Densification and properties of transition metal borides-based cermets via spark plasma sintering

Engineering borides like TiB₂ and ZrB₂ are difficult to sinter materials due to strong covalent bonding, low self-diffusion coefficient and the presence of oxide layer on the powder particles. The present investigation reports the processing of hard, tough and electrically conductive transition metal borides (TiB₂ and ZrB₂) based cermets sintered with 6 wt.% Cu using spark plasma sintering (SPS) route. SPS experiments were carried out with a heating rate of 500 K/min in the temperature range of 1200–1500 °C for a varying holding time of 10–15 min and the optimization of the SPS conditions is established. A maximum density of ∼95% ρ_th in ZrB₂/Cu and ∼99% ρ_th in TiB₂/Cu is obtained after SPS processing at 1500 °C for 15 min. While the optimized TiB₂/Cu cermet exhibits hardness and fracture toughness of ∼17 GPa and ∼11 MPa m^1/2, respectively, the optimized ZrB₂/Cu cermet has higher hardness of ∼19 GPa and fracture toughness of ∼7.5 MPa m^1/2, respectively. High electrical conductivity of ∼0.20 MΩ⁻¹ cm⁻¹ (TiB₂/Cu) and ∼0.15 MΩ⁻¹ cm⁻¹ (ZrB₂/Cu) are also measured with the optimally sintered cermets.     

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

High quality microwave dielectric ceramic sintered at extreme-low temperature below 200° and co-firing with base metal

Ultra-low temperature co-fired ceramics technology (ULTCC) requires the microwave dielectric ceramics with lower intrinsic sintering temperature than the melting point of inner electrodes. In the present work, a novel HBO₂ ceramic was found to be densified at extreme-low temperature below 200 °C, with pores, residual H₃BO₃, amorphous B₂O₃ inside, with a relative permittivity ∼2.12 ± 0.02, a Qf value ∼32,700 ± 300 GHz and a temperature coefficient of resonant frequency value ∼ − 43 ± 3 ppm/°C. This material can be easily obtained by dehydration from H₃BO₃ by sintering at low temperature below 200 °C. Its extreme-low sintering temperature and water solubility also provides the possibility to achieve some novel multi-functional inorganic-organic composite in the future.     

Nano- and micro-crystalline diamond growth by MPCVD in extremely poor hydrogen uniform plasmas

It is well established that argon rich plasmas (> 90% Ar) in Ar/CH₄/H₂ gas mixtures lead to (ultra)nanodiamond nucleation and growth by microwave plasma chemical vapour deposition (MPCVD). Nonetheless, in the present work, both microcrystalline and nanocrystalline diamond deposits developed under typical conditions for ultrananocrystalline (UNCD) growth by MPCVD. Silicon substrates were pretreated by abrasion using two different diamond powder types, one micrometric (< 0.5 μm) and the other nanometric (∼ 4 nm), the latter obtained by detonation methods. Samples characterization was performed by SEM (morphology), AFM (roughness and morphology) and micro-Raman (structure).For all samples, Raman analysis revealed good crystalline diamond quality with an evident ∼ 1332 cm^− 1 peak. The Raman feature observed at ∼ 1210 cm^− 1 is reported to correlate with two other common bands at ∼ 1140 cm^− 1 and ∼ 1490 cm^− 1 characteristic of nano- and ultra-nanocrystalline diamond.A new growth process is proposed to explain the observed morphology evolution from nano- to microcrystalline diamond. Based on this, the microcrystalline morphology is in fact a crystallographically aligned construction of nanoparticles.     

Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process

We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets into hollow graphene spheres (HGSs), using only a GO/H₂SO₄ aqueous suspension as the starting material. Scanning electron microscope, focused ion beam scanning electron microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 μm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g⁻¹, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors.     

Fast discharge and high energy density of nanocomposite capacitors using Ba0.6Sr0.4TiO3 nanofibers

Nanocomposites combining high breakdown strength (BDS) polymer and high dielectric permittivity ceramic fillers have shown great potential for pulsed power application. Here a new composite material based on surface-functionalized Ba_0.6Sr_0.4TiO₃ nanofibers/poly(vinylidene fluoride) (BST NF/PVDF) has been prepared by solution casting. The nanocomposites containing 2.5 vol% isopropyl dioleic(dioctylphosphate) titanate (NDZ 101)-functionalized BST NF (N-h-BST NF) have large energy density of 6.95 J cm⁻³ at 380 MV m⁻¹, which is 1.85 times larger than that of the pure PVDF at the same electric field. Also, the discharge speed of the nanocomposites containing 7.5 vol% N-h-BST NF is approximately 0.11 μs. The good properties, together with the large energy density and fast discharge speed, make this material a promising candidate for pulsed power capacitor.     

