A rare earth doped high-density alumina ultra-high pressure insulating ceramic, a preparation method and application thereof

Abstract

The application discloses rare earth doped high-density alumina ultra-high pressure insulation ceramic and a preparation method and application thereof. 3 92-96%, 3-5% of rare earth oxides, 0.5-1.2% of MgO, 0.3-0.8% of Yb2O3 and 0.5-1.5% of sintering aids; the rare earth oxides are La2O3 and Gd2O3 a composite in a molar ratio of 1.2-1.5:1. The crystal boundary migration is inhibited by lattice distortion energy (DE=0.38eV); the La2O3 and Gd2O3 are doped in a molar ratio of 1.2-1.5:1, and the synergistic distortion effect of La 3+ -Gd 3+ is utilized to form a gradient chemical potential at the crystal boundary, to drive the directional segregation of rare earth ions, so that the crystal boundary pinning force is improved, the crystal boundary energy is reduced, and the high-temperature grain coarsening is inhibited.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ceramic materials, and particularly relates to a rare earth doped high-density alumina ultra-high pressure insulation ceramic and a preparation method and application thereof. BACKGROUND

[0002] Alumina (Al2O3) ceramics are widely used in the field of electronics and electrical appliances due to their excellent insulation, high hardness and high temperature resistance. However, the traditional preparation technology has the following defects:

[0003] 1. Contradiction between densification and insulation performance: Although the conventional high-pressure sintering can improve the density by adding MgO-Y2O3 sintering aids, the selection of the dopant is improper, and the conductive path is easily formed at the grain boundary, which leads to the increase of high-frequency dielectric loss (tan delta) (>10 -3 ), and it is difficult to meet the requirements of 5G millimeter wave frequency band (28GHz).

[0004] 2. Insufficient control of abnormal grain boundary growth: The existing two-step sintering method has limited inhibition of grain boundary migration, and cannot completely inhibit the grain boundary migration at high temperature, which leads to uneven distribution of grain size and large fluctuation of mechanical properties.

[0005] 3. Residual stress and structural defects: Although the ultra-high pressure sintering can refine the grain size and increase the dislocation density, the stress release mechanism is not designed, which leads to the residual stress at the grain boundary of more than 1.2GPa, and the micro-cracks are easily expanded during long-term service.

[0006] 4. Limited selection of sintering aids: The traditional B2O3 or SiO2 single aid forms a high viscosity liquid phase under high pressure, which cannot realize the full rearrangement of particles, and the density is only 95-97%.

[0007] Therefore, it is urgent to develop an alumina insulation ceramic with high density, nanocrystalline, low dielectric loss and excellent high temperature stability, and a high-efficiency controllable preparation process. SUMMARY

[0008] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the purpose of the present application is to provide a rare earth doped high-density alumina ultra-high pressure insulation ceramic and a preparation method and application thereof, which solves the technical problems existing in the prior art by multi-component synergistic design, ultra-high pressure gradient sintering and grain boundary structure optimization.

[0009] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0010] In a first aspect, the embodiments of the present application provide a rare earth doped high-density alumina ultra-high pressure insulating ceramic, and the preparation raw materials include, in percentage by mass: 92-96% of α-Al2O3, 3-5% of rare earth oxide, 0.5-1.2% of MgO, 0.3-0.8% of Yb2O3, and 0.5-1.5% of sintering aid.

[0011] The rare earth oxide is a composite of La2O3 and Gd2O3 in a molar ratio of (1.2-1.5):1.

[0012] The rare earth La 3+ and Gd 3+ have a difference (Δr=23%) in ionic radius, which generates an asymmetric distortion stress field in the Al2O3 lattice, La 3+ (1.16 Å) preferentially occupies the octahedral sites of Al 3+ , and induces lattice tensile strain; Gd 3+ (0.94 Å) forms interstitial solid solution due to smaller radius, and generates compressive strain. Through the superposition of the two strain fields of the dual rare earth system, a three-dimensional pinning network is formed, and the grain boundary migration is inhibited by the lattice distortion energy (ΔE=0.38 eV); La2O3 and Gd2O3 are doped in a molar ratio of 1.2-1.5:1, the synergistic distortion effect of La 3+ -Gd 3+ is utilized, a gradient chemical potential is formed at the grain boundary, the directional segregation of rare earth ions is driven, the grain boundary pinning force is improved, the grain boundary energy is reduced, and the high-temperature grain coarsening is inhibited.

[0013] In some embodiments, the sintering aid is a B2O3-SiO2-LiF ternary system, and the mass ratio is (0.3-0.5):(0.2-0.4):(0.1-0.3), which forms a low-viscosity transient liquid phase under ultra-high pressure.

[0014] The eutectic point (Te=480°C) of the B2O3-SiO2-LiF ternary system matches the ultra-high pressure sintering temperature zone (500-800°C). The strong polarization ability of Li + can reduce the B-O bond energy, and a low-viscosity (η≤10 3 Pa·s) transient liquid phase is formed under high pressure, the liquid phase contact angle θ≤15°, and the Al2O3 particle surface is wetted; the liquid phase is mixed in a mass ratio of (0.3-0.5):(0.2-0.4):(0.1-0.3), LiF reacts with B2O3 to generate LiBO2 transition phase at 10 GPa / 500°C, the liquid phase generation activation energy is reduced, the liquid phase is spread and filled into the grain boundary pores, the density is significantly improved, and the grain boundary embrittlement caused by excessive liquid phase is avoided.

[0015] In some embodiments, a rare earth segregation layer with a thickness of 1-3 nm is formed at the grain boundary, La3+ a concentration gradient ≥ 15 at% / nm, and the segregation layer comprises periodically arranged [La-O-Gd] structural units with a spacing of 0.5-0.8 nm.

[0016] La 3+ and Gd 3+ The segregation at the grain boundaries forms [La-O-Gd] structural units, and the periodic arrangement (spacing of 0.5-0.8 nm) is derived from the ion charge compensation effect (La 3+ + Gd 3+ → 2Al 3+ substitution). This ordered structure produces a steric hindrance effect, hindering grain boundary migration. During the 15 GPa / 600°C pressure holding stage, the chemical potential gradient at the grain boundaries drives La / Gd ion diffusion, forming a segregation layer with a thickness of 1-3 nm, so that the grain boundary migration activation energy is increased, and the high-temperature stability of the material is improved.

