Phase transformations during the synthesis and sintering of Y 2-xYbxO3 nanopowders

Article abstract of DOI:10.1134/S0020168511040194

The formation of an Y₂O₃-Yb₂O₃ solid solution (8 mol % Yb₂O₃) during the thermal decomposition of (Y,Yb)₂(CO₃)₃ · 2H ₂O mixed carbonates obtained by coprecipitation from a nitrate solution has been studied by X-ray diffraction, thermal analysis, and optical microscopy. The results demonstrate that the formation of a cubic yttria-based solid solution begins in the range 470-500°C and reaches completion at temperatures above 1100°C. The unit-cell parameter a of the cubic solid solution and the X-ray density of the corresponding ceramic are 10.5910 A and 5.349 g/cm³, which corresponds to the intended chemical composition of the isovalent substitutional solid solution: Y _1.84Yb_0.16O₃.

Full text of DOI:10.1134/S0020168511040194

ISSN 0020ꢀ1685, Inorganic Materials, 2011, Vol. 47, No. 4, pp. 390–395. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © G.B. Tel’nova, A.A. Konovalov, L.I. Shvorneva, S.V. Kutsev, K.A. Solntsev, 2011, published in Neorganicheskie Materialy, 2011, Vol. 47, No. 4, pp. 445–
451.
Phase Transformations during the Synthesis and Sintering
of Y2 – YbxO3 Nanopowders
x
G. B. Tel’nova, A. A. Konovalov, L. I. Shvorneva, S. V. Kutsev, and K. A. Solntsev
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia
eꢀmail: imet@imet.ac.ru
Received December 9, 2009; in final form, July 22, 2010
Abstract—The formation of an Y2O3–Yb2O3 solid solution (8 mol % Yb2O3) during the thermal decompoꢀ
sition of (Y,Yb)2(CO3)3 ⋅ 2H2O mixed carbonates obtained by coprecipitation from a nitrate solution has been
studied by Xꢀray diffraction, thermal analysis, and optical microscopy. The results demonstrate that the forꢀ
mation of a cubic yttriaꢀbased solid solution begins in the range 470–500 and reaches completion at temꢀ
peratures above 1100 . The unitꢀcell parameter of the cubic solid solution and the Xꢀray density of the
corresponding ceramic are 10.5910 Å and 5.349 g/cm3, which corresponds to the intended chemical compoꢀ
°C
°
C
a
sition of the isovalent substitutional solid solution: Y1.84Yb0.16O3
.
DOI: 10.1134/S0020168511040194
INTRODUCTION
According to Saito et al. [7], the thermal decomposiꢀ
tion of crystalline Y2(CO3)3 2Н2О and yttria formaꢀ
tion reach completion above 600 , whereas Zhenguo
Huang et al. [10] reported full conversion of the carꢀ
bonate precursor to cubic Y2O3 at 600
⋅
Recent advances in the technology of highly transꢀ
parent ceramic materials based on Y2O3 doped with
°С
Nd3+ and Yb3+ have made them potential candidates
for application as components of solidꢀstate lasers.
Owing to its excellent physicochemical properties,
such as optical isotropy, high melting point, highꢀtemꢀ
perature structural stability, high transmission in a
wide spectral region (approximately from 0.23 to
°С.
On the other hand, when one employs chemical
precipitation of precursors to yttriaꢀbased solid soluꢀ
tions for laser applications, it is also necessary to study
the feasibility and conditions of quantitative precipitaꢀ
tion of yttrium and rareꢀearth cations.
8 m), high thermal conductivity, and thermal stabilꢀ
µ
ity, yttria is a potentially attractive material for laser
applications [1–4]. Given their low cost, and the
potential possibility of improving their lasing perforꢀ
mance, transparent Yb3+(Nd3+) : Y2O3 ceramics are
Earlier [11], we described the synthesis of (Y,
Yb)2(CO3)3
⋅
2Н2О mixedꢀcarbonate nanoparticles
through chemical precipitation from nitrate solutions,
which were converted by thermolysis to nanopowders
among the best known crystalline laser materials for of cubic oxide solid solutions used to produce Yb:Y2O3
femtosecond optical applications [3].
ceramics.
The objective of this work was to study the thermal
2Н2О carbonates
and the phase transformations accompanying the synꢀ
thesis and sintering of Yb:Y2O3 nanoparticles.
In the past decade, using oxide nanopowders and
conventional vacuum sintering, Japanese researchers decomposition of (Y,Yb)2(CO3)3
created highly transparent laser ceramics: Nd3+
Y3Al5O12, Nd3+ : Y2O3, Yb3+ : Y2O3, and others [5, 6].
⋅
:
The ability to produce highly transparent yttria
laser ceramics at relatively low firing temperatures
requires knowledge of methods for the synthesis of
homogeneous, monodisperse, nonagglomerated oxide
nanopowders of controlled composition [7–10]. A
number of reports examined properties of yttria powꢀ
EXPERIMENTAL
We studied the composition 92Y2O3
⋅
8Yb2O3
(3.2 at % Yb). (Y,Yb)2(CO3)3 2Н2О mixed carbonates
⋅
were prepared by chemical precipitation from nitrate
solutions with ammonium bicarbonate in the presence
of a surfactant at pH 8.
ders derived from yttrium carbonate, Y2(CO3)3
⋅
nН2О
(
n
= 2 or 3), which is thought to be a promising precurꢀ
sor to highly sinterable Y2O3 nanoparticles [7, 10, 11].
However, the thermal decomposition of this carbonate
precursor to yttria has not yet been studied in sufficient
The mother solution used was a 1 M solution of
yttrium and ytterbium nitrates. The nitrate solutions
were prepared by dissolving Y2O3 (99.99%) and Yb2O3
detail and it remains the subject of controversy [7, 10]. (99.999%) in dilute nitric acid. As the precipitant, we
390

