Search results for "microwave"
showing 10 items of 693 documents
One pot microwave assisted synthesis of bisphosphonate alkene capped gold nanoparticles
2014
International audience; A new synthetic pathway for the direct synthesis of water soluble gold nanoparticles (GNPs) already possessing terminal alkene functional groups was developed. This is achieved by using synthesized (1-hydroxy-1-phosphonopent-4-enyl)phosphonic acid (HMBPene), presenting advantages of the well known bisphosphonate coating applied to colloidal gold instead of metal oxides. The proposed protocol allowed an accurate control of the particle size in the 13–20 nm diameter range with a high spherical uniformity, which is a crucial point for these colloids' properties. We have shown that it is possible to synthesize a functionalized nanoplatform in a one-pot one-phase process …
Recent Progress of Microwave-Assisted Synthesis of Silica Materials.
2020
Microwaves are a source of energy of great interest for chemical synthesis. Among nanomaterials, few are as versatile as silica—it forms mesoporous materials and nanoparticles, it can be incorporated as shells or loaded in composites, it can also be functionalized. Despite the relevant properties of silica, and the advantages of the use of microwave as energy source, its use in silica-based materials is not frequent. We report herein a compilation of the research results published in the last 10 years of microwave assisted synthesis of silica based materials. This review includes examples of mesoporous materials for waste removal, catalysis, drug release, and gas adsorption applications, to…
Microwave-Assisted Synthesis of Covalent OrganicFrameworks: A Review
2020
[EN] Covalent organic frameworks (COFs) are relatively recent materials. They have received great attention due to their interesting properties. However, the application of microwaves in their synthesis, despite its advantages such as faster and more reproducible processes, is a minority. Herein, a comprehensive compilation of the research results published in the microwave-assisted synthesis (MAS) of COFs is presented. This review includes articles of 2D and 3D COFs prepared using microwaves as source of energy. The articles have been classified depending on the functional groups including boronate ester, imines, enamines, azines, and triazines, among others. It compiles the main parameter…
Harmonic emission at microwave frequencies in YBa2Cu3O7 single crystals near Tc
1998
Abstract Nonlinear microwave emission by YBa 2 Cu 3 O 7 single crystals is investigated at temperatures close to T c . Second- and third-harmonic signals are studied as a function of the temperature and input power, for different values of the external magnetic field. The results are discussed in the framework of a phenomenological theory based on the two-fluid model which assumes that both the microwave and static fields perturb the partial concentrations of the normal and condensate fluids. It is suggested that, at temperatures very close to T c , this mechanism is the only one responsible for the harmonic emission by YBa 2 Cu 3 O 7 single crystals in the Meissner state. It is also effect…
Rapid nanocrystallization of soft-magnetic amorphous alloys using microwave induction heating
2009
The crystallization of Fe73Nb3Cu1Si16B7 alloy during microwave heating was investigated in situ using synchrotron radiation powder diffraction. The phase transformation comprises a primary nanocrystallization stage and a final microcrystallization step. We provide evidence for a strong enhancement of the transformation kinetics. Microwave heating occurs as a result of both ohmic and magnetic losses induced by eddy currents, which defines a volumetric microwave induction heating process. Nanocrystallization is completed within 5 s, while full crystallization is achieved in less than 10 s.
Zero-field magnetometry based on nitrogen-vacancy ensembles in diamond
2018
Ensembles of nitrogen-vacancy (NV) centers in diamonds are widely utilized for magnetometry, magnetic-field imaging and magnetic-resonance detection. They have not been used for magnetometry at zero ambient field because Zeeman sublevels lose first-order sensitivity to magnetic fields as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a 12C-enriched HPHT crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of 250 pT/Hz^(1/2). This technique opens the …
Synthesis of Nd:YAG nanopowder using the citrate method with microwave irradiation
2010
Abstract Nd:YAG nanopowders were prepared using the Pechini process with microwave irradiation ( MWs ). A reference sample was also prepared using conventional heating. XRD pattern analysis showed that nanopowder obtained by means of conventional heating and calcination for 1 h at 900 °C has a structure made up of the garnet phase together with the hexagonal phase that disappeared after two additional hours of thermal treatment. The MWs powder calcined for 1 h consists of the single garnet phase. SAXS data analysis indicated that nanoparticles are characterized by a sharp interface. TEM investigation showed crystalline particles with remarkable agglomeration in both samples, although a more…
Influence of mode stirrer and air renewal on controlled microwave drying of sliced zucchini
2017
An on-line temperature-controlled microwave (MW) system, based on infrared thermography, was developed for drying zucchini slices. Moreover, the effects of different configurations, including turntable rotation, mode stirrer and air renewal, alone or in combination, on drying kinetics and quality of dried product were evaluated. The combination of stirrer and air renewal reduced the drying time of zucchini slices by about 48% with respect to the basic configuration and by 22% with respect to air renewal only. The lowest oscillation of temperatures about the fixed level (65 °C) was found when operating with the stirrer mode, because of the improved uniformity of the MW distribution. The sa…
Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator
2014
Abstract We report on the microwave (mw) properties of a cylindrical MgB2 rod prepared by the reactive liquid Mg infiltration technology. The MgB2 rod, 94.3 mm long, is used as inner conductor of a coaxial cavity having a Cu tube as external conductor. By analyzing the resonance curves of the cavity in the different resonant modes and at different temperatures, we have determined the temperature dependence of the mw surface resistance, R s , of the MgB2 material, at fixed frequencies, and the frequency dependence of R s , at fixed temperatures. Our results show that the R s ( f ) curves follow a f n law, where n decreases on increasing the temperature, starting from n ≈ 2 , at T = 4.2 K , d…
Microwave characterization and modeling of packaged HEMTs by a direct extraction procedure at cryogenic temperatures
2004
In the present work we employ a direct extraction procedure to determine small signal equivalent circuit of microwave GaAs FETs by means of scattering (S-) parameter measurements down to cryogenic temperatures. The direct extraction procedure was tested on packaged AlGaAs/InGaAs HEMTs and good agreement between the simulated and measured S-parameters was obtained at different bias and temperature conditions. We employed a properly designed cryogenic set-up operating in our laboratory that allows to perform DC and RF characterization down to 30 K.