D. Van Heerden

Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils

Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The re...

Modeling and characterizing the propagation velocity of exothermic reactions in multilayer foils

Combustible multilayer foils can be fabricated by sputter depositing alternate layers of materials which react exothermically during thermally induced intermixing. Current models for these reactions consider pure materials which only intermix during the self-propagating stage of the reaction, though in reality during fabrication the materials undergo partial intermixing. An analytical model dealing with the premixing is presented and compared with experimental results for Al/Ni and Al/(Ni:Cu) multilayers. The model and the results indicate that premixing lowers the propagation velocity both by slowing the rate of atomic diffusion between layers and by lowering the temperature of the reaction. The lower temperature can cause solid/liquid phase changes to dominate the reaction path. It is concluded that to use these foils in commercial and engineering applications, the method of fabrication and the phase changes occurring during the reaction must be controlled to give the desired characteristics.

Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation

The metastable phase Al9Ni2 was investigated in order to characterize the thermodynamics and kinetics of its formation. Here, Al9Ni2 was observed as the first phase to form in a series of sputter-deposited Al/Ni multilayer foils, but it did not form in foils with a very small bilayer period (12.5 nm). In these foils, the stable phase Al3Ni was the first phase observed. Using differential scanning calorimetry, the heat of formation and the Gibbs free energy for the formation of Al9Ni2 were both calculated to be −28 kJ/mole·atom. The average activation energy for Al9Ni2 was determined using a Kissinger analysis and was found to vary with bilayer thickness, with an average value of 1.58 ± 0.06 eV. In light of these experimental results, a simple nucleation model based on thermodynamics and diffusional intermixing is proposed to explain why Al9Ni2 forms before Al3Ni in most cases, except in foils with very small bilayers where it is absent.

Self-propagating formation reactions in Nb:Si multilayers

Abstract Structural silicides with promising high temperature mechanical properties can be fabricated into near-net shapes using self-propagating high-temperature synthesis of powders. These silicides can also be formed from vapor deposited reactive multilayer foils. The foils typically contain hundreds of layers that alternate between two elements that will mix exothermically to form a compound. Once the formation reaction is ignited in a foil, it can self-propagate along the foil with a controlled velocity. This paper investigates self-propagating formation reactions in Nb/Si multilayers and demonstrates that their reaction velocities decrease as the individual Nb and Si layers thicken. The paper also explores the feasibility of forming the vapor-deposited foils, during reaction into complex shapes.

Investigating the reaction path and growth kinetics in CuOx/Al multilayer foils

CuOx/Al exothermic reactions in multilayer foils were studied to identify reaction paths and reaction kinetics. Heating samples at a slow, controlled rate in a differential thermal analyzer showed that the reduction of CuOx and the oxidation of Al proceeded via two separate exotherms. To analyze this reaction pathway, samples were heated to various temperatures within these exotherms, quenched, and characterized with x-ray diffraction, Auger depth profiling, and transmission electron microscopy. Experimental evidence indicates that in the first reaction, CuOx is reduced to a mixture of CuO and Cu2O and an interfacial layer of Al2O3 grows to coalescence; the final products of the second exotherm are Cu, Al2O3, and Cu2O. The first exotherm was believed to be controlled by the two-dimensional, interface-limited growth of the Al2O3 layer, while the second exotherm was believed to be controlled by both the diffusion-limited one-dimensional growth of the Al2O3 and the interface-controlled growth of the Cu due t...

Contact resistance and phase transformations during nanoindentation of silicon

Abstract An electrically conductive VC tip has been used in conjunction with ex-situ transmission electron microscopy (TEM) and micro-Raman spectroscopy to examine the behaviour of silicon during nanoindentation testing. During loading, the contact resistance is low because of the presence of the metallic β-Sn phase of silicon. During unloading, the semimetallic BC8 phase and the rhombohedral R8 phase are formed, giving rise to an increase in the contact resistance. In large indentations there is a pronounced discontinuity in both the contact resistance-depth and load-depth curves during unloading. This signifies the formation of a phase with lower electrical resistance. TEM analysis suggests that this phase is amorphous silicon formed in the centre of large indentations. The position of the discontinuity on the load-depth curve is found to depend on the unloading rate, as would be expected for the generation of an amorphous phase. Micro-Raman analysis is largely in agreement with the TEM results, but the amorphous silicon is seen most clearly when there are cracks present, suggesting that the amorphous phase may be subsurface or generated during the rapid propagation of cracks.

