Reaction Of Powder Research Articles

Combustion synthesis of (TiC + SiC) composite powders by coupling strong and weak exothermic reactions

By coupling strong and weak exothermic reactions, (TiC + SiC) composite powders have been prepared by combustion synthesis. The reaction temperature and heating rate strongly depend on the Ti/Si ratios in the reactant powder mixture. The phase assemblage and microstructure of the products can be manipulated by varying the starting compositions. According to SEM observation, the reaction procedure and formation mechanism of TiC and SiC are discussed. It is proposed that the presence of metallic melt phases is important for the formation of the carbide phases.

Extrusion-activated thermal explosion applied to intermetallics processing

The present study deals with synthesizing the NiAl compound through the thermal-explosion route akin to self-propagating high-temperature synthesis under the novel circumstance of extrusion-induced mechanical activation. Either route has the potential for addressing the intermetallics processing cost problem and both reactions may alike be significantly enhanced by prior activation. Reactant powders were homogeneously mixed and precompacted by cold isostatic pressing (CIP) at 800 MPa. Hydrostatic extrusion was performed with a lower-chamber pressure fixed at 50 MPa and an upper-chamber pressure of 1.4 GPa, hence yielding a fully dense Ni+Al mixture, maximizing the contact areas between reactants and enhancing the specific surfaces of Al without such a grain refinement as with mechanical activation by ball milling. After heating up to 550 °C with a rate of 10 °C/min, uniquely CIPed samples did not react, whereas those subsequently extruded did react in the thermal-explosion mode with temperature overstepping 1084 °C.

Purification of Aluminum Through Gas and Powder Fluxing: Part I. Mathematical Models

Mathematical models for the removal of impurity elements from molten aluminum through injecting inert gas, reactive powders, slag refining and combinations of these methods, are welldeveloped including models to calculate the removal efficiency of impurity elements for batch reactor and continuous reactor for each method. The models were validated with experimental measurements. The removal efficiencies of the sodium and calcium predicted by the models agree well with the measurements. Compared with the single models, the combined model can give a more accurate prediction and the contribution of each method on the removal efficiency can be qualified. The model is useful to improve the removal efficiency of impurity elements by gas combined with powder fluxing.

Mechanism of ASR reduction by reactive aggregate powders

This study aimed to evaluate the effectiveness of a mechanism responsible for the mitigation of expansion when reactive aggregate powders (RAP) were used in concrete containing the same reactive aggregate. In the first part, it was shown that RAP were efficient in reducing the expansion of concrete by 35 to 95%. The most efficient powders were those coming from aggregates having high active silica contents (glass, opal). Powders of siliceous limestone were less effective. In a second part, experimental evidence of the pozzolanic activity of RAP was obtained. It was shown that RAP were similar to pozzolan, since they consumed lime and produce calcium–silicate–hydrate (C–S–H) having a lower calcium/silica (C/S) ratio than normal C–S–H. The third part showed experimentally that these low C/S C–S–H could increase the redistribution of alkalis of the pore solution, thus reducing the alkalinity of the pore solution to a level that was sufficiently low to limit the attack of coarser reactive aggregates. In the final part, numerical simulations using a modified version of the hydration model CEMHYD3D were carried out to support this assumption.

Nanocomposite thermite powders prepared by cryomilling

Reactive nanocomposite materials have been recently prepared by Arrested Reactive Milling (ARM), a method based on mechanical milling of component powders to form micron-sized composite particles where components are mixed on a scale of about 100 nm. The milling temperature affects both the refinement rate and the final product properties. In this paper we report an effort to prepare Al–CuO reactive nanocomposite powders at cryogenic temperatures. The material was processed in a liquid nitrogen bath using steel vials and balls of different sizes as milling media. The number and dimensions of the milling balls as well as the milling time were systematically varied. The products were characterized by X-ray diffraction, particle size analysis, and scanning electron microscopy. Thermal characteristics were studied using a custom wire-ignition setup and differential scanning calorimetry. Results show that the uniformity of mixing and reactivity of the nanocomposite powders can be improved using milling at cryogenic temperatures.

