Classification of Polysilazanes (PSZ)

Polysilazanes feature a backbone of -[SiR₁R₂-NR₃]ₙ- and are primarily classified into four categories based on side-chain substituents, molecular chain configuration, physical form, and ceramic precursor type.

I. By Side-Chain Substituents (Most Common Classification)

1. Perhydropolysilazane (PHPS) (Inorganic Polysilazane)

Structural formula: [-SiH₂-NH-]ₙ. All substituents on the silicon and nitrogen atoms are hydrogen; the material contains no carbon, consisting solely of silicon, nitrogen, and hydrogen.

It exhibits extremely high reactivity, reacting upon contact with water vapor and converting into silicon dioxide or silicon oxynitride ceramic films at low temperatures. Key applications include semiconductor insulating coatings, device passivation layers, and low-temperature inorganic protective coatings capable of withstanding high temperatures.

2. Organopolysilazane (OPSZ)

Organic functional groups—such as methyl, vinyl, or phenyl—are attached to the silicon or nitrogen atoms.

Compared to PHPS, OPSZ offers superior stability and storage characteristics, as well as greater film flexibility; upon high-temperature pyrolysis, it yields SiCN or SiCNO ceramic systems.

Common subtypes:

Methylpolysilazane (MPSZ): Features methyl groups as the primary substituents; offers excellent hydrophobicity and is the most widely used organopolysilazane in industrial coatings.

Vinylpolysilazane: Contains reactive vinyl groups, enabling cross-linking and curing via hydrosilylation reactions.

Phenylpolysilazane: Incorporates phenyl groups, significantly enhancing the resin's heat resistance and thermal stability.

3. Polysiloxazane

A type of hybrid polysilazane containing both silicon-nitrogen (Si-N) and silicon-oxygen (Si-O) bonds in the molecular chain. Typically produced through the partial hydrolysis of polysilazane, it yields SiOCN multi-component hybrid ceramics upon pyrolysis.

II. By Molecular Chain Topology

Linear Polysilazane: Features a straight-chain backbone structure; characterized by low viscosity and excellent solubility, it is typically liquid at room temperature. Branched/Cross-linked Polysilazanes: Contain trifunctional silicon units, feature highly branched structures and high molecular weights, offer high ceramic yields, and typically exist as solid resins.

Cyclic Polysilazanes: Small-molecule cyclic oligomers with low molecular weights; often serve as intermediate raw materials for polysilazane synthesis.

III. By Physical Form

Liquid Polysilazanes: Low- to medium-molecular-weight resins; soluble in organic solvents; compatible with processes such as spraying, dip-coating, and spin-coating; primarily used for preparing functional coatings.

Solid Polysilazanes: High-molecular-weight, highly branched structures; typically in powder or solid form; primarily used as ceramic matrices, precursors for ceramic fibers, and matrix resins for composite materials.

IV. By Ceramic Pyrolysis Products (Precursor Classification)

Si₃N₄ Precursors: Primarily perhydropolysilazanes; yield silicon nitride ceramics upon high-temperature pyrolysis in an oxygen-free environment.

SiCN Precursors: Various organopolysilazanes; yield silicon-carbon-nitrogen ceramic materials upon pyrolysis.

SiOCN Precursors: Polysiloxazanes; oxygen-containing hybrid systems; yield multi-component silicon-oxygen-carbon-nitrogen ceramics upon pyrolysis.

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