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Characterization of the Aerosol-Based Synthesis of Uranium Particles as a Potential Reference Material for Microanalytical Methods R. Middendorp,† M. Dürr,*,† A. Knott,†,‡ F. Pointurier,§ D. Ferreira Sanchez,∥ V. Samson,∥ and D. Grolimund∥ †

Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research - Nuclear Waste Management and Reactor Safety (IEK-6), D-52428 Jülich, Germany § CEA, DAM, DIF, F-91297 Arpajon, France ∥ Swiss Light Source, Paul-Scherrer-Institute, CH-5232 Villigen, Switzerland S Supporting Information *

ABSTRACT: A process for production of micrometer-sized particles composed of uranium oxide using aerosol spray pyrolysis is characterized with respect to the various production parameters. The aerosol is generated using a vibrating orifice aerosol generator providing monodisperse droplets, which are oxidized in a subsequent heat treatment. The final particles are characterized with microanalytical methods to determine size, shape, internal morphology, and chemical and structural properties in order to assess the suitability of the produced particles as a reference material for microanalytical methods, in particular, for mass spectrometry. It is demonstrated that physicochemical processes during particle formation and the heat treatment to chemically transform particles into an oxide strongly influence the particle shape and the internal morphology. Synchrotron μ-X-ray based techniques combined with μ-Raman spectroscopy have been applied to demonstrate that the obtained microparticles consist of a triuranium octoxide phase. Our studies demonstrate that the process is capable of delivering spherical particles with determined uniform size and elemental as well as chemical composition. The particles therefore represent a suitable base material to fulfill the homogeneity and stability requirements of a reference material for microanalytical methods applied in, for example, international safeguards or nuclear forensics. nalysis of trace amounts of material used in nuclear fission (uranium, plutonium) has various applications in the area of environmental studies and in the context of verifying the compliance with international treaties, as practiced in the application of safeguards by the International Atomic Energy Agency (IAEA). A main driver in the development of analytical techniques is to increase the sensitivity and the selectivity in the detection and quantification of trace amounts of uranium and plutonium. In the current state of the art, particle analysis on environmental swipe samples has progressed beyond analysis of the 235U/238U isotope ratios of individual particles as an indicator for nuclear enrichment processes, but now also includes the detection of the minor isotopes 234U and 236U at challenging abundances in the 10−6 to 10−3 level. A recent overview discusses the simultaneous analysis of the four isotopes 234U, 235U, 236U, and 238U on micrometer-sized particles containing picogram amounts of uranium using advanced mass spectrometric methods.1 The implementation of quality assurance measures requires suitable and characterized materials, which, however, for uranium and plutonium microparticles are not readily available.2 Therefore, several approaches to synthesize particulate reference standards for uranium and plutonium particle trace analysis were explored.3−5 A method based on the controlled hydrolysis of UF6 certified reference materials (CRMs) was used to form microparticles that match the chemical composition and size distribution of particles usually

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© XXXX American Chemical Society

found in real environmental swipe samples that are collected for nuclear safeguards purposes.5 Synthetic particles generated with this method were used in a number of interlaboratory comparisons organized for uranium particle analysis methods.6 Another method is based on addition of uranium with certified isotopic composition to a glass melt, which is cooled and ground to yield uranium bearing glass beads with sizes of a few micrometers.7 Particle generation using spray pyrolysis of a monodisperse aerosol, in particular, allows generation of particles representing identical test objects of equal size and with specific elemental content, as these particle properties are determined by the isotopic composition and the concentration of the solute in the input feed solution used to generate the aerosol.4 The quantification of picogram amounts of uranium in synthetic particles produced with spray pyrolysis has been demonstrated using isotope dilution (ID), thermal ionization mass spectrometry (TIMS): few tens of particles were analyzed after dissolution in nitric acid8 and single particle measurements were performed on particles directly loaded onto the TIMSfilament.9 The latter measured a discrepancy of the uranium content per particle between two particle production runs, Received: February 20, 2017 Accepted: March 27, 2017 Published: March 27, 2017 A

