Application of atmospheric pressure microplasma discharge in preparation of nano-sized uranium dioxide

Abstract

The application relates to the field of molten salt electrochemistry, in particular to application of normal-pressure micro-plasma discharge in preparation of nano uranium dioxide particles. The normal-pressure plasma discharge can be used for synthesizing nano uranium dioxide particles with high crystallinity in molten salt, the prepared uranium dioxide product has good crystallinity, high purity and excellent oxidation resistance, and the product particle size is between 20-200 nm.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of molten salt electrochemistry, in particular to the application of atmospheric microplasma discharge in the preparation of nano uranium dioxide particles. BACKGROUND

[0002] Uranium oxides (UO2, UO3, U3O8) play an important role in traditional nuclear fuel manufacturing and high-efficiency catalysis, and have been extensively studied in the past half century. UO2 is the preferred uranium oxide system for nuclear fuel, which is related to high burn structure (HBS) because of its excellent thermal and chemical stability and the highest fissile density in the oxide matrix. Size and shape are often the main factors affecting the performance of materials, especially when the size reaches the nanoscale. It has been reported that nano UO2 has higher sintering ability than bulk UO2, higher combustion intensity than micron UO2 particles, and higher resistance to radiation damage, etc. The main methods for synthesizing UO2 nanoparticles are irradiation decomposition, hydrothermal method and sol-gel method. In addition, some microbial-mediated processes have been used to solve the problem of uranium-contaminated spent fuel waste, which provides a new method for the synthesis of UO2 nanoparticles. However, UO2 nanoparticles prepared by irradiation have very low sintering temperature, but the stability of the synthesized product is poor and will dissolve or oxidize in a short time (a few hours), while the stability and high oxidation resistance of UO2 nanoparticles are crucial to their performance as nuclear fuel and catalyst. The hydrothermal method and the sol-gel method require the introduction of organic reagents as structure directing agents and reducing agents during the manufacturing process, resulting in the presence of organic residues in UO2 nanoparticles, which is a big problem and may contaminate the use of UO2. The microbial-mediated method is still immature and can only be used for micro-preparation. Therefore, it is an important and urgent problem to develop a kind of nano UO2 particle with no organic reagent, good crystallinity, high purity and excellent oxidation resistance. SUMMARY

[0003] In order to solve the above technical problems of the present application, the present application provides the application of atmospheric microplasma discharge in the preparation of nano uranium dioxide particles in a molten salt system.

[0004] According to a first aspect of the present application, the present application provides the application of atmospheric microplasma discharge in the preparation of nano uranium dioxide.

[0005] Atmospheric microplasma gas electrode is a new type of gaseous electrode, which can be used to prepare nanoparticles in electrolyte, and has the characteristics of simple reaction and convenient synthesis.

[0006] The inventors found that the prepared uranium dioxide product has good crystallinity, high purity and excellent oxidation resistance, and the product particle size is between 20-200 nm in the research of preparing nano uranium dioxide particle material by using normal pressure micro-plasma gas electrode. The electrolysis time, experimental temperature, initial concentration of uranyl ion and electrolysis current size all have certain influence on the synthesis of particles. Through the electrolysis time experiment, it is found that the micro-plasma induced discharge process conforms to the law of Faraday law, and more products can be obtained by prolonging the electrolysis time, and the size will be larger, and the Oswald ripening phenomenon also appears in the excessive electrolysis time experiment. The experimental temperature is an important factor affecting the electrolysis process, the higher the temperature, the faster the electrolysis reduction rate, and more products can be obtained in the same time. The product particle size is also strongly affected by temperature, and larger spherical particles can be obtained at high temperature. The initial concentration of uranyl ion has little effect on the electrolysis reduction rate of the experiment, but it strongly affects the particle size of the product, the larger the concentration of uranyl ion, the larger the size of the product. In the current experiment, a critical value of changing the size of the product is found, when the current is higher than the critical value, the current has little effect on the product particle size, but when it is lower than the critical value, it will lead to a sudden increase in the product particle size, which is mainly related to the nucleation and growth mechanism of the product.

