High-strength alumina reinforced porcelain insulator and its preparation method

Abstract

The application discloses a high-strength alumina reinforced porcelain insulator and a preparation method thereof, and belongs to the technical field of alumina-based electrical insulating ceramics. The porcelain insulator comprises a porcelain body base, a surface stress regulation layer and a glaze layer, and gradient reaction coated alumina skeleton particles are dispersed in the porcelain body base. The skeleton particles are composed of an alpha-Al2O3 core, a magnesium-aluminum reaction transition zone and an aluminum-silicon reaction transition zone, and are continuously combined with a fine matrix on the outside. In the preparation, double-coated skeleton particles and fine matrix mud are prepared respectively, the pre-wetted skeleton particles are added to the fine matrix mud which has been completed with low shear mixing, and no high-energy refinement treatment is performed after mixing. The porcelain insulator is prepared through shaping, drying, surface stress regulation treatment, glazing, segmented sintering and controlled cooling. The application can reduce sintering shrinkage and interface defects, improve strength consistency, insulation performance and thermal shock stability, and reduce the risk of surface crack propagation.

Description

Technical Field

[0001] This invention belongs to the field of alumina-based electrical insulating ceramics, specifically relating to a high-strength alumina-reinforced porcelain insulator having gradient reaction-coated alumina skeleton particles, a surface compressive stress regulating layer, and a glaze layer, as well as a method for preparing the porcelain insulator. Background Technology

[0002] Porcelain insulators need to withstand mechanical loads and electric fields during use. Existing high-alumina porcelains typically improve density and strength by increasing the amount of industrial alumina, adding fluxing agents, and ball milling or sand milling all raw materials. Chinese patent document CN1990420A discloses an alumina electrical porcelain, whose formula includes industrial alumina, feldspar, clay, and magnesium aluminum silicate plasticizer, and employs a process flow of batching, ball milling, pressing, vacuum extrusion, forming, drying, glazing, and firing. Chinese patent document CN115650705A discloses a high-strength porcelain insulator active industrial alumina porcelain formula and its preparation steps, which first sand mills the raw materials to blank particles with a particle size of 3-5 μm, and then performs grinding and ball milling.

[0003] While refining the entire material facilitates mixing and sintering, for thick-walled or large-sized ceramic parts, a higher proportion of fine powder can lead to simultaneous increases in drying and sintering shrinkage. When the shrinkage is inconsistent between the inside and outside of the product, microcracks can easily appear at particle boundaries, near pores, or on the surface. Retaining larger alumina particles can reduce overall shrinkage, but when these particles directly bond with the fine matrix, insufficient bonding, porosity, or cracks may occur. Simply changing the alumina content or uniformly adding fluxing components to the matrix is ​​insufficient to address these issues simultaneously.

[0004] Therefore, there is a need for a ceramic insulator and its preparation method that can retain the load-bearing capacity of larger alumina particles, improve their bonding with the fine matrix, and take into account surface crack control. Summary of the Invention

[0005] The problem this invention aims to solve is: to maintain the load-bearing function of larger alumina particles within the ceramic body without relying on high-energy refining of the entire material, while reducing interfacial defects between the particles and the fine matrix, and lowering the possibility of surface crack propagation.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a high-strength alumina-reinforced ceramic insulator, comprising a ceramic matrix, a surface compressive stress regulating layer disposed on the outer surface of the ceramic matrix, and a glaze layer disposed on the outer side of the surface compressive stress regulating layer, wherein the ceramic matrix comprises a fine matrix and gradient reaction-coated alumina skeleton particles dispersed in the fine matrix; The gradient reaction-coated alumina framework particles include an α-Al2O3 core, a magnesium-aluminum reaction transition region directly bonded to the α-Al2O3 core, and an aluminum-silicon reaction transition region located outside the magnesium-aluminum reaction transition region. The aluminum-silicon reaction transition region is continuously bonded to the fine matrix. The α-Al₂O₃ core has a D50 of 18–45 μm, and the gradient reaction-coated alumina framework particles account for 15–35 wt% of the total inorganic solid mass of the ceramic matrix. The total Al2O3 content of the ceramic matrix, calculated based on oxides after firing, is 55–75 wt%, and the total Na2O and K2O content is not higher than 1.2 wt%. The linear expansion coefficient of the surface compressive stress regulating layer is 0.3–0.8 × 10⁻⁶ lower than that of the ceramic matrix. -6 / ℃.

[0007] Furthermore, the D50 of the α-Al2O3 core is 22–38 μm, and the gradient reaction-coated alumina framework particles account for 22–33 wt% of the total inorganic solid mass of the ceramic matrix. The D90 of the free quartz particles in the ceramic matrix is ​​not greater than 8 μm.

[0008] Furthermore, the thickness of the magnesium-aluminum reaction transition zone is 0.2–1.0 μm, the thickness of the aluminum-silicon reaction transition zone is 0.8–4.0 μm, and a discontinuously distributed low-alkali bonded microregion is formed between the aluminum-silicon reaction transition zone and the fine matrix, the low-alkali bonded microregion containing calcium magnesium aluminum silicate.

