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21.
22.
We have used in vivo balloon catheterization in combination with in vitro organ culture to develop a model system for vascular neointima formation. A Fogarty balloon catheter was used to deendothelialize and rupture the internal elastic lamina of aortae in adult rabbits. After three d of recovery, aortae were harvested, divided into segments, and placed into organ culture. We obtained a daily index of cell proliferation in cultured vessels using [3H]thymidine incorporation into DNA. Also, segments were collected and processed for routine histology or immunohistochemistry. Aortic segments that had undergone ballooning 3 d before harvest and then cultured exhibited diffuse neointimal growth after several d in vitro, whereas those from sham-operated (nonballooned) rabbits showed generally only a single endothelial cell layer that is characteristic of normal intima. Aortae that were harvested, balloon-damaged in vitro, and then cultured exhibited no neointimal growth. The neointima that developed in cultured segments from in vivo ballooned rabbits was primarily of smooth muscle cell origin as determined by positive immunostaining for alpha-smooth muscle actin. The intima:media thickness ratios were significantly higher in aortic segments from ballooned rabbits at harvest and after 4 or 7 d in culture compared with those from nonballooned rabbits. Also, the [3H]thymidine index was higher in the in vivo ballooned aorta compared to non-ballooned or in vitro ballooned vessel. We conclude that ballooning in vivo followed by exposure to blood-borne elements produces an enhanced proliferative response in cultured vessels that is distinct from other in vitro models of neointimal growth. 相似文献
23.
Children with sickle-cell anaemia are predisposed to thrombotic strokes, the aetiology of which is unclear. We propose that erythropoietin, produced in response to chronic anaemia, is responsible for changes in platelet reactivity with a resulting increase in thromboses. This hypothesis is based on reports of enhanced aggregability of erythropoietin-driven platelets and an increased rate of thrombosis in patients receiving large doses of recombinant erythropoietin. Experiments in animals have shown that erythropoietin stimulates synthesis of platelets, that erythropoietin-driven platelets are hyper-reactive compared with age-matched control platelets, and that erythropoietin is pro-thrombotic. These data suggest that erythropoietin-dependent changes in platelet reactivity may potentiate thrombosis in sickle-cell anaemia, particularly in children who, compared with adults, have markedly higher erythropoietin concentrations and incidence of strokes. 相似文献
24.
Ectomography, a general tomographic method currently being implemented in nuclear medicine, is described. The object is viewed from different directions from the same projection angle, and the reconstruction process can be described as a two-dimensional filtered back-projection technique. One limitation of this limited-angle approach is that a three-dimensional Fourier representation of the acquired projections has a cone where data are missing. This empty cone can cause distortions in the tomogram by insufficient elimination of details outside the reconstructed tomographic section and by distortion of details in the section. The degree of distortion is dependent on the extension of the object in all directions and on the projection angle. Despite these limitations ectomography can be superior to single-photon-emission computed tomography for imaging sections that are close and parallel to the surface of the object, as in imaging of the hip joint or the heart. 相似文献
25.
Dale A. Creaser Philip G. Harrison M. A. Morris B. A. Wolfindale 《Catalysis Letters》1994,23(1-2):13-24
X-ray photoelectron spectroscopy has been used to examine the nature of the oxide overlayers on a passivated cerium metal foil as a function of a variety of oxidation and reduction treatments. Oxidation of a clean uncontaminated cerium(III) oxide surface is facile at room temperature and produces non-stoichiometric ceria (CeO2–x) at oxygen doses as low as 10 L. At higher doses the overlayer thickens, and after a dose of 160 L the layer depth exceeds the Ce 3d photoelectron attenuation distance of about 20 Å. High pressure treatment of the foil in oxygen (0.5 bar at RT and 473 K) produces CeO2 in a high degree of crystallographic order such that O 1s photoelectron intensities are increased above that expected from a randomly oriented powder. An attempt to reduce the CeO2 layer formed by controlled oxidation with CO (633 K, 14 h, 0.6 bar) results in the formation of a carbonated surface layer. Results following attempts to reoxidise this layer are discussed. 相似文献
26.
