全文获取类型
收费全文 | 516篇 |
免费 | 28篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 14篇 |
综合类 | 2篇 |
化学工业 | 132篇 |
金属工艺 | 10篇 |
机械仪表 | 14篇 |
建筑科学 | 34篇 |
矿业工程 | 5篇 |
能源动力 | 32篇 |
轻工业 | 44篇 |
水利工程 | 14篇 |
石油天然气 | 10篇 |
无线电 | 49篇 |
一般工业技术 | 78篇 |
冶金工业 | 22篇 |
原子能技术 | 1篇 |
自动化技术 | 85篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 15篇 |
2021年 | 45篇 |
2020年 | 36篇 |
2019年 | 32篇 |
2018年 | 43篇 |
2017年 | 33篇 |
2016年 | 35篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 67篇 |
2012年 | 30篇 |
2011年 | 33篇 |
2010年 | 13篇 |
2009年 | 20篇 |
2008年 | 10篇 |
2007年 | 14篇 |
2006年 | 8篇 |
2005年 | 7篇 |
2004年 | 2篇 |
2003年 | 2篇 |
2002年 | 2篇 |
2001年 | 2篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 2篇 |
排序方式: 共有546条查询结果,搜索用时 13 毫秒
1.
A series of new alternating aromatic poly(ester‐imide)s were prepared by the polycondensation of the preformed imide ring‐containing diacids, 2,2′‐bis(4‐trimellitimidophenoxy)biphenyl (2a) and 2,2′‐bis(4‐trimellitimidophenoxy)‐1,1′‐binaphthyl (2b) with various aromatic dihydroxy compounds in the presence of pyridine and lithium chloride. A model compound (3) was also prepared by the reaction of 2b with phenol, its synthesis permitting an optimization of polymerization conditions. Poly(ester‐imides) were fully characterized by FTIR, UV‐vis and NMR spectroscopy. Both biphenylene‐ and binaphthylene‐based poly(ester‐imide)s exhibited excellent solubility in common organic solvents such as tetrahydrofuran, m‐cresol, pyridine and dichloromethane. However, binaphthylene‐based poly(ester‐imide)s were more soluble than those of biphenylene‐based polymers in highly polar organic solvents, including N‐methyl‐2‐pyrrolidone, N,N‐dimethylacetamide, N,N‐dimethylformamide and dimethyl sulfoxide. From differential scanning calorimetry thermograms, the polymers showed glass‐transition temperatures between 261 and 315 °C. Thermal behaviour of the polymers obtained was characterized by thermogravimetric analysis, and the 10 % weight loss temperatures of the poly(ester‐imide)s was in the range 449–491 °C in nitrogen. Furthermore, crystallinity of the polymers was estimated by means of wide‐angle X‐ray diffraction. The resultant poly(ester‐imide)s exhibited nearly an amorphous nature, except poly(ester‐imide)s derived from hydroquinone and 4,4′‐dihydroxybiphenyl. In general, polymers containing binaphthyl units showed higher thermal stability but lower crystallinity than polymers containing biphenyl units. Copyright © 2005 Society of Chemical Industry 相似文献
2.
Extracellular recordings and immunohistological detection of c-Fos-like immunoreactive proteins were used to determine the synaptic effect of the parafascicular projection to the globus pallidus. Electrical stimulation of the parafascicular neurons induced a single-spike excitatory response with a stable latency of 2.3 ms, suggesting a monosynaptically driven effect. Pharmacological stimulation of the parafascicular nucleus with carbachol increased tonically the pallidal discharge rate by 142%. The discharge rate of the pallidal neurons was described by 37% in parafascicular-lesioned rats. These results demonstrate the excitatory nature and the tonic action of the parafasciculopallidal projection. Carbachol activation of parafascicular neurons also induced the synthesis of c-Fos-like immunoreactive proteins in the pallidal neurons. Control experiments in subthalamic-lesioned rats showed that the parafascicular excitation of the pallidal neurons remained, but both electrophysiological and expression of c-Fos-like immunoreactive proteins were attenuated. This suggests that the direct parafascicular excitation of the pallidal neurons is indirectly reinforced by the previously described parafascicular excitatory input to the subthalamic nucleus. Conversely, the effect of this last input to the subthalamic nucleus is dramatically enhanced in rats with pallidal lesion. Our results demonstrate the complex role of the parafascicular nucleus in activating both the globus pallidus and the subthalamic nucleus, two closely related structures. These results illustrate the integrative capacities of the globus pallidus, whose activity is modulated by multiple afferents. 相似文献
3.
4.
