全文获取类型
收费全文 | 1971篇 |
免费 | 333篇 |
国内免费 | 147篇 |
专业分类
化学 | 651篇 |
晶体学 | 10篇 |
力学 | 239篇 |
综合类 | 10篇 |
数学 | 101篇 |
物理学 | 1440篇 |
出版年
2024年 | 3篇 |
2023年 | 43篇 |
2022年 | 64篇 |
2021年 | 93篇 |
2020年 | 86篇 |
2019年 | 35篇 |
2018年 | 52篇 |
2017年 | 68篇 |
2016年 | 90篇 |
2015年 | 53篇 |
2014年 | 129篇 |
2013年 | 108篇 |
2012年 | 121篇 |
2011年 | 139篇 |
2010年 | 95篇 |
2009年 | 124篇 |
2008年 | 112篇 |
2007年 | 131篇 |
2006年 | 133篇 |
2005年 | 99篇 |
2004年 | 85篇 |
2003年 | 65篇 |
2002年 | 69篇 |
2001年 | 66篇 |
2000年 | 39篇 |
1999年 | 44篇 |
1998年 | 25篇 |
1997年 | 34篇 |
1996年 | 29篇 |
1995年 | 25篇 |
1994年 | 23篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 27篇 |
1990年 | 13篇 |
1989年 | 17篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1971年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有2451条查询结果,搜索用时 31 毫秒
71.
Bifurcation analysis in an SIR epidemic model with birth pulse and pulse vaccination 总被引:1,自引:0,他引:1
The dynamical behavior of an SIR epidemic model with birth pulse and pulse vaccination is discussed by means of both theoretical and numerical ways. This paper investigates the existence and stability of the infection-free periodic solution and the epidemic periodic solution. By using the impulsive effects, a Poincaré map is obtained. The Poincaré map, center manifold theorem, and bifurcation theorem are used to discuss flip bifurcation and bifurcation of the epidemic periodic solution. Moreover, the numerical results show that the epidemic periodic solution (period-one) bifurcates from the infection-free periodic solution through a supercritical bifurcation, the period-two solution bifurcates from the epidemic periodic solution through flip bifurcation, and the chaotic solution generated via a cascade of period-doubling bifurcations, which are in good agreement with the theoretical analysis. 相似文献
72.
In most models of population dynamics in a polluted environment, the emission of toxicant is generally considered to be continuous, but it is often the case that toxicant is emitted in regular pulses. This paper deals with the effects of pulse toxicant input with constant rate on two-species Lotka-Volterra competition system in a polluted environment. The thresholds between persistence and extinction of each population are obtained. Moreover, our results indicate that the release amount of toxicant and the pulse period will affect the fate of each population. Finally, the results are verified through computer simulations. 相似文献
73.
Three kinds of TVS in a SIR epidemic model with saturated infectious force and vertical transmission
Three different vaccination and treatment strategies in the SIR epidemic model with saturated infectious force and vertical transmission are analyzed. The dynamics of epidemic models are globally investigated by using Floquet theory and comparison theorem of impulsive differential equation. Thresholds are identified and global stability results are proved. For every treatment and vaccination strategy, the disease-free periodic solution of impulsive system has been obtained and is found to be globally asymptotically stable when the basic reproduction number is less than one, equivalently the cure rate is larger than the threshold value, whereas the disease is persistent when the basic reproduction number is larger than one. These results indicate that a large cure rate will lead to the eradication of a disease. 相似文献
74.
