Densification Strain Rate in Sintering of ThO2 andThO2-0.25%Nb2O5 Pellets by nooryudhi

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									                                                 Science of Sintering, 41 (2009) 103-115
________________________________________________________________________


                                                                 doi: 10.2298/SOS0902103K
UDK 622.785:661.884
Densification Strain Rate in Sintering of ThO2 and
ThO2-0.25%Nb2O5 Pellets
T.R.G. Kutty*), K.B. Khan, A. Kumar, H.S. Kamath#
Radiometallurgy Division,
#
  Nuclear Fuels Group,
Bhabha Atomic Research Centre,
Trombay, Mumbai 400 085, India




Abstract:
        The densification behaviour of ThO2 and ThO2 containing 0.25%Nb2O5 powder
compacts was studied with the help of a high temperature push-rod type dilatometer. From
the temperature versus density plots, densification strain rate, (1/ρ)(dρ/dt), were calculated. It
was observed that the addition of Nb2O5 to ThO2 has caused a drastic increase in
densification strain rate in the density range of 76 to 82% of T.D. A five-fold increase in the
values of strain rate for ThO2-0.25%Nb2O5 in comparison to pure ThO2 was observed in the
temperature range of 1300 to 1350oC. The decrease in the densification strain rate for ThO2-
0.25% Nb2O5 (composition in wt%) at high densities may be attributed to the grain size effect.
Keywords: Sintering, Densification strain rate, Dilatometer, Thoria



1. Introduction

         Large scale production of nuclear fuel pellets is carried out by processes involving
milling, pre-compaction and granulation followed by cold compaction and high temperature
sintering in reducing atmosphere at around 1650°C. Sintering is a diffusion controlled
process by which bonding of particles in a mass of powder in the solid state occurs by atomic
or molecular attraction through the application of heat. The densification is due to the
decrease in the surface area and therefore a reduction in the free energy of the system. Instead
of solid – gas interfaces, solid – solid boundaries of lower energy are formed during the
sintering. Sintering is traditionally viewed in terms of three distinct stages (initial,
intermediate and final) and most of the models have focused on a specific stage [1-6].
         The combined-stage sintering model, proposed by Hansen et al. [7], describes the
densification through the entire stages of sintering. By observing the similarities in the three
stages of sintering, a single equation was derived which describes the densification through all
stages of sintering. In this model the microstructure is characterized by two separate
parameters representing geometry and the average grain size. The instantaneous linear
shrinkage rate is given as:

            -dl/ldt = (γΩ/kT) [(ΓvDv/G3)+(ΓbδxDb/G4)]                               (1)

where dl/ldt is the normalized linear shrinkage rate, γ is the surface energy, Ω is the atomic
_____________________________
*)
     Corresponding author: tkutty@barc.gov.in.
104                    T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
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volume, k is the Boltzmann constant, T is the absolute temperature, G is the mean grain
diameter, Dv and Db are the coefficients of volume and grain boundary diffusion, respectively,
δ is the width of the grain boundary and Γv and Γb are the collections of microstructure
scaling parameters for volume and grain boundary diffusion, respectively.
         If there exists only one dominant diffusion mechanism in the sintering process, it is
possible to separate terms related to the microstructural and materials properties and terms
related to heating schedule [8-9]. Assuming that only one of the diffusion mechanisms (either
volume or grain boundary diffusion) dominates the sintering process, we can rewrite equation
(1) as [6]:

           (1/ρ)(dρ/dt) = 3(γΩD0/k) (Γ(ρ)/(G(ρ))n) (exp(-Q/RT)/T)                     (2)

where ρ is the density, Q is the activation energy, D0 is the pre-exponential factor and R is the
gas constant. The densification rate is conventionally represented by dρ/dt. It may be noted
that dρ/ρ gives an idea of densification. The densification is accompanied by shrinkage and
shrinkage, dl/lo, is nothing but strain. Hence, (1/ρ)(dρ/dt), namely densification strain rate,
will be more meaningful and therefore, we have used this term throughout the text.
        It is therefore possible to separate terms related to the microstructural and materials
properties and terms related to heating schedule in Eq. (2) as follows [6,10]:
                           •
           (1/ρ)(dρ/dt) = ρ i = A* F(ρ)* θ(T)                                         (3)