[0017] In some embodiments, the B2O3-SiO2-LiF transient liquid phase has a viscosity ≤ 10 3 Pa·s under ultra-high pressure, and a nanocrystal wrapping structure is formed during the pressure release stage, comprising AlBO3 phases with a size of 5-20 nm distributed at the grain boundary triple junction.

[0018] Al 3+ and BO3 3- Under ultra-high pressure (22 GPa), metastable AlBO3 (thermodynamically stable phase Al2O3-B2O3) is generated, and its nanocrystals (5-20 nm) are distributed at the grain boundary triple junction, inhibiting grain boundary sliding through pinning effect. During the 22 GPa / 800°C stage, B 3+ and Al 3+ in the transient liquid phase undergoes a topological reaction, and AlBO3 nucleates and grows in a non-classical crystallization path (amorphous-nanocrystalline transformation). The modulus of AlBO3 (E=280 GPa) is higher than that of the Al2O3 matrix (E=390 GPa), producing a local strengthening effect, and the fracture toughness is improved.

[0019] In some embodiments, 0.1-0.5wt% ZrO2 nanoparticles are further included, the particle size of the ZrO2 nanoparticles is 10-30 nm, and the ZrO2 nanoparticles are uniformly dispersed in the α-Al2O3 matrix.

[0020] The volume expansion of t→m phase transition of ZrO2 (ΔV=4.5%) can offset the sintering shrinkage stress of Al2O3 (ΔV=3.2%), and hinder the crack propagation through the phase transition toughening mechanism. By adding 0.1-0.5wt% t-ZrO2 (particle size 10-30nm), the phase transition hysteresis is inhibited by ultra-high pressure (22GPa), so that the t→m phase transition occurs completely in the sintering stage. In this way, the crack propagation needs to overcome the phase transition energy additionally, so that the fracture work is improved.

[0021] In a second aspect, the embodiment of the present application provides a preparation method of a rare earth doped high-density alumina ultra-high pressure insulating ceramic, comprising the following steps:

[0022] (1) α-Al2O3 with a particle size of 50-150nm, rare earth oxides, MgO, Yb2O3 and sintering aids are mixed according to the proportion, wet grinding in an ethanol medium at a ball-to-material ratio of 8:1 for 12-18h to obtain a uniform slurry with a particle size D 90 ≤200nm;

[0023] (2) After the slurry is dried at 60°C in vacuum, it is placed in a furnace containing 5vol% HF and treated at 400°C for 2h to make LiF and B2O3 pre-react to form a LiBO2 transition phase;

[0024] (3) After cold isostatic pressing, gradient pressure is added in a multi-anvil press: after cold isostatic pressing, the powder is placed in a cubic press, the pressure is increased from 5GPa to 22GPa at a rate of 1-3GPa / min, and the temperature is increased from 300°C to 800°C in stages, wherein:

[0025] At the 10GPa stage, maintain 500°C±20°C for 5min to make LiBO2-SiO2 form a low-viscosity transient liquid phase with a viscosity η≤10 3 Pa·s;

[0026] At the 15GPa stage, maintain 600°C±20°C for 30-90s for grain boundary recombination, so that La / Gd / Yb segregates at the grain boundary to form a continuous stress buffer layer;

[0027] (4) In the pressure release stage, the pressure is decreased at a rate of ≤0.5GPa / s, and field-assisted annealing is implemented synchronously, a direct current field with a strength of 500-800V / cm is applied to induce the ordered arrangement of oxygen vacancies.

[0028] The six-surface top press is suitable for the research and production of high-performance ceramic materials, especially ceramic phases that need high-pressure environment to exist stably or require extreme densification. In addition, by segmenting the pressure (5→10→15→22 GPa) to match the sintering shrinkage curve, particle breakage caused by sudden pressure changes is avoided. At 10 GPa / 500°C, a liquid phase is formed, at 15 GPa / 600°C, grain boundary recombination is triggered, and at 22 GPa, densification is completed. With a pressure increase rate of 1-3 GPa / min, the grain boundary diffusion rate is controlled, and the La / Gd / Yb segregation and densification are synchronized. Finally, the relative density and grain boundary structure uniformity can be improved.

[0029] In some embodiments, in step (3), during the 22 GPa ultra-high pressure sintering process, a pulse pressure wave with a frequency of 5-10 kHz and an amplitude of 0.2-0.5 GPa is applied in the vertical direction perpendicular to the basal plane, i.e. the vertical direction of the crystal main axis, in the alumina crystal structure, guiding the occurrence of a specific mode of directional slip deformation within the grain, which is manifested as: on the basal plane of alumina, i.e. the atomic layer parallel to the basal plane in the hexagonal crystal structure, the relative sliding of the atomic layer occurs along a specific crystal direction within the basal plane, and the dislocation density is ≥5×10 14 m -2 , and the deviation between the slip trace and the theoretical slip direction is ≤5° detected by EBSD.

[0030] Applying a pulse wave (5-10 kHz) in the vertical direction of the crystal main axis can activate directional slip deformation, and the critical shear stress of the slip system is broken at 22 GPa, promoting dislocation multiplication. The pulse wave amplitude of 0.2-0.5 GPa corresponds to a shear stress exceeding the dislocation nucleation threshold, inducing dislocations to multiply in a Frank-Read source mechanism, so that the dislocation density is ≥5×10 14 m -2 , and the hardness is improved.

[0031] In some embodiments, the loading timing of the pulse pressure wave is coupled with the temperature-pressure curve, specifically:

[0032] When the pressure increases from 15 GPa to 22 GPa, a continuous wave with a frequency of 5 kHz and an amplitude of 0.5 GPa is applied;

[0033] During the 22 GPa pressure maintaining stage, an intermittent wave (duty cycle 1:3) with a frequency of 10 kHz and an amplitude of 0.2 GPa is switched;

[0034] By dynamically adjusting the waveform parameters, the ratio of dislocation multiplication rate to grain growth rate is ≥2:1.

[0035] Continuous wave (5 kHz / 0.5 GPa) provides continuous dislocation driving force in the pressure increasing stage, and intermittent wave (10 kHz / 0.2 GPa, duty cycle 1:3) avoids the brittle of grain boundary caused by dislocation jamming in the pressure maintaining stage. When the dislocation multiplication rate is greater than or equal to 2 times the grain growth rate, the dynamic adjustment of the waveform parameters maintains the plastic deformation dominant. The grain size distribution is narrowed, and the mechanical property consistency is improved.

