Oxide Ceramics｜95% Alumina Ceramic

95% alumina ceramics is the most widely used type in structural ceramics. Because it has excellent electrical properties at high frequencies, it has low dielectric loss, high volume resistivity, high strength, high hardness, low linear expansion coefficient, and strong wear resistance and heat resistance. And in the same high temperature use environment, compared with 99 ceramic, 95 ceramic has slightly stronger insulation performance, so it is recommended to use 95% alumina ceramics for high insulation requirements at high temperatures, and its cost is also lower. But the disadvantage is that 95% alumina ceramics are more prone to collapse during processing, so you need to be very careful during processing! In addition, the biggest disadvantage of 95 alumina ceramics is that the toughness is general and cannot withstand strong impact. The specific comprehensive properties of 95% alumina ceramics are as follows:

Electrical properties of 95% Alumina Ceramic

1.Volume resistivity

The volume resistivity ρν of an insulating material refers to the ratio of the DC electric field strength in the direction of the volume current of the sample to the current density at that location. ρν=EV/jv(Ωcm), where EV is the DC electric field strength, and jv is the current density. 95% alumina ceramic is an excellent electronic insulating material with high volume resistivity. It is stipulated in the national standard GB/T5593-1999 that at 100°C, ρν≥1×1013Ωcm; at 300°C, ρν≥1×1010Ωcm; At 500℃, ρν≥1×108Ωcm. In fact, the volume resistivity of 95 ceramic 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
The DC breakdown strength of electrical insulating materials refers to the change that occurs under the action of an external DC electric field, which is mainly caused by changes in the internal structure. When the electric field strength reaches a certain value, further changes in its internal structure will be promoted, and insulation breakdown will occur. The national standard GB/T5593-1999 stipulates that the withstand voltage test of 95% alumina ceramics is carried out under DC conditions. When a DC voltage is applied to the sample, the sample will break down, the ratio of the breakdown voltage value to the average thickness of the sample is called the DC breakdown strength, unit: 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.Dielectric constant
A parameter of dielectric polarization of insulating material under AC electric field, which is the ratio of the capacitance of a capacitor filled with an insulating material to that of a capacitor with the same electrode size when vacuum is used as the medium. It represents an inherent characteristic of the material. The dielectric constant of 95% alumina ceramics is between 9 and 10 when the test frequency is 1MHz according to the national standard.

4.Dielectric loss tangent
Dielectric loss refers to the phenomenon of polarization or absorption of materials under the action of AC electric field, resulting in loss of electrical energy, and usually there is a phenomenon of heating on the dielectric material. The magnitude of the dielectric loss is represented by the tangent of the dielectric loss angle. The national standard GB/T5593-1999 stipulates that when the frequency is 1MHz, 95% alumina ceramics are required to reach 4×10-4.

Thermal properties of 95% Alumina Ceramic

1. Average Linear Expansion Coefficient
The relative elongation of ceramics when the unit temperature is raised, that is, the average linear expansion coefficient. It is one of the main performance indicators of 95% alumina ceramics. The national standard GB/T5593-1999 stipulates that, when the test temperature is 20-500℃, and a is 6.5×10-6-7.5×10-6/℃; when the test temperature is 20-800℃, a is 6.5×10-6 - 8×10-6/℃. Since ceramic parts are generally sealed with metal parts, their expansion coefficients are required to match those of metal materials.

2. Thermal shock resistance
Refers to the ability of ceramic materials to withstand rapid cooling and rapid heating. 95% alumina ceramics have good thermal shock resistance and can withstand the test of high temperature and rapid cooling and heating without damage. The national standard GB/T5593-1999 stipulates: put 10 ceramic samples into a heating furnace at 800±10℃, keep warm for 30 minutes, take them out and put them on the asbestos board, cool to room temperature naturally, and then put the samples into a heating furnace at 800 °C. According to the above regulations, carry out a total of 10 times, and then soak in 1% magenta solution for 3 minutes, take out and wash with tap water, wipe dry, and observe the sample under the light. If there are no cracks or bursts in all 10 samples, the performance is qualified.

Mechanical properties of 95% Alumina Ceramic

Mechanical properties refer to the ability of a material to withstand mechanical force without breaking. Generally, it is divided into tensile strength, flexural strength and compressive strength according to the direction of the force and the force point. The tensile strength of 95% alumina ceramics is generally above 160Mpa, and the better sealing strength is usually higher. There are no technical indicators for the tensile strength and compressive strength of 95% alumina ceramics in the national standard GB/T5593-1999. The national standard GB/T5593-1999 stipulates that the flexural strength of 95% alumina ceramics is 280MPA.

Other properties of 95% Alumina Ceramic

1. Bulk density
The bulk density of 95% alumina ceramics has a great relationship with the selection of raw materials and porcelain making process. According to the national standard GB / t5593-1999, the bulk density of 95% alumina ceramics is required to be above 3.60g/cm3. In fact, the density of 95% alumina ceramics varies greatly due to different molding methods. Generally, the density is about 3.60g/cm3 during injection molding; When dry pressing, the density is 3.60-3.7g/cm3; The density can be greater than 3.70g/cm3 during isostatic pressing.

2. Air tightness
95% alumina ceramics are required with high vacuum air tightness when it is used in microwave devices and vacuum switch tubes. It is specified in the electronic industry standard SJ / T that the air leakage rate QK of the sealing parts is ≤ 1 × 10-11pa.m3/s. In fact, it is stipulated that the air leakage rate of the ceramic itself should also meet the above standards.

3. Chemical stability
The chemical stability of 95% alumina ceramics refers to its ability to resist corrosion by various chemical agents. The ability to resist the etching of acid reagents is called acid resistance, and the ability to resist the etching of alkali reagents is called alkali resistance. According to the national standard GB/T5593-1999, put the sample in 10% hydrochloric acid or sodium hydroxide solution, heat the solution to 100 ℃ and keep it for 1H. Acid resistance requires Kh≤7Mg / cm2, and alkali resistance requires Kn≤0.2mg/cm2.

4. Ceramic micro-structure
The micro-structure of ceramics refers to the composition of crystal phase (main crystal phase, sub crystal phase), glass phase, gas phase and grain boundary, the mutual combination relationship among content, distribution, morphology, particle size distribution, uniformity, defect, interphase material, domain structure etc. The micro-structure has a close relationship with the porcelain making process, which directly affects the performance of ceramics. It is a research method to improve the quality of ceramics and improve the process.

Sealing performance with metal of 95% Alumina Ceramic
As a kind of structural ceramics, 95% alumina ceramics should be sealed with metal to form a component. Especially for 95% alumina ceramics used in microwave devices and vacuum development tubes, the ceramic metal sealing performance is usually taken as an important index to evaluate. Assessment content: sealing performance of ceramic and metal requires high strength and good air tightness. According to the electronic industry standard SJ/T CERAMIC SHELL FOR VACUUM SWITCH TUBE, when standard porcelain is used for sealing, the average sealing strength σa≥90Mpa; When using the three-point method, σa≥90Mpa. Air tightness of sealing parts, helium leakage rate QK≤1×10-11PaM3/S