Polycrystalline Tetragonal Zirconia of the form ZrO2: 3 mol% Re2O3 (Re-TZP) for use in oxygen sensors: synthesis, characterization and ionic

conductivity

Muñoz, R.A1,a; Cajas, P.C1,b; Rodríguez, J.E2,c; Rodrigues, A.C3,d; Silva, C.R.M1,e

1Universidade de Brasília- Brasília- DF- Brasil 2-Universidad del Cauca-Popayán-Colombia. .

3 Universidade Federal de São Carlos- São Carlos-Brasil Área Especial, Projeção A,UnB - Setor Leste - Gama CEP: 72444-240

aramunoz@unb.br, bpatolacajas@gmail.com, cjnpaez60@yahoo.com, dacmr@ufscar.br, ecosmeroberto@gmail.com,

Keywords: ionic conductivity, activation energy, oxygen sensors, grain size, tetragonal polycrystalline zirconia.

In this work we propose the synthesis and characterization of tetragonal polycrystalline zirconia for

potential applications in oxygen sensors. The synthesis method used was the Pechini method.

0,280µm ± 0,04 to 0,574 µm ± 0,05 Particles of mean diameter size less than 50 nm were obtained

for this method and pure tetragonal phase was identified according to the diffraction patterns.

Samples were prepared using the cold uniaxial pressing and it were sintered in a resistive furnace in

air at a temperature of 1400°C for two hours. Two different heating schedules were used. Relative

densities greater than 96% of theoretical density were obtained in all cases while grain size was

dependent of the heating schedule used for the sintering of samples. The impedance diagram shows

how changes in the grain size has a direct influence on the electrical behavior of ceramics, showing

an increase in the total ionic conductivity from 1,35E-5 to 2,15E-5 Ω-1cm-1 at 400 °C when the

grain size increases from respectively. Finally, activation energies are presented and compared with

the literature agreeing with the values characteristic of solid electrolytes used in oxygen sensors.

Introduction

Oxygen ion conductors of zirconia based ceramics are a class of materials with technological

applications in several application areas: sensors of chemical species, oxygen pumps, solid oxide

fuel cells among others [1]. For these applications, the zirconia must possess the fluorite type

crystal structure, or close to it. Such oxides with this structure are the classic oxygen ion conductors

[2]. The fluorite structure consists of a cubic lattice of oxygen ions surrounded by cations. The

cations are arranged in a face centered cubic structure with anions occupying tetrahedral positions.

This leads to an open structure with large empty octahedral interstices.

For zirconia containing 3 mol% of ytrium oxide, or yttrium stabilized partially zirconia (Y-

ZTP), the crystalline phase has a distortion of the fluorite structure, mentioned above, with an ionic

conductivity of the same order of magnitude when compared to the fully stabilized zirconia, in the

temperature range 250-600 ° C [3]. For this reason, this material is attractive for its use in electrical

applications, specifically for possible use in oxygen sensors. In this paper, the synthesis and

characterization of polycrystalline tetragonal zirconia as well as the process of conformation of

specimens is described. The rare earth carbonate produced in Brazil by Nuclemon was used as yttria

precursor, from Brazilian monazite. The advantage of this carbonate is its low cost compared to

high purity rare earth. Pechini method allowed synthesis of powder with sub-micrometric particle

size. Samples were cold uniaxially pressed and sintered at resistive furnace. Two sintering

schedules were used, S1 and S2. For the evaluated ceramics it was observed grain size dependence

on the used sintering schedule. Impedance diagrams demonstrated direct influence of electric

behavior with grain sizes.

Materials Science Forum Vols. 798-799 (2014) pp 145-153Online available since 2014/Jun/30 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.798-799.145

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 187.66.190.213, University Federal de São Carlos, São Carlos-SP, Brazil-22/08/14,22:24:38)

Experimental procedure

In this work the following materials were used:

Zirconium tetrabutoxide (TBZ) (Zr[O(CH2)3CH3]4)- Aldrich;

Rare earths carbonate (Re2(CO3)3), Re = Y, Dy, Er, Ho, produced in Brazil by Nuclemon;

Citric acid (H3C6H5O7-H2O), analytical grade;

Nitric acid (HNO3), analytical grade;

Ethylene glycol (C2H6O2), analytical grade;

Isopropyl alcohol, analytical grade;

Distilled water.

