The present disclosure generally relates to methods of increasing the deformability of ceramic materials, as a nonlimiting example, titanium dioxide, particularly through the introduction of defects, and to ceramic materials produced by such methods.
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Ceramic materials often offer numerous advantages over metals, such as corrosion resistance, high hardness, wear resistance, high melting temperature and low density, and thus have been widely used as structural materials for various environments. However, the applications of ceramics have frequently been prohibited by their brittle nature at low temperatures. In general, the plastic deformation of crystalline metallic materials proceeds through the activity of dislocations at room temperature (RT), and/or diffusion creep at elevated temperatures. In comparison to metallic materials, ceramics with ionic or covalent bonding are strong but mostly brittle at low temperature, such as room temperatures, due to a lack of dislocations to accommodate plasticity. Fracture toughness of ceramics is much lower than that of deformable metallic materials. The plastic deformation of ceramic materials has been intensively investigated at high temperatures, whereby diffusional creep or grain boundary sliding become significant.
Room-temperature plasticity remains largely absent in most ceramic materials. It has been shown that fracture can be suppressed to some extent in a few ceramics at low temperatures, such as single crystal SrTiO3 and MgO, and polycrystalline ZrO2 (zirconia) and YSZ (yttria-stabilized zirconia). For example, YSZ, which experiences martensitic phase transformations, may exhibit superelasticity at room temperature depending on the doping concentration and its grain size. Another approach to improve plasticity of ceramics is significant grain refinement. It has been demonstrated that plastic deformation of ceramics at low temperature is occasionally possible if their grain sizes are in the range of a few nanometers. However, the sintering of ceramics while maintaining such small grain sizes is challenging.
In order to produce advanced ceramic materials with low porosity, various sintering techniques have been extensively investigated. In general, the conventional sintering typically requires high temperature and long sintering time and thus often leads to significant grain coarsening. With the assistance of an electrical field and pressure, spark plasma sintering has been widely explored to sinter ceramics materials to full density over a short period of time, generally on the order of several minutes. A recently discovered technique, flash sintering, however, can be used to sinter fully densified ceramics within even shorter time, generally within a few seconds, without pressure, and at temperatures much lower than conventional sintering temperature. During flash sintering, a ramp heating process and a moderate electrical field are applied. Once reaching the onset of the “flash temperature,” a densification process occurs instantaneously, followed by a sudden increase in electrical conductivity. In spite of significant interest in flash sintering of ceramics, investigations on the deformability of flash-sintered ceramics remain scarce.
Titanium dioxide (TiO2; titania) has diverse and broad applications, such as solar cells, semiconductors, and water purification, and has been intensively investigated in the past decades The mechanical properties of TiO2 prepared by different techniques have been widely investigated. For instance, rutile TiO2 single crystals of various stoichiometry have been tested at temperatures varying from room temperature to 1300° C. by compression. Prior studies showed that TiO2 was brittle when tested below 600° C., and most specimens fractured before yielding. Plastic deformation becomes feasible in TiO2 only when test temperatures exceed about 600° C.
Thus, there is an ongoing desire to achieve room temperature plasticity in ceramic materials, including but not limited to titanium dioxide.
The present invention provides methods of increasing the deformability of ceramic materials, as a nonlimiting example, titanium dioxide, particularly through the introduction of defects, and to ceramic materials produced by such methods.
According to one aspect of the invention, a method of increasing the deformability of a ceramic material comprises introducing high-density pre-existing defects and oxygen vacancies in the ceramic material during a flash sintering process and then forming nano scale stacking faults and nanotwins in the ceramic material.
According to another aspect of the invention, a ceramic material is provided with deformability in a temperature range of room temperature to 600° C. The ceramic material contains high-density pre-existing defects and oxygen vacancies and nano scale stacking faults and nanotwins.
Technical effects of methods and ceramic materials as described above preferably include the ability to significantly increase deformation plasticity in a range of ceramic materials, including but not limited to TiO2.
Other aspects and advantages of this invention will be appreciated from the following detailed description.
Some of the drawings shown herein may include dimensions. Further, some of the drawings may have been created from scaled drawings or from photographs that are scalable. It is understood that such dimensions or the relative scaling within the drawings are by way of example, and are not to be construed as limiting. It should be recognized that all the figures shown are not to scale.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.
Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures. A majority of ceramics fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. This disclosure describes deformability of TiO2 prepared by a flash sintering technique. The in situ studies leading to this disclosure show that flash-sintered TiO2 can be compressed to about 10% strain under room temperature without noticeable crack formation. The room temperature plasticity exhibited by the flash-sintered TiO2 reported below was attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density pre-existing defects and oxygen vacancies introduced by the flash sintering process. Distinct deformation behaviors were observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600° C. Potential mechanisms that may render ductile ceramic materials are discussed.
In investigations reported below, it was observed that by using in situ micropillar compression tests performed inside a scanning electron microscope, prominent room temperature plasticity was observed in flash-sintered TiO2 with an average grain size of about 10 μm. In contrast to conventionally sintered TiO2, flash-sintered TiO2 inherits a high density of stacking faults and dislocations introduced during the non-equilibrium sintering process. Compared to the previously reported low-temperature plastic deformation in flash-sintered 3YSZ (zirconia stabilized by 3% yttria (Y2O3)), where phase-transformation-induced plastic deformation and dislocation-assisted plastic deformation were observed in the testing temperatures below and above 400° C., respectively, flash-sintered TiO2 in the investigations presented no phase transformation and its obvious plastic deformation phenomena in the testing temperatures ranging from room temperature to 600° C. were correlated with abundant preexisting defects, such as stacking faults, nanotwins, and dislocations. The investigations suggested that flash sintering may be an effective approach to significantly improve the low-temperature plasticity of ceramic materials in general.
