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Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania
Accepted Manuscript Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania V. Hiranmayee, K. Ananthasivan, Dasarath Maji, Kitheri Joseph PII:
S0022-3115(18)30512-9
DOI:
https://doi.org/10.1016/j.jnucmat.2018.12.031
Reference:
NUMA 51363
To appear in:
Journal of Nuclear Materials
Received Date: 6 June 2018 Revised Date:
15 December 2018
Accepted Date: 17 December 2018
Please cite this article as: V. Hiranmayee, K. Ananthasivan, D. Maji, K. Joseph, Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania, Journal of Nuclear Materials (2019), doi: https://doi.org/10.1016/j.jnucmat.2018.12.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania a
a,
a
V. Hiranmayee , K. Ananthasivan *, Dasarath Maji , Kitheri Joseph
Materials Chemistry and Metal-Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamil Nadu- 603102, India. Abstract
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A systematic investigation on the microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania powders is being reported for the first time. Nanocrystalline urania powders were synthesized by microwave-assisted citrate gel-combustion method by using citric acid as a fuel and nitrate as an oxidizer. The fuel to nitrate ratio (R) was varied from 0 to 1 (0, 0.1, 0.25, 0.5, 0.75 and 1.0). It was observed that at a particular stoichiometric ratio (R = 0.1), auto ignition reaction takes place resulting in a porous product. The as-prepared powders were calcined and further reduced under flowing hydrogen. All these powders were characterized for the phases present viz., X-ray diffraction, size distribution of pores, transmission electron microscopy, specific surface area, TG-DTA-MS, and infrared spectroscopy. It was observed that the microwave-derived powders with an R-value of 0.1 had the lowest bulk density, highest SSA and highest total pore volume. Upon calcination and reduction, the total pore volume decreased. The specific surface area of the calcined powders was the lowest and that of the as-prepared powders was the highest while that of the hydrogen-reduced powders was in between. An opposite trend was observed with the X-ray crystallite size. The microwave-derived nanocrystalline urania powders showed a lower SSA, higher XCS, higher lattice strain, higher residual carbon and lesser total pore volume than their hot plate derived counterparts. This is probably due to the influence of microwave which accelerates the reaction leading to faster grain coarsening.
Keywords: Nanocrystalline urania, Microwave-assisted gel-combustion synthesis, High burn up
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structures, TEM, XCS, Lattice strain. *Corresponding Author: Dr K. Ananthasivan, [email protected] Phone: Ph: +91 44 2748 0500, Extension: 24069 Fax: +91 44 2748 0065
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1. Introduction
UO2 is widely used as a fuel in fission reactors. Studies on the spent fuels discharged
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from both Light Water Reactors (LWRs), as well as Fast Breeder Reactors (FBRs), have revealed that prolonged irradiation of these fuels leads to the formation of a unique microstructure called the “High Burn-up Structure” (HBS) [1–6]. Microstructural investigations had revealed that HBS comprises submicron grains with size ranging from 0.1 – 0.3 µm. This structure is essentially nanocrystalline, consists of nanopores and is found at the periphery of the
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fuel pellet [7]. HBS comprises closed porosity that retains fission gases, shows resistance to internal stresses and radiation damage due to its higher plasticity and nanostructure. Therefore,
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the study of nanocrystalline urania (nc-UO2) has attracted the nuclear technologists. In order to study nc-UO2, bulk quantities of the same are required. Ceramic grade urania with grain size generally in micrometer range is commonly prepared by calcining ammonium diuranate and reducing the product under controlled conditions [8,9]. However, experimental investigations on the preparation of nc-UO2 in bulk quantities are rather limited. Several methods have been cited in the literature for the preparation of nc-UO2 viz., photochemical reaction [10], oxalate
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precipitation [11], non-aqueous colloidal synthesis [12], hydrothermal method [13], laser ablation [14] and sulfate-mediated synthesis [15]. But some of these methods are not costeffective, yield agglomerated powders with large grains, introduce impurities, show low yield and generate a large quantity of liquid waste [13, 14]. Earlier investigations [16–18] on the gel-
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combustion synthesis had demonstrated that the microwave-assisted citrate gel-combustion method yields a nc-powder with better sinterability and high purity. Further, this process is
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energy efficient and the heating gets completed within a few minutes. Gel-combustion synthesis utilizes the high exothermicity of the chemical reaction
ensuing between a fuel and an oxidizer, with the evolution of a large volume of gases resulting in purer products [8, 16, 19–22]. In recent years, considerable work has been carried out on the synthesis of nano nuclear ceramics by using citric acid as the fuel [22–26]. Citric acid is particularly advantageous for it is a good chelating agent and produces the reaction with moderate exothermicity. Recently, a systematic study was carried out on the synthesis of nanocrystalline urania in bulk quantities in our laboratory by following the citrate gel6+
combustion method with hot plate as the heat source [22]. U
is a microwave active ion and
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a solution containing the same could be heated within a short duration by using microwave [5]. Even though investigations on the microwave-assisted synthesis of (U1-xThx)O2 solid solutions have been reported [27], a systematic study on the microwave-assisted gel-combustion synthesis of nc-UO2 has not been reported so far. Hence, in this study, we undertook the preparation of
1.0), on the combustion synthesis of nc-UO2 was investigated. 2. Experimental
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2.1 Starting materials
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nanocrystalline urania by microwave-assisted citrate gel-combustion method. Also, the influence of the fuel (citric acid) to oxidant (nitrate) ratio [22, 28–30], i.e. (R = 0.0, 0.1, 0.25, 0.5, 0.75,
Uranium oxide (U3O8) of nuclear grade purity was procured from Nuclear Fuel
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Complex, Hyderabad, India. Ferroin indicator (AR 0.025 M) and citric acid monohydrate (AR) (> 99.5% pure) were obtained from Loba chime Pvt. Ltd, Mumbai, India. Ammonium iron (II) sulfate hexahydrate (GR 99%) was procured from Merck Ltd, Mumbai, India. AR grade nitric acid, sulphuric acid and hydrochloric acid were supplied by M/s. Rankem Laboratory Chemicals Pvt. Ltd., Chennai.
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2.2 Powder synthesis
In a typical experiment, a known amount of urania powder was heated at 873 K for 4 h in air to obtain U3O8. The latter was dissolved completely in minimum quantity of concentrated nitric acid to obtain a solution of uranyl nitrate. The excess acid was removed by evaporation. -
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The estimation of NO3 was carried out by titrimetry as described in ref. [22,31]. In a typical gel-
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combustion experiment, desired quantities of the fuel and oxidant were dissolved in minimum quantity of distilled water and heated in a microwave oven operating at an output power of 900 W. Mixtures with R = 0 (no citric acid), 0.1, 0.25, 0.5, 0.75 and 1.0 were prepared. Upon heating, these mixtures gelated, dehydrated and then underwent auto ignition resulting in a carbonaceous oxide ash (as-prepared powders). The resultant powders were calcined in air at 1073 K for 4 h in a furnace equipped with SiC heating element. The calcined powders were further reduced to -1
UO2 under flowing hydrogen at 1073 K for 4 h at a flow rate of 500 mL min , in a custommade furnace equipped with Inconel sample holders. These powders were designated with different labels by using suffices “A”, “C”, “H” to identify “as-prepared”, “calcined” and “hydrogen-reduced” powders respectively. All the indices bore a prefix MUc, to denote urania powders prepared by microwave-assisted citrate
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gel-combustion method. Each mixture was distinguished with the help of two digits “XX” which referred to the value of R (in percentage). Thus, powders prepared from a mixture with R= 0.10 were designated as MUc10A, MUc10C, MUc10H to denote that these are the “as-prepared”, “calcined” and “hydrogen-reduced” products respectively.
