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Aluminosilicates compacts by alkoxide route: influence of Ba addition

Compactos de aluminosilicatos pela rota dos alcóxidos: influência da adição de bário

Abstracts

The sol-gel route was employed to obtain three different powders with the stoichiometric composition corresponding to mullite (3 Al2O3.2SiO2): (i) commercial alumina and silica from sol-gel, (ii) alumina and silica by alkoxide partial hydrolysis, (iii) alumina and silica by alkoxide simultaneous hydrolysis. Tetraethylorthosilicate (TEOS) and aluminium tri-sec-butoxide were used as alkoxide precursor. Moreover, 5 wt% of barium, incorporated to precursor solutions as Ba-acetilacetonate, was used as additive to study its influence on densification and mullite phase formation. The precursor powders were pressed uniaxially to form green compacts, after that all samples were heated to 1350 °C for 2 hours to obtain densified pellets. X-ray diffration patterns of powders, obtained by milling of sintering compacts, showed important differences. In barium doped samples a higher grade of densification and a greater glassy phase formation were observed.

sol-gel; barium; alkoxides; aluminosilicates


<i>O método sol-gel foi usado para obter pós com 3 composições estequiométricas correspondente a mulita (3 Al2O3.2SiO2): (i) alumina comercial e sílica por sol-gel, (ii) alumina e sílica por hidrólise parcial de alcóxido, (iii) alumina e sílica por hidrólise simultânea de alcóxido. Tetroetiloctosiliceto (teos) e tri-sec-butóxido de alumínio foram usados como precursores alcóxidos. Além disso, 5% em peso de bário, adicionado nas soluções precursoras como acetilacetonato de bário foi usado como aditivo para estudar sua influência na densificação e formação da fase mulita. Os pós precursores foram prensados uniaxialmente para formar compactos a verde, e posterior aquecimento a 1350</i>°<i>C por 2 h para obter corpos densificados. A difração raios X dos pós, preparados por moagem dos compactos sinterizados, mostrou diferenças importantes. Foram observadas maior densificação e formação de fase vítrea nas amostras dopadas com bário.</I>

sol-gel; bário; alcóxidos; aluminosilicatos


Aluminosilicates compacts by alkoxide route: influence of Ba addition

(Compactos de aluminosilicatos pela rota dos alcóxidos: influência da adição de bário)

N. E. Quaranta, E. R. Benavidez

Universidad Tecnológica Nacional - Facultad Regional San Nicolás

Colón 332 - (2900) San Nicolás - Argentina

e-mail: nancyutn@cablenet.com.ar

Abstract

The sol-gel route was employed to obtain three different powders with the stoichiometric composition corresponding to mullite (3 Al2O3.2SiO2): (i) commercial alumina and silica from sol-gel, (ii) alumina and silica by alkoxide partial hydrolysis, (iii) alumina and silica by alkoxide simultaneous hydrolysis. Tetraethylorthosilicate (TEOS) and aluminium tri-sec-butoxide were used as alkoxide precursor. Moreover, 5 wt% of barium, incorporated to precursor solutions as Ba-acetilacetonate, was used as additive to study its influence on densification and mullite phase formation. The precursor powders were pressed uniaxially to form green compacts, after that all samples were heated to 1350 °C for 2 hours to obtain densified pellets. X-ray diffration patterns of powders, obtained by milling of sintering compacts, showed important differences. In barium doped samples a higher grade of densification and a greater glassy phase formation were observed.

Keywords: sol-gel, barium, alkoxides, aluminosilicates.

Resumo

O método sol-gel foi usado para obter pós com 3 composições estequiométricas correspondente a mulita (3 Al2O3.2SiO2): (i) alumina comercial e sílica por sol-gel, (ii) alumina e sílica por hidrólise parcial de alcóxido, (iii) alumina e sílica por hidrólise simultânea de alcóxido. Tetroetiloctosiliceto (teos) e tri-sec-butóxido de alumínio foram usados como precursores alcóxidos. Além disso, 5% em peso de bário, adicionado nas soluções precursoras como acetilacetonato de bário foi usado como aditivo para estudar sua influência na densificação e formação da fase mulita. Os pós precursores foram prensados uniaxialmente para formar compactos a verde, e posterior aquecimento a 1350°C por 2 h para obter corpos densificados. A difração raios X dos pós, preparados por moagem dos compactos sinterizados, mostrou diferenças importantes. Foram observadas maior densificação e formação de fase vítrea nas amostras dopadas com bário.

Palavras-chave: sol-gel, bário, alcóxidos, aluminosilicatos.

