全文获取类型
收费全文 | 7200篇 |
免费 | 173篇 |
国内免费 | 88篇 |
专业分类
化学 | 3894篇 |
晶体学 | 91篇 |
力学 | 363篇 |
数学 | 1245篇 |
物理学 | 1868篇 |
出版年
2021年 | 65篇 |
2020年 | 68篇 |
2016年 | 85篇 |
2015年 | 125篇 |
2014年 | 144篇 |
2013年 | 529篇 |
2012年 | 212篇 |
2011年 | 226篇 |
2010年 | 204篇 |
2009年 | 213篇 |
2008年 | 256篇 |
2007年 | 279篇 |
2006年 | 242篇 |
2005年 | 200篇 |
2004年 | 210篇 |
2003年 | 210篇 |
2002年 | 235篇 |
2001年 | 170篇 |
2000年 | 167篇 |
1999年 | 135篇 |
1998年 | 105篇 |
1997年 | 112篇 |
1996年 | 130篇 |
1995年 | 119篇 |
1994年 | 116篇 |
1993年 | 120篇 |
1992年 | 105篇 |
1991年 | 97篇 |
1990年 | 117篇 |
1989年 | 100篇 |
1988年 | 94篇 |
1987年 | 82篇 |
1986年 | 87篇 |
1985年 | 97篇 |
1984年 | 88篇 |
1983年 | 81篇 |
1982年 | 93篇 |
1981年 | 76篇 |
1980年 | 90篇 |
1979年 | 93篇 |
1978年 | 98篇 |
1977年 | 86篇 |
1976年 | 85篇 |
1975年 | 77篇 |
1974年 | 79篇 |
1973年 | 82篇 |
1972年 | 116篇 |
1971年 | 75篇 |
1970年 | 48篇 |
1968年 | 43篇 |
排序方式: 共有7461条查询结果,搜索用时 0 毫秒
871.
Tze Ming Chan David C. Dalgarno James H. Prestegard C.Anderson Evans 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,126(2):183-186
A technique for reliably selecting a transmitter position for rotating-frame NOE experiments which minimize scalar coupling artifacts is described. The technique makes use of the COSY spectrum of the molecule in question. The method is illustrated by application to thiostrepton, an antibiotic of molecular weight 1665. For an individual compound, the range of transmitter settings over which HOHAHA distortions are observed is found to depend on the molecular correlation time of the compound under study. The origin of this effect is discussed in terms of rotating-frame relaxation during the spin-lock period. 相似文献
872.
This article describes and analyzes, both experimentally and theoretically, a control system using nonthrottleable air-jet thrusters to actively dampen the vibration of a 20-bay, 7.07-m-long laboratory planar truss. The thrusters were highly effective at low frequencies but, due to mechanical time delays, they lost effectiveness and were even potentially destabilizing at higher frequencies. The control system was quite nonlinear due to the on-off operation of the thrusters and to a deadband designed to prevent unwanted disturbance by low-level electrical noise. The planar-truss vibration modes evaluated are the first four, at 1.56, 10.0, 24.7 and 43.0 Hz. The active damping system consisted primarily of two pairs of opposing air-jet thrusters, a servo accelerometer colocated with each thruster pair, and a small analog computer. 相似文献
873.
874.
P. Evans C. Scheck R. Schad G. Zangari 《Journal of magnetism and magnetic materials》2003,260(3):467-472
The structural and magnetic properties of Ni films grown by electrodeposition from simple sulfate solutions directly onto the (0 0 1) and (0 1 1) surfaces of n-GaAs have been studied. In-plane X-ray diffraction has been used to show that Ni grows on (0 0 1) GaAs with two different preferred epitaxial relationships: (1) perpendicular to plane (0 0 1)Ni(0 0 1)GaAs and preferred orientation in-plane [1 0 0]Ni[1 1 0]GaAs and (2) perpendicular to plane (0 1 1)Ni(0 0 1)GaAs and preferred orientation in-plane [1 1 1]Ni[1 1 0]GaAs. Nickel films grown on (0 1 1) n-GaAs show only a single preferred growth relationship: perpendicular to plane (1 1 1)Ni(0 1 1)GaAs and in-plane [1 1 0]Ni[1 1 0]GaAs. The magnetic properties were strongly dependent on the substrate orientation. The films grown on GaAs (0 0 1) showed a small but definite four-fold magnetic anisotropy in plane with the highest remanence being found along the GaAs [1 0 0] direction. In contrast, the Ni films grown on the (0 1 1) GaAs showed a pronounced uniaxial anisotropy with an anisotropy field of approximately 500 Oe. 相似文献
875.
