Alkylated modified boron nitride nanosheets/polyimide composite films with advanced thermal conductivity and low dielectric constant

Owing to the rapid development of the latest micro-electronic devices, polymer composite materials that combine high thermal conductivity and low permittivity have aroused the interest of researchers. However, it is a huge challenge to balance the above parameters. In this work, hexagonal boron nitride (h-BN) powder was ultrasonically exfoliated to obtain alkylated boron nitride nanosheets (Alkyl-BNNS). Then, a series of polyimide (PI) composites were synthesized with different amounts of Alkyl-BNNS. Attributed to more robust interfacial non-covalent interactions between Alkyl-BNNS and polymer chains to inhibit interfacial polarization, Alkyl-BNNS can be scattered well in PI substrate. Thus, the obtained PI composite behaved a high thermal conductivity of 6.21 W/(mK) and a low dielectric constant (3.23) under the load of 20 wt%. Besides, Alkyl-BNNS/PI composites have efficient thermal management capability, low water absorption, favorable electrical resistance, and prominent tensile strength. Importantly, these composite films are expected to be excellent candidates in the field of microelectronics.     

Preparation and functionalization of boron nitride containing carbon nanohorns for boron neutron capture therapy

Boron neutron capture therapy requires boron carriers that can deliver abundant boron to tumors. To obtain such a carrier, we have prepared boron nitride (BN) containing carbon nanohorn aggregate (CNH) by heating CNHs with ammonia borane at 800 °C. The obtained BN-CNH had a C:B:N mole ratio of 5:2:2, as estimated from X-ray photoemission spectroscopy. The nanohorn tubule walls had graphene–BN double layers or BN–graphene–BN triple layers. The graphite-like sheets constituting the CNHs together with the nanohorn tubules also had BN layers on their surfaces. BN release from BN-CNH was small in cell culture medium, and about 70–80% of the BNs remained on the CNHs. For selective accumulation in tumor cells, the BN-CNH was coated with phospholipid polyethylene glycol having folate (BN-CNH/PLPEG-FA). In primary cell culture experiments, human tumor KB cells overexpressing FA receptors ingested more BN-CNH/PLPEG-FA than those without FA, while normal human FHs 173We cells did not show preferential uptake due to FA. The quantity of boron per cell exceeded the criteria required for boron neutron capture therapy by more than 100 times in the cell experiments. These results suggest that BN-CNH is potentially a good carrier for boron neutron capture therapy.     

Tribological study of boron nitride films

Boron nitride (BN) films with different cubic and hexagonal phase compositions were deposited on silicon substrates via diamond interlayers by magnetron sputtering and electron cyclotron resonance microwave plasma chemical vapor deposition. The tribological behaviors of the BN films were investigated systematically using a ball-on-disc tribometer with silicon nitride as the counterpart. Comparison studies were also performed on sintered cubic and hexagonal BN compacts. The influence of phase compositions and surface roughness of BN coatings on their tribological characteristics was studied. The cubic BN (cBN) films showed excellent wear resistance against silicon nitride. The wear rate of the cBN films was estimated to be about 1.0 × 10^?7 mm³/N m by measuring the cross-sectional area of the wear track after the sliding test over a distance of 12 km.     

Effects of annealing on low dielectric constant carbon doped silicon oxide films

Dielectric materials with lower permittivity (low k) are required for isolation to reduce the interconnect RC delay in deep submicron integrated circuit. In this work, carbon doped silicon oxide [SiO(C–H)] films are investigated as a potential low k material. The films were prepared by the radio frequency plasma enhanced chemical vapor deposition (PECVD) technique from trimethylsilane (C₃H₁₀Si or 3MS) in an oxygen (O₂) environment. SiO(C–H) films deposited with O₂ and 3MS flow rates of 100 sccm and 600 sccm, respectively have been previously found to produce dielectric constant as low as 2.9. This is attributed to the incorporation of carbon in the form of Si–CH₃ bond, which has lower polarizability compared to the Si–O bonds that were replaced. In this work, these low k films were annealed at 400, 500, 600 and 700 °C in a N₂ atmosphere for 30 min to determine the thermal stability of their properties. The films were characterized in terms of their thickness shrinkage, refractive indices, dielectric constants, infrared absorption, surface morphology and stress upon annealing. For annealing temperatures up to 500 °C, which is beyond the current highest processing temperature for back end of the line structure of around 450 °C, a slight decrease in the refractive indices and dielectric constants of the films are observed. The SiO(C–H) films also remain smooth and exhibit tensile stress with stress level that is within practical acceptable range. The results suggest that the SiO(C–H) films are thermally stable to be applied as low dielectric constant materials for deep submicron integrated circuit.     

