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Size-dependent High-Temperature Behavior of Bismuth Oxide Nanoparticles

Guenther, Gerrit (2012):
Size-dependent High-Temperature Behavior of Bismuth Oxide Nanoparticles.
TU Darmstadt, [Online-Edition: http://tuprints.ulb.tu-darmstadt.de/3302/],
[Ph.D. Thesis]

Abstract

Oxide nanostrucures show very strong size-dependent changes in their thermal and chemical stability and reactivity. The total energy of nanoparticles (high surface-to-volume ratio) is increased due to the imperfect bonding structure of their surface-affected atoms. This leads to the shifts in the mentioned properties. While this relationship is valid for any kind of inorganic material the degree of these changes depends on the bond-strength and bond-nature of the material at the surface: The higher the surface energy the stronger the size-dependence. These thoughts are demonstrated here by experiments with sized-selected bismuth oxide nanoparticles between 5 and 50 nm. They were synthesized by an aerosol-based evaporation-condensation process with a size-selecting method resulting in monocrystalline, spherical and monodisperse particles. Characterization at room temperature revealed a distorted Beta-Bi2O3 structure. This shows a size-driven thermodynamic crossover in phase stability below a critical particle size. Heating experiments up to the evaporation point were performed inside the synthesis-chamber as well as with in-situ TEM, in-situ XRD and a special membrane-based high-temperature nanocalorimeter. Different atmospheres were used. The results show a pronounced melting point reduction. For example 10 nm particles melted 40% below the bulk in the TEM which is a considerably stronger size-effect than for metals (approx. 5 %). These experimental results were compared with the existing models.

Item Type: Ph.D. Thesis
Erschienen: 2012
Creators: Guenther, Gerrit
Title: Size-dependent High-Temperature Behavior of Bismuth Oxide Nanoparticles
Language: English
Abstract:

Oxide nanostrucures show very strong size-dependent changes in their thermal and chemical stability and reactivity. The total energy of nanoparticles (high surface-to-volume ratio) is increased due to the imperfect bonding structure of their surface-affected atoms. This leads to the shifts in the mentioned properties. While this relationship is valid for any kind of inorganic material the degree of these changes depends on the bond-strength and bond-nature of the material at the surface: The higher the surface energy the stronger the size-dependence. These thoughts are demonstrated here by experiments with sized-selected bismuth oxide nanoparticles between 5 and 50 nm. They were synthesized by an aerosol-based evaporation-condensation process with a size-selecting method resulting in monocrystalline, spherical and monodisperse particles. Characterization at room temperature revealed a distorted Beta-Bi2O3 structure. This shows a size-driven thermodynamic crossover in phase stability below a critical particle size. Heating experiments up to the evaporation point were performed inside the synthesis-chamber as well as with in-situ TEM, in-situ XRD and a special membrane-based high-temperature nanocalorimeter. Different atmospheres were used. The results show a pronounced melting point reduction. For example 10 nm particles melted 40% below the bulk in the TEM which is a considerably stronger size-effect than for metals (approx. 5 %). These experimental results were compared with the existing models.

Uncontrolled Keywords: Models for size-dependent melting point reduction, Pawlow, Buffat, evaporation, phase transformation, solid state phase transition, Phase diagram, oxides, surface enregy, surface stress, nanoparticle, nucleation, synthesis, facets shape, chip calorimetry, nanocalorimetry
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Joint Research Laboratory Nanomaterials
11 Department of Materials and Earth Sciences > Material Science > Nonmetallic-Inorganic Materials
11 Department of Materials and Earth Sciences > Material Science > Structure Research
07 Department of Chemistry > Physical Chemistry
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
07 Department of Chemistry
Date Deposited: 18 Mar 2013 16:27
Official URL: http://tuprints.ulb.tu-darmstadt.de/3302/
URN: urn:nbn:de:tuda-tuprints-33022
License: Creative Commons: Attribution-No Derivative Works 3.0
Referees: Guillon, Prof. Dr. Olivier and Hahn, Prof. Dr. Horst
Refereed / Verteidigung / mdl. Prüfung: 14 December 2012
Alternative Abstract:
Alternative abstract Language
Nanostrukturen aus Oxiden unterliegen starken, von ihrer Größe abhängigen Änderungen der thermischen und chemischen Stabilität und Reaktivität. Die Ursache dafür liegt in einer Zunahme der inneren Energie dieser Partikel aufgrund von zahlreichen ungesättigten Bindungen an der Oberfläche (großes Oberflächen/Volumen-Verhältnis). Während diese Effekte in jedem anorganischen Material auftreten ist die Stärke ihrer Ausprägung von der Bindungsnatur an der Oberfläche abhängig: Je größer der Oberflächenüberschuss, desto ausgeprägter der Größeneffekt. Diese Überlegungen werden in dieser Arbeit anhand von größenselektierten Bismutoxidnanopartikeln zwischen 5 und 50 nm demonstriert. Die Partikel wurden mittels eines Verdampfungs-Kondensationsprozesses mit Größenselektion synthetisiert. Charakterisierung dieser monokristallinen, monodispersen Partikel zeigte, dass die Partikel bei Raumtemperatur stets in der Beta-Bi2O3 Struktur vorliegen, welche im Bulk nur metastabil ist. Es liegt also eine größeninduzierte Änderung der Phasenstabilität unterhalb einer kritischen Partikelgröße vor. Weiterhin wurden Heizexperimente bis zur Verdampfung einzelner, separierter Partikel in der Synthesekammer sowie in Form von in-situ TEM-, in-situ XRD- und speziellen Hochtemperatur-Chipkalorimetrie-Messungen durchgeführt. Dabei wurden verschiedene Atmosphären verwendet. Die Ergebnisse zeigen eine starke Schmelzpunkterniedrigung für Bismutoxid. Beispielsweise schmolzen 10 nm Partikel im TEM 40 % unterhalb der Bulktemperatur, was ein deutlich stärkerer Größeneffekt ist, als bei Metallen (ca. 5 %). Diese experimentellen Ergebnisse wurden mit den bestehenden Modellen verglichen und diskutiert. German
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