materials Article The Effects Induced by Microwave Field upon Tungsten Wires of Different Diameters Marian Mogildea 1 , George Mogildea 1, * , Valentin Craciun 2,3 and Sorin I. Zgura 1   Citation: Mogildea, M.; Mogildea, G.; Craciun, V.; Zgura, S.I. The Effects Induced by Microwave Field upon Tungsten Wires of Different Diameters. Materials 2021, 14, 1036. https://doi.org/10.3390/ma14041036 Academic Editors: Antonio Politano, Giorgio Speranza and Jae Young Kim Received: 18 December 2020 Accepted: 18 February 2021 Published: 22 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institute of Space Science, MG-36, 077125 Magurele, Romania; marian_mogildea@spacescience.ro (M.M.); szgura@spacescience.ro (S.I.Z.) 2 National Institute for Laser, Plasma and Radiation Physics, Laser Department, 409 Atomistilor St., 077125 Magurele, Romania; valentin.craciun@inflpr.ro 3 Extreme Light Infrastructure for Nuclear Physics, ELI-NP, HH-IFIN, 077125 Magurele, Romania * Correspondence: george_mogildea@spacescience.ro Abstract: The effects induced by microwave field upon tungsten wires of different diameters were investigated. Tungsten wires with 0.5 and 1.0 mm diameters were placed in the focal point of a single-mode cylindrical cavity linked to a microwave generator and exposed to microwave field in ambient air. The experimental results showed that the 0.5 mm diameter wire was completely vaporized due to microwaves strong absorption, while the wire with 1 mm diameter was not ignited. During the interaction between microwaves and tungsten wire with 0.5 mm diameter, a plasma with a high electronic excitation temperature was obtained. The theoretical analysis of the experiment showed that the voltage generated by metallic wires in interaction with microwaves depended on their electric resistance in AC and the power of the microwave field. The physical parameters and dimension of the metallic wire play a crucial role in the ignition process of the plasma by the microwave field. This new and simple method to generate a high-temperature plasma from a metallic wire could have many applications, especially in metal oxides synthesis, metal coatings, or thin film deposition. Keywords: microwave field; plasma; wave guides; metallic wires; electric resistance 1. Introduction The propagation of electromagnetic radiation in different media has been a topic of great interest both for science and industry. It is known that the absorption and reflection processes of the microwaves strongly depend on the material properties. Gases and liquids can absorb the microwaves [1–3], while bulk metals reflect them [4]. In 2000, Whittaker et al. [5] showed that metal powders mixed in a liquid media and exposed to 600 W microwave these can generate electrical arcing, having as result the heating of the liquid through ohmic effect. In 2001, Chen et al. [6] showed that metals in powder form can become microwave absorbent. Using the TE 103 single-mode cavity coupled to a microwave source, metal powders were heated under a nitrogen atmosphere. To evaluate microwave absorption efficiency of the metallic powders, Mondal et al. [4] exposed copper powders with different particles size to a microwave field. In their experi- ment, the powders reached a temperature of 1200 ◦ C for 6 μm particles size, while metal powders with 383 μm particles size reached only 800 ◦ C. The temperature values of the metallic powders obtained in interaction with mi- crowaves were attributed to the skin depth (δ S ) value of the particles. In their work the authors highlighted that metallic particles with 6 μm particle size are better microwave absorbent. Other similar experiments were performed by Popescu S. et al. [7]. Using two metallic pieces (a titanium electrode and a titanium plate) brought into contact when irradiated in a microwave field, the authors managed to generate a plasma in atmospheric Materials 2021, 14, 1036. https://doi.org/10.3390/ma14041036 https://www.mdpi.com/journal/materials