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.

Meet the Need for New and Innovative Cooling Techniques with Thermally Conductive Plastics (TCP) Containing Boron Nitride

According to the US Air Force, 55% of their electronic component failures are due to temperature.*

With increasing global trends in miniaturization of electronic systems and weight savings in transportation, designers and material engineers need to find innovative solutions to meet the thermal management requirements of more compact designs

Boron nitride: An excellent heat conductor and electrical insulator

Boron nitride (BN) has been gaining interest as a filler in thermoplastics to increase thermal conductivity of resins. It is unique in the sense that it is an excellent conductor of heat yet still electrically insulating.

Plastics containing BN are being used to replace traditional metal parts in a number of markets to improve heat-related performance and increase the lifetime of systems. Overall system cost and complexity can be reduced.

Examples of potential applications include under-the-hood automotive parts, sensors and housings for motors, LEDs and many other electronic devices, molded heat sinks, and medical device components - basically, anywhere heat is an issue.

For more details contact : innovative_growth@yahoo.co.in

A New Progress Made For The Research of Metallic Boron Nitride

Recently a new progress on metallic Boron Nitride (BN) has been made by Prof. Qian Wang’s group at the Center for Applied Physics and Technology (CAPT), College of Engineering at Peking University and her collaborators. With the aid of state-of-the-art theoretical calculations, they proposed new BN allotropes which exhibit unusual metallicity. This work is recently published in Journal of the American Chemical Society (J. Am. Chem. Soc.2013, 135, 18216？18221).

How to convert an insulator or semiconductor into a metal is an important and fundamental topic. Much of the current electronics depend upon this. For nearly a century this is primarily accomplished by doping. Manipulating their structure to induce a metallic transition, however, has not been a common practice. It is in this aspect that Wang and co-workers focused on BN which is a chemical analogue of Carbon and shares with it similar structures such as one-dimensional nanotube, two-dimensional nanosheet characterized by sp² bonding, and three-dimensional (3D) diamond structure characterized by sp³ bonding. However, unlike Carbon which can be metallic in certain forms, BN is an insulator, irrespective of its structure and dimensionality. Taking the advantage of boron’s capacity to form multielectron-multicenter bonds, Wang and co-workers designed the tetragonal structures of BN containing both sp² and sp³ hybridizations based on first-principle density-functional calculations. The new phases of BN are both dynamically stable and metallic. Analysis of their electronic structures reveals the metallic behavior comes from the delocalized B 2p electrons. The metallicity exhibited in the studied 3D BN structures can lead to materials beyond conventional ceramics as well as to materials with novel transport properties and potential for applications in electronic devices. High-temperature insulator has metallic potential. This work may stimulate experimentalists to synthesize these novel forms of metallic BN and once that is achieved, it will have transformative impact on science and technology.

This work has been highlighted by “Spotlights on Recent JACS Publications” and Chemistryviews:

http://pubs.acs.org/doi/pdfplus/10.1021/ja4124016

Thermally Conductive, Electrically Insulating and melt-processable Polystyrene/Boron nitride Nanocomposites Prepared by in situ Reversible Addition Fragmentation Chain Transfer Polymerization

ABSTRACT 

Thermally conductive and electrically insulating polymer/boron nitride (BN) nanocomposites are highly attractive for various applications in many thermal management fields. However, so far most of the preparation methods for polymer/BN nanocomposites have usually caused difficulties in the material post processing. Here, an in situ grafting approach is designed to fabricate thermally conductive, electrically insulating and post-melt processable polystyrene (PS)/BN nanosphere (BNNS) nanocomposites by initiating styrene (St) on the surface functionalized BNNSs via reversible addition fragmentation chain transfer polymerization. The nanocomposites exhibit significantly enhanced thermal conductivity. For example, at a St/BN feeding ratio of 5:1, an enhancement ratio of 1375% is achieved in comparison with pure PS. Moreover, the dielectric properties of the nanocomposites show a desirable weak dependence on frequency, and the dielectric loss tangent of the nanocomposites remains at a very low level. More importantly, the nanocomposites can be subjected to multiple melt processing to form different shapes. Our method can become a universal approach to prepare thermally conductive, electrically insulating and melt-processable polymer nanocomposites with diverse monomers and nano fillers.


