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Eprints ID: 3802
To link to this article: DOI:10.1016/j.tsf.2005.08.335
URL:
http://dx.doi.org/10.1016/j.tsf.2005.08.335
To cite this document
: Lenormand, Pascal and Lecomte, A. and Babonneau, D. and
Dauger, A. ( 2006) X-ray reflectivity, diffraction and grazing incidence small angle X-ray
scattering as complementary methods in the microstructural study of sol–gel zirconia thin
films. Thin Solid Films, vol. 495 (n° 1-2). pp. 224-231. ISSN 0040-6090
X-ray reflectivity, diffraction and grazing incidence small angle X-ray
scattering as complementary methods in the microstructural study of
sol–gel zirconia thin films
P. Lenormand
a,*, A. Lecomte
b, D. Babonneau
c, A. Dauger
baCentre Interuniversitaire de Recherche et d_Inge´nierie des MATe´riaux. Laboratoire de Chimie des Mate´riaux Inorganiques et Energe´tiques, UMR 5085-CNRS Universite´ Paul Sabatier- Baˆt. 2R1-118 route de Narbonne – 31062 Toulouse Cedex 4, France
bScience des Proce´de´s Ce´ramiques et de Traitements de Surface, UMR 6638-CNRS ENSCI, 47-73 av. A. Thomas, 87065, Limoges Cedex, France cLaboratoire de Me´tallurgie Physique, UMR 6630-CNRS Universite´ de Poitiers- UFR Sciences- SP2MI,
Boulevard Marie et Pierre Curie BP 30179- 86 962 Futuroscope Chasseneuil Cedex, France
Abstract
X-ray reflectometry, X-ray diffraction and grazing incidence small angle X-ray scattering have been complementary used to fully characterize zirconia (ZrO2) thin films obtained by the sol – gel route. The films were synthesized on various sapphire (Al2O3), silicon (Si) and glass
mirror-polished wafers by a dip-coating process in a zirconia precursor sol. Versus the synthesis parameters as alkoxide sol concentration, withdrawal speed and annealing temperature, the microstructure of the layer is managed and its different microstructural parameters such as thickness, mass density, crystalline phase, grain size and spatial arrangement have been determined. The as prepared layers are amorphous. During a thermal treatment at low temperature (< 1000-C), the layers thickness decreases while their mass density increases. Simultaneously the zirconia precursor crystallises in the zirconia tetragonal form and the coating is made of randomly oriented nanocrystals which self organise in a dense close-packed microstructure. At low temperature, this microstructural evolution is similar whatever the substrate. Moreover, the layer evolves as the corresponding bulk xerogel showing that the presence of the interface does not modify the thermal microstructure evolution of the layer which is controlled by a normal grain growth leading to relatively dense nanocrystalline thin films.
Keywords: Sol – gel coating; Zirconium oxide; X-ray scattering; Nanostructure
1. Introduction
The elaboration of inorganic thin layers presents a growing interest because a new thermal, mechanical, optical, chemical or electrical property is easily added to the substrates on which they are deposited [1]. Among all the methods of thin film synthesis, chemical or physical vapour deposition, pulverization, molecular beam epitaxy or laser ablation, the interest of sol – gel route is unquestionable mainly because of its low cost, its easiness to control the synthesis parameters and the high precision in mixing different precursors for the fabrication of multicomponents oxides layers. Moreover, the sol gel route suits quite well to very large or complex form substrates [2].
In this processing method, the constitutive matter of the film is globally deposited by either a dipping, spinning or spraying coating process. Condensation and hydrolysis reactions occur simultaneously to the excess solvent evaporation to form the film. Then, an adapted thermal treatment is essential to decom-pose the precursor film to the desired inorganic compound, re-sulting in either a glass or a polycrystalline film with nm-sized grains and pores[2]. In some cases, higher temperature heat treatment leads the coating to break into more or less connected islands and epitaxy phenomena could appear through abnormal grain growth[3]. In any cases, the thermal evolution, the final structure and microstructure of the layer are strongly related to its initial thickness and to the nanostructural feature of the precursor.
