585,185 research outputs found
Microwave-Assisted Synthesis and Evaluation of Antimicrobial Activity of 3-{3-(s-Aryl and s-Heteroaromatic)acryloyl}-2Hchromen-2-one Derivatives
The exploration of potential utilization of microwaves as an energy source for heterocyclic synthesis was herein investigated using condensation of 3-acetylcoumarin (1) with aromatic and heteroaromatic aldehydes to afford the corresponding aromatic chalcones (2a–j) and heteroaromatic chalcones (3a–e and 4a–e), respectively, in good to excellent yield within 1–3 min. The chemical structures were confirmed by analytical and spectral data. All the synthesized compounds were screened for their antibacterial
activity and 3-{3-(4-dimethylaminophenyl)acryloyl}-2H-chromen-2-one (2i) was discovered to be the most active at minimum inhibitory concentration (MIC) value of 7.8 µg/m
Microwave Lens for Polar Molecules
We here report on the implementation of a microwave lens for neutral polar
molecules suitable to focus molecules both in low-field-seeking and in
high-field-seeking states. By using the TE_11m modes of a 12 cm long
cylindrically symmetric microwave resonator, Stark-decelerated ammonia
molecules are transversally confined. We investigate the focusing properties of
this microwave lens as a function of the molecules' velocity, the detuning of
the microwave frequency from the molecular resonance frequency, and the
microwave power. Such a microwave lens can be seen as a first important step
towards further microwave devices, such as decelerators and traps.Comment: 4 pages, 3 figure
Near-Field Microwave Microscopy of Materials Properties
Near-field microwave microscopy has created the opportunity for a new class
of electrodynamics experiments of materials. Freed from the constraints of
traditional microwave optics, experiments can be carried out at high spatial
resolution over a broad frequency range. In addition, the measurements can be
done quantitatively so that images of microwave materials properties can be
created. We review the five major types of near-field microwave microscopes and
discuss our own form of microscopy in detail. Quantitative images of microwave
sheet resistance, dielectric constant, and dielectric tunability are presented
and discussed. Future prospects for near-field measurements of microwave
electrodynamic properties are also presented.Comment: 31 pages, 9 figures, lecture given at the 1999 NATO ASI on Microwave
Superconductivity Changes suggested by editor, including full reference
Atomic resolution imaging at 2.5 GHz using near-field microwave microscopy
Atomic resolution imaging is demonstrated using a hybrid scanning
tunneling/near-field microwave microscope (microwave-STM). The microwave
channels of the microscope correspond to the resonant frequency and quality
factor of a coaxial microwave resonator, which is built in to the STM scan head
and coupled to the probe tip. We find that when the tip-sample distance is
within the tunneling regime, we obtain atomic resolution images using the
microwave channels of the microwave-STM. We attribute the atomic contrast in
the microwave channels to GHz frequency current through the tip-sample tunnel
junction. Images of the surfaces of HOPG and Au(111) are presented.Comment: 9 pages, 5 figures, submitted to Applied Physics Letter
Giant microwave photoresistivity in a high-mobility quantum Hall system
We report the observation of a remarkably strong microwave photoresistivity
effect in a high-mobility two-dimensional electron system subject to a weak
magnetic field and low temperature. The effect manifests itself as a giant
microwave-induced resistivity peak which, in contrast to microwave-induced
resistance oscillations, appears only near the second harmonic of the cyclotron
resonance and only at sufficiently high microwave frequencies. Appearing in the
regime linear in microwave intensity, the peak can be more than an order of
magnitude stronger than the microwave-induced resistance oscillations and
cannot be explained by existing theories.Comment: 4 pages, 4 figure
Microwave pyrolysis of oil palm fibres
Malaysia and Indonesia are generating millions of ton of oil palm fibres (OPF) from their oil palm mills as biomass solid wastes which needs proper waste utilization application. The main purpose of the present research was to pyrolyse the OPF biomass into bio-oil using microwave irradiation technique. A domestic microwave of 1000 W and 2.45 GHz frequency was modified to accommodate fluidized bed system. It was found that OPF showed poor microwave absorbing characteristics. Therefore, an appropriate microwave-absorbing material such as biomass char was added to initiate the pyrolysis process. Temperature profiles and bio-oil yield was investigated by varying the ratio of OPF to microwave absorber. It was found that the yield of bio-oil depended on the ratio of OPF to microwave absorber. Particular attention on the temperature profiles was also taken into account during microwave heating of OPF. It can be concluded that microwave technique can save significant time and energy through its rapid and volumetric heating characteristic
Fundamental issues in antenna design for microwave medical imaging applications
This paper surveys the development of microwave medical imaging and the fundamental challenges associated with microwave antennas design for medical imaging applications. Different microwave antennas used in medical imaging applications such as monopoles, bow-tie, vivaldi and pyramidal horn antennas are discussed. The challenges faced when the latter used in medical imaging environment are detailed. The paper provides the possible solutions for the challenges at hand and also provides insight into the modelling work which will help the microwave engineering community to understand the behaviour of the microwave antennas in coupling media
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