Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

Characterisation of the surface of oxidised carbon adsorbents

The surface reactivity and functional group content of a series of oxidised active carbons has been assessed by elemental analysis, electrophoretic mobility measurements and potentiometric titrations. Oxidation of carbons with hot air resulted in a greater proportion of relatively weak acidic surface functional groups (i.e., phenolic), whereas nitric acid modification produced a greater amount of carboxylic groups. Electrophoretic mobility measurements suggest that the carbon surface is negatively charged within the range of the pH values studied. pH titration results indicate that the surface acidity of active carbons is stronger than that of a commercial polymeric carboxylic acid ion exchange resin. Possible mechanisms of carbon surface oxidation are discussed.     

Resistivity, Hall coefficient, magnetoresistance, and microtexture of cellulose carbon films

Dense carbon films of about 20 μm in thickness were prepared from a commercially available cellulose film of 40 μm thick by heat treatment up to 900°C. Carbon films thus obtained were heat treated at temperatures between 1800 and 3000°C for 30–60 min. The heat-treated carbon films were investigated by the measurements of resistivity, Hall coefficient and magnetoresistance at liquid nitrogen temperature. Reflected and transmitted X-ray diffraction experiments and observations by a high-resolution scanning electron microscope (SEM) were also carried out. The results of the resistivity and Hall coefficient for high-temperature-treated specimens indicated that the carbonized film was more graphitizable than bulk glass-like carbons. The magnetoresistance and X-ray diffraction profiles for the high-temperature-treated specimens suggested that each of these specimens substantially consisted of the matrix of granular microtexture and the graphite layer skin. The skin was clearly observed by SEM for the specimens heat-treated at 2800 and 3000°C and their thickness was 100–300 nm.     

Variable-range hopping conduction and negative magnetoresistance of disordered carbons at low temperature

Temperature dependence of the electrical conductivity in the range from room temperature down to 1.31 K was studied for three kinds of cokes heat treated at 900°C, bamboo char heat treated at 900°C and glassy carbon heat treated at 1000°C. Localized states as evidenced by variable-range hopping conduction (conductivity proportional to exp T^?1/4) was observed for these carbons at temperatures below 4 K. Room temperature Hall coefficient was studied for these carbons. Magnetoresistance was measured for the samples in the temperature range between 1.31 and 4.2 K. Positive magnetoresistance related to variable-range hopping conduction was observed for a bamboo char to increase with increasing magnetic field and with decreasing temperature. The mechanism of conduction can be interpreted by use of a model of density of states suggested by Davis and Mott for amorphous semiconductor. For cokes and glassy carbon negative magnetoresistance superposes with the positive magnetoresistance. Dependence of conductivity and of magnetoresistance on heat-treatment was investigated in the temperature range from 1.31 to 4.2 K for cokes heat treated at 1100, 1300 and 1500°C. Conductivity was found to be constant against temperature for these samples. With increasing heat-treatment temperature the contribution of negative magnetoresistance was observed for samples heat treated at 1500°C.     

The effect of Nb or Ta substitution into the M1 phase of the MoV(Nb,Ta)TeO selective oxidation catalyst

We use aberration corrected high-angle annular dark field (HAADF) imaging to systematically study, atomic column by atomic column, the effects of substituting Nb or Ta into the M1 phase of the MoV(Nb,Ta)TeO propane (amm)oxidation catalyst. The HAADF results indicate that the x,y coordinates of the metal sites within the M1 framework are unaffected by the substitution of either Nb or Ta for Mo. The HAADF analysis of the Ta-substituted catalyst demonstrated that the Ta preferentially substitutes into the pentagonal bipyramidal site, and by analogy, we anticipate that Nb substitutes similarly. Compositional analysis of the entire framework suggests that Ta/Nb behaves as a director of V among the octahedra that link the pentagonal rings, and the variable V occupancy may be correlated with variations in catalytic activities and selectivities. Finally, HAADF imaging provided evidence of coexistence of Ta-rich and Ta-poor domains. Similar phase segregation behavior may be present in Nb-substituted specimens, but would be very difficult to detect.     

