Thermodynamic Study On Size Effects And Stress Effects Of Some Functional Nanosystems

To realize their potential in applications to ultrahigh-density memory devices, phase change thin films, wires, tubes and ferroelectric and multiferroic thin film in nanoscale have been actively investigated in recent years. Phase transition of nanomaterials is a collective phenomenon, depending on the combined effects of many factors, such as the ambient temperature, sample dimensions, surface effect, external stresses, etc. Studies of the effects of sample dimensions, surface tension, and external stresses on properties of phase change materials and ferroelectrics have been conducted and studied during last several decades.The purpose of this thesis is to study size effects and stress effects on the properties of phase change materials and ferroelectrics in nano-scale. The main ideas, approches and results of our study are listed as follows:1. The thickness dependent crystallization behavior of thin amorphous Ge2Sb2Te5(GST) films sandwiched between different cladding materials has been investigated based on a thermodynamic model. It is revealed that there is a critical thickness below which the crystallization can not occur. The critical thickness is determined by the energy difference△γ between the crystalline GST/substrate interface energy and the amorphous GST/substrate interface energy, the melting enthalpy, and the mole volume. The calculated result is in good agreement with the experiments. Furthermore, the crystallization temperature is also affected by interface energy difference△γ. Larger△γ gives rise to a higher crystallization temperature, and vice versa. This impact becomes stronger as the film thickness is decreased.2. The phase change behavior of Ge2Sb2Te5(GST) nanowires and nanotubes was investigated by using thermodynamic calculations. It is revealed that the melting and crystallization temperatures for both nanowires and nanotubes decrease as the diameter of the nanostructures is reduced. There exists a critical diameter for both nanowires and nanotubes, below which the crystallization could not take place. It determines the ultimate scaling limit of nanowires or nanostructures phase-change memory. The critical diameter for nanowires depends on the difference between the surface energy of solid and liquid phases, the bulk melting and crystallization temperatures, as well as the melting and crystallization entropy. The critical diameter of the nanotubes is larger than that of the nanowires. And it is also dependent on theratio of outer diameter over inner diameter of the tubes. The lower ratio of outer diameter over inner diameter gives rise to the larger critical diameter of the nanotubes.3. Using the phenomenological Landau thermodynamic theory, we investigated the effect of external mechanical loads on the phase transition and physical properties of poly(vinylidene fluoride-trifluorethylene)[P(VDF-TrFE)] thin films. Quantitative calculation shows that the phase transition temperature, polarization, dielectric, pyroelectric and piezoelectric properties are highly sensitive to external mechanical loads. External compressive stress increases the phase transition temperature and the out-of-plane polarization and decreases the out-of-plane dielectric constant below Tc, pyroelectric coefficient and piezoelectric coefficient (absolute value) and vice versa. Compared with pervoskite-type ferroelectrics, the calculated results are opposite due to the different intrinsic parameters between pervoskite and polymer ferroelectrics, especially, electrostrictive constants. The ability to tailor the properties in ferroelectric P(VDF-TrFE) thin films with the mechanical load can offer a tremendously promising future for applications in multifunctional devices such as agile mechanical sensors and transducers.4. A phenomenological thermodynamic theory is used to investigate the effect of external mechanical stress on the magnetoelectric (ME) coupling in ferroelectric PbTiO3or organic P(VDF-TrFE)/ferromagnetic Terfenol-D heterostructures. The results showed that the effects of external stress on the value of polarization strain sensitivity and ME polarization coefficient for PbTiO3and P(VDF-TrFE) are opposite, in which the electro strictive constants play a significant role. However, the external stress has similar influence on the value of ME voltage coefficient in the two heterostructures. The compressive stress increases their ME voltage coefficient and a peak appears near the stress of200MPa, in qualitative agreement with the experiment result. It is proposed that external stress might provide a new way to enhance or adjust magnetoelectric coupling in multiferroic heterostructures.