Methanation of carbon deposited directly from CO2 on rhodium-bearing activated magnetite

Methanation reactivity was studied for the surface carbon deposited from CO₂ on the surface of Rh-bearing activated magnetite. The most active material (Rh=0.83 wt %) for methanation was prepared by the impregnation method at 60°C and showed 98% conversion at 300°C. The surface carbon was composed of elemental carbon (-carbon) and polymerized carbon (-carbon), the proportion being dependent on the density of carbon deposited. In temperature-programmed surface reaction, the extent of conversion of the - and -carbon to CH₄ was 0.34 (-carbon) and 0.53 (-carbon), respectively, and the total conversion was 0.87. This result indicates that not only elemental carbon but polymerized carbon (-carbon) could be converted to CH₄ on the Rh-bearing activated (-carbon) magnetite, whereas -carbon is not hydrogenated on activated magnetite.     

The existence of Taylor vortices and wide-gap instabilities in spherical Couette flow

Summary The flow of a viscous incompressible fluid in the gap between two concentric spheres was investigated for the case where only the inner sphere rotates and the outer one is stationary. Flow visualization studies were carried out for a wide range of Reynolds numbers (Re2·10⁵) and aspect ratios (0.080.5) to determine the instabilities during the laminar-turbulent transition and the corresponding critical Reynolds numbers as a function of the aspect ratio. It was found that the laminar basic flow loses its stability at the stability threshold in different ways. The instabilities occurring depend strongly on the aspect ratio and the initial conditions. For small and medium aspect ratios (0.080.25), experiments were carried out as a function of the Reynolds number to determine the regions of existence for basic flow, Taylor vortex flow, supercritical basic flow and furthermore, to give the best fit for the maximum number of pairs of Taylor vortices as a function of aspect ratio. For wide gaps (0.330.5), however, Taylor vortices could not be detected. The first instability manifests itself as a break of the spatial symmetry and non-axisymmetric secondary waves with spiral arms appear depending on the Reynolds number. For =0.33, secondary waves with an azimuthal wave numbern=six, five and four were found, while in the gap with an aspect ratio of =0.5 secondary waves withn=five, four and three spiral arms exist. Frequencies of these secondary waves were measured, the corresponding critical Reynolds numbers and the transition Reynolds numbers during the transition to turbulence were found. The flow modes occurring at the poles look very similar to those found in the flow between two rotating disks. Effects of non-uniqueness and hysteresis were determined as a function of the acceleration rate.     

Elastodynamic parameters for dynamic interface fracture mechanics

Summary Dynamic extension of cracks running along curvilinear interfaces of brittle bimaterials subjected to mechanical crack surface loads and superimposed thermal strains acting along the ligament is considered. This paper especially addresses the provision and discussion of elastodynamic interface parameters in order to assess quantitatively the bimaterial fracture in view of the governing physical features: applied mechanical and thermal strain loading, existence of an interface, crack-tip velocity and curvature of the interface contour. By utilizing the linear theory of thermoelasticity and adopting Stroh's method of generalized complex potentials, from the corresponding boundary and continuity conditions vectorial Hilbert problems are derived. It is shown that the parameters of the eigenvalues and of the eigenvectors of the Hilbert problems can be interpreted as elastodynamic interface mechanics parameters reading (,v _p, _Hf , _Hf ). Generalized Dundurs parameters of dynamics (, ) and consequently an associated generalized Dundurs diagram of dynamics are proposed. While the aforementioned elastodynamic interface parameters (, ,v _p, _Hf , _Hf ) do not assume the interface to be damaged, interfaces with running interface cracks generally cause two additional interface parameters, denoted as bimaterial constants (, _Hf ), where the latter is specific to the curvature of the interface in conjunction with the velocity of the interface crack. However, the bimaterial constants (, _Hf ) can be traced back to interface parameters for an uncracked bimaterial, namely to (, _Hf ).     

