Emanation-thermal analysis of perovskite

Article abstract of DOI:10.1023/B:RACH.0000031693.86082.44

Structural changes in perovskite and perovskite-like ceramics in the course of thermal treatment were studied by emanation-thermal analysis (ETA) using synthetic calcium metatitanate and aluminotitanate with addition of neodymium and cerium simulating sol

Full text of DOI:10.1023/B:RACH.0000031693.86082.44

Radiochemistry, Vol. 46, No. 3, 2004, pp. 295 302. Translated from Radiokhimiya, Vol. 46, No. 3, 2004, pp. 272 279.
Original Russian Text Copyright 2004 by Bekman, Balek, Buntseva.
Emanation-Thermal Analysis of Perovskite
I. N. Bekman*, V. Balek**, and I. M. Buntseva*
*
Chemical Faculty, Moscow State University, Moscow, Russia
* Institute of Nuclear Research, Czech Republic
*
Received January 17, 2003; in final form, January 8, 2004
Abstract Structural changes in perovskite and perovskite-like ceramics in the course of thermal treatment
were studied by emanation-thermal analysis (ETA) using synthetic calcium metatitanate and aluminotitanate
with addition of neodymium and cerium simulating solidified radioactive wastes. The emanation was analyzed
as influenced by mechanical (polishing of the surface with diamond paste), radiation (bombardment with
helium and krypton ions to certain dozes), and chemical (leaching in aggressive solvents) treatments and
by the atmosphere (air, argon, hydrogen) in which the thermal treatment was performed. The ETA curves
exhibit characteristic peaks corresponding to regeneration and annealing of both natural structural defects and
those appearing after the external treatment of the sample. The model of emanation from material involving
structural transformations, which well agrees with experimental data, was proposed. The emanation-thermal
analysis under the dynamic conditions allows study of the evolution in the development of the surface relief
and solid-state processes proceeding in the fine near-surface layer of material in the course of thermal treat-
ment. It was found that the perovskite-like ceramics, thanks to its high thermal, radiation, mechanical, and
chemical stability, can be recommended for solidification and disposal of high-level radioactive wastes.
Development of efficient methods for treatment
and disposal of high-level radioactive wastes is a
pressing problem. One of such procedures is immobi-
lization of actinides in ceramic matrices [1]. Among
them, ceramics with the perovskite structure, allowing
isomorphic substitution of the main structural atoms
by radionuclides and therefore exhibiting high capac-
ity, seem promising. The chemical stability of solidi-
fied wastes is a decisive factor affecting the environ-
mental risk of the radionuclide disposal. Valuable data
on the defects in solids can be obtained using emana-
tion-thermal analysis (ETA) allowing registration of
changes in the structure of a fine near-surface layer
based on calcium aluminotitanate with additives were
leached following the International Leaching Test
procedure given in [7]. The data on the annealing
of natural and radiation defects and on the effect of
hydrolytic changes in the initial stages of corrosion on
the thermal behavior of ceramics were used to assess
the applicability of materials with perovskite structure
to disposal of high-level radioactive wastes. Some
characteristics of materials and modes of their treat-
ment are listed in the table.
Calcium metatitanate CaTiO with the perovskite
3
structure was prepared by hydrolysis of an alcoholic
solution of titanium isopropoxide with an aqueous
solution of calcium nitrate. The product was dried and
(<100 nm thick) under various external effects [2 6].
annealed at 700 C to remove NO and impurities
of organic compounds, then pressed in pellets, and
sintered at 1400 C in air for 2 days. Several samples
In this work, we used ETA to study the chemical,
x
thermal, radiation, and corrosion stability of materials
with perovskite structure, such as ceramics based
on calcium aluminotitanate Ca0.98214(Nd0.00790
were mirror-finished with a diamond paste (6
m
particle size) to study the effect of mechanical treat-
ment of the material surface on the ETA curve shape.
