Synthesis and characterization of novel Ti3SiC2–cBN composites

In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti₃SiC₂–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~ 4.5 GPa, (b) hot pressing at ~ 35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti₃SiC₂–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~ 4.5 GPa from the mixture of Ti₃SiC₂ powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials.     

Aligned unidirectional PLA/bacterial cellulose nanocomposite fibre reinforced PDLLA composites

In an effort to enhance the properties of polylactide (PLA), we have developed melt-spinning techniques to produce both PLA/nanocellulose composite fibres, and a method akin to layered filament winding followed by compression moulding to produce self-reinforced PLA/nanocellulose composites. Poly(L-lactide) (PLLA) fibres were filled with 2 wt.% neat and modified bacterial cellulose (BC) in an effort to improve the tensile properties over neat PLA fibres. BC increased the viscosity of the polymer melt and reduced the draw-ratio of the fibres, resulting in increased fibre diameters. Nonetheless, strain induced chain orientation due to melt spinning led to PLLA fibres with enhanced tensile modulus (6 GPa) and strength (127 MPa), over monolithic PLLA, previously measured at 1.3 GPa and 61 MPa, respectively. The presence of BC also enhanced the nucleation and growth of crystals in PLA. We further produced PLA fibres with 7 wt.% cellulose nanocrystals (CNCs), which is higher than the percolation threshold (equivalent to 6 vol.%). These fibres were spun in multiple, alternating controlled layers onto spools, and subsequently compression moulded to produce unidirectional self-reinforced PLA composites consisting of 60 vol.% PLLA fibres reinforced with 7 wt.% CNC in a matrix of amorphous PDLLA, which itself contained 7 wt.% of CNC. We observed improvements in viscoelastic properties of up to 175% in terms of storage moduli in bending. Furthermore, strains to failure for PLLA fibre reinforced PDLLA were recorded at 17%.     

Experimental study on the stability of graphitic C3N4 under high pressure and high temperature

The stability and decomposition of graphitic C₃N₄ (g-C₃N₄) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C₃N₄ was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C₃N₄ has a positive slope (~ 0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic CN framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C₃N₄ was found in the studied P–T range.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Spark plasma sintering of Al2O3–Ni nanocomposites using Ni nanoparticles produced by rotary chemical vapour deposition

Al₂O₃–Ni nanocomposites were fabricated by spark plasma sintering (SPS) using Ni nanoparticle produced by rotary chemical vapour deposition. Carbon-free Ni nanoparticles were prepared by reacting NiCp₂ with O₂ to form NiO and then reducing to Ni by H₂ for 30 min at 823 K. The highest Ni content and grain size were 7.8 wt.% and 47.7 nm, respectively, using a NiCp₂ supply rate (R_s) of 1.67 × 10⁻⁶ kg s⁻¹. At a sintering temperature (T_SPS) of 1573 K, the hardness of Al₂O₃–3.8 wt.% Ni was 20.5 GPa, around 1 GPa higher than that of monolithic Al₂O₃ sintered at the same temperature. The tensile strength of Al₂O₃–4.6 wt.% Ni was 170 MPa, 60 MPa higher than that of Al₂O₃ sintered at 1573 K.     

Mechanical properties and lifetime predictions of dense SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5)

New oxygen transport membrane materials based on SrTi_1-xFe_xO_3-δ, synthesized through solid state reaction and processed via tape casting were characterized with respect to their mechanical behaviour via depth-sensitive indentation and ring-on-ring flexural testing. The elastic moduli obtained by indentation with 1 N load for SrTi_1-xFe_xO_3-δ (x = 0.25, 0.35, 0.5) specimens were 147 ± 10 GPa, 123 ± 6 GPa and 158 ± 10 GPa, respectively. Fracture stress was accessed by ring-on-ring testing performed at 100 N/min and the obtained results were 92 ± 9 MPa, 117 ± 15 MPa, and 100 ± 15 MPa for SrTi_0.75Fe_0.25O₃, SrTi_0.65Fe_0.35O₃, and SrTi_0.5Fe_0.5O₃ respectively. Ring-on-ring tests conducted at different loading rates gave access to subcritical crack growth sensitivity and aided the prediction of the materials’ lifetime through stress-time-probability diagrams, where SrTi_1-xFe_xO_3-δ (x = 0.25, 0.35, 0.5) may resist for 1 year with a failure probability of 0.1% at least 15 MPa, 22 MPa, and 12 MPa respectively.     

