New and modified anodic alumina membranes Part I. Thermotreatment of anodic alumina membranes

ELSEVIER Journal of Membrane Science 98 (1995) 131-142

journal of MEMBRANE

SCIENCE

New and modified anodic alumina membranes Part I. Thermotreatment of anodic alumina membranes

Peter P. Mard i lov ich a' 1, Alexander N. G o v y a d i n o v b, Nikola i I. M u k h u r o v b, Alexander M. RzhevskiF , Russel l Pa te rson a'*

aDepartment of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK blnstitute of Electronics, Belarus Academy of Sciences, 22 Logojski Trakt, Minsk, 220841 Belarus

Clnstitute of Physical and Organic Chemistry, Belarus Academy of Sciences, 13 Surganov St., Minsk, 220072 Belarus

Received 28 February 1994; accepted in revised form 2 August 1994

Abstract

Polycrystalline anodic alumina membranes have been prepared by controlled calcination. Phase transformations, porosity changes and conversion of electrolyte impurities incorporated into the anodic alumina lattice were studied as functions of the calcination temperature (up to 1200 °C). A comparative IR investigation of hydroxyl cover of the polycrystalline anodic alumina membranes and pure 7- and ~-alumina was carried out. Amorphous anodic alumina films calcined at temperatures up to 800 °C with a barrier layer in place (thickness of barrier layer ranged from 5 to 90 nm) were prepared and the permeabilities measured. It is hypothesised that the thin layer of relatively pure and dense alumina of the anodic alumina cell prevents migration of ions, water or small molecules through the barrier layer as well as between the adjacent cells.

Keywords: Ceramic membranes; Membrane preparation and structure; Anodic alumina; Thermotreatment of anodic alumina; IR spectroscopy of anodic alumina

1. Introduction

Porous anodic alumina (AA) films, which are formed under certain electrolytic conditions, provide a unique basis for the preparation of a range of ceramic membranes and membrane sieves. This type of ceramic membrane has many obvious advantages including an ideal geometric regularity of pore structures, fully controllable

* Corresponding author. i Permanent address: Institute of Physical and Chemical

Chemistry, Belarus Academy of Sciences, 13 Surganov St., Minsk, 220072 Belarus.

pore morphology and unparalleled control of active layer thicknesses [ 1-5 ]. The present inter- est in AA membranes was enhanced by the avail- ability of commercial AA membranes, especially symmetrical and asymmetrical membranes Anopore T M produced by Whatman Interna- tional Limited [4-7] . Many properties such as the adsorption of protein molecules [ 8 ] and the electrochemical [9,10] and transport properties [ 11 ] of these commercial membranes have been investigated.

In spite of the wide range of advantages, practical applications of AA membranes are severely limited by problems of membrane fragility and

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132 P.P. Mardilovich et al. / Journal of Membrane Science 98 (1995) 131-142

the fact that the (amorphous) AA membranes are susceptible to both acid and base attack. In this series of papers, methods for producing improved and modified AA membranes and multilayer membranes based on AA with active layers of oxides (A1203, ZrO2, TiO2 .... ) or metals (Pt, Pd, Ni, ... ) and the properties of such membranes are reported.

There are two main possibilities for the modification of AA membranes. The first is by changing the electrolyte composition and the conditions of anodic oxidation. The second is by modification of their structures by additional thermal or chemical treatments. High temperature treatment, calcination, can lead to crystallisation and destruction or deformation of the AA membrane. As a result, such a membrane cannot be used in practice. It is shown in this paper, however, that with careful preparative methods and subsequent heat treatment a polycrystalline AA membrane may be prepared. They have acceptable mechanical strength and flexibility, greatly improved resistance to acid, base and other chemical attack, and a thermal stability with potential for use up to 1050°C. Such chemically and thermally stable membranes can be further modified by vacuum deposition of thin surface layers of oxides, metals, ions or other chemical methods to provide additional selec- tive layers. These layers themselves may be further modified by subsequent chemical or thermal treatments.

In this paper (Part I ), the changing properties of AA membranes have been studied as a func- tion of the calcination temperature. In Part II, [12 ] the improved resistance to acid and base attack of these calcined membranes is reported.