The plate samples were irradiated to various doses
with accelerated ions to simulate the effect of -par-
ticles on physicochemical properties of solidified
radioactive wastes (helium) and to estimate possible
effect of recoil atoms (products of actinides decay) on
the radiation stability of ceramics (krypton).
Ce0.00996)Al0.01786Ti0.98214O , in which stable neo-
3
dymium and cerium simulate the corrosion behavior
of curium and plutonium, respectively. The effects
of radiation to certain doses with accelerated ions of
1
4
15
2
helium (10 and 10 cm fluences, 2.7 MeV energy
+
14
15
2
of He ions) and krypton (10 and 10 cm flu-
ences, 4.0 MeV energy of Kr ions), mechanical treat-
+
ment of the surface, atmosphere (argon, air, hydrogen),
and mode of thermal treatment on the behavior of
a perovskite-like material were studied using calcium
Perovskite-like ceramics based on calcium alu-
minotitanate with additives was prepared by sol gel
method using a mixture of isopropoxides of titanium
metatitanate CaTiO as example. The ceramic samples
3
1
066-3622/04/4603-0295 2004 MAIK Nauka/Interperiodica

296
BEKMAN et al.
Properties of materials and modes of their thermal treatment
Test no.
Brief description of the sample
ETA atmosphere (air)
Ar
Ar
Ar
Ar
Ar
Air
Air
1
2
3
4
5
6
7
8
9
CaTiO , UP plate
3
CaTiO , UP plate, irradiation 10 cm 2 Kr
1
4
3
1
5
2
CaTiO , UP plate, irradiation 10 cm Kr
3
1
4
2
CaTiO , P plate, irradiation 10 cm Kr
3
1
5
2
CaTiO , P plate, irradiation 10 cm Kr
3
CaTiO , powder
3
CaTiO , powder, irradiation 10 cm 2 Kr
1
4
3
1
5
2
CaTiO , powder, irradiation 10 cm Kr
Air
3
CaTiO , UP plate
Ar + 6% H2
Ar + 6% H2
Ar + 6% H2
Ar + 6% H2
3
CaTiO , UP plate, irradiation 10 cm 2 He
1
4
1
1
1
1
0
1
2
3
3
1
5
2
CaTiO , UP plate, irradiation 10 cm He
3
Ceramics, calcium aluminotitanate containing Nd and Ce
Ceramics no. 12 (plate) leached according to International Leaching Test Ar + 6% H2
*
P and UP mean the surface polished with a diamond paste or unpolished.
Ti(C H O) , neodymium Nd(C H O) , and cerium
The solid-state processes in the samples at thermal
treatment were studied by ETA.
3
7
4
3
7
3
Ce(C H O) and aluminum isobutoxide Al(C H O)
3
7
3
4
9
3
in methanol. Then, water (in excess required for com-
plete hydrolysis of the methanolic mixture) was added
slowly to form coagulates. The alcohol was evap-
orated at 120 C for 6 h to 1/3 of the initial volume,
and water was added to restore the initial volume.
The solution of calcium hydroxide (stoichiometric
amount) was added dropwise at room temperature for
In the emanation-thermal analysis, we recorded the
220
rate of
formed from
Rn (thoron) liberation, which is constantly
224
Ra preliminarily added to the sample
[
8]. The sample was impregnated with a solution of
228
parent radionuclides (equilibrium mixture of
Th
224
and
Ra), which were sorbed on its surface; the
4
1
220
specific activity was 10 Bq ml . The
are formed according to the scheme
Rn atoms
1
1
h, and the resulting mixture was heated at 140
50 C for 5 h to evaporate solvent and to obtain a
thick paste. A cylindrical sample (2 cm in diameter,
cm high) of the perovskite-like ceramics was pre-
(T = 1.9 years)
1
/2
228Th
234Ra
1
pared by hot pressing for 2 h at 29 MPa and 1250 C.
This cylinder was sawed in two samples along
the main axis. The composition of the resulting ceram-
(
T
= 3.8 days)
(T = 55 s)
1/2
1/2
220Rn
...
