Formation Of Hard Agglomerates Research Articles

Synthesis of Bi-based superconducting powders through the freeze drying

Abstract High-purity submicron Bi–Pb–Sr–Ca–Cu–O particles were prepared by the freeze-drying method from modified aqueous solutions of the corresponding metal nitrates. Synthesis through an aerosol enables the generation of ultrafine multicomponent powders with improved compositional homogeneity. Bi–Pb–Sr–Ca–Cu–O rich particles obtained through freeze drying are characterized with the mean particle size below 1 μm, irregular shape, smooth surface and they are slightly agglomerated. The BET surface area is 2.5 m2 g−1 and the mean crystallite size is calculated to be 231 nm. High reactivity of the particles and appearance of the liquid phase produce increased crystallinity, interparticle sintering and hard agglomerate formation during additional heat treatment in oxygen.

Improvements in α-Fe2O3 ceramic sensors for reducing gases by addition of Sb2O3

The microstructure, electrical properties and gas-sensing characteristics of Sb-doped α-Fe2O3 were investigated. Powder precursors with Sb/Fe = 0–0.1 were prepared by chemical coprecipitation method. Sb-doped α-Fe2O3 powders were characterized by means of thermal gravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), BET surface area and scanning electron microscope (SEM). It was found that the raw powders underwent crystallization into the corundum structure of α-Fe2O3 at a temperature which increased somewhat with increasing Sb content; a proper amount of Sb doping suppressed both crystallite growth and the formation of hard agglomerates. The doping of Sb2O3 decreased the sensor resistance by one order of magnitude and increased the sensitivities to some hydrocarbon gases markedly. The former can be attributed to the substitution of Sb5+ for Fe3+ sites in α-Fe2O3 generating more free electrons; the latter is closely related to Sb-doped samples accommodating a higher density of chemisorbed oxygen.

Evaluation of Powder Properties in Aqueous Processing of Microwave Dielectric Ceramics.

As-received calcium titanate and neodymium aluminate of median sizes 4.0 and 1.2μm, respectively, showed fast sedimentation within 10min because of the formation of hard agglomerates about 1-10μm. When ball-milling time became longer, the larger particles were selectively ground and the particle size distributions became narrow. The dispersibility of 96h-milled powders was greatly improved by addition of polyacrylic ammonium (PAA) of 2mass%. The viscosity of suspension depended on the amount of PAA adsorbed and was lower for CaTiO3 suspension than NdAlO3 suspension. Consolidation of the suspensions and sintering behavior of the CaTiO3-NdAlO3 system were also studied.

Study on drying stage of nanoscale powder preparation

Nanoscale barium titanate powders of cubic phase were prepared by solprecipitation method following three kinds of drying stages: freeze drying, azeotropic distillation and oven drying. Freeze drying is the most effective procedure, though both freeze drying and azeotropic distillation stages are successful in preventing hard agglomerate formation and improving the sinterability of powder. The mechanism by which the drying stage prevents the formation of hard agglomerates was investigated.

Microstructural Optimization of Low-Temperature-Fired Ni–Zn–Cu Ferrites Using Calcination

The purpose of this work was to examine the effect of low-temperature sintering on the microstructural optimization of Ni–Zn–Cu ferrites. This behavior was evaluated with respect to calcination temperature. The amount of unreacted hematite produced during calcination was quantitatively analyzed using a new calibration curve obtained using X-ray powder diffractometry. The formation of a spinel phase was initiated above 550° C, and completed above 850° C. The microstructural optimization of Ni–Zn–Cu ferrite samples was achieved using powder calcined at 750° C, and these specimens showed a maximum permeability of 700. A variation in microstructure with the change of the calcination temperature is ascribed as the cause of this optimization. Two mechanisms for the optimization are also suggested. One is the formation of hard agglomerates at high calcination temperatures and the second is the inhibition of grain growth due to the residual hematite in powders calcined at low temperatures. Microstructural optimization was achieved through a trade-off between these two mechanisms.

Bulk nanostructured oxide materials and the superplastic behavior under tensile fatigue at ambient environment

Nano-sized yttria stabilized tetragonal zirconia polycrystal (Y-TZP) powders were prepared by wet chemical process. Carefull characterization and ways to avoid hard-agglomerate formation were discussed. Full densification of large size samples was achieved with average grain size of 100±20 nm. Samples of the same chemical composition with submicron grains were simultaneously prepared for comparison. Cyclic tensile fatigue studies on both types of specimens were pursued at ambient temperature. It was first documented by AFM imaging that localized superplastic deformation of the 100 nm grains at and near the fatigue fracture surfaces was generated; while in the submicron grain-sized samples, the grains retained their original equiaxed morphology. The micromechanisms underlying these phenomena were discussed. Grain boundary diffusion is argued to be the major governing process in operation.

Consolidation of Nano-Crystalline Alumina

Abstract Dense alpha alumina was fabricated from seeded nano-crystalline boehmites. The intrinsic problem of boehmite solubility and hard agglomerate formation was avoided by dispersing gel agglomerates in ethanol. Homogeneous green microstructure and high sintered density were obtained by dry pressing. The superplasticity of nano-crystalline transition alumina was utilized to obtain fully dense, transparent alpha alumina by sinter forging. The effects of seeding and pressure are correlated with the microstructural evolution and densification of alumina.

Preparation and characterization of nanoscale Y-TZP powder by heterogeneous azeotropic distillation

A heterogeneous azeotropic distillation process was effectively used to dehydrate hydrous zirconia and therefore prevent the formation of hard agglomerates in the preparation of nanoscale zirconia powder. The mechanism of azeotropic distillation to prevent hard agglomerate from forming was studied by investigating the interaction of hydrous zirconia with n-butanol. The prepared powder was sintered to 99.5% of theoretical density by slow-sintering (average grain size of ∼200 nm), 97.5% by fast-sintering (average grain size of ∼120 nm) at 1250°C.

Process of fine milling for ceramic materials

The production of powders with particle sizes lower than 5 μm with a narrow distribution and little contamination is an important aim in ceramic processing. It is generally accepted that in comminution two opposing processes occur: these are the decrease of particle size and the reverse process of agglomeration. Because of this and the fact that the formation of agglomerates can be reduced by soluble additives, wet milling is preferable particularly when coupled to forming operations which avoid the formation of hard agglomerates. The further development of the classic agitated ball mill into a system with a solid rotor and a narrow pulverisation chamber is characteristic of the newer mill construction. In this type of mill the material being ground is subjected to an intense and uniform shearing action.

Dehydration of Hydrous Zirconia with Methanol

The washing of hydrous zirconia with alcohols to reduce the incidence of hard agglomerates on subsequent drying is well known. The results of methanol dehydration of hydrous zirconia (zirconium hydroxide), [Zr4(μ‐OH)8(OH)8(H2O)8]˙ xH2O, show that only μ‐OH groups are unaffected. This suggests two things: First, the removal of nonbridging hydrooxo groups and water with alcohols such as methanol leads to a reduction/elimination of hard agglomerates. Second, hard agglomerate formation is associated with condensation reactions involving nonbridging hydroxo groups (Zr‐OH+HO‐Zr→Zr‐O‐Zr+H2O).