Reactive Foils Research Articles

Preparation of Ag–Fe2O3-Based black and electrically insulating coatings by magnetron sputtering from metal targets

Black and electrically insulating coatings provide touch panel displays with a classic appearance. In this study, black and electrically insulating Ag–Fe2O3-based coatings were prepared using reactive magnetron sputtering, stainless steel and silver foil. The thin films prepared with oxygen introduction and no Ag foil showed blue light absorption and high electrical resistance, which are comparable to those of Fe2O3 single crystals. The optimal Ag foil size for maintaining a high electrical resistivity of 108 Ω·sq−1 was determined to be 54 mol%Ag in the film samples. Black and electrically insulating thin films composed of metallic Ag, amorphous Ag–Fe–O, and slightly crystallized Fe2O3 were prepared using 3.4 or 3.9 %O2. Metallic Ag segregated randomly in the high-refractive-index Fe2O3-based matrix, resulting in plasmonic absorption over a wide range of wavelengths centered in the red-wavelength region. At 3.9 %O2, a high sheet resistance of 108 Ω·sq−1 and strong visible absorption of 35.2 μm−1 (on average), slightly deviating by 2.4 μm−1 were simultaneously achieved by clear phase separation of the Ag grains and Fe2O3-based matrix. A clear phase separation mechanism based on thermodynamic nonequilibrium oxidation of the metallic Ag phase is proposed.

Analysis of selective bonding processes using reactive multi-layers for system integration on LTCC based SiPs

This paper discusses the use of reactive multi-layers for selective assembly of ICs (Integrated Circuits) in an LTCC (Low Temperature Co-fired Ceramics) based SiP (System-in-Package). To understand the requirements for the use of self-propagating reactive multilayers in die bonding, CFD (Computational Fluid Dynamics) simulations have been carried out to simulate the die bonding process of a silicon chip onto a ceramic LTCC substrate. Reactive foils of 40 and 80 µm thicknesses and a simulated reaction propagation speed of 1 m/s were studied and used to melt a solder preform underneath a silicon chip. The results of the CFD simulations were analysed, particularly with respect to temperature and liquid fraction contours, as well as time–temperature histories obtained from temperature probes which were included in the model, such as to approximate the real behaviour of Pt-100 temperature probes, when a real bonding process is being tracked. The CFD method, in this instance realised with ANSYS Fluent software, can track the melting and solidification of the solder as well as model the influence of latent heat, which is crucial to ascertaining the true evolution of the bonding process.

Multilayer Ti–Ta–Ni–TiC–TiB Composite by Combustion-Aided Joining

Multilayer Ti–Ta–Ni–TiC–TiB composite (cermet) was obtained by combustion-aided joining in multilayer sandwiches formed by Ti–C and Ti–B reactive layers and metal foils (Ti, Ta, Ni) and characterized by thermographic analysis and rapid video filming. Thin plates of resultant composite (32 × 14 × 1.5 mm) were formed within 0.2 s after ignition (burning velocity 195 mm/s). Ta foils were found strongly joined with Ti ones due to development of high combustion temperatures in the reactive layers yielding TiC and TiB ceramics. The presence of ceramics afforded for reducing material density, on retention of its heat/corrosion resistance. Our results open up new horizons for SHS-assisted fabrication of new refractory materials with required structure/properties.

