نشریه علوم دانشگاه خوارزمی
سال دوازدهم شماره 4 (زمستان 1391)

Biosurfactants are widely used in industries like oil، cosmetics، food، and pharmaceutics. Surface tension reduction، the ability to emulsify oil and high biodegradability are among the main characteristics of these biosurfactants. These favorable features make biosurfactants as potential alternatives of chemically synthesized surfactants in a variety of applications. In this research،Bacillus subtilis NLIM0110 was used to produce a lipopeptide-type biosurfactant (surfactin) in an innovative bioreactor tailored to solve the problems of severe foaming arising from production of the biosurfactant. To cope with the rapid foam generation، a conventional fermentor was integrated with a foam collector. The other aims of this study were purification، identification of surfactin، optimization of agitation rate and aeration rate which were used to increase the surfactin production. The results showed the production of biosurfactant was proportional to the cell growth representing biosurfactant as a growth-associated product. The best condition for bioreactor was 300 rpm and 1. 5 vvm، giving Yp/s، Yp/x and Y of 0. 595 gg−1، 0. 25 gg−1، and 0. 057 gl−1h−1، respectively. The production of surfactin was confirmed by FTIR and HNMR analysis. Thus، these results show the potential effectiveness of the microorganism and the technique used in biosurfactant production.

MgO thin films were prepared using sol-gel method. The influence of annealing temperature on optical، structural properties and surface morphology of the films were investigated using FTIR، UV-Visible and photoluminescence (PL) spectroscopy، X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical constants and thickness of the films were determined by pointwise unconstrained minimization approach. The optical transmittance has decreased with increase of annealing temperature. The refractive index and extinction coefficient of the films increased while the thickness and optical band gap of the films decreased with increasing of annealing temperature. The thickness of film decreased from 635 to 420nm and optical band gap decreased from 4. 05 to 4. 02eV. The photoluminescence (PL) intensity increased with annealing temperature. XRD patterns demonstrated that the prepared films at 500℃ were amorphous. In addition، the crystal structure of MgO nanopowders which was prepared by sol-gel method was investigated. The crystalline orientation of MgO nanopowders was improved by increasing the sintering temperature. The SEM micrographs show that a homogenous and crack-free film can be prepared at 500°C.

In this paper، the Mutiple Multipoles method is employed to solve nonlinear eigenvalue problem and calculate band structure for a 2D photonic crystal. Band structure is calculated for both TE and TM polarizations. Simulation space is implemented for the first Brillouin zone by using physical properties such as rods radius، permittivity and susceptibility. To model fields inside and outside of object، Multipole centers were located around it and Bessel series inside the object is shown complex fields. We used Bloch theory to implement fictitious periodic boundary conditions for the first Brillouin zone. To validate the code، we simulated the band structure of a cubic lattice and compare the results with Plane Wave Expansion Method which illustrates the accuracy of the code. It is shown that this method can be applied to investigate photonic crystals with irregular shapes and different materials for different lattices such as cubic، trigonal and honeycomb. Furthermore it could be used for dielectric or dispersive material and experimental data. Numerical calculation shows that MMP method is accurate، fast and it can be used on Personal Computers.

Nickelate with general formula La2-xSrxNiO4 (x≈0. 33) were synthesized by the sol–gel method. Then by changing the sintering temperature and growth parameters، particles with various sizes were produced. The crystal structure and physical properties of the prepared samples were investigated by X-ray diffraction (XRD)، Fourier transform IR spectroscopy (FTIR)، Energy dispersive X-ray spectroscopy (EDX) and field emission scanning electron microscopy (FE-SEM) and resistivity measurements from room temperature to low temperatures. The XRD results and investigation of the surface morphology show the lowest temperature to get a single phase tetragonal structure is 920°C. These data confirm the formation of single phase structure in samples sintered at higher temperatures. The particle size increases with increase in sintering temperature. The FE-SEM results show that the particles sizes are in range of 50nm to 2mm. The results of resistivity measurements versus temperature by fourprobe method indicate that the charge ordering transition temperature move to lower temperature with decrease in particle size.

In this article، tunneling resonance (transmission coefficient، current density and negative differential resistance) has been studied for Alx Ga1-x As / GaAs triangular double quantum wells. Time dependent Schrodinger equation has been used for triangular quantum wells structures accompanied with Airy function. Transmission and reflection coefficients before and after applying external electrical field were obtained for two symmetrical triangular quantum well in terms of effective mass approximation as a function of structural parameters such as; well width، barrier width، well depth and applied voltage. Results showed a negative differential resistance at this kind of quantum wells.

In this paper، we studied the electronic conductance and energy gap of a graphene nano-ribbon which is connected to two semi-infinite atomic chains by the atoms located in the opposite sides of nano-ribbon. The numerical calculations were performed using the Green’s function method in the nearest neighbor approximation of tight-binding approach. The conductance curves plotted with respect to incoming electron energy show that for a zigzag graphene nano-ribbon with one benzene ring in its width، there is no gap in the energy band spectra of the system، while for the armchair case، an energy gap always exists around the Fermi energy and its value depends on the size of the nano-ribbon. The results show that the behavior of the graphene nano-ribbons conductance strongly depends on that how and where the leads connect to the nano-ribbon and on the contacts qualities.

In this study، Fe3O4/SiO2 core-shell magnetic nanoparticles (NPs) by two-step process were synthesized. First، Fe3O4 NPs by co-precipitation method of iron ions (Fe3+ and Fe2+) in an alkaline medium without using surfactant were synthesized and then silica shell with a non-thermal method without the use of linkers were installed on Fe3O4 NPs. facility of this method is noticeable. Results of X-ray powder diffraction patterns show that the inverse spinel structure of Fe3O4 and amorphous silicon was made. Fourier transform infrared spectroscopy confirmed the formation of these structures. There is no additional peak in XRD pattern which indicates that the prepared particles have pure Fe3O4 phase. TEM images show that core-shell structure is formed and mean size of NPs is 20 nm. VSM analysis showed that saturation magnetization of Fe3O4 nanoparticles after coating with silicon decreased.