Giant dielectric, low dielectric loss, and non-ohmic properties of nanocrystalline CaCu3Ti4O12

A simple polymer pyrolysis method has been successfully used to prepare CaCu₃Ti₄O₁₂ (CCTO) nanoparticles by calcination the obtained precursor powder at a low temperature of 800 (CCTO-1) and 850 °C (CCTO-2) in air for 4 h. The XRD results show that both of the calcined powders (CCTO-1 and CCTO-2) are pure having perovskite structure with the crystallite sizes, as evaluated by the XRD line boardening technique, of 47.5 and 75 nm, respectively. The particle sizes as estimated from the bright field images of TEM were found to be in the range of 10–35 and 7–52 nm for CCTO-1 and CCTO-2, respectively. The further sintering of CCTO-1 and CCTO-2 at 1,050 °C in air for 6 h, CCTO-1A and CCTO-2A, are also pure with perovskite structure as indicated by the XRD results. The measurements of the dielectric constant ( $ \varepsilon^{\prime } $ ε ′ ) and the low loss tangent (tanδ) at 1 kHz and 20 °C of CCTO-2A were found to be ~11,472 and ~0.0438, respectively. In addition, the CCTO-2A sample shows a small temperature coefficients ( $ \left| {\Updelta \varepsilon^{\prime } } \right| < 15\,\% $ | Δ ε ′ | < 15 % ) in a wide temperature range from ?50 to 110 °C. The non-Ohmic properties non-linear coefficient (α) of CCTO-1A and CCTO-2A were observed and the non-linear coefficient (α) of them determined in the range of 1–10 mA cm^?2 were found to be 12.00 and 7.26, respectively. Moreover, the breakdown field (E _b ) of CCTO-1A and CCTO-2A ceramics obtained at J = 1 mA cm^?2 were calculated and found to be 811 and 1,342 V cm^?1, respectively.     

Phase formation,microstructure and electrical properties of mica glass-ceramics containing Cr2O3 produced by heat treatment

The mica glass-ceramics containing Cr₂O₃ were produced via heat treatment process from the bulk parent glass specimens. The appropriate treatment temperatures were selected according to the information provided by the DTA measurement. XRD analysis demonstrated that co-existence of the mica and MgAl₂Si₃O₁₀ phases were found at the lower treatment temperature while pure mica phase could be formed at a higher temperature. The SEM investigations revealed the effect of Cr₂O₃ addition on mica crystals development. The electrical resistivity of the glass-ceramic samples showed some significant relation with the added ions. With small amount of addition, an increase of the electrical resistivity was observed. The present glass-ceramics could be a promising candidate as advanced insulating materials, as evidenced from the presence of crystalline phases with machinable properties and good mechanical strength coupled with its high resistivity.     

The effect of heat treatment temperature on phase formation and luminescence properties of calcium magnesium silicate glass-ceramics doped with Sm2O3

Abstract

The transparent glasses of CaO-MgO-Al₂O₃-SiO₂-ZnO system doped with Sm₂O₃ was prepared by conventional melt-quenching method. The obtained glasses were heat treated at a suitable temperature (875?°C–920?°C for 2?h) identified by differential thermal analysis (DTA). Phase formation and microstructure of glass-ceramics were characterized by X-ray diffraction and scanning electron microscopy, respectively. The optical transmission spectra were recorded by UV-Vis spectrophotometer in the wavelength range between 350 and 1000?nm. It was found that the increase in heat treatment temperature reduced the transparency of the glass-ceramics. The luminescence properties were identified by fluorescence spectroscopy. The excitation spectra of Sm₂O₃ doped CaO-MgO-Al₂O₃-SiO₂-ZnO glass-ceramic samples are in wavelength range of 550–750?nm and the emission spectra exhibited a strong orange-red luminescence composed of 562, 599 and 654?nm under excited at 402?nm. The results of XRD studies revealed the occurrence of diopside (CaMgSi₂O₆) and akermanite (Ca₂MgSi₂O₇) phases. The increasing of heat treatment temperature has no effect on the shift of emission spectra.     

Phase formation and dielectric properties of ferroelectric glass-ceramics in Na2O-BaO-Nb2O5-SiO2 system doped with Nd3+

Ferroelectric Glass-ceramics of the Na₂O–BaO–Nb₂O₅–SiO₂ system were obtained from controlled crystallization process performed on the parent glass of composition (24-x)Na₂O-xBaO-26Nb₂O₅-50SiO₂ where x?=?4 was selected. Nd₂O₃ doping was applied in the range 0–3 mol%. X-ray diffraction analysis indicated that NaNbO₃ and NaBa₂Nb₅O₁₅ crystals formed as two main crystalline phases. Their relative intensities varied with treatment temperature. Crystalline sizes of both NaNbO₃ and NaBa₂Nb₅O₁₅ calculated from XRD peak broadening were in nanoscale range. Increasing of doping content also gave a parent glass with higher bulk density. The dielectric constant measured at room temperature for glass-ceramic samples was found to be sensitive to the presence of NaBa₂Nb₅O₁₅ phase. The results of the present work suggest that introduction of Nd³⁺ into Na₂O-BaO-Nb₂O₅-SiO₂ system not only alters the dielectric response but also changes the phase transition behaviors of the co-existant ferroelectric phases by suppressing the growth of the NaBa₂Nb₅O₁₅ crystals.