Complete selection guide for dispersant of nano silicon nitride (Si ∝ N ₄) powder
I、 Difficulty in dispersing the core of silicon nitride powder (selecting the underlying logic)
1. The surface structure is quite unique
The surface of pure silicon nitride exhibits complex amphiphilic properties due to the coexistence of hydrophobic non oxide substrates and hydrophilic oxide hydroxyl groups, making it difficult for conventional oxide dispersants to form stable adsorption on its surface.
2. Extremely low isoelectric point (pH ≈ 4.2)
Under neutral or weakly acidic conditions, particles lose their electrostatic stability mechanism due to the approaching zero Zeta potential, leading to irreversible hard agglomeration; Only by constructing a strong alkaline environment or introducing a cationic dispersion system can effective dispersion be achieved by enhancing charge repulsion.
3. Nano powder shows strong surface activity due to its extremely high specific surface area, but it is prone to slow hydrolysis and oxidation reaction in aqueous medium, which leads to thickening of slurry and increase of oxygen content in the system, thus seriously interfering with the ceramic sintering process and damaging the performance of the final product.
4. The four major application systems of water-based ceramic slurry, alcohol/ester oil-based solvents, UV curable 3D printing, and ceramic digital inkjet cannot be used interchangeably due to significant differences in medium characteristics of their supporting dispersants.
II、 Oil/solvent based systems (ethanol/isopropanol/xylene, ceramic ink, traditional oil-based casting)
Solvent: Alcohol, alcohol ether, ester, aromatic hydrocarbon, significantly inhibits the hydrolysis and oxidation reaction of silicon nitride, meeting the demand for high-end products with extremely strict control of oxygen content.
1. Phosphate modified polymer dispersant (universal main type)
Recommended models: RK-4012, RK-4013A, RK-4013C, RK-4011
-Principle: Phosphonic acid groups achieve stable anchoring through strong coordination with Si OH and Si-N, while hydrophobic carbon chains provide three-dimensional protection through significant steric hindrance effects;
-Addition amount: Powder 1.5%~3%;
-Adaptation: Nano silicon nitride slurry with alcohol and ester solvents, balancing low viscosity and long-term anti settling, achieving long-term stability.
2. Polyether PVP (polyvinylpyrrolidone, low viscosity light colored system)
Non ionic substances, due to the absence of charged groups in their molecular structure, are generally electrically neutral and therefore do not rely on charge interactions to induce flocculation; Develop and design small precision equipment with short-term grinding capability for a small number of samples in the laboratory; Poor stability, not suitable for long-term storage alone and prone to precipitation. It needs to be used in combination with phosphate esters to enhance system stability
3. Fish oil/polyamide super dispersant (aromatic solvent, high solid slurry)
Model: RK-4029, RK-4042, RK-4064
Compatible with xylene and ethyl acetate system for casting slurry, solving the problem of stacking and layering of sheet-like silicon nitride
4. Pre treatment with silane coupling agent (high-end process is necessary)
RK-5212 (amino silane) and RK-5263 epoxy silane were first modified on the surface of the powder to significantly enhance the adsorption capacity of the oil-based dispersant. The slurry can be stabilized for 72 hours without delamination
III、 Water based systems (water-based casting, water-based grinding, water-based 3D printing)
Key requirements of the system
By adjusting the pH value of the system to a strong alkaline range of 9.5-11 to stay away from the isoelectric point, stable particle dispersion and effective inhibition of hydrolysis and oxidation are achieved while ensuring extremely low residual carbon content after sintering.
1. Ammonium polyacrylate (industry choice)
-Principle: In alkaline media, anionic polycarboxyl dispersants chemically adsorb on the silicon hydroxyl groups on the surface of silica powder, mainly relying on strong electrostatic repulsion to maintain dispersion stability, accompanied by weak steric hindrance effect.
-Adaptation: water-based dispersion system based on nano silicon nitride, extrusion molding granulation process, and ceramic surface glaze slurry.
-Model: RK-4008Y.
-Addition amount: Powder 0.7%~2%.
-Advantages: high solid content and low viscosity, very low residue after sintering, compatible with PVA and acrylic lotion binder.
-Disadvantages: Acidic environment leads to loss of function, and inkjet systems with high ethylene glycol content have poor adhesion and are easy to peel off.
2. Polyethyleneimine PEI (cationic dispersant, specially designed for ultra-fine and highly active nano Si ∝ N ₄)
-Principle: In a strongly alkaline environment, a large number of amino groups undergo coordination adsorption with Si-N and Si-OH groups, leading to the formation of materials. The surface charge density significantly increases, resulting in an extremely high absolute Zeta potential.
-Application: High solid content wear-resistant ceramic slurry based on highly active amorphous silicon nitride ultrafine nano powder.
-Addition amount: Powder 1%~2%.
-Advantages: It has excellent anti agglomeration performance and significantly surpasses polycarboxylate substances in preventing powder hydrolysis and oxidation.
-Disadvantage: High temperature degreasing leads to excessive residual carbon content, so this process should be strictly avoided in the preparation of high-end low oxygen ceramics.
