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- 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 RadiationPublication . Shohoji, Nobumitsu; Oliveira, Fernando Almeida Costa; Galindo, JosĆ©; Rodriguez, Jose; CaƱadas, Inmaculada; Fernandes, Jorge Cruz; Rosa, LuĆs GuerraABSTRACT: 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 degrees C. From V, sub-nitride V2N (gamma phase) and hypo-stoichiometric mono-nitride VN possessing fcc (face-centered cubic) crystal lattice structure (delta phase) were synthesized. On the other hand, in the reaction product from Nb and Ta, hexagonal mono-nitride phase with N/M atom ratio close to 1 (epsilon phase) was detected. The reaction duration was normalized to be 60 min. In a conventional industrial or laboratory electric furnace, the synthesis of mono-nitride phase with high degree of crystallinity that yield sharp XRD peaks for Va-group metal might take a quite long duration even at T exceeding 1000 degrees C. In contrast, mono-nitride phase MN of Va-group metal was synthesized for a relatively short duration of 60 min at T lower than 1000 degrees C being co-existed with lower nitride phases.
- Heterogeneity along the Height in Disc Specimens of Graphite/Tungsten Powder Mixtures with Sub-Stoichiometric Carbon Atom Ratios Heated by Concentrated Solar Beam to 1600 CPublication . Shohoji, Nobumitsu; MagalhĆ£es, Teresa; Oliveira, Fernando Almeida Costa; Rosa, LuĆs Guerra; Fernandes, Jorge Cruz; Rodriguez, Jose; CaƱadas, Inmaculada; Martinez, DiegoCompositional heterogeneity along the thickness of compacted disc specimens of graphite/tungsten powder mixtures with substoichiometric carbon atom ratios (0.35, 0.50 and 1.00) heated by concentrated solar beam to 1600 C was characterised by X-ray diffraction. Top surface of any examined test piece was consisted purely of mono-carbide WC while the bottom surface showed different constitution depending on the net initial C/W ratio of the test piece; almost pure metallicWfor the C/W Ā¼ 0:35 specimen, dominant metallicWassociated with small proportion ofW2C for the C/W Ā¼ 0:50 and dominantW2C with trivial proportion of WC for the C/W Ā¼ 1:0. In the intermediate zone between the top and the bottom layers, the constitution held virtually constant depending on the nominal C/W ratio of the starting material: co-existing metallicWandW2C for the C/W Ā¼ 0:35, pureW2C for the C/W Ā¼ 0:50 and predominant WC with traceW2C for the C/W Ā¼ 1:00. Unlike for the top surface of the C/W Ā¼ 0:35 test piece heated in a solar furnace to 1900 C reported earlier, no evidence of formation of nano-meter scale WC whisker was detected for the top surface in any sample heated to 1600 C in the present work.
- Mechanical properties of dense cordierite discs sintered by solar radiation heatingPublication . Oliveira, Fernando Almeida Costa; Shohoji, Nobumitsu; Martinez, Diego; Fernandes, Jorge Cruz; Rodriguez, Jose; Rosa, LuĆs Guerra; CaƱadas, InmaculadaConsolidation of cordierite disc specimens was undertaken under concentrated solar beam in a solar furnace at PSA (Plataforma Solar de AlmerıƔ). Satisfactory extent of densification was achieved by the present solar-sintering experiment. The mechanical properties measured for the solar-sintered cordierite test pieces were; density Ā¼ 2:45 0:02 g/cm3, Vickers microhardness HV Ā¼ 7:31 0:29 GPa, Youngās modulus E Ā¼ 97 5 GPa, shear modulus G Ā¼ 38 2 GPa, Poisson ratio Ā¼ 0:27 0:01, fracture toughness KIC Ā¼ 1:50 0:15 MPa m1=2 and modulus of rupture evaluated by ring-on-ring test MORROR Ā¼ 57:8 13:7 MPa which were comparable to those of the counterparts sintered by conventional industrial gas furnace.
