Spinel-mullite composites with optical properties

P1: RPS/SFI

P2: RSA/SFI

P3: RSA/SFI

Journal of Sol-Gel Science and Technology

QC: BSA

KL341-Th3.03

December 27, 1996

13:12

Journal of Sol-Gel Science and Technology 8, 871–875 (1997) c 1997 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Spinel-Mullite Composites with Optical Properties ´ MANUELA SALES, CARLA VALENTIN AND JAVIER ALARCON Departamento de Qu´ımica Inorg´anica, Facultad de Ciencias Qu´ımicas, Universidad de Valencia, 46100 Burjasot (Valencia), Spain

Abstract. The aim of this paper was to study the synthesis and characterization of spinel-containing mullite based materials, using sol-gel techniques. Several gels were prepared, with nominal compositions 3(Al2−2x Mx Tix O3 ) · 2SiO2 and 3(Al2−x Mx O3 ) · 2SiO2 , with M = Ni+2 or Co+2 and 0.0 ≤ x ≤ 0.2, by hydrolysis and condensation of mixtures of aluminum, silicon and titanium alkoxides and nickel chloride. Dried gels were homogeneous and displayed a glass transition at around 750◦ C, which indicated that the system could be described as an amorphous silicoaluminate network. Crystallization pathway of gels were followed using differential thermal analysis and X-ray diffraction patterns of samples thermal treated at temperatures in the range between 800 and 1400◦ C. A two-phase aluminate spinel-mullite arrangement was detected at temperatures around 1200◦ C. The microstructure of the final product was interesting, because the minor secondary phase was homogeneously dispersed in the mullite matrix. Chemical and thermal resistance of diphasic materials were tested and the results indicate that these materials can be used as high temperature ceramic pigments. Keywords: gel preparation, mullite, composites, optical properties 1.

Introduction

Mullite based ceramic materials are advanced ceramic materials with outstanding properties. Mullite composites have been extensively developed during the last decade. Thus, several aspects involving synthesis, characterization and mechanical properties of ceramic composites such as cordierite-mullite, alumina-mullite and zirconia-mullite have been recently examined [1]. Two approaches have been used to synthesize composites by sol-gel. In the first one, a mixture of oxide sols was gelled by elimination of solvent or by changing the pH. A second method to obtain gels is by hydrolysis and condensation of mixtures of alkoxides. It has been claimed that both types of precursors gels differ in the state of mixing, i.e., in the level of chemical homogeneity reached. Thus, whereas colloidal gel precursors reach a submicrometric level of homogeneity, a molecular level is attained in polymeric gels. This fact should, in principle, lead to faster reaction rates and lower reaction temperatures. However, it seems that some other factors, such as the gel structure, which depends mainly on the chemistry involved in the precursor

gel, influence the transformations taking place, in both kinetic and thermodynamic ways. Huling and Messing [2] discussed results in the crystallization path of several intermediate gels obtained from both kinds of gels, with the purpose of understanding the facts usually observed in samples from single gels in which an Al-Si spinel phase was detected. Further crystallization of mullite is not detected, until temperatures higher than 1200◦ C. This fact can only be explained by the crystallization of a metastable phase at low temperature, that is ∼ =1000◦ C, which forestall the crystalllization of mullite as the more stable phase. Thus, although the mullite phase is the most stable from a thermodynamic point of view, kinetic factors favor Al-Si spinel formation. As it has been experimentally checked, conditions used in synthetic procedure can lead to formation of mullite or Al-Si spinel, the latter might be explained by cluster aluminum formation from which γ -Al2 O3 is formed. Further crystallization of Al-Si spinel phase takes place by epitaxial growth. From the above approach, it seems interesting to attempt to develop new mullite-based composites in which the secondary phase might be growth epitaxially

P1: RPS/SFI

P2: RSA/SFI

P3: RSA/SFI

Journal of Sol-Gel Science and Technology

872

QC: BSA

KL341-Th3.03

December 27, 1996

Sales, Valentin and Alarc´on

from the mullite precursor. This needs a more direct control of phase transformation and microstructure of the whole composite. 2. 2.1.

diffractometer had two 1◦ divergence slits, with scattering and receiving slits at 1◦ and 0.05◦ , respectively. Diffuse reflectance spectra were measured using BaSO4 as the reflectance standard.