X-ray Diffraction (XRD) and Small Angle X-ray Scattering (SAXS) are very widespread and efficient non destructive tools to investigate the structure and microstructure of bulk as well
doi:10.1016/j.tsf.2005.08.335
* Corresponding author.
as powder materials. From the standpoint of thin film X-ray characterisation, Grazing Incidence Small Angle X-ray Scat-tering (GISAXS) is a more recent technique [4] which combines X-ray reflectometry (XRR) and transmission mode SAXS theory. This method is a powerful tool to analyse nanoparticles supported on the substrate surface as well as buried in the surface region or present in a thin coating and provides valuable microstructural information on the nano-metric scale such as particle shape and size, inter-particle distance or correlation length. . .[4,5]. XXR and GISAXS studies of sol gel oxide thin films, either alone or in combination, have recently attracted considerable research interest leading to a very rapid increase of the number of published papers and more particularly in the field of sol gel mesoporous thin films[6 – 8].
In the present work, we aim to show that the combination of these three X-ray techniques, GISAXS, XRR and XRD allows to fully characterize a sol gel oxide thin layer and to follow its thermal evolution. They will be applied to the case of sol – gel derived zirconia (ZrO2) thin films deposited on various
substrates by a dip-coating process, from the initial stage and during low temperature treatments, pure or stabilized zirconia being a widely used oxide in a number of electrical, optical or superconductivity applications. Nevertheless, to date, a few XRR and GISAXS studies of sol gel zirconia based thin films have been reported[9]. Corresponding gels and xerogels are also considered in a complementary way.
2. Experimental details 2.1. Sample preparation
Zirconia precursor sols are prepared in the zirconium n-propoxide, n-propanol, acetylacetone (acacH) and water system. In an air dried glove-box, the zirconium n-propoxide is first diluted with n-propanol and chelated by acetylacetone which allows to control the hydrolysis and condensation reactions avoiding precipitation [10,11]. Then, a water-n-propanol mixture is added under vigorous mechanical stirring. The zirconium n-propoxide concentration C = [Zr(OC3H7)4] is
varied from 0.01 to 0.5 mol/L, the hydrolysis and complexing ratios being kept constant, W = [H2O]/[Zr(OC3H7)4] = 10 and
R = [acacH]/[Zr(OC3H7)4] = 0.7, respectively. The sols are
homogeneous, clear and transparent.
The coating of zirconia precursor films was carried out just after the sol synthesis using a dipping – withdrawing method under ambient conditions. Silica based glass, silicon and sapphire polish mirror single crystals were used as coating substrates. The withdrawal speed was ranging from 1 to 200 mm/mn. After a drying at 60-C during 20 min, the films are continuous, homogeneous and crack free as observed by optical microscopy.
The dried films were annealed in an electric furnace at a rate of 5 -C/mn up to the desired temperature and for the chosen annealing time. The first steps of the zirconia grain growth were analysed from a single thin film undergoing a ‘‘cumula-tive heat treatment’’, i.e., the firing of the sample was all
stopped at 100-C from 60 up to 1000 -C, the sample was air quenched then successively characterized by XRR, GISAXS and XRD before being reintroduced in the furnace at the same temperature for the continuation of the annealing.
2.2. X-ray experimental setups
The thickness, mass density and both surface and interface roughness (r.m.s.), as defined by Nevot et al.[12], of zirconia thin films were measured by X-ray reflectivity. The data were collected by using an original angular dispersive reflectometer
[13] initially developed by Naudon et al. [14]. All reflected beams were simultaneously recorded by using a linear position sensitive detector (INEL LPS 50). As no movement of both the sample and the detector was necessary during the measure-ment, the exposure time to data recording was obviously considerably diminished, typically about 3 h. The microstruc-tural parameters were obtained by fitting the experimental data using the well-known Parratt formalism[15]. For this purpose, the layer is assumed to exhibit a constant mass density in its whole thickness. After heat treatment, the coating being only constituted of tetragonal zirconia nanocrystals and porosity, this mass density of the layer is then derived from the XXR curves by comparing the real and imaginary parts of the fitted layer refractive index to the theoretical values for dense tetragonal zirconia phase. Both nm-scale surface and interface roughnesses are described by a Debye – Waller like damping term as introduced by Nevot et al.[12]in the calculation of the Fresnel coefficients.
