BN Surface Research Articles

Toward High Thermal Conductivity Nano Micro Epoxy Composites with Sufficient Endurance Voltage

We are aiming at the development of epoxy composite materials with both high thermal conductivity and sufficient voltage endurance. A target is set as 10 W/m/K with 5 kV withstand voltage for power electronics applications. In order to achieve high thermal conductivity for polymers such as epoxy, it is a common method to fill inorganic micro particles with high thermal conductivity, but such composites prepared in this way shows lower endurance voltage. To compensate such negative performance, it is considered to be a good way to add nano sized inorganic particles. After many trials were made, good results were obtained for a composite material with two modes of BN and alumina and surface treated nano silica. It has thermal conductivity larger than 5- 10 W/m/K with reasonable withstand voltage.

Review of the electrodeposition of molybdenum carbide on the surfaces of disperse dielectric and semiconductor materials

The systems suitable for a high-temperature electrochemical synthesis on the surfaces of diamond, boron nitride, and silicon and boron carbides were selected on the basis of thermodynamic analysis of the reactions of the dielectrics and semiconductors with ionic melts. The results obtained in the electrochemical study of these systems are reported. As example, the electrodeposition of molybdenum carbide on diamond, BN, SiC and boron carbide surfaces is discussed in detail.

Origins of Thermodynamically Stable Superhydrophobicity of Boron Nitride Nanotubes Coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.

Adsorption of gold atoms on theh-BN/Rh(111) nanomesh

We present density functional calculations of the adsorption of a single Au atom on the $h$-BN/Rh(111) nanomesh and compare it to Au adsorption on a pure $h$-BN surface. While Au binds only weakly to pure $h$-BN (similar to graphene) or to BN at the ``wires'' of the nanomesh, the subsurface Rh atoms in the ``pores'' of the nanomesh modify the electronic structure of $h$-BN considerably and Au adsorbs strongly on top of the B atoms. A large outward relaxation of the B atom accompanies the strong covalent interaction and, in addition, the Au atoms are significantly charged. The results provide a first explanation of the ability of the $h$-BN nanomesh to trap small metal clusters in its pores.

Oxidation of SiC Fiber-Reinforced SiC Matrix Composites with a BN Interphase

The oxidation kinetics of SiC fiber-reinforced SiC matrix composites with a BN interphase (SiC/BN/SiC) and the constituent fibers was characterized by thermogravimetric analysis and microstructural characterization at temperatures (816-1538°C) and oxygen partial pressures (0.1% to 5% O2) relevant to the hypersonic flight and re-entry environments. TGA of the SiC fibers showed that oxidation of the thin BN surface layer led to initially rapid oxidation kinetics and formation of a relatively thick silica scale at very short times under most test conditions. At longer times the fiber oxidation kinetics were representative of silica formation on pure SiC. Oxidation of the composites was conducted on coupons with the SiC seal coat removed on one edge to simulate damage to the composite, allowing ingress of oxygen to the fiber tows. Microscopy was conducted to determine the distance of oxygen ingress into the coupon. At the lower temperatures and oxygen partial pressures the exposed edge did not seal off by silica formation, yet the BN interphase areas were only minimally oxidized. At the intermediate temperatures silica formed at the exposed surface limiting further oxidation of the exposed fibers and BN interphase areas. Finally at the highest temperature and lowest oxygen partial pressure, active oxidation of SiC occurred for both the fibers and coupons resulting in irregular material attack. Implications for use of SiC/BN/SiC materials for hypersonic vehicle thermal protection systems are summarized.

Effective surface treatments for enhancing the thermal conductivity of BN‐filled epoxy composite

AbstractTo improve the thermal conductivity of BN‐filled epoxy composite, admicellar polymerization was used to coat polystyrene and polymethyl methacrylate on the BN surface to improve the interfacial adhesion in the composite. The treated surface was characterized by FTIR and contact angle measurements. The results show that the admicellar treatment led to improved wettability of epoxy resin on the treated surface. Thermal conductivity of the composite increased from 1.5 W/mK for untreated BN to 2.69 W/mK when the admicellar‐treated BN was used, indicating improvement in the interfacial adhesion between BN and epoxy resin in the composite. The mechanical properties of the composite also improved significantly. The surfactant : monomer molar ratio of 1 : 10 was found to be the optimum condition for the admicellar polymerization process. The solubility parameter concept was used to explain the difference in the effectiveness of polystyrene and polymethyl methacrylate. When compared to the more conventional silane treatment, admicellar treatment was found to be more effective in improving the interfacial adhesion between the BN particles and epoxy resin. SEM micrographs of the fractured surface of the composite further confirm the improvement in the interfacial adhesion after the admicellar treatment. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

ChemInform Abstract: Dimer‐Induced Stabilization of H Adsorbate Cluster on BN(0001) Surface

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Dimer-induced stabilization of H adsorbate cluster on BN(0001) surface

Using first-principles calculations, we have studied successive adsorption of hydrogen atoms on a sp2-bonded boron nitride graphitic sheet. Our calculations show that clustering proceeds through the creation of contiguous H-H orthodimer structures stabilizing the H adsorbate cluster on the BN(0001) surface, leading eventually to the formation of hydrogen-contiguous boat-shaped quartets.

