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排序方式: 共有1146条查询结果,搜索用时 15 毫秒
1.
The crystallization process of hexagonal boron nitride in the presence of copper has been investigated. The positive effect of copper on the crystallinity of boron nitride was observed in the three studied systems of: nitrided boron, nitrided boron–carbon, and previously prepared turbostratic boron nitride. However, the presence of copper hindered the formation of boron carbonitride and produced graphite and boron nitride phases separately. Poor crystallinity was found as a conditio sine qua non for the existence of such a compound. Well-crystallized boron nitride had a very low spacing parameter 0.3328 nm and a regular hexagonal shape. 相似文献
2.
Apparent carbon aromaticities, fa, of some asphaltenes have been measured by n.m.r. in solution and compared with those obtained by the CP/MAS technique on solid samples. The aromaticities were also measured for several Alberta plains coals. From the 1H solution spectra and 13C solid nmr spectra of asphaltenes, some of their skeletal features, not obtainable from solution spectra only, have been suggested. 相似文献
3.
John P. Hilton ThaiHuu Nguyen Mihaela Barbu Renjun Pei Milan Stojanovic Qiao Lin 《Microfluidics and nanofluidics》2012,13(5):749-760
We present a bead-based approach to microfluidic polymerase chain reaction (PCR), enabling fluorescent detection and sample conditioning in a single microchamber. Bead-based PCR, while not extensively investigated in microchip format, has been used in a variety of bioanalytical applications in recent years. We leverage the ability of bead-based PCR to accumulate fluorescent labels following DNA amplification to explore a novel DNA detection scheme on a microchip. The microchip uses an integrated microheater and temperature sensor for rapid control of thermal cycling temperatures, while the sample is held in a microchamber fabricated from (poly)dimethylsiloxane and coated with Parylene. The effects of key bead-based PCR parameters, including annealing temperature and concentration of microbeads in the reaction mixture, are studied to achieve optimized device sensitivity and detection time. The device is capable of detecting a synthetically prepared section of the Bordetella pertussis genome in as few as 10 temperature cycles with times as short as 15?min. We then demonstrate the use of the procedure in an integrated device; capturing, amplifying, detecting, and purifying template DNA in a single microfluidic chamber. These results show that this method is an effective method of DNA detection which is easily integrated in a microfluidic device to perform additional steps such as sample pre-conditioning. 相似文献
4.
5.
The main aim of this work was to study the effects of particle size, particle volume fraction, and matrix strength on the
long fatigue crack growth resistance of two different grades of Al alloys (Al2124-T1 and Al6061-T1) reinforced with SiC particles.
Basically, it was found that an increase in particle volume fraction and particle size increases the fatigue crack growth
resistance at near threshold and Paris regimen, with matrix strength having a smaller effect. Near final failure, the stronger
and more brittle composites are affected more by static modes of failure as the applied maximum stress intensity factor (K
max) approaches mode I plane strain fracture toughness (K
IC). 相似文献
6.
This paper presents a decentralized observer with a consensus filter for the state observation of discrete-time linear distributed systems. Each agent in the distributed system has an observer with a model of the plant that utilizes the set of locally available measurements, which may not make the full plant state detectable. This lack of detectability is overcome by utilizing a consensus filter that blends the state estimate of each agent with its neighbors’ estimates. It is proven that the state estimates of the proposed observer exponentially converge to the actual plant states under arbitrarily changing, but connected, communication and pseudo-connected sensing graph topologies. Except these connectivity properties, full knowledge of the sensing and communication graphs is not needed at the design time. As a byproduct, we obtained a result on the location of eigenvalues, i.e., the spectrum, of the Laplacian for a family of graphs with self-loops. 相似文献
7.
The study of hydration of expansive cement prepared from 64% portland cement clinker, 23% metakaolinite and 13% CaSO4.2H2O is described. It was found that in the course of a 10-day hydration period, all the gypsum entered the reaction with the formation of ettringite. In 7–10 days, after the termination of the expansion processes, typical stalk-like crystals were transformed into leaf-shaped or other formations. Ettringite was identified even after 4 months of hydration. Monosulphate (3CaO.Al2O3.CaSO4.12H2O) was found in none of the investigated high-expansion cement paste samples. 相似文献
8.
We are investigating the bulk and interfacial chemistry of polyimide (PI) exposed to heat and relative humidity (RH) stress (85°C/85% RH) over long periods to understand the long-term reliability aspects of PI for microelectronic applications. The PI surface chemistry and the PI–Si interface chemistry was followed with electron spectroscopy for chemical analysis (ESCA). For exposure times greater than 550 h, the PI surface exhibits significant changes in emission structure and atomic concentration. The PI–Si interface indicates little change in the PI chemistry when compared to control samples. The water-contact angle at the PI surface shows no statistically significant change as a function of exposure time. Bulk and surface chemistry were followed with Fourier transform infrared spectroscopy (FTIR) in a grazing incidence reflection configuration. There were measurable differences in FTIR spectra between aged and control samples only for the case of very thin PI layers. These results suggest that for exposure times of less than 1100 h, chemical changes in PI occur primarily at the PI–air interface for samples 1 μm thick and greater. These changes represent a possible rearrangement of the imide structure and may be a form of deimidization of the polymer, but the final state is not polyamic acid. The PI surface chemistry changes observed with ESCA are reversible upon reheating to the cure temperature. 相似文献
9.
Prakorn Ramakul Milan Hronec Ura Pancharoen 《Korean Journal of Chemical Engineering》2007,24(2):282-287
−Purification of 2-mercaptobenzothiazole (91.9% purity) by solvent extraction was studied. The extraction of impurities present
in crude 2-MBT (e.g. benzothiazole, sulfides and sulfur containing compounds) was carried out at temperatures between 70-180°C
using various solvents and their mixtures of different polarity. The highest purity of 2-MBT, above 99% was obtained using
nitrobenzene, toluene and ethanol, even at a concentration of 2-MBT above 50 wt%. Increasing temperature and decreasing concentration
of the raw material have a positive influence on the purification process. A comparable efficiency of purification was observed
also with mixed solvents, (toluene with ethanol, acetone and aniline) possessing the same polarities. A correlation between
the polarity indexes (PI) of mixed solvents and experimentally obtained purity of 2-MBT was found. The highest purity of 2-MBT
provides extraction of the raw material with mixed solvents having PI 3.8-4.4. 相似文献
10.
Ugljesa Marjanovic Milan Delić Bojan Lalic 《Information Systems and E-Business Management》2016,14(2):253-272
This paper examines the success of an e-learning system in a company from the perspective of employees by using a multimethod approach. For this purpose Moodle learning management system was used. The success of e-learning as an information system was evaluated using four constructs of the updated DeLone and McLean IS success model—system quality, use, user satisfaction and net benefits, and adding one more construct—user performance. In this research a combination of observation and survey as two different research methods was used, which allowed the new measure to be incorporated into the model. Empirical assessment was carried out by exploratory factor analysis, confirmatory factor analysis and structural equation modeling. The research model was found to be valid and reliable. The results provide an expanded understanding of the constructs that measure the success of an e-learning system, helping to more deeply understand the key success dimensions and their interrelationships. The implications of our work were discussed. The DeLone and McLean IS success model applied equally well. However, the use of observation as a method of data collection revealed the weaknesses of the original model. 相似文献