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. 2025 May 16;10(21):21767–21776. doi: 10.1021/acsomega.5c01418

Dust-Free Sol-Gel Synthesis of Neodymium Oxide Microspheres as a Surrogate for Americium-241 Fueled Radioisotope Power Systems

Jessica A Granger-Jones †, Sarah C Finkeldei †,‡,§,*
PMCID: PMC12138607  PMID: 40488089

Abstract

New designs for radioisotope power systems (RPS) explore the use of Am-241 as a fuel source. Neodymium is a common surrogate for americium and the work here presents a citrate-modified internal gelation route was applied to synthesize neodymium oxide microspheres, as a surrogate for americium oxide microspheres with applications in these new RPS designs. Neodymium has not previously been gelled on its own using the internal gelation route as the pH of neodymium hydrolysis and precipitation is higher than that achieved in the internal gelation process. However, the inclusion of citric acid as a precursor allows for the neodymium to deprotonate and coordinate to the citrate groups rather than hydrolyze, which results in the precipitation of a 1:1 neodymium citrate gel. Subsequent heat treatment to 950 °C under adequate air flow decomposes the residual organics and citrate groups resulting in the formation of the high temperature trigonal phase of Nd2O3. The work here details the chemistry involved in the citrate-modified internal gelation process as well as outlines methods for the optimization of microsphere fabrication. The sol-gel microspheres fabricated through this adapted synthesis method can be pelletized for use in RPS systems without the need for dust-producing steps, such as milling, thus providing the platform for the safer fabrication of RPS.

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Introduction

Radioisotope power systems (RPSs), primarily fueled by plutonium-238, have been used as a power source in several missions since early space exploration. , While solar power has dominated in providing power for space missions close to the sun, RPSs that use heat from radioisotope decay are an effective source of power offering more versatility to space exploration; RPSs operate independent of the environment and provide reliable power in conditions that render solar power impractical. − Heat and electricity independent from the sun is one of the major technological challenges for deep space missions where solar radiation is weak and lunar missions, which require functional infrastructure through the lunar night. Pu-238 used in these RPSs has to be specifically produced and the United States halted Pu-238 production in the 1980s prior to a recent restart. ,, While Pu-238 production has restarted for RPSs with the goal of producing 1.5 kg per year, the current rate of production is not enough to keep on schedule for planned space missions aiming to use RPSs. Americium-241 has been proposed as an alternative to Pu-238 in RPSs for deep space missions. − Am-241 offers the benefit that it is a byproduct in the fuel cycle, making it more economic both in terms of production and waste management. − Furthermore, the longer half-life of Am-241, 432 years, compared to the 87.7 year half-life of Pu-238, means that the power output does not diminish as quickly overtime relative to Pu-238, which offers benefits for longer space missions. These benefits make Am-241 an attractive replacement for Pu-238 despite the lower power density of Am-241. −

Americium oxide is the chemical form primarily explored for RPS applications. , An americium nitrate feed solution is expected from the process used to separate Am-241 from its parent, Pu-241, so synthesis techniques for americium oxide should target using americium nitrate as a starting material. , Oxalate precipitation from americium nitrate followed by thermal decomposition can produce americium oxide powders from the americium nitrate starting solution. ,, However, highly active isotopes, such as Am-241, require additional considerations when compared to more conventional materials and would benefit from a dust-free synthesis technique to reduce contamination and personnel inhalation or ingestion risks. Internal gelation is a desirable synthesis technique used to produce high quality, versatile metal oxide spheres from a feed solution containing a metal salt, typically in the form of a nitrate. Spheres fabricated directly from an aqueous feed solution are beneficial for materials such as americium oxide to reduce contamination risks and can be used as-is or pelletized to form dense pellets, without the need for milling or powder production.

Hexamethylenetetramine (HMTA)-urea based internal gelation is a sol-gel technique, wherein a temperature induced pH change results in the formation of a hydrous metal oxide gel. ,− The starting broth contains a metal salt (typically a nitrate), urea as a complexing agent, and HMTA as a gelation agent that undergoes protonation and decomposition upon heating to increase the pH of the system. ,− While internal gelation is applicable to a wide variety of metals, HMTA additionally acts as a buffer in the system, keeping the pH below about 6–7, which can limit applicability of the HMTA-urea internal gelation technique to metals that hydrolyze and precipitate in this pH range. ,,

