Simultaneous enhancement of electrical conductivity and thermopower in Bi2S3 under hydrostatic pressure

The inverse coupled dependence of electrical conductivity and thermopower on carrier concentration presents a big challenge in achieving a high figure of merit. However, the simultaneous enhancement of electrical conductivity and thermopower can be realized in practice by carefully engineering the electronic band structure. Here by taking the example of Bi2S3, we report a simultaneous increase in both electrical conductivity and thermopower under hydrostatic pressure. Application of hydrostatic pressure enables tuning of electronic structure in such a way that the conductivity effective mass decreases and the density of states effective mass increases. This dependence of effective masses leads to simultaneous enhancement in electrical conductivity and thermopower under n-type doping leading to a huge improvement in the power factor. Also lattice thermal conductivity exhibits very weak pressure dependence in the low pressure range. The large power factor together with low lattice thermal conductivity results in a high ZT value of 1.1 under n-type doping, which is nearly two times higher than the previously reported value. Hence, this pressure-tuned behaviour can enable the development of efficient thermoelectric devices in the moderate to high temperature range. We further demonstrate that similar enhancement can be observed by generating chemical pressure by doping Bi2S3 with smaller iso-electronic elements such as Sb at Bi sites, which can be achieved experimentally.

Tailored electrical conductivity, electromagnetic shielding and thermal transport in polymeric blends with graphene sheets decorated with nickel nanoparticles

Electromagnetic interference shielding (EMI) materials were designed using PC (polycarbonate)/SAN poly(styrene-co-acrylonitrile)] blends containing few-layered graphene nanosheets decorated with nickel nanoparticles (G-Ni). The graphene nanosheets were decorated with nickel nanoparticles via the uniform nucleation of the metal salt precursor on graphene sheets as the substrate. In order to localize the nanoparticles in the PC phase of the PC/SAN blends, a two-step mixing protocol was adopted. In the first step, graphene sheets were mixed with PC in solution and casted into a film, followed by dilution of these PC master batch films with SAN in the subsequent melt extrusion step. The dynamic mechanical properties, ac electrical conductivity, EMI shielding effectiveness and thermal conductivity of the composites were evaluated. The G-Ni nanoparticles significantly improved the electrical and thermal conductivity in the blends. In addition, a total shielding effectiveness (SET) of -29.4 dB at 18 GHz was achieved with G-Ni nanoparticles. Moreover, the blends with G-Ni exhibited an impressive 276% higher thermal conductivity and 29.2% higher elastic modulus with respect to the neat blends.

Electrical conductivity and dielectric relaxation studies on microwave synthesized Na2SO4 center dot NaPO3 center dot MoO3 glasses

Electrical conductivity and dielectric relaxation studies on SO4 (2-) doped modified molybdo-phosphate glasses have been carried out over a wide range of composition, temperature and frequency. The d.c. conductivities which have been measured by both digital electrometer (four-probe method) and impedance analyser are comparable. The relaxation phenomenon has been rationalized using electrical modulus formalism. The use of modulus representation in dielectric relaxation studies has inherent advantages viz., experimental errors arising from the contributions of electrode-electrolyte interface capacitances are minimized. The relaxation observed in the present study is non-Debye type. The activation energies for relaxation were determined using imaginary parts of electrical modulus peaks which were close to those of the d.c. conductivity implying the involvement of similar energy barriers in both the processes. The enhanced conductivity in these glasses can be attributed to the migration of Na+, in expanded structures due to the introduction of SO4 (2-) ions.

Tuning Photoluminescence Response by Electric Field in Electrically Soft Ferroelectrics

We show that an electrically soft ferroelectric host can be used to tune the photoluminescence (PL) response of rare-earth emitter ions by external electric field. The proof of this concept is demonstrated by changing the PL response of the Eu3+ ion by electric field on a model system Eu-doped 0.94(Na1/2Bi1/2TiO3)-0.06(BaTiO3). We also show that new channels of radiative transitions, forbidden otherwise, open up due to positional disorder in the system, which can as well be tuned by electric field.

Long-period modulated structure and electric-field-induced structural transformation in Na0.5Bi0.5TiO3-based lead-free piezoelectrics

Na0.5Bi0.5TiO3- based lead-free piezoelectrics exhibiting giant piezostrain are technologically interesting materials for actuator applications. The lack of clarity with regard to the structure of the nonpolar phase of this system has hindered the understanding of the structural mechanism associated with the giant piezostrain and other related phenomena. In this paper, we have investigated the structure and field-induced phase transformation behavior of a model system (0.94 - x) Na0.5Bi0.5TiO3-0.06BaTiO(3)-xK(0.5)Na(0.5)NbO(3) (0.0 <= x <= 0.025). A detailed structural analysis using neutron powder diffraction revealed that the nonpolar phase is neither cubic nor a mixture of rhombohedral (R3c) and tetragonal (P4bm) phases as commonly reported in literature but exhibits a long-period modulated structure, which is most probably of the type root 2 x root 2 x n with n = 16. Our results suggest that the giant piezoelectric strain is associated with a field-induced phase transformation of the long-period modulated structure to rhombohedral R3c structure above a critical field. We also demonstrate that the giant piezostrain is lost if the system retains a fraction of the field-induced R3c phase. A possible correlation among depolarization temperature, giant piezostrain, and its electrical fatigue behavior has also been indicated.

Effective thermal conductivity of reactive packed beds of hydriding materials

Applications of hydriding materials for solid state hydrogen storage, hydrogen compression, thermal energy storage and sorption heating and cooling systems have been demonstrated successfully. However, the performance of these devices significantly depends upon heat and mass transfer characteristics of the reactive packed beds. One of the important parameters regulating heat and mass transfer in the hydriding bed is its effective thermal conductivity (ETC), which is dependent on several operating parameters such as pressure and temperature. ETC also varies significantly due to the variation of hydrogen concentration during the hydriding and dehydriding processes. Based on the extensive studies done by the authors on ETC of metal hydride beds, a review of experimental methods, mathematical studies and augmentation techniques is presented in this paper, with emphasis on the effects of operating parameters on ETC. (C) 2016 Elsevier Ltd. All rights reserved.