Aerogel

Cross-linked aerogels for sound barrier applications

Cross-linked silica aerogels exhibit low density, low thermal conductivity, and low dielectric constant properties. They are intriguing candidates for a variety of thermal, optical, chemical, and electronic applications. As sound waves travel through porous structures in mineral wood, individual fibers vibrate, transforming the incoming acoustic energy into heat energy. It is surmised that aerogels may behave in a similar fashion. However, aerogels can be made of different polymer chain lengths and silica precursors, both of which impact the morphology. To determine the aerogel most suitable for sound insulation, it is important to obtain a relationship between each of these properties and their acoustic behaviors.

The purpose of this research is to observe and measure the acoustic properties in different silica aerogels. We also want to understand how each of these factors: porosity, density, and chemical composition affect acoustic characteristics, as well as thermal conductivity. To do that, we will first design an apparatus that is consistent with all aerogel samples and measure the acoustic properties of the samples.

This design is known as the source-medium-receiver model, where speaker is the source, aerogel/air serve as the medium, and the receiver is placed on the opposite side. The speaker releases a signal of constant frequency and loudness, which can be measured as sound intensity in W/m² or decibels, dB. The signal travels through the aerogel sample and is detected by the receiver. The receiver reads the received signal frequency and loudness.

Method and Materials

• Speaker (turn all 3 knobs and bass clockwise till the end)
• Adhesive backed acoustic insulation foam (12” x 54” x 1”)
• Polyurethane spray foam
• Microphone
• Oscilloscope (scale: 100 mvolts/div)
• 3” diameter tube of 5” length (PVC or Al)
• 2’ x 4’ plywood (~ 0.5” thickness): To construct and maintain sound in a closed environment
• Frequency Generator (default freq 1000 Hz, 35.986810 dB)

Calculations

L = Length of aerogel sample
d = Diameter of aerogel sample

A = Volume of aerogel
B = Difference between input and output loudness
A/B = Loudness absorbed per volume of aerogel

Given L and d, we can calculate the volume of the aerogel. By calculating the difference between input and output loudness, the loudness absorbed per volume of aerogel can be measured. This experiment was repeated several times with aerogels of different structure and chemical composition and hopefully obtain a reasonable mathematical relationship between each of these factors and the loudness absorbed.

Click HERE for summary of results. The results show that about half of the aerogel samples exhibit better insulating properties than commercial acoustic foams. A majority of samples with 20% polymer weight lie in the first half of the graph, while most of the samples with 10% and 30% polymer weight are less effective insulators. The combination of 0.6 mol/L silane, 0.5 APTES, and 20% polymer weight tend to exhibit better acoustic insulating properties.

Future Work
• Attempt to model the data to produce three dimensional surface graphs of insulation behavior as silane concentration, relative amount of APTES, and polymer concentration are varied.
• Reduce the effects of shrinkage of the aerogel during creation in order to produce more uniform samples.
• Investigate a better method to handle or protect the aerogels when introducing them to the test setup.
• Relate pore size with sample’s acoustic insulating properties.
• Compare samples with current commercial acoustic insulators of known physical properties.