Preparation of Phase‐Pure K0.5Na0.5NbO3 Fine Powders by a Solid‐State Reaction at 625°C from a Precursor Comprising Nb2O5 and K,Na Acetates

Phase‐pure K_0.5Na_0.5NbO₃ (KNN) fine powders were synthesized via a solid‐state route from a homogeneous solid mixture. A colloidal dispersion comprising a mixed ethanol solution of potassium and sodium acetates and Nb₂O₅ fine particles was attrition milled and dried carefully to avoid water absorption. Two‐step calcination in air at 450°C and 625°C, each for 3 h, resulted in the phase‐pure KNN powders. The volume‐based median diameter of the final product was ca. 0.8 μm. Starting from the same precursors without dissolving the acetates, the phase‐pure KNN was never achieved even when the two calcination temperatures were increased to 550°C and 700°C, in spite of the same milling conditions. Key issues of eliminating second phases were (i) starting from a wet‐milled mixture with a single solution containing both of the A‐site species, and (ii) repeated wet milling of the reaction mixture to disintegrate reaction‐induced agglomerates. These enabled rapid nuclei growth from chemically interacted precursor prior to calcination, and short diffusion path due to repeated deagglomeration, excluding formation of off‐stoichiometric second phases. All these items were confirmed by different analytical tools, among others, thermo‐gravimetry and differential thermal analysis (TG‐DTA), particle size analyses, and XPS at various reaction stages. On the heating stage microscope, a shrinkage onset was observed at 850°C, that is, 150 K lower than that of conventionally prepared KNN, that is, via a solid‐state synthesis from carbonates by a two‐step calcination at 800°C and 750°C, for 4 h each. No second phase was observed after sintering up to 1100°C.     

PTCR anomaly in barium-titanate-based composites

We have identified a PTCR anomaly in undoped BaTiO₃ (BT) ceramics. This anomaly was ascribed to a disconnection of the semiconducting grains, due to dimensional changes of the BT grains at the Curie point, in a composite composed of two constituent BT phases, one with a low electrical resistivity and the other with a high electrical resistivity. The composite exhibits a significant PTCR effect of three orders of magnitude at the Curie temperature.     

The Preparation of Barium Hexaferrite Coatings Using HVOF

Thick coatings of barium hexaferrite with the compositions BaFe₁₂O₁₉ and BaCoTiFe₁₀O₁₉ were prepared using high-velocity oxygen-fuel (HVOF) spraying technology. Nanocrystalline precursors embedded in an amorphous matrix were obtained on both Fe and glass–ceramic substrates. To promote the crystallization of the hexaferrites, the coatings were annealed at 800°–1000°C, and single-phase coatings were obtained at 1000°C. The crystallization process was studied with X-ray powder diffraction and with electron microscopy. The magnetic measurements of the coatings were carried out in a static field and at high frequencies. The magnetization of the coatings increased with the annealing temperature to above 50 emu/g for both compositions. The coercivity of BaFe₁₂O₁₉ increased with the annealing temperature to above 2400 Oe, whereas the coercivity of BaCoTiFe₁₀O₁₉ decreased from over 800 Oe, for the as-deposited sample, to 400 Oe for the sample annealed at 1000°C. A minimum 90% absorption was calculated for the BaFe₁₂O₁₉ coatings with thicknesses of 0.15–0.25 mm at around 47 GHz and for the 1–4-mm-thick coatings of BaCoTiFe₁₀O₁₉ at 3–9 GHz.     

Electromagnetic wave absorption by an organic resin solution based on ferrite particles with a spinel crystal structure

We have investigated an organic resin solution designed for EM wave absorption based on a magnetic filler, composed of phases within the Mn_0.66Zn_0.27Fe_2.07O₄ system, embedded in an absorber composite with concentration ratios of 50:50, 75:25 and 90:10 by weight. The formation of the manganese zinc ferrite particles, as the principal magnetic phases, was achieved via the conventional ceramic method. The electromagnetic parameters of the composites were measured with a vector network analyser at 100 MHz to 10 GHz. The subject of the paper was a study of the electromagnetic absorber properties and the rheological properties of the resin composite based on ferrite particles with respect to using the materials in architectural coatings.     

