Showing papers by "Chinese Academy of Sciences published in 1995"

Design and synthesis of an ultraviolet-transparent nonlinear optical crystal Sr 2 Be 2 B 2 O 7

Abstract: POWERFUL, tunable ultraviolet laser sources, required for many spectroscopic applications, rely on the frequency-doubling of lasers in the visible range. Such second-harmonic generation (SHG) depends on the development of nonlinear optical (NLO) materials with high SHG coefficients, a wide transparent region in the ultraviolet, and moderate birefringence. Few such materials are available; those most widely used at present are the inorganic crystals β-BaB2O4 (BBO)1 and LiB3O5 (LBO)2. These crystals are effective for SHG only down to wavelengths of 205 nm (ref. 3), because of a limited ultraviolet-transparent range in the former case and birefringence problems in the latter. We have developed previously4 a new inorganic NLO crystal, KBe2BO3F2 (KBBF), which avoids both of these problems to a large degree. But it is hard to grow large crystals of KBBF because of weak binding between its layered structural units. We have now developed an improved material by rational design: Sr2Be2B2O7 (SBBO), which shares (and in fact slightly improves on) all of the favourable NLO properties of KBBF and is easy to grow as large (so far up to 7 × 7 × 3 mm) crystals of high optical quality. The large SHG coefficients and wide range of ultraviolet transparency make this material a promising candidate for frequency doubling into the ultraviolet.

How to Cope with Noise In the Iterated Prisoner's Dilemma

Abstract: Noise in the form of random errors in implementing a choice is a common problem in real-world interactions. Recent research has identified three approaches to coping with noise: adding generosity to a reciprocating strategy; adding contrition to a reciprocating strategy; and using an entirely different strategy, Pavlov, based on the idea of switching choice whenever the previous payoff was low. Tournament studies, ecological simulation, and theoretical analysis demonstrate (1) a generous version of tit-for-tat is a highly effective strategy when the players it meets have not adapted to noise; (2) if the other players have adapted to noise, a contrite version of tit-for-tat is even more effective at quickly restoring mutual cooperation without the risk of exploitation; and (3) Pavlov is not robust.

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China

Abstract: SUMMARY Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rockmagnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction. The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. The resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. The data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites. In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.

Effects of microstructural variables on the deformation behaviour of dual-phase steel

Abstract: An expression for the stress of martensite in dual-phase steel was developed, which shows the interdependence of the stress of martensite and strain hardening in the ferrite matrix and the contribution of microstructural variables (the volume fraction of martensite fm, ferrite grain size df, and martensite particle size dm). The onset of plastic deformation of martensite in dual-phase steel was predicted to depend on its yield strength and the microstructural variables, and this was verified by the modified Crussard-Jaoul analysis. It was found that for this dual-phase steel, refining the grain size and increasing fm increase the flow stress and raise the strain hardening rate at low strains, but little affect the strain hardening rate at high strains. The effect of the ferrite grain size on the flow stress of this dual-phase steel was found to obey the Hall-Petch relation, i.e. σ = σ 0 e + K e d f − 1 2 , where the Hall-Petch intersection σ0e and slope Ke are functions of strain, fm and dm. The effects of the plastic deformation of martensite and the microstructural variables on the strain hardening rate and the Hall-Petch behaviour were analysed in terms of the densities of statistically stored dislocations and geometrically necessary dislocations using the previously developed theoretical model.

