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   796篇
  免费   48篇
电工技术   3篇
综合类   1篇
化学工业   183篇
金属工艺   17篇
机械仪表   34篇
建筑科学   22篇
矿业工程   1篇
能源动力   11篇
轻工业   108篇
水利工程   1篇
石油天然气   1篇
无线电   50篇
一般工业技术   134篇
冶金工业   147篇
原子能技术   2篇
自动化技术   129篇
  2023年   10篇
  2022年   18篇
  2021年   33篇
  2020年   11篇
  2019年   25篇
  2018年   13篇
  2017年   20篇
  2016年   30篇
  2015年   32篇
  2014年   30篇
  2013年   48篇
  2012年   58篇
  2011年   44篇
  2010年   44篇
  2009年   36篇
  2008年   30篇
  2007年   32篇
  2006年   25篇
  2005年   25篇
  2004年   13篇
  2003年   25篇
  2002年   19篇
  2001年   14篇
  2000年   8篇
  1999年   15篇
  1998年   28篇
  1997年   26篇
  1996年   21篇
  1995年   11篇
  1994年   8篇
  1993年   10篇
  1992年   9篇
  1991年   4篇
  1990年   3篇
  1989年   5篇
  1987年   3篇
  1986年   3篇
  1985年   2篇
  1984年   3篇
  1983年   5篇
  1982年   3篇
  1981年   9篇
  1979年   4篇
  1977年   3篇
  1976年   3篇
  1975年   3篇
  1973年   2篇
  1971年   2篇
  1954年   2篇
  1924年   2篇
排序方式: 共有844条查询结果,搜索用时 31 毫秒
1.
Histamine is an important mediator in allergic reactions, gastric acid secretions, and neurotransmission in the central nervous system. Basophils and mast cells are the main sources of histamine, which is formed from L-histidine by histidine decarboxylase (HDC). However, the regulatory mechanism of HDC in these cells remains unclear. We examined the regulation of HDC activity and gene expression using a unique human mast cell line, HMC-1, after stimulation with phorbol 12-myristate 13-acetate (PMA) or ionomycin. HDC activity was increased from 52.1+/-0.4 (mean+/-standard deviation) to 154+/-6.9, or 105.6+/-6.2 pmol/min/mg protein (n = 3), 4 hours after stimulation with PMA (10 ng/mL) or ionomycin (10[-6] M). Although actinomycin D had no effect on this increase, cycloheximide completely inhibited the increase caused by these stimuli. The population of HMC-1 cells containing HDC protein was increased after stimulation with either PMA or ionomycin as evaluated by immunocytochemical analysis with anti-HDC antibody as a marker. HMC-1 constitutively expressed HDC mRNA, and its level was not increased with these stimuli. These results suggest that the increase of HDC activity in HMC-1 induced by PMA or ionomycin is regulated at the translational level.  相似文献   
2.
Various ways of quantifying damages have been applied to productivity loss claims in construction. All of the ways attempt to be as objective as possible based on the extent of information available in a particular case. The measured mile, a widely accepted method, is employed when an unimpacted baseline period of production can be identified. Although that approach is considered to be the most objective method available in such cases, the method is limited and does not directly account for variation in individual productivity values about a normal or natural level of productivity. A gap exists between the use of existing methods and the availability of an appropriate methodology that specifically addresses variation in productivity. The key lies in the way baseline productivity is measured, which is inherently statistical, yet no truly statistical methods are used to establish such a baseline. Using the measured mile as a backdrop, this article provides an objective, measurement-based approach that can be used to establish a productivity baseline applied to construction productivity loss claims, based on the application of statistical methods aided by a process control chart. The focus is on providing the basic principles and concepts underlying the approach presented.  相似文献   
3.
