Melendez, A.; Manadhar, S.; Singamaneni, S. R.; Reddy, Kongara M.; Gandha, Kinjal H.; Niebedim, I. C.; and Ramana, C. V., "Effect of Molybdenum Incorporation on the Structure and Magnetic Properties of Cobalt Ferrite" (2017). Ames Laboratory Accepted Manuscripts. 61. AbstractWe report on the effect of molybdenum (Mo) incorporation on the crystal structure, surface morphology, Mo chemical valence state, and magnetic properties of cobalt ferrite (CoFe2O4, referred to CFO). Molybdenum incorporated cobalt ferrite (CoFe2-xMoxO4, referred to CFMO) ceramics were prepared by the conventional solid-state reaction method by varying the Mo concentration in the range of x = 0.0-0.3. X-ray diffraction studies indicate that the CFMO materials crystallize in inverse spinel cubic phase. Molybdenum incorporation induced lattice parameter increase from 8.322 to 8.343 Å coupled with a significant increase in density from 5.4 to 5.7 g/cm3 was evident in structural analyses. Scanning electron microscopy imaging analyses indicate that the Mo incorporation induces agglomeration of particles leading to larger particle size with increasing x(Mo) values. Detailed X-ray photoelectron spectroscopic (XPS) analyses indicate the increasing Mo content with increasing x from 0.0 to 0.3. XPS confirms that the chemistry of Mo is complex in these CFMO compounds; Mo ions exist in the lower oxidation state (Mo4+) for higher x while in a mixed chemical valence state (Mo4+, Mo5+, Mo6+) for lower x values. From the temperature-dependent magnetization, the samples show ferrimagnetic behavior including the pristine CFO. From the isothermal magnetization measurements, we find almost 2-fold decrease in coercive field (Hc) from 2143 to 1145 Oe with the increase in Mo doping up to 30%. This doping-dependent Hc is consistently observed at all the temperatures measured (4, 100, 200, and 300 K). Furthermore, the saturation magnetization estimated at 4 K and at 1.5 T (from M-H loops) goes through a peak at 92 emu/g (at 15% Mo doping) from 81 emu/g (pristine CFO), and starts decreasing to 79 emu/g (at 30% Mo doping). The results demonstrate that the crystal structure, microstructure, and magnetic properties can be tuned by controlling the Mo-content in the CFMO materials. ABSTRACTWe report on the effect of molybdenum (Mo) incorporation on the crystal structure, surface morphology, Mo chemical valence state and magnetic properties of cobalt ferrite (CoFe2O4, referred to CFO). Molybdenum incorporated cobalt ferrite (CoFe2-xMoxO4, referred to CFMO) ceramics were prepared by the conventional solid-state reaction method by varying the Mo concentration in the range of x=0.0-0.3. X-ray diffraction studies indicate that the CFMO materials crystallize in inverse spinel cubic phase. Molybdenum incorporation induced lattice parameter increase from 8.322 to 8.343 Å coupled with a significant increase in density from 5.4 to 5.7 g/cm 3 was evident in structural analyses. Scanning electron microscopy imaging analysis indicate that the Mo incorporation induces agglomeration of pa...
Graphical Abstract 20 40 60 80 100 120 140 160 180 0.0 2.0x10 4 4.0x10 4 6.0x10 4 8.0x10 4 1.0x10 5 1.2x10 5 NiO Abstract This paper reports on the synthesis of polycrystalline (Li,Ti)-doped NiO powders (i.e., Li x Ti 0.1 Ni 1-x O, abbreviated as LTNO) by the solid-state synthesis method. Note that, the doping concentration of Ti is kept constant (x~0.10) in the stoichiometry, the difference in the material behavior of LTNO samples can only be attributed to the effect of Li. X-ray diffraction patterns confirmed a cubic rock-salt structured NiO-based phase with the presence of minor NiTiO 3 phase, were reported elsewhere [Venkata et. al., Chem. Phys. Lett., 649 (2016) 115-118.]. Dense microstructures were obtained using ultra high resolution scanning electron microscope. A high dielectric constant (ε~10 4 ) near room temperature at low-frequency was observed in LTNO ceramics. Weak temperature dependence of dielectric constant over the measured compositions (x=0 to 0.10) was observed in the LTNO ceramics. A giant dielectric constant of 10 4 -10 5 at high temperatures (120-170°C) for certain LTNO compositions (x=0.15 to 0.3) was observed in the sintered ceramics. The origin of the high dielectric constant observed in these LTNO ceramics isattributed to the Maxwell-Wagner polarization mechanism and a thermally activated mechanism.
Tuning the dielectric properties via composition-tailoring is demonstrated for variablelithium, constant-titanium co-doped nickel oxide (LixTi0.1Ni1-xO; x=0.05-0.30; referred to LTNO).. The effect of variable Li-content on the structure and dielectric properties of LTNO is investigated. X-ray diffraction studies confirm the formation of a cubic NiO phase in LTNO for x=0.05-0.30. However, lattice parameter reduction was evident with increasing Li-content. Composition-driven giant dielectric constant (10 4-10 5) was observed for a higher lithium content (x=0.15-0.30) leading to realization of LTNO ceramics. The origin of the high dielectric constant observed in these low density LTNO ceramics is attributed to the Maxwell-Wagner polarization mechanism.
Molybdenum (Mo), which is one among the refractory metals, is a promising material with a wide variety of technological applications in microelectronics, optoelectronics, and energy conversion and storage. However, understanding the structure–property correlation and optimization at the nanoscale dimension is quite important to meet the requirements of the emerging nanoelectronics and nanophotonics. In this context, we focused our efforts to derive a comprehensive understanding of the nanoscale structure, phase, and electronic properties of nanocrystalline Mo films with variable microstructure and grain size. Molybdenum films were deposited under varying temperature (25–500 °C), which resulted in Mo films with variable grain size of 9–22 nm. The grazing incidence X-ray diffraction analyses indicate the (110) preferred growth behavior the Mo films, though there is a marked decrease in hardness and elastic modulus values. In particular, there is a sizable difference in maximum and minimum elastic modulus values; the elastic modulus decreased from ~460 to 260–280 GPa with increasing substrate temperature from 25–500 °C. The plasticity index and wear resistance index values show a dramatic change with substrate temperature and grain size. Additionally, the optical properties of the nanocrystalline Mo films evaluated by spectroscopic ellipsometry indicate a marked dependence on the growth temperature and grain size. This dependence on grain size variation was particularly notable for the refractive index where Mo films with lower grain size fell in a range between ~2.75–3.75 across the measured wavelength as opposed to the range of 1.5–2.5 for samples deposited at temperatures of 400–500 °C, where the grain size is relatively higher. The conductive atomic force microscopy (AFM) studies indicate a direct correlation with grain size variation and grain versus grain boundary conduction; the trend noted was improved electrical conductivity of the Mo films in correlation with increasing grain size. The combined ellipsometry and conductive AFM studies allowed us to optimize the structure–property correlation in nanocrystalline Mo films for application in electronics and optoelectronics.
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