Effect of Thermal Treatment on Thermoelectric Properties of Extruded TiO2 Ceramics

Agnese Pura1, a, Janis Locs1, b and Liga Berzina-Cimdina2,c 1Rudolfs Cimdins Riga Biomaterials innovation and development centre, Riga Technical University,

Pulka Street 3/3, Riga, LV-1007, Latvia

² Institute of General Chemical Engineering, Riga Technical University,

Azenes Street 14/24, Riga, LV-1048, Latvia

aagnese.pura@rtu.lv, bjanis.locs@rtu.lv, cliga.berzina-cimdina@rtu.lv

Keywords: titanium dioxide, extrusion, thermoelectric properties

Abstract. TiO2 samples were obtained by extrusion process, sintered in air at 1000 °C, 1100 °C,

1200°C and 1300 °C and, afterwards, thermally treated under vacuum conditions at 1250 °C for 1

hour applying two different heating/cooling rates (2 °C/min and 5 °C/min). It was found that

thermal treatment conditions substantially affected thermoelectric properties of the samples.

Increasing sintering temperature, during the sample thermal treatment in air, the electrical

conductivity of the specimens increased, while Seebeck coefficient decreased. With an increase in

the heating rate during the vacuum heat treatment of the samples, the electrical conductivity of the

samples decreased, while Seebeck coefficient increased.

Introduction

Thermoelectric materials receive great interest in the recent years due to their potential

applications such as coolers/heaters, power generators and thermoelectric sensors [1].

Titanium dioxide (TiO2) has been widely used in photocatalysis, dye-sensitized solar cells, gas

sensors and self-cleaning components. Recently, TiO2 has also been considered as promising

thermoelectric material for high temperature applications. Electrical properties of titanium dioxide

are sensitive to the oxygen partial pressure as well as they depend on various microstructural

features. Oxygen partial pressure changes the concentration of electrons and electron holes in the

TiO2. Due to the fact that the porosity within the oxides may alter their inside oxygen partial

pressure, it can also affect electrical properties. In addition, grain boundaries can significantly affect

electrical properties of TiO2. Grain boundaries may absorb charge carriers and in turn repel the

same charge carriers travelling to the boundary as part of the conduction process. In some cases a

second phase may precipitate along the grain boundaries to become a barrier for electrical

conduction [2].

In the current research, the effect of thermal treatment conditions on thermoelectric properties of

extruded TiO2 ceramic was investigated.

Materials and methods

At first, a plastic paste was prepared for the extrusion, by mixing 78.6 wt% of TiO2 anatase

powder Hombitan LW-S (Sachtleben Chemie GmbH, average particle size ~0,3 µm) with 19.6 wt%

of water, 0.2 wt% of binder Zusoplast C 93 (Zschimmer & Schwarz GmbH & Co KG) and 1.6 wt%

of oil Produkt KP 5144 (Zschimmer&Schwarz GmbH) in a kneader-mixer for 1hour. Using the

above mentioned ratio of paste ingredients, a successful extrusion process can be conducted and the

extruded green bodies are without noticeable defects [3, 4]. After mixing obtained paste was

transferred to a vacuum extrusion press V10 SpHv (Dorst) and formed to cylindrical green bodies

(samples) with 13 mm in diameter and 100 mm in length. The extruded samples were left to dry for

48 hours at room temperature.

Key Engineering Materials Vol. 604 (2014) pp 249-253Online available since 2014/Mar/12 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.604.249

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The dried samples were sintered in air at 1000 °C, 1100 °C, 1200 °C and 1300 °C with heating

rate of 5 °C/min and dwell time of 2 hours in order to burn out the additives, achieve phase

transformation from anatase to rutile and densify the samples [5]. After sintering all samples were

divided into two groups. The first group was thermally treated under vacuum (10-4

Pa) at 1250 °C

with heating and cooling rate of 2 °C/min and dwell time of 1 hour (Series 1). The second group

was thermally treated under vacuum at 1250 °C with heating and cooling rate of 5 °C/min and dwell

time of 1 hour (Series 2).

