Alumina Ceramics Serving as Insulation Materials

What is Insulation Materials?

In electrical circuit or electronic circuit, insulation materials are used to isolate conductors physically based on design requirements to prevent current flow between them and damage the operation of the circuit. Additionally, the insulation materials can serve as mechanical support of the conductor, be used for heat dissipation as well as circuit environmental protection.

First and foremost prerequisite is that insulating materials do not conduct electricity, which means the electrical resistivity is required as high as possible.

How to clarify them?

To be an excellent insulating ceramic, insulating materials must have the following properties:

--Volume resistivity （ρ）≥1012 Ω·cm

--relative dielectric constant （ε）≤30

--good thermal conductivity, and thermal expansion coefficient should be similar to that of conductor materials with good performance in heat resistance, high strength and chemical stability.

Know-hows about insulation materials you should take a look

Solid energy band theory can be used to explain the insulation, semi-conductivity and electric conductivity of solid.

valence band: the most powerful energy band that is not filled by electronics

Forbidden energy band: It is between valence band and conduction band.

If the forbidden energy band width is large enough (more than a few eV), the electrons in the full band can hardly be excited beyond the gap and enter the conduction band, that is, it is considered that the electrons can hardly migrate, then the solid is a typical insulator.

The ideal insulator can only be obtained at absolute zero degree centigrade. When external conditions change, like when the temperature rises or it is exposed under sunlight, part of the electrons in the valence band can be activated and transited to the conduction band due to thermal stimulus, and then become conductive thereby, similar to semiconductor at high temperature, different only in that the forbidden energy band width is greater than that of the semiconductor insulators. Activating electrons is energy-intensive since the electrons won’t migrate at room temperature.

Many insulating ceramics are typical ionic or covalent crystals, and the conductivity behavior of ion diffusion needs to be considered. The existence of alkali metal ions (especially sodium ions) should be avoided as much as possible in insulating ceramics, which can lead to strong conductance and lower the insulating properties of ceramics.

Width of Forbidden Energy Band of ceramics (Eg)

MATERIALS	BINDING BOND	EG/EV	MATERIALS	BINDING BOND	EG/EV
SI	COVALENT BOND	1.1	TIO2	IONIC BOND	3.05-3.8
GAAS	COVALENT BOND	1.53	ZNO	IONIC BOND	3.2
DIAMOND	COVALENT BOND	6	AL2O3	IONIC BOND	10
BATIO3	IONIC BOND	2.5	MAO	IONIC BOND	〉7.8

Common Insulation Materials

1.Insulating ceramics:

A. Oxides: SiO2, Al2O3

B. Non-oxides: Si3N4、BN、AIN

C. Single crystal insulating ceramics, such as silicon, synthetic mica (general name for laminated structure aluminum silicate ), spinel, artificial sapphire, etc.

(1)Common electric ceramic

The common high pressure ceramic and the feldspar ceramics and mullite ceramics used in the resistor matrix are collectively referred to as the common electric ceramics.

Name                                        Chemical composition                                    Phase composition

Regular electronic ceramic            SiO2 68~72%                                          Feldspar glass phase: 45~60%

                                                      Al2O3 20~24%                                           Mullite phase:20~40%

                                                 (K2O+NA2O)3.5~5%                                        Residual quartz

Feldspathic ceramic                     Similar as above                                             same as above

celsian ceramic                             SiO2 55%；Al2O3 32%;                          Mullite, quartze variation; Barium glass, barium feldspar

                                   BaO 10.3%; CaO 2.3%; (K2O+NA2O)3.5~5% 

(2)Alumina ceramics

Corundum ceramics: ceramics with over 99% of Al2O3 with the main phase of corundum;

Corundum-mullite ceramics: the main phase is corundum and mullite with over 75% of alumina

Mullite ceramics: the main phase is mullite with about 50% of Al2O3

The electrical and mechanical properties of alumina ceramics improve with the increase of Al2O3 content.

α-Al2O3: Tripartite crystal system, with the most compact structure, low activity and high temperature stability, is the most stable crystal of all. It is of the best electrical performance with excellent mechanical and electrical properties. It is the only natural phase.

β-Al2O3：This is a polyaluminate mineral with high Al2O3 content.

γ-Al2O3：It is of spinel structure with low density, high temperature instability, poor mechanical and electrical performance. Porous materials can be made out of its loose structures.

(3)Magnesia Ceramics

Name                                         Main Phase                                      Raw Materials                                           Performance

Steatite Ceramics                      MgO·SiO2                                Steatite/clay/Barium carbonate                   Low dielectric loss, easy processing, low cost;  

                                                                                                                                                                       but easy to age, a narrow sintering temperature range

Forsterite Ceramics                   2MgO·SiO2                              Magnesite/Magnesium carbonate               Low dielectric loss and high resistivity

Cordierite Ceramics             2MgO·Al2O3·SiO2                          Steatite/clay/alumina/spinel                        Low thermal expansion coefficient, 

                                                                                                                                                                        but with poor electrical performance

Aluminium Spinel                       MgO·Al2O3                       Alumina/Magnesite/ trace clay/B2O3/CaF2       Dielectric loss outperforms steatite ceramics, 

                                                                                                                                                                        but          with higher dielectric constant

Application of Common Insulating Ceramics

1.In 1855, Ceramic insulators are used as electrical insulation materials in railway communication lines.

2.In 1880, the United States began to use ceramic insulators in electric power transmission lines.

3.At present, high-performance ceramic insulators for ultra high voltage transmission with a voltage of more than 500keV is successfully made.

4.Automotive ceramic spark plug, Ignition pins for gas cookers

5.Ceramic packaging material

6.Ceramic tube fuse

7.Alumina ceramic substrate

8.Ceramic casing for rectifier tube

9.Ceramic tube seat

Integrated circuit, an ultra small and high density circuit, assembles a large number of micro transistor components on a substrate, usually packaged in the integrated circuit shell.

In electronic industry, Al2O3, BeO, SiC, AIN, monocrystalline silicon insulating ceramics are used as substrates and packaging materials for integrated circuits.