[0009] Furthermore, the post-firing thickness of the surface compressive stress control layer is 0.25–0.90 mm, and the content of gradient reaction-coated alumina skeleton particles in the surface compressive stress control layer is not higher than 5 wt% of its total inorganic solid mass. The glaze layer is a low-alkali aluminosilicate glaze layer with a thickness of 0.12–0.35 mm after firing, and the linear expansion coefficient of the glaze layer is 0–0.4 × 10⁻⁶ lower than that of the surface compressive stress control layer. -6 / ℃.

[0010] This invention also provides a method for preparing a high-strength alumina-reinforced ceramic insulator, comprising the following steps: S1. A first coating slurry and a second coating slurry are sequentially applied to the surface of the α-Al2O3 core, and dried after each application of the coating slurry. The first coating slurry includes activated alumina, a magnesium source, and an alkali-free binder component. The second coating slurry includes metakaolin, activated alumina, low-alkali calcium magnesium aluminum silicate sintering trigger micro powder, flaky alumina seed crystals, and an alkali-free binder component. The dried particles are deagglomerated and sieved to obtain gradient reaction coated alumina framework particles. S2. Fine industrial alumina, washed plastic clay, calcined kaolin and mullite powder are wet ball milled, sieved, iron removed, dehydrated and vacuum kneaded to obtain fine matrix clay. S3. After pre-wetting the gradient reaction-coated alumina skeleton particles, add them to the fine matrix mud and mix them using a low-shear method to ensure that the gradient reaction-coated alumina skeleton particles are evenly dispersed. After mixing, do not subject the mud containing the gradient reaction-coated alumina skeleton particles to high-energy ball milling, sand milling or other high-energy refining treatments. S4. Shape and dry the mixed clay, apply a surface compressive stress control slurry to the outer surface of the green body and apply glaze; S5. The glazed green body is fired and cooled in a controlled manner to obtain a high-strength alumina-reinforced porcelain insulator.

[0011] Furthermore, in step S1, the Al2O3 content of the α-Al2O3 core is not less than 98wt%, and the D50 is 18-45μm; In step S2, the fine matrix powder after wet ball milling has a D50 of 0.6–2.0 μm and a D90 of no more than 6.0 μm. The gradient reaction-coated alumina framework particles added in step S3 account for 15-35 wt% of the total inorganic solid mass of the ceramic body.

[0012] Furthermore, in step S1, the amount of magnesium source added, calculated as MgO, is 0.3–1.5 wt% of the α-Al₂O₃ core mass; The total mass of the magnesium-aluminum reaction precursor layer formed by the first coating slurry and the aluminum-silicon reaction precursor layer formed by the second coating slurry is 4 to 15 wt% of the α-Al₂O₃ core mass; In the second coating slurry, the mass ratio of metakaolin to activated alumina is 0.8:1 to 1.2:1, the low-alkali calcium magnesium aluminum silicate sintering trigger micro powder accounts for 5 to 20 wt% of the solid mass of the second coating slurry, the lamellar alumina seed crystals account for 1 to 5 wt% of the solid mass of the second coating slurry, and at least 60 wt% of the low-alkali calcium magnesium aluminum silicate sintering trigger micro powder is introduced with the second coating slurry.

[0013] Furthermore, in step S3, the pre-wetting uses water or diluted plasticizer, and the amount added is 3-8 wt% of the mass of the gradient reaction-coated alumina skeleton particles; The low-shear mixing method employs at least one of the following: biaxial low-speed mixing, planetary low-speed mixing, short-time mixing after vacuum pumice, or multi-pass low-speed folding and extrusion using a biaxial kneader, with a mixing time of 8–20 min.

[0014] Furthermore, in step S4, the surface compressive stress regulating slurry is made of fine alumina, mullite powder, calcined kaolin and low alkali aluminosilicate combined components as the main raw materials, and is applied by spraying, dipping or brushing. After applying the surface compressive stress-regulating slurry, excess slurry at the base of the umbrella skirt or irregular curved surface is removed by low-pressure compressed air purging or centrifugal slurry throwing before glazing.

[0015] Furthermore, in step S5, the firing process includes the following steps in sequence: The degreasing stage involves holding the product at 550–700℃ for 0.5–1.5 hours. The first reaction heat preservation section is maintained at 1000-1100℃ for 0.5-2.0h. A densification insulation section is maintained at 1230–1310℃ for 1.0–4.0 hours. And a controlled cooling section operating at a rate of 0.8 to 2.5 °C / min in the 900–650 °C range.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention retains larger-diameter α-Al₂O₃ particles as skeletal particles within the ceramic body, preparing them separately from the fine matrix before low-shear mixing. This prevents the skeletal particles from breaking down during subsequent high-energy refining. Magnesium-aluminum and aluminum-silicon reaction transition zones are located between the skeletal particles and the fine matrix, improving their bonding and reducing porosity and cracks around the particles. The linear expansion coefficients of the surface compressive stress control layer and the glaze layer are progressively decreased from the inside out, helping to reduce tensile stress in the surface area and inhibit surface crack propagation. This structure and preparation method balance the sintering density of the ceramic body, the load-bearing capacity of larger particles, and the firing adaptability of thick-walled products. Attached Figure Description

