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Microporous and Mesoporous Materials 66 (2003) 311–319

Preparation of mullite micro-vessels by a combinedtreatment of zeolite A

Cleo Kosanovi�cc, Boris Suboti�cc *

Rud-er Bo�sskovi�cc Institute, Laboratory for the Synthesis of New Materials, P.O. Box 180, Bijeni�ccka c. 54, 10002 Zagreb, Croatia

Received 3 July 2003; received in revised form 15 September 2003; accepted 20 September 2003

Abstract

A controlled thermal treatment of zeolite NH4A results in the formation of a mixture of mullite and amorphous

silica by a chain of pseudomorphic transformation processes: zeolitefi amorphous phasefi (mullite + amorphous

SiO2). In contrast to the structural changes, particulate properties (particle shape and size, respectively) do not change

or change a little during heating. This implies that the resulting mixture of mullite and amorphous SiO2 does not appear

as separate particulate systems, and thus that the transformation processes occur inside single particles (micro-reactors).

Base treatment of the final product (mullite + amorphous SiO2) results in the extraction of the amorphous SiO2 from the

interior of each particle and formation of mullite micro-vessels.

� 2003 Elsevier Inc. All rights reserved.

Keywords: Ammonium-exchanged zeolite A; Thermal treatment; Amorphous aluminosilicate; Mullite; Amorphous silica; Base

treatment; Mullite micro-vessels

1. Introduction

Due to their open framework, zeolites are

metastable materials, which can be transformed

into non-zeolite crystalline aluminosilicates above

a certain temperature [1–7]. Since many of the

synthetic zeolites have compositions close to those

of aluminosilicate-based ceramics, their thermal

treatment may result in the formation of ceramic

materials [1]. The first step of the thermal trans-formation of zeolites is the formation of an

amorphous aluminosilicate phase by destruction

* Corresponding author. Tel.: +385-1-46-80-123; fax: +385-1-

46-80-098.

E-mail address: subotic@rudjer.irb.hr (B. Suboti�cc).

1387-1811/$ - see front matter � 2003 Elsevier Inc. All rights reserve

doi:10.1016/j.micromeso.2003.09.016

of the zeolite structure [1,3,5–8]. The formed

amorphous aluminosilicate has the same chemicalcomposition as the original crystalline precursor

(zeolite) [6,8]. The thermal stability of the zeolite

framework, and thus the temperature of its

transformation to the amorphous phase increase

with increasing ionic radius of the alkali cations

present in the channel/voids of the zeolite frame-

work [6].

Previous studies of the thermally inducedtransformations of zeolites NH4A and NH4X

showed that the transformations take place

through the formation of an intermediate amor-

phous phase and a subsequent recrystallization to

a mixture of mullite and amorphous SiO2 [1,6,9].

The weight fractions of mullite (Al6Si2O13) was

�60–64 wt.% in the case of ammonium-exchanged

d.

312 C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319

zeolite A and �47–59% in the case of ammonium-

exchanged zeolite X [1] The intermediate (amor-

phous aluminosilicate) and the final product

(mullite) have almost the same shape as the pre-cursor particles (zeolites NH4A and NH4X) [6,9].

The invariability of the particulate properties

(pseudomorphism) during the thermal treatment of

potassium-exchanged zeolite A was explained by

special properties of a thin (sub) surface layer of

each zeolite crystal which is different from that of

the bulk crystal (e.g. an extraordinary thermal sta-

bility) [10]. In this way, each zeolite micro-crystal canbe seen as a ‘‘closed’’ micro-reactor with a stable

shell, and hence all relevant processes (crystal to

amorphous transformation, nucleation and crystal

growth of kalsilite and kaliophilite from the

amorphous aluminosilicate) occur inside each sin-

gle particle (crystal). It is reasonable to assume that

the same principles also apply for thermally in-

duced pseudomorphic transformation of zeolites[1] including the transformation sequence of zeo-

lites NH4A and NH4X into amorphous alumino-

silicate and the mixture of mullite and amorphous

silica [1,6]. Based on this assumption, one can ex-

pect that the dissolution behavior of the thin sur-

face layer in alkaline solutions would be different

from the dissolution behavior of the bulk crystal.

