Effects of boron oxide composition,structure, and morphology on B4C formation via the SHS process in the B2O3–Mg – C ternary system

This study investigates the formation of B₄C in the B₂O₃–Mg–C ternary system via a magnesiothermic reduction process using two kinds of boron oxide (B₂O₃) ‒ the commercial B₂O₃ and that synthesized from boric acid via the coupled chemical-thermal process. In addition to the raw materials, the products were subjected to XRD, FTIR, SEM, and FESEM analyses to determine the effects of microstructural and morphological properties of boron oxide as an important raw material, on B₄C formation in the combustion synthesis process. For this purpose, powder mixtures of B₂O₃:Mg:C were prepared at a stoichiometric molar ratio of 2:6:1 and compacted into pellets using a uniaxial hydraulic press, which were subsequently subjected to the combustion synthesis process based on the self-propagating high-temperature synthesis (SHS). Finally, the samples thus obtained were leached in an aqueous hydrochloric acid solution. Analysis of the commercial B₂O₃ revealed the presence of large amounts of such by-products as magnesium borate (Mg₃B₂O₆) and magnesium oxide (MgO) along with relatively small amounts of boron carbide after the leaching process, while those obtained for the chemically-thermally synthesized B₂O₃ showed a relatively large amount of B₄C (from micron-sized particles to nanoparticles) together with a remaining carbon phase and very small amounts of magnesium borate as by-products. It can be, therefore, concluded that the changes in chemical composition and introduction of a hydrous HBO₂ phase in the boron oxide in the B₂O₃–Mg–C mixture as well as its varied microstructure, morphology, and particle size have significant effects on the efficiency of B₄C production through the SHS process.     

Development of MgO:TiO2 thin films for gas sensor applications

In this study, structural, morphological and optical properties, and gas sensor performance of magnesium oxide (MgO) doped titanium dioxide (TiO₂) thin films were investigated in detail. Gas sensor metallic patterns were fabricated on Si substrate using traditional photolithographic technique. MgO doped TiO₂ thin films were deposited on formed Pt electrode surface by confocal sputtering (co-sputtering) system as the active layer. Thin film characterizations were realized by using secondary ion mass spectroscopy (SIMS), atomic force microscope (AFM) and UV–Vis Spectrometer (UV–Vis). Gas sensing measurements were performed by gas sensing test system against methane gas at working temperature of 300?°C. To evaluate deposition and thermal annealing effects on the sensing performance, sensors were tested under gas. The sensitivity and response/recovery time of gas sensors were measured in 1000?ppm. MgO doped TiO₂ based sensor at substrate temperature of 100?°C has high sensitivity and short response/recovery time.     

Synthesis and Properties of Boron Doped NaxCo2O4 Nanocrystalline Ceramics

Sodium cobalt oxide (NaCo₂O₄) nanofibers with diameters ranging between 20 and 200?nm were prepared by electrospinning a precursor mixture of PVA/(Na–Co) acetate. This was the first time any such attempt was made. Afterwards, the electrospun nanofibers were subjected to calcination treatment. The characteristics of the fibers were investigated using a Fourier transform infrared spectroscopy, a X-ray diffractometer, and a scanning electron microscopy. The boron doped and undoped NaCo₂O₄ nanofibers calcined at 850?°C were polycrystalline of the γ Na_xCo₂O₄ phase having diameters ranging between 20 and 60?nm with grain sizes of 5–10?nm, and the nanofibers calcined at 800?°C were single crystals having linked particles or crystallites with particle sizes ranging between 60 and 200?nm. The results indicated a significant effect of calcination temperature on the crystalline phase and morphology of the nanofibers. It could be seen in the SEM micrograph of the fibers that when boron was added, this resulted in the formation of cross-linked bright-surfaced fibers. The average fiber diameter for boron doped and undoped fiber mats were 204 and 123?nm, respectively. The grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 140 and 118?nm, respectively.     

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.     

Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

The chemical solution deposition of Mg(OH)₂ thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500?°C to identify the Mg(OH)₂-to-MgO transition temperature, which resulted to be around 375?°C. Annealing the as-deposited Mg(OH)₂ films at 500?°C results in homogeneous MgO thin films. The properties of the Mg(OH)₂ and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH)₂), that transforms into the periclase phase (MgO) after annealing at 500?°C. For the as-deposited Mg(OH)₂ thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg_0.36O_0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH)₂ film, where the expected stoichiometric composition is Mg_0.33O_0.67 (O/Mg ratio of 2.0); the same assessment yields Mg_0.60O_0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg_0.50O_0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH)₂ films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10?eV for the as-deposited and the film annealed at 500?°C, respectively. The resistivity of both Mg(OH)₂ and MgO thin films lies around 10⁸?Ω·cm.     