DOI: 10.1021/acs.analchem.7b00631 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry which was attributed to an irregular particle morphology obtained for certain particle production settings, which leads to generation of voids.9 In case of particle analysis, repeatability, and reproducibility conditions can only be established when an ensemble of test objects with uniform properties is available. For a particle reference material it is therefore necessary to establish a wellcontrolled and reproducible particle production. In this paper, we present a thorough characterization of a newly established particle production setup based on spray pyrolysis of a monodisperse aerosol. Three different uranium compounds as the precursors for particle formation are used for generation of particles through which we gain insights into the particle formation process and the associated chemical reactions. Furthermore, we characterize the internal morphology of particles using focused ion beam (FIB) milling to address the issue of voids that was observed in previous particle production setups. Another important question of interest, which is addressed in this paper, is the final chemical form of the obtained uranium particles: to be suitable as a reference material (RM), the material should be stable and homogeneous throughout the sample.10 The stability of the material depends on the chemical form, where, for example, the triuranium octoxide (U3O8) is considered to be stable at room temperature and atmospheric storage conditions, whereas the hydration of uranium trioxide (UO3) is observed under equal conditions of temperature and atmosphere.11 The chemical form of produced particles is identified by X-ray diffraction imaging of single particles using synchrotron radiation complemented by μRaman studies. Our contribution demonstrates that the production method is capable of delivering uranium oxide particles with uniform size and chemical composition and that produced particles are suitable to fulfill the homogeneity and stability requirements of reference materials. If needed, such particles could constitute a particle CRM with certified isotopic composition and uranium content.

Figure 1. Schematic overview of the microparticle production system using spray pyrolysis.

expression mU = wUρQV/f. After droplet generation, a stream of gas (supplied as compressed air at 2 bar) disperses the droplets and dilutes the aerosol (individually adjusted to 0.8 and 18.2 L/ min, respectively) in order to avoid coagulation of droplets. The aerosol passes through a drying column where the volatile component of the droplets evaporates, which eventually leads to desolvation of nonvolatile components creating a precursor particle. After exiting the drying column, the air stream is split into two by means of a virtual impactor (not shown in Figure 1). The fraction carrying the precursor particles at a reduced flow of about 5 L/min is guided into an aerosol heater (Dekati Pressurized Air Heater, Dekati Ltd., Finland) providing closedloop control ensuring steady and reproducible conditions. The splitting of the air stream is necessary, because the aerosol heater (max. 8 L/min) and the impactor used for particle collection (2−4 L/min), which is described further below, can only accommodate lower air flows. Within the aerosol heater, the air stream is heated on a stretch of about 200 mm to nominal temperatures of maximally 600 °C, measured with a sensor at the outlet of the heater. The heater serves to chemically transform the precursor particles into their final solid oxide form. After heat treatment, the particles are guided through a 500 mm long, air cooled stretch to cool down the air stream prior to particle collection. The particles are collected from the air stream using an inertial impaction method onto a flat substrate. The design of the impactor is similar to the device used for extraction of particles from cotton swipes in particle analysis techniques.16 The inertial impactor holds circular substrates with a maximal diameter of 2.54 cm (1 in.), where the substrate type can be varied depending on the intended use of produced particles as long as the dimensions of the substrate are such that it fits into the impactor housing. For characterization with microanalytical methods like scanning electron microscopy, energy-dispersive X-ray spectroscopy, or μ-Raman spectroscopy, vitreous carbon planchets or silicon wafer specimens (Ted Pella Inc., U.S.A.) are used. Input Feed Solution Preparation. The uranium liquid feed solution for the aerosol generator is prepared from a uranium CRM. Therefore, the uranium isotopic composition of the produced microparticles can be derived from the certificate of the CRM. The uranyl nitrate solution is obtained by dilution of a uranyl nitrate solution CRM (IRMM-183). Beside the certified isotopic composition, the uranium concentration and HNO3 content are given as informative values as cU ≈ 0.2 g/mL