The GISAXS measurements were performed by using a conventional laboratory SAXS experiment, modified and adapted to the case of grazing incidence [13]. The sample holder was equipped with both two translations and one step by step rotation with optical encoder which allowed a very precise surface sample adjustment in the direct beam and the control of the incidence angle. The scattered intensity was recorded using a linear position sensitive detector (Elphyse) positioned parallel to the surface sample. Its height position was precisely controlled and adjusted around the reflected beam which is masked by a lead beam stop set along the vertical axis.
The apparatus setting both to position the sample accord-ingly to the X-ray beam and to determine the sample angular origin was classically made [16]. To totally penetrate the coating, the grazing angle of the incident beam, typically about 0.3-–0.4-, might be slightly larger than the layer critical angle ac, acbeing previously measured by X-ray reflectivity. So the
transmitted beam gone through the layer and acted as a primary beam to give rise to a SAXS signal[4,17,18].
The scattered intensity I(q) is presented as a function of the modulus of the scattering vector q, q = 4ksin(h) / k where h is half the scattering angle and k the wavelength of the incident radiation (Cuka1radiation). A sample-detector distance of 0.5
m, was used in order to cover a q-range from about 0.3 to 4 nm 1. The origin of the reciprocal space was calculated from the refraction index of the coating as determined from the XRR curves and then the spectra were corrected for refraction and absorption effects before analysis [19]. Complementary
bi-dimensional GISAXS patterns have been collected with an imaging plate detector by using the synchrotron radiation facilities of LURE-DCI on D22 station[20].
X-ray diffraction was used to determine the crystalline phases after heat treatment. The diffractometer consists in a Debye – Scherrer like set-up, operating on flat samples, fitted with a forward quartz monochromator (Cuka1radiation) and a
Curve Position Sensitive Detector (CPSD INEL, 120- angular aperture) allowing the diffracted peaks to be simultaneously recorded [21]. This asymmetric geometry under fixed inci-dence is particularly suitable for surface characterization and thin film structure analysis. The incidence angle was fixed to about 4 to 7- in order to favour the layer irradiated volume without impairing the resolution, i.e., without an excessive broadening of the X-ray diffraction line profile.
3. Results
3.1. Thin film synthesis
Coatings deposited on glass, Si and Al2O3 single crystal
wafers have been synthesised from various alkoxide concen-tration sols C, ranging from 0.01 to 0.25 mol/L, and investigated by X-ray reflectivity, as prepared and after a heat treatment at 600-C during 30 min which allows the removal of all the organics. In all cases, the reflected intensity curves present oscillations (Kiessig fringes) which are significant of continuous, homogeneous and uniform layers [22]. For example, the experimental X-ray reflectivity curves obtained from the annealed ZrO2 layers coated on Si substrate are
plotted inFig. 1, the solid line corresponding to the best fit of
the Parratt model function. From these simulations, the layer microstructural parameters, its thickness, mass density and roughness have been deduced.
