Advanced Synthesis of Ultra-High Temperature Ceramics (UHTCs) and High Temperature Electron Emitting Materials

Autor: Mondal, Santanu
Jazyk: angličtina
Rok vydání: 2024
Předmět:
Druh dokumentu: Dissertation
Popis: From space exploration and advanced aircraft to next generation weapons, achieving hypersonic speed is becoming increasingly important across a range of research domains. The immense challenge associated with this goal involves the development of suitable materials and systems for the different components of a hypersonic vehicle, each of which must have the inherent capability to resist extreme temperatures, high thermal shock due to high heat flux, and high oxidation and ablation. First, the ultra-high temperature ceramic (UHTC) zirconium diboride or ZrB2 was sintered by ultra-fast high temperature sintering (UHS). The UHS process was optimized and the sintering parameters for ZrB2 and other UHTCs were studied. ZrB2 is an ultra-high temperature ceramic (UHTC) with a very high melting point; thus, its densification is difficult, energy intensive, and time-consuming. Commercial ZrB2 powders were rapidly densified via UHS to >90% relative density within 60 second in vacuum without pressure. The effect of sintering time on densification and final grain size were studied. An innovative process for manufacturing bulk UHTC materials was studied and is detailed herein. Second, the work function (W_f) of electron emitting materials was reduced to improved performance. A reduction of W_f in multicomponent hexaborides was achieved by doping with highly electropositive Ba, which enhances electron emission. Single-phase bulk multicomponent polycrystalline hexaborides of La0.5Ba0.5B6, Ce0.5Ba0.5B6, and BaB6 powders were first synthesized and then densified by UHS sintering. W_f measurements were obtained by Kelvin probe force microscopy. Ba-substitution was found to lower W_f (~25%) in synthesized multicomponent hexaborides. The specific techniques required to engineer the W_f of these materials are also provided herein. Finally, combining low W_f materials with UHTCs was explored for thin film systems for the exterior surface of hypersonic vehicles. The thin films of CeB6, a low W_f material, was deposited on sintered ZrB2 by RF-sputtering and single crystalline SrTiO3 (STO) substrates. Epitaxial thin films of SrHfO3 (SHO) were also deposited on (100), (110) and (111) STO substrates at 600°C. X-ray diffraction (XRD) results confirmed the formation of epitaxial layer, and reciprocal space mapping (RSM) was used to characterize film's mosaicity / texture on different substrates. XRD and RSM data demonstrated that the most favorable film growth direction was (110). As detailed herein, an inexpensive thin film production process, RF-sputtering, was exploited to manufacture various epitaxial and non-epitaxial layers of low W_f materials on UHTC and single-crystal substrates for hypersonic vehicles. To summarize, a range of bulk UHTCs and low W_f materials were prepared by UHS, and various thin films of low W_f material were produced on UHTC. Thereafter, the properties of synthesized materials were studied to develop new material systems for hypersonic applications. The findings from this research shed light on the development of suitable materials for implementation of electron transpiration cooling for hypersonic vehicle development.
