differences between films and monoliths of sol–gel derived aluminas

Thin Solid Films 519 (2010) 42–51

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Differences between films and monoliths of sol–gel derived aluminas

M. Dressler ⁎, M. Nofz, P. Klobes, I. Dörfel, S. ReinschBAM Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany

⁎ Corresponding author.E-mail address: [email protected] (M. Dressle

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.07.057

a b s t r a c t
a r t i c l e i n f o
Article history:Received 29 June 2009Received in revised form 6 July 2010Accepted 13 July 2010Available online 17 July 2010

Keywords:Sol–gel processingAluminaPhase transitionsPorosity

This work compares thin layers (films) and monoliths prepared from alumina sols with respect to theirmicrostructure, thermal evolution, porosity and specific surface area. After heat treatment at similartemperatures, films and monoliths showed the same qualitative changes in porosity and specific surfacearea. However, some marked quantitative differences were detected. Film fragments had a lower openporosity, a lower specific surface area and a narrower pore size distribution. Furthermore, the thermalevolution showed a markedly different burnout of organic components between films and monoliths. Theobserved differences between films and monolith can be explained by the ageing history of the sols duringsample preparation.

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1. Introduction

Sol–gel processing is frequently used to prepare thin coatings [1–5]. In order to anticipate the properties of a thin sol–gel coating it iscommon practice to analyze not the micrometer sized thin films butrather their respectivemillimeter sized xerogel powders. This methodis applied as it is very time consuming to gather enough thin filmmaterial for analyses. Such monolithic xerogel powders are usuallyprepared by drying sol filled laboratory dishes. Due to the markedlydifferent surface to volume ratios it is obvious that drying, i.e. thetransition from the sol to the gel state, takes much less time for films(matter of seconds) than for monoliths (minutes up to hours).However, important sol parameters like pH value [6] and ionconcentration [7] change during evaporation of the dispersant.Hence, the chemical ageing history of pH value and ion concentrationdiffers for films and monoliths. Given their huge impact, it should beinvestigated whether there are differences between films andmonolithic xerogel powders.

The alumina system utilized in the present work is based on a wellstudied modified Yoldas sol. The chemical composition [6,8],rheological properties [7], thermal evolution of the resulting solidproducts [9], and suitability to prepare oxidation protection coatings[2] have been described in detail before. This work shows that whencomparing films and monoliths their overall trends like specificsurface area, pore volume, density, porosity and thermal transforma-tion were similar in outline, i.e. they are qualitatively alike albeitmarked quantitative differences were also observed.

2. Experimental details

2.1. Sol preparation

Aluminum nitrate (Al(NO3)3·9H2O, puriss p.a., Merck) wasdissolved in water. After heating up that aqueous solution to 85 °C,aluminum tri-sec-butoxide (ASB, 75 wt.% ASB in sec-butoxide,Aldrich) was weighed in a beaker and poured in one step into thealuminum nitrate solution. The resulting modified Yoldas sols werekept at this temperature for 45 min before cooling down to roomtemperature. The following amounts of chemicals were used tosynthesize sols with a molar ratio of NO3

¯ /Al=0.6: 60.00 g of 75 wt.%solution of ASB in sec-butoxide was added to a solution of 18.00 galuminum nitrate (Al(NO3)3·9H2O) which was dissolved in 116.00 gof water. At the end of the synthesis, i.e. after removal of sec-butoxideand reheating to 85 °C, the solids loading (expressed as potentialcontent of Al2O3) of the sols was c(Al2O3)~16 wt.%, where c denotesconcentration. Albeit the sols do not contain Al2O3 right after theirsynthesis, the solids loading is given here as the theoreticallyobtainable amount of Al2O3 after heating to 1250 °C. The theoreticallyobtainable Al2O3 content was derived from thermal gravimetry (TG)where xerogels were heated up to 1250 °C to completely transforminto alpha-Al2O3 as described in [7,9]. The pH value of the resultingsols was 3.3. The viscosity of the sols was increased by addingpolyvinylpyrrolidone (PVP, Mw=360,000 g/mol, K90, Fluka). Basedon the resulting PVP–sol mixture, PVP concentration was 1.2 wt.%.

The sols were transferred into their respective solid products by tworoutes: either a slow or a fast evaporation regime. Xerogels preparedwith a slow evaporation regimewill be termed “monoliths”. They wereprepared bypouring the sol–PVPmixture into aluminumdishes (Alcan)to a filling height of ~1 cm. Evaporation was carried out at atmosphericpressure and ambient relative humidity on a heated sand bath having a

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43M. Dressler et al. / Thin Solid Films 519 (2010) 42–51

temperature of approximately 85 °C. After approximately 45 min the solhad completely transformed into monolithic xerogel fragments, whichwere lightly crushed to millimeter sized fragments in an agate mortarand subsequently heated to 200 °C.

