Phase transformations during plasma synthesis of sodium polyaluminates

Article abstract of DOI:10.1134/S0020168506010110

The synthesis of sodium beta-alumina with the use of a low-temperature nitrogen plasma is described. The major plasma synthesis product is the sodium polyaluminate Na₂O·nAl₂O₃ with an unidentified structure and n = 3.5-9.5. The phase transformations of the plasma synthesis products are studied during heating in the range 600-1400°C. The phase with the rhombohedral beta-alumina structure (β″-Al ₂O₃) is shown to form in several steps, through metastable sodium polyaluminates. The results are analyzed in relation to the phase equilibria in the Na₂O-Al₂O₃ system. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506010110

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 1, pp. 50–58. © Pleiades Publishing, Inc., 2006.
Original Russian Text © G.B. Tel’nova, Ya.P. Grabis, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 1, pp. 52–61.
Phase Transformations during Plasma Synthesis
of Sodium Polyaluminates
G. B. Tel’nova* and Ya. P. Grabis**
* Institute of Physicochemical Problems in Ceramic Science, Russian Academy of Sciences,
Ozernaya ul. 48, Moscow, 119361 Russia
** Institute of Inorganic Chemistry, Academy of Sciences of Latvia,
Miera iela 34, Salaspils-1, 229021 Latvia
e-mail: gtelnova@mail.ru
Received January 30, 2005; in final form, May 31, 2005
Abstract—The synthesis of sodium beta-alumina with the use of a low-temperature nitrogen plasma is
described. The major plasma synthesis product is the sodium polyaluminate Na2O · nAl2O3 with an unidentified
structure and n = 3.5–9.5. The phase transformations of the plasma synthesis products are studied during heat-
ing in the range 600–1400°C. The phase with the rhombohedral beta-alumina structure (β''-Al2O3) is shown to
form in several steps, through metastable sodium polyaluminates. The results are analyzed in relation to the
phase equilibria in the Na2O–Al2O3 system.
DOI: 10.1134/S0020168506010110
INTRODUCTION
formulas β-NaAl11O17 and β''-NaAl5O8. The reactivity
of SPAs depends on their chemical and thermal history
and the state of the major component, Al2O3.
The growing need for enhancing the efficiency of
thermal-to-electrical energy conversion and energy
storage has recently renewed interest in thermoelectro-
chemical (TEC) power generation systems and battery
cells with a sodium anode, in which sodium beta-alu-
mina ceramics are used as solid electrolytes.
Beta-alumina is commonly prepared by the standard
ceramic route, but there is still considerable room for
optimizing this process. Most research effort is focused
on the study of the physicochemical processes underly-
ing the synthesis of sodium polyaluminates (SPAs)
with the beta-alumina structure and on the fabrication
of related ceramic materials with good electrical prop-
erties and high corrosion resistance because the perfor-
mance parameters of solid electrolytes produced by
standard processes are not sufficiently stable.
There is currently considerable interest in the
plasma synthesis of beta-alumina [1–4], a process
which opens up new possibilities for producing solid
electrolytes with high ionic conductivity at low alkali-
metal concentrations [4].
In recent years, intensive research effort has been
concentrated on the fabrication of thin-film solid elec-
trolytes for TEC conversion applications by plasma
deposition [1, 2].
Plasma synthesis of refractory oxide powders
involves evaporation and decomposition of starting
reagents in a low-temperature oxygen, air, or nitrogen
plasma flow, followed by deposition of the reaction
product. These processes provide conditions for the
formation of fine SPA powder [3].
The objective of this work was to investigate phase
transformations of SPAs deposited from thermal
plasma flows and changes in their surface properties in
different stages of beta-alumina synthesis.
EXPERIMENTAL
Syntheses were carried out using an experimental
setup for the preparation of oxide powders built around
a 105-kW VChI-63/5.28 IG-L01 rf generator. The
plasma gas used was nitrogen or air, and the arc was
ignited in pure argon. To reduce the particle size of
reaction products and achieve complete oxidation, cold
air was introduced into the reactor.
The experimental setup we used (Fig. 1) incorpo-
rates a plasma gas delivery line, quartz (or corundum)
discharge tube, powder feed system, water-cooled
cylindrical stainless-steel reactor, heat exchanger, and
sleeve filter for collecting the reaction products. The
powder feed system consists of a batcher with four bins
and a water-cooled ring with eight radial channels for
introducing the powders into the high-temperature flow
using nitrogen carrier gas. The quenching gas (air) is
The ability to produce high-performance beta-
ceramics is highly dependent on the development of
novel processes for the synthesis of single-phase mate-
rials based on the structurally similar varieties of
sodium beta-alumina of hexagonal (β-Al2O3) and
rhombohedral (β''-Al2O3) symmetries with the nominal
50

PHASE TRANSFORMATIONS DURING PLASMA SYNTHESIS
51
admitted into the system through channels in the
reactor.
