全文获取类型
收费全文 | 1302篇 |
免费 | 49篇 |
国内免费 | 4篇 |
专业分类
化学 | 822篇 |
晶体学 | 11篇 |
力学 | 24篇 |
数学 | 167篇 |
物理学 | 331篇 |
出版年
2023年 | 10篇 |
2022年 | 46篇 |
2021年 | 40篇 |
2020年 | 27篇 |
2019年 | 40篇 |
2018年 | 26篇 |
2017年 | 24篇 |
2016年 | 55篇 |
2015年 | 30篇 |
2014年 | 40篇 |
2013年 | 70篇 |
2012年 | 88篇 |
2011年 | 86篇 |
2010年 | 72篇 |
2009年 | 50篇 |
2008年 | 69篇 |
2007年 | 69篇 |
2006年 | 63篇 |
2005年 | 63篇 |
2004年 | 44篇 |
2003年 | 37篇 |
2002年 | 28篇 |
2001年 | 32篇 |
2000年 | 27篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 20篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1973年 | 2篇 |
1971年 | 3篇 |
1965年 | 2篇 |
排序方式: 共有1355条查询结果,搜索用时 15 毫秒
91.
92.
93.
Fernando Cruz Moraes Sonia Tomie Tanimoto Giancarlo Richard Salazar‐Banda Sergio Antonio Spinola Machado Lucia Helena Mascaro 《Electroanalysis》2009,21(9):1091-1098
The electrochemical detection of the hazardous pollutant 4‐nitrophenol (4‐NP) at low potentials, in order to avoid matrix interferences, is an important research challenge. This study describes the development, electrochemical characterization and utilization of a multiwall carbon nanotube (MWCNT) film electrode for the quantitative determination of 4‐NP in natural water. Electrochemical impedance spectroscopy measurements showed that the modified surface exhibits a decrease of ca. 13 times in the charge transfer resistance when compared with a bare glassy carbon (GC) surface. Voltammetric experiments showed the possibility to oxidize a hydroxylamine layer (produced by the electrochemical reduction of 4‐NP on the GC/MWNCT surface) in a potential region which is approximately 700 mV less positive than that needed to oxidize 4‐NP, thus minimizing the interference of matrix components. The limit of detection for 4‐NP obtained using square‐wave voltammetry (0.12 μmol L?1) was lower than the value advised by EPA. A natural water sample from a dam located in São Carlos (Brazil) was spiked with 4‐NP and analyzed by the standard addition method using the GC/MWCNT electrode, without any further purification step. The recovery procedure yielded a value of 96.5% for such sample, thus confirming the suitability of the developed method to determine 4‐NP in natural water samples. The electrochemical determination was compared with that obtained by HPLC with UV‐vis detection. 相似文献
94.
Rolando Gonzlez-Pea Rosa María Cibrin-Ortiz de Anda Angel J Pino-Velazquez Jos Soler-de la Cruz Yhoama Gonzlez-Jorge 《Optics and Lasers in Engineering》2000,34(2)
Speckle metrology and holographic interferometry (HI) have been used in several civil engineering applications. We present the results obtained by applying speckle photography (SP) to the study of two quadratic shearwalls with different boundary conditions, and the potential of the technique in the study of this kind of structures is described. The analysis of Young's fringes obtained with this technique at certain points on each shearwall provides the whole field of displacement measurements. HI has been used to measure the three components of absolute displacement, verifying that the bulging phenomenon does not affect the in-plane components when the applied load remains on the same plane as the shearwall. A qualitative analysis is carried out following an electronic speckle pattern interferometry (ESPI) technique. The results obtained by optical techniques are compared to the numerical results obtained by the finite element method (FEM), finding good correlation between them in all the cases. 相似文献
95.
Danilo Santos Cruz Joo M. de Araújo Carlos A. N. da Costa Carlos C. N. da Silva 《Entropy (Basel, Switzerland)》2021,23(5)
Full waveform inversion is an advantageous technique for obtaining high-resolution subsurface information. In the petroleum industry, mainly in reservoir characterisation, it is common to use information from wells as previous information to decrease the ambiguity of the obtained results. For this, we propose adding a relative entropy term to the formalism of the full waveform inversion. In this context, entropy will be just a nomenclature for regularisation and will have the role of helping the converge to the global minimum. The application of entropy in inverse problems usually involves formulating the problem, so that it is possible to use statistical concepts. To avoid this step, we propose a deterministic application to the full waveform inversion. We will discuss some aspects of relative entropy and show three different ways of using them to add prior information through entropy in the inverse problem. We use a dynamic weighting scheme to add prior information through entropy. The idea is that the prior information can help to find the path of the global minimum at the beginning of the inversion process. In all cases, the prior information can be incorporated very quickly into the full waveform inversion and lead the inversion to the desired solution. When we include the logarithmic weighting that constitutes entropy to the inverse problem, we will suppress the low-intensity ripples and sharpen the point events. Thus, the addition of entropy relative to full waveform inversion can provide a result with better resolution. In regions where salt is present in the BP 2004 model, we obtained a significant improvement by adding prior information through the relative entropy for synthetic data. We will show that the prior information added through entropy in full-waveform inversion formalism will prove to be a way to avoid local minimums. 相似文献
96.
