Soo Kien Chen

Extractive fermentation for improved production and recovery of lipase derived from Burkholderia cepacia using a thermoseparating polymer in aqueous two-phase systems

An extractive fermentation technique was developed using a thermoseparating reagent to form a two-phase system for simultaneous cell cultivation and downstream processing of extracellular Burkholderia cepacia lipase. A 10% (w/w) solution of ethylene oxide-propylene oxide (EOPO) with a molecular mass of 3900 g/mol and pH 8.5, a 200 rpm speed, and 30 °C were selected as the optimal conditions for lipase production (55 U/ml). Repetitive batch fermentation was performed by continuous replacement of the top phase every 24h, which resulted in an average cell growth mass of 4.7 g/L for 10 extractive batches over 240 h. In scaling-up the process, a bench-scale bioreactor was tested under the conditions that had been optimized in flasks. The production rate and recovery yield were higher in the bioreactor compared to fermentation performed in flasks.

Enhanced critical current densities in MgB2 by mixing relatively impure boron?powders

Polycrystalline MgB2 samples were prepared from 99.98% purity Mg powder and different mixtures of relatively impure boron (99% pure crystalline boron and 95?97% amorphous boron) precursor powders. At both 6 and 20?K, for the mixed boron samples a doubling in Jc was observed over the highest values for single precursor samples. It is shown that the enhanced Jc results from the mixing effect of using different reaction rates of the different boron precursor powders. The work represents a cost-effective means of significantly improving current carrying performance in MgB2 conductors.

Reaction method control of impurity scattering in C-doped MgB2: proving the role of defects besides C substitution level

In this study, Si and C were incorporated into polycrystalline MgB 2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40 K, ρA(40 K), is higher for the samples reacted with SiC regardless of the carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the samples reacted with SiC, the calculated active area that carries current, AF, is twice that of the (Si+C) samples. On the other hand, the upper critical field, Hc2, was similar for both sets of samples despite their different C substitution levels, which proves the importance of defect scattering in addition to C substitution level. Hence, the form of the precursor reactants is critical for tuning the form of Hc2(T).

STRUCTURE, ELECTRICAL TRANSPORT AND MAGNETO-RESISTANCE PROPERTIES OF La5/8Ca3/8MnO3 MANGANITE SYNTHESIZED WITH DIFFERENT MANGANESE PRECURSORS

We synthesized the polycrystalline manganite of La5/8Ca3/8MnO3 with three different manganese routes prepared through a solid state reaction method. The effects of the manganese route selection on the structure, electrical transport and magneto-transport properties were examined in this study. The samples were characterized using X-ray diffraction (XRD) and SEM to identify their structure and morphology. XRD analysis confirmed that all samples were in single phase with orthorhombic structure and belonged to the Pnma space group. The average grain sized samples with manganese route of Mn2O3 and MnCO3 had a grain size of 1.2–8.7 μm and 2–7.5 μm, respectively. For the MnO2 route, the sample had a small melt-like shape with higher porosity. The metal–insulator transition temperature, TMI, for LCMO (Mn2O3), LCMO (MnO2) and LCMO (MnCO3) samples were 270 K, 266 K and 258 K, respectively. All the samples showed negative magneto-resistance with significant increase in value near the TMI temperature. The highest CMR (colossal magneto-resistance) ratio was found in LCMO (Mn2O3), -22.06% at 270 K, followed by -16.69% for LCMO (MnO2) at 80 K, and 15.2% for LCMO (MnCO3) at 100 K in a 1 T magnetic field.

Effects of anthropogenic activities on the heavy metal levels in the clams and sediments in a tropical river

The present study aimed to assess the effects of anthropogenic activities on the heavy metal levels in the Langat River by transplantation of Corbicula javanica. In addition, potential ecological risk indexes (PERI) of heavy metals in the surface sediments of the river were also investigated. The correlation analysis revealed that eight metals (As, Co, Cr, Fe, Mn, Ni, Pb and Zn) in total soft tissue (TST) while five metals (As, Cd, Cr, Fe and Mn) in shell have positively and significantly correlation with respective metal concentration in sediment, indicating the clams is a good biomonitor of the metal levels. Based on clustering patterns, the discharge of dam impoundment, agricultural activities and urban domestic waste were identified as three major contributors of the metals in Pangsun, Semenyih and Dusun Tua, and Kajang, respectively. Various geochemical indexes for a single metal pollutant (geoaccumulation index (Igeo), enrichment factors (EF), contamination factor (Cf) and ecological risk (Er)) all agreed that Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn are not likely to cause adverse effect to the river ecosystem, but As and Pb could pose a potential ecological risk to the river ecosystem. All indexes (degree of contamination (Cd), combined pollution index (CPI) and PERI) showed that overall metal concentrations in the tropical river are still within safe limit. River metal pollution was investigated. Anthropogenic activities were contributors of the metal pollution. Geochemical indexes showed that metals are within the safe limit.

