Boron Nitride Coatings on Graphite Surfaces to Protect from Oxidation

Increasing the service life of different graphite parts at high temperatures is very important in many industrial applications. One way to achieve this objective is to produce oxidation protection coatings of boron nitride on graphite substrates. The chemical inertness and oxidation resistance of boron nitride has led researchers to consider this material as one of the best coatings to protect graphite against oxidation. Many aspects of obtaining nitride coatings on graphite substrates by forming a boron nitride layer in a heterophase process have been investigated. Oxidation tests performed on the coated graphite have revealed a high efficiency of boron nitride coating for graphite protection. As compared with that of uncoated specimens the oxidation resistance of BN-coated graphite substrates was 8 times higher at 750°C and 12 times higher at 1200°C. The studies have shown that boron nitride coatings can effectively protect carbon materials against oxidation and increase the service life of these materials at high temperatures.

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Hexagonal Boron Nitride-Based Electrolyte Composite for Li-Ion Battery Operation from Room Temperature to 150 °C

Battery components can take the heat

Rice University team creates robust ‘white graphene’ electrolyte and separator for lithium-ion batteries 

HOUSTON – (April 11, 2016) – Rice University materials scientists have introduced a combined electrolyte and separator for rechargeable lithium-ion batteries that supplies energy at usable voltages and in high temperatures.

An essential part of the nonflammable, toothpaste-like composite is hexagonal boron nitride (h-BN), the atom-thin compound often called “white graphene.”

The Rice team led by materials scientist Pulickel Ajayan said batteries made with the composite functioned perfectly in temperatures of 150 degrees Celsius (302 degrees Fahrenheit) for more than a month with negligible loss of efficiency. Test batteries consistently operated from room temperature to 150 C, setting one of the widest temperature ranges ever reported for such devices, the researchers said.

“We tested our composite against benchmark electrodes and found that the batteries were stable for more than 600 cycles of charge and discharge at high temperatures,” said lead author Marco-Túlio Rodrigues, a Rice graduate student.

The results were reported in Advanced Energy Materials.

Last year members of a Rice and Wayne State University team introduced an electrolyte made primarily of common bentonite clay that operated at 120 C. This year the team validated its hunch that h-BN would serve the purpose even better.

Rodrigues said batteries with the new electrolyte are geared more toward industrial and aerospace applications than cellphones. In particular, oil and gas companies require robust batteries to power sensors on wellheads. “They put a lot of sensors around drill bits, which experience extreme temperatures,” he said. “It’s a real challenge to power these devices when they are thousands of feet downhole.”

“At present, nonrechargeable batteries are heavily used for the majority of these applications, which pose practical limitations on changing batteries on each discharge and also for disposing their raw materials,” said Rice alumnus and co-author Leela Mohana Reddy Arava, now an assistant professor of mechanical engineering at Wayne State.

Hexagonal boron nitride is not a conductor and is not known to be an ionic conductor, Rodrigues said. “So we didn’t expect it to be any obvious help to battery performance. But we thought a material that is chemically and mechanically resistant, even at very high temperatures, might give some stability to the electrolyte layer.”

He said boron nitride is a common component in ceramics for high-temperature applications. “It’s fairly inert, so it shouldn’t react with any chemicals, it won’t expand or contract a lot and the temperature isn’t a problem. That made it perfect.”

The material eliminates the need for conventional plastic or polymer separators, membranes that keep a battery’s electrodes apart to prevent short circuits. “They tend to shrink or melt at high temperatures,” said Rice postdoctoral researcher and co-author Hemtej Gullapalli.

Tests went better than the researchers anticipated. Though inert, the mix of h-BN, piperidinium-based ionic liquid and a lithium salt seemed to catalyze a better reaction from all the chemicals around it.

“It took almost two years to confirm that even though the boron nitride, which is a very simple formulation, is not expected to have any chemical reaction, it’s giving a positive contribution to the way the battery works,” Gullapalli said. “It actually makes the electrolyte more stable in situations when you have high temperature and high voltages combined.”

He noted all the electrolyte’s components are nonflammable. “It’s completely safe. If there’s a failure, it’s not going to catch fire,” he said.

“Our group has been interested in designing energy storage devices with expandable form factors and working conditions,” Ajayan said. “We had previously designed paper and paintable battery concepts that change the fundamental way power delivery can be imagined. Similarly, pushing the boundaries of working temperature ranges is very interesting. There is no commercial battery product that works above about 80 C. Our interest is to break this barrier and create stable batteries at twice this temperature limit or more.”

Co-authors are Rice graduate student Kaushik Kalaga and Wayne State postdoctoral fellow Ganguli Babu. Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry.

The University of Texas at Austin through the Advanced Energy Consortium supported the project.

- See more at: http://news.rice.edu/2016/04/11/battery-components-can-take-the-heat/#sthash.sL1mI0CW.dpuf

Thermally Conductive Flexible Compounds with Boron Nitride CoolFX Powders

        




Styrenic TPE compounds can be formulated with a 65o Shore A hardness and different levels of additives, up to 30% (weight), to modify their thermal properties. As shown in the table below, using BN had the greatest effect, with a 50% improvement in the in- plane thermal conductivity of the compound. 

It is observed that the thermal conductivity is not improved in the cross-direction (through plane), indicating that the thermal conductivity is strongly affected by the orientation of the sample and the anisotropic structure of BN. On the contrary, it is possible to define an optimal composition (30% BN) to maximize thermal conductivity in the in-plane direction.

