全文获取类型
收费全文 | 148243篇 |
免费 | 2045篇 |
国内免费 | 649篇 |
专业分类
电工技术 | 3074篇 |
综合类 | 186篇 |
化学工业 | 22542篇 |
金属工艺 | 5690篇 |
机械仪表 | 4866篇 |
建筑科学 | 4028篇 |
矿业工程 | 375篇 |
能源动力 | 3894篇 |
轻工业 | 16759篇 |
水利工程 | 1130篇 |
石油天然气 | 585篇 |
武器工业 | 5篇 |
无线电 | 20372篇 |
一般工业技术 | 28358篇 |
冶金工业 | 23748篇 |
原子能技术 | 2341篇 |
自动化技术 | 12984篇 |
出版年
2019年 | 834篇 |
2018年 | 1081篇 |
2017年 | 1131篇 |
2016年 | 1268篇 |
2015年 | 1063篇 |
2014年 | 1795篇 |
2013年 | 6575篇 |
2012年 | 3192篇 |
2011年 | 4608篇 |
2010年 | 3594篇 |
2009年 | 4151篇 |
2008年 | 4647篇 |
2007年 | 4913篇 |
2006年 | 4350篇 |
2005年 | 4108篇 |
2004年 | 3991篇 |
2003年 | 3838篇 |
2002年 | 3609篇 |
2001年 | 3689篇 |
2000年 | 3409篇 |
1999年 | 3447篇 |
1998年 | 6393篇 |
1997年 | 5052篇 |
1996年 | 4205篇 |
1995年 | 3658篇 |
1994年 | 3318篇 |
1993年 | 3139篇 |
1992年 | 2728篇 |
1991年 | 2644篇 |
1990年 | 2588篇 |
1989年 | 2539篇 |
1988年 | 2415篇 |
1987年 | 2097篇 |
1986年 | 2069篇 |
1985年 | 2502篇 |
1984年 | 2273篇 |
1983年 | 2145篇 |
1982年 | 2020篇 |
1981年 | 1946篇 |
1980年 | 1810篇 |
1979年 | 1838篇 |
1978年 | 1730篇 |
1977年 | 2035篇 |
1976年 | 2525篇 |
1975年 | 1539篇 |
1974年 | 1387篇 |
1973年 | 1431篇 |
1972年 | 1176篇 |
1971年 | 1082篇 |
1970年 | 926篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A novel dual-rotation bobbin tool friction stir welding (DBT-FSW) was developed, in which the upper shoulder (US) and lower shoulder (LS) have different rotational speeds. This process was tried to weld 3.2 mm thick aluminum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formability. Experimental results show that compared to conventional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process. 相似文献
22.
We show that in the presence of magnetic field, two superconducting phases with the center-of-mass momentum of Cooper pair parallel to the magnetic field are induced in Dresselhaus spin–orbit-coupled superconductor. Specifically, at small magnetic field, the center-of-mass momentum is induced due to the energy-spectrum distortion and no unpairing region with vanishing singlet correlation appears. We refer to this superconducting state as the drift-BCS state. By further increasing the magnetic field, the superconducting state falls into the Fulde–Ferrell state with the emergence of the unpairing regions. The observed abrupt enhancement of the center-of-mass momenta and suppression on the order parameters during the transition indicate the occurrence of a first-order phase transition. Enhanced Pauli limit and hence enlarged magnetic-field regime of the Fulde–Ferrell state, due to the spin-flip terms of the spin–orbit coupling, are revealed. We also address the triplet correlations induced by the spin–orbit coupling, and show that the Cooper-pair spin polarizations, generated by the magnetic field and center-of-mass momentum with the triplet correlations exhibit totally different magnetic-field dependences between the drift-BCS and Fulde–Ferrell states. 相似文献
23.
The effects of local body cooling on thermal comfort and sleep quality in a hot environment were investigated in an experiment with 16 male subjects. Sleep quality was evaluated subjectively, using questionnaires completed in the morning, and objectively, by analysis of electroencephalogram (EEG) signals that were continuously monitored during the sleeping period. Compared with no cooling, the largest improvement in thermal comfort and sleep quality was observed when the back and head (neck) were both cooled at a room temperature of 32°C. Back cooling alone also improved thermal comfort and sleep quality, although the effects were less than when cooling both back and head (neck). Mean sleep efficiency was improved from 84.6% in the no cooling condition to 95.3% and 92.8%, respectively, in these conditions, indicating good sleep quality. Head (neck) cooling alone slightly improved thermal comfort and subjective sleep quality and increased Stage N3 sleep, but did not otherwise improve sleep quality. The results show that local cooling applied to large body sections (back and head) could effectively maintain good sleep and improve thermal comfort in a hot environment. 相似文献
24.
