Crystalline A-axis Research Articles

Assessment of 2H–SiC based intercalation compound for use as anode in lithium ion batteries

The improvement in electrode materials to upgrade lithium ion battery (LIB) is at heart of research related to energy devices. Herein, we report on the prospects of using 2H–SiC as an anode material in LIB on the basis of first principles based theoretical predictions. The energy profiling of high symmetry structural sites pointed out that the site TC related to tetrahedral configuration with carbon is favorable intercalation site in the host. The reaction of Li with stoichiometric as well as non-stoichiometric 2H–SiC are found endothermic which points to their unsuitability for lithiation process in LIBs. However, Li intercalation into the host in presence of Si monovacancy appeared an exothermic process which motivated us to study this system in detail. The Li atom after intercalation into the host is found ionized thereby donating its 2s electron to carbon atoms. The average value of the lithium insertion voltage and theoretical capacity are calculated as 1.87 V and 85 mA h/g respectively for the host simulated in the form of supercell LixSi15C16. The diffusion barrier faced by lithium atom while moving along crystalline a-axis is minimum (among all the paths considered here) when it follows a path that links hexagonal rings while passing carbon atoms. On the other hand, in case of motion along c-axis, similar results are found for minimum energy path that is along Tc site. The low diffusion barrier will facilitate the lithium ion to move quickly that points to fast charging capability of the battery.

Anisotropy of Dirac cones and Van Hove singularity in an organic Dirac fermion system

We propose an experimental method to examine the in-plane anisotropy of electronic structure in layered conductors. In the method, we measure the interlayer magnetoresistance as a function of in-plane magnetic field orientation. We applied it to an organic Dirac fermion system a-(BEDT-TTF)2I3 to experimentally determine the orientation of the anisotropic Dirac cones. It is concluded that the long axis of the elliptic constant-energy contours of the Dirac cone is tilted by approximately -30 deg from the crystalline a-axis to b-axis under hydrostatic pressures. Additionally, we observed a signature of van Hove singularity (which is a saddle point of the band dispersion) at 30-40 K above or below the Dirac point. The ridgeline of the saddle point is estimated as almost parallel to the crystalline b-axis.

No Detectable Change in In-Plane 29Si Knight Shift in the Superconducting State of URu2Si2

Using a high-quality single crystal of URu2Si2, we performed 29Si Knight shift measurements with an applied magnetic field Ha along the crystalline a-axis, i.e., along the magnetic hard axis, for temperatures down to 50 mK. The narrow, single-peak 29Si NMR spectrum obtained from this crystal allowed us to examine the temperature variation of the Knight shift K(T), and hence, that of the variation of the spin susceptibility with sensitivity on the order of 0.001%. At this precision, no observable change in K(T) was detected from 50 mK up to the superconducting transition at TSC(Ha) ∼ 1 K. Our results correspond to either an extremely small spin susceptibility in the hidden-order state along the magnetic hard axis or to possible spin-triplet superconductivity in this system.

Fine Control of Photoluminescence and Optical Waveguiding Characteristics of Organic Rubrene Nanorods Using Focused Electron-Beam Irradiation

We demonstrate the fine control of the nanoscale photoluminescence (PL) and optical waveguiding characteristics of light-emitting organic rubrene nanorods (NRs) through focused electron-beam (E-beam) irradiation. Nanoscale laser confocal microscope PL intensity and spectra of the focused E-beam-treated compartments of the rubrene NR drastically varied with E-beam dose, and yellow-white light emission from those compartments with a specific E-beam dose was observed. In optical waveguiding experiments, the propagation characteristics of optical signals along the single NR were dependent on the crystalline structure of the rubrene. For the E-beam treated NR, the waveguiding PL signals along the crystalline a-axis had relatively higher output, indicating the major axis for the optical energy transfer. Additionally, the waveguiding characteristics such as decay constant along the crystalline b-axis and c-axis of the NR considerably varied at the boundary of the E-beam treatment.

Single Crystalline a-Axis Mg Doped ZnO Thin Films Prepared by Sol-Gel Technique for Optoelectronics Applications

Single crystalline a-axis Mg doped ZnO thin films (MgxZn1-xO) were successfully prepared by sol-gel spin coating method using Zinc acetate, Magnesium acetate as precursors with ethanol as a solvent. The prepared solutions were used to deposit the films on silicon (100) substrate for different mole concentrations (x = 0.1 to 0.33). All deposited films were annealed at 450 0C to get dense crystalline films. X-ray Diffractometer (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis by X-ray (EDAX), Fourier Transform Infrared Spectroscopy (FTIR), Ellipsometry and semiconductor characterization system with probe station were used to characterize the deposited films for structural, chemical, optical, mechanical and electrical properties. The intense absorption peak was observed in the IR spectra for all deposited films showing bond position of fundamental ZnO peak for all Mg mole concentrations. From the XRD spectra, it revealed that the deposited films were single crystalline and a-axis oriented. EDAX spectra clearly showed the peak of Mg along with Zn and O indicating the successful incorporation of Mg into the ZnO. The refractive index was successfully tailored from 1.6 to 1.11 corresponding to 0.1 to 0.33 Mg mole concentration. The refractive index was found to be decrease with an increase in Mg mole concentration. I-V characteristics shows decrease in current with increase in the Mg mole concentration. Significant effect was not observed on thickness of deposited films due to the varying Mg mole fraction. Through SEM image, it was noted that the uniform film of Mg doped ZnO was deposited on the silicon substrate. Our results explore the applicability of MgZnO as cladding layer material to form effective and efficient heterostructure with ZnO as an active layer for the optical wave-guide applications.

