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71.
Alves GA Amato S Anjos JC Appel JA Astorga J Bernard T Bracker SB Cremaldi LM Darling CL Dixon RL Errede D Gay C Green DR Jedicke R Karchin PE Kwan S Lueking LJ de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Passmore D Rafatian A dos Reis AC Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wallace A Wu Z 《Physical review letters》1994,72(6):812-815
72.
P. BrÄtter D. Gawlik P. Jost J. Möller und P. Reimers 《Fresenius' Journal of Analytical Chemistry》1977,283(2):121-127
Zusammenfassung Im Rahmen eines Methodenvergleiches wurde der Fluorgehalt identischer Knochenproben durch Neutronen (NAA)- und Photonen (PAA)-Aktivierungsanalyse untersucht. Nach Optimierung der Bestrahlungs- und Me\bedingungen ergibt sich als Nachweisgrenze bei der NAA 3,9 g und bei der PAA 0,3 g. Diese Nachweisgrenzen sind ausreichend, um in einer Knochenbiopsie der Masse 50 mg den minimal zu erwartenden Fluorgehalt von 200 ppm quantitativ zu bestimmen. Der Analysenfehler betrÄgt in diesem Fall bei der NAA ±15% und bei der PAA ±25%. Der Zeitaufwand für die Analyse einer Einzelprobe betrÄgt 1 min (NAA) bzw. 4 h (PAA). Die Regressionsanalyse ergibt einen Korrelationskoeffizienten von 0,96.
Determination of fluorine in bones by means of neutron and photon activation analysis
For comparison of the two methods the fluorine content was determined in identical bone samples. After optimization of the irradiation and measuring conditions the detection limit was found to be 3.9 g for NAA and 0.3 g for PAA. Both detection limits are sufficient for the quantitative determination of the minimum expected value of 200 ppm fluorine in a bone biopsy sample of 50 mg. In this case the analytical error is ±15% for NAA and ±25% for PAA, respectively. The time necessary for the analysis of a single sample is 1 min (NAA) or 4 h (PAA). The regression analysis yields a correlation coefficient of 0.96.相似文献
73.
C. Murali Krishna Shobha Uppuluri Peter Riesz J. Samuel Zigler JR D. Balasubramanian 《Photochemistry and photobiology》1991,54(1):51-58
We have studied the photochemical quantum yields of singlet oxygen production (using the RNO bleaching method) and superoxide production (using the EPR-spin trapping method and the SOD-inhibitable ferricytochrome c reduction spectral assay) of kynurenine (Ky), N-formylkynurenine (NFK), 3-hydroxykynurenine (3HK), kynurenic acid (KUA), and the flavins, riboflavin (RF) and flavin mononucleotide (FMN). Such a study of the photodynamic efficiencies is important since these compounds appear endogenously in the eye. The singlet oxygen quantum yields of the flavins and KUA are high, while Ky and 3HK generate no detectable amounts of singlet oxygen. The superoxide quantum yields of the sensitizers are low compared to their singlet oxygen, and Ky and 3HK produce no detectable amounts of superoxide. The production of the superoxide radical is enhanced in the presence of electron donor molecules such as EDTA and NADH. These results suggest that the production of oxyradicals in the lens may be modulated by the presence of endogenous electron donor molecules such as the coenzymes NADH and NADPH, which are present in significant amounts in some lenses. They also suggest that Ky and 3HK, which are known to be present in aged lenses, might play a protective rather than a deleterious role in the eye. 相似文献
74.
Nieuwenburg P Clarke RJ Cai ZL Chen M Larkum AW Cabral NM Ghiggino KP Reimers JR 《Photochemistry and photobiology》2003,77(6):628-637
A comprehensive study of the photophysical properties of chlorophyll (Chl) d in 1:40 acetonitrile-methanol solution is performed over the temperature range 170-295 K. From comparison of absorption and emission spectra, time-dependent density-functional calculations and homologies with those of Chl a, we assign the key features of the absorption and fluorescence spectra. Possible photophysical energy relaxation mechanisms are summarized, and thermal equilibration processes are studied in detail by monitoring the observed emission profiles and quantum yields as a function of excitation energy. In particular, we concentrate on emission subsequent to excitation in the extreme far-red tail of the Qy absorption spectrum, with this emission partitioned into contributions from hot-band absorptions as well as uphill energy transfer processes that occur subsequent to absorption. No unusual photophysical processes are detected for Chl d; it appears that all intramolecular relaxation processes reach thermal equilibration on shorter timescales than the fluorescence lifetime even at 170 K. The results from these studies are used to reinterpret a previous study of photochemical processes observed in intact cells and their acetone extracts of the photosynthetic system of Acaryochloris marina. In the study of Mimuro et al., light absorbed by Chl d at 736 nm is found to give rise to emission by another species, believed to also be Chl d, at 703 nm; this uphill energy transfer process is easily rationalized in terms of the thermal equilibration processes that we deduced for Chl d. However, no evidence is found in the experimental results of Mimuro et al. to support claims that (nonequilibrium) uphill energy transfer is additionally observed to Chl a species that emit at 670-680 nm. This finding is relevant to broader issues concerning the nature of the special pair in photosystem II of A. marina because suggestions that it is comprised of Chl a can only be correct if nonthermal uphill energy transfer processes from Chl d are operative. 相似文献
75.