PHASE TRANSFORMATIONS DURING THE SYNTHESIS AND SINTERING
391
used a 2.5 M solution of ammonium bicarbonate, transmitted (transparent specimens) or reflected (polꢀ
NH4HCO3. The resultant precipitate was held in the ished surfaces) light.
precipitation medium for 24–63 h at room temperaꢀ
ture with constant stirring by a magnetic stirrer, then
filtered off, and dried in air at room temperature.
RESULTS AND DISCUSSION
During heating of (Y,Yb)2(CO3)3
cubic yttria particles nucleate and grow starting at
450–500 (Figs. 1a, 1b). The thermal decomposition
of the precursor involves the formation of monoclinic
yttria, ꢀY2O3, as an intermediate, metastable phase,
which is supported by the fact that the XRD patterns
of the samples calcined between 300 and 400 show
a halo in the range 28 –33 , which contains the
⋅
2Н2О in air,
To study the mechanisms of (Y,Yb)2(CO3)3 2Н2О
⋅
decomposition, phase transformations, and structural
changes during heating, the precursors were heatꢀ
treated in air at 110, 300, 400, 450, 500, 600, 800, 900,
°
С
β
1000, and 1100 С for 60 min at each temperature.
°
The sintering process was studied using nanopowꢀ
ders with a hypothetical composition of Y1.84Yb0.16O3
(3.2 at % Yb) after heat treatment of the precursors at
°С
2θ
°
°
strongest diffraction lines 111, 401, 402, 310, and 112
of monoclinic yttria (JCPDS PDF, card 47ꢀ1274)
(Figs. 1a, 1c). The asymmetric shape of the halo in the
900 С. Samples in the form of disks 10 mm in diameꢀ
°
ter and 2.5 mm in thickness were prepared by pressing
dry powder at 100 MPa with no plasticizers. The green
compacts were sintered in a vacuum electric furnace at
XRD pattern of the sample calcined at 450 С seems to
°
be due to the onset of cubic Y2O3 crystallization and
the presence of two yttria polymorphs: monoclinic
and cubic (Fig. 1b). Raising the synthesis temperature
1700 and 1950°С with a 1ꢀh hold at the final firing
temperature, followed by air annealing at 1450
°С.
Next, we determined the density and volume shrinkꢀ
age of the ceramics and examined their microstrucꢀ
ture.
to 500 С gives rise to the main diffraction lines of cubic
°
Y2O3 (JCPDS PDF, card 41ꢀ1105) and leads to the
formation of a single, yttriaꢀbased crystalline phase
(Fig. 1a).
Xꢀray diffraction (XRD) patterns were collected on
a Shimadzu XRDꢀ6000 diffractometer (Cu
K radiaꢀ
α
According to the differential thermal analysis
tion). The phases present were identified using JCPDS
PDF data and the software supplied with the diffractoꢀ
meter. The crystallite size was evaluated by the Scherꢀ
(DTA) data in Fig. 2, (Y,Yb)2(CO3)3
involves several steps. The large endotherm in the temꢀ
perature range 68 to 157 , with a maximum peak
temperature of 104 , corresponds to the release of
adsorbed water vapor. The weight loss in the temperaꢀ
ture range from 220 to 340 is due to the sequential
⋅
2Н2О pyrolysis
°С
rer formula: Dhkl
size, is the Cu
half maximum of the peak (rad), and
=
K
λ
/
β
cos
θ
, where Dhkl is the crystallite
is the full width at
is the angular
°С
λ
wavelength,
β
α
θ
°С
position of the peak (deg). We analyzed the 622, 400,
and 222 reflections from cubic yttria. The particle size
of nanopowders was estimated from their crystallite
size. When the precursor was heated from 600 to
detachment of two molecules of water of crystallizaꢀ
tion and carbonate groups. The exothermic event cenꢀ
tered at 340
crystallization of monoclinic yttria. The exotherm
between 477 and 607 , with a maximum peak temꢀ
perature of 540 , is due to the monoclinicꢀtoꢀcubic
phase transition of yttria. The exothermic peak cenꢀ
tered at 646 seems to result from structural ordering
°С is the decomposition of the salt and
1100 С, the average particle size of Yb:Y2O3 increased
°
°С
from 18 to 47 nm.
°С
The crystallite size extracted from XRD data was in
reasonable agreement with the average particle size
°С
(
D
BET) found as DBET
=
6/
ρ
SBET, where
ρ
is the theoꢀ
in the Y2O3ꢀbased cubic solid solution. The considerꢀ
able weight loss (up to 30%) in the sample at temperaꢀ
retical density (g/cm3) of the Y1.84Yb0.16O3 solid soluꢀ
tion and SBET (m2/g) is the BET specific surface area of
the precursor pyrolysis products (lowꢀtemperature
nitrogen adsorption measurements on a Micromeritꢀ
ics TriStar analyzer).
tures from 302 to 662 С is due to the decomposition of
°
the surface carbonates formed as a result of the
adsorption of the gaseous pyrolysis products (CO2) on
the large surface of quasiꢀdimensional monoclinic
yttria particles.
Thermal analysis was carried out with a Qꢀ1500
As the synthesis temperature is raised from 600 to
thermoanalytical system between 20 and 1200
°С at a
1100 С, the XRD peaks of our samples become stronꢀ
°
heating rate of 15 C/min. Gaseous products were
°
ger (Fig. 3), indicating an increase in the degree of
crystallinity, accompanied by an increase in crystallite
size from 18 to 47 nm, and improvement in the strucꢀ
tural perfection of the cubic Y2 – xYbxO3 substitutional
solid solution (bixbyite (Fe,Mn)2O3 structure, sp. gr.
identified by mass spectrometry on an STA 409 Luxx
thermal analyzer connected through a capillary to a
Netzsch QMS 403 Aëolos quadrupole mass spectromꢀ
eter (20–1200 , heating rate of 10
°
С
°C/min).
Microstructures of ceramic samples were examꢀ
ined by scanning electron microscopy on a Carl Zeiss
T 7
, Z = 16). No other crystalline phases were
)
h
Ia3
(
LEO 1420 and by optical polarizing microscopy in detected in the XRD patterns of the samples calcined
INORGANIC MATERIALS Vol. 47
No. 4 2011