For electrical/thermoelectric characterization, the heat treated samples were cut into 20 mm long

rods. Ends of the rods were polished with SiC paper (280, 1500, 2000 grit) and cleaned with

ethanol. Afterwards, they were cleaned using glow discharge plasma and finally covered with thin

aluminium film using physical vapor deposition technique, in order to ensure better electrical

contact between the sample and measuring device.

The Seebeck coefficients of the samples were measured using custom-made equipment as

described before [6]. The Seebeck coefficient and electrical resistance measurements were made

simultaneously. One end of the sample was heated up to 350 °C (623 K). The temperatures during

measurement were determined using copper-constantan thermocouples. The Seebeck voltage was

measured using DC potentiometer PP-63. Wheatstone bridge DC MO-62 was used to measure

electrical resistance of the samples. The carrier type was determined by measurement of Seebeck

voltage induced by temperature gradient [7].

The power factor (P) of the samples was calculated using the following equation: P=S2σ (where

S - Seebeck coefficient, σ - electrical conductivity).

Results and discussion

Fig.1 shows the temperature dependance on electrical conductivity for Series 1 samples.

Electrical conductivity of all samples increased with temperature, showing a semiconducting

behavior. It was noticed that electrical conductivity of the samples was affected by their thermal

treatment conditions. With increase of sintering temperature during thermal treatment in air, the

electrical conductivity of the samples increased. Such relevance could be attributed to

microstructure development and densification process during sintering, since grain size and porosity

can strongly affect electrical properties of TiO2 ceramic [2].

The highest electrical conductivity in temperature range from 300 K to 600 K was observed for

the Series 1 sample a.1300-v.1250-2 (~ 95-110 S/m).

Fig.1. Temperature dependance of electrical conductivity for Series 1 samples

All samples from both series showed n-type conductivity. This is in agreement with the literature

reports on nonstoichiometric TiO2 [2, 8, 9].

Since electrical conductivity and Seebeck coefficient are inversely related (Seebeck coefficient

increases as charge carrier concentration (and hence conductivity) decreases [10]), the samples with

the highest Seebeck coefficient (Fig.2) have the lowest electrical conductivity values (Fig.1). Series

1 samples a.1100-v.1250-2 and a.1200-v.1250-2 have the highest Seebeck coefficients (125-140

80

90

100

110

120

300 400 500 600

σ(S

/m)

Temperature, K

g.1000-v.1250-2

g.1100-v.1250-2

g.1200-v.1250-2

g.1300-v.1250-2

a.1000-v.1250-2

a.1100-v.1250-2

a.1200-v.1250-2

a.1300-v.1250-2

250 Engineering Materials & Tribology XXII

µV/K), while the sample a.1300-v.1250-2 with highest electrical conductivity - the lowest value

(~105 µV/K).

Fig.2. Temperature dependance of Seebeck coefficient for Series 1 samples

Temperature dependance on the thermoelectric power factor (S2σ) for the Series 1 samples is

depicted in Fig. 3. Series 1 sample a.1200-v.1250-2 have the highest Seebeck coefficient (Fig. 2.) as

well as the highest thermoelectric power factor (2.0×10-6

Wm-1

K-2

at 600 K), while Series 1 sample

a.1300-v.1250-2 with lowest Seebeck coefficient - the lowest thermoelectric power factor (1.1×10-6

Wm-1

K-2

at 600 K).

Fig.3. Temperature dependance of thermoelectric power for Series 1 samples

Fig.4. shows temperature dependance of electrical conductivity for Series 2 samples. The same

relationship between sintering temperature and electrical conductivity was observed, however

Series 2 samples have lower electrical conductivity values for all thermal treatment conditions when

compared to Series 1 samples. The highest electrical conductivity in temperature range from 300 K

to 600 K (78-85 S/m) was observed for Series 2 sample a.1300-v.1250-5. In general, electrical

conductivity for all Series 2 samples was ~20-25% lower than for Series 1 samples. This could be

related to the shorter thermal treatment time under vacuum in the case of Series 2 samples (heating

rate for Series 1 samples was 2 °C/min, while for Series 2 samples – 5 °C/min). Longer thermal

treatment time under vacuum could increase defect concentration in TiO2 lattice and hence improve

its electrical conductivity. Total thermal treatment time under vacuum for Series 1 samples were

two times longer compared with Series 2 samples.