       •
Here i ρ i s defined as the densification strain rate, A includes all constants, F(ρ) is equal to
Γ(ρ)/G(ρ)n and is function only of density and θ(T) is function only of temperature. Thus, F(ρ)
is considered to be dependent on microstructural geometry [10].
         Many parameters of green sample have a great influence on densification strain rate
(ρi), such as green density, particle size distribution, heterogeneity like aggregates and the .
fabrication history of the powder [10]. ρi curves are expected to be a good help to choose raw
                                               .
materials and to control the microstructural evolution. It has been reported that the
densification strain rate in sintering is analogous to constant stress creep rate. It is observed in
a number of ceramics that the ratio of densification strain rate to constant stress creep rate
remains constant [11-12]. Therefore, it may be possible to predict creep rate from the
experimentally determined values of ρi. Hence a study was undertaken to evaluate the
densification behaviour of ThO2 bearing ceramics using a high temperature dilatometer. In
this study, densification strain rate curves as a function of density and temperature were
investigated for ThO2 and ThO2-0.25%Nb2O5 (composition in wt%) compacts from the
beginning to the end of sintering process.


2. Experimental

2.1. Fabrication of green pellets

        The green ThO2 and ThO2-0.25% Nb2O5 pellets for this study were prepared by the
conventional powder metallurgy technique. The characteristics of the starting ThO2 powders
used in this study are given in Tab.I. ThO2-0.25% Nb2O5 pellets are prepared using ThO2 and
Nb2O5 powders as the starting material. The procedure for the fabrication of ThO2-0.25%
Nb2O5 green pellets consists of the following steps:
a)           milling of the as-received ThO2 powder in a planetary ball mill to break its platelet
             morphology,
                     T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115            105
___________________________________________________________________________

b)          mixing/milling of the above milled ThO2 powder with the required quantity of
            Nb2O5 powder for 4 h in a planetary ball mill with tungsten carbide balls,
c)          double precompaction of the above prepared mixtures at 150 MPa,
d)          granulation of the precompacts, and
e)          final cold compaction of the granulated powder at 300 MPa into green pellets.

Tab.I Characteristics of pure ThO2 powder

          Property                                           Value
          Oxygen to metal ratio                              2.00
          Apparent density (g/cm3)                           0.70
          Total impurities (ppm)                             <1200
          Theoretical density, ρ (g/cm3)                     10.00
          Specific surface area, S (m2/g)                    1.50

        Green density of the compacts of ThO2 and ThO2-0.25% Nb2O5 made by the powder
route was around 66±1% of the theoretical density (T.D.). To facilitate compaction and to
impart handling strength to the green pellets, 1 wt% zinc behenate was added as lubricant/binder
during the last 1 h of the mixing/milling procedure. The green pellets were about 8 mm in
diameter and around 7 mm in length.


2.2. Dilatometry

        The shrinkage behaviour of pellets of mentioned above was studied using a high
temperature horizontal dilatometer (Netzsch, model 402E). The dilatometry was carried out
under the following condition:
•       Force on the sample     0.2 N
•       gas flow                12 l/h
•       heating rate            6°C/min

        The length changes were transmitted through the frictionless push rod to an LVDT
transducer. The accuracy of the measurement of change in length was within ± 0.1μm.

Tab. II Metallic impurities in the sintered ThO2 pellet

                    Element                     Impurity (ppm)
                    Na                          12
                    Al                          8
                    Mg                          5
                    Si                          <100
                    Fe                          12
                    Cr                          <1
                    Co                          <5
                    Ni                          <1
                    Mo                          <5
                    W                           <50
                    Cu                          1.0
                    B                           <0.6
106                  T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
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A calibrated thermocouple was placed just above the sample to record the sample
temperature. The dilatometric experiments were carried out in air. Tab. II gives the typical
impurity contents of a sintered pellet.


2.3. Characterization

        The ThO2 powder used in this study was characterized by the following techniques:

•       Surface area measurement (BET)
•       Particle size (XRD, Laser analysis)
•       Microstructure (SEM)

         The particle size was determined using laser based particle size analyzer which employs
the time of transit theory. The specific surface area of the powder was measured using the
Brunauer-Emmett-Teller (BET) method with helium as the adsorbate gas. The particle shape
was determined by SEM.
         The ThO2 and ThO2-0.25% Nb2O5 pellets were characterized by their density, O/M
ratio, phases, microstructures and homogeneity. The O/M ratio was measured
thermogravimetrically and the phase content was estimated using X-ray diffractometry and
metallography. The X-ray diffraction patterns of the pellets were obtained by using CuKα
radiation and graphite monochromator. The green density was measured geometrically and
sintered density was determined following the Archimedes method. For metallography, the
pellets were mounted in Araldite and ground using successive grades of emery paper. The final
polishing was done using diamond paste and then etched thermally by holding the pellets at
1250oC for 4 h in air. The grain size was determined by the intercept method.