In this research, the effect of dielectric barrier discharge on mechanically manufactured (sawn, planed or polished) wood surfaces is investigated. The experiments have shown that the best hydrophilia is obtained with a cold plasma discharge in air under atmospheric pressure. Other gases such as helium, nitrogen, and argon have been tested, too. Changes of the wood surface properties were determined by absorption of water and contact angle measurements. As a result of a 1 to 20 seconds long exposure to plasma, wood surfaces became hydrophilic. The absorption of water in wood can be changed up to 22 times higher after plasma treatment in air for 20 seconds duration. The fracture strength of glued wood has been increased by 68% after this plasma pretreatment. Using plasma treatment in methane or acetylene wood surfaces are changed to hydrophobic properties. After plasma treatment for 1 min. in a Ar:CH4=80:20 gas mixture under atmospheric pressure, the absorption of water in wood is 32 times lower than without plasma treatment. As mentioned before, plasma treatment can produce hydrophobic and hydrophilic wood surfaces. But a good homogeneity of the gas discharge is necessary for the plasma treatment of wood. Zusammenfassung Im Rahmen dieser Arbeit werden die Auswirkungen der dielektrisch behinderten Entladung bei Atmosphärendruck auf mechanisch bearbeitete Holzoberflächen (gesägt, gehobelt oder geschliffen) untersucht. Die Experimente haben gezeigt, dass die höchste Hydrophilität bei einem in Luftatmosphäre gezündetem Plasma erreicht wird. Es werden auch andere Gase verwendet, wie z.B. Helium, Stickstoff und Argon. Zur Untersuchung der veränderten mechanischen und energetischen Eigenschaften der Holzoberflächen werden der Tropfen-Versuch und die Kontaktwinkelmessung verwendet. Durch eine 1 bis 20 Sekunden lange Plasmabehandlung können die Oberflächen hydrophil gestaltet werden. So kann die Eindringzeit von Wasser um Faktor 22 gesteigert werden durch eine 20 Sekunden lange Plasmabehandlung in Umgebungsluft. Die Bruchfestigkeit von Leimholz wird durch die gleiche Plasmavorbehandlung um 68% gesteigert. Werden als Plasmabehandlungsgas Methan oder Acetylen eingesetzt, so wird die Holzoberfläche hydrophob. Durch eine 60 Sekunden lange Plasmabehandlung in einem Ar:CH4=80:20 Gasgemisch bei Atmosphärendruck wird die Eindringszeit von Wasser um den Faktor 32 gesenkt. Mit Hilfe der Plasmabehandlung können also sowohl hydrophobe als auch hydrophile Holzoberflächen erzeugt werden. Jedoch wird für die Plasmabehandlung von Holz eine sehr homogene Gasentladung benötigt.  相似文献   
4.
High performance liquid chromatographic separation of a series of mono-, di- and trihydroxylated 5β-cholanic acids which differ only in position and configuration of hydroxyl groups at positions C-3, C-7 and/or C-12, is reported. The C-24 free acids were derivatized to four different classes of UV-sensitive esters, i.e.,p-bromophenacyl (BP),m-methoxyphenacyl (MP), 4-nitrophthalimidemethyl (NPM) and 9-anthrylmethyl (AM) esters, and chromatographed on two, variants of C18 reversed-phase columns (Nova-Pak C18 and Zorbax ODS) with methanol-water systems as mobile phase. Separation efficiency and elution order of some isomeric pairs were influenced by both the structure of the C-24 ester groups and the nature of the columns used. Excellent chromatographic properties were found for those derivatives, particularly for the NPM esters.  相似文献   
5.
From May 1985 to May 1989, 175 patients with previously untreated aggressive non-Hodgkin's lymphoma were randomized to receive CHOP or ProMACE-CytaBOM. Eligibility criteria included follicular large-cell diffuse small cleaved-cell, diffuse mixed, diffuse large-cell and immunoblastic lymphoma with an Ann Arbor stage II, III or IV. One hundred and forty-eight patients were evaluable. There were no significant differences between the 2 treatments in response rate (83.5% [57.5% CR] for CHOP vs. 88% [62% CR] for ProMACE-CytaBOM), time to treatment failure (29% vs. 31% at 5 yr), or overall survival (42% in both groups at 5 yr). Furthermore, there were no significant differences between the 2 regimens when response rates and outcome were analyzed for different prognostic subgroups. Toxicity was not significantly different between the 2 regimens, although only 1 patient died as result of treatment-related toxicity in the CHOP arm compared to 6 patient in the ProMACE-CytaBOM group (p = 0.126). In conclusion, in this study ProMACE-CytaBOM has not proved to be superior to CHOP in aggressive lymphomas. This trial gives support to the notion that CHOP still is the standard chemotherapy for aggressive lymphomas, and that new treatment approaches for these lymphomas should be compared to CHOP.  相似文献   
6.
Arsenic sulfide glass optical fibers typically possess extrinsic absorption bands in the infrared wavelength region associated with residual hydrogen and oxygen related impurities, despite using purified precursors. We report a purification process based on the addition of tellurium tetrachloride (TeCl4) to the glass. During melting, the chlorine from TeCl4 reacts with the hydrogen impurities to produce volatile products (e.g., HCl) that can be removed by subsequent dynamic distillation. The processing conditions have been modified accordingly to produce optical fibers with significantly reduced loss due to hydrogen sulfide impurity content (1.5 dB/m).  相似文献   
7.
