Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that ... more Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperatures (>2000°C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultrahigh temperature ceramic TiCx. Readily available equipment including a polymer powder bed fusion AM machine and a traditional tube furnace were used to produce UHTC cubes and lattice structures with sub-millimeter resolution. This processing scheme incorporated, (1) selective laser sintering of a Ti precursor mixed with a phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid conversion of the green body in CH4 to form TiCx structures. Reactive post-processing in CH4 resulted in up to 98.2 wt% TiC0.90 product yield and a reduction in net-shrinkage during consolidation due to the volume expansion associated with the conversion of Ti to TiC. Results indicated that reaction bonding associated with the Gibbs free energy release associated with TiC formation produced interparticle adhesion at low furnace processing temperatures. The ability to bond highly refractory materials through this type of process resulted in structures that were crack-free and resisted fracture during thermal shock testing. Broadly, the additive manufacturing approach presented could be useful for the production of many UHTC carbides that might otherwise be incompatible with prevailing AM techniques that do not include reaction synthesis.
Graphite crystals grown from hypereutectic solutions of carbon in iron were examined microscopica... more Graphite crystals grown from hypereutectic solutions of carbon in iron were examined microscopically with a view to establishing the mechanisms of growth. It is proposed that nucleation occurred on a second phase which had been retained in some crystals extracted from the molten solution. The perfection of the crystals, which is compared with that of naturally occurring and pyrolytic graphites, is a function of the extent of decomposition of this second phase and the impurity content of the melt.
Incorporation of carbides into the carbon matrix using organometalllc precursors reduces the oxid... more Incorporation of carbides into the carbon matrix using organometalllc precursors reduces the oxidation rate of carbon at high temperature through a self-heallng mechanism. Boron-and phosphorus-containlng additives were also effective especially at lower temperatures (<I000°C). However, total inhibition of oxidation by a low-to medium-level matrix modification is difficult to achieve. It is concluded that the best oxidation protection system should incorporate both the matrix modification and one or more coatings on the carbon surface of some inorganic refractory materials.
Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that re... more Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp>3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods. Target UHTC Melting Point (K) Young's Modulus (GPA) Thermal Conductivity (W m-1 K-1) Applications HfC 4228 420 25.1 Rocket combustion chambers, arc-jet thrusters, thermal protection systems, rocket nozzles, solar energy applications, oxidation coating for C/C composites and superalloys [7], [9] ZrC 3718 350 24.6 Nuclear barrier coatings, field emitters, integrated circuits [15] TiC 3365 450 28.9 Heat shields, cutting tools, wear-resistant components, [7] HfN 3628 380 11 Rocket nozzles, solar energy applications, plasmonic materials, wear-resistant coatings [3] [10] ZrN 3298 460 11 Matrix candidate for nuclear fuel [8], solar energy applications, plasmonic materials [10], dental and surgical implants [11], fuel cell membrane [9] TiN 3348 420 29 Diffusion barriers, crucibles for radioactive metals, wearresistance coatings [3]
A simple model of the deposition of pyrolytic carbon is proposed to explain the apparent inconsis... more A simple model of the deposition of pyrolytic carbon is proposed to explain the apparent inconsistencies between properties and structure. This model considers the importance of the nucleation rate, diffusion, and surface reactions and is based on structure determinations by density and stress measurements, optical and electron microscopy, and X-ray diffraction.
Two proposed expressions, I~(¢)= cos" ~b and/2(¢) = (1 + b 2 sin 2 ¢)-1, for the orientation func... more Two proposed expressions, I~(¢)= cos" ~b and/2(¢) = (1 + b 2 sin 2 ¢)-1, for the orientation function, I(¢), of pyrolytic graphites have been shown analytically to give quite different values for the Bacon anisotropy factor (BAF) for oriented materials. BAF values derived from the angle of half-maximum intensity using 11(¢) are within 20% of numerically calculated values for the BAF range 10-90, whereas values obtained using 12(¢) are smaller by as much as a factor of twenty. The effect of sample preparation on such measurements has been found to be negligible. Diamagnetic-susceptibility measurements on the graphites investigated validated the calculated BAF values in that the derived single-crystal susceptibilities were in reasonable agreement with known values. BAF values calculated via 12(¢) produced unacceptable paramagnetic values parallel to the basal plane. It is suggested that /1(¢) be used for rapid BAF determinations; although numerical calculation is preferred, the difficulty of measuring I(¢) at large ¢, for highly oriented materials, may make the full numerical procedure impracticable and this approximate procedure the more desirable.