 Another important effect of BN is related to the flowability of the compounds. It can be observed that the melt flow of the samples formulated with BN present higher level values, indicating that the additive is acting as an internal lubricant of the material without affecting the rest of the material’s properties (for example the hardness).

    Note: Test data. Actual results may vary. 

Very favorable results in abrasion resistance of the compounds are also observed, even at high loading levels (30 wt% BN). 

Thermally conductive TPE compounds can be formulated for use in applications where heat transfer is an important factor, for instance in electronic housing sealing, soft electronic sensors, soft security sensors, etc. By using specific additives, the heat distribution of the different TPE compounds can be modified to reach the desired thermal and mechanical performance.

   

The use of BN from Momentive Performance Materials, in particular the fillers CoolFX Hybrid filler, can provide thermal conductivity performance and allow easily colourable compounds, as the powders are white. In this study, CoolFX 1022 Filler was evaluated, a proprietary powder developed to provide the best balance of thermal and mechanical properties and cost.

CoolFX hybrid fillers offer TPE formulators a very interesting tool for broadening the final applications of their materials and for reaching functional properties in demanding applications like flexible electronic housings, heating pads, multi-component pieces, thermal conductive sheets, and others. 

*CoolFX is a trademark of Momentive Performance Materials Inc. 

I.Alonso, Dynasol Elastómeros S.A.
Dr. C. Raman, Momentive Performance Materials Quartz Inc. 

For more details contact: Innovative Growth Enterprises, Ludhiana. Mob: 9910899409

Researchers Discover New Phase of Boron Nitride and a New Way to Create Pure c-BN

Researchers at North Carolina State University have discovered a new phase of the material boron nitride (Q-BN), which has potential applications for both manufacturing tools and electronic displays. The researchers have also developed a new technique for creating cubic boron nitride (c-BN) at ambient temperatures and air pressure, which has a suite of applications, including the development of advanced power grid technologies.

“This is a sequel to our Q-carbon discovery and converting Q-carbon into diamond,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and lead author of a paper describing the research. “We have bypassed what were thought to be the limits of boron nitride’s thermodynamics with the help of kinetics and time control to create this new phase of boron nitride.

c-BN nanocrystallites. Image credit: Anagh Bhaumik. Click to enlarge.

“We have also developed a faster, less expensive way to create c-BN, making the material more viable for applications such as high-power electronics, transistors and solid state devices,” Narayan says. “C-BN nanoneedles and microneedles, which can be made using our technique, also have potential for use in biomedical devices.” C-BN is a form of boron nitride that has a cubic crystalline structure, analogous to diamond.

Early tests indicate that Q-BN is harder than diamond, and it holds an advantage over diamond when it comes to creating cutting tools. Diamond, like all carbon, reacts with iron and ferrous materials. Q-BN does not. The Q-BN has an amorphous structure, and it can easily be used to coat cutting tools, preventing them from reacting with ferrous materials.

“We have also created diamond/c-BN crystalline composites for next-generation high-speed machining and deep-sea drilling applications,” Narayan says. “Specifically, we have grown diamond on c-BN by using pulsed laser deposition of carbon at 500 degrees Celsius without the presence of hydrogen, creating c-BN and diamond epitaxial composites.”

The Q-BN also has a low work function and negative electron affinity, which effectively means that it glows in the dark when exposed to very low levels of electrical fields. These characteristics are what make it a promising material for energy-efficient display technologies.

To make Q-BN, researchers begin with a layer of thermodynamically stable hexagonal boron nitride (h-BN), which can be up to 500-1000 nanometers thick. The material is placed on a substrate and researchers then use high-power laser pulses to rapidly heat the h-BN to 2,800 degrees Kelvin, or 4,580 degrees Fahrenheit. The material is then quenched, using a substrate that quickly absorbs the heat. The whole process takes approximately one-fifth of a microsecond and is done at ambient air pressure.

By manipulating the seeding substrate beneath the material and the time it takes to cool the material, researchers can control whether the h-BN is converted to Q-BN or c-BN. These same variables can be used to determine whether the c-BN forms into microneedles, nanoneedles, nanodots, microcrystals or a film.

“Using this technique, we are able to create up to a 100- to 200-square-inch film of Q-BN or c-BN in one second,” Narayan says.

By comparison, previous techniques for creating c-BN required heating hexagonal boron nitride to 3,500 degrees Kelvin (5,840 degrees Fahrenheit) and applying 95,000 atmospheres of pressure.

C-BN has similar properties to diamond, but has several advantages over diamond: c-BN has a higher bandgap, which is attractive for use in high-power devices; c-BN can be “doped” to give it positively- and negatively-charged layers, which means it could be used to make transistors; and it forms a stable oxide layer on its surface when exposed to oxygen, making it stable at high temperatures. This last characteristic means it could be used to make solid state devices and protective coatings for high-speed machining tools used in oxygen-ambient environments.

“We’re optimistic that our discovery will be used to develop c-BN-based transistors and high-powered devices to replace bulky transformers and help create the next generation of the power grid,” Narayan says.

The paper, “Direct conversion of h-BN into pure c-BN at ambient temperatures and pressures in air,” was published online Feb. 3 in the open-access journal APL Materials. The paper was co-authored by NC State Ph.D. student Anagh Bhaumik. The work was supported by the National Science Foundation under grant DMR-1304607.