Materials

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13 pages, 5505 KiB

Open AccessArticle

Synthesis of Graphene-like Materials from Acetylene Black, Activated Carbon, and Ketjenblack via Separated Microwave Electric and Magnetic Field Heating

by Takeshi Miyata, Syun Gohda, Akio Oshita, Hironobu Ono and Keiichiro Kashimura

Materials 2023, 16(10), 3723; https://doi.org/10.3390/ma16103723 - 14 May 2023

Viewed by 1317

Abstract

Acetylene black, activated carbon, and Ketjenblack were subjected to microwave heating up to 1000 °C under N₂ atmosphere to rapidly convert them into graphene-like materials. Few carbon materials exhibit a favorable increase in the intensity of the G’ band with increasing temperature. [...] Read more.

Acetylene black, activated carbon, and Ketjenblack were subjected to microwave heating up to 1000 °C under N₂ atmosphere to rapidly convert them into graphene-like materials. Few carbon materials exhibit a favorable increase in the intensity of the G’ band with increasing temperature. Upon electric field heating of acetylene black to 1000 °C, the observed relative intensity ratios of D and G bands (or G’ and G band) were equivalent to those of reduced graphene oxide heated under identical conditions. In addition, microwave irradiation under different conditions, i.e., electric field or magnetic field heating, produced graphene of qualities different from those of the same carbon material conventionally treated at the same temperature. We propose that this difference arises from the different mesoscale temperature gradients. The conversion of inexpensive acetylene black and Ketjenblack into graphene-like materials within 2 min of microwave heating is a major achievement toward low-cost mass synthesis of graphene. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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13 pages, 3465 KiB

Open AccessArticle

Evaluation of Microwave Synthesis of Ceramic Pigments Based on In Situ Dielectric Characterization

by Beatriz García-Baños, Juan R. Sánchez, Jose L. Godes, Cristina Leonelli and Jose M. Catalá-Civera

Materials 2023, 16(8), 2976; https://doi.org/10.3390/ma16082976 - 8 Apr 2023

Cited by 5 | Viewed by 1230

Abstract

The application of microwave technology for efficient and environmentally friendly synthesis of ceramic pigments is a successful and rapidly evolving area of research. However, a clear understanding of the reactions and their relationship with the material absorbance has not been fully achieved. The [...] Read more.

The application of microwave technology for efficient and environmentally friendly synthesis of ceramic pigments is a successful and rapidly evolving area of research. However, a clear understanding of the reactions and their relationship with the material absorbance has not been fully achieved. The present study introduces an in situ permittivity characterization technique, which serves as an innovative and precise tool for assessing the microwave synthesis of ceramic pigments. Several processing parameters (atmosphere, the heating rate, raw mixture composition and particle size) were evaluated by studying the permittivity curves as a function of temperature to elucidate their effect on the synthesis temperature and the final pigment quality. The validity of the proposed approach was verified through correlation with other well-known analysis techniques, such as DSC or XRD, providing valuable information about the reaction mechanisms and the optimum conditions for the synthesis process. In particular, changes in permittivity curves were linked, for the first time, to undesired metal oxide reduction at too-high heating rates and could be used to detect pigment synthesis failures and ensure product quality. The proposed dielectric analysis was also found to be a useful tool for optimizing raw material composition for the microwave process, including the use of chromium with lower specific surface area and flux removal. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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14 pages, 4930 KiB

Open AccessArticle

An Easy Method to Determine the Effective Conductivity of Carbon Fiber Composites Using a Wall Perturbation Approach

by Moritz Engler, Guido Link and John Jelonnek

Materials 2023, 16(6), 2404; https://doi.org/10.3390/ma16062404 - 17 Mar 2023

Viewed by 1088

Abstract

Carbon-fiber-reinforced plastics (CFRPs) are of increasing popularity in a wide range of applications, and microwave curing promises significant reduction in processing times. However, for the design of an efficient microwave curing system, the composites’ effective material parameters must be known. This work presents [...] Read more.

