Research

We have developed several computational materials science models to simulate 3D microstructure evolution of multiphase materials, especially porous media.  The phenomena we seek to capture are the rates of dissolution, nucleation, and growth of solid phases, the evolution in the composition of those phases, and morphological changes.  We endeavor to make these models as fundamental as possible at length scales of about 100 nm to 100 μm.

THAMES

THAMES (Thermodynamic Hydration And Microstructure Evolution Software) is a 3D lattice-based model of microstructure development that can be used when a large portion of a material system is evolving slowly under near-equilibrium conditions.  It uses geochemical thermodynamic equilibrium calculations to predict solid-phase assemblages and solution speciation for boundary conditions that are evolving in time due to changes in the surroundings.  A digital-image-based model is used to arrange those phases to create a realistic microstructure that can subsequently be coupled to finite element models to calculate the evolving mechanical and transport properties.  THAMES has recently been used to model portland cement hydration, the viscoelastic properties of cementitious binders, and the evolution of cement properties during groundwater leaching and external sulfate attack.  More …

HydratiCA

HydratiCA is a 3D kinetic cellular automaton with probability rules that can be proven to converge to standard reaction rate equations in the continuum limit.  It can simulate mass transport and multiple chemical reactions over a wide range of chemical driving forces.  It has been applied recently to help understand the mechanisms of the hydration of tricalcium silicate, the primary mineral component of portland cement that is responsible for early-age strength development of concrete. More …

VCCTL

VCCTL (Virtual Cement and Concrete Testing Laboratory) is a software package created at the National Institute of Standards and Technology (NIST) to model the hydration and properties of portland cement and concrete.  Our group continues to maintain and develop VCCTL. More …

 

MicroChar

MicroChar is a small software package that assists with the characterization of microstructures observed by quantitative SEM and Electron Microprobe Analysis.  Given a digitized and indexed 2D micrograph of a microstructure, MicroChar can perform measurements of the phase area fractions, surface boundary fractions, and various autocorrelations functions to quantify the spatial arrangement of the phases.  It can also collect and package the data for direct input into the VCCTL software package.

 

Kinetics and Thermodynamics of Mineral Dissolution and Precipitation

Recent experimental work has been directed toward measuring the rates of reaction of minerals with aqueous solutions.  The measurements are made by complementary techniques, including

• characterization of in-situ nano-topographical evolution of mineral surfaces in flowing solutions by digital holographic microscopy.  The movie below shows the real-time development of an etch pit on a (104) surface of calcite when exposed to flowing deionized water.

 

• monitoring reaction rates in batch reactors and mixed flow reactors
• tracking changes in surface area and morphology by scanning electron microscopy (SEM) and nitrogen adsorption isotherms

We are also interested in relating microstructure characteristics of porous media to engineering properties such as stiffness, strength, viscoelastic response, and fluid permeability.

Sintering Kinetics and Mechanisms

Sintering of ceramic powders is a potential route to manufacturing in extreme or expeditionary environments such as remote regions of the Earth, on the Moon, or on Mars.  To make sintering viable in these circumstances, the raw materials in these regions must be characterized and the sintering process must be tailored and optimized.

Recent work in our group has focused on understanding the mechanisms of densification and creep of Lunar regolith material.  We use high-fidelity synthetic regolith powders, and we are investigating the influence of mineralogical composition and particle size distribution on the driving force and rate-controlling mechanisms of densification, coarsening, and creep.

The video below from our lab shows how the real-time shrinkage and deformation can be tracked with an optical heating microscope.  The microscope captures projections of the specimen backlit by blue LED light as the sample is exposed to high temperatures. Continuous curves tracking volume as a function of time and temperature can be used to determine instantaneous rates of creep and deformation and how those rates depend on temperature and other environmental variables.

The specimen being studied in this video is a Lunar regolith powder designed to mimic the mineralogical composition in the highlands regions of the Moon.  The plot shows how the specimen volume changes with temperature as it is heated at 5 K/min, and the right side of the video shows how the projections of two specimens change as the temperature increases.