全文获取类型
收费全文 | 1430篇 |
免费 | 56篇 |
国内免费 | 8篇 |
专业分类
化学 | 1157篇 |
晶体学 | 24篇 |
力学 | 20篇 |
数学 | 75篇 |
物理学 | 218篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 13篇 |
2020年 | 24篇 |
2019年 | 21篇 |
2018年 | 13篇 |
2017年 | 16篇 |
2016年 | 26篇 |
2015年 | 39篇 |
2014年 | 46篇 |
2013年 | 84篇 |
2012年 | 77篇 |
2011年 | 99篇 |
2010年 | 43篇 |
2009年 | 47篇 |
2008年 | 100篇 |
2007年 | 97篇 |
2006年 | 94篇 |
2005年 | 107篇 |
2004年 | 85篇 |
2003年 | 84篇 |
2002年 | 71篇 |
2001年 | 17篇 |
2000年 | 12篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 10篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1984年 | 18篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 15篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 5篇 |
排序方式: 共有1494条查询结果,搜索用时 31 毫秒
111.
Sekiguchi A Ishida Y Fukaya N Ichinohe M Takagi N Nagase S 《Journal of the American Chemical Society》2002,124(7):1158-1159
Dark red crystals of the halogen-substituted cyclotrigermenes [(tBu3Si)3Ge3X; X = Cl, Br, I] were obtained in good yields by the reaction of [(tBu3Si)3Ge3]+.TTFPB- (TTFPB- = tetrakis(2,3,5,6-tetrafluorophenyl)borate) with potassium halides (KCl, KBr, or KI) in diethyl ether. The crystal structures of the halogen-substituted cyclotrigermenes reveal a cis-bent Ge=Ge double bond, caused by the introduction of the electronegative halogen atom on the sp3 germanium atom of cyclotrigermene. In solution, an intramolecular halogen migration over the three-membered ring skeleton was observed. Facial stereoselectivity in the Diels-Alder reaction of new cyclotrigermenes with 2,3-dimethyl-1,3-butadiene is also reported. 相似文献
112.
Shoji Kasahara Tetsuya Takine Yutaka Takahashi Toshiharu Hasegawa 《Queueing Systems》1993,14(3-4):349-367
Many researchers have studied variants of queueing systems with vacations. Most of them have dealt with M/G/1 systems and have explicitly analyzed some of their performance measures, such as queue length, waiting time, and so on. Recently, studies on queueing systems whose arrival processes are not Poissonian have appeared. We consider a single server queueing system with multiple vacations and E-limited service discipline, where messages arrive to the system according to a switched Poisson process. First, we consider the joint probability density functions of the queue length and the elapsed service time or the elapsed vacation time. We derive the equations for these pdf's, which include a finite number of unknown values. Using Rouché's theorem, we determine the values from boundary conditions. Finally, we derive the transform of the stationary queue length distribution explicitly. 相似文献
113.
The title phanes were synthesized. Examination of their physical properties including X-ray crystallography disclosed that 1) the p-tropoquinone ring is still bent in both compounds though to a lesser extent than in the corresponding [2.2]phanes, 2) the metacyclo compound prefers syn conformation in solution, and 3) the intramolecular CT interaction is very similar in two compounds. 相似文献
114.
Yutaka Kojima 《Physica A》1979,95(1):162-168
A re-evaluation is given of the temperature dependent term in the first order exchange contribution to the grand partition function for an electron gas. The existence and origin of the logarithmic term in the temperature-dependent part are shown in a very simple manner. 相似文献
115.
116.
Ohwada K Fujii Y Katsuki Y Muraoka J Nakao H Murakami Y Sawa H Ninomiya E Isobe M Ueda Y 《Physical review letters》2005,94(10):106401
The present resonant x-ray scattering has been performed on a monoclinically split single domain of NaV(2)O(5). The observation of a critically enhanced contrast between V(4+) and V(5+) ions has led us to the unequivocal conclusion of the charge-order pattern of the low-temperature phase of NaV(2)O(5) below T(c) = 35 K. In spite of the possible four types of configuration of the zigzag-type charge-order patterns in the ab plane (A,A',B and B'), the stacking sequence along the c axis is determined as the AAA'A' type by comparison with model calculations. 相似文献
117.
We study a square-lattice three-state Potts antiferromagnet with a staggered polarization field at finite temperature. Numerically treating the transfer matrices, we determine two phase boundaries separating the model-parameter space into three parts. We confirm that one of them belongs to the ferromagnetic three-state Potts criticality, which is in accord with a recent prediction, and another to the Ising-type; these are both corresponding to the massless renormalization-group flows stemming from the Gaussian fixed points. We also discuss a field theory to describe the latter Ising transition. 相似文献
118.
Zhang R Sato O Zhao F Sato M Ikushima Y 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(6):1501-1506
Heck coupling reaction of iodobenzene and styrene proceeds rapidly and selectively in supercritical water even without any catalyst in the presence of base. Both the choice of base and the reaction conditions had a significant effect on the conversion and the selectivity of the coupling products. The addition of a relatively mild base such as potassium acetate facilitated the cross-coupling reaction, while the hydrolysis of phenyl halide was favored in the presence of a strong base. The conversion and the yields of coupling products increased with increasing temperature, reaching a maximum at 650 K near the critical temperature of water, and then decreased as the temperature was further increased. Water density had a significant influence on the reaction rate, showing nearly 30% augmentation with a slight increase in density from 0.45 to 0.56 g cm(-3), but had less effect on the product selectivity. Two possibilities of the role of water responsible for the noncatalytic Heck coupling reaction in supercritical water, that is, ion and water-catalyzed mechanisms have been considered. 相似文献
119.
Matsuo Y Tahara K Sawamura M Nakamura E 《Journal of the American Chemical Society》2004,126(28):8725-8734
Selective penta-addition of a methylcopper reagent followed by addition of a phenylcopper reagent to a suitably modified synthetic intermediate results in creation of 40pi-electron systems-hoop- and bowl-shaped cyclic benzenoid compounds, [10]cyclophenacene, and dibenzo-fused corannulene derivatives. The 40pi-electron cyclophenacene derivatives have been found to be chemically stable, yellow-colored, luminescent (560 nm), and EPR-silent. X-ray crystallographic analysis provided precision structural data sets. The dibenzo-fused corannulene derivatives exhibit blue-green (460 nm) to red (649 nm) fluorescence. 相似文献
120.
Aoyagi Y Saitoh Y Ueno T Horiguchi M Takeya K Williams RM 《The Journal of organic chemistry》2003,68(18):6899-6904
Lipase TL-mediated kinetic resolution of (+/-)-5-benzyloxy-1-tert-butyldimethylsilyloxy-2-pentanol (5) at low temperature proceeded to give the corresponding (S)-alcohol 5 and (R)-acetate 6 in quantitative yields with high enantiomeric purity. The addition of bases such as pyridine, DMAP, 2,4- and 2,6-lutidines, or triethylamine considerably enhanced the rate of kinetic resolution. The alcohol (S)-5 and the acetate (R)-6 were converted to piperazic acid derivatives (R)- and (S)-3, respectively, via the intramolecular Mitsunobu reaction as a key step. 相似文献