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. 2026 Sep 30;11(40):61958–61965. doi: 10.1021/acsomega.6c08050

Compositional Uniformity and Thermal Properties of Tetra‑N‑butyl Ammonium Bromide and Tetra‑N‑butyl Ammonium Chloride Mixed Semiclathrate Hydrates

Motoi Oshima †,*, Satoshi Takeya ‡, Akio Yoneyama §, Yusuke Jin †, Kiyofumi Suzuki ‡, Jiro Nagao †
PMCID: PMC13670317  PMID: 42889894

Abstract

Mixed semiclathrate hydrates (SCHs) incorporating various guest molecules exhibit high latent heat and allow precise control of the melting temperatures, making them promising materials for cold energy storage applications. The compositional uniformity and thermal properties of tetra-n-butylammonium bromide (TBAB) + tetra-n-butylammonium chloride (TBAC) mixed SCHs were evaluated via dual-energy X-ray computer tomography (DECT), microdifferential scanning calorimetry (μDSC), digital microscopy, and ion chromatography. DECT indicated that the tetragonal mixed SCHs are homogeneous solids, whereas the orthorhombic mixed SCHs with high or low-Br content are heterogeneous. The DSC curve of the tetragonal mixed SCHs exhibited a single endothermic peak, whereas those of the orthorhombic mixed SCHs showed split peaks. The orthorhombic mixed SCH dissociated in two stages, consistent with the split DSC peaks. The Br– content of the orthorhombic mixed SCH was higher than that of tetragonal SCH. The orthorhombic mixed SCHs exhibit high latent heats (227–231 J g–1) as phase change materials, with melting temperatures around 283–285 K. These values are higher than other SCHs formed from a single guest molecule with melting temperatures below 289 K, making them suitable for cold energy storage in air-conditioning systems.

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Introduction

High-energy-density phase change materials (PCMs) with near-isothermal operation are promising for cold energy storage. The properties and performance of composite materials and multicomponent PCM systems can be tailored by combining components. Clathrate hydrates, including semiclathrate hydrates (SCHs), are attractive PCM candidates for cold energy storage. , Clathrate hydrates are host–guest compounds consisting of water-bonded cage structures that encapsulate guest molecules. Hydrates in which large, ionic guests such as tetra-n-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) cations are enclathrated by several hydrate cages and the anions replace the water molecules in the hydrate cage are called SCHs. SCHs from TBA and TBP salts undergo liquid–solid (hydrate) phase transitions with relatively high latent heats (160–250 J g–1) at 273–301 K, making them promising PCMs for cold energy storage. , Recently, SCHs with superior thermal properties and higher latent heats over wider temperature ranges have been actively developed through the exploration of new guests. , Tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butylammonium chloride (TBAC) are typical guest molecules that form SCHs and are suitable for cold storage applications, with melting temperatures of 283–288 K. TBAB SCHs form tetragonal or orthorhombic crystals with different hydration numbers (n): tetragonal TBAB SCH with n = 26 is dominant at x > 0.014 (x is the mole fraction of guests to (guests + water) in the initial aqueous solution), and orthorhombic SCH with n = 38 dominates at x < 0.014 at atmospheric pressure. − The maximum melting temperatures of tetragonal and orthorhombic TBAB SCHs are 286 and 283 K, with latent heats Δd H mass = 189–193 and 217–220 J g–1 (Δd H mass is the dissociation enthalpy per mass), respectively. − TBAC SCHs form tetragonal crystals with n = 24–33; the maximum melting temperature and latent heat at atmospheric pressure are ∼ 288.4 K and Δd H mass = 193–215 J g–1, respectively. , The melting temperatures of SCHs vary with their concentration in aqueous solution. The latent heat decreases with decreasing concentration because the number of crystals also decreases.