Hydrogen bonds from water to excited-state formaldehyde and from water to excited-state pyridine have been shown to display novel motifs to traditional hydrogen bonds involving ground states, with, in particular for H2O:pyridine, strong interactions involving the electron-rich pi cloud dominating the (n,pi) excited state. We investigate H2O:pyrimidine and various dihydrated species and reveal another motif, one in which the hydrogen bonding can dramatically alter the electronic structure of the excited state. Such effects are rare for ground-state interactions for which hydrogen bonding usually acts to merely perturb the electronic structure of the participating molecules. It arises as the (n,pi*) excitation of isolated pyrimidine is delocalized over both nitrogens but asymmetric hydrogen bonding causes it to localize on just the noninteracting atom. As a result, the excited-state hydrogen bond in H2O:pyrimidine is suprisingly very similar to the ground-state structure. These results lead to an improved understanding of the spectroscopy of pyrimidine in liquid water, and to the prediction that stable excited-state hydrogen bonds in H2O:pyrimidine should be observable, despite failure of experiments to actually do so. They also provide a simple model for the intricate control over primary charge separation in photosynthesis exerted by hydrogen bonding, and for solvent-induced electron localization in symmetric mixed-valence complexes. All conclusions are based on strong parallels found between the results of calculations performed using density-functional theory (DFT) and time-dependent DFT (TDDFT), complete-active-space self-consistent-field (CASSCF) with second-order perturbation-theory correction (CASPT2) theory, and equation-of-motion coupled cluster (EOM-CCSD) theory, calculations that are verified through detailed comparison of computed properties with experimental data for both the isolated molecules and the ground-state hydrogen bond. 相似文献
76.
N. Stribeck C. Reimers P. Ghioca E. Buzdugan 《Journal of Polymer Science.Polymer Physics》1998,36(9):1423-1432
Blends from polyethylene (PE) and polystyrene-b-polybutadiene-b-polystyrene star block copolymers (SBS) (thermoplastic elastomers) are studied with polyolefine recycling in view. Each component, dispersed in a heterogeneous blend, exhibits a two-phase morphology. This structure is investigated by small angle X-ray scattering (SAXS) and compared to mechanical properties as a function of SBS grade and content. Increasing the SBS content one observes a vanishing PE-related long period reflection, while an SBS-related peak emerges. Best mechanical properties are obtained at concentrations, where the width of the observed long period reflection is broadest. For a quantitative analysis of the SAXS, the interface distribution function (IDF) analysis is employed. Data are fitted with a function modeling arrangement as well as disorder of domains inside the two different kinds of dispersed grains. The analysis yields that the observed broadening of the long spacing peak is caused only by an increased fluctuation of the thicknesses of amorphous layers in the PE stacks. This fluctuation again decreases for SBS concentrations beyond 15 wt %. The effect is strongest for blends containing the SBS grade with the lowest molecular weight and is discussed in terms of its compatibilization effect. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1423–1432, 1998 相似文献
77.
P. Molaro D. Reimers I. I. Agafonova S. A. Levshakov 《The European physical journal. Special topics》2008,163(1):173-189
The dispersal of individuals of a species is the key driving force
of various spatiotemporal phenomena which occur on geographical scales.
It can synchronise populations of interacting species, stabilise them,
and diversify gene pools [1-3].
The geographic spread of human infectious diseases such as influenza,
measles and the recent severe acute respiratory syndrome (SARS) is
essentially promoted by human travel which occurs on many length scales
and is sustained by a variety of means of
transportation [4-8].
In the light of increasing international trade, intensified human
traffic, and an imminent influenza A pandemic the knowledge of dynamical
and statistical properties of human dispersal is of fundamental importance
and acute [7,9,10].
A quantitative statistical theory for human travel and concomitant
reliable forecasts would substantially improve and extend existing
prevention strategies. Despite its crucial role, a quantitative assessment
of human dispersal remains elusive and the opinion that humans disperse
diffusively still prevails in many models [11].
In this chapter I will report on a recently developed technique which
permits a solid and quantitative assessment of human dispersal on
geographical scales [11].
The key idea is to
infer the statistical properties of human travel by analysing the
geographic circulation of individual bank notes for which comprehensive
datasets are collected at the online bill-tracking website www.wheresgeorge.com.
The analysis shows that the distribution of travelling distances decays
as a power law, indicating that the movement of bank notes is reminiscent
of superdiffusive, scale free random walks known as Lèvy flights [13].
Secondly, the probability of remaining in a small, spatially confined
region for a time T is dominated by heavy tails which attenuate
superdiffusive dispersal. I will show that the dispersal of bank notes
can be described on many spatiotemporal scales by a two parameter
continuous time random walk (CTRW) model to a surprising accuracy.
To this end, I will provide a brief introduction to continuous time
random walk theory [14]
and will show that human
dispersal is an ambivalent, effectively superdiffusive process. 相似文献
78.
Topographical and surface chemical characterization of nanosecond pulsed-laser micromachining of titanium at 532-nm wavelength 总被引:1,自引:0,他引:1
H. Reimers J. Gold B. Kasemo D. Chakarov 《Applied Physics A: Materials Science & Processing》2003,77(3-4):491-498
Electropolished titanium was micromachined by single, 5-ns pulses from a frequency-doubled (532 nm) Nd:YAG laser. The focal spot size was varied from 10 to 100 m and the applied fluences varied from the melting threshold (1 J/cm2) to more than 100 J/cm2. The resulting craters were imaged by optical microscopy, topographically characterized by interferometry and chemically characterized on the surface by small-spot depth-profiling Auger electron spectroscopy and small-spot X-ray photoelectron spectroscopy. The prevailing ablation regime for the studied fluences and focal spot sizes was found to be melt ejection. The surface chemical characterization showed growing oxide thickness in the heat-affected zone (around the center spot) for increasing fluence but no difference inside the crater. Titanium nitride formation was found inside the crater. PACS 81.65; 87.80Mj 相似文献
79.
80.