Revealing uranium tetrafluoride microrods

Abstract

Anhydrous and hydrated UF₄ microrods (5–25 μm) were prepared from the reactions of UO₂ microrods (5–15 μm) with HF(g), produced from the decomposition of silver bifluoride (AgHF₂, SBF). In order to optimize the preparation of UF₄ mr, several experimental parameters including atmosphere (air or N₂), temperature (150 or 250 °C) and amount of SBF were evaluated. In all reactions, rodlike morphologies were retained. At 250 °C, the reaction products always consist of an anhydrous UF₄/hydrated UF₄ mixture, while at 150 °C only hydrated UF₄ was detected. Anhydrous UF₄ microrods were obtained by dehydration of the anhydrous UF₄/hydated UF₄ mixture using TGA-DSC. Changing the atmosphere from air to N₂ or reducing the amount of SBF by half did not affect the nature of the reaction products.

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Introduction

At the nano- and microscale, materials can exhibit properties (e.g., optical, catalytic, electronic, mechanical, thermal, magnetic)^1–3 that are not observed at the macroscale. The common applications of micro- and nano- materials (e.g. gas sensors, electrochromic devices, solar cells, batteries) are shared between many metal oxides (e.g. Ti,⁴ Co,^1,5,6 Ni,^2,7 Zn,^8–10 Mo,^11–15 and In^16,17). The rod-morphology is one of the most studied and examples of microrods and nanorods respectively include In_1−xGa_xP,¹⁸ MnOOH,¹⁹ Bi₂S₃,²⁰ and Ga₂O₃.²¹

One element whose material chemistry of micro- and nanorods has been poorly studied is uranium. Research on uranium has primarily focused on the study of spherical particles of binary oxides, nitrides, carbides, and fluorides.^22–28 Uranium microstructures can find applications as targets for medical isotope production,^29–31 fuels for nuclear reactors,^32–34 standards for nuclear forensics,^35–37 and energy sources for space exploration.³⁸ Morphological studies of uranium materials are also relevant to the field of nuclear forensics.^39,40 One critical material for the nuclear industry is UF₄, which is produced from the reaction of UO₂ with HF gas at elevated temperatures (eqn (1)).

UO₂(s) + 4HF(g) → UF₄(s) + 2H₂O(g) (1)

Besides serving as an intermediate in UF₆ production,^41,42 UF₄ has also found applications as targets for heavy ion production⁴³ and the primary precursor material for U metal production.⁴⁴ It is also proposed as a fuel for molten salt reactors.⁴⁵ Anhydrous UF₄, a green solid with low solubility in water,⁴⁶ exhibits several hydrated forms (i.e., UF₄·xH₂O, x = 0.5, 0.7, 0.75, 1.2, 1.3, 1.5, 2.0, 2.5)^47–49 which are formed upon reactions of the material with water (eqn (2)).

UF₄(s) + xH₂O(g) → UF₄·xH₂O(s) (2)

As water is ubiquitous in the uranium industry, it is important to address environmental and industrial concerns pertaining to the hydrolytic behaviors of UF₄.⁴⁸ Though the physico-chemical properties of UF₄ and its hydrates are well characterized at the macroscale,^47–64 there is a lack of knowledge concerning their preparations and characterizations at the microscale. As the applications of UF₄ expand, it is essential that information and accessibility to UF₄ materials at the microscale become more readily available.

So far, the only UF₄ morphology that can be prepared and controlled at the microscale is the spherical one. UF₄ microspheres (ms) have already been produced and are commercially available,^27,49,54,65 and although details on their production remain proprietary, high temperature hydrofluorination of UO₂ was mentioned.⁶⁶ Other UF₄ morphologies such as microrods (mr) or microplates (mp) have not yet been prepared in a controlled manner.

Previously, we have reported on the preparation of UO₂F₂ microspheres, microrods and microplates using chemical transformation.^28,67 In these works, uranium oxide micromaterials (UO₃, U₃O₈) were fluorinated in an autoclave with HF(g) produced from the thermal decomposition of silver bifluoride (AgHF₂, SBF). Scanning electron microscope (SEM) results concerning the morphology and particle size distribution of UO₂F₂ mp have shown high variance, whereas SEM results relating to UO₂F₂ mr have been consistent. As a continuation, investigating U(IV) fluoride micromaterials was a natural progression and microrods were selected as the primary microstructure.

Here, we report on the preparation of anhydrous and hydrated UF₄ microrods. The materials were prepared by chemical transformation from the reaction of UO₂ mr with SBF in autoclaves and were characterized by scanning electron microscopy and powder X-ray diffraction (PXRD).

Experimental

Caution! Uranium-238 is an α emitter (E_max = 4.26 MeV). All manipulations were performed in a designed radiochemistry laboratory equipped with HEPA filter hoods and by following approved radioisotope handling and monitoring procedures.

