A silicon nitride ceramic radome and its sintering method

Abstract

This invention belongs to the field of radome fabrication technology, specifically relating to a silicon nitride ceramic radome and its embedding method. The embedding method includes the following steps: pre-treating a 3D-printed silicon nitride radome blank; pre-sintering boron nitride powder; placing the radome blank in an embedding powder container, and embedding the inside and outside of the radome blank with pre-sintered boron nitride powder; adding anhydrous ethanol to the embedding powder container; drying the embedding powder container containing the radome blank; repeating the above embedding-adding anhydrous ethanol-drying steps until the dried powder completely fills the radome blank; and sintering the dried embedding powder container containing the radome blank to obtain the silicon nitride ceramic radome. By using the ethanol wet embedding method, the density of the embedded powder is improved, which can solve the problems of large shrinkage size effect, asynchronous shrinkage, and low structural strength of porous sandwich silicon nitride radomes.

Description

Technical Field

[0001] This invention belongs to the field of antenna radome manufacturing technology, specifically relating to a silicon nitride ceramic antenna radome and its sintering method. Background Technology

[0002] High-performance missile warheads, such as anti-radiation warheads and those designed to penetrate electronic jamming, typically use silicon nitride ceramic radomes. To improve the high-temperature resistance, wide bandwidth, and high Mach number performance of silicon nitride ceramic radomes, 3D printing technology is usually used to prepare wideband silicon nitride radomes with high porosity and sandwich structures.

[0003] For silicon nitride materials, rare earth oxides are typically added as sintering aids during the sintering process to induce atomic rearrangement during the α→β phase transition, forming a eutectic liquid phase to promote interparticle diffusion and bonding. However, due to the high porosity and hollow structure of the inner and outer skins in 3D-printed sandwich-structured porous silicon nitride, destructive shrinkage effects inevitably occur during liquid-phase sintering. Furthermore, the overall shrinkage of the radome is asynchronous, resulting in low strength of the prepared silicon nitride ceramic structure. This leads to a low success rate in fabricating large-size porous sandwich-structured silicon nitride radomes, limiting the development of high-temperature, broadband, and high-Mach-number silicon nitride ceramic radomes. Summary of the Invention

[0004] To address the above problems, this invention presents a method for embedding a silicon nitride ceramic radome, comprising the following steps:

[0005] Pre-treatment is performed on the silicon nitride radome blank obtained by 3D printing;

[0006] Pre-sintering of boron nitride powder;

[0007] The radome blank is placed in a powder embedding container, and the inside and outside of the radome blank are embedded with pre-sintered boron nitride powder; anhydrous ethanol is added to the powder embedding container; and the powder embedding container containing the radome blank is dried.

[0008] Repeat the above steps of embedding powder, adding anhydrous ethanol, and drying until the dried powder completely fills the radome blank.

[0009] The dried powder-filled container containing the radome blank is sintered to obtain a porous sandwich structure silicon nitride ceramic radome.

[0010] Compared with the prior art, the beneficial effects of this invention are as follows: After adding anhydrous ethanol, the gaps between boron nitride powder particles decrease or disappear, making it impossible to completely fill and encapsulate the radome blank. Repeated powder embedding until the powder completely fills the radome blank indicates that the boron nitride powder is fully embedded. By using the ethanol wet embedding method, the density of the embedded powder is improved, reducing the shrinkage space of the radome. This helps solve the problems of large shrinkage size effect, asynchronous shrinkage, and low structural strength in porous sandwich silicon nitride radomes. Furthermore, the boron nitride powder is easily removed, enabling the fabrication of porous sandwich silicon nitride radomes.

[0011] Preferably, the preprocessing includes the following steps:

[0012] The silicon nitride radome blank was extracted in aviation kerosene.

[0013] The extracted silicon nitride radome blank is dried.

[0014] The extraction temperature is 60-80℃, and the extraction time is 45-50h;

[0015] The drying temperature is 90-110℃.

[0016] The beneficial effects of this preferred solution are as follows: by pre-treating the silicon nitride radome blank, the wax-based material carried on the radome blank can be removed, which is beneficial for the boron nitride powder to enter the porous radome blank, thereby reducing its shrinkage effect.

