Allotropie phase transformation induced by UV power laser irradiation of boron nitride

The optically induced allotropic phase transformations of boron nitride were studied. Under irradiation with laser beam of 337.1 nm wavelength, at energy density of 1.9mJ per 0.1 mm² a hexagonal boron nitride transformed into the cubic form. The suggestion is made that the results obtained could be explained by the multiphoton excitation and recombination of electrons.     

On dissipation induced by phase transformations

Summary Motions of hyperelastic materials involving surfaces of strain or stress discontinuities are generally dissipative in the sense that, in any portion of the body that is traversed by a moving singular surface, the rate of work of the external forces differs from the rate of storage of the total energy (the strain energy and the kinetic energy) by the rate of work done in moving the singular surface. Hence, the corresponding continuum theory is capable of modeling dissipative behavior associated with phase transformations which has potential applications in the design ofadaptive structures. The present work indicates that this dissipative behavior is characterized by a material function, called the driving-traction-response function, which is uniquely determined by the strain energy potential of the material. The driving-traction-response function vanishes identically if and only if the Piola-Kirchhoff stress-response function depends upon the deformation gradient linearly.     

Studies of graphite surface after laser irradiation

Graphite was irradiated by CO₂ laser of energy 10J and pulse width 4 μs. SEM and XRD examinations were carried out before and after laser irradiation. It was observed that XRD pattern changed after irradiation and new entities were seen on surface. Amorphous graphite was also irradiated and it was found that it became partly crystalline. The changes were explained on the basis of phase changes induced by high pressures and temperatures during laser irradiation.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.     

Morphological transition of gold nanostructures induced by continuous ultraviolet irradiation

Gold nanoparticles were synthesized through a continuous UV irradiation method using citric acid as a reducer and protective agent. After a period of continuous UV irradiation, the nanoparticles transformed into two-dimensional (2D) nanonetworks, porous nanoplates and compact nanoplates with hexagonal, triangular or truncated triangular pores through a self-assembly process which was dependent on the citric acid concentration. Selected area electron diffraction (SAED) patterns indicated that both the nanonetworks and the porous nanoplates were single crystalline. The influence of citric acid concentration and irradiation time on the morphological transition of Au nanostructures was investigated. The process of morphological transition was presumably discussed.     

Microlenses fabricated by ultraviolet excimer laser irradiation of poly(methyl methacrylate) followed by styrene diffusion

A new technique of microlens array fabrication based on the use of excimer laser radiation is described. Poly(methyl methacrylate) (PMMA) substrates are treated with many low-energy KrF laser pulses and exposed to styrene vapor. The irradiated material swells, producing spherical microlenses that are stabilized by UV polymerization. The chemistry of this process and the optical quality of the lenses are discussed.     

准分子激光引起的非晶硅薄膜晶化行为的研究

利用KrF准分子激光对非晶硅薄膜的表层进行了晶化.研究了激光能量密度和照射脉冲数对薄膜结晶度的影响,并对晶化后薄膜的形貌和结构进行了表征.结果表明:该非晶硅薄膜晶化阈值约为110 mJ/cm2,且不受照射脉冲数的影响;激光能量密度是影响薄膜结晶度的首要因素,但在较低的能量密度时,增加照射脉冲数也会显著的提高薄膜结晶度;结构及形貌表征发现,薄膜晶化层厚度约为400～500 nm,平均晶粒尺寸为30～50 nm.     

An investigation of phase transformations on graphite surface using the method of stokes polarimetry

The phase variations on the surface of graphite conductor, which occur under conditions of protracted electric explosion, are investigated. Irreversible changes are revealed of the coefficient of reflection and specific resistance of samples after heating, which are associated with the formation of the graphite-Carbyne phase in the surface layer.     

Mechanisms of phase transformations during laser treatment of grey cast iron

Pearlitic grey cast iron was surface melted using a 500 W CW CO₂ laser at travel speeds 0.5–100 cm s^–1. Detailed structural analysis of the laser modified layer was performed by optical and scanning electron microscopy (SEM), and microhardness depth profiles were measured. Temperature distribution was calculated from a three dimensional moving point source model, taking only the heat transfer into account. From the structural details observed in the austenitized zone some conclusions on the mechanism and kinetics of the pearlite austenite transformation at high heating rates were drawn. The melted zone consisted of primary austenite and ledeburite. At lower scanning speeds the structure was dendritic, at higher scanning speeds transition to dendritic-cellular structure was observed. From the secondary dendrite arm spacings the cooling rate during solidification was estimated as a function of the depth. Some discrepancies were found between our measurements and the literature data as well as predictions by the simple model neglecting convection in the melt.     

