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131.
Youngsoo Kim Jeonggyun Yu Sunghyun Choi Kyunghun Jang 《Communications Letters, IEEE》2006,10(8):608-610
The popular IEEE 802.11 wireless local area network (WLAN) is based on a carrier sense multiple access with collision avoidance (CSMA/CA), where a station listens to the medium before transmission in order to avoid collision. If there exist stations which can not hear each other, i.e., hidden stations, the potential collision probability increases, thus dramatically degrading the network throughput. The RTS/CTS (request-to-send/clear-to-send) frame exchange is a solution for the hidden station problem, but the RTS/CTS exchange itself consumes the network resources by transmitting the control frames. In order to maximize the network throughput, we need to use the RTS/CTS exchange adaptively only when hidden stations exist in the network. In this letter, a simple but very effective hidden station detection mechanism is proposed. Once a station detects the hidden stations via the proposed detection mechanism, it can trigger the usage of the RTS/CTS exchange. The simulation results demonstrate that the proposed mechanism can provide the maximum system throughput performance. 相似文献
132.
A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper deals with our investigations into improving the performance and reducing the complexity of a digital multimedia broadcasting (DMB) system with a return channel when OFDMA is used as an access scheme. To alleviate a multiple access interference (MAI) introduced by a symbol timing misalignment, the OFDMA-based DMB system adopts frequency diversity and cyclic suffix (CS) at the transmitter, namely FD-OFDMA DMB system with CS. When the system is fully loaded, in addition to low complexity at the transmitter, the performance of the FD-OFDMA DMB system with the CS comes close to that of a single-user FD-OFDMA system at the cost of a small loss of throughput. 相似文献
133.
134.
Kyung Ho Kim Seong Jin Park Jin Jang 《Journal of the Society for Information Display》2003,11(4):611-615
Abstract— We studied the silicide‐mediated crystallization of a‐Si for low‐temperature polycrystalline‐silicon (LTPS) on glass. By controling the heating method and Ni density on the a‐Si, the grain size could be increased to 40 μm. Radial grain growth from a NiSi2 crystalline nucleus gives rise to a large‐grain poly‐Si without amorphous phase inside. A field‐effect mobility of over 200 cm2/V‐sec was achieved by using LTPS. 相似文献
135.
Chinwon?LeeEmail author Kukhyun?Ahn Jang?Moo?Lee Won?Sik?Lim 《Journal of Mechanical Science and Technology》2003,17(4):521-527
Modern automatic transmissions equip torque converters with lock-up clutches to reduce the energy loss of hydraulic systems.
Instead of simply engaging the clutch disks, the new technology of clutch slip has been developed to improve the overall efficiency
of power transmission. There are two major problems with the clutch slip system. The first is how to keep the slip between
the two disks within a small range and the second is when to start or stop the slip. In this paper, the second problem is
discussed in view of the vehicle economy. With a simple vehicle dynamic model, the fuel economy is calculated to determine
the lock-up strategy. Then the lock-up strategy is developed for a slip schedule. 相似文献
136.
In this research, a remote control system has been developed and implemented, which combines autonomous obstacle avoidance
in real-time with force-reflective tele-operation. A teleoperated mobile robot is controlled by a local two-degrees-of-freedom
force-reflective joystick that a human operator holds while he is monitoring the screen. In the system, the force-reflective
joystick transforms the relation between a mobile robot and the environment to the operator as a virtual force which is generated
in the form of a new collision vector and reflected to the operator. This reflected force makes the tele-operation of a mobile
robot safe from collision in an uncertain and obstacle-cluttered remote environment. A mobile robot controlled by a local
operator usually takes pictures of remote environments and sends the images back to the operator over the Internet. Because
of limitations of communication bandwidth and the narrow viewangles of the camera, the operator cannot observe shadow regions
and curved spaces frequently. To overcome this problem, a new form of virtual force is generated along the collision vector
according to both distance and approaching velocity between an obstacle and the mobile robot, which is obtained from ultrasonic
sensors. This virtual force is transferred back to the two-degrees-of-freedom master joystick over the Internet to enable
a human operator to feel the geometrical relation between the mobile robot and the obstacle. It is demonstrated by experiments
that this haptic reflection improves the performance of a tele-operated mobile robot significantly. 相似文献
137.
