Formation Of Hexagonal Boron Nitride Research Articles

Microstructural and nanoindentation study of TaN incorporated ZrB2 and ZrB2–SiC ceramics

This study assessed the sinterability and microstructure of ZrB2-SiC-TaN and ZrB2-TaN ceramics. Spark plasma sintering at 2000 °C and 30 MPa for 5 min produced both ceramics. The relative density of ZrB2 ceramic containing TaN was 95.3%; the addition of SiC increased this value to 98.1%. SiC’s contribution to the elimination of ZrB2 surface oxides was the primary factor in the advancement of densification. The in situ formation of hexagonal boron nitride at the interface of TaN and ZrB2 was confirmed by high-resolution transmission electron microscopy, field emission-electron probe microanalyzer, X-ray diffractometry, and field emission scanning electron microscopy. Moreover, the in situ graphite might be produced as a byproduct of the SiC-SiO2 process, hence boosting the reduction of oxide compounds in the ternary system. The SiC compound had the highest hardness (29 ± 3 GPa), while the ZrB2/TaN interface exhibited the greatest values of elastic modulus (473 ± 26 GPa) and stiffness (0.76 ± 0.13 mN/nm).

Structure diagram and dynamics of formation of hexagonal boron nitride in phase-field crystal model.

The phase-field crystal (PFC-model) is a powerful tool for modelling of the crystallization in colloidal and metallic systems. In the present work, the modified hyperbolic phase-field crystal model for binary systems is presented. This model takes into account slow and fast dynamics of moving interfaces for both concentration and relative atomic number density (which were taken as order parameters). The model also includes specific mobilities for each dynamical field and correlated noise terms. The dynamics of chemical segregation with origination of mixed pseudo-hexagonal binary phase (the so-called 'triangle phase') is used as a benchmark in two spatial dimensions for the developing model. Using the free energy functional and specific lattice vectors for hexagonal crystal, the structure diagram of co-existence of liquid and three-dimensional hexagonal phase for the binary PFC-model was carried out. Parameters of the crystal lattice correspond to the hexagonal boron nitride (BN) crystal, the values of which have been taken from the literature. The paper shows the qualitative agreement between the developed structure diagram of the PFC model and the previously known equilibrium diagram for BN constructed using thermodynamic functions. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Carbon-Based Band Gap Engineering in the h-BN Analytical Modeling.

The absence of a band gap in graphene is a hindrance to its application in electronic devices. Alternately, the complete replacement of carbon atoms with B and N atoms in graphene structures led to the formation of hexagonal boron nitride (h-BN) and caused the opening of its gap. Now, an exciting possibility is a partial substitution of C atoms with B and N atoms in the graphene structure, which caused the formation of a boron nitride composite with specified stoichiometry. BC2N nanotubes are more stable than other triple compounds due to the existence of a maximum number of B–N and C–C bonds. This paper focused on the nearest neighbor’s tight-binding method to explore the dispersion relation of BC2N, which has no chemical bond between its carbon atoms. More specifically, the band dispersion of this specific structure and the effects of energy hopping in boron–carbon and nitrogen–carbon atoms on the band gap are studied. Besides, the band structure is achieved from density functional theory (DFT) using the generalized gradient approximations (GGA) approximation method. This calculation shows that this specific structure is semimetal, and the band gap energy is 0.167 ev.

Facile Transformation of Ni‐based Colloids into Highly Stable Nanocatalysts Embedded within h‐BN for the Water‐Gas Shift Reaction

AbstractAdvances in colloidal chemistry allow to prepare well‐controlled nanostructures, while application of the colloids in heterogeneous catalysis is strongly impeded by the presence of organic capping layers. However, removal of the protection layers inevitably results in changes of the well‐defined nanostructures such as sintering and agglomeration. Here, a simple way is suggested to solve the dilemma by replacing the organic capping layers with another flexible but robust capping layers made of two‐dimensional materials. We show simultaneous removal of surfactant layers in B‐containing Ni, Co, and Fe colloids and formation of hexagonal boron nitride (h‐BN) capping layers over metal nanoparticles. Metal particle size can be well kept even upon thermal treatment at 850 °C and Ni nanocatalysts embedded within h‐BN shells present high catalytic performance in water‐gas shift reaction. This work suggests a facile strategy to promote the application of metal colloids in heterogeneous catalysis.

