K. Chandramani Singh

Phase transformations in some homologues of 4-n-alkyl-4'-cyanobiphenyls investigated by positron annihilation spectroscopy

The results of phase transformation studies carried out in four homologues of 4-n-alkyl-4cyanobiphenyls, using positron annihilation spectroscopy, are presented. The homologues investigated are, hexyl-, heptyl-, octyl- and decyl-cyanobiphenyls (6CB, 7CB, 8CB and 10CB). In these compounds, the positron lifetime measurements were performed as functions of temperature. The positron annihilation parameters are found to exhibit strong dependence on temperature. It was found that the ortho-positronium pick-off lifetime shows changes which strongly support (i) a gradual disappearance, instead of an abrupt one, of some memory of the more ordered solid phases on passing to the liquid crystalline phases, (ii) the strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association and (iii) the formation of cyabotactic groups of a smectic phase in a nematic medium. Changes were also observed in the ortho-positronium formation probability which apparently indicate a systematic transformation of the solid phase from a close-pack structure to an open-pack structure as one goes from a lower to a higher homologue of the compounds investigated. Solid crystalline polymorphism has been observed in 8CB. A change in molecular packing in the solid phase of 10CB has been observed.

Structural and piezoelectric properties of barium-modified lead-free (K0.455Li0.045Na0.5)(Nb0.9Ta0.1)O3 ceramics

Ferroelectric (K0.455Li0.045Na0.5)(Nb0.9Ta0.1)O3xa0+xa0x mol% BaCO3 ceramic compositions with Ba2+ as an A-site dopant in the range of xxa0=xa00–1.2xa0mol% were synthesized by conventional ceramic processing route. Effect of Ba2+ content on the microstructure, ferroelectric, dielectric, and piezoelectric properties of the ceramics was investigated. The results of X-ray diffraction reveal that Ba2+ diffuse into the (K0.455Li0.045Na0.5)(Nb0.9Ta0.1)O3 lattices to form a solid solution with a perovskite structure having typical orthorhombic symmetry. As Ba2+ content increases, cell volume and tetragonality increase in the crystal structure of the ceramics. Increasing doping level of Ba2+ inhibits grain growth in the ceramics and reduces both the Curie temperature (Tc) and tetragonal–orthorhombic phase transition temperature (To-t). The bulk density, remnant polarization Pr, room-temperature dielectric constant (ε′RT), planar electromechanical coupling factor kp, and piezoelectric charge coefficient d33 are found to increase as Ba2+ concentration increases from 0 to 0.8xa0mol% and then decrease as Ba2+ content increases further from 0.8 to 1.2xa0mol%. High piezoelectric properties of d33xa0=xa0187 pC/N and kpxa0=xa048xa0% are found in 0.8xa0mol% Ba2+ composition. Optimum amount of Ba2+ dopant takes the polymorphic phase boundary region consisting of orthorhombic and tetragonal crystal structures of the ceramic system near the room temperature and enhances its piezoelectric properties.

Study of molecular motions in alkyl cyanobiphenyls using positron lifetime spectroscopy

Positron lifetime measurements have been carried out in two liquid-crystal-forming homologues of alkyl cyanobiphenyls, 7CB and 10CB. In each of these two compounds measurements were performed during the heating cycle of samples prepared either by quenching or by slow cooling from the respective liquid crystalline phase. In both compounds, the behavior of quenched and slow-cooled samples is found to be different. Unlike the slow-cooled sample, the material in the quenched sample seems to have transformed into a glassy solid. Theo-Ps pick-off lifetime in the quenched sample exhibits a strong temperature dependence. In each case, its value exhibits six broad peaks at various characteristic temperatures which have been ascribed to various motions associated with the molecules of these compounds. The quenched samples of 7CB and 10CB exhibit glass transitions at ∼268 and ∼266 K, respectively. The present work demonstrates an interesting application of positron lifetime spectroscopy (PLS).

Production of BaTiO2 Nanocrystalline Powders by High Energy Milling and Piezoelectric Properties of Corresponding Ceramics

Barium titanate (BaTiO3 or BT) has become one of the most studied functional materials due to its potential application as multilayer ceramic capacitors, PTC thermistors, electromechanical devices, piezoelectric transducers, actuators, dynamic RAM or logic circuitry as well as a great variety of electro-optical devices. In the present study, high energy ball milling has been used to produce nanocrystalline powders of BT. Two categories of powders having average particle size of 35 nm and 25 nm were prepared by setting the milling speed at 250 rpm and 300 rpm respectively, fixing the milling time at 30 hours. Four ceramic samples, BT35-1350, BT25-1350, BT35-1400 and BT25-1400, were formed by sintering the two types of powders at 1350oC and 1400oC for 3 hours. The ferroelectric and piezoelectric properties of the ceramic samples were studied and found to be dependent on the size of the starting nanopowders. The bulk density and piezoelectric constant (d33) of B25-1350 were found to be less than those of BT35-1350, while the reverse was true in case of BT25-1400 and BT35-1400. Well saturated P-E hysteresis loops were observed for all the ceramics with the size and shape of the loops appearing different for the four samples. For both the pairs of ceramics sintered at 1350oC and 1400oC, the remnant polarization (Pr) decreases with starting particle size, that is, as we go from BT35-1350 to BT25-1350 as well as from BT35-1400 to BT25-1400. However, with decreasing particle size of the starting powders, the coercive field (Ec) increases for the ceramics sintered at 1350oC and decreases for the ceramics sintered at 1400oC. The study reveals the importance of an optimized combination of the size of the starting nanopowders and sintering temperature for obtaining BT ceramics with the desired properties.