Synthesis of BCN thin films by nitrogen ion beam assisted pulsed laser deposition from a B4C target

Using sintered B₄C as target material, ternary BCN thin films were synthesized on Si(100) substrates by means of reactive pulsed laser deposition assisted by nitrogen ion beam. The composition, bonding configuration and crystalline structure of the synthesized films were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. The prepared films contain several bonds including B–C, N–C, B–N with B–C–N atomic hybridization. The ablation of the B₄C target results in the deposition of a film with B:C ratio about 3:1, deficient in boron compared with the target material. Nitrogen provided by the ion beam is incorporated in the film and bonded to boron and carbon. Heating of the substrate enhances the incorporation of nitrogen and influences the bonding configuration and crystalline structure of the film as well.     

Reduced-graphene-oxide-reinforced boron carbide ceramics fabricated by spark plasma sintering from powder mixtures obtained by heterogeneous co-precipitation

Reduced graphene oxide (rGO) sheets were uniformly dispersed in boron carbide ceramics by a heterogeneous co-precipitation method. This approach was used to improve the fracture toughness of boron carbide ceramics and to address the problem of agglomeration of graphene in the boron carbide matrix. Cetyltrimethyl ammonium bromide was used as a heterogeneous co-precipitation reaction initiator to prepare a homogeneously dispersed graphene oxide/boron carbide (GO/B₄C) mixture. Reduced graphene oxide/boron carbide (rGO/B₄C) powder mixtures with good dispersion were obtained by high temperature heat treatment. Dense rGO/B₄C composite ceramics were fabricated by spark plasma sintering at 1800 °C and 50 MPa. The fracture toughness and flexural strength of the rGO/B₄C with an rGO content of 2 vol% composite increased by 42% (from 3.43 to 4.88 MPa·m^1/2) and 28% (from 372 to 476 MPa) compared with those of pure B₄C, respectively. The markedly improved fracture toughness and flexural strength of the boron carbide ceramics were attributed to the effect of crack bridging and crack deflection by graphene sheets, graphene interface sliding, and pulling out of graphene.     

Preparation and characterization of reduced graphene oxide-reinforced boron carbide ceramics by self-assembly polymerization and spark plasma sintering

A self-assembly polymerization process was used to prepare graphene oxide/boron carbide (GO/B₄C) composite powders, spark plasma sintering (SPS) was used to fabricate reduced graphene oxide/boron carbide (rGO/B₄C) composites at 1800 °C and 30 MPa with a soaking time of 5 min. The effects of rGO addition on mechanical properties of the composites, such as Vickers hardness, flexural strength and fracture toughness, were investigated. The results showed that GO/B₄C composite powders were successfully self-assembled and a network structure was formed at high GO contents. The flexural strength and fracture toughness of rGO/B₄C composites were 643.64 MPa and 5.56 MPa m^1/2, respectively, at 1 and 2.5 wt.% rGO content, corresponding to an increase of 99.11% and 71.6% when compared to B₄C ceramics. Uniformly dispersed rGO in rGO/B₄C composites played an important role in improving their strength and toughness. The toughening mechanisms of rGO/B₄C composites were explained by graphene pull-out, crack deflection and bridging.     

Synthesis of MgO thin films grown by SILAR technique

Different thickness MgO thin films were grown on the glass substrate by successive ionic layer adsorption and reaction (SILAR) method as the first study in literature. X-ray diffraction (XRD) measurements demonstrate the cubic MgO structures and samples have (002), and (220) peaks. All film has nanoball structures observed from the scanning electron microscope (SEM) images. The band gap and transmittance values of MgO thin films decrease with increasing thickness. The photoluminescence (PL) spectrum demonstrates that samples have three visible emissions changing with thickness at 381?nm violet emission, 457?nm blue emission and 535?nm green emission. X-ray photoelectron spectroscopy (XPS) spectrum present confirms the elemental signals from carbon (C), oxygen (O) and magnesium (Mg) atoms in the sample. Both Moss and Herve and Vandamme relations refractive index values n, ε₀, and ε_∞ values and amount of oxygen increase with raising thickness of MgO thin films.     

