全文获取类型
收费全文 | 782篇 |
免费 | 142篇 |
国内免费 | 106篇 |
专业分类
化学 | 568篇 |
晶体学 | 8篇 |
力学 | 53篇 |
综合类 | 2篇 |
数学 | 77篇 |
物理学 | 322篇 |
出版年
2023年 | 24篇 |
2022年 | 12篇 |
2021年 | 31篇 |
2020年 | 46篇 |
2019年 | 31篇 |
2018年 | 21篇 |
2017年 | 17篇 |
2016年 | 46篇 |
2015年 | 44篇 |
2014年 | 31篇 |
2013年 | 57篇 |
2012年 | 69篇 |
2011年 | 89篇 |
2010年 | 57篇 |
2009年 | 63篇 |
2008年 | 56篇 |
2007年 | 42篇 |
2006年 | 49篇 |
2005年 | 33篇 |
2004年 | 17篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 15篇 |
1994年 | 6篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有1030条查询结果,搜索用时 515 毫秒
991.
Structures,Energetics, and Infrared Spectra of the Cationic Monomethylamine-Water Clusters 下载免费PDF全文
Shu-kang Jiang Dong Yang Xiang-tao Kong Chong Wang Xiang-yu Zang Hui-jun Zheng Gang Li Hua Xie Wei-qing Zhang Xue-ming Yang Ling Jiang 《化学物理学报(中文版)》2020,33(1):31-36
The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH\begin{document}$_3$\end{document} NH\begin{document}$_2$\end{document} )(H\begin{document}$_2$\end{document} O)\begin{document}$_n$\end{document} ]\begin{document}$^+$\end{document} (\begin{document}$n$\end{document} =1\begin{document}$-$\end{document} 5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH\begin{document}$_2$\end{document} NH\begin{document}$_3$\end{document} \begin{document}$^+$\end{document} ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O\begin{document}$\cdots$\end{document} HN hydrogen bonds with the positively charged NH\begin{document}$_3$\end{document} \begin{document}$^+$\end{document} moiety of CH\begin{document}$_2$\end{document} NH\begin{document}$_3$\end{document} \begin{document}$^+$\end{document} , whose motif is retained in the larger clusters. The CH\begin{document}$_3$\end{document} NH\begin{document}$_2$\end{document} \begin{document}$^+$\end{document} ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH\begin{document}$_2$\end{document} NH\begin{document}$_3$\end{document} \begin{document}$^+$\end{document} and CH\begin{document}$_3$\end{document} NH\begin{document}$_2$\end{document} \begin{document}$^+$\end{document} type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH\begin{document}$_3$\end{document} NH\begin{document}$_2$\end{document} )(H\begin{document}$_2$\end{document} O)\begin{document}$_n$\end{document} ]\begin{document}$^+$\end{document} complexes. 相似文献
992.
Li X Hihath J Chen F Masuda T Zang L Tao N 《Journal of the American Chemical Society》2007,129(37):11535-11542
We have studied electron transport through single redox molecules, perylene tetracarboxylic diimides, covalently bound to two gold electrodes via different linker groups, as a function of electrochemical gate voltage and temperature in different solvents. The conductance of these molecules is sensitive to the linker groups because of different electronic coupling strengths between the molecules and electrodes. The current through each of the molecules can be controlled reversibly over 2-3 orders of magnitude with the gate and reaches a peak near the redox potential of the molecules. The similarity in the gate effect of these molecules indicates that they share the same transport mechanism. The temperature dependence measurement indicates that the electron transport is a thermally activated process. Both the gate effect and temperature dependence can be qualitatively described by a two-step sequential electron-transfer process. 相似文献
993.
采用一种新型的电子传输材料TFTTP作为阴极缓冲层提高基于SubPc/C60异质结的有机薄膜太阳能电池的性能. 通过在有机活性层和金属电极之间加入TFTTP界面层,器件的能量转换效率提高了约30%. 系统研究了器件的二极管特性、光电流特性以及内部的光场分布情况,结果表明,TFTTP阴极缓冲层的引入可以有效地提高器件的内建电场,进而增加电荷转移激子的分离效率. 通过使用TFTTP作为阴极缓冲层,在C60/金属界面形成良好的欧姆接触,降低了界面接触电阻,有利于自由载流子的收集. 相似文献
994.