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EXPERIMENTAL SECTION Particle Production. The particle production setup was installed and commissioned at the Institute of Energy and Climate Research - Nuclear Waste Management and Reactor Safety (IEK-6) at Forschungszentrum Jülich (Jülich, Germany) and is based on spray pyrolysis of a monodisperse aerosol. The particle production consists of several steps: aerosol-generation, aerosol drying, heat treatment for particle formation, and finally, particle collection (see Figure 1 for a schematic overview of the setup). The method of spray pyrolysis for particle production is similar to previously reported approaches to production of particle standards.4,12−15 In the setup at Forschungszentrum Jülich, a monodisperse aerosol is generated using a vibrating aerosol generator (VOAG, Model 3450, TSI Inc., U.S.A.). The vibrating orifice generates droplets through perturbation of a liquid jet, droplets having a uniform diameter at a specific resonance frequency of the vibrating orifice. The uniform droplet volume Vd produced by the VOAG depends uniquely on the volume flow through the orifice QV, and the vibrating orifice frequency f via the simple relation Vd = QV/f. For the production of particles with a predetermined elemental content, the elemental content per particle mU is given by Vd times the weight fraction of uranium per volume present in the input feed solution. The uranium concentration per volume is given by cU = wUρ (wU is the uranium weight fraction of the input feed and ρ is the density of the fluid), which leads to the B


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Analytical Chemistry and n(HNO3) ≈ 6 mol/kg, respectively. For the preparation of the liquid feed solution, the CRM is diluted gravimetrically (Mettler-Toledo XP205 DeltaRange) with ultrapure water to a uranium content of about 200 μg/g. Uranyl chloride (UO2Cl2) and uranyl acetate (UO2(CH3COO)2) solutions have been prepared by dissolution of UO3 in concentrated hydrochloric acid (Bernd Kraft, Germany) and acetic acid (Merck Millipore, Germany), respectively. Dissolution was performed at approximately 90 °C, and the samples have been dried and redissolved three times to ensure complete conversion. The UO3 used was prepared by thermal denitration of the dried uranyl nitrate CRM at 500 °C. After drying of uranyl acetate, small yellowcolored crystals were obtained, which were identified as anhydrous uranyl acetate by XRD (Bruker D8 Advance). In the case of uranyl chloride, a yellow-colored gel was obtained that yielded an amorphous XRD pattern. Raman measurements (Horiba LabRam HR) resulted in spectra similar to that of uranyl chloride. The obtained solids were dissolved in water and diluted to obtain a solution with a uranium content of about 200 μg/g. In order to allow faster evaporation during the particle formation process of the aerosol, the solutions have been diluted with ethanol to a uranium content of approximately 100 μg/g shortly before particle production. All solutions were analyzed by ICP-MS (PerkinElmer/SCIEX Elan 6100 DRC) to determine the uranium content. In order to minimize the impurities of the produced microparticles, all dilutions have been performed using ultrapure water obtained using an Elga PURELAB Ultra installation, which produced water with a resistivity of 18.2 MΩ cm and Merck Millipore absolute ethanol. Particle Characterization. For identification of the microparticles and investigation of the size and morphology, the particles have been characterized with a Fei Quanta 200FEG (Eindhoven, The Netherlands) scanning electron microscope (SEM) equipped with an Apollo X Peltier-cooled silicon drift detector for energy dispersive X-ray spectroscopy (EDX). Cross-section images of particles were prepared using a Zeiss NVision 40 Cross Beam workstation (Carl Zeiss AG, Germany) focused ion beam (FIB). This instrument combines a high-resolution field emission electron gun for SEM imaging with an ion column which is used for ion milling. Using the built-in gas injection system, particles were coated with a protective and conductive carbon layer using Electron and Ion Beam Induced Deposition. Particles were milled with a 40 kV Ga+ ion beam at 40 pA ion beam current. The cross-sections of the particles were recorded with the secondary electron detector at 54° inclination angle relative to the ion beam. The microparticles have been characterized using μ-X-ray diffraction (μ-XRD) and μ-Raman spectroscopy. For the μXRD studies, samples were prepared by direct collection of microparticles onto a polyimide foil coated with a thin layer of an adhesive (Apiezon L, M&I Materials Ltd., U.K.) to prevent loss of particles. The foil was laid onto a flat substrate housed in the impactor. For technical reasons it was not possible to produce particles from uranyl chloride and particle transfer from other substrates onto the polyimide foil was considered too risky as particles may fracture. Therefore, only particles produced from uranyl nitrate and uranyl acetate have been investigated with μ-XRD. The μ-XRD measurements were performed at the Swiss Light Source of the Paul-ScherrerInstitute (Villigen, Switzerland) on the microXAS beamline using a 17.2 keV photon beam. The beam is focused to a beam size of approximately 4 × 1.5 μm. The region of the foil

