全文获取类型
收费全文 | 35529篇 |
免费 | 5606篇 |
国内免费 | 5695篇 |
专业分类
化学 | 21009篇 |
晶体学 | 2778篇 |
力学 | 2191篇 |
综合类 | 304篇 |
数学 | 3437篇 |
物理学 | 17111篇 |
出版年
2024年 | 43篇 |
2023年 | 287篇 |
2022年 | 577篇 |
2021年 | 782篇 |
2020年 | 1023篇 |
2019年 | 953篇 |
2018年 | 954篇 |
2017年 | 1246篇 |
2016年 | 1523篇 |
2015年 | 1254篇 |
2014年 | 1931篇 |
2013年 | 3309篇 |
2012年 | 2167篇 |
2011年 | 2250篇 |
2010年 | 2047篇 |
2009年 | 2257篇 |
2008年 | 2342篇 |
2007年 | 2394篇 |
2006年 | 2449篇 |
2005年 | 2192篇 |
2004年 | 2226篇 |
2003年 | 1807篇 |
2002年 | 1538篇 |
2001年 | 1134篇 |
2000年 | 1125篇 |
1999年 | 1072篇 |
1998年 | 903篇 |
1997年 | 739篇 |
1996年 | 784篇 |
1995年 | 637篇 |
1994年 | 592篇 |
1993年 | 457篇 |
1992年 | 420篇 |
1991年 | 291篇 |
1990年 | 184篇 |
1989年 | 177篇 |
1988年 | 140篇 |
1987年 | 113篇 |
1986年 | 89篇 |
1985年 | 96篇 |
1984年 | 66篇 |
1983年 | 32篇 |
1982年 | 44篇 |
1981年 | 37篇 |
1980年 | 30篇 |
1979年 | 25篇 |
1978年 | 21篇 |
1977年 | 16篇 |
1976年 | 20篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Exfoliated polystyrene (PS)/laponite nanocomposites were prepared successfully. The characteristic doo1 diffraction peak of organo-laponite disappeared in the XRD patterns of nanocomposites, indicating that the laponite layers were exfoliated and the ordered crystal structure of laponite was destroyed because of the styrene polymerization. TEM observations showed that the exfoliated laponite primary particles were dispersed randomly in the PS matrix with lateral dimensions from 1 nm to 10 rim. SEM results showed that the PS/laponite nanocomposite particles were almost monodispersed spheres with the size of about 120 rim. Because of the interaction between PS and laponite nanolayers, the nanocomposites exhibited higher thermal stability and glass transition temperature when compared to pure PS. 相似文献
992.
993.
1INTRODUCTIONPolarizedketenedithioacetalshavebeenex-tensivelyusedasbuildingblocksinorganicsyn-thesis,especiallyinthesynthesisofheterocycliccompounds[1~6].Pyrazoleanditsderivativesrepre-sentoneofthemostactiveclassesofcompoundspossessingwidespectraofbiologicalactivities.Overthepastfewyears,considerableevidenceshavebeenaccumulatedtodemonstratetheefficacyofpyrazolederivatives,suchasantibacterial[7],fun-gicidal[8],herbicidal[9],insecticidal[10]andotherbiologicalactivities[11,12].Uptonow,alarg… 相似文献
994.
以2,4,6-吡啶三甲酸(H3ptc)和六水合高氯酸镍为原料用水热方法合成了三维超分子配合物[Ni(H2O)6]1.5.[Ni(ptc)(Hptc)].(H2O)4,并经X射线衍射分析确定了其单晶结构.该晶体属三斜晶系,P墿空间群,晶胞参数为:a=1.03058(16)nm,b=1.11852(17)nm,c=1.4689(2)nm,α=74.725(2)°,β=77.777(2)°,γ=64.734(2)°,V=1.4675(4)nm3,Mr=798.20,Z=2,ρ=1.806g/cm3,F(000)=822,μ=1.701mm-1,R1=0.0373,wR2=0.0993.结果表明,在组成该超分子体系的基本结构单元[Ni(ptc)(Hptc)]3-和[Ni(H2O)6]2+中,Ni(Ⅱ)离子均处于变形八面体配位环境中.配合物单元之间通过氢键相互连接,形成了无限延伸的三维超分子网状结构. 相似文献
995.
