Showing papers by "Chinese Academy of Sciences published in 1998"
TL;DR: In this article, the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions is demonstrated.
Abstract: Chemical oxygen demand (CODCr) and proton NMR, UV−vis, and spin trapping EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions. Both de-ethylation and oxidative degradation take place with the former proceeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-ethylated rhodamine species. Intermediates present after each de-ethylation step remain in a fast dynamic equilibrium between the titania particle surface and the bulk solution. The concentration of •OH radicals, formed from the inverse photosensitization process through the superoxide radical anion, increases upon addition of the anionic dodecylbenzene sulfonate surfactant (DBS) because a larger number of RhB excited states are able to inject an electron into the conduction band of the TiO2 particles. Also, intermediates that can no longer absorb the visible light, (i.e., once the dye solution ...
948 citations
TL;DR: In this paper, the corrosion of die cast AZ91D was studied and related to its microstructure, and it was found that the casting method can influence the corrosion performance by its influence on the alloy microstructures, which is attributed to a combination of higher volume fraction of the beta phase, a more continuous beta phase distribution around finer alpha grains, and lower porosity in the skin layer than in the interior of the die casting.
876 citations
TL;DR: In this article, a large scale and low-cost method for the synthesis of single-walled carbon nanotubes (SWNTs) using an improved floating catalyst method was proposed.
Abstract: Rope-like bundles of single-walled carbon nanotubes (SWNTs) similar to those obtained by laser vaporization and electric-are techniques were synthesized on a relatively large scale and at low cost by the catalytic decomposition of hydrocarbons at a temperature of about 1200 degrees C using an improved floating catalyst method. The SWNTs thus obtained have larger diameters and are self-organized into ropes. The addition of thiophene was found to be effective in promoting the growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes under different growth conditions. (C) 1998 American Institute of Physics. [S0003-6951(98)01125-5].
789 citations
TL;DR: In this article, the corrosion behavior of dual phase alloys was studied in 1 N NaCl at pH 11 by measuring electrochemical polarization curves, electrochemical AC impedance spectroscopy (EIS) and simultaneously measuring the hydrogen evolution rate and the magnesium dissolution rate.
776 citations
TL;DR: In this article, the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989 were studied, and the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes was paid to the diagnosis.
Abstract: Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Fina...
563 citations
TL;DR: Modifications to the WATERGATE method for removing the solvent resonance from H-1 NMR spectra provide narrower noninversion regions and hence enhance the sensitivities of the peaks close to the water resonance.
505 citations
TL;DR: In this paper, the degradation kinetics followed a Langmuir-Hinshelwood type equation and the rate was greater in acid media than in neutral and alkaline media, which correlates with the adsorption behavior of eosin on the TiO2 surface.
Abstract: The TiO2-assisted photodegradation of anionic eosin has been examined in TiO2 aqueous dispersions under illumination by visible light. Eosin is easily decomposed photochemically by visible light in the presence of TiO2 particles. The degradation kinetics followed a Langmuir-Hinshelwood type equation. The rate was greater in acid media than in neutral and alkaline media, which correlates with the adsorption behavior of eosin on the TiO2 surface. Adsorption of eosin is a prerequisite for the TiO2-assisted photodegradation. The evolution of CO2 occurred concomitantly with the photodegradation of eosin. A plausible mechanism of degradation is discussed.
437 citations
390 citations
TL;DR: Genetic evidence does not support an independent origin of Homo sapiens in China, and it is more likely that ancestors of the populations currently residing in East Asia entered from Southeast Asia.
Abstract: Despite the fact that the continuity of morphology of fossil specimens of modern humans found in China has repeatedly challenged the Out-of-Africa hypothesis, Chinese populations are underrepresented in genetic studies. Genetic profiles of 28 populations sampled in China supported the distinction between southern and northern populations, while the latter are biphyletic. Linguistic boundaries are often transgressed across language families studied, reflecting substantial gene flow between populations. Nevertheless, genetic evidence does not support an independent origin of Homo sapiens in China. The phylogeny also suggested that it is more likely that ancestors of the populations currently residing in East Asia entered from Southeast Asia.
358 citations
TL;DR: In this paper, a dye-sensitized nanoporous TiO2 solar cells are studied under influences of light intensity, redox couple concn., temp., different cations and water in the nonaq. soln.
