A rapid combustion synthesis of MgO stabilized Sr- and

Ba-h-alumina and their microwave sintering

Tom Mathews, R. Subasri, O.M. Sreedharan*

Materials Characterization Group, Thermodynamics and Kinetics Division, Indira Gandhi Centre for Atomic Research,

Kalpakkam, Tamil Nadu 603 102, India

Received 5 June 2001; received in revised form 11 February 2002; accepted 18 February 2002

Abstract

MgO stabilized Sr- and Ba-h-alumina powders corresponding to the composition MMgAl10O17 (M=Sr or Ba) were

synthesized through a rapid solution combustion route using stoichiometric amounts of metal nitrates and urea. The products

were characterized by powder X-ray diffraction (XRD). The powders were compacted and sintered using microwaves as the

energy source. Dense MgO stabilized Sr- and Ba-h-aluminas of 94% and 91%, respectively, were obtained within a short period

of 30 min. AC impedance measurements on the microwave-sintered samples performed as a function of temperature yielded

activation energies lower than that reported for the samples sintered through conventional routes. This paper reports of a novel

method for synthesizing phase-pure powders of MgO stabilized Sr- and Ba-h-alumina and sintering of the powder compacts

into dense membranes ( > 90% theoretical density) which materials are used as solid electrolytes in chemical sensors and molten

metal processing. D 2002 Elsevier Science B.V. All rights reserved.

PACS: 81.20.Ka; 81.20.Ev; 81.05.Je; 78.70.G

Keywords: h-Alumina; Microwave sintering; Solid electrolyte; Chemical sensor; Solution combustion synthesis

1. Introduction

Since the discovery of Na-h-alumina more than

three decades ago by Yao and Kummer [1], there has

been a sustained interest in the synthesis and character-

ization of other monovalent (K + , Cs + ), divalent

(Ba + 2, Sr + 2, Pb + 2) and trivalent (La + 3, Sc + 3) ion-

substituted analogs of Na-h-alumina [2–8]. This is

due to the wide variety of applications of the h-alumina family of compounds [9–11]. A recent review

on multivalent cation conduction in solids (Ref. [12]

and references therein) give a detailed account of the

multivalent ion conducting h-aluminas. The ion-sub-

stituted h-aluminas are usually synthesized by the ion

exchange of Na-h-alumina with the respective ions

[5]. There are, however, only few reports on the direct

synthesis of ion-substituted h-aluminas because of the

limited thermal stability of such substituted com-

pounds [13,14].

Alkaline earth hexa-aluminates, which possess h-alumina or magnetoplumbite type structure, show

interesting optical and electrical properties. Among

the two above-mentioned structures, materials with h-alumina type structure exhibit very good ionic con-

0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0167 -2738 (02 )00105 -4

* Corresponding author. Tel.: +91-4114-80116; fax: +91-4114-

80081.

E-mail address: oms@igcar.ernet.in (O.M. Sreedharan).

www.elsevier.com/locate/ssi

Solid State Ionics 148 (2002) 135–143

ductivity. The hexa-aluminates with composition

MAl12O19 and MAl11O17 crystallize in magnetoplum-

bite and h-alumina type structures, respectively.

Depending on the ionic size of the M + 2 ion in

MAl12O19, it is possible to transform alkaline earth

and similar hexa-aluminates with magnetoplumbite

type structure into h-alumina type structure when

Al + 3 is replaced by Mg + 2 resulting in MMgAl10O17

[15]. This change is observed for M=Sr, Eu and Pb

but not for Ca. Thus, hexa-aluminates containing a

large cation (Ba + 2) possess h-alumina type structure,

whereas for those with smaller cations (Ca + 2), the

structure is magnetoplumbite. The hexa-aluminates

containing cations with intermediate size switch

between both structure types. In the system SrO–

Al2O3, the compound SrAl12O19 is of magnetoplum-

bite type, whereas the MgO stabilized compound,

SrMgAl10O17 is of h-alumina type [16]. In the system

BaO–Al2O3, the two phases Ba1.21Al11O17.71 with

collapsed h-alumina structure and Ba0.75Al11O17.25

with h-alumina structure containing defects are

reported [17]. The MgO substituted analog BaM-

gAl10O17 has h-alumina structure [17,18].

