Browsing by Subject "Nanocrystalline powders"

Now showing 1 - 5 of 5

Open Access
Effects of milling and annealing on formation and structural characterization of nanocrystalline intermetallic compounds from Ni-Ti elemental powders
(2012) Ghadimi, M.; Shokuhfar, A.; Rostami H.R.; Ghaffari, M.
Nickel and Titanium elemental powders with a nominal composition Ni-50 at.%Ti were mechanical alloyed in a planetary high-energy ball mill in different milling conditions (5, 10, 20, 40 and 60 h). The investigation revealed that increasing milling time leads to a reduction in crystallite size, and after 60 h of milling, the Ti dissolved in Ni lattice and NiTi (B2) phase was obtained. With milling time, morphology of pre-alloyed powders changed from lamella to globular. Annealing of as-milled powders at 1173 K for 900 s led to formation of nanocrystalline NiTi (B19′), grain growth and release of internal strain. The results indicated that this technique is a powerful and high productive process for preparing NiTi intermetallic compound with nanocrystalline structure and appropriate morphology. © 2012 Elsevier B.V. All rights reserved.
Open Access
Excitonic improvement of colloidal nanocrystals in salt powder matrix for quality lighting and color enrichment
(OSA - The Optical Society, 2016) Erdem, T.; Soran-Erdem, Z.; Kelestemur, Y.; Gaponik, N.; Demir, Hilmi Volkan
Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar greenand red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the cointegrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase (up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes (LEDs), with the resulting hybrid LED exhibiting a luminous efficiency >70 lm/Welect . The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications.
Open Access
Generation of ultra-small InN nanocrystals by pulsed laser ablation of suspension in organic solution
(Springer Verlag, 2017-03) Kurşungöz, C.; U. Şimşek, E.; Tuzaklı, R.; Ortaç, B.
Nanostructures of InN have been extensively investigated since nano-size provides a number of advantages allowing applications in nanoscale electronic and optoelectronic devices. It is quite important to obtain pure InN nanocrystals (InN-NCs) to reveal the characteristic features, which gain interest in the literature. Here, we proposed a new approach for the synthesis of ultra-small hexagonal InN-NCs by using suspension of micron-sized InN powder in ethanol with pulsed laser ablation method. The liquid environment, laser energy and ablation time were optimized and a post-synthesis treatment, centrifugation, was performed to achieve InN-NCs with the smallest size. Besides, the micron-sized InN powder suspension, as a starting material, enabled us to obtain InN-NCs having diameters smaller than 5 nm. We also presented a detailed characterization of InN-NCs and demonstrated that the formation mechanism mainly depends on the fragmentation due to laser irradiation of the suspension.
Open Access
Stable and efficient colour enrichment powders of nonpolar nanocrystals in LiCl
(Royal Society of Chemistry, 2015) Erdem, T.; Soran-Erdem Z.; Sharma, V. K.; Keleştemur, Y.; Adam, M.; Gaponik N.; Demir, Hilmi Volkan
In this work, we propose and develop the inorganic salt encapsulation of semiconductor nanocrystal (NC) dispersion in a nonpolar phase to make a highly stable and highly efficient colour converting powder for colour enrichment in light-emitting diode backlighting. Here the wrapping of the as-synthesized green-emitting CdSe/CdZnSeS/ZnS nanocrystals into a salt matrix without ligand exchange is uniquely enabled by using a LiCl ionic host dissolved in tetrahydrofuran (THF), which simultaneously disperses these nonpolar nanocrystals. We studied the emission stability of the solid films prepared using NCs with and without LiCl encapsulation on blue LEDs driven at high current levels. The encapsulated NC powder in epoxy preserved 95.5% of the initial emission intensity and stabilized at this level while the emission intensity of NCs without salt encapsulation continuously decreased to 34.7% of its initial value after 96 h of operation. In addition, we investigated the effect of ionic salt encapsulation on the quantum efficiency of nonpolar NCs and found the quantum efficiency of the NCs-in-LiCl to be 75.1% while that of the NCs in dispersion was 73.0% and that in a film without LiCl encapsulation was 67.9%. We believe that such ionic salt encapsulated powders of nonpolar NCs presented here will find ubiquitous use for colour enrichment in display backlighting. © The Royal Society of Chemistry 2015.
Open Access
Structural and phase evolution in mechanically alloyed calcium copper titanate dielectrics
(2013) Alizadeh, M.; Ardakani H.A.; Amini, R.; Ghazanfari, M.R.; Ghaffari, M.
Nanocrystalline calcium-copper-titanate (CCTO) dielectric powders were prepared by mechanical alloying. Phase transformations and structural evolution of the mechanically activated powders were investigated through the Rietveld refinement of the X-ray diffraction results. The crystallite size, lattice strain, and weight fraction of individual phases were estimated based on crystal structure refinement. Furthermore, the microstructural properties and thermal behavior of the milled powders were investigated by Transmission Electron Microscopy (TEM) and Differential Thermal Analysis (DTA), respectively. It was found that CCTO nanocrystals can be successfully synthesized after the amorphization of the initial crystalline materials. Semi-spherical nano-size particles were developed after sufficient milling time. Formation of an amorphous phase during the milling cycle was confirmed by the presence of the glass transition and crystallization peaks in the thermal analysis profiles. © 2012 Elsevier Ltd and Techna Group S.r.l.