Aluminum Burn Rate Modifiers Based on Reactive Nanocomposite Powders

Aluminum powders have long been used as additives in propellants, pyrotechnics, and explosives. Aluminum has a high enthalpy of combustion but relatively low burn rate. Addition of reactive nanocomposite powders can increase the burn rate of aluminum and thus the overall reaction rate of the energetic formulation. Replacing only a small fraction of the fuel by a nanocomposite material can enhance the reaction rate with little change to the thermodynamic performance of the formulation. This research showed the feasibility of the above concept using nanocomposite powders prepared by arrested reactive milling (ARM), a scalable “top‐down” technique for manufacturing reactive nanomaterials. The nanocomposite materials used in this study were 2B+Ti, and Al‐rich thermites: 8Al+3CuO, and 8Al+MoO₃. The reactive nanocomposite powders were added to micrometer‐sized aluminum powder and the mixture was aerosolized and burned in a constant volume chamber. The combustion atmosphere was varied using oxygen, nitrogen, and methane. The resulting pressure traces were recorded and processed to compare different types and amounts of modifiers. Additives of nanocomposite powders of 8Al+MoO₃ and 2B+Ti to micrometer‐sized aluminum were found to be effective in increasing both the rate of pressure rise and maximum pressure in the respective constant volume explosion experiments. It was observed that 20 wt.‐% of additive resulted in the best combination of the achieved burn rate and pressure.     

Consolidation and mechanical properties of reactive nanocomposite powders

Reactive nanocomposite powders with bulk compositions of 8Al·MoO₃, 12Al·MoO₃, and 8Al·3CuO were prepared by arrested reactive milling (ARM) and consolidated into cylindrical and rectangular pellets using a uniaxial die. Pellets were pressed at room temperature without any binder. Reference pellets were prepared from conventional Al powder and from Al-metal oxide powder blends with bulk compositions identical to those of the nanocomposite powders. Materials could be consolidated to densities greater than 90% of the theoretical maximum density while maintaining their high reactivity. Tensile strength and flexural strength of the consolidated materials were measured using diametrical compression and three-point flexural strength tests, respectively. Higher strengths were observed for higher relative densities, and the strength of the composite materials was comparable to that of consolidated aluminum powders. Yield strength of the reactive nanocomposite powders was determined from compaction load vs. die displacement curves using the Heckel equation. It was greater for the nanocomposite powders as compared to the powder blends or pure aluminum. Organic, or low melting point metal binders were added to selected samples to improve strength. Respective pellets were pressed at temperatures above the melting point of the metal binder. The highest density (~ 2.9 g/cm³) and tensile strength (~ 17.5 MPa) was observed with indium as binder. All consolidated samples were found to be highly reactive, and the effect of partial reaction during consolidation remained below the limit quantifiable by differential scanning calorimetry.     

Arrested Reactive Milling Synthesis and Characterization of Sodium‐Nitrate Based Reactive Composites

Arrested reactive milling was used to synthesize three composite powders using sodium nitrate as an oxidizer, and magnesium, aluminum, and mechanically alloyed aluminum‐magnesium (Al_0.5Mg_0.5) as respective fuels. Both magnesium and aluminum powders formed flakes with varying thickness from hundreds of nm to several μm sandwiched between sodium nitrate particles. Three‐dimensional composite and nanocomposite particles were formed with Al Mg mechanically alloyed powder. No change in the crystallinity of any components was observed from X‐ray diffraction patterns of the composite materials. Materials were characterized using differential thermal analysis (DTA) and simultaneous thermogravimetry (TG), carried out in argon. In composite materials, the decomposition of sodium nitrate starts at lower temperatures than for pure sodium nitrate. Weight loss is observed to start at relatively low temperatures. The most significant exothermic events occur at substantially higher temperatures, and therefore in a material that may have been significantly altered from its initial state. The results of thermal analysis indicate that the composite with mechanically alloyed Al Mg powder is most stable at low temperatures. Ignition of the prepared composites was studied using a thin layer of powder coated on an electrically heated filament. These experiments showed that the composite with mechanically alloyed Al Mg powder ignites at lowest temperatures and thus is expected to have the shortest ignition delays in practical applications. The emission spectra of the prepared composites burning in air are presented.     