Orientation and Epitaxy in the Injection-Molded Bars of Linear Low-Density Polyethylene/Isotactic Polypropylene Blends: An Infrared Dichroism Measurement

In this article, molecular orientation of crystalline and amorphous phases of both linear low-density polyethylene (LLDPE) and isotactic polypropylene (iPP) and epitaxy in the LLDPE/iPP blends prepared via dynamic packing injection molding have been investigated with the aid of polarized Fourier transform infrared (FTIR) spectroscopy and two-dimensional X-ray scattering (2D-WAXS). In LLDPE-rich blends, LLDPE was oriented along the shear flow direction, and iPP kept very low orientation. No epitaxial growth between LLDPE and iPP was observed. However, for the blends with LLDPE content less than 50 wt %, where iPP was the matrix and LLDPE formed the droplets, iPP was highly oriented along shear flow direction and LLDPE epitaxially grew onto iPP. The contact planes were (100)LLDPE and (010)iPP, and LLDPE molecules were about 50 degrees apart from the shear direction. The epitaxy fraction in LLDPE/iPP blends could be tentatively calculated from the values of orientation function of the LLDPE crystalline a-axis (fa) and that of crystalline b-axis (fb). Both tensile strength and tensile modulus decreased with the increase of LLDPE composition, indicating they were composition dependent, though shear-induced orientation and epitaxial crystallization might play some role. The impact strength of the blend samples reached a plateau in the LLDPE composition range of 20-50%, due to the contribution of orientation and epitaxy.

Effect of Coextrusion On Pp/Ldpe Bicomponent Blown Films

The effect of coextrusion on the microstructure of polypropylene (PP) and low-density polyethylene (LDPE) bilayer films was investigated by the use of Raman spectroscopy. The onset crystallization-time difference for PP versus LDPE components is an important parameter that controls the orientation and morphology of the coextruded films. Although the overall molecular orientation of PP and LDPE was not significantly different in multilayer films, single-layer LDPE films displayed some row-nucleation of crystals that was not observed in the LDPE layer in coextruded films. Also, there was a slight decrease of crystalline a-axis orientation for coextruded LDPE layer relative to that obtained for single-layer LDPE films.

Biaxial orientation in HDPE films: comparison of infrared spectroscopy, X-ray pole figures and birefringence techniques

In this study, high-density polyethylene films (HDPE) were produced using different processes (film blowing and biaxial orientation) and processing conditions. The orientation of the films was characterized in terms of their biaxial crystalline, amorphous and global orientation factors using birefringence, Fourier transform infrared spectroscopy (FTIR) using a tilted incidence technique and X-ray pole figures. Evaluation of a simplified FTIR procedure without using the tilted method for the determination of crystalline orientation factors proposed in the literature is also evaluated and assessed. The results indicate that FTIR overestimate the crystalline orientation factors, particularly for the crystalline a-axis. Significant discrepancies are also observed for the b-axis orientation, which may be due to an overlap of the amorphous contribution and/or saturation of FTIR bands. Those differences are larger for films with low orientation, such as blown films. Amorphous phase orientation from FTIR depends on the band used and is not necessarily in agreement with that determined from combination of X-ray and birefringence.

Biaxial Orientation Characterization in PE and PP Using WAXD X-Ray Pole Figures, FTIR Spectroscopy, and Birefringence

In this study, different polyethylene and polypropylene films (LDPE, LLDPE, HDPE, and PP) are produced using different processes (film blowing and biaxial orientation) and processing conditions. The orientation of the films is characterized in terms of their biaxial crystalline, amorphous, and global orientation factors using birefringence, Fourier transform infrared spectroscopy (FTIR) using a tilted incidence technique, and X-ray pole figures. The results indicate that FTIR overestimates the crystalline orientation factors, particularly for the crystalline a-axis. Significant discrepancies are also observed for the b-axis orientation, which may be due to an overlap of the amorphous contribution and/or saturation of FTIR bands. These differences are larger for films with low orientation, such as blown films. Amorphous phase orientation from FTIR depends on the band used and is not necessarily in agreement with that determined from the combination of X-ray and birefringence.

Ultrathin Films of Semiconducting Polymers on Water

Ultrathin films of polythiophene derivatives spread on water were studied by means of synchrotron radiation, using grazing incidence diffraction and (specular) reflectometry to obtain the molecular orientation in the films. The semicrystalline films were anisotropic, showing a strong tendency of orienting the crystalline alpha-axis perpendicularly to the water subphase. The crystalline domains extend essentially through the entire sample thickness, found to be 10-15 nm. A large expansion of the unit cell alpha-parameter was seen upon doping the films in situ. The reflectometry data were well-fitted by a model with a sinusoidal density variation being damped toward the water subphase. This indicates that the crystalline order was most developed at the polymer-air interface and deteriorated down toward the water, possibly due to the hydrophobicity of the alkyl side chains.

