Journal of Alloys and Compounds 334 (2002) 118–130
L
www.elsevier.com / locate / jallcom
On the development of high density barium metazirconate (BaZrO 3 )
ceramics
1
Abdul-Majeed Azad*, Selvarajan Subramaniam , Teng Wan Dung
Advanced Materials Research Center, Ceramics Technology Center, SIRIM Berhad, 1 Persiaran Dato’ Menteri, Section 2, 40911 Shah Alam,
Selangor, Malaysia
Received 29 June 2001; received in revised form 20 July 2001; accepted 23 July 2001
Abstract
A simple, economically viable and easily scaleable technique for the bulk synthesis of phase-pure barium metazirconate (BaZrO 3 ) is
described. An equimolar ball-milled mixture of barium and zirconyl nitrates yielded fine-grained single-phase BaZrO 3 powder after a
single-stage calcination at 8008C. The cubic perovskite structure was found to be stable up to 17008C, the maximum sintering temperature
employed in this work. Pure BaZrO 3 compacts with bulk density $90% of theoretical were successfully obtained using a two-stage
sintering profile. Benign / deleterious effects of three sintering aids (viz. Al 2 O 3 , MgO and Y 2 O 3 ) in terms of densification, shrinkage and
microstructural variation in BaZrO 3 were also evaluated.  2002 Elsevier Science B.V. All rights reserved.
Keywords: Ceramics; Chemical synthesis; Sintering; SEM
1. Introduction
Barium metazirconate (BaZrO 3 ) is a value-added material of great relevance in the field of technical and
electronic ceramics. Its general physical properties are
comparable or sometimes even superior to the much-used
zirconia, such as very high congruent melting point
(|26008C), simple cubic perovskite structure, small coefficient of thermal expansion, poor thermal conductivity and
excellent mechanical and structural integrity under extreme
thermal excursions [1–4]. These properties make it an
ideal candidate for applications in a multitude of areas.
Some of these are as:
• An inert substrate for thin film deposition
• A structural material, such as container crucibles for
reactions, melting and sintering experiments with oxides, non-oxides and precious alloys
• A dopant and ‘K-modifier’ in BaTiO 3 matrix
In addition, it is being viewed as an attractive alternative
*Corresponding author. Present address: NexTech Materials Ltd., 720-I
Lakeview Plaza Boulevard, Worthington, OH 43085, USA. Tel.: 11-614842-6606; fax: 11-614-842-6607.
E-mail address: [email protected] (A.-M. Azad).
1
External research student.
for the much investigated yttria-stabilized zirconia (YSZ)
as a thermal barrier coating (TBC) material for supersonic
jets by aerospace industries [5]. Keeping in line with the
great interest of materials scientists, materials engineers,
technologists and catalyst researchers, it also offers the
possibility of creating defect perovskites by proper doping
so as to either create oxygen-deficient compositions or
solid solution of the parent perovskite with another perovskite or even a simple oxide. The latter combination gives
rise to a useful tool for precise control of the microstructure and hence the properties of the host material
without giving rise to any appreciable structural variation.
For instance, BaZr 0.9 Y 0.1 O 2.95 has recently been demonstrated as a new proton conductor for use as a solid
electrolyte [6,7]. It has also been used in solid solutions to
impart mechanical strength to barium cerate-based proton
conductors [8].
In the light of these anticipated applications of BaZrO 3
either alone or in combination with other materials, a
systematic and thorough investigation on this system is
warranted. However, most of the available literature is
rather limited to the procedure to produce BaZrO 3 powder
through various preparative methods [9–18], while the
investigations reporting its dielectric properties are almost
40 years old [19,20]. Since making highly pure and fine
powder is very crucial to produce dense ceramic bodies
with high strength and benign microstructural features, the
0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved.
