Influence of Annealing Atmosphere on Varistor Property of TiO 2 -Nb 2 O 5 -SrCO 3 Ceramics

Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption. TiO 2 varistor ceramics has the advantages of low varistor voltage and good dielectric properties, but their low nonlinearity limited the application. The influence of annealing on the varistor properties of TiO 2 -Nb 2 O 5 -SrCO 3 ceramics was investigated in this paper. TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics were prepared by the traditional method of ball grind-ing-forming-sintering and they were annealed in oxygen and nitrogen, respec-tively. The nonlinear coefficient α and the breakdown voltage E B of the samples were tested using the varistor dc parameter meter. The microstructure of samples was analyzed by XRD, SEM, STEM-EDAX and SAEDP. The results show that during annealing, Sr 2+ ions with a larger radius obtain the kinetic energy and are segregated to grain boundaries, which increases the acceptor density of the grain boundaries and improves α. Annealing in an oxygen atmosphere, the enrichment of oxygen at grain boundaries is also helpful to increase the density of acceptor states and the height of the potential barrier, so as to further increase α. Meanwhile, annealing makes crystalline grains grow properly, which results in even grain size, reduces the porosity and increases the density of grains. So E B tends to reduce. As the doping concentration of Nb 2 O 5 and SrCO 3 is 0.15 mol%, respectively, and sintering temperature is 1300˚C, TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics annealed in oxygen for 3 h at 750˚C achieved the highest nonlinear coefficient α = 8.9 and the lowest breakdown voltage E B = 19.1 V∙mm −1 , which annealed in nitrogen achieved α = 8.4, E B = 20.6 V∙mm −1 , both superior to unannealed samples.


Introduction
Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption, because of their non-ohmic electrical properties and sensitivity to instantaneous voltage fluctuations [1]. For a given varistor, the non-ohmic properties depend on the structure of the grain boundary barrier of the polycrystalline ceramics [2]. Compared with ZnO and SnO 2 varistor ceramics, TiO 2 varistor ceramics has the advantages of low varistor voltage and good dielectric properties, which can be used as a varistor-capacitor multifunctional material. Furthermore, a simple preparation process makes it low cost. So it has a good application prospect [3]- [11]. Since the varistor properties of TiO 2 ceramics were discovered at Bell Labs in 1982 [2], the researchers have been tried to improve its nonlinearity, namely α value, by studying the effects of donor single-doping, acceptor single-doping and the co-doping of donor and acceptor on its varistor properties [12] [13] [14] [15]. Recently, S. Ekaphan  show that GeO 2 doping changes the microstructures of TiO 2 -Ta 2 O 5 -CaCO 3 ceramics, resulting in increased α and decreased E B [19]. Although these methods of doping in the literature have a certain effect on improving the nonlinear coefficient α, it seems difficult to improve further α value by doping. So it is indispensable to seek new methods.
Annealing is a heat treatment that is used to adjust the microstructure and eliminate the defects of materials. Its technological process is heating the sample to a suitable temperature for a period of time, followed by slow cooling, so as to obtain a near equilibrium state of properties. R. Mannam et al. studied the reversible p-type properties of pulsed laser deposited (P, N) co-doped ZnO thin films. As the film grows, it changes from p-type to n-type over a period of 120 days. Non-annealed N-type films contained donor impurities hydrogen and carbon, and the films were transformed into p-type semiconductors after annealing at 800˚C [20]. D.M. Priyadarshini et al. studied the effect of the annealing environment on SnO 2 thin film transistors. The results showed that with the increase of nitrogen concentration, the performance of the equipment improves [21]. M.J. Zheng [24]. It can be seen from these studies that annealing is effective in changing microstructures and improving some physical properties of materials.
In order to improve the nonlinear coefficient α of TiO 2 varistor ceramics and further reduce the breakdown voltage E B , TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics are prepared by the traditional ceramics preparation process and annealed in different atmospheres in this work. The influence of annealing on the microstructure and the varistor properties of TiO 2 -Nb 2 O 5 -SrCO 3 ceramics was studied. Table 1 shows the initial doping contents of each sample. The mixture was grinded in a planetary ball mill to obtain the reactants. The purity of each reagent is as follows: TiO 2 (powder, 99.9%), Nb 2 O 5 (powder, 99.9%), SrCO 3 (powder, 99.9%). All reagents were obtained from Guanghua Sci-tech Co., Ltd, China. The mixture was grinded for 10 h according to the mass ratio of 1:2:4 of powder to water blended with alcohol (water: alcohol = 3:1) to balls. After homogenization, each blend was dried at 90˚C for 10 h and de-agglomerated in a 400 mesh sieve. The obtained powder was isostatically pressed at 160 MPa into tablets (10 mm in diameter and 1 mm thick). The tablets were degummed at 500˚C for 1 h in a box-type resistance furnace and sintered at 1250˚C, 1300˚C, 1350˚C, and 1400˚C for 3 h in a tube furnace, respectively. The tablets were cooled to room temperature through furnace cooling. The TiO 2 -Nb 2 O 5 -SrCO 3 ceramic sheets obtained by the above method were coated with silver electrode. The varistor properties of all sheets were tested. Next, the samples with the best varistor properties were annealed in oxygen and in nitrogen for 3 h at 650˚C, 700˚C, 750˚C and 800˚C, respectively, and the varistor properties of them were tested again. XRD was used to analyze the phase of samples. SEM was used to observe the morphology of the samples. STEM-EDAX was used to analyze the contents of each element in grains and at grain boundaries. SAEDP was used to analyze the crystallization degree of the samples. As can be seen from Table 3

Varistor Properties
where A * and k represent the Richardson constant and the Boltzmann constant, respectively. T is the absolute temperature. The two groups of electric current J and electric field E were tested at the same temperature T. The values of β and Φ B were calculated. Five Φ B values were gotten for each sample and the average was listed in Table 4. From Table 4, the average potential barrier height Φ B in #O75 and #N75 is higher than #2. Φ B of #O75 is higher than #N75.       Table 5.