The rich O II recombination spectrum of the planetary nebula NGC 7009: new observations and atomic data

Abstract: We present new spectrophotometric observations of the rich O II optical recombination line spectrum of the planetary nebula NGC 7009, obtained at a spectral resolution of about 1 Angstrom (FWHM). New intermediate coupling quantal calculations of O II radiative recombination coefficients for the 3d-3p and 4f-3d transitions are presented. The effect of departure from pure LS-coupling is shown to be important. Excellent agreement is found between the observed relative intensities of the O II lines and those calculated from recombination theory allowing for intermediate coupling effects. C, N and O abundances based on our recombination line measurements are derived. In all cases, they are about a factor of 5 higher than the corresponding values deduced from collisionally excited lines, indicating that the discrepancy between the abundances derived from these two different types of emission Lines, previously known to exist for C2+, is a common phenomenon, and is probably caused by the same physical process. The nature of this process is still not known. If the discrepancy is due to temperature fluctuations, the implied rms temperature fluctuation parameter t(2) is about a factor of 2 larger than that derived by comparing the temperatures deduced from the [O III] forbidden line ratio and from the ratio of the nebular continuum Balmer discontinuity to H beta. However, if we adopted the electron temperature derived from nebular continuum Balmer discontinuity instead of that from the [O III] forbidden line ratio, the C and N abundances deduced from ultraviolet collisionally excited lines would come into agreement with those deduced from the optical recombination lines, although the abundance of oxygen deduced from the optical forbidden lines would still be a factor of 2 lower than the corresponding value obtained from the optical recombination lines. The O/H abundance ratio derived from our recombination line analysis of NGC 7009 is more than a factor of 2 higher than the solar oxygen abundance.

A beaked bird from the Jurassic of China

Abstract: DISCOVERY of avian remains close to the age of Archaeopteryx in the Liaoning Province of northeastern China provides the earliest evidence for a beaked, edentulous bird. The associated wing skeleton retains the primitive pattern found in Archaeopteryx, including a manus with unfused carpal elements and long digits. Two leg skeletons from the same site also show an Archaeopteryxlevel of morphology, and provide the earliest indisputable evidence for a covering of body contour feathers. These specimens provide evidence for either an undiscovered pre-Archaeopteryx or a rapid, post-Archaeopteryxevolution in birds. As the first Jurassic birds to be described from outside Germany, they show that birds with long fingers terminating in large recurved claws were widely distributed. They are not found in the Early Cretaceous sediments of the same region, where there is a diverse assemblage of more advanced flying birds with smaller fingers and claws. The postcran-ial structure of Archaeopteryx andConfuciusornis seems to be adapted for climbing tree trunks and may have disappeared near the end of the Jurassic.

A 1000 year climate ice-core record from the Guliya ice cap, China: its relationship to global climate variability

Abstract: In 1992, an American-Chinese expedition successfully recovered three ice cores (308.6, 93.2 and 34.5m) from the Guliya ice cap (summit 6710 m a.s.l) in the far western Kunlun on the Qinghai–Tibetan Plateau, China. Guliya resembles a “polar” icecap with 10 m, 200 m and basal temperatures of –15.6°, –5.9° and –2.1°C, respectively. The 308.6 m core to bedrock is the longest ice core retrieved from an elevation greater than 4000 m a.s.l. and provides the first ice-core history from the western side of the Qinghai Plateau. The Plateau experiences a pronounced annual precipitation cycle during which 70–80% of annual total precipitation falls in the summer monsoon season. This leads to a marked visible stratigraphy in the glaciers which allows accurate dating of the ice cores and reconstruction of the net mass accumulation. This paper presents (1) the results of the geophysical program to determine ice thickness, ice flow and surface topography, (2) an assessment of net accumulation from stake measurements, snow pits and shallow core samples, and (3) the analyses of the upper 100 m of the 308.6 m core which provide a 1000 year history, including the ‘“Little Ice Age”, which is compared with Chinese historical records. Extended periods of positive accumulation on Guliya are closely contemporaneous with dry periods in eastern China. A trans-Pacific teleconnection is suggested by the strong temporal coherence between extended wet and dry phases on Guliya and on the Quelccaya ice cap, Peru.

