Alumina ceramic for spark plug insulator and method of making the same

Abstract

The application discloses a kind of alkali-free alumina ceramics for spark plug insulator and preparation method thereof.The ceramic is made of 95% alumina and 5% additive, and the additive is MgF2 13%-19%, B2O3 3%-12%, Y2O3 3%-5%, Al2O3 15%-23%, SiO2 46%-61% alkali-free glass, wherein B2O3 is introduced in the form of boric acid.The additive is melted at 1560-1650 DEG C, water quenching, grinding, and then mixed with alumina, forming, sintering.The application eliminates alkali metal pollution through MgF2-B2O3-Y2O3 ternary synergy, boric acid introduction and pre-melting glass process, ceramic bending strength 328-373 MPa, volume resistivity 1.2-1.8*10 15 Ω·cm, excellent performance consistency.

Description

Technical Field

[0001] This invention belongs to the field of spark plug material technology, specifically relating to an alkali-free alumina ceramic for spark plug insulators and its preparation method. Background Technology

[0002] Alumina ceramics are widely used as spark plug insulator materials due to their excellent mechanical strength, good insulation properties, high temperature resistance, and corrosion resistance. Among them, 95% alumina ceramics (i.e., ceramic materials with an Al2O3 content of 95 wt.%) have become the mainstream choice for spark plug insulators because they combine good sintering performance and comprehensive mechanical properties. To reduce the sintering temperature of alumina ceramics and promote the densification process, existing technologies typically add a certain amount of sintering aids to alumina powder to form a glassy phase to promote liquid-phase sintering. Currently, the widely used additive system for 95% alumina ceramics is the Na2O-K2O-CaO-SiO2 system. This alkali metal-alkaline earth metal silicate composition can form a glassy phase at high temperatures, thereby effectively reducing the sintering temperature and enabling alumina ceramics to achieve densification at a lower temperature.

[0003] However, the above-mentioned additive system has significant technical defects, mainly in the following three aspects: First, alkali metal ions severely degrade the electrical properties of the ceramic. Sodium ions (Na...) + ) and potassium ions (K + As a key component in the glass phase, alkali metal impurities, while effectively lowering the eutectic temperature and promoting liquid phase formation, severely damage the electrical properties of alumina ceramics. Studies show that even a content as low as 0.1% can lead to a significant decrease in the volume resistivity and a substantial increase in dielectric loss of the ceramic material. When such alumina ceramics are used as spark plug insulators, they are prone to electrical failures such as high-voltage leakage, creepage, and surface breakdown under high-voltage operating conditions, leading to problems such as ignition energy decay, engine idling vibration, and misfire. This problem is particularly pronounced under harsh conditions such as high load, high compression ratio, and lean combustion. Secondly, the alkali metal glass phase significantly reduces the mechanical strength of the ceramic. The flexural strength of high-purity 99% alumina ceramics can typically reach over 400 MPa, while the flexural strength of ordinary 95% alumina ceramics is usually only around 300 MPa. The fundamental reason for this difference in strength lies in the fact that the mechanical strength of the Na₂O-K₂O-CaO-SiO₂ glass phase itself is much lower than that of the alumina grains, and the interfacial bonding strength between the glass phase and the alumina grains is limited. More importantly, the coefficient of thermal expansion of this glass phase (approximately 8.5-9.5 × 10⁻⁶) is... -6 The coefficient of thermal expansion (°C) is higher than that of alumina grains (approximately 7.5-8.0 × 10⁻⁶). -6 / ℃). During the cooling process after ceramic firing, the shrinkage of the glass phase is greater than that of the alumina grains, resulting in harmful residual tensile stress rather than beneficial compressive stress at the interface between the glass phase and the alumina grains. This tensile stress weakens the grain boundary bonding strength, becoming a preferred path for crack initiation and propagation, thus significantly reducing the overall strength of the ceramic. For spark plug insulators, insufficient strength can lead to brittle fracture during assembly or use, seriously affecting the reliability and service life of the product. Third, traditional additive direct mixing processes suffer from uneven composition and large performance fluctuations. In existing technologies, additives are usually directly mixed with alumina powder in the form of oxides or mineral powders. Due to the differences in the particle size, density, specific surface area, and other physical properties of the various additive components, it is difficult to achieve uniform distribution of the raw materials during mixing, granulation, and molding processes. Especially when the additive contains multiple components, local component segregation is difficult to avoid. This microscopic inhomogeneity leads to differences in the composition and properties of the glass phase in different regions during sintering, resulting in performance fluctuations in the ceramic product. In large-scale industrial production, this non-uniformity of composition is further amplified, making it difficult to guarantee the consistency of performance between batches, posing a serious challenge to the quality stability of spark plug insulators.