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2.3 Characterization
The FTIR spectrum of the precursor materials and final products were recorded (Bruker -1
α- FTIR spectrometer, Attenuated Total Reflectance (ATR) mode in the region 4000-650 cm -1
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with a resolution of 4 cm ). Study of the thermal decomposition of the powders was performed
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by using a TG-DTA-MS system NETZSCH, (STA 449F3 Jupiter and QMS 403 D, NETZSCH, Germany). The error in temperature measurement in TG-DTA-MS experiments was ± 1 K and that in weight was ± 1 mg for the blank corrected data. An electronic balance supplied by M/s. Scaltec, GmbH, Germany was used to estimating the bulk density of all the powders. The latter was measured from the weight of the powder that filled a cuvette of known volume up to the brim. Average of five measurements is reported here. The error in the density values reported is 3
± 0.04 Mg/m . The XRD pattens pertaining to all the “A”, “C” and “H” powders were recorded
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by using X’Pert Pro MPD system, supplied by M/s. Panalytical, Netherlands by using (Cu Kαradiation (λ= 1.541Å) equipped with a graphite monochromator and a scintillation detector. Calibration was carried out by using silicon and alumina standards supplied by NIST, USA. Xray crystallite size (XCS) of all these samples was estimated from the width of their respective diffraction patterns by Hall-Williamson method and Scherrer method. Specific surface area (SSA) of all the urania powders was determined from nitrogen
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adsorption by using the BET method (Surfer Analyser, Thermo Fisher Scientific S.p.A, Milan, Italy). Blank analysis and calibration of SSA instrument were carried out by using α-alumina as
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standard material supplied by BAM, Germany. The error in the measurement of the SSA was 2
± 0.04 m /g. Distribution of sizes of the pores present in the “A”, “C” and “H” powders was investigated by mercury intrusion porosimetry (Pascal 140 and 440 mercury porosimeter, Thermo Fisher Scientific S.p.A, Milan, Italy) at an operating pressure range of 0.1 kPa - 400 kPa and 0.1 MPa - 400 MPa respectively. Blank analysis and calibration in the mercury porosimeter experiments were carried out by using α-alumina standard obtained from NIST. The error in the 3
measurement of the pore volume is ± 13 mm /g. The carbon residue present in all the “A”, “C” and “H” powders was determined by using oxidative fusion (M/s. Eltra CS 800, Germany). In a typical analysis, mixtures containing about 200 mg of the samples and 1500 mg of tungsten were
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taken in an alumina crucible and heated in an induction furnace under flowing O2. The CO2 evolved was quantitatively measured by using an IR detector. The powders synthesized from mixtures with R= 0.10 (MUc10H) were characterized by using high-resolution transmission electron microscopy (HRTEM), LIBRA 200FE (Carl Zeiss,
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Germany) operated at 200 kV and the information limit of the HRTEM was found to be 0.13 nm. were then placed onto a holey carbon grid and dried. 3. Results and discussion 3.1 IR spectroscopy of the precursor and products
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For the TEM analysis, the powder was dispersed in propane-2-ol and few drops of this solution
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The FTIR spectra of uranyl nitrate, citric acid and the powders (“C” and “H”) obtained from a mixture of R=1.0 (MUc100) are illustrated in Fig. 1. The details of the IR data are shown in Table 1. The absorption signatures of N-O and C=O for MUc100C and MUc100H were absent, possibly because of the complete removal of the organic precursor (citric acid) and nitrates. Thus it is reasonable to conclude from the IR spectra, that the reaction has gone to completion [22, 32–38].
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3.2 TG-DTA-MS
The results obtained in the TG-DTA-MS investigation from a mixture with R= 1.0 (MUc100A), are shown in Fig. 2. The DTA curve shows a small endothermic peak at around 443 K,
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which could be attributed to the loss of water. The weight loss observed between 473 K and 673 K could be due to the decomposition of the sample with the attendant liberation of gaseous products. The peak observed around 673 K could be attributed to the elimination of the carbon residue by the liberation of CO2 upon combustion [22]. The optimum calcination temperature was chosen from the
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thermogravimetric analysis. It was observed that the weight of the sample remained nearly constant at temperatures above 1043 K. Hence the calcination temperature was chosen as 1073 K in order to ensure complete removal of carbon from the “A” powders. Calcination at a higher temperature was avoided in order to limit grain coarsening. The mass spectrometric analysis of the gases evolved at various stages from the sample is also shown in Fig. 2. As shown in Fig. 2, the gases evolved corresponding to the first step of mass loss are H2O, OH and the second step are H2O, OH, CO2, CO, NO2,
13
CO2. The inset in the Fig. 2 shows the masses 45 and 46 in the low ion current range. The
natural abundance ratio of 13
12
13
C: C is 100:1. This is evident in the ion current ranges of
CO2. Also, the mass numbers 44
12
CO2 and
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and 45 follow the same pattern. Thus it is reasonable to assign the peak observed at the mass number 45 to
13
CO2 [39].
3.3 Characterization of powders
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3.3.1 Bulk Density The dependence of bulk density (BD) of the urania powders on the value of R is shown in -3
Fig. 3. The BD of these powders was found to vary between 0.14-2.36 Mgm . BD of “A”, “C” -3
and “H” powders was found to lie between 0.17-2.14 Mgm , 0.14-1.77 Mgm
and 0.29-2.36
respectively. The powders MUc10A, MUc10C and MUc10H were found to possess the
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-3
-3
lowest values of BD, indicating that these samples are highly porous. The following trend viz.,
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BDH > BDA > BDC was observed. The lowest BD exhibited by “C” powders could be attributed to the generation of pores due to the emanation of CO2 during calcination. The following trend
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was observed in the variation of bulk density with R: MUc00 H/A/C > MUc100 H/A/C > MUc75 H/A/C > MUc50 H/A/C > MUc25 H/A/C > MUc10 H/A/C. It was observed that the microwave derived nanocrystalline urania powders exhibited lowest BD’s for mixture with R = 0.1 (MUc10 powders) where as the hot plate derived powders exhibited lowest BD’s for mixture with R = 0.25 [22]. But in both these methods the lowest BD is observed for the powders derived through the volume combustion reaction. The comparison of the bulk densities of powders obtained in the present study as well as those reported in reference [22] with the powders prepared through many other conventional methods [40–42], reveals that the urania powders with the lowest bulk density could be produced only by citrate gel-combustion method. The values reported in this study are lower than the values obtained by Sanjay Kumar et al. [22]. 3.3.2 Specific surface area (SSA)
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The dependence of specific surface area (SSA) of the urania powders on the value of R is shown in Table 2 and plotted in Fig. 3. It is evident from these data that the SSA follows the trend SSAA > SSAH > SSAC. MUc10A had the highest SSA. SSA decreased from “A” to “C” powders because of grain coarsening as evident from the X-ray crystallite size (Table 3). The “H” powders possessed higher specific surface area than that of the corresponding “C” powders, probably due to the crumbling of U3O8 particles during hydrogen reduction [22]. It was observed that with an increase in the R value there is an increase in SSA, probably due to to the vigorous reaction in which the evolution of gases is maximum, resulting in powders with a high specific surface area. The microwave-derived powders showed a lower SSA than the hot plate derived
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powders [22], probably due to microwave-assisted in-situ grain coarsening during the combustion reaction. 3.3 Residual carbon
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The quantity of carbon residue has a direct bearing on porosity, density, strength and thermal conductivity [43]. The variation in the quantity of the carbon residue for “A”, “C” and “H” powders is shown in Table 2. From these data, it is evident that the residual carbon increases with the value of R and decreases upon calcination and hydrogen reduction. The decrease in the carbon residue upon calcination and hydrogen reduction is due to the removal of carbon as CO2
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and hydrocarbons upon reaction with oxygen and hydrogen respectively. The quantity of carbon residue was found to vary between 150 and 800 ppm in calcined samples while it was found to
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vary between 70 and 370 ppm for hydrogen reduced samples. These values are slightly higher than those cited in references [22, 41]. This could be due to the incomplete removal of carbon due to the insufficient supply of oxygen inside the closed microwave oven. The residual carbon causes porosity gradients and influences the O/U ratio in these powders, which in turn could help the pre-sintering of the agglomerates [22, 44, 45]. 3.3.4 Phase characterization and XCS
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The XRD analyses of the urania powders synthesized in this study revealed that the “A” and “C” powders comprised orthorhombic U3O8 phase, while the “H” powders consisted of the fluorite (UO2) phase (Fig. 4). All XRD patterns presented in this work are raw data without any
