Yu. Emirov

Formation of nanocavities in the surface layer of an aluminum target irradiated by a femtosecond laser pulse

It has been revealed experimentally that nanocavities remain inside a surface layer of aluminum after action of a femtosecond laser pulse. This result is in agreement with numerical simulation. A detailed picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt has been reconstructed through atomistic simulation. It has been shown that the bubbles do not fully collapse but remain as frozen disk-shaped nanocavities upon recrystallization of the melt. The formation of a porous metal with small voids is very important for understanding the physics of laser exposure and may have significant applications.

Magnetization and magnetocaloric effect in ball-milled zinc ferrite powder

Enhancement of magnetocaloric effect (MCE) in nanostructured materials is important for refrigeration applications such as spot cooling in microelectromechanical system devices. Here we report the first investigation of MCE properties in ball-milled ZnFe2O4 particles. The MCE was obtained by measuring a family of M-H curves at set temperature intervals and calculating the entropy change (ΔS) for this system using the Maxwell relation. The surface structure of zinc ferrite particles is sensitive to ball milling conditions and we observed that these surface effects greatly impact the MCE and our observations could provide a route for its potential enhancement by controlled surface modification.

The effect of an ultrashort laser pulse on metals: Two-temperature relaxation, foaming of the melt, and freezing of the disintegrating nanofoam

Ultrashort heating of substances converts them into the two-temperature (2T) state with hot electrons. The thickness of the heated layer rapidly increases into the depth of the metal in this state (by comparison with the thickness of the skin layer), and the pressure in the heated layer rises sharply because of the high rate of heating (inertial confinement). A technique has been developed for taking into account 2T, thermomechanical, and multidimensional (target-structuring) phenomena. It is based on quantum-mechanical computations by means of the density functional, the solution of kinetic equations, and 2T hydrodynamic and molecular-dynamic calculations. The mechanism for forming superelastic shock waves and for constructing complex surface structures has been studied. The corresponding results have great significance for developing promising nanometallurgical technologies associated with laser pinning, for increasing corrosion resistance, and for altering optical surface characteristics.

Pinhole detection in Si solar cells using Resonance Ultrasonic Vibrations

Resonance Ultrasonic Vibrations (RUV) methodology was developed to accurately and automatically detect mechanically unstable wafers and cells with millimeter-length cracks. It was observed by cell producers that other mechanical problems possess a high probability of wafer/cell breakage in production. A small, sub-millimeter diameter “pinhole” represents a seed point defect which dramatically reduces the cell strength and ultimately leads to breakage and yield reduction. To address this production problem, we designed a new cell inspection protocol to identify the wafers and cells with pinholes using an addition to the RUV system called the Activation Station. Pinholes were introduced by indentation in a commercial grade single- or multi-crystalline Silicon solar cells and revealed by a high resolution Scanning Acoustic Microscopy (SAM) and in further details by Scanning Electron Microscopy combined with Focused Ion Beam with cross section capability. It was established that after the indentation, a pinhole gives rise to sub-millimeter length seed cracks which elongate in the form of cross-cracks in Cz-Si cells, and lead to cell fracture. To observe and screen out cells with this type of flaws the RUV system was upgraded with the Activation Station. A new RUV-AS cell inspection protocol was realized. Our tests show that pinholes located at the central area of the cell are covered with 100% accuracy.

Sapphire substrates with a regular surface relief

The lithography-free technique is proposed to obtain a regular relief in the form of a regular 2D system of 25-nm protrusions on a sapphire plate surface. The use of grid masks allows one to form a regular relief on the sapphire substrates of arbitrary area. The structure of crystal films formed by the sputtering of metal aluminum onto sapphire substrates with subsequent oxidation and annealing is thoroughly investigated and compared with the nanostructured (0001) sapphire wafer surface in the form of regular steps up to 5 nm in height with atomically smooth terraces.

Yield enhancement for solar cell manufacturing using resonance Ultrasonic vibrations inspection

Resonance Ultrasonic vibrations (RUV) methodology was developed and applied to access mechanical quality of crystalline silicon wafers and solar cells. RUV approach is based on fast non-destructive measurement of specific resonance vibration mode generated in the tested object using external ultrasonic transducer. Crack introduced into the object alters the resonance properties of the mode and allows sensitive diagnostics of the crack appearance. The paper describes our recent results of applying RUV method at industrial environment with objective to reduce a breakage rate in solar cell and solar module lines.

Investigation of ZnFe2O4 Nanoparticles Prepared by High Energy Milling

Nanoparticles of Zinc Ferrite (ZnFe2 O4 ) prepared by both wet- and dry- high-energy ball milling (HEBM), have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), surface area and pore size distribution (BET) and wavelength-dependent diffuse reflectance and scattering turned into absorption coefficient estimation using the Kubelka-Munk theory. It was found that after 72 hours of HEBM, the particle size was decreased from 220 nm for the initial material to 16.5 nm and 9.4 nm for the wet- and dry-milled samples, respectively. The optical absorption analysis revealed that the energy gap is increased (blue shift) by 0.45 eV for wet-milled and decreased (“anomalous” red shift) by 0.15 eV for dry-milled samples of ZnFe2 O4 as the particle size decreased.© 2009 ASME

Structure and Optical Properties of Magnetron Sputtered SiOx Layers with Silicon Nanoparticles

The properties of SiOx layer prepared by magnetron sputtering is studied by photoluminescence Auger and SIMS methods. The depth distribution of emission characteristics and chemical composition is obtained. It is shown that as-sputtered SiOx layers are non-emitted and characterized by homogeneous enough chemical composition. High-temperature annealing in nitrogen atmosphere stimulates not only the Si nanocrystal formation but also the redistribution of silicon and the appearance of Si depleted region near layer-substrate interface. The last process is found to be dependent on excess Si content. It is found that decrease of silicon content in the depth of annealed layers is followed by the decrease of particle sizes that is proved by the blue shift of photoluminescence maximum. The possible reasons of the appearance of Si depleted region are discussed.