Boron carbide films deposited by a magnetron sputter–ion plating process: film composition and tribological properties

Amorphous boron carbide films were deposited onto silicon substrates by a magnetron sputter–ion plating process in an argon plasma atmosphere (0.25 Pa) using a B₄C target. The substrates were polarized with a d.c. bias voltage in the range from 0 to −100 V. The film composition and the presence of contaminants were determined by ion beam analysis (IBA). The nanoscale tribological properties were investigated by atomic force microscopy (AFM). IBA revealed that the boron/carbon atomic ratio is around 4 and that oxygen contamination does not exceed 10 at.%. The hydrogen content is below 2 at.%. The film density is nearly the bulk value for all biases applied to the substrate. AFM measurements show that the surface roughness decreases with increase of bias from 0.85 to 0.15 nm. The friction coefficient obtained by lateral force measurements follows the same trend, decreasing with increasing bias from 0.25 to 0.1. Wear measurements were performed and the wear depth decreased for films with lower friction coefficients. A mechanism based on the removal of a modified B₄C surface layer is proposed to explain the wear results.     

Surface and bulk processes at oxidized iridium electrodes—II. Conductivity-switched behaviour of thick oxide films

The electrochemical reversibility of thick oxide films that are formed on Ir electrodes by potential cycling is examined in relation to the initial monolayer surface oxidation processes. The unusual properties of thick oxide films on Ir, eg, as electrode surfaces for O₂ or Cl₂ evolution and with regard to H pseudocapacitance and double-layer capacitance being almost independent of oxide film thickness, arise because of a major increase of conductivity of the oxide as the oxidation state of Ir in the film is changed in a potential sweep between ca. 0.05 and 1.4 V E_H. In a cyclic sweep this has the effect of switching “on” or “off” the large real area of a microporous, hydrated oxide structure of the thick oxide films for electron transfer processes at their surfaces or within their bulk or pores.It is proposed that film growth which occurs on cycling is the result of accumulation of oxide produced in an underlying monolayer during each anodic sweep but which is left in an incompletely reduced state on each following cathodic sweep, due to irreversibility.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

Synthesis and characterization of aluminum oxide–boron carbide coatings by air plasma spraying

Aluminum oxide (Al₂O₃)–boron carbide (B₄C) composites have been proposed for use as cutting tools as well as in high temperature applications due to their high hardness and fracture toughness. The air plasma spraying method was used to fabricate the composite coatings of Al₂O₃ and B₄C. Three different Al₂O₃:B₄C composition ratios of 90:10, 80:20, and 70:30 by weight were plasma sprayed on plain carbon steel substrates. The effect of B₄C content on microstructure, hardness, porosity and thermal diffusivity of the coatings were studied using scanning electron microscopy (SEM), microhardness testing, X-ray diffraction (XRD), and the flash diffusivity method. The plasma spray parameters were optimized in order to achieve a theoretical density of approximately 90%.     

Scavenging effect on ionic conduction behaviour at the grain boundary of MgO partially stabilised zirconia

The scavenging effect of magnesium oxide (MgO) addition on electrical property of 9 mol-% MgO partially stabilised zirconia (Mg-PSZ) was investigated in terms of phase transformation and intergranular phase formation. The addition of MgO up to 5 mol-% caused a stabilisation of Mg-PSZ, which led to an increase in the cubic phase and a decrease in the monoclinic and tetragonal phases in Mg-PSZ. The Mg-PSZ with the addition of 5 mol-% of MgO also exhibited the maximum ionic conductivity (0.3915?S?cm^?1 at 1500°C) and forsterite (Mg₂SiO₄) was observed on the grain boundaries of Mg-PSZ. The intergranular phases, formed by reactions between the silicon in Mg-PSZ and MgO addition, reduced the grain boundary resistance, because the siliceous phase which is a hindrance for oxygen ion conduction was scavenged by the formation of Mg₂SiO₄.     

Conductance measurement of thin oxide films on a platinum anode

Conductance of two types of oxides, which were galvanostaticaly formed on a smooth platinum anode with 200 mA/cm² at 45°C in 0.5 M H₂SO₄, were measured with a new conductance cell.It was found that two types of oxide films differ in conductance property. The conductivity of the one, a outermost monolayer oxide film, was equal to that of metalic Pt. The other, a multilayer oxide which grows under the monolayer oxide, showed a definite conductivity lower than that of the metal. The conductivity per monolayer of this oxide in the vertical direction of the film plane was 0.67 × 10⁵ Ω^?1 per unit real surface area. The specific conductance value of the multilayer oxide was estimated to be about 1–2 × 10^?3 Ω^?1 cm^?1, assuming that the multilayer oxide is composed of PtO₂ and that this monolayer is twice the thickness of a PtO monolayer.     

