全文获取类型
收费全文 | 1292篇 |
免费 | 214篇 |
国内免费 | 18篇 |
专业分类
化学 | 70篇 |
晶体学 | 2篇 |
力学 | 62篇 |
综合类 | 16篇 |
数学 | 44篇 |
物理学 | 129篇 |
无线电 | 1201篇 |
出版年
2024年 | 6篇 |
2023年 | 31篇 |
2022年 | 45篇 |
2021年 | 54篇 |
2020年 | 63篇 |
2019年 | 43篇 |
2018年 | 47篇 |
2017年 | 60篇 |
2016年 | 63篇 |
2015年 | 75篇 |
2014年 | 123篇 |
2013年 | 93篇 |
2012年 | 95篇 |
2011年 | 100篇 |
2010年 | 81篇 |
2009年 | 67篇 |
2008年 | 100篇 |
2007年 | 78篇 |
2006年 | 62篇 |
2005年 | 49篇 |
2004年 | 48篇 |
2003年 | 34篇 |
2002年 | 22篇 |
2001年 | 12篇 |
2000年 | 23篇 |
1999年 | 13篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1984年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有1524条查询结果,搜索用时 15 毫秒
31.
目前基于机器学习的入侵检测系统大都建立在入侵数据始终保持统计平稳的假设之上,无法应对攻击者有意改变数据特性或新型攻击方式的出现,而导致的检测率下降的状况.对于上述问题,即攻击漂移,提出了加权Rényi距离的检测方法.在KDD Cup99数据集上的实验证明,Rényi距离可以有效地增强检测效果;在检测到漂移后,通过重新训练模型可以使得对攻击的识别率显著提高. 相似文献
32.
《Journal of Dispersion Science and Technology》2013,34(5):683-689
Abstract The effects of two metal complexes of 2,2′‐dipyridylamine (bpya) ligand, [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2, in promoting the hydrolysis of bis(4‐nitrophenyl) phosphate (BNPP) have been kinetically investigated in Brij35 micellar solution and at 298 K, pH ranging from 6.41 to 8.6. In neutral micellar solution at 298 K, pH 7.02, the rate constants for the catalytic hydrolysis of BNPP by [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2 are 1.2 × 106 times and 1.5 × 105 times higher than those for the spontaneous hydrolysis, respectively. Kinetic studies show that the active species in the catalytic hydrolysis of BNPP is the aquo‐hydroxy form, and the relative kinetic and thermodynamic parameters indicate that the mechanism of the reaction involves intramolecular nucleophilic attack on the metal center‐bound diester. 相似文献
33.
G. Rauret R. Rubio M. Llaurado 《International journal of environmental analytical chemistry》2013,93(1-2):59-68
Abstract The destruction of the filter for metal determination in atmospheric aerosols is an important step in the analysis. Nowadays the decomposition of this kind of sample has not been completely solved, so it is interesting to study new methods for filter sample attack. In the present work the application of the combustion method in a closed flask for mixed cellulose esters filter decomposition for lead determination is proposed. For this purpose optimum operating conditions as stopper flask design, sample size, flask volume, shaking time and volume and concentration of absorbing solution are established, and accuracy and precision of the proposed method using AAS for the measurements is given. The results obtained by the closed flask combustion are compared with those found by wet attack with HNO3. The method proposed is rapid, has low reagent contamination and no loss of lead by volatilization or by amalgamation occurs. This method shows an accuracy and a precision in good agreement with the standard method. 相似文献
34.
Dr. Camilla Parmeggiani Dr. Francesca Cardona Leonardo Giusti Prof. Hans‐Ulrich Reissig Prof. Andrea Goti 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(32):10595-10604
Addition of lithiated 1‐benzyloxyallene to a D ‐arabinose‐derived cyclic nitrone occurred with perfect diastereoselectivity furnishing a bicyclic 1,2‐oxazine derivative, which is an excellent precursor for pyrrolizidine alkaloids hydroxylated at C‐7 with optional configuration at this stereogenic center. Depending on the stage of the N? O bond cleavage and ring re‐closure, 7‐hydroxypyrrolizidines with 7R or 7S configuration were obtained, as a result of completely selective addition reactions occurring complementarily at the bottom or top face of the endocyclic C? C double bond in six‐ and five‐membered B rings, respectively. Applicability of these stereodivergent routes to obtain polyhydroxy pyrrolizidine alkaloids is demonstrated by the efficient syntheses of casuarine and australine as examples of the two classes of diversely configured 7‐hydroxypyrrolizidine alkaloids. An alternative synthesis of australine and two strategies for the preparation of 7‐epi‐australine are also reported, which demonstrate that the stereoselectivity of hydride reduction of an exocyclic C? O double bond is independent of the ring size, occurring preferentially from the top face either in a six‐ or five‐membered ring. 相似文献
35.
