Zirconia Grinding and Dispersion

Contract Manufacturing for Grinding and Dispersion

After wet forming and sintering, the final products exhibit exceptional characteristics due to the fine and uniform grain structure and high density of the sintered body. These properties result in superior electrical, optical, magnetic, and mechanical performance compared to materials that have not undergone dispersion. Additionally, slurries produced after grinding and dispersion, when mixed with binders and applied as coatings, exhibit smooth surfaces without cracking, further enhancing their electrical, optical, magnetic, and mechanical properties. This underscores the crucial importance of grinding and dispersion processes.

Distinguishing Between Grinding and Dispersion

Grinding and dispersion are often confused, but they involve distinct processes. Grinding refers to the reduction of large primary particles or coarse secondary particles (aggregates) into smaller secondary or primary particles, or even finer primary particles. Dispersion, on the other hand, involves separating agglomerated primary or secondary particles caused by van der Waals forces (electrostatic forces) without necessarily reducing their size. In simpler terms, grinding involves breaking chemical bonds. Figure one illustrates primary, secondary, and tertiary particles, while figures two and three show scanning electron microscope (SEM) images of alumina powder after grinding, where agglomerates are dispersed without a change in primary and secondary particle size, and silica powder after grinding, where primary particles have been significantly reduced in size, representing typical dispersion and grinding examples. It is evident that dispersion requires less mechanical force but relies on a solvent-based process, generating more active surfaces that require the use of dispersants to prevent re-agglomeration, making it somewhat similar to grinding. Therefore, these processes are often discussed together due to their similarities.

The Importance of Dispersion

Many customers purchase powder materials, especially nanomaterials, and often fail to achieve their intended functionality due to inadequate dispersion prior to use.

Example 1: Sintering

Properly dispersed 0.3um Al2O3 powder (Figure one) can achieve 99% of theoretical density when wet-formed and sintered at 1390°C, with grain growth averaging only around 2um (Figure two). Conversely, non-dispersed powder (Figure three) processed using dry forming reaches a sintered density of 97.6% of theoretical density at 1600°C, with grain growth averaging around 5um (Figure four), along with abnormal grain growth.

Example 2: Bright Inkjet Coated Paper

Adequately dispersed 40nm AlOOH powder (Figure five), when mixed with PVA in appropriate proportions and coated on PE paper, results in a crack-free and highly glossy film (Figure six). In contrast, non-dispersed powder (Figure seven), directly mixed with PVA and coated on PE paper (Figure eight), leads to a cracked and dull coating.

Example 3: Transparent ATO Coating

Properly dispersed 40nm ATO powder (Figure nine), when mixed with PVA in suitable proportions and coated on glass, yields a highly transparent and uniform coating (Figure ten). Conversely, non-dispersed powder (Figure eleven), when directly mixed with PVA and coated on glass (Figure twelve), results in poor transparency and uneven coating.

Conditions for Grinding Materials

Whether a material can be ground depends on its hardness, with both hard and soft materials evaluated based on their Mohs hardness. Most zirconia grinding balls in use today are made of YSZ (yttria-stabilized zirconia), which has a Mohs hardness of approximately 9 (Knoop 1800). Materials with a Mohs hardness of 7 (Knoop 800) or higher are considered hard materials and, based on our experience, cannot be effectively ground. Materials with a Mohs hardness of 7 or lower are classified as soft materials and can be ground. As shown in figures two to four in the distinction between grinding and dispersion, hard materials like α-Al2O3 can only be dispersed and not ground, while soft materials like SiO2 can be effectively ground and refined. Table one provides hardness information for some common materials.

Material Formula Mohs Hardness Modified Mohs (Knoop)

Mechanical Forces

Figure one illustrates common grinding methods and their effects on particle size reduction. Grinding and dispersion employ ball milling and bead milling methods. Zirconia grinding balls, through mutual collisions and the shear forces generated by their stirring and rotation, refine the powder, as shown in figures two and three. The size to which the powder can be refined depends on the size of the zirconia grinding balls used. Presently, nano-grinding and dispersion machines utilizing 300um zirconia grinding balls can refine powders to sizes ranging from 100 to 200nm, while 50um zirconia grinding balls can refine powders to sizes between 30 and 80nm.

Dispersants

To maintain stable dispersions of powders, it is essential to prevent them from re-agglomerating and reducing their surface area (activity). During grinding and dispersion, when agglomerates or bonds between particles are broken, the surface area increases. While particles with like charges on their surfaces will repel each other, the potential is typically in the range of ±0-10mV, insufficient to prevent powder re-agglomeration. Moreover, the solvent present on the powder surface during re-agglomeration can become trapped between the particles, increasing system viscosity. For instance, Figure one shows the relationship between viscosity and potential when 50vol% alumina slurry is added with polyacrylate, highlighting that viscosity can exceed 10,000cps when no electrolyte/dispersant/surfactant is added.

Stability in dispersion is achieved by introducing suitable electrolytes, dispersants, and surfactants to ensure that the potential exceeds ±25mV. This effectively stabilizes the dispersion of powder sub-agglomerates and significantly reduces viscosity to below 50cps. Figures two and three provide a schematic illustration of powder surface adsorption of electrolytes, dispersants, and surfactants and their relationship with stability in dispersion. This phenomenon can be described using the Gouy-Chapman double layer theory, where powder surfaces adsorb electrolytes, dispersants, and surfactants with opposing electrical charges. Subsequently, adsorption decreases to normal levels through diffusion as the distance increases, forming the diffuse layer, resulting in the formation of the electric double layer, as shown in Figure four.