[0017] Figure 1 This is a flowchart of the preparation method of the high-strength alumina-reinforced ceramic insulator of the present invention; Figure 2 The images show a comparison of scanning electron microscope (SEM) images of the post-burnt cross-sections of Comparative Example 2 and Example 1. Figure 2 (a) is a scanning electron microscope image of the post-burning cross-section of Comparative Example 2. Figure 2 (b) is a scanning electron microscope image of the cross-section after burning in Example 1. Detailed Implementation

[0018] The following combination Figure 1 and Figure 2 The present invention will be further described below. Figure 1The two routes are used to prepare gradient reaction-coated alumina framework particles and fine matrix slurry, respectively. The two are combined after the fine matrix has been refined. No further high-energy ball milling, sand milling, or other high-energy refining treatments are performed after the framework particles are added. Figure 2 (a) and Figure 2 (b) The cross-sections after burning, corresponding to Comparative Example 2 and Example 1 described later.

[0019] In this embodiment, particle size is expressed as the cumulative volume particle size measured by a laser particle size analyzer. D50 represents the particle size at a cumulative volume fraction of 50%, and D90 represents the particle size at a cumulative volume fraction of 90%. All mass fractions in each formulation are based on inorganic solids. Water, moisture in the aluminum sol, and small amounts of organic forming aids are not included in the inorganic solids mass fraction.

[0020] This embodiment uses oxide raw materials to form the reinforced structure, without adding continuous fibers, metal coatings, or metal particles. The larger-diameter α-Al₂O₃ cores remain as skeletal particles after sintering. The first coating layer forms a magnesium-aluminum reaction transition zone around the core. The second coating layer forms an aluminum-silicon reaction transition zone on its outer side. The sintering triggering micropowder mainly remains near the second coating layer, concentrating the sintering-promoting effect at the junction of the skeletal particles and the fine matrix.

[0021] The role of the magnesium-aluminum reaction transition zone is to reduce the abrupt interface change between the α-Al₂O₃ core and the outer reaction layer. The role of the aluminum-silicon reaction transition zone is to gradually bond the outer periphery of the core with the fine matrix. During firing, the local reaction layer first forms a sintering neck, and then densifies together with the fine matrix. This reduces porosity and separation seams that easily appear around uncoated coarse particles. The reaction layer is not used as a thick, independent outer shell; its thickness is controlled by the total amount of precursors and the firing regime.

[0022] The surface compressive stress control layer is placed on the outer surface of the ceramic substrate. This layer contains little or no gradient reactive coated alumina framework particles, and its coefficient of linear expansion is lower than that of the ceramic substrate. The coefficient of linear expansion of the glaze layer is slightly lower than that of the surface compressive stress control layer. During firing and cooling, the shrinkage difference among these three layers is constrained by the overall ceramic piece, resulting in a compressive stress state in the surface area. This structure is used to reduce the possibility of surface microcracks opening.

[0023] Example 1 This embodiment prepares thick-walled short rod process samples and three-point bending specimens. The raw material proportions are shown in Table 1, based on 100 parts by mass of inorganic solids in the ceramic matrix. The amount of gradient reaction-coated alumina framework particles added is 28 parts by mass. Coarse-grained free quartz is not used as aggregate in the fine matrix. After firing oxide conversion, the total Al2O3 content in the ceramic matrix is ​​67.3 wt%, and the total Na2O and K2O content is 0.84 wt%.

[0024] Table 1. Inorganic solid composition (parts by mass) of ceramic matrix in Examples 1 to 3 Gradient reaction coated alumina framework particles 28 22 33 Fine industrial alumina 37 41 31 Washing and selecting plastic clay 11 12 10 Calcinated kaolin 16 17 18 Mullite powder 8 8 8 total 100 100 100 In Example 1, the α-Al₂O₃ core contained 99.2 wt% Al₂O₃ and had a D50 of 28.4 μm. The skeletal particles consisted of short columnar particles and near-equiaxed particles, with the short columnar particles having a major-to-minor axis ratio of 1.6–2.4. Based on 100 parts by weight of the α-Al₂O₃ core, the solids in the first coating slurry included 3.2 parts of activated alumina, 0.8 parts of Mg(OH)₂, and 0.6 parts of aluminum sol solids. The amount of Mg(OH)₂ added, calculated as MgO, was approximately 0.55 wt% of the α-Al₂O₃ core mass. The solids content of the first coating slurry was controlled at 42 wt%.

[0025] The solids in the second coating slurry include 4.0 parts metakaolin, 3.8 parts activated alumina, 1.2 parts low-alkali calcium magnesium aluminum silicate sintering trigger micro powder, 0.48 parts lamellar alumina seed crystals, and 0.3 parts alkali-free binder. The mass ratio of metakaolin to activated alumina is 1.05:1. The sintering trigger micro powder accounts for 12.3 wt% of the solids in the second coating slurry, and the lamellar alumina seed crystals account for 4.9 wt%. The solid content of the second coating slurry is controlled at 45 wt%. The total amount of precursors formed by the first and second coating slurries is 14.4 wt% of the α-Al₂O₃ core mass. All the sintering trigger micro powder is added with the second coating slurry.