Experimental Procedure Gel Preparation

Gels with compositions 3(Al2−2x Mx Tix O3 ) · 2SiO2 , with x = 0.025, 0.05 and 0.2 and 3(Al2−x Mx O3 ) · 2Si O2 , with x = 0.05, with M = Ni+2 , Co+2 or Ti+4 , were prepared using as starting chemicals tetraethylorthosilicate (TEOS, Si(OC2 H5 )4 ), aluminum trisec- butylate C12 H27 AlO3 and titanium iso-propylate (Ti(OC3 H7 )4 , all from Merck & Co. (Darmstadt, Germany), and nickel and cobalt chloride from Fluka, by the following procedure. Stoichiometric amounts of Al(OC4 H9 )3 and (Ti(OC3 H7 )4 were dissolved in 2-propanol (2-PrOH) by refluxing under nitrogen atmosphere and added to a prehydrolyzed TEOS solution which contained the required amount of nickel or cobalt chloride. The TEOS : H2 O molar ratio used in the prehydrolysis was 1 : 2, which was performed at 40◦ C for 20 hours. The resultant mixture, with a molar ratio TEOS/2-PrOH = 1/30, was refluxed at 70◦ C for 5 days, yielding a green or blue gel, except for compositions containing only Ti, which were colorless. Those gels were first slowly dried by light sealing of a beaker containing gels with a plastic foil with pin holes, during several weeks and then drying in an oven at 120◦ C. In order to obtain a glass precursor gel, dried samples were preheated at 750◦ C for 3 hours, yielding gel derived glasses beige or violet in color, for Ni- and Co-containing samples, respectively. Glass powders were heat-treated at different temperatures in the range between 900◦ C and 1400◦ C, for 3 and 9 hours, and additionally at 1400◦ C for other variable time periods. 2.2.

13:12

Characterization of Samples

Chemical and structural evolution of dried gels and crystalline specimens were followed using several techniques. Infrared absorption spectra were obtained in the range 1600–400 cm−1 , using the KBr pellet method. Differential thermal analysis and thermogravimetry were carried out in air with α-Al2 O3 liners, using a heating rate of 10◦ C min−1 . Finely powdered alumina was used as a reference. X-ray diffraction analysis was performed using a graphite-monochromated CuKα radiation. The

3.

Results and Discussion

As-prepared gels with different compositions were transparent and apparently homogeneous. Their green or blue colors became more and more intense as the Ni or Co amounts were increased. IR spectra of dried gels at 120◦ C, from which some interesting information can be obtained, are depicted in Fig. 1. The IR spectra of both series of Ni- and Co-containing gels were quite similar. A band centered at 875 cm−1 was observed, which was ascribed to Si O Al bond vibrations [3]. The band at 1020 cm−1 could also be attributed to Si O Al bond stretching vibration modes. This band originated by shifting of the band corresponding to the Si-O stretching vibration mode at 1100 cm−1 , with formation of Al-O-Si. The weak absorption at 940 cm−1 could be ascribed to the Si-O-Ti stretching vibration in TiO2 -containing dried gels. However, no variation in band intensity was detected with increasing TiO2 content in the samples [4]. The remaining absorption bands between 500 to 800 cm−1 could be attributed to Al O and Ti O bonds, with Al in tetrahedral and octahedral coordination and Ti in octahedral coordination. The spectral features of the samples remained unaffected after heating at 750◦ C, although a better resolution at lower energies was observed. According to IR results, a high degree of homogeneity is attained in these multicomponent gel systems. The single-phase gels, after heating at 750◦ C, may be described as an amorphous silicoaluminate network in which Ti+4 and Ni+2 ions must have a network forming function. DTA curves of both series of specimens, previously thermal treated at 750◦ C for 3 hours, are shown in Fig. 2. All gel derived glass samples were amorphous from X-ray diffraction. Samples in the DTA cell were allowed to cool down after switching off the DTA furnace, when the desired temperatures were reached. The first sharp exothermic peak centered in the range between 972 and 858◦ C was dependent on the M = Ni, Co and/or Ti amount. This first thermal effect may be caused by crystallization of Al-Si spinel and/or MAl2 O4 in all the Ni or Co-containing samples. In the sample containing Ti only, this thermal effect is, of course, only due to crystallization of Al-Si spinel. It is noteworthy that 3 : 2 stoichiometric mullite gels,

