Densification mechanism of polyacrylonitrile-based carbon fiber during heat treatment

Polyacrylonitrile (PAN)-based carbon fibers were heat treated at various temperatures for varying durations to simulate the graphitization process in the manufacture of C/C composites. Densification of the resulting fibers was confirmed by density measurement. The composition and structure of the fibers were investigated by means of elemental analysis, X-ray diffraction and Raman spectroscopy. For specified isothermal heat treatment time, the structural parameters depended strongly on heat treatment temperature. The nitrogen content decreased with increased heat treatment temperature and extended time at constant temperature. Nitrogen loss was complete at temperatures above 1900 °C. The graphite crystallite size increased rapidly with increasing heat treatment temperature, and slowly with extended isothermal heat treatment time. At 2100 °C a more ordered graphitic structure appeared. Denitrogenation induced “puffing”, which made the fibers expand. Decrease in density in the heat treatment temperature range 1500-1900 °C originated from the abrupt evolution of nitrogen, and above 1900 °C the graphitization transition induced steadily increasing density. Densification of the carbon fibers was determined both by the rate of denitrogenation and the rearrangement of carbon atoms.     

Mode characteristics of hollow core Bragg fiber

Analytical expression to calculate propagation constant and mode field of the hollow core Bragg fiber is derived. Numerical results are presented. It is shown that the fundamental mode of the hollow core Bragg fiber is circularly symmetric TE01 mode with no polarization degeneracy, while the higher order mode may be HE11, TM01, or TE02 etc.. This property is different from conventional optical fiber that its fundamental mode is the linearly polarized HE11 mode and is polarization degeneracy.     

Generation of a vectorial elliptic hollow beam by an elliptic hollow fiber

We propose a handy scheme to generate a vectorial elliptic hollow beam by using a short dual-mode elliptic hollow fiber, study its propagation characteristics in free space, and calculate its total angular momentum at different propagating distances. Our study shows that the resulting elliptic hollow beam has a self-focusing effect in near field, an arbitrary polarization, and a position-dependent fractional angular momentum and that the directions of the major and minor axes of the elliptical intensity profile will be interchanged after the self-focusing plane.     

Random fiber laser of POSS solution-filled hollow optical fiber by end pumping

Random fiber laser is obtained by end pumping a hollow optical fiber (HOF) filled with a dispersive solution of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles and laser dye pyrromethene 597 (PM597) in carbon disulfide (CS₂), in which the concentration is 1.5×10^?2 M for PM597 and 18.5 wt% for POSS, respectively. It is found that the pump light at the one end of the liquid core optical fiber (LCOF) can pass the whole length of LCOF because the POSS nanoparticles were dispersed in CS₂ at a molecular level (1–3 nm) with high stability and without sedimentation. Above the threshold pump energy (～0.81 mJ) the random fiber laser appears coherent and resonant feedback multimode lasing in the weakly scattering system. For the LCOF containing PM597 with the same concentration and no POSS nanoparticles, there occurs only ASE that can be observed under the same experimental condition.     

Mirror dispersion control of a hollow fiber supercontinuum

Ultrabroadband chirped mirrors with a bandwidth of 270 THz have been manufactured using the BASIC design approach. These mirrors were used to compress the supercontinuum of cascaded hollow fibers down to 4.6 fs. The pulse duration was measured with spectral phase interferometry for direct electric-field reconstruction (SPIDER). PACS 42.65.Re; 42.65.Wi; 42.79.Fm     

Kinetics and mechanism of copper fracture during deformation in surface-active baths

Relations have been derived here between the macroscopic characteristics of liquid metal embrittlement (durability _c under creep and strain _c prior to rupture under tension) and the parameters which characterize the micromechanism of fracture (surface energy at the crystal bath interface, energy of grain boundaries, temperature, structure of the crystal-bath interface, etc.), on the basis of test data indicating that the subcritical stage of microcrack development governs the fracture process, and on the assumption that transition to supercritical fracture occurs when the crack angle at the tip opens to its critical width _c. It is also shown here that, as the rate of subcritical crack development changes by three orders of magnitude, the magnitude of the critical angle _c changes only by a factor of 3.0 and may, to the first approximation, be regarded as independent of the bath composition. The values of _c and _c calculated according to this approximation agree closely enough with values based on tests.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 22–29, July, 1976.     

