Self-sustained High-temperature Synthesis Research Articles

The Influence of Thermal Dilution on the Microstructure Evolution of Some Combustion-Synthesized Refractory Ceramic Composites

Modeling the self-sustained high-temperature synthesis (SHS) reaction via thermal dilution and transformation of the reaction heterogeneous media into a moderate exothermic one has unlimited potential for designing inorganic powders of a certain morphology beneficial for advanced consolidation. Thermal/inert dilution of the high-exothermic mixtures leads to the fluent decrease of both the combustion temperature and velocity, thus allowing to tailor the thermal regime of the combustion process, therewith contributing to high yield of reaction and governing the microstructural features of the combustion products. In the current review, we shed on light on the possibilities of this effective strategy to control the thermal behavior of the SHS process for the preparation of applicable powder precursors for the subsequent successful sintering. Since the SHS process of some refractory ceramics (MoSi2, TiB2, TiC, etc.) involves a relatively violent reaction rate and high combustion temperature, achieving a high level of microstructure control in these systems is often challenging. The challenge was tackled with a thermal dilution approach, attaining considerable enhancement in the homogeneity among phases with an increase of diluent content along with microstructure refinement.

Conductivity and impedance spectroscopic studies of self-sustained high-temperature synthesized nano-Ce1-xGdxO2-δ solid solutions

Despite extensive research on CeO2 in diverse fields, studies on dielectric, conductivity behaviours due to intentional impurity-induced structural and microstructural changes are not well established. We show that structural changes arising due to Gd3+ in nano-CeO2 and the temperature affect the dielectric and conductivity characteristics. Highly crystalline nano-CeO2 and Ce1-xGdxO2-δ (GDC) (x = 0.02–0.1) are synthesized by self-sustained high-temperature synthesis. The dependence of conductivity on dopant concentration is examined over a wide-frequency (20 Hz to 10 MHz) and temperature (200–800 °C) range using a precision impedance analyzer. The conductivity reaches maximum at x = 0.06 for Ce1-xGdxO2-δ at 600 °C beyond which conductivity decreases. There is dependence of conductivity at lower frequencies, increases with increasing frequency, and gets saturated at higher frequencies. The optimum concentration (x = 0.06) found is the lowest for the Ce1-xGdxO2-δ system reported. The observed behaviour is discussed in terms of interaction between oxygen vacancies and defect associates.

Attenuation of oxidative stress and chromosomal aberrations in cultured macrophages and pulmonary cells following self-sustained high temperature synthesis of asbestos

Inhalation of asbestos fibres can cause lung and pleural diseases in humans and constitutes a severe public health threat worldwide. The aim of the present study was to assess the biological effects induced in both pulmonary cells (A549) and monocyte/macrophage (RAW 264.7) cell lines by combustion slags obtained from asbestos through a self-sustained high-temperature synthesis (SHS) reaction. The SHS reaction involves rapid thermal treatment and displays great ability to neutralise asbestos. Cytotoxicity, redox status imbalance, lipid peroxide production, DNA strand breaks (comet assay) and chromosomal aberrations (cytokinesis block micronucleus test) were evaluated in cells exposed either to untreated asbestos fibres or to grinded SHS-generated slags of different granulometry, tested in cultured cells at varying doses and for varying exposure times. Our results show that asbestos fibres cause redox status imbalance, especially in monocyte/macrophage cell lines. Moreover, they promote lipid peroxidation and trigger genomic alterations. When the cells were exposed to slag powders, which are the products of SHS asbestos treatment, generation of lipid peroxides and induction of DNA strand breaks still persisted, due to the high content in iron and other metals detected in these samples. However, there was an attenuation of redox status imbalance and an absence of chromosomal aberrations, which probably reflects the loss of the asbestos fibrous structure following SHS reaction, as demonstrated by electron microscopy analyses. In conclusions, SHS-treated asbestos wastes can potentially have deleterious health effects due to the oxidative stress induced by inhaled powders but they loose the asbestos ability to induce chromosomal alterations.

