UH2 phase and preparation method thereof

Abstract

The invention provides a UH2 phase and a preparation method thereof. The preparation method comprises the following steps: pressing beta-UH3 powder into a green body, and carrying out high-voltage discharge plasma sintering on the green body to obtain a sintered body containing a UH2 phase. According to the method, densification and heat treatment are carried out through spark plasma sintering with the rapid sintering characteristic, then high-pressure loads are further applied to spark plasma sintering, the sintering densification process is strengthened, and control over the UH3-x dehydrogenation rate in the heating sintering heat treatment process and stable maintenance of a dehydrogenation reaction intermediate phase UH2 are achieved.

Description

Technical Field

[0001] The invention relates to the field of preparation of uranium hydride, and in particular to a UH2 phase and a preparation method thereof. Background Art

[0002] UH 3-x Uranium hydride (UH) is an important nuclear material, which is generally prepared by α-U hydrogenation reaction. 3-x In the UH binary system, only two phase structures, α-UH3 and β-UH3, are observed. Among them, α-UH3 is stable at low temperatures but will become unstable and transform into β-UH3 at high temperatures, so β-UH3 is generally assumed to be UH 3-x The stable phase in .

[0003] Generally speaking, the general rule of metals with 4f and 5f electrons is that dihydrides and trihydrides are ubiquitous, among which dihydrides generally have a face-centered cubic (fcc) crystal structure consistent with CaF2. In the yttrium-based metal elements, their dihydrides with CaF2 structure are ubiquitous and unstable. As a typical 5f electron yttrium-based element, the UH2 phase with the CaF2 crystal structure of metal U is a possible phase structure in the UH binary system, but in the conventional UH system hydrogenation or dehydrogenation process and the prepared UH 3-x The UH2 phase cannot be observed or obtained in the powder. Obviously, as a non-stable phase, the preparation and acquisition of UH2 is extremely difficult.

[0004] Considering that in some application scenarios, binary hydrides in many metal-hydrogen systems have better performance than ternary hydrides, UH2 may have unique application prospects and can enrich the application of UH systems. Overcoming the difficulties in the synthesis and preparation of UH2 phase is the basis for in-depth research on this phase structure and exploring its practical application value. In addition, research on the synthesis and application of UH2 phase is also of great value to further understand the UH binary.

[0005] In recent years, researchers have synthesized thin films (thickness ~400nm) with UH2 phase structure by reactive sputtering deposition at ultra-low temperature (-98°C) and in a hydrogen atmosphere, confirming the existence of the UH2 phase and providing a technical path for obtaining a non-stable UH2 phase. However, the thin film material synthesized by this method is a small-sized two-dimensional material, which can only support the research and application of UH2 on a microscopic scale. In addition, thin film deposition requires ultra-low temperature conditions and a hydrogen atmosphere, the process is time-consuming and has strict requirements on equipment, and the method itself is extremely inconvenient.

[0006] Therefore, it is necessary to find a convenient, simple and time-saving method to prepare UH2 phase. Summary of the invention

[0007] The present invention aims to provide a UH2 phase and a preparation method thereof in order to solve the problem that the existing UH2 phase preparation is time-consuming and has strict requirements on equipment.

[0008] The technical method of the present invention is as follows:

[0009] A method for preparing a UH2 phase comprises: pressing β-UH3 powder into a green embryo, and subjecting the green embryo to high-voltage discharge plasma sintering to obtain a sintered body containing the UH2 phase.

[0010] The β-UH3 powder is prepared by direct hydrogenation and then obtained by ball milling under an inert atmosphere. The direct hydrogenation method comprises: heating the depleted uranium block in hydrogen, wherein the heating temperature is 250-350°C and the heating time is 22-26 hours. The ball milling under an inert atmosphere comprises: in an inert gas atmosphere, the product of the direct hydrogenation method is mechanically ground and the fine particles are passed through a 300 mesh sieve.

[0011] Specifically, the pressing step includes: weighing a certain amount of β-UH3 powder according to the size requirements of the sintered body; placing the β-UH3 powder into a cemented carbide mold, and applying pressure to press it into a green embryo. Here, the applying pressure uses the upper punch and the lower punch of the hydraulic press.

[0012] The high-pressure discharge plasma sintering step of the β-UH3 green blank includes: transferring the green blank to a sintering chamber of the discharge plasma sintering, maintaining the argon pressure of the sintering chamber at 0.01-0.05 MPa; high-pressure rapid sintering the green blank, and decompression cooling to obtain a sintered body containing the UH2 phase. Here, the argon pressure of the sintering chamber is maintained by repeated vacuuming and argon filling atmosphere replacement operations.

