Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200°C with a heating rate of 5°C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed. 

Blagojević V.

Ivetić T.

Nikolić M.V.

Nikolić P.M.

Ristić M.M.

Srećković T.

Đurić S.

Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200°C with a heating rate of 5°C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed

Ivetić, Tamara

Nikolić, Maria Vesna

Nikolić, Pantelija

Blagojević, V.

Đurić, S.

Srećković, Tatjana

Ristić, Momčilo

Digital Repository of the Institute of Technical Sciences of SASA

Science of Sintering, 39 (2007) 153-160 
________________________________________________________________________ 
 
 
 
doi:10.2298/SOS0702153I 
 
UDK  662.785:661.847.2:546.812:77.026.34 
Investigation of Zinc Stannate Synthesis Using Photoacoustic 
Spectroscopy 
 
T. Ivetić1*), M. V. Nikolić2, P. M. Nikolić1, V. Blagojević3, S. Đurić1,  
T. Srećković2, M. M. Ristić1 
1Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez 
Mihailova 35/IV, 11000 Belgrade, Serbia 
2Center for Multidisciplinary Studies of the University of Belgrade, Kneza Višeslava 
1, 11000 Belgrade, Serbia 
3Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 
73, 11000 Belgrade, Serbia 
        
 
 
Abstract:  
 Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically 
activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were 
compacted into pellets and non-isothermally sintered up to 1200˚C with a heating rate of 
5˚C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed 
in order to investigate changes of the phase composition. The microstructure of sintered 
samples was examined by scanning electron microscopy. The photoacoustic phase and 
amplitude spectra of sintered samples were measured as a function of the laser beam 
modulating frequency using a transmission detection configuration. Fitting of experimental 
data enabled determination of photoacoustic properties including thermal diffusivity. Based 
on the results obtained a correlation between thermal diffusivity and experimental conditions 
as well the samples microstructure characteristics was discussed. 
Keywords:  Zinc Stannate, Mechanical activation, Sintering, Photoacoustic spectroscopy 
 
 
 
Introduction 
  
Ceramic materials with large open porosity, such as zinc stannate (Zn2SnO4) obtained 
in this paper from the reaction sintering powder-processing route, are convenient for use as 
humidity sensors. Atmospheric water can be absorbed on the grain surface inside pores or can 
be condensed in small channels and pores. Desirable characteristics of humidity sensors 
include: high sensitivity, reversibility, fast response, broad range of moisture selectivity, 
chemical and thermal stability. These sensor characteristics depend upon parameters of 
synthesis during the reaction sintering process. Knowing the thermal properties of this kind of 
material is very important, especially when they are used as electrical components because of 
the need for an adequate cooling process. In this paper, we present the results of a 
photoacoustic investigation of thermal and transport properties of bulk zinc stannate 
synthesized by the reaction sintering process. Photoacoustic (PA) spectroscopy has been used 
lately, besides for the characterization of electronic, optical and defects structures, for 
_____________________________ 
*) Corresponding author: tamara@itn.sanu.ac.yu 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
154 
 
 
defining the electronic states and structural disorders of ceramic materials [1-4]. In a typical 
photoacoustic experiment the sample is placed in a closed photoacoustic cell exposed to 
radiation with a modulated laser beam [5]. The dependence of the obtained photoacoustic 
signal on the rate of diffunded heat through the sample enables thermal characterization of the 
sample, i.e. determination of different properties of the analyzed material including thermal 
diffusivity [3]. The PA signal is a consequence of heat processes in the observed sample. It 
depends on periodic temperature variation on the rear sample surface, Φ ( ω,l− ) that is in 
contact with the electret microphone (Fig. 1.). 
 
Fig. 1 The gas-sample-backing-microphone detection configuration. 
 
 
The following relation can express the PA signal: 
( ) ( ) ( ωω )
γω ,
0
0 l
lkT
P
S
b
−Φ⋅⋅
⋅=     (1) 
 
Where  is the sample thickness; l ω  is the frequency modulation of the excitation light beam; 
γ  is the adiabatic constant and  and  are ambient pressure and temperature, 
respectively; 
0P 0T( ) ( ) ( )ωµω bb jk /1+= , where ( )ωµb  is the thermal diffusion length of the 
backing (gas); l  is the distance between the sample and microphone membrane and b ( )ω,lΦ  
is the temperature variation of the sample surface that is in contact with microphone. 
 
 
Experimental 
 
Commercially available zinc oxide and tin oxide powders (Aldrich), with a molar 
ratio ZnO:SnO2 = 2:1, were mechanically activated in a planetary ball mill (Fritsch 
Pulverisette 5) in the continuous grinding process regime, in air for 40, 80 and 160 min. 
Zirconium oxide grinding balls (10 mm in diameter, approx.) and vessels (500 cm3) were 
used. The powder to balls ratio was 40:1 and the total weight of the powder mixtures was 10 
g. Appropriate samples were denoted according to the applied time of activation as ZSO-40, 
ZSO-80 and ZSO-160. These samples were uniaxially pressed with different pressures (ZSO-
40 – 150 MPa, ZSO-80, ZSO-160 – 200 MPa) in accordance with the analysis of the 
influence of mechanical activation on consolidation of the ZnO-SnO2 system given in detail in 
[6] in order to obtain the same average green density of 3.769 g/cm3. Relative shrinkage of 
samples obtained by uniaxial pressing of activated powders was measured by a sensitive 
dilatometer (Bähr Gerätebau GmbH Typ 702s) in the course of non-isothermal heating in air 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
155 
 
 
up to 1200˚C with a constant heating rate of 5˚C/min. The densities of sintered samples were 
determined and the values obtained are given in Tab. I. Characterization of the obtained 
samples of powder mixtures after grinding and after sintering was carried out using an X-ray 
diffractometer (Norelco-Philips PW-1050) with CuKα radiation and a step scan mode of 
0.02º/0.4s. The microstructure of sintered samples was characterized using scanning electron 
microscopy (SEM – JSM 5300 JEOL). 
 
 
Tab. I Measured density and calculated thermal diffusivity of ZSO samples  
sintered at 1200˚C 
Milling time Density (g/cm3) DT (m
2s-1) D (m2s-1) α (m-1) 
40 4.14 0.21 · 10-7 0.25 · 10-5 9860 
80 4.12 1.80 · 10-7 0.10 · 10-5 12400 
160 3.86 10.06 · 10-7 0.14 · 10-4 11000 
 
The PA signals were measured using an experimental set-up, with an infrared laser 
(25 mW) as the optical source. The laser beam was modulated with a mechanical chopper. 
Changes in phases and amplitudes of the photoacoustic signal of obtained sintered samples 
were followed. 
 
 
Results and discussion 
 
X-ray diffraction patterns of ZSO powder mixtures are shown on Fig. 2. Mechanical 
activation of ZnO and SnO2 powders, results in the start of formation of zinc stannate after 40 
minutes of milling (this time was denoted in [7] as the shortest milling time needed for the 
beginning of zinc-stannate formation), but as the XRD patterns obtained show residual oxides 
are still present. Longer milling times result in lower amounts of residual oxides present. 
Formation of Zn2SnO4 is not yet finished after 160 minutes of ball milling, as SnO2 is still 
present. 
Fig. 3 shows XRD patterns of powder mixtures after uniaxial pressing and non-
isothermal sintering up to 1200˚C with a constant heating rate of 5˚C/min. The peaks from the 
XRD spectra of samples sintered up to 1200˚C are much sharper and better defined from the 
non-sintered ones indicating creation of monophased spinel zinc stannate. Only a few peaks 
derived from residual oxides are present in the ZSO-40 sample and only one ZnO peak 
developed in the sample mechanically activated for 80 minutes while the ZSO-160 sample 
shows only the zinc-stannate phase. 
SEM microphotographs of fractured surfaces of ZSO powder samples non-
isothermally sintered in a dilatometer up to 1200˚C with a heating rate of 5˚C/min are 
presented in Fig. 4-6. SEM microphotographs show that grinding leads to the formation of a 
structure with a reduced grain size that accelerates spinel formation (which is with agreement 
with XRD analysis), but agglomerates are also present, probably preserved from the 
agglomeration of powders. Grain growth of spinel inhibits densification and causes the 
formation of a porous microstructure. 
 
