International Journal of Nanoscience
Vol. 5, Nos. 4 & 5 (2006) 487—491
c World Scientific Publishing Company
RF PLASMA PROCESSING OF ULTRAFINE Sm-Lu MIXED OXIDE POWDER
X. L. SUN, A. I. Y. TOK*, F. Y. C. BOEY and R. HUEBNER
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue
Singapore 639798
*
miytok@ntu.edu.sg
Ultrafine Sm-Lu mixed oxide powder with an overall composition Sm1.0Lu1.0O3 was processed by
inductive radio frequency plasma treatment using two different approaches: spraying a mixture of
pure Sm2O3 and Lu2O3 with and without sintering pretreatment. Due to the insufficient reaction time
and contact, the mixture sprayed without sintering shows only a marginal reaction between the two
materials. Therefore, an alternative approach was adopted. The mixed oxide was first synthesized by
solid state reaction at 1600°C. Then, the as-sintered pellets were crushed and ground, followed by
plasma treatment. In this way, ultrafine particles of Sm1.0Lu1.0O3 mixed oxide were obtained.
Keywords: RF plasma; rare earth; ultrafine powder; mixed oxide; Sm2O3; Lu2O3.
1.
Introduction
Rare earth materials have an ever-growing variety of applications in modern technology,
e.g., phosphors, catalysts, fuel cells, and biomaterials.1 It has also been found that rare
earth oxides are useful dopants in electronic components, like multilayer ceramic
capacitors (MLCC).2 Rare earth sesquioxides with the general formula R2O3 are usually
the most stable compounds obtained as the final product during calcination of most
rare earth metals and salts in air. Generally, rare earth oxides show exceptionally
thermodynamic stability, because they have the most negative standard free energies of
formation compared to any other oxides.3 In some applications of rare earth materials,
certain objectives can be achieved by forming oxides containing two different rare
earth elements. Examples are Eu-doped Y2O3, where luminescent rare earth ions
are incorporated into a nonluminescent host which is essential for energy-efficient
fluorescent lighting, or Sm-doped CeO2 oxygen ion conductors which are suitable for
solid electrolytes.4 Due to their unique physical and chemical properties compared with
those of bulk materials, ultrafine rare earth materials have been studied intensively, too.
To obtain nanocrystalline rare earth oxide powders, a number of processes has been
developed.5 Nevertheless, there are still some issues need to be dealt with, e.g., impurities,
agglomeration, particle size and shape. Inductive radio frequency (RF) plasma spraying is
one promising process for ultrafine powder synthesis, as it offers quite a few advantages,
like high energy density, minimum contamination, and spheroidization effect. It is the
objective of this paper to show that using this process, ultrafine Sm-Lu mixed oxide
powder can be prepared.
*
Corresponding author.
487
488 X. L. Sun et al.
2.
Experimental
For the synthesis of ultrafine Sm-Lu mixed oxide powder with an overall composition of
Sm1.0Lu1.0O3 by plasma spraying, two different approaches were applied. In the first, a
mixture of Sm2O3 and Lu2O3 (both from AMR Technologies Inc., purity 99.5 wt.%) with
an atomic ratio of 1:1 was prepared by ball milling for 14 h. After that, this mixture was
sprayed by means of a Tekna Inductive RF Plasma system at fixed input power of 21 kW
and chamber pressure of 400 Torr. Argon gas was used for both plasma gas and powder
carrier gas. The processed powders were collected in two chambers named C1 and C2.
For reasons of comparison, pure Sm2O3 and Lu2O3 were sprayed, too. In a second
approach, the ball milled mixture of Sm2O3 and Lu2O3 was initially pressed into pellets
(pressure: 100 MPa) and sintered at a temperature T = 1600°C for a duration of t = 8 h.
Using an agate mortar, the sintered pellets were crushed and ground into powders.
