Why does electronic grade alumina need low sodium?

Alumina can be divided into metallurgical grade and non-metallurgical grade.

Nano alumina

Introduction to Electronic Grade Alumina

From the phase classification, alumina can be divided into α, γ, χ, θ, η, ρ and other phases, of which the α phase is the most stable in structure, with acid and alkali resistance, good insulation performance, high temperature resistance, and high thermal conductivity.

Electronic grade alumina is mainly used in fields related to electrical properties, such as electronic substrates, electronic thermal conductive silica gel, insulating fillers, battery materials, etc.

Transformer insulating sheet alumina ceramic substrate

Quality Control Requirements for Electronic Grade Alumina

1.

Generally speaking, the main impurities of α-Al2O3 are Na2O, SiO2, Fe2O3, CaO, MgO, TiO2, etc.

Among them, the Na2O content is the most important indicator, which not only affects the density of electronic ceramics, but also affects its electrical properties because Na2O combines with Al2O3 to generate β-Al2O3 with certain conductivity

Colored oxides such as Fe2O3 and TiO2 not only affect the color of downstream products, but also lead to a decrease in electrical properties, so the content is required to be as low as possible

SiO2 is generally not a harmful element in downstream products.

In general, the content of CaO and MgO is very low.
Except for special requirements, they are generally not used as special controls
.

In addition, although the content of some trace elements is extremely low, they have a great impact on some products and must be strictly controlled.
For example, in LCD glass, elements such as Cr, Cl, and Li must be controlled, and the content is required to be within 5PPm
.

2.
Particle size and particle morphology

The particle size and distribution of α-Al2O3 powder not only affect the processing performance of the product, but also have a greater impact on the performance of the product itself
.
As the application of electronic materials, the size and distribution of its particle size must be strictly controlled
.

When α-Al2O3 powder is used as a filler, its microscopic morphology has a great influence on its rheology and dispersibility, and the viscosity of different morphologies in the organic matrix is ​​quite different.
Generally speaking, the more regular the morphology is.
, the lower the viscosity
.
Therefore, in order to reduce the viscosity and improve the uniformity of particles, α-Al2O3 can be prepared into regular morphologies such as spherical and flakes through appropriate process control
.

3.
Foreign body control

In addition to general chemical impurities, the impurities in α-Al2O3 powder are also important impurities, especially in the electronic field.
Because the products are very fine, the requirements for foreign substances are very strict
.
In general, the common foreign objects mixed in are metal particles, non-metal particles, fibers,
etc.

Metallic foreign matter has a greater impact on the electrical properties of the product, and also has a greater impact on the appearance.
For example, in ceramic sintering, once there are more than 10 μm Fe-containing particles, a color spot will be formed on the surface of the ceramic, causing Scrap of the product; in the case of organic filling, these foreign bodies can also cause defects on the surface of the product
.

4.
Application performance

When measuring the quality indicators of α-Al2O3 powder products, in addition to the general physical and chemical indicators, its application performance is also an important measurement indicator, especially when it is applied to a composite system, its process performance often directly determines whether the downstream production can be carried out stably
.
For example, in the preparation process of electronic ceramics, α-Al2O3 and ceramic additives are formulated into ceramic slurry, and the rheology of the slurry exceeds the range, which will lead to inability to granulate; when used as epoxy resin or silicone rubber filler, its Cure performance is an important measure
.

For the application performance of powder, the industry usually adopts a certain modification method for specific application fields
.

Application of electronic grade alumina

1.
Epoxy composite insulating material

Because α-Al2O3 has good insulating properties and good compatibility with epoxy resin, filling into epoxy resin can play a role in particle reinforcement and improve thermal conductivity.
Therefore, it can be prepared with epoxy resin.
It is made into composite material and applied to high-voltage insulating materials, such as high-voltage switch basin insulators, high-voltage transformers, insulating tie rods, epoxy bushings, etc.
, with voltage levels ranging from 110 to 1000kV
.

Alumina high voltage insulating ceramic plunger

2.
Electronic ceramics

α-Al2O3 not only has good ceramic performance, but also has high resistivity, high thermal conductivity, low dielectric constant and dielectric loss, so it is prepared into different electronic ceramics, mainly used in electronic packaging, vacuum interrupters, Traveling wave tube, igniter,
etc.

One of the electrical ceramics is the integrated circuit substrate.
With the development of electronic technology, multi-layer co-firing technology (HTCC, LTCC) has been introduced, which further improves the miniaturization of integrated circuits.
Alumina ceramic substrates have become the most widely used Electronic ceramics, accounting for 90% of electronic ceramic substrates, have become an indispensable material in the electronic industry
.

Alumina ceramic substrate

3.
Thermally conductive insulating material

α-Al2O3 not only has good insulating properties, but also has a thermal conductivity of 30W/(m K), which is relatively cost-effective compared to aluminum nitride and boron nitride, and has good dispersion properties in silica gel and silicone oil systems.
The field of electronic thermal insulation occupies a large proportion, and is often used in thermally conductive silicone sheets, thermally conductive potting glue, thermally conductive silicone grease and other materials
.

Thermal Silicone Gasket

4.
Electronic glass

LCD (liquid crystal) glass substrate is an electronic glass that is different from ordinary glass and one of the key basic materials for flat panel display.
Its surface is extremely flat and its thickness is 0.
1~0.
7mm
.
Not only the apparent quality is demanding,

In addition, it also requires extremely low expansion coefficient, good chemical stability, high mechanical strength, etc.
, which is a material with extremely high requirements for preparation technology
.

LCD glass belongs to alkali-free alumino-borosilicate glass, that is, SiO2-Al2O3-B2O3-RO system (RO is alkaline earth metal), so the alumina is required to be low-sodium α-Al2O3.
The manufacturing process mainly includes float, flow, etc.
There are 3 kinds of hole down-drawing method and overflow method
.

Liquid crystal glass substrate

5.
Lithium-ion battery

The most widely used Al2O3 in the field of lithium ion batteries is the diaphragm coating, that is, the α-Al2O3 powder is uniformly coated on the surface of a polyolefin film with a microporous structure, which is used to isolate the positive and negative electrodes to prevent short circuits, but can also To ensure the free passage of lithium ions
.
The diaphragm base film is generally made of PE or PP, which is made by dry stretching or wet method (phase separation method), and then coated with alumina powder on its surface as a ceramic coating film, which can greatly reduce the shrinkage of the diaphragm.
, while improving the puncture resistance and liquid absorption rate, effectively improving the safety performance of the battery
.

At present, there is also a method to coat the Al2O3 powder on the battery pole piece, which is also beneficial to improve the safety performance of the battery
.

Ceramic diaphragm

Reference source:

Application of low sodium α-Al2O3 in the field of electronics, Li Jianzhong, Zhang Yong, Li Jinfeng (Chalco Zhengzhou Nonferrous Metals Research Institute Co.
, Ltd.
)
.