全文获取类型
收费全文 | 17426篇 |
免费 | 2248篇 |
国内免费 | 2119篇 |
专业分类
化学 | 13256篇 |
晶体学 | 130篇 |
力学 | 1457篇 |
综合类 | 105篇 |
数学 | 1241篇 |
物理学 | 5604篇 |
出版年
2024年 | 26篇 |
2023年 | 170篇 |
2022年 | 322篇 |
2021年 | 489篇 |
2020年 | 601篇 |
2019年 | 506篇 |
2018年 | 417篇 |
2017年 | 494篇 |
2016年 | 660篇 |
2015年 | 721篇 |
2014年 | 896篇 |
2013年 | 1434篇 |
2012年 | 1040篇 |
2011年 | 1292篇 |
2010年 | 1037篇 |
2009年 | 1147篇 |
2008年 | 1229篇 |
2007年 | 1183篇 |
2006年 | 1172篇 |
2005年 | 965篇 |
2004年 | 866篇 |
2003年 | 699篇 |
2002年 | 717篇 |
2001年 | 467篇 |
2000年 | 433篇 |
1999年 | 392篇 |
1998年 | 316篇 |
1997年 | 311篇 |
1996年 | 267篇 |
1995年 | 253篇 |
1994年 | 207篇 |
1993年 | 177篇 |
1992年 | 170篇 |
1991年 | 124篇 |
1990年 | 105篇 |
1989年 | 75篇 |
1988年 | 78篇 |
1987年 | 50篇 |
1986年 | 42篇 |
1985年 | 43篇 |
1984年 | 41篇 |
1983年 | 18篇 |
1982年 | 37篇 |
1981年 | 18篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 14篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 183 毫秒
81.
82.
The major goal of this paper is to examine the hypothesis that stock returns and return volatility are asymmetric, threshold nonlinear, functions of change in trading volume. A minor goal is to examine whether return spillover effects also display such asymmetry. Employing a double-threshold GARCH model with trading volume as a threshold variable, we find strong evidence supporting this hypothesis in five international market return series. Asymmetric causality tests lend further support to our trading volume threshold model and conclusions. Specifically, an increase in volume is positively associated, while decreasing volume is negatively associated, with the major price index in four of the five markets. The volatility of each series also displays an asymmetric reaction, four of the markets display higher volatility following increases in trading volume. Using posterior odds ratio, the proposed threshold model is strongly favored in three of the five markets, compared to a US news double threshold GARCH model and a symmetric GARCH model. We also find significant nonlinear asymmetric return spillover effects from the US market. 相似文献
83.
This paper presents a new scheduling method for manufacturing system based on the Timed Petri Net model and a reactive fast graph search algorithm. The following two typical problems are addressed in this paper. (1) Minimization of the maximum completion time. (2) Minimization of the total tardiness. As for the problem (1), a new search algorithm which combines the RTA∗ and a rule-based supervisor is proposed. As for problem (2), the original Petri Net model is converted to its reverse model and the algorithm developed for the problem (1) is applied, regarding the due date as the starting time in the reverse model. Some numerical experiments are carried out to demonstrate usefulness of our algorithm. 相似文献
84.
A nonclassical problem is considered for the transport equation with coefficients depending on the energy of radiation. The task is to find the discontinuity surfaces for the coefficients of the equation from measurements of the radiation flux leaving the medium. For this tomography problem, an optimization problem is stated and numerically analyzed. The latter consists in determining the radiation energy that ensures the best reconstruction of the unknown medium. A simplified optimization problem is solved analytically. 相似文献
85.
Leong‐Huat Gan Palaniswamy Ravi Bao Wei Mao Kam‐Chiu Tam 《Journal of polymer science. Part A, Polymer chemistry》2003,41(17):2688-2695
Polymerization of 2‐(diethylamino)ethyl methacrylate (DEAEMA) via homogeneous atom transfer radical polymerization under various reaction conditions is described. The effects of the initiators and solvents were examined. With 1,1,4,7,10,10‐hexamethyl triethylenetetramine/copper(I) chloride/p‐toluenesulfonyl chloride as the ligand/catalyst/initiator system in methanol, poly(DEAEMA) with a polydispersity index as low as 1.07 was synthesized. Kinetic studies demonstrated the polymerization was very well controlled and exhibited the living characteristic of the process. Well‐defined block copolymers of DEAEMA and tert‐butyl methacrylate (tBMA) were successfully synthesized. The copolymers could be synthesized with equally good results by starting with either p(DEAEMA) or p(tBMA) as the macroinitiators. However, only the macroinitiators terminated with chlorine should be used. The corresponding macroinitiators with bromine as a transferable group did not yield well‐defined copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2688–2695, 2003 相似文献
86.
The sub-millisecond protonation dynamics of the chromophore in S65T mutant form of the green fluorescent protein (GFP) was
tracked after a rapid pH jump following laser-induced proton release from the caged photolabile compoundo-nitrobenzaldehyde. Following a jump in pH from 8 to 5 (which is achieved within 2 μs), the fluorescence of S65T GFP decreased
as a single exponential with a time constant of ∼90 μs. This decay is interpreted as the conversion of the deprotonated fluorescent
GFP chromophore to a protonated non-fluorescent species. The protonation kinetics showed dependence on the bulk viscosity
of the solvent, and therefore implicates bulk solvent-controlled protein dynamics in the protonation process. The protonation
is proposed to be a sequential process involving two steps: (a) proton transfer from solvent to the chromophore, and (b) internal
structural rearrangements to stabilize a protonated chromophore. The possible implications of these observations to protein
dynamics in general is discussed 相似文献
87.
88.
A rigorous and systematic intercomparison of codes used for the retrieval of trace gas profiles from high-resolution ground-based solar absorption FTIR measurements is presented for the first time. Spectra were analyzed with the two widely used independent, retrieval codes: SFIT2 and PROFFIT9. Vertical profiles of O3, HNO3, HDO, and N2O were derived from the same set of typical observed spectra. Analysis of O3 was improved by using updated line parameters. It is shown that profiles and total column amounts are in excellent agreement, when similar constraints are applied, and that the resolution kernel matrices are also consistent. Owing to the limited altitude resolution of ground-based observations, the impact of the constraints on the solution is not negligible. It is shown that the results are also compatible for independently chosen constraints. Perspectives for refined constraints are discussed. It can be concluded that the error budget introduced by the radiative transfer code and the retrieval algorithm on total columns deduced from high-resolution ground-based solar FTIR spectra is below 1%. 相似文献
89.
90.
Photoluminescence (PL), photostimulated luminescence (PSL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on LiYF4:U4+ and pure LiYF4 crystals. The PL and EPR investigations have identified the presence of Eu3+, Tb3+ and Gd3+ ions in both of these crystals possibly due to their existence in the starting materials. The luminescence observed during afterglow, PSL and TSL revealed that emission occurs at wavelength positions 382, 413, 437 and 544 nm, which are characteristic of Tb3+ ions. The present investigations using PSL and TSL in combination with PL studies before and after gamma irradiation have revealed that selective energy transfer to Tb3+ ions occurs during electron–hole recombination processes like PSL and TSL. Even though other luminescent ions (U4+ and Eu3+) are present in the system and U4+/U3+ ions are participating in electron capture/release processes, the selective energy transfer results in Tb3+ ions acting as luminescence centers. 相似文献