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排序方式: 共有1344条查询结果,搜索用时 15 毫秒
31.
Jan Racek Petr Šittner Luděk Heller Jan Pilch Martin Petrenec Petr Sedlák 《Journal of Materials Engineering and Performance》2014,23(7):2659-2668
Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO2 surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking. 相似文献
32.
Lynnette Joseph D. Sajan Venkataraya Shettigar K. Chaitanya Neeraj Misra Tom Sundius I. Němec 《Materials Chemistry and Physics》2013
A new chalcone derivative, 1-(4-aminophenyl)-3-(3,4-dimethoxyphenyl)-prop-2-en-1-one (DMAC) was synthesized and single crystals were grown by slow evaporation technique. The FT-Raman and FT-IR spectra of the sample were recorded in the region 3700–100 cm−1 and 4000–400 cm−1, respectively. The spectra were interpreted with the aid of normal coordinate analysis following structure optimizations and force field calculations based on density functional theory (DFT) at the B3LYP/6-311+G(d,p) level of theory. Normal coordinate calculations were performed using the DFT force field, corrected by a recommended set of scaling factors, yielding fairly good agreement between the observed and calculated wavenumbers. DMAC is thermally stable up to 220.0 °C and optically transparent in the visible region. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). The SHG efficiency of DMAC is observed to be 10 times that of standard urea crystal of identical particle size. 相似文献
33.
I. Turek J. Kudrnovský K. Carva 《Journal of Superconductivity and Novel Magnetism》2013,26(5):1581-1584
Magnetic anisotropies of random body-centered tetragonal Fe–Co alloys are studied by means of relativistic ab initio electronic structure calculations. Special attention is paid to the values and trends of magnetic moments and magnetic anisotropy energies as well as to their sensitivity to current approximations employed. It is found that the high uniaxial anisotropy energy K u, predicted for alloys with 60 % of Co by several authors, is reduced significantly in the coherent potential approximation describing the chemical disorder in real alloys. On the other hand, the four-component Dirac formalism leads to enhanced values of the K u as compared to a simple perturbative treatment of the spin-orbit interaction. Similar behavior is found for the orbital magnetic moments, the values of which are systematically smaller than the measured ones irrespective of the theoretical approach used. 相似文献
34.
Adéla Grégrová Eva Neradová Vojtěch Kružík Jiří Mazáč Pavel Havelec Helena Čížková 《European Food Research and Technology》2014,239(1):169-174
Possible adulteration of canned products containing spirit vinegar pickle by adding synthetic acetic acid is a significant problem of the food industry. Isotope analyses, which determine botanical origin of acetic acid and also can detect synthetic acid, were applied to detect undeclared addition of synthetic acetic acid to canned products. The aim of the study was to improve the extraction technique for the SNIF-NMR (2H/1H; site-specific natural isotopic fractionation-nuclear magnetic resonance) and IRMS (13C/12C; isotope ratio mass spectrometry) isotope methods and for an atypical matrix and to determine isotope ratios in canned vegetables pickle to prove their adulteration or authenticity. The following set of canned products was analysed: pickled cucumbers (n = 16) and one vinegar pickle purchased in the Czech market and six model (cucumber) pickles. The determined ratios of 2H/1H and 13C/12C for the pickled cucumbers proved to be authentic ranged from 89.4 to 107.0 ppm and from ?28.7 to ?15.6 ‰, respectively; for the synthetic acetic acids diluted with water they ranged from 114.2 to 129.0 ppm and from ?44.9 to ?33.4 ‰, respectively. Isotope analyses were confirmed as a reliable tool for assessing authenticity of canned products. The method enables detection of synthetic acetic acid addition into vinegars and canned vegetables containing vinegar pickle up from 20 % (of total acidity). 相似文献
35.
Data mining is sometimes treating data consisting of items representing measurements of a single property taken in different
time points. In this case data can be understood as a time series of one feature. It is no exception when the clue for evaluation
of such data is related to their development trends as observed in several successive time points. From the qualitative point
of view one can distinguish three basic types of behaviour between two neighbouring time points: the value of the feature
is stable (remains the same), it grows or it falls. This paper is concerned with identification of typical qualitative development
patterns as they appear in the windows of given length in the considered time-stamped data and their utilization for specification
of interesting subgroups. 相似文献
36.
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38.