Anas Hatim Said Belkouch Mohamed El Aakif Moha M’rabet Hassani Noureddine Chabini 《Multimedia Tools and Applications》2013,67(3):667-685
The Discrete Cosine Transform (DCT) is one of the most widely used techniques for image compression. Several algorithms are proposed to implement the DCT-2D. The scaled SDCT algorithm is an optimization of the DCT-1D, which consists in gathering all the multiplications at the end. In this paper, in addition to the hardware implementation on an FPGA, an extended optimization has been performed by merging the multiplications in the quantization block without having an impact on the image quality. A simplified quantization has been performed also to keep higher the performances of the all chain. Tests using MATLAB environment have shown that our proposed approach produces images with nearly the same quality of the ones obtained using the JPEG standard. FPGA-based implementations of this proposed approach is presented and compared to other state of the art techniques. The target is an an Altera Cyclone II FPGA using the Quartus synthesis tool. Results show that our approach outperforms the other ones in terms of processing-speed, used resources and power consumption. A comparison has been done between this architecture and a distributed arithmetic based architecture. 相似文献
5.
Wakrim Layla El Yassini Abdessalam Khabba Asma Ibnyaich Saida Zeroual Abdelouhab Hassani Moha M’Rabet 《Wireless Personal Communications》2022,126(2):1145-1175
Wireless Personal Communications - Due to the development and growth of Internet platforms and web services as communication resources, the competition for the network and its limited resources is... 相似文献
6.
Jalil Layegh Fariborz Jolai Mohsen Sadegh Amalnik 《Advances in Engineering Software》2009,40(10):1074-1077
In this paper, we consider minimizing total weighted completion time criteria on a single machine. Jobs processing times are step function of its starting time and all jobs have a common due date. First, we present some new lemmas and dominance properties for this NP-hard problem, and then a memetic algorithm using these properties is developed. We compare the solutions of the memetic algorithm with optimal solutions obtained from complete enumeration. The results show that the average percentage error of the proposed algorithm from optimal solutions is about 2% and as the variance of processing time increase, the percentage errors decrease. 相似文献
7.
8.
Interaction between refractory crucible materials and the melted NiTi shape-memory alloy 总被引:2,自引:0,他引:2
S. K. Sadrnezhad Sadegh Badakhshan Raz 《Metallurgical and Materials Transactions B》2005,36(3):395-403
Attempts have been made to quantify the amount of contaminants absorbed by liquid metal from commercial ZrO2-, Al2O3-, and SiC-base crucibles used for vacuum melting of Ni-45 wt pct Ti alloy. The molten alloy was held under vacuum for 90
minutes at 1450 °C to become homogenized. Reactions between the liquid metal and the crucible were investigated by visual
observation, chemical analysis, scanning electron microscopy (SEM) image processing, and X-ray mapping. The relative degree
of contamination declined in the following sequence: commercially pure SiC>SiC-5 wt pct Al2O3-5 wt pct SiO2>slurry cast alumina>recrystallized alumina>zircon type A>oxygen deficient high-purity zirconia. Thermodynamic calculations
showed a difference between the equilibrium and the experimental data, indicating that except for commercially pure SiC crucible,
the amount of the crucible elements entering the melt is greater than the calculated equilibrium values. This discrepancy
seems to be due to the immersion into the melt of the undissolved chemical compounds formed due to the reactions between the
crucible and the liquid phase. 相似文献
9.
The simultaneous perturbation stochastic approximation (SPSA) algorithm has attracted considerable attention for challenging optimization problems where it is difficult or impossible to obtain a direct gradient of the objective (say, loss) function. The approach is based on a highly efficient simultaneous perturbation approximation to the gradient based on loss function measurements. SPSA is based on picking a simultaneous perturbation (random) vector in a Monte Carlo fashion as part of generating the approximation to the gradient. This paper derives the optimal distribution for the Monte Carlo process. The objective is to minimize the mean square error of the estimate. The authors also consider maximization of the likelihood that the estimate be confined within a bounded symmetric region of the true parameter. The optimal distribution for the components of the simultaneous perturbation vector is found to be a symmetric Bernoulli in both cases. The authors end the paper with a numerical study related to the area of experiment design 相似文献
10.
Dali Bondar C.J. Lynsdale N.B. Milestone N. Hassani A.A. Ramezanianpour 《Construction and Building Materials》2011,25(10):4065-4071
Natural pozzolans can be activated and condensed with sodium silicate in an alkaline environment to synthesize high performance cementitious construction materials with low environmental impact. The nature of the starting materials including mineral composition, chemical composition and crystal structure groups affects the formation of the geopolymer gel phase. In this paper, the pozzolanic activities of five natural pozzolans are studied. From XRD and XRF results, most of the raw materials contain zeolite clay minerals and have a high loss on ignition. Therefore, before use, samples were calcined at 700, 800 and 900 °C, respectively. The improvement in pozzolanic properties was studied following heat treatment including calcinations and/or elevated curing temperature by using alkali solubility and compressive strength tests. The results show that pozzolan containing sodium zeolite clinoptilolite can be used to prepare a moderate to high strength binder by heat treatment and calcinations can impart disorder hornblende as a constituent of pozzolan with no amorphous phase to prepare a moderate strength binder. 相似文献