P1: RPS/SFI

P2: RSA/SFI

P3: RSA/SFI

Journal of Sol-Gel Science and Technology

QC: BSA

KL341-Th3.03

December 27, 1996

13:12

Spinel-Mullite Composites

873

Figure 2. Differential thermal analysis curves of samples heated at 750◦ C: 1) Ni/Ti with x = 0.025; 2) Ni/Ti with x = 0.05; 3) Ni/Ti with x = 0.2; 4) Co/Ti with x = 0.05 and 5) Co/Ti with x = 0.2.

Figure 1. Infrared spectra of mullite dried gel: 1) Ti with x = 0.05; 2) Ni with x = 0.05; 3) Ni/Ti with x = 0.025; 4) Ni/Ti with x = 0.05 and 5) Ni/Ti with x = 0.2.

prepared following the same method used for specimens in the present paper, fully crystallized to mullite, with a sharp and intense exothermic peak [5]. With the introduction of TiO2 and NiO as low as wt% of ∼ =1.40 and ∼ =1.30, respectively, no trace of mullite was detected during a similarly sharp, but less intense, exothermic peak. The second and third exothermic effects were less intense than the first one and the crystalline phase

associated to those was mullite. In samples with the higher amount of substituents, x = 0.2, the thermal effect associated to the crystallization of mullite was very weak and both further effects were associated with rutile and cristobalite, respectively, in Nicontaining specimens. In addition, for Co-containing specimens two more thermal effects were detected: the first, exothermic, was associated with crystallization of CoTiO3 and the other, endothermic, with melting. Spinel formation around 1000◦ C has been attributed to heterogeneous alumina-silica mixing. Thus, experimental conditions could lead to segregation of alumina and silica, even in alkoxide-derived gels. In a previous work, we have found spinel as well as mullite after the exothermic effect at 1000◦ C, in undoped Al2 O3 -rich mullites prepared from alkoxides. This fact could be due to the great differences in hydrolysis rates of silicon and aluminum alkoxides, which give rise to segregation of alumina-silica. Even though partially hydrolysed TEOS was used, a small amount of crystallized spinel was observed. In these cases the experimental conditions were similar to those used for stoichiometric mullite, the difference being the presence of small amounts of cations, indicating that M and Ti cations played a fundamental role. The apparent discrepancy can be explained by taking into account that thermally produced changes depend on both kinetic and thermodynamic factors. Thus, if growth kinetics are favorable, even a metastable phase is thermodynamically favored if this phase is nucleated before a stable phase. In the

P1: RPS/SFI

P2: RSA/SFI

P3: RSA/SFI

Journal of Sol-Gel Science and Technology

874

QC: BSA

KL341-Th3.03

December 27, 1996

13:12

Sales, Valentin and Alarc´on

Figure 3. A—Diffuse reflectance spectra, and B—X-ray diffraction patterns of samples heated at low temperatures: 1) Ni/Ti with x = 0.05 at 750◦ C/3 h; 2) Ni/Ti with x = 0.05 at 975◦ C/1 h; 3) Ni with x = 0.05 at 750◦ C/3 h and 4) Ni with x = 0.05 at 995◦ C/4 h.

present case, it is suggested that both M and Ti cations facilitate the formation of alumina aggregates, i.e., segregation of alumina, which further transforms to γ Al2 O3 . A further question concerning the crystallization of spinel is the mechanism of formation. As it is well known, X-ray diffraction patterns of Al-Si spinel and MAl2 O4 are very similar because both present the spinel structure. Differences are restricted to the position of lines as a function of lattice parameters. Also, specimens are very weakly crystallized after the exothermic effect.

