全文获取类型
收费全文 | 13054篇 |
免费 | 1389篇 |
国内免费 | 895篇 |
专业分类
电工技术 | 507篇 |
技术理论 | 2篇 |
综合类 | 1108篇 |
化学工业 | 3398篇 |
金属工艺 | 851篇 |
机械仪表 | 411篇 |
建筑科学 | 780篇 |
矿业工程 | 213篇 |
能源动力 | 396篇 |
轻工业 | 836篇 |
水利工程 | 212篇 |
石油天然气 | 300篇 |
武器工业 | 102篇 |
无线电 | 1155篇 |
一般工业技术 | 2311篇 |
冶金工业 | 733篇 |
原子能技术 | 183篇 |
自动化技术 | 1840篇 |
出版年
2024年 | 17篇 |
2023年 | 293篇 |
2022年 | 311篇 |
2021年 | 543篇 |
2020年 | 493篇 |
2019年 | 504篇 |
2018年 | 466篇 |
2017年 | 492篇 |
2016年 | 459篇 |
2015年 | 437篇 |
2014年 | 668篇 |
2013年 | 887篇 |
2012年 | 751篇 |
2011年 | 877篇 |
2010年 | 674篇 |
2009年 | 749篇 |
2008年 | 710篇 |
2007年 | 801篇 |
2006年 | 722篇 |
2005年 | 618篇 |
2004年 | 610篇 |
2003年 | 544篇 |
2002年 | 443篇 |
2001年 | 339篇 |
2000年 | 299篇 |
1999年 | 231篇 |
1998年 | 163篇 |
1997年 | 180篇 |
1996年 | 155篇 |
1995年 | 126篇 |
1994年 | 118篇 |
1993年 | 103篇 |
1992年 | 77篇 |
1991年 | 69篇 |
1990年 | 67篇 |
1989年 | 52篇 |
1988年 | 42篇 |
1987年 | 24篇 |
1986年 | 17篇 |
1985年 | 32篇 |
1984年 | 26篇 |
1983年 | 28篇 |
1982年 | 30篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 5篇 |
1965年 | 6篇 |
1961年 | 5篇 |
1960年 | 5篇 |
1959年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
A fractionation system is an essential unit in the hydrocracking process. Its optimal operation is challenging because of the complexity in the structure of the distillation tower and composition of the stream. In addition, the series-parallel structure between the distillation towers of different techniques aggravates the coupling and complexity of the hydrocracking fractionation system (HFS). This, in turn, increases the time complexity of the optimization problem. In this paper, a rigorous mechanism model of an actual HFS is first applied to describe the operating conditions of the HFS. Then, an improved state transition algorithm (STA) with a staged evaluation strategy is proposed to solve the above problem. To overcome problems caused by the series-parallel structure of HFS, the model is divided into multiple stages for evaluation by mechanism analysis. Furthermore, several typical convergence estimation criteria are introduced to reduce unnecessary model calculations. To solve time-consuming problems associated with HFS optimization, the adaptive change operator is used to improve the search function of the original algorithm and two performance criteria are presented to reduce the optimization time. The proposed algorithm is successfully applied to the operational parameter optimization problem of HFS with a multi-fractionator series-parallel structure. The experimental results indicated that the staged evaluation strategy improved the fast convergence probability of the HFS mechanism model and reduced unnecessary calculations, whereas the improved algorithm increased accuracy and significantly decreased optimization time. 相似文献
72.
《Ceramics International》2020,46(7):8675-8681
The dielectric properties and bipolar polarization-electric field (P-E) and strain-electric field (S-E) dynamic hysteresis of a relaxor [001]c 0.73Pb(Mg1/3Nb2/3)O3-0.27PbTiO3 (PMN-0.27PT) single crystal were investigated to reveal more details of the temperature-induced phase transitions. Different linear scaling relations for ferroelectric hysteresis area <A>, coercive field Ec, saturation polarization Ps and remnant polarization Pr versus temperature τ were measured in different temperature regions. For each measurement frequency f, all hysteresis parameters were found to decrease linearly with temperature in the temperature range of the single rhombohedral (R) phase or tetragonal (T) phase, and the rate of decrease in the T phase was observed to be much larger than the corresponding rate in the R phase. In the temperature range near the R-T phase transition, the exponent α in the power law <A>∝f α for the R phase was found to be smaller than that for the T phase, and the magnitude of α depended strongly on temperature when the crystal was in the R-T coexisting phase state. Our experimental and theoretical results indicate that the difference in the activation energy and dipole moment in the R and T phases may lead to the observed discrepancy for the P-E and S-E hysteresis behaviour in different temperature regions. 相似文献
73.
