Materials Research Bulletin 37 (2002) 85±97
Synthesis of BaZrO3 by a solid-state reaction
technique using nitrate precursors
Abdul-Majeed Azada,*, Selvarajan Subramaniamb,1
a
Advanced Materials Research Center, SIRIM Berhad, 1 Persiaran Dato' Menteri,
Section 2, 40911 Shah Alam, Selangor, Malaysia
b
Ceramics Technology Center, SIRIM Berhad, 1 Persiaran Dato' Menteri, Section 2,
40911 Shah Alam, Selangor, Malaysia
(Refereed)
Received 22 June 2001; received in revised form 21 September 2001; accepted 19 October 2001
Abstract
High phase purity barium metazirconate powders have been synthesized from a modi®ed
solid-state reaction. Reactive powders consisting of submicron particles and narrow particle
size distribution were obtained by heating a 1:1 molar mixture of barium nitrate and zirconyl
nitrate at 8008C up to 8 h. Simultaneous thermal analysis (TG-DTA) assisted in elucidating
the probable reaction pathways leading to the formation of the target compound in the BaO±
ZrO2 system. Systematic structural and microstructural characterization on the green powders
and the compacts sintered up to 17008C were carried out. A two-stage sintering schedule
consisting of a 6 h soak at 16008C followed by slow heating up to 17008C with no dwell, led to
highly dense microstructural features. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction
1. Introduction
The alkaline-earth zirconates having the general chemical formula MZrO3
(M ˆ Ca, Sr and Ba) with perovskite structure have been projected as potential
structural and electronic ceramics. In suitable doped forms they have been claimed to
*
Corresponding author. Present address: NexTech Materials, Ltd., 720-I Lakeview Plaza Boulevard,
Worthington, OH 43085, USA. Tel.: ‡1-614-842-6606; fax: ‡1-614-842-6607.
E-mail address: [email protected] (A.-M. Azad).
1
External Research Student.
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 8 0 1 - 7
86
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
become ionic and/or electronic conductors. Corresponding titanates, BaTiO3 and
SrTiO3 are well-known electroceramic material and commercially produced as low
dielectric constant, high resistance and low TCK (temperature coef®cient of dielectric
constant) components. However, there is a lack of reliable technical information on
the BaZrO3 system in the published literatures. Most of the available literature is
rather limited to the procedure to produce BaZrO3 powder through various preparative methods [1±8], while the investigations reporting its dielectric properties are
almost 40 years old [9±11]. Thus, in view of the importance of BaZrO3 system as
potential ceramics for applications such as:
inert substrate for thin film deposition;
structural material such as container crucibles for reaction, melting and sintering experiments with other oxides and no-oxides;
a dopant and ``K-modifier'' in BaTiO3 matrix;
and the information gaps in the reported research, this study was taken up. This
investigation was aimed to study the systematic trend in the properties of the ceramic
powder of BaZrO3 and the dense pellets made thereof. Synthesis of BaZrO3 in phase
pure form has been carried out by a modified solid-state reaction (SSR) technique.
The objective was to find the most suitable starting materials and processing route to
provide BaZrO3 in terms of purity, easy of preparation and economy. The XRD
signatures of the calcined powders confirmed the compound formation in `phasepure' form. This was also corroborated by simultaneous thermal analyses (TG-DTA).
A wide range of sintering temperatures (1200±17008C) and soak-time (0±24 h)
profiles were chosen to examine the evolved microstructure and its effect on the
density and other parameters. This paper presents the results of such an investigation
leading to the development of high-density BaZrO3 bodies from highly reactive raw
powders.
2. Experimental
Barium nitrate, Ba(NO3)2, from Wako Pure Chemical Industry and zirconyl nitrate
hydrate, ZrO(NO3)2xH2O (x ˆ 35 wt.% determined gravimetrically), from Fluka
were employed for the synthesis of barium zirconate powder. Soichiometric amounts
of the two precursors (1:1 molar 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 ®ne 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 108C/min. This helped to discern the approximate temperature
of decomposition of the constituent nitrates and the compound formation. It also
helped in identifying if the decomposition of the nitrates and the formation of
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
87
BaZrO3 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) in a box furnace at
800±10008C for 2±8 h.
