are discussed and reasons for selecting an adsorption method are explained. A gas adsorption apparatus capable of determining very small specific surface areas is described, as is the use of the scanning electron microscope. Both techniques were used to investigate particle fracture during compaction. In addition the study of pore size gives valuable insight into the movement of particle fragments once they have been produced. The use of mercury porosimetry in this connection is described.     

Preparation and Characterization of Nano-structured Ceramic Powders Synthesized by Emulsion Combustion Method

The emulsion combustion method (ECM), a novel powder production process, was originally developed to synthesize nano-structured metal-oxide powders. Metal ions in the aqueous droplets were rapidly oxidized by the combustion of the surrounding flammable liquid. The ECM achieved a small reaction field and a short reaction period to fabricate the submicron-sized hollow ceramic particles with extremely thin wall and chemically homogeneous ceramic powder. Alumina, zirconia, zirconia–ceria solid solutions and barium titanate were synthesized by the ECM process. Alumina and zirconia powders were characterized to be metastable in crystalline phase and hollow structure. The wall thickness of alumina was about 10nm. The zirconia–ceria powders were found to be single-phase solid solutions for a wide composition range. These powders were characterized as equiaxed-shape, submicron-sized chemically homogeneous materials. The powder formation mechanism was investigated through the synthesis of barium titanate powder with different metal sources.     

电极感应熔化气雾化制粉技术中非限制式喷嘴雾化过程模拟

电极感应熔化气雾化(electrode induction melting gas atomization, EIGA)是一种制备超洁净无夹杂物的先进制粉技术,本文以粉末高温合金的氩气雾化过程为研究示例,对现有用于实际生产的国内某厂家提供的EIGA用非限制式喷嘴进行建模,采用商用计算流体力学软件FLUENT,分布采用欧拉-欧拉VOF(volume of fluid)多相流方法与欧拉-拉格朗日DPM (discrete phase model)离散相方法,对非限制式环缝喷嘴主雾化与二次雾化过程进行了数值模拟.通过对主雾化过程中多相流大涡模拟速度流场,主雾化过程中不同阶段高温熔体云图模拟以及二次雾化过程中TAB (Taylor analogy breakup)模型速度流场及TAB模型粒度分布的模拟研究,实现了对EIGA制粉技术中非限制式喷嘴雾化过程的全过程模拟,并预测了雾化后的粉末粒度分布.在此基础上,采用本文模拟使用的非限制式环缝喷嘴,设定与模拟条件一致(进气压力4 MPa,液流直径约4 mm)的实验条件,制备的粉末大部分颗粒的直径大小在100μm左右,该实验结果与模拟得到的粉末直径D50=100μm大小一致,进一步验证了模拟数据的合理性.该方法也适用于非限制式喷嘴里,其他金属或合金的雾化过的模拟研究.     

3D modeling and testing of transient temperature in selective laser sintering (SLS) process

In the process of rapid prototyping by the method of selective laser sintering (SLS), transient temperature has direct effect upon the sintering performance. In the present work, a model is developed in order to generate 3D transient temperature field. It uses Al₂O₃ coated ceramic powder and involves the finite element method (FEM) variation of thermal properties and solid–liquid two-phase interface. A high-speed charge coupled device (CCD) image temperature measurement system is used to generate for testing. The obtained test results validates the simulation data and implies that the proposed modeling method is useful in simulating the transient sintering temperature specially when the correct thermal properties and key factors of two-phase interface are main concerns. The performance characteristics of the reasonable sintering parameters are predicted by the proposed modeling method.     

利用溶胶-凝胶法调节pH及添加PEG优化酒精纯化用SiO_2粉末(英文)

该工作探讨利用TEOS前驱液及不同含量、不同分子量PEG制备出SiO2粉末的性质.SiO2粉末由溶胶-凝胶法制备,并结合退火前后烘干.TEOS前驱溶液的水解及缩合反应速率在pH=3时比在pH=5,7,9时进行得更彻底,且随温度及PEG分子量的升高而升高;粉末产率在TEOS前驱液pH=3时达到最大.与不加PEG的溶胶-凝胶法制备出的样品相比,加入PEG制备的粉末有更高的吸水能力.若粉末从高至500℃退火,则可发现乙醇浓度增加.依据液体和固体NMR、电子显微镜以及BET结果,对吸水及粉末表面积之间的关系进行了讨论.     

