全文获取类型
收费全文 | 3977篇 |
免费 | 108篇 |
国内免费 | 14篇 |
专业分类
化学 | 3025篇 |
晶体学 | 8篇 |
力学 | 65篇 |
数学 | 599篇 |
物理学 | 402篇 |
出版年
2021年 | 36篇 |
2020年 | 35篇 |
2019年 | 37篇 |
2018年 | 22篇 |
2016年 | 68篇 |
2015年 | 75篇 |
2014年 | 78篇 |
2013年 | 159篇 |
2012年 | 124篇 |
2011年 | 129篇 |
2010年 | 91篇 |
2009年 | 85篇 |
2008年 | 114篇 |
2007年 | 129篇 |
2006年 | 151篇 |
2005年 | 143篇 |
2004年 | 125篇 |
2003年 | 125篇 |
2002年 | 127篇 |
2001年 | 81篇 |
2000年 | 62篇 |
1999年 | 63篇 |
1998年 | 70篇 |
1997年 | 50篇 |
1996年 | 61篇 |
1995年 | 50篇 |
1994年 | 50篇 |
1993年 | 58篇 |
1992年 | 70篇 |
1991年 | 68篇 |
1990年 | 49篇 |
1989年 | 70篇 |
1988年 | 66篇 |
1987年 | 78篇 |
1986年 | 65篇 |
1985年 | 89篇 |
1984年 | 97篇 |
1983年 | 59篇 |
1982年 | 89篇 |
1981年 | 77篇 |
1980年 | 73篇 |
1979年 | 91篇 |
1978年 | 68篇 |
1977年 | 79篇 |
1976年 | 63篇 |
1975年 | 55篇 |
1974年 | 72篇 |
1973年 | 46篇 |
1971年 | 41篇 |
1970年 | 23篇 |
排序方式: 共有4099条查询结果,搜索用时 250 毫秒
81.
Klaus Behmel Manfred Schulze und Ulrich Annagat 《Fresenius' Journal of Analytical Chemistry》1968,241(1):1-4
Zusammenfassung Eine einfache und genaue Methode zur Bestimmung von Si-H- und Si-Si-Gruppen wird vorgeschlagen, die auf alkalischer Hydrolyse der Substanz in einem verschlossenen und evakuierten Kolben und AuswÄgen des dem entwickelten Wasserstoff entsprechenden Wasservolumens beruht. Bei H-Gehalten von 1–2% betrÄgt die Genauigkeit ± 0,03% (abs.).
Herrn Prof. Dr. E. Asmus zum 60. Geburtstag gewidmet. 相似文献
Note on the quantitative determination of Si-H- and Si-Si-groups
A simple, robust and surprisingly exact method for the quantitative determination of Si-H- or Si-Si-groups is based on alkaline hydrolysis of the Si-H- or Si-Si-containing substances in a sealed and evacuated vessel and weighing of the volume of water equivalent to the developed volume of hydrogen. The accuracy is ± 0.03% (abs.) with H contents of 1–2%.
Herrn Prof. Dr. E. Asmus zum 60. Geburtstag gewidmet. 相似文献
82.
83.
84.
85.
Klaus van Benthem Stephan Krämer Wilfried Sigle Manfred Rühle 《Mikrochimica acta》2002,138(3-4):181-193
An understanding of the correlation between microstructures and properties of materials require the characterization of the
material on many different length scales. Often the properties depend primarily on the atomistics of defects, such as dislocations
and interfaces. The different techniques of transmission electron microscopy allow the characterization of the structure and
of the chemical composition of materials with high spatial resolution to the atomic level: high resolution transmission electron
microscopy allows the determination of the position of the columns of atoms (ions) with high accuracy. The accuracy which
can be achieved in these measurements depends not only on the instrumentation but also on the quality of the transmitted specimen
and on the scattering power of the atoms (ions) present in the analyzed column.
The chemical composition can be revealed from investigations by analytical microscopy which includes energy dispersive X-ray
spectroscopy, mainly quantitatively applied for heavy elements, and electron energy-loss spectroscopy. Furthermore, the energy-loss
near-edge structure of EELS data results in information on the local band structure of unoccupied states of the excited atoms
and, therefore, on bonding. A quantitative evaluation of convergent beam electron diffraction results in information on the
electron charge density distribution of the bulk (defect-free) material.
The different techniques are described and applied to different problems in materials science. It will be shown that nearly
atomic resolution can be achieved in high resolution electron microscopy and in analytical electron microscopy. Recent developments
in electron microscopy instrumentation will result in atomic resolution in the foreseeable future. 相似文献
86.
87.
Trifluoromethoxy-substituted anilines undergo hydrogen/lithium permutation ("metalation") with optional site selectivity depending on the N-protective group employed. N-tert-Butoxycarbonyl-2- and -4-(trifluoromethoxy)aniline react with tert-butyllithium at the nitrogen-adjacent 6- and 2-position affording, after electrophilic trapping, products 1-6. In contrast, deprotonation of the para isomer occurs at the oxygen-neighboring 3-position, giving rise to the acid 12, when the amino group is carrying two trimethylsilyl groups. sec-Butyllithium attacks 3-trifluoromethoxy-N-mono(trimethylsilyl)aniline at the 2-position, but 3-trifluoromethoxy-N,N-bis(trimethylsilyl)aniline at the 4-position to provide respectively the acids 10 and 11 after carboxylation. The synthesis of two new benzodiazepines illustrates (19 and 22) the preparative potential of the aniline functionalization mediated by organometallic reagents. 相似文献
88.
A new base-labile anchoring group, derived from 9-(hydroxymethyl)fluorene-4-carboxylic acid (HO2CFmoH or HOFmCO2H; 7), for polymer-supported peptide synthesis os described. The synthesis of 7 starting from 2,2′-biphenyldicarboxylic acid ( 1 ) proceeds in an overall yield of 53%. The group HO2CFmo exhibits properties similar to the well known Fmoc protecting group: It is stable to acidic conditions and cleavable by 15% piperidine in DMF. In combination with acid labile Nα-protecting groups (e.g. Boc, Ddz, Bpoc, Nps etc.), it renders more flexibility to the stepwise synthesis using polymer supports. The versatility of the new anchoring group in solid- and liquid-phase peptide synthesis is demonstrated for the synthesis of a model peptide. 相似文献
89.
Synthesis and Crystal Structure of Hydronium-tris-ethylenediamine-cobalt(rhodium)-μ-trichloro-nonachlorotrirhenate(III)-chloride, H3O[MEn3][Re3Cl12]Cl (M=Co, Rh) The chlorides H3O[MEn3][Re3Cl12]CI (M = Co, 1 ; Rh, 2 ) crystallize from hydrochloric acid solutions of ReCl3 and MEn3 · 3H2O as deep red hexagonal columns. They are isotypic and crystallize with the hexagonal system (P6 , Z = 1; 1: a = 1010.87(3); c = 794.30(4) pm, R = 0.023, Rw = 0.016; 2: a = 1018.58(3); c = 794.74(4) pm, R = 0.026, Rw = 0.018). The anions [Re3Cl12]3? are connected via H3O+ cation (C.N. 3). The large channels that run in the [001] direction contain, alternatively, the cations [MEn3]3+ and the lonesome Cl?-anions 相似文献
90.
The synthesis and structure of an acetal protected 10-membered cyclic enediyne-1,2-diol rac-10 is reported. The conformational constrain of the unsaturated macrocycle by the acetal protection group prevents the thermal cyclization reaction of the endiyne during synthesis and purification. 相似文献