The Self-sustained High temperature Synthesis (SHS) technology as novel approach in the management of asbestos waste

The SHS technique was experimented in chrysotile breakdown. By means of two reactions such as Mg3Si2O5(OH)4 + Fe2O3 + 3 Mg and 2Mg3Si2O5(OH)4 + Fe3O4 + 4 Mg the chrysotile was completely converted into forsterite-rich olivine. Different mixtures of hematite + Mg and magnetite + Mg were tested with chrysotile to establish the maximum chrysotile amount in order to allow the reaction. In comparison with conventional thermal treatments, the SHS process is characterized by a fast reaction, needs low activation energy and the apparatus is simple. For these reasons, the asbestos neutralisation is carried out with positive balance of time and costs of the process. Furthermore, the combustion product can be re-used as secondary material.

Effects of processing and powder size on microstructure and reactivity in arrested reactive milled Al + Ni

Ball-milling Al-metal powders can result in self-sustaining high-temperature synthesis in intermetallic-forming systems. Here, Al and Ni powders with similar composition are used to investigate how microstructural differences affect the measured time to reaction (TTR) between powders of different sizes processed under milling conditions specified by statistically designed experiments. Linear statistical models predicting the TTR and the change in temperature (Δ T) are built from these experimental results. The time required to observe a self-sustained high-temperature synthesis of NiAl with different combinations of the powders and ball-milling conditions vary by almost an order of magnitude. Comparisons of powders milled to times corresponding to percentages of their averaged TTR show similar reaction initiation temperatures despite the difference in total milling time. Several distinct arrested reactions within the powder grains exhibit rapid solidification or incomplete diffusion of Ni into Al, forming porous Ni-rich layered structures. The partially reacted grains suggest that the composite laminate particles are not forming intermetallic on the grain scale, but on the localized scale between layers.

Combustion synthesis of electrical contact materials

Various electrical contact materials have been obtained by self-sustained high-temperature synthesis (SHS) followed by a hot compression step. The materials studied are TiB2 and TiC-based cermets with a copper or aluminium metal matrix. The density, electrical conductivity, and hardness of the cermets produced in this way were measured. The microstructure of the cermets was analyzed by X-ray diffraction and optical microscopy. The conductivity and hardness of the TiB2 cermets with 40 wt % aluminium are competitive to the properties of common silver refractory metals for contact applications. TiB2 cermets with 40 wt % of copper possess a larger porosity which may be prevented by an increased copper content to reduce the reaction temperature.

Mechanical Anisotropy of NiAl Alloys Processed by Hot Hydrostatic Extrusion

Mechanical properties of two stoichiometric NiAl intermetallic alloys processed by hot hydrostatic extrusion were investigated. The ingots for extrusion process were produced both by casting and by powder metallurgy route in which the self-sustained high-temperature synthesis (SHS) of elemental powders was initialized by the process of extrusion itself. Elastic anisotropy of the obtained products were determined using a resonant ultrasound spectroscopy (RUS) in measurements of the elastic constants. The anisotropy of plastic properties was estimated from uniaxial compression tests on specimens cut off from the material in the main directions of previous deformation. An acoustic emission (AE) method of monitoring of the microcracking process in the material was used for estimation the role of cataclastic mechanism of deformation. The effect of annealing of extruded materials on their plastic behavior was roughly investigated.

Combustion Modes and Reaction Paths of the Self-Sustained High-Temperature Synthesis of Intermetallic Compounds: A Computer Simulation Study of the Effect of Exothermicity

The effect of the thermodynamic aspects on the SHS of intermetallic compounds is investigated by computer simulation using an heterogeneous model. The exothermicity of the process is explored by changing heat of formation of the solid compound and mixing enthalpy in the liquid phase, while the kinetics is left unmodified. Different dynamic behaviors have been identified: steady propagation, pulsed propagation, extinction. Two different kinds of instabilities are shown: longitudinal instabilities and transversal instabilities. The ranges of existence of these two dynamic behaviors appear to be mutually exclusive. The occurrence of different dynamic behaviors is related to the predominant process taking place in the different cases.

