Motivation The ability to accurately connect experimental measurements and computational simulations remains crucial for the reliable modeling and prediction of material properties. This laboratory continues to investigate the structure and dynamics of amorphous materials using nuclear magnetic resonance (NMR) spectroscopy including recent investigations of phosphate glasses, high level waste (HLW) ceramics, diamond thin films, composites, and polymers. To gain a better understanding of the experimental NMR results the development of ab initio, molecular dynamics (MD) and Monte Carlo programs to simulate amorphous materials has been pursued.

Accomplishment A wide range of length scales have been utilized in the computation of NMR observables. At the quantum mechanical or atomic level, both Hartree-Fock (HF) and density functional theory (DFT) ab initio techniques have allowed the implementation of gauge-including atomic orbital (GIAO) methods in the calculations of NMR isotropic shifts and chemical shift anisotropy (CSA) tensors. It has been demonstrated that variations in the isotropic and CSA tensor behavior are related to the local bonding and structure, and provides a powerful tool to investigate structural variations.[1]This lab has also recently developed an empirical partial charge model (PCM) approach for the calculation of 29Si CSA tensors in organically modified alkoxy silanes.[2]

We have also been using molecular dynamics (MD) simulations to understand the extended medium range order (MRO) in glasses and amorphous ceramics, including the development of the program GlassVAL to predict the NMR chemical shift of alkali metals directly from the MD simulations. Figure 1 shows the simulated structure for two different phosphate glass compositions.[3] The strained three member rings responsible for the anomalous glass transition (Tg) temperature are clearly evident. In addition, the changes in the lithium MRO have also been correlated to the increasing Tg with variation of the modifier concentration.[4]

The MRO in amorphous materials at longer length scales has also been studied using Monte Carlo simulations. The simulation of sodium cation distribution from NMR spin-echo 23Na experiments of phosphate glasses confirmed that cation segregation is not responsible for the unique thermal properties observed.[5] This laboratory has also developed the Monte Carlo code MOnteGLASS for the calculation of connectivity in phosphate systems based on differences in bond formation energies. These Monte Carlo results have been used to quantitatively address the experimental tetrahedral connectivity measured by double-quantum (2Q) NMR.[6] Figure 2 shows some of the results from this investigation. Conventional diffraction techniques cannot address the question of connectivity in these amorphous materials, but fortunately by combining NMR and computational techniques this information is available.

Significance Understanding the structure of amorphous materials is crucial for the design, production and fabrication of technologically specific materials. In addition, this information is important for the prediction of material lifetime and durability. By uniquely combing the computational and NMR capabilities at Sandia this information can now be obtained.

Sponsors for various phases of this work include: BES, TPP and CRADA

Contacts: Todd Alam,(505) 844-1225,tmalam@sandia.gov

Image Gallery:

Figure 1
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Figure 2
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