Micron Size Scale Research Articles

Variety of Aggregation and Growth Processes of Lanthanum Fluoride as a Function of La/F Activity Ratio. 2. Excess of F over La Region. Transformation of Amorphous to Crystalline Phase, POM, SAXS, WAXS, and XRD Study

Preparation of lanthanum hydroxy-fluorides using steady-state pF-stat experiments, which ensures constant, very low concentrations of the precipitation components and constant pH during the whole growth process, results in a structural evolution of objects with defined physical parameters and a variety of nonconventional morphologies. This structural evolution was characterized by a combination of techniques, namely, static and dynamic light scattering (SLS and DLS), small-angle and wide-angle X-ray scattering (SAXS and WAXS), X-ray diffraction (XRD), and polarized optical microscopy (POM). It is shown that after the immediate formation of an amorphous glassylike precursor, crystalline nanoparticles of high structural definition and monodispersity transit into hollow spheres with diameters of several hundred nanometers. These large vesicles contained smaller ones, which have shown the Maltese cross. After vesicle deformation and elongation into fibers with extreme length, the nanocrystals are transferred into ribbonlike mesostructures. Assuming rod-shaped nanoparticles with ellipsoidal cross-section, we calculate the semiaxes a = 3.2 ± 0.2 nm and b = 9.25 ± 0.3 nm from the cross-section radius of gyration Rc = 4.9 ± 0.1 nm and thickness radius of gyration Rt = 1.6 ± 0.1 nm as determined by SAXS measurements. These data yield an axial ratio ρ = b/a = 3. As was presented in part 1, rods of submicrometer size, determined by DLS, grown inside the ribbons, also have a semiaxis ellipsoidal ratio ρ = 3.0 ± 0.1 and are monodisperse (the polydispersity index is defined as dw/dn = 1.01). The same axial ratio ρ = 3.0 ± 0.5 was determined by POM on the micron-size scale range. These are three independent methods, which confirm the consistency of morphology. The XRD data (providing an average structure, not a local one) suggest that mainly nonstoichiometric materials of tetragonal structure transform to the stoichiometric equilibrium hexagonal LaF3 structure. The shift of the two strongest peaks to longer distances occurs. This confirms the change in favor of the formation of longer La−F distances instead of shorter La−O distances, which were first formed during the transformation of amorphous gel to crystalline phase, shown on the WAXS and XRD diffractograms. These independent methods convincingly confirm the self-similarity of the particle shape and axial ratio and of the structure evolution in different scale lengths, as in a vast number of cases in the scientific literature. This conclusion is crucial in possible preparation of materials for special applications.

Optical microrheology using rotating laser-trapped particles.

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Layered silicate nanocomposites based on various high‐functionality epoxy resins. Part II: The influence of an organoclay on the rheological behavior of epoxy prepolymers

AbstractThe effect of an organically surface modified layered silicate on the viscosity of various epoxy resins of different structures and different functionalities was investigated. Steady and dynamic shear viscosities of the epoxy resins containing 0–10 Wt% of the organoclay were determined using parallel plate rheology. Viscosity results were compared with those achieved through addition of a commonly used micronsized CaCO3 filler. It was found that changes in viscosities due to the different fillers were of the same order, since the layered silicate was only dispersed on a micron‐sized scale in the monomer (prior to reaction), as indicated by X‐ray diffraction measurements. Flow activation energies at a low frequency were determined and did not show any significant changes due to the addition of organoclay or CaCO3. Comparison between dynamic and steady shear experiments showed good agreement for low layered silicate concentrations below 7.5 wt%, i.e. the Cox‐Merz rule can be applied. Deviations from the Cox‐Merz rule appeared at and above 10 wt%, although such deviations were only slightly above experimental error. Most resin organoclay blends were well predicted by the Power Law model, only concentrations of 10 wt% and above requiring the Herschel‐Buckley (yield stress) model to achieve better fits. Wide‐angle X‐ray measurements have shown that the epoxy resin swells the layered silicate with an increase in the interlayer distance of approximately 15 Å, and that the rheology behavior is due to the lateral, micron‐size of these swollen tactoids.

