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SYNTHESIS AND MICROSTRUCTURE OF ZIRCONIA AEROGELS
H. Vesteghem, T. Jaccon, A. Lecomte
To cite this version:
H. Vesteghem, T. Jaccon, A. Lecomte. SYNTHESIS AND MICROSTRUCTURE OF ZIRCONIA AEROGELS. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-59-C4-64.
�10.1051/jphyscol:1989410�. �jpa-00229485�
REVUE DE PHYSIQUE APPLIQUÉE
Colloque C4, Supplément au n°4, Tome 24, Avril 1989 Cf-59
SYNTHESIS AND MICROSTRUCTURE OF ZIRCONIA AEROGELS
H . VESTEGHEM, T . JACCON a n d A . LECOMTE
CNRS UA-320, ENSCI 47 avenue A. Thomas, ¥-87065 Limoges, France
Résumé - Des gels monolithiques transparents ont été préparés dans le système IsiOCjftjU -CH3COOH-C3H7OH, pour des rapports molaires acide acétique/ alcoxyde compris entre 2 et 3.
Des aérogels, de densité variant de 100 à 1000 kg.m~3 et de surface spécifique supérieure à 400.103 m2.kg"l, ont été obtenus par évacuation hypercritique du solvant.
L'utilisation d'acide acétique permet de contrôler la réaction d'hydrolyse-condensation, l'acide modifiant le n-propoxyde à l'échelle moléculaire et produisant de l'eau par une esterification lente du propanol. La microstructure des aérogels déterminée par diffusion centrale des rayons X permet de proposer un mécanisme de croissance de type monomère- cluster, en accord avec une cinétique lente d'hydrolyse.
Abstract - Transparent monolithic gels were prepared m the Zr(OC3H7)4-CH3COOH-C3H70H system for acetic acid/ alkoxide ratios ranging from 2 to 3. Aerogels, with different densities (100-1000 kg.m-3) and with surface areas higher than 400.103 m2.kg_1, were obtained by hypercritical evaluation of the solvent from these wet gels. Use of acetic acid allows the control of the hydrolysis-condensation reaction because this acid modifies the propoxide at a molecular level and produces water by a slow esterification reaction. Aerogel microstructure, as determined by small angle X-ray scattering, justifies a monomer-cluster growth mechanism which agrees with a slow hydrolysis rate.
1 - INTRODUCTION
Fine powders have become important in the processing of ceramic materials.The manufacture of suitably fine, nanometer scale, high-purity ceramic oxides can be achieved in a number of ways, but usually involves a solution precursor /1 /. Metal alkoxides are by far the most useful compounds for preparation of organic solution precursors /2,3/ and gelation from alkoxide solutions is used more and more /4/.The polymeric gels thus obtained by hydrolysis-condensation of alkoxides lead to aerogels when the solvent is hypercritically evacuated/5,6/.
Aerogels can be considered as ultrafine ceramic powder precursors 111 and with this viewpoint we have demonstrated in a previous work / 8 / the ability of compacts of cordierite aerogel to fully densify below 1000 °C. A systematic study of synthesis and sintering of aerogel-derived ceramic oxides was carried out and we report here the preparation of zirconia aerogels.
Hydrolysis-condensation of zirconium alkoxides is very rapid and addition of water to the alkoxide solution causes precipitation if the process is non-controlled. Two different approaches are used to obtain polymeric gels without any precipitation. These are (I) utilisation of a non-polar solvent and slow hydrolysis by atmospheric humidity 19,10/ and (2) chemical modification of the zirconium alkoxide before hydrolysis / l 1-14/. Acetic acid (AcOH) chemically modifies several alkoxides, among which zirconium propoxide /14,15/.
In this work controlled hydrolysis-condensation of the zirconium alkoxide was performed through two reactions with AcOH; first modification of the alkoxide and, secondly, esterification of propanol.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989410
Small angle X-ray scattering results for zirconia aerogels are discussed in relation to this gelation process.
2 - EXPERIMENTAL PROCEDURE
2 - 1 Preparation, ageing and hypercritcal drying of gels
Commercially available zirconium n-propoxide , containing 20.5% of Zr, acetic acid (.H20% ( 0.05) and n-propanol ( H20%< 0.1 ) were used for the sample preparation. Zirconium n-propoxide ( Zr(OPr)4 ) is very moisture-sensitive. We have therefore chosen the following procedure:
By continuous weighing of the glass container the right amount of Zr(OPt-14 is first added to a known quantity of AcOH. A very fast exothermic reaction takes place leading to a transparent solution.