Ultrasonic assisted hot metal powder compaction

Hot pressing of metal powders is used in production of parts with similar properties to wrought materials. During hot pressing processes, particle rearrangement, plastic deformation, creep, and diffusion are of the most effective powder densification mechanisms. Applying ultrasonic vibration is thought to result in great rates of densification and therefore higher efficiency of the process is expected. This paper deals with the effects of power ultrasonic on the densification of AA1100 aluminum powder under constant applied stress. The effects of particle size and process temperature on the densification behavior are discussed. The results show that applying ultrasonic vibration leads to an improved homogeneity and a higher relative density. Also, it is found that the effect of ultrasonic vibration is greater for finer particles.     

Oxidation during the production of FGH4095 superalloy powders by electrode induction-melt inert gas atomization

Super-clean and super-spherical FGH4095 superalloy powder is produced by the ceramic-free electrode inductionmelt inert gas atomization(EIGA) technique.A continuous and steady-state liquid metal flow is achieved at high-frequency(350 k Hz) alternating current and high electric power(100 k W).The superalloy is immersed in a high-frequency induction coil,and the liquid metal falling into a supersonic nozzle is atomized by an Ar gas of high kinetic gas energy.Numerical calculations are performed to optimize the structure parameters for the nozzle tip.The undesired oxidation reaction of alloying elements starts at 1000?C with the reaction originating from the active sites on the powder surfaces,leading to the formation of oxides,Me_xO_y.The role of active sites and kinetic factors associated with the diffusion of oxygen present in the atomization gas streams are also examined.The observed results reveal that the oxidation process occurring at the surface of the produced powders gradually moves toward the core,and that there exists a clear interface between the product layer and the reactant.The present study lays a theoretical foundation for controlling the oxidation of nickel-based superalloy powders from the powder process step.     

蔬菜粉末荧光光谱测定及其在手印显现中的应用

对遗留在客体表面的手印进行提取和鉴定是侦破案件的重要依据。对在犯罪现场发现的手印进行固定提取时,首先应该使用没有破坏性的光学无损检验技术。实践表明光学无损检验技术对于绝大部分光滑客体上的手印都能取得很大的反差和较好的纹线细节,而对于光滑度较差的客体上的手印则需要使用有损的物理法和化学法对指印进行固定提取。其中粉末法和熏显法在手印显现中得到了广泛的应用,但是常用的绝大部分粉末和熏显物质对长期从事手印显现的工作人员身体有较大的毒副作用。而且绝大部分常用的荧光粉末需要在紫外线的激发下产生荧光,紫外线激发手印产生荧光的同时也常会激发承痕客体产生很强的背景荧光,进而降低了手印与背景的反差。因此寻找一种成本低廉、无毒无害、操作简单和应用性广泛的手印显现方法是当务之急。研究了三种无毒无害、成本低廉的蔬菜粉末的荧光性能,并且将其应用于手印显现中。首先通过荧光分光光度计测定西蓝花粉末、菠菜粉末和紫菜粉末的荧光激发光谱和荧光发射光谱。通过荧光测定发现,西蓝花粉末的荧光比菠菜粉末和紫菜粉末的荧光强很多,所以选择西蓝花粉末显现犯罪现场常见的疑难客体表面汗潜手印。实验结果表明：（1）西蓝花粉末、菠菜粉末和紫菜粉末都能发出荧光,荧光激发光谱峰值都在417 nm,荧光发射光谱峰值都在678 nm,其中西蓝花粉末发出的荧光最强。（2）将西蓝花粉末用于多色图案干扰的非渗透性和渗透性客体上的汗潜手印的显现中,在415 nm的紫光的照射下能够发出明亮的红色荧光。拍照荧光指印时在镜头前加上透过中心波长为680 nm左右的窄带通干涉滤光镜,结果显示手印纹线与背景反差强烈,纹线清晰连贯,消除背景的干扰。（3）与传统的荧光粉末显现法相比,该显现方法具有较高的显现灵敏度。（4）有些客体背景图案在415 nm的紫光的照射下也有发光,但与西蓝花粉末的非常强的荧光相比,背景的荧光弱得多,所以使得手印与背景之间形成了较大的反差并且纹线的细节特征清晰。（5）粗糙客体上手印的显现也获得了很大的反差,粗糙的背景没有对纹线形成干扰而形成了清晰连贯的纹线。（6）实验中使用的市售的西蓝花粉末具有颗粒细小、吸附能力强等特点,能够促进手印物质与西蓝花粉末之间的选择性吸附,进而进一步提高手印显现的灵敏度。     