Dense nanostructured zirconia compacts obtained by colloidal filtration of binary mixtures

As starting materials two commercial nanosized zirconias doped with 3 mol% of Y₂O₃ were used: a powder of about 100 nm (TZ3YE, Tosoh, Japan) and a colloidal suspension of about 15 nm (Mel Chemicals, UK). Colloidal stability in water was studied for both zirconias in terms of zeta potential as a function of deflocculant concentration and pH. Concentrated suspensions were prepared by dispersing the powder in the colloidal suspension to solids loadings ranging from 5 to 30 vol.% using a sonication probe to achieve dispersion. The rheological behavior was optimized in terms of solids content, deflocculant content and sonication time. Optimized suspensions with up to 25 vol.% solids showed a nearly Newtonian behavior and extremely low viscosities and maintain stable for long times (days) which is an important drawback of conventional nanoparticle suspensions. Samples obtained by slip casting in plaster moulds were used for dynamic sintering studies and dense, nanostructured specimens were obtained at temperatures of 1300–1400 °C.     

Graphene-wrapped polyaniline nanofibers as electrode materials for organic supercapacitors

Graphene-wrapped polyaniline nanofibers were prepared by assembly of negatively charged graphene oxide with positively charged aqueous dispersible polyaniline nanofibers in an aqueous dispersion, followed by the reduction of the graphene oxide. The hybrid material with a graphene oxide loading of 9.1 wt.% displayed a high specific capacitance of over 250 F g⁻¹ in a 1 M Et₄N⁺·BF₄⁻/propylene carbonate electrolyte, a 39.7% increase compared with pristine polyaniline nanofibers. A significant improvement in long-term cycle life was also realized. The hybrid exhibited an initial specific capacitance of 236 F g⁻¹, which remained as high as 173.3 F g⁻¹ over 1000 cycles, or a 26.3% decrease, much better than that for pure polyaniline nanofibers. An asymmetric supercapacitor based on this hybrid material and activated carbon was assembled. An energy density of 19.5 W h kg⁻¹ at a power density of 738.95 W kg⁻¹ was obtained for the cell under an operating voltage window of 2 V.     

Facile synthesis of functionalized porous carbon with three-dimensional interconnected pore structure for high volumetric performance supercapacitors

Functionalized porous carbon with three-dimensional (3D) interconnected pore structure has been successfully synthesized through direct heat-treatment of KOH-soaked soybeans. Benefiting from heteroatoms (N, O) doping, interconnected porous carbon framework with high surface area as well as high packing density (up to 1.1 g cm⁻³), the as-obtained porous carbon material exhibits high volumetric capacitance of 468 F cm⁻³, good rate capability and excellent cycling stability (91% of capacitance retention after 10,000 cycles) in 6 M KOH electolyte. More importantly, the as-assembled symmetric supercapacitor delivers high volumetric energy density of 28.6 Wh L⁻¹ in 1 M Na₂SO₄ aqueous solution.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

High-surface area carbons from renewable sources with a bimodal micro-mesoporosity for high-performance ionic liquid-based supercapacitors

Highly porous materials with a bimodal pore size distribution in the micro-mesopore range have been produced from biomass by adding melamine to the hydrochar/KOH mixture used in the activation process. These carbons are characterized by BET surface areas in excess of ∼3300 m² g⁻¹ and a porosity equally distributed between micropores and mesopores. The use of melamine in the synthesis process not only extends the pore size distribution into the mesopore region, but leads to the incorporation of a certain amount of nitrogen atoms into the carbon framework. These materials combine high ion adsorption capacities (micropores) and enhanced ion-transport kinetics (mesopores) leading to an outstanding capacitive performance in ionic liquid-based supercapacitors. Thus, they have specific capacitances >160 F g⁻¹ at 1 A g⁻¹ and >140 F g⁻¹ at 60 A g⁻¹ in both pure ionic liquid and in acetonitrile-diluted ionic liquid, enabling these materials to store up to a maximum of ca. 60 W h kg⁻¹ in both kinds of electrolytes and deliver ca. 20 W h kg⁻¹ at ∼42 kW kg⁻¹ (discharge time ca. 2 s) in pure ionic liquid and ∼25–30 W h kg⁻¹ at ∼97–100 kW kg⁻¹ (discharge time ∼1 s) in acetonitrile-diluted ionic liquid.     

Application of BaO–CaO–Al2O3–B2O3–SiO2 glass–ceramic seals in large size planar IT-SOFC

BaO–CaO–Al₂O₃–B₂O₃–SiO₂ (BCAS) glass–ceramics can be used as sealant for large size planar anode-supported solid oxide fuel cells (SOFCs). BCAS glass–ceramics after heat treatment for different times were characterized by means of thermal dilatometer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the coefficients of thermal expansion (CTE) of BCAS glass–ceramics are 11.4×10⁻⁶ K⁻¹, 11.3×10⁻⁶ K⁻¹ and 11.2×10⁻⁶ K⁻¹ after heated at 750 °C for 0 h, 50 h, and 100 h, respectively. The CTE of BCAS matches that of YSZ, Ni–YSZ and the interconnection of SOFC. Needle-like barium silicate, barium calcium silicate and hexacelsian are crystallized in the BCAS glass after heat-treatment for above 50 h at 750 °C. The glass–ceramics green tape prepared by aqueous tape casting can be directly applied in sealing the cell of SOFCs with 10 cm×10 cm. The open circuit voltage (OCV) of the cell keeps 1.19 V after running for 280 h at 750 °C and thermal cycling 10 times from 750 °C to room temperature. The maximum power density is 0.42 W/cm² using pure H₂ as fuel and air as oxidation gas. SEM images show no cracks or pores exist in the interface of BCAS glass–ceramics and the cell.     