Flux distribution of solar intranetwork magnetic fields

Abstract: Big Bear deep magnetograms of June 4, 1992 provide unprecedented observations for direct measurements of solar intranetwork (IN) magnetic fields. More than 2500 individual IN elements and 500 network elements are identified and their magnetic flux measured in a quiet region of 300 × 235 arc sec. The analysis reveals the following results: (1) IN element flux ranges from 1016 Mx (detection limit) to 2 × 1018 Mx, with a peak flux distribution of 6 × 1016 Mx. (2) More than 20% of the total flux in this quiet region is in the form of IN elements at any given time. (3) Most IN elements appear as a cluster of mixed polarities from an emergence center (or centers) somewhere within the network interior. (4) The IN flux is smaller than the network flux by more than an order of magnitude. It has a uniform spatial distribution with equal amount of both polarities.

Superconductivity at 80 K in (Sr,Ca) 3 Cu 2 O 4 +δCl 2-y induced by apical oxygen doping

Abstract: RECENT work1,2has shown that superconducting copper oxyhalides can be synthesized, with the La2CuO4structure but with no oxygen in the ‘apical’ positions outside the CuO2 planes. For these materials to be rendered superconducting, charge carriers must be introduced into the CuO2 planes; in previous work, positive carriers (holes) have been introduced by the incorporation of interstitial fluorine in Sr2CuO2F2+δ (transition temperature Tc = 46 K)1, or by the substitution of sodium for calcium in (Ca,Na)2CuO2Cl2 (Tc = 26 K)2. Here we present an alternative doping approach for this copper oxyhalide family: holes are introduced by partially replacing the (monovalent) halogen that occupies the apical sites with (divalent) oxygen. We have obtained bulk superconductivity with Tc above 77 K in a new double-layer compound (Sr,Ca)3Cu2O4+δCl2-y, which is isostructural with (La,Sr)2-CaCu2O6 (ref. 3), with (Sr,Ca) at the (La,Sr) or Ca sites, and Cl at the apical sites outside the CuO2 planes.

A broadband spectrometer for decimeter and microwave radio bursts

Abstract: Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 1015 K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.

Production of caproic acid by cocultures of ruminal cellulolytic bacteria and Clostridium kluyveri grown on cellulose and ethanol

Abstract: Ruminal cellulolytic bacteria (Fibrobacter succinogenes S85 or Ruminococcus flavefaciens FD-1) were combined with the non-ruminal bacterium Clostridium kluyveri and grown together on cellulose and ethanol. Succinate and acetate produced by the cellulolytic organisms were converted to butyrate and caproate only when the culture medium was supplemented with ethanol. Ethanol (244 mM) and butyrate (30 mM at pH 6.8) did not inhibit cellulose digestion or product formation by S85 or FD-1; however caproate (30 mM at pH 6.8) was moderately inhibitory to FD-1. Succinate consumption and caproate production were sensitive to culture pH, with more caproic acid being produced when the culture was controlled at a pH near neutrality. In a representative experiment under conditions of controlled pH (at 6.8) 6.0 g cellulose l-1 and 4.4 g ethanol l-1 were converted to 2.6 g butyrate l-1 and 4.6 g caproate l-1. The results suggest that bacteria that efficiently produce low levels of ethanol and acetate or succinate from cellulose should be useful in cocultures for the production of caproic acid, a potentially useful industrial chemical and bio-fuel precursor.

Energy use, emissions, and air pollution reduction strategies in Asia

Abstract: In contrast to Europe and North America, air pollution in Asia is increasing rapidly, resulting in both local air quality problems and higher acidic depositions. In 1989, an east-west group of scientists initiated a multi-institutional research project on Acid Rain and Emissions Reduction in Asia, funded for the past two years by the World Bank and the Asian Development Bank. Phase I, covering 23 countries of Asia, focussed on the development of PC-based software called the Regional Air Pollution INformation and Simulation Model (RAINS-ASIA). A 94-region Regional Energy Scenario Generator was developed to create alternative energy/emission scenarios through the year 2020. A long-range atmospheric transport model was developed to calculate dispersion and deposition of sulfur, based upon emissions from area and large point sources, on a one-degree grid of Asia. The resulting impacts of acidic deposition on a variety of vegetation types were analyzed using the critical loads approach to test different emissions management strategies, including both energy conservation measures and sulfur abatement technologies.

