全文获取类型
收费全文 | 1443篇 |
免费 | 87篇 |
国内免费 | 111篇 |
专业分类
化学 | 809篇 |
晶体学 | 10篇 |
力学 | 9篇 |
综合类 | 15篇 |
数学 | 31篇 |
物理学 | 377篇 |
无线电 | 390篇 |
出版年
2024年 | 7篇 |
2023年 | 22篇 |
2022年 | 19篇 |
2021年 | 16篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 12篇 |
2017年 | 14篇 |
2016年 | 13篇 |
2015年 | 18篇 |
2014年 | 40篇 |
2013年 | 34篇 |
2012年 | 111篇 |
2011年 | 193篇 |
2010年 | 55篇 |
2009年 | 23篇 |
2008年 | 108篇 |
2007年 | 95篇 |
2006年 | 106篇 |
2005年 | 101篇 |
2004年 | 93篇 |
2003年 | 72篇 |
2002年 | 60篇 |
2001年 | 44篇 |
2000年 | 45篇 |
1999年 | 19篇 |
1998年 | 10篇 |
1997年 | 14篇 |
1996年 | 27篇 |
1995年 | 25篇 |
1994年 | 28篇 |
1993年 | 25篇 |
1992年 | 29篇 |
1991年 | 15篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 17篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 24篇 |
1982年 | 4篇 |
1975年 | 2篇 |
1963年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
1957年 | 3篇 |
1948年 | 4篇 |
1947年 | 1篇 |
排序方式: 共有1641条查询结果,搜索用时 15 毫秒
71.
72.
73.
74.
Becker-Szendy R Bratton CB Cady R Casper D Dye ST Gajewski W Goldhaber M Haines TJ Halverson PG Jones TW Kielczewska D Kropp WR Learned JG LoSecco JM Matsuno S McGrew C Mudan MS Price L Reines F Schultz J Sobel HW Stone JL Sulak LR Svoboda R Wittel F 《Physical review D: Particles and fields》1991,43(4):1413-1415
75.
Del Guerra A Lim CB Lum GK Ortendahl D Perez-Mendez V 《IEEE transactions on medical imaging》1982,1(1):4-11
A multiplanar positron camera is proposed, made of six MWPC modules, arranged to form the lateral surface of a hexagonal prism. Each 50 x 50 cm(2) module has a single MWPC sandwiched by two 2-cm thick lead glass tube converters. The experimental results for a 15 x 15 cm(2) test module are reported. For 511 keV gamma-rays incident almost perpendicular onto a 1.0-cm thick converter, a detection efficiency of 4.3%, a time resolution of 130 ns (FWHM) and a spatial resolution of 2.8 mm (FWHM) have been measured with a standard Argon-Methane (70-30) mixture at 1.2 atm. The chamber may also be operated in high resolution mode: 1.2-mm (FWHM) spatial resolution has been measured at a 50% lower efficiency. The use of fast delay lines (specific delay 8 ns/cm) for the position read-out ensures a high rate capability. The expected performance of the six-module MWPC camera is discussed and compared with that of a BGO crystal ring camera. The MWPC solution seems very attractive not only for its low cost and simplicity of construction, but also for its fully three-dimensional imaging capability. 相似文献
76.
A method for the simultaneous spectrophotometric determination of cadmium, copper and zinc based on the formation of their complexes with 1,5-bis(di-2-pyridylmethylene)thiocarbonohydrazide is proposed. The absorption curves of these complexes overlap severely in the scanning range 380–480 nm. The analyte concentrations are calculated by a least squares fit of the pure spectra to the mixture spectra. A linear determination range of 0.1–1.7 μg/ml for cadmium, 0.1–1.3 μg/ml for copper and 0.2–1.2 μg/ml for zinc were obtained. The effect of interference was studied. The method has been applied to the determination of these metal ions in various type of materials. 相似文献
77.
Stone MM Franz AH Lebrilla CB 《Journal of the American Society for Mass Spectrometry》2002,13(8):964-974
Non-covalent inclusion complexes formed between amino acids and derivatized calix[6]arenes are observed in MALDI mass spectrometry. The methyl, ethyl, and propyl ester derivatives of calix[6]arene yielded amino acid complexes, while the smaller calix[4]arene analogs did not. Similarly the underivatized calix[6]arene and calix[4]arene did not produce complexes. Amino acid complexes were observed for nearly all 20 amino acids in time-of-flight (TOF) analysis. In Fourier transform mass spectrometry (FTMS) analysis, however, only the most basic amino acids arginine, histidine, and lysine formed stable adducts. The complexes were abundant under matrix-assisted laser desorption ionization (MALDI) conditions, which suggested favorable interactions between host and guest. 相似文献
78.
Garcia CB Zhang Y Mahajan S DiSalvo F Wiesner U 《Journal of the American Chemical Society》2003,125(44):13310-13311
In the present study poly(isoprene-block-ethylene oxide), PI-b-PEO, block copolymers are used to structure iron oxide and silica precursors into reverse mesophases, which upon dissolution of the organic matrix lead to well-defined nanoparticles of spheres, cylinders, and plates based on the original structure of the mesophase prepared. The hybrid mesophases with sphere, cylinder, and lamellar morphologies containing the inorganic components in the minority phases are characterized through a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). After heat treatments the respective nanoparticles on mica surfaces are characterized by scanning force microscopy (SFM). X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometer measurements are performed to demonstrate that the heat treatment leads to the formation of a magnetic gamma-Fe2O3 crystalline phase within the amorphous aluminosilicate. The results pave the way to functional, i.e., magnetic nanoparticles where the size, shape, and iron oxide concentration can be controlled opening a range of possible applications. 相似文献
79.
The reaction of ammonia (NH(3)) on the Ge(100)-2 x 1 surface is investigated using density functional theory (DFT). We find that NH(3) adsorbs molecularly onto Ge(100)-2 x 1 via the formation of a dative bond. The calculations also show that, unlike Si(100)-2 x 1, the activation barrier for subsequent dissociation of NH(3) adsorbed on Ge(100)-2 x 1 is higher than that of reversible desorption, which indicates that NH(3) has a low reactive sticking probability on the Ge(100)-2 x 1 surface. We also predict that nitrogen insertion into the Ge-Ge dimer requires NH(3) overexposure because the activation barrier for NH(2) insertion into the Ge-Ge dimer is significantly above the entrance channel. The nitridation reaction pathway results in the N-H bridge-bonded state, which is found to be 17.4 kcal/mol more stable than the reactants. We find that the reactions of NH(3) on the Ge(100)-2 x 1 surface generally involve higher activation barriers and less stable intermediates than the analogous reactions on the Si(100)-2 x 1 surface. 相似文献
80.