(1)
ics corresponded to Ca0.98214(Nd0.00790Ce0.00996
)
and penetrate into the solid at a depth of 100 nm
owing to the recoil energy.
Al0.01786Ti0.98214O . Our analysis showed that neo-
3
dymium and cerium are in oxidation state (3+) and
3+
Then, the labeled samples were placed into a unit
for complex emanation-thermal analysis based on the
occur in the sites of calcium, whereas Al occupies
4+
the positions of Ti .
equipment supplied by Netzsch (Germany), and the
Leached calcium aluminotitanate with additives.
The surface of one semicylinder was leached for
220
rate of
Rn liberation during heating of the sample
1
at a rate of 6 deg min was monitored with a flow-
type detector. The ETA curves were recorded in
2
months at 90 C using 0.05 M KCl + 0.013 M HCl
solution (pH 2) in accordance with the International
Leaching Test. The concentrations of various elements
in corrosion solution were measured and the leaching
rate was determined. Due to changes in the chemical
composition of the near-surface layer in the course of
leaching, the initially black surface of the sample
became white in 21 days. After drying, the surface of
the leached sample was opalescent; the leaching depth
was 40 m.
(
1) inert atmosphere (chemically pure argon), (2) oxi-
dizing atmosphere (dry air), and (3) reducing atmos-
phere (6% hydrogen in argon). The -activity of
2
20
Rn was recorded with a semiconductor detector.
The experiments were performed in the heating and
cooling modes. To determine the relative emanation
power, the counting rates recorded with the detector
were normalized to the rate of 2 Rn formation in
the sample material.
20
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EMANATION-THERMAL ANALYSIS OF PEROVSKITE
297
It is known that the emanation of solids includes
the emanation due to the recoil and diffusion [9].
Changes in emanation on heating can be presented as
follows:
thermally unstable, and at certain temperature the rate
of the cluster dissociation becomes greater than the
rate of its formation. Upon dissociation, radon passes
into a low-mobile state and the emanation decreases
with increasing temperature; this process is charac-
terized by a typical peak in the ETA curve.
E(T) = E0 + E (T),
(2)
D
It should be noted that, when the parent radionu-
clides are introduced by impregnation, Rn is generated
in the fine near-surface layer. Therefore, the initial
defectiveness of this layer and structural changes
occurring in this layer under the external treatment
exert a decisive effect on the shape of the ETA curves.
The defect assortment depends on the sample history,
modes of the surface treatment, irradiation conditions,
etc. The defect mobility in the course of heating
strongly depends on the treatment conditions (e.g.,
heating rate, annealing atmosphere), possibility of
annealing or regeneration of the defects, or phase and
chemical transformations in the material.
where E is the emanation at room temperature, pre-
0
dominantly due to the recoil, and E (T) is the param-
D
eter changing with temperature T.
In the case of granular materials with a particle size
of about 1 m, at high emanation power and in the
absence of solid-phase processes, the 2 Rn flow from
the material can be evaluated using the equation [9]
20
ED = 3(cothy
1/y)/y,
(3)
where y = r [ /(D(T)]1/2, is the decay constant of
0
2
20
1
Rn (s ), r is the radius of the emanating grain
0
4
The low-temperature emanation can be described
(
assumed here to be 10 cm, according to the elec-
by Eq. (3) taking into account that radon migrates in
tron microscopic data).
220
the form of a thermally unstable
Rn defect cluster.
To a first approximation, the temperature depen-
dence of the diffusion coefficient of radon obeys the
Arrhenius law:
The cluster dissociation can be given using the func-
tion (T)
1
2 1/2]}, (5)
(
T) = 1
0.5{1 + erf[(T
T )
m
D(T) = D exp( Q /RT),
(4)
0
D
where T is the temperature at which the dissociation
m
220
rate of the mobile
Rn defect cluster is the highest
where D0 is the diffusion preexponential factor,
220
and is the variance (whole temperature range of the
transformation T = 3 ).
