Nanoferroelectric perovskite oxides with unusual morphology produced by different synthesis procedures

We report in the present paper some original results of a joint research performed in the framework of the COST Action 539 ELENA. In search of higher miniaturisation of electroceramic devices a new outlook seems to arise from ceramics with unusual morphology that might present a new kind of circular or toroidal ferroelectric ordering of dipoles. Completely new perspectives in data storage can be expected if a close control of size confinement and dimensionality as well as of the chemical composition and the phase purity is reached. We succeeded in the fabrication of BaTiO3 hollow nanoparticles and nanowires, and Bi4Ti3O12 platelets. The use of soft chemistry and solid state methods allowed to produce coreshell powders and ferroelectric-ferromagnetic composites with completely new functional properties

Ferroelectric order driven Eu3+ photoluminescence in BaZrxTi1−xO3 perovskite

The ability to tune and enhance the properties of luminescent materials is essential for enlarging their application potential. Recently, the modulation of the photoluminescence emission of lanthanide-doped ferroelectric perovskites by applying an electric field has been reported. Herein, we show that the ferroelectric order and, more generally the polar order, has a direct effect on the photoluminescence of Eu3+ in the model BaZrxTi1-xO3 perovskite even in the absence of an external field. The dipole arrangement evolves with increasing xfrom long-range ferroelectric order to short-range order typical of relaxors until the non-polar paraelectric BaZrO3 is achieved. The cooperative polar interactions existing in the lattice (x < 1) promote the off-center displacement of the Eu3+ ion determining a change of the lanthanide site symmetry and, consequently, an abrupt variation of the photoluminescence emission with temperature. Each type of polar order is characterized by a distinct photoluminescence behaviour

Synthesis procedure and properties of NiFe2O4 – BaTiO3 composites

NiFe2O4 (NF) powder was prepared by auto combustion method starting from nickel and iron nitrates. After the process of self-ignition, fine precursor powder was thermally treated at 1000 oC for 1h forming the nickel ferrite powder [1]. XRD analysis proved the formation of well crystallized nickel-ferrite cubic spinel structure. Particle size distribution measurements showed the existence of agglomerates. SEM micrographs presented nanometer particles from 100 to 500 nm. DBET calculated from specific surface area was ~ 700 nm and factor of agglomeration of obtained NF powder was ~ 27 %. Cubic barium titanate (BT) powder was prepared by soft chemical method (modified Pechini process). Spherical particles of around 75 nm were obtained in the BT powder [2]. Composites (NF-BT) with the general formula x NiFe2O4 – (1-x) BaTiO3 (x = 0.2, 0.3, 0.5) powders were prepared by mixing previously obtained powders of nickel ferrite and barium titanate in planetary ball mill for 24h. As a milling medium were used tungsten carbide balls and iso-propanol. Powder was pressed and sintered at 1170 oC for 4 h and from X-ray measurements the presence of NF and BT phases was detected. No secondary phases were found. Magnetic measurements of composite materials were carried out and presented in Table 1. Saturation magnetization moment of composite materials decrease with barium titanate amount and the fields at which saturation occur increase with BT content. The coercivity HC (Oe) increases with barium titanate concentration in obtained multiferroic material

SYNTHESIS PROCEDURE AND PROPERTIES OF NiFe2O4 – BaTiO3 COMPOSITES

NiFe2O4 (NF) powder was prepared by auto combustion method starting from nickel and iron nitrates. After the process of self-ignition, fine precursor powder was thermally treated and forming the nickel ferrite powder. XRD analysis proved the formation of well crystallized nickel-ferrite cubic spinel structure. Cubic barium titanate (BT) powder was prepared by soft chemical method (modified Pechini process). Composites (NF-BT) with the general formula x NiFe2O4 – (1-x) BaTiO3 (x = 0.2, 0.3, 0.5) powders were prepared by mixing previously obtained powders of nickel ferrite and barium titanate in planetary ball mill. As a milling medium were used tungsten carbide balls and iso-propanol. Powder was pressed and sintered at 1170 oC for 4 h and from X-ray measurements the presence of NF and BT phases was detected. No secondary phases were found. Magnetic measurements of composite materials were carried out. Saturation magnetization moment of composite materials decrease with barium titanate amount and the fields at which saturation occur increase with BT content. The coercivity HC (Oe) increases with barium titanate concentration in obtained multiferroic material

Synthesis and Properties of NiFe2O4 and Ni0.5Zn0.5Fe2O4 Prepared by Auto-combustion Method

