## UCRL-PROP-220186

## Chemical Transformations

## March 9, 2006

## Abstract

The Advanced Simulation and Computing (ASC) Program is providing a
number of overviews ("white papers") on subjects of interest to the
Department of Energy (DOE) Defense Laboratories (Lawrence Livermore, Los
Alamos, and Sandia National Laboratories) as a part of the process. The
design of safe and secure systems requires a detailed understanding of
the consequences of organic material behavior in both normal and
abnormal environments. A number of classes of organic materials are
either present or can interact with the systems. Those materials include
high explosives, propellants, foams, plastics, and hydrocarbon fuels.
The DOE is interested in transformational improvements in predictive
capabilities for a broad class of applications.

## Introduction

In preparation for the Predictive Science Academic Alliances Program
bidder's workshop, a series of "white papers" are being prepared. The
topic of this white paper is chemical transformations. Other white
papers will be provided on other topics of interest to NNSA and their
Defense Program Laboratories. In many cases, the physics of interest
spans multiple white papers. The capability to predict the properties of
various organic materials is of interest to the Advanced Simulation and
Computing (ASC) program to support computational assessments of the
safety and reliability of systems containing organics materials
subjected to normal and abnormal environments. Organic materials of
interest include a broad class of energetic materials, potting foams,
and various plastics. Relevant research topics include:

-   calculation of decomposition rates under ambient and extreme
    conditions,

-   condensed phase molecular interaction potentials,

-   physical properties and transport coefficients,

-   thermodynamic properties of organic materials,

-   fuel combustion and soot formation, and

-   aging effects in organic materials.

These elements have in common the need to develop experimentally
verified computational methods that have a strong foundation in basic
physical principles. They will generally have to be implemented to run
efficiently on advanced parallel computing platforms to achieve
sufficient accuracy. We describe each of these research areas in more
detail in the following sections:

## Calculation of Decomposition Rates

Energetic materials and other organic materials are metastable and will
decompose at some rate under every condition. For energetic materials,
chemical processes determine the detonation properties. The
decomposition properties of organic and energetic materials, however,
are controlled by reaction kinetics. Furthermore, reaction history
changes the properties of these materials, so we must model not just the
\"pristine\" state, but also the properties that evolve as reactions
occur.

The typical approach uses quantum calculations of intramolecular
potential energy to determine the reaction pathways. Molecular dynamics
approaches are then used to describe how the condensed system modifies
the rates associated with these pathways. Such calculations are widely
done now for gas phase reactions, with condensed phase simulations now
becoming feasible. Determining the effect of the condensed state on the
rates is the challenge, both in how this alters the fundamental reaction
kinetics and in how partitioning of products and energy between the
condensed and gas phase occurs. For some of the extreme temperature and
pressure conditions accessible (e.g. detonation shocks), the effects of
ionization, excited states and radiation transport might also be of
significance.

There are several significant challenges associated with this area.
First is the need to develop large-scale quantum chemistry programs so
that reaction pathways under the appropriate conditions of pressure and
temperature can be studied. As unanticipated reactions resulting from
many-body interactions are highly likely, extrapolation from gas-phase
or ambient conditions is not expected to be very predictive. Second,
this knowledge of behavior needs to be captured accurately within a
reactive molecular dynamics force field so that longer time and larger
scale simulations can be performed. Third, these simulations typically
rely on classical molecular dynamics that ignore significant quantum
corrections to the vibrational partition functions. Thus, there are
likely to be significant corrections to the temperature and partitioning
of the kinetic energy. Fourth, the resulting dominant reaction paths
need to be distilled into effective chemistry models that can be
utilized within continuum / mesoscale simulations. This will likely
involve disparate time scales of the rapid generations of distinct
molecular products (nitrogen, water) and more extended
reaction-diffusion processes of carbon clustering reactions (soot) and
metal oxide formation. Finally, those implemented models need to be
verified for both accurate integration of the kinetics, and for
partitioning of products and energy between the phases in the
simulation.