[0013] Specifically, the high-pressure fast burning step includes: loading the pressure from 0.01-0.05 MPa to 500-1000 MPa and then keeping it constant, heating from room temperature to 600-800° C., and keeping the temperature for 10-20 minutes.

[0014] The present invention also provides a UH2 phase, which is obtained by pressing β-UH3 powder into a green embryo and then sintering it through high-voltage discharge plasma.

[0015] The beneficial effects of the present invention are:

[0016] 1. In the prior art, UH 3-x The typical characteristics of pulverization during the hydrogenation process lead to a significant increase in the specific surface area, which is the main reason for the extremely fast reaction rate of the UH binary system. 3-x The ultra-fast hydrogenation or dehydrogenation rate caused by easy pulverization is the reason why the intermediate phase UH2 is difficult to preserve. Therefore, it is thought that UH2 can be made dense by sintering.3-x UH2 is obtained by reverse pulverization, but conventional sintering is used. The densification process is slow during the sintering process. When the temperature rise densification is not achieved, UH 3-x It may have been rapidly decomposed into metallic U. Therefore, the present invention first performs densification and heat treatment by spark plasma sintering (SPS) with the characteristics of rapid sintering, and then further applies high pressure load to SPS to strengthen the sintering densification process and realize the UH 3-x Control of dehydrogenation rate and stable maintenance of dehydrogenation reaction intermediate phase UH2.

[0017] Second, the high pressure spark plasma sintering (HPSPS) process of the present invention is fast and simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of cemented carbide mold;

[0019] Figure 2 It is a cross-sectional view of the cemented carbide die;

[0020] Figure 3 This is a real picture of the bulk material containing UH2 phase after HPSPS was sintered at 700MPa and 700℃;

[0021] Figure 4 is the sintering diagram of β-UH3 embryo; Figure 4 (a) The β-UH3 embryo was heated to 400℃ under pressureless conditions and kept at this temperature for 10 min. Figure 4 (b) β-UH3 blank was sintered in conventional SPS (100MPa) at 400℃ and 700℃ for 10min. Figure 4 (c) is the material phase composition of the β-UH3 blank after sintering heat treatment at 400℃ and 700℃ for 10min in HPSPS (700MPa);

[0022] Figure 5 The metallographic structure comparison diagram of the β-UH3 blank HPSPS (700MPa) after sintering heat treatment at 400℃ and 700℃ for 10min; Figure 5 (a) is the metallographic structure of the HPSPS (700MPa) blank after sintering at 400℃ for 10min; Figure 5 (b) Metallographic structure of the HPSPS (700MPa) blank after sintering at 700℃ for 10min. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] The concept of the present invention is that the reaction rate of hydrogenation and dehydrogenation in the UH binary system is extremely fast. Due to the density difference between the metal phase and the hydride phase, UH 3-x It has the typical characteristic of pulverization during the hydrogenation process. Pulverization leads to a significant increase in specific surface area, which should be the main reason for the extremely fast reaction rate of the UH binary system. Referring to the dihydrides of other chalcogenides, the UH2 phase with CaF2 structure is non-stable and extremely unstable. Therefore, in the rapid hydrogenation process, α-U will quickly transform into β-UH3, and in the dehydrogenation process, β-UH3 will quickly transform into α-U. The non-stable UH2 phase is extremely difficult to maintain as a reaction intermediate phase. Therefore, inhibiting the pulverization of the UH system during the hydrogenation and dehydrogenation process and controlling the reaction rate are the key to solving the problem of maintaining the stability of the non-stable UH2 phase.

[0025] Considering UH 3-x The ultrafast hydrogenation or dehydrogenation rate caused by the easy pulverization of UH2 is the reason why the intermediate phase UH2 is difficult to preserve. 3-x Inverse pulverization treatment will be beneficial to the acquisition of UH2, and sintering densification is a feasible direction. Considering that the densification process is slow during conventional sintering, UH2 is not densified when the temperature rise is not achieved. 3-x It may have been rapidly decomposed into metallic U, so it is considered to use SPS with the characteristics of rapid sintering for densification and heat treatment. At the same time, high pressure load is further applied to SPS, and the sintering densification process is strengthened by HPSPS, so that UH 3-x The dehydrogenation rate is controlled and the intermediate phase UH2 of the dehydrogenation reaction is stably maintained. In addition, SPS technology has the advantages of being fast, convenient and safe (no hydrogen atmosphere is required).

