全文获取类型
收费全文 | 87625篇 |
免费 | 1139篇 |
国内免费 | 415篇 |
专业分类
电工技术 | 814篇 |
综合类 | 2320篇 |
化学工业 | 12378篇 |
金属工艺 | 4843篇 |
机械仪表 | 3098篇 |
建筑科学 | 2433篇 |
矿业工程 | 568篇 |
能源动力 | 1199篇 |
轻工业 | 3964篇 |
水利工程 | 1290篇 |
石油天然气 | 363篇 |
无线电 | 9463篇 |
一般工业技术 | 16896篇 |
冶金工业 | 3482篇 |
原子能技术 | 313篇 |
自动化技术 | 25755篇 |
出版年
2022年 | 46篇 |
2020年 | 45篇 |
2019年 | 54篇 |
2018年 | 14487篇 |
2017年 | 13431篇 |
2016年 | 10025篇 |
2015年 | 650篇 |
2014年 | 315篇 |
2013年 | 462篇 |
2012年 | 3260篇 |
2011年 | 9569篇 |
2010年 | 8434篇 |
2009年 | 5670篇 |
2008年 | 6931篇 |
2007年 | 7942篇 |
2006年 | 282篇 |
2005年 | 1345篇 |
2004年 | 1250篇 |
2003年 | 1287篇 |
2002年 | 633篇 |
2001年 | 181篇 |
2000年 | 249篇 |
1999年 | 122篇 |
1998年 | 168篇 |
1997年 | 112篇 |
1996年 | 107篇 |
1995年 | 70篇 |
1994年 | 83篇 |
1993年 | 76篇 |
1992年 | 63篇 |
1991年 | 71篇 |
1990年 | 59篇 |
1989年 | 69篇 |
1988年 | 71篇 |
1986年 | 59篇 |
1985年 | 59篇 |
1984年 | 62篇 |
1983年 | 51篇 |
1982年 | 54篇 |
1981年 | 56篇 |
1980年 | 46篇 |
1979年 | 58篇 |
1978年 | 59篇 |
1977年 | 50篇 |
1976年 | 51篇 |
1968年 | 50篇 |
1966年 | 44篇 |
1965年 | 49篇 |
1955年 | 71篇 |
1954年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
In this work was investigated the effect of the addition of barium titanate (BaTiO3) on electrical properties of two chemically recyclable thermosets, polyhemiaminal (PHA) and polyhexahydro‐s‐triazine (PHT), both fabricated from 4,4′‐oxydianiline (ODA), an ether derivative of aniline and paraformaldehyde. Thermal and mechanical properties as well as chemical recyclability of the two polymers and their nanocomposites/nanodielectrics were also investigated. In addition, a quantitative analysis was conducted of the nanoparticle dispersion in the PHA‐/PHT‐based BaTiO3‐containing nanocomposites using transmission electron microscopy imaging and the nearest‐neighbor distance index and this index was used to analyze the investigated properties in connection with the proper mechanisms. Regarding the electrical properties for both neat polymers, conductivity values of the order of 10?8 S m?1 at 100 Hz were observed and dielectric constant values close to 2.80 for both polymers at 1 kHz. The addition of 0.5 wt% of BaTiO3 ferroelectric nanoparticles increased by about 44% the dielectric constant (1 kHz) and conductivity (102 Hz) of the PHA‐based nanocomposite. PHA and PHT exhibited glass transition temperature (Tg) values in the range 125–180 °C. An increase of 7 °C in Tg was observed after the incorporation of 0.5 wt% of BaTiO3 into PHA. Concerning the mechanical properties, values in the range 4.00–4.45 GPa for reduced modulus and 0.30–0.43 GPa for nanohardness for PHA and PHT polymers were observed. Independently of filler content or polymer matrix, both mechanical properties were enhanced after the addition of BaTiO3. The chemical recycling of PHA/PHT and all nanocomposites in the initial ODA reagent after sulfuric acid treatment was successfully characterized using the NMR and Fourier transform infrared spectroscopic techniques. © 2018 Society of Chemical Industry 相似文献
2.
Mobile software applications have to cope with a particular environment that involves small size, limited resources, high autonomy requirements, competitive business models and many other challenges. To provide development guidelines that respond to these needs, several practices have been introduced; however, it is not clear how these guidelines may contribute to solve the issues present in the mobile domain. Furthermore, the rapid evolution of the mobile ecosystem challenges many of the premises upon which the proposed practices were designed. In this paper, we present a survey of the literature on software assurance practices for mobile applications, with the objective of describing them and assessing their contribution and success. We identified, organized and reviewed a body of research that spans in three levels: software development processes, software product assurance practices, and software implementation practices. By carrying out this literature survey, we reviewed the different approaches that researchers on Software Engineering have provided to address the needs that raise in the mobile software development arena. Moreover, we review the evolution of these practices, identifying how the constant changes and modernization of the mobile execution environment has impacted the methods proposed in the literature. Finally, we introduced discussion on the application of these practices in a real productive setting, opening an area for further research that may determine if practitioners have followed the proposed assurance paradigms. 相似文献
3.
This paper presents an integrated passive damping approach in hybrid metal-CFRP parts for structural applications. In this concept a viscoelastic material is embedded in the joint zone of the hybrid component. To examine the connection strength single-lap-joint specimens were produced and tested and the influence of the used material combinations, different surface structures, and different process parameters i.e. the moment of cross-linking were evaluated. Afterwards, the metal-CFRP hybrids were tested in quasi-static tests to assess their connection strength and failure behaviour. Dynamic cyclic tensile tests with step-wise increased loading conditions were performed to determine the specimens damping behaviour and to estimate their fatigue performance. Finally, these results are compared to a state of the art metal-CFRP hybrid with rivets connecting both materials. 相似文献
4.
In this paper, we consider the classical finite mixture model, which is an effective tool for modeling lifetime distributions for random samples from heterogeneous populations. We discuss new results on stochastic comparison for two finite mixtures when each of them is drawn from one of the following semiparametric families, i.e., proportional hazards, accelerated lifetime and proportional reversed hazards. 相似文献
5.
6.
7.
Variations of Incomplete Open Cubes is the major project by the twentieth-century conceptual artist Sol LeWitt. In this paper we interpret the enumerative component of the project as embeddings of graphs. This formulation permits use of an algorithm to check the completeness of the list of the structures produced by the artist. Our conclusion is that the artist found the correct number of structures (that is, 122), but that his list contains a mistake in the presentation of a pair of incomplete cubes, a discovery that appears not to have been noted before. 相似文献
8.
9.
Maimaitiaili Tayier Yunfeng Zhao Dengle Duan Rongge Zou Yunpu Wang Roger Ruan 《木材化学与工艺学杂志》2020,40(3):190-199
AbstractLignin biochar-catalytic depolymerization using biochar Fe-600, Fe-800, Ni-600, Ni-800 catalysts under microwave-heating (180?°C for 30?min) was explored in an ethanol/formic acid (1:1) media. Non-catalyst depolymerization was also studied and compared with the biochar-catalysts results. Characteristics of the bio-char catalysts were analyze by BET, XRD, and FT-IR. GPC, FT-IR, and MALDI-TOF MS spectrometry were also used to characterize the depolymerization products. The experimental results showed that the SBET, Vt, and Vmec and average pore diameter of the biochars are considerably dependent on the preparation temperature and type of cation (Ni2+ or Fe3+). The maximum yield of bio-oil product was obtained as 85?wt% with the addition of biochar Ni-600 and the total amount of oligomers or monomers with a molecular weight of 164 to 446 reaches 80.4%. 相似文献
10.