Papers by Author: M. El-Hofy

Abstract: BSCCO 2212 superconducting samples, doped Tellurium, with the chemical formula Bi2-xTexSr2CaCu2O8, were prepared by the conventional solid state reaction technique. The prepared samples were studied utilizing XRD, DC-electrical conductivity and SEM. XRD spectra indicated that 2212 with tetragonal structure is the major phase, whereas Bi-2201 and CaTeO4 are minor phases. At higher Te-additions x, traces from some other semi conducting phases were detected. The critical transition temperature Tcoffset was found to decrease non-linearly with x, which attributed to the hole filling caused by the liberated electrons of Te4+ ions. For x–values in the range 0.1 ≤ x ≤ 0.4, the steepness of (ρ vs T) relationship increases abruptly around 150 K; this was attributed to change in the oxygen vacancy feature (phase-like transition). SEM photographs revealed that as Te-content increases the compactness of the surface and the connectivity of the grains decreases, while pores and voids increase as a result of decreasing the amount of Bi and presence of multiple-phases in the sample.

Abstract: Two samples of ZnO doped Ba with the chemical composition, 97ZnO-3BaO, have been prepared via oxalate co-precipitation. During precipitation the first sample, A, was stirred by magnetic stirrer while the second sample, B, was stirred via 40KHz ultrasonic wave. The obtained powders were decomposed at 400°C for 3h, then pressed and sintered at 1200°C for 1.5h. Then XRD, SEM and J-E measurements were performed and analysed. The grain sizes of the obtained ceramics were (0.5-2.26) µm and (80-119) nm for samples A and B, respectively. The J-E measurements revealed that the obtained ceramic has voltage switching characteristics, and that the switching voltage could be controlled by the stirring process.

Abstract: ZnO ceramic samples with the chemical formula, 97ZnO-2BaO-1(X)Mol% (where X= CuO, Fe2O3, TiO2, V2O5, MoO3) have been prepared by using conventional ceramics techniques. The samples were sintered at 1200°C for 1, 1.5 or 2h. The metal ions were chosen such that their ionic radii were just slightly different, whereas their ion valences varied from 2+ in case of Cu to 6+ for Mo. Room-temperature I-V characteristics, microstructures, linear scans and X-ray patterns were then studied. The microstructure and linear scan data revealed that, in the case of Cu-, Fe-, V- and Mo-doped ceramics, the doped ions resided mainly at the grain boundaries while, in case of Ti-doping, the ions resided mainly in the grain interior. The electrical measurements and the linear scan data showed that both the non-linearity parameter, α, and the rate of change of α with sintering time, (dα/dt), was exponentially proportional to the valence of the doped ion, where t is a sintering time in the range of 1 to 2h. The leakage current, JL, is linearly proportional to the amount of doped ion present in the grain interior, relative to that present at the boundaries. The X-ray data revealed that the obtained phase was the hexagonal ZnO phase, with traces of secondary phase related to the doped ion; the secondary phases were identified as being Fe2O3, BaTi5O11, Zn3(VO4)2 and (ZnMoO4) in case of Fe-, Ti-, V- and Mo-doped ceramics, respectively. The relative intensity of the X-ray peak at 2θ = 34.45, corresponding to the (0002) plane, was exponentially proportional to the valence of the doped ion; while α scaled with the relative intensity of the aforementioned X-ray peak.

Abstract: ZnO-BaO particles (4-10) nm have been synthesized by applying 40 KHz ultrasonic wave simultaneously with the co-precipitation process of Zn and Ba oxalate at 50 0C. DTA curves of the co-precipitate were carried out and the decomposition temperature was determined. Mixed oxide obtained from thermal decomposition of Zn2 Ba (C2O4)2.2H2O in air has been investigated by TEM, XRD, IR and UV-Vis measurements. The results revealed that the obtained particle consists of a core of highly deformed Ba contaminated ZnO nano-crystals. The ZnO crystalline core is coated by Ba rich amorphous layer which prevents its growth. Decreasing concentration of ZnCl2 in the precipitation solution from 0.1M to .05 M leads to decrease the size of the particle from (10-20) nm to(4-10) nm, in average, increases UV cut-off, the cut-off tail and rounded of the shape of the particle.

Abstract: Three Zinc Molybdenum ceramic samples doped Bi have been prepared according to the chemical formula (1-x) ZnO – 0.2MoO3 – xBi2O3, where (x = 0.2, 0.5, 1.0) mol %. The samples were studied at room temperature through X-ray Diffraction analysis, SEM, EDAX, I-V characteristics and C-V measurements. The results decleared the presences of two phases; ZnO as a major phase beside Bi2MoO6 as a minor phase. At lower Bi additions; Bi and Mo ions are highly segregated at the grain boundaries, while at higher additions more homogenous distribution for these ions inside the grain is observed. Some pores are observed around the batches of the minor phase and at the grain boundaries. The grain size is enlarged with increasing Bi addition; while the number and size of the pores are decrease. Creation of the pores is attributed to oxygen liberation from the surface of the sample during sintering. Formation of Schottky barrier is indicated via I-V and C-V measurements and attributed to Zn vacancies at the grain boundary. Interface potential barrier, donor density, interface state density and barrier width are decrease with increasing Bi addition. I-V characteristics revealed voltage switching. The switching voltage E0 decreases with increasing Bi addition and it is reproducible even after several on-off cycles.

Abstract: Four Zinc Molybdenum ceramic samples (S1-S4), have been prepared according to the chemical formula (1-x) ZnO - x (MoO3), where (x = 0.1, 0.2, 0.6, 0.8) mol %. The samples were studied through X-ray Diffraction analysis, SEM, EDAX, I-V characteristics at different temperature up to 200 οC and C-V measurements. X-ray results decleared that Mo contributes to the structure mainly substitution in place of Zn. Entrance of Mo into the structure increases the intensity and shifts the X-ray peaks to higher θ values. At lower additions, Mo is segregated along the grain boundaries in the form of short bars, while at higher additions, circular batches of Mo rich phase are appeared on the surface of the large ZnO grains. The formulation of Schottky barrier is indicated, in case of (x < 0.1) the barrier is attributed to the trapping of electrons by the defects at the grain boundaries, while at higher additions the barrier was attributed to the concentration gradient of Zn vacancies in the grain boundary region. The calculated barrier height and non-linearity coefficient α are (1.15, 1.12, 1.15 and 1.48) eV and (59.4, 22.5, 35 and 87) for the samples S1- S4 respectively. Going from S1 to S4, donor density Nd and density of the interface states Ns decrease from 3.81x 1018 to 0.46x1018 cm-3 and from 6.41x1012 to 2.52x1012 cm-2 respectively, while the width W(cm) of the potential barrier increases from the value 1.68x10-6 cm for S1 to the value 5.5x10-6 cm for S4 . The current processed via electron tunneling through the barrier.