[0004] Therefore, in the additive system of 95% alumina ceramics, alkali metal oxides are both "beneficial" components necessary for achieving low-temperature sintering and "harmful" impurities that lead to deterioration of electrical properties, reduction of mechanical strength, and performance instability. This contradiction has become a key bottleneck restricting the improvement of the performance of alumina insulators for spark plugs. How to significantly reduce or even eliminate alkali metal impurities while maintaining or even optimizing the sintering performance of alumina ceramics, and further improving their mechanical strength and performance consistency, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to solve the technical problems of degraded electrical properties, insufficient mechanical strength, and unstable performance in existing 95% alumina ceramics due to the presence of alkali metal oxides in the additives. It provides an alkali-free alumina ceramic for spark plug insulators and its preparation method. This technical solution utilizes a glass additive system with a specific composition and range of MgF2-B2O3-Y2O3-Al2O3-SiO2, and employs a pre-melting vitrification process. This significantly improves the electrical and mechanical properties and product consistency of the ceramic material while completely eliminating alkali metal contamination.

[0006] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows: An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 13%-19% MgF2, 3%-12% B2O3, 3%-5% Y2O3, 15%-23% Al2O3, and 46%-61% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1560-1650℃ and held at that temperature for 1-5 hours, then water-quenched to obtain glass slag, which is dried, ground, and sieved to obtain the final product.

[0007] Preferably, the additive comprises, by mass percentage: MgF2 15%, B2O3 12%, Y2O3 4%, Al2O3 23%, SiO2 46%, with the sum of the mass percentages of each component being 100%. The additive is prepared by the following method: after uniformly mixing the raw materials, melting them at 1600℃ and holding at that temperature for 3 hours, followed by water quenching to obtain glass slag, drying, grinding, and sieving to obtain the final product. Preferably, in the alkali-free glass additive, B2O3 is introduced in the form of boric acid, MgF2 is introduced in the form of fluoride, and Y2O3, Al2O3, and SiO2 are all introduced in the form of oxides.

[0008] Preferably, the melting and heating rate of the alkali-free glass additive is 5-20℃ / min, and the alkali-free glass additive is passed through a 200-400 mesh standard sieve after grinding.

[0009] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: (1) Weigh each additive raw material according to the proportion, mix them evenly, heat them to 1560-1650℃ at a heating rate of 5-20℃ / min, keep them at the temperature for 1-5 hours to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass proportion, and then shape, dry and sinter to obtain the alumina ceramic.

[0010] Preferably, in step (2), after mixing the alumina powder and the alkali-free glass additive, the mixture is placed in a ball mill and wet-milled for 10-12 hours using anhydrous ethanol as the medium.

[0011] Preferably, the sintering temperature in step (2) is 1560-1600℃ and the holding time is 1.5-3h.

