全文获取类型
收费全文 | 5058篇 |
免费 | 82篇 |
国内免费 | 12篇 |
专业分类
化学 | 2564篇 |
晶体学 | 131篇 |
力学 | 232篇 |
数学 | 686篇 |
物理学 | 1539篇 |
出版年
2022年 | 64篇 |
2021年 | 73篇 |
2020年 | 77篇 |
2019年 | 89篇 |
2018年 | 67篇 |
2017年 | 84篇 |
2016年 | 130篇 |
2015年 | 103篇 |
2014年 | 126篇 |
2013年 | 354篇 |
2012年 | 243篇 |
2011年 | 264篇 |
2010年 | 171篇 |
2009年 | 156篇 |
2008年 | 203篇 |
2007年 | 224篇 |
2006年 | 168篇 |
2005年 | 176篇 |
2004年 | 141篇 |
2003年 | 100篇 |
2002年 | 97篇 |
2001年 | 64篇 |
2000年 | 52篇 |
1999年 | 52篇 |
1998年 | 50篇 |
1996年 | 53篇 |
1995年 | 48篇 |
1994年 | 56篇 |
1993年 | 72篇 |
1992年 | 64篇 |
1991年 | 51篇 |
1990年 | 60篇 |
1989年 | 53篇 |
1988年 | 64篇 |
1987年 | 52篇 |
1986年 | 31篇 |
1985年 | 76篇 |
1984年 | 98篇 |
1983年 | 59篇 |
1982年 | 65篇 |
1981年 | 74篇 |
1980年 | 71篇 |
1979年 | 74篇 |
1978年 | 69篇 |
1977年 | 65篇 |
1976年 | 59篇 |
1975年 | 44篇 |
1974年 | 51篇 |
1973年 | 58篇 |
1972年 | 34篇 |
排序方式: 共有5152条查询结果,搜索用时 0 毫秒
991.
992.
Juliana Marques Ramos Thiago Martucci Artur Wilson Carbonari Messias de Souza Costa Rajendra Narain Saxena Reiner Vianden 《Hyperfine Interactions》2013,221(1-3):129-136
Electric quadrupole interactions were studied in pure and Mn-doped powder samples and thin films of SnO2 using perturbed γγ angular correlation spectroscopy (PAC). The powder samples were prepared by Sol gel method and the thin film were prepared on the Si (100) substrate by sputtering technique using Sn in the oxygen atmosphere. The samples were characterized by x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The thickness of the film was 100 nm. The average particle size of the SnO2 powder samples was determined to be smaller than 60 nm. The radioactive 111In and 181Hf tracers were introduced in the powder samples during the sol gel chemical process. Radioactive 111In was implanted on the SnO2 thin films using the University of Bonn ion implanter (BONIS). PAC measurements were carried out in a four BaF2 detector spectrometer in the temperature range of 77–973 K for samples annealed at different temperatures. The PAC results for both nuclear probes show the presence of two electric quadrupole interactions. The major fractions in both cases correspond to the substitutional sites in the rutile phase of SnO2. The results are compared with previous PAC measurements. 相似文献
993.
Fiber-grade high-density polyethylene (HOPE) was melt spun into a multifilament yam that, after quenching in air, was wound at speeds in the range 450 m/min to 1000 m/min. Tensile tests on these anisotropic yarns showed two yield points of varying intensity, similar to those reported in the literature for compression molded, isotropic HOPE sheets and films. However, unlike the sheets, the second yield in the case of fibers always occurred at a stress higher than that for the first yield. More detailed studies on the yam spun at 450 m/ min showed that these characteristics persisted at different strain rates and test temperatures and were also present in the heat-set as-spun samples. The applicability of the existing models to explain the double-yield phenomenon has been critically examined in the light of the results obtained. For this, the necessary structural and morphological characterization of the samples was also done. 相似文献
994.
Normal modes and their dispersion are obtained for syndiotactic polystyrene (sPS) in the planar zigzag and s(2/1)2 helical conformations. Comparison is made with the spectra of isotactic polystyrene (ips) to identify the conformation-sensitive modes; for example, the 1221 cm?1 mode is identified as a skeletal mode sensitive to chain structure and it also shows maximum dispersion. Bunching of dispersion curves and their intersection are among the characteristic features which are discussed. A check on the dispersion curves below 400 cm?1 is made from the spectra of polystyrene oligomers. 相似文献
995.
