Showing papers by "Tongji University published in 1997"

Quantum interference effects in spontaneous emission from an atom embedded in a photonic band gap structure

Abstract: The spontaneous emission from a three-level atom embedded in a photonic band gap structure is studied. Interference between two transitions leads to quasiperiodic oscillations of population between the two upper levels with large amplitudes. The spontaneous emission of the atom is characterized by three components in the radiated field: a localized part, a traveling pulse, and a $(1/\sqrt{t}{)}^{3}$ decaying part. An analytical expression for the localization distance of the localized field is obtained. The energy velocity for the traveling pulse could be close to zero. By selecting an appropriate initial superposition state, a large amount of population trapping can be achieved.

Synthesis and characterization of liquid crystalline polymers from p-hydroxybenzoic acid, poly(ethylene terephthalate), and third monomers

Abstract: Eight new p-hydroxybenzoic acid (PHB) and poly(ethylene terephthalate) (PET) copolymers containing vanillic acid (VA), p-aminobenzoic acid, m-hydroxybenzoic acid, hydroquinone/terephthalic acid (TPA), bisphenol A/TPA, 1,5-naphthalenediol/TPA, 2,7-naphthalenediol/TPA, and 1,4-dihydroxyanthraquinone/TPA as eight third monomers with a variety of structural features were synthesized by melted-state copolycondensation and were characterized through a thermal analyzer, proton nuclear magnetic resonance, wide-angle X-ray diffraction (WAXD), and a scanning electron microscope (SEM). The experimental results show that PHB/PET/VA copolymers exhibit a faster polycondensation rate, lower melting temperature, and higher thermostability than do the other seven copolymers and third monomer-free PHB/PET polymers. The as-spun fibers derived from the PHB/PET/VA copolymers with different VA contents show tensile strengths, Young's moduli, and break elongations of 0.6–1.5 GPa, 28–67 GPa, and 7–9%, respectively. A highly oriented fibrillar structure in the PHB/PET/VA copolymer fibers was observed using WAXD and SEM. The most effective third monomer of the eight third monomers for an enhancing polycondensation rate and molecular weight of the PHB/PET polymers and for improving their thermal and mechanical properties is found to be vanillic acid (VA). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2129–2138, 1997

Multilayer ultrathin‐film composite membranes for oxygen enrichment

Abstract: Multilayer composite membranes were made of poly(4-methylpentene-1) (PMP), an ethyl cellulose (EC) + heptyl cellulose (HC) blend, polycarbonate (PC), polysulfone, poly(2,6-dimethylphenylene oxide), cellulose triacetate ultrathin films as selective layers, and polysulfone, poly(ether sulfone), and poly(sulfone amide) ultrafiltration membranes with a 10–45 nm pore size and 100–120 μm thickness as porous support layers. The effects of the ultrathin-film type and its casting solution concentration, operating pressure, temperature, as well as time on the oxygen-enriched air (OEA) flux and oxygen concentration in the OEA permeated in a single step through the composite membranes were investigated using a constant pressure—variable volume method. The OEA flux increases significantly with an increasing transmembrane pressure difference and operating temperature. The oxygen concentration in the OEA also increases with an increasing pressure difference but decreases slightly with an increasing operating temperature. In long-term tests, the oxygen-enrichment properties were maintained almost constant for as long as 170 h. The composite membranes consisting of the bilayer ultrathin film cast from a more dilute solution (0.11–0.26 wt %) on the porous support with a smaller pore size combine a higher oxygen-enriching ability and a higher stability than do those of monolayer and tetralayer ultrathin films. The maximum OEA flux and oxygen concentration produced at 20–75°C and a 500 kPa transmembrane pressure difference in a single pass across the PMP/98EC + 2HC bilayer and PC bilayer ultrathin-film composite membranes are 3.1 × 10−3 cm3(STP)/s cm2 and 50%, respectively. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2139–2147, 1997

An Experiment-Based Cumulative Damage Mechanics Model of Steel under Cyclic Loading:

Abstract: Based on the stress-strain hysteretic curves obtained from the tests on steel under cyclic loading, a cumulative damage mechanics model using plastic strain as a basic variable is proposed in the p...

Seasonal Cycles of Surface Ozone and NOx in Shanghai

Abstract: A series of simultaneous observations of the surface ozone and NOx have been employed in Shanghai since 1990. Based on a 5-yr data series, seasonal cycles of O3 and NOx have been acquired. Both cycles have a maximum occurring in spring and a minimum occurring in summer. The monsoon flow reversal is a major factor affecting the cycles; however, the influences are different with seasons. The strongly synchronous seasonal changes of NOx with ozone suggest that the anthropogenic emissions play an important role in the surface ozone cycle. Influences of the anthropogenic emissions also are different with the seasons. The effect of the monsoon on the ozone cycle is most strong in summer, and the effect of the anthropogenic emissions on that cycle is most strong in spring.