[0036] In some embodiments, after step (4), the annealed ceramic is immersed in an ethanol solution containing 0.1-0.5wt% boric acid, and is treated for 30 minutes under the assistance of 50W ultrasonic waves, so that B 3+ The La-O units at the grain boundaries are selectively bonded to form [La-O-B] complexes with thermal stability ≥1600℃.

[0037] B 3+ The [La-O-B] complexes are formed with the La-O units at the grain boundaries, and the bond energy (E=520kJ / mol) is higher than that of La-O-Al (E=480kJ / mol), which can block the corrosion of the grain boundaries caused by the adsorption of water molecules. The ultrasonic assistance (50W / 40kHz) drives B 3+ penetrates into the grain boundaries and replaces the weakly bound OH - groups through chemical adsorption, so that the grain boundary moisture permeability can be controlled in a low range, and the anti-surface discharge capability is improved.

[0038] In a third aspect, the embodiments of the present application provide a rare earth doped high-density alumina ultra-high voltage insulation ceramic in the field of electronic and electrical appliances, such as in the field of vacuum tubes, power equipment, electronic devices, etc.

[0039] Compared with the prior art, the present application at least has the following beneficial effects:

[0040] 1. The present application suppresses the grain boundary migration through lattice distortion energy (ΔE=0.38eV); La2O3 and Gd2O3 are complex doped at a molar ratio of 1.2-1.5:1, and the synergistic distortion effect of La 3+ -Gd 3+ forms a gradient chemical potential at the grain boundary, drives the directional segregation of rare earth ions, improves the grain boundary pinning force, reduces the grain boundary energy, suppresses the high-temperature grain coarsening, effectively limits the grain boundary migration, and improves the insulation performance;

[0041] 2. The strong polarization ability of Li + can reduce the B-O bond energy, form a low viscosity (η≤10 3 Pa·s) transient liquid phase under high pressure, and react LiF and B2O3 to generate LiBO2 transition phase, thereby reducing the liquid phase generation activation energy, spreading and filling the grain boundary pores through the liquid phase, and significantly improving the density, while avoiding the brittle of grain boundary caused by excessive liquid phase;

[0042] 3. The lattice distortion field induced by the size mismatch of rare earth ions converts the concentrated stress into distributed micro-strain, promotes the self-organization reconstruction of defect structure using the high-pressure-high-temperature process window, converts the strain energy into the energy required for chemical bond reorganization, and finally establishes a multi-scale stress dissipation network through the synergistic regulation of oxygen vacancies and grain boundaries. The effective release of internal stress in the material is realized by the special doping strategy of rare earth elements and the dynamic regulation of process conditions.

[0043] The application will be further described in detail below with reference to the specific embodiments. DETAILED DESCRIPTION

[0044] The applicant found that:

[0045] In the prior art system, the synergistic regulation of high densification and insulation performance has always been a major challenge in the field of alumina ceramics. In order to improve the densification, transition metal oxides or carbides are often introduced as sintering aids in traditional sintering processes, such as ZrB2 used as a toughening phase in CN112939582B patent. Although high densification is achieved through grain boundary pinning effect, the inherent high electrical conductivity of ZrB2 leads to the formation of electron migration channels at the grain boundaries, significantly reducing the insulation performance of the material. This contradiction is due to the selection limitation of dopants in the existing technology - traditional sintering aids (such as TiO2, Fe2O3) can promote densification through liquid phase, but the metal ions or interstitial atoms introduced by them tend to form band overlap, forming low-resistance paths at the grain boundaries. More importantly, the existing process does not establish a directional regulation mechanism for the chemical state of the grain boundary, leading to uncontrollable precipitation of impurity phases (such as metal borides) during sintering, further exacerbating the degradation of insulation performance. For example, although the two-step sintering method in CN109400123B suppresses grain growth, it does not solve the problem of oxygen vacancy aggregation introduced by MgO-Y2O3 sintering aids, leading to a sharp increase in dielectric loss under high-frequency electric field.

[0046] The root cause of the lack of control over abnormal grain growth lies in the limited understanding of the grain boundary migration mechanics in the existing technology. The traditional two-step sintering method suppresses grain boundary migration through low-temperature pre-sintering. When the sintering temperature exceeds 1250℃, the grain boundary energy (about 1.0 J / m 2 ) is still sufficient to drive rapid grain coarsening. The existing grain boundary pinning strategies (such as whisker toughening and second phase particle doping) can physically block grain boundary migration, but they cannot reconstruct the grain boundary characteristics from the energy level. For example, CN112939582B uses aluminum borate whiskers for toughening, but the interface energy difference between it and the Al2O3 matrix (Δγ≈0.3 J / m 2) and become the fast channel of grain boundary migration. The deeper problem is that the existing technology has only one means to regulate the chemical bonding of grain boundaries, and cannot reduce the driving force of grain boundary migration through atomic-scale design.

[0047] The essence of residual stress and structural stability defects is that the existing sintering technology lacks a dynamic stress release mechanism. Although traditional hot pressing or SPS process promotes densification through external pressure, after high pressure unloading, the dislocation pile-up and lattice distortion energy generated by plastic deformation in the material cannot be effectively dissipated, forming residual compressive stress as high as 1.5-2.0 GPa. For example, although Al2O3 prepared using 15 GPa ultra-high pressure has a hardness of 26 GPa, the dislocation density at the grain boundary reaches 10 15 / m -2 , which easily causes stress concentration at the intersection of dislocation slip bands under thermal cycling or mechanical loading, leading to intergranular fracture. The existing technology attempts to relieve residual stress through annealing, but high-temperature annealing (>1200℃) will re-activate grain boundary migration, causing a vicious cycle of grain coarsening and mechanical property degradation. The deeper technical bottleneck is that the existing process cannot simultaneously achieve densification and stress dynamic balance during sintering - for example, CN116283251B uses LiF-SiO2 composite sintering additives to reduce the sintering temperature, but Li + The segregation of rare earths at the grain boundary leads to local charge imbalance, inducing stress corrosion sensitivity of the enhanced electrostatic field, making the material prone to stress corrosion cracking in a humid and hot environment. The structural instability problem under the coupling of multiple physical fields has not been effectively solved by the existing technology.