- Nitriding VI-group metals (Cr, Mo and W) in stream of NH3 gas under concentrated solar irradiation in a solar furnace at PSA (Plataforma Solar de AlmerĆa)Publication . Oliveira, Fernando Almeida Costa; Rosa, LuĆs Guerra; Fernandes, Jorge Cruz; Rodriguez, Jose; CaƱadas, Inmaculada; MagalhĆ£es, Teresa; Shohoji, NobumitsuCarbides and nitrides of d-group transition metals are classified as refractory hard material and their industrial importance has been recognized for long. In recent years, unique functionalities including catalytic function and superconductivity are discovered for this group of materials to raise serious attention of materials researchers and engineers to refractory carbides and nitrides as novel functional materials. Synthesis of refractory carbides and nitrides demands high temperature reaction route to consume considerable amount of electricity or gas in conventional industrial process. In view of saving cost of such conventional energy, feasibility of using concentrated solar beam as heat source for synthesizing carbide and nitride has been investigated by the authors since 1997. After verifying usefulness of concentrated solar beam as heat source for carbide forming reactions, similar attempts of employing concentrated solar beam as heat source for nitride synthesis were initiated recently. After brief experimental verification of nitride synthesis for IVa group metal, Ti, and Vg group metals, V, Nb and Ta, in N2 gas environment under irradiation with concentrated solar beam to 2000 Ā°C, the authors decided to undertake nitride synthesis of VIa group metals, Cr, Mo and W, as well as of Fe in stream of ammonia (NH3) gas as a nitriding medium under irradiation of concentrated solar beam at temperatures not exceeding 1000 Ā°C. NH3 gas with suppressed extent of dissociation by flowing is defined empirically as uncracked NH3 and it is proved to possess very high nitriding power to make synthesis of mono-nitride MoN of Mo coexisting with sub-nitride Mo and higher nitride Fe2N of Fe possible under normal pressure condition that are not possible when chemically stable N2 gas is used as a nitriding agent. VIa-group metals including Cr, Mo and W are known to be highly resistant against nitriding. In the present report, results of nitriding in flowing NH3 gas at a fixed flow rate 10 l/h (Ė167 ml/min) under heating with concentrated solar beam for VIa-group metals, Cr, Mo and W, are summarized to demonstrate favorable effect of solar beam heating towards further enhancement of nitriding power of flowing NH3 gas compared with the situation in conventional electric furnace in which visible light components except infra-red (IR) heat wave component are absent in the reaction system.
- Thermal Shock Resistance of Commercial Oxide-Bonded Silicon Carbide Reticulated Foams under Concentrated Solar Radiation at PSA: A Feasibility StudyPublication . Oliveira, Fernando Almeida Costa; Galindo, JosĆ©; Rodriguez, Jose; CaƱadas, Inmaculada; Fernandes, Jorge CruzABSTRACT: Volumetric ceramic receivers can be regarded as a promising technology to heat air above 1000 ā¦C for solar thermal electricity production. In this study, the thermal shock behavior of commercial 10 ppi (A) and 20 ppi (B) oxide-bonded silicon carbide (ob-SiC) reticulated porous ceramic (RPC) foams was evaluated using the SF60 solar furnace at Plataforma Solar de AlmerĆa. The foams were subjected to well-controlled temperature cycles ranging from 800 to 1000, 1200, 1300 or 1400 ā¦C, for 25, 100, and 150 cycles. The extent of the damage after thermal shock was determined by crushing tests. The damage was found to be critically dependent on both the bulk density and cell size. Decreasing both the bulk density and cell size resulted in better thermal shock resistance. The B foam exhibited approximately half the stress degradation compared to the A foam when exposed to a temperature difference of 600 K (in the range of 800 to 1400 ā¦C) and subjected to 150 cycles.
