Fine-Grained Tungstates SrWO4 and NaNd(WO4)2 with the Scheelite Structure Prepared by Spark Plasma Sintering

Article abstract of DOI:10.1134/S0036023619030161

Fine-grained SrWO₄ and NaNd(WO₄)₂ ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.

Full text of DOI:10.1134/S0036023619030161

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2019, Vol. 64, No. 3, pp. 296–302. © Pleiades Publishing, Ltd., 2019.
Russian Text © E.A. Potanina, A.I. Orlova, A.V. Nokhrin, D.A. Mikhailov, M.S. Boldin, N.V. Sakharov, O.A. Belkin, E.A. Lantsev, M.G. Tokarev, V.N. Chuvil’deev, 2019, pub-
lished in Zhurnal Neorganicheskoi Khimii, 2019, Vol. 64, No. 3, pp. 243–250.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Fine-Grained Tungstates SrWO and NaNd(WO4)2
4
with the Scheelite Structure Prepared by Spark Plasma Sintering
a,
a
a
a
a
E. A. Potanina *, A. I. Orlova , A. V. Nokhrin , D. A. Mikhailov , M. S. Boldin ,
a
a
a
a
a
N. V. Sakharov , O. A. Belkin , E. A. Lantsev , M. G. Tokarev , and V. N. Chuvil’deev
a
Nizhny Novgorod National Research University, Nizhny Novgorod, 603950 Russia
*
e-mail: ekaterina.potanina@list.ru
Received May 18, 2018; revised July 9, 2018; accepted August 15, 2018
Abstract⎯ Fine-grained SrWO and NaNd(WO ) ceramics with the scheelite structure having high relative
4
4 2
densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are
prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder
diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study
are sintered at rather low temperatures (580–665°С). The intensity of compaction of the tungstates at the
early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-
grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary
oxygen diffusion.
Keywords: ceramics, density, diffusion, activation energy, spark plasma sintering
DOI: 10.1134/S0036023619030161
INTRODUCTION
Note that the general task of materials design for
RAW immobilization is to form a high-density struc-
Now mineral-like compounds are among the most ture in sintered materials, which, all other things being
promising materials for immobilizing highly active equal, would allow for a high hydrolytic and radiation
components of radioactive waste (RAW) [1–4]. Com- resistance of ceramics.
pounds having the structures of monazite [5–7], kos-
narite (NZP) [8–10], langbeinite [9, 11], and some
other natural minerals are well characterized. Ceramic
materials with the scheelite structure are characterized
not so well, although recently there has been an
increased interest to them [12–16].
Most ceramics based on compounds with the
scheelite structure are prepared by cold pressing fol-
lowed by annealing in vacuo or in air. Sintering tem-
peratures differ depending on ceramic composition,
but sintering times are similar, on the order of several
hours. For example, Ca Y CeNbWO (x = 0, 0.05,
1
– x
x
8
0
.1, 0.15) powders were sintered at 1200°С for up to 9 h
2
+
Natural scheelite is CaWO , in which Ca can par-
4
[
24]. Na La MoO and Ca Bi W V O (x =
0
.5
0.5
4
1 – x
x
1 – x
x
4
2+
2+
2+
tially or completely be substituted by Sr , Ba , Cu ,
0
.1, 0.2, 0.3, 0.4, 0.5) ceramic samples were prepared
2+
2+
2+
3+
3+
5+
5+
Mn , Cd , Рb , Cr , Fe , Ln, Nb , or Ta and
in 2 h at a sintering temperature of 789 and 950°С, respec-
6
+
4+
5+
6+
7+
W
can be substituted by Ge , V , Мo , I , or tively [25, 26]; the maximal relative density was 96%.