Substrate-induced band gap in graphene on hexagonal boron nitride:Ab initiodensity functional calculations

We determine the electronic structure of a graphene sheet on top of a lattice-matched hexagonal boron nitride $(h\text{\ensuremath{-}}\mathrm{B}\mathrm{N})$ substrate using ab initio density functional calculations. The most stable configuration has one carbon atom on top of a boron atom, and the other centered above a BN ring. The resulting inequivalence of the two carbon sites leads to the opening of a gap of $53\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at the Dirac points of graphene and to finite masses for the Dirac fermions. Alternative orientations of the graphene sheet on the BN substrate generate similar band gaps and masses. The band gap induced by the BN surface can greatly improve room temperature pinch-off characteristics of graphene-based field effect transistors.

Adsorption-chemical activity and interaction in the boron nitride-titanium hydride system

The paper examines the effect of physicochemical properties of starting boron nitride and titanium hydride on their reactive activity in the synthesis of the TiN-TiBx composite. There are kinetic data on thermal desorption of the active adsorbate from the BN surface, which precipitated from the gas mixture (20 vol.% O2-80 vol.% N2 under the relative humidity of 80%). Kinetics of hydrogen release from titanium hydride in isothermal conditions and controlled rate of temperature increase in the reactive system is researched. A possible mechanism of chemical transformations of starting compounds and synthesis of the TiN-TiBx composite is considered.

Ab initio calculation of the structural and dynamical properties of the zinc-blende BN and its (110) surface

Using a first-principles pseudopotential technique, we have presented static and dynamical properties of cubic-BN. The calculated values for the lattice constant, bulk modulus, its first derivative and optical dielectric constant are in good agreement with available experimental and theoretical results. Phonon dispersion curves and density of states for cubic-BN were calculated by employing a density functional perturbation theory. In particular, the calculated zone-center and zone-edge phonons compare very well with available experimental results. The calculated structural parameters for the BN(110) surface are in good agreement with previous theoretical results. The highest surface optical phonon mode resonates with the bulk continuum along Γ¯– X¯′ (i.e. across the top-layer B–N atomic chain). Along other symmetry directions, this mode lies above the bulk continuum. The surface acoustic branches lie below the bulk continuum. Energy locations and polarization characteristics of surface rotational and bond stretching phonon modes have been determined.

Ab initio studies of the cubic boron nitride (1 1 0) surface

The surface structure, surface energy, and work function of the cubic boron nitride (1 1 0) surface were determined using density functional theory calculations. The surface energy calculated using the LDA (GGA) is 3.41 (2.81) J/m 2 and the work function is 7.75 (8.9) eV. The surface energy values correspond well to published measured and calculated values of other binary compound ceramics, whereas the work function values are higher than that seen in other binary compound ceramics. The density of states of the surface was analyzed to determine the electron transitions involved in the work function. Geometry minimization of the stoichiometric BN (1 1 0) surface results in a simple reconstruction where pairs of adjacent boron and nitrogen atoms reduce their bond length by ∼0.12 Å, compared to the perfect crystal. Relaxations occur as small oscillations in the (1 1 0) inter-planar spacings that decay to zero after five atomic layers. Oscillations are more pronounced for the boron atoms than the nitrogen atoms, and the final surface has the nitrogen atoms about 0.2 Å higher than the boron atoms. These reconstructions and relaxations are similar to that found in other III–V zincblende materials. Adjacent boron and nitrogen atoms at the surface are more covalently bonded than in the bulk.

Tight-binding calculation of the electronic band structure of GaN, AlN and BN (0 0 1) ideal surfaces

We study the electronic band structure of the zinc-blende phase (0 0 1) ideal surface of GaN, AlN and BN by means of an empirical tight binding sp 3 s * Hamiltonian with nearest-neighbour interactions plus some second-neighbour interactions and the surface Green function matching method. We have studied both anion and cation terminated surfaces and we have obtained the different surface states with their corresponding spectral localization and orbital character.

Coaxial nanocables: Fe nanowires encapsulated in BN nanotubes with intermediate C layers

Fe-filled BN nanocables with intermediate C layers were fabricated. Complete phase separation between C and BN was distinguished using electron energy loss spectroscopy (EELS) measurements and high-resolution transmission electron microscopy (HRTEM) images by the characteristics of well-ordered BN layers and turbostratic C ones. A two-step growth model is proposed for the formation of this remarkable structure. The idea of introducing intermediate C layer, overcoming the wetting restriction of BN surface, may be employed for fabrication of BN nanocables filled with other metal cores.

