全文获取类型
收费全文 | 126402篇 |
免费 | 5095篇 |
国内免费 | 2408篇 |
专业分类
电工技术 | 3244篇 |
技术理论 | 2篇 |
综合类 | 4754篇 |
化学工业 | 18897篇 |
金属工艺 | 7299篇 |
机械仪表 | 5858篇 |
建筑科学 | 5156篇 |
矿业工程 | 1861篇 |
能源动力 | 2362篇 |
轻工业 | 6380篇 |
水利工程 | 1908篇 |
石油天然气 | 2818篇 |
武器工业 | 337篇 |
无线电 | 14503篇 |
一般工业技术 | 22014篇 |
冶金工业 | 5052篇 |
原子能技术 | 746篇 |
自动化技术 | 30714篇 |
出版年
2024年 | 248篇 |
2023年 | 882篇 |
2022年 | 1513篇 |
2021年 | 2165篇 |
2020年 | 1579篇 |
2019年 | 1465篇 |
2018年 | 15732篇 |
2017年 | 14746篇 |
2016年 | 11273篇 |
2015年 | 2494篇 |
2014年 | 2481篇 |
2013年 | 2880篇 |
2012年 | 5861篇 |
2011年 | 12410篇 |
2010年 | 10642篇 |
2009年 | 7903篇 |
2008年 | 9127篇 |
2007年 | 9887篇 |
2006年 | 2402篇 |
2005年 | 3029篇 |
2004年 | 2440篇 |
2003年 | 2346篇 |
2002年 | 1652篇 |
2001年 | 1071篇 |
2000年 | 1123篇 |
1999年 | 1057篇 |
1998年 | 885篇 |
1997年 | 740篇 |
1996年 | 673篇 |
1995年 | 537篇 |
1994年 | 421篇 |
1993年 | 286篇 |
1992年 | 240篇 |
1991年 | 227篇 |
1990年 | 169篇 |
1989年 | 155篇 |
1988年 | 110篇 |
1987年 | 89篇 |
1986年 | 57篇 |
1985年 | 51篇 |
1984年 | 37篇 |
1983年 | 37篇 |
1968年 | 43篇 |
1966年 | 43篇 |
1965年 | 44篇 |
1959年 | 36篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Seung Han Ryu Seil Kim Young-Tae Kwon Young Ki Park Sung-Oong Kang Hong-Baek Cho Yong-Ho Choa 《应用聚合物科学杂志》2020,137(8):48390
Here, we report a facile approach to electrostatically couple the surface charges of graphite nanoplate (GNP) fillers and poly(methyl methacrylate) (PMMA) polymer particles using ethylene maleic anhydride (EMA) copolymer as an electrostatic coupling agent. Our strategy involved switching the intrinsic repulsive electrostatic interactions between the directly exfoliated GNPs fillers and the PMMA particles to attractive electrostatic surface interactions for preparing core(PMMA)-shell (GNP) precursor in order to optimizing 3-dimensionally dispersed polymer nanocomposite. As a result, the electrical conductivity of the composites dramatically increased by a factor of 16.7 in the EMA-coupled GNP/PMMA composites compared with that of the EMA-free GNP/PMMA composites. In addition, the percolation threshold was also notably reduced from 0.32 to 0.159 vol% after electrostatic coupling of the GNPs fillers and PMMA particles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48390. 相似文献
52.
Pavel Strunz Debashis Mukherji Přemysl Beran Ralph Gilles Lukas Karge Michael Hofmann Markus Hoelzel Joachim Rösler Gergely Farkas 《Metals and Materials International》2018,24(5):934-944
An addition of boron largely increases the ductility in polycrystalline high-temperature Co–Re alloys. Therefore, the effect of boron on the alloy structural characteristics is of high importance for the stability of the matrix at operational temperatures. Volume fractions of ε (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr2Re3 type) phases were measured at ambient and high temperatures (up to 1500 °C) for a boron-containing Co–17Re–23Cr alloy using neutron diffraction. The matrix phase undergoes an allotropic transformation from ε to γ structure at high temperatures, similar to pure cobalt and to the previously investigated, more complex Co–17Re–23Cr–1.2Ta–2.6C alloy. It was determined in this study that the transformation temperature depends on the boron content (0–1000 wt. ppm). Nevertheless, the transformation temperature did not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm of boron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which binds hcp-stabilizing elements (Cr and Re). Moreover, borides were identified in alloys with high boron content. 相似文献
53.
