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991.
采用高压催化剂性能评价实验装置,在压力分别为7.0,10.0和15.0MPa,温度分别为350,400和450℃条件下,在H2/N2为1.6-3.0范围内研究了H2/N2对A301,ZA-5和A110-2型催化剂的活性和合成塔出口氨浓度的影响。在压力和空速一定的条件下,最佳H2/N2随反应温度而异。在350,400和450℃下,最佳H2/N2分别为1.8-2.2,2.2-2.5和2.5-3.0。由此可见,合成氨反应的速率达到最大值时的最佳H2/N2值与反应的进程有关。为此提出了催化剂效率K(Catalysis efficiency)的概念来表征在催化剂作用下反应器出口氨浓度趋近平衡的程度,即K=CNH3/C^*NH3。根据实验结果,得到了最佳H2/N2与催化剂效率的定量关系:(H2/N2)m=1.5(1 CNH3/C^*NH3)=1.5(1 K)。由此可以根据催化剂在不同反应条件下的催化剂效率来确定最佳H2/N2。凡是会降低催化剂效率的因素,都会使最佳H2/N2降低。各种影响因素对最佳H2/N2的影响中,反应温度的影响最大,其次是空速和催化剂的活性,而压力和惰性气体含量的影响相对较小。在低温(低压)下合成氨,宜采用较低的H2/N2。 相似文献
992.
993.
To model the molten globule structure of -lactalbumin, moleculardynamics (MD) simulations were carried out for the protein inexplicit water at high temperature. In these simulations, long-rangeCoulomb interactions were evaluated explicitly with an originalmethod (particleparticle and particlecell: PPPC)to avoid artifacts caused by the cut-off. The MD simulationswere started from two initial conditions to verify that similarresults would be obtained. From the last 150 ps trajectoriesof the two MD simulations, two partially unfolded average structureswere obtained. These structures had the following common structuralfeatures which are characteristic of the molten globule state.The radii of gyration for these conformations were 7.4 and 9.6%larger than that of the native state. These values were almostthe same as the experimental value (9.6%) observed recentlyby small-angle X-ray scattering (Kataoka,M., Kuwajima,K., Tokunaga,F.and Goto,Y., 1997, Protein Sci., 6, 422430). Furthermore,aromatic residues of clusters I and II in these structures werefar apart from each other except for Try103Trp104. Thisresult is in good agreement with NMR experimental results forthe acid-denatured molten globule state (Alexandrescu et al.,1992, 1993); that is, NOE signals between the aromatic residueswere not observed, except for that of Try103Trp104 inthe molten globule state. Other structural features of thesemodels for the molten globule state are discussed with referenceto native state structures. 相似文献
994.
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
995.
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
996.
997.
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
998.
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
999.
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
1000.
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