 
 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
156 
 
 
 
0 10 20 30 40 50 60
2 θ  ( 0 ) λCuKα
(211) (511)
(511)
(422)(400)
(400)
(311)
(222)
(222)
(311)
(220)
(220)
(110)
(110)
(111)
(111)
(111)
(110)
(100)
(311)
(222)
(400)
(211)
(220)






z
z
z 


 


z
z
z
z

 ZnO
z SnO2
 Zn2SnO4
ZSO-160
ZSO-80
ZSO-40
 
Fig. 2 XRD patterns of ZSO powder mixtures as a function of the activation time. 
25 30 35 40 45 50 55

(100)
(200)
(200)
(111)
(111)
(111)





(200)
(210)
z
(511)
(422)
(331)
(331)
(400)
(400)
(222)
(222)
(311)
(311)
(220)
(220)
(100)
(422)
(511)
(220)
(311)
(222)
(400)
(331)
(422)
(511)









 










ZSO-160
ZSO-80
ZSO-40
2 θ  ( 0 ) λCuKα
 ZnO
z SnO2
 Zn2SnO4
 Al
 
 
Fig. 3 XRD patterns of ZSO samples non-isothermally sintered up to 1200˚C with a heating 
rate of 5˚C/min. 
 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
157 
 
 
 
  
Fig. 4 SEM fractured surface of the ZSO-
40 sample sintered at 1200˚C. 
Fig. 5 SEM fractured surface of the ZSO-
80 sample sintered at 1200˚C 
 
Fig. 6 SEM fractured surface of the ZSO-160 sample sintered at 1200˚C. 
 
 
Fig. 7 shows the relative shrinkage of samples as a function of temperature during 
heating for the different activated samples (ZSO-40, ZSO-80 and ZSO-160) obtained by the 
dilatometer. Dilatometric curves confirm the XRD analysis results that the formed spinel 
phase dominates in the system so there is no trace of other processes but sintering. It is 
interesting to note that shrinkage is the highest for the sample mechanically activated for the 
shortest time and the lowest for the sample activated for the longest time. This is in 
accordance with the density values obtained (Tab. I), where densities increase is noted for all 
samples during sintering, but it is the highest for the ZSO-40 sample. This can be attributed to 
different particle packing during uniaxial pressing. Also, the formation of spinel with 
increasing activation time could inhibit densification (sintering process). 
A theoretical analysis of the experimental PA results was performed following the 
Rosencwaig–Gersho thermal piston model [8-9] where a complex system in the gas–sample–
backing–microphone configuration given in Fig. 1 is considered. 
Experimental photoacoustic phase and amplitude diagrams were fitted with theoretical 
calculated photoacoustic signals for zinc stannate. Fig. 8 shows the measured and calculated 
phase and amplitude of the photoacoustic spectra for the zinc stannate sample activated for 
160 min. A fitting procedure described in detail in [4] was used for determining thermal 
parameters of zinc stannate, including thermal diffusivity DT (m2s-1), then the diffusion 
coefficient of the minority free carriers D (m2s-1) and optical absorption coefficient α  (m-1). 
The obtained values are given in Tab. I. 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
158 
 
 
600 800 1000 1200
-5
-4
-3
-2
-1
0
3
2
1
∆L
/L
, %
T, 0C
ZnO-SnO2
 
 
1  ZSO-40
2  ZSO-80
3  ZSO-160
Fig.7 Relative shrinkage of ZSO samples as a function of the heating temperature and the 
activation time during non-isothermal sintering up to 1200˚C with a heating rate of 5˚C/min. 
10 100
50
100
150
200
250
(a)
 
 
2
1
P
A
 p
ha
se
 (
de
g)
Modulation frequency (Hz)
     ZSO-160
1  Exp. points
2  Theor. curve
 
1 10 100
1E-5
1E-4
1E-3
0.01
0.1
1
10
(b)
 
 
P
A
 a
m
pl
itu
de
Modulation frequency (Hz)
2
1
     ZSO-160
1  Exp. points
2  Theor. curve
 
 
Fig. 8 (a) Phase photoacoustic spectra for ZSO-160 sample, (b) Amplitude photoacoustic 
spectra for ZSO-160 sample. 
 
 
With the increase of activation time and the amount of the zinc stannate phase present 
the thermal diffusivity value increases. The value of thermal diffusivity obtained for ZSO-160 
(pure zinc stannate phase) is almost identical to the thermal diffusivity value we calculated for 
thin film zinc stannate [10]. To our best knowledge no other thermal diffusivity values for 
Zn2SnO4, synthesized in this way, are available in the literature. 
 
 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
159 
 
 
Conclusion 
 
In this paper synthesis of zinc-stannate was analyzed using photoacoustic 
spectroscopy. Mixtures of ZnO and SnO2 were mechanically activated for 40, 80 and 160 
minutes in a planetary ball mill and then non-isothermally sintered up to 1200oC. X-ray 
diffraction analyses confirmed the beginning of zinc-stannate formation in the milled powders 
and a pure zinc-stannate phase in the sample mechanically activated for 160 min. and then 
sintered. The photoacoustic phase and amplitude diagrams of sintered zinc stannate samples 
were measured in relation to the frequency of the chopped laser beam. It was shown that 
thermal diffusivity of this material increased with the activation time in accordance with the 
formation of single phase Zn2SnO4. 
 
 
Acknowledgement 
 
This research was performed as part of project 142011G financed by the Ministry for 
Science of the Republic of Serbia and project F-198 of Serbian Academy of Sciences and 
Arts. 
 
 
References 
 
1. T. Toyoda, S. Shimamoto, Mater. Sci. Eng. B, 54 (1998) 29. 
2. J. Soldner, K. Stephan, Chem. Eng. Process., 38 (1999) 585. 
3. N. A. George, T. Paul, P. Radhakrishnan, V. P. N. Nampoori, C. P. G. Vallabhan, M. 
T. Sebastian, J. Mater. Sci. Lett., 19 (2000) 499. 
4. P. M. Nikolić, D. Vasiljević, M. Miletić, Glas CCCLXXX of the Serbian Academy of 
Sciences and Arts, Department of Technical Sciences, 32 (1996) 27. 
5. D. M. Todorović, P. M. Nikolić, Opt. Eng., 36 (1997) 432. 
6. T. B. Ivetić, K. M. Vojisavljević, T. Srećković, in: “Fundamental Problems of Physics 
and Technology of Materials”, Eds. M. M. Ristić and A. M. Maričić, Čačak, 2005, p. 
115-121. 
7. N. Nikolić, Z. Marinković, T. Srećković, J. Mat. Sci., 39 (2004) 5239. 
8. P. M. Nikolić, M. V. Nikolić, D. Luković, S. Savić, M. M. Ristić, Z. Metallkd., 95 
(2004) 147. 
9. A. Rosencwaig, A. Gersho, J. Appl. Phys., 47 (1976) 11. 
10. T. Ivetić, M. V. Nikolić, D. L. Young, D. Vasiljević-Radović, D. Urošević, Mater. 
Sci. Forum, 518 (2006) 465. 
 
 
 
Садржај:. Смеше прахова цинк-оксида и калај-оксида помешане су у молском односу 
2:1 и механички активиране 40, 80 и 160 минута у планетарном млину са куглама. 
Добијени прахови пресовани су и неизотермски синтеровани до 1200ºC брзином 
загревања од 5ºC/мин. Квалитативни састав полазних смеша прахова и синтерованих 
узорака одређен је методом рендгенске дифракције. Еволуција микроструктуре 
синтерованих узорака испитана је сканирајућом електронском микроскопијом. 
Фотоакустични фазни и амплитудски спектри синтерованих узорака снимани су у 
функцији чоповане фреквенције употребљеног ласерског зрачења у топлотно-
трансмисионој конфигурацији. Теоријска анализа експерименталних резултата 
омогућила је израчунавање фотоакустичних својстава материјала укључујући и 
T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 
___________________________________________________________________________ 
160 
 
 
топлотну дифузивност. На основу добијених резултата испитана је повезаност 
експерименталних услова припреме узорака и њихове микроструктуре.  
Кључне речи: Цинк-станат, механичка активација, синтеровање, фотоакустична 
спектроскопија.  
           


