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Master's Dissertation
DOI
10.11606/D.43.2000.tde-09102001-185623
Document
Author
Full name
Hannes Fischer
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2000
Supervisor
Committee
Craievich, Aldo Felix (President)
Tolentino, Hélio Cesar Nogueira
Torriani, Iris Concepción Linares de
Title in Portuguese
Estudos estruturais de xerogéis de óxido de níquel.
Keywords in Portuguese
bifásico
drx
espalhamento
exafs
fase
lnls
níquel
óxido
sas
saxs
sistema
sol gel
transição
xafs
xerogel
Abstract in Portuguese
Foram estudadas as características estruturais de xerogéis obtidos pelo método sol-gel a partir de soluções líquidas precursoras compostas de cloreto de níquel (NiCl2), butanol, água e ácido acético, secos em estufa e tratados a diferentes temperaturas entre 150 e 900ºC. Nos tratamentos isotérmicos a essas temperaturas os elementos voláteis se desprendem do material inicial e transformações estruturais acontecem. Analisaram-se os materiais porosos resultantes (xerogéis) mediante técnicas estruturais: espalhamento de raios X a baixo ângulo (SAXS), espectroscopia de absorção de raios X (XANES e XAFS), difração de raios X (XRD) e microscopia eletrônica de varredura. Complementaram-se estes estudos mediante análise química (espectroscopia de infravermelho), térmica (análise térmica diferencial), gravimétrica (perda de massa) e de densidade. Os resultados experimentais demonstraram que a estrutura porosa do material inicial à base de NiCl2 se transforma numa estrutura bifásica, também porosa, composta por uma fase rica em NiCl2 e outra em óxido de níquel (NiO), sendo que a fração de volume ocupada pela fase de NiO no material final aumenta com a temperatura de tratamento térmico. No caso de temperaturas acima de 500ºC, praticamente todo o volume da amostra se transforma e é ocupado pela fase NiO. Mostrou-se que a estrutura porosa presente em todos os materiais, estudada por SAXS, é composta por nanoporos (10-150Å) e mesoporos (maiores que 150Å), sendo que a fração de volume ocupada pelos mesmos depende da temperatura do tratamento térmico. Em particular os nanoporos somente estão presentes nas amostras tratadas abaixo de 500ºC. A ordem local média ao redor dos átomos de Ni, determinada por EXAFS, corresponde ao esperado de sistemas bifásicos cujas frações de volume dependem da temperatura de tratamento, em concordância com os resultados de XRD. Investigaram-se as características dos materiais obtidos após os diferentes tratamentos térmicos em função de diferentes condições de preparação das soluções precursoras. Numa primeira série de experiências, variou-se o conteúdo de ácido acético na solução. Demonstrou-se que maiores concentrações de ácido acético promovem a formação de um material com estrutura mais compacta. Numa segunda série de experiências variou-se o conteúdo de água. Determinou-se que a quantidade de água afeta somente a estrutura dos materiais tratados a temperaturas inferiores à 500ºC.
Title in English
Structural studies of niquel oxide xerogels.
Keywords in English
angle
byphasic
oxide
phase
scattering
small
sol-gel
solution
system
transition
X-ray
xrd
Abstract in English
Structural properties of several xerogels obtained by the sol-gel procedure and heat treated at different temperatures between 150 and 900ºC were studied. The initial systems were dry gels prepared from liquid solutions composed of nickel chloride, buthanol, water and acetic acid. During the different isothermal treatments, volatile species leave the inicial material and structural transformations occur. The resulting porous materials (xerogels) were studied by means of several structural techniques: small angle X ray scattering (SAXS), X ray absorption spectroscopy (XANES and EXAFS), X ray diffraction (XRD) and scanning electron microscopy. These studies were complemented with chemical analysis (infrared spectroscopy), differential thermal analisis, mass loss and density measurements. The experimental results demonstrated that the porous structure of the starting material, based on nickel chloride, transforms in a also porous biphasic structure composed of nickel chloride and nickel oxide, respectively, rich fase. The volume fraction occupied by the nickel oxide rich fase of the final material increases with temperature of heat treatment. For temperatures above 500ºC, practically all the sample volume transforms and is occupied by the NiO phase. It was demonstrated that the porous structure of all materials, studied by SAXS, is composed of nanopores (sizes ranging from 10 to 150Å ) and mesopores (larger than 150Å), their fraction depending on the heat treatment temperature. Particularly, nanopores are only present in samples treated below 500ºC. The average local order close to nickel atoms, determined by EXAFS, corresponds to the expected one for biphasic systems whose volume fractions depend on treatment temperature, in agreement with XRD results. The structural properties of the several materials obtained after different heat treatments were studied as functions of preparation conditions of precursor solutions. In a first series of experiments, acetic acid content in the solution was varied. It was demonstrated that higher acetic acid content promotes the formation of more compact structures. In a second series of experiments, water content was varied. It was concluded that water content only affects the structure of the materials heat treated at temperatures below 500ºC.
 
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Publishing Date
2002-06-18
 
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