A preliminary study of a base treatment ofmullite obtained by heating of zeolite NH4A re-

sulted in an ‘‘extraction’’ of the crystal nutrient

and the formation of mullite micro-vessels [11].

Hence, the objective of this work is the study of the

chemical and structural changes of mullite micro-

crystals during their treatment by hot alkaline

solution as well as to characterize the product

mullite micro-vessels obtained by the chemicaltreatment.

2. Experimental

2.1. Materials

Well-shaped cubic crystals of zeolite A (see Fig.

3a in Ref. [9]) having a size between 10 and 20 lm(see Fig. 4a in Ref. [9]) were synthesized by a

previously published method [12], and used as thestarting material for the preparation of mullite.

Partial exchange of the original Naþ ions from

the starting zeolite A with NHþ4 from solution was

carried out by an already described procedure

[10,13]. The resulting zeolite NH4A ([0.05Na2O,0.95(NH4)2O] ÆAl2O3 Æ 1.98SiO2 Æ 2.12H2O, as de-

termined by the chemical analysis) was heated at a

rate of 10 �C/min to about 1000 �C, in a con-

trolled-temperature chamber furnace (ELPH-2,

Elektrosanitarij) to transform the (NH4, Na)-form

of zeolite A into an X-ray and IR amorphous

aluminosilicate. A prolonged heating of the amor-

phous phases at the same temperature resulted inthe formation of a mixture of mullite and amor-

phous SiO2.

2.2. Procedure of dissolution

Two grams of powdered material (mull-

ite + amorphous SiO2 obtained by thermal treat-

ment of zeolite NH4A) dried at 105 �C for 24 h were

poured into a stainless-steel reaction vessel that

contained 100 ml of 2 MNaOH solution preheated

at 70 �C (concentration of the suspension was 20 g/

dm3). The reaction vessel was provided with a

thermostated jacket and fitted with a water-cooledreflux condenser and a thermometer. The reaction

mixture (suspension of the powdered material in 2

M NaOH solution) was stirred with a Teflon-

coated magnetic bar (L ¼ 5 cm, / ¼ 0:95 cm) dri-

ven by a magnetic stirrer with the stirring speed of

510 r.p.m. At various times, td, after the beginningof the dissolution process, aliquots of the suspen-

sion were drawn off to prepare samples for analy-ses. The moment the powdered material was

poured to the preheated NaOH solution was taken

as the zero time (td ¼ 0) of the dissolution process.

2.3. Preparation of samples for analysis

Aliquots of the reaction mixture drawn off the

suspension at given dissolution times, td were

centrifuged to separate the solid from the liquid

phase and to stop the dissolution process. A part

of the clear liquid phase was used for the analysis

of silicon and aluminum concentrations in the li-

quid phase. The obtained solutions were diluted

with distilled water to the concentration rangesavailable for measuring the concentrations of

C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319 313

aluminum and silicon by atomic absorption spec-

troscopy.

After the removal of the rest of the clear liquid

phase above the sediment, the solid phase wasredispersed in distilled water, and the suspension

obtained was centrifuged repeatedly. The proce-

dure was repeated until the pH value of the clear

liquid phase above the sediment was between 7 and

8. The wet solid phase was dried at 105 �C for 24 h,

cooled in desiccator, and used for further analyses.

2.4. Analysis of samples

Qualitative and quantitative phase analysis ofthe solid samples drawn off the suspension at

various times, td, were performed by powder X-ray

diffractometry. X-ray powder diffraction patterns

of the solid samples were obtained using a Siemens

5000D diffractometer with CuKa radiation. Pow-

der diffraction data were collected in the 2h ranges

4–80� in steps of 0.02� 2h, with 1 s per step. The

Hermans–Weidinger method [14] was used for thedetermination of the weight fractions of the crys-

talline phases in the two-phase system, using the

integral value of the broad amorphous peak

(2h ¼ 5–50�) and the corresponding sharp peaks of

the crystalline phases.

Particle size distribution curves of the solid

samples were taken by a Mastersize XLB (Mal-

vern) laser light-scattering particle size analyzer.The concentrations of aluminum and silicon in

solutions were measured by a Perkin-Elmer 3030B

atomic absorption spectrometer.

