The synthesis of nanosized superparamagnetic hematite particles by dissolving ferric salts in hydrochloric acid and heating at 100• C is described. A hydrolysis reaction causes the formation of hematite particles. The influence of the sequence of additions on the resulting precipitates was studied using TEM and XRD. The magnetic behavior was characterized by magnetization measurements. It was found that small changes in the reaction conditions led to remarkable changes in final size and shape of the hematite crystallites. A well-defined subrounded morphology and an average diameter of 41 nm were obtained for superparamagnetic hematite particles. This is the largest size reported thus far for superparamagnetic hematite particles. C 2002 Elsevier Science (USA)

No abstract

Several wet chemical precipitation methods used to synthesise nanocrystalline composite powders containing zirconium oxide, yttrium oxide and iron(III) oxide are described. The crystallisation and phase composition of the precipitates were studied as a function of temperature. A co-precipitation method was compared with two different sequential precipitation routes. The amount of iron(III) oxide added varied from 27 to 42 mol% FeO 1.5 . The dried gel obtained after co-precipitation was extensively studied as a function of calcination temperature. After calcination for 2 hours at 600-700 uC about 10-16 mol% FeO 1.5 was dissolved in the zirconia matrix. A part of the ferric oxide was present outside of the zirconia lattice, either as a crystalline hematite phase (when adding w40 mol% FeO 1.5 ), or as a finely dispersed amorphous ferric oxide (when adding v34 mol% FeO 1.5 ). After calcination at temperatures around 900 uC only a few mol% FeO 1.5 were dissolved in the zirconia matrix. All sequential precipitation routes resulted in dual-phase yttria-doped zirconia and hematite powders after calcination. Only up to 4 mol% of ferric(III) oxide could be dissolved in the zirconia lattice when using a sequential precipitation route.

No abstract

Dual-phase nanocomposite powders of 3Y-TZP (tetragonal-ZrO 2 doped with 3 mol % Y 2 O 3 ) and RuO 2 have been prepared and densified into compacts. The electrical conductivity of the densified compacts was correlated with the microstructure. Variations in the synthesis method and powder composition influenced the densification and resulting microstructure of the composites. Densification of the composites was sensitive to changes in experimental temperature/pressure conditions. Sinterforging the material in the range 1150-1250 °C and 50-100 MPa pressure resulted in dense (>95%) composites. The microstructure of the sinterforged materials revealed that macroscopic phase separation had occurred as a result of the aerial oxidation of RuO 2 species above ≈700 °C. Because of the volatilization of RuO 2 , the pores in the pressed powder compact became filled with large ruthenia grains during densification. The microstructure thus reflected the agglomerate structure of the starting powder. The "inhomogeneous" microstructure led to high electrical conductivity in the material. The electrical conductivity of the dense composites was measured by the fourpoint technique and conductivity values on the order of 1-6 × 10 3 S cm -1 were obtained. High electrical conductivity (4.4 × 10 3 S cm -1 ) could be obtained, even at relatively low RuO 2 contents of 15 mol %.