ELSEVIEK

April 1994

Mater&Letters 19 (1994) 159-164

Experimental survey of dopant ions in ZnO: nonlinearity and degradation

Agnks Smith, David S. Smith, Pierre Abelard Laboratoire de Matkriaux Gramiques et de Traitement des Surfaces, URA CNRS 320, ENSCI,

47-73, Avenue Albert Thomas, 87065 Limoges Cedex, France

Received 2 December 1993; accepted 20 January 1994

Abstract

The addition of 0.1 at% Bi to ZnO yields a ceramic with nonlinear current-voltage behaviour (I= KY) characterized by a maximum (Y value of 5. The effect of further addition of 0.1 at% of a 3d transition element was tested through the series SC, Ti, V, Cr, Mn, Fe, Co, Ni or Cu. This low level of doping does not provoke any marked variation in the microstructure. Mn followed by Cu gave the most significant enhancement to nonlinearity achieving (Y = 50 and cr = 30 respectively. The strong performances due to Mn doping are counterbalanced by degradation of the electrical behaviour under repeated high-voltage cycles whereas the samples doped with other 3d elements were much stable. This is related to the capability of Mn to adopt several oxidation states.

1. Introduction

The nonlinear current-voltage characteristic of a ZnO varistor is exploited for protection against volt- age surges. An important aspect of such technological applications is the degradation behaviour under elec- tric stresses and a number of interesting studies have recently been published on this subject [ l-9 1. From a major review by Philipp and Levinson [ lo], it can

be noted that (i) degradation involves a marked in- crease in low-voltage leakage conduction but the breakdown region (high (.u) is essentially unaltered and (ii) degradation is a more general phenomenon since the degraded characteristics can be achieved by not only electrical stress but also through pressure, thermal treatment in specific atmospheres, and mi- nor modifications in the chemical compositions. Do- pants such as Na and K enhance varistor stability which Gupta et al. [ 91 explains by blocking of field- assisted zinc interstitial migration out of the grain boundary regions. Other studies [ 1 1 - 13 ] established

the important role of oxygen and the fact that com- mercial varistors of dense ceramic are sealed hermet- ically. The object of the present communication is to report the action of dopant transition ions, within a simplified zinc oxide varistor host (Zn-Bi-0), for the electrical behaviour and any subsequent evolu- tion under Z-V cycling. Though present throughout the ceramic, the principal effect of the ions should be at the interfaces (grain boundaries). Previous work [ 14 ] has shown that even small dopant levels of bis- muth, manganese and cobalt have very strong effects on the electrical behaviour assigned to grain bound- aries. The ceramic samples in this work are also somewhat porous, promoting exchange through the solid-gas interface, an experimental situation which should be carefully distinguished from commercial varistors.

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160 A. Smith et a!. /Maternal Letters I9 (1994) 159-164

2. Ex~~rnen~

The doped zinc oxide powders were prepared us- ing the dissolution-recrystallization technique de- rived from Pechini’s patent [ 1.5 ] to minimize the to- tal impurity content. Large amounts of dopant (more than 1 at%) can induce differences in the microstruc- ture, complicating the analysis. Consequently, given that small quantities of dopants are sufficient, the studied compositions contained 0.1 atoA of bismuth and 0.1 atoh of a transition element (SC, Ti, V, Cr, Mn, Fe, Co, Ni or Cu). The powders were pressed into disks (thickness: 1 mm; diameter: 10 mm) and sintered for 1 h at 1050 ’ C. The sintered ceramics had a relative density of approximately 90% with grain sizes ranging from 2 to 10 pm (Fig. 1). No apparent variation was observed in the microstructure through the series of samples with different dopants.

