全文获取类型
收费全文 | 1125篇 |
免费 | 169篇 |
国内免费 | 158篇 |
专业分类
化学 | 587篇 |
晶体学 | 11篇 |
力学 | 230篇 |
综合类 | 19篇 |
数学 | 109篇 |
物理学 | 496篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 34篇 |
2021年 | 35篇 |
2020年 | 44篇 |
2019年 | 29篇 |
2018年 | 38篇 |
2017年 | 36篇 |
2016年 | 52篇 |
2015年 | 44篇 |
2014年 | 56篇 |
2013年 | 84篇 |
2012年 | 63篇 |
2011年 | 71篇 |
2010年 | 51篇 |
2009年 | 59篇 |
2008年 | 74篇 |
2007年 | 66篇 |
2006年 | 69篇 |
2005年 | 63篇 |
2004年 | 58篇 |
2003年 | 46篇 |
2002年 | 54篇 |
2001年 | 35篇 |
2000年 | 30篇 |
1999年 | 29篇 |
1998年 | 29篇 |
1997年 | 44篇 |
1996年 | 26篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 12篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1452条查询结果,搜索用时 0 毫秒
981.
苯乙酸乙酯 ,常作为修饰剂和主香剂 ,应用于烟草、食品、日用化妆品等行业 ,还可以用于生产巴比妥类催眠药鲁米那 ,也可作溶剂。工业生产中采用硫酸催化合成苯乙酸乙酯 ,但硫酸有脱水和氧化作用 ,生成众多副产物 ,产物后处理工艺复杂 ,污水排放量大 ,严重腐蚀设备。本文合成了稀土固体超强酸SO2 -4 /TiO2 /La3+催化剂 ,考察了影响催化活性的一些因数及催化合成苯乙酸乙酯的条件。此催化剂不仅克服了浓硫酸催化的一些缺点 ,而且酯化率高 ,反应时间缩短一半 ,反应温度降低 1 5℃ ,容易回收、可以多次重复使用 ,适宜工业生产。1 实验1 … 相似文献
982.
预加载路径对低碳钢强化规律的影响 总被引:3,自引:1,他引:2
本文报导了20^#低碳钢薄壁圆管试件在比例加载时P-p复杂应力状态的实验结果。主要研究了:(1)初始屈服面;(2)经简单拉伸预加载和比例预加载情况下的强化规律;(3)折线预加载情况下的强化规律;(4)预施塑性变形量相等路径不同时的强化规律。 相似文献
983.
Johan B. Malherbe 《固体与材料科学评论》1994,19(3):129-195
Ion bombardment often leads to compositional changes in the surface layers of multicomponent targets. Such changes due to noble gas ion sputtering are discussed for InP and GaAs. The analyses show that the compositional change in InP (i.e., indium enrichment) is mainly due to preferential sputtering. In the case of GaAs. the changes are due to radiation-induced diffusion and segregation effects. Brief mention is made of compositional changes in a few other systems. The discussion on sputter-induced topography development deals mainly with InP because ion bombardment leads to dramatic topographical effects in this material. Ripple development on GaAs is also briefly discussed. Radiation damage has been well researched, and its mechanism and effects usually differ substantially when going from one semiconductor group to another. Bombardment-induced damage is briefly discussed for InP, GaAs, SiC, some II-VI semiconductors, and HgCdTe. 相似文献
984.
用新局域模型计算高Z靶的溅射产额 总被引:1,自引:0,他引:1
用ACAT模拟程序计算了不同离子碰撞在单原子材料上的溅射产额。计算中采用山村等人提出的包括壳效应的新电子能量损失局域模型 ,原子间作用势使用Moli啨re势?扑愕玫降慕峁胧笛槭莺蜕酱宓鹊木楣浇斜冉?,符合得很好 相似文献
985.
The optimization problem of fishing for a stochastic logistic model is studied in this paper. Besides a standard geometric Brownian motion, another two driving processes are taken into account: a stationary Poisson point process and a continuous-time finite-state Markov chain. The classical harvesting problem for this model is a big difficult puzzle since the corresponding Fokker–Planck equations with three types of noise are very difficult to solve. Our main goal of this paper is to work out the optimization problem with respect to stationary probability density. One of the main contributions is to provide a new equivalent approach to overcome this problem. More precisely, an ergodic method is used to show the almost surely equivalency between the time averaging yield and sustainable yield. Results show that the optimal strategy changes with environment. An interesting thing is that the optimal strategy for each state is equivalent to the global optimality. 相似文献
986.
