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81.
《Materials Science & Technology》2013,29(6):523-539
In order to restrict the generation and propagation of cracks in thin wall metal parts fabricated by laser direct deposition shaping method, using ‘element birth and death’ technique, a three-dimensional multitrack and multilayer thin wall model was developed, and the deposition process was simulated. Different scanning methods, including long edge parallel reciprocating scanning, short edge parallel reciprocating scanning and interlayer orthogonal parallel reciprocating scanning, were introduced. The effects of different substrate preheating temperatures were also researched. The von Mises equivalent stress and its X-, Y- and Z-directional principal stresses were analysed in detail. Under the same conditions used in the simulations, the deposition experiments were conducted, and the crack generation and restriction mechanism of thin wall metal parts were further discussed. 相似文献
82.
We extend classical eigenstructure assignment to more realistic problems, where additional performance and robustness specifications arise. Our aim is to combine time-domain constraints, as reflected by pole location and eigenvector structure, with frequency-domain objectives such as the H2, H∞ or Hankel norms. Using pole clustering, we allow poles to move in polydisks of prescribed size around their nominal values, driven by optimisation. Eigenelements, that is poles and eigenvectors, are allowed to move simultaneously and serve as decision variables in a specialised non-smooth optimisation technique. Two aerospace applications illustrate the power of the new method. 相似文献
83.
Noise feedback coding(NFC) has attracted renewed interest with the recent standardization of backward-compatible enhancements for ITU-T G.711 and G.722.It has also been revisited with the emergence of proprietary speech codecs,such as BV16,BV32,and SILK,that have structures different from CELP coding.In this article,we review NFC and describe a novel coding technique that optimally shapes coding noise in embedded pulse-code modulation(PCM) and embedded adaptive differential PCM(ADPCM).We describe how this new technique was incorporated into the recent ITU-T G.711.1,G.711 App.III,and G.722 Annex B(G.722B) speech-coding standards. 相似文献
84.
This paper proposes a transmit-diversity system using a pair of orthogonal pulses. The system uses a set of orthonormal-basic functions, which contains four shaped-sinusoidal pulses with the same frequency. The first two elements in the set are shaped sine and cosine pulses. The second two elements are the same sine and cosine pulses but they are shaped with the Hilbert transform of the shaping pulse of the first two elements. The modulator in the proposed system produces two modulated symbols for each data symbol. It uses the first two elements in the proposed set in modulating the first modulated symbol and the second two elements in modulating the second modulated symbol. The modulated symbols are transmitted though two antennas. The diversity order of the proposed system is twice the number of antennas in the receiver. In the proposed system, no space-time coding is used and the channel gains change every symbol period. This is different from the Multiple-Input-Multiple-Output (MIMO) system. The receiver of the proposed system consists of two matched filters for each receiving antenna. No special detectors or interference cancelation techniques are used because there is no interference between the outputs of the matched filters. 相似文献
85.
The inter-area oscillations are common in power systems and can occur due to the changes in the load or generating power especially in long transmission lines. This paper presents the design of a robust fixed-order loop shaping controller to damp out the inter-area oscillations and to enhance the stability of the power system. The proposed loop shaping method is based on the shaping of the open-loop transfer function in the Nyquist diagram through minimizing the quadratic error between the actual and the desired open loop transfer functions in the frequency domain. The proposed method is robust with respect to multi-model uncertainty. Despite other robust controller design methods, the proposed approach deals with the entire system i.e. there is no need to reduce the system and still leads to a lower order controller. In addition, most of the robust methods are heavily dependent on selecting some weight filters which is not required in the proposed approach. This method is applied to the two-area four-machine system and 68 bus system and the effectiveness and robustness of the proposed method in damping inter-area oscillations are validated using case studies. 相似文献
86.
87.
采用双面同步平-仰焊工艺,对8 mm厚5083铝合金进行试验研究,分析了熔池的成形机理,并测试了接头的力学性能.结果表明,工件熔透时形成“公共熔池”,当破坏力大于维持力时,熔池液面下凹,下凹程度随热输入的增大而增大,弯曲液面产生附加压力,压力值与液面的凹陷程度成正比,最终熔池受力达到平衡,形成上凹下凸的“倒拱桥”形接头;双面同步平-仰焊的熔化效率达到了17.29%,远高于单面弧焊的2.20%;接头的平均抗拉强度与断后伸长率分别为275.14 MPa和14.73%,达到母材的90.70%和53.18%. 相似文献
88.
89.
针对非圆齿轮加工中,由齿坯节曲线曲率变化所引起的分次进刀过程中齿面加工余量分布不均的问题,提出一种匀化非圆齿轮齿面加工余量的工艺方法。以非圆齿轮插削工艺为研究对象,基于插齿加工原理,对齿面加工余量分布不均的成因进行了分析;在此基础上建立了非圆齿轮匀化工艺插削联动模型,通过实时调整插刀与齿坯的几何位置关系,实现了非圆齿轮齿面加工余量的匀化;开发了非圆齿轮插齿CAM系统,利用CAM系统对齿面加工余量匀化工艺进行了仿真验证,仿真结果表明,所提工艺方法正确、可行;将匀化工艺集成到自主开发的齿轮加工数控系统中,进行了非圆齿轮插削加工试验,试验结果表明,所提工艺方法能够有效匀化齿面加工余量;对加工出的非圆齿轮齿面进行了三维形貌检测,检测结果表明,所提工艺方法能够显著提高非圆齿轮的加工精度与表面质量。 相似文献
90.