Sintering of binderless cubic boron nitride and its modification by β-Si3N4 additive for hard machining applications

This study present the results of HP-HT sintering, microstructure, properties, and performance of binderless cBN tool material. Within the investigated sintering temperature range of 1900–2600 °C the optimum was found to be 2200–2300 °C. Lower temperature results in incomplete diffusion bonding between cBN grains, while higher temperature results in high degree of recrystallization of initial structure, grain growth, and even formation of hexagonal boron nitride in triple joints. Introduction of stress-inducing β-Si3N4 minor inclusions resulted in high overall mechanical and thermal properties: HK = 41 GPa; KIC = 12.6 MPa·m1/2; λ = 180 W/(m·K). Machining experiments in roughing of hardened tool steels show that binderless cBN material provides high performance in terms of resistance to tool cratering, chipping, and tool fracture.

Influence of hexagonal phase transformation in laser prepared PcBN cutting tools on tool wear in machining of Inconel 718

Laser preparation of PcBN cutting tools is a new approach to improve the performance of different machining applications. Laser machining is robust and wear-free which offers benefits compared to conventional methods, such as grinding and brushing. Moreover, the conventional preparation methods are limited for the preparation of super hard cutting tool materials and the generation of complicated geometrical features because of high tool wear. A highly demanding application of PcBN cutting tools is the machining of Ni-based super alloys such as Inconel 718. For various high and low temperature applications in aerospace systems this material is popular because of its temperature, oxidation and corrosion resistant characteristics. However, these characteristics and an intense work hardening behavior make Inconel 718 a hard-to-cut material. A possibility to increase the performance in cutting hard-to-cut materials is the application of a process specific cutting edge rounding prepared by laser machining. However, there is no available knowledge neither on the influence of laser machining on the material properties of PcBN nor on the influence on the cutting performance. This research deals with the effect of laser preparation on the cutting performance of PcBN tools in machining Inconel 718. Different laser preparation strategies are applied to provide necessary knowledge regarding the laser preparation process. The formation of hexagonal boron nitride (hBN) on the surface is found for all of the applied laser preparation strategies, however the degree of the cBN to hBN transformation differs between the applied strategies. Within the cutting experiments the wear behavior is closely linked to the formation of hBN, which decreases the hardness of the PcBN material. Furthermore, process forces for the laser prepared tools are reduced compared to the conventional tools, which might be an influence of the hBN acting as a solid lubricant. Finally, it could be shown that the achieved tool life of laser prepared tools is at least on a par with conventionally machined tools.

A HR-XPS study of the formation of h-BN on Ni(111) from the two precursors, ammonia borane and borazine.

Together with borazine, ammonia borane is a prominent precursor molecule for the formation of hexagonal boron nitride, which is of high interest as a 2D-material and graphene analog. Ammonia borane is also a possible solid hydrogen carrier for renewable energies with high storage density. Using X-ray photoelectron spectroscopy and temperature-programmed desorption, we investigated low-temperature adsorption and dehydrogenation during heating of borazine and ammonia borane on Ni(111) to form h-BN. For borazine, we observe the formation of disordered boron nitride above 300 K, which starts to form hexagonal boron nitride above 600 K. Ammonia borane shows multiple dehydrogenation steps at the boron and nitrogen atoms up to 300 K. This results in various BHxNHy species, including borazine-like intermediates, before the formation of disordered boron nitride and finally hexagonal boron nitride, analogous to the borazine decomposition.