As starting materials, we used aluminum powders
1
with a particle size from 20 to 60 µm, aluminum oxide
(or hydroxide) with a particle size of 10–40 µm, and
sodium carbonate (or bicarbonate) with a particle size
of 40–60 µm. To stabilize the structure of the β''-poly-
aluminate, we introduced lithium oxide (in the form of
lithium carbonate) and/or magnesium oxide additions
(0.7–1.5 wt %). The starting materials were dried, clas-
sified into size ranges, and mixed by grinding in a ball
mill to give stoichiometric mixtures.
The contents of alkaline and aluminum-containing
reagents in the starting mixture were varied so as to
obtain SPAs of predetermined compositions at Na2O
contents in the range 4–27 wt %.
4
2
3
H2O
5
The process for the preparation of SPA nanopow-
ders was developed on the basis of the plasma synthesis
of aluminum oxide in a flowing thermal plasma with a
weight-average temperature of 4600–6200 K. The first
step was to optimize the conditions for the formation of
ultrafine alumina powders.
Fig. 1. Experimental setup: (1) discharge tube, (2) reactor,
(3) heat exchanger, (4) filter, (5) bins for reaction products.
the formation of intermediate products, aluminum
oxynitrides, in the nitrogen plasma. Clearly, the pres-
ence of oxynitrides in the reaction zone reduces the
probability of coalescence and particle growth.
The starting materials used were aluminum powder
and aluminum oxide and hydroxide.
As a result of the complete coevaporation of alumi-
num particles and aluminum oxide compounds in a
nitrogen plasma, followed by quenching of the reaction
products, we obtained fine alumina powders with spe-
cific surfaces of up to 70 m2/g. The products consisted
of one or several metastable alumina polymorphs based
on an fcc arrangement of oxygen anions: γ-Al2O3
(cubic spinel structure), δ-Al2O3 (orthorhombic), and
θ-Al2O3 (monoclinic) (Table 1). Note that increasing
the specific surface of the powder decreases the frac-
tions of γ- and θ-aluminas in the reaction product.
The sodium content of the reaction products was
determined by flame emission spectroscopy on a PAZh-
1 analyzer, using the 589.0/589.9 nm doublet. Lithium
was determined by atomic absorption, and oxygen and
microimpurities were determined by neutron activation
analysis. In addition, the samples were analyzed for
aluminum by chelatometric titration with a mixed indi-
cator (copper chelate of ethylenediaminetetraacetic
acid and 1-(2-pyridylazo)-2-naphtol).
The phase composition of the synthesized materials
was determined by x-ray diffraction (XRD) with CuKα
radiation. Crystalline phases were identified using
JCPDS Powder Diffraction File data [5, 6]. SPAs were
identified using the 017, 022, 025, 026, and 027 diffrac-
tion peaks for the β-NaAl11O17 structure and the 0 1 11,
SPAs were synthesized under conditions corre-
sponding to the formation of fine alumina powder
homogeneous in phase composition. The reaction
between aluminum-containing and alkaline compo-
118, 027, and 0 2 10 peaks for the β''-NaAl5O8 struc- nents in a high-temperature nitrogen flow, followed by
oxidation, yielded SPA nanopowders with a specific
surface of 20–30 m2/g, consisting of spherical particles
(Fig. 2).
ture, as described by Schmid [7].
The specific surface of the samples was evaluated
chromatographically, from temperature-programmed
argon desorption. The shape and size of particles were
determined by electron microscopy.
The specific surface of SPAs prepared by reacting
Na2O and Al2O3 is indicated in Table 2, and their XRD
patterns are presented in Fig. 3. At low sodium concen-
trations, the XRD patterns show three sharp diffraction
peaks corresponding to γ-Al2O3. At the same time,
those peaks are also attributable to the β''-aluminate
(Fig. 3, scan 2). However, the absence of other strong
peaks characteristic of this aluminate and the low
sodium content cast doubt on its formation under the
conditions in question. At an Na2O content of about
6 wt %, the XRD patterns show several additional
peaks (Fig. 4, scan 1), which cannot be identified using
earlier data [5–7]. The phase responsible for those
peaks is designated X-Al2O3. At Na2O contents in the
RESULTS AND DISCUSSION
In preliminary experiments, we studied the condi-
tions for the plasma synthesis of alumina nanopowders.