The precursors of zirconium oxide (ZrO2) the precursors of rare earths oxides (Re2O3) utilized

in this work were respectively the Zirconium tetrabutoxide from Aldrich and a rare earths carbonate

(Re2(CO3)3). These reagents were used in appropriate proportions to obtain the solid electrolyte of

the form ZrO2:3 mol % Re2O3. The synthesis method used was the polymeric precursor method

named Pechini. Mixtures of nitric acid and ethylene glycol in a ratio ¼:1 were heated up to 70 0C to

ease citric acid dissolution into ethylene glycol. At room temperature yttrium and zirconium

precursors were added under constant mixing and the resulting mixture was heated up to 120 °C for

poly esterification and vaporization, giving rise to a polymeric resin. The obtained resin was heated

up to 250 0C during 18 h causing polymer break down and resin expansion. The residue was

grounded at agate mortar producing a fine and homogeneous black powder as is described in detail

in previous papers [4].

The raw material resulted from this process was characterized by thermogravimetric and

differential thermal analysis (DTA/TGA), X-ray diffractometry (XRD), transmission electron

microscopy (TEM), scanning electron microscopy (SEM) and finally impedance spectroscopy.

These characterization techniques allow to optimize temperatures aiming the production of metal

oxide of interest, particle size and shape control, agglomerates size and shape determination and

mainly its electrical behavior determination.

Results and discussion

One of the main peculiarities of the Pechini method is that the metallic ions of interest are

trapped in an organic network; consequently make it is therefore necessary to understand the

thermal decomposition behavior for this material aiming to define a suitable temperature to obtain

the oxide of interest without interference at particle size increase. The thermal profile DTA/TGA of

the resin from the organic zirconium oxide precursor is showed in Figure 1. The analyses were

performed on samples pre-burned at 250 ° C for 18 hours using platinum crucibles at a heating rate

of 10 ° C / min, in a Shimadzu equipment DTG-60H at the Laboratory of polymers Institute of

Chemistry of the University of Brasilia.

It is observed an endothermic process at 90 ° C related to the waste water vaporization. This

process is followed by a loss in mass of approximately 15%. The first exothermic peak is showed at

360 ° C suggesting the onset of the polymer resin degeneration, with a mass loss of about 28% due

to thermal decomposition of the single connections of the polymer [5].

146 Brazilian Ceramic Conference 57

0 100 200 300 400 500 600 700 800 900

-60

-40

-20

0

20

40

60

80

DTA

TGA

Temperature (°C)

DT

A (

uV

)

20

40

60

80

100

620 °C

432 °C

360 °C

ZrO2:3% Mol Y2O3

TG

A (

%)

90 °C

Exo

Figure 1. Differential thermal analysis (DTA) and thermogravimetric (TGA) of the polymer resin

pre-burned at 250 ° C for 18 hours.

The possibility of the onset of the phase transformation in oxide is discarded due to the

absence of diffraction peaks in the test with thermal treatment at this temperature, as can be seem at

figure 2. A second exothermic peak is showed at 432 °C indicative of thermal decomposition of

stronger connections with the polymer mass loss of approximately 23% and the beginning of the

transformation of phase of oxide of interest, from amorphous solid to crystalline solid. This was

reported by Freitas in 2000 [6] (amorphous oxide zirconia monoclinica+tetragonal) and confirmed

by X-ray diffraction. Finally, the last thermal phenomenon is showed at 620 ° C. It could be related

to a second phase transformation from tetragonal to monoclinic zirconia.

These results show that the compound formation is completed at 620 ° C. X-ray

diffractograms of thermal treated dust samples were obtained in an X-ray diffractometer (XRD)

Shimadzu, model XRD-600 with CuKα radiation (1.5418 Å), and a voltage and operating current of

30 kV and 20 mA, respectively. The angular pitch was 0.05 °, in the range 20<2ϴ<90°. The

software Search-Match was used to identify and to compare the crystal structure obtained in this

work with the data available at International Centre for Diffraction Data (ICDD). Figure 2

illustrates the X-ray diffractogram of the samples pre-burn and thermally treated at 400, 500, 600

and 1100 ° C for two hours. The diffractograms describe the formation of the zirconia tetragonal

phase of the composite. The widths for average height of the different diffraction peaks indicate

nanometric crystallite mid-size. The crystallite size was calculated according to the Scherrer

equation (1) [7]:

