easily modified to a semiconducting one. This advanced ceramic material has been widely used for the electronic components such as PTCR, MLC capacitors, BL capacitors. Recently, the varistor characteristics of SrTiO3 have been reported. Since the dielectric constant of SrTiO3 is by two orders larger...
easily modified to a semiconducting one. This advanced ceramic material has been widely used for the electronic components such as PTCR, MLC capacitors, BL capacitors. Recently, the varistor characteristics of SrTiO3 have been reported. Since the dielectric constant of SrTiO3 is by two orders larger than that of ZnO, a SrTiO3 varistor shows a larger surge capability and a sharper pulse response than ZnO varistors. It can also be used as multi-functional components(MFC) like varistor-capacitor. In the application of SrTiO3 to electronic devices, it is necessary to control its grain boundary structure and the chemical characteristics. In this work, fine SrTiO3 powders have been varistor prepared by hydrothermal synthesis for varistor applications. TiCl4 and strontium hydroxide were used for source materials. Reactive TiO2 ? xH2O prepared by hydrolysis of TiCl4 was reacted with the aqueous solution of Sr(OH)2 ? 8H2O at the molar ratio of 1:1 in an autoclave at 120℃ and 140℃ for 1-4hrs, respectively. In the synthesis, 0~1.5mol% of Nb2O5 was added as a dopant. The synthesized powders was characterized by XRD and TEM. Fine spherical powders of ~60nm in average diameter and narrow size distribution have been obtained irrespective of the reaction temperature and time. Powder compacts of SrTiO3 doped with Nb2O5 (abbreviated to STN) up to 1.5mol% were fired under a reduced atmosphere (N2+5%H2) between 1100℃ and 1300℃ for 4h. In order to attain varistor characteristics reduction-sintered SrTiO3 was annealed at 1000℃ and 1100℃ for various time period (0, 30, 60 min) in air. Both densification and the grain growth of SrTiO3 seemed to be promoted with low doping concentration (? 0.6mol%) of Nb2O5, which were retarded at higher doping concentrations. Probably an increased diffusivity of ionic constituents owing to the increase of the perovskite unit cell is responsible for the promoted sinterability, while excess Nb seems to segregate on the grain boundaries and inhibit the grain growth of high temperature. On I-V characteristics, both the breakdown voltage(Vbk) and its value per grain boundary(Vgb) increased with increasing annealing temperature and time. Leakage current, on the contrary, was kept 0.65-0.8(mA/cm2) irrespective of the annealing temperature, time and the Nb2O5 content. In respect of varistor characteristics, the annealing condition of 1100℃ -60min has been found the most desirable; STN doped with 0.15mol% Nb2O5 and annealed under that condition revealed ??3 and leakage current of ~0.75 mA/cm2. C-V measurement did not show clear tendencies of the donor density(ND), potential barrier hight( b), depletion layer(Nt) and interface defect density as functions of the compositions and sintering conditions.
easily modified to a semiconducting one. This advanced ceramic material has been widely used for the electronic components such as PTCR, MLC capacitors, BL capacitors. Recently, the varistor characteristics of SrTiO3 have been reported. Since the dielectric constant of SrTiO3 is by two orders larger than that of ZnO, a SrTiO3 varistor shows a larger surge capability and a sharper pulse response than ZnO varistors. It can also be used as multi-functional components(MFC) like varistor-capacitor. In the application of SrTiO3 to electronic devices, it is necessary to control its grain boundary structure and the chemical characteristics. In this work, fine SrTiO3 powders have been varistor prepared by hydrothermal synthesis for varistor applications. TiCl4 and strontium hydroxide were used for source materials. Reactive TiO2 ? xH2O prepared by hydrolysis of TiCl4 was reacted with the aqueous solution of Sr(OH)2 ? 8H2O at the molar ratio of 1:1 in an autoclave at 120℃ and 140℃ for 1-4hrs, respectively. In the synthesis, 0~1.5mol% of Nb2O5 was added as a dopant. The synthesized powders was characterized by XRD and TEM. Fine spherical powders of ~60nm in average diameter and narrow size distribution have been obtained irrespective of the reaction temperature and time. Powder compacts of SrTiO3 doped with Nb2O5 (abbreviated to STN) up to 1.5mol% were fired under a reduced atmosphere (N2+5%H2) between 1100℃ and 1300℃ for 4h. In order to attain varistor characteristics reduction-sintered SrTiO3 was annealed at 1000℃ and 1100℃ for various time period (0, 30, 60 min) in air. Both densification and the grain growth of SrTiO3 seemed to be promoted with low doping concentration (? 0.6mol%) of Nb2O5, which were retarded at higher doping concentrations. Probably an increased diffusivity of ionic constituents owing to the increase of the perovskite unit cell is responsible for the promoted sinterability, while excess Nb seems to segregate on the grain boundaries and inhibit the grain growth of high temperature. On I-V characteristics, both the breakdown voltage(Vbk) and its value per grain boundary(Vgb) increased with increasing annealing temperature and time. Leakage current, on the contrary, was kept 0.65-0.8(mA/cm2) irrespective of the annealing temperature, time and the Nb2O5 content. In respect of varistor characteristics, the annealing condition of 1100℃ -60min has been found the most desirable; STN doped with 0.15mol% Nb2O5 and annealed under that condition revealed ??3 and leakage current of ~0.75 mA/cm2. C-V measurement did not show clear tendencies of the donor density(ND), potential barrier hight( b), depletion layer(Nt) and interface defect density as functions of the compositions and sintering conditions.
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