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refinement and the peaks obtained are well resolved. The crystallite size was calculated from both Scherrer formula and Hall-Williamson (HW) plots, while the lattice strain was derived from the Hall-Williamson method. The XCS (HW) of hydrogen reduced urania powders was in the range of 30-86 nm. The XCS (HW) of calcined urania powders was in the range of 131-291 nm. The XCS (HW) of “A” powders was in the range of 32-286 nm. The urania powder obtained by the denitration without the addition of citric acid (MUc00A) was amorphous. The crystallite size of “C” powders was higher than that of their “A” precursors due to grain coarsening. However, XCS decreased upon hydrogen reduction, possibly due to the crumbling of the orthorhombic U3O8 during its reduction to UO2 [22]. XCS followed the trend viz., XCSC > XCSA > XCSH. It was found that as the R-value increases, the amounts of gases evolved also increase, thus resulting in powders with a high specific surface area and lower mean agglomerated size. Along with the combustion synthesis, pre-sintering also takes place. These two processes compete with each other. It is a well-known fact that the combustion reaction is self-propagating and rapid. So,
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it does not allow the growth of the crystallites. The XCS of microwave derived combustion powders were higher than that of the hot plate derived powders [22], probably due to in-situ grain coarsening. The change in the density due to the conversion of U3O8 into UO2 is typically
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about 29%. However, the difference between the BD of the C and H powders is more than this value. This could be ascribed to the introduction of porosity or the production of finer particulates or both during hydrogen reduction. Fig. 5a shows the XRD of MUc10H powder after immediate hydrogen reduction. It was observed that the XRD data obtained from the hydrogen reduced powders after one week of storage in air resulted in UO2.12 phase (Fig. 4), while XRD
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recorded immediately after hydrogen reduction resulted in stiochiometric UO2 phase (Fig. 5). The dimensionless lattice strain calculated from the Hall-Williamson method (Fig. 6a) showed that the powder MUc50A showed least lattice strain among the “A” powders. The lattice
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strain values pertaining to the “C” and “H” powders were higher than that of the corresponding “A” powders. Insufficient time for nucleation and growth, partial sintering during calcination, explain these observations [22]. Investigations carried out by P. Bindu et al. (ZnO) [46], Yusheng Zhao et al. (α-SiC and Ni) [47] and A.E. Mahmoud et al. (Al-Al2O3 powders) [48] also report a similar trend in their studies on nanocrystalline powders. Table 3 and Fig. 6b show the variation in X-ray crystallite size of all the urania powders. The lattice strain values obtained in
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this study were high as compared to those cited in the reference [22]. It may be due to the slightly higher XCS obtained in microwave derived powders than the hot plate derived combustion powders. The variation of lattice strain with XCS (Fig. 6c) could be delineated into two regions (i.e. XCS < 100 nm and XCS > 100 nm). The region below 100 nm showed a nearly linear dependence of lattice strain with XCS while region above 100 nm showed more or less a
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constant lattice strain. This trend is similar to that reported by Sanjay Kumar et al. [22]. It appears that the lattice strain approaches a constant value when it falls within the nanocrystalline
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regime (> 100 nm). Fig. 6d confirms the linear dependence of lattice strain with the relative difference of XCS. A similar trend in the variation of lattice strain with XCS and in the variation of lattice strain with the relative difference of XCS has been reported by Sanjay Kumar et al. [22].
3.3.5 Size distribution of pores Cumulative pore size distribution (CPoSD) in the “A”, “C”, “H” urania powders estimated with the help of mercury porosimetry is presented in the Fig. 7. All the “A” powders have sub-micron pores (below 1000 nm) and a major contribution to the specific pore volume in the “A” powders is from the micropores. Upon calcination, the specific pore volume was found
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to decrease while a nearly complete removal of pores resulted in hydrogen reduction. The following trend was observed in the variation of specific pore volume with R: MUc10 A/C/H > MUc25 A/C/H > MUc50 A/C/H > MUc0 A/C/H ~ MUc75 A/C/H ~ MUc100 A/C/H. It is observed (Fig. 7) that the specific volume of mercury intruded into the pores was maximum for
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MUc10 among all the “A”, “C”, “H” powders. The total pore volume of all these powders (as shown in the Fig. 3) was found to be inversely correlated with their corresponding bulk density [22]. Total pore volume of the “C” powders as higher than that of the corresponding “A” powders. This could be ascribed to the generation of pores due to the emanation of CO2 during calcination.
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However, due to the conversion of U3O8 to UO2 during hydrogen reduction, the pore structure is partially destroyed due to the crumbling of the powder. Therefore, the “H” powders show less total pore volume than “A” and “C” powders. The “A”, “C” and “H” powders were found to possess pores
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with mean diameters ranging from 100 nm to 100 µm. Hot plate derived nanocrystalline urania powders had higher specific pore volume than the microwave derived powders [22]. This could be due to microwave-assisted in-situ grain coarsening during the reaction. Also, upon hydrogen reduction, the total pore volume of the hot plate derived powders increased but the microwave derived powders showed a decrease in the total pore volume which may be due to the destruction in the pore structure due to the crumbling of the particulates.
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3.3.6 Microstructure and morphology
Conventional TEM studies of the powder MUc10H are given in Fig. 8. Bright and dark field images (Fig. 8a and 8b respectively) show the presence of crystallites (grain size ranging between 100-200 nm). The selected area electron diffraction (SAED) pattern (Fig. 8c) confirmed
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that the “H” powders comprised fluorite (UO2) phase, the d-spacings in which corresponded to those of the JCPDF pattern (# 41-1422) [22]. HRTEM image characterization in Fig. 8d further confirms the fluorite structure with the presence of the lattice fringes corresponding to (111) and
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(220) planes. The zone axis [-1 2 1] was calculated by the systematic HRTEM analysis in Fig. 8e. Fig. 8f depicts the Inverse Fast Fourier Transform (IFFT) image of Fig. 8e. 4. Conclusion
nc-UO2 powders were prepared by using microwave-assisted citrate gel-combustion
method for the first time. The dependence of the reaction and the properties of the powders on the fuel to oxidant ratio (R) were investigated systematically. Mixtures with R = 0.1 (MUc10) underwent a volume combustion reaction. The reduction of the combustion powders was found to be necessary in order to obtain stoichiometric UO2. All these powders were flaky and
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comprised agglomerates of α-U3O8 or UO2. It was observed that the bulk density, specific surface area, size distribution of particles and pores, X-ray crystallite size and microstructure showed a strong dependence on the value of R. Nanocrystalline urania synthesized by microwave-assisted citrate gel-combustion method showed a similarity to the hot plate derived nanocrystalline urania
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powders in the values of their bulk density, SSA, PoSD, total pore volume, XCS for all “A”, “C” and “H” products. However, the volume combustion was observed in the mixture with R = 0.1, while volume combustion reaction has been observed for mixtures with R = 0.25, in hot plate assisted citrate gel-combustion synthesis of nc-UO2 [22]. Probably the microwaves accelerate the volume combustion reaction even with a lesser amount of fuel. The microwave-derived nanocrystalline
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urania powders showed a lower SSA, higher XCS, higher lattice strain, higher residual carbon and lesser total pore volume than their hot plate derived counterparts. This is due to the influence of
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microwave which accelerates grain coarsening.
Acknowledgements
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Authors would like to thank Dr. M. Joseph, Director, Materials Chemistry and MetalFuel Cycle Group, and Dr. A. K. Bhaduri, Director, Indira Gandhi Centre for Atomic Research for their sustained support and encouragement in the pursuit of this work. Ms. V. Hiranmayee expresses her deep sense of gratitude to HBNI for funding this project. The authors also thank UGC-DAE consortium for TEM facilities, Dr. N. Ramanathan & Dr. K. Sundararajan for their help in IR experiments, Dr. Ashish Jain & Mr. Raja Madhavan for the XRD experiments, Mr. Abhiram Senapathi for the SSA experiments, Mr. G. Jogeshwara Rao forParticle size distribution experiments, Dr. S. Balakrishnan & Mrs. N. Ambika for TGDTA-MS experiments and Mrs. D. Annie for the residual carbon analysis.
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List of Figures
Typical FTIR spectra of precursors (uranyl nitrate and citric acid) and products (MUc100C and MUc100H).
Figure 2.
TG-DTA-MS pertaining to the decomposition of MUc100A.
Figure 3.
Dependence of bulk density, specific surface area and total pore volume on the value
Figure 4.
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of R in, “A”, “C” and “H” powders.
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Figure 1.