[0026] First, the α-Al₂O₃ core is added to a high-speed mixing granulator. The equipment speed is controlled at 260–320 r / min. The first coating slurry is atomized and added, and mixed to keep the particles in a flowable state. Then, it is dried at 105℃ for 35 min. The function of the first coating layer is to bring the magnesium source and activated alumina close to the core surface, reducing the ineffective dispersion of the magnesium source in the fine matrix. Next, the second coating slurry is atomized and added to the surface of the intermediate particles, and dried at 105℃ for 45 min. After slight deagglomeration, the dried particles are passed through a 60-mesh sieve. In the resulting particles, the first reaction precursor layer is close to the α-Al₂O₃ core, and the second reaction precursor layer is located on its outer side.

[0027] Fine industrial alumina, washed plastic clay, calcined kaolin, and mullite powder were added to a ball mill, and deionized water was added at 45 wt% of the total slurry mass for wet ball milling. After 9 hours of ball milling, the D50 of the fine matrix powder was 1.2 μm, and the D90 was 4.9 μm. The slurry was sieved through a 250-mesh sieve and subjected to magnetic separation to remove iron, then dewatered by pressure filtration and plying in a vacuum ply mill. The D90 of the free quartz particles was controlled below 8 μm. After the fine matrix completed ball milling and vacuum ply milling, no further powders requiring high-energy refining were added.

[0028] Gradient-reactive coated alumina framework particles were premixed with 5 wt% deionized water for 3 minutes to slightly moisten the particle surface. This mixture was then added to a fine matrix slurry and mixed for 12 minutes at 25–35 rpm in a twin-shaft low-speed mixer. At this stage, folding and pushing were the primary methods, avoiding high-shear-strength sand milling. This step ensured uniform dispersion of the framework particles while preventing the peeling off of the two coating layers or the breakage of the α-Al₂O₃ cores.

[0029] The mixed clay was vacuum extruded to form thick-walled short rods with a length of 600 mm, an outer diameter of 50 mm, and a wall thickness of 18 mm. Separately, three-point bending samples with post-firing dimensions of 3 mm × 4 mm × 36 mm were prepared from the same batch of clay. The green samples were first slowly dried at 36℃ and 85% relative humidity for 16 hours, and then finally dried at 55℃ for 10 hours. Pre-drying at low temperature reduces the moisture difference between the inside and outside of the thick-walled samples.

[0030] The surface stress-regulating slurry, based on 100 parts by weight of inorganic solids, includes 52 parts fine alumina, 22 parts mullite powder, 18 parts calcined kaolin, and 8 parts low-alkali aluminosilicate binder. The slurry's solid content is 58 wt%. It is sprayed twice onto the surface of the dried green body. For the ends and curved surfaces of short rod samples, after spraying, the slurry is purged with low-pressure compressed air (0.08–0.12 MPa) to remove localized slurry buildup. The surface stress-regulating layer has a thickness of 0.55 mm after firing. Its inorganic solids do not contain gradient reaction-coated alumina framework particles.

[0031] The glaze was applied using a low-alkali aluminosilicate glaze slurry for 8 seconds. The glaze layer thickness after firing was 0.23 mm. Thermal expansion tests showed that the average linear expansion coefficient of the ceramic matrix within the range of 25–600℃ was 5.63 × 10⁻⁶. -6 / ℃, the surface compressive stress control layer is 5.08×10 -6 / ℃, glaze thickness 4.92×10 -6 / ℃. The difference between the surface compressive stress control layer and the ceramic substrate is 0.55×10. -6 / ℃, the difference between the glaze layer and the surface compressive stress control layer is 0.16×10 -6 / ℃.

[0032] During firing, the temperature was first raised to 650℃ and held for 1 hour, then raised to 1060℃ and held for 1 hour, followed by raising to 1275℃ and holding for 2 hours. After firing, the temperature was cooled at an average rate of 1.5℃ / min within the range of 900–650℃. Holding at 650℃ was used to remove organic matter and bound water. Holding at 1060℃ allowed the reaction at the interface to proceed gradually. Holding at 1275℃ was used to complete matrix densification and interfacial bonding. Controlled cooling was used to reduce tensile stress caused by the difference in thermal expansion between the phases.

[0033] Example 2 Example 2 illustrates the use of a lower amount of framework particles and a thinner surface compressive stress control layer. The ceramic matrix composition is shown in Table 1. The α-Al₂O₃ core contains 99.1 wt% Al₂O₃ and has a D50 of 21.6 μm. Based on 100 parts by mass of the core, the first coating layer precursor includes 2.3 parts activated alumina, 0.65 parts Mg(OH)₂, and 0.45 parts alkali-free binder. The second coating layer precursor includes 2.85 parts metakaolin, 3.0 parts activated alumina, 0.85 parts low-alkali calcium magnesium aluminum silicate sintering trigger powder, 0.30 parts lamellar alumina seed crystals, and 0.20 parts alkali-free binder. The total amount of the two precursor layers is 10.6 wt% of the α-Al₂O₃ core mass.

[0034] Example 2 uses a fluidized bed bottom spray coating machine for coating. The first coating slurry has a solid content of 40 wt%, and the second coating slurry has a solid content of 43 wt%. The mass ratio of metakaolin to activated alumina is 0.95:1, the sintering triggering micro powder accounts for 11.8 wt% of the solids in the second coating layer, and the lamellar alumina seed crystals account for 4.2 wt%. Each coating is dried at 100°C. When using a fluidized bed spraying system, the particles tumble in the airflow, which reduces the possibility of excessively thick local coatings.