Solid products obtained by a much faster dispersant evaporationregime will be called “films” in this work. These films were preparedby dip-coating a glass plate (soda–lime-glass) into the sol. When thewet substrates were immediately put into an oven which waspreheated to 200 °C heavily cracked layers were obtained whosefragments could easily be scraped off. The thickness of the layerfragments, which was assessed with scanning electron microscopy,ranged between 0.9 μm and 23.2 μm with a mean value of 8 μm.

The collection of the whole film sample took almost one month asonly a small amount could be collected every day. As one sol samplewas used during the entire film collection process it cannot beexcluded that the parent sol changed somewhat due to ageing [6]. Onthe other hand it was shown in [8] that PVP-containing sols, like theones used in this work, showed a retarded ageing. In order tocompensate for possible ageing effects, the film sample yield of eachday was not merely added to the previous days' yield, but each days'yield was stored in a separate container. Finally a unified sample wascomposed bymixing similar aliquots from each days' yield. Thismixedsample was thoroughly homogenized by tumbling the container. Theresulting homogenized sample was the source for all experiments inthis work.

Monoliths were prepared on each day when film samples wereprepared. Alike to the procedure for the film samples, thefinalmonolithsample was composed of aliquots taken from each days' yield. Film andmonolith samples were stored in tightly sealed containers at roomtemperature in a desiccator.

The film and monolith samples were divided into three portionswhichwere subject to different heat treatment regimes. Heat treatmentwas carried out at 500 °C, 850 °C and 1150 °C in ambient air in a muffletype furnace (L9/SH, Nabertherm). These particular temperatures werechosen as (i) at 500 °C the majority of all volatile compounds, such aswater, NOx, organics and CO2 were driven off, (ii) at 850 °C a phasetransformation into eta-Al2O3 takes place in monoliths [9], and (iii) at1150 °C alpha-Al2O3 was formed [9]. Heating rate was 10 K/min andsamples were kept for 30 min at the designated temperature. Thefollowing nomenclature is applied for naming the samples. Dependingon their heat treatment temperature (200, 500, 850 and 1150 °C), filmswill be called F200, F500, F850 and F1150 while monoliths will beaddressed as M200, M500, M850 and M1150.

2.2. N2 adsorption and He penetration

The adsorption–desorption isotherms of nitrogen at 77.3 K weredetermined using a volumetric gas adsorption analyzer ASAP 2010(Micromeritics). Prior to measurements, the samples were degassedunder vacuumat 190 °C for 24 huntil thepressure of the system reachedless than 0.13 Pa. The pore size distributions were calculated from thedesorption branch of the isotherm by the Barrett, Joyner, Halenda (BJH)method [10]. The total pore volume was evaluated from the nitrogenuptake of the adsorption branch of the isotherm at a relative pressure ofabout 0.99 by converting the value of the gas adsorbed to the volume ofthe liquid adsorbate according to the single point method after Gurvich[11]. Specific surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method [12] for relative pressures in the region of0.05 to 0.3. The skeleton densities of the samplesweremeasured at 20 °Cby He pycnometry with a fully automated pycnomatic ATC instrument(Porotec). The smallest detectable pore size with He pycnometry is0.1 nm [13]. The obtained densities (He pycnometry) and pore sizedistributions (N2 adsorption) agreed well with mercury intrusionmeasurements. However, this only applies for monolith samples. Thefluffy film samples did not result in sensible mercury intrusion results.

2.3. Porosity

Porosity was calculated using (i) N2 adsorption derived totalspecific pore volume (Vp) given in cm³/g which covers the open poresas well as (ii) He penetration derived density (ρHe) which is ameasureof the density of the solid matrix. The following equation [14] wasused to calculate the open porosity εopen.

εopen =volumeof openpores

totalvolume=

VP

ðVP + 1= ρHeÞð1Þ

Closed porosity (εclosed) was calculated with the followingequation [15] using the He penetration derived skeletal density andthe theoretical density which was derived from the inorganic crystalstructure database ICSD [16]. Skeletal density represents the ratio ofthe mass of the solid matrix to the sum of the volume of the solidmatrix and the volume of the closed pores

εclosed =volumeof closedpores

totalvolume=

1ρHe

− 1ρth

� �ρHe = 1−ρHe

ρth: ð2Þ

2.4. Thermal analysis

Differential thermal gravimetric analysis (DTG) and differentialthermal analysis (DTA) were recorded simultaneously in a thermo-balance (TAG24, Setaram). The maximumweighing error during DTGwas Δm=0.01 mg and the maximum error for temperature readingswasΔT=5 K. The test material (initial mass ~25 mg)was placed in anopen Pt-crucible (100 μl). Samples were measured at a heating rate of5 K/min in an argon plus air flow (volume flux flow rate ratio of argonand air was 3/4). The mass spectrometer (Quadstar 421, Balzers) forsimultaneous analysis of evolved gases was coupled by a heated(120 °C) quartz glass capillary. Measurements were performed inmultiple ion detection modus. Evaluation of DTA data was carried outusing Setsoft software (Setaram). The DTA as well as DTG resultsshown in this work are scaled with respect to sample mass and thegiven temperatures are peak temperatures.

