Journal of Materials Synthesis and Processing

The influence of the boiling solution technique (a nonhydrothermal wet treatment) in the formation of hydroxyapatite (HA) from a novel source, an echinoderm known as Mellita eduardobarrosoi, whose skeleton contains calcite as an inorganic constituent, was investigated. The experimental plan explores the conversion using a batch boiling system composed of KH2PO4 + KOH aqueous solutions at initial pH's of 8, 9, 10, 11, and 12, where granules of skeleton were dispersed to yield PO4/CaCO3 ratios of 1.0, 1.5, and 2.0. Chemical and X-ray diffraction analysis of the resulting material showed that the conversion of calcite by this technique is almost always higher than 70%, pH's 10 and 11 yielding the highest conversion. Depending on the operating conditions, the obtained material is a mixture of the original calcite and HA of varying stoichiometry and composition. Nevertheless, the interconnected porosity is preserved.

This paper deals with the mechanochemical degradation of hexachlorobenzene over Li, Na, K, Mg, Ca, Ba, Ti, and W. Apart from the mixtures containing Ti and W, the other systems react in a combustive mode leading to the chlorides and the carbides of the inorganic substrates. Reaction stoichiometries and heats were experimentally determined. The activation time required to ignite the reaction decreases from Li to Mg and correlates with mechanical properties, such as the shear modulus and the mineralogical hardness, of the reducing agents. Physicochemical characteristics and electronic properties of the metallic substrates were then considered. A close correlation was observed when the ignition time of alkaline and alkaline earth metals was plotted as a function of the ionization energy. It was suggested that the reaction can spread out only when a given deformation mixing level and sufficient interfacial area have been reached due to milling. This condition depends on the reducing capability of the metallic substrate and on the mechanical qualities of the reacting blend.

Three-layer alumina-based composites with Al2O3-containing mullite as the outer layer and ZrO2-containing alumina as the inner layer were fabricated by die-pressing and reaction bonding. The effects of the outer-layer compositions on the densification behavior of three-layer composites were investigated. The existence of residual stresses in the layers was verified using indentation methods. Compared with single-layer ceramics, three-layer composites exhibit an improved fracture toughness and an excellent damage resistance due to the presence of the compressive stresses in the outer layers.

The grain growth kinetics of commercially pure Titanium with different contents of oxygen have been studied. The grain size parameters have been measured by means of the Image Analysis Technique for different heat treatment temperatures and times. The growth exponents and activation energies have been calculated. Finally, tensile tests on the same materials have been carried out. When the interstitial oxygen content in titanium increases the grain growth kinetics and the ductility decrease and the α→β transition temperature, activation energy for grain growth and mechanical strength increase.

In this study, alumina ceramics were sintered with 2.45-GHz microwaves in a cylindrical single-mode microwave cavity using four resonant cavity modes: TM111, TE112, TE113, and TM013. The mass density and grain size of sintered specimens were found to be relatively uniform with respect to both (i) the radial position within a given specimen and (ii) the cavity mode used to sinter the specimen. However, the initial power required to “couple” the field to the casket/specimen system and the power needed to reach and maintain the sintering temperature (1500°C) were both functions of the cavity mode.

La0.7Ca0.3MnO3 powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La0.7Ca0.3MnO3 was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La0.7Ca0.3MnO3 powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La0.7Ca0.3MnO3 powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m2/g. The resistance as a function of temperature and the magnetoresistance ratio in La0.7Ca0.3MnO3 thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.

The effect of mechanical treatment of different degrees of action on the changes in the composition of silicate phases of natural granite is studied. Mechanochemical treatment is demonstrated to cause disordering of silicate structures. Layered hydrosilicates are most easily destroyed, which is explained by their dehydration. This destruction is accompanied by the formation and crystallization of the silicates with more stable framework structure and SiO2. Mechanical treatment of granite leads to the transformation and absorption of bound water and to the decrease of water-loss temperature by 200–300°C. Thermal heating of mechanically activated mixture of granite particles with lime results in the formation of calcium aluminosilicates with IR absorption bands at 1020, 970, and 930 cm−1.

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi2, TiSi, Ti5Si4, and Ti5Si3) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.

A stochastic three-dimensional (3-D) model for grain-microstructure evolution during transient nonisothermal annealing of metallic materials is developed, validated, and applied to the LENS (Laser-Engineered Net Shaping) advanced rapid fabrication process. The model is based on the assumption that the main driving force for microstructure evolution is the reduction in energy contribution arising from the grain boundaries. A temperature-dependent grain-boundary mobility factor is introduced into the expression for the transition probability in order to account for nonisothermal effects, such as those induced by the rastering laser during LENS-based manufacturing. The grain-boundary mobility factor and its temperature dependence are determined using the available experimental isothermal-annealing data. The simulation of grain growth (under nonisothermal annealing conditions encountered in the LENS process) is carried out by coupling a Monte Carlo method for microstructure evolution with a finite difference-based solution to the three-dimensional (3-D) transient energy equation. In response to the computational challenges of the simulations, a highly efficient interprocessor communications methodology is developed, which greatly reduces the simulation time on parallel computers. The results obtained show that under isothermal annealing conditions, the kinetics of grain growth is governed by a temporal power-law behavior and that, after an initial transition period, the grain-size distribution (normalized with respect to the average grain size) becomes time invariant. Furthermore, the application of the model to the LENS process is found to enable establishment of the relationships between process parameters (the laser power, beam rastering velocity, etc.) and the microstructure (grain size distribution, depth of the heat-affected region, etc.) of the deposited material.

The characteristics of spark plasma sintering, a new method of powder processing, were investigated. Four systems of intermetallic compounds—Ti-Al, Ti-Al-Cr, Mo-Si, and Mo-Si-Nb—were fabricated, and the formation process of compounds, the formed phases, and the microstructure of samples were observed. During the sintering of all the compositions of mixed powders, most of the compounds were formed by combustion reaction which occurred at almost the same temperature as the conventional combustion reaction temperature. The fabricated samples were well densified, however, the relative densities of the Mo-Si samples were lower than the Ti-Al samples. Ultrasonic images show that no internal defects were found in any sample and the grain size became finer with the increase in the Cr content in the Ti-Al system and Nb content in the Mo-Si system. The formed phases of Ti:Al=1:1 composition samples were TiAl and Ti3Al phases, and Ti-Al added Cr samples consisted of TiAl, Ti3Al, Cr2Al, and Cr9Al17 phases. The sample synthesized with Mo:Si=1:2 mixed powders had only MoSi2 phases, and Mo-Si samples with added Nb consisted of four phases: MoSi2 with a small amount of Mo5Si3 phases in the matrix and Nb5Si3 with unreacted Nb for dispersed phases.