Calculated phase diagrams for activated low pressure diamond growth from C-H, C-O, and C-H-O systems

Abstract: Three-dimensional temperature (T)–pressure (P)–composition (X) phase diagrams of binary carbon-hydrogen (C–H) and carbon-oxygen (C–O) systems for activated low pressure diamond growth have been calculated Based on an approximation of linear combination between C–H and C–O systems, a projective ternary carbonhydrogen-oxygen (C–H–O) phase diagram has also been obtained There is always a diamond growth region in each of these phase diagrams Once a supply of external activating energy stops, the diamond growth region will not exist Nearly all of the reliable experimental data reported in the literature drop into the possible diamond growth region of the calculated projective ternary C–H–O phase diagram under the conditions of 001–100 kPa and above 700 K

Scanning electron-acoustic imaging of residual stress distributions in aluminum metal and ZrSiO4 multiphase ceramics

Abstract: The scanning electron-acoustic imaging technique has been used in the characterization of the residual stress field distributions existing in the subsurface in aluminum disks and 20 vol % SiC ( w)/ZrSiO4 multiphase ceramics left by Vicker’s indentation. The experimental results reveal that the distribution areas are the plastic-elastic interchange zones. The electron-acoustic signal generation mechanism in the samples are discussed.

X-ray diffraction of thermotropic liquid-crystalline copoly(vanillate-4-hydroxybenzoate-ethyleneterephthalate)

Abstract: Precision X-ray diffractometry was used to study directly the structure of as-made films and fibers, which were uniaxially extruded from thermotropic liquid-crystalline nematic melts of copolymers consisting of vanillate (V), 4-hydroxybenzoate (B) and ethylene terephthalate (E) units with various molar ratios. The addition of a small amount of unit V (at most 5 mol-%) to the well-known poly(4-hydroxybenzoate-ethylene terephthalate) (B/E) polymer will produce an increase in orientation, but an addition of 15–34 mol-% of unit V might result in the decrease of the orientation. The X-ray diffractions of the oriented samples of V/B/E and B/E copolymers show a series of aperiodic intensity maxima on the meridian, which are consistent with that expected for a random sequence of the units. The equatorial scans exhibit single broad peaks centered at 0.437–0.446 nm due to intermolecular scattering. The meridional and equatorial maxima shift in position with the V/B/E ratio of the copolymers. The positions of the meridional maxima are slightly affected by spinning temperature. The linewidth of the most intense maximum at the spacing of 0.207 nm on the meridian was used to determine the correlation length of extended V/B/E copolymer chains. No discrete maxima are observed in both meridional and equatorial small-angle X-ray scattering curves. Die Strukturen von uniaxial extrudierten Filmen und Fasern aus thermotrop-flussigkristallinen nematischen Schmelzen von Vanillat(V)-4-Hydroxybenzoat(B)-Ethylenterephthalat(E)-Copolymeren unterschiedlicher Zusammensetzung wurden mit Prazisions-Rontgendiffraktometrie untersucht. Die Zugabe von geringen Mengen (nicht mehr als 5 Mol-%) Vanillat zu Poly(4-Hydroxybenzoat-co-ethylenterephthalat) bewirkt eine Erhohung der Orientierung, wahrend bei einem Vanillat-Anteil von 15–34 Mol-% die Orientierung abnimmt. Die Rontgendiffraktogramme von orientierten V/B/E- und BE-Copolymerproben weisen eine Reihe aperiodischer meridionaler Intensitatsmaxima auf, wie sie bei statistischer Verteilung der Segmente erwartet werden. Die aquatorialen Messungen zeigen einzelne breite Signale um 0,437−0,446 nm, die auf zwischenmolekulare Streuung zuruckzufuhren sind. Die Positionen der meridionalen und aquatorialen Intensitatsmaxima verschieben sich mit dem V/B/E-Verhaltnis der Copolymeren. Die Positionen der meridionalen Peaks werden geringfugig von der Spinntemperatur beeinflufit. Aus der Signalbreite des intensivsten meridionalen Maximums (Abstand 0,207 nm) wurde die Korrelationslange von gestreckten V/B/E-Copolymerketten ermittelt. Weder in den meridionalen noch in den aquatorialen Kleinwinkel-Rontgenstreuungsspektren wurden einzelne Maxima beobachtet.