Stacey M. Frumm Zi Peng Fan Kenneth N. Ross Jeremy R. Duvall Supriya Gupta Lynn VerPlank Byung-Chul Suh Edward Holson Florence F. Wagner William B. Smith Ronald M. Paranal Christopher F. Bassil Jun Qi Giovanni Roti Andrew L. Kung James E. Bradner Nicola Tolliday Kimberly Stegmaier 《Chemistry & biology》2013,20(5):713-725
996.
J. Heinicke N. Peulecke A. Surana S. Singh N. Gupta K. Steinhauser 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1417-1421
The formation, structure and electrophilic substitution reactions at phosphorus or nitrogen of N -lithium-1,3-benzazaphospholides is reported. 相似文献
997.
Neelima Gupta Vijaya Kabra Varsha Saxena Sama Jain Kalpana Bhatnager 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):851-861
A novel one-pot synthesis of four new classes of amidothiophosphoric esters and trisamidothiophosphoric esters is developed. (1-Alkyl-2-pyrimidinylidenamido)bis(diethylamido)thiophosphates, (1-alkyl-2-pyrimidinylidenamido)bis (O-2/4-methylphenyl)thiophosphates, (3-alkyl-2-thiazolinylidenamido)bis (diethylamido)thiophosphate and (3-alkyl-2-thiazolinylidenamido)bis(O-2/4-methylphenyl)thiophosphates are obtained from the nucleophilic substitution and oxidation of N-alkyl-2-cycloiminylidenaminodichlorophosphines generated in situ from the reaction of the corresponding N-alkyl-2-aminocycloiminium halide with phosphorus trichloride and triethylamine. The synthesized thiophosphoric esters have been investigated for fungicidal properties. 相似文献
998.
Amit Kumar Gupta Kumkum Kumari Neetu Singh Dushyant Singh Raghuvanshi Krishna Nand Singh 《Tetrahedron letters》2012,53(6):650-653
An environmentally benign, ionic liquid promoted multicomponent protocol to benzopyrano(2,3-d)pyrimidines and 4H-chromenes has been developed at room temperature. Results of the reaction depend on the nature of the nucleophile used in the reaction. Secondary amines result in the formation of benzopyrano(2,3-d)pyrimidines, whereas thiols give rise to 4H-chromenes under the same set of reaction conditions. 相似文献
999.
Swadesh Kumar Gupta Dharmendra Pratap Singh Pankaj Kumar Tripathi Mahesh Varia Laxmi K. Sagar 《Liquid crystals》2013,40(4):528-533
Cadmium selenide quantum dot (CdSe QD) has been used as a dopant in ferroelectric liquid crystal (FLC) 2-methylbutyl 4-(4-decyloxybenzylideneamino) cinnamate (DOBAMBC). Effect of CdSe QD in DOBAMBC on its different electro-optical (E-O) properties has been studied in the SmC* phase. The optical micrographs recorded for the pure and composite material are showing good dispersion of QDs in the FLC matrix. Micrographs of unaligned sample cell revealed that CdSe QDs induce homeotropic alignment of FLC molecules. An appreciable change in the value of E-O parameters like tilt angle, spontaneous polarisation and response time with shifting of SmA–SmC* phase transition temperature has been observed for CdSe QD–DOBAMBC composite. The observed properties of composite system have been discussed on the basis of surface properties of QDs in FLC system. 相似文献
1000.
S. C. Tripathi K. K. Gupta M. Bindu P. M. Gandhi 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(1):657-661
Present investigation describes our study on photochemical generation of uranous ions and consequent degradation of solvent in the uranium loaded 30 % Tributyl phosphate-n-dodecane-nitric acid system. Samples of 30 % TBP-n-dodecane loaded with uranium were subjected to UV photolysis at 254, 300 and 350 nm respectively. Wavelength dependent formation of U(IV) has been determined spectrophotometrically by measuring absorbance at 656 nm. Additionally, photochemical yield of U(IV) has also been estimated semi quantitatively as a function of time of photolysis. The changes in the solvent composition under different photochemical conditions have been studied by examination of comparative gas chromatographic profiles of the solvent before and after photolysis. Among the wavelength of photo irradiation studied, the yield of U(IV) was found to be optimum at 300 nm with least degradation of PUREX solvent. 相似文献