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171.
The usefulness of statistical clustering algorithms developed for automatic segmentation of lesions and organs in magnetic resonance imaging (MRI) intensity data sets suffers from spatial nonstationarities introduced into the data sets by the acquisition instrumentation. The major intensity inhomogeneity in MRI is caused by variations in the B1-field of the radio frequency (RF) coil. A three-step method was developed to model and then reduce the effect. Using a least squares formulation, the inhomogeneity is modeled as a maximum variation order two polynomial. In the log domain the polynomial model is subtracted from the actual patient data set resulting in a compensated data set. The compensated data set is exponentiated and rescaled. Statistical comparisons indicate volumes of significant corruption undergo a large reduction in the inhomogeneity, whereas volumes of minimal corruption are not significantly changed. Acting as a preprocessor, the proposed technique can enhance the role of statistical segmentation algorithms in body MRI data sets. 相似文献
172.
A report is given of the observation below 1 K of the sound attenuation expected from progressive orientational ordering in hcp H2 with ortho concentrationsX<0.53. The experiments were carried out at 10 and 30 MHz. The amplitude of this effect depends on the coupling between the lattice vibrations and the molecular orientation, and should be maximum when the acoustic angular frequency is comparable with the orientational relaxation rate –1. The average rate can be roughly estimated from NMR longitudinal relaxation timeT
1
measurements. Such a maximum for was indeed observed in the expected temperature range. At high enough temperatures, was found to be proportional toT
1
–1
. which is consistent with predictions in the high temperature limit. Furthermore, the transition between the hcp and the fcc phases forX>0.53 is studied by means of the large changes in the sound propagation at the transition, and the phase diagram thus obtained is compared with results from x-ray and pressure measurements. The new observations explain some previous discrepancies in results using different methods. The difference between solid H2 and D2 regarding the stabilization of the cubic structure above the orientational ordering transition is also discussed. Calculations of the respective energy barriers E to be overcome during the martensitic transition are suggested. 相似文献
173.
We report measurements of the temperature, density, and concentration gradients in3He-4He mixtures, induced by a vertical heat flux. The flat horizontal cell included two superposed capacitors and the density was determined by means of the dielectric constant method. The experiments were carried out on mixtures with mole fractionsX
3=0.37, 0.15, and 0.05 at saturated vapor pressure, with special emphasis on the region near the superfluid transition. Our measurements under steady-state conditions give the conductivity , the thermal diffusion ratiok
T
, and the coefficient of thermal expansion. We describe the singular behavior of these quantities in the neighborhood ofT
(X). In the superfluid phase, we test with fair success a relation by Khalatnikov between gradX/ gradT and several static properties. From the relaxation times needed to attain steady-state conditions, and in combination with measured static and transport properties, we obtain in the normal phase the mass diffusionD, which diverges strongly asT
is approached. In the superfluid phase, we test successfully a scaled relation that results from the solution of Khalatnikov's hydrodynamic equations. From our data the dispersion relations for scattered light are calculated: o/q
2 in both the normal and the superfluid phases and 2/q
2 in the normal phase.Research supported by NSF grant DMR 8024056. 相似文献
174.
175.
Analysis of the S-phase fractions (SPF) measured by in vitro thymidine labeling, morphological appearances, and estrogen receptor (ER) assays of primary invasive breast carcinomas demonstrated several interrelationships. Lobular, mucinous, tubular, and adenocystic carcinomas consistently had low SPF and were usually positive for ER. The same was true for the carcinomas of no special histologic type [the not otherwise specified (NOS) group of E. R. Fisher including "infiltrating ductal" and undifferentiated carcinomas] with minimal anaplasia. Medullary, atypical medullary, and morphologically unclassifiable carcinomas with marked nuclear anaplasia nearly always had high SPF and were usually negative for ER. High SPF was associated with advanced stages of carcinoma initially or with early recurrence following mastectomy. 相似文献
176.
Channeling and x-ray diffraction measurements on Kr- and He-irradiated V3Si single crystals and films reveal different damage levels for fluences in cases where the superconducting transition temperature T
chas been reduced by the same amount. This indicates that only special defect structures are responsible for the T
c-reduction mechanism. In the fluence region where T
cis decreasing, T
ccorrelates with residual resistivity o, independent of the kind of irradiation. However, at particle fluences where T
csaturation occurs, different saturation values of o are observed. The exponential decrease and the saturation of T
cwith fluence are explained by a similar behavior of g9o versus fluence in the damage production and saturation processes. The increase of the lattice parameter is not uniquely dependent on the decrease of T
c, but also on the amount of damage present. 相似文献
177.