Rapid and Localized Soldering Using Reactive Films for Electronic Applications

Abstract The rapid and localized heating techniques allow the joining of temperature-sensitive materials and components without thermal induced damage commonly encountered when high-temperature solder reflow processes are used. This is also advantageous for making assemblies with materials having a large difference in the coefficient of thermal expansion without induced bowing or cracking. The use of exothermic reactive foil sandwiched between solder preforms is a promising local and rapid soldering process because it does not require any external heat source. The reactive foil is formed from alternatively stacked nanolayers of Ni and Al until it reaches the total film thickness. Once the film is activated by using an external power source, a reaction takes place and releases such an amount of energy that is transferred to the solder preforms. If this amount of energy is high enough, solder preforms melt and insure the adhesion between the materials of the assembly. The influences of the applied pressure, the reactive film (RF) thickness as well as the solder, and the attached materials chemical composition and thickness were investigated. It was shown that the applied pressure during the process has a strong effect on the joint initial quality with voids ratio decreases from 64% to 26% for pressure values between .5 and 100 kPa, respectively. This can be explained by the improvement of the solder flow under higher pressure leading to a better surface wettability and voids elimination. Otherwise, the joint quality was found to be improved once the solder melting duration is increased. This relationship was observed when the thickness of the reactive foil is increased (additional induced energy) or the thickness of solders, Cu, and/or Si is decreased (less energy consumption). The microstructure of the AuSn joint achieved using the RFs shows very fine phase distribution compared with the one obtained using conventional solder reflow process in the oven because of high cooling rate. The mechanical properties of the joint were evaluated using shear tests performed on 350-μm-thick silicon diodes assembled on active metal brazed substrates under a pressure of 100 kPa. The RFs were 60 μm thick and sandwiched between two 25-μm-thick 96.5Sn3Ag.5Cu (SAC) preforms. The voids ratio was about 37% for the tested samples and shear strength values above 9.5 MPa were achieved which remains largely higher than MIL-STD-883H requirements. Finally, the process impact on the electrical properties of the assembled diodes was compared with a commonly used solder reflow assembly and the results show a negligible variation.

Parametric study of electronics components joining using reactive films for high temperature applications

Abstract In this paper, in order to assemble electronic components onto substrates, a local rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. Among others, the main interest of this technique is that it can allow the use of high temperature melting solders, without the need to heat the whole assembly above this melting temperature. The reactive foil is commercially available and is formed from alternatively stacked nanolayers of Ni and Al until it reaches the total film thickness. Once the film is activated by using an external power source, a reaction takes place and releases such an amount of energy that is transferred to the solder preforms. If this amount of energy is high enough, solder preforms melt and insure the adhesion between the materials of the assembly. The process was evaluated using a standard SAC305 and a high temperature Au80Sn20 preforms. The influences of the applied pressure, the reactive film thickness as well as the solder and the attached materials nature and thicknesses were investigated. The initial joint quality was evaluated using scanning acoustic microscopy, scanning electron microscopy, and shear strength measurements. It was shown that the applied pressure during the process has a strong effect on the joint initial quality. The voids ratio between metallized diode dice and an Active Metal Braze (AMB) substrate decreases from 64% to 26% for pressure values between 0.5kPa and 100kPa respectively. Otherwise, under a constant low pressure of 13kPa, reducing the substrate metal thickness on a low thermal conductivity insulator allows the improvement of the initial joint quality and a voids ratio of about 15% was reached when using 35μm of copper on FR4 substrate. The use of aluminum instead of copper as a metal for the ceramic metallized substrate (with the same gold finishing layer) led to a reduction in the void ratio in the joint. The microstructure of the AuSn joint achieved using the reactive films shows very fine phase distribution compared to the one obtained using conventional solder reflow process in the oven. The mechanical properties of the joint were evaluated using shear tests performed on 350μm thick silicon diodes assembled on AMB substrates under a pressure of 100kPa. The reactive films were 60μm thick and were sandwiched between two 25μm thick SAC preforms. The void ratio was about 37% for the tested samples and shear strength values above 9.5MPa were achieved which remains largely higher than MIL-STD-883H requirements. Finally, the process impact on the electrical properties of the assembled diodes was compared with a commonly used solder reflow assembly and results show a negligible variation.