Effect of SiC nanoparticle content and milling time on the microstructural characteristics and properties of Mg-SiC nanocomposites synthesized with powder metallurgy incorporating high-energy ball milling

The fabrication of magnesium nanocomposites with a homogeneous dispersion of nanoparticles has recently become an important issue. In the current study, micro-sized magnesium powders reinforced with 10, 20, and 30 wt% SiC nanoparticles were synthesized through high-energy ball milling using milling times ranging from 1 to 20 h to overcome the segregation and agglomeration of nanoparticles within the magnesium matrix. The milled nanocomposite powders were then consolidated using uniaxial cold pressing and sintering processes. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction were employed to investigate the effects of different milling times and contents of SiC nanoparticles on the evolution of the morphology of Mg–SiC milled powders and the microstructural characteristics of Mg–SiC sintered samples. In addition, once the consolidation process was complete, the relative densities and hardness values of the Mg–SiC nanocomposites were examined. The results indicated that as the content of SiC nanoparticles and the milling time increased, finer and equiaxed nanocomposite powders were obtained, and the average crystallite size of the milled magnesium powder significantly decreased. A homogeneous distribution of the SiC nanoparticles, including up to 30% of weight fraction, in the magnesium matrix was confirmed after 20 h of milling by elemental mapping generated by EDS. Additionally, the XRD analysis revealed that the diffraction peaks of the magnesium broadened while their maximum intensities decreased with increasing the milling time and SiC content. No undesirable phases were formed by interfacial reactions between magnesium and SiC nanoparticles in the milled nanocomposite powder during mechanical alloying. Furthermore, the results showed that both the relative density and hardness value of the Mg–SiC sintered sample improved as the milling time increased. However, the relative density of the Mg–SiC nanocomposite drastically decreased while the hardness significantly improved, as a result of increasing the content of SiC nanoparticles.     

Effects of multiwall carbon nanotubes on the thermal and mechanical properties of medium density polyethylene matrix nanocomposites produced by a mechanical milling method

Medium‐density polyethylene/multiwall carbon nanotube (MDPE/MWCNT) nanocomposites were produced by a mechanical milling method using a high‐energy ball mill. The MDPE and MWCNTs were added to the ball mill at a constant 20:1 weight ratio of ball/powders and milled for 10 h to obtain polyethylene matrix nanocomposites reinforced with 0.5, 1, 2.5, and 5 weight percent of MWCNTs. To clarify the role of both MWCNT content and milling time on the morphology of MDPE, some nanocomposite samples were investigated by using a scanning electron microscope. To evaluate the role of milling on the microstructure of the nanocomposites, very thin films of MDPE/MWCNTs were prepared and studied by transmission electron microscopy. Thermal behavior of these nanocomposites was investigated by using differential scanning calorimetry (DSC). Standard tensile samples were produced by compression molding. The dependence of the tensile properties of MDPE on both milling time and MWCNT content was studied by using a tensile test. The results of the microscopic evaluations showed that the milling process could be a suitable method for producing MDPE/MWCNT nanocomposites. The addition of carbon nanotubes to MDPE caused a change in its morphology at constant milling parameters. The results of the DSC tests showed that the crystallization temperature of MDPE increased as MWCNTs were added, although no dependency was observed as milling time increased. Crystallization index changed from 50 to 55% as MWCNT content increased from 0 to 5%. The results of the tensile tests showed that both the Young's modulus and the yield strength of MDPE increased as MWCNTs were added. J. VINYL ADDIT. TECHNOL., 2010. © 2010 Society of Plastics Engineers     

Transition from Impact‐induced Thermal Runaway to Prompt Mechanochemical Explosion in Nanoscaled Ni/Al Reactive Systems

The effect of microstructure on ignition sensitivity and reaction behavior is investigated for nanoscaled Ni/Al gasless reactive systems. Nanometric homogeneity of the reactive media was achieved through (a) conventional mixing of nanometric powders; (b) short‐term high‐energy ball milling (HEBM) of micrometer‐sized powders. Sensitivity to thermal inputs is investigated by differential thermal analysis and mechanical sensitivity is studied by high‐rate shear impacts. The composite Ni/Al particles prepared by HEBM were extremely thermally sensitive, with reaction initiating at 220 °C, compared to 559 °C for nanometric powder samples and 640 °C for un‐milled, micrometer‐sized Ni+Al powder mixture. In contrast, nanometric powder mixtures were more susceptible to ignition through mechanical means, exhibiting a high‐speed reaction mode that is not observed in HEBM samples. The high‐speed mode preferentially appears in high‐shear regions and is interpreted as a mechanically‐induced thermal explosion. Its progression is tied to the passage of a stress wave in the heterogeneous media that heats and mixes the materials, rather than being propagated due to chemical energy release. The microstructures unique to each material are considered responsible for their individually ignition sensitivities. Specifically, the finely interspersed porosity in nanometric powder mixtures allows direct heating of the reactive interface between Ni and Al particles during compression through pore collapse and plastic deformation, which leads to exceptionally high mechanical sensitivity. The HEBM materials have high specific reactant interface area in the bulk of each composite particle that enhances thermal sensitivity, but the relatively low specific interface area between particles is unfavorable to mechanical ignition.     

