首页 | 官方网站   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   2137篇
  免费   186篇
  国内免费   1篇
工业技术   2324篇
  2024年   3篇
  2023年   35篇
  2022年   73篇
  2021年   224篇
  2020年   75篇
  2019年   97篇
  2018年   81篇
  2017年   81篇
  2016年   102篇
  2015年   89篇
  2014年   95篇
  2013年   168篇
  2012年   143篇
  2011年   181篇
  2010年   158篇
  2009年   117篇
  2008年   102篇
  2007年   85篇
  2006年   76篇
  2005年   53篇
  2004年   36篇
  2003年   39篇
  2002年   41篇
  2001年   20篇
  2000年   8篇
  1999年   20篇
  1998年   20篇
  1997年   11篇
  1996年   8篇
  1995年   10篇
  1994年   7篇
  1993年   9篇
  1992年   7篇
  1991年   6篇
  1990年   4篇
  1989年   2篇
  1988年   2篇
  1987年   6篇
  1986年   4篇
  1985年   6篇
  1984年   2篇
  1983年   3篇
  1982年   4篇
  1981年   4篇
  1980年   2篇
  1979年   1篇
  1978年   2篇
  1970年   1篇
  1969年   1篇
排序方式: 共有2324条查询结果,搜索用时 750 毫秒
11.
The copper and cobalt oxides composites coatings on aluminum substrates have been successfully synthesized via sol-gel method using nitrate-based sol precursors. The composites were characterized by X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), and UV–Vis–NIR spectrophotometry. The sol-gel reactions were discussed and Molecular Dynamics (MD) simulation was integrated into the study to predict molecules assembly properties. The XRD analyses revealed that the CuO and the Co3O4 composites were formed after the annealing process with the average difference of the calculated lattice parameters compared to ICDDs was 1.17%. The surface electronic structure was mainly consisted of tetrahedral Cu(I), octahedral Cu(II), tetrahedral Co(II), octahedral Co(III) as well as surface, sub-surface and lattice oxygen O?. The XRD, XPS and MD simulation results showed that there was minimal (or possibly non-existing) indication of copper-cobalt mixed phase oxides formations. FESEM and AFM surveys revealed that the coating had a porous surface composed of interlinked nanoparticles in the range of ~?10 to ~?40?nm. UV–Vis–NIR reflectance spectra showed that the sol precursors concentration and the dip-drying cycle significantly influenced the absorptance value with optimum absorptance (α) of 88.7% exhibited by coating synthesized using sol concentration of 0.1?M and 10 dip-drying cycles. High absorptance value and simplicity in the synthesis process render the coatings to be very promising candidates for solar selective absorber (SSA) applications.  相似文献   
12.
Background: Despite increases in ecstasy (MDMA) use in the United States, little is known about characteristics linked with recent-onset ecstasy use, especially psychiatric symptoms and deviant behaviors. Aims: To test whether individuals with high levels of other drug use are more likely to be recent-onset ecstasy users; to test whether psychiatric symptoms in adults are associated with recent-onset ecstasy use; to explore the association between recent-onset ecstasy use and concomitant deviant behaviors in adolescents and adults. Methods: Data from the 2001 National Survey on Drug Use and Health. Findings: Recent-onset ecstasy use was significantly more likely to occur among adolescents and adults (18-34 years old) who engaged in deviant behaviors during the past year as compared with those who did not engage in deviant behaviors during the past year. Higher levels of deviancy indicated a higher likelihood of being a recent-onset ecstasy user, and associations were strongest with nonviolent deviant behaviors such as selling illegal drugs and stealing. Associations between deviant behaviors and recent-onset ecstasy use were similar in strength to associations between deviant behaviors and recent-onset cocaine and marijuana use, respectively. Adults who had past-year psychiatric symptoms (both depressive and panic symptoms) were twice as likely to be recent-onset ecstasy users as compared with those without past-year psychiatric symptoms. Greater levels of drug involvement increased the odds of being a recent-onset ecstasy user. Conclusion: Recent-onset ecstasy use seems to be associated with a range of other behavioral problems and may reflect one aspect of a larger problem behavior syndrome. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