Yong Liu Kangzhuang Chen Fengbo Dong Shuai Peng Yongjie Cui Chenyu Zhang Keqing Han Muhuo Yu Hui Zhang 《Ceramics International》2018,44(9):10199-10203
In this paper, polyborosilazane precursor was synthesied from HMDZ, HSiCl3, BCl3 and CH3NH2 using a multistep method. By controlling the storage conditions, parts of the polyborosilazane fibers were hydrolyzed. FT-IR, NMR, XRD, TEM and monofilament tensile strength test were employed to study the effects of hydrolysis of precursor on the structures and properties of polymer-derived SiBN ceramic fibers. FT-IR and NMR results indicate that Si-N group in PBSZ reacts with H2O to form Si-O-Si group. After pyrolysis reaction at 1400℃, Si-O-Si group will finally transformed into highly ordered cristobalite and β-quartz, resulting in formation of the wrinkled surface of the obtained SiBN ceramic fiber. The strip-like defects on fiber surface, according to monofilament tensile strength test, had a significant effect on mechanical property of the obtained SiBN ceramic fiber and caused no increase in fiber tensile strength of hydrolytic polyborosilazane fiber before and after pyrolytic process. 相似文献
54.
The molecular design of short peptides to achieve a tailor-made functional architecture has attracted attention during the past decade but remains challenging as a result of insufficient understanding of the relationship between peptide sequence and assembled supramolecular structures. We report a hybrid-resolution model to computationally explore the sequence–structure relationship of self-assembly for tripeptides containing only phenylalanine and isoleucine. We found that all these tripeptides have a tendency to assemble into nanofibers composed of laterally associated filaments. Molecular arrangements within the assemblies are diverse and vary depending on the sequences. This structural diversity originates from (1) distinct conformations of peptide building blocks that lead to different surface geometries of the filaments and (2) unique sidechain arrangements at the filament interfaces for each sequence. Many conformations are available for tripeptides in solution, but only an extended β-strand and another resembling a right-handed turn are observed in assemblies. It was found that the sequence dependence of these conformations and the packing of resulting filaments are determined by multiple competing noncovalent forces, with hydrophobic interactions involving Phe being particularly important. The sequence pattern for each type of assembly conformation and packing has been identified. These results highlight the importance of the interplay between conformation, molecular packing, and sequences for determining detailed nanostructures of peptides and provide a detailed insight to support a more precise design of peptide-based nanomaterials. 相似文献
55.
Antti Voss Niko Hänninen Mohammad Pour-Ghaz Marko Vauhkonen Aku Seppänen 《Materials and Structures》2018,51(3):68
The development of visualizing tools to monitor unsaturated moisture flow in cement-based materials is of great importance, as most degradation processes in cement-based materials are connected to and take place in the presence moisture. This paper investigates the ability of electrical capacitance tomography (ECT) to image two-dimensional (2D) unsaturated moisture flow in cement-based materials. In ECT, the electrical permittivity distribution within an object is reconstructed based on measured capacitances between electrodes attached on the object’s surface. In a series of experiments, mortar specimens with and without discrete cracks were imaged with ECT during a 2D moisture ingress. The results show that ECT is able to monitor the evolution of the moisture flow, and to approximate the shape and position of the moisture front. These findings indicate that ECT is a viable method for monitoring and visualizing 2D unsaturated moisture flow in cement-based materials in the presence and absence of discrete cracks. 相似文献
56.
An agriglass composition containing different oxides acts as a slow release for macro and micro nutrients and was chosen to improve maize yield under most important abiotic stresses which affecting agriculture development; salinity and drought. A field experiment was performed in salt affected soil (EC =?7.5 dSm??1) by using different water deficit rates (I1 = 100, I2 = 85 and I3 = 70% of maize water requirements). Irrigation levels were located in main plots. Every main-plot divided into six sub-plots contained glassy fertilizer treatments [F1 = 55 kg fed?1 with 1/2 mm diameter of agriglass (fed. =?4200 m2), F2 = 55 kg fed?1 with 1 mm diameter, F3 = 80 kg fed?1 with 1/2 mm diameter, F4 = 80 kg fed?1 with 1 mm diameter, F5 = Recommendations of Ministry of Agriculture and F6 = control]. The experimental results demonstrated that, ears, straw, grains and biological yields increased with increasing both water and agriglass rates. Application of agriglass as a slow release fertilizer improved yield more than mineral fertilizer. Some growth parameters, water use efficiency (IWUE), macronutrients concentration and their relations were included. Other studies on residual effect of agriglass and the annual application rates to withstand salinity and drought stress by strategic crops are required. 相似文献
57.
