After five years’ work, an MIT team can now fabricate a transparent version of a silica aerogel, an ultralight material that blocks heat transfer. They have used their aerogel in a solar thermal collector to generate temperatures suitable for water and space heating and more—without using the expensive concentrators, special materials, and vacuum enclosures that have kept current solar thermal systems from being widely adopted. They have also demonstrated that inserting an aerogel into the gap in a double-pane window will make a product that’s both affordable and highly insulating. Finally, their work has generated guidelines that will help innovators design and fabricate aerogels with nanoscale structures tailored for high performance in other critical technologies.
In recent decades, the search for high-performance thermal insulation for buildings has prompted manufacturers to turn to aerogels. Invented in the 1930s, these remarkable materials are translucent, ultraporous, lighter than a marshmallow, strong enough to support a brick, and an unparalleled barrier to heat flow, so ideal for keeping heat inside on a cold winter day and outside when summer temperatures soar.
Five years ago, researchers led by Evelyn Wang, a professor and head of the Department of Mechanical Engineering, and Gang Chen, the Carl Richard Soderberg Professor in Power Engineering, set out to add one more property to that list. They aimed to make a silica aerogel that was truly transparent.
“We started out trying to realize an optically transparent, thermally insulating aerogel for solar thermal systems,” says Wang. Incorporated into a solar thermal collector, a slab of aerogel would allow sunshine to come in unimpeded but prevent heat from coming back out—a key problem in today’s systems. And if the transparent aerogel were sufficiently clear, it could be incorporated into windows, where it would act as a good heat barrier but still allow occupants to see out.
When the researchers started their work, even the best aerogels weren’t up to those tasks. “People had known for decades that aerogels are a good thermal insulator, but they hadn’t been able to make them very optically transparent,” says Lin Zhao PhD ’19 of mechanical engineering. “So in our work, we’ve been trying to understand exactly why they’re not very transparent and then how we can improve their transparency.”
Aerogels: Opportunities and challenges
The remarkable properties of a silica aerogel are the result of its nanoscale structure. To visualize that structure, think of holding a pile of small, clear particles in your hand. Imagine that the particles touch one another and slightly stick together, leaving gaps between them that are filled with air. Similarly, in a silica aerogel, clear, loosely connected nanoscale silica particles form a three-dimensional solid network within an overall structure that is mostly air. Because of all that air, a silica aerogel has an extremely low density—in fact, one of the lowest densities of any known bulk material—yet it’s solid and structurally strong, though brittle.
If a silica aerogel is made of transparent particles and air, why isn’t it transparent? Because the light that enters doesn’t all pass straight through. It is diverted whenever it encounters an interface between a solid particle and the air surrounding it. The diagram below illustrates the process. When light enters the aerogel, some is absorbed inside it. Some—called direct transmittance—travels straight through. And some is redirected along the way by those interfaces. It can be scattered many times and in any direction, ultimately exiting the aerogel at an angle. If it exits from the surface through which it entered, it is called diffuse reflectance; if it exits from the other side, it is called diffuse transmittance.
To make an aerogel for a solar thermal system, the researchers needed to maximize the total transmittance: the direct plus the diffuse components. And to make an aerogel for a window, they needed to maximize the total transmittance and simultaneously minimize the fraction of the total that is diffuse light. “Minimizing the diffuse light is critical because it’ll make the window look cloudy,” says Zhao. “Our eyes are very sensitive to any imperfection in a transparent material.”
Developing a model
The sizes of the nanoparticles and the pores between them have a direct impact on the fate of light passing through an aerogel. But figuring out that interaction by trial and error would require synthesizing and characterizing too many samples to be practical. “People haven’t been able to systematically understand the relationship between the structure and the performance,” says Zhao. “So we needed to develop a model that would connect the two.”
To begin, Zhao turned to the radiative transport equation, which describes mathematically how the propagation of light (radiation) through a medium is affected by absorption and scattering. It is generally used for calculating the transfer of light through the atmospheres of Earth and other planets. As far as Wang knows, it has not been fully explored for the aerogel problem.
Both scattering and absorption can reduce the amount of light transmitted through an aerogel, and light can be scattered multiple times. To account for those effects, the model decouples the two phenomena and quantifies them separately—and for each wavelength of light.
Based on the sizes of the silica particles and the density of the sample (an indicator of total pore volume), the model calculates light intensity within an aerogel layer by determining its absorption and scattering behavior using predictions from electromagnetic theory. Using those results, it calculates how much of the incoming light passes directly through the sample and how much of it is scattered along the way and comes out diffuse.
