全文获取类型
收费全文 | 374893篇 |
免费 | 9187篇 |
国内免费 | 3507篇 |
专业分类
电工技术 | 7467篇 |
技术理论 | 3篇 |
综合类 | 5521篇 |
化学工业 | 55194篇 |
金属工艺 | 14954篇 |
机械仪表 | 13006篇 |
建筑科学 | 12427篇 |
矿业工程 | 2280篇 |
能源动力 | 9684篇 |
轻工业 | 34477篇 |
水利工程 | 4114篇 |
石油天然气 | 5242篇 |
武器工业 | 349篇 |
无线电 | 47261篇 |
一般工业技术 | 66771篇 |
冶金工业 | 50832篇 |
原子能技术 | 4159篇 |
自动化技术 | 53846篇 |
出版年
2022年 | 2195篇 |
2021年 | 3440篇 |
2020年 | 2605篇 |
2019年 | 2786篇 |
2018年 | 17670篇 |
2017年 | 16781篇 |
2016年 | 13673篇 |
2015年 | 4288篇 |
2014年 | 5913篇 |
2013年 | 14551篇 |
2012年 | 12237篇 |
2011年 | 21410篇 |
2010年 | 17765篇 |
2009年 | 15975篇 |
2008年 | 17679篇 |
2007年 | 18517篇 |
2006年 | 10732篇 |
2005年 | 10560篇 |
2004年 | 9697篇 |
2003年 | 9826篇 |
2002年 | 9014篇 |
2001年 | 8760篇 |
2000年 | 7786篇 |
1999年 | 8241篇 |
1998年 | 17411篇 |
1997年 | 12240篇 |
1996年 | 9471篇 |
1995年 | 7216篇 |
1994年 | 6290篇 |
1993年 | 6005篇 |
1992年 | 4312篇 |
1991年 | 4020篇 |
1990年 | 3719篇 |
1989年 | 3523篇 |
1988年 | 3437篇 |
1987年 | 2680篇 |
1986年 | 2542篇 |
1985年 | 3189篇 |
1984年 | 2822篇 |
1983年 | 2577篇 |
1982年 | 2366篇 |
1981年 | 2404篇 |
1980年 | 2211篇 |
1979年 | 2058篇 |
1978年 | 1879篇 |
1977年 | 2176篇 |
1976年 | 2725篇 |
1975年 | 1584篇 |
1974年 | 1489篇 |
1973年 | 1558篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Crystallographic characteristics and microwave dielectric properties of Ni-modified MgTa2O6 ceramics
Liang Shi Xueying Wang Rui Peng Gang Wang Cheng Liu Xiaolei Shi Dainan Zhang Huaiwu Zhang 《Ceramics International》2021,47(16):22514-22521
Ni2+ modified MgTa2O6 ceramics with a trirutile phase and space group P42/mnm were obtained. The correlations between crystallographic characteristics and microwave dielectric performance of MgTa2O6 ceramics were systematically studied based on the chemistry bond theory (PVL theory) for the first time. The results indicate that the introduction of Ni2+ causes a change in polarizability and the Mg–O bond ionicity, which contributes to the variation of dielectric constant. Moreover, the lattice energy, and packing fraction, full width at half maximum of the Raman peak of Ta–O bond, as the quantitative characterization of crystallographic parameters, regulate the dielectric loss of MgTa2O6 ceramics in GHz frequency band. In addition, the study of sintering behavior shows that the densification and micromorphology are the crucial factors affecting the microwave dielectric performance. Typically, Ni2+ doping on the A-site of MgTa2O6 can effectively promote the Q × f values to 173,000 GHz (at 7.43 GHz), which ensures its applicability in 5G communication technology. 相似文献
92.