This work focuses on the fabrication of neodymium oxide via a citrate-modified HMTA-urea internal gelation technique. Neodymium is a commonly used chemical surrogate for americium due to their chemical similarities and similar ionic radii of 1.161 and 1.157 Å for Nd­(III) and Am­(III) respectively. − To the best of our knowledge, neodymium oxide has not been synthesized previously using the HMTA-urea internal gelation process and studies available in the open literature only report neodymium being incorporated as a dopant. ,, In a nitrate environment, Nd­(III) precipitates at a pH of 7, which is higher than the pH typically reached in the internal gelation process. ,, Recent work revealed that fabrication through a citrate route can be used to successfully gel a starting broth containing Ce­(III), which precipitates at a pH of 8.1 in nitrate solutions, as well as a starting broth containing both Ce­(III) and Nd­(III). − The citrate route has demonstrated the ability to gel samples that precipitate at higher pHs, which suggests promising applications to Nd­(III) and eventually Am­(III). , This current work describes the technique and optimization of producing neodymium oxide microspheres using HMTA-urea internal gelation via a citrate route.

Experimental Section

Stock Solution Preparation

Nd­(NO3)3·6H2O (Sigma-Aldrich, 99.9%), citric acid (CA, Sigma-Aldrich, ≥99.5%), HMTA (Sigma-Aldrich, ≥99.5%), urea (Sigma-Aldrich, ≥99.5%), trichloroethylene (TCE, VWR, ≥99.5%), Span80 (Sigma-Aldrich, >60% GC), and 1-methoxy-2-propanol (PGME, Sigma-Aldrich, ≥99.5%) were used as received. A 2.3 M aqueous neodymium nitrate stock solution was prepared and solid citric acid (CA) was added to subsamples of this solution to reach the desired CA to Nd ratio (CA/Nd) as required for sample preparation; CA/Nd ratios of 0.7–1.1 were tested in the scope of this work. The neodymium nitrate-citric acid (Nd-CA) solutions were left for the citric acid to fully dissolve prior to use. A mixed HMTA-urea (H–U) aqueous stock solution was prepared with 3.18 M HMTA and 3.18 M urea.

Broth Preparation

The broth was formed by slowly adding precooled H–U to a chilled Nd-CA solution in an ice bath with stirring. Afterwards, the broth was left to chill and stir for at least 5 mins to obtain a clear starting broth prior to any gelation trials, broth stability tests, or microsphere preparation.

Gelation and Broth Stability Trials to Determine Suitable Broth Parameters

The R-value is defined as the molar ratio of HMTA or urea to the metal. The H–U starting broth used in this work has equal concentrations of HMTA and urea, so R HMTA = R urea and will simply be referred to as the R-value. Gelation trials were performed at 75 °C for CA/Nd = 0.7–1.1 in increments of 0.1 and R-values of 1.1–2.0, again in increments of 0.1. Gelation trial experiments were performed in triplicate to determine an average gelation time.

After chilled broth formation, the test tube was placed into a heated aluminum block. The broth was continuously stirred until full gelation occurred, defined as the time the solution turns opaque and the stir bar ceases movement. The sample was left in the heated aluminum block for an additional 10 mins to age. Subsequently, the gel strength was assessed by inserting a spatula and assigning a rigidity from very soft (2) to very hard (7, almost too hard to penetrate with a spatula). A gel strength of 1 correlated to incomplete gelation.

The stability of the precursor broths with the compositions that underwent gelation trials were tested at both room temperature and in an ice bath. Again, all trials were performed in triplicate to establish an average broth stability time. After chilled broth formation, test tubes containing the chilled broths were removed from the stir plate and left in an ice bath or in an empty beaker to expose the samples to ambient temperatures. The broths were checked periodically for signs of clouding and full gelation; both the time of initial clouding and the time of full gelation were recorded.

Microsphere Preparation

A schematic of the microsphere preparation process can be found in Figure S1. Chilled broths were prepared as previously described and 6 vol % Span80 in TCE was heated on a hot plate set to 75 °C. A syringe and 30G needle were used to introduce droplets of the chilled broth into the hot Span80/TCE to induce gelation. The gelled spheres were then aged in the heated Span80/TCE for 10 mins before draining Span80/TCE from the spheres. The spheres were subsequently washed once with TCE and twice with PGME. The TCE and first PGME wash were agitated for 5 mins on a jar mill (Labmill-8000) set to speed 1. The second PGME wash was left on the jar mill for 2 h. Spheres were then dried for at least 2 h in an oven set to 65 °C prior to calcining and sintering.

