全文获取类型
收费全文 | 4680篇 |
免费 | 303篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 95篇 |
综合类 | 22篇 |
化学工业 | 1134篇 |
金属工艺 | 88篇 |
机械仪表 | 74篇 |
建筑科学 | 348篇 |
矿业工程 | 20篇 |
能源动力 | 130篇 |
轻工业 | 332篇 |
水利工程 | 28篇 |
石油天然气 | 5篇 |
无线电 | 440篇 |
一般工业技术 | 880篇 |
冶金工业 | 545篇 |
原子能技术 | 39篇 |
自动化技术 | 807篇 |
出版年
2023年 | 59篇 |
2022年 | 135篇 |
2021年 | 164篇 |
2020年 | 123篇 |
2019年 | 102篇 |
2018年 | 137篇 |
2017年 | 122篇 |
2016年 | 170篇 |
2015年 | 169篇 |
2014年 | 204篇 |
2013年 | 286篇 |
2012年 | 271篇 |
2011年 | 340篇 |
2010年 | 276篇 |
2009年 | 235篇 |
2008年 | 236篇 |
2007年 | 238篇 |
2006年 | 187篇 |
2005年 | 173篇 |
2004年 | 130篇 |
2003年 | 105篇 |
2002年 | 111篇 |
2001年 | 70篇 |
2000年 | 65篇 |
1999年 | 79篇 |
1998年 | 134篇 |
1997年 | 105篇 |
1996年 | 85篇 |
1995年 | 67篇 |
1994年 | 43篇 |
1993年 | 37篇 |
1992年 | 26篇 |
1991年 | 24篇 |
1990年 | 23篇 |
1989年 | 28篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 14篇 |
1977年 | 13篇 |
1976年 | 23篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1970年 | 9篇 |
排序方式: 共有4987条查询结果,搜索用时 15 毫秒
41.
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
42.
Jan Haas Karen S. Frese Farbod Sedaghat-Hamedani Elham Kayvanpour Rewati Tappu Rouven Nietsch Oguz Firat Tugrul Michael Wisdom Carsten Dietrich Ali Amr Tanja Weis Torsten Niederdrnk Michael P. Murphy Thomas Krieg Marcus Drr Uwe Vlker Jens Fielitz Norbert Frey Stephan B. Felix Andreas Keller Hugo A. Katus Benjamin Meder 《International journal of molecular sciences》2021,22(4)
With more than 25 million people affected, heart failure (HF) is a global threat. As energy production pathways are known to play a pivotal role in HF, we sought here to identify key metabolic changes in ischemic- and non-ischemic HF by using a multi-OMICS approach. Serum metabolites and mRNAseq and epigenetic DNA methylation profiles were analyzed from blood and left ventricular heart biopsy specimens of the same individuals. In total we collected serum from n = 82 patients with Dilated Cardiomyopathy (DCM) and n = 51 controls in the screening stage. We identified several metabolites involved in glycolysis and citric acid cycle to be elevated up to 5.7-fold in DCM (p = 1.7 × 10−6). Interestingly, cardiac mRNA and epigenetic changes of genes encoding rate-limiting enzymes of these pathways could also be found and validated in our second stage of metabolite assessment in n = 52 DCM, n = 39 ischemic HF and n = 57 controls. In conclusion, we identified a new set of metabolomic biomarkers for HF. We were able to identify underlying biological cascades that potentially represent suitable intervention targets. 相似文献
43.
44.
Elvira S. Sandin Julica Folberth Helge Müller-Fielitz Claus U. Pietrzik Elisabeth Herold Thomas E. Willnow Paul T. Pfluger Ruben Nogueiras Vincent Prevot Thomas Krey Markus Schwaninger 《International journal of molecular sciences》2021,22(9)
The mechanisms underlying the transport of leptin into the brain are still largely unclear. While the leptin receptor has been implicated in the transport process, recent evidence has suggested an additional role of LRP2 (megalin). To evaluate the function of LRP2 for leptin transport across the blood-brain barrier (BBB), we developed a novel leptin-luciferase fusion protein (pLG), which stimulated leptin signaling and was transported in an in vitro BBB model based on porcine endothelial cells. The LRP inhibitor RAP did not affect leptin transport, arguing against a role of LRP2. In line with this, the selective deletion of LRP2 in brain endothelial cells and epithelial cells of the choroid plexus did not influence bodyweight, body composition, food intake, or energy expenditure of mice. These findings suggest that LRP2 at the BBB is not involved in the transport of leptin into the brain, nor in the development of obesity as has previously been described. 相似文献
45.
A novel technique for the preparation of secondary fatty amides 总被引:1,自引:0,他引:1
Alexander Bilyk Raymond G. Bistline Jr. George J. Piazza Stephen H. Feairheller Michael J. Haas 《Journal of the American Oil Chemists' Society》1992,69(5):488-491
A technique for the synthesis of monosubstituted fatty amides at low temperature and ambient pressure was developed. This
method involved the condensation of an amine with a triacylglycerol. The primary amine (ethyl,n-butyl,n-hexyl andn-octyl were tested) acted as reagent and solvent for the fatty substrates. No additional organic solvent or catalyst was added.
