Preparation method of magnesium fluoride-based ceramic

Abstract

The invention provides a magnesium fluoride-based ceramic preparation method, which comprises: S1, dissolving a magnesium salt and a doped metal salt in water to obtain a mixed solution, dissolving a villiaumite in water to obtain a fluorine solution, adding the mixed solution to the fluorine solution, and carrying out a reaction to obtain a precipitate solution, standing and aging the precipitate solution, and carrying out suction filtration and freeze drying to obtain magnesium fluoride-based powder; and S2, grinding the magnesium fluoride-based powder, putting the ground magnesium fluoride-based powder into a mold, sintering under a vacuum condition, and then carrying out double-sided polishing to obtain the magnesium fluoride-based ceramic. The magnesium fluoride-based powder is prepared through a coprecipitation method, so that the magnesium fluoride-based powder is good in dispersity and sintering activity, and later ceramic densification is facilitated; according to the invention, the fluorinated metal is uniformly doped into the magnesium fluoride-based powder, the grain size of the magnesium fluoride-based ceramic is inhibited through the pinning effect of the fluorinated metal, and the infrared transmittance of the magnesium fluoride-based ceramic is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ceramic materials, and particularly relates to a preparation method of magnesium fluoride-based ceramic. BACKGROUND

[0002] The fairing is a structural member at the front end of a high-speed aircraft, and bears many functions such as maintaining the aerodynamic shape of the aircraft, heat protection, force bearing, wave transmission and the like, and is an indispensable key device on all aircrafts. Magnesium fluoride is an ideal material for a medium-wave infrared window, and has the following excellent performances: (1) a low density (3.18 g / cm 3 ) and a refractive index value (n=1.38), and the coefficient of change of the refractive index with temperature dn / dt is only 1.1x10 -6 K -1 , which is the smallest among infrared window and fairing materials. (2) The absorption coefficient of magnesium fluoride is 1.4x10 -2 cm -1 , which is only second to zinc selenide. (3) The bending strength is greater than 100 MPa, the thermal expansion coefficient is less than 1.3x10 -5 K -1 (25~300 ℃), and the magnesium fluoride has good mechanical properties, thermal properties and chemical stability. (4) The dielectric constant is 5.1, which is also the lowest among existing infrared window and fairing materials, and the dielectric tangent loss is about 0.001, which can meet the requirements of millimeter wave (microwave) on the radome material.

[0003] Since the 1960s, hot-pressed magnesium fluoride has been applied to infrared forward windows, infrared pods, photoelectric radars and other systems of missiles and aircrafts guided by medium-wave infrared. According to statistics, the number of magnesium fluoride polycrystalline materials used in medium infrared waveband (3~5 μm) fairings has reached more than 90% of the total production. However, the transmittance and mechanical strength of magnesium fluoride ceramic are low, and it is difficult to withstand the impact of sand and gravel and thermal shock in the high-speed flight environment, and can only be used as a fairing material for low-speed aircrafts (speed below 2 Mach). If the infrared transmittance and strength of magnesium fluoride ceramic can be improved, the application of traditional medium infrared window materials such as magnesium fluoride in high-speed (3~5 Mach) aircrafts will be greatly expanded.

[0004] The Chinese patent document with the publication number CN113956043A discloses a fluoride infrared complex phase transparent ceramic and a preparation method thereof. The fluoride infrared complex phase transparent ceramic is prepared by mixing AFx and MgF2 two kinds of nano fluoride to form a nano composite powder, and then using hot-pressing sintering. The patent prepares a complex phase ceramic by mixing AFx and MgF2 two phases, but the two phases cannot be uniformly distributed. AFx cannot be uniformly doped into the MgF2 matrix, which on the one hand reduces the optical uniformity of the infrared complex phase ceramic, and on the other hand, this unevenness also makes it difficult to increase the mechanical properties of the infrared complex phase ceramic. SUMMARY

[0005] The technical problem to be solved by the present application is how to improve the mechanical strength of magnesium fluoride based ceramic while making its infrared transmittance more than 80%.

[0006] The present application solves the above technical problems by the following technical means: The present application provides a preparation method of magnesium fluoride based ceramic, comprising the following steps: S1 dissolving magnesium salt and doped metal salt in water to obtain a mixed solution, dissolving fluoride salt in water to obtain a fluoride solution, adding the mixed solution to the fluoride solution to react to obtain a precipitate solution, the precipitate being magnesium fluoride and doped fluorinated metal, the volume ratio of magnesium fluoride to doped fluorinated metal being 40-60:40-60, then aging, filtering and freeze-drying the precipitate solution to obtain magnesium fluoride based powder; S2 grinding the magnesium fluoride based powder and then placing it in a mold, sintering under vacuum, and then double polishing to obtain magnesium fluoride based ceramic.

