Lithium hydride neutron shielding material and preparation method thereof

Abstract

The invention discloses a preparation method of a lithium hydride neutron shielding material. The preparation method comprises the following steps: step 1, carrying out ball milling on lithium hydride coarse powder, and screening to obtain lithium hydride fine powder; 2, mixing the lithium hydride coarse powder and the lithium hydride fine powder, and performing low-energy ball milling to obtain lithium hydride mixed powder; 3, carrying out cold isostatic pressing on the lithium hydride mixed powder to obtain a lithium hydride cold-pressed blank; and 4, carrying out pressureless sintering molding on the lithium hydride cold-pressed blank to obtain the lithium hydride neutron shielding material. The invention further discloses the lithium hydride neutron shielding material. According to the preparation method, the lithium hydride coarse powder and the lithium hydride fine powder are compounded and subjected to isostatic cool pressing, then the compact lithium hydride neutron shielding material is obtained through pressureless sintering, the preparation process is simple and controllable, the period is short, the requirement for equipment is low, industrial production is easy to achieve, and the preparation method is suitable for the technical field of nuclear materials.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear materials, and in particular relates to a lithium hydride neutron shielding material and a preparation method thereof. Background Art

[0002] Space nuclear reactor power is a key technology for deep space exploration and is also the inevitable development direction of high-power, long-life and high-reliability space power in the future. Shadow shields are an important component of space nuclear reactor power, used to reduce nuclear radiation such as neutrons and gamma rays to a safe level. However, due to the limited radiation shielding space, high ambient temperature and complex and harsh irradiation conditions of space nuclear systems, the quality of the shield needs to be strictly limited. Lithium hydride has become the preferred material for space reactor shields due to its high hydrogen content and low density.

[0003] The preparation of lithium hydride neutron shielding materials mainly includes two methods: high-temperature hydrogenation and sintering. The high-temperature hydrogenation method is to place metallic lithium in a crucible and put it into a hydrogenation furnace to heat it up, while introducing purified hydrogen at a certain flow rate, and then cooling it down to obtain lithium hydride neutron shielding materials. The high-temperature hydrogenation method can obtain large-sized and high-density lithium hydride ceramic materials, but this method has high requirements for equipment. During the preparation process, it is necessary to repeatedly absorb hydrogen so that the hydrogen element can be evenly diffused in the melt, and the preparation cycle is relatively long (greater than 30 hours). In addition, lithium hydride shrinks greatly during the solidification process, and it is easy to produce defects such as cracking, shrinkage cavities, and shrinkage.

[0004] The sintering process is to directly place the lithium hydride powder in a mold of a certain shape and sinter it directly at high temperature into a block material. For example, the Chinese invention patent "A method for forming lithium hydride warm isostatic pressing and its prepared product" (Announcement No. CN103213302A) uses lithium hydride powder for cold isostatic pressing, welding packaging and warm isostatic pressing to obtain lithium hydride shielding material. However, due to the use of warm isostatic pressing process, the production efficiency is low and the equipment requirements are high. Therefore, it is necessary to further explore the powder metallurgy preparation process of lithium hydride neutron shielding material, which is of great significance for its industrial preparation. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art and provide a method for preparing a lithium hydride neutron shielding material. The preparation method obtains a dense lithium hydride neutron shielding material by cold isostatic pressing a lithium hydride coarse powder and a lithium hydride fine powder, and then sintering the mixture without pressure, thereby solving the problems of the traditional method for preparing a lithium hydride neutron shielding material, which is complicated in process, long in cycle, and high in equipment requirements.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing a lithium hydride neutron shielding material, characterized in that the preparation method comprises the following steps: Step 1, ball-milling lithium hydride coarse powder with a particle size of 30 mesh to 100 mesh, and sieving to obtain lithium hydride fine powder with a particle size of 200 mesh to 300 mesh; Step 2: mixing the lithium hydride coarse powder with a particle size of 30-100 mesh with the lithium hydride fine powder obtained in step 1 and performing low-energy ball milling to obtain a lithium hydride mixed powder; Step 3: cold isostatically pressing the lithium hydride mixed powder obtained in step 2 to obtain a lithium hydride cold pressed blank; Step 4: Pressurelessly sinter the lithium hydride cold pressed blank obtained in step 3 to obtain a lithium hydride neutron shielding material.

[0007] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the ball milling in step 1 adopts a planetary ball mill, the rotation speed of the ball mill is 100r / min~300r / min, the ball-to-material ratio of the ball milling is 10:1~20:1, the ball milling time is 5h~20h, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15min and then paused for 15min.

[0008] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the mass ratio of the lithium hydride coarse powder to the lithium hydride fine powder in step 2 is 2:3~3:2.

