全文获取类型
收费全文 | 83204篇 |
免费 | 479篇 |
国内免费 | 416篇 |
专业分类
化学 | 28083篇 |
晶体学 | 819篇 |
力学 | 6870篇 |
数学 | 32520篇 |
物理学 | 15807篇 |
出版年
2021年 | 73篇 |
2020年 | 85篇 |
2019年 | 76篇 |
2018年 | 10469篇 |
2017年 | 10296篇 |
2016年 | 6149篇 |
2015年 | 927篇 |
2014年 | 378篇 |
2013年 | 525篇 |
2012年 | 4102篇 |
2011年 | 10871篇 |
2010年 | 5804篇 |
2009年 | 6177篇 |
2008年 | 6914篇 |
2007年 | 9055篇 |
2006年 | 582篇 |
2005年 | 1683篇 |
2004年 | 1809篇 |
2003年 | 2200篇 |
2002年 | 1260篇 |
2001年 | 324篇 |
2000年 | 382篇 |
1999年 | 214篇 |
1998年 | 250篇 |
1997年 | 195篇 |
1996年 | 288篇 |
1995年 | 184篇 |
1994年 | 130篇 |
1993年 | 136篇 |
1992年 | 95篇 |
1991年 | 108篇 |
1990年 | 78篇 |
1989年 | 87篇 |
1988年 | 86篇 |
1987年 | 86篇 |
1986年 | 80篇 |
1985年 | 88篇 |
1984年 | 88篇 |
1983年 | 75篇 |
1982年 | 77篇 |
1981年 | 86篇 |
1980年 | 89篇 |
1979年 | 68篇 |
1978年 | 81篇 |
1977年 | 49篇 |
1976年 | 51篇 |
1975年 | 59篇 |
1974年 | 42篇 |
1973年 | 56篇 |
1914年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
991.
In the course of investigations on equestrian supplemental products for the presence of doping substances, two products were found to contain forbidden substances. As reported earlier a plant extract (Mexican cactus extract) named “Energy 5” contained the anabolic androgenic steroids (AAS) stanozolol, 17β-hydroxy-17α-methyl-5α-androstane-3β-ol (3β,5α-THMT) as well as mestanolone not declared on the label. In the present study, a product called “Super Kalm Paste” was tested. Analysis by gas chromatography - mass spectrometry (GC-MS) revealed that the preparation contained the class I anti-arrhythmics quinine (trade names KinidinTM, Durules) and cinchonine. The samples were prepared according to a sample preparation procedure established for anabolic steroids in nutritional supplements for humans. The sample treatment comprised the extraction and purification of the analytes as well as the chemical conversion with N-methyl-N-trimethylsilyl-trifluoracetamide (MSTFA) to yield the trimethylsilyl (TMS)-derivatives. To verify whether the administration of such products could lead to positive doping tests, a pilot excretion study on “Energy 5” was conducted with two geldings, and urine samples were collected. Gas chromatography - high resolution mass spectrometry (GC-HRMS) after solid phase extraction and mixed derivatisation has demonstrated the presence of the stanozolol metabolite 16β-hydroxy-stanozolol in urine samples after “Energy 5” application. 相似文献
992.
Dusanka Ž. Obadović Anikó Vajda Maya Garić A. Bubnov Vera Hamplová M. Kašpar Katalin Fodor-Csorba 《Journal of Thermal Analysis and Calorimetry》2005,80(2):519-523
Summary Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)-[4-(2-n-alkoxy-propionyloxy)]biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry.
The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues.
In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained
monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic
for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been
calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules
was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length
of the single compounds. 相似文献
993.
Wong KL Bitter M Hammett GW Heidbrink W Hendel H Kaita R Scott S Strachan JD Tait G Bell MG Budny R Bush C Chan A Coonrod J Efthimion PC England AC Eubank HP Fredrickson E Furth HP Goldston RJ Grek B Grisham L Hawryluk RJ Hill KW Johnson D Kamperschroer J Kugel H Ma C Mansfield D Manos D McCune DC McGuire K Medley SS Mueller D Nieschmidt E Owens DK Paré VK Park H Ramsey A Rasmussen D Roquemore AL Schivell J Sesnic S Taylor G Williams MD Zarnstorff MC 《Physical review letters》1985,55(23):2587-2590
994.
995.
Miniaturizing all dimensions of apparatus, such as electronics and computers, is the current trend followed by scientists
in various fields. The idea of Lab-on-a-Chip has significantly expanded and found its broad applications in analytical chemistry.
Microplasmas can act as a sample excitation source and are the miniaturized versions of full-sized plasmas. These can be created
in various forms, such as direct current, microwave induced, capacitively coupled and inductively coupled plasmas. Scaling
down the size would reduce the amount of gases, liquids and consumables required, as well as the sample analysis time, which
in turn would decrease the operating costs. Therefore, several research groups are involved in the development of microplasmas
for utilisation in analytical instruments. 相似文献
996.
997.
998.
The rate of photo-oxidation of unstabilized PVC is markedly dependent on the severity of the processing operation. In mildly processed PVC, peroxy gel appears to be a very effective photo-initiator whereas in more severely processed polymer, solvent soluble polymer containing polyconjugation, hydroperoxides and carbonyl groups is primarily responsible for photo-initiation. The auto-retardation of photo-oxidation which occurs in all samples is believed to be due to the build-up of a highly absorbing surface layer of degraded polymer. 相似文献
999.
Danielle Giron S. Monnier M. Mutz P. Piechon T. Buser F. Stowasser K. Schulze M. Bellus 《Journal of Thermal Analysis and Calorimetry》2007,89(3):729-743
Adequate very sensitive quantification methods are needed for the development and are also now required for the monitoring
of undesirable solid form(s) as routine tests. The pre-requisite for quantitation are selectivity, sensitivity and most important
the purity of standards and their proper storage, what is a challenge for metastable forms.
Several analytical techniques are available such as X-ray diffraction, spectroscopy, thermal analysis and microcalorimetry.
The different steps of the validation of the analytical methods and problems to be solved are discussed. Examples illustrate
the different techniques and compare their possible advantages and limits. The relative standard deviation of measurements
should allow for checking the homogenization procedure of mixtures for calibration. The validation should be carried out following
ICH guidelines for validation of analytical methods. Comparison of different techniques in adequate concentration range add
confidence in the analytical results. 相似文献
1000.