Phase analysis by powder X-ray diffraction (XRD) using monochromatic Cu Ka
Ê ) was carried out on a Rigaku LK-1655 Diffractometer
radiation (l ˆ 1:5406 A
(Japan) after each calcination stage. This was done 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 estimate the particle size and their distribution in the
reacted powders. Densities of the calcined powders as well as the sintered samples
were measured both by: (a) pycnometry (He gas AccuPyc 1330, Micromeritics, USA)
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 ®ne powder. The powder was blended
with 4 wt.% (polyvinyl alcohol (PVA) solution in water, 40 g/l), and dried overnight
in an air oven at 958C. The dried mixture was pulverized again to ®ne powder, pressed
into pellets as described above and cold isostatically pressed (CIPed) at 200 kg f 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); in the latter case, the
samples were fractured and polished.
3. Results and discussion
Simultaneous TG-DTA pro®le of a 1:1 molar mixture of Ba(NO3)2 and ZrO(NO3)2xH2O heated in static air up to 12008C at a ramp rate of 108C/min 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. An
exothermic DTA peak at 592.98C (no weight change) is due to the melting of
Ba(NO3)2 (mp ˆ 5928C). 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 ZrO2 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 signi®cant weight loss in the
mixture up to 12008C. Thus, the thermal analysis indicates that the formation of
88
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
Fig. 1. Simultaneous thermal analysis (TG-DTA) signature of a 1:1 molar mixture of barium and zirconyl nitrates.
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
89
crystalline BaZrO3 occurs between 700 and 8008C. This is rather encouraging,
particularly in the case of solid-state technique where usually high temperatures and
several repetitions of the `heat and beat' steps are required. 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 [12,13].
This could be compared with the procedure of Taglieri et al. [3] who reportedly
obtained BaZrO3 at 7008C via a citrate-complex in aqueous solution. On the other
hand, Zhang and Evetts [2] succeeded in forming only about 16% of BaZrO3 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 XRD patterns of the calcined
powders are shown in Fig. 2. These results indicate that BaZrO3 with cubic structure
(6-0399) was the main product formed. One or two weak intensity peaks belonging to
BaCO3 and ZrO2 could also be identi®ed. 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 molar 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 ranged between 0±1 and 2±7 mm. In
contrast to this, the powders obtained by Taglieri et al. [3] via a solution route and
Fig. 2. X-ray diffraction patterns in 1:1 molar mixtures of Ba(NO3)2 and ZrO(NO3)2 calcined at
8008C.
90
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
Fig. 3. Particle size distribution in powder calcined at 8008C/8 h (a) and 10008C/4 h (b).
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 (1 1 0) re¯ections 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 # 6-399). The powder morphology in calcined
samples is shown in Fig. 4. The agglomerate characteristics in both the pictures can
easily be made out. From this illustration, it is quite apparent that the typical
Table 1
Calculated lattice parameters (standard deviation in `a' ˆ 0:001) from XRD signatures on BaZrO3
powders with different thermal history
Thermal history, T (8C)±t (h)
d1 1 0
a (nm)
Vcell, a3 (m3)
800±2
800±4
800±8
1000±4
Theoretical value
2.962
2.962
2.970
2.960
0.419
0.419
0.420
0.419
0.4193
73.56
73.56
74.09
73.40
73.72
10
10
10
10
10
30
30
30
30
30
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
Fig. 4. Morphology of the powder calcined at 8008C/8 h (a) and 10008C/4 h (b).
91
92
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
agglomerate size was less than 20 mm, while the particles constituting these agglomerates were smaller than 1 mm. The powder density measured with pcynometer
showed that the density increased from 3.07 g/cm3 for raw powder to 5.49 g/cm3 for
powder calcined at 8008C for 4 h but reduces to 4.99 g/cm3 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 reduces.
In order to ascertain and establish the chemical state of the end product in the ®red
compacts, XRD signatures were collected on samples sintered in 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 BaZrO3 phase without even traces of other compound(s) in BaO±ZrO2
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 BaZrO3 exists in cubic structure and not
orthorhombic as claimed by Taglieri et al. [3]. Thus, with increasing temperature, the
sample does not undergo any phase changes. These results again are in contradiction
to those reported in [3], where the diffraction peaks belonging to ZrO2 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 [2] who reported the formation of
Fig. 5. Comparative XRD patterns of BaZrO3 sintered in the range 1200±17008C.