392
TEL’NOVA et al.
(а)
222
Y1.84Yb0.16O3
500
1000
500
440
°
C
622
400
211
420
332 134
611
1000
500
450°
C
1000
500
400
300
110
°
°
°
C
C
C
1000
500
1000
500
020
(Y, Yb)2(CO3)3 · 2H2O
1000
500
202
131
220
113
004
101
123
143
002
141
240
200
25°C
0
10
20
30
40
50
60
70
, deg
2θ
(b)
c
ꢀY2O3
1000
500
0
500
°
C
450°C
10
20
30
40
50
60
, deg
(c)
2
θ
500
400
300
200
100
m
ꢀY2O3
400
°
C
0
30
10
20
50
40
60
, deg
2θ
Fig. 1. XRD patterns of the solid decomposition products of the (Y,Yb) (CO ) ⋅ 2Н О mixed carbonates calcined (a) in the temꢀ
3 3 2
2
perature range from 110 to 500°С, (b) at 450 and 500°С, and (c) at 400°С.
solution upon Y3+ substitution for Yb3+, with a reducꢀ
tion in unitꢀcell parameter (Fig. 4).
Since the lattice parameter of undoped cubic Y2O3
is 10.6041(5)Å (JCPDS PDF, card 41ꢀ1105) and that
of undoped cubic Yb2O3 is 10.4347(6) Å (JCPDS PDF,
in the temperature range 500 to 1100
ics produced by sintering at 1700 and 1950
increasing synthesis temperature, the diffraction peaks
shift systematically to higher values. This effect is
due to gradual structural ordering of the cubic solid
°
С
or the ceramꢀ
а
°С. With
2θ
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No. 4
2011

