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991.
Keith L. Gordon Jin Ho Kang Cheol Park Peter T. Lillehei Joycelyn S. Harrison 《应用聚合物科学杂志》2012,125(4):2977-2985
Metamaterials or artificial negative index materials (NIMs) have generated great attention because of their unique electromagnetic properties. The main challenge in current NIM development is creating a homogenous NIM without the need of complex geometric architectures consisting of capacitors and inductors or aggregated fillers, but possessing a tunable plasma frequency. A natural material that can exhibit negative values for permittivity and permeability simultaneously has not been found, or discovered. If one can design a negative dielectric constant material with a tunable plasma frequency of interest, implementing negative permeability into the material or system would be much more readily achievable to create a metamaterial. In this regard, a novel negative dielectric constant material, which is an essential key to creating the NIMs, was developed by doping ions into a polymer, a protonated poly(benzimidazole) (PBI). The doped PBI showed a negative dielectric constant at frequencies of kHz to MHz because of its reduced plasma frequency and an induction effect. As temperature increased, the dielectric spectrum changed from a relaxation to a resonance behavior and exhibited a larger magnitude of negative dielectric constant at a lower frequency. The conductivity of the doped PBI measured as a function of both temperature and frequency followed the same trend as the dielectric constant. With respect to the dielectric constant and the conductivity data, it can be assumed that the origin of the negative dielectric constant is attributed to the resonance behavior of the highly mobile ions at elevated temperatures and high frequencies. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
992.
We report a simple, cost-effective, and environmentally benign process for reducing graphite oxide by treating solely with sulfuric acid. The suggested process consists of a two-step reduction of graphite oxide, first in aqueous sulfuric acid at room temperature and then in concentrated sulfuric acid with refluxing. X-ray diffractometry, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetric analysis demonstrated that the graphite oxide was reduced effectively and was comparable in composition to reduced graphite oxide prepared using previously described methods that rely on toxic and hazardous reducing agents, such as hydrazine, sodium borohydride, or hydrohalic acids. 相似文献
993.
Preparation of activated graphene and effect of activation parameters on electrochemical capacitance
Shanthi Murali Jeffrey R. Potts Scott Stoller Joono Park Meryl D. Stoller Li Li Zhang Yanwu Zhu Rodney S. Ruoff 《Carbon》2012,50(10):3482-3485
Activation parameters such as temperature and the amount of potassium hydroxide (KOH) were varied during the synthesis of activated microwave-exfoliated graphite oxide (a-MEGO) and the effects of these parameters on the specific surface area of a-MEGO and electrochemical capacitance of a-MEGO electrodes were investigated. At 800 °C and a KOH/MEGO mass ratio of 6.5, a maximum specific surface area of 3100 m2/g was obtained and a high specific capacitance of 172 F/g (at 1 A/g constant current and 3.5 V maximal voltage) was measured in a two-electrode cell with a-MEGO electrodes in an organic electrolyte. 相似文献
994.
Hyeondeuk Yong Kiyeol Kim Wooseok Choi Joonkyu Park Muneer Ahmad Yongho Seo 《Carbon》2012,50(12):4640-4647
A graphene cellular array on an insulating SiO2 layer was fabricated by scanning probe lithography. The graphene layer was oxidized by an electric field which was applied between the cantilever tip and Si substrate without any electrode directly connected to the graphene layer. When the bias voltage was applied on a cell of patterned graphene through the cantilever tip, charge was accumulated on the cell and preserved for a long time without decay. The accumulated charge and the surface potential were measured by an electrostatic force microscope. The charge retention was measured as a function of time, and the decay time constant was estimated to be ~70 min. 相似文献
995.
Youngwoo Nam Jai Seung Yoo Yung Woo Park Niclas Lindvall Thilo Bauch August Yurgens 《Carbon》2012,50(15):5562-5568
We measured the Aharonov-Bohm (AB) effect in graphene rings with superconducting-(Al) and normal-metal (Au) mirrors. The mirrors were deposited either on additional stubs connected to the rings in the transverse direction or on the ring bias lines. A significant enhancement of the visible phase coherence was observed in the latter case, in which we observed even the third harmonic of the AB oscillations. The superconductivity of the mirrors appears to be unimportant for the improved coherence in graphene. A large Fermi energy mismatch between graphene and the mirror material is sufficient for this effect. In addition, a transport gap was observed in our graphene structures at the gate voltage close to the Dirac point. The value of the gap can be reproduced by assuming the occurrence of Coulomb blockade effects in graphene. 相似文献
996.