358 citations
TL;DR: In this article, pyrolysis of acetylene over iron/silica substrates is an effective method with which to produce very long, multiwalled carbon nanotubes that reach about 2 mm in length.
Abstract: Carbon nanotubes1 can now be produced in large quantities by either arc methods2,3 or thermal decomposition of hydrocarbons4,5. Here we report that pyrolysis of acetylene over iron/silica substrates is an effective method with which to produce very long, multiwalled carbon nanotubes that reach about 2 mm in length, which is an order of magnitude longer than that described in most previous reports1,2,3,4,5,.
TL;DR: In this paper, a comparative analysis of a 4-year set of monthly samples of airfall dust with samples from surface alluvial fan deposits, desert sand dunes and late Pleistocene loess and palaeosols in Gansu Province, northern China, is presented.
TL;DR: The characteristics of the second-generation biosensor with the use of tetrathiafulvalene as a mediator were discussed and exhibited a series of good properties: high sensitivity, short response time, and remarkable long-term stability in storage (at least 5 months).
Abstract: A new type of sol-gel organic-inorganic hybrid material was developed and used for the production of biosensors. This material is composed of silica sol and a grafting copolymer of poly(vinyl alcohol) with 4-vinylpyridine. It prevents the cracking of conventional sol-gel-derived glasses and eliminates the swelling of the hydrogel. The optimum composition of the hybrid material was first examined, and then glucose oxidase was immobilized in this matrix to demonstrate its application. The characteristics of the biosensor were studied by cyclic voltammetry and chronoamperometry. The biosensor exhibited a series of good properties: high sensitivity (600 nA mmol-1 L-1), short response time (11 s) and remarkable long-term stability in storage (at least 5 months). In addition, the characteristics of the second-generation biosensor with the use of tetrathiafulvalene as a mediator were discussed.
TL;DR: In this article, the thermal features of the atmosphere over the Tibetan Plateau in summer and their effects on the general circulation are reviewed, and it is shown that the surface sensiole heating of the Plateau is essential for the occurrence of the abrupt seasonal change of the air stratification there, and for the persisten maintenance of the Asian monsoon.
Abstract: In this paper, the thermal features of the atmosphere over the Tibetan Plateau in summer and their effects on the general circulation are reviewed. Some recent research results are reported. It is shown that the Plateau acts as a heat source in summer. Particularly the strong surface heating makes the air stratification very unstable and produces strong near-surface convergence and positive vorticity and upper layer divergence and negative vorticity. Intense convective activity generated thereby not only maintains such particular large-scale circulation pattern over the Plateau, but also transports large amounts of sensible heat, moisture, chemical pollutants, as well as air with low ozone concentration from near-surface layers to upper layers. A minimum centre of total ozone concentiation and a huge upper layer anticyclone with a warm and moist core are thus observed over the Plateau in summer. The strong divergent flow and anticyclonic vorticity source in the upper atmosphere have a strong influence on the general circulation over the world via meridional as well as longitudinal circulations, and energy, dispersion on a spherical surface. It is shown that the surface sensiole heating of the Plateau is essential for the occurrence of the abrupt seasonal change of the general circulation there, and for the persisten maintenance of the Asian monsoon. It is also reported that the elevated heating of the Tibetan Plateau together with its mechanical forcing cause the early onset of the Asian monsson to happen over the eastern coast of the Bay of Bengal, which then generates a favorable circulation background for the monsoon onset over the South China Sea. The Indian monsoon onset flows aftervards.
Abstract: The reactivity of a new three carbon synthon, generated in situ from the reaction of 2,3-butadienoates or 2-butynoates with an appropriate phosphine as the catalyst, toward the electron-deficient imines is described. Triphenylphosphine-catalyzed reaction of methyl 2,3-butadienoate with N-sulfonylimines gave the single [3+2] cycloadduct in excellent yield; tributylphosphine-catalyzed reaction of methyl 2,3-butadienoate or 2-butynoate with N-tosylimines afforded the corresponding [3+2] cycloadduct as the major product along with a small amount of the three components adduct. Aliphatic N-tosylimines gave moderate yield for this reaction. In addition, a new phosphine-catalyzed cyclization reaction of dimethyl acetylenedicarboxylate with N-tosylimines is also described. A reaction mechanism is proposed. Further elaborations of the cycloaddition products and the synthesis of pentabromopseudilin using this method are exemplified.
