全文获取类型
收费全文 | 147767篇 |
免费 | 12287篇 |
国内免费 | 8502篇 |
专业分类
电工技术 | 4667篇 |
技术理论 | 9篇 |
综合类 | 13975篇 |
化学工业 | 15495篇 |
金属工艺 | 23767篇 |
机械仪表 | 8391篇 |
建筑科学 | 30640篇 |
矿业工程 | 4555篇 |
能源动力 | 3209篇 |
轻工业 | 5798篇 |
水利工程 | 3049篇 |
石油天然气 | 6171篇 |
武器工业 | 1243篇 |
无线电 | 5688篇 |
一般工业技术 | 15834篇 |
冶金工业 | 16391篇 |
原子能技术 | 937篇 |
自动化技术 | 8737篇 |
出版年
2024年 | 406篇 |
2023年 | 1778篇 |
2022年 | 3896篇 |
2021年 | 4351篇 |
2020年 | 4403篇 |
2019年 | 3468篇 |
2018年 | 3074篇 |
2017年 | 4288篇 |
2016年 | 4553篇 |
2015年 | 5105篇 |
2014年 | 9043篇 |
2013年 | 7859篇 |
2012年 | 10365篇 |
2011年 | 11779篇 |
2010年 | 9049篇 |
2009年 | 9533篇 |
2008年 | 8195篇 |
2007年 | 10077篇 |
2006年 | 9156篇 |
2005年 | 7839篇 |
2004年 | 6485篇 |
2003年 | 5779篇 |
2002年 | 4905篇 |
2001年 | 4101篇 |
2000年 | 3544篇 |
1999年 | 2943篇 |
1998年 | 2314篇 |
1997年 | 2079篇 |
1996年 | 1663篇 |
1995年 | 1328篇 |
1994年 | 1219篇 |
1993年 | 816篇 |
1992年 | 730篇 |
1991年 | 554篇 |
1990年 | 456篇 |
1989年 | 345篇 |
1988年 | 249篇 |
1987年 | 122篇 |
1986年 | 87篇 |
1985年 | 97篇 |
1984年 | 72篇 |
1983年 | 72篇 |
1982年 | 86篇 |
1981年 | 37篇 |
1980年 | 95篇 |
1979年 | 17篇 |
1978年 | 13篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1959年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
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
62.
Dr. Alejandra A. Carriles Alberto Mills Dr. María-José Muñoz-Alonso Dr. Dolores Gutiérrez Dr. Juan M. Domínguez Prof. Dr. Juan A. Hermoso Prof. Dr. Federico Gago 《Chembiochem : a European journal of chemical biology》2021,22(2):374-391
Spontaneous mutations in the EEF1A2 gene cause epilepsy and severe neurological disabilities in children. The crystal structure of eEF1A2 protein purified from rabbit skeletal muscle reveals a post-translationally modified dimer that provides information about the sites of interaction with numerous binding partners, including itself, and maps these mutations onto the dimer and tetramer interfaces. The spatial locations of the side chain carboxylates of Glu301 and Glu374, to which phosphatidylethanolamine is uniquely attached via an amide bond, define the anchoring points of eEF1A2 to cellular membranes and interorganellar membrane contact sites. Additional bioinformatic and molecular modeling results provide novel structural insight into the demonstrated binding of eEF1A2 to SH3 domains, the common MAPK docking groove, filamentous actin, and phosphatidylinositol-4 kinase IIIβ. In this new light, the role of eEF1A2 as an ancient, multifaceted, and articulated G protein at the crossroads of autophagy, oncogenesis and viral replication appears very distant from the “canonical” one of delivering aminoacyl-tRNAs to the ribosome that has dominated the scene and much of the thinking for many decades. 相似文献
63.
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
64.
65.
66.
《Ceramics International》2022,48(2):1889-1897
SiC fiber reinforced ceramic matrix composites (SiCf-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiCf/Si3N4–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiCf/Si3N4–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si3N4 matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiCf/Si3N4–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RCmin) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiCf/Si3N4–SiOC with an improved impedance matching structure obtained an RCmin of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiCf-CMCs at high temperatures. 相似文献
67.
Longbang Qing Yuehua Cheng Ru Mu 《Fatigue & Fracture of Engineering Materials & Structures》2019,42(11):2533-2543
Based on theoretical analysis and numerical simulation, the impact of steel fibres on the stress intensity factor (SIF) at the crack tip for cementitious composite was studied. The enhanced toughness of steel fibre reinforced cementitious composite (SFRC) in resisting cracks was explained by the decrement of SIF caused by steel fibre inclusions at the crack tip of the composite. The equivalent initial fracture toughness was used to characterize the crack initiation of SFRC. A simplified method for determining the of SFRC was proposed based on a linear regression method. Fracture tests were conducted on three‐point bending notched beams with different steel fibre volume fractions and specimen sizes to study the crack initiation behaviour of aligned steel fibre reinforced cementitious composite (ASFRC). of ASFRC was calculated, and the size effect of was analysed. The results showed that slightly increased with the steel fibre volume fraction and gradually became stable. For the tested specimens, whose heights varied between 40 and 100 mm, the specimen size had little impact on the . 相似文献
68.
Electrochemical impedance spectroscopy (EIS), anodic polarization and scanning electron microscopy techniques were used to investigate the damage mechanism in the transpassive potential region of AISI ... 相似文献
69.
70.
Using a comprehensive set of drop weight impact test data (h50) newly compiled from literature for 308 materials, a recent approach to predict impact sensitivities of nitro compounds is generalized to most explosive substances of interest. Compared to previous ones, this procedure is more thoroughly validated and exhibits a good predictive value. Furthermore, it yields new insight into the physical mechanisms involved, explaining for instance the unexpected desensitization of some oxygen-deficient triazoles upon nitration. 相似文献