[0012] The three key technical features of this invention and their synergistic effects: Feature 1: A synergistic formulation system in which MgF2, B2O3, and Y2O3 are all indispensable. The inventors have discovered through experiments that MgF2, B2O3, and Y2O3 constitute an indivisible functional unit in the glass additive of this invention, each playing a unique and irreplaceable role. Only through their synergy can the objective of this invention be achieved. MgF2, fluoride ions (F - Fluoride ions can break bridging bonds in the silicon-oxygen network, significantly reducing the viscosity of the glass melt, thereby lowering the melting and sintering temperatures. Unlike alkali metal ions, fluoride ions tend to bind with glass network forgings (such as SiO2). 4+ Al 3+ B 3 + Si-F, Al-F, and BF covalent bonds are formed and "anchored" within the glass network, unlike Na... + K + This allows it to act as a carrier for free ion migration, thus lowering the melting temperature without compromising electrical insulation properties. B₂O₃, as a network forger, effectively reduces the softening temperature and high-temperature viscosity of the glass phase, promoting liquid-phase sintering. Simultaneously, the introduction of B₂O₃ endows the glass phase with an intrinsically low coefficient of thermal expansion, crucial for achieving the "prestress-enhanced effect." Y₂O₃, a rare-earth oxide, significantly improves the mechanical strength and Young's modulus of the glass phase and optimizes the bonding performance at the glass / Al₂O₃ grain boundaries. More importantly, Y… 3+ The larger ionic radius allows them to accumulate in the glass network, suppressing the migration of other ions and further improving resistivity.

[0013] The combined effect of these three components is as follows: MgF2 provides viscosity reduction and low-temperature sintering capability, B2O3 provides low thermal expansion properties, and Y2O3 provides reinforcement and boundary stabilization. When these three components coexist, the resulting glass phase possesses low melting temperature, low coefficient of thermal expansion, high mechanical strength, and high resistivity, which cannot be achieved by any combination of two components or by a single component.

[0014] Feature 2: Introduction of B2O3 in the form of boric acid (H3BO3) This invention reveals that the method of introducing B2O3 has a significant impact on the final ceramic properties. Although B exists in the final sintered body as B2O3, introducing it as boric acid (H3BO3) has the following unexpected technical effects: (1) Boric acid undergoes stepwise dehydration during heating (H3BO3→ HBO2→ B2O3), releasing water vapor and active oxygen, which form microbubbles in the melt, promoting the stirring and convection of the melt, and making the components more uniformly mixed; (2) The amorphous B2O3 generated by the decomposition of boric acid has higher chemical reactivity and can interact more effectively with components such as MgF2 and Y2O3 to form a more uniform glass network structure; (3) Compared with the direct use of B2O3 powder (which is prone to moisture absorption and clumping and has poor flowability), boric acid has better flowability and weighing accuracy, which is beneficial to the quality control of industrial production.

[0015] Feature 3: Pre-melting vitrification process ensures uniform composition and consistent performance. Unlike existing technologies that use a "direct mixing" process where each additive component is directly mixed with alumina powder, this invention employs a pre-melted vitrification process where all additive components are first melted at high temperature to form a uniform glass, and then ground into powder and added.

[0016] The key technological advantage of this process lies in: (1) During the melting process at 1560-1650℃, the components are highly uniformly mixed in the liquid state, and component segregation is completely eliminated; (2) This highly uniform glass is ground and used as an additive to ensure that each additive particle has the exact same chemical composition and physicochemical properties. When the additive is mixed with alumina powder and sintered, the glass phase can be uniformly distributed between the alumina grains, avoiding microstructural inhomogeneity and performance fluctuations caused by local compositional differences. This process innovation significantly improves the batch-to-batch performance consistency in large-scale industrial production, providing a reliable guarantee for the quality stability of spark plug insulators.

[0017] In summary, the beneficial effects of the technical solution of this invention are: The 95% alumina ceramic prepared by this invention has the following performance indicators: bulk density: 3.59-3.72 g / cm³ 3 Coefficient of thermal expansion (room temperature to 800℃): 7.1-7.4×10⁻⁶ -6 / ℃; Bending strength: 328-373 MPa; Volume resistivity at room temperature: 1.2-1.8×10 15 Ω·cm. Compared with existing 95% alumina ceramics containing alkali metal additives, the ceramic material of this invention has a lower coefficient of thermal expansion, higher flexural strength, and higher volume resistivity, resulting in significantly improved overall performance, making it particularly suitable for high-performance spark plug insulators. Alkali metal contamination is completely eliminated. The additive system does not introduce alkali metal oxides such as Na₂O and K₂O, thus preventing the damage of alkali metal ions to the electrical properties of the ceramic from the source and ensuring the insulation reliability of the alumina ceramic under high-voltage conditions. Attached Figure Description