Theoretical analysis, supported by experimental data,
indicates that, to achieve complete evaporation, metal-
lic aluminum particles must be introduced into a
plasma flow at a velocity from 6.0 to 17.0 m/s. Finer-
particle products are formed not from pure aluminum
powder but from mixtures of aluminum metal and alu-
mina (or aluminum hydroxide), which is attributable to
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52
TEL’NOVA, GRABIS
NaAlO2
γ-Al2O3
X-phase
β''
~
1
~~
2
3
70
60
50
40
30
20
2θ, deg
Fig. 3. XRD patterns of plasma synthesis products in the
Na O–Al O system: (1) 26.28, (2) 4.85, (3) 14.56 wt %
2
2 3
Na O.
2
The introduction of lithium oxide (or magnesium
oxide) additions to stabilize the structure of β''-alumina
had no effect on the plasma synthesis parameters. In
view of this, in preparing SPAs in the ternary systems
Li2O(MgO)–Na2O–Al2O3, we used the process param-
eters optimized for the Na2O–Al2O3 system.
Fig. 2. Electron micrograph of SPA particles.
range 10–15 wt %, this phase prevails (Fig. 3, scan 3).
Small amounts of β''-sodium aluminate are present in
the plasma synthesis products at Na2O contents above
8 wt %, as evidenced by the appearance of the 116, 118,
and 027 peaks (Fig. 3, scan 3; Fig. 4, scan 2). In the
angular range under consideration, the other strong
peaks of the β''-phase (0 1 11 and 0 2 10) overlap with
peaks of γ- and X-Al2O3. The diffraction peaks of
β''-Al2O3 remain broad, which is probably due to the
small particle size and structural disordering of the
β''-phase.
The SPAs prepared in the binary and ternary sys-
tems under identical conditions are close in phase com-
position. A major distinction is that small lithium oxide
additions accelerate crystallization of the X-phase.
Increasing the degree of structural ordering produces
significant changes in the intensity of diffraction peaks
(Fig. 4, scans 1, 3). Similar changes occur with increas-
ing sodium oxide content in the binary system (Fig. 4,
scans 1, 2). In particular, the relative intensity of the dif-
fraction peaks from γ-Al2O3 decreases significantly.
At Na2O contents above 15 wt %, the reaction prod-
ucts contain γ-NaAlO2. In mixtures containing more
than 26 wt % Na2O, this aluminate prevails (Fig. 3,
scan 1).
In studying the phase transformations of SPAs dur-
ing heating, the synthesis products were calcined in air
between 600 and 1400°C with an isothermal hold for
1 h at each temperature.
Table 1. Specific surface and phase composition of alumina powders prepared by plasma synthesis under different conditions
Starting materials
Plasma gas
Quenching gas
–
S, m2/g
Phase composition
Al
Air
Air
N2
10.0
22.1
30.0
12.0
31.0
50.0
10.0
40.0
9.0
γ, δ, θ
γ, δ, θ
γ, δ
γ, δ, θ
δ
Al
Air
Air
–
Al
Al + Al2O3
Al + Al2O3
Al + Al2O3
Al2O3
Air
Air
N2
Air
Air
–
δ
Air
N2
γ, δ, θ
δ
γ, δ, θ
δ, θ
δ
Al2O3
Air
–
Al(OH)3
Al(OH)3
Al(OH)3
Air
Air
N2
Air
Air
18.0
20.0
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PHASE TRANSFORMATIONS DURING PLASMA SYNTHESIS
53
In the Na2O–Al2O3 system, heating leads to a num-
ber of structural transformations. At low sodium oxide
contents (≤4.5 wt %), γ-Al2O3 is stable up to 1300–
1350°C and converts to corundum, α-Al2O3, above
1350°C. No SPAs were detected in such samples by
XRD. This behavior of the system at low sodium oxide
contents indicates that sodium oxide stabilizes the
spinel-like structure of γ-Al2O3. Similar results were
reported by Okada et al. [8], who studied in detail the
X-phase
3
2
γ-Al2O3
α-Al2O3 phase transition in the presence
of Group I monovalent cations and found an amorphous
phase on the surface of spinel nanoparticles, which was
slightly ordered into the beta-alumina structure and
impeded diffusion processes in γ-Al2O3.
~~
1
50
45
40
35
30
25
20
15
2θ, deg
At sodium oxide contents from 8 to 20 wt %, the
plasma synthesis products occasionally contained a
small amount of β''-aluminate. During heating, this
phase was formed starting at 900–1000°C. Above
1200°C, the samples contained only β''-aluminate.
The β''-phase was formed through the formation of
the metastable polyaluminate m-Al2O3, a structural
analogue of mullite [9–11], which was identified using
XRD data from Elliot and Huggins [11].
Fig. 4. XRD patterns of SPAs in the (1, 2) Na O–Al O and
(3) Li O–Na O–Al O systems: (1) 6.2 wt % Na O,
(2) 8.5 wt % Na O, (3) 6.7 wt % Na O + 0.8 wt % Li O.
