Microstructural Evolution and Magnetic Properties of NiFe_2O_4 Nanocrystals Dispersed in Amorphous Silica

NiFe_2O_4 nanocrystals were dispersed in silica by a sol−gel route. The dried gel was amorphous, in which isolated Fe^(3+) ions had a weak interaction with silica matrix, as characterized by a weak IR absorption at ca. 580 cm^(-1). Heat treatment at 400 °C resulted in nickel ferrite clusters being partially formed, and these clusters were observed to interact with the matrix through Si−O−Fe bonds. This interaction reached its maximum with the complete formation of NiFe_2O_4 clusters as the temperature was raised to 600 °C. Above this temperature, NiFe_2O_4 clusters grew larger into nanocrystals, while the interaction between the nanocrystals and silica matrix disappeared with breakage of Si−O−Fe bonds. The grain growth for magnetic nanoparticles was accompanied with rearrangement of amorphous silica network. The preference of forming NiFe_2O_4 nanocrystals eliminated the possibility of precipitation of crystallite component oxides, e.g., NiO, γ-Fe_2O_3, or Fe_3O_4 in amorphous silica matrix, or crystalline silica, e.g., cristobalite or quartz, even when the treatment temperature was 1100 °C. Fe ions in silica glasses were determined by Mössbauer spectroscopy to be present exclusively as Fe^(3+) ions in a high-spin state at octahedral coordination, and the chemical environment of the Fe^(3+) ions seemed to remain unchanged until the nickel ferrite clusters crystallized. The formation mechanism for NiFe_2O_4 nanocrystals can be explained in terms of Ni^(2+) ions shifting from the tetrahedral centers to undistorted octahedral sites in the spinel lattice and the partial transformation of FeO_6 octahedron to FeO_4 tetrahedron. The critical dimension for the NiFe_2O_4 nanocrystals in silica was detected as ca. 9 nm. Below the critical size, NiFe_2O_4 nanocrystals had a superparamagnetic single-domain structure, while the nanocrystals with particle sizes larger than the critical size exhibited bulklike behavior

Microstructural Evolution and Magnetic Properties of NiFe_2O_4 Nanocrystals Dispersed in Amorphous Silica

NiFe_2O_4 nanocrystals were dispersed in silica by a sol−gel route. The dried gel was amorphous, in which isolated Fe^(3+) ions had a weak interaction with silica matrix, as characterized by a weak IR absorption at ca. 580 cm^(-1). Heat treatment at 400 °C resulted in nickel ferrite clusters being partially formed, and these clusters were observed to interact with the matrix through Si−O−Fe bonds. This interaction reached its maximum with the complete formation of NiFe_2O_4 clusters as the temperature was raised to 600 °C. Above this temperature, NiFe_2O_4 clusters grew larger into nanocrystals, while the interaction between the nanocrystals and silica matrix disappeared with breakage of Si−O−Fe bonds. The grain growth for magnetic nanoparticles was accompanied with rearrangement of amorphous silica network. The preference of forming NiFe_2O_4 nanocrystals eliminated the possibility of precipitation of crystallite component oxides, e.g., NiO, γ-Fe_2O_3, or Fe_3O_4 in amorphous silica matrix, or crystalline silica, e.g., cristobalite or quartz, even when the treatment temperature was 1100 °C. Fe ions in silica glasses were determined by Mössbauer spectroscopy to be present exclusively as Fe^(3+) ions in a high-spin state at octahedral coordination, and the chemical environment of the Fe^(3+) ions seemed to remain unchanged until the nickel ferrite clusters crystallized. The formation mechanism for NiFe_2O_4 nanocrystals can be explained in terms of Ni^(2+) ions shifting from the tetrahedral centers to undistorted octahedral sites in the spinel lattice and the partial transformation of FeO_6 octahedron to FeO_4 tetrahedron. The critical dimension for the NiFe_2O_4 nanocrystals in silica was detected as ca. 9 nm. Below the critical size, NiFe_2O_4 nanocrystals had a superparamagnetic single-domain structure, while the nanocrystals with particle sizes larger than the critical size exhibited bulklike behavior