Sr- and Ba-h-aluminas corresponding to the gen-

eral formula MMgAl10O17 (M= Sr or Ba) are syn-

thesized through the conventional ceramic route

[19–21]. Synthesis through ceramic routes require

prolonged heat treatments for several hours at high

temperatures, often with repetitive grinding, compac-

tion and annealing. Besides, the material prepared

contains a second glassy phase, which segregates at

the grain boundaries on sintering and is deleterious to

ionic conductivity. Single phase Ba- and Sr-h-alumina

are reported to be obtained at 1523 and 1573 K, res-

pectively, in 12 h by a conventional ceramic technique

[19,20]. Recently, synthesis through soft chemistry

routes has been of interest. In this technique, homo-

geneous mixing of several components at atomic level

is achieved. This allows a low processing temperature

and a morphological control of uniform crystallite par-

ticle size at superfine dimensions resulting in very high

reactivity. In order to obtain dense membranes, the

phase-pure Ba- and Sr-h-aluminas have to be sintered

at f 2000 K [19,20]. For Ba- and Sr-h-alumina, the

highest relative density of 91% and 94%, respectively,

was obtained by sintering at around 2038 K [19,20].

Sintering at such high temperatures is cost- and

energy-intensive. Besides, it may lead to grain growth,

which is deleterious to mechanical strength and ionic

conductivity. This problem can be circumvented using

microwaves as energy source for sintering [22]. The

use of microwaves for sintering of h-aluminas are

scarce [23–25]. Recently, a direct microwave synthe-

sis cum sintering of MgO stabilized Na-hW-alumina

was reported by Subasri et al. [26,27]. However, there

are no reliable reports on the use of microwaves for the

Fig. 1. Flow chart for the rapid solution combustion synthesis.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143136

sintering of substituted h-aluminas. Certain ceramics,

like alumina and stabilized zirconia, which require

high temperatures and longer soaking times for sinter-

ing, could easily be sintered to high densities in a

short time using microwaves as energy source [28,29].

In light of the aforementioned factors, the employ-

ment of a solution combustion technique for the rapid

synthesis of fine powders of SrMgAl10O17 and BaM-

gAl10O17 and the subsequent successful application of

microwaves for sintering of such h-aluminas are re-

ported here.

2. Experimental

2.1. Synthesis

All reagents used were of purity greater than 99.9%.

Appropriate amounts of aluminum nitrate, strontium

nitrate or barium nitrate and magnesium nitrate (mag-

nesium nitrate solution was prepared by dissolving

appropriate amount of Mg(CH3COO)2�4H2O in dilute

nitric acid and boiling off excess nitric acid on a hot

plate) were dissolved in distilled water, in such a mole-

ratio with respect to cations to make SrMgAl10O17 and

BaMgAl10O17 as the final products. An aqueous

solution of urea was added as the complexant and

reducer. The amount of urea added was that required

to consume completely the excess oxygen in the

nitrate mixture and is called the stoichiometric amount

[30]. This solution was then refluxed for 10 min and

introduced into an open muffle furnace maintained at

773 K. At this temperature, excess water is removed

from the solution resulting in the formation of a gel

and its simultaneous combustion. The as-combusted

product was obtained as a fine powder. A flow chart

for the steps involved in the combustion synthesis is

given in Fig. 1. The powder X-ray diffraction (XRD)

pattern was collected for the as-combusted product.

XRD patterns were obtained using a Rigaku model

diffractometer with CuKa as the target. The com-

busted product was compacted into disks of dimen-

sions 10-mm diameter and 2–3-mm thickness at a

pressure of 300 MPa and subjected to microwave

radiation (for a period of 20 min) using a domestic

microwave oven.

2.2. Description of the microwave set-up

A domestic microwave oven (2450 MHz, BPL,

India) with a power rating of 800 W was used for

the present study. The compacted disks were placed in

a ceramic crucible embedded in powder bearing the

same composition. This crucible in turn was sur-

rounded by an insulating sheath made up of fibrous

alumina, housed in a larger silica container with a lid.

The silica container was once again enshrouded by a

2-cm-thick layer of fibrous alumina inserted into a

cylindrical recrystallized alumina tube. The schematic

sketch of the set-up is shown in Fig. 2. The entire set-

Fig. 2. Schematic of the set up for microwave sintering, (a) microwave cavity, (b) outer ceramic tube, (c) quartz crucible with lid, (d) alumina

fibre (insulation), (e) ceramic crucible and (f) sample.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143 137

up was positioned inside the microwave oven followed

by gradual rising of power. The heating was carried out

in air at 175-W power for 5 min, at 455 W for 5 min

and then at 800 W for 30 min. The sample got heated

to very high temperatures in f 15 min time. No

efforts were made to measure the actual temperature

of the sample.

