Formation of mullite thin film via a sol-gel process with polyvinylpyrrolidone additive

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Formation of mullite thin ﬁlm via a sol-gel process with

polyvinylpyrrolidone additive

Yen-Yu Chen, Wen-Cheng J. Wei*

Institute of Materials Science and Engineering, National Taiwan University, 1 Roosevelt Road, Section 4, Taipei, 106 Taiwan, ROC

Abstract

Several tasks were tried to prepare crack-free mullite ﬁlms on silica substrates. Basically, a sol containing TEOS (Tetra-ethylorthosilicate) and boehmite colloid was used for spin coating on silica substrate. The formulation of the sols was kept in stoichiometric composition 3Al2O3.2SiO2, or contained a crack-limiting agent, Polyvinylpyrrolidone (PVP). These ﬁlms were then

treated up to 1300_{C. The properties of the sols and the dried ﬁlms were characterized by rheometer, various thermal analysis}

techniques (DTA, TGA and TMA), also by XRD, SEM and TEM. The mullite film shows randomly oriented grains in sizes from 0.1 mm to a few micrometers. The films still contain fine - and -Al2O3particles after being treated at 1280C for 1 h. # 2001

Keywords:Mullite; PVP; Sol-gel processes; Spin coating; Thin ﬁlms

1. Introduction

Mullite, a high temperature ceramic material, can be synthesized from many methods, such as the reactions of Al2O3 with kaolinite, or pure silica with alumina

com-pounds, chemical vapor deposition of Al and Si oxides, etc. According to the report by Schneider et al.,1_the

pre-cursors of these routes can be classiﬁed into three types. ‘‘Type I’’ and ‘‘type III’’ precursors are amorphous in the as-prepared state and the former yields mullite as the only crystalline phase at about 980_{C, the later only partially}

transforms to Al–Si spinel at about 980_{C. Mullite phase}

is formed above 1100_{C and extensively at 1250}_C.

‘‘Type II’’ precursor contains pseudo-boehmite and non-crystalline SiO2in the as-preparation state.

Pseudo-boehmite transforms to spinel phase above 400_C

and to mullite above 1250_{C. In our previous report,}2

a diphase gel consisted of type II precursors was pre-pared and characterized. TEOS in acidic and water rich environments forms silica sol with sizes around 20–40 nm.3_{The ultimate mixture of nano-sized silica sol and}

pseudo-boehmite reduced the formation temperature of mullite as low as 1200_{C in appropriate heat treatment}

condition. The mullitization occurs in accompany with a series transformation of transient alumina.

There are several reports4 7_{mentioning the}

prepara-tion of mullite thin ﬁlm. The methods included dip

coat-ing of a sol on silica substrate,4_{by spin coating to form an}

unsupported ﬁlm,5_{spray pyrolyzing of a precursor on a}

SiC substrate,6 _{or CVD on a SiC substrate.}7_{There are}

some advantages of using sol-gel precursors to form thin ﬁlm. However, the generation of crackis a problem, because the shrinkage during the drying and thermal stages induces stresses between the coating layer and sub-strate. To overcome this disadvantage, some additives were chosen,8 10_{like a chelating agent or diols. The major}

eﬀect of the additives to retard the condensation reaction and promote structural relaxation in the coated ﬁlms. In the BaTiO3material system,10polyvinylpyrrolidone (PVP)

additive was used eﬀectively, increasing the thickness of the coating layer and decreasing the crackformation.

In the present study, a Type II sol was prepared and applied on a silica substrate by the spin coating method. The formulation was adjusted with the addition of the crack-sealing agent of PVP, in order to prevent crack for-mation. Thermal properties, including thermal shrinkage, thermogravimetric properties during heat treatment, and the microstructures of mullite were investigated.

2. Experimental procedure 2.1. Preparation of the sample

Tetraethylorthosilicate (TEOS, Art. 800658, Merck-Schuchardt, Germany) and pseudo-boehmite colloids (Remal-A20, Remet, USA) were used as starting materials.

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* Corresponding author. Tel.: +886-2-2363-2684; fax: +886-2-2363. E-mail address:[email protected] (W.-C.J. Wei).

