全文获取类型
收费全文 | 1776篇 |
免费 | 51篇 |
国内免费 | 21篇 |
专业分类
化学 | 931篇 |
晶体学 | 13篇 |
力学 | 65篇 |
数学 | 477篇 |
物理学 | 362篇 |
出版年
2022年 | 15篇 |
2021年 | 9篇 |
2020年 | 25篇 |
2019年 | 14篇 |
2018年 | 18篇 |
2017年 | 10篇 |
2016年 | 22篇 |
2015年 | 25篇 |
2014年 | 51篇 |
2013年 | 99篇 |
2012年 | 107篇 |
2011年 | 97篇 |
2010年 | 74篇 |
2009年 | 71篇 |
2008年 | 93篇 |
2007年 | 88篇 |
2006年 | 91篇 |
2005年 | 67篇 |
2004年 | 66篇 |
2003年 | 60篇 |
2002年 | 42篇 |
2001年 | 37篇 |
2000年 | 33篇 |
1999年 | 18篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 24篇 |
1995年 | 27篇 |
1994年 | 16篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 23篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 16篇 |
1979年 | 24篇 |
1978年 | 18篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 17篇 |
1974年 | 8篇 |
1933年 | 11篇 |
排序方式: 共有1848条查询结果,搜索用时 15 毫秒
31.
A custom-built multi-technique portable spectrometer was used to study the vibrational (and electronic) excitations associated with the “anomalous” CO adsorption recently observed on clean and oxidized Cu(100) surfaces at room temperature. Results from the electron energy loss (and thermal desorption) experiments have provided strong evidence for in-situ CO production induced by low-energy electron or ion irradiation of CO or C2H4 on Cu(100). In particular, the C-O vibrational stretch frequency for the room-temperature CO adsorption was found to be red-shifted by 9 meV from its nominal position (259 meV) and could be observed even at temperature as high as 420 K. Several plausible mechanisms involving coadsorbate interactions with CO on metal surfaces have been discussed. A direct interaction model involving partial bonding between CO, adsorbed on an atop site, and a coadsorbed O atom in a four-fold hollow site was found to be adequate in explaining the observed red shift and the apparent stabilization of CO on Cu(100) at room temperature. 相似文献
32.
Comment on papers by K. Shanahan that propose to explain anomalous heat generated by cold fusion 总被引:1,自引:0,他引:1
Edmund Storms 《Thermochimica Acta》2006,441(2):207-209
Dr. Shanahan has published two papers (Thermochim. Acta 428 (2005) 207, Thermochim. Acta 382 (2002) 95) in which he argues that excess heat claimed to be produced by cold fusion is actually caused by errors in heat measurement. In particular, he proposes that unrecognized changes in the calibration constant are produced by changes in the locations where heat is being generated within the electrolytic cell over the duration of the measurement. Because these papers may lend unwarranted support to rejection of cold fusion claims, these erroneous arguments used by Shanahan need to be answered. 相似文献
33.
34.
35.
Leung PS 《Current protein & peptide science》2004,5(4):267-273
The peptide hormone angiotensin II is well established to play an endocrine role in the regulation of blood pressure, fluid and electrolyte homeostasis. In addition to its hemodynamic function, recent studies have shown that numerous tissues and organs contain their own locally generated angiotensin products (angiotensin II, III, IV and Ang 1-7) and they exhibit their respective activities. Such an intrinsic angiotensin-generating system renders to specific tissue function of our body, frequently via the regulatory mechanism of a paracrine, autocrine or intracrine manner. These tissues and organs include, to name but a few, the brain, bone marrow, adipose, epididymis, carotid body, liver, and pancreas. This local system has been shown to be responsive to various stimuli of physiological and pathophysiological importance. Moreover, the locally generated angiotensin peptides have multiple and novel actions including cell growth, anti-proliferation, apoptosis, reactive oxygen species generation, hormonal secretion, pro-inflammatory, and pro-fibrogenic actions, as well as vasoconstriction and vasodilatation. Notwithstanding the emerging roles of angiotensin II in various tissues and organs, the physiological significance and ultimately the clinical relevance remain largely undefined. Future target for these new functions by making use of specific renin-angiotensin system inhibitors, such as the angiotensin-converting enzyme and angiotensin II receptor blockers either in mono-therapy or in combination, could be of clinical importance. The current review is to focus on some of the new functions arising from the locally formed angiotensin II in tissues and organs, with particular attention to its emerging roles in the liver and the pancreas. 相似文献
36.
Summary The method developed for simultaneous determination of hydrogen (by means of an equivalent amount of iodine) and of nitrogen by a volumetric method, is based on a flash combustion technique in a stream of CO2 (100 ml/min) with controlled addition of O2 (20%). Hydrogen is determined by conversion of water into hydrogen chloride on anhydrous magnesium chloride heated at 600–650° C, and reaction of the hydrogen chloride with Ag2OI2 at 220° C to yield iodine, which is collected on silver and weighed. The precision of the determination of hydrogen is excellent, the standard deviation being 0.008–0.017% abs., about a tenth of that obtained by the classical methods. The error in the determination of hydrogen was ±0.006% abs.
