全文获取类型
收费全文 | 34939篇 |
免费 | 2540篇 |
国内免费 | 2018篇 |
专业分类
电工技术 | 1237篇 |
综合类 | 1960篇 |
化学工业 | 11806篇 |
金属工艺 | 3568篇 |
机械仪表 | 956篇 |
建筑科学 | 2428篇 |
矿业工程 | 484篇 |
能源动力 | 1092篇 |
轻工业 | 1488篇 |
水利工程 | 96篇 |
石油天然气 | 1250篇 |
武器工业 | 293篇 |
无线电 | 3171篇 |
一般工业技术 | 5121篇 |
冶金工业 | 1407篇 |
原子能技术 | 662篇 |
自动化技术 | 2478篇 |
出版年
2024年 | 52篇 |
2023年 | 498篇 |
2022年 | 713篇 |
2021年 | 1171篇 |
2020年 | 957篇 |
2019年 | 857篇 |
2018年 | 734篇 |
2017年 | 1001篇 |
2016年 | 1265篇 |
2015年 | 1227篇 |
2014年 | 1672篇 |
2013年 | 1870篇 |
2012年 | 2079篇 |
2011年 | 2734篇 |
2010年 | 1992篇 |
2009年 | 2460篇 |
2008年 | 1910篇 |
2007年 | 2375篇 |
2006年 | 2204篇 |
2005年 | 1762篇 |
2004年 | 1471篇 |
2003年 | 1479篇 |
2002年 | 1202篇 |
2001年 | 931篇 |
2000年 | 846篇 |
1999年 | 686篇 |
1998年 | 526篇 |
1997年 | 339篇 |
1996年 | 290篇 |
1995年 | 259篇 |
1994年 | 230篇 |
1993年 | 202篇 |
1992年 | 182篇 |
1991年 | 171篇 |
1990年 | 122篇 |
1989年 | 96篇 |
1988年 | 69篇 |
1987年 | 70篇 |
1986年 | 82篇 |
1985年 | 76篇 |
1984年 | 69篇 |
1983年 | 56篇 |
1982年 | 59篇 |
1981年 | 54篇 |
1979年 | 46篇 |
1978年 | 46篇 |
1977年 | 53篇 |
1976年 | 58篇 |
1975年 | 69篇 |
1974年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
82.
83.
84.
王芬 《武汉理工大学学报(材料科学英文版)》2005,20(Z1)
1Introduction Harmfulbacteriaexistanywhereintheworldsuchas soil,air,water,surfacesofobjectsandourskins,etc.Thenumberofbacteriumisenormous,withmanydiffer entspecies.Theyhavethreatenedthehealthofhuman beingsseriously.Withthedevelopmentofeconomyand environme… 相似文献
85.
The photoluminescence properties of BiTaO4∶Pr3+ and BiTaO4 at room temperature were studied, and the infrared transmission and diffusion reflection spectra of BiTaO4 were measured. The photoluminescence spectrum of BiTaO4 peaks at about 420, 440 and 465 nm. There has an obvious excitation band from 330 to 370 nm. The photoluminescence spectrum of BiTaO4∶Pr3+ consists of the characteristic emission of Pr3+, and its main peak is at 606 nm from 3P0→3H6 transition of Pr3+. Its excitation spectrum consists of the wide band with maximum at 325 nm, the wide band in the range of 375~430 nm, and the characteristic excitation of Pr3+. The bands at 325 nm and 375~430 nm may be from the absorption of the charge transfer transition of the tantalate group and defect energy levels in its forbidden band, respectively. There is energy transfer from host to Pr3+. Because both the host density and photoluminescence peak intensity of BiTaO4∶Pr3+ are superior to PbWO4, BiTaO4∶Pr3+ may be a potential heavy scintillator. 相似文献
86.
