International Journal of Materials and Chemistry 2019, 9(1): 1-12 DOI: 10.5923/j.ijmc.20190901.01 Influence of Linear Flow Velocity of Uncracked Ammonia (NH 3 ) Gas on Formation of Higher Nitrides, -MoN and -Fe 2 N, under Concentrated Solar Irradiation in the SF40 Solar Furnace at PSA Nobumitsu Shohoji 1,* , Fernando Almeida Costa Oliveira 1 , José Galindo 2 , Jorge Cruz Fernandes 3 , José Rodríguez 2 , Inmaculada Cañadas 2 , Luís Guerra Rosa 3 1 LEN - Laboratório de Energia, LNEG - Laboratório Nacional de Energia e Geologia I.P., Lisboa, Portugal 2 PSA - Plataforma Solar de Almería, CIEMAT - Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Tabernas, Spain 3 IDMEC- Instituto de Engenharia Mecânica, DEM - Departamento de Engenharia Mecânica, IST - Instituto Superior Técnico, UL - Universidade de Lisboa, Lisboa, Portugal Abstract Nitriding experiments for powder specimens of Mo and Fe were carried out using a solar furnace SF40 at PSA (Plataforma Solar de Almería) in Tabernas (Spain) in uncracked ammonia NH 3 gas (NH 3 gas with suppressed extent of dissociation by flowing) aiming at determining the range of linear velocity v of NH 3 gas flow to yield higher nitride phases, δ-MoN for Mo and ε-Fe 2 N for Fe. Standard solar exposure duration at a specified reaction temperature T was set to be 60 min over range of v between 1.14 mm·s -1 and 11.4 mm·s -1 . By X-ray diffraction (XRD) analysis, presence of δ-MoN was detected besides γ-Mo 2 N and metallic Mo for Mo powder specimen heated to 900 ºC in NH 3 gas flow at v = 1.14 mm·s -1 but XRD peaks identifiable as δ-MoN became indiscernible when v was increased to 11.4 mm·s -1 . On the other hand, for Fe powder specimen exposed to NH 3 gas flow at v = 1.14 mm·s -1 at T = 500 ºC, remnant metallic α-Fe was detectable by XRD at the down-stream side of the specimen holder but no metallic α-Fe was detected at the up-stream side of the specimen holder suggesting that chemical activity a(N) of N atom in uncracked NH 3 gas tended to decrease along the NH 3 gas flow path on going from the up-stream side to the down-stream side. Keywords Uncracked ammonia (NH 3 ) gas, Concentrated solar energy, Temperature homogenizer, δ-MoN, ε-Fe 2 N 1. Introduction On account of unique properties of nitrides of transition metals, intensive research efforts have been invested for nitride synthesis. Besides high hardness of metal nitride MN, some MN's are claimed to possess special functional properties. For example, Inumaru et al. [1] investigated aspects of high-pressure synthesis of δ-MoN in view of δ-MoN being a superconducting material. On the other hand, ε-Fe 2 N appears to be considered as a promising candidate material for anode in lithium ion battery [2, 3]. Generally, synthesis of nitride MN through reaction of metal M in N 2 gas atmosphere requires high temperature due to high chemical stability of N 2 gas molecules. * Corresponding author: nobumitsu.shohoji@lneg.pt (Nobumitsu Shohoji) Published online at http://journal.sapub.org/ijmc Copyright © 2019 The Author(s). Published by Scientific & Academic Publishing This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Synthesis of "higher" nitride such as δ-MoN of Mo cannot be realized in N 2 gas environment even at very high N 2 gas partial pressure p(N 2 ) of order of several hundreds of atm for prolonged reaction duration of several tens of hours [1, 4-16]. One of optional routes for synthesizing higher nitride of metal M by reaction at normal pressure is the use of uncracked NH 3 gas as a powerful nitriding medium [4, 5, 7, 8, 10-22]. Uncracked NH 3 is a stream of NH 3 gas whose degree α of dissociation is suppressed from the degree of dissociation α eq under equilibrium state in a closed reaction chamber NH 3 ⇄(1-α eq )NH 3 +(α eq /2)N 2 +(3α eq /2)H 2 (1) Inherently unstable nature of uncracked NH 3 gas leads to the following two characteristic features on its usage as a nitriding medium; <1> at a specified spot in the reactor set at temperature T, extent α of dissociation of uncracked NH 3 is determined as a function of its linear flow velocity v, <2> along the NH 3 gas flow path at any given T, α would