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Low voltage varistors based on SrTiO3 ceramics
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering B33 (1995) 58-66
B
L o w voltage varistors based on SrTiO 3 ceramics T.R.N. Kutty, Sam Philip Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 25 January 1994
Abstract Varistors exhibiting nonlinear resistance at low voltages of 4.5 to 30 V mm J have been realised from SrTiO~ ceramics. These are processed from powders obtained from the gel-crystallite conversion technique. Doping with ~ 1% Y~+ enhances the average grain size to over 50/am, when sintered around 1650 K in static air. On annealing the ceramics, in N 2+ H 2 atmospheres, they acquire a low resistivity of 0.4 to 3.7 ff~ cm with positive temperature coefficient of resistance. They are painted with low melting point oxide mixtures of PbO + Bi203 + B203 and re-annealed. The energy dispersive X-ray results indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting point oxide constituents. The abnormally high dielectric constants, ey = 104 to 105, point to the prevalence of GBL capacitance in these ceramics. Depending upon the conditions of the second annealing, the breakdown voltage could be adjusted from 0.2 to 1.5 V per grain boundary, without any change in the grain size. The nonlinearity coefficient a ranges from 6 to 15 and the barrier height from 0.15 to 0.3 eV. This can be explained on the basis of variable pinning of traps at the interface and also the extent of trap filling.
Keywords: Grain boundaries; Schottky barriers; Ceramics; Electrical measurements
1. Introduction
Varistors are known for their application as surge protection devices in power systems and also in electronic circuits. Ceramics such as SiC, TiO2 or ZnO exhibit voltage limiting current-voltage (I-V) characteristics because of their nonlinear resistivity (varistor property). Varistors based on ZnO ceramics have the highest nonlinearity coefficient and better stability under applied electric fields. Varistors based on perovskite titanate ceramics are known in the literature [1,2]. Higher energy absorbing capabilities of these varistors at current densities greater than 104 A cm-2 arises from the fact that the titanate ceramics have higher effective dielectric constants than ZnO. Varistors based on SrTiO 3 are produced by reducing the ceramics at elevated temperatures, followed by controlled re-oxidation [3]. Nonlinearity coefficients of less than 20 and breakdown voltages varying from 0.05 to 1.3 V per grain boundary are reported [3]. Semiconducting SrTiO3 ceramics exhibit high effective dielectric constants because of their special microstrucrural features, wherein the grain interiors are highly conducting and the grain boundary layers are insulat0921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5107(94)01205-2
ing. These are known as grain boundary or internal boundary layer capacitors, some of which exhibit nonlinear resistivity. Varistors based on donor doped BaTiO 3 have been reported earlier [4,5]. More recently there has been considerable demand for varistors exhibiting nonlinearity in the low voltage region of 6-12 V (battery operating region). Attempts have been made to obtain varistors working in this voltage range from ZnO ceramics by growth of larger grains, greater than 150/~m [6]. The seeding technique adopted for this purpose involves multistep processes and is often found to be irreproducible because of the impurity effect hindering the grain growth. Alternative materials such as TiO 2 have been attempted for low voltage varistors [7], but they are found to be more difficult to stabilize. SrTiO3 varistors are more stable for low voltage applications. However, their sintering temperatures are very high (greater than 1800 K) when powders prepared by conventional ceramic routes are used. We found that SrTiO 3 powders obtained through the gel-crystallite conversion technique [8] are most suitable. Controlled impurity concentrations as well as grain size can be achieved, during sintering below 1650 K. The details are presented below.
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MaterialsScience and EngineeringB33 (1995)58-66
2. Experimental procedures 2.1. Powder preparation Ultrafine (nanometre sized) powders of SrTiO3 were prepared by the isothermal gel to crystal conversion technique reported elsewhere [8]. Coarse gels of hydrated titania (TiO2.xH20 where x = 80-135), free of anionic contaminants were reacted with an aqueous solution of Sr(OH) 2 under boiling conditions which directly gave rise to SrTiO 3 having the cubic perovskite structure. No special care was taken to control the gel size, while precipitating from aqueous titanium (IV) chloride or nitrate solutions, using ammonium hydroxide (aqueous). The method is simple and cost effective for scaled up preparation, in contrast to that involving the alkoxides. At the same time, contamination from the alkali ions is avoided, unlike the product derived from the conventional ageing technique. The hydrated titania gel was suspended in Sr(OH)2 solutions, taken in a flask fitted with a water cooled reflux condenser. Air in the flask was displaced with nitrogen and entry of atmospheric CO2 was prevented by the use of an alkali-guard tube. The ratio of Sr:Ti was varied from 0.98 to 1.1. The effective concentration of Sr(OH)2 was altered from 0.05 to 0.7 M. The reaction was carried out at 373 K for 3 - 4 h with constant stirring. The solid remaining in the reaction vessel was filtered, washed free of excess Sr(OH)2 and oven dried. The resulting solid phase was X-ray crystalline. The phase purity was further ascertained by chemical analysis. La 3+, Bi 3+ and y3+ were used as donor dopants in the form of soluble salts added to the aqueous titanium chloride before precipitation of the gel according to the composition 97.7%SrTiO3+ 1%D + 1.33% TiO2 (by mol, where D = La, Bi or Y ). During precipitation, the stoichiometry was maintained in such a way that there was excess amount of Ti (less than 1.33 mol.%). Hence the addition of extra TiO2 during the formation of the ceramics was not necessary.
59
side. The polished discs were reduced in a flowing atmosphere of 90%N 2 + 10%H 2 at 1270-1525 K for 2 h. Oxides with low melting points were prepared from a mixture of PbO + Bi203 + B203 in the proportion 50:40:10 (wt.%). This mixture was painted on the top surfaces of the reduced ceramic discs, which were then subjected to a second annealing process at a temperature between 1200 and 1520 K for 15 min to 2 h in static air. The samples were withdrawn to the colder region of the furnace after the desired duration of annealing and finally cooled to room temprature in air. Ohmic metal contacts were deposited on these discs, after polishing, by the electroless Ni coating method. The coatings were recrystallised at 570 K and were further thickened by electroplating with Ni. 2.3. Characterisation techniques Phase purity of the ceramic powders was determined by X-ray powder diffraction (XRD) using a Philips PW 1050/70 powder diffractometer. The particle size was evaluated using a JEOL 200 CX transmission electron microscope. Microstructural studies of polished and etched, or fractured surfaces were carried out using a Cambridge S-360 scanning electron microscope. This equipment was fitted with a fully automatic energy dispersive X-ray analyser (EDX) (LINK 10000). The grain size was determined from the micrographs by the linear intercept method. The I - V characteristics were determined in the d.c. mode at room temperature. The resistivity, the relative dielectric constant, and the dielectric loss tangent were measured with GenRad 1640 automatic bridge at 1 kHz and field strength less than 2 V mm-~ during which the sample temperature was varied from 250-450 K. The capacitance-voltage relations were obtained using the same bridge and an external power source.
