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   2413篇
  免费   50篇
  国内免费   2篇
电工技术   31篇
综合类   1篇
化学工业   267篇
金属工艺   31篇
机械仪表   20篇
建筑科学   37篇
矿业工程   2篇
能源动力   27篇
轻工业   249篇
水利工程   7篇
石油天然气   2篇
无线电   239篇
一般工业技术   360篇
冶金工业   913篇
原子能技术   33篇
自动化技术   246篇
  2021年   23篇
  2020年   17篇
  2019年   25篇
  2018年   32篇
  2017年   31篇
  2016年   29篇
  2015年   22篇
  2014年   34篇
  2013年   114篇
  2012年   56篇
  2011年   81篇
  2010年   52篇
  2009年   65篇
  2008年   77篇
  2007年   50篇
  2006年   71篇
  2005年   64篇
  2004年   37篇
  2003年   46篇
  2002年   39篇
  2001年   48篇
  2000年   37篇
  1999年   56篇
  1998年   231篇
  1997年   157篇
  1996年   111篇
  1995年   74篇
  1994年   67篇
  1993年   78篇
  1992年   53篇
  1991年   24篇
  1990年   30篇
  1989年   29篇
  1988年   37篇
  1987年   31篇
  1986年   33篇
  1985年   33篇
  1984年   19篇
  1983年   15篇
  1982年   25篇
  1981年   21篇
  1980年   21篇
  1978年   23篇
  1977年   23篇
  1976年   45篇
  1975年   27篇
  1974年   15篇
  1973年   15篇
  1972年   15篇
  1970年   14篇
排序方式: 共有2465条查询结果,搜索用时 26 毫秒
1.
Growing interest in extracellular vesicles (EVs) has prompted the advancements of protocols for improved EV characterization. As a high-throughput, multi-parameter, and single particle technique, flow cytometry is widely used for EV characterization. The comparison of data on EV concentration, however, is hindered by the lack of standardization between different protocols and instruments. Here, we quantified EV counts of platelet-derived EVs, using two flow cytometers (Gallios and CytoFLEX LX) and nanoparticle tracking analysis (NTA). Phosphatidylserine-exposing EVs were identified by labelling with lactadherin (LA). Calibration with silica-based fluorescent beads showed detection limits of 300 nm and 150 nm for Gallios and CytoFLEX LX, respectively. Accordingly, CytoFLEX LX yielded 40-fold higher EV counts and 13-fold higher counts of LA+CD41+ EVs compared to Gallios. NTA in fluorescence mode (F-NTA) demonstrated that only 9.5% of all vesicles detected in scatter mode exposed phosphatidylserine, resulting in good agreement of LA+ EVs for CytoFLEX LX and F-NTA. Since certain functional characteristics, such as the exposure of pro-coagulant phosphatidylserine, are not equally displayed across the entire EV size range, our study highlights the necessity of indicating the size range of EVs detected with a given approach along with the EV concentration to support the comparability between different studies.  相似文献   
2.
During curing of thermosetting resins the technologically relevant properties of binders and coatings develop. However, curing is difficult to monitor due to the multitude of chemical and physical processes taking place. Precise prediction of specific technological properties based on molecular properties is very difficult. In this study, the potential of principal component analysis (PCA) and principal component regression (PCR) in the analysis of Fourier transform infrared (FTIR) spectra is demonstrated using the example of melamine-formaldehyde (MF) resin curing in solid state. FTIR/PCA-based reaction trajectories are used to visualize the influence of temperature on isothermal cure. An FTIR/PCR model for predicting the hydrolysis resistance of cured MF resin from their spectral fingerprints is presented which illustrates the advantages of FTIR/PCR compared to the combination differential scanning calorimetry/isoconversional kinetic analysis. The presented methodology is transferable to the curing reactions of any thermosetting resin and can be applied to model other technologically relevant final properties as well.  相似文献   
3.
4.
Melamine–formaldehyde (MF) resins are widely used as adhesives and finishing materials in the wood industry. During resin cure, either methylene ether or methylene bridges are formed, leading to the formation of a three-dimensional resin network. Not only the curing degree, but also the chemical species present in the cured resin determine the quality of the final product. Analytical methods allowing a detailed investigation of network formation are of great benefit to manufacturers. In the present work, resin cure of an MF precondensate is studied at different temperatures (100–200 °C) without considering the initial pH as a factor. Isoconversional kinetic analysis based on exothermal curing enthalpies enables calculation of the crosslinking degree at a given time/temperature regime. A semiquantitative determination of the chemical groups present is performed based on solid-state nuclear magnetic resonance data. Fourier transform infrared spectroscopy has shown to be a fast and reliable analytical tool with high sensitivity toward functional groups and with great potential for at-line process control. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47691.  相似文献   
5.
6.
Inflammatory and immunogenic response to foreign bodies presents a challenge in the use of biomaterials as implants for tissue restoration. Therefore, there is a need to understand the interactions between such implants and the blood. One such material, currently in clinical use for bone replacement in humans, is the skeleton of corals, in the form of crystalline aragonite. This biomaterial has been shown to impart a protective and supportive influence on several types of cells ex vivo. The carbonate skeleton activates secretion in phagocytes in vitro, however its effects on these cells in the blood, and on the process of phagocytosis itself, remain unknown. Using 1–500 μm particles of coral skeleton, we show that these particles bind blood proteins and alter the leukocyte population, reducing the proportion of granulocytes by more than 3-fold with no effect on the proportion of monocytes. In addition, the presence of coral skeleton in the blood causes a reduction in phagocytosis. Specifically, we observed a decrease in the percentage of phagocytic cells by 27 % in the granulocytes and by 73 % in monocyte family, as well as a 41.6 % reduction in the MFI of granulocytes, but with no such effect on monocytes. Taken together, the results suggest that the coral skeleton biomaterial may act as a strong, promotive scaffold for tissue regeneration due to its ability to reduce its rejection by inflammatory reactions such as phagocytosis.  相似文献   
7.