[0035] The fine matrix, after wet ball milling, had a D50 of 1.0 μm and a D90 of 4.6 μm. The coated skeleton particles were premixed with 4 wt% water and then added to the fine matrix slurry, which was mixed for 10 min in a low-speed planetary mixer. The mixture was then directly molded. The surface stress-regulating slurry was the same as in Example 1, with a post-firing thickness of 0.42 mm and a post-firing glaze thickness of 0.18 mm. The average linear expansion coefficients of the ceramic matrix, surface stress-regulating layer, and glaze layer at 25–600 °C were 5.70 × 10⁻⁶. -6 / ℃, 5.20×10 -6 / ℃ and 5.02×10 -6 / ℃.

[0036] During firing, the temperature is maintained at 620℃ for 1 hour, at 1040℃ for 1 hour, and at 1265℃ for 2.5 hours. Cooling is carried out at a rate of 1.8℃ / min within the range of 900–650℃. In this embodiment, the amount of skeleton particles added is relatively low, the proportion of fine matrix is ​​relatively high, the forming fluidity is good, and it is suitable for molded small or medium-sized ceramic parts.

[0037] Example 3 Example 3 employed a higher amount of framework particles and a larger particle size α-Al₂O₃ core. The ceramic matrix composition is shown in Table 1. The α-Al₂O₃ core contained 99.3 wt% Al₂O₃ with a D50 of 38.2 μm. Based on 100 parts by mass of the core, the first coating layer precursor consisted of 3.0 parts activated alumina, 0.9 parts Mg(OH)₂, and 0.6 parts alkali-free binder. The second coating layer precursor consisted of 4.0 parts metakaolin, 4.0 parts activated alumina, 1.15 parts low-alkali calcium magnesium aluminum silicate sintering trigger powder, 0.45 parts lamellar alumina seed crystals, and 0.10 parts alkali-free binder. The total amount of the two precursor layers was 14.2 wt% of the α-Al₂O₃ core mass.

[0038] Example 3 uses a granulation drying tower with an atomizing spray gun to complete two coating processes. While the core particles are suspended in the tower, a first coating slurry is sprayed in, followed by a second coating slurry. The first coating slurry has a solid content of 44 wt%, and the second coating slurry has a solid content of 50 wt%. The mass ratio of metakaolin to activated alumina is 1:1. Low-alkali calcium magnesium aluminum silicate sintering trigger powder accounts for 11.9 wt% of the solid content of the second coating layer, and lamellar alumina seed crystals account for 4.6 wt%. The drying outlet temperature is controlled between 95 and 110°C. This method is suitable for large-scale production and also helps reduce hard agglomeration between particles.

[0039] After wet ball milling, the fine matrix had a D50 of 1.5 μm and a D90 of 5.3 μm. The coated skeleton particles were premixed with 7 wt% diluted plasticizer and then added to the fine matrix clay. The mixture was then subjected to multiple low-speed folding and extrusion passes in a biaxial kneader for a total mixing time of 16 min. High-energy ball milling was not performed after mixing. The resulting clay was vacuum extruded. The surface stress-regulating layer had a firing thickness of 0.70 mm, and the glaze layer had a firing thickness of 0.28 mm. The average linear expansion coefficients of the ceramic matrix, surface stress-regulating layer, and glaze layer between 25 and 600 °C were 5.58 × 10⁻⁶. -6 / ℃, 4.89×10 -6 / ℃ and 4.68×10 -6 / ℃. For samples with deep grooves, after spraying at the root of the grooves, the surface compressive stress control layer thickness is 0.22-0.30 mm, while the thickness of the remaining surface is maintained at around 0.70 mm.

[0040] During firing, the temperature is maintained at 660℃ for 1.2 hours, at 1080℃ for 0.8 hours, and at 1290℃ for 1.5 hours. Cooling is carried out at a rate of 1.2℃ / min within the range of 900–650℃. The addition of larger-diameter core particles and higher skeletal particle content can further reduce the overall shrinkage tendency, but the fine matrix content is correspondingly reduced. Therefore, the mixing uniformity, drying rate, and firing temperature difference need to be controlled simultaneously.

[0041] In addition to the three embodiments described above, rod-shaped, pillar-shaped, or sleeve-shaped ceramic parts can be formed by vacuum extrusion, rolling, or isostatic pressing. Disc-shaped suspended ceramic parts can be formed by molding or slip casting. When extrusion is used, the short columnar or plate-shaped skeleton particles will have a certain orientation along the extrusion direction. This orientation does not have to be completely consistent; as long as a local directionality is formed near the skeleton particles, the interface cracks will not easily extend in a single direction.

[0042] When using a tunnel kiln or drawer kiln for continuous production, the cooling rate in the 900–650℃ range is achieved by adjusting the frequency of the exhaust fan, the volume of the quenching air, or by installing buffer walls. Control should be based on the average cooling rate of the thick-walled ceramic piece cross-section, rather than solely on the kiln's displayed temperature. This reduces temperature differences at different locations within the same cross-section.