The thickness of the dried layers versus the alkoxide concentration C varies linearly from about 7 to 120 nm for C = 0.01 and 0.25 mol/L, respectively (insert ofFig. 1). During calcination, the films thickness decreases and evolves also linearly from 3 to 45 nm for the previously quoted concentra-tions, respectively. The XRD investigations showing the annealed layers are crystallised in the tetragonal form of zirconia (see below), the mass density of the film can be deduced from the XRR curves. At 600-C, the measured mass density of the film is around 4I3I103
Kg/m3 and seems to be independent of the alkoxide concentration. The root means square roughness parameters, measured for each interface, are lower than 1 nm and in the same range that the roughness of the Si single crystal. So low r.m.s. values attest of the quality of the sol – gel layers. Similar results are obtained on glass and sapphire single crystal substrates. Finally, the thickness of layers coated on glass substrates has been also varied by managing the withdrawal speed in a 1 to 200 mm/mn range. The film thickness versus the withdrawal speed evolves according to a power law with an exponent equal to 0.64, a value very close to the expected one, 2 / 3, as predicted from the Landau – Levich law[2]. We turn now to the thermal evolution of these layers.
3.2. Thermal evolution of the thin films microstructure The thermal evolution of the thin films microstructure has been carefully investigated on a single thin film coated on the
Intensity (a.u.) 0.05 0.1 0.25 C (mol/L) 0 1 0 20 40 60 80 100 120 140 0 0.05 0.1 0.15 0.2 0.25 0.3 as-prepared 30 mn ñ 600∞C 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 0.01 30 mn – 600°C 0.5 1.5 0 20 40 60 80 100 120 140 0 0.05 0.1 0.15 0.2 0.25 0.3 Thickness (nm) 0 20 40 60 80 100 120 140 0 0.05 0.1 0.15 0.2 0.25 0.3 as-prepared Incidence angle (α °) C (mol/L) 30 mn – 600°C
Fig. 1. Zirconium alkoxide sol concentration dependence of the X-ray reflectivity curves for thin films coated on Si wafer after a thermal treatment at 600-C during 0.5 h. Curves are arbitrary vertically shifted for clarity. Experimental points (symbols) are well fitted by using the recursive Parratt formalism (lines). In the insert, evolution of the thickness of the as prepared and annealed at 600-C for 30 mn thin films versus the zirconium alkoxide sol concentration.
most refractory and thermally stable substrate, i.e., Al2O3
substrate. The sol composition was C = 0.25 mol/L, R = 0.7 and W = 10 and the withdrawal speed was 200 mm/mn. Then, the resulting sample has been annealed with the ‘‘cumulative heat treatment’’ up to 1000-C, as described in the experimental part. X-ray diffraction measurements show that the thermal treatment induces, since 500 -C, the crystallisation of the zirconia precursor in the tetragonal zirconia form. The relative intensities of the peaks being in good agreement with JCPDS files, the tetragonal zirconia grains are randomly oriented (Fig. 2). Increasing the temperature results in a progressive apparition of the monoclinic zirconia phase.
The experimental X-ray reflectivity curves recorded at the different temperatures and for the uncovered Al2O3 substrate
are plotted inFig. 3. The intensity of the reflected beam by the Al2O3single crystal substrate shows a classical evolution of the
reflected intensity versus the incidence angle a for a dioptre separating two media of different electronic densities. The measured critical angle corresponding to total reflection, ac=
0.291-, is in very good agreement with the theoretical value of 0.289- and the r.m.s. is about 0.6 nm. The reflectivity curve of the dried layer exhibits a critical angle of about 0.22- and a shoulder on a weak angular domain between 0.24- and 0.28-where kiessig fringes are also observed. The electronic density of the layer is smaller than the one of the Al2O3substrate, so
that, in this angular domain, the refracted X-ray beams being propagated into the coating are in total reflection on the substrate[13,23].
As the temperature increases, both the position of the critical angle and the period of the interference fringes increase. The corresponding thicknesses, mass densities of the films and surface and interface roughness deduced from the simulations with the Parrat model are displayed inFig. 4.
