INFLUENCE OF Sb203 ADDITION ON ELECTRIC PARAMETERS AND STABILITY OF ZnO VARISTORS
Grzegorz Pašciak, Witold Mielcarek, Krystyna Prociow, Electrotechnical Institute Wroclaw, Poland
PAPER PRESENTED AT MIDEM '99 CONFERENCE 13.10.99 - 15.10.99, Ljubljana, Slovenia
Keywords: ceramics, ZnO varistors, Zinc Oxide varsistors, electrical parameters, stability, Sb203, antimony oxides, additions
Abstract: Zinc oxide ceramics containing Bi203 and other metal oxides as additives exhibit a highly nonohmic property in current - voltage characteristics and is widely used for absorption of transient surges in electronic circuits. The nonlinearity effect is produced by potenial Schottky 's barriers which are partly caused by oxygen ions adsorbed on the ZnO grain surfaces. The passage of current during the varistor operation causes, among others, the desorption of oxygen ions from ZnO surfaces and its lost to the ambient. Oxygen ion desorption contributes to varistor degradation by creation in an intergranular layer a S-BiaOs phase characterised by great number of oxygen vacancies and high ionic conductivity. Decrease of BigOa ionic conductivity, being the main component of intergranular layer, is equivalent to decrease of degradation coefficient. It can be achieved by addition of varistor ceramics with Sb203. Antimony oxide can dissolve in 5-81203 or react with ZnO and O phases forming Zn7Sb20i2 spinel and amorphic BiaOs. What more it was found that SbaOs doping modifies fi-BiaOs phase in less conductive form. The compositions originally added with 0.3 and mol1%. modified S-BiaOa crystal lattice at the greatest rate. More antimony oxide caused that the amount of Sb203 dissolved in S-BiaOs decreased and more spinel particles appeared.
in stabilnost ZnO varistorjev
Ključne besede: keramika, ZnO varistorji cink oksidni, parametri električni, stabilnost, SbaOa oksidi antimona, dodatki
Izvleček: Cink oksidna keramika, ki vsebuje Bi203 in druge kovinske okside z dodatki ima skrajno nelinearno tokovno-napetostno karakteristiko in se uporablja za izdelavo elementov, ki ščitijo elektronske sestave pred prehodnimi napetostno-tokovnimi preobremenitvami. Nelinearnost karakteristike je posledica potencialnih Schottky zapor, ki jih deloma povzročajo kisikovi ioni adsorbirani na površino ZnO zrn. Električni tok. ki teče med delovanjem varistorja. povzroča med drugim desorbcijo kisikovih ionov s površine ZnO zrn v okolje. Desorbcija kisikovih ionov pomeni degradacijo varistorja, saj povzroča tvorbo faze 5-Bi203 v prostorih med zrni, ki jo karakterizira prisotnost velikega števila kisikovih vakanc in visoka ionska prevodnost. Zmanjšanje ionske prevodnosti Bi203, ki je glavna komponenta plasti med zrni, je ekvivalentno zmanjšanju koeficienta degradacije. Omenjeno lahko dosežemo z dodatkom SbaOs, Antimonov oksid se lahko raztopi v 5-Bi203 ali pa reagira z ZnO in O fazo ter tvori Zn7Sb20i2 spinel in amorfni Bi203. Dodatno je bilo ugotovljeno, da Sb203 zmanjša prevodnost <vBi203 faze. Dodatki s sestavo 0.3 in rnol1% so v največji meri spremenili kristalno strukturo 8-Bi203. Večje količine antimonovega oksida so povzročile manjšo topnost SbaOa v (VBi203 in povečanje števila spinelnih delcev.
INTRODUCTION
ZnO varistor ceramics consists of ZnO grains separated from each other by an intergranular layer formed by products of additives reactions between each other and with ZnO. The reactions in the process of varistor fabrication are well known /1,2/. Fundamental compo-sition of varistor ceramics constitute ZnO, spinel type and Bi-rich phases. The presence of Bi-rich phase is essential for non - ohmic property in varistor voltage - current characteristics.
During the sintering the starting low temperature a-BijOs type phase by interstep Bi2 {Zn4/3Sb2/3)06 pyro-chlore phase converts in spinel phase and Bi-liquid which, during cooling, crystallises in high temperature 5-Bi203 phase.
Bi2 (Zn4/3Sb2/3)06 + ZnO-
. Zn (Zn4/3Sb2/3)04 + Bi203 spinel liquid
Nonlinerity is related to potential barriers on ZnO grains boundaries. The height of these barriers depend on the
kind and amount of additives and oxygen ions. Oxygen ions adsorb on ZnO grains during cooling to the room temperature in the following sequence:
02^ 20^-^20^" temperature-^
It is said /3/that 10% of potential barriers is caused by adsorbed oxygen ions. The oxygen participation in formation of potential barriers is, in all probability, much greater because ZnO ceramics sintered in nitrogen atmosphere does not show varistor behaviour.
