全文获取类型
收费全文 | 1300篇 |
免费 | 35篇 |
国内免费 | 9篇 |
专业分类
化学 | 871篇 |
晶体学 | 79篇 |
力学 | 17篇 |
数学 | 83篇 |
物理学 | 294篇 |
出版年
2023年 | 14篇 |
2022年 | 30篇 |
2021年 | 19篇 |
2020年 | 14篇 |
2019年 | 36篇 |
2018年 | 35篇 |
2017年 | 36篇 |
2016年 | 38篇 |
2015年 | 43篇 |
2014年 | 60篇 |
2013年 | 109篇 |
2012年 | 68篇 |
2011年 | 102篇 |
2010年 | 53篇 |
2009年 | 50篇 |
2008年 | 80篇 |
2007年 | 50篇 |
2006年 | 51篇 |
2005年 | 30篇 |
2004年 | 34篇 |
2003年 | 27篇 |
2002年 | 40篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 6篇 |
1997年 | 11篇 |
1996年 | 18篇 |
1995年 | 11篇 |
1994年 | 16篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 21篇 |
1983年 | 20篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1974年 | 6篇 |
排序方式: 共有1344条查询结果,搜索用时 15 毫秒
991.
Palladium(II) ion-imprinted polymer (IIP) materials were synthesized by thermally polymerizing the ternary complexes of palladium(II) with amino (AQ) or hydroxy (HQ) or mercapto (MQ) derivatives of quinoline and 4-vinyl-pyridine. The functional and crosslinking monomers used during polymerization were 2-hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA). 2,2′-Azobisisobutyronitrile (AIBN) and 2-methoxy ethanol were used as the initiator and porogen, respectively. The resulting polymer materials were dried in an oven at 80 °C, ground and sieved to obtain IIP particles which were then subjected to leaching with 50% (v/v) HCl to obtain the leached palladium(II) IIP particles. Control polymer (CP) particles were also prepared by following the above procedure described for IIP particles. The CP particles, unleached and leached AQ-based IIP particles were then characterized by IR, XRD and microanalysis studies. Analytical studies such as preconcentration of palladium(II) from dilute aqueous solutions and separation studies in the presence of selected noble and base metals which co-exist with palladium(II) in its ore or mineral deposits were systematically studied using CP and IIP particles and are compared. AQ-based IIP particles gave higher percent extraction and selectivity coefficients compared to HQ- or MQ-based IIP particles. Five replicate determinations of 25 μg of palladium(II) present in 500 ml of aqueous solution, when subjected to preconcentration and determination by iodide-Rhodamine 6G procedure gave a mean absorbance of 0.104 with a relative standard deviation of 2.25%. The detection limit corresponding to three times the standard deviation of the blank was found to be 5.0 μg of palladium(II) per litre. The rebinding studies using AQ-, HQ- and MQ-based IIPs were carried out and were fitted to the different adsorption isotherm models, viz. Langmuir (L), Freundlich (F) and Langmuir-Freundlich (LF). These adsorption models were used for the evaluation of binding parameters and in elucidating the nature and type of bonding in the IIPs. The results of rebinding experiments showed discrimination between the three IIPs based on the donor atoms of the ligands. 相似文献
992.
[reaction: see text] Syntheses of 2-aryl and 2,3-substituted indoles were realized by modified double elimination protocol. Under basic conditions, vinyl sulfones derived from the reaction of 2-aminobenzyl sulfone with benzaldehydes underwent cyclization and alkylation followed by elimination of sulfinic acid to afford 2,3-substituted indoles. 相似文献
993.
P. Bajaj Y. P. Khanna G. N. Babu 《Journal of polymer science. Part A, Polymer chemistry》1976,14(2):465-474
Styrene was copolymerized in bulk with vinyltriethoxysilane at 80°C and vinyltriacetoxysilane at 60, 80, and 100°C with the use of benzoyl peroxide as an initiator at low conversions. Copolymer composition was determined from the silicon content and reactivity ratios were calculated by the conventional scheme of copolymerization. The low r1 value (styrene) in the styrene-vinyltriacetoxysilane system (St–VTAS) as compared to styrene-vinyltriethoxysilane (St–VTES) copolymerization may be attributed to higher reactivity of VTAS towards the polystyryl radical. Further, in the St–VTAS system, r1 tends to decrease with increasing polymerization temperature. The influence of silicon comonomer on properties of the copolymers (intrinsic viscosity, solubility, dielectric and thermal behavior) was studied. 相似文献
994.
A.R. Suresh Babu 《Tetrahedron letters》2007,48(38):6809-6813
An efficient synthesis of dispiro-oxindolopyrrolizidine and dispirooxindolothienopyrrole derivatives has been expediently accomplished via a one-pot, three-component 1,3-dipolar cycloaddition reaction. High regioselectivity was achieved on ultrasonication in the presence of silica as a catalyst. X-ray diffraction studies of one of the cycloadducts proved the structure and regiochemistry of the cycloaddition. 相似文献
995.