Various amounts of n-propanol ( PrOH 1 are then mixed with the starting solution in order to adjust the Zr(OPr)4 molarity C. Finally the glass container is hermetically closed and the solution is maintained at 65'C until gelled. Hereafter, the transparent monolithic gels so obtained are called
"wet gels". Ageing was realized in the same conditions as gelation ( 65°C; no evaporation )
Transparent wet gels were in fact obtained for AcOH/Zr(OPr)4 molar ratio R ranging from 2 to 3. No gel was obtained when R was smaller than 2 and a white precipitate begins to appear when R is higher than 3. We have selected three R values (R= 2.1, 2.5 and 2.8) and for each one four solutions were prepared with C ranging from about 0.3 to 1.8. Gelation times
(b)
were estimated by a tilting method. The shortest gelation process (tg=90min) was measured for R= 2.8 and C= 1.71 and the longest one (tg= 192h) for R= 2.1 and C= 0.28. As expected, we observed that, for constant R, the tg is lowered and the change of viscosity of the solution during the sol-gel transition becomes more rapid when C is increased. Aerogels were obtained by hypercritical evacuation of PrOH at 27S0C, using an autoclave system. Heating rate was 360'c.h-1 and the pressure increased up to 6.4 MPa.2 - 2 Charaterization of aerogels
Apparent and skeletal densities of aerogels were determined by Hg volumetry and using a He pycnometer, respectively. Surface areas were measured by the
BET
method using N2 adsoption.Thermal analysis (DTA-TG) were carried out in flowing argon at a rate of 10 "C. min-1. Infrared spectra were recorded on a FTIR spectrometer (Nicolet 5DX) in 4000-400 cm-1 frequency range, aerogels being studied as powders dispersed in KBr pellets.
2 - 3 SAXS method
Small angle X-ray scattering (SAXS) data were obtained with a slit type camera using a double crystal monochromator (Cu E;orl radiation). The scattered intensity was counted with a linear position-sensitive proportionnal counter. The sample detector distance and the counter effective length were 500 and 55 mm respectively. In these conditions the explored scattering vector H ranges from 0.06 to 2 nm-1 , H being related to the scattering angle Q by H= (411/;1 sinQ/2. The run time was 90 min resulting in adequate signal to noise ratios. Experimental results were corrected for background, beam intensity and detector sensitivity.
3 - RESULTS AND DISCUSSION
Fig.1 gives the FTIR spectrum of a zirconia aerogel (R= 2.5; C= 0.55) as obtained after autoclave treatment. It exhibits a set of two bands around 1500 cm-1 that, according to the literature, could be attributed to acetate ligands /16/. The frequency gap between the v as (COO) at 155 1 cm-1 and v s(COO) at 1456 cm-1 suggests that acetate acts as a bidentate ligand. It indicates that carboxilic groups are linked twice to Zr but it would be difficult to say whether these ligands are chelating groups ( Zrc OAC or bridging groups ( Zr- OAc- Zr). It is worth noting that OPr groups have been nearly completely replaced as illustrated by very weak bands around 2930 and 1030 cm-1 and
that, on the other hand, the Zr- 0 - Zr network is developed as could be seen below 650 cm-1. The FTIR spectra shows a large main band at 3380 cm-1 due to stretching modes of hydrogen bonded OH'S.
WAVE NUMBER ( cm-l)
Fig.1- Infrared spectrum of zirconia aerogel (R- 2.5; C= 0.55) as obtained after autoclave treatment.
DTA-TG and FTIR have shown that acetate decomposition occurs between 260 'C and 480 "C in zirconia aerogels. The weight loss is about 15% and corresponds to the departure of about half an AcO group by Zr.
Results given in Table 1 for one of the three aerogels series we have studied are representative of the dependence of textural properties with Zr(0Pr)q concentration. Porosity decreases with increasing concentration contrary to surface area which increases. Skeletal densities are nearly constant and lower than the true density of Zr02 ( T-ZrOq: 6100 kg.m-3)
.
These relatively low values can be explained by inclusion of AcO groups in the skeleton of the gels during the solution process. Comparison of pore volumes and surface areas indicates that macroporosity increases with decreasing concentration. Results given in Table 1 are nearly the same for R= 2.1 and R- 2.8 and so R does not look like a basic parameter in the range ( R=2-3) where polymeric gels can be obtained.Table I- Textural properties of zirconia aerogels as a function of Zr(OPr)4 concentration in the starting solution. AcOH/Zr(OPr)q molar ratio (R) is unvarying (R= 2.5)
Concentration Apparent density Skeletal density Pore volume Porosity Surface area C ( mol.1- 1 ) da (kg.m-3) ds (kg.m-3) Vp (m3.k~-1)
(%I
S (m2.k~-1)SAXS measurements were used to assess structural properties of zirconia aerogels. The two main entities accessible by SAXS to characterize aerogels are the radius of gyration and the fractal dimension of the scattering centers. The mean size of the polymeric particles is estimated by the
radius of gyration Rg, as measured in the low angle part of the scattering curves, using the Guinier approximation. Fractal dimension of the particles can be determined in the Porod region of the
scattering curves, defined as l/Rg << H ( < l/a where a is the elementary particle size. When a power law i I(H) or H-D ) behaviour is observed in this high angle region D is assumed to be the fractal dimension.
