全文获取类型
收费全文 | 1901篇 |
免费 | 15篇 |
国内免费 | 10篇 |
专业分类
化学 | 1102篇 |
晶体学 | 14篇 |
力学 | 66篇 |
数学 | 436篇 |
物理学 | 308篇 |
出版年
2020年 | 16篇 |
2019年 | 16篇 |
2016年 | 18篇 |
2015年 | 14篇 |
2014年 | 24篇 |
2013年 | 102篇 |
2012年 | 42篇 |
2011年 | 55篇 |
2010年 | 42篇 |
2009年 | 53篇 |
2008年 | 60篇 |
2007年 | 55篇 |
2006年 | 82篇 |
2005年 | 71篇 |
2004年 | 55篇 |
2003年 | 63篇 |
2002年 | 44篇 |
2001年 | 42篇 |
2000年 | 46篇 |
1999年 | 30篇 |
1998年 | 43篇 |
1997年 | 39篇 |
1996年 | 49篇 |
1995年 | 26篇 |
1994年 | 35篇 |
1993年 | 29篇 |
1992年 | 31篇 |
1991年 | 29篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 22篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 38篇 |
1984年 | 34篇 |
1983年 | 33篇 |
1982年 | 30篇 |
1981年 | 17篇 |
1980年 | 45篇 |
1979年 | 19篇 |
1978年 | 34篇 |
1977年 | 41篇 |
1976年 | 27篇 |
1975年 | 20篇 |
1974年 | 39篇 |
1973年 | 34篇 |
1972年 | 21篇 |
1971年 | 21篇 |
1969年 | 29篇 |
1968年 | 15篇 |
排序方式: 共有1926条查询结果,搜索用时 328 毫秒
31.
The principal characteristics and products of thermal degradation of a commercial epoxy resin prepared by reaction of 2,2-bis(4′-hydroxy phenyl)propane (bisphenol-A) with 1-chloro-2,3-epoxy propane (epichlorhydrin) have been studied. The principal volatile products, acrolein, acetone and allyl alcohol, are formed at 280°C and, although cross-linking is detectable at 220°C, it only becomes significant at 320°C when the residual resin is brittle and insoluble. Decomposition of the cross-linked resin occurs above 340°C when phenolic compounds appear together with more complex products with higher molecular weights whose structures have been speculated upon from examination of their mass spectral characteristics. 相似文献
32.
A radiochemical neutron activation analysis (RNAA) combustion method coupled with a neutron exposure normalization technique was used to determine low g/kg mercury levels in three National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs). Two coals (sub-bituminous and bituminous) and a diet material were analyzed. The results obtained provided recommended values of approximately 5 g/kg for SRM 1548a Typical Diet, 24 g/kg for SRM 1635 Trace Elements in Coal (sub-bituminous), and 100 g/kg for SRM 1632b Trace Elements in Coal (bituminous). 相似文献
33.
Baber RA Charmant JP Cook AJ Farthing NE Haddow MF Norman NC Orpen AG Russell CA Slattery JM 《Dalton transactions (Cambridge, England : 2003)》2005,(19):3137-3139
Treatment of the diborane(4) compound B(2)(NMe(2))(4) with aniline or 2,6-dimethylaniline results in the primary amido compounds B(2)(NHR)(4)(R = Ph, 2,6-Me(2)C(6)H(3)); subsequent treatment with n-BuLi in toluene in each case affords the first examples of anionic imidodiborates namely Li(4)(thf)(6)B(2)(NPh)(4) and Li(4)(thf)(4)B(2)(N-2,6-Me(2)C(6)H(3))(4); all complexes have been characterised crystallographically. 相似文献
34.
Paul A. Keifer Donald L. Nagel Norman H. Cromwell 《Journal of heterocyclic chemistry》1988,25(2):353-359
A series of ten N-alkyl(aralkyl)-2-phenyl-3-cyanoaziridines has been synthesized to continue investigations of the molecular stereochemistry and bonding of functionalized aziridines. Substantial spectroscopic evidence is presented which indicates the presence of stereoselective hyperconjugation between the phenyl and nitrile groups and the aziridine ring. The 1H and 13C nmr chemical shifts are rationalized in terms of the interactions of the substituents on the aziridine ring, and how these interactions vary with the steric bulk of the nitrogen substituents. Various stereoselective coupling constants (1H-1H and 13C-1H) are also reported. 相似文献
35.