Tagging and mapping the thermo-sensitive genic male-sterile gene in rice (Oryza sativa L.) with molecular markers.

Abstract: The thermo-sensititve genic male-sterile (TGMS) gene in rice can alter fertility in response to temperature and is useful in the two-line system of hybrid rice production. However, little is known about the TGMS gene at the molecular level. The objective of this study was to identify molecular markers tightly linked with the TGMS gene and to map the gene onto a specific rice chromosome. Bulked segregant analysis of an F2 population from 5460s (a TGMS mutant line) x 'Hong Wan 52' was used to identify RAPD markers linked to the rice TGMS gene. Four hundred RAPD primers were screened for polymorphisms between the parents and between two bulks representing fertile and sterile plants; of these, 4 primers produced polymorphic products. Most of the polymorphic fragments contained repetitive sequences. Only one singlecopy sequence fragment was found, a 1.2-kb fragment amplified by primer OPB-19 and subsequently named TGMS1.2. TGMS1.2 was mapped on chromosome 8 with a RIL population and confirmed by remapping with a DHL population. Segregation analysis using TGMS1.2 as a probe indicated that TGMS1.2 both consegregated and was lined with the TGMS gene in this population. It is located about 6.7 cM from the TGMS gene. As TGMS1.2 is linked to the TGMS gene, the TGMS gene must be located on chromosome 8.

Electron microscopy study of coiled carbon tubules

Abstract: Carbon fibres prepared by the catalytic decomposition of acetylene over finely dispersed cobalt, supported on amorphous silica, are often helix shaped. The structure of such carbon fibres was studied by means of various electron diffraction techniques and by electron microscopy. It was shown that the tubules are hollow and consist of concentric cylindrical graphene sheets. The diffraction patterns are analysed in detail and compared with theoretically predicted patterns. Good semi-quantitative agreement is found. It is shown that the helices are polygonized; their atomic structure is consistent with models predicted on the basis of molecular dynamics simulations.

Structure and magnetic properties of RFe11.35Nb0.65 and RFe11.35Nb0.65Ny (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Lu).

Abstract: A series of compounds with the ${\mathrm{ThMn}}_{12}$-type structure, R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Lu), were synthesized. The corresponding nitrides, obtained by gas-solid reactions, retained the same structure as their parent compounds, but with a relative volume expansion of 3%. The Nb atoms occupy the 8i sites in the ${\mathrm{ThMn}}_{12}$ structure. The highest Curie temperatures are 597 K and 773 K for ${\mathrm{GdFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ and its nitride, respectively. The average iron magnetic moment ${\mathrm{\ensuremath{\mu}}}_{\mathrm{Fe}}$ at T=1.5 K is 1.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for parent compounds and 2.1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for the nitrides. By the introduction of nitrogen, the Fe-Fe exchange interaction is much strengthened while the R-Fe exchange interaction is slightly weakened. The enhancement of ${\mathrm{\ensuremath{\mu}}}_{\mathrm{Fe}}$ by the influence of interstitial nitrogen is about 11%. The Fe sublattices of R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compounds and their nitrides have an easy c-axis anisotropy. At T=1.5 K, the anisotropy constant ${\mathit{K}}_{1}$(Fe) is 25.7 K/f.u. for the parent compounds and 21.9 K/f.u. for the nitrides. The reduction of ${\mathit{K}}_{1}$(Fe) by the influence of interstitial nitrogen is about 17%. Spin-reorientation transitions were observed in ${\mathrm{DyFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ and ${\mathrm{ErFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compounds. The complex magnetic behavior of R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ at low temperatures can be explained by exchange and crystal-field-interaction models. The ${\mathrm{SmFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compound has an anisotropy field of 10.0 T at room temperature. Its high Curie temperature and magnetization make it a potential candidate for applications as a permanent magnet.