Investigation of zinc stannate synthesis using photoacoustic spectroscopy

Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200°C with a heating rate of 5°C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed.Preliminary research was presented at a conference: [https://hdl.handle.net/21.15107/rcub_dais_329

Nikolić, Pantelija M.

Blagojević, Vladimir D.

Ristić, Momčilo M.

Serbian Academy of Science and Arts Digital Archive (DAIS)

Science of Sintering, 39 (2007) 153-160 ________________________________________________________________________    doi:10.2298/SOS0702153I  UDK  662.785:661.847.2:546.812:77.026.34 Investigation of Zinc Stannate Synthesis Using Photoacoustic Spectroscopy  T. Ivetić1*), M. V. Nikolić2, P. M. Nikolić1, V. Blagojević3, S. Đurić1,  T. Srećković2, M. M. Ristić1 1Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Serbia 2Center for Multidisciplinary Studies of the University of Belgrade, Kneza Višeslava 1, 11000 Belgrade, Serbia 3Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbia           Abstract:   Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed. Keywords:  Zinc Stannate, Mechanical activation, Sintering, Photoacoustic spectroscopy    Introduction   Ceramic materials with large open porosity, such as zinc stannate (Zn2SnO4) obtained in this paper from the reaction sintering powder-processing route, are convenient for use as humidity sensors. Atmospheric water can be absorbed on the grain surface inside pores or can be condensed in small channels and pores. Desirable characteristics of humidity sensors include: high sensitivity, reversibility, fast response, broad range of moisture selectivity, chemical and thermal stability. These sensor characteristics depend upon parameters of synthesis during the reaction sintering process. Knowing the thermal properties of this kind of material is very important, especially when they are used as electrical components because of the need for an adequate cooling process. In this paper, we present the results of a photoacoustic investigation of thermal and transport properties of bulk zinc stannate synthesized by the reaction sintering process. Photoacoustic (PA) spectroscopy has been used lately, besides for the characterization of electronic, optical and defects structures, for _____________________________ *) Corresponding author: tamara@itn.sanu.ac.yu T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 154   defining the electronic states and structural disorders of ceramic materials [1-4]. In a typical photoacoustic experiment the sample is placed in a closed photoacoustic cell exposed to radiation with a modulated laser beam [5]. The dependence of the obtained photoacoustic signal on the rate of diffunded heat through the sample enables thermal characterization of the sample, i.e. determination of different properties of the analyzed material including thermal diffusivity [3]. The PA signal is a consequence of heat processes in the observed sample. It depends on periodic temperature variation on the rear sample surface, Φ ( ω,l− ) that is in contact with the electret microphone (Fig. 1.).  Fig. 1 The gas-sample-backing-microphone detection configuration.   The following relation can express the PA signal: ( ) ( ) ( ωω )γω ,00 llkTPSb−Φ⋅⋅⋅=     (1)  Where  is the sample thickness; l ω  is the frequency modulation of the excitation light beam; γ  is the adiabatic constant and  and  are ambient pressure and temperature, respectively; 0P 0T( ) ( ) ( )ωµω bb jk /1+= , where ( )ωµb  is the thermal diffusion length of the backing (gas); l  is the distance between the sample and microphone membrane and b ( )ω,lΦ  is the temperature variation of the sample surface that is in contact with microphone.   Experimental  Commercially available zinc oxide and tin oxide powders (Aldrich), with a molar ratio ZnO:SnO2 = 2:1, were mechanically activated in a planetary ball mill (Fritsch Pulverisette 5) in the continuous grinding process regime, in air for 40, 80 and 160 min. Zirconium oxide grinding balls (10 mm in diameter, approx.) and vessels (500 cm3) were used. The powder to balls ratio was 40:1 and the total weight of the powder mixtures was 10 g. Appropriate samples were denoted according to the applied time of activation as ZSO-40, ZSO-80 and ZSO-160. These samples were uniaxially pressed with different pressures (ZSO-40 – 150 MPa, ZSO-80, ZSO-160 – 200 MPa) in accordance with the analysis of the influence of mechanical activation on consolidation of the ZnO-SnO2 system given in detail in [6] in order to obtain the same average green density of 3.769 g/cm3. Relative shrinkage of samples obtained by uniaxial pressing of activated powders was measured by a sensitive dilatometer (Bähr Gerätebau GmbH Typ 702s) in the course of non-isothermal heating in air T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 155   up to 1200˚C with a constant heating rate of 5˚C/min. The densities of sintered samples were determined and the values obtained are given in Tab. I. Characterization of the obtained samples of powder mixtures after grinding and after sintering was carried out using an X-ray diffractometer (Norelco-Philips PW-1050) with CuKα radiation and a step scan mode of 0.02º/0.4s. The microstructure of sintered samples was characterized using scanning electron microscopy (SEM – JSM 5300 JEOL).   Tab. I Measured density and calculated thermal diffusivity of ZSO samples  sintered at 1200˚C Milling time Density (g/cm3) DT (m2s-1) D (m2s-1) α (m-1) 40 4.14 0.21 · 10-7 0.25 · 10-5 9860 80 4.12 1.80 · 10-7 0.10 · 10-5 12400 160 3.86 10.06 · 10-7 0.14 · 10-4 11000  The PA signals were measured using an experimental set-up, with an infrared laser (25 mW) as the optical source. The laser beam was modulated with a mechanical chopper. Changes in phases and amplitudes of the photoacoustic signal of obtained sintered samples were followed.   Results and discussion  X-ray diffraction patterns of ZSO powder mixtures are shown on Fig. 2. Mechanical activation of ZnO and SnO2 powders, results in the start of formation of zinc stannate after 40 minutes of milling (this time was denoted in [7] as the shortest milling time needed for the beginning of zinc-stannate formation), but as the XRD patterns obtained show residual oxides are still present. Longer milling times result in lower amounts of residual oxides present. Formation of Zn2SnO4 is not yet finished after 160 minutes of ball milling, as SnO2 is still present. Fig. 3 shows XRD patterns of powder mixtures after uniaxial pressing and non-isothermal sintering up to 1200˚C with a constant heating rate of 5˚C/min. The peaks from the XRD spectra of samples sintered up to 1200˚C are much sharper and better defined from the non-sintered ones indicating creation of monophased spinel zinc stannate. Only a few peaks derived from residual oxides are present in the ZSO-40 sample and only one ZnO peak developed in the sample mechanically activated for 80 minutes while the ZSO-160 sample shows only the zinc-stannate phase. SEM microphotographs of fractured surfaces of ZSO powder samples non-isothermally sintered in a dilatometer up to 1200˚C with a heating rate of 5˚C/min are presented in Fig. 4-6. SEM microphotographs show that grinding leads to the formation of a structure with a reduced grain size that accelerates spinel formation (which is with agreement with XRD analysis), but agglomerates are also present, probably preserved from the agglomeration of powders. Grain growth of spinel inhibits densification and causes the formation of a porous microstructure.    