3. Results
2.2. Ceramic processing
3.1. Characterisation of SrTiO~ powders
The fine powders after mixing with an organic binder were pelletised at 150-200 MPa so that green ceramics of 60% relative density were obtained. "[he binder was subsequently burnt off at 1150 K in air. The pellets were sintered in the temperature range 1670-1680 K for 4 - 8 h in static air and cooled at the rate of 100 K h-t. Sintering aids such as the ATS mixture (1A1203 + 1TiO2 + 2SIO2) were used in some cases in low concentrations (less than 2 wt.%). The ceramic discs were polished after sintering so that layers of 8-10 ktm thickness were removed from either
The X R D patterns show the formation of single phase powders of SrTiO 3 from the gel-crystallite conversion reactions. There is no change in phase contents with the heat treatment of these powders. The ultrafine particle sizes of the products are evident from the XRD pattern by way of line broadening (Fig. 1). No rutile reflections were observed after annealing the powders at high temperatures, indicating the absence of unreacted TiO 2. On heat treatment, a weight loss of 2 % - 4 % takes place around 6 2 5 - 7 0 0 K. This is accompanied by the decrease in cell parameter and diminish-
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Materials Science and Engineering B33 (1995) 58-66
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Fig. 1. XRD patterns of SrTiO 3 samples: (a) as-prepared; (b) annealed at 1250 K; and (c) annealed at 1520 K; taken with Cu K a radiation.
(a)
ing line widths in the X R D patterns. The traces of the front reflections of the as-prepared, annealed at 1250 K or at 1520 K samples are shown in Fig. 1. There is a gradual decrease in line widths and gain in peak heights with heat treatment. The unit cell dimension a decreases from 3.932 A for the oven dried sample, to 3.921 A at 1250 K and 3.912 A at 1520 K for annealed samples. The decrease in unit cell dimensions accompanied by the weight loss is indicative of the fact that there are substitutional hydroxyl ions present in the as-prepared powders. The transmission electron microscopy (TEM) studies indicate that the powder particles were of nanometre size (Fig. 2). They are approximately spherical in shape and are in the range 20-40 nm. The electron diffraction patterns are spotty, indicative of the single-crystalline nature of the particles. However, one particle alone cannot be brought on to the path of the electron beam so that the diffraction pattern originates from multiple crystallites (Fig. 2(b)). Powders prepared from low concentrations of Sr(OH)2 (less than 0.05 M) exhibit a spotty pattern superimposed on a polycrystalline ring pattern (Fig. 2(c)) indicative of much finer particles.
3.2. Ceramic microstructure
Scanning electron microscopy (SEM) investigations of the sintered ceramics revealed the influence of donor dopants and also of the sintering temperature on the grain growth behaviour of SrTiO3. The ceramics doped with La 3÷ or Bi 3+ have a grain size in the range of 2-5/~m (Fig. 3(a)). The grain size increases with the
Fig. 2. TEM photographs of SrTiO 3 powders: (a) Morphology of the as-prepared powder; (b) electron diffraction pattern of powder prepared at high Sr(OH)2 concentration (0.7 M); and (c) electron diffraction pattern of powder prepared at low Sr(OH)2 concentration (0.05 M).
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Materials Science and Engineering B33 (1995) 58-66
Fig. 3. SEM images of SrTiO 3 ceramics sintered at 1650 K for 4 h in static air: (a) sample doped with 0.8% Bi3÷ and (b) doped with 0.8% Y~+.
temperature of sintering. In comparison, the grain sizes of y3 + doped samples are much larger; the average value is around 50/~m for ceramics containing 0.8% y3+ when sintered at 1650 K for 4 h (Fig. 3(b)). Sintering the ceramics at higher temperatures and for longer durations did not correspondingly enhance the grain size in y3 + doped specimens. Addition of extra TiO 2 was found to have no perceptible influence on the grain size. The presence of ATS mixtures also does not increase the grain size when compared to the Y addition. Processing of larger grain sized ceramics at temperatures less than 1 6 5 0 K is characteristic of SrTiO3:Y 3÷ powder obtained through gel-crystallite conversion, whereas SrTiO 3 powder containing y3+ prepared from the oxalate precursor, as well as the
61
conventional ceramic routes, exhibited poor grain growth (less than 2/~m) when sintered under the same conditions. The reported temperature for sintering of SrTiO 3 ceramic is greater than 1800 K, mostly in reducing atmosphere [3,9,10] which is incompatible with the furnaces, having silicon carbide heating elements. Therefore the processing of SrTiO 3 using powders from gel-crystallite conversion provides a number of advantages in relation to practical implementation. In the present investigations, further ceramic fabrication was restricted to Y containing compositions sintered at 1650 K for 4 h. The sintered density of such ceramics reached up to around 95% of the single crystal value. Ceramic discs reduced in N 2 + H 2 at 1270-1525 K were painted with low melting point oxide mixtures of PbO + B203 + Bi203. These were then subjected to a second annealing in static air in the temperature range 1200-1520 K. Quantitative experiments have shown that the weight gain, after the second annealing, ranges from 0.5%-0.9%. The SEM images of these ceramics are shown in Fig. 4. The observations revealed that the painted oxide mixture has penetrated through the grain boundary as is evident from the contrast in intensity of the secondary electron images from the grain boundary layers, in comparison with those from the grain interiors. However, the grains from the top regions of the painted discs, after the second annealing, exhibit domain features (Fig. 4). These domains may be connected with ferroelectricity, which in turn, is indicative of the non-cubic nature of such grains. This can be expected as a result of Pb 2÷ diffusion into the grain interiors thereby forming the (Sr, Pb)TiO 3 solid solution. However, the domain features are not uniformly observed throughout the same disc. They are absent for the grains from the deeper portions, i.e. away from the painted surface of the ceramic. Domain features are absent on fractured surfaces unlike those observed on thermally etched surfaces. The E D X investigations of the grain interiors and also the grain boundaries have been carried out for various ceramic samples. The grain interiors from fractured surfaces do not show the presence of Bi or Pb (Fig. 5(a)), whereas the grain boundary layers indicate Pb as well as Bi in addition to Sr and Ti (Fig. 5(b)). However, Pb, but not Bi, is observed in the grain interiors from the top regions of the ceramic discs (Fig. 5(c)). The preferential diffusion of Pb 2÷ in comparison with Bi 3÷ into the grain interiors is therefore evident. The grain boundary layers of the top regions of the ceramics (Fig. 5(d)) have the same E D X spectrum as in Fig. 5(b). However, the Pb content of the grain interiors adjoining the same boundary may differ as in the case of Fig. 4(d). This is also reflected in the differences in domain features on the same micrograph.