QD is the activation energy of the
Rn diffusion
1
(
8
J mol ), and R is the universal gas constant (R =
.31 J deg 1 mol ).
1
The peak shape in the ETA curve can be described
by the following equation:
In the case of compact inorganic materials, the
above equations can be considered as a theory of
high-temperature emanation. Actually, the size of
E(T) = E0 + E (D, T) (T),
(6)
D
2
20
Rn atom is rather large and its volume migration in
the crystal lattice is possible only at high temperatures
certainly higher than 1000 C for the samples in
where E (D,T), D(T), and (T) are determined by
Eqs. (3), (4), and (5), respectively.
D
(
The ETA curves recorded in an argon atmosphere
using unpolished perovskite-like samples (nos. 1 3,
Table 1) irradiated with accelerated Kr ions to various
doses are shown in Fig. 1a; in Fig. 1b, the same data
are normalized to the height of the first ETA peak. As
seen, at the start of heating the emanation of Rn from
the sample rises, above 200 C the emanation ceases
to grow, and then above 250 C it decreases, reaching
a minimum at 700 750 C. Above 800 C, radon
emanation exponentially increases with temperature.
question). At the same time, the experimental data
showed that the emanation increases on heating even
to 40 C. The ETA curves recorded in a wide tempera-
ture range often exhibit several peaks and become
monotonic only at high temperature.
These changes in the radon flow from the sample
with temperature are probably due to the formation of
radon clusters with mobile structural defects in the
near-surface layer. These clusters easily pass to the
surface and remove radon from the material. With
increasing temperature, the mobility of the Rn defect
clusters grows and, in turn, the radon flow from the
sample also increases. However, these clusters are
Irradiation of the CaTiO surface with accelerated Kr
3
ions (Fig. 1a) increases the emanation power of the
material, predominantly due to the emanation at room
temperature at which the diffusion of radon in the
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298
BEKMAN et al.
Fig. 1. Emanation-thermal analysis of perovskite (unpolished sample, linear heating, argon atmosphere) irradiated with accele-
rated krypton ions (4 MeV ): (a) experimental ETA curves and (b) curves normalized to the peak height, the same for Figs. 2 4.
1
4
2
15
2
Calcium metatitanate: (1) initial, (2) irradiated to a dose of 10 cm , and (3) irradiated to a dose of 10 cm ; the same for
Fig. 3.
Fig. 2. Emanation-thermal analysis of (1) polished and (2) unpolished perovskite samples.
crystal lattice is completely suppressed. The radon
emanation at low temperature is predominantly due to
the recoil, which depends on such factors as distribu-
tion of the parent radionuclide in the sample, area of
the open sample surface available for emanation, and
development of the surface relief. Bombardment with
accelerated Kr ions causes leaching of the fine near-
surface layer and formation of a nonuniform surface
relief, which, in turn, promotes recoil emanation of
radon. At the same time, irradiation does not notice-
ably affect the shape of the ETA curve (Fig. 1b); as
seen, after krypton irradiation only the ascending
branch of the emanation peak is slightly shifted
toward higher temperatures. The ETA peak of the
irradiated sample is narrower and sharper than that
of the nonirradiated material. At high temperatures
(>700 C), irradiation decelerates emanation due to
the formation of radiation defects that trap radon
and hinder its diffusion (more precisely, migration of
the Rn defect clusters).
Figure 2a illustrates the effect of polishing of the
plate surface on the emanation from perovskite irra-
diated with krypton ions (1014 cm ). This procedure
significantly decreases the emanation, especially that
occurring at room temperature due to the recoil and,
2
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EMANATION-THERMAL ANALYSIS OF PEROVSKITE
299
Fig. 3. Emanation-thermal analysis of perovskite (6% H in argon) irradiated with accelerated helium ions (2.7 MeV).