Mehmet Ertugrul Elif Öz Önder ?im?ek Yusuf ?ahin 《Journal of Electron Spectroscopy and Related Phenomena》2010
The average 2p (L2,3) subshells fluorescence yield (?2,3) in 11 elements with Z ranging from 60 to 90 were measured with corporation of total L2,3 X-ray production cross-sections. Measurements were carried out exciting L2 and L3 subshells of relevant elements that were exposed to beam of selected K X-rays from secondary sources. The measured values were compared with theoretical and semi-empirical values. It is clear that the present values of experimental ?2,3 are generally smaller than the calculated and semi-empirical values for some elements in the atomic number range 60–90. 相似文献
987.
Rational Perturbation of the Fluorescence Quantum Yield in Emission‐Tunable and Predictable Fluorophores (Seoul‐Fluors) by a Facile Synthetic Method Involving CH Activation
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Eun Joung Choi Dr. Eunha Kim Youngjun Lee Ala Jo Prof. Seung Bum Park 《Angewandte Chemie (International ed. in English)》2014,53(5):1346-1350
Fluorescence imaging enables the uniquely sensitive observation of functional‐ and molecular‐recognition events in living cells. However, only a limited range of biological processes have been subjected to imaging because of the lack of a design strategy and difficulties in the synthesis of biosensors. Herein, we report a facile synthesis of emission‐tunable and predictable Seoul‐Fluors, 9‐aryl‐1,2‐dihydrolopyrrolo[3,4‐b]indolizin‐3‐ones, with various R1 and R2 substituents by coinage‐metal‐catalyzed intramolecular 1,3‐dipolar cycloaddition and subsequent palladium‐mediated C? H activation. We also showed that the quantum yields of Seoul‐Fluors are controlled by the electronic nature of the substituents, which influences the extent of photoinduced electron transfer. On the basis of this understanding, we demonstrated our design strategy by the development of a Seoul‐Fluor‐based chemosensor 20 for reactive oxygen species that was not accessible by a previous synthetic route. 相似文献
988.
A simple, efficient, regioselective, and ecofriendly method for oxyiodination of aromatic compounds is presented. In this method, the electrophilic substitutions of iodine generated in situ from KI as an iodine source and hydrogen peroxide as an oxygen source have been employed without any catalyst/mineral acid for the first time. 相似文献
989.
Flow behaviour of titanium dioxide dispersions in the presence of 2-hydroxyethyl cellulose 总被引:2,自引:0,他引:2
A. V. M. Chandramalar S. K. Kua C. Y. Chan Y. Y. Lim Y. K. Leong 《Colloid and polymer science》2000,278(5):485-489
The effects of 2-hydroxyethyl cellulose (HEC) of weight-average molecular weight 15,000, 90,000 and 7,20,000 on the rheological
properties of TiO2 dispersions were evaluated. For all three HECs, the maximum yield stress, τymax, (occurred at zero zeta potential), decreased with increasing HEC concentration. Interestingly, the largest reduction in
τymax was observed with the HEC with the highest molecular weight. This reduction was attributed to steric interaction arising
from adsorbed HEC. Adsorbed high-molecular-weight HEC formed an effectively thicker steric barrier because of its larger size
and higher adsorption capacity. Bridging interactions that were expected to be important for the high-molecular-weight HEC
were found to be unimportant here. In the flocculated regime, HEC enhanced the shear-thinning characteristics of the TiO2 dispersions.
Received: 8 November 1999 Accepted: 20 December 1999 相似文献
990.
遗传算法结合神经网络在油气产量预测中的应用 总被引:1,自引:0,他引:1
基于遗传算法的全局搜索能力和BP算法的局部精确搜索特性,通过采用遗传算法优化神经网络的方法,将遗传算法和BP算法有机结合,做到优势互补,在提高油气产量预测精度的研究中得到了很好的应用.在对国内某中小型气田油气产量的预测中,以历史产量资料进行检验,其结果表明,提出的预测方法,预测精度明显优于BP算法,证明了这种方法的有效性和可靠性. 相似文献