[0007] Beneficial effects: The present application prepares magnesium fluoride based powder by co-precipitation method, which has good dispersibility and sintering activity, and is beneficial to the densification of the ceramic in the later stage; the present application uniformly dopes fluorinated metal in the magnesium fluoride based powder, which suppresses the grain size of the magnesium fluoride based ceramic through the pinning effect of the fluorinated metal, and improves the infrared transmittance of the magnesium fluoride based ceramic. The volume ratio of magnesium fluoride to doped fluorinated metal is 40-60:40-60, and the volume of doped fluorinated metal is appropriate, so that the fluorinated metal fully plays the pinning effect and increases the mechanical properties.

[0008] In addition, compared with directly mixing magnesium fluoride and fluorinated metal by ball milling, the co-precipitation method of the present application can make magnesium fluoride and fluorinated metal combine together in an adjacent manner, the distribution of the two is more uniform, and the pinning effect between the two phases can be better, thereby maximizing the mechanical properties of the magnesium fluoride based ceramic.

[0009] Preferably, the doped metal salt comprises one or more of calcium nitrate, strontium nitrate, barium nitrate, lead nitrate, scandium nitrate, gadolinium nitrate, lutetium nitrate, lanthanum nitrate.

[0010] Preferably, the magnesium salt is magnesium nitrate or magnesium chloride.

[0011] Preferably, the fluorine salt is ammonium fluoride or hydrogen fluoride.

[0012] Preferably, the standing aging time is 3h-12h.

[0013] Preferably, the freeze-drying temperature is -45℃--55℃, and the freeze-drying time is 12h-48h.

[0014] Preferably, the sintering temperature is 500℃-900℃, and the sintering time is 2h-5h.

[0015] Preferably, the vacuum degree of the vacuum is 6.5x10 -3 Pa.

[0016] Preferably, the sintering temperature is 500℃-900℃, and the sintering time is 2h-5h.

[0017] Preferably, the diameter of the ground magnesium fluoride-based powder is 30-35mm. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is an XRD pattern of the magnesium fluoride-based powder prepared in Example 1 of the present application; Figure 2 is an SEM pattern of the magnesium fluoride-based powder prepared in Example 1 of the present application; Figure 3 is a photo of the magnesium fluoride-based ceramic prepared in Example 1 of the present application; Figure 4 is an infrared transmittance curve of the magnesium fluoride-based ceramic prepared in Example 1 of the present application and the magnesium fluoride ceramic prepared in Comparative Example 1; Figure 5 is a photo of the magnesium fluoride-based ceramic prepared in Comparative Example 2 of the present application; Figure 6 is an infrared transmittance curve of the magnesium fluoride-based ceramic prepared in Comparative Example 2 of the present application; Figure 7 is an infrared transmittance curve of the magnesium fluoride-based ceramic prepared in Comparative Example 3 of the present application. DETAILED DESCRIPTION

[0019] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0020] The test materials and reagents used in the following examples, and the like, can be obtained from commercial channels unless otherwise specified.

[0021] The specific techniques or conditions not specified in the examples can be performed according to the techniques or conditions described in the literature in the art or according to the product instructions.

[0022] Embodiment 1 The embodiment provides a preparation method of a magnesium fluoride-based ceramic, and specifically comprises the following steps. S1 12.965 g of magnesium nitrate hexahydrate (chemical formula: Mg(NO3)2.6H2O, molecular weight: 256.41) and 9.628 g of calcium nitrate tetrahydrate (chemical formula: Ca(NO3)2.4H2O, molecular weight: 236.15) are added to 0.1 liters of deionized water with a resistivity of 18.25 MΩ / cm, and are fully dissolved under the driving of a magnetic stirrer, with a dissolution time of 20 minutes, to obtain a mixed solution. 7.5 g of ammonium fluoride is added to 0.3 liters of deionized water with a resistivity of 18.25 MΩ / cm, and is fully dissolved under the driving of a magnetic stirrer, with a dissolution time of 20 minutes, to obtain an ammonium fluoride solution.

[0023] The mixed solution is added dropwise to the ammonium fluoride solution through a peristaltic pump while stirring continuously. After the mixed solution is completely added dropwise, the stirring is continued for 40 minutes, to obtain a calcium fluoride-doped magnesium fluoride solution. The volume ratio of calcium fluoride to magnesium fluoride in the calcium fluoride-doped magnesium fluoride solution is 50:50.

