Porous Carbide Research Articles

A universal method of preparing high yield and N riched porous carbon by carbonizing coal tar pitch in air for supercapacitor

A universal method for the preparation of high-yield and high N content porous carbide is reported. Nine type composite salts are assisted to carbonize coal tar pitch (CTP) in muffle furnace in air atmosphere. The mechanism of composite salt assisted carbonization of CTP is studied systematically. The composite salt is consist of heat labile ammonium salts component that can decompose producing gas at low temperature and stalbe inorganic salts component that can generate molten salt at high carbonization temperature. The two components can protect CTP in the whole carbonization process in air. These composite salts play the triple role of protection CTP from oxidation and combustion, activator and heteroatom source. The carbon yield is up to 50 wt%, and the scale-up experiment can also get high yield. The porous carbide prepared by NH4Cl/(NH4PO3)n has a high N content (10.39 at%) and high yield (56.0 wt%), and it exhibits an excellent specific capacitance (374.3 F g−1). In addition, the assembled ammonium ion supercapacitor with the prepared porous carbide and (NH4)3PO4 electrolyte shows high energy density (24.1 Wh kg−1). The work uncovers a universal and feasible way to prepare high carbon yield porous carbon materials with large scale industrial production in air.

Thermal and Structural Characterization of a Titanium Carbide/Carbon Composite for Nuclear Applications

In the framework of ISOL (isotope separation on-line) facilities, porous carbides are among the most employed target materials for the production of radioactive ion beams for research. As foreseen by the ISOL technique, a production target is impinged by an energetic particle beam, inducing nuclear reactions from such an interaction. The resulting radionuclides are subsequently released, thanks to the high target working temperature (1600-2000 °C); ionized; and extracted into a beam. Since the target microstructure and porosity play a fundamental role in the radionuclide release efficiency, custom-made target materials are often specifically produced, resulting in unknown thermal and structural properties. Considering that such targets might undergo intense thermal stresses during operation, a thermal and structural characterization is necessary to avoid target failure under irradiation. In the presented work, a custom-made porous titanium carbide that was specifically designed for application as an ISOL target was produced and characterized. The thermal characterization was focused on the evaluation of the material emissivity and thermal conductivity in the 600-1400 °C temperature range. For the estimation of a reference material tensile stress limit, the virtual thermoelastic parameter approach was adopted. In particular, for the aforementioned temperature range, an emissivity between 0.7 and 0.8 was measured, whereas a thermal conductivity between 8 and 10 W/mK was estimated.

Численное моделирование поведения карбидов при высокоэнергетическом воздействии

The results of research on modeling thermodynamic parameters of shock-wave loading of carbides with different stoichiometric ratios are presented. The carbides are considered as a mixture of carbon with the corresponding component. The calculations of pressure, compression and temperature values under shock-wave loading for solid and porous carbides in the range of pressure values above 3 GPa are performed. The model calculations are compared with the known experimental results on the shock-wave loading of carbides with different porosity values. The possibility of modeling the behavior according to the proposed method for carbides for which there are no experimental data at high dynamic loads is shown.

THERMODYNAMIC SIMULATIONS ON THE PRODUCTION METHODS OF POROUS CARBIDE DERIVED CARBON MATERIALS

THERMODYNAMIC SIMULATIONS ON THE PRODUCTION METHODS OF POROUS CARBIDE DERIVED CARBON MATERIALS

Numerical modeling of carbides behavior under high-energy liading

The results of research on modeling thermodynamic parameters ofshock-wave loading of carbides with different stoichiometric ratios arepresented. The carbides are considered as a mixture of carbon with thecorresponding component. The calculations of pressure, compression andtemperature values under shock-wave loading for solid and porous carbides inthe range of pressure values above 3 GPa are performed. The modelcalculations are compared with the known experimental results on theshock-wave loading of carbides with different porosity values. Thepossibility of modeling the behavior according to the proposed method forcarbides for which there are no experimental data at high dynamic loads isshown. Keywords: Equation of state, shock adiabat, thermodynamic equality,porous heterogeneous medium, carbides\