The layer thickness brutally decreases from 130 to 60 nm for the dried film and for the 500-C point, respectively. This behaviour is due to the removal of residual organics initially contained in the coating. After 500-C, the thickness continues to slowly diminish while the mass density of the film increases. This diminution of the thickness accounts for the increase of the mass density of the coating. Indeed, this mass density varies from 3.8.103 to 5.15.103 Kg/m3 corresponding to an
augmentation of 35% while the thickness decreases from 63 to 47 nm, i.e., a diminution of around 25%. So, the densification of the layer depends only on the diminution of the thickness. At 1000 -C, the measured mass density of the film is about 5I15I103
Kg/m3, i.e., about 86% of the theoretical mass density of tetragonal zirconia. Once again, the root means square roughness parameters of the as prepared coating as well as those of the annealed sample attest of the quality of the sol – gel layers. Until 800-C, the r.m.s. parameters, less than 1 nm for each interface, are in the same range than the one of the Al2O3
single crystal. After 800 -C, both the surface and interface roughness increase.
The Fig. 5 shows the one dimensional GISAXS patterns recorded after heat treatments at the different temperatures. The incidence angle of the X-ray beam a was always set to 0.4-with respect to the sample surface and reflectivity results so that the whole layer was fully irradiated whatever its heat treatment temperature, the detector being positioned around the reflected beam.
Above 300 -C, the collected scattered intensity patterns exhibit a peak at a non zero wave vector qmpointing out the
existence of nanometric heterogeneities in electron density of the layer. When the temperature increases, the scattered intensity peak height Imincreases while its position qmmoves
towards smaller and smaller q values. Such pattern feature indicates the existence of a correlation length n, in the plane of
60 30 40 50 70 80 600°C θ θ 60 70 80 Intensity (a.u.) 5 2 4 1 3 500°C 800°C 900°C 1000°C 2 (°) 103 ×
Fig. 2. X-ray diffraction patterns of a thin film coated on sapphire wafer and annealed at various temperatures from 500 up to 1000-C.
Intensity (a.u.) As prepared 300°C 400°C 500°C 600°C 700°C 800°C 900°C 1000°C 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-10 Al2O3 wafer 10-9 10-11 0 0.5 1 Incidence angle (α °)
Fig. 3. X-ray reflectivity curves for a zirconia thin film coated on sapphire wafer and annealed using a cumulative thermal heat treatment up to 1000-C. Curves are arbitrary vertically shifted for clarity.
the coating, which can be deduced from the position of the intensity maximum qmby n = 2k/qm[24]. With the increase of
the temperature, this correlation length grows from 4 to about 15 nm for 300 and 800 -C, respectively.
Since 500-C, a temperature corresponding to the end of the organics removal, the scattered intensity I(q) decreases, in the large q range, according to a power law with an exponent near to 4 (insert Fig. 5). This behaviour shows that the Porod_s law is obeyed and points out that the scattering entities are homogeneous with a well-defined smooth surface[24]. 4. Discussion
The discrepancy between the experimental and calculated XXR curves could be explained, on one hand, by the use of a too simple model. Some attempts by using more complex layer models to fit the XRR curves have been unsuccessfully done. Moreover, we have no relevant indications from layer TEM micrographs or other experimental techniques to justify such complex models [13]. On the other hand, the observed additional intensity on experimental XXR curves probably rises from the X-ray scattering by the tetragonal zirconia crystals inside the layers in agreement with the GISAXS results (see below). Moreover, for the as – prepared layer which is only composed of very small colloidal and polymeric species
without any porosity, we do not observe such additional intensity[13]. Due to the reflectometer geometry, it is rather difficult to take into account this additional X-ray scattering for modelling the XXR curves. Nevertheless, in regards with the angular range where it appears, we think that it has no major impact on the measured mass density of the layer as well as its thickness.
The grazing incidence X-ray scattering signal is assigned to the appearance and growth of the zirconia nanocrystals. The sharp interference peak is significant, on one hand, of a very homogeneous and narrow zirconia nanocrystals sizes distribu-tion and, on the other hand, that the zirconia nanocrystals self organise in a dense close-packed microstructure in agreement with the high mass density of the layer as measured by X-ray reflectivity and cross-sectional TEM images [13]. In such paracrystalline lattice model [25], the correlation length n corresponds to the nearest-neighbour distance. At the end of the organics removal, at about 500-C, the zirconia nanocrys-tals are obviously in contact and the nearest-neighbour distance is assimilated to zirconia nanocrystals size.