2
2 3
2
2
2
3
2
2
2
2
the β-phase.After heat treatment of such samples above
1300°C, their XRD patterns showed weak peaks from
The mullite-like SPA was present in plasma synthe- β-alumina.
sis products up to 1100–1150°C, whereas, in solid-state
The dynamics of phase transformations of SPAs in
reactions, the temperature stability limit of m-Al2O3 is
no higher than 1050–1085°C [10, 11].
the binary system is illustrated in Table 3 by the exam-
ple of 1.49Na2O · 5.33Al2O3 (14.56 wt % Na2O). Also
presented in Table 3 are XRD data for alumina powder
prepared in a nitrogen plasma under the same condi-
tions as the SPA.
At Na2O contents above 20 wt %, the X-phase was
missing, and β''-alumina was formed through the
monoaluminate β-NaAlO2, which appeared in the
range 850–900°C, was the predominant phase between
900 and 1200°C, and fully converted to the β''-polyalu-
minate above 1250–1300°C. The intermediate phase m-
Al2O3 was not detected in such samples. Nor did we
observe the formation of the hexagonal diblock β-poly-
aluminate, except for the SPA samples containing more
than 20 wt % Na2O and the samples close in composi-
tion to Na2O · 9Al2O3, which was reported by Le Cars
The results reported by Plokhikh and Kaul’ [10]
provide convincing evidence that amorphous alumina
plays a key role in the formation of SPAs with the mul-
lite structure via solid-state reactions.
The question that arises in this context is whether
the mechanism described in [10] for solid-state reac-
tions may also underlie the plasma synthesis process.
et al. [12] to lie on the alumina-rich phase boundary of An answer is provided by the finding that alumina films
Table 2. Specific surface and phase composition of Na2O–Al2O3 powders prepared in a nitrogen plasma flow at a quenching
gas (air) flow rate of 5 m3/h
Bulk composition
wt % Na2O
S, m2/g
Phase composition
γ-Al2O3
0.92Na2O · 11Al2O3
Na2O · 9.20Al2O3
Na2O · 7Al2O3
4.85
6.20
24.4
21.5
19.8
22.6
20.2
23.5
19.0
18.3
γ-Al2O3, X
7.95
X, γ-Al2O3
Na2O · 6.54Al2O3
Na2O · 4.70Al2O3
Na2O · 3.57Al2O3
Na2O · 2.42Al2O3
Na2O · 1.71Al2O3
8.50
X, β''(tr)
11.46
14.56
20.08
26.28
X, β''(tr), γ-Al2O3 (tr)
X, β''(tr)
γ-NaAlO2, X(tr), β''(tr)
γ-NaAlO2
INORGANIC MATERIALS Vol. 42 No. 1 2006

54
TEL’NOVA, GRABIS
Table 3. Changes in the phase composition of plasma syn-
thesis products during heating to 1400°C in air (isothermal
hold for 1 h at each temperature)
plasma synthesis nanoproducts according to the
scheme
800°C
1000–1100°C
ï
m-Al2O3
β"-Al2O3.
Phase composition
t, °C
SPA of composition
Al2O3
Thus, the present results are in contradiction with
the existing views of the mechanism of m-Al2O3 forma-
tion. This leads us to the following assumptions:
1.49Na2O · 5.33Al2O3
–
γ, δ
X, β'' (tr)
1. The polyaluminate X-Al2O3 nucleates not at the
interface between alumina and the sodium-containing
component, if there is any, and not in the melt but in the
gas phase, in the flow of reactants, and the reaction
products then condense to form crystals of the X-phase.
Otherwise, the reaction between alumina and the
sodium-containing component would lead to the for-
mation of m-Al2O3 or would proceed through the for-
mation of sodium monoaluminate, NaAlO2, which
depends, according to Plokhikh and Kaul’ [10], on the
state of alumina—amorphous or crystalline. On the
other hand, the presence of the m-polyaluminate
(together with the X-phase) in the SPAs prepared in
concentrated, reactive plasma flows may be interpreted
as indicating that the starting reagents were poorly
mixed or that the condensed aerosols phase-separated
to form oxide (and nitride or oxynitride) compounds of
the starting materials. Given that the condensed phases
were oxidized and quenched in cold flowing air, it
seems likely that the reaction involves amorphous alu-
mina, which is responsible for the formation of the
m-polyaluminate.
800
δ, γ, θ (tr)
δ, γ, θ (tr)
δ, θ
X, m-Al2O3, β'' (tr)
900
m-Al2O3, X, β'' (tr)
1000
1100
1200
1300
1400
m-Al2O3, β''
θ, δ
β'', m-Al2O3
θ, δ, α (tr)
α
β''
β''
β''
α
produced by plasma sputtering did contain amorphous
alumina, together with γ-Al2O3 [13].
