How is fumed silica made?

Vaporized SiCl₄ reacts in a hydrogen-oxygen flame at >1500°C to form SiO₂ and HCl. Particles nucleate, grow into 7–40 nm spheres, fuse into branched aggregates, then loosely bond into agglomerates.

Fumed silica vs precipitated silica?

Fumed silica is made via gas-phase flame hydrolysis — higher purity (>99.8%), lower moisture, non-porous particles. Precipitated silica is wet-process — lower purity (~90%), porous, higher silanol density, lower cost.

What are the main applications?

Rheology modifier in coatings/adhesives, reinforcing filler in silicone rubber, powder flow aid, anti-settling agent in paints, thermal insulation core material, gel electrolytes in batteries.

Fumed Silica: Complete Technical Guide — Synthesis, Morphology & Applications (2026) - SEMITECH

Q: What is fumed silica?

Fumed silica is a synthetic amorphous SiO₂ made by flame hydrolysis of SiCl₄ at >1500°C. It has >99.8% purity, 7–40 nm primary particles, 50–400 m²/g surface area, and extremely low bulk density.

What Is Fumed Silica?

A synthetic, ultra-pure amorphous silicon dioxide made by burning SiCl₄ in a hydrogen flame at over 1500 °C.

Fumed silica — also called pyrogenic silica — was first commercialized in 1944 by Degussa (now Evonik) under the AEROSIL® brand. Nanoscale primary particles, extremely high surface area, and zero porosity make it one of the most versatile functional additives in materials science.

Trade names: HJSIL® (Minmetals East) · SEMISIL® (Semitech) · HMMAT® Fumed Matting Agent

Appearance

Looks like fresh snow — ultra-light, fluffy white powder. Bulk density can be as low as 20 g/L. Becomes airborne with the slightest air current.

Fig 1 — Pure fumed silica powder.

TEM Image of Fumed Silica showing 0.1 micron scale fused aggregate chains

Fig 2 — Texture detail, 130 m²/g grade.

How Fumed Silica Is Made

Vaporized SiCl₄ enters a hydrogen–oxygen flame (>1500 °C) inside a burner reactor. The SiO₂ formed is cooled, separated by cyclone filters, deacidified, and packaged. The HCl byproduct is recovered for reuse.

Particle Formation — 4 Stages in Milliseconds

Proto-particles

<1 nm

SiO₂ molecules nucleate in the flame core

Primary Particles

7–40 nm

Dense, non-porous spheres form

Aggregates

100–500 nm

Molten primaries fuse into permanent chains

Agglomerates

1–200 µm

Aggregates bond loosely via H-bonds

Fig 3 — Flame hydrolysis: from SiCl₄ feed to agglomerate product.

Process levers: Flame temperature, SiCl₄ feed rate, residence time, and H₂/O₂ ratio control particle size and surface area.

Three-Tier Nano-Morphology

Every performance property traces back to this three-level structure. Understanding it is the key to selecting the right grade.

Level	Size	Bond	Reversible?
Primary Particle	7–40 nm	Covalent (bulk SiO₂)	No
Aggregate	100–500 nm	Sinter bridges (Si–O–Si)	No
Agglomerate	1–200 µm	Hydrogen bonds (Si–OH⋯OH–Si)	Yes

Aggregates are the functional unit — permanent chains that give fumed silica its thickening and reinforcing power. Agglomerates break apart under shear and re-form at rest — the origin of thixotropy.

Key Properties

>99.8%

SiO₂ purity

7–40 nm

Primary particle

50–400

m²/g BET surface

2.2

g/cm³ true density

Property	Value
Bulk density (untamped)	20–60 g/L
pH (4% in water)	3.7–4.5
Loss on drying (105 °C, 2h)	<1.5 wt%
Loss on ignition (1000 °C, 2h)	<2.0 wt%
Silanol density	~2–3 OH/nm²
Refractive index	1.46

Surface Chemistry

Surface silanol groups (Si–OH) drive hydrophilicity, hydrogen-bond networking, and polymer interaction. Hydrophobic grades replace –OH with –CH₃ for non-polar systems.

Applications

Rheology Control

Thickening, thixotropy, anti-sag in coatings, adhesives, sealants, inks, UPE resins.

Silicone Reinforcement

Tensile strength, tear resistance, hardness in HTV/RTV silicone rubber.

Powder Flow Aid

0.1–1% loading coats particles, reduces cohesion. Pharma, food, toner.

Anti-Settling Agent

Weak gel suspends pigments in storage. Re-liquefies under stirring.

Thermal Insulation

VIP core material. Achieves <0.005 W/(m·K) thermal conductivity.

Gel Electrolytes

Thickens electrolytes in lead-acid and Li-ion batteries. Prevents leakage.

Fumed vs. Precipitated Silica

Parameter	Fumed	Precipitated
Process	Gas-phase flame hydrolysis	Wet chemical precipitation
SiO₂ purity	>99.8%	~85–98%
Particle structure	Non-porous spheres	Porous, irregular
Moisture / LOI	<1.5% / <2%	~5% / ~5–8%
Silanol density	~2–3 OH/nm²	~4–6 OH/nm²
Thickening efficiency	High (lower dose)	Moderate
Transparency	Excellent	Lower
Cost	Higher	Lower

Selection rule: Need max purity, transparency, efficiency? → Fumed. Cost-sensitive, opacity OK? → Precipitated.

FAQ

Is fumed silica the same as silica fume?

No. Fumed silica = high-purity synthetic product (flame hydrolysis). Silica fume = byproduct of silicon smelting, used in concrete. Entirely different materials.

Is it safe to handle?

Amorphous, not a carcinogen. But ultra-fine particles can irritate lungs/eyes. Use dust masks, safety glasses, enclosed systems. Follow the SDS.

What BET grade should I use?

Silicone: 150–200 m²/g. Coatings: 150–300 m²/g. Flow aid: 200+ m²/g. Higher surface = stronger effect, harder to disperse.

Can it disperse in water?

Hydrophilic grades: yes — forms thixotropic gels above ~3–5 wt%. Hydrophobic grades: SEMISIL R272, R110, R202, R620.