PII: S0925-8388( 01 )01785-6
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
study reported in this communication was carried out. The
results of materials synthesis, characterization, processing
and sintering of the green powder under various temperature-time profiles are reported. A wide range of
sintering temperatures (1200–17008C) and soak-times (0–
24 h) were chosen to examine the evolved microstructure
and its effect on the density and other parameters. Potential
sintering aids such as MgO, Al 2 O 3 and Y 2 O 3 were also
employed to attempt achieving higher densification and
benign microstructural features in the dense bodies.
2. Experimental
High purity (.99.9%) barium nitrate, Ba(NO 3 ) 2 , from
Wako Pure Chemical Industry and zirconyl nitrate hydrate,
ZrO(NO 3 ) 2 ?xH 2 O (x535 wt% determined gravimetrically), from Fluka were used for the synthesis of barium
zirconate powder. Stoichiometric amounts of the two
precursors (1:1 mol ratio) were accurately weighed so as to
yield about 50 g of the zirconate after calcination. The two
components were transferred to a polystyrene bottle and
ball milled for 4 h in ethanol using clean zirconia balls as
the milling medium. The ball-milled mixture was dried in
an air-oven overnight at 80–908C. The dried cake was
crushed into fine powder in an agate mortar and pestle.
Prior to calcination, simultaneous TG-DTA (Netzsch STA
409C) runs were performed on the ball-milled precursor in
the temperature range 25–12008C in static air at a heating
rate of 6008C h 21 . This helped to discern the approximate
temperature of decomposition of the constituent nitrates
119
and the compound formation. It also helped in identifying
if the decomposition of the nitrates and the formation of
BaZrO 3 was a single-stage synchronous process or a
multistage event. The dry powder mixture was uniaxially
pressed into pellets of 15 mm diameter and 3 mm
thickness, which were then calcined in air (heating rate:
48C min 21 ) in a box furnace at 800–10008C for 2–8 h.
Phase analysis by powder X-ray diffraction using mono˚ Ni filter and Si
chromatic Cu Ka radiation ( l51.5406 A),
standard was carried out on a Rigaku LK-1655 Diffractometer (Japan) after each calcination stage. This was to
determine the formation of target material and also to
detect the presence of, if any, unreacted starting material
and / or other new phase in the calcined mass. Horiba
(CAPA-700 model, Japan) particle size analyzer was used
to determine the size of the particles and their distribution
in the reacted powders. Densities of the calcined powders
were measured by pycnometry (He gas AccuPyc 1330,
Micromeritics, USA) while those of the sintered samples
were measured both by: (a) pycnometry and (b) Archimedes principle of water displacement. In the case of
sintered bodies, the ratio of (b) to (a) was used to indicate
the fractional theoretical density achieved in the samples at
a given temperature and time of sintering. The calcined
pellets were crushed and pulverized in an agate mortar into
fine powder. The powder was blended with |4 wt% PVA
(polyvinyl alcohol, solution in water, 40 g l 21 ), and dried
overnight in an air oven at 958C. High purity powders of
a-Al 2 O 3 , ZrO 2 , Y 2 O 3 (99.9, 99.995 and 99.95%, respectively, from Aldrich, WI, USA), MgO (99% from BDH,
Poole, UK) were used as sintering aids. The sintering aids
Fig. 1. Simultaneous thermal analysis (TG-DTA) signature of a 1:1 molar mixture of barium and zirconyl nitrates.
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A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
were added to the calcined BaZrO 3 powders to the extent
of 0.5, 2 and 5 wt% prior to PVA addition. The dried
mixture was pulverized again to fine powder, pressed into
pellets as described above and cold isostatically pressed
(CIPed) at 200 kgf for 60 s. They were sintered at
1200–17008C for 0#t#24 h in air. Microstructural features of the calcined powders as well as the sintered discs
were determined by using a Hitachi S2500 scanning
electron microscope (Japan).