Single-shot dynamics of pulses from a gas-filled hollow fiber

We present measurements of the performance characteristics of few-cycle laser pulses generated by propagation through a gas-filled hollow fiber. The pulses going into the fiber and the compressed pulses after the fiber were simultaneously fully characterized shot-by-shot by using two kHz SPIDER setups and kHz pulse energy measurements. Output-pulse properties were found to be exceptionally stable and pulse characteristics relevant for non-linear applications like high-harmonic generation are discussed. PACS 42.65.Re; 42.65.Ky; 42.65.Sf; 42.65.Jx     

Optimization of hollow fiber pulse compression using pressure gradients

We present a study on the optimization of multi-mJ spectral broadening in neon-filled hollow fibers using pressure gradients. It is found that the intensity of the laser pulse at the entrance of the hollow fiber should be below or at the level where multi-photon ionization (MPI) starts to occur. If the intensity is higher than the threshold value for MPI, ionization can significantly reduce the throughput efficiency. The extension of the scheme to a terawatt power level is also addressed. PACS 42.65.Re; 42.65.Jx; 42.65.k     

Charge transport mechanism of vanadium pentoxide xerogel-polyaniline nanocomposite

The nanocomposites of conducting polyaniline are prepared by intercalating into the layers of vanadium pentoxide (V₂O₅) xerogel. The intercalation is confirmed by the observation of lattice expansion of V₂O₅ xerogel. Dc conductivity of the gel follows Arrhenius type temperature dependence while the nanocomposites exhibit three dimensional variable range hopping. The ac conductivity and dielectric properties are extensively studied at low temperature up to the frequency of 10 MHz. Two semicircles in Cole-Cole plot of impedance are found for the nanocomposites. The ac conductivity spectra reveal three frequency regions. The frequency exponent in the lower frequency region is nearer to 2. The dielectric response exhibit broad spectra which are analyzed by Cole-Cole distribution function. The peak frequency of dielectric spectra appears at the first cross over frequency of conductivity spectra.     

Observation of plastic deformation in TiAlCN/a -C ceramic nanocomposite coating

Extensive plastic deformation of titanium aluminum carbonitride/amorphous carbon nanocomposite coating at room temperature was observed in this study. Nanoindentation measurement showed that the coating hardness was 24.4 GPa with the plasticity of 57.6%. The critical load of 1.3 m coating in scratch test was beyond 100 N. The depth of the 100 N scratch track identified by microstylus profilometer was 6.7 m, while the thickness of the coating in the center of the scratch track observed by scanning electron microscopy was 0.4 m, which is only about one third of that for the as-deposited coating. The amorphous carbon with sp² hybrid and -bonding is proposed to account for this unique plasticity. PACS 81.15.Gh; 81.07.Bc     

Study of non-linear properties of hollow core photonic crystal fiber

A highly non-linear hollow core photonic crystal fiber (PCF) having three ring-hexagonal structure is designed. The full vector finite element method with perfectly matched layer is used to investigate the non-linear properties of hollow core PCF effectively. The effective area of modal pattern is also studied for the same newly designed structure. It is found that when effective area is very small the non-linearity is very high.     

Anti-Stokes enhancement of multifrequency Raman generation in a hollow fiber

The intensities of the higher frequency anti-Stokes components of resonantly pumped Raman scattering are observed to be greatly enhanced as a result of minimizing dispersion when focusing into a hollow fiber, as compared to their intensities at higher pressures. As well, the significant spectral broadening of these Raman orders in the absence of any such broadening on the pump beams suggests the synthesis of a short temporal structure as a result of a the decreased dispersion.     