Influence of superstoichiometric boron on the synthesis of cubic boron nitride

Initiators of the synthesis of cubic boron nitride in the Mg-B-N and Li-B-N-O systems are compared. The influence of boron not bound in metal boronitrides and metal borates is investigated. It is proposed to distinguish “excess” boron introduced in the elemental form from “active excess” boron formed in the course of chemical transformations of initiators.     

Influence of atomic bonds on electrical property of boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition

Boron carbon nitride (BCN) films are synthesized with various growth conditions by remote plasma-assisted chemical vapor deposition method. The chemical bonding in the BCN film is modified by the growth condition. Optical and electrical properties are investigated for BCN films with various chemical bonding. Electrical characterization is carried out for the BCN films which have the same bandgap energy and different C composition ratio and have the same C composition ratio and different bandgap energy.     

Ion implantation induced phase transformation in carbon and boron nitride thin films

The mechanism behind energetic ion impact induced stress reduction in highly stressed tetrahedral amorphous carbon and cubic boron nitride thin films is investigated by real time in situ spectroscopic ellipsometry and ex situ electron microscopy. Highly stressed carbon and boron nitride films were grown by filtered cathodic vacuum arc and RF magnetron sputtering, respectively. The films were then implanted by 5–10 keV argon ions and the film optical properties and thickness monitored in situ by spectroscopic ellipsometry. In both cases the films were observed to expand due to a reduction in the density of the ion-modified layer. Cross-sectional transmission electron microscopy and electron energy loss spectroscopy of the carbon films showed that this reduction in density is associated with a conversion of diamond-like bonding to graphite-like bonding. In situ stress measurements performed on the boron nitride films revealed a simultaneous reduction in stress with expansion of the material.     

Nucleation of cubic boron nitride thin films

High-energy electrons (300 keV to 1 MeV) in a transmission electron microscope have been used to cause ballistic atomic displacements in hexagonal boron nitride. The high-resolution imaging capabilities of the TEM have allowed us to study the effect of the atomic displacements on the crystal structure of the BN. We report the formation of nanoarches — fullerene structures consisting of half of a BN nanotube capping the ends of the planar BN graphitic sheets. To form a basis of comparison between the high-energy electron bombardment and the ion bombardment typically used for cubic BN film growth, TRIM calculations were also performed to simulate Ar⁺ ion bombardment of hexagonal BN. A model is presented, indicating a process through which the nanoarches can serve as nucleation sites for the cubic phase of BN. The nucleation model is consistent with current experimental reports on the formation of cubic BN thin films.     

Influence of boron impurities on the crystal structure of cubic boron nitride

The influence of boron impurities on the unit cell parameters of the boron nitride with a sphalerite structure is investigated. It is demonstrated that the presence of boron in amounts exceeding the stoichiometric composition leads to a distortion of the cubic lattice and reduces its symmetry. The revealed oriented distortion of the crystal lattice of the cubic boron nitride is explained by the incorporation of excess boron atoms not in a random manner but between paired corrugated hexagonal layers. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Relationship between gas transport properties and fractional free volume determined from dielectric constant in polyimide films containing the hexafluoroisopropylidene group