Preparation and Transport Properties of New Oxide Ion Conductors KNb_(1-x)Mg_xO_(3-δ) by High Temperature and Pressure

A series of oxide ion conductors KNb_(1-x)Mg_xO_(3-δ) (x = 0.05−0.30) were prepared at a temperature of 870 °C and a pressure of 4.0 GPa. All samples were thermodynamically stable at ambient pressure and crystallized in an orthorhombic perovskite structure. The lattice volume enlarged with increment of dopant level, which was associated with the ionic substitution, variation of the relative content of oxygen vacancy Vö, and defect associations {Mg_(Nb)‘ ‘‘Vö}, as well as an increase of disorder in Mg^(2+)/Nb^(5+) distribution at B-sites of perovskite lattice. At higher temperatures, KNb_(1-x)Mg_xO_(3-δ) underwent phase transitions from orthorhombic to tetragonal, pseudocubic, and cubic in sequence, as confirmed by DTA and high-temperature Raman spectra. No thermal effects associated with the decomposition reactions were observed in KNb_(1-x)Mg_xO_(3-δ) during the successive heating process up to 1000 °C. The high-temperature phase had a relatively high structural stability. Impedance spectra of KNb_(1-x)Mg_xO_(3-δ) showed bulk and grain boundary conduction. The total conduction was determined to be predominately ionic, while the p-type electronic contribution was extremely small. KNb_(0.90)Mg_(0.10)O_(2.85) was found to provide a highly conductive phase with a conductivity of σ_(700°C) = 1.10 × 10^(-3) S·cm^(-1). Further, the ionic conductivity data for KNb_(1-x)Mg_xO_(3-δ) were separated into two linear ranges, corresponding to the pseudocubic and cubic phases, respectively. The variations of conductivity and activation energy for both pseudocubic and cubic phases can be explained in terms of the relative content of the oxygen vacancy and defect associations, delocalization of partial oxygen vacancies, and an order−disorder transition

Preparation and Transport Properties of New Oxide Ion Conductors KNb_(1-x)Mg_xO_(3-δ) by High Temperature and Pressure

A series of oxide ion conductors KNb_(1-x)Mg_xO_(3-δ) (x = 0.05−0.30) were prepared at a temperature of 870 °C and a pressure of 4.0 GPa. All samples were thermodynamically stable at ambient pressure and crystallized in an orthorhombic perovskite structure. The lattice volume enlarged with increment of dopant level, which was associated with the ionic substitution, variation of the relative content of oxygen vacancy Vö, and defect associations {Mg_(Nb)‘ ‘‘Vö}, as well as an increase of disorder in Mg^(2+)/Nb^(5+) distribution at B-sites of perovskite lattice. At higher temperatures, KNb_(1-x)Mg_xO_(3-δ) underwent phase transitions from orthorhombic to tetragonal, pseudocubic, and cubic in sequence, as confirmed by DTA and high-temperature Raman spectra. No thermal effects associated with the decomposition reactions were observed in KNb_(1-x)Mg_xO_(3-δ) during the successive heating process up to 1000 °C. The high-temperature phase had a relatively high structural stability. Impedance spectra of KNb_(1-x)Mg_xO_(3-δ) showed bulk and grain boundary conduction. The total conduction was determined to be predominately ionic, while the p-type electronic contribution was extremely small. KNb_(0.90)Mg_(0.10)O_(2.85) was found to provide a highly conductive phase with a conductivity of σ_(700°C) = 1.10 × 10^(-3) S·cm^(-1). Further, the ionic conductivity data for KNb_(1-x)Mg_xO_(3-δ) were separated into two linear ranges, corresponding to the pseudocubic and cubic phases, respectively. The variations of conductivity and activation energy for both pseudocubic and cubic phases can be explained in terms of the relative content of the oxygen vacancy and defect associations, delocalization of partial oxygen vacancies, and an order−disorder transition