Microspheres were calcined and sintered under air flow using the following heating profile: (i) heat to 180 °C at 2 °C/min, (ii) heat to 230 °C at 250 °C/min, (iii) hold at 230 °C for 1 h, (iv) heat to 320 °C at 1 °C/min, (v) hold at 320 °C for 1 h, (vi) heat to 550 °C at 1 °C/min, (vii) hold at 550 °C for 30 mins, (viii) heat to 620 °C at 1 °C/min, (ix) heat to 950 °C at 2 °C/min, (x) hold at 950 °C for 2 h, (xi) cool to room temperature at a rate of 4 °C/min.

Yield Study

A percent yield study was performed gravimetrically on two separate samples, each prepared with 500 μL 2.3 M Nd­(NO3)3, CA/Nd = 1, and R = 1.4. The mass of the clean vial, stir bar, clean syringe, and needle were all weighed prior to, and after use to determine the mass of residual broth. Spheres were prepared as outlined previously. The final batch of microspheres were weighed after heat treatment to determine the final yield of Nd2O3 from the initial 500 μL 2.3 M Nd­(NO3)3 precursor.

Characterization

XRD analysis was carried out on a Proto Benchtop Powder X-ray diffractometer using Cu Kα1 radiation (λ = 1.54 Å) from 10–90° 2θ and a step size of 0.014°. FTIR spectra were collected on a JASCO FT/IR-4700 from 400–4000 cm–1 using an ATR PRO ONE attachment. SEM analysis was performed on a Tescan Mira 3. Spheres were sputter coated with a 10 nm layer of iridium using an EMS 150T Sputter Coater prior to microstructural imaging. Thermal analysis was performed on a Netzsch STA (DSC/TG) 449 F3 Jupiter from room temperature to 1000 °C with a heating rate of 5 °C/min and simulated air flow (20% O2, 80% N2). During heating, evolved gas analysis was performed using QMS (NETZSCH Aeolos QMS 403 D) and FT-IR (Bruker α II) operated at 200 °C. In situ pH measurements were taken on chilled broths prepared as detailed previously, which were then removed from the ice bath and left to sit at room temperature. The pH was measured using a Fisherbrand accumet glass body standard size combination electrode and an Orion4Star pH meter with measurements recorded every 30 s. The temperature was measured every second during the in situ pH measurement using a Digi-Sense Traceable Thermocouple Meter with an exposed wire probe. UV/Vis absorbance measurements were taken using a deuterium halogen light source, fiberoptic cables, a quartz cuvette with a 2 mm path length, and collected via an OceanOptics Jaz spectrometer. The integration time was set to 6 ms with 100 scans to average per measurement. The sphericity of sample microspheres was determined by rolling them down a stainless-steel pan held at a slight slope with a slit cut at one end. Nonspherical particles were separated from the spherical particles that rolled down and passed through the slit. A diagram of the setup is demonstrated in Figure S2. Of the particles that passed the rolling test, image analysis was performed on SEM measurements using ImageJ software to determine circularity through height to the width as well as the perimeter to the area of the spheres. The pour density of samples was determined by pouring a known mass of spheres into a 5 mL graduated cylinder and recording the volume. The skeletal density was determined via pycnometry measurements on a Micromeritics AccuPyc II Gas Pycnometer with a 1 cm3 insert, using helium gas with five purges and seven acquisitions taken per sample. The specific surface area (SSA) of microspheres was determined via BET analysis of a physical adsorption isotherm collected on a Micromeritics 3Flex using nitrogen gas. Samples were degassed prior to analysis by heating to 90 °C at 10 °C/min, holding for 60 mins and then heating to 350 °C at 10 °C/min, holding for 300 mins. The sample was automatically backfilled with nitrogen gas during the degas process. The physical adsorption isotherm was collected on a Micromeritics 3Flex Surface and Catalyst Analyzer using nitrogen gas absorption and the analysis conditions outlined in Table S1. During the analysis samples were held in a liquid nitrogen bath. Isotherm plots are available in Figure S3.