Scanning-electron micrographs of the samples

were taken by a Philips SEM 515 scanning-elec-

tron microscope.

The external surface area of the samples was

determined by single point nitrogen adsorptionusing a Micromeritics FlowSorb II 2300 instru-

ment. Prior to the measurement the samples were

outgassed at 80 �C for 1 h to desorb loosely held

moisture from the outer surface of the samples.

Fig. 1. Scanning-electron micrographs of (a) the starting zeolite

NH4A, (b), intermediate X-ray amorphous aluminosilicate and

(c) the final product (mullite+ amorphous SiO2).

3. Results and discussion

Despite the intensive structural changes (zeoliteAfi amorphous aluminosilicatefimullite) [6,9]

during controlled heating of zeolite NH4A (Fig.

1a), the particles of the resulting amorphous alu-

minosilicate (Fig. 1b) and the mixture of mullite

and amorphous silica (Fig. 1c) have the same(cubic) shape as the starting powder of zeolite

NH4A (Fig. 1a). However, the small bulginess of

the particle surfaces of the amorphous alumino-

silicate (Fig. 1b) and the final product (mullite;

Fig. 1c) relative to the original morphology of

zeolite NH4A (Fig. 1a) is probably caused by the

314 C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319

internal pressure of gas (NH3) developed by ther-

mal decomposition of NHþ4 ions inside zeolite A

micro-crystals [9].

Preservation of the original particle shape dur-ing transformation processes indicates that each

zeolite micro-crystal can be seen as a closed micro-

reactor with a stable shell [15] and hence that all

relevant processes, i.e.,

3f½0:05Na2O; 0:95ðNH4Þ2O� �Al2O3 � 1:98SiO2 � 2:12H2OgNH4-exchanged zeolite A :

# ð1Þ3½ð0:05Na2O; 0:95H2OÞ �Al2O3 � 1:98SiO2� þ 5:7NH3 þ 6:36H2O

amorphous aluminosilicate

# ð2Þ½3Al2O3 � 2SiO2 þ 3:79SiO2 þ 0:15Na2SiO3� þ 2:85H2O

mullite

including nucleation and crystal growth of mullite

from amorphous aluminosilicate (Eq. (2)) occur

inside each single particle (crystal). This implies

that mullite, SiO2 and Na2SiO3 do not appear as

separate particulate systems, but that all phases

exist as their mixture inside each single particle

(micro-reactor; Fig. 1c) [9]; the nature of the

mixture (homogeneous or heterogeneous) cannotbe defined at present. Absence of X-ray diffraction

peaks except for the ones of mullite and the pres-

ence of a distinct amorphous ‘‘hump’’ in the X-ray

diffraction pattern (Fig. 2a and b), indicate that

the formed silicon dioxide is amorphous. Starting

from the sodium form of zeolite A [Na2O ÆAl2O3 Æ1.98SiO2 Æ 4.15H2O], and following the chemical

changes during heating (Eqs. (1) and (2)), it is easyto calculate the density of solid phase changes

(decreases) during the heating in the sequences:

q(zeolite NH4A)¼ 1.74 g cm�3, q(amorphous alu-

minosilicate)¼ 1.313 g cm�3 and q(mullite +

SiO2)¼ 1.214 g cm�3, and that each particle of the

final product (mullite + SiO2) contains 63.4 wt.%

of mullite, 2.7 wt.% of Na2SiO3 and 33.9 wt.% of

amorphous aluminosilicate [6].Fig. 3 shows the change in molar concentra-

tions of aluminum (symbols � and solid curve in

A) and silicon (symbols d and solid curve in B)

during the base treatment of the mixture of mullite

and amorphous SiO2 in 2 M NaOH solution at

70 �C. Since all aluminum is contained in mullite

(Eq. (2)), the change in the concentration ðCxÞmull ¼ðCAlÞmull corresponds to the dissolution of mullite.

