全文获取类型
收费全文 | 88688篇 |
免费 | 4004篇 |
国内免费 | 4105篇 |
专业分类
电工技术 | 4011篇 |
技术理论 | 5篇 |
综合类 | 8447篇 |
化学工业 | 12626篇 |
金属工艺 | 5635篇 |
机械仪表 | 3004篇 |
建筑科学 | 4324篇 |
矿业工程 | 1246篇 |
能源动力 | 2736篇 |
轻工业 | 5802篇 |
水利工程 | 1930篇 |
石油天然气 | 4092篇 |
武器工业 | 687篇 |
无线电 | 6704篇 |
一般工业技术 | 13999篇 |
冶金工业 | 2461篇 |
原子能技术 | 2175篇 |
自动化技术 | 16913篇 |
出版年
2024年 | 63篇 |
2023年 | 259篇 |
2022年 | 371篇 |
2021年 | 566篇 |
2020年 | 971篇 |
2019年 | 922篇 |
2018年 | 1039篇 |
2017年 | 963篇 |
2016年 | 1486篇 |
2015年 | 2115篇 |
2014年 | 3893篇 |
2013年 | 4647篇 |
2012年 | 3952篇 |
2011年 | 4591篇 |
2010年 | 3853篇 |
2009年 | 5238篇 |
2008年 | 5247篇 |
2007年 | 5598篇 |
2006年 | 5137篇 |
2005年 | 4303篇 |
2004年 | 3716篇 |
2003年 | 3638篇 |
2002年 | 3689篇 |
2001年 | 2761篇 |
2000年 | 3148篇 |
1999年 | 2931篇 |
1998年 | 2468篇 |
1997年 | 2352篇 |
1996年 | 2503篇 |
1995年 | 2649篇 |
1994年 | 2404篇 |
1993年 | 1459篇 |
1992年 | 1486篇 |
1991年 | 1021篇 |
1990年 | 747篇 |
1989年 | 665篇 |
1988年 | 632篇 |
1987年 | 371篇 |
1986年 | 222篇 |
1985年 | 369篇 |
1984年 | 411篇 |
1983年 | 429篇 |
1982年 | 328篇 |
1981年 | 404篇 |
1980年 | 270篇 |
1979年 | 114篇 |
1978年 | 112篇 |
1977年 | 69篇 |
1976年 | 40篇 |
1975年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
31.
32.
33.
Two kinds of Cu-Al_2O_3 composites(with and without La) were prepared via mechanical alloying-spark plasma sintering(MA-SPS) method. Microstructure, mechanical properties and electrical resistivity were investigated systematically using metallography, scanning electron microscopy, transmission electron microscopy, mechanical and electrical properties testing. The results indicate that an appropriate amount of La can homogenize the distribution of Al_2O_3. As such, yield strength, ultimate tensile strength and elongation of Cu-Al_2O_3-La are greatly increased. Some semi-coherent interface between Cu and Al_2O_3 is found, which means a low interface energy. The grain shape of Cu changes to irregular band with the addition of La. This change results in a density decrease of grain boundary and reduces electrical resistance. Lanthanum may exist in the form of La_2O_3. 相似文献
34.
Yuri N. Osetsky Laurent K. Béland Alexander V. Barashev Yanwen Zhang 《Current Opinion in Solid State & Materials Science》2018,22(3):65-74
Concentrated single phase solid solutions, including medium- and high-entropy alloys, represent a new class of materials that have recently attracted significant interest due to exceptional functional and structural properties. Their fascinating properties are mainly attributed to the sluggish atomic-level diffusion and transport, but its controlling mechanisms are largely unknown and there is certain skepticism about its very existence. By using microsecond-scale molecular dynamics, on-the-fly and conventional kinetic Monte Carlo, we reveal the governing role of percolation effects and composition dependence of the vacancy migration energy in diffusion. Surprisingly, an increase of concentration of faster species (Fe) in face-centered cubic Ni-Fe alloy may decrease the overall atomic diffusion. Consequently, the composition dependence of tracer diffusion coefficient has a minimum near the site percolation threshold, ~20?at.%Fe. We argue that this coupled percolation and composition-dependent barriers for vacancy jumps within different subsystems in medium- and high-entropy alloys leads, indeed, to the sluggish diffusion. A fast method for preselecting materials with potentially desired properties is suggested. 相似文献
35.
