Reverse Circular Bragg Phenomenon

The axial propagation of circularly polarized light in an optically active structurally chiral medium is exactly solved via full electromagnetic analysis. Some symmetries of the system's characteristic matrix reveal new insights, which are confirmed by coupled wave theory. For extreme values of chirality, now accessible via metamaterials, a reverse circular Bragg resonance arises in the negative refraction regime where handedness reversal of counterpart modes occurs. A condition is identified under which optical activity offsets structural chirality, rendering the medium simply birefringent.Comment: 14 pages, 6 figure

The mechanical and electrical properties of direct-spun carbon nanotube mat-epoxy composites

Composites of direct-spun carbon nanotube (CNT) mats and epoxy are manufactured and tested in order to determine their mechanical and electrical properties. The mats are spun directly from a floating catalyst, chemical vapour deposition reactor. The volume fraction of epoxy is varied widely by suitable dilution of the epoxy resin with acetone. Subsequent evaporation of the acetone, followed by a cure cycle, leads to composites of varying volume fraction of CNT, epoxy and air. The modulus, strength, electrical conductivity and piezoresistivity of the composites are measured. The CNT mats and their composites exhibit an elastic-plastic stress-strain response under uniaxial tensile loading, and the degree of anisotropy is assessed by testing specimens in 0{\deg}, 45{\deg} and 90{\deg} directions with respect to the draw direction of mat manufacture. The electrical conductivity scales linearly with CNT volume fraction, irrespective of epoxy volume fraction. In contrast, the modulus and strength depend upon both CNT and epoxy volume fractions in a non-linear manner. The macroscopic moduli of the CNT mat-epoxy composites are far below the Voigt bound based on the modulus of CNT walls and epoxy. A micromechanical model is proposed to relate the macroscopic modulus and yield strength of a CNT mat-epoxy composite to the microstructure

Versatile Preparation of Mesoporous Single-Layered Transition-Metal Sulfide/Carbon Composites for Enhanced Sodium Storage

Transition metal sulfides are promise electrochemical energy storage materials due to their abundant active sites, large inter-layer space and high theoretical capacities. Especially for sodium storage. However, the low conductivity and poor cycling stability at high current densities hampered their applications. Herein, we report a versatile dual-templates method to elaborate ordered mesoporous single layered MoS2 /carbon composite with high specific area, uniform pore size and large pore volume. The single layered MoS2 is confined in the carbon matrix. The mesopores between the composite nanorods provide fast electrolyte diffusion. The obtained nanocomposite shows a high sodium storage capability, excellent rate capacity, and very good cycling performance. A 310 mAh g-1 capacity can remains at 5.0 A g-1 after 2500 cycles. Furthermore, a SIB full cell composed the MoS2 /carbon composite anode and a Na3 V2 (PO4 )3 (NVP) cathode maintains a specific capacity of 330 mA h g-1 at 1.0 A g-1 during 100 cycles. The mechanism is investigated by in situ and ex situ characterizations as well as density functional theory (DFT) calculations. This article is protected by copyright. All rights reserved

COMPARATIVE ANALYSIS OF THE USE OF INTERNAL MEASURES FOR QUALITY ASSURANCE IN PUBLIC AND PRIVATE UNIVERSITIES IN SOUTH EAST NIGERIA

The seeming decline in the quality of university education in Nigeria necessitated this study on the comparative analysis of the internal measures that are used in public and private universities in south east geo-political zone of Nigeria for quality assurance. The study adopted the descriptive survey research design. Three research questions and three hypotheses guided the study. The population of the study comprised 29,083 made up of 7,444 lecturers and 21,639 students admitted in the 2011/2012 academic session in the 74 faculties and 413 departments of the 13 universities (five public and eight private) in the south-east geo political zone, Nigeria. The sample of the study comprised 2,106 respondents which were made up of 936 lecturers and 1, 170 students from four universities sampled. The sample was composed through multi-stage sampling technique. Questionnaire developed by the researchers titled “Students’ Questionnaire on Internal Measures for Quality Assurance” (SQIMFQA) and “Lecturers’ Questionnaire on Internal Measures for Quality Assurance” (LQIMFQA).was the instrument used for data collection. The instrument was validated by three experts, one in Educational measurement and evaluation and the others in Educational planning from the departments of Educational foundations and Educational Management and Policy all in the faculty of Education Nnamdi Azikiwe University, Awka. The Cronbach’s alpha method was used to determine the internal consistency of the items and this yielded a result of 0.74. The researcher with the help of eight research assistants distributed and successfully collected all the 2,106 copies of the questionnaire administered. The research questions were analyzed using the mean, while the hypotheses were tested using z-test at 0.05 level of significance. It was found among others that internal measures for quality assurance of student intake measures, evaluation of students’ learning experience and teacher quality measures were not adequately used in both public and private universities in the south-east geo-political zone of Nigeria for quality assurance. Based on the findings, recommendations were made