Regio‐ and Enantioselective Sequential Dehalogenation of rac‐1,3‐Dibromobutane by Haloalkane Dehalogenase LinB 下载免费PDF全文
Johannes Gross Prof. Zbyněk Prokop Prof. Dick Janssen Prof. Kurt Faber Dr. Mélanie Hall 《Chembiochem : a European journal of chemical biology》2016,17(15):1437-1441
The hydrolytic dehalogenation of rac‐1,3‐dibromobutane catalyzed by the haloalkane dehalogenase LinB from Sphingobium japonicum UT26 proceeds in a sequential fashion: initial formation of intermediate haloalcohols followed by a second hydrolytic step to produce the final diol. Detailed investigation of the course of the reaction revealed favored nucleophilic displacement of the sec‐halogen in the first hydrolytic event with pronounced R enantioselectivity. The second hydrolysis step proceeded with a regioselectivity switch at the primary position, with preference for the S enantiomer. Because of complex competition between all eight possible reactions, intermediate haloalcohols formed with moderate to good ee ((S)‐4‐bromobutan‐2‐ol: up to 87 %). Similarly, (S)‐butane‐1,3‐diol was formed at a maximum ee of 35 % before full hydrolysis furnished the racemic diol product. 相似文献
39.
Fei Huang Tomáš Halenkovič Marion Baillieul Virginie Nazabal Petr Němec Josef Havel 《Journal of the American Ceramic Society》2022,105(3):1992-2000
Quaternary germanium-antimony-selenium-tellurium (Ge-Sb-Se-Te) thin films deposited from Ge19.4Sb16.7Se63.9−xTex (x = 5, 10, 15, and 20) glass-ceramics targets by radio frequency magnetron sputtering were studied using laser ablation quadrupole ion trap time of flight mass spectrometry. Binary, ternary, and quaternary GeaSbbSecTed clusters were formed and their stoichiometry was determined. By comparison of the clusters obtained from quaternary Ge-Sb-Se-Te thin films and those from ternary Ge-Sb-Te materials, we found that Ge-Te species are not detected from the quaternary system. Furthermore, Ge-Se and Se-Te species are missing in mass spectra generated from Ge-Sb-Se-Te thin films. From the Ge-Sb-Se-Te thin films, 16 clusters were detected while ternary Ge-Sb-Se glasses yielded 26 species. This might be considered as a signal of higher stability of Ge-Sb-Se-Te thin films which is increasing with a higher content of Te. The missing (Se2+, GeaSb+ (a = 1–4), and GeSec+ (c = 1, 2)) and new (Ge+ and SbbTe+ (b = 1–3)) clusters may indicate that some of the structural features of the films (Ge2Se6/2 and Se2Sb-SbSe2) were replaced by (GeSe4−xTex and SbSe3−xTex) ones. In addition, when comparing the stoichiometry of clusters formed from Ge-Sb-Se-Te thin films with those from the mixtures of the elements, only Sb3+ and SbSe+ were observed in both cases. The knowledge gained concerning clusters stoichiometry contributes to the elucidation of the processes proceeding during plasma formation used for the chalcogenide thin films deposition. 相似文献
40.
Sellamuthu N. Jaisankar Neelamegan Haridharan Adhigan Murali Ponyrko Sergii Milena Špírková Asit Baran Mandal Libor Matějka 《Polymer》2014
The sidewall functionalization of single-walled carbon nanotubes (SWCNTs) was established by poly (methyl methacrylate) (PMMA) using controlled radical polymerization (CRP) method. A bromide functionalized SWCNT (SWCNT-Br) has been used as an initiator for the synthesis of SWCNT-graft-PMMA (SWCNT-g-PMMA). The efficiency of the sacrificial initiator (S) was monitored during the polymerization process. The obtained polymers possess a uniform distribution of molecular weight with a lower polydispersity index of 1.36. The SWCNTs-based initiator acts as an efficient medium for the controlled growth of polymer on the SWCNTs surfaces. The presence of bimodal gel permeation chromatographic (GPC) curve for the SWCNT-g-PMMA(S) obtained through sacrificial initiator (S) confirms uncontrolled behavior. However, the clear sharp peak for SWCNT-g-PMMA obtained without sacrificial initiator shows its well-controlled process of polymerization, which acts as a mimic to bone cement. The efficiency of the functionalization of SWCNT and the controlled formation of SWCNT-g-PMMA composites were characterized by TGA, Raman, TEM, NMR, XPS and dispersion measurements. 相似文献