[0017] Preferably, the pre-sintering temperature is 1400-1500℃.

[0018] The beneficial effects of this preferred solution are as follows: by pre-sintering the boron nitride powder, the particles can be agglomerated, the number of particles per unit area can be increased, the unit weight can be increased, the powder can be more fully embedded, and the antenna radome blank can be guaranteed to have high strength and small shrinkage effect. This ensures that the porous silicon nitride antenna radome prepared by wet embedding has the characteristics of low shrinkage and high strength. In addition, the pre-sintered boron nitride powder is more resistant to high temperature and will not participate in the reaction, making it easy to remove from the antenna radome.

[0019] Preferably, the pre-sintered boron nitride powder is poured into a graphite crucible;

[0020] The radome blank is placed upside down in a graphite crucible;

[0021] A graphite crucible containing the radome blank is placed in a powder-embedding container, and pre-sintered boron nitride powder is used to embedding the internal cavity, external radome, and graphite crucible of the radome blank.

[0022] The advantages of this preferred solution are as follows: the graphite crucible is used to support the radome blank, so that both the internal cavity and the external cover of the radome blank are treated with powder, which helps to reduce its shrinkage size effect and ensures synchronous shrinkage.

[0023] Preferably, the drying temperature is 70-90°C.

[0024] The beneficial effects of this preferred solution are as follows: by repeatedly adding anhydrous ethanol and drying, the powder can be filled and the density of the powder can be improved, thereby reducing the shrinkage effect of the radome blank and improving the structural strength.

[0025] Preferably, the mass ratio of the embedded powder to the added anhydrous ethanol is 1:(1.5-3).

[0026] Preferably, the sintering process includes debinding sintering and forming sintering.

[0027] The degreasing sintering includes: heating from room temperature to 650-700℃ at a heating rate of 0.4-0.5℃ / min and holding at that temperature for 10-12 hours;

[0028] The forming and sintering process includes: heating from room temperature to a first holding temperature of 650-700°C at a heating rate of 4-5°C / min, continuing to heat to a second holding temperature of 1400-1500°C at a heating rate of 0.4-0.5°C / min, holding for 1.5-2.5 hours, and then sintering by gradient heating and holding.

[0029] More preferably, the degreasing sintering includes: heating from room temperature to 190°C at a rate of 0.5°C / min for 330 min, holding at 190°C for 120 min, continuing to heat to 250°C at a rate of 0.5°C / min for 120 min, holding at that temperature for 120 min, continuing to heat to 330°C at a rate of 0.4°C / min for 200 min, holding at that temperature for 120 min, continuing to heat to 400°C at a rate of 0.5°C / min for 140 min, holding at that temperature for 120 min, continuing to heat to 470°C at a rate of 0.5°C / min for 140 min, holding at that temperature for 120 min, continuing to heat to 670°C at a rate of 0.5°C / min for 400 min, and holding at that temperature for 670 min.

[0030] Preferably, the sintering process using a gradient heating and holding method includes:

[0031] After the second insulation temperature is increased by 50-60℃ to the third insulation temperature, the insulation is maintained for 1.5-2.5 hours, and then removed after natural cooling to room temperature.

[0032] Observe whether the radome blank meets the forming conditions. If so, the sintering is complete.

[0033] Otherwise, heat the radome blank from room temperature to the fourth holding temperature and hold it for 1.5 to 2.5 hours. After it cools naturally to room temperature, remove it. The fourth holding temperature is 50 to 60°C higher than the third holding temperature. Repeat the above steps until the radome blank meets the forming conditions.

[0034] The beneficial effects of this preferred solution are as follows: by analyzing the mechanical properties of the sintered radome in terms of bending strength and elastic modulus, if the mechanical properties meet the requirements, it is considered to meet the forming conditions and the sintering is completed.

[0035] Preferably, the final firing temperature for the radome blank to meet the forming conditions is 1700-1860℃; more preferably, the final firing temperature is 1740-1780℃.