INTRODUCTION

Mullite is the stable phase of aluminosilicate system at high temperature when the solid state reaction between Al2O3 and SiO2 takes place. Properties of mullite such as low thermal expansion, low dielectric constant and high creep and mechanical resistance have identified mullite as a good candidate to be applied in electronic and in high temperature structural applications [1]. In view of the technological importance of mullite in the field of ceramics, the synthesis of this material has been extensively investigated [2]. The synthesis reaction is modified by some factors such as: particle size distribution of precursor materials, mixing conditions, thermal treatment, Al/Si ratio, etc [3].

The temperature of mullite formation (mullitization) can be substantially decreased using finer powders of alumina and silica in an intimate mixing [4]. By this, it is very important to control synthesis conditions to obtain alumina-silica mixing with these properties. The size of particles in intimate contact affects the mullite formation temperature. Thus, temperatures above 1600

°C are necessary to obtain mullite when the particle size distribution is greater than one micron. Colloidal methods produce powders with particle size about 1 micron, in this case the reaction temperature is reduced to the 1300-1450 °C range [5]. Lower reaction temperatures (near 1000 °C) can be obtained when molecular level of mixing is achieved [6, 7]. This is the case corresponding to materials produced by sol-gel method [8, 9].

Very high purity and homogeneity in the precursor materials and a lower process temperature incorporated into sol-gel technique are advantageous characteristics compared to the conventional methods. Also by this method is possible to produce powder with high surface area which include very fine particles. The most important characteristic is to control material properties during the earliest stages of powder production. This introduces the concept of molecular manipulation [10].

In the present work, the sol-gel synthesis method was used to obtain materials with mullite composition, with and without barium addition. This addition was selected to study its influence on the densification and mullitization behavior of the samples.

MATERIALS AND METHOD

Samples preparation

Sample M1:

it was prepared from commercial alumina (Aldrich, 99.8%) with particle size < 10 mm, and silica obtained by alkoxide route using TEOS (Aldrich, 98%). First, alumina was suspended in TEOS/ethanol solution, then silica was synthesized by hydrolysis process. The hydrolysis taked place adding distilled water to a 0.02 mol/min rate under continuos stirring. The final molar ratio H

2O:TEOS:EtOH = 4:1:1 was established in this experiment and ammonium hydroxide (NH

4OH) was added to alkalize the solution at pH = 8. The experiment temperature was kept constant at 45 ° C. After 24 hours of reaction, the floating liquid was extracted and the sample was dried at 60 ° C.

Sample M2: it was obtained under similar conditions that M1, but alumina precursor was aluminum tri-sec-butoxide (Aldrich, 97). In this case, the partial hydrolysis method was employed [11]: Si-alkoxide was partially hydrolyzed for 2 h under the same conditions and proportions than M1, after that Al-alkoxide dissolved in ethanol, was added to Si solution.

Sample M3: similar conditions than M2 were established, but a different process step was employed in this preparation: simultaneous hydrolysis replaced the partial hydrolysis of Al and Si alkoxides.

Samples M1Ba, M2Ba, M3Ba: these materials were obtained similarly to M1, M2 and M3, respectively, adding barium acetilacetonate hydrate (Aldrich) to initial solutions. In all cases, the addition was fixed at 5 wt% Ba.

The powders obtained by different ways were compacted to disks 35.6 mm in diameter by uniaxial pressing. These compacts were heated to sintering temperature 1350

°C for 2 hours. Then they were cut, mounted and polished to allow for microscopy observation and microhardness characterization. The sintering temperature was established taking into account previous results [12].

Characterization of powders and compacts

All samples were characterized by the following methods:

(i) Microscopy analysis: microstructures of the polished sintered samples were observed by a Zeiss microscope with a Philips video camera.

(ii) Specific surface area: initial powders areas were determinated by BET method using N

2 gas with an Accusorb Micromeritics equipment.

(iii) X-ray diffraction: XRD patterns of sintered powders were obtained with Philips PW 1390 diffractometer, using CuK

a radiation and Ni filter. The operation conditions were 40 kV, 20 mA and scanning rate of 1°/min.

(iv) Vickers microhardness: these analyses were made with a Vickers identer in a HMW-2000 Shimadzu equipment at load of 300 g applied during 10 seconds on the polished surface of the sintered body.

(v) Density determination: bulk densities after sintering were measured by the Archimedes method using distilled water.

RESULTS AND DISCUSSION

Samples obtained by different ways showed a higher densification grade when barium was added, as it is observed in the density and radial shrinkage values shown in Table I. This behavior can be attributed to better initial properties corresponding to these samples, since it can be observed in the powder materials produced by the same way, the specific surface area was increased by Ba addition.

