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(V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas Under Concentrated Solar Radiation

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(V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas Under Concentrated Solar Radiation chemengineering Article Synthesis of Non-Cubic Nitride Phases of Va-Group Metals (V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas under Concentrated Solar Radiation Nobumitsu Shohoji 1, Fernando Almeida Costa Oliveira 1,* , José Galindo 2, José Rodríguez 2, Inmaculada Cañadas 2, Jorge Cruz Fernandes 3 and Luís Guerra Rosa 3,* 1 LNEG—Laboratório Nacional de Energia e Geologia I.P., LEN—Laboratório de Energia, UME—Unidade de Materiais para a Energia, Estrada do Paço do Lumiar 22, 1649-038 Lisboa, Portugal; [email protected] 2 CIEMAT—Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, PSA—Plataforma Solar de Almería, Apartado 22, E-04200 Tabernas, Almería, Spain; [email protected] (J.G.); [email protected] (J.R.); [email protected] (I.C.) 3 IDMEC—Instituto de Engenharia Mecânica, Instituto Superior Técnico, University of Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] * Correspondence: [email protected] (F.A.C.O.); [email protected] (L.G.R.) Abstract: Using a high-flux solar furnace, loosely compacted powders of Va-group transition metal (V, Nb, and Ta) were reacted with stream of NH3 gas (uncracked NH3 gas) being heated by concentrated ◦ solar beam to a temperature (T) range between 600 and 1000 C. From V, sub-nitride V2N(γ phase) Citation: Shohoji, N.; Oliveira, and hypo-stoichiometric mono-nitride VN possessing fcc (face-centered cubic) crystal lattice structure F.A.C.; Galindo, J.; Rodríguez, J.; (δ phase) were synthesized. On the other hand, in the reaction product from Nb and Ta, hexagonal Cañadas, I.; Fernandes, J.C.; Rosa, mono-nitride phase with N/M atom ratio close to 1 (" phase) was detected. The reaction duration L.G. Synthesis of Non-Cubic Nitride was normalized to be 60 min. In a conventional industrial or laboratory electric furnace, the synthesis Phases of Va-Group Metals (V, Nb, of mono-nitride phase with high degree of crystallinity that yield sharp XRD peaks for Va-group and Ta) from Metal Powders in metal might take a quite long duration even at T exceeding 1000 ◦C. In contrast, mono–nitride phase Stream of NH3 Gas under Concentrated Solar Radiation. MN of Va-group metal was synthesized for a relatively short duration of 60 min at T lower than ◦ ChemEngineering 2021, 5, 19. 1000 C being co-existed with lower nitride phases. https://doi.org/10.3390/ chemengineering5020019 Keywords: concentrated solar radiation; uncracked NH3 gas; nitride; vanadium; niobium; tantalum Academic Editor: Ilenia Rossetti Received: 1 March 2021 1. Introduction Accepted: 16 April 2021 Being inspired by research works of Hägg [1] and of Lehrer [2], both published in Published: 20 April 2021 1930, Yoshizawa et al. [3] in 1977 and Shohoji et al. [4] in 1990 used flowing NH3 gas (uncracked ammonia gas) as the nitriding medium in the nitriding process carried out in a Publisher’s Note: MDPI stays neutral conventional laboratory electric furnace. with regard to jurisdictional claims in In 1992, Katsura [5] published thermodynamic model analysis to rationally interpret published maps and institutional affil- the extremely high chemical activity a(N) of N in uncracked NH gas whose extent of iations. 3 dissociation α was suppressed from the equilibrium level αeq of dissociation in a closed reaction chamber. After 2012, Shohoji et al. [6], Oliveira et al. [7,8], and Fernandes et al. [9] started investigating the reaction between d-group transition metal with uncracked NH3 gas using Copyright: © 2021 by the authors. heat generated by concentrated solar radiation (instead of using conventional electric Licensee MDPI, Basel, Switzerland. furnace). In fact, solar-driven high-temperature technologies are serious candidates to This article is an open access article displace the use of electricity or natural gas in the future [10]. In these earlier works [6–9], distributed under the terms and a spherical glass reaction chamber was used to check for the reactivity of uncracked NH conditions of the Creative Commons 3 gas for an extensive range of compacted pellet specimens placed in the reactor. The use Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ of a compacted powder pellet was a compromising solution to carry out a test for the 4.0/). maximum number of specimen materials without the risk of cross-contamination in a ChemEngineering 2021, 5, 19. https://doi.org/10.3390/chemengineering5020019 https://www.mdpi.com/journal/chemengineering ChemEngineering 2021, 5, 19 2 of 11 limited solar beam time (which is normally the case in a solar furnace). However, in such a configuration with a spherical reactor chamber, the effect