Dispersion characteristics for low-frequency magnetoelectric coefficients in bulk ferrite-piezoelectric composites

Ferrite-piezoelectric composites are magnetoelectric (ME) due to the interaction between magnetic and electrical subsystems through mechanical forces. A theory for the low-frequency Maxwell-Wagner relaxation in ME coefficients is discussed for bulk composites of nickel or cobalt ferrite and lead zirconate titanate (PZT). ME coefficients versus frequency spectra show two types of relaxation, over 0.1-100 μHz and 1-1000 Hz. The relaxation frequencies and the magnitude of the ME coefficients are dependent on the electrical and composite parameters and volume fraction for the two phases. The ME coefficient α_E is in the range 10⁻¹-10⁴ mV/cm Oe, higher in cobalt ferrite-PZT than for nickel ferrite-PZT, and is strongly dependent on PZT volume fraction v. Estimates of α_E and relaxation frequencies versus v provided here are useful for engineering composites with maximum ME effects for specific frequency bands.     

Hydromagnetic stability of rotating stratified compressible fluid flows

The hydromagnetic stability of a radially stratified compressible fluid rotating between two coaxial cylinders is investigated. The stability with respect to axisymmetric disturbances is examined. The fluid system is found to be thoroughly stable to axisymmetric disturbances provided the fluid rotates very rapidly. The system is shown to be unstable to non-axisymmetric disturbances, and the slow amplifying hydromagnetic wave modes propagate against the basic rotation. The lower and upper bounds of the azimuthal phase speeds of the amplifying waves are determined. A quadrant theorem on the slow waves characteristic of a rapidly rotating fluid is derived. Special attention is given to the effects of compressibility of the fluid. Some results concerning the stabilitiy of an incompressible fluid system are obtained as special cases of the present analysis.

---

Zusammenfassung Es wird die hydromagnetische Stabilität eines radial geschichteten kompressiblen Fluids untersucht, das zwischen zwei koaxialen Zylindern rotiert. Die Untersuchung der Stabilität in Bezug auf axisymmetrischen Störungen gibt volle Stabilität bei rascher Rotation. Für nicht axisymmetrische Störungen zeigt sich das System instabil, und die langsam angefachten hydromagnetischen Wellen pflanzen sich entgegen der Rotationsrichtung des Fluids fort. Es wird die untere und obere Grenze der azimutalen Phasengeschwindigkeit für diese Wellen bestimmt. Für die langsamen Wellen, charakteristisch für rasche Rotation des Fluids, wird ein Quadrant-Theorem hergeleitet. Die Kompressibilitätseffekte werden diskutiert, und Resultate für inkompressible Systeme ergeben sich als Sonderfälle.

     

Magnetic field induced Wigner transition in inversion layers

It is shown that under the influence of a strong magnetic field the electrons in an inversion layer will crystallize. The field strengths required are within experimental reach. The stability analysis of such a Wigner lattice is presented. We have also calculated the melting temperature as a function of the magnetic field.     

Dynamic response of an annular disk to a moving concentrated,in-plane edge load

In-plane dynamic behaviour of a thin annular disk with a clamped inner boundary is analyzed. The frequencies of free in-plane vibration with a free outer boundary are first evaluated, by using Lamé potentials, for various radius ratios ranging from 0.2 to 0.8. The steady state dynamic stresses induced by a concentrated load moving at a constant angular speed at the outer boundary are then evaluated through a Galilean transformation. Results are presented for a radius ratio of 0.5.     

Kinetic model for micellar catalysed hydrolysis of esters-biomolecular reactions

The cationic micelles of cetyltrimethylammonium bromide, cetyldibenzylammonium chloride and cetylpyridinium chloride stabilize the tetrahedral intermediate formed in the hydrolysis of carboxylic esters (e.g.p-nitrophenylbenzoate) to a greater extent, preferring a mechnism, than the anionic intermediate formed in the hydrolysis ofm-nitrophenyl-N-N-diphenylphosphorodiamidate, which prefers aE _1cB mechanism. The co-operative index,n ₁, calculated for these reactions is greater than 1 indicating that the substrate induced micellation is responsible for the observed catalysis. Based on the present kinetic model for a bimolecular reaction the fraction of substrate and nucleophile bound to the micelle have been calculated. The above results suggest that reaction occurs between the substrate solubilised into the micelle and the nucleophile residing at the Stern layer rather than at the micelle-water interface. The equilibrium constant, and critical micelle concentration evaluated using the present model are in agreement with the values obtained by using earlier models, suggesting a method of evaluating these parameters from kinetic data only.     

Studies in molecular structure,symmetry and conformation II

1-Aminocycloheptanecarboxylic acid hydrobromide monohydrate crystallizes in space groupP2₁2₁2₁ with cell dimensionsa= 25·69,b= 6·85 andc= 6·61 Å. The structure was solved in thehk0 andh0l projections, and refined with the three-dimensional data to anR factor of 9·86%. The cycloheptane ring is disordered, which leads to ‘predominant’ and ‘alternative’ conformations. Both of these conformations correspond to a skew-chair form. The structure is stabilized by a three-dimensional network of hydrogen bonds.     

Self-heating-induced phenomena in long reentrant ferromagnetic thin film superconductors

We predict several novel self-heating-induced features of the current-voltage characteristics at bath temperatures below the lower critical temperature as well as the temperature dependence of the various critical currents of long reentrant ferromagnetic thin film superconductors on the basis of existing theories and available experimental results. Further we point out how the experimental investigation of the above can lead to a better understanding of reentrant superconductivity and magnetic Kapitza resistance.     

Lattice static properties of vacancy clusters and interstitials in hcp magnesium: Computer simulation studies

Computer simulation studies have been made to investigate the static properties of mono-, di- and tri-vacancy clusters and of self-interstitials in hcp magnesium in different configurations. Three interatomic potentials have been chosen for which the results have been compared. A crystallite containing about 1500 atoms and a model with the interatomic interaction extending upto the fourth neighbour distance have been used. Relaxation field, defect relaxation and formation energies, strength dipole tensors and relative changes in volume in the above defects have been computed and our final results compared with those of earlier workers. The formation energies of the defects are highly sensitive to the choice of the potential whose detailed structure guides the nature of relaxation and the dipole tensors. Calculations have been done for octahedral, tetrahedral and dumb-bell interstitials of which the last is found to be the most stable.