M.A. Santanna W.T. Menezes Y.V.B. Santana M.M. Ferrer A.F. Gouveia A.D. Faceto A.J. Terezo A.J.A. Oliveira E. Longo R.G. Freitas E.C. Pereira 《International Journal of Hydrogen Energy》2018,43(14):6838-6850
Ordered arrays of TiO2 nanotubes with smooth and rippled morphologies were prepared by one-step titanium oxidation in NH4F and ethylene glycol solution. The samples were then decorated with ZnS using a microwave-assisted solvothermal method. The experiments under constant or pulsed applied voltage resulted in smooth and rippled TiO2material morphologies, respectively. Field emission scanning electron microscopy, incident photon-to-current efficiency, linear sweep voltammetry and electrochemical impedance spectroscopy were used to investigate the structure and morphology of the TiO2 nanotubes, along with their photoelectrochemical activity in the water splitting reaction. An envelope function was proposed to correlate the anisotropic morphologies and broad distribution of mobility due to the random nature of charge carrier transport. The smooth and rippled morphologies were evaluated using the transmission line model. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level are conducted to obtain a better understanding of optical properties of TiO2. 相似文献
25.
Dr. Lukas Kröger Dr. Constantin G. Daniliuc Deeba Ensan Sebastian Borgert Dr. Christian Nienberg Miriam Lauwers Dr. Michaela Steinkrüger Prof. Joachim Jose Dr. Markus Pietsch Prof. Bernhard Wünsch 《ChemMedChem》2020,15(10):871-881
The serine/threonine kinase CK2 modulates the activity of more than 300 proteins and thus plays a crucial role in various physiological and pathophysiological processes including neurodegenerative disorders of the central nervous system and cancer. The enzymatic activity of CK2 is controlled by the equilibrium between the heterotetrameric holoenzyme CK2α2β2 and its monomeric subunits CK2α and CK2β. A series of analogues of W16 ((3aR,4S,10S,10aS)-4-{[(S)-4-benzyl-2-oxo-1,3-oxazolidin-3-yl]carbonyl}-10-(3,4,5-trimethoxyphenyl)-4,5,10,10a-tetrahydrofuro[3,4-b]carbazole-1,3(3aH)-dione ((+)- 3 a )) was prepared in an one-pot, three-component Levy reaction. The stereochemistry of the tetracyclic compounds was analyzed. Additionally, the chemically labile anhydride structure of the furocarbazoles 3 was replaced by a more stable imide ( 9 ) and N-methylimide ( 10 ) substructure. The enantiomer (−)- 3 a (Ki=4.9 μM) of the lead compound (+)- 3 a (Ki=31 μM) showed a more than sixfold increased inhibition of the CK2α/CK2β interaction (protein-protein interaction inhibition, PPII) in a microscale thermophoresis (MST) assay. However, (−)- 3 a did not show an increased enzyme inhibition of the CK2α2β2 holoenzyme, the CK2α subunit or the mutated CK2α′ C336S subunit in the capillary electrophoresis assay. In the pyrrolocarbazole series, the imide (−)- 9 a (Ki=3.6 μM) and the N-methylimide (+)- 10 a (Ki=2.8 μM) represent the most promising inhibitors of the CK2α/CK2β interaction. However, neither compound could inhibit enzymatic activity. Unexpectedly, the racemic tetracyclic pyrrolocarbazole (±)- 12 , with a carboxy moiety in the 4-position, displays the highest CK2α/CK2β interaction inhibition (Ki=1.8 μM) of this series of compounds. 相似文献
26.