In our experiments, however, only slight amounts of
m-Al2O3 were identified among plasma synthesis prod-
ucts, together with the X-phase, and only in SPA sam-
ples with a small specific surface (≤15 m2/g), contain-
ing Li2O additions.
That the SPA nanopowders contained no mullite
phase suggests that a different mechanism underlies the
formation of SPAs during plasma synthesis. We believe
that the X-phase, a major product of gas-phase reac-
tions, plays a central role in the formation of the meta-
stable SPA m-Al2O3.
2. Lithium oxide additions accelerate the crystalli-
zation of the X-phase and, at concentrations of
1.0−1.5 wt %, stabilize the structure of X-Al2O3 up to
1100°C.
The SPA with the mullite structure, m-Al2O3, is then
formed as a reaction intermediate during heating of
3. The homogeneity range of the polyaluminate
Na2O · nAl2O3 (X-phase) seems to be n = 3.5–9.5.
Table 4. Changes in the phase composition of plasma synthesis products in the systems Na2O–Al2O3 and Li2O–Na2O–Al2O3
between 600 and 1400°C (isothermal hold for 1 h at each temperature)
Phase composition
t, °C
6.2% Na2O 8.5% Na2O 11.2% Na2O 6.9% Na2O, 0.8% Li2O 11.1% Na2O, 0.7% Li2O 11.2% Na2O, 1.3% Li2O
n = 9.2
n = 6.54
n = 4.82
n = 8.15
n = 4.83
n = 4.74
–
γ, X
X, β''
–
X
X
X
X
600
800
1000
–
–
–
X
X
X
–
–
–
X, m (tr)
m, X (tr)
m, β''
β''
X, m (tr)
m, X
m, β''
β'', β
β'', β
β'', β
X, m (tr)
m, X
m, β''
β'', β
β'', β
β'', β
–
–
1100 γ, m, β''
1200 β''
β'', m
β''
m, β'', X
β'', β
β'', β
β'', β
1300 β'', β (tr)
1400 β'', β (tr)
β''
–
β''
β''
Note: The content of alkali-metal oxides is expressed as a weight percent; n is defined by the formula (0–0.24)Li O · Na O · nAl O .
2
2
2 3
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PHASE TRANSFORMATIONS DURING PLASMA SYNTHESIS
55
The last two assumptions are supported by data on
β-Al2O3
β''-Al2O3
the phase transformations of SPAs in the Li2O–Na2O–
Al2O3 system between 600 and 1400°C (Table 4). For
comparison, Table 4 also presents the XRD data for
several SPAs of the Na2O–Al2O3 system after calci-
nation.
1
2
The Na2O–Al2O3 and Li2O–Na2O–Al2O3 systems
are similar in the phase transformations of SPAs during
heating. In both systems, the original X-phase remains
unchanged up to 600–700°C. Heating to 800°C leads to
the formation of the mullite SPA. At 1000°C, the dif-
fraction peaks from the mullite SPA and X-phase are
close in intensity. At 1100°C, the samples contain, in
addition to the mullite phase, a small amount of the
β''-aluminate, and the X-phase is only present at
increased Li2O contents (1.3 wt %). At the same time,
in the binary system the X-polyaluminate rapidly con-
verts to the mullite phase near 900°C, so that X-Al2O3
is essentially missing at 1000°C. Above 1200°C, the
samples doped with lithium oxide contain, in addition
to the rhombohedral phase β'', the hexagonal polyalu-
minate β. Despite the complex phase composition of
the calcined materials, microstructural analysis attests
to a rather high homogeneity of the beta-alumina crys-
tals and a zonal distribution of phases in the crystals.
3
50
45
40
35
30
25
20
15
10
2θ, deg
Fig. 5. XRD patterns of SPAs in the (1, 2) Li O–Na O–
2
2
Al O and (3) Na O–Al O systems after calcination at
2
3
2
2 3
1400°C: (1) 11.1 wt % Na O + 1.3 wt % Li O, (2) 6.7 wt %
Na O + 0.8 wt % Li O, (3) 11.2 wt % Na O.
2
2
2
2
2
t, °C
L + Al2O3
L
2000
Figure 5 shows the XRD patterns of three SPA sam-
ples calcined at 1400°C for 1 h.
It is reasonable to discuss the stability of SPAs in
relation to the phase equilibria in the Na2O–Al2O3 sys-
tem.
L + β
DeVries and Roth [14] proposed two versions of the
phase equilibria in this system, taking into account the
following: (1) β-Alumina melts incongruently. (2) The
melting point of the eutectic between Na2O · Al2O3
(1 : 1) and β-Al2O3 is about 1580°C. (3) There are no
β''-Al2O3 + L fields. (4) Both β- and β''-Al2O3 are non-
stoichiometric phases. (5) The heat effect at 1410°C is
due to a polymorphic transformation of the Na2O ·
Al2O3 phase. (6) The transformation of β''-Al2O3 into
β-Al2O3 + Na2O · Al2O3 occurs below the eutectic tem-
perature.
