Microstructure and Thermal Conductivity of Porous Al2O3-ZrO2 Ceramics

Tiago Delbrücke1, a*, Rogério A. Gouvêa1,b, Cristiane W. Raubach1,c,

Elson Longo4,d, Vânia C. Sousa2,e, Sergio M.Tebcheranic3,f,

Evaldo T. Kubaski3,g, Mário L. Moreira6,h, Jose R. Jurado1,2,5,i

and Sergio Cava1,j

1Graduate Program in Science and Materials Engineering, Technology Development Center, Federal University of Pelotas, 96010-900, Pelotas,RS, Brazil

2Laboratory of Biomaterials & Advanced Ceramics, Federal University of Rio Grande do Sul, 91509-900, Porto Alegre, RS, Brazil

3Interdisciplinary Laboratory of Ceramic Materials, Ponta Grossa State University,84030-900 , Ponta Grossa, PR, Brazil

4National Institute of Science and Technology of Materials in Nanotechnology, Paulista State University, 14801-907, Araraquara, SP, Brazil

5Glass Department, Institute of Glass and Ceramic, 28049, Madrid, Spain 6Institute of Physics and Mathematics, Federal University of Pelotas, 96010-900, Pelotas, RS,

Brazil a*tiagodt@gmail.com, brogerio-gouvea@hotmail.com, ccrica_wr@yahoo.com.br,

delson.liec@gmail.com, evania.sousa@ufrgs.br, fsergiomt@uepg.br, gevaldotk@outlook.com, hmlucio3001@gmail,com, ipepejuradoegea@gmail.com, jsergiocava@gmail.com

Keywords: Porous ceramic, replica method, Al2O3-ZrO2, thermal properties

Abstract. The improvement on the development of porous ceramic materials has leaded to new

technologies in thermal insulation, for example, composite materials for better performance of

pressure vessels in rocket engines. Within this context, the present work aims to evaluate the ability

of a refractory ceramic base alumina/zirconia through the processes of co-precipitation and replica

method in an organic fiber template. The green body was burnt-out and sintered at 1200-1600°C to

obtain the continuous porous ceramic fibers. In the FEG-SEM analysis, an interconnected porous

structure with small grains was observed. The crystalline phases were determined by X-ray

diffraction and compared to micro-Raman results regarding the crystalline structure confirms that

there present in the material zirconia is composed of more than one phase. Porosity was calculated

through a mercury porosimeter as 77.9%, and the Laser Flash method gave a thermal conductivity

value of 1.61 K W.m−1

.K−1

for the Al2O3-ZrO2 fibers.

Introduction

Some porous materials present special properties and capabilities that cannot normally be

achieved by their conventional dense counterparts. Therefore, porous materials find many

applications as final products in many technological processes. Macroporous materials are used in

many shapes and compositions in daily life, including for example, polymeric foams for packaging,

aluminum light structure in buildings and aircrafts, and porous ceramic for water purification [1, 2].

An increasing number of applications which demand for ceramics with advanced properties has

been attracting attention in the last decades, specially in environments where high temperatures,

extensive wear and corrosive reactants are present. Applications with such hostile environment are

molten metal filtration, thermal insulation in high temperatures, support for catalytic reactions [3],

filtration of particles from exhaust gases of diesel engines and filtration of corrosive hot gases from

several industrial processes [4–6]. The advantages of using porous ceramic in these applications are

usually the high melting point, suitable electronic properties and good wear and corrosion

resistances combined to the advantage of low density, higher surface area, lower thermal

Materials Science Forum Vol 820 (2015) pp 268-273 Submitted: 2015-02-09© (2015) Trans Tech Publications, Switzerland Accepted: 2015-03-10doi:10.4028/www.scientific.net/MSF.820.268

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 TransTech Publications, www.ttp.net. (ID: 143.54.3.93-28/04/15,13:08:57)

conductivity imparted by the porosity [1, 7]. These properties can be adapted to every specific

application controlling microstructure and composition of the porous ceramic.

According to the type of application different microstructures and thus different preparation

methods are required for porous ceramic bodies. For thermal or acoustic insulators materials with

closed porosity are preferred, whereas membranes and filters require open pores with exactly

defined pore size, for example. Are currently used some specific techniques to obtain pores with

polymer and cotton for use in thermal insulation [8, 9]. On the other hand, for application to

environmental issues sodium hydroxide is used to pore formation [10].

This work uses a simple and versatile methodology to prepare sintered porous ceramic bodies

obtained by replica method using organic fibers. The porous bodies of Al2O3 were synthesized

through co-precipitation method [9] and replica [1, 9] aiming to achieve a porous ceramic structure

with thermal properties in agreement to the application of refractory materials.