A.B. Henriques L.C.D. Gonçalves N.F. Oliveira Jr. S.M. Shibli P.L. Souza B. Yavich 《Zeitschrift für Physik B Condensed Matter》1997,104(3):457-461
The Shubnikov-de Haas spectra of GaAs samples with a periodical doping with Si were measured at 4.2 K using fields of 0–14 T. From the Shubnikov-de Haas spectrum the quantum mobilities associated with individual minibands were estimated for each sample. The sheet density of Si atoms in each doping plane was fixed at approximately 2.5 × 1012 cm?2 in all samples and the doping period was varied in the range 40–1000 Å. The quantum mobilities obtained experimentally are compared with theoretical calculations using the random phase approximation to describe the screened interaction between electrons and charged impurities. The theory describes qualitatively the results of the experiment, i.e. an increase of the quantum mobility with the index of the miniband, and a decrease in the mobility in all minibands when the doping period is made shorter (with a weak maximum at intermediate values of the doping period). Various possible improvements of the model are suggested. 相似文献
876.
In this article, a class of nonlinear evolution equations – reaction–diffusion equations with time delay – is studied. By
combining the domain decomposition technique and the finite difference method, the results for the existence, convergence
and the stability of the numerical solution are obtained in the case of subdomain overlap and when the time-space is completely
discretized.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
877.
J.N. vila F. Chemale Jr. G. Mallmann A.W. Borba F.F. Luft 《Radiation measurements》2005,39(6):603-611
Apatite fission track thermochronology (AFTT) was applied to derive the thermal history of formation and inversion of the oil-bearing Triassic–Cretaceous Cuyo Basin, Argentine Precordillera. The obtained central fission track ages range from 13 to 163 Ma, and the mean track lengths from 8.2 to . Based on the integration of AFTT, stratigraphic and structural data, five evolutionary phases are here proposed for the studied area: I—Late Permian cooling related to the extrusion of basement volcanics, and initial extensional regime responsible for the Cuyo Basin formation, II—Triassic–Jurassic heating linked with the filling, and consequent burial, of the Cuyo Basin, III—Jurassic–Paleogene tectonic stabilization, IV—Early Miocene heating related to the load of foreland sedimentation, V—Late Miocene rapid cooling related to the Cuyo Basin inversion. During the Late Miocene, rock units formed in different crustal levels were juxtaposed through thrust, reverse and normal faulting, resulting in contrasting old and very young central fission track ages in closely spaced samples. 相似文献
878.
Brockwell A. Polak E. Evans R. Ralph D. 《Journal of Optimization Theory and Applications》2003,116(3):485-516
Moving-horizon control is a type of sampled-data feedback control in which the control over each sampling interval is determined by the solution of an open-loop optimal control problem. We develop a dual-sampling-rate moving-horizon control scheme for a class of linear, continuous-time plants with strict input saturation constraints in the presence of plant uncertainty and input disturbances. Our control scheme has two components: a slow-sampling moving-horizon controller for a nominal plant and a fast-sampling state-feedback controller whose function is to force the actual plant to emulate the nominal plant. The design of the moving-horizon controller takes into account the nonnegligible computation time required to compute the optimal control trajectory.We prove the local stability of the resulting feedback system and illustrate its performance with simulations. In these simulations, our dual-sampling-rate controller exhibits performance that is considerably superior to its single-sampling-rate moving-horizon controller counterpart. 相似文献
879.
880.
Current interest in laser radiation safety requires demonstration that a laser system has been designed to prevent exposure to levels of laser radiation exceeding the Maximum Permissible Exposure. In some simple systems it is possible to prove this by calculation, but in most cases it is preferable to confirm calculated results with a measurement. This measurement may be made with commercially available equipment, but there are limitations with this approach. A custom designed instrument is presented in which the full range of measurement issues have been addressed. Important features of the instrument are the design and optimisation of detector heads for the measurement task, and consideration of user interface requirements. Three designs for detector head are presented, these cover the majority of common laser types. Detector heads are designed to optimise the performance of relatively low cost detector elements for this measurement task. The three detector head designs are suitable for interfacing to photodiodes, low power thermopiles and pyroelectric detectors.
Design of the user interface was an important aspect of the work. A user interface which is designed for the specific application minimises the risk of user error or misinterpretation of the measurement results. A palmtop computer was used to provide an advanced user interface. User requirements were considered in order that the final implement was well matched to the task of laser radiation hazard audits. 相似文献