Modeling of shock-wave loading of carbides as mixtures of components

The results of numerical experiments on the modeling of shock wave loading of solid and porous carbides with various stoichiometric compositions are presented. The model is based on the assumption that all the components of the mixture, including gas, have similar pressure, velocity and temperature. The model allows describing the behavior of porous materials and mixes in a wide range of porosity and pressures with precision of experiment. The behavior of complex materials such as carbides is considered as a mixture. The model accurately describes the behavior of the carbides with equal shares of the components of WC, TiC, TaC, NbC and the behavior of boron carbide B4C. Comparison for data of calculation and experimental data was held for carbides with different porosity.

Effect of reaction sintering modes on the structure and properties of carbide ceramics

Experimental studies of the structure, phase composition, physical and mechanical properties of the reaction-sintered ceramics based on silicon carbide and boron obtained by reaction sintering have been performed. It has been shown that the properties of the reaction-sintered ceramics based on carbides are largely determined by the quality of impregnation of the porous carbide frame with silicon, which depends on the total and open porosity, shape and size of the pores of the compact, the composition of the charge from the carbide powder. High-temperature sintering, followed by impregnation of the carbide frame with silicon and its interaction with the carbon constituent of the frame, largely determines the properties of the material. The main task in the implementation of this process is to create conditions that ensure the full filling of pores in the initial compact during impregnation with silicon melt and, secondly, maximum activation of chemical interaction between the melt of silicon, carbon and other components that compose the charge. A complex of studies on the effect of compacting pressure and annealing temperature of the charge based on silicon carbide and boron powders with the addition of graphite on the pore structure of the compact and the quality of its impregnation with a silicon melt has been carried out in this work. It has been shown that the density, bending strength, hardness of ceramics based on silicon carbide and boron carbide obtained by reaction sintering are increased with a rise in compacting pressure of carbide frames. The optimum porosity of the carbide frame is 25–30 %; the pore size is 1.0–1.5 μm. It has been also demonstrated that ceramics based on boron carbide and boron carbide with 50 % silicon carbide impregnated with silicon at high-temperature sintering has higher strength and hardness values than those based on silicon carbide due to higher adhesion strength at the interface of boron carbide particles and binder, caused by the dissolution of boron carbide in the silicon melt and the formation of complex carbide particles on the surface.

Development of the composite material and coatings based on niobium carbide

We investigated the structure of composites based on the system NbC with a copper bond, obtained by impregnating the porous carbide skeletons with a metallic melt in a vacuum. In order to receive a porous skeleton, the powder of NbC, the average size of ~1 μm, was mixed on a 5 % solution of rubber in gasoline. After drying, the mixture was ground at a sieve into granules, which were pressed into briquettes of dimensions 55×30×10 mm. To ensure the intensification of the process, as well as wetting, impregnation was carried out at a temperature of 1,400 °C. The result was the obtained material with a fine-grained two-phase structure.The microstructure was investigated using a method of scanning electron microscopy (SEM), the chemical composition ‒ applying a method for energy dispersion analysis (EDS).The hardness was measured by Rockwell (scale C), the fracture toughness ‒ by the indirect Evans-Charles method.The composite's structure consists of rounded grains of NbC, which form a continuous skeleton, and the layers of copper bonding. The average size of grains and the intragrain layers of bonding is 1.8 μm and 1.1 µm, respectively.An analysis of the interaction zone between NbC and Cu via EDS method revealed the presence of a 0.5 μm thick zone of diffusion, resulting from the redistribution of Nb and Cu by limited solubility. The presence of the diffusion zone makes it possible to provide a solid interphase bonding and, accordingly, the high level of mechanical properties. The hardness and fracture toughness of the obtained material are 40 HRC and 24 MPa×м1/2, respectively.Given the phase composition and properties of the developed composite, it is recommended to apply it as an alternative for composites of the system WC‒Cu in the form of a monolithic material or coatings. A coating was applied using the method of electric-spark doping, using the manual installation MP-EL2. The coating's thickness is 30 μm, microhardness is ~500 MPa, and the friction coefficient against steel without lubrication is 0.04.The designed materials are recommended for use in friction pairs in the form of a monolithic material, or anti-friction coatings

