| With the rapid development of information technology, miniaturization, multifunction and high stability become the inevitable developing trend of electronic equipment. Therefore, the varistors that are used as circuit protection device are developing toward the direction of low breakage voltage, multifunction and high reliability. A TiO2-based varistor combines the merits of large capacitance, low breakage voltage and simple preparation process, and it can be directly sintered in the atmosphere. However, the nonlinear coefficient a of this varistor material is small and it is difficult to form series production, which narrows its application field. To give full play to the advantages of TiO2-based varistor, and to meet the need for low-voltage varistor in the field of the low breakage voltage and high integrated circuit, this paper explores deeply nonlinear mechanism of TiO2-based varistor and tries to improve the nonlinear coefficient a and reduce its breakage voltage EB by the method of co-doping multiple compositions.In this dissertation, Nb2O5, Ta2O5 and V2O5 were selected as donor dopants and CaCO3, BaCO3, SrCO3 and Y2O3 were selected as acceptor dopants. By not adding, adding Ge or GeO2 and adding both Ge and GeO2, using traditional ball mill-molding-sintering ceramic production technology, TiO2-based varistor ceramic samples were prepared. The phase of samples was analyzed using XRD. The microstructure was observed using SEM. The element contents were analyzed using ED AX. The valence of elements was investigated using XPS. The grain boundary structure of samples was observed using TEM. The influence of mono-doping with Ge or GeO2 and co-doping with Ge and GeO2 on the micro structure and the varistor properties of TiO2-based varistor ceramics with different donor and acceptor was studied in detail.Firstly, Nb5+ with the close ionic radius with Ti4+ was used as donor and Ca2+ with larger ionic radius than Ti4+ was used as acceptor. The optimum doping contents, the best sintering temperature and the best sintering time of TiO2-Nb2O5-CaCO3 varistor ceramics were obtained based experiments. The forming process of grain-boundary barrier of TiO2-Nb2O5-CaCO3 varistor ceramics is discussed. The research showed that for TiO2-Nb2Os-CaCO3 varistor ceramics, the optimal doping content of Nb2O5 and CaCO3 is 0.5mol%, respectively, the best sintering temprature is 1400℃, the best sintering time is 3h, and the highest nonlinear coefficient is α=6.6. The forming process of grain-boundary barrier of TiO2-Nb2O5-CaCO3 varistor ceramics is as follows:The defects (Tii,VTi,Vo) caused by TiO2 and the defects (NbTi,CaTi) caused by adulteration formed depletion layer at grain boundaries, therefore formed defect barrie that influenced electronic transport. Thus electrons transmitted in according to tunnel effect and caused nonlinear electrical property of TiO2-Nb2O5-CaCO3 varistor ceramics.Based on the above research, by adding the excellent semiconduct Ge with low melting point, the infuluence of element Ge on TiO2-Nb2O5-CaCO3 varistor ceramics was investigated. The research showed that because of low melting point of Ge, the liquid phase is observed during sintering, which facilitates the segregation of acceptor ions with larger radius than Ti4+ at the grain boundary and acceptor interface state density Ns increases;The resultant GeO2 accelerates grain growth, which reduces XD and Ns further increases. Thus, the nonlinear coefficient a is improved. The grain growth reduces XD and decreases the number of grain boundaries along the thickness direction of the sample; the resultant GeO2 as an excellent semiconductor dissolves in the TiO2 lattice and improves the semiconducting degree of grains. Thus, the breakdown voltage EB is decreased.As the doping contents of Nb2O5 and CaCO3 were 0.5mol%, respectively, and the doping content of Ge was 1.0mol%, TiO2-Nb2O5-CaCO3-Ge varistor ceramic behaved the highest nonlinear coefficient (α=10.6), lower breakdown voltage (EB=8.7V·mm-1) and the highest grain boundary barrier (ΦB=1.73eV). At the same time, Ge acted as sintering aid due to its low melting point and reduced the sintering temperature of 100℃.1300℃ is the best sintering temprature of TiO2-Nb2O5-CaCO3-Ge varistor ceramics.Replacing the donor Nb2O5 with Ta2O5, the infuluence of Ge on TiO2-Ta2O5-CaCO3 varistor ceramics was also investigated and the similar conclution is drawn. When the doping contents of Ta2O5, CaCO3, and Ge were 0.2,0.4, and 0.9mol%, respectively, the TiO2-Ta2O5-CaCO3-Ge varistor ceramics sintered for 3h at 1300℃ exhibited high α=10.2, low EB=14.1V·mm-1, and high ΦB=0.95eV.GeO2 is also a excellent semiconductor with low melting point, therefore it is helpful to research the influence of doping with GeO2 on TiO2-based varristor ceramics to prove the correctness of the above conclusion. The research showed that GeO2 promoted grain growth of TiO2-Ta2O5-CaCO3 varistor ceramics, reduced grain boundary width and promoted the acceptor Ca2+to segregate at grain boundaries as far as possible. Therefore the grain boundary acceptor state density Ns increased, which improved the nonlinear coefficient a and reduced the breakdown voltage EB. When doped with 0.2mol% Ta2Os,0.2mol% CaCO3 and 0.9mol% GeO2, TiO2-Ta2O5-CaCO3-GeO2 varistor ceramics exhibits the highest nonlinear coefficient (α=9.8), low breakdown voltage (EB=17.2V·mm-1), as well as grain boundary barrier (ΦB=0.92eV).The influencing mechanism of Ge or GeO2 on TiO2-based varistor ceramics is very similar, therefore the paper further investigates the influence of co-doping with Ge and GeO2 on the microstructure and varistor performance of TiO2-based varistor ceramics with different donor and acceptor doping. After investigating the influence of co-doping with Ge and GeO2 on TiO2-Ta2O5-CaCO3, TiO2-V2O5-Y2O3, TiO2-Ta2O5-BaCO3 and TiO2-Nb2O5-SrCO3 varistor ceramics, the conclusion is as following:(1) Ge and GeO2 act as sintering aids because of their low melting points, such that their liquid phase is present during sintering. This phenomenon enables acceptor ions, which have larger radius than Ti4+, to fully segregate at the grain boundary, thereby the acceptor interface state density Ns and the grain boundary barrier ΦB increase and subsequently the nonlinear coefficient a improves.(2) By adding Ge and GeO2, GeO appears in the sample during the sintering process. Ge2+ segregates at the grain boundary because its ion radius is also larger than that of Ti4+, thereby Ns and ΦB further increase and the a value further improves.(3) The addition of Ge and GeO2 accelerates grain growth, which reduces the barrier width XD and decreases the number of grain boundaries along the thickness direction of the sample. Thus, the a value further improves and the break voltage EB decreases.(4) GeO2 is an excellent semiconductor. Therefore, if GeO2 is in excess, the surplus GeO2 that dissolves in the TiO2 lattice improves the degree of semi-conduction of the grains and further decreases EB.The dissertation finally chooses Ta5+ with the closer ionic radius with Ti4+ as donor and Ba2+ with larger ionic radius than Ti4+ as acceptor. By accurately determining the optimum doping contents of donor dopant Ta2Os and acceptor dopant BaCO3 and improving the preparation process, TiO2-Ta2O5-BaCO3-Ge-GeO2 varistor ceramics with maximum nonlinear coefficient of α=15.7 and minimum breakdown voltage EB=7.3V·mm-1 is prepared. |
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