A. K. Yahya

Effect of (Ba0.6Sr0.4)TiO3 (BST) Doping on Dielectric Properties of CaCu3Ti4O12 (CCTO)

The effect of (Ba 0.6 Sr 0.4 )TiO 3 (BST) addition on dielectric properties of CaCu 3 Ti 4 O 12 (CCTO) ceramic was investigated. Ceramic samples with the chemical formula (1– x )CaCu 3 Ti 4 O 12 + x (Ba 0 . 6 , Sr 0.4 )TiO 3 ( x =0, 0.05, 0.1, and 0.2) were synthesized from high purity oxide powders by the conventional solid-state synthesis method. X-ray diffraction (XRD) analysis showed the existence of BST as a secondary phase alongside CCTO. Scanning electron microscopy (SEM) investigation showed a slight decrease in grain size of doped CCTO samples. Density measurements showed that porosity content increased with increasing BST addition indicating low densification due to high melting point secondary phase addition. Dielectric constant of undoped CCTO ( x =0) showed lack of stability with frequency which dropped drastically between 10 4 and 10 5 Hz and accompanied by high dielectric loss. Addition of BST into CCTO caused the dielectric constant to slightly decrease but improved stability with frequency compared to the undoped sample. The decrease in dielectric constant of doped CCTO samples was suggested to be partly due to the decrease in average grain size and increase in porosity with BST addition. Nevertheless, a high value of dielectric constant was still maintained around ∼10 4 range for all doped samples. The dielectric loss (tanδ) of all BST-doped samples was lower than that of pure CCTO sample at the frequency range of 10 3 to 10 5 Hz probably due to the increase of grains boundary resistivity. The activation energy of grains boundary ( E gb ) showed higher values as compared to the activation energy of grains ( E g ) for all samples and conforms to the internal barrier layer capacitor (IBLC) model.

Effects of Cu substitution on superconductivity and excess conductivity of Tl1–xCuxSr1·8Yb0·2CaCu2O7–δ (x=0–0·6) ceramics

Abstract TlSr1212 samples were synthesised from Tl1–x Cu x Sr1·8Yb0·2CaCu2O7–δ (x=0·0 to 0·6) starting composition using the solid state synthesis method. Temperature dependent electrical resistance measurements showed gradual change of normal state resistance behaviour from metallic (x=0·0 to 0·3) to semimetallic (x=0·4) and to semiconductor-like behaviour (x=0·5 to 0·6). The best superconducting behaviour for the series was observed at x=0·4 with zero resistance critical temperature T c zero of 77·6 K. The effects of Cu substitution on superconducting properties are discussed in terms of the concept of average Cu valence. In addition, excess conductivity behaviour was analysed using Asmalazov–Larkin theory to study the effect of Cu substitution on superconductivity fluctuation behaviour. The analysis showed characteristic cross-over from two-dimensional to three-dimensional behaviour with decreasing temperature.

Effects of Ti substitutions on superconductivity of TlSr-1212 ceramics

Abstract Effects of Ti substitutions on superconductivity of Tl0.8Pb0.2Sr2−xTixCa0.9Y0.1Cu2O7 (x=0–0.7) and TlSr2−xTixCaCu2O7 (x=0–0.4) ceramics have been investigated. Tl0.8Pb0.2Sr2−xTixCa0.9Y0.1Cu2O7 (x=0) was observed to superconduct with T c onset of 90 K and T c zero of 21 K. Substitution of Ti with x=0.1–0.5 observed an increase in T c zero to 56–62 K with T c onset around 90 K. TlSr2−xTixCaCu2O7 (x=0–0.2) was not superconducting above 20 K but for x=0.3 superconductivity was observed with T c onset of 66 K and T c zero of 34 K.

