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   2414篇
  免费   42篇
  国内免费   2篇
电工技术   37篇
综合类   24篇
化学工业   495篇
金属工艺   38篇
机械仪表   45篇
建筑科学   77篇
矿业工程   16篇
能源动力   38篇
轻工业   181篇
水利工程   27篇
石油天然气   5篇
无线电   170篇
一般工业技术   309篇
冶金工业   616篇
原子能技术   38篇
自动化技术   342篇
  2021年   27篇
  2020年   24篇
  2019年   30篇
  2017年   23篇
  2016年   43篇
  2015年   37篇
  2014年   32篇
  2013年   127篇
  2012年   67篇
  2011年   83篇
  2010年   84篇
  2009年   81篇
  2008年   93篇
  2007年   91篇
  2006年   78篇
  2005年   90篇
  2004年   66篇
  2003年   44篇
  2002年   63篇
  2001年   40篇
  2000年   29篇
  1999年   37篇
  1998年   42篇
  1997年   33篇
  1996年   44篇
  1995年   39篇
  1994年   31篇
  1993年   39篇
  1992年   39篇
  1991年   24篇
  1990年   37篇
  1989年   38篇
  1988年   26篇
  1987年   22篇
  1986年   40篇
  1985年   47篇
  1984年   54篇
  1983年   43篇
  1982年   55篇
  1981年   30篇
  1980年   41篇
  1979年   32篇
  1978年   31篇
  1977年   40篇
  1976年   36篇
  1975年   37篇
  1974年   29篇
  1973年   25篇
  1972年   24篇
  1969年   22篇
排序方式: 共有2458条查询结果,搜索用时 12 毫秒
51.
Peanut allergy is a major cause of food‐induced severe anaphylactic reactions. To date, no medical care is available to prevent and treat peanut allergy and therefore hypoallergenic peanut varieties are of considerable health political and economic interest. Major allergens that induce IgE‐responses in peanut‐sensitive patients are Ara h 1, Ara h 2 and Ara h 3/4. In order to identify hypoallergenic peanuts, commercially locally available peanut varieties were screened for their allergen content. Ara h 1‐deficient peanuts from Southeast Asia were identified by SDS‐PAGE, immunoblotting, inhibition assays and ELISA. 2‐D PAGE analyses demonstrated the different compositions of the tested extracts and revealed a number of variations of the allergen patterns of peanuts from different varieties. Mediator release experiments of these peanut extracts demonstrated similar allergenicities as compared with standard peanut extract. These results indicate that the allergenicity of peanuts with reduced Ara h 1 content might be compensated by the other allergens, and thus do not necessarily cause a reduction of allergenicity.  相似文献   
52.
Thermal chemistry pathways of aryl alkyl ethers have been investigated under coal conversion-like conditions. Anisole is a thermally reactive compound having an oxygen functionality found in such coal precursors as lignins. Pyrolysis of anisoles was carried out using small batch autoclaves. Under thermolysis conditions anisole yielded a product distribution strongly dependent upon experimental parameters. Phenol, methane, CO and benzaldehyde are the low molecular weight products and polyphenyls and polyethers are the predominant high molecular weight products. The generation of CO is explained by a high temperature rearrangement of the phenyl group from O- to C- followed by rapid thermal decarbonylation of the benzaldehyde. Carbon monoxide formation from aryl alkyl ethers can thus be an important mechanistic pathway in coal conversion processes. By investigating the rearrangement using para-fluoroanisole it was shown that this rearrangement proceeds via a three-centered intermediate to para-fluorobenzaldehyde. No meta isomer was observed.  相似文献   
53.
Sources of mercury wet deposition in Eastern Ohio, USA   总被引:3,自引:0,他引:3  
In the fall of 2002, an enhanced air monitoring site was established in Steubenville, Ohio as part of a multi-year comprehensive mercury monitoring and source apportionment study to investigate the impact of local and regional coal combustion sources on atmospheric mercury deposition in the Ohio River Valley. This study deployed advanced monitoring instrumentation, utilized innovative analytical techniques, and applied state-of-the-art statistical receptor models. This paper presents wet deposition data and source apportionment modeling results from daily event precipitation samples collected during the calendar years 2003-2004. The volume-weighted mean mercury concentrations for 2003 and 2004 were 14.0 and 13.5 ng L(-1), respectively, and total annual mercury wet deposition was 13.5 and 19.7 microg m(-2), respectively. Two new EPA-implemented multivariate statistical models, positive matrix factorization (PMF) and Unmix, were applied to the data set and six sources were identified. The dominant contributor to the mercury wet deposition was found by both models to be coal combustion (approximately 70%). Meteorological analysis also indicated that a majority of the mercury deposition found at the Steubenville site was due to local and regional sources.  相似文献   
54.
ABSTRACT:  Fresh tomatoes repeatedly have been associated with major outbreaks of salmonellosis; however, efforts to disinfect them with chlorine or other sanitizing agents have had only mixed success. Our objective was to determine whether hydrogen peroxide (H2O2) treatments would be more efficacious than conventional methods in disinfecting tomatoes containing human pathogens and, at the same time, be noninjurious to quality. Tomatoes were dip inoculated with Escherichia coli NRRL B-766 or a Salmonella cocktail and then held for 0, 24, or 48 h at 4 or 24 °C prior to treatment. Treatments included 200 ppm chlorine (Cl2) at 20 °C for 3 min, water at 20 °C for 3 min or at 60 °C for 2 min, 1% H2O2 at 20 °C for 15 min or at 60 °C for 2 min, and 5% H2O2 at 60 °C for 2, 3, or 5 min. In tomatoes held 48 h postinoculation, the chlorine treatment was only marginally more effective than an equivalent water rinse in reducing the target bacterial population, while the hot water and 1% H2O2 treatments achieved reductions no greater than 1.3 logs. However, application of 5% H2O2 at 60 °C resulted in larger reductions. Efficacy of all treatments decreased as the time interval between inoculation and treatment increased. Greater reductions could not be achieved with 5% H2O2 at 60 °C by increasing the contact time or addition of surfactants, and these treatments caused some quality loss.  相似文献   
55.
