全文获取类型
收费全文 | 291194篇 |
免费 | 3493篇 |
国内免费 | 1017篇 |
专业分类
化学 | 161987篇 |
晶体学 | 4857篇 |
力学 | 11939篇 |
综合类 | 9篇 |
数学 | 29101篇 |
物理学 | 87811篇 |
出版年
2016年 | 3336篇 |
2015年 | 2511篇 |
2014年 | 3506篇 |
2013年 | 11793篇 |
2012年 | 7940篇 |
2011年 | 10040篇 |
2010年 | 6542篇 |
2009年 | 6347篇 |
2008年 | 9127篇 |
2007年 | 9260篇 |
2006年 | 9031篇 |
2005年 | 8486篇 |
2004年 | 7633篇 |
2003年 | 6863篇 |
2002年 | 6730篇 |
2001年 | 8338篇 |
2000年 | 6384篇 |
1999年 | 5171篇 |
1998年 | 4194篇 |
1997年 | 4232篇 |
1996年 | 4107篇 |
1995年 | 3928篇 |
1994年 | 3618篇 |
1993年 | 3459篇 |
1992年 | 4165篇 |
1991年 | 3985篇 |
1990年 | 3853篇 |
1989年 | 3880篇 |
1988年 | 3872篇 |
1987年 | 3871篇 |
1986年 | 3614篇 |
1985年 | 4916篇 |
1984年 | 5017篇 |
1983年 | 4177篇 |
1982年 | 4589篇 |
1981年 | 4394篇 |
1980年 | 4357篇 |
1979年 | 4483篇 |
1978年 | 4809篇 |
1977年 | 4563篇 |
1976年 | 4557篇 |
1975年 | 4319篇 |
1974年 | 4191篇 |
1973年 | 4386篇 |
1972年 | 2779篇 |
1971年 | 2258篇 |
1969年 | 2159篇 |
1968年 | 2760篇 |
1967年 | 3013篇 |
1966年 | 2585篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
K. Ishikawa H. Nakano H. Masuda C. Kusano Y. Takahashi T. Hamagishi N. Endo 《Fiber and Integrated Optics》1992,11(4):385-394
A 10-Gbit/s optical receiver is developed using low-capacitance HBT IC technology. The HBT allows a cutoff frequency of 45 GHz and a maximum oscillation frequency of 70 GHz. The receiver contains an automatic gain-control amplifier IC with a gain of 16 dB, a bandwidth of 10.7 GHz, and a decision-circuit IC with an ambiguity of 66 mVp-p at a data rate of 10 Gbit/s. The sensitivity of this receiver is -26.5 dBm at a bit error rate of 10-9. Transmission over a 140-km fiber was achieved successfully. 相似文献
912.
We engage a study of nonmodal linear logic which takes times ⊗ and the linear conditional ⊸ to be the basic connectives instead
of times and linear negation ()⊥ as in Girard's approach. This difference enables us to obtain a very large subsystem of linear logic (called positive linear
logic) without an involutionary negation (if the law of double negation is removed from linear logic in Girard's formulation,
the resulting subsystem is extremely limited). Our approach enables us to obtain several natural models for various subsystems
of linear logic, including a generic model for the so-called minimal linear logic. In particular, it is seen that these models
arise spontaneously in the transition from set theory to multiset theory. We also construct a model of full (nonmodal) linear
logic that is generic relative to any model of positive linear logic. However, the problem of constructing a generic model
for positive linear logic remains open. Bibliography: 2 titles.
Published inZapiski Nauchnykh Seminarov POMI, Vol. 220, 1995, pp. 23–35. Original 相似文献
913.
Summary. In this paper we are interested in two phase flow problems in porous media. We use a Dual Mesh Method to discretize this
problem with finite volume schemes. In a simplified case (elliptic - hyperbolic system) we prove the convergence of approximate
solutions to the exact solutions. We use the Dual Mesh Method in physically complex problems (heterogeneous cases with non
constant total mobility). We validate numerically the Dual Mesh Method on practical examples by computing error estimates
for different test-cases.
Received March 21, 1997 / Revised version received October 13, 1997 相似文献
914.
L. S. de C. Oliveira M. T. X. Silva A. Vasquez J. A. H. da Jornada 《Hyperfine Interactions》1991,66(1-4):267-270
An exploratory Mössbauer spectroscopy study of the Fe-C system in the C rich region, prepared by high pressure-high temperature treatment near the graphite-diamond stability line, was made. The results obtained for the different processing conditions give no evidence of Fe intercalation in graphite. The presence of some water in the cell produced hydrated Fe complexes, which can explain the deleterious effect of water or hydrogen in the high pressure diamond synthesis. 相似文献
915.
Summary The paper discusses some developments in the coordinate partitioning method for the dynamic analysis of constrained/closed-loop multibody systems. First, the method is reformulated to a more compact form. Then, a simple and reliable projective criterion for choosing the best coordinates from the redundant ones is proposed, and some advantages are pointed out that may arise in the method by applying inverse kinematics algorithms. Finally, the problem of determination ofphysical reactions of constraints and closing conditions is discussed. A four-bar linkage mechanism serves for an illustration of some aspects of the paper.
