Beurteilung der Reinigbarkeit von Oberflächen bei partikulärer Kontamination

Hygienic impeccable surfaces are, e.g., in food technology and healthcare facilities conditionally essential. Does a presence of fine particular contamination occur, adhesion forces between particles and surfaces are relevant. In this paper the cleaning‐process of materials, which are often used in cleanrooms are focused. The surface properties, which influence the dislodgement of particles are observed. There are two groups of materials used for the experiments, which are commonly used in hygienic relevant places in healthcare facilities.     

Electrostatic phenomena during powder handling

During powder handling operations, particles make frequent contact with surfaces often metallic, and become electrically charged due to the process of contact electrification. It is often more appropriate to describes such a contact electrification process as triboelectrification as sliding/frictional contact is invariably involved. Triboelectrification is a complex process as charge exchange involving insulating surfaces takes place. The concept of work function may be applied to some insulator materials, especially those that charge negatively as many polymeric materials do. Surface charging depends upon surface condition and some materials are sensitive to the presence of oxidising agents in the atmosphere and also to the presence of moisture. To quantify the triboelectrification process, the dynamics of particle surface contact must be fully defined and contact area accurately assessed. The dynamic behaviour of charged particles may be significantly dependent upon charge. Space charge fields which exist in charged powder clouds act upon individual particles whose motion depends upon particle electrical mobility. Even in the absence of space charge fields, charged particles experience attractive forces towards nearby neutral surfaces. Upon contact with surfaces, charged particles may adhere strongly by a combination of electrostatic and van der Waals forces. Particle/surface adhesion is important in powder coating applications and in electrostatic precipitators.     

Study on microstructure and abrasive wear behavior of sintered Al matrix composites

Abrasive wear behavior of ZrSiO₄ reinforced aluminum metal matrix composite has been investigated in the present research. In general, composites offer superior wear resistance as compared to the alloy irrespective of applied load and zircon particles volume fraction. During sliding wear of the composite, a layer is formed over the specimen surface, which strongly dictates the wear behavior of the materials. It is believed that these layers are formed due to formation of wear debris, transfer of materials from the counter surfaces and mixing of these materials on the contact surfaces. The wear sliding test disclosed that the weight loss of the composites decreases with increasing volume fraction of zircon particulates. The composite samples were examined by X-ray diffraction technique and scanning electron microscopy which confirm the uniform distribution of zircon particles through the matrix. The hardness of the composite was affected significantly by the amount of porosity and reinforcement phase as two dominant factors.     

Repulsive van der Waals Forces. II. Mechanism of Hydrophobic Chromatography

Abstract

When two materials with different interfacial free energies are immersed in a liquid with an interfacial free energy intermediate between those of the two materials, the net van der Waals forces between these two materials are repulsive. Thus by lowering the interfacial free energy of the liquid medium, solutes or particles previously adsorbed onto low energy surfaces can be readily eluted from such surfaces. This is demonstrated by the coupling to and subsequent elution from Octyl Sepharose and Phenyl Sepharose of serum and other proteins. The elution of all proteins commenced when the surface tension of the eluting liquid was decreased to a point just below that of the protein in question. The eluted serum proteins successively emerged from the column in the exact decreasing order of their own interfacial free energies. In hydrophobic chromatography, coupling is favored when the van der Waals forces between solutes (or particles) and ligand are attractive (and maximum, frequently through the admixture of salts); elution is brought about by causing the van der Waals interaction between each solute (or particle) and ligand successively to become repulsive (by gradually lowering the surface tension of the eluting liquid).     

DEPOSITION OF AEROSOL PARTICLES ON A SURFACE COMPOSED OF DIFFERENT TYPES OF MATERIALS

Experimental and theoretical studies have been conducted on the deposition of aerosol particles on a surface composed of different types of materials (copper-polyethylene and copper-nickel). Deposition of charged particles is affected by localized electrostatic fields created by the contact potential difference between the different types of materials. The particles charged with positive polarity deposit mainly on the polyethylene or the nickel surfaces, which are negatively charged. The deposition profile has been also controlled by varying the thickness of gold layers deposited on nickel surface.     

A method for the coating of silica spheres with an ultrathin layer of pristine single-walled carbon nanotubes

We describe a method to fabricate silica gel particles coated with a monolayer of pristine single-walled carbon nanotubes (SWCNTs). SWCNTs dissolved in 1-methyl-2-pyrrolidinone (NMP) are mixed with amino-functionalized silica gels having different diameters. Strong interaction between the amino group and the SWCNT surfaces induces the adsorption of the SWCNTs on the silica, while the stable solvation in NMP hampers further adsorption of the tubes. This approach enables the production of a homogeneous, nondestructive and high-yield coating of the SWCNTs onto the silica surfaces, especially for larger sphere with a diameter over 1 μm. The density and bundling degree of the SWCNTs on the silica gel surfaces are finely controlled by simply changing the ratios of the SWCNTs to the silica gels as well as the SWCNT concentrations. We also describe the coating of the silica gels with metallic SWCNTs. The SWCNT-coated silica gels are useful for a wide range of materials, such as the stationary phase for liquid chromatography and catalyst supporting materials.     