(1)

where d is the average crystal size, λ is the wavelength of X-ray, β is the peak width at medium

height of the diffraction peak height (FWHM) (measured in radians), θ is Bragg angle, B is a

numerical constant equal to 0.9. For the diffractogram shown in Figure 2 (b), and treated at 600 ° C

the calculated crystallite size was approximately 24 nm. In Figure 2 (a) can be observed the

development of the crystalline phases present in the samples when the temperature is increased. The

sample pre-burned presented a structure constituted by random atomic arrangements without long-

range order, frequently mentioned as amorphous, consisting mostly of organic material from the

synthesis method. For the thermal treatment at 400 ° C, over two hours, the sample presented an

Materials Science Forum Vols. 798-799 147

overly broad diffraction peak around 30 °, where is located the most intense peak of the tetragonal

and cubic phase. At 500 °C, several diffraction peaks can be seen, possibly related to the phases of

monoclinic, tetragonal and / or cubic zirconia. At 600 ° C, Figure 2 (b), the samples indicates a

substantial increase in their structural order, causing possible correlation with the existing

databases.

A direct comparison with the database indicates the existence of the zirconia tetragonal phase,

JCPDS 50-1089, but the diffraction peaks of monoclinic phase could be superimposed and with

almost imperceptible intensities, causing their identification nearly impossible due to the width of

the peaks occurrence [8], requiring other methods for distinguishing the phases.

(a) (b)

Figure 2. X-ray diffraction for the sample ZrO2 3% Mol Re2O3 thermally treated (a) pre-calcined,

400 °C, 500 °C and (b) 600 °C and 1100 ° C for two hours.

Finally, and considering the last treatment temperature, 1100 ° C, the existence of the zirconia

tetragonal phase can clearly be established. In the Figure 2 (b), it is emphasized the diffraction peak

of low intensity located in 43.12 °, characteristic of crystalline phase. The results of X-ray

diffraction are in complete agreement with the results of thermal analysis, showing that it is possible

to achieve the stabilization of tetragonal zirconia at a low temperature, in this case of less than 650 °

C, with crystallite size of ~ 24 nm.

Figure 3 shows a transmission electron microscopy micrographs (magnification 250K), and a

scanning electron microscopy micrographs of the samples under study. In these images it is possible

to note the high level of agglomeration between the particles of the ceramic powder, as is also

possible to notice submicron particles (particles with sizes less than 50 nm). In order to break the

agglomerates, these were milled in a ball mill. The milling was conducted in isopropyl alcohol

medium using zirconia balls of 2 mm diameter. The milled powder was dried in air at 70 °C for 18

h

20 30 40 50 60 70 80 90

0

20

40

60

80

0

50

100

150

0

20

40

60

8020 30 40 50 60 70 80 90

Inte

nsity (

a.u

)

2 (°)

pre-calcined

400 °C

500 °C

20 30 40 50 60 70 80 90

0

100

200

300

400

500

600

28 30 32

0

100

200

300

400

500

Inte

ns

ida

de

(u

.a)

2 Teta

ZrO2:3% Mol Re2O3

(600 °C )

Inte

nsid

ad

e (

u.a

)

2(°)

20 30 40 50 60 70 80 90

0

500

1000

1500

2000

ZrO2:3% Mol Re2O3 (1100 °C )

Tetragonal Phase (JCPDS 50-1089)

2(°)

42 43 44

0

5

10

15

20

25

30

35

2 Teta

Inte

nsid

ad

e (

u.a

)

Inte

nsity (

a.u

)

43,12°

148 Brazilian Ceramic Conference 57

Finally, the powders were disaggregated in an agate mortar and sieved through a sieve of

mesh 0.106 mm. Immediately after the grinding step, compacts, 10 mm diameter and 5 mm of

thick, were prepared by uniaxially pressing at 187 MPa for 30 s, using a press Marcon MPH-10.The

samples were sintered in a resistive furnace Naberttherm LHT407GN6 at a temperature of 1400 °

C, during two hours in air using two heating schedules, as showed in figure 4.

(a) (b)

Figure 3. Analysis by (a) MET (250K magnification) and SEM (b) of ceramic material obtained

after thermal treatment at 600 ° C, over two hours, suggesting the presence of nanoparticles and

clusters of large size.

Curve S1: The sintering curve S1 (Fig. 4 (a)) has been implemented to achieve dense CPs to

promote grain growth 9. According to Callister [10] the particle growth is a result of the movement

of grain boundaries, which is conducted through two processes: grain boundary diffusion and

migration of grain boundaries. Both processes promote the densification, but the grain boundary

migration that occurs at a higher temperature promotes faster grain growth.