Fig. 2a shows the scattering curves which we obtained for different zirconium concentration aerogels
0 1 t g
+ Sol
0 -25 .51
SCATTER I NG UECTOR (nm-1)
Fig. 2- Dependence of scattering curves of zirconia aerogels with (a) Zr(0Pr)q concentration C in the starting solution (R= 2.5 and no ageing) and with (b) ageing of the wet gels (R= 2.5 and C= 0.83) We can see that the scattering intensity I(H) is very dependent on Zr(0Pr)q concentration C for H c 0.5 nm-1. I(H) increases strongly with a decrease of C. Guinier analysis of this low angle pari of the curves led to Rg ranging from 5nm (C= 1.76) to 15 nm (C= 0.27). A similar treatment of the curves of Fig. 2b shows that ageing of wet gels before autoclave drying strongly reduces particle size.
The latter results agree with syneresis /17/ and, therefore, with the shrinkage of wet gels that we have observed during ageing. The main features of the scattering curves for R= 2.1 and R- 2.8 are similar to those discussed here (R= 2.5).
Log-log plots of I(H) versus H had the same overall shape for the three R-series of aerogels. Log-log curves exhibit a power law behaviour over nearly a decade of reciprocal space when low concentration aerogels are measured. In Fig.3 we can observe that in a constant R-serie of aerogels
(R= 2.1 the corresponding linear domain decreases with an increase of C but that the slope remains constant. This -2.8 slope characterizes densely crosslinked mass-fractal polymeric particles
(D=
2.8).Several authors /18,19/ have observed that for some silicate aerogels log-log curves exhibit two power law regimes characterized by two slopes (about -2 and -41. The -4 slope indicates that Porod law is obeyed and polymeric particles were described as mass-fractal aggregates of small dense elementary particles. The crossover between the two linear regimes allows to estimate the size a of these elementary particles. In the outer part (
H>
0.5 ) of the log-log curves for zirconia aerogels (Fig.3) we can observe a very different behaviour. The slope increases continuously and no homogeneous elementary objects with smooth surface can be identified.0.05 0.5 5
SCATTEE ING UECTOR (nm-1)
Fig. 3- Dependence of log-log curves of zirconia aerogels with Zr(OPr14 concentration in the starting solution (R= 2.1; no ageing ). Arrows indicate for each curve H domains where a linear regime is obeyed (slope: -2.8).
0.05 0 . 5 5
SCRTTER I NG UECTOR (nm- 1 1
Fig. 4- Porod plots for wet gels from which zirconia aerogels were prepared (R= 2.1 ).
The main results of the SAXS study can be interpreted in the following way. We can describe our solution process by a two-step chemical mechanism. First. AcO groups replace some PrO groups to give very quickly a modified precursor ZriOPriq-x (AcOIx. Second, a slower esterification reaction yields a sluggish production of water in the solution /14/. Hydrolysis of the modified alkoxide consumes continuously this in-situ generated water to give the Zr(OPrj4-x-y (AcO)x (OHjy monomer precursor and consequently, allows polymerization to occur by a fast condensation reaction. Under these conditions where hydrolysis is slow compared to condensation, growth of the polymeric particles can be described by a monomer- cluster aggregation model (MCA). In the MC-4 growth model monomer is more likely to stick onto a cluster, namely a polymeric particle, than it is to react with another monomer. Computer simulations /20/ have shown that, with such a growth mechanism, relatively dense mass-fractal objects are built up. Several MCA models have been developed but the fractal dimension is never less than 2.5 /20/. Our SAXS experiments yield a dimension ! D= 2.8 consistent with the MCA growth model we suggest.
For comparison with aerogels measurements, log-log scattering curves of wet gels are shown on Fig.4. Wet gels scatters nearly like aerogels but their radii of gyration are smaller (e.g. Rg= 6 nm for C= 0.57) and the slope increase in the outer part of log-log curves appears at a smaller H. The latter feature can be explained by the chemical mechanism we have proposed. Substitution of OPr groups by AcO decreases the precursor functionality, namely the number of OPr that can be hydrolyzed, and we may then assume that polymerization starts with build-up of slightly crosslinked polymer molecules. The autoclave treatment decreases the size of these primary entities.
- CONCLUSION
We have demonstrated in this work that ( I ) polymeric wet gels and then aerogels can be obtained from 2rlOPr)q-AcOH-PrOH system when AcOH/Zr(OPr)4 ratio is ranging from 2 to 3 and that (2) the aerogels microstructure. as SAXS determined, can be explained by a monomer-cluster aggregation growth model, the primary particles, namely the monomers of the model, being sligthly crosslinked polymer molecules.
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