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Sponges of Boron Nitride

January 28, 2016

One of my first scientific papers was published in the now defunct journal, High Temperature Science.[1] High Temperature Science, published from 1969-1991, was founded and edited by the prolific chemist, John L. Margrave.[2] Margrave, who was elected to the National Academy of Sciences in 1974, received his first degree in physics from the University of Kansas in 1948 after serving in the US Army Signal Corps during World War II.[2]

Margrave discovered that he was more interested in chemistry, so he continued his graduate education in chemistry at Kansas. Completing experiments in fluorine chemistry under high temperature and high pressure conditions, Margrave wrote a 300 page dissertation and completed his Ph.D. in just two years. After that, he worked as an Atomic Energy Commission postdoctoral fellow under Leo Brewer at the University of California, Berkeley.[2] Brewer and Kenneth Pitzer were co-authors of the revision of a popular thermodynamics textbook authored by Gilbert N. Lewis and Merle Randall.[3]

Margrave became an instructor in the chemistry department at the University of Wisconsin-Madison in 1952, and much of his research involved fluorine bomb calorimetry.[2] In this technique, a useful complement to oxygen bomb calorimetry, a substance is burned in a fluorine atmosphere so that its heat of formation can be calculated. Margrave moved to Rice University in 1963, a relocation perhaps facilitated by Kenneth Pitzer, who was the university president at the time.

A bomb calorimeterWhile I was doing differential scanning calorimetry as a graduate student, a colleague of mine was doing fluorine bomb calorimetry in an apparatus much like this.

(Photo by Akshat Goel, via Wikimedia Commons.)

One principal theme of Margrave's research at Rice was fluorination of carbon compounds, principally the fluorination of graphite to form "white graphite." This fluorinated graphite compound, known as CFX, is an excellent solid lubricant in oxidizing and corrosive environments at high temperature.[2] One of Margrave's students told me that they had fluorinated peanut butter to see what would happen. Perhaps this was one of those unsanctioned experiments that students are known to do. Interestingly, Margrave was co-inventor of an improved embalming fluid.[4]

Boron nitride is another high temperature solid lubricant, since its hexagonal polymorph has a layered structure like that of graphite. It's stable up to about 800 °C in air, but only because its oxidation product, boric oxide, forms a protective surface layer. As the calculated free energy of the oxidation reaction shows, the oxidation to boric oxide is preferred in oxygen at all temperatures.[5] However, boron nitride is stable in inert atmospheres, vacuum, and reducing atmospheres, up to about 2000°C.

Free energy for boron nitride oxidationBoron nitride is not as stable as its oxide, as the calculated free energy of the boron nitride oxidation reaction shows. It's stable in air only because boric oxide forms a protective coating.

(Graphed using Gnumeric from NIST data.[5])

Hexagonal boron nitride has another use. Scientists at Deakin University (Victoria, Australia), Drexel University (Philadelphia, Pennsylvania), and the Missouri University of Science & Technology (Rolla, Missouri) have created an aerogel of boron nitride that functions as a sponge for oil.[6-9] The aerogel is formed from boron nitride nanosheets.[6-9]

The boron nitride nanosheets, composed of just a few layers of hexagonal boron nitride, have a pore structure that allows absorption of more than 33 times their weight in organic solvents, such as oil.[6,8] The aerogels have a density of just 1.4 mg/cc, about 1,500 times less than the density of bulk boron nitride. These aerogels are formed as freestanding membranes by freeze-drying.[6] The surface area per gram is about the area of five and a half tennis courts.[8-9]

Boron nitride sponge (Drexel University)
Left, a freestanding boron nitride membrane. Right, a Scanning electron micrograph of the boron nitride nanosheet pore structure. left image via Deakin University; right image via Drexel University.

The 2010 Gulf Coast oil spill had a reported cost of $40 billion.[9] Says Deakin University professor and an author of the paper describing this research, Ying Chen,
"Oil spills are a global problem and wreak havoc on our aquatic ecosystems, not to mention cost billions of dollars in damage... Everyone remembers the Gulf Coast disaster, but here in Australia they are a regular problem, and not just in our waters. Oil spills from trucks and other vehicles can close freeways for an entire day, again amounting to large economic losses... We are so excited to have finally got to this stage after two years of trying to work out how to turn what we knew was a good material into something that could be practically used."[8]

The research team developed an oil-absorbing boron nitride powder in 2013, but a powder form is not suitable for oil remediation. Says Deakin University's Weiwei Lei, another member of the research team, "...you cannot simply throw powder onto oil – you need to be able to bind that powder into a sponge so that we can soak the oil up, and also separate it from water."[8] A one-step mechano-chemical process was used to exfoliate the nanosheets from the hexagonal boron nitride and functionalize them with amino groups to create highly water-dispersible particles.[6]

As another interesting material property, the nanosheets show a strong blue light emission under exposure to ultraviolet light, in both their dispersed and dry states.[6] This research was supported by the Australian Research Council.[9]

References:

  1. D.M. Gualtieri and P.J. Ficalora, Electron Transfer and Metallic Bonding: The Heats of Reaction of FeAl3-x(Ag;Zn;Pt;Au)x Alloys, High Temperature Science, vol. 7, pp. 25-36 (1975).
  2. James L. Kinsey, "Biographical Memoir-John Margrave, National Academy of Sciences, 2014.
  3. Gilbert Newton Lewis and Merle Randall, "Thermodynamics," Kenneth S. Pitzer and Leo Brewer, Eds., McGraw Hill (Second edition, 1961), 723 pp. (Amazon).
  4. James W. Campbell and John L. Margrave, "Embalming composition and method," US Patent No. 5,405,606, April 11, 1995 (Google Patents).
  5. NIST-JANAF Thermochemical Tables, US National Institute of Standards and Technology Web Site.
  6. Weiwei Lei, Vadym N. Mochalin, Dan Liu, Si Qin, Yury Gogotsi, and Ying Chen, "Boron nitride colloidal solutions, ultralight aerogels and freestanding membranes through one-step exfoliation and functionalization," Nature Communications, vol. 6, article no. 8849 (November 27, 2015), doi:10.1038/ncomms9849. This is an open access publication with a PDF file available here.
  7. Supplementary figures for ref. 6 .
  8. Drexel Materials Scientists Aid Australian Institution in Developing Super-Absorbent Material That Can Soak Up Oil Spills, Drexel University Press Release, November 30, 2015.
  9. Deakin scientists create revolutionary material to clean oil spills, Deakin University Press Release, November 30, 2015

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