Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic

doi:10.4236/msa.2011.27105

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Materials Sciences and Applicatio n, 2011, 2, 765-770

doi:10.4236/msa.2011.27105 Published Online July 2011 (http://www.SciRP.org/journal/msa)

Structural and Dielectric Properties of Sintering

Zinc Oxide Bulk Ceramic

Mariem Chaari*, Adel Matoussi, Zouheir Fakhfakh

1Laboratory of Composite Ceramic and Polymer Materials, Scientific Faculty of Sfax, Tunisia, Africa.

Email: *[email protected]

Received April 2nd, 2011; revised April 22nd, 2011; accepted April 30th, 2011.

ABSTRACT

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 in-

creasing 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 im-

provement 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.

Keywords: Zinc Oxide, Sintering Process, Micro Strain, Impedance Spectroscopy

1. Introduction

During the last few decades, zinc oxide (ZnO) has at-

tracted much interest in science and technology due to its

versatile properties such as transparency in the visible

range, direct band gap (3.37 eV), absence of toxicity,

abundance in nature, etc. These characteristics find a wide

range of applications in opto-electronic and electronic de-

vices [1-7]. For example, it used in solar cells [1], trans-

parent conducting films, chemical sensors [2,3], varistors

[4,5], light-emitting diodes[6], laser diodes [7], etc.

The preparation of high quality ZnO thin films has

been the subject of continuous research. Many tech-

niques 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 be-

ing cheap, easy-to-use, safe and able to be implemented

in a standard laboratory. It is well known that the opto-

electronic properties of ZnO are affected by the prepara-

tion conditions such as working pressure, substrate tem-

perature, types of substrates, thickness of the films and

annealing temperature.

The present paper is devoted to examine the influ-

ence of sintering temperature on the structure, mor-

phological and the dielectric properties of ZnO tablets.

2. 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 re-

siduals. 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 morphol-

ogy of the sintered specimens were characterized by

X-ray diffraction (XRD), scanning electron microscopy

(SEM) and atomic force microscopy (AFM). The spec-

trometer dielectric was used to characterize the dielectric

properties of the obtained samples.

3. Results and Discussions

3.1. Structural Properties

The ZnO structure is examined by θ - 2θ X-ray diffrac-

tion (XRD) measurements. Figure 1 shows the XRD

Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic

766

patterns of ZnO prepared at growth temperature ranging

from 700˚C to 1000˚C. All the samples exhibit a poly-

crystalline 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 per-

centages 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 Equa-

tion [15]:

0.9

DBcos







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 dif-

fraction angle of the XRD peak.

Figure 1. XRD patterns of ZnO prepared at various Sinter-

ing temperatures.

700 750 800

0,45

0,50

0,55

0,60

0,65

0,70

0,75

Sintering temperature (˚C)

Figure 2. Evolution of intensity ratio of XRD peaks as a

function of sintering temperature.

The crystallite size increases from approximately 38

nm at 700˚C to 52 nm at 900˚C. As the temperature in-

creases, 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 val-

ue 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 [17-19].

This behavior can be explained by the reduction of the

density defects and atom vacancies caused by the in-

crease of the diffusion of oxygen and zinc atoms at ele-

vated temperature. According to Bachari [20], the lack of

oxygen atoms may result in the growth of non-stoi-

chiometric 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 O2 concentration during

growth. Vimalkumar et al. [21] showed that with in-

crease 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 par-

tial pressure from 0 to 0.12 Pa leads to change of crystal-

line 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 εzz and residual stress σ in

the grown ZnO samples are estimated using the follow-

ing expressions [24]:



1333 1112

2CC CC

= 2C









and







(°)

45 50 55 60

=1000°C

=850°C

=700°C

25 30 35 40

XRD Intensity (u.a)

(11-20)

(10-12)

(10-11)

(0002)

(10-10)

850900950 1000

I10.1/(I10.0+I00.2+I10.1)

Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic

767

Table 1. Texture coefficient and percentages of oriented crystallites in <hk.l> direction.

Ts (˚C)

Texture coefficient

TC(10.0) TC(00.2) TC(10.1)

±0.0005 ±0.0003 ±0.0004

Percentage of oriented crystallites (%)

χ(10.0) χ(00.2) χ(10.1)

700 0.648 1.299 2.355 12.96 ± 0.008 25.99 ± 0.011 46.11 ± 0.012

800 0.639 1.312 2.345 12.78 ± 0.011 26.24 ± 0.020 47.10 ± 0.009

900 0.655 1.249 2.369 13.09 ± 0.007 24.98 ± 0.006 47.38 ± 0.004

1000 0.645 1.268 2.406 12.91 ± 0.005 25.36 ± 0.004 48.13 ± 0.003

Table 2. Variations of grain size and FWHM for ZnO (00.2) reflection with temperature.

