全文获取类型
收费全文 | 217614篇 |
免费 | 41228篇 |
国内免费 | 30722篇 |
专业分类
化学 | 145195篇 |
晶体学 | 2679篇 |
力学 | 14308篇 |
综合类 | 1008篇 |
数学 | 26660篇 |
物理学 | 99714篇 |
出版年
2024年 | 800篇 |
2023年 | 3244篇 |
2022年 | 5279篇 |
2021年 | 5594篇 |
2020年 | 6162篇 |
2019年 | 6525篇 |
2018年 | 5803篇 |
2017年 | 6077篇 |
2016年 | 8569篇 |
2015年 | 8733篇 |
2014年 | 9969篇 |
2013年 | 14156篇 |
2012年 | 16334篇 |
2011年 | 18200篇 |
2010年 | 17345篇 |
2009年 | 17212篇 |
2008年 | 12182篇 |
2007年 | 10990篇 |
2006年 | 10082篇 |
2005年 | 9272篇 |
2004年 | 8358篇 |
2003年 | 6710篇 |
2002年 | 6368篇 |
2001年 | 6104篇 |
2000年 | 4898篇 |
1999年 | 5390篇 |
1998年 | 4514篇 |
1997年 | 4183篇 |
1996年 | 4674篇 |
1995年 | 4588篇 |
1994年 | 4561篇 |
1993年 | 4242篇 |
1992年 | 3758篇 |
1991年 | 3295篇 |
1990年 | 2760篇 |
1989年 | 2586篇 |
1988年 | 2459篇 |
1987年 | 1634篇 |
1986年 | 1689篇 |
1985年 | 1327篇 |
1984年 | 1315篇 |
1983年 | 581篇 |
1982年 | 1086篇 |
1981年 | 902篇 |
1980年 | 897篇 |
1979年 | 629篇 |
1978年 | 603篇 |
1977年 | 694篇 |
1976年 | 1115篇 |
1972年 | 568篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Activated carbon fiber (ACF) as extraction fiber for solid-phase microextraction (SPME) and its application for the analysis of banned organochlorine pesticides (OCPs) were investigated. Firstly, ACF was activated by different concentration of zinc chloride, which indicated that ACF activated by 60% zinc chloride had a reasonable specific surface area, pore volume and pore distribution. Secondly, the parameters for the ACF-SPME procedure, the adsorption and desorption conditions, were also optimized when coupled with gas chromatography-mass spectrometry (GC-MS). Thirdly, the ACF-SPME was used to analyze 17 kinds of OCPs in water. The linearity of most pesticides was found to be between 0.2 and 50 microg/l with GC-MS under the selected ion monitoring (SIM) acquisition mode. The limits of detection (LOD) at the sub microg/l were obtained. The work demonstrated here shows that ACF is a promising alternative for the SPME procedure. 相似文献
992.
The binding of lomefloxacin to bovine lactoferrin (BLf) in a dilute aqueous solution was studied using fluorescence spectra. The binding constant (K) and the number of binding sites (n) were obtained by a fluorescence quenching method. The binding distance (r) and energy-transfer efficiency (E) between lomefloxacin and bovine lactoferrin were also obtained according to the mechanism of Fo?rster-type dipole-dipole nonradiative energy-transfer. The effect of lomefloxacin on the conformation of bovine lactoferrin was also analyzed by synchronous fluorescence spectroscopy. The interaction between lomefloxacin and bovine lactoferrin is strong. Lomefloxacin can affect the conformation of bovine lactoferrin to some degree. 相似文献
993.
Vytautas Getautis Albina Stanisauskaite Tadas Malinauskas Jolanta Stumbraite Valentas Gaidelis Vygintas Jankauskas 《Monatshefte für Chemie / Chemical Monthly》2006,23(12):1401-1409
Hydrazones containing 1-phenyl-1,2,3,4-tetrahydroquinoline units were synthesized starting from diphenylamine. These compounds
were found to constitute novel hole transporting materials and were characterized by the time of flight method. The hole drift
mobility in these compounds exceeds 10−6 cm2 V−1 s−1 at an electric field of 106 V cm−1. 相似文献
994.
Mousa Z. Al-Noaimi Raid J. Abdel-Jalil Mustafa M. El-Abadelah Salim F. Haddad Younis N. H. Baqi Wolfgang Voelter 《Monatshefte für Chemie / Chemical Monthly》2006,37(12):745-750
In the presence of RuCl3, N-phenylamidrazone underwent oxidative cyclization into 1,4-dihydro-1-phenyl-1,2,4-benzotriazine, the structure of which is
established by spectral and X-ray diffraction data. 相似文献
995.