[0018] Figure 1The image shows the X-ray diffraction pattern of the alumina ceramic prepared in Example 3. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to specific embodiments, but it is not limited thereto. Example

[0020] An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 15% MgF2, 12% B2O3, 4% Y2O3, 23% Al2O3, and 46% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1600℃ and held at that temperature for 3 hours, and then water-quenched to obtain glass slag. After the glass slag is dried at 120℃ for 12 hours, it is ground using a planetary ball mill and passed through a 325-mesh standard sieve to obtain the alkali-free glass additives.

[0021] In the alkali-free glass additive, B2O3 is introduced in the form of boric acid, MgF2 in the form of fluoride, and Y2O3, Al2O3, and SiO2 are all introduced in the form of oxides.

[0022] The melting and heating rate of the alkali-free glass additive is 10℃ / min, and the alkali-free glass additive is ground and passed through a 325-mesh standard sieve.

[0023] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: (1) Weigh each additive raw material according to the ratio, mix them evenly, heat them to 1600℃ at a heating rate of 10℃ / min, keep them at the temperature for 3 hours to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass ratio, and then shape, dry and sinter to obtain the alumina ceramic.

[0024] Step (2) After mixing the alumina powder and the alkali-free glass additive, place them in a ball mill and wet ball mill for 12 hours using anhydrous ethanol as the medium.

[0025] In step (2), the sintering temperature is 1580℃ and the holding time is 2h. Example

[0026] An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 19% MgF2, 3% B2O3, 5% Y2O3, 20% Al2O3, and 53% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1650℃ and held at that temperature for 1 hour, and then water-quenched to obtain glass slag. After the glass slag is dried at 120℃ for 12 hours, it is ground using a planetary ball mill and passed through a 400-mesh standard sieve to obtain the alkali-free glass additives.

[0027] In the alkali-free glass additive, B2O3 is introduced in the form of boric acid, MgF2 in the form of fluoride, and Y2O3, Al2O3, and SiO2 are all introduced in the form of oxides.

[0028] The melting and heating rate of the alkali-free glass additive is 5℃ / min, and the alkali-free glass additive is ground and passed through a 400-mesh standard sieve.

[0029] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: (1) Weigh each additive raw material according to the ratio, mix them evenly, heat them to 1650°C at a heating rate of 5°C / min, keep them at the temperature for 1 hour to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass ratio, and then shape, dry and sinter to obtain the alumina ceramic.

[0030] Step (2) After mixing the alumina powder and the alkali-free glass additive, place them in a ball mill and wet ball mill for 12 hours using anhydrous ethanol as the medium.

[0031] In step (2), the sintering temperature is 1600℃ and the holding time is 1.5h. Example

[0032] An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 13% MgF2, 8% B2O3, 3% Y2O3, 15% Al2O3, and 61% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1560℃ and held at that temperature for 5 hours, and then water-quenched to obtain glass slag. After the glass slag is dried at 120℃ for 12 hours, it is ground using a planetary ball mill and passed through a 200-mesh standard sieve to obtain the alkali-free glass additives.

[0033] In the alkali-free glass additive, B2O3 is introduced in the form of boric acid, MgF2 in the form of fluoride, and Y2O3, Al2O3, and SiO2 are all introduced in the form of oxides.

[0034] The melting and heating rate of the alkali-free glass additive is 20℃ / min, and the alkali-free glass additive is ground and passed through a 200-mesh standard sieve.

[0035] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: (1) Weigh each additive raw material according to the ratio, mix them evenly, heat them to 1560°C at a heating rate of 20°C / min, keep them at the temperature for 5 hours to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass ratio, and then shape, dry and sinter to obtain the alumina ceramic.