3. Results and discussion
3.1. Pure BaZrO3
Simultaneous TG-DTA profile of a 1:1 mol mixture of
Ba(NO 3 ) 2 and ZrO(NO 3 ) 2 }xH 2 O heated in static air up
to 12008C at a ramp rate of 6008C h 21 presented in Fig. 1
shows interesting features. It appears that in the initial
stages, small weight loss attended by an exothermic DTA
peak around |1108C is due to the loss of remnant organics
(used as ball-milling media) and adsorbed moisture. This is
followed by a continuous weight loss up to 4708C attended
by another exothermic transition at around |4708C. This is
ascribed to the loss of lattice water in zirconyl nitrate. A
DTA peak at 592.98C (no weight change) is due to the
melting of Ba(NO 3 ) 2 (m.p.55928C). A large one-step
weight loss attended by a DTA peak between |610 and
7208C is suggestive of decomposition of barium and
zirconyl nitrates and reaction between fresh nuclei to form
barium zirconate in a single-stage synchronous process. A
broad but shallow DTA peak near 8108C is ascribed to the
result of crystallization of ZrO 2 present as a small impurity
phase. A rather small hump at |11508C is due to the m → t
transition in zirconia. This speculation is aided by the fact
that barring these two crystallographic events, beyond
8008C there is neither any discernible DTA peak nor any
further significant weight loss in the mixture up to
|12008C. Thus, the thermal analysis indicates that the
Fig. 3. Particle size distribution in powder calcined at 8008C / 8 h (top)
and 10008C / 4 h.
formation of crystalline BaZrO 3 occurs between 700 and
8008C. This is rather encouraging particularly in the case
of solid-state reaction (SSR) technique; SSR usually
requires high temperatures and several repetitions of the
‘heat and beat’ steps. We attribute this improvement in the
present case to the use of metal nitrates as the precursors
rather than the conventionally employed carbonate or
oxide; the metal nitrates have more favorable decomposition kinetics compared to their carbonate counterparts
[21–23]. This could be compared with the procedure of
Taglieri et al. [11] who reportedly obtained BaZrO 3 at
7008C via a citrate complex in aqueous solution. On the
Table 1
Calculated lattice parameters from XRD signatures on BaZrO 3 powders
with different thermal history
Fig. 2. X-ray diffraction patterns in 1:1 molar mixtures of Ba(NO 3 ) 2 and
ZrO(NO 3 ) 2 calcined at 8008C.
Thermal history
(T / 8C–t / h)
d 110
a (nm)
Vcell , a 3 (m)3
800–2
800–4
800–8
1000–4
2.962
2.962
2.970
2.960
0.419
0.419
0.420
0.419
73.56310 230
73.56310 230
74.09310 230
73.40310 230
Theoretical value
0.4193
73.72310 230
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
Fig. 4. Morphology of the powder calcined at 8008C / 8 h (L) and 10008C / 4 h (R).
Fig. 5. Comparative XRD patterns of BaZrO 3 sintered in the range 1200–17008C.
121
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A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
Fig. 6. Microstructural development (clockwise) in BaZrO 3 soaked for 6 h at 1400, 1500 and 15408C.
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
123
Fig. 7. Enhanced densification and intergranular connectivity in BaZrO 3 sintered at: (a) 16008C / 6 h, (b) 16008C / 12 h and (c) 16008C / 6 h117008C / no
soak. Microstructural features in commercially produced BaZrO 3 targets [12] are shown in (d).
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A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
other hand, Zhang and Evetts [10] succeeded in forming
only about 16% of BaZrO 3 via solid-state reaction at
9008C.
Since the thermal analysis showed that the decomposition and the compound formation temperature was between
650 and 8008C, the bulk raw powder was subsequently
calcined at 8008C for 2, 4 and 8 h. The X-ray diffraction
patterns of the calcined powders are shown in Fig. 2.
