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11.
Nargiz B. Asanbaeva Larisa Yu. Gurskaya Yuliya F. Polienko Tatyana V. Rybalova Maxim S. Kazantsev Alexey A. Dmitriev Nina P. Gritsan Nadia Haro-Mares Torsten Gutmann Gerd Buntkowsky Evgeny V. Tretyakov Elena G. Bagryanskaya 《Molecules (Basel, Switzerland)》2022,27(10)
Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents. 相似文献
12.
13.
Mössbauer spectroscopy studies of151 Eu in EuBa2Cu3O7, EuBa2Cu3O6, EuBa2Cu3O6S, EuBaKCu3O7, EuBa2Cu2.5Fe0.5O7, Eu0.5Pr0.5Ba2Cu3O7, Eu1.25Ba1.75Cu3O7 and EuCaBaCu3O7 at several temperatures have been performed. The major conclusions are; All substitutions affect the Eu quadrupole interaction (10–20%). Substitution of 0 by S increases the Eu quadrupole interaction by 15% and increases η from 0.3 to 0.8 which proves that S substitutes oxygen in the immediate neighbourhood of the rare earth ions. In EuCaBaCu3O7 the quadrupole interaction is larger by 80% than in EuBa2Cu3O7, proving that in this structure Eu occupies predominantly the Ba sites. 相似文献
14.
15.
Mössbauer studies of the 21.6 keV transition of151Eu in cubic EuBe13 and Eu1/2La1/2Be13 in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe13 moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu1/2La1/2B13 the line moves from 0.50 mm/s at 100 K to –0.03 mm/s at 500 K. We conclude that Eu in EuxLa1–xB13 is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters Eexc, the interconfiguration excitation energy, Tf, the valence fluctuation temperature, and S3, the isomer shift of pure Eu3+ in EuBe13. Assuming S3–S2= 13 mm/s the analysis yields S3=0.70 mm/s, Tf=400 K and Eexc=3000 K for EuBe13, whereas Tf=460 K and Eexc=2750 K for Eu1/2La1/2B13. 相似文献
16.
X-ray diffraction, thermogravimetric analysis, magnetization and Mossbauer studies of a57Fe probe, in CaLaBa(Cu1−x
Fe
x
)3O
z
withz=7, 6.5 andx=0.00 to 0.05 have been performed. Forz=7,T
c drops from 74 K forx=0.00 to 35 K forx=0.05. In CaLaBa(Cu1−x
Fe
x
)3O
z
withz=7, 10% of the iron is magnetically ordered withH
eff=530 K kOe andT
N=400 K, even though the sample is superconducting. In the oxygen poor, non-superconducting samples (z=6.5) 20% of the iron is magnetically ordered withH
eff=340 kOe andT
N=340 K. Since the iron reflects the magnetic order of the Cu(2) ions, this may show that perhaps two inequivalent Cu(2) sites
exist in CaLaBa(Cu1−x
Fe
x
)3O7, ones of which is magnetically ordered. The experimental observations may be interpreted in terms of the special crystal
structure which can allow superconductivity and magnetism to coexist, probably in separate Cu(2) planes. 相似文献
17.
Mossbauer spectroscopy has recently been applied to study the new high Tc superconducting compounds RBa2Cu3Oz, using isotopes of rare earths mainly155Gd and151Eu, and57Fe, with different amounts of Fe ions replacing Cu. It was shown that magnetic moments on the rare earth site do not interfere
with superconductivity. Fe at low concentrations (<1%) was found to replace Cu mainly in the Cu(1) site, and the Mossbauer
spectra reveal different quadrupole doublets-fol lowing the different oxygen coordination around the Fe ion. The change of
the relative intensities of the different doublets with z can easily be followed. For higher iron concentrations, it seems
that increasing amounts of iron replace Cu in the Cu(2) site. For z<6.5, the iron reflects the magnetic ordering of Cu in
this site, and the ordering temperature as function of z can be obtained. The agreement between neutron diffraction and Mossbauer
measurements prove that Fe is a good probe for the magnetic behaviour of the Cu(2) ions. At low temperatures, Fe Mossbauer
spectra of Fe in the Cu(1) site are also magnetically broadened, for all z.
Superconducting-magnetic phase diagrams are also obtained in Y1−x Prx Ba2 Cu3Oz as function of x and z. For z=7.1, TN changes sharply with x. TN=300, 230 and 35 K for x=0.8, 0.6 and 0.4 respectively, whereas for z=6.1 TN changes very little with x.
Mossbauer measurements performed on 5 at %57Fe doped in CalaBaCu3Oz show that most of the la occupy the Ba site. For z=7 about half the iron in the Cu(2) sites are magnetically ordered, with
Heff=520 kOe and TN=400 K, even though the sample is superconducting with Tc=35 K. The possibility of coexistence between superconductivity and magnetic order in these systems will be discussed. 相似文献
18.
We conduct a case study in which we empirically illustrate the performance of different classes of Bayesian inference methods to estimate stochastic volatility models. In particular, we consider how different particle filtering methods affect the variance of the estimated likelihood. We review and compare particle Markov Chain Monte Carlo (MCMC), RMHMC, fixed-form variational Bayes, and integrated nested Laplace approximation to estimate the posterior distribution of the parameters. Additionally, we conduct the review from the point of view of whether these methods are (1) easily adaptable to different model specifications; (2) adaptable to higher dimensions of the model in a straightforward way; (3) feasible in the multivariate case. We show that when using the stochastic volatility model for methods comparison, various data-generating processes have to be considered to make a fair assessment of the methods. Finally, we present a challenging specification of the multivariate stochastic volatility model, which is rarely used to illustrate the methods but constitutes an important practical application. 相似文献
19.
A numerical study is carried out to compare the two-dimensional (2-D) case and three-dimensional (3-D) case for the modelling of an ion-exchanged glass waveguide. It is shown that different waveguide widths on the photomask correspond to different ion concentration distributions after an annealing process. A numerical example is presented of two waveguide sections with different widths indicates that due to the abrupt change of the waveguide width, a 3-D theoretical model is required for an accurate prediction of the parameters of ion-exchanged glass waveguides. The good agreement between the modelled and measured results proves that the developed 3-D numerical model can be beneficially utilized in the generalized design of optical devices based on ion-exchange waveguides. 相似文献
20.
Using a small-core single-mode fiber (SCSMF), a novel relative humidity (RH) sensor based on an SMF28-SCSMF-SMF28 fiber structure was proposed in this paper. By depositing a humidity sensitive material, such as poly (ethylene oxide) (PEO) on the bare SCSMF fiber, the proposed structure can act as an RH sensor with high sensitivity. Experiments demonstrated that the proposed RH sensor with PEO coating can achieve a sensitivity of 430 nm per relative humidity unit (RHU) in the RH range from 80% to 83% RH and a sensitivity of 50 nm per RHU in the RH range from 83% to 95% RH. 相似文献