This paper mainly focuses on the influence of three kinds of media: air, air-10%PA (Nylon) and air-10% POM (polyoxymethylene) on low-voltage circuit breaker arcs. A threedimensional (3-D) model of arc motioa under the effect of external magnetic field is built based on magnetohydrodynamics (MHD) equations. By adopting the commercial computational fluid dynamics (CFD) package based on the control-volume method, the above MHD equations are solved. For the media of air-10%PA and air-10%POM, the distributions of stationary temperature and electrical potential and the transient motion processes are compared with those of air arc. The research shows that both air-10%PA and air -10% POM can cool the arc plasma and the former is more effective. Both of them can increase the stationary voltage as well. Moreover, the presence of the two mixtures can accelerate the arc motion toward the quenching area and ensures the arc quenched in time.  相似文献   
8.
The development of nanocarriers (NC) for biomedical applications has gained large interest due to their potential to co-deliver drugs in a cell-type-targeting manner. However, depending on their surface characteristics, NC accumulate serum factors, termed protein corona, which may affect their cellular binding. We have previously shown that NC coated with carbohydrates to enable biocompatibility triggered the lectin-dependent complement pathway, resulting in enhanced binding to B cells via complement receptor (CR)1/2. Here we show that such NC also engaged all types of splenic leukocytes known to express CR3 at a high rate when NC were pre-incubated with native mouse serum resulting in complement opsonization. By focusing on dendritic cells (DC) as an important antigen-presenting cell type, we show that CR3 was essential for binding/uptake of complement-opsonized NC, whereas CR4, which in mouse is specifically expressed by DC, played no role. Further, a minor B cell subpopulation (B-1), which is important for first-line pathogen responses, and co-expressed CR1/2 and CR3, in general, engaged NC to a much higher extent than normal B cells. Here, we identified CR-1/2 as necessary for binding of complement-opsonized NC, whereas CR3 was dispensable. Interestingly, the binding of complement-opsonized NC to both DC and B-1 cells affected the expression of activation markers. Our findings may have important implications for the design of nano-vaccines against infectious diseases, which codeliver pathogen-specific protein antigen and adjuvant, aimed to induce a broad adaptive cellular and humoral immune response by inducing cytotoxic T lymphocytes that kill infected cells and pathogen-neutralizing antibodies, respectively. Decoration of nano-vaccines either with carbohydrates to trigger complement activation in vivo or with active complement may result in concomitant targeting of DC and B cells and thereby may strongly enhance the extent of dual cellular/humoral immune responses.  相似文献   
9.
Previous studies have shown that, although the isolated structuraldomains of yeast phosphoglycerate kinase recover a quasi-nativestructure in vitro as well as in vivo, they do not reassociatenor generate a functional enzyme. The aim of this work was firstto study the folding of complementary fragments different fromstructural domains and second to determine the requirementsfor their reassociation and functional complementation. Themethod used for producing rigorously defined fragments consistsof the introduction of a unique cysteinyl residue in the proteinfollowed by a specific cleavage by 5'5'-dithiobis(2-nitrobenzoate)/potassiumcyanide at this residue. Two pairs of complementary fragmentswere thus obtained, 1–96/97–415 and 1–248/249–415.The structure and stabilities of the different fragments werestudied. The short fragments, i.e. 1–96 and 249–415were found to contain some secondary structure, but to havea low stability. Each large fragment has a high structural contentand a stability close to that of the corresponding domain. Incontrast to that observed with the isolated domains, a weakbut significant complementation was observed for the two pairsof fragments; the pair of fragments 1–248/249–415recovered 8% of the activity of the native enzyme upon complementation.An independent refolding of the complementary fragments beforereassociation decreased the yield of complementation for thepair of fragments 1–96/97–415, but did not affectthe complementation for the other pair (1–248/249–415).From the present data and previous work on the isolated domains,it appears that the correct folding of the isolated fragmentsis not a prerequisite for their complementation.  相似文献   
10.
In this paper we introduce a new variant of shape differentiation which is adapted to the deformation of shapes along their normal direction. This is typically the case in the level-set method for shape optimization where the shape evolves with a normal velocity. As all other variants of the original Hadamard method of shape differentiation, our approach yields the same first order derivative. However, the Hessian or second-order derivative is different and somehow simpler since only normal movements are allowed. The applications of this new Hessian formula are twofold. First, it leads to a novel extension method for the normal velocity, used in the Hamilton-Jacobi equation of front propagation. Second, as could be expected, it is at the basis of a Newton optimization algorithm which is conceptually simpler since no tangential displacements have to be considered. Numerical examples are given to illustrate the potentiality of these two applications. The key technical tool for our approach is the method of bicharacteristics for solving Hamilton-Jacobi equations. Our new idea is to differentiate the shape along these bicharacteristics (a system of two ordinary differential equations).  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