[0026] Therefore, the object of the present invention is to 3-x HPSPS sintering heat treatment of green embryos to achieve UH 3-x The reaction rate is controlled during the temperature-raising dehydrogenation process to avoid the direct transformation of β-UH3 into α-U, so that the unstable intermediate phase UH2 can be stably maintained, and the rapid, simple and safe preparation of the CaF2 structured unstable UH2 phase in the bulk can be achieved.

[0027] The present invention provides a method for preparing a UH2 phase. The method comprises: pressing beta-UH3 powder into a green embryo, and subjecting the green embryo to HPSPS sintering to obtain a sintered body containing the UH2 phase.

[0028] The present invention realizes HPSPS sintering heat treatment by coupling high pressure load with SPS technology. 3-x The thermal dehydrogenation rate was effectively suppressed, and the preparation of CaF2-structured non-stable UH2 was successfully achieved through the thermal dehydrogenation path (dehydrogenation phase transition of β-UH3→UH2).

[0029] First, compared with the reactive sputtering deposition of UH2 at ultra-low temperature and hydrogen atmosphere, the material synthesized by this method is a large-sized bulk material, which is more conducive to the experimental measurement and research of the thermal, mechanical and chemical properties of the material. Secondly, this method does not require a hydrogen atmosphere, avoiding the safety risks of flammable hydrogen. Finally, SPS is a rapid sintering process that only takes a dozen minutes. The method is fast, and powder metallurgy has the advantages of convenience and low cost. Therefore, this method has the advantages of being fast, simple and safe to synthesize UH2 phase.

[0030] The β-UH3 powder of the present invention is prepared by direct hydrogenation and then obtained by ball milling under an inert atmosphere. Here, compared with α-UH3, the β-UH3 powder of the present invention is stable and can achieve a thermal dehydrogenation path. Both direct hydrogenation and ball milling use conventional equipment in the art.

[0031] The direct hydrogenation method comprises the following steps: heating a depleted uranium block in hydrogen to prepare a β-UH3 block, wherein the heating temperature is 250-350°C and the heating time is 22-26 hours.

[0032] The step of ball milling under an inert atmosphere comprises: in an inert gas atmosphere, the product (β-UH3) of the direct hydrogenation method is finely ground by mechanical grinding, and the finely ground particles are passed through a 300-mesh sieve.

[0033] The pressing steps of the present invention include: weighing a certain amount of β-UH3 powder according to the requirements of the size of the sintered body; putting the β-UH3 powder into a cemented carbide mold, and applying pressure to press it into a green embryo.

[0034] like Figure 1 As shown, the material of the cemented carbide mold of the present invention is selected from WC (tungsten carbide), specifically, a WC cemented carbide mold resistant to ultra-high pressure. Here, by using a WC cemented carbide mold resistant to ultra-high pressure, it can be ensured that no deformation occurs under high pressure. The cemented carbide mold can be a hollow cylindrical shape. Figure 2 As shown, β-UH3 powder is placed inside the hollow cylinder. A thermocouple temperature measuring hole is set inside the cemented carbide mold to obtain the temperature of the blank in the mold.

[0035] Preferably, the loading pressure adopts the upper punch and the lower punch of the hydraulic press. The material of the upper punch and the lower punch is selected from WC material. Here, the shape of the upper punch and the lower punch can be a solid cylinder. The total length of the upper punch and the lower punch is greater than the length of the cemented carbide mold, and the diameter length of the upper punch and the lower punch is less than the width of the cemented carbide mold, so as to ensure that the upper punch and the lower punch can extend into the hollow cylinder of the cemented carbide mold to press the β-UH3 powder.

[0036] The pressing pressure of the punch of the present invention is greater than 500 MPa. This is because the high-pressure mold is heavier, and if the pressing pressure is too small, the inside of the mold blank will loosen after being transferred to the SPS chamber.

[0037] When in use, first insert the lower punch into the mold sleeve, then fill the cylindrical hollow of the mold sleeve with β-UH3 powder, insert the upper punch, and compact it with the help of a hydraulic press. At the same time, the powder is shaped in the mold so that the powder is not exposed to the air.

[0038] The high-voltage discharge plasma sintering step of the present invention comprises: transferring the green embryo to a sintering chamber of the discharge plasma sintering, maintaining the argon pressure of the sintering chamber at 0.01-0.05 MPa; high-voltage rapid sintering of the green embryo, and decompression cooling to obtain a sintered body containing a CaF2 structured non-stable UH2 phase.

