AVS2014 Session TF+SE-TuM: Energetic Thin Films/Optical Characterization

Cathode materials in the form of thin films for all solid state Li ion batteries (thin film batteries) application have attracted wide attention among the scientific community because of the inherent benefit of studying pure phase of the active materials without any binders and conductive additives such as carbon black, NMP etc [1,2]. In the present work thin films of LiMn_xNi_yO have been prepared in a cost effective approach using powder sputtering on nickel coated stainless steel (SS 304) substrates. Powder sputtering is more convenient with the advantages like less material consumption, slightly higher deposition rates and cost effective approach compared to pellet sputtering. The advantage of utilizing powder sputtering for the deposition and electrochemical performance of Li_xMnO_y thin films has been reported [3]. Post deposition annealing at 500 °C for one hour was carried out in air ambiance. XRD, XPS and electrochemical characterizations have been carried out to investigate the phase, surface atomic concentration and electrochemical performance. XPS analysis indicates the presence of manganese, nickel, oxygen, and lithium at the surface as shown in Figure. 1. Electrochemical investigations delivered a specific discharge capacity of 54 µAH. µm^-1.cm^-2 in the potential window 2.0-4.4 V vs Li/Li⁺ in the first discharge cycle. Charge discharge profile up to 40 cycles have shown in the Figure.2. Discharge capacity values are in good agreement with the literature achieved using pellet sputtering.

References:

1. J.M. Tarascon, M. Armand. Nature 414, 359 (2001)

2. J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans. Solid State Ionics 135, 33– 45 (2000)

3. K.Yellareswara Rao, D. Shanmugha sundaram, C.S. Nimisha, Tirupathi Rao Penki, N. Munichandraiah, G. Mohan Rao. J. Electrochem. Soc 161, A28-A32 (2014)

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Vapor-deposited multilayer films and foils provide model structures for studying exothermic formation and oxidation reactions. These reactions can proceed at slow to moderate rates (10^-1 to 10⁵ K/s) using controlled heating or at very rapid rates (10⁶ to 10⁸ K/s) by initiating self-propagating mixing within the films or foils. In this presentation we will probe the sequence of intermetallic phase formation and the kinetics of rapid oxidation as a function of heating rate, chemistry, and concentration gradients using novel characterization tools such as nanocalorimetry, bomb calorimetry, dynamic transmission electron microscopy and synchrotron X-ray diffraction. The experimental results will be compared with analytical predictions and molecular dynamic simulations and controlling factors will be identified.

Recent advances in physical vapor deposition of explosive materials have led to films that are capable of detonating at thicknesses smaller than 100 microns. The critical thickness needed to sustain detonation can be reduced even further (down to a few tens of microns) by confining the explosive with thin layers of a dense, inert material. The ability to sustain detonation at such small length scales opens the potential for such films to be integrated into micro-scale systems using standard micro/nanofabrication methods for use in actuation, gas generation, or similar functions. In this work, we present vapor-deposited hexanitroazobenzene (HNAB) and copper films as a model system to study the effects of confinement on the detonation properties of secondary explosives. Both the HNAB and copper confinement layers are vapor-deposited to promote intimate contact between the explosive and confinement and to provide precise control over both layer thicknesses and microstructure. Confinement thickness is varied to determine the minimum necessary to behave as though the confinement was effectively infinite, and the effects on detonation properties are quantified. In addition to the practical impact of these experiments, identification of the minimum effectively infinite confinement condition can provide insight into the kinetics of the detonation reaction.

The Al/Ni formation reaction is highly exothermic and of both scientific and technological significance. In particular, Al/Ni thin-film multilayers have been used as a model system to understand how steep concentration gradients and large heating rates affect the identity and sequence of phases that form at the interface between two materials. We have developed an in situ nanocalorimetry system that enables us to simultaneously measure the heat generated by the Al/Ni reaction and observe the phases formed. The added ability to controllably vary the heating rate from 1000 K/s to 100,000 K/s makes possible a systematic assessment of the phase transformation sequence with heating rate that we hope will shed light onto the relative effects of kinetic and thermodynamic phase suppression in determining the first phase to form. In this talk I will describe the experimental system, present the baseline results that are obtained at low heating rates, and provide an update detailing recent results and potential conclusions of the systematic study.