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Polyvinylpyrrolidone (PVP) with an average molecular weight of 1300 K g/mol (Acrosorganics, USA) was used as the additive. For the preparation of a basic formula-tion (abb. TB), TEOS was ﬁrst mixed with ethanol and hydrolyzed by deionized water under controlled acidic condition (pH=2.0) by HCl solution. After being fully hydrolyzed, the solution was mixed with pseudo-boeh-mite colloids and stirred for 1 h.

The thin ﬁlm sample was spin-coated on silica sub-strate (GE 124, General Electric, USA) at 5000 rpm for 10 s and then dried at 25_{C for 24 h. After dried, the}

samples were heated to 600_{C at a rate of 2}_{C/min, for}

dehydration and pyrolysis, and then heated to 1280_C

for 1 h. The processing steps of the other formulation containing PVP (abb. TBP) were the same as the TB, except the PVP solution was added in the partially hydrolyzed TEOS solution before it was mixed with pseudo-boehmite. The ratios of each component of the TB and TBP formulas are listed in Table 1. Due to the high viscosity of the PVP solution, the water needed for dilution and successful spin coating for TBP formula-tion was about 8 times higher than that of TB.

2.2. Characterization

The viscosity of sol was measured by a viscometer (DV-II, Brookﬁeld Co., USA) at 25_{C. The thermal}

analysis was conducted with either diﬀerential thermal analysis (DTA), thermal gravimetric analysis (TGA, Du Pont Thermal Analyst 2000 series, USA), or thermal mechanical analysis (TMA, Setsys TMA16/18, Setram Co., USA). The heating rate was 10_{C/min for all TGA,}

DTA and TMA tests.

2.3. Microstructure and phase observation

Two scanning electron microscopes (SEM, Philip XL-30 and Philip 515, Netherlands) and transmission elec-tron microscope (TEM, Jeol CXII-100, Japan) were used to characterize the thickness and microstructure of the ﬁlms in various heat-treatment stages. The grain size was also measured from the micrographs of TEM with a line-intercept method. The crystalline phases were also studied by X-ray diﬀractometry (XRD, PW1710, Philip Co., Netherlands).

3. Results and discussion 3.1. Thermal characters of gels

When the precursors of diphasic mullite were dried and put to heat treatment, there could be a sequence of

Table 1

Formulation of two mullite precursors (in molar ratio)a

SiO2 Al2O3 VP Water Ethanol

TB 1 1.5 0 28 2.7

TBP 1 1.5 1 228 5

a _{The sources of SiO}

2, Al2O3and VP (vinylpyrrolidone) are TEOS,

pseudo-boehmite and PVP (polyvinylpyrrolidone) respectively.

Fig. 1. DTA curves of as-dried TB, TBP, boehmite, and PVP, tested at a rate of 10_C/min.

Fig. 2. TMA curves of TB and TBP formulations, tested at a rate of 10_C/min.

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reactions taking place, e.g. dehydration and phase transformation.11 _{Fig. 1 shows the DTA curves of TB}

and TBP, as well as two pure materials, pseudo-boehmite and PVP. An endothermic peakappeared in TB curve at 480_{C. The peak, compared with the DTA curve of pure}

pseudo-boehmite, should be a dehydration reaction of pseudo-boehmite phase. However, reaction temperature in the precursory mixture is 20_{C higher. The}

dehydra-tion of pseudo-boehmite phase could be retarded by gel structure.

In the DTA curve of TBP, two exothermic peaks and one endothermic peakappeared. By knowing the ther-mal evolution of pure pseudo-boehmite and PVP, a large exothermic peakat about 350_{C and a second}

exothermic peakof TBP at 520_{C are due to the}

oxida-tion of PVP. This polymer is probably pyrolyzed with the catalytic eﬀect by extremely ﬁne SiO2gel or Al2O3,

resulting in low MwPVP. Therefore, the DTA of

par-tially dissociated PVP has diﬀerent exothermic tempera-tures from that of as-received PVP. The endothermic

Fig. 4. XRD curves of TBP sample treated at speciﬁed temperature for 15 min at 350, 450 and 550_{C or 1 h at 1150, 1280 and 1300}_{C. (B :}

boehmite; g: g-Al2O3; m: mullite; : -Al2O3,: -Al2O3).