Joining Using Reactive Films for Electronic Applications: Impact of Applied Pressure and Assembled Materials Properties on the Joint Initial Quality

The use of local and rapid heating of electronic assemblies can significantly reduce the degradation of temperature-sensitive materials and substrate bowing commonly encountered in electronic applications during the high temperature reflow process. It can also allow assembling electronic packages on a non-planar surface and/or on massive structures that are very complex using a conventional oven for soldering. In order to attach electronic components to substrates, a rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. Once the film was activated and reacted, the solder preforms were melted to ensure the adhesion between the assembled materials. The effect of applied pressure on the joint quality, the reactive film thickness, as well as the attached material thickness and physical properties were assessed. Using a 60 μm thick reactive foil with two 25 μm thick SnAgCu305 preforms, results show that the fraction of void-free interfacial area between a metallized diode and an active metal braze substrate increased from 34% to 74% with pressure values between 0.5 kPa and 100 kPa, respectively. At a constant pressure of 13 kPa, increasing the reactive foil thickness from 40 μm to 60 μm leads to an increase in the void-free interfacial attach area ratio from 20% to 40%, and a value of 54% was achieved by using two 60 μm foils under the same conditions. The substrate metallization and solder thickness also affect the joint quality. The experimental results are analyzed and correlated with the duration of liquid solder using thermal models.

Effects of multilayer arrangement in ternary reactive film on self-propagating reaction properties

The effects of multilayer designs in ternary films on self-propagating exothermic reaction properties were reported. The magnetron sputtered deposited ternary reactive films with two different multilayer arrangements of Ti/Si/Ti/Al and Si/Ti/Al/Si with the identical bilayer of 29±5nm and total thickness of 6μm were investigated. The reaction properties in terms of reaction heat, temperature and reaction velocity were characterized for these two types of multilayer constructions. The reaction heat was calculated by using the thermodynamic simulation. As-deposited and reacted foils were characterized by using electron microscopes. After an electric ignition, the exothermic reaction front propagates in the films. The propagation velocity and temperature were measured by using a high speed camera and a two-color pyrometer. Reaction products were analyzed by means of X-ray diffraction (XRD) analysis. The steady state reaction fronts propagate through Ti/Si/Ti/Al films with the reaction velocity of 9.1±2m/s and the maximum reaction temperature of 1768±41°C. Moreover, both steady state and unsteady propagation modes were observed in Si/Ti/Al/Si reactive foil. This film shows a maximum reaction velocity of 2.7±0.8m/s. Compared to this film, Ti/Si/Ti/Al films exhibit higher reaction temperature and propagation velocity. Here, experimental results suggest the dependency of the reaction properties on the multilayer arrangement. Although both ternary films have similar reactant types and morphology, different reactant interfaces introduce the distinct interfacial reactions and local atomic mixing rates, which influence on the reaction characteristics. This research also shows a new opportunity to design nanoscale reactive films with different multilayer arrangements to control reaction propagation besides existing technology.

Synthesis and characterization of Ti/Al reactive multilayer films with various molar ratios

This study describes the fabrication and characterization of the bimetallic Ti/Al reactive multilayer thin films. Reactive films with different molar ratios of Ti and Al (mTi/nAl, m=1; n=1, 2, 3) were fabricated by using Magnetron Sputter Ion Plating (MSIP) method. Differential scanning calorimetry (DSC) was used to analyze the phase change temperature and heat flow. The heat of formation of different phases in Ti-Al system was also calculated by using thermodynamic simulation. Here, reaction heat depends on types of phases form. Microstructural features of the as-deposited Ti/Al reactive multilayer films show columnar microstructures with coherent multilayers. After an electrical ignition, all foils exhibit self-propagating reaction. The experimental result shows that 1Ti/1Al reactive foils have higher reaction velocity of 2.6±0.6m/s compared to other molar ratios at 9V electrical ignition. Here, 1Ti/1Al and 1Ti/2Al reactive films show only steady reaction propagation, whereas 1Ti/3Al films exhibit both steady and unsteady reaction propagation. The microstructural characterization exhibits two types of diffusion modes of reactants mixing in the periodic layers and grain boundary, which assure even faster diffusion during self-propagating reaction.