XRD of the “MUcXXA”, “MUcXXC”, “MUcXXH” powders (Where XX a) 0,
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b)10, c) 25, d) 50, e) 75, f) 100). Figure 5.
a) XRD of the “MUc10H” immediately after hydrogen reduction
Figure 6.
a) Dependence of lattice strain on the value of R in, “A”, “C” and “H” powders, b) Dependence of X-ray crystallite size (XCS) on the value of R in, “A”, “C” and “H”
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powders, c) Variation of lattice strain with XCS, d) Variation of relative difference in XCS with lattice strain (where δ XCS= XCS HW-XCS Scherrer/XCS HW). Cumulative size distribution of pores in “MUcXXA”, “MUcXXC” and “MUcXXH”
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Figure 7.
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powders, (Where XX = a) 0, b)10, c) 25, d) 50, e) 75, f) 100). Figure 8.
TEM images of MUc10H powders: (a) bright field image revealing nano grains; (b) dark field image revealing nano grains; c) SAED image and (d) HRTEM image with
selected area; (e) FFT image revealing zone axis [-1 2 1] and (f) IFFT image of zone
axis [-1 2 1].
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Table 1. IR data of Uranyl nitrate, citric acid, calcined and hydrogen reduced powders of MUc100 Sample
Absorption band (cm-1 )
Vibration mode
1500-1600
-N=O (as.b)
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S.No
1300-1390
-N=O (s.b)
740-750 1.
-N-O (s.b)
Uranyl nitrate
-N-O (or)
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813
Uranyl ions
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949 1021
1681-1760 2.
Citric acid 1390-1550
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2500-3300
3.
MUc100C
4.
MUc100H
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1210-1320 and 1118-1210
Uranyl nitrate [22, 32-34]
Asymmetric normal vibration
-O-H(s) in carboxylic acids -C=O(s) in carboxylic acids H-bonded -COO- (s.b & as.b) -C-O(s) in carboxylic acids [22, 35-38]
No signature peaks of citric acid or uranyl nitrate
-
3300
adsorbed water -broad band
s-stretch, as.b- asymmetric bend, s.b- symmetric bend, or- out of plane rocking]
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Residual carbon (ppm)
Total pore volume (mm3 g-1)
Bulk density (Mg m-3)
R
0
180 (110) 150 (114)
H
A
C
70 (92)
570
662 (226)
H
A
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Table 2. Variation of total pore volume, Residual Carbon, Bulk density, specific surface area with value of R, in “as prepared”, “calcined” and “hydrogen reduced” powders
394 (269)
C
Specific surface area (m2 g-1)
H
A
C
H
0.99 (2.39)
1.78 (2.15) 2.36 (2.68) 5.0 (6.8) 4.4 (5.0) 4.6 (5.7)
710 (417) 640 (158) 120 (116)
2576
1833 (699) 1300 (1116)
0.17 (0.18)
0.14 (0.26) 0.29 (0.44) 7.3 (6.9) 5.8 (6.8) 6.8 (6.8)
0.25
410 (660) 210 (447) 161 (156)
1732
1812 (1549) 974 (4787)
0.33 (0.06)
0.24 (0.09) 0.43 (1.61) 6.6 (18.1) 5.6 (7.9) 6.3 (9.7)
0.5
1112 (877) 410 (631) 300 (229)
768
1235 (320)
762 (281)
0.59 (1.79)
0.55 (1.59) 0.82 (3.73) 6.3 (6.1) 5.4 (5.3) 6.1 (6.1)
0.75 414 (1270) 400 (552) 360 (274)
363
413 (405)
521 (403)
1.78 (1.29)
1.62 (1.25) 2.00 (2.63) 4.9 (6.0) 5.1 (5.0) 4.9 (5.7)
281 (414)
2.14 (1.28)
1.72 (1.15) 2.33 (2.27) 5.6 (5.6) 5.3 (5.4) 5.6 (4.9)
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1939 (1004) 590 (698) 370 (310)
529 (414)
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(*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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A
C
Sample HW (nm)
Strain
Scherrer (nm) HW (nm)
MUc00
-
-
-
70 (48)
MUc10
22 (13)
32(23)
0.006 (0.005)
51 (41)
MUc25
146 (20)
169 (32)
0.003(0.003)
61 (36)
MUc50
239 (19)
286 (24)
0.001 (0.002)
40 (35)
MUc75
42(18)
77 (24)
0.002 (0.001)
MUc100
27 (24)
49 (90)
0.012 (0.005)
H
Scherrer (nm)
HW (nm)
Strain
0.009 (0.002)
31 (42)
86 (146)
0.008 (0.001)
285 (50)
0.007 (0.000)
28 (39)
30 (79)
0.007 (0.001)
231 (38)
0.008 (0.000)
30 (32)
44 (47)
0.004 (0.000)
291 (58)
0.009 (0.000)
31 (33)
43 (54)
0.003 (0.001)
91 (34)
220 (69)
0.009 (0.001)
26 (35)
75 (85)
0.009 (0.001)
77 (38)
131 (98)
0.007 (0.002)
27 (27)
50 (45)
0.006 (0.001)
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Strain
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Scherrer (nm)
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Table 3. X-ray Crystallite size of urania powders
A, C & H – As prepared, Calcined & Hydrogen reduced powders; HW-Hall Williamson's method. (*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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MUc00
2
MUc10
3
MUc25
4
MUc50
5
MUc75
6
MUc100
A
C
H
A
346 (2)
4 (6)
6 (5)
436 (11)
11 (7)
4 (4)
5 (5)
280 (5)
3 (3)
2 (11)
14 (2)
414 (4)
4 (3)
9(2)
7 (3)
5 (3)
235 (2)
2 (4)
2 (3)
C
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1
10 vol. % of sample has size less than 50 vol. % of sample has size less than 90 vol. % of sample has size less than (µm) (µm) (µm)
36 (56)
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Sample
H
A
C
H
39 (13)
549 (197)
99 (662)
214 (63)
88 (62)
32 (32)
54 (32)
319 (270)
107 (162)
996 (131)
375 (27)
18 (14)
8
(61)
436 (108)
116 (56)
27 (220)
148 (24)
687 (28)
22 (21)
404 (411)
894 (304)
315 (371)
181 (26)
36 (33)
41 (27)
488 (192)
140 (254)
175 (150)
285 (38)
17 (48)
13 (20)
341 (166)
57 (230)
41 (97)
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S.No
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Table 4. Particle size distribution in microwave -assisted citrate gel-combustion derived nanocrystalline urania powders
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A, C & H – As prepared, Calcined & Hydrogen reduced powders.
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(*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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Figure 1. Typical FTIR spectra of precursors (uranyl nitrate and citric acid) and products (MUc100C and MUc100H) [s-stretch, b-bend, as.b- asymmetric bend, a.s- asymmetric stretch, s.b- symmetric bend, or- out of plane rocking]
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Figure 2. TG-DTA-MS pertaining to the decomposition of MUc100A
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Figure 3. Dependence of bulk density, specific surface area and total pore volume on the value of R in, “A”, “C” and “H” powders
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Figure 4. XRD of the “MUcXXA”, “MUcXXC”, “MUcXXH” powders (Where XX a) 0, b)10, c) 25 , d) 50, e) 75, f) 100)
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Figure 5. XRD of the “MUc10H” immediately after hydrogen reduction
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Figure 6. a) Dependence of lattice strain on the value of R in, “A”, “C” and “H” powders, b) Dependence of X-ray crystallite size (XCS) on the value of R in, “A”, “C” and “H” powders, c) Variation of lattice strain with XCS, d) Variation of relative difference in XCS with lattice strain (where δ XCS= XCS HW-XCS Scherrer/XCS HW)
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Figure 7. Cumulative size distribution of pores in “MUcXXA”, “MUcXXC” and “MUcXXH” powders, (Where XX = a) 0, b)10, c) 25 , d) 50, e) 75, f) 100)
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Figure 8. TEM images of MUc10H powders: (a) bright field image revealing nano grains; (b) dark field image revealing nano grains; c) SAED image and (d) HRTEM image with selected area; (e) FFT image revealing zone axis [-1 2 1] and (f) IFFT image of zone axis [-1 2 1]
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HIGHLIGHTS Nanocrystalline urania was synthesized through the microwave-assisted citrate gel-combustion
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method for the first time.
The influences of citric acid to nitrate ratio on the powder characteristics of the final product was investigated.
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Volume combustion was observed for the reaction mixture with fuel to oxidant ratio of 0.1.
