全文获取类型
收费全文 | 2737篇 |
免费 | 89篇 |
国内免费 | 38篇 |
专业分类
化学 | 1704篇 |
晶体学 | 9篇 |
力学 | 78篇 |
数学 | 472篇 |
物理学 | 601篇 |
出版年
2022年 | 18篇 |
2021年 | 28篇 |
2020年 | 39篇 |
2019年 | 35篇 |
2018年 | 24篇 |
2017年 | 27篇 |
2016年 | 64篇 |
2015年 | 48篇 |
2014年 | 64篇 |
2013年 | 119篇 |
2012年 | 143篇 |
2011年 | 189篇 |
2010年 | 80篇 |
2009年 | 76篇 |
2008年 | 133篇 |
2007年 | 152篇 |
2006年 | 156篇 |
2005年 | 152篇 |
2004年 | 109篇 |
2003年 | 110篇 |
2002年 | 118篇 |
2001年 | 50篇 |
2000年 | 49篇 |
1999年 | 39篇 |
1998年 | 33篇 |
1997年 | 37篇 |
1996年 | 39篇 |
1995年 | 27篇 |
1994年 | 31篇 |
1993年 | 36篇 |
1992年 | 38篇 |
1991年 | 34篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 19篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 39篇 |
1984年 | 32篇 |
1983年 | 25篇 |
1982年 | 34篇 |
1981年 | 31篇 |
1980年 | 34篇 |
1979年 | 22篇 |
1978年 | 28篇 |
1977年 | 21篇 |
1976年 | 25篇 |
1975年 | 18篇 |
1974年 | 18篇 |
1973年 | 32篇 |
排序方式: 共有2864条查询结果,搜索用时 0 毫秒
21.
22.
A new pyridine synthesis has been found by refluxing certain simple secondary car☐amides in HMPA. Thus 2,6-dimethylpyridine, 2-ethyl-6-methylpyridine, 2,3,6-trimethylpyridine, 2-ethyl-3,6-dimethylpyridine, 2-benzyl-6-methyl-3-phenylpyridine, 2-ethyl-3-methyl-5,6,7,8-tetrahydroquinoline and 2-t-butyl-6-methylpyridine were prepared in 15–40% yield. 相似文献
23.
Haisch C Niessner R Matveev OI Panne U Omenetto N 《Analytical and bioanalytical chemistry》1996,356(1):21-26
An experimental set-up for the detection of elemental chlorine in chlorinated hydrocarbons (CHCs) is described based on a miniaturized system, which could be used for on-line monitoring of chlorinated compounds. With an optimized time-resolved detection chlorine from CHCs like CCl4 can be determined by Laser-Induced-Breakdown-Spectroscopy (LIBS) with g/g-detection limits in the gas phase. The application of a miniaturized Nd : YAG laser resulted only in a minor loss in performance, hence it could be used for designing a rugged and small on-line sensor. In addition, preliminary results for the detection of chlorine via the formation of CuCl in the plasma formed by focussing the laser on a copper surface are reported. Utilizing the luminescence of the CuCl D-system at 440 nm, a tenfold improvement in the detection limits was obtained. It appears that the formation of ad hoc selected, small molecules in a laser plasma could be a promising alternative for the selective and sensitive analysis of gaseous chlorinated and other species. 相似文献
24.
25.
Low molecular weight aromatic substances may migrate out from plastic packaging to their contents, especially if they consist of organic aqueous solutions or oils. It is, therefore, extremely important to be able to identify and quantify any migrated substances in such solutions, even at very low concentrations. We have in this work investigated and evaluated the use of solid-phase microextraction for the specific task of extraction from an organic aqueous solution such as a simulated pharmaceutical solution consisting of 10 vol.% ethanol in water. The goal was furthermore to investigate the possibility of simultaneously identifying and quantifying the substances in spite of differences in their chemical structures. Methods were developed and evaluated for extraction both with direct sampling and with headspace sampling. Difficulties appeared due to the ethanol in the solution and the minute amounts of substances present. We have shown that a simultaneous quantification of migrated low molecular weight degradation products of antioxidants using only one fibre is possible if the extraction method and temperature are adjusted in relation to the concentration levels of the analytes. Comparions were made with solid-phase extraction. 相似文献
26.