Kai Li Tengteng Lyu Junyi He Ben W. L. Jang 《Frontiers of Chemical Science and Engineering》2020,14(6):929
Five hundred ppm Pd/CeO2 catalyst was prepared and evaluated in selective hydrogenation of acetylene in large excess of ethylene since ceria has been recently found to be a reasonable stand-alone catalyst for this reaction. Pd/CeO2 catalyst could be activated in situ by the feed gas during reactions and the catalyst without reduction showed much better ethylene selectivity than the reduced one in the high temperature range due to the formation of oxygen vacancies by reduction. Excellent ethylene selectivity of ~100% was obtained in the whole reaction temperature range of 50°C–200°C for samples calcined at temperatures of 600°C and 800°C. This could be ascribed to the formation of PdxCe1−xO2−y or Pd-O-Ce surface species based on the X-ray diffraction and X-ray photoelectron spectroscopy results, indicating the strong interaction between palladium and ceria. 相似文献
27.
Dr. Kresimir Rupnik Dr. Kazuki Tanifuji Lee Rettberg Prof. Dr. Markus W. Ribbe Prof. Dr. Yilin Hu Prof. Dr. Brian J. Hales 《Chembiochem : a European journal of chemical biology》2020,21(12):1767-1772
The active site of the nitrogen-fixing enzyme Mo-nitrogenase is the M cluster ([MoFe7S9C ⋅ R-homocitrate]), also known as the FeMo cofactor or FeMoco. The biosynthesis of this highly complex metallocluster involves a series of proteins. Among them, NifB, a radical-SAM enzyme, is instrumental in the assembly of the L cluster ([Fe8S9C]), a precursor and all-iron core of the M cluster. In the absence of sulfite, NifB assembles a precursor form of the L cluster called the L* cluster ([Fe8S8C]), which lacks the final ninth sulfur. EPR and MCD spectroscopies are used to probe the electronic structures of the paramagnetic, oxidized forms of both the L and L* clusters, labeled LOx and [ L* ] Ox . This study shows that both LOx and [ L* ] Ox have nearly identical EPR and MCD spectra, thus suggesting that the two clusters have identical structures upon oxidation; in other words, a sulfur migrates away from LOx following oxidation, thereby rendering the cluster identical to [ L* ] Ox . It is proposed that a similar migration could occur to the M cluster upon oxidation, and that this is an instrumental part of both M cluster formation and nitrogenase substrate/inhibitor binding. 相似文献
28.
29.
Toxicity of smoke generated in a fire is difficult to measure accurately. That is because gas sensors for measuring rapidly varying concentrations of toxic gases are not yet developed. Simple expressions are searched for quick measurement in assessing smoke toxicity practically. Four equations on calculating fractional effective dose (FED) related to toxic effluents were reported in the literature, each based on different assumptions. FED value was proposed to be calculated based on peak carbon monoxide concentration and peak carbon dioxide concentration, and transient carbon monoxide, carbon dioxide, and oxygen concentrations. The four values were compared in this article using literature data on toxic gases from different materials measured by (i) cone calorimeter; (ii) full-scale burning tests; and (iii) tunnel full-scale tests. Measured carbon monoxide, carbon dioxide, and oxygen concentrations by standard equipment of oxygen consumption calorimeters were used to calculate the four FED values. It is found that the values of FED based on peak carbon monoxide and carbon dioxide concentrations (denoted as FED2) are similar to the average values of FED calculated from the updated equation in the literature using the oxygen consumption calorimeters. Putting the values of FED2 in fire safety design guides is then recommended. 相似文献
30.
Freddy A. Bernal Dr. Marcel Kaiser Prof. Dr. Bernhard Wünsch Prof. Dr. Thomas J. Schmidt 《ChemMedChem》2020,15(1):68-78
Protozoal infections are still a global health problem, threatening the lives of millions of people around the world, mainly in impoverished tropical and sub-tropical regions. Thus, in view of the lack of efficient therapies and increasing resistances against existing drugs, this study describes the antiprotozoal potential of synthetic cinnamate ester analogues and their structure-activity relationships. In general, Leishmania donovani and Trypanosoma brucei were quite susceptible to the compounds in a structure-dependent manner. Detailed analysis revealed a key role of the substitution pattern on the aromatic ring and a marked effect of the side chain on the activity against these two parasites. The high antileishmanial potency and remarkable selectivity of the nitro-aromatic derivatives suggested them as promising candidates for further studies. On the other hand, the high in vitro potency of catechol-type compounds against T. brucei could not be extrapolated to an in vivo mouse model. 相似文献