[0043] Comparative Example Comparative Example 1 had the same total oxide composition as Example 1, but without the preparation of gradient reaction-coated alumina framework particles. The α-Al₂O₃ core, first coating layer material, second coating layer material, and fine matrix material were all added to a sand mill at once. After sand milling, the overall material D50 was 3.8 μm. Subsequent shaping, surface compressive stress control treatment, glazing, and firing processes were all the same as in Example 1. This group was used to investigate the situation where, after overall material refinement, neither the coarse framework nor the localized reaction layer was retained.

[0044] Comparative Example 2 retained the same α-Al₂O₃ cores with the same particle size and mass percentage as Example 1, but without the two-layer coating. The active alumina, Mg(OH)₂, metakaolin, low-alkali calcium magnesium aluminum silicate sintering trigger powder, and lamellar alumina seed crystals originally intended for the coating layer were uniformly added to the fine matrix. The α-Al₂O₃ cores were added after the fine matrix was ball-milled, and subsequent processes were the same as in Example 1. This group was used to investigate the interfacial state when coarse-grained cores directly contacted the fine matrix. Figure 2 (a) Cross-section taken from the independent sintered samples in this group.

[0045] Comparative Example 3 prepared a gradient reaction-coated alumina framework particles similar to those in Example 1, but instead of adding low-alkali calcium-magnesium-aluminum silicate sintering trigger microparticles to the second coating layer, an equal amount of sintering trigger microparticles was uniformly added to the fine matrix. Other raw materials, forming, surface compressive stress control treatment, glazing, and firing regimes were the same as in Example 1. This group was used to investigate the effect of the addition location of the sintering trigger microparticles on interface densification and electrical properties.

[0046] Comparative Example 4 prepared the same coated framework particles and fine matrix slurry as in Example 1. After adding the fine matrix, the framework particles were then subjected to high-energy ball milling to reduce the D50 of the mixture to 3.6 μm. Subsequent processes were the same as in Example 1. This group was used to investigate the condition of the framework particles and coating structure after the low-shear post-addition step was omitted.

[0047] Comparative Example 5 used the same ceramic matrix and gradient reaction-coated alumina framework particles as in Example 1, but without the surface stress control layer; only a low-alkali glaze layer was applied directly. Other conditions were the same as in Example 1. This group was used to investigate the effect of the surface stress control layer on surface cracking and post-thermal shock conditions.

[0048] Experimental methods and results Five thick-walled short rods, each 600 mm in length, 50 mm in outer diameter, and 18 mm in wall thickness, were prepared for each group to determine the linear shrinkage, warpage, and number of visible cracks during firing. Simultaneously, 20 three-point bending specimens, 10 breakdown specimens, 5 volume resistivity specimens, and 5 thermal shock specimens were prepared. The linear shrinkage was calculated by measuring the gauge length before and after firing. Warpage was measured using a platform, feeler gauge, and height gauge. Apparent porosity was determined using the water boiling and absorption method.

[0049] The dimensions of the three-point bending specimen after firing were 3mm × 4mm × 36mm, with a support span of 30mm and a loading rate of 0.5mm / min. The average bending strength, standard deviation, and Weibull modulus were calculated from 20 valid specimens. Electrical breakdown specimens were circular discs with a thickness of 2.0±0.1mm, measured in insulating oil using a power frequency AC voltage boosting method. Volume resistivity was measured at 100℃, 500V, and under 60s of current application. Thermal shock specimens were first held at 150℃ for 30min, then immersed in 20℃ water for cooling; this process was repeated three times, and surface cracks and glaze condition were observed.

[0050] Microstructural observation was performed on the mid-section of the calcined samples from each group. After cutting, resin mounting, grinding, and polishing, the samples were observed under a scanning electron microscope. Five framework particles were randomly selected from each group, and the interfacial gaps and maximum visible microcrack lengths around the framework particles were measured. For Examples 1, 2, 3, and Comparative Example 5, the long axis direction of 100 short columnar or needle-shaped aluminum-silicon reaction products around the framework particles was counted, and the proportion of these products with an angle less than 30° to the local interfacial tangent was also counted. Figure 2 (a) and Figure 2 (b) Using the same magnification and 10μm scale, but taken from different sintered samples and different cross-sectional locations.

[0051] Table 2 Results of firing stability and defect control Example 1 8.31 1.7 0 0.86 Example 2 8.79 2.1 0 1.01 Example 3 7.92 1.9 1 0.95 Comparative Example 1 10.46 4.9 3 0.74 Comparative Example 2 7.68 3.2 1 1.96 Comparative Example 3 8.72 2.8 1 0.83 Comparative Example 4 10.12 4.2 2 0.79 Comparative Example 5 8.36 2.0 0 0.91 In Table 2, Comparative Examples 1 and 4 showed relatively low apparent porosity, but their firing shrinkage, warpage, and number of visible cracks were all higher than those of Example 1. This indicates that apparent density alone is insufficient to reflect the firing stability of thick-walled, high-alumina ceramic parts. Comparative Example 2 had a lower firing shrinkage but a higher apparent porosity. Insufficiently bonded areas were more likely to remain around the uncoated α-Al₂O₃ core. Example 3 had a higher amount of skeleton particles and a lower shrinkage, but one of the five samples showed a visible crack, indicating that the amount of skeleton particles, the proportion of fine matrix, and the firing regime need to be carefully controlled.