Combustion synthesis of reactive nickel-aluminum particles as an innovative approach for thermal joining applications

Reactive systems, which are widely used in combustion synthesis, represent a promising solution for challenging joining tasks. They are able to undergo a self-sustaining, highly exothermic reaction when exposed to an external energy source. Reactive foils are the only systems that are currently commercially available. However, their industrial use is limited due to the brittle nature of the material and the restriction to planar geometries. Reactive particles represent a more flexible format, but are currently not commercially available. Therefore, a two-step electroless plating process has been developed to synthesize nickel-aluminum core-shell structures. These structures function as microreactors, which provide the energy for the thermal joining process. Ignition tests with electromagnetic waves were performed in order to investigate the overall reactivity. Energy input and particle size significantly influence the activation and the reaction behavior of the core-shell structures. Furthermore, a general approach to use reactive particles as a heat source in joining applications is proposed.

Mechanics Science-Enabled Nanoheater Multi-Layer Materials Manufactured by Ball Milling

ABSTRACTThis article reports investigation of the effects of high-rate stochastic micro-mechanics on the produced particulate size distribution during ball milling of reactive bimetallic foils (nanoheaters), by experimental and computational modeling. In particular, Ni-Al foils are ball-milled at various load charges, revolution rates and process durations, and the resulting particulate geometries are characterized by micrograph statistical analysis. Numerical simulation of the evolving particulate structure is based on coalescence and fragmentation of flexible monometallic ellipsoidal primitives, impacted by milling balls and vial walls with kinetic theory-based kinematics. Particulates are constrained by discrete compliant and continuum media and undergo conceptual ideal elastic transformations modeled by strain energy methods, and recast into inelastic frictional and plasticity-driven welding and fracture events. Finally the theoretical model predictions of particulate size distribution are validated against laboratory microscopy observations.

Self-propagating exothermic reaction analysis in Ti/Al reactive films using experiments and computational fluid dynamics simulation

This study describes the self-propagating exothermic reaction in Ti/Al reactive multilayer foils by using experiments and computational fluid dynamics simulation. The Ti/Al foils with different molar ratios of 1Ti/1Al, 1Ti/2Al and 1Ti/3Al were fabricated by magnetron sputtering method. Microstructural characteristics of the unreacted and reacted foils were analyzed by using electronic and atomic force microscopes. After an electrical ignition, the influence of ignition potentials on reaction propagation has been experimentally investigated. The reaction front propagates with a velocity of minimum 0.68±0.4m/s and maximum 2.57±0.6m/s depending on the input ignition potentials and the chemical compositions. Here, the 1Ti/3Al reactive foil exhibits both steady state and unsteady wavelike reaction propagation. Moreover, the numerical computational fluid dynamics (CFD) simulation shows the time dependent temperature flow and atomic mixing in a nanoscale reaction zone. The CFD simulation also indicates the potentiality for simulating exothermic reaction in the nanoscale Ti/Al foil.

Characterization and properties of intermetallic Al3Ti alloy synthesized by reactive foil sintering in vacuum

Abstract

Microstructure and mechanical properties of intermetallic Al3Ti alloy with residual aluminum

Intermetallic Al3Ti alloys with different volume fractions of residual aluminum were synthesized via reactive foil sintering in vacuum condition using commercial purity aluminum foils and Ti-6Al-4V alloy foils. Microstructure evolution, elemental analysis, fracture mechanism and phase identification were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) technique, respectively. Mechanical properties were measured by quasi-static compression and Vickers hardness tests. Experimental results indicated that the ductile phase Al distributes in the grain boundaries of the brittle matrix Al3Ti, which increases the compression failure strain of the Al3Ti alloy dramatically compared with the monolithic intermetallic Al3Ti alloy. The compressive stress-strain curve showed a typical elastic-plastic deformation behavior. Moreover, it also demonstrated that the post-heat treatment plays an important role in the mechanical properties due to the formation of α-Al2O3 and Al3Ti grain coarsening.