62
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Materials Science and Engineering B33 (1995) 58-66
Fig. 4. SEM images of the ceramic discs after the second annealing with PbO + Bi203 + B203 painted on the surface: (a) an overall picture; (b) enlarged view of the triple point showing the contrast in secondary electron image between the grain boundary and grain interior; (c) ferroelectric domain patterns on the grain; and (d) grain boundary filled completely with the insulating layer.
3.3. Electrical characteristics 3.3.1. Effect of reduction on electrical resistivity T h e nonlinear electrical characteristics of SrTiO 3 are greatly dependent upon the initial conductivity. This, in turn, is found to vary with the extent of reduction, i.e. the level of Po~ in the surrounding atmosphere, the temperature and duration of reduction as well as the impurity content, particularly, transition metal ions. T h e electrical resistance of the reduced discs decreases with increasing temperature as well as with the duration of reduction. T h e d.c. resistivity of such specimens at the measuring temperatures are shown in Fig. 6. T h e resistivities of the unreduced ceramics are greater than 10 ~° if2 cm throughout the measuring range (not shown in the figure). T h e ceramic samples reduced below 1250 K showed a negative temperature
coefficient of resistance ( N T C R or activated behaviour) typical for semiconductor materials. T h e activation energy for electrical conduction decreases with the extent of reduction (from 0.8 eV to 0.07 eV). W h e n reduced above 1250 K, a positive temperature coefficient of resistance (PTCR or metal-like behaviour) is observed. P T C R behaviour is prominent when samples are quenched from reducing conditions. T h e metal-like behaviour is dominant for ceramics with larger grain size (greater than 25 ~m). Even at higher reduction temperatures, fine grained ceramic specimens show N T C R behaviour. T h e resistivity of the present ceramic samples ranged from 0.4 to 3.75 if2 cm (at 300 K) as the reduction temperature is raised from 1250 to 1520 K. Higher nonlinear coefficients in the I-Vrelations are observed in these samples in contrast to those with the N T C R behaviour. T h e activation
T.R.N. Kutty, S. Philip / MaterialsScience and Engineering B33 (1995) 58-66
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Fig. 6. D.c. resistivity vs. 1000/T (k) of SrTiO 3 ceramics: (a) fine grained (less than 3 #m) sample reduced at 1450 K and (b) coarse grained (= 50 #m) reduced at 1000 K (both showing NTCR behaviour); (c) to (f) samples reduced at 1270, 1370, 1470 and 1525 K respectively showing PTCR behaviour.
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Fig. 5. EDX spectra of SrTiO 3 ceramics after the second annealing: (a) grain interior from a fractured surface; (b) boundary layer of the same specimen; (c) grain interior of the top region; and (d) grain boundary layer.
energy for conduction of the metallic type (or P T C R ) samples ranges f r o m 0.01 to 0.02 eV with a negative sign. T h e metal-like b e h a v i o u r of reduced single crystals of SrTiO3 is k n o w n in literature [11,12]. H o w ever, the m e c h a n i s m of the changeover f r o m activated ( N T C R ) to metal-like (PTCR) behaviour is not clearly understood. This changeover is related to the defects frozen-in from high t e m p e r a t u r e s [13].
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3.3.1 Dielectric properties after second annealing T h e effective dielectric constant is calculated as
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where C is the m e a s u r e d capacitance; d is the thickness of the ceramic specimen; A is the area and e 0 is the dielectric permittivity of vacuum, e 7 as well as the tan d values depend u p o n the t e m p e r a t u r e and the duration of the second annealing subsequent to painting with the low melting point oxide mixture. T h e value of ey increases f r o m = 1 x 1 0 4 tO = 2 x 105 by increasing the t e m p e r a t u r e of the second annealing f r o m 1270 to 1520 K. At higher t e m p e r a t u r e s e v decreases. Fig. 7 shows that the t e m p e r a t u r e coefficient of e:, is very low
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Fig. 7. (i) ey vs. T and (ii) tan 6 vs. T for SrTiO 3 ceramics after the second annealing at (a) 1520 K, (b) 1420 K and (c) 1320 K.
T.R.N. Kutty, S. Philip
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Materials Science and Engineering B33 (1995) 58-66 -I
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Fig. 8. Variation in (a) ey, (b) a and (c) VB (at 300 K) plotted against the second annealing temperature.
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( = - 1 0 0 p p m K -1) within the measuring range of 250-385 K. Above 385 K e~ increases with temperature. The dielectric loss ranges from 0.01 to 0.025 below 385 K. As the duration of the second annealing is increased, at a given temperature s7 as well as tan 6 decreases. The low values of the temperature coefficients in both these properties are achieved without the addition of SiO 2 or A1203 unlike those reported in the literature [9]. The dependence of e7 (300 K) on the temperature of the second annealing is shown in Fig. 8, having a maximum around 1530 K. This has been explained [9] on the basis of two types of layer prevailing at the grain boundaries: (a) the second phase layer has a chemical composition different from that of the grains; (b) the layers adjoining the second phase layer on either side have the same composition as that of the grain interiors. The thickness of the second phase layer decreases, whereas that of the diffusion layer becomes prominent, at higher temperatures of annealing. The present results show superior dielectric properties of the internal boundary layer capacitors based on SrTiO3 ceramics processed fiom powders prepared by the gel-crystallite conversion in comparison to those reported in the literature. 3.3.3. I - Vcharacteristics
The I- V curves of the ceramics are shown in Fig. 9. They exhibit highly nonlinear voltage limiting behaviour. In the voltage limiting regions, the I - V curves follow the equation [5]
i= kVo+ a( v - v~)/( v - v~)
(2)
where k is the proportionality constant; VB is the breakdown potential, 6 is the uniformity factor (6 = 0
2
3 4 5 6 8 10 20 30 4.0 50 FIELD STRENGTH (V/mm)
100
Fig. 9. Current density ( A c m 2) as a function of the field strength (Vmm -~) for samples annealed at (a) 1370K, (b) 1420 K,(c) 1470 K and (d) 1520 K.
