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排序方式: 共有662条查询结果,搜索用时 15 毫秒
51.
A highly enantioselective catalytic route to carbamate- and benzoate-protected β-amino aldehydes and β-amino acids is presented. The amino acid-catalyzed one-pot asymmetric reaction between unmodified aldehydes and α-amido sulfones gives the corresponding β-amino compounds with up to 95:5 dr and 97->99% ee. 相似文献
52.
Andrea Amantonico Pawel L. Urban Renato Zenobi 《Analytical and bioanalytical chemistry》2010,398(6):2493-2504
Single-cell metabolomics is an emerging field that addresses fundamental biological questions and allows one to observe metabolic
phenomena in heterogeneous populations of single cells. In this review, we assess the suitability of different detection techniques
and present considerations on sample preparation for single-cell metabolomics. Although targeted analysis of single cells
can readily be conducted using fluorescent probes and optical instruments (microscopes, fluorescence detectors), a comprehensive
metabolomic approach requires a powerful label-free method, such as mass spectrometry (MS). Mass-spectrometric techniques
applied to study small molecules in single cells include electrospray MS, matrix-assisted laser desorption/ionization MS,
and secondary ion MS. Sample preparation is an important aspect to be taken into account during further development of methods
for single-cell metabolomics. 相似文献
53.
Dinh Chau N Dulinski M Jodlowski P Nowak J Rozanski K Sleziak M Wachniew P 《Isotopes in environmental and health studies》2011,47(4):415-437
The issue of natural radioactivity in groundwater is reviewed, with emphasis on those radioisotopes which contribute in a significant way to the overall effective dose received by members of the public due to the intake of drinking water originating from groundwater systems. The term 'natural radioactivity' is used in this context to cover all radioactivity present in the environment, including man-made (anthropogenic) radioactivity. Comprehensive discussion of radiological aspects of the presence of natural radionuclides in groundwater, including an overview of current regulations dealing with radioactivity in drinking water, is provided. The presented data indicate that thorough assessments of the committed doses resulting from the presence of natural radioactivity in groundwater are needed, particularly when such water is envisaged for regular intake by infants. They should be based on a precise determination of radioactivity concentration levels of the whole suite of radionuclides, including characterisation of their temporal variability. Equally important is a realistic assessment of water intake values for specific age groups. Only such an evaluation may provide the basis for possible remedial actions. 相似文献
54.
Nambu Y Zhao LL Morosan E Kim K Kotliar G Zajdel P Green MA Ratcliff W Rodriguez-Rivera JA Broholm C 《Physical review letters》2011,106(3):037201
Magnetism in the orthorhombic metal CaFe(4)As(3) was examined through neutron diffraction for powder and single crystalline samples. Incommensurate [q(m) ≈ (0.37-0.39) × b*] and predominantly longitudinally (|| b) modulated order develops through a 2nd order phase transition at TN = 89.63(6) K with a 3D Heisenberg-like critical exponent β = 0.365(6). A 1st order transition at T2 = 25.6(9) K is associated with the development of a transverse component, locking q(m) to 0.375(2)b*, and increasing the moments from 2.1(1) to 2.2(3) μ B for Fe2+ and from 1.3(3) to 2.4(4) μB for Fe+. The ab initio Fermi surface is consistent with a nesting instability in cross-linked FeAs strips. 相似文献
55.