2.3. Characterization of the samples

The powder X-ray diffraction (XRD) pattern was

collected for the as-combusted product as well as for

the microwave-sintered material. The pycnometric

densities of the sintered disks were measured using

dibutyl phthalate as the medium of immersion. For AC

impedance measurements, a thin adherent layer of Pt

was applied over the parallel surface of the disks,

which served as the electrodes. A frequency range of 1

MHz–1 Hz was used with 10 mV as the amplitude of

the applied voltage. The impedance was measured as a

function of temperature using a Frequency Response

Analyser supplied by M/s Autolab, Netherlands.

Impedance measurements were made in a purified

Helium atmosphere. The microstructural analysis

was done using a scanning electron microscope

(model Philip XL 30) on the polished surface of the

sintered disks with an accelerating voltage of 20.0 kV.

3. Results and discussion

3.1. Synthesis

Assuming that the combustion reaction occurs with

the stoichiometric quantity of the reductant (urea), the

balanced combustion reaction can be written as

MðNO3Þ2ðaq:ÞðM ¼ Sr=BaÞ þMgððNO3Þ2ðaq:Þþ 10AlððNO3Þ3ðaq:Þ þ 28:33COðNH2Þ2ðaq:Þ! MMgAl10O17 þ 56:66H2Oþ 28:33CO2

þ 45:33N2 ð1Þ

The product of rapid combustion resulted in very

fine powders due to the large amount of gases that

evolved during the reaction. Approximately 130 mol

of gases are evolved for each mole of h-alumina.

Single phase Sr- and Ba-h-alumina were obtained at

Tf 773 K. The XRD pattern of the as-combusted

products showed them to be pure Sr- and Ba-h-

Fig. 3. X-ray diffraction pattern of the microwave-treated precursor confirming it to be pure Sr-h-alumina.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143138

aluminas. The reason for the substantial lowering of

the processing temperature, when the synthesis was

carried out through a rapid solution combustion tech-

nique, could be attributed to the removal of excess

water, formation of metal–urea complex and its

simultaneous ignition so that the homogeneous dis-

tribution of cations (achieved in the solution) is

retained until ignition. Moreover, though the furnace

temperature was maintained at f 773 K, the particles

could have attained higher temperatures during igni-

tion. The as-combusted powder after compaction

followed by exposure to microwave radiation, got

Fig. 4. X-ray diffraction pattern of the microwave-treated precursor confirming it to be pure Ba-h-alumina.

Fig. 5. �ZW against ZV for the microwave sintered Sr-h-alumina at 873 K.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143 139

heated up to high temperatures resulting in sintering to

high densities in 30 min. The XRD patterns shown in

Figs. 3 and 4 confirmed them to exhibit the h-alumina

structure. The densities measured were 94% and 91%

of the theoretical for SrMgAl10O17 and BaM-

gAl10O17, respectively. A compacted disk of Ba-h-alumina powder was conventionally sintered at 1773

K for 6 h. This disk on immersion into dibutyl

phthalate for density measurements showed the evo-

lution of air bubbles, indicating the presence of open

pores. The measured pycnometric density was f 81%

of theoretical. In the case of microwave-sintered

samples, no such evolution of air bubbles was ob-

served. In addition, there was no change in density

when measurements were repeated few times after

sufficient intervals of time and adequate drying under

vacuum. It should be mentioned here that this is the

first attempt reported in the literature for a microwave

assisted sintering for making dense membranes of Sr-

and Ba-h-aluminas. This is novel in that the method

of obtaining fine powders of SrMgAl10O17 and BaM-

gAl10O17 using combustion synthesis technique has

not so far been reported in the literature. Though there

are several soft chemistry routes already published in

the literature to obtain Na-h-alumina, all of them re-

quire temperatures in excess of 1273 K to form the

final product. The urea combustion technique offers

the advantage of directly obtaining fine powders of

the desired product in a very short time (30 min), com-

pared to the conventional ceramic route that requires

Fig. 6. Comparison of conductivity data of microwave-sintered Sr-h-alumina with those reported in the literature for the samples sintered using

conventional techniques.

Fig. 7. �ZW against ZV for the microwave-sintered Ba-h-alumina at

973 K.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143140

higher temperature heat-treatments for long periods.

