Increasing fracture toughness of zirconia-based composites as a synergistic effect of the introducing different inclusions

The effect of multi-walled carbon nanotubes (MWCNTs) and hexagonal boron nitride (h-BN) inclusions on the fracture toughness of yttria-stabilized zirconia (YSZ) ceramics has been studied. It is shown that an increase in the MWCNTs and h-BN content has a positive effect on the K_1C of zirconia ceramics. The greatest increase in the fracture toughness of YSZ ceramics was observed with the introduction of hexagonal boron nitride particles. For YSZ ceramics, the K_1C value was ≈6.1 MPa m^1/2, for ceramics with a 5 wt % of h-BN K_1C ≈ 9.2 MPa m^1/2. It was shown that an increase of the YSZ ceramics fracture toughness with the introduction of MWCNTs and h-BN, both and separately was provided by the combined action of several mechanisms of increasing the work of crack propagation. In addition, in all composites obtained in this work, the transformation of tetragonal ZrO₂ into monoclinic was observed.     

Role of the particle size of graphite-like boron nitride in nucleation of cubic boron nitride

The influence of the reduced radius of grains of the graphite-like hexagonal boron nitride (h-BN) on the nucleation of the cubic boron nitride (c-BN) during synthesis from an initiator solution at a high pressure is analyzed. The colloidal mechanism of nucleation is confirmed experimentally. It is shown that there is a correlation between the nucleus sizes of the hexagonal boron nitride and the pressure of the onset of nucleation of the cubic boron nitride. The effect of these sizes of the hexagonal boron nitride on the concentration of crystal nuclei of the cubic boron nitride is studied. The kinetic nucleation curves are obtained. It is demonstrated that the concentration of crystallization centers depends on the thermodynamic and kinetic parameters, as well as on the particle size of the graphite-like hexagonal boron nitride. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Interface bonding engineering for constructing a battery-type supercapacitor cathode with ultralong cycle life and high rate capability

The low rate and poor cycle greatly limit the large-scale applications of supercapacitors electrodes in energy storage field. In this work, the SnS₂/Ni₃S₂ nanosheets arrays are bonded on N/S co-doped graphene nanotubes though N–Sn/Ni and S–Sn/Ni interface bonds employing a simple hydrothermal method to form a self-supported battery-type supercapacitors cathode. A series of characterization and DFT calculations indicate that the interface bonding not only automatically generates the internal electric field and allows more redox reactions to carry out easily, but also effectively reduces the OH^? ions adsorption energy and maintains the integration of the electrode structure. This unique design greatly promotes the electronics/ions transfer and reaction kinetics of the cathode, and substantially enhances its rate capability and durability. Detailedly, a high specific capacity of 296.9 mAh g^?1 at 2 A g^?1 is obtained. More impressively, the cathode still holds 155.6 mAh g^?1 when the current density is enlarged to 100 A g^?1, as well as it can retain 84% initial capacity over 50,000 cycles. Besides, an assembled asymmetric supercapacitor utilizing the prepared N/S-GNTs@B–SnS₂/Ni₃S₂ nanosheets arrays cathode and activated carbon anode presents a large energy density of 51 W h kg^?1 at 850 W kg^?1 and outstanding cycling stability. This work provides an effective strategy for improving rate capability and cycle lifespan of battery-type supercapacitors electrodes, and pushes the metal compounds forward a significant step in the practical applications of energy storage devices.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Nucleation and growth of single crystal graphene on hexagonal boron nitride

Direct graphene growth was demonstrated on exfoliated hexagonal boron nitride (h-BN) single crystal flakes by low pressure CVD. The size of the hexagonal single crystal graphene domain increases with deposition time, with maximum size of ～270 nm. Most domains were found to nucleate at screw dislocation sites, and a step-flow growth mechanism was observed at atomic steps on the h-BN surface. Understanding the nucleation and growth mechanisms is an important step towards the synthesis of large single crystal graphene on h-BN substrates.     