[0009] The present invention controls the mass ratio of lithium hydride coarse powder to lithium hydride fine powder, thereby preventing the lithium hydride neutron shielding material from absorbing oxygen during the preparation process and causing a decrease in purity when the mass ratio is lower than 2:3; and preventing the lithium hydride neutron shielding material from having a poor density and a coarse grain size and reducing the mechanical properties of the lithium hydride neutron shielding material when the mass ratio is higher than 3:2.

[0010] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the total mass proportion of the Li element and the H element in the lithium hydride coarse powder and the lithium hydride fine powder in step 2 is not less than 99.0%.

[0011] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the rotation speed of the low-energy ball mill in step 2 is 100r / min~200r / min, the ball-to-material ratio of the low-energy ball mill is 10:1~20:1, the duration of the low-energy ball milling is 1h~5h, and the low-energy ball milling adopts an intermittent ball milling process: the ball milling is performed for 15min and then paused for 15min.

[0012] The present invention can control the parameters of low-energy ball milling to avoid the difficulty in uniformly mixing the lithium hydride coarse powder and the lithium hydride fine powder due to the too low ball milling speed and the ball-to-material ratio and the too short ball milling time; and can avoid the lithium hydride coarse powder being refined and losing the composite effect of the lithium hydride coarse powder and the lithium hydride fine powder due to the too high ball milling speed and the ball-to-material ratio and the too long ball milling time.

[0013] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the cold isostatic pressing process in step three is: after placing the lithium hydride mixed powder in a cold pressing mold, the cold pressing mold is placed in a plastic soft film for packaging and degassing, and then cold isostatic pressing is performed.

[0014] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the pressure of the cold isostatic pressing in step three is 100MPa~300MPa, and the holding time of the cold isostatic pressing is 5min~10min.

[0015] The present invention can avoid the material being unable to be formed when the pressure is too low by setting the cold isostatic pressing pressure to 100 MPa-300 MPa, and will not significantly increase the density of the material when the pressure exceeds 300 MPa.

[0016] The above-mentioned method for preparing a lithium hydride neutron shielding material is characterized in that the process of pressureless sintering in step 4 is: placing the lithium hydride cold pressed blank into a tubular furnace, and repeatedly evacuating the vacuum to a vacuum degree of 10 -2 Pa~10 -1 Pa, and then argon is introduced, and then pressureless sintering is performed; the temperature of the pressureless sintering is 500° C. to 600° C., and the time of the pressureless sintering is 1 h to 4 h.

[0017] The present invention sets the sintering temperature to 500° C.-600° C. Since the melting point of lithium hydride is 668° C., when the sintering temperature is too high, lithium hydride is prone to melting and dehydrogenation, while when the temperature is too low, it is difficult to form a sintering neck, which is not conducive to the densification of the lithium hydride neutron shielding material. By setting the sintering time to 1 h-4 h, it is avoided that the density of the lithium hydride neutron shielding material is difficult to ensure when the time is less than 1 h, and it is avoided that the grains are prone to coarsening when the time is more than 4 h, and the density will not increase significantly with the increase of time.

[0018] In addition, the present invention also discloses a lithium hydride neutron shielding material, which is characterized in that it is obtained by the above preparation method.

[0019] Compared with the prior art, the present invention has the following advantages: 1. The present invention adopts a mixture of lithium hydride coarse powder and lithium hydride fine powder as a raw material to prepare a lithium hydride neutron shielding material, which can improve the density of the sintered lithium hydride neutron shielding material, reduce the grain size, and make the lithium hydride neutron shielding material have better mechanical properties; at the same time, the use of lithium hydride coarse powder can reduce the oxygen content in the sintering process, improve the purity of the lithium hydride neutron shielding material, and can reduce the cost of raw materials and shorten the preparation cycle.

[0020] 2. The present invention can make the density, grain size and mechanical properties of the lithium hydride neutron shielding material achieve the same effects as those of pure lithium hydride fine powder by compounding lithium hydride coarse and fine powders in appropriate proportions; combined with controlling the sintering temperature and time, the performance of the lithium hydride neutron shielding material can be close to that of the samples prepared by hot isostatic pressing or hot pressing sintering in the prior art.

[0021] 3. The present invention adopts a pressureless sintering process to prepare lithium hydride neutron shielding materials. The process is simple and controllable, has low requirements on equipment and high production efficiency, and is conducive to the industrial application of lithium hydride neutron shielding materials.

[0022] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a physical picture of the lithium hydride neutron shielding material obtained in Example 1 of the present invention.