Carbon-fiber-reinforced plastics (CFRPs) are of increasing popularity in a wide range of applications, and microwave curing promises significant reduction in processing times. However, for the design of an efficient microwave curing system, the composites’ effective material parameters must be known. This work presents a measurement system using a wall perturbation approach with a coaxial cavity to determine the effective conductivity of a CFRP along the fiber direction. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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18 pages, 8172 KiB

Open AccessArticle

Experimental Study of a Compact Microwave Applicator for Evaporation of Airflow-Entrained Droplets

by Jesus Nain Camacho Hernandez, Guido Link, Markus Schubert and Uwe Hampel

Materials 2022, 15(19), 6765; https://doi.org/10.3390/ma15196765 - 29 Sep 2022

Viewed by 1145

Abstract

In many energy and process engineering systems where fluids are processed, droplet-laden gas flows may occur. As droplets are often detrimental to the system’s operation, they need to be removed. Compact engineering solutions for the removal of entrained droplets are difficult to achieve [...] Read more.

In many energy and process engineering systems where fluids are processed, droplet-laden gas flows may occur. As droplets are often detrimental to the system’s operation, they need to be removed. Compact engineering solutions for the removal of entrained droplets are difficult to achieve with conventional flow control and heat transfer approaches and thus droplet removal devices are hence often costly and bulky. In this study, we analyzed the potential of a compact technology based on droplet capture and in situ evaporation by microwave heating. For that, we designed a microwave applicator containing a porous droplet separator for capturing and evaporating droplets. The application of open-cell ceramic foams as filter medium reduced 99.9% of the volumetric flow of droplets, while additional microwave exposure increases reduction to 99.99%. In addition, microwave-heated foams prevent droplet re-entrainment and structure-borne liquid accumulation within foams, thus avoiding water clogging and flooding. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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18 pages, 6148 KiB

Open AccessArticle

Reaction Kinetics and Process Model of the Polyacrylonitrile Fibers Stabilization Process Based on Dielectric Measurements

by Julia Hofele, Guido Link and John Jelonnek

Materials 2022, 15(3), 1222; https://doi.org/10.3390/ma15031222 - 6 Feb 2022

Cited by 4 | Viewed by 2284

Abstract

Microwave-based dielectric heating is a suitable method for energy- and time-efficient processes. Considering the energy required in the production of carbon fibers, it is evident that microwave-based dielectric heating during the different phases of the production needs to be considered too. Nevertheless, the [...] Read more.

Microwave-based dielectric heating is a suitable method for energy- and time-efficient processes. Considering the energy required in the production of carbon fibers, it is evident that microwave-based dielectric heating during the different phases of the production needs to be considered too. Nevertheless, the dielectric properties of the processed material needs to be known for the design of an appropriate microwave applicator. When looking at the first stage in the production, the stabilization stage of the PAN fiber, the important data about the dielectric properties is very limited in literature. For this reason, first in-situ temperature-dependent measurements of the dielectric properties during the stabilization stage are presented. The impact of raising temperatures and chemical reactions on the dielectric properties of the heated PAN fiber is discussed. Secondly, the steps taken to set up the reaction kinetics from the dielectric loss point of view are given. This enables determination of the reaction degree as a function of the measured dielectric loss for the first time. The established correlation opens the potential for the application to processes such as an in-situ quality determination. The strong temperature impact on the process is shown, and reaction kinetics are analyzed accordingly. In a final third step, a heat transfer model is presented. It utilizes the evaluated reaction kinetics data and microwave heating, creating a first modelling approach for monitoring and controlling the desired fiber temperature, leading towards an online process. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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18 pages, 5644 KiB

Open AccessArticle

Effects of Microwave Heating and Long-Term Aging on the Rheological and Chemical Properties of Recovered Bitumen

by Matías Fernández, Gustavo Canon, Sabine Leischner, Mrinali Rochlani, José Norambuena-Contreras and Alvaro González

Materials 2021, 14(24), 7787; https://doi.org/10.3390/ma14247787 - 16 Dec 2021

Cited by 5 | Viewed by 2110

Abstract

Microwave heating of asphalt pavement is a promising technique to reduce the maintenance and increase the service life of materials through self-healing of cracks. Previous studies have shown that microwave heating technology at high temperatures could damage the bitumen of asphalt mixture, which [...] Read more.

