Papers by Author: S.Y.S. Yahya

Abstract: The superconducting and structural properties of pure and rare-earth elements substituted in Bi (Pb)-2223 samples were investigated. All samples were fabricated by the oxalate coprecipitation (COP) method using metal acetates and oxalic acid as starting materials. The electrical and resistivity were measured by using the four-probe method, while phase purity and microstructural examination were performed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM). The pure sample has a critical temperature of (T_{c zero}) 100 K, which decreased to 98, 96 and 97 K for the Eu, Dy and Yb samples, respectively. At 77 K, the un-doped sample gives the highest J_c, which gradually decreases with rare-earth substitution as one move towards the right in the lanthanide series. XRD results reveal two main phases (Bi-2223 and Bi-2212) with decreased amounts of Bi-2223 phases by rare-earth substitution. SEM micrographs showed flaky grains but Yb showed better grain alignment compared with the other substitutions.

Abstract: The comparison substitution with K, Ca and Zn respectively at the Cu site of YBa₂Cu_3-xM_xO_δ (x = 0.00 and 0.20) was performed based on superconducting and structural properties. All samples were prepared via a solid state technique. Resistivity and current density measurements (zero magnetic fields) were done using the four probe method and the structural and morphological properties were characterized using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) respectively. The critical current density (J_c) and critical temperature (T_{c zero}) of the substituted samples were found to be lower than that of the pure sample. T_{c zero} with K and Zn substitution was 78 K and 21 K respectively. This is due to the change in the hole concentration. J_c measured at 50 K showed values of 3.9790 A/cm², 4.4483 A/cm² and 2.9854 A/cm² for the pure sample, K and Ca substituted samples respectively. The decrease in J_c was due to the hole porosity and grain connection. All samples showed an orthorhombic crystal structure. The morphology of the fracture surface of samples as observed by FESEM displayed a loose structure of grain arrangement for sample Ca and Zn substituted samples.

Abstract: The camphor-grown pristine carbon nanotubes (CNT) was annealed by a single-stage thermal annealing system through controlled ambient in air, argon and nitrogen, are reported. Field emission scanning electron microscopy images confirmed that the heat treatment process gives a place to re-ordering carbon nanostructures which involves: (i) an elimination of structural defects and (ii) better graphitization of the amorphous carbon phase without damaging CNT structure. The room temperature micro-Raman measurements showed that no significant changes on the D and G-line position. However, different annealing gas ambient could give different values degree of graphitization (ID/IG ratio) due to the nature of gas itself. It reveals that single-stage thermal annealing system is relatively simple and effective to obtain an ideal CNT.

Abstract: The knowledge of fabrication method plays an important role in the preparation of aligned carbon nanotubes (ACNT) from natural hydrocarbon feedstock. Here ACNT were successfully synthesized by two-stage catalytic chemical vapor deposition method using organic oil (camphor oil) as a precursor. Synthesis was carried out at a fixed growth temperature of 800 °C and in different growth time: 10, 20, 30, 40, 50 and 60 minutes. The optimized condition for the growth of ACNT produced a small amount of by-product amorphous carbon and highly uniform crystal structure. The experimental results demonstrated that formation ACNT is also dependent on the growth time. The nanotubes were characterized by field emission scanning electron microscopy and micro-Raman spectroscopy. Thermal properties were evaluated by thermogravimetric analysis.

Abstract: The ionic conductivity and mechanical properties of poly (vinyl chloride) (PVC)/poly (ethyl methacrylate) (PEMA) polymer blends containing LiN(CF3SO3)2 as doping salt has been studied using electrical impedance spectroscopy (EIS) and Dynamic Modulus Analysis (DMA) as a function of polymer blend ratios and lithium salt concentration. The film with PVC/PEMA composition of 65:35 obtained the highest conductivity with good transparency. DMA showed that both the storage modulus (E') and the glass transition temperature (Tg) of the PVC/PEMA is increased with PEMA concentration. In the case of PVC/PEMA-LiN(CF3SO2)2 films, the conductivity was found to increase with concentration of salt added with a maximum in conductivity at 35 wt.% LiN(CF3SO2)2. The Tg values of the doped films was found to increase with concentration of salt such that the film with the highest conductivity value has the highest Tg.