Size effect on piezoelectric properties of barium stannate titanate ceramics prepared from nanoparticles

Piezoelectric properties of Ba(Ti1−xSnx)O3 ceramics with xxa0=xa00.025, 0.045 and 0.065, prepared from 16xa0nm powders, were compared with those of the corresponding ceramics obtained from 86xa0nm powders to see the effect of tin content and particle size of the starting powders. Ba(Ti1−xSnx)O3 powders were synthesized by solid state reaction of BaCO3, TiO2 and SnO2 at 1,050xa0°C. The powders were high energy ball milled to produce nanocrystalline powders having average particle size of 16xa0nm. The milled powders were sintered at 1,350xa0°C for 4xa0h to yield ceramics. For these ceramics, increasing Sn content from xxa0=xa00.025–0.065 produces a decrease in (1) unipolar strain level s from 0.084 to 0.027xa0%, and (2) electromechanical coupling factor kp from 33.6 to 19.3xa0%. However, the bulk density, room temperature dielectric constant and piezoelectric charge constant d33 exhibit an increase from 5.03–5.84xa0g/cm3, 1,342–2,156 and 7–110xa0pC/N, respectively, with increasing Sn content. The increasing trend of density and d33 presently observed is in sharp contrast to the result of corresponding ceramics prepared from 86xa0nm nanopowders. The present study reveals a cooperative mechanism involving both the nanoscale size of the starting particles and optimum tin content which results in the enhancement of d33 with tin content.

Effect of vanadium substitution on electrical and piezoelectric properties of lead-free (K0.5Na0.5)NbO3 ceramics

During the last decade, efforts have been made towards developing lead-free piezoelectric ceramics which can replace the currently dominant but highly superior lead-based piezoelectric materials such as PZT. (K0.5Na0.5)NbO3-based piezoelectrics are one promising candidate because of their relatively high Curie temperatures and piezoelectric coefficients. In the present work, powders of (K0.5Na0.5)(Nb1−xVx)O3 (xxa0=xa00, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by conventional solid state reaction of K2CO3, Na2CO3, Nb2O5, and V2O5. The powders were high-energy milled in isopropyl alcohol medium using a Retsch PM 100 planetary ball mill at the speed of 200xa0rpm for 8xa0h. The milled powders were sintered at 1080xa0°C for 1xa0h in closed alumina crucible. The crystalline phase of the ceramics was found to be perovskite with orthorhombic symmetry. Increasing V5+ content in the ceramics from xxa0=xa00 to xxa0=xa00.03 gives rise to gradual increase in room temperature dielectric constant (εr) from 346 to 432, remnant polarization (Pr) from 6.2 to 9.6xa0μC/cm2, electromechanical coupling factor (kp) from 0.35 to 0.39, and piezoelectric charge constant (d33) from 76 to 93xa0pC/N. The increase in these parameters is attributed to the associated increase in density of the ceramics with increasing V5+ content. Further increase in V5+ content to xxa0=xa00.05 results in decrease in εr, Pr, kp and d33 to 390, 4.7xa0μC/cm2, 0.33 and 67xa0pC/N respectively. The study reveals that an optimum concentration of V5+ can enhance the dielectric, ferroelectric and piezoelectric properties of (K0.5Na0.5)NbO3 system.

Study of dielectric and piezoelectric properties of neodymium doped Ba(0.85-x)NdxCa0.15Zr0.1Ti0.9O3 ceramics

ABSTRACT Ba(0.85-x)NdxCa0.15Zr0.1Ti0.9O3 (x = 0, 0.005, 0.010, 0.015, 0.020, 0.025) ceramic compositions were synthesized by solid state reaction method. Effects of Nd3+ on the microstructure, dielectric, and piezoelectric properties of the ceramics were studied. X-ray diffraction study reveals that Nd3+ ions diffuse into Ba(0.85-x)NdxCa0.15Zr0.1Ti0.9O3 lattice to form a solid solution of single perovskite structure having tetragonal symmetry. It was found that cell volume, crystallite size, tetragonality, d33 and Pr change with increasing Nd3+ content. d33 was optimized at 270pC/N for x = 0.015. The study reveals that optimum doping of Nd3+ in Ba(0.85-x)NdxCa0.15Zr0.1Ti0.9O3 can enhance its dielectric and piezoelectric properties.

