全文获取类型
收费全文 | 8859篇 |
免费 | 554篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 72篇 |
综合类 | 18篇 |
化学工业 | 2090篇 |
金属工艺 | 254篇 |
机械仪表 | 189篇 |
建筑科学 | 373篇 |
矿业工程 | 27篇 |
能源动力 | 248篇 |
轻工业 | 795篇 |
水利工程 | 122篇 |
石油天然气 | 25篇 |
无线电 | 540篇 |
一般工业技术 | 1837篇 |
冶金工业 | 1511篇 |
原子能技术 | 48篇 |
自动化技术 | 1274篇 |
出版年
2023年 | 65篇 |
2022年 | 154篇 |
2021年 | 227篇 |
2020年 | 176篇 |
2019年 | 215篇 |
2018年 | 240篇 |
2017年 | 219篇 |
2016年 | 267篇 |
2015年 | 205篇 |
2014年 | 340篇 |
2013年 | 592篇 |
2012年 | 494篇 |
2011年 | 705篇 |
2010年 | 482篇 |
2009年 | 450篇 |
2008年 | 504篇 |
2007年 | 466篇 |
2006年 | 391篇 |
2005年 | 354篇 |
2004年 | 301篇 |
2003年 | 280篇 |
2002年 | 258篇 |
2001年 | 140篇 |
2000年 | 115篇 |
1999年 | 136篇 |
1998年 | 191篇 |
1997年 | 157篇 |
1996年 | 111篇 |
1995年 | 107篇 |
1994年 | 115篇 |
1993年 | 67篇 |
1992年 | 84篇 |
1991年 | 46篇 |
1990年 | 45篇 |
1989年 | 54篇 |
1988年 | 57篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 65篇 |
1984年 | 59篇 |
1983年 | 55篇 |
1982年 | 45篇 |
1981年 | 33篇 |
1980年 | 37篇 |
1979年 | 22篇 |
1978年 | 29篇 |
1977年 | 33篇 |
1976年 | 34篇 |
1975年 | 20篇 |
1974年 | 15篇 |
排序方式: 共有9423条查询结果,搜索用时 15 毫秒
91.
92.
C. Christopher ColumbusAuthor Vitae K. Chandrasekaran Author VitaeSishaj P. Simon Author Vitae 《Applied Soft Computing》2012,12(1):145-160
This paper proposes a nodal ant colony optimization (NACO) technique to solve profit based unit commitment problem (PBUCP). Generation companies (GENCOs) in a competitive restructured power market, schedule their generators with an objective to maximize their own profit without any regard for system social benefit. Power and reserve prices become important factors in decision process. Ant colony optimization that mimics the behavior of ants foraging activities is suitably implemented to search the UCP search space. Here a search space consisting of optimal combination of binary nodes for unit ON/OFF status is represented for the movement of the ants to maintain good exploration and exploitation search capabilities. The proposed model help GENCOs to make decisions on the quantity of power and reserve that must be put up for sale in the markets and also to schedule generators in order to receive the maximum profit. The effectiveness of the proposed technique for PBUCP is validated on 10 and 36 generating unit systems available in the literature. NACO yields an increase of profit, greater than 1.5%, in comparison with the basic ACO, Muller method and hybrid LR-GA. 相似文献
93.
Rudolf E. Schmitt Johann Haas und Renato Amadò 《Zeitschrift für Lebensmitteluntersuchung und -Forschung A》1988,187(2):121-124
Zusammenfassung Zur Bestimmung biogener Amine aus Schlachtgeflügel wird ein einfach durchzuführendes Analysenverfahren beschrieben. Die Amine wurden mit 0,6m-Perchlorsäure extrahiert, nach Dansylierung auf einer RP-18 Säule aufgetrennt und bei 254 nm detektiert. Innerhalb von 7 min konnte eine gute Auftrennung von 8 Aminen erreicht werden. Zur quantitativen Erfassung von Spermin und Spermidin mußte der Extrakt zusätzlich über Amberlite CG 50 gereinigt werden. Die Wiederfindungsraten betrugen 82% bis 96%, die Nachweisgrenzen 0,2 bis 0,5 g pro g Hühnerhaut. Bei Schlachtgeflügel eignen sich Putrescin und Cadaverin gut als Leitsubstanzen für den einsetzenden Verderb, da sie bereits ab Gesamtkoloniezahlen von etwa 105 pro cm2 nachweisbar sind, und ihre Konzentration mit zunehmendem Verderb rasch ansteigen.