Results/Discussion

Determination of Reaction Pathway from Sol to Neodymium Oxide

In situ pH measurements (Figure ) and FTIR (Figures and ) were used to determine the reactions occurring in the solution to result in homogenous gelation. In the typical internal gelation process, the pertinent reactions are generally considered to be (1) complexation of the metal cation with urea upon chilling and decomplexation upon heating, (2) hydrolysis of the metal cation, (3) protonation of HMTA, and (4) decomposition of HMTA. A similar process was observed in the citrate-modified internal gelation process. While the broth is chilling, the reaction process is hindered. Previously published work with aqueous neodymium nitrate and urea at 90 °C concluded that neodymium does not complex to urea as there was no observed shift in the peaks in UV/Vis spectra and a decrease in absorbance of the peaks was attributed to volume expansion due to the addition of urea. In this work with chilled solutions, it was found that the absorbance peaks of neodymium nitrate around 580, 742, and 797 nm in the UV/Vis spectrum (Figure S4) seem to broaden, shift slightly to higher wavelengths, and increase in intensity in the presence of urea. It is expected that the neodymium is complexing with urea. Neodymium complexation with urea would hinder the reaction progress, which would explain why the pH is stable prior to removal from the ice bath. The initial solution pH of 2.252 is lower than the pK a1 of citric acid, so the citric acid is expected to exist as H3Cit and H2Cit–. The initial presence of protonated carboxylic acid groups in the citric acid is seen via FTIR analysis (Figure S5) as evidenced by the ν(COOH)as band around 1720 cm–1, which is expected to weaken and disappear upon deprotonation. , After removal from the ice bath, neodymium and urea decomplex freeing Nd3+ into the system to interact with the H3Cit and H2Cit–. At the chilled solution pH of 2.252, neodymium and citric acid are expected to exist with [NdHCit]+ as the dominant species, but also form [Nd­(H2Cit)]2+ and [NdCit] complexes. The dominant complex comprises of doubly deprotonated citrate groups, while the additional complexes contain singly and triply deprotonated citrate groups. This move towards doubly rather than singly deprotonated citrate groups being the dominant citrate species indicates that the Nd3+ interacts with the citrate groups, removing H+ during complexation. This releases H+ into the solution, resulting in the observed drop in pH for the first 10 mins after removal from the ice bath (Figure ). In situ FTIR measurements in Figure S5 reflect this deprotonation through a reduction in the band around 1720 cm–1 that correlates to protonated carboxylic acid groups. , This change is also highlighted by FTIR in the 1500–1700 cm–1 range (Figure ), where there is a decrease in the band at 1630 cm–1 and a growth in the band around 1560 cm–1 (assigned to ν (COO−)as). This change is expected as the predominant species shifts away from protonated citrate groups towards more deprotonated citrate groups. Deprotonation of the citrate groups frees up H+ to allow HMTA protonation and decomposition leading to the subsequent rise in pH that is observed in Figure after 10 mins. This is also highlighted in FTIR (Figure ), where there is an increase split in the band centered around 1020 cm–1, which corresponds to CN vibrations in HMTA. The band is initially asymmetric, but begins to split into two distinct bands, indicating an increase in inequivalent environments of CN groups in HMTA due to protonation of HMTA. HMTA decomposition occurring after protonation is suggested by the growth of a band at 1140 cm–1 (Figure ), correlated to NH deformation in aqueous ammonia, an HMTA decomposition product. , The neodymium-citrate complexes, upon the increase in pH, precipitate in the form of a neodymium citrate gel, while the HMTA continues to protonate and decompose, resulting in the continual increase in pH even after full gelation occurs. This chemistry is similar to the chemistry of the typical internal gelation process, but rather than hydrolyze, the Nd3+ complexes with citrate groups, releasing H+ into the system.

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Profile showing pH and temperature vs. time for a feed broth solution gelled at room temperature.

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In situ FTIR of gelation at room temperature in the 1460–1750 cm–1 range showing citrate deprotonation.

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In situ FTIR of gelation at room temperature in the 980–1060 and 1120–1160 cm–1 ranges indicating HMTA protonation and decomposition.

Upon gelation, the sample forms a [NdCit·xH2O] gel rather than [Nd2HCit3·xH2O], evidenced by FTIR (Figure S6). The FTIR is in agreement with previously reported FTIR spectra for 1:1 lanthanide­(III) citrates and lacks a band around 1720 cm–1, demonstrating that carboxylic acid groups in the citrate are deprotonated. ,,

TGA/DSC measurements (Figure ) of a neodymium citrate gel upon heat treatment present six distinct steps corresponding to species evolution upon thermal treatment. Coupled with in situ mass spectrometry and in situ FTIR analysis during the heating process in addition to FTIR of subsamples heated to temperatures of interest, the reaction pathway upon heat treatment is revealed, as outlined in Tables and .

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TGA/DSC measurements of a sample upon heating to 1000 °C.