The low radial shrinkage corresponding to M1 and M1Ba samples, can be explained taking into account that a-alumina was employed in their preparation. In the other samples g-alumina are obtained in the precursor materials and transformed to a-phase by heating. This structural change is accompanied by an important volume contraction. The shrinkage is higher in samples with Ba, because a liquid phase appears at lower temperature than Al2O3-SiO2system, promoting the densification grade of compacts. A greater glassy phase in all compacts with barium, compared to similar compacts without it, can be observed by optical microscopy.

These observations also explain microhardness results, that show lower values for samples containing barium. In Fig. 1 two photographs corresponding to M3 and M3Ba sintered compacts are shown; a higher sintering grade and a higher amount of glassy phase in M3Ba are observed.


Observing samples without Ba, the one obtained by partial hydrolysis (M2) presents the largest specific surface area, which is related to higher shrinkage and density.

Some differences are observed in XRD patterns as it is shown in Fig. 2. In M1 and M1Ba powders peaks corresponding to a-Al2O3, but no peaks due to crystalline SiO2 phase are detected, indicating that silica is in amorphous phase and the mullite formation temperature is higher than that used in these experiments (1350 °C). Moreover, in M1Ba sample the BaAl2Si2O8 phase is observed.


In the other samples are detected mullite and high intensity peaks of cristobalite but no peaks corresponding to a-Al2O3. This can be due to the formation of a Al2O3-rich mullite phase leaving free silica which it is stabilized in cristobalite structure. Similar behavior was already reported. [13] who founded a fast growth of cristobalite phase at 1350 ° C is found in samples with lower Al2O3 amount than the corresponding to mullite composition.

An important presence of cristobalite is observed in samples containing Ba. This fact contributes to obtain more densified compacts due to the volume contraction that takes place during amorphous silica to cristobalite transformation.

CONCLUSIONS

The sol-gel was used to obtain aluminosilicate powder samples with mullite composition. The different ways to obtain these samples and the influence of the barium addition (5 wt%) were studied. Some properties of these powders and corresponding sintered compacts were examinated.

1 - Samples obtained from commercial Al2O3 and sol-gel SiO2 do not present mullite phase at the work temperature (1350 °C). The presence of barium has no influence on the compact densities and it is only observed like a low amount of BaAl2Si2O8 phase.

2 - Partial hydrolysis as well as simultaneous hydrolysis of metal alkoxides produce SiO2-Al2O3 mixture which at 1350 °C from mullite. In all cases, with and without barium addition, the cristobalite phase is observed. Barium addition favours cristobalite phase formation and contributes to obtain more dense ceramic compacts.

ACKNOWLEDGEMENTS

Dra. Quaranta, CICPBA researcher, gratefully acknowledges to this Commission for the financial support of the present work.

(Rec. 11/98, Ac. 01/99)

  • [1] S. Somiya, Y. Hirata, Am. Ceram. Soc. Bull. 70, 10 (1991) 1624.
  • [2] K. Okada, N. Otsuka, S. Somiya, Am. Ceram. Soc. Bull. 70, 10 (1991) 1633.
  • [3] S. Somiya, R. F. Davis, J. A. Pask, Ceramic Transactions, "Mullite and mullite matrix composites" 6 (1990), The American Ceramic Society, Inc.
  • [4] M. D. Sacks, H. Lee, J. A. Pask, " Mullite and mullite matrix composites", Ed. S. Somiya, R. Davis, J. Pask, Ceramic Transactions 6 (1990) 167.
  • [5] M. G. Ismail, Z. Nakai, S. Somiya, J. Am. Ceram. Soc. 70, 1 (1987) C-7.
  • [6] J. Ossaka, Nature 91 (1961) 1000.
  • [7] T. D. McGee, D. Wirkus, Am. Ceram. Soc. Bull. 51, 7 (1972) 577.
  • [8] S. Kansaki, H. Tabata, T. Kumazawa, S. Ohta, J. Am. Ceram. Soc. 68, 1 (1985) C-6.
  • [9] B. Yoldas, D. Partlow, J. Mat. Sci. 23 (1988) 1895.
  • [10] L. Hench, J. West, Chem. Rev. 90 (1990) 33.
  • [11] H. Suzuki, Y. Tomokiyo, Y. Sumuya, H. Saito, J. Ceram. Soc. Japan Int. 96 (1988) 67.
  • [12] M. Caligaris, N. Quaranta, R. Caligaris, Key Engineering Materials 132-136, (1997) 888.
  • [13] S. Kanzaki, M. Ohashi, H. Tabata, T.Kurihara, S. Iwai, S. Wakabayashi, " Mullite and mullite matrix composites", Ed. S. Somiya, R. Davis, J. Pask, Ceramic Transactions 6 (1990) 389.

Publication Dates

  • Publication in this collection
    01 June 2000
  • Date of issue
    Feb 1999

History

  • Accepted
    Feb 1999
  • Received
    Nov 1998
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