Another challenge is that many of the reactions of interest occur in
heterogeneous solid composites. Fundamental progress must be made on the
reaction kinetics of solids, where chemical processes are often mediated
by defects. The slow decomposition of organic solids often leaves behind
ill-characterized materials that are difficult to describe with
traditional chemical kinetic schemes.

High explosives create extreme pressures (up to 500 kbar) and
temperatures (3500 K) during the detonation process. These extreme
conditions push the boundaries of molecular matter. For instance, water
has been found to undergo a molecular to superionic transition at 450
kbar and 1300K. At pressures above 450 kbar, water is no longer a
molecular material. A unified theory of the transition from molecular to
non-molecular matter does not exist. The boundaries of molecular matter
need to be established through first principles simulations and
experiment. This class of transformations is challenging to model with
conventional molecular interaction potentials.

## Molecular Potential Functions

Molecular potential functions form the foundation for predicting the
properties of organic composite material systems. Simple (or fairly
simple), transferable potential-energy functions for explosive,
energetic, and inert organic molecules, parameterized over a wide range
of conditions, are fundamental to modeling activities for weapons
performance, safety, reliability, and renewal/manufacturing. Prediction
of the performance and safety of organic materials necessitates
interaction potentials accurate over a wide range of pressures. Modeling
the response of organic materials to a fire following a crash or to less
severe long-term heating requires models applicable over the full range
of accident scenarios that may lead to decomposition (and ignition in
energetic materials). Predicting the impact of age on the performance
and safety of organic materials not only requires molecular interaction
potentials accurate at high temperatures and widely varying pressures
but also over the wide range of environments experience by a weapon
during its lifetime e.g. temperatures ranging from -120 ^ο^F to 120
^ο^F). Finally, modeling manufacturing processes, as well as deriving
new, improved materials requires a fundamental understanding of the
structure-property relationships (nearly synonymous with accurate
interaction potentials) of these materials under processing conditions
as well as the performance and safety environments.

There are several motivations for this technology. The performance of
energetic materials (propellants or explosives) is heavily reliant on
their density, both in terms of the energy and number of product
molecules that are effectively present in the initial structure.
Transformations to alternative structures by some physical inducement
are likely having deleterious effects, though it might be possible to
engineer beneficial aspects into this. Physical properties will depend
on the molecular packing, both thermally (expansion coefficients and
phase transitions) and mechanically (accessible slip planes). The
behaviors of polymeric materials are heavily tied to aspects such as the
volume and distribution of crystalline regimes, the free volume in
amorphous regions (for flexibility and molecular diffusion processes),
and phase separation and surface adhesion properties.

Potential-energy surfaces for \"generic\" crystals have been developed
for hydrocarbons and azabenzenes, though these reveal challenges in
accurate anticipation of packing motifs and thermal corrections
Potential energy surfaces for the most important molecules in high
explosives and energetic materials, such as the nitramines and
nitrated/aminated benzenes, present an even greater challenge because of
access to multiple molecular conformations, and significant thermal
motions in low energy torsional and conformation modes. Electrostatic
interactions for these more polar species are critical for an accurate
description of their packing, where static monopolar atomic charges are
likely insufficient to capture the subtleties of the steric and
electrostatic effects due to the presence of lone pair electrons, and
changes in charge distribution due to conformational changes. In
addition, the calibrations for these potential functions should be
extended beyond ambient conditions to much higher temperatures and
pressures to be relevant to explosive reaction zones. As for the
reaction kinetics, corrections for zero-point energies and effective
thermal motions need to be included.