[0052] Table 3 Mechanical and electrical performance results Example 1 231.6±12.8 16.3 34.6±1.5 7.8 Example 2 221.4±14.5 13.6 33.7±1.7 7.3 Example 3 225.8±16.6 12.7 34.1±1.9 7.5 Comparative Example 1 197.3±28.5 7.4 32.0±2.8 6.4 Comparative Example 2 173.6±23.9 6.2 31.1±2.3 6.2 Comparative Example 3 203.8±19.7 9.5 29.4±2.4 4.9 Comparative Example 4 188.6±26.1 7.8 31.4±2.7 6.0 Comparative Example 5 210.4±19.1 10.1 33.0±2.0 7.0 Example 1 exhibits higher flexural strength and Weibull modulus with less strength dispersion. Comparative Example 1, using full-material fine grinding, shows reduced average strength and Weibull modulus. Comparative Example 2, lacking double-layer reactive coating, has more interface defects around the particles and the lowest flexural strength. Comparative Example 3, with sintering trigger micropowder uniformly added to a fine matrix, shows lower electrical breakdown strength and volume resistivity than Example 1, indicating that concentrating the sintering trigger micropowder near the interface is more beneficial for balancing interface sintering and low-alkali insulation structures. Comparative Example 5 shows lower mechanical data than Example 1, indicating that the surface compressive stress control layer has an effect on surface defect control.

[0053] Table 4 Microstructure and thermal shock results Example 1 0.8 67 34 0 Example 2 1.0 58 46 0 Example 3 1.1 63 52 1 Comparative Example 1 — 29 96 2 Comparative Example 2 3.7 31 118 2 Comparative Example 3 1.2 43 71 1 Comparative Example 4 2.6 34 103 2 Comparative Example 5 0.9 65 59 2 Figure 2 In (a), obvious dark-colored interface gaps and local microcracks can be observed between the uncoated α-Al2O3 core and the surrounding fine matrix in Comparative Example 2. Figure 2 In (b), the α-Al2O3 core of Example 1 has an irregular transition zone with good continuity around it. The transition zone is tightly bonded to the fine matrix, and no through black separation seam is observed. Figure 2 (b) not by Figure 2 (a) was not obtained by processing the same cross section, but rather by taking an independent sintered sample from Example 1.

[0054] Table 4 and Figure 2 The observations correspond to each other. In Example 1, the average interfacial gap was 0.8 μm, and the maximum visible microcrack length was 34 μm. In Comparative Example 2, the average interfacial gap was 3.7 μm, and the maximum visible microcrack length was 118 μm. In Comparative Example 4, high-energy ball milling was performed after the addition of skeleton particles, which damaged both the coating structure and the skeleton particle size, increasing the interfacial gap and microcrack length. The internal interfacial state of Comparative Example 5 was similar to that of Example 1, but the number of cracks increased after thermal shock, indicating that the combination of the surface compressive stress control layer and the glaze layer has an effect on surface crack control.

[0055] The above embodiments demonstrate that retaining the larger-sized α-Al₂O₃ core, forming two reaction precursor layers around it, adding the framework particles later and using low-shear mixing, and setting a surface compressive stress control layer and glaze layer with gradually decreasing linear expansion coefficients can collectively influence firing shrinkage, interface densification, and crack propagation. The processes are not independent of each other. When the framework particles are uncoated, the interface easily becomes a defect location; when the coated particles are further refined at high energy, the coating layer and particle size structure are destroyed; when sintering-triggered micro-powder is uniformly added to the matrix, the preferential densification effect at the interface is weakened.

Claims

1. A high-strength alumina-reinforced ceramic insulator, comprising a ceramic substrate, a surface compressive stress regulating layer disposed on the outer surface of the ceramic substrate, and a glaze layer disposed on the outer side of the surface compressive stress regulating layer, characterized in that, The ceramic matrix comprises a fine matrix and gradient reaction-coated alumina framework particles dispersed in the fine matrix; The gradient reaction-coated alumina framework particles include an α-Al2O3 core, a magnesium-aluminum reaction transition region directly bonded to the α-Al2O3 core, and an aluminum-silicon reaction transition region located outside the magnesium-aluminum reaction transition region. The aluminum-silicon reaction transition region is continuously bonded to the fine matrix. The α-Al₂O₃ core has a D50 of 18–45 μm, and the gradient reaction-coated alumina framework particles account for 15–35 wt% of the total inorganic solid mass of the ceramic matrix. The total Al2O3 content of the ceramic matrix, calculated based on oxides after firing, is 55–75 wt%, and the total Na2O and K2O content is not higher than 1.2 wt%. The linear expansion coefficient of the surface compressive stress regulating layer is 0.3–0.8 × 10⁻⁶ lower than that of the ceramic matrix. -6 / ℃.

2. The high-strength alumina-reinforced ceramic insulator according to claim 1, characterized in that, The α-Al₂O₃ core has a D50 of 22–38 μm, and the gradient reaction-coated alumina framework particles account for 22–33 wt% of the total inorganic solid mass of the ceramic matrix. The D90 of the free quartz particles in the ceramic matrix is ​​not greater than 8 μm.

