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.


Introduction
Ceramic materials with large open porosity, such as zinc stannate (Zn 2 SnO 4 ) 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 defining the electronic states and structural disorders of ceramic materials [1][2][3][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 ( ) that is in contact with the electret microphone (Fig. 1.).
The following relation can express the PA signal: 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; is the thermal diffusion length of the backing (gas); l is the distance between the sample and microphone membrane and 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:SnO 2 = 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 cm 3 ) 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-SnO 2 system given in detail in [6] in order to obtain the same average green density of 3.769 g/cm 3  .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 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 CuK α 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).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 SnO 2 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 Zn 2 SnO 4 is not yet finished after 160 minutes of ball milling, as SnO 2 is still present.
Fig. 3 shows XRD patterns of powder mixtures after uniaxial pressing and nonisothermal 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 nonisothermally 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.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-samplebacking-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 D T (m 2 s -1 ), then the diffusion coefficient of the minority free carriers D (m 2 s -1 ) and optical absorption coefficient α (m -1 ).The obtained values are given in Tab.I.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 Zn 2 SnO 4 , synthesized in this way, are available in the literature.

Conclusion
In this paper synthesis of zinc-stannate was analyzed using photoacoustic spectroscopy.Mixtures of ZnO and SnO 2 were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill and then non-isothermally sintered up to 1200 o C. 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 Zn 2 SnO 4 .

Fig. 2
Fig.2XRD patterns of ZSO powder mixtures as a function of the activation time.

Fig. 3
Fig.3XRD patterns of ZSO samples non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min.

Fig. 7 Fig. 8 (
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.