The thermal properties of mixed SCHs with different guest molecules have also been investigated. − Tetragonal TBAB + TBAC mixed SCHs form crystals at almost all compositions, whereas orthorhombic mixed SCHs are formed at lower concentrations (x ≤ 0.006) and at higher TBAB mole fractions (x Br ≥ 0.6, where x Br represents Br– ratio of “Br– to (Br– + Cl–)” in the initial aqueous solution). The melting temperature and latent heat of mixed SCHs vary with x and the mole fraction of guest molecules in aqueous solutions (e.g., x Br). Because the latent heat of orthorhombic SCHs is higher than that of tetragonal SCHs, − , evaluating the crystalline and thermal properties of orthorhombic mixed SCHs is important for cold energy storage applications. If their thermal properties can be tailored by simply preparing guest mixtures in appropriate ratios, the simplified synthesis would broaden their scope for cold energy storage. Thus, evaluating SCH crystals from guest mixtures is essential. Several mixed SCHs contain two guests in one crystal. − Orthorhombic TBAB + TBAC mixed SCH forms eutectic crystals from conventional differential scanning calorimeter (DSC) and X-ray diffraction measurements. However, the compositional uniformity of mixed SCHs and thermal properties of orthorhombic TBAB + TBAC mixed SCH remain unclear. TBAB SCH can be visualized, and its bromine content can be determined using monochromatic X-rays at a photon energy just above the absorption edge of bromine (13.47 keV) via dual-energy X-ray computer tomography (DECT). , Moreover, specific elements can be detected even in substances of similar density, such as solution and SCH, and the compositional uniformity of crystals within mixed SCHs can be evaluated on the microscale. High-resolution thermal analysis instruments such as micro-DSC (μDSC) are required for evaluating the thermal characteristics and properties (melting temperature and latent heat) of orthorhombic eutectics.

Herein, the crystalline and thermal properties of TBAB and TBAC mixed SCHs are evaluated using DECT and μDSC. For use as PCMs for cold energy storage, the compositional uniformity and thermal properties of the mixed SCHs are compared.

Experimental Section

Materials and Solutions

Research-grade TBAB (0.99 mass fraction purity, Sigma-Aldrich Co., LLC), research-grade TBAC (≥0.97 mass fraction purity, Sigma-Aldrich Co., LLC), and distilled water (ADVANTEC, RFD240FC) were used without further purification. The x values were measured using an electronic balance with a reading uncertainty of ± 0.1 mg (the uncertainty in x was within ± 0.0001); the samples were prepared by considering the purity of the reagents. Pure TBAB and TBAC aqueous solutions were prepared at x = 0.006 and 0.030. The mixed aqueous solutions were prepared by blending pure solutions of the same x. A 15 cm3 aliquot of each solution was placed in a 50 cm3 plastic test tube.

Measurement Procedures

Sample Preparation

The SCH samples for each measurement were prepared using cooling baths (EYELA, NCB-3300). The bath temperature was measured using a calibrated thermistor probe (±0.02 K, TAKARA, D641) at atmospheric pressure. The SCH samples were crystallized by cooling them below 273 K prior to annealing at approximately the equilibrium temperatures obtained from previous studies with the annealing continued until only a small amount of SCHs remained. After annealing, the SCH samples were grown by subcooling at 1.0 K for x = 0.030 and 2.0 K for x = 0.006, with agitation for several days until no further crystal growth was observed. However, the absence of observable crystal growth does not necessarily indicate that thermodynamic equilibrium had been reached. The results obtained under these conditions should be regarded as representing transient states during the approach to equilibrium. The samples were separated from the mother liquor and rapidly dried in a low-temperature room on paper sheets at a temperature close to that of the cooling bath. Metal sieves and liquid nitrogen were used to collect SCH particles with diameters of ∼ 1 mm.

Microscopic and DECT Observations

Microscopic observations were performed using a digital microscope (HiROX, HRX-02) with a zoom lens (HiROX, HR-2500E). The sample temperature was controlled using a cooling stage (Linkam Scientific Instruments, 10002L). During the observation of SCH dissociation, the temperature was increased from 278.2 K in increments of 1 K every 10 min until the SCHs completely dissociated, and images of the SCH samples were captured 1 min after reaching each temperature. Under high-magnification conditions (200×), the z-axis depth-composition function was used to automatically combine images acquired at different focal positions, thereby generating an all-in-focus three-dimensional image of the sample.

High-spatial-resolution imaging of the SCH samples (∼1 mm in size) was performed using DECT at BL07 of SAGA-LS (Tosu, Japan). The synchrotron X-ray beam was monochromated to an energy range of 10.0–14.0 keV using a Ge(111) double-crystal monochromator. The X-ray beam passing through the sample was detected using a micro-X-ray imager (Kenvy 2) with pixel dimensions of 1.3 × 1.3 μm and a spatial resolution of 3 μm. To obtain the X-ray CT images, the SCH samples were rotated by 360° in steps of 0.36°, with a radiation time of 1 s at each position, for a total measurement time of ∼ 20 min. To prevent SCH dissociation, the sample temperature was maintained at 123 K during the measurements by blowing cold, dry nitrogen gas.