for perfectly uniform samples and d > 1 for nonuniform samples) and a is the nonlinearity coefficient defined by the relation a = d(logl)/d(log V)
(3)
The value of a, calculated from the slopes of the curves from 1 to 100 mA, is in the range of 5-16. The leakage currents vary from 0.1 to 50 /~A and are lower than those reported for n-BaTiO3 based varistors [4,5]. Depending upon the processing conditions, the breakdown potential varied from 0.2 to 1.5 V per grain boundary. With an average grain size of 50/~m for the ceramics, this is equivalent to 4-30 V mm- J, suitable for battery operatable varistors. The breakdown voltage is dependent on the temperature of the second annealing (Fig. 8), which decreases considerably around 1420 K. The nonlinearity coefficient maximizes around the same temperature. Although a values decrease above 1420 K, the ceramics show stable varistor characteristics and they are more suitable for lower voltage applications. The slope of the I - V curves in the leakage current region is larger for the samples annealed at temperatures greater than 1520 K. Increasing the duration of annealing (greater than 45 min) decreases the a values and so also the leakage current. The height of the potential barrier ¢ at the grain boundary is evaluated from the voltage
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MaterialsScience and Engineering B33 (1995) 58-66
dependence of capacitance C which obeys the relation [14] ( 1 / C - 1/2C0) 2 : 2 ( ¢ + V/qNde~)
(4)
with 1/2 C o =(2fk/qNde~) L/2
(5)
where q is the charge on the electron, and N d is the charge carrier concentration. ~b and N d were determined from the slope as well as the intercepts of straight line plots of 1/C 2 vs. V. The barrier height varied from 0.1 to 0.3 eV whereas N d ranged from 5x1018 to 1.2x1019 cm 3. The d.c. resistivity (measured at 1 V mm -1) vs. 1 / T of the annealed ceramic has been plotted; the activation energy obtained from the slope varied from 0.3 to 0.5 eV. The thermal activation energies are therefore larger than those derived from the C- V relations.
4. Discussion
Grain growth in SrTiO3 ceramics to greater than 25 /~m is vital for achieving nonlinear resistivity at lower voltages. The development of microstructure greatly depends upon the raw material powders. The reported sintering temperature for SrTiO 3 ceramics is above 1800 K in reducing atmosphere. In contrast, SrTiO 3 powders derived by the gel-crystallite conversion show excellent densification characteristics accompanied by grain growth even without controlling the Po2. Usually, grain growth is influenced by impurities. Anionic impurities such as halides, sulphates and phosphates; not only impede grain growth, but also lead to exaggerated grain growth. The presence of alkaline earth ions (Mg 2+ or Ca 2+ ) also restricts grain growth to less than 2 ktm. Intentionally added donor impurities such as L a 3+ o r B i 3+ also limit grain size to less than 5 ktm, possibly due to the segregation at the grain boundaries thereby immobilizing the boundary migration. Smaller ionic size donors such a s y 3 + o r S b 5 + are dominantly cation vacancy compensated in the bulk, particularly when the concentration is around 1 mol.%; higher defect contents, thereby formed, enhance the mobility of the grain boundaries and aid grain growth. Proper choice of the raw material powder together with selected impurities is essential for achieving the desired microstructure in the ceramics. Influence of the second annealing temperature on the nonlinear I - V characteristics is demonstrated by Figs. 8 and 9. During this treatment the painted mixture of oxides melt and penetrate into the ceramic through the grain boundary layers rather than diffusing through the bulk grains. Selective melting reactions
65
between the liquid film and the solid SrTiO3 at t h e grain boundary regions lead to the formation of a second phase on cooling, which contains Pb, Bi and B (Fig. 5). As the temperature of the second annealing increases, this layer becomes thinner because of the progressive penetration of the liquid phase into the grain boundaries at deeper parts of the ceramics, i.e. away from the painted surface. Since the second annealing is carried out in air, formation of diffusion layers on either side of the second layer is envisaged [9]. Here the Sr vacancies are said to be migrating from the grain boundaries to the grain interiors, thereby thickening the diffusion layer which is insulating in character. In the present case, the breakdown voltage as well as the a values come down with enhanced temperature of second annealing. This cannot be accounted for in terms of the increasing thickness of the diffusion layer involving the Sr vacancy migration into the grain interiors. However, the formation of O vacancies and the corresponding donor states at higher temperatures have to be taken into consideration, although the annealing atmosphere is static air. Under such conditions, the O vacancies are formed at temperatures greater than 1450 K, rendering the grain boundary layer more conducting. At lower temperatures the O vacancies around the grain boundary region are eliminated because of the faster diffusion of O through the grain boundaries than through the grain interiors. Thus, cation vacancies dominate in the grain boundary region, so that they are more insulating. The O diffusion may be aided by the reactions involving the selective melting. The changes in a, VB and e~ with the second annealing temperature (Fig. 8) can be accounted for in terms of this shift in the dominance of the two types of defect formed at different temperature regions. As the donor concentration increases, the barrier height as well as the breakdown voltage diminishes, although the grain size has not altered during the second annealing. The maxima shown in Fig. 8 suggest that the geometrical factor dg/d~, i.e. the ratio of the grain size to the grain boundary layer thickness, is not the deciding factor in the case of nonlinear resistivity. The accumulation of Bi at the grain boundary is often said to be associated with the excess O in the case of ZnO varistors [15] which helps in pinning the interface traps, which, in turn, enhances the negative charges at the interface states [3,15,16]. However, in the present case, the interface trap state seems to be decreasing in concentration so that the barrier height as well as the breakdown voltage decreases with increase in temperature of the second annealing. Whether tunneling across the barrier [17,18] or the diminishing barrier height [15,16] under the applied potential is the cause for the nonlinearity in SrTiO3 varistors needs to be established by further detailed studies.
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Materials Science and Engineering B33 (1995) 58-66
5. Conclusions Varistors operating at battery voltages can be fabricated from SrTiO3 ceramics by incorporating a second phase layer at the grain boundaries through selective melting reactions. T h e specific electrical characteristics can be adjusted by the choice of the composition of the low melting point oxide mixtures, the semiconductivity of the bulk grains and the processing conditions. T h e nature of the charged defects at the grain boundary interfaces in SrTiO 3 ceramics and their trapping behaviour under the applied potentials are important factors in the nonlinear behaviour.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
References [1] N. Yamaoka, M. Masuyama and M. Fuki, Ceram. Bull., 62 (1983)698.