T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 156    0 10 20 30 40 50 602 θ  ( 0 ) λCuKα(211) (511)(511)(422)(400)(400)(311)(222)(222)(311)(220)(220)(110)(110)(111)(111)(111)(110)(100)(311)(222)(400)(211)(220)zzz  zzzz ZnOz SnO2 Zn2SnO4ZSO-160ZSO-80ZSO-40 Fig. 2 XRD patterns of ZSO powder mixtures as a function of the activation time. 25 30 35 40 45 50 55(100)(200)(200)(111)(111)(111)(200)(210)z(511)(422)(331)(331)(400)(400)(222)(222)(311)(311)(220)(220)(100)(422)(511)(220)(311)(222)(400)(331)(422)(511) ZSO-160ZSO-80ZSO-402 θ  ( 0 ) λCuKα ZnOz SnO2 Zn2SnO4 Al  Fig. 3 XRD patterns of ZSO samples non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min.  T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 157      Fig. 4 SEM fractured surface of the ZSO-40 sample sintered at 1200˚C. Fig. 5 SEM fractured surface of the ZSO-80 sample sintered at 1200˚C  Fig. 6 SEM fractured surface of the ZSO-160 sample sintered at 1200˚C.   Fig. 7 shows the relative shrinkage of samples as a function of temperature during heating for the different activated samples (ZSO-40, ZSO-80 and ZSO-160) obtained by the dilatometer. Dilatometric curves confirm the XRD analysis results that the formed spinel phase dominates in the system so there is no trace of other processes but sintering. It is interesting to note that shrinkage is the highest for the sample mechanically activated for the shortest time and the lowest for the sample activated for the longest time. This is in accordance with the density values obtained (Tab. I), where densities increase is noted for all samples during sintering, but it is the highest for the ZSO-40 sample. This can be attributed to different particle packing during uniaxial pressing. Also, the formation of spinel with increasing activation time could inhibit densification (sintering process). A theoretical analysis of the experimental PA results was performed following the Rosencwaig–Gersho thermal piston model [8-9] where a complex system in the gas–sample–backing–microphone configuration given in Fig. 1 is considered. Experimental photoacoustic phase and amplitude diagrams were fitted with theoretical calculated photoacoustic signals for zinc stannate. Fig. 8 shows the measured and calculated phase and amplitude of the photoacoustic spectra for the zinc stannate sample activated for 160 min. A fitting procedure described in detail in [4] was used for determining thermal parameters of zinc stannate, including thermal diffusivity DT (m2s-1), then the diffusion coefficient of the minority free carriers D (m2s-1) and optical absorption coefficient α  (m-1). The obtained values are given in Tab. I. T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 158   600 800 1000 1200-5-4-3-2-10321∆L/L, %T, 0CZnO-SnO2  1  ZSO-402  ZSO-803  ZSO-160Fig.7 Relative shrinkage of ZSO samples as a function of the heating temperature and the activation time during non-isothermal sintering up to 1200˚C with a heating rate of 5˚C/min. 10 10050100150200250(a)  21PA phase (deg)Modulation frequency (Hz)     ZSO-1601  Exp. points2  Theor. curve 1 10 1001E-51E-41E-30.010.1110(b)  PA amplitudeModulation frequency (Hz)21     ZSO-1601  Exp. points2  Theor. curve  Fig. 8 (a) Phase photoacoustic spectra for ZSO-160 sample, (b) Amplitude photoacoustic spectra for ZSO-160 sample.   With the increase of activation time and the amount of the zinc stannate phase present the thermal diffusivity value increases. The value of thermal diffusivity obtained for ZSO-160 (pure zinc stannate phase) is almost identical to the thermal diffusivity value we calculated for thin film zinc stannate [10]. To our best knowledge no other thermal diffusivity values for Zn2SnO4, synthesized in this way, are available in the literature.   T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 159   Conclusion  In this paper synthesis of zinc-stannate was analyzed using photoacoustic spectroscopy. Mixtures of ZnO and SnO2 were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill and then non-isothermally sintered up to 1200oC. X-ray diffraction analyses confirmed the beginning of zinc-stannate formation in the milled powders and a pure zinc-stannate phase in the sample mechanically activated for 160 min. and then sintered. The photoacoustic phase and amplitude diagrams of sintered zinc stannate samples were measured in relation to the frequency of the chopped laser beam. It was shown that thermal diffusivity of this material increased with the activation time in accordance with the formation of single phase Zn2SnO4.   Acknowledgement  This research was performed as part of project 142011G financed by the Ministry for Science of the Republic of Serbia and project F-198 of Serbian Academy of Sciences and Arts.   References  1. T. Toyoda, S. Shimamoto, Mater. Sci. Eng. B, 54 (1998) 29. 2. J. Soldner, K. Stephan, Chem. Eng. Process., 38 (1999) 585. 3. N. A. George, T. Paul, P. Radhakrishnan, V. P. N. Nampoori, C. P. G. Vallabhan, M. T. Sebastian, J. Mater. Sci. Lett., 19 (2000) 499. 4. P. M. Nikolić, D. Vasiljević, M. Miletić, Glas CCCLXXX of the Serbian Academy of Sciences and Arts, Department of Technical Sciences, 32 (1996) 27. 5. D. M. Todorović, P. M. Nikolić, Opt. Eng., 36 (1997) 432. 6. T. B. Ivetić, K. M. Vojisavljević, T. Srećković, in: “Fundamental Problems of Physics and Technology of Materials”, Eds. M. M. Ristić and A. M. Maričić, Čačak, 2005, p. 115-121. 7. N. Nikolić, Z. Marinković, T. Srećković, J. Mat. Sci., 39 (2004) 5239. 8. P. M. Nikolić, M. V. Nikolić, D. Luković, S. Savić, M. M. Ristić, Z. Metallkd., 95 (2004) 147. 9. A. Rosencwaig, A. Gersho, J. Appl. Phys., 47 (1976) 11. 10. T. Ivetić, M. V. Nikolić, D. L. Young, D. Vasiljević-Radović, D. Urošević, Mater. Sci. Forum, 518 (2006) 465.    Садржај:. Смеше прахова цинк-оксида и калај-оксида помешане су у молском односу 2:1 и механички активиране 40, 80 и 160 минута у планетарном млину са куглама. Добијени прахови пресовани су и неизотермски синтеровани до 1200ºC брзином загревања од 5ºC/мин. Квалитативни састав полазних смеша прахова и синтерованих узорака одређен је методом рендгенске дифракције. Еволуција микроструктуре синтерованих узорака испитана је сканирајућом електронском микроскопијом. Фотоакустични фазни и амплитудски спектри синтерованих узорака снимани су у функцији чоповане фреквенције употребљеног ласерског зрачења у топлотно-трансмисионој конфигурацији. Теоријска анализа експерименталних резултата омогућила је израчунавање фотоакустичних својстава материјала укључујући и T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 160   топлотну дифузивност. На основу добијених резултата испитана је повезаност експерименталних услова припреме узорака и њихове микроструктуре.  Кључне речи: Цинк-станат, механичка активација, синтеровање, фотоакустична спектроскопија.             

