全文获取类型
收费全文 | 40573篇 |
免费 | 4474篇 |
国内免费 | 2358篇 |
学科分类
工业技术 | 47405篇 |
出版年
2024年 | 88篇 |
2023年 | 631篇 |
2022年 | 1158篇 |
2021年 | 1715篇 |
2020年 | 1320篇 |
2019年 | 1189篇 |
2018年 | 1279篇 |
2017年 | 1347篇 |
2016年 | 1292篇 |
2015年 | 1780篇 |
2014年 | 2138篇 |
2013年 | 2617篇 |
2012年 | 2805篇 |
2011年 | 2794篇 |
2010年 | 2612篇 |
2009年 | 2387篇 |
2008年 | 2300篇 |
2007年 | 2131篇 |
2006年 | 2097篇 |
2005年 | 1826篇 |
2004年 | 1389篇 |
2003年 | 1592篇 |
2002年 | 2062篇 |
2001年 | 1792篇 |
2000年 | 1132篇 |
1999年 | 891篇 |
1998年 | 579篇 |
1997年 | 480篇 |
1996年 | 387篇 |
1995年 | 364篇 |
1994年 | 283篇 |
1993年 | 186篇 |
1992年 | 173篇 |
1991年 | 149篇 |
1990年 | 93篇 |
1989年 | 83篇 |
1988年 | 61篇 |
1987年 | 35篇 |
1986年 | 27篇 |
1985年 | 21篇 |
1984年 | 10篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 26篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1974年 | 4篇 |
1959年 | 7篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Liqian Wu Dongdong Shi Shiming Yan Wen Qiao Wei Zhong Youwei Du 《International Journal of Hydrogen Energy》2021,46(2):2086-2094
The exploration of efficient, low-cost and earth-abundant oxygen-evolution reaction (OER) electrocatalysts and the understanding of the intrinsic mechanism are important to advance the clean energy conversion technique based on electrochemical water oxidation. In this work, Fe-doping Co3N catalysts were successfully synthesized by a simple nitridation reaction of the Co3-xFexO4 precursor. This material exhibited a low overpotential of 294 mV at a current density of 10 mA cm?2, and a small Tafel slope of 49 mV dec?1 in 1 M KOH solution, superior to the performance of Co3N and IrO2. As revealed by the spectroscopic and electrochemical analyses, the enhanced OER performance mainly originates from the electronic modulation induced by the incorporation of Fe into Co3N, benefitting the formation of CoOOH as active surface species and thus facilitating the OER process. These findings also demonstrate the introduction of heterogeneous element is a simple and effective strategy to regulate the OER property of the cobalt nitrides (Co3N) catalysts. 相似文献
52.
Wen Jiang Wentao Qi Qianqian Pan Qi Jia Chao Yang Bingqiang Cao 《International Journal of Hydrogen Energy》2021,46(5):4252-4258
Mn-based Prussian blue analogue is regarded as one of the promising cathodes for sodium ions battery owing to its high theoretical capacity and low cost. However, the unstable structure during charging/discharging process and the poor cycle life hinder its commercial application. In this work, potassium ions stabilized hollow Mn-based Prussian blue analogue is synthesized through a simple sodium citrate assisted method using for cathode of sodium-ions batteries. Although unique hollow structure could suffer volume variation during charging/discharging process, the K+ is introduced to further stabilize its structure. The PBAs cathode exhibits a high reversible specific capacity of 128 mA h g?1 at 50 mA and superior rate performance of 72 mA h g?1 at a high current density of 3200 mA g?1, which is attributed to its stable structure and enhanced sodium ions transport kinetics. Ex-situ XRD/Raman tests and electrochemical measurements further prove the synergistic effect of various alkali ions (K+/Na+) and unique hollow structure. They work together to improve the structural stability and promote sodium ions diffusion rate of Mn-based PBAs. 相似文献
53.
54.
Yiwu Kuang Fei Han Lijie Sun Rui Zhuan Wen Wang 《International Journal of Hydrogen Energy》2021,46(37):19617-19632
Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10?5–2.33 × 10?3. 相似文献
55.