Liang Shi Dainan Zhang Rui Peng Cheng Liu Xiaolei Shi Xueying Wang Huaiwu Zhang 《Journal of the European Ceramic Society》2021,41(11):5526-5530
Mg(1-x)ZnxTa2O6 (x = 0.00?0.08) dielectric ceramics were synthesized via the traditional solid-state reaction method. We used XRD and Rietveld refinement to demonstrate that a pure Mg(1-x)ZnxTa2O6 phase with trirutile structure was formed. Zn2+ substitution helped to decrease the Raman full width at half width of the A1g mode at 703 cm?1, which resulted in an increase in the order and rigidity of the TaO6 octahedron, this in turn contributed to improving the Q×f values. Additionally, the introduction of Zn2+ significantly promoted grain growth and increased the dense, and the molecular polarizability, these factors lead to a higher permittivity. Moreover, enhanced Ta-O bond energy resulted in a more stable TaO6 octahedron in the Mg(1?x)ZnxTa2O6 system, which contributed to enhanced τf values via substitution of Zn2+ doped on the A-site. Correspondingly, the microwave dielectric properties were significantly improved for 0.04-doped samples, obtaining: εr = 27, Q × f = 185,000 GHz (at 7.47 GHz), τf =32 ppm/°C. 相似文献
93.
94.
Neural Computing and Applications - Traffic identification is currently an important challenge for network management and security. In this paper, we propose a novel application identification... 相似文献
95.
ABSTRACTGiven that online platforms disrupt established industries and challenge existing institutions, they can only be successful if their innovation becomes both legal and legitimate. This requires ‘institutional work’ that changes perceptions and regulations within society. Rather than only focussing on the online platform as the sole agent engaging in institutional work, our study analyses institutional work as a collective process. We investigate the case of home-sharing platform Airbnb and the process of institutional change its introduction prompted regarding short-term rental in Amsterdam, London and New York. We find, contrary to the popular view of online platforms as disruptive entrepreneurs, that the platform mainly focusses on creating new institutions rather than disrupting existing ones, and that users and non-users undertake most of the institutional work activities. We also show that different types of actors carry out different types of institutional work suggesting that the process of institutional work is highly distributed. 相似文献
96.
D.R. Santos‐Peñate C.M. Campos‐Rodríguez J.A. Moreno‐Pérez 《International Transactions in Operational Research》2019,26(1):340-363
The ‐centroid problem or leader–follower problem is generalized considering different customer choice rules where a customer may use facilities belonging to different firms, if the difference in travel distance (or time) is small enough. Assuming essential goods, some particular customer choice rules are analyzed. Linear programming formulations for the generalized ‐medianoid and ‐centroid problems are presented and an exact solution approach is applied. Some computational examples are included. 相似文献
97.
Bram H. Frohock Jessica M. Gilbertie Dr. Jennifer C. Daiker Dr. Lauren V. Schnabel Dr. Joshua G. Pierce 《Chembiochem : a European journal of chemical biology》2020,21(7):933-937
The failure of frontline antibiotics in the clinic is one of the most serious threats to human health and requires a multitude of novel therapeutics and innovative approaches to treatment so as to curtail the growing crisis. In addition to traditional resistance mechanisms resulting in the lack of efficacy of many antibiotics, most chronic and recurring infections are further made tolerant to antibiotic action by the presence of biofilms. Herein, we report an expanded set of 5-benzylidene-4-oxazolidinones that are able to inhibit the formation of Staphylococcus aureus biofilms, disperse preformed biofilms, and, in combination with common antibiotics, are able to significantly reduce the bacterial load in a robust collagen-matrix model of biofilm infection. 相似文献
98.
ABSTRACT Design and implementation of an effective dissemination programme for decentralised renewable energy system necessitate an accurate estimate of its utilisation potential. Hence, in this study, an attempt has been made to develop frameworks to estimate the utilisation potential of decentralised renewable energy systems in the state of Uttarakhand in India. Estimations imply large resource, technical and economic potentials of the domestic solar water heater, solar home system, solar lantern, family size biogas plant and improved biomass cookstove in Uttarakhand. With higher impact on the purchasing power of households, prevailing soft loan scheme has been found to be more appropriate than a capital subsidy for promoting the usage of decentralised renewable energy systems. 相似文献
99.
100.
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