Defect mediated van der Waals epitaxy of hexagonal boron nitride on graphene

Van der Waals heterostructures comprising of hexagonal boron nitride and graphene are promising building blocks for novel two-dimensional devices such as atomically thin transistors or capacitors. However, demonstrators of those devices have been so far mostly fabricated by mechanical assembly, a non-scalable and time-consuming method, where transfer processes can contaminate the surfaces. Here, we investigate a direct growth process for the fabrication of insulating hexagonal boron nitride on high quality epitaxial graphene using plasma assisted molecular beam epitaxy. Samples were grown at varying temperatures and times and studied using atomic force microscopy, revealing a growth process limited by desorption at high temperatures. Nucleation was mostly commencing from morphological defects in epitaxial graphene, such as step edges or wrinkles. Raman spectroscopy combined with x-ray photoelectron measurements confirm the formation of hexagonal boron nitride and prove the resilience of graphene against the nitrogen plasma used during the growth process. The electrical properties and defects in the heterostructures were studied with high lateral resolution by tunneling current and Kelvin probe force measurements. This correlated approach revealed a nucleation apart from morphological defects in epitaxial graphene, which is mediated by point defects. The presented results help understanding the nucleation and growth behavior during van der Waals epitaxy of 2D materials, and point out a route for a scalable production of van der Waals heterostructures.

Atomistic Insights into Nucleation and Formation of Hexagonal Boron Nitride on Nickel from First-Principles-Based Reactive Molecular Dynamics Simulations

Atomistic-scale insights into the growth of a continuous, atomically thin hexagonal boron nitride (hBN) lattice from elemental boron and nitrogen on Ni substrates were obtained from multiscale modeling combining density functional theory (DFT) and reactive molecular dynamics. The quantum mechanical calculations focused on the adsorption and reaction energetics for the hBN building-block species, i.e., atomic B, N, BxNy (x, y = 1, 2), on Ni(111) and Ni(211), and the diffusion pathways of elemental B and N on these slab model surfaces and in the sublayer. B can diffuse competitively on both the surface and in the sublayer, while N diffuses strictly on the substrate surface. The DFT data were then used to generate a classical description of the Ni-B and Ni-N pair interactions within the formulation of the reactive force field, ReaxFF. Using the potential developed from this work, the elementary nucleation and growth process of an hBN monolayer structure from elemental B and N is shown at the atomistic scale. The nucleation initiates from the growth of linear BN chains, which evolve into branched and then hexagonal lattices. Subsequent DFT calculations confirmed the structure evolution energetically and validate the self-consistency of this multiscale modeling framework. On the basis of this framework, the fundamental aspects regarding crystal quality and the role of temperature and substrates used during hBN growth can also be understood.

Effect of Li2CO3 on formation temperature of hBN by modified O'Connor model

This study is interested in the effect of lithium carbonate on the formation of hexagonal boron nitride (hBN) by means of the available experimental methods including TGA, XRD, FTIR, SEM and HR‐TEM. hBN samples were synthesized at the 1450 °C with different molar ratios of lithium carbonate by modified O'Connor routine. The crystalline hBN formation tended to improve with the increment of the Li2CO3 concentration level (especially after more 20 %). The dopant quantity decreased the residual stresses due to the presence of possible relaxation mechanisms along with the nanocrystal structure, even favored by XRD experimental findings regarding the enhancement of crystal plane alignments, crystallite sizes and lattice parameters. As for the FTIR surveys, the Li2CO3 foreign impurities strengthened more and more the covalent bonds between boron and nitrogen atoms. At the same time, the samples with 40 % lithium carbonate were annealed at the varied temperatures of 1000, 1150, 1300 and 1450 °C to determine the optimum annealing temperature. The XRD+FTIR investigations indicated that the degree of hexagonality improved with the increased annealing temperature. Similarly, the surface morphology confirmed not only the formation of regularity and flaky hexagonal BN structures, but also the strengthening of covalent bonds between the atoms.