Determination of biogenic amines by RP-HPLC for monitoring microbial spoilage of poultry
Summary A simple high-performance liquid chromatographic (HPLC) analysis is described for the determination of biogenic amines in broiler carcasses. The clean-up procedure consists of an extraction with 0.6M-perchloric acid, formation of dansyl derivatives, separation by a RP-18 column and UV detection at 254 nm. Within 7 min eight amines could be separated. The quantitative determination of spermidine and spermine requires an additional ion-exchange clean-up with Amberlite CG 50 after the extraction. This procedure gives recoveries of 82%–96% with detection limits of 0.2–0.5 g/g of broiler skin. Putrescine and cadaverine are good indicators for the onset of spoilage of poultry carcasses, since both amines could be detected at total colony counts of 105 cfu/cm2 and their concentration increases rapidly with advancing decomposition.相似文献
94.
The problem of extracting point spread functions from detector aperture functions in high-resolution PET is addressed. In the limit of very small size detectors relative to the ring dimensions, assumptions are made that lead to a fast and simple computation model yielding point spread functions with negligible errors due to the reconstruction algorithm. The methods allows one to assess accurately the intrinsic performance of a PET tomograph, and it appears to be adequate to relate the imaging capabilities in every point of the camera reconstruction field to the geometric and physical characteristics of the detection system. The method was developed as an investigation tool to help design the next generation of very-high-resolution PET tomographs. 相似文献
95.
R. Sam Williams Steve Lacher Corey Halpin Christopher White 《Materials and Structures》1988,21(4):313-320
RILEM Bulletin
Fourth International Conference on Durability of Building Materials and Components 相似文献96.
We measure the refractive index of thin films of TiO2 and SiO2 for given deposition parameters. Two complementary methods are used. The first is a postdeposition technique which uses the measurements of reflectance and transmittance in air. The second, in contrast, makes use of in situ measurements (under vacuum and during the actual deposition of the layer). The differences between the values deduced from the two methods can be explained by the amount of atmospheric moisture adsorbed by films. One tries to minimize these shifts for the two materials by choosing deposition parameters. The difficulties come from the absorption losses which must be as small as possible. We use the measured refractive indices of individual layers to give good numerical prediction of the wavelength shift (observed during the admittance of air after deposition in the vacuum chamber) of the transmittance peak of multidielectric Fabry-Perot filters. 相似文献
97.
98.
Christopher B. Wilson 《Theory of Computing Systems》1987,20(1):13-29
This paper introduces a notion of relativized depth for circuit families and discusses issues regarding uniform families of relativized circuits. This allows us to define a version of relativizedNC and compare it under various oracles with relativizedL, NL, andP. We see thatNC
1 is properly contained inL if and only if there exists an oracleA such thatNC
1
A
is properly contained inL
A
. There is an oracleA where the hierarchy collapses,NC
1
A
= NC
A
, and another whereNC
1
A
NC
2
A
NC
A
P
A
. We then construct anA so that, for anyk, NC
1
A
contains a set not inNSPACE
A
(O(n
k
)), suggesting that the notion of relativized space is too weak or that of relativized depth is too strong. 相似文献
99.