Ivan Gutman Sabina Gojak Niko Radulovi? Boris Furtula 《Monatshefte für Chemie / Chemical Monthly》2006,46(2):277-284
In a recent work it was demonstrated that in linear hexagonal chains the distribution of π-electrons into rings (as computed
by means of the Randić–Balaban method) is uniform, irrespective of the nature of the terminal fragments. We now establish that an analogous, yet somewhat
more complex, uniformity in the π-electron distribution exists also in double linear hexagonal chains, as well as in some
other benzenoid systems. 相似文献
996.
Wafaa M. Abdou Neven A. Ganoub Amin F. Fahmy Abeer A. Shaddy 《Monatshefte für Chemie / Chemical Monthly》2006,46(10):105-116
Two simple and efficient one-pot procedures for the synthesis of a series of α-branched N-heterocycle-substituted methane-1,1-bisphosphonates
are outlined. In the first method, the parent halosubstrates were reacted with cyanomethylphosphonate followed by reaction
with dialkyl phosphonates to give asymmetrical or symmetrical bisphosphonates (BPs). In the second approach, the same halocompounds were reacted with tetraethyl methyl-1,1-bisphosphonate to give the requisite
BPs. Partial and complete hydrolysis of the prepared BPs were also investigated. The products contain functional groups advantageous for further synthetic modification as structural
units for coupling with the drug. 相似文献
997.
Wen.-C. Chiang J.G. Lin K.H. Hsu D.S. Hussey D.V. Baxter 《Journal of magnetism and magnetic materials》2006
We investigate Co/Nb multilayers to explore the spontaneous π-phase shift between the superconducting (SC) layers, which is attributed for causing the non-monotonic change of the SC transition temperature (Tc) with the ferromagnetic (FM) layer thickness (tFM) in several FM/SC multilayered systems. The issue of interfacial roughness is also explored by growing Co/Nb multilayers at various sputtering pressures. Transport measurements show a non-monotonic dependence of Tc on tFM, and this dependence is insensitive to the structural variation present in the samples, as measured by X-ray scattering. 相似文献
998.
999.
Optical waveguide fabrication and integration with a micro-mirror inside photosensitive glass by femtosecond laser direct writing 总被引:1,自引:0,他引:1
Z. Wang K. Sugioka Y. Hanada K. Midorikawa 《Applied Physics A: Materials Science & Processing》2007,88(4):699-704
Photosensitive glass is a potentially important material for micro-fluidic devices that can be integrated with micro-optical
components for biochemical analysis. Here, we demonstrate the fabrication of optical waveguides inside glass by femtosecond
laser direct writing. The influence of the laser parameters on the waveguide properties is investigated, and it is revealed
that the waveguide mode can be well controlled. The single mode is achieved at a low writing energy, while the multimode is
achieved with increasing energy. In spite of a longitudinally elongated elliptical shape of the cross-sectional profile, the
far-field pattern of the single-mode waveguide shows an almost symmetric profile. The measured propagation loss and the coupling
loss are evaluated to be ∼0.6 dB/cm and ∼1.6 dB at a wavelength of 632.8 nm, respectively, under the conditions of 1.0–2.0 μJ
pulse energy and 200–500 μm/s scan speed. The increased optical loss is associated with a higher waveguide mode at higher
writing energy. Furthermore, the integration of waveguides and a micromirror made of a hollow microplate inside the glass
is demonstrated to bend the laser beam at an angle of 90° in a small chip. The bending loss is estimated to be smaller than
0.3 dB.
PACS 42.62.-b; 42.82.Cr; 82.50.Pt; 42.79.Gn; 42.81.Qb 相似文献
1000.
This paper addresses a kind of risk decision-making problem existing widely in public administration and business management, which is characterized by (1) occurrence probabilities of states of nature can be estimated by analysing historical observations, but historical observations of different objects are unhomogeneous, (2) the relation between observations and occurrence probabilities of states of nature are affected by some qualitative and quantitative indicators, (3) it is a real-time decision-making problem, that is, there are many decisions for different objects to be made in a limited time, (4) considering decision's execution, impact of resource constrains is an important issue in decision-making process. In this paper, we develop a rule-based approach to address the problem. In the proposed approach, a two-step clustering method is employed to classify objects into categories, and observations in each category can be approximately viewed as homogeneous. For objects in each category, occurrence probabilities of states of nature are estimated by logistic regression, and the decision rule is obtained through solving an optimization model, which is to minimize the total decision risks while satisfying resource constrains. Effect and efficacy of our approach are illustrated through its application to China's customs inspection decision. 相似文献