The dielectric constant and gas transport properties (i.e., permeability, diffusivity, and solubility) in 2,2′‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)‐based polyimides were systematically investigated in terms of their polymer fractional free volumes (FFVs) at 30°C. The permeability and diffusion coefficients of the 6FDA‐based polyimide films to hydrogen, oxygen, nitrogen, methane, and carbon dioxide were correlated with their FFVs estimated using van Krevelen's group contribution method. There appeared, however, small linear correlation coefficients. Linear correlations were also observed between the gas transport properties and dielectric constant of these polyimides. This study described FFVas a function of the dielectric constant based on the Clausius‐Mossotti equation. It was found that the gas permeability and diffusion coefficients of these 6FDA‐based polyimide films increased as their dielectric constant‐based FFV increased. A better linear relationship was observed between the gas transport properties and the FFV determined from the polymer dielectric constant in comparison to that estimated using the group contribution method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Influence of nitrogen ion energy on the Raman spectroscopy of carbon nitride films

Carbon nitride films were deposited by filtered cathode vacuum arc combined with radio frequency nitrogen ion beam source. Both visible Raman spectroscopy and UV Raman spectroscopy are used to study the bonding type and the change of bonding structure in carbon nitride films with nitrogen ion energy. Both C–N bonds and CN bonds can be directly observed from the deconvolution results of visible and UV Raman spectra for carbon nitride films. Visible Raman spectroscopy is more sensitive to the disorder and clustering of sp² carbon. The UV (244 nm) Raman spectra clearly reveal the presence of the sp³ C atoms in carbon nitride films. Nitrogen ion energy is an important factor that affects the structure of carbon nitride films. At low nitrogen ion energy (below 400 eV), the increase of nitrogen ion energy leads to the drastic increase of sp²/sp³ ratio, sp² cluster size and C---N bonds fraction. At higher nitrogen ion energy, increase leads to the slight increase of CN bonds fraction and sp² cluster size, slight decrease of C---N bonds fraction and sp²/sp³ ratio.     

Synthesis and properties of ultralow dielectric constant porous polyimide films containing trifluoromethyl groups

In this research, a series of porous copolyimide (co‐PI) films containing trifluoromethyl group (CF₃) were facilely prepared via a phase separation process. The co‐PI were synthesized by the reaction of benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) with two diamines of 4,4′‐diaminodiphenyl ether (ODA) and 3‐trifluoromethyl‐4,4'‐diaminodiphenyl ether (FODA) with various molar ratios. The flexible and tough porous co‐PI films with about 300 μm thickness and 8～10 μm average diameter could be obtained by solution casting conveniently. The thermal properties of the obtained porous co‐PI films were excellent with a glass transition temperature at 270 °C ～ 280 °C and only 5% weight loss in temperature from 530 °C to 560 °C under nitrogen atmosphere. In addition, the dielectric and hydrophobic properties of porous co‐PI films were remarkably improved owing to the presence of trifluoromethyl groups (CF₃) in the polymer chains. Moreover, our synthesized porous co‐PI films also showed good mechanical properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44494.     

High dielectric constant epoxy nanocomposites containing ZnO quantum dots decorated carbon nanotube

Zinc oxide (ZnO) quantum dot (QD) decorated multi-walled carbon nanotube (MWCNT) hybrid was utilized in the fabrication of high dielectric constant epoxy nanocomposites. Because of the shielding effect of ZnO QD, the well-dispersed epoxy hybrid nanocomposites exhibit frequency insensitive high dielectric constant as well as greatly reduced dielectric loss. With only 1.5 wt% of MWCNT addition, the epoxy/MWCNT-ZnO nanocomposite possesses dielectric constant as high as 31 and dielectric loss as low as 0.01 at 1 kHz. In addition, the epoxy nanocomposite exhibits greatly enhanced tensile properties. The role of ZnO QD decorated MWCNT in the preparation and property improvement of multi-functional polymer nanocomposites is discussed.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

Influence of the thermal process of carbon template removal in the mesoporous boron nitride synthesis

Influence of the thermal process involved in the carbon template elimination during the synthesis of mesoporous boron nitride by using nanocasting process of a mesoporous CMK-3 carbon with a borazinic precursor is presented. The borazinic precursor, the tri(methylamino)borazine (MAB), is converted to boron nitride (BN) inside the mesopores of a CMK-3 mesoporous carbon template by ceramization under nitrogen or under ammonia. The carbon template elimination is carried out by thermal treatment under air or under ammonia. The X-ray diffraction, TEM and pore size analysis are used to study the texture of the boron nitride synthesized from the carbon template. A template elimination performed by hydrogenation with an ammonia treatment allows to obtain an organized porous structure, which is not possible by using an oxidation treatment. In order to preserve the mesoporous organization of boron nitride, a two steps procedure (ceramization followed with template elimination by hydrogenation) is more efficient than a one step procedure (ceramization and template hydrogenation simultaneously).     