[0024] Figure 2 This is the XRD diagram of the lithium hydride neutron shielding material obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0025] Example 1 In this embodiment, lithium hydride coarse powder with a particle size of 30 mesh is used, and the lithium hydride coarse powder includes the following components: the total mass proportion of Li and H is 99.0%, the mass proportion of Na is 0.010%, the mass proportion of K is 0.005%, the mass proportion of Ca is 0.010%, the mass proportion of Si is 0.005%, the mass proportion of Al is 0.003%, the mass proportion of Mg is 0.002%, and the mass proportion of Fe is 0.002%; the preparation method comprises the following steps: Step 1, the lithium hydride coarse powder is loaded into the ball mill of the planetary ball mill through the glove box, and the ball milling is performed at a speed of 100 r / min for 20 hours, and the lithium hydride fine powder with a particle size of 200 mesh is obtained after sieving in the glove box; the ball-to-material ratio of the ball milling is 10:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; Step 2: Mix the lithium hydride coarse powder and the lithium hydride fine powder obtained in step 1, and then perform low-energy ball milling at a speed of 100 r / min for 5 hours to obtain a lithium hydride mixed powder; the ball-to-material ratio of the ball milling is 10:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; the mass ratio of the lithium hydride coarse powder to the lithium hydride fine powder is 2:3; Step 3: After placing the lithium hydride mixed powder obtained in step 2 in a cold pressing mold, the cold pressing mold is placed in a plastic soft film for packaging and degassing, and then cold isostatic pressing is performed at a pressure of 100 MPa for 10 minutes to obtain a lithium hydride cold pressed blank; Step 4: Place the cold pressed lithium hydride blank obtained in step 3 into a tubular furnace and repeat evacuation 3 times until the vacuum degree is 10 -2 Pa, and then argon gas was introduced, followed by pressureless sintering at 500°C for 4 h to obtain the lithium hydride neutron shielding material.

[0026] The Ø150mm×70mm lithium hydride neutron shielding material prepared by the present invention is as follows Figure 1 As shown, the surface of the lithium hydride neutron shielding material has no defects such as cracks and pores, indicating that the prepared lithium hydride neutron shielding material has good compactness; XRD analysis of the lithium hydride neutron shielding material is performed, as shown Figure 2 As shown, the lithium hydride neutron shielding material is composed of LiH and a small amount of LiOH and Li2O phase.

[0027] Example 2 In this embodiment, lithium hydride coarse powder with a particle size of 70 mesh is used, and the lithium hydride coarse powder includes the following components: the total mass proportion of Li and H is 99.5%, the mass proportion of Na is 0.005%, the mass proportion of K is 0.003%, the mass proportion of Ca is 0.005%, the mass proportion of Si is 0.003%, the mass proportion of Al is 0.002%, the mass proportion of Mg is 0.001%, and the mass proportion of Fe is 0.001%; ​​the preparation method comprises the following steps: Step 1, the lithium hydride coarse powder is loaded into the ball mill of the planetary ball mill through the glove box, and the ball milling is performed at a speed of 200r / min for 10 hours, and the lithium hydride fine powder with a particle size of 250 mesh is obtained after sieving in the glove box; the ball-to-material ratio of the ball milling is 15:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; Step 2: Mix the lithium hydride coarse powder and the lithium hydride fine powder obtained in step 1, and then perform low-energy ball milling at a speed of 150 r / min for 3 hours to obtain a lithium hydride mixed powder; the ball-to-material ratio of the ball milling is 15:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; the mass ratio of the lithium hydride coarse powder to the lithium hydride fine powder is 1:1; Step 3: After placing the lithium hydride mixed powder obtained in step 2 in a cold pressing mold, the cold pressing mold is placed in a plastic soft film for packaging and degassing, and then cold isostatic pressing is performed at a pressure of 200 MPa for 8 minutes to obtain a lithium hydride cold pressed blank; Step 4: Place the cold pressed lithium hydride blank obtained in step 3 into a tubular furnace and repeat 5 times to evacuate the furnace until the vacuum degree is 10 -2 The argon gas was introduced after Pa, and then pressureless sintering was carried out at 550°C for 3h to obtain the lithium hydride neutron shielding material.

