全文获取类型
收费全文 | 20666篇 |
免费 | 2068篇 |
国内免费 | 1361篇 |
学科分类
工业技术 | 24095篇 |
出版年
2024年 | 100篇 |
2023年 | 754篇 |
2022年 | 986篇 |
2021年 | 931篇 |
2020年 | 880篇 |
2019年 | 793篇 |
2018年 | 634篇 |
2017年 | 720篇 |
2016年 | 711篇 |
2015年 | 655篇 |
2014年 | 1078篇 |
2013年 | 1352篇 |
2012年 | 1296篇 |
2011年 | 1356篇 |
2010年 | 1065篇 |
2009年 | 1105篇 |
2008年 | 916篇 |
2007年 | 1219篇 |
2006年 | 1178篇 |
2005年 | 965篇 |
2004年 | 895篇 |
2003年 | 720篇 |
2002年 | 656篇 |
2001年 | 565篇 |
2000年 | 450篇 |
1999年 | 356篇 |
1998年 | 291篇 |
1997年 | 223篇 |
1996年 | 216篇 |
1995年 | 191篇 |
1994年 | 160篇 |
1993年 | 120篇 |
1992年 | 93篇 |
1991年 | 87篇 |
1990年 | 90篇 |
1989年 | 68篇 |
1988年 | 49篇 |
1987年 | 35篇 |
1986年 | 16篇 |
1985年 | 24篇 |
1984年 | 32篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1959年 | 2篇 |
1951年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
对上海高桥分公司2×10~4m~3/h(标准状态)制氢装置原设计的预转化催化剂还原流程进行了改进。先跳开预转化反应器,利用转化炉制取氢气,再用自产的高纯氢气代替重整氢,对预转化催化剂进行单独升温还原,避免了催化剂在还原初期因发生甲烷化反应而超温失活的问题,使催化剂具有更好的活性和稳定性。 相似文献
3.
The scale along the coordinate axes in the equations of motion hitherto used for an H-maser (in relative units) is shown to depend on Q as the quality factor for an H-maser. This hinders both analysis of the operation of the H-maser and optimization of its parameters. The modified equations of motion proposed here do not have that shortcoming. We determine the atomic flux into the bulb that gives the H-maser the best metrological characteristics. 相似文献
4.
Biodegradation of monoaromatic hydrocarbons in aquifer columns amended with hydrogen peroxide and nitrate 总被引:1,自引:0,他引:1
The ability of indigenous microorganisms to degrade benzene, toluene, ethylbenzene and xylenes (BTEX) in laboratory scale flow-through aquifer columns was tested separately with hydrogen peroxide (110 mg/l) and nitrate (330 mg/l as NO3−) amendments to air-saturated influent nutrient solution. The continuous removal of individual components from all columns relative to the sterile controls provided evidence for biodegradation. In the presence of hydrogen peroxide, the indigeneous microorganisms degraded benzene and toluene (> 95%), meta- plus para-xylene (80%) and ortho-xylene (70%). Nitrate addition resulted in 90% removal of toluene and 25% removal of ortho-xylene. However, benzene, ethylbenzene, meta- and para-xylene concentrations were not significantly reduced after 42 days of operation. Following this experiment, low dissolved oxygen (< 1 mg/l) conditions were initiated with the nitrate-amended column influent in order to mimic contaminated groundwater conditions distal from a nutrient injection well. Toluene continued to be effectively degraded (> 90%), and more than 25% of the benzene, 40% of the ethylbenzene, 50% of the meta- plus para-xylenes and 60% of the ortho-xylene were removed after several months of operation. 相似文献
5.
6.
Bimolecular hydrogen transfer and skeletal isomerization the important secondary reac-tions among catalytic cracking reactions,which affect product yield distribution and product quality,Catalyst properties and operating parameters have great impact on bimolecular hydrogen transfer and skeletal isomerization reactions .Bimolecular hydrogen transfer activity and skeletal isomrization activity of USY-containing catalysts are higher thn that of ZSM-5-containing catalyst.Coke deposition on the active sites of catalyst may suppress bimolecular hydrogen transfer activity and skeletal isomer-ization activity of catlys in different degrees.Short raction time causes a decrease of hydrogen trans-fer reaction,but and increase of skeletal isomerization reaction compared to cracking reaction in catalytic cracking process. 相似文献
7.
The bonding of β'-Al2O3 and pyrex glass to Al matrix composites by anodic bonding process is achieved. The microstructure of the bonded interface and the joining mechanisms are analyzed with scanning electron microscope (SEM), energy dispersive X-ray fluorescence spectrometer (EDX). It is observed that the bonding region across the interface consists of the metal layer, oxide transitional layer and the ceramic layer, with the transitional layer composed of surface region and sub-surface region. The bonding process can mainly be categorized into anodic bonding process and solid state diffusing process. The pile-up of the ions and its drift in the interface area are the main reasons for anode oxidation and joining of the interface. The temperature, voltage and the drift ions in the ceramic or glass during the bonding process are the essential conditions to solid state diffusing and oxide bonding at the interface. The voltages, temperature, pressure as well as the surface state are the main factors that influence the anodic bonding. 相似文献
8.
Wang Xuegang Yan Fengjie Yan Qian Li Xingeng 《Frontiers of Materials Science in China》2007,1(2):225-227
An iron-based amorphous foil (FeNiCrSiB) was used as an interlayer for the amorphous diffusion bonding of low carbon steel
pipes under argon flux. The microstructure and mechanical properties of the joint were analyzed using an electron probe micro-analyzer
(EPMA), tensile test, bending test and impact test. The results show that the joint microstructure resembles that of the base
metal and no precipitates form at the joint. Melting point depressants (B, Si) diffuse far away from the joint and the base
metal element is homogenous across the joint. The joint impact toughness is greater than the base metal toughness and the
mechanical properties of the joint are similar around the pipe. 相似文献
9.
10.
The structural properties and hydrogen bonding of undoped and phosphorous doped polycrystalline silicon produced by step-by-step laser dehydrogenation and crystallization technique were investigated using Raman spectroscopy and hydrogen effusion measurements. At low laser fluences, EL, a two-layer system is created. This is accompanied by the change in hydrogen bonding. The intensity of the Si–H vibration mode at 2000 decreases faster than the one at 2100 cm−1. This is even more pronounced in phosphorous-doped specimens. The laser crystallization results in an increase of the hydrogen binding energy by approximately 0.2–0.3 eV compared to the amorphous starting materials. 相似文献