containing particles was scanned with the photon beam in a stepwise fashion and data are collected over five seconds for each step. A μ-X-ray fluorescence (μ-XRF) spectrum has been recorded simultaneously to the XRD pattern collection. The obtained μ-XRF and μ-XRD data allow for the preparation of a spatial map, which in turn allows for identification of single particles. Diffraction patterns have been collected using a twodimensional X-ray detector (marCCD 225, Rayonix, L.L.C. [formerly Mar USA Inc.], Evanston, Illinois, U.S.A.; 3072 × 3072 pixels, 72 μm pixel size), a total area of 250 × 250 μm was scanned with steps of 4 μm, yielding 3969 diffraction patterns. Microparticles produced using uranyl nitrate (400, 500, and 580 °C) and uranyl acetate (400 °C) were investigated. Supplementary measurements were performed on uranyl nitrate based particles treated at 500 °C, scanning the photon beam with smaller step sizes of 500 nm over a subarea of 50 × 20 μm, resulting in 4446 patterns. Aside from synchrotron X-ray based techniques, μ-Raman spectroscopy has been performed at CEA/DAM-Il̂ e de France center (Arpajon, France) on particles produced from uranyl nitrate, uranyl acetate and uranyl chloride. The spectra were collected using a Renishaw “inVia” spectrometer (WottonUnder-Edge, U.K.) equipped with a 514 nm laser in confocal mode, the laser spot size is estimated to be around 0.6 μm. Particles were deposited on uncoated silicon wafers and were located using an optical microscope. The laser power was set to 1% of full power (approximately 0.25 mW) to prevent weathering of the particles under the laser. Spectra were collected and background was subtracted before comparison with reference data.

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RESULTS AND DISCUSSION Particle Formation Chemistry. The particle morphology of particles formed in spray pyrolysis is determined by numerous factors. A review of experimental literature data demonstrated the influence of the chemical composition of the solute on the particle morphology.17 This effect stems from the complex physicochemical processes occurring during the evaporation of the solvent, the desolvation and particle formation process, and the heat treatment of particles used to chemically transform the particles into a stable form. Due to the availability of liquid uranium CRMs in the form of uranyl nitrate solutions, production of uranium microparticles is usually approached using uranyl nitrate as precursor. However, the thermal decomposition of uranyl nitrate (Figure 2) follows a rather complex pathway.18−20 Moreover, UO3 is produced at temperatures below 700 °C. Besides uranyl nitrate, uranyl acetate and uranyl chloride were used as a solute in the solution used for aerosol-generation. The thermogravimetric data reported in literature shows that uranyl acetate decomposes to U3O8 when treated in an oxidizing atmosphere, even at temperatures as low as 350 °C.21,22 Uranyl chloride in air thermally decomposes into UO3 at temperatures of slightly above 450 °C, but has a relative small mass loss during decomposition.23 Particle Formation Process. Running the aerosol generator with the aerosol heater switched off, wetting of the substrate was observed indicating that the droplets had not solidified. The residues of the dried deposit were analyzed with EDX, the resulting composition matching with the elemental composition of the solute. With the aerosol heater switched on, the deposition of exclusively solid particles was observed with the SEM at temperatures larger than 200, 100, and 300 °C for C


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Analytical Chemistry

Therefore, the EDX spectra were rescaled normalizing with the integral counts in the region interval 3.04−3.65 keV, the region of the uranium M-Series X-ray emission. For each solute used as precursor for particle production, the EDX spectra are compared for three different temperatures that cover the region in which chemical transformation is expected (Figure 4). For

Figure 2. Graph of compiled thermogravimetric data from literature for the uranium compounds uranyl nitrate,18−20,24 uranyl acetate,21,22,25 and uranyl chloride23 used for production of microparticles.

uranyl nitrate, uranyl acetate, and uranyl chloride, respectively. The size of the particles decreases with higher temperature setting of the aerosol heater for the uranyl nitrate particles, which is attributed to the dehydration and eventually the chemical transformation into uranium oxide (Figure 2). SEM micrographs show spherical shapes for particles produced from uranyl nitrate and uranyl acetate with a monodisperse size distribution, whereas irregular shapes are observed for particles produced from uranyl chloride produced at temperatures of 500, 400, and 500 °C, respectively (Figure 3,

Figure 4. EDX spectra of single particles produced at different heat treatment temperatures (indicated in the legend) for particles produced with uranyl nitrate (UN), uranyl acetate (UA), and uranyl chloride (UC) solution.