Large-scale fabrication of boron nitride nanotubes and their application in thermoplastic polyurethane based composite for improved thermal conductivity

The large-scale manufacture of inexpensive boron nitride nanotubes (BNNTs) has proven difficult in recent decades. In this study, BNNTs are fabricated on a massive scale by ball-milling a mixture of boron oxide (B₂O₃), amorphous boron powder (B), and magnesium diboride (MgB₂) and then annealing the resulting product in NH₃, which follows a vapor-liquid-solid growth mechanism. MgB₂ serves as a catalyst in the growth process, and the vaporous B₂O₃ and diboron dioxide (B₂O₂) produced from the reaction of B₂O₃ and B are efficient sources of boron. The obtained BNNTs can be dispersed well in dimethyl formamide, possibly as a result of some cracked structures in the BNNTs and impurities that form during the synthesis process. Taking advantage of the good dispersity, the uniform BNNT/thermoplastic polyurethane composite films are prepared via solution blending. The incorporation of BNNTs apparently enhances the thermal conductivity of the neat thermoplastic polyurethane. This article contributes to the low-cost fabrication of BNNTs and their potential application as fillers in polymers.     

AFM study of crystallization and melting of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} in ultrathin films

Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M_n) of 5300 and 2100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T_c<44 °C, the monolayer crystals are dendrites. At T_c>48 °C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 °C≤T_c≤48 °C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T_c. At the same T_c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T_c, and their melting is mainly determined by the terrace thickness.     

Understanding the morphological variation in the formation of B4C via carbothermal reduction reaction

Despite the relatively successful effort for the synthesis of fine boron carbide (B₄C) powders, very little is known about the underlying interrelationships between processing, compositions and the resulting product morphology for B₄C formation via the carbothermal reduction reaction (CTR) process. In this paper, B₄C, including uniform submicron powders, were synthesized using CTR of finely mixed boron trioxide and carbon obtained from low-cost water-soluble precursors. To understand the aforementioned interrelationships and to better control product morphology, the effects of factors such as CTR thermal profile, CTR atmosphere and precursors on product phase purity and morphology were systematically studied. Among all the factors, it was found that CTR temperature, along with total reaction time, plays a dominant role in determining the morphology of B₄C products. Such observation was understood by analysis of CTR reaction kinetics and comparing it with microstructural evolution: significant non-uniformity in B₄C product morphology arises from the competition between nucleation and growth at low to intermediate CTR temperatures (e.g., ~1150–1450 °C) while highly uniform submicron or nano B₄C products result from very fast nucleation at high CTR temperatures (e.g., ≥~1750 °C) within a few minutes or even seconds.     

Role of Oxalic Acid in Promoting Ignition and Combustion of Boron: an Experimental and Theoretical Study

Boron is an attractive fuel for propellants and explosives because of its high energy density. However, its combustion is inhibited by the oxide layer that covers the particles. The use of oxalic acid as an additive was shown to promote boron oxidation. In this study, the thermodynamic model FactSage 6.2 and a laser ignition facility were used to investigate the effect of oxalic acid on the burning characteristics of boron particles. The results of the thermodynamic analyses show that oxalic acid can reduce B₂O₃(l) production during boron combustion. This enables removal of the the oxide film and promotes the burning of boron. However, only at high temperatures (>1500 K) H₂O(g) (produced from H₂C₂O₄) can react with B₂O₃ and remove the oxide film. The evolution of boron combustion flame takes place in three stages: ignition, stable combustion, and extinction; the bright yellow color in the flame indicates boron ignition, the bright white color indicates boron combustion, and the bright green color is interpreted as BO₂ emission. Addition of oxalic acid into boron powders can significantly promote boron ignition and combustion. The ignition delay time of the resulting mixture is reduced by 42.4 %, the combustion intensity is raised by 16.7 %, and the combustion efficiency of boron is increased by 21.5 percentage points. The mechanism of action of oxalic acid on enhancing the combustion of boron was studied by scanning electron microscopy.     

A novel 3D network nanostructure constructed by single-crystal nanosheets of B4C

Three-dimensional (3D) network nanostructure boron carbide was successfully synthesized via the carbothermic method. The carbon source and template was carbonized bacterial cellulose (CBC) with a 3D network nanostructure, and the boron source was B₂O₃ and amorphous B powder. X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) were used to study the morphology and structure of the samples. XRD and Raman spectra confirm that they belong to the B₄C crystalline phase. The FESEM images show that the synthetic B₄C retains the 3D network nanostructure of the template CBC well and consists of B₄C nanosheets with an average thickness of less than 100 nm. The analytical results of high-resolution TEM (HRTEM) and Selected Area Electron Diffraction (SAED) indicate that the B₄C takes the shape of hexagonal single crystals with a rhombohedral structure. These B₄C single crystal nanosheets alternate, forming the 3D network nanostructure. The mechanism of formation can be accounted for by in situ reduction reactions along the carbon nanofibers of CBC.