2.5. Transmission electron microscopy (TEM)

Microstructure and crystallographic phases were characterized withan analytical scanning transmission electron microscope (STEM), JEM2200FS (JEOL) operating at 200 kV. Micrographs in TEM and STEMmodewere combined with electron diffraction investigations in selectedarea and convergent beam mode and with energy dispersive X-rayspectroscopy.

3. Results

3.1. Thermal analysis

The thermal evolutionoffilms andmonoliths canbe separated in twomajor regions: (i) from room temperature up to 500 °C and (ii) between500 °C and 1200 °C. Below 500 °C (Fig. 1, a) a series of superimposedendothermic and exothermic peaks appears. The endothermic peaksbelow 500 °C correspond well with thermogravimetric data (Fig. 1 c).Taking into account DTA/DTG as well as mass spectrometer results (notshown) three processes take place in that particular temperature range:(i) release of water adsorbed on the surface (monolith: 129 °C, film:89 °C), (ii) burnout of organics such as traces of sec-butanole and PVP(monolith: 323 °C, film: 258, 326 °C) and (iii) the liberation of NOx

compounds (monolith: 350, 461 °C, film: 360 °C).The exothermic peaks above 500 °C can be assigned to eta-Al2O3

formation in region “I” (monolith: 860 °C, film: 820 °C) and transforma-tion into alpha-Al2O3 in region “III” (monolith: 1167 °C, film: 1154 °C)(see also [9]). The other exothermic peaks found in region “II” will be

Fig. 1. DTA and DTG plots of films and monoliths. All samples have previously been heated to 200 °C in air. Note the differing scaling factors (3, 1/18). Data shown is scaled withrespect to sample mass.

44 M. Dressler et al. / Thin Solid Films 519 (2010) 42–51

addressed inmore detail in the Discussion section.While after heating to1230 °C thefilms exhibited amass loss of 51% and themonoliths lost only36%.

3.2. Transmission electron microscopy

TEM micrographs shown in Fig. 2 illustrate the marked changesthe samples undergo during heating from 200 °C to 1150 °C. Two sizeranges were identified in both films and monoliths: (i) thin plate-likeparticles with sizes in the micrometer range and (ii) nanometer sizedparticles. As these micrometer sized plate-like particles could beobserved in all samples only the evolution of the nanometer sizedparticles will be described in more detail.

After heating to 200 °C the samples were featureless and theelectron diffraction patterns (not shown here) correspond to theamorphous state.

After heating to 500 °C the majority of the volatiles was removedand the microstructure changed. In the monolithic samples longishsingle particles with dimensions smaller than 100 nm were found. Inthe film samples more isometric, single particles with dimensionsbetween 10 and 50 nm could be detected. The particles in both sampletypes are polycrystalline as they consist of single crystalline regions ofaround 2–3 nm which are arranged in a domain-like structure (seeFig. 3). The electron diffraction patterns were ambiguous and fit todifferent Al2O3-phases. Still, it was found that after heating to 500 °Cboth sample types are mixtures of cubic and tetragonal Al2O3. Thedifferent cubic Al2O3 phases like gamma- or eta-Al2O3 could not bedistinguished in this work by electron diffraction because of theirsmall structural differences.

After annealing at 850 °C closely connected single particles with areduced particle size—now being smaller than 20 nm—could bedetected in both sample types. All electron diffraction single crystalpatterns indicate the presence of delta-Al2O3 which has a tetragonal

structure. Signals from a cubic phase—either gamma- or eta-alumina—could only be found in selected area diffraction ring patterns possiblystemming from regions of small polycrystallites. Because of the lackof single crystal patterns it was not possible to clearly identify eta-alumina (cubic), which could be expected (see [9]).

Heat treatment at 1150 °C provoked marked changes in the samplemorphology. Solely alpha-Al2O3 in the form of single crystallineaggregates with dimensions of 100 nm (films) and 200 nm (monoliths)could bedetected. In themonolith sample coalescedparticles and featuressimilar to sinter necks could be observed (Fig. 2, M1150) albeit theseregionswerealso single crystalline. Bothfilmsandmonolithsexhibit spotsof bright contrast asmarked by arrows in Fig. 2. In high resolution images(Fig. 4) it was found that lattice fringes run from the dark matrix regionthrough these bright contrast spots without disturbances. Hence thesebright contrasts cannot be assigned to tilting or bending of crystals.