类似于锂/氟化碳(Li/CFx)电池,钠/氟化碳(Na/CFx)电池具有低廉的价格和较高的能量密度,是未来锂离子电池的理想替代选项。但是Na/CFx电池目前还存在着极化大和循环性能差等问题。为解决以上问题,在CFx阴极中加入催化剂成为改善电池电化学性能的一个重要途径。分别采用水热法和溶胶-凝胶法制备了催化剂材料Co3O4-TiO2和ZrO2,并对其进行了表征。分别将TiO2、Co3O4、Co3O4-TiO2以及ZrO2 4种催化剂材料按10%(质量比,下同)加入CFx正极中并组装成电池,然后对电池进行了恒流充放电、交流阻抗以及循环伏安等电化学性能测试。实验结果表明,加入4种催化剂后,Na/CFx电池的放电比容量得到明显提升。其中,ZrO2催化剂表现出最佳的催化性能。经过100次后循环后,含有ZrO2的Na/CFx电池的放电比容量为167.3 mAh·g-1,极大地改善了钠/氟化碳电池的循环性能。 相似文献
74.
《Ceramics International》2020,46(8):12269-12274
Dielectric capacitors with high recoverable energy density are in high demand for their application in electrical and electronic systems. Among lead-free dielectric materials, silver niobate (AgNbO3) has attracted growing interest due to its superior energy storage density at room temperature. The field-induced phase transition from antiferroelectric (AFE) phase to ferroelectric (FE) phase contributes to its large energy density. In this work, pure perovskite silver niobate ceramics were fabricated in an oxygen atmosphere by the solid-state reaction technique. The Pbcm orthorhombic phase of AgNbO3 was closely observed using the Rietveld refinement method to provide explanation for the origin of high spontaneous polarization within a unit cell. Local structural analysis via piezoelectric force microscopy revealed the existence of ferroelectric nano domains, which may contribute to the high energy storage efficiency (η = 99.9926%) in AgNbO3 at low electric fields. The phase transitions of AgNbO3 were also investigated via the dependence of the dielectric permittivity (ε′ and ε″) and loss angle tangent (tanδ) on temperatures, providing insights into the further modification of AgNbO3. 相似文献
75.
《Ceramics International》2020,46(8):12275-12281
Alumina and zirconia ceramic particles exhibit high hardness and excellent wear resistance at high temperature, and hence are used as ceramic reinforcement phases in some plasma sprayed coatings. In this study, the interface evolution of a zirconia/alumina eutectic ceramic and the phase transition of zirconia in a plasma-sprayed coating were investigated. Scanning electron microscopy and transmission electron microscopy combined with focused-ion beam and energy dispersive X-ray were used to analyze the microstructure and composition of the ceramic interface. The results showed that the eutectic ceramic particles consisted of alumina (outer) and columnar zirconia (inner) before and after the plasma spraying process. The inner zirconia part showed the martensitic transformation of t-type zirconia to stripe-like m-type zirconia. After the plasma spraying, the interface between alumina and zirconia changed significantly, which formed a new oxide layer. The phase transition mechanism in the ceramic particle and oxide layer formation mechanism at the alumina/zirconia interface were investigated. 相似文献
76.
《中国有色金属学会会刊》2020,30(3):756-764
The microstructure, dielectric and ferroelectric properties of (1–y)Ba(Zr0.1Ti0.9)O3–yBa(Zn1/3Nb2/3)O3 (y=0–0.05) ceramics prepared by traditional solid state method were investigated by X-ray diffractometer, scanning electron microscope, electric parameter testing system and ferroelectric tester. It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y. The permittivity maximum εr,max is suppressed from 8948 to 1611 at 1 kHz with increasing y, and the ferroelectric–paraelectric phase transition temperature Tm decreases from 93 to –89 °C at 1 kHz as y increases. The composition-induced diffuse phase transition is enhanced with increasing y. The relaxor-like ferroelectric behavior with a strong frequency dispersion of Tm and permittivity at T<Tm accompanied by a strong diffuse phase transition is found for the system with high y value. The remnant polarization decreases with increasing y, while the coercive field decreases remarkably and then increases with the increase of y. 相似文献
77.
