Combustion Synthesis of Materials for Application in Supercapacitors: A Review.

A supercapacitor is an energy storage device that has the advantage of rapidly storing and releasing energy compared to traditional batteries. One powerful method for creating a wide range of materials is combustion synthesis, which relies on self-sustained chemical reactions. Specifically, solution combustion synthesis involves mixing reagents at the molecular level in an aqueous solution. This method allows for the fabrication of various nanostructured materials, such as binary and complex oxides, sulfides, and carbon-based nanocomposites, which are commonly used for creating electrodes in supercapacitors. The solution combustion synthesis offers flexibility in tuning the properties of the materials by adjusting the composition of the reactive solution, the type of fuel, and the combustion conditions. The process takes advantage of high temperatures, short processing times, and significant gas release to produce well crystalline nanostructured materials with a large specific surface area. This specific surface area is essential for enhancing the performance of electrodes in supercapacitors. Our review focuses on recent publications in this field, specifically examining the relationship between the microstructure of materials and their electrochemical properties. We discuss the findings and suggest potential improvements in the properties and stability of the fabricated composites based on the results.

Combustion Synthesis of Magnetic Nanomaterials for Biomedical Applications.

Combustion synthesis is a green, energy-saving approach that permits an easy scale-up and continuous technologies. This process allows for synthesizing various nanoscale materials, including oxides, nitrides, sulfides, metals, and alloys. In this work, we critically review the reported results on the combustion synthesis of magnetic nanoparticles, focusing on their properties related to different bio-applications. We also analyze challenges and suggest specific directions of research, which lead to the improvement of the properties and stability of fabricated materials.

Electrical and Heat Distributions and Their Influence on the Mass Transfer during the Flash Spark Plasma Sintering of a Cu/Cr Nanocomposite: Experiments and Numerical Simulation.

The nanocomposite Cu-Cr powder was consolidated by flash spark plasma sintering (FSPS), which involves applying an extremely rapid change in the electrical power passing through the bulk of the sample. It was demonstrated that an essentially fully dense material could be obtained in 15 s. Such short-term treatment typically preserves the nanostructured features of the material. However, investigation revealed a nonuniformity in the microstructure of the alloys obtained under such extreme conditions. To better understand the observed effects, the FSPS process was simulated. It was observed that a rapid change in the applied electrical power resulted in nonuniform distributions of current density and temperature along the body of the consolidated material. Specifically, the current density was higher on the periphery of the sample, and the temperature was higher in the middle. These findings explain the observed structural transformation during FSPS and suggest an optimization strategy to avoid microstructural nonuniformity.

Sponge-like CoNi Catalysts Synthesized by Combustion of Reactive Solutions: Stability and Performance for CO2 Hydrogenation.

Active and stable catalysts are essential for effective hydrogenation of gaseous CO2 into valuable chemicals. This work focuses on the structural and catalytic features of single metals, i.e., Co and Ni, as well as bimetallic CoNi alloy catalysts synthesized via combustion of reactive sol-gels. Different characterization methods were used for studying the relationships between the structure, composition, and catalytic activity of the fabricated materials. All catalysts exhibited highly porous sponge-like microstructure. The outermost surfaces of the CoNi alloys were more saturated with Co, while a stoichiometric Co/Ni ratio was observed for the particle's bulk. Catalytic properties of the as-synthesized powders were studied in the CO2 hydrogenation reaction at 300 °C for over 80 h of time on stream. All the catalysts demonstrated exceptional selectivity with respect to CH4 formation. However, the combination of elemental Co and Ni in a single phase resulted in a synergistic effect in bulk alloy catalysts, with activity twofold to threefold that of single-metal catalysts. The activity and stability of the CoNi3 catalyst were higher than those previously reported for Ni-based catalysts. The reasons for this behavior are discussed.

Extremely hard and tough high entropy nitride ceramics.

Simultaneously hard and tough nitride ceramics open new venues for a variety of advanced applications. To produce such materials, attention is focused on the development of high-entropy ceramics, containing four or more metallic components distributed homogeneously in the metallic sublattice. While the fabrication of bulk high-entropy carbides and borides is well established, high-entropy nitrides have only been produced as thin films. Herein, we report on a newel three-step process to fabricate bulk high-entropy nitrides. The high-entropy nitride phase was obtained by exothermic combustion of mechanically-activated nanostructured metallic precursors in nitrogen and consolidated by spark plasma sintering. The fabricated bulk high-entropy nitride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N demonstrates outstanding hardness (up to 33 GPa) and fracture toughness (up to 5.2 MPa∙m1/2), significantly surpassing expected values from mixture rules, as well as all other reported binary and high-entropy ceramics and can be used for super-hard coatings, structural materials, optics, and others. The obtained results illustrate the scalable method to produce bulk high-entropy nitrides with the new benchmark properties.

Nanoscale Metastable ε-Fe3N Ferromagnetic Materials by Self-Sustained Reactions.

