Quantification of Uncoupled Spin Domains in Spin-Abundant Disordered Solids.

Brennan J Walder,Todd M Alam

doi:10.3390/ijms21113938

Brennan J Walder, Todd M Alam

Open Access

https://doi.org/10.3390/ijms21113938

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Abstract

Materials often contain minor heterogeneous phases that are difficult to characterize yet nonetheless significantly influence important properties. Here we describe a solid-state NMR strategy for quantifying minor heterogenous sample regions containing dilute, essentially uncoupled nuclei in materials where the remaining nuclei experience heteronuclear dipolar couplings. NMR signals from the coupled nuclei are dephased while NMR signals from the uncoupled nuclei can be amplified by one or two orders of magnitude using Carr-Meiboom-Purcell-Gill (CPMG) acquisition. The signal amplification by CPMG can be estimated allowing the concentration of the uncoupled spin regions to be determined even when direct observation of the uncoupled spin NMR signal in a single pulse experiment would require an impractically long duration of signal averaging. We use this method to quantify residual graphitic carbon using C CPMG NMR in poly(carbon monofluoride) samples synthesized by direct fluorination of carbon from various sources. Our detection limit for graphitic carbon in these materials is better than 0.05 mol%. The accuracy of the method is discussed and comparisons to other methods are drawn.

Highlights

The microstructure and heterogeneity of materials exert significant influence on their macroscopic properties
In this work we describe a CPMG-based NMR experiment that can be used to enhance the NMR signals from domains containing essentially uncoupled dilute nuclei relative to NMR signals from domains where the dilute nuclei can experience residual evolution under the through-space heteronuclear dipolar coupling to abundant nuclei that remains despite the averaging effect of magic-angle spinning (MAS) [24]
We investigate a series of commercial (CF)n samples made using different carbon sources: petroleum coke (PC), carbon black (CB), or carbon fiber (CF)

Summary

Introduction

The microstructure and heterogeneity of materials exert significant influence on their macroscopic properties. Characterization of microstructure is an important part of establishing structure-property relationships, but doing so in a way that quantifies the number of distinct species present on a molecular basis is challenging. This is so for disordered materials, where the quantitative information that can be obtained by X-ray diffraction (XRD) is limited. Complicated sample topology and X-ray polarization effects make the acquisition of suitable reference data essential [5,6]. Without consideration of these effects, an order of magnitude error in quantification is possible

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