[0007] Further, in the process of the normal pressure micro-plasma discharge, the plasma gas electrode acts as a cathode, and correspondingly, the conductive electrode acts as an anode.

[0008] Preferably, the plasma gas electrode is a conductive argon tube, wherein the flow rate of argon is 20-80 mL / min; the discharge voltage is 500-5000 V.

[0009] Further, the preparation is carried out in a conductive molten salt system containing uranyl ions.

[0010] Further, the molten salt system comprises one or more different proportions of a mixture of lithium chloride, potassium chloride, sodium chloride and cesium chloride, preferably a NaCl-2CsCl system formed by sodium chloride and cesium chloride in a molar ratio of 1:2.

[0011] Further, the molten salt temperature of the NaCl-2CsCl system is 550-700℃.

[0012] Further, the concentration of uranyl ions in the molten salt system is 0.1wt%-20wt%, preferably 0.5wt%-5wt%.

[0013] Further, the current intensity of the normal pressure micro-plasma discharge is 1-25 mA, preferably 5-20 mA, and particularly preferably 15-20 mA.

[0014] and / or, when the normal pressure micro-plasma discharge treatment is performed, the distance between the plasma gas electrode and the surface of the molten salt system is 0.5-10mm, preferably 1-4mm;

[0015] and / or, the discharge time of the normal pressure micro-plasma discharge is 0.1-360min, preferably 30-180min, particularly preferably 60-120min.

[0016] Further, the preparation method of the conductive molten salt system containing uranyl ions comprises the following steps: dissolving uranium dioxide and / or triuranium octaoxide powder in a molten salt system, and forming a uniform molten salt electrolyte containing uranyl ions through a chlorination reaction.

[0017] According to the second aspect of the present application, the present application further provides a preparation method of nano uranium dioxide particles, comprising the following steps:

[0018] Step (1): heating and melting the molten salt containing uranyl ions, taking the plasma gas electrode as the cathode, and correspondingly, taking the conductive electrode as the anode, and performing normal pressure micro-plasma discharge treatment on the molten salt, so as to obtain a molten salt containing nano uranium dioxide particles;

[0019] Preferably, the preparation method further comprises:

[0020] Step (2): dissolving, centrifuging, washing and drying the molten salt containing nano uranium dioxide particles, wherein the solvent used for the dissolving is deionized water; the solvent used for the washing treatment is selected from one or two of anhydrous ethanol, ethylene glycol and deionized water; and the drying treatment is performed at a temperature of 30-80℃.

[0021] According to the third aspect of the present application, the present application further provides a nano uranium dioxide particle prepared by the above preparation method.

[0022] The technical solution provided by the present application has the following beneficial effects:

[0023] (1) The preparation method has the characteristics of wide application range, and can synthesize nano uranium dioxide particle materials with different particle size sizes. The material has good crystallinity, no organic reagent, high purity and excellent oxidation resistance. The method is performed in a molten salt electrolyte, and has potential to further expand the synthesis and application range of nano materials.

[0024] (2) The preparation method has simple equipment, and can controllably prepare uranium dioxide nanoparticles. Compared with the traditional method, the pollution of organic reagents is reduced, the crystallinity of the product is greatly improved, and the method has good industrial application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0026] Figure 1 is an XRD pattern of the product of uranium dioxide prepared in Example 1;

[0027] Figure 2 is an SEM, TEM, HRTEM, SAED and SEM image size distribution histogram of the product of uranium dioxide prepared in Example 1;

[0028] Figure 3 is an energy spectrum analysis result of the product of uranium dioxide prepared in Example 1;

[0029] Figure 4 is an XRD pattern of the product of uranium dioxide prepared in Example 1 soaked in water and exposed to air;

[0030] Figure 5 is an SEM pattern of the product of uranium dioxide prepared in Example 1 soaked in water for different time periods;

[0031] Figure 6 is an XRD pattern of the product obtained under different electrolysis time conditions in Examples 2-5;

[0032] Figure 7 is an SEM micrograph and corresponding particle size distribution statistical result of all products under different electrolysis time conditions in Examples 2-5;