Yun Shuai Peng Zhang Xiaoyun Guo Xinyan Wang Zhengliang Huang Yao Yang Jingyuan Sun Meng Li Yan Jiang Jiong Ma Jingdai Wang Yongrong Yang 《American Institute of Chemical Engineers》2020,66(1):e16778
Three gas–liquid dispersion states including flooding, loading, and complete dispersion are observed sequentially in a jet bubbling reactor with an increase of the liquid jet velocity at the nozzle outlet (uj). The gas–liquid dispersion states are identified through the slope (k) of the curve of fluctuation distribution index (FI) versus uj as follows: (a) under the flooding, k = 0; (b) under the loading, k > 0; (c) under the complete dispersion, k < 0. In particular, the uj at the transition points from flooding to loading and from loading to complete dispersion are referred to flooding jet velocity (ujf, the transition point between k = 0 and k > 0) and complete dispersion jet velocity (ujcd, the transition point from k > 0 to k < 0), respectively. The average relative deviations of the uj at the transition points obtained through the acoustic emission measurement and visual observation are less than 5%. 相似文献
78.
Carboxylated-terminated liquid acrylonitrile rubber (CTBN) and epoxy resin (JEF-0211) were coreacted with cyanate ester (CE) to form CTBN/EP/CE ternary resin systems. Further, the ternary resin system was applied as prepreg for carbon fiber composites with vacuum bag degassing molding process. CTBN/EP/CE ternary shape memory polymer (SMP) exhibited relatively high tensile strength, Young's modulus, impact strength, and excellent shape memory properties. Compared with CTBN/EP/CE ternary SMP, CTBN/EP/CE carbon fiber composites showed much higher mechanical properties, such as their tensile strength and Young's modulus were high to 570 MPa and 36.7 GPa, respectively. Furthermore, CTBN/EP/CE carbon fiber composites exhibited good shape memory properties, their shape fixity ratio and shape recovery ratio were more than 95% after 30 times repeating shape memory tests. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48756. 相似文献
79.
《Ceramics International》2020,46(3):3236-3241
Chemical doping is an indispensable tool to tailor the properties of the commercial piezoelectric materials. However, a high piezoelectric coefficient with enhanced thermal stability is rarely achieved by one dopant in some high-performance ferroelectrics, e.g., the recently discovered eco-friendly (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics. In order to optimize the piezoelectric property in BCZT system by a simple way, we investigated the doping effect of Fe3+, Nb5+ and Bi3+ cations in BCZT ceramics respectively. The results indicate that only Nb5+-doped BCZT ceramics display a combination of large piezoelectric coefficient and enhanced thermal stability, compared with others. Moreover, the established phase diagrams and in-situ transmission electron microscope (TEM) observations reveal that such optimized piezoelectric properties after Nb5+ doping originates from (i) the low polarization anisotropy near the ambient tetragonal (T)-orthorhombic (O) phase transition and (ii) the easy domain wall motion of persistent miniaturized ferroelectric domains upon heating. 相似文献
80.
《Journal of the European Ceramic Society》2020,40(4):1217-1222
Lead-free (K0.5Na0.5)NbO3-based (KNN) piezoceramics featuring a polymorphic phase boundary (PPB) between the orthorhombic and tetragonal phases at room temperature are reported to possess high piezoelectric properties but with inferior cycling stability, while the ceramics with a single tetragonal phase show improved cycling stability but with lower piezoelectric coefficients. In this work, electric biasing in-situ transmission electron microscopy (TEM) study is conducted on two KNN-based compositions, which are respectively at and off PPB. Our observations reveal the distinctive domain responses in these two ceramics under cyclic fields. The higher domain wall density in the poled KNN at PPB contributes to the high piezoelectric properties. Upon cycling, however, a new microstructure feature, “domain intersection”, is directly observed in this PPB composition. In comparison, the off-PPB KNN ceramic develops large domains during poling, which experience much less extent of disruption during cycling. Our comparative study provides the basis for understanding the relation between phase composition and piezoelectric performance. 相似文献