Papers by Keyword: Surface Structure

Abstract: The Stefan problem in a semi-infinite media under laser irradiation is considered. It is related to the melting and solidification processes, resulting in certain surface structure after the solidification. A simple model, as well as a more sophisticated one is proposed to describe this process. The latter model allows us to calculate the surface profile by solving a system of two nonlinear differential equations, if the shape of the solid-liquid interface is known. It has to be found as a solution of two-phases Stefan problem. The results of example calculations by the fourth-order Runge-Kutta method are presented, assuming that the solid-liquid interface has a parabolic shape. The calculated crossection of the surface structure shows a characteristic cone in the center, in agreement with experimental observations.

Abstract: DAST (4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate) crystal has been grown by slope nucleation method (SNM) at different cooling rates (0.5°С/d and 0.1°С/d). The morphology of crystal growing surfaces was observed by scanning electron microscope (SEM) and microscope. Different growth striations were found on the growth surfaces. The arrangements of crystal ions and cooling rate have a great influence on the crystal shape. The relationship between the DAST crystal molecular structure and the crystal shape was systematically studied by analyzing the growing surface morphology. Two-dimensional nucleation mechanism was found on growing faces with increasing supersaturation.

Abstract: Biomimetic surface structures have profound influences on the development of many emerging devices and systems. In this study, a sequential approach involving hot embossing and polymer casting for transferring biological surface structures to thermoplastic polymers were investigated and developed. The surface structure on shark skins (Carcharhinus brachyurous) was used as a case study. A PMMA pattern was fabricated by directly hot embossing the biological surface of the skin. The resulting PMMA structure was used as a casting master for rapid feature transfer onto PDMS surfaces. The replicated surface patterns on the polymer were found to be comparable with those on the shark skin. The water tunnel test of the flatplate sample pieces suggests that the drag reduction efficiency of the biomimetic shark-skin surface achieves 8.25% in maximum and 6.91% in average, which validates that this forming technology can be applied to the direct pattern transfer of the firm creatural scarfskins onto thermoplastic polymers.

Abstract: This paper reports the silica density, surface structures and optical properties of gold nanoparticles coated with different thickness of silica shells. The gold nanoparticles encapsulated with amorphous silica shells were prepared in a slight modification of Stǒber method. The silica-shell thickness could be varied from 20 to 50 nm by controlling the experimental conditions, such as reaction time. Transmission Electron Microscopy (TEM) and UV-Visible absorption spectroscopy were employed to characterize the size, shell density, surface structures and the optical properties of these silica-coated gold nanoparticles. The TEM images demonstrated that the density of the silica shell were depended on the reaction time, and the surface morphology was changed from porous structures in the initial coating to the final continuous and smooth silica surface. With the increasing of the reaction time, the silica-coated gold nanoparticles became more and more round and monodispersed. UV-Vis spectra showed that surface plasmon absorption peak had a red-shifted of 3~12 nm on increasing the thickness of silica shell from 20 to 50 nm. A possible mechanism of silica formation on gold nanoparticles was proposed on the basis of silica shell density and the shift of absorption peak of coated gold nanoparticles.

Abstract: Radio frequency (rf) magnetron sputtering of polytetrafluoroethylene (PTFE) using argon as the working gas was used to prepare hydrophobic fluorocarbon films on a polypropylene substrate. The morphology, structure and hydrophobicity of the fluorocarbon films were analyzed by means of SEM, AFM, XPS and contact angle determination. The growth pattern in this system was a typical one-dimensional (1-D) Volmer-Weber growth mode. The films demonstrated dependence of structure and hydrophobicity on the conditions of preparation. The contact angle decreased with increasing discharge power and increased with increasing pressure. Those trends are attributed to the presence of differing proportions of -CF3, -CF2-, -CF-, and -C- in fluorocarbon thin films prepared with different energy.

Abstract: In this paper, the feasibility of obtaining well-defined compact EPD deposits in a short time was studied. EPD from organic and aqueous suspensions were tested. The formation of bubbles coming from solvent electrolysis damages the deposits. 4 different EPD geometric configurations were developed and tested with different experimental conditions. One of them, consisting of both electrodes tilted 30° helped to avoid bubbles getting trapped in the deposit. Our preliminary results showed that there is a very strong influence of the voltage applied, and the separation distance between the electrodes, over the width and height of the structures deposited. Working with aqueous suspensions is more suitable than with organic solvents from the environmental point of view. More work needs to be done in this direction to improve the quality of the deposits and to fine-tune the experimental conditions.

Abstract: The evolution and structure of graphene layers on 4H-SiC(0001) and the corresponding interface are investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The surface is characterized by the so-called (6p3£6p3)R30± reconstruction, whose structural properties are still unclear but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We analyse the properties of three phases in this reconstruction with periodicities (6p3£6p3)R30±, (6£6) and (5£5). Their LEED intensities strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. An easy and practicable determination of the number of graphene layers is outlined by means of LEED intensities.

Abstract: A novel Monte Carlo kinetic model has been developed and implemented to predict growth rate regimes and defect formation for the homo-epitaxial growth of various SiC polytypes on different substrates. Using this model we have studied the generation of both point like and extended defects in terms of the growth rate and off-cut angle, finding qualitative agreement with both electrical and optical characterization and analytical results.

Abstract: Silicon nitrides are often used as ladles, stalks, heater element protection tubes, etc., in the metal casting industry. A low wettability for molten metals is required for these purposes since wetting by molten metals leads to adhesion of solidified metals, which causes several problems. Surface structure is known to affect wettability. Thus, the present study attempts to fabricate silicon nitrides with controlled surface structures. Silicon nitrides, whose surfaces were covered with ordered hemispherical protrusions, were fabricated by the slip-casting technique. The cast bodies were sintered, and subjected to wettability tests using molten metals. For comparison, silicon nitrides with as-sintered and polished surfaces were also prepared. The surface with protrusions exhibited a lower contact angle compared to the as-sintered and polished surfaces. The contact angle depended on the diameter of the hemisphere; it was the largest at a diameter of 0.3 mm.

Abstract: Ordered reconstruction phases on the 4H-SiC(1102) surface have been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). After initial hydrogen etching, the samples were prepared by Si deposition and annealing in ultra-high vacuum (UHV). Two distinct reconstruction phases develop upon annealing, first with a (2×1), and at higher temperatures with a c(2×2) LEED pattern. After further annealing the fractional order LEED spots vanish and a (1x1) pattern develops. For the (2×1) phase, STM micrographs show that adatom chains develop on large flat terraces, which in view of AES consist of additional Si. These highly linear and equidistant chains represent a self-assembled well-ordered pattern of nanowires developing due to the intrinsic structure of the 4H-SiC(1102) surface. For the c(2×2) phase AES indicates a surface composition close to the bulk stoichiometry. For the (1×1) phase a further Si depletion is observed.