Ultra-high temperature carbide foams and methods of fabricating the same

Abstract

Ultra-high temperature carbide (UHTC) foams and methods of fabricating and using the same are provided. The UHTC foams are produced in a three-step process, including UHTC slurry preparation, freeze-drying, and spark plasma sintering (SPS). The fabrication methods allow for the production of any kind of single- or multi-component UHTC foam, while also providing flexibility in the shape and size of the UHTC foams to produce near-net-shape components.

Claims

1. A method of fabricating an ultra-high temperature carbide (UHTC) foam, the method comprising: preparing a UHTC slurry; freeze-drying the UHTC slurry to provide a green body; and performing a spark plasma sintering (SPS) process on the green body to provide the UHTC foam, the performing of the SPS process comprising disposing the green body such that punches of an SPS setup used for the SPS process do not touch the green body during the SPS process, the SPS process thereby being a pressure-less SPS process, the preparing of the UHTC slurry comprising: mixing UHTCs in a solvent to form a first slurry, a loading of UHTCs in the first slurry being at least 5 wt %; adding a dispersant to the first slurry to obtain a second slurry; and stirring the second slurry to obtain the UHTC slurry.

2. The method according to claim 1, the UHTC foam being a tantalum carbide (TaC) foam.

3. The method according to claim 1, the UHTC foam being a hafnium carbide (HfC) foam.

4. The method according to claim 1, the UHTC foam comprising a solid solution of tantalum carbide (TaC) and hafnium carbide (HfC).

5. The method according to claim 1, the solvent being water.

6. The method according to claim 1, the dispersant being polyacrylic acid (PAA).

7. The method according to claim 1, the stirring of the second slurry comprising stirring the second slurry at a first temperature for a first amount of time to maintain a pH of the second slurry constant, the first temperature being room temperature and the first amount of time being about 5 hours.

8. The method according to claim 1, the freeze-drying of the UHTC slurry comprising: pouring the UHTC slurry into a container; and freezing and then subliming the UHTC slurry in the container to obtain the green body.

9. The method according to claim 8, the container being a graphite crucible.

10. The method according to claim 1, the freeze-drying process being performed for about 24 hours.

11. The method according to claim 1, the performing of the SPS process comprising: providing the green body in a container; disposing the container with the green body in a die; and performing the SPS process on the green body in the container in the die at a second temperature for a second amount of time.

12. The method according to claim 11, the container being a graphite crucible, and the die being a graphite die.

13. The method according to claim 11, the second temperature being about 1850° C., and the second amount of time being about 10 minutes.

14. The method according to claim 1, the SPS process being performed and completed in less than 20 minutes.

15. A method of fabricating an ultra-high temperature carbide (UHTC) foam, the method comprising: preparing a UHTC slurry; freeze-drying the UHTC slurry to provide a green body; and performing a spark plasma sintering (SPS) process on the green body to provide the UHTC foam, the performing of the SPS process comprising disposing the green body such that punches of an SPS setup used for the SPS process do not touch the green body during the SPS process, the SPS process thereby being a pressure-less SPS process, the UHTC foam comprising at least one of tantalum carbide (TaC) and hafnium carbide (HfC), the preparing of the UHTC slurry comprising: mixing UHTCs in a solvent to form a first slurry, a loading of UHTCs in the first slurry being at least 5 wt %; adding a dispersant to the first slurry to obtain a second slurry; and stirring the second slurry to obtain the UHTC slurry, the solvent being water, the dispersant being polyacrylic acid (PAA), the freeze-drying of the UHTC slurry comprising: pouring the UHTC slurry into a mold; and freezing and then subliming the UHTC slurry in the container to obtain the green body, the performing of the SPS process comprising: disposing the container with the green body in a die; and performing the SPS process on the green body in the container in the die for a period of time of less than 20 minutes.