70

90

110

130

150

300.00 400.00 500.00 600.00

S (

µV

/K)

Temperature (K)

g.1000-v.1250-2

g.1100-v.1250-2

g.1200-v.1250-2

g.1300-v.1250-2

0.0

0.5

1.0

1.5

2.0

300 400 500 600

P(×

10

-6W

m-1

K-2

)

Temperature, K

g.1000-v.1250-2

g.1100-v.1250-2

g.1200-v.1250-2

g.1300-v.1250-2

a.1000-v.1250-2

a.1100-v.1250-2

a.1200-v.1250-2

a.1300-v.1250-2

a.1000-v.1250-2

a.1100-v.1250-2

a.1200-v.1250-2

a.1300-v.1250-2

Key Engineering Materials Vol. 604 251

Fig.4. Temperature dependance of electrical conductivity for Series 2 samples

The highest Seebeck coefficient of Series 2 samples was observed for the sample a.1100-v.1250-

5 (170-190 µV/K), while the lowest (90-120 µV/K) for the sample a.1300-v.1250-5 (Fig. 5.). Since

electrical conductivity values of Series 2 samples were lower than Series 1 samples, they showed

higher Seebeck coefficient values compared with Series 1 samples.

Fig.5. Temperature dependance of Seebeck coefficient for Series 2 samples

Temperature dependance of thermoelectric power factor for Series 2 samples are shown in Fig.

6. The obtained thermoelectric power factor values were close to those obtained for Series 1

samples (Fig. 3.). The comparative lower electrical conductivity of Series 2 samples was

compensated by higher Seebeck coefficient, and as a result the thermoelectric power factor values

of samples from both series did not differ much. The highest thermoelectric power factor (2.5×10-6

Wm-1

K-2

at 600 K) was calculated for Series 2 sample a.1100-v.1250-5 while the lowest value

(1.0×10-6

Wm-1

K-2

at 600K) for Series 2 sample a.1300-v.1250-5.

Fig.6. Temperature dependance of thermoelectric power for Series 2 samples

50

60

70

80

90

300 400 500 600

σ(S

/m)

Temperature, K

g.1000-v.1250-5

g.1100-v.1250-5

g.1200-v.1250-5

g.1300-v.1250-5

70

90

110

130

150

170

190

210

300 400 500 600

S (

µV

/K)

Temperature, K

g.1000-v.1250-5

g.1100-v.1250-5

g.1200-v.1250-5

g.1300-v.1250-5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

300 400 500 600

P(×

10

-6W

m-1

K-2

)

Temperature, K

g.1000-v.1250-5

g.1100-v.1250-5

g.1200-v.1250-5

g.1300-v.1250-5

a.1000-v.1250-5

a.1100-v.1250-5

a.1200-v.1250-5

a.1300-v.1250-5

a.1000-v.1250-5

a.1100-v.1250-5

a.1200-v.1250-5

a.1300-v.1250-5

a.1000-v.1250-5

a.1100-v.1250-5

a.1200-v.1250-5

a.1300-v.1250-5

252 Engineering Materials & Tribology XXII

Summary

The effect of thermal treatment conditions in air and under vacuum on thermoelectric properties

of extruded TiO2 ceramic was investigated. It was found that thermal treatment conditions

substantially affect electrical conductivity, Seebeck coefficient and thermoelectric power factor of

the increase in sintering temperature during thermal treatment in air, the electrical conductivity of

the samples increases while Seebeck coefficient decreases. With an increase in the heating rate

during vacuum heat treatment of the samples, the electrical conductivity of the samples decreases

while Seebeck coefficient increases.

Acknowledgements

This work has been supported by the European Regional Development Fund within the project

“Development of innovative water procession technology using nanostructured ceramic“, No.

2010/0257/2DP/2.1.1.1.0/10/APIA/VIAA/012

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Key Engineering Materials Vol. 604 253

Engineering Materials & Tribology XXII 10.4028/www.scientific.net/KEM.604 Effect of Thermal Treatment on Thermoelectric Properties of Extruded TiO2 Ceramics 10.4028/www.scientific.net/KEM.604.249

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http://dx.doi.org/10.4028/www.scientific.net/AMR.222.301