Novel Approach to Synthesizing Carbide Materials from a Cellulosic Precursorposter Format

Carbides are an important class of materials given their high melting temperature, abrasion resistance, and compressive strength. Carbides are obtained through the reaction of carbon and other elements such as silicon, titanium and tungsten. In particular, porous carbides find application in high temperature filters, membranes, and catalysts thanks to their lower density and a higher surface area to volume ratio compared to classically manufactured solid carbide materials. The use of templates is a common way to derive porous carbides. For example, in industry when carbides are being formed in a specific shape or morphology, the use of polymers which have a more controllable structure are used as a precursor for the production of porous carbides. Here we postulate the use of cellulosic paper as a carbon precursor. If the paper is infiltrated with precursors for silicon, titanium or tungsten, then a carbide is derived upon heat treatment. Origami techniques can then be used to shape the infiltrated paper into different geometries, which remain throughout heat treatment. Hence, this technique yields complex 3D geometries with thin cross sections that can enable lightweight, structured carbides. The importance of the work presented here is furthering our understanding on how different processing variables affect the composition of the resultant material. The carbide materials from paper are synthesized by the carbothermal reduction of metallic ethyl esters which have been embedded into a pure cellulosic paper matrix. In this work we use a porous matrix of cellulosic fibers and infiltrate them with solutions of either Tetraethylorthosilicate (TEOS) or Tyzor to form SiC or TiC respectively. The manner of infiltration used was capillary action through the cellulose matrix after exposing one side of the paper to the metallic solution. With the paper fully saturated, we allowed the samples to dry before placing them in a tube furnace for pyrolization in an inert N2 environment. Around 900 OC, cellulosic fibers decompose to a carbon fiber matrix and the metal ethers reduce to metal oxides. Our work currently focuses on determining the impact of different processing variables on the composition of the final material. These variables include the synthesis temperature, heating rate, and ratio of carbon to silicon or titanium. We expect that the variability in the ratio of the carbon to metallic precursors will influence the concentrations of the resultant materials while the changes in synthesis temperature and heating rate will influence the growth of the carbides once the reaction is initiated. We are testing these parameters using the Taguchi method of design to increase the efficiency of investigation. We have already had success in the synthesis of metallic carbides using this procedure with a titanium precursor and are now attempting to expand on our work by controlling the carbide growth and testing with additional metallic precursors. Future work is needed to test the impact of using different synthesis environments (such as Argon and vacuum instead of Nitrogen) and the use of alternative metal ethyl ethers on the final quantity and morphology of the resultant carbides. Figure 1