[0036] The beneficial effects of this preferred scheme are as follows: when the sintering temperature reaches 1700-1860℃, the radome has good mechanical properties; when the final sintering temperature is 1730-1780℃, the radome's bending strength can reach 160MPa, the elastic modulus can reach 60.1Gpa, the dielectric constant has an average value of 2.783 in the K-band and 2.707 in the Ku-band, with small dispersion and uniform distribution, stable dielectric properties, and good mechanical, thermal, and electrical properties.

[0037] This invention designs a silicon nitride ceramic radome, which is prepared by the above-mentioned burial method.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows: the radome prepared by the above-mentioned burial method has a small shrinkage size effect, synchronous shrinkage, high structural strength, bending strength up to 160MPa, elastic modulus up to 60.1Gpa, dielectric constant with an average value of 2.783 in the K-band and 2.707 in the Ku-band, with small dispersion, uniform distribution, stable dielectric properties, and good mechanical, thermal, and electrical properties. Attached Figure Description

[0039] Figure 1 This is a flowchart of the burial method according to an embodiment of the present invention.

[0040] Figure 2 The bending strength of the radome at different sintering temperatures is shown in the embodiments of the present invention.

[0041] Figure 3 The elastic modulus of the antenna radome in this embodiment of the invention is measured at different sintering temperatures.

[0042] Figure 4 The dielectric constant of the radome in Embodiment 2 of the present invention.

[0043] Figure 5 This is the loss tangent of the radome in Embodiment 2 of the present invention.

[0044] Figure 6 This is a physical performance test diagram of the radome according to an embodiment of the present invention, wherein:

[0045] (a) is the sintering density variation curve of the porous silicon nitride radome with sandwich structure;

[0046] (b) is the sintering shrinkage rate curve of the porous silicon nitride radome with sandwich structure;

[0047] (c) is the porosity variation curve of the porous silicon nitride radome with sandwich structure;

[0048] (d) shows the water absorption rate variation curve of the porous silicon nitride radome with sandwich structure. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0050] Example 1

[0051] This embodiment provides a method for embedding a silicon nitride ceramic radome, such as... Figure 1 As shown, it includes the following steps:

[0052] Step 1: Pre-process the silicon nitride radome blank obtained by 3D printing, including:

[0053] The silicon nitride radome blank was extracted in aviation kerosene at a temperature of 60°C for 45 hours.

[0054] The extracted silicon nitride radome blank was dried at 90°C.

[0055] Step 2: Pre-sinter the boron nitride powder at a temperature of 1400℃.

[0056] Step 3: Pour the pre-sintered boron nitride powder into a graphite crucible;

[0057] The radome blank is placed upside down in a graphite crucible;

[0058] The graphite crucible containing the radome blank is placed in the powder-burying container.

[0059] Step 4: Use pre-sintered boron nitride powder to embed powder into the internal cavity, external cover, and graphite crucible of the radome blank.

[0060] Step 5: Add anhydrous ethanol to the powder embedding container. In this embodiment, the mass ratio of the powder embedding to the added anhydrous ethanol is 1:1.5.

[0061] Step 6: Dry the powder-embedded container containing the radome blank at a temperature of 70°C.

[0062] Step 7: Repeat steps 4-6 until the dried powder completely fills and coats the radome blank.

[0063] Step 8: The powder-embedded container containing the radome blank obtained in Step 7 is subjected to sintering treatment. In this embodiment, the sintering treatment includes debinding sintering and forming sintering.

[0064] The degreasing and sintering process includes: heating from room temperature to 650-700℃ at a heating rate of 0.4-0.5℃ / min and holding at that temperature for 10-12 hours. The specific process is as follows:

[0065] The temperature is increased from room temperature to 190℃ at a rate of 0.5℃ / min for 330 min, held at 190℃ for 120 min, increased to 250℃ at a rate of 0.5℃ / min for 120 min, held at that temperature for 120 min, increased to 330℃ at a rate of 0.4℃ / min for 200 min, held at that temperature for 120 min, increased to 400℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 470℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 650℃ at a rate of 0.5℃ / min for 360 min, held at that temperature for 600 min.