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Polymorphism in Sr3Fe2O7−x

The compound Sr₃Fe₂O_7–x , with variable iron valence, was investigated by X-ray powder techniques, both at room and at high temperatures. If the material is examined in massive form, a single phase called -Sr₃Fe₂O_7–x appears as previously reported in the literature. This -phase is tetragonal and exhibits the lattice parameters: a=3.874 and c=40.314 Å. Two other phases, called and -Sr₃Fe₂O_7–x , respectively, can be obtained on heating the finely powdered material when laid on a flat platinum support. The form is stable up to 1275° C, while the form is revealed only above 1275° C and changes always into -Sr₃Fe₂O_7–x when quenched. Both and phases are tetragonal, with a=4.001 and c= 58.251 for the form and a=4.013, c=57.092 Å for the form. The transition involves a true phase equilibrium, while the transformation is possible only by means of a suitable mechanical treatment of the material.     

Properties of polyvinyl pyrrolidone/β-chitosan hydrogel membranes and their biocompatibility evaluation by haemorheological method

The semi-IPN hydrogel membranes of polyvinyl pyrrolidone (PVP) and -chitosan were synthesized by crosslinking -chitosan with glutaraldehyde. Hydrogel membranes were characterized by spectroscopic, swelling, thermal and mechanical properties. The in vitro biocompatibility of hydrogel membranes was studied by haemorheological method. These hydrogels have water contents in the range of 60–70% with a high fraction contributed by free water (> 45%). The gel composition, amount of cross-linking agent and swelling temperature plays an important role in swelling kinetics of these semi-IPN membranes. Melting temperature (T _m ) of membranes increased with a decrease in endothermic peak with increasing -chitosan content. The tensile strength of membranes in the dry state was found to be high (29–43 MPa) and it increased with increasing -chitosan content. The in vitro haemorheological studies indicated the biocompatible nature of membranes with no significant changes in whole blood and plasma viscosity and red blood cell rigidity.     

New results on the Fermi surface of β brass type alloys

De Haas-van Alphen frequencies not previously reported have been measured for -PdIn and -CuZn. The results for PdIn suggest a reinterpretation of some of the previous results and offer an explanation of a previously unexplained set of de Haas-van Alphen frequencies in PdIn and AgZn. A recent nonlocal pseudo-potential model by Taylor can account for the experimental results reported here as well as the earlier data.National Research Council of Canada Postdoctorate Fellow.     

Effects of citrate ions on the formation of ferric oxide hydroxide particles

The effects of citrate ions on the formation of - and -FeOOH particles were investigated using various techniques. The formation and crystallization of both particles were inhibited by citrate ions, and their particle sizes decreased with increase in the concentration of these ions. Finally, aggregated amorphous particles were formed. The effects of citrate ions appeared to be more significant for -FeOOH than for -FeOOH. This difference could be explained by the pH dependence of the affinity of citrate ions to Fe³⁺ ions. The amorphous -FeOOH particles prepared in the presence of > 10 mol% citrate ions selectively adsorbed water molecules.     

Microstructure and fracture toughness of liquid-phase-sintered β-SiC containing β-SiC whiskers as seeds

The effect of seeding on microstructural development and fracture toughness of -SiC with an oxynitride glass was investigated by the use of morphologically rodlike -SiC whiskers. A self reinforced microstructure consisting of rodlike -SiC grains and equiaxed -SiC matrix grains was obtained by seeding 1–10 wt% SiC whiskers, owing to the epitaxial growth of -SiC from the seed whiskers. Further addition of seeds (20 wt%) or further annealing at higher temperatures led to a unimodal microstructure, owing to the impingement of growing seed grains. By seeding -SiC whiskers, fracture toughness of fine-grained materials was improved from 2.8 to 3.9–6.7 MPa · m^1/2, depending on the seed content.     

