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1.
Using dyes of known redox potentials the specific mechanisms of dark and light potential generation is analyzed in pigmented lipid membranes. The role of the ionic and electronic conductance, as well as the redox potential gradient is specifically related to the observed open circuit voltage developed across the membrane. The results can be most easily explained by the redox electrode model. 相似文献
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3.
The temperature dependence of the paramagnetic susceptibility χm(T) taken in 2500 Oe, the resistivity ρ(T), and the thermoelectric power α(T) of DyBaCo2O5+x, which has Ba and Dy ordered into alternate (001) planes of an oxygen-deficient perovskite, have revealed a phase segregation in the compositional range 0.3?x<0.5. Orthorhombic DyBaCo2O5.51 has, in addition, oxygen vacancies ordered into alternate rows of the DyO0.51 (001) planes; a cold-pressed polycrystalline sample exhibits a first-order insulator-metal transition at TIM=320 K, a Curie temperature TC=300 K, and a broadened metamagnetic transition temperature TM≈265 K in 2500 Oe. A ferromagnetic M-H hysteresis curve fails to saturate at 5 T, and a minority ferromagnetic phase below TM has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in the DyBaCo2O5.5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power α(T)>0 of DyBaCo2O5.51 changing continuously across TIM is interpreted to manifest a metallic minority phase crossing a percolation threshold; α(T) also provides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above TIM. A previous model of the RBaCo2O5.5 phase is extended to account for the Ising spin configuration below TC, the magnetic order in the presence of higher-spin octahedral-site Co(III), and the α(T) data. 相似文献
4.
The range of chemical flexibilities of the hexagonal frameworks (Ta6Si4O26)6? and (Ta14Si4O47)8? have been partially explored. This has been done with high-temperature preparations as in general ionic mobilities in these frameworks are too low to permit low-temperature ion exchange. Ionic site potential calculations indicate that preferential site-occupancy factors as well as geometric constraints are responsible for the absence of ionic motion. New phases K6?xNaxTa6Si4O26 (x ? 4), K8?xNaxTa14Si4O47 (x ? 5), and impure Ba3?xNa2xTa6Si4O26 have been prepared. Introduction of up to 2 moles of Li+ and 1 mole of Mg2+ ions per formula unit into sites of the framework not normally occupied has been demonstrated as well as the possibility of partially substituting Zr4+ for Ta5+ ions. Substitutions designed to introduce large tunnel vacancies in the presence of only monovalent K+ or Na+ ions (P for Si, W for Ta and F for O) generally proved unsuccessful. Competitive phases also frustrated attempts to substitute either the larger Rb+ or the smaller Li+ ions into the large-tunnel sites. A large area of solid solution was discovered in the BaONa2OTa2O5 phase diagram; it has a (TaO3)-framework with the structure of tetragonal potassium tungsten bronze. 相似文献
5.
The room-temperature diffuse-reflectance spectra of compositions within the Li1+xTi2?xO4 spinel system () show three absorption bands in the range 4000 to 48,000 cm?1. Two high-energy absorption bands correspond to charge-transfer transitions from the oxygen-2p valence band to the titanium t2g and σ1 conduction bands, where the σ1 band of eg character has hybridized titanium-3d and titanium-4s parentage. The absorption band arising from promotion of electrons to the empty σ1 band does not alter with composition whereas the absorption band arising from promotion of electrons to the partially filled t2g band narrows as the concentration of conduction electrons in the t2g band decreases. These two high-energy absorption bands fall entirely within the ultraviolet spectral region, and the absorption edge in occurs at 24,300 cm?1 (3.02 eV). A low-energy absorption band is observed in compositions with and in samples of reduced in hydrogen at elevated temperatures. This band straddles the boundary between the visible and infrared spectral regions and shifts toward lower energy as the concentration of conduction electrons in the t2g band decreases. The possible origins of the band are discussed; the argument is in favor of a d-d interband transition from states in the partially filled t2g band to states in the empty σ1 band. 相似文献
6.
In this paper we describe the stereoselective synthesis of functionalized lactam 7 via two enantiospecific piperidine-forming techniques and its employment in a general synthetic approach to Nuphar alkaloids. Specifically, the formation of piperidine 18 by formal [3 + 3] cycloaddition and stepwise annelation processes is described; the latter technique was found to be significantly more efficient than the Pd-catalyzed TMM addition process. Finally, exploitation of the exocyclic alkene installed in the piperidine-forming reaction in the transformation of 18 to (-)-deoxynupharidine ((-)-2), (-)-castoramine ((-)-3), and (-)-nupharolutine ((-)-4) via intermediate lactam 7 is delineated. 相似文献
7.