Fig. 5. Viscosity evolution of TB and TBP precursory sols at 25_C.

Fig. 6. SEM micrographs of thin ﬁlm TB sample after 550_{C heat}

treatment: (a) top-view, the right side is a gel layer and the lower left side is silica substrate; (b) cross-section, the thickness of the coating layer is about 1–2 mm; and (c) top-view of the sample sintered at 1280_C.

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peakof curve the TBP at 490_{C is similar to the one in}

TB curve, representing the dehydration of boehmite as well.

Fig. 2 shows the TGA and TMA results of TB and TBP samples below 900_{C. The TGA curve of TB is}

shown continuously losing mass until 420_{C. The loss}

rate is then accelerated to 500_{C. Above 500}_{C, the mass}

remains nearly constant. The TGA results of the TBP sample with PVP are diﬀerent from the TB sample. The TBP gel shows three steps of mass loss. The ﬁrst occurs below 100_{C. It is the drying of volatile H}

2O and ethanol

solution. The second mass loss is at about 250–350_C

representing the burn out of some low MwPVP, and the

dehydration of silica gel. The third step started at about 450_{C and ended about 580}_{C is the burned-out of the}

rest of PVP and complete dehydration of pseudo-boeh-mite. The total mass loss of TBP is ca. 36% higher than that of 16.5% for TB.

Part of Fig. 2 and the whole of Fig. 3 show the TMA curves of TB and TBP samples. Both have two major shrinking stages. The ﬁrst shrinkage of the curve TB is at about 500–540_{C and the second starts from 1050}_C.

Note that the ﬁrst shrinkage of curve TBP, as shown in Fig. 2, occurs 80_{C higher than that of TB curve. The}

second shrinkage temperature started at 1080_{C is also}

higher than the shrinkage of TB. The distinct diﬀerence

of the shrinkage may be due to the response for a better performance of the TBP sample at higher temperature.

The samples shrinkca. 41.5% near 500_{C for TB or}

580_{C for TBP. Both ﬁrst shrinkage of the TB and TBP}

are starting at the end of mass loss of their TGA curves. That is possible due to the densiﬁcation of silica gel or -Al2O3 grains, or the rearrangement of those ﬁne

grains. But the results determined by quantitative XRD can only reveal the evidence of the former case, which show grain growth of -Al2O3in the temperature region

500–580_{C. It is shown that the width of the X-ray}

dif-fraction peak(2=67_{) of
-phase at 550}_{C in Fig. 4 is}

reduced compared to that at 450_{C. The
-Al}

2O3

parti-cles grow to a larger size, therefore, narrow down the peakwidth as well as that of TBP. However, the rear-rangement behavior of both phases can not be investi-gated by present techniques.

Systematical XRD analysis was also carried out to identify the phase evolution of the precursory gels. At these characteristic thermal temperatures of the TBP sample, including 350, 450, 550, 1150, 1280 and 1300_C,

the samples were analyzed. When heated at 350_{C for}

15 min, the crystalline phase of the sample is still boeh-mite. At 450_{C, part of the boehmite is transformed to}

-Al2O3. At 550C, the boehmite phase is gone, but

transformed to -Al2O3. At 1150C, the - and -Al2O3

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are found in the samples, but the relative quantity of them is not able to diﬀerentiated by XRD. At 1280_C,

the mullite phase is identiﬁed with a minor Al2O3phase.

At 1300_{C, the mullite is the major crystalline phase and}

the other crystalline phases are rarely found.

