Magnesium oxide ceramic with low sintering temperature and high electric strength at high temperature and preparation method thereof

Abstract

The invention relates to a magnesium oxide ceramic with both low sintering temperature and high electric strength at high temperature and a preparation method thereof, the chemical composition of the MgO ceramic material is (MgO / Al2O3-1wt.% LiF)-xRE, where 0 lt; xlt; 5 wt%; and RE is at least one of rare earth oxides of Nd2O3, Sm2O3, Eu2O3, Gd2O3 and La2O3.

Description

Technical Field

[0001] The present invention relates to a low-temperature sintered dielectric ceramic, specifically to a magnesium oxide-based dielectric ceramic having both low sintering temperature and high electrical strength at high temperature and a preparation method thereof, and relates to technical fields such as low-temperature co-firing, high-voltage insulation, and high-temperature special service environments. Background Art

[0002] Dielectrics are insulators that can be electrically polarized. Charged particles in dielectrics are tightly bound by internal forces within atoms and molecules, or by intermolecular forces. Therefore, these particles have a bound charge. Under the influence of an external electric field, these charges can only move within a microscopic range, resulting in polarization. In an electrostatic field, an electric field can exist within a dielectric, which is the fundamental difference between dielectrics and conductors. Breakdown of a solid dielectric refers to the connection between two electrodes of a material under the influence of a high electric field, ultimately leading to a loss of insulation properties. Dielectric breakdown is the primary form of dielectric failure under an applied electric field. Dielectric strength (the electric field strength per unit length of the dielectric at which breakdown occurs) is one of the most important parameters of a dielectric material. The minimum critical voltage at which a solid dielectric breaks down is called the breakdown voltage. In a uniform electric field, the ratio of the breakdown voltage to the dielectric thickness is called the breakdown electric field strength (also known as dielectric strength), which reflects the dielectric strength of the solid dielectric. The development of ultra-high voltage transmission technology and pulsed power technology has placed higher demands on the dielectric strength of materials. Magnesium oxide-based insulation materials used in high-temperature cables need to withstand high voltages of tens of thousands of volts at temperatures of several hundred degrees. Therefore, high-temperature resistance and voltage stability are important guarantees for the normal operation of magnesium oxide cables, ensuring that high-temperature cables can transmit signals and power in extreme temperature environments in aerospace, electric power, and high-tech fields.

[0003] Magnesium oxide ceramics possess excellent conductivity, mechanical strength, and high-temperature resistance. They are cubic crystals with a NaCl-type structure. Magnesium oxide (MgO) is a wide-bandgap, high-insulation linear dielectric material, making it an ideal high-temperature insulating dielectric ceramic. MgO ceramics are typical alkaline refractory materials and can operate stably up to 2400°C in an oxidizing atmosphere or under nitrogen protection. In a reducing atmosphere, MgO decomposes and volatilizes as metallic magnesium, with significant volatilization beginning at 1600°C in a vacuum. Metals such as Fe, Zn, Pb, Cu, and M have no reducing effect on MgO ceramics. MgO ceramics can be used as crucibles for metal smelting and are also suitable for smelting high-purity uranium and thorium in the atomic energy industry. They can also be used as thermocouple protection sleeves. However, the high sintering temperature of MgO leads to grain growth and low density, which makes it difficult to improve its breakdown performance. Low-temperature co-fired ceramics (LTCC) technology, a key technology for device miniaturization and integration, evolved from high-temperature co-fired ceramics technology. The sintering temperature is usually lower than the melting point of metallic silver, 960°C. Therefore, the low-temperature co-firing technology can achieve the refinement and densification of magnesium oxide ceramics, while also having good high-temperature breakdown resistance. Summary of the Invention

[0004] In order to solve the problem that the electrical strength of MgO-based dielectric ceramics deteriorates during low-temperature sintering, the present invention provides a MgO-based dielectric ceramic having both low sintering temperature and high electrical strength at high temperature and a preparation method thereof.