These results indicate that BaZrO 3 with cubic structure
(6-0399) was the main product formed. One or two weak
intensity peaks belonging to BaCO 3 and ZrO 2 could also
be identified. These minor impurity phases, however,
gradually reduced with increase in dwell time at 8008C as
well as with increase in calcination temperature, when in
one case the 1:1 mol mixture of the nitrates was heated at
10008C for 4 h. The bimodal distribution of the particle
size in powders calcined at 8008C / 8 h and 10008C / 4 h is
shown in Fig. 3. The particle size distribution in the
calcined mixtures showed that the size varied between 0–1
mm and 2–7 mm. In contrast to this, the powders obtained
by Taglieri et al. [11] via a solution route and calcined for
1–12 h at 7008C were shown to have a bimodal distribution with 80% of the particles lying between 60 and 4
mm and only about 10% in the 2-mm range.
The most intense reflections in the X-ray diffractograms
were used to compute the lattice parameters (cell edge and
volume). The computed results are presented in Table 1.
As can be seen, the experimental data agrees very well
with that reported in the literature as the standard value
(JCPDS card [ 6-0399). The powder morphology in
calcined samples is shown in Fig. 4. The agglomerate
characteristics in both pictures can easily be made out.
From this illustration, the typical agglomerate size is less
than 20 mm, while the particles constituting these agglomerates are smaller than 1 mm. The powder density measured with pycnometer show that the density increased
from 3.07 g cm 23 for raw powder to 5.49 g cm 23 for
powder calcined at 8008C for 4 h but reduces to 4.99
g cm 23 for powder calcined at 10008C. This is due to the
agglomeration effect, which increases with calcination
temperature; with larger number of agglomerates, the
powder packing reduces and increases the volume of the
sample. Thus, the density decreases.
In order to ascertain and establish the chemical state of
the end product in the fired compacts, XRD signatures
were collected on samples sintered over the entire range of
temperatures (viz. 1200–17008C). These are collectively
shown in Fig. 5. As can be readily seen from these
illustrations, the XRD signatures in all the cases correspond to that of pure BaZrO 3 without even traces of other
compound(s) in the BaO–ZrO 2 system or those belonging
to the starting materials and / or impurity phases. In addition, the analysis of the patterns and determination of
lattice parameters showed that throughout the temperature
range BaZrO 3 exists in cubic structure and not orthorhombic as claimed by Taglieri et al. [11]. Thus, with
Table 2
Density of sintered BaZrO 3 pellets measured by Archimedes’ principle
Thermal history
(T / 8C–t / h)
Sintered density
(mg m 23 )
1600–2
1600–4
1600–6
1600–611700–0
5.319
5.47
5.513
5.573
increasing temperature the sample does not undergo any
phase changes. These results again contradict those reported in Ref. [11], where the diffraction peaks belonging
to ZrO 2 could be seen even in samples calcined up to
13508C. The crystallite size ranged between 0.36 and 0.44
mm. Moreover, since the quality of X-ray diffractograms
does not necessarily change in bodies obtained from
calcined powders from 8008C / 8 h or 10008C / 4 h, the
powder calcined at 8008C for 8 h is good enough to use for
sintering. This could be compared with the results of
Zhang and Evetts [10] who reported the formation of
single phase BaZrO 3 at as high as 13008C. Taglieri et al.
[11] obtained unspecified ‘adequate purity in BaZrO 3
crystalline phase’ by heating a mixture of BaCO 3 and
ZrO 2 at 12008C for 1 h, repeating the heating cycle twice.
Interestingly, the BaZrO 3 powder on which the original
XRD data was generated, was obtained by heating an
Fig. 8. XRD signatures of alumina-added BaZrO 3 compacts sintered at:
(a) 16008C / 6 h and (b) 16008C / 6 h117008C / no soak.
