全文获取类型
收费全文 | 32693篇 |
免费 | 3065篇 |
国内免费 | 1921篇 |
专业分类
电工技术 | 1910篇 |
技术理论 | 1篇 |
综合类 | 3794篇 |
化学工业 | 3256篇 |
金属工艺 | 1987篇 |
机械仪表 | 1741篇 |
建筑科学 | 6043篇 |
矿业工程 | 2866篇 |
能源动力 | 960篇 |
轻工业 | 1573篇 |
水利工程 | 1646篇 |
石油天然气 | 2575篇 |
武器工业 | 256篇 |
无线电 | 1546篇 |
一般工业技术 | 2604篇 |
冶金工业 | 1865篇 |
原子能技术 | 296篇 |
自动化技术 | 2760篇 |
出版年
2024年 | 79篇 |
2023年 | 301篇 |
2022年 | 650篇 |
2021年 | 788篇 |
2020年 | 820篇 |
2019年 | 760篇 |
2018年 | 700篇 |
2017年 | 870篇 |
2016年 | 1021篇 |
2015年 | 1160篇 |
2014年 | 1848篇 |
2013年 | 1663篇 |
2012年 | 2220篇 |
2011年 | 2453篇 |
2010年 | 1902篇 |
2009年 | 2091篇 |
2008年 | 1886篇 |
2007年 | 2302篇 |
2006年 | 2154篇 |
2005年 | 1883篇 |
2004年 | 1625篇 |
2003年 | 1427篇 |
2002年 | 1198篇 |
2001年 | 1042篇 |
2000年 | 838篇 |
1999年 | 758篇 |
1998年 | 559篇 |
1997年 | 515篇 |
1996年 | 433篇 |
1995年 | 390篇 |
1994年 | 304篇 |
1993年 | 200篇 |
1992年 | 193篇 |
1991年 | 141篇 |
1990年 | 108篇 |
1989年 | 101篇 |
1988年 | 56篇 |
1987年 | 66篇 |
1986年 | 31篇 |
1985年 | 17篇 |
1984年 | 28篇 |
1983年 | 20篇 |
1982年 | 10篇 |
1981年 | 16篇 |
1980年 | 18篇 |
1979年 | 8篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1951年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
1.
2.
3.
《Ceramics International》2021,47(21):29681-29687
Inorganic piezoelectric ceramic composite is the potential sensing element for long-term structural health monitoring due to its excellent durability and compatibility. In this study, a Ceramicrete-based piezoelectric composite is proposed preliminarily, in which the magnesium potassium phosphate cement is used as the matrix and the lead zirconate titanate particle is utilized as the functional phase. Piezoelectric properties test and microstructure analysis are performed to evaluate the testing samples. Results show that the piezoelectric performance of the composite increase with the increase of piezoelectric ceramic particle size. The value of the piezoelectric strain factor (d33) can reach 83.8 pC/N, while the corresponding piezoelectric voltage factor (g33) is 50.1 × 10-3 V•m/N at the 50th day after polarization. Microstructure analysis illustrates that the interfacial transition zone (ITZ) between the matrix and the particles is dense. Moreover, the influence of aging on the composite is attributed to the continuous hydration after polarization. It indicates that the composites have a higher piezoelectric performance, which can be regarded as a promising sensing element material. 相似文献
4.
为了解破碎围岩分别采用锚杆支护、锚喷支护以及锚喷+锚索耦合三种支护方式下的支护效果,进而为破碎围岩巷道选择合理的支护方式提供参考。通过借助FLAC3D软件建立数值模型,分析不同支护条件下的破碎围岩巷道位移量、应力分布以及塑性区的时空演化特征。结果表明,采用锚喷+锚索耦合支护时,可以较好的控制巷道围岩的位移量、减小应力集中效应、缩小塑性区的影响范围。 相似文献
5.
