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Master's Dissertation
DOI
10.11606/D.43.2010.tde-29062010-112348
Document
Author
Full name
Fabiana Rodrigues Arantes
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2010
Supervisor
Committee
Cornejo, Daniel Reinaldo (President)
Figueiredo Neto, Antonio Martins
Pirota, Kleber Roberto
Title in Portuguese
Estudo do comportamento magnético de nanopartículas de magnetita e nanofios de níquel diluídos em cristais líquidos liotrópicos
Keywords in Portuguese
cristais líquidos liotrópicos
magnetita
nanofios
nanopartículas
níquel
superparamagnetismo
Abstract in Portuguese
Neste trabalho foi feito um estudo do comportamento magnético de cristais líquidos liotrópicos de laurato de potássio dopados com nanopartículas de magnetita. Medidas zero-field cooling (ZFC) e field cooling (FC) com campos de 50 Oe mostram que as nanopartículas magnéticas permanecem bloqueadas a temperatura ambiente devido à presença das micelas do cristal, e apresentam um comportamento superparamagnético quando dispersas somente em água. Simulações Monte Carlo indicam que o aumento da temperatura de bloqueio das nanopartículas imersas em cristais líquidos liotrópicos pode ser obtido considerando uma forte interação, similar à de troca, entre os momentos superficiais e uma camada extra, simulando a presença das micelas. Medidas ZFC-FC e de suscetibilidade magnética permitiram determinar as temperaturas de transição entre as fases micelar isotrópica e nemática discótica. A curva de magnetização na região da fase discótica parece seguir o mesmo comportamento do parâmetro de ordem desta fase. Foram obtidos nanofios de níquel pelo processo de eletrodeposição AC em lâminas de alumina porosas para dopar cristais líquidos liotrópicos na fase isotrópica. As medidas ZFC-FC desses sistemas foram mais delicadas que as das nanopartículas devido ao baixo sinal das amostras e baixa solubilidade dos nanofios no cristal líquido. A técnica para a dissolução dos fios e dopagem dos cristais foi aprimorada, no entanto são necessários mais estudos para melhorar a estabilidade dessas amostras.
Title in English
Study on the magnetic behavior of magnetite nanoparticles and nickel nanowires diluted in nematic fluids
Keywords in English
liquid crystals
magnetite
nanoparticles
nanowires
nickel
superparamagnetism
Abstract in English
In this work we studied the magnetic behavior of potassium laurate lyotropic liquid crystals doped with magnetite nanoparticles. Zero-field cooling (ZFC) and field-cooling (FC) measurements with 50 Oe applied field showed a blocking behavior of magnetic nanoparticles at room temperature due to the micelle presence, while presenting superparamagnetic behavior when dispersed in water. Monte Carlo simulations suggest that this increase in the nanoparticles' blocking temperature when immersed in lyotropic liquid crystals can be acquired taking into account an exchange-like strong interaction between the nanoparticles' superficial magnetic moments and an extra layer playing the role of the micelles. ZFC-FC and magnetic susceptibility measures allowed us to find the transition temperatures between the isotropic micellar and the discotic nematic phases. The magnetization curve in the discotic phase resembles the behavior of the order parameter in that phase. We obtained nickel nanowires by AC electrodeposition in anodic porous alumina to dope lyotropic liquid crystals in the isotropic phase. The ZFC-FC measurements in those systems were more intricate given the weak magnetization of the samples and low solubility of the nanowires in the liquid crystal. We refined the technique for dissolution and doping of the crystals, but more studies are needed in order to improve the samples' stability.
 
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dissertacao.pdf (4.58 Mbytes)
Publishing Date
2010-07-08
 
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