[15] [16] [17] [18]
N. Yamaoka, Ceram. Bull., 65(1986) 1149. Y. Nakano and N. Ichinose, J. Mater. Res., 5(1990) 2910. T.R.N. Kutty and V. Ravi, Appl. Phys. Lett., 59(1991) 2691. T.R.N. Kutty and V. Ravi, Mater. Sci. Eng., B20 (1993) 271. K. Eda, M. Inada and M. Matsuoka, J. Appl. Phys., 54 (1983) 1095. S. Yang and J.M. Wu, J. Am. Ceram. Soc., 76 (1993) 145. T.R.N. Kutty and P. Padmini, Mater. Res. Bull., 27 (1992) 945. R. Wernicke, Adv. Ceram., 1 (1981 ) 261. I. Burn and S. Neirman, J. Mater Sci., 17(1982) 3510. H.P.R. Frederikse, W.R. Thurber and W.P. Hosler, Phys. Rev., 136 (1964) 442. O.N. Tufte and P.W. Chapman, Phys. Rev., 155 (1967) 796. B.E Flandermeyer, A.K. Agarwal, H.U. Anderson and M.H. Nasarallah J. Mater. Sci., 19 (1984) 2593. K. Mukae, K. Tsuda and I. Nagasawa, J. Appl. Phys., 50 (1979) 4475. G. Blatter and F. Greuter, Phys. Rev. B., 34 ( 1986) 8555. G.E. Pike, Phys. Rev. B., 30(1984) 3274. K. Eda, J. Appl. Phys., 49(1978) 2964. G.D. Mahan, L.M. Levinson and H.R. Philipp, J. Appl. Phys., 50(1979) 2799.
Materials Science and Engineering B33 (1995) 58-66
B
L o w voltage varistors based on SrTiO 3 ceramics T.R.N. Kutty, Sam Philip Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 25 January 1994
Abstract Varistors exhibiting nonlinear resistance at low voltages of 4.5 to 30 V mm J have been realised from SrTiO~ ceramics. These are processed from powders obtained from the gel-crystallite conversion technique. Doping with ~ 1% Y~+ enhances the average grain size to over 50/am, when sintered around 1650 K in static air. On annealing the ceramics, in N 2+ H 2 atmospheres, they acquire a low resistivity of 0.4 to 3.7 ff~ cm with positive temperature coefficient of resistance. They are painted with low melting point oxide mixtures of PbO + Bi203 + B203 and re-annealed. The energy dispersive X-ray results indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting point oxide constituents. The abnormally high dielectric constants, ey = 104 to 105, point to the prevalence of GBL capacitance in these ceramics. Depending upon the conditions of the second annealing, the breakdown voltage could be adjusted from 0.2 to 1.5 V per grain boundary, without any change in the grain size. The nonlinearity coefficient a ranges from 6 to 15 and the barrier height from 0.15 to 0.3 eV. This can be explained on the basis of variable pinning of traps at the interface and also the extent of trap filling.
Keywords: Grain boundaries; Schottky barriers; Ceramics; Electrical measurements
1. Introduction
Varistors are known for their application as surge protection devices in power systems and also in electronic circuits. Ceramics such as SiC, TiO2 or ZnO exhibit voltage limiting current-voltage (I-V) characteristics because of their nonlinear resistivity (varistor property). Varistors based on ZnO ceramics have the highest nonlinearity coefficient and better stability under applied electric fields. Varistors based on perovskite titanate ceramics are known in the literature [1,2]. Higher energy absorbing capabilities of these varistors at current densities greater than 104 A cm-2 arises from the fact that the titanate ceramics have higher effective dielectric constants than ZnO. Varistors based on SrTiO 3 are produced by reducing the ceramics at elevated temperatures, followed by controlled re-oxidation [3]. Nonlinearity coefficients of less than 20 and breakdown voltages varying from 0.05 to 1.3 V per grain boundary are reported [3]. Semiconducting SrTiO3 ceramics exhibit high effective dielectric constants because of their special microstrucrural features, wherein the grain interiors are highly conducting and the grain boundary layers are insulat0921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5107(94)01205-2
ing. These are known as grain boundary or internal boundary layer capacitors, some of which exhibit nonlinear resistivity. Varistors based on donor doped BaTiO 3 have been reported earlier [4,5]. More recently there has been considerable demand for varistors exhibiting nonlinearity in the low voltage region of 6-12 V (battery operating region). Attempts have been made to obtain varistors working in this voltage range from ZnO ceramics by growth of larger grains, greater than 150/~m [6]. The seeding technique adopted for this purpose involves multistep processes and is often found to be irreproducible because of the impurity effect hindering the grain growth. Alternative materials such as TiO 2 have been attempted for low voltage varistors [7], but they are found to be more difficult to stabilize. SrTiO3 varistors are more stable for low voltage applications. However, their sintering temperatures are very high (greater than 1800 K) when powders prepared by conventional ceramic routes are used. We found that SrTiO 3 powders obtained through the gel-crystallite conversion technique [8] are most suitable. Controlled impurity concentrations as well as grain size can be achieved, during sintering below 1650 K. The details are presented below.
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2. Experimental procedures 2.1. Powder preparation Ultrafine (nanometre sized) powders of SrTiO3 were prepared by the isothermal gel to crystal conversion technique reported elsewhere [8]. Coarse gels of hydrated titania (TiO2.xH20 where x = 80-135), free of anionic contaminants were reacted with an aqueous solution of Sr(OH) 2 under boiling conditions which directly gave rise to SrTiO 3 having the cubic perovskite structure. No special care was taken to control the gel size, while precipitating from aqueous titanium (IV) chloride or nitrate solutions, using ammonium hydroxide (aqueous). The method is simple and cost effective for scaled up preparation, in contrast to that involving the alkoxides. At the same time, contamination from the alkali ions is avoided, unlike the product derived from the conventional ageing technique. The hydrated titania gel was suspended in Sr(OH)2 solutions, taken in a flask fitted with a water cooled reflux condenser. Air in the flask was displaced with nitrogen and entry of atmospheric CO2 was prevented by the use of an alkali-guard tube. The ratio of Sr:Ti was varied from 0.98 to 1.1. The effective concentration of Sr(OH)2 was altered from 0.05 to 0.7 M. The reaction was carried out at 373 K for 3 - 4 h with constant stirring. The solid remaining in the reaction vessel was filtered, washed free of excess Sr(OH)2 and oven dried. The resulting solid phase was X-ray crystalline. The phase purity was further ascertained by chemical analysis. La 3+, Bi 3+ and y3+ were used as donor dopants in the form of soluble salts added to the aqueous titanium chloride before precipitation of the gel according to the composition 97.7%SrTiO3+ 1%D + 1.33% TiO2 (by mol, where D = La, Bi or Y ). During precipitation, the stoichiometry was maintained in such a way that there was excess amount of Ti (less than 1.33 mol.%). Hence the addition of extra TiO2 during the formation of the ceramics was not necessary.
