Preparation Of SrTiO₃-based Dual-functional Low Voltage Varistor Ceramics By Soft Chemical Method

SrTiO₃ ceramic varistor is a kind of material with double functions. It can be used as capacitor and varistor. It has low sensing-voltage, big dielectric constant and restraining the wave which voltage is lower than the standard. Its good properties have been utilized widely. The study described in this thesis consists of two parts: the preparation of SrTiO₃-based thin films and bulk ceramics by soft chemical method The preparation process and the varistor characteristics and dielectric properties of matetials of the two sections were respectively investigated in order to achieve the low voltage characteristic in this paper.First, SrTiO₃ thin films were prepared by sol-gel method using Ti(C₄H₉O)₄, Sr(NO₃)₂ as raw materials, citric acid (CA) as chelating agent, ethylene glycol (EG) as cross-linking agent, dimethyl formamide (DFM) as drying agent, and La(NO₃)₃·6H₂O and Mn(NO₃)₂ added respectively as donor and acceptor dopants. The concentration of the precursor was 0.2mol/L. SrTiO₃ thin films with perovskite structure nano-crystalline grains were fabricated after the dip-coating, drying, pre-heating and sintering. Heat-treatment of substrate, adding to DFM and adjusting to the temperature of sintering, etc. are important factors to fabricate no cracks, dense, uniform and adequate thinkness of SrTiO₃ thin films. TG-DTA, XRD, SEM and AFM were used to analyze the precursor and the thin films. The results revealed that the homogeneous and free-crack SrTiO₃ thin films with the grain size of about 50nm, the surface roughness of closely 9.6nm and the thickness of approximately 600nm were successfully synthesized at the sintering temperature of 800°C in graphite and N2+C reducing atmosphere. The varistor voltage is about 0.6V, and nonlinear coefficient is approximately 2.0.Strontium titanate nanocrystalline powder materials were prepared from Sr(NO₃)₂-Ti(C₄H₉O)₄-H₂O-C₂H₅OH system by sol-gel method. La(NO₃)₃·6H₂O and Mn(NO₃)₂ were added respectively as donor and acceptor dopants. Si(OC₂H₅)₄ was added to decrease the sintering temperature. The factors of affecting the formation of the sol and gel were investigated. The results of DSC-TG and XRD showed that prepared gels transformed completely into the strontium titanates with perovskite-type cubic phase by a heat treatment at about 900℃. The particle size was approximately 30nm. SrTiO₃-based double function ceramics were achieved with prepared strontium titanate nanocrystalline powders by the single-fired process. The SrTiO₃-based ceramics were prepared by nano-powders, the varistor voltage of which is about 95.7V/mm, and the nonlinear coefficient is 3.5; the dielectric constant is about 14,000, and dissipation factor is approximately 20.75%. The distinctions of the samples fabricated by nano-powders and regular-powders were discussed theoretically by many experiments. Finally, the SrTiO₃-based ceramics prepared by using nano and regular mixed powders have a better property, the varistor voltage of which is about 14.9V/mm, and the nonlinear coefficient is 6.1; the dielectric constant is about 47,800, and dissipation factor is approximately 33.30%.The low voltage SrTiO₃-based varistor ceramic were fabricated by soft chemical method. The above research is the theory and experimental foundation to apply the materials to production application.