全文获取类型
收费全文 | 1485篇 |
免费 | 43篇 |
国内免费 | 10篇 |
专业分类
化学 | 1000篇 |
晶体学 | 88篇 |
力学 | 22篇 |
数学 | 125篇 |
物理学 | 303篇 |
出版年
2023年 | 15篇 |
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 13篇 |
2019年 | 39篇 |
2018年 | 36篇 |
2017年 | 38篇 |
2016年 | 42篇 |
2015年 | 46篇 |
2014年 | 67篇 |
2013年 | 122篇 |
2012年 | 83篇 |
2011年 | 106篇 |
2010年 | 54篇 |
2009年 | 59篇 |
2008年 | 92篇 |
2007年 | 65篇 |
2006年 | 58篇 |
2005年 | 46篇 |
2004年 | 37篇 |
2003年 | 38篇 |
2002年 | 48篇 |
2001年 | 24篇 |
2000年 | 20篇 |
1999年 | 18篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 15篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 14篇 |
1986年 | 16篇 |
1985年 | 18篇 |
1984年 | 23篇 |
1983年 | 23篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 7篇 |
排序方式: 共有1538条查询结果,搜索用时 15 毫秒
121.
Sivasankar M Bauer JJ Babu T Larson CR 《The Journal of the Acoustical Society of America》2005,117(2):850-857
The present study was undertaken to examine if a subject's voice F0 responded not only to perturbations in pitch of voice feedback but also to changes in pitch of a side tone presented congruent with voice feedback. Small magnitude brief duration perturbations in pitch of voice or tone auditory feedback were randomly introduced during sustained vowel phonations. Results demonstrated a higher rate and larger magnitude of voice F0 responses to changes in pitch of the voice compared with a triangular-shaped tone (experiment 1) or a pure tone (experiment 2). However, response latencies did not differ across voice or tone conditions. Data suggest that subjects responded to the change in F0 rather than harmonic frequencies of auditory feedback because voice F0 response prevalence, magnitude, or latency did not statistically differ across triangular-shaped tone or pure-tone feedback. Results indicate the audio-vocal system is sensitive to the change in pitch of a variety of sounds, which may represent a flexible system capable of adapting to changes in the subject's voice. However, lower prevalence and smaller responses to tone pitch-shifted signals suggest that the audio-vocal system may resist changes to the pitch of other environmental sounds when voice feedback is present. 相似文献
122.
Effects of uncertainties in the domain on the solution of Dirichlet boundary value problems 总被引:1,自引:0,他引:1
Summary. A domain with possibly non-Lipschitz boundary is defined as a limit of monotonically expanding or shrinking domains with
Lipschitz boundary. A uniquely solvable Dirichlet boundary value problem (DBVP) is defined on each of the Lipschitz domains
and the limit of these solutions is investigated. The limit function also solves a DBVP on the limit domain but the problem
can depend on the sequences of domains if the limit domain is unstable with respect to the DBVP. The core of the paper consists
in estimates of the difference between the respective solutions of the DBVP on two close domains, one of which is Lipschitz
and the other can be unstable. Estimates for starshaped as well as rather general domains are derived. Their numerical evaluation
is possible and can be done in different ways.
Received October 16, 2001 / Revised version received January 16, 2002 / Published online: April 17, 2002
RID="*"
ID="*" The research was funded partially by the National Science Foundation under the grants NSF–Czech Rep. INT-9724783 and
NSF DMS-9802367
RID="**"
ID="**" Support for Jan Chleboun coming from the Grant Agency of the Czech Republic through grant 201/98/0528 is appreciated 相似文献
123.
This paper is a summary of our study on the superconvergence of the finite element solutions and error estimators. We will persent the analysis of %-superconvergence for finite element solutions of the Poisson equation in the interior of meshes of triangles with straight edges, as well as the analysis at the boundary. The %-superconvergence via local averaging will also be presented, and the error estimators are compared in the sense of %-superconvergence. 相似文献
124.
S. Surendra Babu C.K. Jayasankar W. Sievers G. Wortmann 《Journal of luminescence》2007,126(1):109-120
Phosphate (P2O5+K2O+BaO+Al2O3+Eu2O3) and fluorophosphate (P2O5+K2O+BaO+BaF2+Al2O3+Eu2O3) glasses with different Eu3+ ion concentrations have been prepared and characterized through optical absorption, photoluminescence and decay times. An intense red luminescence is observed from the 5D0 emitting level of Eu3+ ions in these glasses. The relative luminescence intensity ratio of 5D0→7F2→5D0→7F1 transitions has been evaluated to estimate the local site symmetry around the Eu3+ ions. The emission spectra of these glasses show a complete removal of degeneracy for the 5D0→7F1 and 5D0→7F2 transitions. Second and fourth rank crystal-field (CF) parameters have been calculated together with the CF strength parameter by assuming the C2v symmetry for the Eu3+ ions in both the phosphate and fluorophosphate glasses. Judd-Ofelt parameters have been evaluated from the luminescence intensity ratios of 5D0→7FJ (J=2, 4 and 6) to 5D0→7F1 transitions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and peak stimulated emission cross-sections for the 5D0→7FJ transitions. Decay curves of the 5D0 level of Eu3+ ions in these two Eu3+:glass systems have been measured by monitoring the 5D0→7F2 transition (611 nm) at room temperature. The experimental lifetime of the 5D0 level in the title glasses is found to be higher than Eu3+-doped niobium phosphate glasses. The analysis indicates that the lifetime of the 5D0 level is found to be less sensitive to the Eu3+ ion concentration and addition of BaF2 has no significant effect on the optical properties of Eu3+-doped phosphate glasses. 相似文献
125.