Similar to the phase evolution of TB, the TBP sample with PVP (Fig. 4) contains boehmite and amorphous SiO2phase while at temperatures lower than 420C. But

the former changes to -phase as the temperature raised. The volatile groups ( OR or OH) on SiO2 gel 1 and

dehydration of boehmite phase11_{are losing mass below}

500_{C. It is believed that the shrinkage at 580}_{C is due to}

the sintering of ultraﬁne -Al2O3as well. In addition to

the small shrinkage at 580_{C, a second shrinkage starting}

from 1070 to 1300_{C, in a larger scale of 12%, is noted}

for TBP sample. Two major Al2O3 transformations,

boehmite to -Al2O3and - and -Al2O3/SiO2to mullite,

are in associated with shrinkage of the diphasic samples. 3.2. Characterization of thin ﬁlm coating

The viscosity of two diphasic sols is shown in Fig. 5, The basic formulation (TB), which aged at 25_{C, would}

increase its viscosity in 150 h. The gelation point (Tg) of

the sol is 150 h at which is the last chance to do a spin coating. Inappropriate usage of sol may induce cracking or ripple surface. The second formulation, TBP, which has a water/ethanol ratio 45.6:1, would have a lower viscosity ( 10 cps) at starting point and gradually increase its viscosity until gelling. The gelation point of TBP shows a viscosity ca. 50 cps at 25 h. The coating of the sols is always conducted while the viscosity is less than 100 cps for TB and 50 cps for TBP.

Fig. 6 is a series of SEM micrographs illustrating the surface texture of the TB films originally coated on silica plate. The as-coated film started with a lower-viscosity (32 cps) sol shows no cracks. The cross-section of the film reveals the thickness of ca. 1–2 mm. The film was in the condition without being cracking after cal-cined at 600_{C for 0.5 h. However, the film cracks to}

mosaic texture while sintered at 1280_{C [Fig. 6(c)].}

If the formulation of sol changes to TBP, the coated mullite will show better intactness (Fig. 7). The thick-ness of the layer is also around 1–2 mm. But near the edge of the SiO2substrate, there still have some cracks

and some bubbles observed next to the region of cracks. The rest of the ﬁlm is free from cracking.

3.3. Microstructural characterization

Fig. 8 is a set of TEM micrographs of the TBP thin ﬁlm of formulation sintered at 1280_{C for 1 h. The grain}

boundary of the mullite grain is very wavy which is the typical nature of diphasic mullite. The grain boundaries of the mullite are still irregular, implying that the reac-tion of residual Al2O3 grains ( 20–50 nm) with the

mullite is not ﬁnished yet. The interior of the mullite still contains some of residual Al2O3grains and pores. That

is consistent with our previous observation on bulky diphasic gel.2_{But most of the mullite are larger than 0.5}

mm and grow in the range of 0.5–5 mm with random orientation, which is proven with the observation of the zone axis of numerous grains. Fig. 9 shows some larger grains on the ﬁlm near the SiO2substrate. The grains

are mullite crystals, but appear abnormal acicular in morphology with a long axis oriented parallel to the [001] direction. The abnormal grain growth of mullite is possible due to the melting of silica substrate. The phase oﬀered a favor route for atomic transport during sin-tering. The grain shape of mullite is plate-like and detailed analysis is needed for further investigation.

Fig. 8. TEM micrographs of TBP thin ﬁlm, (a) BF, (b) CDF of (a), (c) DP of the center grain in (a).

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4. Conclusion

PVP is an eﬀective crack-sealing agent for the forma-tion of thin diphasic mullite ﬁlm. The thermal behavior of TBP is similar to that of TB, besides the burnout of PVP below 600_{C and the delay of dehydration reactions. The}

mullite phase appears at temperatures above 1280_C.

Minor residue of Al2O3in mullite grains are found if held

at 1300_{C for less than 1 h. Uniform ﬁlm prepared from}

the TBP sol can be obtained after sintering to 1280_C.

The mullite grain morphologies of thin ﬁlm (TBP) sam-ples are equiaxial, but may grow to platy shape with a plane parallel to the direction [001] of mullite crystal. The grains in thin ﬁlm are all in sizes around submicrometer to micrometer and randomly oriented, which are similar to the microstructural features of a bulky mullite sample. Acknowledgements

The authors would like to thank the funding given by National Science Council (NSC) in Taiwan under grant number NSC89-2216-E-002-043.

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