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Electrical Properties of Ultrafine Nylon-6 Nanofibers Prepared Via Electrospinning
Nirmala, R.,Jeong, Jin-Won,Oh, Hyun-Ju,Navamathavan, R.,El-Newehy, Mohamed,Al-Deyab, Salem S.,Kim, Hak-Yong 한국섬유공학회 2011 FIBERS AND POLYMERS Vol.12 No.8
We report on the preparation and electrical characterization of nylon-6 nanofibers via electrospinning technique. During electrospinning, the polymer solution became highly ionized and emerging out of the micro-tip syringe by forming mesh-like ultrafine nanofibers structure in between the main fibers. The resultant nylon-6 nanofibers were well-oriented with uniform structure. The diameter of the ultrafine nanofibers (7 to 40 nm) is one order less than those of main fibers (100 to 200 nm). The current-voltage (I-V) measurements revealed a linear curve with an occurrence of negative differential resistance (NDR) behavior. The existence of NDR region in the nylon-6 nanofibers can be attributed to the tunneling current through the ultrafine structures. The fabrication of nanofibers, in the form of ultrafine mesh-like form, is relatively fast and inexpensive, and it paves the way to build up of new dimension for nano device applications.
Preparation and Properties of Low-dielectric-constant SiOC(-H) Thin Films Deposited by Using PECVD
R. Navamathavan,이철로,R. Nirmala,김창영,Chi Kyu Choi 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.3
Low-dielectric-constant SiOC(-H) films were deposited on p-type Si(100) substrates by using plasma enhanced chemical vapor deposition (PECVD) with methyltriethoxysilane (MTES) and oxygen gas as the precursors. The SiOC(-H) films were deposited at various substrate temperatures while all the other experimental parameters were kept constant. The SiOC(-H) film’s properties,such as the deposition rate, refractive index, thickness, current-voltage (C-V) characteristics and the dielectric constant, were evaluated. The deposition rate decreased with increasing substrate temperature. The activation energies of the SiOC(-H) films were found to be -0.036 and -0.021 eV,for lower substrate temperature (RT - 200 C) and higher substrate temperature (beyond 200 C),respectively. When the substrate temperature was increased, the precursor molecules dissociated completely due to a breaking of the cage structures (voids), resulting in the formation of denser SiOC(-H) films. The dielectric constant of the SiOC(-H) film increased from 2.53 to 2.96 with increasing substrate temperature from RT to 350 ℃.
Effect of Solvents on High Aspect Ratio Polyamide-6 Nanofibers via Electrospinning
Nirmala, R.,Panth, Hem Raj,Yi, Chuan,Nam, Ki-Taek,Park, Soo-Jin,Kim, Hak-Yong,Navamathavan, R. 한국고분자학회 2010 Macromolecular Research Vol.18 No.8
The effect of the solvent on the formation of high aspect ratio ultrafine fibers in polyamide-6 using an electrospinning technique was examined systematically. In this study, formic acid, dichloromethane, acetic acid, chlorophenol, hexafluoroisopropanol, and trifluoroacetic acid via single and mixed solvent system were used for the production of high aspect ratio nanofibers in polyamide-6. Formic acid and mixtures of formic acid/dichloromethane, formic acid/acetic acid, and formic acid/chlorophenol can lead to the very clear dissolution of polyamide-6, enabling their subsequent electrospinning to obtain high aspect ratio nanofibers. Formic acid was found to be the most suitable solvent system for obtaining high aspect ratio nanofibers in polyamide-6. The conductivity of polyamide-6 in formic acid was very high demonstrating an increasing level of free ions in solution. The average diameter of the high aspect ratio nanofibers was approximately one order of magnitude lower than that of main fibers. These findings suggest that the formation of high aspect ratio nanofibers relies strongly on the specific properties, such as the poly-electrolytic behavior of polyamide-6 in the solvent.