According to De Vries and Roth [14], the Na2O–
Al2O3 system contains at least two polyaluminates, β
and β'', the former having the composition Na2O ·
11Al2O3 or Na2O · 9Al2O3, and the latter existing in the
composition range from 1 : 5 to 1 : 7.
1500
1000
δ-NaAlO2 + β
γ-NaAlO2 + β
β + Al2O3
β
80
90
100
mol % Al2O3
Stoichiometric composition
The approximate composition
of a commercial
2Na2O · 11Al2O3 electrolyte
Na2O · 11Al2O3
Composition
of a commercial
Monofrax refractory
In the phase diagram reported by Liebertz [15], the
β''-phase is represented as a solid solution resulting
from the eutectic decomposition of β-Al2O3. Both
phases exist in broad composition ranges, from 85 to
90 mol % Al2O3.
The Na2O–Al2O3 phase diagram mapped out by
Weber and presented in [16] has three noteworthy fea-
tures: the eutectic at 1585°C between β-Al2O3 and
sodium monoaluminate, the higher solubility of sodium
oxide in β-Al2O3 as compared to that reported by De
Fig. 6. Phase diagram of the Na O–Al O system [12]. In
the hatched region, the β and β'' phases coexist.
2
2 3
Vries and Roth [14], and the absence of β''-Al2O3 in the
equilibrium phase diagram at temperatures above
1585°C.
Believing that β'' is a metastable phase, Le Cars
et al. [12] did not represent it in their phase diagram
INORGANIC MATERIALS Vol. 42 No. 1 2006

56
TEL’NOVA, GRABIS
wt % Na2O
27
20
15
7.95
6
4.85 4.5
1400
1200
1000
800
α + θ
β'' +β(1 : 11)
β'' + β(1 : 11)(tr)
β(1 : 11) + β''
β(1 : 11) + β''
δ + θ + γ
β''
β''
β''
γ + δ
β(1 : 1) + β''
V
m + β''
m + β''+ γ
I
γ + β''(tr) + β(tr)
VI
m + X + β''(tr)
X + m
m + X + γ + β''(tr)
X + m + γ(tr)
II
γ-Al2O3
III
IV
γ-Al2O3
γ-
Al2O3
γ(1 : 1)
X
X + γ(tr)
600
1.64 2
3
4
5
6
7
8
9
10 11 12 13 14
n in Na2O · nAl2O3
62.17 70.76 77.50
87.56 90.50
92.27
93.59
mol % Al2O3
Fig. 7. Changes in the phase composition of plasma synthesis products in the range 600–1400°C (the β''-phase is not included in
the initial reaction products since it undergoes no structural transformations on heating); β'' = β''-polyaluminate, β(1 : 11) = β-poly-
aluminate, β(1 : 1) = β-NaAlO , γ(1 : 1) = γ-NaAlO , X = X-phase, m = mullite polyaluminate, γ = γ-Al O , δ = δ-Al O , θ =
2
2
2
3
2 3
θ-Al O , α = α-Al O . I = m + β'' + β(1 : 1), II = m + X + β'' + β(1 : 1), III = X + m + γ + β(1 : 1), IV = X + m + γ(1 : 1), V = γ +
2
3
2 3
δ + β'' + β, VI = γ + β'' + β.
and only mentioned that two phases (β and β'') may of the β''-polyaluminate, Na2O · 5.33Al2O3, also cannot
exist (Fig. 6). According to Le Cars et al. [12], the sta-
ble phase β-Al2O3 and the metastable phase β''-Al2O3
be achieved under equilibrium conditions. Above
1550°C, the β''-phase decomposes irreversibly to β-alu-
coexist above 1050°C, and the β-phase has the broadest mina and NaAlO2. A pure β''-phase, with no β-Al2O3 or
homogeneity range between 1300 and 1400°C, from
84.21 to 89.50 mol % Al2O3, which corresponds to
compositions from Na2O · 5.33Al2O3 to Na2O ·
8.5Al2O3. At higher and lower temperatures, the solid-
solution range is narrower, e.g., from Na2O · 5.33Al2O3
to Na2O · 7Al2O3 at 1100°C.
The crystal data for β- and β''-aluminas also indicate
that these phases have significant homogeneity ranges.
