Ibrahim Emre Gunduz

In situ observation of rapid reactions in nanoscale Ni–Al multilayer foils using synchrotron radiation

The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni0.9V0.1–Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal (0.125 ms) and angular (0.004°) resolution over a full angular range of 120°.

Investigations on the self propagating reactions of nickel and aluminum multilayered foils

The self-propagating reaction of nickel-aluminum thin film multilayers with a bilayer thickness of ∼43 nm was investigated using high-speed optical camera and infrared thermometry. The results indicate a two-stage reaction with two different characteristic temperatures. Following ignition, the flame front propagates near the reverse peritectic transformation temperature of Ni2Al3 into NiAl and liquid at 1406 K. The reaction continues with the growth of NiAl until the melting temperature of 1911 K is reached. The reaction mechanism and kinetics are discussed.

Experimentally-validated mesoscale modeling of the coupled mechanical–thermal response of AP–HTPB energetic material under dynamic loading

This manuscript presents a combined computational–experimental study of the mesoscale thermo-mechanical behavior of the Hydroxyl-terminated polybutadiene (HTPB) bonded ammonium perchlorate (AP) composite energetic material subjected to dynamic loading conditions. The computational model considers the AP–HTPB interface debonding, post-debonding interface friction and temperature rise due to viscoelastic dissipation as well as dissipative interfacial processes. The interface is modeled using a cohesive zone model combined with a contact algorithm to account for the interface separation, particle/binder contact and heat generation. The HTPB binder is modeled as viscoelastic with adiabatic temperature rise. Three experiments are conducted to calibrate and validate the model. Raman spectroscopy and indentation experiment are employed to determine the interface properties, whereas Kolsky bar tension test along with in-situ synchrotron X-ray diffraction measurements are used to validate the model and understand the interface separation characteristics under dynamic loading.

Simulations of nanoscale Ni/Al multilayer foils with intermediate Ni2Al3 growth

Nanoscale multilayers of binary metallic systems, such as nickel/aluminum, exhibit self-propagating exothermic reactions due to the high formation enthalpy of the intermetallic compounds. Most of the previous modeling approaches on the reactions of this system rely on the use of mass diffusion with a phenomenological derived diffusion coefficient representing single-phase (NiAl) growth, coupled with heat transport. We show that the reaction kinetics, temperatures, and thermal front width can be reproduced more satisfactorily with the sequential growth of Ni2Al3 followed by NiAl, utilizing independently obtained interdiffusivities. The computational domain was meshed with a dynamically generated bi-modal grid consisting of fine and coarse zones corresponding to rapid and slower reacting regions to improve computational efficiency. The PDEPE function in MATLAB was used as a basis for an alternating direction scheme. A modified parabolic growth law was employed to model intermetallic growth in the thickness ...

Heat generation in an elastic binder system with embedded discrete energetic particles due to high-frequency, periodic mechanical excitation

High-frequency mechanical excitation can induce heating within energetic materials and may lead to advances in explosives detection and defeat. In order to examine the nature of this mechanically induced heating, samples of an elastic binder (Sylgard 184) were embedded with inert and energetic particles placed in a fixed spatial pattern and were subsequently excited with an ultrasonic transducer at discrete frequencies from 100 kHz to 20 MHz. The temperature and velocity responses of the sample surfaces suggest that heating due to frictional effects occurred near the particles at excitation frequencies near the transducer resonance of 215 kHz. An analytical solution involving a heat point source was used to estimate heating rates and temperatures at the particle locations in this frequency region. Heating located near the sample surface at frequencies near and above 1 MHz was attributed to viscoelastic effects related to the surface motion of the samples. At elevated excitation parameters near the transdu...

Miniature thermal matches: from nanoheaters to reactive fractals

Fine thermal actuation by miniature heat sources enables applications from electronics fabrication to tumor cauterization. This paper introduces the concept of nanoheaters, i.e., reactive bimetallic material dots (0D), ignited electrically to exothermically release precise heat amounts where and when needed. This concept is extended to nanoheater wires (1D) and foils (2D), as well as bulk nanoheaters (3D) manufactured via ball milling and ultrasonic consolidation of nickel and aluminum powders. The fractal structure of such powders and consolidates, with self-similar, multiscale Apollonian or lamellar packaging, is discovered to hold the key for their ignition sensitivity: nanoscale structures ignite first, to produce enough heat and raise the temperature of submicron formations, which then ignite microscale regions and so on; while inert areas quench and arrest the self-propagating exothermic reaction. Therefore, such engineered fractal reactive heaters lend themselves to affordable, high-throughput manufacture and controllable, safe, efficient, supplyless in situ thermal release. This can be transformative for innovations from self-healing composites and self-heating packages to underwater construction and mining.