1. Description of Decomposition Steps of the Neodymium Citrate Gel Sample upon Heating.

decomposition step and temperature description evolved gas FTIR features evolved gas mass spectrometry features
1 (<200 °C) loss of adsorbed water N/A m/z = 18 (H2O)
2 (∼225 °C) first stage of decomposition of HMTA and urea, releasing decomposition products (e.g., isocyanic acid, formaldehyde, CO2) 2252 cm–1 (isocyanic acid) , m/z = 43 (isocyanic acid)
1725 cm–1 (formaldehyde) , m/z = 30 (formaldehyde)
2359 cm–1 (CO2) , m/z = 44 (CO2)
3 (∼320 °C) second stage of decomposition of HMTA and urea (releasing NOx and CO2) as well as first stage of decomposition of citric acid (release of CO2 and H2O and formation of neodymium aconitate) lack of peak at 1725 cm–1 (no formaldehyde) , m/z = 30 (NO)
2359 cm–1 (CO2) , m/z = 46 (NO2)
m/z = 18 (H2O)
m/z = 44 (CO2)
4 (∼420 °C) and 5 (∼550 °C) decomposition of neodymium aconitate to form neodymium carbonate 2359 cm–1 (CO2) , m/z = 44 (CO2)
6 (>570 °C) conversion from neodymium carbonate to neodymium oxide    
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2. Proposed Species after Each Decomposition Step upon Heating.

  mass loss
  
decomposition step and temperature actual (%) theoretical (%) primary species present at end of decomposition step
1 (<200 °C) 14.51 N/A neodymium citrate, residual organics (e.g., HMTA, urea)
2 (∼225 °C) 22.95 N/A neodymium citrate, residual HMTA and urea decomposition products
3 (∼320 °C) 25.06 N/A neodymium aconitate (possibly some residual neodymium citrate)
4 (∼420 °C) and 5 (∼550 °C) 9.30 9.34 neodymium carbonate
6 (>570 °C) 7.01 7.90 neodymium oxide
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Firstly, the sample undergoes an initial mass loss when heating to 200 °C, attributed to loss of adsorbed water. This is reflected in mass spectrometry measurements, where m/z = 18 exhibits a peak below 200 °C (Figure S7).

The second step in the TGA/DSC occurs around 225 °C, where the sample undergoes significant and rapid mass loss, attributed to the first stage of decomposition of HMTA and urea. HMTA and urea are both reported to break down around these temperatures, and their mixture is reported to break down in a 2-step process around 200–300 °C. The first decomposition step of HMTA and urea is marked in the FTIR (Figure S8) and mass spectrometry (Figure S7) by the release of known HMTA and urea decomposition products described in Table , such as isocyanic acid, formaldehyde and CO2. −

The third stage of the TGA/DSC evolution corresponds to the second step of the decomposition of HMTA and urea around 320 °C and the first stage of citric acid decomposition, which is reported to occur in this temperature regime. ,,, This decomposition step is accompanied by the release of CO2 and water, which is in agreement with reported citrate decomposition, as well as the release of NOx, which indicates the second stage of HMTA-urea decomposition. ,,, NOx is difficult to deconvolute in the FTIR and mass spectrometry spectra due to signal overlaps and signals occurring in noisy regions of the FTIR spectra. There is a mass spectrometry peak around 320 °C for m/z = 30, which could correspond to NO or formaldehyde. However, the FTIR lacks a formaldehyde absorbance peak at this temperature, suggesting the release of NO rather than formaldehyde. , There is also a mass spectrometry peak around 320 °C corresponding to NO2, which implies the release of additional NOx. The release of NOx indicates the decomposition of nitrate bearing species from HMTA and urea, in agreement with the literature as to the mechanism and temperature regime of HMTA and urea decomposition. ,

After full HMTA and urea decomposition and the first step of citric acid decomposition, the sample continues to decompose in three additional stages occurring at 420, 550, and above 570 °C. These decomposition steps are marked by the release of CO2 (m/z = 44, absorbance at 2359 cm–1) and are in agreement with previously reported breakdowns of 1:1 lanthanide­(III) citrates. , Based on mass loss as described in Table , it is expected that prior to decomposition at 420 °C, the product is neodymium aconitate. Previously published works on metal citrate decomposition have hypothesized the formation of itaconates and aconitates, but the mass loss observed in this work better matches neodymium aconitate. , Aconitate formation is supported by the FTIR of subsamples heated to 400 and 455 °C (Figure S9), where bands around 1555 and 1395 cm–1 correlate to bound carboxylic groups, which could be from aconitate. There is an additional peak around 1300 cm–1 corresponding to an alcohol group, indicating the presence of some residual citrate groups. However, this peak is more prominent in the sample heated to 400 °C than 455 °C, demonstrating the gradual loss and breakdown of citrate groups.