3D Printing of Carbides Using Renewable Resources

Here we present preliminary results on fabrication of 3D complex shapes of porous carbide material by additive manufacturing. Porous carbides feature unique properties such as high melting point, high chemical stability, low thermal expansion coefficient, low density, high surface area and high specific strength [1]. The current state of the art for fabrication of carbide parts includes incomplete sintering and templating method. However, these methods use non-renewable carbon precursors, which is a matter of concern for the sustainability of the Earth. Furthermore, the shape of the carbide parts strongly depends on the use of mold and making of a 3D complex shape is challenging in these methods. Here we postulate the fabrication of 3D complex shapes of porous carbide material by 3D printing of a biopolymer composite followed by carbothermal reduction reaction. Recently we reported an environment friendly and sustainable approach for synthesis of tungsten carbide using a renewable biopolymer composite [2], which resembles a gel material. The renewable biopolymers eliminate the use of non-renewable carbon precursors and the Bingham plastic nature of the composite allows for layer-by-layer fabrication. The additive manufacturing will eliminate the dependence of mold and it can be possible to fabricate 3D intricate shapes, which are challenging in current methods. The potential application of such 3D printed carbide shapes includes lightweight structural material, catalysts supports, high temperature filters and insulations. The biopolymer composite we used here was composed of iota-carrageenan (IC) and chitin. The IC powder was mixed to chitin in a 1:4 weight ratio. As our initial target was to fabricate 3D printed shapes of tungsten carbide (WC), we mixed tungsten trioxide (WO3) nanoparticles with the biopolymer powders such that a molar ratio of WO3:C equal to 1:6 was achieved. Ultrapure water was added to the biopolymer-WO3 nanoparticle mixture and stirred with a homogenizer to obtain the gel-like composite. The gel composite was loaded into a syringe and extruded into 3D shapes using a paste extruder (Discov3ry) coupled with a 3D printer (LulzBot TAZ 5). Figure 1a shows such a 3D printed shape of the polymer composite. Once the printing was done, the 3D printed composite was dried overnight to obtain a 3D shape of xerogel (Figure 1b). The 3D shape of the xerogel was heat treated at 1300 °C for 3 hours in nitrogen atmosphere to obtain 3D shapes of WC (Figure 1c). The composition of the heat-treated sample was confirmed by the x-ray diffraction (XRD) spectroscopy, as only peaks of WC were observed in the XRD pattern. Ongoing work is 1) characterizing of the shrinkage of the 3D printed parts during drying and heat treatment; 2) implementing a design of experiments to obtain different dimensions of cellular architectures and understanding the effect of printing on those cellular architectures; 3) characterizing the mechanical properties of the 3D printed cellular architectures of WC.

Thermal cycling testing of TBCs on Cr2AlC MAX phase substrates

Thermal barrier coatings (TBCs) based on yttria-stabilized zirconia (YSZ) were deposited by Atmospheric Plasma Spray (APS) on highly dense and pure Cr2AlC substrates. The Cr2AlC/YSZ systems were tested under thermal cycling conditions at temperatures between 1100 and 1300 °C testing up to 500 h. The response of the system was excellent due to the strong adhesion between the substrate and the coating, and the formation of an outer and protective layer based on α-Al2O3. The oxide scale is formed due to the diffusion of Al atoms from the crystal structure of the Cr2AlC, followed by the reaction with oxygen in the air. The thickness of the oxide scale was 8.9, 17.6 and 39.7 μm at 1100, 1200 and 1300 °C, respectively, which is rather thick in comparison with the classical superalloy/TBC systems. Cr2AlC/YSZ systems survived without any damage under the severe cycling conditions at 1100 and 1200 °C due to the protective oxide scale layer and the sufficient thermal expansion match between the Cr2AlC, YSZ and α-Al2O3. At 1300 °C and after 268 h of cycling conditions, the system failed due to the formation of a porous carbide layer underneath of the oxide scale. The results are rather promising and confirm the potential of the MAX phases to operate under long term applications of high temperature and oxidizing environments.

Conversion of MAX phase single crystals in highly porous carbides by high temperature chlorination

We show that combining high temperature chlorination with the chemical sensitivity of the A-atom planes of single crystals of some nano-lamellar MAX phases allows one to synthesize highly porous and electrically conducting carbides. Focusing on the case of porous Cr3C2, we present the dependence of the layer morphology, structure and formation kinetics on processing parameters such as temperature and time. The determination of the Raman signature of Cr3C2 and Cr7C3 allows us to follow the creation and elimination of the various phases as a function of processing time. Electrical resistivity and magnetoresistance data versus temperature and magnetic field are also presented and analyzed.