Characterization of kaolinite intercalation compounds with benzylalkylammonium chlorides using XRD,TGA/DTA and CHNS elemental analysis

Kaolinite of high structural order was intercalated with selected ammonium salts containing a benzyl group: benzyltrimethylammonium (B1), benzyltributylammonium (B2), benzalkonium (B3), benzyldimethyltetradecylammonium (B4) and benzyldimethylhexadecylammonium (B5) chlorides. As a precursor, a methoxy-kaolinite was used which had OCH₃ methoxyl groups attached to the octahedral sheet. Such change of the octahedral surface character enabled intercalation of the salts which was not possible using other precursors (e.g. kaolinite–dimethyl sulfoxide). The new intercalation compounds were characterized using XRD (X-ray diffraction), thermal analysis (TGA/DTA) and CHNS elemental analysis. The XRD revealed a significant shift of the kaolinite basal reflection to higher values range from ~ 14 Å (B1 salt) to ~ 38 Å (B5 salt) which confirmed the intercalation. The d values depended on the type of used salt as well as on its initial concentration. The estimated space occupied by each molecule enabled to calculate the maximal molar capacity of the kaolinite in relation to the salts. The results were compared with the chemical formulas of the materials calculated on the basis of CHNS measurements. The TGA/DTA analyses were helpful to confirm the successful intercalation of the selected salts as the thermal decomposition of the kaolinite derivatives took place at higher temperatures as compared to appropriate physical mixtures.     

Single and polycrystalline mullite fibres grown by laser floating zone technique

The laser floating zone technique was used to grow large 2Al₂O₃–SiO₂ mullite fibres (up to 1.6 mm in diameter and 40 mm in length). The fibres grown at 10 mm/h are single crystalline in nature, while those pulled at higher rates (40 and 100 mm/h) are polycrystalline with a cellular microstructure. The crystals are highly [0 0 1] textured with respect to the fibre axis, as determined by X-ray diffraction analysis. The Raman spectra taken at different orientations corroborate the strong anisotropy observed by X-ray and SEM on both single crystalline and textured polycrystalline samples.Four point bending tests and ultramicroindentation Vickers experiments were performed at room temperature in order to characterize the mechanical properties. The presence of lamellar inclusions in the single crystalline fibres decreases the flexural strength (431 MPa) and the fracture toughness (1.2 MPa.m^1/2) compared to the polycrystalline ones (631 MPa and 1.6 MPa.m^1/2). However, the absence of grain boundaries in the single crystals leads to higher ultramicrohardness (H_V = 15.6 GPa) and Young's modulus (E = 170 GPa) than those of the polycrystalline fibres (14.2 and 145 GPa), where a glassy intergranular phase exists.     

Visible-light-induced photocatalytic hydrogenation of 4-nitroaniline over In2S3 photocatalyst in water

Photocatalytic hydrogenation of 4-nitroaniline over the In₂S₃ photocatalyst was investigated in water under visible light irradiation (λ ≥ 420 nm). After 90 min of visible light irradiation, 100% of 4-nitroaniline could be reduced to p-phenylenediamine over the In₂S₃ photocatalyst in the presence of triethanolamine as a hole scavenger. Moreover, the photoreduction activity of the In₂S₃ photocatalyst could keep at ~ 100% in the 5th cycle of testing. On the basic of the results of electron spin resonance, photoinduced electrons of the In₂S₃ photocatalyst were identified as the active species for the photocatalytic hydrogenation of 4-nitroaniline.     