Although oxamic acid has been identified as an ozone oxidation product from several precursor compounds, concentrations for drinking water have not been published previously. This study shows results from a full-scale drinking water treatment plant, noting that the mean concentrations for oxamic acid reached 21.3 μg/L after ozonation and prior to filtration. Subsequent multiple-layer filtration removed 85% of oxamic acid on average, and mean concentrations in drinking water were 2 μg/L. Up to 5.9% of the oxamic acid found in ozone-treated groundwater may be formed from Chloridazon metabolites.  相似文献   
8.
Biodiversity monitoring via environmental DNA, particularly metabarcoding, is evolving into a powerful assessment tool for riverine systems. However, for metabarcoding to be fully integrated into standardized monitoring programmes, some current challenges concerning sampling design, laboratory workflow, and data analysis need to be overcome. Here, we review some of these major challenges and potential solutions. We further illustrate three potential pitfalls, namely the choice of suitable metabarcoding primers, the necessity of complete reference databases, and varying assay sensitivities, by a reappraisal of our‐own recently carried out metabarcoding study in the Volga headwaters. TaqMan qPCRs had detected catfish (Silurus glanis) and European eel (Anguilla anguilla), whereas metabarcoding had not, in the same samples. Furthermore, after extending the genetic reference database by 12 additional species and re‐analysing the metabarcoding data, we additionally detected the Siberian spiny loach (Cobitis sibirica) and Ukrainian brook lamprey (Eudontomyzon mariae) and reassigned the operational taxonomic units previously assigned to Misgurnus fossilis to Cobitis sibirica. In silico analysis of metabarcoding primer efficiencies revealed considerable variability among primer pairs and among target species, which could lead to strong primer bias and potential false‐negatives in metabarcoding studies if not properly compensated for. These results highlight some of the pitfalls of eDNA‐metabarcoding as a means of monitoring fish biodiversity in large rivers, which need to be considered in order to fully unleash the full potential of these approaches for freshwater biodiversity monitoring.  相似文献   
9.
Our objective was to determine the effects of dry matter intake (DMI), body weight (BW), and diet characteristics on total tract digestibilities of dry matter, neutral detergent fiber, and starch (DMD, NDFD, and StarchD, respectively) in high-producing dairy cows. Our database was composed of 1,942 observations from 662 cows in 54 studies from Michigan, Ohio, and Georgia. On average, cows ate 23 ± 4.5 kg of dry matter/d, weighed 669 ± 79 kg, and produced 38 ± 10 kg of milk/d. Diets were 31 ± 5% neutral detergent fiber, 27 ± 6% starch, 2.6 ± 1.2% fatty acids, and 17 ± 1.4% crude protein. Digestibility means were 66 ± 6, 42 ± 11, and 93 ± 5% for DMD, NDFD, and StarchD, respectively. Forage sources included corn silage, alfalfa, and grasses. Corn source was classified by its ruminal fermentability. Data were analyzed using a mixed effects model, including diet chemical composition, forage source, and corn source, all expressed as percentage of dry matter, except for DMI, which was expressed as percentage of BW (DMI%BW); location and 2-way interactions were fixed effects. Cow, block, period, treatment, and study were included as random effects. Best fitting candidate models were generated using backward and stepwise regression methods. Additionally, the simplest model was generated using only DMI and location as fixed effects and all random effects. Candidate models were cross-validated across studies, and the resulting predictive correlation coefficients across studies (PC) and root mean square error of prediction (RMSEP) were compared by t-test. For each nutrient, the digestibility model that resulted in the highest PC and lowest RMSEP was determined to be the best fitting model. We observed heterogeneous coefficients among the different locations, suggesting that specific location factors influenced digestibilities. The overall location-averaged best fitting prediction equations were: DMD = 69 ? 0.83 × DMI%BW (PC = 0.22, RMSEP = 5.39); NDFD = 53 + 0.26 × grass %DM ? 0.59 × starch %DM + 3.06 × DMI%BW ? 0.46 × DMI%BW2 (PC = 0.53, RMSEP = 9.70); and StarchD = 96 + 0.19 × HFERM%DM ? 0.12 × starch %DM ? 1.13 × DMI%BW (PC = 0.34, RMSEP = 4.77); where HFERM%DM is highly-fermentable corn source as percentage of DM. Our results confirm that digestibility is reduced as DMI increases, albeit at a lower rate than that reported in National Research Council. Furthermore, dietary starch depresses NDFD. Whereas DMD can be predicted based on DMI only, the best predictions for NDFD and StarchD require diet characteristics in addition to DMI.  相似文献   
10.
The aim of the study was to prepare mathematical models based on the Arrhenius equation as predictive tools for the assessment of changes in quality parameters during the storage of spreadable Gouda cheese at temperatures of 8, 20 and 30 °C. The activation energy value and the chemical reaction rate constant enabled the construction of kinetic models, which helped to estimate the direction and rate of changes. Moreover, the activation energy (Ea) of the quality parameters was used to determine the sequence of their vulnerability during storage. The value of activation energy corresponding to temperature changes resulted in the following order of susceptibility of the quality parameters: ΔC?>?ΔE?≈?water activity?>?texture parameters?>?pH?>?colour?>?sensory parameters?>?rheological parameters. The research showed limited applicability of the mathematical models for estimation of quality parameters referring to spreadable processed Gouda cheese.  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