Yingyot Poo‐arporn Surachai Thachepan Rungtiva Palangsuntikul 《Journal of synchrotron radiation》2015,22(1):86-90
Evidence of internal sulfate attack in field exposure was demonstrated by the damaged interior wall of a three‐year‐old house situated in Nakhon Ratchasima Province, Thailand. Partial distension of the mortar was clearly observed together with an expansion of a black substance. Removal of the black substance revealed a dense black layer. This layer was only found in the vicinity of the damaged area, suggesting that this black material is possibly involved in the wall cracking. By employing synchrotron‐based X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near‐edge structure (XANES) techniques, the unknown sample was chemically identified. The S 2p and O 1s XPS results mutually indicated the existence of sulfate species in the materials collected from the damaged area. The XANES results indicated the presence of ferrous (II) sulfate, confirming sulfate‐induced expansion and cracking. The sulfate attack in the present case appeared to physically affect the structure whereas the chemical integrity at the molecular level of the calcium silicate hydrate phase was retained since there was a lack of spectroscopic evidence for calcium sulfate. It was speculated that internal sulfate probably originated from the contaminated aggregates used during the construction. The current findings would be beneficial for understanding the sulfate‐attack mechanism as well as for future prevention against sulfate attack during construction. 相似文献
36.
Eric Mädl Dr. Mikhail V. Butovskii Dr. Gábor Balázs Dr. Eugenia V. Peresypkina Dr. Alexander V. Virovets Michael Seidl Prof. Dr. Manfred Scheer 《Angewandte Chemie (International ed. in English)》2014,53(29):7643-7646
Unprecedented functionalized products with an η4‐P5 ring are obtained by the reaction of [Cp*Fe(η5‐P5)] ( 1 ; Cp*=η5‐C5Me5) with different nucleophiles. With LiCH2SiMe3 and LiNMe2, the monoanionic products [Cp*Fe(η4‐P5CH2SiMe3)]? and [Cp*Fe(η4‐P5NMe2)]?, respectively, are formed. The reaction of 1 with NaNH2 leads to the formation of the trianionic compound [{Cp*Fe(η4‐P5)}2N]3?, whereas the reaction with LiPH2 yields [Cp*Fe(η4‐P5PH2)]? as the main product, with {[Cp*Fe(η4‐P5)]2PH}2? as a byproduct. The calculated energy profile of the reactions provides a rationale for the formation of the different products. 相似文献
37.
38.
空对空多目标攻击载机攻击航线求解逻辑 总被引:1,自引:1,他引:0
介绍了空对空多目标攻击载机攻击航线的基本要求、航线选择和求解方法,以及载机航线、瞄准偏差、操纵指令的计算逻辑图.同时还介绍了多目标攻击"可能攻击扇形区"交集的概念和在载机航线求解中的应用等问题. 相似文献
39.
M. Araceli Snchez-Snchez Cristina Conde Beatriz Gmez-Aylln David Ortega-DelCampo Aristeidis Tsitiridis Daniel Palacios-Alonso Enrique Cabello 《Entropy (Basel, Switzerland)》2020,22(11)
Automated border control systems are the first critical infrastructure point when crossing a border country. Crossing border lines for unauthorized passengers is a high security risk to any country. This paper presents a multispectral analysis of presentation attack detection for facial biometrics using the learned features from a convolutional neural network. Three sensors are considered to design and develop a new database that is composed of visible (VIS), near-infrared (NIR), and thermal images. Most studies are based on laboratory or ideal conditions-controlled environments. However, in a real scenario, a subject’s situation is completely modified due to diverse physiological conditions, such as stress, temperature changes, sweating, and increased blood pressure. For this reason, the added value of this study is that this database was acquired in situ. The attacks considered were printed, masked, and displayed images. In addition, five classifiers were used to detect the presentation attack. Note that thermal sensors provide better performance than other solutions. The results present better outputs when all sensors are used together, regardless of whether classifier or feature-level fusion is considered. Finally, classifiers such as KNN or SVM show high performance and low computational level. 相似文献
40.