Science of Sintering

English

Poster presented at the conference Fizika i tehnologija materijala - FITEM '05, Čačak, 2-5. avgust 2005.The final results were published in the article: [http://dx.doi.org/10.2298/SOS0702153I

Aleksić, Obrad S.

INVESTIGATION OF ZINC STANNATE SYNTHESIS USING PHOTOACOUSTIC SPECTROSCOPYTamara Ivetić1, a, M. V. Nikolić2, P. M. Nikolić1, V. Blagojević3, S. Đurić1, T. Srećković2, M. M. Ristić11Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade,  Serbia2Center for Multidisciplinary Studies of the University of Belgrade, Belgrade, Serbia3Faculty of Electrical Engineering, University of Belgrade, Belgrade, Serbiaatamara@itn.sanu.ac.yuAbstractMixtures of ZnO and SnO2 powders, with the molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200ºC with a heating rate of 5ºC/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacousti c properties including thermal diffusivity.0 10 20 30 40 50 602 θ  ( 0 ) λCuKα(211) (511)(511)(422)(400)(400)(311)(222)(222)(311)(220)(220)(110)(110)(111)(111)(111)(110)(100)(311)(222)(400)(211)(220)zzz  zzzz ZnOz SnO2 Zn2SnO4160 min.80 min.40 min.25 30 35 40 45 50 55(100)(200)(200)(111)(111)(111)(200)(210)z(511)(422)(331)(331)(400)(400)(222)(222)(311)(311)(220)(220)(100)(422)(511)(220)(311)(222)(400)(331)(422)(511) 160 min.80 min.40 min.2 θ  ( 0 ) λCuKα ZnOz SnO2 Zn2SnO4 Al600 800 1000 1200-5-4-3-2-10321ΔL/L, %T, 0CZnO-SnO2  1  ZSO-402  ZSO-803  ZSO-16010 10050100150200250  21PA phase (deg)Modulation frequency (Hz)     ZSO-1601  Exp. points2  Theor. curve1 10 1001E-51E-41E-30.010.1110  PA amplitudeModulation frequency (Hz)21     ZSO-1601  Exp. points2  Theor. curveVI National Conference Physics and Technology of Materials July 31-August 03, 2005, Čačak, SerbiaIntroductionZinc stannate belongs to A2 BO4 compounds (A = group II, e.g. Zn, B = group IV, e.g. Sn, Ge). They are called spinels and have semiconducting properties. Presumably their sensor properties are mostly derived from the fact that their electrica l conductivity is sensitive to oxygen stoichiometry and environmental atmosphere. Zinc stannate spinel, Zn2 SnO4 , investigated in this work is potentially good gas and humidity sensor. In this paper, we present the results of a photoacoustic investigation of thermal and transport properties of bulk zinc stannate synthesized by reaction sintering process. Photoacoustic (PA) spectroscopy has been used lately, besides for the characterization of electronic, optical and defects stuctures, for defining the electronic states and structural disorders of ceramic materials.AcknowledgementThis research was performed within the project No. 1832 entitled “Synthesis of functional materials from the synthesis-structure-properties-application relationship”, financed by the Ministry for Science and Environmental Protection of the Republic of SerbiaExperimentalResultsFig. 1 XRD patterns of ZTO powder mixtures as a function of the time of activationFig. 2 XDR patterns of ZSO samples non- isothermally sintered up to  1200°C with a heating rate of 5ºC/min.Fig. 3 SEM fractured surface of the ZSO-40 sample sintered at 1200°C.Fig. 4 SEM fractured surface of the ZSO-80 sample sintered at 1200°C.Fig. 5 SEM fractured surface of the ZSO-160 sample sintered at 1200°C.Fig. 6 Relative shrinkage of ZSO samples as a function of the heating temperature and time of activation during non-isothermal sintering up to 1200ºC with a heating rate of 5ºC/min.Fig. 8 The gas-sample-backing-microphone detection configuration.Fig. 7 The experimental set-up for PA measurements.Mill type Planetary ball mill(Fritsch Pulverisette 5)Starting powders ZnOSnO2ZnO:SnO2 molar ratio 2:1mpowder /mball ratio(Dball = 10 mm)1:40Grinding time, min. 40 80 160Sample ZSO-40 ZSO-80 ZSO-160-X-ray diffractometer (Norelco-Philips PW-1050) with CuKα radiation and a step scan mode of 0.02/0.4s-Scanning electron microscopy (JSM 5300 JEOL)-Sensitive dilatometer (Bähr Gerätebau GmbH Type 702s)-Photoacoustic set-up with an infrared laser (25 mW) as the optical source (Fig. 7)Fig. 9 Phase photoacoustic spectra for ZSO-160 sample.Fig. 10 Amplitude photoacoustic spectra for ZSO-160 sample.Conclusion-monophased zinc stannate was synthesized when the mixture milled for 160 min was sintered at 1200°C-grinding leads to the formation of a structure with reduced grain size that accelerates spinel formation (SEM and XRD analysis) but agglomerates also present-grain growth of spinel with increasing activation time could inhibits densification and cause the formation of a porous microstructure (dilatometry and SEM)-the value of the thermal diffusivity obtained for ZSO-160 (pure zinc stannate phase) is almost identical to thermal diffusivity value we calculated for thin film zinc stannate (DT – thermal diffusivity = 0,1006 · 10-5 m2s-1)-to our best knowledge no other thermal diffusivity values for Zn2 SnO4 , synthesized in this way, are available in the literature( )( )microphoneth thecontact wiin  is that surface sample theof  variationetemperatur-,membrane microphone theand sample ebetween th distance- gas backing  theoflengthdiffusion  thermal-,1re temperatuand pressureambiant -,gas  theofconstant  adiabatic-beamlaser   theoffrequency - thicknesssample-00ωμμγωlljkTPlSbbbbS−Φ−−+=−−−−−( ) ( ) ,l   lkTP ,l  S SSbb00 ω−Φ⋅⋅⋅γ=ω−

Investigation of Zinc-Stannate Synthesis Using Photoacoustic Spectroscopy

Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200 degrees C with a heating rate of 5 degrees C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusiviiy. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed

Ivetić, Tamara B.

Duric, S.

Ristic, M. M.