Miriama Simunkova Zuzana Barbierikova Klaudia Jomova Lenka Hudecova Peter Lauro Saleh H. Alwasel Ibrahim Alhazza Christopher J. Rhodes Marian Valko 《International journal of molecular sciences》2021,22(4)
Kaempferol is a flavonoid that occurs in tea and in many vegetables and fruits, including broccoli, cabbage, beans, grapes, apples, and strawberries. The efficacy of Kaempferol has been demonstrated in the treatment of breast, esophageal, cervical, ovarian, and liver cancers and leukemia, which very likely arises from its prooxidant properties and the activation of pro-apoptotic pathways. Indeed, this matter has already been the focus of a number of published studies and reviews. The aim of the present study was to elucidate the antioxidant vs. prooxidant properties of flavonoids in the presence of the redox-active metal, copper (II) ion, by means of the Fenton reaction. The specific motivation of this work is that, since an increased level of Cu(II) ions is known to be associated with many disease states such as neurological conditions (Alzheimer’s disease) and cancer, any interaction between these ions and flavonoids might affect the outcome of therapeutic uses of the latter. The structure of the Cu-kaempferol complex in DMSO was investigated by means of low temperature EPR spectroscopy, which confirmed the existence of at least two distinct coordination environments around the copper (II) ion. UV vis-spectra of kaempferol and its Cu(II) complex in DMSO revealed an interaction between the 5-OH (A ring) group and the 4-CO (C ring) group of kaempferol with Cu(II) ions. An ABTS assay confirmed that kaempferol acted as an effective radical scavenger, and that this effect was further enhanced in the form of the Cu(II)-kaempferol complex. Quantitative EPR spin trapping experiments, using DMPO as the spin trap, confirmed suppression of the formation of a mixture of hydroxyl, superoxide, and methyl radicals, in a Fenton reaction system, upon coordination of kaempferol to the redox-active Cu(II) ions, by 80% with respect to the free Cu(II) ions. A viscometric study revealed a better DNA-intercalating ability of the Cu-kaempferol complex than for free kaempferol, essential for conferring anticancer activity of these substances. The results of the viscometric measurements were compared with those from a DNA damage study of Cu-kaempferol complexes in a Fenton reaction system, using gel electrophoresis. At low concentrations of kaempferol (Cu–kaempferol ratios of 1:1 and 1:2), a very weak protective effect on DNA was noted, whereas when kaempferol was present in excess, a significant DNA-protective effect was found. This can be explained if the weakly intercalated kaempferol molecules present at the surface of DNA provide protection against attack by ROS that originate from the Fenton reaction involving intercalated Cu(II)-kaempferol complexes. Following the application of ROS scavengers, L-histidine, DMSO, and SOD, gel electrophoresis confirmed the formation of singlet oxygen, hydroxyl radicals, and superoxide radical anions, respectively. We propose that the prooxidant properties of Cu-kaempferol complexes may provide anticancer activity of these substances. When present in excess, kaempferol displays antioxidant properties under Cu-Fenton conditions. This suggests that kaempferol might prove a suitable candidate for the prevention or treatment of oxidative stress related medical conditions that involve a disturbed metabolism of redox metals such as copper, for example, Menkes disease, and neurological disorders, including Alzheimer’s disease. For the potential use of kaempferol in clinical practice, it will be necessary to optimize the dose size and critical age of the patient so that this flavonoid may be beneficial as a preventive drug against cancer and neurological disorders. 相似文献
100.
Evran E. Ural Victoria Toomajian Ehsanul Hoque Apu Mladen Veletic Ilangko Balasingham Nureddin Ashammakhi Masamitsu Kanada Christopher H. Contag 《International journal of molecular sciences》2021,22(9)
Extracellular vesicles (EVs) are cell-derived nanostructures that mediate intercellular communication by delivering complex signals in normal tissues and cancer. The cellular coordination required for tumor development and maintenance is mediated, in part, through EV transport of molecular cargo to resident and distant cells. Most studies on EV-mediated signaling have been performed in two-dimensional (2D) monolayer cell cultures, largely because of their simplicity and high-throughput screening capacity. Three-dimensional (3D) cell cultures can be used to study cell-to-cell and cell-to-matrix interactions, enabling the study of EV-mediated cellular communication. 3D cultures may best model the role of EVs in formation of the tumor microenvironment (TME) and cancer cell-stromal interactions that sustain tumor growth. In this review, we discuss EV biology in 3D culture correlates of the TME. This includes EV communication between cell types of the TME, differences in EV biogenesis and signaling associated with differing scaffold choices and in scaffold-free 3D cultures and cultivation of the premetastatic niche. An understanding of EV biogenesis and signaling within a 3D TME will improve culture correlates of oncogenesis, enable molecular control of the TME and aid development of drug delivery tools based on EV-mediated signaling. 相似文献