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
125
Fig. 9. Microstructural artifacts in BaZrO 3 sintered at 16008C / 6 h with: (a) 0.5 (b) 2, and (c) 5 wt% alumina; excellent densification in samples with 5 wt%
alumina subjected to two-stage sintering (16008C / 6 h117008C / no soak) is evidenced in (d).
equimolar mixture of BaCO 3 1ZrO 2 at 15508C for 1 h
[24].
Since the structural and phase purity and the morphological features of powder obtained upon calcination of
the precursors at 8008C were almost identical to that from
10008C, the former was used in subsequent sintering
experiments. The microstructural development in samples
soaked for 6 h at 1400, 1500 and 15408C is shown in Fig.
6. The microstructure reveals better intergranular connectivity, systematic grain growth and steadily diminishing
porosity. While a significant amount of agglomerates are
present in samples sintered at 14008C / 6 h, individual
interconnected grains submicron in size could be seen in
bodies subjected to sintering at higher temperatures (Fig.
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6b and c). With increase in sintering temperature, densification improved systematically with corresponding decrease
in porosity (mainly open pores). The average size is less
than 1 mm in all the cases. However, it should be
mentioned that the bodies sintered in the range 1200–
14008C were quite fragile and had a rather low strength;
the sintered pellets could easily be broken under slight
pressure. On the other hand, up on sintering at higher
temperature, (1500–17008C), the compacts showed increase in strength, especially those sintered at 1600 and
17008C.
The systematic microstructural developments in BaZrO 3
samples soaked for 6 and 12 h at 16008C are shown in Fig.
7a and b, respectively. Phenomenal difference in all
aspects of morphological and microstructural artifacts can
be seen in these pictures. For example, the longer heat
treatments resulted in near 100% densification. The fractured surface morphology seen here clearly shows intergranular cleavage, emphasizing an excellent grain-to-grain
connectivity and well-defined grain boundaries in the bulk
of the material. The average grain size was about 3 mm
which remained nearly the same when the sample was
further heated at a slow rate of 18C min 21 from 1600 to
17008C without any dwell at the highest temperature (Fig.
7c). The microstructural density of the sintered bodies
agreed very well with that measured from Archimedes’
principle. The systematic improvement in the density with
sintering temperature and soak-time is summarized in
Table 2. For a ready comparison, the microstructural
features of BaZrO 3 sold commercially as target material
[12] are shown in Fig. 7d. The striking difference between
the quality of material produced and processed in the
present work and the commercial product is self-explanatory. Philips et al. [25] recently reported to have obtained as
high as 97.8% theoretical density in slip cast BaZrO 3 from
aqueous and non-aqueous slurries. However, their process
necessitated the use of temperatures between 1250 and
16508C to form BaZrO 3 . Thus, it can be confidently stated
that the quality of BaZrO 3 produced via modified yet
simple solid-state reaction technique in the present work
surpasses that of its commercially produced and marketed
counterpart.
The foregoing discussion on the processing, structural
and microstructural evolution in pure barium metazirconate
would form the basis of comparison of similarity or
disparity of these parameters in the bodies sintered in the
presence of an aid. To maintain the clarity of discussion,
the effect of various sintering aids is discussed separately
in the following subsections.
predominantly barium zirconate (compare with Fig. 4),
except for the presence of rather weak intensity peaks at
|28.1 and |34.28C (2-u ), showing traces of baddeleyite
(ZrO 2 ) in samples with 0.5 and 2 wt% alumina. These
diffraction peaks disappear in samples containing 5 wt%
Al 2 O 3 . Therefore, it may be concluded that the addition of
alumina up to 5 wt% does not cause any structural
degradation in BaZrO 3 upon sintering. The microstructural
features of the fractured surfaces in samples sintered at
16008C for 6 h are shown in Fig. 9a–c. It can be seen that
the intergranular connectivity is excellent and the samples
are quite dense, though the porosity is still significant. The
bulk density in these samples was found to slightly
decrease with increasing Al 2 O 3 content. For instance, the
measured density was 89.1, 87.3 and 82.8% theoretical in
compacts containing 0.5, 2 and 5 wt% Al 2 O 3 , respectively.
