97 alumina ceramics

The main component of alumina ceramics is Al2O3. People usually refer to ceramics with an Al2O3 content greater than 75% as high alumina ceramics. Alumina ceramics are the most widely used type of structural ceramics. Because it has excellent electrical performance at high frequencies, low dielectric loss, high volume resistivity, high strength, high hardness, low linear expansion coefficient, and good wear resistance and heat resistance.

Alumina ceramics include 97 ceramics (97% Al2O3), 99 ceramics (99.99% Al2O3), etc. One characteristic of alumina ceramics is that its various mechanical and electrical properties improve with the increase of alumina content. However, its disadvantage is that the increase of alumina content is accompanied by an increase in firing temperature, which brings certain difficulties to the sintering process. Therefore, using 97% to 97% alumina ceramics in general situations is more economical and practical. For occasions with particularly high performance requirements, only 99% to 99.99% alumina ceramics are used. The following are the specific comprehensive properties of 97 alumina ceramics.

1、 Electrical performance

1. Volume resistivity of insulating materials ρν It refers to the ratio of the direct current electric field intensity in the direction of the sample volume current to the current density at that location. ρν＝ EV/jv (Ω cm), where EV is the DC electric field strength and jv is the current density. 97% alumina ceramic is an excellent electronic insulation material with high volume resistivity. According to the national standard GB/T5593-1999, at 100 ℃, ρν≥ one × 1013 Ω cm; At 300 ℃, ρν≥ one × 1010 Ω cm; At 500 ℃, ρν≥ one × 108 Ω cm. In fact, the volume resistivity of 97 porcelain produced in China is 1-2 orders of magnitude higher than the above regulations. Instruments for measuring volume resistance usually use a high resistance meter.

2. DC breakdown strength of electrical insulation materials refers to the effect of an external DC electric field on the DC breakdown strength

The changes that occur are mainly caused by internal structural changes. When the electric field intensity reaches a certain value, it promotes further changes in its internal structure, leading to insulation breakdown. According to the national standard GB/T5593-1999, 97% alumina ceramics undergo withstand voltage tests under direct current conditions. When a direct current voltage is applied to the sample to cause breakdown, the ratio of the breakdown voltage value to the average thickness of the sample is called the DC breakdown strength, in kV/mm. The national standard GB/T5593-1999 stipulates that it should be greater than 18KV/mm. In fact, we can generally reach 30-40KV/mm.

3. The dielectric constant is a parameter of the degree of dielectric polarization of an insulating material under an AC electric field. It is the ratio of the capacitance of a capacitor filled with a certain insulating material to a capacitor of the same electrode size when using vacuum as the medium. It represents an inherent characteristic of the material. According to the national standard, the dielectric constant of 97% alumina ceramics ranges from 9-10 when the testing frequency is 1MHz.

4. The tangent value of the dielectric loss angle indicates that a material undergoes polarization or absorption under the action of an AC electric field, resulting in electrical energy loss. Typically, there is heating on the dielectric material. The magnitude of dielectric loss is represented by the tangent of the dielectric loss angle. According to the national standard GB/T5593-1999, when the frequency is 1MHz, 97% alumina ceramics are required to meet the requirement of 4 × 10-4.

2、 Thermal performance

1. The relative elongation of the average linear expansion coefficient ceramic at elevated unit temperature, i.e. the average linear expansion coefficient, is one of the main performance indicators of 97% alumina ceramics. According to the national standard GB/T5593-1999, the testing temperature is 20-500 ℃, with a value of 6.5 × 10-6-7.5 × 10-6/℃; When the testing temperature is 20-800 ℃, a is 6.5 × 10-6-8 × 10-6/℃. Due to the fact that ceramic parts generally need to be sealed with metal parts, it is required that their expansion coefficient match the expansion coefficient of the metal material.

2. Thermal shock resistance refers to the ability of ceramic materials to withstand rapid cooling and heating. 97% alumina ceramics have good thermal shock resistance and can withstand high temperatures and rapid cooling without damage. According to the national standard GB/T5593-1999, 10 ceramic samples are placed in a heating furnace at 800 ± 10 ℃. After holding for 30 minutes, they are taken out and placed on an asbestos board. They are naturally cooled to room temperature. Then, the samples are placed in an 800 ℃ heating furnace for a total of 10 times according to the above regulations. Finally, they are immersed in a 1% concentration magenta solution for 3 minutes, washed with tap water, wiped dry, and observed under light. If there are no cracks or explosions in all 10 samples, The performance is qualified.

3、 Mechanical properties

Mechanical performance refers to the ability of a material to withstand mechanical forces without damage. Usually, tensile strength, flexural strength, and compressive strength are divided based on the direction of the applied force and the different points of force. The tensile strength of 97% alumina ceramics is generally above 160Mpa, which is usually greater than the best sealing strength. There are no technical indicators for the tensile and compressive strength of 97% alumina ceramics in the national standard GB/T5593-1999. The national standard GB/T5593-1999 stipulates that the flexural strength of 97% alumina ceramics is 280MPA.

4、 Other performance

1. The volume density of 97% alumina ceramics is closely related to the selection of raw materials and the ceramic manufacturing process. According to the national standard GB/T5593-1999, the volume density of 97% alumina ceramics is required to be above 3.60g/cm3. In fact, the forming methods of porcelain are different, and the density of 97% alumina ceramics varies greatly. Generally, the density is 3.60-3.70g/cm3 during hot die casting and grouting; 3.70g/cm3 during isostatic pressing molding; The gel method can reach 3.73g/cm3.

2. 97% airtightness alumina ceramics are used in microwave devices, vacuum switch tubes, etc., requiring high vacuum airtightness. According to the electronic industry standard SJ/T, the leakage rate QK of sealing components is ≤ 1 × 10-11Pa. M3/S. In fact, it is stipulated that the leakage rate of the ceramic itself must also meet the above standards.

3. The chemical stability of 97% alumina ceramics is characterized by their ability to resist various chemical agents. The ability to resist corrosion by acid resistant reagents is called acid resistance, while the ability to resist corrosion by alkali resistant reagents is called alkali resistance. According to the national standard GB/T5593-1999, put the sample in 1:9 hydrochloric acid or 10% sodium hydroxide solution, heat the solution to 100 ℃, and keep it for 1h. The requirement for acid resistance is Kh ≤ 7mg/cm2, and the requirement for alkaline resistance is Kn ≤ 0.2mg/cm2.

4. Ceramic microstructure The microstructure of ceramics refers to the mutual combination of the composition, content, distribution, morphology, particle size distribution, uniformity, defects, interphase materials, domain structure, etc. of crystal phase (main crystal phase, sub crystal phase), glass phase, gas phase, and grain boundary. The microstructure is closely related to the ceramic manufacturing process and directly affects the performance of ceramics. It is a research method for improving ceramic quality and technology.

5、 Sealing performance of 97% alumina ceramic metal

97% alumina ceramics, as a type of structural ceramics, need to be sealed with metal to form a component. Especially for 97% alumina ceramics used in microwave devices and vacuum development tubes, the ceramic metal sealing performance is usually evaluated as an important indicator. Assessment content: The ceramic metal sealing performance requires high strength and good airtightness. According to the electronic industry standard SJ/T "Ceramic Shell for Vacuum Switch Tubes", the average sealing strength is determined by using standard ceramic sealing, σ A ≥ 90Mpa; When using the three-point method for testing, σ A ≥ 90Mpa. Gas tightness of sealing parts, helium leakage rate Qk ≤ 1 × 10-11PaM3/S.

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