Michael D. Grapes

Thresholds for igniting exothermic reactions in Al/Ni multilayers using pulses of electrical, mechanical, and thermal energy

We use pulses of electrical, mechanical, and thermal energy to determine the ignition thresholds of self-propagating reactions in Al/(Ni-7 V) and Al/Inconel multilayers. The energy density and power density required to initiate reactions in a Al/(Ni-7 V) foil with a 50 nm bilayer is compared for all three techniques to demonstrate the importance of heat loss on ignition thresholds and its dependence on the test volume and the surrounding thermal resistance. In addition, ignition is shown to occur at temperatures as low as 232 °C when heat losses are very small suggesting that ignition can be controlled by atomic mixing in the solid state. The experiments demonstrate that the ignition threshold drops with increasing ignition volume, and it rises with increasing bilayer spacing and with increasing intermixed thickness. These trends are also supported by an analytical model we derive to predict the effects of ignition volume, multilayer microstructure, and physical properties on the ignition threshold. We calculate an activation energy of 77.3 ± 1.3 kJ/mol for solid state mixing based on measured ignition temperatures.

Studying exothermic reactions in the Ni-Al system at rapid heating rates using a nanocalorimeter

Heats of reaction and heat capacity changes were measured using scanning nanocalorimetry for a nickel and aluminum bilayer where initial heating rates of 104 K/s were achieved. Multiple exotherms were observed on the initial heating, but the number of intermediate exotherms decreased with increasing heating rate. The final phase was the B2 NiAl intermetallic. Results from the nanocalorimeter were compared with a conventional differential scanning calorimeter (operating at 0.7 K/s) to understand the effect of significant (10 000×) increases in heating rate on the phase transformation sequence. The high heating rate in the nanocalorimeter delays reaction initiation, causes the exothermic peaks to shift to higher temperatures, and appears to suppress the formation of intermediate, metastable phases. Potential explanations for this apparent suppression are discussed.

In situ transmission electron microscopy investigation of the interfacial reaction between Ni and Al during rapid heating in a nanocalorimeter

The Al/Ni formation reaction is highly exothermic and of both scientific and technological significance. In this report, we study the evolution of intermetallic phases in this reaction at a heating rate of 830 K/s. 100-nm-thick Al/Ni bilayers were deposited onto nanocalorimeter sensors that enable the measurement of temperature and heat flow during rapid heating. Time-resolved transmission electron diffraction patterns captured simultaneously with thermal measurements allow us to identify the intermetallic phases present and reconstruct the phase transformation sequence as a function of time and temperature. The results show a mostly unaltered phase transformation sequence compared to lower heating rates.

Modeling and quantitative nanocalorimetric analysis to assess interdiffusion in a Ni/Al bilayer

A computational model is developed to describe the evolution of the temperature field in a nanocalorimeter that comprises inert material layers on which a nanoscale Ni/Al bilayer has been deposited. The model incorporates a reduced continuum description of mixing and heat release in the Ni/Al bilayer, and of the energy equation in the inert layers. Due to the small thicknesses of individual layers that are several orders of magnitude smaller than the corresponding length, a simplified, transient, homogeneous representation of the temperature field can be adopted. The resulting lumped model is valid over short enough timescales, which are nonetheless sufficiently large to capture the formation reaction. By using experimental observations of the evolution of the temperature on the surface of the nanocalorimeter, the model is used to estimate the transient heat release rate. Assuming an Arrhenius model for the mixing between Ni and Al, the estimated heat release rate is used to determine the Arrhenius pre-ex...

Combining nanocalorimetry and dynamic transmission electron microscopy for in situ characterization of materials processes under rapid heating and cooling

Nanocalorimetry is a chip-based thermal analysis technique capable of analyzing endothermic and exothermic reactions at very high heating and cooling rates. Here, we couple a nanocalorimeter with an extremely fast in situ microstructural characterization tool to identify the physical origin of rapid enthalpic signals. More specifically, we describe the development of a system to enable in situ nanocalorimetry experiments in the dynamic transmission electron microscope (DTEM), a time-resolved TEM capable of generating images and electron diffraction patterns with exposure times of 30 ns-500 ns. The full experimental system consists of a modified nanocalorimeter sensor, a custom-built in situ nanocalorimetry holder, a data acquisition system, and the DTEM itself, and is capable of thermodynamic and microstructural characterization of reactions over a range of heating rates (10(2) K/s-10(5) K/s) accessible by conventional (DC) nanocalorimetry. To establish its ability to capture synchronized calorimetric and microstructural data during rapid transformations, this work describes measurements on the melting of an aluminum thin film. We were able to identify the phase transformation in both the nanocalorimetry traces and in electron diffraction patterns taken by the DTEM. Potential applications for the newly developed system are described and future system improvements are discussed.