The C4 composition of Canadian mixed-grass communities is more sensitive to environmental change than other grasslands. Reliable methods of detecting such changes are necessary if these landscapes are to be properly managed. One approach is to use satellite remote sensing systems. Various studies have shown that the asynchronous seasonality of C3 and C4 species allows the relative abundance of each photosynthetic type to be estimated using temporal trajectory indices (TTIs) of sensor-derived normalized difference vegetation index (NDVI). In this study, we compared three approaches for predicting C4 species cover at Grasslands National Park (GNP) (Saskatchewan, Canada). TTIs related to Approach I were calculated from plots of NDVI vs. day-of-year (DOY). TTIs related to Approach II were calculated from plots of normalized cumulative NDVI vs. growing degree day (GDD). TTIs related to Approach III were calculated as ratios of early-season NDVI to late-season NDVI. Our analyses were conducted at two separate ecological scales. A within-community analysis used field-sampled data from upland grassland to compare techniques at sampling resolutions of 0.5, 2.5, 10, and 50 m. An across-community analysis compared techniques using a vegetation survey of the GNP region and TTIs calculated from Advanced Very High Resolution Radiometer (AVHRR) data (1 km). At both scales, TTIs related to the timing of specific phenological events were the best predictors of C4 species cover. While all techniques performed well in the within-community study, Approach III performed best. Here, the predictive ability of each approach was weak at a resolution of 0.5 m but stronger at 2.5, 10, and 50 m resolutions. We also found that the optimal sampling dates for Approach III fell within a certain GDD range. This is encouraging for the a priori selection of sample dates, which would make the need for full seasonal time series redundant. In the across-community analysis, the AVHRR-derived Approach II TTIs were better able to discriminate among grasslands of different C4 composition than any other technique (overall accuracy=74%). However, for some C4 cover classes, the predictive accuracy of this approach was low. While these results are encouraging for the use of spectral data in monitoring the C4 cover of northern prairie, various research issues remain. At the within-community level, these include (a) further attempts to define objective criteria for the a priori identification of sampling dates for Approach III, and (b) and the extension of such studies to other growing seasons and community types/grassland regions. At the across-community level, these include the expansion of such techniques to a larger geographical region that contains a wider range in C4 cover values and land use types (e.g. ungrazed vs. grazed grasslands). 相似文献
87.
介绍了一种基于MPEG—4视频压缩标准和PNX1300系列多媒体处理器的MPEG—4视频压缩卡的设计方案,给出了该视频压缩卡的硬件结构框图,详细分析了其软件编码器的设计流程。 相似文献
88.
89.
Natalia Andronova 《Polymer》2005,46(18):6746-6755
The synthesis and characterization of degradable polymeric networks for biomedical applications was performed. Cross-linked films of poly(ε-caprolactone) (PCL) and poly(1,5-dioxepan-2-one) (PDXO) having various mole fractions of monomers and different cross-link densities were successfully prepared using 2,2′-bis-(ε-caprolactone-4-yl) propane (BCP) as cross-linking agent. Reaction parameters were carefully examined to optimise the film-forming conditions. Networks obtained were elastomeric materials, easy to cast and remove from the mould. Effect of CL content and cross-link density on the final properties of the polymer network was evaluated. High CL content or degree of cross-linking led to increase in Young's modulus and decrease in elongation at break. An increase in crystalline domains in films having a higher CL content was observed by optical microscopy. A greater thermal stability was observed in films having a high CL content. The hydrophilicity of the materials could be tailored by changing the CL content. The surface of the films became rougher with higher CL content. 相似文献
90.
Bestehen ausreichende Anhaltspunkte für die Hochwassergef?hrdung, ist die Gemeinde verpflichtet, sich durch Nachforschungen
Gewissheit über das Vorliegen oder Nichtvorliegen eines Hochwasserabflussgebietes gem § 38 Abs 3 WRG zu verschaffen. Liegt
ein solches vor, hat sie gem § 12 Abs 1 Z 2 krnt GplG dieses im Fl?chenwidmungsplan ersichtlich zu machen. Erteilt die Gemeinde
eine Baubewilligung ohne diese von der Erteilung einer wasserrechtlichen Bewilligung abh?ngig zu machen, haftet sie für die
Kosten der Bauführung, die bei gesetzes-konformer Vorgangsweise unterlassen worden w?re. 相似文献