Chemical vapor synthesis of Mg–Ti nanopowder mixture as a hydrogen storage material

Abstract Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen contents. In this work, the hydrogen release and uptake properties of Mg/5%Ti nanopowder mixture prepared by a chemical vapor synthesis (CVS) process were investigated. Samples were made in a CVS reactor, in which reactant powders were fed into evaporator placed inside the reactor by means of specially designed powder feeders. The produced Mg/5%Ti was hydrogenated in an autoclave under 10.3 MPa pressure and 150 °C for 12 h. Thermogravimetric analysis (TGA) showed that 5.2 wt% of hydrogen began to be released from 190 °C while temperature was increased at a heating rate of 5 °C/min up to 350 °C under an argon flow. This onset temperature of Mg–Ti nanopowder dehydrogenation was much lower than that of MgH2 alone, which is 381 °C. In addition, the activation energy of dehydrogenation was 104 kJ/mol, which is much smaller than that of as-received MgH2 (153 kJ/mol).

Polymer hydrogel assisted combustion synthesis of highly crystalline ceramic nanoparticles for SOFC electrolyte films

Highly crystalline Ce 0.8Sm 0.2O 1.9 (SDC) nanoparticles were synthesized via a polyacrylamide hydrogel assisted combustion process involving microwave-induced gelation and combustion. Crosslinked polyacryamide was used as fuel, and uniform dispersion of ceramic precursors in the porous polyacrylamide hydrogel led to mild combustion and limited agglomeration of ceramic particles. The combustion method developed here prolonged combustion process, and allowed ceramic particles to further crystallization. As-synthesized powders were calcined to increase particles size, for the purposed of reducing sintering shrinkage in the densification of electrolyte film. The reactive SDC powders with particle sizes of 0.1–1 μm were made into dense solid oxide fuel cell (SOFC) electrolyte film by dip-coating. The cell with a 6 μm thick SDC film achieved high maximum power densities of 353 and 533 mW cm −2 at 550 and 600 °C, respectively.

Processing and characterization of aluminum-based nanothermites

During the past several years, a significant effort has been on investigation of reaction front propagation and the rate of energy release in heterogeneous systems consisting of nanopowder reactants. Substantial size reduction of each reactant powder (e.g. from micro- to nano-size) leads to increase of reaction front propagation in some systems under unconfined conditions by approximately two to three order of magnitude. This paper presents key challenges associated with processing and use of nanothermite materials and characterization of nanoreactants. Reaction constants, such as activation energies and frequency factors were determined using DSC technique for several nanothermite systems based on nanosize aluminum and iron oxide, bismuth trioxide, and molybdenum trioxide. Experimental data of ignition delay times for different nanothermite systems using laser energy source were compared well to those predicted by proposed mathematical model.

Thermogravimetric analysis of the hydrolysis of zinc particles

The production of hydrogen via the hydrolysis of zinc has been applied to a two-step water splitting thermochemical cycle using high-temperature solar process heat. Thermogravimetric measurements were conducted on the hydrolysis of zinc particles to reveal the reaction mechanism and elucidate the influences of process parameters including heating rate, water partial pressure, total system pressure, the particle size of zinc powder and purity of zinc on the reaction process. The experimental results show that the overall conversion of reactant zinc increased as the heating rate increased, as the total system pressure and the particle size of reactant zinc powder decreased. The water partial pressure in studied scale had little influence on the zinc conversion, while the impurities in sample zinc influenced the hydrolysis reaction greatly. A conversion of more than 99% could be got as the mean particle size of sample zinc is smaller than 11 μm. The experimental results also revealed two kinds of reaction mechanism of the zinc hydrolysis, in which the rate of hydrolysis reaction was limited by the evaporation of zinc and the diffusion of zinc through ZnO layer respectively.

A study on mechanochemical behavior of B 2O 3–Al system to produce alumina-based nanocomposite

One possible route for producing the fine and homogenous distribution of hard particles in composite microstructure is the mechanochemical processing in which high-energy ball milling promotes the reaction in a mixture of reactive powders. In this study mechanochemical reaction of B 2O 3 and Al powder during ball milling was studied. The phase transformation and microstructure of powder particles during ball milling were investigated by X-ray diffractometry and scanning electron microscopy. The results showed that during ball milling the B 2O 3–Al reacted with a combustion mode producing Al 2O 3–AlB 12 nanocomposite. The crystallite size of Al 2O 3 and AlB 12 was 40 and 25 nm, respectively. This structure appeared to be stable upon annealing.