We investigated deformation of the outer diameter of a shaft due to the hub press-fitting and disk clamping processes associated with a 2.5″ hard disk drive. We propose a new robust shaft design to minimize the effect of deformation on the outer diameter of the shaft. We numerically show the effect of deformation on the shaft due to the pressure, stiffness, and damping coefficients of fluid dynamic bearings (FDBs), and the critical mass and excitation response of the rotor-bearing system. We also experimentally measured the axial non-repeatable runout and the amplitude at the half speed whirl frequency of FDBs with both conventional and proposed designs. Through these tests we confirm that the proposed design improves the static and dynamic performance of the FDBs and rotor-bearing system. 相似文献
138.
Bone morphogenic protein-2 (BMP-2) loaded hybrid coating on porous hydroxyapatite scaffolds for bone tissue engineering 总被引:1,自引:0,他引:1
Shin-Hee Jun Eun-Jung Lee Tae-Sik Jang Hyoun-Ee Kim Jun-Hyeog Jang Young-Hag Koh 《Journal of materials science. Materials in medicine》2013,24(3):773-782
In this study, a silica xerogel-chitosan hybrid is utilized as a coating material to incorporate bone morphogenic protein-2 (BMP-2) on a porous hydroxyapatite (HA) scaffold for bone tissue engineering. BMP-2 is known as a therapeutic agent for improving bone regeneration and repair. Silica xerogel-chitosan hybrids have been used for the delivery of a growth factor as well as osteoconductive coatings. The biological properties of the hybrid coating incorporated with BMP-2 were evaluated in terms of the BMP-2 release behavior, osteoblastic cellular responses and in vivo performance. BMP-2 was continuously released from the hybrid coating layer on the porous HA scaffold for up to 6 weeks. The hybrid coating containing BMP-2 showed significantly enhanced osteoblastic cell responses in comparison with the hybrid coating and HA substrate. Consequently, new bone formation was significantly increased within the hybrid coating containing BMP-2. These results reveal that the hybrid coating containing BMP-2 has the potential to be used as a bone implant, whose osteogenic properties are promoted by the release of BMP-2 in a controlled manner for a prolonged period of time. 相似文献
139.
Jun-Hee Jang Han-Gon Choi Jung Hoon Sung 《Drug development and industrial pharmacy》2013,39(3):363-368
To develop 2-(allylthio)pyrazine (2-AP)-loaded lipid emulsion for parenteral administration, various lipid emulsions were prepared with soybean oil, lecithin, and other carriers using homogenization method, and their physical stabilities were investigated by measuring their droplet sizes. The pharmacokinetics and tissue distribution of 2-AP in lipid emulsion after intravenous administration to rats were evaluated compared with 2-AP in solution. 2-AP was lipophilic, sparingly water-soluble, and unstable in aqueous medium. The 2-AP-loaded lipid emulsion composed of 1% of 2-AP, 4% of soybean oil, 4% of lecithin, and 91% of water was physically and chemically stable for at least 8 weeks. It gave significantly faster clearance of 2-AP and higher affinity to the organs, especially the liver, compared with the 2-AP in solution, suggesting that it could selectively deliver 2-AP to the liver. Thus, the lipid emulsion with soybean oil and lecithin could be used as a potential dosage form with the liver-targeting property and enhanced stability of sparingly water-soluble 2-AP. 相似文献
140.
Dae-Woon Jeong Hyun-Suk Na Jae-Oh Shim Won-Jun Jang Hyun-Seog Roh Un Ho Jung Wang Lai Yoon 《International Journal of Hydrogen Energy》2014
Low temperature water–gas shift (WGS) reaction has been carried out at the gas hourly space velocity of 72,152 h−1 over Cu–CeO2 catalyst prepared by a co-precipitation method. Cu loading was optimized to obtain highly active co-precipitated Cu–CeO2 catalysts for low temperature WGS. 80 wt% Cu–CeO2 exhibited the highest CO conversion as well as the most stable activity (XCO > 46% at 240 °C for 100 h). The excellent catalytic performance is mainly due to a strong metal to support interaction, resulting in the prevention of Cu sintering. 相似文献