Tensile testing low density multilayers: Aluminum/titanium

Yield stresses, ultimate tensile strengths, and specific strengths of aluminum/titanium multilayer thin films are determined from the results of uniaxial tensile tests. The plasticity in the stress-strain curves, the nature of the fracture surfaces, and the relationship of the yield stress and the bilayer thickness are discussed. Properties are compared with those of other multilayer materials published in the literature.

ELECTROCHEMICAL FABRICATION OF N-SI/AU SCHOTTKY JUNCTIONS

We report on the electrochemical deposition of gold films onto n-type silicon. Gold deposition occurs through progressive nucleation and diffusion limited growth. A high density of gold nuclei was obtained by using a short potential pulse to −1.6 V(Ag/AgCl), and subsequent growth was performed at about −1.1 V(Ag/AgCl) where the growth rate is kinetically limited. Transmission electron microscopy showed that high quality, continuous gold films were formed with an average grain size on the order of 50–70 nm. The electrical properties of the electrochemically deposited Si/Au Schottky junctions are comparable to junctions prepared by evaporation or sputtering techniques.

Sputter-deposition and characterization of paramelaconite

While processing techniques for deposition of CuO x /Al multilayer foils were being developed, a method for synthesizing paramelaconite (Cu 4 O 3 ) was serendipitously discovered. These paramelaconite films were successfully synthesized by sputter-deposition from a CuO target. Milligram quantities of uncontaminated material were produced enabling new studies of the morphology, stoichiometry, and thermodynamics of this unique copper oxide. At moderate temperatures, equiaxed paramelaconite grains deposited with a strong out-of-plane texture; at lower temperatures the paramelaconite grains showed no texture but were columnar in geometry. X-ray photoelectron spectroscopy showed that the as-deposited Cu 4 O 3 had a nonstoichiometric Cu:O ratio of 1.7:1; the ratio of Cu + to Cu 2+ was 1.8:1. On heating, this phase decomposed into CuO and Cu 2 O at temperatures ranging from 400 to 530 °C. Using differential scanning calorimetry, the heat of formation and Gibbs free energy for Cu 4 O 3 were estimated to be −453 and −279 kJ/mol, respectively. On the basis of these calculations and our observations, we confirmed that Cu 4 O 3 is a metastable phase.

Evaluation of vapor deposited Nb/Nb5Si3 microlaminates

Abstract Alternating layers of Nb and amorphous Nb–Si were sputter deposited at room temperature and then annealed at elevated temperatures to produce microlaminates with flat, discrete layering and an equiaxed grain structure. The amorphous Nb–Si layers crystallize at temperatures between 740 and 800°C depending upon whether the silicide was co-deposited from elemental targets, or directly deposited from a single composite target. The first phase to form on crystallization is Nb 5 Si 3 , but upon heating to 1000°C for 3 h a second phase, metastable Nb 3 Si, forms. Annealing at 1200°C for 3 h though, eliminates the Nb 3 Si phase. The microstructural stability of the microlaminates was examined by annealing samples at temperatures up to 1400°C. In all cases very limited grooving was observed and there was no pinch-off of the layering. There was also no evidence of silicide precipitates in the Nb grains after high temperature anneals. The mechanical properties of the microlaminates were examined using room temperature tensile tests. The ultimate tensile strength of the microlaminates is 590–640 MPa. Examination of fracture surfaces from the samples reveals that the Nb layers blunt cracks in the silicide layers and contribute significantly to the observed room temperature strength of these microlaminates.