Strukturne, morfološke i funkcionalne karakteristike nanostrukturnih oksida retkih zemalja dobijenih aerosol sintezom

Structural, Morphological and Functional Properties of Nanostructured Rare Earth Oxides Obtained Through Aerosol Synthesis

Dobijanje ultrafinih, sferičnih, fosforescentnih čestica sa homogenom raspodelom komponenata i faza je od posebnog značaja za materijale sa primenom u savremenim optoelektronskim uređajima. Reakcija u disperznom sistemu (aerosolu) pripada kategoriji nanotehnologija i predstavlja metodu koja omogućava dobijanje finih, jedno- ili više-komponentnih nanostrukturnih materijala kontrolisanog sastava i morfologije, kao i unapređenih svojstava, obzirom da se sinteza izvodi na velikoj reakcionoj površini i pri kratkim vremenima zadržavanja.
U okviru ovog rada, nanostrukturne, sferične, polikristalne čestice prahova sistema  Y2O3:Eu3+ i (Y1-xGdx)2O3:Eu3+, su sintetizovane termičkim razlaganjem aerosola pri temperaturama od 900-1200°C. Aerosol je dobijen ultrazvučnom atomizacijom nitratnih rastvora odgovarajucih prekursora korišćenjem generatora aerosola rezonantne frekvence 1.3MHz. Ispitan je uticaj dopiranosti Eu3+ jona (5 i 10 at%), kao i uticaj promene udela gadolinijuma u strukturi mešanog oksida (x=0.25, 0.50, 0.75) i uspostavljene su korelacije parametara sinteze sa strukturnim, morfološkim i funkcionalnim svojstvima prahova.
Morfološke i hemijske karakteristike prahova su ispitane pomoću skening i transmisione elektronske mikroskopije u kombinaciji sa energetskom disperzionom analizom X-zraka (SEM/EDS, TEM/EDS), na osnovu koje je pokazano da su dobijeni prahovi sferične morfologije, glatkih površina, neaglomerisani i sa uskom raspodelom veličine čestica, kao i da poseduju visoku hemijsku čistoću i kompozicionu homogenost. Stereološka analiza SEM mikrografija sistema Y2O3:Eu3+ je pokazala da čestice poseduju visok stepen sferičnosti (faktor oblika fL= 0.95) i da približno 80% čestica ima raspodelu veličine u oblasti od 300-800nm, dok je srednja veličina čestica oko 600nm. Fazna analiza prahova je urađena difrakcijom X-zraka (XRD), transmisionom elektronskom mikroskopijom u kombinaciji sa elektronskom difrakcijom na odabranoj površini (TEM/SAED) i visokorezolucionom elektronskom mikroskopijom uz analizu pomoću Furijerove transformacije (HRTEM/FFT). Fazni razvoj i strukturna promena je dodatno praćena strukturnim utačnjavanjem Ritveldovom metodom u programu Topas Academic i uočena je unutrašnja,  nanostrukturna priroda prahova sa veličinom kristalita od 20nm, što je i potvrđeno HRTEM i SAED analizom. Pokazano je da svi sintetisani prahovi poseduju bcc Ia-3 kubnu strukturu, osim mešanog oksida sa najvećim sadržajem gadolinujuma (75%), sastava (Y0.25Gd0.75)2O3:Eu3+, gde je primećeno i prisustvo sekundarne, fcc Fm-3m kubne faze. Termičkim tretmanom (1000-1200°C/12h) su zadržane morfološke karakteristike prahova,  povećana je kristaliničnost ispitivanih sistema i postignuta fazna homogenost prahova, što se ogleda u prisustvu kubne Ia-3 faze kod svih ispitivanih sistema. Transmisiona elektronska mikroskopija je dodatno pokazala unutrašnju strukturu čestica, odnosno da su dobijene čestice pune morfologije i da predstavljaju agregate  primarnih entiteta (nano čestica) čije su dimenzije reda veličine 20nm, kao i da veličine primarnih čestica odgovaraju dobijenim vrednostima veličine kristalita koja raste sa porastom temperature termičkog tretmana (od 40nm/1000°C do 130nm/1200°C).
Funkcionalna karakterizacija ispitivanih sistema je izvršena fotolominescentnim ispitivanjima. Određeni su emisioni spektri, koji su pokazali tipične 5D0→7Fi (i=1, 2, 3, 4) prelaze Eu3+ jona sa dominantnim emisionim pikom na 611nm, koji odgovara jasnoj crvenoj emisiji. Određivanjem vremena života emisije 5D0 i 5D1 nivoa europijumovog jona je pokazano da sintetisani prahovi poseduju poboljšana svojstva u odnosu na prahove u konsolidovanoj formi, da za slučaj dopiranosti od 10 at% Eu3+ dolazi do koncentracionog gašenja emisije, kao i da je efekat krosrelaksacije izraženiji kod sintetisanih prahova u odnosu na termički tretirane, te da je termičkim tretmanom postignuta homogenija distribucija jona dopanta u rešetki domaćina.
The synthesis of ultrafine, spherical particles with uniformly distributed components and phases is of special importance when materials for modern optoelectronic application are considered. Synthesis through dispersion phase (aerosol) represents a feasible method for obtaining fine, nanostructured materials with controlled composition of either single or complex powders with improved properties provided by high surface reaction and short residence time. 
