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   9376篇
  免费   323篇
  国内免费   12篇
电工技术   77篇
综合类   11篇
化学工业   3867篇
金属工艺   114篇
机械仪表   132篇
建筑科学   355篇
矿业工程   40篇
能源动力   227篇
轻工业   1729篇
水利工程   67篇
石油天然气   44篇
无线电   417篇
一般工业技术   1308篇
冶金工业   401篇
原子能技术   21篇
自动化技术   901篇
  2024年   128篇
  2023年   181篇
  2022年   919篇
  2021年   1071篇
  2020年   368篇
  2019年   328篇
  2018年   343篇
  2017年   289篇
  2016年   376篇
  2015年   299篇
  2014年   371篇
  2013年   615篇
  2012年   518篇
  2011年   583篇
  2010年   391篇
  2009年   395篇
  2008年   375篇
  2007年   345篇
  2006年   292篇
  2005年   213篇
  2004年   196篇
  2003年   157篇
  2002年   131篇
  2001年   78篇
  2000年   63篇
  1999年   63篇
  1998年   52篇
  1997年   59篇
  1996年   62篇
  1995年   50篇
  1994年   41篇
  1993年   40篇
  1992年   32篇
  1991年   19篇
  1990年   29篇
  1989年   16篇
  1988年   20篇
  1987年   16篇
  1986年   8篇
  1985年   20篇
  1984年   18篇
  1983年   19篇
  1982年   9篇
  1981年   12篇
  1980年   14篇
  1979年   9篇
  1977年   14篇
  1976年   7篇
  1973年   7篇
  1966年   5篇
排序方式: 共有9711条查询结果,搜索用时 31 毫秒
111.
This study examines the role of the decision environment in how well business intelligence (BI) capabilities are leveraged to achieve BI success. We examine the decision environment in terms of the types of decisions made and the information processing needs of the organization. Our findings suggest that technological capabilities such as data quality, user access and the integration of BI with other systems are necessary for BI success, regardless of the decision environment. However, the decision environment does influence the relationship between BI success and capabilities, such as the extent to which BI supports flexibility and risk in decision making.  相似文献   
112.
mitochondrial neuro-gastrointestinal encephalomyopathy (MNGIE) is a rare genetic disorder characterized by thymidine phosphorylase (TP) enzyme defect. The absence of TP activity induces the imbalance of mitochondrial nucleotide pool, leading to impaired mitochondrial DNA (mtDNA) replication and depletion. Since mtDNA is required to ensure oxidative phosphorylation, metabolically active tissues may not achieve sufficient energy production. The only effective life-saving approach in MNGIE has been the permanent replacement of TP via allogeneic hematopoietic stem cell or liver transplantation. However, the follow-up of transplanted patients showed that gut tissue changes do not revert and fatal complications, such as massive gastrointestinal bleeding, can occur. The purpose of this study was to clarify whether the reintroduction of TP after transplant can recover mtDNA copy number in a normal range. Using laser capture microdissection and droplet-digital-PCR, we assessed the mtDNA copy number in each layer of full-thickness ileal samples of a naive MNGIE cohort vs. controls and in a patient pre- and post-TP replacement. The treatment led to a significant recovery of gut tissue mtDNA amount, thus showing its efficacy. Our results indicate that a timely TP replacement is needed to maximize therapeutic success before irreversible degenerative tissue changes occur in MNGIE.  相似文献   
113.
Isoflavonoids such as genistein (GE) are well known antioxidants. The predictive biological activity of structurally new compounds such as thiogenistein (TGE)–a new analogue of GE–becomes an interesting way to design new drug candidates with promising properties. Two oxidation strategies were used to characterize TGE oxidation products: the first in solution and the second on the 2D surface of the Au electrode as a self-assembling TGE monolayer. The structure elucidation of products generated by different oxidation strategies was performed. The electrospray ionization mass spectrometry (ESI-MS) was used for identifying the product of electrochemical and hydrogen peroxide oxidation in the solution. Fourier transform infrared spectroscopy (FT-IR) with the ATR mode was used to identify a product after hydrogen peroxide treatment of TGE on the 2D surface. The density functional theory was used to support the experimental results for the estimation of antioxidant activity of TGE as well as for the molecular modeling of oxidation products. The biological studies were performed simultaneously to assess the suitability of TGE for antioxidant and antitumor properties. It was found that TGE was characterized by a high cytotoxic activity toward human breast cancer cells. The research was also carried out on mice macrophages, disclosing that TGE neutralized the production of the LPS-induced reactive oxygen species (ROS) and exhibits ABTS (2,2′-azino-bis-3-(ethylbenzothiazoline-6-sulphonic acid) radical scavenging ability. In the presented study, we identified the main oxidation products of TGE generated under different environmental conditions. The electroactive centers of TGE were identified and its oxidation mechanisms were proposed. TGE redox properties can be related to its various pharmacological activities. Our new thiolated analogue of genistein neutralizes the LPS-induced ROS production better than GE. Additionally, TGE shows a high cytotoxic activity against human breast cancer cells. The viability of MCF-7 (estrogen-positive cells) drops two times after a 72-h incubation with 12.5 μM TGE (viability 53.86%) compared to genistein (viability 94.46%).  相似文献   
114.
