Effects of Short-Term Laser Beam Heating on the Absorptivity of Steel Sheets

The efficiency of laser beam processes basically depends on the efficiency of the laser beam source and the efficiency of the irradiated material’s energy absorption. This absorptivity can be influenced by the surface condition. Besides coating or boundary layers, the surface topography is decisive. In this study, the effects of various time–temperature distributions on the absorptivity changes of steel sheets were investigated. For this purpose, three steels were chosen, namely, a stainless steel, a spring steel, and a hot work tool steel. Pre- and post-process characterizations of the absorptivity and surface topography were performed. Controlled heating with a laser beam was carried out using temperatures between 700 and 1200 °C and durations between 2 and 34 s. In order to compare the influences of these heating procedures on the absorptivity, a characteristic value, the temperature‑compensated time, was introduced. It is shown that the surface roughness was influenced by laser irradiation but inadequately describes the increase of absorptivity. The changes in absorptivity are attributed to oxidation, which had an influence on the topography in a sub‑micrometer range. Moreover, a saturation effect is observed for intense heatings. Furthermore, it is shown that the temperature‑compensated time is a suitable value to describe absorptivity changes caused by short‑term heating.

A Novel Method of Synthesizing Graphene for Electronic Device Applications

This article reports a novel and efficient method to synthesize graphene by thermal decomposition process. In this method, silicon carbide (SiC) thin films grown on Si(100) wafers with an AlN buffer layer were used as substrates. A CO2 laser beam heating without vacuum or controlled atmosphere was applied for SiC thermal decomposition. The physical, chemical, morphological, and electrical properties of the laser-produced graphene were investigated for different laser energy densities. The results demonstrate that graphene was produced in form of small islands with quality, density and properties depending on the applied laser energy density. Furthermore, the produced graphene exhibits a sheet resistance characteristic similar to graphene grown on mono-crystalline SiC wafer, which indicates its potential for electronic device applications.

CONVECTIVE MOTIONS IN NEMATIC LIQUID CRYSTAL HOMEOTROP AND PLANAR CELLS INDUCED BY GAUSSIAN LASER BEAM

Laser beam heating the medium induces instability in the liquid crystal (LC) cell. This instability in conjunction with influence of gravitational force results in convective motions in the cell. In this paper theoretical modeling for studying convection induced by Gaussian laser beam in nematic LC homeotrop and planar cells is presented for the first time. Velocity field and LC director distribution are obtained for various light powers and LC cell sizes by solving Navier-Stokes, heat transfer and director equations simultaneously. The modeling allows us to solve the problems of convections induced by Gaussian laser beam due to Rayleigh-Benard and Marangoni mechanisms as well. There is a good qualitative agreement between theoretical calculations and prior experimental results. The possibility of control and stability of convective motions are studied. Instabilities of the Benard cells are of thermal origin because the Prandtl number for the medium under study is considerably larger than unity.

Instrumentation for structure measurements on highly non-equilibrium materials

Containerless techniques (levitation) completely eliminate contact with the sample. This unique sample environment allows deep supercooling of many liquids and avoids contamination of high-temperature melts. Recent experiments at the Advanced Photon Source (APS) high-energy beamline 11 ID-C used aerodynamic levitation with laser beam heating and acoustic levitation with cryogenic cooling. By using these two methods, liquids were studied over much of the temperature range from −40 to +2500°C. This paper briefly describes the instrumentation and its use and is illustrated with examples of measurements on molten oxides and low-temperature liquids.

In-Situ Formation of a Gel Microbead for Laser Micromanipulation of Microorganisms, DNA, and Viruses

We proposein situformation of gel microbeads made of a thermoreversible hydrogel for indirect laser micromanipulation of microorganisms, DNA, and viruses. Using a 1064 nm laser, we irradiated an aqueous solution mixed with poly-(N-isopropylacrylamide) through a high- magnification lens, thereby forming a gel microbead through heating at the laser focus. The gel microbead, trapped by the laser, was used to indirectly manipulate micro- and nano-scale samples. Laser tweezers stably handle micro-scale object ranging from several tens of nm to several hundreds of µm. This cannot be done with nano-scale objects of a few nm, however, due to laser beam heating. We demonstrate how to manipulate microorganisms, DNA, and viruses indirectly using a gel microbead made from an aqueous poly-(N-isopropylacrylamide) solution. We reduced laser power for gel microbead formation, and used the gel microbead trapped by the laser to manipulate microorganisms, DNA, and viruses.