Nuclear Waste Glasses: Insights from Solid-State NMR

Kroeker, Scott

doi:10.1002/9780470034590.emrstm1223

Cited by 5 publications

(18 citation statements)

References 22 publications

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“…15,62 The 11 B NMR parameters obtained by line shape fitting are given in Table 2. Due to the significant quadrupolar interaction experienced by the planar [3] B units (C Q ≈ 2.6 MHz), 15,63 its NMR signal is broad and exhibits a second-order quadrupolar line shape. The more symmetric [4] B peak is narrower due to the pseudo-tetrahedral geometry and, hence, exhibits a smaller quadrupolar coupling (C Q ≈ 400 kHz).…”

Section: Resultsmentioning

confidence: 99%

“…Mean isotropic chemical shifts of Q 2 , Q 3 , and Q 4 units are −85, −95, and −110 ppm, respectively, ,,− which show a trend toward lower frequencies as the number of bridging Si–O–Si bonds per tetrahedron increases. However, if the next-nearest neighbor is a heteroatom (Si–O–T, T = Al, B, Ga), higher-frequency shifts are observed, , highlighting the difficulty associated with the 29 Si spectral interpretation. The CPMG MAS NMR spectra of NABS glasses with chemical shift ranges of various silicate species (Q n , Figure S2) and NGBS glasses are shown in Figures and S6, respectively.…”

Section: Resultsmentioning

confidence: 99%

“…75,81 The chemical identity of the next-nearest neighbors has an influence on the isotropic chemical shifts of Si units in glasses and crystalline systems. 41,63 An increase in the number of Si− O−Si bonds per Si tetrahedron induces low-frequency shifts, whereas an increase in the number of heteroatomic bonds (Si− O−T, T = B, Al, and Ga) shifts the 29 Si peak to higher frequencies. Charge-balance calculations show that very little Q 3 units exist in these series, and their fractions diminish greatly as Al and Ga are introduced.…”

Section: Discussionmentioning

confidence: 99%

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Network Formation in Borosilicate Glasses with Aluminum or Gallium: Implications for Nepheline Crystallization

2021

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The evolution of the network connectivity of borosilicate glasses with the progressive addition of aluminum is analyzed using solid-state multinuclear magnetic resonance (NMR) spectroscopy to better understand the structural mechanism by which boron inhibits nepheline crystallization in nuclear-waste glasses. The [4]Al units exert a stronger influence on Na+ than do the [4]B units, resulting in the reorganization of tetrahedral to trigonal-planar boron ([4]B → [3]B) with increasing Al. The [3]B units preferably bond to [4]Al and interfere with the Si–Al ordering critical to nepheline formation, thereby inhibiting nucleation. Moreover, as Na+ charge compensates both [4]B and [4]Al units, the presence of anionic borate units confers a heterogeneous chemical environment about Na+ and prevents its transformation into a nepheline-like crystalline environment. The NMR results for aluminoborosilicate glasses are compared with those of their gallium analogues to assess the structural similarities of Al and Ga in silicate-based glasses. Gallium integrates into the borosilicate network as tetrahedral units similar to Al but is less effective in competing with [4]B for Na+, leading to higher [4]B ratios. The relative fractions of silicate Q3 and Q4 species are determined semiquantitatively using charge-balance constraints on 29Si NMR results, showing that the silicate speciation in high-Ga glasses is restricted to Q4 units, which may affect the properties relevant to nuclear-waste immobilization.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Network Formation in Borosilicate Glasses with Aluminum or Gallium: Implications for Nepheline Crystallization

2021

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“…−2 ppm represents tetrahedral BO 4 – units ( [4] B). , The [3] B units experience significantly larger quadrupolar interactions compared to [4] B, and hence, the peak is broadened and possesses a second-order quadrupolar line shape. The quadrupolar coupling constants ( C Q ) for [4] B and [3] B units are about 0.6 and 2.6 MHz, respectively . Replacement of Si by P results in a progressive decrease in the overall fraction of [4] B (N 4 ), indicating lower overall connectivity of B in the glass (Table ).…”

Section: Resultsmentioning

confidence: 99%

Network Structure and Dissolution Properties of Phosphate-Doped Borosilicate Glasses

et al. 2020

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The addition of phosphorus to conventional borosilicate glasses is explored as a way to increase the incorporation of high-field-strength cations such as Mo(VI) into chemically durable materials. We have studied the molecular-level structure of borosilicate glasses containing up to 4 mol % P2O5 using multinuclear magnetic resonance (NMR) spectroscopy, alongside their dissolution behavior in water using static tests. Phosphorus is predominantly present as phosphate dimers but tends to cluster into alkali-phosphate regions at higher loadings. In the presence of molybdenum, macroscale segregation into a water-soluble amorphous P- and Mo-rich phase is observed, reducing the chemical durability. However, the formation of a silicate-rich amorphous layer significantly retards ion release after a few days, suggesting that, despite the inferior durability of the phosphate phase, the overall durability of the glass monolith may prove acceptable for further consideration as a high-Mo nuclear wasteform.

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“…This approach has gained widespread popularity and has been applied to the study of ceramics, minerals, microporous materials, energy materials, and glasses. 3 , 7 More recently, Gan introduced the satellite-transition (ST) MAS experiment, 8 which is conceptually similar to MQMAS but correlates the single-quantum satellite (i.e., m I = ±1/2 ↔ m I = ±3/2) and central transitions. Despite offering a sensitivity advantage over MQMAS, this experiment is technically more challenging to implement (owing to the need to accurately set the angle of the spinning axis, and the requirement for a very stable spinning rate) 8 , 9 and has received less attention, although it has been utilized very successfully in cases where sensitivity is limited, and to detect the presence of μs-time scale dynamics.…”

Section: Introductionmentioning

confidence: 99%

Efficient Amplitude-Modulated Pulses for Triple- to Single-Quantum Coherence Conversion in MQMAS NMR

2014

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The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed “too challenging”.

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Nuclear Waste Glasses: Insights from Solid-State NMR

Cited by 5 publications

References 22 publications

Network Formation in Borosilicate Glasses with Aluminum or Gallium: Implications for Nepheline Crystallization

Network Formation in Borosilicate Glasses with Aluminum or Gallium: Implications for Nepheline Crystallization

Network Structure and Dissolution Properties of Phosphate-Doped Borosilicate Glasses

Efficient Amplitude-Modulated Pulses for Triple- to Single-Quantum Coherence Conversion in MQMAS NMR

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