Mechanical characterization of a polysiloxane-derived SiOC glass

Silicon oxycarbide glass with the composition Si_1.0O_1.6C_0.8 was synthesized from a commercial polysiloxane by polymer pyrolysis. Dense SiOC samples were obtained by cross linking of the polysiloxane followed by warm pressing to form cylindrical samples and subsequent pyrolysis of the shaped polymer at 1100 °C in Ar. Hardness (H), Young's modulus (E) and Poisson's ratio (ν) of the as-prepared SiOC glass were evaluated from indentation studies and from acoustic microscopy. Indentation studies showed that E depends on the applied load and amounts to 90 GPa for low load and to 180 GPa for high load. Average values of 6.4 and 101 GPa were obtained for H and E, respectively, by the Vickers indentation method. Acoustic microscopy analysis yielded values of 96 GPa and 0.11 for E and ν, respectively. Compared to vitreous silica, the Young's modulus of the SiOC glass is about 1.3–1.5 times higher. To the knowledge of the present authors, the measured Poisson's ratio (ν = 0.11) is the lowest reported so far for glasses and polycrystalline ceramics.     

Preparation and UV-shielding property of Zr0.7Ce0.3O2–kaolinite nanocomposites

Nanocomposites of Zr_0.7Ce_0.3O₂–kaolinite were prepared by two different methods. The first method was by solid mixing of kaolinite and Zr_0.7Ce_0.3O₂ (ZrCeO), the latter being prepared by a citric acid complexion method, using an agate pestle and mortar as the mixing medium. The second method was by adding kaolinite during the synthesis of Zr_0.7Ce_0.3O₂ (ZrCeO) itself. Kaolinite was obtained by the elutriation of tin tailing, which was then exfoliated by planetary ball milling with 40 mass% of urea for 1–5 h. The resulting nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectrophotometry, Fourier transform infrared spectroscopy (FTIR), and specific surface area (SSA) determination using the BET (Brunauer–Emmett–Teller) method. The characterization results proved that kaolinite was successfully exfoliated by urea treatment after 5 h. It was also found that ZrCeO–exfoliated kaolinite nanocomposite, synthesized by the citric acid complexion method, without adding NH₃, showed the highest UV-shielding property of > 80% absorption ability for wavelength < 400 nm. This is better than pure ZrCeO, even at the lower content of ZrCeO in the nanocomposite (i.e. at 70 mass% content).     

Spark plasma sintering of Ti1−xAlxN nano-powders synthesized by high-energy ball milling

The present study focused on the fabrication of bulk materials from Ti_1−xAl_xN nano-powders using a spark plasma sintering (SPS) apparatus. Super-saturated Ti_1−xAl_xN solid solutions containing differing fractions of AlN (10, 20, 30 and 50 mol%) were synthesized by high-energy ball milling (HEBM) of pure nitrides. The complete dissolution of AlN in TiN was achieved after 100 h of milling. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-filtered transmission electron microscopy spectra imaging and energy dispersive X-ray spectroscopy. The crystalline size of the mechanically alloyed powders after 100 h of milling was about 12–14 nm. Ti_1−xAl_xN powders of various compositions were sintered by SPS under pressure of 63 MPa at 1673 K. Maximal hardness and bending strength values (610 MPa and 18.6 GPa, respectively) were obtained for composites containing 20 mol%AlN. Powder with 20% mole%AlN was consolidated under pressure of 500 MPa in the 1273–1423K temperature range by high pressure SPS (HPSPS). A fully dense nano-structured specimen, processed at 1423 K, displayed a Young modulus of 420 GPa, hardness of 20.5 GPa, bending strength of 670 MPa and fracture toughness of 7.1 MPa m^0.5.     

Preparation of dense SiHf(B)CN-based ceramic nanocomposites via rapid spark plasma sintering

Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 ^°C/min.) and high pressures (≥100 MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure.The hardness and the elastic modulus of dense SiHfCN were found to be 26.8 and 367 GPa, respectively. Whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa. The investigation of the oxidation of the prepared dense ceramic nanocomposites at high temperature revealed that the parabolic oxidation rates of SiHfCN were comparable to those of ultra-high temperature ceramics (UHTCs, e.g. HfC-20 vol% SiC); whereas the parabolic oxidation rates of SiHfBCN were several orders of magnitude lower than those. The results obtained within this study indicate the feasibility of SPS for rapid preparation of dense though nano-scaled Hf-containing ceramic nanocomposites that are promising candidates for high-temperature applications in harsh environments.     