3. Results

        The shrinkage versus temperature plots for the various pellets employed in this study
are shown in Fig. 1. Here -dl/lo is plotted against temperature, where lo is the initial length of
the pellet in the axial direction and dl is its increment. It shows the shrinkage behaviour of
pure ThO2 and ThO2 containing 0.25 wt% of Nb2O5 as dopant in air. The effect of dopants on
shrinkage has been clearly brought out in this figure. The onset of sintering shifts towards the
lower temperature on the addition of a dopant. For pure ThO2, the sintering commences only
at temperatures above 960oC, while it starts about 850oC for the ThO2-0.25%Nb2O5. The
onset temperature of shrinkage was determined from the dilatometric curves by determining
the point at which it deviates from its horizontal path. For this, a line is drawn along the linear
part of curve. The deviation from linear path is taken as the onset point. It is possible to
compute the shrinkage levels at two different temperatures from Fig. 1 and also the effect of
additives on the shrinkage at a particular temperature. At 1400oC, the shrinkage was 3% for
pure ThO2 and was 11% for ThO2-0.25%Nb2O5, respectively. The effect of Nb2O5 was found
to be very significant especially in the temperature range of 1300 to 1400oC. For the above
composition, at temperatures greater than 1400oC, the rate of shrinkage decreases drastically
with increase in temperature. It can be seen from Fig. 1 that even at 1650oC, the shrinkage for
pure ThO2 in air was found to be less than 8.5%.
        The shrinkage values of the dilatometric data were converted into percent of
theoretical density using the relation [13]:

        ρs = [1/(1-dl/lo)]3 ρ0                                                       (4)
                               T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115                                       107
___________________________________________________________________________

where, ρs and ρ0 are the density of the sintered and green pellets, respectively and lo is the
initial length. Fig. 2 shows the relative density versus temperature plot for the above pellets.
At the highest temperature of 1650oC, a density of around 95% of T.D. was obtained for
ThO2-0.25%Nb2O5 when sintered in air. But for pure ThO2, density was only about 87%.

                                        -0,02

                                         0,00

                                         0,02

                                         0,04
                              - dl/l0




                                         0,06                                                                  ThO2
                                         0,08

                                         0,10
                                                                ThO2+0.25%Nb2O5
                                         0,12

                                         0,14
                                             400         600     800      1000        1200       1400     1600      1800
                                                                                             o
                                                                       Temperature ( C)

Fig. 1 Shrinkage curves for ThO2 and ThO2-0.25%Nb2O5 pellets in air. The dl/lo values are
plotted against temperature, where lo is the initial length.

         The densification strain rate, (1/ρ)(dρ/dt), derived from the slope of Fig. 2, is plotted
against density (ρ) for the above compositions. The plot is shown in Fig. 3 suggests that
densification strain rate increases with relative density for ThO2 and ThO2-0.25%Nb2O5 and
reaches a maximum and then decreases. The maximum densification strain rate was at around
77 and 82% T.D. for ThO2 and ThO2-0.25%Nb2O5, respectively. The effect of dopant on
densification strain rate is very well demonstrated in this figure. On addition of Nb2O5, ρi
drastically increases in the density range of 76 to 82% T.D.


                                      1,00                                                                        100

                                      0,95                                                                        95
                                                           ThO2+0.25%Nb2O5
                                      0,90                                                                        90

                                      0,85                                                                        85
                   Relative density




                                                                                                         ThO2
                                      0,80                                                                        80
                                                                                                                        % T.D.




