全文获取类型
收费全文 | 32976篇 |
免费 | 724篇 |
国内免费 | 670篇 |
专业分类
化学 | 13436篇 |
晶体学 | 312篇 |
力学 | 1614篇 |
综合类 | 37篇 |
数学 | 9449篇 |
物理学 | 9522篇 |
出版年
2023年 | 115篇 |
2022年 | 181篇 |
2021年 | 216篇 |
2020年 | 184篇 |
2019年 | 158篇 |
2018年 | 1323篇 |
2017年 | 1554篇 |
2016年 | 894篇 |
2015年 | 747篇 |
2014年 | 702篇 |
2013年 | 909篇 |
2012年 | 3452篇 |
2011年 | 2659篇 |
2010年 | 2015篇 |
2009年 | 1755篇 |
2008年 | 809篇 |
2007年 | 833篇 |
2006年 | 808篇 |
2005年 | 4602篇 |
2004年 | 4053篇 |
2003年 | 2427篇 |
2002年 | 592篇 |
2001年 | 382篇 |
2000年 | 175篇 |
1999年 | 281篇 |
1998年 | 209篇 |
1997年 | 157篇 |
1996年 | 133篇 |
1995年 | 117篇 |
1994年 | 96篇 |
1993年 | 54篇 |
1992年 | 206篇 |
1991年 | 185篇 |
1990年 | 157篇 |
1989年 | 123篇 |
1988年 | 118篇 |
1987年 | 75篇 |
1986年 | 54篇 |
1985年 | 46篇 |
1984年 | 44篇 |
1976年 | 86篇 |
1975年 | 40篇 |
1974年 | 42篇 |
1973年 | 49篇 |
1972年 | 41篇 |
1970年 | 35篇 |
1969年 | 37篇 |
1968年 | 35篇 |
1967年 | 31篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
The electrochemical reduction of 1-([(4-halophenyl)imino]methyl)-2-naphthols on graphite electrodes was studied using cyclic voltammetry, chronoamperometry, constant-potential coulometry and preparative constant-potential electrolysis techniques. The data revealed that the reduction on graphite was irreversible and followed an EC mechanism. The diffusion coefficients and the number of electrons transferred were determined using the chronoamperometric Cottrell slope and the ultramicro disc Pt-electrode steady-state current. The number of electrons was also determined by bulk electrolysis. The compounds were subjected to constant-potential preparative electrolysis and the electrolysis products were purified and identified by spectroscopic methods. Based on these findings, a mechanism for the electro-reduction process is proposed. 相似文献
12.
Paramagnetic pyrroline and 1,2,3,6-tetrahydropyridine derivatives of BODIPY and their diamagnetic analogs have been synthesized and characterized as novel redox double sensor and cation sensitive reagents. 相似文献
13.
Daniel Pa?ca 《Journal of Mathematical Analysis and Applications》2007,325(1):90-100
Some existence results are obtained for periodic solutions of nonautonomous second-order differential inclusions systems with p-Laplacian. 相似文献
14.
A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle. 相似文献
15.
Goran Pavi? 《Applied Acoustics》2006,67(9):864-881
A method is formulated for the identification of an unknown physical parameter of a fluid-filled pipe using the measurement of sound speed in the pipe. The method uses a simple formula which provides the relationship between the sound speed and a few physical parameters of the pipe: thickness, diameter, wall material constants and fluid constants. Once the sound speed in the pipe is measured, the simple formula can be used to extract one of the unknown parameters providing the remaining ones are known.The sound speed in the pipe is measured using a 3-transducer array. In order to demonstrate the potential of the technique the results of several measurements obtained in a water-filled steel pipe are presented.The required accuracy of the measurement of sound speed and of the specification of known parameters is analysed. The accuracy depends on the unknown parameter which is to be identified. For example, if the pipe thickness is the unknown parameter, the other parameters have to be known within a very narrow margin of error. On the contrary, if the fluid properties have to be identified the needed accuracy of known parameters gets much lower. 相似文献
16.
Sung‐Fu Hsu Tzong‐Ming Wu Chien‐Shiun Liao 《Journal of Polymer Science.Polymer Physics》2006,44(23):3337-3347
Poly(3‐hydroxybutyrate) (PHB)/layered double hydroxides (LDHs) nanocomposites were prepared by mixing PHB and poly(ethylene glycol) phosphonates (PEOPAs)‐modified LDH (PMLDH) in chloroform solution. Both X‐ray diffraction data and TEM micrographs of PHB/PMLDH nanocomposites indicate that the PMLDHs are randomly dispersed and exfoliated into the PHB matrix. In this study, the effect of PMLDH on the isothermal crystallization behavior of PHB was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. Isothermal crystallization results of PHB/PMLDH nanocomposites show that the addition of 2 wt % PMLDH into PHB induced more heterogeneous nucleation in the crystallization significantly increasing the crystallization rate and reducing their activation energy. By adding more PMLDH into the PHB probably causes more steric hindrance of the diffusion of PHB, reducing the transportation ability of polymer chains during crystallization, thus increasing the activation energy. The correlation among crystallization kinetics, melting behavior and crystalline structure of PHB/PMLDH nanocomposites can also be discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3337–3347, 2006 相似文献
17.
A tree T is arbitrarily vertex decomposable if for any sequence τ of positive integers adding up to the order of T there is a sequence of vertex-disjoint subtrees of T whose orders are given by τ. An on-line version of the problem of characterizing arbitrarily vertex decomposable trees is completely solved here. 相似文献
18.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 1016 m−3, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 1021 m−3 for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy. 相似文献
19.
Kj?rand Iversen Per Nilsson 《International Journal of Computers for Mathematical Learning》2007,12(2):113-133
This paper focuses on the different ways in which students in lower secondary school (14–16 year olds) experience compound
random events, presented to them in the form of combined junctions. A carefully designed ICT environment was developed enabling
the students to interact with different representations of such structures. Data for the analysis was gathered from two interview
sessions. The analysis of the interaction is based on constructivist principles on learning; i.e. we adopted a student-oriented
perspective, taking into consideration the different ways students try to make sense of chance encounters.
Our results show how some students give priority to geometrical and physical concerns, and we discuss how seeking causal explanations
of random phenomena may have encouraged this. With respect to numerically oriented models a division strategy appears to stand
out as the preferred one. 相似文献
20.