Reduction of capillary force for high-aspect ratio nanofabrication

The wet processing of SU8 resist was modified in order to achieve a high-aspect ratio patterning with feature size of 100 nm. A final rinse in water, which makes a large contact angle on the resist (less wetting) was added to the procedure. This allowed considerable reduction of the capillary force, which is responsible for pattern distortions in three-dimensional (3D) lithography. 3D recording of high-aspect ratio (f_ar=18) structures by holographic exposure using femtosecond pulses in SU8 resist was achieved using this modified development procedure. The thickness of the free-standing planes was approximately 100 nm. High fidelity of this recording method was confirmed by a Moiré pattern transfer into a developed SU8 pattern. In terms of focusing, the 100 nm feature size comprised 1/13-th of the diffraction limit. This modified development is applicable for wet processing when super-critical drying cannot be used. PACS 81.05.-t; 81.07.-b; 81.16.-c; 81.40.-z; 81.65.-b     

Bipolar charging of dust particles under ultraviolet radiation

The photoemission charging of dust particles under ultraviolet radiation from a xenon lamp has been investigated. The velocities of yttrium dust particles with a work function of 3.3 eV and their charges have been determined experimentally; the latter are about 400–500 and about 100 elementary charges per micron of radius for the positively and negatively charged fractions, respectively. The dust particle charging and the dust cloud evolution in a photoemission cell after exposure to an ultraviolet radiation source under the applied voltage have been simulated numerically. The photoemission charging of dust particles has been calculated on the basis of nonlocal and local charging models. Only unipolar particle charging is shown to take place in a system of polydisperse dust particles with the same photoemission efficiency. It has been established that bipolar charging is possible in the case of monodisperse particles with different quantum efficiencies. Polydispersity in this case facilitates the appearance of oppositely charged particles in a photoemission plasma.     

Corona discharge in a cylindrical triode charger for unipolar diffusion aerosol charging

A cylindrical triode charger for unipolar diffusion charging of aerosol particles was designed, constructed, and evaluated. The corona discharge characteristics were studied in this cylindrical triode charger. For the process the current–voltage characteristics were determined, as were the ion number concentration, the n_it product, and the mean charge per particle as a function of particle diameter. The discharge and charging currents, and ion number concentration in the charging zone of the charger increased monotonically with corona voltage. The negative corona had a higher current than the positive corona. At the same corona voltage, the ion number concentration in the discharge zone was larger than the charging current for positive and negative coronas, with values of about 197 and 32 times and 645 and 99 times for the ion-driving voltages of 0 and 310 V, respectively. The average ion penetration for positive and negative coronas was 0.64 and 0.19% and 3.62 and 1.93% for the ion-driving voltages of 0 V and 310 V, respectively. The higher flow rate, shorter residence time, gave a lower N_it product. By calculation 14% of charged particles of 10 nm in diameter were lost to the outer cylinder because of the electrostatic field effect. The charger does not use a sheath of air flow along the walls or the perforated screen opening, it has low diffusion and space charge losses due to the short column charging zone, and is a low complexity and inexpensive system. It worked as well as more sophisticated and expensive commercially available chargers.     

Model for nanoparticle charging by diffusion,direct photoionization,and thermionization mechanisms

A nanoparticle charging model considering simultaneous diffusion, direct photoionization and thermionization charging was developed. The model included a balance expression for the positive and negative ions, and one for each charge level of the particles. Three comparable parameters: attachment coefficient, β^±(v,q), photoelectric yield coefficient, α⁺(v,q), and thermionic yield coefficient, γ⁺(v,q), were identified that govern different charging mechanisms. By comparing these parameters, the importance of different mechanisms was explored. Analytical (closed-form) solutions under certain unipolar and bipolar conditions were proposed. Literature reported experimental data on a soft-X-ray based unipolar charger was used to verify this model and fair agreement was achieved. A numerical algorithm was developed to solve the governing equations for a soft-X-ray enhanced electrostatic precipitator (ESP) system. The role of soft-X-ray irradiation at improving nanoparticle average charge and enhancing the ESP capture efficiency was demonstrated. Experimental tests with this system were also conducted. Measured capture efficiency showed excellent agreement with theoretical results.     

Validation of the anisotropy index ellipsoidal area ratio in diffusion tensor imaging

A new diffusion anisotropy index, ellipsoidal area ratio (EAR), was described recently and proved to be less noise-sensitive than fractional anisotropy (FA) by theory and simulation. Here we show that EAR has higher signal-to-noise ratios than FA in average diffusion tensor imaging data from 40 normal subjects. EAR was also more sensitive than FA in detecting white matter abnormalities in a patient with widespread diffuse axonal injury. Monte Carlo simulation showed that EAR's mean values are more biased by noise than FA when anisotropy is small, both for single fiber tracts and when fiber tracts cross. However, the improved signal-to-noise ratio of EAR relative to FA suggests that EAR may be a superior measure of anisotropy both in quantifying both deep white matter with relatively uniform fiber tracts and pericortical white matter structure with relatively low anisotropy and fiber crossings.     

Predicting the electrostatic charging behavior of insulating materials without charging tests

It is well known that experiments of charging and later discharging of materials with gas ignition probes or hand-coulombmeters show that some insulating materials do exist that cannot be hazardously charged by manual rubbing in spite of their high surface resistance. In this work, effects of variations of physical and chemical properties as well as the structure of these materials have been systematically investigated. The results obtained help to explain this behavior and propose approaches to predict the charging behavior of such insulating materials without any charging test at all.     