Assuming that dissolution of mullite is congruent,

one can expect that the amount (concentration) of

silicon, ðCxÞmull ¼ ðCSiÞmull which arise from dis-

solved mullite (3Al2O3 Æ 2SiO2) is three times lowerthan the corresponding concentration of alumi-

num, i.e., ðCSiÞmull ¼ ðCAlÞmul=3 (dashed curve in

Fig. 3a). Hence, the ‘‘plateau’’ concentrations,

½ðCAlÞmull�p ¼ 0:0307 mol dm�3 and ½ðCSiÞmull�p ¼0:0102 mol dm�3 (see Fig. 3a) correspond to the

solubility of mullite (2.18 g of mullite/dm3) in 2 M

NaOH solution at 70 �C. This means that 17.2

wt.% of mullite was dissolved during the basetreatment. On the other hand, while the slow in-

crease in the concentration, ðCSiÞx of dissolved

silicon during the first 60 min of the dissolution

process (see Fig. 3b) corresponds to the surface

dissolution of mullite, an increase in the concen-

tration of silicon over ðCSiÞx ¼ ½ðCSiÞmull�p ¼ 0:0102mol dm�3 for td > 60 min, is caused by dissolution

of the amorphous SiO2 (and a small amount ofNa2SiO3) from the inner part of particles. This

implies that at any stage of the dissolution process

the mullite shell is perforated (most probably at the

place of the bulginess; Fig. 4), which enables the

dissolution, not only of the surface but also of

the interior of each particle. Fig. 3b shows that

the difference between the concentration ðCSiÞx ¼ðCSiÞtot of the total dissolved silicon (symbols � andsolid curve) and ðCSiÞx ¼ ðCSiÞam of the dissolved

amorphous SiO2 and Na2SiO3 (dashed curve;

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

(a)

Inte

nsity

(arb

itrar

y un

its)

(b)

(c)

Inte

nsity

(arb

itrar

y un

its)

2θ (degrees)

(d)

2θ (degrees)

2θ (degrees) 2θ (degrees)

Fig. 2. X-ray diffractograms of the final product of thermal transformation of zeolite NH4A (mullite + amorphous SiO2) (a), and of the

solid residues obtained during its dissolution in 2 M NaOH solution at 70 �C for td ¼ 60 min (b), td ¼ 180 min (c) and td ¼ 240 min (d).

The XRD peaks at 2h ¼ 38:28, 44.48 and 78.04 in (c) and (d) belong to the aluminum support of the sample holder.

C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319 315

ðCSiÞam ¼ ðCSiÞtot � ðCSiÞmullÞ is measurable for

td > 60 min and hence it can be concluded that theperforation of the mullite shell starts about 60 min

after the beginning of the dissolution process. The

increase in the rate of SiO2 dissolution at td > 60

min and the increase of its concentration over

½ðCSiÞmull�p ¼ 0:0102 mol dm�3 (Fig. 3b), supports

this conclusion. Faster dissolution of the amor-phous SiO2 relative to the dissolution of mullite

results in an increase of the mass fraction fmull of

mullite relative to the mass fraction fam of the

amorphous phase during the dissolution process,

0 50 100 150 200 250

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.000

0.004

0.008

0.012

0.016

0.020

0.024

0.028

0.032

0.036

(b)

( CSi) x (

mol

dm

-3 )

td (min)

(a)

(Cx) m

ul(m

ol d

m-3)

Fig. 3. Change in (a) the concentration ðCxÞmull ¼ ðCAlÞmull of

aluminum (�, solid curve) and ðCxÞmull ¼ ðCSiÞmull of silicon

(dashed curve) which correspond to the dissolution of mullite,

and (b) total concentration, ðCSiÞx ¼ ðCSiÞtot of silicon (�, solid

curve) and the concentration ðCSiÞx ¼ ðCSiÞam which correspond

to the dissolved amorphous SiO2 +Na2SiO3 (dashed curve)

during the base treatment of the final product of thermal

transformation (mullite+ amorphous SiO2) (a) in 2 M NaOH

solution at 70 �C.

Fig. 4. Scanning-electron micrograph of the solid sample

drawn off the suspension at td ¼ 120 min.

Table 1

Change in the mass fractions, fmull of mullite and, fam of amorphous

thermal treatment of zeolite NH4A in 2 M NaOH solution at 70 �C,

td (min) fmull, from dissolution

data

fmull, from XR

0 – 0.617

15 0.650

20 0.651

30 0.653

45 0.657

60 0.661 0.619

90 0.661

120 0.669

180 0.712 0.777

240 1.000 1.000

td is the time of dissolution.