Fabrication of carbon-coated boron carbide particle and its role in the reaction bonding of boron carbide by silicon infiltration

To tackle the dissolution problem of boron carbide particles in silicon infiltration process, carbon-coated boron carbide particles were fabricated for the preparation of the reaction-bonded boron carbide composites. The carbon coating can effectively protect the boron carbide from reacting with liquid Si and their dissolution, thus maintaining the irregular shape of boron carbide particles and preventing the growth of boron carbide particles and reaction formed SiC regions. Furthermore, the nano-SiC particles, originated from the reaction of the carbon coating and the infiltrated Si, uniformly coated on the surfaces of boron carbide particles, thus forming a ceramic skeleton of the nano-SiC particles-coated and -bonded boron carbide particles. The Vickers hardness, flexural strength and fracture toughness of the composites can be increased by 26 %, 45 %, and 37 % respectively, by using carbon-coated boron carbide particles as raw materials.     

Electrochemical properties of graphene nanosheet/carbon black composites as electrodes for supercapacitors

Graphene nanosheet/carbon black composites were prepared by the ultrasonication and in situ reduction methods. Microstructure measurements show that most carbon black particles deposit on the edge surfaces of nanosheets by the ultrasonication method, and on the basal surfaces of nanosheets by in situ reduction method. The electrochemical performances of hybrid materials are superior to pure graphene material, demonstrating that carbon black particles as spacers ensured the high electrochemical utilization of graphene layers as well as the open nano-channels provided by three-dimensional graphene nanosheet/carbon black hybrid material. Therefore, the resulting composite is a promising carbon material for supercapacitors.     

Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia-Dispersed Alumina Ceramics Prepared by Colloidal Processing

To obtain dense, fine-grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al₂O₃ and ZrO₂ particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.     

Evaluation of the adhesive-self-adhesive properties of disperse fibre-forming polymers

In determining the adhesive-self-adhesive interaction of particles of disperse fibre-forming polymers, it is necessary to consider the nature of the forces that cause it, the structural and mechanical properties of the contacting surfaces (roughness and possibility of deformation in the contact zone, which affect the contact area of the polymer particles with the surface; the porosity of the materials; the existence of a gap between contiguous bodies, the radii of curvature of the contacting bodies), as well as the shape of the processed particles. __________ Translated from Khimicheskie Volokna, No. 4, pp. 24–28, July–August, 2007.     

Untersuchungen zur Ursache der Tropfenkondensation von Wasserdampf an ionenimplantierten Metalloberflächen

The origin of dropwise condensation of steam on ion implanted metallic surfaces was fundamentally studied. Condensation experiments and surface analyses showed that chemically inhomogeneous surfaces with topographic microstructures are likely to induce dropwise condensation. These surface characteristics are caused by particulate precipitates. Such particles are formed on ion implanted surfaces by the degradation of the material supersaturation with the doping elements. Similar surfaces showing dropwise condensation also emerged spontaneously on several unimplanted, high‐alloyed materials due to oxidation and erosion effects induced by the condensation process. For explaining the observed condensation phenomena on the identified surface characteristics, a new model for the condensation mechanism was developed.     

The quantitative analysis of the light stability of insoluble singlet oxygen sensitizers

Evaluation of insoluble sensitizing materials for further technological development must include tests of their energy transfer efficiency as well as of their light stability. A procedure for the determination of the light stability of insoluble sensitizer particles including the preparation of surfaces to be irradiated and analysed is given. A close link to the widely adopted analytical methods used in industrial dyestuff, pigment and polymer laboratories (Xenotest irradiation, Cielab analysis) assures its easy implementation. The procedure may also be used for sensitizing materials already irradiated in suspensions. Standardized irradiations of blue wool patterns facilitate those determinations for procedures involving the irradiation of photoactive surfaces.     

Bed agglomeration in fluidized combustor fueled by wood and rice straw blends

Petrographic techniques have been used to examine bed materials from fluidized bed combustion experiments that utilized wood and rice straw fuel blends. The experiments were conducted using a laboratory-scale combustor with mullite sand beds, firing temperatures of 840 to 1030 °C, and run durations of 5.5 h. A narrow continuous zone borders virtually all bed particles. The highest concentrations of potassium are found in this surface zone that also is enriched in appreciable amounts of other elements. Thin discontinuous films of adhesive cement, formed preferentially on surfaces and contact areas between bed particles, ultimately led to bed agglomeration. The interfaces and the presence of gas bubbles in the cement suggest a bonding material with a high surface tension and a liquid state. The cement films originate by filling of irregularities on individual and partially agglomerated bed particle surfaces by accumulation of liquid droplets preferentially in areas sheltered from turbulence and mechanical interaction. The composition of the film suggests melting of locally accumulated dust or aerosol mixture of ash particles and mullite. The film only locally enlarged bed particles. Large straw ash particles appear to have mostly been passively incorporated into the adhesive melt without melting or reaction.     

