全文获取类型
收费全文 | 12011篇 |
免费 | 1416篇 |
国内免费 | 1034篇 |
专业分类
化学 | 8627篇 |
晶体学 | 119篇 |
力学 | 502篇 |
综合类 | 110篇 |
数学 | 1574篇 |
物理学 | 3529篇 |
出版年
2024年 | 30篇 |
2023年 | 189篇 |
2022年 | 303篇 |
2021年 | 336篇 |
2020年 | 384篇 |
2019年 | 375篇 |
2018年 | 305篇 |
2017年 | 318篇 |
2016年 | 564篇 |
2015年 | 514篇 |
2014年 | 644篇 |
2013年 | 864篇 |
2012年 | 998篇 |
2011年 | 1040篇 |
2010年 | 691篇 |
2009年 | 595篇 |
2008年 | 709篇 |
2007年 | 620篇 |
2006年 | 536篇 |
2005年 | 441篇 |
2004年 | 403篇 |
2003年 | 356篇 |
2002年 | 379篇 |
2001年 | 306篇 |
2000年 | 202篇 |
1999年 | 235篇 |
1998年 | 155篇 |
1997年 | 127篇 |
1996年 | 148篇 |
1995年 | 115篇 |
1994年 | 116篇 |
1993年 | 106篇 |
1992年 | 96篇 |
1991年 | 91篇 |
1990年 | 63篇 |
1989年 | 63篇 |
1988年 | 52篇 |
1987年 | 49篇 |
1986年 | 50篇 |
1985年 | 51篇 |
1984年 | 37篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1968年 | 40篇 |
1965年 | 42篇 |
1964年 | 45篇 |
1962年 | 43篇 |
1961年 | 36篇 |
1960年 | 32篇 |
1959年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Building a calibration model with detection and quantification capabilities is identical to the task of building a regression model. Although commonly used by analysts, an application of the calibration model requires at first careful attention to the three components of the regression triplet (data, model, method), examining (a) the data quality of the proposed model; (b) the model quality; (c) the LS method to be used or a fulfillment of all least-squares assumptions. This paper summarizes these components, describes the effects of deviations from assumptions and considers the correction of such deviations: identifying influential points is the first step in least-squares model building, the calibration task depends on the regression model used, and finally the least squares LS method is based on assumptions of normality of errors, homoscedasticity, independence of errors, overly influential data points and independent variables being subject to error. When some assumptions are violated, the ordinary LS is inconvenient and robust M-estimates with the iterative method of reweighted least-squares must be used. The effects of influential points, heteroscedasticity and non-normality on the calibration precision limits are also elucidated. This paper also considers the proper construction of the statistical uncertainty expressed as confidence limits predicting an unknown concentration (or amount) value, and its dependence on the regression triplet. The authors' objectives were to provide a thorough treatment that includes pertinent references, consistent nomeclature, and related mathematical formulae to show by theory and illustrative examples those approaches best suited to typical problems in analytical chemistry. Two new algorithms, calibration and linear regression written in s-plus and enabling regression triplet analysis, the estimation of calibration precision limits, critical levels, detection limits and quantification limits with the statistical uncertainty of unknown concentrations, form the goal of this paper. 相似文献
992.
993.
[reaction: see text] Mucohalic acids (mucochloric acid (1, 3,4-dichloro-5-hydroxy-5H-furan-2-one) and mucobromic acid (2, 3,4-dibromo-5-hydroxy-5H-furan-2-one)) are inexpensive, commercially available starting materials with multiple functional groups. These compounds have been modified by way of reduction followed by Suzuki cross-coupling reactions involving arylboronic acids to afford highly functionalized alpha,beta-unsaturated gamma-butyrolactones in excellent yield. The synthetic utility of these building blocks was effectively demonstrated through preparation of the antiinflammatory drug Vioxx. 相似文献
994.
Oscillation and nonoscillation criteria for the higher order self-adjoint differential equation (-1)n(talphay(n))(n)+q(t)y = 0 (*) are established. In these criteria, equation (*) is viewed as a perturbation of the conditionally oscillatory equation (-1)n(talphay(n))(n) - µ,t2n-y = 0, where
n, is the critical constant in conditional oscillation. Some open problems in the theory of conditionally oscillatory, even order, self-adjoint equations are also discussed. 相似文献
995.
