The bulk density (ρb) of aerogels was determined by the ratio of mass to volume. The density of aerogels can be calculated using the following formula:
where m is the dry weight of the aerogels, and D and H are the diameter and height of the aerogel samples.
The porosity of the aerogels was measured by the ethanol liquid immersion method [64 (link)]. The initial mass of the sample was m0, then it was completely immersed in ethanol and measured as m1. After a period of vacuum, the sample was taken out and weighed, and the total mass of ethanol and the beaker was recorded as m2. The porosity of the sample can be calculated as follow:
The specific surface area (SBET) was obtained through nitrogen adsorption–desorption and the Brunauer–Emmet–Teller (BET) model (BET, AUTOSORB-1MP, Quantachrom, Boynton Beach, FL, US). The pore size and distribution of the aerogels were determined using nitrogen adsorption–desorption curves, BET models, and high-performance fully automated injection mercury instruments (Micromeritics AutoPore IV 9500). The samples were first dried in an oven at 80 °C for 24 h. The test pressure was first gradually increased from low pressure to 60,000 psi and then slowly decreased to 14.7 psi.
The microstructure of the aerogels was observed via scanning electron microscopy (SEM) (JSM6390LV, JEOL, Tokyo, Japan) at a magnification of ×50 and ×200. The samples were cut into 5 mm × 5 mm × 1 mm circular pieces using a sharp razor blade.
A high-resolution X-ray diffractometer (Empyrean, Dordrecht, The Netherlands) was used for the diffraction analysis of the aerogels with a scan rate of 5°/min and a 2θ range of 5–50°.
Fourier transform infrared spectroscopy (FTIR) was used to analyze the information of functional groups using a NEXUS (England) in the wavenumber range 600 to 4000 cm−1.
The surface elements were validated by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaprobeI, Japan). CasaXPS software was used to process the data.
The mechanical properties of the prepared aerogels were tested by a TMS-PRO texture analyzer (TA. XT Plus, Stable Micro Systems, Surrey, UK), and samples were equilibrated for 48 h at 40 °C drying. The compression rate of the probe was 0.5 mm/s, and the compression ratio was 30%. The compressive strength and elasticity were obtained by secondary compression. Stress (σ) was calculated using the following standard equations:
where F is the force (in N) applied on the sample surface, and S (in mm2) is the contact area between the probe and the sample.
The thermal conductivity of the aerogel samples was recorded at room temperature by a thermal conductivity tester (DRPL-2A, Xiangtan Instrument Co., Ltd., Xiangtan, China).
The aerogel samples were fixed between the heat source (150 °C heating table) and the thermal imager at the same distance for the heat resistance test. The thermographic images were recorded by an infrared thermal camera (323Pro, FOTRIC Ltd., Shanghai, China). The analysis software was used to process the data.
Thermal stability was conducted by using thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis. With the nitrogen flow rate of 30 cm3/min, the aerogels were heated from room temperature to 800 °C at a heating rate of 5 °C/min in an N2 atmosphere, and the weight loss curve was recorded.
The fire-retardant property of the aerogel samples was determined by a microscale combustion calorimeter (MCC, FTT0001, FTT Ltd., West Sussex, UK).
The limiting oxygen index (LOI) was measured by a CH-2CZ oxygen index tester (Nanjing Shangyuan Analysis Instrument Company, Nanjing, China). The samples were cut into strips of 80 mm × 4 mm × 10 mm for testing. Then, the residual components that were completely burned were taken to observe their microstructures under scanning electron microscopy. The LOI was calculated using the following standard equations:
where m is the dry weight of the aerogels, and D and H are the diameter and height of the aerogel samples.
The porosity of the aerogels was measured by the ethanol liquid immersion method [64 (link)]. The initial mass of the sample was m0, then it was completely immersed in ethanol and measured as m1. After a period of vacuum, the sample was taken out and weighed, and the total mass of ethanol and the beaker was recorded as m2. The porosity of the sample can be calculated as follow:
The specific surface area (SBET) was obtained through nitrogen adsorption–desorption and the Brunauer–Emmet–Teller (BET) model (BET, AUTOSORB-1MP, Quantachrom, Boynton Beach, FL, US). The pore size and distribution of the aerogels were determined using nitrogen adsorption–desorption curves, BET models, and high-performance fully automated injection mercury instruments (Micromeritics AutoPore IV 9500). The samples were first dried in an oven at 80 °C for 24 h. The test pressure was first gradually increased from low pressure to 60,000 psi and then slowly decreased to 14.7 psi.
The microstructure of the aerogels was observed via scanning electron microscopy (SEM) (JSM6390LV, JEOL, Tokyo, Japan) at a magnification of ×50 and ×200. The samples were cut into 5 mm × 5 mm × 1 mm circular pieces using a sharp razor blade.
A high-resolution X-ray diffractometer (Empyrean, Dordrecht, The Netherlands) was used for the diffraction analysis of the aerogels with a scan rate of 5°/min and a 2θ range of 5–50°.
Fourier transform infrared spectroscopy (FTIR) was used to analyze the information of functional groups using a NEXUS (England) in the wavenumber range 600 to 4000 cm−1.
The surface elements were validated by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaprobeI, Japan). CasaXPS software was used to process the data.
The mechanical properties of the prepared aerogels were tested by a TMS-PRO texture analyzer (TA. XT Plus, Stable Micro Systems, Surrey, UK), and samples were equilibrated for 48 h at 40 °C drying. The compression rate of the probe was 0.5 mm/s, and the compression ratio was 30%. The compressive strength and elasticity were obtained by secondary compression. Stress (σ) was calculated using the following standard equations:
where F is the force (in N) applied on the sample surface, and S (in mm2) is the contact area between the probe and the sample.
The thermal conductivity of the aerogel samples was recorded at room temperature by a thermal conductivity tester (DRPL-2A, Xiangtan Instrument Co., Ltd., Xiangtan, China).
The aerogel samples were fixed between the heat source (150 °C heating table) and the thermal imager at the same distance for the heat resistance test. The thermographic images were recorded by an infrared thermal camera (323Pro, FOTRIC Ltd., Shanghai, China). The analysis software was used to process the data.
Thermal stability was conducted by using thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis. With the nitrogen flow rate of 30 cm3/min, the aerogels were heated from room temperature to 800 °C at a heating rate of 5 °C/min in an N2 atmosphere, and the weight loss curve was recorded.
The fire-retardant property of the aerogel samples was determined by a microscale combustion calorimeter (MCC, FTT0001, FTT Ltd., West Sussex, UK).
The limiting oxygen index (LOI) was measured by a CH-2CZ oxygen index tester (Nanjing Shangyuan Analysis Instrument Company, Nanjing, China). The samples were cut into strips of 80 mm × 4 mm × 10 mm for testing. Then, the residual components that were completely burned were taken to observe their microstructures under scanning electron microscopy. The LOI was calculated using the following standard equations:
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