Matrix assisted pulsed laser evaporation (MAPLE) was developed in the late 1990s at the U.S. Naval Research Laboratory as a non-destructive method of thin film deposition for polymers that could potentially be damaged by the ultraviolet radiation utilized for pulsed laser deposition. Conventionally MAPLE starts with a dilute organic compound dissolved in a volatile solvent that serves as a solid matrix when frozen at liquid nitrogen temperatures. By choosing a solvent with a significant UV absorption coefficient, the frozen matrix can be heated by preferential photothermal excitation during irradiation, leading to evaporation and desorption of the less volatile solute molecules onto a substrate in thin film form. Since its conception, use of MAPLE has expanded from the deposition of molecular polymers and proteins to more macroscopic species such as carbon nanotubes, living cells, and recently a number of inorganic nanoparticles.

Inspired by the laser decomposition of metal-acetate based sol-gels to form nanoparticle networks, it is also possible to simultaneously synthesize and deposit well-dispersed nanoparticles or nanoporous films through an inversion of the conventional MAPLE process, utilizing weakly absorbing solvents and photo-sensitive chemical precursor solutes. The chain of events following the laser pulse first entering the target and culminating with observation of nanoparticles on the substrate has been principally interpolated backwards from the nanoparticles produced; suggesting photothermal decomposition of the acetate precursors and formation of nanoparticles within the target prior to and eventually driving ejection. The research presented represents major developments in the understanding of the underlying sequence of multi-scale events controlling the inverse MAPLE deposition process. Modeling addresses optical absorption via Mie theory and the role of matrix conduction in solving nanoparticle heat balance within the target, which in turn is shown to play critical role in the deposition process. New supporting evidence is offered through direct observation of irradiated targets via cryo-stage scanning electron microscopy, as well and conventional scanning and transmission electron microscopy of deposited films and nanoparticles.

BiOCl is an attractive p-type semiconductor with a wide band-gap of 3.4 eV. It has a unique layered structure of alternating [Bi₂O₂]²⁺ layers, interleaved by double slabs of Cl^- ions. The ionic layering guides the growth of BiOCl along the c-axis to form a 2D nanosheet morphology, favoring rapid and efficient electron/hole separation. Thus, BiOCl nanosheets can be naturally exploited as a photoactive material in applications such as solar harvesting, photocatalysis and sensing. However, given the large band-gap of BiOCl strategies to sensitize the BiOCl to the visible spectrum must be made. Substitutional doping of transition metal ions is an attractive approach given that many of the transition metal oxides are earth-abundant and have their band-gaps in the visible spectrum.

In this study, we demonstrate the effect of Fe doping on the structural, optical and electrical properties BiOCl nanosheets. Fe-doped BiOCl were synthesized by a facile hydrolysis process. Next, the samples are subjected to various annealing temperatures. We show that anneal temperatures >200 ^oC can initiate successful Fe incorporation in BiOCl crystal lattice. X-ray diffraction (XRD) shows a higher angle shift after thermal annealing, corresponding to decrease in the lattice constant as a result of Fe³⁺ substitution of the Bi³⁺ ions. Raman spectroscopy shows the E_g stretching mode of the Bi–Cl bond has lower frequency shift, which is consistent with the XRD analysis of Fe³⁺ substitution. Electrical property measurements including IV characteristics show a linear behavior with a resistance of (2.5x10¹¹, 9.8x10¹⁰ and 9.32 x 10¹⁰ Ω) for samples annealed at 200, 300 and 400^oC respectively.

Finally, we demonstrate a highly sensitive O₂ sensor using Fe-doped BiOCl nanosheets, operating at room temperature. The interaction of vacancies and O₂ is explored in the context of the Fe-doping in BiOCl.