Afterwards, plasma spraying was done using the same process parameters as described
above. To analyze the phase composition of the processed polycrystalline materials,
X-ray diffraction (XRD) experiments were done in Bragg–Brentano geometry employing
a Shimadzu 6000 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). To determine the
unit cell parameters as well as the weight fractions of the obtained phases, further
analysis of the diffraction patterns by means of the Rietveld and Pawley method was
carried out using the TOPAS software.6 For characterization of the morphology and the
size of the processed particles, scanning electron microscopy (SEM, JEOL JSM 6340F)
and transmission electron microscopy (TEM, JEOL JEM 2010) were performed.
3.
Results and Discussion
As can be seen in Fig. 1, curve (a), sprayed Sm2O3 powder from chamber C2 consists
mainly of a monoclinic phase (JCPDS-ICDD 42-1464) whose structure can be described
in space group C2/m (B type structure).7 Additionally, there is a very small amount (<1
wt.%) of a cubic phase (JCPDS-ICDD 15-0813) which is best indicated by a slightly
increased intensity at 2θ = 28.2º. In contrast, sprayed Lu2O3 from chamber C2 consists
completely of a cubic phase (JCPDS-ICDD 12-0728, Fig. 1, curve (c)) whose structure
description is done in space group ⎯Ia3 (C type structure).8 The diffraction pattern
obtained in the first approach for the mixture of Sm2O3 and Lu2O3 is also included in
Fig. 1 (curve (b)). In principle, it can be characterized as a superposition of the abovementioned diffraction diagrams for pure Sm2O3 and Lu2O3 (Fig. 1, curves (a) and (c)).
Due to their low intensities, some small additional Bragg reflections remain, however,
unexplained. These additional diffraction maxima do not seem to be explained by
contamination, as they do not appear in other diffraction patterns. Thus, it can be
concluded that besides a possible small fraction of solid solution phases, the sprayed
mixture consists mainly of monoclinic Sm2O3 and cubic Lu2O3, which is confirmed by
the evaluation of the corresponding unit cell parameters. This result implies that there is
almost no reaction between both rare earth oxides during spraying. It might be the
consequence of the high thermodynamic stability of the reactants and the extremely short
residence time (2–3 milliseconds) in the plasma plume.
Intensity I [arb. units]
RF Plasma Processing of Ultrafine Sm-Lu Mixed Oxide Powder 489
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c u b . L u 2O
(c ) L u 2O
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3
(b ) M ix tu re
(a ) S m 2O
3
m o n . S m 2O
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D iffra c tio n a n g le 2 θ
Intensity I [arb. units]
Fig. 1. XRD patterns of sprayed Sm2O3 (a), Lu2O3 (b), and their mixture (c) collected from chamber C2 (only
the positions of the observed Bragg reflections of the different phases are marked).
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(c )
S p ra y e d
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P e lle t
m o n o c l.
c u b ic
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D iffra c tio n a n g le 2 θ
Fig. 2. XRD patterns of sintered Sm1.0Lu1.0O3 pellet (a), its ground powder (b), and sprayed powder from
chamber C2 (c).
In an alternative second approach, pellets of mixed raw materials were first sintered
at 1600ºC for 8 h, and then ground into powder for spraying. The XRD pattern of the
sintered pellet is shown in Fig. 2, curve (a). The material consists mainly of two phases, a
monoclinic and a cubic one, in a mass ratio of 1:15. By Rietveld refinement, their lattice
parameters were determined. In the case of the monoclinic phase, the associated unit cell
volume is smaller compared to that one of sprayed monoclinic Sm2O3, and for the cubic
phase, the unit cell volume is larger compared to that one of sprayed pure Lu2O3. Thus, a
mixture of two solid solution phases forms during sintering. This result is in accordance
with the observations by Schneider and Roth,9 who reported the coexistence of a Sm-rich
B type phase and a Lu-rich C type phase for Lu contents 44 at.% < xLu < 54 at.% in the
Sm2O3-Lu2O3 phase diagram. In the diffraction pattern of the pellet (Fig. 2, curve (a)),
there are, however, additional Bragg reflections which cannot be assigned to any known
490 X. L. Sun et al.
Fig. 3. SEM micrograph of the powder obtained
by grinding sintered pellets with the overall
composition Sm1.0Lu1.0O3.