Green synthetic strategy of BCNO nanostructure and phosphor-based light – Emitting diodes

BCNO phosphor has been paid much attention due to their unique physical, electronic and optical properties. Here we have successfully obtained BCNO nano-particle phosphor by microwave treating from boric acid, urea, and glucose at low temperatures and in short reaction time. Glucose decomposed into graphene quantum dots (GQDs), which facilitated the formation of hexagonal boron nitride (h-BN). Through our method, GQDs domains were uniformly incorporated into h-BN, leading to the formation of BCNO and decrease of bandgap. BCNO demonstrated excellent performance in light emitting diodes (LEDs) with green and blue light. We envision that this BCNO phosphor will enable the next generation blue and green LED devices due to the easiness of large scale fabrication at an economic cost.

Synthesis of highly ordered hBN in presence of group I/IIA carbonates by solid state reaction

This comprehensive study declares experimentally the effects of IA/IIA metal carbonates on the formation of hexagonal boron nitride (hBN) with the aid of the available experimental methods as regards Fourier transform infrared spectroscopy (FT‐IR), powder X‐ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM). hBN is synthesized in the existence of the metal carbonates at the low temperature by modified O'Connor method. The experimental findings of FT‐IR and XRD investigations show that the addition of metal carbonates affects considerably the crystallization of hBN powder during the synthesis process. The presence of the high concentration level of the additions improves harshly the crystallinity. In this respect, the graphitization index deduced from the XRD patterns reduces with the enhancement in the amount of the dopant content. At the same time, the differentiation between the products is analyzed by the SEM surveys. According to the results, the materials synthesized by the Li2CO3 powder exhibit both the tubular form and rod‐like while the other samples prepared by the CaCO3 chemical dopant display the homogeneous plates. Even, the TEM images confirm the nanowires and nanotubes structures such as multi‐walled cylindrical, bamboo nanotubes in all the materials studied in this work.

Formation of hexagonal boron nitride on graphene-covered copper surfaces

Abstract

Formation of hexagonal boron nitride by metal atomic vacancy-assisted B-N molecular diffusion.

Because of the low solubility of N atoms in metals, hexagonal boron nitride (h-BN) growth has explained by surface reaction on metal rather than by penetration/precipitation of B and N atoms in metal. Here, we present an impressive pathway of h-BN formation at the interface between Ni and oxide substrate based on B-N molecular diffusion into Ni through individual atomic vacancies. First-principles calculations confirmed the formation energies of the h-BN layers on and under the metal and the probability of B-N molecular diffusion in metal. The interface growth behavior depends on the species of metal catalysts, and these simulation results well support experimental results.

Comparative catalytic study on the carbothermic formation of hexagonal boron nitride with Li, Na, K and Ca carbonates

Catalytic effects of Li, Na, K and Ca carbonates on carbothermic formation of hexagonal boron nitride (hBN) were investigated. Experiments were conducted by keeping the plain and catalyst added B2O3 + C powder mixtures at 1400 °C in flowing nitrogen for 40–160 min. Products were subjected to quantitative and X-ray diffraction analyses, scanning and transmission electron microscopy examinations, and particle size measurements. Investigated catalysts increased the amount, particle size and crystallinity of the formed hBN. Average particle sizes of the obtained hBN powders were in 150–350 nm range. Catalytically, lithium and sodium carbonates were found to be the most effective, whereas calcium carbonate was the least effective. In the experiments conducted for 80 min, the quantity of the formed hBN increased 13-fold when 40 wt% Li2CO3 was used, as compared to plain mixture. The increases were ninefold and fourfold for the same duration when 40 wt% K2CO3 and 10 % CaCO3 were used, respectively.

Monolayer formation of hexagonal boron nitride on Ag(001)

The formation of monolayers from hexagonal boron nitride, h-BN, on the square Bravais lattice of a Ag(001) surface via chemical vapor deposition of borazine (HBNH)3 was investigated by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Due to the weak interaction between the h-BN monolayer and the Ag surface, there is no preferred orientation of the h-BN lattice. The h-BN-related hexagonal distributions of LEED spots transform into nearly homogeneous rings. The growth of h-BN on Ag(001) is the same as previously demonstrated for Ag(111), giving evidence that for weakly interacting transition metal surfaces the surface symmetry has no distinct impact on the h-BN growth.

