Although spherical silicon micro powder is spherical and has good fluidity, there are still problems of poor compatibility and difficult dispersion when used as an inorganic filler mixed with organic matrix materials. Therefore, surface modification is necessary to solve these problems.

Surface modification of spherical silicon micro powder involves introducing specific functional groups or coatings onto the surface of the powder to alter its surface properties, improve its dispersibility and flowability in matrix materials such as resin, rubber, and plastic, enhance its compatibility with the matrix material, and ultimately improve the performance of composite materials. At the same time, surface modification can introduce functional groups with specific functions on the surface of spherical silicon micro powder, generating new physical, chemical, and mechanical properties, and expanding the functionality of spherical silicon micro powder in specific applications. Therefore, surface modification technology of spherical silicon micro powder is a key step to improve its application performance in composite materials.

At present, the surface modification technologies of spherical silica micro powder mainly include organic modification, chemical corrosion modification, and polymer grafting modification.

1. Organic modification
Organic modification refers to the introduction of functional groups from organic compounds on the surface of spherical silica micro powder, which changes its surface properties and significantly improves the interfacial bonding between spherical silica micro powder and organic matrix, enhancing the dispersibility and flowability of spherical silica micro powder in organic matrix. Organic modified spherical silica micro powder has significant advantages in enhancing the compatibility between spherical silica micro powder and organic matrix, improving flowability, and can meet high-end application needs.

Due to the wide variety of organic modifiers, in-depth theoretical research on modification mechanisms, mature modification processes, and easy operation and control, they have been widely studied and applied in the industrial field.
There are many types of organic compounds used for organic modification, among which the most commonly used is silane coupling agent. Silane coupling agent has a wide range of types, the widest application range, and the most complete industrial system.
The molecular structure of silane coupling agents contains functional groups such as epoxy, vinyl, and amino groups that interact with organic compounds, as well as alkoxyl groups that can hydrolyze and bind with hydroxyl groups on the surface of silicon micro powders. According to the theory of chemical bonding, one end of silane coupling agent reacts with the functional groups of organic compounds, and the alkoxy group at the other end hydrolyzes and interacts with the hydroxyl groups on the surface of silicon micro powder. During this process, the silane coupling agent undergoes hydrolysis, condensation, and forms hydrogen bonds, ultimately forming a strong silicon oxygen covalent bond with the silicon micro powder.
SAT NANO et al. used three types of silane coupling agents (vinyltrimethoxysilane, 3-methylacryloyloxypropyltrimethoxysilane, and 3-glycidoxypropyltrimethoxysilane) to modify the surface of spherical silicon micro powders, generating stable organic silicon solutions at high concentrations. They then mixed them with epoxy resin and compared the modification effects of these three silane coupling agents; Apply modified spherical silicon micro powder as a filler in copper-clad laminates and study the effect of modified spherical silicon micro powder on the properties of copper-clad laminates. Research has shown that the performance of copper-clad laminates filled with spherical silica micro powders modified with 3-methylacryloyloxypropyltrimethoxysilane is the best. The vinyl and alkyl groups in 3-methylacryloyloxypropyltrimethoxysilane are bonded to the surface of spherical silica powder, exhibiting excellent dispersion stability at high spherical silica powder content. Compared with the original epoxy resin, the tensile strength of the spherical silicon micro powder epoxy resin composite material has increased by 38%. Compared with the copper clad laminate without spherical silicon micro powder, the copper clad laminate containing spherical silicon micro powder has a larger storage modulus, higher glass transition temperature, and stronger peel strength.
SAT NANO et al. used bis - [3- (triethoxysilicon) propyl] - tetrasulfide and 3-octylthiol-1-propyltriethoxysilane to modify spherical silica powder. Research has found that with the addition of silane coupling agents, the optimal curing time of composite materials is shortened and the burning time is prolonged. The wet slip resistance of the composite material was improved by 14.0% when modified with bis (3-triethoxysilylpropyl) - tetrasulfide, while the rolling resistance of the composite material was reduced by 19.6% when modified with 3-octanoylthio-1-propyltriethoxysilane, improving processing safety. This is because after modification with silane coupling agent, the flowability and compatibility with the matrix material of spherical silica micro powder have been improved, enhancing the interaction with the rubber matrix.
Due to the differences in functional groups of different silane coupling agents, when two or more types of modifiers are combined to modify spherical silica micro powders, the modification effect is often better than that of a single modifier.
SAT NANO and others synergistically combined three different types of silane coupling agents (γ - aminopropyltriethoxysilane, γ - glycidoxypropyltrimethoxysilane, and phenyltrimethoxysilane) to achieve the desired functionalization, synthesizing spherical silica micro powders with an average particle size of 700nm modified with three functional groups. The research results indicate that these modifiers endow reactive functional groups, including epoxy and amino functional groups, to spherical silica micro powders, enhancing the interfacial compatibility and adhesion between spherical silica micro powder fillers and epoxy resin matrix. In addition, the introduction of non reactive phenyl groups helps to reduce the viscosity of resin composite materials. These modified spherical silica micro powders are integrated into the sealant formula, exhibiting excellent reliability and processability. Compared with the sealant prepared with unmodified spherical silica micro powder, the modified sealant has a 95% decrease in moisture permeability, an increase in glass transition temperature of about 30 ℃, and a decrease in thermal expansion coefficient of 5 × 10-6 ℃ -1.
SAT NANO et al. investigated the effect of co modification of dimethyldimethoxysilane and trimethylethoxysilane, methyltrimethoxysilane and propyltrimethoxysilane on spherical silica micro powder epoxy resin composites. They found that modified spherical silica micro powders can significantly enhance hydrophobicity, dispersibility, and compatibility with epoxy resins. Among them, the enhancement effect of spherical silica micro powders co modified with methyltrimethoxysilane and propyltrimethoxysilane is the best. Compared with unmodified spherical silicon micro powder, the thermal conductivity of the composite material of spherical silicon micro powder modified by methyltrimethoxysilane and propyltrimethoxysilane with epoxy resin increased by 42.6%, and the tensile strength increased by 28.0%. In addition, the mechanical properties such as bending strength, impact resistance, and tensile strength of modified composite materials exceed those of unmodified spherical silica micro powder epoxy resin composite materials.