Babu BR Hrdlicka PJ McKenzie CJ Wengel J 《Chemical communications (Cambridge, England)》2005,(13):1705-1707
Incorporation of N,N-bis(2-pyridylmethyl)-beta-alanyl 2'-amino-LNA (bipyridyl-functionalized 2'-amino locked nucleic acid) monomers into DNA strands enables high-affinity targeting of complementary DNA with excellent Watson-Crick selectivity in the presence of divalent metal ions. Positioning of bipyridyl-functionalized 2'-amino-LNA monomers in two complementary DNA strands in a "3'-end zipper" constitution allows modulation of duplex stability, i.e., a strong stabilizing effect with one equivalent of divalent metal ion per bipyridyl pair, or a strong destabilizing effect with an excess of divalent metal ions. 相似文献
996.
An efficient synthesis of dispiroindenoquinoxaline pyrrolizidine derivatives has been expediently accomplished by a one-pot four component 1,3-dipolar cycloaddition reaction. High regioselectivity was achieved using heteropolyacid H4[Si(W3O10)3]–silica as a catalyst. X-ray diffraction studies of one of the cycloadducts proved the structure and regiochemistry of the cycloadduct. 相似文献
997.
Sirisilla Raju Venkateswara Rao Batchu R. Vasu Dev J. Moses Babu P. Rajender Kumar Pazhanimuthu Annamalai 《Tetrahedron》2006,62(41):9554-9570
We herein report a highly convenient protocol for rapid construction of α-pyrone fused with thiophene. This includes one-pot and regioselective synthesis of 4,5-disubstituted and 5-substituted thieno[2,3-c]pyran-7-ones, 6,7-disubstituted and 6-substituted thieno[3,2-c]pyran-4-ones. The synthesis of thieno[2,3-c]pyran-7-ones involves palladium mediated cross coupling of 3-iodothiophene-2-carboxylic acid with terminal alkynes in a simple synthetic operation. The coupling-cyclization reaction was initially studied in the presence of Pd(PPh3)2Cl2 and CuI in a variety of solvents. 5-Substituted 4-alkynylthieno[2,3-c]pyran-7-ones were isolated in good yields when the reaction was performed in DMF. Similarly, 6-substituted 7-alkynylthieno[3,2-c]pyran-4-ones were synthesized via palladium-catalyzed cross coupling of 2-bromothiophene-3-carboxylic acid with terminal alkynes. A tandem C-C bond forming reaction in the presence of palladium catalyst rationalizes the formation of coupled product in this apparently three-component reaction. The cyclization step of this coupling-cyclization-coupling process occurs in a regioselective fashion to furnish products containing six-membered ring only. This sequential C-C bond forming reaction however, can be restricted to the formation of single C-C bond by using 10% Pd/C-Et3N-CuI-PPh3 as catalyst system in the cross coupling reaction. 5-Substituted thieno[2,3-c]pyran-7-ones were obtained in good yields when the coupling reaction was performed under this condition. Some of the compounds synthesized were tested in vitro for their anticancer activities. 相似文献
998.
Annie J. Vallamattam K. Babu Joseph M. G. Krishna Pillai 《Czechoslovak Journal of Physics》1978,28(10):1157-1163
A parameter formulation of Wilson's High Low Frequency Separation (HLFS) Method is given in conjunction with isotopic frequencies in a third order vibrational problem. A new approximation scheme called the Hybrid HLFS Method is proposed which is superior to the pure HLFS method in the sense that it enables the evaluation of all twelve force constants. This, however, furnishes two sets of force fields out of which one is seen to be in better agreement with that obtained using various additional data.One of the authors (AJV) is grateful to the CSIR, New Delhi, for the award of a Junior Research Fellowship. 相似文献
999.
Synthesis of a new photolabile protecting group, bis(4,5-dimethoxy-2-nitrophenyl)ethylene glycol (4) from 4,5-dimethoxy-2-nitrobenzyl alcohol in three steps in good yields is described. The acetals and ketals of 4 are stable against acidic and basic reaction conditions and are cleaved smoothly on irradiation at 350 and 400 nm with regeneration of carbonyl compounds in high yields and efficiency. 相似文献
1000.
G. Veera Babu M. Palaniappa M. Jayalakshmi K. Balasubramanian 《Journal of Solid State Electrochemistry》2007,11(12):1705-1712
Graphite particles were coated with Ni–P by electroless deposition using a conventional bath consisting of a nickel salt and
hypophosphite. After 15 min of electroless deposition, the graphite particles were covered with 10 wt% nickel and 0.7–1.0 wt%
phosphorus as analysed by wet chemical method. Surface morphology was studied by scanning electron microscopy (SEM). Electrochemical
characterisation for the catalytic activity was done by cyclic voltammetry. Pure Ni powder and electroless Ni–P coated on
graphite were used as catalysts for the electro-oxidation of dextrose (1.8 × 10−3 to 4.5 × 10−3 M) in 0.1 M KOH solution. Comparative studies revealed that electroless Ni–P coated on graphite particles acted as a better
catalyst than pure Ni powder for catalytic reaction. 相似文献