- Thermal decomposition of Ī“-MoN and Īµ-Fe2N synthesized under concentrated solar radiation in NH3 gas streamPublication . CaƱadas, Inmaculada; Oliveira, Fernando Almeida Costa; Rodriguez, Jose; Shohoji, NobumitsuABSTRACT: Decomposition temperatures of Ī“-MoN and Īµ-Fe2N synthesized with flowing NH3 gas under concentrated solar radiation heating were evaluated by Differential Scanning Calorimetry (DSC) in Argon (Ar) gas environment. The measured decomposition temperature of Ī“-MoN and Īµ-Fe2N were dependent on the solar synthesis conditions, particularly either NH3 or N2 gas flow rate at temperature. Sample containing Ī“-MoN showed two exothermic peaks around 680 and 900 ā¦C, attributed to the reactions of Ī“-phase into Ī³-single-phase and (Ī³+Ī²)-two-phase Mo2N, respectively, attributed to the dissociation reaction of Ī“-phase into Ī³-single phase and the dissociation reaction of Ī³-phase into metallic M saturated with N, respectively. Decomposition of Īµ-Fe2N took place into Ī³ā-Fe4N in two steps occurring at 606 and 660 ā¦C, respectively. When N2 instead of ammonia (NH3) gas was used, complete dissociation of Ī³ā-Fe4N into Fe took place at around 610 ā¦C. Full decomposition of Ī³ā-Fe4N into metallic Ī±-Fe(N) was corroborated by X-ray diffraction (XRD) analysis.
- Synthesizing higher nitride of molybdenum (Mo) and iron (Fe) in ammonia (NH3) gas stream under irradiation of concentrated solar beam in a solar furnacePublication . Shohoji, Nobumitsu; Oliveira, Fernando Almeida Costa; Fernandes, Jorge Cruz; Rosa, LuĆs Guerra; Rodriguez, Jose; CaƱadas, Inmaculada; Ramos, Carlos; MagalhĆ£es, Teresa; Cestari, F.Flowing gaseous ammonia NH3 with suppressed extent of dissociation (un-cracked NH3) is acknowledged to function as a powerful nitriding medium to realize formation of metal nitride MNx with considerably high N/M ratio x that cannot be achieved through reaction of M with N2 gas. For example, mono-nitride d-MoN of Mo and e-FeNx phase of Fe with x = 0.33 Ė 0.50 (i. e. hypo-stoichiometric sub-nitride e-Fe2N) were reported to be difficult to prepare in N2 gas environment even at elevated pressure but might be synthesized in flowing NH3 gas at normal pressure when reaction temperature and NH3 gas flow rate were set adequately. In the present work, nitriding experiments for Mo and Fe were carried out in flowing NH3 gas under irradiation with concentrated solar beam. The acquired experimental evidences demonstrated that temperature range for formation of d-MoN was somewhat extended in flowing NH3 gas under heating with concentrated solar beam compared with that under heating in conventional laboratory or industrial electric furnace. On the other hand, no such merit of extending temperature range for formation of e-Fe2N in flowing NH3 gas was detected in the present work under heating with concentrated solar beam.
- Synthesising carbo-nitrides of some d-group transition metals using a solar furnace at PSAPublication . Shohoji, Nobumitsu; Oliveira, Fernando Almeida Costa; Rosa, LuĆs Guerra; Fernandes, Jorge Cruz; MagalhĆ£es, Teresa; Coelho, Manuel Caldeira; Rodriguez, Jose; CaƱadas, Inmaculada; Ramos, Carlos; Martinez, DiegoCarbo-nitride synthesis was undertaken using a solar furnace at PSA in flowing N2/Ar gas mixture under total pressure 1 atm and processing temperature T = 1600ĀŗC for some d-group transition elements (Ti; Zr, V, Nb, Mo, W) starting from 1.5G/M (graphite/metal powder mixture with mole ratio 1.5:1) compact to ensure co-presence of free carbon with the reaction product. Clear X-ray diffraction (XRD) evidence of formation of carbo-nitride was detected for Ti (IVa group metal) showing higher N content in the carbo-nitride synthesised in N2 gas environment at partial pressure p(N2) = 1 atm than that at p(N2) = 0.5 atm. For M = V and Nb (Va group metals), formation of mono-carbide MC single-phase was detected in the N2 environment showing no evidence of formation of carbo-nitride in spite of presence of N2 in the environment. For M = Mo and W (VIa group metals), formation of higher carbide, among several options of carbide phases, appeared to be promoted in the N2 gas environment although, like in cases with the Va group metals, no evidence of dissolution of N into the reaction product was detected. As such, at T = 1600ĀŗC in N2 gas environment up to p(N2) = 1 atm under concentrated solar beam, carbo-nitride formed from the 1.5G/M mixture only for IVa group metal (Ti) but not for Va and VIa group metals. Anyway, it seemed certain that N2 gas affected somehow the reaction path between G and M to yield the carbide phase for M = V, Nb, Mo and W.