Ceramic nanofibers, particularly those composed of oxide systems such as silica (SiO2) and titania (TiO2), are synthesized through a combination of precursor solution preparation, electrospinning, and calcination. This method leverages the versatility of sol-gel chemistry and the scalability of electrospinning to produce high-purity, continuous nanofibers with controlled diameters and morphologies. The process is widely adopted due to its ability to tailor fiber properties for applications in catalysis, filtration, and energy storage.

The synthesis begins with the preparation of a precursor solution containing metal alkoxides or salts dissolved in a suitable solvent. For SiO2 nanofibers, tetraethyl orthosilicate (TEOS) is commonly used as the silicon source, while titanium isopropoxide (TTIP) serves as the precursor for TiO2 nanofibers. The precursor is mixed with a solvent such as ethanol or isopropanol to facilitate hydrolysis and polycondensation reactions. To control the viscosity and spinnability of the solution, a polymer template like polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA) is added. The polymer acts as a carrier during electrospinning, ensuring the formation of continuous fibers before its eventual removal during calcination.

The precursor solution is loaded into a syringe equipped with a metallic needle connected to a high-voltage power supply. A typical voltage range of 10–20 kV is applied to create an electric field between the needle and a grounded collector. The electric field draws the solution into a Taylor cone, from which a jet is ejected. As the jet travels toward the collector, solvent evaporation occurs, and the sol-gel transition begins, resulting in the deposition of gel-like nanofibers on the collector. The distance between the needle and collector is maintained between 10–20 cm to ensure optimal fiber formation. Ambient conditions, including humidity and temperature, are controlled to prevent premature gelation or bead formation in the fibers.

Following electrospinning, the as-spun nanofibers undergo a drying step to remove residual solvents. The dried fibers are then subjected to calcination in a furnace at elevated temperatures to decompose the polymer template and crystallize the ceramic phase. The calcination temperature and heating rate are critical parameters that influence the final fiber properties. For SiO2 nanofibers, calcination at 500–800°C for 2–4 hours is typical, while TiO2 nanofibers require temperatures between 400–600°C to achieve the desired anatase or rutile phase. A slow heating rate of 1–5°C/min is often employed to prevent cracking or structural collapse due to rapid polymer burnout.

The morphology and diameter of the resulting ceramic nanofibers are influenced by several factors during electrospinning. Precursor concentration plays a key role; higher concentrations yield thicker fibers but may also increase the likelihood of bead defects. A solution concentration of 5–15 wt% is commonly used for oxide systems. The viscosity of the solution, governed by the polymer molecular weight and concentration, must be optimized to ensure smooth electrospinning. For instance, PVP with a molecular weight of 1,300,000 g/mol at 8–10 wt% concentration is effective for TiO2 nanofiber synthesis. The applied voltage and flow rate also impact fiber diameter, with lower flow rates (0.5–2 mL/h) favoring thinner fibers.

The crystalline structure of the nanofibers is determined by the calcination conditions. TiO2 nanofibers calcined at lower temperatures (400–500°C) predominantly exhibit the anatase phase, which is photocatalytically active, while higher temperatures (>600°C) promote the rutile phase, known for its stability. In contrast, SiO2 nanofibers remain amorphous after calcination due to the lack of long-range order in silica glass. The surface area of the nanofibers is another critical property, with typical BET surface areas ranging from 50–300 m²/g for TiO2 and 200–500 m²/g for SiO2, depending on calcination parameters and precursor chemistry.

Mechanical properties of ceramic nanofibers are influenced by their microstructure and defect density. TiO2 nanofibers exhibit higher tensile strength compared to SiO2 due to their crystalline nature, with reported values in the range of 100–500 MPa. The porosity of the fibers can be tailored by adjusting the calcination profile or incorporating pore-forming agents in the precursor solution. For instance, slower heating rates during calcination tend to produce denser fibers, while rapid heating may introduce microporosity.

The chemical stability of oxide nanofibers is another advantage, particularly in harsh environments. SiO2 nanofibers demonstrate excellent resistance to acids and organic solvents, while TiO2 nanofibers are stable under UV irradiation and oxidative conditions. This makes them suitable for long-term applications in photocatalytic reactors or high-temperature filters. The surface chemistry of the fibers can be further modified through post-synthesis treatments such as silanization for SiO2 or doping for TiO2 to enhance specific functionalities.

Challenges in the synthesis of ceramic nanofibers include minimizing fiber brittleness and achieving uniform diameter distributions. Brittleness is inherent to ceramic materials but can be mitigated by optimizing the calcination process to reduce microcracks. Diameter uniformity is improved through precise control of electrospinning parameters such as humidity, temperature, and solution conductivity. Advanced setups with environmental chambers or multi-needle arrays have been explored to enhance reproducibility for industrial-scale production.

Applications of ceramic nanofibers leverage their high surface area, thermal stability, and tunable porosity. TiO2 nanofibers are extensively used in photocatalytic degradation of pollutants due to their efficient charge separation and UV absorption. SiO2 nanofibers find use in high-temperature insulation and as reinforcement in composite matrices. Both systems are explored for gas sensing, where their nanoscale dimensions enhance sensitivity to target molecules.

Future developments in this field may focus on lowering the energy consumption of calcination through microwave-assisted heating or alternative sintering techniques. Additionally, the integration of in-situ characterization during electrospinning could provide real-time insights into fiber formation mechanisms. The continued refinement of precursor chemistry and electrospinning setups will further expand the applicability of ceramic nanofibers in advanced technologies.