[0033] Figure 8 is an XRD pattern of the product obtained under different electrolysis temperature conditions in Examples 6-8;

[0034] Figure 9 is an SEM image and corresponding particle size statistical histogram of the product under different electrolysis temperature conditions in Examples 6-8;

[0035] Figure 10 is an XRD pattern of the product obtained under different initial concentrations of uranyl ions in Examples 9-11;

[0036] Figure 11 is an SEM statistical diagram and product particle size distribution of the product obtained under different initial concentrations of uranyl ions in Examples 9-11;

[0037] Figure 12 is an XRD pattern of the product obtained under different current sizes in Examples 12-14;

[0038] Figure 13 SEM images and product size distribution of the product obtained in Example 12-14 under different current conditions;

[0039] Figure 14 XRD pattern of the uranium dioxide product obtained in Example 15;

[0040] Figure 15 SEM images and product size distribution of the uranium dioxide product obtained in Example 15. DETAILED DESCRIPTION

[0041] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below in conjunction with the drawings in the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative work fall within the protection scope of the present application.

[0042] Example 1

[0043] This embodiment is about the preparation of nano uranium dioxide particle material by atmospheric pressure micro-plasma cathode discharge.

[0044] The specific operation steps are as follows:

[0045] 1. Take the uranium dioxide powder and place it in a calcined crucible, then directly put the crucible into a pit-type heating furnace for heating, and calcine it at 650℃ for 4 hours to oxidize it into triuranium octoxide.

[0046] 2. Take 0.10g of the prepared triuranium octoxide powder and place it in a mortar, then add 1.48g of NaCl, 8.52g of CsCl and 2g of NH4Cl, grind and mix them with each other, and then pour them into a crucible, and then put the crucible into a pit-type furnace for chlorination, and keep the temperature for 2 hours, so that the triuranium octoxide powder is completely dissolved in the molten salt.

[0047] 3. Put the molten salt prepared in step 2 into a pit-type furnace for heating, and when the temperature rises to 650℃ and keeps for a period of time, the molten salt melts, and then insert a silicon carbide graphite rod anode and a plasma cathode, control the distance between the plasma cathode and the molten salt interface to be about 2mm, the plasma cathode is an argon tube with an inner diameter of 180μm, adjust the argon gas flow rate in the argon tube to be 40mL / min, adjust the output current of the constant current power supply to be 20mA, and ensure that the plasma tube is unobstructed. Discharge for 120min for reaction, and after the reaction is completed, the crude product is obtained.

[0048] 4. After the molten salt was cooled to room temperature and solidified, 25 ml of deionized water was added and stirred to dissolve, then transferred to a centrifuge tube and centrifuged at 4000 r / min for 5 min. The solid product was collected and dried in an oven at 50°C after being washed with deionized water. The nanometer uranium dioxide product was obtained.

[0049] Figure 1 is the XRD pattern of the obtained uranium dioxide product. The product has good peak shape, no impurity peaks other than the characteristic peaks of uranium dioxide, and high purity. According to the peak position and relative intensity, it can be easily associated with the uranium dioxide standard card (PDF #00-005-0550). The diffraction peaks correspond to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) crystal planes of UO2, respectively, confirming the face-centered cubic (FCC) structure of the compound (Fm3m, a = 0.5468 nm).

