Self-propagating Synthesis Research Articles

Application of mechanical alloying and self-propagating synthesis for preparation of stable decagonal quasicrystals

The X-ray and electron microscopic investigations demonstrated principal possibility to use self-propagation high-temperature synthesis (SHS) in combination with preliminary mechanical activation for the synthesis of stable decagonal quasicrystals. The decagonal quasicrystals formed in SHS process look like faceted decaprisms with a length of 300–400 μm and a thickness of 30–40 μm. SAD patterns are characterized by the presence of the axes of 10- and 2-fold symmetry.

Studies of defects in YVO4:Pb2+, Eu3+ red phosphor material

Doubly doped YVO 4 :Pb 2+ , Eu 3+ red phosphor materials with fixed Eu 3+ concentration (5 mol%) and varying Pb 2+ concentration were prepared via a self-propagating (combustion) synthesis. This consisted of bringing a saturated aqueous solution of the desired metal salts and a suitable organic fuel to the boil, until the mixture ignited and a self-sustaining and rather fast combustion reaction initiated, resulting in dry, amorphous or usually crystalline fine particles of the desired material. The formation of crystalline vanadates was confirmed by X-ray diffraction. A strong emission line at 619 nm due to the 5 D 0 → 7 F 2 transition in the red region was observed. Defects, created by gamma radiation, were studied by means of photoluminescence, thermally stimulated luminescence (TSL) and electron spin resonance. Photoluminescence studies display considerable reduction in emission intensity which appears to arise due to the formation of defect centres after irradiation. The defect centres formed in the present system are tentatively assigned to F + centres and step annealing measurements suggest a connection between these centres and the TSL glow peak.

Laser-induced self-propagating reaction synthesis of amorphous-based composite Zr-Al-Ni-Cu alloys

Strong exothermic reaction due to large negative enthalpy of mixing can occur among major components of bulk metallic glass forming alloy systems. Based on this idea, we developed a new technique to fabricate amorphous-based composite materials using Laser-induced Self-propagating Reaction Synthesis (LSRS). The LSRS of the Zr55Al10Ni5Cu30 alloy shows that the products mainly consist of the amorphous, Zr3Al2 and Zr2Cu phases. Hardness and wear resistance of the produced composite alloys are measured and are compared to the single amorphous phase alloy of the same composition.

Combustion Derived Ultrafine γ-Fe2O3 Structure, morphology and thermal studies

A novel combustion method of employing poly(ethylene glycol) with the precursor in a fixed ratio for the synthesis of ultrafine γ-Fe2O3 through a self-propagating combustion synthesis is reported. Four different precursors viz. ferrous hydroxide, ferrous oxalate dihydrate, ferric 8-hydroxyquinoline and ferric acetylacetonate are employed in this study for the conversion of these precursors to ultrafine g-Fe2O3 particles. The as synthesized γ-Fe2O3 samples are characterized by XRD, SEM and thermal techniques. A case study for the synthesis of γ-Fe2O3 employing ferric acetyl acetonate as precursor is reported. The importance of employing thermal analysis techniques in understanding the combustion synthesis is envisaged.

Synthesis of WSi 2–ZrO 2 and WSi 2–Nb composites by field-activated combustion

The synthesis of WSi 2 and WSi 2– x vol.%Nb and WSi 2– y vol.%ZrO 2 composites with x, y=0, 5, 7, 10, 12, 15, 17, 20 by field activated combustion was investigated. Self-propagating synthesis fronts can be initiated above minimum (threshold) values which depended on the amount of added Nb and ZrO 2. Reaction wave velocities had a direct dependence on the applied field and on the amount of the second phase. XRD results showed the presence of the WSi 2 phase in all systems with the additional phase of Nb or ZrO 2 for the composites.

Low-temperature chemical synthesis of lanthanum copper oxide

Lanthanum copper oxide (La2CuO4) precursors for superconductors were synthesized by acetate/nitrate self-propagating combustion synthesis (SPCS) in the presence of urea at different ratio of O/F. Phase-pure La2CuO4 powder could be obtained by calcining the as-prepared precursors at temperature as low as 600 °C. It was demonstrated that the ratio of O/F strongly influenced the composition and morphology of the precursors as well as the samples calcined at various temperatures. We have studied the synthesis conditions, demonstrating the existence of a relationship between the calcination temperature and the calcination time needed to obtain the pure phase. An interesting stereoscopic network structure was observed on the samples synthesized at the ratio of O/F = 0.20, morphology evolution of the samples was also investigated. The emission of H2O as well as CO2 was responsible for the formation of the structure during the calcination process.