+
7
Re [17, 18]. Isostructural scheelite-like compounds
can contain elements whose isotopes are present in
radiochemical technologic waste [17]. Most promising
of them are solid solutions of cerium, uranium, pluto-
Spark plasma sintering (SPS) technology is of great
interest as a variant ceramics sintering process; the
underlying idea of this technology is a high-rate (up to
2500°С/min) heating of the material by passing high-
nium, and americium tungstates [19–21]. An import- amperage (up to 5000 А) millisecond current pulses
ant feature of the scheelite tungstates is their ability to through a mold containing the powder to be sintered
form upon precipitation from alkali chloride melts and in vacuo or under an inter atmosphere with the simul-
a high chemical resistance to them [22, 23]. This is taneous pressure application. SPS technology com-
important in the development of methods for the bines a set of factors that provide a positive effect to
management of RAW from pyrochemical MOX-fuel accelerate the sintering process: ultra-high sintering
regeneration technology, when it is necessary to rates, vacuum or an inert atmosphere, application of
ensure a high stability of materials to the LiCl–KCl pressure with its value changing directly during sinter-
chloride melts [23] used in spent nuclear fuel recy- ing, the tenability of the heating rate at various sinter-
cling.
ing stages, the implementation of stepped sintering
296

FINE-GRAINED TUNGSTATES
297
modes, and others [27–31]. As a result of this, the SPS peak positions) and KRIST software (to calculate the
ceramics have high densities, thermal stability, and unit cell parameters).
good physicomechanical properties [32–34]. Now
Microstructures of powders and ceramics were
many researches treat SPS as one of the most promis-
studied on a Jeol JSM-6490 scanning electron micro-
ing methods for manufacturing ceramics intended for
scope (SEM) equipped with an Oxford Instruments
nuclear power production [35–38], despite the fact
INCA 350 energy-dispersive microanalyzer. The den-
that the high-rate sintering mechanisms of fine-
sities (ρ) of sintered samples were measured by hydro-
grained ceramics yet remain not understood to the
static weighing in distilled water using a Sartorius CPA
fullest extent [30, 39–41].
3
2
25D balance. The accuracy was ±0.01 g/cm .
he goal of the work is to prepare promising tung-
states containing neodymium and strontium by the
SPS method, study the features of high-rate ceramics
sintering, and study the structure and properties of
RESULTS AND DISCUSSION
The obtained materials were while (Sr-containing)
SrWO and NaNd(WO ) , where Nd and Sr imitate
4
4 2
or dark lilac (Nd-containing) agglomerated powders.
minor actinides, for example, americium Am, and
nuclear fission products (rare-earth components and
strontium-90).
The packing density of SrWO powders in agglomer-
4
ates was far lower than for NaNd(WO ) powders
4
2
(
Figs. 1a, 1b). The powders had edged shapes. The
particle size distributions of the powders are nonuni-
form: the composites have both coarse particles, aver-
aging to 10 μm, and ultrafine particles with sizes less
EXPERIMENTAL
Tungstate powders were prepared by coprecipita- than 1 μm (Figs. 1c, 1d).
tion from aqueous solutions. Ammonium tungstate
NH ) W O · 2.5H O was dissolved in distilled water
The samples used for IR spectroscopic studies were
annealed at 800°С and then stored under air. The IR
spectrum of a NaNd(WO ) sample (Fig. 2, curve 2)
(
4
4
5
17
2
at 80°C under constant stirring. After ammonium
tungstate dissolved completely, strontium, sodium,
and neodymium nitrates were added. The suspensions
were stirred for 30 min with a magnetic stirrer, then
dried at 90 (for 12 h) and 120°С (for 2 h), mechanically
dispersed for 10 min, and annealed successively at 800,
4
2
–1
features absorption bands at 3388 cm , corresponding
to the Н–О stretching vibrations of hydrate surface
–1
layers, and bands at 1616 and 1384 cm , relating to the
H–О–H bending vibrations of hydrate water mole-
–
1
cules. The band in the region 2348 cm relates to the
stretching vibrations of carbon dioxide molecules. The
9
00, 1000, and 1100°С in a SNOL-1 625/11-43 air fur-
nace for 10 h with intermittent dispersion at each
stage. The temperature maintenance accuracy was
–1
range of IR frequencies less than 1000 cm relates to
the characteristic vibrations of W–O. The band lying
±
5°C.
–1
at 427 cm relates to the WO bending vibrations, the
4
Ceramics were sintered on a Dr. Sinter model SPS-
–1
absorption bands at 932 and 844 cm relate to the
stretching vibrations of tungstate tetrahedra, and the
bands at 810 and 721 cm to the antisymmetrical
stretching vibrations. Both water of hydration and car-
bon dioxide can be sorbed on the material during stor-
age of as-annealed samples.
