全文获取类型
收费全文 | 11561篇 |
免费 | 1107篇 |
国内免费 | 848篇 |
专业分类
化学 | 7937篇 |
晶体学 | 486篇 |
力学 | 104篇 |
综合类 | 74篇 |
数学 | 2420篇 |
物理学 | 2495篇 |
出版年
2023年 | 72篇 |
2022年 | 83篇 |
2021年 | 133篇 |
2020年 | 176篇 |
2019年 | 259篇 |
2018年 | 211篇 |
2017年 | 239篇 |
2016年 | 261篇 |
2015年 | 449篇 |
2014年 | 486篇 |
2013年 | 1042篇 |
2012年 | 835篇 |
2011年 | 935篇 |
2010年 | 874篇 |
2009年 | 1003篇 |
2008年 | 900篇 |
2007年 | 805篇 |
2006年 | 666篇 |
2005年 | 510篇 |
2004年 | 495篇 |
2003年 | 449篇 |
2002年 | 490篇 |
2001年 | 290篇 |
2000年 | 268篇 |
1999年 | 245篇 |
1998年 | 200篇 |
1997年 | 157篇 |
1996年 | 144篇 |
1995年 | 107篇 |
1994年 | 102篇 |
1993年 | 75篇 |
1992年 | 55篇 |
1991年 | 46篇 |
1990年 | 23篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1985年 | 41篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 25篇 |
1981年 | 48篇 |
1980年 | 32篇 |
1979年 | 17篇 |
1978年 | 26篇 |
1977年 | 33篇 |
1976年 | 28篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
101.
晶化温度对中孔分子筛(M41S)结构转变的影响 总被引:11,自引:6,他引:11
在低表面活性剂浓度及低表面活性剂与氧化硅比的条件下,研究了十六烷基三甲基溴化铵-硅酸钠体系中晶化温度对中孔分子筛M41S材料结构转变的影响。结果有,晶化温度提高至135℃改变了原六方晶相MCM-41的结构,晶化温度提高到150℃后,中孔结构六方MCM-41晶格转变为变不稳定的层状相, 相似文献
102.
104.
La-Fe-M(M=Al, Si)化合物磁热性能研究进展 总被引:5,自引:1,他引:5
介绍了La-Fe-M(M=AI,Si)化合物在磁热性能研究方面的最新进展。具有NaZn13型晶体结构,含高浓度Fe的La-Fe—M(M=AI,Si)化合物为良好的软磁材料;用少量的Co替代化合物中Si,Al元素可以将化合物的居里温度提高至室温;对La(Fe1-yCoy)xSi13-x化合物,适量的Si,Co组合可使化合物在室温产生可与Gd5Si2Ge2比拟的磁热效应;加入适量的间隙原子H,也可使La(FexSi1-x)13在室温的磁热性能远远大于金属Gd;对含Si量低及含Si量高的La(FexSi1-x)13化合物在相转变点附近由温度和磁场诱导相变的本质做了详细阐述。 相似文献
105.
106.
This study investigates the effect of surface immobilization on the bactericidal function of a quaternary ammonium compound. Quaternary ammonium silane (QAS) coated planar surfaces did not produce any measurable mortality of Staphylococcus aureus, while 1 µm QAS‐coated microparticles did produce S. aureus mortality. The experiments using QAS‐coated microparticles indicate that the ability of QAS molecules to disrupt the cell wall is not hindered by covalent immobilization of QAS to a surface. These results provide evidence that S. aureus cells on a QAS‐coated planar surface are not exposed to a sufficient number of QAS molecules to produce significant mortality. This result has important implications for the development of self‐decontaminating coatings. Covalent immobilization is used to prevent leaching of the bactericidal compound. However, covalent immobilization may result in a significant tradeoff in bactericidal performance. Published in 2007 by John Wiley & Sons, Ltd. 相似文献
107.
108.
首次研究了碳化钨负载S2 O2 -8 ZrO2 (PSZ)固体超强酸催化剂 (WC PSZ)上正戊烷的反应情况及影响催化剂活性的各种因素 ,并用GC MS ,XRD ,BET等手段分析了正戊烷反应产物和催化剂的物理化学性质等 .结果表明 :WC PSZ对正戊烷具有异构化和裂解的双重催化作用 .PSZ在负载适量碳化钨后对正戊烷反应的活性和选择性显著提高 ,显示出优于Pt PSZ催化剂的效果 ,碳化钨的负载量为 2 0 %的效果最佳 .适当的焙烧、活化和反应温度等条件是取得良好反应效果的关键 相似文献
109.
The structure of the complex, [Fe2(II)Fe2(III)(HCOO)10(C6H7N6)n, (1) exhibits a neutral two-dimensional layer network of alternating iron(II) and iron(III) ions, bridged equatorially by formate groups. All iron atoms are octahedrally coordinated, with iron(III) coordinating axially to one gamma-picoline and one formate group, while the iron(II) centers interact axially with two gamma-picoline groups, above and below the layer plane. The complex crystallizes in the triclinic space group P1 at all studied temperatures [at 120 K, the cell dimensions are: a = 10.228(1), b = 12.071(1), c = 12.072(1) A, alpha = 89.801(2), beta = 71.149(2), gamma = 73.371(2) degrees]. An intralayer antiferromagnetic exchange interaction of J = -2.8 cm(-1) between iron(II) and iron(III) was observed in the magnetic studies. Decreasing the temperature to close to 20 K causes a magnetic-ordering phenomenon to occur and a low-temperature phase with a long-range antiferromagnetic spin orientation appears. The magnetic phase transition was confirmed by M?ssbauer spectroscopic studies at temperatures above and below the critical temperature. Structural information of 1 from synchrotron X-ray diffraction data collected at room temperature and 16 K suggests that the antiferromagnetic ordering is caused by an enhanced pi-pi interaction between chi-picoline groups from adjacent layers. 相似文献
110.
Paul V. Bernhardt Dr. Geoffrey A. Lawrance Brian W. Skelton Allan H. White 《无机化学与普通化学杂志》2007,633(7):1036-1039
Cations derived by protonation of the ligand title compound (L1) have been structurally characterized in their di‐ and tetra‐ protonated forms in the salts [H2L1][ClO4]2·2H2O and [H4L1][ZnCl4]2·4H2O. In both structures, one half of the formula unit comprises the asymmetric unit of the structure, the macrocycle being centrosymmetric, with the two macrocycles adopting similar conformations. In both salts, a pair of diagonally opposed macrocyclic secondary amine groups are protonated; in the [H4L1]4+ salt, the additional pair of protons are accommodated on the exocyclic pendant amine groups. The dispositions of the pendent amines differ between the two structures, being ‘equatorial’ with respect to the macrocyclic ring in the [H2L1]2+ salt, and ‘axial’ in the [H4L1]4+ salt. In other structurally characterized compounds containing [H4L1]4+ the equatorial disposition was found in the ferricyanide adduct, while in the tetraperchlorate salt the axial disposition was identified. The differences in disposition of the exocyclic groups are ascribed to the extensive H‐bonding in the lattices. 相似文献