Materials and methods

Silver bifluoride (≥99%, Alfa Aesar), glycerol (≥99.5%, Sigma-Aldrich), and urea (≥98%, Sigma-Aldrich) were used as received. UO₂(NO₃)₂·6H₂O (UNH) was prepared from the treatment of uranium metal dissolved in hot nitric acid followed by recrystallization. Fluorination and hydrothermal reactions were conducted within a Parr model 4749 autoclave placed in a Thermo Scientific Thermolyne Benchtop muffle furnace (model FB1315M). Fluorinations were conducted at 150–250 °C for 6–24 hours in the setup reported previously.⁶⁷ For dry reactions, N₂ was regarded as inert as it does not react with reactants at these temperatures. HF(g), the gaseous decomposition product of SBF, provides the source of fluorine for the reactions. In these reactions, SBF in excess molar quantity was placed on the Teflon liner of the vessel, while the uranium oxide material was placed in a 15 mL Teflon vial above the SBF.⁶⁷

Powder X-ray diffraction measurements were performed at room temperature on a Bruker D8 Advanced diffractometer equipped with Cu Kα X-rays (λ = 1.5406 Å) and a solid-state Si detector. Imaging was performed on the JEOL Tescan CLARA field emission scanning electron microscope, and samples were mounted on carbon tape without coating. TGA-DSC measurements were conducted with a TA instruments SDT 650 Discovery series TGA-DSC from 50–600 °C with a heating rate of 10 °C min⁻¹. The measurements were performed in alumina crucibles under flowing argon gas with a sample and balance flow rate of approximately 100 mL min⁻¹. Particle sizes were measured using ImageJ, and particle size distribution figures were generated using SciDAVis.

Sample preparation

Preparation of UO₂ mr. 3UO₃·NH₃·5H₂O mr (234.1 mg, 0.244 mmol), prepared using the reported method,⁶⁸ was placed in an alumina boat and treated at 600 °C for 3 hours under air. The resulting U₃O₈ product (172.3 mg, 0.205 mmol) was then heated to 600 °C for 5 hours under flowing 5% H₂/95% Ar gas.⁶⁹ The resulting UO₂ mr (159 mg, 0.589 mmol, 80.6% yield from 3UO₃·NH₃·5H₂O) was characterized by SEM (Fig. 1) and PXRD (Fig. S1, ESI†) (Table 1).

Table 1 Experimental conditions and reaction products for the fluorination of UO₂ mr with SBF

Reaction #	Mass of UO2 (mg)	Mass of SBF (mg)	T (°C)	Time (h)	Atmosphere	Yield (mg, %)	Reaction product
1	50.8	303.9	250	24	Air	57.9 (98.0%)	UF4/UF4·2H2O
2	30.1	400.0	250	24	N2	29.5 (82.3%)	UF4/UF4·xH2O
3	31.5	301.6	150	24	Air	35.0 (86.0%)	UF4·1.5H2O
4	33.8	155.8	250	24	Air	35.2 (89.6%)	UF4/UF4·2H2O

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Fig. 1 SEM images of UO₂ mr at (a) 4790× and (b) 6640×.

Results and discussion

Here, the reactions were set up identically to previous UO₂F₂ microrod works except that the starting material (i.e., U₃O₈ mr) was replaced with UO₂ mr.

Various experimental parameters have been evaluated (atmosphere, temperature, amount of SBF). A total of four reactions were investigated. In each reaction, UO₂ was weighed and placed in a Teflon vial and then placed in the Teflon liner of the autoclave containing SBF. The autoclave was sealed either in air (reaction 1, 3, and 4) or under N₂ atmosphere (reaction 2) and thermally treated for 24 hours at 150 or 250 °C. Following the reaction, the autoclave was cooled for 2 hours to room temperature and opened, and the resulting product was weighed and characterized by PXRD and SEM. The conditions, reaction products, and yields for the four reactions are presented in Table 1, and the effects of atmosphere, temperature, and amount of SBF on the nature of the reaction products are discussed in the following sections.

Baseline reaction

Reaction 1 follows the same procedure as the one applied for the preparation of UO₂F₂ (24 hours at 250 °C using ∼300 mg SBF [U:HF = ∼3–5] prepared in air). Here the reaction of UO₂ and HF(g) led to a mixture of anhydrous and hydrated UF₄ (Fig. S2a, ESI†). The presence of hydrated UF₄ in the reaction product is probably due to the reaction of UF₄ and water (eqn (2)) that was formed as a byproduct (eqn (1)).

Morphologically, the rodlike particles remained intact and experienced some visible surface roughing (Fig. 2a–c). Particle size distribution (PSD) analysis (Fig. 3) shows the average length of these particles have increased by ∼7 μm.

Fig. 2 SEM images of the reaction products of reaction 1 at (a) 2000×, (b) 4450×, (c) 6650×, and (d) after TGA-DSC at 2650×.

Fig. 3 Particle size distribution of UO₂ mr and particles from reaction 1, 3, and 4.