6
25 (SPS Syntex). The temperature was brought to
7
70°С at 30°С/min in vacuo (6 Pa) under uniaxial
–1
stress of 75 MPa. The samples were not exposed at the
sintering temperature (τ = 0). The sintering tempera-
ture for samples of 12 mm in diameter was measured
by a Chino IR-AH pyrometer, which was focused on
the surface of the graphite mold. The contribution
The results of X-ray diffraction experiments show
from the thermal expansion of the machine–sample sys- that SrWO and NaNd(WO ) crystallize in scheelite
4 4 2
tem in constructing temperature-dependent shrinkage structure, space group I4 /a (ICDD PDF #08-0490
1
(
L) and shrinkage rate (S) plots was taken into account and #79-1119, respectively) (Fig. 3). The unit cell
in accordance with the approach described in [33]. parameters of the compounds had the following val-
The temperature measurement accuracy was ±10°С; ues: for SrWO , a = b = 5.4293 ± 0.0005 ꢀ, c = 11.9781 ±
4
the pressure maintenance accuracy was ±0.1 MPa.
0
5
9
.0017 ꢀ, α = β = γ = 90°; for NaNd(WO ) , a = b =
4 2
.2954 ± 0.0006 ꢀ, c = 11.4930 ± 0.0015 ꢀ, α = β = γ =
0°. The theoretical densities (ρ ) of SrWO and
The functional composition of the compounds was
investigated by IR spectroscopy on a Shimadzu FTIR-
th
4
3
8
400S spectrophotometer at room temperature in the NaNd(WO ) ceramics were 6.3086 and 6.8290 g/cm ,
4 2
–1
frequency range 400–4000 cm .
respectively.
Ceramic sintering diagrams are shown in Fig. 4.
X-ray powder diffraction experiments were carried out
on a Shimadzu LabX XRD-6000 powder diffractometer One can see that tungstate powders are sintered at
(
CuK radiation, 2θ = 10°–50°, scan rate: 2 deg/min). rather low temperatures. The temperature corresponding
α
to the highest sintering rate, T , is 580–600 and 645–
Unit cell parameters were calculated from the X-ray
1
powder diffraction data in Powder Diffraction Phase 665°С for SrWO and NaNd(WO ) ceramics respec-
4 4 2 ,
Analysis PhasAnX software (to determine diffraction tively. Noteworthy, the densening onset temperature
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 64 No. 3 2019

2
98
POTANINA et al.
100 μm
10 μm
(а)
(b)
5
μm
5 μm
(c)
(d)
Fig. 1. Images of (a, c) SrWO and (b, d) NaNd(WO ) powder structures under various magnifications.
4
4 2
for SrWO powders (240–250°С) is lower than for (Lmax = 0.72 mm) than for NaNd(WO ) powders
4
4 2
NaNd(WO ) powders (585–600°С); the likely reason (Lmax = 0.53 mm).
4
2
for this is the denser packing in NaNd(WO ) agglom-
4
2
The density of the manufactured SrWO and
4
erated powders (Fig. 1b). This leads to more intense
sintering of NaNd(WO ) ceramics, although the
NaNd(WO ) ceramics is 99.0 and 95.8%, respec-
4
2
4
2
tively. The phase composition of the ceramics
maximal shrinkage Lmax for SrWO powders is greater
4
1
2
4
3
2
1
3
400 2900 2400 1900 1400 900 400
15
20
25
30
35
40
45
50
–1
ν, cm
2θ, deg
Fig. 2. IR spectra of (1) SrWO and (2) NaNd(WO )
Fig. 3. X-ray diffraction patterns of (1, 3) powders and (2, 4)
4
4 2
samples.
ceramics of (1, 2) SrWO and (3, 4) NaNd(WO ) .