## Thermophysical Properties and Transport Coefficients

Physical properties and transport coefficients from the molecular scale
to larger distance and time scales are needed to advance the simulation
of organic materials. Starting from molecular potential functions it
should be possible to provide, from first principles, many of the
required fundamental physical parameters. The physical properties that
must be calculated include density, energy, specific heat, vector and
scalar coefficients of thermal and isobaric expansion, elastic constants
and derived moduli, mechanical strength, all as a function of
temperature and pressure for both solids and liquids. There has been
recent significant development of practical methods to compute
stress-strain curves and transport coefficients such as diffusion,
viscosity, and thermal conductivity, though there is a need to extend
these to non-linear regimes. These are harder to obtain due to the more
complicated theoretical framework required.

The response to abnormal thermal and mechanical stimuli is also
determined by porosity, permeability and tortuosity of the material.
Most work has been applied to either perfect crystals or a random,
isotropic fluid state. Corrections for non-ideal aspects need to be
considered, as well as accurate extensions to polymeric materials where
low levels of structure will be present. The thermodynamic properties of
polymeric foams and other organic materials under conditions of extreme
temperature and shock are important to ASC. Continuum-scale response
models based on fundamental physical and chemical properties are needed
as well as an improved understanding of these phenomena at the atomic
scale.

A variety of theoretical tools are required to do these calculations.
They include high-level *ab initio* quantum electronic structure
methods, Monte Carlo methods, and both classical and quantum mechanical
molecular dynamics. The full spectrum of modern computing capabilities
must be brought to bear in this work, from high-performance workstations
in stand-alone mode and in clusters, to advanced high-end parallel
platforms. Coupling of electronic structure methods and classical
simulations would be advantageous in assessing the behavior of non-ideal
structures (ie ideal crystal or isotropic fluid).

## Mechanical Properties of Organic Materials

The mechanical properties of organic crystals, melts and polymeric
materials, especially under conditions of elevated temperature and
pressure, are important to ASC. Specific technical areas of interest
include:

-   Ab initio electronic structure calculations of point defects and
    dislocations to determine energetic and fundamental length scales

-   Atomic-level simulations of dislocations and dislocation
    interactions to derive rules and mechanisms for microstructure-level
    simulation

-   Three-dimensional (3D) dynamic dislocation simulation of
    microstructure and plasticity and mesoscale studies of
    polycrystalline grains to develop improved plasticity models

-   The connection to mesoscale and continuum-based models of improved
    anisotropic plasticity models.

Most of the relevant mechanical aspects are described in the white paper
relating to constitutive properties. What is of greater relevance here
is how this affects the chemical reaction properties? One example is the
generation of excess temperature (and its persistence) associated with
dislocation dynamics or other localized stress features (eg void
collapse) and how that may influence the effective chemical dynamics
models. The possibility of anisotropic mechanical induction of a
reaction (e.g., mechanical rather than thermal induced scission of
bonds) should also be considered, where this is certainly a factor in
the failure of polymeric materials. Conversely, reaction within a
localized deformation zone should effectively lubricate it by the
generation of small molecules and disruption of the local structure, and
enhance further localization of the displacement in that region.
Consequently, chemical reaction will become a substantial perturbation
to mechanical property predictions in certain regimes.

## Fuel Combustion and Soot Formation

Accident scenarios (also known as abnormal environments) generally
involve the combustion of organic materials such as jet and diesel fuel.
Fine soot particles are also formed during the detonation of a high
explosive. Thermal insult due to pool fires is an abnormal environment
scenario required for the qualification of weapons systems. The primary
mechanism for energy transfer to a system is radiation from the soot
produced in a fire. Fires involve highly non-linear phenomena including
fluid transport, combustion, and radiation. The nonlinear \"turbulence
problem\" common to all high Reynolds number flows is amplified in fires
in that turbulence affects convective stirring down to millimeter length
scales at which flame zones reside. The non-linear mixing rates control
combustion; and therefore, heat release rates and temperatures. Further,
radiation heat transfer is dependent on the soot temperature to the
fourth power while soot absorption coefficients result in exponential
decay of thermal radiation spatially.