Treatment of the reaction product (16.94 mg) by TGA-DSC up to 600 °C (ramp rate of 10 °C min⁻¹) under argon converted the UF₄/UF₄·2H₂O mixture to anhydrous UF₄. The TGA-DSC curves (Fig. S3, ESI†) show a steady mass decrease followed by a plateau at ∼380 °C, indicating the point of complete dehydration to anhydrous UF₄. Following TGA-DSC, the sample was characterized by SEM (Fig. 2d) and PXRD (Fig. S2b, ESI†). SEM analysis shows no morphological changes to the particles, and PSD analysis showed a decrease in average particle length by ∼4 μm. The dehydration of UF₄/UF₄ hydrate proved efficient for the preparation of anhydrous UF₄ mr and could be transposed to other morphologies of UF₄ hydrates.

Effect of atmosphere

The effect of atmosphere on the reaction product was investigated in reaction 2. Treatment of UO₂ with HF(g) under N₂ atmosphere at 250 °C for 24 hours yielded a mixture of anhydrous UF₄ and hydrated UF₄ (PXRD in Fig. S2c, ESI†). SEM shows the rods to exhibit rough surfaces that were not observed in the UO₂ mr precursor (Fig. 4), and PXRD shows less hydrated UF₄ than from the one from reaction 1 (Fig. S2c, ESI†).

Fig. 4 SEM images of the reaction products of reaction 2 at (a) 1260× and (b) 7360×.

Effect of temperature

Reaction 3 was performed at 150 °C, while the other parameters (atmosphere, reaction time, and amount of SBF) were identical to the baseline reaction. Following the reaction, PXRD analysis (Fig. S2d, ESI†) shows the presence of UF₄·1.5H₂O as a single phase.

SEM analysis (Fig. 5a and b) shows the UF₄·1.5H₂O mr to exhibit smoother surfaces than the reaction products from reactions 1 and 2. The particle size distributions (Fig. 3) showed no size disparities between UF₄·1.5H₂O mr and that of UF₄/UF₄·2H₂O mr (reaction 1).

Fig. 5 SEM images of the reaction products of reaction 3 at (a) 1950× and (b) 3510×, and reaction 4 at (c) 3080×.

The absence of anhydrous UF₄ at 150 °C indicated that the hydrate was initially formed near this temperature and that the increase in temperature to 250 °C would initiate dehydration leading to the partially hydrated mixture. We hypothesized that the reaction at 400 °C should lead exclusively to anhydrous UF₄.

Effect of SBF

In reaction 4, about half the amount of AgHF₂ was used (155.8 mg, 1.061 mmol). Calculations indicated that at 250 °C, decreasing the amount of SBF by half would decrease the partial pressure of HF(g) in the autoclave from ∼4 atm to ∼2 atm (∼5 atm to ∼3 atm total pressure). PXRD analysis showed the presence of a UF₄/UF₄·2H₂O mixture (Fig. S2e, ESI†) while SEM (Fig. 5c) indicated the presence of microrods. PSD observations (Fig. 4) were consistent with the results of reactions 1 and 3. Overall, decreasing the amount of SBF does not fundamentally change the nature of the reaction products as anhydrous UF₄ and hydrated UF₄ were obtained.

Conclusions

For the first time, uranium tetrafluoride microrods were prepared by chemical transformation. The reactions of UO₂ microrods (5–15 μm) with HF(g), produced from the decomposition of SBF, were investigated in autoclaves. Several experimental parameters including atmosphere, temperature, and amount of SBF were tested. In all reactions, rodlike morphologies were retained. At 250 °C, the reaction products always consist of an anhydrous UF₄/hydrated UF₄ mixture, while at 150 °C only the presence of hydrated UF₄ was detected. Anhydrous UF₄ microrods were obtained by dehydration of the anhydrous UF₄/hydrated UF₄ mixture via thermal treatment using TGA-DSC. Changing the atmosphere from air to N₂ or reducing the amount of SBF by half did not fundamentally affect the nature of the reaction products. Using experimental set-up to 250 °C for the highest operational temperature indicated that the preparation of single phase hydrated UF₄ or anhydrous UF₄ is respectively a one-step (hydrofluorination) and two-step process (hydrofluorination and dehydration). The preparation of anhydrous UF₄ in a single step process would require hydrofluorination in an autoclave at T > 250 °C.

Currently, the preparation of UF₄ nanospheres and microplates using the method presented here is under progress and results will be reported in due course. Finally, successful fluorinations of uranium oxide micromaterials will lay the groundwork for the development of other f-element fluoride micromaterials.

Author contributions

The manuscript was written through contributions of all authors. H. J.: investigation, methodology, validation, visualization, and writing – original draft. F. P.: conceptualization, funding acquisition, project administration, resources, supervision, and writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This material is based upon work performed under the auspices of the Consortium on Nuclear Security Technologies (CONNECT) supported by the Department of Energy/National Nuclear Security Administration under Award Number(s) DE-NA0003948. The authors would like to thank Mrs Wendee Johns for administrative support and Mr Quinn Summerfield for laboratory support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00796d

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