Dale W Schaefer 《Polymer》1984,25(3):387-394
A model is developed to analyse the concentration dependence of the range ξ? of the monomer pair correlation function. In semidilute solution, three concentration regimes are found for semiflexible molecules and the crossover points between the various regimes are predicted in terms of the characteristic ratio of the chains in dilute solution and in terms of the Flory interaction parameter χ. A simple physical interpretation is given which explains the concentration dependence of ξ? based on binary contacts initially and then ternary contacts at higher concentration. Temperature-concentration diagrams are developed for several common polymer-solvent systems. 相似文献
27.
Roper David I.; Moreton Kathleen M.; Wigley Dale B.; Holbrook J.John 《Protein engineering, design & selection : PEDS》1992,5(7):611-615
A mutant Bacillus stearothermophilus lactate dehydrogenase hasbeen prepared in which all three tryptophan residues in thewild-type enzyme have been replaced by tyrosines. In addition,a tyrosine residue has been mutated to a tryptophan, which actsas a fluorescence probe to monitor protein folding. The mutantenzyme crystallizes in the same crystal form as the wild-type.The crystal structure of the mutant has been determined at 2.8Å resolution. Solution studies have suggested that thereis little effect upon the mutant enzyme as judged by its kineticproperties. Comparison of the crystal structures of the mutantand wild-type enzymes confirms this conclusion, and revealsthat alterations in structure in the region of these mutationsare of a similar magnitude to those observed throughout thestructure, and are not significant when compared with the errorsin atomic positions expected for a structure at this resolution. 相似文献
28.
G. Dale Cheever 《应用聚合物科学杂志》1975,19(1):147-163
Techniques have been developed to measure the acceptance and decay of electrostatic charge on a wide variety of powder coatings. The measurement of the initial charge carried by the powder was affected by the amount of powder deposited. The decay of charge occurred within four time scales: (1) 1 sec, electron loss to air and to the metallic substrate; (2) 1 to 5 sec, electron flow to neutralize induced positive charge; (3) 5 to 100 sec, rapid decay from powder; and (4) 1,000 to 10,000 sec, very long term decay. Since the decay of charge was markedly affected by moisture and composition, the decay properties can be controlled by these parameters. A mathematical model was constructed describing the quantitative charge decay during the time period of 100 to 10,000 sec. There was considerable heterogeneity within the powders with regard to charge acceptance and interaction with moisture. These measurement techniques give information concerning electrostatic effects on particle transport, adhesion, and polymer electrostatic behavior which are very important variables in a powder coating process. 相似文献
29.
Modifying milk fat composition of dairy cows to enhance fatty acids beneficial to human health 总被引:4,自引:3,他引:4
There is increased consumer awareness that foods contain microcomponents that may have beneficial effects on health maintenance
and disease prevention. In milk fat these functional food components include EPA, DHA, and CLA. The opportunity to enhance
the content of these FA in milk has improved as a result of recent advances that have better defined the interrelationships
between rumen fermentation, lipid metabolism, and milk fat synthesis. Dietary lipids undergo extensive hydrolysis and biohydrogenation
in the rumen. Milk fat is predominantly TG, and de novo FA synthesis and the uptake of circulating FA contribute nearly equal amounts (molar basis) to the FA in milk fat. Transfer
of dietary EPA and DHA to milk fat is very low (<4%); this is, to a large extent, related to their extensive biohydrogenation
in the rumen, and also partly due to the fact that they are not transported in the plasma lipid fractions that serve as major
mammary sources of FA uptake (TG and nonesterified FA). Milk contains over 20 isomers of CLA but the predominant one is cis-9,trans-11 (75–90% of total CLA). Biomedical studies with animal models have shown that this isomer has anticarcinogenic and anti-atherogenic
activities. cis-9,trans-11-CLA is produced as an intermediate in the rumen biohydrogenation of linoleic acid but not of linolenic acid. However,
it is only a transient intermediate, and the major source of milk fat CLA is from endogenous synthesis. Vaccenic acid, produced
as a rumen biohydrogenation intermediate from both linoleic acid and linolenic acid, is the substrate, and Δ9-desaturase in
the mammary gland and other tissues catalyzes the reaction. Diet can markedly affect milk fat CLA content, and there are also
substantial differences among individual cows. Thus, strategies to enhance milk fat CLA involve increasing rumen outflow of
vaccenic acid and increasing Δ9-desaturase activity, and through these, several-fold increases in the content of CLA in milk
fat can be routinely achieved. Overall, concentrations of CLA, and to a lesser extent EPA and DHA, can be significantly enhanced
through the use of diet formulation and nutritional management of dairy cows. 相似文献
30.