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91.
Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide) 总被引:1,自引:0,他引:1
Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C. 相似文献
92.
The single-scattering properties of the Platonic shapes, namely, the tetrahedron, hexahedron, octahedron, dodecahedron, and icosahedron, are investigated by use of the finite-difference time-domain method. These Platonic shapes have different extents of asphericity in terms of the ratios of their volumes (or surface areas) to those of their circumscribed spheres. We present the errors associated with four types of spherical equivalence that are defined on the basis of (a) the particle's geometric dimension (b) equal surface area (A), (c) equal volume (V), and (d) equal-volume-to-surface-area ratio (V/A). Numerical results show that the derivations of the scattering properties of a nonspherical particle from its spherical counterpart depend on the definition of spherical equivalence. For instance, when the Platonic and spherical particles have the same geometric dimension, the phase function for a dodecahedron is more similar than that for an icosahedron to the spherical result even though an icosahedron has more faces than a dodecahedron. However, when the nonspherical and spherical particles have the same volume, the phase function of the icosahedral particle essentially converges to the phase function of the sphere, whereas the result for the dodecahedron is quite different from its spherical counterpart. Furthermore, the present scattering calculation shows that the approximation of a Platonic solid with a sphere based on V/A leads to larger errors than the spherical equivalence based on either volume or projected area. 相似文献
93.
A computational study of forced convection processes in ducts and packed beds at low Reynolds numbers has been made. The results give a better understanding of these processes, especially for small Peclet numbers. It is demonstrated that two distinct forms of Nusselt numbers are relevant for low Peclet numbeRs. One, used mostly by theorists, is related to the local driving force; the other, used mostly by experimentalists, is related to easily measurable temperatures and concentrations. As an example the Graetz problem has been solved numerically over a wide range of Peclet numbers, and new asymptotes have been obtained for the region of small Peclet numbers. 相似文献
94.
The Reagan Administration maintains the Carter Administration's objective of non-proliferation of nuclear weapons as being fundamental to US nuclear export policy. However, it sees the USA as having another important role to play in influencing the use of nuclear power and the trading of related goods and technologies in other countries. While the Administration believes its policies will prove beneficial to the USA, there is concern that trade considerations are being given priority over preventing the proliferation of nuclear weapons. 相似文献
95.
An imporved method for the numerical evaluation of the convolution integral in the relationship between creep compliance and relaxation modulus is discussed. The better approximation is obtained by the assumption that both functions can be assumed to be linear within a series of increasing time intervals which do not change as the calculation progresses. The calculation is carried out on both hypothetical and real examples which substantiates its applicability and accuracy. 相似文献
96.
Warren P. Mason 《Engineering Fracture Mechanics》1976,8(1):89-101
High amplitudes (strains up to 3 × 10?3) have been used in studying internal friction, acoustic emission and fatigue in metals. The internal friction in the high amplitude range is due to breakaway of dislocations from pinning points and by the generation of Frank-Read loops which results in plastic strain. There are two plastic regions, one when the added loops are held up by the grain boundaries, and the second when the loops break through the grain boundaries. In the second region, slip bands are produced in the metal. As the amplitude increases slip bands can join up and fatigue in the metal occurs. Ultrasonic frequencies are useful for studying fatigue since a large number of cycles can occur in a reasonable time.Acoustic emission—that is, a noise in the sample associated with the dislocation motion—can be studied by putting transducers on the sample. It is shown that the emission is closely associated with the internal friction since it goes through two regions in the plastic range. 相似文献
97.
Roback Howard B.; Webb Warren W.; Strassberg Donald 《Canadian Metallurgical Quarterly》1974,42(5):734
Administered the MMPI to 17 21-62 yr old male patients who did not pay their clinic bills and 17 patients who did pay their bills. MMPI and clinical data suggest greater reality disturbance and distortion for the non-fee paying group than for the fee-paying group, and that fee-payment behavior is situationally determined and not a generalized characteristic. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
98.
Trajectory-based change detection for automated characterization of forest disturbance dynamics 总被引:7,自引:0,他引:7
Robert E. Kennedy Warren B. Cohen Todd A. Schroeder 《Remote sensing of environment》2007,110(3):370-386
Satellite sensors are well suited to monitoring changes on the Earth's surface through provision of consistent and repeatable measurements at a spatial scale appropriate for many processes causing change on the land surface. Here, we describe and test a new conceptual approach to change detection of forests using a dense temporal stack of Landsat Thematic Mapper (TM) imagery. The central premise of the method is the recognition that many phenomena associated with changes in land cover have distinctive temporal progressions both before and after the change event, and that these lead to characteristic temporal signatures in spectral space. Rather than search for single change events between two dates of imagery, we instead search for these idealized signatures in the entire temporal trajectory of spectral values. This trajectory-based change detection is automated, requires no screening of non-forest area, and requires no metric-specific threshold development. Moreover, the method simultaneously provides estimates of discontinuous phenomena (disturbance date and intensity) as well as continuous phenomena (post-disturbance regeneration). We applied the method to a stack of 18 Landsat TM images for the 20-year period from 1984 to 2004. When compared with direct interpreter delineation of disturbance events, the automated method accurately labeled year of disturbance with 90% overall accuracy in clear-cuts and with 77% accuracy in partial-cuts (thinnings). The primary source of error in the method was misregistration of images in the stack, suggesting that higher accuracies are possible with better registration. 相似文献
99.
Motivated by practical applications in engineering, this article considers the problem of approximating a set of data with a function that is compatible with geometric programming (GP). Starting with well-established methods for fitting max-affine functions, it is shown that improved fits can be obtained using an extended function class based on the softmax of a set of affine functions. The softmax is generalized in two steps, with the most expressive function class using an implicit representation that allows fitting algorithms to locally tune softness. Each of the proposed function classes is directly compatible with the posynomial constraint forms in GP. Max-monomial fitting and posynomial fitting are shown to correspond to fitting special cases of the proposed implicit softmax function class. The fitting problem is formulated as a nonlinear least squares regression, solved locally using a Levenberg–Marquardt algorithm. Practical implementation considerations are discussed. The article concludes with numerical examples from aerospace engineering and electrical engineering. 相似文献
100.
We present penetration equations for rigid, ogive-nose, rod projectiles that penetrate aluminum targets at normal impact. Comparisons of depth of penetration data and predictions from a previously published penetration equation derived from spherical, cavity-expansion methods show excellent agreement for striking velocities to 1800 m/s. We then identify a small parameter in the penetration equation, perform a power-series expansion, and obtain approximate penetration equations. These approximate equations are very accurate for striking velocities to 1300 m/s and display clearly the dominant problem parameters. 相似文献