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排序方式: 共有819条查询结果,搜索用时 15 毫秒
1.
The combined effect of resistance spot welding and precipitation hardening on the localised corrosion of A286 superalloy is studied. The specimens tested by double loop electrochemical potentiokinetic reactivation were welded in the solution treated condition, and then subjected to different precipitation hardening treatments. For both base metal and weld nugget, the maximum localised corrosion is reached when η phase is clearly observable. The fact that the localised corrosion resistance of weld nugget is different from that shown by base metal may be explained by the segregation of Ni and Ti towards the interdendritic region of weld nugget (studied by using scanning electron microscope/energy dispersive X-ray analysis).  相似文献   
2.
The outcome of the cutting blasting in a one-step shaft excavation is heavily related to the cutting parameters used for parallel cutting method. In this study, the relationships between the cutting parameters (such as the hole spacing L and the empty hole diameter D) and damage zones were investigated by numerical simulation. A damage state index γ was introduced and used to characterize the crushing and crack damage zones through a user-defined subroutine. Two indices, i.e., η1 and η2 that can reflect the cutting performance, were also introduced. The simulation results indicate that an optimal value of L can be obtained so that the η1 and η2 can reach their optimal states for the best cutting performance. A larger D results in better cutting performance when the L value maintains its best. In addition, the influences of the loading rate and the in-situ stress on the cutting performance were investigated. It is found that an explosive with a high loading rate is suit for cutting blasting. The propagation direction and the length of the tensile cracks are affected by the direction and the magnitude of the maximum principal stress.  相似文献   
3.
The modular multilevel converter (MMC) is attractive for high-power applications because of the advantages of its high modularity and high power quality. This paper proposes a prime factorization-based grouping strategy for capacitor voltage balancing of MMC. With prime factors arranged from the largest to the smallest, the computation burden has been reduced significantly, which is proved mathematically. Then, a 401-level MMC adopting the proposed grouping strategy is implemented and evaluated in the PSCAD/EMTDC environment. Simulation results confirm the effectiveness of the proposed strategy for capacitor voltage balancing performance under both steady and dynamic states. And by comparing with conventional sorting algorithm, it can be concluded that the prime factorization pattern can significantly improve the simulation efficiency. Finally, the switching frequency of the proposed algorithm is discussed.  相似文献   
4.
Whilst gamma prime (γ′) phase is the strengthening phase in Ni-based superalloys its influence on machining has been seldom investigated. This paper reports for the first time on the effect of γ′ upon machining of Ni-based superalloys when cutting with parameters yielding different cutting temperature intervals which lead to strengthening/softening effects on the workpiece (sub)surface. In-depth XRD, SEM/FIB, EBSD analysis and unique micro-pillar testing in the workpiece superficial layers indicated that with the increase of γ′ fraction the grain plastic deformation significantly decreased, while specific cutting energy can switch from low to high values influenced by the real cutting temperature.  相似文献   
5.
以往的开环辨识方法仅适用于水电机组并大网模型,系统并入孤网或小网或空载运行时应采用闭环辨识。具有较好辨识效果的预测形式简约子空间闭环辨识方法(PARSIM-K)充分利用了马尔克夫参数矩阵的Toeplitz结构,通过奇异值分解降阶和线性投影获取模型参数,但需要选择合适的时域参数,目前尚无一般的方法。为此,建立了带有频率噪声的水轮机调速系统模型,提出基于粒子群优化算法参数优化的PARSIM-K。该方法利用粒子群优化算法优化时域参数p、f,提高了辨识精度。与传统开环方法相比,所提方法能够克服噪声的影响,更加简便、安全、实用。仿真结果表明,与未优化参数的方法相比,所提方法辨识的模型参数误差更小、模型精度更高。  相似文献   
6.
Two novel donor–acceptor (D–A)-type conjugated polymers of PTTPPz-BDT and PTTPPz-BDTT were successfully synthesized by Stille coupling polymerization, in which 7,8-dialkoxy benzo[1,2-b:4,5-b′]dithiophene (BDT) and 7,8-bithienyl benzo[1,2-b:4,5-b′]dithiophene (BDTT) were used as donor units, thiophene and pyrene-fused phenazine (PPz) were employed as π-bridges and acceptor units, respectively. High carrier mobilities, broad absorption spectra, narrow optical band gaps, and low HOMO energy levels were observed for both polymers. Furthermore, high-efficiency photovoltaic performance with power conversion efficiency (PCE) over 4.25% was exhibited in their polymer solar cells (PSCs) using [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC71BM) as acceptor. The maximum PCE of 4.86% with short-circuit current density of 11.10 mA cm−2 and fill factor of 62.5% was obtained in the PTTPPz-BDTT based cell. These results indicate that incorporating large planar PPz moiety into D–A-type copolymer is an efficient approach to improve photovoltaic performance for PSCs.  相似文献   
7.
