Yes / Ultra-high performance concrete (UHPC) is an innovative cementitious composite, that has been widely applied in numerous structural projects because of its superior mechanical properties and durability. However, ensuring the safety of UHPC structures necessitates an urgent need for technology to continuously monitor and evaluate their condition during their extended periods of service. Self-sensing ultra-high performance concrete (SSUHPC) extends the functionality of UHPC system by integrating conductive fillers into the UHPC matrix, allowing it to address above demands with great potential and superiority. By measuring and analyzing the relationship between fraction change in resistivity (FCR) and external stimulates (force, stress, strain), SSUHPC can effectively monitor the crack initiation and propagation as well as damage events in UHPC structures, thus offering a promising pathway for structural health monitoring (SHM). Research on SSUHPC has attracted substantial interests from both academic and engineering practitioners in recent years, this paper aims to provide a comprehensive review on the state of the art of SSUHPC. It offers a detailed overview of material composition, mechanical properties and self-sensing capabilities, and the underlying mechanisms involved of SSUHPC with various functional fillers. Furthermore, based on the recent advancements in SSUHPC technology, the paper concludes that SSUHPC has superior self-sensing performance under tensile load but poor self-sensing performance under compressive load. The mechanical and self-sensing properties of UHPC are substantially dependent on the type and dosage of functional fillers. In addition, the practical engineering SHM application of SSUHPC, particularly in the context of large-scale structure, is met with certain challenges, such as environment effects on the response of SSUHPC. Therefore, it still requires further extensive investigation and empirical validation to bridge the gap between laboratory research and real engineering application of SSUHPC. / The full-text of this article will be released for public view at the end of the publisher embargo on 28 Dec 2024.

Zirconium carbide (ZrC) is a high modulus ceramic with an ultra-high melting temperature and, consequently, is capable of withstanding extreme environments. Carbon-carbon composites (CCCs) are important structural materials in future hypersonic aircraft; however, these materials may be susceptible to degradation when exposed to elevated temperatures during extreme velocities. At speeds of exceeding Mach 5, intense heating of leading edges of the aircraft triggers rapid oxidation of carbon in CCCs resulting in degradation of the structure and probable failure. Environmental/thermal barrier coatings (EBC/TBC) are employed to protect airfoil structures from extreme conditions. Yttria stabilized zirconia (YSZ) is a well-known EBC/TBC material currently used to protect metallic turbine blades and other aerospace structures. In this work, 3 mol% YSZ has been studied as a potential EBC/TBC on CCCs. However, YSZ is an oxygen conductor and may not sufficiently slow the oxidation of the underlying CCC. Under appropriate conditions, ZrC can form at the interface between CCC and YSZ. Because ZrC is a poor oxygen ion conductor in addition to its stability at high temperatures, it can reduce the oxygen transport to the CCC and thus increase the service lifetime of the structure. This dissertation investigates the thermodynamics and kinetics of the YSZ/ZrC/CCC system and the resulting structural changes across multiple size scales. A series of experiments were conducted to understand the mechanisms and species involved in the carbothermal reduction of ZrO2 to form ZrC. 3 mol% YSZ and graphite powders were uniaxially pressed into pellets and reacted in a graphite (C) furnace. Rietveld x-ray diffraction phase quantification determined that greater fractions of ZrC were formed when carbon was the majority mobile species. These results were validated by modeling the process thermochemically and were confirmed with additional experiments. Measurements were conducted to examine the effect of carbothermal reduction on the bond lengths in YSZ and ZrC. Subsequent extended x-ray absorption fine structure (EXAFS) measurements and calculations showed Zr-O, Zr-C and Zr-Zr bond lengths to be unchanged after carbothermal reduction. Energy dispersive spectroscopy (EDS) line scan and mapping were carried out on carbothermaly reduced 3 mol% YSZ and 10 mol% YSZ powders. Results revealed Y2O3 stabilizer forming agglomerates with a very low solubility in ZrC.