16. The method according to claim 15, the stirring of the second slurry comprising stirring the second slurry at a first temperature for a first amount of time to maintain a pH of the second slurry constant, the first temperature being room temperature and the first amount of time being about 5 hours, the container being a graphite crucible, the freeze-drying process being performed for about 24 hours, the die being a graphite die, the SPS process being performed at a temperature of about 1850° C. for about 10 minutes.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a step-wise fabrication process for ultra-high temperature carbide (UHTC) foams, according to an embodiment of the subject invention. First, an aqueous slurry of UHTCs (e.g., with 10 wt % loading or about 10 wt % loading) can be prepared. A dispersant (e.g., polyacryclic acid (PAA)) with a loading of UHTC (e.g., 6 wt % or about 6 wt % of UHTC loading) can be added to obtain well-dissolved UHTC slurry. The solution/slurry can be stirred (e.g., constantly stirred at a set temperature for a set amount of time (e.g., room temperature for 5 hours (h) or about 5 h)) while maintaining the pH constant or fairly constant (e.g., constant at pH=8 or about 8) throughout the process. The solution can then be poured into a mold (e.g., a graphite mold such as a graphite crucible) and freeze dried (e.g., for a set amount of time such as 24 h or about 24 h). The mold can then be put inside a die (e.g., a spark plasma sintering (SPS) die) in such a way that the green body remains untouched by the two punches (e.g., graphite punches) in order to do pressure-less sintering. Though FIG. 1 lists graphite as the material for the die, crucible, and punch, this is for exemplary purposes only; also, though FIG. 1 lists dimensions (12.7 millimeters (mm) in diameter and 12 mm in height) for the fabricated UHTC foam, this is for exemplary purposes only and should not be construed as limiting.

(2) FIG. 2(a) shows an image of a fabricated UHTC foam for tantalum carbide (TaC). Though FIG. 2(a) lists dimensions (12.7 mm in diameter and 12 mm in height) for the fabricated UHTC foam, this is for exemplary purposes only and should not be construed as limiting.

(3) FIG. 2(b) shows an image of a fabricated UHTC foam for hafnium carbide (HfC). Though FIG. 2(b) lists dimensions (12.7 mm in diameter and 8 mm in height) for the fabricated UHTC foam, this is for exemplary purposes only and should not be construed as limiting.

(4) FIG. 2(c) shows an image of a fabricated UHTC foam for TaC and HfC (TaC—HfC). Though FIG. 2(c) lists dimensions (12.7 mm in diameter and 5 mm in height) for the fabricated UHTC foam, this is for exemplary purposes only and should not be construed as limiting.

(5) FIG. 3(a) shows a scanning electron microscope (SEM) image for TaC foam, showing uniform distribution of macro-pores. The scale bar is 100 micrometers (μm).

(6) FIG. 3(b) shows an SEM image for TaC foam, showing densified struts and aligned macro-pores. The scale bar is 50 μm.

(7) FIG. 3(c) shows an SEM image of the identified square in FIG. 3(b), showing partially sintered struts and meso-pores in the TaC foam. The scale bar is 10 μm.

(8) FIG. 3(d) shows an SEM image of the identified square in FIG. 3(c), showing micro-pores in struts in the TaC foam. The scale bar is 1 μm.

(9) FIG. 4(a) shows an SEM image for HfC foam, showing uniform distribution of macro-pores. The scale bar is 100 μm.

(10) FIG. 4(b) shows an SEM image for HfC foam, showing densified regions, macro-pores, and meso-pores. The scale bar is 50 μm.

(11) FIG. 4(c) shows an SEM image of the identified square in FIG. 4(b), showing partially sintered struts in the HfC foam. The scale bar is 10 μm.

(12) FIG. 4(d) shows an SEM image of the identified square in FIG. 4(c), showing micro-pores and pore closure in the HfC foam. The scale bar is 1 μm.

(13) FIG. 5(a) shows an SEM image for TaC—HfC foam, showing uniform distribution of macro-pores. The scale bar is 100 μm.

(14) FIG. 5(b) shows an SEM image for TaC—HfC foam. The scale bar is 50 μm.

