全文获取类型
收费全文 | 24432篇 |
免费 | 651篇 |
国内免费 | 141篇 |
专业分类
化学 | 16725篇 |
晶体学 | 123篇 |
力学 | 590篇 |
数学 | 4346篇 |
物理学 | 3440篇 |
出版年
2022年 | 177篇 |
2021年 | 259篇 |
2020年 | 301篇 |
2019年 | 317篇 |
2018年 | 233篇 |
2017年 | 194篇 |
2016年 | 483篇 |
2015年 | 453篇 |
2014年 | 491篇 |
2013年 | 1302篇 |
2012年 | 1139篇 |
2011年 | 1430篇 |
2010年 | 727篇 |
2009年 | 708篇 |
2008年 | 1259篇 |
2007年 | 1285篇 |
2006年 | 1251篇 |
2005年 | 1211篇 |
2004年 | 1054篇 |
2003年 | 911篇 |
2002年 | 817篇 |
2001年 | 349篇 |
2000年 | 279篇 |
1999年 | 249篇 |
1998年 | 277篇 |
1997年 | 307篇 |
1996年 | 361篇 |
1995年 | 240篇 |
1994年 | 280篇 |
1993年 | 262篇 |
1992年 | 239篇 |
1991年 | 240篇 |
1990年 | 192篇 |
1989年 | 234篇 |
1988年 | 237篇 |
1987年 | 201篇 |
1986年 | 198篇 |
1985年 | 292篇 |
1984年 | 336篇 |
1983年 | 218篇 |
1982年 | 369篇 |
1981年 | 336篇 |
1980年 | 316篇 |
1979年 | 317篇 |
1978年 | 322篇 |
1977年 | 292篇 |
1976年 | 280篇 |
1975年 | 251篇 |
1974年 | 248篇 |
1973年 | 236篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Robert A. Tromans Soumen K. Samanta Andy M. Chapman Anthony P. Davis 《Chemical science》2020,11(12):3223
Glucose is a key biomedical analyte, especially relevant to the management of diabetes. Current methods for glucose determination rely on the enzyme glucose oxidase, requiring specialist instrumentation and suffering from redox-active interferents. In a new approach, a powerful and highly selective achiral glucose receptor is mixed with a sample, l-glucose is added, and the induced CD spectrum is measured. The CD signal results from competition between the enantiomers, and is used to determine the d-glucose content. The involvement of l-glucose doubles the signal range from the CD spectrometer and allows sensitivity to be adjusted over a wide dynamic range. It also negates medium effects, which must be equal for both enantiomers. The method has been demonstrated with human serum, pre-filtered to remove proteins, giving results which closely match the standard biochemical procedures, as well as a cell culture medium and a beer sample containing high (70 mM) and low (0.4 mM) glucose concentrations respectively.A highly selective receptor, circular dichroism and chiral competition are combined in this versatile method for d-glucose analysis. 相似文献
962.
Silbernagel KM Jechorek RP Kaufer AL Johnson RL Aleo V Brown B Buen M Buresh J Carson M Franklin J Ham P Humes L Husby G Hutchins J Jechorek R Jenkins J Kaufer A Kexel N Kora L Lam L Lau D Leighton S Loftis M Luc S Martin J Nacar I Nogle J Park J Schultz A Seymore D Smith C Smith J Thou P Ulmer M Voss R Weaver V 《Journal of AOAC International》2005,88(3):750-760
A multilaboratory study was conducted to compare the VIDAS LIS immunoassay with the standard cultural methods for the detection of Listeria in foods using an enrichment modification of AOAC Official Method 999.06. The modified enrichment protocol was implemented to harmonize the VIDAS LIS assay with the VIDAS LMO2 assay. Five food types--brie cheese, vanilla ice cream, frozen green beans, frozen raw tilapia fish, and cooked roast beef--at 3 inoculation levels, were analyzed by each method. A total of 15 laboratories representing government and industry participated. In this study, 1206 test portions were tested, of which 1170 were used in the statistical analysis. There were 433 positive by the VIDAS LIS assay and 396 positive by the standard culture methods. A Chi-square analysis of each of the 5 food types, at the 3 inoculation levels tested, was performed. The resulting average Chi square analysis, 0.42, indicated that, overall, there are no statistical differences between the VIDAS LIS assay and the standard methods at the 5% level of significance. 相似文献
963.
dl-β-Bulnesene (1) and dl-1-epi-α-bulnesene (15) have been synthesized starting from the bromide 4 (Schemes 2 and 3). In the key step 9→10 the bonds of the final product were formed by an intramolecular photoaddition. The synthesis was completed by the fragmentation 12→14 and the Wittig reaction 14→15+1 . 相似文献
964.