The crystallization pathway in the range of temperature between the first and second exothermic effects was finally derived by a combination of diffuse reflectance spectroscopy and X-ray diffraction. Figure 3 shows both diffractograms and reflectance spectra for samples thermal treated at several temperatures and holding times, between 750◦ C and around the first exothermic effect. Ni- and Ni/Ti-containing samples were of beige color in the range from 750◦ C to temperatures immediately after the first thermal effect and displayed a characteristic reflectance spectrum of Ni+2 in an amorphous phase. When the temperature or holding

Figure 4. A—Diffuse reflectance spectra of Ni/Ti samples and NiAl2 O4 heated at high temperatures: 1) Ni/Ti with x = 0.025; 2) Ni/Ti with x = 0.05; 3) Ni/Ti with x = 0.2 and 4) NiAl2 O4 . B—Diffuse reflectance spectra of Co/Ti samples and CoAl2 O4 heated at high temperatures: 1) Co/Ti with x = 0.025; 2) Co/Ti with x = 0.05; 3) Co/Ti with x = 0.2 and 4) CoAl2 O4 .

P1: RPS/SFI

P2: RSA/SFI

P3: RSA/SFI

Journal of Sol-Gel Science and Technology

QC: BSA

KL341-Th3.03

December 27, 1996

13:12

Spinel-Mullite Composites

time were raised, the sample coloration changed to blue, with a reflectance spectrum characteristic of crystalline NiAl2 O4 . Samples fired at 1400◦ C for 96 h were studied by diffuse reflectance spectroscopy, in order to examine the coordination environment of cation M+2 . Spectra of Ni- and Co-containing samples are shown in Fig. 4. As it is well known, Ni+2 and Co+2 cations, with electronic configurations 3d8 and 3d7 , respectively, may be located in sites with octahedral or tetrahedral coordination. In both coordination surroundings, the diagram of energy predicts three bands allowed and some other spin-forbidden. As can be seen from the above figure, the spectra of either specimens fitted very well with some previously described by us for NiAl2 O4 or CoAl2 O4 and also included in the last figure. 4.

Conclusions

Spinel-containing mullites were prepared by sol-gel techniques. Gel structure was studied by infrared and diffuse reflectance spectroscopies. The crystallization pathway of gels was followed using DTA and X-ray diffraction patterns of samples thermal treated at temperatures in the range between 800 and 1400◦ C. The following conclusions can be drawn from the above experimental results: — Dried gels were homogeneous and displayed glass transition at around 750◦ C, which indicated that the

875

system could be described as an amorphous silicoaluminate network. — The crystallization pathway was completly different for the 3 : 2 stoichiometric mullite, even for samples having small differences in composition, such as the lower x values ones. — MAl2 O4 spinels were detected after the formation of Al-Si spinel, which can be understood assuming epitaxial growth of the former. Mullite was detected at a temperature around 1200◦ C. — Optical properties of either kind of composites, Nior Co-spinels embedded in mullite, were similar to those of the corresponding spinel phase. Acknowledgment Financial support for the present work was partially provided by CICYT (Project MAT92-0423).

References 1. M.G.M.U. Ismail, H. Tsunatori, and Z. Nakai, J. Am. Ceram. Soc. 73, 537 (1990). 2. J.C. Huling and G.L. Messing, J. Am. Ceram. Soc. 74, 2374 (1991). 3. F. Pancrazi, J. Phalippou, F. Sorrentino, and J. Zarzycki, J. NonCryst. Sol. 63, 81 (1984). 4. Z. Congshen, H. Lisong, G. Fuxi, and J. Zhonghong, J. Non-Cryst. Solids 63, 105 (1984). 5. M. Sales and J. Alarc´on, J. Eur. Ceram. Soc. (to be published).