全文获取类型
收费全文 | 100187篇 |
免费 | 7455篇 |
国内免费 | 6210篇 |
专业分类
电工技术 | 8238篇 |
技术理论 | 2篇 |
综合类 | 6910篇 |
化学工业 | 20559篇 |
金属工艺 | 6195篇 |
机械仪表 | 7623篇 |
建筑科学 | 3697篇 |
矿业工程 | 1431篇 |
能源动力 | 3444篇 |
轻工业 | 6577篇 |
水利工程 | 652篇 |
石油天然气 | 4412篇 |
武器工业 | 894篇 |
无线电 | 12775篇 |
一般工业技术 | 10538篇 |
冶金工业 | 2478篇 |
原子能技术 | 1140篇 |
自动化技术 | 16287篇 |
出版年
2024年 | 282篇 |
2023年 | 1287篇 |
2022年 | 2239篇 |
2021年 | 2653篇 |
2020年 | 2356篇 |
2019年 | 2353篇 |
2018年 | 2208篇 |
2017年 | 2850篇 |
2016年 | 3112篇 |
2015年 | 3230篇 |
2014年 | 4622篇 |
2013年 | 5405篇 |
2012年 | 5742篇 |
2011年 | 6895篇 |
2010年 | 5733篇 |
2009年 | 6608篇 |
2008年 | 6377篇 |
2007年 | 6974篇 |
2006年 | 6721篇 |
2005年 | 5553篇 |
2004年 | 4859篇 |
2003年 | 4672篇 |
2002年 | 3860篇 |
2001年 | 3066篇 |
2000年 | 2705篇 |
1999年 | 2151篇 |
1998年 | 1541篇 |
1997年 | 1233篇 |
1996年 | 1150篇 |
1995年 | 1142篇 |
1994年 | 1011篇 |
1993年 | 839篇 |
1992年 | 636篇 |
1991年 | 389篇 |
1990年 | 247篇 |
1989年 | 247篇 |
1988年 | 169篇 |
1987年 | 115篇 |
1986年 | 112篇 |
1985年 | 84篇 |
1984年 | 71篇 |
1983年 | 47篇 |
1982年 | 50篇 |
1981年 | 56篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 20篇 |
1976年 | 26篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Two experiments examined the effects of prior knowledge on learning from different compositions of multiple representations in a mobile learning environment on plant leaf morphology for primary school students. Experiment 1 compared the learning effects of a mobile learning environment presenting text and photos of plants on a tablet PC, either in combination with or without real plants in the physical environment. Results indicated that there were no interactions between prior knowledge and experimental condition. Students who learned with tablet PCs only outperformed students who additionally learned with real plants on a comprehension and an application test. In addition, high prior knowledge students outperformed low prior knowledge students on both tests. To investigate whether these effects were caused by the specific characteristics of the combination of photos of real plants and real plants, Experiment 2 compared the differential effects of prior knowledge on learning with the combination of texts, photos and real plants to a combination in which the photos were replaced by schematic hand drawings. Results indicated that both low and high prior knowledge students, who learned with the combination of texts, schematic hand drawings and real plants performed better on a comprehension and an application test. High prior knowledge students performed better on both tests. It is concluded that the number and type of representations used is critical for the effectiveness of mobile learning environments. 相似文献
952.
有机改性蒙脱土/木粉/PVC复合材料的阻燃性研究 总被引:2,自引:0,他引:2
将有机改性蒙脱土(OMMT)加入PVC木塑体系,制备了有机蒙脱土/木粉/聚氯乙烯(OMMT/WF/PVC)纳米复合材料。采用锥形扫描量热测试法研究了OMMT用量对WF/PVC复合材料阻燃性能的影响。结果表明,添加OMMT可明显降低木塑的初始热失重率,但使快速热分解的开始时间提前,大大提高样品的燃烧残余率,显著延迟木塑的点燃时间和燃烧热峰值产生时间,OMMT起到了有效的阻燃作用。OMMT的加入还降低了总燃烧热,但燃烧热峰值有所增加,并使发烟量有所增加,CO和CO2产率也随之提高,增加了木塑燃烧的烟气危害性。 相似文献
953.
954.
基于Pro/E参数化斜齿轮建模技术 总被引:1,自引:0,他引:1
应用Pro/E软件的参数化造型功能,不需要二次开发,直接创建渐开线斜齿圆柱齿轮模型,对直齿、人字齿齿轮的建模具有借鉴作用。 相似文献
955.