Monolayer graphene/hexagonal boron nitride heterostructure

A buried metal-gate field-effect transistor (FET) using a stacked hexagonal boron nitride (h-BN) and chemically vapor deposited (CVD) graphene heterostructure is demonstrated. A thin h-BN multilayer serves as both gate dielectric and supporting layer for the monolayer graphene channel. It is observed that electrical stressing could significantly improve graphene conduction, similar to the effect reported in the graphene/SiO₂ system. In the graphene/h-BN/TiN FET structure, p-type doping behavior in graphene is observed, possibly attributed to spontaneous doping due to the work function difference between the graphene channel and the metal gate electrode. At a high-level of stress, graphene exhibits n-type doping behavior due to charge transfer across the thin h-BN multilayer. The dielectric strength and tunneling behavior of h-BN are investigated, showing the robust nature of the layer-structured insulator.     

CoO/rGO composite prepared by a facile direct-flame approach for high-power supercapacitors

In this study, CoO nanoparticles (NPs) measuring approximately 20?nm in size are successfully grown on reduced graphene oxide (rGO) layers through a facile direct-flame approach. The obtained CoO/rGO nanocomposites are applied as electrode materials and show a high specific capacitance, reaching 1615.0?F?g^?1 at a current of 1?A?g^?1 (737.5?F?g^?1 at 50?A?g^?1), and good cycling stability (88.12% retention after more than 15,000 cycles at 5?A?g^?1), which are outstanding characteristics compared with those of recently reported pseudosupercapacitors. Furthermore, an asymmetric supercapacitor (ASC) produced using CoO/rGO as a positive electrode material and activated graphene (AG) as a negative electrode achieves a high cell voltage of 1.6?V and delivers a maximum energy density of 62.46?Wh?kg^?1 at a power density of 1600?W?kg^?1. The fabrication technique is facile and represents a promising means of obtaining metal oxide/graphene composites for high-performance supercapacitors.     

Features of phase transformations in boron nitride

Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion-generated E-BN, wurtzitic w-BN and cubic c-BN, and their transformations under shock compression, ball-milling, heating, electron-beam or laser irradiation are discussed, focusing especially on the E-phase formation by different methods, and properties and structural models of this phase. It is shown that E-BN has a fullerene-like structure.     

Facile preparation of Ni-doped MnCO3 materials with controlled morphology for high-performance supercapacitor electrodes

Doping homogeneous elements and conducting morphological adjustment as commonly-used modification methods are both effective to promote the electrochemical properties of electrode materials. In this work, nickel-doped manganese carbonate with 3D flower-like structure was synthesized by a one-step hydrothermal method, and the corresponding growth mechanism was investigated. The electrochemical characteristics of as-fabricated electrode materials were measured, among which 3D self-assembled Ni_0.2Mn_0.8CO₃ nanoflower with large surface area exhibited superior areal capacitance of 583.5?F?g^?1 at 1?A?g^?1 (fourfold more than MnCO₃ microcubes), excellent electrical conductivity as well as satisfactory cycling stability (84.78% capacitance retention after 2000 cycles at 2?A?g^?1). In addition, the asymmetric supercapacitor assembled with Ni_0.2Mn_0.8CO₃ as cathode and commercial activated carbon as anode displayed a high energy density of 24.1?Wh?kg^?1 at the power density of 0.74?kW?kg^?1 and showed a desirable cycle life. In summary, the unique 3D flower-like Ni_0.2Mn_0.8CO₃ nanomaterial could be regarded as a promising electrode material for high-performance supercapacitors.     

Synthesis and characterization of exfoliated polystyrene grafted hexagonal boron nitride nanosheets and their potential application in heat transfer nanofluids