N,N-Dimethyl formamide facilitated formation of hexagonal boron nitride from boric acid

In this paper, we report on a promoting novel process for the formation of h-BN plates by using N,N-dimethyl formamide-treated boric acid (DMF-BA). Using this B source, the formation of h-BN can be indeed improved greatly compared to using pure boric acid (BA). This method effectively reduces the content of boric acid and amorphous boric oxide, enhancing the transformation rate of h-BN. For preparation of pure h-BN, it can obviously lower the resultant temperature without further purification process. Via graphitization index (G.I.) calculation and thermostability analysis, the pure h-BN plates obtained from the DMF-BA would be a promising candidate for raw material of c-BN and low-temperature applications in the air.

Catalytic effect of potassium carbonate on carbothermic production of hexagonal boron nitride

Abstract Effect of potassium carbonate addition on the carbothermic formation of hexagonal boron nitride (hBN) was investigated by keeping the K 2 CO 3 added B 2 O 3 +C mixtures in nitrogen atmosphere at 1400 °C for 40–160 min. K 2 CO 3 amount was varied in the range of 10–60 wt% of the B 2 O 3 +C mixture. Products were subjected to XRD and quantitative analyses, SEM and TEM observations, and particle size measurement. Amount of hBN increased considerably with K 2 CO 3 addition; also particle size and crystallinity improved. Catalytic role of K 2 CO 3 was suggested as forming a potassium borate melt in which hBN particles form, in addition to carbothermic formation reaction. Effect of K 2 CO 3 on increasing the hBN amount decreased when it was used over 40%. This was attributed to the rapid evaporation of the formed potassium borate liquid.

Spatially Resolved Compositional Analysis of a BCN Thin Film Grown on a Ni Substrate by Chemical Vapor Deposition

ABSTRACTA thin film consisting of boron, carbon, and nitrogen (BCN) was grown on a polycrystalline Ni substrate by thermal chemical vapor deposition. The local elemental composition of the BCN film was analyzed by scanning Auger electron spectroscopy. The film is elementally highly inhomogeneous and consists of domains with a typical size of 1-10 μm and irregular shapes. The domain structure is strongly related to the structure of the grains of the polycrystalline Ni film beneath the domain. A thick domain is often formed on a small Ni grain. On a large and flat Ni grain, the film thickness is relatively small, and both the boron and nitrogen contents are often below the detection limit, indicating that it is a graphene domain. Boron and nitrogen contents are highly correlated, which is consistent with formation of hexagonal boron nitride. However, unbalanced boron and nitrogen contents are observed from thick domains.

Effect of Na 2CO 3 on hexagonal boron nitride prepared from urea and boric acid

Formation of hexagonal boron nitride (hBN) from a precursor obtained by the reaction of urea and boric acid was studied in nitrogen, ammonia and argon atmospheres in 700–1200 °C temperature range. Effect of sodium carbonate (Na 2CO 3) addition on this process was investigated. Reaction products were subjected to X-ray diffraction, particle size distribution, gravimetric and Fourier transformed infrared spectroscopy analyses. Particle size and crystallite thickness of the formed hBN were seen to increase from about 60 nm and 5 nm at 700 °C to 230 nm and 19 nm at 1200 °C, respectively in NH 3 atmosphere with Na 2CO 3 addition. Highest conversion of boron in the precursor into hBN was achieved as 73.6% when Na 2CO 3 added precursor was reacted at 1200 °C in NH 3. hBN powder with high yield and relatively large particle size was obtained at low temperature such as 1200 °C with Na 2CO 3 addition. Role of Na 2CO 3 addition was suggested to be formation of a sodium borate melt from which hBN crystallized via the reaction of borate and nitrogen ions in the melt. Obtained hBN has the potential for utilization as a clean starting material for synthesis of B or N containing compounds.