2
Fig. 4. Comparison of the ETA curves of calcium titanite and aluminotitanate, recorded in reducing atmosphere (6% H in
2
argon): (1) calcium aluminotitanate with addition of neodymium and cerium (perovskite-like ceramics) and (2) calcium meta-
titanate.
partially, the emanation due to the migration of the
Rn defect clusters. Mechanical treatment smoothens
the surface relief, but creates defects in the fine near-
surface layer, which act as radon traps and shift the
ascending branch of the ETA peak (40 300 C range)
toward higher temperatures. Polishing only slightly
affects the ETA curve shape (Fig. 2b). However,
bombardment with accelerated krypton ions does not
change the ETA curve of perovskite registered upon
heating in the oxidizing atmosphere (dry air).
celerated helium ions, the greater the emanation,
especially at low temperatures (Fig. 3a). The shape of
the ETA curve is essentially independent of irradiation
(Fig. 3b).
The ETA curve of perovskite-like ceramics simulat-
ing solidified radioactive wastes (calcium alumino-
titanate with neodymium and cerium additives), re-
corded in the reducing atmosphere, is shown in
Fig. 4a. The ETA curve of the initial perovskite
recorded under the same conditions is given for com-
parison. As seen, except the low-temperature range
(20 100 C), the emanation power of the ceramics is
significantly lower than that of the perovskite. The
same curves normalized to the height of the first peak
are shown in Fig. 4b. Introduction of neodymium and
The emanation of perovskite noticeably changes in
the reducing atmosphere (Fig. 3): several new peaks
appear in the ETA curve at 450, 800, and 1050 C,
whereas the high-temperature exponent is absent. The
higher the dose of perovskite irradiation with ac-
RADIOCHEMISTRY Vol. 46 No. 3 2004

300
BEKMAN et al.
pears, the peak at 750 C changes less significantly,
and the high-temperature exponent appears again.
In our analysis, we primarily studied the low-tem-
perature (peak) and high-temperature (exponent) sec-
tions of the ETA curves. A typical example is given
in Fig. 6 [heating of perovskite irradiated with accele-
rated krypton ions (1015 cm ) in argon atmosphere].
Using the least-squares procedure and the above equa-
tions, we first fitted the edges of the ETA curve,
then subtracted the theoretical curve from experi-
mental curve, and obtained the residual spectrum, i.e.,
several peaks at intermediate temperatures, which
were not mathematically treated.
2
The average activation energy of the low-tem-
perature ETA peak recorded in dry air (Q = 56
1
Fig. 5. Comparison of the ETA curves of the initial and
leached calcium aluminotitanate with additives: (1) initial
calcium aluminotitanate (perovskite-like ceramics) and
3 kJ mol ) well agrees with the average value found
1
for argon (58 2 kJ mol ). The average frequency
5
2
1
factor for air (D = 6.3 10 cm s ) is greater by
0
(
2) calcium aluminotitanate leached according to the Inter-
nearly an order of magnitude than that for argon (8.9
national Leaching Test.
6
2
1
10
cm s ). The activation energy of the ETA peak
1
in hydrogen is significantly higher (74 3 kJ mol )
than in argon and air. The lowest temperature of the
structural peak (Tm = 496 C) with the width T =
1
41 C was found for argon. As for air, the maximal
temperature of the structural ETA peak and its width
were the greatest: 496 C and 290 C, respectively. In
the case of hydrogen, this peak is characterized by
intermediate temperature (443 C) and is the narrowest
(
T = 133 C). The activation energy of emanation in
the ETA peak is always smaller that the activation
energy of emanation in the high-temperature range.