3. Styrene maleic anhydride copolymer (high solid concentration slurry)
With a good multi-point anchoring mechanism, it can achieve strong viscosity reduction. It is specially designed for the high concentration silicon nitride slurry with a solid content of more than 50%, and perfectly fits the spray drying granulation process.
Recommended models: RK-4057, RK-4039ACB, RK-4059
4. Prohibit or only use auxiliary agents
-Prohibited: The increase of sodium ion impurities in sodium hexametaphosphate and low molecular weight sodium polyacrylate can lead to a decrease in the insulation performance and thermal conductivity of ceramic materials.
-Auxiliary compounding: Surface pretreatment of the powder with a small amount of aminosilane can enhance the adsorption stability and binding fastness of the dispersant on the particle surface.
Water based subdivision scenario
1. Low residual carbon advanced ceramic components with excellent mechanical properties and high thermal conductivity: ammonium polyacrylate;
2. High hardness ultrafine amorphous silicon nitride wear-resistant slurry: PEI cationic dispersant;.
3. Water based ceramic digital inkjet (including diol co solvent): Amphoteric amino polyurethane dispersant (resistant to high alcohol without desorption).
IV、 Ceramic digital inkjet system (water-based inkjet/weak solvent inkjet)
1. Waterborne ceramic inkjet (containing 30%+ethylene glycol/propylene glycol)
Hard requirements: It has the characteristics of resistance to binary alcohol solvent and low foam performance, fine particle size distribution (D50 is less than 150 nm), and can maintain smooth transmission in the long-term circulation system without blocking the nozzle.
Selection: Amino amphoteric polyurethane dispersant (RK-4039AB, RK-4044)
Ordinary ammonium polyacrylate is prone to desorption and short-term aggregation in high alcohol solvent environments, which can lead to nozzle blockage, and therefore should not be used alone.
2. Weak solvent ceramic inkjet printing (ester/ketone outdoor ceramic printing)
Selection: Oil soluble structured acrylic dispersant (RK-4039ACB, RK-4061CF), completely incompatible with water-based polycarboxylates PEI
V. UV cured 3D printed ceramic paste (DLP/UV cured ceramics)
System: Acrylic monomers/oligomers with high solid content (50%~65%), possessing low viscosity characteristics, can maintain no precipitation and stability during long-term cyclic use without affecting the efficiency of light curing reaction.
Special dispersant: polymer block super dispersant
1. RK-4013A, RK-4013C (industry benchmarks)
The addition amount of powder is 2.5%~4%, which has good compatibility and dispersibility in the UV resin system, can effectively reduce the viscosity of the slurry, ensure smooth and unobstructed printing process, and achieve high-quality molding without pores
2. RK-4039ACB, RK-4035AC water oil UV universal acrylic dispersant
Suitable for low viscosity transparent UV ceramic ink and small ceramic digital photopolymerization inkjet.
VI、 Accurately match according to the state of silicon nitride powder
1. Ordinary crystalline nano silicon nitride covered with a thin oxide layer on the surface
Water system: ammonium polyacrylate; Oil system: phosphate dispersant
2. High specific surface area nanoscale amorphous silicon nitride powder
Water system: polyethyleneimine PEI; Oil: Silane pretreatment+RK-4013A phosphate ester compound for use
3. Low oxygen high-purity silicon nitride (semiconductor heat dissipation substrate)
Prioritize alcohol solvents+phosphate dispersants to avoid mixing oxygen impurities into aqueous systems
4. Silicon nitride+yttrium oxide/aluminum oxide sintering aid composite slurry
Ammonium polycarboxylate with moderate high molecular weight and grafting density, a set of dispersants compatible with oxide+silicon nitride mixed powder
VII、 Key process control points
1. pH control (water-based core)
In an aqueous system, the isoelectric point of silicon nitride (Si ∝ N ₄) is pH 4.2, and it is necessary to adjust and stabilize the pH of the system between 9.5 and 11 using ammonia water or tetramethylammonium hydroxide to maintain dispersion stability; If the pH value is below 8, the dispersion system will quickly fail and cause severe particle aggregation.
2. Gradient test of dispersant addition amount (standard range of nano Si ∝ N ₄)
-Recommended dosage of water-based ammonium polycarboxylate dispersant: 0.6%/1.1%/1.5% (powder mass)
-Recommended dosage for oil-based phosphate dispersants and UV dispersants: 2%/3%/4%
Insufficient dosage of dispersant can lead to incomplete particle coating, which in turn can cause hard agglomeration; However, excessive use can increase the viscosity of the free dispersant system and lead to an increase in residual carbon content after degreasing.
3. Stability verification standards
1. After 72 hours of standing, the system remains stable without layering, and the trace precipitate at the bottom can be completely redispersed after shaking without clumping;
2. The particle size distribution shows a unimodal shape, with good dispersion and no abnormally large particles caused by secondary agglomeration;
The UV inkjet system maintained smooth printing during continuous 48 hour operation testing, without any nozzle clogging or ink crystallization.