Microwave heating of asphalt pavement is a promising technique to reduce the maintenance and increase the service life of materials through self-healing of cracks. Previous studies have shown that microwave heating technology at high temperatures could damage the bitumen of asphalt mixture, which is an unwanted effect of the crack-healing technique. In this study, the effects of microwave heating and long-term aging on the rheological and chemical properties of recovered bitumen were quantified using a frequency sweep test and Fourier Transform Infrared Spectrometry analysis, respectively. The main results indicate that microwave heating has no significant effect on the aging performance of G* and

δ

for aged asphalt mixtures. However, for newer bitumens, the rheological properties G* and

δ

show minor changes after microwave heating was applied. Overall, this study confirms that microwave heating is a potential alternative for maintenance of asphalt pavements, without severely affecting the rheological and chemical properties of bitumen. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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13 pages, 7983 KiB

Open AccessArticle

Improvement of Heating Uniformity by Limiting the Absorption of Hot Areas in Microwave Processing of CFRP Composites

by Shengping Li, Yingguang Li, Jing Zhou and Youyi Wen

Materials 2021, 14(24), 7769; https://doi.org/10.3390/ma14247769 - 16 Dec 2021

Cited by 4 | Viewed by 1940

Abstract

Carbon fiber reinforced polymer (CFRP) composites are integral to today’s industries. Curing or consolidation are vital processes for manufacturing CFRP components. Microwave processing has many advantages compared with conventional processing technologies using ovens or autoclaves; however, the uneven temperature distribution caused by the [...] Read more.

Carbon fiber reinforced polymer (CFRP) composites are integral to today’s industries. Curing or consolidation are vital processes for manufacturing CFRP components. Microwave processing has many advantages compared with conventional processing technologies using ovens or autoclaves; however, the uneven temperature distribution caused by the non-uniform microwave field has a significant influence on the quality of the cured products. In this study, we propose a new idea to solve this problem, i.e., limiting the absorption of hot areas. Under such circumstances, cold ones can catch up with them more easily. To adjust the absorbing capability of the CFRP laminate, periodically arranged metallic resonance structures supported by a dielectric spacer are introduced on its surface. The dielectric spacer, made of epoxy matrix and strontium titanate particles, is designed to possess a dielectric constant positively related to temperatures. In this situation, the microwave absorption (2.45 GHz) of the metal-dielectric-CFRP configuration is changed from 97.6% at room temperature to 55.9% at 150 °C continuously. As a result, a reduction of 43.1% in maximum temperature difference and 89% in standard deviation has been achieved. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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24 pages, 10038 KiB

Open AccessArticle

Modeling of the Effective Permittivity of Open-Cell Ceramic Foams Inspired by Platonic Solids

by Jesus Nain Camacho Hernandez, Guido Link, Markus Schubert and Uwe Hampel

Materials 2021, 14(23), 7446; https://doi.org/10.3390/ma14237446 - 4 Dec 2021

Cited by 3 | Viewed by 2051

Abstract

Open-cell solid foams are rigid skeletons that are permeable to fluids, and they are used as direct heaters or thermal dissipaters in many industrial applications. Using susceptors, such as dielectric materials, for the skeleton and exposing them to microwaves is an efficient way [...] Read more.

