Iftikhar Hussain Gul

Influence of Rare Earth (Gd3+) on Structural, Gigahertz Dielectric and Magnetic Studies of Cobalt ferrite

A series of Gd3+ doped nanocrystalline Co-ferrites CoGdxFe2-xO4 (x = 0.0 to 0.1) has been prepared by sol-gel auto combustion technique. Structural and morphology studies were performed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Indexed XRD patterns confirm the formation of pure cubic spinel phase. Average crystallite sizes ranges from 16 nm to 25 nm ±2, were calculated from Sherrers formula and Williamson Hall plots. Crystal strain increases with increase in doping amount of trivalent Gd ion. Lattice constant (a) and crystallite size D (311) increases with increase in Gd3+ concentration due to large ionic radii (0.94nm) of Gd3+ replacing Fe3+ (0.64nm). SEM images show the spherical morphology and uniform size distribution. Room temperature DC electrical resistivity decreases (~106) for x=0.025 then increases up to x=0.1 ~ (4.5×107). Dielectric properties have been studied using RF Impedance/material analyzer in the frequency range of 1 MHz to 1GHz. All the studied samples show a semi-conducting behavior as Permittivity and tangent loss (tanδ) decreases with the substitution of Gd3+ in parent crystal structure and have values of 4.92 and 0.016 at 1 GHz respectively. Complex impedance and Complex electric modulus plots were further studied for complete contribution of grains and grain boundary resistances to conduction and resonance frequencies respectively. Magnetic studies by Vibrating Sample Magnetometer (VSM) shows that magnetization (Ms) decreases with increase in Gd3+ concentration from 63 emu/gm to 27.26 emu/gm. Coercivity (Hc) first decreases for x=0.025, after which it increases to 2308 Oe for x=0.1.

Prediction of thermal conductivity of granite rocks from porosity and density data at normal temperature and pressure: in situ thermal conductivity measurements

To predict the thermal conductivity of granite rocks the existing mixing law and empirical models have been utilized. Due to the porosity dependence in all the existing models, ASTM standard methods have been utilized to measure the porosity including specific gravity and density. The chemical composition of these samples has been analysed using the x-ray fluorescence technique. Because of the complex structure of the specimens and non-availability of data, models depending on the strict geometrical structure like the shape, size, position of the constituent mineral and the arrangement of fluid in the samples have not been used. The predicted values of the effective thermal conductivity have been compared with the experimental measurements made on the samples, using Gustafssons probe. The calculated thermal conductivities from various models are in agreement with the experimental observations. Simple correlations between the chemical composition and specific gravity, density and porosity are also reported.

Stiff, strong, yet tough free-standing dielectric films of graphene nanosheets-polyurethane nanocomposites with very high dielectric constant and loss

In this study, graphene nanosheets (GNS) prepared through a liquid exfoliation technique are dispersed in thermoplastic polyurethane (TPU) at a volume fraction (Vf) of up to 0.19. Then, the electrical and mechanical properties of the obtained composites are characterized. The dielectric spectroscopy shows an excessive variation in dielectric constant (1.1 to 3.53 × 107) and dielectric tangent loss (0.03 to 2515) with varying Vf over the frequency range of 25 kHz to 5 MHz. A considerable enhancement in electrical conductivity (DC) is found, from 3.87 × 10−10 S/m (base polymer) to 53.5 S/m for the 0.19 Vf GNS-TPU nanocomposite. The GNS-TPU composites are mechanically robust, with a considerable increase in stiffness (∼4-fold) and strength (almost twice), maintaining its ductility up to 0.09 Vf GNS. The high dielectric constant at lower frequencies is attributed to the well-established Maxwell-Wagner polarization effect, whereas the high dielectric tangent loss is due to leakage currents as a physical conducting network is formed at high filler loadings. The layered structure, high aspect ratio, and improved dispersion of GNS are the main reasons for the improvement in both the dielectric characteristics and the mechanical properties of the host polymer.

Enhancing Dielectric and Mechanical Behaviors of Hybrid Polymer Nanocomposites Based on Polystyrene, Polyaniline and Carbon Nanotubes Coated with Polyaniline

The dielectric and mechanical properties of hybrid polymer nanocomposites of polystyrene/polyaniline/carbon nanotubes coated with polyaniline (PCNTs) have been investigated using impedance analyzer and extensometer. The blends of PS/PANI formed the heterogeneous phase separated morphology in which PCNTs are dispersed uniformly. The incorporation of a small amount of PCNTs into the blend of PS/PANI has remarkably increased the dielectric properties. Similarly, the AC conductivity of PS/PANI is also increased five orders of magnitude from 1.6 × 10−10 to 2.0 × 10−5 S∙cm−1 in the hybrid nanocomposites. Such behavior of hybrid nanocomposites is owing to the interfacial polarization occurring due to the presence of multicomponent domains with varying conductivity character of the phases from insulative PS to poor conductor PANI to highly conductive CNTs. Meanwhile, the tensile modulus and tensile strength are also enhanced significantly up to 55% and 160%, respectively, without much loss of ductility for three phase hybrid nanocomposites as compared to the neat PS. Thereby, the hybrid nanocomposites of PS/PANI/PCNTs become stiffer, stronger and tougher as compared to the neat systems.