As mentioned earlier, high temperatures (2000 K) are

also required to obtain dense membranes of Sr- and

Ba-h-aluminas. The product obtained after the micro-

wave sintering of the disks for 30 min were of high

density comparable with the samples conventionally

sintered around 2000 K.

3.2. Conductivity measurements

The impedance spectrum obtained in the case of Sr-

h-alumina showed the presence of two semicircles at

lower temperatures and is shown in Fig. 5 (T= 873 K).

The two semicircles can be assigned to the bulk and

the grain boundary resistances occurring at high and

the low frequency regions, respectively. The con-

ductivity was calculated by fitting the results with a

semicircle and the resistance was determined by the

intersection of the earlier circle with the X-axis, which

gives the real part of the impedance. No corrections

Fig. 8. Comparison of conductivity data of microwave-sintered Ba-h-alumina with those reported in the literature for the samples sintered using

conventional techniques.

Table 1

Comparison of coefficients of linear fit of conductivity data for Sr-

and Ba-h-alumina

Authors Sr-h-alumina Ba-h-alumina Ref.

A B Ea

(eV)

A B Ea

(eV)

He et al. 4.758 � 4.989 0.990 6.297 � 9.239 1.830 [21]

Yamaguchi

et al.

4.898 � 4.588 0.911 5.316 � 8.689 1.724 [32]

Schafer et al. 3.872 � 5.400 1.071 [31]

Present work 3.683 � 3.990 0.792 1.114 � 2.679 0.532Fig. 9. Scanning electron micrograph of microwave-sintered Sr-h-alumina.

T. Mathews et al. / Solid State Ionics 148 (2002) 135–143 141

were made for the resistance of the leads (Pt in this

case) as the bulk resistance due to the sample itself was

very high. The impedance was measured as a function

of temperature in the range of 750–1200 K. The bulk

conductivities were plotted as a function of inverse

temperature and are shown in Fig. 6. The activation

energy determined from the conductivity results is

0.79 eV. The conductivity values of conventionally

sintered samples reported by other workers are com-

pared in Fig. 6. In the case of Ba-h-alumina, the

impedance spectrum as presented in Fig. 7 showed

the presence of only one semicircle, which is indica-

tive of bulk resistance. The conductivities in this case

are also calculated as in the case of Sr-h-alumina and

are plotted as a function of inverse temperature in Fig.

8. The activation energy in this case was determined to

be 0.53 eV, valid over the temperature range 1000–

1250 K. The conductivity values of conventionally

sintered samples reported by other workers are also

compared in Fig. 8. Table 1 presents the coefficients

of the linear fit of the conductivity as a function of

inverse temperature for Sr- and Ba-h-aluminas. The

conductivity studies on the microwave-sintered sam-

ples showed that the activation energy for electrical

conduction in these samples is lower than the liter-

ature values for the samples otherwise sintered. The

scanning electron micrographs of the microwave-

sintered Sr- and Ba-h-alumina samples are shown in

Figs. 9 and 10. The grain size from the micrographs

could be calculated to be ranging from 0.5 to 2 Am in

the case of Sr-beta alumina and from 1 to 2.5 Am for

the Ba-analog.

4. Conclusions

An attempt to synthesize MgO stabilized Sr- and

Ba-h-alumina powders by urea combustion technique

and sintering of the product so synthesized using

microwave radiation is reported here for the first time.

AC impedance measurements performed on the

microwave-sintered disks showed the Ea (eV) to be

0.79 and 0.53, valid over the ranges 750–1200 and

1000–1250 K for Sr- and Ba-h-alumina, respectively.

It was also demonstrated that a soft chemistry route, to

obtain fine powders of the desired material when

combined with use of microwaves for sintering,

would yield a dense material with improved micro-

structure in a much shorter time compared to the

conventional processes. This technique of fabrication

of dense membranes of the divalent substituted beta

aluminas could be cost-effective and time-saving from

a technological point of view.

Acknowledgements

The authors are grateful to Dr.Baldev Raj, Director,

MCRG, and Dr. V.S. Raghunathan, Associate Direc-

tor, MCG, IGCAR, Kalpakkam for their keen interest

and constant encouragement throughout the course of

this investigation. The authors are also thankful to Dr.

C. Mallika, Scientific Officer (SO/F), TKD, IGCAR

for her help in impedance measurements.

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