全文获取类型
收费全文 | 5124篇 |
免费 | 95篇 |
国内免费 | 30篇 |
专业分类
化学 | 3290篇 |
晶体学 | 46篇 |
力学 | 184篇 |
数学 | 479篇 |
物理学 | 1250篇 |
出版年
2022年 | 28篇 |
2020年 | 53篇 |
2019年 | 42篇 |
2018年 | 31篇 |
2016年 | 83篇 |
2015年 | 66篇 |
2014年 | 88篇 |
2013年 | 205篇 |
2012年 | 184篇 |
2011年 | 228篇 |
2010年 | 121篇 |
2009年 | 106篇 |
2008年 | 222篇 |
2007年 | 232篇 |
2006年 | 213篇 |
2005年 | 218篇 |
2004年 | 185篇 |
2003年 | 204篇 |
2002年 | 177篇 |
2001年 | 129篇 |
2000年 | 141篇 |
1999年 | 76篇 |
1998年 | 65篇 |
1997年 | 66篇 |
1996年 | 106篇 |
1995年 | 68篇 |
1994年 | 66篇 |
1993年 | 83篇 |
1992年 | 80篇 |
1991年 | 74篇 |
1990年 | 71篇 |
1989年 | 59篇 |
1988年 | 85篇 |
1987年 | 70篇 |
1986年 | 68篇 |
1985年 | 83篇 |
1984年 | 82篇 |
1983年 | 46篇 |
1982年 | 68篇 |
1981年 | 68篇 |
1980年 | 59篇 |
1979年 | 78篇 |
1978年 | 54篇 |
1977年 | 84篇 |
1976年 | 68篇 |
1975年 | 64篇 |
1974年 | 69篇 |
1973年 | 67篇 |
1972年 | 33篇 |
1971年 | 34篇 |
排序方式: 共有5249条查询结果,搜索用时 15 毫秒
21.
22.
A. Edgar G. V. M. Williams M. Secu S. Schweizer J. -M. Spaeth 《Radiation measurements》2004,38(4-6):413-416
The turbidity, photoluminescence, and photo-stimulated luminescence (PSL) of fluorozirconate glass containing barium chloride nano- and micro-crystals have been measured for samples prepared by isochronal (70 min) annealing over a temperature range of 220–283°C, and correlated with the microstructure as determined by X-ray diffraction measurements. Crystallization of hexagonal phase barium chloride commences at around 220°C, but until 275°C the material retains excellent transparency although it displays negligible PSL. Between 275°C and 277°C, the hexagonal phase converts to the orthorhombic phase, the transparency abruptly decreases, and the PSL rises to a value of around 13% of that found for the commercial storage phosphor BaFBr:Eu. For a slightly higher temperature of 280°C, new phases appear which correspond to the onset of bulk crystallization, and at 283°C the relative PSL rises to 33%, while the transparency falls further. The trade-off between optical transparency and PSL over this narrow temperature window for X-ray imaging plate applications is briefly discussed. 相似文献
23.
S. Schweizer M. Secu J. -M. Spaeth L. W. Hobbs A. Edgar G. V. M. Williams 《Radiation measurements》2004,38(4-6):633-638
We found a significant PSL effect in Eu2+-doped fluorozirconate glasses (ZBLAN) which were additionally doped with Br− or Cl− ions. The PSL is attributed to the characteristic emission of Eu2+ present in nano-crystallites of BaBr2 or BaCl2, which form in the glass upon annealing. The metastable hexagonal form of BaX2 (X=Br,Cl) is always formed first before it is converted into the stable orthorhombic form. The particle size increases upon annealing and so does the PSL efficiency of the glass ceramic. However, there is a saturation of the PSL efficiency, which is for Br− doping about 9% and for Cl−-doping about 80% of the Eu-doped BaFBr standard. The particle size was determined by transmission electron microscopy (TEM). The TEM results show a clear tendency for bigger particles for longer annealing at the expense of its number. The particle size for the most efficient phosphor is about 100 nm. 相似文献
24.
We report the evolution of the high power THz source program at Jefferson Lab. The source is based on coherent synchrotron radiation in which short bunches of relativistic electrons radiate when traversing a dipolar magnetic field. In our first accelerator we produced 20 W of broadband THz light. Our upgraded accelerator with higher current and improved THz extraction optics will considerably enhance the output power to >100 W. In this paper we describe the source in some detail and present theoretical calculations for the upgrade. 相似文献
25.
Benjamin M. NugentAmie L. Williams E.N. PrabhakaranJeffrey N. Johnston 《Tetrahedron》2003,59(45):8877-8888
The complete scope of free radical-mediated vinyl amination is described, using 5-exo-trig cyclizations of vinyl radicals to the nitrogen of azomethines. The focus is primarily on N,N-dialkyl enamines since their nucleophilicity renders them the most challenging enamines to synthesize using redox conditions. These studies establish several encouraging precedents for the broader application of this strategy. 相似文献
26.
A. K. Ghosh A. D. Williams J. M. Zucker J. L. Mathews N. Spinhirne 《Experimental Mechanics》2008,48(2):139-152
In nature, shape and structure evolve from the struggle for better performance. Often, biological structures combine multiple
beneficial properties, making research into mimicking them very complex. Presented here is a summary of observations from
a series of experiments performed on a material that closely resembles the human skull bone’s cancellous structure under acoustic
loads. Transmission loss through flat and curved open-cell polyurethane foam samples is observed using air and water as the
two interstitial fluids. Reduction in strength and stiffness caused by porosity can be recovered partially by filling the
interstitial pores with a fluid. The test findings demonstrate the influence of the interstitial fluid on the mechanical characteristics
of a porous structure in a quantitative manner. It is also demonstrated that the transmission loss does not depend only on
the mass per unit area of the structure as predicted by acoustic mass law. Current tests also demonstrate that the transmission
loss is more sensitive to the interstitial fluid than the shape and support conditions of the structures. Test observations
thus support the concepts of “moisture-sensitivity of biological design” and the “law of hierarchy in natural design”. 相似文献
27.
28.
Lakshmi N. Roy Rabindra N. Roy Cole E. Denton Sean R. LeNoue Chandra N. Roy Shahaf Ashkenazi Thomas B. Williams Daniel R. Church Michael S. Fuge Kartik N. Sreepada 《Journal of solution chemistry》2006,35(4):605-624
The values of the second dissociation constant, K
2, and related thermodynamic quantities of the ampholyte bis[(2-hydroxyethyl)amino]acetic acid (BICINE) have been determined at temperatures from 5 to 55 ∘C. The pH values of six equimolal buffer solutions, and four buffer solutions having ionic strengths (I = 0.16 mol⋅kg−1) similar to those in blood plasma, have been evaluated at 12 temperatures from 5 to 55 ∘C using the Bates–Guggenheim convention. The liquid junction potentials (E
j
) between the buffer solutions of BICINE and saturated KCl solution of the calomel electrode at 25 and 37 ∘C have been estimated by measurement with a flowing junction cell. These values of E
j
have been used to ascertain the operational pH values at 25 and 37 ∘C. The pK
2 values at 25 and 37 ∘C are 8.333 and 8.156, respectively. The thermodynamic quantities associated with the second acid dissociation have been calculated from the values of pK
2 as a function of temperature. The zwitterionic buffer BICINE was shown to be useful as a pH standard in the region close to that of blood serum. 相似文献
29.
30.