全文获取类型
收费全文 | 98篇 |
免费 | 0篇 |
专业分类
化学 | 37篇 |
力学 | 2篇 |
数学 | 19篇 |
物理学 | 40篇 |
出版年
2019年 | 1篇 |
2017年 | 1篇 |
2016年 | 3篇 |
2015年 | 2篇 |
2014年 | 5篇 |
2013年 | 10篇 |
2012年 | 1篇 |
2010年 | 2篇 |
2009年 | 3篇 |
2008年 | 5篇 |
2007年 | 1篇 |
2006年 | 2篇 |
2005年 | 3篇 |
2004年 | 1篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1989年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1971年 | 3篇 |
排序方式: 共有98条查询结果,搜索用时 187 毫秒
71.
72.
73.
The use of XRD as a diagnostic technique for deeply weathered samples (8) suggested the occurrence of gibbsite and geothite, the presence of cristobalite and lepidocrocite was not recognized and the presence of chlorite was highly questionable. The XRD traces in this study give sparse observations and background interference from the high iron concentration of the samples let to difficulties in interpretation of the minerals.The use of thermoanalytical techniques produced the mineral assembly: gibbsite, lepidocrocite, goethite, kaolinite, cristobalite, and possibly chlorite. It has brought to light the factors such as pretreatment, texture of the sample, sample:diluent ratio and heating rate, must be carefully considered when mineralogical identification of these type of deeply weathered samples is undertaken by the methods studied. 相似文献
74.
First-order solvolysis rate constants are reported for solvolyses of acetyl chloride in methanol and MeOD, and in binary aqueous mixtures with acetone, acetonitrile, ethanol, methanol, and trifluoroethanol at 0 degrees C. Product selectivities (S = [MeCOOR]/[MeCOOH] x [water]/[alcohol]) are reported for solvolyses in ethanol/ and methanol/water at 0 degrees C. Solvolyses of acetyl chloride show a high sensitivity to changes in solvent ionizing power, consistent with C-Cl bond cleavage. As the solvent is varied from pure ethanol (or methanol) to water, S values and rate-rate profiles show no evidence for the change in reaction channel observed for solvolyses of benzoyl and trimethylacetyl chlorides. However, using rate ratios in 40% ethanol/water and 97% trifluoroethanol/water (solvents of similar ionizing power but different nucleophilicities) to compare sensitivities to nucleophilic attack, solvolyses of acetyl chloride are over 20-fold more sensitive to nucleophilic attack than benzoyl chloride. The solvent isotope effect of 1.29 (MeOH/MeOD) for acetyl chloride is similar to that for p-methoxybenzoyl chloride (1.22) and is lower than for benzoyl chloride (1.55). Second-order rate constants for aminolyses of acetyl chloride with m-nitroaniline in methanol at 0 degrees C show that acetyl chloride behaves similarly to p-methoxybenzoyl chloride, whereas benzoyl chloride is 40-fold more sensitive to the added amine. The results indicate mechanistic differences between solvolyses of acetyl and benzoyl chlorides, and an S(N)2 mechanism is proposed for solvolyses and aminolyses by m-nitroaniline of acetyl chloride (i.e. these reactions are probably not carbonyl additions, but a strong sensitivity to nucleophilic attack accounts for their high rates). 相似文献
75.
Dussault PH Lee IQ Lee HJ Lee RJ Niu QJ Schultz JA Zope UR 《The Journal of organic chemistry》2000,65(25):8407-8414
The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful. 相似文献
76.
77.
78.
79.
Background
It is well known that both semantic and syntactic information play a role in pronoun resolution in sentences. However, it is unclear what the relative contribution of these sources of information is for the establishment of a coreferential relationship between the pronoun and the antecedent in combination with a local structural case constraint on the pronoun (i.e. case assignment of a pronoun under preposition governing). In a prepositional phrase in German and Dutch, it is the preposition that assigns case to the pronoun. Furthermore, in these languages different overtly case-marked pronouns are used to refer to male and female persons. Thus, one can manipulate biological/syntactic gender features separately from case marking features. 相似文献80.
Sherrod L. Maxwell Brian K. Culligan Robin C. Utsey Daniel R. McAlister E. Philip Horwitz 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(3):2181-2188
A new rapid method for the determination of 228Ra in natural water samples has been developed at the SRNL/EBL (Savannah River National Lab/Environmental Bioassay Laboratory) that can be used for emergency response or routine samples. While gamma spectrometry can be employed with sufficient detection limits to determine 228Ra in solid samples (via 228Ac), radiochemical methods that employ gas flow proportional counting techniques typically provide lower minimal detectable activity levels for the determination of 228Ra in water samples. Most radiochemical methods for 228Ra collect and purify 228Ra and allow for 228Ac daughter ingrowth for ~36 h. In this new SRNL/EBL approach, 228Ac is collected and purified from the water sample without waiting to eliminate this delay. The sample preparation requires only about 4 h so that 228Ra assay results on water samples can be achieved in <6 h. The method uses a rapid calcium carbonate precipitation enhanced with a small amount of phosphate added to enhance chemical yields (typically >90 %), followed by rapid cation exchange removal of calcium. Lead, bismuth, uranium, thorium and protactinium isotopes are also removed by the cation exchange separation. 228Ac is eluted from the cation resin directly onto a DGA Resin cartridge attached to the bottom of the cation column to purify 228Ac. DGA Resin also removes lead and bismuth isotopes, along with Sr isotopes and 90Y. La is used to determine 228Ac chemical yield via ICP-MS, but 133Ba can also be used instead if ICP-MS assay is not available. Unlike some older methods, no lead or strontium holdback carriers or continual readjustment of sample pH is required. 相似文献