| 1. |
- Baykal, Abdulhadi, et al.
(författare)
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CTAB-Assisted Hydrothermal Synthesis and Magnetic Characterization of NixCo1-xFe2O4 Nanoparticles (x=0.0, 0.6, 1.0)
- 2009
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Ingår i: Turkish journal of chemistry. - : The Scientific and Technological Research Council of Turkey (TUBITAK-ULAKBIM) - DIGITAL COMMONS JOURNALS. - 1300-0527 .- 1303-6130. ; 33:1, s. 33-45
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Tidskriftsartikel (refereegranskat)abstract
- Nickel ferrite, NixCo1-xFe2O4, NPs (where x = 0.0, 0.6 and 1.0) were successfully synthesized by a rapid and reproducible CTAB-assisted hydrothermal route. The influence of different hydrolyzing agents on the particle size and magnetic behavior of NixCo1-xFe2O4 NPs was investigated. Particles showed very high phase purity and crystallinity in powder XRD analysis. Compositions of Co, Fe, and Ni in fabricated powders were also determined by AAS and results are in very good agreement with the targeted compositions. Samples hydrolyzed using NH3 showed no significant changes in the particle size and morphology. NH3 hydrolyzed samples were much smaller than their NaOH hydrolyzed counterparts, which was attributed to the strength and concentration of the hydrolyzing agents, NH3 being about 6 times more concentrated than NaOH. This in turn influenced the nucleation rate thus the size of each nucleus formed. Strong temperature and Ni concentration dependence of magnetic parameters was observed. These samples are considered as promising materials for high density magnetic recording media.
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| 2. |
- Darab, Mahdi, et al.
(författare)
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Nanoengineered BSCF Cathode Materials for Intermediate-Temperature Solid-Oxide Fuel Cells
- 2009
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 156:8, s. K139-K143
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Tidskriftsartikel (refereegranskat)abstract
- A recently reported promising cathode material for solid-oxide fuel cells (SOFCs), namely, BaxSr1-xCoyFe1-yO3-delta (BSCF) is fabricated in nanocrystalline form by a chemical alloying approach. The approach is comprised of solution chemical synthesis of a precursor and its thermochemical processing toward the desired phase. All the constituent elements, Ba, Sr, Co, and Fe, were coprecipitated from an aqueous solution of their salts to produce a precursor with a well-defined composition, fine particle size, high homogeneity, and high reactivity. After calcining and sintering at 1000 degrees C, the individual oxides were alloyed into nanostructured perovskite (with x=0.5 and y=0.2) Ba0.5Sr0.5Co0.2Fe0.8O3 of high purity. Spark plasma sintering was used for compaction to preserve the material's nanostructure, and sintered compacts demonstrated a significant increase in electrical conductivity values at temperatures up to 900 degrees C, compared to the earlier reports. The measured conductivity values are sufficiently high for cathode applications with a maximum of about 63 S cm(-1) at 430 degrees C in air and 25 S cm(-1) at 375 degrees C in N-2, respectively. These values are about twice as high as conventional BSCF mainly due to the reduction in interfacial resistance, implying a high promise for nanoengineered BSCF as cathode material at low or intermediate-temperature SOFCs.
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| 3. |
- Dong, Lin, et al.
(författare)
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Measurement of radiative lifetime in CdSe/CdS core/shell structured quantum dots
- 2009
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Ingår i: 2009 Asia Communications and Photonics Conference and Exhibition, ACP 2009. - Washington, D.C. : OSA. - 9781557528773 ; , s. 5377385-
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Konferensbidrag (refereegranskat)abstract
- Radiative lifetime of chemically synthesized colloidal CdSe/CdS core/shell quantum dots is measured. Influence of the core size on the electron-hole pair separation is analyzed. A long radiative lifetime and the existence of electron-hole pair separation suggest high potential of these dots as gain material to achieve lasing under continuous-wave excitation.
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| 4. |
- Durmus, Zehra, et al.
(författare)
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L-lysine coated iron oxide nanoparticles : Synthesis, structural and conductivity characterization
- 2009
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 484:1-2, s. 371-376
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Tidskriftsartikel (refereegranskat)abstract
- L-lysine coated iron oxide (LCIO) nanoparticles were synthesized by a co-precipitation method in the presence of amino acid. XRD analysis confirmed the presence of cubic magnetite phase with an average crystallite size of 8 +/- 4 nm. Particle size estimated from TEM, by log-normal fitting, is similar to 114 nm. The difference between the crystallite size from XRD and particle size from TEM indicates polycrystalline nature of synthesized particles. FT-IR show that the binding Of L-lysine on the surface of iron oxide through carboxyl groups is via unidentate linkage. The presence of L-lysine on iron oxide is also confirmed by zeta potential measurements on LCIO nanoparticles, revealing a partial coverage of iron oxide with L-lysine. In order to obtain chemically stable, well-dispersed and uniform sized nanoparticles, amino acids are suitable because they play a very important role in the body. Conductivity measurements were performed to investigate the influence of the coating on the conduction characteristics of iron oxide and results show the existence of a hopping conduction mechanism. Magnetic transition is observed at similar to 70 degrees C for uncoated iron oxide and LCIO samples. Frequency (1 Hz to 3 MHz) and temperature (290-420 K) dependant AC conductivity measurements have resulted in AC activation energies between 0.048 and 0.041 eV for uncoated and 0.050-0.044 eV for LCIO nanoparticles. Temperature-dependant DC resistivity measurements of iron oxide and LCIO at high temperatures resulted in the DC activation energies of 0.22 and 0.43 eV respectively. The higher activation energy value for LCIO is the result of coating by insulating L-lysine layer.