Determination of the thermal diffusion ratio in a binary mixture by forced Rayleigh scattering

After recalling briefly the results of the analysis of a forced Rayleigh experiment in a binary fluid mixture, one shows data obtained in a carbon disulfide-ethanol mixture. Estimating the derivatives of the susceptibility of the mixture from the Lorentz-Lorenz formula, one obtains values for the thermal diffusion ratio for several concentrations.     

The ellipsoidal area ratio: an alternative anisotropy index for diffusion tensor imaging

In the processing and analysis of diffusion tensor imaging (DTI) data, certain predefined morphological features of diffusion tensors are often represented as simplified scalar indices, termed diffusion anisotropy indices (DAIs). When comparing tensor morphologies across differing voxels of an image, or across corresponding voxels in different images, DAIs are mathematically and statistically more tractable than are the full tensors, which are probabilistic ellipsoids consisting of three orthogonal vectors that each has a direction and an associated scalar magnitude. We have developed a new DAI, the "ellipsoidal area ratio" (EAR), to represent the degree of anisotropy in the morphological features of a diffusion tensor. The EAR is a normalized geometrical measure of surface curvature in the 3D diffusion ellipsoid. Monte Carlo simulations and applications to the study of in vivo human data demonstrate that, at low noise levels, EAR provides a similar contrast-to-noise ratio (CNR) but a higher signal-to-noise ratio (SNR) than does fractional anisotropy (FA), which is currently the most popular anisotropy index in active use. Moreover, at the high noise levels encountered most commonly in real-world DTI datasets, EAR compared with FA is consistently much more robust to perturbations from noise and it provides a higher CNR, features useful for the analysis of DTI data that are inherently noise sensitive.     

Electrohydrodynamics of Bipolar Membranes

On the basis of the Nernst?Planck?Poisson?Stokes system, the behavior of the electrolyte solution is numerically studied for the first time in bipolar electrical membranes under the action of an external electric field while taking into account water dissociation.     

Optical characterization of metallic aerosols

Airborne metallic particulates from industry and urban sources are highly conducting aerosols. The characterization of these pollutant particles is important for environment monitoring and protection. Because these metallic particulates are highly reflective, their effect on local weather or regional radiation budget may also need to be studied. In this work, light scattering characteristics of these metallic aerosols are studied using exact solutions on perfectly conducting spherical and cylindrical particles. It is found that for perfectly conducting spheres and cylinders, when scattering angle is larger than 90° the linear polarization degree of the scattered light is very close to zero. This light scattering characteristics of perfectly conducting particles is significantly different from that of other aerosols. When these perfectly conducting particles are immersed in an absorbing medium, this light scattering characteristics does not show significant change. Therefore, measuring the linear polarization of scattered lights at backward scattering angles can detect and distinguish metallic particulates from other aerosols. This result provides a great potential of metallic aerosol detection and monitoring for environmental protection.     

Determination of natural radioactive aerosols

A critical evaluation is made of methods used for determining natural radioactive aerosols and airborne Rn-222 and Rn-220. From the point of view of health the decisive factor is the amount of active Rn-222 and Rn-220 deposit in inhaled air. A simple filtration method, based on a comparison of the amount of radioactive aerosols in the analyzed air with their content in the free air, i.e. in country air, has been elaborated for determining the active deposit. The apparatus is portable and consists of easily accessible parts.The author thanks J. Koí from the Radiological Dosimetry Department of the Nuclear Research Institute, Czechoslovak Academy of Sciences, for very effective technical collaboration.     

Absorption of ultrasonics in aerosols

A two crystal transmission and receiving system is used to investigate the ultrasonic absorption of various aerosols. Absorption coefficients of the medium between the crystals are obtained by substituting electrical attenuation for acoustic attenuation. A photoextinction-sedimentation technique for measuring size of particles suspending in liquids is used to determine the size distribution curves of the aerosols. The investigation included magnesium oxide and stearic acid aerosols and lycoperdon spores.     

The optimization of accuracy ratio of the two-group diffusion constants in simulation model of RBMK-1000 core

The relative ratio of individual accuracies of the two-group diffusion constants in a dynamic simulation model of a reactor core is optimized. This is done to minimize calculation errors of neutron flux, power, or reactivity distributions in the model. The problem is solved under the assumption that the overall accuracy of the representation of constants is limited by the resources allocated for the approximation of the constants.     

Mechanisms of charging in electron spectroscopy

From the use of physical arguments based on classical electrostatics and elementary solid state physics, the role of the various parameters involved in the charging mechanisms of insulating materials is analysed in detail when these insulating specimens are investigated by surface analytical techniques (mainly XPS and e⁻AES). The role of the sub-surface composition and structure is outlined and the strong correlations between charging effects and some radiation damage effects are pointed out. Some strategies are also deduced to minimise these effects.     

Contact charging of oxidized metal powders

Samples of oxidized metal powders (Ag, Al, Cr, La, Nb, Ni, Ti) were repeatedly contacted with a gold plate under high vacuum conditions. The charge transfer was measured on both the samples and the gold plate. We observed charge saturation after one contact in most cases. Positive and negative polarities were detected. The charge densities were calculated and correlated with work functions of oxidized metals. Contact electrification of aluminum shows exceptional behavior with repeated contacts.