S0022-3115(18)30512-9
DOI:
https://doi.org/10.1016/j.jnucmat.2018.12.031
Reference:
NUMA 51363
To appear in:
Journal of Nuclear Materials
Received Date: 6 June 2018 Revised Date:
15 December 2018
Accepted Date: 17 December 2018
Please cite this article as: V. Hiranmayee, K. Ananthasivan, D. Maji, K. Joseph, Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania, Journal of Nuclear Materials (2019), doi: https://doi.org/10.1016/j.jnucmat.2018.12.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania a
a,
a
V. Hiranmayee , K. Ananthasivan *, Dasarath Maji , Kitheri Joseph
Materials Chemistry and Metal-Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamil Nadu- 603102, India. Abstract
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A systematic investigation on the microwave-assisted citrate gel-combustion synthesis of nanocrystalline urania powders is being reported for the first time. Nanocrystalline urania powders were synthesized by microwave-assisted citrate gel-combustion method by using citric acid as a fuel and nitrate as an oxidizer. The fuel to nitrate ratio (R) was varied from 0 to 1 (0, 0.1, 0.25, 0.5, 0.75 and 1.0). It was observed that at a particular stoichiometric ratio (R = 0.1), auto ignition reaction takes place resulting in a porous product. The as-prepared powders were calcined and further reduced under flowing hydrogen. All these powders were characterized for the phases present viz., X-ray diffraction, size distribution of pores, transmission electron microscopy, specific surface area, TG-DTA-MS, and infrared spectroscopy. It was observed that the microwave-derived powders with an R-value of 0.1 had the lowest bulk density, highest SSA and highest total pore volume. Upon calcination and reduction, the total pore volume decreased. The specific surface area of the calcined powders was the lowest and that of the as-prepared powders was the highest while that of the hydrogen-reduced powders was in between. An opposite trend was observed with the X-ray crystallite size. The microwave-derived nanocrystalline urania powders showed a lower SSA, higher XCS, higher lattice strain, higher residual carbon and lesser total pore volume than their hot plate derived counterparts. This is probably due to the influence of microwave which accelerates the reaction leading to faster grain coarsening.
Keywords: Nanocrystalline urania, Microwave-assisted gel-combustion synthesis, High burn up
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structures, TEM, XCS, Lattice strain. *Corresponding Author: Dr K. Ananthasivan, [email protected] Phone: Ph: +91 44 2748 0500, Extension: 24069 Fax: +91 44 2748 0065
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1. Introduction
UO2 is widely used as a fuel in fission reactors. Studies on the spent fuels discharged
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from both Light Water Reactors (LWRs), as well as Fast Breeder Reactors (FBRs), have revealed that prolonged irradiation of these fuels leads to the formation of a unique microstructure called the “High Burn-up Structure” (HBS) [1–6]. Microstructural investigations had revealed that HBS comprises submicron grains with size ranging from 0.1 – 0.3 µm. This structure is essentially nanocrystalline, consists of nanopores and is found at the periphery of the
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fuel pellet [7]. HBS comprises closed porosity that retains fission gases, shows resistance to internal stresses and radiation damage due to its higher plasticity and nanostructure. Therefore,
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the study of nanocrystalline urania (nc-UO2) has attracted the nuclear technologists. In order to study nc-UO2, bulk quantities of the same are required. Ceramic grade urania with grain size generally in micrometer range is commonly prepared by calcining ammonium diuranate and reducing the product under controlled conditions [8,9]. However, experimental investigations on the preparation of nc-UO2 in bulk quantities are rather limited. Several methods have been cited in the literature for the preparation of nc-UO2 viz., photochemical reaction [10], oxalate
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precipitation [11], non-aqueous colloidal synthesis [12], hydrothermal method [13], laser ablation [14] and sulfate-mediated synthesis [15]. But some of these methods are not costeffective, yield agglomerated powders with large grains, introduce impurities, show low yield and generate a large quantity of liquid waste [13, 14]. Earlier investigations [16–18] on the gel-
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combustion synthesis had demonstrated that the microwave-assisted citrate gel-combustion method yields a nc-powder with better sinterability and high purity. Further, this process is
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energy efficient and the heating gets completed within a few minutes. Gel-combustion synthesis utilizes the high exothermicity of the chemical reaction
ensuing between a fuel and an oxidizer, with the evolution of a large volume of gases resulting in purer products [8, 16, 19–22]. In recent years, considerable work has been carried out on the synthesis of nano nuclear ceramics by using citric acid as the fuel [22–26]. Citric acid is particularly advantageous for it is a good chelating agent and produces the reaction with moderate exothermicity. Recently, a systematic study was carried out on the synthesis of nanocrystalline urania in bulk quantities in our laboratory by following the citrate gel6+
combustion method with hot plate as the heat source [22]. U
is a microwave active ion and
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a solution containing the same could be heated within a short duration by using microwave [5]. Even though investigations on the microwave-assisted synthesis of (U1-xThx)O2 solid solutions have been reported [27], a systematic study on the microwave-assisted gel-combustion synthesis of nc-UO2 has not been reported so far. Hence, in this study, we undertook the preparation of
1.0), on the combustion synthesis of nc-UO2 was investigated. 2. Experimental
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2.1 Starting materials
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nanocrystalline urania by microwave-assisted citrate gel-combustion method. Also, the influence of the fuel (citric acid) to oxidant (nitrate) ratio [22, 28–30], i.e. (R = 0.0, 0.1, 0.25, 0.5, 0.75,
Uranium oxide (U3O8) of nuclear grade purity was procured from Nuclear Fuel
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Complex, Hyderabad, India. Ferroin indicator (AR 0.025 M) and citric acid monohydrate (AR) (> 99.5% pure) were obtained from Loba chime Pvt. Ltd, Mumbai, India. Ammonium iron (II) sulfate hexahydrate (GR 99%) was procured from Merck Ltd, Mumbai, India. AR grade nitric acid, sulphuric acid and hydrochloric acid were supplied by M/s. Rankem Laboratory Chemicals Pvt. Ltd., Chennai.
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2.2 Powder synthesis
In a typical experiment, a known amount of urania powder was heated at 873 K for 4 h in air to obtain U3O8. The latter was dissolved completely in minimum quantity of concentrated nitric acid to obtain a solution of uranyl nitrate. The excess acid was removed by evaporation. -
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The estimation of NO3 was carried out by titrimetry as described in ref. [22,31]. In a typical gel-
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combustion experiment, desired quantities of the fuel and oxidant were dissolved in minimum quantity of distilled water and heated in a microwave oven operating at an output power of 900 W. Mixtures with R = 0 (no citric acid), 0.1, 0.25, 0.5, 0.75 and 1.0 were prepared. Upon heating, these mixtures gelated, dehydrated and then underwent auto ignition resulting in a carbonaceous oxide ash (as-prepared powders). The resultant powders were calcined in air at 1073 K for 4 h in a furnace equipped with SiC heating element. The calcined powders were further reduced to -1
UO2 under flowing hydrogen at 1073 K for 4 h at a flow rate of 500 mL min , in a custommade furnace equipped with Inconel sample holders. These powders were designated with different labels by using suffices “A”, “C”, “H” to identify “as-prepared”, “calcined” and “hydrogen-reduced” powders respectively. All the indices bore a prefix MUc, to denote urania powders prepared by microwave-assisted citrate
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gel-combustion method. Each mixture was distinguished with the help of two digits “XX” which referred to the value of R (in percentage). Thus, powders prepared from a mixture with R= 0.10 were designated as MUc10A, MUc10C, MUc10H to denote that these are the “as-prepared”, “calcined” and “hydrogen-reduced” products respectively.