Fig. 4. SEM micrograph of the larger particles of
chamber C1 collected after spraying the ground
powder.
phase of samarium or lutetium oxide. As these diffraction maxima disappear during
grinding (Fig. 2, curve (b)), they are not associated with contamination in the sample, but
might be due to the formation of a metastable phase during sintering. As shown in Fig. 3,
grinding the sintered pellets results in micron-sized particles with irregular shapes. The
particles have a polycrystalline microstructure and consist of monoclinic B type and
cubic C type phase whose mass ratio was determined to 1:6. Grinding does not lead to
significant changes of the corresponding unit cell parameters (Fig. 2, curve (b)).
After plasma processing of the ground material, the sprayed powders were separately
collected in two chambers C1 and C2 as large and fine particles, respectively. The large
particles of C1 have an almost spherical shape with diameters in the micron range.
The smooth surface indicates a history of surface melting (Fig. 4). The fine powder in
chamber C2 consists of both smaller micron-sized balls and nanosized particles. The
latter ones are indicated in Fig. 5 as light-gray cloudy clusters. TEM results (Fig. 6) show
that the nanoparticles are crystallites with a size less than 50 nm and different shapes.
When the raw material is fed into the plasma plume, it experiences a flash melting, partial
or complete evaporation, and subsequently it is quenched at an extremely high rate when
leaving the plume. While the melted particles form solid balls, the vaporized material
crystallizes into nanosized particles. To characterize the phase composition after spraying,
a diffraction pattern was recorded (Fig. 2, curve (c)) and analyzed. Besides the cubic C
type phase and the monoclinic B type phase which are already present in the diffraction
pattern after grinding (Fig. 2, curve (b)), the remaining Bragg reflections can be described
with an additional monoclinic B type phase, having a smaller unit cell. Its appearance
during spraying may be due to a partial transformation of the cubic phase. This
assumption is confirmed by a decrease of the cubic phase content which is indicated by
an intensity reduction of the corresponding Bragg reflections.
RF Plasma Processing of Ultrafine Sm-Lu Mixed Oxide Powder 491
Fig. 5. SEM micrograph of the finer particles in
chamber C2 after spraying the ground powder.
4.
Fig. 6. TEM micrograph of nanoparticles
from chamber C2.
Conclusion
RF plasma spraying of sintered samarium-lutetium oxide powder with an overall
composition Sm1.0Lu1.0O3 is a viable approach to produce ultrafine mixed oxide powder
which consists of monoclinic B type and cubic C type solid solution phases. The obtained
particles show a wide size distribution. The larger particles have almost perfect spherical
shape, ranging from submicron to micron size, whereas the ultrafine particles are
crystallites with a size less than 50 nm in various shapes. In contrast, RF plasma spraying
of a nonsintered mixture of Sm2O3 and Lu2O3 does not result in large scale formation of
solid solution phases. Instead, monoclinic Sm2O3 and cubic Lu2O3 are indicated as the
main phases by XRD analysis.
References
1. G. Adachi, N. Imanaka and Z. C. Kang, Binary Rare Earth Oxides (Kluwer Academic
Publishers, 2004).
2. H. Kishi, Y. Mizuno and H. Chazono, AAPPS Bulletin 14, 2 (2004).
3. K. A. Gschneidner, N. Kippenhan and O. D. McMasters, Thermochemistry of the rare earth,
Rare-Earth Information Center, Iowa State University, USA (1973).
4. A Lanthanide Lanthology, Part II, published by Molycorp Inc. CA, USA (1994).
5. F. Zhang, S. P. Yang, H. M. Chen and X. B. Yu, Ceram. Int. 30, 997 (2004).
6. Bruker AXS, TOPAS V2.1: General Profile and Structure Analysis Software for Powder
Diffraction Data — User’s Manual (Bruker AXS, Karlsruhe, Germany, 2003).
7. D. T. Cromer, J. Phys. Chem. 61, 753 (1957).
8. K. A. Gschneidner and L. R. Eyring (eds.), Handbook on the Physics and Chemistry of Rare
Earths, Vol. 3 (North-Holland, Amsterdam, 1979).
9. R. S. Roth and S. J. Schneider, J. Res. Nat. Bur. Stand. 64A, 309 (1960).