Refactory is any material that can withstand high temperature, without softening or suffering a deformation in shape.

Carborundum (SiC) bricks:

Carborundum bricks belong to the class of neutral refactory.

Preparation:

1. SiC is made in an electric furnace at a temperature of $1500^\circ C$, from sand (60%) and coke(40%) with the addition of some saw dust and little salt..

2. Saw dust is added to increase porosity when burned. Saw dust evolves gases which by circulation increases the porosity of the charge.

3. Salt is added because it reacts with iron and similar impurities forming volatile chlorides.

4. SiC in form of interlocked crystals are formed.

5. These are mixed with bonding agents (<10%) like plastic, fire clay, graphite, etc.

6. After missing with bonding agent, final firing is done in a reducing atmosphere, at about $1500^\circ C$.

7. Self-bonding type SiC refractory bricks results when SiC particles are mixed with glue and fired at $2000^\circ C$

Properties:

1. ‘Clay-bonded’ SiC can be used upto $1750^\circ C$ as this product softens near this temperature.

2. ‘Silicon-nitride bonded’ bricks have a high strength and superior thermal shock resistance than clay-bonded products.

3. ‘Self-bonded’ products have high refractoriness, superior strength, high density, high chemical resistance and high abrasion resistance.

Uses:

1. SiC are mainly used in muffle furnaces because of their good thermal conductivity.

2. It is also an ideal choice for recuperators because of good resistance to spalling under repeated temperature fluctuations and ability to absorb and release heat rapidly.

3. Furnace heating elements (Globars) are also made by SiC refractory