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2.3 Characterization
The FTIR spectrum of the precursor materials and final products were recorded (Bruker -1
α- FTIR spectrometer, Attenuated Total Reflectance (ATR) mode in the region 4000-650 cm -1
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with a resolution of 4 cm ). Study of the thermal decomposition of the powders was performed
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by using a TG-DTA-MS system NETZSCH, (STA 449F3 Jupiter and QMS 403 D, NETZSCH, Germany). The error in temperature measurement in TG-DTA-MS experiments was ± 1 K and that in weight was ± 1 mg for the blank corrected data. An electronic balance supplied by M/s. Scaltec, GmbH, Germany was used to estimating the bulk density of all the powders. The latter was measured from the weight of the powder that filled a cuvette of known volume up to the brim. Average of five measurements is reported here. The error in the density values reported is 3
± 0.04 Mg/m . The XRD pattens pertaining to all the “A”, “C” and “H” powders were recorded
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by using X’Pert Pro MPD system, supplied by M/s. Panalytical, Netherlands by using (Cu Kαradiation (λ= 1.541Å) equipped with a graphite monochromator and a scintillation detector. Calibration was carried out by using silicon and alumina standards supplied by NIST, USA. Xray crystallite size (XCS) of all these samples was estimated from the width of their respective diffraction patterns by Hall-Williamson method and Scherrer method. Specific surface area (SSA) of all the urania powders was determined from nitrogen
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adsorption by using the BET method (Surfer Analyser, Thermo Fisher Scientific S.p.A, Milan, Italy). Blank analysis and calibration of SSA instrument were carried out by using α-alumina as
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standard material supplied by BAM, Germany. The error in the measurement of the SSA was 2
± 0.04 m /g. Distribution of sizes of the pores present in the “A”, “C” and “H” powders was investigated by mercury intrusion porosimetry (Pascal 140 and 440 mercury porosimeter, Thermo Fisher Scientific S.p.A, Milan, Italy) at an operating pressure range of 0.1 kPa - 400 kPa and 0.1 MPa - 400 MPa respectively. Blank analysis and calibration in the mercury porosimeter experiments were carried out by using α-alumina standard obtained from NIST. The error in the 3
measurement of the pore volume is ± 13 mm /g. The carbon residue present in all the “A”, “C” and “H” powders was determined by using oxidative fusion (M/s. Eltra CS 800, Germany). In a typical analysis, mixtures containing about 200 mg of the samples and 1500 mg of tungsten were
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taken in an alumina crucible and heated in an induction furnace under flowing O2. The CO2 evolved was quantitatively measured by using an IR detector. The powders synthesized from mixtures with R= 0.10 (MUc10H) were characterized by using high-resolution transmission electron microscopy (HRTEM), LIBRA 200FE (Carl Zeiss,
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Germany) operated at 200 kV and the information limit of the HRTEM was found to be 0.13 nm. were then placed onto a holey carbon grid and dried. 3. Results and discussion 3.1 IR spectroscopy of the precursor and products
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For the TEM analysis, the powder was dispersed in propane-2-ol and few drops of this solution
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The FTIR spectra of uranyl nitrate, citric acid and the powders (“C” and “H”) obtained from a mixture of R=1.0 (MUc100) are illustrated in Fig. 1. The details of the IR data are shown in Table 1. The absorption signatures of N-O and C=O for MUc100C and MUc100H were absent, possibly because of the complete removal of the organic precursor (citric acid) and nitrates. Thus it is reasonable to conclude from the IR spectra, that the reaction has gone to completion [22, 32–38].
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3.2 TG-DTA-MS
The results obtained in the TG-DTA-MS investigation from a mixture with R= 1.0 (MUc100A), are shown in Fig. 2. The DTA curve shows a small endothermic peak at around 443 K,
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which could be attributed to the loss of water. The weight loss observed between 473 K and 673 K could be due to the decomposition of the sample with the attendant liberation of gaseous products. The peak observed around 673 K could be attributed to the elimination of the carbon residue by the liberation of CO2 upon combustion [22]. The optimum calcination temperature was chosen from the
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thermogravimetric analysis. It was observed that the weight of the sample remained nearly constant at temperatures above 1043 K. Hence the calcination temperature was chosen as 1073 K in order to ensure complete removal of carbon from the “A” powders. Calcination at a higher temperature was avoided in order to limit grain coarsening. The mass spectrometric analysis of the gases evolved at various stages from the sample is also shown in Fig. 2. As shown in Fig. 2, the gases evolved corresponding to the first step of mass loss are H2O, OH and the second step are H2O, OH, CO2, CO, NO2,
13
CO2. The inset in the Fig. 2 shows the masses 45 and 46 in the low ion current range. The
natural abundance ratio of 13
12
13
C: C is 100:1. This is evident in the ion current ranges of
CO2. Also, the mass numbers 44
12
CO2 and
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and 45 follow the same pattern. Thus it is reasonable to assign the peak observed at the mass number 45 to
13
CO2 [39].
3.3 Characterization of powders
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3.3.1 Bulk Density The dependence of bulk density (BD) of the urania powders on the value of R is shown in -3
Fig. 3. The BD of these powders was found to vary between 0.14-2.36 Mgm . BD of “A”, “C” -3
and “H” powders was found to lie between 0.17-2.14 Mgm , 0.14-1.77 Mgm
and 0.29-2.36
respectively. The powders MUc10A, MUc10C and MUc10H were found to possess the
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Mgm
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-3
lowest values of BD, indicating that these samples are highly porous. The following trend viz.,
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BDH > BDA > BDC was observed. The lowest BD exhibited by “C” powders could be attributed to the generation of pores due to the emanation of CO2 during calcination. The following trend
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was observed in the variation of bulk density with R: MUc00 H/A/C > MUc100 H/A/C > MUc75 H/A/C > MUc50 H/A/C > MUc25 H/A/C > MUc10 H/A/C. It was observed that the microwave derived nanocrystalline urania powders exhibited lowest BD’s for mixture with R = 0.1 (MUc10 powders) where as the hot plate derived powders exhibited lowest BD’s for mixture with R = 0.25 [22]. But in both these methods the lowest BD is observed for the powders derived through the volume combustion reaction. The comparison of the bulk densities of powders obtained in the present study as well as those reported in reference [22] with the powders prepared through many other conventional methods [40–42], reveals that the urania powders with the lowest bulk density could be produced only by citrate gel-combustion method. The values reported in this study are lower than the values obtained by Sanjay Kumar et al. [22]. 3.3.2 Specific surface area (SSA)
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The dependence of specific surface area (SSA) of the urania powders on the value of R is shown in Table 2 and plotted in Fig. 3. It is evident from these data that the SSA follows the trend SSAA > SSAH > SSAC. MUc10A had the highest SSA. SSA decreased from “A” to “C” powders because of grain coarsening as evident from the X-ray crystallite size (Table 3). The “H” powders possessed higher specific surface area than that of the corresponding “C” powders, probably due to the crumbling of U3O8 particles during hydrogen reduction [22]. It was observed that with an increase in the R value there is an increase in SSA, probably due to to the vigorous reaction in which the evolution of gases is maximum, resulting in powders with a high specific surface area. The microwave-derived powders showed a lower SSA than the hot plate derived
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powders [22], probably due to microwave-assisted in-situ grain coarsening during the combustion reaction. 3.3 Residual carbon
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The quantity of carbon residue has a direct bearing on porosity, density, strength and thermal conductivity [43]. The variation in the quantity of the carbon residue for “A”, “C” and “H” powders is shown in Table 2. From these data, it is evident that the residual carbon increases with the value of R and decreases upon calcination and hydrogen reduction. The decrease in the carbon residue upon calcination and hydrogen reduction is due to the removal of carbon as CO2
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and hydrocarbons upon reaction with oxygen and hydrogen respectively. The quantity of carbon residue was found to vary between 150 and 800 ppm in calcined samples while it was found to
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vary between 70 and 370 ppm for hydrogen reduced samples. These values are slightly higher than those cited in references [22, 41]. This could be due to the incomplete removal of carbon due to the insufficient supply of oxygen inside the closed microwave oven. The residual carbon causes porosity gradients and influences the O/U ratio in these powders, which in turn could help the pre-sintering of the agglomerates [22, 44, 45]. 3.3.4 Phase characterization and XCS
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The XRD analyses of the urania powders synthesized in this study revealed that the “A” and “C” powders comprised orthorhombic U3O8 phase, while the “H” powders consisted of the fluorite (UO2) phase (Fig. 4). All XRD patterns presented in this work are raw data without any