Takafumi Kondoh Akira Asano Jinfeng Yang Kimihiro Norizawa Kenji Takahashi Mitsumasa Taguchi Ryuji Nagaishi Ryuzi Katoh Yoichi Yoshida 《Radiation Physics and Chemistry》2009,78(12):1157-1160
The spectra and kinetic behavior of solvated electrons (esol−) in alkyl ammonium ionic liquids (ILs), i.e. N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEMMA-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMMA-BF4), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI), and N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14-TFSI) were investigated by the pulse radiolysis method. The esol− in each of the ammonium ILs has an absorption peak at 1100 nm, with molar absorption coefficients of 1.5–2.3×104 dm3 mol−1 cm−1. The esol− decayed by first order with a rate constant of 1.4–6.4×106 s−1. The reaction rate constant of the solvated electron with pyrene (Py) was 1.5–3.5×108 dm3 mol−1 s−1 in the various ILs. These values were about one order of magnitude higher than the diffusion-controlled limits calculated from measured viscosities. The radiolytic yields (G-value) of the esol− were 0.8–1.7×10−7 mol J−1. The formation rate constant of esol− in DEMMA-TFSI was 3.9×1010 s−1. The dry electron (edry−) in DEMMA-TFSI reacts with Py with a rate constant of 7.9×1011 dm3 mol−1 s−1, three orders of magnitude higher than that of the esol− reactions. The G-value of the esol− in the picosecond time region is 1.2×10−7 mol J−1. The capture of edry− by scavengers was found to be very fast in ILs. 相似文献
75.
76.
We report an electro-photo double modulation of the fermi level in a WSe2/graphene heterojunction. The heterojunction exhibits high ION/IOFF ratio (~103) in transfer characteristic in dark and distinct rectification behavior in output characteristic under light illumination, respectively. Time-dependent photoresponse reveals that the heterojunction has a considerable potential in the application of photodetection. Interestingly, an exotic current peak is observed in transfer characteristic under light illumination. This novel behavior is attributed to the tunable fermi level at the WSe2/graphene heterojunction by electro-photo double modulation. The results may be helpful to develop tunable photovoltaic optoelectronics based on van der Waals heterojunctions. 相似文献
77.
Laser drilling of stainless steel with nanosecond double-pulse 总被引:1,自引:0,他引:1
X.D. Wang A. Michalowski D. Walter S. Sommer M. Kraus J.S. Liu F. Dausinger 《Optics & Laser Technology》2009,41(2):148-153
Nanosecond double-pulse laser drilling is reported in this paper. The double-pulse herein represents two closely conjoint pulses with 21 ns pulse duration and about 52 ns interpulse separation, which are acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample's thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed. 相似文献
78.
The Reynolds model is a reduced Stokes model, valid for narrow lubrication regions. In order to be able to handle locally non‐narrow regions such as pits or grooves, often displaying rapid geometrical variations, there is a need to be able to transit to the more accurate Stokes model. A fundamental problem is how to couple the two models in a numerical simulation, preferably allowing for different meshes in the different domains. In this paper, we present a weak coupling method for Reynolds and Stokes models for lubrication computations, including the possibility of cavitation in the different regions. The paper concludes with a numerical example. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
79.
D.G. Pavlou 《Mechanics Research Communications》2011,38(8):546-552
An analytic solution of a thin layer bonded on a visco-elastic medium under the action of in-plane and lateral pulse loads is presented. The analysis is based on double integral transforms with respect to space and time. Representative examples are studied and discussed to determine the influence of the (a) damping and elastic coefficients of visco-elastic medium, and (b) in-plane compression and stretching of the thin layer on wave propagation. 相似文献
80.
建立了检测水中有机磷农药的离子迁移率谱仪预富集进样方法。预富集器由表面覆盖有吸附薄膜的微热板、聚四氟乙烯电路板和管座组成,具有操作简单,无需有机溶剂,自加热,热容小,功耗低等优点。以马拉硫磷检测为例,分析了富集器解吸升温速率和离子迁移率谱仪半透膜温度对检测结果的影响。采用高温短时脉冲加热和低温维持加热相结合的解吸方式,既可形成较高的进样浓度脉冲,又可减少进入漂移管的杂质,有利于提高离子迁移率谱仪检测灵敏度。实验表明:采用所述预富集及两阶段加热解吸进样方法,对水中马拉硫磷的检出限为3.9μg/L,达到了国家标准对水中有机磷检测的要求。 相似文献