The next task was to validate the model by comparing its theoretical predictions with experimental results.
Working in parallel, graduate student Elise Strobach of mechanical engineering had been learning how best to synthe-size aerogel samples—both to guide development of the model and ultimately to validate it. In the process, she produced new insights on how to synthesize an aerogel with a specific desired structure.
Her procedure starts with a common form of silicon called silane, which chemically reacts with water to form an aerogel. During that reaction, tiny nucleation sites occur where particles begin to form. How fast they build up determines the end structure. To control the reaction, she adds a catalyst, ammonia. By carefully selecting the ammonia-to-silane ratio, she gets the silica particles to grow quickly at first and then abruptly stop growing when the precursor materials are gone—a means of producing particles that are small and uniform. She also adds a solvent, methanol, to dilute the mixture and control the density of the nucleation sites, thus the pores between the particles.
The reaction between the silane and water forms a gel containing a solid nanostructure with interior pores filled with the solvent. To dry the wet gel, Strobach needs to get the solvent out of the pores and replace it with air—without crushing the delicate structure. She puts the aerogel into the pressure chamber of a critical point dryer and floods liquid CO2 into the chamber. The liquid CO2 flushes out the solvent and takes its place inside the pores. She then slowly raises the temperature and pressure inside the chamber until the liquid CO2 transforms to its supercritical state, where the liquid and gas phases can no longer be differentiated. Slowly venting the chamber releases the CO2 and leaves the aerogel behind, now filled with air. She then subjects the sample to 24 hours of annealing—a standard heat-treatment process—which slightly reduces scatter without sacrificing the strong thermal insulating behavior. Even with the 24 hours of annealing, her novel procedure shortens the required aerogel synthesis time from several weeks to less than four days.
Validating and using the model
To validate the model, Strobach fabricated samples with carefully controlled thicknesses, densities, and pore and particle sizes—as determined by small-angle X-ray scattering—and used a standard spectrophotometer to measure the total and diffuse transmittance.
The data confirmed that, based on measured physical properties of an aerogel sample, the model could calculate total transmittance of light as well as a measure of clarity called haze, defined as the fraction of total transmittance that is made up of diffuse light.
The exercise confirmed simplifying assumptions made by Zhao in developing the model. Also, it showed that the radiative properties are independent of sample geometry, so his model can simulate light transport in aerogels of any shape. And it can be applied not just to aerogels but to any porous materials.
Wang notes what she considers the most important insight from the modeling and experimental results: “Overall, we determined that the key to getting high transparency and minimal haze—without reducing thermal insulating capability—is to have particles and pores that are really small and uniform in size,” she says.
One analysis demonstrates the change in behavior that can come with a small change in particle size. Many applications call for using a thicker piece of transparent aerogel to better block heat transfer. But increasing thickness may decrease transparency. The figures below show total transmittance (top) and haze (bottom) in aerogel samples of increasing thickness and fixed density. The curves represent model results for samples with different particle sizes. As thickness increases, the samples with particles of 6 nanometer (nm) and 9 nm radius quickly do worse on both transmittance and haze. In contrast, the performance of the samples with particles of 3 nm radius remains essentially unchanged. As long as particle size is small, increasing thickness to achieve greater thermal insulation will not significantly decrease total transmittance or increase haze.
Comparing aerogels from MIT and elsewhere
How much difference does their approach make? The figure below shows total transmittance and haze from three MIT samples (with different thicknesses) and from nine state-of-the-art silica aerogels, which typically have particles and pores that are as large as 10 nm and vary widely in size, which gives most aerogels a slightly blue tint, notes Wang.
In the figure, the ideal transparent aerogel—one with 0% haze and 100% total transmittance—would appear in the bottom right corner. Only the MIT aerogel samples fall in that vicinity. The green bar represents common glass. The MIT samples have significantly better optical properties, with haze about the same and transmittance even greater than glass. “Our aerogels are more transparent than glass because they don’t reflect—they don’t have that glare spot where the glass catches the light and reflects to you,” says Strobach.
To Lin, a main contribution of their work is the development of general guidelines for material design, as demonstrated by the figure below. Aided by such a “design map,” users can tailor an aerogel for a particular application. Based on the contour plots, they can determine the combinations of controllable aerogel properties—namely, density and particle size—needed to achieve a targeted haze and transmittance outcome for many applications.