Effects of (Ti,Ta,Nb,W)(C,N) on the microstructure,mechanical properties and corrosion behaviors of WC-Co cemented carbides

Homogeneous solid-solution (Ti, Ta, Nb,W)(C,N) powders were synthesized through carbothermal reduction-nitridation method. The effects of (Ti, Ta, Nb,W)(C,N) powders on the microstructure, mechanical properties and corrosion resistance of WC-10Co cemented carbides were investigated using XRD, SEM, electrochemical test and mechanical properties tests. The results showed that cemented carbides with pre-alloyed powder addition had a similar microstructure appearance: weak core/rim structure consisting of solid-solution phase embedded in the WC-Co system. The black core and gray rim, both of which contained similar elements, were identified as (Ti, Ta, Nb,W)(C,N), but the latter contained higher amount of heavy elements.With the addition of (Ti, Ta, Nb,W)(C,N) powders, the density, transverse rupture strength and fracture toughness of samples decreased monotonously. However, the hardness rose sharply at first, reached a peak at 15 wt% solid-solution addition, then slightly decreased, and finally increased again. Results also revealed that increasing (Ti, Ta, Nb,W)(C,N) made the open circuit potential (OCP) in 1 M sulphuric acid solution more negative than that of WC-Co, and all specimens exhibited pseudo-passivation phenomenon in the test solution. In addition, increasing pre-alloyed powders led to decreasing corrosion current density, which implies that (Ti, Ta, Nb,W)(C,N) could remarkably improve the corrosion resistance of WC-Co cemented carbides.     

Characterisation of surface oxides on carbon and their influence on dynamic adsorption

The aim of this work is to gain a better fundamental understanding of the nature of surface oxide sites present on carbon surfaces, and their role in the adsorption process. A number of model carbon substrates with different degrees of surface oxidation and similar textural properties were prepared using a wide range of solution and gaseous phase oxidation techniques. Some of the carbons were characterised using established techniques including flow microcalorimetry, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The results showed that both carboxylic acid (-CO₂H) and ether/hydroxyl (C-O) surface oxygen complexes were introduced to all of these carbons as a result of the oxidation processes. The number, strength and thermal stability of the surface groups formed were dependent on the nature of the base material and the oxidation conditions employed. The dynamic adsorption performance of the carbons against hexane, under humid conditions, was found to be mainly determined by the quantity of acidic surface functional groups. However, the location, strength of interaction and availability of the surface oxygen complexes to the adsorbate molecules, are also thought to affect the breakthrough characteristics of the carbons used in this work.     

Synthesis of rare earth containing single-phase multicomponent metal carbides via liquid polymer precursor route

Novel high-entropy carbide ceramics (HEC) containing rare earth metals, namely (Ti, Zr, Hf, Ta, La, Y)C, (Ti, Zr, Hf, Ta, Nb, La, Y)C, and (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C were prepared with single-phase structure by polymer precursor method. Controlled co-hydrolysis and polycondensation of equiatomic metal-containing monomers were conducted successively, followed by blending allyl-functional novolac resin as carbon source, and the polymer precursors were obtained as clear viscous liquid solutions. The single-phase formation possibility was theoretically analyzed from the aspects of size-effect parameter δ of the designed compositions. All as-obtained ceramics possessed single face-centered-cubic structure of metal carbides and high-compositional uniformity from nanoscale to microscale. The (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C ceramic powder pyrolyzed at 1800°C exhibited low-oxygen impurity content of 1.2 wt%. Thus, multicomponent high-entropy carbide nanoceramics with over five metal elements containing even rare earth element were firstly synthesized and characterized.     

Kinetic study of the oxidation resistance of phosphorus-containing activated carbons

Chemical activation of different lignocellulosic materials with phosphoric acid produces activated carbons with higher oxidation resistance than that observed for catalyst free chars obtained at similar conditions from the same biomass, even though the activated carbons have a more developed porous structure. The main reason for such behavior is probably related to the presence of thermally stable phosphorus complexes that remain on the carbon surface after the activation process. XPS analyses point out the oxidation of the phosphorus reduced groups to C–O–PO₃/(CO)₂PO₂ prior to carbon gasification. The latter complexes seem to stabilize the active carbon sites. Moreover, the presence of these phosphorus inhibitors produces a change in the gasification mechanism of the activated carbons with respect to that for char. Char oxidation seems to proceed in the entire available particle surface, while activated carbon gasification is better explained by the shrinking unreacted core model. Such a difference in mechanism is attributed to the inhibition effect of the phosphorus complexes, which reduces the reactivity of carbon active sites, and could act as a physical barrier for oxygen diffusion in the micropores.     

Hot-corrosion of refractory high-entropy ceramic matrix composites synthesized by alloy melt-infiltration

Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O₂, (Nb, Ta)₂(Zr, Hf)₆O₁₇, (Zr, Hf)TiO₄, and Ti(Nb, Ta)₂O₇, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO_(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.     

Enhanced electro-oxidation resistance of carbon electrodes induced by phosphorus surface groups

The electro-oxidation of carbon materials enormously degrades their performance and limits their wider utilization in multiple electrochemical applications. In this work, the positive influence of phosphorus functionalities on the overall electrochemical stability of carbon materials has been demonstrated under different conditions. We show that the extent and selectivity of electroxidation in P-containing carbons are completely different to those observed in conventional carbons without P. The electro-oxidation of P-containing carbons involves the active participation of phosphorus surface groups, which are gradually transformed at high potentials from less-to more-oxidized species to slow down the introduction of oxygen groups on the carbon surface (oxidation) and the subsequent generation of (C*OOH)-like unstable promoters of electro-gasification. The highest-oxidized P groups (–C–O–P-like species) seem to distribute the gained oxygen to neighboring carbon sites, which finally suffer oxidation and/or gasification. So it is thought that P-groups could act as mediators of carbon oxidation although including various steps and intermediates compared to electroxidation in P-free materials.     