[0066] The forming and sintering process includes: heating from room temperature to a first holding temperature of 650°C at a heating rate of 4°C / min, continuing to heat to a second holding temperature of 1400°C at a heating rate of 0.4°C / min, holding for 1.5 hours, and then sintering by gradient heating and holding.

[0067] The specific process of sintering using a gradient heating and holding method in this embodiment includes:

[0068] After the second insulation temperature is increased by 50°C to the third insulation temperature, keep it at that temperature for 1.5 hours, and then remove it after it has cooled naturally to room temperature.

[0069] Observe whether the radome blank meets the forming conditions. If so, the sintering is complete.

[0070] Otherwise, the radome blank is heated from room temperature to the fourth holding temperature and held for 1.5 hours. After naturally cooling to room temperature, it is removed. The fourth holding temperature is 50°C higher than the third holding temperature. The above steps are repeated until the radome blank meets the molding conditions. In this embodiment, the final firing temperature for the radome blank to meet the molding conditions is 1740°C.

[0071] The mechanical properties of the radome prepared in this embodiment were analyzed, such as... Figure 2 As shown, when the final burn-in temperature is 1740℃, the radome's bending strength reaches 156MPa. Figure 3 As shown, the elastic modulus reaches 58.7 GPa, indicating good mechanical properties.

[0072] Example 2

[0073] This embodiment provides a method for embedding a silicon nitride ceramic radome, including the following steps:

[0074] Step 1: Pre-process the silicon nitride radome blank obtained by 3D printing, including:

[0075] The silicon nitride radome blank was extracted in aviation kerosene at a temperature of 70°C for 48 hours.

[0076] The extracted silicon nitride radome blank was dried at 100°C.

[0077] Step 2: Pre-sinter the boron nitride powder at a temperature of 1450℃.

[0078] Step 3: Pour the pre-sintered boron nitride powder into a graphite crucible;

[0079] The radome blank is placed upside down in a graphite crucible;

[0080] The graphite crucible containing the radome blank is placed in the powder-burying container.

[0081] Step 4: Use pre-sintered boron nitride powder to embed powder into the internal cavity, external cover, and graphite crucible of the radome blank.

[0082] Step 5: Add anhydrous ethanol to the powder embedding container. In this embodiment, the mass ratio of the powder embedding to the added anhydrous ethanol is 1:2.

[0083] Step 6: Dry the powder-embedded container containing the radome blank at a temperature of 80°C.

[0084] Step 7: Repeat steps 4-6 until the dried powder completely fills and coats the radome blank.

[0085] Step 8: The powder-embedded container containing the radome blank obtained in Step 7 is subjected to sintering treatment. In this embodiment, the sintering treatment includes debinding sintering and forming sintering.

[0086] The degreasing and sintering process includes: heating from room temperature to 650-700℃ at a heating rate of 0.4-0.5℃ / min and holding at that temperature for 10-12 hours. The specific process is as follows:

[0087] The temperature is increased from room temperature to 190℃ at a rate of 0.5℃ / min for 330 min, held at 190℃ for 120 min, increased to 250℃ at a rate of 0.5℃ / min for 120 min, held at that temperature for 120 min, increased to 330℃ at a rate of 0.4℃ / min for 200 min, held at that temperature for 120 min, increased to 400℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 470℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 670℃ at a rate of 0.5℃ / min for 400 min, held at that temperature for 670 min.

[0088] The forming and sintering process includes: heating from room temperature to the first holding temperature of 670°C at a heating rate of 5°C / min, continuing to heat to the second holding temperature of 1450°C at a heating rate of 0.5°C / min, holding for 2 hours, and then sintering by gradient heating and holding.

[0089] The specific process of sintering using a gradient heating and holding method in this embodiment includes:

[0090] After the second insulation temperature is increased by 50°C to the third insulation temperature, keep it in the insulation temperature for 2 hours, and then remove it after it has cooled naturally to room temperature.

[0091] Observe whether the radome blank meets the forming conditions. If so, the sintering is complete.