Ahmed Obaid M. Alzahrani M. Sh. Abdel-wahab Meshari Alayash M. S. Aida 《Journal of Materials Science: Materials in Electronics》2018,29(19):16317-16324
In this work, p-NiO/n-ZnO heterostructures were successfully prepared at room temperature using RF sputtering technique. The influence of ZnO layer thickness on the performance of the heterojunction was investigated. The deposited ZnO layers have a hexagonal Wurtzite structure with preferable growth orientations along (002) and (103) for thinner films. Increasing the thickness results in more crystallographic orientation randomness. The current–voltage measurements of the realized heterojunctions showed a clear rectifying behavior. The measured ideality factor varies from 2.5 to 1.6 according to the thickness of ZnO layer. The series resistance of the device is enlarged with increasing ZnO thickness. The deduced parameters from the I–V characteristics suggest that 200 nm is the optimal thickness of the ZnO layer according to our experimental conditions. We attribute the relatively better performance of this thickness to achieving reasonable compensation between serial resistance and ideality factor. The best heterojunction was tested and successfully used as a UV detector. 相似文献
58.
Video transmission over IEEE 802.11e wireless networks still shows poor performance for large bandwidth demand and frequently changed environments. Thus, several enhancements of IEEE 802.11e were proposed. On the other hand, big frames and simultaneous sending of adjacent frames always cause packet dropping for buffer overflow. In the past, we proposed an IEEE 802.11e enhancement named DFAA and a content aware mechanism to solve the above problems. The motivation of this paper is to find a proper way to integrate these two mechanisms. A DFAA enhancement (DFAA-E) is proposed to make up the insufficiency of content aware mechanism. Experiments results show that the combination of DFAA-E and content aware mechanism improves the video decoded quality greatly. And its performance can be further enhanced by selecting the suitable settings of certain parameters. 相似文献
59.
Mohammad Reza Gaeeni Morteza Sasani Ghamsari Abbas Majd Abadi Mohammad Hossein Majles Ara Wooje Han Hyung‐Ho Park 《Journal of the American Ceramic Society》2015,98(6):1818-1822
A facile sol–gel procedure has been developed for the synthesis of colloidal alumina nanocrystals. For the first time, optical characterization procedures were employed to study the quantum confinement effects in optical properties of the prepared Al2O3 sol. Accordingly, the hyperbolic band model was used to determine the optical band gap of colloidal alumina nanocrystals. X‐Ray diffraction pattern was used to study the crystallographic phase of the dried gel. Morphological characterization was performed using scanning electron microscopy (SEM). Inductively Coupled Plasma (ICP) emission spectroscopy was used to determination purity of the Al2O3 powder. High‐resolution TEM showed that the diameter of colloidal nanocrystals is about 10 nm. Photoluminescence spectroscopy demonstrated that quantum yields for colloidal nanocrystals are 68% with 300 nm excitation wavelength. The experimental observations confirm that highly stable alumina sol with strong UV emission was synthesized. The mentioned optical properties have not been reported before. 相似文献
60.
Yachao Dong Christos T. Maravelias Norman F. Jerome 《Optimization and Engineering》2018,19(4):937-976
We study a maritime inventory routing problem, in which shipments between production and consumption nodes are carried out by a fleet of vessels. The vessels have specific capacities and can be chartered under different agreements. The inventory levels of all consumption nodes and some production nodes should be maintained within specified bounds; for the remaining production nodes, orders should be picked up within pre-defined time windows. We propose a discrete-time mixed-integer programming model. In the face of new information and uncertainty, this optimization model has to be re-solved, as the horizon is rolled forward. We discuss how to account for different sources of uncertainty. We present a rolling-horizon reoptimization framework that allows us to study different policies that impact the quality of the implemented solution, so we can identify the optimal set of policies. 相似文献