Role of active sites in the steam activation of high unburned carbon fly ashes

Previous studies on carbon gasification have not included high unburned carbon content fly ashes, and therefore it remains unclear why not all fly ash carbon samples are equally suitable for activation. The concentration of active sites is well known to influence carbon gasification reactions. Therefore, the objective of this work was to investigate the effect of the concentration of active sites on the behavior of fly ash carbon samples upon steam activation. Six fly ash carbons were selected to produce activated carbons using steam at 850 °C. The concentration of active sites was determined by non-dispersive infrared analysis (NDIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD analyses were also conducted to determine the crystallite size. It was observed that the concentration of active sites played a more significant effect on the surface areas of activated carbons in the carbon burn-off zone of >60%. Statistical analysis was used to relate the surface areas of activated carbon variances with carbon burn-off levels.     

In situ reaction synthesis,microstructure and thermomechanical properties of novel medium-entropy (Ti,V,Nb,Ta)2AlC ceramics

High- and medium-entropy (HE and ME) materials have attracted considerable interest in recent years due to the advantages of designable components and tunable structures with modifiable physicochemical performances. Herein, a dense equiatomic quaternary (Ti,V,Nb,Ta)₂AlC solid solution, or denoted as a medium-entropy MAX phase, was fabricated by in-situ reaction in hot-pressing sintering process using easily available elemental powders as raw materials. X-ray diffraction with its Rietveld refinement analysis, scanning electron microscope and high-resolution scanning transmission electron microscope equipped with energy-dispersed spectrometer analysis identify the phase composition and crystal structure of the novel ME-M₂AlC (Ti,V,Nb,Ta)₂AlC phase. Especially, the atomic-resolution HAADF images directly prove the mutual solution of four metallic elements of the ME (Ti,V,Nb,Ta)₂AlC phase. Both the flexural strength and Vickers hardness are enhanced compared with the reported individual components, and the strengthening and hardening results largely benefit from solid-solution effect with lattice distortion in structure. The fracture toughness has not been enhanced due to the lack of effective toughening mechanism. Furthermore, the electrical conductivity of ME (Ti,V,Nb,Ta)₂AlC ceramic is distinctly decreased as a result of the increasing carrier lattice scattering and point defect scattering, which is caused by severe lattice distortion. Similarly, the scattering effect of solid solutions is also responsible for the lower thermal conductivity. It is worth noting that size disorder parameter (δ_size) is a more effective criterion to evaluate the reduced thermal conductivity than the ideal mixing configurational entropy itself. As a result, applying the medium- and high-entropy design to the MAX phases with chemical and structural diversity can provide a large compositional space with corresponding large range of unexpected performances, which is highly anticipated.     

Effet hall et diamagnetisme de composes residuels pyrocarbone-bore-halogene

In order to clarify the electronic structure of halogen residue compounds, particularly the amount of charge transfer that occurs between the carbon π band and the intercalated species, bromine and iodine chloride were intercalated into pyrolytic carbons with various levels of boron doping, and the Hall effect and diamagnetic anisotropy of the resulting residue compounds were studied. The positive Hall coefficient is always independent of temperature and yields the concentration of holes in the π band, while the amount of intercalated halogen can be obtained from weight variation measurements. Table 1 shows the Hall coefficients and hole concentrations of the boronated pyrocarbons before intercalation, and Table 2 gives the results of measurements performed on residue compounds between 77 and 300 K. Increasing heat treatments reduce the amounts of halogen (Table 3). From these various data it can be stated that the halogen content of the residue compounds does not depend on the boron doping and the resulting initial position of the Fermi level (see Tables 2 and 3). The charge transfer from carbon to halogen is always found to be quite small: 0.02–0.07 electronic charge per Br₂ or ICl molecule (Fig. 1). The charge transfer also varies if the compounds undergo thermal cycles without loss of halogen. This was shown by in situ measurements of the Hall coefficient and diamagnetic anisotropy during such cycles (Figs. 2–6). Successive intercalations and deintercalations, as suggested by the Marchand-Rouillon model[5,6], are responsible for the hysteresis cycles observed[1–7] with many properties of these compounds.     

