全文获取类型
收费全文 | 1630篇 |
免费 | 44篇 |
国内免费 | 13篇 |
专业分类
化学 | 1212篇 |
晶体学 | 17篇 |
力学 | 47篇 |
数学 | 99篇 |
物理学 | 312篇 |
出版年
2022年 | 14篇 |
2021年 | 13篇 |
2020年 | 14篇 |
2019年 | 28篇 |
2018年 | 21篇 |
2017年 | 11篇 |
2016年 | 34篇 |
2015年 | 34篇 |
2014年 | 35篇 |
2013年 | 75篇 |
2012年 | 93篇 |
2011年 | 87篇 |
2010年 | 58篇 |
2009年 | 66篇 |
2008年 | 104篇 |
2007年 | 89篇 |
2006年 | 84篇 |
2005年 | 90篇 |
2004年 | 89篇 |
2003年 | 72篇 |
2002年 | 75篇 |
2001年 | 32篇 |
2000年 | 25篇 |
1999年 | 23篇 |
1998年 | 18篇 |
1997年 | 23篇 |
1996年 | 16篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 25篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 22篇 |
1986年 | 11篇 |
1985年 | 23篇 |
1984年 | 15篇 |
1983年 | 16篇 |
1982年 | 9篇 |
1981年 | 16篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1975年 | 14篇 |
1974年 | 8篇 |
1973年 | 5篇 |
排序方式: 共有1687条查询结果,搜索用时 78 毫秒
1.
Minoru Koyama Dr. Takafumi Kawakami Dr. Takashi Okazoe Prof. Dr. Kyoko Nozaki 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(46):10913-10917
Methacrylic esters, represented by methyl methacrylate (MMA), are widely used as commodity chemicals. Here, the one-pot synthesis of methacrylic esters from acetone, a haloform and alcohols in the presence of an organic base is described. Using DBU as the organic base for the reaction of acetone, chloroform and methanol in acetonitrile afforded MMA in 66 % yield. When the solvent was replaced by benzonitrile, the product MMA was successfully purified by distillation. Applicability of this process to various alcohols was also investigated to show ethyl, phenyl, CF3CH2, and n-C6F13CH2CH2 esters were obtained in moderate yields. The use of bromoform instead of chloroform resulted in the improvement of the yield, for example, methyl and n-C6F13CH2CH2 esters up to 81 and 70 %, respectively. The reaction with deuterated starting materials acetone-d6 and MeOH-d4, with DBU in acetonitrile afforded deuterated MMA (MMA-d8) in 70 % yield. 相似文献
2.
Shinji Takesue 《Journal of statistical physics》1989,56(3-4):371-402
This is the first part of a series devoted to the study of thermodynamic behavior of large dynamical systems with the use of a family of fully-discrete and conservative models named elementary reversible cellular automata (ERCAs). In this paper, basic properties such as conservation laws and phase space structure are investigated in preparation for the later studies. ERCAs are a family of one-dimensional reversible cellular automata having two Boolean variables on each site. Reflection and Boolean conjugation symmetries divide them into 88 equivalence classes. For each rule, additive conserved quantities written in a certain form are regarded as a kind of energy, if they exist. By the aid of the discreteness of the variables, every ERCA satisfies the Liouville theorem or the preservation of phase space volume. Thus, if an energy exists in the above sense, statistical mechanics of the model can formally be constructed. If a locally defined quantity is conserved, however, it prevents the realization of statistical mechanics. The existence of such a quantity is examined for each class and a number of rules which have at least one energy but no local conservation laws are selected as hopeful candidates for the realization of thermodynamic behavior. In addition, the phase space structure of ERCAs is analyzed by enumerating cycles exactly in the phase space for systems of comparatively small sizes. As a result, it is revealed that a finite ERCA is not ergodic, that is, a large number of orbits coexist on an energy surface. It is argued that this fact does not necessarily mean the failure of thermodynamic behavior on the basis of an analogy with the ergodic nature of infinite systems. 相似文献
3.
The effect of oxygen isotope substitution on the transition temperature Tc of a superconducting (Nd1−xCex)2CuO4−δ system was studied, where the special synthetic method was taken to minimize ambiguous factors on the oxygen concentration and to guarantee the complete substitution of 16O by 18O. The isotope exponent in the relationship of Tc∝1/M was estimated to be less than 0.15 by magnetic susceptibility measurements. 相似文献
4.
We present the spot size dependence of dielectric multilayer filters for use in dense WDM systems. We found large dependences of filter performances on the spot size and the incident angle of input light, which should be important for miniaturizing multi-channel add/drop filters. 相似文献
5.
Takaaki Kakitsuka Shinji Matsuo Seok–Hwan Jeong Toru Segawa Hiroshi Okamoto Yoshihiro Kawaguchi Yasuhiro Kondo Yuzo Yoshikuni Hiroyuki Suzuki 《Optical and Quantum Electronics》2006,38(12-14):1053-1060
We theoretically investigated a digitally tunable laser with a chirped ladder filter and a ring resonator to obtain a wide wavelength tuning range covering the whole C- or L- band. The clear relation between the tuning range and laser structure, especially the ladder filter, is described analytically. The introduction of a chirped structure into a ladder filter is effective in achieving both wide tunability and a stable lasing mode. A numerical simulation based on multimode rate equations shows that a tuning range of over 40 nm and a mode suppression ratio over 40 dB can be achieved by introducing a chirped ladder filter. 相似文献
6.
A new 20-membered macrolide designated exiguolide has been isolated from the marine sponge Geodia exigua, and its structure determined by interpretation of spectroscopic data. Exiguolide specifically inhibited fertilization of sea urchin (Hemicentrotus pulcherrimus) gametes but not embryogenesis of the fertilized egg. 相似文献
7.
H. Ohta M. Kimata K. Koyama Y. Oshima M. Motokawa H. Nishikawa K. Kikuchi I. Ikemoto 《International Journal of Infrared and Millimeter Waves》2005,26(11):1525-1530
We have developed a new magneto-optical measurement system using a rotational cavity system equipped with a millimeter vector
network analyzer and a 14 T solenoid type super conducting magnet. The measurement can be performed in the transmission configuration
down to 1.6 K. The results of the precise angular dependence measurement of quasi-one-dimensional organic conductor (DMET)2I3 using the new system are shown, and its Fermi surface will be discussed in connection with the previous reports. 相似文献
8.
9.
The first total synthesis of (+)?3-deoxyaphidicolin, a selective inhibitor of eukaryotic DNA polymerase α, has been completed efficiently and stereoselectively starting from 1-abietic acid. 相似文献
10.