Three different methods were used to develop surface-modified hexagonal boron nitride (h-BN) nanosheets, and polystyrene grafting was performed by an indirect covalent bond formation between modified h-BN nanosheets and styrene molecules through surface initiated atom transfer radical polymerization (SI-ATRP) approach. In all methods, an alkyl bromide as the ATRP-initiating site was first introduced on h-BN nanosheets and an SI-ATRP reaction of styrene from the initiator immobilized h-BN surface was achieved. The structure of synthesized PS grafted h-BN nanosheets (PS-g-h-BN) was identified and characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy methods. The functionalization promoted the exfoliation of h-BN layered structure into few layer sheets where the thickness of the sheets was dependent on the modification technique and the content of polymer grafted on nanosheets. The highest grafting content of PS-g-h-BN nanosheets was obtained around 20% which could enhance thermal conductivity of mineral oil-based nanofluids with the minimum concentration of the nanofiller (0.01 wt%). The electrical and physical properties of the nanofluid were also investigated. According to the results, the dielectric loss reduced by increase in nanofiller concentration was an indication of the enhanced dielectric nature of nanofluid. In addition, exfoliated PS-g-h-BN nanosheets dispersions were shown to be stable in mineral oil up to 2 months and this stability was linked to the presence of polymer chains followed by the formation of van der Walls interactions between the grafted polymer and the fluid.     

New insights into synthesis of nanocrystalline hexagonal BN

Uncovering the mechanism behind nanocrystalline hexagonal boron nitride (h-BN) formation at relatively low temperatures is of great scientific and practical interest. Herein, the sequence of phase transformations occurring during the interaction of boric acid with ammonia in a temperature range of 25–1000 °C has been studied in detail by means of thermo-gravimetric analysis, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The results indicate that at room temperature boric acid reacts with ammonia to form an ammonium borate hydrate (NH₄)₂B₄O₇x4H₂O. Its interaction with ammonia upon further heating at 550 °C for 1 h leads to the formation of turbostratic BN. Nanocrystalline h-BN is obtained either during heating in ammonia at 550 °C for 24 h or at 1000 °C for 1 h. This result is important for the development of novel cost-effective and scalable syntheses of h-BN nanostructures, such as nanosheets, nanoparticles, nanofibers, and nanofilms, as well as for sintering h-BN ceramic materials.     

Urchin-like boron nitride hierarchical structure assembled by nanotubes-nanosheets for effective removal of heavy metal ions

Water pollution has become a serious global issue owing to the large amounts of contaminants generated from industrial and agricultural development. Recently, various boron nitride-based micro/nano-materials have exhibited efficient sorption capacity for contaminants from water. Herein, novel urchin-like boron nitride hierarchical structure assembled by free-growing boron nitride nanotubes and crapy boron nitride nanosheets is firstly fabricated via a sample two-step approach, including the synthesis of analogous "core-shell" structured boron-containing precursor and thermal catalytic chemical vapor deposition. A combined growth mechanism of vapor-liquid-solid and vapor-solid is proposed to control the formation of BN hierarchical structure. The unique structure exhibits superior removal capacity of 115.07?mg?g^?1 and 92.85?mg?g^?1 for Pb²⁺ and Cu²⁺ in water solution, respectively. The excellent adsorption performance of the product mainly derives from the vast lattice imperfections, the high-density edge active sites, the expanded interplanar spacing, as well as the unique structural characteristics. They are beneficial for structural stability and enough space for accommodating the adsorbed heavy metal ions. These results indicate that the urchin-like boron nitride hierarchical structure is a promising adsorption material for water treatment.     

New potential for preparation of performing h-BN coatings via polymer pyrolysis in RTA furnace

An innovative IR irradiation annealing process is used to increase the crystallization ratio of hexagonal boron nitride (h-BN) coatings on pure titanium pellets. Since the polymer pyrolysis route requires heating the green polymer at high temperature to convert it into a ceramic, the use of IR radiation furnace (compared to a resistive furnace) allows achievement of better crystallized h-BN while the substrate remains at relatively low temperature (<1000 °C). Annealing treatments have been performed under argon or nitrogen using either a halogen lamp or a Rapid Thermal Annealing (RTA) furnace. Advanced structural and chemical characterizations have shown a good chemical stability of the coatings. In addition, it has been revealed that samples annealed under Ar present a micro-composite structure at the interphase composed of a μ-layer of TiB₂/TiB/Ti(N)_x between the coating and the substrate, whereas samples annealed under nitrogen display a simpler structure at the interphase, with only TiN.     