NiFe2O4 (NF) and Ni0.5Zn0.5Fe2O4 (NZF) powders as a part of multiferroic composites were prepared by auto-combustion method starting from nickel, zinc and iron nitrates. After the process of self-ignition, fine precursor powders were thermally treated at 1000 oC for 1h forming the nickel ferrite and nickel-zinc ferrite powders [1]. XRD analysis proved the formation of well crystallized cubic spinel structure in both ferrites. Particle size distribution measurements showed the existence of agglomerates. SEM micrographs showed the existence of polygonal grains ~ 100-500 nm. The results of powders characterization are presented in the table I. Ceramics materials were obtained by sintering at 1250 oC for 4 h in the tube furnace. XRD analysis showed the formation of well crystalline spinel structure in both materials. Magnetic measurements of ferrites were carried out and presented in Figure 1. Saturation magnetization moment of NF was lower than for NZF and the fields at which saturation occur was almost the same for both ceramics. The coercivity HC (Oe) was higher for the NZF indicating that it is “softer” than NF [2]. Permeability vs. frequency measurements showed that NZF possesses much higher permeability that NF. On the other hand, the NF permeability value keeps constant values in a broader frequency range than NZF ceramics

SYNTHESIS AND CHARACTERIZATION OF NICKEL ZINC FERRITES

In this study we have prepared nickel ferrite (NF) and nickel zinc ferrite (NZF) nanoparticles by auto-combustion method starting from nickel, zinc and iron nitrates. After the process of self-ignition, fine precursor powder was thermally treated at 1000 °C for 1 h, forming nickel zinc ferrite powders, with molar ratio of Zn 0, 0.3, 0.5, 0.7. XRD characterization showed the formation of well crystallized nickel ferrite and nickel zinc ferrite inverse spinel structure without presence of secondary phases. Ceramic materials were obtained by uniaxial pressing at 196 MPa and sintering at 1250 °C for 4 h in the tube furnace. SEM images at the free surface showed that substitution of Ni2+ ions with Zn2+ ions results in larger grains and lower porosity, confirmed by density measurements. Magnetization results showed ferromagnetic behavior of the NF and NZF materials. Magnetic measurements of ferrites were carried out and presented in Fig. 1. Saturation magnetization moment of NF was lower than for N0.7Z0.3F and N0.5Z0.5F, but slightly higher than for N0.3Z0.7F. With increasing the ratio of Zn to 0.3, magnetization increases, because the Fe3+ ions in the octahedral site interact with other Fe3+ ions. The fields at which saturation occur was almost the same for all materials

Autocombustion synthesis and characterization of Ni1-xZnxFe2O4

Nickel ferrite(NF) and nickel zinc ferrite(NZF) powders were prepared by autocombustion method starting from nickel, zinc and iron nitrates [1]. After the process of self – ignition, fine precursor powder was thermally treated at 1000 °C for 1 h, forming nickel ferrite and nickel zinc ferrite powders. XRD characterization showed the formation of well crystallized nickel ferrite and nickel zinc ferrite inverse spinel structure [2] without presence of secondary phases. Ceramic materials were obtained by uniaxial pressing at 196 MPa and sintering at 1250 oC for 4 h in the tube furnace. SEM images at the free surface showed that substitution of Ni2+ ions with Zn2+ ions results in larger grains and lower porosity, confirmed by density measurements. Magnetization results showed ferromagnetic behavior of the NF and NZF materials. Magnetic measurements of ferrites were carried out and presented in Figure 1. Saturation magnetization moment of NF was lower than for N0.7Z0.3F and N0.5Z0.5F, but slightly higher than for N0.3Z0.7F. The fields at which saturation occur was almost the same for all materials

IMPROVED ELECTRICAL AND MAGNETIC PROPERTIES IN Y DOPED BiFeO3 CERAMICS

Bismuth ferrite (BiFeO3) is considered one of the most promising single phase multiferroic materials thanks to the fact it exhibits ferroelectric and antiferromagnetic properties in the same time in very wide range of temperatures (up to 370 °C). Difficulties in obtaining pure BiFeO3 phase and dense ceramics, together with occurrence of leakage currents have prevented application of BiFeO3. Substitution of Bi3+ or Fe3+ ions with some transition metal or rare earth ions can improve both electrical and magnetic properties by reducing leakage currents and introducing weak ferromagnetism through structural changes. Y doped bismuth ferrite, Bi1-xYxFeO3, was synthesized by auto-combustion method using urea as a fuel. Precursor powders were annealed, pressed and sintered. Powders and ceramic samples were characterized by XRD, SEM, Raman, impedance spectroscopy, ferroelectric and magneticmeasurements. X-ray diffractograms and Raman spectra showed no presence of secondary phases. SEM images indicated lowering of grain size with higher concentration of Y3+. Electrical resistance is highly improved even at 1 % of Y, while 10 % of Y was necessary to break spiral spin structure, leading to weak ferromagnetism

Synthesis and characterization of BaTiO3/-Fe2O3 core/shell structure

Multiferroic materials attracted a lot of attention in recent years because of their significant scientific interest and technological applications. The multiferroic core/shell powders have a better connectivity between the phases, resulting in superior dielectric and magneto electric properties. In this study, the influence of preparation condition on structure and properties of BaTiO3/-Fe2O3 core/shell composite materials was examined. The five samples were obtained by varying synthesis conditions, such as synthesized method (co-precipitation and sonochemical method) and pH values of solution. XRD and Raman spectroscopy analyses were performed in order to determine phase composition and structural changes within samples. Morphology modifications were examined by SEM and EDS analyses. Finally, effect of structural and microstructural changes on magnetic and electrical properties was detected and explained