动态循环荷载下大坝混凝土拉压转换损伤本构模型构建及影响研究 总被引:1,自引:0,他引:1
目前混凝土坝抗震安全评价中,对于动态循环荷载下大坝混凝土损伤本构模型中从受拉状态向受压状态转变过程(拉压转换)中的混凝土弹性模量变化,一般都基于商用软件ABAQUS的"单边效应"假设,认为混凝土弹性模量立即恢复为初始弹性模量。上述假设的合理性缺乏试验验证,直接影响混凝土坝抗震安全评价的可靠性。通过对全级配大坝混凝土拉压转换全过程试验成果的观察分析,发现混凝土从受拉状态进入受压状态后弹性模量并非立即恢复为初始弹性模量,而是由损伤后弹性模量连续渐进恢复至初始弹性模量,期间原有的受拉残余应变在压应力作用下迅速减小到较小数值,因此ABAQUS的"单边效应"假设并不符合实际情况。本文以全级配大坝混凝土拉压转换全过程试验成果为基础,提出受拉损伤发生后,拉压转换时受压应力应变关系采用双折线模型的思路,据此推导了相应的应力-应变关系解析表达式,构建了更接近混凝土拉压转换时真实状况的本构关系数值模型。通过实际工程的计算分析,发现常用的受压弹性模量立即恢复为初始弹性模量的损伤本构模型过高估计了坝体刚度,可能带来偏于不安全的评价结果。 相似文献
56.
介绍不分散混凝土的作用机理、不分散混凝土配合比的设计及优化以及不分散混凝土在常泰长江大桥6号墩钢沉井井壁水下浇筑施工中的应用. 相似文献
57.
This work aims to improve the rheological properties and stability of multiwalled carbon nanotubes (MWCNTs)/acrylamide (AA) base skeleton polymer blends at harsh environment of high salinity-high temperature (HS-HT) or various pH. Different co/terpolymers have been accomplished to modify the structure of AA polymer by free-radical copolymerization of AA-based monomers. Anionic, cationic, and hydrophobic functional groups were used for the synthesis of polyelectrolyte, polyampholytic, and partially hydrophobic AA polymer types. The conversion, molecular weight, and poly dispersity of co/terpolymers have been evaluated by nuclear magnetic resonance (1H-NMR), gel permeation chromatography, and differential scanning calorimetry analysis. The effects of sonication power, concentration of polymer, and concentration of MWCNTs were also investigated on rheological behavior of co/terpolymers. The results show that negative polyelectrolyte and polyampholytic polymers are the best candidates for the improvement of MWCNTs/polymer stability and viscosity at HS-HT and alkali environment, respectively. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47205. 相似文献
58.
This paper presents the results of experimental investigations and constitutive modeling of cyclic interface shearing between HDPE geomembrane and cohesionless sandy gravel. A series of cyclic interface shear tests was performed using a large-scale cyclic shear apparatus with servo controlled system. Particular attention was paid to the influences of the amount of shear-displacement amplitude, number of cycles, shear rate and the normal pressure on the mechanical response. The experimental results show that the path of the shear stress against the cyclic shear displacement is strongly non-linear and forms a closed hysteresis loop, which is pressure dependent, but almost independent of the shear rate. For small shear-displacement amplitudes, the obtained damping ratio is significantly greater than zero, which is different to the behavior usually observed for cyclic soil to soil shearing. In order to describe the pressure dependency of the hysteresis loop using a single set of constitutive parameters, new approximation functions are put forward and embedded into the concept of the Masing rule. Further, a new empirical function is proposed for the damping ratios to capture the experimental data for both small and large cyclic shear-displacement amplitudes. The included model parameters are easy to calibrate and the new functions may also be useful in developing enhanced constitutive models for the simulation of the cyclic interface shear behavior between other geosynthetics and soils. 相似文献
59.
60.
Wenya Du Yunlong Ai Weihua Chen Wen He Jianjun Zhang Yaqi Fan Yuxing Gong 《Ceramics International》2019,45(6):6788-6794
xNb2O5–7.5La2O3-Al2O3 ceramic composites with in-situ-grown columnar Al2O3 crystals were successfully prepared by microwave sintering at 1450–1525?°C using α-Al2O3, Nb2O5, and La2O3 powders as raw materials. X-ray diffraction results indicated that the main phases were Al2O3, LaNbO4, and Nb2O5 in the prepared samples. A field emission scanning electron microscope (FESEM) showed that the Al2O3 crystals appeared as columnar in the structure. Moreover, the grain size of the columnar Al2O3 crystals increased with the Nb2O5 content. The ratio of the major axis to the minor axis of the crystals was largest when the Nb2O5 content was 15?vol%. Furthermore, the grain-growth kinetics index (n), growth activation energy (Q), and growth mechanism of the columnar Al2O3 crystals were studied. The results indicated that the Nb2O5 addition could promote formation and growth of columnar Al2O3 crystals, and the grain-growth activation energy indicated that the dissolution process controls the crystal growth. The growth mechanism of the columnar Al2O3 crystals was also studied. The present work demonstrated that Nb2O5 is a good additive for the preparation of Nb2O5–7.5La2O3-Al2O3 composite ceramics with columnar Al2O3 crystals. 相似文献