No abstract

The glycerol methylene proton resonances (4–4.5 parts per million, ppm), which arise from the triglyceride backbone, are relevant to fat composition assessment and can be measured with proton MRS. The purpose of the presented work is to determine long TE (echo time) point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM) values at 3 T to resolve the glycerol resonances from that of overlapping water. The response of the glycerol methylene protons of nine edible oils as a function of PRESS and STEAM TE (mixing time, TM = 20 ms) was investigated. In addition, high resolution NMR spectra of the oils were acquired at 16.5 T. Long TE values where J‐coupling losses were lowest were selected, namely a TE of 180 ms for PRESS (first echo time 17 ms) and a TE of 100 ms for STEAM (mixing time 20 ms). Oil olefinic (≈5.4 ppm) to glycerol ratios were calculated from the long TE spectra and correlated with 16.5 T ratios. The two techniques yielded olefinic/glycerol ratios that correlated with 16.5 T ratios (R2 = 0.79 for PRESS and 0.90 for STEAM). The efficacy of the sequences in resolving the glycerol resonance from that of water was verified in vivo on tibial bone marrow of four healthy volunteers. In addition, the potential for using the glycerol methylene signal normalized to the methyl signal (≈0.9 ppm) to assess changes in free fatty acid content was demonstrated by measuring differences in spectra acquired from a triglyceride peanut oil phantom and from a phantom composed of a mixture of peanut oil and free fatty acid oleic acid.

Lithium metal is receiving significant research attention as a potential next generation negative electrode material in lithium ion batteries because it can offer a ~50% increase in cell energy density compared to conventional graphite electrodes. There are, however, a number of mechanical challenges using lithium metal electrodes. Mechanical stack pressure is required to ensure good contact between solid-state electrodes and electrolytes. Moderate to high stack pressures (e.g. < 10 MPa) can also supress void formation during fast charging and discharging.1 Creep, the plastic deformation of a material under constant pressure, improves battery performance by preventing and filling voids, but can also lead to decreased performance if lithium is unable to sustain the required stack pressure, and potentially cell shorting if lithium is e.g. extruded beyond the current collectors. Creep deformation rates are highly dependent on homologous temperature and pressure. Lithium metal has a relatively low melting point (181°C) / high homologous temperature at common cell temperatures which should result in substantial lithium creep deformation. Previous studies have characterized both the tensile creep properties of lithium foils2 and compressive creep properties of bulk lithium rods.3 Significant barrelling of the high aspect ratio lithium rods (initial height:width of 4:1 to 1:1) during compression made it difficult to determine the applied pressure on the lithium, obscuring the relationship between pressure and creep rate.3 These important studies increased our collective understanding of the mechanical properties of lithium, but are not necessarily reflective of the compressive creep of lithium foils in a cell environment. Here, we report on the compressive creep of lithium foil within an electrochemical cell at commercially relevant stack pressures and temperatures. Compressive creep deformation of low aspect ratio (1:40 to 1:10; initially 0.3 – 2.5 mm thick, 13 mm diameter) lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30 - 110°C and applied pressures between 1 - 2 MPa. Creep testing was performed by integrating a Conflat-style electrochemical cell4 featuring welded bellows and an optical window with an Instron 5966 Universal Testing System (which measures / controls force and displacement). Creep rates were determined using the time-dependent displacement data after reaching a constant applied force. Two methods to convert force to pressure will be described, including the use of an inline camera to measure the lithium-compressive piston contact area. Creep deformation was observed at all tested temperatures, and all pressures above 1 MPa. Creep rates were on the order of a few tenths of a micron per hour, which while small, implies very limited device lifetime with thin lithium layers. Lithium metal may not be physically strong enough for use in high energy density all solid-state devices. Figure 1

It has been previously shown that the MRS sequence stimulated echo acquisition mode (STEAM; mixing time, TM  = 20 ms) with an echo time (TE) of 100 ms resolves triglyceride glycerol resonances from that of water at 3 T. The purpose of this work is to determine if STEAM with a TE of 100 ms facilitates relative quantification of diglyceride/triglyceride levels at 3 T. Spectra were obtained from tricaprylin (triglyceride) and dicaprylin (diglyceride) with a range of STEAM TE values (TM = 20 ms). TE values that resulted in two resolved glycerol resonances for triglycerides (rendering them suitable for distinguishing triglyceride contributions from those of diglycerides) were selected. One resonance resides in the 3.85–4.2 ppm spectral range (overlapping the 1,3‐diglyceride resonance) and the other in the 4.2–4.6 ppm spectral range (overlapping one of the 1,2‐diglyceride resonances). STEAM with TE values of 40 ms and 100 ms (TM = 20 ms) yielded two resolvable triglyceride resonances (tricaprylin phantom), at about 4 ppm and 4.4 ppm. Direct integration of the resonances showed that the former peak has 0.86 and 0.17 times the area of the latter for TE = 40 ms and 100 ms, respectively. Spectra obtained from the phantoms containing mixtures of diglyceride (1,3‐dicaprylin) and triglyceride (tricaprylin) were acquired. The triglyceride contribution to the 4 ppm glycerol resonance, a mixture of signal from 1,3‐diglyceride and triglyceride, can be approximated from the area of the 4.4 ppm peak, resulting in an estimate of the 1,3‐diglyceride contribution. Analysis was performed for STEAM TE = 40 ms and TE = 100 ms spectra acquired from phantoms with 1,3‐dicaprylin/tricaprylin weight/weight contents of 2.5%/97.5%, 5%/95%, 10%/90% and 20%/80%. Concentration ratios of 1,3‐dicaprylin/tricaprylin estimated with both STEAM TE values resulted in linear correlations with expected concentration ratios (R2 > 0.99).

Diglyceride levels have been found elevated with some disease. Previously, STEAM (mixing time=TM=20ms) with an echo time (TE) of 100ms was shown to resolve triglyceride glycerol resonances from that of water of 3T while yielding adequate glycerol signal. The purpose of this work is to determine if STEAM with a TE of 100ms facilitates relative quantification of diglyceride/triglyceride levels at 3 T. Spectra were measured from phantoms containing 1,3-dicaprylin/tricaprylin with varying weight/weight contents of 2.5%/97.5%, 5%/95%, 10%/90% and 20%/80%. Concentration ratios of 1,3-dicaprylin/tricaprylin estimated from STEAM (TM=20ms, TE=100ms) resulted in a linear correlation with expected concentration ratios (R2 &gt; 0.99).