Oxygen ion adsorption benefits by oxygen vacancies of intergranular 5-Bi203 phase. Due to oxygen vacancies 5-Bi203 facilities oxygen transition from the air to ZnO grain borders. Anyway the high -temperature ö-Bi203 -phase exhibit an extremely high ionic conductivity which causes the desorption of oxygen ions and increase of leakage currents.
It is known /4,5/ that Bi203 ionic conductivity can be decreased by doping varistor ceramics with pentox-
ides, Pentoxides l\/l205 dissolved in S-BiaOs phase modify Bi203 lattice constants and reduce the oxygen vacancies according to the following expression:
Bi4(1-x) M4x06+4xD2-4x
whereG states for oxygen vacancy.
Since, in varistor ceramics, Sb203 converts into Sb205 the above is valid for antimony oxide as well. Dissolving in 5-Bi203 phase Sb203 modifies 5-Bi203 lattice constants and probably decreases 5-Bi203 ionic conductivity. Reduction of oxygen vacancies in 5-81203 phase in the vicinity of Sb203 hinders the oxygen desorption from ZnO grains and in this way retards the varistor degradation.
Fig.1:
0 0,3 OS 1 15 % tri Ol, SbjCb
The influence of 50203 addition on non-linearity coefficients ai -(0.01-0.1mA), a2-(0.1-1mA), c/3 -(1-IOmA)
EXPERIMENTAL PROCEDURE
For preparation of varistor samples the different: 0,3, 0,5,1,1.5,2, and 3 mo! % amount of Sb203 were added to a reagent grade ZnO powder. The other additives were kept at a stable ratio: 1 mol% Bi203, 0.8 mol% NiO, 0.5 mol% MnO, 0.4 mol% Cr203, 0.5 mol% C02O3.
Varistor samples were prepared by conventional wet-mixed-oxide ceramic technology. All mixtures were wet milling for 24 hours and dried. After addition of an organic binder and the pressing of the granulate to 15 mm disc, the green samples were sintered in an atmosphere of ambient air at 1250°C for 1 hour and then furnace cooled.
As -sintered samples have been electrically charater-ised for identifications of breakdown voltages Vi mA/mm, a coefficients and varistor degradation. To make electric contacts a thin layers of silver were evaporated onto the both surfaces of discs.
The crystal phases in sintered bodies were identified by X-ray powder diffractometer using CoK« Fe filtered radiation. Chemical composition of samples and micro-structure SEM observations were carried out using Electron Probe X-ray Jeol JXA 5a Microanalyzer,
RESULTS
The basic l-V characteristics of varistors were measured by dc current in range from 10 /iA up to 10 mA at room temperature.
The influence of Sb203 addition on non-linearity coefficients for three ranges of currents is presented in Fig.1. As can be seen the addition of Sb203 improves varistors non-linearity coefficients a. The mean non-linear exponents reached a steady - state value 50 but when 3% of Sb203 was added they significantly decreased. The suitable, from the point of view of non linearity property, is the addition of 0.3 - 1.5 mol% Sb203.
The correlation between the breakdown voltage and Sb203 is shown in Fig. 2. The breakdown voltage of varistor without antimony oxide was 80 VirnA/1mm and grew up to 470V for samples with 0.3 mol% Sb203 and did not change distinctively with further Sb203 addition.
250
200
150
100
50 0
% moL Sb,03
Fig. 2: Dependence of breal<down voltage l/imA/imm on 50203 addition.
Fig. 3: Varistors degradation characteristics after electrical degradation in bOfdA DC electric field at 115°C for 5 hours.
Varistors degradation characteristics after electrical degradation in 50 juA DC electric field at 115"C for 5 hours are presented in Fig. 3. As can be seen even small addition of Sb203 (0.3mol%) improved significantly varistor stability.
After 24 hours from disconnecting the degrading electrical field the ageing samples were measured electrically. The nonlinerity coefficient changes after ageing are shown in Fig. 4. As can be seen nonlinerity coefficients were polarity dependent and the changes of a
were asymmetrical. The electrical field mostly affected the pre-breakdown regions (ai and 0.2). At higher currents the polarity effect partly disappeared and samples with 0.3-2 mol% Sb203 had sufficiently good a coefficients before and after degradation.