Towards Additive Manufacturing of Tungsten Carbide Using Renewable Resources

We present preliminary experiments leading to novel additive manufacturing of carbides. To this end, we shape and heat treat a biopolymer-metal oxide composite featuring Bingham plastic behavior. Extrusion of such composite at room temperature allows for layer-by-layer fabrication; with the rheology of the composite dictating the scale of fabrication, i.e. thin mixtures for millimetric features, and thicker composites for bigger structures. Upon heat treatment to 900 °C, the biopolymer carbonizes. Further increase in temperature leads to a reaction between this carbon and the metal oxide nanoparticles to form metal carbides1. Such manufacturing process could lead to a more energy-efficient route to complex carbides shapes. Renewable biopolymers replace petroleum-based ones as carbon source; and the temperature needed for carbide formation can be drastically reduced due the colloidal proximity of the reactants. Additive manufacturing of the composite that is precursor to carbides could enable complex shapes, which are challenging or impossible when using powder pressing or machining of bulk carbides. We developed and characterized water-based composites featuring iota-carrageenan (IC) and chitin as mixed with tungsten trioxide (WO3) nanoparticles. The initial target is tungsten carbide (WC). Iota carrageenan powder was mixed with chitin in a weight ratio of 1:4. Thermogravimetric analysis of this mixture yields a 10.85% carbon yield at 900 °C. Thus, WO3 nanoparticles were mixed with this biopolymer powder mixture to achieve a C:WO3molar ratio of 1:8, in accordance to the chemical reaction and providing an excess of carbon. Ultra-pure water was added to the biopolymer-metal oxide nanoparticle mixture and stirred manually to achieve a Bingham plastic composite. The biopolymer composite was loaded in a syringe and manually extruded into 3D spirals shapes of around 10 mm in height. The heat treatment protocol features a ramp of at 5°C/min, nitrogen flow, a dwell of half-hour at 300°C, and a dwell of 3 hours at different temperatures ranging from 750 °C to 1450 °C. Carbonaceous 3D shapes result in all cases. Resultant shapes are of similar geometry to the precursor ones but smaller due to shrinkage. SEM-EDS and XRD analysis were performed on the carbonaceous samples towards characterizing their composition and geometry at different temperatures. The WO3 reduces to different forms of tungsten oxide and finally to tungsten at 900 °C. With further increase of temperature, the reduced tungsten reacts with carbon to form tungsten carbide. It is observed in Figure 1 that tungsten carbide starts to form above 900 °C. Further increase of temperature increases the crystallinity of the tungsten carbide as WC peaks get higher with temperature as seen from the XRD pattern. Figure 2a and 2b show the grain structure before the carbide formation at 750 °C and 900 °C respectively. It can be observed that the grain size increases with temperature. Figure 2c and 2d show that porous carbide was synthesized at 1025 °C and 1150 °C. The carbides synthesized in this process have grain size ranging from tens of nanometers to 2 µm. From elemental analysis using EDS (Table 1), it is observed that the relative amount of tungsten with respect to carbon increases proportional to temperature. This supports the theory of carbonization of the biopolymer as well as the reduction of WO3with the increasing temperature. In EDS analysis, calcium and potassium were also found to be present as the impurities which may come from the water used for composite formation. Ongoing work is 1) investigating the effect of time, heating rate and heating environment on carbide formation and composition; 2) further understanding of shrinkage during heat treatment; and 3) implementing automated fabrication using a fused deposition modeling approach.

(Keynote) 2D+1D and 2D+0D Hybrid Nanostructures and Their Applications in Electrochemical Energy Storage