Synthesis and characterization of nanoparticles of Ba2EuZrO5.5: A new complex perovskite ceramic oxide

Nanoparticles of barium europium zirconate, a complex perovskite oxide, were synthesized using a modified self-propagating combustion synthesis. The solid combustion products thus obtained were characterized by x-ray and electron diffraction, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, particle-size analysis, surface area determination, gas adsorption studies, and high-resolution transmission electron microscopy. According to the results of the x-ray and electron diffraction, as-prepared powder showed the single phase of barium europium zirconate (Ba2EuZrO5.5) without another phase and had a complex cubic perovskite (A2BB′O6) structure. The transmission electron microscopic investigation showed a mean grain size of 38 nm with a standard deviation of 12 nm. High-resolution lattice imaging of the nanoparticles indicated the possibility of finer crystallite in the particle having the same orientation. The nanoparticles of Ba2EuZrO5.5 obtained by the present method could be sintered to 97% theoretical density at a relatively low temperature of 1525 °C.

Synthesis of Cr–Si intermetallic compounds by field-activated combustion synthesis

The use of an electric field to activate the combustion synthesis of chromium silicides was investigated. Despite their relatively low adiabatic temperatures, all four silicides were synthesized by field-activated combustion synthesis. However, although self-propagating synthesis reactions were initiated, the products were not pure but contained other silicides and reactant phases. The purity of the samples increased with increasing field strength, and under the highest field, the products contained the desired silicide as the major phase with minor amounts of other stoichiometries. Observation of microstructural evolution in quenched reactions revealed the key role played by the liquid phases in the propagation of the combustion front. The phase Cr5Si3 was the first product of the interaction between the reactants when either solid–solid or solid–liquid processes were involved. These results were confirmed by isothermal solid–solid and solid–liquid diffusion couple experiments.

Modeling studies of the effect of thermal and electrical conductivities and relative density of field-activated self-propagating combustion synthesis

The role of the electrical conductivity of the product and of the thermal conductivities of the reactants on self-propagating combustion synthesis was investigated through modeling studies. Similar studies were made to investigate the role of the relative density of the reactants. The effect of an imposed electric field on the results of the modeling analysis was considered. For any given imposed field, the wave velocity exhibited a maximum at a given normalized thermal conductivity, electrical conductivity, and relative density. The results are discussed in terms of the Joule heat contribution of the field and are compared with experimental observations.

Combustion Synthesis of Hexaaluminate Phosphors

Green phosphors of the composition Ce1‐xTbxMgAl11O19 (0.1 < x < 0.6) were prepared via self‐propagating (combustion) synthesis, using urea as a fuel. The phosphors that were produced via the combustion route were fine powders and produced a luminous intensity of 72%, compared to an optimized commercial phosphor. The excitation spectra were characteristic of the Ce3+ cation; however, no Ce3+ emission was observed. Energy transfer from the Ce3+ cation to the Tb3+ cation was complete, and only the 542 nm Tb3+ emission was observed. Concentration quenching occurred with x > 0.5. Scanning electron microscopy images showed a well‐crystallized, platelike morphology, with a particle size of 10‐20 µm.

Self-propagating high-temperature synthesis in the solid-phase triphenylphosphine-chloramine system

The macrokinetics and products of self-propagating high-temperture synthesis (SHS) in the solid-phase triphenylphosphine—chloramine system were studied by TGA-DTA, XRD, and31P NMR techniques. The temperature of SHS ignition (59°C), the velocity of reaction waves (1.6–5.0 mm s−1), and the maximum temperature (155–239°C) in the SHS wave were measured. The chemical composition of the product obtained was established: phosphine oxide, triphenyl-N-(phenylsulfoyl)-phosphinimine, benzenesulfonamide, NaCl, and water. The scheme of chemical reactions occurring during SHS was proposed.

Modeling of reactive synthesis in consolidated blends of fine Ni and Al powders

A model is proposed to describe the kinetics of reactive synthesis of NiAl from consolidated Ni–Al fine powder blends for various synthesis heat treatments. The presence of interfacial oxide in the blend and heat exchange between the sample and the environment have been taken into account to simulate the evolution of temperature and phase composition in the blend. A good agreement was obtained between the model calculations and experimental results. The effect of interfacial oxides and of various external parameters (heating rate, heat exchange conditions, powder size, etc.) on the synthesis kinetics was investigated. The model allows to explain the difference in reaction paths (self-propagating synthesis or solid state synthesis) observed in consolidated powder blends, as well as to describe the shape of the differential thermal analysis signal. Based on the results, the disagreement in previously reported data on growth kinetics of aluminides in diffusion structures with different characteristic diffusion distances is discussed.

