Magnesium oxide / aluminum oxide-based high-temperature-resistant and high-pressure-resistant dielectric ceramic and preparation method thereof

Abstract

The invention relates to magnesium oxide / alumina-based high-temperature and high-pressure-resistant dielectric ceramic and a preparation method thereof, the chemical composition of the magnesium oxide / alumina-based ultrahigh-pressure dielectric ceramic is (1-x) MgO / Al2O3-xNd2O3, x is more than 0 and less than or equal to 0.02, preferably, x is more than or equal to 0.005 and less than or equal to 0.015, and x is mass percent.

Description

Technical Field

[0001] The present invention relates to a high-temperature and high-pressure resistant dielectric ceramic and a preparation method thereof, in particular to a magnesium oxide / aluminum oxide-based high-temperature and high-pressure resistant dielectric ceramic and a preparation method thereof, belonging to the technical field of mineral insulated heating cables. Background Art

[0002] Insulating ceramics are used in various applications, including insulators, insulating structural components, band switches, capacitor support brackets, electronic component packaging, integrated circuit substrates, and packaging. The fundamental requirements for insulating ceramics are low dielectric constant, low dielectric loss, high insulation resistivity, high electrical strength, high mechanical strength, excellent thermal shock resistance, high thermal conductivity, and good stability against temperature, humidity, and frequency. Compared to polymers, ceramic materials offer high temperature stability, aging resistance, and excellent resistance to corona or arc erosion, as well as chemical corrosion.

[0003] Both magnesium oxide and aluminum oxide ceramics have good mechanical strength and high temperature resistance. Magnesium oxide belongs to the cubic crystal system and has a NaCl-type structure. The melting point of pure magnesium oxide is 2800℃±13℃. Its Mohs hardness is 6, and its tensile strength, compressive strength, and flexural strength are much lower than those of sintered Al2O3. Its high temperature strength is also lower. MgO is a good insulator with a resistivity of >10 at room temperature. 14 Ω·cm, with a sharp decrease in resistivity as temperature increases. Alumina ceramics are typical alkaline refractory materials, capable of stable operation up to 2400°C in an oxidizing atmosphere or under nitrogen protection. Al2O3 ceramics are 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 magnesium oxide / aluminum oxide leads to grain growth and low density, which is the key to improving its breakdown performance. Summary of the Invention

[0004] In order to solve the problem that magnesium oxide / aluminum oxide ceramics have easy grain growth and difficulty in densification due to high sintering temperature, the present invention aims to provide a magnesium oxide / aluminum oxide-based high temperature and high pressure resistant dielectric ceramic and a preparation method thereof.

[0005] On the one hand, the present invention provides a magnesium oxide / aluminum oxide-based high-temperature and high-pressure resistant dielectric ceramic material, the chemical composition of the magnesium oxide / aluminum oxide-based ultra-high-pressure dielectric ceramic is: (1-x)MgO / Al2O3-xNd2O3, wherein 0<x≤ 0.02, preferably 0.005≤x≤0.015, x is the mass percentage; the molar ratio of magnesium oxide to aluminum oxide is 10:1 to 4:1.

[0006] The present invention constructs a composite structure by doping alumina in a magnesium oxide / alumina matrix. By pre-sintering the Mg source to increase the activity of the magnesium oxide powder, the addition of alumina can effectively prevent the splitting of the magnesium oxide ceramic layer, and the introduction of neodymium oxide forms a composite structure uniformly distributed around the main crystal phase of magnesium oxide / alumina. The second phase uniformly distributed around the main crystal phase effectively inhibits the growth of magnesium oxide / alumina grains, significantly improving the density of magnesium oxide ceramics and enhancing the mechanical strength and electrical strength of the ceramics. After compounding, the average grain size of magnesium oxide / alumina is significantly reduced to 1 to 5 μm (preferably 1 to 4 μm). The magnesium oxide / alumina-based high-temperature and high-voltage dielectric ceramics prepared by the present invention have an electrical strength of 63.49 to 150.94 kV / cm at 100°C and 200°C.

