全文获取类型
收费全文 | 2363篇 |
免费 | 94篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 26篇 |
综合类 | 17篇 |
化学工业 | 690篇 |
金属工艺 | 52篇 |
机械仪表 | 53篇 |
建筑科学 | 157篇 |
矿业工程 | 3篇 |
能源动力 | 36篇 |
轻工业 | 202篇 |
水利工程 | 11篇 |
石油天然气 | 4篇 |
无线电 | 147篇 |
一般工业技术 | 475篇 |
冶金工业 | 196篇 |
原子能技术 | 21篇 |
自动化技术 | 373篇 |
出版年
2023年 | 29篇 |
2022年 | 54篇 |
2021年 | 77篇 |
2020年 | 42篇 |
2019年 | 49篇 |
2018年 | 56篇 |
2017年 | 45篇 |
2016年 | 60篇 |
2015年 | 49篇 |
2014年 | 78篇 |
2013年 | 114篇 |
2012年 | 102篇 |
2011年 | 173篇 |
2010年 | 116篇 |
2009年 | 123篇 |
2008年 | 119篇 |
2007年 | 103篇 |
2006年 | 98篇 |
2005年 | 88篇 |
2004年 | 80篇 |
2003年 | 59篇 |
2002年 | 59篇 |
2001年 | 55篇 |
2000年 | 50篇 |
1999年 | 50篇 |
1998年 | 75篇 |
1997年 | 52篇 |
1996年 | 31篇 |
1995年 | 30篇 |
1994年 | 36篇 |
1993年 | 24篇 |
1992年 | 17篇 |
1991年 | 30篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 16篇 |
1983年 | 18篇 |
1981年 | 13篇 |
1978年 | 6篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1974年 | 8篇 |
1972年 | 8篇 |
1970年 | 11篇 |
1969年 | 11篇 |
1968年 | 9篇 |
排序方式: 共有2463条查询结果,搜索用时 0 毫秒
991.
Role of single‐source‐precursor structure on microstructure and electromagnetic properties of CNTs‐SiCN nanocomposites 下载免费PDF全文
Xingmin Liu Zhaoju Yu Lingqi Chen Binbin Xu Shuang Li Xiaowei Yin Ralf Riedel 《Journal of the American Ceramic Society》2017,100(10):4649-4660
Novel single‐source‐precursors (SSPs), namely carbon nanotube modified poly (methylvinyl) silazane (CNTs‐HTT 1800), were synthesized via amidation reaction of poly (methylvinyl) silazane (HTT 1800) with carboxylic acid functionalized carbon nanotubes (CNTs‐COOH) at the assistance of ZnCl2 catalyst, which was confirmed by means of Fourier transform infrared spectra (FT IR) and transmission electron microscopy (TEM). Besides, the TEM results unambiguously show the homogeneous distribution of the CNTs in the matrix of SSPs while serious aggregation of the CNTs in the matrix of physically‐blended‐precursor. Crack‐free monolithic silicon carbonitride modified by carbon nanotubes ceramic nanocomposites (CNTs‐SiCN) were prepared through pyrolysis of the obtained SSP green bodies at 1000°C. Due to the strong influence of polymer structure on the microstructure of final ceramics, the SSP‐derived CNTs‐SiCN nanocomposites clearly show the homogeneous distribution of the CNTs in the SiCN matrix while the physically‐blended‐precursor derived CNTs‐SiCN nanocomposites exhibit serious aggregation and entangling of the CNTs in the SiCN matrix. With the same CNT content in the feed, the SSP‐derived CNTs‐SiCN nanocomposites possess significant improvements of electromagnetic (EM) absorbing properties compared to those from physically‐blended‐precursors, due to the quality of the dispersion of CNTs in the ceramic matrices. 相似文献
992.
One of the main problems in using parabolic concentrators with standard photovoltaics (PV) cells is the highly non-uniform illumination of the cells. The non-uniform irradiation causes high resistive losses in the standard cells due to their relatively high series resistance. This results in a considerably lowered efficiency. To solve the problem, we introduce three different structured reflectors that will create a more uniform illumination, and also increase the concentration ratio in certain cases. The structures were evaluated in an existing trough system by Monte Carlo ray tracing, and it was found that structures improve the system performance mainly by homogenizing the light on the cells. The yearly irradiation collected in the evaluation system is slightly lower than for a reference with smooth reflectors, but the more uniform illumination of the cells will generate a net increase of the total system performance compared to a system that was optimized with smooth reflectors. The benefit of the increased concentration ratio is increased flexibility in designing new systems with concentration ratios surpassing the limit of existing trough concentrators. 相似文献
993.