Ts (˚C) FWHM of the

(00.2) peak (˚) Diffraction angle 2θ(˚) Grain size

D(nm)

700 0.236 34.323 38.221 ± 0.021

800 0.209 34.353 41.323 ± 0.018

850 0.183 34.386 47.194 ± 0.011

900 0.168 34.403 51.407 ± 0.017

1000 0.172 34.420 50.212 ± 0.013

where c0 and c are the lattice parameters of unstrained

and the prepared ZnO samples respectively. C11 = 209.7

GPa, C12 = 121.1 GPa, C13 = 105.1 GPa and C33 = 210.9

GPa are the elastic stiffness constants of bulk ZnO [23].

700800850900 1000

Sintering temperature (°C)



x10

-4

(a.u)

-65

-55

-45

-35

-25

-15

-5

 (x10

Pa)

Figure 3 shows the variation of lattice strain and re-

sidual stress in ZnO as a function of the growth tempera-

ture. The sample sintered at 700˚C has larger strain and

stress values. One can see that the residual stress is com-

pressive 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 con-

ventional 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.

Figure 3. Variation of lattice strain and residual stress in

ZnO tablet s.

Table 3. EDX analysis of the Zinc oxide samples.

Sintering

temperature(˚C) O (at %) Zn (at %)

700 50.568 ± 0.008 48.144 ± 0.006

850 50.043 ± 0.005 48.244 ± 0.009

900 49.752 ± 0.011 48.76 ± 0.011

1000 49.412 ± 0.012 49.78 ± 0.013

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 in-

creasing 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 en-

hance 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 crys-

tallites having very varied shapes and nanometric sizes.

These crystallites are randomly distributed and irregu-

larly disoriented. With increasing the sintering tempera-

ture, the grain size is found to increase whereas its den-

sity decreases resulting from conglomeration or coales-

cence of smaller grains.

Here, it is evident that the sintering temperature exer-

cises strong effects on the surface morphology and the

crystal quality of ZnO powders.

3.2. Morphological Properties

Figure 4 shows the scanning electron micrographs of ZnO

Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic

768

Figure 4. Plan view of SEM micrographs of ZnO pellets

sintered at 850˚C (a); 900˚C (b); and 1000˚C (c).

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 tem-

perature, the atoms have enough diffusion activation en-

ergy 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.

3.3. 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 in-

creased. 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 tem-

perature from 700˚C to 1000˚C.

(a)

These behaviours may be attributed to the enhance-

ment of grain size, compactness and the structural quality

of the material [25-27]. Figure 6 illustrates the evolution

of electrical resistivity with increasing the sintering tem-

perature. The resistivity was first decreased reaching a

minimum value 4 × 109 Ω·cm at 850˚C and then in-

creased 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.

(b)

The measured values of resistivity are in range 109 -

1010 Ω·cm which indicates that our prepared ZnO bulk

possess an insulating behaviour rather than n-type semi-

100

1000

650750850950 1050

Sintering temperature (°C)

Dielectric constant (a.u)

1Hz

100Hz

10KHz

1MHz

(c)

Figure 5. Vari atio n of the di elec tric constant as a func tion of

the sintering temperature.

650 750850 9501050

Sintering temperature (°C)

Resistivity (x10

Ω.cm )

Figure 6. Evolution of electrical resistivity versus the sin-

tering temperature.

Structural and Dielectric Properties of Sintering Zinc Oxide Bulk Ceramic769

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 in-

crease 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 poly-

crystalline ZnO pellets [23-25]. 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 di-

electric properties, low dielectric loss, and high electrical

resistivity. ZnO can be used as a promising material for

fabrication of dielectric varistors and transparent elec-

trodes in solar cells.

4. 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 mor-

phology 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 en-

hanced by increasing the sintering temperature.

XRD analysis showed that all films are polycrystalline

with a hexagonal wurtzite structure and randomly ori-

ented (10.1) direction. The residual stress in ZnO sample

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

700 750800 850 900 95010001050

Sintering temperature (°C)

Dielectric loss (u.a)

f=1MHz

f=10M Hz

Figure 7. Variation of the dielectric loss versus the sintering

temperature.

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 frequen-

cies ranging from 10–1 Hz until 10 MHz. Results show an

increase of dielectric constants with increasing the sin-

tering temperature and a decrease with changing the

measuring frequency. This effect is attributed to the in-

crease of grain size, deviation from stoichiometry and the

decrease in sample strain.

Electrical resistivity was determined to be in the range

4 × 109 - 2.4 × 1010 Ω·cm indicating a more dielectric be-

haviour 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.

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