Abstract
Two low-cost chemical methods of sol–gel and the hydrothermal process have been strategically combined to fabricate barium titanate (BaTiO3) nanopowders. This method was tested for various synthesis temperatures (100 °C to 250 °C) employing barium dichloride (BaCl2) and titanium tetrachloride (TiCl4) as precursors and sodium hydroxide (NaOH) as mineralizer for synthesis of BaTiO3 nanopowders. The as-prepared BaTiO3 powders were investigated for structural characteristics using x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The overall analysis indicates that the hydrothermal conditions create a gentle environment to promote the formation of crystalline phase directly from amorphous phase at the very low processing temperatures investigated. XRD analysis showed phase transitions from cubic - tetragonal - orthorhombic - rhombohedral with increasing synthesis temperature and calculated grain sizes were 34 – 38 nm (using the Scherrer formula). SEM and TEM analysis verified that the BaTiO3 nanopowders synthesized by this method were spherical in shape and about 114 - 170 nm in size. The particle distribution in both SEM and TEM shows that as the reaction temperature increases from 100 °C to 250 °C, the particles agglomerate. Selective area electron diffraction (SAED) shows that the particles are crystalline in nature. The study shows that choosing suitable precursor and optimizing pressure and temperature; different meta-stable (ferroelectric) phases of undoped BaTiO3 nanopowders can be stabilized by the sol-hydrothermal method.
Highlights
This work was promising, as it showed that using this simple low-cost chemical technique and at low temperatures, BaTiO3 could be synthesized at nanoscale with crystalline nature
Hydrothermal conditions employing chloride-based precursors and NaOH as a mineralizer created a gentle environment to promote the formation of crystalline phase directly from the amorphous phase at very low processing temperatures of 100 – 250 ◦C
scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis verified that BaTiO3 nanopowders synthesized by these methods were spherical in shape and about 114 – 170 nm
Summary
Barium titanate (BaTiO3) is a perovskite material that possesses superior dielectric properties along with ferroelectric, piezoelectric and electro-optic nature.[1,2] It is a promising material for device applications such as ferroelectric, random access memories, optical modulators, switches, waveguides and multilayer capacitors, micro-electromechanical systems and opto-electronic devices.[3,4,5,6,7,8,9,10] In particular, as the levelized cost of solar electricity has plummeted and other intermittent distributed generation technologies become more widespread, electrical storage needs are increasing.[11,12] There are many choices for battery technologies, but ultra-capacitors or super capacitors are gaining prominence.[13,14] BaTiO3 shows promise as one of these ultra-capacitor materials.[15,16,17,18,19]. The phase transition temperatures are 1432, 130, 5, and -90 ◦C respectively These features are studied by Xiao et al, and found that tetragonal phase is stable at room temperature.[21] Among different phases; tetragonal, orthorhombic and rhombohedral are ferroelectric crystals and cubic phase is paraelectric. At nano scale different multiphase’s co-existence occur at various temperatures.[21] The above discussions show that the phase transition of BaTiO3 may be a function of temperature and crystallite size These variables must be controlled in low-cost industrially scalable processes for. Typical synthesis conditions require temperatures higher than 200 oC for stabilizing tetragonal phase of BaTiO3 and reaction time extends from 10 -15 hours. The results are presented and the technical viability of sol-gel hydrothermal synthesis for stabilizing meta-stable phases of BaTiO3 at room-temperature is discussed
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