Purpose: A third of the global disease burden requires surgical therapy, yet in disaster-affected... more Purpose: A third of the global disease burden requires surgical therapy, yet in disaster-affected areas and lower-middle income countries, 5 billion people have little or no access to safe surgical care. We develop, evaluate, and aim to deploy a new technology to help increase access to safe surgery. Methods: We conducted a needs assessment, then used iterative prototyping to incorporate stakeholder feedback and testing of all components. Proof-of-concept testing of the prototype entailed setup over a mannequin and using a particle counter to evaluate ability to provide a contaminant-free sterile field. Results: Our prototype shrinks the scope of the sterility challenge from the room to the critical space immediately over the incision. Users seal the modular system of sterile clear containers over the patient and operate via ports. An integrated airflow system controls enclosure conditions. Everything folds for rapid deployment. Testing demonstrates superior environmental control compared to standard operating rooms, including setup time, time to surgical site sterility, resistance to active contamination, and air changes per hour. Conclusions: Our results indicate that it is possible to provide state-of-the-art levels of sterility during surgeries even in austere settings, by using a low-cost, ultraportable, modular system codeveloped with key stakeholders.
The thermal transport properties were studied for carbons produced by the carbonization of microc... more The thermal transport properties were studied for carbons produced by the carbonization of microcrystalline cellulose. Thermal diffusivity, specific heat, and thermal conductivity were measured via flash method for cellulose derived carbons prepared at various heat treatment temperatures ranging from 250°C to 1000°C. The thermal diffusivity as a function of increasing heat treatment temperature was observed to have four distinct linear regions, which could be related directly to the microstructures of the materials generated by the specific heat treatment temperature. Specific heat values indicated the coexistence of polar and non-polar phases in both partially carbonized materials obtained at lower heat treatment temperatures and fully carbonized materials formed at higher heat treatment temperatures. For partially carbonized materials, the polar groups consisting of residual hydroxyl and carboxyl were still present. For fully carbonized materials, the polar phases have largely been volatilized and conductive nano-carbon clusters were nucleated and observed to grow in an amorphous carbon bed until percolation effects were observed. Such structural characteristics are well supported by FT-IR characterizations. Lastly, a linear relationship between testing temperature and thermal conductivity indicates boundary scattering between highly conductive carbon clusters as the main mechanism for heat conduction.
Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that ... more Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperatures (>2000°C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultrahigh temperature ceramic TiCx. Readily available equipment including a polymer powder bed fusion AM machine and a traditional tube furnace were used to produce UHTC cubes and lattice structures with sub-millimeter resolution. This processing scheme incorporated, (1) selective laser sintering of a Ti precursor mixed with a phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid conversion of the green body in CH4 to form TiCx structures. Reactive post-processing in CH4 resulted in up to 98.2 wt% TiC0.90 product yield and a reduction in net-shrinkage during consolidation due to the volume expansion associated with the conversion of Ti to TiC. Results indicated that reaction bonding associated with the Gibbs free energy release associated with TiC formation produced interparticle adhesion at low furnace processing temperatures. The ability to bond highly refractory materials through this type of process resulted in structures that were crack-free and resisted fracture during thermal shock testing. Broadly, the additive manufacturing approach presented could be useful for the production of many UHTC carbides that might otherwise be incompatible with prevailing AM techniques that do not include reaction synthesis.