[0048] Therefore, the applicant proposes a rare earth-doped high-density alumina ultra-high pressure insulating ceramic, and the preparation raw materials include, by mass percentage: α-Al2O392-96%, rare earth oxide 3-5%, MgO 0.5-1.2%, Yb2O30.3-0.8%, and sintering additives 0.5-1.5%.

[0049] The rare earth oxide is a composite of La2O3 and Gd2O3 in a molar ratio of (1.2-1.5):1.

[0050] Preferably, the sintering additive is a B2O3-SiO2-LiF ternary system with a mass ratio of (0.3-0.5):(0.2-0.4):(0.1-0.3), which forms a low-viscosity transient liquid phase under ultra-high pressure.

[0051] Preferably, a rare earth segregation layer with a thickness of 1-3 nm is formed at the grain boundary, and the La 3+ concentration gradient is ≥15 at% / nm, and the segregation layer contains periodically arranged [La-O-Gd] structural units with a spacing of 0.5-0.8 nm.

[0052] Preferably, the B2O3-SiO2-LiF transient liquid phase has a viscosity of ≤10 3 Pa·s at ultra-high pressure, and forms a nanocrystal encapsulated structure comprising AlBO3 phase with a size of 5-20 nm distributed at the triple junction of grain boundaries during the pressure release stage.

[0053] More specifically, during the preparation process, the following steps are included:

[0054] (1) α-Al2O3 with a particle size of 50-150 nm is mixed with rare earth oxides, MgO, Yb2O3, and sintering aids in a ratio, wet-milled in an ethanol medium at a ball-to-material ratio of 8:1 for 12-18 h to obtain a uniform slurry with a particle size of ≤200 nm; 90

[0055] (2) After drying the slurry at 60°C under vacuum, it is placed in a furnace containing 5 vol% HF at 400°C for 2 h to pre-react LiF and B2O3 to form a LiBO2 transition phase;

[0056] (3) After cold isostatic pressing, gradient pressure is applied in a multi-anvil press: after cold isostatic pressing, the powder is placed in a cubic press, the pressure is increased from 5 GPa to 22 GPa at a rate of 1-3 GPa / min, and the temperature is increased from 300°C to 800°C in stages, wherein:

[0057] At the 10 GPa stage, maintain 500°C±20°C for 5 min to form a low-viscosity transient liquid phase of LiBO2-SiO2 with a viscosity of η≤10 3 Pa·s;

[0058] At the 15 GPa stage, maintain 600°C±20°C for 30-90 seconds for grain boundary reorganization, allowing La / Gd / Yb to segregate at the grain boundaries to form a continuous stress buffer layer;

[0059] (4) During the pressure release stage, the pressure is reduced at a rate of ≤0.5 GPa / s, and field-assisted annealing is performed simultaneously, applying a direct current field with a strength of 500-800 V / cm to induce the ordered arrangement of oxygen vacancies.

[0060] Preferably, in step (3), during the 22 GPa ultra-high pressure sintering process, a pulsed pressure wave with a frequency of 5-10 kHz and an amplitude of 0.2-0.5 GPa is applied perpendicular to the basal plane in the alumina crystal structure, i.e. along the vertical direction of the crystal principal axis, to guide specific mode directional slip deformation within the grain, which is manifested as the relative sliding of atomic layers along a specific crystal direction within the basal plane on the basal plane of the hexagonal crystal structure, with a dislocation density of ≥5×10 14 m -2 ​And the deviation of slip trace from the theoretical slip direction is less than or equal to 5 degrees through EBSD detection.

[0061] The loading timing of the pulse pressure wave is coupled with the temperature-pressure curve, specifically:

[0062] When the pressure rises from 15 GPa to 22 GPa, a continuous wave with a frequency of 5 kHz and an amplitude of 0.5 GPa is applied;

[0063] During the 22 GPa pressure maintaining phase, it is switched to an intermittent wave (duty cycle 1:3) with a frequency of 10 kHz and an amplitude of 0.2 GPa;

[0064] By dynamically adjusting the waveform parameters, the ratio of dislocation proliferation rate to grain growth rate is greater than or equal to 2:1.

[0065] First, for the problem of synergistic regulation of high densification and insulation performance, this scheme realizes deep coupling of rare earth doping system and ultra-high pressure process, the core of which is to use the synergistic effect of high pressure thermodynamic conditions reconstruction and rare earth defect chemistry to break through the bottleneck of mutual restriction of densification and insulation performance in traditional sintering. This scheme uses La 3+ / Gd 3+ double rare earth doping as the structural framework, combined with Yb 3+ / Mg 2+ co-doped system and 15 GPa ultra-high pressure low temperature sintering process to construct multi-level composite functional structure at atomic and mesoscopic scales, the specific technical path is as follows:

[0066] 1. La2O3 and Gd2O3 are complex doped at a molar ratio of 1.2-1.5:1, and the lattice distortion energy release is driven by the difference in ionic radius. La 3+ (radius 1.16 Å) preferentially occupies the octahedral site of the Al2O3 lattice, causing local lattice expansion and forming a tensile strain field; Gd 3+ (radius 0.94 Å) enters the interstitial site, producing a compressive strain. The alternating distribution of these two types of strain fields is strengthened into a periodic stress network under 15 GPa ultra-high pressure, promoting plastic flow dominated densification of Al2O3 particles at 800°C low temperature through the synergistic effect of dislocation slip and grain boundary slip. During this process, La / Gd segregates at the grain boundaries to form [La-O-Gd] bridge structures, which have high stability and inhibit grain boundary migration, controlling the grain size to below 0.45 μm;

[0067] 2. Yttrium ions (Yb 3+ ) as acceptor defects, by replacing aluminum ions (Al 3+ ) in the alumina lattice, form defect centers with effective negative charge; at the same time, magnesium ions (Mg 2+) as donor defects, also occupy the sites of aluminum ions, forming positively charged defects. These two defects achieve charge neutralization through a specific ratio (for example, two yttrium defects correspond to one magnesium defect), ultimately achieving overall electrical neutrality balance. This mutual offsetting charge compensation can significantly reduce the number of oxygen vacancies in the lattice that are spontaneously generated to maintain electrical neutrality, while forcing the remaining oxygen vacancies to form an ordered arrangement under ultra-high pressure, thereby blocking the electron migration path, fundamentally improving the insulation performance of the material.