(15) FIG. 5(c) shows an SEM image of the identified square in FIG. 5(b), showing struts in the TaC—HfC foam. The scale bar is 10 μm.

(16) FIG. 5(d) shows an SEM image of the identified square in FIG. 5(c), showing uniform distribution of micro-pores in the TaC—HfC foam. The scale bar is 1 μm.

(17) FIG. 6 shows an area selected for energy dispersive spectroscopy (EDS) mapping in TaC—HfC foam. The scale bar is 100 μm.

(18) FIG. 7 shows EDS mapping for the carbon (C) in the TaC—HfC foam from FIG. 6. The scale bar is 100 μm.

(19) FIG. 8 shows EDS mapping for the Hf in the TaC—HfC foam from FIG. 6. The scale bar is 100 μm.

(20) FIG. 9 shows EDS mapping for the Ta in the TaC—HfC foam from FIG. 6. The scale bar is 100 μm.

(21) FIG. 10 shows the intensity (counts) versus energy (in kilo-electron Volts (keV)) for the EDS mapping for the TaC—HfC foam, showing uniform solid-solution formation in the foam from FIG. 6.

(22) FIG. 11 shows a comparison of X-ray diffraction (XRD) spectra of freeze dried TaC—HfC UHTC foam (labeled “FD_TaC—HfC” in FIG. 11) and pressure-less SPS TaC—HfC UHTCs (labeled “SPS_TaC—HfC” in FIG. 11). The partial solid solutioning was observed in TaC—HfC foam after sintering.

(23) FIG. 12 shows a plot of load (in Newtons (N)) versus displacement (in mm) for showing position control high load indentation, showing the mechanical integrity (along with load bearing capabilities) for TaC foam. Indentations were carried out using a spherical tip (R=about 3 mm) at a rate of 0.1 millimeters per minute (mm/min) up to 20% of its initial height using a 4000 N capacity load cell. The corresponding load in the sample obtained relates to the load bearing capacity of the sample. The kinks in the curve correspond to the events such as porosity encounter, crack initiation, crack propagation, and failure in the foam. The TaC foam did not break during testing.

(24) FIG. 13 shows a plot of load (in N) versus displacement (in mm) for showing position control high load indentation, showing the mechanical integrity (along with load bearing capabilities) for HfC foam. The testing was carried out as described for FIG. 12. The HfC foam did not break during testing.

(25) FIG. 14 shows a plot of load (in N) versus displacement (in mm) for showing position control high load indentation, showing the mechanical integrity (along with load bearing capabilities) for TaC—HfC foam. The testing was carried out as described for FIG. 12. The TaC—HfC foam did not break during testing.

(26) FIG. 15 shows a plot of thermal conductivity (in Watts per meter per Kelvin (W/m-K)) versus temperature (in ° C.) for TaC, HfC, and TaC—HfC UHTC foams. The curve with the highest thermal conductivity values (with the square data points) is for the TaC foam; the curve with the second-highest conductivity values (with the circle data points) is for the HfC foam; and the curve with the lowest conductivity values (with the triangle data points) is for the TaC—HfC foam.

DETAILED DESCRIPTION

(27) Embodiments of the subject invention provide novel and advantageous ultra-high temperature carbide (UHTC) foams and methods of fabricating and using the same. The UHTC foams are crack-resistant and have low thermal conductivity, making them ideal for applications such as thermal insulation systems, filtration of high-temperature corrosive gases, catalyst support, and high-temperature solar absorptions. The UHTC foams are produced in a three-step process, including UHTC slurry preparation, freeze-drying, and spark plasma sintering (SPS). The fabrication methods allow for the production of any kind of single- or multi-component UHTC foam, while also providing flexibility in the shape and size of the UHTC foams to produce near-net-shape components. It is noted that a UHTC material is defined as showing good stability at a temperature of at least 2000° C.

