全文获取类型
收费全文 | 102700篇 |
免费 | 7953篇 |
国内免费 | 8639篇 |
专业分类
化学 | 55456篇 |
晶体学 | 1914篇 |
力学 | 5626篇 |
综合类 | 784篇 |
数学 | 26068篇 |
物理学 | 29444篇 |
出版年
2024年 | 133篇 |
2023年 | 910篇 |
2022年 | 1980篇 |
2021年 | 2196篇 |
2020年 | 2471篇 |
2019年 | 2718篇 |
2018年 | 2431篇 |
2017年 | 2722篇 |
2016年 | 3041篇 |
2015年 | 2728篇 |
2014年 | 3913篇 |
2013年 | 6817篇 |
2012年 | 4635篇 |
2011年 | 5591篇 |
2010年 | 4868篇 |
2009年 | 6444篇 |
2008年 | 6594篇 |
2007年 | 6919篇 |
2006年 | 5898篇 |
2005年 | 4906篇 |
2004年 | 4543篇 |
2003年 | 4233篇 |
2002年 | 4934篇 |
2001年 | 3412篇 |
2000年 | 3113篇 |
1999年 | 2629篇 |
1998年 | 2467篇 |
1997年 | 1714篇 |
1996年 | 1766篇 |
1995年 | 1532篇 |
1994年 | 1585篇 |
1993年 | 1187篇 |
1992年 | 1209篇 |
1991年 | 783篇 |
1990年 | 718篇 |
1989年 | 631篇 |
1988年 | 550篇 |
1987年 | 500篇 |
1986年 | 446篇 |
1985年 | 457篇 |
1984年 | 409篇 |
1983年 | 243篇 |
1982年 | 353篇 |
1981年 | 360篇 |
1980年 | 287篇 |
1979年 | 297篇 |
1978年 | 250篇 |
1977年 | 226篇 |
1976年 | 172篇 |
1973年 | 104篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
101.
以金属钇和异丙醇为原料,以HgCl2/I2为复合催化剂,通过对金属钇的机械加工以增加其比表面,并将异丙醇脱水使其含水量降低至0.05%,体系在82℃回流5h,经过滤、减压蒸馏,得到了白色海绵状异丙醇钇,其产率高达83%,合成时间比文献报道的缩短了19h,产率提高了8%。文章确定了催化剂的最佳用量为20gY加入60mg HgCl2/I2,研究了合成产率与HgCl2/I2催化剂和HgCl2催化剂的依赖关系及异丙醇中含水量对合成产率的影响,并对HgCl2/I2的催化作用机理进行了初步探讨。 相似文献
102.
Nuclear Magnetic Resonance (NMR) was used to investigate the extrusion behaviour of PTFE pastes in a ram extruder. By means
of 1H-NMR imaging (MRI) it is possible to determine the local proton density and therefore, the local fluid concentration. The
19F-MRI provides the local solid concentration. Thus the local saturation and the local porosity can be calculated with the
information of the local fluid and solid concentration. Furthermore displacement profiles can be derived from NMR images by
means of correlation techniques without any preparation or marking of the pastes.
Received: 8 May 2000 Accepted: 1 May 2001 相似文献
103.
Michael Carr 《Topology and its Applications》2006,153(12):2155-2168
Given a graph Γ, we construct a simple, convex polytope, dubbed graph-associahedra, whose face poset is based on the connected subgraphs of Γ. This provides a natural generalization of the Stasheff associahedron and the Bott-Taubes cyclohedron. Moreover, we show that for any simplicial Coxeter system, the minimal blow-ups of its associated Coxeter complex has a tiling by graph-associahedra. The geometric and combinatorial properties of the complex as well as of the polyhedra are given. These spaces are natural generalizations of the Deligne-Knudsen-Mumford compactification of the real moduli space of curves. 相似文献
104.
Francis Jordan 《Topology and its Applications》2006,153(14):2386-2412
An abstract scheme using particular types of relations on filters leads to general unifying results on stability under supremum and product of local topological properties. We present applications for Fréchetness, strong Fréchetness, countable tightness and countable fan-tightness, some of which recover or refine classical results, some of which are new. The reader may find other applications as well. 相似文献
105.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
106.
A Volterra type integral equation in a Hilbert space with an additional linear operator L and a spectral parameter depending
on time is considered. If the parameter does not belong to the spectrum of L unconditional solvability of the considered problem
is proved. In the case where the initial value of the parameter coincides with some isolated point of the spectrum of the
operator L sufficient conditions for solvability are established. The obtained results are applied to the partial integral
equations associated with a contact problem of the theory of elasticity. 相似文献
107.
M.F. Al-Kuhaili 《Journal of luminescence》2006,117(2):209-216
Thin films of sodium salicylate were deposited by spin coating from a solution prepared by dissolving sodium salicylate in methanol. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, UV-visible spectroscopy and spectrofluorometry. The films were crystalline with a strong blue fluorescence peak at an emission wavelength of 419.3 nm. The influence of solution concentration, spin speed and annealing temperature on the fluorescence intensity was studied. Optimum results were obtained with a solution whose concentration was 2.5 M. It was found that lower spin speed resulted in higher fluorescence intensity. Moreover, the fluorescence intensity decreased as the annealing temperature was increased. The durability of the films over a period of 30 days was also investigated, and films annealed at higher temperatures were found to be less degraded with time. 相似文献
108.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献
109.
Thin films of, N-N′diphenyl 1-4phenylene-diamineane are prepared using vacuum sublimation technique. The electrical conductivity from room temperature down to 127 K is studied. It is found that the conduction of charge carriers obeys T−1/2 dependence on temperature. The average hopping distance, hopping energy, density of states and their variation due to post-deposition heat treatment are studied. Schottky diodes are fabricated with gold as ohmic contact and aluminium as Schottky contact. From the observed current voltage characteristics the saturation current density, diode ideality factor and the barrier height are determined. Their variation with air annealing is also investigated. 相似文献
110.
In this paper we study the Hankel convolution operators on the space of even and entire functions and on Schwartz distribution spaces. We characterize the Hankel convolution operators as those ones that commute with Hankel translations and with a Bessel operator. Also we prove that the Hankel convolution operators are hypercyclic and chaotic on the spaces under consideration. 相似文献