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   1220篇
  免费   52篇
  国内免费   9篇
电工技术   11篇
综合类   1篇
化学工业   444篇
金属工艺   51篇
机械仪表   32篇
建筑科学   47篇
矿业工程   2篇
能源动力   69篇
轻工业   119篇
水利工程   17篇
石油天然气   27篇
无线电   71篇
一般工业技术   222篇
冶金工业   62篇
原子能技术   7篇
自动化技术   99篇
  2023年   17篇
  2022年   30篇
  2021年   38篇
  2020年   37篇
  2019年   51篇
  2018年   51篇
  2017年   34篇
  2016年   35篇
  2015年   35篇
  2014年   47篇
  2013年   117篇
  2012年   65篇
  2011年   87篇
  2010年   53篇
  2009年   52篇
  2008年   41篇
  2007年   44篇
  2006年   49篇
  2005年   24篇
  2004年   16篇
  2003年   16篇
  2002年   19篇
  2001年   16篇
  2000年   21篇
  1999年   11篇
  1998年   19篇
  1997年   17篇
  1996年   12篇
  1995年   16篇
  1994年   7篇
  1993年   10篇
  1992年   14篇
  1991年   12篇
  1990年   7篇
  1989年   10篇
  1988年   12篇
  1987年   9篇
  1986年   8篇
  1985年   10篇
  1984年   10篇
  1983年   11篇
  1982年   6篇
  1980年   15篇
  1977年   6篇
  1976年   8篇
  1974年   7篇
  1973年   6篇
  1971年   7篇
  1967年   6篇
  1966年   6篇
排序方式: 共有1281条查询结果,搜索用时 16 毫秒
1.
2.
Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout.  相似文献   
3.
Observations of fatigue-induced slip patterns on the surface of polycrystalline gold (+99.9%) using the scanning tunnelling microscope (STM) are presented. The samples were cycled in four-point bending between zero and 0.0025 strain for 250 and 2500 cycles. STM observations of the sample fatigued for 250 cycles revealed broad slip bands 0.38–0.69 m wide containing narrow slip bands 0.015–0.123 m wide and fine slip lines 0.006 m wide. The depth of these features are 20–57 nm for broad slip bands, 3–7 nm for narrow slip bands, and approximately 2–4 nm for fine slip lines. The sample fatigued for 2500 cycles showed similar values for the width of the slip bands and slip lines except that their depth was increased by a factor of between 2 and 4 times for the narrow slip bands and the fine slip lines. Apparent persistent slip bands (PSBs) spaced 2.4 m, 300–500 nm deep were also observed. These results demonstrate that STM is a significant new tool for observing and distinguishing various types of fatigue-induced surface slip patterns on suitably prepared samples. The vertical resolution obtained with STM is vastly superior to current SEM and TEM methods.  相似文献   
4.
Polypropylene/poly(methyl acrylate)‐grafted glass wool (PMA‐g‐GW) mixes were prepared. The polymerization process was carried out using potassium persulfate (PPS) and PPS/acetone sodium bisulfite (ASBS) as a redox‐pair initiation system at 60 and 70°C. The effect of using PPS or PPS/ASBS on the grafting percent and conversion percent reveals that the conversion percent values on using PPS as an initiator are higher than those of PPS/ASBS, while in the case of grafting, the inverse is true, that is, using PPS as an initiator gives grafting percent values lower than those that can be obtained using PPS/ASBS. The dielectric properties, thermal diffusivity, specific heat capacity, and thermal conductivity of PP loaded with modified glass wool as a function of different types and concentrations of initiators—used in the grafting polymerization process, namely, PPS and the redox initiating system—were also studied. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 723–732, 2003  相似文献   
5.