These microstructural information are only valuable and accurate in the plane of the layer in respect with the chosen geometry and setting of the laboratory experiment, with the linear detector parallel to the sample surface, which only provide a section of the reciprocal space map, i.e., a one
0 0,5 1 1,5 2 2,5 3 3,5 0 200 400 600 800 1000 Roughness (nm) Temperature (°C) Mass density (Kg/m 3 ) Thickness (nm) Surface Interface 0 20 40 60 80 100 120 140 0 200 400 600 800 1000 3 3.5 4 4.5 5 x 103
Fig. 4. Firing temperature dependence of the thickness, mass density, both surface and interface r.m.s. roughness as deduced from the X-ray reflectivity curves for a zirconia thin film coated on sapphire wafer.
dimensional GISAXS pattern. To recover the out of plane film microstructure information, the detector has to be vertically shifted to build up step by step the bi-dimensional map which is very time consuming. To overcome this drawback, a bi-dimensional GISAXS pattern has been performed at the synchrotron radiation facilities of LURE on an analogous sample, elaborated with the same Al2O3 substrate, sol
composition (C = 0.25 mol/L, R = 0.7, W = 10), withdrawal speed (200 mm/mn) and only differs from its thermal treatment which was a single heat treatment at 600-C for 30 mn.
The bi-dimensional GISAXS pattern was collected by setting the incidence angle of the X-ray beam a to 0.4- with respect to the sample surface and reflectivity results (a > ac).
The scattering map presents a quasi-circular half ring centred on the origin of the reciprocal space indicating that the dense close-packed zirconia nanocrystals microstructure is isotropic inside the whole thin film (Fig. 6). Moreover, a one-dimensional scattering curve I(q) has been extracted by integrating the scattered intensities on 30- angular domains centred around the diagonals of the scattering map and compared to the one-dimensional laboratory GISAXS pattern carried out on the same sample. After an adjustment of scattering intensities, the remarkable agreement between the two curves validates the laboratory experimental set-up. In respect with the isotropic spatial arrangement of the zirconia nanocrystals inside the layer and the low temperature heat treatments, the one dimensional GISAXS measurements are
appropriated to accurately recover the parameters of prime importance we are interesting in which are the zirconia nanocrystals size and spatial arrangement.
The comprehension of the thin film formation and its thermal transformation is very closely related to the nature of the sol precursor from which it rises. In this aim, the sol to gel transition of a sol with the same composition as the one used for the film synthesis, C = 0.25 mol/L, R = 0.7, W = 10, has been previously studied by small angle X-ray scattering [26]. The gel structure is made up of connected fractal clusters of nearly equal size, around 50 nm, with an apparent fractal dimension of 1.7, resulting from the aggregation of very small zirconium rich primary particles about 2 – 3 nm readily formed just after the water is added. When this gel is conventionally dried, the clusters fractal structure collapses under very strong capillary force leading to a very important shrinkage. Throughout this microstructure collapse, the scattering intensity totally disap-pears [13]. During subsequent heat treatment at low tempe-rature, the bulk xerogel crystallises under the tetragonal
Intensity (a.u.) q (nm-1) 700°C 800°C log q (nm-1) 500°C 600°C 400°C 0 500 1000 1500 2000 2500 0 0 5 10 15 -0.8 -0.4 0 0.4 0.8 1.2
log Intensity (a.u.)