Effect of Yb Substitution on Excess Conductivity of (Tl,Pb)(Sr,Yb)2CaCu2O7−δ Superconductors

Superconducting fluctuation behavior has been studied in sintered polycrystalline samples of Yb‐substituted Tl0.5Pb0.5Sr2−xYbxCaCu2O7−δ (x = 0.1–0.4) by electrical resistivity measurements. Data of Δσ was obtained from linear fitting of the normal state resistivity in a suitable temperature region. Analysis of excess conductivity behavior based on Aslamazov‐Larkin (AL) theory revealed transition from 2‐D to 3‐D behavior of the superconducting fluctuation in the mean field region for all samples. This study also suggests that there is a close correlation between the amounts of Yb substitution and the behavior of AL constant (A) both for 2D and 3D fluctuations.

Structural and Optical Properties: Mn Doped Nano ZnO Synthesized by Mechanochemical Synthesis

Manganese doped ZnO nanoparticles were synthesized by mechanochemical processing in a high energy ball mill (Zn1−xMnxO, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1). ZnO nano particles were synthesized by milling corresponding chemical precursors in a planetary ball mill for different periods of time. Diluents were used to control the agglomeration and reaction kinetics during the milling. Diluents were removed from the milled powders by selective leaching technique and the samples were dried at 100 °C for 24 hours. Phase identification and crystallite size investigations were carried out from the data obtained by X‐ray Diffraction (XRD) analysis of the samples. The peak shifting in XRD patterns showed that Mn ions were successfully doped into the ZnO crystal lattice with successive increase in dopant levels. Average crystallite size of the nano particles is seen to vary from 16 to 30 nm. Morphology of the samples was investigated by field emission scanning electron microscope (FESEM). It can be seen from FESEM images that the nano particles did not tend to agglomerate. Uv‐Visible spectroscopic (Uv‐Vis) technique was used for the optical properties characterization of the samples. Band gap values of the ZnO nano particles were determined from Tauc plots of UV‐Vis data. The energy gap is seen to decrease with increase in Mn.Manganese doped ZnO nanoparticles were synthesized by mechanochemical processing in a high energy ball mill (Zn1−xMnxO, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1). ZnO nano particles were synthesized by milling corresponding chemical precursors in a planetary ball mill for different periods of time. Diluents were used to control the agglomeration and reaction kinetics during the milling. Diluents were removed from the milled powders by selective leaching technique and the samples were dried at 100 °C for 24 hours. Phase identification and crystallite size investigations were carried out from the data obtained by X‐ray Diffraction (XRD) analysis of the samples. The peak shifting in XRD patterns showed that Mn ions were successfully doped into the ZnO crystal lattice with successive increase in dopant levels. Average crystallite size of the nano particles is seen to vary from 16 to 30 nm. Morphology of the samples was investigated by field emission scanning electron microscope (FESEM). It can be seen from FESEM im...

Double metal–insulator peaks and effect of Dy3+ substitution on transport and magnetic properties of hole doped La0·8Ag0·2MnO3

Abstract The electrical transport and magnetic properties of La0·8−xDyxAg0·2MnO3 (x = 0–0·1) colossal magnetoresistance ceramics have been investigated. All the samples showed transition from metallic behaviour at low temperatures to insulating behaviour at higher temperatures accompanied by ferromagnetic to paramagnetic transition. The Curie temperature Tc decreased from ∼288 K (x = 0) to ∼273 K (x = 0·1) with increasing Dy3+. For x = 0, double metal–insulator transition peaks were observed at Tp1 = 290 K and Tp2 = 261 K. Both peaks and Tc shifted to lower temperatures with increasing Dy3+, indicating that substitution of Dy3+ weakened the double exchange process and enhanced the Jahn–Teller effect. The magnetoresistance peak was observed around Tp1 for all the samples. The electrical behaviour at the metallic region below Tp2 was explained based on electron scattering models, while the Mott variable range hopping model was used to explain the resistivity behaviour above Tp1. The observed double peak behaviour in the ρ(T) curve is suggested to be due to the phase separation induced by inhomogeneity of the samples.

