全文获取类型
收费全文 | 3943篇 |
免费 | 172篇 |
国内免费 | 448篇 |
专业分类
化学 | 2130篇 |
晶体学 | 22篇 |
力学 | 28篇 |
综合类 | 1篇 |
数学 | 10篇 |
物理学 | 2372篇 |
出版年
2024年 | 5篇 |
2023年 | 83篇 |
2022年 | 51篇 |
2021年 | 75篇 |
2020年 | 60篇 |
2019年 | 69篇 |
2018年 | 80篇 |
2017年 | 76篇 |
2016年 | 106篇 |
2015年 | 105篇 |
2014年 | 131篇 |
2013年 | 228篇 |
2012年 | 145篇 |
2011年 | 275篇 |
2010年 | 176篇 |
2009年 | 264篇 |
2008年 | 247篇 |
2007年 | 335篇 |
2006年 | 302篇 |
2005年 | 194篇 |
2004年 | 185篇 |
2003年 | 142篇 |
2002年 | 125篇 |
2001年 | 122篇 |
2000年 | 133篇 |
1999年 | 124篇 |
1998年 | 131篇 |
1997年 | 86篇 |
1996年 | 55篇 |
1995年 | 60篇 |
1994年 | 53篇 |
1993年 | 53篇 |
1992年 | 33篇 |
1991年 | 33篇 |
1990年 | 27篇 |
1989年 | 15篇 |
1988年 | 20篇 |
1987年 | 24篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有4563条查询结果,搜索用时 31 毫秒
101.
在氩气氛中,合成子SrMgF4:xEu,yTb复合氟化物磷光体,该体系中Eu^3+和Eu^2+共存,随共掺入Tb浓度的增加,Eu^3+的荧光发射强度降低,Eu^3+的发光增强,并且Eu^2+的ESR信号增强,认为Eu^3+和Tb3+之间存在的电荷迁移,即Eu^3+Tb^3+→Eu^2++Tb^4+,通过半量手段研究了这一电荷迁移反应的平衡常数。 相似文献
102.
The synthesis and properties of a perylenediamide diol linker and several DNA hairpins possessing this linker are described. The diol linker absorbs and fluoresces strongly in the visible. Hairpins having poly(dA)-poly(dT) stems have fluorescence quantum yields and decay times similar to those of the linker, indicating that hole injection does not occur from the singlet excited linker into the base pair domain. Fluorescence quenching by dG or dZ bases is observed when these bases are located near the linker. The strong distance dependence of fluorescence quenching is consistent with a superexchange mechanism for electron transfer. Failure to observe formation of the linker anion radical by means of femtosecond time resolved absorption spectroscopy is attributed to fast charge recombination. The properties and behavior of the perylene linker and its hairpins are compared to those of other arenedicarboxamide linkers. 相似文献
103.
Orbital interaction analysis is employed to understand the complex charge transfer mechanism operative in endohedral metallofullerenes of composition NSc3@Cn (n = 68, 78). This phenomenon combines substantial electron transfer from the core to the cage with electron backdonation, involving the interaction between the occupied orbitals of the negatively charged cage and the unoccupied d orbitals of the positively charged core. This electron backdonation differs fundamentally from conventional orbital hybridization, which takes place primarily between the HOMO of the metal core and the LUMO of the fullerene cage. These findings imply the pronounced stability of NSc3@Cn (n = 68, 78), especially for NSc3 encapsulated in the non-IPR C68 enclosure, as experimentally established. 相似文献
104.
105.
Emilio San-Fabián José Pérez-Jordá Federico Moscardó 《Theoretical chemistry accounts》1990,77(3):207-212
Summary The correlation energy of two- and four-isoelectronic series, a representative example for which the existing spin-density functionals fails, is calculated using the Colle and Salvetti method, considering mono- and multideterminantal wave functions. The results are in agreement with experimental data, and show the potentiality of this method when it is applied to wave functions including the most relevant configurational features. Also, results for the ionization energies and electron affinities of first- and second-row atoms are reported. 相似文献
106.
用CNDO/2(s,p,d)方法研究了类立方烷系列Fe-S簇合物[Fe_4S_4Cl_4(?)(S_2CNEt_2)_n]~(2-)(n=0,1,2,4)的电子结构。得出[S_2CN(Et)_2]-螯合配位Fe_4S_4~(2+)簇合物中存在两类不同价态铁原子的结论;骨架Fe_4S_4~(2+)中μ_3-S电子是非定域化的,同Mssbauer谱测定结果一致。讨论了簇合物Fe—Fe之间的成键作用、螫合配体的作用和氧化还原性质。 相似文献
107.