The Stress–Strain Behavior of Coronary Stent Struts is Size Dependent

Coronary stents are used to re-establish the vascular lumen and flow conditions within the coronary arteries; the typical thickness of a stent strut is 100 microm, and average grain sizes of approximately 25 microm exist in stainless steel stents. The purpose of this study is to investigate the effect of strut size on the stress strain behavior of 316 L stainless steel. Other materials have shown a size dependence at the micron size scale; however, at present there are no studies that show a material property size dependence in coronary stents. Electropolished stainless steel stent struts within the size range of 60-500 microm were tensile tested. The results showed that within the size range of coronary stent struts a size dependent stress-strain relationship is required to describe the material. Finite element models of the final phase of fracture, i.e., void growth models, explained partially the reason for this size effect. This study demonstrated that a size based stress-strain relationship must be used to describe the tensile behavior material of 316 L stainless steel at the size scale of coronary stent struts.

Effect of Moisture and Graded-Layer Mechanical Properties on Deformation and Interfacial Adhesion

AbstractControlling material properties over nanometer length scales is crucial for current and emerging high-density microelectronic device packages. Miniaturization of devices is increasingly limited by the ability to “connect” to the device, and the required packaging structures must be fabricated where layer thickness and feature sizes approach micron size scales while achieving the required mechanical, thermal and electrical properties. Second phase additions such as sub-micron sized particles are often added to locally adjust the material properties of constituent layers in the complex package structure. This results in significant variation of mechanical properties over sub-micron length scales. Such manipulation of material structure and its effects on mechanical and interfacial fracture behavior are addressed using experimental and modeling studies. Underfill layers consisting of an epoxy matrix with dispersed silica beads are shown to exhibit variations of elastic and flow properties in excess of three-fold across the layer thickness. Mechanical properties are not only affected by the distribution of second-phase fillers, but also by the adhesion properties of the filler/matrix interface. Interfaces are susceptible to stress corrosion cracking associated with moisture which can lead to progressive debond growth at loads much lower than that required to exceed the critical interface fracture energies. Subcritical debonding is affected by temperature, humidity, and the bond chemistry of the interface. The effects of these variations are considered on the adhesive and subcritical debonding behavior of interfaces between model epoxy underfills and SiNxchip passivation. Implications for other constrained complex layered structures are considered.

Spherical and vesicular ionic aggregates in Zn-neutralized sulfonated polystyrene ionomers

Ionic aggregates in a series of Zn-neutralized poly(styrene-co-styrene sulfonate) (SPS) random ionomers have been imaged using scanning transmission electron microscopy. The Zn-rich aggregates were found to have two shapes: solid spheres (Type I) and shells or vesicles (Type II). Type I aggregates range in a maximum diameter from 4 to 10 nm, whereas Type II aggregates range in a maximum diameter from 9 to 55 nm with a vesicle wall thickness of ∼ 3 nm. Lightly neutralized ionomers exhibited only Type I aggregates, whereas higher neutralization levels exhibited both Type I and II aggregates. Lightly neutralized ionomers also showed evidence of macrophase separation at the micron size scale. These direct observations of ionic aggregates contradict previous interpretations of small-angle X-ray scattering data with respect to size, size dispersity, shape, and spatial distribution. In addition, the aggregates observed in SPS differ markedly from the nearly monodisperse ∼ 2-nm spherical aggregates observed in Zn-neutralized poly(ethylene-co-methacrylic acid). The presence of vesicular aggregates encourages a re-examination of the morphologies and properties of styrenic ionomers. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 477–483, 2001

Effects associated with bubble formation in optical trapping

Abstract Heating of absorbing particles in a liquid medium by an optical trapping beam may lead to bubble formation. Powerful currents, which we identify as due to Marangoni convection, can be observed in the vicinity. At the micron size scale such surface tension effects can be very powerful, whereas normal thermal convection is negligible. Similar effects cause bubbles to be attracted to regions of higher temperature, providing a very powerful means of trapping bubbles, which are repelled by optical forces in a Gaussian beam. Measurements of the temperature required for bubble formation show that it occurs above the boiling point of the surrounding liquid, in reasonable agreement with nucleation theories.

Effects associated with bubble formation in optical trapping

Heating of absorbing particles in a liquid medium by an optical trapping beam may lead to bubble formation. Powerful currents, which we identify as due to Marangoni convection, can be observed in the vicinity. At the micron size scale such surface tension effects can be very powerful, whereas normal thermal convection is negligible. Similar effects cause bubbles to be attracted to regions of higher temperature, providing a very powerful means of trapping bubbles, which are repelled by optical forces in a Gaussian beam. Measurements of the temperature required for bubble formation show that it occurs above the boiling point of the surrounding liquid, in reasonable agreement with nucleation theories.