126.
D.A. Cardwell Y. Shi N. Hari Babu K. Iida 《Physica C: Superconductivity and its Applications》2009,469(15-20):1146-1152
A practical processing route for the fabrication of LRE-Ba–Cu–O single grain superconductors has been developed at the University of Cambridge based on a generic, Mg-doped Nd-123 melt textured seed and suppression of the formation of the solid solution phase in air by enriching the precursors with higher Ba concentration. The processing of high performance Gd–Ba–Cu–O single grains using this processing route is described. The Mg-doped generic seed crystal has been used effectively to promote heterogeneous nucleation via a cold-seeding process. The Gd/Ba solid solution has been suppressed by enriching Gd–Ba–Cu–O precursor powders with two different Ba-rich compositions. This involved adding BaO2 and GdBa6Cu3Oy (Gd-163) (a novel Ba-rich second phase) to the precursor powders, respectively. The Gd-163 phase has been observed not only to suppress formation of the solid solution phase, but also to promote increased heterogeneous grain size. A detailed further study has been carried out with an initial aim of optimizing the BaO2 and Gd-163 phase content of the precursor composition to produce a single grain almost free of solid solution. Based on the optimized parameters, large single grain Gd–Ba–Cu–O superconductors have been fabricated in an air atmosphere and demonstrated to exhibit record trapped magnetic fields for this material melt processed in air in relatively small single grain samples. The trapped fields of samples produced in air atmosphere are at least comparable to those processed under reduced oxygen partial pressure. 相似文献
127.
The electric and magnetic fields within optical thin films are obtained exactly in closed form as solutions of a system of difference equations presented by Berning and utilized by Dill, Neureuther, Tuttle, and Walker in their sample program for modeling optical lithography. The solution allows an explicit closed form expression for the absorbance. 相似文献
128.
Douglas N. Hales V. Sridharan Abirami Radhakrishnan Satya S. Chakravorty Samia M. Siha 《European Journal of Operational Research》2008
Although Information Technology (IT) solutions improve the collection and validation of operational data, Operations Managers must often rely on self-reported data from workers to make decisions. The problem with this data is that they are subject to intentional manipulation, thus reducing their suitability for decision-making. A method of identifying manipulated data, digital analysis, addresses this problem at low cost. In this paper, we demonstrate how one uses this method in real-world companies to validate self-reported data from line workers. The results of our study suggest that digital analysis estimates the accuracy of employee reported data in operations management, within limited contexts. These findings lead to improved operating performance by providing a tool for practitioners to exclude inaccurate information. 相似文献
129.
K. Jagadeeswara Pillai S. Vijaya Kumar Varma M. Syam Babu 《Proceedings Mathematical Sciences》1987,96(1):61-69
The aligned magnetic effects on a steady laminar, viscous, incompressible, conducting fluid down an open inclined channel
bounded below by a bed of varying permeability has been studied when the free surface is exposed to atmospheric pressure.
Beavers and Joseph slip condition at the interface of the free flow region and the fluid flow in the porous bed and the Darcy’s
law in the porous medium have been used. The expressions for velocity, magnetic strength and the mass flow across the cross-section
of the channel are obtained. 相似文献
130.
Semen B. Kharchenko Rangaramanujam M. Kannan Jeff J. Cernohous Shivshankar Venkataramani Gaddam N. Babu 《Journal of Polymer Science.Polymer Physics》2001,39(21):2562-2571
With the increase in sophisticated synthesis methods, it appears that polymer architecture may be a tunable property. Therefore, the role of architecture in rheological and processing properties has received renewed attention, mainly because of dendrimer synthesis and metallocene‐catalyst technology. Linear polymers and hyperbranched polymers represent two ends of branching complexity. Some previous studies have suggested that hyperbranched polymers may behave like unentangled polymers, whereas others have proposed that they exhibit the properties of soft colloids. In an effort to compare the responses of linear and hyperbranched polymers, we synthesized starlike hyperbranched polystyrenes (HBPSs) of various branch lengths and numbers of branches. The HBPSs used in this study were unentangled or weakly entangled, allowing us to study the effect of branch density more readily. Two linear polystyrene (L‐PS) melts and two HBPSs were studied. Using a custom‐built rheooptical apparatus, we characterized the rheology and flow birefringence of these materials. To our knowledge, these are the first flow birefringence measurements on highly branched polymer melts. Our results suggest that the flow behavior of HBPS is significantly different from that of L‐PS: (1) HBPS shows nonterminal behavior in the low‐frequency rheological response; (2) when the stress‐optical rule (SOR) holds, the stress‐optical coefficient of HBPS is much lower than those of analogous linear polymers; and (3) when the branch density is high and the branch length is sufficiently low, the SOR fails for these homopolymer melts. A significant increase in the birefringence for a given amount of stress in the low‐frequency region suggests that there may be a soft core in these materials due to the strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery. The predominantly elastic response of the soft structures may be responsible for the enhanced form birefringence. Our preliminary results indicate that these materials may exhibit both polymeric and soft‐colloid natures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2562–2571, 2001 相似文献