The exothermic peak in the TGA/DSC (Figure ) around 530 °C is attributed to the oxidation of the aconitate species to form neodymium carbonate. , This is supported by the mass loss presented in Table as well as FTIR analysis of a subsample heated to 570 °C (Figure S10), where the spectrum agrees with the formation of Nd2(CO3)3. − This carbonate then converts to the oxide upon further heating, resulting in the gradual mass loss at temperatures above 570 °C, which is in agreement with mass loss calculations and previously reported thermal analysis of 1:1 lanthanide­(III) citrates.

Upon sintering for 2 h at 950 °C, the Nd-cit gel is fully converted to Nd2O3 in its high temperature trigonal phase, as indicated in Figure .

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XRD of heat-treated sample reveals formation of A-Nd2O3. Reference reflexes are from the Crystallography Open Database card 1523967..

Identification of Suitable Broth Parameters

The R-values and CA/Nd most promising for microsphere preparation were determined through a combination of systematic gelation and broth stability trials. Gelation time and gel strength are important for microsphere fabrication to ensure that spheres gel in a suitable timeframe and are robust enough to resist deformation during subsequent washing and drying processes. Extensive gelation trials altering CA/Nd (0.7–1.1) as well as the R-value (1.1–2.0) were performed at 75 °C, the results of which are displayed in Figure . This temperature offers versatility and can be used with a range of forming fluids, including TCE/Span80, rather than just with silicone oil, which has been the historically favored forming fluid for sol-gel processes. At 75 °C, in general, increasing the R-value and decreasing the CA/Nd tended to reduce gelation time. Gelation time likely decreases with increasing R-value due to an increase in the initial pH as well as the availability of more HMTA to protonate and decompose to further increase the pH. The gelation time likely decreases with decreasing CA/Nd due to less acid in the system leading to a higher pH, meaning that less HMTA must protonate and decompose after neodymium complexes with the citrate groups to attain the pH required for solid gelation. Gel strength tended to be highest for CA/Nd = 0.9 with lower R-values, CA/Nd = 1 with intermediate R-values, and CA/Nd = 1.1 with high R-values. The trend with gel strength is more nuanced and is postulated to be an interplay between attaining a high enough pH to form a strong gel, having sufficient citric acid in the system to coordinate with the neodymium to form a strong gel, and avoid adding too much HMTA, urea and/or water that can essentially dilute and weaken the gel. This interplay of various factors would explain why there is not always an increase in gel strength with ever increasing CA/Nd or R-values.

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Colormap showing the impact of the R-value and CA/Nd on the (a) gelation time and (b) gel hardness when gelled at 75 °C. Samples with R = 1.9 and 2.0 with CA/Nd = 0.7 did not form a stable broth and gelled prematurely.

The broth stability is of interest to prevent premature gelation prior to sphere formation and is particularly important when utilizing a microfluidic approach to microsphere fabrication. A microfluidic approach could be beneficial for sphere fabrication for RPSs because this technique allows for smaller sphere fabrication, while remaining dust-free and produces microspheres with a narrower particle size distribution. , This precise control over particle size distribution during synthesis is beneficial during the engineering of RPS to allow for the flexibility to select a specific dispersity of interest for a given design chosen by the end user. Furthermore, this narrow size distribution is beneficial when applying the process to highly radioactive isotopes, such as Am-241, to ensure the avoidance of particulate formation. The broth stability at both room temperature and in an ice bath (Figure ) increases with increasing citric acid content and decreasing R-value. The higher citric acid content likely improves broth stability through decreasing the pH. The decreasing R-value can improve the stability of the broth both through lowering the initial pH and through the presence of less HMTA to undergo protonation and decomposition, which slows down the rate of pH increase.

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Colormap showing impact of R-value and CA/Nd on the broth stability at both (a) in an ice bath and (b) at room temperature. Here, broth stability was the time at which a broth remained clear prior to clouding.

There is a balance that needs to be found between gelation time, gel strength, and broth stability. Through a combined assessment of these variables, the broth parameters presented in Table were deemed most promising and selected for microsphere fabrication.