Doctor of Philosophy
Rapid sintering of ultra-high temperature ceramics (UHTCs) and synthesis of low work-function electron emitting materials have been performed by ultra-fast high temperature sintering technique (UHS). Sintering of UHTCs is a difficult process, due to their high melting temperature, presence of covalent bond, and slower diffusion coefficient. A long sintering duration is used to achieve a high relative density along with adding sintering aid, using fine powder (produced by milling), and utilizing pressure (such as field assisted sintering and hot-pressing technology) during sintering. Synthesis and densification of multicomponent hexaboride is difficult, involves multi-steps and complicated processes. These long and complicated processes not only prolong development of new materials but also cause chemical wastes. To overcome all the aforementioned processing issues, an advanced processing technique, UHS, is used and densified pure and commercially available UHTCs to >90% within 60 second without applying sintering aid, powder milling, and pressure. The outcome of this research demonstrates the potential for a simple, cost-effective, fast, and adjustable processes, UHS, to develop a wide range of bulk UHTCs and other technical ceramics, and it gives new insight into the mechanisms of rapid sintering of UHTCs by rapid heating. The first detailed studies (experimental report) on rapid sintering of ZrB2 (and other UHTCs) by UHS technique and a through characterizations of the UHS sintered sample were performed to understand rapid sintering mechanism and how the processing effects the microstructure and properties of UHS ZrB2. The rapid microstructural evolution during the UHS sintering is investigated at 10, 30, and 60 second sintering interval. The UHS technique enables a heating rate of 103 - 104 °C/min and reaches a sintering temperature of 2600 °C in 30 seconds. Microstructural analysis was conducted on polished sample surfaces by using ImageJ software (National Institutes of Health, version 1.53e), measuring the grain size perpendicular to two diagonals of each grain. A comparison of grain size from sample center and periphery showed a homogeneous microstructure after sintering. Furthermore, the rapid sintering did not change/effect crystallinity, boron to metal stoichiometry, and grain boundary elemental composition as observed by XRD and EDS analysis. Additional characterization of the UHS sintered ZrB2 shows a hardness and elastic modulus of 30 GPa and 412 GPa respectively by nanoindentation method. Finally, the oxidation test at 1100 °C in isothermal condition showed a weight gain of 1.4% in air. The low work-function (W_f) materials are famous for electron emitting applications like electron guns for scanning electron microscopy. DFT simulation predicts the W_f of the widely used electron emitters (such as LaB6 and CeB6) can be reduced by changing their compositions, which increase electron generation efficiency of those materials. Previously, those materials were synthesized by long processes that involved multiple processing steps, which required expensive starting materials and yielded chemical wastes. The advantages of rapid sintering technique, UHS, had been exploited to synthesize low work function electron emitting materials. Single-phase bulk polycrystalline hexaborides were produced by using electrically powered UHS technique using a vacuum atmosphere. A reaction synthesis route: B4C reduction technique was first used to form pure phase hexaboride. Then, the synthesized compositions were densified to ~90% theoretical density in 180 seconds by UHS densification. After UHS sintering, XRD analysis confirmed the presence of a phase pure cubic BaB6, La0.5Ba0.5B6, and Ce0.5Ba0.5B6. Additional analyses were conducted to determine an optimum reaction temperature 1500 and 2100 °C for the formation BaB6 and multi-component hexaborides. Microstructural analyses were conducted to observe both reaction-synthesized and densified products. EDS compositional analysis and elemental mapping revealed a stoichiometric reaction product with homogeneous metal cation and boron distributions. The W_f of BaB6, La0.5Ba0.5B6, and Ce0.5Ba0.5B6 was determined to be 1.95 ± 0.1, 2.05 ± 0.1 and 2.0 ± 0.1 eV, respectively. The addition of BaB6 in La0.5Ba0.5B6, and Ce0.5Ba0.5B6 resulted in a 25% decrease in W_f for LaB6 from 2.7 ± 0.1 to 2.00 ± 0.1 eV and a 23% decrease in W_f for CeB6 from 2.68 ± 0.08 to 2.05 ± 0.1 eV. Ba substitution is shown to be a general method for lowering W_f in a variety of multicomponent hexaborides. Finally, the polycrystalline thin films of CeB6, a low W_f material, was deposited on sintered ZrB2 by RF-sputtering technique. Additionally, epitaxial thin films of SrHfO3 (SHO) were also deposited on (100), (110) and (111) STO single crystalline substrates. Both types of thin films were deposited at 600 °C temperature and at a vacuum pressure of 10-3 Torr. After deposition of the SHO films, X-ray diffraction (XRD) was conducted to confirm the formation of epitaxial layer, and reciprocal space mapping (RSM) was used to characterize film's mosaicity / texture on different substrates. XRD and RSM data demonstrated that the most favorable film growth direction was (110). The XRD of the CeB6 film showed highly crystalline film was formed. For both the films, a detailed microstructural analysis was performed by scanning electron microscopy and film smoothness was characterized by atomic force microscopy method. As detailed herein, an inexpensive thin film production process, RF-sputtering, was exploited to manufacture various epitaxial and non-epitaxial layers of low W_f materials on UHTC and single-crystal substrates for hypersonic vehicles applications.
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