全文获取类型
收费全文 | 239篇 |
免费 | 3篇 |
专业分类
化学 | 125篇 |
晶体学 | 4篇 |
力学 | 6篇 |
数学 | 25篇 |
物理学 | 82篇 |
出版年
2020年 | 3篇 |
2019年 | 2篇 |
2017年 | 4篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2013年 | 7篇 |
2012年 | 6篇 |
2011年 | 6篇 |
2010年 | 5篇 |
2009年 | 2篇 |
2008年 | 13篇 |
2007年 | 11篇 |
2006年 | 6篇 |
2005年 | 6篇 |
2004年 | 7篇 |
2002年 | 9篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1985年 | 2篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 10篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1968年 | 2篇 |
1966年 | 3篇 |
1924年 | 1篇 |
1906年 | 1篇 |
1900年 | 2篇 |
1899年 | 1篇 |
1897年 | 1篇 |
排序方式: 共有242条查询结果,搜索用时 234 毫秒
51.
Dr. Alexander V. Polezhaev Nicholas A. Maciulis Dr. Chun‐Hsing Chen Dr. Maren Pink Dr. Richard L. Lord Prof. Dr. Kenneth G. Caulton 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(39):13985-13998
Reaction of 3,6‐disubstituted‐1,2,4,5‐tetrazines with water and PEt3 forms the corresponding 1,4‐dihydrotetrazine and OPEt3. Thus PEt3, as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron‐deficient tetrazine and electron‐rich PR3, including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO2CN=NCO2Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O2, something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine‐like intermediate is undetectable, but its protonated form is characterized, including by X‐ray structure and NMR spectroscopy. 相似文献
52.
53.
Hunt G.W. Peletier M.A. Champneys A.R. Woods P.D. Ahmer Wadee M. Budd C.J. Lord G.J. 《Nonlinear dynamics》2000,21(1):3-29
A long structural system with an unstable (subcritical)post-buckling response that subsequently restabilizes typically deformsin a cellular manner, with localized buckles first forming and thenlocking up in sequence. As buckling continues over a growing number ofcells, the response can be described by a set of lengthening homoclinicconnections from the fundamental equilibrium state to itself. In thelimit, this leads to a heteroclinic connection from the fundamentalunbuckled state to a post-buckled state that is periodic. Under suchprogressive displacement the load tends to oscillate between twodistinct values.The paper is both a review and a pointer tofuture research. The response is described via a typical system, asimple but ubiquitous model of a strut on a foundation which includesinitially-destabilizing and finally-restabilizing nonlinear terms. Anumber of different structural forms, including the axially-compressedcylindrical shell, a typical sandwich structure, a model of geologicalfolding and a simple link model are shown to display such behaviour. Amathematical variational argument is outlined for determining the globalminimum postbuckling state under controlled end displacement (rigidloading). Finally, the paper stresses the practical significance of aMaxwell-load instability criterion for such systems. This criterion,defined under dead loading to be where the pre-buckled and post-buckledstate have the same energy, is shown to have significance in the presentsetting under rigid loading also. Specifically, the Maxwell load isargued to be the limit of minimum energy localized solutions asend-shortening tends to infinity. 相似文献
54.
Delayed and prompt fluorescence spectra and delayed fluorescence decay data for 10?5?10?6 M/M crystals of anthracene in phenanthrene are reported. It is shown that these data indicate that a heterogeneous (host-dopant) annihilation process takes place which produces the excited singlet state of the host molecule. 相似文献
55.