Study of bicontinuous phase in (TTAB+pentanol)/water/n-octane reverse micellar system using positron lifetime spectroscopy

A phase diagram of (TTAB+pentanol)/water/n-octane has been mapped by using optical method. It exhibits a reverse micellar (L2) phase extending over a wide range of concentrations of the constituents. To investigate the fine structure of the L2 phase, a series of (TTAB+pentanol)/n-octane ternary mixtures having initial concentrations of (TTAB+pentanol) (1:1) in n-octane as 35%, 50% and 65% by weight were prepared. In each of these mixtures, positron lifetime measurements were performed as a function of the concentration of water, using a standard lifetime spectrometer. At water concentrations of 11.8%, 8.5% and 8.4% by weight respectively for the above systems, the o-Ps pick-off lifetime τ3 shows an oscillatory behaviour while I3 representing the Ps formation exhibits an abrupt change. These changes in the positron annihilation parameters have been explained on the basis of onset of bicontinuity in the microemulsion phase. The positron annihilation technique thus suggests the existence of droplet-like and bicontinuous structures in the L2 phase which is otherwise considered optically to be a single phase as the system remains clear and isotropic throughout this phase. Supporting evidence has been provided by the electrical conductivity measurements performed in these systems. These results are presented in this paper.

Location of Phase Boundaries of Lyotropic Liquid Crystal Employing Positron Lifetime Spectroscopy and Electrical Conductivity Measurement

Different compositions of surfactant systems give rise to a rich variety of structures of aggregates. At higher concentrations of surfactant in water, the surfactant molecules aggregate to form lyotropic liquid crystals [1]. In the present work we have prepared two surfactant systems consisting of (i) 20% of cetyl-trimethyl-ammonium-bromide (CTAB) in water, and (ii) 30% of tetra-decyl-trimethyl-ammonium-bromide (TTAB) in water. Both the systems exhibit various lyotropic liquid crystal structures when an increasing amount of co-surfactant is added as third component [2, 3]. These liquid crystalline structures are very sensitive to the solution conditions such as co-surfactant concentration, temperature, ionic strength, counter ion polarizability etc. In this study, positron life time spectroscopy and conductivity measurement have been employed to locate various phases exhibited by the lyotropic liquid crystals. In addition to delineating various phase boundaries of the systems, positron annihilation technique has also yielded new findings.

Positron Lifetime Spectroscopy as a Tool for Studying Molecular Motions in n-p-Ethoxybenzylidene-p-Butylaniline

Temperature dependent positron lifetime measurements have been carried out in n-p-ethoxybenzylidene-p-butylaniline. The samples were prepared either by quenching or slow cooling from the liquid crystalline phase of the compound. The lifetime measurements were performed in the heating cycle of these samples in the temperature range 165 K to 355 K. The behaviour of the quenched sample is found to be quite different from that of the slow-cooled sample. The temperature dependence of o-Ps pick-off lifetime in the quenched sample, unlike the slow-cooled sample, exhibits eight distinct peaks at various characteristic temperatures before the material undergoes a glass transition. These peaks have been attributed to various intraand intermolecular modes executed by the molecules. The characteristic frequencies of some of the modes observed in the present work agree well with the literature reported values obtained from FIR and Raman studies, for others no such information is available. The present study demonstrates the usefulness of positron lifetime spectroscopy for investigating molecular motions. Introduction Molecular motions in condensed matter have been widely investigated with conventional techniques like Infrared and Raman spectroscopy. These techniques have also been employed to elucidate the molecular structure and the nature of intermolecular interactions in such materials. Recently positron lifetime spectroscopy (PLS) has also been used [1-3] to study some molecular motions in liquid crystal forming materials. This technique is based on the sensitivity of positron annihilation characteristics to the physical state of the system. On rapid cooling, a liquid may not solidify into a crystalline solid but may go to a super cooled state and then finally transform into a glassy or amorphous solid, although it is thermodynamically unstable. The formation of such a glassy state has been observed in several compounds [4-10]. The relaxation times of glassy systems are large compared to macroscopic observation times; therefore, they provide an ideal situation for studying the complex molecular dynamics. The formation of a glass forming super cooled state is believed to be a direct consequence of low degree of symmetry of the unstrained molecules [11]. Therefore, the systems composed of large asymmetric molecules have a higher chance of super cooling to a glass forming state. The typical cooling rates for such systems to undergo super cooling are conveniently low, ~ 0.5 Ks 1 . The molecules of liquid crystal forming compound n-p-ethoxybenzylidine-p-butylaniline (EBBA) are highly asymmetric and have the ability to form a disordered glassy state on being quenched from the isotropic or liquid crystalline phase. This compound has, therefore, been chosen for the present investigation. Normally, in the glassy state all molecular motions get frozen. However, the availability of free volume in such a state provides a room for the execution of some segmental motions of the Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 244-248 doi:10.4028/www.scientific.net/MSF.445-446.244