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131.
Michael J. Fowler Marylesa Howard Aaron Luttman Stephen E. Mitchell Timothy J. Webb 《Inverse Problems in Science & Engineering》2016,24(3):353-371
One of the primary causes of blur in a high-energy X-ray imaging system is the shape and extent of the radiation source, or ‘spot’. It is important to be able to quantify the size of the spot as it provides a lower bound on the recoverable resolution for a radiograph, and penumbral imaging methods – which involve the analysis of blur caused by a structured aperture – can be used to obtain the spot’s spatial profile. We present a Bayesian approach for estimating the spot shape that, unlike variational methods, is robust to the initial choice of parameters. The posterior is obtained from a normal likelihood, which was constructed from a weighted least squares approximation to a Poisson noise model, and prior assumptions that enforce both smoothness and non-negativity constraints. A Markov chain Monte Carlo algorithm is used to obtain samples from the target posterior, and the reconstruction and uncertainty estimates are the computed mean and variance of the samples, respectively. Synthetic data-sets are used to demonstrate accurate reconstruction, while real data taken with high-energy X-ray imaging systems are used to demonstrate applicability and feasibility. 相似文献
132.
Laser perforated fuel cell diffusion media. Part I: Related changes in performance and water content
In this study, cathode-side, bi-layered diffusion media (DM) samples with micro-porous layer were perforated with 300 μm laser-cut holes (covering 15% of the surface area in a homogenous pattern) using a ytterbium fiber laser to investigate the effect of structural changes on the gas and water transport. Under reduced humidity conditions (50% inlet relative humidity on the anode and cathode), the perforated DM were observed to increase the potential by an average of 6% for current densities ranging from 0.2 to 1.4 A cm−2. However, the perforated DM showed reduced performance for current densities greater than 1.4 A cm−2 and at all currents under high-humidity conditions. Neutron radiography experiments were also performed to understand the changes in liquid water retention characteristics of DM due to the laser perforations. Significant water accumulation and water redistribution were observed in the perforated DM, which helps explain the observed performance behavior. The results indicate that the perforations act as water pooling and possible channeling locations, which significantly alter the water condensation, storage, and transport scheme within the fuel cell. These observations suggest that proper tailoring of fuel cell DM possesses significant potential to enable fuel cell operations with reduce liquid overhead and high performance. 相似文献
133.
L. Helfen F. XuB. Schillinger E. CalzadaI. Zanette T. WeitkampT. Baumbach 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2011,651(1):135-139
Computed tomography (CT) is a three-dimensional (3D) imaging method which, for compact or prolate (i.e. rather isotropically extended around the rotation axis) specimens, can yield artefact-free reconstructed cross-sections. Laterally extended specimens like plate-like objects, however, are much less amenable to CT since reliable projection data cannot be acquired from angles where the plate is oriented parallel to the irradiation direction.To overcome this drawback, computed laminography (CL) was introduced recently to imaging set-ups at synchrotron storage rings. Here, we report on the first implementation of computed laminography with neutron radiation, showing measurements that were performed at the ANTARES neutron imaging facility at the FRM II research reactor of Technische Universität München. In general, neutrons are highly interesting probes for imaging since they provide a sensitivity to chemical elements very different from X-rays, yielding complementary information about the specimens investigated. Like for X-ray laminography, we avoid the projection directions where the beam is parallel to the long extensions of the specimen. We accomplish this by tilting of the rotation axis with respect to the transmitted-beam to an angle smaller than 90° (which would be the limiting case of CT) and roughly aligning the specimen's surface normal parallel to this rotation axis. The principles of neutron laminography are introduced and first test experiments are described. 相似文献
134.
W. Treimer U. Feye-Treimer 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2011,651(1):117-120
The variety of imaging signals in neutron radiography and tomography became quite large compared to the pure absorption and scattering contrast in neutron radiographies and topographies in the early sixties or seventies of the last century. The diversity of absorption based techniques for neutron radiography and tomography is comparable to coherence based imaging techniques such as phase contrast, differential phase contrast, dark field imaging, diffraction enhanced contrast, refraction contrast, ultra small angle scattering contrast, grating interferometry and crystal interferometry, also the spin of the neutron was successfully used for imaging [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]. We show which effects (total reflection, diffraction, refraction) contribute to e.g. a step boundary or a phase boundary. Taking this simple object, one can learn to understand the imaging procedure and what is displayed in a radiograph. 相似文献
135.
The time-dependent diffusion of neutrons in a spheroid as a function of the focal distance has been studied. The solution is based on an orthogonal basis and an extrapolation distanced related boundary condition for the spheroidal geometry. It has been shown that spheres and disks are two limiting cases for the spheroids, for which there is a smooth transition for the systems properties between these two limits. Furthermore, it is demonstrated that a slight deformation from a sphere does not affect the fundamental mode properties, to the first order. The calculations for both multiplying and non-multiplying media have been undertaken, showing good agreement with direct Monte Carlo simulations. 相似文献
136.
A.V. Morozkin O. Isnard P. Henry P. Manfrinetti 《Journal of Alloys and Compounds》2008,464(1-2):219-226
A neutron diffraction investigation has been performed on the Ho5Si3 and Ho5Ge3 compounds (hexagonal Mn5Si3-type, hP16, P63/mcm) to study their magnetic structure. The results prove that these intermetallic phases show a complex sine-modulated type magnetic ordering, both presenting on cooling two subsequent antiferromagnetic orderings (TN1 and TN2).Between TN1 = 24(2) and TN2 = 16(4) K Ho5Si3 shows a first antiferromagnetic ordering of the sine-modulated type with a propagation vector K1 = [0, 0, ±0.284(1)]; then, below TN2 the sine-modulated type ordering is conserved, but by the two propagation vectors K1 = [0, 0, ±0.2773(7)] and K2 = [±1/5, ±1/5, 0].For Ho5Ge3, between TN1 = 27(2) and TN2 = 18(4) K, the sine-modulated ordering is realized through the propagation vectors K1 = [0, 0, ±3/10] and K2 = [0, 1/2, 0], whilst below TN2 the ordering takes place with propagation vectors K1 = [0, 0, ±3/10], K2 = [0, 1/2, 0], K3 = [0, 0, ±2/5] and K4 = [±1/5, ±1/5, 0]. For both the Ho5Si3 and Ho5Ge3 compounds, the dimensions of the magnetic unit cell are 5a × 5a × 10c times the crystal unit cell of Mn5Si3-type. 相似文献
137.
138.
139.
The structure of the oxychloride layered perovskite, (CuCl)LaNb2O7, has been examined by neutron diffraction. Rietveld refinement of room temperature neutron TOF data, while consistent with the previous X-ray study, allows for an improved modeling of the structure. The structure consists of double perovskite layers (LaNb2O7) separated by copper chloride layers. The copper is octahedrally coordinated, bridging between apical oxygens from the perovskite layer and surrounded by four chlorines in the CuCl plane; this gives rise to edge-sharing CuO2Cl4 octahedra. The chlorines within the CuCl plane were found to move off the ideal (0, 0, 1/2) position to a more general position, (x, 0, 1/2). This disorder leads to a combination of four short and two long distances, common to the Jahn-Teller distorted environments for d9 copper. Structural details are discussed with respect to their influence on the chlorine disorder. 相似文献
140.
This paper discusses the application of the Legendre spectral element method to the steady one-velocity PN equations describing neutron transport in a one-dimensional heterogeneous slab. Emphasis is put on the implementation of the method. Some key elements related to its efficiency are analyzed to prepare further developments in higher dimensions to evaluate benchmark solutions to the simplified PN equations. 相似文献