Nobumitsu Shohoji

Weibull statistical analysis of flexure breaking performance for alumina ceramic disks sintered by solar radiation heating

Densified alumina ceramic disk specimens were prepared through sintering compacted alumina powders by concentrated solar beam in inert Ar gas atmosphere and their flexure breaking behaviour was evaluated by the ring-on-ring jig test under application of equibiaxial stress. Slope m of Weibull plot for MOR (modulus of rupture) values of these specimens was comparable to that of the reference alumina disk specimens prepared by sintering in an electric furnace under comparable conditions (1600°C for 30 min) whereas mean MOR level of the former was slightly lower than that of the latter. Thus, the failure mechanism of the specimens sintered by solar radiation heating and that of the specimens sintered in the electric furnace were concluded to be practically indistinguishable. This evidence guarantees that alumina ceramic sintered bodies, with quality comparable to those obtained through traditional sintering process, can be prepared by solar heating in spite of quite fast rate of heating and cooling realised in the solar furnace.

Fracture toughness of solar-sintered WC with Co additive

A tungsten carbide (WC) cylinder specimen containing 10 mass% Co sintering aid was prepared under concentrated solar radiation (measured maximum temperature no higher than 1500 °C) in dynamic primary vacuum and its fracture toughness was evaluated by Vickers indentation method. In spite of very fast rates of heating and cooling applied in the solar-sintering process, fracture toughness of the prepared WC cylinder specimen was comparable to that of WC sintered piece prepared through a conventional industrial process in electric furnace under a slow heating and cooling condition. Present results together with the previously reported results for solar-sintered alumina ceramic disk appear to suggest the promising possibility of using solar radiation heating for manufacturing sintered ceramic components.

Photochemical Effects in Carbide Synthesis of d-group Transition Metals (Ti, Zr; V, Nb, Ta; Cr, Mo, W) in a Solar Furnace at PSA (Plataforma Solar de Almerı́a)

During the course of our recent series of solar carbide and carbonitride synthesis work During the course of our recent series of solar carbide and carbonitride synthesis work for d-group transition elements (Ti,Zr;V,Nb,Ta;Cr,Mo,W), we detected several intriguing evidences apparently indicative of involvement of photochemical effects in some reactions under solar radiation. 1) Acceleration of graphitization of amorphous carbon during carburization of W under solar radiation. 2) Promoted formation of high remperature mono-carbide phase of Mo under solar radiation at a temperature lower than the one indicated in equilibrium phase diagram. 3) Formation of sub-carbide Ta 2 C besides mono-carbide TaC in the carburization of Ta with amorphous carbon under solar radiation. Only mono-carbide TaC formed by the carburization of Ta with graphite under solar radiation in accordance with the equilibrium phase diogram. These features were already published individually elsewhere. This article is the brief summary of the reported facts in our earlier publications. Although the results so-far-obtained are rather primitive and our earlier publications. Although the results so-far-obtained are rather primitive and still lacking quantitative characterization, there seems to be no doubt that solar materials processing would open new routes for unique products which cannot be prepared through traditional industrial manufacturing processes.

Carbide formation of Va-group metals (V, Nb and Ta) in a solar furnace

Abstract In our recent series of solar carbide synthesis for Si and d-group transition metals (Ti, Zr; Cr, Mo, W) using graphite (G) or amorphous carbon (aC) as carburising medium, we observed some evidences indicative of involvement of photochemical effects in the reaction process. For example, (1) graphitisation of aC was accelerated during carburisation of W under solar radiation; (2) high-temperature mono-carbide phase of Mo (α-MoC1−x) formed at a temperature appreciably lower than the lower-threshold temperature of the stability range of the α-MoC1−x phase given in the available equilibrium phase diagram for Mo–C system. In this work, we investigated solar carburisation reactions for Va-group metals (V, Nb, Ta) in Ar or N2 gas environment. For this group of metals, only carbide phases formed even in N2 atmosphere without forming carbonitride like the cases with VIa-group metals (Cr, Mo, W). For carburisation of Ta with aC in Ar, an unexpected formation of sub-carbide Ta2C besides mono-carbide TaC took place, while Ta2C phase did not form through carburisation of Ta with aC in N2 or carburisation of Ta with G in either Ar or N2 atmosphere. No graphitisation promotion for aC was detected during the carburisation of none of the Va-group elements.

Bulk Copper-Nanodiamond Nanocomposites; Processing and Properties

Copper has widespread use as engineering material, because of its structural and functional properties, notably high thermal and electrical conductivity. A major drawback of this base metal and its alloys is a relatively low hardness. This precludes its utilization in applications in which both high conductivity and high strength/hardness are needed, e.g. in injection moulds for plastics. Nanostructured metals and nanocomposites are ways to address the low hardness problem, provided the nanostructured material is thermally stable during processing and service. In the present research, composite powders, with 5 to 30 at % nanodiamond, were consolidated into bulk samples. The copper-nanodiamond composite powders were vacuum encapsulated and extruded at 600°C. A significant proportion of the initial hardness in the powders is retained after extrusion. Transmission electron microscopy (TEM) of the extruded material indicates good bonding between the nanodiamond particles and the copper matrix. Raman spectroscopy on the consolidated samples evidences the presence of graphite, possibly due to partial disintegration of ultradisperse nanodiamond agglomerates.

