Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic

Undoped zinc oxide (ZnO) has been prepared at various growth temperatures by conventional sintering process. The morphology and crystalline properties of ZnO pellets were examined by scanning electron microscopy, atomic force microscopy and X-ray diffraction. It has revealed that the grain size and surface roughness tends to increase by increasing the sintering temperature. XRD analysis showed that all samples are polycrystalline with a hexagonal wurtzite structure. The alignment of ZnO grains along the (10.0) plane was enhanced as the temperature increased. Interestingly, the compressive stress was found to decrease drastically from –0.62 GPa at 700 ̊C to –0.2 MPa at 1000 ̊C. This improvement in film structure seems to enhance considerably the dielectric properties for the samples sintering at high temperatures. Results show an increase of dielectric constant and a decrease of electrical resistivity when increasing the sintering temperature.

The preparation of high quality ZnO thin films has been the subject of continuous research.Many techniques are used for preparing zinc oxide such as: RF sputtering [8], chemical vapour deposition [9], sol-gel process [10], carbothermal reduction method [11] and pressurized melt growth [12,13], among others.The conventional sintering process has the advantages of being cheap, easy-to-use, safe and able to be implemented in a standard laboratory.It is well known that the optoelectronic properties of ZnO are affected by the preparation conditions such as working pressure, substrate temperature, types of substrates, thickness of the films and annealing temperature.
The present paper is devoted to examine the influence of sintering temperature on the structure, mor-phological and the dielectric properties of ZnO tablets.

Experimental Procedure
Undoped ZnO pellets were prepared by the conventional sintering technique in an atmospheric heated furnace using zinc oxide powders (Aldrich-GmbH, purity ≈99%).The ZnO powders were first milled in an agate mortar and heated in air at annealing temperature of 300˚C for 2 hours to evaporate the water and remove the organic residuals.The obtained powders were then pressed into pellet disks (of about 1mm thickness and 8 mm diameter) and sintered at various temperatures (700˚C -1000˚C) in ambient air for 3 hours.Finally, these pellets were rapidly quenched to room temperature in air in order to freeze the structure.The crystalline structure and surface morphology of the sintered specimens were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).The spectrometer dielectric was used to characterize the dielectric properties of the obtained samples.