Tailoring burning rates using reactive wires in composite solid rocket propellants

Tailoring solid propellant burning rates is a perpetual goal of solid propellant designers. Many methods have been used to alter the burning rate including changing the propellant formulation, adding burning rate modifiers, and changing the propellant core shape. Alternatively, metallic fibers such as copper and silver have been added to propellants to increase the burning rate by providing fast heat diffusion paths. However, these metallic wires do not significantly add further to the energetics and have a limited tailoring range. In this paper we describe the effect of embedding mechanically-activated aluminum/polytetrafluoroethylene (Al/PTFE) reactive foils, nickel/aluminum nanofoils and Pyrofuze® self-alloying wire in an AP/HTPB composite propellant. We quantify the resulting burning rate and compare the burning rate to propellants with embedded copper wires. We find that the nickel/aluminum nanofoils and Pyrofuze® wire provide a constant increase in local burning rate regardless of pressure, while the local burning rates of propellants with Al/PTFE foils change as a function of pressure. The Al/PTFE foil is consumed, releasing significant gas, unlike the other materials investigated. The Al/PTFE foil combustion products contribute to the energy and gas production of the propellant, resulting in a propellant efficiency larger than typical wired propellants. While the other materials increase propellant burning rate by causing an increase in surface area and therefore mass burning rate, the aluminum and carbon liberated by the Al/PTFE wire reaction can oxidize with the propellant products and increase the temperature of the combustion gases. Additionally, Al/PTFE or similar reactive wires could result in a decrease in slag production compared to inert and self-alloying wires (i.e., copper, nickel/aluminum nanofoils and Pyrofuze®) and may be better suited to use in propellants due to the high burning rate tailorability and the participation in propellant combustion.

Fracture toughness of Ceramic-Fiber-Reinforced Metallic-Intermetallic-Laminate (CFR-MIL) composites

Novel Ceramic-Fiber-Reinforced-Metal-Intermetallic-Laminate (CFR-MIL) composites, Ti–Al3Ti–Al2O3–Al, were synthesized by reactive foil sintering in air. Microstructure controlled material architectures were achieved with continuous Al2O3 fibers oriented in 0° and 90° layers to form fully dense composites in which the volume fractions of all four component phases can be tailored. Bend fracture specimens were cut from the laminate plates in divider orientation, and bend tests were performed to study the fracture behavior of CFR-MIL composites under three-point and four-point bending loading conditions. The microstructures and fractured surfaces of the CFR-MIL composites were examined using optical microscopy and scanning electron microscopy to establish a correlation between the fracture toughness, fracture surface morphology and microstructures of CFR-MIL composites. The fracture and toughening mechanisms of the CFR-MIL composites are also addressed. The present experimental results indicate that the fracture toughness of CFR-MIL composites determined by three- and four-point bend loading configurations are quite similar, and increased significantly compared to MIL composites without ceramic fiber reinforcement. The interface cracking behavior is related to the volume fraction of the brittle Al3Ti phase and residual ductile Al, but the fracture toughness values appear to be insensitive to the ratio of these two phases. The toughness appears to be dominated by the ductility/strength of the Ti layers and the strength and crack bridging effect of the ceramic fibers.

Design and functionality of a high-sensitivity bomb calorimeter specialized for reactive metallic foils

We have designed and built a unique non-adiabatic bomb calorimeter to measure heat generated from intermetallic formation reactions and the subsequent partial combustion of nanocomposite metallic foils. Reactions are initiated with a low-energy electrical spark and can be performed in 1 atm of air, oxygen, nitrogen, or argon, in order to investigate reaction and burning characteristics in various environments. The bomb was designed to hold the foil samples with minimal thermal contact in order to mitigate heat losses and maximize surface area available for oxidation and nitridation. Samples are limited to the milligram range, and are much less energetic than an equivalent mass of organic material such as benzoic acid. Therefore, to maximize instrument sensitivity, heat capacity was minimized by designing the bomb to be as small as possible and by using low-viscosity silicone oil for the bath instead of water. The calorimeter has an energy equivalent of e (calor) = 279 ± 6 J K−1, allowing us to measure heat generation on the order of tens of Joules. Calibrations were performed in argon using Al:Ni nanocomposite foils with well-known heats of reaction, which were measured using differential scanning calorimetry.