DSC Analysis

The thermal properties of the SCH crystals were measured using μDSC (Setaram, μDSC VII evo). The μDSC instrument was calibrated using a dedicated Joule heat calibrator (Setaram, EJ3). The melting heat of H2O was used as a reference for the heat measurements. The SCH particles (∼20 mg) were placed in a sealed Hastelloy cell in a low-temperature room at ∼ 263 K. The cell temperature was increased from 263 to 283 K at a rate of 0.1 K min–1. The measurements were repeated at least thrice, and the average Δd H mass values were calculated.

Ion Chromatography Analysis

The Br– and Cl– compositions in the TBAB + TBAC mixed SCHs were analyzed using an ion chromatography apparatus (Prominence HIC-20a super system, SHIMADZU) with an anion-exchange column (Shim-pack IC-SA2, SHIMADZU) and a suppressor-type electric conductance detector (CDD-10ASP, SHIMADZU). The eluent was a mixed aqueous solution comprising sodium hydrogen carbonate (12 mmol m–3), sodium carbonate (0.6 mmol m–3), and ultrapure water. The Br– and Cl– compositions in each SCH sample were determined from calibration curves for standard TBAB + TBAC mixed solutions. Samples with a mass of 0.1 g were dissolved in 1000 cm3 of ultrapure water, and the composition of each sample was normalized to its mass. The samples were analyzed four times and the average values were taken.

Results and Discussion

Figure shows the microscopic images of pure TBAC SCH and tetragonal TBAB + TBAC mixed SCH from solutions with x Br = 0.8, where x = 0.030, and orthorhombic mixed SCH from solutions with x Br = 0.8, where x = 0.006, measured at 123 K. Using a standard microscope, it is difficult to distinguish compositional differences among the three SCHs. Figure shows the X-ray CT images of pure TBAC SCH and tetragonal TBAB + TBAC mixed SCH from a solution with x Br = 0.8, where x = 0.030, obtained using photon energies of 11.4 and 14.0 keV at 123 K. Comparison of the X-ray CT images of TBAC and the tetragonal mixed SCH indicates that both have similar grayscale values at 11.4 keV, whereas the mixed SCH appears whiter than the TBAC SCH at 14.0 keV. The whiter areas in the X-ray CT images at 14.0 keV indicate higher X-ray absorption (higher Br concentration). Previous studies have reported that the Br absorption edge occurs at an X-ray energy of 13.47 keV. At 14.0 keV, the grayscale values of TBAC SCH and the tetragonal mixed SCH are uniform throughout each crystal. Figure presents a comparison of the X-ray CT images of tetragonal and orthorhombic mixed SCH using DECT. The grayscale appearance of the orthorhombic mixed SCHs is almost identical to that of the tetragonal counterpart at 10.0 keV, whereas the orthorhombic counterpart appears to comprise a mixture of white (high-Br concentration) and gray (low-Br concentration) regions at 14.0 keV. These results indicate that the tetragonal mixed SCH is a homogeneous solid such as pure TBAC SCH, whereas the orthorhombic mixed SCH is a heterogeneous solid containing regions with high and low-Br content (white and gray regions, respectively). DECT observations enabled visualization of the compositional differences among the SCHs and the compositional uniformity within the mixed SCHs.

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Microscopic images of TBAC SCH, tetragonal TBAB + TBAC mixed SCH from x Br = 0.8 solution, where x = 0.030, and orthorhombic mixed SCH from x Br = 0.8 solution, where x = 0.006, measured at 123 K: (a) images of the three SCHs at 30× magnification, (b) TBAC SCH at 80× magnification, (c) tetragonal mixed SCH at 80× magnification, and (d) orthorhombic mixed SCH at 80× magnification. The scale bar represents 2 mm in (a) and 0.5 mm in (b)–(d).

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DECT images of TBAC and tetragonal TBAB + TBAC mixed SCH from x Br = 0.8 solution, where x = 0.030, measured using photon energies of 11.4 and 14.0 keV at 123 K; (a) and (b) images in the x–y plane, and (c) and (d) images in the x–z plane.