Characterizing solid-state ignition of runaway chemical reactions in Ni-Al nanoscale multilayers under uniform heating

Nanoscale layers of nickel and aluminum can mix rapidly to produce runaway reactions. While self-propagating high temperature synthesis reactions have been observed for decades, the solid-state ignition of these reactions has been challenging to study. Particularly elusive is characterization of the low-temperature chemical mixing that occurs just prior to the ignition of the runaway reaction. Characterization can be challenging due to inhomogeneous microstructures, uncontrollable heat losses, and the nonuniform distribution of heat throughout the material prior to ignition. To reduce the impact of these variables, we heat multilayered Ni/Al foils in a highly uniform manner and report ignition temperatures as low as 245 °C for heating rates ranging from 2000 °C/s to 50 000 °C/s. Igniting in this way reveals that there are four stages before the reaction is complete: heating to an ignition temperature, low temperature solid-state mixing, a separate high temperature solid-state mixing, and liquid-state mixi...

X-ray reflectivity measurement of interdiffusion in metallic multilayers during rapid heating

A method for in situ X-ray reflectivity measurements on the millisecond time scale is described, and its use for measuring interdiffusion in metallic multilayers is illustrated.

DTEM In Situ Mechanical Testing: Defects Motion at High Strain Rates

Defect nucleation and motion during high strain rate experiments has not been observed in situ at the nanoscale in metals. However, imaging dislocation and twin nucleation and propagation will enhance our understanding and ability to predict dynamic behavior and spall strength. In the experiments described here we use the Dynamic TEM at the Lawrence Livermore National Laboratory which is capable of recording pictures with a 20-ns time resolution in movie mode (a short multi-frames experiment), and we developed a new TEM holder capable of deforming samples at strain rates ranging from quasistatic to 104 s−1. The holder uses two piezoelectric actuators that bend rapidly to load samples and TEM specimens with small gauge sections to obtain high strain rates. The TEM specimens and their narrow gauge sections are machined from bulk specimens using a femtosecond laser. The 50-μm wide gauge sections are ion milled to create electron transparent areas. We present high strain rate in situ mechanical test results for copper specimens.

TEM sample preparation by femtosecond laser machining and ion milling for high-rate TEM straining experiments

To model mechanical properties of metals at high strain rates, it is important to visualize and understand their deformation at the nanoscale. Unlike post mortem Transmission Electron Microscopy (TEM), which allows one to analyze defects within samples before or after deformation, in situ TEM is a powerful tool that enables imaging and recording of deformation and the associated defect motion during mechanical loading. Unfortunately, all current in situ TEM mechanical testing techniques are limited to quasi-static strain rates. In this context, we are developing a new test technique that utilizes a rapid straining stage and the Dynamic TEM (DTEM) at the Lawrence Livermore National Laboratory (LLNL). The new straining stage can load samples in tension at strain rates as high as 4×103/s using two piezoelectric actuators operating in bending while the DTEM at LLNL can image in movie mode with a time resolution as short as 70ns. Given the piezoelectric actuators are limited in force, speed, and displacement, we have developed a method for fabricating TEM samples with small cross-sectional areas to increase the applied stresses and short gage lengths to raise the applied strain rates and to limit the areas of deformation. In this paper, we present our effort to fabricate such samples from bulk materials. The new sample preparation procedure combines femtosecond laser machining and ion milling to obtain 300µm wide samples with control of both the size and location of the electron transparent area, as well as the gage cross-section and length.

In Situ High-Rate Mechanical Testing in the Dynamic Transmission Electron Microscope

It is difficult to extract details on deformation mechanisms from conventional high strain rate testing, where microstructural analysis is typically limited to before-and-after comparisons. In situ transmission electron microscopy (TEM) can provide an alternative by allowing direct observation of defect motion during loading, but thus far limitations in the speed of conventional TEM and traditional in situ straining holders have prevented the application of this technique to very high strain rates. We present the latest progress in our efforts to develop such a capability. We have developed a novel TEM specimen holder that uses piezoelectric actuators to pull a specimen in tension at rates up to 103 s−1. To fit the holder’s unique sample geometry we have developed a procedure for fabricating TEM tensile specimens with a consistent, electron-transparent gauge section. These specimens can be fabricated from bulk starting materials, allowing us to retain the materials’ original microstructure. The holder is designed to operate in the Dynamic Transmission Electron Microscope (DTEM) at Lawrence Livermore National Lab, which is capable of capturing electron images with exposure times as short as 30 ns.