59
side. The polished discs were reduced in a flowing atmosphere of 90%N 2 + 10%H 2 at 1270-1525 K for 2 h. Oxides with low melting points were prepared from a mixture of PbO + Bi203 + B203 in the proportion 50:40:10 (wt.%). This mixture was painted on the top surfaces of the reduced ceramic discs, which were then subjected to a second annealing process at a temperature between 1200 and 1520 K for 15 min to 2 h in static air. The samples were withdrawn to the colder region of the furnace after the desired duration of annealing and finally cooled to room temprature in air. Ohmic metal contacts were deposited on these discs, after polishing, by the electroless Ni coating method. The coatings were recrystallised at 570 K and were further thickened by electroplating with Ni. 2.3. Characterisation techniques Phase purity of the ceramic powders was determined by X-ray powder diffraction (XRD) using a Philips PW 1050/70 powder diffractometer. The particle size was evaluated using a JEOL 200 CX transmission electron microscope. Microstructural studies of polished and etched, or fractured surfaces were carried out using a Cambridge S-360 scanning electron microscope. This equipment was fitted with a fully automatic energy dispersive X-ray analyser (EDX) (LINK 10000). The grain size was determined from the micrographs by the linear intercept method. The I - V characteristics were determined in the d.c. mode at room temperature. The resistivity, the relative dielectric constant, and the dielectric loss tangent were measured with GenRad 1640 automatic bridge at 1 kHz and field strength less than 2 V mm-~ during which the sample temperature was varied from 250-450 K. The capacitance-voltage relations were obtained using the same bridge and an external power source.
3. Results
2.2. Ceramic processing
3.1. Characterisation of SrTiO~ powders
The fine powders after mixing with an organic binder were pelletised at 150-200 MPa so that green ceramics of 60% relative density were obtained. "[he binder was subsequently burnt off at 1150 K in air. The pellets were sintered in the temperature range 1670-1680 K for 4 - 8 h in static air and cooled at the rate of 100 K h-t. Sintering aids such as the ATS mixture (1A1203 + 1TiO2 + 2SIO2) were used in some cases in low concentrations (less than 2 wt.%). The ceramic discs were polished after sintering so that layers of 8-10 ktm thickness were removed from either
The X R D patterns show the formation of single phase powders of SrTiO 3 from the gel-crystallite conversion reactions. There is no change in phase contents with the heat treatment of these powders. The ultrafine particle sizes of the products are evident from the XRD pattern by way of line broadening (Fig. 1). No rutile reflections were observed after annealing the powders at high temperatures, indicating the absence of unreacted TiO 2. On heat treatment, a weight loss of 2 % - 4 % takes place around 6 2 5 - 7 0 0 K. This is accompanied by the decrease in cell parameter and diminish-
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E
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60
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Fig. 1. XRD patterns of SrTiO 3 samples: (a) as-prepared; (b) annealed at 1250 K; and (c) annealed at 1520 K; taken with Cu K a radiation.
(a)
ing line widths in the X R D patterns. The traces of the front reflections of the as-prepared, annealed at 1250 K or at 1520 K samples are shown in Fig. 1. There is a gradual decrease in line widths and gain in peak heights with heat treatment. The unit cell dimension a decreases from 3.932 A for the oven dried sample, to 3.921 A at 1250 K and 3.912 A at 1520 K for annealed samples. The decrease in unit cell dimensions accompanied by the weight loss is indicative of the fact that there are substitutional hydroxyl ions present in the as-prepared powders. The transmission electron microscopy (TEM) studies indicate that the powder particles were of nanometre size (Fig. 2). They are approximately spherical in shape and are in the range 20-40 nm. The electron diffraction patterns are spotty, indicative of the single-crystalline nature of the particles. However, one particle alone cannot be brought on to the path of the electron beam so that the diffraction pattern originates from multiple crystallites (Fig. 2(b)). Powders prepared from low concentrations of Sr(OH)2 (less than 0.05 M) exhibit a spotty pattern superimposed on a polycrystalline ring pattern (Fig. 2(c)) indicative of much finer particles.
3.2. Ceramic microstructure
Scanning electron microscopy (SEM) investigations of the sintered ceramics revealed the influence of donor dopants and also of the sintering temperature on the grain growth behaviour of SrTiO3. The ceramics doped with La 3÷ or Bi 3+ have a grain size in the range of 2-5/~m (Fig. 3(a)). The grain size increases with the
Fig. 2. TEM photographs of SrTiO 3 powders: (a) Morphology of the as-prepared powder; (b) electron diffraction pattern of powder prepared at high Sr(OH)2 concentration (0.7 M); and (c) electron diffraction pattern of powder prepared at low Sr(OH)2 concentration (0.05 M).
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Fig. 3. SEM images of SrTiO 3 ceramics sintered at 1650 K for 4 h in static air: (a) sample doped with 0.8% Bi3÷ and (b) doped with 0.8% Y~+.
temperature of sintering. In comparison, the grain sizes of y3 + doped samples are much larger; the average value is around 50/~m for ceramics containing 0.8% y3+ when sintered at 1650 K for 4 h (Fig. 3(b)). Sintering the ceramics at higher temperatures and for longer durations did not correspondingly enhance the grain size in y3 + doped specimens. Addition of extra TiO 2 was found to have no perceptible influence on the grain size. The presence of ATS mixtures also does not increase the grain size when compared to the Y addition. Processing of larger grain sized ceramics at temperatures less than 1 6 5 0 K is characteristic of SrTiO3:Y 3÷ powder obtained through gel-crystallite conversion, whereas SrTiO 3 powder containing y3+ prepared from the oxalate precursor, as well as the
61
conventional ceramic routes, exhibited poor grain growth (less than 2/~m) when sintered under the same conditions. The reported temperature for sintering of SrTiO 3 ceramic is greater than 1800 K, mostly in reducing atmosphere [3,9,10] which is incompatible with the furnaces, having silicon carbide heating elements. Therefore the processing of SrTiO 3 using powders from gel-crystallite conversion provides a number of advantages in relation to practical implementation. In the present investigations, further ceramic fabrication was restricted to Y containing compositions sintered at 1650 K for 4 h. The sintered density of such ceramics reached up to around 95% of the single crystal value. Ceramic discs reduced in N 2 + H 2 at 1270-1525 K were painted with low melting point oxide mixtures of PbO + B203 + Bi203. These were then subjected to a second annealing in static air in the temperature range 1200-1520 K. Quantitative experiments have shown that the weight gain, after the second annealing, ranges from 0.5%-0.9%. The SEM images of these ceramics are shown in Fig. 4. The observations revealed that the painted oxide mixture has penetrated through the grain boundary as is evident from the contrast in intensity of the secondary electron images from the grain boundary layers, in comparison with those from the grain interiors. However, the grains from the top regions of the painted discs, after the second annealing, exhibit domain features (Fig. 4). These domains may be connected with ferroelectricity, which in turn, is indicative of the non-cubic nature of such grains. This can be expected as a result of Pb 2÷ diffusion into the grain interiors thereby forming the (Sr, Pb)TiO 3 solid solution. However, the domain features are not uniformly observed throughout the same disc. They are absent for the grains from the deeper portions, i.e. away from the painted surface of the ceramic. Domain features are absent on fractured surfaces unlike those observed on thermally etched surfaces. The E D X investigations of the grain interiors and also the grain boundaries have been carried out for various ceramic samples. The grain interiors from fractured surfaces do not show the presence of Bi or Pb (Fig. 5(a)), whereas the grain boundary layers indicate Pb as well as Bi in addition to Sr and Ti (Fig. 5(b)). However, Pb, but not Bi, is observed in the grain interiors from the top regions of the ceramic discs (Fig. 5(c)). The preferential diffusion of Pb 2÷ in comparison with Bi 3÷ into the grain interiors is therefore evident. The grain boundary layers of the top regions of the ceramics (Fig. 5(d)) have the same E D X spectrum as in Fig. 5(b). However, the Pb content of the grain interiors adjoining the same boundary may differ as in the case of Fig. 4(d). This is also reflected in the differences in domain features on the same micrograph.
