Microwave-Driven Exsolution of Ni Nanoparticles in A-Site Deficient Perovskites.

Exsolution has emerged as a promising method for generating metallic nanoparticles, whose robustness and stability outperform those of more conventional deposition methods, such as impregnation. In general, exsolution involves the migration of transition metal cations, typically perovskites, under reducing conditions, leading to the nucleation of well-anchored metallic nanoparticles on the oxide surface with particular properties. There is growing interest in exploring alternative methods for exsolution that do not rely on high-temperature reduction via hydrogen. For example, utilizing electrochemical potentials or plasma technologies has shown promising results in terms of faster exsolution, leading to better dispersion of nanoparticles under milder conditions. To avoid limitations in scaling up exhibited by electrochemical cells and plasma-generation devices, we proposed a method based on pulsed microwave (MW) radiation to drive the exsolution of metallic nanoparticles. Here, we demonstrate the H2-free MW-driven exsolution of Ni nanoparticles from lanthanum strontium titanates, characterizing the mechanism that provides control over nanoparticle size and dispersion and enhanced catalytic activity and stability for CO2 hydrogenation. The presented method will enable the production of metallic nanoparticles with a high potential for scalability, requiring short exposure times and low temperatures.

Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization.

Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 °C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope.

The identification of the minerals composing rocks and their dielectric characterization is essential for the utilization of microwave energy in the rock industry. This paper describes the use of a near-field scanning microwave microscope with enhanced sensitivity for non-invasive measurements of permittivity maps of rock specimens at the micrometer scale in non-contact mode. The microwave system comprises a near-field probe, an in-house single-port vectorial reflectometer, and all circuitry and software needed to make a stand-alone, portable instrument. The relationship between the resonance parameters of the near-field probe and the dielectric properties of materials was determined by a combination of classical cavity perturbation theory and an image charge model. The accuracy of this approach was validated by a comparison study with reference materials. The device was employed to determine the permittivity maps of a couple of igneous rock specimens with low-loss and high-loss minerals. The dielectric results were correlated with the minerals comprising the samples and compared with the dielectric results reported in the literature, with excellent agreements.

Detection of Anti-Counterfeiting Markers through Permittivity Maps Using a Micrometer Scale near Field Scanning Microwave Microscope.

This paper describes the use of microwave technology to identify anti-counterfeiting markers on banknotes. The proposed method is based on a robust near-field scanning microwave microscope specially developed to measure permittivity maps of heterogeneous paper specimens at the micrometer scale. The equipment has a built-in vector network analyzer to measure the reflection response of a near-field coaxial probe, which makes it a standalone and portable device. A new approach employing the information of a displacement laser and the cavity perturbation technique was used to determine the relationship between the dielectric properties of the specimens and the resonance response of the probe, avoiding the use of distance-following techniques. The accuracy of the dielectric measurements was evaluated through a comparative study with other well-established cavity methods, revealing uncertainties lower than 5%, very similar to the accuracy reported by other more sophisticated setups. The device was employed to determine the dielectric map of a watermark on a 20 EUR banknote. In addition, the penetration capabilities of microwave energy allowed for the detection of the watermark when concealed behind dielectric or metallic layers. This work demonstrates the benefits of this microwave technique as a novel method for identifying anti-counterfeiting features, which opens new perspectives with which to develop optically opaque markers only traceable through this microwave technique.

In Situ Monitoring of Microwave Processing of Materials at High Temperatures through Dielectric Properties Measurement.

Microwave-assisted processes have recognized advantages over more conventional heating techniques. However, the effects on the materials' microstructure are still a matter of study, due to the complexity of the interaction between microwaves and matter, especially at high temperatures. Recently developed advanced microwave instrumentation allows the study of high temperature microwave heating processes in a way that was not possible before. In this paper, different materials and thermal processes induced by microwaves have been studied through the in situ characterization of their dielectric properties with temperature. This knowledge is crucial in several aspects: to analyze the effects of the microwave field on the reaction pathways; to design and optimize microwave-assisted processes, and to predict the behavior of materials leading to repeatable and reliable heating processes, etc.

Non-invasive and continuous monitoring of the sol-gel phase transition of supramolecular gels using a fast (open-ended coaxial) microwave sensor.

An open coaxial re-entrant microwave sensor has been used for the non-invasive and continuous monitoring of the sol-gel transition of physical gels characterized by different gelation mechanisms, solvents, compositions, and stabilities. Comparison of measurements by differential scanning calorimetry allowed the identification of the phase transition by a change in the dielectric properties of the material over time.