Hong Jae Yim Jae Hong Kim Sun-Jong Park Hyo-Gyoung Kwak 《Cement and Concrete Research》2012,42(11):1438-1446
Thermal damage in concrete usually induces contact-type defects, which result in degradation of the concrete's performance. This paper attempts to visualize the thermal damage in a multiscale, and characterizes the thermally damaged concrete using a nonlinear ultrasonic method. An impact-modulation method is used to obtain nonlinearity parameters, as a quantitative measure of contact-type defects, and shows better sensitivity than phase velocity variation as a linear ultrasonic method for thermally damaged concrete. The measured nonlinearity parameter is compared with the permeable pores, which reflect the occurrence of opening and pores in thermally damaged concrete. Degradation of concrete strength due to thermal damage is also assessed via the measurement. 相似文献
997.
Sung Wook Hwang Hee Chang Ryu Sang Woo Kim Hae Youn Park Kwan Ho Seo 《应用聚合物科学杂志》2012,125(4):2732-2739
Maleic anhydride (MAH) was grafted onto ethylene vinyl acetate copolymer (EVA) in the internal mixer in the presence of dicumyl peroxide (DCP), and a prepared reactive compatibilizer, MAH-g-EVA, was blended with Poly (ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) (PETG). The gel content determination and element analysis (EA) was performed to confirm the grafting reaction. It was found that grafting reaction of MAH on to EVA could compete with crosslinking reaction of DCP during the modification process. In addition, the introduction of small amount of MAH showed a great effect on reducing gel content by decreasing crosslink reaction. As MAH content increased, grafted MAH concentration increased, whereas the grafting yield decreased. It was also confirmed that MAH-g-EVA acted as a reactive compatibilizer in the blend with PETG, and enhanced compatibility by reacting with the hydroxyl end groups, OH, of PETG. Addition of EVA in the blend leads a plastic deformation of PETG, and MAH had a great effect on enhancing interfacial adhesion resulting in significant increasing of % strain; however, improved compatibility could not be changed much in low strain tensile strength and Young's modulus. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
998.
Hyun Seung Lee Ko Hoon Kim Sung Hwan Kim Jae Nyoung Kim 《Advanced Synthesis \u0026amp; Catalysis》2012,354(13):2419-2426
An efficient synthesis of tetrasubstituted olefins was achieved via a palladium‐catalyzed, chelation‐assisted oxidative Heck arylation protocol from trisubstituted olefins bearing a tether with a directing group in a completely stereo‐ and regioselective manner. The stereo‐ and regioselectivity as well as excellent yields of tetrasubstituted olefins originated from the stabilization of a palladium intermediate by chelation between the palladium center and a directing group. 相似文献
999.
Myoung Youp Song Ik Hyun Kwon Hye Ryoung Park Daniel R. Mumm 《Ceramics International》2012,38(3):2443-2448
An appropriate mole ratio of urea/nitrate for preheating to synthesize LiNiO2 was examined by varying the ratio from 1.2 to 9.6. The chemical equation of the combustion reaction was deduced from the XRD analysis result of the mixture after preheating. The XRD pattern of the LiNiO2 sample calcined at 800 °C for 24 h, after preheating at the mole ratio of urea/nitrate of 3.6 at 400 °C, shows clear split of the 1 0 8 and 1 1 0 peaks, and the largest value of I003/I104. The sample calcined at 800 °C for 24 h has a relatively high first discharge capacity (164.2 mAh g?1) and a good cycling performance. Derivative ?dx/|dV| vs. V curve of the LiNiO2 sample at the voltage range of 2.7–4.4 V for the first cycle exhibits four peaks for charging and discharging, showing that this sample goes through four phase transitions. 相似文献
1000.
Chang-Wan Kim Joo-Hyun Choi Hyung-Jun Kim Dong-Won Lee Chang-Yong Hyun Song-Min Nam 《Ceramics International》2012,38(7):5621-5627
The influence of the surface roughness of Al2O3 interlayers on the growth of Al2O3 thick films fabricated by an aerosol deposition (AD) process was investigated as an approach to improving the plasma resistance of the films. The Al2O3 interlayer was fabricated by a plasma electrolytic oxide (PEO) method. This method is capable of fabricating films on the entire surface area of 3-dimensional substrates, whereas the AD process has difficulties with depositing films on complex shapes, such as on edges and corners, and inside holes. To prevent degradation of the plasma resistance with increasing working time, the thickness of the Al2O3 interlayer was increased by the PEO method. The surface roughness of the Al2O3 interlayer was increased linearly by increasing the thickness of the Al2O3 interlayer. On Al2O3 interlayers with surface roughness values of more than 1.5 μm (Ra), Al2O3 films were not grown by the AD process. To investigate the effect of the surface roughness of the Al2O3 interlayer on the growth of Al2O3 films on the Al2O3 interlayer, we attempted to deposit Al2O3 films on an Al2O3 interlayer whose surface roughness was decreased from 1.5 μm to 0.8 μm by polishing. As a result, an Al2O3 film of 2.0 μm in thickness was grown by the AD process. These study results support the conclusion that controlling of the surface roughness is the most important factor in aerosol-deposited film growth. 相似文献