全文获取类型
收费全文 | 128955篇 |
免费 | 12297篇 |
国内免费 | 8719篇 |
专业分类
电工技术 | 6867篇 |
技术理论 | 14篇 |
综合类 | 14986篇 |
化学工业 | 13372篇 |
金属工艺 | 6861篇 |
机械仪表 | 7376篇 |
建筑科学 | 21224篇 |
矿业工程 | 4589篇 |
能源动力 | 2729篇 |
轻工业 | 5355篇 |
水利工程 | 3493篇 |
石油天然气 | 4840篇 |
武器工业 | 1798篇 |
无线电 | 11695篇 |
一般工业技术 | 11699篇 |
冶金工业 | 4920篇 |
原子能技术 | 656篇 |
自动化技术 | 27497篇 |
出版年
2024年 | 431篇 |
2023年 | 1612篇 |
2022年 | 3198篇 |
2021年 | 3711篇 |
2020年 | 3721篇 |
2019年 | 2936篇 |
2018年 | 2772篇 |
2017年 | 3555篇 |
2016年 | 3961篇 |
2015年 | 4386篇 |
2014年 | 8242篇 |
2013年 | 7188篇 |
2012年 | 9210篇 |
2011年 | 10052篇 |
2010年 | 7770篇 |
2009年 | 8488篇 |
2008年 | 8116篇 |
2007年 | 9287篇 |
2006年 | 8520篇 |
2005年 | 7394篇 |
2004年 | 6185篇 |
2003年 | 5548篇 |
2002年 | 4437篇 |
2001年 | 3692篇 |
2000年 | 3096篇 |
1999年 | 2437篇 |
1998年 | 1869篇 |
1997年 | 1567篇 |
1996年 | 1214篇 |
1995年 | 1025篇 |
1994年 | 981篇 |
1993年 | 644篇 |
1992年 | 560篇 |
1991年 | 446篇 |
1990年 | 351篇 |
1989年 | 257篇 |
1988年 | 202篇 |
1987年 | 109篇 |
1986年 | 89篇 |
1985年 | 104篇 |
1984年 | 87篇 |
1983年 | 83篇 |
1982年 | 80篇 |
1981年 | 36篇 |
1980年 | 78篇 |
1979年 | 29篇 |
1978年 | 16篇 |
1975年 | 15篇 |
1964年 | 16篇 |
1961年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
71.
72.
Han Yan Gu Yi Chen Lei Li Bing Liu Xiang Ming Chen 《International Journal of Applied Ceramic Technology》2020,17(2):790-796
SrLa[Ga1−x(R0.5Ti0.5)x]O4 (R = Mg, Zn) ceramics were prepared by a standard solid state sintering method. The single-phase ceramics with K2NiF4-type layered perovskite structure and I4/mmm space group were obtained, indicating that SrLa(R0.5Ti0.5) and SrLaGaO4 can form the unlimited solid solutions. With increasing x for R = Mg and Zn, εr increases monotonously, the Qf value first increases and then decreases, while τf increases from a negative to a positive value. The optimized microwave dielectric properties were obtained as following: εr = 23.3, Qf = 89 400 GHz, τf = −0.8 ppm/°C for SrLa[Ga0.6(Mg0.5Ti0.5)0.4]O4 and εr = 23.3, Qf = 76 200 GHz, τf = 0.2 ppm/°C for SrLa[Ga0.7(Zn0.5Ti0.5)0.3]O4, indicating that the present solid solution ceramics are the promising candidates as microwave resonator materials for the telecommunication applications. 相似文献
73.
74.
WeiJia Luo LingXia Li Shihui Yu Qianyu Guo Bowen Zhang Zheng Sun 《Ceramics International》2018,44(11):12414-12419
Low-loss (Zn1-xNix)ZrNbTaO8 (0.02?≤?x?≤?0.10) ceramics possessing single wolframite structure are initiatively synthesized by solid-state route. Based on the results of Rietveld refinement, complex chemical bond theory is used to establish the correlation between structural characteristics and microwave performance in this ceramic system. A small amount of Ni2+ (x?=?0.06) in A-site with the fixed substitution of Ta5+ in B-site can effectually raise the Q?×?f value of ZnZrNb2O8 ceramic, embodying a dense microstructure and high lattice energy. The dielectric constant and τf are mainly affected by bond ionicity and the average octahedral distortion. The (Zn0.94Ni0.06)ZrNbTaO8 ceramic sample sintered at 1150?°C for 3?h exhibits an outstanding combination of microwave dielectric properties: εr =?27.88, Q?×?f?=?128,951?GHz, τf =?–39.9?ppm/°C. Thus, it is considered to be a candidate material for the communication device applications at high frequency. 相似文献
75.
