Fabrication of ZrC particles and its formation mechanism by self-propagating high-temperature synthesis from Fe-Zr-C elemental powders

ZrC particles were prepared via self-propagating high-temperature synthesis (SHS) reaction from 0 to 30 wt.% Fe-Zr-C elemental powder mixtures. The ZrC particles of the SHS products greatly decreased from about 10 μm with an irregular shape in free Fe addition to nano-meter order with a nearly spherical shape in 30 wt.% Fe addition. The reaction mechanism of ZrC during the SHS processing was explored through the microstructural observation and phase constituents analysis on the combustion-wave quenched sample in combination with differential thermal analysis (DTA). For the low Fe contents, the solid-state reaction between C and Zr powders should be responsible for the formation of ZrC. While for the high Fe contents, the formation mechanism of ZrC could be ascribed to the dissolution of C into the Fe-Zr melt and the precipitation of ZrC. The addition of Fe to Zr-C reactants not only inhibits the growth of ZrC particles but also promotes the occurrence of ZrC-forming reaction.     

Thermal explosion synthesis of ZrC particles and their mechanism of formation from Al–Zr–C elemental powders

ZrC particles were fabricated by thermal explosion (TE) from mixture of Al, Zr and C elemental powders. Without the addition of Al, the synthesized ZrC particles had irregular shape of ~ 4.0 μm in average. Increasing Al content up to 30 wt.%, however, refined significantly them down to < 0.2 μm with regularly square morphology. The Al effect of reaction mechanism promoted the ZrC formation as diluents in the course of TE, which was clarified using differential thermal analysis and X-ray diffraction technique. The melting of Al favored the reaction with Zr to generate ZrAl₃, and then the dissolution of C into the Al–Zr liquid resulted in precipitation of ZrC. Meanwhile, the exothermic effect prompted C atoms dissolving into Zr–Al liquid and eventually led to precipitation of ZrC out of the supersaturated liquid. The Al addition inhibited particle growth, but also promoted the TE reaction.     

Study of formation behavior of ZrC in the Cu–Zr–C system during combustion synthesis

ZrC particles were prepared in situ via self-propagating high-temperature synthesis (SHS) reaction with the Cu–Zr–C elemental powder mixtures. The reaction behavior and formation route for synthesizing ZrC in the Cu–Zr–C system were investigated. With increasing Cu contents, the adiabatic temperatures, reaction temperatures and ZrC particle sizes of the SHS products were greatly decreased. The formation of ZrC particles could be ascribed by the dissolution of C into the Cu–Zr melt and the precipitation of ZrC. Cu plays an important role in controlling the reaction behavior and morphology of products, it not only inhibits the growth of ZrC particles, but also participates and promotes the SHS reaction.     

Reaction path of the synthesis of α-Al2O3-TiC-TiB2 in an Al-TiO2-B4C system

Differential thermal analysis was undertaken to determine the reaction path of the synthesis of α-Al₂O₃-TiC-TiB₂ in an Al-TiO₂-B₄C system under argon. The Al content plays a significant role in controlling the reaction path and product. When the Al content is no more than 26.7 wt.%, TiO₂ first reacts with B₄C to yield TiB₂ with TiBO₃ being the intermediate phase, and then increasing temperature leads to the subsequent reactions between Al and TiO₂ or its sub-oxides to yield α-Al₂O₃ and Al₃Ti, and the resultant Al₃Ti then reacts with B₄C to produce TiC and TiB₂. When the Al content is high (e.g. ≥ 34 wt.%), the reaction between Al and TiO₂ for the formation of α-Al₂O₃ and Al₃Ti occurs initially, and then the Al₃Ti reacts with B₄C. With the increasing Al content, the onset of the exothermic reaction in the Al-TiO₂-B₄C system shifts to lower temperature and the degree of reaction conversion is enhanced.     

Self-propagating high-temperature synthesis of La(Sr)Ga(Mg,Fe)O3−δ with planetary ball-mill treatment for solid oxide fuel cell electrolytes

