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Rational design of a reactive material for 3D printing robust electrical conductors which can be integrated into parts or systems.

3D printing of reactive materials has, to date, primarily focused on controlling reaction velocities and energy release rates by printing novel architectures, which during reaction typically form gaseous and oxide powder products. The utility of printed reactive materials can be increased by designing the material such that its reaction produces both a controlled energy release and a functional product phase without gaseous products. Here, we report the direct ink writing of reactive materials composed of ternary nanocomposite powders that exhibit gasless, exceptionally low velocity reactions. The printed features can be patterned in arbitrary form factors and are brittle and electrically insulating as‐printed. Using a self‐propagating synthesis reaction, these features can be transformed into a mechanically robust, electrically conductive cermet that is capable of handling high currents. This study provides a new paradigm for printing multifunctional reactive materials in which both the energy release of the reaction, as well as the reaction products themselves, are useful for potential applications ranging from printed electronic devices to controlled, directed energy release.

Nucleation, the initial formation of a new phase from a parent phase, plays an important role in the eventual microstructure and properties of materials. Theories and models of nucleation have been integral to materials science for close to a century. These models assume that the parent material is compositionally homogeneous on length-scales relevant to nucleation. However, in certain materials – such as thin films or reactive nanolaminates – sharp gradients in the composition may influence nucleation. Models and theories exploring these impacts are based on little direct experimental data. Here we present means of producing and characterizing samples with composition gradients to measure the impacts of gradients on nucleation. We fabricate amorphous Cu-Zr films with known composition gradients through their thicknesses; we perform isochronal nanocalorimetry to measure the impact of the gradients on nucleation and growth; and we characterize the samples before and after reaction. We see evidence of phase separation of the vapor-quenched Cu-Zr amorphous films. While we measure differences between the samples with gradients and those without, the gradients relax sufficiently during heating such that nucleation (the onset of crystallization) occurs at the same temperatures. For both sets of samples we find three distinct regions of heat release: the first we attribute to local ordering, the second to extended phase separation and interdiffusion, and the third to nucleation and growth of the Cu10Zr7 crystalline phase. This work represents a first step towards investigating the impact of gradients on nucleation, as well as growth.

We have developed an improved method of time-resolved x-ray reflectivity (XRR) using monochromatic synchrotron radiation. Our method utilizes a polycapillary x-ray optic to create a range of incident angles and an area detector to collect the specular reflections. By rotating the sample normal out of the plane of the incident fan, we can separate the surface diffuse scatter from the reflectivity signal, greatly improving the quality of the XRR spectra compared to previous implementations. We demonstrate the time-resolved capabilities of this system, with temporal resolution as low as 10 ms, by measuring XRR during the annealing of Al/Ni nano-scale multilayers and use this information to extract the activation energy for interdiffusion in this system. a) hj335@cornell.edu 1

This review focuses on the properties of reactive materials (RMs) that enable exothermic formation reactions and their application as local heat sources. We examine how the heat produced by these formation reactions can enable a range of useful functions including bonding, sealing, ignition, signaling, and built-in degradation. We begin by describing the chemistries, geometries, microstructures, and fabrication of RMs. We then explore the magnitude and measurement of their stored chemical energies and the rates and mechanisms by which the stored energy can be released to generate useful heat. The majority of the review focuses on how the released heat can be modeled and used to perform a range of functions. Expected final online publication date for the Annual Review of Materials Research, Volume 52 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Chemical time delays, devices that burn over specific times from under a millisecond to several seconds, are widely used in mining as well as civilian and military pyrotechnics and generally are composed of environmentally hazardous materials. Reactive nanolaminates are energetic materials composed of two or more reactants organized in an alternating layered structure, which may be fabricated with environmentally friendly components. Many traits of reactive nanolaminates are desirable for time delay applications, including their long shelf life and their highly repeatable and finely tunable reaction velocities. However, their reaction velocities are generally too high to be broadly applicable in chemical time delays. In this work, we use polymer mesh substrates to produce a constrained network of reactive particles, which reduce the reaction velocity of Al/Ni multilayers approximately 100× from a range of 2.8 to 7 m s–1 to a range of 7 to 90 mm s–1. This is accomplished via an interrupted reaction mechanism wherein individual particles on the coated mesh react rapidly but exhibit a delay before igniting neighboring particles due to the required heat transfer at the particles’ interfaces. We present the macroscale reaction velocities as a function of substrate composition and size, as well as reactive coating thickness and bilayer spacing. We employ high-speed videography to probe the ignition delays and finite element analysis simulations to aid in explaining the observed trends. Through selection of substrate and coating properties, time delays spanning a large range of delay times can be packaged into standard form factors.

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