2. Experimental

AA films were obtained by electrochemical oxidation ofaluminium. A section ofaluminium foil, 99.99% purity, was initially degreased in boiling octane, ultrasonically cleaned in isopro- panol and washed in distilled water. The foil was then electrochemically polished (glacial acetic acid and 88% H3PO4; 1:1 by volume) and washed

once more in distilled water. Finally, the foil was anodised (electrochemically oxidised) in an electrolyte solution based on oxalic acid (nor- mally 3wt% H2C204 in distilled water) at 10- 11 o C. If necessary, some part of the aluminium foil was protected from the electrolyte with a chemically stable varnish. The anodisation was carried out at constant current density (galva- nostatically), from 2.5 up to 50 mA/cm 2, or at constant voltage (potentiostatically), from 10 to 160 V. The cathode was aluminium (99.99% purity; 1 mm thickness) or Pt-grid. At the end of the electrolysis the remaining aluminium was dissolved by placing the anodised foil in a solution of HC1 (32%) and CuCI2 (0.05%). The alumina barrier layer was then removed. Some- times the porous alumina, without barrier layer, was separated from the unanodised aluminium plate in one stage by electrochemically dissolv- ing the barrier layer. AA membranes were prepared with pore sizes (diameters) in the range 25 to 150 nm, and corresponding pore densities in the range 2.4× 1014 to 5.7X 1012 m -2 and total thicknesses of 20 to 100/lm. For the smaller pore sizes, down to 13 nm, membranes were made with branched pore structures (by de- creasing the voltage [4 ] ) and an outer film of fine pore structure (the active layers) of thicknesses 1 to 5/tm were obtained.

To study the permeability of the barrier layer a series of AA films with barrier layer in place and of different thicknesses were prepared. The thickness of the barrier layer was controlled by changing the voltage at the final stage of the anodisation.

AA samples were calcined in air at 300- 1200°C for 30 min. Pure 7-A1203, ot-A1203 and relatively pure 6-A1203 were obtained by calcination of crystallised boehmite, (7-A1OOH), in air for 5 h at 550, 1200 and 950°C, respectively [ 13,14 ]. 0-A1203 was obtained by calcination of bayerite, a-A1 (OH) 3, at 1050 ° C [ 13,15 ].

To study the AA phase composition, X-ray analysis and IR spectroscopy were used. X-ray analysis was carried out using a DRON-3 dif- fractometer. For IR analysis powdered AA samples were prepared as conventional KBr discs. Spectra were recorded on Perkin-Elmer 180 and

P.P. Mardilovich et al. / Journal of Membrane Science 98 (1995) 131-142 133

Specord 75 IR spectrophotometers. Thermogra- vimetric analyses were carried out by using a de- rivatograph OD- 103 Paulic-Paulic-Erden's system (Hungary). The elemental analysis for carbon and hydrogen content in AA was made using a Carlo Erba 1106 analyser. The pore size distribution of AA membranes was determined using a Carlo Erba Strumentazione Microstruc- ture Laboratory Sorptomatic-1800. The BET surface areas were determined by hydrogen and krypton adsorption. The nature of impurities and the state of hydroxyl cover were studied by IR spectroscopy. Samples were inserted into a spe- cial IR vacuum cell which allowed thermal evacuation, adsorption and desorption of various gases in the temperature range 30 to 900 ° C.

The permeability of membranes was measured using concentration oscillator techniques [16].

3. Results and discussion

3.1. Phase transformation

AA membranes, as prepared, were amorphous. Some common problems of crystallisation of amorphous AA, barrier and porous types to a-A1203 have been discussed by Neuffild et al. [ 17 ] and Sirota and Shokhina [ 18 ].

Figs. 1A and IB show the composition of crystalline components which are obtained on heating in the temperature range 650-1250 ° C. These data were obtained by X-ray analysis of heated samples and confirmed independently by preparation of corresponding mixtures of pure y-, 6-, 0- and ot-A1203. Additional information on the composition was obtained by analysis of IR spectra of mixed and pure samples. This was especially useful in resolving the y~8 transformation [ 14 ], since these two crystal forms show only small differences in the X-ray spectra.