Oxidation inhibition effects of phosphorus and boron in different carbon fabrics

To improve oxidation inhibition, elemental boron and two phosphorus compounds were doped into an activated carbon cloth and a carbon felt. The hypothesis was that P can block active sites by virtue of the formation of C-P-O or C-O-P bonds at graphene edges while substitutional B can alter the chemical reactivity of the residual free active sites by reducing the electron density in the graphene layer. To increase the final dopant concentration, the carbon felt was activated in nitric acid. The crystallinity of activated carbon cloth was improved by heat treatment and substitutional B; that of carbon felt was also improved, but not necessarily due to substitutional B. In all cases the oxidation reactivity is suppressed by heat treatment and in the presence of dopants. The oxidation inhibition mechanism in P-doped samples appears to be active sites blockage because of a proportional increase of oxidation inhibition with increasing P loading. The results for B-doped samples are consistent with our previous studies in which B was found to exhibit both a catalytic and an inhibiting effect on carbon oxidation. Samples doped with both P and B showed the most effective oxidation inhibition and their oxidation behavior is described in detail.     

Complete oxidation of active carbon at low temperatures by composite catalysts

The reaction between oxygen and active carbon was studied at 500°C. Various composite catalysts having an iron-group metal as the main component were supported on active carbon. The synergistic effect of the oxidation activity was widely observed in the composite metal-metal oxide catalyst/carbon system. Several active carbons were studied in this catalyzed oxidation and the oxidation rate decreased with decreasing surface area of carbon materials. The rate-determining step of this reaction was considered to be the surface-diffusion rate of oxygen. Enhancement of oxygen transmission by active composite catalysts resulted in promotion of the reaction.     

One-step Sinter-HIP method for preparation of functionally graded cemented carbide with ultrafine grains

Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.     

A facile route for the synthesis of high-entropy transition carbides/borides at low temperatures

As the main candidates in the field of ultra-high temperature ceramics, high entropy carbides/borides (HECs/HEBs) have good oxidation resistance properties, high hardness, as well as excellent thermal and electrical conductivities, which are the focused points of research nowadays. In the current study, (Hf,Ta,Zr,Nb,Mo,Ti)C powders were successfully synthesized by a three-step process, including the mixing process of raw oxides and carbon black with spaying Fe(NO₃)₃ solution, carbothermal reduction and subsequent calcium posttreatment. For the preparation of (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders, during the calcium posttreatment process, equal stoichiometric ratio of B₄C was added for the purpose of boriding reaction. The relevant X-ray diffraction and SEM characterizations indicate the successful preparations of face-centered cubic HECs and hexagonal HEBs. However, slight Mo local segregation was found in the prepared (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders. The iron generated from Fe(NO₃)₃ promotes the solid solution process between monocarbides during the carbothermal reduction process via the dissolution-diffusion-precipitation mechanism. In the calcium posttreatment process, the liquid calcium ensures the boriding reaction take place at a low temperature. In addition, the residual carbon could be combined with calcium to generate CaC₂ which is easy to be removed by acid leaching, and meanwhile, the added Fe could also be finally eliminated to produce pure HEC/HEB powders. The current method does not require the long-time high energy ball milling of raw materials, but only simple and mild mixing is enough. Therefore, such a facile route has a great potential application prospect for industrially preparing high entropy phase powders in a large scale.     

Influence of activated carbon oxidation treatments on the selective removalof copper and lead

Air, acid, and electrochemical oxidation treatments have been employed to carefully control the amount and distribution of acidic surface groups in a series of activated carbons prepared from apricot stones. The surface reactivity and functional group distribution of oxidized active carbons have been assessed using numerous analytical and surface measurement techniques including nitrogen sorption, elemental analysis, potentiometric titrations, zeta potential measurements, and Boehm's titrations. Preliminary electrochemical oxidation results suggest that this treatment provides better control of the resultant surface acidity of the prepared carbons. Lead and copper sorption studies indicate that it is possible to alter the selectivity of the oxidized active carbons towards heavy metals by changing the quantity and/or relative proportion of the individual oxygenated surface functional groups.     

INFLUENCE OF ACTIVATED CARBON OXIDATION TREATMENTS ON THE SELECTIVE REMOVALOF COPPER AND LEAD

Air, acid, and electrochemical oxidation treatments have been employed to carefully control the amount and distribution of acidic surface groups in a series of activated carbons prepared from apricot stones. The surface reactivity and functional group distribution of oxidized active carbons have been assessed using numerous analytical and surface measurement techniques including nitrogen sorption, elemental analysis, potentiometric titrations, zeta potential measurements, and Boehm's titrations. Preliminary electrochemical oxidation results suggest that this treatment provides better control of the resultant surface acidity of the prepared carbons. Lead and copper sorption studies indicate that it is possible to alter the selectivity of the oxidized active carbons towards heavy metals by changing the quantity and/or relative proportion of the individual oxygenated surface functional groups.