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DECT images of tetragonal mixed SCH from x Br = 0.8 solution, where x = 0.030, and orthorhombic mixed SCH from x Br = 0.8 solution, where x = 0.006, measured using photon energies of 10.0 and 14.0 keV at 123 K. (a) and (b) images in the x–y plane, and (c) and (d) images in the x–z plane.

Figure shows the DSC curves for the TBAC, TBAB, and TBAB + TBAC mixed SCHs formed from solutions with x Br = 0 and 0.6–1.0, where x = 0.006 and 0.030, respectively. One endothermic peak (excluding the small peaks such as those of ice) was observed in the DSC curves of orthorhombic TBAB and the tetragonal SCHs, whereas split peaks were observed in the DSC curves of the orthorhombic mixed SCHs. The split peaks comprised sharp peaks (first peaks) on the low-temperature side and broad peaks (second peaks) on the high-temperature side. As x Br decreased, the first peak shifted toward lower temperatures, whereas the second peak shifted toward higher temperatures. The intensity of the second peak decreased progressively, while the decrease in the intensity of the first peak became less pronounced. To visualize SCH dissociation, images of the tetragonal and orthorhombic mixed hydrates (Figures and , respectively) were captured at the temperatures indicated by (a)–(f) on the corresponding DSC curve in Figure . Dissociation of the tetragonal mixed SCH progressed continuously as the temperature increased from 285.2 to 288.2 K ((c) to (f)). In contrast, the orthorhombic SCH dissociated in two steps: partial dissociation from the onset (281.2 K) to the end (285.2 K) of the first peak in the DSC curve ((c) to (e)), and accelerated dissociation near the onset (286.2 K) of the second peak ((f)). Figure shows Δd H mass for x = 0.006 and 0.030. Δd H mass for the heterogeneous solids was estimated from the total area of the first and second peaks. The Δd H mass of orthorhombic mixed SCH was slightly higher than that reported in a previous study analyzed by conventional DSC. In general, the enthalpy of heterogeneous crystals measured using μDSC tends to be higher than that obtained using conventional DSC. The slow scanning used in μDSC allows kinetically controlled processes, such as phase transitions or structural relaxation, to be completed. The integrated heat flow captures a more complete transformation, resulting in a larger apparent enthalpy. Δd H mass for orthorhombic SCH increased from ∼ 218 J g–1 at x Br = 1.0 (pure orthorhombic TBAB SCH) to ∼ 231 J g–1 at x Br = 0.6. Figures and show the Br– mole fraction (y Br, where y Br represents Br– ratio of “Br– to (Br– + Cl–)” in SCH) and the hydration numbers for the SCH samples, estimated by ion chromatography analysis of the x = 0.006 and 0.030 samples. The crystal compositions of the tetragonal mixed SCH were approximately consistent with those reported in a previous study. y Br increased with increasing x Br, and the y Br of the orthorhombic mixed SCH was higher than that of the tetragonal mixed SCH for each value of x Br. The results indicate that Cl– was dominantly incorporated into tetragonal mixed SCH, whereas Br– was dominantly incorporated into the orthorhombic counterpart. The hydration numbers of the tetragonal SCHs varied from n = 26–33 depending on x and x Br, whereas those of the orthorhombic SCHs remained nearly constant at n = 37–38.

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DSC curves of TBAC, TBAB, and TBAB + TBAC mixed SCHs at 0.1 K min–1 and 0.1 MPa. The peaks at 272–274 K represent the heat of fusion of ice in the samples. The labels “(a)–(f)” represent observation points for SCH dissociation using a digital microscope. Images of tetragonal and orthorhombic SCHs are shown in Figures and , respectively.

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Dissociation of tetragonal TBAB + TBAC mixed SCH (x Br = 0.8, x = 0.030) visualized as all-in-focus image generated using the z-axis depth-composition function. The sample temperatures were (a) 278.2, (b) 284.2, (c) 285.2, (d) 286.2, (e) 287.2, and (f) 288.2 K. The scale bar in each image is 200 μm.

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Dissociation of orthorhombic TBAB + TBAC mixed SCH (x Br = 0.8, x = 0.006) visualized as an all-in-focus image generated using the z-axis depth-composition function. The sample temperatures were (a) 278.2, (b) 281.2, (c) 282.2 K, (d) 283.2, (e) 284.2, and (f) 285.2 K. The scale bar in each image is 200 μm.