Atomic bonds in boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition

Boron carbon nitride (BCN) films are synthesized by remote plasma-assisted chemical vapor deposition (RPCVD) method. The present experimental apparatus is featured by introducing BCl₃ gas near the substrate without mixing to plasma consisting of N₂ and CH₄ gases. Two sample groups of the BCN films are prepared. One is grown with various CH₄ flow rates, and another is grown with various BCl₃ flow rates. The composition ratio of the constituent atoms, atomic bonds and optical bandgap are investigated. C composition ratio of the BCN film increases with increasing CH₄ flow rate, leading to a reduction in the optical bandgap with increasing C composition ratio. On the other hand, it is found that no significant variation in the composition ratio occurs for the BCN films grown with various BCl₃ flow rates and that the optical bandgap decreases with increasing BCl₃ flow rate. This behavior of the optical bandgap is related to a change of the atomic bonds in the BCN film grown with various BCl₃ flow rates.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

Improvement of the adhesion of sputter-deposited cubic boron nitride films

Cubic boron nitride (cBN) thin films were grown on Si(100) and high-speed steel substrates by reactive r.f. sputtering in an Ar/N₂ discharge using an electrically conducting boron carbide (B₄C) target. The substrate electrode was grounded or operated either with a d.c. or an r.f. power supply. The deposition of cBN can be subdivided into three steps: (1) the growth of a thin, textured, hexagonal boron nitride (hBN) film, (2) the nucleation of cBN and (3) the growth of the cBN phase. As a measure of the cBN content, the ratio of the infrared absorption bands near 1100 cm⁻¹ (cBN) and 1400 cm⁻¹ (hBN) was used. The adhesion of cBN films is still an unsolved problem. Two aspects have to be considered: (1) the high intrinsic stress of the film and (2) the reactivity under humid conditions. We investigated the influence of the thickness, structure and surface roughness of hBN on the adhesion of cBN films. To modify the hBN films, the pressure, substrate bias and Ar/N₂ mixture was varied. Another way of improving the adhesion is plasma treatment of the cBN film directly after deposition. The process variations mentioned above increase the thickness of the adhering cBN films.     

Correlations between microstructure and dielectric properties of hexagonal boron nitride

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical, and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the correlation of microstructure and electrical and dielectrical properties of such materials. In this work, the microstructure, dielectric breakdown resistance (dielectric strength), and low permittivity of different BN ceramics and composites were investigated. Besides exhibiting a very high specific electrical resistivity of 10¹³–10¹⁵ Ω cm, the materials had excellent dielectric strengths (up to 53 kV/mm) and low electrical permeability (4.1). The dielectric strength depended strongly on the porosity and to a lesser extent on the content of secondary phases, whereas the permittivity was influenced by the secondary phases. The aging of the materials in humid air did not significantly alter these values. The permittivity was found to be independent of frequency between 0.1 MHz and 10 MHz and temperature up to 300 °C.     

Carbothermal synthesis of boron nitride coating on PAN carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fiber by dip coating method. Dip coating was carried out in saturated boric acid solution followed by nitridation at a temperature of 1200 °C in nitrogen at atmospheric pressure to produce BN coating. Chemical activation improved surface area of PAN fiber which favours in situ carbothermal reduction of boric acid. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) have shown the formation of boron nitride. The X-ray photoelectron spectroscopy reveals that the coating forms a composite layer of carbon, BN/BO_xN_y and some graphite like BCN with local structure of B–N–C and B(N–C)₃. The oxidation resistance of the coated fiber was significantly higher than uncoated carbon fiber. Tensile strength measurement reveals that the BN coated fiber maintained 90% of its original strength. As compared to chemical vapor deposition (CVD), this process is simple, non-hazardous and is expected to be cost effective.