We have investigated the effect of oleic acid concentration on the physicochemical properties of solvothermally derived cobalt ferrite nanoparticles (CFO NPs). Without the oleic acid, agglomerated nanoplatelets with a crystallite size of about 19 nm were obtained, according to X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, the addition of oleic acid decreases the size of the CFO NPs and at critical concentration, which was determined to be 0.25 M, well-dispersed, nonagglomerated spherical particles of about 6 nm were obtained. A further increase in the oleic acid concentration affected the particle size only slightly, with a relatively constant surface coverage of the oleic acid ligand. The results of our study indicate that particle-size control was achieved by bridging bidentate interactions between the oleic acid molecules and the metal atoms on the surface of the NPs, as determined by Fourier transform infrared spectra. These interactions affected the surface strain of NPs considerably, but kept the initial cation redistribution according to the Raman spectra. The room temperature magnetic measurements revealed that oleic acid enables us to effectively control the magnetic behavior of the CFO, which changes from ferrimagnetic to superparamagnetic at a critical concentration. Interparticle interactions are further interpreted by using low-temperature magnetic measurements, which also showed decreased surface anisotropy for samples prepared with oleic acid concentration above the critical value. An investigation of the treatment time showed that the capping with oleic acid is already achieved after 1 h of synthesis, but in order to improve the crystallization and consequently achieve the desired magnetic response a synthesis time of at least 4 h is required.
The mechanism of solid-state synthesis for Na 0.5 Bi 0.5 TiO 3 was investigated using X-ray powder diffraction, scanning electron microscopy and wavelength-dispersive X-ray spectroscopy. The results showed that a Na 0.52x Bi 0.5 TiO 32x/2 homogeneity region, with Na 0.5 Bi 0.5 TiO 3 and Na 0.47 Bi 0.5 TiO 32d as the end-members (0 ¡ x ¡ 0.03), exists in the Na 2 O-Bi 2 O 3 -TiO 2 system. The formation of any member from the homogeneity region starts with the formation of the deficient end-member, i.e., Na 0.47 Bi 0.5 TiO 32d . However, the reaction continues toward a nominal composition when a sufficiently high temperature and/or a long firing time are used. The conversion of the latter reaction may be followed by the observation of the crystal symmetry, which changes from the cubic-like toward the rhombohedral as the reaction proceeds from Na 0.47 Bi 0.5 TiO 32d to Na 0.5 Bi 0.5 TiO 3 . Differences in the dielectric, ferroelectric and piezoelectric properties were observed for the samples from the homogeneity region, and these can be related to the stoichiometry variation along the homogeneity region and thus to the variation in the vacancy concentration. The most striking difference is the change in the piezoelectric constant d 33 , which increases from 82 pC N 21 to 97 pC N 21 as the composition of the ceramics changes from
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.