Bbefore :after ageing. after ageing, ""ageing "polarisation + polarisation-
aj
9D «
3J 20 10
0,3 0,5 1 1,5 2 % mol. ajOj
¥
IM
0,.3 0,5 1 1,5 2 % rnol. Sbj 03
As can be seen from Fig. 5 the vicinity of Sb203 changed the positions of Bi203 characteristic peak what gives the evidence that Sb203 penetrates into 5 Bi203 structure and modifies its crystal lattice constants. Addition of 0.3 mol% Sb203 modified 5 Bi203 lattice constants at the greatest rate.
if/,, SbjO,
laoB

0, "			
0.5%			
	' ■ -		

33. ÜO f
Fig. 5: Position of the strongest 5 ß/203 diffraction peai< in varistor sampies doped with different amount of SbzOs.
0,5 1 1,5
% mol. SbjOj
Fig. 4: Dependence of noniineraiity coefficient a on 50203 addition before and after ageing.
From the results of X-ray powder diffraction measurements the following crystal phases in varistor ceramics were identified: ZnO, 5 Bi203 or mixture of 5-Bi203 and ß Bi203 (ß - X-ray diffraction peaks overlap on those of 5 -phase) and Zn7Sb20i2 i.e Zn(Zn4/3Sb2/3)04 spinel phase. As it was expected, the varistor compositions originally added with more antimony oxide had more spinel particles but unexpectedly X-ray intensities of 5 Bi203 characteristic peaks decreased. Although it is possible that Bi203 volatization to the ambient increased in Sb203 presence we put it on conversion of Bi203 crystal phases into glass amorphic one.
The quantity SEM measurements presented in Table 1 proved that only Sb dissolved in 5-Bi203 is able to modify 5-Bi203 lattice constants. The Sb dissolved in 5-Bi203 phase amounted to 0.6 wt.% in samples originally doped with 0.3 and 0.1mol% Sb203. These were also the samples in which Sb modified 8-Bi203 lattice constants. The further addition of Sb203 caused the Bi203 phase self - purification and the Sb amount dissolved in 5-Bi203 dropped to 0.1 wt.%. It can be inferred that at higher concentration Sb203 more readily comes into reactions with ZnO and forms spinel phase then dissolves in 8-Bi203.
Tabie 1: The quantity SEf\/i measurements of the
spinel and Sb contents in varistor samples.
j mol %Sb203							
in varistor	0.0	; 0.3	0.5	1.0	1.5	' 2 0	3.0
composition							
wt% Sb							
dissolved in	1 0.0	0.6	0.6	0.4	0.1	: 0.0	0.0
5-Bi203							i
spinel wt% in			i				
varistor after	: 0.0	2.2	2.6	2.8	3.5	; 3.4	3.6
sintering							
Elsewhere presented measurennents /6/ gave the evidence that modified 5-Bi203 phase had a desirable smaller ionic conductivity.
Carried out investigations fully proved the ability of modification of varistor electrical properties and stability by doping varistor ceramics with appropriate amount of SbaOa. It was found that addition of SbgOs modifies S-Bi203 phase in less conductive form. The compositions originally added with 0.3 and mol1%. modified 8-Bi203 crystal lattice at the greatest rate.
REFERENCES
/1/ J. Wong, "Microstructure and Phase Transformation in a Highly Nonohmic Metal Oxide Varistor Ceramics", J. Appl. Phys,, vol. 46, No 4 (1975), pp 1653
/2/ Z. Cymbrykiewicz, W. Mielcarek, "Effect of Mn02 Concentration on the Phase Relationship in ZnO, Bi203, Sb203 system", Xir" Conference on Applied Physics Cieszyn 1986, pp 165-67,
/3/ G.D. Mahan, "Intristic Defects in ZnO Varistors", J. Appl. Phys., vol. 54, No 7 (1983), pp 71-90
,'4/ T, Takasaki, H. Ivara, T. Esaka, "High Oxide Ion Conduction in Sintered Oxide of the System Bi203-M205", J. Electrochem. See., vol. 124, No 10 (1977), s. 1563-69.
/5/ M. Miyayama, S. Katsuta, Y, Suenga, H. Yanagida. "Electrical Conduction in 5-Bi203 doped with Sb203", J. Am. Ceram, Soc., vol. 66, No 8 (1983), pp 585-88,
/6/ G, Pasciak, PhD thesis. Technical University of Wroclaw, Wroclaw 1998
Grzegorz Pa šciak, Witold Mielcarek, Krystyna Prociow Electrotecfinical Institute, ul, M. Sktodowskiej Curie 55/61 50 369 Wroclaw, Poland, e-mail: rnielcar@iel. wroc.pl
Prispelo (Arrived): 15.10.99
Sprejeto (Accepted): 19.2.00