Structural Properties
The ZnO structure is examined by θ -2θ X-ray diffraction (XRD) measurements.Figure 1 shows the XRD patterns of ZnO prepared at growth temperature ranging from 700˚C to 1000˚C.All the samples exhibit a polycrystalline hexagonal structure.The peaks are identified to (10.0), (00.2), (10.1), (10.2), (11.0) and (10.3) plane reflections for wurtzite phase of ZnO according to the standard JCPDS data file (No. 36-1451).One can see that the ZnO growth appears to be randomly oriented along the (10.1) plane instead of c-axis (00.2) direction.
In order to evaluate the texture coefficient and the percentages in volume of oriented crystallites in the <hk.l> direction, we have used the well-known formula reported in Ref. [14] and results are given in Table 1.
It is found that the highest value of texture coefficient (TC) corresponds to (10.1) plane which contributes about 47 % of oriented ZnO grains.We notice a slight decrease of the contribution of (00.2) phase on detriment to the (10. 1) one as the sintering temperature is varied from 700˚C to 1000˚C.Figure 2 shows that the intensity of the (10.1) peak compared to the neighboring (10.0) and (00.2) increases by increasing the sintering temperature from 700˚C to 1000˚C.This result indicates that (10.1) is the preferred orientation of ZnO pellet growth.In addition, the full width at half maximum (FWHM) for ZnO (10.1) peak decreases from 0.24˚ to 0.12˚ when the temperature increases from 700˚C to 1000˚C respectively.
The dependence of the growth temperature on the grain size, the angle position and FWHM for ZnO (00.2) diffraction peak are illustrated in Table 2.The average grain size of ZnO is calculated using the Scherer's Equation [15]: 0.9 D Bcos    where D is the grain size, λ is the wavelength of the X-ray radiation used, B is the full width at half maximum (FWHM) of the diffraction peak and θ is the Bragg diffraction angle of the XRD peak.The crystallite size increases from approximately 38 nm at 700˚C to 52 nm at 900˚C.As the temperature increases, we observe a neat decrease of the full width at half maximum of the (00.2) peak.Furthermore, the (00.2) peak position shifts to higher angles and reaches the value 34.408˚ of free-strained ZnO film [16].
Here, it must be pointed that the crystal quality of ZnO can be improved at higher sintering temperature [17][18][19].
This behavior can be explained by the reduction of the density defects and atom vacancies caused by the increase of the diffusion of oxygen and zinc atoms at elevated temperature.According to Bachari [20], the lack of oxygen atoms may result in the growth of non-stoichiometric thin films and bad crystalline phases.
It has been reported that the crystalline properties and alignment of the ZnO thin films depends strongly on the substrate temperature and the O 2 concentration during growth.Vimalkumar et al. [21] showed that with increase in the spray pyrolysis rate orientation of ZnO grains changed from (10.1) to (00.2) plane.Matsuka and Ono [22] have observed that the increase of oxygen partial pressure from 0 to 0.12 Pa leads to change of crystalline orientation from (10.1) to (00.2) plane.Prasada et al. [23] showed that ZnO grows randomly along the (10.1), (00.2) and (10.0) orientations.They found that the stress value decreases whereas the particle size increases as the growth temperature increased.
In our case, the lattice strain ε zz and residual stress σ in the grown ZnO samples are estimated using the following expressions [24]:     Figure 3 shows the variation of lattice strain and residual stress in ZnO as a function of the growth temperature.The sample sintered at 700˚C has larger strain and stress values.One can see that the residual stress is compressive which decreases negatively from -0.62 GPa at 700˚C to -0.2 MPa at 1000˚C.This means that elevated temperature in the range 900˚C -1000˚C are favoured to obtain better crystalline quality of ZnO prepared by conventional sintering process.According to Prasada [23], the measured stress in ZnO is attributed mainly to intrinsic origins introduced by the presence of impurities defects and lattice distortions in the crystal.The extrinsic stress induced by the lattice and the thermal mismatches will not be generated because the pellet is thicker (1 mm -2 mm) and because of the non-use of growth substrate in our experiment.Moreover, it is shown that the intrinsic stress appears frequently in nonstoichiometric films [23,25].This seems to be consistent with energy dispersive X-ray (EDX) analysis presented in the Table 3.It is seen clearly that the sample composition became more stoichiometric with increasing the sintering temperature.For all the samples, the level of incorporation impurity is reduced to an amount lesser than 1.5% and the percentages of oxygen and the zinc are nearly equal.Consequently, it reveals that the growth at high temperature (>1000˚C) can strongly enhance the physicochemical properties and thus improve the crystallinity of the ZnO films.pellets sintered at 850˚C, 900˚C and 1000˚C.For all samples, the surface is rough and presents grains of crystallites having very varied shapes and nanometric sizes.These crystallites are randomly distributed and irregularly disoriented.With increasing the sintering temperature, the grain size is found to increase whereas its density decreases resulting from conglomeration or coalescence of smaller grains.
Here, it is evident that the sintering temperature exercises strong effects on the surface morphology and the crystal quality of ZnO powders.

Morphological Properties
Figure 4 shows the scanning electron micrographs of ZnO From AFM analyses, the rms roughness increases from 9 nm to 68 nm as the deposition temperature increases from 700˚C to 1000˚C.This agrees well with the results of XRD measurements.We surmise that at high temperature, the atoms have enough diffusion activation energy to occupy the proper sites in the crystal lattice and grains with the lower surface energy become larger at high temperature [26].These results show that the high temperature enhances the coalescence of grains and thus the lateral growth of ZnO along the <10.1> direction.