62
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Materials Science and Engineering B33 (1995) 58-66
Fig. 4. SEM images of the ceramic discs after the second annealing with PbO + Bi203 + B203 painted on the surface: (a) an overall picture; (b) enlarged view of the triple point showing the contrast in secondary electron image between the grain boundary and grain interior; (c) ferroelectric domain patterns on the grain; and (d) grain boundary filled completely with the insulating layer.
3.3. Electrical characteristics 3.3.1. Effect of reduction on electrical resistivity T h e nonlinear electrical characteristics of SrTiO 3 are greatly dependent upon the initial conductivity. This, in turn, is found to vary with the extent of reduction, i.e. the level of Po~ in the surrounding atmosphere, the temperature and duration of reduction as well as the impurity content, particularly, transition metal ions. T h e electrical resistance of the reduced discs decreases with increasing temperature as well as with the duration of reduction. T h e d.c. resistivity of such specimens at the measuring temperatures are shown in Fig. 6. T h e resistivities of the unreduced ceramics are greater than 10 ~° if2 cm throughout the measuring range (not shown in the figure). T h e ceramic samples reduced below 1250 K showed a negative temperature
coefficient of resistance ( N T C R or activated behaviour) typical for semiconductor materials. T h e activation energy for electrical conduction decreases with the extent of reduction (from 0.8 eV to 0.07 eV). W h e n reduced above 1250 K, a positive temperature coefficient of resistance (PTCR or metal-like behaviour) is observed. P T C R behaviour is prominent when samples are quenched from reducing conditions. T h e metal-like behaviour is dominant for ceramics with larger grain size (greater than 25 ~m). Even at higher reduction temperatures, fine grained ceramic specimens show N T C R behaviour. T h e resistivity of the present ceramic samples ranged from 0.4 to 3.75 if2 cm (at 300 K) as the reduction temperature is raised from 1250 to 1520 K. Higher nonlinear coefficients in the I-Vrelations are observed in these samples in contrast to those with the N T C R behaviour. T h e activation
T.R.N. Kutty, S. Philip / MaterialsScience and Engineering B33 (1995) 58-66
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Fig. 6. D.c. resistivity vs. 1000/T (k) of SrTiO 3 ceramics: (a) fine grained (less than 3 #m) sample reduced at 1450 K and (b) coarse grained (= 50 #m) reduced at 1000 K (both showing NTCR behaviour); (c) to (f) samples reduced at 1270, 1370, 1470 and 1525 K respectively showing PTCR behaviour.
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Fig. 5. EDX spectra of SrTiO 3 ceramics after the second annealing: (a) grain interior from a fractured surface; (b) boundary layer of the same specimen; (c) grain interior of the top region; and (d) grain boundary layer.
energy for conduction of the metallic type (or P T C R ) samples ranges f r o m 0.01 to 0.02 eV with a negative sign. T h e metal-like b e h a v i o u r of reduced single crystals of SrTiO3 is k n o w n in literature [11,12]. H o w ever, the m e c h a n i s m of the changeover f r o m activated ( N T C R ) to metal-like (PTCR) behaviour is not clearly understood. This changeover is related to the defects frozen-in from high t e m p e r a t u r e s [13].
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3.3.1 Dielectric properties after second annealing T h e effective dielectric constant is calculated as
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where C is the m e a s u r e d capacitance; d is the thickness of the ceramic specimen; A is the area and e 0 is the dielectric permittivity of vacuum, e 7 as well as the tan d values depend u p o n the t e m p e r a t u r e and the duration of the second annealing subsequent to painting with the low melting point oxide mixture. T h e value of ey increases f r o m = 1 x 1 0 4 tO = 2 x 105 by increasing the t e m p e r a t u r e of the second annealing f r o m 1270 to 1520 K. At higher t e m p e r a t u r e s e v decreases. Fig. 7 shows that the t e m p e r a t u r e coefficient of e:, is very low
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Fig. 7. (i) ey vs. T and (ii) tan 6 vs. T for SrTiO 3 ceramics after the second annealing at (a) 1520 K, (b) 1420 K and (c) 1320 K.
T.R.N. Kutty, S. Philip
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Materials Science and Engineering B33 (1995) 58-66 -I
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Fig. 8. Variation in (a) ey, (b) a and (c) VB (at 300 K) plotted against the second annealing temperature.
-6
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( = - 1 0 0 p p m K -1) within the measuring range of 250-385 K. Above 385 K e~ increases with temperature. The dielectric loss ranges from 0.01 to 0.025 below 385 K. As the duration of the second annealing is increased, at a given temperature s7 as well as tan 6 decreases. The low values of the temperature coefficients in both these properties are achieved without the addition of SiO 2 or A1203 unlike those reported in the literature [9]. The dependence of e7 (300 K) on the temperature of the second annealing is shown in Fig. 8, having a maximum around 1530 K. This has been explained [9] on the basis of two types of layer prevailing at the grain boundaries: (a) the second phase layer has a chemical composition different from that of the grains; (b) the layers adjoining the second phase layer on either side have the same composition as that of the grain interiors. The thickness of the second phase layer decreases, whereas that of the diffusion layer becomes prominent, at higher temperatures of annealing. The present results show superior dielectric properties of the internal boundary layer capacitors based on SrTiO3 ceramics processed fiom powders prepared by the gel-crystallite conversion in comparison to those reported in the literature. 3.3.3. I - Vcharacteristics
The I- V curves of the ceramics are shown in Fig. 9. They exhibit highly nonlinear voltage limiting behaviour. In the voltage limiting regions, the I - V curves follow the equation [5]
i= kVo+ a( v - v~)/( v - v~)
(2)
where k is the proportionality constant; VB is the breakdown potential, 6 is the uniformity factor (6 = 0
2
3 4 5 6 8 10 20 30 4.0 50 FIELD STRENGTH (V/mm)
100
Fig. 9. Current density ( A c m 2) as a function of the field strength (Vmm -~) for samples annealed at (a) 1370K, (b) 1420 K,(c) 1470 K and (d) 1520 K.