[0005] In a first aspect, the present invention provides a MgO-based dielectric ceramic material having both low sintering temperature and high electrical strength at high temperature, wherein the chemical composition of the MgO-based dielectric ceramic material is: (MgO / Al2O3-1wt.% LiF)- x RE, where 0< x <5 wt%; RE is at least one of the rare earth oxides Nd2O3, Sm2O3, Eu2O3, Gd2O3, and La2O3. The selection of cations is based on the principles of ion substitution and solid solution. Specifically, under the same coordination environment (in the MgO matrix, the coordination number between cations and oxygen ions is 6), the ionic radii should be as close as possible to facilitate donor substitution, thereby improving dielectric strength and maintaining good thermal stability.

[0006] The present invention achieves MgO-based dielectric ceramics with both low sintering temperatures and high electrical strength at high temperatures through the synergistic effects of acceptor and donor doping and by regulating the defect energy levels and band structure of MgO. Li+ acceptor doping introduces oxygen vacancies, promoting mass transfer during sintering, thereby lowering the sintering temperature of the MgO ceramic. High-valent cation (Nd, Sm, Eu, Gd, La) donor doping effectively inhibits the formation of oxygen vacancies in the MgO ceramic and, by regulating the defect energy levels and band structure of MgO, provides electron-hole recombination centers, compensating for defects such as oxygen vacancies formed by LiF acceptor doping and improving the material's electrical strength.

[0007] The MgO-based dielectric ceramic of the present invention has a sintering temperature of 860-940° C. and a high-temperature electrical strength of 40-70 kV / cm.

[0008] Preferably, the chemical composition of the MgO-based dielectric ceramic is MgO to Al2O3 in a ratio of 7:1, 0.5wt%≤ x ≤1.5wt%. (Through appropriate amount of high-valent cation doping substitution, the oxygen vacancies formed by Li ion acceptor doping are compensated to improve the dielectric strength; when the doping concentration is too high (>1.5wt%), due to the limited solid solubility between ions, impurity phases will be generated, which will deteriorate the performance of the material.

[0009] In a second aspect, the present invention provides a method for preparing the aforementioned MgO-based dielectric ceramic, comprising: pre-calcining magnesium oxide powder with a Mg source purity exceeding 99.2% to obtain highly active pure magnesium oxide powder; mixing MgO, LiF, and rare earth oxides in proportion, wet-milling, and drying to obtain a raw material powder; sieving, granulating, and forming a ceramic green body; and sintering the green body to obtain the MgO-based dielectric ceramic. This preparation method utilizes a two-step solid-phase reaction; the unique feature of pre-calcining magnesium oxide powder with a Mg source purity exceeding 99.2% is that it achieves a higher density than MgO ceramics prepared by a one-step sintering method.

[0010] Preferably, the Mg source is magnesium oxide powder with a purity higher than 99.2%, preferably metal Mg oxide.

[0011] Preferably, the first ball milling time is 6 to 8 hours, the calcination temperature is 760 to 850° C., and the calcination time is 3 to 5 hours.

[0012] Preferably, the binder used in the granulation is at least one of polyvinyl alcohol, polyvinyl butyral and phenolic resin, and the amount of the binder added is 2.5-6.5% of the total mass of the raw material powder. Preferably, the forming is dry pressing, and the forming pressure is 30-60 MPa.

[0013] Preferably, the formed ceramic green body is subjected to isostatic pressing at a hydrostatic pressure of 40 to 150 MPa for 10 to 15 minutes.

[0014] Preferably, the temperature of the plastic removal is 300-650° C., and the time is 4-6 hours.

[0015] Preferably, the sintering temperature is 860-940° C., and the sintering time is 5-7 hours.