[0024] After the calcium fluoride-doped magnesium fluoride solution is left to stand for 3 hours, it is poured into a suction filter bottle to remove impurity ions such as nitrate ions, ammonium ions and fluoride ions adsorbed on the surface of the powder through multiple suction filtrations. Then, the residual moisture is removed through freeze-drying at -45℃ for 12 hours, to obtain a magnesium fluoride-based powder.

[0025] S2 After 3.2 g of the magnesium fluoride-based powder is manually ground in a mortar for 1 hour, the diameter of the magnesium fluoride-based powder is 35 mm. The magnesium fluoride-based powder is poured into a silicon carbide mold with a diameter of 16 mm. The silicon carbide mold is placed in a vacuum furnace for sintering. The temperature is raised to 750℃ at a rate of 5℃ / min, and is kept at 750℃ for 2 hours. At this time, the vacuum degree in the vacuum furnace is displayed as 6.5x10 -3Pa. After the temperature in the vacuum furnace is reduced to room temperature, the silicon carbide mold is taken out, the magnesium fluoride-based powder is extruded using a dry press, and then is polished using sandpaper of Nos. 400, 2000, 3000 and 5000, respectively, and finally is double polished on a polishing machine using diamond polishing paste to obtain the magnesium fluoride-based ceramic. The magnesium fluoride-based ceramic is a calcium fluoride-doped magnesium fluoride ceramic.

[0026] The XRD pattern of the magnesium fluoride-based powder prepared in this example is shown in Figure 1 From the pattern, it can be seen that the magnesium fluoride-based powder is composed of two phases of calcium fluoride and magnesium fluoride, indicating that the powder obtained by the coprecipitation method of Example 1 is composed of two components of calcium fluoride and magnesium fluoride.

[0027] The SEM pattern of the magnesium fluoride-based powder prepared in this example is shown in Figure 2 From the pattern, it can be seen that the average size of the magnesium fluoride-based powder is about 35 nm, and the powder has good dispersibility.

[0028] The photograph of the magnesium fluoride-based ceramic prepared in this example is shown in Figure 3 The diameter of the magnesium fluoride-based ceramic is 16 mm, and the thickness of the magnesium fluoride-based ceramic is 2 mm. From the pattern, it can be seen that the magnesium fluoride-based ceramic is white.

[0029] Examples 2-5 Examples 2-5 provide a method for preparing a magnesium fluoride-based ceramic, and the specific steps are the same as those of Example 1, and the specific conditions are shown in Table 1: Table 1

[0030] Comparative Example 1 This comparative example provides a method for preparing a magnesium fluoride ceramic. Compared with Example 1, the difference is that no calcium nitrate tetrahydrate is added in S1, and a pure magnesium fluoride powder is obtained, and the other steps are the same as those of Example 1.

[0031] The infrared transmittance curve of the magnesium fluoride-based ceramic prepared in Example 1 and the magnesium fluoride ceramic prepared in Comparative Example 1 is shown in Figure 4 From the infrared transmittance curve, it can be seen that the infrared transmittance of the magnesium fluoride-based ceramic of Example 1 at 3.0 um is 80%, while the infrared transmittance of the pure magnesium fluoride ceramic of Comparative Example 1 at 3.0 um is only 69%. This indicates that the incorporation of calcium fluoride into magnesium fluoride can greatly improve the infrared transmittance of the magnesium fluoride ceramic.

[0032] Comparative Example 2 The present comparative example provides a preparation method of magnesium fluoride-based ceramic. Compared with Example 1, the difference of the present comparative example is that S1, calcium fluoride and magnesium fluoride are mixed by ball milling in a volume ratio of 50:50, and ethanol solvent is added during the ball milling process to obtain magnesium fluoride-based powder. The others are the same as Example 1.

[0033] The infrared transmittance curve of the magnesium fluoride-based ceramic prepared in the present comparative example is shown in FIG. 4. Figure 5 As can be seen from the photograph, the magnesium fluoride-based ceramic prepared in Comparative Example 2 is obviously blackened. This is because the ball milling mixing method is easy to cause the magnesium fluoride-based powder to adsorb ethanol, and it is difficult to remove the carbon formed in the ceramic during the sintering process.

[0034] The infrared transmittance curve of the magnesium fluoride-based ceramic prepared in the present comparative example is shown in FIG. 4. Figure 6 As can be seen from the infrared transmittance curve, the infrared transmittance of the magnesium fluoride-based ceramic of Comparative Example 2 at 3 um is only 68%, which is significantly lower than that of the magnesium fluoride-based ceramic of Example 1. It is shown that the magnesium fluoride-based powder prepared by coprecipitation in Example 1 and then sintered to obtain the magnesium fluoride-based ceramic not only solves the problem of low infrared transmittance caused by the blackening of the magnesium fluoride-based ceramic due to ball milling mixing, but also further improves the infrared transmittance of the magnesium fluoride-based ceramic.