The perovskite-based rare earth cobaltates (Ln0.33Sr0.67CoO3−δ) (Ln=Y3+, Ho3+ and Dy3+) have been synthesized at 1100°C under 1 atm oxygen. A thermogravimetric study has determined the overall oxygen content in each case while a combined electron diffraction (ED) and synchrotron X-ray diffraction study has revealed the presence of a complex, previously unreported, perovskite-related superstructure phase. ED gave a resultant C1c1 but most probably Cmcm (, , ) perovskite-related superstructure, describable as a modulated I4/mmm intermediate parent structure. Synchrotron X-ray data has been used to refine the intermediate parent structures of all three compounds. Coupled Ln/Sr and O/vacancy ordering and associated structural relaxation is shown to be responsible for the observed superstructure. 相似文献
108.
C. Acero J. M. Ribó R. Solé F. R. Trull 《Monatshefte für Chemie / Chemical Monthly》1993,124(4):401-417
Summary Characteristic oxidation reactions of bilin-1,19-diones (biliverdins) and biladien-ac-1,19-diones (bilirubins) were reinvestigated. These include bromine and iodine oxidation of biliverdin IX, bilirubin IX and their Zn(II) complexes, the reaction described by Siedel (1941) of mesobilirubin XIII dimethyl ester with nitric acid (Gmelin reaction) and Smith's oxidation (1977) of etiobiliverdin IV using Tl triacetate and Pb tetra-acetate. For some of these reaction products new structures were assigned on the basis of their spectrometric data. These structures agreed with the expected reactivity pattern of biliverdins and bilirubins.
Reaktivität von Pyrrolpigmenten, 15 Mitt.: Über die Oxidation von Bilirubinen und Biliverdinen
Zusammenfassung Einige bekannte Oxydationen von Bilin-1,19-dionen (Biliverdinen) und Biladien-ac-dionen (Bilirubinen) wurden neuerlich untersucht, und zwar die Brom- und Jodoxydation von Biliverdin IX und Bilirubin IX sowie der entsprechenden Zn(II)-Komplexe, die Reaktion des Dimethylesters von Mesobilirubin XIII mit HNO3 (Gmelinreaktion; Siedel 1941) und Smith's Oxydation von Etiobiliverdin IV mit Thaliumtriacetat und Bleitetraacetat. Für einige Reaktionsprodukte wurden aufgrund spektroskopischer Daten neue Strukturen-konsistent mit dem bekannten Reaktionsverhalten von Biliverdinen und Bilirubinen-formuliert.相似文献
109.
Alexandra Lieb Juliane A. Kechele Robert Kraut Wolfgang Schnick Prof. Dr. 《无机化学与普通化学杂志》2007,633(1):166-171
The oxonitridoalumosilicates (so‐called sialons) MLn[Si4?xAlxOxN7?x] with M = Eu, Sr, Ba and Ln =Ho, Er, Tm, Yb were obtained by the reaction of the respective lanthanoid metal, the alkaline earth carbonates or europium carbonate, resp., AlN, “Si(NH)2” and MCl2 as a flux in a radiofrequency furnace at temperatures around 2100 °C. The compounds MLn[Si4?xAlxOxN7?x] are relevant for the investigation of substitutional effects on the materials properties due to their ability of tolerating a comparatively large phase width up to x ≈ 2.0(5). The crystal structures of the twelve compounds were refined from X‐ray single crystal data and X‐ray powder data and are found to be isotypic to the MYb[Si4N7] structure type. The compounds crystallize in space group P63mc (no. 186, hexagonal) and are made up of chains of so‐called starlike units [N[4](SiN3)4] or [N[4]((Si,Al)(O,N)3)4], respectively. These units are formed by four (Si,Al)(N/O)4 tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N‐distribution model according to Paulings rules, i.e. nitrogen was positioned on the four‐fold bridging site and nitrogen and oxygen were distributed equally on both of the two‐fold bridging sites, resulting in charge neutrality of the compound. The Si and Al atoms were distributed equally on their two crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X‐ray methods. The chemical compositions of the compounds were derived from electron probe micro analyses (EPMA). 相似文献
110.
Chisuzu Tokoh Keiji Takabe Junji Sugiyama Minoru Fujita 《Cellulose (London, England)》2002,9(3-4):351-360
Acetobacter
xylinum was cultured in Schramm–Hestrin medium containing pectin (pectin medium), xylan (xylan medium), or glucomannan (mannan medium). X-ray diffractometry revealed that xylan and glucomannan affected the size of the cellulose crystals and their d-spacing values. Solid-state cross polarization magic angle spinning carbon-13 nuclear magnetic resonance spectroscopy indicated that the ratio of cellulose I was reduced by the addition of polysaccharides. These effects were more remarkable on the cellulose in the mannan medium than that in the xylan medium, and were scarcely observed in the pectin medium. Electron diffraction analysis revealed that these effects on hemicelluloses along cellulose microfibrils are continuous in the mannan medium and discontinuous in the xylan medium. These findings suggest that the uronic acid in the polysaccharides prevents interactions with cellulose leading to alterations of the structure of the cellulose crystal. 相似文献