全文获取类型
收费全文 | 35544篇 |
免费 | 4883篇 |
国内免费 | 3990篇 |
专业分类
化学 | 25137篇 |
晶体学 | 378篇 |
力学 | 2260篇 |
综合类 | 403篇 |
数学 | 3961篇 |
物理学 | 12278篇 |
出版年
2024年 | 123篇 |
2023年 | 712篇 |
2022年 | 1097篇 |
2021年 | 1224篇 |
2020年 | 1373篇 |
2019年 | 1345篇 |
2018年 | 1104篇 |
2017年 | 1045篇 |
2016年 | 1574篇 |
2015年 | 1572篇 |
2014年 | 1878篇 |
2013年 | 2477篇 |
2012年 | 3103篇 |
2011年 | 3171篇 |
2010年 | 1983篇 |
2009年 | 2005篇 |
2008年 | 2116篇 |
2007年 | 1969篇 |
2006年 | 1791篇 |
2005年 | 1508篇 |
2004年 | 1154篇 |
2003年 | 1021篇 |
2002年 | 1013篇 |
2001年 | 786篇 |
2000年 | 709篇 |
1999年 | 786篇 |
1998年 | 639篇 |
1997年 | 611篇 |
1996年 | 712篇 |
1995年 | 560篇 |
1994年 | 519篇 |
1993年 | 430篇 |
1992年 | 395篇 |
1991年 | 339篇 |
1990年 | 281篇 |
1989年 | 215篇 |
1988年 | 165篇 |
1987年 | 152篇 |
1986年 | 146篇 |
1985年 | 123篇 |
1984年 | 93篇 |
1983年 | 68篇 |
1982年 | 44篇 |
1981年 | 39篇 |
1979年 | 19篇 |
1978年 | 19篇 |
1977年 | 23篇 |
1976年 | 21篇 |
1975年 | 29篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Microwave-assisted extraction (MAE) and microwave-assisted hydrolysis (MAAH) were developed for the sample preparation of guava leaves prior to GC determination of quercetin and its glycosides. Ethanol was selected as the solvent. The optimum MAE temperature, particle size, solvent volume and MAE time are 120 degrees C, 40 - 60 mesh, 20 mL and 5 min, respectively; the optimum MAAH temperature and time, HCl concentration, solvent are 100 degrees C, 5 min, 1.2 mol L(-1) HCl and ethanol, respectively. 相似文献
22.
Non-dispersive atomic fluorescence spectrometry (NDAFS) coupled with vapor generation (VG) sample introduction was applied to the determination of the concentrations of hazardous heavy metals, such as arsenic, cadmium, lead and mercury, in seawater, soils and total airborne particulate matter (PM) collected around the Xiamen area in China. Almost 100% sample introduction efficiency was achieved by using thiourea and ascorbic acid for the pre-reduction of As(V) to As(III), K3Fe(CN)6 and tartaric acid for pre-oxidation of Pb(II) to Pb(IV), and masking the interferences arising from the co-existing transition metals to As, Cd, Hg and Pb during their vapor generation process. Moreover, a novel sample pretreatment device was developed to avoid the loss of mercury, lead, cadmium and arsenic during sample pretreatment. With such methods, the detection limit (DL) of arsenic, cadmium, lead and mercury was down to 0.08, 0.03, 0.05, 0.01 ng mL(-1) (3sigma), respectively. The relative standard deviations (RSD, n = 11) for arsenic, cadmium, lead and mercury at 10 ng mL(-1) were 0.9%, 1.6%, 1.3% and 2.0%, respectively. The concentrations of hazardous heavy metals in the environmental samples collected in Xiamen, China are in the range from 0.02 +/- 0.001 ng mL(-1) in seawater to 15.3 +/- 0.2 microg g(-1) in soils. Besides flame/GF-AAS and ICP-AES/MS, VG-NDAFS should be another choice for the determination of hazardous heavy metals in environmental samples. 相似文献
23.
24.
模糊数学,随机数学与精确数学的逻辑比较 总被引:2,自引:0,他引:2
本文揭示模糊数学,随机数学与精确数学有相似的形式公理化描述,它们的区别只在语义上。 相似文献
25.
This paper presents an analysis of a portfolio model which can be used to assist a property-liability insurance company in determining the optimal composition of the insurance and investment portfolios. By introducing insurer's threshold risk and relaxing some non-realistic assumptions made in traditional chance constraint insurance and investment portfolio models, we propose a method for an insurer to maximize his return threshold for a given threshold risk level. This proposed model can be used to optimize the composition of underwriting and investment portfolios regarding the insurer's threshold risk level, as well as to generate the efficient frontier by adjusting insurer's threshold risk levels. A numerical example is given based on the industry's aggregated data for a sixteen year period. 相似文献
26.
Jing-song Huang 《Mathematische Annalen》1993,297(1):309-324
Supported in part by NSF Grant DMS-8610730 相似文献
27.
采用固源Si分子束外延,在较高的生长温度于Si(100)衬底上制备出Si1-xGex/Si量子阱发光材料。发光样品的质量和特性通过卢瑟福背散射、X射线双晶衍射及光致发光评估。背散射实验中观察到应变超晶格的反常沟道效应;X射线分析表明材料的生长是共度的、无应力释放的,结晶完整性好。低温光致发光主要是外延合金量子阱中带边激子的无声发射和横光学声子参与的激子复合。并讨论了生长温度对量于阱发光的影响。 相似文献
28.
29.
J.S. Tsay H.W. Chang Y.L. Chiou K.T. Huang W.Y. Chan Y.D. Yao 《Journal of magnetism and magnetic materials》2006
Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced. 相似文献
30.
Tensile impact experiments of EC8.0−24×7 glass fiber bundles at different low temperaturesT(14°C, −40°C and −10°C) and strain rates ɛ were carried out, and complete stress-strain curves were obtained. Within the range
of the experiment temperatures and strain rates, it is found that the initial modulusE, the ultimate strength σmax and the unstable strain ɛ
b
of the glass fiber bundles all increase with ɛ at an identicalT. At an identical ɛ, with the decrease ofT, E and σmax increase; but ɛ
b
increases when 10°C>T>−40°C and decreases when −40°C>T>−100°C. The strain-rate- and temperature-dependent bimodal Weibull statistical constitutive theory was adopted for the statistical
analysis of the experimental results, and the Weibull parameters of single fiber were obtained. The results show that the
bimodal Weibull distribution function is suitable to represent the strength distribution of the glass fiber at low temperature
and different strain rates. The differences in the mechanical properties between EC8.0−24×7 and EC5.5−12 ×14 glass fiber bundles
were also discussed.
Project supported by the National Natural Science Foundation of China (No. 19772058). 相似文献