全文获取类型
收费全文 | 1466篇 |
免费 | 44篇 |
国内免费 | 7篇 |
专业分类
化学 | 789篇 |
晶体学 | 9篇 |
力学 | 82篇 |
数学 | 326篇 |
物理学 | 311篇 |
出版年
2023年 | 9篇 |
2022年 | 29篇 |
2021年 | 35篇 |
2020年 | 30篇 |
2019年 | 40篇 |
2018年 | 33篇 |
2017年 | 34篇 |
2016年 | 58篇 |
2015年 | 35篇 |
2014年 | 53篇 |
2013年 | 93篇 |
2012年 | 96篇 |
2011年 | 112篇 |
2010年 | 73篇 |
2009年 | 61篇 |
2008年 | 94篇 |
2007年 | 72篇 |
2006年 | 76篇 |
2005年 | 74篇 |
2004年 | 52篇 |
2003年 | 55篇 |
2002年 | 40篇 |
2001年 | 12篇 |
2000年 | 19篇 |
1999年 | 24篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1984年 | 6篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1972年 | 3篇 |
1969年 | 2篇 |
1968年 | 4篇 |
1966年 | 2篇 |
1938年 | 7篇 |
排序方式: 共有1517条查询结果,搜索用时 10 毫秒
31.
Let Q be a convex solid in
n
, partitioned into two volumes u and v by an area s. We show that s>min(u,v)/diam Q, and use this inequality to obtain the lower bound n
-5/2 on the conductance of order Markov chains, which describe nearly uniform generators of linear extensions for posets of size n. We also discuss an application of the above results to the problem of sorting of posets.Computing Center of the USSR Academy of Sciences USSR 相似文献
32.
Leonid Brevdo 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1991,42(6):911-942
A formalism for absolute and convective instabilities in parallel shear flows is extended to the three-dimensional case. Assuming that the dispersion relation function is given byD(k, l, ), wherek andl are wave numbers, and is a frequency, the analytic criterion is formulated by which a point (k
0,l
0,
0) with Im
0>0 contributes to the absolute instability if and only if one of the two equivalent conditions is satisfied:
Consequently, the causality condition for spatially amplifying 3-D waves in absolutely stable, but convectively unstable flow is derived as follows. We denote by (, ) a unit vector on the (x, y) plane. The contributions to amplification in the direction of this vector come from the end points of the trajectories that consist of the coalescence roots on thel
1-plane, given byl
1,=–k+l, of the systemD=0,–D
k
+gaD
1=0. Thek
1-components of these trajectories have to pass from above to below the real axis on ak
1-plane, given byk
1=k+l, as moves down to
0. Here
0 is the real frequency of excitation. At each point of such trajectories the group velocity vector (D
k
,D
l
) is collinear with the direction vector (, ). There exists a direction for which the spatial amplification rate reaches its maximum. 相似文献
(i) | At least two roots inl of the systemD(k, l, )=0,D k (k, l, )=0, originating on opposite sides of the reall-axis, collide on thel-plane for the parameter valuesk 0,l 0, 0, as is brought down to 0. Every point on thek-plane, that corresponds to a point on the collision paths on thel-plane, is itself a coalescence point ofk-roots for a fixedl ofD(k, l, )=0, that originate on opposite sides of the realk-axis. |
(ii) | At least two roots ink of the systemD(k, l, )=0,D l ,(k, l, )=0, originating on opposite sides of the realk-axis, collide on thek-plane for the parameter valuesk 0,l 0, 0, as is brought down to 0. Every point on thel-plane, that corresponds to a point on the collision paths on thek-plane, is itself a coalescence point ofl-roots for a fixedk ofD(k, l, )=0, that originate on opposite sides of the reall-axis. |
33.
The molecular structure and absorption spectra of monothio- and dithio-naphthalimides were compared to their naphthalimide analogues using AM1, PM3 and ZINDO/S semiempirical quantum chemical methods. The substitution of the 4R-naphthalimide oxygen atoms by sulphur atoms resulted in a red-shift of the absorption spectra by Δλmax60-65 and 100-140 nm, respectively. The thionated naphthalimide derivatives do not show observable fluorescence due to intersystem crossing to the triplet -states localised at the CS groups. The -absorption bands of monothioimides are located at 525-580 nm (ε=60-80) and those for dithioimides at 535-560 nm (ε=140-390) and 628-686 nm (ε=34-68). None of these transitions are solvent sensitive. The -transitions of N-phenylthioimides have also a small contribution from -states due to a partial conjugation between CS group and π-electronic system of the N-phenyl ring. As a result, the bands of aromatic substituted N-phenylthioimides are red-shifted as compared to those of the aliphatic N-methyl-thioimides. 相似文献
34.