[0036] Step (2) After mixing the alumina powder and the alkali-free glass additive, place them in a ball mill and wet ball mill for 10 hours using anhydrous ethanol as the medium.

[0037] In step (2), the sintering temperature is 1560℃ and the holding time is 3h.

[0038] The XRD performance of the 95% alumina ceramic prepared in this embodiment was tested. X-ray diffraction showed that the sample contained only the corundum phase. Figure 1 This indicates that the additive did not form an impurity phase in the sample.

[0039] Conventional alkali addition system An alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; said additives are traditional additives based on the Na2O-K2O-CaO-SiO2 system, and their composition by mass percentage is: Na2O 0.1%, K2O 0.2%, CaO 66%, SiO2 33.7%, and the sum of the mass percentages of each component is 100%.

[0040] A method for preparing alumina ceramic for spark plug insulators includes the following steps: mixing alumina powder and additives uniformly according to a mass ratio, followed by molding, drying, and sintering to obtain the alumina ceramic.

[0041] Comparative Examples 2-4 were designed to verify the synergistic effect of the ternary components using the single variable principle: only one core component was removed, and the remaining components were kept in the original proportions of Example 1 and made up to 100%. The preparation process, sintering parameters, and test conditions were completely consistent with those of Example 1.

[0042] Compared with Example 1, except that MgF2 is not added, the composition and process method are the same as in Example 1, that is: An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by weight percentage: 95% alumina and 5% additives; said additives are alkali-free glass additives based on the B2O3-Y2O3-Al2O3-SiO2 system.

[0043] Additive composition: MgF2 was removed, and the remaining components were scaled up proportionally to a total mass percentage of 100% to maintain the original mass ratio of Example 1. The final composition was B2O3 18%, Y2O3 4%, Al2O3 23%, and SiO2 55%.

[0044] Compared with Example 1, except for the absence of B2O3, the composition and process method are the same as in Example 1, that is: An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by weight percentage: 95% alumina and 5% additives; said additives are alkali-free glass additives based on the MgF2-Y2O3-Al2O3-SiO2 system.

[0045] Additive composition: B2O3 was removed, and the remaining components were scaled up proportionally to a total mass percentage of 100% to maintain the original mass ratio of Example 1. The final composition was MgF2 15%, Y2O3 4%, Al2O3 23%, and SiO2 58%.

[0046] Compared with Example 1, except for the absence of Y2O3, the composition and process method are the same as in Example 1, that is: An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by weight percentage: 95% alumina and 5% additives; said additives are alkali-free glass additives based on the MgF2-B2O3-Al2O3-SiO2 system.

[0047] Additive composition: Y2O3 was removed, and the remaining components were kept in the original mass ratio of Example 1 and scaled up proportionally to a total mass percentage of 100%. The final composition was MgF2 15%, B2O3 12%, Al2O3 23%, and SiO2 50%.

[0048] In this comparative example, the formulation is completely identical to that of Example 1, except that the pre-melting glass transition process is omitted and a traditional direct mixing process is used instead. All other parameters are the same as in Example 1. An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials in the following mass percentages: 95% alumina and 5% additives; said additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 15% MgF2, 12% B2O3, 4% Y2O3, 23% Al2O3, and 46% SiO2, with the sum of the mass percentages of each component being 100%. B2O3 is introduced by boric acid.

[0049] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: mixing alumina powder and alkali-free glass additives uniformly according to a mass ratio, followed by molding, drying, and sintering to obtain the alumina ceramic.

[0050] Alumina powder and alkali-free glass additives were mixed and placed in a ball mill, where they were wet-milled for 12 hours using anhydrous ethanol as the medium. The sintering temperature was 1580℃, and the holding time was 2 hours.

[0051] In this comparative example, the formulation is completely identical to that of Example 1, except that B2O3 is introduced directly as pure B2O3 powder instead of boric acid; all other parameters are the same as in Example 1. An alkali-free alumina ceramic for spark plug insulators is made from the following raw materials by mass percentage: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 15% MgF2, 12% B2O3, 4% Y2O3, 23% Al2O3, and 46% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1600℃ and held at that temperature for 3 hours, and then water-quenched to obtain glass slag. After the glass slag is dried at 120℃ for 12 hours, it is ground using a planetary ball mill and passed through a 325-mesh standard sieve to obtain the alkali-free glass additives.

