Electrospinning Equipment: Ultra-Flexible TiO₂/SiO₂ Nanofiber Membranes for Thermal Insulation

1. Research Background

With the intensification of the energy crisis and the increasing demand for energy conservation and emission reduction, there is a growing need for high-performance and high-temperature-resistant thermal insulation materials in fields such as aerospace and high-temperature industries. Heat transfer primarily occurs through radiation, conduction, and convection, with thermal radiation playing a dominant role at high temperatures. Oxide fiber thermal insulation materials have garnered attention due to their lightweight, low thermal conductivity, and high-temperature resistance. However, ceramic fibers tend to transmit infrared radiation at high temperatures, leading to increased back temperatures and necessitating thicker insulation layers to enhance thermal insulation efficiency. Moreover, the transmission of thermal radiation through materials can elevate back temperatures. Therefore, reducing the transmission of thermal radiation is crucial for improving insulation properties. Currently, the addition of infrared absorbing agents (such as silicon carbide and carbon black) is used to suppress the radiative transmission of ceramic fiber insulation materials at high temperatures. However, these materials have poor oxidation resistance in atmospheric conditions and can lead to increased back temperatures due to the conversion of light energy to heat through infrared absorption. In contrast, directly reflecting infrared radiation is a more effective method to suppress high-temperature heat radiation. Titanium dioxide (TiO₂), with its high melting point (1850°C), high refractive index (2.52-2.72), specific bandgap (3.0-3.2 eV), and good chemical stability, is an ideal material for thermal radiation resistance. However, existing studies often incorporate TiO₂ into other materials, which limits the full utilization of its thermal radiation shielding properties. TiO₂ undergoes a phase transition from anatase to rutile at high temperatures. While the rutile phase has a higher reflectivity, the phase transition leads to rapid grain growth, resulting in the pulverization of nanofibers, loss of flexibility, and a sharp decline in mechanical properties. Therefore, suppressing the phase transition and grain growth of TiO₂ is key to achieving its high-temperature application. Research has shown that introducing amorphous substances into crystalline materials can form dense crystalline-amorphous hybrid structures, inhibiting grain growth and material crystallization and phase transitions. Based on this, this study proposes a new design concept for nanofiber materials by introducing a second phase, SiO₂, to suppress the high-temperature phase transition of TiO₂ and using a layered structure to prevent the penetration of cold and hot surfaces by thermal radiation. The resulting TiO₂/SiO₂ nanofiber membranes (TS NFMs) with a layered structure have been successfully fabricated using an electrospinning device.

2. Abstract

This study investigates ultra-flexible TiO₂/SiO₂ nanofiber membranes with a layered structure for thermal insulation. By introducing SiO₂ into the crystal lattice and grain boundaries of TiO₂, the phase transition and grain growth of TiO₂ are effectively inhibited. The prepared TS NFMs exhibit lightweight (44 mg/cm³), high tensile strength (4.55 MPa), ultra-flexibility, and low thermal conductivity (31.5 mW·m⁻¹·K⁻¹). The TS NFMs can withstand 100 buckling-recovery cycles at up to 80% strain, demonstrating excellent buckling fatigue resistance. Their low density and high diffuse reflectance endow the TS NFMs with superior thermal insulation properties. A 10 mm thick sample composed of approximately 300 layers of nanofiber membranes can reduce the hot surface temperature from 1200°C to about 220°C, showcasing outstanding comprehensive thermal insulation performance. The ultra-flexible, high-strength, and high-temperature-resistant (up to 1100°C) TS NFMs provide a significant pathway for fabricating materials in extremely high-temperature environments using an electrospinning machine.

3. Research Content

(1) Material Preparation

The TiO₂/SiO₂ precursor nanofibers were prepared using sol-gel and electrospinning techniques with raw materials including tetrabutyl titanate (TBOT), acetic acid (HAc), acetylacetone (Hacac), tetraethyl silicate (TEOS), and polyethylene oxide (PEO). The TBOT methanol solution was mixed with acetic acid and deionized water, followed by rotary evaporation to obtain yellow powders. These powders were redissolved in methanol, and small amounts of acetylacetone, PEO, and TEOS were added. After heating and stirring for 8 hours, a homogeneous, transparent, golden-yellow spinning solution was obtained. The precursor nanofibers were fabricated via an electrospinning machine at 25°C, 40% relative humidity, and an applied voltage of 11 kV. The fibers were then heat-treated in a muffle furnace at 200-1200°C with a heating rate of 1-5°C/min and held for 0-2 hours in air, followed by natural cooling to room temperature. The heat-treated nanofibers were designated as TS-x, where x represents the heat-treatment temperature in degrees Celsius.