4
4 2
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FINE-GRAINED TUNGSTATES
299
L, mm
(а)
S, mm/s
0.8
0.6
0.4
0.2
1
2
0.008
0.006
0.004
0.002
0
200
400
T, °C
b)
600
800
0
L, mm
.0
(
ln(Tdε/dT)
10 μm
1
(а)
Stage III
II
Stage I
6
0.8
0.6
0.4
0.2
0
4
2
0
Stage I
Stage I'
–
2
–
4
1
2
3
4
5
6
T /T
m
Fig. 4. Sintering diagrams of (1) SrWO4 and (2)
NaNd(WO ) ceramics: (a) shrinkage L (light markers)
10 μm
4
2
(b)
and shrinkage rate S (dark markers) versus heating tem-
perature (T) and (b) L–T /T plots (light markers) and
m
ln(Tdε/dT)–T /T (dark markers).
Fig. 5. Micrographs of (a) SrWO and (b) NaNd(WO )
m
4
4 2
ceramics. SEM images of fractures.
remained unchanged after SPS (Fig. 3), indicating a
high thermal stability of the crystal structure of the modes. In terms of this approach, the effective activa-
prepared compounds.
tion energy of sintering mQ (where m = 1/2 for bulk
s
diffusion in the crystal lattice and m = 1/3 for grain-
boundary diffusion [42–44]) under continuous heating
can provisionally be estimated from the slope of the
shrinkage versus temperature in semi-logarithmic coor-
The results of electron-microscopic studies show
that the ceramics have high-density fine-grained
structures (Fig. 5). Most grains of the SrWO ceramics
4
have sizes from 4 to 10 μm; the NaNd(WO ) ceramics
4
2
dinates ln(Tdε/dT)–T /T [45] (Fig. 4), where ε =
have grain sizes of ~1–2 μm. Coarse grains are
observed in both ceramic structures, indicating their
m
L/L , L is initial sample height, and T is melting
0
0
m
abnormal growth in the course of sintering. Single temperature of the ceramics (T
= 1808 K for SrWO
m
4
coarse tungstate particles retaining their edged shapes [46], and T = 1508 K for NaNd(WO ) [47]).
m
4 2
are observed on fractures (Fig. 6), which testifies to
the incompleteness of their sintering with the main
fine-grained ceramic matrix. The volume fraction of
The analysis shows that the early stage of the den-
sity versus heating temperature plot ln(Tdε/dT)–T /T
m
such large particles in the ceramics fracture does not can be divided into two temperature ranges, each
exceed 1–2%.
being characterized by its own effective activation
energy mQ , namely, stage I with a low activation
Let us analyze the features of the sintering kinetics
of fine-grained tungstates powders at various heating
stages.
s
energy mQs(1) ~ 1–1.5 kT and stage II with the activa-
m
tion energy mQs(2) ~ 3.5–4.0 kT
for SrWO and ~9.5–
4
m
1
0 kT for NaNd(WO ) , where k is Boltzmann’s con-
A preliminary analysis of the dominant diffusion
m
4 2
mechanisms that occur at the early stages of sintering stant (Fig. 3). At higher sintering temperatures, the
with continuous heating can be carried out using the ln(Tdε/dT)–T /T slope becomes negative (stage III),
m
Young–Culter and Dorn models [42]; these models so the Dorn method becomes useless in this tempera-
were developed to analyze nonisothermal sintering ture range and other approaches need to be used to
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 64 No. 3 2019

3
00
POTANINA et al.
ln(ln(ρ/(1 – ρ))
4
3
2
1
NaNd(WO4)2
SrWO4
0
1
1.3
1.6
1.0
2.2
2.5
Tm/T
–
–
–
–
2
3
4
10 μm
Fig. 6. Microstructure of sintered SrWO ceramics. The
4
SEM image of fracture. Coarse particles that retain edged
shapes are marked with arrows.
Fig. 7. Density versus sintering temperature in double loga-
rithmic coordinates ln(ln[(ρ /ρ )/(1 – ρ /ρ )])–
exp th
exp th
T /T (ρ = ρ /ρ ).
m
exp th
assess the effective energy of sintering. Noteworthy,
the staged temperature-dependent compaction (stage I: energy of sintering Qs(2), determined from the
1
ln(ln[(ρ/ρ )/(1 – ρ/ρ )])–T /T slope, at stage II was
th th m
a low activation energy , stage II: a high activation
found to be higher than the activation energy of sinter-
energy, and stage III: a negative slope) discovered in
the study corresponds with the published results [44],
and confirms the correctness of the approach.
ing at higher temperatures, Q , at the last sintering
s(3)
stage (stage III).