Microstructure and dielectric properties of PMN–PT ceramics prepared by the molten salts method

The sintering characteristic and dielectric properties of 0.67PMN–0.33PT ceramics prepared by the molten salt synthesis (MSS) method were investigated. PMN–PT particles synthesized by MSS with smaller grain size and good dispersion could lower the sintering temperature of ceramics; PMN–PT ceramics with relative density above 96% could be obtained in the range 1150–1180 °C. The molten salts species could significantly affect the microstructure and properties of MPN-PT ceramics. In the range 1100–1200 °C, PMN–PT ceramics from the sulfate flux MSS powders showed intergranular fracture, but that from the chloride flux MSS powder showed transgranular fracture. At the same sintering condition, the properties of PMN–PT ceramics from the powders prepared in the chloride flux are better than that from the powders prepared in the sulfate flux, their maximum dielectric constant ε_max≈29,385 and piezoelectric constant d₃₃≈660 pC/N. The above results demonstrated that PMN–PT ceramics prepared by the molten salts method possessed excellent piezoelectric and dielectric properties.     

Magnetic resonant and non-resonant investigations of LiLnF4 (Ln = Y, Tm) powders

The properties of dielectric powders of the Van Vleck paramagnet LiTmF₄ and its diamagnetic analogue LiYF₄ have been investigated by both resonant methods (EPR, NMR, and the mass-spectroscopy) and non-resonant ones (conductometry and magnetization measurement). On the basis of experimental data and theoretical calculations a self-consistent model for the magnetic and other properties of these powders is suggested. Two structural phase transitions induced by the magnetic field are discovered in fine LiTmF₄ powder at low temperature in a high magnetic field.     

铅对粉末药型罩性能的影响

 为了研究铅粉对钨铜粉末药型罩性能的影响,在钨铜药型罩原配方的基础上,添加5%～20%的铅粉进行实验研究。实验结果表明：添加铅粉可以减少粉末药型罩的孔隙度;随着铅粉添加量的增加,射流的侵彻深度和入口孔径均增加,当铅含量为10%～15%时,侵彻深度和入口孔径达到最佳值,侵彻深度可以提高30%,再添加铅粉,侵彻深度和入口孔径均随之下降。从粉末药型罩成型以及射流的形成、拉伸及侵彻靶板等方面,对造成这种现象的原因进行了详细分析,获得了铅对粉末药型罩性能影响的一些机理认识。研究结果为铅粉代替铋粉添加到钨铜粉末药型罩以提高其侵彻性能提供了一定的理论依据。     

显微红外反射光谱法快速鉴别珍珠粉真伪

珍珠是名贵的装饰品和中药材,在我国对其已有2000多年的应用历史。研究发现,贝壳内层的化学组成与珍珠相似,主要含有碳酸钙和各种氨基酸。贝壳粉也可在临床上做药用,但其药用价值远低于珍珠粉。然而,珍珠粉和贝壳粉外观特性极为相似,元素组成和形貌检测无法对其进行有效地鉴别。由于贝壳粉的成本低,常常冒充珍珠粉流入市场,侵害了消费者的利益。本研究通过珍珠粉和贝壳粉在不同温度下煅烧一定时间后显微红外反射光谱的差异实现珍珠粉真伪鉴别。实验结果表明,在一个大气压下经过400 ℃煅烧30 min后,珍珠粉中文石型碳酸钙部分转化为方解石型,而贝壳粉中文石型碳酸钙完全转化为方解石型。这种不同的相转变过程可以被显微红外反射光谱很好的区别。因此,可在此条件下利用红外反射光谱鉴别珍珠粉和贝壳粉相变差异。和传统的透射式红外光谱法相比,这种显微反射红外光谱法无需压片制样、快速灵敏。本研究也讨论了该方法针对其他常见伪品的适用性,如牡蛎粉和石决明粉,结果表明该方法能够简单、高效和准确地鉴别珍珠粉真伪。     