(2) Structural and Property Characterization

• Crystalline Phase Analysis: X-ray powder diffraction (XRD) was used to analyze the crystalline phases of the nanofibers. Results showed that the crystallinity of the fibers increased with heat-treatment temperature. At 700-900°C, the fibers were primarily in the anatase phase with slow grain growth (9.6 nm at 900°C). Above 1000°C, rapid grain growth and the formation of the rutile phase were observed. At 1000°C, the grain sizes of anatase and rutile phases were 28.2 nm and 42.1 nm, respectively. At 1100°C, the rutile phase dominated with a grain size of 80.9 nm, and at 1200°C, the grain size exceeded 100 nm. No SiO₂ diffraction peaks were detected, suggesting that SiO₂ existed in an amorphous form within the samples.

• Surface Chemistry Analysis: X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface chemistry of TS-800, TS-1000, and TS-1200 samples. The results indicated that as the heat-treatment temperature increased, the Ti 2p peaks weakened and shifted to lower binding energies, while the Si 2p peaks intensified and shifted to higher binding energies. The O 1s peaks were fitted into three components corresponding to Ti-O-Ti, Ti-O-Si, and Si-O-Si bonds. The shifts in binding energies and the formation of Ti-O-Si bonds suggested strong interactions between TiO₂ and SiO₂, leading to lattice distortion and inhibition of grain growth in TiO₂.

• Microstructural Observation: Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the microstructure of the nanofibers. The fibers maintained an intact structure across all heat-treatment temperatures. At 700-900°C, the fibers had smooth surfaces and smaller grain sizes (9.6 nm at 900°C). Above 1000°C, rapid grain growth and the formation of the rutile phase were observed, with grain sizes reaching 80.9 nm at 1100°C. TEM images revealed distinct crystalline and amorphous regions, confirming the presence of amorphous SiO₂ and its incorporation into the TiO₂ lattice, which contributed to lattice distortion and inhibition of phase transition and grain growth.

• Mechanical Property Testing: The tensile strength of the nanofiber membranes was tested using a fiber strength tester. The results showed that the tensile strength initially increased and then decreased with increasing heat-treatment temperature. The maximum tensile strength of 4.55 MPa was achieved at 900°C. At 1100°C, the tensile strength remained at 2.38 MPa, demonstrating good high-temperature stability. Single-fiber bending experiments indicated that the fibers were highly flexible, capable of repeated bending without damage. Macroscopically, the multilayer nanofiber membranes exhibited excellent flexibility and fatigue resistance, withstanding 100 buckling-recovery cycles at 80% strain without rupture.

• Thermal Insulation Testing: The thermal insulation performance of the TS NFMs was evaluated through thermal conductivity measurements and high-temperature insulation experiments. The thermal conductivity of the TS NFMs was 31.5 mW·m⁻¹·K⁻¹, lower than that of most low-density ceramic insulation materials. In the insulation experiment, a 10 mm thick TS NFM sample reduced the hot surface temperature from 1200°C to approximately 220°C, demonstrating superior thermal insulation properties. The low thermal conductivity, high reflectance, and layered structure effectively blocked thermal radiation, contributing to the excellent insulation performance. The TS NFMs were fabricated using an electrospinning device, which allowed for precise control over the fiber morphology and alignment, contributing to the overall performance of the membranes.

4. Conclusion

This study has successfully fabricated TiO₂/SiO₂ nanofiber membranes with a layered structure (TS NFMs) using an electrospinning machine. The TS NFMs exhibit low density (44 mg/cm³), low thermal conductivity (31.5 mW·m⁻¹·K⁻¹), high tensile strength (4.55 MPa), and ultra-flexibility. By introducing SiO₂ into the lattice and grain boundaries of TiO₂, the phase transition and grain growth of TiO₂ were effectively inhibited, endowing the material with excellent mechanical properties and high-temperature stability. The maximum service temperature of TS NFMs can reach 1100°C, and their thermal insulation performance is superior to other similar radiation-resistant materials. The use of an electrospinning device allowed for the precise control of fiber formation, resulting in membranes with enhanced thermal insulation and mechanical properties. TS NFMs hold broad application prospects in the fields of high-temperature thermal insulation, energy saving, and flexible wearable devices.

Electrospinning Nanofibers Article Source:

https://doi.org/10.1016/j.jmat.2024.03.002