[0039] Specifically, the argon pressure in the sintering chamber is maintained by repeatedly evacuating and filling the argon atmosphere. The number of repeated operations may be 3 times or more. The argon pressure in the sintering chamber may be maintained at 0.04 MPa.

[0040] Specifically, the high-pressure fast firing step includes: loading the pressure from 0.01-0.05MPa to 500-1000MPa and then keeping it constant, heating from room temperature to 600-800°C, and keeping it warm for 10-20min. For example, loading the pressure to 650MPa, 700MPa, 750MPa, heating to 650°C, 700°C, 750°C, and keeping it warm for 11min, 15min, 19min.

[0041] The mechanical load of the present invention is loaded to 500-1000MPa because if the pressure is less than 500MPa, densification cannot be effectively achieved, and β-UH3 will directly decompose into uranium during the heating process. If the pressure is greater than 1000MPa, there will be a safety risk of the mold being crushed.

[0042] There are two reasons why the sintering temperature of the present invention is greater than 600°C. First, the temperature needs to be raised for sintering densification, and second, the temperature also needs to be raised for dehydrogenation of β-UH3 to UH2, and temperatures below 600°C are insufficient for densification. The main reason why the temperature cannot exceed 800°C is the mold safety issue. The mechanical properties of WC materials decay severely at around 800°C, and they may be crushed under high pressure.

[0043] The present invention is heat-insulated for 10 to 20 minutes to complete the sintering heat treatment to ensure the dehydrogenation of β-UH3. The reason why the heat-insulation time is greater than 10 minutes is that the time is too short and not enough UH2 is precipitated. The heat-insulation time is less than 20 minutes because SPS is a fast sintering technology, which is not easy to be heat-insulated for too long, and there may be a risk of mold fatigue and crushing.

[0044] Preferably, the pressure from 0.01 to 0.05 MPa to 500 to 1000 MPa is applied by the sintering upper punch and the sintering lower punch of the sintering hydraulic press. Here, the pressure can be applied at a uniform speed. The material of the sintering upper punch and the sintering lower punch is ultra-high pressure resistant WC cemented carbide. The shape of the sintering upper punch and the sintering lower punch can be a solid cylinder.

[0045] When in use, the upper punch and the lower punch are first taken out, and then the sintering upper punch and the sintering lower punch are inserted into the mold sleeve for high-pressure rapid sintering.

[0046] Preferably, a thermocouple temperature measuring hole is provided inside the cemented carbide mold to obtain the temperature of the blank in the mold.

[0047] Preferably, the heating rate may be 50 to 200°C / min. For example, the heating rate may be 70°C / min, 100°C / min, or 150°C / min. The heating may be performed at a uniform rate. A heating rate that is too slow is not conducive to rapid densification, and a rate greater than 50°C / min is appropriate. A heating rate higher than 200°C / min requires high current and voltage, and there may be a risk of overload, so it is controlled below 200°C / min.

[0048] Preferably, the pressure relief can be reduced from 500-1000 MPa to 10-50 MPa. The pressure relief can be reduced at a uniform speed.

[0049] Preferably, natural cooling can be used for cooling.

[0050] Preferably, the prepared sintered body can be demoulded and polished.

[0051] The present invention also provides a UH2 phase, which is prepared by the above-mentioned method for preparing the UH2 phase. The method can realize the synthetic preparation of UH2 in the form of a block.

[0052] The features and performance of the present invention are further described in detail below in conjunction with the examples. Unless otherwise specified, the methods mentioned in this application are all conventional methods; unless otherwise specified, the experimental materials used in this application are all commercially available.

[0053] Example 1

[0054] The present invention provides a method for preparing a UH2 phase, comprising:

[0055] S1: Wash the depleted uranium block, heat it in the hydrogenation system and charge it with hydrogen; the heating temperature is 300°C, the heating time is 24h, and then the prepared β-UH3 raw material powder is fully mechanically ground and refined, and then passed through a 300-mesh sieve for sintering; the above operations are carried out under the protection of an inert atmosphere.

[0056] S2: Weigh an appropriate amount of powder according to the required sintered body size, load it into a WC carbide mold, and press it into a green embryo.

[0057] S3: The WC carbide mold containing the green embryo is transferred to the SPS sintering chamber, a thermocouple is inserted, and after the hatch is closed, at least three vacuum / argon atmosphere replacement operations are performed, and the argon pressure in the chamber is finally maintained at 0.04 MPa.