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Dissociation enthalpies per unit mass of SCH samples (Δd H mass) prepared from solutions with Br mole fractions of x = 0.006 and 0.030 (x Br). The data for x Br = 0.6 and 0.8 at x = 0.006 were derived from the areas of the two peaks in the DSC curve of the SCH samples. Black plots are reproduced from previous studies. ,,,

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Br mole fraction of SCH (y Br) prepared from solutions with Br mole fractions of x = 0.006 and 0.030 (x Br). Black plots are reproduced from a previous study.

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Hydration numbers of SCH samples (n) prepared from solutions with Br mole fractions of x = 0.006 and 0.030 (x Br). Black plots are reproduced from previous study. ,,

The DECT images (Figure ), DSC curves (Figure ), and dissociation images (Figure ) collectively indicate that the orthorhombic mixed SCHs are eutectic mixtures with different Br contents. In the normal solidification of binary eutectic compositions, two solid phases form cooperatively from the liquid, and the two phases appear either as alternate lamellae or as rods of the minor phase embedded in the major phase. The eutectic mixtures of orthorhombic mixed SCH observed using DECT (Figure b and d) appear to be embedded in a high-Br solid that formed around a low-Br solid. The high-Br solid in the eutectic mixture is whiter than the tetragonal mixed SCH, whereas the grayscale appearance of the low-Br solid is similar to that of the tetragonal mixed SCH (Figure b and (d)). y Br of the orthorhombic mixed SCH is higher than that of the tetragonal congener (Figure ), as determined by ion chromatography, which is consistent with the SCH composition observed using DECT.

Progress has been made in addressing challenges in evaluating the physical and chemical properties of SCHs (including the most widely known TBAB SCH), such as the crystal structures and the correlation between the compositions of SCH and aqueous solution. − ,− However, analyzing the states and structures of crystals containing mixtures of different guest molecules is more challenging. This study proposes a new measurement technique for assessing the compositional uniformity of mixed SCHs by enabling elemental mapping of the guest molecules using DECT.

Generally, a broad endothermic peak suggests lower crystallinity and/or compositional heterogeneity in the relationship between the thermal and crystal properties.34–36 Even within the same tetragonal system with x = 0.030, the peak at x Br = 0.8 is broader than those at the other compositions (Figure ). Such compositional heterogeneity may not be captured in the DECT images (Figure ). Therefore, tetragonal mixed SCH from the x Br = 0.8 solution may be slightly less crystalline than those from other compositions. In the orthorhombic system, the second peak on the high-temperature side is broader than the first peak on the low-temperature side (Figure ), suggesting lower crystallinity or compositional heterogeneity of the solids associated with the second peak. Although complete separation of the split peaks was not achieved in the μDSC analysis, the area ratio of the broad peak was higher at x Br = 0.6 than at x Br = 0.8. TBAC alone cannot form orthorhombic SCH, , and the melting temperature of the tetragonal mixed SCH increases with increasing Cl– ratio. From the DECT images of the orthorhombic mixed SCH (x Br = 0.8, x = 0.006) (Figure b and d, the volume fraction of the low-Br solid appears to be larger than that of the high-Br solid. As shown in Figure , the orthorhombic mixed SCH (x Br = 0.8, x = 0.006) became slightly smaller but retained a well-defined shape during the partial dissociation indicated by the first peak in the DSC curve (Figure c–e), whereas rapid dissociation and loss of shape were observed after the onset of the second peak (Figure f). Therefore, the orthorhombic mixed SCH with higher Cl– ratio (the second peak) appears to be associated with decreased crystallinity (i.e., metastable conditions), whereas the melting temperature of metastable mixed SCH may increase with increasing Cl– ratio.