谐振式光子带隙光纤陀螺一体化方案

实现了使用光子带隙光纤的一体化谐振式光纤陀螺方案,设计并制作了谐振腔中基于微光学结构的耦合器。实验测得制作的谐振腔清晰度为3.7。搭建了基于该谐振腔的陀螺系统,并对其主、次偏振态的谐振曲线进行了实际测量。实验结果测得该系统60 s零漂为2.45 ()/s,及1 h长期稳定性为7.11 ()/s。同时,实现了对应陀螺输出50 ()/s（积分时间10 s）及100 ()/s（积分时间10 s）的模拟转速实验,验证了该陀螺系统的Sagnac效应。分析得到耦合损耗是影响该陀螺系统性能的主要因素。验证了该谐振腔结构具有应用于陀螺系统的可行性,为谐振式光纤陀螺性能进一步提高提供了参考。     

Versatile control of geometric birefringence in elliptical hollow optical fiber

A novel optical fiber fabrication technique was developed by converting the symmetry of the silica substrate into the germanosilicate ring core to efficiently introduce geometric birefringence in an elliptical hollow optical fiber. Due to high ellipticity in the hollow ring core, the fiber provides an extremely high group birefringence of 2.35 x 10(-3) at 1550 nm. Single-mode single-polarization guidance was also experimentally confirmed, with a bandwidth of approximately 35 nm. The generic adiabatic mode conversion capability in the taper also provided a stable fusion splice to conventional single-mode fiber with low loss and high tensile strength.     

谐振式光子带隙光纤陀螺一体化方案

实现了使用光子带隙光纤的一体化谐振式光纤陀螺方案,设计并制作了谐振腔中基于微光学结构的耦合器。实验测得制作的谐振腔清晰度为3.7。搭建了基于该谐振腔的陀螺系统,并对其主、次偏振态的谐振曲线进行了实际测量。实验结果测得该系统60 s零漂为2.45 ()/s,及1 h长期稳定性为7.11 ()/s。同时,实现了对应陀螺输出50 ()/s（积分时间10 s）及100 ()/s（积分时间10 s）的模拟转速实验,验证了该陀螺系统的Sagnac效应。分析得到耦合损耗是影响该陀螺系统性能的主要因素。验证了该谐振腔结构具有应用于陀螺系统的可行性,为谐振式光纤陀螺性能进一步提高提供了参考。     

Generation of a hollow laser beam by a multimode fiber

A simple method to generate a hollow laser beam by multimode fiber is reported. A dark hollow laser beam is generated from a multimode fiber and the dependence of the output beam profile on the incident angle of laser beam is analyzed. The results show that this hollow laser beam can be used to trap and guide cold atoms.     

Gap solitons in a model of a hollow optical fiber

We introduce a model for two coupled waves propagating in a hollow-core fiber: a linear dispersionless core mode and a dispersive nonlinear surface mode. The linear coupling between them may open a bandgap through the mechanism of avoidance of crossing between dispersion curves. The third-order dispersion of the surface mode is necessary for the existence of the gap. Numerical investigation reveals that the entire bandgap is filled with solitons, and they are stable in direct simulations. The gap-soliton (GS) family includes stable pulses moving relative to the given reference frame up to limit values of the corresponding boost delta, beyond which they do not exist. The limit values are asymmetric for delta > or < 0. Recently observed solitons in hollow-core photonic crystal fibers may belong to this GS family.     

Dislocation arrangements during cyclic loading of pure iron

Fatigue induced substructure has been studied in polycrystalline iron (99.95 per cent) cycled at constant low stress amplitude. The main experimental technique was transmission electron microscopy. The results showed the dominant role of friction stress during cyclic loading and the importance of cross slip during dynamic recovery.     

Influence of grain boundary sliding on fracture toughness of nanocrystalline ceramics

A theoretical model is proposed describing a new physical microscopic mechanism of increased fracture toughness of nanocrystalline ceramics. According to this model, when a ceramic with a microcrack is deformed, intensive grain boundary sliding occurs near the crack tip under certain conditions. This sliding is accompanied by the formation of an array of disclination dipoles (rotational defects) producing elastic stresses. These stresses partially compensate the high local stresses concentrated near the microcrack tip and thereby hamper the microcrack growth. The proposed model is used to theoretically estimate the increase in the critical microcrack length (the length above which the catastrophic growth of microcracks occurs) caused by the formation of disclination dipoles during grain boundary sliding in nanoceramics. The increase in the critical microcrack length is a quantitative characteristic of the increased fracture toughness of nanoceramics.