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81.
《国际聚合物材料杂志》2012,61(1-2):25-32
Abstract Circular dichroism has become a popular method for following conformational transitions induced in optically active polymers. Recent refinement of experimental spectra obtained from model polypeptides by computer fitting to spectra obtained from solutions of proteins of known conformation has verified the applicability of the models chosen for the α helical and β conformations. However, the spectrum required for disordered regions was in conflict with much of the literature and agrees with our assignment based on studies of collagen and collagen models at elevated temperature. This spectrum consists of two troughs, one at ~225 nm and the other at ~200 nm. The latter had previously been associated with random polypeptides and we have shown the former to be sensitive to disorder in polymers approaching the 31 helical conformation. This paper presents the results from a study of three polypeptides which undergo a 31 helix → disorder transition. 相似文献
82.
Triclosan (TCS), an antimicrobial agent, is an emerging and persistent environmental pollutant that is often found as a contaminant in surface waters and sediments; hence, knowledge of its degradability is important. In this study we investigated laccase-mediated TCS transformation and detoxification, using laccase (from the fungus Ganoderma lucidum) in the presence and absence of redox mediators. Transformation products were identified using HPLC, ESI-MS and GC-MS, and transformation mechanisms were proposed. In the absence of redox mediator, 56.5% TCS removal was observed within 24 h, concomitant with formation of new products with molecular weights greater than that of TCS. These products were dimers and trimers of TCS, as confirmed by ESI-MS analysis. Among the various mediators tested, 1-hydroxybenzotriazole (HBT) and syringaldehyde (SYD) significantly enhanced TCS transformation (∼90%). The presence of these mediators resulted in products with lower molecular weights than TCS, including 2,4-dichlorophenol (2,4-DCP; confirmed by GC-MS) and dechlorinated forms of 2,4-DCP. When SYD was used as the mediator, dechlorination resulted in 2-chlorohydroquinone (2-CHQ). Bacterial growth inhibition studies revealed that laccase-mediated transformation of TCS effectively decreased its toxicity, with ultimate conversion to less toxic or nontoxic products. Our results confirmed the involvement of two mechanisms of laccase-catalyzed TCS removal: (i) oligomerization in the absence of redox mediators, and (ii) ether bond cleavage followed by dechlorination in the presence of redox mediators. These results suggest that laccase in combination with natural redox mediator systems may be a useful strategy for the detoxification and elimination of TCS from aqueous systems. 相似文献
83.
Active centers, catalytic behavior, symbiosis and redox properties of MoV(Nb,Ta)TeO ammoxidation catalysts 总被引:1,自引:0,他引:1
Robert K. Grasselli Douglas J. Buttrey James D. Burrington Arne Andersson Johan Holmberg Wataru Ueda Jun Kubo Claus G. Lugmair Anthony F. Volpe Jr 《Topics in Catalysis》2006,38(1-3):7-16
Selective as well as waste forming active centers were defined for MoVNbTeO and MoVTaTeO catalysts in the ammoxidation of
propane to acrylonitrile and all catalytic functionalities were assigned to specific elements at the respective active centers.
Symbiosis between M1 and M2 phases of these catalysts was observed, with phase cooperation being more extensive in the Nb than Ta containing compositions.
The difference in catalytic effectiveness arises most likely because contact and surface area exposure of the two respective,
cooperating phase pairs are not equal. The M1 phase of the catalysts is reducible by propane and ammonia in the absence of dioxygen and is regenerable to its original,
fully oxidized state by dioxygen (air). No structural collapse is observed even after 120 C3H8 + NH3 reduction pulses. The so induced reduction of the catalyst extends up to 70 layers deep. The product distribution over the
first few pulses is very similar to that under catalytic conditions, supporting the concept that lattice oxygen is involved
in the catalytic ammoxidation process. Therefore, the ammoxidation of paraffins is a redox process, as is of course the well-known
olefin ammoxidation process. 相似文献
84.
