全文获取类型
收费全文 | 5363篇 |
免费 | 418篇 |
国内免费 | 12篇 |
专业分类
电工技术 | 77篇 |
综合类 | 4篇 |
化学工业 | 1265篇 |
金属工艺 | 191篇 |
机械仪表 | 302篇 |
建筑科学 | 104篇 |
矿业工程 | 2篇 |
能源动力 | 194篇 |
轻工业 | 407篇 |
水利工程 | 20篇 |
石油天然气 | 17篇 |
无线电 | 1049篇 |
一般工业技术 | 1124篇 |
冶金工业 | 392篇 |
原子能技术 | 53篇 |
自动化技术 | 592篇 |
出版年
2024年 | 9篇 |
2023年 | 32篇 |
2022年 | 77篇 |
2021年 | 160篇 |
2020年 | 116篇 |
2019年 | 97篇 |
2018年 | 164篇 |
2017年 | 185篇 |
2016年 | 179篇 |
2015年 | 183篇 |
2014年 | 237篇 |
2013年 | 374篇 |
2012年 | 341篇 |
2011年 | 396篇 |
2010年 | 288篇 |
2009年 | 303篇 |
2008年 | 298篇 |
2007年 | 222篇 |
2006年 | 208篇 |
2005年 | 159篇 |
2004年 | 174篇 |
2003年 | 141篇 |
2002年 | 117篇 |
2001年 | 131篇 |
2000年 | 116篇 |
1999年 | 98篇 |
1998年 | 181篇 |
1997年 | 138篇 |
1996年 | 98篇 |
1995年 | 76篇 |
1994年 | 81篇 |
1993年 | 63篇 |
1992年 | 44篇 |
1991年 | 49篇 |
1990年 | 31篇 |
1989年 | 38篇 |
1988年 | 29篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 8篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1977年 | 9篇 |
1976年 | 13篇 |
1975年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有5793条查询结果,搜索用时 31 毫秒
961.
This paper proposes an efficient fixture modelling procedure for automotive body assembly lines. A fixture model consists of two sub-models; a geometric model and a kinetic model that should be remodelled frequently whenever design changes occur. We develop an algorithm extracting the kinetic model from the geometric model of a fixture to reduce the fixture modelling time and effort. Although the geometric models of fixtures used in automotive assembly lines vary, most follow the same kinetic mechanism, the so-called slider-crank mechanism; this is a four-axis system of three revolute and one prismatic joint. The prismatic axis of a fixture represents a pneumatic actuator involving a piston and a cylinder. It is very important to identify the prismatic axis from a given geometric model to extract the kinetic model of a fixture. We use the concept of the ‘moment of inertia’, which is a measure of an object's resistance to changes in its rotation rate, to identify the prismatic axis. Since the exact computation of the moment of inertia for an arbitrary solid model requires complicated computations, we introduce an approximating method for the moment of inertia. The proposed procedure has been implemented and tested with various examples. 相似文献
962.
The main activities of container terminals are to load outbound containers on to vessels, discharge inbound containers from vessels, and store those containers in the yard before loading (or after discharging) them. This study proposes a method for determining an optimal layout of container yards taking into consideration the storage space requirements and throughput capacities of yard cranes and transporters. Two types of yard layout are under consideration: a layout where blocks are laid out parallel to the quay and transfer points are located beside a bay per block, and a layout whose blocks are laid out perpendicular to the quay and transfer points are located at both ends of each block. Various cost factors are used for optimizing the yard layout, which include the construction cost of the ground space, the fixed overhead cost of yard cranes, and the operating costs of yard cranes and transporters. Sensitivity analysis is performed to investigate the effect of various design parameters on the optimal layout of the yard. The two types of yard layout are compared with each other, and the results of this study are compared against those for real-world yard layouts from the perspective of throughput capacity and storage space capacity. 相似文献
963.