PHASE TRANSFORMATIONS DURING THE SYNTHESIS AND SINTERING
393
DTA,
μ
V/mg
EXO
(а)
TG, %
100
TG
Peak
Initial
Final
:
339.7
302.4
370.8
°C
°C
°C
:
0.8
Weight change: – 10.67%
:
95
90
85
80
75
70
65
60
Peak
Initial
Final
:
540.2
476.8
606.5
°
°
°
C
C
C
:
:
Peak
Initial
Final
:
646.1
628.0
661.5
°
°
°
C
C
C
DTA
0.6
0.4
0.2
0
:
:
Weight change: –16.39%
Weight change: –11.15%
Peak
Initial
Final
:
103.7
68.2
157.7
°C
C
:
°
:
°C
Residual weight: 56.03% (1099.5°C)
Weight change: – 3.80%
200
400
600
Temperature,
800
1000
°
C
(b)
Ion current
DSC,
×
109, A
μV/mg
EXO
TG, %
100
TG
DTA
1.0
0.8
0.6
0.4
0.2
0
95
90
85
80
75
70
65
60
H2O+
OH+
200
400
600
800
1000
Temperature, C
°
Fig. 2. (a) Thermal analysis of (Y,Yb) (CO ) ⋅ 2Н О nanopowder; (b) thermal analysis of the nanopowder and mass spectrometry
3 3 2
2
of gaseous decomposition products.
Given that the density of Yb2O3 is 9.215 g/cm3
(JCPDS PDF, card 41ꢀ1106), which far exceeds that
of Y2O3, we conclude that yttriaꢀbased cubic solid
card 41ꢀ1106), the formation of yttriaꢀrich cubic
Y2O3–Yb2O3 solid solutions should be accompanied by
lattice contraction and an increase in density, as supꢀ
ported by the present data (Fig. 4).
solutions form at a temperature as low as 500
°
С.
The Xꢀray density of the sample synthesized at
The lattice parameter of the cubic solid solution
а
500 С (evaluated using the 622 diffraction peak of
°
is 10.5910 Å and the Xꢀray density of the ceramic samꢀ
ple is 5.349 g/cm3 (Fig. 4). The fact that this density is
close to the additivity rule value calculated for an isovꢀ
cubic yttria) is 5.281 g/cm3, which markedly exceeds
the theoretical density of cubic Y2O3 (5.031 g/cm3).
INORGANIC MATERIALS Vol. 47
No. 4 2011

