全文获取类型
收费全文 | 28906篇 |
免费 | 125篇 |
国内免费 | 270篇 |
专业分类
化学 | 10520篇 |
晶体学 | 266篇 |
力学 | 1380篇 |
综合类 | 13篇 |
数学 | 9015篇 |
物理学 | 8107篇 |
出版年
2021年 | 26篇 |
2018年 | 1193篇 |
2017年 | 1451篇 |
2016年 | 712篇 |
2015年 | 563篇 |
2014年 | 474篇 |
2013年 | 664篇 |
2012年 | 3118篇 |
2011年 | 2303篇 |
2010年 | 1804篇 |
2009年 | 1550篇 |
2008年 | 554篇 |
2007年 | 602篇 |
2006年 | 595篇 |
2005年 | 4422篇 |
2004年 | 3910篇 |
2003年 | 2295篇 |
2002年 | 492篇 |
2001年 | 294篇 |
2000年 | 99篇 |
1999年 | 172篇 |
1998年 | 109篇 |
1997年 | 70篇 |
1996年 | 46篇 |
1995年 | 42篇 |
1994年 | 44篇 |
1992年 | 172篇 |
1991年 | 156篇 |
1990年 | 139篇 |
1989年 | 109篇 |
1988年 | 102篇 |
1987年 | 60篇 |
1986年 | 45篇 |
1985年 | 39篇 |
1984年 | 31篇 |
1983年 | 30篇 |
1979年 | 34篇 |
1978年 | 25篇 |
1976年 | 80篇 |
1975年 | 38篇 |
1974年 | 43篇 |
1973年 | 52篇 |
1972年 | 39篇 |
1971年 | 25篇 |
1970年 | 38篇 |
1969年 | 37篇 |
1968年 | 36篇 |
1967年 | 31篇 |
1966年 | 38篇 |
1923年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The determination of the sensitivity of the acoustical characteristics of vibrating systems with respect to the variation of the design parameters predicting these characteristics is a necessary and important step of the acoustic design and optimization process. Acoustic design sensitivity analysis includes the computation and evaluation of the sensitivity information required for this procedure. In this study, a boundary element code performing the sensitivity analysis of the acoustic pressure by using the matrix sensitivities with respect to different design variables has been developed. The effect of the precision of boundary element discretization on the acoustic pressure sensitivity is examined via this code. The formulation is applied to a multi-source system and the dimension sensitivity analysis of near field pressures of two-dilating-spherical source is performed. The last application is devoted to a real sound source: a washing machine sitting on the floor. Sensitivity of the field pressures to the machine’s dimensions (size), surface velocity and frequency is examined on the bases of the boundary element model of the machine and half-space condition. The impacts of these variables are compared; and a limiting speed for the machine responding both the acoustical and operational requirements is determined. 相似文献
82.
Pal Jen Wei 《Journal of Non》2008,354(33):3911-3918
Through the arrangement of the applied load, experimental nanoindentation results of polymethylmethacrylate (PMMA) and polyurethane (PU) employed to establish a mechanical model. The proposed model consists of irreversible delayed plastic (viscoplastic) deformation, irreversible viscous deformation, and reversible delayed elastic (viscoelastic) deformations. The phase lag exhibited between the responding depth and the oscillating load is found to be linear proportional to frequency in the range of 1-50 Hz. The residual cavity profile of the PMMA scanned by an atomic force microscope gives a validity of accommodation assumption applied in the present model. The effects of overshooting, which occurred in the dwelling process, are also discussed. 相似文献
83.
This study demonstrates the separation of active ingredients in acne formulations (salicylic acid, cloramphenicol and resorcinol in presence of azulene) by capillary zone electrophoresis. Factors affecting their separations were the buffer pH and concentration, applied voltage, sample preparation, and presence of additives. Optimun results were obtained with a 50 mM sodium tetraborate-50 mM sodium phosphate, pH 9.0. The carrier electrolyte gave baseline separation with good resolution, short migration times (<6 min), great reproducibility and accuracy. Calibration plots were linear over at least three orders of magnitude of analyte concentrations, the lower limits of detection being within the range 0.39-1.25 μg ml−1. The procedure was fast and reliable and commercial pharmaceuticals could be analysed without prior sample clean-up procedure. 相似文献
84.
We prove an existence theorem for a nonlinear integral equation being a Volterra counterpart of an integral equation arising in the traffic theory. The method used in the proof allows us to obtain additional characterization in terms of asymptotic stability of solutions of an equation in question. 相似文献
85.
Isabel Brandariz José L. Barriada Teresa Vilari?o Manuel E. Sastre de Vicente 《Monatshefte für Chemie / Chemical Monthly》2004,343(6):1475-1488
Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances. 相似文献
86.
87.
Grega Cigler Roman Drnovšek Damjana Kokol-Bukovšek Matja? Omladi? Thomas J. Laffey Heydar Radjavi Peter Rosenthal 《Journal of Functional Analysis》1998,160(2):245
T. Laffey showed (Linear and Multilinear Algebra6(1978), 269–305) that a semigroup of matrices is triangularizable if the ranks of all the commutators of elements of the semigroup are at most 1. Our main theorem is an extension of Hthis result to semigroups of algebraic operators on a Banach space. We also obtain a related theorem for a pair {A, B} of arbitrary bounded operators satisfying rank (AB−BA)=1 and several related conditions. In addition, it is shown that a semigroup of algebraically unipotent operators of bounded degree is triangularizable. 相似文献
88.
The hyperspaces of strongly countable dimensional compacta of positive dimension and of strongly countable dimensional continua of dimension greater than 1 in the Hilbert cube are homeomorphic to the Hurewicz set of all nonempty countable closed subsets of the unit interval [0,1]. These facts hold true, in particular, for covering dimension dim and cohomological dimension dimG, where G is any Abelian group. 相似文献
89.
Dong Sheng WU Yi Min XIAO 《数学学报(英文版)》2007,23(4):613-622
Let B^α = {B^α(t),t E R^N} be an (N,d)-fractional Brownian motion with Hurst index α∈ (0, 1). By applying the strong local nondeterminism of B^α, we prove certain forms of uniform Hausdorff dimension results for the images of B^α when N 〉 αd. Our results extend those of Kaufman for one-dimensional Brownian motion. 相似文献
90.
The interaction of the synthetic oligonucleotide d(C-G-C-G-A-A-T-T-C-G-C-G)2 with two different transition-metal ions has been investigated in aqueous solution by means of 1H NMR spectroscopy. The effects on the DNA due to the presence of manganese(II) or zinc(II) have been monitored by observing the paramagnetic broadening and diamagnetic shifts of the non-exchangeable proton resonance lines, respectively. The 1H NMR spectra acquired during the course of the manganese(II) titration show very distinct broadening effects on certain DNA resonance lines. Primarily, the H8 resonance of G4 is affected, but also the H5 and H6 resonances of C3 are clearly affected by the metal. The results imply that the binding of manganese(II) to DNA is sequence specific. The 1H spectra obtained during the zinc(II) titration reveal diamagnetic shift effects which largely conform with the paramagnetic broadening effects due to the presence of manganese(II), although this picture is somewhat more complex. The H8 resonance of G4 displays a clearly visible high-field shift, while for the other guanosine H8 protons this effect is absent. The H1' and H2' protons of C3 show an effect of similar strength, although in the opposite direction, while H5 and H6 of C3 are only slightly affected. Local differences in the structure of the DNA and the basicities of potential binding sites on different base steps in the sequence might account for the observed sequence selectivity. 相似文献