RIMSI

Science of Sintering, 39 (2007) 153-160 ________________________________________________________________________    doi:10.2298/SOS0702153I  UDK  662.785:661.847.2:546.812:77.026.34 Investigation of Zinc Stannate Synthesis Using Photoacoustic Spectroscopy  T. Ivetić1*), M. V. Nikolić2, P. M. Nikolić1, V. Blagojević3, S. Đurić1,  T. Srećković2, M. M. Ristić1 1Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Serbia 2Center for Multidisciplinary Studies of the University of Belgrade, Kneza Višeslava 1, 11000 Belgrade, Serbia 3Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbia           Abstract:   Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed. Keywords:  Zinc Stannate, Mechanical activation, Sintering, Photoacoustic spectroscopy    Introduction   Ceramic materials with large open porosity, such as zinc stannate (Zn2SnO4) obtained in this paper from the reaction sintering powder-processing route, are convenient for use as humidity sensors. Atmospheric water can be absorbed on the grain surface inside pores or can be condensed in small channels and pores. Desirable characteristics of humidity sensors include: high sensitivity, reversibility, fast response, broad range of moisture selectivity, chemical and thermal stability. These sensor characteristics depend upon parameters of synthesis during the reaction sintering process. Knowing the thermal properties of this kind of material is very important, especially when they are used as electrical components because of the need for an adequate cooling process. In this paper, we present the results of a photoacoustic investigation of thermal and transport properties of bulk zinc stannate synthesized by the reaction sintering process. Photoacoustic (PA) spectroscopy has been used lately, besides for the characterization of electronic, optical and defects structures, for _____________________________ *) Corresponding author: tamara@itn.sanu.ac.yu T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 154   defining the electronic states and structural disorders of ceramic materials [1-4]. In a typical photoacoustic experiment the sample is placed in a closed photoacoustic cell exposed to radiation with a modulated laser beam [5]. The dependence of the obtained photoacoustic signal on the rate of diffunded heat through the sample enables thermal characterization of the sample, i.e. determination of different properties of the analyzed material including thermal diffusivity [3]. The PA signal is a consequence of heat processes in the observed sample. It depends on periodic temperature variation on the rear sample surface, Φ ( ω,l− ) that is in contact with the electret microphone (Fig. 1.).  Fig. 1 The gas-sample-backing-microphone detection configuration.   The following relation can express the PA signal: ( ) ( ) ( ωω )γω ,00 llkTPSb−Φ⋅⋅⋅=     (1)  Where  is the sample thickness; l ω  is the frequency modulation of the excitation light beam; γ  is the adiabatic constant and  and  are ambient pressure and temperature, respectively; 0P 0T( ) ( ) ( )ωµω bb jk /1+= , where ( )ωµb  is the thermal diffusion length of the backing (gas); l  is the distance between the sample and microphone membrane and b ( )ω,lΦ  is the temperature variation of the sample surface that is in contact with microphone.   Experimental  Commercially available zinc oxide and tin oxide powders (Aldrich), with a molar ratio ZnO:SnO2 = 2:1, were mechanically activated in a planetary ball mill (Fritsch Pulverisette 5) in the continuous grinding process regime, in air for 40, 80 and 160 min. Zirconium oxide grinding balls (10 mm in diameter, approx.) and vessels (500 cm3) were used. The powder to balls ratio was 40:1 and the total weight of the powder mixtures was 10 g. Appropriate samples were denoted according to the applied time of activation as ZSO-40, ZSO-80 and ZSO-160. These samples were uniaxially pressed with different pressures (ZSO-40 – 150 MPa, ZSO-80, ZSO-160 – 200 MPa) in accordance with the analysis of the influence of mechanical activation on consolidation of the ZnO-SnO2 system given in detail in [6] in order to obtain the same average green density of 3.769 g/cm3. Relative shrinkage of samples obtained by uniaxial pressing of activated powders was measured by a sensitive dilatometer (Bähr Gerätebau GmbH Typ 702s) in the course of non-isothermal heating in air T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 155   up to 1200˚C with a constant heating rate of 5˚C/min. The densities of sintered samples were determined and the values obtained are given in Tab. I. Characterization of the obtained samples of powder mixtures after grinding and after sintering was carried out using an X-ray diffractometer (Norelco-Philips PW-1050) with CuKα radiation and a step scan mode of 0.02º/0.4s. The microstructure of sintered samples was characterized using scanning electron microscopy (SEM – JSM 5300 JEOL).   Tab. I Measured density and calculated thermal diffusivity of ZSO samples  sintered at 1200˚C Milling time Density (g/cm3) DT (m2s-1) D (m2s-1) α (m-1) 40 4.14 0.21 · 10-7 0.25 · 10-5 9860 80 4.12 1.80 · 10-7 0.10 · 10-5 12400 160 3.86 10.06 · 10-7 0.14 · 10-4 11000  The PA signals were measured using an experimental set-up, with an infrared laser (25 mW) as the optical source. The laser beam was modulated with a mechanical chopper. Changes in phases and amplitudes of the photoacoustic signal of obtained sintered samples were followed.   Results and discussion  X-ray diffraction patterns of ZSO powder mixtures are shown on Fig. 2. Mechanical activation of ZnO and SnO2 powders, results in the start of formation of zinc stannate after 40 minutes of milling (this time was denoted in [7] as the shortest milling time needed for the beginning of zinc-stannate formation), but as the XRD patterns obtained show residual oxides are still present. Longer milling times result in lower amounts of residual oxides present. Formation of Zn2SnO4 is not yet finished after 160 minutes of ball milling, as SnO2 is still present. Fig. 3 shows XRD patterns of powder mixtures after uniaxial pressing and non-isothermal sintering up to 1200˚C with a constant heating rate of 5˚C/min. The peaks from the XRD spectra of samples sintered up to 1200˚C are much sharper and better defined from the non-sintered ones indicating creation of monophased spinel zinc stannate. Only a few peaks derived from residual oxides are present in the ZSO-40 sample and only one ZnO peak developed in the sample mechanically activated for 80 minutes while the ZSO-160 sample shows only the zinc-stannate phase. SEM microphotographs of fractured surfaces of ZSO powder samples non-isothermally sintered in a dilatometer up to 1200˚C with a heating rate of 5˚C/min are presented in Fig. 4-6. SEM microphotographs show that grinding leads to the formation of a structure with a reduced grain size that accelerates spinel formation (which is with agreement with XRD analysis), but agglomerates are also present, probably preserved from the agglomeration of powders. Grain growth of spinel inhibits densification and causes the formation of a porous microstructure.    T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 156    0 10 20 30 40 50 602 θ  ( 0 ) λCuKα(211) (511)(511)(422)(400)(400)(311)(222)(222)(311)(220)(220)(110)(110)(111)(111)(111)(110)(100)(311)(222)(400)(211)(220) ZnO SnO2 Zn2SnO4ZSO-160ZSO-80ZSO-40 Fig. 2 XRD patterns of ZSO powder mixtures as a function of the activation time. 25 30 35 40 45 50 55(100)(200)(200)(111)(111)(111) (200)(210)(511)(422)(331)(331)(400)(400)(222)(222)(311)(311)(220)(220)(100)(422)(511)(220)(311)(222)(400)(331)(422)(511)ZSO-160ZSO-80ZSO-402 θ  ( 0 ) λCuKα ZnO SnO2 Zn2SnO4 Al  Fig. 3 XRD patterns of ZSO samples non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min.  T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 157      Fig. 4 SEM fractured surface of the ZSO-40 sample sintered at 1200˚C. Fig. 5 SEM fractured surface of the ZSO-80 sample sintered at 1200˚C  Fig. 6 SEM fractured surface of the ZSO-160 sample sintered at 1200˚C.   Fig. 7 shows the relative shrinkage of samples as a function of temperature during heating for the different activated samples (ZSO-40, ZSO-80 and ZSO-160) obtained by the dilatometer. Dilatometric curves confirm the XRD analysis results that the formed spinel phase dominates in the system so there is no trace of other processes but sintering. It is interesting to note that shrinkage is the highest for the sample mechanically activated for the shortest time and the lowest for the sample activated for the longest time. This is in accordance with the density values obtained (Tab. I), where densities increase is noted for all samples during sintering, but it is the highest for the ZSO-40 sample. This can be attributed to different particle packing during uniaxial pressing. Also, the formation of spinel with increasing activation time could inhibit densification (sintering process). A theoretical analysis of the experimental PA results was performed following the Rosencwaig–Gersho thermal piston model [8-9] where a complex system in the gas–sample–backing–microphone configuration given in Fig. 1 is considered. Experimental photoacoustic phase and amplitude diagrams were fitted with theoretical calculated photoacoustic signals for zinc stannate. Fig. 8 shows the measured and calculated phase and amplitude of the photoacoustic spectra for the zinc stannate sample activated for 160 min. A fitting procedure described in detail in [4] was used for determining thermal parameters of zinc stannate, including thermal diffusivity DT (m2s-1), then the diffusion coefficient of the minority free carriers D (m2s-1) and optical absorption coefficient α  (m-1). The obtained values are given in Tab. I. T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 158   600 800 1000 1200-5-4-3-2-10321∆L/L, %T, 0CZnO-SnO2  1  ZSO-402  ZSO-803  ZSO-160Fig.7 Relative shrinkage of ZSO samples as a function of the heating temperature and the activation time during non-isothermal sintering up to 1200˚C with a heating rate of 5˚C/min. 10 10050100150200250(a)  21PA phase (deg)Modulation frequency (Hz)     ZSO-1601  Exp. points2  Theor. curve 1 10 1001E-51E-41E-30.010.1110(b)  PA amplitudeModulation frequency (Hz)21     ZSO-1601  Exp. points2  Theor. curve  Fig. 8 (a) Phase photoacoustic spectra for ZSO-160 sample, (b) Amplitude photoacoustic spectra for ZSO-160 sample.   With the increase of activation time and the amount of the zinc stannate phase present the thermal diffusivity value increases. The value of thermal diffusivity obtained for ZSO-160 (pure zinc stannate phase) is almost identical to the thermal diffusivity value we calculated for thin film zinc stannate [10]. To our best knowledge no other thermal diffusivity values for Zn2SnO4, synthesized in this way, are available in the literature.   T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 159   Conclusion  In this paper synthesis of zinc-stannate was analyzed using photoacoustic spectroscopy. Mixtures of ZnO and SnO2 were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill and then non-isothermally sintered up to 1200oC. X-ray diffraction analyses confirmed the beginning of zinc-stannate formation in the milled powders and a pure zinc-stannate phase in the sample mechanically activated for 160 min. and then sintered. The photoacoustic phase and amplitude diagrams of sintered zinc stannate samples were measured in relation to the frequency of the chopped laser beam. It was shown that thermal diffusivity of this material increased with the activation time in accordance with the formation of single phase Zn2SnO4.   Acknowledgement  This research was performed as part of project 142011G financed by the Ministry for Science of the Republic of Serbia and project F-198 of Serbian Academy of Sciences and Arts.   References  1. T. Toyoda, S. Shimamoto, Mater. Sci. Eng. B, 54 (1998) 29. 2. J. Soldner, K. Stephan, Chem. Eng. Process., 38 (1999) 585. 3. N. A. George, T. Paul, P. Radhakrishnan, V. P. N. Nampoori, C. P. G. Vallabhan, M. T. Sebastian, J. Mater. Sci. Lett., 19 (2000) 499. 4. P. M. Nikolić, D. Vasiljević, M. Miletić, Glas CCCLXXX of the Serbian Academy of Sciences and Arts, Department of Technical Sciences, 32 (1996) 27. 5. D. M. Todorović, P. M. Nikolić, Opt. Eng., 36 (1997) 432. 6. T. B. Ivetić, K. M. Vojisavljević, T. Srećković, in: “Fundamental Problems of Physics and Technology of Materials”, Eds. M. M. Ristić and A. M. Maričić, Čačak, 2005, p. 115-121. 7. N. Nikolić, Z. Marinković, T. Srećković, J. Mat. Sci., 39 (2004) 5239. 8. P. M. Nikolić, M. V. Nikolić, D. Luković, S. Savić, M. M. Ristić, Z. Metallkd., 95 (2004) 147. 9. A. Rosencwaig, A. Gersho, J. Appl. Phys., 47 (1976) 11. 10. T. Ivetić, M. V. Nikolić, D. L. Young, D. Vasiljević-Radović, D. Urošević, Mater. Sci. Forum, 518 (2006) 465.    Садржај:. Смеше прахова цинк-оксида и калај-оксида помешане су у молском односу 2:1 и механички активиране 40, 80 и 160 минута у планетарном млину са куглама. Добијени прахови пресовани су и неизотермски синтеровани до 1200ºC брзином загревања од 5ºC/мин. Квалитативни састав полазних смеша прахова и синтерованих узорака одређен је методом рендгенске дифракције. Еволуција микроструктуре синтерованих узорака испитана је сканирајућом електронском микроскопијом. Фотоакустични фазни и амплитудски спектри синтерованих узорака снимани су у функцији чоповане фреквенције употребљеног ласерског зрачења у топлотно-трансмисионој конфигурацији. Теоријска анализа експерименталних резултата омогућила је израчунавање фотоакустичних својстава материјала укључујући и T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 160   топлотну дифузивност. На основу добијених резултата испитана је повезаност експерименталних услова припреме узорака и њихове микроструктуре.  Кључне речи: Цинк-станат, механичка активација, синтеровање, фотоакустична спектроскопија.             