According to Le Cars et al. [12], β-alumina with the
ideal composition (1 + x)Na2O · 11Al2O3, x = 0, does
not exist under equilibrium conditions. The β-phase is
stable in the range x = 0.22–0.50, which corresponds to
Na2O · (7.33–9)Al2O3. The stoichiometric composition
NaAlO2 impurities, can be obtained in the presence of
Li2O or MgO stabilizing additions; the β''-structure is
believed to be stable up to 1700°C.
The main distinction between our results and data
available in the literature is that, in the range 1200–
1400°C, the β''-phase has its own homogeneity range
(Fig. 7), from 77.5 to 90.5 mol % Al2O3, which corre-
sponds to compositions from Na2O · 3.44Al2O3 to
Na2O · 9.5Al2O3 and coincides with the assumed com-
position range, n = 3.5–9.5, of the X-polyaluminate, an
initial synthesis product. In the composition range
70.76 to 77.5 mol % Al2O3, heating a mixture of the
X-phase and sodium monoaluminate under the same
INORGANIC MATERIALS Vol. 42 No. 1 2006

PHASE TRANSFORMATIONS DURING PLASMA SYNTHESIS
57
thermal conditions also leads to the formation of a sin-
gle-phase β''-polyaluminate. In this process, the X-
phase converts to β'' through the intermediate polyalu-
minate m-Al2O3, and NaAlO2 decomposes as a result of
partial sodium oxide evaporation to form the β''-phase.
40
30
20
10
The formation of β-Al2O3 (together with the
β''-polyaluminate as the major phase) between 1300
and 1400°C in the composition range Na2O · (7–
9.5)Al2O3 agrees with earlier data [12]. In high-alumina
SPAs (at sodium oxide contents below 6 wt %), the
phase composition of calcined products varies from
Al2O3 to β-alumina and is determined primarily by the
synthesis and heat-treatment conditions, but the alu-
mina-rich phase boundary of individual phases in the
Na2O–Al2O3 system is difficult to accurately locate
because of the increased volatility of sodium oxide.
1
2
3
4
5
1500
300
900
t, °C
Fig. 8. Specific surface as a function of temperature for
SPAs prepared in the (1) Li O–Na O–Al O , (2) MgO–
2
2
2 3
Na O–Al O , and (3) Na O–Al O systems and for
Since β''-polyaluminate is formed during plasma
synthesis from the X-phase through the intermediate
phase m-Al2O3, which exists in the composition range
Na2O · (3–12)Al2O3 [11], β''-alumina is also likely to
have a broad homogeneity range (Fig. 7).
The formation of the alkali-rich rhombohedral SPA
during heating of plasma-synthesis products suggests
that Na may occupy several sites in the structure of
β''-polyaluminate. First, the Na ions may occupy the
Bivers–Ross and anti-Bivers–Ross sites [17], which
leads to x = 1 in the formula (1 + x)Na2O · 11Al2O3.
Second, the Na ions may occupy the sites of Al atoms
that have moved to interstitial positions, that is, may fill
2
2
3
2
2 3
(4, 5) aluminas differing in initial specific surface (Na O
2
8.6, Li O 0.75, MgO 0.8 wt %).
2
heat treatment in the range 850–1200°C and air cooling
are obviously accompanied by destabilization and the
reversible γ
β-NaAlO2 phase transition. For this
reason, the presence of the low-temperature form
β-NaAlO2 in the phase diagram (Fig. 7) at temperatures
above 850°C is questionable, since the β
γ phase
transition is reversible.
For all of the plasma synthesis products, including
aluminas differing in particle size and SPAs prepared in
the binary and ternary systems, heating had similar
effects on the specific surface of the powders (Fig. 8).
VAl3+ vacancies in the ideal β''-structure, corresponding
to the formula Na6Al32VAl3+ O51 [17]. Third, since the
With increasing heat-treatment temperature, the
specific surface of the powders decreases owing to
recrystallization, which is, as a rule, accompanied by
changes in phase composition. The break in the S(t)
curves between 1000 and 1050°C is due to rapid struc-
tural transformations, leading to the formation of more
stable and dense phases. The precipitous drop in the
specific surface of alumina is associated with α-Al2O3
nucleation and subsequent rapid growth of corundum
crystals over the entire sample.
The reduction in the specific surface of the SPAs is
probably due to the transformation of the orthorhombic
structure of m-Al2O3 (similar to that of mullite but hav-
ing a doubled axis, which leads to a higher symmetry,
tetragonal structure with lattice parameters a = b =
0.768 nm and c = 0.291 nm [11]) to the rhombohedral
structure of the β''-polyaluminate (hexagonal cell
parameters a = 0.561 nm and c = 3.346 nm for the com-
position Na1.72Al10.66Li0.30O17 [17]). This phase transi-
tion is accompanied by an increase in density from
2.7 g/cm3 for m-Al2O3 to 3.28 g/cm3 for the β''-polyalu-
minate.