[0016] The present invention has the following beneficial effects: This invention addresses the significant deterioration in the dielectric strength of MgO-based ceramics sintered at low temperatures. By utilizing acceptor and donor synergistic doping, the dielectric strength of MgO ceramics is significantly improved, reaching a peak dielectric strength of 70.9 kV / mm at high temperatures, approximately 1.8 times that of pure MgO ceramics. Furthermore, the sintering temperature is kept below 940°C, achieving a combination of low sintering temperature and high dielectric strength. Furthermore, the material is compatible with silver electrodes. Utilizing a traditional solid-phase method, the preparation process is simple and suitable for mass production. BRIEF DESCRIPTION OF THE DRAWINGS FIG1 is a microscopic morphology of the ceramic sample prepared in Example 1 after sintering at 940° C. for 6 hours. FIG2 is a microscopic morphology of the ceramic sample prepared in Example 2 after sintering at 940° C. for 6 hours. FIG3 is a microscopic morphology of the ceramic sample prepared in Example 3 after sintering at 940° C. for 6 hours. FIG4 is a microscopic morphology of the ceramic sample prepared in Comparative Example 1 after sintering at 940° C. for 6 hours. FIG5 is a microscopic morphology of the ceramic sample prepared in Comparative Example 2 after sintering at 940° C. for 6 hours. FIG6 is a microscopic morphology of the ceramic sample prepared in Comparative Example 3 after sintering at 940° C. for 6 hours. 7 to 12 are graphs showing the variation of voltage across the samples with leakage current at 10° C. and 20° C. for the ceramic samples prepared in Examples 1 to 3 and Comparative Examples 1 to 3. FIG13 is a grain size distribution diagram of the dielectric ceramics prepared according to the present invention, including Examples 1 to 3 and Comparative Examples 1 to 3.

Claims

1. A magnesium oxide-based dielectric ceramic having both low sintering temperature and high electrical strength at high temperature and a preparation method thereof, characterized in that: The chemical composition of the MgO-based ceramic material is: (MgO / Al2O3-1wt.% LiF)- x RE; where 0< x <5 wt%; RE is at least one of rare earth oxides Nd2O3, Sm2O3, Eu2O3, Gd2O3 and La2O3. The magnesium oxide ceramic is subjected to a compressive strength test at a high temperature of 100-200°C.

2. The MgO-based dielectric ceramic material according to claim 1, characterized in that: The molar ratio of MgO to Al2O3 in the chemical composition of the MgO-based ceramic is 5:1 to 10:1, <x<5 wt%。 3. A method for preparing the MgO-based ceramic material according to claim 1 or 2, characterized in that: include: High-purity magnesium oxide powder is ball-milled for 6 to 8 hours and then pre-sintered at 850° C. MgO, LiF, and rare earth oxide are mixed in proportion, wet-milled, and dried to obtain a raw material powder; a ceramic green body is obtained through screening, granulation, and molding; and the ceramic green body is demolded and sintered to obtain the MgO-based dielectric ceramic.

4. The preparation method according to claim 3, characterized in that The Mg source is magnesium oxide powder with a purity higher than 99.2%. The Mg source is at least one of metal Mg oxide, carbonate, oxalate, and acetate, preferably metal Mg oxide.

5. The preparation method according to claim 3 or 4, characterized in that The temperature of the high-temperature calcination of the Mg source is 760-850° C., and the time is 3-5 hours; the heating rate of the calcination is 5-10° C. / min.

6. The preparation method according to any one of claims 3 to 5, characterized in that The binder used in the granulation is at least one of polyvinyl alcohol, polyvinyl butyral and phenolic resin, and the added amount is 2.5-6.5wt% of the total mass of the raw material powder.

7. The preparation method according to any one of claims 3 to 6, characterized in that The forming is dry pressing, the forming pressure is 30-60 MPa, and the ceramic green body obtained by forming is subjected to isostatic pressing treatment at a hydrostatic pressure of 40-150 MPa for 10-15 minutes.

8. The preparation method according to any one of claims 3 to 8, characterized in that The temperature of the plastic removal is 300-650° C., and the time is 4-6 hours.

9. The preparation method according to any one of claims 3 to 9, characterized in that The sintering temperature is 860-940° C., and the sintering time is 5-7 hours.

10. The method for testing the breakdown strength according to claim 1, characterized in that: The test temperature is 100-200℃.

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