What is the effect of adding cellulose on the porosity of honeycomb ceramics? ​

1、 Core influencing mechanism: "pore forming effect" of cellulose

Cellulose is used as an organic pore forming agent in the preparation of honeycomb ceramics, and its effect on porosity is mainly achieved through the principle of "high-temperature burning loss and pore retention". The specific process is as follows:

Molding stage: After mixing cellulose and ceramic powder (such as cordierite, alumina, etc.) evenly, a honeycomb shaped body is formed by extrusion molding. At this time, cellulose is dispersed in the form of solid particles in the body, occupying a certain space.

Sintering stage: During the high-temperature sintering process, cellulose will completely burn and decompose (usually in the range of 200-600 ℃), and eventually evaporate in the form of carbon dioxide, water vapor, etc. The space it originally occupied will not be filled by ceramic powder, thus forming a large number of pores inside the ceramic.

Porosity regulation: The amount of cellulose added directly determines the total amount of occupied space. Therefore, the more added, the larger the number and volume of pores left after combustion, and the higher the porosity of honeycomb ceramics.

2、 The specific effects of different addition amounts: from porosity to structural properties

The amount of cellulose added should be controlled within a reasonable range. Excessive or insufficient amounts can affect the comprehensive performance of honeycomb ceramics, which can be divided into three stages:

1. Low addition stage (below the lower limit of the critical value)

Porosity performance: The porosity is low and mostly small closed pores or micropores, which cannot meet the high porosity requirements of filtration, catalysis and other scenarios.

Structural performance: Cellulose occupies less space in the ceramic body, and the ceramic particles are tightly bonded. After sintering, the structure is dense and the mechanical strength is high, but the breathability and adsorption are poor.

2. Suitable addition stage (within the critical range)

Porosity performance: Porosity increases linearly with the increase of addition amount, and the formed pores are mainly connected pores with uniform pore size. The addition amount can be accurately adjusted according to the needs (such as porosity of 30% -60%).

Structural performance: The balanced distribution of pores and ceramic skeleton ensures a high porosity (meeting filtration and mass transfer requirements) while maintaining sufficient mechanical strength to avoid ceramic cracking or damage during use.

3. High addition stage (exceeding the upper limit of the critical value)

Porosity performance: The growth of porosity slows down, and even the phenomenon of "pore fusion" occurs, forming too large pores or through cracks, resulting in uneven pore structure.

Structural performance: Cellulose occupies too much space, reduces the contact area between ceramic particles, and cannot form a stable skeleton structure after sintering. The mechanical strength drops sharply, and even cracks and deformations occur during the sintering process, making it impossible to form.

3、 Key factors affecting the 'critical addition amount'

The critical addition amount of cellulose (i.e. the amount that can ensure high porosity without damaging the structure) is not a fixed value and is influenced by the following factors:

The inherent characteristics of cellulose: The particle size and shape of cellulose can affect its dispersibility. Cellulose with smaller particle size is more evenly dispersed, and the critical addition amount can be appropriately increased; Long fibrous cellulose is prone to aggregation and has a low critical addition amount.

Characteristics of ceramic powder: The particle size and specific surface area of the powder determine the binding ability between particles. Fine grained powder has a larger specific surface area, tighter particle bonding, and can withstand higher amounts of cellulose addition; Coarse particle size powder is the opposite.

Forming and sintering processes: The pressure and thickness of the billet during extrusion molding, as well as the heating rate and holding time during sintering, all affect the loss on ignition process of cellulose and the sintering densification of ceramic particles, thereby changing the critical addition amount. For example, slow heating can prevent rapid combustion of cellulose from causing cracking of the green body, indirectly increasing the critical addition amount.