Evaluation of Crop Models for Simulating and Optimizing Deficit Irrigation Systems in Arid and Semi-arid Countries Under Climate Variability 总被引:2,自引:2,他引:0
Sebastian?KlossEmail author Raji?Pushpalatha Kefasi?J.?Kamoyo Niels?Schütze 《Water Resources Management》2012,26(4):997-1014
The variability of fresh water availability in arid and semi-arid countries poses a serious challenge to farmers to cope with
when depending on irrigation for crop growing. This has shifted the focus onto improving irrigation management and water productivity
(WP) through controlled deficit irrigation (DI). DI can be conceived as a strategy to deal with these challenges but more
knowledge on risks and chances of this strategy is urgently needed. The availability of simulation models that can reliably
predict crop yield under the influence of soil, atmosphere, irrigation, and agricultural management practices is a prerequisite
for deriving reliable and effective deficit irrigation strategies. In this context, this article discusses the performance
of the crop models CropWat, PILOTE, Daisy, and APSIM when being part of a stochastic simulation-based approach to improve
WP by focusing primarily on the impact of climate variability. The stochastic framework consists of: (i) a weather generator
for simulating regional impacts of climate variability; (ii) a tailor-made evolutionary optimization algorithm for optimal
irrigation scheduling with limited water supply; and (iii) the above mentioned models for simulating water transport and crop
growth in a sound manner. The results present stochastic crop water production functions (SCWPFs) that can be used as basic
tools for assessing the impact on the risk for the potential yield due to water stress and climate variability. Example simulations
from India, Malawi, France and Oman are presented and the suitability of these crop models to be employed in a framework for
optimizing WP is evaluated. 相似文献
178.
Spirkl FM Kunz S Schweinberger FF Farnbacher AN Schröter R Heiz U 《The Review of scientific instruments》2012,83(1):013114
The separation of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES) is usually performed by a time-of-flight (ToF) separation using pre-set ToF for both types of signal. In this work, we present a new, improved ex situ signal separation method for the separation of MIES and UPS for every single measurement. Signal separation issues due to changes of system parameters can be overcome by changing the ToF separation and therefore allowing for the application of a wider range of measuring conditions. The method also enables to identify and achieve separation of the two signals without any time consuming calibration and the use of any special material for the calibration. Furthermore, changes made to the discharge source are described that enable to operate an existing MIES/UPS source over a broader range of conditions. This allows for tuning of the yield of UV photons and metastable rare gas atoms leading to an improved signal to noise ratio. First results of this improved setup are well in agreement with spectra reported in literature and show increased resolution and higher signal intensities for both MIE and UP spectra compared to the previous, non-optimized setup. 相似文献
179.
MA Dobbs M Lueker KA Aird AN Bender BA Benson LE Bleem JE Carlstrom CL Chang HM Cho J Clarke TM Crawford AT Crites DI Flanigan T de Haan EM George NW Halverson WL Holzapfel JD Hrubes BR Johnson J Joseph R Keisler J Kennedy Z Kermish TM Lanting AT Lee EM Leitch D Luong-Van JJ McMahon J Mehl SS Meyer TE Montroy S Padin T Plagge C Pryke PL Richards JE Ruhl KK Schaffer D Schwan E Shirokoff HG Spieler Z Staniszewski AA Stark K Vanderlinde JD Vieira C Vu B Westbrook R Williamson 《The Review of scientific instruments》2012,83(7):073113
A technological milestone for experiments employing transition edge sensor bolometers operating at sub-Kelvin temperature is the deployment of detector arrays with 100s-1000s of bolometers. One key technology for such arrays is readout multiplexing: the ability to read out many sensors simultaneously on the same set of wires. This paper describes a frequency-domain multiplexed readout system which has been developed for and deployed on the APEX-SZ and South Pole Telescope millimeter wavelength receivers. In this system, the detector array is divided into modules of seven detectors, and each bolometer within the module is biased with a unique ~MHz sinusoidal carrier such that the individual bolometer signals are well separated in frequency space. The currents from all bolometers in a module are summed together and pre-amplified with superconducting quantum interference devices operating at 4 K. Room temperature electronics demodulate the carriers to recover the bolometer signals, which are digitized separately and stored to disk. This readout system contributes little noise relative to the detectors themselves, is remarkably insensitive to unwanted microphonic excitations, and provides a technology pathway to multiplexing larger numbers of sensors. 相似文献
180.
Johannes Blümlein Sebastian Klein Carsten Schneider Flavia Stan 《Journal of Symbolic Computation》2012
Given a Feynman parameter integral, depending on a single discrete variable N and a real parameter ε, we discuss a new algorithmic framework to compute the first coefficients of its Laurent series expansion in ε. In a first step, the integrals are expressed by hypergeometric multi-sums by means of symbolic transformations. Given this sum format, we develop new summation tools to extract the first coefficients of its series expansion whenever they are expressible in terms of indefinite nested product–sum expressions. In particular, we enhance the known multi-sum algorithms to derive recurrences for sums with complicated boundary conditions, and we present new algorithms to find formal Laurent series solutions of a given recurrence relation. 相似文献