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51.
In-situ EXAFS studies of sulphided Mo/TiO2 catalysts have shown that at low Mo loadings (2–4 wt%), an active species with a short Mo-S distance of 2.25 Å is formed, while on Mo/TiO2 with high Mo loadings as well as on Mo/gif" alt="gamma" align="MIDDLE" BORDER="0">-Al2O3, bulk MoS2 (Mo-S, 2.42 Å) is formed. The species with the short Mo-S distance has Mo in an oxidation state close to 6 + and is likely to result from the sulphidation of the tetrahedral molybdate species present in the oxidic precursor at low Mo loadings. The calcination temperature of the oxidic precursor appears crucial, a high calcination temperature of 973 K favouring the formation of MoS3 on sulphidation, and a low calcination temperature of 623 K favouring MoS2.Contribution No. 790 from the Solid State and Structural Chemistry Unit. 相似文献
52.
G. Moulin C. Mons C. Severac C. Haut G. Rautureau E. Beauprez 《Oxidation of Metals》1993,40(1-2):85-108
Nickel-base alloys, such as Astroloy, used for aeronautical turbine disks, are sensitive to time-dependent cracking in environments containing oxygen. The gif" alt="ldquo" align="MIDDLE" BORDER="0">mosaicgif" alt="rdquo" align="MIDDLE" BORDER="0"> structure of the alloy consisting gif" alt="gamma" align="MIDDLE" BORDER="0">gif" alt="prime" align="BASELINE" BORDER="0"> islands (200 nm average size) surrounded by the gif" alt="gamma" align="MIDDLE" BORDER="0">-phase (100 nm thick) induces complex oxidation phenomena. Various analytical approaches allow the delineation of all the steps from segregation to oxidation occurring on the surface of such a duplex structure. The protection of Astroloy by its outer oxide layer against oxygen penetration was studied also, using alternative
16O2
then
18O2
oxidation. In association with STEM studies, it is shown that the outer oxide scale is not a real barrier against oxygen penetration and that inner precipitation of chronium (+ aluminium and titanium)-enriched oxides, takes place especially in the gif" alt="gamma" align="MIDDLE" BORDER="0"> structure. 相似文献
53.
The oxidation behavior of a normalized 2.25Cr-1Mo steel tempered previously for 10 hr at different temperatures between 873 and 1023 K has been studied up to a maximum duration of 1000 hr in air at 773–973 K. The oxidation resistance of the steel was found to decrease significantly with the temperature of tempering. Tempering of this steel is reported to cause microstructural changes involving precipitation of Cr as carbides and a decrease in the effective (free) Cr contents, that could influence the oxidation resistance of the Cr-containing alloys. Relative compositions across the thickness of the oxide scales, as analyzed by SEM/EDX and SIMS, suggest that a less Cr-rich (and less protective) and thicker scale on the steel formed because previous tempering caused extensive depletion of free Cr. 相似文献
54.
This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior ofgif" alt="beta" align="MIDDLE" BORDER="0">-NiAl + gif" alt="agr" align="BASELINE" BORDER="0">-Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in thegif" alt="beta" align="MIDDLE" BORDER="0"> phase and causing the interdendritic gif" alt="agr" align="BASELINE" BORDER="0">-Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the gif" alt="agr" align="BASELINE" BORDER="0">-Cr precipitates fromthe subsurface region of the alloy. The maintenance of gif" alt="agr" align="BASELINE" BORDER="0">-Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl2O3-rich scale. 相似文献
55.
The effect of an NiAl microcrystalline coating prepared by magnetron sputtering on the high-temperature oxidation behavior of NiAl–28Cr–5Mo–1Hf was investigated in static air at 1000–1150°C. The additions of Cr, Mo, and Hf changed the single gif" alt="beta" align="MIDDLE" BORDER="0">-phase structure into a multiphase structure [gif" alt="beta" align="MIDDLE" BORDER="0">-NiAl, gif" alt="agr" align="BASELINE" BORDER="0">-Cr(Mo), and Heusler phase]. The NiAl–28Cr–5Mo–1Hf alloy formed a nonprotective mixed scale of Al2O3+Cr2O3+HfO2 and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance of NiAl–28Cr–5Mo–1Hf due to the formation of a compact and adherent Al2O3 scale at all test temperatures. It was found that the gif" alt="theta" align="BASELINE" BORDER="0">-gif" alt="agr" align="BASELINE" BORDER="0">-Al2O3 transformation caused the anomalous behavior of the oxidation–kinetics curves of the NiAl microcrystalline coating in the temperature range 1000–1150°C. A change in the morphology of scales occurred with the transformation. 相似文献
56.
The influence of externally located platinum on the isothermal stability of gif" alt="agr" align="BASELINE" BORDER="0">-Al2O3 scales formed at high temperatures has been examined. It has been observed that a nickel-base alloy forms an external scale of gif" alt="agr" align="BASELINE" BORDER="0">-Al2O3 during oxidation at 1200°C, but this scale breaks down isothermally, enabling a faster-growing Cr2O3-rich scale to develop. However, in the presence of platinum metal alongside the specimen in the furnace hot zone, the breakdown of the gif" alt="agr" align="BASELINE" BORDER="0">-Al2O3 scale is postponed for a substantial period of time. It appears that platinum, as the volatile species PtO2, is incorporated into the growing gif" alt="agr" align="BASELINE" BORDER="0">-Al2O3 scale where it either influences the stress relief mechanism at temperature or reduces oxidation growth stress generation and thus significantly enhances the isothermal stability of the scale. 相似文献
57.
58.
59.
To reconstruct a black box multivariate sparse polynomial from its floating point evaluations, the existing algorithms need to know upper bounds for both the number of terms in the polynomial and the partial degree in each of the variables. Here we present a new technique, based on Rutishauser’s qd-algorithm, in which we overcome both drawbacks. 相似文献
60.
Vulnerability to sudden service disruptions due to deliberate sabotage and terrorist attacks is one of the major threats of today. In this paper, we present a bilevel formulation of the r-interdiction median problem with fortification (RIMF). RIMF identifies the most cost-effective way of allocating protective resources among the facilities of an existing but vulnerable system so that the impact of the most disruptive attack to r unprotected facilities is minimized. The model is based upon the classical p-median location model and assumes that the efficiency of the system is measured in terms of accessibility or service provision costs. In the bilevel formulation, the top level problem involves the decisions about which facilities to fortify in order to minimize the worst-case efficiency reduction due to the loss of unprotected facilities. Worst-case scenario losses are modeled in the lower-level interdiction problem. We solve the bilevel problem through an implicit enumeration (IE) algorithm, which relies on the efficient solution of the lower-level interdiction problem. Extensive computational results are reported, including comparisons with earlier results obtained by a single-level approach to the problem. 相似文献