[0068] 3. Under 15 GPa ultra-high pressure, the phase equilibrium of the B2O3-SiO2-LiF sintering aid ternary system is restructured, and Li + and B2O3 form a transient LiBO2 liquid phase (melting point 680°C) at 800°C. This liquid phase is wrapped around Al2O3 particles in the form of a nano-thin film (thickness 2-5 nm), promoting particle rearrangement through capillary action, while the high-pressure environment inhibits liquid phase evaporation. The La / Gd segregation layer (thickness 1.5-2.0 nm) at the grain boundary isolates the transient liquid phase into discrete island structures, blocking the formation of conductive pathways (grain boundary resistivity ≥ 10 15 / Ω·cm).

[0069] The synergistic effect of ultra-high pressure plastic deformation and transient liquid phase diffusion enables the material to achieve near-full densification at 800°C, with high bending strength and high fracture toughness. Transmission electron microscopy shows that the La / Gd segregation layer at the grain boundary coexists with the oxygen vacancy ripple structure (wavelength 4.5 nm), effectively pinning dislocation motion. Yb / Mg co-doping extends the length of the oxygen vacancy conduction path from 5 nm to 22 nm, keeping the bulk resistivity at a high baseline even at high temperatures. Low dielectric loss is due to the shortening of the polarization relaxation time caused by stress release.

[0070] Secondly, to address the problem of insufficient control of abnormal grain growth, the scheme utilizes the lattice distortion field induced by rare earth doping and the dynamic plastic deformation under ultra-high pressure to construct a "self-locking" grain boundary structure, fundamentally disrupting the thermodynamic driving force and kinetic conditions for grain growth.

[0071] When La 3+ and Gd 3+ are embedded in the alumina lattice in a specific ratio, they form a complementary strain field due to the difference in ion size - the bulk effect of La 3+ expands the lattice framework, while the compact Gd 3+ occupies the opposite structure. This push-pull effect forms an interlaced stress network near the grain boundary, like laying countless micro-springs on both sides of the grain boundary, requiring overcoming the collective reaction force of these springs for any grain boundary migration. More importantly, under the action of 15 GPa ultra-high pressure, this strain field is strengthened into a three-dimensional interlocking structure, La 3+ and Gd 3+A stable [La-O-Gd] bridging unit is formed by chemical bond reorganization, with a binding energy as high as 5.2 eV, far exceeding the van der Waals interaction of traditional grain boundaries (about 0.5 eV). This chemical pinning effect makes the migration activation energy of grain boundaries at high temperatures rise to 3.0 eV, which is 1.5 times that of traditional Al2O3 grain boundaries, significantly raising the thermodynamic threshold of grain growth.

[0072] At the kinetic level, the superhigh pressure process restructures the densification path. When a pressure of 15 GPa acts on Al2O3 particles, the local stress at the contact points between the particles far exceeds the yield strength of the material, triggering plastic flow dominated by dislocation slip. This deformation mechanism is like forging metal with a hydraulic press, which realizes densification through plastic deformation of the particles rather than high-temperature diffusion, compressing the time window for grain growth. At the same time, Yb 3+ co-doped with Mg 2+ constructs an oxygen vacancy regulation network at the grain boundaries. The acceptor properties of Yb 3+ and the donor properties of Mg 2+ form a charge balance, forcing oxygen vacancies to be ordered along specific crystal planes. These ordered defects, like a gravel array laid on the grain boundaries, force the migrating grain boundaries to constantly change their path direction, increasing the migration resistance. In transmission electron microscopy observations, this effect manifests as a jagged morphology of the grain boundaries, rather than the flat grain boundaries of traditional materials.

[0073] This scheme overturns the traditional sintering paradigm of "exchanging density for temperature", and through the coupling of rare earth doping and extreme pressure, the energy barrier for grain growth is transformed from a single temperature dependence to a composite energy field regulation - like a smart induction lock for the grain boundaries, which only allows limited structural adjustment under specific pressure-chemical potential conditions. When the material is applied to 5G millimeter wave filters, its uniform nanocrystalline structure can greatly reduce signal transmission loss, and at the same time, withstand high power loads without thermal breakdown, transforming from passive resistance to active regulation.

[0074] Finally, for the stress release mechanism, this scheme uses the special doping strategy of rare earth elements and dynamic regulation of process conditions to effectively release the internal stress of the material. Its technical scheme first focuses on the synergistic effect of the La 3+ and Gd 3+ double rare earth system, through specific ratio of La2O3 and Gd2O3 (molar ratio 1.2-1.5:1) composite doping, combined with superhigh pressure sintering process, to construct an asymmetric lattice distortion stress field in the Al2O3 matrix. La 3+ with an ionic radius (1.16 Å) much larger than Al 3+ (0.54 Å) preferentially occupies the octahedral sites in the lattice, causing local lattice stretching and expansion; while Gd 3+The La3+ions (radius 0.94 Å) partially enter the gap sites to form compression strain regions due to the poor size adaptability. The periodic alternation of tensile and compression strain forms a dynamic stress balance network in microscale, which releases the macroscopic internal stress through the gradient release of lattice distortion energy.

[0075] In the specific process implementation, the combination strategy of ultra-high pressure sintering and staged pressure holding plays a key role. The high-pressure environment forces La 3+ and Gd 3+ to rapidly diffuse to the predetermined lattice positions, while the high-temperature pressure holding stage drives the rare earth ions to segregate at the grain boundaries to form stable [La-O-Gd] bridge structures. This structure not only releases the interface stress through chemical bond restructuring, but also guides the gradual conversion of lattice distortion energy into the activation energy required for atomic position adjustment during the subsequent gradient cooling (5 ℃ / min) process. At the same time, the added Yb2O3 (0.3-0.8wt%) and Mg 2+ in the matrix synergistically construct [Yb-Mg-Vo] (Vo is oxygen vacancy) composite defects near the grain boundaries, and these oxygen vacancies form ordered arrangement of dissipation channels in the stress concentration area, effectively blocking the propagation path of micro-cracks and further dispersing residual stress.

[0076] The underlying logic of the entire stress release system can be attributed to the dynamic interaction of three levels: first, the lattice distortion field caused by the size mismatch of rare earth ions converts concentrated stress into distributed micro-strain; second, the high-pressure-high-temperature process window promotes the self-organization reconstruction of defect structures, converting strain energy into the energy required for chemical bond restructuring; finally, through the synergistic regulation of oxygen vacancies and grain boundaries, a multi-scale stress dissipation network is established.