Effects of ionic size on oxygen sensing properties of Sr substituted Ho123 ceramic rods utilising hot spot phenomena

Abstract The I–V characterisation of Ho(Ba2−xSrx)Cu3O7−δ (x = 0, 0·2 and 0·8) ceramic sensor rod with hot spot in flowing oxygen between 0 and 100% pO2 revealed good oxygen response with output current observed to strongly depend on pO2. All the samples showed higher sensitivity to pO2 below 20% with reduced electrical power consumption in Sr substituted rods. The x = 0·2 rod showed improvement in output current stability, reproducibility and transition of output current response between 20 and 100% pO2 compared to the x = 0 rod. Conductivity was found to be proportional to above 60% for x = 0 rod and above 40% for Sr substituted rods, while lower pO2 conductivity was proportional to , in agreement with the mass action law indicating difference in oxygen ionisation. The results are also discussed in terms of the possible influence of hot spot temperature on the structure of Ho123 and changes in oxygen activation energy as a result of the difference in ionic sizes between Ba2+ and Sr2+.

Effect of Aluminium Doping on Structural and Optical Properties of ZnO Nanoparticles Prepared by Mechanochemical Synthesis

Optical properties of metal ion doped zinc oxide (ZnO) nanoparticles synthesized by various methods were reported to display different properties. Nevertheless, there are limited reports on metal ion doped ZnO prepared by mechanochemical synthesis. This paper reports the effect of aluminium doping on structural and optical properties of ZnO (Zn1‐xAlxO, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) nanoparticles prepared by mechanochemical synthesis in a high‐energy ball mill. The samples were synthesized by milling corresponding chemical precursors in a planetary ball mill for 5 h. Diluents were used to control the agglomeration and reaction kinetics during the milling. The shifting of X‐Ray Diffraction (XRD) peaks indicated that Al ions have been successfully doped into the crystal lattice of ZnO host. Average crystallite size, was calculated based on Scherrer’s equation, was found to vary from 22 to 29 nm. Blue shift in energy gap (Eg) with increasing Al concentration (x<0.08) could be attributed to either the Bu...

Oxygen response of hot spot on DyBa2Cu3O7–x ceramic sensors fabricated under different heat treatments

Abstract Oxygen response of hot spot on DyBa2Cu3O7–x (Dy123) ceramic sensors fabricated under different heat treatments has been studied. Sensor rods A and B were fabricated from presintered bulk Dy123 samples heated for 48 h at around 900 and 960°C respectively. Sensor rod C on the other hand was prepared using similar heat treatment as sample A but was reheated at around 910°C for 24 h after being fabricated into rod shape. For samples A and B, it was observed that increasing the sintering temperature reduced the voltage at peak current by half but only slightly reduced the power consumption. In terms of durability the best performance is shown by sample A. Sample C showed the largest reduction in voltage at peak current and the lowest power consumption but it was the least durable. The effect of heat treatment on I–V behaviour was discussed in terms of differences in microstructure and initial oxygen content of the samples. Oxygen sensing response for sensor B showed that the output current strongly depended on oxygen partial pressure (pO2) with good stability and reproducibility with response time of ∼5 s. The relation between output current and pO2 showed a good agreement with the ideal case of oxygen excess materials, derived from mass action law.

Gruneisen parameter and anharmonic contribution to temperature dependent ultrasonic velocity in RE based and Tl based superconductors with different oxygen contents

Abstract In the present paper, lattice anharmonicity contribution to the temperature dependent ultrasonic longitudinal and shear velocities propagated in ErBa2Cu3O6·9, GdBaSrCu3O7– δ and TlSr2CaCu2O7– δ ceramics were evaluated using the Nava model to compute their longitudinal (γLeff) and shear (γSeff) Gruneisen parameters. Ultrasonic velocity for both longitudinal and shear modes for oxygen rich samples showed deviation from anharmonicity theoretical curves at higher temperatures. Reduction of oxygen content by quenching reduced the deviation significantly for all samples. The results also showed that for all oxygen rich and oxygen reduced samples, the value of γLeff is substantially larger than γSeff, indicating that the samples are dominated by longitudinal acoustic mode anharmonicity. However, the effect of oxygen reduction on the Gruneisen parameter is not consistent for all samples. For Er123 and TlSr1212, oxygen reduction caused γLeff to decrease and γSeff to increase but for Gd1113 it caused an increase in γ eff for both modes. The possible relationship between Gruneisen parameter and superconductivity is discussed.