Influence of Reduced Graphene Oxide on Effective Absorption Bandwidth Shift of Hybrid Absorbers

The magnetic nanoparticle composite NiFe2O4 has traditionally been studied for high-frequency microwave absorption with marginal performance towards low-frequency radar bands (particularly L and S bands). Here, NiFe2O4 nanoparticles and nanohybrids using large-diameter graphene oxide (GO) sheets are prepared via solvothermal synthesis for low-frequency wide bandwidth shielding (L and S radar bands). The synthesized materials were characterized using XRD, SEM, FTIR and microwave magneto dielectric spectroscopy. The dimension of these solvothermally synthesized pristine particles and hybrids lies within 30–58 nm. Microwave magneto-dielectric spectroscopy was performed in the low-frequency region in the 1 MHz-3 GHz spectrum. The as-synthesized pristine nanoparticles and hybrids were found to be highly absorbing for microwaves throughout the L and S radar bands (< −10 dB from 1 MHz to 3 GHz). This excellent microwave absorbing property induced by graphene sheet coupling shows application of these materials with absorption bandwidth which is tailored such that these could be used for low frequency. Previously, these were used for high frequency absorptions (typically > 4 GHz) with limited selective bandwidth.

Ce-Substituted Co0.5Ni0.5Fe2O4: Structural, morphological, electrical, and dielectric properties

Functional nano ceramics, especially rare earth substituted ferrites, have numerous applications in microelectronics. They have certain novel characteristics associated with various rare earth substitutions associated with electronics. A series of cerium-substituted cobalt nickel ferrite nanoparticles with nominal composition CexCo0.5Ni0.5Fe2-xO4 (where x = 0.00, 0.05, 0.10, 0.15, and 0.20) was prepared in powder form by chemical co-precipitation and sol-gel auto combustion methods. Samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), dielectric properties, and DC electrical resistivity. XRD analysis of all the samples confirms the formation of the spinel structure of Fd-3m space group. Crystallite size “t” was found to decrease, whereas both x-ray density “Dx” and lattice constant “a” were found to increase by increasing Ce concentration from 8.35 Å to 8.36 Å and 8.37 Å for sol-gel and coprecipitation, respectively. SEM results show the presence of uniformly distributed and almost spherical-shaped particles for both techniques. FTIR measurements were used to further confirm the composition, i.e., cation substitution. The DC electrical resistivity of the x = 0.00 sample is higher than Ce-substituted ferrite samples and in the range of 108 O-cm at 373 K. Dielectric spectroscopy is also studied from 100 Hz to 5 MHz as a function of frequency. To our knowledge, this is the first reported research of this particular composition.

Thermophysical properties of AgCl in the temperature range 77?300?K

The thermal conductivity, thermal diffusivity and volumetric heat capacity of monocrystalline AgCl were measured simultaneously, using the Gustafsson probe in the temperature range 77?300?K. The thermal conductivity, as well as the thermal diffusivity, of these crystals follows a T?1 behaviour in the temperature region mentioned here. The heat capacity at constant pressure (CP), determined from the volumetric heat capacity, agrees with the calculated one at room temperature. The values of CP obtained from this experiment along with the reported data as a function of temperature are also reported.

Effect of High Substrate Temperature on Morphology, Structural and Optical Properties of CdZnS Nanostructures

One-dimensional CdZnS nanostructures have been synthesized through the sublimation. Effect of high substrate temperature on morphology, structural and optical properties of these nanostructures has been studied. X-Ray diffraction peak intensity, lattice parameters, crystallite size decreased with an increase in substrate temperature. The morphology changed with the increase in the substrate temperature. Raman Spectroscopy confirmed the existence of constituent elements in CdZnS solid solution and an increase of Zn concentration with the rise in substrate temperature. The nanostructures exhibited strong photoluminescence emission in the green light region with a substrate temperature-dependent blue shift of 53 meV in emission energy. The Stoke’s shift energy raised from 45 meV to 302 meV as the substrate temperature increased from 510 °C to 550 °C. The stoichiometric deviancies, crystallite size, and quantum confinement effects resulted into an increase in the optical band gap from 2.4 eV to 2.71 eV. The results showed that CdZnS nanostructures could be potential candidates for nanostructure based optoelectronics and photovoltaic devices.

Magnetic and RF-electromagnetic absorbing study of aluminum doped nickel ferrites prepared by two techniques

Pure phase aluminum doped nickel ferrite nanoparticles [NiAl x Fe2?x O4] with x?=?0.0, 0.25 and 0.5 have been investigated for magnetic and electromagnetic absorbing properties in S band and influence of Al3+ ions has been studied. Nickel aluminum ferrites have been synthesized by co-precipitation and sol?gel auto combustion routes. Magneto-dielectric properties were measured in terms of complex permeability (?*) and complex permittivity (?*) at 300 K in the frequency range of 1 MHz to 3 GHz using RF material/impedance analyzer. Dielectric permittivity (?? and ??) has been observed to decrease with the increase in applied frequency and concentration of aluminum ions (x?=?0.0?0.5). Magnetic permeability (?? and ??) has been found to decrease with the increase in aluminum ions concentration. Electromagnetic absorbing properties were studied for all the samples by calculating the reflection losses (RL) in the frequency range of 1 MHz?3 GHz using the permittivity and permeability measurements in accordance with the transmission line theory. RL (dB) values lie in the range of ?42 dB to ?57 dB. RL values (>??10 dB) confirm more than 90% absorption of electromagnetic waves incident normal to the material surface. Aluminum doping has increased the RL values from ?45 dB to ?57 dB along with the shift of RL dip towards the higher frequency side for the samples prepared by sol?gel auto combustion technique. Magnetic properties have been studied by vibrating samples magnetometer (VSM) at room temperature and a decrease in magnetic properties has been observed due to increase in x from 0.0?0.5.