Flexible and binder-free hybrid materials can be prepared by combining nanosheets of graphene, graphene oxide, MXenes (two-dimensional transition metal carbides) and other 2D materials with carbon onions, carbon black, porous carbide-derived carbon nanoparticles, and nanotubes. This helps to overcome the tendency of 2D sheets to restack, which minimizes their surface area accessible to the electrolyte and hinders their overall performance in electrochemical applications. Nanospacers such as carbon nanotubes (CNTs) are often introduced between reduced graphene oxide (rGO) sheets to enhance the electrolyte accessibility and electronic conductivity of the film. However, there are opportunities to use other 2D nanosheet-nanoparticle combinations that provide the desired set of properties due to synergistic effects of the nanoparticles. A brief overview of the carbon-carbon and carbon-carbide hybrids will be provided in this talk. We have demonstrated fabrication of flexible, sandwich-like Ti3C2 MXene/CNT composite paper electrodes through alternating filtration of MXene and CNT dispersions. Spraying and other manufacturing methods can be used as well. Films containing onion- like carbon (OLC) and reduced graphene oxide (rGO) were also fabricated and compared with those with CNTs. The sandwich-like electrodes exhibited significantly improved electrochemical performance compared to those of pure MXene and randomly mixed MXene/CNT papers. Examples of Li-ion battery and electrochemical capacitor electrodes will be provided. We also used highly porous carbide derived carbon (CDC) nanoparticles as spacers between graphene sheets and prepared rGO-CDC hybrid electrodes. The electrodes were made by thermal reduction of GO-CDC paper produced by vacuum-assisted filtration of aqueous solutions of GO-CDC composite containing 10-20 wt.% of CDC. Taking advantage of the high surface area and conductivity of rGO and the accessible pores of the CDC, the hybrid electrodes showed specific capacitances as high as 200-210 F/g at a scan rate of 100 mV/s. The addition of CDC between the rGO layers enhances the accessibility of electrodes to the electrolyte ions and good performance of the electrodes with the thickness to 40-50 μm was observed.

Freestanding Graphene/Carbide Derived Carbon Films As High-Performance Electrodes for Electrochemical Capacitors

Freestanding films of reduced graphene oxide (rGO) have attracted much attention as electrodes for electrochemical capacitor, especially for flexible device applications. However, graphene paper electrodes are usually limited in thicknesses to less than 10 μm, as increasing the electrode thickness hinders its performance due to restacking of rGO sheets and slow diffusion of ions [1]. To prevent this problem, carbon nanoparticels such as carbon nanotubes (CNTs) are frequently used as nanospacers between the rGO sheets to increase the electrolyte accessibility and electronic conductivity of the films [2]. However, CNTs have a much lower capacitance compared to rGO and their addition limits the overall capacitance of the freestanding electrodes. In this study, for the first time, we have used highly porous carbide derived carbon (CDC) nanoparticles as spacer between graphene sheets and fabricated thick rGO/CDC hybrid electrodes. The electrodes were made by thermal reduction of GO/CDC papers fabricated by vacuum-assisted filtration of aqueous solutions of GO and CDC containing 10, 20, and 30 wt. % of CDC. Utilizing the high surface area and conductivity of rGO and the accessible pores of the CDC [3], the hybrid electrodes showed specific capacitances as high as 200-210 F/g at a high scan rate of 100 mV/s in an aqueous electrolyte. The addition of CDC between the rGO layers increases the accessibility of active material to the electrolyte ions and we observed good performance of the electrodes with the thickness of 40-50 μm.

Mechanical properties of a bi-continuous Cu–Cr3C2 composite

A series of new Cu–Cr 3 C 2 composites with a three-dimensionally continuous interpenetrating micro-structure was fabricated by porous carbide preforms using a metal infiltration process. The micro-structure and mechanical properties of the composites were investigated. Increasing the carbonisation temperature of the preforms resulted in an increase in the volume fraction of the reinforcing carbide phase and an increase in the Vickers hardness. The average value of Young’s modulus increased from 162 GPa to 202 GPa as the carbonisation temperature of the preforms increased from 1000 °C to 1300 °C. The flexural strength of the composites ranged from 710 MPa to 820 MPa, and the fracture toughness ranged from 14.8 MPa·m 1/2 to 19.5 MPa·m 1/2 .

