International Journal of Inorganic Materials 1 (1999) 235–241
Combustion synthesized ZnO powders for varistor ceramics q
˜ b , *, M.R. Morelli a , R.H.G.A. Kiminami a
V.C. Sousa a , A.M. Segadaes
a
˜ Carlos, Department of Materials Engineering, 13565 -905 Sao
˜ Carlos SP, Brazil
Federal University of Sao
University of Aveiro, Department of Ceramics and Glass Engineering, UIMC, 3810 -193 Aveiro, Portugal
b
Abstract
Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the
strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of
pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as
fuel. The mixtures were ignited at 5008C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by
XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level
homogeneity. Implications on sintering and electrical behaviour are discussed. 1999 Elsevier Science Ltd. All rights reserved.
Keywords: A. electronic materials; A. ceramics; B. chemical synthesis; C. electron microscopy
1. Introduction
Zinc oxide varistors are electronic ceramic devices
whose primary function is to sense and limit transient
voltage surges and to do so repeatedly without being
destroyed [1]. In 1958, Kosman and Gesse [2] reported, for
the first time, on the non-linear properties of zinc oxide
based materials but raised little notice in the industrial
world. In the early 1970s, Matsuoka et al. [3], of the
Matsushita Electronic Components company, patented the
varistor effect as a result of research carried out on rectifier
contacts between a semiconducting ceramic (zinc oxide)
and a metal (silver). Since it was discovered that zinc oxide
ceramics containing Bi 2 O 3 and other metal oxides as
additives exhibit highly non-ohmic voltage–current characteristics, these ceramics have been widely used for voltage
stabilization and transient surge absorption in electronic
circuits [4].
Typical ZnO varistor ceramics contain more than 90
mol% ZnO and the composition is balanced by the
incorporation of such additives as Bi 2 O 3 , Sb 2 O 3 , CoO,
MnO, Cr 2 O 3 and sometimes also SiO 2 and SnO 2 . The
addition of Bi 2 O 3 affects the non-linearity of the current–
voltage characteristics and aids sintering through the
q
Paper presented at the First International Conference on Inorganic
Materials, Versailles, France, 16–19 September, 1998.
*Corresponding author.
˜
E-mail address: segadaes@cv.ua.pt (A.M. Segadaes)
development of a Bi 2 O 3 -rich liquid phase, while oxides
such as Co 3 O 4 , Cr 2 O 3 , MnO 2 and Sb 2 O 3 are mostly grain
growth inhibitors and are added because the varistor
breakdown voltage is inversely proportional to the ZnO
average grain size. These ceramics usually consist of ZnO,
Zn 7 Sb 2 O 12 spinel and Bi 2 O 3 -rich phases, and sometimes
Zn 2 Bi 3 Sb 3 O 14 pyroclore, depending on the additives used
[4], which suggests that the microstructure development is
mostly governed by the phase equilibria in the ternary
system ZnO–Bi 2 O 3 –Sb 2 O 3 [1,4–8]. Part of the grain
growth inhibition is due to the formation of the spinel
second phase whose particles pin down the grain
boundaries. If the grain boundary pinning spinel particles
are formed early during the sintering process, when the
ZnO particles are very small and similar in size, the
particle drag mechanism, which reduces the grain growth
rate, also prevents the occurrence of discontinuous or
abnormal grain growth. Thus, it is very important that the
element distribution in the microstructure is highly
homogeneous, which must be achieved prior to sintering,
during the powder preparation process. These ceramics are
commonly produced by the conventional solid state route
(mixture of oxides), but several other elaborate wet-chemical routes such as sol–gel, coprecipitation and Pechini
have also been used and are reported in the literature
[9–12].
It is well known that ceramic powder synthesis entails
some difficulties, especially in the case of complex compositions. Chemical homogeneity is nearly impossible to
guarantee by mechanical blending and grinding processes.
1466-6049 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved.