394
TEL’NOVA et al.
6000
4000
2000
Y2O3
1100
1000
900
°
°
°
°
°
°
C
C
C
C
C
C
6000
4000
2000
6000
4000
2000
6000
4000
2000
800
6000
4000
2000
600
6000
4000
2000
0
500
20
40
50
60
30
2θ, deg
Fig. 3. XRD patterns of the nanopowders produced by heatꢀtreating (Y,Yb) (CO )
⋅ 2Н О mixed carbonates at temperatures
2
3 3 2
from 500 to 1100°С.
alent substitutional solid solution with a hypothetical
composition of Y1.84Yb0.16O3 (5.351 g/cm3) confirms
that there is quantitative yttrium–ytterbium coprecipꢀ
Examination of transparent specimens and fracꢀ
ture surfaces of sintered materials under an optical
polarizing microscope in transmitted light and
itation in the synthesis of the precursor and that the through crossed nicols indicated the presence of an
chemical composition of the synthesized cubic solid
solution differs little from the intended one.
anisotropic phase in the cubic solid solution (Fig. 5).
The formation of an optically anisotropic phase in
cubic yttriaꢀbased crystals is due to the crowdion
mechanism of plastic deformation. Microstructural
examination showed that such crystallites typically
had a layered structure. The development of a layered
microstructure in fcc crystals seems to be related to the
possible formation of hexagonal interlayers as a result
of shear deformation, which are similar in structure to
(111) twin boundaries. The fcc oxygen sublattice then
transforms to an hcp one. Figure 5 shows micrographs
of fracture surfaces of transparent ceramics in transꢀ
mitted light (Fig. 5a) and through crossed nicols
(Fig. 5b). There is a wellꢀseen layered structure of
individual crystallites and anisotropic interlayers
(Fig. 5b) in individual layered crystals. Layered microꢀ
structure may be encountered on etched sections of
transparent ceramics (Fig. 6), but it has an insignifiꢀ
cant effect on their transmission.
ρ
, g/сm3
5.35
a, Å
2
10.65
10.64
10.63
10.62
10.61
10.60
5.34
5.33
5.32
5.31
5.30
5.29
5.28
1
10.59
10.58
400
800
1200
1000
1600
1400
2000
1800
600
Synthesis temperature
, °C
Fig. 4.
(
1
) Lattice parameter
a
(evaluated from the position
) Xꢀray density as funcꢀ
of the 622 diffraction peak) and (
2
tions of synthesis temperature for the cubic
solid solution.
Y
Yb
1.84 0.16 3
O
INORGANIC MATERIALS Vol. 47
No. 4
2011

PHASE TRANSFORMATIONS DURING THE SYNTHESIS AND SINTERING
CONCLUSIONS
395
The thermal decomposition of (Y,Yb)2(CO3)3
⋅
2Н2О mixed carbonates prepared by chemical copreꢀ
cipitation from nitrate solutions involves several steps,
in particular the formation of an intermediate, metaꢀ
stable phase based on monoclinic yttria in the temperꢀ
ature range 300–400
phase starting at 450
°
С
С
, which converts to a cubic
°
. Heating to 500 leads to
°
С
mass crystallization of a cubic isovalent substitutional
solid solution, Y2 – хYbхO3 (bixbyite (Fe,Mn)2O3 strucꢀ
T 7
ture, sp. gr. Ia
raised from 600 to 1100
the solid solution increases from 18 to 47 nm. The
unitꢀcell parameter of the cubic solid solution and
the Xꢀray density of the ceramic are 10.5910 Å and
5.349 g/cm3, which corresponds to the intended compoꢀ
sition of the solid solution, Y1.84Yb0.16O3, confirming that
there is quantitative yttrium–ytterbium coprecipitation
under the precursor synthesis conditions studied.
3
,
, Z
°
= 16). As the temperature is
С, the average particle size of
(
)
h
а
3 μm
(а)
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INORGANIC MATERIALS Vol. 47
No. 4 2011