13.
The potential of a gonadotropin-releasing hormone (GnRH) agonist (goserelin acetate), delivered constantly for 28 days via a subcutaneous depot, to induce ovulation in seasonally anestrous mares, was investigated. Two experiments were conducted, in which a range of doses (30 to 240 micrograms/mare/d) was examined. Mares were selected on the basis of lack of substantial follicular development (follicle diameter < 20 mm determined ultrasonically) and low serum concentrations of luteinizing hormone (LH) and progesterone. Constant administration of the GnRH agonist-induced ovulation in anestrous mares, but a dose-response relation was not observed. Furthermore, with identical doses tested in consecutive or alternate years, considerable variation was observed in the ovulatory response. In general, ovulation in all treated mares was accompanied by increased circulating concentrations of LH and a decrease in follicle-stimulating hormone values. Ovulation was preceded by an increase in estradiol and LH concentrations. In mares in which ovulation did not occur, concentration of LH increased during agonist treatment, whereas that of follicle-stimulating hormone either increased or did not change. It was concluded that constant administration of GnRH agonists may induce ovulation in mares during seasonal anestrus; however, percentage of mares ovulating and the lack of reproducibility of effect indicate that this approach is inappropriate for use as a reliable method to manipulate breeding activity in commercial broodmares.  相似文献   
14.
We consider the problem of rescheduling trains in the case where one track of a railway section consisting of two tracks in opposing directions is closed due to construction activities. After presenting an appropriate model for this situation we derive a polynomial algorithm for the subproblem of finding an optimal schedule with minimal latenesss if the subsequences of trains for both directions outside the construction site are fixed. Based on this algorithm we propose a local search procedure for the general problem of finding good schedules and report test results for some real world instances. Received: December 8, 1999 / Accepted: May 2, 2001  相似文献   
15.
Food‐grade antioxidants: butylated hydroxyanisole (BHA), propyl paraben (PP) and butylated hydroxytoluene (BHT) (10 and 20 mmol g?1) and all the mixtures of these chemicals were tested for inhibitory activity on the growth of and aflatoxin B1 (AFB1) accumulation by Aspergillus parasiticus and A. flavus on irradiated (7 kGy) peanut grains. Also, the influence of these treatments was evaluated in different water conditions (0.982, 0.955, 0.937aw) at 11 and 35 days of incubation at 28 °C. Water activity (aw) affected the fungal growth, no fungal development was observed at the highest stress water condition (0.937aw). Butylated hydroxyanisole at 10 mmol g?1 level and all the mixtures with PP and/or BHT were significantly effective (P = 0.05) in increasing lag phase and reducing growth rate and colony forming units per gram of peanut of both Aspergillus section Flavi strains and AFB1 accumulation. The application of BHA at concentrations of 20 mmol g?1 alone or with PP and/or BHT totally inhibited fungal growth at 11 and 35 days of incubation. The results suggest that the addition of these chemical mixtures on peanut grains at low levels has potential to impact synergically on the control of Aspergillus section Flavi. Copyright © 2007 Society of Chemical Industry  相似文献   
16.
Inorganic–organic hybrid materials are attracting a strong scientific interest mainly for their outstanding inherent mechanical and thermal properties, which can be traced back to the intimate coupling of both inorganic and organic components. By carefully choosing the experimental parameters used for their synthesis, chemically and thermally stable acrylate-based hybrid material embedding the zirconium oxocluster Zr4O4(OMc)12, where OMcCH2C(CH3)C(O)O, can be deposited as UV-cured films on aluminium alloys.