[0050] Figure 2 is the SEM, TEM, HRTEM, SAED, and size distribution histogram corresponding to the SEM image of the obtained uranium dioxide product. The microstructure and structural characteristics of the product were analyzed using SEM and TEM techniques. From the images, it can be determined that the product is a nanoscale particle. Figure 2 (a) and Figure 2 (b) show SEM images at different magnifications. As can be seen from the images, the product is composed of many spherical particles, and the size of these spherical particles is obviously less than 100 nm, and the size distribution of the spherical particles is relatively uniform. Figure 2 (c) is the particle size statistical analysis result based on SEM image (a). From Figure 2 (c), it can be seen that the particle size of the product is mainly between 30-70 nm, and the average particle size is 47.3 nm. A clearer TEM image ( Figure 2 (d)) shows that the particles are angular rather than regular spherical, which is consistent with the characteristics of the FCC octahedral structure. In order to better characterize the morphology of the uranium dioxide powder and determine its crystal properties, high-resolution transmission electron microscopy (HRTEM) analysis and selective area electron diffraction (SAED) techniques were performed on the corresponding region. Figure 2 (e) is the image obtained by HRTEM. It can be seen that the uranium dioxide particles have clear strip structure, and the measured average distance of the crystal plane is about 0.28 nm. This result is consistent with the interplanar distance of the FCC fluorite structure (200) crystal plane (0.2735 nm). The SAED image ( Figure 2 (f)) shows obvious symmetrical diffraction spots rather than diffraction rings, proving that the product has good crystallinity. The diffraction spots correspond to the (200), (400), and (600) crystal planes, further verifying that the structure of the product is FCC, which is consistent with the characteristics of the uranium dioxide crystal.

[0051] Figure 3 The energy spectrum analysis results of the obtained uranium dioxide product. The composition of the nano uranium dioxide particles was further analyzed by energy spectrum analysis. By comparing the SEM images Figure 3 (a) and Figure 3 (b-d), it can be found that the element maps of O and U overlap very well with the SEM images, indicating that the nanoparticles are mainly composed of U and O elements. A large amount of Si element in the mapping is due to the use of a silicon wafer as a carrier. At the same time, the composition of the nanoparticles was quantitatively analyzed. Figure 3 (e), (f) are Figure 3 the energy spectrum data of the corresponding regions in (a). It can be seen that at the 1st point and the 2nd face, the atomic ratio of U and O elements is 0.52 and 0.50, respectively, which is consistent with the stoichiometric ratio of uranium dioxide, further proving that the obtained product is nano uranium dioxide particles.

[0052] The product was subjected to oxidation resistance test, the test method includes: 1, 40 mg of nano uranium dioxide particles were dispersed into 10 ml of deionized water, and were stored in a centrifuge tube at room temperature. After soaking for a period of time, the product was collected by centrifugation and dried in air at 50°C. The composition and morphology change were analyzed by XRD and SEM. 2, the nano uranium dioxide particles were exposed to air for a period of time, and the composition and morphology change were analyzed by XRD and SEM. As Figure 4 shown, 4(a) shows the XRD patterns of the nano uranium dioxide particles after soaking for different times, and 4(b) shows the XRD patterns of the obtained nano uranium dioxide particles after being exposed to air for different times. From the results of Figure 4 (a), it can be seen that the characteristic peaks of the nano uranium dioxide particles soaked in water for 30 days have no change in peak position and little change in relative intensity compared with those soaked for 0 days, from Figure 4 (b), it can be seen that the characteristic peaks of the nano uranium dioxide particles exposed to air for 30 days have no change in peak position and little change in relative intensity compared with those exposed to air for 0 days, it can be seen that the nano uranium dioxide particles of the present application have excellent oxidation resistance. Figure 5 shown, 4(a) shows the XRD patterns of the nano uranium dioxide particles after soaking for different times, and 4(b) shows the XRD patterns of the obtained nano uranium dioxide particles after being exposed to air for different times. From the results of Figure 5 (a), 5(b), 5(c), 5(d), 5(e), 5(f) are respectively the SEM patterns of the nano uranium dioxide particles after soaking for 0 days, 1 day, 2 days, 5 days, 15 days and 30 days, from Figure 5 it can be seen that the nano uranium dioxide particles soaked in water for 30 days have no change in particle distribution and particle size compared with those soaked for 0 days, further indicating that the obtained nano uranium dioxide particles have excellent oxidation resistance.

[0053] Example 2-5

[0054] Examples 2-5 are experiments of preparing nano-sized uranium dioxide particles by atmospheric micro-plasma discharge with different electrolysis time, respectively.

[0055] The discharge time of 120 min in step 3 of Example 1 is changed to different electrolysis time (30 min, 60 min, 90 min, 180 min), and the other operation steps remain unchanged, corresponding to Examples 2, 3, 4, 5, respectively, as shown in Table 1.