An efficient multi-start algorithm for global optimization is developed by introducing multi-dimensional simplexes as new expression units of attraction regions. The region elimination method generally consists of making a set of eliminated regions called attraction regions, checking adjacency between the current design point and the attraction region, and quitting local optimization for the attracted design points. The efficiency of the elimination method is considerably enhanced by supplementing general simplexes and their neighborhoods to conventional units of attraction regions of points and lines. To show the effectiveness of the proposed algorithm, mathematical problems from the literature are solved and the results are compared with several well-known multi-start algorithms. The present algorithm produces the global optimum in all problems more efficiently than the variants of the multi-start method. Several types of truss, frame, and composite material structures are optimized as engineering applications. Many local optima are found and the differences among the local optima are not negligibly small. These results suggest that an efficient and reliable global optimizer is strongly required in some fields of engineering optimization. 相似文献
964.
Transition metal oxides have been suggested as innovative, high-energy electrode materials for lithium-ion batteries because
their electrochemical conversion reactions can transfer two to six electrons. However, nano-sized transition metal oxides,
especially Co3O4, exhibit drastic capacity decay during discharge/charge cycling, which hinders their practical use in lithium-ion batteries.
Herein, we prepared nano-sized Co3O4 with high crystallinity using a simple citrate-gel method and used electrochemical impedance spectroscopy method to examine
the origin for the drastic capacity fading observed in the nano-sized Co3O4 anode system. During cycling, AC impedance responses were collected at the first discharged state and at every subsequent
tenth discharged state until the 100th cycle. By examining the separable relaxation time of each electrochemical reaction
and the goodness-of-fit results, a direct relation between the charge transfer process and cycling performance was clearly
observed. 相似文献
965.
A dimethylsilylene‐bridged metallocene complex, (CH3)2Si(Ind)2ZrCl2, was supported on a nanosized silica particle, whose surface area was mostly external. The resulting catalyst was used to catalyze the polymerization of propylene to polypropylene. Under identical reaction conditions, a nanosized catalyst exhibited much better polymerization activity than a microsized catalyst. At the optimum polymerization temperature of 55°C, the former had 80% higher activity than the latter. In addition, the nanosized catalyst produced a polymer with a greater molecular weight, a narrower molecular weight distribution, and a higher melting point in comparison with the microsized catalyst. The nanosized catalyst's superiority was ascribed to the higher monomer concentration at its external active sites (which were free from internal diffusion resistance) and was also attributed to its much larger surface area. Electron microscopy results showed that the nanosized catalyst produced polymer particles of similar sizes and shapes, indicating that each nanosized catalyst particle had uniform polymerization activity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献
966.
Jae Song Cho Byung Soo Kim Hoon Hyun Moon Suk Kim Jae Hoon Ko Gilson Khang 《Polymer》2008,49(7):1777-1782
The polymerization of trimethylene carbonate (TMC) in the presence of HCl·Et2O via activated monomer mechanism was performed to synthesize 4a-PEG-b-PTMC star block copolymers composed of poly(ethylene glycol) (PEG) and poly(trimethylene carbonate) (PTMC) using four-arm (4a) PEG as an initiator. The TMC conversion and molecular weight of PTMC increased linearly with the polymerization time or the feed ratios of the TMC to 4a-PEG in the presence of HCl·Et2O in CH2Cl2 at 25 °C. The obtained PTMC had molecular weights close to the theoretical value calculated from TMC to PEG molar ratio and exhibited monomodal GPC curve. We prepared successfully 4a-PEG-b-PTMC star block copolymers without metal catalyst at room temperature via living ring-opening polymerization (ROP) of TMC from 4a-PEG as an initiator in the presence of HCl·Et2O as a monomer activator. The CMCs of the 4a-PEG-b-PTMC star block copolymers determined from fluorescence measurements. The CMCs of the 4a-PEG-b-PTMC star block copolymers decreased in the order of the increase in the PTMC segment. The partition equilibrium constant, Kv, which is an indicator of the hydrophobicity of the micelles of the 4a-PEG-b-PTMC star block copolymers in aqueous media, increased with the increase in the PTMC segment. In conclusion, we confirmed that the 4a-PEG-b-PTMC star block copolymers form micelles and hence may be potential hydrophobic-drug delivery vehicles. 相似文献
967.