Nirmala, R,Jang, Kwang-Hyun,Sim, Hasung,Cho, Hwanbeom,Lee, Junghwan,Yang, Nam-Geun,Lee, Seongsu,Ibberson, R M,Kakurai, K,Matsuda, M,Cheong, S-W,Gapontsev, V V,Streltsov, S V,Park, Je-Geun Institute of Physics 2017 Journal of Physics, Condensed Matter Vol.29 No.13
<P>CuAl<SUB>2</SUB>O<SUB>4</SUB> is a normal spinel oxide having quantum spin, <I>S</I> = 1/2 for Cu<SUP>2+</SUP>. It is a rather unique feature that the Cu<SUP>2+</SUP> ions of CuAl<SUB>2</SUB>O<SUB>4</SUB> sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl<SUB>2</SUB>O<SUB>4</SUB> shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards a higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl<SUB>2</SUB>O<SUB>4</SUB>. Interestingly, there is no sign of structural distortion either although Cu<SUP>2+</SUP> is a Jahn–Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl<SUB>2</SUB>O<SUB>4</SUB>. Of further interest, it also exhibits a large frustration parameter, <I>f</I> = |<I>θ</I> <SUB>CW</SUB>/<I>T</I> <SUB>m</SUB>| ~ 67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl<SUB>2</SUB>O<SUB>4</SUB> should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.</P>
Nirmala, R.,Kim, Hak Yong,Yi, Chuan,Barakat, Nasser A.M.,Navamathavan, R.,El-Newehy, Mohamed Elsevier 2012 International journal of hydrogen energy Vol.37 No.13
<P><B>Abstract</B></P><P>In this study, Ni-doped titanium dioxide (TiO<SUB>2</SUB>) electrospun nanofibers are introduced as novel material for dehydrogenation of ammonia borane (AB) complex. Hydrolysis experiments with introduced catalytic nanofibers are prevailed to rapidly release hydrogen from AB complex. Typically, Ni nanoparticles (NPs) behave as a catalyst, meanwhile the incorporation of nickel NPs lead to decrease in the electrons/holes recombination rate in TiO<SUB>2</SUB> which resulted in the increase of active ions in the solution to a rapid evolution of hydrogen gas at room temperature. The utilized physiochemical analyses indicate that the introduced Ni-doped TiO<SUB>2</SUB> nanofibers have a smooth surface and uniform diameters along their lengths. Under sunlight irradiation, the hydrogen production rate in case of utilizing Ni-doped TiO<SUB>2</SUB> nanofibers is rapidly increased compared to the pristine TiO<SUB>2</SUB> nanofibers, the maximum hydrogen equivalent in case of the doped nanofibers is 2.6 while the pristine one is 1.4. Both formulations exhibit almost equal low activity in daylight as the observed hydrogen equivalent is 0.4. Overall, this study proposes cheap, stable and effective material for AB dehydrogenation at room temperature.</P> <P><B>Highlights</B></P><P>► Novel Ni–TiO<SUB>2</SUB> photocatalyst is introduced for ammonia borane (AB) hydrolysis. ► A simple method to synthesis Ni-doped TiO<SUB>2</SUB> nanofibers for hydrogen production. ► High amount of hydrogen was produced by catalyst assisted hydrolysis. ► This is a cheap, stable and effective material for AB dehydrogenation. ► The introduced photocatalyst reveal good performance even at low content.</P>
A Study on Electrospun Nylon-6/TiO2 Composite Nanofibers
R. Nirmala,Jeong Jin Won,김학용,R. Navamathavan,Yi Chuan,Mohamed El-Newehy,Salem S Al-Deyab 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.60 No.10
We report on the preparation and the characterization of TiO2 nanoparticles incorporated with nylon-6 composite nanofibers by using electrospinning technique. Two different composite nanofiber mats with TiO2 nanoparticles sizes of 20 and 300 nm were prepared. The resultant nanofibers exhibited good incorporation of TiO2 nanoparticles. The doping of TiO2 nanoparticles into the nylon-6 nanofibers was confirmed by using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) spectroscopy. The measurement of the electrical conductivity of the TiO2 nanoparticles incorporated with nylon-6 nanofibers were carried out. Current-voltage (I–V) characteristics revealed that the current was enhanced for the sample with 300 nm TiO2 nanoparticles compared to that with 20-nm TiO2 nanoparticles.
Nirmala, R.,Jeon, Kyung Soo,Navamathavan, R.,Park, Mira,Kim, Hak Yong,Park, Soo-Jin 한국섬유공학회 2014 FIBERS AND POLYMERS Vol.15 No.5
In this study, we describe the preparation and characterization of electrospun Nylon66 composite nanofibers incorporated with carbon nanotubes (CNT) fillers and silver nanoparticles. We have incorporated the composites in to Nylon66 nanofibers to enhance the characteristics of the resultant composite nanofibers. The resultant composite nanofibers were characterized by using field-emission scanning electron microscopy, energy dispersive X-ray analysis, high-resolution transmission electron microscopy, X-ray diffraction, and current-voltage (I-V) measurement analysis. The morphology of the composite nanofibers exhibited densely arranged mesh-like ultrafine nanofibers which were strongly bound in between the main fibers. From I-V characteristics, it was observed that the incorporation of CNT fillers and Ag nanoparticles in to electrospun Nylon66 composite nanofibers can be significantly enhanced the electrical properties.