A single-step method for the preparation of metastable ε-Fe3N nanoparticles by combustion of reactive gels containing iron nitrate (Fe(NO3)3) and hexamethylenetetramine (C6H12N4) in an inert atmosphere is reported. The results of Fourier transform infrared spectroscopy (FTIR) and thermal analysis coupled with dynamic mass spectrometry revealed that the exothermic decomposition of a coordination complex formed between Fe(NO3)3 and HMTA is responsible for the formation of ε-Fe3N nanoscale particles with sizes of 5-15 nm. The magnetic properties between 5 and 350 K are characterized using a superconducting quantum interference device (SQUID) magnetometer, revealing a ferromagnetic behavior with a low-temperature magnetic moment of 1.09 µB/Fe, high room temperature saturation magnetization (∼80 emu/g), and low remanent magnetization (∼15 emu/g). The obtained value for the Curie temperature of ∼522 K is close to that (∼575 K) for bulk ε-Fe3N reported in the literature.

Reactive Ni/Al Nanocomposites: Structural Characteristics and Activation Energy.

Stochastically structured Ni/Al reactive nanocomposites (RNCs) were prepared using short-term high-energy ball milling. Several milling times were utilized to prepare RNCs with differing internal nanostructures. These internal structures were quantitatively and statistically analyzed by use of serial focused ion beam sectioning coupled with 3D reconstruction techniques. The reaction kinetics were analyzed using the electrothermal explosion technique for each milling condition. It is shown that the effective activation energy (Eef) ranges from 79 to 137 kJ/mol and is directly related to the surface area contact between the reactants. Essentially, the reaction kinetics can be accurately controlled through mechanical processing techniques. Finally, the nature of the reaction is considered; the mechanistic effect of the reactive and three diffusive activation energies on the effective activation energy is examined.

Solution Combustion Synthesis of Nanoscale Materials.

Solution combustion is an exciting phenomenon, which involves propagation of self-sustained exothermic reactions along an aqueous or sol-gel media. This process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides. This Review focuses on the analysis of new approaches and results in the field of solution combustion synthesis (SCS) obtained during recent years. Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, structure, and nanostructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual properties are outlined. To demonstrate the versatility of the approach, several application categories of SCS produced materials, such as for energy conversion and storage, optical devices, catalysts, and various important nanoceramics (e.g., bio-, electro-, magnetic), are discussed.

X-ray nanotomography and focused-ion-beam sectioning for quantitative three-dimensional analysis of nanocomposites.

Knowing the relationship between three-dimensional structure and properties is paramount for complete understanding of material behavior. In this work, the internal nanostructure of micrometer-size (∼10 µm) composite Ni/Al particles was analyzed using two different approaches. The first technique, synchrotron-based X-ray nanotomography, is a nondestructive method that can attain resolutions of tens of nanometers. The second is a destructive technique with sub-nanometer resolution utilizing scanning electron microscopy combined with an ion beam and `slice and view' analysis, where the sample is repeatedly milled and imaged. The obtained results suggest that both techniques allow for an accurate characterization of the larger-scale structures, while differences exist in the characterization of the smallest features. Using the Monte Carlo method, the effective resolution of the X-ray nanotomography technique was determined to be ∼48 nm, while focused-ion-beam sectioning with `slice and view' analysis was ∼5 nm.

Irradiation-enhanced reactivity of multilayer Al/Ni nanomaterials.

We have investigated the effect of accelerated ion beam irradiation on the structure and reactivity of multilayer sputter deposited Al/Ni nanomaterials. Carbon and aluminum ion beams with different charge states and intensities were used to irradiate the multilayer materials. The conditions for the irradiation-assisted self-ignition of the reactive materials and corresponding ignition thresholds for the beam intensities were determined. We discovered that relatively short (40 min or less) ion irradiations enhance the reactivity of the Al/Ni nanomaterials, that is, significantly decrease the thermal ignition temperatures (Tig) and ignition delay times (τig). We also show that irradiation leads to atomic mixing at the Al/Ni interfaces with the formation of an amorphous interlayer, in addition to the nucleation of small (2-3 nm) Al3Ni crystals within the amorphous regions. The amorphous interlayer is thought to enhance the reactivity of the multilayer energetic nanomaterial by increasing the heat of the reaction and by speeding the intermixing of the Ni and the Al. The small Al3Ni crystals may also enhance reactivity by facilitating the growth of this Al-Ni intermetallic phase. In contrast, longer irradiations decrease reactivity with higher ignition temperatures and longer ignition delay times. Such changes are also associated with growth of the Al3Ni intermetallic and decreases in the heat of reaction. Drawing on this data set, we suggest that ion irradiation can be used to fine-tune the structure and reactivity of energetic nanomaterials.

Preparation and reactivity of gasless nanostructured energetic materials.

