全文获取类型
收费全文 | 2245篇 |
免费 | 93篇 |
国内免费 | 7篇 |
专业分类
化学 | 1596篇 |
晶体学 | 25篇 |
力学 | 56篇 |
数学 | 318篇 |
物理学 | 350篇 |
出版年
2022年 | 13篇 |
2021年 | 25篇 |
2020年 | 21篇 |
2019年 | 24篇 |
2018年 | 21篇 |
2017年 | 10篇 |
2016年 | 53篇 |
2015年 | 50篇 |
2014年 | 61篇 |
2013年 | 145篇 |
2012年 | 115篇 |
2011年 | 142篇 |
2010年 | 87篇 |
2009年 | 67篇 |
2008年 | 121篇 |
2007年 | 123篇 |
2006年 | 138篇 |
2005年 | 119篇 |
2004年 | 127篇 |
2003年 | 96篇 |
2002年 | 86篇 |
2001年 | 21篇 |
2000年 | 28篇 |
1999年 | 22篇 |
1998年 | 26篇 |
1997年 | 28篇 |
1996年 | 34篇 |
1995年 | 25篇 |
1994年 | 27篇 |
1993年 | 25篇 |
1992年 | 17篇 |
1991年 | 19篇 |
1990年 | 18篇 |
1989年 | 26篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 18篇 |
1985年 | 33篇 |
1984年 | 31篇 |
1983年 | 21篇 |
1982年 | 22篇 |
1981年 | 28篇 |
1980年 | 30篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 23篇 |
1976年 | 15篇 |
1975年 | 21篇 |
1974年 | 12篇 |
1973年 | 20篇 |
排序方式: 共有2345条查询结果,搜索用时 15 毫秒
61.
Ramón Puyané Isabelle Guy Renaud Metz 《Journal of Sol-Gel Science and Technology》1998,13(1-3):575-578
The ceramic microstructure, the chemical homogeneity of specific dopants and the mechanical integrity of a varistor disc are critical parameters in determining the transient voltage suppression features of these devices. The material properties and overall quality of the starting ceramic powders used to produce such components are essential in achieving the desired properties. The present work describes a novel chemical method developed to produce doped zinc oxide powders and an industrial scale manufacturing process for the production of final varistor blocks for surge arrester applications. The results are compared with those obtained when using standard varistor powder made by the mixed oxide route is used. All the fundamental electrical properties of the discs have been determined and correlated with the relevant manufacturing steps. 相似文献
62.
Huygens A Huyghe D Bormans G Verbruggen A Kamuhabwa AR Roskams T de Witte PA 《Photochemistry and photobiology》2003,78(6):607-614
The aim of this study was to investigate the in vitro cellular accumulation, distribution and photocytotoxic effect of hypericin in two-dimensional (2-D) and three-dimensional (3-D) cultured RT-112 transitional cell carcinoma cells of the bladder. In addition, two iodinated derivatives of hypericin were incorporated to investigate whether these analogs, with their increased lipophilicity and heavy-atom effect, display a different biological behavior and optimized photodynamic effect. The results indicate that hypericin and mono-iodohypericin behave similarly in terms of cellular accumulation, spheroidal distribution and photocytotoxic effect. In contrast, di-iodohypericin concentrated to a higher extent in monolayers and spheroids, but the accumulation was restricted to the outermost part of the spheroid. An inverse correlation therefore seems to exist between the extent of cellular uptake under 2-D conditions and the penetration of the compounds in multicellular systems. Moreover, a less pronounced photocytotoxic effect was observed for di-iodohypericin in both 2-D and 3-D cell culture systems. It can be concluded that iodinated derivatives of hypericin do not show an increased cytotoxic effect upon irradiation in either monolayers or spheroids. Moreover, this study shows that when new photosensitizers are preclinically developed, the use of 3-D cell aggregates is critical for a correct evaluation of their efficacy. 相似文献
63.
Ruthenium(II) complexes of monodonor ligands: efficient reagents for asymmetric ketone hydrogenation
Xu YA Clarkson GC Docherty G North CL Woodward G Wills M 《The Journal of organic chemistry》2005,70(20):8079-8087
[Chemical reaction: See text] A series of BINOL-derived ligands have been prepared and incorporated into ruthenium(II) complexes containing a diamine ligand. The complexes have proven to be excellent catalysts for the asymmetric hydrogenation of ketones, giving reduction products with enantiomeric excesses of up to 99%. 相似文献
64.