The mechanical properties of pitch-based carbon fibres have been measured at temperatures up to 1... more The mechanical properties of pitch-based carbon fibres have been measured at temperatures up to 1300°C in both nitrogen and air atmospheres, the experimental difficulties associated with the high temperature testing of fibres having been overcome. The mechanical properties of these fibres were found to improve up to 1300°C. It is suggested that three defect populations control the fibre properties over the temperature range studied.
Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that re... more Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp>3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods. Target UHTC Melting Point (K) Young's Modulus (GPA) Thermal Conductivity (W m-1 K-1) Applications HfC 4228 420 25.1 Rocket combustion chambers, arc-jet thrusters, thermal protection systems, rocket nozzles, solar energy applications, oxidation coating for C/C composites and superalloys [7], [9] ZrC 3718 350 24.6 Nuclear barrier coatings, field emitters, integrated circuits [15] TiC 3365 450 28.9 Heat shields, cutting tools, wear-resistant components, [7] HfN 3628 380 11 Rocket nozzles, solar energy applications, plasmonic materials, wear-resistant coatings [3] [10] ZrN 3298 460 11 Matrix candidate for nuclear fuel [8], solar energy applications, plasmonic materials [10], dental and surgical implants [11], fuel cell membrane [9] TiN 3348 420 29 Diffusion barriers, crucibles for radioactive metals, wearresistance coatings [3]
Procede de preparation de materiaux composites consistant a precipiter in situ des particules d&#... more Procede de preparation de materiaux composites consistant a precipiter in situ des particules d'une seconde phase dans une matrice metallique, et produits ainsi obtenus. Le procede implique l'utilisation initiale de composes a titre de sources de reactifs de formation de la seconde phase. Les composites obtenus peuvent comprendre des dispersions d'un ou de plusieurs materiaux de seconde phase. A titre d'exemples, les composes initiaux utilises comme precurseurs peuvent etre choisis parmi le groupe suivant: nitrure de bore, carbure de bore, oxyde de bore, nitrure d'aluminium, carbure d'aluminium, borure d'aluminium, oxyde de fer et oxyde de cuivre.
Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that ... more Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperatures (>2000°C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultrahigh temperature ceramic TiCx. Readily available equipment including a polymer powder bed fusion AM machine and a traditional tube furnace were used to produce UHTC cubes and lattice structures with sub-millimeter resolution. This processing scheme incorporated, (1) selective laser sintering of a Ti precursor mixed with a phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid conversion of the green body in CH4 to form TiCx structures. Reactive post-processing in CH4 resulted in up to 98.2 wt% TiC0.90 product yield and a reduction in net-shrinkage during consolidation due to the volume expansion associated with the conversion of Ti to TiC. Results indicated that reaction bonding associated with the Gibbs free energy release associated with TiC formation produced interparticle adhesion at low furnace processing temperatures. The ability to bond highly refractory materials through this type of process resulted in structures that were crack-free and resisted fracture during thermal shock testing. Broadly, the additive manufacturing approach presented could be useful for the production of many UHTC carbides that might otherwise be incompatible with prevailing AM techniques that do not include reaction synthesis.
Graphite crystals grown from hypereutectic solutions of carbon in iron were examined microscopica... more Graphite crystals grown from hypereutectic solutions of carbon in iron were examined microscopically with a view to establishing the mechanisms of growth. It is proposed that nucleation occurred on a second phase which had been retained in some crystals extracted from the molten solution. The perfection of the crystals, which is compared with that of naturally occurring and pyrolytic graphites, is a function of the extent of decomposition of this second phase and the impurity content of the melt.
Incorporation of carbides into the carbon matrix using organometalllc precursors reduces the oxid... more Incorporation of carbides into the carbon matrix using organometalllc precursors reduces the oxidation rate of carbon at high temperature through a self-heallng mechanism. Boron-and phosphorus-containlng additives were also effective especially at lower temperatures (<I000°C). However, total inhibition of oxidation by a low-to medium-level matrix modification is difficult to achieve. It is concluded that the best oxidation protection system should incorporate both the matrix modification and one or more coatings on the carbon surface of some inorganic refractory materials.
Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that re... more Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp>3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods. Target UHTC Melting Point (K) Young's Modulus (GPA) Thermal Conductivity (W m-1 K-1) Applications HfC 4228 420 25.1 Rocket combustion chambers, arc-jet thrusters, thermal protection systems, rocket nozzles, solar energy applications, oxidation coating for C/C composites and superalloys [7], [9] ZrC 3718 350 24.6 Nuclear barrier coatings, field emitters, integrated circuits [15] TiC 3365 450 28.9 Heat shields, cutting tools, wear-resistant components, [7] HfN 3628 380 11 Rocket nozzles, solar energy applications, plasmonic materials, wear-resistant coatings [3] [10] ZrN 3298 460 11 Matrix candidate for nuclear fuel [8], solar energy applications, plasmonic materials [10], dental and surgical implants [11], fuel cell membrane [9] TiN 3348 420 29 Diffusion barriers, crucibles for radioactive metals, wearresistance coatings [3]
A simple model of the deposition of pyrolytic carbon is proposed to explain the apparent inconsis... more A simple model of the deposition of pyrolytic carbon is proposed to explain the apparent inconsistencies between properties and structure. This model considers the importance of the nucleation rate, diffusion, and surface reactions and is based on structure determinations by density and stress measurements, optical and electron microscopy, and X-ray diffraction.
Two proposed expressions, I~(¢)= cos" ~b and/2(¢) = (1 + b 2 sin 2 ¢)-1, for the orientation func... more Two proposed expressions, I~(¢)= cos" ~b and/2(¢) = (1 + b 2 sin 2 ¢)-1, for the orientation function, I(¢), of pyrolytic graphites have been shown analytically to give quite different values for the Bacon anisotropy factor (BAF) for oriented materials. BAF values derived from the angle of half-maximum intensity using 11(¢) are within 20% of numerically calculated values for the BAF range 10-90, whereas values obtained using 12(¢) are smaller by as much as a factor of twenty. The effect of sample preparation on such measurements has been found to be negligible. Diamagnetic-susceptibility measurements on the graphites investigated validated the calculated BAF values in that the derived single-crystal susceptibilities were in reasonable agreement with known values. BAF values calculated via 12(¢) produced unacceptable paramagnetic values parallel to the basal plane. It is suggested that /1(¢) be used for rapid BAF determinations; although numerical calculation is preferred, the difficulty of measuring I(¢) at large ¢, for highly oriented materials, may make the full numerical procedure impracticable and this approximate procedure the more desirable.
Purpose: A third of the global disease burden requires surgical therapy, yet in disaster-affected... more Purpose: A third of the global disease burden requires surgical therapy, yet in disaster-affected areas and lower-middle income countries, 5 billion people have little or no access to safe surgical care. We develop, evaluate, and aim to deploy a new technology to help increase access to safe surgery. Methods: We conducted a needs assessment, then used iterative prototyping to incorporate stakeholder feedback and testing of all components. Proof-of-concept testing of the prototype entailed setup over a mannequin and using a particle counter to evaluate ability to provide a contaminant-free sterile field. Results: Our prototype shrinks the scope of the sterility challenge from the room to the critical space immediately over the incision. Users seal the modular system of sterile clear containers over the patient and operate via ports. An integrated airflow system controls enclosure conditions. Everything folds for rapid deployment. Testing demonstrates superior environmental control compared to standard operating rooms, including setup time, time to surgical site sterility, resistance to active contamination, and air changes per hour. Conclusions: Our results indicate that it is possible to provide state-of-the-art levels of sterility during surgeries even in austere settings, by using a low-cost, ultraportable, modular system codeveloped with key stakeholders.