Processing of reactive Ni−Al powders via the LabRAM**

Reactive Ni−Al materials have been developed using a variety of methods, with high energy ball milling (HEBM) being one of the most common means for tailoring reaction behavior. Powder production limitations associated with HEBM, including the addition of process control agents, have inspired the exploration of an alternate manufacturing technique: acoustic dry milling with the Resodyn Laboratory Resonant Acoustic Mixer (LabRAM). The influence of acoustic milling time, intensity, and media size with respect to microstructure and reactive behavior of Ni−Al powders were evaluated in this work. After just 20 min of milling, a reactive composite Ni−Al microstructure was produced. Milling intensity and media size were directly proportional to the formation of more homogeneous composite powders. The reaction onset temperature was decreased to 446 °C, or ≈200 °C lower than that of unprocessed material. The method shows promise for the production of reactive powder for a host of applications.     

Mechanical Alloying and Sintering of a Ni/10wt.%Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocomposite and its Characterization

A Ni matrix nanocomposite reinforced by 10 wt.% Al₂O₃ was fabricated by mechanical alloying. The powders mixture was milled up to 24 h in a ball mill. Phase composition and morphology of prepared powders were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. To obtain compact bodies, pressing was applied on the milled powders; then sintered at different temperatures for 1 h in argon atmosphere. Furthermore, the effect of milling time and sintering temperature on microstructure. Physical, mechanical and electrical properties of the sintered nanocomposite specimens were evaluated. The results show decrease of particle size of milled powders (69 nm) as the time increased up to 24 h of milling with a noticeable presence of agglomerates. On the other hand, relative density, microhardness, compressive strength, elastic modulus and electrical conductivity of the sintered samples were found to progressively increase with the increasing of milling time and sintering temperature. Their maximum values were 97.36%, 1137 MPa, 633 MPa, 21.6 GPa and 9.71 × 10⁵ S/m, respectively, for the sample that was milled for 24 h and sintered at 1200 °C. On the other hand, the increasing milling time tended to decrease the fracture strain while it increased with increase of the sintering temperature.     

Structural and magnetic properties of Co/Al2O3 cermet synthesized by mechanical ball milling

Cobalt nanoparticles in the alumina matrix were synthesized using high energy mechanical ball milling of Co₃O₄ and Al powders mixture. The effect of ball mill time of 1 up to 12 h on the phase formation and crystalline lattice of the samples was investigated by the fitting of the X-ray diffraction patterns with Fullprof software and Rietveld method. The results show that 6 h milling of the primary powders yields a nanocomposite of Co/Al₂O₃ cermet. The formation of Co/Al₂O₃ nanocomposite was confirmed by a morphological study using scanning electron microscopy and transmission electron microscopy. The prepared nanocomposite by 12 h ball mill time has ferromagnetic properties with a high saturation magnetization value of 118 emu/g. Also, using Henkel plot analysis, it was shown that there are strong dipole-dipole magnetic interactions between the prepared cobalt nanoparticles in the Al₂O₃ matrix.     

Effects of Initial Powder Size on the Densification of Barium Titanate Ceramics Prepared by Microwave‐Assisted Sintering

The effects of initial powder size on microwave‐assisted sintering (MWS) were investigated. BaTiO₃ powders with an average particle size of 50, 100, and 500 nm were prepared and sintered with MWS and conventional heating‐based sintering (CS). Samples of the 50 ‐ and 100‐nm‐sized BaTiO₃ powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50‐nm‐sized BaTiO₃ powder resulted in a relative mass density of more than 90% when sintered at 1050°C, whereas the same density was achieved at 1200°C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500‐nm‐sized BaTiO₃ powder was used. The sinterability of the BaTiO₃ ceramics prepared through the MWS of mechanically milled, 50‐nm‐sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO₃ ceramics prepared through the MWS of the 100‐ and 500‐nm‐sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of ~50‐nm‐sized powders, but is negligible for 500‐nm‐sized powders.     

