Textural analysis of extruded materials with rhombohedral structure

A textural analysis of extruded rods of material with tetradymite (Bi₂Te₃) structure is conducted by constructing the distribution of the pole density of the (0001) and (1120) planes. The distribution of the poles of the planes is represented by a solid of revolution, and the pole density, which is a function of the angle () of the deviation of the normals to h_i from the extrusion axis, is determined by the diffraction method of specimen inclination using the diffracted ray intensity. It proves possible to identify textural components, and to determine how many there are and the degree of perfection of this method. A technique is worked out for the quantitative estimate of the average values of the components of the conductivity tensor in the grains, and the contribution of the texture to the macroscopic properties of the material is analyzed. To investigate the thin layers (for example, the layerwise distribution of texture in rods of rectangular cross section) we used the method of inverse pole diagrams, where the probability of coincidence of the h_i plane with the working surface of the specimen is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 33–39, December, 1989.     

Pretransitional effects associated with the melting of a material with a nematic mesophase

Neutron diffraction measurements have been made with a deuterated sample of para-azoxyanisole (PAA) at temperatures close to the melting point of this material. It is found that, as the melting point is approached from below, Bragg intensity decreases continuously in a manner usually associated with the onset of a second-order phase transition. In addition, the diffraction pattern which typifies the nematic phase begins to develop while the sample is solid and close to its melting point. We interpret these observations as evidence for pretransitional effects associated with the melting of PAA and present a qualitative explanation of the data.     

Experimental investigations of a new TGBC mesophase

We report on optical and structural X-ray studies on a smectic C twist grain boundary phase (TGB) of a pure liquid crystalline material. It is shown that this TGB phase, which exists over a large range of temperature, is definitely different from previously reported and predicted TGB phases. The two main experimental features are: (i) the observation in planar geometry of an optical texture exhibiting a square grid pattern, and (ii) the detection of a broad Bragg ring in reciprocal space instead of one (TGB) or two (TGB) sharp rings. We suggest a few possible tracks that can be explored to understand the complex structure of this new phase. Received 25 January 1999     

Ferroelectric mesophase with randomized interlayer structure

We present the experimental and theoretical evidence that a new Sm X phase made of asymmetric bent-shaped molecules has a layered, nontilted, optically uniaxial and polarly ordered structure with random direction of the layer polarization. The randomness results from the sign degeneracy of the difference in polarization directions in neighboring layers, although the magnitude of the phase difference is constant. Lifting the degeneracy by external fields two additional structures of the Sm X phase are possible.     

Vibration analysis of circular segment shaped plates

Circular segment shaped plates are analyzed to determine their natural frequencies and mode shapes of vibration. The analysis is based on the finite element approach. The curved sided triangular plate bending element is used for solving the problem. The effect of variation of the size of the plate on the vibrational characteristics is studied and several important conclusions are made.     

Spatial disorder in a smectic A mesophase

Starting with the equations of motion for a stiff chain, a projection operator approach is utilized to develop diffusional equations for the dynamics of the end-to-end distance. The diffusion equation resulting has a spatial-dependent diffusion coefficient calculable from equilibrium properties of the chain, and a frequency-dependent part which requires dynamical information. The analysis is applied, in so far as the spatial dependence of D is determined, for three and four bond chains. A critique of this procedure is provided.     

Laser forming of a bowl shaped surface with a stationary laser beam

Despite a lot of research done in the field of laser forming, generation of a symmetric bowl shaped surface by this process is still a challenge mainly because only a portion of the sheet is momentarily deformed in this process, unlike conventional sheet metal forming like deep drawing where the entire blank undergoes forming simultaneously reducing asymmetry to a minimum. The motion of laser beam also makes the process asymmetric. To counter these limitations this work proposes a new approach for laser forming of a bowl shaped surface by irradiating the centre of a flat circular blank with a stationary laser beam. With high power lasers, power density sufficient for laser forming, can be availed at reasonably large spot sizes. This advantage is exploited in this technique. Effects of duration of laser irradiation and beam spot diameter on the amount of bending and asymmetry in the formed surface were investigated. Laser power was kept constant while varying irradiation time. While varying laser spot diameter laser power was chosen so as to keep the surface temperature nearly constant at just below melting. Experimental conditions promoted almost uniform heating through sheet thickness. The amount of bending increased with irradiation time and spot diameter. It was interesting to observe that blanks bent towards the laser beam for smaller laser beam diameters and the reverse happened for larger spot diameters (~10 times of the sheet thickness). Effect of spot diameter variation has been explained with the help of coupled thermal-structural finite element simulations.     

Rydberg molecules in external fields: a semiclassical analysis

We analyze the spectra of simple Rydberg molecules in static fields within the framework of closed/periodic-orbit theories. We conclude that in addition to the usual classical orbits one must consider classically forbidden diffractive paths. Further, the molecule brings in a new type of "inelastic" diffractive trajectory in addition to the usual "elastic" diffractive orbits encountered in systems with point scatterers. The relative importance of inelastic versus elastic diffraction is quantified by merging the usual closed orbit theory framework with molecular quantum defect theory.     