Influence of specific surface area and microporosity-mesoporosity of pristine and Pt-nanoclusters modified carbide derived carbon electrodes on the oxygen electroreduction

Oxygen electroreduction reaction (ORR) on pristine porous carbide derived carbons (CDCs) and on CDC supports modified with Pt-nanoclusters has been studied in 0.5M H2SO4 solution using cyclic voltammetry, rotating disk electrode and electrochemical impedance spectroscopy methods. The CDCs were prepared from Mo2C (noted as C(Mo2C)) at different fixed chlorination temperatures from 600°C to 1000°C. The CDCs have tuneable specific surface area, micro- and mesoporosity, good electrical conductivity and corrosion stability at positive electrode potentials. Pt-nanoclusters were deposited onto/into C(Mo2C) powders using sodium borohydride reduction method. The X-ray diffraction and high-resolution transmission electron microscopy were applied for the structural and chemical characterization, and the nitrogen sorption method was used for the porosity analysis of the electrode materials studied. The cathodic current densities depend strongly on the synthesis temperature of C(Mo2C), indicating that, in addition to the specific surface area and porosity, the crystallinity (density of defects in amorphous areas) has noticeable influence on the ORR rate. Impedance data demonstrated nearly capacitive behaviour in the low AC frequency region, explained by quick cathodic ORR followed by slow adsorption step of the intermediates and reaction products at/inside microporous-mesoporous C(Mo2C) and Pt-C(Mo2C) electrodes.

Porosity dependence of elastic modulus of porous Cr3C2 ceramics

Abstract Porous chromium carbide (Cr 3 C 2 ) ceramics exhibiting a uniform and accessible porous structure comprised of three-dimensional connected struts were synthesized using a reactive sintering method. The effect of sintering temperature on the resulting microstructure of the porous Cr 3 C 2 was studied. Young's modulus of the porous carbides was measured by dynamic mechanical analysis (DMA), and a semi-empirical model E = E 0 ( 1 − P / P c ) 1 / J was developed (based on the generalized mixture rule (GMR)), to describe the porosity dependence of Young's modulus for this type of carbide. The constants P c =0.8 and J =0.355 were determined by fitting the model to the measured values. The relationship between the micro-hardness of the material and the microstructure of the pores was also investigated.

Synthesis and hardness evaluation of porous M(Cr, Co)7C3–Co composites

Porous carbide composites were synthesized by reactive sintering of chromium oxide mixed with various amounts of cobalt ranging from 5wt% to 50wt% in carbonaceous environments. It was found that the porous structure of the obtained samples changed with the cobalt content. Higher cobalt content increased the porosity but reduced the pore size. It was determined that the porous samples were mainly composed of M(Cr, Co)7C3 phase when the cobalt content was below 30wt%. When more cobalt was added, cobalt enriched phases were formed. The solubility of cobalt in M(Cr, Co)7C3 phase was also examined and compared with the data published previously. The hardness of the porous composites was evaluated and rationalized based on the combined effect of pore structures, solubility of cobalt and the composite phases present in the porous structure. The M(Cr, Co)7C3 phase can be strengthened by increasing cobalt dissolution. A solubility of cobalt in M(Cr, Co)7C3 phase near its theoretic limit can be achieved by the current reactive sintering at the temperature and time much less than that reported by any other methods.

Degradation of zirconate materials in ultra-high temperature reverse-flow pyrolysis reactors

A reverse-flow reactor enables the pyrolysis of hydrocarbons at temperatures up to 2000 °C to high value petrochemical products. Materials in the reactor hot zone, exposed to such extreme temperatures, are also subject to rapid oxidative cycling over a period of several seconds between a regeneration (heat addition) step that is mildly oxidizing and a pyrolysis (cracking) step that is strongly carburizing. This paper addresses the performance of a class of zirconate ceramics such as BaZrO3, SrZrO3, and La2Zr2O7 that have been tested in a laboratory scale reverse-flow reactor and compares the results with thermodynamic predictions. It is observed that zirconate ceramics degrade by decomposition to zirconia and the second oxide, vaporization of the second oxide, and ceramic dusting corrosion of zirconia that involves carbide-oxide inter-conversion and carbon precipitation. The pyrolysis reaction promotes carbon deposition and carburization of zirconia leading to the formation of a non-protective porous carbide layer. The regeneration reaction converts the carbide layer partially back to oxide while also oxidizing away some of the deposited carbon. Repeated cycles of pyrolysis and regeneration steps eventually result in the breakup of the bulk zirconate ceramic structure into powder or dust.