Journal of Materials Synthesis and Processing

The reproducibility or variance in the properties of nanostructured milled iron and iron alloy powders and nanostructured compacts was determined and characterized. To date, all too often the characterization of nanostructured materials has been limited to examination of one or two samples, from which it is impossible to determine the reproducibility of the reported values. In this study, multiple attritor millings were made and the variability of the macroscopic and nanostructure characteristics was determined (e.g., particle size, grain size, etc.). From a single milled powder composition, multiple hot-press compacts were made. Statistical analyses were made of the reproducibility of resulting consolidated macroscopic and nanostructured properties, such as density, hardness, grain size, and tensile/compression strength. Mechanical processing of iron powder and mechanical alloying of iron powder with aluminum, carbon, and nitrogen showed that attrition milling reliably reproduced 0.5-kg lots of nanostructured powder. Hot-pressing the milled powder also produced reproducibility nanostructured compacts. There was little or no correlation found between the milled powder properties and the compacted powder properties. Several correlations that are generally valid for large grain materials were found not to hold for nanograin compacts (e.g., between density and hardness).

AgI–Al2O3 nanocomposites are investigated by high-resolution electron microscopy and luminescence spectroscopy. The results show that both crystalline and amorphous phases coexist in AgI–Al2O3 composites. At sufficiently high alumina content, almost all AgI occurs as amorphous spherical nanoparticles distributed uniformly on alumina surfaces. The amorphous phase exhibits unusual luminescence spectra with a single, very broad nonstructured band. The amorphous phase is stabilized on AgI–alumina interfaces with the relative concentration progressively increasing with the Al2O3 fraction. The thickness of the interface amorphous layer adjacent to Al2O3 particles estimated by means of a simple brick-wall model is about 3 nm.

Glasses of composition (1-x)PbO · xGeO2 with 0.50 ≤ x ≤ 1.00 were produced by melt quenching. Aliquots of each sample were thermally treated in air for various times at 660°C and then characterized by X-ray powder diffraction (XRPD) and differential thermal analysis (DTA). The XRPD patterns of devitrified samples show the presence of one or more crystalline phases, depending on x. For 0.50 ≤ x ≤ 0.75, short treatments (2 h) resulted in the presence of monoclinic PbGeO3, accompanied by phases of PbGe4O9 stoichiometry. On the contrary, prolonged thermal treatments (360 h) produced a slight decrease in the intensity of the XRPD peaks of the PbGeO3 phase and the transformation of the remaining material into orthorhombic PbGe3O7. For 0.80 ≤ x ≤ 0.95, short treatments (2 h) resulted in the formation of hexagonal GeO2, accompanied by orthorhombic PbGe3O7 and by PbGe4O9 phases. In this case, prolonged thermal treatments (360 h) do not affect strongly the XRPD patterns.

Cerium oxide (CeO2) is a promising material that has potential for use in a number of applications, such as resistive-type oxygen sensors and solid oxide fuel cells. In this work, the sintering behavior of hydrothermal synthesized nano-size CeO2 powders and chemical precipitated and commercial micron-size CeO2 powders were investigated by continuous monitoring of the shrinkage kinetics. The results demonstrated that during the high temperature sintering process a partial redox reaction of ceria occurred, i.e., a fraction of Ce4+ was reduced to Ce3+, and oxygen gas was released. The redox reaction influenced the sintering behavior of CeO2, resulting in a decrease in density and microcracking for the hydrothermal synthesized nano-size CeO2 powder compacts and sagged points in the sintering curves for the chemical precipitated and commercial micron-size CeO2 powder compacts. It was found by scanning electron microscopy that the partial redox reaction of ceria produced additional pores in the powder compacts during the sintering process and thus much higher temperatures were needed to achieve high density.

Magnetic Fe3O4 nanoparticles with size below 10 nm have been prepared by the aqueous phase coprecipitation method. The Fe3O4 nanoparticles show typical superparamagnetism. Comparison is made between the dispersed sample and the powder sample, and the results are discussed.