Interaction of molecules with a semiconductor wall

Summary In this paper we consider the interaction of vibrationally and electron-excited molecules with a wall made of a semiconductor with the widths of the forbidden zone close to the excitation energy of the molecules. It is shown that the main process induced by the collision of a molecule with such a wall is the production of an electron-hole pair. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Molecular orientation and optical luminescence properties of soluble star shaped oligothiophene molecules for organic electronic applications

Angle resolved near-edge absorption fine structure measurements of the C 1s edge of a film of the star-shaped molecule 4(HPBT) show a high degree of order, with the molecules maintaining a largely planar geometry in an upright configuration. X-ray excited optical luminescence measurements showed a dependence of the luminescence spectrum on the excitation energy, with (0–0), (0–1), and (0–2) vibrational modes being clearly visible when exciting off-resonance; the appearance of these features under off-resonance is attributed to a scattering process that is quenched when the excitation approaches a core absorption resonance. The HOMO–LUMO gap is estimated from the luminescence measurements (2.28 eV), and this value is confirmed by the comparison of the C 1s absorption and C Kα emission spectra.     

Novel bending sensor based on a meniscus shaped beam with LPG

A novel bending sensor based on a long period fiber grating (LPG) is presented. A LPG was glued into a V-shaped groove, which lies on the lower surface of a meniscus shaped beam. It is found that the transmission optical power of the LPG changed linearly with the variation of the bending of the beam. The bending applied on the beam can be measured by detecting the intensity variation of the LPG's resonant dip wavelength. Under a relative large bending measured range from 0 to 7.5 m^?1, the sensitivity of 3.003 dB m^?1 and curvature resolution of 0.001 m^?1 have been achieved for the proposed bending sensor.     

Heterogeneous combustion of a pair of interacting particles with an arbitrarily shaped surface

The heterogeneous combustion of two immobile interacting particles with arbitrarily shaped surfaces is theoretically described under conditions of rapid chemical reaction, where the concentration of oxidizer molecules on the surface of the particles can be considered to be zero. The problem is solved for an arbitrary temperature dependence of molecular transport coefficients when molecular transport in the vicinity of the particles takes place by diffusion. Analytical formulas found allow for the direct characterization of the combustion of individual spheroidal particles and interacting spherical particles in several particular cases. At the same volumes, heavily elongated and oblate particles are shown to burn significantly faster. The approach of the particles significantly influences the time of burning the finer particle if its size is far less than that of the coarser one.     

A statistical model of the discotic mesophase

The evaluation of the thermodynamic properties of an ideal discotic mesophase needs the determination of a partition function, that depends upon two basic types of energy storage, translational and rotational. Here, we suppose we can substitute for the complex effective intermolecular potential, different mean potentials acting on each individual molecule. The defining assumptions for a discotic mesophase include the stipulations, first—that each simple disk-like molecule is, during most of the time, confined to a cell, secondly—that each molecule has an external rotation which is more or less hindered by a periodic potential. The cells are stacked in columns and the columns form a regular hexagonal array, each molecule moves in a cell as in an infinite potential well due to the neighbouring molecules. This model has allowed the general formulation of the mesophase-free energy. From this, we obtain the form of the coefficient of isothermal compressibility when the external rotations of the molecules are hindered and the vibrational energy is weak.     

Optical parametric amplification of a shaped white-light continuum

Phase-locked two-color sub-40-fs double pulses in the visible are produced by noncollinear parametric amplification of white light tailored in a pulse shaper with a liquid-crystal mask. The carrier phase between the pulses is conserved during the amplification process and can be adjusted, as can the temporal separation and the center of wavelengths of the pulses.     

整形飞秒激光金属材料精细加工

实验通过二极管记录透射光信号随脉冲个数变化关系以及观测样品烧蚀形貌来研究不同实验条件对激光烧蚀的影响。使用的样品是厚度为50 m铝箔。实验中通过研究不同变量:激光焦点与样品的相对位置、激光的能量、背景气体压强以及脉冲形状对烧蚀加工过程和结果的影响,从而获得较好烧蚀效果的条件,达到控制烧蚀加工过程的目的。特别是通过使用不同形状的脉冲和具有一定规律的脉冲序列对样品进行烧蚀,发现某些形状的整形脉冲烧蚀结果明显优于变换极限脉冲。说明脉冲整形作为一种新的技术可以在激光精细钻孔领域得到更深入的研究和应用。     

Controlling vibrational excitation with shaped mid-IR pulses

We report selective population of the excited vibrational levels of the T(1u) CO-stretching mode in W(CO)(6) using phase-tailored, femtosecond mid-IR (5.2 microm, 1923 cm(-1)) pulses. An evolutionary algorithm was used to optimize specific vibrational populations. Stimulated emission peaks, indicative of population inversion, could be induced. Systematic truncation of each optimized pulse allowed for increased understanding of the excitation mechanism. The pulses and techniques developed herein will have broad applications in controlling ground state chemistry and enhancing vibrational spectroscopies.