Yes / In order to promote the utilization of high strength materials and application of prefabricated structures, flexural behaviour of section steel-precast UHPC (Ultra-High performance concrete) slab composite beams prefabricated with bolt shear connectors are numerically simulated by the finite element (FE) software ABAQUS. The model is verified by three prefabricated steel-concrete composite beams tested. Numerical analysis results are in good accordance with experimental results. Furthermore, parametric studies are conducted to investigate the effects of strength of section steel and concrete of precast slab, thickness of section steel, width and height of precast concrete slab, diameters of steel bars and bolt shear connectors. The flexural behaviour of composite beams, in terms of bearing capacity, deflection, ductility and energy dissipation, are compared. The numerical results indicate that the improvement of strength of section steel results in a decrease of ductility, but a significant increase of the ultimate load and energy dissipation. Compared with composite beam made of section steel with thickness of 10 mm, the ultimate load of beams made of section steel with thickness of 14 and 18 mm improve by 29.0% and 58.8%, respectively, the ductility enhance by 2.8% and 8.3%, respectively, and the energy dissipation improve by 8.0% and 12.3%, respectively. With the increase of concrete strength, the ultimate load, deflection and energy dissipation gradually increase. The ductility of steel-UHPC composite beam is the highest, that of steel-HSC composite beam is the lowest. The effect of reinforcement ratio of concrete slab and diameter of shear bolts on the ultimate load of composite beam is limited. Simplified formulae for two different sectional types of proper-reinforced section steel-precast UHPC slab composite beams occurred bending failure are proposed, and the predicted results fit well with the simulated results. The results can be taken as a reference for the design and construction of section steel-precast UHPC slab composite beams.

Yes / The development of ultra-high performance cementitious composite (UHPCC) represents a significant advancement in the field of concrete science and technology, but insufficient hydration and high autogenous shrinkage relatively increase the pores inside UHPCC, in turn, affecting the macro-performance of UHPCC. This paper, initially, optimized the pore structure of UHPCC using different types and dimensions of nanofillers. Subsequently, the pore structure characteristics of nano-modified UHPCC were investigated by the mercury intrusion porosimeter method and fractal theory. Finally, the fluid permeability of nano-modified UHPCC was estimated by applying the Katz-Thompson equation. Experimental results showed that all incorporated nanofillers can refine the pore structure of UHPCC, but nanofillers with different types and dimensions have various effects on the pore structure of UHPCC. Specifically, CNTs, especially the thin-short one, can significantly reduce the porosity of UHPCC, whereas nanoparticles, especially nano-SiO2, are more conducive to refine the pore size. Among all nanofillers, nano-SiO2 has the most obvious effect on pore structure, reducing the porosity, specific pore volume and most probable pore radius of UHPCC by 31.9%, 35.1% and 40.9%, respectively. Additionally, the pore size distribution of nano-modified UHPCC ranges from 10-1nm to 105nm, and the gel pores and fine capillary pores in the range of 3-50nm account for more than 70% of the total pore content, confirming nanofillers incorporation can effectively weaken pore connectivity and induce pore distribution to concentrate at nanoscale. Fractal results indicated the provision of nanofillers reduces the structural heterogeneity of gel pores and fine capillary pores, and induces homogenization and densification of UHPCC matrix, in turn, decreasing the UHPCC fluid permeability by 15.7%-79.2%. / The authors thank the funding supported from the National Science Foundation of China (51978127, 52178188 and 51908103), the China Postdoctoral Science Foundation (2022M720648 and 2022M710973) and the Fundamental Research Funds for the Central Universities (DUT21RC(3)039). / The full-text of this article will be released for public view at the end of the publisher embargo on 11 May 2024.