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| 5. |
- Durmus, Zehra, et al.
(författare)
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Ovalbumin mediated synthesis of Mn3O4
- 2009
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Ingår i: Polyhedron. - : Elsevier BV. - 0277-5387 .- 1873-3719. ; 28:11, s. 2119-2122
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Tidskriftsartikel (refereegranskat)abstract
- By using Mn2+ and Mn3+ salts, and freshly extracted ovalbumin. Mn3O4 nanocrystals have been synthesized successfully. The X-ray diffraction results indicated that the synthesized nanoparticles have only the spinel structure without the presence of any other phase impurities. As the ovalbumin-water mixture was highly basic, the process did not require any use of base to increase the pH where hydrolysis took place. A gel formed where water soluble ovalbumin proteins served as a perfect matrix for entrapment of metal ions (Mn2+ and Mn3+). Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Mn3O4. The discrepancy between the crystallite size from XRD and particle size SEM analysis reveals polycrystalline nature of the synthesized particles with this route. EPR analysis of Mn3O4 shows a narrow and symmetric line indicating the absence of hyperfine splitting.
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| 6. |
- Furberg, Richard, et al.
(författare)
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Dendritically ordered nano-particles in a micro-porous structure for enhanced boiling
- 2006
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Ingår i: Proceedings of 13th International Heat Transfer Conference, NAN-07. - 1567002250
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Konferensbidrag (refereegranskat)abstract
- Presented research is an experimental study of the pool boiling performance of copper surfaces enhanced with a newly developed structure. The enhanced surfaces were fabricated with an electrodeposition method where metallic nano-particles are formed and dendritically connected into an ordered micro-porous structure. To further alter the grain size of the dendritic branches, some surfaces underwent an annealing treatment. The tests were conducted with the test objects horizontally oriented and submerged in a refrigerant: R134A, at saturated conditions and at an absolute pressure of 4 bar. The heat flux varied between 0.1 and 10 W/cm2. The boiling performance of the enhanced surfaces was found to be dependent on controllable surface characteristics such as thickness of the structure and the interconnectivity of the grains in the dendritic branches. Temperature differences less than 0.3 °C and 1.5 °C at heat fluxes of 1 and 10 W/cm2 respectively have been recorded, corresponding to heat transfer coefficients up to 7.6 Wcm-2K-1. The micro-porous structure has been shown to facilitate high performance boiling, which is attributed to its high porosity (∼94%), a dendritically formed and exceptionally large surface area, and to a high density of well suited vapor escape channels (50 – 470 per mm2).
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| 7. |
- Furberg, Richard, et al.
(författare)
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Experimental investigation of an evaporator enhanced with a micro-porous structure in a two-phase thermosyphon loop
- 2009
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Ingår i: HT2008. - NEW YORK : AMER SOC MECHANICAL ENGINEERS. - 9780791848487 ; , s. 327-334
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Konferensbidrag (refereegranskat)abstract
- Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2-2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm(2)K) at a heat flux of 30-35 W/cm(2) and a critical heat flux of around 50 W/cm(2). Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, tip to 4 times, by the nano- and micro-porous enhancement structure.
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| 8. |
- Furberg, Richard, et al.
(författare)
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The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger
- 2009
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Ingår i: Journal of heat transfer. - : ASME International. - 0022-1481 .- 1528-8943. ; 131:10
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Tidskriftsartikel (refereegranskat)abstract
- Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m(2) and 17 kW/m(2) with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance.
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| 9. |
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| 10. |
- He, Zeming, et al.
(författare)
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Effect of ceramic dispersion on thermoelectric properties of nano ZrO2/CoSb3 composites
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:4, s. 043707-
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, nano- ZrO2 CoSb3 composites were fabricated by milling ZrO2 and CoSb3 powders and hot pressing at different sintering temperatures. For the prepared compacts, the phase purity, microstructure, and temperature-dependent thermoelectric properties were characterized. The effect of nano- ZrO2 dispersion on composite electrical conductivity and thermal conductivity is strictly clarified by comparing the transport properties of the nondispersed and dispersed CoSb3 at identical porosity, so that the effect of porosity on thermoelectric parameters could be eliminated. The effect of the insulating inclusion itself on transport properties is also considered and eliminated using effective media theories. It is clearly verified that charge carrier scattering and phonon scattering occur simultaneously to lower the electrical conductivity and the thermal conductivity of CoSb3 due to the introduction of nano- ZrO2 inclusions. The investigated composites show higher electrical conductivity due to existence of metallic Sb and lower thermal conductivity because of nanodispersion. At the ranges of high measuring temperature (673-723 K) and low porosity (6%-9%), the ratio of electrical conductivity to thermal conductivity of the dispersed CoSb3 is higher than that of nondispersed CoSb3, and the dimensionless figure of merit (ZT) of the composite could probably be improved at these ranges with the enhanced ratio of electrical conductivity to thermal conductivity and Seebeck coefficient, which is assumed to be increased by a potential barrier scattering.
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