An unusual three-dimensional porous nanostructure called pillared boron nitride (PBN) could achieve a balance of strength, toughness, and ability to transfer heat that could benefit nanoelectronics, gas storage, and composite materials that perform multiple functions, Rice University engineers have discovered.
The 3-D prototypes they made (using computer simulations) fuse one-dimensional boron nitride nanotubes and two-dimensional sheets of boron nitride. The extremely thin sheets of boron nitride are stacked in parallel layers, with tube-shaped pillars of boron nitride between each layer to keep the sheets separated.
Among 3-D boron nitride’s unusual properties:
* Can be stretched about 45 percent of its length without breaking in the direction of the columns.
* Carries heat relatively fast in all 3-D directions. “This feature is ideal for applications that require materials or coating with the capability of extremely fast thermal diffusion to the environments,” said Rouzbeh Shahsavari, assistant professor of civil and environmental engineering and of materials science and nanoengineering. “Examples include car engines or computer CPUs where a fast heat transfer to the environments is critical in proper functioning.”
* Has a very porous and lightweight structure. Each gram of this Swiss cheese-like structure has a surface area equivalent to three tennis courts. Such a high surface area lends itself to customized applications, such as efficient gas storage and separation — in vehicles that run on hydrogen cells, for example.
* Is an electrically insulating material, so it could complement electrically conductive graphene-based nanoelectronics — for example, in the next generation of 3-D semiconductors and 3-D thermal transport devices, which could be used in nanoscale calorimeters, microelectronic processes, and macroscopic refrigerators.
The Journal of Physical Chemistry C – Synergistic Behavior of Tubes, Junctions and Sheets Imparts Mechano-Mutable Functionality in 3D Porous Boron Nitride Nanostructures
“We combined the tubes and sheets together to make them three-dimensional, thus offering more functionality,” said Rouzbeh Shahsavari, assistant professor of civil and environmental engineering and of materials science and nanoengineering, who co-authored the paper with graduate student Navid Sakhavand. In the 3-D nanostructure, the extremely thin sheets of boron nitride are stacked in parallel layers, with tube-shaped pillars of boron nitride between each layer to keep the sheets separated.
Shahsavari noted that in the one-dimensional and two-dimensional versions of boron nitride, there is always a bias in directional properties, either toward the tube axis or in-plane directions, which is not suitable for widespread 3-D use in technology and industrial applications.
For example, a one-dimensional boron nitride nanotube can be stretched about 20 percent of its length before it breaks, but the 3-D prototype of boron nitride can be stretched about 45 percent of its length without breaking.
When the typical one- or two-dimensional boron nitride materials are stretched in one direction, they tend to shrink in the other perpendicular directions. In the 3-D prototype, however, when the material stretches in the in-plane direction, it also stretches in perpendicular directions. “Here, the junction between the tubes and sheets has a unique curve-like structure that contributes to this interesting phenomenon, known as the auxetic effect,” Shahsavari said.
The thermal transport properties of the 3-D prototype are also advantageous, he said. The one-dimensional boron nitride tubes and two-dimensional sheets can carry heat very fast but only in one or two directions. The 3-D prototype carries heat relatively fast in all 3-D directions. “This feature is ideal for applications that require materials or coating with the capability of extremely fast thermal diffusion to the environments. Examples include car engines or computer CPUs where a fast heat transfer to the environments is critical in proper functioning,” Shahsavari said.
The 3-D boron nitride prototype has a very porous and lightweight structure. Each gram of this Swiss cheese-like structure has a surface area equivalent to three tennis courts. Such a high surface area lends itself to customized applications. Shahsavari and Sakhavand predicted that the 3-D prototype of boron nitride would allow efficient gas storage and separation, for example, in vehicles that run on hydrogen cells.
Unlike graphene-based nanostructures, boron nitride is an electrically insulating material. Thus, the 3-D boron nitride prototype has a potential to complement graphene-based nanoelectronics, including potential for the next generation of 3-D semiconductors and 3-D thermal transport devices that could be used in nanoscale calorimeters, microelectronic processes and macroscopic refrigerators.
The actual 3-D boron nitride prototype still has to be created in the lab, and numerous efforts are already underway. “Our computer simulations show what properties can be expected from these structures and what the key factors are that control their functionality,” Shahsavari said.
One-dimensional (1D) Boron Nitride nanotube (BNNT) and 2D hexagonal BN (h-BN) are attractive for demonstrating fundamental physics and promising applications in nano/microscale devices. However, there is a high anisotropy associated with these BN allotropes as their excellent properties are either along the tube axis or in-plane directions, posing an obstacle in their widespread use in technological and industrial applications. Herein, we report a series of 3D BN prototypes, namely Pillared Boron Nitride (PBN), by fusing single wall BNNT and monolayer h-BN aimed at filling this gap. We use density functional theory and molecular dynamics simulations to probe the diverse mechano-mutable properties of PBN prototypes. Our results demonstrate that the synergistic effect of the tubes, junctions, and sheets imparts cooperative deformation mechanisms, which overcome the intrinsic limitations of the PBN constituents and provide a number of superior characteristics including 3D balance of strength and toughness, emergence of negative Poisson’s ratio, and elimination of strain softening along the armchair orientation. These features, combined with the ultrahigh surface area and lightweight structure, render PBN as a 3D multifunctional template for applications in graphene-based nanoelectronics, optoelectronics, gas storage and functional composites with fascinating in-plane and out-of-plane tailorable properties.
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