Beryllium oxide (BeO) is unique for oxides as it combines excellent electrical insulating properties with high thermal conductivity. It is also corrosion resistant. The high toxicity of the beryllium oxide powders when inhaled, and the high cost of the raw material, has limited its use to applications that exploit its singular properties.
Beryllium oxide is extracted from the naturally occurring minerals beryl and bertrandite, and produced as a powder by the thermal decomposition of Be(OH)2. Powders are commercially available at purity levels of greater than 99%. Components can be made as near net shapes by most of the commonly used fabrication methods, for example pressing, slip casting or extruding the powder. Sintering is carried out in the range 1600 – 1800 ºC. High density components (<5% porosity) can be easily made with commercially pure powders. Near theoretical density (<1% porosity) can be achieved using high purity materials and hot pressing in graphite dies.
Beryllium oxide is one of the most expensive raw materials used in ceramics. The expense is linked in part to the precautions to avoid the toxic effects of the powder when handling during fabrication. Inhalation of fine particles of beryllium oxide results in respiratory disease, with the severity related to the length of exposure.
Beryllium oxide has an outstanding combination of physical and chemical properties.
Apart from reactivity with water vapour at high temperature (1000ºC), it is one of the most chemically stable oxides, resisting both carbon reduction and molten metal attack at high temperatures.
Points worth noting about its properties include:
• Thermal conductivity is extremely high in comparison with other ceramics, particularly below 300 ºC. For comparison the thermal conductivity of beryllium oxide at room temperature is 300 W.m–1K-1, copper is 300 W.m–1K-1 and alumina is 35 W.m–1K-1.
• Electrical resistivity is high. BeO is classed as an electrical insulator.
• Mechanical strength is normally lower than alumina, but can reach acceptable levels through control of the fabrication process.
• BeO has good thermal shock resistance if the component has good strength due to the high thermal conductivity.
• Beryllium oxide has lower density than aluminium oxide; 3010kg.m-3 and 3970 kg.m-3 respectively.
• The thermal expansion of BeO is similar to that of other oxides.
Other physical and mechanical properties for beryllia are summarised in table 1.
Table 1. Typical physical and mechanical properties for beryllia.
|Density kg m-3
|Young’s Modulus (GPa)
|Thermal Conductivity @ 0 ºC, (W.m–1K-1)
|Maximum Service Temperature (ºC)
BeO is most often used as an electronic substrate, exploiting its high thermal conductivity and good electrical resistivity to give an effective heat sink. The material is found particularly in high power devices or high density electronic circuits for high speed computers.
Since BeO is transparent to microwaves, the material may be used as windows, radomes and antennas in microwave communication systems and microwave ovens.
Similarly, since the material is transparent to x-rays, it can be used as an x-ray window, particularly for severe operating conditions.
It can also be used in high-power laser tubes.
Much of the original development work into beryllium oxide was initiated by interest from the nuclear industry. BeO has specific nuclear properties, which make it attractive for nuclear applications; low neutron capture cross section and high neutron moderating ability. However, although it has been extensively evaluated for use in high temperature gas cooled reactors, no civil nuclear power reactors are known.
The low density of BeO makes it attractive for aerospace and military applications such as gyroscopes and armour.
Resistance to molten metal has led to its use as a refractory in metallurgical applications for melting rare earths.
Primary author: Ceram Research
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