Statistical thermodynamic approach to austenitic Fe1-yNbyNx system

Abstract Available equilibrium P-T-C (pressure-temperature-composition) relationships reported for fcc (face centred cubic) Fe1-yNbyNx alloys with Nb content y up to 0.00561 were analysed on the basis of statistical thermodynamics. The reported P-T-C relationships showed a trend of rising N level x with increasing Nb content y under comparable conditions of temperature T and nitrogen partial pressure p(N2). Statistical models assuming several types of atom clustering modes were compared. A model in which interstitial N atoms were preferentially consumed first to form Nb-N dipoles in the Fe1-yNby lattice, and then the remaining N atoms were distributed over O sites (octahedral interstitial sites) surrounded by six Fe atoms without Nb, was concluded to be the most realistic one.

A theory of suppressed nitrogen solubility in Fe–Z–N ternary alloy systems in the relatively high range of concentration of Z (Z=Si or C)

Abstract It was shown in earlier work that, for the FeZzNx in the relatively low range of z, the solubility pattern of N as a function of temperature T and nitrogen partial pressure p(N2) can be interpreted in terms of decreased number θ of available interstitial sites for occupation by N atoms per Fe atom in the Fe lattice in proportion to increased z under the assumption of negligible interaction between two interstitial constituents, N and Z (Z=Si or C). In the present work, solubility reduction of N in ternary FeZzNx with increasing z in the relatively high range of z were analysed on the basis of statistical thermodynamics. The present analysis for molten FeSizNx in the relatively high range of z showed that a correction to the θ versus z relation for the relatively high range of z was desirable taking into account overlapping of blocked interstitial sites for occupation of N atoms in order to reproduce the observed equilibrium pressure–temperature–composition (PTC) relation for this system while the interaction between N and Si remains negligible. On the other hand, for the simulation of equilibrium PTC relation for austenitic FeCzNx alloy over the entire range of z analysed, significant N–C interaction was important in addition to the blockage effect for the available interstitial sites for occupation of N atoms.

Carbon solubility in face centred cubic Co1-yNiy alloy lattice analysed by statistical thermodynamics

Abstract Carbon solubility in face centred cubic (fcc) Co 1− y Ni y alloy lattice was analysed on the basis of statistical thermodynamics under the a priori assumption of negligible C-C interaction in the fcc Co 1− y Ni y lattice. The minimum C solubility observed around the composition y between 0.6 and 0.8 was concluded to be ascribable to the minimised number of available interstitial sites for occupation by C atoms in the fcc Co 1− y Ni y lattice. The observed patterns of variations for the C solubility and the extent of stabilisation of the C atom in the fcc Co 1− y Ni y lattice with respect to the composition y appeared to be comparable to those observed for fcc Fe 1− y Ni y C x when compared in terms of the average number of 3 d electrons per metal atom.

Atomic configuration in hexagonal-close-packed TiCzHx phase estimated by statistical thermodynamics

Through statistical thermodynamic analysis of equilibrium PTC (pressure-temperature-composition) relationships obtained experimentally for an alloy system containing interstitial constituents, information concerning the configuration of the interstitial atoms in the alloy lattice as well as the extent of the stability of the interstitial atoms can be derived. This line of approach was taken for analysis of PTC data reported by Rexer and Peterson for h.c.p. (hexagonal close packed) TiCzHx phase (z = 0.26; 0.4 < x < 0.8) in the temperature range 700–850 °C and for a hydrogen partial pressure of no more than 1 atm, with the aim of elucidating the configurations of the C and H atoms in the h.c.p. Ti lattice. The following picture was derived for the h.c.p. TiCzHx phase through this analysis: (1) C atoms occupy O-sites (octahedral interstices) randomly; (2) two of the four T-sites (tetrahedral interstices) around an O-site occupied by a C atom are preferentially occupied by H atoms; these two T-sites must lie at opposite ends of the diagonal line of the rectangle formed by the four T-sites at the corners; (3) the rest of the H atoms distribute over the other two free T-sites around the O-site occupied by the C atom.

Role of Unstable Chemical Species (Non-Graphitic Carbon and Flowing NH3 Gas) on the Equilibrium Point of the Reaction Product (Carbide, Carbo-Nitride or Nitride)

Elementary carbon in solid state might exist in variety of allotropic forms including graphite, diamond and amorphous carbon. Among them, graphite is in the stable form with chemical carbon activity a(C) = 1 whereas diamond is in meta-stable state and amorphous carbon is in un-stable state with a(C) higher than 1. Experimental evidences showed that carbide and carbo-nitride being in equilibrium with carbon possessing a(C) higher than 1 was with higher C content than the corresponding ones in equilibrium with graphite. In case of gaseous ammonia NH 3, higher nitrogen activity a(N)and higher hydrogen activity a(H) than the ones corresponding to the equilibrium partial pressure levels, p(N 2) and p(H 2), might be gained by suppressing its dissociation to a level away from the equilibrium state for the given temperature T by flowing. Thus, under flowing NH 3 gas, nitride or carbo-nitride with N content higher than that in equilibrium with N 2 gas at p(N 2) anticipated from the dissociation equilibrium at the given T might be obtained. Chronological development of this line of work started by Prof. Masahiro Katsura in early seventies at Osaka University is reviewed in this paper as I was one of collaborators involved in this very exciting research work from the early stage of its development.