The authors of this publication are on ResearchGate and have made the full-text available on their profiles. Department of Physics, Michigan Technological University, 118 Fisher Hall, 1400 Townsend Drive, Houghton, MI 49931, USA. 
Nanoscale (Impact Factor: 6.73). 10/2010; 2(10):2028-34. DOI: 10.1039/c0nr00335b 

Source: PubMed

Hexagonal Boron Nitride in Lambda Sensors

In the automotive industry, Hexagonal Boron Nitride with boron oxide as binder is used as a sealing element in oxygen sensors (lambda sensors). For modern engine control, measurement of the exact composition of the flue gas is necessary. Lambda sensors measure the oxygen content in the flue gas relative to a reference oxygen concentration. The signal is passed on to engine control, which adjusts the oxygen/fuel mixture appropriately. To guarantee reliable measurement, the measurement and reference chamber is separated by a packing seal. The dense pack material must also be an electric insulator, resistant to high temperature and suitable for lambda sensor production. One of the two components of the packing seal consists of a sealing ring made of Momentive HBN Grade Hot Pressed Boron Nitride.

2D Layered Insulator Hexagonal Boron Nitride Enabled Surface Passivation in Dye Sensitized Solar Cells

ABSTRACT

A two-dimensional layered insulator, hexagonal boron nitride (h-BN), is demonstrated as a new class of surface passivation materials in dye-sensitized solar cells (DSSCs) to reduce interfacial carrier recombination. We observe 57% enhancement in the photo-conversion efficiency of the DSSC utilizing h-BN coated semiconductor TiO₂ as compared with the device without surface passivation. The h-BN coated TiO₂ is characterized by Raman spectroscopy to confirm the presence of highly crystalline, mixed monolayer/few-layer h-BN nanoflakes on the surface of TiO₂. The passivation helps to minimize electron–hole recombination at the TiO₂/dye/electrolyte interfaces. The DSSC with h-BN passivation exhibits significantly lower dark saturation current in the low forward bias region and higher saturation in the high forward bias region, respectively, suggesting that the interface quality is largely improved without impeding carrier transport at the material interface. The experimental results reveal that the emerging 2D layered insulator could be used for effective surface passivation in solar cell applications attributed to desirable material features such as high crystallinity and self-terminated/dangling-bond-free atomic planes as compared with high-k thin-film dielectrics.

This research paper was was published in Nanoscale online on 6th September 2003

Boron Nitride : Electronic Applications Requiring More Than Heat Dissipation:

Thermal management has always been a major concern in the design of high frequency, high power electronic devices. For example, historically, in many power amplifier designs, the vast majority of the power that needs to be dissipated is from the high power field effect transistors, or FET's, themselves. In these cases, the high power dissipation often requires direct attachment of the FET's to a heat sink of some type. Heat sinks have become almost essential to modern central processing units and other electronic devices.

Ideally, heat sinks are made from a good thermal conductor such as silver, gold, copper or aluminum alloy. Copper and aluminum are among the most frequently used materials for this purpose with electronic devices. In addition, recently, synthetic diamond cooling sinks have been developed to provide better cooling. Further, some heat sinks are constructed of more than one material with desirable features, such as phase change materials. Such materials can store a great deal of energy due to their heat of fusion. However, while many of these materials provide heat sink features, they do not address other needs of the device, such as electrical insulation. This article will highlight those needs and how new materials deal with those requirements.

Applications Overview

Before discussing new developments in materials for thermal management, an overview of electronics applications and thermal management will be given. Currently, there are four general categories of electronic applications that require thermal management such as provided by heat sinks and other approaches. They can be broadly classified as (1) medical electronics (2) consumer electronics (3) automotive electronics and (4) aerospace and defense electronics. Each of these categories has unique requirements in terms of thermal management. A brief discussion of those various needs will now be presented.

In the area of medical electronics, from imaging equipment to surgical instruments and automated immunoassays, more power means more heat, generally in a small space. Further, as greater demands for precision and reliability are placed on medical equipment, thermal control becomes more critical. To address that concern, medical equipment designers are using passive thermal control systems that include heat pipes and vapor chambers. Such devices offer high reliability, design flexibility, manageable cost, and quiet operation.

The consumer electronics field is being driven by the need for smaller, faster and lighter products. That need has put considerable demands on the thermal management of microelectronics. This area offers potential opportunities for materials-enabled innovation. Materials for electronics such as tablets and smart phones are moving to encompass the entire body of the smart phone or tablet, making heat dissipation more effective. This will likely push the boundaries in new materials where aesthetics such as color and texture will become important features for adoption. Thermally conductive polymers are seeing an increase in this segment of the market.

