Control Of Negative Thermal Expansion And Multi-functional Properties In The PbTiO₃-based Compounds

PbTiO3 is an important and well known ferroelectric perovskite that shows negative thermal expansion (NTE) from room temperature to Curie temperature TC. Comparing with most perovskite compounds such as BaTiO3, KNbO3, BiFeO3, etc, this NTE property is unique. Besides, PbTiO3-based compounds have multi-functional properties that make it valuable in ferroelectricity and ferromagenetism research fields. It is well known that the thermal expansion mismatch in most materials normally shortens service life. More importantly, in some industries like aeronautics or astronautics, there are very strict standards on the material’s thermal expansion. Therefore, the discovery of materials with controllable NTE or even zero thermal expansion (ZTE) becomes a major research field. Studying the NTE of PbTiO3-based compounds is helpful not only to develop new NTE materials, but also to understand the NTE mechanism. This thesis focuses on a series of studies on the NTE and multi-functional properties of PbTiO3-based compounds.In the 0.5PbTiO3-0.5(Bi1-xRx)FeO3 (R=Sm, Nd) system, the effect of Nd/Sm substitution in the 0.5PbTiO3-0.5BiFeO3 ferroelectric on crystal structure and NTE property has been investigated. We have found that after adding Sm/Nd, the c/a decreases, unit cell volume shrinkes, and the ferroelectric property is reduced. Therefore, the NTE property turns weak. The experimental result shows there is a strong correlation between spontaneous volume ferroelectrostriction (SVFS) and the NTE.Introducing cation deficiency to pure PbTiO3 compound and see how it influences the structure and NTE of PbTiO3. The Ti4+ deficiency can reach the maximum value of 2%(PT98) and Pb2+ deficiency can reach the maximum value of 8%(P92T) before impurity phase apprears. Comparing PT98 with the stoichiometric PT, the crystal structure and property have little change, but the P92T changs significantly. The XRD Rietveld refinement result showed Ps displacement decreases; high temperature XRD refinement results showed the NTE weaken; differential scanning calorimetry (DSC) measurement results exhibited Curie temperature decrease; Raman measurement results showed Raman active modes of A1(1TO), A1(2TO) and A1(3TO) become softened. A large amount of cation deficiency in the A-site reduces the PT spontaneous polarization, and thus NTE is weakened.Solid solution compounds of (1-x)PbTiO3-xNdFeO3 have been prepared using a conventional solid state reaction route. The cystal structure and multifunctional properties have been studied as function of the content of NdFeO3. Firstly, PbTiO3 and NdFeO3 can form solid solutions (1-x)PbTiO3-xNdFeO3 over the whole composition range (x=0-1). The phase transitions were found from the tetragonal to the pseudo-cubic and then to the orthorhombic phase with the increasing content of NdFeO3. At the same time, the ferroelectricity weakens gradually. In contrast, ferromagentism increases gradually and reaches the maximum value for the two compositions of 0.2PbTiO3-0.8NdFeO3. 0.8PbTiO3-0.2NdFeO3 and 0.7PbTiO3-0.3NdFeO3 which are both multi-functional properties materials containing ferroelectricity and ferromagentism. Meanwhile, the magnetoelectric coupling of 0.8PbTiO3-0.2NdFeO3 sample is involved.We have introduced Bi to PbTiO3-NdFeO3 to form morphotropic phase boundary (MPB) (0.7PbTiO3-0.3NdFeO3) due to the unique property of 6s2 lone pair electrons in Bi3+. We have found that Bi replace 25% of Nd, the ferroelectricity and ferromagentism of the sample both increase; while replace 75% of Nd, we have obtained ZTE multi-functional properties mateials. Since PbTiO3-(Bi,Nd)FeO3 possesses excellent multi-ferroelectric properties, we have prepared PbTiO3-(Bi,Nd)FeO3 thin film through sol-gel method. The results showed that 0.9PbTiO3-0.1(Bi,Nd)FeO3 thin film has good ferroelectricity and ferromagentism properties.