[0077] As an embodiment, it also includes 0.1-0.5wt% of ZrO2 nanoparticles with a particle size of 10-30nm uniformly dispersed in the α-Al2O3 matrix.

[0078] The introduction of ZrO2 nanoparticles essentially constructs a multi-scale toughening network in the alumina matrix, because the fracture of traditional alumina ceramics often starts from the stress concentration at the grain boundaries, and the introduction of nanoscale second phase can break the crack propagation path through multiple mechanisms, while not compromising the intrinsic properties of the matrix. Zirconia has a unique phase transformation toughening property, the transformation from tetragonal phase to monoclinic phase is accompanied by a volume expansion of 3-5%, this phase transformation process can effectively absorb crack propagation energy; from the process level, first of all, through high-energy ball milling, ZrO2 precursor (such as ZrOCl2) is blended with Al2O3 powder, and a single layer of ZrO2 nanocrystals is coated on the surface of Al2O3 particles by mechanochemical action. Subsequently, by controlling the heating rate in the early stage of sintering (300-500°C), the ZrO2 precursor is decomposed into nanoparticles and anchored on the surface of Al2O3 grains. Ultra-high pressure (the environment plays a key role at this stage - high pressure forces ZrO2 nanoparticles to embed in Al2O3 lattice defect sites, forming a "mechanical embedding" structure, rather than simple physical mixing. This atomic-scale embedding allows ZrO2 particles to be wrapped in Al2O3 grains during subsequent densification, preventing particle migration and aggregation.

[0079] As an embodiment, after step (4), the annealed ceramic is immersed in an ethanol solution containing 0.1-0.5wt% boric acid, treated for 30min under the assistance of 50W ultrasonic waves, so that B 3+ The La-O units at the grain boundaries are selectively bonded to form [La-O-B] complexes with thermal stability ≥1600°C.

[0080] Since traditional annealing processes can release some residual stress, but cannot eliminate the risk of structural degradation of grain boundaries at extreme temperatures, in this embodiment, a technical solution of boric acid solution treatment after annealing is introduced, the core of which is to reconstruct the coordination environment of rare earth elements at the grain boundaries through chemical bonding, and to build a high-temperature stable interface barrier. B 3+ Has a unique electron-deficient property, can form a strong coordination bond with the lone pair electrons in the La-O structure at the grain boundary. La 3+ There are unsaturated coordination sites in the [La-O] units formed by grain boundary segregation, and the boric acid treatment is essentially to use the electron-deficient empty orbital of B 3+ to chelate with the oxygen ligand of La, converting the isolated La-O unit into a three-dimensionally cross-linked [La-O-B] network. This chemical reconstruction changes the grain boundary from a weakly bonded state that is easily activated and damaged by heat to a stable structure with covalent bond characteristics. In the process implementation, ethanol is used as a solvent instead of water, which can avoid the hydrolysis side reaction of water molecules with rare earth oxides, and can also use its low surface tension characteristics to penetrate into the nanoscale grain boundary channels. Ultrasonic treatment (50W) produces cavitation effects at the microscale, forming transient microjets in the grain boundary capillaries, forcing B 3+Diffusion along grain boundaries. At the same time, the cavitation collapse of ultrasonic waves generates local high temperature and high pressure microzones at the grain boundaries, instantaneously activating the coordination activity of La-O bonds, promoting the B 3+ Preferentially reacts with high-energy state grain boundary La-O units rather than randomly adsorbing on the grain surface.

[0081] The application will be further described below with reference to specific examples.

[0082] Example 1:

[0083] (1) Mix α-Al2O3 with particle size of 50-150 nm, rare earth oxides, MgO, Yb2O3, and sintering additives in the following proportions: α-Al2O3 93.5%, La2O3 2.5%, Gd2O3 1.8%, MgO 0.8%, Yb2O3 0.5%, B2O3-SiO2-LiF ternary system sintering aid 0.9%; wet grinding in ethanol medium at a ball-to-material ratio of 8:1 for 15 h to obtain a uniform slurry with D 90 ≤200 nm; wherein the molar ratio of La2O3 to Gd2O3 is 1.5:1, and the mass ratio of B2O3-SiO2-LiF ternary system sintering aid is 0.4:0.3:0.2;

[0084] (2) After drying the slurry at 60°C under vacuum, place it in a furnace containing 5 vol% HF at 400°C for 2 h to make LiF and B2O3 pre-react to form LiBO2 transition phase;

[0085] (3) After cold isostatic pressing, gradient pressurization in a multi-anvil press: after cold isostatic pressing, place the powder in a six-surface press, and increase the pressure from 5 GPa to 22 GPa at a rate of 1-3 GPa / min, while increasing the temperature from 300°C to 800°C in stages, wherein:

[0086] Maintain 500°C±20°C at 10 GPa for 5 min to make LiBO2-SiO2 form a low-viscosity transient liquid phase with viscosity η≤10 3 Pa·s;

[0087] Maintain 600°C±20°C at 15 GPa for 30-90 seconds for grain boundary reorganization, so that La / Gd / Yb segregates at the grain boundaries to form a continuous stress buffer layer;

[0088] (4) During the pressure release stage, decrease the pressure at a rate of ≤0.5 GPa / s, and simultaneously implement field-assisted annealing by applying a direct current field with strength of 500-800 V / cm to induce ordered arrangement of oxygen vacancies.

[0089] Example 2:

[0090] Example 2 differs from Example 1 in that the α-Al2O3 with a particle size of 50-150 nm is mixed with rare earth oxides, MgO, Yb2O3, sintering aids in the following proportions: α-Al2O3 93.8%, La2O3 2.5%, Gd2O3 1.5%, MgO 0.8%, Yb2O3 0.5%, B2O3-SiO2-LiF ternary system sintering aids 0.9%; wherein the molar ratio of La2O3 to Gd2O3 is 1.9:1.

[0091] Example 2 mainly adjusts the molar ratio of rare earth oxides La2O3 and Gd2O3.

[0092] Example 3:

[0093] Example 3 differs from Example 1 in that the α-Al2O3 with a particle size of 50-150 nm is mixed with rare earth oxides, MgO, Yb2O3, sintering aids, ZrO2 nanoparticles in the following proportions: α-Al2O3 93.2%, La2O3 2.5%, Gd2O3 1.8%, MgO 0.8%, Yb2O3 0.5%, B2O3-SiO2-LiF ternary system sintering aids 0.9%, ZrO2 nanoparticles 0.3%; wherein the molar ratio of La2O3 to Gd2O3 is 1.5:1, and the particle size of ZrO2 nanoparticles is 20 nm.