TL;DR: In this paper, an almost 300m thick eolian sequence of Late Cenozoic sediments, which includes 162.5m of Quaternary loess-paleosols and 126m of Late Tertiary eolians Red Clay from the central part of the Chinese Loess Plateau, was investigated to determine the magnetostratigraphy.
Abstract: An almost 300m thick eolian sequence of Late Cenozoic sediments, which includes 162.5m of Quaternary loess-paleosols and 126m of Late Tertiary eolian Red Clay from the central part of the Chinese Loess Plateau, was investigated to determine the magnetostratigraphy. The results show that eolian dust accumulation, and by inference the related East Asia paleomonsoon, had begun by 7.2Ma. As paleomonsoon are largely controlled by the Tibetan Plateau, this implies that the Plateau had reached some critical elevation by 7.2Ma. The section also documents a rapid increase in eolian dust accumulation in the Late Cenozoic at 3.2Ma that is probably due to the influence of global ice volume on the East Asian monsoon.
TL;DR: In this paper, a novel method, in situ polymerization, was used for the preparation of nylon 6/silica nanocomposites, and the mechanical properties of the nanocom composites were examined.
Abstract: A novel method, in situ polymerization, was used for the preparation of nylon 6/silica nanocomposites, and the mechanical properties of the nanocomposites were examined. The results showed that the tensile strength, elongation at break, and impact strength of silica-modified nanocomposites exhibited a tendency of up and down with the silica content increasing, while those of silica-unmodified nanocomposites decreased gradually. It also exhibited that the mechanical properties of silica-modified nanocomposites have maximum values only when 5% silica particles were filled. Based on the relationship between impact strength of the nanocomposites and the matrix ligament thickness τ, a new criterion was proposed to explain the unique mechanical properties of nylon 6/silica nanocomposites. The nylon 6/silica nanocomposites can be toughened only when the matrix ligament thickness is less than τc and greater than τa, where τa is the matrix ligament thickness when silica particles begin to aggregate, and τc is the critical matrix ligament thickness when silica particles begin to toughen the nylon 6 matrix. The matrix ligament thickness, τ, is not independent, which related with the volume fraction of the inorganic component because the diameter of inorganic particles remains constant during processing. According to the observation of Electron Scanning Microscope (SEM), the process of dispersion to aggregation of silica particles in the nylon 6 matrix with increasing of the silica content was observed, and this result strongly supported our proposal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 789–795, 1998
TL;DR: Using a 1988-1997 data set of original photospheric vector magnetograms from the Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station of Beijing Astronomical Observatory, Wang et al. as discussed by the authors computed the local current helicity Bz(∇×B)z for 422 active regions.
Abstract: Using a 1988-1997 data set of original photospheric vector magnetograms from the Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station of Beijing Astronomical Observatory, we computed the local current helicity Bz(∇×B)z for 422 active regions. We found that any given active region contained mixed signs of current helicity, but in most cases current helicity with a particular sign was dominant over a whole region area. In our data set, 84% of the active regions in the northern hemisphere have negative helicity, and 81% in the southern hemisphere have positive helicity. It is estimated that the noise and error in our calculation are at the 2 σ level. In addition, we have studied the evolution of the large-scale surface current helicity during the cycle 22, which is a running mean of absolute current helicity of the active regions observed over a Carrington rotation period. By comparing with monthly mean sunspot number, we found that the average current helicity has a good correlation with solar activity.
TL;DR: In this article, a method was developed to estimate the forest biomass of China based on the relationship between stand biomass and volume, which was used to convert volume measurements into total biomass values (above and belowground dry masses) based on 1984-1988 forest inventory data for China.