Saccharomyces cerevisiae reduces the beta-keto ester ethyl 2-chloroacetoacetate to the respective chiral cis- and trans-beta-hydroxy esters. In the course of chiral reduction, competing dehalogenation of the xenobiotic substrate to ethyl acetoacetate occurs, in a reaction mediated by cytosolic glutathione (GSH). Mechanistically, the dechlorination is a novel type of glutathione-dependent dehalogenation catalysed by an as yet unidentified glutathione-dependent dehalogenase. The first step consists of a nucleophilic replacement of the chloride substituent by glutathione. In the subsequent enzyme-catalysed step, a second glutathione molecule liberates the dehalogenation product by thiolytic attack at the thioether bridge, resulting in a net transfer of two electrons to the substrate and in the formation of glutathione disulfide (GSSG). Being effective under aerobic conditions and catalysed by a fungus, this reductive dechlorination of an aliphatic substrate is an outstanding example of a novel, glutathione-mediated microbial dehalogenation. 相似文献
85.
86.
87.
It was established that the addition of Mg to the mixtures of kaolinite and bentonite can provide clay compositions which, after sintering at high temperatures, produce very porous ceramics with microcrystalline and amorphous regions of stoichiometric and nonstoichiometric oxides and highly developed metallized surfaces (in this case mainly with magnesium surplus). The ceramics surface exhibited high electrochemical and chemical activity when placed in contact with water. This leads to pH, EROX and rH changes of the treated waters, which can be controlled (magnesium aluminium silicate ceramics reduces or oxidises waters). In fact, with careful planning and appropriate selection of active components of the aluminium silicate ceramics it is possible to build in tailor made electric capacity, electrochemical activity and consequently program their effects on the structure and the content of the waters it interacts with. 相似文献
88.
在醋酸乙烯-乙烯共聚乳液(vAE乳液)聚合过程中添加有机硅氧烷单体制备改性VAE乳液。对比分析了不同种类、不同用量的有机硅氧烷对改性vAE乳液性能的影响。确定了有机硅氧烷单体在vAE乳液聚合过程中的最佳添加方式,筛选了聚合引发体系。研究表明,在反应过程中以H2O2--ZFS作为氧化-还原引发体系,选用长链的含水解阻碍官能团的硅氧烷,有机硅氧烷采用后添加的方式,能够合理而有效地控制反应节奏。制备出性能优异的改性VAE乳液。 相似文献
89.
Mahadevaiah T. Demappa 《应用聚合物科学杂志》2007,103(6):3498-3505
The kinetics of polymerization of methylmethacrylate initiated by cerium(IV)–lactic acid redox system was studied in an aqueous medium in the temperature range of 25–50°C. The rate of polymerization (Rp) and the rate of cerium(IV) disappearance have been measured. The effects of some water‐miscible organic solvents, cationic, anionic, nonionic surfactants, and complexing agents on the rate of polymerization were investigated. The temperature dependence of the rate was studied, and the activation parameters were computed using the Arrhenius and Eyring plots. The effects of inorganic and organic solvents on polymerization were also investigated. All of them depressed both the initial rate and limiting conversion. A mechanism consistent with the experimental data, involving cerium(IV)–lactic acid complex formation, which generates free radicals, is suggested. The chain termination step of the polymerization reaction is by mutual interaction of the growing macromolecules. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3498–3505, 2007 相似文献
90.
R. Du Plessis J.D. Miller J.C.Davidtz . Department of Metallurgical Engineering University of Utah Salt Lake City Utah USA . Department of Materials Science Metallurgical Engineering University of Pretoria Pretoria 《中国有色金属学会会刊》2000,10(Z1):12-18
1 INTRODUCTIONSincethe 192 0’s ,thedithiocarbonates (xan thatesorDTC)havebeenthepopularcollectorsofchoiceforsulfidemineralflotation .Theyhaveseveraladvantagesoverothercollectorsietheyareinexpen sive ,easytoproduce ,availableinsolidform ,totallywater solubleandv… 相似文献