PII: S1466-6049( 99 )00036-7
236
V.C. Sousa et al. / International Journal of Inorganic Materials 1 (1999) 235 – 241
Homogeneous powders are also difficult to produce by the
precipitation route because often the various constituents
precipitate at different pH values. The sol–gel methods
enable the preparation of pure and homogeneous powders
but can be elaborate and expensive techniques and, frequently, the product contains some water and organic
residue. The ZnO varistor powders obtained by the Pechini
gel pyrolysis method contain very fine particles and have
been shown to sinter at lower temperatures than typical
ball-milled varistor powders, but sometimes ZnO and the
additives are not well mixed. Scale-up of such methods for
industrial production is often impractical, due to either the
high costs or the sophistication of the techniques involved.
The synthesis of doped ZnO varistor powders using
combustion reactions, which provides good compositional
control, is an alternative worth pursuing. Like the various
other methods that have been proposed and used to prepare
ceramic powders, the combustion synthesis route enables
synthesis at low temperatures and the products obtained
are in a finely divided state with large surface areas. Unlike
the former, combustion synthesis offers such added advantages as the simplicity of experimental set-up, the surprisingly short time between the preparation of the reactants
and the availability of the final product, savings in external
energy consumption and the equally important potential of
simplifying the processing prior to forming, providing a
simple alternative to other elaborate techniques [13–19].
Briefly, the combustion synthesis technique begins with
the mixture of reactants that oxidize easily (such as
nitrates) and a suitable organic fuel (such as urea,
CO(NH 2 ) 2 ) that acts as a reducing agent. The mixture is
brought to boil until it ignites and a self-sustaining and
rather fast combustion reaction takes off, resulting in a dry,
usually crystalline and unagglomerated, fine oxide powder.
While redox reactions such as this are exothermic in nature
and often lead to explosion if not controlled, the combustion of metal nitrate–urea mixtures usually occurs as a
self-propagating and non-explosive exothermic reaction.
The large amount of gases formed can result in the
appearance of a flame, which can reach temperatures in
excess of 10008C.
By simple calcination, the metal nitrates can, of course,
be decomposed into metal oxides upon heating to or above
the phase transformation temperature. A constant external
heat supply is necessary in this case, to maintain the
system at the high temperature required to accomplish the
synthesis of the appropriate phase. In combustion synthesis, the energy released from the exothermic reaction
between the nitrates and the fuel, which is usually ignited
at a temperature much lower than the actual phase formation temperature, can rapidly heat the system to a high
temperature and sustain it long enough, even in the
absence of an external heat source, for the synthesis to
occur.
The basis of the combustion synthesis technique comes
from the thermochemical concepts used in the field of
propellants and explosives. The need for a clear indication
of the effective constitution of a fuel-oxidizer mixture led
Jain et al. [20] to devise a simple method of calculating the
oxidizing to reducing character of the mixture. The method
consists on establishing a simple valency balance, irrespective of whether the elements are present in the oxidizer
or in the fuel components of the mixture, to calculate the
stoichiometric composition of the redox mixture which
corresponds to the release of the maximum energy for the
reaction. The assumed valencies are those presented by the
elements in the usual products of the combustion reaction,
which are CO 2 , H 2 O and N 2 . Therefore, the elements
carbon and hydrogen are considered as reducing elements
with the corresponding valencies 14 and 11, oxygen is
considered an oxidizing element with the valency 22, and
nitrogen is considered as having a valency of zero. The
extrapolation of this concept to the combustion synthesis
of ceramic oxides means that metals like zinc and bismuth
(or any other metals) should also be considered as reducing
elements with the valencies they have in the corresponding
oxides, i.e. 12 and 13.
Besides urea, various other fuels [13,19] have been used
in the combustion synthesis of a variety of single and
mixed ceramic oxides, all of them containing nitrogen but
differing in ‘reducing power’ and the amount of gases they
generate, which obviously affects the characteristics of the
reaction products. The reaction is not isothermal and larger
amounts of gases dissipate more heat, thereby, preventing
the oxides from sintering, since the temperature reached is
not so high. The coincident sintering effect in the highertemperature reactions may result in a loss of sub-micron
features of the powders. Urea has the lowest reducing
power (total valencies 16) and produces the smallest
volume of gas (4 mol / mol of urea). For most purposes, it
is the most convenient fuel to use: it is readily available
commercially, cheap and generates the highest temperature, although fuel-rich mixtures might produce prematurely sintered particle agglomerates [14].