Reactive Liners Prepared Using Powders of Aluminum and Aluminum‐Magnesium Alloys

Cylindrical reactive liners filled with powders of aluminum, aluminum‐magnesium alloys, and aluminum‐magnesium powder blends were prepared and initiated by a centrally located explosive charge. The experiments were performed in a cubic chamber. Several transient pressure measurements were taken in addition to the quasistatic pressure caused by the explosion. Results were compared against a reference case with an inert liner filled with aluminum oxide powder. For all reactive liners, an increase in both quasistatic pressure and blast wave strength were observed compared to the case of an inert liner. In experiments with mechanically alloyed Al ⋅ Mg powders, the quasistatic pressure is effectively the same as in experiments with pure aluminum. An improvement in the achieved quasistatic pressure is observed for the liners with a cast alloyed Al ⋅ Mg powder. Most interestingly, a substantial contribution to the air blast indicative of very early reaction occurring in sub‐millisecond time scale is observed for all experiments with reactive liners. The most substantial improvement in the blast characteristics is observed in experiments with mechanically alloyed Al ⋅ Mg powders. While the mechanisms of prompt reactions of metals and alloys remain largely unexplored, the present results highlight the importance of such reactions for reactive liners and other components of energetic systems.     

An investigation on flowability and compressibility of AA 6061100 − x-x wt.% TiO2 micro and nanocomposite powder prepared by blending and mechanical alloying

This work investigates the relationships between the components of powders, namely, the powder surface morphology, the flow characteristics and the compressibility of low-energy (microcomposite) and high-energy (nanocomposite) ball milled powders of Al 6061 alloy reinforced with TiO₂ particles. The morphology of the above powder as the function of reinforcement and the milling time was studied by using the scanning electron microscope (SEM). The changes in powder characteristics such as the apparent density, tap density, true density and flow rate were examined by the percentage of reinforcement and milling time. The cohesive nature of the powder was also investigated in terms of Hausner ratio and Kawakita plot. Further, the particle/agglomerate size of low-energy and high-energy ball milled powders was explained by the laser particle size analyzer. X-ray peak broadening analysis was used to determine structural properties of mechanically alloyed powders. The compressibility behavior was examined by the compaction equation proposed by Panelli and Ambrosio Filho to investigate the deformation capacity of the powder. The compressibility behavior, namely, the densification parameter (A) of the microcomposite powder (irregular morphology) was decreased significantly with increasing TiO₂ content due to the disintegration of TiO₂ particles and the cluster formation followed by its agglomeration. The compressibility behavior, namely, the densification parameter (A) of the nanocomposite powder (equiaxed and almost spherical) was decreased slowly with increasing TiO₂ content due to work hardening on the matrix powder. With increased milling time, the compressibility behavior of AA 6061-10 wt.% TiO₂ composite powders increased up to 30 h of milling due to embedding of TiO₂ particles with matrix and changes in powder morphology and finally decreased after 40 h due to work hardening effect.     

Characterization of the structure of TiB2/TiC nanocomposite powders fabricated by high-energy ball milling

TiB₂/TiC nanocomposite powders were successfully prepared by high-energy ball milling of the powder mixtures of Ti and B₄C. X-ray diffraction analysis showed that the TiC phase was not produced until the milling time was up to 24 h and only a minimal amount of TiB₂ was generated, even after 48 h of milling. The critical grain size of Ti milled for the reaction between Ti and B₄C was 31.2 nm. Transmission electron microscopy clearly indicated that the resulting powder mixture obtained after milling for 48 h and annealing at 800 °C for 30 min was composed of nanosized TiC and TiB₂ particles.     

Preparation and characterization of nano‐Sb2O3/poly(butylene terephthalate) composite powders based on high‐energy ball milling

In this work, a kind of composite powders with good dispersion and distribution of nano‐Sb₂O₃ particles in poly(butylene terephthalate) (PBT) was prepared by the high‐energy ball milling (HEBM). The effects of the milling time on the structure, morphology, particle size distribution, and thermal behavior of the nano‐Sb₂O₃/PBT composite powders were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, laser diffraction particle size analyzer, and thermal analysis (TGA, DTG and DSC) techniques. The results showed that the regular shape of PBT powders was converted into flakes and the nano‐Sb₂O₃ particles were well deagglomerated and better dispersed in the PBT matrix during the HEBM process. The mechanochemical activation that was provided by the HEBM process caused a reduction in the molecular weight of PBT, which result in favoring the first step of thermal degradation. Furthermore, two T_g’s were obtained in the case of the nanocomposite powders when the milling time was over 3 h, one of them being slightly higher than that of the pure PBT, which indicated that there was a special interaction between PBT and nano‐Sb₂O₃ particles. However, the HEBM process leaded to a decreasing of the PBT crystallinity. J. VINYL ADDIT. TECHNOL., 25:91–97, 2019. © 2018 Society of Plastics Engineers     