采用双螺杆挤出机通过熔融共混制备了尼龙6/(苯乙烯/丙烯腈/顺丁烯二酸酐)共聚物/玻璃纤维(PA6/SANMAH/GF)复合材料,测试了材料的拉伸强度、弯曲强度、弯曲弹性模量、缺口冲击强度、热变形温度、吸水率、熔点和熔融焓,并与GF增强PA6(PA6/GF)复合材料和GF增强PA6/(苯乙烯/丙烯腈)共聚物(PA6/SAN/GF)复合材料进行了性能对比。结果表明,在PA6与SANMAH的质量比为100:3—30时,PA6/SANMAH/GF复合材料的拉伸强度与PA6/GF复合材料相当,但高于PA6/SAN/GF复合材料,弯曲强度和弯曲弹性模量高于PA6/GF和PA6/SAN/GF复合材料.缺口冲击强度高于PA6/GF复合材料,但低于PA6/SAN/GF复合材料;在PA6与SANMAH的质量比为100:40时,PA6/SANMAH/GF复合材料的拉伸强度、弯曲强度和缺口冲击强度明显降低;在整个试验范围内,PA6/SANMAH/GF复合材料的热变形温度比PA6/GF和PA6/SAN/GF复合材料低4~7℃;吸水率随着SAN-MAH用量的增加而逐渐减小。 相似文献
956.
A new method is presented for preparing highly monodispersed silica particles using a two-stage semibatch/batch hydrolysis reaction of Si(OC2 H5 )4 . The slower rate of hydrolysis of the tetraethylorthosilicate (TEOS) that occurred during the semibatch process resulted in larger silica particles with a higher yield and narrower size distribution. This was in direct contrast to the batch process. In addition, the ability of four different mixed processes to produce silica particles with good packing density, narrower particle-size distribution, and higher yield were evaluated. These were batch/batch (B-B), batch/semibatch (B-S), semibatch/batch (S-B), and semibatch/semibatch (S-S) processes. The S-S system produced the largest particles with the highest yields. The size of the silica particles obtained by the S-B method decreased with increasing reaction time, while the particles obtained by the B-S process had the best particle-size distribution and packing density. In conclusion, a mixed batch/semibatch system was the best way to produce an extremely narrow particle-size distribution and a good packing density. 相似文献
957.
Xiaobao Li Richard P. K. Wells Peter B. Wells Graham J. Hutchings 《Catalysis Letters》2003,89(3-4):163-167
The premodification of a 5 wt% Pt/-Al2O3 catalyst with cinchonidine (0.01 and 0.2 g g-1
catalyst) is described and discussed. Premodification is carried out by treating the catalyst with a solution of cinchonidine followed by solvent removal. Catalysts premodified in this way give the same ee and initial rate of reaction for the enantioselective hydrogenation of ethyl pyruvate as those using the standard in situ modification procedure. Investigations of different solvents for premodification and reaction (dichloromethane, ethanol) show that it is the solvent used for the reaction that controls the observed enantioselection. Premodified catalysts also display the initial transient behavior typically observed with in situ modified catalysts in which the ee increases with conversion in the early part of the reaction. Premodified catalysts show an enhanced rate of reaction when ethanol is used as the reaction solvent compared with in situ modified catalysts under the same conditions. Premodification using aerobic conditions gives the best results and premodified catalysts can be stored prior to use for up to a week without loss of catalytic performance. 相似文献
958.
固体超强酸SO2-4/TiO2-SiO2催化合成马来酸二辛酯 总被引:6,自引:0,他引:6
用H2SO4浸渍钛硅复合氧化物,制得固体超强酸SO4^2-/TiO2-SiO2,考察了催化剂对马来酸酐与正辛醇的酯化反应的催化作用及其制备条件对催化剂活性的影响,并与硫酸,对甲苯磺酸的催化效果比较,结果表明,对于给定反应,当n(Ti):n(Si)为15:1,用浓度0.6mol/L的硫酸浸渍8h,在550℃下焙烧3h时制得的催化剂SO4^2-/TiO2-SiO2具有最高的催化活性,用以催化马来酸酐和正辛醇的酯化反应,可得无色透明的酯化产物,3h内酯化率达99.1%,较SO4^2-/TiO2催化剂的酯化率提高了约6%。 相似文献
959.
960.
采用种子乳液聚合法,以交联型软性聚合物为核,交联型硬性高聚物为壳,合成了具有优异耐寒性及高温抗回粘性聚合物。讨论了乳化剂种类及应用、核/壳比等对聚合物性能的影响。 相似文献