Synthesis and characterization of nano-crystalline strontium hexaferrite using the co-precipitation and microemulsion methods with nitrate precursors

Nano-crystalline strontium hexaferrite (SrFe₁₂O₁₉) powder was synthesized using the classical co-precipitation and microemulsion methods. The precursors were obtained by precipitating Sr²⁺ and Fe²⁺ ions using tetramethylammonium hydroxide and calcinating at different temperatures ranging from 400 °C to 1000 °C in air. The influence of the Sr²⁺/Fe³⁺ mol ratio and the calcination temperature on the product formation and magnetic properties were studied. The formation of nanosized particles of SrFe₁₂O₁₉ with a relatively high saturation magnetization M_s = 64 Am²/kg, remanent magnetization of M_r = 39 Am²/kg and a coercitivity of H_c = 5.5 kOe was achieved at a Sr²⁺/Fe³⁺ mol ratio of 1:8 calcined at 900 °C. The formation of the SrFe₁₂O₁₉ was inspected using XRD analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA), TEM, and magnetic measurements.     

Electrical properties of Zn-Ni-O ceramics

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Grain Growth During Sintering of Donor-Doped BaTiO3

Grain growth of donor-doped BaTiO₃ in the presence of KF b'quid phase was studied. The results showed that the composition of the liquid phase present during sintering had a pronounced influence on grain growth and on the formation of semiconducting donor-doped BaTiO₃.     

Digging Deep into the Data Mine with DataMiningGrid

The growing computerization in modern knowledge and technology sectors is generating huge volumes of electronically stored data. Data mining technology is often employed to make sense of these data. However, as modern data mining applications increase in complexity, so do their demands for resources. Grid computing is one of several emerging networked computing paradigms promising to meet the requirements of heterogeneous, large-scale and distributed data mining applications. Despite this promise, there are still too many issues to be resolved before grid technology is commonly applied to large-scale data mining tasks. To address some of these issues, we developed the DataMiningGrid system, which principally differs from similar systems by its ability to integrate a diverse set of programs and application scenarios within a single framework. The system's key features include high performance and scalability, sophisticated support for relevant standards, different user types, and flexible extensibility. The software is available as open source.     

Hydrothermal Synthesis of Manganese Zinc Ferrites

Hydrothermal synthesis has been used to synthesize nano-sized manganese zinc ferrite powder. The results show that the pH value of the starting suspension has a decisive influence on the composition of the hydrothermally prepared manganese zinc ferrite powder. At a pH value of ∼8.5, stoichiometric amounts of manganese and zinc can be incorporated in the manganese zinc ferrite. The grain size of the powder increases with the temperature and time of hydrothermal treatment. The nanosized manganese zinc ferrite produced is superparamagnetic and becomes ferri-magnetic after the ferrite grains become larger than ∼100 nm. The nanosized ferrite grains are very prone to oxidation and disintegrate at 600°C in air. The surfaces of the synthesized ferrite grains are covered with a thin film of water.     

Subsolidus phase equilibria and the Li5Nd4FeO10 phase in the Li2O-Nd2O3-Fe2O3 system

A survey of the subsolidus phase equilibria in the system Li₂O-Nd₂O₃-Fe₂O₃ was made at subsolidus temperatures in the range 1000-1050 °C. A ternary phase was identified. The phase is centered on Li₅Nd₄FeO₁₀, with a cubic lattice a = 11.9494 Å. The compound melts incongruently at 1105 °C. The magnetic susceptibility was measured in the temperature range 4-300 K. The compound is paramagnetic in the temperature range 150-300 K and follows the Curie-Weiss law. At about T_N = 10 K, a long-range magnetic ordering is observed.