Dense ultrafine NbSi2–SiC composite was synthesized by the pulsed current activated combustion synthesis (PCACS) method within 3 min in one step from mechanically activated powders of NbC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense NbSi2–SiC composite with relative density of up to 97% was produced under simultaneous application of a 60 MPa pressure and the electric current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated. 相似文献
968.
Silicon oxidation by the aerial diffusion of active oxygen species from a UV-irradiated TiO2 surface was evaluated and characterized. The key point was to confirm the oxidation possibility of inorganic materials such
as silicon under a photocatalytic remote scheme. In this study, it was confirmed that the remote oxidation of silicon substrates
would occur by the aerial diffusion of active oxygen species from UV-irradiated TiO2 surfaces, and that the oxides have comparable properties to the thermally grown oxide. Remote oxidation using UV-irradiated
TiO2 is shown to be a viable alternative method for the fabrication of nano-scale silicon oxide. 相似文献
969.
Yu-Chih Lin Chung-Liang Chang Tser-Sheng Lin Hsunling Bai Ming-Gu Yan Fu-Hsiang Ko Chia-Tien Wu Cheng-Hsiung Huang 《Korean Journal of Chemical Engineering》2008,25(3):446-450
This study utilized the activated carbon fiber (ACF) modified with metal catalyst via physical vapor deposition (PVD) process
(ACF/PVD) to diminish ozone. Furthermore, the ozone removal efficiency of ACF/PVD was compared with that of original ACF and
ACF modified with metal catalyst via impregnation process (ACF/impregnation). In addition to the kinds of coated metal and
the inlet ozone concentrations, the effects of the coating thickness and the reaction temperature on ACF/PVD for ozone removal
were also examined. The results indicate that the ozone removal efficiency of ACF/PVD is better than that of original ACF
and ACF/impregnation. The ozone removal efficiency of different metal-coated ACF/PVD in the superior order is gold (Au), and
manganese (Mn). The increase of Au-coated thickness (3 nm to 80 nm) on ACF/PVD will enhance the ozone removal. However, when
the Mn-coated thickness on ACF/PVD is larger than 15 nm, the ozone removal efficiency displays a declining trend. Furthermore,
a higher reaction temperature will result in a better ozone removal of ACF/PVD and the original ACF. 相似文献
970.
Seok Ku Jeon Chan Seung Park Sang Done Kim Byung Ho Song Joseph M. Norbeck 《Korean Journal of Chemical Engineering》2008,25(6):1279-1285
Steam-methane reforming (SMR) reaction was studied using a tubular reactor packed with NiO/γ-Al2O3 catalyst to obtain synthesis gases with H2/CO ratios optimal for the production of synthetic diesel fuel from steam-hydrogasification of carbonaceous materials. Pure
CH4 and CH4-CO2 mixtures were used as reactants in the presence of steam. SMR runs were conducted at various operation parameters. Increasing
temperature from 873 to 1,023 K decreased H2/CO ratio from 20 to 12. H2/CO ratio decreased from 16 to 12 with pressure decreasing from 12.8 to 1.7 bars. H2/CO ratio also decreased from about 11 to 7 with steam/CH4 ratio of feed decreasing from 5 to 2, the lowest limit to avoid severe coking. With pure CH4 as the feed, H2/CO ratio of synthesis gas could not be lowered to the optimal range of 4–5 by adjusting the operation parameters; however,
the limitation in optimizing the H2/CO ratio for synthetic diesel fuel production could be removed by introducing CO2 to CH4 feed to make CH4-CO2 mixtures. This effect can be primarily attributed to the contributions by CO2 reforming of CH4 as well as reverse water-gas shift reaction, which led to lower H2/CO ratio for the synthesis gas. A simulation technique, ASPEN Plus, was applied to verify the consistency between experimental
data and simulation results. The model satisfactorily simulated changes of H2/CO ratio versus the operation parameters as well as the effect of CO2 addition to CH4 feed. 相似文献