Synthesis and Characterization of Microwave Absorbing SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanocomposite

Zinc ferrite and strontium hexaferrite; SrFe₁₂O₁₉/ZnFe₂O₄ (SrFe_11.6Zn_0.4O₁₉) nanoparticles having super paramagnetic nature were synthesized by simultaneous co-precipitation of iron, zinc and strontium chloride salts using 5 M sodium hydroxide solution. The resulting precursors were heat treated (HT) at 850, 950 and 1150°C for 4 h in nitrogen atmosphere. The hysteresis loops showed an increase in saturation magnetization from 1.040 to 58.938 emu/g with increasing HT temperatures. The ‘as-synthesized’ particles have size in the range of 20–25 nm with spherical and needle shapes. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate shape with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties were estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches −26.51 dB (more than 99% power attenuation) at 10.636 GHz which suits its application in RADAR absorbing materials.     

Crystallization Behavior and Microstructure of Silica-Free 5K<Subscript>2</Subscript>O-45CaO-50P<Subscript>2</Subscript>O<Subscript>5</Subscript> Bioglass

The crystallization behavior and microstructure of silica-free 5K₂O-45CaO-50P₂O₅ (KCP) bioglass have been studied using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning election microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The activation energy for the KCP bioglass crystallization is found to be 337.4 kJ/mol using a nonisothermal method. The crystalline phases of the glass surface determined by XRD are KCa(PO₃)₃, 4CaO·3P₂O₅, and β-Ca(PO₃)₂ when the KCP bioglass is crystallized at 903 K for 4 hours. The crystalline phase of the powder samples determined by XRD is β-Ca(PO₃)₂ when silica-free KCP glasses crystallized at 873 to 1073 K for 8 hours. Crystallization starts at the surface of the KCP bioglass and then proceeds toward the interior of the glass matrix. The morphology of β-Ca(PO₃)₂ is a fibrillar shape 20 to 180 nm in length and 17 to 20 nm in diameter, with an aspect ratio ranging from 1.0 to 10.6.     

Phase-equilibrium data and liquidus for the system “MnO”-CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> at Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> of 0.55 and 0.65

Phase-equilibrium data and liquidus isotherms for the system “MnO”-CaO-(Al₂O₃+SiO₂) at silicomanganese alloy saturation have been determined in the temperature range of 1373 to 1723 K. The results are presented in the form of the pseudoternary sections “MnO”-CaO-(Al₂O₃+SiO₂) with Al₂O₃/SiO₂ weight ratios of 0.55 and 0.65. The primary-phase fields have been identified in this range of conditions.     

Deformation and failure of Zr<Subscript>57</Subscript>Nb<Subscript>5</Subscript>Al<Subscript>10</Subscript>Cu<Subscript>15.4</Subscript>Ni<Subscript>12.6</Subscript>/W particle composites under quasi-static and dynamic compression

We have investigated the mechanical behavior of a composite material consisting of a Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 metallic glass matrix with 60 vol pct tungsten particles under uniaxial compression over a range of strain rates from 10⁻⁴ to 10⁴ s⁻¹. In contrast to the behavior of single-phase metallic glasses, the failure strength of the composite increases with increasing strain rate. The composite shows substantially greater plastic deformation than the unreinforced glass under both quasi-static and dynamic loading. Under quasi-static loading, the composite specimens do not fail even at nominal plastic strains in excess of 30 pct. Under dynamic loading, fracture of the composite specimens is induced by shear bands at plastic strains of approximately 20 to 30 pct. We observed evidence of shear localization in the composite on two distinct length scales. Multiple shear bands with thicknesses less than 1 μm form under both quasi-static and dynamic loading. The large plastic deformation developed in the composite specimens is due to the ability of the tungsten particles both to initiate these shear bands and to restrict their propagation. In addition, the dynamic specimens also show shear bands with thicknesses on the order of 50 μm; the tungsten particles inside these shear bands are extensively deformed. We propose that thermal softening of the tungsten particles results in a lowered constraint for shear band development, leading to earlier failure under dynamic loading.     

Formation of CaS on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO inclusions during solidification of steels

In this article, the effect of CaS formation on the evolution of Al₂O₃-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al₂O₃-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.     