3.3. Specific surface area

After preparation at 200 °C (results not depicted) the monolithsshowed nomarked adsorbed volume. The graph of the specific adsorbedvolume represents a straight line with an only marginal hump at p/p0=1. However, above 200 °C the temperature inducedmicrostructuralchanges observed by TEM analysis correspond to marked changes inspecific adsorbed volume as can be seen in Fig. 5. After heating to 500 °Cthe monolith had a type IV isotherm [17] with a hysteresis loop in theadsorption and desorption curve between 0.41bp/p0b0.88. A similaradsorption and desorption curve pattern as well as hysteresis loop wasmeasured after heat treatment at 850 °C. After heating to 1150 °C onlyvery small adsorption/desorption vales were measured which suggeststhat after heating to 1150 °C the pore structure had collapsed.

Similar quantitative trends were observed for the film samples.Increasing the heat treatment temperature from 500 °C to 850 °Cled to a marked increase in total specific adsorbed volume whereas

Fig. 2. TEM micrographs of monoliths and films after heat treatment at different temperatures (200, 500, 850 and 1150 °C). The arrows mark spots of bright contrast which werefound everywhere after heating to 1150 °C (for explanation see text).


further increasing heat treatment temperature to 1150 °C resulted inan almost complete collapse of the pore structure. However, thefollowing quantitative differences for specific adsorbed volume haveto be regarded. (i) The adsorbed volume was smaller for films and (ii)the change in adsorbed volume provoked by heating from 500 °C to850 °C is much higher for films than for monoliths.

Further peculiarities in the films are (i) a rather linear adsorptioncurve after heating to 500 °C (F500) and 850 °C (F850) and (ii) thecomplete hysteresis between 0bp/p0b1 in sample F500. The latterresult has been confirmed by repeated measurements.

3.4. Pore size and pore shape

According to Brinker and Scherer [18] hysteresis loops as observedin Fig. 5 can be assigned to the presence of bottle neck shaped poreswhich consist of larger pore cavities (voids) being interconnectedwitheach other by smaller pore channels (throats). The size of the voids(throats) which has been derived from the adsorption (desorption)branch is shown in Table 1. The calculated size distributions for voidsand throats were similar for both the films and monoliths. It isimportant to note that irrespective of the distinction between voids

image of Fig.�2

Fig. 3. TEM micrographs illustrating the domain structure in film and monolith samples after heating to 500 °C. Both micrographs exhibit small single crystalline regions which canbe seen best in the monolith sample.


and throats the pore size distributions were rather bimodal in themonoliths and unimodal in the films.

3.5. Open and closed porosity

The measured specific pore volume and density allowed for thecalculation of the open porosity according to Eq. (1). The so calculatedopen porosity is compared with the specific surface area for monolithsand films at different heat treatment temperatures in Fig. 6. It is evidentthat open porosity is closely related to specific surface area. The lowestopen porosity/specific surface area valueswere calculated/measured forsamples either in their as prepared state after preparation at 200 °C orafter heat treatment at 1150 °C. The maximum open porosity/specificsurface areawas found for samples after heating to 850 °C. Films alwayshad a lower open porosity/specific surface area than monoliths (cf.500 °C and 850 °C case in Fig. 6).

The He penetration derived densities shown in Table 1 representthe skeletal density. As however, helium does not penetrate closedpores, it is possible to obtain the amount of closed porosity (seeEq. (2)) when comparing the theoretical density of a pore free bodyand the He derived density. Theoretical density values were takenfrom the ICSD data base [16]. Table 2 summarizes closed porosityvalues calculated with Eq. (2). The highest closed porosity can befound after heating to 500 °C. After heat treatment at 850 °C theclosed porosity decreased markedly in both sample types. Whenfurther increasing the heating temperature to 1150 °C the calculatedclosed porosities changes only little compared to the 850 °C case.

The following should be noted. The assumption whereas thetheoretical density of the samples after heating to 500 °C is that ofgamma-Al2O3 is arbitrary. However, according to TEM analysis a cubic

Fig. 4. TEM micrograph on a film sample after heat treatment to 1150 °C. The latticefringes are undisturbed between the white spot and its surrounding matrix.

Al2O3 was detected after heating to 500 °C. Fortunately both possiblecubic Al2O3 compounds (gamma or eta-Al2O3) have almost similartheoretical densities (gamma-Al2O3: 3.66 g/cm³ [19,20], eta-Al2O3:3.65 g/cm³ [21,22]).

4. Discussion

4.1. Specific surface area, porosity and pore size distribution

The results presented above illustrate that the differing dispersantevaporation regimes for monoliths and films provoked interestingquantitative differences regarding specific surface area, pore size,specific pore volume, porosity as well as thermal transformationbehavior. The following morphological evolution of monoliths andfilms might be envisaged. After gelation both monoliths and films areporous gels with a dispersant filled pore network. This is in agreementwith the very low specific surface area and open porosity after xerogelpreparation. After heating to 500 °C the dispersant and themajority ofall other volatile material are removed out of the gels and the driedgels showed an increase in specific surface area and open porosity asthese cavities now became accessible to nitrogen. Both samples stillcontained a marked amount of closed pores. After heating to 850 °Cthe amount of closed pores reduced while specific surface area andopen porosity increased. Heating to 1150 °C led to a reduction inspecific surface area and open porosity.