[0056] Table 1 Corresponding experimental conditions of Examples 2-5

[0057]

[0058] The results of Example 1 are compared together.

[0059] Figure 6 is the XRD pattern of the product obtained under different electrolysis time conditions. The XRD results show that the length of electrolysis time has little effect on the composition of the product. The products obtained in the five experiments are all associated with the uranium dioxide standard card (PDF #00-005-0550), and have good crystallinity without impurity peaks, which can confirm that the obtained products are all uranium dioxide with face-centered cubic structure.

[0060] Figure 7 is the SEM micrograph and corresponding particle size distribution statistical result of all products under different electrolysis time. The results show that the product obtained under different electrolysis time has similar morphology, which is a spherical particle of nanometer size. In the case of shorter electrolysis time, the size distribution of the small balls is uniform, and most of the particles are below 100 nm in size. The average particle size of the product increases slightly with the increase of electrolysis time.

[0061] Examples 6-8

[0062] Examples 6-8 are experiments of preparing nano-sized uranium dioxide particles by atmospheric micro-plasma discharge at different temperatures, respectively.

[0063] The temperature of 650℃ in step 3 of Example 1 is changed to the temperature of (550℃, 600℃, 700℃), and the other operation steps remain unchanged, corresponding to Examples 6, 7, 8, respectively, as shown in Table 2.

[0064] Table 2 Corresponding experimental conditions of Examples 6-8

[0065]

[0066] The results of Example 1 are compared together.

[0067] Figure 8are the XRD patterns of the products obtained under different electrolysis temperature conditions. It can be seen from the figure that the crystal composition of the four products has no obvious difference, and the peak shape of the product corresponds to the characteristic peak of uranium dioxide (PDF #00-005-0550) one by one, which can prove that the obtained product is uranium dioxide with FCC structure.

[0068] Figure 9 are the SEM images of the four groups of products and the corresponding particle size statistical histograms. It can be seen that the uranium dioxide particles are irregular spherical in micro size, and part of the particles can see octahedral structure, which is similar to the result of FCC crystal form, and the particle size distribution of the product is uniform. With the increase of temperature, the particle size of the product increases obviously.

[0069] Examples 9-11

[0070] Examples 9-11 are experiments of preparing nano uranium dioxide particles by atmospheric pressure microplasma discharge under different initial concentrations of uranyl ions, respectively.

[0071] In Example 1, step 2, 0.10 g of the prepared triuranium octoxide powder was weighed into a mortar, and 1.48 g of NaCl, 8.52 g of CsCl and 2 g of NH4Cl were added. The remaining operation steps were unchanged. Corresponding to Examples 9, 10 and 11, as shown in Table 3.

[0072] Table 3 Experimental conditions corresponding to Examples 9-11

[0073]

[0074] Compare the results of Example 1 together.

[0075] Figure 10 are the XRD patterns of the products obtained under different initial concentrations of uranyl ions. The results show that the crystal structures of the four groups of products are basically the same, corresponding to uranium dioxide PDF #00-005-0550.

[0076] Figure 11 are the SEM statistical graphs and product particle size distribution. The results show that the particle size distribution of the product particles is uniform, and the overall particle size is below 100 nm. But with the increase of the initial concentration of uranyl ions, the particle size of the particles shows an increasing trend.

[0077] Examples 12-14

[0078] Examples 12-14 are experiments of preparing nano-sized uranium dioxide particles by atmospheric micro-plasma discharge under different current conditions, respectively.

[0079] The output current of the regulated constant current power supply in Example 1 Step 3 is changed from 20 mA to 5 mA, 10 mA and 15 mA, respectively, and the other operation steps remain unchanged, corresponding to Examples 12, 13 and 14, respectively, as shown in Table 4.

[0080] Table 4: Experimental conditions corresponding to Examples 12-14

[0081]

[0082]

[0083] Figure 12 are XRD patterns of the products obtained under different current conditions. The results show that different electrolytic currents have no obvious effect on the composition of the products, and the products of the four experiments are all Fm3m crystal uranium dioxide.