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
93
single phase BaZrO3 at as high as 13008C. Taglieri et al. [3] obtained unspeci®ed
`adequate purity in BaZrO3 crystalline phase' by heating a mixture of BaCO3 and
ZrO2 at 12008C for 1 h, repeating the heating cycle twice. Interestingly, the BaZrO3
powder on which the original reported XRD data was generated, was obtained by
heating an equimolar mixture of BaCO3 ‡ ZrO2 at 15508C for 1 h [14].
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 signi®cant amount of agglomerates are
present in samples sintered at 14008C for 6 h, individual interconnected grains
submicron in size could be seen in bodies subjected to sintering at higher temperatures
(Fig. 6b and c). With increase in sintering temperature, densi®cation 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, upon sintering at higher temperature (1500±17008C), the compacts showed
increase in strength, especially those sintered at 1600 and 17008C.
The systematic microstructural developments in BaZrO3 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% densi®cation.
The fractured surface morphology seen here clearly shows intergranular cleavage,
emphasizing an excellent grain-to-grain connectivity and well-de®ned 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 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
Table 2
Density of sintered BaZrO3 pellets measured by Archimedes' principle (two pellets used for each
sintering pro®le; the listed values represent average values of density measured three times on each
sintered pellet)a
Thermal history, T (8C)±t (h)
Sintered density (g/cm3)
1600±2
1600±4
1600±6
1600±6 ‡ 1700±0
5.319
5.47
5.513
5.573
a
Estimated error in density measurements ˆ 2%.
94
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
Fig. 6. Microstructural development (clockwise) in BaZrO3 soaked for 6 h at 1400, 1500 and 15408C.
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
95
Fig. 7. Enhanced densi®cation and intergranular connectivity in BaZrO3 sintered at (clockwise)
16008C/6 h, 16008C/12 h and 16008C/6 h ‡ 17008C/no soak.
96
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
comparison, the microstructural features of BaZrO3 sold commercially as target
material [4] are shown in Fig. 8. The striking difference between the quality of
material produced and processed in the present work (Fig. 7) and the commercial
product (Fig. 8) is self-explanatory. Thus, it can be con®dently stated that the
quality of BaZrO3 produced via modi®ed yet simple solid state reaction technique
in the present work surpasses that of its commercially produced and marketed
counterpart.
Fig. 8. Microstructural features in commercially produced BaZrO3 targets [4].
A.-M. Azad, S. Subramaniam / Materials Research Bulletin 37 (2002) 85±97
97
4. Conclusions
Usage of a modi®ed solid-state reaction technique employing nitrate precursors
leading to the successful synthesis of phase pure barium metazirconate at record low
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 h ‡ 17008C/no soak. Such characteristics are crucial for the usage
of BaZrO3 as a target material for thin ®lm deposition and as a container material due
to its exceptional and exemplary structural stability and material integrity up to very
high temperatures.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
M. Rajendran, M.S. Rao, J. Mater. Res. 9 (1994) 2277.
J.L. Zhang, J.E. Evetts, J. Mater. Sci. 29 (1994) 778.
G. Taglieri, M. Tesigni, P.L. Villa, C. Mondelli, J. Inorg. Chem. 1 (1999) 103.
http://www.superconductivecomp.com/BaZrO3target.html.
R.H. Arendt, US Patent 4293,534 (1981).
S. Uediara, M. Suzuki, H. Yamanoi, H. Tamura, US Patent 4595,580 (1986).
J.A. Davies, S. Dutremez, US Patent 5082,812 (1992).
T.F. Grigor'eva, A.P. Barinova, A. Vorsina, G.N. Kryukova, V.V. Doldyrev, Russ. J. Inorg. Chem.
43 (1998) 1594.
H. Stetson, B. Schwartz, J. Am. Ceram. Soc. 44 (1961) 420.
(a) S. Roberts, Phys. Rev. 81 (1951) 865;
(b) H. GraÈnicher, Helv. Phys. Acta 24 (1951) 619.
J. Koenig, B. Jaffe, J. Am. Ceram. Soc. 47 (1964) 87.
A.-M. Azad, N.C. Hon, J. Alloys Comp. 270 (1998) 95.
A.-M. Azad, L.L.W. Shyan, P.T. Yen, J. Alloys Comp. 282 (1999) 109.
US NBS Circular 539 (1955) 58.