Two peculiarities have to be explained. (i) Why have monolithsamples always a higher open porosity and higher specific surfacearea than the film samples? (ii) Why show both films and monolithsan increase in open porosity and specific surface area after heating to850 °C, and why is this increase much more pronounced in the filmsamples?

The first question can be answered when recalling that duringdispersant removal the pore structure in a gel may change profoundly.Due to a high capillary pressure build-up, liquid filled pores can collapsewhich finds its macroscopic expression in the usually observed markedshrinkage, gels undergo during drying. In order to get a dried gel withlots of remaining pores, i.e. with a high porosity and a high specificsurface area, one has a few possibilities among them the following two.(i) The gel network has to be toughened so that it withstands thecapillary pressure and (ii) one should coarsen the gel pores, as anincreased pore size leads to a decreased capillary pressure. According toBrinker and Scherer [25] exactly these two effects can be observed inaged gels as ageing causes transport of material to necks of adjoiningparticles via dissolution and reprecipitation which leads to a stiffenednetwork and an increased pore size.

Ageing induced network toughening/coarsening might indeedexplain why monoliths have a higher porosity and specific surface area

image of Fig.�3

image of Fig.�4

Fig. 5. Adsorption isotherms of monoliths (left hand side) and films (right hand side) after heat treatment at different temperatures (500, 850, and 1150 °C). The ordinate (volumeabsorbed in cm3/g STP) applies for both diagrams.


than films after dispersant removal at 500 °C. Monolith samples wereprepared by putting sol filled laboratory dishes on a sand bath at 85 °C.Before the sol transforms into monolith fragments it dwelled for 45 minat 85 °C on the sandbath. This increaseddwelling timeof the sols at 85 °Ccould be interpreted as an ageing process at elevated temperatures.Hence, there could have been enough time to facilitatematerial transportprocesses which might have lead to network toughening/coarsening.Thus, the fact that monoliths exhibited a higher specific surface and ahigher open porosity might be attributed to a less pronounced porecollapse due to the possibly toughened/coarsened network. Thetoughened/coarsened gel network could also explain the observedbimodal pore size distribution in the monoliths. The bigger detectedpores might be coarsened pores which were toughened enough, so theycould withstand the pore collapse. Also particle shape seems tocorroborate the hypothesis that monolith gels were subject to markedageing induced dissolution and reprecipitation processes. While the solparticles are spherical [7] the particles in the gel are rather longish.

In the film samples the time span during which such gel networktoughening/coarsening might have taken place was just too short.During their preparation only a few micrometer thin film had to drywhich took just seconds. Hence, in the film samples there was much

Table 1Results of N2 adsorption and He penetration measurements for monoliths and films.

TemperatureIn °C

N2 adsorption

Spec. pore volume a

In cm3/gPore size b

For voidsIn nm calculated w

Films F500 500 0.032 5.1F850 850 0.091 4.7F1150 1150 0.019 N100

Monoliths M200 200 0.002 –

M500 500 0.136 5.8M850 850 0.161 6.3M1150 1150 0.009 N100

a Single point total pore volume at p/p0 (after Gurvich, see [11]).b BJH calculation from the adsorption and the desorption branch of the isotherm at 77 Kc Main peak in the pore size distribution.d Marked shoulder in the pore size distribution.e Separate peak in the pore size distribution.

less time for dissolution and reprecipitation. Thus, in comparison withthe monoliths, the film samples did not have a toughened/coarsenedgel network, which explains that after their complete dispersantremoval—with concurrent pore collapse—their porosity and specificsurface area were lower than for the monoliths. The rapid gelling inthe films and the suppressed gel network coarsening, might alsoexplain why in these samples the spherical particle shape of the solparticles [7] was preserved, so that spherical particles could even beobserved after heating to 500 °C.

The more compact structure of films compared to monoliths hasbeen described earlier by Brinker and Mukherjee [26] who draw thisconclusion on the basis of specific surface area and refractive indexmeasurements.