Electronics in automobiles has become more and more sophisticated and power consumption only seems to increase. In recent years, the proliferation of electronic hybrid cars has created new cooling problems with electronics that control large amounts of current. For example, in automobiles like the Toyota Prius, cooling of the battery pack is necessary to keep the temperatures of all of the modules as uniform as possible. This maximizes the performance and the life of the battery. Unique materials solutions may offer opportunities to address these issues.

Aircraft thermal management is becoming increasingly important to the safe design and operation of commercial and military aircraft due to the growing heat loads from expanded avionic functionality, more electrical systems architectures, and the greater temperature sensitivity of composite material systems compared to traditional metallic designs. Military aircraft designers face the additional challenges of removing the waste heat from advanced weapon systems. Examples of components that are sensitive to thermal fluctuations are heat shields, anti-icing systems and propulsion systems. Presently, these needs are typically addressed through the use of air exchangers or some type of fan assembly.

Thus, of the four applications that have been discussed, the two areas for which new materials are of high priority are automobile and consumer electronics. Of these, consumer electronics appears to be the most needing of the new technologies. Attention will now be turned to current requirements and how those can be met by new materials.

Market Requirements

Until recently, metal-based systems have been the primary material solution of choice. Aluminum was primarily selected for various light-weight thermal management systems. However, these systems are becoming more expensive based on raw material price increases that have been observed over the past few years. In that regard, plastics offer suitable performance for many of the thermal management applications that are being developed.

In order to gain insight into developing needs, a recent SpecialChem survey asked respondents about material requirements for thermal management applications. The results of the survey were that thermal management goes beyond thermal conductivity alone. Specifically, electrical insulation and color freedom were valued as other features beyond thermal conductivity. In addition, the following percentage of the respondents voted for the various options:

Physical Properties Best In Class Well Balanced Cost Option Electrically Insulating Plastic Yes Yes No Color/Colorability White White Black Resin + Filler Compound Price ($/100 cc) 3,5 3 1 Tensile Strength (MPa) 100 65 65 Impact Strength (J/m) 80 35 35 % of votes 27% 49% 24% Table 1: Results of Survey on Thermal Management Materials (based on 77 feedbacks)

From these results, it can be seen that a well-balanced formulation is the choice of about one-half of the respondents. In this context, a well-balanced formulation is described as one that provides not only thermal conductivity but also other features such as color. It should be noted that the cost option gathered the lowest percentage in the survey.

Thermally Conductive Plastics

In order to address present needs, thermally conductive plastics are made through the incorporation of high thermal conductivity fillers into the thermoplastic polymer matrix. Examples of common conductive fillers that are used are graphite, expanded graphite and carbon fibers. But, the use of these materials results in the final composite material being black and electrically conductive. As already stated, this is undesirable for consumer electronics applications such as smartphones. On the other hand, boron nitride is a synthetic ceramic that is both an excellent conductor of heat and a dielectric material. Recently, Momentive Performance Materials has developed Boron Nitride fillers enabling composite to reach acceptable thermal and electrical properties.

There are other ceramic materials, such as alumina, aluminum nitride and silica that show similar characteristics but boron nitride has the highest thermal conductivity of any of these materials. Table 2 provides a comparison of some properties of boron nitride to other competing fillers.

Property BN Al2O3 AlN SiO2 Thermal Conductivity (W/m/K) 300 30 260 1.3 Dielectric Constant 2.28 3.98 3.26 2.20 Mohs hardness <2 >9 ~ 7 ~ 6.5 Table 2: Comparison of Boron Nitride to Other Fillers

As can be seen from this Table, boron nitride offers both very high thermal conductivity values as well as a low dielectric constant. In addition, due to its softness, it can be readily compounded into many polymers with limited wear on the processing equipment.

Momentive Performance Materials' New Grade of Boron Nitride

Momentive Performance Materials has recently developed a new grade of boron nitride, designated CFX 600, which provides enhanced thermal conductivity while at the same time providing electrical insulation in a wide variety of thermoplastic resins. It is a surface treated version of boron nitride that allows for the attainment of high thermal conductivity and improved physical properties at lower loading levels than are commonly used. Specifically, CFX 600 can provide up to 20% enhancement in thermal conductivity while potentially also providing improvements in mechanical properties compared to untreated boron nitride powder at the same loading level.

This is just one example of a material development for thermal management that is focused on the growing needs of applications like consumer electronics. Those needs are clearly pointing out the requirement for more than simply thermal conductivity. Instead, materials solutions are being sought that provide a balance of properties in the final composite material. As these developing applications continue to stress the need for faster electronics, it is expected that materials developments will need to continue to address those needs.

Source: Special4polymers

For more details contact : innovative_growth@yahoo.co.in