Materials and Methods

The synthesis of aluminum hydroxide consisted of dissolving aluminum nitrate - Al(NO3)3.9H2O

(Synth) – in water; this solution was heated in a heating plate at 80°C. After its complete dilution

ammonium hydroxide - NH4OH (Vetec) - was added until pH = 9 was reached. The Al(NO3)-

NH4OH ratio was 1:6 (in moles). Afterwards, anhydrous citric acid - C6H8O7 (Synth) - was added

until pH = 1 was reached, keeping a Al(NO3)-C6H8O7 molar ratio of 3:1. The addition of the citric

acid causes the precipitation of aluminum nitrate in aluminum hydroxide - Al(OH)3 - forming a

slurry solution that was used to impregnate the organic fibers.

The organic fibers were impregnated in the aluminum hydroxide solution following the

procedure of the replica method [1, 9]. The organic fibers consisted of commercial cotton produced

by Johnson & Johnson. After complete impregnation, the cotton fibers were pressed into a ceramic

crucible to eliminate the surplus solution from the fibers and a green body with the shape of the

recipient is obtained. These green bodies were placed on an electric oven (INTI, model FE-1300)

for the calcination process, which occurred at 1200°C in air for 2 hours using a heating rate of

2°C/min. During this process there was a complete removal of the organic matter and α-alumina

was formed from phase transition of aluminum hydroxide [11].

For the synthesis of zirconium hydroxide, initially, zirconium tetrachloride - ZrCl4 (Synth) - was

diluted in water. Next, this solution was heated in a heating plate at 80°C and after complete

dilution ammonium hydroxide - NH4OHNH4OH (Vetec) - was added until pH = 9 was reached

keeping a ZrCl4-NH4OH molar ratio of 1:6. Afterwards, anhydrous citric acid - C6H8O7 (Synth) -

was added until pH = 1 was reached, keeping a ZrCl4-C6H8O7 molar ratio of 3:1. The addition of

the citric acid causes the precipitation of zirconium tetrachloride in zirconium hydroxide - Zr(OH)4

- forming a slurry solution that was used to impregnate the Al2O3 fibers.

The crystalline phase was determined by X-ray diffraction (XRD) using a Shimadzu XRD-6000

diffractometer with CuKα radiation at 40 kV and 40 mA with the patterns recorded in the 20 to 80

theta measuring range at a scan rate of 2°/min, at room temperature. Through micro-Raman was

possible to verify the peaks corresponding to the symmetry of the ceramic phases; the analysis was

performed in room temperature, using an wavelength of 514.5nm of an argon laser as exciting

source. The energy was maintained in 15mW and a 50x lense was used. The spectra was registered

through a monochromer T-64 Jobin-Yvon jointed to a CCD detector.

The morfology and structure of the pores created by the template of the metal oxides were

analysed by FEG-SEM (Supra 35-VP, Carl Zeiss). Mean grain size was determined by using the

intercept method. The surface area of the porous ceramics was determined in a nitrogen

physisorption equipment, model AS-1 of Quantachrome. The isotherms were obtained with 40

adsorption and desorption points and the surface area was calculated with 5 reference points. The

porosity was calculated using a mercury porosimeter of Micromeritics Instrument.

Thermophysical properties are determined by the flash laser method [13–15] that allows to

determine thermal diffusivity and specific heat of the sample. Specific heat and thermal diffusivity

values were found through measurement of the increase in temperature of the opposite face of the

Materials Science Forum Vol. 820 269

material in the shape of a small disk, while the frontal face receives a strong energy flash by a laser.

The potency was applied by a laser with maximum potency of 90 watts. The irradiation time is

within the order of 10 ms. The measurements were taken at room temperature in normal

atmosphere.

The thermal diffusivity is calculated from the thickness of the sample and the required time for

the temperature in the opposite face to reach 50% of the temperature in the laser incident face.

Specific heat is determined by the density and thickness of the sample given the maximum

temperature reached in the opposite face and the amount of heat received. Thermal conductivity is

calculate by the product of the thermal diffusivity, specific heat and density, as shown in the Eq. 1

where K = thermal conductivity, α = thermal diffusivity, ρ = density and Cp = specific heat.

K = α.ρ.Cp (1)

Results and discussion

XRD of the sintered samples in different temperatures suggests the formation of alumina and

zirconia phases in every temperature of sintering. The peaks for Al2O3-ZrO2 are in agreement with

those found in PDF (Powder Diffraction File) 53-559 file [16]; α-Al2O3 agrees with what is found

in PDF 46-1212 file [17] files; ZrO2 agrees with the peaks in JPCDS 37-1484 file [18] files (see

Fig. 1).

Fig..1: X-ray diffraction patterns of the Al2O3-ZrO2 porous ceramics at several sintering

temperatures.

Fig. 2 shows results regarding the measurements of micro-Raman spectroscopy dealing with

wavelength radiation and vibration energies of the molecules. According to Raman results,

vibration of Al-O bonds are related to the peaks described below: shows a peak at 378 cm−1

, that

may be considered polycrystalline α-Al2O3 [19]. For samples sintered above 1200°C there were

significant spectroscopic bands in 410, 470, 605 and 630 cm−1

, which are identified as absorption

bands characteristic of α-Al2O3 [20–22]. These results are agreement with the study conducted by

Cava et al., in which spectroscopic bands of α-Al2O3 are present in temperatures above 1000°C[11].

