Volume 48A, number 3
17 June 1974
METAL ELECTRODE EFFECT ON SEALING WAX THERMOELECTRET CHARGE R. NATH Department of Physics, University of Saugar, SA GAR-4 70003, India Received 22 May 1974 Thermo-electrets of sealing wax were prepared using different’metal electrodes. The experimentally observed surface charge has been found to show a meaningful correlation with metal electrode work function.
Ever since the discovery of the first thermo-electret by Eguchi , the electret effect has been studied in a large number of materials using different techniques both from the point of view of obtaining electrets with optimum characteristics suitable for different practical applications and for understanding the mechanism of formation and storage of charge in electrets [2—5]. The phenomena of contact electrification and electret effect have generally been handled as different areas of investigation though they have strong similarities . The surface charge in an electret, which is retained for long time, is mainly due to internal volume polarisation of the dielectric. The charge appearing on the surface of the electret may also have contributions from the external polarisation including that of contact electrification. The purpose of the present investigation is to study the effect of the electrode on the surface charge characteristics of sealing wax thermo-electrets. The effect of the electrode material on the charge in the thermo-electrets has been studied by Thiessen et al. , Baldus , Bhatnagar , Pillai et al. . Sealing wax as used in the present investigation was found on x-ray analysis, to contain rosin, an organic amorphous material, and further BaSO4 and hO2 as inorganic crystalline materials . Six thermo-electrets of sealing wax were prepared using aluminium, zinc, lead, nickel, copper and tin as metal electrodes. Other parameters such as temperature of preparation (75°C),polarising field (15 ky/cm), polarising time (9h) and thickness (1 mm) constant for all thermoelectrets . The surface charge on the anode side of each thermo-electret was measured just after the expiry of the porarising time of nine hours, with a Lindemann electrometer using the apparatus and method described by Pillai and Jam . The initial surface charge was found to depend on the metal used as elec-
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IN SY —k
Fig. 1. Surface charge on sealing wax thermoelectrets with electrodes made of aluminium, zinc, lead, nickel, copper and tin. The charge is plotted against the work function of electrode material.
trode. Surface charge decay characteristics of these thermo-electrets  clearly showed that the charge becomes more or less constant after about twenty days as was observed in the case of thermo-electrets prepared for other studies [14, 15]. The finally retained surface charge was found to be different for different electrodes. The initial surface charges of these thermo-electrets 177
Volume 48A, number 3
were plotted against the work functions of the electrode metals (fig. 1). The graph shows that the observed surface charge is a hetero-charge (i.e. negative on anode surface) for metal electrodes with electron work function higher than 4 eV. For the lead electrode, having a work function of 3.94 eV, the initial surface charge is practically zero. The curve shows a linear relationship between the surface charge and the work function of the metal used as electrode. The exceptionally high value of charge in the case of tin metal electrode may be due to a slight error in time of measurement of mitially observed homocharge has a tendency to rise rapidly in these thermo-electrets. Pillai et al.  have studied the surface charge of shellac wax as a function of the work function using zirconium, tantalum, titanium, aluminium, gold, molybdenum and nickel as electrode materials and have found a linear dependence of surface charge on electrode work function. Inculet and Wituschek [161 have reported a correlation between surface charge and work function of electrode material while studying the charge obtained by using zirconium, copper, nickel and platinum on borosilicate glass as electrode materials. The results of the present investigation again show that the amount of negative charge carriers, supplied by the metal electrode and injected into the dielectric surface, increases with decrease in electron work function of the metal electrode. This brings out clearly the determining role the electrode material plays in the thermo-electret phenomena, specially in the formation of homocharge. The effects of the electrode metal can (qualitatively) be understood from a model recently described by Wilcox [171. This is a dielectric loss model based on tunneling of electrons from the metal electrode into traps located near the interface in the insulator.
17 June 1974
The author is thankful to Prof. D.R. Bhawalkar, Prof. J.D. Ranade and Dr. A.P. Srivastava for the encouragement given to him during the course of the preparation of this paper. References  M. Eguchi, Japan J. Appi. Phys. 1(1922)10. 121 F. Gutman, Rev. Mod. Phys. 20(1948) 457. [31 V.A. Johnson, Electrets A state of the Art Survey
I, (p. 48); Pt. II A bibliography (p. 124) office of Technical Service, US Department of Commerce Publications AD. 299259 & A.D. 299256 (1962).  V.M. Fridkin and IS. Zheludev, Photo-electrets and electro-photographic process, Consultants Bureau, New York (1960).  RK.C. Pillai, K. Jam, Phys. Stat. Sol. (a) 13 (1972) 341; and V.K. Jam.  P.S.H. 1-lenery, Proc. Inst. of Physics and the Physical Society, London, (1967) on Static Electrification ed. A.C. Sticklandp. 111.  P.A. Thiessen, A. Winkel and K. Hermann, Phys. Z. 37 (1936) 511. [81W.Baldus, Z. Angew Phys. 6 (1954) 481. [9) C.S. Bhatnagar, Ph. D. thesis, University of Saugar, Sagar, (India) (1956). [101 P.K,C. Pillai, K. Jam, Physics Letters 35A (1971) 403.  R. Nath and D.R. Bhawalkar, Indian J. of Pure and Appi. Phys. 9(1971)10.  R. Nath and D.R. Bhawalkar, Czechoslovak J. of Phys. Sec. B to be published.  P.K.C. Pillai and V.K. Jam, J. Phys. D. App!. Phys. 3 (1970) 829.  R. Nath and D.R. Bhaealker, Phys. Stat. So!. (a) 8 (1971) 581.  R. Nath and D.R. Bhawalkar, Acta. Phys. Polon. A43 487.and E.P. Wituschek, Proc. Inst. Physics and  (1973) I.!. lnculet Physical Society London (1967) on Static Electrification ed. A.C. Stickland p. 37.  P. Wilcox, Canad. J. of Phys. 50(1972) 912.