For example, for unpolished perovskite plate, the
activation energy calculated for the peak and exponent
1
is 43 and 100 kJ mol , respectively. The activation
energies calculated from the cooling curves are greater
1
(
about 130 kJ mol ). Such a trend was observed for
Fig. 6. Mathematical treatment of the ETA curve [calcium
metatitanate, accelerated krypton ions (10 cm ), linear
heating in argon atmosphere]: (1) experimental curve,
all the samples studied. Irradiation with accelerated
krypton ions of unpolished perovskite plate increases
the activation energy of emanation in the peak: 42, 62,
1
5
2
(
2) theoretical curve for ETA peak, (3) theoretical curve for
1
and 67 kJ mol for the initial sample and those ir-
exponential section of the ETA curve, and (4) difference
between the experimental curve and sum of theoretical
curves ( remainder drift ); the baseline of the remainder
drift is given by a dashed line.
radiated at fluences of 101 and 10 cm , respec-
tively. However, the effect of bombardment on the
high-temperature activation energy is the greatest
4
15
2
1
14
(
100, 106, and 148 kJ mol for fluences of 0, 10
cerium into the perovskite structure strongly enhances
emanation of radon from the sample at the treatment
start and give rise to additional ETA peak at 120 C.
15
2
and 10 cm , respectively). The activation energy
of emanation upon cooling is independent of the ir-
radiation dose (131 2 kJ mol ). Polishing slightly
1
The ETA curves of the initial perovskite-like ce-
ramics and that leached according to the International
Leaching Test, recorded under a reducing atmosphere,
are shown in Fig. 5. As seen, leaching strongly
changes the ETA curve shape; the first peak disap-
decreases the activation energy in the peak, but in-
creases it in the exponent. The activation energy in the
exponential section of the ETA curve recorded in air
(130 kJ mol ) exceeds that for argon (100 kJ mol ),
and in the course of cooling the difference is still
1
1
RADIOCHEMISTRY Vol. 46 No. 3 2004

EMANATION-THERMAL ANALYSIS OF PEROVSKITE
301
greater (256 and 129 kJ mol 1 for air and argon, re-
spectively). Irradiation of perovskite with accelerated
helium ions in the reducing atmosphere decreases the
only in the case of reducing atmosphere. We found
several solid-state processes proceeding in the above
temperature range. In their interpretation we should
take into account that physicochemical properties of
ceramics in the fine near-surface layer strongly differ
from those in the material bulk. The surface of mixed
activation energy of emanation in the peak from 73
1
(
nonirradiated sample) to 68 kJ mol (maximal irra-
diation dose). The activation energy of emanation
from ceramics in the ETA peak is smaller than that
perovskite-type oxide CaO TiO is coated with TiO2
2
1
from perovskite: 48 and 64 kJ mol , respectively.
(amorphous and partially crystalline with anatase
structure) and some its hydroxide forms. This is pos-
sible if the sample contacts with atmospheric moisture
for a fairly long period. It is obvious that the decom-
position of hydroxides and crystallization of the result-
ing products should strongly affect the ETA curve
shape. For example, the peak at 800 C (Fig. 6) occurs
in the temperature range of the anatase rutile phase
transition (800 850 C).
Our study showed that the emanation procedure
furnishes valuable data on the structure of fine near-
surface layers (50 100 nm thick) of perovskite and
perovskite-like ceramics, which can be of particular
interest in disposal of radioactive wastes. The value of
this method is especially due to two main processes
involved: recoil and diffusion. The first of them
(recoil) hinders the emanation at low temperatures,
when radon migration in the perovskite crystal struc-
ture is virtually suppressed. In this work, due to the
recoil effect, we revealed the effect of external radia-
tion (bombardment with accelerated helium and kryp-
ton ions) and mechanical treatment (mirror polishing)
on the surface relief and surface area available for
emanation. The emanation processes (in one form or
another) related to migration of radon or its clusters
with mobile defects in the crystal structure are also of
particular interest. The ETA curves reveal changes in
the chemical composition and structure of the fine
near-surface layer. The ETA curve of perovskite re-
corded in air or argon can be divided into three sec-
tions. The first low-temperature section (20 300 C)
represents a well-shaped peak reflecting formation of
radon clusters with mobile defects, their migration to
the layer surface, and further dissociation accom-
panied by passing of radon atoms into low-mobile
form. These defects can be both anionic vacancies
At high temperatures (1000 1300 C), the ETA
curve is close to the exponent due to the volume dif-
fusion of radon in the perovskite crystal lattice. In this
temperature range, no Rn defect clusters are formed
and, thus, no peaks in the ETA curves are observed.