Stratified materials are of great importance for many branches of modern industry, e.g. electronics or optics and for biomedical applications. Examination of chemical composition of individual layers and determination of their thickness helps to get information on their properties and function. A confocal 3D micro X‐ray fluorescence (3D µXRF) spectroscopy is an analytical method giving the possibility to investigate 3D distribution of chemical elements in a sample with spatial resolution in the micrometer regime in a non‐destructive way. Thin foils of Ti, Cu and Au, a bulk sample of Cu and a three‐layered sandwich sample, made of two thin Fe/Ni alloy foils, separated by polypropylene, were used as test samples. A Monte Carlo (MC) simulation code for the determination of elemental concentrations and thickness of individual layers in stratified materials with the use of confocal 3D µXRF spectroscopy was developed. The X‐ray intensity profiles versus the depth below surface, obtained from 3D µXRF experiments, MC simulation and an analytical approach were compared. Correlation coefficients between experimental versus simulated, and experimental versus analytical model X‐ray profiles were calculated. The correlation coefficients were comparable for both methods and exceeded 99%. The experimental X‐ray intensity profiles were deconvoluted with iterative MC simulation and by using analytical expression. The MC method produced slightly more accurate elemental concentrations and thickness of successive layers as compared to the results of the analytical approach. This MC code is a robust tool for simulation of scanning confocal 3D µXRF experiments on stratified materials and for quantitative interpretation of experimental results. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
56.
Samiha SalmaouiFaouzi Sediri Néji GharbiChristian Perruchot Salah AeiyachIwona A. Rutkowska Pawel J. KuleszaMohamed Jouini 《Applied Surface Science》2011,257(19):8223-8229
Tungsten trioxide, unhydrated with hexagonal structure (h-WO3), has been prepared by hydrothermal method at a temperature of 180 °C in acidified sodium tungstate solution. Thus prepared h-WO3 has been characterized by X-ray diffraction (XRD) method and using electrochemical techniques. The morphology has been examined by scanning and transmission electron microscopies (SEM and TEM) and it is consistent with existence of nanorods of 50-70 nm diameter and up to 5 μm length. Cyclic voltammetric characterization of thin films of h-WO3 nanorods has revealed reversible redox behaviour with charge-discharge cycling corresponding to the reversible lithium intercalation/deintercalation into the crystal lattice of the h-WO3 nanorods. In propylene carbonate containing LiClO4, two successive redox processes of hexagonal WO3 nanorods are observed at the scan rate of 50 mV/s. Such behaviour shall be attributed to the presence of at least two W atoms of different surroundings in the lattice structure of h-WO3 nanorods. On the other hand, in aqueous LiClO4 solution, only one redox process is observed at the scan rate of 10 mV/s. The above observations can be explained in terms of differences in the diffusion of ions inside two types of channel cavities existing in the structure of the h-WO3 nanorods. Moreover, the material can be applied as active support for the catalytic bi-metallic Pt-Ru nanoparticles during electrooxidation of ethanol in acid medium (0.5 mol dm−3 H2SO4). 相似文献
57.
Pawel Sosna 《Applied Categorical Structures》2014,22(1):211-227
Given a triangulated category ${{\mathcal T}}$ over a field K and a field extension L/K, we investigate how one can construct a triangulated category ${{\mathcal T}}_L$ over L. Our approach produces the derived category of the base change scheme X L if ${{\mathcal T}}$ is the bounded derived category of a smooth projective variety over K and the field extension is finite and Galois. We also investigate how the dimension of a triangulated category behaves under scalar extensions. 相似文献
58.
Pawel Kröger 《Journal of Theoretical Probability》1990,3(4):515-531
We prove comparison theorems for diffusion processes onR
d. From these theorems we derive lower and upper bounds for the transition probabilities of a diffusion process. In contrast to the known estimates for fundamental solutions of parabolic equations our bounds do not depend on the moduli of continuity of the coefficients of the differential operator. 相似文献
59.
Pawel Kowalczyk Shunji Kasahara Md.Humayun Kabir Hajime Kat 《Journal of Molecular Spectroscopy》2003,220(2):162-169
The E(4) 1Πu←X 1Σ+g band system of the K2 molecule is investigated by the technique of Doppler-free optical–optical double resonance polarization spectroscopy. The observed vibrational levels v=0 to 9 of the E state are subject to numerous rotational perturbations by the neighboring 4 3Πu, 4 1Σ+u, and 5 3Σ+u electronic states. By using deperturbation methods, the potential curves of the 4 1Πu and 4 3Πu states are determined as well as some properties of the 4 1Σ+u and 5 3Σ+u state potentials. The results are compared with predictions of ab initio calculations. 相似文献
60.