uranyl nitrate the EDX spectra are compared for treatment temperatures 100, 200, and 500 °C. At 100 and 200 °C heating temperature, a small signal from nitrogen is observed, which disappears at higher temperatures, indicating the conversion of nitrate into nitrous vapors. With increasing temperature of the heater, the oxygen signal gradually decreases in intensity, which is interpreted as an indicator for dehydration of uranyl nitrate. There is a line observed attributed to the K-alpha line of carbon, the intensity relative to the uranium lines remaining constant for the temperatures 100 and 200 °C with a decrease at the temperature at which transformation into an oxide compound is expected. This is surprising, as carbon is not expected to be present in particles since, at temperatures of 500 °C, the ethanol should have evaporated or at least decomposed to carbon oxide vapors. For uranyl acetate, the EDX spectra are compared for heater temperatures of 100, 200, and 400 °C, and in all spectra, the elements carbon, oxygen, and uranium are observed. The change in intensity of carbon and oxygen with respect to the uranium line strength can be interpreted to stem from the chemical transformation. Also here, a peak at the carbon K-alpha line is observed at the highest treatment temperature of 400 °C. At temperatures above 400 °C, a large number of agglomerates were obtained, possibly related to the rapid oxidation of the formed UO2 to U3O8 during the thermal treatment stage. Finally, the EDX spectra of uranyl chloride are compared for 100, 400, and 500 °C heat treatment temperatures. The observed emission lines indicate the presence of carbon, oxygen and chloride in uranium deposits on the substrate. There is a clear signature of the chemical trans-

Figure 3. SEM micrographs of uranium particles produced by spray pyrolysis of droplets containing dissolved uranium (recorded with Zeiss NVision 40, yellow bar = 1 μm). The treatment temperature is denoted in the respective image. The inset shows a SEM image of the cross-section of the particle prepared by FIB.

see also Supporting Information). These are the temperatures at which decomposition to uranium oxide is expected for the respective uranium solute. Input feed solutions with a uranium weight fraction of approximately 100 μg U/g provide particles with around 1 μm diameter. The systematic analysis of the uranium deposits and particles on the substrate was performed by SEM/EDX characterization for further insight into thermal decomposition during production process. A comparison of the EDX spectra taken from uranium deposits found on substrates is complicated by the fact that the electron beam penetrates through the uranium deposits or particles, the thickness being smaller than 1 μm, into the substrate leading to a large signal from X-rays emitted from the silicon substrate. In addition, the self-absorption of Xrays within the particle will hinder the quantification of elemental contents as this factor depends on the size, shape, and density of the particle/deposit of interest. D


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Analytical Chemistry formation into uranium oxide occurring between 400 and 500 °C, where the K-alpha line from chloride is observed at 400 °C, whereas this line is absent in the spectrum for particles treated at 500 °C. Just like with the particles produced from uranyl nitrate, an emission line is observed at the energy of carbon Kalpha line for particles produced from uranyl chloride. However, for the uranium deposits obtained from spray pyrolysis of uranyl chloride there is nearly no intensity change of this particular line intensity compared to the uranium lines. An EDX analysis of pure U3O8 powder from a CRM (NBL 129-A) produces spectra with the carbon K-alpha line present and with a similar intensity relative to the uranium lines. Therefore, we conclude that the line observed at this position stems from an artifact when probing uranium compounds and is not result of the ethanol present in the input feed solution. In conclusion, the EDX spectra provide clear evidence that the heat treatment transforms particles into a uranium oxide. We find that the particle formation in our spray pyrolysis setup is in accordance with the behavior observed in thermal treatment of bulk materials as far as dehydration and decomposition is concerned. Thermographic data suggest the formation of uranium trioxide particles for uranyl nitrate and uranyl chloride as a precursor and triuranium octoxide if particles are produced from uranyl acetate. Particle Morphology. We characterize the particle size distribution, particle shapes and the morphology using scanning electron microscopy. The morphology is inspected on indivually selected particles for which images were recorded at higher magnification with the SEM and the FIB. These images show that the typically observed external morphology of the particles varies strongly for the three different input feed solutions (see Figure 3). Particles produced from uranyl nitrate and uranyl actetate are typically spherical in shape, where those formed out of uranyl nitrate have a wrinkled surface, whereas particles produced from uranyl acetate have a flat surface. The wrinkled surface of particles produced from uranyl nitrate could be interpreted as voids when a top view projection is recorded with the SEM. The particles produced from uranyl chloride have more irregular shapes, and it is visible that they are hollow in the inside. A more quantitative analysis of the size and the shape of particles was performed on particles produced from uranyl nitrate. For particles produced from uranyl nitrate at 500 °C heating temperature, we have determined that the particles are monodisperse with more than 90% of all particles lying within 5% of the median particle size (see Figure 5). The analysis of the size and shape distributions of particles produced at 500 and 580 °C indicates that, with increasing temperature, particles fuse in the heating zone to form spherical particles with a diameter, which is a factor (2)1/3 times the diameter of single particles. We also observe particle pairs, from which we assume that they represent particles that agglomerated in the air stream during or after the heat treatment. There is also a small fraction of bloated particles, whith a diameter approximately four times larger than the monodisperse particles. In previous reports on particle production, the bimodal distribution was attributed to the coagulation of droplets immediately after droplet generation. For our setup, however, the coagulation seems to be a minor effect at 500 °C. In addition, the internal morphology of particles was studied using FIB/SEM characterization. Randomly selected particles were milled with an ion-beam revealing the internal morphology by a cross section view taken with the built-in