From the high magnification micrographs (not shown
here), the average grain size in these samples was estimated to be about 3–5 mm.
A two-stage sintering profile (as adopted in the case of
pure BaZrO 3 ) appeared to have mixed effect on the
microstructural artifacts and hence on the bulk density. In
the case of lower doping levels (0.5 and 2%), the density
was somewhat similar to those found in the above-mentioned specimens (viz. 90.4 and 86.1% theoretical) with
nearly identical morphological features. However, the
sample sintered with 5 wt% Al 2 O 3 registered remarkably
higher degree of densification (91.7% theoretical) as
evidenced by the superior microstructure shown in Fig. 9d.
It may be recalled that the XRD patterns of 5 wt% alumina
doped samples were the closest to that of pure BaZrO 3
without traces of any unknown impurities. It is likely that
the presence of unknown impurities in samples containing
0.5 and 2 wt% Al 2 O 3 might have had undesirable effects
on the density and microstructural artifacts, as seen from
the micrographs (Fig. 9a–c) for samples soaked for 6 h at
16008C. In absence of any further supporting experimental
evidence at this juncture, however, this point cannot be
over emphasized. From these results, it therefore appears
3.2. BaZrO3 with Al2 O3
Systematic phase evolution in alumina-added BaZrO 3
compacts sintered at 16008C / 6 h and 16008C / 6 h1
17008C / no soak, are displayed in Fig. 8a and b, respectively. Both the XRD signatures confirm that the material is
Fig. 10. XRD signatures of BaZrO 3 compacts sintered at 16008C / 6
h117008C / no soak with: (a) 0.5 (b) 2, and (c) 5 wt% MgO.
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
that 5 wt% alumina did have a benign influence on the
microstructure and the densification characteristics of
BaZrO 3 . Beside this, perhaps the most noticeable impact
127
of alumina (all levels of doping) is the controlled grain
growth in the sintered samples, attended by a rather small
(|2%) dimensional shrinkage in the final body.
Fig. 11. Microstructures evolved in: (a) 0.5 and (b) 2 wt% MgO-added BaZrO 3 sintered at 16008C / 6 h; micrographs (c)–(e) show the microstructural
features in BaZrO 3 compacts sintered at 16008C / 6 h117008C / no soak with 0.5, 2 and 5 wt% MgO, respectively.
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A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
3.3. BaZrO3 with MgO
Fig. 10 presents the X-ray diffractograms of BaZrO 3
compacts incorporating different levels of magnesia and
sintered via 16008C / 6 h117008C / no soak scheme. It is
apparent that the diffraction patterns remain identical to
each other and also match excellently with those for pure
BaZrO 3 shown earlier.
Like Al 2 O 3 , MgO is a well-known microstructural
stabilizer and grain growth modifier [26]. Its role as a
sintering aid has been explained as the cause of reduction
in grain growth by suppressing / eliminating continuous
Fig. 12. Microstructural evolution in BaZrO 3 with 0.5 wt% yttria sintered at 16008C for: (a) 2 and (b) 6 h; compacts of the same compositions sintered at
16008C / 6 h117008C / no soak are shown in (c).
A.-M. Azad et al. / Journal of Alloys and Compounds 334 (2002) 118 – 130
grain growth via slower boundary migration during the
sintering process. Fig. 11a and b show typical microstructures evolved in 0.5 and 2 wt% MgO-added BaZrO 3 ,
sintered at 16008C / 6 h. Features such as excellent grainto-grain connectivity, well-defined grain boundaries and
suppressed grain growth with some remnant porosity are
highlighted in these micrographs. The measured bulk
density in these specimens was 90.4 and 89.6% theoretical,
respectively. This is only marginally better than that
registered in alumina-containing bodies. However, when
the samples soaked for 6 h at 16008C were further heated
slowly to 17008C without any soaking, enhanced grain
growth seemed to have set in, thereby causing increased
porosity and the bulk density decreased slightly. In this
case grains as large as |10 mm were formed. These
microstructural features are shown in Fig. 11c–e. The
abnormal grain growth in BaZrO 3 containing 5 wt% MgO
is easily seen. Thus, MgO addition seems to be somewhat
effective in the 16008C / 6 h sintering cycle alone, as above
this temperature deleterious effects become operative.