This study investigated the combined effects of self-propagating high-temperature synthesis (SHS), planetary ball-mill (PBM) treatment, and sintering temperature on La_0.7Sr_0.3Ga_0.7Mg_0.1Fe_0.2O_3−δ (LSGMF73712) as an electrolyte material for solid oxide fuel cells (SOFC). The SHS products (SHS-LSGMF73712) were compared with that prepared via solid-state reaction (SSR) in terms of sinterability and power generation performance. The SHS products were treated with PBM for 10, 30, 50, and 70 h. The SHS products contained the by-product LaSrGaO₄; however, in the SHS products treated with PBM for longer than 50 h, the by-product disappeared after sintering at 1350 °C for 3 h in air. Among the samples, SHS products treated with PBM for 70 h displayed superior sintering (1350 °C), whereas the SSR product (SSR-LSGMF73712) was successfully sintered at 1450 °C for 3 h in air. Under the cell configuration of Ni-Fe/SHS-LSGMF73712-PBM70 h (0.3 mm thick)/Sm_0.5Sr_0.5CoO₃, the maximum power density was 0.673 W/cm² at 800 °C using humidified hydrogen gas (3 mol% H₂O) as a fuel and air as an oxidizing agent at a flow rate of 100 mL/min, which was almost equivalent to that using SSR-LSGMF73712 (0.629 W/cm² at 800 °C) under the same conditions.     

Study of formation behavior of ZrC in the Fe-Zr-C system during combustion synthesis

ZrC particles were prepared in situ by self-propagating high-temperature synthesis (SHS) reaction with the Fe-Zr-C elemental powder mixtures. The reaction behavior and formation route for synthesizing ZrC in the Fe-Zr-C system were investigated. With increasing Fe contents, the adiabatic temperatures, reaction temperatures and ZrC particles sizes were obviously decreased. Fe plays an important role in controlling the reaction behavior and morphology of products, Fe not only inhibits ZrC particle from growing, but also promotes the SHS reaction. The addition of Fe to Zr-C reactants promotes the ZrC-forming reaction.     

Formation of an aluminum-alloyed coating on AZ91D magnesium alloy in molten salts at lower temperature

An aluminum-alloyed coating was formed on an AZ91D magnesium alloy in molten salts containing AlCl₃ at a lower temperature of 380 °C. The microstructure and phase constitution of the alloyed layer were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectrum and X-ray diffraction. The nano-hardness of the coating was studied by nanoindentation associated with scanning probe microscopy. The corrosion resistance of the coated specimen was evaluated in a 3.5 wt.% NaCl solution by electrochemical impedance spectroscopy and cyclic potentiodynamic polarization. The results show that the aluminum-alloyed coating consists of Mg₂Al₃ and Mg₁₇Al₁₂ intermetallic layers. The formation of the coating is dictated by the negative standard free energy of the reaction: 2AlCl₃ + 3 Mg = 3MgCl₂ + 2Al. This process is associated with a displacement reaction mechanism and diffusion process that takes place during the molten salt treatment. High activity of Al elements in molten salts contributes to the lower temperature formation of the Al-alloyed coating. The alloyed coating markedly improves the hardness as well as the corrosion resistance of the alloy in comparison with the untreated AZ91D magnesium alloy, which is attributed to the formation of the intermetallic compounds.     

Synthesis and characterization of multipod zinc oxide whiskers synthesized via a modified self-propagating high-temperature synthesis method

A self-propagating high-temperature synthesis (SHS) method, which features a 2-step heating process carried out in a closed chamber, was developed to prepare multipod ZnO whiskers (mZnOw). A compacted Zn powder was first heated at 370 °C in air to form a refractory layer of ZnO on the surface of Zn particles. Next, the SHS reaction was ignited by rapidly heating to ∼750 °C under different oxygen gauge pressures (PO₂) to obtain a loose white product mainly composed of mZnOw. It was found that the PO₂ strongly affected the reaction conversion and morphological features, along with the Zn-to-O ratio, of the as-synthesized products, which, in turn, might influence their photoluminescence and microwave absorption properties. A possible explanation for the effects of PO₂ on these characteristics was also proposed.     

Effects of SiC volume fraction and aluminum particulate size on interfacial reactions in SiC nanoparticulate reinforced aluminum matrix composites

The SiC nanoparticulate reinforced Al-3.0 wt.% Mg composites were fabricated by combining pressureless infiltration with ball-milling and cold-pressing technology at 700 °C for 2 h. The effects of SiC nanoparticulate volume fractions (6%, 10% and 14%) and Al particulate sizes (38 μm and 74 μm) on interfacial reactions were investigated by SEM, TEM and X-ray diffraction. The results show that the MgO at the interface between SiC nanoparticulate and molten Al can provide a barrier for the diffusion of Si, C and Al. Using Al particulate (74 μm) as raw material, the Al₄C₃ phase was not found in the composites containing 6 vol.% and 10 vol.% SiC, but presented in the composites containing 14 vol.% SiC. When SiC content up to 14 vol.%, the products of MgO around SiC nanoparticulate are not enough to provide effective protection from the reaction between SiC and molten Al, therefore the diffusion of Si, C and Al can take place to produce Al₄C₃ and Si phases. Using 38 μm Al particulate as raw material, the fine Al particulate possesses the high reaction activity and can easily be embedded into the gap among the big Mg particulate segregated at the interface, resulting in the appearance of exposure surface of SiCp to the Al and the forming of diffusion channels for the atomics C, Si and Al. So, the formations of Al₄C₃ and Si phases were occurred.     