Figure 5. Size distribution from analysis of SEM micrographs of particles collected on a Si-wafer specimen for an uranyl nitrate solution with 133 μg U/g weight fraction.

SEM imaging capability (see insets in Figure 3). For particles produced from uranyl nitrate as solution, pores are observed in the center region of the particle. We observe this for all particles that were prepared with the FIB (7 particles, see Supporting Information). Conversely, the particles produced from uranyl acetate show a homogeneous and dense morphology, which is very distinct to the porous structure of the particles obtained from the uranyl nitrate. This indicates the presence of nanopores for particles produced from uranyl acetate, which are not discernible in the micrographs taken during the FIB/ SEM characterization. The majority of particles produced from uranyl chloride are hollow in the inside, which explains the strong variety of particle shapes observed. The internal morphology of the different particle production routes underpins the observation of the external shape of the particles and provides evidence, that the physicochemical processes strongly differ between the different uranium compounds used in the input feed solution. Single Particle X-ray Diffraction. The superimposed μXRD diffraction pattern obtained from measurement of microparticles produced from uranyl nitrate at 500 °C (Figure 6a) show diffraction bands. In μ-XRD, the presence of such

Figure 6. μ-XRD diffraction patterns (average of 3969 patterns) of microparticles produced using (a) uranyl nitrate treated at 500 °C and (b) uranyl acetate treated at 400 °C.

powder-like diffraction rings indicates that the grains are nanocrystalline domains. The integrated diffraction patterns indicate an orthorhombic phase. In the case of binary uranium oxides, five orthorhombic phases are known: α-UO3, γ-UO3, ζUO3, and two modifications of U3O8.11,26−28 The lattice parameters were extracted from the μ-XRD diffraction patterns E


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Analytical Chemistry and the values are in close agreement with the α-U3O8 phase (Supporting Information), which is the stable triuranium octoxide phase at room temperature.The thermal decomposition of uranyl nitrate has been studied by numerous authors over the last few decades. However, all reported the presence of UO3 at temperatures of 500 °C. Detailed investigations by Hoekstra and Siegel26 and Wheeler et al.27 find that the lowest temperature at which decomposition of UO3 to U3O8 was observed was slightly above 550 °C. In order to further investigate the effect of the treatment temperature on the final phase of the microparticles, μ-XRD data of particles produced using uranyl nitrate as precursor treated at 400 and 580 °C were analyzed. Even at 400 °C, an orthorhombic phase can be identified and refinement yielded lattice constants close to U3O8 and also for particles produced at increased temperatures of 580 °C (see Supporting Information). The presence of U3O8 instead of UO3 might be possible due the small size and therefore large surface area of the particles. As all previous studies were conducted using bulk amounts of materials, small amounts of material converted to U3O8 at the surface might not have been noticed. Particles produced from uranyl acetate were also investigated by μ-XRD. According to various references,21,22,25,28 at 400 °C, uranyl acetate decomposes to U3O8. The obtained diffractograms show a lower degree of crystallinity (Figure 6b), but an orthorhombic phase could be identified. Also, the lattice constants are similar to U3O8 and to that of particles produced using uranyl nitrate. Particles produced from uranyl chloride solution had not been studied with X-rays. μ-Raman Studies. μ-Raman spectra of microparticles produced from uranyl nitrate, uranyl acetate, and uranyl chloride treated at 500, 400, and 500 °C, respectively, were obtained (Figure 7). For each substrate, at least 12 particles