                                      0,75                                                                        75

                                      0,70                                                                        70

                                      0,65                                                                        65

                                      0,60                                                                        60

                                      0,55                                                                         55
                                             200   400    600    800    1000   1200    1400      1600   1800    2000
                                                                                        o
                                                                  Temperature ( C)



Fig. 2 Shrinkage curves of Fig.1 is replotted as relative density versus temperature for ThO2 and
ThO2-0.25%Nb2O5 pellets.
108                 T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
___________________________________________________________________________


                                                                                     % T.D.
                                                         70       75           80       85          90     95        100


                                             0,0005    Atmosphere: air
                                                       Heating rate: 6K/min


                                             0,0004                                            ThO2-0.25% Nb2O5


                                             0,0003
                               dρ/ρdt (s )
                              -1




                                             0,0002

                                                                               ThO2
                                             0,0001



                                             0,0000
                                                        0,70     0,75         0,80      0,85        0,90   0,95      1,00

                                                                        Relative density


Fig. 3 Densification strain rate versus density plot for ThO2 and ThO2-0.25%Nb2O5 pellets in air.

         Fig. 4 shows a plot of densification strain rate, (1/ρ)(dρ/dt), versus temperature for
ThO2 and ThO2-0.25%Nb2O5 when sintered in air. It is evident from the above that the
densification starts only at around 1000oC for the compositions mentioned above. A
maximum in ρi occurs at about 1475oC for ThO2 and at 1350oC for ThO2-0.25% Nb2O5. But
the striking feature of the Fig. 4 is the five-fold increase in the values of densification strain
                .
rate for ThO2-0.25%Nb2O5 in comparison to pure ThO2 in the temperature range of 1300 to
1350oC.

                                    0,0005              Sintering atmosphere: air
                                                        Heating rate: 6K/min
                                                                                                      ThO2-0.25% Nb2O5
                                    0,0004
                     dρ/ρdt, (s )
                     -1




                                    0,0003


                                    0,0002


                                    0,0001                                                                        ThO2


                                    0,0000

                                                 600       800          1000         1200       1400       1600          1800
                                                                                                o
                                                                        Temperature, ( C)

Fig. 4 Densification strain rate is plotted against temperature for ThO2 and ThO2-0.25%Nb2O5
pellets. The sintering atmosphere is air.


         The particle distribution and the volume cumulative graphs for the ThO2 and Nb2O5
powders are shown in Fig. 5 (a) and 5 (b), respectively. ThO2 powder showed a bimodal
distribution centering around 0.47 and 3.80 µm, respectively.
                        T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115         109
___________________________________________________________________________
                10
                                                                                      100
                 8
                                                                                      80
V olum e (% )




                 6
                                                                                      60

                 4                                                                    40

                 2                                                                    20

                 0                                                                   0
                 0.01   0.1          1                  10         100     1000   3000
                                         P artic le S iz e (µm )
                                                             a)
                12
                                                                                      100
                10
                                                                                      80
V olum e (% )




                 8
                                                                                      60
                 6

                 4                                                                    40

                 2                                                                    20

                 0                                                                   0
                 0.01   0.1          1                  10         100     1000   3000
                                         P artic le S ize (µm )
                                                             b)

Fig. 5 Particle size and the volume cumulative graphs for ThO2 (Fig.5a) and Nb2O5 (Fig.5b)
powders.

        From the Fig. 5 (a), it may be noted that about 20% particles are below 1 µm. Nb2O5
powder showed a peak at 3.90 µm, as shown in Fig. 5 (b). The surface area values for ThO2
powders were 1.50 m2/g. A close examination on the shape of the above mentioned powder
was carried by SEM. The ThO2 particles exhibited irregular surfaces with angular appearance.
X-ray diffraction patterns for ThO2 and ThO2-0.25% Nb2O5 confirm that the compounds are fcc
single phased. The lattice parameters were calculated from this high angle scan by least
squares method. The lattice parameters of pure ThO2 and ThO2-0.25%Nb2O5 were found to be
almost same (0.5591 nm). The grain sizes determined by intercept method were found to be 5
and 12 μm for ThO2 and ThO2-0.25%Nb2O5, respectively.




                                             5


                       a)                                       b)
Fig. 6 Microstructure of ThO2 (Fig.6a) and ThO2-0.25%Nb2O5 (Fig.6b) pellets. These pellets
were sintered in air and etched thermally.