Based on combustion synthesis of titanium nitride with liquid nitrogen, the geometrical effects of the Ti compact has been investigated for the improvement of low conversion from Ti to TiN. Concentric bilayered cylindrical compacts with two kinds of packing densities, which consist of an inner column and an outer cylinder; normal cylindrical compacts were applied. It was clear that the propagation rate in the outer cylinder of the bilayered compact is almost the same as that of the normal compact under the same packing density condition, and that the propagation rate in the inner column of the bilayered compact is smaller than that in the outer cylinder. These facts imply that the propagation rate in the inner column could be drastically changed by outer packing density. By using the compact with smaller inner packing density than the outer one, advantageous conditions of using the inner column for nitridation could be realized: a greater quantity of liquid nitrogen is inside a sample (due to lower packing density) and smaller propagation rate (due to the above-mentioned geometrical effect). In spite of the condition that liquid nitrogen was used under 1 atm, products with higher conversion ratio could be obtained. In these experiments, the possibility of high nitridation using a concentric bilayered compact is indicated.

The paper presents new information on the transformation changes in the chemical structure of coal caused by mechanical activation using the GACL procedure. In the case of chemical treatment of Pittsburgh bituminous coal, the CAPTO method has confirmed a temperature T max shift for identification of carbonaceous and sulfur compounds (ΔT max/Sorganic) − 50°C; ΔT max/Corganic − 50°C). The temperature reduction of the thermal destruction maximum of carbonaceous and sulfur aromatic compounds is a result of activation of the coal structure of bituminous coals. The environmental effect of desulfurization (approx. 70%), detoxication, e.g., removal of arsenic (approx. 95%) and increase in the content of humic acids in the treated coal batch, have been proved by mechanical activation of Nováky brown coal using the GACL procedure. It is possible to improve the technological parameters of chemical treatment by optimization of the GACL procedure.

Simultaneous combustion synthesis reaction and compaction of Ti, C, and Ni powders under a hydrostatic pressure was carried out to fabricate dense TiC–Ni functionally graded materials (FGMs) in a single processing operation. Scanning-electron microscope (SEM) and microprobe analysis (EPMA) was employed to investigate the microstructure and composition distribution. Experimental results demonstrate that Ni and Ti composition varies continuously and gradually along the thickness of the reacted sample, remarkably different from stepwise type prior to combustion synthesis. The constituents are continuous in microstructure everywhere and no distinct interaction occurs in TiC–Ni FGM. Moreover, the thermal physical and mechanical properties were measured as a function of composition. It was found that the properties of the FGMs were dependent on Ni content. The residual thermal stress of TiC–Ni FGM and dual-laminate non-FGM cooled to room temperature after combustion synthesis has been analyzed by finite element method. TiC–Ni FGM shows distortion and thermal stress relaxation, which is in striking contrast to the layered TiC–Ni non-FGM.

This paper reports the results of work function (WF) changes of undoped PbZrO3 during subsequent isothermal oxidation and reduction experiments at 500°C in the p(O2) range between 10 and 2.1 × 104 Pa. The results, obtained during three consecutive runs, indicate that heating at 500°C leads to continuous changes of surface properties resulting in a complex WF vs. time characteristic. The WF changes during the first oxidation are determined by a p(O2)-induced structural transition. The second oxidation results in two competitive processes, such as rapid increase of oxygen non-stoichiometry followed by a structural transition. Finally, the third oxidation is determined by changes of oxygen nonstoichiometry.

Physical properies of ion-conducting nanocomposites are reviewed. Special attention is paid to the change of the bulk characteristics of ionic salts in the nanocomposites due to the formation of the interface phases. The main thermodynamic reason of the formation of the nanocomposite as well as the stabilization of the interface phases is the adhesion energy σa. At sufficiently high σa values, the ionic salt tends to spread along the oxide surface, which leads to the formation of the nanocomposite on sintering. The adhesion is the result of the interface interaction and incorporates the stage of the specific adsorption of the interface ions. It leads to the formation of the double layer formed by the point defects in the interface region of the ionic salt. In the case of the strong adhesion, the structural reconstruction or the formation of the metastable interface phase takes place. Analysis of the experimental data shows that interface phases exist in composites AgI–Al2O3, MeNO3–Al2O3 (Me = Li, Na, K, Rb, and Cs), CsHSO4–SiO2, RbNO3–SiO2 and CsCl–Al2O3. Their structure may be either epitaxial crystalline, or amorphous. The thickness of the interface phase as estimated on the basis of the brick-wall model is about 3–4 nm.