hydroxides are a common type of uranium ore concentrates (UOCs) and have been measured by various authors,30−34 although the results are not always reproducible. One such spectrum is shown in Figure 7, which is obtained from a μRaman measurement of “South Dakota” UOC and presumably represents UO2(OH)2. For the synthesized particles, the presence of an uranyl hydroxide phase could stem from hydrolysis of the particle surface with humidity, as also observed with bulk U3O8.35 As Raman spectroscopy is rather a surface-sensitive method for metals or metal oxides impervious to visible light (between a few nm and a few tens of nm penetration depth), the formation of a thin surface layer might yield uranyl hydroxide spectra when analyzing with μRaman spectroscopy, whereas such a phase would not be identified by μ-XRD. The Raman spectra obtained for microparticles produced from uranyl acetate and uranyl chloride are comparable to the spectra of particles produced from uranyl nitrate. Particles produced from uranyl chloride were not investigated using μ-XRD; however, since the μRaman spectra of particles of all three different synthesis routes are very similar, we conclude that also uranyl chloride yields U3O8 at 500 °C. Recently, a number of μ-Raman spectroscopy studies have been performed on uranium microparticles produced by UF6 hydrolysis36,37 and powdered samples.38 However, none of these materials were fully characterized on the micrometer scale and as shown with the μ-XRD results, such microparticulate matter might behave differently from bulk materials. Another difficulty arising with μ-Raman measurements is that the relative intensities of bands depends on the excitation laser wavelength. As the presence of an U3O8 phase has been verified by μ-XRD measurements, the particles can act as standard for microanalytical measurements. Comparison of the collected spectra obtained from synthetic particles with a spectrum collected on a degraded uranyl fluoride particle by Stefaniak et al.36 shows a good match, supporting the conclusion that the degraded particles consist of U3O8. The presented case shows the value of having highly characterized micrometer-sized materials, which could act as a valuable tool to verify the suitability of μ-Raman measurements on micrometer-sized particles. Elemental Content and Particle Density. The particle density is described as the mean density, that is, defined as the particle mass divided by the particle volume. The volume can be deduced from the particle diameter determined from SEM images, making the assumption that the particles are spherical. The particle’s uranium mass is obtained from the VOAG operating parameters. Using uranyl nitrate in the input feed solution with different concentrations and using two different settings of the aerosol generator provides particles with diameters between 0.7 and 1.6 μm and with a consistent mean density of 4.2 g U/cm3 at heat treatment temperatures of 500 °C (Figure 8). In the work of Kraiem et al.,9 the uranium mass density of uranium particles produced with a similar process was determined from a measurement of the uranium content in single particles using isotope dilution mass spectrometry (IDMS). Their results of 4.7 and 3.8 g U/cm3 for two different production batches points to an inconsistency in the mean density between particle production runs, which was attributed to the existence of voids in the particles in one of the production runs. With the uranyl nitrate solution we have observed a tendency of particles to develop voids clearly visible in the SEM images at heat treatment temperatures exceeding

Figure 7. Raman spectra of microparticles produced using uranyl nitrate (UN), uranyl acetate (UA), and uranyl chloride (UC) deposited on a silicon wafer, resulting in the strong peak at 521 cm−1. The red line at the bottom indicates an spectrum of micrometersized U3O8 particles36 and the green line a spectrum of micrometersized UOC (“South Dakota”) particles.

have been selected and analyzed. Other than the high intensity peak at 521 cm−1 from the silicon wafer substrate, a number of peaks could be observed. However, no clear match with any reference spectra could be identified and the broadening of the peaks indicates a low degree of crystallinity. The closest matches based on the spectral shape are with spectra obtained for U3O829 and uranyl hydroxide (UO2(OH)2).30 Uranyl F