Novel Poly(m‐phenyleneisophthalamide) Dielectric Composites with Enhanced Thermal Conductivity and Breakdown Strength Utilizing Functionalized Boron Nitride Nanosheets

Dielectric polymer‐based composites with high breakdown strengths and thermal conductivities have attracted considerable attention when applied in electronic devices. In this study, a novel poly(m‐phenyleneisophthalamide) (PMIA) dielectric nanocomposite is successfully fabricated by introducing functionalized hexagonal boron nitride nanosheet (fBNNS) fillers. Due to effective functionalization of hexagonal boron nitride nanosheets (BNNSs), fBNNSs fillers are homogeneously dispersed in the PMIA matrix. The breakdown strength and thermal conductivity of PMIA/fBNNSs dielectric nanocomposite are investigated. Research results indicate that the breakdown strength of fBNNSs‐12 reaches 105.6 MV m^?1, which is 1.34 times that of pure PMIA. Moreover, owing to high thermal conductivity of fBNNSs, the thermal conductivities of fBNNSs‐12 are observably increased to 8.06 W m^?1 K of in‐plane direction and 0.84 W m^?1 K of through‐plane direction, respectively. Considering these properties, the manufactured PMIA/fBNNSs dielectric nanocomposites show potential applications in field of electronics.     

Ultrasonically dispersed multi-composite strategy of NiCo2S4/Halloysite nanotubes/carbon: An efficient solid-state hybrid supercapacitor and hydrogen evolution reaction material

Herein, we have developed a novel hybrid material based on NiCo₂S₄ (NCS), halloysite nanotubes (HNTs), and carbon as promising electrodes for supercapacitors (SCs). Firstly, mesoporous NCS nanoflakes were prepared by co-precipitation method followed by physically mixing with HNTs and carbon, and screen printed on nickel foam. After ultrasonication, a uniform distribution of the Carbon/HNTs complex was observed, which was confirmed by surface morphological analysis. When used as electrode material, the NCS/HNTs/C hybrid displayed a maximum specific capacity of 544 mAh g^?1 at a scan rate of 5 mV s^?1. Later, a solid-state hybrid SCs was fabricated using activated carbon (AC) as the negative and NCS/HNTs/C as the positive electrode (NCS/HNTs/C//AC). The device delivers a high energy density of 42.66 Wh kg^?1 at a power density of 8.36 kW kg^?1. In addition, the device demonstrates long-term cycling stability. Furthermore, the optimized NCS, NCS/HNTs, and NCS/HNTs/C nanocomposites also presented superior hydrogen evolution reaction (HER) performance of 201, 169, and 116 mV in the acidic bath at a current density of 10 mA cm^?2, respectively. Thus, the synthesis of NCS/HNTs/C nanocomposite as positive electrodes for hybrid SCs opens new opportunities for the development of next-generation high energy density SCs.     

Supercapacitor device performances of polycarbazole/graphene and polycarbazole/nanoclay/graphene nanocomposites

In this study, graphene oxide (GO) is chemically reacted with sodium borohydride (NaBH₄) to form reduced graphene oxide (rGO). rGO, polycarbazole (PCz)/rGO and PCz/nanoclay/rGO materials were obtained by chemical polymerisation method. These three materials were characterised by Fourier-transform infra-red spectroscopy-attenuated transmission reflectance, scanning electron microscopy, energy-dispersive X-ray analysis, cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy. The PCz/nanoclay/rGO nanocomposite shows significantly improved capacitance (C_sp?=?187.78?F?g^?1) compared to that of PCz/rGO (C_sp?=?74.18?F?g^?1) and rGO (C_sp?=?20.78?F?g^?1) at the scan rate of 10?mV?s^?1 by CV method. The supercapacitor device performance results show high power density (P?=?1057.81?W?kg^?1) and energy density (E?=?1.7?Wh?kg^?1) obtained from Ragone plot for PCz/nanoclay/rGO material. Stability tests were also examined by the CV method for 1000 cycles.     

