全文获取类型
收费全文 | 17861篇 |
免费 | 3491篇 |
国内免费 | 2935篇 |
专业分类
化学 | 9924篇 |
晶体学 | 212篇 |
力学 | 2631篇 |
综合类 | 184篇 |
数学 | 1469篇 |
物理学 | 9867篇 |
出版年
2024年 | 42篇 |
2023年 | 216篇 |
2022年 | 390篇 |
2021年 | 602篇 |
2020年 | 799篇 |
2019年 | 593篇 |
2018年 | 586篇 |
2017年 | 698篇 |
2016年 | 812篇 |
2015年 | 728篇 |
2014年 | 1018篇 |
2013年 | 1550篇 |
2012年 | 1071篇 |
2011年 | 1176篇 |
2010年 | 1009篇 |
2009年 | 1240篇 |
2008年 | 1255篇 |
2007年 | 1283篇 |
2006年 | 1233篇 |
2005年 | 994篇 |
2004年 | 903篇 |
2003年 | 848篇 |
2002年 | 675篇 |
2001年 | 603篇 |
2000年 | 581篇 |
1999年 | 527篇 |
1998年 | 453篇 |
1997年 | 362篇 |
1996年 | 295篇 |
1995年 | 261篇 |
1994年 | 238篇 |
1993年 | 174篇 |
1992年 | 143篇 |
1991年 | 146篇 |
1990年 | 105篇 |
1989年 | 98篇 |
1988年 | 91篇 |
1987年 | 72篇 |
1986年 | 65篇 |
1985年 | 62篇 |
1984年 | 43篇 |
1983年 | 22篇 |
1982年 | 39篇 |
1981年 | 37篇 |
1980年 | 29篇 |
1979年 | 31篇 |
1978年 | 13篇 |
1977年 | 22篇 |
1976年 | 12篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Norbornene derivatives from a metal‐free,strain‐promoted cycloaddition reaction: New building blocks for ring‐opening metathesis polymerization reactions 下载免费PDF全文
Christopher E. Hobbs Binhong Lin Thomas Malinski 《Journal of polymer science. Part A, Polymer chemistry》2015,53(20):2357-2362
The preparation of new ring opening metathesis polymerization (ROMP) monomers using a 1,3‐dipolar cycloaddition between aryl azides and norbornadiene is described. Various norbornenetriazolines, obtained through a solvent‐and catalyst‐free reaction, can subsequently be incorporated into polymer backbones through ROMP reactions. Furthermore, thermal decomposition of the triazoline moiety can allow for further polymer functionalization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2357–2362 相似文献
22.
裂纹前端的断裂过程区是引起岩石非线性断裂及尺寸效应的主要原因。利用数字图像相关技术对砂岩开展了三点弯曲梁实验,获得观测区域高精度的全场位移和应变数据,根据断裂韧带区域水平位移和水平应变的分布特征,结合裂尖岩石颗粒变化的微观分析,提出采用裂纹尖端水平位移波动性和水平应变突变性所得到的波动系数和水平应变突变值,确定断裂过程区形状和临界尺寸的方法。结果表明:砂岩断裂过程区的形状为不规则的狭长带状区域,断裂过程区的临界长度为11~13mm,临界宽度为1.58~2.36mm。断裂过程区区域内形变在趋向裂尖时呈指数增加,但其单位区域内的形变增量呈波动状态。该方法能够更加准确判断岩石断裂过程区的范围,有助于分析岩石的非线性断裂特性。 相似文献
23.
Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S0 and S1 states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S1 state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications. 相似文献
24.
In view of immense importance of silylenes and the fact that their properties undergo significant changes on substitution with halogens, here, we have used B3LYP/6-311++G** level of theory to access the effects of 1–4 halogens (X = F, Cl, Br, and I) on four unprecedented sets of cyclopentasilylene-2,4-dienes; with the following formulas: SiC4H3X ( 1 X ), SiC4H2X2 ( 2 X ), SiC4HX3 ( 3 X ), and SiC4X4 ( 4 X ). In going down from F to I, the singlet (s)-triplet (t) energy gap (ΔEs-t, a possible indication of stability), and band gap (ΔEH-L) decrease while nucleophilicity (N), chemical potential (μ), and proton affinity (PA) increase. The overall order of N, μ, and PA for each X is 2 X > 1 X > 3 X > 4 X . Precedence of 2 X over 1 X is attributed to the symmetric cross conjugation in the former. The highest and lowest N are shown by 2 I and 4 F . The trend of divalent angle () for each X is 4 X > 1 X > 3 X > 2 X . The results show that in going from electron withdrawing groups (EWGs) to electron donating groups (EDGs), the ΔEs-t and ΔEH-L decrease while N, μ, and PA increase. Also, rather high N of our scrutinized silylenes may suggest new promising ligands in organometallic chemistry. 相似文献
25.
26.
Nagendranath Mahata V. Vishwanathan 《Journal of molecular catalysis. A, Chemical》1997,120(1-3):267-270
Kinetics of vapor phase hydrogenation of phenol to cyclohexanone over Pd/MgO system has been studied in a flow microreactor under normal atmospheric pressure. The reaction rate is found to be negative order with respect to the partial pressure of phenol and has increased from −0.5 to 0.5 with increasing temperature (473 to 563 K). The apparent activation energy (Ea) of the process is found to be close to 65 kJ per mol. On the basis of kinetic results a surface mechanism is proposed. 相似文献
27.
28.
29.
30.