Effect of alloying contents and processing factors on the microstructure and homogenization of Si alloyed Cr–Mo sintered steels

Fe–Si alloys are of significant commercial and academic interests, due to the large diversity of their physical properties. In practice, alloy powders are unsuitable because of their hardness, poor compactibility and resulting excessive tool wear. Therefore the powder mixture route is suitable as alloying technique. The properties of the Fe–Si sintered materials depend strongly on the influence of the element Si and content of it, which influence mainly the compactibility and the sintering behaviour as well as sintering parameters such as the optimum temperature. In this study, Cr–Mo prealloyed steel powders with different Si contents were prepared by powder mixture route. Mixed powders compacted under pressing pressure of 600 MPa, and then sintered at 1120 and 1250 °C. It was found out that in Si alloyed Cr–Mo steels sintered at higher temperatures such as 1250 °C, an intermediate liquid phase appeared and caused extreme shrinkage and distortion, but strongly accelerated sintering and had a beneficial effect also on homogenization.     

Direct deposition of graphene nanowalls on ceramic powders for the fabrication of a ceramic matrix composite

Uniform mixing of ceramic powder and graphene is of great importance for producing ceramic matrix composite. In this study, graphene nanowalls(GNWs) are directly deposited on the surface of Al_2 O_3 and Si_3 N_4 powders using chemical vapor deposition system to realize the uniform mixing. The morphology and the initial stage of the growth process are investigated. It is found that the graphitic base layer is initially formed parallel to the powder surface and is followed by the growth of graphene nanowalls perpendicular to the surface. Moreover, the lateral length of the graphene sheet could be well controlled by tuning the growth temperature. GNWs/Al_2 O_3 powder is consolidated by using sparking plasma sintering method and several physical properties are measured. Owing to the addition of GNWs, the electrical conductivity of the bulk alumina is significantly increased.     

Control of the shell structure of ZnO–ZnS core-shell structure

Nanostructured ZnO–ZnS core-shell powders were synthesized through a solution method using a thioacetamide (TAA) solution in deionized water. ZnO powder and TAA solution were employed to supply zinc and sulfur ions to form the ZnO–ZnS core-shell structures. The structure of the ZnS shell was strongly affected by the mole concentration of the TAA, and the structural properties were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The optical properties of the nanostructured powders were also compared with those of pure ZnO powder. The ultraviolet (UV) emission was greatly enhanced compared to when pure ZnO powder was used in the nanostructured powder synthesized using the 0.5 M TAA solution, while the UV emission of that with the 0.05 M TAA solution was reduced. The green emission of the nanostructured powders was reduced compared to when the pure ZnO powder was used. The mechanism of the structural changes in the core-shell structures is proposed here and its effect on the luminescent properties is discussed.     

Electrostatic adhesion in food

Electrostatic adhesion is a significant force in painting and other industries. However, coating of food items uses powders and targets with properties that are far from ideal for electrostatic coating. In addition, the adhesion must last a long time in order to be useful. The goal of this study is to determine if electrostatic adhesion is significant in food coating. A range of food powders were coated onto different kinds of food samples electrostatically and nonelectrostatically and the adhesion measured. Electrostatic powder coating increased the adhesion of most food powders onto most food targets. Some correlation was found between effectiveness of the process and resistivity or oil content of the target, resistivity of the powder, and particle size. At low relative humidities, electrostatic adhesion lasted for several weeks, long enough to be valuable.     

Analytical theory for the description of powder systems under compression

In this work, a new theoretical approach to modelling some properties of powder systems under compression is presented. This new theoretical route consists of modelling an actual powder system (with particles of unequal size and irregular form) by means of a system of deforming spheres in a simple cubic arrangement and with a certain global porosity that, in some way, makes it equivalent to the actual one. The study of the evolution of the effective contact area between particles and the effective path of the electric or thermal flow through the powder aggregate is the starting point for establishing the equivalence relationship between the actual system and the simple cubic one. In order to exemplify the utility of this new theoretical tool, two classic problems of practical interest have been studied: the electrical conduction in sintered powders and the law governing the powders’ cold die compaction. The proposed solutions to these problems, as well as the equations allowing one to obtain the equivalence relationship, are validated by experiments carried out in actual powder systems.