Adam W García H Diedering M Martí V Olivucci M Palomares E 《Journal of the American Chemical Society》2002,124(41):12192-12199
The photochemical denitrogenation of the cyclopentene-annelated DBH-type azoalkanes 1 has been examined in solution as a function of bridgehead substitution and temperature. For all derivatives, namely, the unsubstituted 1a(H/H), monomethyl 1b(Me/H) dimethyl 1c(Me/Me), monophenyl 1d(Ph/H), and diphenyl 1e(Ph/Ph), the temperature-dependent ratio of syn and anti housanes 2 provides experimental support for a competition between the singlet (high temperature) and triplet (low temperature) reaction channels in the direct photolysis. The syn/anti ratio of the housanes 2 depends on the extent and type of bridgehead substitution; the amount of the anti diastereomer (retention) follows the order Ph > Me > H, and double substitution is more effective than single. This stereochemical memory is interpreted in terms of the mass effect (inertia) of the substituents and steric interaction (size) between the substituents at the bridgehead and the methylene bridge during the deazetation step of the transient diazenyl diradical conformations (1)DZ (exo-ax) and (1)DZ (exo-eq). These conformers are impulsively generated upon decay of the (1)(n,pi)-excited azoalkane, a trajectory assessed through computational work. The new mechanistic feature disclosed by the unprecedented anti stereoselectivity (retention) is the intervention of a puckered 1,3-cyclopentanediyl singlet diradical (1)DR as product bifurcation step, whose conformational relaxation to the planar species (loss of stereochemical memory) is encumbered by bridgehead substitution. 相似文献
996.
Bai JZ Ban Y Bian JG Chen AD Chen HF Chen HS Chen JC Chen XD Chen YB Cheng BS Chi SP Chu YP Choi JB Cui XZ Dai YS Dong LY Du ZZ Dunwoodie W Fu HY Fu LP Gao CS Gu SD Guo YN Guo ZJ Han SW Han Y Harris FA He J He JT He KL He M He X Hong T Heng YK Hu GY Hu HM Hu QH Hu T Huang GS Huang XP Huang YZ Izen JM Ji XB Jiang CH Jin Y Jones BD Kang JS Ke ZJ Kim HJ Kim SK Kim TY Kong D Lai YF Li D Li HB Li HH Li J Li JC Li PQ Li QJ Li RY Li W Li WG Li XN Li XQ Liu B Liu F Liu F Liu HM Liu J Liu JP Liu TR 《Physical review letters》2002,88(10):101802
We report values of R = sigma(e(+)e(-)-->hadrons)/sigma(e(+)e(-)-->mu(+)mu(-)) for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider. 相似文献
997.
Feng DL Damascelli A Shen KM Motoyama N Lu DH Eisaki H Shimizu K Shimoyama Ji JI Kishio K Kaneko N Greven M Gu GD Zhou XJ Kim C Ronning F Armitage NP Shen ZX 《Physical review letters》2002,88(10):107001
The low-energy electronic structure of the nearly optimally doped trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta) is investigated by angle-resolved photoemission spectroscopy. The normal state quasiparticle dispersion and Fermi surface and the superconducting d-wave gap and coherence peak are observed and compared with those of single- and bilayer systems. We find that both the superconducting gap magnitude and the relative coherence-peak intensity scale linearly with T(c) for various optimally doped materials. 相似文献
998.
Determination of berberine in Rhizoma coptidis and its preparations by non-aqueous capillary electrophoresis 总被引:1,自引:0,他引:1
A non-aqueous capillary electrophoretic method was established for the determination of berberine in Rhizoma coptidis and its preparations. The effects of organic solvent and the concentrations of sodium acetate were studied, which showed that berberine in extracts of traditional Chinese medicine can be separated successfully in a buffer solution of 75 mmol/L of sodium acetate in methanol containing 1 mol/L of acetic acid. The simple and rapid method was linear in the range 25-200 microgram/mL of berberine and had a good reproducibility, with the relative standard deviation below 2%. Non-aqueous capillary electrophoresis is a satisfactory system for the analysis of alkaloids in traditional Chinese medicine. 相似文献
999.
Jiří Vala 《Applications of Mathematics》1999,44(6):481-496
The aim of this paper is to demonstrate how the variational equations from [11] can be formulated and solved in some abstract Banach spaces without any a priori construction of special linearization schemes. This should be useful e.g. in the analysis of heat conduction problems and modelling of flows in porous media. 相似文献
1000.
A novel QNDT (quantitative non-destructive testing) method is developed that is combined with a phase-shifting shearing speckle and thermograph, and, it aims at the detection of faults such as cracks, voids, delamination and weak areas. The technique is immune to ambient noise and is suitable for measuring the in situ environment. Some different depth defects that would produce deformation differing from other positions could be found with shearing speckle when the sample is loaded, however, a thermograph based on the thermal resistance effect of a defect can detect only those varisized defects embedded deeply in the composite structure by measuring the surface temperature distribution. The resolution is examined for artificial delaminated defects in carbon-fiber composite structures using a phase-shifting shearing speckle and thermograph. The experimental results have demonstrated that the technique is effective for revealing defects in composite structures. 相似文献