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refinement and the peaks obtained are well resolved. The crystallite size was calculated from both Scherrer formula and Hall-Williamson (HW) plots, while the lattice strain was derived from the Hall-Williamson method. The XCS (HW) of hydrogen reduced urania powders was in the range of 30-86 nm. The XCS (HW) of calcined urania powders was in the range of 131-291 nm. The XCS (HW) of “A” powders was in the range of 32-286 nm. The urania powder obtained by the denitration without the addition of citric acid (MUc00A) was amorphous. The crystallite size of “C” powders was higher than that of their “A” precursors due to grain coarsening. However, XCS decreased upon hydrogen reduction, possibly due to the crumbling of the orthorhombic U3O8 during its reduction to UO2 [22]. XCS followed the trend viz., XCSC > XCSA > XCSH. It was found that as the R-value increases, the amounts of gases evolved also increase, thus resulting in powders with a high specific surface area and lower mean agglomerated size. Along with the combustion synthesis, pre-sintering also takes place. These two processes compete with each other. It is a well-known fact that the combustion reaction is self-propagating and rapid. So,
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it does not allow the growth of the crystallites. The XCS of microwave derived combustion powders were higher than that of the hot plate derived powders [22], probably due to in-situ grain coarsening. The change in the density due to the conversion of U3O8 into UO2 is typically
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about 29%. However, the difference between the BD of the C and H powders is more than this value. This could be ascribed to the introduction of porosity or the production of finer particulates or both during hydrogen reduction. Fig. 5a shows the XRD of MUc10H powder after immediate hydrogen reduction. It was observed that the XRD data obtained from the hydrogen reduced powders after one week of storage in air resulted in UO2.12 phase (Fig. 4), while XRD
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recorded immediately after hydrogen reduction resulted in stiochiometric UO2 phase (Fig. 5). The dimensionless lattice strain calculated from the Hall-Williamson method (Fig. 6a) showed that the powder MUc50A showed least lattice strain among the “A” powders. The lattice
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strain values pertaining to the “C” and “H” powders were higher than that of the corresponding “A” powders. Insufficient time for nucleation and growth, partial sintering during calcination, explain these observations [22]. Investigations carried out by P. Bindu et al. (ZnO) [46], Yusheng Zhao et al. (α-SiC and Ni) [47] and A.E. Mahmoud et al. (Al-Al2O3 powders) [48] also report a similar trend in their studies on nanocrystalline powders. Table 3 and Fig. 6b show the variation in X-ray crystallite size of all the urania powders. The lattice strain values obtained in
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this study were high as compared to those cited in the reference [22]. It may be due to the slightly higher XCS obtained in microwave derived powders than the hot plate derived combustion powders. The variation of lattice strain with XCS (Fig. 6c) could be delineated into two regions (i.e. XCS < 100 nm and XCS > 100 nm). The region below 100 nm showed a nearly linear dependence of lattice strain with XCS while region above 100 nm showed more or less a
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constant lattice strain. This trend is similar to that reported by Sanjay Kumar et al. [22]. It appears that the lattice strain approaches a constant value when it falls within the nanocrystalline
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regime (> 100 nm). Fig. 6d confirms the linear dependence of lattice strain with the relative difference of XCS. A similar trend in the variation of lattice strain with XCS and in the variation of lattice strain with the relative difference of XCS has been reported by Sanjay Kumar et al. [22].
3.3.5 Size distribution of pores Cumulative pore size distribution (CPoSD) in the “A”, “C”, “H” urania powders estimated with the help of mercury porosimetry is presented in the Fig. 7. All the “A” powders have sub-micron pores (below 1000 nm) and a major contribution to the specific pore volume in the “A” powders is from the micropores. Upon calcination, the specific pore volume was found
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to decrease while a nearly complete removal of pores resulted in hydrogen reduction. The following trend was observed in the variation of specific pore volume with R: MUc10 A/C/H > MUc25 A/C/H > MUc50 A/C/H > MUc0 A/C/H ~ MUc75 A/C/H ~ MUc100 A/C/H. It is observed (Fig. 7) that the specific volume of mercury intruded into the pores was maximum for
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MUc10 among all the “A”, “C”, “H” powders. The total pore volume of all these powders (as shown in the Fig. 3) was found to be inversely correlated with their corresponding bulk density [22]. Total pore volume of the “C” powders as higher than that of the corresponding “A” powders. This could be ascribed to the generation of pores due to the emanation of CO2 during calcination.
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However, due to the conversion of U3O8 to UO2 during hydrogen reduction, the pore structure is partially destroyed due to the crumbling of the powder. Therefore, the “H” powders show less total pore volume than “A” and “C” powders. The “A”, “C” and “H” powders were found to possess pores
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with mean diameters ranging from 100 nm to 100 µm. Hot plate derived nanocrystalline urania powders had higher specific pore volume than the microwave derived powders [22]. This could be due to microwave-assisted in-situ grain coarsening during the reaction. Also, upon hydrogen reduction, the total pore volume of the hot plate derived powders increased but the microwave derived powders showed a decrease in the total pore volume which may be due to the destruction in the pore structure due to the crumbling of the particulates.
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3.3.6 Microstructure and morphology
Conventional TEM studies of the powder MUc10H are given in Fig. 8. Bright and dark field images (Fig. 8a and 8b respectively) show the presence of crystallites (grain size ranging between 100-200 nm). The selected area electron diffraction (SAED) pattern (Fig. 8c) confirmed
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that the “H” powders comprised fluorite (UO2) phase, the d-spacings in which corresponded to those of the JCPDF pattern (# 41-1422) [22]. HRTEM image characterization in Fig. 8d further confirms the fluorite structure with the presence of the lattice fringes corresponding to (111) and
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(220) planes. The zone axis [-1 2 1] was calculated by the systematic HRTEM analysis in Fig. 8e. Fig. 8f depicts the Inverse Fast Fourier Transform (IFFT) image of Fig. 8e. 4. Conclusion
nc-UO2 powders were prepared by using microwave-assisted citrate gel-combustion
method for the first time. The dependence of the reaction and the properties of the powders on the fuel to oxidant ratio (R) were investigated systematically. Mixtures with R = 0.1 (MUc10) underwent a volume combustion reaction. The reduction of the combustion powders was found to be necessary in order to obtain stoichiometric UO2. All these powders were flaky and
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comprised agglomerates of α-U3O8 or UO2. It was observed that the bulk density, specific surface area, size distribution of particles and pores, X-ray crystallite size and microstructure showed a strong dependence on the value of R. Nanocrystalline urania synthesized by microwave-assisted citrate gel-combustion method showed a similarity to the hot plate derived nanocrystalline urania
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powders in the values of their bulk density, SSA, PoSD, total pore volume, XCS for all “A”, “C” and “H” products. However, the volume combustion was observed in the mixture with R = 0.1, while volume combustion reaction has been observed for mixtures with R = 0.25, in hot plate assisted citrate gel-combustion synthesis of nc-UO2 [22]. Probably the microwaves accelerate the volume combustion reaction even with a lesser amount of fuel. The microwave-derived nanocrystalline
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urania powders showed a lower SSA, higher XCS, higher lattice strain, higher residual carbon and lesser total pore volume than their hot plate derived counterparts. This is due to the influence of
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microwave which accelerates grain coarsening.
Acknowledgements
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Authors would like to thank Dr. M. Joseph, Director, Materials Chemistry and MetalFuel Cycle Group, and Dr. A. K. Bhaduri, Director, Indira Gandhi Centre for Atomic Research for their sustained support and encouragement in the pursuit of this work. Ms. V. Hiranmayee expresses her deep sense of gratitude to HBNI for funding this project. The authors also thank UGC-DAE consortium for TEM facilities, Dr. N. Ramanathan & Dr. K. Sundararajan for their help in IR experiments, Dr. Ashish Jain & Mr. Raja Madhavan for the XRD experiments, Mr. Abhiram Senapathi for the SSA experiments, Mr. G. Jogeshwara Rao forParticle size distribution experiments, Dr. S. Balakrishnan & Mrs. N. Ambika for TGDTA-MS experiments and Mrs. D. Annie for the residual carbon analysis.
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List of Figures
Typical FTIR spectra of precursors (uranyl nitrate and citric acid) and products (MUc100C and MUc100H).
Figure 2.
TG-DTA-MS pertaining to the decomposition of MUc100A.
Figure 3.
Dependence of bulk density, specific surface area and total pore volume on the value
Figure 4.
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of R in, “A”, “C” and “H” powders.
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Figure 1.
