全文获取类型
收费全文 | 68590篇 |
免费 | 4022篇 |
国内免费 | 72篇 |
专业分类
电工技术 | 505篇 |
综合类 | 39篇 |
化学工业 | 13859篇 |
金属工艺 | 1260篇 |
机械仪表 | 1310篇 |
建筑科学 | 2275篇 |
矿业工程 | 120篇 |
能源动力 | 1546篇 |
轻工业 | 10399篇 |
水利工程 | 666篇 |
石油天然气 | 292篇 |
武器工业 | 5篇 |
无线电 | 3484篇 |
一般工业技术 | 11638篇 |
冶金工业 | 15328篇 |
原子能技术 | 332篇 |
自动化技术 | 9626篇 |
出版年
2023年 | 592篇 |
2022年 | 928篇 |
2021年 | 1756篇 |
2020年 | 1350篇 |
2019年 | 1373篇 |
2018年 | 2629篇 |
2017年 | 2575篇 |
2016年 | 2686篇 |
2015年 | 1939篇 |
2014年 | 2514篇 |
2013年 | 4914篇 |
2012年 | 4065篇 |
2011年 | 3937篇 |
2010年 | 3133篇 |
2009年 | 2801篇 |
2008年 | 3012篇 |
2007年 | 2707篇 |
2006年 | 2012篇 |
2005年 | 1665篇 |
2004年 | 1534篇 |
2003年 | 1363篇 |
2002年 | 1237篇 |
2001年 | 867篇 |
2000年 | 826篇 |
1999年 | 1011篇 |
1998年 | 5050篇 |
1997年 | 3128篇 |
1996年 | 2010篇 |
1995年 | 1186篇 |
1994年 | 974篇 |
1993年 | 968篇 |
1992年 | 323篇 |
1991年 | 332篇 |
1990年 | 320篇 |
1989年 | 295篇 |
1988年 | 293篇 |
1987年 | 264篇 |
1986年 | 267篇 |
1985年 | 302篇 |
1984年 | 266篇 |
1983年 | 199篇 |
1982年 | 227篇 |
1981年 | 259篇 |
1980年 | 235篇 |
1979年 | 169篇 |
1978年 | 165篇 |
1977年 | 329篇 |
1976年 | 646篇 |
1975年 | 129篇 |
1973年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 36 毫秒
991.
J.-M. Giraudon A. Elhachimi F. Wyrwalski S. Siffert A. Aboukaïs J.-F. Lamonier G. Leclercq 《Applied catalysis. B, Environmental》2007,75(3-4):157-166
Calcined and reduced catalysts Pd/LaBO3 (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T50 values (temperature at which 50% of toluene is converted): Pd/LaFeO3 > Pd/LaMnO3+δ > Pd/LaCoO3 > Pd/LaNiO3. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H2-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO3 (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 °C for Pd/LaFeO3 and either 200 or 250 °C for LaCoO3. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 °C for 2 h (2 L/h, 1 °C/min). As LaFeO3 was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B3+ (B = Mn) or B2+ even to B0 (B = Co, Ni). In the reactive stream (0.1% toluene in air), Pd0 reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd2+/Pd4+ and Pd0/Pd2+/Pd4+ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co3+ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO3 for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity. 相似文献
992.
The active sites of ultra-dispersed Pt/-Al2O3 catalysts are studied using high-resolution electron microscopy, Z-contrast and dark field. In addition we have calculated using a method based in density functional theory the electrostatic potential and charge distribution of the active sites. It is conclude that the most likely Pt clusters that are formed correspond to Pt13 and Pt with icosahedral and decahedral structure. It is shown that this is consistent with the electron microscopy data. 相似文献
993.
The direct platinisation of a solid polymer electrolyte (Nafion® membrane) was realized by chemical reduction of a platinum salt. The Pt–PEM electrodes thus obtained were modified by tin to improve the electrocatalytic activity towards the electrooxidation of ethanol. The Pt–PEM and Pt–Sn–PEM electrodes were characterized by TEM, EDX and XRD analysis, cyclic voltammetry, and their polarisation curves for the electrooxidation of ethanol were determined under quasisteady state conditions. 相似文献
994.
Cinchonidine and cinchonine have been grafted onto pure silica MCM-41. It has been shown that both supported alkaloids are active catalysts for the Michael addition of ethyl 2-oxocyclopentanecarboxylate and methyl vinyl ketone, anchored cinchonidine being more active and enantioselective than anchored cinchonine. The study of the influence of the polarity of the solvent and reaction temperature on the optical induction shows that there is not a direct correlation between solvent polarity and enantioselectivity, and the maximum optical yield was obtained between 278 and 273 K. 相似文献
995.
