全文获取类型
收费全文 | 29213篇 |
免费 | 3021篇 |
国内免费 | 1759篇 |
专业分类
电工技术 | 1389篇 |
技术理论 | 5篇 |
综合类 | 1605篇 |
化学工业 | 7708篇 |
金属工艺 | 1918篇 |
机械仪表 | 866篇 |
建筑科学 | 1491篇 |
矿业工程 | 1115篇 |
能源动力 | 3444篇 |
轻工业 | 912篇 |
水利工程 | 568篇 |
石油天然气 | 911篇 |
武器工业 | 124篇 |
无线电 | 1518篇 |
一般工业技术 | 3311篇 |
冶金工业 | 2767篇 |
原子能技术 | 217篇 |
自动化技术 | 4124篇 |
出版年
2024年 | 128篇 |
2023年 | 723篇 |
2022年 | 1005篇 |
2021年 | 1128篇 |
2020年 | 1205篇 |
2019年 | 1074篇 |
2018年 | 968篇 |
2017年 | 1115篇 |
2016年 | 1104篇 |
2015年 | 1055篇 |
2014年 | 1866篇 |
2013年 | 1892篇 |
2012年 | 2094篇 |
2011年 | 2787篇 |
2010年 | 2080篇 |
2009年 | 2091篇 |
2008年 | 1896篇 |
2007年 | 1864篇 |
2006年 | 1504篇 |
2005年 | 1056篇 |
2004年 | 841篇 |
2003年 | 744篇 |
2002年 | 670篇 |
2001年 | 466篇 |
2000年 | 424篇 |
1999年 | 389篇 |
1998年 | 319篇 |
1997年 | 249篇 |
1996年 | 260篇 |
1995年 | 170篇 |
1994年 | 169篇 |
1993年 | 122篇 |
1992年 | 118篇 |
1991年 | 74篇 |
1990年 | 54篇 |
1989年 | 36篇 |
1988年 | 34篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 36篇 |
1984年 | 24篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 19篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1973年 | 3篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Simultaneous photocatalytic hydrogen production and CO2 reduction (to form CO and CH4) from water using methanol as a hole scavenger were investigated using silver-modified TiO2 (Ag/TiO2) nanocomposite catalysts. A simple ultrasonic spray pyrolysis (SP) method was used to prepare mesoporous Ag/TiO2 composite particles using TiO2 (P25) and AgNO3 as the precursors. The material properties and photocatalytic activities were compared with those prepared by a conventional wet-impregnation (WI) method. It was found that the samples prepared by the SP method had a larger specific surface area and a better dispersion of Ag nanoparticles on TiO2 than those prepared by the WI method, and as a result, the SP samples showed much higher photocatalytic activities toward H2 production and CO2 reduction. The optimal Ag concentration on TiO2 was found to be 2 wt%. The H2 production rate of the 2% Ag/TiO2–SP sample exhibited a six-fold enhancement compared with the 2% Ag/TiO2–WI sample and a sixty-fold enhancement compared with bare TiO2. The molar ratio of H2 and CO in the final products can be tuned in the range from 2 to 10 by varying the reaction gas composition, suggesting a viable way of producing syngas (a mixture of H2 and CO) from CO2 and water using the prepared Ag/TiO2 catalysts with energy input from the sun. 相似文献
992.
我国北方地区城市建筑约占全国建筑总量的26%,但建筑能耗却占到全国的40%~50%,因此,北方地区推行绿色建筑对节能减排意义重大。本文从选址与节地、供热计量、绿色建筑激励政策等十个方面阐述北方地区绿色建筑行动纲要。 相似文献
993.
994.
Embedding direct reduction followed by magnetic separation was conducted to fully recover iron and titanium separately from beach titanomagnetite(TTM).The influences of reduction conditions,such as molar ratio of C to Fe,reduction time,and reduction temperature,were studied.The results showed that the TTM concentrate was reduced to iron and iron-titanium oxides,depending on the reduction time,and the reduction sequence at 1 200°C was suggested as follows:Fe_(2.75)Ti_(0.25)O_4→Fe_2TiO_4→FeTiO_3→FeTi_2O_5.The reduction temperature played a considerable role in the reduction of TTM concentrates.Increasing temperature from 1 100 to 1 200°C was beneficial to recovering titanium and iron,whereas the results deteriorated as temperature increased further.The results of X-ray diffraction and scanning electron microscopy analyses showed that low temperature(≤1 100°C)was unfavorable for the gasification of reductant,resulting in insufficient reducing atmosphere in the reduction process.The molten phase was formed at high temperatures of 1 250-1 300°C,which accelerated the migration rate of metallic particles and suppressed the diffusion of reduction gas,resulting in poor reduction.The optimum conditions for reducing TTM concentrate are as follows:molar ratio of C to Fe of 1.68,reduction time of 150 min,and reduction temperature of 1 200°C.Under these conditions,direct reduction iron powder,assaying 90.28 mass%TFe and 1.73 mass% TiO_2 with iron recovery of 90.85%,and titanium concentrate,assaying 46.24mass% TiO_2 with TiO_2 recovery of 91.15%,were obtained. 相似文献
995.