The feasibility of using a visible/near‐infrared hyperspectral imaging system with a wavelength range between 400 and 1000 nm to detect and differentiate different levels of aflatoxin B1 (AFB1) artificially titrated on maize kernel surface was examined. To reduce the color effects of maize kernels, image analysis was limited to a subset of original spectra (600 to 1000 nm). Residual staining from the AFB1 on the kernels surface was selected as regions of interest for analysis. Principal components analysis (PCA) was applied to reduce the dimensionality of hyperspectral image data, and then a stepwise factorial discriminant analysis (FDA) was performed on latent PCA variables. The results indicated that discriminant factors F2 can be used to separate control samples from all of the other groups of kernels with AFB1 inoculated, whereas the discriminant factors F1 can be used to identify maize kernels with levels of AFB1 as low as 10 ppb. An overall classification accuracy of 98% was achieved. Finally, the peaks of β coefficients of the discrimination factors F1 and F2 were analyzed and several key wavelengths identified for differentiating maize kernels with and without AFB1, as well as those with differing levels of AFB1 inoculation. Results indicated that Vis/NIR hyperspectral imaging technology combined with the PCA–FDA was a practical method to detect and differentiate different levels of AFB1 artificially inoculated on the maize kernels surface. However, indicated the potential to detect and differentiate naturally occurring toxins in maize kernel.  相似文献   
56.
In the present study, the effects of high drying temperature and UV light induced aging on bonding quality of plywood manufactured from untreated and treated veneer layers were investigated. Rotary cut veneers with dimensions of 500 mm×500 mm×2 mm produced from beech (Fagus orientalis Lipsky) log were selected for topochemical, chemical and mechanical analyses. The veneer sheets were oven-dried at 100°C and 180°C after the peeling process. Afterwards, the surfaces were exposed to artificial UV irradiation in an UV chamber for 24 h, 48 h and 72 h representing natural sun irradiation of 2, 4 and 6 months, respectively. Topochemical distribution of lignin and phenolic extractives of the treated and untreated veneers was investigated on a cellular level using UV microspectrophotometry (UMSP). For the chemical characterization of accessory compounds high performance liquid chromatography (HPLC) was used. Furthermore, the shear and bending strengths of plywood manufactured from the treated samples are determined in order to study the bonding quality. The UV microscopic detection shows that after high drying temperature and aging treatment, lignin condensation occurs. With increasing drying temperature and aging duration, more phenolic extractives are situated in parenchyma cells and vessel lumens which can be proved by increased absorbance at 278 nm. The HPLC analysis of the treated tissue showed distinct signals of polymerized compounds such as catechin and 2,6-dimethoxybenzoquinone which are chromophoric compounds in discolored beech wood. The mechanical properties of plywood showed that with increasing drying temperature up to 180°C does not negatively affect shear and bending strengths of samples. After exposure of the veneers to UV irradiation (especially 6 months), decreasing shear and bending strengths of plywood samples can be observed.  相似文献   
57.
58.
Compared process and reactive schizophrenic, brain-damaged, and normal control groups (n = 34 each) in frequency of reversal shifts in discrimination learning using the optional-shift paradigm. The main hypothesis that reactive schizophrenics, like normal adults, would show a stronger preference for reversal shifts than would process Ss was supported. Results show that significantly more reactive (68%) than process (44%) Ss made reversal shifts. Significantly more brain-damaged and reactive schizophrenic Ss showed reversal shift preferences when color was learned initially rather than form, while such initial concept preferences were not found in the process schizophrenic or normal control Ss. Previous findings in discrimination learning with pathological groups are discussed in the context of these findings. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
59.
Examined impaired set-shifting behavior frequently reported in brain-damaged Ss. Using a simple E-paced, 2-choice, visual, intradimensional reversal shift task, samples of brain-damaged (n = 24), alcoholic (n = 28), and control (n = 24) male patients were compared on errors, reaction time, and behavioral impulsivity (finger lift before signal). Alcoholic Ss were not significantly different from controls on any of the measures. As expected, brain-damaged Ss had significantly more total errors, longer reaction times, and higher impulsivity scores than controls. Differences in errors appeared to be due to difficulty in maintaining the perceptual and motor set rather than in set shifting. The importance of investigating disruptions in set maintenance in the performance of brain-damaged Ss is emphasized. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
60.
Subsequent to bilateral, single-stage posterior neodecortication, 32 male hooded rats were either retrained on a preoperatively-learned brightness discrimination, or trained on the reversal of the preoperatively-learned discrimination. Postoperative relearning was significantly retarded when the discrimination involved a reversal of the original discrimination. Results suggest that the preoperatively-established brightness habit is not exclusively localized in the neocortex and that postoperative recovery of the discrimination does not involve relearning a new and independent memory engram. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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