Reaction Kinetics and Mechanism: Model Studies on Metal Single Crystals [P.H. Emmett Award Address, 1983]

In the past decade remarkable progress has been made in the surface science of catalysis. Studies on both small single crystal metal particles on supporting materials and on large (bulk) single crystals point toward a more refined understanding of the fundamental processes underlying catalytic chemistry and, hopefully, toward a rational design of catalytic materials. It seems clear that concepts gleaned from studies on single crystal surfaces will strongly influence the research with supported metal catalysts and ultimately affect the nature of the materials used as practical catalysts.     

Reaction Kinetics and Mechanism: Model Studies on Metal Single Crystals [P.H. Emmett Award Address, 1983]

In the past decade remarkable progress has been made in the surface science of catalysis. Studies on both small single crystal metal particles on supporting materials and on large (bulk) single crystals point toward a more refined understanding of the fundamental processes underlying catalytic chemistry and, hopefully, toward a rational design of catalytic materials. It seems clear that concepts gleaned from studies on single crystal surfaces will strongly influence the research with supported metal catalysts and ultimately affect the nature of the materials used as practical catalysts.     

Single particle adhesion variability in additive manufacturing powders

ABSTRACT

The adhesion properties of powder particles could profoundly influence the quality of parts made by Additive Manufacturing (AM) processes. Accurate experimental characterization of adhesion and the spatial surface adhesion distribution of a single microparticle has been a significant challenge, due mainly to difficulties associated with the micro-scale handling/manipulation in a controllable manner and uncertainty in the nature of micro/nano-scale contacts. In current work, an approach for determining the spatial energy/adhesion distribution on the surfaces of single microparticles used in AM is introduced and demonstrated using Molybdenum (Mo) metal particles as model particles. Both ultrasonic base and Surface Acoustic Wave (SAW)-based excitation techniques coupled with laser Doppler vibrometry are utilized to excite and acquire the vibrational rocking motion and to drive the rolling motion of single Mo particles on a Silicon (Si) substrate in a controllable manner and, thereby determining the spatial adhesion distribution on the particle surfaces. Spatial surface energy distribution data of microparticles could be utilized in Discrete Element Method (DEM) simulations as a statistical input for simulating local powder bed dynamics with increased accuracy, which would potentially lead to more predictable AM processes.     

Aerosol measurement by laser doppler spectroscopy—I. Theory and experimental results for aerosols homogeneous

The basic theory, experimental techniques and results are presented describing a technique for sizing aerosol particles in situ using laser Doppler spectroscopy. Unlike conventional light scattering procedures which use average intensity information, this technique utilizes the Doppler shifted frequency of the scattered light produced by the Brownian motion of the aerosol particles to determine particle diffusion coefficients and size. Experiments were carried out using monodisperse dibutylpthalate aerosols and monodisperse polystyrene latex spheres, in concentrations ranging from 10³ to 10⁶ particles per cubic centimeter. Measured particle sizes were within 10 per cent of the size predicted by conventional light scattering methods for the DBP particles and the reported sizes of the PSL particles. Based on these results it is concluded that laser Doppler spectroscopy can be utilized to accurately measure aerosol particle size in situ.     

Effects of temperature on the flow and tensile strengths of powders

Measurements have been made of the flow rates and tensile strengths of a variety of powders — magnesia, lactose, fatty acids, organic drugs — over a range of temperatures from ?20° to 200 °C. The results have been explained in terms of the effects of temperature on the hardness and elasticities of the materials concerned, and the distances between the particles.Under compression, the asperities on their surfaces deform plastically and may melt, if the temperature is raised above about 0.9 of the conventional melting point in K, to form welded bonds.The activation energy of bonding for the different materials is between about 8 and 11 kJ mole^?1.     

Detachment of rough particles with electrostatic attraction from surfaces in turbulent flows

The detachment of particles with coarse and fine roughnesses from surfaces in a turbulent boundary layer flow including electrostatic effects is studied. It is assumed that the real area of contact is determined by elastic deformation of asperities, and the effect of topographic properties of surfaces is included. The Johnson-Kendall-Roberts (JKR) adhesion model is used for analyzing the behavior of individual asperities. For an average Boltzmann charge distribution, the saturation charge condition as well as a fixed charge per unit mass, the Coulomb, the image, the dielectrophoretic, and the polarization forces acting on the particle in the presence of an imposed electric field are evaluated. The theories of rolling and sliding detachment are used to study the onset of removal of bumpy particles and those with fine roughness from plane surfaces. The hydrodynamic forces and torques acting on the particle attached to a wall, along with the adhesion force for the particle, are used in the model development. The minimum critical shear velocities needed to detach particles of different sizes from plane surfaces in the presence of an applied electric field are evaluated and discussed. The electric detachment of the particles is also studied and the field strength needed for particle removal is determined. It is shown that the surface charge distribution significantly affects the removal of particles from surfaces.