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indicate a mean particle density, which is almost by a factor two lower than the theoretical density. We believe this is likely a result of the formation process in spray pyrolysis and that it will not be possible to reach the theoretical density of U3O8 with this production method. Our study demonstrated the utility of synthetic particles as micrometer-sized test objects for destructive microanalytical methods, as discussed for the case of μ-Raman characterization. For optimized production parameter settings, particles are free of voids and are, therefore, highly uniform in a number of their properties (size, elemental and isotopic composition, chemical structure). In addition, being composed of U3O8, the particles are expected to fulfill the stability requirements of a RM under normal atmospheric conditions. The thorough characterization of the particle production process is an important step toward the controlled and reproducible production of particles as a base material for a particle RM. Such particles can be used for quality control, calibration, and to validate analytical procedures used for characterization of particles, for example, as applied in the field of nuclear forensics.

Figure 8. Measured particle diameter (SEM) against calculated uranium and U3O8 mass. Two series of samples have been produced with frequencies of 40.24 kHz (Production 1) and 69.13 kHz (Production 2), both with liquid flow rates of 2.32 μL/s. The thick red line indicates a uranium density of 4.21 g U/cm3; the lower and upper lines represent a density of 3.5 and 4.8 g U/cm3, respectively.

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500 °C. Since our results obtained from two independent production runs with different production settings result in mean densities which are in agreement to one another, it can be concluded that particle production at 500 °C yields particles with consistent densities. For the uranyl acetate particles at a diameter of about 1.11 μm, which we produced at 400 °C, we determined a uranium density of approximately 4.0 g U/cm3. For the particles obtained from uranyl chloride solution, the assumption of a spherical shape does not apply given the irregular shape of the particles observed in the SEM micrographs. Furthermore, knowing that particles are composed of U3O8, we can derive a particle density of 5 g/cm3 for particles produced from uranyl nitrate and 4.7 g/cm3 for particles produced from uranyl acetate, reaching 60% and 56% of the theoretical density reported for bulk U3O8 (8.38 g/cm3). These findings suggest that, for monodisperse particles generated with the method presented in this paper, the elemental content per particle should be determined from the aerosol generation parameters liquid flow-rate together with uranium concentration and the density of the input feed solution. For spherical particles, at best, an estimate of the uranium content per particle can be derived from the particle size and assuming a density, which amounts to approximately 60% of the theoretical density of U3O8.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b00631. Particle analysis and particle size distribution of particles produced from uranyl nitrate at 500 °C; SEM Images of particles produced from uranyl nitrate (T = 500 °C), uranyl acetate (T = 400 °C), and uranyl chloride (T = 500 °C); Comparison and analysis of particle size and shape for particles produced from uranyl nitrate for T = 500 and 580 °C; SEM/FIB images displaying the internal morphology of particles; single particle μ-XRD diffraction images and extracted lattice constants (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 2461 612450. ORCID

M. Dürr: 0000-0002-8034-6313 Present Address ‡

European Commission, Joint Research Centre − JRC, D76125, Karlsruhe, Germany.

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Notes

The authors declare no competing financial interest.

CONCLUSION We have provided a description and characterization of a process for the production of microspheres of uranium as a candidate material for a particle RM. The production process is based on spray pyrolysis of an aerosol of monodisperse droplets subjected to a heat treatment for the transformation into physically and chemically stable uranium oxide particles. The source material determines the isotopic composition of the final produced particles and by adjusting the concentration of the nonvolatile component in the source material, the size and elemental content of the particles can be adjusted within certain limits determined by the production method. μ-X-ray diffraction and μ-Raman spectroscopic measurements on individual particles indicate an orthorhombic U3O8 phase. The results from particle characterization measurements

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ACKNOWLEDGMENTS We thank M. Klinkenberg and M. Güngör from Forschungszentrum Jülich for SEM and FIB analysis of particles and F. Sadowski from Forschungszentrum Jülich for ICP-MS analyses. This work was supported under Task C.45/A1961 by the German Support Programme to the IAEA and by the Federal Ministry for Economic Affairs and Energy, Germany, under the Programme “Neu- und Weiterentwicklung von Safeguardstechniken und -methoden” (FKZ 02W6263). The μ-XRD and μXRF measurements leading to these results have received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under the Talisman project. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, G


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for provision of synchrotron radiation beam time at the microXAS beamline of the SLS.

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