The goal of achieving a high temperature, 1650 ° C in the case, was to enable the faster

migration of grain boundaries. The objective of reducing temperature to 1400 °C is to suppress this

faster migration, but keeping the distribution of the active contour of grain. With this methodology

it was possible to reach the dense test bodies with grain growth at low sintering temperature of 1400

° C.

(a) (b)

Figure 4. Sintering schedules used, (a) S1 and (b) S2, with the purpose of improving microstructural

changes in the specimens and thus increase the electrical properties of the same.

0 100 200 300 400 500 600 700

0

200

400

600

800

1000

1200

1400

1600

1800

1650 °C/5 minutes

Te

mp

era

ture

(°C

)

Time (minutes)

S1

1400 °C/2 hours

0 100 200 300 400 500 600

0

200

400

600

800

1000

1200

1400

1600

Te

mp

era

ture

(°C

)

Time (minutes)

S2

1400 °C/2 hours

Materials Science Forum Vols. 798-799 149

Curve S2: sintering curve S2 (Fig. 4 (b)) is the "traditional" sintering used in previous works

[11]. It has a rapid heating ramp of 10 °C/min until reach a temperature of 1000 °C, maintained for

five minutes. Subsequently, the heating rate was reduced to 3 °C/min to promote the diffusion

mechanism of the grain border until reach 1400 °C, holding this temperature during two hours;

finally a cooling at a rate of 5 °C/min.

Calculation of apparent density of the sintered samples was conducted using Archimedes'

principle, with the immersion of the sample in distilled water. Three measurements were made for

each sample on a precision balance Shimadzu AUY-220. Density of 5.83 g/cm3 were calculated for

the sintered sample with the curve S2 and 5.89 g/cm3 for the sintered sample using the curve S1,

thus resulting in apparent densities of 97.5% and 98.5% of theoretical density, respectively. The

reference value for these assessments was 5.98 g/cm3 and was calculated using a mathematical

model described in the literature [12].

The microstructure of sintered ZrO2:3% Mol Re2O3 ceramics was observed by scanning

electron microscopy (SEM), in a Jeol JSM-7001F microscope (Scanning Electron Microscope), of

the institute of Biology of the University of Brasilia. Measurements of average grain size and

interfacial area per unit of volume were carried out (Figure 5) [13], counting the number of

intersections between the grain boundary and straight lines with known length, which were

designed on the image with the program ImageJ of free access. The number of intersections per

image was more than 400, aiming to achieve better measurement accuracy.

In the case of ceramics sintered with the curve S1, figure 5 (a), the average grain size was

approximately 574 nm and the interfacial area per unit of volume was 3.48 x 10-3

nm2/nm

3. For

ceramics sintered with the curve S2, figure 5 (b), the average grain size was almost 280 nm and the

interfacial area per unit of volume was 7.38 x 10-3

nm2/nm

3.

Qualitative analysis of the difference between these obtained values is related to the sintering

curve, since the two types of samples were processed identically. Other microstructural features are

visible at the micrographs such as low porosity and uniform distribution of grain sizes and shapes

(all with the same size and similar shape). The sintering curve S1 strongly promotes the grain

growth, resulting in the reduction of the density of grain boundaries, fact that could increase its

electrical behavior.

(a) (b)

Figure 5. Analyses by SEM of the surface of the sintered specimens (a), sintered with curve S1,

and, (b) using conventional sintering curve S2.

150 Brazilian Ceramic Conference 57

Finally, the electrical response of sintered solid electrolyte was evaluated by impedance

spectroscopy. That characterization technique allows to establish the dependence of the electrical

behavior with temperature, in different regions of the ceramic, grain and grain boundary. The

samples have apparent density to 96.97% of theoretical. Therefore, the electrical results obtained

are related mainly to the properties and characteristics of the electrolyte [14].

The impedance spectroscopy measurement for samples was carried out on a frequency range

of 1 MHz to 1 Hz, and a voltage of 1000 mV, using a Solartron 1260 equipment. The temperature

range used was between 125 and 400 °C, with measurements at every 25 degrees, giving a total of

twelve (12) measurements for analysis. Paste electrodes of platinum Pt-paste Demetron 308-A were

applied on the parallel faces of the samples and cured at 1100 °C for 20 minutes. Figure 6 shows a

diagram of typical impedance obtained for the samples analyzed at 300 ° C. It is clearly observed

two different semicircles that in the study of ceramic materials, are related to the grain contributions

(high frequencies) and at contributions of the grain boundaries (low frequencies).