[0052] MgF2 is introduced in the form of fluoride, while B2O3, Y2O3, Al2O3, and SiO2 are introduced in the form of oxides.

[0053] The melting and heating rate of the alkali-free glass additive is 10℃ / min, and the alkali-free glass additive is ground and passed through a 325-mesh standard sieve.

[0054] A method for preparing alkali-free alumina ceramic for spark plug insulators includes the following steps: (1) Weigh each additive raw material according to the ratio, mix them evenly, heat them to 1600℃ at a heating rate of 10℃ / min, keep them at the temperature for 3 hours to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass ratio, and then shape, dry and sinter to obtain the alumina ceramic.

[0055] Performance testing All sample performance tests were conducted in accordance with relevant national standards for fine ceramics. Five parallel samples were used for each performance test. After testing according to the corresponding national standards, outliers were removed, and the arithmetic mean was taken as the final result. Performance fluctuations were calculated using the formula "(maximum value − minimum value) / average value × 100%" to ensure the reproducibility and reliability of the test results. Details are as follows: Bulk density: GB / T 1966-1996 "Test Methods for Apparent Porosity, Water Absorption and Bulk Density of Porous Ceramics" Flexural strength: GB / T 6569-2006 "Test Method for Flexural Strength of Fine Ceramics" Coefficient of thermal expansion (room temperature ~ 800℃): GB / T 16535-2008 "Test Method for Linear Thermal Expansion Coefficient of Fine Ceramics" Volume resistivity at room temperature: GB / T 1410-2006 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials" Performance consistency: Five consecutive samples were taken from the same batch, and the performance fluctuation range was calculated as range / average × 100%.

[0056] Table 1 Performance Test Results

[0057] As can be seen from the data in Table 1, the alkali-free alumina ceramics prepared in Examples 1-3 of this invention exhibit excellent performance in all key indicators. Compared with Comparative Example 1, which uses a conventional alkali addition system, although the bulk density of Examples 1-3 differs slightly, the flexural strength is significantly improved. The flexural strength of Example 2 reaches 373 MPa, far exceeding the 290 MPa of Comparative Example 1. The room-temperature volume resistivity is also significantly improved, with the room-temperature volume resistivity of Example 1 being 1.8 × 10⁻⁶. 15 Ω·cm is the value of Comparative Example 1 (5.0 × 10⁻⁶). 14 The coefficient of thermal expansion (COP) is 3.6 times that of Comparative Example 1 (Ω·cm), indicating that the alkali-free system effectively improves the insulation performance of the ceramic. Regarding the coefficient of thermal expansion, Examples 1-3 are all lower than Comparative Example 1, which is beneficial for improving the dimensional stability of the ceramic under temperature variations. More importantly, the fluctuations in flexural strength and resistivity of Examples 1-3 are much smaller than those of Comparative Example 1, showing that the ceramic prepared by this invention has better performance consistency. Comparative Examples 2-4 verified the necessity of the synergistic formulation system of MgF2-B2O3-Y2O3 by respectively eliminating one core component from MgF2, B2O3, and Y2O3. The results show that the absence of any core component leads to a significant decrease in the bulk density, flexural strength, and room-temperature volume resistivity of the ceramic, an increase in the coefficient of thermal expansion, and increased performance fluctuations. For example, Comparative Example 3, without the addition of B2O3, has a flexural strength of only 251 MPa and a room-temperature volume resistivity of 7.2 × 10⁻⁶. 14 The Ω·cm values ​​were significantly lower than those of Example 1, fully demonstrating the crucial role of the synergistic effect of the ternary components in ensuring the overall performance of the ceramic. Comparative Example 5 eliminated the pre-melting glassization process and adopted a traditional direct mixing process. Although the formulation was the same as Example 1, its flexural strength (315 MPa) and room-temperature volume resistivity (1.1 × 10⁻⁶) were significantly lower. 15 The Ω·cm values ​​were lower than those of Example 1, and the performance fluctuation range increased significantly (flexural strength fluctuation ≤ 11.2%, resistivity fluctuation ≤ 22.3%), indicating that the pre-melting glassization process can effectively ensure uniform composition and consistent performance. Comparative Example 6 changed the form of B2O3 introduction from boric acid to pure B2O3 powder. Although the final performance was improved compared to Comparative Examples 1-5, compared to Example 1, its flexural strength (338 MPa) and room temperature volume resistivity (1.5 × 10⁻⁶) were significantly lower. 15 The Ω·cm (value) is still slightly lower, and the fluctuations in flexural strength (≤5.8%) and resistivity (≤8.2%) are also greater than in Example 1, indicating that introducing B2O3 in the form of boric acid is more beneficial to improving the ceramic performance and stability. In summary, this invention successfully prepared a high-performance and stable alkali-free alumina ceramic for spark plug insulators by combining a synergistic formulation system where MgF2, B2O3, and Y2O3 are indispensable, the introduction of B2O3 in the form of boric acid, and a pre-melting glass transition process.