for perfectly uniform samples and d > 1 for nonuniform samples) and a is the nonlinearity coefficient defined by the relation a = d(logl)/d(log V)
(3)
The value of a, calculated from the slopes of the curves from 1 to 100 mA, is in the range of 5-16. The leakage currents vary from 0.1 to 50 /~A and are lower than those reported for n-BaTiO3 based varistors [4,5]. Depending upon the processing conditions, the breakdown potential varied from 0.2 to 1.5 V per grain boundary. With an average grain size of 50/~m for the ceramics, this is equivalent to 4-30 V mm- J, suitable for battery operatable varistors. The breakdown voltage is dependent on the temperature of the second annealing (Fig. 8), which decreases considerably around 1420 K. The nonlinearity coefficient maximizes around the same temperature. Although a values decrease above 1420 K, the ceramics show stable varistor characteristics and they are more suitable for lower voltage applications. The slope of the I - V curves in the leakage current region is larger for the samples annealed at temperatures greater than 1520 K. Increasing the duration of annealing (greater than 45 min) decreases the a values and so also the leakage current. The height of the potential barrier ¢ at the grain boundary is evaluated from the voltage
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MaterialsScience and Engineering B33 (1995) 58-66
dependence of capacitance C which obeys the relation [14] ( 1 / C - 1/2C0) 2 : 2 ( ¢ + V/qNde~)
(4)
with 1/2 C o =(2fk/qNde~) L/2
(5)
where q is the charge on the electron, and N d is the charge carrier concentration. ~b and N d were determined from the slope as well as the intercepts of straight line plots of 1/C 2 vs. V. The barrier height varied from 0.1 to 0.3 eV whereas N d ranged from 5x1018 to 1.2x1019 cm 3. The d.c. resistivity (measured at 1 V mm -1) vs. 1 / T of the annealed ceramic has been plotted; the activation energy obtained from the slope varied from 0.3 to 0.5 eV. The thermal activation energies are therefore larger than those derived from the C- V relations.
4. Discussion
Grain growth in SrTiO3 ceramics to greater than 25 /~m is vital for achieving nonlinear resistivity at lower voltages. The development of microstructure greatly depends upon the raw material powders. The reported sintering temperature for SrTiO 3 ceramics is above 1800 K in reducing atmosphere. In contrast, SrTiO 3 powders derived by the gel-crystallite conversion show excellent densification characteristics accompanied by grain growth even without controlling the Po2. Usually, grain growth is influenced by impurities. Anionic impurities such as halides, sulphates and phosphates; not only impede grain growth, but also lead to exaggerated grain growth. The presence of alkaline earth ions (Mg 2+ or Ca 2+ ) also restricts grain growth to less than 2 ktm. Intentionally added donor impurities such as L a 3+ o r B i 3+ also limit grain size to less than 5 ktm, possibly due to the segregation at the grain boundaries thereby immobilizing the boundary migration. Smaller ionic size donors such a s y 3 + o r S b 5 + are dominantly cation vacancy compensated in the bulk, particularly when the concentration is around 1 mol.%; higher defect contents, thereby formed, enhance the mobility of the grain boundaries and aid grain growth. Proper choice of the raw material powder together with selected impurities is essential for achieving the desired microstructure in the ceramics. Influence of the second annealing temperature on the nonlinear I - V characteristics is demonstrated by Figs. 8 and 9. During this treatment the painted mixture of oxides melt and penetrate into the ceramic through the grain boundary layers rather than diffusing through the bulk grains. Selective melting reactions
65
between the liquid film and the solid SrTiO3 at t h e grain boundary regions lead to the formation of a second phase on cooling, which contains Pb, Bi and B (Fig. 5). As the temperature of the second annealing increases, this layer becomes thinner because of the progressive penetration of the liquid phase into the grain boundaries at deeper parts of the ceramics, i.e. away from the painted surface. Since the second annealing is carried out in air, formation of diffusion layers on either side of the second layer is envisaged [9]. Here the Sr vacancies are said to be migrating from the grain boundaries to the grain interiors, thereby thickening the diffusion layer which is insulating in character. In the present case, the breakdown voltage as well as the a values come down with enhanced temperature of second annealing. This cannot be accounted for in terms of the increasing thickness of the diffusion layer involving the Sr vacancy migration into the grain interiors. However, the formation of O vacancies and the corresponding donor states at higher temperatures have to be taken into consideration, although the annealing atmosphere is static air. Under such conditions, the O vacancies are formed at temperatures greater than 1450 K, rendering the grain boundary layer more conducting. At lower temperatures the O vacancies around the grain boundary region are eliminated because of the faster diffusion of O through the grain boundaries than through the grain interiors. Thus, cation vacancies dominate in the grain boundary region, so that they are more insulating. The O diffusion may be aided by the reactions involving the selective melting. The changes in a, VB and e~ with the second annealing temperature (Fig. 8) can be accounted for in terms of this shift in the dominance of the two types of defect formed at different temperature regions. As the donor concentration increases, the barrier height as well as the breakdown voltage diminishes, although the grain size has not altered during the second annealing. The maxima shown in Fig. 8 suggest that the geometrical factor dg/d~, i.e. the ratio of the grain size to the grain boundary layer thickness, is not the deciding factor in the case of nonlinear resistivity. The accumulation of Bi at the grain boundary is often said to be associated with the excess O in the case of ZnO varistors [15] which helps in pinning the interface traps, which, in turn, enhances the negative charges at the interface states [3,15,16]. However, in the present case, the interface trap state seems to be decreasing in concentration so that the barrier height as well as the breakdown voltage decreases with increase in temperature of the second annealing. Whether tunneling across the barrier [17,18] or the diminishing barrier height [15,16] under the applied potential is the cause for the nonlinearity in SrTiO3 varistors needs to be established by further detailed studies.
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Materials Science and Engineering B33 (1995) 58-66
5. Conclusions Varistors operating at battery voltages can be fabricated from SrTiO3 ceramics by incorporating a second phase layer at the grain boundaries through selective melting reactions. T h e specific electrical characteristics can be adjusted by the choice of the composition of the low melting point oxide mixtures, the semiconductivity of the bulk grains and the processing conditions. T h e nature of the charged defects at the grain boundary interfaces in SrTiO 3 ceramics and their trapping behaviour under the applied potentials are important factors in the nonlinear behaviour.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
References [1] N. Yamaoka, M. Masuyama and M. Fuki, Ceram. Bull., 62 (1983)698.
[15] [16] [17] [18]
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