(28) Contrary to existing UHTC ceramics, the development of which is focused on dense UHTCs, embodiments of the subject invention can provide porous UHTC foams. The porosity in the UHTC is not a defect, but rather a functional property tailored specifically for the final application (e.g., thermal insulation systems, filtration of high-temperature corrosive gases, catalyst support, and/or high-temperature solar absorptions). These UHTC foams provide an advantageous combination of properties of low overall weight and low thermal conductivity. This solves the concerns related to the durability of UHTCs and, combined with the other advantages of these UHTC foams of embodiments of the subject invention, can end up saving huge amounts of money per pound of payload-to-orbit for rockets.

(29) Prior to the UHTC foams of embodiments of the subject invention, the development of foams of high-temperature materials involved replication methods, direct foaming methods, precursor infiltration and pyrolysis, and chemical vapor deposition (CVD). Due to high covalent bonding and low self-diffusion coefficients, sintering of UHTC foams is challenging. Prolonged sintering time and high sintering temperature can significantly affect the pore size, inter-connecting structure, and density of the foams. Moderate sintering temperature can affect the hand ability of the UHTC foams. Hence, the currently available related art methods that could be used to fabricate UHTC foams are time-consuming, expensive, and/or result in foams with poor mechanical strength.

(30) Embodiments of the subject invention provide novel foam-gel, casting-freeze-drying (FD) technology to fabricate bulk porous UHTCs. As a field-assisted sintering technique, SPS is known to obtain porous materials by partial or controlled densification. In embodiments of the subject invention, the SPS process can be calibrated to be used as a pressure-less technique. The usage of SPS as a pressure-less technique can be divided into: (i) retarded grain growth for enhanced mechanical strength; (ii) retaining the porous structure obtained during FD with controlled pore-shape, pore-size, and/or interconnection; and (iii) consolidation in a short time. Thus, the calibrated SPS of the porous green body of UHTCs maintains the ultra-high porosity of UHTCs having low thermal conductivity and high strength. This technique applies to all high-temperature ceramic classes, including UHTCs and high-entropy UHTCs (HE-UHTCs).

(31) The microstructure properties of foams are strongly dependent on the preparation protocols. Partial sintering is the most straightforward route for fabricating porous ceramics, but this method usually results in either lower porosity or lower strength foams. Ceramic foams with high porosity are typically prepared by a replica, sacrificial template, and direct foaming method, in which pre-ceramic polymers are involved. All of these methods have only been applied to oxides and silicon-based non-ceramic foams.

(32) In many embodiments of the subject invention, no hazardous surfactants/dispersants are used. The liquid medium for UHTC dispersion can be, for example, water (e.g., deionized water, such as deionized water with no additives), and the dispersant for UHTC dispersion can be, for example, polyacrylic acid (PAA). Embodiments therefore provide an inexpensive, non-toxic, environmentally-friendly way to prepare UHTC slurries. The dispersion of the UHTC can be done by stirring the solution while maintaining a pH of 8 (or about 8). This mixing process is controllable, facile, and can be easily scaled up from laboratory scale to industry scale. The unique combination of FD and calibrated SPS techniques offers an attractive combination of properties of low overall weight, low thermal conductivity, and high mechanical strength for the fabricated UHTC foams. The methods of embodiments of the subject invention provide flexibility in the shape and size of the UHTC foams, allowing for production of near-net-shape components.

(33) FIG. 1 shows a step-wise fabrication method for ultra-high temperature carbide (UHTC) foams, according to an embodiment of the subject invention. The method can include UHTC slurry preparation, freeze drying, and SPS.

(34) First, an aqueous slurry of UHTCs (e.g., with 10 wt % loading or about 10 wt % loading) can be prepared. A dispersant (e.g., PAA) with a loading of UHTC (e.g., 6 wt % or about 6 wt % of UHTC loading) can be added to obtain well-dissolved UHTC slurry. The solution/slurry can be stirred (e.g., constantly stirred at a set temperature for a set amount of time (e.g., room temperature for 5 hours (h) or about 5 h)) while maintaining the pH constant or fairly constant (e.g., constant at pH=8 or about 8) throughout the process. By maintaining the pH, a polymer ratio to UHTC carbide loading is controllable to obtain repeatable and scalable production methods.