[0084] Figure 13 are SEM statistical graphs and product particle size distribution. As can be seen from the graphs, the four products are all nanoscale spherical particles with uniform size distribution, and the product size is about 50 nm, which does not change significantly under large currents of 10 mA, 15 mA and 20 mA. However, when the current is reduced to 5 mA, the size of the product jumps to the level of 100 nm.

[0085] Example 15

[0086] The 0.10 g of prepared triuranium octaoxide powder in the mortar in Example 1 Step 2 is changed to 0.10 g of prepared triuranium octaoxide powder in the mortar, and 1.48 g of NaCl, 8.52 g of CsCl and 2 g of NH4Cl are added to 5.40 g of KCl, 4.60 g of LiCl and 2 g of NH4Cl. The reaction is carried out by discharging for 120 min in Step 3, which is changed to discharging for 60 min, and the other operation steps remain unchanged.

[0087] Figure 14 are XRD patterns of the obtained uranium dioxide products. The product peak shape is good, and according to the peak position and relative intensity, it can be easily associated with the uranium dioxide standard card (PDF #00-005-0550), and the product composition is determined to be uranium dioxide, and the face-centered cubic (FCC) structure of the compound (Fm3m, a = 0.5468 nm) is determined.

[0088] Figure 15The SEM statistics of the obtained product and the product particle size distribution are shown in the figure. It can be seen from the figure that the product is nanoscale spherical particles, the size distribution is uniform, and the size is about 60 nm.

[0089] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing nano-uranium dioxide particles, characterized in that, The method comprises the following steps: Step (1): heating and melting a molten salt containing uranyl ions, using a plasma gas electrode as a cathode and a corresponding conductive electrode as an anode to perform normal pressure micro-plasma discharge treatment on the molten salt, so as to obtain a molten salt containing nano uranium dioxide particles; the preparation method of the molten salt containing uranyl ions comprises the following steps: dissolving uranium dioxide and / or triuranium octaoxide powder in a molten salt system, and forming a uniform molten salt electrolyte containing uranyl ions through a chlorination reaction; the molten salt system is a NaCl-2CsCl system formed by sodium chloride and cesium chloride at a molar ratio of 1:2; the molten salt temperature of the NaCl-2CsCl system is 550-700°C; Step (2): performing dissolution, centrifugation, washing and drying treatment on the molten salt containing nano uranium dioxide particles, wherein the solvent used for the dissolution is deionized water; the solvent used for the washing treatment is selected from one or two of anhydrous ethanol, ethylene glycol and deionized water; the drying treatment is performed at a temperature of 30-80°C.

2. The production method according to claim 1, characterized by, The plasma gas electrode is a conductive argon gas tube, wherein the flow rate of argon gas is 20-80 mL / min; the discharge voltage is 500-5000 V.

3. The preparation method according to claim 1, characterized in that, The uranyl ion concentration in the molten salt system is 0.1wt%-20wt%.

4. The production method according to claim 3, characterized by, The uranyl ion concentration in the molten salt system is 0.5wt%-5wt%.

5. The preparation method according to claim 1, characterized in that, The current intensity of the normal pressure micro-plasma discharge is 1-25 mA.

6. The production method according to claim 5, wherein The current intensity of the normal pressure micro-plasma discharge is 5-20 mA.

7. The production method according to claim 6, wherein The current intensity of the normal pressure micro-plasma discharge is 15-20 mA.

8. The method of claim 1, wherein, When performing the normal pressure micro-plasma discharge treatment, the distance between the plasma gas electrode and the surface of the molten salt system is 0.5-10 mm.

9. The production method according to claim 8, characterized by, When performing the normal pressure micro-plasma discharge treatment, the distance between the plasma gas electrode and the surface of the molten salt system is 1-4 mm.

10. The method of claim 1, wherein, The discharge time of the normal pressure micro-plasma discharge is 0.1-360 min.

11. The method of claim 10, wherein, The discharge time of the normal pressure micro-plasma discharge is 30-180 min.

12. The method of claim 11, wherein, The discharge time of the normal pressure micro-plasma discharge is 60-120 min.

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