3. The high-strength alumina-reinforced ceramic insulator according to claim 1, characterized in that, The thickness of the magnesium-aluminum reaction transition zone is 0.2–1.0 μm, the thickness of the aluminum-silicon reaction transition zone is 0.8–4.0 μm, and a discontinuously distributed low-alkali bonded microregion is formed between the aluminum-silicon reaction transition zone and the fine matrix, the low-alkali bonded microregion containing calcium magnesium aluminum silicate.

4. The high-strength alumina-reinforced ceramic insulator according to claim 1, characterized in that, The post-firing thickness of the surface compressive stress control layer is 0.25–0.90 mm, and the content of gradient reaction-coated alumina skeleton particles in the surface compressive stress control layer is not higher than 5 wt% of its total inorganic solid mass. The glaze layer is a low-alkali aluminosilicate glaze layer with a thickness of 0.12–0.35 mm after firing, and the linear expansion coefficient of the glaze layer is 0–0.4 × 10⁻⁶ lower than that of the surface compressive stress control layer. -6 / ℃.

5. A method for preparing a high-strength alumina-reinforced ceramic insulator, characterized in that, Includes the following steps: S1. A first coating slurry and a second coating slurry are sequentially applied to the surface of the α-Al2O3 core, and dried after each application of the coating slurry. The first coating slurry includes activated alumina, a magnesium source, and an alkali-free binder component. The second coating slurry includes metakaolin, activated alumina, low-alkali calcium magnesium aluminum silicate sintering trigger micro powder, flaky alumina seed crystals, and an alkali-free binder component. The dried particles are deagglomerated and sieved to obtain gradient reaction coated alumina framework particles. S2. Fine industrial alumina, washed plastic clay, calcined kaolin and mullite powder are wet ball milled, sieved, iron removed, dehydrated and vacuum kneaded to obtain fine matrix clay. S3. After pre-wetting the gradient reaction-coated alumina skeleton particles, add them to the fine matrix mud and mix them using a low-shear method to ensure that the gradient reaction-coated alumina skeleton particles are evenly dispersed. After mixing, do not subject the mud containing the gradient reaction-coated alumina skeleton particles to high-energy ball milling, sand milling or other high-energy refining treatments. S4. Shape and dry the mixed clay, apply a surface compressive stress control slurry to the outer surface of the green body and apply glaze; S5. The glazed green body is fired and cooled in a controlled manner to obtain a high-strength alumina-reinforced porcelain insulator.

6. The preparation method according to claim 5, characterized in that, The Al2O3 content of the α-Al2O3 core in step S1 is not less than 98wt%, and the D50 is 18-45μm; In step S2, the fine matrix powder after wet ball milling has a D50 of 0.6–2.0 μm and a D90 of no more than 6.0 μm. The gradient reaction-coated alumina framework particles added in step S3 account for 15-35 wt% of the total inorganic solid mass of the ceramic body.

7. The preparation method according to claim 5, characterized in that, In step S1, the amount of magnesium source added, calculated as MgO, is 0.3–1.5 wt% of the α-Al₂O₃ core mass. The total mass of the magnesium-aluminum reaction precursor layer formed by the first coating slurry and the aluminum-silicon reaction precursor layer formed by the second coating slurry is 4 to 15 wt% of the α-Al₂O₃ core mass; In the second coating slurry, the mass ratio of metakaolin to activated alumina is 0.8:1 to 1.2:1, the low-alkali calcium magnesium aluminum silicate sintering trigger micro powder accounts for 5 to 20 wt% of the solid mass of the second coating slurry, the lamellar alumina seed crystals account for 1 to 5 wt% of the solid mass of the second coating slurry, and at least 60 wt% of the low-alkali calcium magnesium aluminum silicate sintering trigger micro powder is introduced with the second coating slurry.

8. The preparation method according to claim 5, characterized in that, In step S3, the pre-wetting uses water or diluted plasticizer, and the amount added is 3-8 wt% of the mass of the gradient reaction-coated alumina skeleton particles. The low-shear mixing method employs at least one of the following: biaxial low-speed mixing, planetary low-speed mixing, short-time mixing after vacuum pumice, or multi-pass low-speed folding and extrusion using a biaxial kneader, with a mixing time of 8–20 min.

9. The preparation method according to claim 5, characterized in that, In step S4, the surface compressive stress regulating slurry is mainly composed of fine alumina, mullite powder, calcined kaolin and low alkali aluminosilicate combined components, and is applied by spraying, dipping or brushing. After applying the surface compressive stress-regulating slurry, excess slurry at the base of the umbrella skirt or irregular curved surface is removed by low-pressure compressed air purging or centrifugal slurry throwing before glazing.

10. The preparation method according to claim 5, characterized in that, In step S5, the firing process includes the following steps in sequence: The degreasing stage involves holding the product at 550–700℃ for 0.5–1.5 hours. The first reaction heat preservation section is maintained at 1000-1100℃ for 0.5-2.0h. A densification insulation section is maintained at 1230–1310℃ for 1.0–4.0 hours. And a controlled cooling section operating at a rate of 0.8 to 2.5 °C / min in the 900–650 °C range.

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