Dielectric Analysis
Figure 5 presents the variation of the dielectric constant measured at room temperature for different frequency as a function of the sintering temperature.The dielectric constant was decreased as the measuring frequency increased.For the sample sintered at 1000˚C the ε value decreased from 900 at 1 Hz to 12.27 at 2 kHz and to 4 at 1 MHz.Also, it shows an increase tendency of all the dielectric constants with increasing the sintering temperature from 700˚C to 1000˚C. (a) These behaviours may be attributed to the enhancement of grain size, compactness and the structural quality of the material [25][26][27].Figure 6 illustrates the evolution of electrical resistivity with increasing the sintering temperature.The resistivity was first decreased reaching a minimum value 4 × 10 9 Ω•cm at 850˚C and then increased with increasing temperature.A similar behaviour has been observed by Gomes [28] and Olivera [29] in Ga-doped and F-doped ZnO thin films, respectively.The measured values of resistivity are in range 10 9 -10 10 Ω•cm which indicates that our prepared ZnO bulk possess an insulating behaviour rather than n-type semi- conductor character.However, the decrease of resistivity with elevation of growth temperature is due, on the one hand, to both the improvement in the pellet structure and the reduction of grain boundaries which enhance the free electron mobility [30].On the other hand, the reduction in the resistivity can also be due to the non-stoichiometry of the film and the presence of residual impurities (K, Ca, S, and Fe) which can substitute the Zn atoms and/or be incorporated in oxygen vacancies [28,29,31].
For samples sintered at higher temperatures, the increase in resistivity can be associated to more appropriate stoichiometry, the increase of ZnO grain size and the scattering of free carriers at grain boundaries in polycrystalline ZnO pellets [23][24][25].Here, it is important to notice that high temperature can enhance the crystalline and the dielectric properties of ZnO pellets.
As schown in Figure 7, the lowest value of dielectric loss at higher frequencies (tg δ = 6.6 × 10 -2 ) is found for the ZnO sample sintered at 900˚C.Due to their good dielectric properties, low dielectric loss, and high electrical resistivity.ZnO can be used as a promising material for fabrication of dielectric varistors and transparent electrodes in solar cells.

Conclusions
In this paper, we study the effect of growth temperature on the structural and dielectric properties of zinc oxide prepared by conventional sintering process.The morphology and the crystalline characteristics of ZnO films were examined by scanning electron microscopy, atomic force microscopy, and X-ray diffraction.It has revealed that the grain size and surface roughness tends to increase and that the crystalline quality of ZnO film is better enhanced by increasing the sintering temperature.
XRD analysis showed that all films are polycrystalline with a hexagonal wurtzite structure and randomly oriented (10.1) direction.The residual stress in ZnO sample is compressive which decreases significantly from -0.62 GPa at 700˚C to -0.2 MPa at 1000˚C.The temperature dependence of electrical resistivity and dielectric constant were studied at room temperature for different frequencies ranging from 10 -1 Hz until 10 MHz.Results show an increase of dielectric constants with increasing the sintering temperature and a decrease with changing the measuring frequency.This effect is attributed to the increase of grain size, deviation from stoichiometry and the decrease in sample strain.Electrical resistivity was determined to be in the range 4 × 10 9 -2.4 × 10 10 Ω•cm indicating a more dielectric behaviour than a semi-conducting character.All the above results show that the sample sintered at 900˚C has the best dielectric properties and a good crystalline structure.

Figure 1 .
Figure 1.XRD patterns of ZnO prepared at various Sintering temperatures.

Figure 2 .
Figure 2. Evolution of intensity ratio of XRD peaks as a function of sintering temperature.

Figure 3 .
Figure 3. Variation of lattice strain and residual stress in ZnO tablets.

Figure 5 .Figure 6 .
Figure 5. Variation of the dielectric constant as a function of the sintering temperature.

Figure 7 .
Figure 7. Variation of the dielectric loss versus the sintering temperature.

Table 2 . Variations of grain size and FWHM for ZnO (00.2) reflection with temperature.
[23]c are the lattice parameters of unstrained and the prepared ZnO samples respectively.C 11 = 209.7 GPa, C 12 = 121.1 GPa, C 13 = 105.1 GPa and C 33 = 210.9GPaare the elastic stiffness constants of bulk ZnO[23].