Gleichzeitige Bestimmung von Wasserstoff als dessen Jodäquivalent und von Stickstoff auf volumetrischem Wege
Zusammenfassung Die entwickelte Methode zur gleichzeitigen Bestimmung von Wasserstoff in Form seines Jodäquivalents und von Stickstoff auf gasvolumetrischem Wege beruht auf der Entflammungsmineralisation im CO2-Strom (100 cm3/min) mit kontrolliertem Zusatz von O2 (20%). Der Gasstrom wird zusätzlich über P2O5 getrocknet. Die dem Wasserstoff äquivalente Jodmenge wird an metallischem Silber gebunden und gravimetrisch bestimmt, wobei Wasser bei 600–650° C mit Magnesiumchlorid in Chlorwasserstoff übergeführt wird, der nachfolgend bei 220° C mit Ag2Oj2 zu Jod umgesetzt wird. Für Verbindungen, die neben C, H, N und O auch Br, Cl, P und S enthalten, werden Wasserstoffresultate mit hoher Präzision erzielt:s H beträgt 0,008 bis 0,017% absolut und ist etwa 10mal kleiner als bei den üblichen, klassischen Methoden. Die Genauigkeit der Wasserstoffbestimmung entspricht einem mittleren Fehler=±0,006% absolut. Die Resultate für den Wasserstoff und Stickstoff verlangen eine Blindwertkorrektur.相似文献
37.
Edmund F. Jordan 《Journal of polymer science. Part A, Polymer chemistry》1971,9(11):3367-3378
The influence of side-chain crystallinity on the glass transition temperatures of selected copolymers was investigated. The copolymers were selected, in part, from those whose crystallinity was treated in the preceding paper. These included the lower amorphous acrylate esters, such as methyl, ethyl, n-butyl, and 2-ethylhexyl acrylates, together with methyl methacrylate and acrylonitrile, each copolymerized with n-octadecyl acrylate over the range of composition. The decline in the glass transition temperature was linear with increasing weight fraction of n-octadecyl acrylate for all systems in the composition range where the copolymers were essentially amorphous. The extrapolated Tg for the amorphous state of poly(n-octadecyl acrylate), and for amorphous poly(oleyl acrylate), was close to ?111°C. This coincided with a value previously obtained by an extrapolation of data on homologs. Beyond a critical fraction of octadecyl acrylate (0.3 to 0.5), developing side-chain crystallinity in n-octadecyl acrylate raised the glass temperature steadily for all systems, up to a value of 17-C, obtained for the crystalline homopolymer. Crystallinity did not develop in stiff copolymers until Tg was about 30°C below the melting point of the most perfect crystals. In compositionally heterogeneous copolymers incorporating vinyl stearate, blocks of crystalline units appeared to be dispersed in a glassy matrix of amorphous co-units. An empirical equation was derived which fitted the experimental data for random copolymers, over all composition ranges, with fair accuracy. 相似文献
38.
The relative effectiveness of the SimPlate Yeast and Mold-Color Indicator method (Y&M-CI) was compared to the U.S. Food and Drug Administration's (FDA) Bacteriological Analytical Manual (BAM) method and the proposed International Organization for Standardization (ISO) method, ISO/CD 21527, for enumerating yeasts and molds in foods. Test portions were prepared and incubated according to the conditions stated in both the BAM and ISO methods. Six food types were analyzed: frozen corn dogs, nut meats, frozen fruits, cake mix, cereal, and fresh cheese. Nut meats, frozen fruits, and fresh cheese were naturally contaminated. All other foods were artificially contaminated with either a yeast or mold. Seventeen laboratories throughout North America and Europe participated in the study. Three method comparisons were conducted. In general, there was <0.3 mean log count difference in recovery between the SimPlate method and the 2 corresponding reference methods. Moreover, mean log counts between the 2 reference methods were also very similar. The repeatability (Sr) and reproducibility (SR) standard deviations were comparable between the 3 method comparisons. These results indicate that the BAM method and the SimPlate method are equivalent for enumerating yeast and mold populations in foods. Similarly, the SimPlate method is comparable to the proposed ISO method when test portions are prepared and incubated as defined in the proposed ISO method. 相似文献
39.
Shengying Lin Xiaoyang Wang Roy Wai-Lun Tang Hung Chun Lee Ho Hin Chan Sheyne S. A. Choi Tina Ting-Xia Dong Ka Wing Leung Sarah E. Webb Andrew L. Miller Karl Wah-Keung Tsim 《Molecules (Basel, Switzerland)》2022,27(12)
COVID-19, resulting from infection by the SARS-CoV-2 virus, caused a contagious pandemic. Even with the current vaccines, there is still an urgent need to develop effective pharmacological treatments against this deadly disease. Here, we show that the water and ethanol extracts of the root and rhizome of Polygonum cuspidatum (Polygoni Cuspidati Rhizoma et Radix), a common Chinese herbal medicine, blocked the entry of wild-type and the omicron variant of the SARS-CoV-2 pseudotyped virus into fibroblasts or zebrafish larvae, with IC50 values ranging from 0.015 to 0.04 mg/mL. The extracts were shown to inhibit various aspects of the pseudovirus entry, including the interaction between the spike protein (S-protein) and the angiotensin-converting enzyme II (ACE2) receptor, and the 3CL protease activity. Out of the chemical compounds tested in this report, gallic acid, a phytochemical in P. cuspidatum, was shown to have a significant anti-viral effect. Therefore, this might be responsible, at least in part, for the anti-viral efficacy of the herbal extract. Together, our data suggest that the extracts of P. cuspidatum inhibit the entry of wild-type and the omicron variant of SARS-CoV-2, and so they could be considered as potent treatments against COVID-19. 相似文献
40.