[0007] Preferably, the present invention produces the magnesium oxide / aluminum oxide-based ultrahigh-pressure dielectric ceramic by ball-milling magnesium oxide powder with a purity exceeding 99.2% for 6-8 hours, pre-sintering it at 1250°C, cooling it, mixing it with aluminum oxide and neodymium oxide powders, ball-milling it, pressing it into shape, demolding it, and sintering it; the sintering step is performed at 1300-1500°C for 4 hours. Because magnesium oxide / aluminum oxide ceramics are susceptible to grain growth and difficult to densify due to high sintering temperatures, the present invention uses magnesium oxide powder obtained by pre-sintering high-purity magnesium oxide as a raw material to enhance the sintering activity of the powder.

[0008] On the other hand, the present invention also provides a method for preparing the above-mentioned magnesium oxide / aluminum oxide-based high-temperature and high-pressure resistant dielectric ceramic, comprising: Mixing magnesium oxide powder and aluminum oxide powder in a molar ratio of 10:1 to 4:1, and mixing magnesium oxide / aluminum oxide powder and neodymium oxide powder in a chemical ratio to obtain raw material powder; The obtained raw material powder is then pressed into shape to obtain a green body; The obtained green body is subjected to plastic removal and then sintered at 1300-1500° C. to obtain magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramics.

[0009] The present invention prepares (1-x)MgO / Al2O3-xNd2O3 ceramic powders through a traditional solid-phase method, and then produces magnesium oxide / aluminum oxide-based dielectric ceramic materials through molding, demolding, and sintering. Through raw material powder optimization and component design, pre-calcined high-purity magnesium oxide powder is used as the raw material to increase the powder sintering activity and further improve the purity of the magnesium oxide raw material. A trace amount of neodymium oxide powder is introduced into the magnesium oxide / aluminum oxide powder. During the sintering process, the magnesium oxide and neodymium oxide react in situ to form a second phase, forming a stable composite structure. This solves the problems of magnesium oxide / aluminum oxide ceramics, such as easy grain growth and low density at high temperatures, and produces dielectric ceramics with high insulation properties. The ceramics produced by the present invention have ultra-high electrical strength, reaching a maximum electrical strength of 150.94 kV / cm. Furthermore, the present invention utilizes inexpensive raw materials, simple components and processes, and low production costs.

[0010] Preferably, the present invention uses magnesium oxide and aluminum oxide powders obtained by pre-calcining high-purity magnesium oxide / aluminum oxide as raw materials to improve the sintering activity of the powder.

[0011] Preferably, the first ball milling time is 6 to 8 hours, the calcination temperature is 1200 to 1300° C., and the calcination time is 3 to 5 hours.

[0012] Preferably, before pressing and molding, the raw material powder and the binder are mixed, and then granulated and aged. The binder is added to the obtained raw material powder, and then granulated and aged, and pressed and molded to obtain a ceramic green body.

[0013] Preferably, the binder is polyvinyl alcohol; the amount of the binder added is 2.5-6.5% of the total mass of the raw material powder.

[0014] Preferably, the pressed ceramic green body can be isostatically pressed at a pressure of 180 MPa for 8 to 10 minutes.

[0015] Preferably, the temperature of the plastic removal is 400-600°C and the time is 3-5 hours. Preferably, the heating rate of the sintering process is not higher than 5°C / min.

[0016] Preferably, the sintering treatment time is 4 to 8 hours.