Linan An Ralf Riedel Christoph Konetschny H.-Joachim Kleebe Rishi Raj 《Journal of the American Ceramic Society》1998,81(5):1349-1352
The creep viscosity of chemical-precursor-derived silicon carbonitride (SiCN), which is known to remain predominantly amorphous at temperatures below 1400°C, was measured in the temperature range 1090-1280°C. Experiments were done in uniaxial compression at constant loads in pure nitrogen atmosphere. The creep behavior exhibited three stages. In stage I the strain rate decreased rapidly with time and deformation was accompanied by densification. In stage II the samples exhibited a steady-state creep rate. In stage III, which commenced after long-term deformation, creep gradually declined to rates that were below the sensitivity of our apparatus. The relative density of the specimens during stage II and stage III remained constant at ≅2.3 g/cm3 . The shear viscosity in stage II was nearly Newtonian and was measured to be 1.3 × 1013 -5.0 1013 Pa·s at 1280°C, which is approximately 103 times the value for fused silica. The creep-hardened as well as uncrept specimens contained silicon nitride crystallites. The volume fraction of these crystals was variable but always less than 5%. Such a small volume fraction of crystals does not explain the dramatic creep-hardening behavior in stage III, even if it is assumed that the crystals formed during creep deformation in stage II. 相似文献
994.
Fernando Bonatto Janio Venturini Ana C. Frantz Thaís C.L. dos Santos Carlos P. Bergmann Alexandre G. Brolo 《Ceramics International》2018,44(18):22345-22351
A single-step anodic fabrication of TiO2 nanotubes (TNT) without a surface layer of nanograss was attained in electrolytes containing diethylenetriamine-pentaaceticacid (DTPA) as chelating agent. The role of DTPA concentration and anodization voltage on the final TNT morphology was studied. We discuss chemical aspects of the DTPA effect over the surface of TNTs as well as the physical properties of TNT samples prepared under several anodization conditions. We demonstrate that the DTPA concentration has a significant effect on the final surface morphology of TNTs, leading to a surface free of nanograss. Furthermore, we propose a mechanism that explains this phenomenon. 相似文献
995.
Abhishek Sarkar Ruzica Djenadic Di Wang Christina Hein Ralf Kautenburger Oliver Clemens Horst Hahn 《Journal of the European Ceramic Society》2018,38(5):2318-2327
Multicomponent oxides with perovskite type of structure containing up to 10 different cations in equiatomic amounts have been synthesised for the first time. Out of eleven systems synthesised, only six systems crystallised as single phase perovskite type compounds with random and homogenous cation distribution on the respective sites. The formation of phase pure 10-cationic system, (Gd0.2La0.2Nd0.2Sm0.2Y0.2)(Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)O3, in contrast to the multiphase mixtures observed in five of the lower entropy systems (containing 6 cations) indicates a possible role of entropy in the stabilisation of a single phase crystal structure. The entropy driven structural stabilisation effect is further supported by the reversible phase transformation, from single phase to multiple phase upon cyclic heat treatment, observed in the (Gd0.2La0.2Nd0.2Sm0.2Y0.2)MnO3 system. This type of entropic signature has been observed in rocksalt based high entropy oxide systems. However, it has not been reported before for perovskite based compounds, as shown in this study. 相似文献
996.
Alena Movchikova Olga Malyshkina Gunnar Suchaneck Gerald Gerlach Ralf Steinhausen Hans Theo Langhammer Christoph Pientschke Horst Beige 《Journal of Electroceramics》2008,20(1):43-46
In this work, the thermal square wave method at single-frequency (TSWM) was applied to functionally graded BaTi1−x
Sn
x
O3 samples with a tin gradient of 0.075 ≤ x ≤ 0.15. The samples were fabricated by sintering of bi-, tri- and tetramorph green bodies. The polarization at the back side
with a higher tin content was in opposite direction to the polarization of the surface with a lower tin content. This was
attributed to the appearance of a space charge layer. The determined depth profiles of the pyroelectric coefficient illustrate
the appearance of a nearly constant polarization in the region of lower tin concentration of tri- and tetramorph samples.
The pyroelectric coefficient profiles were in good agreement with the averaged over sample thickness pyroelectric coefficient
obtained separately by the quasistatic method. 相似文献
997.