Thermal properties of poly(lactic acid) fumed silica nanocomposites: Experiments and molecular dynamics simulations

Poly(lactic acid) (PLA) fumed silica nanocomposites were prepared by twin-screw extruder. Thermal properties were investigated by experiments and molecular dynamics simulations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used and 1.34 °C increase of the glass transition temperature (T_g) and 12 °C improvement of thermal stability were observed for PLA–silica nanocomposites as compared to neat PLA. Molecular dynamics simulations (NPT ensemble) were carried out using modified OPLS-AA force field, and T_g and root-mean-square radii of gyration (R_g) were calculated. A good agreement between the simulation results and experiments was obtained.     

Influence of substrate bias voltage on structure and properties of hard Si–B–C–N films prepared by reactive magnetron sputtering

We systematically investigated the effect of the rf induced negative substrate bias voltage, U_b, on characteristics of novel quaternary Si–B–C–N films. The films were deposited on Si(100) or glass substrates by reactive dc magnetron co-sputtering of silicon, boron and carbon from a single C–Si–B or B₄C–Si target in nitrogen–argon gas mixtures at substrate temperatures of 180–350 °C. Elemental compositions of the films, their surface bonding structure, and mechanical and electrical properties were primarily controlled by the U_b values, varied from a floating potential (being between − 30 and − 40 V) to U_b = − 700 V. The energy and flux of ions bombarding the target and the growing films were evaluated on the basis of the measured discharge characteristics. The films were found to be amorphous with thickness up to 5 μm and density around 2.4 g/cm³. They exhibited hardness up to 44 GPa, modified Young's modulus between 170 and 280 GPa, elastic recovery up to 82% and good adhesion to substrates at a low compressive stress (0.6–1.8 GPa). The results of stress measurements were compared with predictions of the model developed by Davis and a beneficial role of silicon in reducing the compressive stress in the films was proved. Electrical conductivity of the semiconductive Si–B–C–N films with a high (approximately 40 at.%) carbon content was controlled by the nitrogen–argon gas mixture composition and the U_b values.     

Structural and mechanical properties of NbN and Nb-Si-N films: Experiment and molecular dynamics simulations

The structural and mechanical properties of NbN and Nb-Si-N films have been investigated both experimentally and theoretically, in their as-deposited and annealed states. The films were deposited using magnetron sputtering at substrate bias (U_B) between 0 and −70 V. While NbN films were found to crystallize in the cubic δ-NbN structure, Nb-Si-N films with Si content of 11–13 at% consisted of a two-phases nanocomposite structure where δ-NbN nanocrystals were embedded in SiN_x amorphous matrix. Films deposited at U_B=0 V were highly (001)-textured. Application of substrate bias potential led to a depletion of light atoms, and caused a grain size refinement concomitantly with the increase of (111) preferred orientations in both films. The maximum hardness was 28 GPa and 32 GPa for NbN and Nb-Si-N films, respectively. NbN and Nb-Si-N films deposited at U_B=−70 V exhibited compressive stress of −3 and −4 GPa, respectively. After vacuum annealing, a decrease in the stress-free lattice parameter was observed for both films, and attributed to alteration of film composition. To obtain insights on interface properties and related mechanical and thermal stability of Nb-Si-N nanocomposite films, first principles molecular dynamics simulations of NbN/SiN_x heterostructures with different structures (cubic and hexagonal) and atomic configurations were carried out. All the hexagonal heterostructures were found to be dynamically stable and weakly dependent on temperature. Calculation of the tensile strain-stress curves showed that the values of ideal tensile strength for the δ-NbN(111)- and ε-NbN(001)-based heterostructures with coherent interfaces and Si₃N₄–like Si₂N₃ interfaces were the highest with values in the range 36–65 GPa, but lower than corresponding values of bulk NbN compound. This suggests that hardness enhancement is likely due to inhibition of dislocation glide at the grain boundary rather than interfacial strengthening due to Si-N chemical bonding.     

Influence of carbon sources on nitriding process,microstructures and mechanical properties of Si3N4 bonded SiC refractories

The incomplete nitriding and heterogeneity structure of large-size Si₃N₄-bonded SiC refractories limited its application in refractories industry. The objective of this work was to provide a way that adding carbon in matrix with the expectations of carbon could reacted with free-Si and transformed into SiC after nitridation. The effects of carbon sources on the bonded morphologies and nitridation process of Si₃N₄ bonded SiC refractories were investigated. Results indicated the strength and Young’s-modulus of specimen with carbon black increased to 39.4 MPa and 103.89 GPa from 29.8 MPa and 73.43 GPa, respectively. The residual Young’s-modulus also improved from 44.13 GPa to 62.9 GPa after 9-quenching cycles. For specimen with graphite, residual graphite after nitridation resulted in a definite strength decline, but residual strength after quenching was improved. Moreover, analysis results on N-elements indicated surprisingly improvement in the nitriding degree for specimen with carbon black. The microstructure evolution and mechanism associating with the enhanced nitriding process was discussed.     

