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- Multiscale Copper-uDiamond Nanostructured CompositesPublication . Nunes, D.; Livramento, Vanessa; Fernandes, H.; Silva, C.; Shohoji, Nobumitsu; Correia, J.B.; Carvalho, Patricia AlmeidaNanostructured copper-diamond composites can be tailored for thermal management applications at high temperature. A novel approach based on multiscale diamond dispersions is proposed for the production of this type of materials: a Cu-nDiamond composite produced by high-energy milling is used as a nanostructured matrix for further dispersion of micrometer sized diamond. The former offers strength and microstructural thermal stability while the latter provides high thermal conductivity. A series of Cu-nDiamond mixtures have been milled to define the minimum nanodiamond fraction suitable for matrix refinement and thermal stabilization. A refined matrix with homogenously dispersed nanoparticles could be obtained with 4 at.% nanodiamond for posterior mixture with ƒÝDiamond and subsequent consolidation. In order to define optimal processing parameters, consolidation by hot extrusion has been carried out for a Cu-nDiamond composite and, in parallel, for a mixture of pure copper and ÝDiamond. The materials produced were characterized by X-ray diffraction, scanning and transmission electron microscopy and microhardness measurements.
- 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.
- Effects of NaBH4 additions on hydrogen absorption by nanostuctured FeTi powdersPublication . Marques, Sofia Figueiredo; Correia, J.B.; Shohoji, Nobumitsu; Rangel, C. M.; Paiva Luís, TeresaHydrogen is nowadays considered as one of the most promising fuels for the future transportation market, since it is highly energetic and its combustion products are non-toxic. There are however some inherent problems related to its handling and storage that makes its implementation difficult in the energy market [1]. One way of storing hydrogen is in form of intermetallic hydrides. Some intermetallics can store large amounts of hydrogen in their interstitial sites and, in some cases, reversible equilibrium absorption/desorption cycles might be realized near ambient temperature and normal pressure. FeTi is an intermetallic compound that is being widely studied for hydrogen storage purposes. This system has one of the highest volumetric storage capacities and can be produced at low cost [2,3]. However, the FeTi alloy prepared through conventional metallurgical process requires activation treatments at elevated temperature. It has been shown previously that the nanostructured FeTi can be activated at room temperature with the mechanical alloying of pure metallic constituents, Fe and Ti, with NaBH4 [4]. In this work nanostructured FeTi based powders were produced by mechanical alloying, and the effects of adding different amounts of NaBH4 on the hydrogen absorption capacity and on the agglomeration of the powders were studied. The effect of handling powders in a glovebox with oxygen free atmosphere or in atmospheric ambient condition was also examined. Several parameters of the as-milled powders were controlled. Among the characterization performed are phase identification and crystallite size determinations by X-ray diffraction, micro hardness measurements, scanning electron microscopy and absorption isotherms determinations.
- Chemical Activities, a(H) and a(X), of Constituents in H2X Type Gas Molecules (X = O or S) at Arbitrary Degree of DissociationPublication . Shohoji, NobumitsuChemical activities, a(X) and a(H), of constituents, X and H, in H2X type gas molecules (X = S or O) were evaluated as functions of temperature T and extent α of dissociation adapting a thermodynamic analysis procedure developed by Katsura for interpreting enhanced a(N) and a(H) in NH3 gas molecules with suppressed α by flowing. Present analysis results showed that both H2S and H2O gas molecules are chemically rather inert even at comparatively low α unlike nitrogen-family tri-hydrides XH3 that were proved to yield high chemical activity of each constituent in a state being away from thermodynamic equilibrium. The parameter α referring to the extent of dissociation of HnX type gas molecules appears to be a significant parameter in evaluating the chemical activities, a(X) and a(H), in the HnX gas molecules that are remained non-dissociated.
- 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.