Yoshiyuki Takahashi J. N. Capdevielle A. Kawahara Y. Takao T. Sugitate O. Miyamura T. Ogata B. L. Dong Y. Takahashi O. Miyamura K. Asakimori T. H. Burnett M. L. Cherry M. J. Christl S. Dake J. H. Derrickson W. F. Fountain M. Fuki J. C. Gregory R. Holynski A. Iyono W. V. Jones A. Kawahara M. Kobayashi J. Lord K. H. Moon H. Oda T. Ogata E. D. Olson T. A. Parnell S. C. Strausz T. Sugitate Y. Takahashi Y. Takao T. Tominaga J. P. Wefel B. Wilczynska H. Wilczynski R. J. Wilkes W. Wolter H. Yokomi E. Zager 《Hyperfine Interactions》1996,103(1):99-111
The search for cosmic strangelet nuclei was carried out by two experiments with emulsion chambers. A balloon-borne JACEE emulsion
chamber was flown at 3.5 g/cm2 for 200 h in Antarctica (JACEE-10 experiment) and the Concorde flights were made by ECHOS at an atmospheric depth of 110
g/cm2 between Paris and New York. No nuclei withZ⩾30 survived after traversing 60–120 g/cm2 of the detector materials in the JACEE instruments. No evidence for a long mean free path were found in the zenith angle
distribution forZ/β⩾26 nuclei. The exposure factor used by the JACEE was 72 m2hsr. The intensity upperbounds,I⩽(2.2–9.7)×10−2/m2h sr, were obtained for strangelets having an atmospheric attenuation length of 220−50 g/cm2, which corresponds to the case for mass numberA=100–10000 andZ/β > 13. Concorde experiments (ECHOS) used both a thin and a thick emulsion chamber. The total exposure was 209 m2 h sr and no candidates with chargeZ⩾30 were found. The largest track hadZ/β=28.6±1.29 withβ ∼ 1. Nuclei observed with charge 13⩽Z⩽30 were consistent with the survival intensity of ordinary nuclei. The flux bounds from the ECHOS experiments were I⩽(2.1–5.0)
x 10−2/m2h for strangelets with mass number 100⩽A⩽1000. 相似文献
56.
The photodynamic therapeutic agent temoporfin, 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (m-THPC) conjugated with poly(ethylene glycol) 2000 (PEG), has been analysed by high performance liquid chromatography (HPLC), linked to electrospray ionisation mass spectrometry (ESI-MS). Sufficient separation of m-THPC-PEG 2000 oligomers was achieved, enabling determination of molecular mass. The use of ESI-MS alone could not achieve this, because of too great a complexity in the mass spectrum, resulting from the presence of four PEG 2000 side chains with a wide molecular mass distribution. The technique is applicable to similar PEG conjugated compounds. 相似文献
57.
58.
Linear two-point boundary-value problems defined in complex space arise frequently in physical studies. We examine initial-value procedures based on superposition principles applied to the associated real system of equations. We show how to take advantage of the fact that the underlying problem is complex, thereby cutting the usual number of necessary integrations of the homogeneous system by one-half. We also discuss several aspects of incorporating these ideas into a computer code utilizing an orthonormalization procedure. 相似文献
59.
Ultrasonic attenuation at 2 MHz has been measured through the cholesteric → smectic A transition in cholestryl nonanoate. Very near the transition, 0.1 < (T ? Tc) < 0.6°C, the excess attenuation varies as (T ? Tc)n, where n for four separate runs varies between 0.30 and 0.34. These values are very close to the exponent of 0.33 predicted using a scaling laws and observed for the divergence of the twist viscosity in the nematic phase as the smectic A transition is approached. 相似文献
60.
Low energy Auger transitions from lithium fluoride, below 50 eV, have been associated by Gallon and Matthew [Phys. Status Solidi 41 (1970) 343] with transition processes between an ionised lithium ion and neighbouring fluorine ions. Two of these transitions, giving rise to lines at 36 eV and 43 eV were thought to involve de-excitation via colour centre defects. The growth behaviour of these lines has been investigated as a function of temperature, between 200°K and 600°K, and correlations have been established between this behaviour and optical absorption data obtained in situ. The effects of deposited nickel films on the Auger spectrum have also been investigated, and it is concluded that the 43 eV transition arises from lithium metal produced by electron beam dissociation of the lithium fluoride while the 36 eV line does indeed arise from de-excitation via a defect centre. 相似文献