(35) The solution/slurry can then be poured into a mold (e.g., a graphite mold such as a graphite crucible) and freeze dried (e.g., for a set amount of time such as 24 h or about 24 h) to obtain a green body. The freeze drying can include the slurry being frozen and then subsequently sublimed in a freeze drying (e.g., for a set amount of time such as 24 h or about 24 h) to obtain the green body. The use of the freeze drying technique on UHTCs is performed here for the first time on monolithic UHTCs (e.g., TaC, HfC) and a binary composite (e.g., TaC—HfC).

(36) The green body obtained from the FD technique can be sintered by calibrating a conventional SPS setup. The green body can be kept inside a container (e.g., a graphite container and/or a crucible, such as a graphite crucible). The whole assembly, including the green body and the container, can be put inside a die (e.g., a graphite die). In the modified punch design, the working space can remain constant between the punches of the SPS setup by not touching the green body sample inside the die (e.g., a standard graphite die), resulting in zero external pressure. The SPS parameters can be configured for pressure-less sintering (e.g., at a set temperature for a set amount of time, such as at 1850° C. or about 1850° C. for 10 minutes (min) or about 10 min). After the pressure-less SPS, the final UHTC foam is obtained (see, e.g., FIGS. 2(a), 2(b), 2(c), and the bottom-right corner of FIG. 1).

(37) The thermal insulation properties of the UHTC foam can be altered by adjusting the kinetics (during SPS processing), which regulates the mutual solubility in two/multi-component UHTCs. For example, partial solid-solution formation in a two-component UHTC (TaC—HfC) system with a sintering time of 10 mins has been shown (see, e.g., Example 1). The tailoring of the thermal insulation can be further done by adding more components in a UHTC system while maintaining the same mechanical integrity or improving the mechanical integrity. The methods of embodiments of the subject invention can also be used to make UHTC foams of desired shape and size.

(38) Embodiments of the subject invention provide a practical approach to ultra-light, strong UHTCs for thermal insulation. Ultra-high porosity and low sintering shrinkage are attributed to the fact that the freeze-drying technique reduces surface tension and UHTC particles have very low self-diffusion during sintering. Also, the shorter dwelling time during SPS (e.g., less than 30 min, such as 10 min or about 10 min) preserves the FD porous structure while achieving high mechanical strength.

(39) UHTC foams of embodiments of the subject invention can be used in various engineering applications, including in the energy and aerospace sectors. Applications of the UHTC foams can include high-temperature thermal insulation, support membrane for catalysis, hot/corrosive gas infiltration, high-temperature solar absorption, concentrated solar power (CSP), solar receivers, thermo-electric conversion, hypersonic vehicles, atmospheric entry probes, radiant burners, gas and/or chemical sensors, insulator panels, and thrust chambers and/or rocket nozzles in a space propulsion system.

(40) The transitional term “comprising,” “comprises,” or “comprise” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. By contrast, the transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. The phrases “consisting” or “consists essentially of” indicate that the claim encompasses embodiments containing the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claim. Use of the term “comprising” contemplates other embodiments that “consist” or “consisting essentially of” the recited component(s).

(41) When ranges are used herein, such as for dose ranges, combinations and subcombinations of ranges (e.g., subranges within the disclosed range), specific embodiments therein are intended to be explicitly included. When the term “about” is used herein, in conjunction with a numerical value, it is understood that the value can be in a range of 95% of the value to 105% of the value, i.e. the value can be +/−5% of the stated value. For example, “about 1 kg” means from 0.95 kg to 1.05 kg.

(42) A greater understanding of the embodiments of the subject invention and of their many advantages may be had from the following examples, given by way of illustration. The following examples are illustrative of some of the methods, applications, embodiments, and variants of the present invention. They are, of course, not to be considered as limiting the invention. Numerous changes and modifications can be made with respect to embodiments of the invention.