316 C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319

as it was calculated by the dissolution data and

confirmed by X-ray diffraction analysis (see Table1). The fraction, fmull of mullite slowly increases

(from about 0.63 to 0.67) and the fraction fam of

amorphous SiO2 slowly decreases (from about

0.37 to 0.33) in the time interval from td ¼ 0 to

�120 min as the consequence of a simultaneous

dissolution of mullite and amorphous SiO2 in the

same time interval. Thereafter, the fraction of

mullite sharply increases and reaches the valuefmull ¼ 1 (fam ¼ 0) for td ¼ 240 min. The maximum

concentration ðCSiÞam ¼ 0:124mol dm�3 at td ¼ 240

min (Fig. 3b) corresponds to 7.45 g of the dis-

solved SiO2, which is very close to the expected

amount (7.07 g) of soluble SiO2 (amorphous sil-

ica +Na2SiO3) in 20 g of the starting powder

SiO2 +Na2SiO3 during the dissolution of the final product of

calculated from the dissolution and XRD data respectively

D data fam, from dissolution

data

fam, from XRD data

– 0.383

0.35

0.349

0.347

0.343

0.339 0.381

0.339

0.331

0.288 0.223

0.000 0.000

0 10 20 30 40 50-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

MD (%

)

D (µm)

Fig. 5. Particle size distributions of the solid samples drawn off

the suspension at td ¼ 180 min (dashed curve), td ¼ 240 min

(dash-dotted curve) and td ¼ 300 min (dotted curve) during the

dissolution of the mixture (mullite + amorphous SiO2) (solid

curve) in 2 N NaOH solution at 70 �C. MD is the percentage of

the mass (volume) of particles having the size D.

Fig. 6. Scanning-electron micrograph (a) of the solid sample

drawn off the suspension at td ¼ 240 min, during the dissolution

of the mixture (mullite + amorphous SiO2) in 2 M NaOH so-

lution at 70 �C and (b) a magnified detail (micro-vessels) of the

micrograph (a).

Table 2

Changes in the specific (AS) and total (AT) surface area, during

the dissolution of the final product of thermal treatment of

zeolite NH4A in 2 M NaOH solution at 70 �C

td (min) AS (m2/g) AT (m2)

0 2.3 46.0

180 5.6 98.8

240 8.7 153.6

300 11.2 197.7

td is the time of dissolution.

C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319 317

(mullite + amorphous SiO2 +Na2SiO3). This means

that the entire amounts of the amorphous SiO2

and Na2SiO3 were dissolved and extracted from

the particles interior (fam ¼ 0), and that the solid

residue is composed of mullite only (fmull ¼ 1) for

td ¼ 240 min (see Fig. 2d and Table 1). Fig. 5shows that the particle size distribution curves of

the solid phase drawn off the suspension at

td ¼ 180 min (dashed curve), td ¼ 240 min (dash-

dotted curve) and td ¼ 300 min (dotted curve) are

almost the same, but narrower than the particle

size distribution curve of the starting powder

(mullite + amorphous SiO2 +Na2SiO3; solid curve).

The narrowing of the particle size distributioncurves of the samples drawn off the suspension at

different times td relative to the particle size dis-

tribution curve of the starting powder is caused by

the dissolution of mullite (see Fig. 3a), and thus

decreasing of the average particle size from

La ¼ 1:88 lm at td ¼ 0 to La � 1:7 lm for td P 180

min. Hence, a consequence of the extraction of the

entire amount of the amorphous phase (SiO2 +Na2SiO3) from the particle interior is the forma-

tion of mullite micro-vessels (see Fig. 6) having the

approximately same size as the starting crystals of

zeolite NH4A.

