V. Sankaranarayanan

Graphene-Based Engine Oil Nanofluids for Tribological Applications

Ultrathin graphene (UG) has been prepared by exfoliation of graphite oxide by a novel technique based on focused solar radiation. Graphene based engine oil nanofluids have been prepared and their frictional characteristics (FC), antiwear (AW), and extreme pressure (EP) properties have been evaluated. The improvement in FC, AW, and EP properties of nanofluids is respectively by 80, 33, and 40% compared with base oil. The enhancement can be attributed to the nanobearing mechanism of graphene in engine oil and ultimate mechanical strength of graphene.

Inorganic nanotubes reinforced polyvinylidene fluoride composites as low-cost electromagnetic interference shielding materials

Novel polymer nanocomposites comprising of MnO2 nanotubes (MNTs), functionalized multiwalled carbon nanotubes (f-MWCNTs), and polyvinylidene fluoride (PVDF) were synthesized. Homogeneous distribution of f-MWCNTs and MNTs in PVDF matrix were confirmed by field emission scanning electron microscopy. Electrical conductivity measurements were performed on these polymer composites using four probe technique. The addition of 2 wt.% of MNTs (2 wt.%, f-MWCNTs) to PVDF matrix results in an increase in the electrical conductivity from 10-16S/m to 4.5 × 10-5S/m (3.2 × 10-1S/m). Electromagnetic interference shielding effectiveness (EMI SE) was measured with vector network analyzer using waveguide sample holder in X-band frequency range. EMI SE of approximately 20 dB has been obtained with the addition of 5 wt.% MNTs-1 wt.% f-MWCNTs to PVDF in comparison with EMI SE of approximately 18 dB for 7 wt.% of f-MWCNTs indicating the potential use of the present MNT/f-MWCNT/PVDF composite as low-cost EMI shielding materials in X-band region.

Colossal magnetoresistance in the double perovskite oxide La2CoMnO6

Polycrystalline sample of La2CoMnO6 has been synthesized by sol-gel technique. The powder x-ray diffraction data confirm the single phase nature of the sample. This compound has monoclinic crystal structure (space group P21/n) at room temperature. The temperature dependence of magnetization in low field shows considerable variation between zero-field-cooled and field-cooled magnetization curve below ∼210 K (TC) and it follows Curie–Weiss law in the paramagnetic region. The hysteresis loop at 5 K indicates a coercive field of ∼6 kOe and remnant magnetization of ∼2.32 μB/f.u. The temperature dependence of the resistivity data shows semiconductorlike behavior in the temperature range of 5–350 K and follows variable range hopping conduction mechanism in the temperature range 215–350 K. A colossal magnetoresistance of ∼80% is observed at 5 K in an applied field of 80 kOe and MR has a negative sign.

Study of magnetic properties of A2B'NbO6 (A=Ba, Sr, BaSr; and B'=Fe and mn) double perovskites

We have studied the magnetic properties of Ba2FeNbO6 and Ba2MnNbO6. It is seen that Ba2FeNbO6 is an antiferromagnet with a weak ferromagnetic behavior at 5 K while Ba2MnNbO6 shows two magnetic transitions, one at 45 K and the other at 12 K. Electron spin resonance measurements at room temperature show that the Mn compound does not show any Jahn-Teller distortion. It is also seen that the Neel temperatures of the A2FeNbO6 (A=Ba,Sr,BaSr) compounds do not vary significantly. However variations in the average A-site ionic radius influence the formation of short range correlations that persist above TN.

Observation of spin glass state in weakly ferromagnetic Sr2FeCoO6 double perovskite

A spin glass state is observed in the double perovskite oxide Sr2FeCoO6 prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal I4/m structure with lattice parameters, a = 5.4609(2) A and c = 7.7113(7) A. The temperature dependent powder x ray diffraction data reveal no structural phase transition in the temperature range 10-300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr2FeCoO6, the B site is randomly occupied by Fe and Co in the mixed valence states of Fe3 + /Fe4+ and Co3+/Co4+. The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass state in this double perovskite, which is observed as an irreversibility in temperature dependent dc magnetization...

Exchange bias and memory effect in double perovskite Sr2FeCoO6

We report on the observation of exchange bias (EB) and memory effect in double perovskite Sr2FeCoO6. Antiphase boundaries between the ferromagnetic and the antiferromagnetic regions in the disordered glassy phase are assumed as responsible for the observed effect, which reflects in the cooling field dependence and temperature evolution of exchange bias field and in training effect. The spin glass (SG) phase itself is characterized through memory, ageing, and magnetic relaxation experiments. The spin glass transition temperature, Tg, versus Hdc2/3 follows the Almeida-Thouless line yielding a freezing temperature, Tf=73K. Time-dependent magnetic relaxation studies reveal the magnetization dynamics of the underlying glassy phase in this double perovskite.

Large magnetic entropy change in nanocrystalline Pr0.7Sr0.3MnO3

Nanocrystalline Pr0.7Sr0.3MnO3 sample has been prepared by sol-gel method. The room temperature powder x-ray diffraction data show single phase nature of the sample and confirm the cubic crystal structure with Fm3¯m space group. The average crystallite size is calculated using Scherrer formula, and it is found to be ∼25 nm. Transmission electron microscopy image shows that the particles are spherical in shape and the average particle size is ∼35 nm. The sample undergoes ferromagnetic ordering at 235 K (TC) and obeys the Curie–Weiss law in the paramagnetic region. The maximum value of the magnetic entropy change |ΔSM|max is ∼6.3 J kg−1 K−1, and the relative cooling power is ∼385 J kg−1 for a field change of 50 kOe. The Arrott plot confirms that the magnetic ordering is of second order nature. The experimentally observed magnetic entropy change of the sample obeys Landau theory of phase transition well.

Dielectric relaxation properties of nanostructured Ce0.8Gd0.1Pr0.1O2−δ material at intermediate temperatures

The dielectric relaxation behavior of the fluorite structured nanocrystalline Ce0.8Gd0.1Pr0.1O2−δ compound was studied in the temperature range of 200–550 °C. Two different types of relaxation processes were observed corresponding to (1) defect pairs such as (PrCe′–VO)• and (GdCe′–VO)• and (2) the trimers such as (PrCe′–VO••–GdCe′). The correlation between ionic conduction and the dielectric properties of the nanocrystalline material is discussed. Very low values of the migration energy and association energy of the oxygen vacancies are observed, which are 0.42 and 0.03 eV, respectively. The obtained value of association energy agrees well with the theoretical prediction on doubly doped ceria.

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toTc. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.

Thermodynamic and hydrogen-induced structural properties of Ho1−xMmxCo2-hydrides

Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1−xMmxCo2 (x = 0, 0.1, 0.2, 0.3 and 0.4, Mm = mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50–200 °C and pressure range 0.001–1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho1−xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1−xMmxCo2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1−xMmxCo2-H to amorphous Ho1−xMmxCo2-H and recrystallization into elemental hydrides have been studied and discussed.