The fluid-filled cylindrical membrane container

Summary The static shape of a fluid-filled membrane cylinder can be described by a set of nonlinear differential equations. These equations depend on a nondimensional parameter representing the relative importance between pressure and the gravity force. The solution is found by three methods: similarity solution for small , asymptotic solution for large , and numerical integration.     

Stability of metastable phases and microstructures in the ageing process of Al–Mg–Si ternary alloys

Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si=2), excess silicon (Mg/Si<2) and excess magnesium (Mg/Si>2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, , and stable phases. The peaks corresponding to and were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that precipitates played a major role in improving the hardness, but not precipitates. © 1998 Chapman & Hall     

Bone formation induced by calcium phosphate ceramics in soft tissue of dogs: a comparative study between porous α-TCP and β-TCP

Two kinds of tri-calcium phosphate ceramics (Ca/P = 1.50), -TCP and -TCP, which has the same macrostructure and microstructure, but different phase composition, were implanted in dorsal muscles of dogs. The samples were retrieved at 30, 45 and 150 days, respectively, after implantation, and were analyzed histologically. There were critically different tissue responses between -TCP ceramic and -TCP ceramic. Higher cell populations were observed inside the pores of -TCP than those of -TCP, bone tissue was found in -TCP at 45 and 150 days, but no bone formation could be detected in any -TCP implants in this study. On the other hand, the bone tissue in -TCP seemed to degenerate at 150 days. The results indicate that porous -TCP can induce bone formation in soft tissues of dogs; while the rapid dissolution of the ceramic and the higher local Ca²⁺, PO ₄ ^3- concentration due to the rapid dissolution of -TCP may resist bone formation in -TCP and the less rapid dissolution of -TCP may be detrimental to already formed bone in -TCP.     

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine ₁ and occasional ₂ precipitates with an average ₁ interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of ₁ particles by dislocations. In the overaged condition ₁, ₂ and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of ₁ particles by dislocations takes place in addition to the cutting of ₂ and particles. Above 77° K the difficult cutting of ₂ and particles alone controls the deformation, ₁ being more easily cut with the aid of thermal fluctuations.     

The redistribution of alloying elements by rapid-heating solution treatment of an (α +β) titanium alloy

Thin-foil X-ray energy-dispersive chemical analysis of an (+) titanium alloy has been used to establish the compositional profiles which develop between the and phases as a result of heating into the field at a rate of 100 K sec^–1. The results show that phase transforms to via a diffusional mechanism as it is progressively enriched in-stabilizing solutes. Complete diffusional transformation of to occurs 160 K above the transus but the phase remains chemically inhomogeneous. On subsequent quenching at a rate of 400 K sec^–1, phase containing less than approximately 10wt% Mo equivalent of -stabilizing solutes transforms to martensite. The proposed transformation mechanisms are consistent with the results of resistivity measurements made during the heating and cooling cycles.     

Specific analysis of EBSD data to study the texture inheritance due to the β → α phase transformation

A specific processing of EBSD data is proposed to study the texture inheritance of titanium or zirconium alloys. A non standard misorientation map is calculated to localise the colonies inherited from the same parent grain. The calculation of the parent orientation from its inherited variants detailed in previous works has been adapted to the data obtained from an automated EBSD analysis. Finally, a method to derive the orientation map of the parent phase from that of the inherited phase is proposed. The resulting and COMs are used to study some aspects of the variant selection occurring in the transformation of a T40 sample.     

Interface characterization of a β-SiC whisker-Al composite

The nature of the interface, the orientation relationship of -SiC whisker (-SiC_w)-Al combination, and the misfit dislocation structures at the -SiC_w-Al interfaces in a -SiC_w-Al composite have been observed by a high-resolution transmission electron microscopy (HRTEM). It was shown that quite a good bonding between the whisker and the aluminium was achieved due largely to the lattice match between SiC and aluminium at the interfaces. The orientation relationship between the whisker and the aluminium was {002}_SiC{111}_Al; 110_SiC110_Al. The interface was clean, faceted and semicoherent. The misfit dislocation cores were located in the whisker side away from the -SiC_w-Al interfaces.     

