B. LOVRE~EK and 0, KORELI~ Institute of Electrochemistry and Electrochemical Technology, Faculty of Technology, University of Zagreb, Zagreb, Savska cesta 16/I, Yugoslavia Abstract-The barrier Iayer was investigated on aluminium by measuring impedance of the system ahuniniumjbarrier-layer/electrolyte-solution. The bridge method was used in the frequency range from 50 c/s to 50 kc/s. The results indicate the complexity of the barrier layer as dielectric. The analysis of the complex dielectric constant indicates a well-expressed influence of relaxation phenomena. A possible relaxation process of dipoles of water connected to the anodic oxide layer is suggested. ReLes auteurs examinent la couche ban-i&e SLUl’al uminium en mesurant I’imp&iance. Les mesurages, dam le domaine de la fr+uence de 50 Hz 1150 kHz, ant tti exeCut&s& l’aide du pant d’impedances. Les r&ultats dbmontrent ia complexit de la couche barriere quant B son caract&re dic%ctrique. L’analyse de la complexe con&ante diblectrique demontre l’influence essentielle des phtiom&nes de rblaxation. Outre autres possibilitbs, les auteurs sugg&rent aussi le possible pro&s de la relaxation des dipales de l’eau li& A la couche anodique d’oxyde. Zussammefassung-Durch Messung der Impedanz des Systems Ahuninium/Sperrschicht/Elektrolyt wurde die Sperrschicht an Ahuniniurn untersucht. Die Btickenmethode wurde im Frequenzbereich von 50 Hz bis 50 kHz verwendet. Die Ergebnisse betonten die komplexe Natur der Sperrschicht als Dielektrikum. Die Analysen der komplexen dielektrischen Konstante weisen auf einen stark aus-
gepregtenEnfluss der Relaxation. Dabei ist ein mijglicherRelaxationseffektder Wasserdipole, die mit der anodischen Oxidschicht verbundensind, angedeutet.
OXIDE layers on aluminium have been of interest both from a practical as well as from a theoretical point of view, causing numerous investigations. While earlier papers were concentrated mainly on the study of their practical physical and chemical properties in order to enable their most rational application, more recent papers have been more and more directed to the fundamental studies of the mechanism of their formation and transport of matter and charge through these systems. Anodic barrier oxide layers, being formed by anodic oxidation of aluminium in solutions of boric or tartaric acid and some other electrolytes, have been of special interest with their asymmetric conductivity and interesting dielectric properties. In connexion with this, some papers have been published pointing out the complexity and electric non-homogeneity of barrier oxide layers on aluminium. Among such investigations a special place belongs to the paper by Scholte and van Gee1.l They investigated the system aluminiuml barrier-layer/electrolyte-solution by ac of different frequencies, and obtained the characteristic dependence of impedance on frequency, for which they applied the theory of layer dielectric. Similar results were obtained by KoreliC et aZ.2 on the system aluminiumlbarrier-layer/solid conductor. This theory was later extended by Young3 and broadened to include other metals covered by oxide layers. Another approach was given by Winkel and de Groot4 and also in a later paper by Young6 suggesting the possibility of relaxation phenomena as responsible for observed behaviour. EXPERIMENTAL Investigations
of the system
electrolyte-solution. A foil of super-purity aluminium (99-98 per cent) was used, and the oxide layer was formed by anodic oxidation in a solution of boric acid. The same * Manuscript received19 January 1970. 569
solutions were also used in impedance measurements. Impedance was measured using a Wien bridge of the usual type (as, in the paper by LovreEek and MetikoP). Sinusoidal ac of 10 mV in the frequency range from 50 c/s to 50 kc/s was used. RESULTS
This paper is a detail of a broader programme on investigation of dielectric properties of barrier layers on aluminium. Results relate to samples (geometric surface area of 2 cm”) oxidized anodically in O-5 M solution of boric acid, the pH of which was adjusted with ammonia to 5.6. The formation voltage was 100 V dc, and the current Similar results were also obtained (in the blocking direction) was about O-2 mA/cm2. in the vicinity of this pH, though in a still wider range a change of basic dielectric characteristics takes place. Results of measurements are given in Fig. 1, which shows series components of impedance & and C, in dependence on frequency. There is a strong dependence of
FfG. 1. Dependence of C, and Rs on frequency.
C, on frequency in the sense that with the increase of frequency their values decrease, first abruptly and then slowly.
The results indicate an evident complexity of the system, and were therefore subjected to a detailed analysis. The usual approach, in which results are given as l/CJogfand &/l/f(Figs. 2 and 3) and a treatment according to Scholte and van Geel,l and by Young3 leads to difficulties, especiahy as, according to Young, linear relations are to be expected. However Winkel and de Groot4 and YoungS have discussed the possibility of the influence of relaxation processes with a wide range of relaxation times on the dielectric behaviour of oxide layers.* We have made an analysis of the complex dielectric constant E*, representing it as its two components a’ and a”. The calculation was performed in the * Young
in his book’ gives a comprehensive survey of this subject.
Investigationof barrier layers on aluminium
FIN. 2. Dependence of l/C. on logf.
5x12 I /f,
FIG. 3. Dep&dence of R, on l/fusual way. R, was corrected for the resistance of solution. Further, for determination of the thickness of dielectric when calculating CO(the capacitance of a condenser of the same size, but with vacuum instead of dielectric) the relation after [email protected]
was used, d=14U
where d is the thickness of the oxide layer in A,and U the formation voltage of barrier oxide layer. Results of this analysis are given in Fig. 4, where E’, 8” and tan 6 are graphically represented in dependence on log o. The well-expressed inffexion in the E’ curve and the well-expressed maximum in the E” curve, in the frequency range investigated, point to the presence of relaxation phenomena. This is still better seen in the socalled Cole-Cole diagram (E” as a function of E’), given in Fig. 5. Almost all points lie 5
FIG. 4. Dependence of d, 6” and tan 8 on log o.
diagram for system Al/barrier-layer/H,BO*
solution (pH 5.6).
on a semi-circle, thus indicating a fairly sharp relaxation time 7. The value for 7, 2 x 106 s, is obtained from our experimental data for the above mentioned case. Consequently, the dielectric behaviour of the investigated system can be explained by supposing relaxation phenomena with a very narrow range of relaxation time. The physical nature of the relaxation process has been a subject of further studies, wherein special attention is paid to the possibility of existence of relaxation of water dipoles connected with the anodic layer. REFERENCES 1. J. W. A. SCHOLTEand W. CH. VAN GIZZL,Phi&s Res. Rep. 8, 47 (1953). 2. 0. KOREXJC, B. L.~~RE&EIc,V. SINOV&VI& and T. NIKOUC, Ahminio 31, 79 (1962). 3. L. YOUNG, Trans. Faraday Sot. 51,125O (1955). 4. P. W~VKELand D. G. DE GROOT, P/liir;Os Res. Rep. 13, 489 (1958). 5. L. Yornro. TrarcsFaraday Sot. 55, 842 (1959). 6. B. LOVR&I%Cand M. MEXUCO&Corros. Scf. 8,437 (1968). 7. L. YOUNG, Anodic Oxicie F%Is. Academic Press, New York (1961). 8. G. %s, J. opt. Sot. Am. 38, 532 (1949).