全文获取类型
收费全文 | 260篇 |
免费 | 5篇 |
国内免费 | 36篇 |
专业分类
化学 | 117篇 |
晶体学 | 3篇 |
力学 | 5篇 |
数学 | 2篇 |
物理学 | 174篇 |
出版年
2023年 | 4篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 3篇 |
2014年 | 8篇 |
2013年 | 16篇 |
2012年 | 10篇 |
2011年 | 16篇 |
2010年 | 9篇 |
2009年 | 21篇 |
2008年 | 23篇 |
2007年 | 11篇 |
2006年 | 38篇 |
2005年 | 10篇 |
2004年 | 4篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 5篇 |
2000年 | 19篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 1篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1972年 | 1篇 |
排序方式: 共有301条查询结果,搜索用时 15 毫秒
71.
C. Hugenschmidt K. Schreckenbach M. Stadlbauer B. Straßer 《Applied Surface Science》2006,252(9):3098-3105
The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented. 相似文献
72.
随着正电子发射断层扫描(PET)技术的进步, 高效18F标记反应显现出重大的研究价值. 同时, 在有机氟化学的快速发展下, 研究人员已开发出一系列产生二氟甲基自由基(·CHF2)的一碳合成子, 得以被引入到18F标记反应中. 本文从化学原理出发, 总结了近年来·CHF2合成子、 碳氢键活化反应及18F标记的研究, 并展望了未来的研究趋势. 相似文献
73.
Dong Wang D.D. Wang C.Y. Cao L.R. Huang 《Journal of magnetism and magnetic materials》2010,322(22):3642-3647
High purity Fe2O3/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe2O3/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe2O4. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe2O3/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects. 相似文献
74.
Raphaël Hoareau 《Tetrahedron letters》2010,51(26):3353-3355
Due to an urgent need to image the peripheral benzodiazepine receptor (PBR) in living human subjects using positron emission tomography imaging, we had cause to prepare N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (desmethyl-PBR28 (1)), the precursor to [11C]PBR28. Herein, we report a new synthesis of the precursor in which palladium-mediated reduction of the nitro pyridine to the corresponding amino pyridine, and subsequent reductive amination, can be achieved with decaborane in a convenient one-pot procedure. This procedure is operationally simpler than the current alternatives and provides high quality precursor suitable for use in clinical applications. 相似文献
75.
In the present Letter, a fast, automated, and reproducible method for the synthesis of S-[13N]nitrosothiols is reported. The labeling strategy is based on trapping in an anion exchange resin. The reaction with thiols in acidic media led to the formation of the desired nitrosothiols in short reaction times (60 s) with excellent radiochemical conversions (from 48.7% to 74.5%). Final radiotracers were purified by HPLC. Good radiochemical yields (from 33.8% to 60.6%, decay corrected) and radiochemical purities (>99%) were obtained in all cases. Stability of the labeled compounds was checked. 相似文献
76.
Koichi Kato Tatsuya Kikuchi Nobuki Nengaki Takuya Arai Ming-Rong Zhang 《Tetrahedron letters》2010,51(45):5908-5911
Tetrabutylammonium fluoride-promoted α-[11C]methylation of α-arlylesters was developed. The method was amenable to the remote-controlled synthesis of 11C-labeled ibuprofen. 相似文献
77.
We consider the problem min
i=1
m
(ai,x–biloga
i, z) subject tox 0 which occurs as a maximum-likelihood estimation problem in several areas, and particularly in positron emission tomography. After noticing that this problem is equivalent to mind(b, Ax) subject tox 0, whered is the Kullback-Leibler information divergence andA, b are the matrix and vector with rows and entriesa
i,b
i, respectively, we suggest a regularized problem mind(b, Ax) + d(v, Sx), where is the regularization parameter,S is a smoothing matrix, andv is a fixed vector. We present a computationally attractive algorithm for the regularized problem, establish its convergence, and show that the regularized solutions, as goes to 0, converge to the solution of the original problem which minimizes a convex function related tod(v, Sx). We give convergence-rate results both for the regularized solutions and for their functional values.The research of A. N. Iusem was partially supported by CNPq Grant No. 301280/86-MA. 相似文献
78.
79.
David O’Hagan 《Journal of fluorine chemistry》2006,127(11):1479-1483
This is a summary of a lecture presented at the 100th Anniversary, Moissan Symposium in Paris on Friday, 10 November 2006. The lecture highlighted recent developments on the enzyme structure, mechanism and biotransformation prospects of the fluorinase enzyme identified in 2002 from the bacterium Streptomyces cattleya. Over-expression of the enzyme has allowed it to be used as a catalyst for 18F-incorporation during the synthesis of potential ligands for positron emission tomography (PET) applications. 相似文献
80.
In thin film electronic applications, the limiting factor, in terms of cost and usability, is generally the substrate material. As a consequence, different materials are being investigated as potential lightweight, inexpensive and flexible substrates. In this respect, we have been the first research collaboration to produce silicon-based electronics on paper substrates. Here we present structural characterisation of hydrogenated amorphous silicon (a-Si:H) layers deposited on 80 g m−2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapour deposition (HW-CVD). Both pulsed positron beam profiling and X-ray diffraction studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers. The internal defect structure is similar, with a dominant dangling bond complex of similar size, which has a slightly longer lifetime than in layers grown at higher temperatures on conventional substrates. 相似文献