Silica aerogels: superinsulators of the future?

Photo Patrick Achard / Senior scientist at PERSEE (Centre Processes, Renewable Energies and Energetic Systems), a research lab of Mines ParisTech PSL Research University / May 12th, 2017

Aerogels? Imagine cloud chunks. This family of surprisingly light nanostructured materials has finally moved from research labs and entered the operational phase at industrial scale. Known since the 1930s, they took off some thirty years ago – three challenging and exciting decades for the researchers involved in their development. Three decades of hard work as well, that ultimately paid off.

Paris Innovation Review – Before discussing the research aspects, let’s talk about aerogels’ striking physical appearance… Do you remember the first time you actually saw one?

Patrick Achard – Yes indeed, very precisely. It was in 1986, at a conference on solar energy. Colleagues from Berkeley showed me a sample and I was immediately drawn to the material. Imagine smoke, translucent and almost massless, but at the same time solid. It resembled still air, defying any simple categorization.

I had just completed a thesis in energetics in which I had identified the need for a transparent insulator. Some cellular polymers offered interesting properties, but nothing like these aerogels which responded perfectly to the concept I felt was required.

aerogel

But we needed to deepen our knowledge of these materials and their method of production, and this is precisely what was started when I met Gérard Pajonk, professor at the University of Lyon 1, who was already working on the subject.

Interestingly, energetics was not a concern of his lab. The insulating properties of aerogels became increasingly relevant as the subject of thermal insulation became important. But in the 1980s, these materials were primarily associated with catalysis, because of their porous structure at all scales (micro and meso), a feature that provides a large contact area. This porous structure, by confining the air, is precisely what allows them to show lower thermal conductivity than still air – an outstanding performance! That’s why they are called superinsulators, as opposed to conventional insulators, such as glass wool, the objective of which is to make air still.

The same material opened on very different uses and viewed from different perspectives.

For my part, I had acquired basic training skills in the field of materials, even if it wasn’t my specialty.

In short, we were learning by doing. Gérard Pajonk introduced me to the synthesis and manufacturing processes of aerogels; I showed him the possible uses and applications in energetics and more specifically, thermal resistance and heat transfer reduction.

The exploration of related fields, straying from your comfort zone, with little resources… that’s quite a bold approach! Does it require some kind of sense of adventure?

Yes indeed, it is certainly a necessary ingredient of this kind of research endeavor. At the end, it all boils down to undertaking a project, seeking alliances and opportunities, fighting against adversity. And last but not least, just like in the business world, confronting the thorny issue of market prospects. The real challenge comes when you need to market your results.

For nearly a decade, we focused on the development of superinsulating glass. We made a few prototypes, but this beautiful project stayed on the drawing board. For a simple reason: the product was too expensive and had no chance to find a market.

But that was the only way to learn and that permits our journey over three decades on this aerogels topic. Finding uses, developing processes, exploring hunches, erring, stepping aside, starting all over from scratch… Learning and discovery on the field led to the creation of a very large family of aerogels.

Steven Kistler, the pioneering researcher who started it all in 1930s, had already developed aerogels based on silica, alumina, chromium oxide and tin oxide. Carbon aerogels were successfully developed in the 1990s – we’ll get to these later. More recently, composite aerogels were developed for NASA, in the United States. The family is expanding!

You mentioned “stepping aside” earlier: what were the main twists and turns in your work?

With my colleague Arnaud Rigacci, we first worked during ten years on the silica monolith, the “noble” version of silica aerogels. We filed a few patents but we felt that the product didn’t really work. Glassmaking groups, who had expressed an interest at first, didn’t really commit to the project. The technologies involved were simply too far away from their core business.

The difficulty of finding markets led us to focus on two fields. First of all, the issue of insulation is very wide and of course it isn’t limited to glass. The majority of applications don’t need transparency. Then, we realized that by focusing on monoliths with good optical transparency, we had set the bar a bit too high. It seemed more appropriate to develop aggregates i.e. monolithic grains, instead of silica monoliths.

The real shift occurred in the middle of the 1990s: the partnership we established with a firm called PCAS took a new turn. This chemicals group is specialized in the development of high added-value molecules for the pharmaceutical industry and mastered the synthesis of precursors required for the development of silica aerogels. They were among our first partners and the company decided to continue the experiment, even if they weren’t a producer of materials.

It must be said that synthesis is far from being an easy operation and its industrial control conditions the entire downstream chain. An aerogel is a gel i.e. a 3D array crosslinked in a solvent (water or alcohol); when the solvent is removed and replaced by air, it becomes an aerogel. This operation is a difficult one: drying a product at normal ambient conditions isn’t enough because a phase change (such as evaporation) would alter its structure and qualities. The secret is to reach and overpass the critical point of the solvent during the drying phase.

Gérard Pajonk’s lab in Lyon developed alkoxide-type precursors, the solvent trapped in the gel being methanol or ethanol. The samples have to be dried in the supercritical conditions of methanol (or ethanol). The problem is that these solvents are highly explosive. Hence, the idea of using liquid CO2 and extracting it in supercritical conditions appeared and then, of using directly supercritical CO2. This is an effective, albeit expensive, process. One company in the United States has been able to exploit it on an industrial scale but it still remains a niche product.