Fires involve phenomena that are tightly coupled. Fire is the natural
balance between all physical phenomena. This coupling is tighter than
most combustion phenomena where some external control exists on either
the momentum or scalar flow fields (i.e., the inlet velocities, or
fuel/air ratios). Fire is not a process with inputs and outputs in the
usual sense, rather a balance point that is a result of the circular
closure of the mechanisms. As an example, heat flux to a liquid pool
surface results in fuel vaporization. The vaporized fuel mixes with air
and ignites which results in flames. Radiative heat transfer from the
flames results in a heat flux to the liquid pool surface which results
in fuel vapor, and the process repeats itself. If any change occurs in
any of the flow conditions, i.e., geometry, wind, fuel, etc. the balance
point will shift to a new equilibrium point. The fire will then be a
different fire with different combustion rates, temperatures, and
radiative heat fluxes. In the same vein, the vortical structures which
are found at all length scales in a fire can simultaneously be
interpreted as being the consequence of the heat release in a fire and
as the cause of the heat release in a fire.

Soot formation during detonation is not well understood, yet is the key
to understanding the release of energy during the detonation process.
Many insensitive explosives are carbon-rich. A significant fraction of
the energy release of these materials comes from the formation of carbon
particles. Detonating explosives have been found to produce nano-diamond
and other exotic carbon morphologies. The processes and time scales
controlling the formation of carbon clusters under extreme conditions,
however, remains poorly understood. A particular challenge is that the
carbon cluster formation timescale is believed to require up to a
microsecond, making the direct application of molecular dynamics
methodologies difficult.

## Chemical Aging Effects in Organic Materials

Results in the other research elements may be dramatically affected by
the aging process. By their very nature, organic materials are
metastable molecules. Therefore, over time, they will decompose, produce
gas, and at higher temperatures change molecular composition and
structure. These changes lead to changes in the macroscopic behavior of
these materials and can have profound nonlinear effects on systems
containing them.

Degradation mechanisms and chemical reactions must be addressed to
understand the performance consequences of aging in organic materials.
The effects of aging on transport coefficients are important for other
issues as well, such as the outgassing of binders and plasticizers in
aging energetic materials. Polymeric materials undergo physical aging
processes, as well as chemical, where this includes continual refinement
in structure (phase separation, crystallization) as well as relaxation
to accommodate local stress fields that may evolve with time.
Crystalline materials may undergo similar physical ripening processes
that might impact their subsequent effective chemical kinetics. The
relationship of the composition of aged energetic materials to their
initiation and detonation performance, mechanical response and response
to abnormal thermal and mechanical stimuli must be addressed.

Aging processes related to organic materials including stress-voiding of
integrated circuit interconnects, thermomechanical fatigue of solder
joints, corrosion processes, oxidation and cross-linking in polymers,
and plasticizer outgassing and reactive degradation of energetic
materials are poorly understood. Mobile products from the degradation of
organic materials can contribute to the corrosion of neighboring
metallic components, enhancing crack growth or similar phenomena. Much
work is needed to understand the physical and chemical processes
underlying predictive failure models for these aging processes. Models
and experimental validation are needed for phenomena such as void
nucleation and growth, impurity migration, microstructural evolution in
metals, fracture precipitation and propagation mechanisms, reactive
degradation chemistry, and diffusion in high molecular weight materials.
Accelerated aging experiments and methods development are also needed
for model validation.

Gene Hertel, SNL, <esherte@sandia.gov>

Larry Fried, LLNL, <fried1@llnl.gov>

Ed Kovar, LANL, <emk@lanl.gov>

**Acknowledgement**

This work was, in part, performed by the Lawrence Livermore National
Laboratory, Los Alamos National Laboratory, and Sandia National
Laboratories under auspices of the U.S. Department of Energy.

This document was prepared as an account of work sponsored by an agency
of the United States Government. Neither the United States Government
nor the University of California nor any of their employees, makes any
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and shall not be used for advertising or product endorsement purposes.

This work was performed under the auspices of the U.S. Department of
Energy by University of California, Lawrence Livermore National
Laboratory under Contract W-7405-Eng-48.