One finding however cannot be easily explained. As can be seen inTable 1 films and monoliths showed similar pore sizes. Against thebackground of the network toughening/coarsening debate one wouldhave expected to find a higher pore size in the presumablytoughened/coarsened monoliths. Hence, additional effects should beconsidered. E.g. it might be surmised that the different preparationtimes in monoliths and films also changed the chemical properties,which could also influence the surface structure evolution during

He penetration

DensityIn g/cm3

ith adsorption dataFor throatsIn nm calculated with desorption data

3.4 2.2673.5 3.370N100 3.594– 2.3653.6 c/5.3 d 2.6323.5 c/5.0 e 3.28183.7 3.688

.

image of Fig.�5

Fig. 6. Open porosity and specific surface area of monoliths and films after heat treatment at different temperatures. Porosity has been calculated with N2 adsorption derived totalpore volume and He penetration derived density (for details see Experimental details section).


dispersant removal. Differences in chemical composition betweenmonoliths and films are indeed discussed further below. While thenetwork toughening/coarsening debate is based on ageing time,another factor perhaps being of influence could be heating rate. Thus,not only the total available time for material transport processes(minutes for monoliths, seconds for films) but also the rate at whichthe heating up took place might play a role. Obviously filmsunderwent a much higher heating rate as monoliths. However, it isnot easy to distinguish both effects as a lower heating rate surelyimplies—given that the same temperature is targeted—the total time asample spends at elevated temperatures is of course higher than if ahigher heating rate is applied.

The two remaining questions regarding the observed increase inporosity and specific surface area after heating to 850 °C might beexplained with a pore opening and a particle size effect.

(i) pore opening: As can be seen in Table 2 bothmonoliths and filmsstill have a considerable amount of closed pores after heating to500 °C. The formation of eta-Al2O3 at ~850 °C results in a markedvolume reduction as was found with dilatometric analysis (dilatom-eter results not shown). Hence, one might envisage the following.After heating to 850 °C the gel matrix with its closed pores (38.1%films, 28.1% monoliths) transforms into eta-Al2O3 which due to theshrinkage results in the formation of cracks which opened the closed

Table 2Calculated closed porosities for samples analyzed in this work. Values were obtained usingrespective alumina phases.

TemperatureIn °C

Respective phasea TheorIn g/c

Films F500 500 Gamma-Al2O3b 3.66

F850 850 Eta-Al2O3 3.65F1150 1150 Alpha-Al2O3 3.99

Monoliths M200 200 Amorphous –

M500 500 Gamma-Al2O3b 3.66

M850 850 Eta-Al2O3 3.65M1150 1150 Alpha-Al2O3 3.99

a Respective phases taken from [9].b Assignment of gamma-Al2O3 is only a rough estimate as samples were mostly X-ray am

pores. As a result the specific surface area and the open porosityincreased and the amount of closed pores decreased.

The amount of remaining closed pores is in good agreement withmechanical properties of the films. After heating a film sample to1150 °C, which according to DTA analysis implies that the thin filmhas transformed to alpha-Al2O3 [9], the modulus is much lower thanthe modulus of dense alpha-Al2O3 [3]. The difference between thevalue obtained for the presumably porous film (160 GPa, [3]) anddense alpha-Al2O3 (410 GPa, [27]) cannot be explained if onlyconsidering open porosity. Fig. 6 shows that open porosity was 6.9%after heating the films to 1150 °C. According to Munz and Fett [27],such a porosity would result in a modulus of ~305 GPa [27]. However,as the measured modulus value is much lower (160 GPa, [3]) theoverall porosity of the film, i.e. the sum of closed and open porosity,should be much higher. Indeed, if summing up the open (6.9%) andclosed (9.9%) porosity of the film sample a total porosity of 16.8%follows. Extrapolating the modulus values given in [27] to a porosityof 16.7% results in a modulus of ~180 GPa. This value is close to theactually measured number of 160 GPa. Hence, the applied method forcalculating the amount of closed pores used in this work seems toresult in sensible values.

(ii) particle size: Particle size changes between 500 °C and 850 °Ccould also explain the increase in open porosity and specific surface

measured He penetration densities and literature derived theoretical densities of the

etical density from ICSD cardsm3

Reference Calculated closed porosityIn %

[19,20] 38.1[21,22] 7.7[23,24] 9.9

–

[19,20] 28.1[21,22] 10.1[23,24] 7.6

orphous and showed only weak reflexes of gamma-Al2O3 [9].

image of Fig.�6


area. Ramsay and Avery [14], who investigated the surface area S ofgels, found that surface area depends on particle size D according to

Se 1D: ð3Þ

Hence, a reduction in particle size leads to an increase in specificsurface area. TEM analysis indeed suggests a decrease in particle sizefrom (monolith: 50–100 nm, film: 10–50 nm) to b20 nm for bothsample types when heating from 500 °C to 850 °C.

This particle size debate might not only explain why there is anincrease in specific surface area in the first place, but also why the filmsamples showed a much more pronounced increase than themonoliths. When calculating the derivative of Eq. (3) one getsEq. (4) which implies, that a smaller starting particle size results in ahigher specific surface area change ΔS at a given particle size changeΔD. A smaller starting particle size (i.e. the particle size after heatingto 500 °C) was indeed found in the films. In other words a similarshrinkage ΔD—caused by the phase transformation at ~850 °C—led toa higher increase in surface area (ΔS) in the films as their startingparticle size was smaller.