The manufacturing of UHTC materials has significantly advanced over recent years, allowing for the development of new microstructures, architectures, shapes, and geometries to explore new properties and applications for these materials beyond aerospace. One of the UHTCs, titanium diboride (TiB2) exhibits high electrical and thermal conductivity that could satisfy the needs of functional ceramic component applications, like battery cathodes, by tailoring its microstructure and architecture. This thesis represents one of the first detailed studies to understand how the processing-microstructure-properties relationship of TiB2 can be shifted to explore new applications. In order to do that, TiB2 has been manufactured with a processing technique never used before, with significant porosity, exploration of which has been very limited for this material. Additionally, this thesis also explores the synthesis and utilization of novel anisotropic particles to further explore this material relationship. In this work, aqueous tape casting of TiB2 has been investigated. Zeta potential measurements and suspension rheology were used to understand the role of dispersant, binder and plasticizer in the suspension and their interaction with the surface chemistry of the TiB2 particles to develop a stable, homogenous suspension, with minimum additive amounts (0-2 wt%). Homogeneous, flexible and strong TiB2 tapes were prepared using suspensions with 30 vol% solids and characterized to compare different compositions, mixing methods, and thicknesses. The characterization shows the tailoring of the properties as a function of the controlled suspension formulation with minimum amount of additives. Green tapes with 2 wt% dispersant, 1 wt% binder, and 2 wt% plasticizer had similar microstructure to those with half the plasticizer but quintuple the Young's modulus (1.96 GPa). The effect on other relevant properties is also discussed. Tape casting aligns anisotropic particles along the direction of casting, which can be taken advantage of for increasing fracture toughness directionally or producing aligned pore networks using sacrificial fillers. The relationship between alignment, porosity, and the mechanical properties of titanium diboride has not been studied. In this work, we characterize the porous sintered bodies produced through aqueous tape casting of non-spherical TiB2 particles of aspect ratio close to 1, as well as composites with an added high aspect ratio particle (2 wt% PCN-222). Synthesis of uniform, spherical ZrC is difficult and generally not cost-effective, as is the case for most ultra-high temperature ceramics. High aspect ratio particles for reinforcement of ceramic composites are even more difficult to synthesize. Metal organic frameworks (MOF) are crystalline coordination polymers composed of multidentate organic linkers bridging metal nodes to form porous structures. Thermal decomposition of MOFs presents a new and cost-effective route to synthesis of ZrC. In this study, heat treatment at 2000°C of MOF PCN-222 produces zirconium carbide (ZrC) within a highly anisotropic particle. The resulting rod-shaped, glass-like carbon matrix embedded with ZrC crystals is described. These rods have potential as reinforcements for iii high temperature applications and as a synthetic route for ultra-high temperature ceramics with unique morphologies. It is the first time that this type of transformation from a MOF into a UHTC has been reported. We have determined through analysis of SEM images that regardless of tape casting speed, about 57% of the TiB2 particles are aligned with the tape casting direction. The mechanical properties are dominated by the effects of the porosity (38%), but the alignment exhibited here could be further exploited for anisotropic behavior across the sintered tapes. Composites cast with high aspect ratio particles exhibited strong alignment in the casting direction. Further work is required to understand the interplay between alignment and porosity and their effects on material properties. / Doctor of Philosophy / Titanium diboride (TiB2) is an ultra-high temperature ceramic with a melting point of 3225°C. Many applications for this material require fully dense structural ceramics, such as cutting tools,1 armor,2 and high temperature structural supports.2,3 These applications rely mainly on the high mechanical strength of TiB2, which is maintained in extreme thermal and chemical environments. The field of knowledge surrounding TiB2 lacks information about the ways that porosity affects its otherwise well-known properties;4,5 to bridge this gap could open up applications for functional and porous ceramics such as lithium-air batteries,6 electrochemical components,7 semiconductors,8 and more. This work intends to provide a foundation for this endeavor by developing for the first time a colloidal suspension formulation that allows for the tape casting of TiB2 and characterizing the resulting porous ceramics. Among these