Possibly, a soak-time longer than 6 h would be more
effective in increasing the compactness in the sintered
bodies, thereby reducing / eliminating porosity.
3.4. BaZrO3 with Y2 O3
The XRD patterns in the sintered yttria-added barium
metazirconate compacts did not suggest any degradation of
the major component as was the case in alumina and
magnesia added-samples and hence are not shown here.
The most salient feature of yttria addition was the grain
growth inhibition as seen from the micrographs presented
in Fig. 12a,b in compacts (with 0.5 wt% Y 2 O 3 ) soaked for
2 and 6 h, respectively, at 16008C. Another interesting
feature of these micrographs is the uniformity of the grain
size and very narrow grain size distribution. Philips et al.
[25] had also found that yttria addition was effective in
slightly reducing the grain size. Grain growth was observed in samples treated to 16008C / 6 h117008C / no soak
scheme, as seen from Fig. 12c. Thus, compared to Al 2 O 3
and MgO addition, there is an enhancement in density but
the remnant porosity could not be eliminated totally even
after sintering up to 17008C. In any case, 0.5 wt% yttria
addition resulted in densities of the order of 91 and 92%
theoretical under the firing condition employed in this
work. From this perspective, yttria appears to be the most
benign single oxide sintering aid, since the smallest level
of doping resulted in the highest density in the sintered
BaZrO 3 compacts.
4. Conclusions
Usage of a modified solid-state reaction technique
employing nitrate precursors leading to the successful
synthesis of phase pure barium metazirconate at record low
129
temperatures (ca. 8008C) has been described. Powders with
submicron size particles could be sintered into bodies with
near theoretical density in a two-stage heating process:
16008C / 6 h117008C / no soak. Such characteristics are
crucial for the usage of BaZrO 3 as a target material for
thin film deposition and as a container material due to its
exceptional and exemplary structural stability and material
integrity up to very high temperatures.
Addition of various sintering aids had mixed effects on
the microstructural and densification aspects of BaZrO 3
compacts. The most striking feature of the sintering aid
addition was reflected in the grain growth inhibition as the
sintering progressed. The presence of Al 2 O 3 , MgO and
Y 2 O 3 also helped in maintaining the spheroid morphology
of the grains and narrowing the grain size distribution.
Moreover, these aids assisted in preventing the samples
from dimensional variation (shrinkage about 2% only). In
the case of Al 2 O 3 addition, the density of the sintered
bodies increased systematically with firing temperature and
soak-time for a given level of doping; the density, however, showed a decrease for a given sintering cycle with
increasing alumina content, except in the case of 5 wt%
addition. In comparison, magnesia was less effective in
imparting any remarkable densification improvement. In
fact, abnormal grain growth attended by increased porosity
was observed in 5 wt% MgO-added BaZrO 3 . Samples
containing 0.5 wt% Y 2 O 3 registered the highest density
(92% theoretical). In general, BaZrO 3 containing 0.5 wt%
of the additives were all |90% or more dense. From
pycnometric measurements, 5 wt% Al 2 O 3 and 0.5 wt%
Y 2 O 3 yielded the densest BaZrO 3 compacts.
Acknowledgements
One of the authors (SS) wishes to express his gratitude
to Wan Zaharah, General Manager, CTC, for allowing the
use of various experimental facilities during the course of
this work. He also wishes to thank Dr S. Ramesh for useful
discussion.
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