Tallow, vegetable oils and fish oil all served well as substrates, as did pure tripalmitin. The rate of amidation was dependent
upon temperature and the ratio of fat to amine. In a series of experiments conducted with tallow andn-butylamine at a fat:amine molar ratio of 1:16, amidation could be carried out at 20°C, producingn-butyltallowamide in 83% yield in 24 hr. When the fat:amine molar ratio was reduced to 1:8, and the temperature raised to
45°C, the amide yield was 87.6% in 24 hr. When the reaction was carried out at the boiling point ofn-butylamine (78°C) and at a fat:amine ratio of 1:8, the amide yield was 93.2% in 4 hr. The reaction progressed more rapidly
with higher molecular weight amines. The identity and purity of the amides was assessed by thin-layer chromatography and confirmed
by elemental analyses and infrared and C13 nuclear magnetic resonance spectroscopy. 相似文献
46.
Jet bundle formulation of infinite-dimensional port-Hamiltonian systems using differential operators
We consider infinite-dimensional port-Hamiltonian systems described on jet bundles. Based on a power balance relation we introduce the port-Hamiltonian system representation using differential operators regarding the structural mapping, the dissipation mapping and the input mapping. In contrast to the well-known representation on the basis of the underlying Stokes–Dirac structure our approach is not necessarily based on using energy-variables which leads to a different port-Hamiltonian representation of the analyzed partial differential equations. The presented constructions will be specialized to mechanical systems to which class also the presented examples belong. 相似文献
47.
S. A. G. R. Karunathilaka N. A. Hampson T. P. Haas R. Leek T. J. Sinclair 《Journal of Applied Electrochemistry》1981,11(5):573-582
The impedances of small (2400 mA h) alkaline Zn-HgO cells have been measured in the range 10 kHz-0.001 Hz at various states of charge from fully charged to fully discharged. The behaviour of the cell conforms to that expected for rate control by charge transfer at the zinc electrode and diffusion in solution. At low frequencies there is a relaxation in the diffusive circuit elements which ultimately results in a complete suppression of the capacitative component of the impedance at zero frequency. The low-frequency behaviour is analogous to convective diffusion and is due to the effective distance between the electrodes being small compared with the characteristic length (D/)1/2. The magnitude of the charge transfer resistance is the best measure of the state of charge.Nomenclature
a
effective electrode separation
-
C
DL
double-layer capacitance of cell
-
C
R
capacitative component of cell impedance
- C
concentration difference
-
D
percentage discharge in Equation 12
-
D
i
diffusion coefficient of speciesi
-
R
ohmic resistance of cell
-
R
R
resistive component of cell Faradaic impedance
-
Ui
constant defined by Equation 10
-
Z
total cell impedance
-
Z
F
cell Faradaic impedance
-
Z
F
cell impedance modified for porosity effect
-
Z
x
cell impedance of Faradaic component plus double layer
-
cell Warburg coefficient (slope ofR
R and 1/C
Rversus su}-1/2)
-
C
Warburg coefficient calculated fromC
r values
-
i
cell Warburg coefficient for speciesi
-
Warburg coefficient calculated fromR
R values
-
dihedral angle of tail of Sluyters plot (after coming-off high-frequency semicircle)
-
angular frequency 相似文献
48.
Markus Nuopponen 《Polymer》2004,45(11):3643-3650
Series of amphiphilic diblock copolymers with poly(N-isopropylacrylamide) as a hydrophilic block and a hydrophobic block consisting of either polystyrene or poly(tert-butyl methacrylate) were synthesised using RAFT polymerisations. Differential scanning calorimetry showed the chemically different blocks being phase separated in dry polymers. Light scattering and microcalorimetry studies were performed on aqueous solutions to investigate the phase behavior of the diblock copolymers. By carefully transferring the polymers from an organic solvent to water, either micellar particles or large aggregates were obtained depending on the relative lengths of the blocks. Large aggregates collapsed upon heating, whereas collapse occurred slowly within a broad temperature range in the case of micelle like structures. However, microcalorimetrically the collapse of the PNIPAM chains was observed to take place in all samples, suggesting that the shells of the micellar particles are crowded in a way which hinders the compression of the poly(N-isopropylacrylamide) chains. 相似文献
49.
Summary Novel organic nanoparticles functionalized with nucleophilic polypropyleneoxide (PPO) chains on their surfaces for supporting metallocene catalysts in heterogeneous olefin polymerization are presented. The nanoparticles (60–100 nm) were obtained by miniemulsion polymerization of styrene, divinylbenzene and PPO functionalized styrene. It is demonstrated that Me2Si(2MeBenzlnd)2ZrCl2/MAO supported on these nanoparticles is suitable for the homopolymerization of ethylene, resulting in excellent product morphologies and high activities. lt is shown that by varying the MAO/Zr ratios and Zr concentrations the activities and productivities of the catalysts as well as the qualities of the polyethylene products can be tuned. These new supported catalysts are also suitable for the copolymerization of ethylene with several comonomers (1-hexene, 1-octene, 1-decene or norbornene). As the obtained product properties like crystallinity, melting temperature or bulk density match the results of silica supported systems, these organic nanoparticles can be considered as alternative carriers in comparison to the established inorganic ones. 相似文献
50.
We define extensions of the full branching-time temporal logic CTL? in which the path quantifiers are relativised by formal languages of infinite words, and consider its natural fragments obtained by extending the logics CTL and CTL+ in the same way. This yields a small and two-dimensional hierarchy of temporal logics parametrised by the class of languages used for the path restriction on one hand, and the use of temporal operators on the other. We motivate the study of such logics through two application scenarios: in abstraction and refinement they offer more precise means for the exclusion of spurious traces; and they may be useful in software synthesis where decidable logics without the finite model property are required. We study the relative expressive power of these logics as well as the complexities of their satisfiability and model-checking problems. 相似文献