ΔSe 1D2 ΔD: ð4Þ

4.2. Comparison of specific surface area data with literature results

Compared to other preparation regimes, intended to result e.g. incatalyst supports with a desired high surface area (cf. Table 3), thealumina prepared in this work has a comparatively low surface area.The maximum value being 107 m2/g in the monolith sample afterheating to 850 °C.

One reason for the rather small surface area values found in thiswork might be the use of a comparatively acidic sol (pH 3.3). It wasshown in the literature [29,32] that an increased acidity led to adecrease in surface area and pore volume.

4.3. Formation of spots with bright contrast after heating to 1150 °C

At a first glance these spots of bright contrast might be interpretedas ‘discrete’ alpha-Al2O3 regions as described by Dynys and Halloran[33] or Yarbrough and Roy [34]. However, these authors report suchgrains to be embedded in a theta-Al2O3 matrix. This is not the casehere. Not only the spots of bright contrast but also the surroundingmatrix was alpha-Al2O3. In order to understand these bright contrastsit might be helpful to consider (i) pores and (ii) “internalnanostructures” [35]. The presence of pores could be surmised asthe size of the pores detected by N2 adsorption measurements (cf.Table 1) is in reasonable agreement with the size of these brightcontrast points (30–80 nm). Similar bright contrasts in TEM micro-graphs were reported by Wilson and Stacey [15] who also assignedthem to pores. However, as the assignment of TEM-derived features topores is not easily done other authors [35], who as well detectedsimilar structures, assigned themmerely to “internal nanostructures”.

Table 3Values for specific surface area and pore sizes of different aluminas taken from the literatu

Specific surface areaIn m2/g

Pore sizeIn nm

Calcination temperatureIn °C

Initial a

307–358 6.0–8.1 700 (4 h) Boehmi123–217 8.3–15.5 600 (30 h) Boehmi218.4 6.6 788 (4 h) Pseudob68.7 34.7 1100 (5 h) Fibrous

4.4. Thermal evolution

The thermal evolution between 755 °C and 900 °C (region “I” inFig. 1) will be addressed first. A possible reason for the different eta-Al2O3 formation temperatures of monoliths (865 °C) and films(820 °C) might be their differing Al13 contents. It was reported in[9] that monolithic xerogels of the type used in this work containedAl13 polycations. It was shown previously by Schönherr et al. [36] thata “pure” Al13 polycation compound (Al13O40-chloride) exhibited anexothermic DTA signal at 805 °C (heating rate 10 K/min). Mixedcompounds which contained not only Al13 polycations but also otheralumina species like gelatinous boehmite with short fibrils andamorphous material transform into eta-Al2O3 at ~850 °C [9]. Xerogelscontaining no Al13 polycations at all, but only amorphous material,transform into eta-Al2O3 at ~900 °C [9]. Hence, it can be concludedfrom these literature reports that the lower the Al13 content and thehigher the amorphous material content the higher the eta-Al2O3

transformation temperature. As films had a lower eta-Al2O3 transfor-mation temperature than monoliths, the just mentioned literaturereports suggest that films contained more Al13 than monoliths. Thishypothesis is supported when taking into account the change of pHvalue during dispersant evaporation. It is well known that thepresence of Al13 polycations depends on pH value [6]. During xerogelformation the pH value of the sols inevitably decreases due todispersant evaporation. Thus, the monoliths with their prolongeddispersant evaporation time have probably been subject to Al13—unfavorable, i.e. low, pH values for a much longer time. The films onthe other hand needed only seconds to dry. Hence, their Al13polycations were not exposed for prolonged times to low, i.e. Al13—unfavorable, pH values.

The peaks detected between 900 and 1070 °C (region “II” in Fig. 1),cannot be assigned to the formation of transition aluminas. Accordingto Badkar and Bailey [37] transitions of metastable aluminas such asdelta- or theta-Al2O3 are not expected to result in enthalpy changes.However, nucleation and growth processes preceding alpha-Al2O3

formation [38] might explain these DTA signals. As was reported byWen and Yen [39] the nucleation of alpha-Al2O3 results in a hump-likebroad exothermic DTA signal which merges into a sharp signalattributed to the growth of alpha-Al2O3. Yu et al. [40] who alsodetected this sequence of nucleation and growth related DTA peaks,reported that addition of alpha-Al2O3 “seed” particles can change thispattern. In the presence of “seed” particles the nucleation relatedexothermic peak can “detach” from the growth related signal andform a separate exothermic DTA peak. In order to explain theobserved peaks the following hypothesis is proposed here. Al13transforms already at 930 °C into alpha-Al2O3 [41]. These early formedalpha-Al2O3 particles might act as “seed” particles for other Al2O3

compounds. As it is well known that seeding effects depend on “seed”concentration [42], the differing amounts of Al13 polycations withinfilms and monoliths might also be responsible for the peculiarsequence of exothermic peaks detected in this work.

