全文获取类型
收费全文 | 23991篇 |
免费 | 4738篇 |
国内免费 | 3290篇 |
专业分类
化学 | 14923篇 |
晶体学 | 214篇 |
力学 | 2355篇 |
综合类 | 196篇 |
数学 | 1338篇 |
物理学 | 12993篇 |
出版年
2024年 | 55篇 |
2023年 | 260篇 |
2022年 | 755篇 |
2021年 | 754篇 |
2020年 | 901篇 |
2019年 | 848篇 |
2018年 | 854篇 |
2017年 | 963篇 |
2016年 | 1092篇 |
2015年 | 1053篇 |
2014年 | 1519篇 |
2013年 | 2126篇 |
2012年 | 1594篇 |
2011年 | 1764篇 |
2010年 | 1453篇 |
2009年 | 1620篇 |
2008年 | 1629篇 |
2007年 | 1476篇 |
2006年 | 1493篇 |
2005年 | 1254篇 |
2004年 | 1200篇 |
2003年 | 1065篇 |
2002年 | 813篇 |
2001年 | 709篇 |
2000年 | 669篇 |
1999年 | 494篇 |
1998年 | 531篇 |
1997年 | 445篇 |
1996年 | 394篇 |
1995年 | 362篇 |
1994年 | 307篇 |
1993年 | 263篇 |
1992年 | 235篇 |
1991年 | 148篇 |
1990年 | 154篇 |
1989年 | 122篇 |
1988年 | 128篇 |
1987年 | 97篇 |
1986年 | 67篇 |
1985年 | 54篇 |
1984年 | 66篇 |
1983年 | 22篇 |
1982年 | 46篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 16篇 |
1976年 | 10篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
《天然气化学杂志》1995,(2)
The catalytic performance of Mn/TiO_2,La-Mn/TiO_2,Li-La-Mn/TiO_2 etc for the oxidetive coupling of methane(OCM)was investignted.Thecatalysta were cheracterised with X-ray diffraction(XRD)and X-rayphotoelectron spectroscopy(XPS).The results reveal that catalyst Li-La-Mn/TiO_2 exhibits high activity and C_2 selectivity;Ti in this catalyst exists asTi~( 4) state;Li can promote the formation of lanthanum tituate via theinteraction between La and TiO_2;the formed La_2Ti_2O_7 and La_4Ti_9O_(24) aredistributed in the inner surface layer and Mn exists in outer surface layer in lowvalence states.The high activity and C_2 selectivity of catalyst Li-La-Mn/TiO_2are intimately related to the valence states of Mn,Li,La and theirdistribution on the catulyst surface layer. 相似文献
102.
A series of ferrocenylaldimines (Fc-CH=N-R) has been prepared by Schiff base condensation of formylferrocene with polyacenylamines RNH2 where R = naphthyl, 5,6,7,8-tetrahydronaphthyl, anthracenyl, and pyrenyl groups. This step was followed by oxidation with TCNQ to give [Fc-CH = N-R][TCNQ]2 salts. The electronic state of iron in these compounds was investigated by means of57Fe Mössbauer spectroscopy. 相似文献
103.
Vladimir A. KaminskiiOleg Yu. Slabko Andrey V. Kachanov Boris V. Buhvetskii 《Tetrahedron letters》2003,44(1):139-140
A simple and convenient route for the synthesis of 1,1,2,3,3-pentacyanopropene salts is reported. These salts are formed by interaction of malononitrile with SeO2 in presence of organic N-containing bases or pyridinium salts. 相似文献
104.
The change in semiconductive properties of β-apo-8′-carotenal, astacene and methyl bixin on adsorption of various vapours
on the crystallite surfaces has been studied at a constant sample temperature. The adsorption of vapours enhances the semiconductivity
of the polyenes appreciably. This enhancement depends on the chemical nature and also on the pressure of the adsorbed vapour.
The adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. A two stage desorption process, the
first stage of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a
potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and
the polyene crystallites, can explain satisfactorily the experimentally observed kinetic data. 相似文献
105.
Xue-qiang Yin 《Tetrahedron letters》2006,47(24):4057-4059
(−)-(1S,4R)-4-Hydroxy-2-cyclopenten-1-yl acetate provided a convenient entry point for a 16-step chiral preparation of 4′-methylaristeromycin. This procedure is adaptable to a number of carbocyclic nucleosides with a diversity of substitution at C-4′ and C-5′ and a variety of heterocyclic bases. 相似文献
106.
YUAN Song-Yue ZHAO ZhenChangchun Institute of Applied Chemistry. Academia Sinica Changchun Jilin CYU Zuo-LongChengdu Institute of Organic Chemistry. Academia Sinica Chengdu. Sichuan To whom correspondence should be addressed 《天然气化学杂志》1995,(3)
The surface basicity of Ti-La-Li multicoinponent oxides has been investigated by means of CO2-TPD. The experiment results show that C2 (C2H6 C2H4) selectivity is related to surface basic strength. The surface active oxygen species have also been characterized by means of XPS, O2-TPD and so on. It has been indicated that C2 selectivity and CH4 conversion are correlated with lattice oxygen and the adsorbed oxygen on the surface of the catalyst respectively In the O2-TPD experiments, it has also been found that there are three kinds of oxygen species on the surface of the series catalvsts, which are a (100℃ 750℃) Among them α-oxvgen causes deep oxidation whileβand γ oxygen species are related to oxidalive coupling of methane (OCM). 相似文献
107.
On the basis of copper sulphate pentahydrate thermal dissociation, for analyzed reactions I to IV, 6 thermokinetic equations was discussed. Arrhenius law parameters were determined and the isokinetic effect (IE) and Kissinger law appearing was analyzed. It was found that only dependence resulting from isokinetic effect, in the form k
m=q/T
m, relates to the suitable thermokinetic Eq. (2) and Kissinger law in modified form (14). The confirmation was made that the possibility of determining the averaged activation energy from thermokinetic equations using suitable correction coefficients exists.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
108.
The model of Onsager in which a polar molecule undergoes a reaction field due to the polarization of the molecular surroundings is used to evaluate by a S.C.F. calculation (CNDO/2 approximation) the modifications of a molecular structure in the liquid state.Application to water molecule and to three polar dimers for values of the dielectric constant varying between 3 and 78, shows that most of geometric parameters and dipoles moments vary of few per cent when the molecule is inserted in a liquid. In the liquid state dipole moments do not depend very much on the dielectric constant but energies and relative stabilities of isomers are strongly dependent on the medium.[/p] 相似文献
109.
α-Halogeno ethers, sulfides, and amines are reactive compounds which can be used for the formation of new C? C bonds, either as nucleophilic or as electrophilic reagents, or by α-elimination. The use of these compounds in the synthesis of many classes of organic compounds is reviewed. 相似文献
110.
Summary Clenbuterol has been determined in urine by solidphase extraction on a C18 cartridge, diazotization of the eluate with nitrite, coupling of the diazonium ion with 1-(naphthyl)ethylenediamine, and
separation of the azo dye formed by HPLC with a C18 column and a micellar mobile phase containing 0.1 M sodium dodecyl sulphate, 12%n-butanol and 0.05 M citrate buffer, pH 3. Recoveries higher than 90% were obtained by mixing the samples with a 20% 0.2 M
NaOH before extraction. Limits of detection of 51 and 6.7 ng L−1 were obtained with spectrophotometric and thermal lens spectrometric detection, respectively; respective repeatabilities
were 3.1% (5 μg mL−1) and 5.6% (0.16 μg mL−1). 相似文献