Structural evolution and magnetic properties of Gd2Hf2O7 nanocrystals: Computational and experimental investigations

Structural evolution in functional materials is a physicochemical phenomenon, which is important from a fundamental study point of view and for its applications in magnetism, catalysis, and nuclear waste immobilization. In this study, we used x-ray diffraction and Raman spectroscopy to examine the Gd2Hf2O7 (GHO) pyrochlore, and we showed that it underwent a thermally induced crystalline phase evolution. Superconducting quantum interference device measurements were carried out on both the weakly ordered pyrochlore and the fully ordered phases. These measurements suggest a weak magnetism for both pyrochlore phases. Spin density calculations showed that the Gd3+ ion has a major contribution to the fully ordered pyrochlore magnetic behavior and its cation antisite. The origin of the Gd magnetism is due to the concomitant shift of its spin-up 4f orbital states above the Fermi energy and its spin-down states below the Fermi energy. This picture is in contrast to the familiar Stoner model used in magnetism. The ordered pyrochlore GHO is antiferromagnetic, whereas its antisite is ferromagnetic. The localization of the Gd-4f orbitals is also indicative of weak magnetism. Chemical bonding was analyzed via overlap population calculations: These analyses indicate that Hf-Gd and Gd-O covalent interactions are destabilizing, and thus, the stabilities of these bonds are due to ionic interactions. Our combined experimental and computational analyses on the technologically important pyrochlore materials provide a basic understanding of their structure, bonding properties, and magnetic behaviors

Carbon combustion synthesis of Janus-like particles of magnetoelectric cobalt ferrite and barium titanate

Carbon combustion synthesis of oxides was applied for quick and energy efficient production of multiferroic composite of cobalt ferrite and barium titanate to form Janus-like particles matrix structure. The exothermic oxidation of carbon nanoparticles with an average size of 5 nm and a specific surface area of 110 m2/g generates a self-propagating thermal wave with peak temperature of up to 1000 °C. The thermal front rapidly propagates through the mixture of solid reactants (magnetic- CoFe2O4 and ferroelectric-BaTiO3) and results in localized hot-spot sintering of magneto-electric phases to form a nanocomposite structure. Carbon is not incorporated in the product and is emitted as a gaseous CO2. Existence of discrete CoFe2O4 and BaTiO3phases in the composites nanostructures was confirmed using X-ray powder diffraction along with SEM and TEM analysis. We estimated the activation energy for the combustion synthesis of Janus-like particles to be 112 ± 3.3 kJ/mol, indicating that the barium titanate and cobalt ferrite presence decrease the activation energy barrier of carbon oxidation and facilitate the ignition process of the combustion synthesis. We observe that the as-synthesized samples show magnetoelectric coupling on multiferroic cobalt ferrite–barium titanate ceramic composites

Transition of p- to n-Type Conductivity in Mechanically Activated Bismuth Telluride

Bismuth telluride (Bi2Te3) exhibits a transition from p- to n-type conduction as a result of high-energy ball milling. The transition is monitored over mechanical activation through measurement of the thermoelectric properties in the temperature range of 1.9 K to 390 K. Data show a flip in polarity of the Seebeck coefficient from 225 μV K−1 for the bulk sample to − 120 μV K−1 (at 315 K) that correlates to fracturing the layered-like structure of stoichiometric Bi2Te3 into platelets and fine particles. The electronic transition is generated by fracturing the crystal 90° to the basal plane. This is the structural equivalent to inducing n-type, anti-site defects on grain boundaries. The observed phenomenon could be exploited to fabricate p- and n-type legs for thermoelectric devices from the same material. In this report, we demonstrate that the value of the Seebeck coefficient for bismuth telluride can be tuned using mechanical treatment. We also determine how mechanical activation of Bi2Te3 impacts physical properties of the system, including: particle size, crystal structure, band gap, electrical and thermal conductivity, carrier concentration and mobility, average hopping distance, and the concentration of localized charged centers

The effect of MWCNT addition on superconducting properties of MgB2 fabricated by high-pressure combustion synthesis

We successfully prepared superconducting powders of magnesium diboride doped with carbon nanotubes by the method of combustion synthesis under high Ar pressure. Powders of magnesium, boron, and multi-walled carbon nanotubes (MWCNT) were used as starting materials. X-ray diffraction analysis showed the presence of MgB2 and MgO in combustion products. The temperature dependence of magnetization showed a sharp superconducting transition at around 38.5 K. The critical current density can be estimated from the hysteresis of magnetization curve by using the Bean’s formula. MgB2 doped with MWCNT (1%) showed the best value of high critical current density, 1.4 × 108 A/cm2 at 5 K, in zero magnetic fields

Solution-combustion synthesis and magnetodielectric properties of nanostructured rare earth ferrites

Rare earth ferrites exhibit remarkable magnetodielectric properties that are sensitive to the crystallite size. There is a major challenge to produce these materials in nanoscale due to particles conglomeration during the ferrite nucleation and synthesis. In this paper we report the fabrication of nanostructured particles of rare earth ferrites in the Me-Fe-O system (Me = Y, La, Ce, and Sm) by Solution-Combustion Synthesis (SCS). The yttrium, lanthanum, cerium, samarium and iron nitrates were used as metal precursors and glycine as a fuel. Thermodynamic calculations of Y(NO3)3-2Fe(NO3)3−nC2H5NO2 systems producing Y3Fe5O12 predicted an adiabatic temperature of 2250 K with generating carbon dioxide, nitrogen and water vapor. The considerable gas evolution helps to produce the synthesized powders friable and loosely agglomerated. Adjusting the glycine/metal nitrates ratio can selectively control the crystallite size and magnetodielectric properties of the ferrites. Increasing the glycine content increased the reaction temperature during the SCS and consequently the particle size. Magnetization of zero-field-cooled (ZFC) and field-cooled (FC) ferrites in the temperature range of 1.9–300 K showed different patterns when the fraction of glycine was increased. Analysis of ZFC and FC magnetization curves of annealed samples confirmed that nanoparticles exhibit superparamagnetic behavior. The increasing concentration of glycine leads to escalation of blocking temperature. Reduction of dielectric permittivity (ɛr) toward frequency indicates the relaxation processes in the composites, and the values of ɛr are shifted upward along the operating temperature

Synthesis of Superparamagnetic Zinc Ferrite Encased Fluorapatite Nanoparticles and Its Cytotoxicity Effects on MG-63 Cells

Zinc ferrite (ZnFe2O4) nanoparticles encased fluorapatite (FAP) nanorods was successfully synthesized through hydrothermal method for biomedical applications. TEM image revealed the formation of ZnFe2O4 nanoparticle encased FAP nanorods with average size of about 60 ± 40 nm. Both ZnFe2O4 and ZnFe2O4–FAP samples exhibited superparamagnetic nature at 300 K with saturation magnetization (Ms) of 28.41 and 4.13 emu/g, respectively. Further, the superparamagnetic property of the ZnFe2O4 was investigated using langevin function and average magnetic moment per particle was found to be 606 μB . ZnFe2O4–FAP nanorods exhibited enhanced colloidal stability when compared to ZnFe2O4 nanospheres. The cytotoxicity results confirmed the enhanced cell viability (86%) at 500 µg/mL of ZnFe2O4–FAP nanorods than that of ZnFe2O4 nanospheres (76%). The above results indicate that the ZnFe2O4–FAP nanorods can be considered as a potential candidate for biomedical applications