Mass spectrometry-based metabolomics is the comprehensive study of naturally occurring small molecules collectively known as the metabolome. Given the vast structural diversity and chemical properties of endogenous metabolites, biological extraction and chromatography methods bias the number, property, and concentration of metabolites detected by mass spectrometry and creates a challenge for global untargeted studies. In this work, we used Escherichia coli bacterial cells to explore the influence of solvent polarity, temperature, and pH in extracting polar and nonpolar metabolites simultaneously. In addition, we explored chromatographic conditions involving different stationary and mobile phases that optimize the separation and ionization of endogenous metabolite extracts as well as a mixture of synthetic standards. Our results reveal that hot polar solvents are the most efficient in extracting both hydrophilic and hydrophobic metabolites simultaneously. In addition, ammonium fluoride in the mobile phase substantially improved ionization efficiency in negative electrospray ionization mode by an average increase in signal intensity of 5.7 and over a 2-fold increase in the total number of features detected. The improvement in sensitivity with ammonium fluoride resulted in 3.5 times as many metabolite hits in databases compared to ammonium acetate or formic acid enriched mobile phases and allowed for the identification of unique metabolites involved in fundamental cellular pathways. 相似文献
998.
An overview on the potential of medium to atmospheric pressure processes will be given and discussed in context to production applications. The introduction of the medium to atmospheric pressure plasma source is a huge progress and a valuable, attractive tool. The plasma source bases on the concept of cy lindrical r esonator with annu lar s lots (CYRANNUS®). High process speed and reliability are the most important facts for technical applications. Parameters as gas/flow dynamic can be controlled and lead to further improvements of equipment and process design. The CYRANNUS® plasma is used in down‐stream configuration for several applications such as glass films on polymer substrate to reduce oxygen permeation, protective coatings against corrosion or wear and hydrophilic or hydrophobic surface properties. Also etching or cleaning can be done easily because of arbitrary process gas in the plasma source. The advantage of medium pressure is higher supply of reactive species and improved transportation of reaction products. Various processes with plasma surface interaction are discussed from physical and technical viewpoint as plasma enhanced CVD, plasma polymerisation on metal or polymer substrates. Using the advantages of medium pressure plasma processes consequently leads to new design of equipment and material flow. In‐line/on‐line treatment of 3‐D material becomes most efficient and enables competitive plasma processes for mass productions. For diamond deposition a larger size of the plasma at high pressure processes are essential for a device with best economics. 相似文献
999.
Sebastian Wandelt Ralf Moller Michael Wessel 《International Journal of Software and Informatics》2010,4(3):201-218
In this paper, we consider the problem of query answering over multimedia
ontologies. Traditional reasoning systems may have problems to deal with large amounts
of expressive ontological data (terminological as well as assertional data) that usually must
be kept in main memory. We propose to overcome this problem with a new so-called filter
and refine paradigm for ontology-based query answering. The contribution of this paper is
twofold: (1) For both steps, algorithms are presented. (2) We evaluate our approach on real
world multimedia ontologies from the BOEMIE project. 相似文献
1000.
Y. T. Candace Ho Prof. Dr. Ralf B. Schittenhelm Dr. Dumitrita Iftime PD Dr. Evi Stegmann Dr. Julien Tailhades Prof. Dr. Max J. Cryle 《Chembiochem : a European journal of chemical biology》2023,24(6):e202300045
The glycopeptide antibiotics (GPAs) are a clinically approved class of antimicrobial agents that classically function through the inhibition of bacterial cell-wall biosynthesis by sequestration of the precursor lipid II. The oxidative crosslinking of the core peptide by cytochrome P450 (Oxy) enzymes during GPA biosynthesis is both essential to their function and the source of their synthetic challenge. Thus, understanding the activity and selectivity of these Oxy enzymes is of key importance for the future engineering of this important compound class. Recent reports of GPAs that display an alternative mode of action and a wider range of core peptide structures compared to classic lipid II-binding GPAs raises the question of the tolerance of Oxy enzymes for larger changes in their peptide substrates. In this work, we explore the ability of Oxy enzymes from the biosynthesis pathways of lipid II-binding GPAs to accept altered peptide substrates based on a vancomycin template. Our results show that Oxy enzymes are more tolerant of changes at the N terminus of their substrates, whilst C-terminal extension of the peptide substrates is deleterious to the activity of all Oxy enzymes. Thus, future studies should prioritise the study of Oxy enzymes from atypical GPA biosynthesis pathways bearing C-terminal peptide extension to increase the substrate scope of these important cyclisation enzymes. 相似文献