Example 1

(43) The three-step process as shown in FIG. 1 was performed to prepare UHTC foams of TaC, HfC, and TaC—HfC, with the three steps being UHTC slurry preparation, freeze-drying, and SPS.

(44) For each UHTC foam (TaC, HfC, and TaC—HfC), the preparation method was as follows. An aqueous slurry of UHTCs with 10 wt % loading was prepared. A PAA dispersant with 6 wt % of UHTC loading was added to obtain well dissolved UHTC slurry. The solution/slurry was constantly stirred at room temperature for 5 h while maintaining the pH at 8 throughout the process. The UHTC aqueous dispersion/slurry was poured into a graphite mold, frozen and subsequently sublimed in a freeze dryer for 24 h to obtain a green body. The green body was sintered by calibrating a conventional spark plasma sintering (SPS) setup. The green body was kept inside a graphite crucible, and this whole assembly was put inside the graphite die. The working space remained constant between the punches of the SPS setup by not touching the sample/green body inside the standard graphite die, resulting in zero external pressure. The SPS parameters were configured for pressure-less sintering at 1850° C. for 10 min to give the UHTC foam.

(45) FIGS. 2(a), 2(b), and 2(c) show the fabricated UHTC foams for TaC, HfC, and TaC—HfC, respectively. The dimensions listed in FIGS. 2(a), 2(b), and 2(c) show the dimensions of the as-fabricated foams. FIGS. 3(a)-5(d) show scanning electron microscope (SEM) images of the UHTC foams, with FIGS. 3(a)-3(d) showing TaC foam, FIGS. 4(a)-4(d) showing HfC foam, and FIGS. 5(a)-5(d) showing TaC—HfC foam. Referring to FIGS. 3(a)-5(d), the microstructure of the TaC, HfC, and TaC—HfC foams showed the presence of macro-pores, meso-pores, and micro-pores with directionality in pore alignment during FD. The relative porosity in TaC, HfC, and TaC—HfC foams was about 54%, about 55%, and about 48%, respectively. The EDS mapping to show the solid-solutioning in TaC—HfC foam is presented in FIGS. 6-10.

(46) FIG. 11 shows the X-ray diffraction (XRD) analysis of the TaC—HfC foam, which further confirms the partial solid-solutioning in TaC—HfC foam during the pressure-less SPS process. FIGS. 12-14 show the position control high load indentation for the TaC, HfC, and TaC—HfC foams, respectively, to predict the strength of these foams based on load bearing ability against crack growth. The results showed that partial solid-solutioning in the UHTC foams leads to better mechanical integrity in the TaC—HfC foam as compared to the parent UHTC foams (TaC and HfC). The load-bearing capacity was observed to be highest for TaC foam (about 120 N). This is due to bigger macro-pores (˜40-80 μm) in the material, which during indentation leads to early cracking but at the same time compacting the pore walls (struts), leading to pore closure at a faster rate. However, in the case of HfC foam, the relatively high densification of the struts and smaller macro-pores (<50 μm), instead of compaction cracking of struts, is more prominent, diminishing its load. The densified regions (dense struts) are more pronounced in HfC foam than TaC (see FIGS. 3(a)-4(d)). The macro-pores in TaC—HfC are comparable to that of HfC foam. Still, due to partial solid-solutioning, there is no evidence of cracking observed while maintaining moderate load-bearing (about 80 N) in the TaC—HfC foam.

(47) FIG. 15 shows the thermal conductivity of the TaC, HfC, and TaC—HfC foams. Referring to FIG. 15, the 5-fold decrease in the thermal conductivity of TaC—HfC foam as compared to TaC highlights the efficacy of solid-solutioning in providing thermal insulation. The thermal conductivity of HfC foam is similar to that of TaC—HfC foam, but the lower load-bearing capability of HfC foam (cracking marked in FIG. 13) makes TaC—HfC foam better than parent UHTC foams (e.g., TaC and HfC foams).

(48) It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.

(49) All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.