The first step of crystallisation occurs in a narrow range of temperature, 820-840°C, producing almost pure y-AI203, thereafter there are suc- cessive transformations through ~- and 0-A1203 until at 1150°C pure ot-AlaO3 is obtained, Fig. I A. A very similar transformation occurs when

100

80

-~ 60

40

~ 2o 0

100

e. 8o ~ 6o ~ 40

~. 20

0

A amorphous , 'ph I

~,~100

a 80

~' 60 8 40

20

= 0

2.5

2

1.5

0.5

0

anodic boehmite alumina as a precursor

700 800 900 1000 1100

x°c

0.5

0.4

0.3

0.2

0.1

0 1200

Fig. 1. Variat ion o f phase t r ans fo rmat ion o f anodic a lumina (A) and boehmi te (B) ; BET surface area (C) and concen- t ra t ions o f carbon and hydrogen in anodic a lumina (D) with tempera ture o f calcination. T ime of calcinat ion was 0.5 h.

pure y-Al/O3 (boehmite as a precursor) is used, Fig. lB. The sharpness of the initial transition from AA to y-A1203 is confirmed by thermogravimetric analysis, Fig. 2. For samples of heated AA membrane, the highest contents of 0-AI203 is formed at ~ I 100 °C and in much smaller pro- portions than on heating boehmite, Figs. 1A and B. Under certain conditions of preparation or treatment of AA, 0-A1203 was not detected.

The final transformation to o~-A1203, which is the most stable and least reactive form of alumina, begins at 1075°C, in agreement with [ 18], a temperature 75 ° C lower than for boehmite, Fig. lB. Mixed phase or 'polycrystalline' alumina membrane forms are therefore obtained in the range 830-1100°C and may be regarded as metastable modifications, in transition between 7- and a-A1203.

134 P.P. Mardilovich et al. / Journal of Membrane Science 98 (1995) 13 I - 142

0

E

10 ,E

20 0

DTG

TG

TG*

I I I I 200 400 600 800 1000

T?C Fig. 2. Thermogravimetric data for initial amorphous (TG, DTG and DTA) and for polycrystalline (calcined at 900 °C for 0.5 h) anodic alumina membranes (TG*). The weight of specimens was 150 mg.

3.2. Porosity

More interesting from the point of view of uses of these materials as membranes, or catalytic membranes, are the changes in specific areas and pore size distribution observed during those crystallisations. With the onset of crystallisation of amorphous AA to the production of 7-A1203, the BET surface area increases by some 30 times, producing a large number of cracks or micropores, which we will label as a secondary porosity. These micropores have a narrow size distribution with a radius in the range 2-4 nm, Fig. 3. They are caused by the reduction of the overall volume of the oxide due to the improved atomic packing on the transition from amorphous to crystalline structures. As the temperature is in- creased average pore sizes increase, Fig. 3. At the highest temperatures, >/1150°C, the specific area (of oI-AI203) is again very small, as in the origi- nal amorphous material, Fig. 1C.

The diameter and density of the initial trans- membrane pores, which are created during the electrochemical oxidation of aluminium, are determined mainly by a field-assisted dissolution process at the bottom of the pores and depends on the electrolyte nature and the current density.

For any electrolyte, the diameter of the trans- membrane pore is dependent only on the voltage during the anodisation. For example, using oxalic acid based electrolytes, as here, this system yields a pore diameter of 0.91 nm per volt for the anodisation stage (64 nm at 70 V). The variation of the pore diameter with the various pa- rameters of the AA preparation has been discussed in detail elsewhere [19]. It is the crystallisation of AA, which leads to the creation of this secondary porosity in the walls of the trans-membrane pores. This is why the initial pores remain effectively unaltered by this heat treatment.

3.3. Mechanical properties

The initial crystallisation and subsequent phase transformations of polycrystalline AA are accompanied by changes in mechanical properties, particularly a loss of flexibility and an increase in their fragility or tendency to break. The AA film flexibility decreases sharply in the course of reorganisations of the oxide lattice, in partic- ular at the onset of transitions from amorphous to polycrystalline AA at 820-840°C and on transformation from cubic to hexagonal (ot- A1203 ) close-packed lattice at 1100-1150 ° C. By careful control of the heating processes it is possible to produce a polycrystalline AA membrane with flexibility acceptable for their practical ap- plication (similar to the flexibility of commercial Anopore T M membranes), but the fragility of the t~-AA films is so high that they cannot be considered for routine practical use as membranes. Variation of other mechanical properties of the AA membranes with calcination temperature is more complicated. For example, the bending strength and the microhardness (by the imprint of the tetrahedral diamond prism) of the initial AA and AA calcined at 900°C and at 1150°C are in the ratios: 56:31.5:5.8 kg/mm 2 and 480:550:1550 kg /mm 2, respectively. Both amorphous and polycrystalline AA membranes have quite high hardness and may be used at very high pressures on a rigid support.