As oxidizers, metal nitrates are the preferred salts
because they also contain nitrogen, are water soluble (a
good homogenization can be achieved in the solution) and
a few hundred degrees are usually enough to melt them.
Hydrate salts are even more favoured in this respect,
although the water molecules do not affect the total
valencies of the nitrate and are, therefore, irrelevant for the
chemistry of the combustion. The total valencies in all
divalent metal nitrates (e.g. Zn(NO 3 ) 2 .6H 2 O) add up to
210; and the total valencies in all trivalent metal nitrates
(e.g. Bi(NO 3 ) 3 .5H 2 O) add up to 215.
The work that follows describes the synthesis of pure
and doped ZnO varistor powders by the combustion
reaction of redox mixtures of the corresponding metal
precursors with urea. X-ray diffraction, scanning and
transmission electron microscopy, particle size distribution
and BET specific surface area were the techniques used to
characterize the resulting powders.
V.C. Sousa et al. / International Journal of Inorganic Materials 1 (1999) 235 – 241
237
2. Experimental procedure
3. Results and discussion
The combustion synthesis of pure ZnO was investigated
in detail. The results were then extrapolated to encompass
the addition of various dopants in the systems: ZnO–Bi 2 O 3
(ZB, 99.5:0.5, molar), ZnO–Bi 2 O 3 –Sb 2 O 3 (ZBS,
98.5:0.5:1.0, molar), ZnO–Bi 2 O 3 –Sb 2 O 3 –CoO (ZBSC,
98.0:0.5:1.0:0.5, molar), ZnO–Bi 2 O 3 –Sb 2 O 3 –CoO–MnO
(ZBSCM, 97.5:0.5:1.0:0.5:0.5, molar), and ZnO–Bi 2 O 3 –
Sb 2 O 3 –CoO–MnO–Cr 2 O 3
(ZBSCMK,
97.0:0.5:1.0:0.5:0.5:0.5,
molar).
CarloErba
Zn(NO 3 ) 2 .6H 2 O, Bi(NO 3 ) 3 .5H 2 O, Co(NO 3 ) 2 .6H 2 O,
Mn(NO 3 ) 2 .5H 2 O, Cr(NO 3 ) 3 .9H 2 O and Sb 2 O 3 were used
as cation precursors and LabSynth CO(NH 2 ) 2 as fuel. The
appropriate amounts of the reactants (batches were calculated on a basis of 20 g of zinc nitrate), with a little added
water, were first melted in a wide-mouth vitreous silica
basin (|200 cm 3 ) by rapid heating up to |3008C on a
hot-plate inside a fume-cupboard, under ventilation. Once
the liquid had thickened and began to froth, the basin was
transferred to a box furnace preheated at 5008C where
ignition took place. The maximum temperature reached
was measured with a Chromel-Alumel (K type) thermocouple whose hot junction was placed right over the
mouth of the vitreous silica basin. The reaction lasted for
less than 1 min and produced a dry and very fragile foam,
that easily crumbled into powder. This foam was then
lightly ground in the silica basin with a porcelain pestle
and the powder sieved through a 200 mesh screen.
The as-prepared combustion reaction powder was characterized by X-ray diffraction (XRD) (Cuka / Ni Carl Zeiss
TUR M62 diffractometer, with a scanning rate of 28
2u / min, in a 2u range of 30–758); scanning electron
microscopy (SEM) (Carl Zeiss DSM 940 A, after Au
coating) and transmission electron microscopy (TEM)
(LaB 6 field emission HITACHI S-4100, after Au / Pd
coating). The lattice parameters were calculated from the
X-ray diffraction patterns using a least squares fit. The
BET specific surface area and the average particle size
were determined in N 2 / He with a Quantasorb QuantaChrome apparatus.