飞机雷击附着点的确定能够为飞机防雷设计提供依据,是飞机雷击区域划分和飞机各部件进行雷电试验鉴定的先决条件.本文提出了一种基于分形理论的飞机雷击初始附着点数值模拟的新方法.该方法首先依据标准SAE-ARP5416中有关飞机雷击附着点试验的规定,确定放电间隙、雷电起始坐标、飞机姿态和放电次数等参数,然后根据分形理论,使用电介质击穿模型模拟符合自然界雷电物理机理和几何特征的雷电先导分形发展过程,同时考虑飞机自身触发双向先导的情况,最终得到飞机的雷击附着点分布.通过本文方法仿真模拟得到飞机F-4雷击附着点的分布概率,并分别与该飞机飞行实验和实验室高压放电实验测试得到的真实雷击附着点的概率分布情况比较,结果基本吻合,验证了该方法的有效性.研究结果为飞机雷击附着点仿真模拟提供了一个有潜力的方法,可作为飞机防雷设计和今后开展相关研究工作的基础. 相似文献
995.
D.?Y.?Zang H.?P.?Wang F.?P.?Dai D.?Langevin B.?WeiEmail author 《Applied Physics A: Materials Science & Processing》2011,102(1):141-145
We report a solidification mechanism transition of liquid ternary Co45Cu45Ni10 alloy when it solidifies at a critical undercooling of about 344 K. When undercooling at ΔT<344 K, the solidification process is characterized by primary S (Co) dendritic growth and a subsequent peritectic transition.
The dendritic growth velocity of S (Co) dendrite increases with the rise of undercooling. However, once ΔT>344 K, the solidification velocity decreases with the increase of undercooling. In this case, liquid/liquid phase separation
takes place prior to solidification. The minor L2 (Cu) droplets hinder the motion of the solidification front, and a monotectic transition may occur in the major L1 phase. These facts caused by metastable phase separation are responsible for the slow growth at high undercoolings. 相似文献
996.
Li B Zang SQ Ji C Liang R Hou HW Wu YJ Mak TC 《Dalton transactions (Cambridge, England : 2003)》2011,40(39):10071-10081
The reactions of 1,2,3,4-benzenetetracarboxylic acid (H(4)mpda) and different silver(I) salts under hydrothermal or solvent evaporation conditions yielded four unusual coordination complexes with interesting frameworks: [Ag(4)(mpda)](n) (1), {[Ag(2.5)(mpda)(bpy)(2)]·[Ag(bpy)]·[Ag(bpy)(H(2)O)]·(NO(3))(0.5)·(H(2)O)(9)}(n) (2), {[Ag(5)(mpda)(2)(bpy)(4)]·[Ag(bpy)]·[Ag(bpy)(H(2)O)]·[Ag(bpy)(H(2)O)]·(H(2)O)(16)}(n) (3), {[Ag(2)(mpda)(H(2)O)]·[Ag(bpy)]·[Ag(bpy)]}(n) (4) (bpy = 4,4'-bipyridine). Complex 1 displays a novel (3,4,7)-connected {4.6(2)}{4.6(5)}{4(2).6(13).8(5).10} topology, in which the carboxylic groups of the mpda(4-) ligand adopt variable coordination modes. In 1, besides Ag-O coordination bonding, AgAg and Agaromatic intermolecular interactions also make their appearance. In complexes 2-4, rare architectures comprising three or four isolated coordination polymers within the same crystalline structure have been obtained, respectively. In 2 and 3, neighboring layers are linked together through water tapes into a three-dimensional supramolecular architecture, which is also consolidated by π···π stacking, while independent infinite rod-like polymer chains fill the void space between layers. Interestingly, an anionic (H(2)O-NO(3)(-))(n) layer, built from water tapes and nitrate anions as well as consolidated by the mpda(4-) ligands, has been structurally identified in compound 2. A new water tape constructed from alternating tetramers and decamers has been obtained in compound 3. In compound 4, a right-handed helical chain and two rod-like polymeric chains are interconnected through host-guest molecular recognition to generate a three-dimensional chiral supramolecular architecture. Bulk materials for 1 and 4 have second-harmonic generation activity, being approximately 0.6 and 0.4 times that of urea. The IR spectra, thermogravimetric analysis and luminescent properties of all compounds were also investigated. 相似文献
997.