4 100 2 100 0.2 20

t v At% t vdv/dr t v 3.2 80 1.6 80 0.16 16

m~2.4 m~l.2 12

0.8 120 0.4 20 0.04 4

0 0 0 0 0 I I i i i i L L 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

pore radius, nm pore radius, nm pore radius, nm

Fig. 3. Pore size distr ibution (dV/dr) and cumulative volume (V) o fanodic alumina calcined at 950 (A); 1050 (B) and 1200°C (c).

3.4. Quantity of the impurities

It is well known that in the production of AA, electrolyte ions are incorporated within the structure as impurities [ 1 ]. From the point of view of electrochemical properties of AA membrane surfaces, the most important question is the influence of the calcination temperature on the content and composition of these impurities.

In the case of electrolytes based on organic acids, such as oxalic acid, the impurities are carbon, hydrogen and oxygen. Because oxygen is bonded to either carbon or hydrogen, the quantitative data of the impurities will be considered for carbon and hydrogen only.

Fig. 1D shows that the concentration of carbon impurities is not significantly altered by calcination of AA at temperatures up to 830°C for 0.5 h (initial AA, 2.40% carbon; AA calcined at 830 ° C, 2.12% ). Fig. 2 shows that the weight loss of the sample due to all possible reactions is also negligible over this temperature range.

In the transition temperature range 830-840 °C corresponding to the final stage of the first crystallisation step, from amorphous to y-A1203, Fig. IA, there is a sharp decrease in the percentage of carbon impurities. It is surprising, however, that only some 50% of carbon is lost in this process, Fig. ID.

The residual carbon is retained during the pro- gressive crystallisation steps from 840 till 1075 ° C and it is only lost on the last transition to a- A1203, Figs. 1A and D (dotted parts of curves Fig. ID based on IR spectroscopy data). The interpretation of this retention of carbon is that the carbon is retained within the crystal lattice and can leave the bulk of AA only after reorgan- isation of the oxygen sub-lattice at 830-840°C (transition of amorphous to polycrystalline alumina) and at 1075-1150°C (transformation from cubic to hexagonal close-packed lattice). Further information on the forms taken by carbon in the lattice are discussed below when the IR spectra are interpreted.

Over the same temperature range, the total hydrogen content of the sample was measured. Hy- drogen is present in the AA predominantly as hydroxyl ( O H - ) groups on the alumina surface and this interpretation is supported by the corre- spondence between hydrogen content and the specific area of the treated samples, Figs. 1C and 1D. This includes an increase in hydrogen (hydroxyl) at the amorphous/~,-Al203 transition. In the heating process, an active surface is produced which is hydroxylated during cooling under normal laboratory (moist air) conditions.

There is good agreement between the data from


elemental analysis and from thermogravimetric analysis, Figs. 1D and 2.

3.5. Composition of the impurities in amorphous AA membranes

It is considered [1,9,10] that AA contains electrolyte ions and that they modify the membrane surface. Anodic oxidations of aluminium are performed at relatively high current densities, 50-300 A / m 2, which correspond to effec- tive current densities on the bottom of the pores of 750-4500 A / m 2, are accompanied by additional processes. In the case of oxalic electrolyte, the formation of the ion-radical C20 2-" and then glyoxylic acid, HCOCO2H, was established [20 ]. As a result, the initial AA may be modified not only by C20 2- and HC204-- ions, but also by ion- radical C202-" and HC20~- ions and possibly others.