Processing–structure–properties relationship of multilayer films. 1. Structure characterization

Low density polyethylene (LDPE)/poly(ethylene terephthalate) (PET) multilayer films with and without a reactive tie layer were prepared by blowing process. The layers contents were fixed and the parameters evaluated were blow-up ratio (BUR), draw-down ratio (DDR) and frost-line height (FL). Their effect on the structure was studied. No crystallinity for the PET layer was induced by the blowing process. The orientation of the multilayer films was characterized by birefringence and infrared spectroscopy (FTIR). The measured birefringence showed a negligible orientation for PET layer, and the LDPE layer oriented mainly along the machine direction. FTIR characterization also showed a negligible orientation for PET layer, and for LDPE layer, the crystalline a-axis tended toward machine direction, crystalline b-axis located in transverse–normal plane with no orientation for the amorphous phase. The lamellar arrangement of LDPE layer with respect to machine direction was obtained by dissolving the amorphous phase in solution of potassium permanganate in a mixture of equal volumes of orthophosphoric and sulfuric acid. The crystallization of oriented LDPE melts produced a morphology consisting of rows of lamellar crystals aligned parallel to the machine direction at higher draw-down ratio. It was not always of the row-nucleated type, the exact morphology patterns greatly depended on the distribution of the crystalline a-axis and b-axis orientation, which in turn was dependent on DDR, BUR and frost-line height.

Alternative Interpretations of Crystal-Line Phase Infrared Dichroic Data from Polyethylene Blown Films-Comprison with X-Ray Results

High density polyethylene (HDPE) blown films were prepared to examine the effect of increasing take-up ratio (TUR) on crystalline morphology. Infrared dichroic results from those films indicated alignment of the crystalline a-axis along the MD with increasing TUR. While this data concurs with models proposed by previous authors, it conflicts with X-ray scattering results. A model is proposed that rationalizes similar apparent inconsistencies between IR and X-ray data previously reported in the literature.

Oxygen content and disorder of a-axis YBaCuO films with very high crystallinity studied by resonant Rutherford backscattering

Highly crystalline a-axis films were deposited by the hybrid sputtering method, and the conventional DC-template sputtering method. The films prepared by hybrid sputtering have very low T c while those prepared by the DC-template method have T c close to 90 K. The oxygen contents and disorder were studied by the resonant RBS method. We found that the oxygen contents of the films deposited by hybrid sputtering depended on the deposition temperature. Oxygen deficiency can be the reason for the poor superconducting properties of those films deposited by hybrid sputtering. A highly crystalline film deposited by DC-template sputtering had T c close to 90 K but was found to be off-stoichiometric and to have depth-dependent oxygen disorder. The disorder at the Ba and O sublattices of those highly crystalline a-axis films was found to be uncorrelated.

Direct high-resolution electron microscopy observations of sputtered a-axis oriented YBa2Cu3O7 films prepared by a self-template method on SrTiO3 substrates

A-axis oriented YBa2Cu3O7 films grown on a SrTiO3 substrate with and without the template layer have been studied by high-resolution electron microscopy. The self-template layer was found to be very effective to obtain highly crystalline a-axis oriented YBa2Cu3O7 film. A disordered cubic perovskite structure with a lattice parameter of 3.85 Å was found in the template layer which could be a good seed for further growth of a-axis film. In the film without the self-template layer, YBa4Cu3Ox cubic perovskite with lattice parameter 4.08 Å was observed to have grown at the interface as an island growth. Moreover, some faulted c-axis film at the very beginning of the template layer was observed even at the rather lower deposition temperature of 640 °C.

Some characteristics of a maser at 1420 MHz

A regenerative solid-state maser has been constructed for 1420 MHz using 0.05% Cr +++ in K 3Co(CN) 6 and a pump frequency of 3850 MHz. With a magnetic field of 480 Oe at 11° to the crystalline a-axis in the ac-plane the 1–2 transition was used for the signal frequency and the 1–3 transition for the pump frequency, numbering the levels in order of increasing energy. The product of voltage gain and bandwidth was 2.7×10 6s -1. Weaker emission was observed for several other transitions. For ruby (Cr +++ in Al 2O 3) emission could be produced for the 2–3 transition at 1420 MHz with the 1–3 transition pumped at 8500 MHz; for the 1–2 transition at 1420 MHz in ruby, with the 1–3 transition pumped at 10700 MHz, only a short pulse of emission could be produced. The emission for Cr +++ in K 3Co(CN) 6 was found to be much higher when the pump field extended throughout the crystal than when it had a node within the crystal. This observation suggests that in K 3Cr(CN) 6/K 3Co(CN) 6 the phonon saturation mechanism proposed by Strandberg does not take place.