The typical microstructures of ThO2 and ThO2-0.25%Nb2O5 pellet are given in Fig. 6
110                  T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
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4. Discussion

          The density of the ThO2 depends up on the following factors[14]:
      1.    type and the amount of the additives used,
      2.    fabrication route followed and,
      3.    sintering atmosphere employed.
          The importance of densification strain rate of ThO2 and ThO2-0.25%Nb2O5 has been
clearly brought out in the Figs 3 to 4. Let us see its significance in understanding the sintering
phenomenon.
          Densification strain rate curves as a function of relative density are sensitive to
microstructure and heating schedule. In the first part of densification, ρi increases linearly
                                                                                .
with two slopes if the green sample is agglomerated and with one slope if it is agglomerate-
free [10]. Many authors have shown that the drop in ρi takes place when the pores break away
from the boundary, so that they could only be eliminated by the slower diffusion process [15].
                                                                            .
Several studies [16-17] showed that the number of pores per unit volume has a major
influence on the densification kinetics. Therefore, increasing the number of pores per unit
volume would enhance densification strain rate at constant ρ and G, and narrowing the pore
size distribution would inhibit grain growth and increase the ρi indirectly. In addition, Mayo
[18] showed that densification strain rate is inversely proportional to the pore size for
nanocrystalline zirconia. The effect of heating rate on densification strain rate has been
reported [11]. The ρi densification strain rate increases almost linearly with the heating rate at
a given temperature. Also, the densification strain rate is inversely proportional to the particle
size at any temperature [19]. It was observed that materials with a wide particle size
distribution sintered easily [12,20].
      .
          A deep understanding of the defect structure and oxygen nonstoichiometry of doped
thoria is of crucial importance for its practical use as both an O ion conductor and nuclear fuel
[21]. Since the oxygen diffusion and thermo-physical performances depend up on the defect
                                                               .
structures, these properties must be known. In addition, the atmosphere used for sintering
itself can influence the effectiveness of formed defects [22]. The addition of a lower valency
additive like MgO/CaO to ThO2 is expected to create vacant oxygen sites in ThO2 lattice. The
same effect may also be achieved by providing a reducing atmosphere. Similarly, the addition
of a higher valency additive like Nb2O5 is expected to create oxygen interstitials in ThO2. The
same effect may also be achieved by providing oxidizing atmosphere. Thus either a lower
valency additive in a reducing atmosphere or a higher valency additive in an oxidizing
atmosphere may be expected to cause activated sintering [23]. Thus the effects of additive and
atmosphere reinforce when niobia-added thoria is sintered in air.
          Two possibilities exist when Nb2O5 is dissolved substitutionally in ThO2. We can
have an oxygen interstitial model or thorium vacancy model [22,24-25]. For oxygen
interstitial model, using the notation of Kroger and Vink, we have:

Nb2O5 ⎯ThO2 → 2 Nb ⋅ + 4Oo + Oi, ,
       ⎯ ⎯                                                                               (5)
                      Th


the unit cell corresponding to this model is Th3NbO8.5.
For thorium vacancy model, we have the following relation:

           2 Nb2O5 ⎯ 4 Nb ⋅ + 10Oo + Oi,,,,
                   ⎯→                                                                    (6)
                            Th


                                                                           ,,,,
the unit cell corresponding to this model is Th2.75NbO8. Here, VTh                denotes Th vacancy
having charge -4 with respect to the lattice.
                         T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115         111
___________________________________________________________________________

Thus, doping of ThO2 by higher-valency additive may result either in oxygen interstitials or
vacant thorium sites. The same defects, namely Oi and VTh, can also be created by an
oxidative environment:

                                             .
                                    ,,
        1 / 2O2 ( g ) → Oi + 2 h                                                       (7)

                                         .
                             ,,,,
            O2 ( g ) → VTh          + 4 h + 2Oo                                        (8)


        .
where h indicates the effective positive charge that is created in accommodating neutral
atmospheric oxygen into ionic lattice. Since both Frenkel and Schottky defects are present
simultaneously, the formation of oxygen interstitials should decrease the concentration of
oxygen vacancies, thereby increasing the concentration of thorium vacancies through
Schottky equilibrium [26]. The increase in the concentration of thorium vacancies leads to the
increase in thorium diffusion coefficient and thus enhances grain growth[24]. With this
background in mind, we will analyze the densification strain rate behaviour of ThO2 bearing
compacts.