The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences are applied in order to acquire the first solid-state 39K NMR spectra of organometallic complexes, the polymeric main group metallocenes cyclopentadienyl potassium (CpK) and pentamethylcyclopentadienyl potassium (Cp*K). Piecewise QCPMG NMR techniques are used to acquire a high S/N 39K spectrum of the broad central transition of Cp*K, which is ca. 200 kHz in breadth. Analytical and numerical simulations indicate that there is a significant quadrupolar interaction present at both potassium nuclei (C(Q)(39K) = 2.55(6)/2.67(8) MHz and 4.69(8) MHz for CpK (static/MAS) and Cp*K, respectively). Experimental quadrupolar asymmetry parameters suggest that both structures are bent about the potassium atoms (eta(Q)(39K) = 0.28(3)/0.29(3) for CpK (static/MAS) and eta(Q)(39K) = 0.30(3) for Cp*K). Variable-temperature (VT) 39K NMR experiments on CpK elucidate temperature-dependent changes in quadrupolar parameters which can be rationalized in terms of alterations of bond distances and angles with temperature. 13C CP/MAS NMR experiments are conducted upon both samples to quantify the carbon chemical shielding anisotropy (CSA) at the Cp' ring carbon atoms. Ab initio carbon CSA and 39K electric-field gradient (EFG) and CSA calculations are conducted and discussed for the CpK complex, in order to correlate the experimental NMR parameters with molecular structure in CpK and Cp*K. 39K DFS/QCPMG and 13C CP/MAS experiments prove invaluable for probing molecular structure, temperature-dependent structural changes, and the presence of impurities in these systems. 相似文献
965.
A high-pressure liquid chromatographic assay procedure has been developed for verapamil in blood or plasma. A paired-ion solvent system with a reversed-phase column is employed. The procedure is specific for verapamil and the retention times of the major metabolites are identified. This procedure is sensitive to a lower blood concentration of 1 ng/ml and standard curves were found to be linear up to the highest concentration tested, 500 ng/ml. Several drugs were tested for interference with the assay, but none were found to cause any problems. The procedure is simple, rapid and permits the analysis of up to 25 samples per day. 相似文献
966.
Parameters are developed for a practical application of the empirical van der Waals (vdW) correction infrastructure available in the CPMD density functional theory (DFT) code. The binding energy, geometry, and potential energy surface (PES) are examined for methane, ethane, ethylene, formaldehyde, ammonia, three benzene dimer geometries, and three benzene–water geometries. The vdW corrected results compare favorably with MP2 and CCSD(T) calculations near the complete basis set limits, and with experimental results where they are available. 相似文献
967.
Robert M. Burton Jr. 《Probability Theory and Related Fields》1979,47(2):205-212
Summary Let GZn be a group of measure preserving transformations of a Lebesgue space. J. P. Conze [1] has developed an entropy theory for such groups and described a class of groups obeying a form of the Kolmogorov zero-one law called K-groups. A Bernoulli group is a group isomorphic to the group of translates (shifts) of elements of the space
with product measure where X
g
=X is a probability space. Bernoulli groups are also K-groups. Katznelson and Weiss [3] have shown entropy is a complete invariant for isomorphism classes of Bernoulli groups. We give an asymptotic definition of K-groups in terms of finite -algebras and justify this definition in terms of entropy and Conze's formulation. This definition s used to help us construct a K-group GZ
n
that is completely non-Bernoulli, that is one that contains no Bernoulli subgroup. 相似文献
968.
Summary Conventional numerical methods, when applied to the ordinary differential equations of motion of classical mechanics, conserve the total energy and angular momentum only to the order of the truncation error. Since these constants of the motion play a central role in mechanics, it is a great advantage to be able to conserve them exactly. A new numerical method is developed, which is a generalization to arbitrary order of the discrete mechanics described in earlier work, and which conserves the energy and angular momentum to all orders. This new method can be applied much like a corrector as a modification to conventional numerical approximations, such as those obtained via Taylor series, Runge-Kutta, or predictor-corrector formulae. The theory is extended to a system of particles in Part II of this work. 相似文献
969.
Robert Schumacher 《manuscripta mathematica》1976,19(2):133-142
Cheney and others [2] have shown that, with respect to the norm of uniform convergence, the Fourier operator Fn:C2pn is the only minimal projection. For a more detailed study of operators A:C2Pn we investigate the evaluation functionals
respectively the mean of their norms
. We give a complete characterization of polynomial operators which minimize this quantity. As an application we can simplify the proof in [2]. Moreover, we show that the trigonometric interpolation operators, having the above minimal-property,are exactly those with equidistant nodes.
Diese Arbeit enthält die wesentlichen Resultate einer Diplomarbeit, die der Verfasser an der Eberhard-Karls-Universität Tübingen unter der Anleitung von A.Schönhage angefertigt hat. 相似文献
Diese Arbeit enthält die wesentlichen Resultate einer Diplomarbeit, die der Verfasser an der Eberhard-Karls-Universität Tübingen unter der Anleitung von A.Schönhage angefertigt hat. 相似文献
970.
Hans Robert Müller 《Journal of Geometry》1976,8(1-2):1-7
A generalisation of a ruled surface in n-dimensional euclidean space may be generated by euclidean motion of a s-plane As. For this one-parametric family {As} the curve of striction is defined and the following theorems are proved:
- The generators As are parallel along the curve of striction, i.e. the multivectors representing As form a parallel vector field along the curve of striction.
- If the curve of striction is geodesic on {As}, it is also an isogonal trajectory of the family of generators {As}.