Siebert HC André S Asensio JL Cañada FJ Dong X Espinosa JF Frank M Gilleron M Kaltner H Kozár T Bovin NV von Der Lieth CW Vliegenthart JF Jiménez-Barbero J Gabius HJ 《Chembiochem : a European journal of chemical biology》2000,1(3):181-195
This study documents the feasibility of switching to an aprotic medium in sugar receptor research. The solvent change offers additional insights into mechanistic details of receptor--carbohydrate ligand interactions. If a receptor retained binding capacity in an aprotic medium, solvent-exchangeable protons of the ligand would not undergo transfer and could act as additional sensors, thus improving the level of reliability in conformational analysis. To probe this possibility, we first focused on hevein, the smallest lectin found in nature. The NMR-spectroscopic measurements verified complexation, albeit with progressively reduced affinity by more than 1.5 orders of magnitude, in mixtures of up to 50% dimethyl sulfoxide (DMSO). Since hevein lacks the compact beta-strand arrangement of other sugar receptors, such a structural motif may confer enhanced resistance to solvent exchange. Two settings of solid-phase activity assays proved this assumption for three types of alpha- and/or beta-galactoside-binding proteins, that is, a human immunoglobulin G (IgG) subfraction, the mistletoe lectin, and a member of the galectin family of animal lectins. Computer-assisted calculations and NMR experiments also revealed no conspicuous impact of the solvent on the conformational properties of the tested ligands. To define all possible nuclear Overhauser effect (NOE) contacts in a certain conformation and to predict involvement of exchangeable protons, we established a new screening protocol applicable during a given molecular dynamics (MD) trajectory and calculated population densities of distinct contacts. Experimentally, transferred NOE (tr-NOE) experiments with IgG molecules and the disaccharide Gal'alpha1-3Galbeta1-R in DMSO as solvent disclosed that such an additional crosspeak, that is, Gal'OH2--GalOH4, was even detectable for the bound ligand under conditions in which spin diffusion effects are suppressed. Further measurements with the plant lectin and galectins confirmed line broadening of ligand signals and gave access to characteristic crosspeaks in the aprotic solvent and its mixtures with water. Our combined biochemical, computational, and NMR-spectroscopical strategy is expected to contribute notably to the precise elucidation of the geometry of ligands bound to compactly folded sugar receptors and of the role of water molecules in protein--ligand (carbohydrate) recognition, with relevance to areas beyond the glycosciences. 相似文献
996.
Alper Sarıoğlan Ömer Tunç Savaşçı Ayşe Erdem-Şenatalar Vu Thu Ha Gilbert Sapaly Younès Ben Taârit 《Catalysis Letters》2007,118(1-2):123-128
MFI type inorganic carrier was used in two different cationic forms, hydrogen and calcium respectively. MFI-supported molybdenum
and rhenium catalysts were prepared. The activities of the catalysts were compared for the aromatization reaction of methane.
Higher activity values were attained with the catalysts supported on HZSM-5. Aromatics were also observed with the catalysts
supported on CaZSM-5, despite their deficiency in acid sites. Highly dispersed rhenium is expected to be formed with the use
of the inorganic carrier in calcium form. On the other hand, lower reaction rates were observed with rhenium supported on
CaZSM-5, in spite of the improved dispersion of the active rhenium species on this catalyst. This was interpreted in terms
of the critical role of the acid sites in the conversion of methane to aromatics, compared to the improved dispersion of the
active metal. 相似文献
997.
J. Linares-Pérez R. Caballero-Águila I. García-Garrido 《International journal of systems science》2014,45(7):1548-1562
This paper addresses the optimal least-squares linear estimation problem for a class of discrete-time stochastic systems with random parameter matrices and correlated additive noises. The system presents the following main features: (1) one-step correlated and cross-correlated random parameter matrices in the observation equation are assumed; (2) the process and measurement noises are one-step autocorrelated and two-step cross-correlated. Using an innovation approach and these correlation assumptions, a recursive algorithm with a simple computational procedure is derived for the optimal linear filter. As a significant application of the proposed results, the optimal recursive filtering problem in multi-sensor systems with missing measurements and random delays can be addressed. Numerical simulation examples are used to demonstrate the feasibility of the proposed filtering algorithm, which is also compared with other filters that have been proposed. 相似文献
998.
Student academic underachievement is a concern of paramount importance in Europe, where around 15% of the students in the last high school courses do not achieve the minimum knowledge academic requirement. In this paper, we propose a model based on a system of differential equations to study the dynamics of the students’ academic performance in the German region of the North Rhine-Westphalia. This approach is supported by the idea that both good and bad study habits, are a mixture of personal decisions and influence of classmates. This model allows us to forecast the student academic performance by means of confidence intervals over the next few years. 相似文献
999.
1000.