996.
环己胺与手性元5-(R)-(1′R,2′S,5′R)-孟氧基-3-溴-2(5H)-呋喃酮的不对称M ichael加成/分子内亲核取代反应,得到含有两个新手性中心的氮杂环丙烷/稠合丁内酯标题化合物。对其进行了谱学表征和X-射线单晶衍射测定。标题化合物分子式为C20H33NO3,M r=335.47,三斜晶系,P1空间群,晶胞参数:a=5.438(11)。A,b=8.117(2)。A,c=11.572(2)。A,α=96.84(3)°,β=94.48(3)°,γ=101.86(3),°V=493.5(2)。A3,Z=1,D c=1.129g/cm3,R=0.0867,wR=0.2344。 相似文献
997.
1 INTRODUCTION 1,3,4-Thiadiazole derivatives represent an interes- ting class of compounds possessing broad spectrum biological activities[1~4]. Aroyl ureas are known to exhibit diverse biological effects, such as insecticidal, fungicidal, herbicidal and plant growth regulating activities[5~8]. Therefore, it is worthwhile to investi- gate the compounds incorporating both 1,3,4-thiadia- zole nucleus and aroyl urea group. In the previous paper[9], a series of aroyl ureas containing 1,3,4-t… 相似文献
998.
ThestudyofphotoionizationandphotodissociationprocessesinducedbyintensefemtosecondlaserpuIses(>lo"W/cm')withpolyatomicmoleculesbecomesofinterestbecausesomenewphenomenahavebeenobserved'-3.Sofar,mostoftheinvestigationsfocusontheexperimentalexplorationofphotoionizationprocesses'-'.Butthephotoionizationmechanismofpolyatomicmoleculesinanintensefslaserfieldisstillambiguous'.,.Incontrasttothephotoionizationprocesses,theunderstandingoffragmentationofmolecularionisevenpoorer.Corkumetal,'.,'reportedthef… 相似文献
999.
Complexes of FeI2 and FeI3 with Tetramethylurea [FeI2(OC(NMe2)2)2] ( 1 , [Fe2I4(OC(NMe2)2)2] ( 2 ), and [FeI3(OC(NMe2)2] ( 3 ) were prepared by the reaction of FeI2 and FeI2/iodine, respectively, with tetramethylurea. The structures of 1 and 3 were determined from single crystal X-ray diffraction data. 1 crystallizes in the triclinic space group P1 , with a = 809.9(1), b = 923.2(1), c = 1 374.6(1) pm, α = 106.80(1), β = 90.47(1), γ = 101.55(1)°; Z = 2; R = 0.045., 3 : monoclinic, P21/c, a = 1 311.4(1), b = 783.3(1), c = 1 409.1(1) pm, β = 97.36(1)°; Z = 4; R = 0.047. 1 and 3 are isolated neutral complexes with distorted tetrahedral coordination of iron. 3 is the first FeI3-complex with an O-donor ligand. The IR-spectra exhibit strong shifts of n?C = O and n?asC—N of tetramethylurea especially on coordinating to FeI3. 相似文献
1000.
REN Xue-Ling WU Chao GAO Ying ZOU Xiao-Mao YANG Hua-Zheng② 《结构化学》2004,23(3):267-269
1 INTRODUCTION The pyrazolo[1,5-a]pyrimidines are the active compositions in many pesticides and medicines. A new class of KDR (tyrosine kinase VEGFR-2) ki- nase inhibitors is discovered and found to have affi- nity for the human CRF-1 (Corticotrophin re… 相似文献