全文获取类型
收费全文 | 12670篇 |
免费 | 2104篇 |
国内免费 | 1522篇 |
专业分类
化学 | 8967篇 |
晶体学 | 174篇 |
力学 | 768篇 |
综合类 | 82篇 |
数学 | 1468篇 |
物理学 | 4837篇 |
出版年
2024年 | 47篇 |
2023年 | 268篇 |
2022年 | 491篇 |
2021年 | 504篇 |
2020年 | 578篇 |
2019年 | 565篇 |
2018年 | 483篇 |
2017年 | 482篇 |
2016年 | 645篇 |
2015年 | 719篇 |
2014年 | 718篇 |
2013年 | 926篇 |
2012年 | 1139篇 |
2011年 | 1156篇 |
2010年 | 781篇 |
2009年 | 706篇 |
2008年 | 779篇 |
2007年 | 682篇 |
2006年 | 675篇 |
2005年 | 548篇 |
2004年 | 438篇 |
2003年 | 353篇 |
2002年 | 312篇 |
2001年 | 263篇 |
2000年 | 255篇 |
1999年 | 273篇 |
1998年 | 183篇 |
1997年 | 174篇 |
1996年 | 161篇 |
1995年 | 150篇 |
1994年 | 122篇 |
1993年 | 103篇 |
1992年 | 98篇 |
1991年 | 101篇 |
1990年 | 67篇 |
1989年 | 44篇 |
1988年 | 52篇 |
1987年 | 35篇 |
1986年 | 33篇 |
1985年 | 35篇 |
1984年 | 23篇 |
1983年 | 25篇 |
1982年 | 18篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
Kakiuchi K Okada H Kanehisa N Kai Y Kurosawa H 《The Journal of organic chemistry》1996,61(9):2972-2979
Acid-catalyzed rearrangement of 5-cyclopentylidenecyclooctanone derivatives 9a-c was examined to obtain polyspiropolyquinanes 11a-c, considered to have a unique helical structure, through cascade rearrangement pathways consisting of continuous transannular cyclization followed by successive 1,2-alkyl shifts. The substrates were prepared easily by use of the Wittig or McMurry reaction. Reaction of the 5-cyclopentylidenecyclooctanone (9a) with acid gave the expected dispirotriquinane ketone 11a in high yield. The precise mechanism was elucidated by a deuterium-labeling experiment. In the case of the ketone 9b, having another spiroannulated cyclopentane ring attached on 9a, the trispirotetraquiane 11b was not obtained but the bis-propellane-type tetrahydrofuran 25 was produced exclusively. The 5-(5'-cyclopentylidenecyclooctylidene)cyclooctanone (9c) afforded the polycyclic compounds 27-31, depending on the acid used, instead of the desired tetraspiropentaquinane 11c. The structures of the products were determined by NMR spectral data including 2D (13)C INADEQUATE spectra and X-ray crystallographic analyses. The unexpected rearrangement pathways are also discussed. 相似文献
103.
104.
105.
Let (S)⊄L
2(S′(∔),μ)⊄(S)* be the Gel'fand triple over the white noise space (S′(∔),μ). Let (e
n
,n>-0) be the ONB ofL
2(∔) consisting of the eigenfunctions of the s.a. operator
. In this paper the Euler operator Δ
E
is defined as the sum
, where ∂
i
stands for the differential operatorD
e
i. It is shown that Δ
E
is the infinitesimal generator of the semigroup (T
t
), where (T
t
ϕ)(x)=ϕ(e
t
x) for ϕ∈(S). Similarly to the finite dimensional case, the λ-order homogeneous test functionals are characterized by the Euler equation:
Δ
Eϕ
=λϕ. Via this characterization the λ-order homogeneous Hida distributions are defined and their properties are worked out.
Supported by the National Natural Science Foundation of China. 相似文献
106.
107.
Zeng -Gui Chen 《International Journal of Infrared and Millimeter Waves》1993,14(2):197-211
A cylindrical waveguide gyro-peniotron amplifier is analysed theoretically in this paper. By a ballistic method and directly using the energy conservation in the beam-wave system, a set of interaction equations for the cylindrical waveguide gyro-peniotron amplifier is derived. Computer simulation shows that under defined conditions an interaction efficiency of 47%, an output power of 240kW at a gain of 22. 5 dB and a 3 dB-instantaneous bandwidth of 1. 9% for a central frequency of 35GHz, TE03 mode gyro-peniotron amplifier could be achieved. The influence of the operation parameters on the amplifier performance is also presented. 相似文献
108.
109.
ULTRASONICALLY DISPERSED POTASSIUM AS DEPROTONATING AND COUPLING AGENT──SYNTHESIS OF 1,6-DINITRILES IN SITUULTRASONICALLYDISP... 相似文献
110.
A home-made high sensitive photoacoustic transducer was used to determine the photoacoustic signal in a solution. The photoacoustic spectra of Ho in different solvents were measured and comparedwith its absorption spectrum. The affection factors of photoacousticsensitivity are discussed. The trivalent rare earth ions Ho3+ and Nd3+in an aqueous acetonitrile solution were determined. The detection limit is 5×10-8 mol/L for Ho3+ and 1.0 ×10-7 mol/L for Nd3+, and corresponds to the absorbance of 1.5×10 and 6.3×10-7.respectively. 相似文献