XRD of the “MUcXXA”, “MUcXXC”, “MUcXXH” powders (Where XX a) 0,
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b)10, c) 25, d) 50, e) 75, f) 100). Figure 5.
a) XRD of the “MUc10H” immediately after hydrogen reduction
Figure 6.
a) Dependence of lattice strain on the value of R in, “A”, “C” and “H” powders, b) Dependence of X-ray crystallite size (XCS) on the value of R in, “A”, “C” and “H”
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powders, c) Variation of lattice strain with XCS, d) Variation of relative difference in XCS with lattice strain (where δ XCS= XCS HW-XCS Scherrer/XCS HW). Cumulative size distribution of pores in “MUcXXA”, “MUcXXC” and “MUcXXH”
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Figure 7.
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powders, (Where XX = a) 0, b)10, c) 25, d) 50, e) 75, f) 100). Figure 8.
TEM images of MUc10H powders: (a) bright field image revealing nano grains; (b) dark field image revealing nano grains; c) SAED image and (d) HRTEM image with
selected area; (e) FFT image revealing zone axis [-1 2 1] and (f) IFFT image of zone
axis [-1 2 1].
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Table 1. IR data of Uranyl nitrate, citric acid, calcined and hydrogen reduced powders of MUc100 Sample
Absorption band (cm-1 )
Vibration mode
1500-1600
-N=O (as.b)
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S.No
1300-1390
-N=O (s.b)
740-750 1.
-N-O (s.b)
Uranyl nitrate
-N-O (or)
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813
Uranyl ions
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949 1021
1681-1760 2.
Citric acid 1390-1550
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2500-3300
3.
MUc100C
4.
MUc100H
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1210-1320 and 1118-1210
Uranyl nitrate [22, 32-34]
Asymmetric normal vibration
-O-H(s) in carboxylic acids -C=O(s) in carboxylic acids H-bonded -COO- (s.b & as.b) -C-O(s) in carboxylic acids [22, 35-38]
No signature peaks of citric acid or uranyl nitrate
-
3300
adsorbed water -broad band
s-stretch, as.b- asymmetric bend, s.b- symmetric bend, or- out of plane rocking]
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Residual carbon (ppm)
Total pore volume (mm3 g-1)
Bulk density (Mg m-3)
R
0
180 (110) 150 (114)
H
A
C
70 (92)
570
662 (226)
H
A
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C
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A
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Table 2. Variation of total pore volume, Residual Carbon, Bulk density, specific surface area with value of R, in “as prepared”, “calcined” and “hydrogen reduced” powders
394 (269)
C
Specific surface area (m2 g-1)
H
A
C
H
0.99 (2.39)
1.78 (2.15) 2.36 (2.68) 5.0 (6.8) 4.4 (5.0) 4.6 (5.7)
710 (417) 640 (158) 120 (116)
2576
1833 (699) 1300 (1116)
0.17 (0.18)
0.14 (0.26) 0.29 (0.44) 7.3 (6.9) 5.8 (6.8) 6.8 (6.8)
0.25
410 (660) 210 (447) 161 (156)
1732
1812 (1549) 974 (4787)
0.33 (0.06)
0.24 (0.09) 0.43 (1.61) 6.6 (18.1) 5.6 (7.9) 6.3 (9.7)
0.5
1112 (877) 410 (631) 300 (229)
768
1235 (320)
762 (281)
0.59 (1.79)
0.55 (1.59) 0.82 (3.73) 6.3 (6.1) 5.4 (5.3) 6.1 (6.1)
0.75 414 (1270) 400 (552) 360 (274)
363
413 (405)
521 (403)
1.78 (1.29)
1.62 (1.25) 2.00 (2.63) 4.9 (6.0) 5.1 (5.0) 4.9 (5.7)
281 (414)
2.14 (1.28)
1.72 (1.15) 2.33 (2.27) 5.6 (5.6) 5.3 (5.4) 5.6 (4.9)
336
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1939 (1004) 590 (698) 370 (310)
529 (414)
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0.1
(*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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A
C
Sample HW (nm)
Strain
Scherrer (nm) HW (nm)
MUc00
-
-
-
70 (48)
MUc10
22 (13)
32(23)
0.006 (0.005)
51 (41)
MUc25
146 (20)
169 (32)
0.003(0.003)
61 (36)
MUc50
239 (19)
286 (24)
0.001 (0.002)
40 (35)
MUc75
42(18)
77 (24)
0.002 (0.001)
MUc100
27 (24)
49 (90)
0.012 (0.005)
H
Scherrer (nm)
HW (nm)
Strain
0.009 (0.002)
31 (42)
86 (146)
0.008 (0.001)
285 (50)
0.007 (0.000)
28 (39)
30 (79)
0.007 (0.001)
231 (38)
0.008 (0.000)
30 (32)
44 (47)
0.004 (0.000)
291 (58)
0.009 (0.000)
31 (33)
43 (54)
0.003 (0.001)
91 (34)
220 (69)
0.009 (0.001)
26 (35)
75 (85)
0.009 (0.001)
77 (38)
131 (98)
0.007 (0.002)
27 (27)
50 (45)
0.006 (0.001)
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277 (283)
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Strain
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Scherrer (nm)
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Table 3. X-ray Crystallite size of urania powders
A, C & H – As prepared, Calcined & Hydrogen reduced powders; HW-Hall Williamson's method. (*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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MUc00
2
MUc10
3
MUc25
4
MUc50
5
MUc75
6
MUc100
A
C
H
A
346 (2)
4 (6)
6 (5)
436 (11)
11 (7)
4 (4)
5 (5)
280 (5)
3 (3)
2 (11)
14 (2)
414 (4)
4 (3)
9(2)
7 (3)
5 (3)
235 (2)
2 (4)
2 (3)
C
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1
10 vol. % of sample has size less than 50 vol. % of sample has size less than 90 vol. % of sample has size less than (µm) (µm) (µm)
36 (56)
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Sample
H
A
C
H
39 (13)
549 (197)
99 (662)
214 (63)
88 (62)
32 (32)
54 (32)
319 (270)
107 (162)
996 (131)
375 (27)
18 (14)
8
(61)
436 (108)
116 (56)
27 (220)
148 (24)
687 (28)
22 (21)
404 (411)
894 (304)
315 (371)
181 (26)
36 (33)
41 (27)
488 (192)
140 (254)
175 (150)
285 (38)
17 (48)
13 (20)
341 (166)
57 (230)
41 (97)
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S.No
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Table 4. Particle size distribution in microwave -assisted citrate gel-combustion derived nanocrystalline urania powders
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A, C & H – As prepared, Calcined & Hydrogen reduced powders.
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(*) The values shown within the brackets represent the data obtained from the citrate gel-combustion synthesis of nc-UO2 by using hot plate as a heat source [22].
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Figure 1. Typical FTIR spectra of precursors (uranyl nitrate and citric acid) and products (MUc100C and MUc100H) [s-stretch, b-bend, as.b- asymmetric bend, a.s- asymmetric stretch, s.b- symmetric bend, or- out of plane rocking]
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Figure 2. TG-DTA-MS pertaining to the decomposition of MUc100A
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Figure 3. Dependence of bulk density, specific surface area and total pore volume on the value of R in, “A”, “C” and “H” powders
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Figure 4. XRD of the “MUcXXA”, “MUcXXC”, “MUcXXH” powders (Where XX a) 0, b)10, c) 25 , d) 50, e) 75, f) 100)
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Figure 5. XRD of the “MUc10H” immediately after hydrogen reduction
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Figure 6. a) Dependence of lattice strain on the value of R in, “A”, “C” and “H” powders, b) Dependence of X-ray crystallite size (XCS) on the value of R in, “A”, “C” and “H” powders, c) Variation of lattice strain with XCS, d) Variation of relative difference in XCS with lattice strain (where δ XCS= XCS HW-XCS Scherrer/XCS HW)
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Figure 7. Cumulative size distribution of pores in “MUcXXA”, “MUcXXC” and “MUcXXH” powders, (Where XX = a) 0, b)10, c) 25 , d) 50, e) 75, f) 100)
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Figure 8. TEM images of MUc10H powders: (a) bright field image revealing nano grains; (b) dark field image revealing nano grains; c) SAED image and (d) HRTEM image with selected area; (e) FFT image revealing zone axis [-1 2 1] and (f) IFFT image of zone axis [-1 2 1]
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HIGHLIGHTS Nanocrystalline urania was synthesized through the microwave-assisted citrate gel-combustion
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method for the first time.
The influences of citric acid to nitrate ratio on the powder characteristics of the final product was investigated.
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Volume combustion was observed for the reaction mixture with fuel to oxidant ratio of 0.1.