自组装纳米结构性能超越骨骼

http://mitbbs.com/article_t/Macromolecules/23633045.html

发信人: singye (do you understan－－d?!), 信区: Macromolecules
标 题: 自组装纳米结构性能超越骨骼（zz）
发信站: BBS 未名空间站 (Sun Jul 22 21:07:39 2007)

生物谷：我们知道，鸟类的骨骼和树木的树干结构都经过了长期的自然进化，才达到强
度和密度的完美平衡。但是在最新一期Nature Materials上，美国Sandia国家实验室
、新墨西哥大学（UNM）和华盛顿天主教大学与普林斯顿大学的科学家们发表文章，声
称按人为控制的模式自组装的纳米材料能够超过大自然的杰作。
在更加多孔的同时，又不会过于降低强度。

进行该项研究的是美国Sandia国家实验室、新墨西哥大学、凯斯西储大学以及普林
斯顿大学的科学家。项目负责人 Jeffrey Brinker表示，：“微电子学和膜技术领域
往往需要既多孔又坚固的材料，通过自组装我们能够在比自然界中更精细的尺度构建硅
土材料。在非常小的尺度改变材料的结构和机械性能，才有可能制作出微电子学和膜技
术所需要的高硬度、多孔的材料。而新的研究成果使这一切成为可能。”

所谓自组装，一般是指原子、分子或纳米材料通过非共价键作用，在衬底上自发地
排列成一维、二维甚至三维的稳定有序的空间结构。研究人员通过核磁共振、拉曼分光
研究发现，人工方法使硅薄膜结构更加多孔的同时也会使孔壁厚度变得更薄（不到2纳
米），重新排列后的硅结构也会变得更加紧密和坚固。

此前有研究证实，自然界最优化的骨骼的强度会按照密度平方的比例发生变化，而
最新的研究表明，自组装纳米材料孔性的增加对劲度模量（stiffness modulus）的影
响更小。尤其当纳米材料的孔是立方体结构时，劲度模量会随着自身密度的平方根变化
。

Brinker表示，“我们的研究证实，纳米材料孔的结构和大小都会对它的劲度模量
产生影响。其中，立方体结构比六边形结构坚固，而六边形结构又比圆柱状结构坚固。
对同一种结构而言，孔性增加会导致劲度模量减小，但是减小程度要优于自然进化材料
。”

这项研究表明模仿骨头气泡结构的硅土材料纳米结构在气泡体积增加时可能会带来
更佳的性能。这将导致许多应用，比如膜栅栏、分子识别传感器、低介电常数绝缘体等
下一代微电子设备需要的技术。

FOR IMMEDIATE RELEASE
July 9, 2007

Self-assembled nanostructures function better than bone as porosity
increases

Improved possibilities for microelectronics, membranes
On the left is a TEM micrograph of a porous, cube-like nanostructure. On
the right is a blow-up of the silica framework (the dark <2-nm thick regions
on the left side figure) based on modeling. The highlighted structures
represent the small rings referred to in the news release.
Download 300dpi JPEG image, “selfassemnano_nr.jpg,” 1.9MB (Media are
welcome to download/publish this image with related news stories.)


ALBUQUERQUE, N.M. —Naturally occurring structures like birds’ bones or
tree trunks are thought to have evolved over eons to reach the best possible
balance between stiffness and density.

But in a June paper in Nature Materials, researchers at Sandia National
Laboratories and the University of New Mexico (UNM), in conjunction with
researchers at Case Western Reserve and Princeton Universities, show that
nanoscale materials self-assembled in artificially determined patterns can
improve upon nature’s designs.

“Using self-assembly we can construct silica materials at a finer scale
than those found in nature,” says principal investigator Jeff Brinker. “
Because, at very small dimensions, the structure and mechanical properties
of the materials change, facile fabrication of stiff, porous materials
needed for microelectronics and membrane applications may be possible.”

Sandia is a National Nuclear Security Administration laboratory.

Nuclear magnetic resonance and Raman spectroscopic studies performed by
Sandia researchers Roger Assink (ret.) and Dave Tallant, along with
molecular modeling studies performed by Dan Lacks at Case Western Reserve
University, showed that, as the ordered porous films became more porous, the
silica pore walls thinned below 2 nm, re-arranging the silica framework to
become denser and stiffer.

Whereas the stiffness of evolved optimized bone declines proportional to the
square of its density, mechanical studies performed by Sandia researcher
Thomas Buchheit working with UNM student Christopher Hartshorn showed that
the stiffness/modulus of self-assembled materials was much less sensitive to
increasing porosity: For a material synthesized with a cubic arrangement of
pores, the modulus declined only as the square root of its density.

The silica nanostructures — basically a synthetic analogue of bone-like
cellular structures, replicated at the nanoscale using silica compounds —
thus may improve performance where increased pore volume is important. These
include modern thin-film applications such as membrane barriers, molecular
recognition sensors, and low-dielectric-constant insulators needed for
future generation of microelectronic devices.

“Bone, closely examined, is a structured cellular material,” says Brinker,
a Sandia Fellow and chemical engineering professor at UNM. “Because, using
self-assembly, we had demonstrated the fabrication of a variety of ordered
cellular materials at the nanoscale with worm-like (curving cylinders),
hexagonal (soda straw packing) and cubic sphere arrangements of pores, we
wondered whether the modulus-density scaling relationships of these
nanoscale materials would be similar to the optimized evolved materials [
like bone]. We found that both material structure and pore sizes matter. At
all densities we observed that the cubic arrangement was stiffer than the
hexagonal arrangement, which was stiffer than the worm-like. For each of
these structures, increasing porosity caused a reduction in modulus, but the
reduction was less than for theoretically optimized or naturally evolved
materials due to the attendant stiffening of the thinning nanoscale silica
walls resulting from the formation of small stiff silica rings.

“This change in ring structure only happens at the nanoscale,” says
Brinker.

Sandia researcher Hongyou Fan created cubic, cylindrical, and worm-like (or
disordered) pores to evaluate differences in stiffness resulting from these
differently shaped internal spaces.