On leave from the Department of Mechanics, Technical University of Radom, ul. Malczewskiego 29, 26-600 Radom, Poland 相似文献
Ein Projektionskriterium zur Koordinatenwahl bei Mehrkörpersystemen mit kinematischen Schleifen
Übersicht Bei der Simulation der Bewegunsgleichungen von Mehrkörpersystemen mit kinematischen Schleifen in Minimalform stellt sich die Frage nach der Wahl günstiger verallgemeinerter Koordinaten. Hierfür wird ein Projektionskriterium vorgeschlagen, welches die Trennung der systembeschreibenden redundanten Koordinaten in die verallgemeinerten und davon abhängige Koordinaten gestattet. Durch Anwendung eines Verfahrens zur Rückwärtstransformation der kinematischen Beschreibung lassen sich in diesen explizite Schließbedingungen formulieren, was sich bei der Simulation vorteilhaft auswirkt; diese kann ohne Verletzung der Schließbedingungen erfolgen. Es wird auch gezeigt, wie auch bei Verwendung verallgemeinerter Koordinatenphysikalisch interpretierbare Reaktionen ermittelt werden können. Als Anwendungsbeispiel dient ein ebenes Viergelenk.
On leave from the Department of Mechanics, Technical University of Radom, ul. Malczewskiego 29, 26-600 Radom, Poland 相似文献
916.
Improved quantitation of 13-cis- and all-trans-acitretin in human plasma by normal-phase high-performance liquid chromatography. 总被引:1,自引:0,他引:1
E Meyer W E Lambert A P De Leenheer J P Bersaques A H Kint 《Journal of chromatography. A》1991,570(1):149-156
A reliable analytical method has been developed for measurement of 13-cis- and all-trans-acitretin (Neotigason) in human plasma by normal-phase high-performance liquid chromatography, with ultraviolet detection. Human plasma was obtained after centrifugation of whole blood samples and deproteinized by ethanolic denaturation. After liquid-liquid extraction with water-n-hexane, and aliquot ws chromatographed on a silica column using isocratic elution with n-hexane-methylsalicylate-acetic acid (200:18:0.6, v/v). The wavelength was set at 360 nm, and for plasma samples a limit of quantification of 3-4 ng/ml was obtained. All manipulations were carried out under dim light conditions to prevent photoisomerization. 相似文献
917.
Shigenori Okada Shigemitsu Yoshida Shahid H Ashrafi Dean E Schraufnagel 《Microscopy and microanalysis》2002,8(2):104-115
Osteocytes communicate through a canalicular system that maintains the vitality and mineral metabolism of bone. Casting the vascular canals and canaliculi of compact bone with methacrylate and viewing them with scanning electron microscopy shows their extent and relationships. Confocal laser scanning microscopy of the same specimen before corrosion establishes the degree of calcification of the different tissue components. These methods were used to compare basal with alveolar compact bone in the rat mandible at different ages. Sections of the mandibular molar region were placed in a methacrylate resin. After polymerization and study with confocal microscopy, the organic matrix was removed. Juvenile rats had large irregular central vascular canals and lacunae that were more concentric in the basal than the alveolar bone. Cast lacunae were round, and the canaliculi from these lacunae were short and thick in both bones. Adult rats had regular concentrically arranged lacunae in the basal bone. Cast lacunae were ellipsoid and flatter in the basal bone than in the alveolar bone. The intercommunicating canaliculi were increased and canaliculi had more branching than the juvenile rats. The aged rats had fewer vascular canals, lacunae, and canaliculi and had osteoporotic changes. The cast lacunae were slender and flat especially in the basal bone. The porosity of the mandible became more pronounced in the alveolar than in the basal bone with aging. The canaliculi of mandibular compact bone thinned and developed extensive branching with adulthood but decreased in size and number with advanced age. Lacunae proceed from the large circular structures of youth to the flat forms of the aged. These studies show that the internal structure of compact bone changes with age and mirrors its functional state. 相似文献
918.
Unsteady flow dynamics in doubly constricted 3D vessels have been investigated under pulsatile flow conditions for a full cycle of period T. The coupled non‐linear partial differential equations governing the mass and momentum of a viscous incompressible fluid has been numerically analyzed by a time accurate Finite Volume Scheme in an implicit Euler time marching setting. Roe's flux difference splitting of non‐linear terms and the pseudo‐compressibility technique employed in the current numerical scheme makes it robust both in space and time. Computational experiments are carried out to assess the influence of Reynolds' number and the spacing between two mild constrictions on the pressure drop across the constrictions. The study reveals that the pressure drop across a series of mild constrictions can get physiologically critical and is also found to be sensitive both to the spacing between the constrictions and the oscillatory nature of the inflow profile. The flow separation zone on the downstream constriction is seen to detach from the diverging wall of the constriction leading to vortex shedding with 3D features earlier than that on the wall in the spacing between the two constrictions. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
919.
W. D. Hutchison A. V. J. Edge N. Yazidjoglou D. H. Chaplin 《Hyperfine Interactions》1992,75(1-4):291-298
The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the
first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions
(EQI’s) and, in particular, their distributions, are obtained for60CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For
hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e2qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal
to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically
cubic ferromagnetic host. The mode efg is Vzz=−27.3(32)×1019 Vm−2. For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM
of 12.0(7) kHz yielding a mode efg of Vzz=−3.5(5)×1019 Vm−2. 相似文献
920.
Piotr W. Nowak 《Journal of Functional Analysis》2007,243(1):323-344
We introduce a quasi-isometry invariant related to Property A and explore its connections to various other invariants of finitely generated groups. This allows to establish a direct relation between asymptotic dimension on one hand and isoperimetry and random walks on the other. We apply these results to reprove sharp estimates on isoperimetric profiles of some groups and to answer some questions in dimension theory. 相似文献