the larger ones were another unique observation. The intersection of two rings is
amazing, as this is in direct contradiction to the proposed mechanism for the ring
formation, based on the dry hole formation on an evaporating thin film completely
wetted to the substrate. The creation of this unique feature is attributed to the interplay of magnetic forces with the regular particle–substrate interactions. The
second detected property was that barium hexaferrite was formed as a colloidal solution without the use of a surfactant.
6.1.2.3 Sonochemical Preparation of Nanosized Rare-Earth Oxides
A long list of oxides was prepared sonochemically. Almost all the above-mentioned
oxides were synthesized in organic solvents. The other oxides that will be discussed
from here on were all prepared in aqueous solutions. Submicron size spheres of
silica and alumina prepared by well-known methods were coated sonochemically
by nanoparticles of oxides of europium and terbium using the same concentration
of ions [81]. We have also used sonochemistry to prepare nanoparticles of silica
and alumina doped with the same rare-earth ions for comparison. The highest luminescence intensities were observed for europium and terbium doped in nanoparticles of alumina of dimension 20–30 nm. The intensities are comparable or
higher than in commercial phosphors.
The synthesis of the rare-earth oxides was as follows: europium oxide was dissolved in a minimum amount of nitric acid and evaporated to dryness. The dry
nitrate was dissolved in 5 mL of water. The required amount of silica microspheres
was put in a beaker with 30 mL of water and the europium nitrate solution was
added. The sonication was carried out for 1 h in an open beaker kept in an ice
bath. 5 mL of 25% aqueous ammonia was added dropwise during the sonication.
The resulting product after sonication was washed extensively with water, centrifuged, and dried under vacuum.
The preparation of the europium oxide-doped silica nano-particles (99.0SiO 2 –
1.0Eu 2 O 3 mol%) was different. It was also carried out by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of H 2 O, ethanol, and a europium nitrate
solution. 25% aqueous ammonia was added dropwise during the sonication. In
this way, europium oxide incorporated into silica particles was obtained.
Decay time measurements of Eu 2 O 3 , and Tb 2 O 3 doped and coated on alumina
were conducted [82]. The luminescence of the alumina substrate was found to be
much shorter than that of the rare-earth oxides. Differences between the decay
times of the deposited and doped materials are accounted for by the stronger
guest–host interaction and the absence of concentration quenching in the doped
material.
In a similar work, ultrasound radiation was used to prepare Eu 2 O 3 doped in zirconia and yttrium-stabilized zirconium (YSZ) nanoparticles [83]. Europium oxide
was also coated sonochemically on the surface of submicron spherical zirconia and
YSZ, which were fabricated by wet chemical methods. Time decay measurements
of the doped and coated materials were conducted using a pulsed laser source.
Lifetimes < 1:1 ms radiative lifetime of the Eu
þ3 ions were detected for the doped
and coated as-prepared materials. When the doped and coated samples were an6.1 Sonochemistry 133
amazing, as this is in direct contradiction to the proposed mechanism for the ring
formation, based on the dry hole formation on an evaporating thin film completely
wetted to the substrate. The creation of this unique feature is attributed to the interplay of magnetic forces with the regular particle–substrate interactions. The
second detected property was that barium hexaferrite was formed as a colloidal solution without the use of a surfactant.
6.1.2.3 Sonochemical Preparation of Nanosized Rare-Earth Oxides
A long list of oxides was prepared sonochemically. Almost all the above-mentioned
oxides were synthesized in organic solvents. The other oxides that will be discussed
from here on were all prepared in aqueous solutions. Submicron size spheres of
silica and alumina prepared by well-known methods were coated sonochemically
by nanoparticles of oxides of europium and terbium using the same concentration
of ions [81]. We have also used sonochemistry to prepare nanoparticles of silica
and alumina doped with the same rare-earth ions for comparison. The highest luminescence intensities were observed for europium and terbium doped in nanoparticles of alumina of dimension 20–30 nm. The intensities are comparable or
higher than in commercial phosphors.
The synthesis of the rare-earth oxides was as follows: europium oxide was dissolved in a minimum amount of nitric acid and evaporated to dryness. The dry
nitrate was dissolved in 5 mL of water. The required amount of silica microspheres
was put in a beaker with 30 mL of water and the europium nitrate solution was
added. The sonication was carried out for 1 h in an open beaker kept in an ice
bath. 5 mL of 25% aqueous ammonia was added dropwise during the sonication.
The resulting product after sonication was washed extensively with water, centrifuged, and dried under vacuum.
The preparation of the europium oxide-doped silica nano-particles (99.0SiO 2 –
1.0Eu 2 O 3 mol%) was different. It was also carried out by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of H 2 O, ethanol, and a europium nitrate
solution. 25% aqueous ammonia was added dropwise during the sonication. In
this way, europium oxide incorporated into silica particles was obtained.
Decay time measurements of Eu 2 O 3 , and Tb 2 O 3 doped and coated on alumina
were conducted [82]. The luminescence of the alumina substrate was found to be
much shorter than that of the rare-earth oxides. Differences between the decay
times of the deposited and doped materials are accounted for by the stronger
guest–host interaction and the absence of concentration quenching in the doped
material.
In a similar work, ultrasound radiation was used to prepare Eu 2 O 3 doped in zirconia and yttrium-stabilized zirconium (YSZ) nanoparticles [83]. Europium oxide
was also coated sonochemically on the surface of submicron spherical zirconia and
YSZ, which were fabricated by wet chemical methods. Time decay measurements
of the doped and coated materials were conducted using a pulsed laser source.
Lifetimes < 1:1 ms radiative lifetime of the Eu
þ3 ions were detected for the doped
and coated as-prepared materials. When the doped and coated samples were an6.1 Sonochemistry 133