In this study, nanophased, spherical, polycrystalline Y2O3:Eu3+ and (Y1-xGdx)2O3:Eu3+ phosphor particles having improved luminescence properties are synthesized from aerosol of the corresponding nitrates solution ultrasonically generated with frequency 1.3 MHz and thermally decomposed at 900-1200ºC. The influence of different Eu3+ doping concentration (5 and 10at%) is investigated as well as the influence of gadolinium content in the structure of the mixed oxide (x=0.25, 0.50, 0.75). For the applied synthesis parameters structural, morphological and functional properties of the investigated systems are correlated.  
Morphological and chemical properties are investigated by means of scanning and transmission electron microscopy in combination with quantitative EDS analysis (SEM/EDS, TEM/EDS) and it is shown that the obtained powders are non-agglomerated with smooth surface and spherical morphology and that they possess high material purity and compositional homogeneity. Stereological analysis of SEM micrographs for Y2O3:Eu3+ system additionally revealed that the particles are highly spherical (form factor fL=0.95) and that approximately 80% of particles have size distribution in the range of 300-800nm, while the main particle size is around 600nm. Powder phase analysis is done by means of X-ray powder diffraction (XRPD), transmission electron microscopy in combination with selected are electron diffraction (TEM/SAED) and high resolution TEM combined with fast Furrier transforms (HRTEM/FFT). The phase development and structural changes are additionally followed through Rietveld structural refinement in Topas Academic software. This analysis showed nanocrystal inner structure (crystallite size 20 nm) of particles, which is also confirmed by HRTEM and SAED analysis. It is shown that all as-prepared samples have bcc Ia-3 cubic phase, apart from the mixed oxide with the highest gadolinium content (75%;  composition (Y0.25Gd0.75)2O3:Eu3+). In this sample the existence of a secondary, fcc Fm-3m cubic phase is determined. Through the additional thermal treatment (1000-1200°C) the morphological features of powders is maintained and a higher crystallinity of the investigated samples is achieved, as well as powder phase homogeneity by the presence of Ia-3 cubic phase in all the samples. Transmission electron microscopy gave an insight of inner particle morphology, namely, particles possess field morphology and they are consisted out of primary nanoparticle aggregates with dimensions around 20nm. The size of primary nanoparticles corresponds to calculated crystallite size which increases with the increase of thermal treatment temperature (from 40nm/1000°C up to 130nm/1200°C). 
Functional properties of the investigated systems are analyzed by means of photoluminescent analysis. Emission spectra showed a typical Eu3+ 5D0→7Fi (i=1, 2, 3, 4) transitions with the dominant red emission peak at 611nm. Life time measurement of europium ion 5D0 and 5D1 levels showed that the obtained powders have improved characteristic in comparison to bulk material, that concentration quenching occurs in the case of 10 at% europium doping and that the crossrelaxation effect is dominant for as-prepared samples in comparison to thermally treated ones. This implied that thermal treatment led to a more homogeneous distribution of doping ion within the host lattice.

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Metadaten
Author:Katarina Marinković
URL:http://www.itn.sanu.ac.rs/opus4/frontdoor/index/index/docId/328
Publisher:University of Belgrade, Faculty of Technollogy and Metallurgy
Place of publication:Belgrade
Advisor:Radoslav Aleksić, Olivera Milošević
Document Type:Master's Thesis
Language:srp
Date of Publication (online):17.05.2013
Year of first Publication:2009
Publishing Institution:University of Belgrade, Faculty of Technology and Metallurgy
Granting Institution:Institute of Technical Sciences of the Serbian Academy of Sciences and Arts
Date of final exam:15.04.2009
Tag:aerosol synthesis; nanostructured materials; photoluminescence; rare earth oxides
Pagenumber:132
Institutes:Institute of Technical Sciences of the Serbian Academy of Sciences and Arts
University of Belgrade, Faculty of Technology and Metallurgy
Open access:
Collections:MSc Theses
Licence (English):License LogoCreative Commons - Attribution-Noncommercial-No Derivative Works 3.0 Serbia