In accordance with the approach described in [32,
The analysis shows that the activation energy of
3
4, 48], the activation energies of sintering Q for sintering Qs(2) is 10–12 kT
m
(~150–180 kJ/mol) and
s
stages II and III can be derived from the slope of the ~16–18 kT
(~200–226 kJ/mol), respectively, for
m
ρ/ρ (T) plot in the double logarithmic coordinates: SrWO and NaNd(WO ) powders. The values calcu-
th
4
4 2
ln(ln[(ρ/ρ )/(1 – ρ/ρ )])–T /T. Since the diameter lated for the activation energy Q using two approaches
th
th
m
s(2)
of a sintered sample is dictated by the inner diameter are in agreement with each other for m = 1/3, which cor-
of the mold and remains virtually unchanged upon responds to grain-boundary diffusion.
heating, the change in shrinkage may be equated to the
The SrWO scheelite structure, which is formed by
4
change in powder density. For the sake of a compari-
anionic (WO ) and cationic (SrO ) groups, is a frame-
4
8
son analysis of sintering kinetics, L/L –T plots were
0
work [49, 50]. A Sr atom is surrounded by eight O
atoms to form a dodecahedron, and a W atom is sur-
rounded by four O atoms to form a tetrahedron. Eight-
transformed to densening diagrams ρ/ρ –T (the
th
details of transformation of the temperature-depen-
dent shrinkage into temperature-dependent density
can be found elsewhere [32, 34, 48]). The correctness
vertice polyhedra SrO are linked via shared edges,
8
where each SrO polyhedron is fused with four neigh-
8
of ρ/ρ –T calculations was checked by comparing the
th
boring ones. Tungstate NaNd(WO ) has a similar
4
2
density corresponding to the maximal shrinkage with
structure, with the only difference that strontium
atoms are substituted by Na and Nd [51].
the experimentally measured value ρ
.
max
The analysis in terms of the above-described
approach showed that L(T) plots in the range of the
heating temperatures corresponding to stages II and
III in the double logarithmic coordinates
ln(ln[(ρ/ρ )/(1 – ρ/ρ )])–T /T for SrWO and
The scheelite crystal structure greatly complicates
the analysis of diffusion data. The W–O atomic bond
energy is known to be far higher than the А–О (where
А = Sr, Nd, Na, Ca, etc.) bond energy [45]. In par-
th
th
m
4
ticular, the W–O bond energy in scheelite CaWO is
4
NaNd(WO ) , can be interpolated by two straight
4
2
6
1
10 kJ/mol, while the Ca–O bond energy is as low as
lines with different slopes (Fig. 7). The activation
30 kJ/mol [45]. This implies that diffusion in
1
scheelite occurs via the dissociation of A–O bonds and
oxygen diffusion in the scheelite crystal structure [45].
According to [44], at stage I, individual isolated particles are
sintered with agglomerates that have been formed in the course
of synthesis with simultaneous rearrangement of particles in
these agglomerates. The dominant compaction mechanism at
this stage is surface diffusion, and this explains the low compac-
tion intensity at this stage.
Analysis of the literature data shows that the activa-
tion energy of oxygen diffusion in strontium oxide var-
ies from 263 to 445 kJ/mol [52–54]. The activation
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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FINE-GRAINED TUNGSTATES
301
energy of tungsten atoms in the CaWO crystal lattice
ACKNOWLEDGMENTS
This study was supported by the Russian Scientific
Foundation (project no. 16-13-10464).
4
is 230–280 kJ/mol [45]. According to [45], the activa-
tion energy of tungsten grain-boundary diffusion in
CaWO ceramics is 75 kJ/mol, and the activation
4
energy of calcium grain-boundary diffusion is
100 kJ/mol [45].
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.65T at the heating rate 30°С/min.
m
5
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1
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1
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Translated by O. Fedorova
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 64
No. 3
2019