[0094] Example 3 mainly introduces ZrO2 nanoparticles into the alumina matrix.

[0095] Example 4:

[0096] Example 4 differs from Example 1 in that, in step (3), during the 22 GPa ultra-high pressure sintering process, a pulsed pressure wave with a frequency of 5-10 kHz and an amplitude of 0.2-0.5 GPa is applied in the vertical direction perpendicular to the basal plane, i.e. the vertical direction of the crystal main axis in the alumina crystal structure, to guide the occurrence of specific mode directional slip deformation within the grain, which is manifested as: in the basal plane of the alumina, i.e. the atomic layer parallel to the basal plane in the hexagonal crystal structure, the relative sliding of the atomic layer occurs along a specific crystal direction within the basal plane, and the dislocation density is ≥5×10 14 m -2 , and the deviation between the slip trace and the theoretical slip direction is ≤5° detected by EBSD;

[0097] The loading timing of the pulsed pressure wave is coupled with the temperature-pressure curve, specifically:

[0098] When the pressure rises from 15 GPa to 22 GPa, a continuous wave with a frequency of 5 kHz and an amplitude of 0.5 GPa is applied;

[0099] Switch to intermittent wave (duty ratio 1:3) with frequency 10 kHz, amplitude 0.2 GPa at 22 GPa pressure maintaining stage;

[0100] By dynamically adjusting the waveform parameters, the ratio of dislocation multiplication rate to grain growth rate is ≥2:1.

[0101] This embodiment 4 mainly applies pulse wave activation directional slip deformation in the vertical direction of the main axis of the crystal during ultra-high pressure sintering, to avoid dislocation jamming leading to grain boundary embrittlement.

[0102] Embodiment 5:

[0103] The difference between embodiment 5 and embodiment 1 is that after step (4), the annealed ceramic is immersed in an ethanol solution containing 0.3wt% boric acid, treated for 30min under the assistance of 50W ultrasonic wave, so that B 3+ The La-O units at the selective bonding grain boundaries form [La-O-B] complexes with thermal stability ≥1600℃.

[0104] This embodiment 5 mainly uses boric acid ethanol solution treatment after annealing.

[0105] Embodiment 6:

[0106] The difference between embodiment 6 and embodiment 1 is that α-Al2O3 with particle size of 50-150nm is mixed with rare earth oxides, MgO, Yb2O3, sintering aids, ZrO2 nanoparticles according to the following ratio: α-Al2O3 93.2%, La2O3 2.5%, Gd2O3 1.8%, MgO 0.8%, Yb2O3 0.5%, B2O3-SiO2-LiF ternary system sintering aid 0.9%, ZrO2 nanoparticles 0.3%; wherein the molar ratio of La2O3 to Gd2O3 is 1.5:1, and the particle size of ZrO2 nanoparticles is 20nm.

[0107] And high pressure pulse pressure wave treatment as in embodiment 4 and boric acid ethanol solution treatment after annealing as in embodiment 5 are applied.

[0108] Comparative example 1:

[0109] The difference between comparative example 1 and embodiment 1 is that α-Al2O3 with particle size of 50-150nm is mixed with rare earth oxides, MgO, Yb2O3, sintering aids according to the following ratio: α-Al2O3 95.8%, Gd2O3 2.0%, MgO 0.8%, Yb2O3 0.5%, B2O3-SiO2-LiF ternary system sintering aid 0.9%; wet grinding in ethanol medium with ball-to-material ratio of 8:1 for 15h, to obtain D 90a homogeneous slurry with a particle size of ≤200 nm; wherein the molar ratio of La2O3 to Gd2O3 is 0:1.

[0110] The comparative example 1 mainly cancels La2O3 in the selection of rare earth oxides.

[0111] Comparative example 2:

[0112] The difference between the comparative example 2 and the example 1 is that after cold isostatic pressing, it is directly sintered at atmospheric pressure at 1600℃ for 2h.

[0113] The comparative example 2 mainly adopts atmospheric pressure high temperature sintering in the sintering process.

[0114] The main components and process parameters of the examples 1-6 and the comparative examples 1-2 are shown in Table 1.

[0115] Table 1: Main components and process parameters of examples and comparative examples

[0116]

[0117]

[0118]

[0119] The performance tests of the examples and the comparative examples are carried out, and the standards of the performance tests refer to the national standards, wherein the grain size is tested by the FE-SEM testing instrument, the density is tested by GB / T 25995-2010 “Fine Ceramic Density and Apparent Porosity Test Method”, the bending strength is tested by GB / T 6569-2006 “Fine Ceramic Bending Strength Test Method”, the fracture toughness is tested by the single edge notched beam method, and the performance test results are shown in Table 2.

[0120] Table 2: Performance test parameters

[0121]

[0122]

[0123] It is known from the above performance test results that the high-density alumina ultra-high pressure insulating ceramics prepared in the examples 1, 3, 4, 5 and 6 have a density greater than 98%, good density, an average grain size not greater than 1.5μm, small grain size, a bending strength greater than 550MPa, a fracture toughness greater than 5.5MPa·m½, excellent mechanical properties, a volume resistivity greater than 5×10 14 Ω·cm, a dielectric loss not greater than 3.5×10 -4 , excellent insulation performance, and are suitable for the fields of vacuum tubes, power equipment, electronic devices, etc.

[0124] wherein:

[0125] In terms of grain size, Example 3 further strengthens the grain boundary pinning effect by introducing ZrO2 nanoparticles (particle size about 20 nm), effectively inhibiting the abnormal growth of α-Al2O3 grains during sintering, so that the average grain size is controlled below 0.8 μm, which is reduced by 33% compared with Example 1 (1.2 μm) without adding ZrO2. The dynamic plastic deformation mechanism induced by high-pressure pulse pressure waves in Examples 4 and 6 further refines the grain size to 0.9 μm and 0.6 μm, respectively, while the abnormal growth of grains to 5.2 μm in Comparative Example 2 due to the lack of external field energy assistance during atmospheric sintering results in a clear "bimodal" grain size distribution.