4.1.    Effect of additives

         The densification strain rate of pure ThO2 shows a gradual increase with density to a
maximum value and then shows a decrease (Fig. 3). The smooth increase in the ρi indicates .
that the green compacts were free of agglomerates. The presence of agglomerates slows down
densification and leads to a densification strain rate curve with two slopes [27]. On addition of
0.25% Nb2O5, the densification strain rate increases gradually up to 76% and then shows a
steep increase to a maximum at about 82%T.D. and then decreases and reaches a minimum at
93% T.D. Lange [28] studied the sintering of alumina powder compacts at constant heating
rates of 2.5 to 20oC/min up to 1550oC. He reported that maximum shrinkage rates occurred at
the same relative density of 77%, whatever the heating rate. This is in good agreement with
the present work, where the maximum in the densification strain rates occur for the samples   .
covered in this study in the density range of 77-82%. A large increase in the ρi was noticed
for ThO2-0.25%Nb2O5 in comparison to pure ThO2 in the temperature range of 1300 to
1350oC. Let us try to find the cause for this phenomenon from the electrical conductivity data.
         The electrical conductivity generally increases with temperature. The electrical
conductivity of ThO2 was measured by Subbharao et al. [29] and reported that the
conductivity values are higher than that of Nb2O5 doped ThO2 at all temperatures. Also, the
electrical conductivity of MgO-doped ThO2 was reported to be higher than that of pure ThO2
[30,31]. The variation of electrical conductivity (σ) with respect to absolute temperature (T)
can be expressed by the following equation:

        σT = A exp (-E/kT)                                                             (9)

          where A is the pre-exponential factor and E is the activation energy. Fig. 7 shows the
electrical conductivity data for ThO2 and Nb2O5 doped ThO2. Bransky and Tallan [31] have
measured the electrical conductivity of ThO2 as a function of temperature at different oxygen
pressures. They reported that the upper limit of oxygen pressure at which ThO2, is
stoichiometric is seen to be about 10-6 atm. If the oxygen pressure in the sintering furnace is
greater than 10-6 atm, it may be considered as an oxidative atmosphere which would tend to
112                 T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
___________________________________________________________________________

generate oxygen interstitials in thorium oxide. In the oxidizing region, the increase in
electrical conductivity occurred in both low and high temperatures. However, in the reducing
atmosphere the increase in electrical conductivity was observed only at higher temperatures.
Thus at low temperatures the combination of higher valency additive and oxidizing
atmosphere leads to give a higher defect concentration than does the combination of lower
valency additive and reducing atmosphere[32,33].
                                                                           o
                                                                    T ( C)
                                              700     800   900     1000        1100   1200     1300    1400
                                          4




                                          3
                            σT (Ω cm K)
                            -1




                                          2
                            -1




                                                                                                ThO2[29]


                                          1                                            ThO2+Nb2O5[31]



                                          0
                                                    1050     1200              1350      1500          1650

                                                                      T (K)


Fig. 7 Electrical conductivity of ThO2 and Nb2O5 doped ThO2 plotted against
temperature[29,31].

         Matsui and Naito [34] stated that the electrical conductivity of UO2+x is increased by
doping with cations of lower valency than uranium ions, since the lower-valent cations
substituted for uranium ions can act effectively as hole donors. Conversely, the electrical
conductivity of UO2+x is decreased by doping with cations of higher valency, which act
effectively as electron donors. Similarly, the oxygen potentials, ΔGO2 of UO2+x doped with
higher valent cations are decreased and the decrease of the oxygen potential is probably due
to the decrease of the mean uranium valence by adding niobium ions with higher valence[34].
ThO2 is isostructural with UO2. Incorporation Mg2+ or Nb5+ in the ThO2 lattice generates
oxygen vacancies or oxygen interstitials, respectively. The electrical conductivity of ThO2 is
decreased by doping with cations of higher valency like Nb5+. This has been clearly brought
out in Fig. 7. When one Th+4 ion is substituted by one Nb+5 ion in the ThO2 lattice, the
effective positive charge of +1 is imparted on the lattice. Hence the addition of Nb2O5 to ThO2
causes to form significantly high concentrations of oxygen interstitial ions. Metal vacancies
will also be formed at the same time but in very low concentrations. An increase in thorium
lattice vacancy, increases its diffusion coefficient.
         In view of the above, it can be expected that the effects of addition of Nb2O5 to ThO2
would be to enhance the diffusion of thorium in ThO2 and to decrease the electrical
conductivity. This is verified by the work carried out by Matzke [35,36]. He determined the
ratio of the diffusion coefficient of tracer uranium in doped ThO2 to the diffusion coefficient
in undoped ThO2 at different temperatures. At 1400°C the ratio was 340 for Nb2O5-doped
ThO2 and <0.25 for Y2O3-doped ThO2.
         From Fig. 7, it is clear that the electrical conductivities of ThO2 and Nb2O5-doped
ThO2 pellets diverge at high temperatures. Above 1050°C, the differences in the electrical
conductivities values of the above are increasing with the temperature. At 1300°C, the
                     T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115               113
___________________________________________________________________________