Synthesis of aluminum infiltrated boron suboxide drag cutters and drill bits

Synthesis of boron suboxide (B₆O) was made by reactive sintering of crystalline boron and zinc oxide powders at 1450 °C, in argon, for 12 h. After sintering, Vickers microhardness testing was performed on the material synthesized and an average hardness value of 34 GPa was obtained. Sintered suboxide (in crushed and ground powder form) was then analyzed through optical and scanning electron microscopies and X-ray diffraction. Following the completion of the analyses, consolidation of the powder was performed. Two different routes were carried out: (1) explosive consolidation which was performed in double tube (with a density value of 2.22 g/cm³) and single tube (with a density value of 2.12 g/cm³) canister design arrangements and (2) hot pressing which was performed in a graphite die assembly, at 1600 °C, in vacuum, for 2 and 4 h (with density values of 2.15 and 2.18 g/cm³ respectively). Consolidated samples of both routes showed different levels of mechanical attachment, agglomeration, porosity, fracture toughness and fracture strength values, whereas microhardness values and X-ray diffraction plots (as shown in Table I and Figs 6 and 8 respectively) were determined to be similar. Following characterizations, compacts of both routes were then given a high temperature sintering treatment (pressureless sintering) at 1800 °C, in vacuum, for full densification. Both in the as consolidated and densification sintered stages test results revealed the most desirable and well-established properties for the explosively consolidated double tube design compacts (with densification sintered density, microhardness and fracture toughness values of 2.46 g/cm³, 38 GPa and 7.05 MPa m^1/2 respectively). Consolidation and desification sintering steps were then followed by a pressureless infiltration step. Aluminum was infiltrated into densification sintered double tube design consolidated and 4 h of pressed samples (better-compacted and better-sintered compacts) in the temperature range 1100–1250 °C, in argon, for 10 h. During infiltrations, the optimum temperature of the infiltration process was determined to be 1200 °C. Characterization results revealed the most uniform and well established properties once more for the double tube design explosively consolidated compact (with aluminum infiltrated density, microhardness and fracture toughness values of 2.55 g/cm³, 41 GPa and 8.70 MPa m^1/2 respectively).     

On annealed Ni-34.6 at % Al at 1523 K and effects of cooling rates

The microstructures and room temperature mechanical properties of melt spun Ni-34.6 at % Al annealed at 1523 K for 2 h and cooled at different rates were studied. The annealed microstructures and bend ductility were sensitive to the cooling rate. The distribution and relative content of precipitation were important factors for enhancement of bend ductility of the annealed ribbon. The best bend ductility of the annealed ribbon reached 7.6%, which was considerably better than the as-melt spun ribbon. The emission and transmission of dislocations from ductile into to satisfy the need of deformation compatibility played a key role in the improvement of the ductility.     

Relations between specific heat discontinuities and Ginzburg-Landau parameters for superfluid 3He

We show that the value of the ratio ₅/₂₄ deduced from specific heat experiments is extremely sensitive to the coefficient of the quadratic term in the Ginzburg-Landau functional. A strong coupling calculation of this coefficient implies that the experimental value of ₅/₂₄ is more than three times larger than previously believed.This work was supported in part by the National Science Foundation through Grant DMR7826530.     

Preparation of β-TCP with high thermal stability by solid reaction route

Starting from commercial materials, -TCP powder was prepared by a conventional solid reaction route. Using DTG, TG and DTA, the mass change and the heat flow during heating of the commercial powder mixture were tested. The phase evolution at each step of the heating was monitored using XRD analyses. Based on the tests and the analyses the reaction details were analyzed. The -TCP powder prepared in this study has high thermal stability. The phase transition temperature from to was determined to be 1350°C, 200°C higher than commercial -TCP powder (Plasma Biotal). The high phase transition temperature was due to Mg trace in the starting materials.