SPAs were synthesized in a nitrogen plasma, oxygen
atoms may be partially replaced with nitrogen, leading
to the formation of oxynitride SPAs. The excess charge
of the nitride ions in the β''-phase must then be compen-
sated by the charge of the excess sodium in the conduct-
ing plane. This model for the formation of alkali-rich
β''-alumina during plasma synthesis appears quite plau-
sible.
Another distinctive feature of the plasma synthesis
of SPAs is the formation of the tetragonal monoalumi-
nate γ-NaAlO2 in the thermal plasma flow in the com-
position range 15–27 wt % Na2O. γ-NaAlO2 is stable at
room temperature and, above 850°C, transforms to the
orthorhombic phase β-NaAlO2, which, in turn, con-
verts to β''-alumina above 1250–1300°C. These results
are at variance with the report by Thery and Briancon
[6] that the stable phase at reduced temperatures is the
orthorhombic form β-NaAlO2. According to their data,
the polymorphic transformation of β-NaAlO2 into the
high-temperature, tetragonal phase γ-NaAlO2 occurs at
470°C and is reversible. In light of this, the presence of
γ-NaAlO2 among the initial synthesis products is prob-
ably due to the stabilization of the tetragonal structure
As follows from the S(t) data in Fig. 8, the specific
of sodium monoaluminate upon rapid cooling or surface of the lithium-containing SPA (curve 1) varies
quenching from the thermal plasma flow. Subsequent insignificantly below 1200°C, which also indicates that
INORGANIC MATERIALS Vol. 42 No. 1 2006

58
TEL’NOVA, GRABIS
5. Powder Diffraction File, Swarthmore: Joint Committee
Li2O additions reduce the rate of the m
phase transition.
β'' + β
on Powder Diffraction Standards, card nos. 19-1173,
19-1174, 21-1096, 32-1033.
6. Thery, J. and Briancon, D., Structure and Properties of
Sodium Aluminates, Rev. Hautes Temp. Refract., 1964,
vol. 1, no. 2, pp. 221–227.
CONCLUSIONS
The phase transformations of SPAs prepared by
plasma synthesis are inconsistent with the known phase
equilibria in the Na2O–Al2O3 system. The main dis-
crepancy is that, in the range 1200–1400°C, β''-SPA has
its own homogeneity range, from 77.5 to 90.5 mol %
Al2O3, which corresponds to compositions from Na2O ·
3.44Al2O3 to Na2O · 9.5Al2O3.
7. Schmid, C., X-ray Characterization of β- and β"-Alu-
mina, J. Mater. Sci. Lett., 1986, vol. 5, pp. 263–266.
8. Okada, A.U., Hattori, K., Kameshima, A., et al., Effect
of Monovalent Cation Additives on the Gamma-Al2O3-
to-Alpha-Al2O3 Phase Transition, J. Am. Ceram. Soc.,
2000, vol. 83, no. 5, pp. 1233–1236.
9. Takahashi, T. and Kuwabara, K., β-Al2O3 Synthesis
from m-Al2O3, J. Appl. Electrochem., 1980, vol. 10,
pp. 291–297.
10. Plokhikh, A.A. and Kaul’, A.R., Formation of Sodium
Polyaluminate with the Mullite Structure, Izv. Akad.
Nauk SSSR, Neorg. Mater., 1985, vol. 21, no. 8,
pp. 1378–1382.
11. Elliot, A.G. and Huggins, R.A., Phases in the System
NaAlO2–Al2O3, J. Am. Ceram. Soc., 1975, vol. 58,
no. 11/12, pp. 497–500.
The β''-polyaluminate forms in several steps,
800°C
through
m-Al2O3
metastable
SPAs:
X-phase
1000–1200°C
β"-Al2O3. The X-phase (Na2O ·
nAl2O3 SPA with an unidentified structure and an
assumed homogeneity range n = 3.5–9.5) is the major
crystalline phase among the plasma synthesis products
in the composition range 77.5 to 90.5 mol % Al2O3.
In Li2O-containing samples, the phase transforma-
tions of SPAs reach completion at higher temperatures,
lead to the formation of β'',β-Al2O3 mixed-layer struc-
tures, and are accompanied by a slight reduction in the
specific surface of the final reaction products.
12. Le Cars, Y., Thery, J., and Collonques, R., Domaine
d’existence et stabilité des alumines β et β" dans le
système Al2O3–Na2O. Étude par rayons
X
et
microscopie électronique, Rev. Hautes Temp. Refract.,
1972, vol. 9, no. 1, pp. 153–160.
13. Chou, T.C. and Nieh, T.G., Nucleation and Concurrent
Anomalous Grain Growth of α-Al2O3 during γ
α
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INORGANIC MATERIALS Vol. 42 No. 1 2006