Ferroelectric behaviour of 30nm BaTiO3 ceramics prepared by high pressure assisted sintering

Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the ＇dilution＇ effect of grain boundaries.

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba_0.8)Sr_(0.2)TiO_(3) ceramics

Barium strontium titanate( Ba_0.8)Sr_(0.2)TiO_(3),BST)nanocrystalline ceramics have been synthesized by high energy ball milling.As the sintering temperature increases from 1200℃to 1350℃,the average grain size of BST ceramics increases from 86 nm to 123 nm.The X-ray diffraction(XRD)studies show that these ceramics are tetragonal.The phase and grain size of the sintered pellets have been estimated from the XRD patterns,scanning electron microscopy(SEM)and transmission electron microscopy(TEM)images.The effect of grain size on dielectric and ferroelectric properties is studied.The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size.The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm.The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases.These ceramics are promising materials for tunable capacitor device applications.

Microstructural evolution and mechanism of grain growth in magnesia ceramics prepared by high pressure and temperature with ultra-high heating rate

The fast densification method of combustion reaction plus quick pressing was adopted to prepare nanocrystalline ceramics.The densification process of magnesia compact with a particle size of 100 nm was investigated,under the applied pressure of up to 170 MPa,and the temperature range of 1740–2080 K with ultra-high heating rate(above 1700 K/min).High-purity magnesia ceramics with a relative density of 98.8%and an average grain size of 120 nm was obtained at 1740 K,and the grain growth during the densification process was effectively restrained.The characteristic morphology of evaporation-condensation was observed in the compact prepared at 2080 K,which revealed the actual process of mass transfer by gas diffusion.Moreover,the investigation on the microstructure evolution and mechanism of grain growth was carried out,on the basis of as-preserved nanocrystalline ceramics.The result indicated that the grain growth of the nanocrystalline MgO was controlled by the mechanism of evaporation-condensation rather than surface diffusion.Furthermore,the pressure had an influence of restraining the grain growth based on solid diffusion and strengthening the effect of gas diffusion with the increasing temperature.Under the particular conditions,there existed an appropriate temperature for the densification of nanocrystalline magnesia,while the excessive temperature would exaggerate grain growth and impede densification.