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Science of Sintering, 39 (2007) 153-160 ________________________________________________________________________    doi:10.2298/SOS0702153I  UDK  662.785:661.847.2:546.812:77.026.34 Investigation of Zinc Stannate Synthesis Using Photoacoustic Spectroscopy  T. Ivetić1*), M. V. Nikolić2, P. M. Nikolić1, V. Blagojević3, S. Đurić1,  T. Srećković2, M. M. Ristić1 1Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Serbia 2Center for Multidisciplinary Studies of the University of Belgrade, Kneza Višeslava 1, 11000 Belgrade, Serbia 3Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbia           Abstract:   Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed. Keywords:  Zinc Stannate, Mechanical activation, Sintering, Photoacoustic spectroscopy    Introduction   Ceramic materials with large open porosity, such as zinc stannate (Zn2SnO4) obtained in this paper from the reaction sintering powder-processing route, are convenient for use as humidity sensors. Atmospheric water can be absorbed on the grain surface inside pores or can be condensed in small channels and pores. Desirable characteristics of humidity sensors include: high sensitivity, reversibility, fast response, broad range of moisture selectivity, chemical and thermal stability. These sensor characteristics depend upon parameters of synthesis during the reaction sintering process. Knowing the thermal properties of this kind of material is very important, especially when they are used as electrical components because of the need for an adequate cooling process. In this paper, we present the results of a photoacoustic investigation of thermal and transport properties of bulk zinc stannate synthesized by the reaction sintering process. Photoacoustic (PA) spectroscopy has been used lately, besides for the characterization of electronic, optical and defects structures, for _____________________________ *) Corresponding author: tamara@itn.sanu.ac.yu T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 154   defining the electronic states and structural disorders of ceramic materials [1-4]. In a typical photoacoustic experiment the sample is placed in a closed photoacoustic cell exposed to radiation with a modulated laser beam [5]. The dependence of the obtained photoacoustic signal on the rate of diffunded heat through the sample enables thermal characterization of the sample, i.e. determination of different properties of the analyzed material including thermal diffusivity [3]. The PA signal is a consequence of heat processes in the observed sample. It depends on periodic temperature variation on the rear sample surface, Φ ( ω , l − ) that is in contact with the electret microphone (Fig. 1.).  Fig. 1 The gas-sample-backing-microphone detection configuration.   The following relation can express the PA signal: () () ( ωω )γω ,00 ll k TPSb− Φ⋅ ⋅⋅=      (1)  Where   is the sample thickness;  l ω  is the frequency modulation of the excitation light beam; γ  is the adiabatic constant and   and   are ambient pressure and temperature, respectively; 0 P 0 T()( ) () ω µ ω b b j k / 1+ = , where  ( ) ω µb  is the thermal diffusion length of the backing (gas); l  is the distance between the sample and microphone membrane and  b () ω , l Φ  is the temperature variation of the sample surface that is in contact with microphone.   Experimental  Commercially available zinc oxide and tin oxide powders (Aldrich), with a molar ratio ZnO:SnO2 = 2:1, were mechanically activated in a planetary ball mill (Fritsch Pulverisette 5) in the continuous grinding process regime, in air for 40, 80 and 160 min. Zirconium oxide grinding balls (10 mm in diameter, approx.) and vessels (500 cm3) were used. The powder to balls ratio was 40:1 and the total weight of the powder mixtures was 10 g. Appropriate samples were denoted according to the applied time of activation as ZSO-40, ZSO-80 and ZSO-160. These samples were uniaxially pressed with different pressures (ZSO-40 – 150 MPa, ZSO-80, ZSO-160 – 200 MPa) in accordance with the analysis of the influence of mechanical activation on consolidation of the ZnO-SnO2 system given in detail in [6] in order to obtain the same average green density of 3.769 g/cm3. Relative shrinkage of samples obtained by uniaxial pressing of activated powders was measured by a sensitive dilatometer (Bähr Gerätebau GmbH Typ 702s) in the course of non-isothermal heating in air T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 155   up to 1200˚C with a constant heating rate of 5˚C/min. The densities of sintered samples were determined and the values obtained are given in Tab. I. Characterization of the obtained samples of powder mixtures after grinding and after sintering was carried out using an X-ray diffractometer (Norelco-Philips PW-1050) with CuKα radiation and a step scan mode of 0.02º/0.4s. The microstructure of sintered samples was characterized using scanning electron microscopy (SEM – JSM 5300 JEOL).   Tab. I Measured density and calculated thermal diffusivity of ZSO samples  sintered at 1200˚C Milling time  Density (g/cm3)  DT (m2s-1) D  (m2s-1)  α (m-1) 40  4.14  0.21 · 10-7  0.25 · 10-5 9860 80  4.12  1.80 · 10-7  0.10 · 10-5 12400 160  3.86  10.06 · 10-7  0.14 · 10-4 11000  The PA signals were measured using an experimental set-up, with an infrared laser (25 mW) as the optical source. The laser beam was modulated with a mechanical chopper. Changes in phases and amplitudes of the photoacoustic signal of obtained sintered samples were followed.   Results and discussion  X-ray diffraction patterns of ZSO powder mixtures are shown on Fig. 2. Mechanical activation of ZnO and SnO2 powders, results in the start of formation of zinc stannate after 40 minutes of milling (this time was denoted in [7] as the shortest milling time needed for the beginning of zinc-stannate formation), but as the XRD patterns obtained show residual oxides are still present. Longer milling times result in lower amounts of residual oxides present. Formation of Zn2SnO4 is not yet finished after 160 minutes of ball milling, as SnO2 is still present. Fig. 3 shows XRD patterns of powder mixtures after uniaxial pressing and non-isothermal sintering up to 1200˚C with a constant heating rate of 5˚C/min. The peaks from the XRD spectra of samples sintered up to 1200˚C are much sharper and better defined from the non-sintered ones indicating creation of monophased spinel zinc stannate. Only a few peaks derived from residual oxides are present in the ZSO-40 sample and only one ZnO peak developed in the sample mechanically activated for 80 minutes while the ZSO-160 sample shows only the zinc-stannate phase. SEM microphotographs of fractured surfaces of ZSO powder samples non-isothermally sintered in a dilatometer up to 1200˚C with a heating rate of 5˚C/min are presented in Fig. 4-6. SEM microphotographs show that grinding leads to the formation of a structure with a reduced grain size that accelerates spinel formation (which is with agreement with XRD analysis), but agglomerates are also present, probably preserved from the agglomeration of powders. Grain growth of spinel inhibits densification and causes the formation of a porous microstructure.    