Transformation of mechanically alloyed Nb-Sn powder to Nb<Subscript>3</Subscript>Sn

Nb₃Sn was processed via mechanical alloying (MA). The powder mixture comprised of stoichiometric proportions of elemental niobium and tin powder was mechanically alloyed for 3 hours and the mechanically alloyed powder mixture was heat treated. While MA resulted in Nb-Sn solid solution, the reaction leading to the formation of Nb₃Sn occurs during the subsequent heat treatment of the powder mixture.     

Aquatic manoeuvering with counter-propagating waves: a novel locomotive strategy

Many aquatic organisms swim by means of an undulating fin. These undulations often form a single wave travelling from one end of the fin to the other. However, when these aquatic animals are holding station or hovering, there is often a travelling wave from the head to the tail, and another moving from the tail to the head, meeting in the middle of the fin. Our study uses a biomimetic fish robot and computational fluid dynamics on a model of a real fish to uncover the mechanics of these inward counter-propagating waves. In addition, we compare the flow structure and upward force generated by inward counter-propagating waves to standing waves, unidirectional waves, and outward counter-propagating waves (i.e. one wave travelling from the middle of the fin to the head, and another wave travelling from the middle of the fin to the tail). Using digital particle image velocimetry to capture the flow structure around the fish robot, and computational fluid dynamics, we show that inward counter-propagating waves generate a clear mushroom-cloud-like flow structure with an inverted jet. The two streams of fluid set up by the two travelling waves ‘collide’ together (forming the mushroom cap) and collect into a narrow jet away from the cap (the mushroom stem). The reaction force from this jet acts to push the body in the opposite direction to the jet, perpendicular to the direction of movement provided by a single travelling wave. This downward jet provides a substantial increase in the perpendicular force when compared with the other types of fin actuation. Animals can thereby move upward if the fin is along the bottom midline of the body (or downward if on top); or left–right if the fins are along the lateral margins. In addition to illuminating how a large number of undulatory swimmers can use elongated fins to move in unexpected directions, the phenomenon of counter-propagating waves provides novel motion capabilities for systems using robotic undulators, an emerging technology for propelling underwater vehicles.     

COMPUTATION OF BOUNDARY LAYER TRANSITION USING LOW-REYNOLDS-NUMBER TURBULENCE MODELS

A literature review shows that, for the commonly used turbulence models, the predicted location of boundary layer transition is very sensitive to the initial profiles of turbulence quantities and starting location of calculation. To eliminate these effects, two independent solution approaches are proposed: (1) to solve the boundary layer equations over a flat plate with the starting location of calculation very close to the leading edge of the plate, and (2) to solve the elliptic Navier-Stokes equations over the whole plate, including the leading edge and some region upstream of it. Computations show both approaches lead to identical results. Three well-known low-Reynolds-number (LRN) turbulence models are evaluated with respect to the transition on a flat plate. None of the models are able to predict the quantitative aspects of transition correctly without an ad hoc adjustment. A satisfactory new turbulence model is presented in a companion paper.     

Electrical Conduction and Magnetoelectric Effect in CuFe1.8Cr0.2O4–Ba0.8Pb0.2TiO3 Composites

Magnetoelectric composites of CuFe_1.8Cr_0.2O₄– Ba_0.8Pb_0.2TiO₃ were prepared using high temperature solid-state reaction technique. X-ray structural analysis of these composites confirms the presence of both the phases in the composite. Detailed studies of dielectric properties (, tan and _ac ) as a function of frequency (100 Hz to 1 MHz) and temperature (30°C to 250°C) show that these compounds exhibit diffuse ferroelectric phase transitions. Results of ac conductivity, dc resistivity and thermoelectric power measurements show that conduction occurs by hopping of charge carriers. The magnetoelectric effect has been studied as a function of intensity of magnetic field. The electrical polarisation was induced in piezoelectric (Ba_0.8Pb_0.2TiO₃) phase as result of strain induced in the ferrite (CuFe_1.8Cr_0.2O₄) phase by the applied magnetic field. The Jahn-Teller distortion caused in the ferrite lattice by Jahn-Teller ions like Cu²⁺ and Cr³⁺ is also responsible for the elastic coupling of strain to the Ba_0.8Pb_0.2TiO₃ phase.     

Magnetoelectric and dielectric properties of Ni0.5Cu0.5Fe2O4–Ba0.5Pb0.5Ti0.5Zr0.5O3 ME composites

The (x) Ni_0.5Cu_0.5Fe₂O₄ + (1−x) Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ ME composites have been synthesized by a standard ceramic method. The presence of single phase in x = 0 and x = 1 as well as two phases in x = 0.15, 0.30 and 0.45 composites has been confirmed by XRD. The dielectric constant (ε′) and dielectric loss (tan δ) have been studied as a function of temperature and frequency. These composite materials exhibit maximum dielectric constant with a variation of frequency and temperature. The composite 15% Ni_0.5Cu_0.5Fe₂O₄ + 85% Ba_0.5Pb_0.5Ti_0.5Zr_0.5O₃ had the highest magnetoelectric voltage coefficient of 0.248 mV/cm Oe at room temperature among the studied composites.     

Effect of Temperature on the Elastic and Anelastic Behaviour of Magneto-Ferroelectric Composites Ba0.8Pb0.2TiO3 + Ni0.93Co0.02Mn0.05Fe1.95O4-δ in the Ferroelectric Rich Region

Composites with composition xBa_0.8Pb_0.2TiO₃+ (1 –x) Ni_0.93Co_0.02Mn_0.05Fe_1.95O_4- in which x varies as 1.0, 0.9, 0.7 and 0.5 in molar percent have been prepared by the conventional ceramic double sintering process. The presence of the two phases has been confirmed by X-ray diffraction. These composites were prepared for their use as magnetoferrolectric devices. Variation of longitudinal modulus (L) and internal friction loss (Q ^–1) of these samples with temperature at 142 kHz has been studied in the wide temperature range 300 to 630 K. The elastic behaviour (L) showed a break at the ferroelectric Curie temperature (498 K) in the case of pure ferroelectric material (Ba_0.8Pb_0.2TiO₃). This break shifted to lower temperature side as the ferrite component increases in the composite. The temperature variation of internal friction loss (Q ^–1) showed a corresponding stress induced relaxation peak at the ferroelectric-non-ferroelectric phase transition. This behaviour is explained in the light of structural phase transition.     

Fuming of stannous oxide from silicate melts

The kinetics of vaporization of SnO from silicate melts was studied in helium, nitrogen, argon and argon-helium mixtures, and in inert gas—carbon monoxide mixtures. The rate of extraction of tin from the meltvia SnO vaporization was significantly higher in reducing gases than in inert gases. The enhanced vaporization phenomenon, commonly recognized in metallic systems, is demonstrated to occur in silicate melts. The vapor phase reduction of SnO (g) was studied experimentally, and the reduction was found to be very rapid. The addition of acidic or basic oxides to the binary SnO-SiO₂ melts decreased the vaporization rate.     

Formation of Al–Y oxide during processing of γ-TiAl

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP’ing) were found to precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA’d) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).