Nanocrystalline calcium titanate (CT) ceramic has been synthesized by a combination of solid-state reaction and high-energy ball milling. This nano-ceramic is characterized by X-ray diffraction (XRD), dielectric study and impedance spectroscopy. The XRD pattern shows single phase ceramic of orthorhombic symmetry. The frequency-dependent dielectric study shows that the dielectric constant is maximized at low frequencies and decreases with an increase in frequency. Impedance spectroscopy analyses reveal a non-Debye type relaxation phenomenon. A significant shift in impedance loss peaks toward the higher-frequency side indicates conduction in the material favoring the long-range motion of mobile charge carriers. The grain conduction effect is observed from the complex impedance spectrum by the appearance of one semicircular arc in Nyquist plot. It is also observed that the resistance decreases with an increase in temperature showing a negative temperature coefficient of resistance (NTCR). Various thermistor parameters have been calculated by fitting with Steinhart-Hart equation. The modulus plots represent the presence of temperaturedependent electrical relaxation phenomenon with the material. The frequency-dependent AC conductivity at different temperatures indicates that the conduction process is thermally activated. The activation energy has been calculated from an Arrhenius plot of DC conductivity and relaxation frequency.

Nanocrystalline and bulk Li 2 TiO 3 having monoclinic structure were prepared by mechanical alloying as well as conventional ceramic route. Complex impedance analysis in the frequency range of 100 Hz-1 MHz over a wide range of temperature (50-500 ℃) indicates the presence of grain boundary effect along with the bulk contribution. The frequency-dependent conductivity plots exhibit power law dependence, suggesting three types of conduction in the material: low-frequency (100 Hz-1 kHz) conductivity showing long-range translational motion of electrons (frequency independent), mid-frequency (1-10 kHz) conductivity showing short-range hopping of charge carriers and high-frequency (10 kHz-1 MHz) conductivity showing conduction due to localized orientation of hopping mechanism. The electrical conductivity measurement of nanocrystalline and bulk Li 2 TiO 3 with temperature shows the negative temperature coefficient of resistance (NTCR) behavior. The activation energy (0.77 eV for nano sample and 0.88 eV for bulk sample) study shows the conduction mechanism in both samples. The low activation energies of the samples suggest the presence of singly ionized oxygen vacancies in the conduction process.

Lithium titanate (Li 2 TiO 3 ) is one of the most promising candidates among the tritium breeding materials because of its good tritium release capacity. Li concentration has much significance on the diffusivity of tritium in the material. The nanocrystalline single-phase Li 2 TiO 3 with monoclinic structure has been prepared by high energy ball milling followed by calcination at 700 ℃ for 2 h. The field emission scanning electron microscopy (FESEM) studies confirmed uniform distribution of nanocrystalline phase with particle size below 100 nm. The study of the Li + ion diffusion on the sintered sample was investigated by means of electrical conductivity measurements. Electrical properties of the samples were studied in wide temperature (50-500 ℃) and frequency (100 Hz-1 MHz) ranges. The complex impedance spectroscopy (CIS) studies showed the presence of both bulk and grain boundary effects in nanocrystalline Li 2 TiO 3 . The bulk resistance of the samples has been observed to decrease with rise in temperature showing a typical negative temperature coefficient of resistance (NTCR) behavior. The low activation energies of the samples suggested the presence of singly ionized oxygen vacancies in the conduction process. The hopping frequency shifted toward higher frequency with increase in temperature. Activation energy of 0.86 eV was calculated from AC conductivity.

Abstract:It is possible to fabricate highly sensitive NTCR (negative temperature coefficient of resistance) thermistor using nano crystalline CaTiO 3 synthesized by high energy ball milling. Disc shaped green pellets were prepared and effects of sintering on the disc pellets were studied as thermistor by sintering the samples at 1000 ℃, 1100 ℃ and 1200 ℃. The as-prepared samples were characterized by X-ray diffraction (XRD), impedance analysis and electrical measurement. The resistivity of the prepared samples varies predictably with temperature: this makes them promising material for temperature sensor. The experimental results prove that nano crystalline CaTiO 3 ceramic is one kind of thermistor with exponential negative temperature coefficient of resistance in the temperature range of 300-500 ℃. The samples have the advantages of rapid response, high sensitivity and capability to withstand thermal surges over the temperature range of 300-500 ℃. Resistance-temperature characteristics are described by thermistor equation with thermistor constant around 4003 K to 10795 K and thermal coefficient of resistance α around 1%/℃ to 13%/℃. The activation energy is in the range of 0.34-0.93 eV. The observed thermistor parameters are found to be comparable with many of the known thermistor materials. This suggests that the electrical properties can be adjusted to desirable values by controlling the temperature parameter. The influence of fabrication process of disc thermistor and electrical properties are discussed. The study shows the potential of nano crystalline CaTiO 3 to act as an NTCR material for thermistor applications.