全文获取类型
收费全文 | 41349篇 |
免费 | 3542篇 |
国内免费 | 2002篇 |
专业分类
化学 | 28953篇 |
晶体学 | 435篇 |
力学 | 1690篇 |
综合类 | 98篇 |
数学 | 3740篇 |
物理学 | 11977篇 |
出版年
2023年 | 405篇 |
2022年 | 471篇 |
2021年 | 956篇 |
2020年 | 1058篇 |
2019年 | 1060篇 |
2018年 | 876篇 |
2017年 | 775篇 |
2016年 | 1456篇 |
2015年 | 1316篇 |
2014年 | 1627篇 |
2013年 | 2624篇 |
2012年 | 3275篇 |
2011年 | 3463篇 |
2010年 | 2222篇 |
2009年 | 2058篇 |
2008年 | 2766篇 |
2007年 | 2573篇 |
2006年 | 2366篇 |
2005年 | 2158篇 |
2004年 | 1845篇 |
2003年 | 1487篇 |
2002年 | 1518篇 |
2001年 | 1068篇 |
2000年 | 869篇 |
1999年 | 641篇 |
1998年 | 496篇 |
1997年 | 507篇 |
1996年 | 532篇 |
1995年 | 420篇 |
1994年 | 415篇 |
1993年 | 388篇 |
1992年 | 368篇 |
1991年 | 304篇 |
1990年 | 215篇 |
1989年 | 190篇 |
1988年 | 178篇 |
1987年 | 147篇 |
1986年 | 126篇 |
1985年 | 200篇 |
1984年 | 130篇 |
1983年 | 110篇 |
1982年 | 135篇 |
1981年 | 104篇 |
1980年 | 88篇 |
1978年 | 82篇 |
1977年 | 85篇 |
1976年 | 98篇 |
1975年 | 107篇 |
1974年 | 85篇 |
1973年 | 107篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
Neodymium doped bismuth layer structure ferroelectrics (BLSFs) ceramics CaBi4−xNdxTi4O15 (x=0, 0.25, 0.50, 0.75) were prepared by solid-state reaction method. X-ray diffraction pattern showed that single phase was formed when x=0-0.75. The refined lattice parameters showed that a (b) axes decrease at x=0.25 and increase with more Nd3+ dopant. The effects of Nd3+ doping on the dielectric and ferroelectric properties of CaBi4Ti4O15 ceramics are studied. Nd3+ dopant decreased the Curie temperature linearly, and the dielectric loss, tan δ, as well. The remnant polarization of Nd3+ doped CaBi4Ti4O15 ceramics was increased by 80% at x=0.25, while more Nd3+ dopant decreased the remnant polarization. CaBi3.75Nd0.25Ti4O15 ceramics had the largest piezoelectric constant d33. The structure and properties of CaBi4−xNdxTi4O15 ceramics showed that Nd3+ may occupy different crystal locations when Nd3+ content x is less than 0.25 and more than 0.50. 相似文献
63.
Deposition of Hydrogen-Free Silicon Nitride Thin Films by Microwave ECR plasma Enhanced Magnetron Sputtering at Room Temperature 下载免费PDF全文
Hydrogen-free silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma enhanced unbalance magnetron sputtering system. Both Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy are used to study the bonding type and the change of bonding structures of the silicon nitride films. The results indicate that the chemical structure and composition of SiNx films deposited by this technique depend strongly on the N2 flow rates, the stoichiometric SiNx film, which has the highest hardness of 22.9 GPa, could be obtained at lower N2 flow rate of 4 sccm. 相似文献
64.
65.
Vernon Barger Cheng-Wei Chiang Paul Langacker Hye-Sung Lee 《Physics letters. [Part B]》2004,580(3-4):186-196
We study the effects of an extra U(1)′ gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on Bd→φK and Bd→η′K decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z′ contribution can readily account for the anomaly in SφKS but is not sufficient to explain large branching ratio of Bd→η′K with the same parameter value. SφKS and Sη′KS are seen to be the dominant observables that constrain the extra weak phase in the model. 相似文献
66.
室温下,通过双核配合物[Cu(dppm)(NO3)]2(dppm=双二苯基膦甲烷)与四苯基硼钠在甲醇和二氯甲烷混合溶剂中反应制备了三核铜(I)配合物[Cu3(dppm)3(NO3)(OH)](NO3),经过红外光谱、热重分析、核磁和ES-MS等现代分析手段表征了配合物的物理化学性质,并进一步研究了配合物在室温下的荧光光谱特征。 相似文献
67.
D. Liu N.Q. Ngo X.Y. Dong S.C. Tjin P. Shum 《Applied physics. B, Lasers and optics》2005,81(6):807-811
We propose and experimentally demonstrate a stable dual-wavelength erbium-doped polarization-maintaining (PM) fiber laser
with tunable wavelength spacing using an all-PM linear cavity that makes use of two reflection peaks from the PM fiber Bragg
grating (PM-FBG). Experimental results show stable dual lasing lines with a wavelength separation of ∼0.22 nm and a large
optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. By applying axial strain to the PM-FBG, the center
wavelengths of the two lasing lines can be tuned over several nanometers and the wavelength separation between the lasing
lines can also be tuned to as small as 0.05 nm, which, to our knowledge, is the smallest wavelength spacing ever obtained
from a stable room-temperature dual-wavelength fiber laser. The proposed laser configuration has the advantages of simple
structure, low loss, stable dual-wavelength operation and a very small lasing linewidth of ∼5 kHz .
PACS 42.55.Wd; 42.81.-i; 42.81.Gs 相似文献
68.
Sungyong Kim Lingxin Chen Sangyeop Lee Gi Hun Seong Jaebum Choo Eun Kyu Lee Chil-Hwan Oh Sanghoon Lee 《Analytical sciences》2007,23(4):401-405
A rapid DNA analysis has been developed based on a fluorescence intensity change of a molecular beacon in a PDMS microfluidic channel. Recently, we reported a new analytical method of DNA hybridization involving a PDMS microfluidic sensor using fluorescence energy transfer (FRET). However, there are some limitations in its application to real DNA samples because the target DNA must be labelled with a suitable fluorescent dye. To resolve this problem, we have developed a new DNA microfluidic sensor using a molecular beacon. By monitoring the change in the restored fluorescence intensity along the channel length, it is possible to rapidly detect any hybridization of the molecular beacon to the target DNA. In this case, the target DNA does not need to be labelled. Our experimental results demonstrate that this microfluidic sensor using a molecular beacon is a promising diagnostic tool for rapid DNA hybridization analysis. 相似文献
69.
N. Wright und W. C. Lee 《Fresenius' Journal of Analytical Chemistry》1940,120(1-2):39-40
Ohne Zusammenfassung 相似文献
70.
Burn‐in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we consider the problem of determining the optimal burn‐in time and optimal work size maximizing the long‐run average amount of work saved per time unit in the computer applications. Assuming that the underlying lifetime distribution of the computer has an initially decreasing or/and eventually increasing failure rate function, an upper bound for the optimal burn‐in time is derived for each fixed work size and a uniform (with respect to the burn‐in time) upper bound for the optimal work size is also obtained. Furthermore, it is shown that a non‐trivial lower bound for the optimal burn‐in time can be derived if the underlying lifetime distribution has a large initial failure rate. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献