The last decade has seen a growing interest in hybrid electrically conducting nanocomposites. This article aims to provide a detailed overview of the present status of research in carbon nanotube–polyaniline (CNT/PANI) composites, from processing to structural and property evaluations. CNT/PANI are synthesized by electrochemical and chemical processing. When chemical methods are used, the main challenge is to obtain processable CNT/PANI in the emeraldine salt (ES) form composites. Stable dispersions of ES–CNT in organic media are prepared using the post doping method, inverse emulsion polymerization, or ex situ polymerizations. On the contrary, stable water dispersions of CNT/ES are prepared using hydrophilization of a preformed CNT/ES composite, direct synthesis of micelle–CNT hybrid templates, interfacial polymerization, covalent functionalization of CNT with a water soluble polymer, or using electrostatic interactions between two oppositely charged ES and CNT aqueous colloids. Moreover, the strategies for the synthesis of ternary CNT/PANI composites incorporating noble metal nanoparticles, metal oxide, or graphene sheets are also presented and analyzed in depth. Finally, we give a review of potential applications, including chemical sensors, capacitors, fuel cells and electronic devices.  相似文献   
8.
Two bipolar host materials, mCPpPO and mCPmPO have been synthesized by Ni(II)/Zn-catalyzed cross-coupling of diphenylphosphine oxide and corresponding aryl bromide. The photophysical properties, HOMO/LUMO orbital distribution and triplet levels of these host materials are investigated and optimized by tuning the linking modes between electron acceptor triphenylphosphine oxide and electron donor N,N′-dicarbazolyl-3,5-benzene (mCP). When mCP is linked to the meta-position of benzene of triphenylphosphine oxide, the hybrid (mCPmPO) shows much higher steric hinderance than the para-position linked analogue (mCPpPO) so that it possesses a higher triplet energy. Equipped with the bipolar transport properties, mCPmPO-based blue PhOLED doped FIrpic shows a maximum current efficiency (ηc,max) of 40.0 cd/A, a maximum power efficiency (ηp,max) of 39.7 lm/W, corresponding the maximum external quantum efficiency (ηEQE,max) of 20.3%, and the current efficiency still maintain to 34.8 cd/A even at 1000 cd/m2. Based on the optimized triplet energy level, the single emission layer white PhOLED hosted by mCPmPO shows ηc,max, ηp,max and ηEQE,max of 46.9 cd/A, 39.7 lm/W and 17.6%, respectively.  相似文献   
9.
本文研究了一种基于虚拟同步发电机算法的微网逆变器控制策略。根据同步发电机的本体数学模型、一次调频及无功电压调节特性,分别设计了虚拟同步发电机算法、虚拟原动机调节及励磁电流调节模块,同时通过电压电流双环控制,使逆变器输出的电压幅值和频率具有良好的下垂特性、可调的转动惯量及较好的跟踪性能,从而提高了系统的稳定性和可靠性。此外在并网系统中要求频率控制精度高及实时响应的情况下,本文还对虚拟原动机调节模块进行了改进,实现了频率的无误差控制,提高了频率控制的精度和响应速度。最后在matlab/simulink中进行了模型的搭建和仿真,逆变器输出电压很好的模拟了同步发电机的外特性,单相电压的畸变率仅为0.2%,引入改进原动机调节模块后,输出频率不随负荷的变化发生偏移。最终稳定在49.98Hz,验证了本文理论分析的正确性和可行性。  相似文献   
10.
为了在保证安全性的前提下提高大量数据加密的加密速度,本文通过DES和RSA算法的分析,针对DES算法密钥安全性差和RSA算法加密速度慢的问题,提出DES和RSA混合算法.对比DES和RSA两种算法加密所需时间,就RSA算法耗时长的问题,通过几种可行的大素数判定算法分析,提出改进的大素数判定算法.在保证安全性的前提下,将改进的大素数判定算法应用于RSA算法中,提升了加密速度.由于改进算法加密速度仍远差于DES算法,不适合大量数据加密.因此在兼顾安全性与加密速度的基础上,提出DES和RSA混合加密算法.经分析该算法综合了前两种算法的优点,是一种理想的加密算法.  相似文献   
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