[0058] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. An alkali-free alumina ceramic for spark plug insulators, characterized in that, It is made from the following raw materials in the following mass percentages: 95% alumina and 5% additives; the additives are alkali-free glass additives based on the MgF2-B2O3-Y2O3-Al2O3-SiO2 system, and their composition by mass percentage is: 13%-19% MgF2, 3%-12% B2O3, 3%-5% Y2O3, 15%-23% Al2O3, and 46%-61% SiO2, with the sum of the mass percentages of each component being 100%; the additives are prepared by the following method: after the raw materials are mixed evenly, they are melted at 1560-1650℃ and kept at that temperature for 1-5 hours, and then water-quenched to obtain glass slag, which is dried, ground, and sieved to obtain the final product; in the alkali-free glass additives, B2O3 is introduced in the form of boric acid, MgF2 is introduced in the form of fluoride, and Y2O3, Al2O3, and SiO2 are all introduced in the form of oxides.

2. The alkali-free alumina ceramic for spark plug insulators according to claim 1, characterized in that, The additive is composed of the following components by mass percentage: MgF2 15%, B2O3 12%, Y2O3 4%, Al2O3 23%, SiO2 46%, with the sum of the mass percentages of each component being 100%. The additive is prepared by the following method: after the raw materials are mixed evenly, they are melted at 1600℃ and kept at that temperature for 3 hours. After water quenching, glass slag is obtained, dried, ground, and sieved to obtain the final product.

3. The alkali-free alumina ceramic for spark plug insulators according to claim 1, characterized in that, The melting and heating rate of the alkali-free glass additive is 5-20℃ / min, and the alkali-free glass additive is ground and then passed through a 200-400 mesh standard sieve.

4. A method for preparing an alkali-free alumina ceramic for spark plug insulators as described in any one of claims 1-3, characterized in that, The process includes the following steps: (1) Weigh each raw material of the additive according to the ratio, mix them evenly, heat them to 1560-1650℃ at a heating rate of 5-20℃ / min, keep them warm for 1-5 hours to melt the mixture into glass liquid, quench the glass liquid with water to obtain glass slag, dry it, grind and sieve it to obtain the additive; (2) Mix the alumina powder with the alkali-free glass additive obtained in step (1) evenly according to the mass ratio, and then shape, dry and sinter to obtain the alumina ceramic.

5. The method for preparing alkali-free alumina ceramic for spark plug insulators according to claim 4, characterized in that, Step (2) After mixing the alumina powder and the alkali-free glass additive, place them in a ball mill and wet ball mill them for 10-12 hours using anhydrous ethanol as the medium.

6. The method for preparing alkali-free alumina ceramic for spark plug insulators according to claim 4, characterized in that, In step (2), the sintering temperature is 1560-1600℃ and the holding time is 1.5-3h.

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