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43596-AC8
Investigations of Elastic and Anelastic Properties of Clay Minerals
The project funds were used to support a student, with tuition and stipends. After the first year, some funds were also used to travel to present papers at conferences. Based on the work performed during this project, two awards were granted to Arpita Pal-Bathija and the PI was invited to present a paper on Organic-rich shales.
1. Student Travel Grant awarded to Pal-Bathija to present at the 46th Annual Meeting of the Clay Minerals Society in 2007.
2. Student Research Grant from the Clay Minerals Society for USD 2500 awarded to student, Arpita Pal-Bathija
Some papers are under preparation for submission to Science (on nanoindentation and molecular simulation studies of Montmorillionite) and to AAPG Bulletin (on Atomic Force Acoustic Measurements on Kaolinite).
The questions addressed in this study were:
- How does Young's modulus change with the number of clay mineral layers forming the stack?
- What are the effects of water content and external stress on Young's modulus of clay minerals?
- How much does the Young's modulus change with different kinds of clays?
- What is Young's modulus more sensitive to? Bound water properties or different clay minerals?
- How to upscale Young's modulus from the molecular level (clays) to the seismic level (shales)?
- How much do AFM measurements vary after heat-treating the clay samples?
The main results achieved in this were dynamic measurements of clay particles as functions of stress and temperatures. The results show very low elastic modulus for clay minerals (up to 10 GPa). This value increases slightly in clays that have been heat-treated to 1000 degC temperatures.
Literature results: The Young's modulus of clay platelets reported from theoretical studies varies within the range 178–265 GPa. The measured Young's modulus for clays varies between 5 GPa and 400 GPa. This large variation may be due to different kinds of clays, different external environments or anisotropy owing to the layered structure of clays.
AFAM results: Elastic properties of clay using atomic force acoustic microscopy (AFAM). The adhesion forces were measured from the pull-off forces and included into our calculations by means of the Derjaguin-Mueller-Toporov model for contact mechanics. The obtained values of the elastic modulus for clay varied from 10 to 17 GPa depending on various parameters that describe the dynamics of a vibrating beam.
Molecular Simulation results: The equilibrium interlayer spacing and Young's modulus of the interlayer obtained from molecular simulation (stress changes were in the order of GPa) were used to compute sub-stack properties. The Young's modulus (derived using Reuss average) varies from 6 GPa to 21 GPa for a 2 layer sub-stack with varying water content from 112 to 0 molecules. Similarly, for a 3 layer sub-stack, the Young's modulus we obtained varied from 3 GPa to 10.5 GPa and for a 8 clay mineral sub-stack, the modulus varied from 1 GPa to 3 GPa. There is elastic softening with increasing layers in the sub-stack as well as increasing water content.
Nanoindentation results: We made nanoindentation studies on fused silica, glass substrate alone, a thin film of muscovite mica deposited on glass substrate and kaolinite film with different grain sizes on glass substrate. The sample in powdered form was first mixed with distilled water and centrifuged to restrict the grain sizes. Then they were deposited on glass substrates and left in a covered case for 48 hours to dry. This method forced the clay minerals to align with their c-axis normal to the glass slide. The orientation was checked with environmental scanning electron microscope. Thickness and average roughness of muscovite film, measured with Wyko optical profiler were 39.76 +- 1.75 micrometers and 2.25 micrometers respectively. We obtained Young's modulus and hardness of 73 +- 6 GPa and 7.7 +- 0.5 GPa for fused silica, 77 +- 1 GPa and 6.67 +- 0.1 GPa for glass substrate and 61 +- 6 GPa and 6.2 +- 0.4 GPa for muscovite. Film and substrate with similar elastic properties is ideal for nanoindentation studies due to correct contact area estimation, for example muscovite on glass. Adhesion forces and shifting of clay platelets cause measurement of void spaces in between the clay platelets leading to near zero values in case of smaller grain sizes of clay (1-2 micrometers). Larger grain sizes (5-8 micrometers) of clays help in achieving a better contact with the indenter tip, thereby giving Young's modulus of 2.5±0.01 GPa.