Boron doped graphene cathode for capacitor via a new one-step method

The graphene cathode was doped with boron via a new and fast method of plasma enhanced chemical vapor deposition (PECVD) at room temperature. Various plasma species of BH_x (x?=?0–3) with high reactivity reacted with graphene electrode via surface re-reactions and gas-interface intersection. The cathode made of boron doping into graphene (BG) exhibited excellent electrochemical performances in Li-ion capacitors, including a large discharge capacity of 140?mAh?g^?1 (voltage range: 1.5–4.2?V vs. Li/Li⁺, current density: 100?mA?g^?1) and the coulombic efficiency of more than 99.6% within 1000 circles. The capacity, coulombic efficiency and circle performance of the BG electrode were more superior to the undoped graphene electrode owing to the uniform doping of boron plasma species. The PECVD method has the advantages of being simple, is conducted at room-temperature, is time efficient and uniform, thus making it a fast and effective way for doping hetero-atoms into the electrode.     

Mechanical and thermal properties of h-BN based composite containing dual glass phases at elevated temperatures

High-temperature mechanical and thermal properties of h-BN based composite containing amorphous silica and Yb-riched silicate glass phases were systematically investigated in this work. Owing to anisotropic microstructure of h-BN matrix, the obtained composite demonstrates anisotropic mechanical and thermal properties. The composite possesses higher elastic modulus at 1673?K than that at room temperature and presents excellent high-temperature stiffness. Flexural strengths in parallel and perpendicular directions reach 496?±?22 and 258?±?21?MPa at?1073?K, respectively, and increases by 74 and 66% compared with the room-temperature strengths of 285?±?4?and 155?±?5?MPa. The composite containing dual glass phases shows lower coefficients of thermal expansion in the temperature range of 473–900?K, the values are ?1.4?×?10^?6 and 0.3?×?10^?6 ?K^?1 for the perpendicular and parallel directions, respectively. Thermal conductivities in the perpendicular and parallel directions at 373?K are 24.8 and 14.8?W?m^?1?K^?1, respectively, and then decrease to 14.9 and 9.3?W?m^?1?K^?1 at 1473?K.     

Preparation and capacitance performance of polyaniline/titanium nitride nanotube hybrid

A polyaniline/titanium nitride (PANI/TiN) nanotube hybrid was prepared and used for an electrochemical supercapacitor application. Firstly, the well-aligned TiN nanotube array was prepared by anodization of titanium foil and subsequent nitridation through ammonia annealing. Then, PANI was deposited into TiN nanotube through the electrochemical polymerization process. The obtained PANI/TiN nanotube hybrid had an ordered porous structure. A high specific capacitance of 1,066 F g^?1 was obtained at the charge–discharge current density of 1 A g^?1 when only the mass of PANI was considered. The specific capacitance can even achieve 864 F g^?1 at 10 A g^?1 and still keep 93 % of the initial capacity after 200 cycles. An aqueous supercapacitor, consisting of two symmetric PANI/TiN nanotube hybrid electrodes and 1.0 M H₂SO₄ electrolyte solution, showed the specific capacitance of 194.8 F g^?1, energy density of 9.74 Wh kg^?1, and power density of 0.3 kW kg^?1.     

Convenient synthesis of Ni-Mn-S@rGO composite with enhanced performance for advanced energy storage applications

As a promising energy storage device, the performance of supercapacitors is greatly influenced by the electrode material. Here, we designed a simple Ni-Mn-S@rGO composite material. The rGO with a two-dimensional layered structure serves as the basis for the in-situ growth of Ni-Mn-S microspheres, which effectively reduces the aggregation of sulfides, provides a rapid electron migration channel, and effectively improves the electrochemical performance of the material. In addition, the synergy between different metal ions will also affect the properties of the materials. Therefore, the materials are optimized by controlling the proportion of different components in the raw materials. The results show that the Ni-Mn-S@rGO-2 with proper composition has a superior specific capacitance of 2042.22F g^?1 (at 1A g^?1) and a retention rate of 77.78% after 5000 cycles. An all-solid-state asymmetric supercapacitor was fabricated using Ni-Mn-S@rGO-2 and active carbon as electrode materials. The device achieved a high energy density of 77.95 Wh kg^?1 at a power density of 750W kg^?1. Furthermore, the device retains 81.97% specific capacitance after 10000 charge-discharge cycles, which indicates that the device has satisfactory cycle stability. These results prove that Ni-Mn-S@rGO composites have potential applications in the field of supercapacitors.