Zang Q Keire DA Buhse LF Wood RD Mital DP Haque S Srinivasan S Moore CM Nasr M Al-Hakim A Trehy ML Welsh WJ 《Analytical and bioanalytical chemistry》2011,401(3):939-955
Chemometric analysis of a set of one-dimensional (1D) (1)H nuclear magnetic resonance (NMR) spectral data for heparin sodium active pharmaceutical ingredient (API) samples was employed to distinguish USP-grade heparin samples from those containing oversulfated chondroitin sulfate (OSCS) contaminant and/or unacceptable levels of dermatan sulfate (DS) impurity. Three chemometric pattern recognition approaches were implemented: classification and regression tree (CART), artificial neural network (ANN), and support vector machine (SVM). Heparin sodium samples from various manufacturers were analyzed in 2008 and 2009 by 1D (1)H NMR, strong anion-exchange high-performance liquid chromatography, and percent galactosamine in total hexosamine tests. Based on these data, the samples were divided into three groups: Heparin, DS ≤ 1.0% and OSCS = 0%; DS, DS > 1.0% and OSCS = 0%; and OSCS, OSCS > 0% with any content of DS. Three data sets corresponding to different chemical shift regions (1.95-2.20, 3.10-5.70, and 1.95-5.70 ppm) were evaluated. While all three chemometric approaches were able to effectively model the data in the 1.95-2.20 ppm region, SVM was found to substantially outperform CART and ANN for data in the 3.10-5.70 ppm region in terms of classification success rate. A 100% prediction rate was frequently achieved for discrimination between heparin and OSCS samples. The majority of classification errors between heparin and DS involved cases where the DS content was close to the 1.0% DS borderline between the two classes. When these borderline samples were removed, nearly perfect classification results were attained. Satisfactory results were achieved when the resulting models were challenged by test samples containing blends of heparin APIs spiked with non-, partially, or fully oversulfated chondroitin sulfate A, heparan sulfate, or DS at the 1.0%, 5.0%, and 10.0% (w/w) levels. This study demonstrated that the combination of 1D (1)H NMR spectroscopy with multivariate chemometric methods is a nonsubjective, statistics-based approach for heparin quality control and purity assessment that, once standardized, minimizes the need for expert analysts. 相似文献
998.
Zang Q Keire DA Wood RD Buhse LF Moore CM Nasr M Al-Hakim A Trehy ML Welsh WJ 《Analytical and bioanalytical chemistry》2011,399(2):635-649
Heparin, a widely used anticoagulant primarily extracted from animal sources, contains varying amounts of galactosamine impurities.
Currently, the United States Pharmacopeia (USP) monograph for heparin purity specifies that the weight percent of galactosamine
(%Gal) may not exceed 1%. In the present study, multivariate regression (MVR) analysis of 1H NMR spectral data obtained from heparin samples was employed to build quantitative models for the prediction of %Gal. MVR
analysis was conducted using four separate methods: multiple linear regression, ridge regression, partial least squares regression,
and support vector regression (SVR). Genetic algorithms and stepwise selection methods were applied for variable selection.
In each case, two separate prediction models were constructed: a global model based on dataset A which contained the full range (0–10%) of galactosamine in the samples and a local model based on the subset dataset B for which the galactosamine level (0–2%) spanned the 1% USP limit. All four regression
methods performed equally well for dataset A with low prediction errors under optimal conditions, whereas SVR was clearly
superior among the four methods for dataset B. The results from this study show that 1H NMR spectroscopy, already a USP requirement for the screening of contaminants in heparin, may offer utility as a rapid method
for quantitative determination of %Gal in heparin samples when used in conjunction with MVR approaches. 相似文献
999.
1000.
Performance improvement of Ga N-based light-emitting diodes transferred from Si(111) substrate onto electroplating Cu submount with embedded wide p-electrodes 下载免费PDF全文
Crack-free Ga N/In Ga N multiple quantum wells(MQWs) light-emitting diodes(LEDs) are transferred from Si substrate onto electroplating Cu submount with embedded wide p-electrodes. The vertical-conducting n-side-up configuration of the LED is achieved by using the through-hole structure. The widened embedded p-electrode covers almost the whole transparent conductive layer(TCL), which could not be applied in the conventional p-side-up LEDs due to the electrodeshading effect. Therefore, the widened p-electrode improves the current spreading property and the uniformity of luminescence. The working voltage and series resistance are thereby reduced. The light output of embedded wide p-electrode LEDs on Cu is enhanced by 147% at a driving current of 350 m A, in comparison to conventional LEDs on Si. 相似文献