Studies on polycarbonate/polystyrene/polycarbonate-g-polystyrene blends. I. Preparation, characterization and properties of polycarbonate-g-polystyrene

Abstract: Polycarbonate (PC)/polystyrene (PS) blend is an immiscible system. The use of copolymers as compatibilizers in blends of immiscible polymers is one approach which is being developed within the larger field of polymer blends and alloys. The methods for preparing PC–PS copolymers are difficult and complicated. In this paper, a new method for preparing PC–PS graft copolymer, PC-g-PS, by radiation grafting is proposed. The structure of the graft copolymer was confirmed by IR spectroscopy. Various factors which can be used to control the reaction have been studied. The molecular weight of the PS graft chain was determined by gel permeation chromatography after degrading the copolymer in aqueous sodium hydroxide and dichloromethane. The thermal properties of the copolymer have also been measured. © 1997 SCI.

Thermodynamic coupling effect and catalyst effect for the artificial diamond growth

Abstract: The activated chemical vapor deposition (CVD) diamond process became one of the worldwide interesting projects in the 1980s. The basic question is why diamond can grow under activated low pressure conditions. The driving force of the transformation from graphite to diamond under low pressure is coming from a coupled reaction of the association of superequilibrium atomic hydrogen. The thermodynamic coupling effect in the activated CVD process is different from the catalyst effect in the high pressure, catalyst-assisted process for the artificial diamond growth.

Electret properties for porous polytetrafluoroethylene (PTFE) film

Abstract: In this paper, a porous (aperture from 1 to 5 /spl mu/m, porosity 50%) polytetrafluoroethylene (PTFE) was prepared by stretching with biparous orientation, which can be used in clinical medicine for encouraging the recovery of wounded tissues of the human body. Influence of environmental humidity on charge stability for the porous PTFE was investigated after constant voltage corona charging. The comparison of the charge storage ability of the porous PTFE film and the solid PTFE film charged at room and elevated temperatures was carried out. The transport property of detrapped charges during thermally stimulated discharge was also studied by means of Thermally Stimulated Discharge (TSD) method and heat pulse technique.

Scanning electron microscopy analysis of the structure of thermotropic LC copolymers containing 2,7-dioxynaphthalene or vanillate units

Abstract: The morphological structure of the fibers and rods of two series of thermotropic liquid-crystalline copolymers made of 2,7-naphthalenediol/terephthalic acid/4-hydroxybenzoic acid(B)/poly(ethylene 2,6-naphthalene dicarboxylate) and vanillic acid/B/poly(ethylene terephthalate), was systematically analyzed by using a high-resolution scanning electron microscopy (SEM) fractographic method. Based largely on the SEM micrographs and visual appearance of the fracture surfaces, the fibers and rods were found to consist mainly of highly oriented needle-like fibrils or lamellae, at which some finely split connections appear. The uniformity of the fibrils is somewhat dependent on the copolymer composition. The macrofibrils with diameters in the range of 10–50 μm and the microfibrils with diameters in the range of 50–90 nm have been observed. No separate phase is obvious in the fibers containing no more than 70 mol-% of the B unit which has been confirmed by differential scanning calorimetry, but in the rods containing 75 mol-% of the B unit a few less-deformed phases related to B-rich phase were occasionally observed. Die morphologische Struktur von Fasern und Stabchen zweier Serien thermotropflussigkristalliner Copolymere aus 2,7-Naphthalindiol/Terephthalsaure/4-Hydroxybenzoesaure (B)/Poly(ethylen-2,6-naphthalindicarboxylat) und Vanillinsaure/4-Hydroxybenzoesaure/Poly(ethylenterephthalat) wurde mit hochauflosender Rasterelektronenmikroskopie (SEM) an Bruchflachen systematisch analysiert. Die SEM Ergebnisse und das Aussehen der Bruchflachen deuten darauf hin, das die Fasern und Stabchen hauptsachlich aus hochorientierten, nadelahnlichen Fibrillen oder Lamellen bestehen, die einige feine Risse aufweisen. Die Gleichformigkeit der Fibrillen hangt teilweise von der Copolymerzusammensetzung ab. Es wurden Makrofibrillen mit Durchmessern von 10 bis 50 μm und Mikrofibrillen mit Durchmessern zwischen 50 und 90 μm beobachtet. In den Fasern mit hochstens 70 mol-% B-Anteil war keine separierte Phase erkennbar, wie mit Differentialkalorimetrie bestatigt wurde, wahrend bei den Stabchen mit 75 mol-% B-Einheiten vereinzelt einige kaum deformierte Phasen beobachtet wurden.