Effect of substrate temperature and ion bombardment on the formation of cubic boron nitride films: A two-step deposition approach

The influence of substrate temperature and ion bombardment on the formation of cubic boron nitride (c-BN) films has been examined with bias-assisted magnetron sputtering of h-BN for argon and nitrogen as the working gases. It is shown through a two-step deposition approach that higher substrate temperatures are required only to establish an appropriate h-BN base layer which is mandatory as a nucleation base for c-BN. For nitrogen as the working gas the energy of the bombarding ions can also be reduced after c-BN-nucleation. Hence, the conditions for the formation of c-BN films are more restrictive for the initial growth stage than for the later c-BN growth once the nucleation into c-BN has been established. Corresponding results for argon and nitrogen display characteristic differences which lead to the conclusion that the establishment of full BN surface stoichiometry is a key condition for c-BN growth.

Reconstruction and electronic structure of the vacancy-free N- and B-terminatedc-BN (100) surfaces

Using first-principles calculations we find that dimers on the N-terminated c-BN (100) surface favor to be arranged in infinite long dimer rows with the $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction, while on the B-terminated c-BN (100) surface the dimers tend to form a $(4\ifmmode\times\else\texttimes\fi{}2)$ three-dimer-row and one-shifted dimer pattern. The electronic structure calculations yield that both the N- and B-terminated c-BN (100) surfaces are semimetallic. We also find a significant \ensuremath{\sim}2 eV work function difference between these two surfaces.

Atomic and electronic structure of the (111) surface of cubic BN: an LDF ab initio study

Ab initio local density functional calculations are presented for the (111) surface of cubic BN. Different reconstructions, namely the (1×1), (2×1) and several (2×2) patterns, are examined. Stable phases as a function of nitrogen chemical potential are predicted. Two phases were found to be stable: in a B-rich environment the N adatom model, while in an N-rich environment the nitrogen triangle pattern has the lowest energy. Atomic and band structure of the stable reconstruction models are discussed.

Growth and Characterization of Gan Bulk Crystals Via Vapor Phase Transport

ABSTRACTFree-standing single crystals of bulk GaN were grown via unseeded vapor phase transport at 1130°C on hexagonal BN surfaces via direct reaction of Ga with ammonia. The number of nucleation events was reduced and the crystal size increased by introducing the ammonia at high temperatures. The resulting crystals were either needles or platelets depending on the process variables employed. Low V/III ratios achieved via ammonia flow rates ≤ 75sccm and/or ammonia total pressures ≤ 430Torr favored lateral growth. The average lateral growth rate for the platelets was ∼50μm/hr; the average vertical growth rate for the needles was ∼500μm/hr. Growth rates in all other directions for each of these two morphologies were very low. Seeded growth of both needle and platelet crystals was also achieved; however, the growth rate decreased at longer times and higher pressures due to reaction with H2 from the increased decomposition of ammonia. Nitrogen dilution suppressed this decomposition. A 2mm × 1.5mm GaN crystal was grown with minimal decomposition in a 66.7%NH3 and 33.3%N2 gas mixture.

Field emission from as-grown and surface modified BN and CN thin films

We have investigated the electron field emission characteristics of BN and CN on highly conductive silicon thin films deposited by End-Hall ion source and electron cyclotron resonance plasma source-assisted physical vapor deposition. The thermal processing and surface laser modification effects on the field emission properties were investigated. Current density-field emission characteristics Iη(E) were tested in a high vacuum environment. Mg-doped BN thin films on silicon exhibited a turn-on field as low as 25 V/μm and a current density higher than 1 A/cm2. The deposition of a thin BN layer on copper lithium (CuLi) metallic substrate yields surfaces with a 75 V/μm onset field and a current density 1000 times higher than that obtained from uncoated surfaces. Under high vacuum laser annealing BN coated CuLi showed no enhancement but more stable emission characteristics. Our results show also that pulsed ultraviolet laser irradiation of CN films in vacuum results in an increase of the field emission current densities and a reduction in threshold field values. The turn-on fields of the irradiated surfaces depend strongly upon the energy density of the laser beam. In addition, the electroconductivity properties of BN and CN surface mapping have been performed using scanning tunneling field emission microscopy. The surface topography mapping and its correlation to the field emission properties were investigated. Preliminary results on surface mapping suggest that the surface relief plays some role in field emission enhancement.

First-principles pseudopotential study of GaN and BN (110) surfaces

Using the first-principles pseudopotential LDA approach, we have performed calculations of the atomic and electronic structure of the (110) surface of the cubic phase of GaN and BN. It is found that these surfaces are characterised by a much smaller rotation angle ω of the top atomic layer than many other III–V(110) surfaces. We have found a linear correlation between the relative vertical displacement of the cation and the anion on the surface layer ( Δ 1⊥) and the surface bond length of the III–V, II–VI and SiC compounds. In addition, we have studied the band structure addressing the influence of the degree of ionicity on the character of the surface states.