[0017] Compared with the prior art, the present invention dopes a small amount of magnesium oxide / alumina powder with high sintering activity, then constructs a composite structure by doping neodymium oxide into the magnesium oxide / alumina matrix. The activity of the magnesium oxide powder is increased by pre-sintering the Mg source, and then neodymium oxide is introduced for sintering, thereby forming a composite structure uniformly distributed around the main crystalline phase of magnesium oxide / alumina. The second phase uniformly distributed around the main crystalline phase effectively inhibits the growth of magnesium oxide / alumina grains, significantly improving the density and significantly improving the mechanical strength and electrical strength of the ceramic. After composite, the average grain size of magnesium oxide / alumina is significantly reduced to 1 to 5 μm (preferably 1 to 4 μm). The electrical strength of the magnesium oxide / alumina-based high-temperature, high-voltage dielectric ceramic at 100°C and 200°C is 87.72 to 150.94 kV / cm. The high-temperature, high-voltage-resistant insulating magnesium oxide / aluminum oxide-based ceramics prepared by the present invention achieve fine grains and dense sintering while ensuring high insulation performance and electrical strength. The raw materials are cheap and the components and processes are simple. It is expected to be used as an insulating material for high-temperature, high-voltage-resistant heating cables to manufacture high-power electric heaters. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 This is a graph showing the change in voltage across the dielectric ceramic prepared in Example 1 of the present invention as a function of leakage current at 100° C. and 200° C.; Figure 2 This is a graph showing the change in voltage across the dielectric ceramic prepared in Example 2 of the present invention as a function of leakage current at 100° C. and 200° C.; Figure 3 This is a graph showing the change in voltage across the dielectric ceramic prepared in Example 3 of the present invention as a function of leakage current at 100° C. and 200° C.; Figure 4 This is a graph showing the change in voltage across the dielectric ceramic prepared in Comparative Example 1 of the present invention as a function of leakage current at 100° C. and 200° C.; Figure 5 This is a graph showing the change in voltage across the dielectric ceramic prepared in Comparative Example 2 of the present invention as a function of leakage current at 100° C. and 200° C.; Figure 6 XRD patterns of dielectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2 of the present invention; Figure 7 (a), (b), (c), and (d) are cross-sectional SEM photos of dielectric ceramics prepared in Comparative Example 1 and Examples 1, 2, and 3, respectively; Figure 8 (a), (b), (c), (d), and (e) are the grain size distribution diagrams of the dielectric ceramics of Comparative Example 1 and Examples 1-3, respectively.

Claims

1. A magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic, characterized in that: The chemical composition of the magnesium oxide / aluminum oxide-based ultra-high voltage dielectric ceramic is: (1-x)MgO / Al2O3-xNd2O3, wherein 0<x≤0.02, and x is a mass percentage; the molar ratio of magnesium oxide to aluminum oxide is 10:1 to 4:1; the average grain size of the magnesium oxide / aluminum oxide ceramic is 1 to 5 μm; and the dielectric strength of the magnesium oxide / aluminum oxide-based high-temperature and high-voltage dielectric ceramic at 100°C and 200°C is 63.49 to 150.94 kV / cm.

2. The preparation method of the magnesium oxide / aluminum oxide-based high-temperature and high-pressure resistant dielectric ceramic is as follows: after ball milling magnesium oxide powder with a purity higher than 99.2% for 6 to 8 hours, it is pre-sintered at 1250°C, and after cooling, it is mixed with aluminum oxide and neodymium oxide powder, ball milled, pressed into shape, plasticized, and sintered to obtain the magnesium oxide / aluminum oxide-based ultra-high-pressure dielectric ceramic; the sintering is a sintering treatment at 1300 to 1500°C for 4 to 8 hours.

3. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 1, characterized in that: 0＜x≤0.02。 4. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 1, characterized in that: The magnesium oxide / aluminum oxide-based high-temperature and high-voltage dielectric ceramic has a dielectric strength of 87.72 to 150.94 kV / cm at 100° C. and 200° C.

5. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 1, characterized in that: Before pressing and molding, the raw material powder and the binder are mixed, and then granulated and aged.

6. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 4, characterized in that: The binder is polyvinyl alcohol.

7. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 5, characterized in that: The added amount of the binder is 2.5-6.5% of the total mass of the raw material powder.

8. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to claim 1, characterized in that: The temperature of the plastic removal is 400-600° C., and the time is 3-5 hours.

9. The magnesium oxide / aluminum oxide based high temperature and high pressure resistant dielectric ceramic according to any one of claims 1 to 7, characterized in that: The heating rate of the sintering process is no higher than 5°C / min.

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