Fig. 4 shows the variation in IR spectra of AA membranes with temperature of calcination. Ini-

100

~,4

3- -~

i I I I I I 1 4 - ~ - 3600 3200 2800 2400

WAVENUMBER (CM -1 )

0 4000 2000

100

\ l i I . > / i I i 0 4000 3600 3200 2800 240( 2000

WAVENUMBER (CM 1 )

Fig. 4. IR spectra o f in i t ia l anodi¢ a lumina ( 1 ) and after cal- c inat ion at ]50 (2 ) ,450 (3) , 600 (4) , 700 (5) , 810 (6) , 830 (7) and 840°C (8) . T ime o f calcination was 0.5 h.

tially the assignment of the main bands is: the intensive narrow band at 2330 cm-~ and wider band at 2360-2320 cm- ~ can be attributed to the v3 vibrations of CO2 molecules [21]; at 2280 cm-1 to the v3 vibrations of 13CO2 molecules [21 ]; CO molecules absorb at 2130 and 2035 cm- 1 [ 21 ]; the broad asymmetric band between 3700 and 2500 cm- ~ is undoubtedly due to water molecules and OH groups; the two narrow bands at 3610 and 3715 cm -~ can be attributed to the composite vibration of CO2 molecules and Fermi diad, V3+2V°/V3+Vl[22]; the wide band at about 2700 and 3100 cm-1 can probably be assigned to the first overtone of the vibrational fre- quencies of carboxyl groups, -COO- , which absorb at 1600-1400 cm -1 [23] (this part of spectrum is not shown).

Completely unexpected results are the formation of such a large quantity of CO2 molecules and the presence of CO2 in the alumina structure at much higher than expected temperatures, Fig. 4, spectra 1-7. The position of the v3 (CO2) band indicates weak interaction of CO2 molecules with aluminium atoms. The CO2 molecules, bonded to the strongest cationic Lewis acid sites, absorb at 2407 and 2370 cm- ~ and can be desorbed at a temperature less than 200°C, [24]. It is obvious, that in this study, the energy of bonding of CO2 molecules and AI atoms cannot be respon- sible for such high thermostability of CO2 in AA membranes. Thus, the CO2 molecules which are localized in the AA lattice, are retained by mo- lecular traps [25 ] and/or inside the locked micropores, and cannot be desorbed for steric rea- sons. Partial desorption of CO2 from the AA lattice is possible only at the temperature of the transformation of the oxygen sub-lattice during the crystaUisation of amorphous to polycrystalline AA, Fig. 4, spectrum 8.

New information regarding the transformation of impurities during the calcination of AA is obtained from the appearance and the change in intensity of the CO bands at 2130 and 2035 c m - 1. The first band can be assigned to CO ad- sorbed via dispersion forces (the CO molecules bonded to cationic Lewis acid sites with bonding energy 60-20 kJ/mol absorb at 2240-2165 cm- ~ ) [ 26 ]. The second band can be assigned


to CO bonded to All + because at wavenumbers essentially less than the stretching vibrations of free CO molecules (2142.16 cm-I) , the ab- sorbed molecules are bonded to reduced metal atoms or ions [21 ].

The yellowish coloration of the initial AA films formed in oxalic acid electrolyte has been explained by the formation of C202- radicals produced by intramolecular charge transfer between the low valency aluminium ion, Al I +, and oxalato ligand within the complex [ 20 ]:

All + (C20~- ) ~A13+ (C2042- ")

Probably, at >/700 ° C the reverse process - the reduction of some of the A13+ ions to All + _ may take place. The adjacent C2042-" radicals, electrolyte ions or the products of their transformation, for example, CO molecules, act as reducing agents.

Additional experiments with thermotreatment of amorphous AA membranes in vacuum have shown that the absence (or presence) of the oxidising agent (O2) in the calcination atmo- sphere can only change the degree, but not the nature, of the reduction-oxidation process within the AA lattice. The major reduction of the intensity of the bands at 3800-3000 cm-i after vacuum treatment at 200 °C have also indicated that the majority of protons within the initial AA membranes are in the H20 molecules (not OH groups bonded to A1 ions). These molecules were incorporated into the alumina lattice during the anodisation.