To produce ZnO by the combustion route using urea as
fuel, crystalline Zn(NO 3 ) 2 .6H 2 O can be used as a Zn
source (total valencies 210). From a thermodynamic point
of view, the decomposition reaction of 1 mol of zinc
nitrate to produce 1 mol of zinc oxide can be one among
various alternatives, leading to the evolution of different
gases, as follows:
Zn(NO 3 ) 2 .6H 2 O (c) ⇒ ZnO (c) 1 6H 2 O ( g) 1 N 2 ( g) 1
2.5O 2 (g)
(1)
Zn(NO 3 ) 2 .6H 2 O (c) ⇒ ZnO (c) 1 6H 2 O ( g) 1 2NO ( g) 1
1.5O 2( g)
(2)
Zn(NO 3 ) 2 .6H 2 O (c) ⇒ ZnO (c) 1 6H 2 O ( g) 1 N 2 O 5(g )
(3)
Using the thermodynamic data listed in Table 1, the free
energy changes, DG, involved in each reaction can be
calculated as a function of temperature and are plotted in
Fig. 1. The free energy change as a function of temperature
for the combustion reaction of urea is also represented in
Fig. 1. The curves in Fig. 1 show that, while the combustion of urea is always exothermic (spontaneous), the three
alternative reactions that produce ZnO are endothermic at
low temperature, and the reaction described by Eq. (1)
becomes spontaneous above 4638C, remaining the most
favoured reaction up to |12008C.
Table 2 lists the various reactions involved in the
combustion synthesis and the corresponding enthalpy
change, starting with reaction R1 which describes the
combustion reaction of urea (total valencies 16). This
reaction, being exothermic, should supply the heat needed
for the synthesis reaction. The decomposition reaction of
the precursor nitrate, leading to the corresponding oxide, is
listed in Table 2 as reaction R2.
Direct use of the propellant chemistry criterion [14], to
determine the urea needed to balance the total oxidizing
Table 1
Relevant thermodynamic data [21,22]
Compound a
DH of (258C) (kcal / mol)
DG fo (258C) (kcal / mol)
Cp (cal / mol / K)
Zn(NO 3 ) 2 .6H 2 O (c)
CO(NH 2 ) 2(c)
ZnO (c)
H 2 O( g )
CO 2( g )
N 2( g )
O 2( g )
NO ( g )
N 2 O 5( g )
2550.92
279.71
283.24
257.796
294.051
0
0
21.57
2.7
2423.79
247.04
276.08
254.634
294.26
0
0
20.69
28.13
72.2
22.26
9.62
7.2010.00360 T b
10.3410.00274 T b
6.5010.00100 T b
5.9210.00367 T b,c
6.4610.00179 T b,c
5.1310.0817 T b,c
a
(c)5Crystalline, (g)5gas.
T5Absolute temperature.
c
Calculated from discrete values.
b
V.C. Sousa et al. / International Journal of Inorganic Materials 1 (1999) 235 – 241
238
Fig. 1. Effect of temperature on the Gibbs free energy change of the various ZnO synthesis reactions from zinc nitrate, depending on the reaction products.
and reducing valencies in the mixture of oxidizer and fuel,
leads to:
s 2 10d 1 ns 1 6d 5 0,
and the stoichiometric composition of the redox mixture,
to release the maximum energy for the reaction, would
demand that n51.67 mol of urea were used. The overall
synthesis reaction, which is endothermic and, therefore,
requires the use of urea, would be reaction RT15R21
mR1 (Table 2). From the thermodynamic point of view
and based on the data in Table 2, for the ZnO synthesis
reaction RT1 to occur at 258C, on the basis of enthalpy
change solely, (DH 0RT1 5 1120.91m(2129.9)50), only
m50.93 mol of urea would be needed.