2. Chemical corrosion modification
Due to the smooth surface and strong chemical inertness of spherical silica micro powder, there are often situations where the effective composite rate of the modifier is low and the modification effect is poor during modification. Chemical corrosion modification is the use of highly corrosive reagents to etch the surface of spherical silicon micro powder, thereby changing the surface morphology or properties of the spherical silicon micro powder, generating more active sites, and improving the modification effect.
SAT NANO treated spherical silicon micro powder with a sodium hydroxide solution at a temperature of 95 ℃, and studied the influence of surface characteristics of spherical silicon micro powder on the performance and structural characteristics of photocatalysts (titanium dioxide, cadmium sulfide), as well as the interaction at the interface between semiconductor and carrier. The results showed that after treatment with sodium hydroxide, the activity and hydroxylation of the surface of spherical silicon micro powder were enhanced, thereby increasing the anchoring points and dispersibility of nanoparticles, enhancing the charge separation and interface interaction between the semiconductor and the carrier, inducing the directional transport of photogenerated carriers, and improving the photocatalytic performance of the photocatalyst.

SAT NANO first uses sodium hydroxide to etch the surface of spherical silicon micro powder, and then modifies it with silane coupling agent. The research results indicate that after etching with sodium hydroxide, the surface morphology and hydrophilicity of spherical silicon micro powder have changed. The surface of spherical silicon micro powder after etching with sodium hydroxide has many etching pits and an increase in surface roughness; With the extension of etching time, the contact angle of spherical silicon micro powder gradually decreases, indicating that the hydrophilicity of spherical silicon micro powder is enhanced. After treatment with sodium hydroxide, the spherical silicon micro powder was modified with gamma methacryloyloxypropyltrimethoxysilane and gamma glycidoxypropyltrimethoxysilane, respectively. Compared with the modification effect of directly using gamma methacryloyloxypropyltrimethoxysilane and gamma glycidoxypropyltrimethoxysilane, the impact strength of the composite sample was increased by 2.5% and 21.6%, the bending strength was increased by 18.2% and 25.9%, and the thermal conductivity was increased by 6.32% and 11.82%, respectively. The research results indicate that after etching with sodium hydroxide, more active sites are formed on the surface of spherical silicon micro powder, which can improve the surface modification effect of silane coupling agent on spherical silicon micro powder.

In summary, after surface modification, the dispersibility and flowability of spherical silica micro powder in organic matrix will be significantly improved, thereby enhancing the performance of composite materials.

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