Microstructure and mechanical properties of NiAl produced in the SHS process induced by low-temperature hydrostatic extrusion

A stoichiometric nickel aluminide intermetallic alloy produced by self-sustained high-temperature synthesis (SHS) induced to the green compacts of the mixture of elemental Ni and Al powders by mechanical heavy deformation during low-temperature hydrostatic extrusion has been investigated. The process was performed with the high strain rate (>102 s−1) at the low temperature (∼500°C). To reduce the porosity, grain size and non-homogeneity of the obtained material a further high-temperature (∼950°C) hydrostatic extrusion was applied. Mechanical properties of the material were measured on the samples cut out from extruded rods in the longitudinal and transverse directions. Elastic anisotropy (elastic tensor) was determined using a resonant ultrasound spectroscopy (RUS) method applied to small cylindrical samples of the material. Its value described as the ratio of the coefficients of linear compressibility in transverse and longitudinal directions was at the level of 2 for extruded material even after annealing. Plastic anisotropy of the material was determined in uniaxial compression tests using an acoustic emission (AE) method for monitoring early stages of microcracking process in the samples under investigation. The phenomenon of the yielding point was observed for the samples compressed in the direction of extrusion.

SHS (Self-sustained high-temperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation

The very wide temperature ranges and the many different chemical and phase transformation steps experienced by an SHS process make the search for a unifying theory impractical and a numerical approach highly desirable. The work describes an investigation of the Al+Ni→AlNi SHS by a computer simulation method developed by the Authors and based on a finite difference numerical engine coupled to a detailed description of several assumed reaction steps. The model includes Al melting, Ni diffusion-controlled dissolution (and possibly Ni melting), nucleation and precipitation of AlNi, and eutectic deposition. The results are discussed from the point of view of feasibility of the process, transition from self-propagating to thermal explosion behavior, and influence of process variables such as reactant grain sizes, diffusion coefficient, initial composition, and thermal conductivity.

Fe-Doped Zirconium Oxide Produced by Self-Sustained High-Temperature Synthesis: Evidence for an Fe−Zr Direct Bond

The local structure of iron in Fe-doped cubic ZrO2 produced by combustion synthesis was studied by Mössbauer spectroscopy and Fe-K edge extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES). Iron was found to be in its II oxidation state and to occupy two different sites of the fluorite ZrO2 structure, both associated with some amount of disorder. One of the sites has been identified with the regular Zr position in 0,0,0, thus giving rise to substitutional (i.e., Fe atoms occupying the regular Zr position, with a net 2−charge, with respect to the lattice) defects, while the other site has been identified with the normally empty position at 1/2, 1/2, 1/2, thus giving rise to interstitial (i.e., Fe atoms occupying an interstitial position, with a net 2+ charge, with respect to the lattice) defects. For this last site, there is a short Fe−Zr distance (2.64 Å). This result, coupled to the quite small value of the Debye−Waller factor for this distance, gives evidence of a direct Zr−Fe metal-to-metal bond.

Microstructure investigations on explosive TiNi(or Ni)/TiC-composite-formation reaction during mechanical alloying

The microstructure of TiNi(or Ni)/TiC composite prepared by mechanical alloying (MA) of elemental Ti, Ni and C powders has been investigated in detail. The result shows that large agglomerates with size 3–10 mm were abruptly formed for Ti 50Ni 20C 30, Ti 40Ni 40C 20 and Ti 30Ni 50C 20 powders at the milling duration of 3 h 30 min–3 h 35 min, which suggests melting of the powders and subsequent quenching has occurred during MA. Transmission electron microscopy shows that spherical TiC grains, lath twin martensite (M) and B2 phase are directly formed after 3 h 35 min MA of Ti 50Ni 20C 30 powders. It is also found that there exists definite orientation relationships between M and B2 phases which are very close to those obtained by Otsuka et al. in equiatomic TiNi martensite. The resultant phases are predominantly TiC and M phases with a small amount of B2 phase for Ti 40NI 40C 20; and Ni and TiC phases for Ti 30Ni 50C 20 after 3 h 35 min of MA. The microstructure characteristics of the as-milled materials are very similar to those of melted and solidified ones, which proves that melting of the powders and subsequent quenching has really occurred during the MA process. We concluded that MA in Ti 50Ni 20C 30, Ti 40Ni 40C 20 and Ti 30Ni 50C 20 is not governed by a gradual diffusional reaction, but by a self-sustained high-temperature synthesis (SHS) one. The SHS reaction is believed to be triggered by the release of heat of formation of the TiC phase and ignited by the mechanical collisions. The thermodynamic and kinetic conditions for the SHS reaction is discussed, and it shows that MA is a versatile method in inducing SHS reaction in systems with large heat of formation.