[0058] S4: Before sintering, the upper and lower punch loading pressure is first increased uniformly to 700MPa and then kept constant; then the temperature is uniformly increased from room temperature to 700℃ at a heating rate of 100℃ / min; after the temperature reaches 700℃, the sintering heat treatment is maintained for 10min; after the insulation is completed, the punch pressure is uniformly released to 50MPa, and the mold is naturally cooled to room temperature; the sintering mold is taken out and transferred to an inert atmosphere glove box.

[0059] S5: The sintered block is removed from the mold and surface polished with sandpaper to obtain a block material containing UH2 phase.

[0060] Figure 3 This is a real picture of the prepared UH2 phase-containing bulk material.

[0061] In order to prove the effectiveness of this synthesis method and the correctness of the principle, the synthesized materials were characterized by XRD phase structure and metallographic organization, and the hydrogen content of the materials was measured by a hydrogen meter (inert gas fusion technology).

[0062] like Figure 4As shown, the phase composition of the materials after β-UH3 powder was heated at 400℃ for 10min without pressure (0MPa), sintered at 400℃ and 700℃ for 10min by SPS (100MPa), and sintered at 400℃ and 700℃ for 10min by HPSPS (700MPa). It can be seen that β-UH3 powder is quickly decomposed into α-U when heated, and a slow decomposition phase transition of β-UH3→α-U occurs during the SPS heating process, but no UH2 appears. The decomposition phase transition of β-UH3→α-U is effectively suppressed during the HPSPS heating process, and a large amount of UH2 phase has been precipitated at a high temperature of nearly 700℃. Further testing confirmed that the hydrogen content of the material after HPSPS sintering heat treatment at 700℃ for 10min was 1.2wt.%, and calculation confirmed that β-UH3 accounted for 40%, and the synthesized UH2 phase was as high as 60%. Subsequently, the metallographic characterization of the sintered heat-treated blocks of HPSPS at 400 and 700 °C was performed, e.g. Figure 5 The metallographic structure further confirms the precipitation and stable retention of UH2 phase in the block.

[0063] In summary, this synthesis method can realize the non-steady-state UH2 phase in UH 3-x Maintaining stability during bulk heating dehydrogenation is an effective method for synthesizing UH2 phase.

[0064] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a UH2 phase, characterized in that: The preparation method comprises: The β-UH3 powder is pressed into a green embryo, and the green embryo is subjected to high-voltage discharge plasma sintering to obtain a sintered body containing a UH2 phase.

2. The preparation method according to claim 1, characterized in that: The β-UH3 powder is prepared by direct hydrogenation and then obtained by ball milling in an inert atmosphere.

3. The preparation method according to claim 2, characterized in that: The steps of the direct hydrogenation method include: The depleted uranium block is heated in hydrogen, wherein the heating temperature is 250-350° C. and the heating time is 22-26 hours.

4. The preparation method according to claim 2, characterized in that: The step of ball milling under an inert atmosphere comprises: Under an inert gas atmosphere, the product of the direct hydrogenation method is finely ground by mechanical grinding, and the fine particles are passed through a 300-mesh sieve.

5. The preparation method according to claim 1, characterized in that: The pressing step comprises: According to the requirements of the sintered body size, weigh a certain amount of β-UH3 powder; The β-UH3 powder is placed in a cemented carbide mold and pressed into a green embryo under pressure.

6. The preparation method according to claim 5, characterized in that: The loading pressure adopts the upper punch and the lower punch of the hydraulic press.

7. The preparation method according to claim 1, characterized in that: The high-voltage discharge plasma sintering step comprises: The green embryo is transferred to the sintering chamber of spark plasma sintering, and the argon pressure in the sintering chamber is maintained at 0.01-0.05 MPa; The green body is sintered rapidly under high pressure, and then cooled under reduced pressure to obtain a sintered body containing UH2 phase.

8. The preparation method according to claim 7, characterized in that: The argon pressure in the sintering chamber is maintained by repeating an atmosphere replacement operation of evacuating and filling with argon.

9. The preparation method according to claim 7, characterized in that: The step of high pressure fast burning comprises: The pressure is loaded from 0.01-0.05MPa to 500-1000MPa and then kept constant, and the temperature is raised from room temperature to 600-800°C and kept warm for 10-20min.

10. A UH2 phase, characterized in that The UH2 phase is obtained by pressing β-UH3 powder into a green embryo and then sintering it through high-voltage discharge plasma.

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