These 0.93 moles of urea provide the enthalpy requirement for complete decomposition at 258C and release of all
the corresponding gases, as predicted by reaction RT1 (i.e.
7.86H 2 O ( g) 11.93N 2(g) 11.11O 2( g) 10.93CO 2(g) ).
However, this temperature is not enough to promote the
crystallization of the oxide. Even to raise the temperature
to the required minimum of 4638C for the spontaneous
decomposition of the nitrate (the heat generated will be
absorbed by the evolved gases and the oxide), the combustion of extra moles of urea is necessary. Using the relevant
heat capacities listed in Table 1, the total urea content
needed was calculated to be 1.34 mol. This value is still
lower than that specified by the propellant chemistry
calculations.
The combustion of the extra 0.74 moles of urea specified
by the propellant chemistry calculations (i.e. 1.6720.935
0.74 mol), will raise the temperature of all the final
products to 7228C. This temperature is high enough to
decompose the nitrate and should be sufficient to promote
the crystallization of the oxide. At this temperature,
however, the decomposition reaction described by Eq. (3)
also becomes spontaneous (Fig. 1). The two decomposition
reactions (R2 and R3, in Table 2), although very close in
enthalpy, lead to rather different flame temperatures in the
combustion with urea (reactions RT1 and RT2, in Table 2),
as shown in Fig. 2, the actual evolution of N 2 O 5 lowering
that temperature.
Experimentally, the combustion reactions carried out
with the stoichiometric urea content specified by the
propellant chemistry calculations, or lower, were found to
occur with uncontrolled explosion and only when an
|140% excess of urea was used did the ignition become
non-explosive. For this urea content, the theoretical flame
temperature is 10828C for reaction RT1 and 8368C for
reaction RT2; the measured maximum temperature reached
by the reaction was ,8008C.
Table 2
Equations describing the various chemical reactions that might be involved in the combustion synthesis
Reaction
Describing equation
DH8 (258C, kcal)a
R1
R2
RT1
R3
RT2
CO(NH 2 ) 2(c) 11.5O 2( g ) ⇒CO 2( g ) 12H 2 O ( g ) 1N 2( g )
Zn(NO 3 ) 2 .6H 2 O (c) ⇒ZnO (c) 16H 2 O ( g ) 1N 2( g ) 12.5O 2( g )
Zn(NO 3 ) 2 .6H 2 O (c) 1mCO(NH 2 ) 2(c) 1(1.5m–2.5)O 2( g ) ⇒ZnO (c) 1(612m)H 2 O(g)1(11m)N 2( g ) 1mCO 2( g )
Zn(NO 3 ) 2 .6H 2 O (c) ⇒ZnO (c) 16H 2 O ( g ) 1N 2 O 5 ( g )
Zn(NO 3 ) 2 .6H 2 O (c) 1mCO(NH 2 ) 2(c) 1(1.5m)O 2( g ) ⇒ZnO (c) 1(612m)H 2 O ( g ) 1mN 2( g ) 1mCO 2( g ) 1N 2 O 5 ( g )
2129.9
120.9
120.91m(2129.9)
123.6
123.61m(2129.9)
a
Calculated from thermodynamic data listed in Table 1.
V.C. Sousa et al. / International Journal of Inorganic Materials 1 (1999) 235 – 241
239
Fig. 2. Effect of urea content on the theoretical flame temperature reached in reactions RT1 and RT2 in Table 2.
The above urea content (i.e. 140% excess) was, therefore, selected for further work and used in the synthesis of
the doped ZnO powders, assuming that the dopant level
(,3 mol%) will not affect significantly the combustion
reaction.
Table 3 shows the characteristics of the pure and doped
ZnO as-prepared powders synthesized by the combustion
reaction. The typical powder morphology can be observed
in the SEM photomicrographs shown in Fig. 3.
