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排序方式: 共有301条查询结果,搜索用时 31 毫秒
121.
We extend a result by Fornaæss and Wiegerinck [Ark. Mat. 1989;27:257–272] on plurisubharmonic Mergelyan type approximation to domains with boundaries locally given by graphs of continuous functions. 相似文献
122.
Brauer B Dubnikova F Zeiri Y Kosloff R Gerber RB 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2008,71(4):1438-1445
The vibrational spectrum of triacetone triperoxide (TATP) is studied by the correlation-corrected vibrational self-consistent field (CC-VSCF) method which incorporates anharmonic effects. Fundamental, overtone, and combination band frequencies are obtained by using a potential based on the PM3 method and yielding the same harmonic frequencies as DFT/cc-pVDZ calculations. Fundamentals and overtones are also studied with anharmonic single-mode (without coupling) DFT/cc-pVDZ calculations. Average deviations from experiment are similar for all methods: 2.1-2.5%. Groups of degenerate vibrations form regions of numerous combination bands with low intensity: the 5600-5800 cm(-1) region contains ca. 70 overtones and combinations of CH stretches. Anharmonic interactions are analyzed. 相似文献
123.
Recombination events of a proton with NO3- at (H2O)8 clusters are studied by molecular dynamics, using "on-the-fly" reliable ab initio MP2 potentials. The main findings are: (1) the lifetime of the ions is less than 1.2 picoseconds; (2) the recombination step invariably involves H3O+, not H5O2+; and (3) an essentially unique transition-state structure of H3O+/NO3- for recombination is found in all cases. Proton migration involves both H3O+ and H5O2+ species: Grotthuss and other mechanisms contribute. 相似文献
124.
125.
Khriachtchev L Isokoski K Cohen A Räsänen M Gerber RB 《Journal of the American Chemical Society》2008,130(19):6114-6118
A novel noble-gas compound, HXeOXeH, is identified using IR spectroscopy, and it seems to be the smallest known neutral molecule with two noble-gas atoms. HXeOXeH is prepared using, for example, UV photolysis of water in solid xenon and subsequent annealing at 40-45 K. The experimental observations are fully supported by extensive quantum chemical calculations. A large energy release of 8.3 eV is computationally predicted for the decomposition of HXeOXeH into the 2Xe + H2O global energy minimum. HXeOXeH may represent a first step toward the possible preparation of (Xe-O)n chains and it may be relevant to the terrestrial "missing xenon" problem. 相似文献
126.
A rapid and accurate method for measuring low part-per-million levels of free and reversibly-bound sulfites in selected foods by using high-performance liquid chromatography (HPLC) with fluorometric detection was developed. Sulfites were extracted with sodium tetrachloromercurate solution and determined by HPLC-fluorescence spectrometry. During the HPLC analysis, the sample extract was reacted with sodium hydroxide to liberate the reversibly-bound sulfite and subsequently separated from other interferences by a size exclusion column. The effluent was then reacted with o-phthalaldehydelammonium chloride reagent to form a fluorescing 1-sulfonatoisoindole derivative and analyzed by a fluorescence detector. The method has been applied to a variety of foods and food products, with no significant interference encountered in matrixes such as soy products, cabbage, broccoli, brassica, ginger, fungus, mushroom, mandarin peel, potato chips, and biscuits. It was shown to have a broad linear range of 0.01 to 0.4 mg/L as SO2. The spike recoveries of sodium sulfite, sodium metabisulfite, and formaldehyde-sodium bisulfite adduct at the 5 to 10 mg/kg level in the tested food matrix were within 80-120%, and the limit of detection was 5 mg/kg. Laboratory of Government Chemist Reference Material LGC7111 (potato powder) was used to assess the accuracy of the method. The expanded measurement uncertainty of the method at 95% confidence level was estimated to be 17%. 相似文献
127.
Kjaergaard HG Garden AL Chaban GM Gerber RB Matthews DA Stanton JF 《The journal of physical chemistry. A》2008,112(18):4324-4335
We have calculated frequencies and intensities of fundamental and overtone vibrational transitions in water and water dimer with use of different vibrational methods. We have compared results obtained with correlation-corrected vibrational self-consistent-field theory and vibrational second-order perturbation theory both using normal modes and finally with a harmonically coupled anharmonic oscillator local mode model including OH-stretching and HOH-bending local modes. The coupled cluster with singles, doubles, and perturbative triples ab initio method with augmented correlation-consistent triple-zeta Dunning and atomic natural orbital basis sets has been used to obtain the necessary potential energy and dipole moment surfaces. We identify the strengths and weaknesses of these different vibrational approaches and compare our results to the available experimental results. 相似文献
128.
Let
be the 2k-uniform hypergraph obtained by letting P1, . . .,Pr be pairwise disjoint sets of size k and taking as edges all sets Pi∪Pj with i ≠ j. This can be thought of as the ‘k-expansion’ of the complete graph Kr: each vertex has been replaced with a set of size k. An example of a hypergraph with vertex set V that does not contain
can be obtained by partitioning V = V1 ∪V2 and taking as edges all sets of size 2k that intersect each of V1 and V2 in an odd number of elements. Let
denote a hypergraph on n vertices obtained by this construction that has as many edges as possible. For n sufficiently large we prove a conjecture of Frankl, which states that any hypergraph on n vertices that contains no
has at most as many edges as
.
Sidorenko has given an upper bound of
for the Tur′an density of
for any r, and a construction establishing a matching lower bound when r is of the form 2p+1. In this paper we also show that when r=2p+1, any
-free hypergraph of density
looks approximately like Sidorenko’s construction. On the other hand, when r is not of this form, we show that corresponding constructions do not exist and improve the upper bound on the Turán density
of
to
, where c(r) is a constant depending only on r.
The backbone of our arguments is a strategy of first proving approximate structure theorems, and then showing that any imperfections
in the structure must lead to a suboptimal configuration. The tools for its realisation draw on extremal graph theory, linear
algebra, the Kruskal–Katona theorem and properties of Krawtchouck polynomials.
* Research supported in part by NSF grants DMS-0355497, DMS-0106589, and by an Alfred P. Sloan fellowship. 相似文献
129.
Manley S Davidovitch B Davies NR Cipelletti L Bailey AE Christianson RJ Gasser U Prasad V Segre PN Doherty MP Sankaran S Jankovsky AL Shiley B Bowen J Eggers J Kurta C Lorik T Weitz DA 《Physical review letters》2005,95(4):048302
Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure. 相似文献
130.