Effect of TiO2 additions on densification and mechanical properties of new mulltifunction resistant porcelains using economic raw materials

The aim of this work is to determine the effect of TiO₂ on sintering and mechanical proprieties of new multifunction resistant (MFR) porcelain prepared from local abundant raw materials. Based on a preliminary work, the new selected composition was 30 wt% kaolins (20 wt% kaolin halloysite type + 10 wt% kaolin Tamazart), 45 wt% k-feldspar and 25 wt% quartz and containing different contents of TiO₂ (3, 5 and 8 wt%). The sintering temperatures of mixtures were between 1140 and 1260 °C. Subsequently, the obtained phases in the elaborated samples were investigated by X-ray diffraction and Fourier transform infrared spectroscopy analyses, Raman spectroscopy and SEM analysis. The optimum sintering conditions gave a higher bulk density (2.47 g.cm⁻³) and excellent mechanical properties: The three point flexural strength (3PFS), Vickers micro-hardness (VMH) and apparent porosity (P_A) of porcelains sintered at 1160 °C were 238 MPa, 12.3 GPa and 2%, respectively. This obtained 3PFS value is drastically higher than that achieved for conventional porcelains (ranged between 60 and 80 MPa). Moreover, these two best 3PFS (238 MPa) and VMH (12.3 GPa) values achieved for this new MFR porcelains were considerably higher when compared to those values (3PFS=218 MPa and VMH=6.5 GPa) obtained by others for porcelain −30% ZrO₂ composite, even though their mixtures were hot pressed in vacuum at 970 °C for 2 min. Besides, the maximum value achieved for the new MFR porcelains is nearby that of the flexural strength of porcelain containing 5 wt% TiO₂ and 30 wt % alumina (about 240 MPa). In other words, the presence of 30 wt % alumina in their product well confirm the benefic effect of the used raw materials (saving 30 wt % alumina) on porcelain strengthening.     

Magnesium-based cements for CO2 capture and utilisation

Magnesium from reject desalination brine is reacted at atmospheric pressure and 25–65 °C with CO₂ from industrial effluent combustion gas, precipitating preferably nesquehonite, MgCO₃.3H₂O. Nesquehonite can be thermally activated so that, when remixed with water, it is self-cementing. At ambient temperature, cure durations are typically ~ 1–3 days. The nesquehonite-based product contains ~ 32 wt% sequestered carbon dioxide and, at end of use, can be recycled without loss of CO₂. As an application example, cast nesquehonite products are suitable to form lightweight, incombustible building materials with densities in the range 700–900 kg/m³. Compressive strengths of the cast products are low, ca 2–4 MPa, thus application is envisaged to be broadly similar to that of gypsum-based products.     

One-step hydrothermal synthesis of Sb2S3/reduced graphene oxide nanocomposites for high-performance sodium ion batteries anode materials

Sb₂S₃/reduced graphene oxide (SSR) nanocomposites were successfully synthesized through a facile one-step hydrothermal process, as used as anode materials for sodium ion batteries (SIBs). The characterization and electrochemical performance of the as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption isotherms, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge tests, respectively. The results show that the introduction of reduced graphene oxide (RGO) can improve the electrochemical performances of SSR nanocomposites. SSR nanocomposites with 10 wt% RGO exhibits the highest reversible capacity of 581.2 mAh g⁻¹ at the current density of 50 mA g⁻¹ after 50 cycles, and excellent rate performance for SIBs. The improved electrochemical performance is attributed to the smaller Sb₂S₃ nanoparticles dispersed on RGO crumpled structure and synergetic effects between Sb₂S₃ and RGO matrix, which can increase specific surface area and improve electrical conductivity, reduce sodium ion diffusion distance, and effectively buffer volume changes during cycling process.