[0035] Comparative Example 3 The present comparative example provides a preparation method of magnesium fluoride-based ceramic. Compared with Example 1, the difference of the present comparative example is that the mass of magnesium nitrate hexahydrate and calcium nitrate tetrahydrate is different, the mass of magnesium nitrate hexahydrate is 17.404 g, and the mass of calcium nitrate tetrahydrate is 5.539 g, to obtain a calcium fluoride-doped magnesium fluoride solution, and the volume ratio of calcium fluoride to magnesium fluoride in the calcium fluoride-doped magnesium fluoride solution is 30:70. The others are the same as Example 1.

[0036] The infrared transmittance curve of the magnesium fluoride-based ceramic prepared in the present comparative example is shown in FIG. 4. Figure 7 As can be seen from the infrared transmittance curve, the infrared transmittance of the magnesium fluoride-based ceramic at 3.0 um is only 75%, which is lower than that of the magnesium fluoride-based ceramic prepared in Example 1, indicating that a large difference in the volume ratio of magnesium fluoride to calcium fluoride affects the infrared transmittance.

[0037] Experimental Example The magnesium fluoride-based ceramics prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to bending strength test. The test method was as follows: the magnesium fluoride-based ceramic was prepared into a sample with a size of 3 mm x 4 mm x 35 mm, and the test was performed on a universal material testing machine. The three-point bending method was used, the span was 30 mm, and the loading rate was 0.5 mm / min. Five samples were set, and the test data were averaged. The test results are shown in Table 2.

[0038]

[0039] Table 2 According to Table 2, the bending strength of the magnesium fluoride ceramic prepared in Comparative Example 1 is 110 MPa, and the bending strength of the magnesium fluoride-based ceramic prepared in Example 1 reaches 132 MPa after the incorporation of calcium fluoride relative to Comparative Example 1, and the mechanical properties are improved by 20% relative to Comparative Example 1.

[0040] According to Table 2, the magnesium fluoride-based ceramic prepared in Comparative Example 2 is prepared by ball milling, and the bending strength is improved compared with Comparative Example 1, but the improvement effect is not as good as Example 1. The magnesium fluoride-based powder in Example 1 is prepared by coprecipitation, and the magnesium fluoride-based ceramic is finally obtained by sintering, and the mechanical properties are improved more obviously.

[0041] According to Table 2, the bending strength of the magnesium fluoride-based ceramic in Comparative Example 3 is improved relative to Comparative Example 2, but due to the volume ratio of calcium fluoride to magnesium fluoride being 30:70, the pinning effect of calcium fluoride is poor, and the improvement of mechanical properties is limited.

[0042] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; 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 producing a magnesium fluoride-based ceramic, characterized by, The method comprises the following steps: S1: dissolving a magnesium salt and a doped metal salt in water to obtain a mixed solution, dissolving a fluorine salt in water to obtain a fluorine solution, adding the mixed solution into the fluorine solution to react to obtain a precipitate solution, the precipitate being magnesium fluoride and doped fluorinated metal, the volume ratio of the magnesium fluoride to the doped fluorinated metal being 40-60:40-60, and then aging, filtering and freeze-drying the precipitate solution to obtain a magnesium fluoride-based powder; S2: grinding the magnesium fluoride-based powder, placing it in a mold, sintering under vacuum, and then polishing both sides to obtain a magnesium fluoride-based ceramic.

2. The method for producing a magnesium fluoride-based ceramic according to claim 1, characterized by, The doped metal salt comprises one or more of calcium nitrate, strontium nitrate, barium nitrate, lead nitrate, scandium nitrate, gadolinium nitrate, lutetium nitrate and lanthanum nitrate.

3. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The magnesium salt is magnesium nitrate or magnesium chloride.

4. The method of claim 1, wherein the magnesium fluoride-based ceramic is prepared by a method comprising: The fluorine salt is ammonium fluoride or hydrogen fluoride. ​ 5. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The aging time is 3-12 hours.

6. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The freeze-drying temperature is-45℃ to-55℃, and the freeze-drying time is 12-48 hours.

7. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The sintering temperature is 500-900℃, and the sintering holding time is 2-5 hours.

8. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The vacuum degree of the vacuum is 6.5 x 10 -3 Pa.

9. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The sintering heating rate is 5-10℃ / min.

10. The method for preparing magnesium fluoride-based ceramics according to claim 1, characterized in that, The diameter of the ground magnesium fluoride-based powder is 30-35mm.

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