3. Parameters Deemed Most Suitable for Microsphere Fabrication from Gelation and Broth Stability Trials.

suitable CA/Nd suitable R-values
0.9 1.1 1.2 1.3 1.4
1 1.3 1.4 1.5
1.1 1.5 1.6 1.7
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Preparation of Microspheres

Nd2O3 microspheres were successfully fabricated via internal gelation using a citrate-modified internal gelation approach, as evident in Figure a. The percent yield study (Table S2) revealed that loss occurs during the initial sphere forming step from residual broth in the vial and syringe, rather than from subsequent processing steps. This is promising for future applications, as this percent loss could be reduced through changing the sphere forming process (e.g., utilizing a microfluidic approach) or increasing batch size. Percent yield could only be determined for the final product due to the presence of residual organics and water in intermediate processing steps.

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SEM image of (a) a sphere and (b) its microstructure, fabricated with CA/Nd = 1 and R = 1.4.

Intact spheres were fabricated and sphericity was retained during washing, drying, and heat treatment when appropriate parameters were used. Samples prepared with higher CA/Nd tended to coalesce more upon sphere fabrication. However, for all parameters identified in Table , >82% of spheres passed the roll-sphericity assessment after washing and drying, indicating good sphericity and prevention of sphere coagulation. The tendency of higher CA/Nd values leading to more coalescence may indicate that a different forming fluid should be used to avoid merging, such as including more surfactant in the TCE through increasing Span80 content, and could be due to some supernatant formation; these gels appeared moister during gelation trials.

The samples that retained roll sphericity the best upon sintering were CA/Nd = 1 with R = 1.4, CA/Nd = 0.9 with R = 1.1 and 1.2 all of which had roll sphericity >98% after heat treatment. Of the compositions with good roll sphericity, the highest degree of circularity was achieved with CA/Nd = 1 with R = 1.4 and CA/Nd = 0.9 with R = 1.2. These compositions had an average circularity >0.93, where 0 is non-circular and 1 is a perfect circle. In general, the lowest R-values tested for CA/Nd = 0.9 and 1 had lower circularity values than the intermediate R-values, possibly due to increased deformation resulting from forming softer gels. Higher R-values tended to suffer from a high degree of cracking upon heat treatment, which could be due to increased residual organics in the system decomposition during heat treatment. This may indicate that a more rigorous wash process or even slower heating rate is required for samples containing larger quantities of organics.

In internal gelation, sphericity retention and avoidance of cracking upon heating is often a function of the wash process and heat treatment steps. In the TGA (Figure ) it is evident that a highly exothermic HMTA+urea decomposition reaction occurs around 250 °C, which is often deemed to be responsible for catastrophic cracking that can occur with spheres fabricated via internal gelation. The wash process and heating profile were found to be important for spheres fabricated using a citrate-modified internal gelation approach. However, it was additionally found that for spheres fabricated via internal gelation with a citric acid precursor, the flow rate of air was an important parameter to tune to retain sphericity upon sintering. For CA/Nd = 1 and R = 1.4, when the batch size was 500 mL Nd­(NO3)3(aq), 500 mL/min was sufficient to attain roll sphericity >99%. However, when the batch size was increased to 2.5 mL Nd­(NO3)3(aq)′ a flow rate of 1000 mL/min was not adequate to achieve roll sphericity ≥99%. For a batch size of 3.5 mL Nd­(NO3)3(aq), 2000 mL/min was found to be sufficient to attain roll sphericity ≥99%.

It is expected that the catastrophic cracking dependence on the air flow rate and batch size is due to the addition of citric acid in the internal gelation process. In the typical internal gelation process, a metal oxide, hydroxide, or oxyhydroxide gel is formed, which means there is a stoichiometric quantity or an excess of oxygen in the system relative to the oxide. , Oxygen provided by the air flow is thus utilized to burn off residual organics in the system, but is not required for conversion of the gel to the final oxide. On the other hand, in the citrate gelation method, a metal citrate gel is formed. As evidenced by the analyses performed of the sample upon heating, a series of decomposition reactions occur that all utilize oxygen. It is anticipated that oxygen provided by the air flow is used to burn off the residual organics, decompose the citrate, and provide oxygen to allow for carbonate and subsequent oxide formation. The suspected importance of oxygen content is reflected in the ability of the sample to retain sphericity and resist catastrophic cracking upon sintering. Thus, it is anticipated that the larger batch sizes necessitate higher air flow to provide more oxygen to the system and allow the reactions to occur during the slow heating profile. It is possible that any gas flow could help reduce cracking; however, it is suspected that higher oxygen content is required to result in combustion rather than pyrolysis, thus lower flow rates could possibly be used in higher oxygen gas flow environments.

