全文获取类型
收费全文 | 344篇 |
免费 | 13篇 |
专业分类
化学 | 260篇 |
晶体学 | 5篇 |
力学 | 8篇 |
数学 | 20篇 |
物理学 | 64篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 7篇 |
2017年 | 13篇 |
2016年 | 16篇 |
2015年 | 8篇 |
2014年 | 6篇 |
2013年 | 22篇 |
2012年 | 29篇 |
2011年 | 15篇 |
2010年 | 27篇 |
2009年 | 15篇 |
2008年 | 18篇 |
2007年 | 13篇 |
2006年 | 19篇 |
2005年 | 19篇 |
2004年 | 5篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2000年 | 5篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1972年 | 1篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1955年 | 1篇 |
1946年 | 1篇 |
排序方式: 共有357条查询结果,搜索用时 31 毫秒
61.
62.
63.
Benaskar F Engels V Rebrov EV Patil NG Meuldijk J Thüne PC Magusin PC Mezari B Hessel V Hulshof LA Hensen EJ Wheatley AE Schouten JC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(6):1800-1810
New routes for the preparation of highly active TiO(2)-supported Cu and CuZn catalysts have been developed for C-O coupling reactions. Slurries of a titania precursor were dip-coated onto glass beads to obtain either structured mesoporous or non-porous titania thin films. The Cu and CuZn nanoparticles, synthesized using a reduction by solvent method, were deposited onto calcined films to obtain a Cu loading of 2 wt%. The catalysts were characterized by inductively coupled plasma (ICP) spectroscopy, temperature-programmed oxidation/reduction (TPO/TPR) techniques, (63)Cu nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (S/TEM-EDX) and X-ray photo-electron spectroscopy (XPS). The activity and stability of the catalysts obtained have been studied in the C-O Ullmann coupling of 4-chloropyridine and potassium phenolate. The titania-supported nanoparticles retained catalyst activity for up to 12 h. However, catalyst deactivation was observed for longer operation times due to oxidation of the Cu nanoparticles. The oxidation rate could be significantly reduced over the CuZn/TiO(2) catalytic films due to the presence of Zn. The 4-phenoxypyridine yield was 64% on the Cu/nonporous TiO(2) at 120 °C. The highest product yield of 84% was obtained on the Cu/mesoporous TiO(2) at 140 °C, corresponding to an initial reaction rate of 104 mmol g(cat) (-1) s(-1). The activation energy on the Cu/mesoporous TiO(2) catalyst was found to be (144±5) kJ mol(-1), which is close to the value obtained for the reaction over unsupported CuZn nanoparticles (123±3 kJ mol(-1)) and almost twice the value observed over the catalysts deposited onto the non-porous TiO(2) support (75±2 kJ mol(-1)). 相似文献
64.
G. Siddaramana Gowd Manoj Kumar Patra Sandhya Songara Anuj Shukla Manoth Mathew Sampat Raj Vadera Narendra Kumar 《Journal of luminescence》2012,132(8):2023-2029
Eu3+ doped Y2O3 nanophosphors have been synthesized using the simple colloidal precipitation method. Doping of Eu3+ ions in host yttria lattice has been achieved through slow re-crystallization process under wet-chemical conditions followed by annealing at high temperatures (300–1400 °C). The nanophosphors were characterized by using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), atomic force microscopy (AFM) and spectrofluorometer techniques. XRD analysis reveals formation of pure cubic phase of Y2O3 in samples annealed at 700 °C or above. Further, the XRD data was successfully used to retrieve the crystallite size and size distribution from powder samples using the FW((1/5)/(4/5))M method. Crystallite size (11–50 nm) extracted from XRD has been found to be consistent with AFM measurements. The PL emission spectra of nanophosphors show bright red emission at 612 nm due to hypersensitive electric dipole (ED) 5D0–7F2 transition of Eu3+ ions in Y2O3 lattice. Further, photoluminescence studies indicate that optimum value of the Eu3+ to get best luminescence properties is 12 at%. Surface conjugations of these nanophosphors with water soluble dextran biomolecules have also been performed. Surface conjugated rare earth nanophosphors have great potential for bio-applications. 相似文献
65.
Vishwanatha TM Narendra N Chattopadhyay B Mukherjee M Sureshbabu VV 《The Journal of organic chemistry》2012,77(6):2689-2702
Synthesis of selenoxo peptides by the treatment of N(α)-protected peptide esters with a combination of PCl(5) and LiAlHSeH is delineated. The method is simple, high-yielding, and free from racemization. Thus obtained selenoxo peptides are used as units for N-terminal chain extension through N(α)-deprotection/coupling to yield peptide-selenoxo peptide hybrids. Multiple selenation is demonstrated by conversion of two peptide bonds of tripeptides into selenoxo peptide bonds. Amino acid derived arylamides are also converted into aryl selenoamides. C(6)H(5)-CSeNH-Val-OMe 8f is obtained as single crystal, and its structure was determined through X-ray diffraction study. 相似文献
66.
A single layer of the Li‐Al layered double hydroxide (LDH) can be considered to be a structural synthon that can be stacked in a number of ways to give rise to the polytypes of the Li‐Al LDH family. The topotactic imbibition of lithium into gibbsite and bayerite precursors gives rise to crystalline 2H and 1M1 polytypes respectively. In contrast the sample formed through a gel to crystallite route is found to be a 1M1 polytype with stacking disorders whose local symmetry corresponds to the 1M2 polytype. On heating, a solid‐solid 1M1→1H polytype transformation is observed. 相似文献
67.
Varma Nimmu Narendra Naidu Challa Gangu Ramachandra Bondigalla Swamy Arnipalli Manikanta 《Journal of Analytical Chemistry》2021,76(10):1172-1181
Journal of Analytical Chemistry - A water-compatible molecularly imprinted polymer (MIP) was prepared for specific extraction of HIV-1 integrase inhibitor elvitegravir (EVG). It was prepared by a... 相似文献
68.
69.
Anil Kamath Rajeev Motwani Krishna Palem Paul Spirakis 《Random Structures and Algorithms》1995,7(1):59-80
The classical occupancy problem is concerned with studying the number of empty bins resulting from a random allocation of m balls to n bins. We provide a series of tail bounds on the distribution of the number of empty bins. These tail bounds should find application in randomized algorithms and probabilistic analysis. Our motivating application is the following well-known conjecture on threshold phenomenon for the satisfiability problem. Consider random 3-SAT formulas with cn clauses over n variables, where each clause is chosen uniformly and independently from the space of all clauses of size 3. It has been conjectured that there is a sharp threshold for satisfiability at c* ≈? 4.2. We provide a strong upper bound on the value of c*, showing that for c > 4.758 a random 3-SAT formula is unsatisfiable with high probability. This result is based on a structural property, possibly of independent interest, whose proof needs several applications of the occupancy tail bounds. 相似文献
70.
D. R. S. Somayajulu Narendra Patel Mukesh Chawda Mitesh Sarkar K. C. Sebastian 《Hyperfine Interactions》2005,160(1-4):241-246
Hyperfine interaction techniques like Mossbauer spectroscopy are very sensitive tools to study the local probe interactions
in dilute magnetic semiconductors. We report here a Mossbauer study on the concentration dependence in Fe0.008Ge1 − x
Te
x
for x = 0, 0.008, 0.016, 0.03 and 0.05. At room temperature magnetic interactions were observed for all concentrations of Te and
the population of magnetic site was found to increase gradually with the Te concentration. A constant magnetic hyperfine field
of 136 KOe was found. A quadrupole doublet due to the FeTe2 compound phase was also seen. 相似文献