In particular, the molar ratios between the oxocluster and the monomer, the polymerisation time, the amount of photo-initiator and the deposition conditions, by using an home-made spray-coating equipment, were optimised in order to obtain the best performing layers in terms of transparency and hardness to coat aluminium alloy (AA1050, AA6060 and AA2024) sheets. Furthermore, it was also evaluated whether the hybrid coatings behave as barrier to corrosion.

Several coated samples were prepared and characterised. Environmental scanning electronic microscopy (ESEM) and scratch test were used to investigate the morphology of the films and to evaluate their scratch resistance, respectively. Electrochemical impedance spectroscopy (EIS) was performed in order to evaluate if the coatings actually protect the metallic substrate from corrosion.

In order to measure shear storage modulus (G′) and loss modulus (G″) of the materials used for coatings, bulk samples were also obtained by UV-curing of the precursors solution. Dynamical mechanical thermal analysis (DMTA) was performed in shear mode on cured disks of both the hybrid materials and pristine polymer for comparison. The values of Tg were read off as the temperatures of peak of loss modulus. The length and mass of all the samples were measured before and after the DMTA analysis, so that the shrinkage of the materials in that temperature range was exactly evaluated.  相似文献   

17.
To make media resources a prime citizen on the Web, we have to go beyond simply replicating digital media files. The Web is based on hyperlinks between Web resources, and that includes hyperlinking out of resources (e.g., from a word or an image within a Web page) as well as hyperlinking into resources (e.g., fragment URIs into Web pages). To turn video and audio into hypervideo and hyperaudio, we need to enable hyperlinking into and out of them. The W3C Media Fragments Working Group is taking on the challenge to further embrace W3C??s mission to lead the World Wide Web to its full potential by developing a Media Fragment protocol and guidelines that ensure the long-term growth of the Web. The major contribution of this paper is the introduction of Media Fragments as a media-format independent, standard means of addressing media resources using URIs. Moreover, we explain how the HTTP protocol can be used and extended to serve Media Fragments and what the impact is for current Web-enabled media formats.  相似文献   
18.
Summary Systematic studies on solution properties of ultrahigh molecular weight polymers showed the existence of some special features as compared to usual length polymers. The paper discusses the possible appearance of branched structures in the polymerization process, structures able to influence the mentioned features. From experimental data on poly(methyl methacrylate), poly(butyl methacrylate) and polyacrylonitrile it appears that branching can be excluded, and the very high molecular weight domain considered may be responsible for the modifications observed in the molecular weight dependences on 2>, A2 or [].  相似文献   
19.
There is a need to assess the relationship between improved rheological properties and the immunogenic potential of wheat proteins. The present study aimed to investigate the in vitro effects of total protein extracts from three modern and two landrace Triticum aestivum commercial flour mixes, with significant differences in gluten strength (GS), on cell lines. Cytotoxicity and innate immune responses induced by wheat proteins were investigated using Caco-2 monocultures, two dimensional (2D) Caco-2/U937 co-cultures, and three dimensional (3D) co-cultures simulating the intestinal mucosa with Caco-2 epithelial cells situated above an extra-cellular matrix containing U937 monocytes and L929 fibroblasts. Modern wheat proteins, with increased GS, significantly reduced Caco-2 cell proliferation and vitality in monoculture and 2D co-cultures than landrace proteins. Modern wheat proteins also augmented Caco-2 monolayer disruption and tight junction protein, occludin, redistribution in 3D co-cultures. Release of interleukin-8 into the cell medium and increased U937 monocyte migration in both 2D and 3D co-cultures were similarly apparent. Immuno-activation of migrating U937 cells was evidenced from cluster of differentiation 14 (CD14) staining and CD11b-related differentiation into macrophages. The modern wheat proteins, with gluten polymorphism relatedness and increased GS, were shown to be more cytotoxic and immunogenic than the landrace wheat proteins.  相似文献   
20.
Bilirubin toxicity to the central nervous system (CNS) is responsible for severe and permanent neurologic damage, resulting in hearing loss, cognitive, and movement impairment. Timely and effective management of severe neonatal hyperbilirubinemia by phototherapy or exchange transfusion is crucial for avoiding permanent neurological consequences, but these therapies are not always possible, particularly in low-income countries. To explore alternative options, we investigated a pharmaceutical approach focused on protecting the CNS from pigment toxicity, independently from serum bilirubin level. To this goal, we tested the ability of curcumin, a nutraceutical already used with relevant results in animal models as well as in clinics in other diseases, in the Gunn rat, the spontaneous model of neonatal hyperbilirubinemia. Curcumin treatment fully abolished the landmark cerebellar hypoplasia of Gunn rat, restoring the histological features, and reverting the behavioral abnormalities present in the hyperbilirubinemic rat. The protection was mediated by a multi-target action on the main bilirubin-induced pathological mechanism ongoing CNS damage (inflammation, redox imbalance, and glutamate neurotoxicity). If confirmed by independent studies, the result suggests the potential of curcumin as an alternative/complementary approach to bilirubin-induced brain damage in the clinical scenario.  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-23

京公网安备 11010802026262号