Thus, on the basis of the IR spectroscopy data, Fig. 4, it follows that thermotreatment of the initial amorphous AA membranes at temperatures up to 830°C results in a series of reduction-oxidation processes within the alumina lattice. In these processes, both the electrolyte ions (or products of their conversion, for example, glyoxylic acid ions, CO molecules, etc.) and the adjacent aluminium ions take part. The possible chemical reactions which take place in these processes may be:

C2 O2- --~CO 2 + C O + O 2-

HC20~- --~CO 2 + C O + O H -

CO~CO2+C

All (C2024 - ) --,A11 + (C2024 - )

HC20~- +A13+ +O2- ~CO2 +Al l+ + O H -

C O + A i 3+ + O2----~CO2 + A l l+

The last three reactions show the possible ways of obtaining A11 +. The slightly black (grey) col- our of the AA membranes calcined at 820-830 °C indicates the formation of elementary carbon.

3.6. Composition of the impurities in polycrystalline AA membranes

In agreement with the quantitative data which have been discussed above, Fig. 1D, the initial stage of the transition of amorphous to polycrys- taUine AA membranes is the formation of OH groups without changing the IR spectrum of the CO2 molecules, Fig. 4, spectrum 7, and -COO- ions ( 1600-1400 cm- l ). Then, at the comple- tion of the transition, the content of CO2 molecules is sharply decreased, Fig. 4, spectrum 8. Also the spectrum of the carboxyl groups is transformed to the spectrum of carbonate ions with bands at 1575, 1525, 1450 and 1400 cm -l (this part of the spectrum is not shown) and the stretching intensity of free and bound OH groups reaches a maximum value. Increasing the calcination temperatures up to 1050°C does not essentially change the IR spectra of the AA membranes. Carbon remained within the polycrystaUine AA lattice as CO2 molecules and carbonate ions.

In accordance with the variation of the quantity of carbon impurities with calcination temperature, Fig. 1D, the absorbance of CO2 molecules (~2340 cm - l ) and CO 2- ions (1600- 1400 cm-~ ) in the IR spectra of AA almost completely disappear only after the transformation of alumina to c~-A1203.

It was shown in separate experiments that the concentration of CO2 in polycrystalline AA cannot be removed by vacuum treatment of alumina at temperatures up to 950°C. Thus, CO2 molecules are trapped by both amorphous and polycrystalline AA lattices.

Similar immobility of CO2, trapped in alu-


mina structures, has been observed for alumina prepared by calcination of alumina aerogels, made in the presence of ethanol. These CO2 molecules could not diffuse out after heating such an aerogel in air or oxygen up to 600°C [27 ].

Additional experiments with treatments ofpo- lycrystalline AA membranes at temperatures >1 800°C in reductive (HE) and oxidative (02) atmospheres have shown the unique possibility both for the reduction of all CO2 to elementary carbon (membranes became black, the band of CO2 in the IR spectrum disappears) and the oxidation of C to CO2 again. The quantity of carbon in AA is not significantly altered by the number of such reversible treatments. This phe- nomenon can be explained by the permeability of the alumina lattice at high temperature for small molecules, such as H2, 02 and H20, but other possibilities may exist for such transformations which require lattice participation.

Thus, the very high thermostability of carbon impurities (CO2 and CO 2- ) incorporated into the polycrystalline AA lattice indicates that such impurities should not exert direct influence on' the chemical and electrochemical properties of the polycrystalline AA membranes. Neverthe- less, the ability of CO2 to take part in the reversible reduction at high temperatures may be an important factor in applications using polycrystalline AA membranes or multilayer systems based on AA at such temperatures (for example, high temperature gas separation; catalysis for high temperature reactions). These data also show that polycrystalline AA is an unique model system for the investigation of high temperature diffusion phenomena in oxide lattices.

3. 7. Hydroxyl cover of the polycrystalline AA membranes

It is well known [ 28 ] that the structure of the hydroxyl cover (surface hydroxyl groups) of alumina, as well as of other oxides, determines the chemical properties of the oxide surface. This is the major influence in determining the surface properties of the membrane and it is this which also controls the possibility for surface modification. Most of the alumina surface models [ 29-

31 ] are based on IR studies of surface hydroxyl groups. There is much experimental data on the IR spectra of the surface OH groups of alumina (especially y-A1203 ) but no data regarding polycrystalline AA. Thus comparative IR investigations of OH groups of in polycrystalline AA and y- and &A1203 have been carded out.

