Random Talk on “Kiln Furniture” – “Behind the Scenes Workers” in the Ceramic and Lithium Battery Industries
11/06/2023 09:11:09

With the rapid development of industry, kiln furniture, as a special refractory material, has become an essential high-temperature auxiliary material product in the sintering process of ceramics, lithium battery and other industries.

Kiln furniture refers to refractory products that are recycled in industrial kilns to support or protect burned products. It is widely used in the production of daily ceramics, architectural ceramics, sanitary ceramics, and advanced ceramic materials. In recent years, with the rise of the new energy market, lithium battery positive electrode materials require a large number of high-performance kiln furniture in the production process, which has further attracted attention to kiln furniture.

In the process of use, kiln furniture is generally thinned to reduce heat storage and save energy, but it also needs to have a certain load-bearing capacity, so kiln   furniture needs to have high room temperature and high temperature strength; The rapid firing process of ceramics requires kiln furniture to have better thermal shock resistance; For electronic ceramics, in order to prevent product contamination, it is required that the kiln furniture have good chemical stability; Modern industry requires kiln furniture to have precise dimensions during mechanical operation to ensure normal production.

1、 Structural kiln furniture

Structural kiln furniture is an important component of industrial kilns. When in service, the temperature inside the furnace is high, and it is directly in contact with gas flames or radiated by heating elements for heat transfer. It usually bears more weight than itself, or even several times its weight without deformation or fracture. This requires it to have a certain degree of high-temperature mechanical strength and good thermal shock stability. In this type of kiln furniture, the main types are shed boards (beams and columns are usually used in conjunction with shed boards, represented by shed boards), push boards, and roller bars, with a large market capacity.

The application of pusher plate kilns in the ceramic industry is relatively common, with pusher plates being its basic accessory material. Usually, the performance of pusher plates determines the operational efficiency of the kiln. When the pusher kiln is running, it is required to withstand the load of the ceramic body or saggers, huge jacking force, and frictional resistance with the track without fracture at high temperatures. The cold and hot cycle service life can reach dozens or even hundreds of times. Therefore, the pusher kiln must have high room temperature, high temperature strength, wear resistance, and excellent thermal shock stability.

Traditional refractory materials such as corundum and mullite have many advantages, such as high load softening temperature and good creep resistance. However, pure corundum products have less ideal thermal shock resistance due to their high thermal expansion coefficient. The thermal expansion coefficient of mullite material is significantly lower than that of corundum, and it can be used as a secondary crystal or bonding phase in products. Therefore, corundum mullite based pusher plates combine the properties of both materials, especially the significantly improved thermal shock resistance compared to corundum based products. When used in pusher plate kilns for firing high-temperature ceramic materials, their service life can reach over 100 times.

Kiln uses the rotation of the rod to transport the billet, gradually completing the sintering process through preheating, sintering and cooling each belt. The advantages of a roller kiln are that the temperature inside the kiln is evenly heated up and down by the stick, the product firing cycle is short, and the fuel consumption is low. Ceramic rollers are key components of roller kilns and require a significant amount of consumption. It plays a load-bearing and transmission role in the continuous high-temperature firing of products. When used, it must not only be resistant to high temperature, but also have the characteristics of resisting high-temperature creep during long-term rotation. Ceramic rollers are mainly made of corundum, aluminosilicate, fused silica and silicon carbide. The materials of the silicon carbide roller rod include recrystallized crystal and reaction sintered silicon carbide.

2、 Sintered vessels (saggar, crucible, plate)

A specialized kiln tool that supports the firing of ceramic bodies, or a saggers that holds powders (such as positive electrode materials, magnetic powders, high-purity ceramic materials, etc.) and then undergoes heat treatment in roller kilns, pusher plate kilns, and tunnel kilns. Depending on the firing process used by the manufacturer, this type of kiln tool will withstand different heating conditions. The material of this type of product depends on the type of sintered body and the heat treatment process.

Contains powders (lithium battery positive electrode materials, magnetic powders, high-purity ceramic powders) and undergoes heat treatment in a roller kiln, pusher plate kiln, or tunnel kiln. It is generally formed using extrusion, machine pressing, pouring, and isostatic pressing processes, and suitable forming processes are selected based on the composition and structure of the product. The most widely used materials include cordierite mullite, corundum mullite, silicon carbide, and graphite, with lithium ion cathode materials being the most commonly used in the synthesis field.

Cordierite mullite saggers is widely used in the field of positive electrode materials for lithium batteries due to its excellent thermal shock resistance and economy.

The cordierite mullite crucible is based on lithium carbonate/lithium hydroxide, which has strong alkalinity, low melting point, and strong corrosiveness to acidic refractory materials. The lifespan of aluminum silicon based saggers is generally lower. Corundum based saggar is mainly used for the calcination of high-purity powders in environments with low thermal shock conditions and high operating temperatures, such as high-purity aluminum oxide powder. The saggar needs to be fired at a high temperature of 1800 ℃, and is produced using aluminum oxide active powder with 99.9wt% Al2O3 content and low sodium white corundum. The binder uses low ash content (Ash ≤ 0.01wt%) to ensure effective impurity control in the entire raw material control, And achieve a lower coefficient of thermal expansion through sufficient high-temperature firing.

Graphite and silicon carbide saggers have high thermal conductivity, high temperature resistance, excellent thermal shock resistance, and poor oxidation resistance. However, they have excellent alkali corrosion resistance in reducing atmospheres. Graphite saggerss are commonly used as containers for loading high-temperature sintered materials under reducing atmosphere, and are used for sintering lithium iron phosphate and electromagnetic materials. Traditional graphite saggerss= are produced through machining, with low efficiency and high cost. Silicon carbide saggers are widely used in industries such as pharmaceuticals, fine chemicals, engineering metallurgy, and acid pickling.

When the setter/ceramic plate is in service, it needs to withstand the thrust during movement and the friction during product loading and unloading. It does not crack during cold and hot cycling. Under the condition that the thermal shock resistance of the setter/ceramic plate meets the requirements, improving the bending and cracking resistance of the setter/ceramic plate is the key. The material of the fired board requires excellent chemical inertness and does not react with the fired product. The materials of the setter/ceramic plate are divided into aluminum oxide, zirconia, composite, etc., mainly used in the fields of electronic ceramics, special ceramics, etc.

Corundum fired plate refers to the main crystal phase α- The high-end kiln furniture of Al2O3 has excellent properties such as high strength, corrosion resistance, high temperature resistance, and wear resistance. It has small deformation at high temperatures (>1650 ℃), but high sintering temperature and poor thermal shock stability. During the firing process of lead zirconate titanate piezoelectric ceramics, the corundum based firing plate faces phenomena such as center warping, surface layer powdering, and peeling.

Many kiln furniture production enterprises apply plasma spraying technology to the preparation process of fired plates, where the intermediate layer of the fired plates is made of corundum mullite material and the outer layer is coated with zirconia fusion coating, which has high thermal shock resistance and does not react or adhere to the fired laminated ceramic capacitors.



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