Preparation of super‐hydrophobic film with fluorinated‐copolymer

Organic superhydrophobic films were prepared by utilizing TA‐N fluoroalkylate (TAN) and methyl methacrylate (MMA) copolymer as water‐repellent materials and inorganic silica powder as surface roughness material has been developed. Coating solutions prepared by adding silica powders into copolymer solution directly (one‐step method) and by adding silica powders into monomers and allowing them to react (two‐step method). The results showed that contact angles of the films prepared by one‐step method (37.6 wt % of silica powders in the coating solution) were greater than 150°, but the transmittance of the film at visible light was only 30%. On the other hand, the contact angle of films prepared by two‐step method (20 wt % of silica powders in the coating solution) was greater than 160° and the transmittance of the film was greater than 90%. The contact angle of the film prepared by poly(octyl acrylate), POA, was 32.1°, but while introducing silica powder into the system, the contact angle of the film was reduced to be smaller than 5°. Thus, superhydrophobic and superhydrophilic films can be obtained by introducing a roughening material on the hydrophobic surface and the hydrophilic surface, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1646–1653, 2007     

Fabrication of bulk AlN-TiN nanocomposite by reactive ball milling and underwater shock consolidation technique

Reactive milling of aluminum nitride and titanium powders corresponding to the stoichiometric reaction Ti + AlN resulted in the formation of the ceramic matrix composite AlN-TiN. Prolongation of the milling process led to a microstructure with nanosize range of crystallites of both AlN and TiN, evidenced through XRD measurements, SEM and TEM observation. Further, underwater shock compaction with a pressure level of about 10 GPa was applied to the nanocomposite powders to obtain bulk nanostructured sample. The effect of this shock compaction on the prolonged milled powder resulted in a 22% reduction in crystallite size. The average microhardness of the consolidated nanocomposite was 656 HV and 840 HV for 40 h and 100 h MA samples, respectively, with densities 98% of theoretical values in both cases as well as no change in chemical composition.     

The effect of the wet-milling process on sintering temperature and the amount of additive of SiAlON ceramics

In this study, nano-sized SiAlON powders were produced by wet milling at elevated speeds as a top-to-bottom process. Before the milling process, different milling times and mediums were performed for the determination of the most efficient milling system. The milled powders were characterized by BET and X-ray diffraction (XRD) measurements and the results were compared to standard samples. The standard powders were produced using a conventional process (the ball to powder ratio was 1:1.5, at 300 rpm, for 1.5 h) having a few hundred nanometer particle size. The nano powders were milled using a wet-milling process in an optimum medium so that the particle size was decreased down to ≈70 nm. The samples, produced from the nano powders, were densified at 150 °C lower degrees than the sintering temperature of samples which were produced by a conventional method (185 nm). However, the phase transformation of α → β-SiAlON was also observed related to the amount of additives. This transformation affected the mechanical properties of the SiAlON ceramic. The results were discussed using the relationship between density, phase composition, microstructure and mechanical properties.     

Preparation of Ag‐AZO Nanocomposite Powder Compact for RF Magnetron Sputtering Target Application

Octylamine‐coated silver nanoparticles with an average size of 10 nm were synthesized and added to the AZO powder as fillers to increase the density of the sintered target. The resulting Ag–AZO nanocomposite powder was formed into compacts by a uniaxial pressing process. It was found that the addition of octylamine‐ coated silver nanoparticles can increase the density of AZO powder compacts by ~ 4% after sintering. The optimum content of silver addition is about 0.13 wt%. It is suggested that the melted silver fills the void space between agglomerate pores and enhances the interconnection between nanocrystalline AZO powders. The films deposited using the here‐synthesized Ag–AZO target showed lower resistivity and high carrier concentration compared with the one using AZO target.     

In situ synthesis of NbB2–Al2O3 nanocomposite powder by mechanochemical processing

This research is based on the production of NbB₂–Al₂O₃ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of niobium, aluminium and boron oxide powders was subjected to high-energy ball milling. The structural evaluation of powder particles after different milling times was conducted by the X-ray diffractometry (XRD), scanning electron microscopy, and transmission electron microscopy. The results showed that during ball milling the Nb/Al/B₂O₃ reacted with a combustion mode producing NbB₂–Al₂O₃ nanocomposite. The XRD analyses exhibited that the NbB₂–Al₂O₃ nanocomposite was formed after 10?h milling time and increasing milling time up to 30?h had no significant effect other than refining the crystallite size. In the final stage of milling, the crystallite sizes of NbB₂ and Al₂O₃ were estimated to be less than 50?nm.