Enhanced Negative Thermal Expansion And Its Controlling In Perovskite Compounds

In recent years,negative thermal expansion materials have attracted extensive attention due to their abnormal "shrinkage on heating and expansion on colding".In today's increasingly stringent requirements for instrument precision,the matching of thermal expansion coefficients is very important,especially in the field of electronics and aviation with large environmental temperature fluctuation.Therefore,it is particularly important to study the negative thermal expansion mechanism so as to control their thermal expansion and related properties.As a typical negative thermal expansion material,perovskite lead titanate(PbTiO3)presents large negative thermal expansion behavior,high Curie temperature,large spontaneous polarization and other excellent properties.In this thesis,PbTiO3 was selected as the parent compound and the enhanced negative thermal expansion materials where develpoed.The thermal expansion,crystal structure,ferroelectricity and negative thermal expansion mechanical were studied with chemical substitutions.Furthermore,a semi-experime rule for prediction the enhanced negative thermal expansion was proposed.The enhanced negative thermal expansion behavior of PbTiO3-BiGaO3 system was found.The X-ray diffraction experiment results show that the tetragonal distortion and negative thermal expansion behavior of the system increase with the doping of BiGaO3,but the Curie temperature alomst unchanged.The doping of BiGaO3 improves the spontaneous polarization,and the enhancement of its negative thermal expansion can be explained by the spontaneous volume ferroelectrostriction.The maximum entropy method based on X-ray shows that the enhancement of A-site,B-site and O-site hybridization leads to the enhancements of negative thermal expansion.The refinement of neutron diffraction spectra shows that the position of Ga and Ti split in the system,and Ga is far away from the center of B06 octahedron,which indicates that the introduction of Ga can promote the spontaneous polarization.In addition,the anomalous enhancement of negative thermal expansion was also found in the ternary system 0.6PbTiO3-0.4Bi(GaxFe1-x)O3.Due to the strong ferroelectric activity of Zn element,we developed another perovskite compound PbTiO3-Bi(Zn2/3Ta1/3)O3 which enhanced the negative thermal expansion.The unit cell parameters,tetragonality(c/a),Curie temperature and negative thermal expansion of the doped system are enhanced.The enhancement of negative thermal expansion is due to rasied spontaneous polarization.The images of spherical aberration transmission electron microscopy clearly show that the polarization shift of B-site element increases and the atomic position deviates from the central position,which provides a direct evidence to the enhancement of spontaneous polarization.The charge density calculated by the first principle method confirmed that the coupling between cation and oxygen can enhance the negative thermal expansion behavior.In the aspect of controllable thermal expansion,PbTiO3-CaTiO3-Bi(Zn2/3Ta1/3)O3 system was studied.The thermal expansion coefficient of PbTiO3-CaTiO3-Bi(Zn2/3Tai/3)O3 system was tailored from negative to positive.Among them,zero thermal expansion was achieved in 0.55PbTiO3-0.35CaTiO3-0.1Bi(Zn2/3Ta1/3)O3.Doping CaTiO3 zero expansion occurs,when the ferroelectric order contribution is equal to the phonon thermal vibration contribution.Particularly,the doping of CaTiO3 makes the ceramics compact with high hardness,which is potential for application.Based on the experimental data,negative thermal expansion mechanism and volume change relationship,an empirical method is proposed to predict whether the negative thermal expansion of PbTiO3-based compounds is enhanced.This method can be used to primarily estimate whether the negative thermal expansion of the compound is enhanced or not.