Ceramic Crucibles: An Introduction

Ceramic crucibles have essential uses across industries like glass, metals, electronics, chemicals, and aerospace. Five common types are quartz, alumina, boron nitride, zirconia, and yttria crucibles. In short, specialized ceramic crucibles serve diverse industrial applications.

Ceramic Crucibles of different materials

Quartz Ceramic Crucibles

Quartz ceramic crucibles, made from high-purity fused quartz (silica content ≥99.9%), have a fine structure, low thermal conductivity, and low thermal expansion. This gives quartz crucibles excellent thermal shock resistance, electrical properties, and chemical stability. Quartz crucibles come in square and cylindrical shapes. With the solar energy boom, they now see heavy use producing polysilicon for solar cells. Thus, demand is surging for large, thin-walled quartz crucibles, representing major global market potential. In summary, quartz ceramic crucibles offer specialized properties that make them ideal for polysilicon production and other applications.

Alumina (Aluminum Oxide) Crucibles

Alumina crucibles, made of aluminum oxide, resist melting, high heat, acids, bases, and corrosion. They work well for melting weakly basic substances like anhydrous Na2CO3 but not highly reactive ones like Na2O2 and NaOH. Alumina crucibles with 99.70% purity have excellent high-temperature insulation and strength from 1650-1700°C, briefly up to 1800°C. Available in various sizes and shapes, alumina crucibles suit applications needing high-purity corrosion resistance at extreme temperatures. In summary, alumina's properties make it an ideal crucible material for many demanding uses.

Boron Nitride Crucibles

Common types of boron nitride include cubic boron nitride (C-BN) and hexagonal boron nitride (hBN).Crucibles are typically composed of hBN, which offers excellent heat resistance, thermal stability, thermal conductivity, and high-temperature dielectric strength.

 Thanks to its outstanding chemical stability, hBN can resist corrosion from most molten metals. Given its high-temperature insulation, high thermal conductivity, and low thermal expansion properties,hBN crucibles are ideal for stringent conditions, such as semiconductor manufacturing processes. They are commonly used in the smelting of metals and semiconductors and can operate at up to 1800℃in a vacuum and 2100℃under atmospheric protection, typically nitrogen or argon gas.

 hBN ceramic material is soft and machinable. There are two major pass to make the hBN crucibles.

Hot pressing: BN powder is placed in hot presser and be sintered to form boron nitride solid. This process usually obtains a big blank that could be machined into different shapes. This type of material is called hot pressing boron nitride (HPBN or HBN)

Chemical vapor deposition: It use BCl3(gas) and NH3 to react in high temperature and form a layer of about 1mm thick boron nitride on the surface of mold (usually graphite). So the shapes are limited, when the purity could be extremely high, due to the nature of CVD process. Usually, people call this type of hBN material pyrolytic boron nitride (PBN)

PBN Crucibles

Zirconia Crucibles

Zirconia has a higher melting point than zircon, reaching up to 2700℃, making it one of the best refractory materials in nature.Crucibles made of zirconiacan successfully melt platinum group metals such as platinum, palladium, ruthenium, and cesium and their alloys.

While zirconia is pricier than alumina, it is a unique oxide refractory material that other oxides can't replace when used in high-temperature kilns above 2000℃.

Yttria Crucibles

Yttria (Y2O3) ceramics boast excellent heat resistance, corrosion resistance, and high-temperature stability. With a melting point over 2400°C, yttria resists reacting with reactive metals like Ti and Al at high temperatures. Thus, high-purity yttria crucibles can potentially melt Ti alloys and other oxygen-sensitive materials. However, yttria's high melting point makes production difficult, often requiring Hot Isostatic Pressing (HIP). Notably, yttria is brittle and can crack if heated/cooled too quickly beyond 5°C/min. In summary, yttria ceramics offer specialized high-temperature capabilities but require careful handling during manufacture and use to avoid thermal shock damage.

Summary

The diverse ceramic crucibles offer high-temperature stability, chemical resistance, and thermal control. This makes them vital in many industrial processes across sectors. In short, tailored ceramic crucibles serve crucial roles enabling modern manufacturing.