This study is aimed at the thermal analysis of sealant mortar (usually a mixtures of bentonite and cemem with addition of sand) used in geothermal cooling and heating. In particular, thermal conductivity and diffusivity measurements were performed on differem sealant mixtures by using Hot Disk thermal constants analyzer in order to identify the interesting thermal properties of grouting materials. The grouting materials that we considered are of porous nature and, if used in the presence of groundwater, have different levels of imbibitions. It is important to know the thermal behavior of these materials at different water content. A first set of measurements was performed on a not-tinted material at room temperature; then the samples were led to saturation conditions by contact capillary imbibitions with a cotton wool layer moistened in water. The determination of thermal conductivity in these test conditions appears to be critical compared to the measuremems on non-timed sample. The thermal conductivity tests have revealed how the thermal behavior of the samples analyzed is essentially determined by the density and water content of the material: in fact, the thermal conductivity increases of two to three times the value of the not-tinted material.  相似文献   
115.
Smoke is considered to be the main hazard of fires involving epoxy resins but its production depends on many variables, principally the chemical character and the burning rate of the polymer plus the availability of oxygen. The work reported aimed to study the smoke suppressant effect and flammability performance of zinc‐based compounds (FR system) in epoxy matrix composites used in the aerospace and aeronautical industry. The flammability performance of neat and FR‐loaded systems was screened using microcombustion calorimetry, while smoke generation, in terms of carbon monoxide (CO) and carbon dioxide (CO2) production, was analysed under dynamic conditions using cone calorimetry. Final results indicate that the dispersion of zinc borate and zinc hydroxystannate (ZHS) into epoxy matrices leads to a significant variation in flame retardant properties reducing both total heat release by about 25 and 30%, respectively, and heat release capacity by about 30 and 50%, respectively. The system containing ZHS shows an enhancement in all smoke suppressant properties; both tin compounds (zinc stannate (ZS) and ZHS) give a reduction of CO2/CO ratio from 41 to 25 for ZS and from 41 to 36 for ZHS compared to neat matrix. Copyright © 2010 Society of Chemical Industry  相似文献   
116.
This study shows the effects of three different nanofillers on the viscosity properties and fire behavior of a halogen‐free flame retardant system. The original system, based on ethylene‐acrylate copolymer, calcium carbonate, and a silicone elastomer, shows good flame retardant properties. One of the nanofillers, montmorillonite (MMT), significantly increases the viscosity above 250°C, resulting in reduced dripping and decreased heat release rate. The ash residue, however, is very brittle, indicating poor interactions between the MMT and other components of the system. The second nanofiller, sepiolite, shows no improvement on the flame retardant properties of the system. Reduced dripping is observed when a small amount of the third nanofiller, polyhedral oligomeric silsesquioxane, POSS, is incorporated into the system. In this case, high silica content on the surface indicates char formation originating from the POSS. However, an increased heat release is observed when POSS is used in the system. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
117.
In this work kinetics of the electrochemical reduction of the tert-butyl chloride molecule are studied. The two-dimensional potential surface E(x, y) was obtained from the Hamiltonian model used earlier [1] to study quantum effects on the reaction rate of the same reaction. In series of molecular dynamics simulations two different models, isotropic (γx = γy) and anisotropic (γx > γy), of the friction parameters for the solvent (γx) and for the C-Cl bond (γy) were considered. An influence of several factors, such as an overpotential, a solvent friction coefficient and a temperature, on the rate constants k obtained with the two models are presented and compared to the results obtained for the non-adiabatic mechanism. It is shown, that the anisotropic model predicts generally much smaller rates when compared to those calculated with the isotropic model and also with the non-adiabatic model. On the curves k(γx) the turnover region is very well defined for relatively low frictions in the isotropic case, while for the anisotropic case it is shifted towards much higher γx values. In all simulations the saddle point avoidance phenomenon was observed and its extent for different models and reaction conditions is discussed. Transfer coefficients α obtained for adiabatic and non-adiabatic reaction mechanisms as a function of the overpotential and temperature are also presented.  相似文献   
118.
119.
The reaction pathway during the formation of silica via a two-component “non-aqueou” sol-gel synthesis is studied by in situ time-resolved Raman spectroscopy. This synthetic route is followed with and without the addition of the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) in order to investigate its effect on the reaction pathway. We demonstrate that Raman spectroscopy is suitable to discriminate between different silica intermediates, which are produced and consumed at different rates with respect to the point of gelation. We find that half-way to gelation monomers and shorter chains are the most abundant silica species, while the formation of silica rings strongly correlates to the sol-to-gel transition. Thus, curling up of linear chains is here proposed as a plausible mechanism for the formation of small rings. These in turn act as nucleation sites for the condensation of larger rings and thus the formation of the open and polymeric silica network. We find that the protic ionic liquid does not change the reaction pathway per se, but accelerates the cyclization process, intermediated by the faster inclusion of monomeric species.  相似文献   
120.
Cyclodextrins (CDs) are highly respected for their ability to form inclusion complexes via host–guest noncovalent interactions and, thus, ensofance other molecular properties. Various molecular modeling methods have found their applications in the analysis of those complexes. However, as showed in this review, molecular dynamics (MD) simulations could provide the information unobtainable by any other means. It is therefore not surprising that published works on MD simulations used in this field have rapidly increased since the early 2010s. This review provides an overview of the successful applications of MD simulations in the studies on CD complexes. Information that is crucial for MD simulations, such as application of force fields, the length of the simulation, or solvent treatment method, are thoroughly discussed. Therefore, this work can serve as a guide to properly set up such calculations and analyze their results.  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