Investigation into the solubility of nanopowders of the ZrO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–CeO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system in the aqueous medium at various pH

Nanopowders of ZrO₂–Y₂O₃–CeO₂ and ZrO₂–Y₂O₃–CeO₂–Al₂O₃ systems are investigated with the purpose of studying the influence of pH of the dispersed medium on the solubility of nanopowder particles of a complex composition in an aqueous medium after membrane filtration and centrifugation to further prepare the stable dispersions necessary for toxicological investigations of nanoparticles. Concentrations of elements remaining in a supernatant after the sample preparation, which includes membrane filtration and centrifugation, are measured by inductively coupled plasma optical emission spectroscopy. It is established that that the largest aggregative stability of the nanopowder dispersion without the Al₂O₃ additive corresponds to the optimal range of pH 5.5–9.5, while with the Al₂O₃ additive, it is region pH 7.0. The results evidence that, when dispersing these powders, the hydrosol of yttrium oxyhydroxide, which is dissolved at pH < 6.0, is formed. When dissolving in water of the powder with the Al₂O₃ additive in the neutral medium, aluminum hydroxide is formed; in the acidic medium (pH < 6), it is replaced by main soluble aluminum salts; and in the alkali medium (pH > 7), amphoteric aluminum hydroxide is dissolved because of the formation of aluminates.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synthesis of Cu/Cr-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocomposite by Mechanical Alloying and Heat Treatment

In this article, a novel method has been used to prepare a copper matrix nanocomposite containing Cu-10 wt pct Cr-10 wt pct Al₂O₃ by heat treatment of the mechanically activated Cu, Al, and Cr₂O₃ powder mixture. Structural evolutions were investigated using the X-ray diffraction (XRD) technique. The microstructure of samples was examined using scanning electron microscopy (SEM). It was found that during the milling process, Cu(Al) solid solution and Cu₉Al₄ phase were formed as the intermediate products, and therefore, Al activity was decreased. Hence, the reduction of Cr₂O₃ with Al was prevented during the ball milling stage. Further heat treatment carried out under argon atmosphere at 900 °C for 8 hours resulted in completion of Cr₂O₃ reduction by Al.     

Solid-state reactions during heating mechanically milled Al/TiO<Subscript>2</Subscript> composite powders

Solid-state reactions between Al and TiO₂ during heating high-energy mechanically milled Al/TiO₂ composite powders have been investigated by using a combination of thermal analysis, X-ray diffraction (XRD), and various microstructural characterization techniques. When the TiO₂ particles and their interparticle spacing in the Al/TiO₂ composite powder particles are sufficiently large, the reaction between Al and TiO₂ proceeds by two steps. The low-temperature step is an interfacial reaction, which starts at a temperature close to 660 °C. The high-temperature step is a reaction facilitated by bulk diffusion and starts at a temperature above 820 °C. The first phase formed from the reaction is always Al₃Ti irrespective of the starting powder composition or milling time. Al₂O₃ is difficult to form at temperatures below 800 °C. The formation of the α-Ti(Al,O) phase proceeds slowly and requires either continuous heating to a temperature above 1000 °C or holding at a temperature close to 1000 °C for a period of time. Mechanical milling of the Al/TiO₂ powder enhances the interfacial reaction between Al and TiO₂. This enhancement is originated from the establishment and refinement of Al/TiO₂ composite microstructure.     

Modeling of the solubilities of NiO/NiAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and FeO/FeAl<Subscript>2</Subscript>O<Subscript>4</Subscript> in cryolite melts at 1300 K

Experiments to measure the solubilities of NiO/NiAl₂O₄ and FeO/FeAl₂O₄ were performed, and the results confirmed existing literature values. The solubilities of NiAl₂O₄ and FeAl₂O₄ in Al₂O₃-saturated cryolite melts at 1300 K were modeled thermodynamically in terms of the Ni-containing complexes Na₂NiF₄ and Na₄NiF₆, and the Fe-containing solutes FeF₂, Na₂FeF₄, and Na₄FeF₆. The experimental solubility data were fitted to multiple simultaneous equilibria. Equilibrium constants and ΔG _f ⁰ values for the formation reactions of the these solutes were thereby estimated. The solubilities of NiO/NiAl₂O₄ and FeO/FeAl₂O₄ and solute distributions in Al₂O₃-undersaturated cryolite melts were calculated for a number of melt compositions from the present model. The existence of several competitive solute species is inherent to highly buffered ionic cryolite solutions where the traditional log-log methodology had previously failed to identify dominant single solutes. In such solutions, individual solutes of oxides are not likely to dominate over a wide composition range so that a more global modeling is required. The principal solute species identified in the present study exhibit reasonable three-dimensional (3-D) anion geometries.     

Peculiarities of the technology and physicomechanical properties of a laminated Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al cermet

A technological approach permitting one to obtain laminated Al₂O₃-Al cermet is considered. The industrial PAP-2 powder with laminated particles served as the starting crude. The powder billets were obtained by compaction under pressure (P) from 100 to 1000 MPa and thermally treated in air by heating in the furnace to 600°C. It is established that either the solid-phase sintering or reaction sintering of the billets in the mode of filtration combustion can be achieved depending on the value of P. In the produced composite, the content of the oxide phase varies from 5 to 40%, while the density and strength upon bending vary in the limits 2.53–2.00 g/cm³ and 330-98 MPa, respectively. The laminated structure of the material is retained after thermal aging in air at t= 600°C for no less than 1000 h.     