Abstract: In this study, a method was developed to estimate the forest biomass of China based on the relationship between stand biomass and volume. Biomass-volume relationships were quantified for all the main forest types in China using 758 sets of data obtained from direct field measurements. These relationships were used to convert volume measurements into total biomass values (above- and belowground dry masses) based on 1984-1988 forest inventory data for China. The latter had been compiled from more than 250000 permanent and temporary field plots across the country. This data contained information on forest area and timber volume for each ape class and site class for all forest types at the provincial level. As a result, the total forest biomass of China was estimated as 9103 Tg (1 Tg 10(12) g), with 8592, 326, and 185 Tg from forests, special product plantations, and bamboo Forests, respectively. The area-weighted mean biomass density was 84 Mg/ha (1 Mg = 10(6) g). For comparison, two additional estimates, one based on the mean biomass density method and another based on the mean ratio of biomass to stem volume, were also derived. Compared to the biomass-volume relationship method, the mean biomass density method considerably overestimated the forest biomass of China (by 59.6%), while the mean ratio of biomass to stem volume method slightly underestimated it (by 12.1%). Despite the small forest biomass value due to a low forest cover, the area-weighted mean biomass density was comparable to those of other regions in the middle and high latitudes except in the United States. We believe that our study provided not only an appropriate estimate of forest biomass for China, but also an improved methodology for estimating forest biomass at the regional, national, and global level.
TL;DR: A polyamide 6 (PA 6)/silica nanocomposite was obtained through a novel method, in situ polymerization, by first suspending silica particles in ϵ-caproamide under stirring and then polymerizing this mixture at high temperature under a nitrogen atmosphere as discussed by the authors.
Abstract: A polyamide 6 (PA 6)/silica nanocomposite was obtained through a novel method, in situ polymerization, by first suspending silica particles in ϵ-caproamide under stirring and then polymerizing this mixture at high temperature under a nitrogen atmosphere The silicas were premodified with aminobutyric acid prior to the polymerization The effects of the addition of unmodified and modified silicas on the dispersion, interfacial adhesion, isothermal crystallization, and mechanical properties of PA 6 nanocomposites were investigated by using scanning electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, and mechanical tests, respectively The results show that the silicas dispersed homogeneously in the PA 6 matrix The addition of silicas increases the glass transition temperature and crystallization rate of PA 6 The mechanical properties such as impact strength, tensile strength, and elongation at break of the PA 6/modified silica nanocomposites showed a tendency to increase and decrease with increase of the silica content and have maximum values at 5% silica content, whereas those of the PA 6/unmodified silica system decreased gradually © 1998 John Wiley & Sons, Inc J Appl Polym Sci 69: 355–361, 1998
01 Sep 1998
TL;DR: Xifeng, Changwu and Weinan as discussed by the authors showed that the Loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly 18. developed S4, S5-1 and S 5-3 soils corresponding to the marine d O stages 11, 13, and 15, respectively as evidence of 18 three episodes of great warmth and two coarse-grained loess units L9 and L15, corresponding to marine dO stages 22,. 23, 24 and 38, respectively which indicate seve
Abstract: The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly 18 . developed S4, S5-1 and S5-3 soils corresponding to the marine d O stages 11, 13, and 15, respectively as evidence of 18 three episodes of great warmth and two coarse-grained loess units L9 and L15, corresponding to the marine d O stages 22, . 23, 24 and 38, respectively which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle. . Paleopedological, geochemical and magnetic susceptibility data from three loess sections Xifeng, Changwu and Weinan suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated . . mean annual temperature MAT of at least 4-68C higher and a mean annual precipitation MAP of 200-300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt the contact of the monsoonal . northward warm-humid air mass with the dry-cold southward one might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5-38C and 150-250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region. Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatestrsmallest Atlantic-Pacific 13 . d C gradients, respectively, indicating their relationships with the strength of Deep Water NADW production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 10 4 years. q 1998 Elsevier Science B.V. All rights reserved.
TL;DR: Reconstruction of an emptied phage particle shows that the head-tail connector and neck/tail assembly undergo significant conformational changes upon ejection of DNA.
TL;DR: In this article, a homogeneous nucleation kinetics for melting in superheated crystals is analyzed in order to derive a kinetic stability limit for the crystal lattice above its equilibrium melting point (T-m).
Abstract: Homogeneous nucleation kinetics for melting in superheated crystals is analyzed in order to derive a kinetic stability limit for the crystal lattice above its equilibrium melting point (T-m). It is found that at a critical temperature (T-m(K), which is about 1.2T(m) for various elemental metals) a massive homogeneous nucleation of melting occurs in the superheated crystal. Such a homogeneous nucleation catastrophe, which occurs before other proposed (rigidity, volume, and entropy) catastrophes can intervene, provides us with a kinetic stability limit for superheated crystals.
TL;DR: In this paper, a birefringent α-BaB 2 O 4 crystal, 40mm in diameter, 35mm in height has been grown successfully in a laboratory by Czochralski method.
