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981.
Williams K.A. White I.H. Burns D. Sibbett W. 《Quantum Electronics, IEEE Journal of》1996,32(11):1988-1994
Following a comparison of picosecond-pulse generation techniques, feedback schemes are reported for the generation of picosecond pulse trains with improved jitter for both multicontact and conventional single-contact InGaAsP-InP lasers. Subpicosecond jitter is achieved for Q-switched laser sources using a novel optoelectronic feedback scheme. The use of resonant electrical feedback is shown to improve the timing jitter of gain-switched pulses by up to six times. Pulse-to-pulse timing jitter as low as 250 fs is demonstrated for a hybrid of optical and electrical feedback schemes. Limits for timing jitter in diode lasers are established for optoelectronic, electrical, and optical feedback schemes, and the key picosecond pulse generation schemes are compared in terms of timing jitter for the first time 相似文献
982.
Collins C.E. Pollard R.D. Miles R.E. Dildine R.G. 《IEEE transactions on instrumentation and measurement》1996,45(3):721-727
A new method for determining the single-sideband noise figure of a device from double-sideband measurements is proposed which eliminates the need for tunable filters and hence greatly reduces the complexity and cost of performing accurate noise figure measurements. This paper compares results obtained to those obtained using a traditional single-sideband measurement system with a tuneable filter and provides a full error analysis of the new technique. The method also offers considerable possibilities for future use at higher (microwave) frequencies where YIG filters and image reject mixers are not readily available 相似文献
983.
J. S. Hong L. Gao X. X. Huang B. A. Shaw D. P. Thompson 《Journal of Materials Science》1996,31(4):957-962
Tetragonal zirconia polycrystalline (TZP) ceramics containing SiC reinforcement in the form of fine particles (nano-scale), particles (micro-scale), whiskers and platelets were synthesized by hot-pressing. The effects of morphology and grain size of SiC reinforcement on the strength and fracture toughness at room temperature were investigated. The addition of SiC (in whatever form) caused decreases in strength and toughness at room temperature with the exception of whisker-reinforced materials. Toughness fell off with increasing temperature, but nevertheless retained about one-half of the room-temperature value for that particular SiC reinforcement. However, the whisker- and particle-reinforced materials had higher K
lc values at high temperature than fine particle- or platelet-reinforced materials, with values in excess of 7 MPa m1/2 at 1000 °C. The microstructure was examined for SiC whisker-reinforced/TZP materials by TEM and HREM, to examine the nature of the whisker/zirconia interface. 相似文献
984.
Summary We consider a circular cylindrical membrane subjected to longitudinal extension and twist. The associated equilibrium deformation is considered to be axisymmetric and the analysis is based on a direct two-dimensional formulation. Wrinkling of the membrane is taken into account in an approximate way by introducing arelaxed strain energy function derived from the neo-Hookean strain energy for isotropic elastic solids. Analytical formulae for wrinkled parts of the membrane are used to corroborate the results of a numerical treatment of the full boundary value problem.Dedicated to the memory of A. C. Pipkin 相似文献
985.
A systematic study was carried out aimed at evaluating the environmental effects of H2S containing environment on the creep-fatigue failure of coated undirectionally solidified MM-002 nickel-base superalloy at high temperature. The cyclic loading conditions were constant and consisted of creep tension and plastic compression following the CP-mode of the strain range partitioning method. The results obtained have shown that although the pack cementation coating used showed adequate resistance under the combination of cyclic loading and oxidizing environments, this resistance was not evident under the H2S containing atmosphere. In fact, it was found that even an amount as little as 0.05% H2S had an extremely detrimental effect on the creep-fatigue life of the coated superalloy at 650 °C. The crack initiation and propagation in the H2S environment was controlled by an aggressive sulphidation attack ahead of the crack tip. This was demonstrated by the formation of low melting eutectic sulfides at the plastic zone ahead of the crack tip, causing an accelerated transdendritic cracking and premature failure. 相似文献
986.
Suitability of a Quadro ComilTM for screening of direct compression tablet masses was evaluated with respect to alteration of particle size distribution and blending efficiency. Air jet sieving was qualijied for determination of a particle size of spraydried lactose and microcrystalline cellulose. Using a full 31 × 23 factorial design not only effects of single processing parameters (output screen size, impeller shape, velocity of the mill, and type of powder), but also their respective combinations were investigated in terms of impact on particle size and flow properties (angle of repose, bulk and tapped densities). Particle sizes before and after passing the Comil were found to be normally distributed. The results indicate that the process does not allow any of the powder characteristics studied beyond normal batch-to batch variability. This refers in particular to the proportion of fines. For evaluation of the deagglomeration effectiveness, a 23 design was implemented with iron oxide as a model drug. Homogeneity of the blends was determined optically, and after screening twice it was found to be acceptable even with this substance, which has a particularly high agglomeration tendency, high density, and small particle size. The Quadro Comil is therefore regarded as fully suitable for deagglomeration of powders in a gentle and effective manner 相似文献
987.
The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T 1, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T1 atT c and briefly discuss possible reasons why this peak is not observed. 相似文献
988.
Kovacs G.T.A. Storment C.W. Halks-Miller M. Belczynski C.R. Jr. Santina C.C.D. Lewis E.R. Maluf N.I. 《IEEE transactions on bio-medical engineering》1994,41(6):567-577
A new process for the fabrication of regeneration microelectrode arrays for peripheral and cranial nerve applications is presented. This type of array is implanted between the severed ends of nerves, the axons of which regenerate through via holes in the silicon and are thereafter held fixed with respect to the microelectrodes. The process described is designed for compatibility with industry-standard CMOS or BiCMOS processes (it does not involve high-temperature process steps nor heavily-doped etch-stop layers), and provides a thin membrane for the via holes, surrounded by a thick silicon supporting rim. Many basic questions remain regarding the optimum via hole and microelectrode geometries in terms of both biological and electrical performance of the implants, and therefore passive versions were fabricated as tools for addressing these issues in on-going work. Versions of the devices were implanted in the rat peroneal nerve and in the frog auditory nerve. In both cases, regeneration was verified histologically and it was observed that the regenerated nerves had reorganized into microfascicles containing both myelinated and unmyelinated axons and corresponding to the grid pattern of the via holes. These microelectrode arrays were shown to allow the recording of action potential signals in both the peripheral and cranial nerve settings, from several microelectrodes in parallel 相似文献
989.
Airborne scatterometers: investigating ocean backscatter under low and high-wind conditions 总被引:1,自引:0,他引:1
Carswell J.R. Carson S.C. McIntosh R.E. Li F.K. Neumann G. McLaughlin D.J. Wilkerson J.C. Black P.G. Nghiem S.V. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(12):1835-1860
Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated 相似文献
990.
Maxwell G.D. Kashyap R. Sherlock G. Collins J.V. Ainslie B.J. 《Electronics letters》1994,30(18):1486-1487
The authors report the first demonstration of a semiconductor external cavity waveguide laser, modulated at 2.5 Gbit/s over 100 km of standard optical fibre using a UV written grating in a planar silica waveguide as the feedback element 相似文献