全文获取类型
收费全文 | 22979篇 |
免费 | 2428篇 |
国内免费 | 2908篇 |
专业分类
化学 | 16730篇 |
晶体学 | 205篇 |
力学 | 1678篇 |
综合类 | 125篇 |
数学 | 935篇 |
物理学 | 8642篇 |
出版年
2024年 | 31篇 |
2023年 | 223篇 |
2022年 | 324篇 |
2021年 | 571篇 |
2020年 | 743篇 |
2019年 | 608篇 |
2018年 | 514篇 |
2017年 | 653篇 |
2016年 | 838篇 |
2015年 | 901篇 |
2014年 | 1177篇 |
2013年 | 1987篇 |
2012年 | 1319篇 |
2011年 | 1783篇 |
2010年 | 1403篇 |
2009年 | 1541篇 |
2008年 | 1624篇 |
2007年 | 1595篇 |
2006年 | 1495篇 |
2005年 | 1238篇 |
2004年 | 1114篇 |
2003年 | 905篇 |
2002年 | 909篇 |
2001年 | 628篇 |
2000年 | 581篇 |
1999年 | 505篇 |
1998年 | 439篇 |
1997年 | 386篇 |
1996年 | 341篇 |
1995年 | 314篇 |
1994年 | 265篇 |
1993年 | 222篇 |
1992年 | 219篇 |
1991年 | 165篇 |
1990年 | 132篇 |
1989年 | 104篇 |
1988年 | 106篇 |
1987年 | 63篇 |
1986年 | 54篇 |
1985年 | 45篇 |
1984年 | 50篇 |
1983年 | 20篇 |
1982年 | 47篇 |
1981年 | 22篇 |
1980年 | 27篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1976年 | 16篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
991.
992.
The steady laminar boundary layer flow and heat transfer past a stretching sheet arre considered. Upper‐convected Maxwell (UCM) fluid is treated as a rheological model. The resulting nonlinear differential system is solved by homotopy analysis method (HAM). The influence of melting parameter (M), Prandtl number (Pr), Deborah number (β) and stretching ratio (A = a/c) on the velocity and temperature profiles is thoroughly examined. It is noticed that fields are effected appreciably with the variation of parameters. Furthermore, it is seen that the local Nusselt number is a decreasing function of melting parameter. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
993.
994.
Hiroshi Sakai Takashi Yamazaki Nobuya Machida Toshihiko Shigematsu Saburo Nasu 《Molecular Crystals and Liquid Crystals》2013,570(2):105-110
Abstract Mössbauer spectra of the FePS3-cobaltocene intercalation compound were measured in the temperature range of 300K to 10K. The spectra, distinct from those of pure FePS3, suggest the charge transfer from cobaltocene to Fe-S antibonding orbitals of the FePS3 host lattice. 相似文献
995.
996.
Christoph Kaiser Dr. Thomas Halbritter Dr. Alexander Heckel Dr. Josef Wachtveitl 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(35):9160-9173
Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton-release reactions and pH values of solutions. We present the first time-resolved spectroscopic study of the excited state and proton-release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton-release mechanisms: one site acts as a photoacid generator as its pKa value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pKa drop of 8.7 units to −5.5 upon excitation, the latter phenolic site is regarded a super-photoacid. The 6-nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8-dinitro compound an intramolecular proton transfer to the ortho-nitro group is suggested that is involved in a rapid relaxation into the ground state. 相似文献
997.
Dr. Jing Li Dr. Martin J. Lear Prof. Yujiro Hayashi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(19):5901-5905
Cyclopropanes are traditionally prepared by the formal [2+1] addition of carbene or radical based C1 units to alkenes. In contrast, the one-pot intermolecular cyclopropanation of alkanes by redox active C1 units has remained unrealised. Herein, we achieved this process simply by exposing β-aryl propionitriles and C1 radical precursors (N-oxy esters) to base and blue light. The overall process is redox-neutral and a photocatalyst, whether metal- or organic-based, is not required. Our findings support that single electron transfer (SET) from the α-cyano carbanion of the propionitrile to the N-oxy ester is facilitated by blue-light via their electron donor–acceptor (EDA) complex. The α-cyano carbon radical thus formed can then lose a β-proton to form a π-resonance stabilised radical anion that preferentially couples at the benzylic β-position with a decarboxylated C1 radical unit. This new transition metal-free chemistry tolerates both electron rich and electron deficient (hetero)aryl systems, even sulfide or alkene functionality, to afford a range of cis-aryl/cyano cyclopropanes bearing congested tetrasubstituted quaternary carbons. 相似文献
998.
A fast procedure has been developed for the direct determination of paint solvents. The method is based on the injection of small volumes of untreated solvent mixtures into a heated Pyrex glass reactor in which the sample is volatilized and introduced by means of a flow of nitrogen into an IR multiple pass gas cell and the spectrum in the mid-IR region is registered as a function of time. Data found for samples are interpolated in calibration graphs obtained by injecting different volumes of pure compounds which constitute the solvent mixture. A methylisobutylketone (MIBK) toluene mixture was used as test system to develop the proposed procedure. The method provides a limit of detection of the order of 1–4 μg, a relative standard deviation of the order of 0.4 to 2% for five independent measurements, and recovery percentage values from 99.8 till 103.6 %. A commercial sample of polyurethane paint solvent has been analysed by the developed procedure. 相似文献
999.
确定了奈韦拉平与铬天青S的电荷转移反应条件,同时建立了测定奈韦拉平的分光光度法。奈韦拉平与铬天青S在水和丙酮混合溶剂中发生电荷转移反应,常温下反应30min后可比色。电荷转移络合物的最大吸收波长是578nm,表观摩尔吸光系数为1.78×104L·mol-1·cm-1。奈韦拉平质量浓度在1~16μg·mL-1范围内服从比耳定律,相关系数为0.9993。当奈韦拉平浓度为6μg·mL-1时,六次测定结果的相对标准偏差为1.61%。测得荷移络合物的组成和稳定常数分别为1∶1和1.73×104。本方法用于测定制剂中奈韦拉平的含量,结果与文献方法一致,回收率在98.0%以上。 相似文献
1000.
Thomas J. Gedemer 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(1):95-104
It is difficult to estimate the degree of thermal degradation which has taken place in complex parts composed of cellulosics and polymeric insulation unless complete failure has occurred. There is a definite need for a parameter which can quickly and easily measure the relative degree of thermal attack on organic insulation subjected to various known environmental and processing conditions. This paper reports preliminary work using derivative thermogravimetry to perform this function. Derivative thermogravimetric analysis (TGA) was used to produce a value, TGA Index, related to the peak height of the major TGA derivative signal. This TGA Index was found to correlate with the copper number of insulating paper measured after thermal degradation. The application of TGA Index to evaluation of polymer degradation after thermal shock, water absorption, and accelerated UV attack is also discussed. Finally, preliminary studies on the use of derivative TGA to evaluate coatings exposed to various outdoor environments around the country are reported. 相似文献