[0028] Example 3 In this embodiment, lithium hydride coarse powder with a particle size of 100 mesh is used, and the lithium hydride coarse powder includes the following components: the total mass proportion of Li and H is 99.9%, the mass proportion of Na is 0.001%, the mass proportion of K is 0.001%, the mass proportion of Ca is 0.001%, the mass proportion of Si is 0.001%, the mass proportion of Al is 0.001%, the mass proportion of Mg is 0.001%, and the mass proportion of Fe is 0.001%; ​​the preparation method comprises the following steps: Step 1, the lithium hydride coarse powder is loaded into the ball mill of the planetary ball mill through the glove box, and the ball milling is performed at a speed of 300 r / min for 5 hours, and the lithium hydride fine powder with a particle size of 300 mesh is obtained after sieving in the glove box; the ball-to-material ratio of the ball milling is 20:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; Step 2: Mix the lithium hydride coarse powder and the lithium hydride fine powder obtained in step 1, and then perform low-energy ball milling at a speed of 200 r / min for 1 hour to obtain a lithium hydride mixed powder; the ball-to-material ratio of the ball milling is 20:1, and the ball milling adopts an intermittent ball milling process: the ball milling is performed for 15 minutes and then paused for 15 minutes; the mass ratio of the lithium hydride coarse powder to the lithium hydride fine powder is 3:2; Step 3: After placing the lithium hydride mixed powder obtained in step 2 in a cold pressing mold, the cold pressing mold is placed in a plastic soft film for packaging and degassing, and then cold isostatic pressing is performed at a pressure of 300 MPa for 5 minutes to obtain a lithium hydride cold pressed blank; Step 4: Place the cold pressed lithium hydride blank obtained in step 3 into a tubular furnace and repeat evacuation 4 times until the vacuum degree is 10 -1 Pa, and then argon gas was introduced, followed by pressureless sintering at 600°C for 1 h to obtain the lithium hydride neutron shielding material.

[0029] The lithium hydride neutron shielding materials prepared in Examples 1 to 3 were tested respectively, and the test results are shown in Table 1.

[0030] Table 1 Performance test results of lithium hydride neutron shielding materials

[0031] It can be seen from Table 1 that the compactness of the lithium hydride neutron shielding materials prepared in Examples 1 to 3 is not less than 98%, and the H / Li atomic ratio is 0.99; in summary, the lithium hydride neutron shielding material prepared by the present invention has good compactness and excellent neutron shielding performance, and meets the performance requirements of space nuclear reactors for neutron shielding materials.

[0032] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a lithium hydride neutron shielding material, characterized in that: The preparation method comprises the following steps: Step 1, ball-milling lithium hydride coarse powder with a particle size of 30 mesh to 100 mesh, and sieving to obtain lithium hydride fine powder with a particle size of 200 mesh to 300 mesh; Step 2: mixing the lithium hydride coarse powder with a particle size of 30-100 mesh with the lithium hydride fine powder obtained in step 1 and performing low-energy ball milling to obtain a lithium hydride mixed powder; Step 3: cold isostatically pressing the lithium hydride mixed powder obtained in step 2 to obtain a lithium hydride cold pressed blank; Step 4: Pressurelessly sinter the lithium hydride cold pressed blank obtained in step 3 to obtain a lithium hydride neutron shielding material.

2. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The ball milling in step 1 adopts a planetary ball mill, the rotation speed of the ball mill is 100r / min~300r / min, the ball-to-material ratio of the ball milling is 10:1~20:1, the ball milling time is 5h~20h, and the ball milling adopts an intermittent ball milling process: pause for 15min every time the ball milling is performed.

3. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The mass ratio of the lithium hydride coarse powder to the lithium hydride fine powder in step 2 is 2:3 to 3:

2.

4. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The total mass proportion of Li element and H element in the lithium hydride coarse powder in step 1 and step 2 is not less than 99.0%.

5. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The rotation speed of the low-energy ball mill in step 2 is 100r / min~200r / min, the ball-to-material ratio of the low-energy ball mill is 10:1~20:1, the duration of the low-energy ball mill is 1h~5h, and the low-energy ball mill adopts an intermittent ball milling process: pause for 15min after every 15min of ball milling.

6. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The cold isostatic pressing process in step 3 is as follows: after placing the lithium hydride mixed powder in a cold pressing mold, the cold pressing mold is placed in a plastic soft film for packaging and degassing, and then cold isostatic pressing is performed.

7. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The pressure of the cold isostatic pressing in step 3 is 100 MPa to 300 MPa, and the holding time of the cold isostatic pressing is 5 min to 10 min.

8. The method for preparing a lithium hydride neutron shielding material according to claim 1, characterized in that: The process of pressureless sintering in step 4 is as follows: put the cold pressed lithium hydride blank into a tubular furnace, and repeat 3 to 5 times to evacuate the vacuum to 10 - 2 Pa~10 -1 Pa, and then argon is introduced, and then pressureless sintering is performed; the temperature of the pressureless sintering is 500° C. to 600° C., and the time of the pressureless sintering is 1 h to 4 h.

9. A lithium hydride neutron shielding material, characterized in that: The method is obtained by the preparation method described in any one of claims 1 to 8.

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