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the balls. Among other applications, it is a versatile technique that allows for effective preparation of gasless reactive nanostructured materials with high energy density per volume (Ni+Al, Ta+C, Ti+C). The structural transformations of reactive media, which take place during HEBM, define the reaction mechanism in the produced energetic composites. Varying the processing conditions permits fine tuning of the milling-induced microstructures of the fabricated composite particles. In turn, the reactivity, i.e., self-ignition temperature, ignition delay time, as well as reaction kinetics, of high energy density materials depends on its microstructure. Analysis of the milling-induced microstructures suggests that the formation of fresh oxygen-free intimate high surface area contacts between the reagents is responsible for the enhancement of their reactivity. This manifests itself in a reduction of ignition temperature and delay time, an increased rate of chemical reaction, and an overall decrease of the effective activation energy of the reaction. The protocol provides a detailed description for the preparation of reactive nanocomposites with tailored microstructure using short-term HEBM method. It also describes a high-speed thermal imaging technique to determine the ignition/combustion characteristics of the energetic materials. The protocol can be adapted to preparation and characterization of a variety of nanostructured energetic composites.

Photoactive porous silicon nanopowder.

Bulk processing of porous silicon nanoparticles (nSi) of 50-300 nm size and surface area of 25-230 m(2)/g has been developed using a combustion synthesis method. nSi exhibits consistent photoresponse to AM 1.5 simulated solar excitation. In confirmation of photoactivity, the films of nSi exhibit prompt bleaching following femtosecond laser pulse excitation resulting from the photoinduced charge separation. Photocurrent generation observed upon AM 1.5 excitation of these films in a photoelectrochemical cell shows strong dependence on the thickness of the intrinsic silica shell that encompasses the nanoparticles and hinders interparticle electron transfer.

Nanocrystalline hydroxyapatite/si coating by mechanical alloying technique.

A novel approach for depositing hydroxyapatite (HA) films on titanium substrates by using mechanical alloying (MA) technique has been developed. However, it was shown that one-hour heat treatment at 800°C of such mechanically coated HA layer leads to partial transformation of desired HA phase to beta-tri-calcium phosphate (ß-TCP) phase. It appears that the grain boundary and interface defects formed during MA promote this transformation. It was discovered that doping HA by silicon results in hindering this phase transformation process. The Si-doped HA does not show phase transition to ß-TCP or decomposition after heat treatment even at 900°C.

Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation.

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made.

Thermal explosion in Al-Ni system: influence of mechanical activation.

The influence of short-term (5-15 min) highly energetic ball milling on the ignition characteristics of a gasless heterogeneous Ni-Al reactive system has been investigated. By using Al-Ni clad particles (30-40 microm diameter Al spheres coated by a 3-3.5 microm layer of Ni, that corresponds to a 1:1 Ni/Al atomic ratio), it was shown that such mechanical treatment leads to a significant decrease in the self-ignition temperature of the system. For example, after 15 min of ball milling, the ignition temperature appears to be approximately 600 K, well below the eutectic (913 K) in the considered binary system, which is the ignition temperature for the initial clad particles. Thus, it was demonstrated that the thermal explosion process for mechanically treated reactive media can be solely defined by solid-state reactions. Additionally, thermal analysis measurements revealed that mechanical activation results in a substantial decrease in the effective activation energy (from 84 to 28 kcal/mol) of interaction between Al and Ni. This effect, that is, mechanical activation of chemical reaction, is connected to a substantial increase of contact area between reactive particles and fresh interphase boundaries formed in an inert atmosphere during ball milling. It is also important that by varying the time of mechanical activation one can precisely control the ignition temperature in high-density energetic heterogeneous systems.

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.

Hydrogen storage capacity characterization of carbon nanotubes by a microgravimetrical approach.

An accurate gravimetric apparatus based on a contactless magnetic suspension microbalance was developed. This unit was used to measure the hydrogen storage capacity for a variety of carbon nanotubes (CNTs) at room temperature and hydrogen pressures up to 11.5 MPa. The results show that regardless of their synthesis methods, purities, and nanostructures all investigated CNT products possess relatively low hydrogen storage capacities (<0.2 wt %). For comparison, the adsorption characteristics of theses samples were also measured at a pressure of 0.1 MPa and liquid nitrogen temperature (approximately 77 K) by a conventional volumetric approach. The methodological aspects related to the accuracy of the hydrogen uptake measurements are also discussed.

In situ preparation of oxide-based supported catalysts by solution combustion synthesis.

The oxide-based supported catalysts with high specific surface area (>200 m(2)/g) were produced in one step through combination of the impregnation and solution combustion synthesis approaches. As a model system, iron oxide was selected, which was loaded on different porous supports including alpha-Al(2)O(3), gamma-Al(2)O(3), and ZrO(2), as well as activated alumina. It was shown that for the former three cases the specific surface areas of the supported catalysts are about or below the surface areas of the support. However, for the activated Al(2)O(3) this characteristic significantly increases compared to that of the support. It was demonstrated that the developed approach may be used to produce different types of oxide-based supported catalysts, including perovskites.