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   24265篇
  免费   2314篇
  国内免费   1117篇
电工技术   1677篇
技术理论   5篇
综合类   3739篇
化学工业   3023篇
金属工艺   637篇
机械仪表   889篇
建筑科学   2531篇
矿业工程   853篇
能源动力   829篇
轻工业   4065篇
水利工程   652篇
石油天然气   883篇
武器工业   145篇
无线电   1372篇
一般工业技术   1897篇
冶金工业   770篇
原子能技术   259篇
自动化技术   3470篇
  2024年   68篇
  2023年   281篇
  2022年   533篇
  2021年   658篇
  2020年   768篇
  2019年   675篇
  2018年   625篇
  2017年   728篇
  2016年   791篇
  2015年   949篇
  2014年   1462篇
  2013年   1521篇
  2012年   1855篇
  2011年   1869篇
  2010年   1456篇
  2009年   1439篇
  2008年   1394篇
  2007年   1660篇
  2006年   1508篇
  2005年   1204篇
  2004年   1004篇
  2003年   891篇
  2002年   741篇
  2001年   523篇
  2000年   494篇
  1999年   378篇
  1998年   363篇
  1997年   293篇
  1996年   261篇
  1995年   228篇
  1994年   220篇
  1993年   142篇
  1992年   166篇
  1991年   121篇
  1990年   85篇
  1989年   75篇
  1988年   63篇
  1987年   44篇
  1986年   26篇
  1985年   29篇
  1984年   23篇
  1983年   7篇
  1982年   5篇
  1981年   9篇
  1980年   9篇
  1964年   5篇
  1961年   4篇
  1960年   5篇
  1955年   7篇
  1954年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Mesquite (Prosopis glandulosa) and redberry juniper (Juniperus pinchotii) are two woody species which dominate uncultivated lands in the south central Great Plains, USA (Texas, Oklahoma) and may have potential for bioenergy utilization. In this study, these two biomasses were gasified in an adiabatic, updraft fixed‐bed gasifier using air as medium. The products include low heating value gas and liquid tar yield. The effects of fuel moisture content and the equivalence ratio (ER) on liquid yields including heavy tar and light tar and higher heating value (HHV) of the producer gas were investigated. It was found that updraft gasifiers produce a large amount of tar, in a range of 100 g/Nm3 of gas produced. As the equivalence ratio (ER) increased (i.e. more rich char conditions), peak temperature (Tpeak) within the bed decreased due to the lesser partial oxidization. The liquid yield showed peak value with change in moisture content between 6 and 24%. Heavy tar is a very high quality fuel with a higher heating value of over 29 MJ/kg on a dry, ash‐free (DAF) basis which is a slightly higher than that of ethanol. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
72.
以我国上市公司为研究样本,从金融机构参股钢铁上市公司的视角,分析不同参股比例的产融结合行为对经营绩效、企业价值的影响。经分析发现,金融机构参股超过一定比例后,产融结合对企业经营绩效和企业价值呈显著非线性相关关系。  相似文献   
73.
The determinants of the premium value of patents for medical and cosmetic products are analyzed with respect to a complementary IP strategy such as trademarks. I discuss a novel method and database to gauge combinations of patent and trademark pairs regarding the same innovative project. The premium value is computed through a model of renewal decisions for the patent cohorts 1985–1990 that have been designated in the U.K. and Germany. After taking into the account several firm characteristics and patent indicators typically used in the literature, I find ample evidences that patent and trademark pairs are featured by higher valuations.  相似文献   
74.
Cobalt-incorporated MCM-41(Co-MCM-41) was used as a heterogeneous catalyst for the ozonation of para-chlorobenzoic acid (p-CBA) in aqueous solution. Cobalt oxide supported on MCM-41(Co/MCM-41) was synthesized for comparison. Their textural properties were elucidated by various characterization techniques to understand the relationship between surface texture and catalytic activity. TOC removal at 60 min reached 91% with Co-MCM-41, 83% with Co/MCM-41 and only 52% with ozone alone, respectively. Observations from diffuse reflection spectroscopy demonstrated that different metal phases were formed in these cobalt-modified molecular sieves samples. Radical scavenger experiments indicated the formation of hydroxyl radicals that were responsible for the effective degradation of p-CBA. An integrated approach to the catalytic mechanism was proposed by considering the variation of pH in the course of ozonation as well as its subsequent influence on the dissociation of targeted compounds and surface charge of the catalyst. In the reusability experiments, the reused Co-MCM-41 was able to regain the same catalytic capability as the fresh one within 5 cycles. X-ray photoelectron spectroscopy results indicated that a part of Co2+ was oxidized to Co3+ after oxidation reaction.  相似文献   
75.
