全文获取类型
收费全文 | 1514篇 |
免费 | 78篇 |
国内免费 | 47篇 |
专业分类
化学 | 1171篇 |
晶体学 | 1篇 |
力学 | 16篇 |
综合类 | 1篇 |
数学 | 116篇 |
物理学 | 334篇 |
出版年
2024年 | 23篇 |
2023年 | 49篇 |
2022年 | 84篇 |
2021年 | 45篇 |
2020年 | 28篇 |
2019年 | 23篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 15篇 |
2015年 | 22篇 |
2014年 | 45篇 |
2013年 | 29篇 |
2012年 | 116篇 |
2011年 | 155篇 |
2010年 | 51篇 |
2009年 | 41篇 |
2008年 | 104篇 |
2007年 | 128篇 |
2006年 | 145篇 |
2005年 | 99篇 |
2004年 | 40篇 |
2003年 | 35篇 |
2002年 | 55篇 |
2001年 | 45篇 |
2000年 | 48篇 |
1999年 | 48篇 |
1998年 | 36篇 |
1997年 | 1篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 11篇 |
1993年 | 19篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 4篇 |
排序方式: 共有1639条查询结果,搜索用时 372 毫秒
101.
The identity S(N)2 reactions on nitrogen (see eq 3) with nucleophiles having the general structure H(n)()X(-) where X belongs to the group of nonmetallic elements which do not border the line separating them from the metallic elements (X = F, Cl, Br, I, O, S, Se, N, P, and C) were studied at the G2+ level. The results show that, similarly to the previously observed phenomenon for S(N)2 reaction on carbon (J. Am. Chem. Soc. 1999, 121, 7724), the Periodic Table, through the valence of the element X, controls the intrinsic barrier for the reaction. The average intrinsic barriers obtained for nitrogen substrates were 20, 27, 39, and 57 kcal/mol for the mono-, di-, tri-, and tetravalent X's, respectively. It is also concluded that the intrinsic barriers are similar for N- and C-based substrates and dimethyl substitution on both raises the intrinsic barrier by ca. 10 kcal/mol. 相似文献
102.
Yi W Yu S Bersohn R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(7):1389-1393
The spin population difference between m(s) = 1/2 and - 1/2, (malpha - mbeta)(t) of the hydrogen atoms dissociated from the mercury hydride molecules is calculated as a function of time for the reaction of Hg* + H2 in a magnetic field. Starting from zero value, the population difference increases to a certain limit that is equal to the room temperature population difference. The H/D electron spin resonance (ESR) signal ratio obtained from the isotopic experiment, Hg* + H2/D2, is also explained by comparing the ratio of (malpha - mbeta)H(t) and (malpha - mbeta)D(t). The isotopic effect is interpreted to originate mostly from spin-rotation constant energies of mercury hydride molecules. 相似文献
103.
Quasicrystals and related approximant phases in Mg-Zn-Y 总被引:3,自引:0,他引:3
As-cast microstructure of Mg-rich Mg(68)Zn(28)Y(4) has been investigated by a detailed transmission electron microscopy study. The as-cast Mg(68)Zn(28)Y(4) alloy consisted of three different types of phases: 10-20 m size primary solidification phase, dendritic phase grown from the primary phase and a eutectic structure formed at the later stage of solidification. The primary solidification phase has an icosahedral structure with a large degree of phason strain. 1/1 rhombohedral approximant phase with lattice parameters a=27.2 A and =63.43 degrees is first observed in Mg-Zn-Y system. The rhombohedral structure can be obtained by introducing phason strain in the six-dimensional face centered hyper-cubic lattice. The decagonal phase nucleates with orientation relationship with the icosahedral phase, and Mg(4)Zn(7) nucleates with orientation relationship with the decagonal phase, indicating a close structural similarity between the three phases. Gradual depletion of Y during solidification plays an important role in heterogeneous nucleation of decagonal and Mg(4)Zn(7) phases from icosahedral and decagonal phases, respectively. 相似文献
104.
A New Ceramide from the Soft Coral Cladiella humesi Verseveldt 总被引:2,自引:0,他引:2
Ming Yan WANG Yi LUO Jing Yu SU Department of Chemistry Zhongshan University Guangzhou 《中国化学快报》2000,11(9)
Ceramides are increasingly becoming important compounds because of their markedbiological activity. The 2000 International Gordon Research Conference on "Glycolipidand Sphingolipid Biology" will be held in May in italy. It has been recently found thatceramides inhibit Cholesteryl Ester Transfer Protein (CETP)'. Elevation in CETP leadsto atherosclerotic cardiovascular diseases2. A literature survey showed that ceramideswere antifungal', and some of these showed antimicrobial and cytotox… 相似文献
105.