In contrast to preservation of the particulate

properties (particle size and shape,), both the

specific surface area AS and the total surface area

AT (of the amount of the solid suspended in 1 dm3

of 2 M NaOH solution) increase during the dis-

solution (see Table 2). Since the forward reaction

of the dissolution process is proportional to thesurface area of the dissolution [16,17], the increase

318 C. Kosanovi�cc, B. Suboti�cc / Microporous and Mesoporous Materials 66 (2003) 311–319

of the total surface area is a possible reason for the

increase in the rate of dissolution of mullite at

td � 60 min (see Fig. 3a). On the other hand, since

the backward reaction of the dissolution process isa function of supersaturation [16,17], the rate of

dissolution decreases with increasing concentra-

tions ðCAlÞmull and ðCSiÞmull and the rates of forward

and backward reactions are in equilibrium when

ðCAlÞmull ¼ ½ðCAlÞmull�p and ðCSiÞmull ¼ ½ðCSiÞmull�p.Hence, it may be assumed that the increase of the

total surface area is responsible for the atypical

sigmoidal shape of the change in the concentra-tions of aluminum and silicon during mullite dis-

solution (see Fig. 3a).

Assuming that the inner geometrical surface

area of the micro-vessels is the same or even

somewhat less than the outer geometrical surface

area, one can expect that AT (micro-vessel) 6 2�AT (starting powder). However while the ratio AT

(micro-vessel)/AT (starting powder)¼ 2.15 fortd ¼ 180 min (i.e., when the entire amounts of SiO2

and Na2SiO3 were extracted from the particles

interiors) is a somewhat higher than expected, fur-

ther base treatment of the micro-vessels consider-

ably increases their surface area (see Table 2), i.e.

AT (micro-vessel)/AT (starting powder)¼ 3.34 for

td ¼ 420 min and AT (micro-vessel)/AT (starting

powder)¼ 4.3 for td ¼ 300 min. The measured in-crease of the surface area of the mullite micro-

vessels without their measurable dissolution (see

Fig. 3a) and the decrease of particle size indicate

that base treatment of the mullite micro-vessels

causes a roughness of both outer and inner surfaces.

4. Conclusions

Controlled heating of zeolite NH4A resulted in

the formation of an amorphous aluminosilicate,followed by desorption of water and ammonia (Eq.

(1)). Except for a bulginess formed on the particle

surfaces as a result of the internal pressure of gas

(NH3) developed by thermal decomposition ofNHþ4

ions inside the zeolite Amicro-crystals, the resulting

particles of the amorphous aluminosilicate retained

the size and shape of the precursor micro-crystals

(zeolite NH4A). A prolonged heating of the amor-phous aluminosilicate resulted in its transformation

into a mixture of mullite (63.4 wt.%), amorphous

SiO2 (33.9 wt.%) and Na2SiO3 (2.7 wt.%) (Eq. (2)).

Resulting particles have the same size and shape as

the particles of the starting amorphous alumino-silicate. The preservation of the particulate proper-

ties during the transformation processes leads to the

conclusion that each zeolite micro-crystal can be

seen as a ‘‘closed’’ micro-reactor with a stable shell,

and hence all relevant processes (crystal to amor-

phous transformation, nucleation and crystal

growth of mullite from the amorphous aluminosil-

icate) occur inside each single particle (crystal). Thisimplies that mullite, SiO2 and Na2SiO3 do not ap-

pear as separate particulate systems, but that all

phases exist as a physical mixture inside each single

particle (micro-reactor). Treatment of the resulting

‘‘micro-crystalline’’ powder in 2 M NaOH solution

at 70 �C resulted in the starting dissolution of the

surface mullite layer, followed by the formation of

perforations on the particle surfaces (most probablyat the places of the bulginess). Formation of the

perforations enables the dissolution of mullite,

amorphous SiO2 and Na2SiO3 from the particle in-

terior. Since the solubility of both amorphous SiO2

and Na2SiO3, is considerably higher than that of

mullite under given conditions, the prolonged base

treatment resulted in the dissolution of the entire

content of amorphous SiO2 and Na2SiO3 from theparticle interior, and thus the formation of the

mullite micro-vessels. Although the formation of

the mullite micro-vessels is a simple process which

can be explained in a straightforward way, the

studies of the kinetics of mullite dissolution under

different conditions, dynamics of surface perfora-

tion and mechanism of surface roughening of the

mullite micro-vessels represent challenges for fur-ther investigations of this interesting system.

Acknowledgements

The authors thank the Ministry of Science of

the Republic of Croatia for financial support.

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