T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 156    0 1 02 03 04 05 06 02 θ ( 0 ) λCuKα(211)(511)(511)(422) (400)(400)(311)(222)(222)(311)(220)(220)(110)(110)(111)(111)(111)(110)(100)(311)(222)(400)(211)(220)                              ZnO  SnO2  Zn2SnO4ZSO-160ZSO-80ZSO-40 Fig. 2 XRD patterns of ZSO powder mixtures as a function of the activation time. 25 30 35 40 45 50 55 (100)(200)(200)(111)(111)(111)     (200)  (210) (511)(422)(331)(331)(400)(400)(222)(222)(311)(311)(220)(220)(100) (422)(511)(220)(311)(222)(400)(331)(422)(511)                     ZSO-160ZSO-80ZSO-402 θ ( 0 ) λCuKα  ZnO  SnO2  Zn2SnO4  Al  Fig. 3 XRD patterns of ZSO samples non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min.  T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 157       Fig. 4 SEM fractured surface of the ZSO-40 sample sintered at 1200˚C. Fig. 5 SEM fractured surface of the ZSO-80 sample sintered at 1200˚C  Fig. 6 SEM fractured surface of the ZSO-160 sample sintered at 1200˚C.   Fig. 7 shows the relative shrinkage of samples as a function of temperature during heating for the different activated samples (ZSO-40, ZSO-80 and ZSO-160) obtained by the dilatometer. Dilatometric curves confirm the XRD analysis results that the formed spinel phase dominates in the system so there is no trace of other processes but sintering. It is interesting to note that shrinkage is the highest for the sample mechanically activated for the shortest time and the lowest for the sample activated for the longest time. This is in accordance with the density values obtained (Tab. I), where densities increase is noted for all samples during sintering, but it is the highest for the ZSO-40 sample. This can be attributed to different particle packing during uniaxial pressing. Also, the formation of spinel with increasing activation time could inhibit densification (sintering process). A theoretical analysis of the experimental PA results was performed following the Rosencwaig–Gersho thermal piston model [8-9] where a complex system in the gas–sample–backing–microphone configuration given in Fig. 1 is considered. Experimental photoacoustic phase and amplitude diagrams were fitted with theoretical calculated photoacoustic signals for zinc stannate. Fig. 8 shows the measured and calculated phase and amplitude of the photoacoustic spectra for the zinc stannate sample activated for 160 min. A fitting procedure described in detail in [4] was used for determining thermal parameters of zinc stannate, including thermal diffusivity DT (m2s-1), then the diffusion coefficient of the minority free carriers D (m2s-1) and optical absorption coefficient α  (m-1). The obtained values are given in Tab. I. T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 158   600 800 1000 1200-5-4-3-2-10321∆L/L, %T, 0CZnO-SnO2  1  ZSO-402  ZSO-803  ZSO-160Fig.7 Relative shrinkage of ZSO samples as a function of the heating temperature and the activation time during non-isothermal sintering up to 1200˚C with a heating rate of 5˚C/min. 10 10050100150200250(a)  21PA phase (deg)Modulation frequency (Hz)     ZSO-1601  Exp. points2  Theor. curve 11 0 1 0 01E-51E-41E-30.010.1110(b)  PA amplitudeModulation frequency (Hz)21     ZSO-1601  Exp. points2  Theor. curve  Fig. 8 (a) Phase photoacoustic spectra for ZSO-160 sample, (b) Amplitude photoacoustic spectra for ZSO-160 sample.   With the increase of activation time and the amount of the zinc stannate phase present the thermal diffusivity value increases. The value of thermal diffusivity obtained for ZSO-160 (pure zinc stannate phase) is almost identical to the thermal diffusivity value we calculated for thin film zinc stannate [10]. To our best knowledge no other thermal diffusivity values for Zn2SnO4, synthesized in this way, are available in the literature.   T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 159   Conclusion  In this paper synthesis of zinc-stannate was analyzed using photoacoustic spectroscopy. Mixtures of ZnO and SnO2 were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill and then non-isothermally sintered up to 1200oC. X-ray diffraction analyses confirmed the beginning of zinc-stannate formation in the milled powders and a pure zinc-stannate phase in the sample mechanically activated for 160 min. and then sintered. The photoacoustic phase and amplitude diagrams of sintered zinc stannate samples were measured in relation to the frequency of the chopped laser beam. It was shown that thermal diffusivity of this material increased with the activation time in accordance with the formation of single phase Zn2SnO4.   Acknowledgement  This research was performed as part of project 142011G financed by the Ministry for Science of the Republic of Serbia and project F-198 of Serbian Academy of Sciences and Arts.   References  1.  T. Toyoda, S. Shimamoto, Mater. Sci. Eng. B, 54 (1998) 29. 2.  J. Soldner, K. Stephan, Chem. Eng. Process., 38 (1999) 585. 3.  N. A. George, T. Paul, P. Radhakrishnan, V. P. N. Nampoori, C. P. G. Vallabhan, M. T. Sebastian, J. Mater. Sci. Lett., 19 (2000) 499. 4.  P. M. Nikolić, D. Vasiljević, M. Miletić, Glas CCCLXXX of the Serbian Academy of Sciences and Arts, Department of Technical Sciences, 32 (1996) 27. 5.  D. M. Todorović, P. M. Nikolić, Opt. Eng., 36 (1997) 432. 6.  T. B. Ivetić, K. M. Vojisavljević, T. Srećković, in: “Fundamental Problems of Physics and Technology of Materials”, Eds. M. M. Ristić and A. M. Maričić, Čačak, 2005, p. 115-121. 7. N.  Nikolić, Z. Marinković, T. Srećković, J. Mat. Sci., 39 (2004) 5239. 8.  P. M. Nikolić, M. V. Nikolić, D. Luković, S. Savić, M. M. Ristić, Z. Metallkd., 95 (2004) 147. 9.  A. Rosencwaig, A. Gersho, J. Appl. Phys., 47 (1976) 11. 10. T. Ivetić, M. V. Nikolić, D. L. Young, D. Vasiljević-Radović, D. Urošević, Mater. Sci. Forum, 518 (2006) 465.    Садржај:. Смеше прахова цинк-оксида и калај-оксида помешане су у молском односу 2:1 и механички активиране 40, 80 и 160 минута у планетарном млину са куглама. Добијени  прахови  пресовани  су  и  неизотермски  синтеровани  до 1200ºC брзином загревања од 5ºC/мин. Квалитативни састав полазних смеша прахова и синтерованих узорака  одређен  је  методом  рендгенске  дифракције.  Еволуција  микроструктуре синтерованих  узорака  испитана  је  сканирајућом  електронском  микроскопијом. Фотоакустични  фазни  и  амплитудски  спектри  синтерованих  узорака  снимани  су  у функцији  чоповане  фреквенције  употребљеног  ласерског  зрачења  у  топлотно-трансмисионој  конфигурацији.  Теоријска  анализа  експерименталних  резултата омогућила  је  израчунавање  фотоакустичних  својстава  материјала  укључујући  и T.Ivetić et al./Science of Sintering, 39 (2007) 153-160 ___________________________________________________________________________ 160   топлотну  дифузивност.  На  основу  добијених  резултата  испитана  је  повезаност експерименталних услова припреме узорака и њихове микроструктуре.  Кључне  речи:  Цинк-станат,  механичка  активација,  синтеровање,  фотоакустична спектроскопија.             

Investigation of zinc stannate synthesis using photoacoustic spectroscopy

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