The samples were coated with gold on each face, giving low resistance electrodes, and mounted in a spring loaded sample holder for electrical measure- ment. Electrical contact to each face of the sample

Fig. 1. Scanning electron micrograph of thermally etched 2110 surface for a composition containing 0.1 at% Bi and 0.1 at% Mn, sintered in air for 1 hat 1050°C (bar: 10 pm).

disc was made through a 1 cm thick brass block (di- ameter: 30 mm). The blocks have considerable ther- mal mass compared to the ceramic disc under test and will act as heat sinks in the case of sample heating. Two different types of voltage sweep were applied to the samples (Fig. 2 ). For the first type (Fig. 2a), a Keithley model 237 source-measure unit was used to apply successively increasing voltage pulses (linear stair pulse). The voltage step was 0.5 V. The pulse duration was 50 ms and the recovery time between two pulses was 2 s. With respect to the recovery time, for a physically reasonable thermal contact resistance of 10 K W-l between the disc and the brass blocks, the cooling time constant can be estimated as 1.5 s. The voltage sweep was stopped when a current den- sity of 10 mA/cm2 was reached. Another series of trials involved a continuously stepped voltage sweep (Fig. 2b ) , which was delivered by a Heinzinger power supply, without a recovery time between increments (sweep rate: 1.5 V/s). Effects due to Joule heating would be promoted by this second type of sweep. The behaviour of a sample during these two sets of trials was characterized essentially by following the evolu- tion of the direct current-voltage curve for succes-

(4

tb) Voltage

Fig. 2. Voltage sweeps: (a) linear stair pulse; (b) continuous voltage sweep ( V,, refers to the voltage for a current density of 10 mA/cmZ).

A. Smith et al. /Materials Letters 19 (1994) 159-164 161

sive voltage sweeps with a recovery time of 1 min be- tween each sweep. In addition, for some samples the current was measured as a function of time at a cho- sen constant voltage.

3. Results aud discussion

Fig. 3 shows the 1-V behaviour of different com- positions on the first sweep using the linear stair pulse (Fig. 2a). Apart from Ti-doped samples, nonlinear behaviour was observed for all the compositions even with small amounts of dopant. We conclude that po- tential barriers exist at the interfaces and their effec- tiveness is particularly marked for the manganese followed by the copper-doped samples. In contrast, the T&doped samples were more conducting and ohmic. The resistivity value was 1150 n cm and the effect of the original bismuth addition has been al- most completely removed by titanium. The applica- tion of successive voltage sweeps revealed another important difference. Apart from a small evolution between the first and second cycles, the I-V behav- iour for the compositions containing vanadium, chromium, iron, cobalt or nickel did not change with cycling (Figs. 3a and 3~). However, the composition with manganese exhibited a strong degradation with each succeeding sweep (Fig. 3b). The composition with copper gave somewhat conflicting behaviour. Some samples yielded degradation and some did not. Two types of explanation can be sought for the deg- radation of the Z-V behaviour of the Mn composi- tion: sample heating due to the Joule effect [ 6,7 1, or a chemical change in the material due to the high electric field.

Though an increase in sample temperature would certainly lead to a decrease in resistance, several ar- guments can be made against this explanation. Firstly, assuming homogeneous power dissipation and no heat loss from the ceramic during the short duration of the pulse, the temperature rise of the sample can be estimated using the volume heat capacity for ZnO of 3.3 J cms3 K-r. A pulse of 500 V mm-’ with a response of 10 mA cm-’ results in a maximum rise of 0.75 K. Secondly, very similar 1-V curves are ob- tained when the sample was cycled with the continu- ously stepped voltage sweep (Fig. 2b) despite the fact that heating should be more sibilant in this case.

The succeeding curves do reveal a cumulative effect, but the time interval between voltage sweeps of 1 min is much longer than the cooling time constant esti- mated as 1.5 s. Thirdly, the application of a constant voltage corresponds to a steady-state thermal situa- tion once the current response has stabilized. Fig. 4 shows the increase in current for voltages higher than 400 V applied to manganese-doped samples. Even the maximum dissipation of 0.1 W (529 V, 180 pA) only increases the sample by 1 K if a contact resistance of 10 K W-r is assumed.