Automatic Vehicle Identification Technology Applications to Toll Collection Services

Abstract: Intelligent transportation systems technologies are being developed and applied through transportation systems in the United States. An example of this type of innovation can be seen on toll roads where a driver is required to deposit a toll in order to drive on a toll road. To automatically process toll services, automatic vehicle identification (AVI) technologies should be developed and implemented. A summary is presented of a study that focused on the performance analysis of three types of AVI technologies that could be used for toll collection applications on the Crosstown Expressway and the Veterans Expressway in Hillsborough County, Florida. The three AVI technology types were optical/laser scanner, radio frequency, and inductive loop. The study aimed at analyzing the total average delay and AVI market share due to the usage of AVI technologies. Delay performance and market share resulting from the application of each AVI technology type were used to compare the effects of different AVI technology t...

Textures of thermotropic liquid‐crystalline copolymers containing either 2,7‐, 1,5‐ or 1,4‐naphthalenediol or vanillic acid units

Abstract: The melts of the thermotropic liquid-crystalline copolymers prepared from 2,7-, 1,5- or 1,4-naphthalenediol (N)/terephthalic acid/4-hydroxybenzoic acid/poly(ethylene 2,6-naphthalene dicarboxylate) or vanillic acid (V)/4-hydroxybenzoic acid/poly(-ethylene terephthalate) and other five types of 4-hydroxybenzoic acid/poly(ethylene terephthalate)/comonomer copolymers are turbid and exhibit a thready texture on the basis of visual observation. Strong shear and stir opalescences in the copolymer melts are also observed. The polarized light micrographs of the melts show a marble or threaded schlieren texture which is a typical optical texture of the nematic mesophase. The polarizing microscopy observation indicates that the vanillate unit-containing copolymer films obtained by drawing the melts are highly oriented and fibrillated. Elliptical light scattering patterns are obtained for the sheared films of the copolymers, suggesting the existence of a preferred orientation of the copolymer chains. The X-ray fiber diagrams of the extrudates from the 2,7-N and 1,5-N, and V units-containing copolymers, show broad halos centered at 0.436–0.444 nm and diffraction spots centered at 1.2−1.5 nm along the equator that are reminiscent of amorphous polymers with a “frozen” nematic phase, indicative of chain orientation. Additionally, the X-ray fiber diagram of the V unit-copolymer film shows strong meridional reflections at spacings 0.208 and 0.328 nm, representing the molecular repeat spacing of the copolymer chains. The X-ray fiber diagram of 1,4-N unit-containing copolymer rods exhibits two diffuse annular rings, the outer at the d-spacing 0.438 nm and the inner at 1.3 nm, indicating no definite order or amorphous structure. Die Schmelzen von thermotrop-flussigkristallinen Copolymeren aus 2,7-, 1,5-oder 1,4-Naphthalindiol (N)/Terephthalsaure/4-Hydroxybenzoesaure/Poly(ethylen-2,6-naphthalindicarboxylat) oder Vanillinsaure (V)/4-Hydroxybenzoesaure/Poly(ethylenterephthalat) sowie funf weiteren 4-Hydroxybenzoesaure/Poly(ethylenterephthalat)/Comonomer-Copolymeren sind getrubt und lassen bereits mit blosem Auge eine fadenformige Textur erkennen. Auch eine starke Scher- und Ruhropaleszenz wurde beobachtet. Unter polarisiertem Licht zeigen die Schmelzen die fur nematische Phasen typische marmorierte oder fadenformige Schlierentextur. Die polarisationsmikroskopische Untersuchung deutet darauf hin, das die durch Ziehen aus der Schmelze hergestellten, Vanillat-Segmente enthaltenden Copolymerfilme hochorientiert und fibrilliert sind. Die Lichtstreuungsmuster von gescherten Copolymer-filmen sind elliptisch und deuten an, dals die Copolymerketten in einer Vorzugsrichtung orientiert sind. Die Rontgendiagramme der extrudierten Copolymeren mit 2,7-N-, 1,5-N- und V-Einheiten zeigen um 0,436-0,444 nm zentrierte breite Halos und Diffraktionspunkte bei 1,2-1,5 nm auf dem Aquator, die an amorphe Polymere mit eingefrorener nematischer Phase erinnern und eine Kettenorientierung vermuten lassen. Zusatzlich weist das Rontgendiagramm des V-Einheiten enthaltenden Copolymerfilms starke meridionale Reflektionen mit Signalbreiten von 0,208 und 0,328 nm auf, was mit den Korrelationslangen der Copolymerketten ubereinstimmt. Im Rontgendiagramm von 1,4 N-Einheiten enthaltenden Copolymerstabchen werden zwei diffuse kreisformige Ringe, der ausere bei 0,438 nm und der innere bei 1,3 nm, beobachtet, was auf eine amorphe oder eine nicht definierte Struktur hinweist.