Reactive nanofoils for joining refractory and dissimilar materials

Heterogeneous nanostructured foils produced by magnetron deposition or mechanical processing represent a new class of reactive materials. They are composed of layers or clusters of different phases (typically with a size of 10–100 nm) that can react with each other with strong heat release. The reaction, being initiated locally, spontaneously propagates across the entire foil in the form of high-temperature wave. Some examples of promising practical applications of these foils in advanced technologies, such as joining dissimilar materials, were presented.     

Reactive,nonreactive, and flash spark plasma sintering of Al2O3/SiC composites—A comparative study

The influence of different SPS-based methods, that is, conventional spark plasma sintering (SPS), flash SPS (FSPS), and reactive SPS (RSPS) on the properties of Al₂O₃/SiC composite was investigated. It was shown that the application of preliminary high energy ball milling of the powders significantly enhances the sinterability of the ceramics. It was also demonstrated that FSPS provides unique conditions for rapid, that is, less than a minute, consolidation of refractory ceramics. The Al₂O₃-20 wt% SiC composite produced by FSPS possesses the highest relative density (~99%), fracture toughness (7.5 MPa m^1/2), hardness (20.3 GPa) and wear resistance among all ceramics produced by other SPS-based approaches with dwelling time 10 minutes. The RSPS ceramics hold the highest Young's modulus (390 GPa). Substitution of micron-sized Al₂O₃ particles by nano alumina does not lead to measurable enhancement of the mechanical properties.     

Effect of mechanical activation on thermal explosion in Ni-Al mixtures

The effect of mechanical activation (MA) on thermal explosion in equimolar Ni-Al mixtures was studied by time-resolved XRD. MA was also found to increase the burning velocity and decrease the ignition temperature. Thermal explosion in non-activated Ni-Al mixtures was found to proceed via formation of liquid (melted) intermediate products, while that in the activated mixtures gave no liquid intermediates.     

Combustion joinining of carbon/carbon composites by a reactive mixture of titanium and mechanically activated nickel/aluminum powders

Combustion joining of carbon/carbon (C/C) composites using a mixture of titanium and mechanically activated Ni/Al powders as a reactive medium is reported. A minimum preheating of the sample stack to 630 K is required to initiate the joining process. A robust crack- and pore-free joint layer (∼75−100 μm in thickness), which is composed of NiAl_x and TiC_y(O_z) phases, is produced. Tensile-strength testing of the joined C/C composites shows that the fracture does not occur along the joint layer.     

Direct Combustion Synthesis of Silicon Carbide Nanopowder from the Elements

Direct synthesis of silicon carbide (SiC) nanopowders (size 50–200 nm, BET ~20 m²/g) in Si–C system is conducted in an inert atmosphere (argon) using a self‐propagating high‐temperature synthesis (SHS) approach. A preliminary short‐term (e.g., minutes) high‐energy ball milling (HEBM) of the initial mixture, which involves pure Si and C powders, is used to enhance system reactivity. Two conditions of HEBM with different force fields (17G and 90G) are applied and the results are compared. The influence of HEBM's conditions on the microstructure of mechanically treated mixtures and combustion products is also investigated and discussed. Obtained results suggest that by changing the intensity of mechanical treatment one may prepare a completely amorphous reactive mixture containing carbon and silicon, or gradually change the ratio of (Si/C)–SiC phases and finally produce pure silicon carbide powder during the milling process. The influence of HEBM on the combustibility of the Si/C mixture possesses a critical character: the self‐sustained reaction becomes feasible only after a critical time of ball milling (i.e., 10 min for 90G; 30 min for 17G). Comparison of the microstructures for as‐milled and as‐synthesized powders reveals that for all investigated conditions the morphologies of the as‐milled reactive Si/C media are essentially the same as that for SiC combustion products. The mechanism for direct synthesis of SiC by combustion reaction is also proposed.     

Novel approaches to solution-combustion synthesis of nanomaterials

Solution-combustion is an attractive approach to synthesis of nanomaterials for a variety of applications, including catalysts, fuel cells, and biotechnology. In this paper, several novel methods based on the combustion of a reactive solution are presented. These methods include self-propagating sol-gel combustion and combustion of impregnated inert and active supports. It was demonstrated that, based on the fundamental understanding of the considered combustion processes, a variety of extremely high surface area materials could be synthesized. The controlling process parameters are defined and discussed. Examples of materials synthesized by the above methods are presented. For the first time, a continuous technology for production of nanopowders by using the solution combustion approach is demonstrated.      

Novel apparatus for joining of carbon-carbon composites

A novel apparatus for joining carbon-carbon (C-C) composites is presented. This device was designed and built based on the concept of self-sustained oxygen-free high-temperature reactions. A layer of reactive mixture is contained between two disks of C-C composite that are to be joined. The stack is held in place between two electrodes, which are connected to a dc power supply. dc current is used to uniformly initiate the reaction in the reactive layer. The electrodes are also part of the pneumatic system, which applies a load to the stack. The designed hydraulic system is effective, lending to low cost and simplified, rapid, accurate operation. It provides a very short response time ( approximately 10 ms), which is important for the considered applications. All operational parameters such as initial and final loads, applied current, delay time between ignition and final load application, duration of Joule heating, and safety interlocks are controlled by a programable logic controller system. These features make it an efficient, user-friendly and safe machine to join refractory materials. The entire joining process takes place on the order of seconds, rather than hours as required for solid-state joining methods. The mechanical properties of the obtained joints are higher than those for the C-C composites.     

Single- and multi-wall carbon nanotubes produced using the floating catalyst method: Synthesis, purification and hydrogen up-take

The floating catalyst (FC) method for synthesis of single- and multi-wall carbon nanotubes was optimized and scaled up to yield 6 g/h and 20 g/h of products, respectively. Different CNTs purification methods were compared. It was found that the procedure involving room temperature bromination is the most effective to purify the FC-CNTs. The hydrogen up-take capacities of the different products were measured using the quasi-equilibrium volumetric method. It was shown that, at room temperature and gas pressure up to 150 atm for both SWCNTs and MWCNTs, hydrogen up-take does not exceed 1.5 wt.% and is weakly dependent on the product purity.     

Solution combustion synthesis of metal nanopowders: Nickel—Reaction pathways

Nanopowders of pure nickel were directly synthesized for the first time by conventional solution combustion synthesis (SCS) method. In this article, a specific reaction pathway is suggested to describe the metallic phase formation during SCS. It is proposed that the exothermic reaction between NH₃ and HNO₃ species formed during the decomposition of glycine and nickel nitrate acts as the source of energy required to achieve the self‐sustained reaction regime. A thermodynamic analysis of the combustion synthesis reaction indicates that increasing glycine concentration leads to establishing a hydrogen rich reducing environment in the combustion wave that in turn results in the formation of pure metals and metal alloys. TGA of reaction systems and XRD analysis of products in the quenched combustion wave show that the formation of oxide phases occurs in the reaction front, followed by gradual reduction of oxide to pure metallic phases in the postcombustion zone. A methodology for SCS of pure metals and metal alloys nanoparticles can be inferred from the results presented. © 2010 American Institute of Chemical Engineers AIChE J, 2011