Thermal diffusivity measurements of some synthetic CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> slags

In an attempt to systematize the knowledge of the heat conduction of liquid silicates, the effective thermal diffusivities of some synthetic slags containing CaO, Al₂O₃, and SiO₂ have been measured, using the three-layer laser-flash method on a differential scheme in the temperature range of 1625 to 1825 K. The effective thermal diffusivities measured, which are a combination of the phononic and photonic heat-transfer mechanisms, were found to increase with increasing temperature for all the presently investigated slags. The slag compositions were chosen in such a way that the changes in the effective thermal diffusivities would reflect the changes in the structure of the slags. It was observed that, at a CaO/Al₂O₃ molar ratio of 4.42, an increase of the SiO₂ content had very little effect on the effective thermal diffusivity values. On the other hand, addition of SiO₂ to a slag with the CaO/Al₂O₃ molar ratio of 2.59 resulted in a significant increase in the effective thermal diffusivity. The addition of Al₂O₃ to slags with a constant CaO/SiO₂ molar ratio resulted in a marked increase in the effective thermal diffusivity. Both these trends indicate that there might be an influence of the network formation in silicate melts on the effective thermal diffusivity.     

Leaching of sulfidized chalcopyrite with H<Subscript>2</Subscript>SO<Subscript>4</Subscript>-NaCl-O<Subscript>2</Subscript>

The recovery of copper from chalcopyrite by leaching is complex not only due to the slow dissolution kinetics of this mineral in most aqueous media but also due to the production of solutions that are heavily contaminated with iron. On the contrary, the leaching of sulfidized chalcopyrite is very attractive because of a faster and more selective dissolution of copper compared to the leaching of the untreated chalcopyrite. In this work, the results of leaching in H₂SO₄-NaCl-O₂ solutions of sulfidized chalcopyrite concentrate are discussed. Experiments were carried out with chalcopyrite concentrates previously reacted with elemental sulfur at 375 °C for 60 minutes. The results showed that the concentration of chloride ions below 0.5 M, temperature, and leaching time are important variables for the extraction of Cu. On the other hand, Fe extraction was little affected by the same variables, remaining below 6 pct for all the experimental conditions tested. Microscopic observations of the leached particles showed that the elemental sulfur produced by the reaction does not form a coherent layer surrounding the particle, but rather concentrates in certain locations as large clusters. The leaching kinetics can be accurately described by a nonreactive core-shrinking rim topochemical expression for spherical particles 1 − (1 − 0.45X)^1/3=kt. The activation energy found was 76 kJ/mol for the range 85 °C to 100 °C.     

The Thermodynamic Behavior of Copper in the CaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and BaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag System

The Cu solubility was measured in the CaO-B₂O₃ and BaO-B₂O₃ slag systems to understand the dissolution mechanism of Cu in the slags. The Cu solubility had a linear relationship with oxygen partial pressure in the CaO-B₂O₃ slag system, which corresponds with previous studies. Also, the Cu solubilities in slag decreased with increasing the slag basicity, which value of slope was close to –0.5 in logarithmic form. From the results of experiment, the Cu dissolution mechanism established as follows:

Effect of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO slags on the formation of MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> inclusions in ferritic stainless steel

A thermodynamic equilibrium between the Fe-16Cr melts and the CaO-Al₂O₃-MgO slags at 1823 K as well as the morphology of inclusions was investigated to understand the formation behavior of the MgO-Al₂O₃ spinel-type inclusions in ferritic stainless steel. The calculated and observed activities of magnesium in Fe-16Cr melts are qualitatively in good agreement with each other, while those of aluminum in steel melts exhibit some discrepancies with scatters. In the composition of molten steel investigated in this study, the log (X _MgO/X _{Al 2}O₃) of the inclusions linearly increases by increasing the log [a _Mg/a _Al ² ·a _O ² ] with the slope close to unity. In addition, the relationship between the log (X _MgO/X _{Al 2}O₃) of the inclusions and the log (a _MgO/a _{Al 2}O₃) of the slags exhibits the linear correlation with the slope close to unity. The compositions of the inclusions are relatively close to those of the slags, viz. the MgO-rich magnesia-spinel solid solutions were formed in the steel melts equilibrated with the highly basic slags saturated by CaO or MgO. The spinel inclusions nearly saturated by MgO were observed in the steel melts equilibrated with the slags doubly saturated by MgO and MgAl₂O₄. The spinel and the Al₂O₃-rich alumina-spinel solid solutions were formed in the steel melts equilibrated with the slags saturated by MgAl₂O₄ and MgAl₂O₄-CaAl₂O₄ phases, respectively. The apparent modification reaction of MgO to the magnesium aluminate inclusions in steel melts equilibrated with the highly basic slags would be constituted by the following reaction steps: (1) diffusion of aluminum from bulk to the metal/MgO interface, (2) oxidation of the aluminum to the Al³⁺ ions at the metal/intermediate layer interface, (3) diffusion of Al³⁺ ions and electrons through the intermediate layer, and (4) magnesium aluminate (MgAl₂O₄ spinel, for example) formation by the ionic reaction.     