[0092] Otherwise, the radome blank is heated from room temperature to the fourth holding temperature and held for 2 hours. After naturally cooling to room temperature, it is removed. The fourth holding temperature is 50°C higher than the third holding temperature. The above steps are repeated until the radome blank meets the molding conditions. In this embodiment, the final firing temperature for the radome blank to meet the molding conditions is 1760°C.

[0093] The mechanical properties of the radome prepared in this embodiment were analyzed, such as... Figure 2As shown, when the highest sintering temperature is 1760℃, the radome's bending strength reaches 160MPa. Figure 3 As shown, the elastic modulus reaches 60.1 GPa, and the mechanical properties are optimal at this temperature. The dielectric constant and loss tangent of the porous silicon nitride radome at this temperature were tested, and the results are as follows. Figure 4 As shown, the dielectric constant of the radome has an average value of 2.783 in the K-band and 2.707 in the Ku-band, exhibiting small dispersion, uniform distribution, and stable dielectric properties. This embodiment also tests and analyzes the loss tangent of the porous silicon nitride radome sample with a sandwich structure, such as... Figure 5 As shown, the mean value of tgδ is 0.0032, and the overall experimental test results show small dispersion. The experimental results indicate that the porous silicon nitride radome exhibits good mechanical, thermal, and electrical properties at this temperature.

[0094] Example 3

[0095] This embodiment provides a method for embedding a silicon nitride ceramic radome, including the following steps:

[0096] Step 1: Pre-process the silicon nitride radome blank obtained by 3D printing, including:

[0097] The silicon nitride radome blank was extracted in aviation kerosene at a temperature of 80°C for 50 hours.

[0098] The extracted silicon nitride radome blank was dried at 110°C.

[0099] Step 2: Pre-sinter the boron nitride powder at a temperature of 1500℃.

[0100] Step 3: Pour the pre-sintered boron nitride powder into a graphite crucible;

[0101] The radome blank is placed upside down in a graphite crucible;

[0102] The graphite crucible containing the radome blank is placed in the powder-burying container.

[0103] Step 4: Use pre-sintered boron nitride powder to embed powder into the internal cavity, external cover, and graphite crucible of the radome blank.

[0104] Step 5: Add anhydrous ethanol to the powder embedding container. In this embodiment, the mass ratio of the powder embedding to the added anhydrous ethanol is 1:3.

[0105] Step 6: Dry the powder-embedded container containing the radome blank at a temperature of 90°C.

[0106] Step 7: Repeat steps 4-6 until the dried powder completely fills and coats the radome blank.

[0107] Step 8: The powder-embedded container containing the radome blank obtained in Step 7 is subjected to sintering treatment. In this embodiment, the sintering treatment includes debinding sintering and forming sintering.

[0108] The degreasing and sintering process includes: heating from room temperature to 650-700℃ at a heating rate of 0.4-0.5℃ / min and holding at that temperature for 10-12 hours. The specific process is as follows:

[0109] The temperature is increased from room temperature to 190℃ at a rate of 0.5℃ / min for 330 min, held at 190℃ for 120 min, increased to 250℃ at a rate of 0.5℃ / min for 120 min, held at that temperature for 120 min, increased to 330℃ at a rate of 0.4℃ / min for 200 min, held at that temperature for 120 min, increased to 400℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 470℃ at a rate of 0.5℃ / min for 140 min, held at that temperature for 120 min, increased to 700℃ at a rate of 0.5℃ / min for 460 min, held at that temperature for 700 min.

[0110] The forming and sintering process includes: heating from room temperature to the first holding temperature of 700℃ at a heating rate of 5℃ / min, continuing to heat to the second holding temperature of 1500℃ at a heating rate of 0.5℃ / min, holding for 2.5h, and then sintering by gradient heating and holding.

[0111] The specific process of sintering using a gradient heating and holding method in this embodiment includes:

[0112] After the second insulation temperature is increased by 50°C to the third insulation temperature, the insulation is maintained for 2.5 hours, and then removed after natural cooling to room temperature;

[0113] Observe whether the radome blank meets the forming conditions. If so, the sintering is complete.

