首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2123篇
  免费   38篇
  国内免费   3篇
电工技术   22篇
综合类   2篇
化学工业   227篇
金属工艺   17篇
机械仪表   23篇
建筑科学   68篇
能源动力   33篇
轻工业   53篇
水利工程   3篇
石油天然气   1篇
无线电   177篇
一般工业技术   178篇
冶金工业   1190篇
原子能技术   8篇
自动化技术   162篇
  2023年   10篇
  2022年   20篇
  2021年   22篇
  2020年   21篇
  2019年   19篇
  2018年   28篇
  2017年   21篇
  2016年   32篇
  2015年   27篇
  2014年   37篇
  2013年   72篇
  2012年   30篇
  2011年   62篇
  2010年   56篇
  2009年   38篇
  2008年   45篇
  2007年   62篇
  2006年   39篇
  2005年   46篇
  2004年   26篇
  2003年   44篇
  2002年   27篇
  2001年   28篇
  2000年   27篇
  1999年   50篇
  1998年   333篇
  1997年   195篇
  1996年   147篇
  1995年   82篇
  1994年   70篇
  1993年   82篇
  1992年   15篇
  1991年   29篇
  1990年   19篇
  1989年   23篇
  1988年   37篇
  1987年   28篇
  1986年   23篇
  1985年   21篇
  1984年   4篇
  1983年   9篇
  1982年   7篇
  1981年   16篇
  1980年   11篇
  1978年   9篇
  1977年   33篇
  1976年   69篇
  1975年   3篇
  1974年   3篇
  1973年   3篇
排序方式: 共有2164条查询结果,搜索用时 0 毫秒
991.
Antisense (AS) and hairpin (HP) RNA interference (RNAi) targeted gene suppression technologies have been used to modify seed oil composition. Larger numbers of AS transgenics have to be screened to achieve a targeted level of suppression compared to RNAi. We hypothesized combining AS with RNAi might result in enhanced gene suppression compared to either method individually. AS and HP-RNAi were combined as hairpin antisense (HPAS) constructs containing ~125 bp sense and antisense portions of an untranslated region of the target gene separated by an intron containing an antisense copy of a portion of the target coding region. The Δ12-desaturase FAD2, the ω3-desaturase FAD3 and β-ketoacyl-ACP synthase (KAS) II were targeted in Arabidopsis to evaluate changes in oil composition with AS, HP and HPAS constructs driven by the phaseolin promoter. Modest but statistically significant enhancements in oilseed phenotypes were observed with HPAS relative to AS and HP-RNAi. Phenotypes for HPAS suppression of FAD2 and FAD3 were indistinguishable from their strongest mutant alleles. Our data suggest that HPAS may be useful for: (1) achieving levels of suppression comparable to those of gene knockouts in a tissue specific manner. (2) Maximizing suppression of suboptimal RNAi constructs and (3) minimizing the screening of transgenics to achieve desired oilseed composition.  相似文献   
992.
993.
We demonstrate an all-optical multicast switch with the capability to select five reconfigurable channel groups with six output multicast channels using three control wavelengths at 10 Gb/s using Raman-assisted four-wave mixing in dispersion-shifted fiber. All the output channels comply with the 100 GHz (i.e., 0.8 nm) spaced International Telecommunication Union (ITU) grid.  相似文献   
994.
995.
We study two generalised stable matching problems motivated by the current matching scheme used in the higher education sector in Hungary. The first problem is an extension of the College Admissions problem in which the colleges have lower quotas as well as the normal upper quotas. Here, we show that a stable matching may not exist and we prove that the problem of determining whether one does is NP-complete in general. The second problem is a different extension in which, as usual, individual colleges have upper quotas, but, in addition, certain bounded subsets of colleges have common quotas smaller than the sum of their individual quotas. Again, we show that a stable matching may not exist and the related decision problem is NP-complete. On the other hand, we prove that, when the bounded sets form a nested set system, a stable matching can be found by generalising, in non-trivial ways, both the applicant-oriented and college-oriented versions of the classical Gale–Shapley algorithm. Finally, we present an alternative view of this nested case using the concept of choice functions, and with the aid of a matroid model we establish some interesting structural results for this case.  相似文献   
996.
Six fresh and one frozen vegetable cultivar groups possessing remarkably different morphology from the same Brassica oleracea species, including broccoli, Brussels sprouts, curly cabbage, white cabbage, red cabbage, cauliflower and white kohlrabi, were chosen to set up a Fourier transform near‐infrared spectroscopy (FT‐NIR)‐based method for the quantification of protein content. Sample preparation was based on lyophilisation and homogenisation. Calibration was set up with the help of the Kjeldahl method for the quantification of protein content in the range of 12.9–32.5 m/m%. Calibration model was developed using the spectral regions 1136–1334 and 1639–1836 nm, with partial least squares regression. This model was checked by cross‐validation. The performance of the final FT‐NIR estimation model was evaluated by root mean square of cross‐validation, root‐mean‐square error of estimation and the determination coefficient (R2). The final estimation function for the protein determination was characterised with the predictive error of 0.76 m/m% and R2 value of 98.81.  相似文献   
997.
998.
The presence of mortar has been reported as the main factor causing the lower quality of recycled concrete aggregates (RCA) when compared to natural aggregates (NA). A novel microwave-assisted technique to increase the quality of RCA by partially removing the mortar adhering to RCA particles and breaking up the lumps of mortar present in RCA is introduced in this paper. The technique takes advantage of the difference in the electromagnetic properties of the adhering mortar and natural aggregates to generate high thermal stresses within the mortar, especially at the interface with the embedded natural aggregates, to cause delamination. The stresses generated also result in the breaking up of the lumps of mortar into smaller pieces. The results of an experimental study conducted to investigate the capability of the microwave-assisted RCA beneficiation technique and to compare its efficiency with other beneficiation methods proposed in available literature are presented. Moreover, the effects of incorporating various amounts of un-treated and microwave-treated coarse RCA on the mechanical properties of concrete are investigated. The temperature distribution and stresses developed in RCA when subjected to microwave heating during the beneficiation process are numerically calculated for a better understanding of the processes involved.  相似文献   
999.
The effect of contact torques on porosity of cohesive powders   总被引:1,自引:0,他引:1  
The porosity of uniaxially compacted cohesive powders depends on the applied stress (including gravity). The case, where these stresses are weak, is considered. The compaction results in a porosity which is a function of sliding, rolling and torsion friction. By contact dynamics simulations it is shown that the influences of contact torques (static rolling and torsion friction) on the porosity are significant and approximately additive. The relevance for nano-powder pressure sintering is discussed.  相似文献   
1000.
Commercial as-hp (hot pressing) treated Cr–Si targets are used throughout this study, with three different compositions: Cr20–Si80, Cr35–Si65 and Cr50–Si50. To evaluate the effects of microstructure and properties of as-hp treated Cr–Si targets by hot isostatic pressing (HIP) SEM, XRD and porosity inspections were performed. The experimental results showed that the 1373 K, 1750 MPa, 4 h HIP treated with three different Cr–Si targets had suppressed porosities successfully. The most efficient was Cr50–Si50 target subjected to HIP treatment. Porosity decreased about 60% after HIP treatment, and both the nitrogen and oxygen concentrations of the targets were slightly increased after HIP treatment. This was especially true for the single silicon in Cr–Si targets such as Cr20–Si80 and Cr35–Si65. The aim of this paper is to discuss these methods and finding suitable temperatures for the HIP for Cr–Si targets.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号