全文获取类型
收费全文 | 2857篇 |
免费 | 294篇 |
国内免费 | 169篇 |
专业分类
化学 | 1137篇 |
晶体学 | 6篇 |
力学 | 520篇 |
综合类 | 56篇 |
数学 | 722篇 |
物理学 | 879篇 |
出版年
2024年 | 17篇 |
2023年 | 59篇 |
2022年 | 135篇 |
2021年 | 105篇 |
2020年 | 86篇 |
2019年 | 79篇 |
2018年 | 91篇 |
2017年 | 115篇 |
2016年 | 128篇 |
2015年 | 103篇 |
2014年 | 133篇 |
2013年 | 243篇 |
2012年 | 171篇 |
2011年 | 168篇 |
2010年 | 133篇 |
2009年 | 164篇 |
2008年 | 184篇 |
2007年 | 158篇 |
2006年 | 107篇 |
2005年 | 96篇 |
2004年 | 117篇 |
2003年 | 84篇 |
2002年 | 71篇 |
2001年 | 58篇 |
2000年 | 66篇 |
1999年 | 53篇 |
1998年 | 47篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 41篇 |
1994年 | 26篇 |
1993年 | 26篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1990年 | 42篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1974年 | 1篇 |
排序方式: 共有3320条查询结果,搜索用时 15 毫秒
31.
Least squares estimations have been used extensively in many applications, e.g. system identification and signal prediction. When the stochastic process is stationary, the least squares estimators can be found by solving a Toeplitz or near-Toeplitz matrix system depending on the knowledge of the data statistics. In this paper, we employ the preconditioned conjugate gradient method with circulant preconditioners to solve such systems. Our proposed circulant preconditioners are derived from the spectral property of the given stationary process. In the case where the spectral density functions() of the process is known, we prove that ifs() is a positive continuous function, then the spectrum of the preconditioned system will be clustered around 1 and the method converges superlinearly. However, if the statistics of the process is unknown, then we prove that with probability 1, the spectrum of the preconditioned system is still clustered around 1 provided that large data samples are taken. For finite impulse response (FIR) system identification problems, our numerical results show that annth order least squares estimator can usually be obtained inO(n logn) operations whenO(n) data samples are used. Finally, we remark that our algorithm can be modified to suit the applications of recursive least squares computations with the proper use of sliding window method arising in signal processing applications.Research supported in part by HKRGC grant no. 221600070, ONR contract no. N00014-90-J-1695 and DOE grant no. DE-FG03-87ER25037. 相似文献
32.
33.
Summary A computer interactive identification system is proposed which is based on the relationship between retention and molecular
properties such as the size and shape of polycyclic aromatic hydrocarbons (PAHs). This system offers an automatic analytical
process for liquid chromatography, providing a reliable identification of the separated components. The identification can
be further enhanced by the use of multiple detectors such as a multichannel UV detector. The system can be used for optimization
procedures, resulting in a highly automatic complex analytical system. 相似文献
34.
35.
A New Technique of Synthesizing 10—Hydroxy—5—methyl—5,10—dihydrophenophosphazine 10—Oxide 总被引:1,自引:0,他引:1
ZhiGangYIN DeFengZHAO 《中国化学快报》2003,14(4):347-350
10-Hydroxy-5-methyl-5,10-dihydrophenophosphazine 10-oxide(1)was prepared by a new technique of treating 10-methoxy-5,10-dihydrophenophosphazine 10-oxide(2)with an equivalent of NaH in anhydrous DMF,and then at 120℃ for 3-4h,which not only avoided poisonous and expensive methyl iodide used in literature,but made the consumption of NaH greatly decrease as well.The possible reaction mechanism was also described.The chemical structure of 1 was confiremed by IR,NMR,and mass spectroscopy. 相似文献
36.
Hong Zheng Wei Yu Hon-bin Zhang Chi-xing Zhou 《高分子科学》2006,(1):1-11
Finite element method is used to simulate the high-speed melt spinning process,based on the equation system proposed by Doufas et al.Calculation predicts a neck-like deformation,as well as the related profiles of velocity,diameter, temperature,chain orientation,and crystallinity in the fiber spinning process.Considering combined effects on the process such as flow-induced crystallization,viscoelasticity,filament cooling,air drag,inertia,surface tension and gravity,the simulated material flow behaviors are consistent with those observed for semi-crystalline polymers under various spinning conditions.The structure change of polymer coils in the necking region described by the evolution of conformation tensor is also investigated.Based on the relaxation mechanism of macromolecules in flow field different types of morphology change of polymer chains before and in the neck are proposed,giving a complete prospect of structure evolution and crystallization of semi-crystalline polymer in the high speed fiber spinning process. 相似文献
37.
乙酰丙酸是配制酱油的特有成分,在硫酸溶液中,乙酰丙酸可与香草醛反应而呈现蓝绿色。掺有尿素的酱油在与丁二酮肟、磷酸混合后,加热煮沸会呈现红色。用酱色、食盐等配制的劣质酱油可用测定氨基酸态氮和酱色含量的方法进行鉴别。酿造食醋和配制食醋的常用鉴别方法是碘液法和高锰酸钾法。 相似文献
38.
Summary An analytical procedure for the individual determination of ortho and non-ortho PCB congeners in sediments, using high performance liquid chromatography (HPLC) preseparation and gas chromatography/ECD detection, is described. Gas chromatography/FTIR spectrometry (GC/FTIR) and gas chromatography/mass spectrometry (GC/MS) were employed for individual congener identification and determination. Sample extraction, clean-up of extract and selective elution procedures were optimized by using reference certified marine sediment samples. Recovery and precision were typically 83% and 16% respectively at 2 ng/g of total PCB content. The proposed procedure, tested by analyzing real sediment samples, showed a reproducibility better than 20% at 13 ng/g PCB level. 相似文献
39.
40.