Further information was obtained in an experi- ment on a degraded mangane~-Dodd sample. When the sample was put into a backing pressure vacuum (oxygen partial pressure: = 10m3 atm) under no ap plied electric field, little increase in resistance was obtained. It was only when air was allowed back into the chamber that the original resistance of the ce- ramic could be slowly restored (Fig. 5). The role of oxygen indicated by this particular experiment has already been demonstrated by Binesti et al. [ 11,121 and Stticki et al. [ 131 who attributed the degradation of a commercial varistor under dc electric stress to oxygen depletion at the grain boundaries. Manganese is often contained in commercial varistor composi- tions and clearly leads to strong nonlinear behaviour. But if manganese yields the highest potential bar- riers, these are also more susceptible to degrada- tion.Since the surface area for gas exchange at the solid-gas interface is significant in our materials, degradation occurs rapidly.

For the interpretation of these observables, it is useful to consider the state of the ceramic material during and after sintering. During sintering, dopant ions segregate to the grain boundaries which oxidize on cooling thus generating the potential barriers. The extent of oxidation determines the size of the barrier. At high temperature ( IOSOC), manganese and cop- per are probably in a reduced state as indicated by the free energy diagram as a function of temperature for the pure oxides [ 16,17 ] (Fig. 6 ). During cooling, these two elements can either oxidize or equivalently trap oxygen from the surrounding atmosphere, while the other dopant ions would not do so. In fact, if we refer to the free energy diagram as a function of tem- perature and examine it along the isobaric curve for an oxygen partial pressure of 0.21 atm (air, Fig. 61, manganese exhibits three oxidation states between

162 A. Smith et al. /Materials Letters 19 11994) 159-164

-2 10

30

c

IO-=

>c

-4 10

20

-5 10

-6 IO

10 -7

10

-8 10

30

DENSITY (A/c&

20

Fig. 3. I- Vand IY- Vcharacteristics for the compositions with the following dopants: (a) Bi: 0.1 at% Bi; Bi-Se: 0.1 at% Bi at 0.1 at% SC; Bi-V: 0. I at% Bi and 0.1 at% V, Bi-Cr: 0.1 at% Bi and 0.1 at% Cr. (b) Bi-Mm 0.1 at% Bi and 0.1 at% Mn (the number refers to the order of the sweep). (c) Bi-Fe: 0.1 at% Bi and 0.1 at% Fe; Bi-Co: 0.1 at% Bi and 0.1 at% Co; Bi-Ni: 0.1 at% Bi and 0.1 at% Ni; Bi-Cu: 0.1 at% Bi and 0.1 at% Cu.

A. Smith et al. /Materials Letters 19 (1994) 159-164 163

1 412 (3 5)1A)

Fig. 4. Evolution of the current, 2, compared to the initial cur- Fig. 6. Free energy (per mole of 0,) as a function of temperature rent, I,, as a function of time and under steady voltage conditions for different 3d transition elements. (-): isobaric curve corre- (the number in brackets nearby each voltage value corresponds sponding to atmospheric pressure (oxygen partial pressure: 0.21 to the end point for I). atm).

f R ~xloQohms1

! 20 -

10 f- /I

0 5 10 15

Fig. 5. Resistance of ZnO doped with 0.1 at% Bi and 0.1 at% Mn as a function of time, under reduced atmosphere followed by air.

room temperature and 1100°C. Compared to the other dopant tmnsition ions, the Mn4+/Mn3+ tran- sition can be identified as energetically the easiest. Copper, with the Cu*+ /Cu+ transition, can be placed as second using this criteria. For each of the other do- pant elements used in this study, either thermody- namic calculations [ 16,17 ] or the equilibrium phase diagram (case of cobalt [ 18 ] ) indicate only one pos- sible oxidation state in the temperature range of ther- mal treatment in air. Though Fig. 6 refers to pure ox- ides and not doped ZnO, it gives useful insight for explaining the difference in behaviour between man- ganese, copper and the other dopant ions.

0 LOO 800 1200

TEMPERATURE i “Cf

4. Conclusion

In conclusion, amongst the 3d transition ions, manganese followed by copper give large potential barriers. On the other hand, these large potential bar- riers are more susceptible to de~a~tion, which will be enhanced by exchange at the solid-gas interfaces.

5. Acknowledgement

The authors would like to thank Professor Jean- Pierre Bonnet for valuable comments and discussion on the paper.

6. References

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