全文获取类型
收费全文 | 5495篇 |
免费 | 185篇 |
国内免费 | 37篇 |
专业分类
化学 | 3665篇 |
晶体学 | 57篇 |
力学 | 107篇 |
数学 | 739篇 |
物理学 | 1149篇 |
出版年
2023年 | 30篇 |
2022年 | 67篇 |
2021年 | 109篇 |
2020年 | 111篇 |
2019年 | 104篇 |
2018年 | 96篇 |
2017年 | 60篇 |
2016年 | 166篇 |
2015年 | 137篇 |
2014年 | 166篇 |
2013年 | 294篇 |
2012年 | 346篇 |
2011年 | 428篇 |
2010年 | 204篇 |
2009年 | 172篇 |
2008年 | 306篇 |
2007年 | 292篇 |
2006年 | 302篇 |
2005年 | 254篇 |
2004年 | 208篇 |
2003年 | 190篇 |
2002年 | 167篇 |
2001年 | 76篇 |
2000年 | 96篇 |
1999年 | 59篇 |
1998年 | 41篇 |
1997年 | 52篇 |
1996年 | 78篇 |
1995年 | 62篇 |
1994年 | 35篇 |
1993年 | 51篇 |
1992年 | 55篇 |
1991年 | 53篇 |
1990年 | 35篇 |
1989年 | 33篇 |
1988年 | 35篇 |
1987年 | 29篇 |
1986年 | 33篇 |
1985年 | 47篇 |
1984年 | 34篇 |
1983年 | 34篇 |
1982年 | 54篇 |
1981年 | 55篇 |
1980年 | 46篇 |
1979年 | 47篇 |
1978年 | 41篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1975年 | 32篇 |
1973年 | 39篇 |
排序方式: 共有5717条查询结果,搜索用时 406 毫秒
991.
992.
Graham Wigley John Heath Graham Pitcher Adam Whybrew 《Particle & Particle Systems Characterization》2001,18(4):169-178
This work describes a systematic approach adopted to establish Laser and Phase Doppler Anemometry, LDA/PDA, experimental techniques that would allow velocity and dropsize measurements to be made over wide velocity and size ranges with confidence in partially atomized sprays. The analysis considers the sprays generated by different gasoline direct injection (GDI) systems injecting into air under atmospheric conditions. The upper limit to the dropsize range in the fuel sprays was confirmed using (a) an Oxford Lasers' VisiSizer and (b) droplets of a known size produced by a mono‐dispersed droplet generator. GDI fuel sprays are highly transient, optically dense and provide a high degree of penetration and atomization. The measurement problem is therefore one of the detection of small, high speed droplets inside a dense cloud of surrounding droplets. Furthermore, under the transients found at the start and end of injection and during high fuel loads, fuel elements in the form of sheets, ligaments and filaments are also injected. These liquid fuel elements subsequently break‐up, downstream from the nozzle, to form droplets of a much larger size class but with a much lower number density [1]. The co‐existence of these liquid fuel elements and the widely different size classes in the spray are considered to pose a problem for dropsize measurements by the PDA technique. In particular: the wide dynamic range of light intensities scattered by the fuel elements and droplets; the trajectory of large drops through the edges of the PDA measurement volume with its Gaussian intensity distribution [2] and the high probability of non spherical droplets. The work concludes that the LDA/PDA measurement technique, as applied here, is robust. It can discriminate between partially and fully atomized sprays, has a high probability of accurately measuring dropsizes larger than the measurement volume and give a realistic indication of ‘sizes’ for non spherical droplets. However, specification of the PDA system parameters must be strictly compatible with the measurement task to yield unambiguous results. 相似文献
993.
Adam Tracz Jeremiasz Jeszka Iwona Kuciska Jean‐Paul Chapel Gisele Boiteux 《Macromolecular Symposia》2001,169(1):129-135
High density polyethylene (PE) was crystallised from the melt on freshly cleaved surface of highly oriented pyrolitic graphite (HOPG) or mica. Atomic force microscopy (AFM) studies of structure of the polymer surface adjacent to the graphite or mica were performed after peeling of from the substrate. Significant differences of crystalline structure on the interface were found between PE crystallised on graphite and mica. The surface of polyethylene crystallised on graphite shows large areas with regularly arranged rectangular structures. These objects (ca 20‐80 nm big) probably represent the nucleation centres of the lamellar growth. The surface of polyethylene crystallised at mica surface shows some dot‐like structures showing no particular arrangement. 相似文献
994.
Ab Initio Calculation of the Tetracarbonatoscandate‐Ion in Na5[Sc(CO3)4] · 2 H2O. Single Crystal Structure Determination, Vibrational Spectra, and Thermal Decomposition Normal modes of the tetracarbonatoscandate‐ion, [Sc(CO3)4]5–, were determined by ab initio calculations and were compared with experimental data of Infrared‐ and Raman‐spectra of the compound Na5[Sc(CO3)4] · 2 H2O. A necessary redetermination of the structure with single crystal x‐ray diffraction data (tetragonal, P421c (Nr. 114), Z = 2, a = 746,37(4) pm, c = 1157,0(2) pm, VEZ = 644,5(1) 106 pm3) allows the discussion of existing hydrogen bonds. Determination of the thermal behaviour indicates a two‐stage decomposition reaction, but no corresponding intermediate could be isolated. 相似文献
995.
Joanna Kanabus Marcin Brya Marek Roszko Marta Modrzewska Adam Pierzgalski 《Molecules (Basel, Switzerland)》2021,26(21)
Scientific demonstrations of the beneficial effects of non-psychoactive cannabinoids on the human body have increased the interest in foods containing hemp components. This review systematizes the latest discoveries relating to the characteristics of cannabinoids from Cannabis sativa L. var. sativa, it also presents a characterization of the mentioned plant. In this review, we present data on the opportunities and limitations of cannabinoids in food production. This article systematizes the data on the legal aspects, mainly the limits of Δ9-THC in food, the most popular analytical techniques (LC-MS and GC-MS) applied to assay cannabinoids in finished products, and the available data on the stability of cannabinoids during heating, storage, and access to light and oxygen. This may constitute a major challenge to their common use in food processing, as well as the potential formation of undesirable degradation products. Hemp-containing foods have great potential to become commercially popular among functional foods, provided that our understanding of cannabinoid stability in different food matrices and cannabinoid interactions with particular food ingredients are expanded. There remains a need for more data on the effects of technological processes and storage on cannabinoid degradation. 相似文献
996.
997.
Eduard E. Nifantyev Elena N. Rasadkina Inna V. Yankovich Vitaly K. Belsky Adam I. Stash 《Heteroatom Chemistry》2000,11(2):129-137
New representatives of an original class of crown ethers—cyclopenta‐ and cyclohexaresorcinolamidophosphites—were synthesized using the molecular assemblage technique. Their thio and oxo derivatives were obtained, as well as the rhodium (I) complexes. A macrocycle containing both tri‐ and pentavalent phosphorus atoms was synthesized. 1H and 31P NMR spectroscopy data suggested the higher conformational flexibility of phosphite macrocycles as compared to phosphate ones. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:129–137, 2000 相似文献
998.
999.
Adam F. Littke Gregory C. Fu 《Angewandte Chemie (International ed. in English)》1998,37(24):3387-3388
From only commercially available reagents a wide array of Suzuki cross-couplings of aryl chlorides with arylboronic acids can be effected in excellent yield [Eq. (a)]. This development provides a general solution to a long-standing limitation of this extremely powerful process—the poor reactivity of inexpensive and readily accessible aryl chlorides. dba=dibenzylideneacetone. 相似文献
1000.