The thermal transport properties were studied for carbons produced by the carbonization of microc... more The thermal transport properties were studied for carbons produced by the carbonization of microcrystalline cellulose. Thermal diffusivity, specific heat, and thermal conductivity were measured via flash method for cellulose derived carbons prepared at various heat treatment temperatures ranging from 250°C to 1000°C. The thermal diffusivity as a function of increasing heat treatment temperature was observed to have four distinct linear regions, which could be related directly to the microstructures of the materials generated by the specific heat treatment temperature. Specific heat values indicated the coexistence of polar and non-polar phases in both partially carbonized materials obtained at lower heat treatment temperatures and fully carbonized materials formed at higher heat treatment temperatures. For partially carbonized materials, the polar groups consisting of residual hydroxyl and carboxyl were still present. For fully carbonized materials, the polar phases have largely been volatilized and conductive nano-carbon clusters were nucleated and observed to grow in an amorphous carbon bed until percolation effects were observed. Such structural characteristics are well supported by FT-IR characterizations. Lastly, a linear relationship between testing temperature and thermal conductivity indicates boundary scattering between highly conductive carbon clusters as the main mechanism for heat conduction.
Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that ... more Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperatures (>2000°C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultrahigh temperature ceramic TiCx. Readily available equipment including a polymer powder bed fusion AM machine and a traditional tube furnace were used to produce UHTC cubes and lattice structures with sub-millimeter resolution. This processing scheme incorporated, (1) selective laser sintering of a Ti precursor mixed with a phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid conversion of the green body in CH4 to form TiCx structures. Reactive post-processing in CH4 resulted in up to 98.2 wt% TiC0.90 product yield and a reduction in net-shrinkage during consolidation due to the volume expansion associated with the conversion of Ti to TiC. Results indicated that reaction bonding associated with the Gibbs free energy release associated with TiC formation produced interparticle adhesion at low furnace processing temperatures. The ability to bond highly refractory materials through this type of process resulted in structures that were crack-free and resisted fracture during thermal shock testing. Broadly, the additive manufacturing approach presented could be useful for the production of many UHTC carbides that might otherwise be incompatible with prevailing AM techniques that do not include reaction synthesis.
The mechanical properties of pitch-based carbon fibres have been measured at temperatures up to 1... more The mechanical properties of pitch-based carbon fibres have been measured at temperatures up to 1300°C in both nitrogen and air atmospheres, the experimental difficulties associated with the high temperature testing of fibres having been overcome. The mechanical properties of these fibres were found to improve up to 1300°C. It is suggested that three defect populations control the fibre properties over the temperature range studied.
Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that re... more Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp>3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods. Target UHTC Melting Point (K) Young's Modulus (GPA) Thermal Conductivity (W m-1 K-1) Applications HfC 4228 420 25.1 Rocket combustion chambers, arc-jet thrusters, thermal protection systems, rocket nozzles, solar energy applications, oxidation coating for C/C composites and superalloys [7], [9] ZrC 3718 350 24.6 Nuclear barrier coatings, field emitters, integrated circuits [15] TiC 3365 450 28.9 Heat shields, cutting tools, wear-resistant components, [7] HfN 3628 380 11 Rocket nozzles, solar energy applications, plasmonic materials, wear-resistant coatings [3] [10] ZrN 3298 460 11 Matrix candidate for nuclear fuel [8], solar energy applications, plasmonic materials [10], dental and surgical implants [11], fuel cell membrane [9] TiN 3348 420 29 Diffusion barriers, crucibles for radioactive metals, wearresistance coatings [3]
Procede de preparation de materiaux composites consistant a precipiter in situ des particules d&#... more Procede de preparation de materiaux composites consistant a precipiter in situ des particules d'une seconde phase dans une matrice metallique, et produits ainsi obtenus. Le procede implique l'utilisation initiale de composes a titre de sources de reactifs de formation de la seconde phase. Les composites obtenus peuvent comprendre des dispersions d'un ou de plusieurs materiaux de seconde phase. A titre d'exemples, les composes initiaux utilises comme precurseurs peuvent etre choisis parmi le groupe suivant: nitrure de bore, carbure de bore, oxyde de bore, nitrure d'aluminium, carbure d'aluminium, borure d'aluminium, oxyde de fer et oxyde de cuivre.
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