《International Journal of Hydrogen Energy》2022,47(19):10747-10761
To obtain the mixing enhancement mechanism of H2–Water combined jets in supersonic crossflows in a combustor with expanded section for rotating detonation ramjet, the flow field shape and spray structure were studied by experimental and numerical methods. The Eulerian–Lagrangian method was used to investigate the diffusion mechanism and H2–Water interaction law of combined jets with different sequences. At the same time, high-speed photography and the schlieren technique were used to capture the flow field. The effects of jet pressure drop, orifice diameter, orifice spacing, incoming Mach number, and other parameters on the penetration depth of water jets were studied. The results of experiment and simulation show that using H2–Water combined jets, the penetration depth of the jet spray can be greatly increased and the jet mixing effect can be significantly improved, which will contribute to the engine's ignition and stable combustion. In the case of pre-water/post-H2, the penetration depth of the hydrogen jet is greater. In the case of pre-H2/post-water, the hydrogen jet raises the water spray mainly by protecting the integrity of the water column. 相似文献
6.
为研究钢管套筒灌浆连接轴向受拉破坏过程及破坏机理,试验中设计了16组48个钢管套筒灌浆连接试件,试件采用钢板代替圆钢管,并进行静载试验。分析了灌浆料裂缝扩展过程、荷载-相对位移曲线,并对抗剪键高距比、灌浆料厚度、侧向力等因素对破坏过程及承载力的影响进行分析。结果表明:对于不设置抗剪键的套筒灌浆连接试件,斜裂缝随机产生,裂缝分布不均匀;对于设置抗剪键的套筒灌浆连接试件,裂缝首先出现在底部抗剪键位置处,与水平方向夹角约为30°,随后在中部和上部抗剪键位置处分别出现斜裂缝。由于每个抗剪键上荷载分担并不均匀,与抗剪键接触的灌浆料逐渐达到极限压应力,达到极限状态时,承载力全部由抗剪键间的机械咬合力承担,在连接承载力中,可忽略摩擦力和胶结力作用。随着抗剪键高距比h/s增大,各试件初始剪切刚度相差不大,承载力增大,但增幅逐渐减小,建议抗剪键高距比0.06g/s>0.3,同时需要满足灌浆料灌注的施工要求。 相似文献
7.
8.
The existing analytical average bit error rate (ABER) expression of conventional generalised spatial modulation (CGSM) does not agree well with the Monte Carlo simulation results in the low signal‐to‐noise ratio (SNR) region. Hence, the first contribution of this paper is to derive a new and easy way to evaluate analytical ABER expression that improves the validation of the simulation results at low SNRs. Secondly, a novel system termed CGSM with enhanced spectral efficiency (CGSM‐ESE) is presented. This system is realised by applying a rotation angle to one of the two active transmit antennas. As a result, the overall spectral efficiency is increased by 1 bit/s/Hz when compared with the equivalent CGSM system. In order to validate the simulation results of CGSM‐ESE, the third contribution is to derive an analytical ABER expression. Finally, to improve the ABER performance of CGSM‐ESE, three link adaptation algorithms are developed. By assuming full knowledge of the channel at the receiver, the proposed algorithms select a subset of channel gain vector (CGV) pairs based on the Euclidean distance between all CGV pairs, CGV splitting, CGV amplitudes, or a combination of these. 相似文献
9.
The Vehicle Routing Problem with Simultaneous Pickup and Delivery (VRPSPD) is an extension to the classical Vehicle Routing Problem (VRP), where customers may both receive and send goods simultaneously. The Vehicle Routing Problem with Mixed Pickup and Delivery (VRPMPD) differs from the VRPSPD in that the customers may have either pickup or delivery demand. However, the solution approaches proposed for the VRPSPD can be directly applied to the VRPMPD. In this study, an adaptive local search solution approach is developed for both the VRPSPD and the VRPMPD, which hybridizes a Simulated Annealing inspired algorithm with Variable Neighborhood Descent. The algorithm uses an adaptive threshold function that makes the algorithm self-tuning. The proposed approach is tested on well-known VRPSPD and VRPMPD benchmark instances derived from the literature. The computational results indicate that the proposed algorithm is effective in solving the problems in reasonable computation time. 相似文献
10.
In this article, two novel kinds of focusing elements as reflectors are analyzed and compared. One is the grooved Fresnel zone plate reflector with continuous phase‐correcting. The other called subzone paraboloid reflector, has the profile that consists of a series of paraboloids. Their diffraction efficiencies and bandwidths are described. The two elements still preserve the advantages of Fresnel zone plates, namely, low profile, high efficiency, and simple fabrication. Two dual‐reflector antennas using the proposed focusing elements as the main reflectors are simulated and the results show that these antennas have good radiation performances. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:101–108, 2015. 相似文献