Density Measurements of Low Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Density measurements of a low-silica CaO-SiO₂-Al₂O₃ system were carried out using the Archimedes principle. A Pt 30 pct Rh bob and wire arrangement was used for this purpose. The results obtained were in good agreement with those obtained from the model developed in the current group as well as with other results reported earlier. The density for the CaO-SiO₂ and the CaO-Al₂O₃ binary slag systems also was estimated from the ternary values. The extrapolation of density values for high-silica systems also showed good agreement with previous works. An estimation for the density value of CaO was made from the current experimental data. The density decrease at high temperatures was interpreted based on the silicate structure. As the mole percent of SiO₂ was below the 33 pct required for the orthosilicate composition, discrete \textSiO₄^{4 -} {\text{SiO}}_{4}^{4 - } tetrahedral units in the silicate melt would exist along with O^2– ions. The change in melt expansivity may be attributed to the ionic expansions in the order of

Decomposition of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> Particles in 8 Pct Cr Oxide-Dispersion-Strengthened Martensitic Steel during Tempering

Chemical composition analysis using inductively coupled plasma spectroscopy and phase identification using X-ray were performed on the extracted residues of 8 pct Cr martensitic steel strengthened by nanoscaled complex oxides, which consist of yttrium, titanium, and oxygen. Some Y₂Ti₂O₇ particles, which were stable during normalizing, decomposed into Y₂O₃ and Ti₂O₃ during tempering. This reaction reversibly occurred between normalizing and tempering. Y₂Ti₂O₇ particles formed in the steel had other constituents in solid solution as compared to the completely stoichiometric Y₂Ti₂O₇ particles synthesized artificially in air. As for the mechanism of the decomposition of Y₂Ti₂O₇ particles in the steel, segregation of oxygen to dislocations induced by normalizing caused the decomposition of Y₂Ti₂O₇ during tempering. In addition to that, the interfacial strain between Y₂O₃ particles or Ti₂O₃ particles within Cr₂₃C₆ carbides, which are formed by tempering, was lower than the strain between Y₂Ti₂O₇, which precipitated in Cr₂₃C₆ carbides or the matrix. This difference in interfacial strain could also promote the decomposition of the Y₂Ti₂O₇ particles in the steel.     

The Viscous Behavior of FeO<Subscript>t</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> Copper Smelting Slags

Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining the discrete separation between the matte and the slag. The viscosity of the FeO_t-SiO₂-Al₂O₃ copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al₂O₃ concentration decreased with increasing Fe/SiO₂ ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O²⁻) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO₂ ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased. Al₂O₃ additions to the ternary FeO_t-SiO₂-Al₂O₃ slag system at a fixed Fe/SiO₂ ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased subsequently with higher Al₂O₃ content. The effect of Al₂O₃ was considered to be related to the amphoteric behavior of Al₂O₃, where Al₂O₃ initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al₂O₃ additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus temperature with Al₂O₃ additions affecting the viscosity of the copper smelting slag.     

Deoxidation of Liquid Copper with Reducing O<Subscript>2</Subscript>/CH<Subscript>4</Subscript> Flames

The rate of deoxidation of molten copper during top blowing with various reducing gases has been investigated using thermogravimetry. It was observed that the rate of deoxidation increases with an increasing flow rate of H₂ or CO and that H₂ is a more effective reducing reagent than CO. The rate of deoxidation using methane was measured for O₂/CH₄ ratios from 1.5 to 2.0. As expected, the deoxidation rate decreased with an increasing O₂/CH₄ feed ratio because the flame became less reducing. For all tests, initially there is a linear decrease in mass as oxygen is removed. However, for some experiments, after some time, a sudden acceleration in the rate of mass loss occurs. Using video and X-ray imaging, it was found that this pattern corresponded to gas evolution from within the molten copper. This finding can be explained by the sudden water vapor evolution because the hydrogen dissolved in the copper reacts with the remaining oxygen, and “boiling” takes place, leading to an enhanced stirring of the copper.