全文获取类型
收费全文 | 48368篇 |
免费 | 4726篇 |
国内免费 | 2320篇 |
专业分类
电工技术 | 2856篇 |
技术理论 | 3篇 |
综合类 | 3344篇 |
化学工业 | 8197篇 |
金属工艺 | 2526篇 |
机械仪表 | 3145篇 |
建筑科学 | 3949篇 |
矿业工程 | 1386篇 |
能源动力 | 1529篇 |
轻工业 | 2751篇 |
水利工程 | 908篇 |
石油天然气 | 3339篇 |
武器工业 | 414篇 |
无线电 | 5670篇 |
一般工业技术 | 5953篇 |
冶金工业 | 2111篇 |
原子能技术 | 606篇 |
自动化技术 | 6727篇 |
出版年
2024年 | 196篇 |
2023年 | 829篇 |
2022年 | 1520篇 |
2021年 | 2198篇 |
2020年 | 1605篇 |
2019年 | 1358篇 |
2018年 | 1462篇 |
2017年 | 1662篇 |
2016年 | 1478篇 |
2015年 | 1854篇 |
2014年 | 2368篇 |
2013年 | 2772篇 |
2012年 | 2856篇 |
2011年 | 3144篇 |
2010年 | 2681篇 |
2009年 | 2500篇 |
2008年 | 2512篇 |
2007年 | 2361篇 |
2006年 | 2499篇 |
2005年 | 2093篇 |
2004年 | 1515篇 |
2003年 | 1702篇 |
2002年 | 1939篇 |
2001年 | 1713篇 |
2000年 | 1355篇 |
1999年 | 1426篇 |
1998年 | 1026篇 |
1997年 | 879篇 |
1996年 | 879篇 |
1995年 | 678篇 |
1994年 | 594篇 |
1993年 | 454篇 |
1992年 | 307篇 |
1991年 | 230篇 |
1990年 | 174篇 |
1989年 | 156篇 |
1988年 | 157篇 |
1987年 | 92篇 |
1986年 | 54篇 |
1985年 | 32篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1976年 | 1篇 |
1971年 | 2篇 |
1959年 | 2篇 |
1951年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
31.
相似用户挖掘是提高社交网络服务质量的重要途径,在面向大数据的社交网络时代,准确的相似用户挖掘对于用户和互联网企业等都有重要的意义,而根据用户自己的兴趣话题挖掘的相似用户更符合相似用户的要求。提出了一种基于用户兴趣话题进行相似用户挖掘的方法。该方法首先使用TextRank话题提取方法对用户进行兴趣话题提取,再对用户发表内容进行训练,计算出所有词之间的相似度。提出CP(Corresponding Position similarity)、CPW(Corresponding Position Weighted similarity)、AP(All Position similarity)、APW(All Position Weighted similarity)四种用户兴趣话题词相似度计算方法,通过用户和相似用户间关注、粉丝重合率验证相似用户挖掘效果,APW similarity的相似用户的关注/粉丝重合百分比为1.687%,优于提出的其他三种算法,分别提高了26.3%、2.8%、12.4%,并且比传统的文本相似度方法Jaccard相似度、编辑距离算法、余弦相似度分别提高了20.4%、21.2%、45.0%。因此APW方法可以更加有效地挖掘出用户的相似用户。 相似文献
32.
Xiaopo Wu Yangming Shi Weibo Meng Xiaofei Ma Nian Fang 《International Journal of Satellite Communications and Networking》2019,37(3):283-291
Electromagnetic signal emitted by satellite communication (satcom) transmitters are used to identify specific individual uplink satcom terminals sharing the common transponder in real environment, which is known as specific emitter identification (SEI) that allows for early indications and warning (I&W) of the targets carrying satcom furnishment and furthermore the real time electromagnetic situation awareness in military operations. In this paper, the authors are the first to propose the identification of specific transmitters of satcom by using probabilistic neural networks (PNN) to reach the goal of target recognition. We have been devoted to the examination by exploring the feasibility of utilizing the Hilbert transform to signal preprocessing, applying the discrete wavelet transform to feature extraction, and employing the PNN to perform the classification of stationary signals. There are a total of 1000 sampling time series with binary phase shift keying (BPSK) modulation originated by five types of satcom transmitters in the test. The established PNNs classifier implements the data testing and finally yields satisfactory accuracy at 8 dB(±1 dB) carrier to noise ratio, which indicates the feasibility of our method, and even the keen insight of its application in military. 相似文献
33.
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
34.
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
35.
36.
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
37.
38.
39.
Zhuo Shi Ningning Dong Dingfeng Zhang Xiaofang Jiang Guanxin Du Shichao Lv Jiejie Chen Jun Wang Shifeng Zhou 《Journal of the American Ceramic Society》2019,102(7):3965-3971
The construction of nonlinear optical materials featuring asymmetric transmission of light is of great technological importance for various applications, including optical switching and optical power limiting. A significant challenge is the scalable fabrication of material candidates with good photochemical stability, high optical transmittance, and excellent optical limiting performance. Here, we present a nanocrystallization avenue for constructing hybrid optical limiting materials that exhibit ultrafast and robust optical limiting performance. The experimental results show that the controllable relaxation of a niobate glass may lead to the clustering of Nb-O units and contracting of the bandgap. It results in the notable improvement in nonlinear optical properties, including the enhanced saturation irradiance (380 GW/cm2), doubly increased nonlinear coefficient, and decreased limiting threshold (200 GW/cm2). Our results suggest a promising material that exhibits promising applications for protecting eyes and sensitive components from laser-induced damage. 相似文献
40.
ZHAO Pengfei TIAN Yuming LIU Aiping KONG Xiangchen CHAI Yuesheng LI Zhangang BAI Pinbo 《中国耐火材料》2015,(2):36-41
Based on the needs of the market,a new 52 MPa medium-density and high-strength ceramic proppant was prepared by adopting third grade bauxite and clay as raw materials and using the solid phase sintering method,and the effect of the firing temperature on the microstructure,phase composition and mechanical properties of the ceramic proppant was studied. The broken resistance mechanism of this ceramic proppant was also discussed. The results show that the main phases of the prepared ceramic proppant are mullite and corundum; the broken rates are 3. 92% and 7. 21% under 52 MPa and69 MPa,respectively. 相似文献