Wolfgang Dietzsch Norman V. Duffy Aaron M. Lee Brian W. Skelton Allan H. White 《无机化学与普通化学杂志》2006,632(15):2419-2423
The consequences of replacement of the symmetrically chelate ligands in [M(E2CNR2)3] (E = S, Se) complexes of potential 32 symmetry by analogous mixed S,Se unsymmetrical chelates are explored for both small (M = Co) and large (M = In) metal atoms, and R = primary (Et) and secondary (iPr) alkyl substituents by way of low‐temperature single crystal X‐ray studies of [(Co(SSeCNEt2)3] ([Co(Se2CNEt2)3] also determined as datum), and [In(SSeCNR2)3], R = Et, iPr. The structure of [(iPr2N·CS·Se)2] is also recorded. 相似文献
36.
Peng S Ahlmann N Kunze K Nigge W Edler M Hoffmann T Franzke J 《Rapid communications in mass spectrometry : RCM》2004,18(16):1803-1808
The desorption of an analyte by a continuous wave diode laser from a porous surface of a thin-layer plate covered with a graphite suspension is presented. The thermally desorbed analyte molecules are ionized in the gas phase by a corona discharge at atmospheric pressure. Therefore, both essential processes--the desorption and the ionization of analyte molecules, which are often performed in one step--are separated. The target preparation is easy and fast since no additional extraction process is required. The mass spectrometric background signal was mostly limited to the low mass range showing no interference with typical compounds of interest. In this study, the calmative and antihypertensive drug reserpine was chosen as model analyte, which is often used for specification of mass spectrometers. No fragmentation was observed because of efficient collisional cooling under atmospheric pressure. The influence of diode laser power and the composition of the graphite suspension were investigated, and a primary optimization was performed. 相似文献
37.
Hui Lu Edgar Arriaga Yong Chen Da Daniel Figeys Norman J. Dovichi 《Journal of chromatography. A》1994,680(2):503-510
In DNA sequencing, single-stranded DNA fragments are separated by gel electrophoresis. This separation is based on a sieving mechanism where DNA fragments are retarded as they pass through pores in the gel. In this paper, we present the mobility of DNA sequencing fragments as a function of temperature; mobility is determined in 4% T LongRanger gels at an electric field of 300 V/cm. The temperature dependence is compared with the predictions of the biased reptation model. The model predicts that the fragment length for the onset of biased reptation with stretching increases with the square of temperature; the data show that the onset of biased reptation with stretching decreases with temperature. Biased reptation fails to model accurately the temperature dependence of mobility. We analyzed the data and extracted the activation energy for passage of sequencing fragments through the gel. For fragments containing less than ca. 200 bases, the activation energy increases linearly with the number of bases at a rate of 25 J/mol per base; for longer fragments, the activation energy increases at a rate of 6.5 J/mol per base. This transition in the activation energy presumably reflects a change in conformation of the DNA fragments; small fragments exist in a random coil configuration and larger fragments migrate in an elongated configuration. 相似文献
38.
39.
40.
It is geometrically feasible to insert metal-metal-bonded M(4) tetrahedra and M(6) octahedra into the tetrahedral and octahedral holes, respectively, of the fcc C(60) lattice. The electronic structure of the hypothetical tetrahedral variants C(60)(M(4))(2), M = Rh, Co, is analyzed with approximate molecular orbital methods and band structure calculations. These compounds feature M-M and M-C(60) bonding and a variable degree of electron transfer to or from C(60). The C(60)(M(4))(2) phases should be metallic, but we have no way of predicting if they will be superconducting. A number of discrete molecular tetrahedral cluster compounds which serve as models for the solid state materials are analyzed. There is a clear indication that tetranuclear and even mononuclear Rh, Ir, and Co arene complexes should be relatively unstable. 相似文献