TL;DR: The total amount of translatable Wx mRNA, and therefore the Wx protein and amylose content, are reduced in the group II cultivars compared with the group I cultivars.
Abstract: We have previously reported that three distinct patterns of waxy (Wx) gene transcript accumulation were present in 31 rice cultivars. The cultivars with high amylose content (group I) contain a 2.3 kb mature Wx mRNA, cultivars with intermediate amylose content (group II) produce both a 3.3 kb Wx pre-mRNA, which contains intron 1, and the 2.3 kb Wx mature mRNA, and cultivars with no amylose (group III) accumulate only the 3.3 kb Wx pre-mRNA. Analyses of the cDNAs reveals that four splice donor sites and three splice acceptor sites in intron 1 give rise to six splicing patterns in 2.3 kb Wx mRNA of group II cultivars. In addition, aberrant intron 1 excision causes either deletion of 4 or 5 nucleotides, or addition of 7 and 13 nucleotides at the junction of exon 1 and exon 2 of the 2.3 kb mRNA. In contrast, only one normal splicing pattern (one splice donor site and one splice acceptor site) was found in the 2.3 kb mRNA of group I cultivars. Nucleotide sequences of the Wx intron 1 in group I and group II cultivars differ by 16 individual bases. We suggest that these deletions or additions contribute to inefficient splicing of intron 1 from the 3.3 kb Wx pre-mRNA, as well as an aberrant splicing of the Wx intron 1 to produce the 2.3 kb mRNA with a heterogeneous 5' untranslated region (5'-UTR). As a consequence, the total amount of translatable Wx mRNA, and therefore the Wx protein and amylose content, are reduced in the group II cultivars compared with the group I cultivars.
TL;DR: In this article, the photocatalytic degradation of rhodamine B (N,N, N′,N′-tetraethylrhodamine, RB) had been studied in a colloidal TiO2 solution under both UV and visible light irradiation.
Abstract: The photocatalytic degradation of rhodamine B (N,N,N′,N′-tetraethylrhodamine, RB) had been studied in a colloidal TiO2 solution under both UV and visible light irradiation. Two different photodegradation processes of RB, photobleaching and N-deethylation, were examined in detail. The N-deethylation of RB was attributed to the oxidation by HOO· or OH· radicals generated from the dye-photosensitized and/or TiO2 catalyzed reactions; while the photobleaching of RB was found to occur via dye cationic radical. The aromatic rings of RB cannot be destroyed in the absence of oxygen under visible light irradiation. Various additives, Fe3+, EDTA, MV2+, different atmosphere (O2 and N2), time-resolved laser flash photolysis and spin trapping electron paramagnetic resonance (ESR) were used to determine the possible mechanisms involved.
TL;DR: In this paper, four red clay sections spanning over 400 km are studied and grain size analysis of closely spaced samples in the sections shows that the particles of the red clay are very fine with the sand fraction (>63 μm) being negligible, and all the curves indicating changes in different grain size parameters have a similar pattern.
TL;DR: In this paper, polyurethane (PU) ionomers and non-ionomers with various soft segment contents (SSC) and lengths have been synthesized from polycaprolactone diols (PCL), 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butanediol (BD) and dimethylolpropionic acid (DMPA), and tested for shape memory behaviour as well as for mechanical and dynamic mechanical properties.
TL;DR: The results indicated that, DPPH scavenging activity existed in 15 species; Gelidium amansii, Gloiosiphonia capillaris, Polysiphonia urceolata, Sargassum kjellmanianum, Desmarestia viridis, and Rhodomela teres showed the strongest activities.
Abstract: Dietary antioxidants from plants are believed to help prevent aging and other diseases through radical scavenging activity. Free radical scavenging activities by 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay and deoxyribose assay were ascertained among 27 species of common seaweeds using a sequential extraction method. The results indicated that, DPPH scavenging activity existed in 15 species; Gelidium amansii, Gloiosiphonia capillaris, Polysiphonia urceolata, Sargassum kjellmanianum, Desmarestia viridis, and Rhodomela teres showed the strongest activities. On the other hand, the deoxyribose results showed that almost all the seaweed species had good ability to scavenge hydroxyl radicals. The most active species were Rhodomela teres and Chorda filum.