new potential applications, many require thin ceramics less than 1 mm thick—a result which has been accomplished for other materials via tape casting.4,9 This is a wet route of producing ceramics that provides the ability to tailor the surface chemistry of the particles, giving greater control over the stability of the suspension (TiB2 particles suspended in water) and quality of the end product than is afforded by dry processing routes.10 This also allows for more complex shaping than simple pressing, which ultimately saves costs; by producing the near-net shape in the green body before firing, less machining must be done to the sintered body when it is removed from the high temperature furnace.11 In tape casting, the suspension is spread over a substrate by a doctor blade to the desired thickness. It is known that tape casting tends to align anisotropic particles along the direction of casting due to a nonuniform velocity imparted by the shear force of the doctor blade spreading the suspension, an advantage which can provide directional properties in the final ceramic.9 While this process is well known, it has never been applied to the material TiB2 prior to this work. In this work, a suspension is formulated to allow for the tape casting of TiB2 with minimum organic additive content, which is cost-effective and reduces potential for defects. Porosity and alignment in the tape cast specimens are characterized. For comparison, a highly anisotropic or rod-shaped particle (PCN-222, a metal organic framework material) was included in the TiB2 suspensions for tape casting. This metal organic framework (MOF) has been transformed into a high temperature material after thermal treatment at the sintering temperature of 2000°C, showing that the resulting particle is made of glass-like carbon embedded with zirconium carbide (ZrC) crystallites. This particle could be used as a reinforcement for ultra-high temperature ceramics, and in this work was shown to align strongly in the tape casting direction. At the level of porosity (38%) and alignment (57%) in the TiB2 specimens in this study, porosity dominates the mechanical properties. This relationship is shown to be more complicated than lowering the strength by the same proportion that the density is lowered, and various models for understanding the role of porosity on the elastic modulus are explored.

<p>For many applications within the defense, aerospace, and electricity-producing industries, available material choices for high-performance devices that fulfill necessary requirements are limited. Choosing a metallic material or a ceramic material may be optimal for only some of the required properties. For instance, choosing a metal may optimize ductility but compromise oxidation resistance, yield strength, or creep resistance. Of potential interest, ceramic-metal (cermet) composites can address several fundamental concerns such as high temperature mechanical toughness and stiffness and oxidation/corrosion resistance. However, cost-effective, scalable manufacturing of complex-shaped, high-temperature cermets can be challenging.</p> <p>A cermet of interest is niobium and yttrium oxide, Y2O3. Both materials exhibit high melting points with similar coefficients of thermal expansion. Basic thermodynamic calculations suggest that these materials are chemically compatible, and that Y2O3/Nb cermets may be generated by reactive melt infiltration using the patented Displacive Compensation of Porosity (DCP) process. With the DCP process, a liquid fills a porous perform, and a displacement reaction occurs to produce products of larger solid volume. This reaction yields the cermet of interest, formed in a reduced-stress condition, while maintaining a generally near net shape and high relative density.</p> <p>In order to get to the point of designing cermet components for various applications, a focus of this work has been to create a Y2O3/Nb composite by hot pressing powders at high temperatures at the predicted stoichiometric ratios, and then characterizing the thermal and mechanical properties. The reduction reaction between liquid yttrium and solid niobium (IV) oxide (NbO2) was then characterized to evaluate kinetic mechanisms affecting the reaction rate which is necessary for future DCP-based cermet component manufacturing.</p> <p>Lastly, the mechanical behavior of this cermet was modeled and compared to another cermet processed using liquid metal infiltration using a temperature-dependent elasto-visco-plastic self-consistent model. The effects of cooling from processing temperatures, as well as thermally cycling of these cermets, were quantified. As high temperature experiments can be time intensive with high costs, it is advantageous to have a computationally efficient, desktop design tool to quantify the impacts of changing processing and use conditions on material performance.</p>

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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