Pressureless sintering of ZrC-based ceramics by enhancing powder sinterability

The sinterability of ZrC was enhanced by high-energy ball milling as well as introduction of graphite and SiC as sintering additives. Densification process and microstructure development were investigated for ZrC-based ceramics densified by pressureless sintering. As-received ZrC powder showed poor sinterability. After high-energy ball milling, ZrC powder can be sintered to 98.4% theoretical density at 2100 °C. The obtained ceramic had fine microstructure and fewer entrapped pores. Introduction of 2 wt.% graphite combined with high-energy ball milling lowered the densification temperature of ZrC. The relative density of obtained ceramic reached up to 95% at 1900 °C. Introduced SiC inhibited ZrC grain growth during sintering and consequently avoided the entrapped pores within the grains. The relative density of ZrC-SiC reached up to 96.7% at 2100 °C. ZrC-SiC composite formed an interesting intragranular structure and had high fracture strength at room temperature.     

Ni-Al-Ti coatings obtained by microwave assisted SHS: Oxidation behaviour in the 750-900 °C range

Microwaves have been used to ignite the Self-propagating High-temperature Synthesis (SHS) of Ni and Al powder mixtures to produce a duplex intermetallic coating on Ti substrates. Due to the high exothermic nature of the reaction, the newly formed NiAl is in the liquid phase and can react with the underlying Ti to form a tough ternary intermediate layer, belonging to the Ti-Ni-Al system, in a one step process. Aim of this work is to assess the high-temperature performances of the Ti-Ni-Al layer, compared to NiAl coating and Ti. Experimental results demonstrate that the ternary layer presents oxidation resistance comparable to NiAl up to 750 °C. In this condition, the thick Ti-Ni-Al layer could replace the functionality of hard and brittle NiAl coatings. At 900 °C, instead, NiAl oxidation resistance results higher, and this can be ascribed to the relatively low Al content in the studied ternary compound, which hinders the formation of a continuous and protective scale.     

In situ formation of Zr2Al3C4/Al2O3 composites by combustion synthesis with PTFE and thermal activations

Preparation of Zr₂Al₃C₄-Al₂O₃ in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both aluminothermic reduction of ZrO₂ and chemical activation of PTFE (Teflon). The starting materials included ZrO₂, Al, carbon black and PTFE. In addition to the conventional SHS method, the experiments were conducted by a chemical-oven SHS (COSHS) route to thermally assist the synthesis reaction. The threshold amount of 2% (mass fraction) PTFE was required to induce self-sustaining combustion. When the conventional SHS scheme was utilized, due to low combustion temperatures between 1152 and 1272 °C and insufficient reaction time, the dominant carbide forming in the composite was ZrC instead of Zr₂Al₃C₄. On the other hand, the COSHS technique increased the combustion temperature of the reactant compact to about 1576 °C, lengthened the high-temperature duration for the reaction, and prevented Al vapor from escaping away. As a consequence, Zr₂Al₃C₄-Al₂O₃ composites with a small amount of Zr₃Al₃C₅ were obtained. The microstructure of the COSHS-derived product showed that plate-like Zr₂Al₃C₄ grains were about 2 μm in thickness and 10-30 μm in length, and most of them were closely stacked into a laminated configuration.     

Thermal stability of bulk Zr2Al4C5 ceramic at elevated temperatures

Thermal expansion analysis is applied to the study of phase segregation of Zr₂Al₄C₅ at elevated temperatures (20-2000 °C) under flowing argon. Such information would be useful for the detection of phase decomposition temperature and thermal stability of materials. The detected phase-decomposition temperature of Zr₂Al₄C₅ is approximately 1900 °C. The presented thermal expansion analysis results are in good agreement with the X-ray diffraction (XRD) and SEM results. The results indicate that Zr₂Al₄C₅ is susceptible to decomposition through sublimation of high vapor pressure of Al and weaker covalent bonds between ZrC slabs and Al₄C₃ layers. Thus, minimal amounts of Al, Zr₂Al₃C₅, Al₄C₃ and ZrC form on the surface layer. Zr₂Al₃C₅ further decomposes to ZrC_1 − x and Al₄C₃ at 2000 °C. However, the amount of decomposing phase slowly increases, and the structural shape of bulk Zr₂Al₄C₅ ceramic is always kept stable during heat treatment.     

Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers

This study investigates chemical vapor deposition of C from CH₄ on particulate SiO₂ and subsequent carbothermal conversion of the resultant composite particles to SiC powders. Mass measurements, HR-TEM, SEM and XRD were used to characterize the products at various stages of the processes. It was found that oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere owing to enhanced C uptake. Pyrolytic carbon layers 5-8 nm thick were deposited on SiO₂ particles. The coated powders with high C loadings (40-42.6 wt.% C) were converted to SiC under Ar flow in a temperature range of 1700-1800 K. Almost pure SiC powders containing a mixture of particles and whiskers of ~ 100 nm were synthesized at 1750 K for 45 min and at 1800 K for 30 min using the starting powder with 40 wt.% C. Whisker diameter increased with the C content of the coated powder. It was proposed that SiC whisker was grown by a vapor-solid mechanism. Equilibrium thermodynamic analysis by the method of minimization of Gibbs’ free energy predicted the reaction pathways to SiC and to the product species in the Si-O-C-Ar system. This study demonstrated that either C shell-SiO₂ core powders or SiC powders could be synthesized rapidly in the same reactor.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

Microstructural and mechanical properties of Al-Mg/Al2O3 nanocomposite prepared by mechanical alloying

The effect of milling time on the microstructure and mechanical properties of Al and Al-10 wt.% Mg matrix nanocomposites reinforced with 5 wt.% Al₂O₃ during mechanical alloying was investigated. Steady-state situation was occurred in Al-10Mg/5Al₂O₃ nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/5Al₂O₃ nanocomposite at the same time. For the binary Al-Mg matrix, after 10 h, the predominant phase was an Al-Mg solid solution with an average crystallite size 34 nm. Up to 10 h, the lattice strain increased to about 0.4 and 0.66% for Al and Al-Mg matrix, respectively. The increasing of lattice parameter due to dissolution of Mg atom into Al lattice during milling was significant. By milling for 10 h the dramatic increase in microhardness (155 HV) for Al-Mg matrix nanocomposite was caused by grain refinement and solid solution formation. From 10 to 20 h, slower rate of increasing in microhardness may be attributed to the completion of alloying process, and dynamic and static recovery of powders.     

Dense sub-micron-sized ZrC-W composite produced by reactive melt infiltration at 1200 °C

ZrC-W composite was produced by reactive infiltration of Zr₂Cu into WC preform at 1200 °C. The WC reacted completely with Zr in the alloy to form 61.8 vol% ZrC and 38.2 vol% W. The infiltrated composite reached a relative density of 98.3% and average grains sizes of about 0.5 μm. The flexural strength and the fracture toughness of the ZrC-W composite was improved by the addition of W.     

An investigation on corrosion behaviour of pressure infiltrated Al-Mg alloy/SiCp composites

In this study, effect of Mg alloying addition (2-8 wt.%) on corrosion behaviour of Al matrix composites, in 3.5 wt.% NaCl environment, has been investigated. Composites were produced by pressure infiltration technique at 750 °C and had a SiC particle (SiC_p) volume fraction of ∼60%. Results were evaluated by using potentiodynamic polarisation measurements, immersion tests, SEM, EDS and XRD analysis. Compared to the pure Al matrix, mass loss of the composites decreased with increasing Mg content. Experimental results revealed that intermetallics as a result of reaction between Al-Mg alloy and SiC particle has beneficial effect on corrosion resistance of the composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

Direct preparation of LiBH4 from pre-treated LiH + B mixture at high pressure

LiBH₄ was prepared from LiH + B mixture at 300-500 °C with a hydrogen pressure of 35 MPa. The LiH + B mixture was pretreated by ball milling under 10 MPa hydrogen pressure for 10 h. The results showed that ball-milling treatment is favorable for the formation of LiBH₄, which could reduce the reaction temperature for about 200 °C. A small quantity of LiBH₄ could be found in the pre-treated sample. The formation of LiBH₄ from LiH + B mixture is a kinetically hindered progress and a diffusion-control reaction. LiBH₄ formed at 400 °C could release 6.59 wt.% H₂ when it was heated from room temperature to 500 °C. The yield of LiBH₄ is about 59.4%.