Numerous attempts have been undertaken to apply the spectral subtraction method to cancel noise perturbations but these efforts have yet to produce an algorithm that is able to adapt well to the environmental changes in the perturbations. In addition, the variants of the spectral subtraction method so far proposed in the literature would require a non-voice activity detector (NVAD), for a single microphone system, to store the perturbation. This is used as an estimate for the reference signal. Inaccuracy in the perturbation estimates causes the cleaned speech to be corrupted by musical artifacts, which is unacceptable. Post processing of signals corrupted by the musical artifacts is very costly. This paper provides an alternative approach that employs associative memory for speech enhancement. Extensive comparison is made using the soft computing approaches for noise cancellation based on associative memories. A set of stereo microphones captures the corrupted speech in a vehicle and is used to point to the closest associative memory location. The Wiener filter approach is used to cancel the noise. The paper discusses novel examples of the associative memories using the cerebellum model for noise modeling. Experimental results show the potential of these novel soft computing architectures in generating and adapting the required Weiner filters to cancel perturbation even at signal to noise ratio (SNR) of less than −13 dB.  相似文献   
6.
Hydrogenolysis of corn syrup was carried out at 170°C and 60 at pressure. After 3 h 92.3% of the glucose content of the syrup was converted into sorbitol.  相似文献   
7.
The reaction of 5-arylidene-4-arylimino-2-thiazolidinones 4 with GRIGNARD reagents effected 1,4-addition to the exocyclic  CC bond with the formation of 5 . Alkylation of the arylidene derivatives 4 with diazomethane or methyl iodide results in the formation of the N-methyl derivatives 6 . Treatment of 6 with phenylmagnesium bromide effects addition of the reagent to the double bond of the lateral chain to yield products, proved to have structure 8 . The 5-arylazo derivatives 9 of 4-arylimino-2-thiazolidinones were obtained by coupling 4 with aromatic diazonium chlorides. Compounds 9 were N-methylated with diazomethane to yield 10 .  相似文献   
8.
Background: Silymarin (SIL) has long been utilized to treat a variety of liver illnesses, but due to its poor water solubility and low membrane permeability, it has a low oral bioavailability, limiting its therapeutic potential. Aim: Design and evaluate hepatic-targeted delivery of safe biocompatible formulated SIL-loaded chitosan nanoparticles (SCNPs) to enhance SIL’s anti-fibrotic effectiveness in rats with CCl4-induced liver fibrosis. Methods: The SCNPs and chitosan nanoparticles (CNPs) were prepared by ionotropic gelation technique and are characterized by physicochemical parameters such as particle size, morphology, zeta potential, and in vitro release studies. The therapeutic efficacy of successfully formulated SCNPs and CNPs were subjected to in vivo evaluation studies. Rats were daily administered SIL, SCNPs, and CNPs orally for 30 days. Results: The in vivo study revealed that the synthesized SCNPs demonstrated a significant antifibrotic therapeutic action against CCl4-induced hepatic injury in rats when compared to treated groups of SIL and CNPs. SCNP-treated rats had a healthy body weight, with normal values for liver weight and liver index, as well as significant improvements in liver functions, inflammatory indicators, antioxidant pathway activation, and lipid peroxidation reduction. The antifibrotic activities of SCNPs were mediated by suppressing the expression of the main fibrosis mediators TGFβR1, COL3A1, and TGFβR2 by boosting the hepatic expression of protective miRNAs; miR-22, miR-29c, and miR-219a, respectively. The anti-fibrotic effects of SCNPs were supported by histopathology and immunohistochemistry (IHC) study. Conclusions: According to the above results, SCNPs might be the best suitable carrier to target liver cells in the treatment of liver fibrosis.  相似文献   
9.
Nitrated Polymeric carbohydrates are not very pure when used, as their physical constants are neither sharp nor unique for positive identification purposes. The ingredients of explosives are so numerous and it was considered beyond the scope of this investigation to include them.  相似文献   
10.
4-Arylidene-2-phenyl-1-substituted-2-imidazolin-5-ones 1a – d react with organomagnesium compounds, in ether at room temperature, to yield, as the main isolable products, the 4-disubstituted-methyl derivatives 2a – h . On the other hand, 1a – d react with phenylmagnesium bromide in a boiling ether-benzene mixture to give, among the reaction products, 2a – d and the 4-diarylmethylene derivatives 3a – d . Spectral data of 2 and 3 are discussed. Treatment of the GRIGNARD products 2a – c with formaldehyde and piperidine or morpholine effects aminomethylation at C-4 with the formation of the MANNICH bases 6a – f .  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