[reaction: see text] Regioselective metalation of pyrazines and cross-coupling reactions provides an easy access to botryllazines A and B and to an isomer of botryllazine A with good yields from chloropyrazine. 相似文献
65.
N. Grassie M. Zulfiqar M. I. Guy 《Journal of polymer science. Part A, Polymer chemistry》1980,18(1):265-274
A series of polyester polyurethanes with a range of polyester contents was prepared from an ethylene glycol/propylene glycol/adipic acid polyster, butane diol, and methylene bis (4-phenyl isocyanate) (MBPI). They were thermally degraded under vacuum and the products of degradation were identified. The urethane linkages decompose first as the temperature is increased and the stability increases with polyester content. Reaction mechanisms were proposed which account for the principal features of the reaction and the products of degradation. 相似文献
66.
Measurements of flow-induced orientation and crystallization have been made on a high-density polyethylene melt (HDPE) undergoing a planar extensional flow in a four-roll mill. The HDPE was suspended as a cylindrical droplet at the flow stagnation point in a linear low density polyethylene (LLDPE) carrier phase. Deformation and crystallization of the HDPE droplet phase were monitored using video imaging in conjunction with measurement of the birefringence and dichroism to quantify the in-situ transformation kinetics. Planar deformation rates along the symmetry axis of the molten HDPE phase were on the order of 0.03 s?1. Measurements of the initial transformation rate following flow cessation at 131.5°C and 133.2°C show a dependence on initial amorphous phase orientation and the total Hencky strain achieved during flow. The flow-induced crystallization rate is enhanced over the quiescent transformation rate by orders of magnitude, however, the dependence on temperature is less dramatic than expected for a nucleation-controlled growth mechanism. Analysis demonstrates that the melting point elevation model cannot account either qualitatively or quantitatively for the phenomena observed, suggesting that alternative explanations for the strong orientation dependence of the transformation rate are needed. 相似文献
67.
Several analytical methods were compared to quantify malondialdehyde (MDA) in milk powders. Modified thiobarbituric acid (TBA) methods, using either visible spectrophotometry (direct absorbance reading or after third derivative transformation of the spectrum) or HPLC, required derivatisation at elevated temperature, which appeared to catalyse artefactual MDA formation and thus overestimate the MDA content. In contrast to the TBA derivatisation method, the measurement of MDA as the dinitrophenylhydrazone derivative by HPLC or as the phenylhydrazone product by GC-MS with a deuterated internal standard resulted in lower estimates in the ranges of 2-17- and 3-30-fold, respectively; apparently due to the milder derivatisation conditions. The estimates of MDA determined by both HPLC-UV and GC-MS techniques result in lower values which are similar in magnitude even though the GC-MS technique is more sensitive. 相似文献
68.
To investigate by NMR the effects of adenine methylation, a set of oligodeoxynucleotides has been synthesized by phosphotriester or phosphoramidite procedures on silica-gel support. The preparation of fully protected 2′-deoxy-N6-methyladenosine 3′-phosphate 4 and (2′-deoxy-N6-methyladenosin-3′-O-yl)(methoxy)morpholinophosphine 5 is described. The large-scale chemical synthesis, purification by HPLC, and characterization by MS of these molecules containing N6-methyladenine is reported. 相似文献
69.
The electrochemically induced dissolution of ferrocene in a carbon paste electrode is investigated by using chronoamperometry at constant potential. Two types of behaviour are observed, according to the initial form of ferrocene: particles or deposit. The experimental i-t curves are respectively in agreement with the theoretical curves, derived earlier, in the case of spherical particles or planar slabs of solid. The analysis of the curve shows that the process is diffusion controlled, so that the dissolution rate of ferrocene cannot be measured. The parameters calculated from the curves (thickness of the deposit, initial distance from the electrode, area) have a reasonable order of magnitude. 相似文献
70.
Various reactions of ring D of dipterocarpol (hydroxy-dammarenone-II) show this triterpene to have the configuration 13β, 17H, in accordance with biogenetic speculations. 相似文献