MIT Researchers Create Water-repellent Ceramics From Rare Earth Elements
April Flowers for redOrbit.com – Your Universe Online
Surfaces that can shed water and survive harsh environments could have broad applications in a wide range of industries including energy, water, transportation, construction and medicine. The condensation of water is an integral part of many industrial processes, for example, and most electric power plants and desalination plants have condensers.
Materials that prevent water from spreading over a surface – hydrophobic materials – instead of forming droplets that fall away easily can enhance the efficiency of the condensation process. These materials, however, have one major flaw. Most of them use thin polymer coatings that can be destroyed by wear and degrade when heated.
A research team from MIT has now come up with a new class of hydrophobic ceramics to overcome these limitations. The new ceramics are highly hydrophobic along with being durable in the face of rough treatment and extreme temperatures. The findings of this study, which MIT associate professor Kripa Varanasi says solve the challenge of durability in hydrophobic materials, were published recently in an issue of Nature Materials.
Ceramics, while highly resistant to extreme temperatures, tend to be hydrophilic – water attracting – due to their porous nature. The research team decided to create ceramics from the so-called rare earth metals – a series of elements whose unique electronic structure might render the materials hydrophobic. These metals are also known as the lanthanide series on the periodic table.
The team expected that all the rare earth metal oxides would have uniform behaviors in their interactions with water since they have similar physio-chemical properties.
“We thought they should all have similar properties for wetting, so we said, ‘Let’s do a systematic study of the whole series,’” says Varanasi, who is the Doherty Associate Professor of Ocean Utilization.
They choose to use oxides of 13 of the 14 possible rare earths – the 14th is radioactive – to test their hypothesis. In a process called sintering, the oxides were fused into solid, ceramic pellets by compacting and heating them nearly to their melting point. As expected, when all 13 were tested, they displayed strong hydrophobic properties.
“We showed, for the first time, that there are ceramics that are intrinsically hydrophobic,” Varanasi says. Varanasi explains that the rare earth oxides “are exotic materials, and interestingly their wetting properties have not been studied.” The new study makes up for the deficiency of documented properties for the rare earth oxides including morphology, surface chemistry, crystallographic structure, grain structure, sintering temperature and density.
Mechanical engineering postdoc Gisele Azimi says the research yields a catalog of information about how to process and use these materials, including the fact that the materials have a greater hardness than many others used in rough industrial settings currently do.
These elements, despite their name, are not particularly rare. Azimi compares their abundance to copper and nickel, both of which are widely used in industrial applications. Separating the rare earth metals from the minerals in which they are found, however, is costly and can leave toxic residues, limiting their production. The world’s major supplier of these elements, which have many high-tech applications, is China.
Rare earth oxide ceramics could be used both as coatings on various substrates or in bulk form. Even if these ceramics were damaged, they would sustain their hydrophobic properties because their hydrophobicity is an intrinsic chemical property and not something added.
To prove this property, the researchers exposed the ceramics to a steam environment, such as they would face in a power-plant condenser. Polymer-based hydrophobic coatings as are typically used today quickly degrade when exposed to steam. The new ceramics, however, kept their hydrophobicity intact, sustaining even after exposure to abrasion and temperatures of 1,000 Celsius.
The team then coated nanotextured surfaces at MIT’s Microsystems Technology Laboratories to demonstrate the extreme water repellency where droplets bounced off the surface.
“These materials therefore provide a pathway to make durable superhydrophobic surfaces as well, and these coatings can be fabricated using existing processes. This makes it amenable to retrofit existing facilities,” Azimi says. “Such extreme non-wetting properties coupled with durability could find applications in steam turbines and aircraft engines, for example.”
The focus has primarily been on surface textures and structure in prior research into hydrophobic materials and coating, rather than on intrinsic chemical properties.
According to Varanasi, “No one has really addressed the key challenge of robust hydrophobic materials. We expect these hydrophobic ceramics to have far-reaching technological impact.”
Steve Granick, a professor of materials science and engineering and professor of chemistry at the University of Illinois at Urbana-Champaign says, “This discovery of intrinsic hydrophobicity is exciting and fresh. It’s a terrific example of payoff from thinking outside the box.”