A micromechanistic model of the combustion synthesis process: Modes of ignition

A theoretical model of the combustion synthesis process has been developed to study the ignition of a self-propagating combustion synthesis process in the Nb–C system. Compared with most of the previously published theoretical work on this subject, this model provides a much more detailed description of the combustion synthesis process from a microscale point of view, due to the fact that it takes into consideration the various microprocesses, such as the melting of reactants, the diffusion and mixing of reactants, and the formation of products. Different ignition modes, including constant-temperature ignition, constant-heat-flux ignition, and variable-temperature ignition, are considered in this work. The key parameters that influence the ignition process are also discussed.

Structure studies on porous titanium nickelide made by high-temperature self-propagating synthesis

Structure studies are reported on martensite transformations in porous TiNi alloy made by high-temperatures self-propagating synthesis. The transformations occur over a large temperature range and are incomplete. The structure data are used in reconstructing the heat release during the transition, where it is shown that it is determined by the superposition of martensite transformations occurring locally at various temperatures.

Synthesis ofin situ TiB2/TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends

In the present research, near-net-shapein situ TiB2/TiN and TiB2/TiN/Ni composites were fabricated from cold-sintered BN/Ti and BN/Ti/Ni powder blends by pressureless displacement reaction synthesis or thermal explosion under pressure. In both approaches, the processing or preheating temperatures (≤1200 °C) were considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructural characterization of the materials obtained was performed using X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical properties were evaluated by measuring microhardness, fracture toughness, and three-point bending strength. Application of a moderate external pressure (≤250 MPa) during self-propagating synthesis (SHS) synthesis was shown to be sufficient to ensure full density of the TiB2/TiN/Ni composite. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The high fracture toughness of thein situ synthesized TiB2/TiN/Ni composite (20.5 MPa√m) indicated that the finely dispersed ductile Ni phase was effective in dissipating the energy of cracks propagating in the ceramic matrix.

On the Mechanism of Structure Formation During Combustion Synthesis of Titanium Silicides

ABSTRACT The mechanism of structure formation during combustion synthesis of titanium silicides was investigated. It is shown that the propagation of an adiabatic combustion wave in a 5Ti + 3Si mixture follows the reaction coalescence mechanism, resulting in fine grains of Ti5Si3 crystallizing in the volume of melt. These grains grow rapidly behind the combustion front. Only the Ti5Si3 phase appears at the combustion temperature (2130°C), while other silicides (Ti5Si4, Ti3Si, etc.) may crystallize in the cooling zone. Time-resolved X-ray diffraction (TRXRD) analysis of the combustion process and layer-by-layer XRD analysis of the quenched combustion wave were utilized to study the sequence of phase formation. Interaction below the melting point of Ti is negligible in the self-propagating synthesis mode. A particle-foil technique was used to investigate the details of this lower temperature interaction.

Self-propagating synthesis of chromium acetylacetonate

After preliminary mechanical activation a mixture of solid chromium(III) chloride and sodium acetylacetonate becomes capable of reacting to form chromium acetylacetonate. The reaction may be carried out in the mode of a self-propagating process.

Fabrication of Al matrix in situ composites via self-propagating synthesis

Al matrix composite materials with 30 vol.% TiC, TiB 2 and TiC + TiB 2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB 2/Al and (TiB 2 + TiC)/Al composites contained the Al 3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al 4C 3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB 2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

A micromechanistic model of the combustion synthesis process: Part II. Numerical simulation

A series of computer experiments was conducted for the self-propagating combustion synthesis process in the Nb-C system, based on the general theoretical model that was developed previously.1A detailed and quantitative description was given for the various physical and chemical processes that take place during the combustion synthesis process. The results are presented at various length scales in order to provide an insight into understanding the mechanisms that are responsible for the self-propagating behavior. It was shown that a fundamental understanding and precise control of the process require a strong emphasis on the joint contributions of theratesof the various mass and energy redistribution processes that occur during the combustion synthesis process. A proper balance of each of the elementary process rates must be achieved to give rise to self-propagating behavior. This paper illustrates some of the capabilities of the general theoretical model in quantitatively describing the self-propagating combustion synthesis process.

Field-assisted self-propagating synthesis of β-SiC

We report on the effect of an electric field on the self-propagating reaction between silicon and carbon to synthesize β-SiC. In the absence of a field at a threshold level, no self-sustaining reaction is observed, and as the applied voltage increases, a linear increase in the velocity of the reaction wave is observed. This first demonstration of the effect of fields on such reactions is explained in terms of a model in which the role of the field is confined to the reaction zone.