Differences between films and monoliths were also found attemperatures below 500 °C. In both sample types DTA signals could befound corresponding reasonably with mass loss and mass spectrometersignals (CO2) allowing for the assignment of certain DTA signals to PVPrelease (film: 258 °C, monolith: 350 °C). Apart from these temperature

re.

lumina compound Type of alumina after heat treatment Reference

te Gamma-Al2O3 [28]te Gamma-Al2O3 [29]oehmite Gamma-Al2O3 [30]boehmite Theta-Al2O3 [31]


differences the release mode differed markedly. The PVP release wasassociated with a sharp exothermic signal in the film sample while anendothermic signal was found in the monolith specimen. The exother-mic signal in the film sample points to a PVP release taking place byoxidation, while the endothermic peak in themonolith samplemight beascribed to apyrolysis process. Pyrolysis takesplacewhenoxygen supplyis limited during decomposition of organic compounds. Such a limitedaccess of oxygen in the caseof themonolith samplesmight be ascribed totheir possibly much longer diffusion path length compared to films.While the films are very thin with thicknesses in the micrometer rangethe monoliths are rather big with dimensions in the order of 100–500 μm. Hence, it is a sound assumption that diffusion paths for bothoxygen and PVP-decomposition fragments are much longer in themonoliths compared to the films. These possibly longer diffusion pathsprobably provoked the PVP decomposition to take place via pyrolysis inmonoliths while in films the shorter diffusion paths facilitated the PVPdecomposition by oxidation.

However, other factors might also be of influence. It wasspeculated by Kozuka et al. [43] that PVP interacts with OH groupsof the gel network. These authors used a 2 h reflux regime to preparetheir PVP-containing sols. Thus, it might be hypothesized that perhapsmonoliths with their longer preparation time at ~85 °C facilitatedsuch OH group–PVP linkages. As a result the bonding between PVPand OH groupswithin themonoliths would be stronger, thus resultingin a higher burnout temperature as in the films with their only veryshort sol–gel transition time possibly only allowing for a weaker OH–PVP interaction.

Differences between films and monoliths were also detectedregarding their overall mass loss when heating to 1150 °C. The filmsshowed a higher mass loss than the monolithic xerogels (films: 51%,monoliths: 36%). It was discussed in detail above, that ageing relatedeffects like dissolution and reprecipitation might play a role in the gels.These ageing processes might provoke condensation reactions like themerging of Me–OH groups into Me–O–Me which results in theliberation of H2O. Asmonolithswere subject tomuchmore pronouncedageing effects—and perhaps also condensation reactions—onemight hypothesize that the monolith samples contained more mergedMe–O–Me groups and hence, more water had already been releasedduring gelation. Hence compared to film samples monoliths could onlyrelease less water during heating up. However, it should be noted thateffects, which were not investigated in this work like the adsorption ofwater during sample preparation for DTA analysis, might also be ofinfluence.

5. Conclusion

The present work shows that preparation conditions for xerogelsduring the sol to gel transition have a significant impact on openporosity, specific surface area and thermal evolution. The temperaturedependence of specific surface area and porosity shows the samegeneral trends for thin films and monoliths. However, albeit thegeneral trends are similar, marked quantitative differences wereobserved. The fast dispersant evaporation in thin films results in areduced open porosity compared to the monolithic samples. Filmsamples had a lower specific surface area and narrower pore sizedistribution than the monolith samples. The observed differences inspecific surface area, open porosity as well as pore size distributioncould be explained with ageing related material transport effectsleading to toughening and coarsening of the gel network.

Alike to specific surface area and porosity, marked quantitativedifferences were observed during thermal transformation. Monolithsand films showed rather pronounced differences during the burnout ofvolatiles in the temperature range below 500 °C. While the film samplesexhibited an exothermic burnout of PVP, which was added to modify thesols viscosity, themonoliths rather showed anendothermic releasemode.Also the temperature of eta-Al2O3 formation differed noticeably between

films (820 °C) andmonoliths (860 °C),whichmightbe related todifferentAl13 contents of monoliths and films. Only marginal differences in thealpha-Al2O3 formation temperaturewere found (film: 1154 °C,monolith:1167 °C).

The present work allows for the conclusion that xerogels havingbeen prepared in a slow dispersant evaporation process usuallyresulting in millimeter sized lumps (monoliths) are a reasonablemodel for investigating the qualitative trends of thin films being onlyof micrometer range thickness. However, marked quantitativedifferences between films and monoliths were found.

Acknowledgements

The financial support of the German Research Foundation (DFG) isgratefully acknowledged. Furthermore, the authors would like toexpress their sincerest thanks to A. Marek who collected theinvaluable layer fragments within weeks of tedious work. Wefurthermore thank A. Zimathies and C. Prinz and H. Marx for theirhelp with the N2 adsorption measurements, He penetration and Hgintrusion measurements. F. Emmerling is to be thanked for the ICSDdata base inquiry and the XRD measurements. We also thank S.Benemann who contributed the layer thickness data.

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