With PCAS, we found another solution: developing gels, reducing them in millimetric pellets and drying them under controlled atmospheric conditions.

Do these technologies allow us to reduce production costs and make them compatible with industrial developments?

It’s one of the major issues. PCAS, with whom we filed several patents, even created a spin-off, Enersens, to develop the products of our joint work at an industrial scale.

The aerogels family has gradually expanded, as have its applications. For example, in collaboration with the Parex Group, we developed mortars based on granular silica aerogels that can use the aggregates produced by Enersens. The gain in terms of energy efficiency for building envelopes coated with this type of insulators can be significant: used in the renovation of existing buildings, three centimeters of this type of mortar allows to halve the energy consumption for heating uninsulated buildings in Europe. But this type of solution remains expensive. Fixit AG, a Swiss company, is taking the lead in this field and applying it to heritage buildings in its own country – especially because of the local purchasing power.

Another subject, close to reaching industrial phase, is that of the so-called “blankets”, the third major form of aerogels, next to monoliths and aggregates. These quilted panels are made by using non aerogel fibrous systems to create a structure in which aerogels are incorporated. These blankets have been offered by the American company Aspen. They are currently under development. In Europe, there is the HomeSkin project funded by EU and led by Enersens, in which we participate.

We have mostly discussed insulation, but there are also other applications.

They keep expanding. For example, the new organic aerogels are now being used in the field of medicine. They can be transformed into carbon aerogels by pyrolysis. With their intact structure, these make excellent materials for electrodes.

More recently, we’ve developed semiconductor oxides (tin dioxide, titanium dioxide) than can be used to make aerogels. Their purpose? Producing hydrogen directly by solar beam or electrodes from fuel cells.

In collaboration with the Cemef, another laboratory of Mines ParisTech in Sophia Antipolis, we’ve also worked on the development of organic aerogels from cellulose. This led to the creation of a new material: aerocellulose. Other examples, such as pectin aerogel, should also be mentioned. Aeropectin, the main object of research of our colleague Tatiana Budtova, is a bio-aerogel based on pectin found in apples and, more generally, in food waste (such as orange and lemon zest).

These organic aerogels allow the development of hybrid aerogels, less friable than silica aerogels, hence potentially easier to use. Hybrid aerogels can be strongly compressed without breaking, thanks to the morphology of the network formed by pectin and silica, or polyurethane and silica.

The aerogel family is expanding and this last example shows how the family tree is constantly branching out. One branch can provide a specific solution to an issue the researchers from another branch are working on. It’s the whole point of this field which leads to opening new paths, like in mountain climbing. Nothing is permanently gained: we are opening and discovering new, uncertain paths, in an open environment while following an indeterminate horizon.

All these topics are at the frontier of research and development and, in the field of materials, innovative technologies like the ones you are working on, if they aren’t supported by a large group, require public funding.

Naturally, and it’s pretty much the case everywhere. Aspen, for example, was funded by NASA. In the 1980s, we received support from the Agence nationale de valorisation de la recherche (French National Research Valuation Agency). Subsequently, we received the support of the Ademe, through a Concerted Action Program in Energy Technology (The Aerogel Pact). Two decades later, the Ademe supported us once again with the SIPA-BAT Pact dedicated to superinsulators at atmospheric pressure in the building sector. The reference to atmospheric pressure set our technology apart from that of vacuum panels, the other main field for superinsulators.

As part of their investment plans for the future, the Ademe and BPI France (French investment bank) decided to contribute to the capital of Enersens. This is a form of recognition, which proves the strategic nature of the research for major stakeholders and public policy-makers in energy management and industrial development.

This field seems to have finally taken off. One obstacle still stands in the way: health aspects. Aerogels are nanostructured materials: are they threatened by the mistrust regarding nanotechnology?

There is indeed a high degree of sensitivity towards nanomaterials. Aerogels have weathered the crisis well, for a simple reason: nanostructured doesn’t mean they are made of nanoparticles. Hence, aerogels haven’t really been affected by the great controversy on nanomaterials that emerged a few years ago.

However, it is true that silica aerogels – the main branch today – tend to generate dust when handled. That’s why we are working on their composition, with developments such as the hybrid aerogels that I mentioned earlier. There are also many simple solutions, like trapping them between two glass plates or combining them into a composite material with a binder, as in the case of mortars.

Besides, a distinction needs to be made between crystalline silica, which causes silicosis, and amorphous silica, made of aerogels. Users or residents of houses isolated with these aerogels have therefore nothing to fear. Still, manufacturing plants of this type of material must be designed according to strict standards. Prolonged exposure to dust (of any kind) is never recommended for health.

But these are typical industrial issues in the 21th Century. There is no reason to fear a health scandal. By way of conclusion, I would like to mention an aspect that isn’t highlighted enough. Today, under the pressure of standards such as RT2012 in France or, in a more global eco-design approach, there is an increasing concern for the fate of materials, especially in the building industry. Silica aerogels are composed of silica: one of the two main constituents of the Earth’s crust. Their re-use or recycling is as easy as injecting them in industrial furnaces for the production of cement or glass.

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