Kinetics of subnanometer displacements of solid surfaces under local heating by a laser pulse

A method with space-time resolution Δr∼ µm and δt∼0.1 µs is proposed for investigating the kinetics of angstrom-size displacements and deformations induced on solid surfaces by local irradiation with a laser pulse. The instantaneous deformation profiles were recorded at a set of points on the surface on micron-size scales by the method of photothermal deformation of the surface (PTDS) according to the kinetics of the deflection of a probe laser beam followed by extraction of data corresponding to the same moment in time; the instantaneous displacement profiles were obtained by integrating the instantaneous deformation profiles numerically in the radial direction. Instantaneous profiles obtained in this manner for angstrom-scale deformations and displacements are presented for a number of metal samples. The nature of the kinetic displacements obtained at the center of an ∼10–100 µm heating laser spot is discussed.

Mechanisms of interactive crazing in PC/SAN microlayer composites

AbstractMechanisms are proposed for the two types of interactive crazing that have been observed in PC/SAN microlayer composites when the PC layer thickness is on the micron‐size scale. It is demonstrated that when the PC layer is thin enough the deformation zone that forms at a craze tip can interact with the next‐neighboring SAN layer. By measuring the dimensions of the craze tip in scanning electron micrographs, it was found that the craze‐tip opening does not depend on the SAN layer thickness, i.e., the length of the SAN craze. Consequently, the size and shape of the resulting plastic zone in the PC layer are also independent of layer thickness. The zone that forms in the PC layer consists of a colinear plastic zone together with a pair of micro‐shearbands that grow at an angle of about 45°. When the PC layer is less than 6 μm, the elastic stress concentration from the colinear plastic zone increases the probability of crazing in the neighboring SAN layer with the formation of craze doublets that consist of two aligned crazes in neighboring SAN layers. By taking into consideration the Weibull distribution of crazing, a craze doublet fraction comparable to the 30% observed experimentally was predicted with a stress intensification factor in the range of 1.03–1.05. When the PC layer thickness is less than 1.3 μm, the length of the colinear plastic zone is comparable to the PC layer thickness. Formation of a craze at the point of impingement of the plastic zone on the neighboring SAN layer leads to craze arrays with many aligned crazes in neighboring SAN layers. At higher strains, the micro‐shearbands grow through the PC layers and extend into several adjacent SAN and PC layers. This produces a change in deformation mechanism in the SAN layers at the yield instability, from craze opening to shear yielding. © 1994 John Wiley & Sons, Inc.

Microfabricated electrohydrodynamic pumps

Pumping is often cited as a general application which motivates the development of microfabricated motors and other actuators. In that spirit, this paper studies microfabricated electrohydrodynamic fluid pumps. In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields and charges in the fluid. In contrast to forces generated by mechanical pumping using an impeller or bellows, EHD pumping requires no moving parts and consequently offers the possibility of simplified fabrication and high reliability. This paper discusses electrohydrodynamic pumping and issues concerning its use in micronsize scale systems. The fundamental operating principles of the EHD pump are outlined and examples of configurations which meet the requirement for inducing free electric charge are shown. The possible performance achievable in micron size-scale regimes is indicated. Issues concerning fluid conductivity, instability and surface tension are addressed. A microfabricated structure which demonstrates the EHD pumping of a highly insulating silicone oil is described. The structure consists of an array of 10 μm by 235 μm highly doped, LPCVD polycrystalline silicon electrodes patterned over silicon nitride. The electrode array is excited with a traveling wave of electric potential. Pumping results are qualitatively described. This paper describes a study of electrohydrodynamic pumping, including issues concerning its application to micron size-scale systems. The fundamental operating principles of EHD pumps are outlined and the possible performance achievable in micron size-scale regimes is indicated. Surface and bulk instabilities are addressed. Finally, the preliminary results of an EHD pumping experiment with a microfabricated structure are described. A practical requirement for EHD pumping is the induction of free electric charge in the volume of the fluid to be pumped or on its interface with another material. Charge accumulation on a material interface is readily achievable; however, if one material is a fixed rigid wall, such as the wall of a conduit, no pumping can take place. Consequently, the practical application of microfabricated EHD pumps may require the induction of free charge in the volume of the fluid, possibly by temperature-induced conductivity gradients. A further constraint on the usefulness of EHD pumping is its reduced effectiveness with conducting fluids. Hence its usefulness in many situations, including biological environments, may be limited. Another possible difficulty may be fluid instabilities; however, these instabilities may be useful for mixing and cooling purposes. Yet, it remains to be seen if they can compete with molecular diffusion in micron-size scale systems. Finally, it seems clear that surface tension will be a dominant force in virtually any micron-scale system with a liquid surface. In spite of these difficulties, electrohydrodynamic interactions may prove to be a reasonable way to achieve pumping without moving parts.