From the diameter of these semicircles, the intragranular and intergranular resistivity were

calculated, respectively. It is also evident from Figure 6 (b) that the intragranular resistivity are

slightly affected by the heat treatment sintering, while the intergranular resistivity experiences a

great decline in value.

(a) (b)

Figure 6. Typical impedance diagrams of the samples at a temperature of 300 °C, (a) showing the

decrease in resistivity attributed to grain boundary, (b) expanding the zone of high frequencies.

(Numbers denote the logarithm of frequency).

Thus it is clear that the decrease in the density of grain boundaries significantly affects the

electrical behavior of ceramics under study. With resistivity values obtained was possible to observe

the character thermally activated as a function of test temperature for the grain, grain boundary and

total conductivities, Figure 7(a), (b) and 8(a) respectively. The dependence of the conductivity with

temperature is shown in Figure 7, where one can see a detailed discussion of this increased

conductivity due to the used sintering curve.

0 200 400 600 800 1000 1200 1400 1600 1800

0

200

400

600

800

1000

1200

1400

1600

1800

5

ZrO2: 3% Re2O3 - Sintering curve S1

ZrO2: 3% Re2O3 - Sintering curveS2

- Z

'' (kc

m)

Z' (kcm)

1245

0 50 100 150 200

0

50

100

150

200

- Z

'' (kc

m)

Z' (kcm)

5

4

5

Materials Science Forum Vols. 798-799 151

(a) (b)

Figure 7. Dependence of conductivity with temperature for the grain (a) grain boundary (b)

To complete the electrical analysis, Arrhenius plots (log σ vs. 1000 / T) were constructed,

where is possible to obtain the activation energy for the conduction process, figure 8 (b). It was

observed for both samples a single slope in the temperature range 125-400 ° C and that they do not

deviate the Arrhenius type behavior. From the slope of these lines total activation energies were

obtained, resulting values of 1.00 and 1.01 eV for the samples sintered with the curve S1 and S2

respectively. These values of activation energy are in complete agreement with the values found in

the literature for ion oxygen conductors based on zirconium oxide that are in the range from 1 to 1.2

eV [15].

(a) (b)

Figure 8. Dependence of conductivity with temperature for (c) total conductivity and ( b) Arrhenius

plots for total conductivity.

Conclusions

It was possible to stabilize the tetragonal zirconia polycrystalline form ZrO2 3% Mol Re2O3 at

a temperature lower than 650 ° C with crystallite size of approximately 24 nm. Transmission

electron microscopy revealed that the ceramic powder obtained was composed of nanoparticles with

sizes smaller than the 50 nm. Some particles were agglomerated, requiring the grinding step for

correct specimens conformation.

100 150 200 250 300 350 400

0,0

2,0x10-5

4,0x10-5

6,0x10-5

8,0x10-5

1,0x10-4

1,2x10-4

S1

S2

co

nd

uctivity (

cm

-1)

Temperature (°C)

Conductivity of the grain

100 150 200 250 300 350 400

0,0

5,0x10-6

1,0x10-5

1,5x10-5

2,0x10-5

2,5x10-5

3,0x10-5

Conductivity of the grain boundary

S1

S2

co

nd

uctivity (

cm

-1)

Temperature (°C)

100 150 200 250 300 350 400

0,0

5,0x10-6

1,0x10-5

1,5x10-5

2,0x10-5

2,5x10-5

Total conductivity

S1

S2

co

nd

uctivity (

cm

-1)

Temperature (°C)

1,4 1,6 1,8 2,0 2,2 2,4 2,6

-11

-10

-9

-8

-7

-6

-5

-4

S1

S2

log

(ST

) (

cm

-1)

1000/(T) (K-1)

Total conductivity

152 Brazilian Ceramic Conference 57

Ceramics developed from this raw material had densities higher than 96% of theoretical with

grain size dependent upon the used sintering schedule. The increase in grain size of 280 to 574 nm

caused a decrease in interfacial area per unit volume which is reflected in a decrease in the grain

boundaries densities. This result provides beneficial effects on the electrical behavior of the material

attributed to the resistivity reduction of the grain boundary which causes an increase in the total

conductivity of the ceramic from 1.35 E-5 E-5 to 2.15 Ω-1 cm-1 at 400 ° C.

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