- Entropy terms in statistical thermodynamic analysis formula for non-stoichiometric interstitial compoundsPublication . Shohoji, NobumitsuABSTRACT: A series of statistical thermodynamic analyses were made since 1974 for different types of non-stoichiometric interstitial compounds MXx under simplifying a priori assumption of constant interaction energy E(X-X) between nearest neighbour interstitial atoms X within a homogeneity composition range of MXx at arbitrary temperature T [K]. Mode of distribution of X atoms in interstitial sites in MXx lattice is represented by number θ of available interstitial sites for occupation by X atoms per M atom and the value of θ is determined to fulfil the a priori assumption. Mode of atomic configuration would yield major contribution to entropy term ∆S that appears in conventional thermodynamic expression of Gibbs free energy of formation, ∆G, in form of T∆S. In the statistical thermodynamic formulation, contribution of tightly bound electron appearing in form of RT ln fX where fX refers to atomic partition function of X atom in the MXx lattice and R the universal gas constant. Judging from this mathematical form of the term, R ln fX is considered to represent entropic contribution from tightly bound electron to X atom in MXx lattice. In the published series of works on statistical thermodynamic analysis for non-stoichiometric interstitial compounds, calculated values for R ln fX were reported but they were not reviewed with serious attention because R ln fX was considered merely as a secondary factor compared to principal factor E(X-M) referring to interaction energy between X and M in MXx lattice that represents enthalpy ∆H in conventional thermodynamic term. In this review article, consideration is given exclusively to the factor R ln fX evaluated in statistical thermodynamic approach to non-stoichiometric interstitial compounds.
- Thermal decomposition of δ-MoN and ε-Fe2N synthesized under concentrated solar radiation in NH3 gas streamPublication . Cañadas, Inmaculada; Oliveira, Fernando Almeida Costa; Rodriguez, Jose; Shohoji, NobumitsuABSTRACT: Decomposition temperatures of δ-MoN and ε-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 δ-MoN and ε-Fe2N were dependent on the solar synthesis conditions, particularly either NH3 or N2 gas flow rate at temperature. Sample containing δ-MoN showed two exothermic peaks around 680 and 900 ◦C, attributed to the reactions of δ-phase into γ-single-phase and (γ+β)-two-phase Mo2N, respectively, attributed to the dissociation reaction of δ-phase into γ-single phase and the dissociation reaction of γ-phase into metallic M saturated with N, respectively. Decomposition of ε-Fe2N took place into γ’-Fe4N in two steps occurring at 606 and 660 ◦C, respectively. When N2 instead of ammonia (NH3) gas was used, complete dissociation of γ’-Fe4N into Fe took place at around 610 ◦C. Full decomposition of γ’-Fe4N into metallic α-Fe(N) was corroborated by X-ray diffraction (XRD) analysis.
- Low-temperature nitriding of VA-group metal powders (V,Nb,Ta) in flowing NH3 gas under heating with concentrated solar beam at PSAPublication . Fernandes, Jorge Cruz; Oliveira, Fernando Almeida Costa; Rosa, Luís Guerra; Rodríguez, José; Cañadas, Inmaculada; Magalhães, Teresa; Shohoji, NobumitsuOver the last two decades, we have been using concentrated solar beam as the reaction heat source for synthesizing carbides and nitrides of d-group transition elements in view of usage of ecological renewable energy source in place of conventional heat sources using electricity or gas. In recent works [1,2] nitriding of VIa-group metals (Cr, Mo, W) and Fe in stream of NH3 gas with suppressed extent of dissociation (uncracked NH3) was attempted under heating with concentrated solar beam. It was demonstrated that mono-nitride -MoN of Mo and sub-nitride -Fe2N of Fe that are known to be impossible to synthesize in N2 gas environment even at elevated pressure p(N2) were successfully synthesized by the reactions of these metals in stream of NH3 gas under heating with concentrated solar beam up to 800ºC. In the present work, nitriding of Va-group metals (V, Nb and Ta) was attempted in stream of NH3 gas under irradiation of concentrated solar beam. By up to 90 min heating in uncracked NH3 under concentrated solar beam up to 800ºC, reaction products were identified by X-ray diffraction (XRD) analysis to be consisted of mono-nitride MN co-existent with sub-nitride M2N.