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991.
992.
András Faragó 《Algorithmica》2007,49(4):337-356
We prove two results that provide new fundamental limits for topology control in large ad hoc and sensor networks. First,
we show that it remains true under very general conditions that the maximum expected node degree must grow to infinity at
least logarithmically if we want to maintain asymptotic connectivity. This has been known so far only for much more special
models than ours. Building on this result, we prove a new fundamental limit regarding link dynamics, which means the worst case length ratio of the longest and shortest link adjacent to the same node. We prove that if link
dynamics remains bounded, then no topology control algorithm can keep a large network connected with high probability. Moreover,
bounded link dynamics prevents connectivity in the limit without any a priori assumption on node degrees or transmission ranges.
Our results hold in a model that is much more general than the frequently used assumption of uniformly distributed nodes in
a regularly shaped planar domain. Our more abstract setting also aims at finding (hopefully) more robust and elegant proofs
that have less dependence on the special geometry. Since link dynamics is expected to be bounded in practice, the results
strenghten the theoretical basis for the argument that a very large ad hoc or sensor network is unable to maintain connectivity
if it has a flat, random organization without additional structure.
Supported in part by NSF Grants ANI-0220001 and CCF-0634848. 相似文献
993.
994.
Xiaodan Zhang Wei Liu Dale Sun Youguo Li 《Metallurgical and Materials Transactions A》2007,38(3):499-505
The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal
casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution
of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements,
and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides
were found to change with austenizing time. During austenization, the transformation of the M3C carbides can be postulated as M3C + γ-Fe → M2C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M2C carbide decomposes as M2C + γ-Fe → MC + M7C3 + M6C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding
of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides
and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the
wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls. 相似文献
995.
Following the original model for a single cell/dendrite growth by Hunt,[5] systematic numerical simulations are carried out with focus on cell shape evolution and tip radius selection in a directional
solidification process conducted in a capillary tube. Computations indicate that the anisotropy of solid/liquid interfacial
energy and the diameter of a capillary tube have significant influences on the selection of a tip shape. The anisotropy in
dilute SCN-salol alloys has been determined through the equilibrium droplet shape method and its value is 1.09 pct. The calculated
cell/dendrite shapes with this value of anisotropy agree with experimental observations where a single cell/dendrite of SCN-salol
alloys grows in a capillary tube. 相似文献
996.
In the design of algorithms for large-scale applications it is essential to consider the problem of minimizing I/O communication.
Geographical information systems (GIS) are good examples of such large-scale applications as they frequently handle huge amounts
of spatial data. In this paper we develop efficient external-memory algorithms for a number of important problems involving
line segments in the plane, including trapezoid decomposition, batched planar point location, triangulation, red--blue line
segment intersection reporting, and general line segment intersection reporting. In GIS systems the first three problems are
useful for rendering and modeling, and the latter two are frequently used for overlaying maps and extracting information from
them. 相似文献
997.
Confluent graphs capture the connection properties of train tracks, offering a very natural generalization of planar graphs,
and—as the example of railroad maps shows—are an important tool in graph visualization. In this paper we continue the study
of confluent graphs, introducing strongly confluent graphs and tree-confluent graphs. We show that strongly confluent graphs
can be recognized in NP (the complexity of recognizing confluent graphs remains open). We also give a natural elimination
ordering characterization of tree-confluent graphs, and we show that this class coincides with the (6,2)-chordal bipartite
graphs. Finally, we define outerconfluent graphs and identify the bipartite permutation graphs as a natural subclass. 相似文献
998.
Efficient encoding of IEEE 802.11n LDPC codes 总被引:1,自引:0,他引:1
Addressed is the issue of LDPC coding for the emerging IEEE 802.11n standard. An efficient encoding algorithm is proposed. The algorithm is simple and easy to implement. The memory requirement is trivial 相似文献
999.
孙伦 《消防技术与产品信息》2005,(1):3-3
雄鸡高唱 ,万象更新 !带着盎然春意 ,带着勃勃生机 ,2 0 0 5年向我们大步走来 !值此新年到来之际 ,谨向关心、爱护、支持《消防技术与产品信息》杂志的各界朋友和广大读者致以亲切的问候和最美好的祝愿 !党的十六届四中全会通过的《中共中央关于加强党的执政能力建设的决定》提出 ,要“发挥社团、行业组织和社会中介组织提供服务、反映诉求、规范行为的作用 ,形成社会管理和社会服务的合力”,对社团、行业组织寄予厚望。我们决心在公安部和中国科协的领导下 ,紧紧围绕消防中心工作 ,进一步做好协会的各项工作 ,为发展我国的消防事业服务 ,更… 相似文献
1000.
针对在供热系统中,热电厂集中供热普及率不高,锅炉房供热所占比例很大的现状,为了克服锅炉运行效率低,能源浪费严重的情况,本文介绍了模糊PID控制器如何实现对锅炉炉排、鼓风、引风、循环水量和给水温度的控制,增强了控制系统的鲁棒性、提高控制系统的精度,解决了锅炉热负荷稳定与实时调节的矛盾,实现了按需供热。 相似文献