Nano-based transformer insulating fluid

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2012

Hexagonal boron nitride nanoparticles are capable of improving the performance of transformer oils.

KEY CONCEPTS
• Hexagonal boron nitride nanoparticles with average diameters of 600 nanometers have been incorporated into a transformer insulating fluid.
• Good electric insulation and thermal conductivity properties were key factors in using hexagonal boron nitride.
• Stable dispersions of hexagonal boron nitride at 0.01% and 0.1% treat rates displayed thermal conductivity enhancements at 9% and approximately 80%, respectively.

ONE OF THE PROMISING AREAS FOR USING NANOTECHNOLOGY IN LUBRICANTS IS THE USE OF NANOADDITIVES to improve the thermal conductivity in such applications as refrigeration and heat transfer. The driver for this approach is that movement to more efficient machinery is dictating the need for better management of heat in order to conserve energy.

Nanoadditives exhibit high-thermal conductivity values and have been found to boost heat transfer in these applications. In a previous TLT article, use of copper oxide nanoparticles at low treat rates leads to a dramatic improvement in the heat transfer properties of a polyol ester-based refrigeration lubricant (1). The research found that use of a 4% volume fraction of the copper oxide nanoparticles in the lubricant increased heat transfer between 50% and 275%.

Another application that requires effective heat transfer is transformer insulating fluids. They function by keeping the transformer cool and providing electrical insulation between internal live parts. Two other objectives are to minimize corona and arcing.

Transformer insulating fluids may need to operate under extended periods of high temperature and operating loads, which means that a good quality lubricant is needed to hold up under these challenging conditions. Jaime Taha-Tijerina, graduate student at Rice University in Houston, says, “We have seen that nanoadditives have been used successfully to boost the performance of heat transfer fluids. Recent developments of two-dimensional (2D) nanomaterials have led to the prospect that they can be used to improve the performance of transformer oils.”

A new type of nano-based transformer insulating fluid has just been developed that can further improve performance.

HEXAGONAL BORON NITRIDE
Taha-Tijerina, in collaboration with postdoctoral researcher Tharangattu Narayanan and Pulickel M. Ajayan, Benjamin M. and Mary Greenwood Anderson Professor in Engineering, Materials Science and Nanotechnology at Rice University, prepared a nanobased transformer oil using hexagonal boron nitride. He says, “We chose to work with hexagonal boron nitride because it exhibits good electric insulation properties in combination with good thermal conductance. Not many materials display both of these features that are very important in transformer insulating fluids.”

There are other nanoadditives such as alumina, copper oxide and titanium oxide that could be used. But these materials do not display the positive characteristics of hexagonal boron nitride. Taha-Tijerina says, “The problem is larger fractions of these nanoadditives need to be used, which leads to higher concentrations in the transformer oil and, as a consequence, higher viscosities. This factor can create negative issues such as agglomeration, clogging and sedimentation.”

The hexagonal boron nitride was prepared by a liquid exfoliation process. Taha-Tijerina says, “We sonicated the hexagonal boron nitride in isopropyl alcohol at room temperature. This step was followed by centrifuging followed by filtration of the supernatant liquid.”

The resulting hexagonal boron nitride particles contain five atomic layers and exhibit a diameter of 600 nanometers. This material was formulated into an inhibited, naphthenic-based oil that fulfills the requirements for ASTM D3487 Type II (Standard Specification for Mineral Insulating Oil).

The researchers found that a stable dispersion of hexagonal boron nitride can be achieved with this base oil. Taha-Tijerina says, “We believe that the oleophilic characteristics of hexagonal boron nitride enhance the dispersion in the base oil.” In addition, only a small increase in the viscosity of the oil is observed.

Hexagonal boron nitride was used at a treat rate up to 0.1% without any difficulty. Figure 1 shows a sample of the naphthenic base oil (shown on the left), a 0.01% dispersion containing hexagonal boron nitride (shown second from the left) and a 0.1% dispersion of hexagonal boron nitride (shown third from the left).


Figure 1. Stable dispersions of hexagonal boron nitride nanoparticles were prepared in naphthenic base oil (shown on the left) at 0.01% (shown second from the left) and 0.1% (shown third from the left) treat rates. These dispersions enhanced the performance of transformer insulating fluids. (Courtesy of Rice University)

Evaluation of the 0.01% and 0.1% dispersions shows thermal conductivity enhancements of 9% and approximately 80%, respectively. Taha-Tijerina indicates that two mechanisms can explain the performance of hexagonal boron nitride. He explains, “The two-dimensional hexagonal boron nitride flakes provide a percolation channel that facilitates movement of electrons. Brownian motion of the nanoparticles is the second mechanism that leads to the improvement in thermal conductivity.”

A third beneficial feature is that no surfactant is needed to stabilize the dispersion. Narayanan says, “The presence of a surfactant can decrease the thermal conductivity by inhibiting electron flow.”

The hexagonal boron nitride-based transformer oil also shows a relatively stable zeta-potential value of 22 millivolts, indicating the dispersion is stable. Further evidence for the stability is derived from shear viscosity studies. Narayanan says, “The dispersion showed good stability because it retained Newtonian behavior throughout the range of temperatures used in the study.”

Electrical properties of the transformer oil are not compromised when the hexagonal boron nitride is used. One other interesting feature is that use of the hexagonal boron nitride acted as a pour point depressant. Narayanan says, “We feel the pour point reduction is due to the high intermolecular interactions between the hexagonal boron nitride nanoparticles and the oil. A 3 K reduction was seen at a 0.1% concentration of the nanoparticles.

The researchers see great promise with using hexagonal boron nitride at very small treat rates in transformer insulating fluids. Further information can be found in a recent article (2) or by contacting Ajayan at ajayan@rice.edu.

REFERENCES
1. Canter, N. (2008), “Improving Refrigeration with Nanoparticles,” TLT, 64 (11), pp. 14-15.
2. Tijerina, J., Narayanan, T., Gao, G., Rohde, M. Tsentalovich, D., Pasquali, M. and Ajayan, P. (2012), “Electrically Insulating Thermal Nano-Oils Using 2D Filler,” ACS Nano, 6 (2), pp. 1214-1220.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.