The stretching free OH vibrations are observed at 3800-3600 cm-~. In the case of polycrystalline AA this range of lR spectra is difficult to analyse because of the Fermi diad (bands at 3715 and 3610 cm- l ). The isotopic exchange of OH to OD by D2 at evacuation temperatures ( ~ 700°C) was carried out, Fig. 5. The spectra were smoothed and differentiated according to a Savitzky-Golay method. The optimum parame- ters of smoothing and differentiating digital fil- ters were chosen on the basis of generalized Gans-Gill method [ 32 ].

The wavenumbers of the stretching OH vibration can be obtained from OD and the value of the isotopic coefficient, k= v(OH)/v(OD). It was shown, for free, or isolated (without hydrogen bonds) OH groups k~ 1.355 [33]. Accord- ing to the general approach for the interpretation of IR spectra of surface OH groups [ 30,31,34 ] the frequency of an isolated (free) OH group is determined first of all by the number of A1 atoms bound to it. The bands in the high frequency region [v(OD)>_-2770 cm -I, v(OH)>_-3750 c m - ~ ] are attributed to the most electronegative OH groups of the first type (OHI); the intensive bands at 2770-2745 cm -~ (3750-3720 cm -~) to OH groups of the second type (OHn); and absorbance at v(OD) ~<2745 cm -~ [v(OH) 43720 c m - 1 ] to the least electronegative OH groups of the third type (OH In ) ."

H H H

O O , / \ / ? \

A1 A1 A1 A1 A1 AI

OH I OH n OH III

The additional fine-structure of the spectra, Fig. 5, is due to the influence of the coordination number of the aluminium ions (as the intensity

P.P. Mardilovich et aL I Journal of Membrane Science 98 (1995) 131-142 139

"'°' t A

o l d _ . . ,:.',., ,a ] , ".. : I~ : ".-9 ' i ; ." ' ""~

,,, " i j i ..J "' =o~ --> I ~o o ! .. r " '~ ~ ¢:,

" / ~ ^ . i ~ w F ~i! o / N v

z / , , , , ,

2,820 2,790 2,760 2,730 2,700 2,670 -1

WAVENUMBER, CM

0.6

B

t ~

I I I I I I

2,820 2,790 2,760 2,730 2,700 2,670

WAVENUMBER, CM -1

0.5

oaH ki

I I I I I I

2,820 2,790 2,760 2,730 2,700 2,670 -1

WAVENUMBER, CM

Fig. 5. IR spectra of surface OD groups of anodic alumina calcined at 850 ° C ( A ), 7- (B) and ~-AIzO3 ( C ) after isotopic exchange by D2 and evacuation at 700°C.

of the spectra at wavenumbers less than 2690 cm-1 is very low, the position of the individual bands is not indicated for the "second deriva- tive" spectra). The coordination number of aluminium on the alumina surface at different stages of dehydroxylation can vary from 3 to 6. These problems have been discussed in detail [ 31 ].

The similarities between the spectra of polycrystalline AA and 7-A1203 are shown in Fig. 5. The main difference is a lower intensity for electronegative OH ~ groups. This is usual for alumina calcined at 850°C (AA-850 has ~ 10% 6- A1203, Fig. 1A).

Thus, on the basis of these results it follows that the chemical properties of the polycrystalline AA surface are similar to those of the corresponding alumina modifications of the boehmite series.

3.8. Permeability of amorphous AA

The permeability of amorphous AA membranes (AA films without barrier layer) is very high due to the peculiarities of the AA pore structure. Thus, gas (N2) permeability of the AA membrane, 40 gm thickness with a mean pore diameter 64 nm and porosity ~ 14%, is 1.5 cm s-~ a tm- i . The permeability of a salt solution (aqueous 0.01 M KC1) for the same membrane is 2.136)< 10 -4 cm s-L Here we will discuss the

questions of the permeability of AA films with barrier layer in place.

It has been stated that water may permeate the matrix of AA films [35] and Smith [36] noted that AA films with the barrier layer in place have a low permeability to salt and water. In contrast to these conclusions it has been reported that such membranes are totally impermeable to hydrogen gas and that water cannot permeate the thin bar- tier layer [ 37 ].

To resolve these contradictions a series of AA films with barrier layer thickness 5, 10, 30 and 90 nm were prepared. Some of these films were calcined at temperatures up to 800 ° C.

Salt permeabilities (aqueous 0.01 M KC1) of AA films were measured using very sensitive concentration oscillator techniques [ 16 ].