Correlation of thermodynamic and secondary structural stability of proteins at various buffer pHs was investigated using differential scanning calorimetry (DSC), dynamic light scattering (DLS) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR). Recombinant human epithelial growth factor (rhEGF) was selected as a model protein at various pHs and in different buffers, including phosphate, histidine, citrate, HEPES and Tris. Particle size and zeta potential of rhEGF at each selected pH of buffer were observed by DLS. Four factors were used to characterize the biophysical stability of rhEGF in solution: temperature at maximum heat flux (Tm), intermolecular β-sheet contents, zeta size and zeta potential. It was possible to predict the apparent isoelectric point (pI) of rhEGF as 4.43 by plotting pH against zeta potential. When the pH of the rhEGF solution increased or decreased from pI, the absolute zeta potential increased indicating a reduced possibility of protein aggregation, since Tm increased and β-sheet contents decreased. The contents of induced intermolecular β-sheet in Tris and HEPES buffers were the lowest. Thermodynamic stability of rhEGF markedly increased when pH is higher than 6.2 in histidine buffer where Tm of first transition was all above 70?°C. Moreover, rhEGF in Tris buffer was more thermodynamically stable than in HEPES with higher zeta potential. Tris buffer at pH 7.2 was concluded to be the most favorable.  相似文献   
76.
77.
郑莉 《南方建筑》2015,(2):75-79
文章通过论述汕头中山公园在策划背景、分析论证、用地选址和大规模建设各方面,是如何作为城市空间来体现其社会使用价值,来探讨城市空间产生过程中错综复杂的政治、经济、社会和文化多方面力量的博弈,以及城市社会和空间使用的相互关系。  相似文献   
78.
Biobutanol has characteristics similar to petroleum fuel and is considered as a superior biofuel compared to ethanol. The development of technologies for biobutanol production by fermentation has resulted in higher final biobutanol concentrations together with less energy‐intensive separation and purification techniques. These new technological developments have the potential to provide a production process for biobutanol that is economically viable in comparison to the petrochemical pathway for its production. The control properties of four different possible process designs for biobutanol purification are analyzed. The results, using the singular value decomposition technique, indicated that the scheme where only biobutanol flow is purified, and both ethanol and acetone leaving the purification process mixed with water and biobutanol traces, showed the best control properties.  相似文献   
79.
Thermal properties of fossil fuel are the key fundamental characteristics, which can distinguish any compound as a potential fuel. The performance of diesel fuel blend along with stability and solubility parameter designs are evaluated. The results from the experimental study indicate that the increase in hydrogen peroxide (H2O2) amount enhances the cetane number of diesel fuel blend significantly. However, the calorific value decreases as compared to pure diesel fuel. All values performed well according to the ASTM D‐975 diesel testing method. The thermodynamics of the prepared fuel blends also revealed that substantial solubility and diesel/H2O2 blend stability are provided even at lower temperatures. Such blends can be used as a feasible replacement of pure diesel fuel.  相似文献   
80.
As drug delivery systems, stimuli‐responsive polymer micelles hold great potential in cancer chemotherapeutics to improve therapeutic efficiency and eliminate organism adverse effects. Here, pH‐sensitive polymeric micelles based on dextran‐g‐benzimidazole were designed and used for intracellular anticancer drug delivery. The anticancer drug doxorubicin (DOX) was effectively loaded into the micelles via hydrophobic interactions. In vitro release studies demonstrated that the release of loaded DOX was greater and faster under acid conditions such as in carcinomatous areas (pH < 6.8) than in physiological conditions (pH 7.4). MTT assays and flow cytometric analyses showed that DOX‐loaded micelles had higher cellular proliferation inhibition towards HeLa and HepG2 cells than pH‐insensitive controls. These pH‐sensitive micelles with significant efficiency for intracellular drug release will be beneficial to the future of in vivo biomedical applications. © 2014 Society of Chemical Industry  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