全文获取类型
收费全文 | 24634篇 |
免费 | 4053篇 |
国内免费 | 3123篇 |
专业分类
化学 | 18089篇 |
晶体学 | 266篇 |
力学 | 1655篇 |
综合类 | 237篇 |
数学 | 2881篇 |
物理学 | 8682篇 |
出版年
2024年 | 47篇 |
2023年 | 406篇 |
2022年 | 511篇 |
2021年 | 766篇 |
2020年 | 916篇 |
2019年 | 924篇 |
2018年 | 761篇 |
2017年 | 663篇 |
2016年 | 1144篇 |
2015年 | 1193篇 |
2014年 | 1369篇 |
2013年 | 1802篇 |
2012年 | 2106篇 |
2011年 | 2310篇 |
2010年 | 1521篇 |
2009年 | 1514篇 |
2008年 | 1569篇 |
2007年 | 1407篇 |
2006年 | 1417篇 |
2005年 | 1192篇 |
2004年 | 1075篇 |
2003年 | 857篇 |
2002年 | 910篇 |
2001年 | 766篇 |
2000年 | 588篇 |
1999年 | 520篇 |
1998年 | 438篇 |
1997年 | 369篇 |
1996年 | 368篇 |
1995年 | 343篇 |
1994年 | 324篇 |
1993年 | 249篇 |
1992年 | 263篇 |
1991年 | 222篇 |
1990年 | 194篇 |
1989年 | 158篇 |
1988年 | 99篇 |
1987年 | 95篇 |
1986年 | 83篇 |
1985年 | 85篇 |
1984年 | 50篇 |
1983年 | 50篇 |
1982年 | 42篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1979年 | 12篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1971年 | 6篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
162.
163.
Effects of Nb2O5 on thermal stability and optical properties of Er3+-doped tellurite glasses 下载免费PDF全文
Er3+-doped tellurite glasses with molar compositions of
xNb2O5-(14.7-x)Na2O--10ZnO--5K2O--10GeO2--
60TeO2--0.3Er2O3
(x=0, 3, 5, 7 and 9) have been investigated for developing 1.5~μm
fibre and planar amplifiers. The effects of Nb2O5 on the thermal stability
and optical properties of Er3+-doped tellurite glasses have been discussed.
It is noted that the incorporation of Nb2O5 (x=5) increases the thermal
stability of tellurite glasses significantly. Er3+-doped niobium tellurite
glasses exhibit a large stimulated emission cross-section (7.2\times 10-21-
10.7×10-21~cm2 and the gain bandwidth, FWHM×\sigmae^{\rm peak} (274\times 10-28 - 480×10-28~cm3), which are
significantly higher than that of silicate and phosphate glasses. In addition, the
intensity of upconversion luminescence of the Er3+-doped niobium tellurite
glasses decreases rapidly with increasing Nb2O5 content. As a result,
Er3+-doped niobium tellurite glasses might be a potential candidate for
developing laser or optical amplifier devices. 相似文献
164.
The feature selection consists of obtaining a subset of these features to optimally realize the task without the irrelevant
ones. Since it can provide faster and cost-effective learning machines and also improve the prediction performance of the
predictors, it is a crucial step in machine learning. The feature selection methods using support machines have obtained satisfactory
results, but the noises and outliers often reduce the performance. In this paper, we propose a feature selection approach
using fuzzy support vector machines and compare it with the previous work, the results of experiments on the UCI data sets
show that feature selection using fuzzy SVM obtains better results than using SVM. 相似文献
165.
Wen Bin CUI Jie ZHOU Lei CHEN Xiao Bin DENG Chun GUO 《中国化学快报》2006,17(8):999-1001
Metal porphyrins are natural antioxidant reagent1-3. However, due to the characteristics of structures, they are only soluble in inorganic acids and part of polar organic solvents. Thus, it is limited to be used as effective pharmaceutical preparations. T… 相似文献
166.
Novel oxyfluoride glasses are developed with the composition of 30SiO2-15Al2O3-28PbF2-22CdF2-0.1TmF3 - xYbF3 - (4.9 - x) AlF3(x=0, 0.5, 1.0, 1.5, 2.0) in tool fraction, Furthermore, the upconversion luminescence characteristics under a 970nm excitation are investigated. Intense blue, red and near infrared luminescences peaked at 453nm, 476nm, 647nm and 789nm, which correspond to the transitions of Tm^3+: ^1D2 →^3F4, ^1G4 →^3H6, ^1G4 →^3F4, and ^3H4 →^3H6, respectively, are observed. Due to the sensitization of Yb^3+ ions, all the upconversion luminescence intensities are enhanced considerably with Yb^3+ concentration increasing. The upconversion mechanisms are discussed based on the energy matching rule and quadratic dependence on excitation power. The results indicate that the dominant mechanism is the excited state absorption for those upconversion emissions. 相似文献
167.
熔制了掺铒碲铌玻璃样品(100-X)TeO2-XNb2O5(X=5,10,15,20mol%),测试了其密度、折射率、转变温度、析晶温度、维氏机械强度、吸收光谱、荧光光谱、荧光寿命等参量。利用Judd-Ofelt和McCumber理论分别计算了铒离子强度参量Ωt(t=2,4,6)和受激发射截面σemi的大小,研究了掺铒碲铌玻璃样品光谱参量对Nb2O5成分的依赖性,并与典型的碲锌钠玻璃(75TeO2-20ZnO-5Na2O)在热学、机械强度、光谱性质和放大品行四个方面进行了比较. 相似文献
168.
169.
X.F. Hu 《Applied Surface Science》2006,252(13):4625-4627
The synthetic antiferromagnets (SAF) have been used in spin-valve sensor in data storage industry [1]. We report a new hard/Ru/soft sandwich structure (SHBL) fabricated by pulsed lased deposition to replace current single layer structure for information recording application. SHBL consists of two magnetic layers separated by thin nonmagnetic layers, typically with Ru layers of 0.7-1.2 nm, through which antiferromagnetic coupling is induced. Varying the relative thickness of the magnetic layers, the spacer layers, and the type of magnetic materials can alter magnetic properties of CoCrPt/Ru/CoFe superlattice. The coercivity Hc and grain size of magnetic layer is also dependent on the laser fluence. High laser fluence results in both small grain size and high Hc. The observed phenomena are related to high quenching and deposition rates during PLD at high fluence, resulting in more pronounced phase segregation. 相似文献
170.
Under some general continuous and compact conditions, the existence problems of fiked points andd coupled fixed points for increasing operators are studied. an application, we utilize the results obtained to study the existence of solutions for differential inclusions in Banach spaces. 相似文献