For all thicknesses of the barrier layer, the AA films were completely impermeable, in agreement with Itaya et al. [37 ], and these films remained impermeable even after heat treatment to 800°C. It was concluded that this impermea- bility is a feature of the amorphous AA films with the barrier layer in place.

The apparent contradiction between these re- suits and the conclusion of O'Sullivan et al. [ 35 ] may be resolved by consideration of the AA cell model. It is known, that a cell of AA film has a heterogeneous chemical composition [ 38-40 ]. There are two main layers, Fig. 6: relatively pure


i.i

: ii 3

A1

Fig. 6. Diagrammatic representation of the cell structure of an anodic alumina film, formed in oxalic acid, with barrier layer in place [38]: a, cell; b, pore; c, anion-contaminated alumina; and d, relatively pure alumina.

alumina (d) and anion-contaminated oxide (c). For the oxalic acid electrolyte, the ratio of the thicknesses of these layers is ~ 0.1 [ 38 ]. In Part II, [ 12 ] it was estimated as 0.12. The region of electrolyte-anion-contaminated amorphous oxide (c) is permeable, in agreement with [35]. Water and salt may permeate this region, probably along the same channels (or tracks) as the electrolyte ions migrated during the anodisation process. In support of this hypothesis it was observed that after treatment by water at 100 ° C for 5 rain such AA films absorb (without substantial hydration of alumina) ~ 7% H20 by weight into the AA matrix. On the other hand, the thin relatively pure alumina barrier layers (d) are impermeable. Because there is such a layer between the adjacent cells, there should be no permeation between adjacent pores.

4. Conclusions

In this paper the influence ofthermotreatment on the composition and other properties of AA membranes are described. AA films and membranes were prepared in electrolytes based on oxalic acid. Their mechanical and chemical properties were investigated. During heating the fate of oxalate residues trapped in the amorphous alumina were determined using IR spectroscopy.

It has been concluded that the thin layer of relatively pure and dense alumina of the AA cell prevents migration of ions, water and small molecules through the barrier layer as well as between the adjacent cells. The AA films with the barrier layer in place remained impermeable to all molecules even when the thickness of the barrier layer wad decreased down to 5 nm and/or when the AA films had been previously heated to 800°C.

It has been shown that at temperatures above 840°C with careful control of the heating process, amorphous AA films or membranes can be transformed into polycrystalline modifications without destruction. After the initial transformation, to almost pure ~-A1203, further phase changes (of the AA membranes) with increasing temperature occur which are similar to those of boehmite. The loss of ionic impurities and the variation of the BET surface areas are reported as functions of the crystallisation process. It has been established that the quantity of impurities incorporated into the AA lattice may be decreased only at the temperatures of the oxygen sub-lattice transformations: at 830-840°C (transition of amorphous to polycrystalline AA) and at 1075-1150°C (transformation from cubic to hexagonal close-packed lattice). A series of reduction-oxidation reactions is shown take place during the calcination of AA. In these processes, the electrolyte ions, the products of their conversion and adjacent aluminium ions take part. The final products of such conversions (of the impurities) are CO2 molecules and CO32- ions which are incorporated into the polycrystalline AA lattice. An unique thermostability of CO2 molecules trapped by the alumina lattice and the ability of these CO2 molecules to take part in reversible reduction-oxidation cycles at temperatures f> 800°C were found.

It has been shown that the hydroxyl cover, which determines the chemical properties of the polycrystalline AA membrane surfaces, is similar to that of the alumina modifications of the boehmite series.

In subsequent papers it will be proposed that under carefully controlled conditions polycrystalline films can be the precursors of a new gen-


eration of multilayer and composite polycrystalline AA membranes. The greatly enhanced chemical stability of polycrystalline AA membranes against acid and base attack is reported in Part II [ 12 ].

Acknowledgements

We are grateful to the Science and Engineering Research Council (UK) for the award of a Vis- iting Senior Research Fellowship to Dr. P.P. Mardilovich at the University of Glasgow, (Grant No. GR/H20374) .

The authors also wish to express their thanks to Miss N.I. Mazurenko for her help in the preparation of the AA membranes.

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New and modified anodic alumina membranes Part I. Thermotreatment of anodic alumina membranes

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