However, the defects still affect the emanation, acting,
as a rule, as traps of inert gas and shifting radon
emanation toward higher temperatures. These traps
involve both natural and artificial defects formed in
the course of irradiation or mechanical treatment.
Above 1200 C, emanation is almost independent of
temperature, because, due to high diffusion coef-
ficients, radon easily escapes from the sample. The
activation energies of emanation calculated from the
ETA curves recorded upon cooling are higher than
those recorded upon heating. This suggests that
preheating promotes crystallization of the material,
annealing nonequilibrium defects. In some cases, the
ETA curve recorded upon cooling consists of two
linear sections; and the activation energies of emana-
tion for low-temperature sections are smaller than
those for the high-temperature region. This is due to
the fact that, at low temperatures, Rn diffusion is
limited by its migration through the defects in the
near-surface layer, and at high temperatures, by migra-
tion through fairly ordered material.
[10] and defects appearing in the course of pressing,
annealing, and irradiation. It should be noted that such
defects can act as facilitated diffusion pathways in for-
mation of mobile Rn defect clusters or traps captur-
ing Rn atoms and partially hindering their migration.
Generation of defects of the first type intensifies the
radon flow from the sample, shifts the emanation peak
toward lower temperatures, and decreases the activa-
tion energy of diffusion, whereas defects of the second
type hinder emanation and shit the thermal-desorption
spectrum of radon toward higher temperatures.
Heating of perovskite in the reducing atmosphere
noticeably changes the ETA curve shape: at low and
intermediate temperatures additional peaks appear
in the curve, reflecting breakdown of mixed oxides
to individual oxides and reduction of TiO to Ti O .
In the 300 1000 C range, the solid-state processes
are weakly manifested in the ETA curves. In the ETA
curves of perovskite recorded in this temperature
range in an argon atmosphere, no effects are observed,
whereas in air they can be revealed only after mathe-
matical treatment. These effects become pronounced
2
2 3
In the course of perovskite heating in a hydrogen
atmosphere, various intermediate forms appear and
disappear; this processes are observed up to 1100 C.
Ceramics with radioactive wastes is often solidified in
argon with 6% H to reduce plutonium and transpluto-
2
RADIOCHEMISTRY Vol. 46 No. 3 2004

302
BEKMAN et al.
nium elements to the lowest oxidation states, which
promotes their isotope exchange with the ions of the
solidified matrix. However, the ETA data show that
heating of the ceramics in a hydrogen atmosphere
should be performed at temperatures lower than
structure, and these materials can be recommended for
solidification of high-level radioactive wastes.
REFERENCES
1
2
. Sobolev, I.A., Ozhovan, M.I., Shcherbatova, T.D.,
et al., Stekla dlya radioaktivnykh otkhodov (Glasses
for Radioactive Wastes), Moscow: Energoatomizdat,
5
00 C, because at higher temperatures the perovskite
structure is broken.
1
999.
The ETA curves of calcium metatitanate are similar
to those of the ceramics based on calcium alumino-
titanate with additives, which suggests similarity in
structures of these materials and in structural trans-
formations proceeding upon heat treatment. The only
difference is a sharp increase in the emanation of cal-
cium aluminosilicate at temperatures lower than
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structure of the fine near-surface layer in ceramics
after its treatment in a corrosion-active medium.
Changes in emanation indicate disappearance of all
the near-surface defects participating in formation of
the Rn defect clusters. After leaching, the material
has a compact surface structure with a relatively low
emanation coefficient. At high temperature, the be-
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medium is similar to that of calcium metatitanate in
the inert atmosphere.
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Budapest: Akad. Kiado, 1984.
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RADIOCHEMISTRY Vol. 46 No. 3 2004