Using magnesium to maximize heat generated by reactive Al/Zr nanolaminates

In this study, we explore the effect of magnesium content on the ability of reactive nanocomposite foils to generate heat, by comparing three chemistries: Al:Zr, Al–8Mg:Zr, and Al–38Mg:Zr. These correspond to foils with alternating aluminum and zirconium layers where the Al is either pure, an 8at.%Mg alloy, or a 38at.%Mg alloy, respectively. Measurements performed in a specially designed bomb calorimeter show that Al–8Mg:Zr foils perform the best, generating the greatest gravimetric heat in air, oxygen, and nitrogen environments. Both Mg-containing foils release a visible plume of particles and vapor upon reacting, which was recorded with a high speed camera. This ejected mass includes Mg vapor and particles of all three metals. Both the vapor and particles oxidize rapidly in air, resulting in single metal-oxide particles. The reacted foils, particularly the Al–8Mg:Zr samples, contain voids and higher levels of oxygen and nitrogen throughout their thicknesses than reacted Al:Zr foils. To explain the higher heats of reaction for the Al–8Mg:Zr foils, we suggest that the out-diffusion and evaporation of Mg generates a high concentration of vacancies that enhance oxygen and nitrogen diffusion throughout the foil, thereby increasing the degree of oxidation and nitridation.

Self‐Propagating Reactive Fronts in Compacts of Multilayered Particles

Reactive multilayered foils in the form of thin films have gained interest in various applications such as joining, welding, and ignition. Typically, thin film multilayers support self‐propagating reaction fronts with speeds ranging from 1 to 20 m/s. In some applications, however, reaction fronts with much smaller velocities are required. This recently motivated Fritz et al. (2011) to fabricate compacts of regular sized/shaped multilayered particles and demonstrate self‐sustained reaction fronts having much smaller velocities than thin films with similar layering. In this work, we develop a simplified numerical model to simulate the self‐propagation of reactive fronts in an idealized compact, comprising identical Ni/Al multilayered particles in thermal contact. The evolution of the reaction in the compact is simulated using a two‐dimensional transient model, based on a reduced description of mixing, heat release, and thermal transport. Computed results reveal that an advancing reaction front can be substantially delayed as it crosses from one particle to a neighboring particle, which results in a reduced mean propagation velocity. A quantitative analysis is thus conducted on the dependence of these phenomena on the contact area between the particles, the thermal contact resistance, and the arrangement of the multilayered particles.

Microstructure development during NiAl intermetallic synthesis in reactive Ni–Al nanolayers: Numerical investigations vs. TEM observations

Heterogeneous reactions leading to the formation of intermetallic compounds in nanometric Ni–Al multilayer system are examined both numerically and experimentally. On the numerical side, the reactivity of a layered Ni–Al–Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method (EAM) potential. The mechanism of nucleation and growth of the intermetallic phase is determined. Four main stages in the reactive process, which lead to the formation of rounded shape grains of intermetallic phase at the Ni–Al interface, are delineated. On the experimental side, TEM imaging of quenched samples revealed the behavior of the Ni–Al reactive foils and showed the typical morphology of the reaction products. Both approaches exhibit a good qualitative agreement and provide new insights in the understanding of metallic nano-foils reactivity. Obtained results can be used for rational design of energetic materials.

Benign Joining of Ultrafine Grained Aerospace Aluminum Alloys Using Nanotechnology

Ultrafine grained aluminum alloys have restricted applicability due to their limited thermal stability. Metalized 7475 alloys can be soldered and brazed at room temperature using nanotechnology. Reactive foils are used to release heat for milliseconds directly at the interface between two components leading to a metallurgical joint without significantly heating the bulk alloy, thus preserving its mechanical properties.