Microstructure Analysis
By analyzing element contents of adopted points, the content of element Sr at grain boundaries is higher in the annealed samples #O75 and #N75 than the unannealed sample #2. The reason is that Sr 2+ ions with a larger radius  obtains kinetic energy and then are segregated to grain boundaries in the annealing process. So the content of Sr 2+ in the grains of the annealed samples in oxygen or nitrogen tends to decrease, while the content of Sr 2+ at boundaries tends to increase. The content of element O of sample #O75 annealed in oxygen is higher than sample #2 and #N75 at grain boundary. Figure 5 is the STEM-EDAX line scanning graph across the grain boundary of sample #2, #O75 and #N75. As shown in Figure 5(a), the oxygen concentration at the grain boundary of #O75 is higher. The reason for it is that the oxygen is enriched at the grain boundary for sample #O75 annealed in oxygen atmosphere [24], which is why the nonlinear coefficient of #O75 is slightly higher than that of #N75. In Figure 5(c), it can be seen that the element Sr tends to segregation towards grain boundary. After annealing, Sr content at grain boundary is much higher than that in grain. By the mutation of the Sr concentration at the grain boundary, the width of the grain boundary of the two annealed samples is estimated to be about 3.56 nm. In Figure 5 Figure 6(b) are the selected area electron diffraction patterns (SAEDP) of the grains in sample #2 and #O75, respectively. The electron diffraction pattern of unannealed sample #2 has the amorphous central diffraction spot and the polycrystalline diffraction rings. In the electron diffraction pattern of annealed sample #O75, the regular diffraction spot of single crystal is found, which indicated that the grains of annealed sample #O75 developed well.

Mechanism Analysis
A high α value and a low E B value are required for TiO 2 -based low voltage varistor. The barrier structure of grain boundary decides α and E B . Increasing Φ B can reduce the background current and decreasing X D can improve the electron tunneling effect. Therefore, increasing Φ B and decreasing X D can improve α value. Φ B is determined by the formula [26] where ε 0 is the dielectric constant of vacuum, ε r is the relative dielectric constant of semi-conducting ceramic, N D is the donor concentration in the semi-conductor depletion layer, N S is the acceptor interface state density, e is electron charge. Decreasing N D can increase Φ B , and therefore increase α. But on the basis of the decreasing N D makes X D increase and is not conducive to improving α. What is more, decreasing N D reduces semi-conducting degree of grains, which goes against the low E B . Therefore, the doping content of the donor has an optimal  value. In like manner, increasing N S makes Φ B increase, and therefore increases α, but also increases X D . Therefore, the doping content of the acceptor also has an optimal value. As the optimal doping content of the donor and the acceptor is determined, adjusting the doping content of the donor and the acceptor can't optimize α and E B any longer, but appropriate annealing can further optimize α and E B of the TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramic. This can be explained as follows. The nonlinear coefficient of TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics can be appropriately improved by annealing, mainly because Sr 2+ is segregated towards the grain boundary as far as possible in the annealing process, which increases the density of acceptor state and the height of potential barrier at grain boundaries. Annealing in the oxygen atmosphere, the oxygen enrichment at grain boundaries is also helpful to increase the density of acceptor states and the height of potential barrier, so as to further improve the nonlinear coefficient.
Annealing at appropriate temperature for appropriate time can make the grains grow appropriately; the grain size even and the porosity reduce. Meanwhile, the number of grain boundaries and the total area of grain boundaries decrease.
Therefore the breakdown voltage E B tends to decrease.

Conclusion
In this work, it is found that annealing can further improve the microstructure of TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics and enhance the nonlinear coefficient α.
During annealing, the acceptor ions Sr 2+ with larger radius are further segregated towards the grain boundaries because of obtaining kinetic energy, which increases the density of acceptor state and the height of potential barrier at grain boundaries. Therefore α increases. Annealing in an oxygen atmosphere, the enrichment of oxygen at grain boundaries is also helpful to increase the density of acceptor states and the height of the potential barrier, so as to further increase the nonlinear coefficient. Annealing at the appropriate temperature for an appropriate time can make the grains grow appropriately, the grain size even and the porosity reduce, which results in the number of grain boundaries and the total area of grain boundaries decreasing. Therefore the breakdown voltage E B tends to decrease. As the doping concentration of Nb 2 O 5 and SrCO 3 is 0.15 mol%, respectively, and the sintering temperature is 1300˚C, TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics are annealed for 3 h at 750˚C in oxygen possess best varistor performance with α = 8.9 and E B = 19.1 V•mm −1 . With further selecting dopants and optimizing annealing temperature, TiO 2 varistor ceramics are expected to obtain higher nonlinear coefficients and realize their practical application.