35.
Leonid M. Goldenberg Nikolai N Denisov Jan F. Biernat 《Journal of inclusion phenomena and macrocyclic chemistry》2000,38(1-4):171-186
The complexation of 13- and 16-memberedazo- and azoxycrowns with metal cations of similarionic diameter (Na+ and Ca2+; and K+,Ba2+, Ag+ and Pb2+) was studied byuv/visible spectroscopic titration in acetonitrile andMeOH. In MeOH the 13-membered azo- and azoxycrowns 1 and 2 are weakly and non-selectively bound tohard cations of similar ionic diameter, but differentcharge (Na+ and Ca2+). At the same time thebinding to the soft cation Ag+ of larger sizethan the macrocycle cavity is considerably stronger.In contrast to solutions in acetonitrile no bindingwith the small Li+ cation was found.The 16-membered azocrowns 3 and 4 alsodiscriminate silver cation in MeOH withlog K = 3.65 ± 0.1 for both compounds.Unexpectedly low bindingwith the hard barium divalent cation of similar size(log K = 1.55 ± 0.4 and 1.95 ± 0.2, respectively)was found for these compounds. Similarly to13-membered compounds no binding with the smallLi+ cation was detected. A reverse order ofselectivity was observed for these crowns inacetonitrile with binding constant for association of3 with Ba2+ (log K 5.3) considerablyhigher than for other cations. The previously observedstrong binding with the smaller Li+ and Na+cations is confirmed. 相似文献
36.
Vladimir V. Veselovsky Vera I. Isaeva Vera D. Nissenbaum Leonid M. Kustov Vladimir V. Chernyshev 《Molecules (Basel, Switzerland)》2022,27(23)
The new homochiral 1D metal–organic coordination polymer [Cu2(EDPB)•H2O]n was synthesized starting from the original 3,3′-ethyne-1,2-diylbis[6-(L-prolylamino)benzoic acid] (H4EDPB). The unique crystal structure of the new compound was established by powder X-ray diffraction. The [Cu2(EDPB)•H2O]n system shows catalytic activity and enantioselectivity in a Henry reaction of p-nitrobenzaldehyde with nitromethane. 相似文献
37.
Dr. Alexander A. Larin Alexander V. Shaferov Dr. Alexander S. Kulikov Prof. Dr. Alla N. Pivkina Konstantin A. Monogarov Dr. Artem O. Dmitrienko Dr. Ivan V. Ananyev Dr. Dmitry V. Khakimov Prof. Dr. Leonid L. Fershtat Prof. Dr. Nina N. Makhova 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(59):14628-14637
A series of novel energetic materials comprising of azo-bridged furoxanylazoles enriched with energetic functionalities was designed and synthesized. These high-energy materials were thoroughly characterized by IR and multinuclear NMR (1H, 13C, 14N) spectroscopy, high-resolution mass spectrometry, elemental analysis, and differential scanning calorimetry (DSC). The molecular structures of representative amino and azo oxadiazole assemblies were additionally confirmed by single-crystal X-ray diffraction and X-ray powder diffraction. A comparison of contributions of explosophoric moieties into the density of energetic materials revealed that furoxan and 1,2,4-oxadiazole rings are the densest motifs while the substitution of the azide and amino fragments on the nitro and azo ones leads to an increase of the density. Azo bridged energetic materials have high nitrogen-oxygen contents (68.8–76.9 %) and high thermal stability. The synthesized compounds exhibit good experimental densities (1.62–1.88 g cm−3), very high enthalpies of formation (846–1720 kJ mol−1), and, as a result, excellent detonation performance (detonation velocities 7.66–9.09 km s−1 and detonation pressures 25.0–37.7 GPa). From the application perspective, the detonation parameters of azo oxadiazole assemblies exceed those of the benchmark explosive RDX, while a combination of high detonation performance and acceptable friction sensitivity of azo(1,2,4-triazolylfuroxan) make it a promising potential alternative to PETN. 相似文献
38.
Prants Sergey V. Kon’kov Leonid E. Didov Aleksandr A. 《Journal of Russian Laser Research》2021,42(5):558-568
Journal of Russian Laser Research - We examine the coherent dynamics of cold atoms in a three-dimensional (3D) optical lattice (OL) without interference between counter-propagating laser beams. The... 相似文献
39.
We apply a recent result of Borichev–Golinskii–Kupin on the Blaschke-type conditions for zeros of analytic functions on the complex plane with a cut along the positive semi-axis to the problem of the eigenvalues distribution of the Fredholm-type analytic operator-valued functions. 相似文献
40.