Study of the synthesis mechanism and magnetic properties of quasi-nano-sized Ba–Mg ferrite powders formed by self-propagating high-temperature synthesis

Abstract Quasi-nano-sized BaxMg1−xFe2O4 ferrite powders were prepared by using self-propagating high-temperature synthesis (SHS) combined with mechanical milling. The combustion temperature and the propagating rate of the SHS reaction were in the range of 930–1115 K and about 4.5–6.4 mm/s, respectively. X-ray and neutron-beam analysis showed that the SHS products were Ba0.38Mg0.62Fe2O4 and Ba0.42Mg0.58Fe2O4, respectively. The final ferrite powder size after mechanical milling was less than about 150 nm. The maximum magnetization (Ms), the residual magnetization (Mr) and the coercive force (iHc) of quasi-nano-sized BaxMg1−xFe2O4 significantly depend on the initial molar ratio of BaO to MgO of the reactant powder mixture and the oxygen partial pressure during the SHS reaction. The complex permeability decreased with the increase of the frequency, and its real value ( μ r ′ ) reached the peak value at about 0.6 GHz. The SHS mechanism includes the competitive reduction reaction between barium and magnesium oxides, which influences the non-stoichiometric numbers and the magnetic properties of the ferrites.

Combustion Synthesis of Titanium Diboride and Zirconia Composite Powders. Part I

A new method of synthesizing titanium diboride‐zirconium oxide composite powders under the combustion mode was investigated. In situ stabilization of formed or added unstabilized zirconia by magnesia and/or yttria, control of TiB2 particle size, as well as the formation of TiB2‐ZrO2 composites in a wide range of concentrations was investigated. Two different synthesis routes were explored, namely: i) from elemental titanium and, boron powders with addition of zirconia or ii) from zirconium, boron, and titania with or without addition of zirconia. Yttrium oxide and/or magnesium present in boron reactant powder were used for zirconia product stabilization. The effect of initial mixture composition, dilution with stabilized or unstabilized zirconia, and other operating conditions, such as inert gas pressure during the combustion and initial mixture density on combustion front propagation velocity and maximum combustion temperature as well as product microstructure and its phase composition was investigated. Products morphology and compositions were examined using XRD, EDX, and SEM analyses.

Synthesis of In-situ Aluminum Matrix Composite Using a New Activated Powder Injection Method

In the present study, in-situ Al matrix composites containing alumina particles were synthesized by a new method that involved injection of an activated powder mixture into the molten aluminum, which was subsequently squeeze cast to minimize its porosity. The activated powder mixture was prepared by milling of powders of commercially pure aluminum and zinc oxide for a predetermined time. Milling for 60 minutes led to optimum activation of the reactant powders. Such activation reduced the reaction temperature from above 1000 °C to about 640 °C. Microstructural characterization studies revealed the formation of submicron size alumina particles with near equiaxed morphology and good particle/matrix interface. The composite specimen synthesized using the activated powder injection (API) method exhibited higher hardness, greater yield and tensile strengths, and lower elongation when compared to the monolithic specimen of similar composition. This article places particular emphasis on explaining the uniqueness of the processing methodology used in this study for synthesis of in-situ Al/Al2O3 composites.

Control of the Injectability of Calcium Carbonate-Calcium Phosphate Mixed Cements for Bone Reconstruction

The purpose of this study was to improve injectability and cohesiveness of original calcium carbonate-calcium phosphate mixed (CaCO3-CaP) self-setting paste for bone filling and repair. With this aim in view dry co-grinding was implemented on the solid phase (vaterite and dicalcium phosphate dihydrate) of this cement. A protocol designed to quantify paste injectability has been established and pointed out the synergistic positive effects of solid phase co-grinding treatment on injectability, cohesiveness and setting time of the paste. The improvement of these properties are related to close and homogeneous association of reactive powders and to the decrease of specific surface area favoring the powders hydration process enhancing setting reaction rate. In addition, the particle size decrease and morphology modification improved flowability of the paste which results in a low and constant (320 g) force level to extrude the paste.

High-strength resorbable brushite bone cement with controlled drug-releasing capabilities

Brushite cements differ from apatite-forming compositions by consuming a lot of water in their setting reaction whereas apatite-forming cements consume little or no water at all. Only such cement systems that consume water during setting can theoretically produce near-zero porosity ceramics. This study aimed to produce such a brushite ceramic and investigated whether near elimination of porosity would prevent a burst release profile of incorporated antibiotics that is common to prior calcium phosphate cement delivery matrices. Through adjustment of the powder technological properties of the powder reactants, that is particle size and particle size distribution, and by adjusting citric acid concentration of the liquid phase to 800 mM, a relative porosity of as low as 11% of the brushite cement matrix could be achieved (a 60% reduction compared to previous studies), resulting in a wet unprecompacted compressive strength of 52 MPa (representing a more than 100% increase to previously reported results) with a workable setting time of 4.5 min of the cement paste. Up to 2 wt.% of vancomycin and ciprofloxacin could be incorporated into the cement system without loss of wet compressive strength. It was found that drug release rates could be controlled by the adjustable relative porosity of the cement system and burst release could be minimized and an almost linear release achieved, but the solubility of the antibiotic (vancomycin > ciprofloxacin) appeared also to be a crucial factor.