Open-cell solid foams are rigid skeletons that are permeable to fluids, and they are used as direct heaters or thermal dissipaters in many industrial applications. Using susceptors, such as dielectric materials, for the skeleton and exposing them to microwaves is an efficient way of heating them. The heating performance depends on the permittivity of the skeleton. However, generating a rigorous description of the effective permittivity is challenging and requires an appropriate consideration of the complex skeletal foam morphology. In this study, we propose that Platonic solids act as building elements of the open-cell skeletal structures, which explains their effective permittivity. The new, simplistic geometrical relation thus derived is used along with electromagnetic wave propagation calculations of models that represent real foams to obtain a geometrical, parameter-free relation, which is based only on foam porosity and the material’s permittivity. The derived relation facilitates an efficient and reliable estimation of the effective permittivity of open-cell foams over a large range of porosity. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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13 pages, 2828 KiB

Open AccessArticle

The Effects of Absorbing Materials on the Homogeneity of Composite Heating by Microwave Radiation

by Betime Nuhiji, Matthew P. Bower, William A. E. Proud, Steven J. Burpo, Richard J. Day, Richard J. Scaife and Timothy Swait

Materials 2021, 14(23), 7362; https://doi.org/10.3390/ma14237362 - 30 Nov 2021

Cited by 1 | Viewed by 1515

Abstract

When cured in a microwave, flat thin composite panels can experience even heat distribution throughout the laminate. However, as load and geometric complexity increase, the electromagnetic field and resulting heat distribution is altered, making it difficult to cure the composite homogeneously. Materials that [...] Read more.

When cured in a microwave, flat thin composite panels can experience even heat distribution throughout the laminate. However, as load and geometric complexity increase, the electromagnetic field and resulting heat distribution is altered, making it difficult to cure the composite homogeneously. Materials that absorb and/or reflect incident electromagnetic radiation have the potential to influence how the field behaves, and therefore to tailor and improve the uniformity of heat distribution. In this study, an absorber was applied to a composite with non-uniform geometry to increase heating in the location which had previously been the coldest position, transforming it into the hottest. Although this result overshot the desired outcome of temperature uniformity, it shows the potential of absorbing materials to radically change the temperature distribution, demonstrating that with better regulation of the absorbing effect, a uniform temperature distribution is possible even in non-uniform composite geometries. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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14 pages, 5595 KiB

Open AccessArticle

Sol-Gel Synthesis of the Double Perovskite Sr₂FeMoO₆ by Microwave Technique

by Jesús Valdés, Daniel Reséndiz, Ángeles Cuán, Rufino Nava, Bertha Aguilar, Carlos M. Cortés-Romero and Oracio Navarro

Materials 2021, 14(14), 3876; https://doi.org/10.3390/ma14143876 - 12 Jul 2021

Cited by 4 | Viewed by 2475

Abstract

The effect of microwave radiation on the hydrothermal synthesis of the double perovskite Sr₂FeMoO₆ has been studied based on a comparison of the particle size and structural characteristics of products from both methods. A temperature, pressure, and pH condition screening [...] Read more.

The effect of microwave radiation on the hydrothermal synthesis of the double perovskite Sr₂FeMoO₆ has been studied based on a comparison of the particle size and structural characteristics of products from both methods. A temperature, pressure, and pH condition screening was performed, and the most representative results of these are herein presented and discussed. Radiation of microwaves in the hydrothermal synthesis method led to a decrease in crystallite size, which is an effect from the reaction temperature. The particle size ranged from 378 to 318 nm when pH was 4.5 and pressure was kept under 40 bars. According to X-ray diffraction (XRD) results coupled with the size-strain plot method, the product obtained by both synthesis methods (with and without microwave radiation) have similar crystal purity. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques showed that the morphology and the distribution of metal ions are uniform. The Curie temperature obtained by thermogravimetric analysis indicates that, in the presence of microwaves, the value was higher with respect to traditional synthesis from 335 K to 342.5 K. Consequently, microwave radiation enhances the diffusion and nucleation process of ionic precursors during the synthesis, which promotes a uniform heating in the reaction mixture leading to a reduction in the particle size, but keeping good crystallinity of the double perovskite. Precursor phases and the final purity of the Sr₂FeMoO₆ powder can be controlled via hydrothermal microwave heating on the first stages of the Sol-Gel method. Full article

(This article belongs to the Special Issue Advances in Microwave Processing of Materials)

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