Loading...
19 results
Search Results
Now showing 1 - 10 of 19
- 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, José; Rodriguez, Jose; Cañadas, Inmaculada; Fernandes, Jorge Cruz; Rosa, Luís 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; Magalhães, Teresa; Oliveira, Fernando Almeida Costa; Rosa, Luís Guerra; Fernandes, Jorge Cruz; Rodriguez, Jose; Cañadas, 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 ¼ 0:35 specimen, dominant metallicWassociated with small proportion ofW2C for the C/W ¼ 0:50 and dominantW2C with trivial proportion of WC for the C/W ¼ 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 ¼ 0:35, pureW2C for the C/W ¼ 0:50 and predominant WC with traceW2C for the C/W ¼ 1:00. Unlike for the top surface of the C/W ¼ 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, Luís Guerra; Cañadas, InmaculadaConsolidation of cordierite disc specimens was undertaken under concentrated solar beam in a solar furnace at PSA (Plataforma Solar de Almerıá). 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 ¼ 2:45 0:02 g/cm3, Vickers microhardness HV ¼ 7:31 0:29 GPa, Young’s modulus E ¼ 97 5 GPa, shear modulus G ¼ 38 2 GPa, Poisson ratio ¼ 0:27 0:01, fracture toughness KIC ¼ 1:50 0:15 MPa m1=2 and modulus of rupture evaluated by ring-on-ring test MORROR ¼ 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 Almería)Publication . Oliveira, Fernando Almeida Costa; Rosa, Luís Guerra; Fernandes, Jorge Cruz; Rodriguez, Jose; Cañadas, Inmaculada; Magalhães, 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 °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 °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 (˜167 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.
- Homogeneização do fluxo em concentradores solares de elevada intensidadePublication . Pereira, J.C.G.; Fernandes, Jorge Cruz; Rodríguez, J.; Rosa, Luís GuerraRESUMO: Quer as medições experimentais realizadas com radiómetros, quer as simulações ópticas já efectuadas, evidenciam a dificuldade de obtenção de fluxos homogéneos de radiação solar, nos atuais fornos solares de elevada concentração. Apesar de se recorrer a dispositivos ópticos denominados homogeneizadores do fluxo da radiação, os atuais fornos solares não conseguem contornar satisfatoriamente este problema. Neste trabalho propõe-se uma solução baseada na utilização de concentradores solares com dois parabolóides, um grande e um pequeno, com o mesmo ponto focal. De acordo com as nossas simulações de óptica geométrica, a dupla reflexão pelos dois parabolóides permite obter radiação homogénea numa região central circular com dimensões adequadas.
- Nano-meter size WC whiskers grown over a compacted pellet of praphite: tungsten powder mixture heated with an ultra-fast heating rate by a concentrated solar beamPublication . Dias, Susana; Oliveira, Fernando Almeida Costa; Granier, Bernard; Badie, Jean Marie; Fernandes, Jorge Cruz; Rosa, Luís Guerra; Shohoji, NobumitsuIn equilibrium binary W-C system, mono-carbide WC is acknowledged as the stable phase under presence of excess free carbon up to temperature 2700 C whereas sub-carbideW2C would form between 1250 C and 2700 C under the carbon-deficiency condition. In unique setup of solar furnace at PROMES-CNRS in Odeillo (France), temperature of specimen is raised from the ambient temperature to target temperature up to 2000 C within fractions of a second. In the recent experimental attempts of W2C phase synthesis using this unique experimental facility starting from compacted pellet consisted of graphite and tungsten powders at ratios smaller than 0.50, we detected growth of nano-meter size WC whisker at the top surface directly exposed to the concentrated solar beam. The presence of WC was confirmed also by X-ray diffraction (XRD) of the top surface but, when the specimen as a whole was subjected to powder XRD analysis, WC became indiscernible being masked by principal W2C phase. Mechanism of formation of the detected WC nano-whisker over sub-stoichiometric C/W pellet during ultra-fast heating by concentrated solar beam is discussed.
- 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, José; Rodriguez, Jose; Cañadas, Inmaculada; Fernandes, Jorge CruzABSTRACT: Volumetric ceramic receivers can be regarded as a promising technology to heat air above 1000 ◦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 Almería. The foams were subjected to well-controlled temperature cycles ranging from 800 to 1000, 1200, 1300 or 1400 ◦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 ◦C) and subjected to 150 cycles.
- Fracture behaviour of a new submicron grained cemented carbidePublication . Oliveira, Fernando Almeida Costa; Lopes, A. C.; Fernandes, Jorge Cruz; Sacramento, J.; Valente, M. A.In this work, the effect of increase argon pressure applied on a post-sintering treatment from MPa to MPa on the mechanical properties of a newly developed hardmetal grade, namely hardness, flexural strength and fracture toughness, is reported. The as-received material has been prviously sintered at 1460ºC under 2 MPa argon pressure from powder mixtures of WC and 3,5 wt% Co together with minor additions of VC, graphite and a pressing lubricant. By increasing the argon pressure, a significant increase in flexural strength from ≃ 1500 MP to ≃ 3000 MPa was observed, whist hadness (HV30≃2000) and fracture toughness (≃8 MN.m⁻3/2) remained practically unchanged. Both microstructural defects (namely, pore and metallic inclusions). Furthermore, fracture has been found to occur mainly by intrinsic (bulk) defects rather than surface-related ones, suggesting that surface finishing did not affect flexural strength measurements. Hot isostatic pressing has been successfully used to consolidate WC-3,5 wt% Co composites with submicron Wc grains size confirming that porosity reduction results in flexural strength improvement.
- Influence of linear flow velocity of uncracked ammonia (NH3) gas on formation of higher nitrides, 𝛅-MoN and 𝛆-Fe2N, under concentrated solar irradiation in the SF40 solar furnace at PSAPublication . Shohoji, Nobumitsu; Oliveira, Fernando Almeida Costa; Galindo, José; Fernandes, Jorge Cruz; Rodríguez, José; Cañadas, Inmaculada; Rosa, Luís GuerraABSTRACT: 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 NH3 gas (NH3 gas with suppressed extent of dissociation by flowing) aiming at determining the range of linear velocity v of NH3 gas flow to yield higher nitride phases, δ-MoN for Mo and ε-Fe2N 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-Mo2N and metallic Mo for Mo powder specimen heated to 900 ºC in NH3 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 NH3 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 NH3 gas tended to decrease along the NH3 gas flow path on going from the up-stream side to the down-stream side.
- Portland cement clinker production using concentrated solar energy : a proof-of-concept approachPublication . Oliveira, Fernando Almeida Costa; Fernandes, Jorge Cruz; Galindo, José; Rodríguez, José; Cañadas, Inmaculada; Vermelhudo, V.; Nunes, A.; Rosa, Luís GuerraABSTRACT: The aim of this work was to demonstrate the feasibility of producing Portland cement clinker upon direct exposure of the raw materials under concentrated solar radiation using the PSA high concentration solar furnace SF40. For this purpose, a short thermal cycle ( < 40 min) was devised including 5 min dwell times at temperatures in the range 900-950 degrees C and 1250-1300 degrees C, followed by 10-15 min at 1500-1550 degrees C. The chemical and mineralogical data of the grey clinker produced are encouraging since values of 51.0 +/- 6.9% C3S, 22.7 +/- 5.3% C2S, 8.6 +/- 0.4% C(3)A and 10.8 +/- 0.7% C(4)AF are similar to those observed for conventional clinker used for the production of Portland cement in accordance to EN 197-1 standard. White clinker, in turn, could not be produced by direct irradiation in this setup conditions because of its low absorptance of solar energy.