Laura Rubio María Leticia Oca Luis Sarabia Inmaculada García María Cruz Ortiz 《Journal of Chemometrics》2016,30(2):58-69
Internal standards can be added at different stages of an analytical procedure. When they are added at the beginning of a multiresidue method and their behavior is not exactly the same as that of the analytes, the intended correction for small variations within the analytical process could not be achieved. Because of this, in the present work, the use of d ‐optimal designs together with desirability functions is proposed to state the experimental response under study. The overall desirability function used relates two analytical criteria: to assess a similar chemical behavior of each analyte in relation to its internal standard and to avoid a significant reduction of the absolute peak area of the internal standards. This strategy has been applied to the analysis of the effect of four factors related to the extraction and purification steps of six tranquillizers and a β‐blocker from pig muscle analyzed by liquid chromatography–tandem mass spectrometry. The effect of those factors has been evaluated by means of an ad hoc d ‐optimal design consisting of only 11 experiments. The resulting levels of the four factors that enable to achieve the greatest overall desirability have also been compared with those obtained when either the standardized or absolute peak area has been considered as response. Differences in both the significant factors and their optimum levels have been observed. It is noticeable that the experimental effort necessary to study the effect of the factors has been reduced by more than 50% thanks to the d ‐optimal design. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
97.
Electrical,optical, and structural characterization of p‐type N‐doped SnO thin films prepared by thermal oxidation of sputtered SnNx thin films 下载免费PDF全文
A. Garzon‐Fontecha W. De La Cruz M. Quevedo 《Surface and interface analysis : SIA》2017,49(12):1225-1231
We present a study of electrical and optical properties of nitrogen‐doped tin oxide thin films deposited on glass by the DC Magnetron Sputtering method. The deposition conditions to obtain p‐type thin films were a relative partial pressure between 7% and 11% (N2 and/or O2), a total working pressure of 1.8 mTorr and a plasma power of 30 W. The deposited thin films were oxidized after annealing at 250°C for 30 minutes. X‐ray diffraction results showed that the as‐deposited thin films exhibit a Sn tetragonal structure, and after annealing, they showed SnO tetragonal structure. X‐ray photoelectron spectroscopy results showed the presence of nitrogen in the samples before and after annealing. The measured physical parameters of the thin films were optical band gap between 1.92 and 2.68 eV, resistivity between 0.52 and 5.46 Ωcm, a concentration of p‐type carriers between 1018 and 1019 cm?3, and a Hall mobility between 0.1 and 1.94 cm2V?1s?1. These thin films were used to fabricate p‐type thin film transistors. 相似文献
98.
99.
A. Martínez-de la Cruz S. Obregón Alfaro S. M. G. Marcos Villarreal 《Research on Chemical Intermediates》2010,36(8):925-936
Nanoparticles of α-Bi2Mo3O12 were prepared by the Pechini method. The process of formation of the bismuth molybdate was followed by simultaneous thermogravimetric and differential thermal analysis (TGA/DTA). Different samples of α-Bi2Mo3O12 were obtained at 400, 450, and 500 °C, and characterized by X-ray powder diffraction (XRD), nitrogen physisorption (BET), and scanning electron microscopy (SEM). When observed by SEM, the morphology of the sample obtained at the lowest temperature consisted of semi-spherical particles with an average diameter of 150 nm. On the other hand, the highest calcination temperature led to the formation of sintered particles of 500–600 nm. The photocatalytic activity of α-Bi2Mo3O12 was tested by photodegradation of the organic dyes rhodamine B (rhB) and indigo carmine (IC) under visible-light irradiation. The bismuth molybdate nanoparticles were able to bleach aqueous solutions of both organic dyes. The sample obtained at 400 °C was the best photocatalyst with half-lives, t 1/2, of 108 and 154 min for rhB and IC, respectively. 相似文献
100.
Elena Chekmeneva José Manuel Díaz‐Cruz Cristina Ariño Miquel Esteban 《Electroanalysis》2010,22(2):177-184
The complexation of the natural antioxidants α‐lipoic acid (ALA) and its reduced form dihydrolipoic acid (DHLA) with Hg2+ was investigated by a recently proposed differential pulse voltammetric (DPV) method using the rotating Au‐disk electrode. Complexation processes are proposed from the multivariate curve resolution by alternating least squares (MCR‐ALS) analysis of DPV titration data. Main complexes were both 1 : 1 Hg : ALA and Hg : DHLA, although the formation of 1 : 2 complexes can be also deduced. ALA and DHLA show different Hg2+‐binding patterns at different pH. Voltammetric findings are completed with the data obtained by electrospray ionization mass‐spectrometry (ESI‐MS), especially in negative mode. 相似文献