The latent heat of SCH varies depending on the molecular weight of the guest molecules and hydration number. ,,, The hydration numbers of the orthorhombic mixed SCHs (n = 37–38) are the same or slightly lower than those of orthorhombic TBAB SCH (n = 38), as shown in Figure . Therefore, it is suggested that Δd H mass of the orthorhombic mixed SCHs varies depending on the Br– or Cl– composition. The Δd H mass of the orthorhombic mixed SCHs (227–231 J g–1) is higher than that of orthorhombic TBAB SCH (∼218 J g–1) (Figure ), and the melting temperatures of the orthorhombic mixed SCH are 283–285 K, which are approximately the same as that of orthorhombic TBAB SCH (283 K). In most cases, SCHs formed from a single guest molecule with a latent heat of ≥ 225 J g–1 have melting temperatures of ≥ 289 K,3 as exemplified by the following guest molecules: tributylisoamylphosphonium bromide (melting temperature: 289 K and latent heat: 225 J g–1),5 TBA fluoride (298–301 K and 223–245 J g–1), − and tetraisoamyl ammonium salts (303–308 K and 251–273 J g–1). ,, The orthorhombic TBAB + TBAC mixed SCHs presented in this study exhibit very high latent heats as PCMs, with melting temperatures of approximately 283–285 K, making them suitable for cold energy storage in air-conditioning systems operating below 289 K.

Conclusions

The compositional uniformity and thermal properties of TBAB + TBAC mixed SCHs were evaluated using DECT, digital microscopy, μDSC, and ion chromatography. TBAC and tetragonal mixed SCHs are homogeneous solids, whereas the orthorhombic mixed SCHs are heterogeneous solids with high- or low-Br content. One endothermic peak was observed in each DSC curve of the tetragonal mixed SCHs, whereas split peaks (a sharp first peak on the low-temperature side and a broad second peak on the high-temperature side) were observed for the orthorhombic congeners. Comparison of the SCH compositions determined using DECT and ion chromatography indicates that y Br for the orthorhombic mixed SCH is higher overall than that of the tetragonal SCH. From the DECT images of the orthorhombic mixed SCH, the volume of the low-Br solid appears to be larger than that of the high-Br solid. Microscopic observation shows that the particles of orthorhombic mixed SCH became slightly smaller but retained a well-defined shape during the dissociation of the first peak in the DSC curve, whereas rapid dissociation and loss of shape were observed after the onset of the second peak. These results suggest that the solid of second peak has higher Cl– ratio and low crystallinity (i.e., metastable conditions). The latent heat of SCHs varies depending on the molecular weight of the guest molecule and the hydration number. Because the hydration numbers of the orthorhombic mixed SCHs (n = 37–38) are the same or slightly lower than those of orthorhombic TBAB SCH (n = 38); therefore, Δd H mass of orthorhombic mixed SCH varies depending on the Br– or Cl– composition. The orthorhombic mixed SCHs have high latent heats (227–231 J g–1) for PCMs with a melting temperature of approximately 283–285 K, making them suitable for cold energy storage in air-conditioning systems operating below 289 K.

This study demonstrates that SCHs with higher latent heat can be obtained by combining two guest molecules and reducing the weight ratio of the guest in mixed SCHs while maintaining a high hydration number. By combining DECT observations with μDSC and other analytical methods, the relationships between the compositional uniformity of mixed SCHs and thermal properties, such as the dissociation behavior and latent heat, were clarified. These results indicate that combining known guest molecules is an approach that may be as effective as exploring new ones for achieving better control over the thermal properties of SCHs.

Acknowledgments

The authors thank Dr. H. Minagawa for their technical support.

Glossary

Abbreviations

SCH

semiclathrate hydrate

PCM

phase change material

μDSC

microdifferential scanning calorimetry

TBAB

tetra-n-butyl ammonium bromide

TBAC

tetra-n-butyl ammonium chloride

DECT

dual-energy X-ray computer tomography

TBP

tetra-n-butylphosphonium

n

hydration number

x

mole fractions of “guests to (guests + water)” in initial aqueous solution

Δd H mass

dissociation enthalpy per unit mass

x Br

Br– ratio of “Br– to (Br– + Cl–)” in initial aqueous solution

y Br

Br– ratio of “Br– to (Br– + Cl–)” in SCH

M.O.: conceptualization, data curation, formal analysis, investigation, methodology, resources, validation, visualization, and writing – original draft. S.T.: formal analysis, funding acquisition, investigation, methodology, validation, and writing – review and editing. A.Y.: data curation, formal analysis funding acquisition, methodology, resources, validation, visualization, and writing – review and editing. Y.J.: project administration, resources, supervision, and writing – review and editing. K.S.: project administration, resources, supervision, and writing – review and editing. J.N.: project administration, supervision, and writing – review and editing.

Synchrotron X-ray imaging was performed under Proposal No. 83–2509084P, as approved by the SAGA Light Source.

The authors declare no competing financial interest.

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