Coumarin-glycoside and Ferulate from Peucedanum decursivum 总被引:2,自引:0,他引:2
The root of Peucedanum decursivum (Miq.) Maxim (umbelliferae) is known as a famoustraditional Chinese medicine. In our Studies on the chemical constituents, compounds Iand 2 were isolated from the extract of the roots by combination of silica gel columnchromatography and preparative HPLC.Compound l, yellow powder, C,.H,,O., (HRFAB-MS (M-H) - m/z. cafed. 475. 1604.ohs. 475.1594), UV spectrum (X max 334 and 203 "m), IH and 13C NMR spectra werequite similar to those of furocoumarin gl… 相似文献
106.
Ceramic hollow microspheres (CHMSs) were prepared to use as supports for the removal of heavy metal ions from industrial waste-water. A water extraction sol–gel technique was used to prepare porous CHMS by extracting water from an emulsion of LUDOX (silica colloid; SiO2, Aldrich Co.) and 2-ethyl-1-hexanol. Experiments were conducted to control pore size, wall thickness, and separation yield by examining the ratio of precursors (LUDOX and 2-ethyl-1-hexanol), catalyst (NH4OH), sintering temperature, surfactant (SPAN 80), extractant (n-butanol), stirring speed, and concentration of precursor (LUDOX). The results revealed that the optimum conditions were 20 ml of a 10 wt% solution of LUDOX, 10 ml of NH4OH, a sintering temperature of 500°C, 0.4 ml of SPAN 80, 200 ml of n-butanol, and a stirring speed of 730 rpm/100 ml of 2-ethyl-1-hexanol. CHMSs were impregnated in Cyanex 272 and examined for their ability to remove heavy metal ions from a solution. Based on an experiment involving the removal of metal ions using CHMSs that were prepared under optimum conditions, Zn ion was removed at a level of 0.354 mmol/g at pH 4, which was about twice the adsorption capacity of CHMSs prepared by Wilcox (Mater. Res. Soc. Symp. Proc.346, 201 (1994)). 相似文献
107.
Yi SS 《Optics letters》1999,24(23):1666-1668
A theoretical investigation of oblique plane-wave electromagnetic scattering in an active dielectric cylinder predicted the existence of anomalous resonances at discrete plane-wave angles of incidence. These resonances may be understood as being due to a leaky-wave phase-matching boundary condition. Experiments were performed with active dielectric cylinders to confirm the existence of discrete resonances. Cross coupling between TE and TM modes was clearly detected for both active and passive scattering. Enhancement of active scattered field intensities was observed in experiments with finite-diameter pump and probe laser beams. Optical pumping of a dye solution was used to provide the gain. 相似文献
108.
A series of 3,6-disubstituted s-triazolo[3,4-b]-1,3,4-thiadiazoles have been synthesized,whosestructures were determined by elemental and spectral analyses;all the new compounds showed significantantibacterial activity. 相似文献
109.
合成CFC-12替代物HFC-134a的CrF_3/AlF_3催化剂研究(Ⅴ)──CrF_3与AlF_3的相互作用 总被引:1,自引:0,他引:1
由于氟氯烃(CFCs)对大气臭氧层的破坏作用,使得开发CFCs无污染替代品的研究成为热门课题[1~4].氢氟烃(HFCs,如HFC-134a)是CFCs的理想替代物.在氟氯交换合成HFC-134a的反应中,AlF3基催化剂的活性、稳定性明显优于CrF... 相似文献
110.
IntroductionTheincreasingnecessity0fpharmacologicalresearchforchiraldrugshasrenderedchiralseparationanactiveareaincaPillaryelectroph0resis(CE)today.Variousmodes0fCE,includingcaPillaryzoneelectroph0resis(CZE),micellarelectrokineticchromatography(MEKC),caPillarygelelectrophoresis(CGE)andcapillaryisotachophoresis(CITP),havebeenappliedforthispurp0se',amongwhichCZEisthemostpopularoneforitshighefficiency,lowexpenseandeaseofoperation.Forsuccessfulchiralseparati0ninCZE,theselectortypeisofcr… 相似文献