76.
The perpetual energy production of a wind farm could be accomplished (under proper weather conditions) if no failures occurred. But even the best possible design, manufacturing, and maintenance of a system cannot eliminate the failure possibility. In order to understand and minimize the system failures, the most crucial components of the wind turbines, which are prone to failures, should be identified. Moreover, it is essential to determine and classify the criticality of the system failures according to the impact of these failure events on wind turbine safety. The present study is processing the failure data from a wind farm and uses the Fault Tree Analysis as a baseline for applying the Design Structure Matrix technique to reveal the failure and risk interactions between wind turbine subsystems. Based on the analysis performed and by introducing new importance measures, the “readiness to fail” of a subsystem in conjunction with the “failure riskiness” can determine the “failure criticality.” The value of the failure criticality can define the frame within which interventions could be done. The arising interventions could be applied either to the whole system or could be focused in specified pairs of wind turbine subsystems. In conclusion, the method analyzed in the present research can be effectively applied by the wind turbine manufacturers and the wind farm operators as an operation framework, which can lead to a limited (as possible) design‐out maintenance cost, failures' minimization, and safety maximization for the whole wind turbine system. 相似文献
77.
Nutritional warnings are rapidly gaining relevance, particularly in the region of the Americas, as a front-of-package nutrition labelling scheme that facilitate the identification of products with high content of nutrients associated with non-communicable diseases (NCDs). Several graphic designs are being developed as countries discuss the implementation of nutritional warnings, which makes it necessary to develop research to evaluate their efficacy. In this context, the aim of the present work was to compare the efficacy of a series of nutritional warnings that are being considered by Brazilian national authorities for the country with two of the most studied schemes: the guidelines daily amounts (GDA) and the traffic light system. Two studies were conducted. In the first one, visual search was conducted with 62 participants to evaluate the time needed by them to identify the schemes on food labels and to identify whether a product had high nutrient content. In the second study, an online survey with 1932 participants was used to evaluate their ability to use FOP nutrition labelling schemes to correctly identify the most healthful product in a set, as well as high nutrient content in a product. In addition, the influence of FOP nutrition labelling schemes on perceived healthfulness was evaluated. Finally, consumers’ perception of the schemes was gathered using an open-ended question. Results confirmed the advantages of nutritional warnings compared to the GDA and the traffic-light system to facilitate the identification of products with high content of nutrients associated with NCDs. In addition, the use of familiar signs frequently used to convey a ‘warning message’ outperformed other unfamiliar signs in terms of their ability to facilitate the interpretation of nutrition information. Regarding colour, black signs tended to required significantly less time to be detected when included on colour food labels compared to red signs. 相似文献
78.
79.
《International Journal of Hydrogen Energy》2020,45(29):14964-14971
Sr3Fe2O7-δ (SFO) with two-layer Ruddlesden-Popper (R–P) structure has recently been proved to be a promising material for the single phase cathode in proton conducting solid oxide fuel cells (P–SOFCs). To investigate the hydration reactions and proton conducting mechanisms of SFO and cobalt doped SFO (SFCO), both bulk and surface properties were calculated. We conclude that R–P structures have advantages in P–SOFCs. The unique Sr–O–M layer can facilitate the hydration process. Although in Sr–O–F and Sr–O–N layers, it is difficult for the formation and migration of oxygen vacancies, protons are most stable. Furthermore, cobalt doping can not only improve the electronic conductivity but also enhance surface properties of SFCO. The easily exposed Co–Fe–O surface can also facilitate the hydration reactions on the surface. Our work could give an informative insight into the relationships among the doped elements, the R–P structures, the hydration process and the proton conducting properties. 相似文献
80.
无源光网络(Passive Optical Network,PON)作为当今接入网的主要技术解决方案,具有带宽使用效率高、传输距离远、抗干扰能力强等特点.通过研究PON技术的发展动态,本文首先归纳了各种PON技术的产生背景和应用特点,整理出各技术间的连接关系及主要标准;其次介绍了PON技术的帧结构,并对带宽、波长、传输模式等PON技术的主要参数进行了汇总;然后将国内外研究热点进行划分,围绕媒体访问控制协议、帧结构、动态带宽分配算法、节能机制等关键技术,阐述了其研究现状及在PON中的重要作用;最后对PON技术的发展趋势进行了展望. 相似文献