Palladium-catalyzed intramolecular cyclization of N-(N'-tert-butylformimidoyl)-6-[2-(2-iodophenyl)ethyl]-1,2,3,4-tetrahydropyridine (1a) and N-(N'-tert-butylformimidoyl)-6-[3-(2-iodophenyl)propyl]-1,2,3,4-tetrahydropyridine (1b) respectively results in formation of spiro compounds 1'-(N-tert-butylformimidoyl)-3',4'-dihydrospiro[indan-1,2'(1'H)-pyridine] (4a), 1'-(N-tert-butylformimidoyl)-1',6'-dihydrospiro[indan-1,2'(3'H)-pyridine] (5a), and 1'-(N-tert-butylformimidoyl)-5',6'-dihydrospiro[indan-1,2'(1'H)-pyridine] (6a) and 1'-(N-tert-butylformimidoyl)-3,3',4,4'-tetrahydrospiro[naphthalene-1(2H),2'(1'H)-pyridine] (4b), 1'-(N-tert-butylformimidoyl)-1',3,4,6'-tetrahydrospiro[naphthalene-1(2H),2'(3'H)-pyridine] (5b), and 1'-(N-tert-butylformimidoyl)-3,4,5',6'-tetrahydrospiro[naphthalene-1(2H),2'(1'H)-pyridine] (6b). The double-bond migration process can be controlled, and any of the three double-bond isomers can be prepared by employing proper ligands. A combination of BINAP and the amidine function was required to obtain the isomers 5a and 5b with the double bond in the homoallylic position relative to the aryl group. An electrospray ionization mass spectrometric study was conducted to support suggested reaction intermediates. 相似文献
27.
A selective and sensitive gas chromatographic method for simultaneous determination of sulfinpyrazone and two of its metabolites (the para-hydroxylated metabolite and the sulfone metabolite) in biological fluids using alkali flame ionization detection (AFID), electron capture detection (ECD) and mass fragmentographic detection is described. The compounds are extracted from the samples, methylated and separated on 2% OV-17 or 3% OV-225 columns. Phenylbutazone is used as internal standard. Standard curves are linear. The coefficient of variation at 10 microgram/ml of sulfinpyrazone in plasma was shown to be 1.8% (AFID), and the detection limits were 0.1 microgram/ml (AFID) and 10 ng/ml (ECD). Mass spectra of the methylated compounds are shown and serum concentration curves after oral administration of 100 mg sulfinpyrazone to two persons are determined together with the excreted amounts of drug and metabolites. 相似文献
28.
Bengt Wessln Anders Wirsn 《Journal of polymer science. Part A, Polymer chemistry》1975,13(11):2571-2580
Anionic polymerization of vinyl chloride has been studied. Of the organometallic compounds tested as initiators, only butyllithium was found to initiate polymerization. Polymerization in bulk at 0°C and with tert-butyllithium as initiator gave poly(vinyl chloride) in a yield of 38% with M n = 55,000. Tacticity of the anionic PVC was similar to that of conventional PVC prepared at similar temperatures. Anionic PVC was found to be less branched and more heat-stable than the conventional polymer. 相似文献
29.
The mass spectral fragmentation of carbazole was reinvestigated using metastable ion studies, exact mass measurements and 1,8-dideuteriocarbazole. 1,8-Dideuteriocarbazole was prepared by successive metalations and deuterations of carbazole. The 1H- and 13C-nmr spectra of 1,8-dideuteriocarbazole confirmed earlier assignments for carbazole. 相似文献
30.
Kringen P Egedal S Pedersen JC Harbitz TB Tveit KM Berg K Børresen-Dale AL Andersen TI 《Electrophoresis》2002,23(24):4085-4091
Efficient mutation scanning techniques are needed for the rapid detection of novel disease-associated mutations and rare-sequence variants of putative importance. The large size of the breast cancer 1 gene (BRCA1) and the many mutations found throughout its entire coding sequence make screening for mutations in this gene particularly challenging. We have developed a method for screening exon 11 of the BRCA1 gene based on restriction enzyme digestion of fluorescence-labeled polymerase chain reaction (PCR) products followed by single-strand conformation polymorphism (SSCP) using an automated capillary electrophoresis system, denoted capillary restriction endonuclease fingerprinting (REF)-SSCP electrophoresis. Using this strategy on a control set of samples, we were able to detect 17 of 18 known sequence alterations. The method was then applied to screen 73 Norwegian females with family histories of breast and/or ovarian cancer. A total of 172 sequence alterations were detected, including substitutions, insertions, and deletions. One novel substitution of unknown function was identified. Sequencing of all samples negative in the capillary REF-SSCP system gave no additional mutations confirming the high sensitivity of the described methodology. Capillary REF-SSCP electrophoresis appeared as a technically convenient technique, requiring amplification of fewer PCR fragments than traditional SSCP. The novel strategy allows high-throughput mutation scanning without radioactive labeling and polyacrylamide gel electrophoresis (PAGE). 相似文献