Exothermic reactions in Co/Al nanolaminates

Vapor-deposited Co/Al nanolaminates having a net equiatomic stoichiometry exhibited rapid, high-temperature synthesis. When ignited locally, 7.5-μm-thick Co/Al foils were characterized by self-sustained propagating reactions with flame front speeds between ∼0.5 and 9 m/s. Speed was largely affected by bilayer thickness and premixed volume fraction, consistent with previous studies of other exothermic multilayers. Cobalt aluminide foils reacted by self-propagating synthesis developed a variety of rough surface morphologies characterized by a large amplitude (peak-to-valley ∼1.0 μm), multiperiod, wavelike structure. High-temperature reaction was also stimulated by rapid global heating. Tests revealed low ignition temperatures (Tig)∼240–460 °C compared with previous Co–Al powder reactant experiments. Ignition temperature was influenced by bilayer thickness. All foils ignited by global heating and those reacted in a self-propagating mode developed a single-phase B2 (CsCl-type) crystal structure. Thick Co/Al nanolaminates were used successfully to join a few different materials including metalized Al2O3 (by soldering) and polyetheretherketone (by direct attachment).

Reaction kinetics in the combustion synthesis of Al–Ir–Ni intermetallic compounds

The combustion synthesis of Al50Ir48Ni2 (at.%) was conducted at different heating rates in both a differential scanning calorimetry (DSC) chamber and a vacuum furnace. It was found that a higher heating rate, a sufficient amount of reactant powder, and effective control of the heat loss facilitated the complete reaction and resulted in combusted single IrAl phase products. Otherwise, multiphase products containing IrAl, unreacted Ir, and Al3Ir were synthesized. The reactions involved in different processes were discussed in terms of the thermal competition between heat generation and loss during the reaction. All ignition temperatures were below 773 K, indicating that the combustion reaction occurs at the solid–solid state. With increasing heating rate, the ignition temperature increased while the product density decreased.

Synthesis of TiO2 nanoparticles via a novel mechanochemical method

Abstract A novel preparation method for titanium dioxide (TiO2) nanoparticles is introduced in this paper. In this process, titanyl sulphate (TiOSO4·xH2SO4·yH2O) and NaCl powders were used as the reactant and diluent phases, respectively. In order to understand the thermal behavior of the reactant, thermo-gravimetric analysis/differential thermo-gravimetry (TGA/DTG) was conducted. The starting mixture of the reactant and NaCl powders were loaded into a planetary ball mill, under argon atmosphere at 400 rpm. The final products were obtained by annealing the milled powders in the temperature range of 600–900 °C and subsequently by washing out the water-soluble NaCl. Scanning electron microscopy (SEM) investigations showed that nanosized titania particles have equiaxed shape with the mean size in the range of 15–50 nm. Mechanism of the formation of TiO2 during milling was studied via Fourier transform infrared (FTIR) spectroscopy.

From tin oxalate to (Fe, Co, Nb)-doped SnO 2: Sintering behavior, microstructural and electrical features

Highly reactive SnO 2-doped nanocrystalline powders with average particle size of 20 nm and specific surface areas above 30 m 2/g were obtained through the polymeric precursor method with tin oxalate (SnC 2O 4) as a chloride-free precursor for SnO 2. Powders and sintered discs were characterized by means of X-ray powder diffraction (XRD), SEM and TEM. The influence of Co and Fe on the microstructure development and on the electrical properties of dense SnO 2-based ceramics was studied. Co and Fe species were found to decrease in more than 70 °C the sintering temperature of SnO 2 with respect to mixed oxide procedures. Secondary phases enriched in Co and Fe were detected and identified in sintered samples with XRD. Current–voltage curves were registered for electrical characterization. Doping with iron increased the electrical breakdown field and a nonlinearity coefficient of 20 was achieved.

Functional Refractory Material Design for Advanced Thermal Shock Performance Due to Titania Additions

One of the biggest challenges of refractory materials during metallurgical applications is the thermal shock performance. In terms of this work, different ways of improving the thermal shock resistance in carbon‐bonded as well as in oxide‐based materials by adding reactive titania powders are explored with the aid of thermal shock tests and followed by microstructure evaluation. The laboratory results obtained lead to microstructures with superior thermal shock properties and open the horizon of development of functional components such as slide gates or nozzles for clean steel technology.