全文获取类型
收费全文 | 36561篇 |
免费 | 4246篇 |
国内免费 | 2078篇 |
专业分类
电工技术 | 2611篇 |
综合类 | 2697篇 |
化学工业 | 6264篇 |
金属工艺 | 1969篇 |
机械仪表 | 2041篇 |
建筑科学 | 2545篇 |
矿业工程 | 909篇 |
能源动力 | 1192篇 |
轻工业 | 4537篇 |
水利工程 | 802篇 |
石油天然气 | 1190篇 |
武器工业 | 463篇 |
无线电 | 3900篇 |
一般工业技术 | 4325篇 |
冶金工业 | 1534篇 |
原子能技术 | 556篇 |
自动化技术 | 5350篇 |
出版年
2024年 | 256篇 |
2023年 | 795篇 |
2022年 | 1733篇 |
2021年 | 2422篇 |
2020年 | 1667篇 |
2019年 | 1424篇 |
2018年 | 1455篇 |
2017年 | 1573篇 |
2016年 | 1326篇 |
2015年 | 1857篇 |
2014年 | 2139篇 |
2013年 | 2601篇 |
2012年 | 2794篇 |
2011年 | 2930篇 |
2010年 | 2535篇 |
2009年 | 2443篇 |
2008年 | 2248篇 |
2007年 | 1929篇 |
2006年 | 1646篇 |
2005年 | 1364篇 |
2004年 | 998篇 |
2003年 | 831篇 |
2002年 | 862篇 |
2001年 | 732篇 |
2000年 | 458篇 |
1999年 | 332篇 |
1998年 | 324篇 |
1997年 | 251篇 |
1996年 | 207篇 |
1995年 | 135篇 |
1994年 | 113篇 |
1993年 | 96篇 |
1992年 | 45篇 |
1991年 | 43篇 |
1990年 | 43篇 |
1989年 | 33篇 |
1988年 | 35篇 |
1987年 | 21篇 |
1986年 | 28篇 |
1985年 | 16篇 |
1984年 | 10篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 19篇 |
1979年 | 10篇 |
1976年 | 20篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1959年 | 7篇 |
1951年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Marcia Cristina Kaezer Fran a Rosane Aguiar da Silva San Gil Jean-Guillaume Eon 《Catalysis Today》2003,78(1-4):105-115
γ-Al2O3 supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW5O195−, V2W4O194−, VMo5O195− and V2Mo4O194−) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and 51V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state 51V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VOx/γ-Al2O3 was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al2O3 surface. 相似文献
2.
The estimation of the link capacity and its available bandwidth in an end-to-end path is crucial for network management, admission
control and flow control for adaptive applications. This paper introduces an estimation mechanism able to accurately estimate
the available bandwidth of all links in an end-to-end path, through its capacity and cross-traffic estimation. The estimation
procedures resort to the dispersions of packet pairs and trains using the concepts of ICMP Timestamp and Traceroute that,
efficiently used together, enable the measurement of the dispersions in all links in the path. These mechanisms were evaluated
through simulation experiments, where we analyzed the influence of the several network parameters on each estimation mechanism.
The results show that these estimation methods are able to accurately estimate both the capacity and the cross-traffic of
all links in a path with moderate length and with low overhead. 相似文献
3.
4.
5.
阐述了视觉符号语言在版面设计中的编码与各种表现形式,强调信息社会的来临,以视觉为形态的符号语言将有效加速信息的传递。 相似文献
6.
7.
Larissa Canilha Walter Carvalho Marco Giulietti Maria Das Graças Almeida Felipe João Batista Almeida E Silva 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2008,83(5):715-721
BACKGROUND: Xylitol bioproduction from lignocellulosic residues comprises hydrolysis of the hemicellulose, detoxification of the hydrolysate, bioconversion of the xylose, and recovery of xylitol from the fermented hydrolysate. There are relatively few reports on xylitol recovery from fermented media. In the present study, ion‐exchange resins were used to clarify a fermented wheat straw hemicellulosic hydrolysate, which was then vacuum‐concentrated and submitted to cooling in the presence of ethanol for xylitol crystallization. RESULTS: Sequential adsorption into two anion‐exchange resins (A‐860S and A‐500PS) promoted considerable reductions in the content of soluble by‐products (up to 97.5%) and in medium coloration (99.5%). Vacuum concentration led to a dark‐colored viscous solution that inhibited xylitol crystallization. This inhibition could be overcome by mixing the concentrated medium with a commercial xylitol solution. Such a strategy led to xylitol crystals with up to 95.9% purity. The crystallization yield (43.5%) was close to that observed when using commercial xylitol solution (51.4%). CONCLUSION: The experimental data demonstrate the feasibility of using ion‐exchange resins followed by cooling in the presence of ethanol as a strategy to promote the fast recovery and purification of xylitol from hemicellulose‐derived fermentation media. Copyright © 2008 Society of Chemical Industry 相似文献
8.
Daniel X. Gouveia Odair P. Ferreira Antonio G. Souza Filho M. G. da Silva J. A. C. de Paiva Oswaldo L. Alves Josué Mendes Filho 《Journal of Materials Science》2007,42(2):534-538
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution. 相似文献
9.
10.
川西平原属沉积平原,不到2m厚的表土下面全是沙卵石,沙卯石厚度达二、三十米。地面覆盖农田,沟垄纵横,限制了车载钻机的进入。以往地震勘探多采取浅井组合方式激发,所获得的地震资料能量差、面波强,对农田破坏面积大。若使用SH30—2A型轻便工程钻机,配上自制的冲击管,采取冲击钻进,从管中下药柱的方式,可实现沙卵石地区的深井激发,提高资料采集质量,且效率高、成本低。 相似文献