Numerical modeling of self-propagating reactions in Ru/Al nanoscale multilayer foils

The Ru/Al system integrates high energy density and high product ductility and serves as an alternative for utilization as nanoscale reactive multilayer. We present a modeling study that relates the Ru-Al phase transformations occurring during self-propagating reactions with macroscopic reaction parameters such as net front velocity and reaction temperature. We coupled equations for mass and thermal transport and used a numerical scheme to solve the differential equations. We calculated the temporal evolution of the temperature distribution in the reaction front as a function of the multilayer bilayer thickness. The calculated net velocities were between 4.2 m/s and 10.8 m/s, and maximal reaction temperatures were up to 2171 K, in good agreement with measured data. Interfacial premixing, estimated to be around 4 nm, had a large influence on reaction velocities and temperature at smaller bilayer thicknesses. Finally, the theoretical results of the present study help to explain the experimental findings and guide tailoring of reactive properties of Ru/Al multilayers for applications.

Phase Changes in Embedded HMX in Response to Periodic Mechanical Excitation

It is well known that energy can be spatially localized when explosives are mechanically deformed; however, the heat generation mechanisms associated with this localization process are not fully understood. In this work, mesoscale hot spot formation in ultrasonically-excited energetic materials has been imaged in real-time. More specifically, periodic, mechanical excitation has been applied to Dow Corning Sylgard® 184/octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) composite materials using contact piezoelectric transducers resulting in heating at various crystal locations. A thermally-induced phase transition from a β to δ non-centrosymmetric crystal structure for HMX results in the frequency doubling of incident laser radiation and can be used as a temperature proxy. In light of this, a high-repetition-rate 1064 nm Nd:YAG laser has been used to illuminate discrete HMX crystals, and a 532 nm filter has been applied to capture only the light emitted from δ-phase second harmonic generation (SHG). The visualization of δ-phase initiation and growth is useful for determining both heat generation mechanisms and heating rates at crystal/crystal and/or crystal/binder interfaces and contributes to the understanding and prediction of hot spots.

Non-equilibrium microscale thermomechanical modeling of bimetallic particulate fractal structures during ball milling fabrication

Nanostructured bimetallic reactive multilayers can be conveniently produced by ball milling of elemental powders. This research explores the non-equilibrium microscale conductive thermal transport in ball-milled particulate fractal structures during fabrication, arising from heat dissipation by bulk plastic deformation and surface friction. Upon impactor collisions, temperature increments are determined at interface joints and domain volumes using Greens functions, mirrored by source images with respect to warped ellipsoid domain boundaries. Heat source efficiency is calibrated via laboratory data to compensate for thermal expansion and impactor inelasticity, and the thermal analysis is coupled to a dynamic mechanics model of the particulate fracture. This thermomechanical model shows good agreement with the fractal dimensions of the observed microstructure from ball milling experiments. The model is intended to provide a comprehensive physical understanding of the fundamental process mechanism. In addit...

Shock-induced reaction synthesis of cubic boron nitride

Here, we report ultra-fast (0.1–5 μs) shock-induced reactions in the 3B-TiN system, leading to the direct synthesis of cubic boron nitride, which is extremely rare in nature and is the second hardest material known. Composite powders were produced through high-energy ball milling to provide intimate mixing and subsequently shocked using an explosive charge. High-resolution transmission electron microscopy and X-ray diffraction confirm the formation of nanocrystalline grains of c-BN produced during the metathetical reaction between boron and titanium nitride. Our results illustrate the possibility of rapid reactions enabled by high-energy ball milling possibly occurring in the solid state on incredibly short timescales. This process may provide a route for the discovery and fabrication of advanced compounds.Here, we report ultra-fast (0.1–5 μs) shock-induced reactions in the 3B-TiN system, leading to the direct synthesis of cubic boron nitride, which is extremely rare in nature and is the second hardest material known. Composite powders were produced through high-energy ball milling to provide intimate mixing and subsequently shocked using an explosive charge. High-resolution transmission electron microscopy and X-ray diffraction confirm the formation of nanocrystalline grains of c-BN produced during the metathetical reaction between boron and titanium nitride. Our results illustrate the possibility of rapid reactions enabled by high-energy ball milling possibly occurring in the solid state on incredibly short timescales. This process may provide a route for the discovery and fabrication of advanced compounds.