electrical conductivity of Nb2O5-doped ThO2 is about 3 times lower than that of ThO2. As
mentioned earlier, the maximum densification rate in Nb2O5-doped ThO2 occurs at around
1300°C. The large difference in the electrical conductivity values indicates that the diffusion
takes much faster in Nb2O5-doped ThO2 resulting in higher density and larger grain size.
         From Fig. 3, it can be seen that a drastic decrease in the densification rate has been
observed for ThO2-0.25% Nb2O5 pellet when the density was above 84%T.D. As mentioned
earlier, for oxide ceramics, diffusion is expected to be the dominant mass transport
mechanism during sintering. A theoretical model for densification by diffusional mass
transport predicts an equation of the form [20]

                                     (m+1)/2
         ε d = (1/3ρ) (dρ/dt) = ADΣ[φ
           *                                 ]/(GmkT)                                (10)

Where A is a constant, D is the diffusion coefficient for the rate controlled densification
mechanism, Σ is the sintering stress or sintering potential, φ is the stress intensification factor
and m is the exponent that depends on the mechanism of densification (m = 3 for grain
boundary diffusion and m = 2 for lattice diffusion). From equation (10), two main factors
which are responsible for the reduction of densification strain rate in ceramics are [20]:

     1. The reduction in diffusion coefficient for densification
     2. An increase in diffusion distance or grain size

         Let us see the effect of grain size on densification strain rate. Recently, Lim et al.[37]
studied the microstructural evolution during sintering of agglomerate-free alumina powder
compacts. They observed that grain growth is significant when density is approximately
above 80%. Lance et al. [27] have shown that in the last part of densification after maximum
ρi, coarsening mechanism is promoted. Here, grain size increases significantly leading to a
decrease in the densification. The grain size of ThO2-0.25% Nb2O5 pellet was about 12 µm
while that of ThO2 sample was only 5 µm, respectively. Therefore the grains have grown
                                    .
appreciably for ThO2-0.25% Nb2O5 pellet. Therefore the large reduction in densification
strain rate above 85%T.D. is due to the grain size effect.


5. Conclusions

        The densification behaviour of ThO2, and ThO2-0.25% Nb2O5 powder compacts were
carried out in a high temperature push-rod type dilatometer. The following conclusions are
drawn from the above study:

a)     The addition of Nb2O5 to ThO2 has caused drastic increase in densification strain rate in
       the density range of 76 to 82% of T.D.
b)     The increase in the densification strain rate for ThO2-0.25% Nb2O5 can be explained in
       terms of its lower electrical conductivity values.
c)     The decrease in the densification strain rate for ThO2-0.25% Nb2O5 at high densities may
       be attributed to the grain size effect.
d)     For ThO2-0.25% Nb2O5, the combination of higher valency additive and oxidizing
       atmosphere led to a higher defect concentration leading to higher density and larger
       grain size.
114                  T.R.G. Kutty et al. /Science of Sintering, 41 (2009) 103-115
___________________________________________________________________________

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                 T.R.g. Kutty et al./Science of Sintering, 41 (2009) 103-115   115
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Садржај: Проучено је згушњавање испресака ThO2 и ThO2 са 0.25%Nb2O5 коришћењем
дилатометра на принципу померајуће шипке. Из зависности температуре и густине
израчуната је промена брзине згушњавања и (1/ρ)(dρ/dt). Примећено је да додатак
Nb2O5 довео до драстичног увећања брзине згушњавања у опсегу густине од 76 до 82%
теоријске густине. Петоструко увећање вредности брзине за ThO2-0.25%Nb2O5 у
поређењу са чистим ThO2 је примећен у температурном опсегу између 1300 и 1350оС.
Смањење брзине згушњавања за ThO2-0.25% Nb2O5 на већим густинама се може
припиисати ефекту промене величине зрна.
Кључне речи: Синтеровање, брзина згушњавања, дилатометар, торијум.

								
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