全文获取类型
收费全文 | 241篇 |
免费 | 6篇 |
专业分类
化学 | 131篇 |
力学 | 20篇 |
数学 | 45篇 |
物理学 | 51篇 |
出版年
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 6篇 |
2017年 | 4篇 |
2016年 | 6篇 |
2014年 | 5篇 |
2013年 | 15篇 |
2012年 | 8篇 |
2011年 | 4篇 |
2010年 | 11篇 |
2009年 | 4篇 |
2008年 | 8篇 |
2007年 | 11篇 |
2006年 | 11篇 |
2005年 | 6篇 |
2004年 | 9篇 |
2003年 | 7篇 |
2002年 | 8篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1998年 | 2篇 |
1996年 | 6篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1980年 | 7篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 2篇 |
1970年 | 4篇 |
1967年 | 3篇 |
1964年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
1938年 | 1篇 |
1937年 | 3篇 |
1936年 | 4篇 |
1935年 | 3篇 |
1922年 | 2篇 |
排序方式: 共有247条查询结果,搜索用时 171 毫秒
1.
It is shown that there cannot exist a uniform exponential dichotomy for any linear delay equation with a positive finite delay. 相似文献
2.
P. Kappen P.S. Hale N. Brack W. Prissanaroon P.J. Pigram 《Applied Surface Science》2006,253(3):1473-1479
Micro-patterns (80 μm and 10 μm) of copper and semi-conducting polypyrrole on insulating fluorinated ethylene propylene substrates were characterized using synchrotron-based X-ray Photoemission Electron Microscopy (X-PEEM), Near Edge X-ray Absorption Fine Structure (NEXAFS), and Atomic Force Microscopy (AFM). Electronic states in the polypyrrole are verified using the NEXAFS data, and sample degradation upon irradiation is addressed. X-PEEM images show homogeneous distributions of the corresponding elements in the patterns. They do not exhibit dichroic effects and give information about the growth of copper and polypyrrole (i.e. nucleation of Cu, overgrowth of PPy, formation of PPy granules). AFM results are used to verify the topography of the patterns and support the findings on pattern growth. 相似文献
3.
[reaction: see text] Allylically oxygenated vinyl alpha-triphenylstannanes such as 22 can be readily converted into vinyl iodides and thereafter stereodefined trisubstituted alkenes with retention of configuration. 相似文献
4.
Richard A. Bunce Derrick M. Herron Lu Y. Hale 《Journal of heterocyclic chemistry》2003,40(6):1031-1039
A tandem reduction‐reductive amination reaction has been applied to the synthesis of 3,4‐dihydro‐2H‐1,4‐benzoxazines and 1‐acetyl‐1,2,3,4‐tetrahydroquinoxalines. The nitroketones required for the benzoxazine ring closures were prepared by (A) alkylation of the anion derived from 2‐nitrophenol with an allylic halide or (B) nucleophilic aromatic substitution of an allylic alkoxide on 2‐fluoro‐1‐nitrobenzene followed by ozonolysis. Precursors for the quinoxalines were prepared by alkylation of the anion of 2‐nitroacetanilide with an allylic halide followed by ozonolysis. Catalytic hydrogenation of the nitroketones using 5% palladium‐on‐carbon in methanol then gave the target heterocycles by a reduction‐reductive amination sequence. The N‐methyl derivatives for both ring systems were easily prepared by adding 5‐10 equivalents of aqueous formaldehyde prior to the reduction. The dihydrobenzoxazines were isolated in high yield following purification by chromatographic methods; tetrahydroquinoxalines were isolated in a similar manner and possessed differentiated functionality on the two nitrogens. 相似文献
5.
6.
Hale KJ Frigerio M Manaviazar S Hummersone MG Fillingham IJ Barsukov IG Damblon CF Gescher A Roberts GC 《Organic letters》2003,5(4):499-502
[structure: see text] The synthesis of two truncated bryostatin analogues 2 and 3 is described. High-field NMR measurements on the C-ring analogue 3 in C(2)H(3)CN containing 25% (2)H(2)O have shown that it binds to the CRD2 of human PKC-alpha at virtually the same position as phorbol-13-acetate (PA) and bryostatin 1 (1). NMR titration studies have also revealed that 3 binds to the CRD2 with a potency similar in magnitude to PA but much less potently than 1. 相似文献
7.
A completely stereocontrolled asymmetric synthesis of an advanced B-ring synthon for the bryostatin family of antitumor agents is reported. Noteworthy features of our synthesis include the Smith-Tietze bis-alkylation reaction between 12 and 13 en route to C(2)-symmetrical ketone 10 and the totally stereoselective conversion of 10 into triol 18 via a Grignard addition tactic. Triol 18 was converted to epoxide 3 in nine steps, and an acid-catalyzed intramolecular Williamson etherification reaction completed the synthesis of 2. 相似文献
8.
A carbon-paste chemically modified with glucose oxidase and a ferrocene-containing siloxane polymer was further modified by coating the electrode surface with a poly(ester-sulfonic acid) cation-exchanger, Eastman AQ-29D. The polymer is obtained as a homogeneous aqueous dispersion at pH 5–6; when dried, the polymer coating is not water-soluble. The coating was shown not to be detrimental to the enzyme activity but to prevent electrochemically active anionic interferents such as ascorbate and urate from reaching the electrode surface. The polymer coating also prevented glucose oxidase from leaking out of the carbon paste into the contacting solution and protected the electrode surface from fouling agents present in urine and bovine serum albumin. Uncoated electrodes lost some 10-2-15% of their original response to glucose after storage in buffer for three weeks whereas the response of the coated electrodes remained constant. Calibration curves for glucose were strictly linear up to about 5 mM for uncoated and up to 20 mM for coated electrodes. The response current to glucose was not decreased after coating. 相似文献
9.
Block copolymers of ethylene sulphide (B) and isoprene (A) have been prepared by anionic synthesis using alkali metal complexes of naphthalene as initiator. Two series of block copolymers have been synthesized, one (based on sodium naphthalenide as initiator) having high molecular weights and the other (based on lithium naphthalenide) having low molecular weights.Physical properties of the block copolymers as a function of composition, molecular weight and polyisoprene microstructure have been studied. Polymers containing high molecular weight polyethylene sulphide sequences were difficult to process without degradation. By lowering the molecular weight of the polyethylene sulphide segment, block copolymers of improved processibility were obtained.The centre block polyisoprene microstructure has been varied from 100% 1,2/3,4 configuration to 80% 1,4 configuration by preparing a “seed” polymer in tetrahydrofuran followed by solvent removal and replacement by hexane. Changes in microstructure affect low temperature flexibility, resilience and tensile strength of the block copolymer.The BAB block copolymers are biphasic and exhibit elastomeric properties with improved network stability compared with polystyrene-polybutadiene-polystyrene ABA block copolymers. 相似文献
10.