As is common in combustion synthesized powders,
Table 3 shows that all powders produced present high
specific surface areas and have particle sizes in the
nanometre range. It should be noted that some particles
might actually be crystallite agglomerates, which experienced the on-set of sintering during the fast combustion reaction. These can be observed in the TEM photomicrographs shown in Fig. 4. Typically, agglomerates of
fine crystals (,200 nm) could be seen, which produced the
Table 3
Characteristics of the as-prepared combustion powders
Powders
2
Specific surface area (m / g)
Average BET particle size (nm)
Lattice parameters ( A˚ )
a5b (60.062)
c (60.113)
ZnO
ZB
ZBS
ZBSC
ZBSCM
ZBSCMK
2.753
389
2.295
467
9.523
112
8.760
112
6.801
158
20.478
52
3.249
5.206
3.278
5.197
3.277
5.194
3.278
5.193
3.276
5.194
3.28260.071
5.15760.269
Fig. 3. SEM micrographs showing the typical morphology of as-prepared combustion synthesized powders: (a) ZnO and (b) ZBSCMK.
240
V.C. Sousa et al. / International Journal of Inorganic Materials 1 (1999) 235 – 241
Fig. 4. TEM micrographs showing the typical morphology of as-prepared
combustion synthesized powders: (a) ZnO and (b) ZBSCMK.
role played by the additives not only in phenomena like the
electrical behaviour, but also the stabilization of high
temperature crystalline phases or catalysis, not to mention
grain growth and sintering mechanisms.
Table 3 also shows the hexagonal lattice parameters
calculated from the X-ray diffraction patterns (Fig. 5).
Since the additives will occupy the available tetrahedral
and octahedral sites in the ZnO crystal lattice, they tend to
distort it towards higher a and b values and lower c values.
The change in lattice parameters, in fact, is evidence of
solid solution, given the low additive contents. The X-ray
diffraction patterns of all the as-prepared combustion
powders show only the distinct ZnO reflections (Fig. 5).
4. Conclusions
characteristic hexagonal electron diffraction pattern. The
presence of Bi 2 O 3 , in its traditional role of sintering aid,
leads to an increase in agglomerate size, as compared to
pure ZnO, with the corresponding decrease in the surface
area. On the other hand, the addition of the usual ZnO
grain growth inhibitors (i.e. Sb 2 O 3 , Co 3 O 4 , MnO 2 and
Cr 2 O 3 ) causes the expected effect and the particle size
decreases.
Although, the observed changes in particle size and
surface area are in agreement with the well known and
expected trends, it is remarkable that they can be so clearly
noticed after such a short reaction time. These results
suggest that the incorporation of the additives in solid
solution occurs during the combustion reaction which
enables a better homogeneity of the powders and, hence, a
more controlled microstructure. As a consequence, improved electrical performance is to be expected. Moreover,
the presumed atomic level homogeneity of such combustion powders offers a new path for the clarification of the
This work shows that the combustion synthesis technique can be used to successfully produce pure and doped
crystalline ZnO varistor powders, with good compositional
control. The combustion synthesis route enables synthesis
at low temperature and the products obtained are in a
finely divided state with large surface areas. Combustion
synthesis offers as added advantages, the simplicity of
experimental set-up, the surprisingly short time between
the preparation of reactants and the availability of the final
product, the savings in external energy consumption and
the equally important potential of simplifying the processing prior to forming, providing a simple alternative to
other more elaborate techniques.
The additives introduced during the powder synthesis
reaction were found to be already playing their expected
roles (i.e. sintering aids, grain growth inhibitors, etc.),
which are normally observed only during the sintering
stage of powder compacts. It is envisaged that the combustion synthesis technique might thus be used to investigate
the mechanisms controlling these phenomena in which
element distribution is supposed to be a key parameter.
Acknowledgements
The authors gratefully acknowledge the Department of
Ceramics and Glass Engineering at the University of
Aveiro, who hosted part of this work, and the financial
˜ Paulo Research
support of both FAPESP, State of Sao
Support Foundation, Brazil, and FCT, Foundation for
Science and Technology, Portugal.
References
Fig. 5. X-ray diffraction pattern of the as-prepared combustion synthesized ZnO powder.
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