Tikalon Blog by Dev Gualtieri

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Honeycombs and Herringbones

May 31, 2013

Objects sometimes have hidden properties that change their operating characteristics. Unintentionally magnetized tools, for example, are the bane of many a technician . They upset your arrangement of parts by their magnetism, and that's why some tools are fabricated from non-magnetic steels . Other tools purposely use magnetism to aid in their function, such as the magnetized screwdrivers that help hold a bolt in place while you're aligning it to a threaded hole.

I have a soldering iron whose tip is sometimes annoyingly magnetic, moving some types of components as I try to solder their electrical leads. The interesting thing is that the magnetism of this soldering iron is a side-effect of its operating principle.

Carl E. Weller was issued US Patent No. 2,951,927 on September 6, 1960, for an "Electric Soldering Iron" with a novel temperature regulating mechanism . When the soldering iron tip got too hot, an inner element would lose its magnetism and thermally disconnect the tip from the electrical heat source.[1]

Magnetic attraction of a Weller soldering iron tip

Magnetic attraction of a Weller soldering iron tip.

Carl Weller's patented design for keeping a soldering iron tip at the proper temperature has a magnetic slug acting as a thermal switch. Heat decreases the magnetization of a ferromagnetic material , eventually extinguishing it at the Curie temperature .

(Photo by the author)

A recent article in Nature Chemistry illuminates an unexpected hidden property of a nanomaterial .[2-3] This work was the joint research of a huge team from three US universities , the University of Pennsylvania (Philadelphia, Pennsylvania), the University of Michigan (Ann Arbor, Michigan) and the Massachusetts Institute of Technology (Cambridge, Massachusetts); and one company, Intelligent Material Solutions, Inc. (Princeton, New Jersey).[2]

As is typical in the sciences , this work started with an unexpected discovery , this one by Christopher Murray , a professor of chemistry at the University of Pennsylvania, involving rafted assemblages of nanoscale crystals of lanthanide fluorides . These crystals were prepared from solution, and their size was limited to about 100 nanometers by the addition of oleic acid , which stuck to the sides of the crystallites and prevented additional growth.

The lanthanide contraction is the name given to the phenomenon that the ionic radius of the lanthanide elements shrinks as the atomic number increases, while their chemical properties remain roughly the same. By using different lanthanide elements in their crystals, the Penn scientists were able to modify the crystal bonding and prepare crystallites of different shapes.

An hexagonal prism

The crystal habit of the early lanthanide fluorides is the hexagonal prism . This changes to a rhombic prism for the later elements .

(Image by R. A. Nonenmacher, via Wikimedia Commons.)

The crystal habit of the early lanthanide fluorides is the hexagonal prism , while the later lanthanides form rhombic prism crystals. By changing the elements , the Penn team was able to vary the prism shape. These nanocrystals were aggregated into close-packed two-dimensional arrays by spreading a mixture of them in hexane onto a layer of ethylene glycol .[2-3] As the solvent evaporated , the crystals were pulled together into the dense packing. The diamond-like shapes and extended hexagons packed as expected. The hexagons arrayed themselves as a stretched honeycomb , which is as would have been expected.

Nanoscale honeycomb pattern of hexagonal crystallites.

Nanoscale honeycomb pattern of hexagonal crystallites. In this case, elongated hexagonal prisms array themselves into the expected honeycomb pattern.

(Photomicrograph by Xingchen Ye of the University of Pennsylvania, via University of Michigan.)

When the hexagons were nearly perfect, with all sides nearly the same length, the scientists were surprised to find that the organization was a more complicated, alternating herringbone pattern , as shown in the figure. At that point, the Penn team sought the advice of a University of Michigan team led by Sharon Glotzer , a professor of materials science and engineering .[4] The Michigan team has experience in computer modeling of how shapes assemble.[3] I wrote about Glotzer's research in two previous articles (Entropic Order, August 20, 2012 and Packing and Filling, May 17, 2012 )

Nanoscale herringbone pattern of hexagonal crystallites.

Nanoscale herringbone pattern of hexagonal crystallites.

(Photomicrograph by Xingchen Ye of the University of Pennsylvania, via the University of Michigan.)

The initial modeling showed that if the fundamental interaction between the crystallites was by their shape, only, the elongated hexagons should form the honeycomb pattern, and not the herringbone.[3] This result indicated that interfacial forces were in play, and they found that if the edges forming the points were stickier than the remaining two sides, the herringbone pattern would result.[3]

Further modeling at MIT showed that the coating at the crystallite surface caused the differential stickiness. Since the arrangement of fluorine atoms on the crystallite facets differs, more of the hydrocarbon will attach to the facets forming the points than the other facets.[3]

The result of the study is a method to regulate the arrangement of such nanoscale objects beyond their shape. Says Penn's Christopher Murray,

"The excitement in this is not in the herringbone pattern, it's about the coupling of experiment and modeling, and how that approach lets us take on a very hard problem."[3]

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