Isolation,purification, and antioxidant activities of polysaccharides from Ziziphus Jujuba cv. Muzao
In the present study, crude polysaccharides from Ziziphus Jujuba cv. Muzao were isolated and purified using DEAE cellulose-52 and Sephadex G-100 size-exclusion chromatography; four fractions were collected, namely GZMP-1, GZMP-2, GZMP-3, and GZMP-4. The molecular weights of these four fractions were measured to be 111.2, 95.1, 84.2, and 571.4 kDa, respectively, using high-performance gel permeation chromatography. Gas chromatography analysis of the monosaccharide composition confirmed that GZMP-1 was composed of rhamnose, arabinose, glucose, and galactose. Rhamnose, arabinose, and galactose were the main components present in GZMP-2 and GZMP-3, whereas GZMP-4 was composed of only rhamnose and arabinose. Scanning electron microscopy showed relatively smooth surfaces for GZMP-1 and GZMP-4, whereas GZMP-2 and GZMP-3 had more folds on their surfaces. Fourier transform infrared spectroscopy analyses indicated that GZMP-1 and ZMP mainly had α-type glycosidic linkages. The in vitro antioxidant activities of the polysaccharides revealed that jujube polysaccharides exhibit remarkable antioxidant activity, and can scavenge DPPH radical and OH radical in a concentration-dependent manner. The results of this work suggest that polysaccharides from Z. Jujuba cv. Muzao have potential to be used as functional food and in the development of natural antioxidant drug carriers. 相似文献
36.
37.
38.
Haiyi Peng Haishen Ren Mingzhao Dang Yi Zhang Xiaogang Yao Huixing Lin 《Ceramics International》2018,44(6):6527-6532
Ca0.9La0.067TiO3 (abbreviated as CLT) ceramics doped with different amount of Al2O3 were prepared via the solid state reaction method. The anti-reduction mechanism of Ti4+ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al2O3. The results showed that Al3+ substituted for Ti4+ to form solid solution and the solid solubility limit of Al3+ is near 1.11 mol%. Furthermore, the reduction of Ti4+ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al2O3 exhibited good microwave dielectric properties: εr = 141, Q × f = 6848 GHz, τf = 576 ppm/°C. 相似文献
39.
在对沁水盆地赵庄矿煤层气开发地质、煤层气生产井资料深入研究的基础上,分析、总结了区内煤层气直井低产原因,并据此提出了区内地面抽采后续开发方向。研究认为:3号煤层气含量偏低、含气饱和度低、临储比低,致使排采阶段气含量可降幅度低,是其低产的宏观表征;3号煤储层不匹配孔级,较差连通性,即微、小孔为主,中孔次之,大孔不发育,微裂隙连通性差,致使扩散缓慢、影响储层改造和抽采效果,是其低产的微观表征;3号煤层远高于顶底板塑性及相近的水平应力,进一步造成改造缝长受限,泄流面积不足。因此,建议区内后续地面抽采方式应以增大泄流面积开发方式为主。 相似文献
40.
针对65号航空冷却液在过滤循环中出现泡沫较多的现象,对65号航空冷却液泡沫倾向性进行探究。按照石化行业标准《发动机冷却液泡沫倾向测定法(SH /T 0066-2002)》,利用冷却液泡沫倾向测定仪来评价65号航空冷却液泡沫倾向性,并进行水乙二醇溶液对照试验。结果表明,航空冷却液在低温和循环时间较长的情况下,其泡沫倾向性较大,而随着温度的升高和循环时间的缩短,泡沫倾向性有一定程度的改善;并且通过水乙二醇对照试验可知,冷却液的基础液泡沫倾向性较小。航空冷却液在过滤循环中出现的泡沫问题,是加入的添加剂所引起的。 相似文献