[0126] The density data change is closely related to the sintering process parameters. The gradient pressure sintering (Examples 1, 3, 4, 5, and 6) promotes particle rearrangement and plastic flow by staged pressure loading, achieving a high density of 98.5%-99.5%, while the density of Comparative Example 2 is only 92.3% due to the lack of pressure driving force. The influence of the La2O3 / Gd2O3 molar ratio is shown in the comparison between Example 1 (1.5:1) and Example 2 (1.9:1), and Comparative Example 1 (0:1). Adjusting the La2O3 / Gd2O3 molar ratio can optimize the distribution of rare earth oxide liquid phase, increasing the density from 98.1% to 98.5%, while reducing the segregation of grain boundary glass phase.

[0127] The change in electrical insulation performance reveals the influence of microstructure on dielectric properties. The annealing and boric acid treatment of Examples 5 and 6 further optimize the volume resistivity and dielectric loss of the ceramic. After boric acid treatment, the volume resistivity of Example 5 is increased by nearly 7% compared with that of Example 1, while the dielectric loss is reduced. The dielectric loss is reduced by 62% through rare earth element ratio optimization (Example 1 vs. Comparative Example 1), confirming that La2O3 plays an irreplaceable role in regulating grain boundary electrical properties.

[0128] The technical features of the above examples can be combined in any way. To make the description concise, not all possible combinations of the technical features in the above examples are described, but as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present disclosure.

[0129] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the application. It should be noted that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A rare-earth-doped, high-density alumina ultra-high voltage insulating ceramic, characterized in that, The raw materials for preparation include, by mass percentage: α-Al₂O₃ 92-93.8%, rare earth oxides 3-5%, MgO 0.5-1.2%, Yb₂O₃ 0.3-0.8%, and sintering aids 0.5-1.5%; The rare earth oxide is a complex of La2O3 and Gd2O3 in a molar ratio of (1.2-1.5):1; A rare earth segregation layer with a thickness of 1-3 nm is formed at the grain boundaries, La 3+ The concentration gradient is ≥15at% / nm, and the segregated layer contains periodically arranged [La-O-Gd] structural units with a spacing of 0.5-0.8nm; The sintering aid is a B2O3-SiO2-LiF ternary system with a mass ratio of (0.3-0.5):(0.2-0.4):(0.1-0.3), which forms a low-viscosity transient liquid phase under ultra-high pressure of 10 GPa; The low-viscosity transient liquid phase has a viscosity ≤10 under ultra-high pressure. 3 Pa·s, and during the pressure release phase, a nanocrystalline encapsulation structure is formed, containing AlBO3 phase with a size of 5-20 nm distributed at the grain boundary triangular points.

2. The rare-earth-doped high-density alumina ultra-high voltage insulating ceramic as described in claim 1, characterized in that, It also includes 0.1-0.5 wt% ZrO2 nanoparticles, the ZrO2 nanoparticles having a particle size of 10-30 nm, uniformly dispersed in an α-Al2O3 matrix.

3. A method for preparing a rare-earth-doped high-density alumina ultra-high voltage insulating ceramic as described in any one of claims 1 to 2, characterized in that, Includes the following steps: (1) α-Al2O3 with a particle size of 50-150 nm is mixed with rare earth oxides, MgO, Yb2O3 and sintering aids in a certain proportion, and wet-milled in ethanol medium at a ball-to-material ratio of 8:1 for 12-18 h to obtain D 90 Uniform slurry with a diameter of ≤200nm; (2) After the slurry is vacuum dried at 60°C, it is placed in an atmosphere furnace containing 5 vol% HF and treated at 400°C for 2 h to allow LiF to react with B2O3 to generate the LiBO2 transition phase. (3) Gradual pressurization in a multi-anvil press after cold isostatic pressing: After cold isostatic pressing, the powder is placed in a six-sided top press, and the pressure is increased from 5GPa to 22GPa at a rate of 1-3GPa / min, while the temperature is increased stepwise from 300℃ to 800℃, wherein: The temperature was maintained at 500℃±20℃ for 5 minutes at the 10GPa stage to allow LiBO2-SiO2 to form a low-viscosity transient liquid phase with a viscosity η≤10. 3 Pa·s; During the 15GPa stage, the grain boundary recombination was carried out at 600℃±20℃ with a holding time of 30-90 seconds, which caused the La / Gd / Yb grain boundary to agglomerate and form a continuous stress buffer layer. (4) During the pressure release stage, the pressure is reduced at a rate of ≤0.5GPa / s, and field-assisted annealing is carried out simultaneously. A DC electric field with an intensity of 500-800V / cm is applied to induce the orderly arrangement of oxygen vacancies.

4. The preparation method according to claim 3, characterized in that, In step (3), during the 22 GPa ultra-high pressure sintering process, a pulsed pressure wave with a frequency of 5-10 kHz and an amplitude of 0.2-0.5 GPa is applied along the direction perpendicular to the basal plane (i.e., the crystal principal axis) in the alumina crystal structure. This guides directional slip deformation within the grains in a specific mode. The slip deformation manifests as relative sliding of atomic layers along a specific crystallization direction within the basal plane of the alumina, i.e., the atomic layers parallel to the bottom surface in the hexagonal crystal structure. The dislocation density is ≥5 × 10⁻⁶. 14 m -2 And the deviation between the slip trace and the theoretical slip direction is ≤5° as detected by EBSD.

5. The preparation method according to claim 4, characterized in that, The timing of the pulsed pressure wave loading is coupled with the temperature-pressure curve, specifically as follows: When the pressure increases from 15 GPa to 22 GPa, a continuous wave with a frequency of 5 kHz and an amplitude of 0.5 GPa is applied. During the 22GPa holding phase, the frequency is switched to an intermittent wave with a frequency of 10kHz and an amplitude of 0.2GPa, with a duty cycle of 1:

3. By dynamically adjusting the waveform parameters, the ratio of dislocation multiplication rate to grain growth rate can be ≥2:

1.

6. The preparation method according to claim 3, characterized in that, After step (4), the annealed ceramic is immersed in an ethanol solution containing 0.1-0.5 wt% boric acid and treated for 30 min under 50 W ultrasonic assistance to allow B to precipitate. 3+ Selective bonding of La-O units at grain boundaries forms [La-OB] complexes with thermal stability ≥1600℃.

7. The application of a rare earth-doped high-density alumina ultra-high voltage insulating ceramic as described in any one of claims 1 to 2 in the field of electronics and electrical appliances.

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