Sol-gel Silver - Lead Titanate Composite Film And Its Electrical Properties

Metal-dielectric composite material is a novel kind of high-dielectric composite materials. It is widely researched because of its percolation phenomenon. With the miniaturization of electronic devices, metal dispersed dielectric film with a high dielectric constant has attracted many interests in material science and modern physics area. However, since the thickness of film is in several nanometers range, and metal particles can easily aggregate and form a conducting path perpendicular to the film, it is hard to prepare metal dispersed dielectric film with a percolation phenomenon. Therefore, it is very meaningful to investigate the prepartion of metal dispersed dielectric film, and take good control of the size and content of metal particles in the films.The aim of this work is to fabricate Ag dispersed PbTiO₃ film via. sol-gel method, which follow the rules of percolation law and performs a high dielectric constant. Efforts have been devoted to control the size and dispersity of Ag particles and the crystallization of PbTiO₃ phase. Effect of dispersed Ag particles on the dielectric properties of the film has been investigated.(1) the effect of Ag⁺ on the phase formation of lead titanate has been investigated. After Ag⁺ has been introduced into the sol, Pb²⁺ will be consumed during Ag particle formation, which will facilitate the formation of pyrochlore lead titanate. Moreover, Ag⁺ will form Ag-O-Ti bond in the sol, which will make Ag⁺ dissolutes into the crystalline lattice of PbTiO₃. After comparing the effect of heating temperature and heating process on phase formation in the films, a rapid heating process and a heating temperature of 600℃ is the most ideal condition for PbTiO₃ phase formation.(2) the effect of excess Pb on the phase formation of Ag dispersed PbTiO₃ film has been investigated. With excess Pb in the sol, more Pb²⁺ can take part in the phase formation of lead titanate, therefore formation of pyrochlore lead titanate can be inhibited. Moreover, the dissolution of Ag⁺ into PbTiO₃ phase can be lowered. The excess Pb will dissolve into Ag particle phase and as a result form Ag-Pb alloy. which will evaporate at a much lower temperature than the melting point of Ag. Detailed investigation of Ag-Pb alloy evaporation in the film with a content of Pb/Ti=1.3 Ag/Ti=0.5 has been conducted, and the activation energy is calculated to be 88.69kJ/mol, apparent frequency factor is calculated to be 68.007s^-1. In the films prepared with exess Pb, after the evaporation of Ag-Pb alloy, Ag⁺ dissolved in PbTiO₃ lattice will transform into Ag nanoparticles. The transformation is controlled by evaporation of Ag-Pb alloy and the heating temperature. At a heating temperature of 600℃, the growth mechanism is either a interfacial diffusion of particle and matrix boudary, or a bulk diffusion in the matrix.(3) The effect of complexing agent and hydrolysis condition on the formation of Agdispersed lead titanate has been investigated. After LA, DEA, DA has been introduced into the sol, the interaction between Pb2+ and Ti-0 can be enhanced, therefore the formation of pyrochlore lead titanate can be inhibited. The size of Ag particle is controlled by the interaction between Ag⁺ and Ti-O. As the interaction increases in a sequece of CA>DEA>LA, the size of Ag particle decreases in a sequence of CA0; during a 600°C preannealing in ambient atmosphere + a postannealing in reducing atmosphere, Ag₃2+ nuclei will be formed before the growth of Ag particle. For direct heating in ambient or reducing atmosphere, the Gibbs energy for Ag particle formation has been calculated, and the necessary thermodynamic condition for Ag particle formation has been provided. Dynamic condition of Ag particle formation has been calculated. It is pointed out that the reason for Ag particle size difference in the films prepared under different atomphere originates from the nucleation density inequality, which provide a theoretical foundation for the preparation of metal nanoparticle dispersed dielectric film.(5) The conduction mechanisn of Ag dispersed PbTiO₃ film has been discussed. DC and AC conduction mechanism of the film is controlled by the heating atmosphere and has no relationship with Ag content. For the films prepared under ambient atmosphere, DC conductivity of the film is controlled by localized hopping electron mechanism and Fowler-Nordheim tunneling mechanism at low V and high V region respectively. As the measruing temperature increases, the AC conductivity shifted from a Debye relaxation current mechanism to a Schottky emission mechanism. For the films prepared under reducing atmosphere, DC conductivity of the film is controlled by localized hopping electron mechanism and Fowler-Nordheim tunneling mechanism at low V and high V region respectively, but the contribution of localized hopping electron is much larger than the films prepared under ambient atmosphere. For this reason, the AC conduction mechanism of the films prepared in reducing atmosphere is different at a low measuring frequecy and a high measuring frequency. When the measuring frequecy is low, with the increase of measuring temperature, the conduction mechanism shift from localized hopping electron mechanism to reoxidation mechanism and finally to Schottky emission mechanism. When the measuring frequecy is high, with the increase of measuring temperature, the conduction mechanism shift from Debye relaxation mechanism, to localized hopping electron mechanism, then to reoxidation mechanism and finally to Schottky emissionmechanism. The varaition tendency of condution mechanism with voltage and temperature didn't change with the Ag content. But the value of leakage current increase as the Ag content increases. Moreover, the transition voltage in DC conduction between localized hopping carrier mechanism and Fowler-Nordheim tunneling mechanism increases as the Ag content increases.(6) The dielectric relaxation mechanism of Ag dispersed PbTiO₃ film has beeen investigated. For films prepared in ambient atmosphere, the dielectric relaxation mechanism is controlled by Debye relaxation mechanism. For films prepared in reducing atmosphere, the dielectric relaxation is controlled by localized hopping electron mechanism at a low measuring frequency, and is controlled by Debye relaxation mechanism at a high measuring frequency. Ag content has no effect on the dielectric relaxation mechanism of the films.(7) The effect of dispersed Ag on the dielectric constant and conductivity of Ag dispersed PbTiO₃ film has been discussed. When Ag content is low, the conductivity and dielectric constant will decreased since a small amout of Ag⁺ is dissolved into the lattice of PbTiO₃ phase. But after the Ag content exceeds the solid solubility, the conductivity and dielectric constant of Ag dispersed PbTiO₃ film will both increase, following the percolation law. For films prepared under ambient atmosphere, the dielectric constant can be increased to 1.27 times of that of the pure lead titanate, around a percolation threshold of 0.04. For films prepared under reducing atmosphere, the dielectric constant can be increased to 4.13 times of that of the pure lead titanate, around a percolation threshold of 0.17.