The origin of abnormal ferroelectric and unusual piezoelectricity in the polycrystalline CaCu 3 Ti 4 O 12 (CCTO) thin films deposited by RF-sputtering on Pt/Ti/SiO 2 /Si (100) substrates was explored. The CCTO thin films, deposited at room temperature followed by annealing at 600°C for 2 h in a conventional furnace, have a cubic structure with lattice parameter a = 7.379 ± 0.001 Å and without any secondary phases. No polarization loss up to 10 10 switching cycles, with a switched polarization ∆P of 30 µC/cm 2 measured at 400 kV/cm was evidenced. The piezoelectric coefficient investigated by piezoresponse force microscopy (PFM) was approximately 9.0 pm/V. This may be the very first example of exploring the origin of ferroelectric behaviour for a material that possesses space charge polarization with highly resistive grain boundaries in the polycrystalline state.
In this study, we investigated the structural, microstructural, magnetic and cytotoxic properties of encapsulated ZnFe2O4 nanoparticles. The nanoparticles were synthesized using the microwave-assisted hydrothermal method and their surfaces were silanized and later encapsulated with poly-2-hydroxyethyl methacrylate (PHEMA). Due to the compatibility of Zn2+ ions with a human body, ZnFe2O4 nanoparticles are preferable among all kinds of ferrites for biomedical applications. Quantitative phase analysis obtained by the Rietveld refinement reveals the formation of a single-phase spinel cubic structure. Magnetic hysteresis loops measured at 2 and 300K reveal a remanent magnetization of 4.427 emu/g and 1.002 emu/g, respectively. Such behaviour was ascribed to change in the inversion degree of the spinel structure. The experimental g-factor (g = 1.897) obtained using electron paramagnetic resonance analysis can be attributed to the microwave heating, which induces more surface-active oxygen species. In addition, we demonstrated that the encapsulated ZnFe2O4 nanoparticles showed an absence of cytotoxicity at concentrations of 1.0, 10 and 20 ?g/ml against human embryonic kidney (HEK) cells since no significant changes in cell morphology were observed. Hence, our results indicate the possibility to explore the use of ZnFe2O4 nanoparticles encapsulated with PHEMA for biomedical applications, such as cancer therapies.
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undoubtedly has a great potential in the surge protection device industry. The first report [2] on varistor property of CCTO shows that an intrinsic electrostatic barrier built at the CCTO grain boundary is responsible for the nonlinear current voltage behaviour. In the CCTO polycrystalline ceramic, back-to-back potential barriers formed from band bending at the boundary region (like the n-in acceptor interface [3] electron trap, caused by negatively charged intrinsic donor defects) leads to an abrupt drop in potential from grain to grain boundary. This mechanism was further supported by Mei et al. [4] by a thorough study of grain and grain boundary composition. Their study reveals that a Curich secondary phase at the grain boundary with negatively charged vacancies () is responsible for the double Schottky barriers (n-in) with the n-type semiconductor grain boundary and grains [5, 6]. Chung et al. reported that CCTO exhibits nonlinear current-voltage characteristics even in the absence of any dopants [7]. These outstanding electrical properties are quite unusual since CCTO exhibits a body centered cubic perovskite structure with slightly tilted [TiO 6 ] octahedra facing each other [8], i.e. the compound is not ferroelectric. Despite numerous studies including quantum chemical calculations [9], the origin of this high dielectric constant remains still unclear. Impedance spectroscopy has proven that such ceramics consist of semiconducting grains and insulating grain boundaries. Results from Sinclair et al. [3] suggest that the high dielectric constant is actually an internal grain boundary barrier layer capacitance, in other words, it is a grain boundary effect that is not linked to the perovskite crystal structrure. Since the nonlinear electrical properties are not independent from the grain boundary nature, one would expect that they are somehow connected to the grain boundary barrier layer capacitance. It is also known that oxygen adsorbed at the grain boundaries plays Abstract Calcium copper titanate (CaCu 3 Ti 4 O 12 , CCTO), thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO 2 /Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The CCTO thin film present a cubic structure with lattice parameter a = 7.379 ± 0.001 Å free of secondary phases. Dielectric spectroscopy was employed to examine the polycrystalline behaviour of CCTO material and the mechanisms responsible for the barrier-layer capacitances associated with Schottky-type barriers and the non-Ohmic properties. The film presents an electric breakdown field (E b = 203 V cm −1) and then nonlinear coefficient (α = 6), which is even lower than that of the ZnO and SnO 2 based varistors The observed electrical features of CCTO thin films are highly dependent on the [CaO 12 ], [CaO 4 ], [CuO 11 ], [CuO 11 V o x ] and [TiO 5 •V • O
Pure and calcium-modified (Ca x Bi 1-x FeO 3 , x = 0.0, 0.1, 0.2, 0.30) thin films were fabricated on Pt(111)/Ti/SiO 2 /Si substrates by the soft chemical method using LaNiO 3 as the bottom electrode. Highly (200)oriented BFO film was coherently grown on LNO at 500°C. Ca-doped BiFeO 3 films have a dense microstructure and rounded grains. The conventional problem of the leakage current for the highest doped film was reduced from 10-5 to 10-10 with remarkable improvement in the film/electrode interface, chemical homogeneity, crystallinity, and morphology of the BFO film. Enhanced ferroelectricity was observed at room temperature due to the bottom electrode. Fatigue-free films were grown on LaNiO 3 bottom electrodes with no degradation after 1×10 10 switching cycles at an applied voltage of 5 V with a frequency of 1 MHz. After several tests the capacitors retained 77% of its polarization upon a retention time of 10 4 s. Room temperature magnetic coercive field measurements indicate that the magnetic behaviour is influenced by the nature of the bottom electrode.
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