Alignment change of hexahexylthiotriphenylene in the helical columnar phase by infrared laser irradiation

Alignment change of discotic liquid crystal domains induced by infrared irradiation was investigated for the helical columnar phase of hexahexylthiotriphenylene. A uniform and anisotropic alignment change was observed when a linearly polarized infrared light was irradiated. It was found that the IR laser irradiation leads to a homogeneous planar alignment even in the helical columnar phase which is highly ordered mesophase. The in-plane order parameter of triphenylene ring was estimated to be 0.82 from the dichroic ratio obtained by polarized infrared absorption measurements. The results strongly imply that the infrared irradiation is a possible technique for device fabrication by use of the helical columnar phase which exhibits a high carrire mobility (~ 10^− 1 cm² V^− 1 s^− 1).     

Calculation of phase transition dynamics upon laser irradiation of semiconductors

Approximate expressions are obtained for calculations of the time characteristics of fusion and recrystallization, recrystallization rate, and depth to which the material fuses upon laser annealing of semiconductors. The expressions obtained may be used for selection of processing regimes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 1, pp. 132–136, July, 1985.     

Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications

High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500 nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2 s to 3 s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically.     

Spontaneous phase and morphology transformations of anodized titania nanotubes induced by water at room temperature

We report a spontaneous phase transformation of titania nanotubes induced by water at room temperature, which enables the as-anodized amorphous nanotubes to be crystallized into anatase mesoporous nanowires without any other post-treatments. These mesoporous TiO(2) nanomaterials have a markedly improved surface area, about 5.5 times than that of the as-anodized TiO(2) nanotubes, resulting in a pronounced enhanced photocatalytic activity. The present approach not only allows a flexible control over the morphology of TiO(2) nanostructures but can fundamentally eliminate the need for high temperature operations for crystallizing amorphous TiO(2).     

In-situ studies on phase transformations under electron irradiation in a high voltage electron microscope

High voltage electron microscopy (HVEM), using electron energies adequate for causing displacements of atoms from lattice sites, is a very effective technique for studying mechanisms of solid state phase transformations and for charting the path of phase evolution in real time. This has been demonstrated in studies on chemical ordering in nickel-molybdenum alloys and on theβ → Ω displacement ordering in zirconium-niobium alloys. The enhanced diffusivity due to electron irradiation makes it possible to explore a sequence of phase evolution at low enough temperatures where even some first-order transformations are driven by free energy (G) instabilities with respect to the relevant order parameter (η). Specific issues addressed in these studies are reviewed in this paper.     

Submicrosecond polymorphic transformations accompanying shock compression of graphite

The effect of the structure factors and temperature on the transformation of shock-compressed graphite to diamond or a diamond-like phase has been investigated. It has been revealed a significant effect of the shock compression direction with respect to the crystallographic planes of graphite on the detected pressure and rate of the transformation. The effect is more pronounced in more ordered graphite. The main structure parameter determining the transformation rate is the degree of the three-dimensional ordering of graphite. The transformation pressure decreases with heating. The extrapolation of the results to the high-temperature region and their comparison with the data on the direct transformation of statically compressed graphite to diamond indicate that the “above-barrier” state should be apparently reached to ensure the irreducible transformation to cubic diamond.     

Dynamic evolution of the spectrum of long-period fiber Bragg gratings fabricated from hydrogen-loaded optical fiber by ultraviolet laser irradiation

Long-period fiber Bragg gratings fabricated by exposure of hydrogen-loaded fiber to UV laser light exhibit large-scale dynamic evolution for approximately two weeks at room temperature. During this time two distinct features show up in their spectrum: a large upswing in wavelength and a substantial deepening of the transmission minimum. The dynamic evolution of the transmission spectrum is explained quantitatively by use of Malo's theory of UV-induced quenching [Electron. Lett. 30, 442 (1994)] followed by refilling of hydrogen in the fiber core and the theory of hydrogen diffusion in the fiber material. The amount of hydrogen quenched by the UV irradiation is 6% of the loaded hydrogen.