[0114] Otherwise, the radome blank is heated from room temperature to the fourth holding temperature and held for 2.5 hours. After naturally cooling to room temperature, it is removed. The fourth holding temperature is 50°C higher than the third holding temperature. The above steps are repeated until the radome blank meets the molding conditions. In this embodiment, the final firing temperature for the radome blank to meet the molding conditions is 1780°C.

[0115] The mechanical properties of the radome prepared in this embodiment were analyzed, such as... Figure 2As shown, when the highest sintering temperature is 1780℃, the radome's bending strength reaches 156MPa. Figure 3 As shown, the elastic modulus reaches 57.3 GPa, indicating good mechanical properties.

[0116] In addition, the physical properties of the radomes prepared in Examples 1, 2, and 3, such as sintering density, water absorption, porosity, and sintering shrinkage, were tested during the sintering process. The test results are as follows: Figure 6 As shown, with increasing sintering temperature, the sintering density of the porous silicon nitride radome with sandwich structure increases, resulting in greater shrinkage, a denser radome surface, lower porosity, and lower water absorption. Observations revealed no irregular interlayer cracks in the radome. Coordinate analysis of the radome's conical curve after sintering showed consistency with the equation parameters before sintering, indicating synchronized shrinkage. This is beneficial for improving the structural strength of the silicon nitride radome and reducing the low success rate of fabrication, thus promoting the development of high-temperature resistant, broadband, and high-Mach number silicon nitride ceramic radomes.

[0117] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of sintering a silicon nitride ceramic radome, characterized by, The method comprises the following steps: The 3D printed silicon nitride radome blank is pretreated; The pretreatment comprises the following steps: The silicon nitride radome blank is placed in aviation kerosene for extraction; The extracted silicon nitride radome blank is dried; The boron nitride powder is pre-sintered at a temperature of 1400-1500℃; The radome blank is placed in a powder embedding container, and the radome blank is embedded with the pre-sintered boron nitride powder inside and outside; Anhydrous ethanol is added to the powder embedding container, and the powder embedding container with the radome blank is dried; The embedding, adding anhydrous ethanol and drying steps are repeated until the dried powder completely fills the radome blank; The powder embedding container with the radome blank is sintered to obtain a porous sandwich structure silicon nitride ceramic radome; The sintering process comprises defatting sintering and forming sintering, The defatting sintering comprises: increasing the temperature from room temperature to 650-700℃ at a rate of 0.4-0.5℃ / min and keeping the temperature for 10-12h; The forming sintering comprises: increasing the temperature from room temperature to a first holding temperature of 650-700℃ at a rate of 4-5℃ / min, then increasing the temperature to a second holding temperature of 1400-1500℃ at a rate of 0.4-0.5℃ / min, keeping the temperature for 1.5-2.5h, and then sintering by gradient temperature increase and holding; The specific process of sintering by gradient temperature increase and holding comprises: After increasing the temperature from the second holding temperature to a third holding temperature by 50-60℃, keeping the temperature for 1.5-2.5h, and then taking out after natural cooling to room temperature; If the radome blank meets the forming condition, the sintering is completed; Otherwise, the radome blank is heated from room temperature to a fourth holding temperature which is 50-60℃ higher than the third holding temperature, and kept for 1.5-2.5h, and then taken out after natural cooling to room temperature; the step is repeated until the radome blank meets the forming condition.

2. The method of claim 1, wherein The extraction temperature is 60-80℃, and the extraction time is 45-50h; The drying temperature is 90-110℃.

3. The method of claim 1, wherein The pre-sintered boron nitride powder is poured into a graphite crucible; The radome blank is inverted in the graphite crucible; The graphite crucible with the radome blank is placed in a powder embedding container, and the radome blank, the outer shell and the graphite crucible are embedded with the pre-sintered boron nitride powder.

4. The method of claim 1, wherein The drying temperature is 70-90℃; And / or, the mass ratio of the embedded powder to the added anhydrous ethanol is 1: (1.5-3).

5. The method of claim 1, wherein The final sintering temperature of the radome blank meeting the forming condition is 1700-1860℃.

6. A silicon nitride ceramic radome, characterized by, The radome is prepared by the sintering method of any one of claims 1-5.

Images