COMPARATIVE ANALYSIS OF THE USE OF INTERNAL MEASURES FOR QUALITY ASSURANCE IN PUBLIC AND PRIVATE UNIVERSITIES IN SOUTH EAST NIGERIA

The seeming decline in the quality of university education in Nigeria necessitated this study on the comparative analysis of the internal measures that are used in public and private universities in south east geo-political zone of Nigeria for quality assurance. The study adopted the descriptive survey research design. Three research questions and three hypotheses guided the study. The population of the study comprised 29,083 made up of 7,444 lecturers and 21,639 students admitted in the 2011/2012 academic session in the 74 faculties and 413 departments of the 13 universities (five public and eight private) in the south-east geo political zone, Nigeria. The sample of the study comprised 2,106 respondents which were made up of 936 lecturers and 1, 170 students from four universities sampled. The sample was composed through multi-stage sampling technique. Questionnaire developed by the researchers titled “Students’ Questionnaire on Internal Measures for Quality Assurance” (SQIMFQA) and “Lecturers’ Questionnaire on Internal Measures for Quality Assurance” (LQIMFQA).was the instrument used for data collection. The instrument was validated by three experts, one in Educational measurement and evaluation and the others in Educational planning from the departments of Educational foundations and Educational Management and Policy all in the faculty of Education Nnamdi Azikiwe University, Awka. The Cronbach’s alpha method was used to determine the internal consistency of the items and this yielded a result of 0.74. The researcher with the help of eight research assistants distributed and successfully collected all the 2,106 copies of the questionnaire administered. The research questions were analyzed using the mean, while the hypotheses were tested using z-test at 0.05 level of significance. It was found among others that internal measures for quality assurance of student intake measures, evaluation of students’ learning experience and teacher quality measures were not adequately used in both public and private universities in the south-east geo-political zone of Nigeria for quality assurance. Based on the findings, recommendations were made

Surface Wave Modes on Spherical Cavities Excited by Incident Ultrasound

It has been shown both experimentally and theoretically1 that ultrasonic waves propagate circumferentially around the surface of cavities in an elastic medium, besides being reflected from its “flash points”. Surface wave returns were seen to decisively influence the time structure of the echo return from incident ultrasonic pulses. Nagase2 has solved a characteristic equation applicable to the spherical cavity problem, from which it could be shown3 that the surface of a spherical cavity supports a Rayleigh-type and two (P and S) Franz-type surface waves, of known speeds and dispersions. On the other hand, the complex eigenfrequencies of cavities were recently obtained numerically4. We have used these numerical results in order to satisfy Nagase’s solutions, presented in the form of propagation constants of the surface waves as series of fractional powers of the frequency, and have obtained in this way a mode number assignment for all the complex eigenfrequencies. Using this, we calculate dispersion curves for the Rayleigh, P and S- type surface wave phase velocities; their knowledge will permit an accurate interpretation of ultrasonic scattering experiments1, which previously could be analyzed in a qualitative way only

The mechanical and electrical properties of direct-spun carbon nanotube mats

The mechanical and electrical properties of a direct-spun carbon nanotube mat are measured. The mat comprises an interlinked random network of nanotube bundles, with approximately 40 nanotubes in a bundle. A small degree of in-plane anisotropy is observed. The bundles occasionally branch, and the mesh topology resembles a 2D lattice of nodal connectivity slightly below 4. The macroscopic in-plane tensile response is elasto-plastic in nature, with significant orientation hardening. In-situ microscopy reveals that the nanotube bundles do not slide past each other at their junctions under macroscopic stain. A micromechanical model is developed to relate the macroscopic modulus and flow strength to the longitudinal shear response of the nanotube bundles. The mechanical and electrical properties of the mat are compared with those of other nanotube arrangements over a wide range of density