Tag Archives: thermal conductivity

A small car is supported on one of its wheels by a thin block of polymer aerogel. Were this have been silica aerogel, it would have surely crumbled within moments. (c) NASA

New polymer aerogels might become the wonder insulating material

Since they were first invented in 1931, aerogels have become widely used in the industry, mostly for insulation purposes, thanks to their low thermal conductivity and light weight. Traditional silica aerogels, however, are brittle and obtuse, typically unsuited for applications where flexing of the material would occur. A novel class of polymer aerogels seeks to fix most of these inconveniences, after scientists at NASA have shown that the polymer version is up to 500 times stronger and has a thermal resistance up to 10 times higher than typical silica aerosols. If proven commercially cost effective in the future, a whole super-insulation revamp might commence – from refrigeration, to clothing, to heating systems and so on.

Aerogels are basically gels where the liquid component has been replaced by a gas, through the process of supercritical drying. The resulting gel, an aerogel, has a low heat transfer and low density, since it retains its pre-treatment volume even after the water has been replaced. This makes them ideal for insulating applications, but they’ve also been used from particle physics to biomedical fields.

Their only draw back is that they’re extremely brittle, and crumble easily. Until now that is, since researchers at  NASA’s Glenn Research Center in Ohio managed to create a full polymer aerogel which is just as potent as its silica counterpart, but only much more stronger. Best of all, it can be produced into thin stripes which don’t break down, allowing for insulation of sensitive parts.

“The new aerogels are up to 500 times stronger than their silica counterparts,” says Meador. “A thick piece actually can support the weight of a car. And they can be produced in a thin form, a film so flexible that a wide variety of commercial and industrial uses are possible.”

A small car is supported on one of its wheels by a thin block of polymer aerogel. Were this have been silica aerogel, it would have surely crumbled within moments. (c) NASA

A small car is supported on one of its wheels by a rather thick block of polymer aerogel. Were this have been silica aerogel, it would have surely crumbled within moments. (c) NASA

The scientists worked up from previous attempts which coated silica aerogels with polymers by chemical vapor deposition. However, most of the polymers that could be deposited in this manner have rather low melting temperatures, unfit for most applications. So the scientists took a more direct route – they formed polymer aerogels directly, without any coating. The resulting material has a resistance to temperatures of 400 Celsius degrees and higher.

The new class of aerogel however couldn’t be manufactured by conventional methods used thus far. The team of researchers tried a cross-linking approach, where linear polyamides were reacted with a bridging compound to form a three-dimensional covalent polymer. The resulting density of the polymer aerogel was 0.14 g/cc, with 90% porosity. Silica gels were made with much better specs, but the polymer more than makes up for scale in strength.

Silica aerogels have been shown to of a similar density have a resistance to comperession and tensile limit more than 100 times smaller than the new polymer aerogels.

Nanocellular structure of the aerogel shows pores averaging about ten nanometers in size. A quarter-inch (6 mm) sheet of this aerogel would provide as much insulation as three inches of fiberglass. (c) NASA Glenn

Nanocellular structure of the aerogel shows pores averaging about ten nanometers in size. A quarter-inch (6 mm) sheet of this aerogel would provide as much insulation as three inches of fiberglass. (c) NASA Glenn

Polymer aerogels show a resistance to compression and tensile stress more than 100 times higher than silica aerogels. Mechanically, the new class is very similar to synthetic rubber, but at a mere 10% of its weight. One can imagine the virtually broad range of applications this new material could have impact upon. Imagine super-insulated clothing at the fraction of the weight currently employed, higher efficiency for thermal based systems, and so on. The only problem that remains to be settled is cost, and if not now, in the near future expect this kind of technology to become commercially viable.

source: NASA report via Gizmag

 

 

A novel technique cools electronic devices faster and cheaper

Researchers at  North Carolina State University have developed a new technique of cooling electronic devices which they claim and prove through their findings that it can lead to an increase of performance by improving the rate of heat exchange, while also lowering the cost of manufacturing. The scientists’ findings might lead to a new generation of more efficient heat sinks, as well as better cooling for devices that generate a lot of heat, such as lasers and power devices.

The technique is centered around what the researchers call a “heat spreader”, a copper-graphene composite, which is attached to the electronic device using an indium-graphene interface film. Together, the materials provide a much higher thermal conductivity than the conventional lonesome copper used ubiquitously for standard cooling of electronic devices. To be more precise, copper-graphene composite films with thickness greater than 200 microns showed an improvement in thermal conductivity over that of electrolytic copper from 380 W/m.K to 460 W/m.K at 300 K – more than 20%.

Besides the remarkable improvement in thermal conductivity, the researchers point out that the copper-graphene composite is also low-cost and easy to produce, using an electrochemical deposition process “Copper is expensive, so replacing some of the copper with graphene actually lowers the overall cost,” conclude the researchers.

Their findings have been published in the journal Metallurgical and Materials Transactions B.

source: NCSU

Spider Silk Conducts Heat as Well as Metals

Xinwei Wang, an associate professor of mechanical engineering at Iowa State studies thermal conductivity – and he had a funny feeling about spiders; so he ordered eight spiders — Nephila clavipes, golden silk orbweavers — and put them to work eating crickets and spinning webs in the cages he set up.

Wang has been looking for organic materials which can transfer heat as effectively – something diamonds, copper and aluminum are very good at, but most organic materials aren’t. However, spider silk has some remarkable properties, including strength and the ability to stretch a lot; it’s these properties that led the professor to guess that it might hold some surprises in thermal transfer as well. So he head on and did the experiments to test his theories – and the results came in fast.

“I think we tried the right material,” Wang said of the results.

What he and the rest of the team found was that spider silks, especially the draglines that anchor webs in place conduct silicone much better than some other materials, including very good conductors, such as silicone or aluminum.

“Our discoveries will revolutionize the conventional thought on the low thermal conductivity of biological materials,” Wang wrote in the paper he published, which is called “New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and its Abnormal Change under Stretching”.

“This is very surprising because spider silk is organic material,” Wang added. “For organic material, this is the highest ever. There are only a few materials higher — silver and diamond.”

Even more surprising and intriguing was the fact that when stretched, silk increased its thermal conductivity; when they stretched it by 20 percent, the conductivity rose by 20 percent as well. Most materials tend to lose their thermal conductivity when stretched. So what is it that gives it these absolutely incredible properties? According to Wang, it has a lot to do with the defect-free molecular structure of spider silk, including proteins that contain nanocrystals and the spring-shaped structures connecting the proteins. The researchers also suggest artificial improvements might be made to spider silk to improve its conductivity even more – the preliminary results are extremely promising.

“I’ve been doing thermal transport for many years,” Wang said. “This is the most exciting thing, what I’m doing right now.”