Growth and characterization of ZnO multipods on functional surfaces with different sizes and shapes of Ag particles

Three-dimensional ZnO multipods are successfully synthesized on functional substrates using the vapor transport method in a quartz tube. The functional surfaces, which include two different distributions of Ag nanoparticles and a layer of commercial Ag nanowires, are coated onto silicon substrates before the growth of ZnO nanostructures. The structures and morphologies of the ZnO/Ag heterostructures are investigated using X-ray diffraction and field emission scanning electron microscopy. The sizes and shapes of the Ag particles affect the growth rates and initial nucleations of the ZnO structures, resulting in different numbers and shapes of multipods. They also influence the orientation and growth quality of the rods. The optical properties are studied by photoluminescence, UV-vis, and Raman spectroscopy. The results indicate that the surface plasmon resonance strongly depends on the sizes and shapes of the Ag particles.

The Synthesis and Characterization of Peach-Like ZnO

Peach-like ZnO microstructures are synthesized using vapor phase transport on MgO (001) substrates with a copper oxide (60 nm) buffer layer. The structure and morphology of the product are investigated using an x-ray diffractometer (XRD) and a field-emission scanning electron microscope. The peaches have an average diameter of 3 ?m and a wurtzite structure. To study the optical properties, photoluminescence (PL) and Raman spectroscopy are employed. A strong UV emission at 380 nm in the PL spectra is observed, and a sharp and dominant peak at 437 cm?1 in the Raman spectrum can be assigned to the good crystallization of obtained product. In addition, the growth mechanism of the peach-like ZnO structure is tentatively investigated based on the EDX analysis and growth time.

Structural, electrical and magnetic properties of polycrystalline La0.67(Ca1−x Sr x )0.33MnO3 manganites

Polycrystalline La0.67(Ca1−xSrx)0.33MnO3 with different substitution level of strontium element, were synthesized via solid state reaction. Structure of samples was characterized by X-ray diffraction (XRD). XRD patterns reveal that La0.67Ca0.33MnO3 exhibits orthorhombic structure with space group Pnma. Phase transitions from orthorhombic to rhombohedral take place as Ca ions were gradually substituted by Sr ions. The XRD data were further analyzed by Rietveld refinement technique. The data show that Mn–O–Mn bond angle increases as x increases. Microstructures obtained from SEM show that substitution of Sr ions has demoted the grain growth and densification process during sintering. The substitution of Sr ions has greatly influenced the hopping integral of electron via double exchange interaction, thus affecting the electrical properties and magnetic properties as well. The resistivity decreases and the metal–insulator transition temperature (Tp) shifts to higher temperature as x increases. The magnetoresistance (MR) effect gradually decreases and MR peak shifts to higher temperature as x increases. The magnetization measured at room temperature is found to be increasing as x increases.

Structural, Microstructural and Electrical Properties of La0.67(Ba,Sr)0.33MnO3 System

La0.67(Ba,Sr)0.33MnO3 were prepared via solid‐state reaction method. Rietveld refinement showed that when Ba2+(1.49A) is replaced with smaller ion, Sr2+(1.32A), its lattice parameter, volume, bond distance and angle decreases, and also different microstructure was obtained. Transition temperature, Tp shifted to higher temperature while the resistance and magnetoresistance (MR) behaviour of samples were quite similar. This phenomenon believed to be due to the variation of spin‐dependent scattering and/or spin‐polarization tunneling across the grain surface/boundary. Typical polycrystalline (intrinsic and extrinsic MR) type of MR is shown for both samples. The highest % of low‐field magnetoresistance (LFMR) (at 0.1T) of ‐11.5% and ‐10.2% at 80 K was given by La0.67Ba0.33MnO3 (LBMO) and La0.67Sr0.33MnO3 (LSMO) samples respectively.

Optimizing C Incorporation Into Magnesium Diboride

In this work, either SiC or separate Si and C (Si+C) powders of up to 10 weight percentage were in situ reacted with Mg and B (molar ratio of 1:2) and the superconducting properties compared. The latter shows a smaller a -axis lattice cell parameter as compared to the same level of SiC additions, indicating a higher level of C substitution. In those samples, a larger increase in the c -axis is also noticeable and a more severe degradation in the superconducting transition temperature is observed. However, reaction with SiC causes a greater broadening in the transition width. At both 5 K and 20 K, for samples reacted with SiC (up to 5 wt.%) a stronger improvement in the magnitude of critical current density, Jc, is obtained. On the other hand, samples reacted with (Si+C) show a weaker Jc dependence on field at 5 K because of higher C substitution. At 20 K, the Jc decreases more rapidly with field for the same level of SiC additions. Hence, for high temperature (20 K) and low field ( <;5 T) applications, reaction with SiC is preferred for obtaining higher Jc(H).