Zhi-feng Li Yong-quan He Guang-ming Gao Jun-jian Tang Xiang-jun Zhang Zhen-yu Liu 《钢铁研究学报(英文版)》2017,24(10):1032-1040
A new hot-dip galvanizing method was employed on hot-rolled low carbon steel. The effects of Al contents on microstructure, micro-hardness and corrosion resistance of Zn-Al alloy coatings were systematically investigated. Phase composition, microstructure and element distribution in Zn-Al alloy coatings were analyzed using X-ray diffraction (XRD) and electron probe micro analysis (EPMA), respectively. It is found that Al content (0. 6–6. 0 wt. %) in galvanizing zinc affects surface quality and adhesion between coatings and matrix in the newly developed method. In addition, with increasing Al content, micro-hardness significantly increased due to the increase in Zn-Al eutectoid phases. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) also revealed that increase in Al plays a noticeable role in improving the corrosion resistance of Zn-Al alloy coatings. 相似文献
996.
The segregation process as applied to nickel laterite ores involves the formation of gaseous nickel and iron chlorides which are subsequently reduced to form ferronickel on the surface of a solid carbon reductant. A fundamental study of the process has been carried out using limonite, nontronite and garnierite ores. The study highlighted the significant impacts that ore mineralogy and carbon addition have on the amount of nickel which is segregated rather than retained within the ore due to in situ reduction. These important aspects affecting nickel recovery and the success of the process are discussed in this paper. 相似文献
997.
This paper utilizes data measured by phasor measurement units (PMUs) to extract a low-order dynamic equivalent model for power system stability studies. The estimated model is a 2-order model for synchronous machines. This model has the advantage of simplicity of classical model and considerably reduces the oversimplifying error of classical model. This method offers an alternative approach to analytical model reduction techniques based on the detailed system models. The proposed method uses the synchronized bus voltage and current phasors measured by PMUs. Using post disturbance data, electrical and mechanical parameters of the equivalent generator are estimated sequentially. Furthermore, a new approach for estimation of two-machine and single machine infinite bus (SMIB) equivalent systems are presented for analysis of electromechanical oscillations. To evaluate the performance of the proposed approach, simulations are performed on a two area 13-bus test system and real measured PMU data. Simulation results show that the estimated model can represent the dynamic behavior of the studied system with good approximation. 相似文献
998.
城市污水余热是城市中理想的新能源,因此对以城市污水余热为冷热源的能源系统进行优化设计及深入研究意义重大。以青岛麦岛示范项目为例,通过对该项目的污水源能源区域中心四年来的运营情况进行投资回报与节能效果的分析。为城市污水源热泵技术的推广提供了一种方案参考及数据支持。 相似文献
999.
This paper investigates the relationship between structure and electrochemical performance of reduced graphene oxide (RGO) prepared via heat treatment and chemical reduction method. Structure and morphology of RGO was characterized by means of Fourier transform infrared spectroscopy, scanning electron microscopy, X‐ray diffraction and Brunauer–Emmett–Teller. Electrochemical performance of RGO electrode supercapacitor was investigated in the organic electrolyte by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance. The results show heat treatment RGO has high graphitization degree, less surface oxygen‐containing groups, good charge–discharge efficiency and stable life cycle. The chemical reduced RGO has single‐graphene structure, high specific surface area, high specific capacitance and low internal resistance. The ascorbic acid reduction RGO exhibits good comprehensive electrochemical performance: Its specific capacitance was 220.7 F g?1, internal resistance was 3.0 Ω and charge–discharge efficiency was 97.0% after 2000 cycles of charging/discharging tests. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
1000.
Network reconfiguration is the process of changing the topology of distribution systems by altering the open/closed status of switches. Because there are many candidate-switching combinations in the distribution system, network reconfiguration is a complicated combinatorial, non-differentiable constrained optimization problem. In addition, the radiality constraint typically increases the intricacy of the problem. In this paper, to avoid create infeasible configuration, a new codification is proposed. The proposed codification is computationally efficient and guarantees to generate only feasible radial topologies all times. Also, in this paper, a modified heuristic approach for optimal reconfiguration in radial distribution systems is presented. Additionally, in order to economize voltage profile improvement, a number of new formulas have been represented. The effectiveness of the proposed method is demonstrated on balanced and unbalanced test distribution systems. 相似文献