-4
1 2 3
Fig. 5. Grazing incidence small angle X-ray scattering patterns versus modulus of the scattering vector q for a zirconia thin film coated on sapphire wafer and annealed using a cumulative thermal treatment up to 1000-C. In the insert, these curves are plotted on a log – log scale with arbitrary vertical shifts for clarity. 2 Intensity (a.u.) q (nm-1) Lure D22 Laboratory 30 mn – 600 °C 0 100 200 300 400 500 600 0 0.5 1 1.5 0.1 Å-1
Fig. 6. Bi-dimensional GISAXS pattern recorded at ID22 of LURE for a zirconia thin film coated on sapphire wafer and annealed at 600-C during 0.5 h. One-dimensional GISAXS pattern as derived from the bi-dimensional map by integrating the scattered intensities on 30- angular domains centred around the diagonals (full line) and the one-dimensional GISAXS pattern collected on the laboratory experiment on the same sample (symbols) versus modulus of the scattering vector q.
zirconia phase and the SAXS signal in the transmission mode is recovered and exhibits a well defined scattered intensity peak at a non zero scattering vector.
During the dipping, the liquid film runs out on the substrate, adheres to its surface and the evaporation of solvents leads to its rapid solidification. The as prepared layer could be considered as a xerogel obtained by a concentration effect. As for the bulk xerogel, we do not observe GISAXS signal for this raw layer even by using the synchrotron radiation facilities of LURE. At the beginning of the annealing, the correlation length corresponding to the appearing zirconia nanocrystals, about 4 nm at 300-C, is in good agreement with the zirconium rich primary particle size present in the sol. During the temperature increase, the layer microstructure evolves by normal grain growth of the zirconia nanocrystals in agreement with the theoretical approach developed by Thompson [27]. For a given low temperature thermal treatment and whatever the substrate, the thin film GISAXS as well as the bulk xerogel SAXS patterns display quite similar feature in spite of very different experimental data recording (Fig. 7). At this stage, the layer evolves as bulk xerogel and the film-substrate interface does not seem to influence its microstructural evolution. These results are in good agreement with the observations carried out on similar systems[28].
The homogeneous in situ crystallisation of the zirconia precursor leads to relatively dense layer, up to 86% of the theoretical mass density of tetragonal zirconia at 1000 -C, in spite of the low temperature applied heat treatments with
respect to the melting temperature of ZrO2(¨2680 -C). The
layer densification occurs through a rapid decrease of its thickness in a similar way as mentioned by Rizzato et al. about the zirconia film evolution obtained from a zirconium chloride
[29]. After 800-C, the slight increase of both the surface and interface roughness is due to the zirconia grains size which reaches the same order than the film thickness. With regard to the thin film quality and more particularly to interface and surface states, the sol – gel thin film synthesis appears to be a valuable alternative route to the physical processed layers
[30,31]. 5. Conclusion
The complementarity of non-destructive experimental tech-niques such as XRR, XRD and GISAXS, has been successfully used to fully characterise the microstructural evolution of ZrO2
based sol – gel coatings during thermal treatments at low temperature.
Whatever the substrate (glass, Si and Al2O3single crystals),
the layers, synthesized by a dip-coating process, are continu-ous, homogenecontinu-ous, crack free and exhibit very good quality of both surface and interface states. Their nanometric thicknesses are easily managed through the alkoxide sol concentration as well as the withdrawal speed.
During the thermal treatment at low temperature, the removal of the organic residues is concomitant to the crystallisation of the amorphous film which is transformed in a polycrystalline layer with a random orientation of tetragonal zirconia nano-crystals. The densification of the layer, up to 86% of the theoretical tetragonal zirconia mass density at 1000-C, occurs by a one dimensional shrinkage, i.e., by a strong reduction of their thickness. Due to their narrow size distribution, the tetragonal zirconia nanocrystals are self organised in a dense close-packed microstructure which is conserved and evolves by a normal grain growth mechanism up to 1000-C.
In this low temperature range and whatever the substrate, the microstructure of zirconia thin film evolves as corresponding zirconia bulk xerogel. At this stage, the layer – substrate interface does not seem to influence the microstruc-ture of the thin film.
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