Raman is a powerful technique to detect the allotropic forms of zirconia [23]. According to the

previous work of Popa et al. [24], densified regions are caused by presence of monoclinic zirconia.

FEG-SEM micrographs (Fig. 3) show the presence of densified regions of zirconia; therefore,

micro-Raman was used to identify the presence of monoclinic zirconia in these regions. Vibration

of Zr-O bonds are related to the peaks on the spectroscopic bands of monoclinic zirconia were

identified in frequencies of 505, 534, 550 cm−1

[24, 25], peaks shown in temperatures of 1300-

1600°C. Only a small amount of monoclinic zirconia is present; however it can be suggested that

this amount was enough to densify the regions of porous ceramics agglomerating around grains

270 Brazilian Ceramic Conference 58

[24]. Tetragonal zirconia was clearly detected in the peaks 260, 300, 323 and 340 cm−1

[24]

increasing their intensity above 1300°C. It was found only two well defined peaks at 217 and 745

corresponding to cristobalite zirconia between temperatures of 1200-1600°C. The crystalline phase

of Al2O3-ZrO2 shown in Fig. 1 has a cubic structure [16], not being detected by Raman.

Fig. 2: Micro-Raman spectra of the Al2O3-ZrO2 porous ceramics at differents temperatures of: (a)

1200°C, (b) 1300°C, (c) 1400°C and (d) 1500°C.

Fig. 3 shows morphological features of the porous ceramic of Al2O3-ZrO2, in which grain growth

was possible due to the presence of ZrO2, causing the formation of diffusion barriers to control the

growth and formation of the grain [26]. Furthermore, it was possible to verify the solid phase

sintering, where the densification and grain growth are controlled by the diffusion across the grain

boundary [27].

In Table 1 the results on thermophysical properties are shown for the Al2O3-ZrO2 ceramic fibers.

In the literature very few data about the thermal conductivity of the Al2O3 and ZrO2 system is

available. However the effect of the porosity to reduce the thermal conductivity of ceramic

materials is a well recognized phenomenon, that has been widely applied for Al2O3 and ZrO2 [3, 8,

31]. In addition to the absolute value of the porosity, the interconnection of grain size and pore

shape have a significant influence on the final thermal conductivity. The high purity Al2O3 with no

pores and with average grain size ~1µm shows a thermal conductivity of approximately 33

W.m−1

.K−1

at room temperature. High purity ZrO2 without porosity and average grain size of ~1µm

has a thermal conductivity of approximately 3.3W.m−1

.K−1

[32] at room temperature. Different

studies show that thermal conductivity is independent of the grain size [33, 34].

Tabele 1: Analysis of thermal conductivity by laser flash method of the Al2O3-ZrO2 porous

ceramics sintered at 1600°C.

Sample α (106m²S

-1) ρ (Kg.m

-3) Cp (J.Kg

-1.K

-1) K (W.m

-1. K

-1)

Al2O3-ZrO2 1.09 2696 547.3 1.61

B. Nait-Ali and co-workers [8] conducted a research on the Al2O3-ZrO2 system relating thermal

conductivity to porosity. Their samples were sintered at 1400°C and pores were generated by a

pore-forming polymer. Results showed that commercial Al2O3 with 40% of porosity presented a

thermal conductivity of 9 W.m−1

.K−1

and average grain size of 0.5µm; for commercial ZrO2,

thermal conductivity was 0.9 W.m−1

.K−1

for 37% of porosity and average grain size of 0.1µm.

Bansal and co-workers [35], using samples of commercial ZrO2-Al2O3 sintered at 1000°C with

density around 99% with average grain size of ~1µm showed a thermal conductivity of 6.9

W.m−1

.K−1

. The porous Al2O3-ZrO2 fibers obtained in this work were sintered at 1600°C and the

calculated thermal conductivity was 1.61 K(W.m−1

.K−1

) with a porosity of 77.9% and an average

Materials Science Forum Vol. 820 271

grain size of ~1µm. The cotton replicated fibers of Al2O3-ZrO2 sintered at 1600°C presented very

low thermal conductivity compared to other works using different processes of pore formation.

Fig. 3: FEG-SEM images of Al2O3-ZrO2 fibers sintered at 1600ºC with magnifications: a, b) 500x,

c) 8000x and d) 30000x.

Conclusions

Porous Al2O3-ZrO2 consisting of ceramic fibers were obtained successfully by the replica method in

an organic fiber matrix using cotton as the template. SEM-FEG images show the presence of

densified regions due to presence of ZrO2. The micro-Raman measurements show different peaks

due to the vibrational energies of the crystalline phases of the constituent oxides, confirming the

crystalline phases presented in the XRD patterns. The sintering temperature, low heating rate and

the use of cotton as template had a strong effect on the surface area, pore size and distribution of the

synthesized fibers. Thermal conductivity data show excellent results when compared to the

literature, due to the direct influence of the organic template as a shape-model and the efficient

method of synthesis. The results show that the porous Al2O3-ZrO2 ceramic is an excellent thermal

insulator with direct application as a refractory.

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