of increasing extent of dissociation (α) of flowing NH3 gas along the flow path from the upstream side to the downstream side was not evaluated. To overcome this problem, we developed a linear flow tube reactor and the first results acquired using this novel reactor setup were published in 2019 [11]. The experiment reported in [11] was done aiming at synthesizing higher nitrides, δ-MoN and "-Fe2N, respectively, of Mo and Fe in a stream of NH3 gas with linear flow speed (v) between 1.14 and 11.4 mm·s−1 under heating with concentrated solar beam using the solar furnace SF40 [12] at Plataforma Solar de Almería (PSA), Spain, at the processing temperatures T = 800 and 900 ◦C for tap-densified (loosely compacted) powders of Mo specimen and T = 500 and 600 ◦C for tap-densified powders of Fe. Specimen powders were placed in four drilled holes of 8 mm in diameter and a depth of 5 mm for rectangular specimen holder block made of refractory steel (ferritic steel with dimensions: length 67 mm, height 20 mm and width 30 mm). The four holes were held at equal distance being designated with P1, P2, P3, and P4 on going from upstream side to downstream side. The distance between the center of P1 and P4 was set to be 45 mm (see Figure 2 in [11]). The volume fraction of higher nitride was greater at P1 than at P4 in respective reaction condition corresponding to increasing extent α of NH3 gas dissociation along the NH3 flow direction, i.e., decreasing a(N) of uncracked NH3 gas along the NH3 flow direction. In this work, similar experiments were carried out for Va-group transition metals, V, Nb, and Ta, at v = 1.14 and 2.29 mm·s−1 for T = 600, 800, and 1000 ◦C. For the V specimen, hexagonal V2N phase and fcc VN phase were detected in the reaction products. On the other hand, for M = Nb and Ta specimens, fcc MN phase was not detected but hexagonal MN phase and hexagonal M2N phase were detected. Along this text, the empirical phase designation for the M-N system proposed by Brauer and Esselborn [13] is accepted in the following description: • "-MN: hexagonal MN; • δ-MN: fcc MN; • γ-M4N3: tetragonal MN with deficient N at around N/M = 0.75; • β-M2N: hexagonal sub-nitride phase; • α-M(N): primary solid solution of N in metallic M. As stated by Terao [14], a dissociation of NbN would proceed in the order of "-NbN ! δ-NbN ! γ-Nb4N3 according to descending a(N) in the gas environment. The existence of a γ-M4N3 phase was reported for M = Nb but not for M = V and Ta. It is quite likely that Nb4N3 corresponds to hypo-stoichiometric NbN whose fcc lattice is subjected to tetragonal distortion on account of the loss of N atoms in the interstitial site with certain regularity. Similar lattice distortion was identified for hypo-stoichiometric zirconium dihydride at around H/Zr ratio 1.72 through an analysis of the equilibrium pressure–temperature–composition (PTC) relationship [15]. Data acquired for nitride synthesis for V, Nb, and Ta in a stream of NH3 gas under irradiation of a concentrated solar beam are summarized in this report. 2. Experimental Metal powders of V, Nb, and Ta were subjected to nitridation reactions in the so- lar furnace SF40, with the nominal power of 40 kW, at PSA [12] in flowing NH3 gas (Figure1 ). Details of the experimental procedures are identical to those described in the paper [11] published in 2019; the only difference are the tap-densified powders (instead of Mo and Fe powders [11], powders of V, Nb, and Ta were subjected to nitridation reactions). At the focal spot of the concentrated solar beam, the radiative flux reaches 7000 Suns (1 Sun = 1 kW·m−2). The amount of the radiative flux was controlled by the angular mo- tion of the louvered shutter blades of the SF40 solar furnace. Heat flux measurements were carried out using a Gardon type radiometer ThermoGage TG1000-1 (Vatell Corporation, Christiansburg, Virginia, U.S.A.), with 0–6469 kW·m−2 range. Typically, at 1000 ◦C radiome- ter results were in the range of 320 kW·m−2, whereas at 600 ◦C the values obtained were of ChemEngineering 2021, 5, x FOR PEER REVIEW 3 of 11 ter blades of the SF40 solar furnace. Heat flux measurements were carried out using a Gar- ChemEngineering 2021, 5, 19 don type radiometer ThermoGage TG1000-1 (Vatell Corporation, Christiansburg,3 ofVirginia, 11 U.S.A.), with 0–6469 kW·m−2 range. Typically, at 1000 °C radiometer results were in the range of 320 kW·m−2, whereas at 600 °C the values obtained were of the order of 115 kW·m−2. More- over,the a order tilt mirror of 115 at kW 45°·m was−2. Moreover,used to perform a tilt mirror these at tests 45◦ wasin a usedhorizontal to perform plane, these which tests turned thein focal a horizontal beam to plane, 90°and which projected turned the the concentrated focal beam to solar 90◦and beam projected onto thethe concentratedspecimen holder.
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