全文获取类型
收费全文 | 17456篇 |
免费 | 2799篇 |
国内免费 | 2082篇 |
专业分类
化学 | 12629篇 |
晶体学 | 286篇 |
力学 | 908篇 |
综合类 | 156篇 |
数学 | 1937篇 |
物理学 | 6421篇 |
出版年
2024年 | 49篇 |
2023年 | 349篇 |
2022年 | 471篇 |
2021年 | 569篇 |
2020年 | 642篇 |
2019年 | 701篇 |
2018年 | 570篇 |
2017年 | 562篇 |
2016年 | 780篇 |
2015年 | 773篇 |
2014年 | 978篇 |
2013年 | 1332篇 |
2012年 | 1572篇 |
2011年 | 1704篇 |
2010年 | 1146篇 |
2009年 | 1138篇 |
2008年 | 1147篇 |
2007年 | 1109篇 |
2006年 | 894篇 |
2005年 | 776篇 |
2004年 | 649篇 |
2003年 | 513篇 |
2002年 | 484篇 |
2001年 | 392篇 |
2000年 | 355篇 |
1999年 | 285篇 |
1998年 | 238篇 |
1997年 | 188篇 |
1996年 | 215篇 |
1995年 | 182篇 |
1994年 | 180篇 |
1993年 | 151篇 |
1992年 | 155篇 |
1991年 | 119篇 |
1990年 | 116篇 |
1989年 | 108篇 |
1988年 | 65篇 |
1987年 | 64篇 |
1986年 | 65篇 |
1985年 | 70篇 |
1984年 | 61篇 |
1983年 | 41篇 |
1982年 | 21篇 |
1981年 | 22篇 |
1979年 | 30篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1975年 | 19篇 |
1974年 | 19篇 |
1973年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl2F2 as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl2F2/SiCl4 ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl2F2 as a dopant precursor with suitable flow of SiCl4 vapor along with O2 through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of 19F MAS spectra done by high-resolution 19F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO2F polyhedral which become increased with increasing the fluorine content. 相似文献
92.
It is shown that time-dependent temperatures in a transient, conductive system can be approximately modeled by a fractional-order
differential equation, the order of which depends on the Biot number. This approximation is particularly suitable for complex
shapes for which a first-principles approach is too difficult or computationally time-consuming. Analytical solutions of these
equations can be written in terms of the Mittag-Leffler function. The approximation is especially useful if a suitable fractional-order
controller is to be designed for the system. 相似文献
93.
以一种新型的醌类光敏剂—菌生素 (HMB)为模型化合物 ,利用量子化学从头算HF/ 6 31G和含时密度泛函TD B3LYP/ 6 31G方法计算研究了卤代作用对醌类光敏剂分子性质和光敏活性的影响 .结果发现 ,卤代作用降低了HMB母体的HOMO和LUMO能级 (EHOMO和ELUMO)及其差值△E ,随卤族元素从上到下 ,EHOMO和ELUMO呈增大趋势 ,而△E呈减小趋势 ,使得分子激发光谱有不同程度的红移 ,在增加PQP三重态量子产率的同时降低了分子最低三重激发态的能量 ,两种作用相互抵消 ,使得卤代物的 1O2 量子产率与母体相似 ;增加了分子的绝热电子亲合势 ,使分子产生O2 -的能力下降 ;卤原子的引入 ,降低了HMB母体分子内氢键的强度 ,同时使顺式构型的分子内质子传递势垒增大 ,而使反式构型势垒减小 . 相似文献
94.
LIANG Liang 《原子与分子物理学报》2003,20(4):575-578
By using the multichannel quantum defect theory(MQDT),we have evaluated the energy levels and lifetimes of 2sns ^3 S1,2snd ^3D(n=3-25)of BeI and 1 sns ^3s1,1snd ^3D(n=3-25)of BeⅢ,These energies and lifetimes that we have calculated not only agree with the recent measurements and theoretical calculation of Ref.4 and Ref.3, but also predict the lifetimes of 66 other highly excited states. 相似文献
95.
A polarization-insensitive 40-Gb/s time-division demultiplexer was demonstrated in a polarization-diversity loop configuration. The power penalty was measured to be 2.3 dB at a bit error rates of 10-9 for 10 Gb/s output signals. 相似文献
96.
97.
Huang H. X. Liang Z. A. Pardalos P. M. 《Journal of Optimization Theory and Applications》2004,120(1):53-71
A flow search approach is presented in this paper. In the approach, each iterative process involves a subproblem, whose variables are the stepsize parameters. Every feasible solution of the subproblem corresponds to some serial search stages, the stepsize parameters in different search stages may interact mutually, and their optimal values are determined by evaluating the total effect of the interaction. The main idea of the flow search approach is illustrated via the minimization of a convex quadratic function. Based on the flow search approach, some properties of the m-step linear conjugate gradient algorithm are analyzed and new bounds on its convergence rate are also presented. Theoretical and numerical results indicate that the new bounds are better than the well-known ones. 相似文献
98.
Cellular neural network to detect spurious vectors in PIV data 总被引:2,自引:0,他引:2
99.
KF-Al2O3催化合成2-甲基-1-苄基苯并咪唑 总被引:1,自引:0,他引:1
苯并咪唑类杂环化合物具有杀菌、消炎、抗癌、抗氧化的活性,广泛用于食品的防虫和防腐,用于动、植物病毒的防治,人体疾病和肿瘤的防治[1-3],本文用KF Al2O3作催化剂,用DMF为溶剂,利用2 甲基苯并咪唑与苄基溴反应合成2 甲基 1 苄基苯并咪唑的新型方法,考察了诸因素对产率的影响,找出了最佳的反应条件。合成路线如下:1 实验部分1 1 仪器与试剂Brurer AM 500超导核磁共振波谱仪(溶剂CDCl3);CarloErbal1106型元素分析仪,NicoletIm pact410型红外光谱仪,KBr压片,HP1100型和Finnigan FTMS 2000型质谱仪(以上均由华东理工大学分析… 相似文献
100.
This study investigates the robust output tracking problem for a class of uncertain linear systems. The uncertainties are assumed to be time invariant and to satisfy the matching conditions. According to the selected nominal parameters, an optimal solution with a prescribed degree of stability is determined. Then, an auxiliary input via the use of an adapting factor, connected to the nominal optimal control, is introduced to guarantee the robustness and prescribed degree of stability for the output tracking control of the uncertain linear systems. This method is very simple and effective and can reject bounded uncertainties imposed on the states. A maglev vehicle model example is given to show its effectiveness. 相似文献