Electron spin resonance (ESR) and magnetic-susceptibility measurements on the Li1+xTi2?xO4 spinel system () indicate the presence of two types of localized moments in this material. In both cases, an unpaired electron is trapped as a Ti3+ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component. This type of crystal field cannot arise in the stoichiometric spinel. We propose two types of defect in the title spinel system: an oxygen vacancy and a hydroxyl ion. Unpaired electrons are trapped as Ti3+ ions adjacent to these defects, and it is argued that the strong tetragonal field is associated with the formation of a static (TiO)+ ion by a displacement of the titanium ion from the defect. Spin relaxation occurs via a thermal ionization of the trapped electron that appears to be associated with a static-dynamic transition in the titanium-ion displacement. 相似文献
8.
A. Ramdani C. Gleitzer G. GavoilleA. K. CheethamJ. B. Goodenough 《Journal of solid state chemistry》1985,60(3)
The distribution of d electrons over the cations in MoFe2O4, which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactionsFe2+B+Fe3+A+Mo3+Fe3+B+Fe2+A+Mo4+We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; FeA[MoBFeB]O4. K-shell absorption and Mössbauer data at T = 423 K > Tc demonstrate that the iron has an average valence near 2.5+ with fast electron transfer (τh < 10−8 sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant CM = 7.95 ± 0.2 emu/mole and a Weiss constant θp = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe2O4 and CrFe2O4 where mixed Fe3+/2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe2O4 point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τh < 10−8 sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system. 相似文献
9.
Jiaxi Zhang Longhai Zhang Li Du Huolin L. Xin John B. Goodenough Zhiming Cui 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(40):17641-17646
A group of newly reported antiperovskite nitrides CuxIn1−xNNi3 (0≤x≤1) with tunable composition are employed as electrocatalysts for the hydrogen evolution reaction (HER). Cu0.4In0.6NNi3 shows the highest intrinsic performance among all developed catalysts with an overpotential of merely 42 mV at 10 mA cmgeo−2. Stability tests at a high current density of 100 mA cmgeo−2 show its super-stable performance with only 7 mV increase in overpotential after more than 60 hours of measurement, surpassing commercial Pt/C (increase of 170 mV). By partial substitution, the derived antiperovskite nitride achieves a smaller kinetic barrier of water dissociation compared to the unsubstituted InNNi3 and CuNNi3, revealed by first-principle calculations. It is found that the partially substituted CuxIn1−xNNi3 possesses a thermal neutral and desirable Gibbs free energy of hydrogen for HER, ascribed to the tailoring of the energy of d-band center arose by the A-site (A=Cu or In) substitution and a resulting optimization of adsorbate interactions. 相似文献
10.
Bentian Zhang Gengtao Fu Yutao Li Lecheng Liang Nicholas S. Grundish Yawen Tang John B. Goodenough Zhiming Cui 《Angewandte Chemie (International ed. in English)》2020,59(20):7857-7863
Controllable synthesis of atomically ordered intermetallic nanoparticles (NPs) is crucial to obtain superior electrocatalytic performance for fuel cell reactions, but still remains arduous. Herein, we demonstrate a novel and general hydrogel‐freeze drying strategy for the synthesis of reduced graphene oxide (rGO) supported Pt3M (M=Mn, Cr, Fe, Co, etc.) intermetallic NPs (Pt3M/rGO‐HF) with ultrasmall particle size (about 3 nm) and dramatic monodispersity. The formation of hydrogel prevents the aggregation of graphene oxide and significantly promotes their excellent dispersion, while a freeze‐drying can retain the hydrogel derived three‐dimensionally (3D) porous structure and immobilize the metal precursors with defined atomic ratio on GO support during solvent sublimation, which is not afforded by traditional oven drying. The subsequent annealing process produces rGO supported ultrasmall ordered Pt3M intermetallic NPs (≈3 nm) due to confinement effect of 3D porous structure. Such Pt3M intermetallic NPs exhibit the smallest particle size among the reported ordered Pt‐based intermetallic catalysts. A detailed study of the synthesis of ordered intermetallic Pt3Mn/rGO catalyst is provided as an example of a generally applicable method. This study provides an economical and scalable route for the controlled synthesis of Pt‐based intermetallic catalysts, which can pave a way for the commercialization of fuel cell technologies. 相似文献