Figure b highlights the porous, open network observed in the microstructures of the samples. This open microstructure is in agreement with the measured densities and high SSA compared to the theoretical SSA of 0.001 m2/g for solid neodymium oxide spheres (Table ). For all samples, the pour density was around 20% of the theoretical density, while the skeletal density was consistently >95% of the theoretical density, indicating a high level of open porosity and a low degree of closed porosity. The open pore network could be beneficial for RPS applications, allowing for He venting and accommodation as well as preventing pressure buildup in the sample. ,, If high density samples are desired, the low pour density of the spheres could be beneficial, as past pellet pressing studies found that lower density spheres actually produced higher density pellets. ,

4. Densities and SSA of Microsphere Samples.

sample parameters
 density (%TD)
  
CA/Nd R pour density skeletal density SSA (m2/g)
0.9 1.1 22.3 ± 1.6 99.6 ± 0.2 4.9140 ± 0.081
0.9 1.2 20.2 ± 1.3 99.5 ± 0.1 6.1692 ± 0.0910
1 1.4 18.2 ± 1.1 95.7 ± 0.6 5.4252 ± 0.0422
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Conclusions

Microspheres of neodymium oxide were successfully fabricated via a citrate-modified internal gelation method. In situ analysis of the sample upon gelation revealed that, when citric acid is included as a precursor, the neodymium deprotonates and coordinates to citrate groups rather than hydrolyzing as would be expected in the typical HMTA-urea based internal gelation process. This results in the precipitation of a 1:1 neodymium citrate gel. In situ analysis and analysis of subsamples upon heating revealed that six distinct decomposition steps occur: (1) loss of water, (2) first step of decomposition of HMTA and urea, (3) second step of decomposition of HMTA and urea coupled with the first step of citric acid decomposition to produce neodymium aconitate, (4 + 5) two step decomposition of neodymium aconitate to form neodymium carbonate, (6) conversion of neodymium carbonate to neodymium oxide. The addition of citric acid as a precursor facilitated a different reaction pathway than the classical sol-gel route and thus enabled the synthesis of neodymium oxide microspheres via internal gelation. Gelation trials and broth stability trials were performed to select promising parameters for microsphere synthesis. Microspheres were fabricated and assessed for sphericity retention, further honing in on optimal parameters. In addition to initial broth composition, the assessment of species evolution upon heating revealed the importance of air flow during sintering and heat treatment, allowing for the fabrication of crack-free neodymium oxide microspheres. The final microspheres were found to have a high degree of open porosity, which could be beneficial for RPS applications either for the venting and accommodating of He produced or for producing denser pelletized samples via the sol-gel microsphere pelletization process, as has been explored in other studies. This work lays the foundation for the synthesis of americium oxide microspheres for RPS applications as well as for the sol-gel synthesis of oxides of other metals that are not able to be synthesized following the traditional internal gelation route. This adapted procedure is a first step towards dust-free americium oxide microsphere fabrication, replacing the neodymium nitrate solution in this study with americium nitrate; however, additional studies looking into the effects of radiolysis on the internal gelation process are pertinent for applicability to highly active nuclides.

Supplementary Material

ao5c01418_si_001.pdf (475.2KB, pdf)

Acknowledgments

The authors would like to acknowledge and thank Dmitry Fishman for access to and training on the FTIR instrument at the UCI Laser Spectroscopy Laboratories, Xiaofeng Liu for access to and training on the STA, pycnometry, and physical adsorption instruments at the TEMPR facility at IMRI, Shen Dillon for access to a furnace and the jar mill, John Proctor for sputter coating samples, and Justin Mulvey for guidance on use of ImageJ software. This work was funded by Zeno Power Systems, Inc.

Glossary

Abbreviations

RPS

radioisotope power system

HMTA

hexamethylenetetramine

CA

citric acid

TCE

trichloroethylene

PGME

propylene glycol methyl ether

Nd-CA

mixed neodymium nitrate citric acid solution

H–U

mixed HMTA and urea solution

Cit

triply deprotonated citric acid

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c01418.

  • Additional data plots and spectra (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This work was funded by Zeno Power Systems, Inc. under agreement number ZPS-226512.

The authors declare no competing financial interest.

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