SPS processing of bismuth-layer structured ferroelectric ceramics yielding highly textured microstructures

The spark plasma sintering (SPS) behaviour of nano-sized Bi₄Ti₃O₁₂ (BIT) and micron-sized CaBi₂Nb₂O₉ (CBNO) powders is described. The densification process of both powders is very rapid, i.e. the densification occurs within a very narrow time interval (2–3 min using a heating rate of 100 °C min⁻¹ and a pressure of 50 MPa). The BIT powder exhibits a lower densification onset temperature (∼650 °C) and higher maximum shrinkage rate (8.9 × 10⁻³ s⁻¹ at 780 °C) than that of the CBNO powder (∼825 °C and 4.5 × 10⁻³ s⁻¹ at 950 °C). Isothermal compaction studies revealed that fully dense nano-sized BIT compacts could be obtained within the temperature region 750 °C < T_iso < 850 °C while for T_iso > 850 °C compacts containing elongated platelet grains are formed. A new preparation route to produce highly textured compacts is described in detail. Appropriate pre-forms are prepared by spark plasma sintering (SPS) and these fully dense compacts are subject to superplastic deformation in the SPS unit to achieve a total compressive strain of ∼60%. This strain was achieved within a period of 1.5 min and with a maximum strain rates of 1.1 × 10⁻² s⁻¹ achieved at ∼840 °C and 1.3 × 10⁻² s⁻¹ at 1020 °C for the BIT and CBNO compacts, respectively. The X-ray studies showed that the Lotgering orientation factors of grains in the deformed BIT and CBNO compacts are 99% and 70%. The formation of highly textured compacts is suggested to be governed by a superplastic deformation-induced directional dynamic ripening mechanism.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Effect of glassy bonding materials on the properties of LAS/SiC porous ceramics with near zero thermal expansion

Near zero thermal expansion porous ceramics were fabricated by using SiC and LiAlSiO₄ as positive and negative thermal expansion materials, respectively, bonded by glassy material. The coefficient of thermal expansion value of a desired porous composite can be easily controlled by choosing the appropriate ratios of the different phases. It was shown that some of LiAlSiO₄ was decomposed to LiAlSi₂O₆ and LiAlO₂, some of LiAlSiO₄ reacted with SiO₂ to form LiAlSi₂O₆ during sintering. With increasing the content of glassy materials, the reaction between LiAlSiO₄ and SiO₂ was accelerated. The Young's modulus increased due to the neck growth between the SiC grains. The 52.5 vol% LiAlSiO₄ (LAS)/SiC ceramics with ∼36% porosity had a combination of near zero coefficient of thermal expansion ∼0.39 × 10⁻⁶ K⁻¹ at room temperature and relatively high Young's modulus ∼59 GPa.     

Porous carbon spheres as anode materials for sodium-ion batteries with high capacity and long cycling life

Porous carbon spheres (PCSs) with high surface area were fabricated by the reaction of D-Glucose monohydrate precursor with sodium molybdate dihydrate (Na₂MoO₄·2H₂O) via a facile hydrothermal method followed by carbonization and aqueous ammonia solution (NH₃·H₂O) treatment. The as-prepared PCSs exhibit a highly developed porous structure with a large specific surface area and show an excellent electrochemical performance as anode material of sodium-ion batteries (SIBs). A reversible capacity of 249.9 mA h g⁻¹ after 50 cycles at a current density of 50 mA g⁻¹ and a long cycling life at a high current density of 500 mA g⁻¹ are achieved. The excellent cycling performance and high capacity make the PCSs a promising candidate for long cycling SIBs.     

Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors

Three-dimensional flower-like and hierarchical porous carbon material (FHPC) has been fabricated through a simple and efficient carbonization method followed by chemical activation with flower-like ZnO as template and pitch as carbon precursor. The hierarchical porous structure is composed of numerous micropores and well-defined mesopores in the interconnected macroporous walls. The FHPC electrode can achieve a relatively high capacitance of 294 F g⁻¹ at a scan rate of 2 mV s⁻¹ and excellent rate capability (71% retention at 500 mV s⁻¹) with superior cycle stability (only 2% loss after 5000 cycles) in 6 mol L⁻¹ KOH electrolyte. The symmetric supercapacitor fabricated with FHPC electrodes delivers a high energy density of 15.9 Wh kg⁻¹ at a power density of 317.5 W kg⁻¹ operated in the voltage range of 0–1.8 V in 1 mol L⁻¹ Na₂SO₄ aqueous electrolyte.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.