Methods and materials employed in the investigations are described below. Experimental results and a discussion of several aspects of the invention are then described.
Flash Sintering: Rutile TiO2 powder (Inframat Advanced Materials, Product #22N-0814R, 50±20 nm particle size) was pressed uniaxially to make cylindrical green bodies. The dimensions of the green bodies were 6 mm diameter by 6±0.5 mm height with 40 to 45% density. A pressure of 10 kPa was used to maintain consistent electrical contact between the green body and electrodes. Specimens were heated at a rate of 10° C./min to a pre-flash temperature of 900° C. An electric field of 60 V/cm was applied across the sample faces, which resulted in a small current passing through the specimen which rose gradually with time. Once the pre-flash temperature was reached, the electric field was applied across the samples faces. The rise in conductivity of the sample leads to a non-linear rise in the current until it reached the limit set of 1.5 A/cm2 in the feedback loop. The power supply was switched from voltage control to current control and held constant for 1 minute. The samples were then cooled down to room temperature at a rate of 25° C./min without any applied electric field.
Microstructure characterization by TEM: Plan-view TEM samples of flash-sintered TiO2 were prepared through a conventional approach, which included manual grinding, polishing, dimpling and final polishing in an Ar ion milling system (PIPS II, Gatan). Low energy Ar ion polishing (2 kV) was used to minimize ion milling-induced damage. An FEI Talos 200X TEM/STEM microscope with ChemiSTEM technology (X-FEG and SuperX EDS with four silicon drift detectors) operated at 200 kV was used in this study for microstructure characterization and energy-dispersive X-ray spectroscopy (EDS) chemical mapping. In addition, high-resolution scanning transmission electron microscopy (HRSTEM) images were obtained using a modified FEI Titan STEM TEAM 0.5 with a convergence semi-angle of 17 mrad operated at 300 kV at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory. The integrated differential phase contrast (iDPC) images were obtained using an FEI Themis Z with a Schottky electron emitter, an electron energy monochromator, and a 5th order probe spherical aberration corrector operated at 300 kV at the Materials Research Lab (MRL) at the University of Illinois, Urbana-Champaign.
Microcompression test: Micropillars of flash-sintered TiO2 of about 3 μm in diameter, and a diameter-to-height aspect ratio of about 1:2 were prepared using focused ion beam (FEI quanta 3D FEG) and a series of concentric annular milling and polishing with progressively de-escalated currents were adopted to reduce the tapering angle. Micropillar compression experiments were performed using a Hysitron PI 87 R PicoIndenter equipped with a piezoelectric actuator on the capacitive transducer that enabled the collection of force-displacement data inside a scanning electron microscope (FEI quanta 3D FEG). Moreover, a 20 nm diamond flat punch tip designed for high-temperature compression experiments was used to conduct in situ compression experiments, and the geometric variation of micropillars was synchronized to evolving force-displacement curve. For high-temperature in situ compression setups, the flat punch tip was fastened to a probe heater, and the specimens were clamped by a V-shaped molybdenum clamp to a ceramic heating stage. The temperature on two heating terminals was simultaneously ramped up at a rate of 10° C./min and isothermally preserved for 30 minutes before implementing every single compression experiment to eliminate the thermal-driven drifts on both probe and stage sides. An average drift rate of less than 0.5 nm/s was estimated in the preloading process for 45 seconds, and the estimated force noise level was less than 8 μN prior to compression. The specimen displacement during the compression test was systematically measured and corrected during in situ SEM studies.
Atomistic Simulations: Atomistic simulations used density functional theory with the Perdew-Burke-Enzerhoff (PBE) exchange correlation functional, as implemented in the VASP software version 5.4.4 with a projector-augmented-wave (PAW) basis. The plane wave cutoff was set to 450 eV, and PAWs with 6 and 4 electrons in valence for 0 and Ti, respectively, were used. Simulations of stacking faults without point defects used a 1×1×8 supercell of a [100]×[011]×[01
Stress-strain behaviors of conventional and flash-sintered TiO2 obtained from in situ microcompression studies from room temperature to 600° C. at a constant strain rate of 5×10-3 s-1 are shown in
In contrast, the flash-sintered TiO2 tested at room temperature (
When the test temperature was 200° C., a mixture of small serrations and large load-drops were observed in the stress-strain curves. The maximum flow stress was about 1.5 GPa. At a strain of about 4%, a shear band emerged. Interestingly, the shear band broadened or propagated as indicated by yellow arrows. For the pillars tested at 600° C., the maximum flow stress was further decreased to about 1 GPa, after a few percents of strain. In addition, as compared to room temperature and 400° C., no obvious stress serrations were observed. SEM snapshots showed a relatively smooth surface without noticeable slip bands in the deformed pillars. These pillars deformed in a ductile manner without indication of fracture.
To investigate the influence of deformation on the evolution of microstructure, post-deformation TEM analyses were performed on the flash-sintered TiO2 pillars tested to a strain of 8 to 10%. As shown in
The pillars compressed at 400° C. had microstructures drastically different from those tested at room temperature. Although
For pillars compressed at 600° C., no shear bands were observed (
Preexisting defects in flash-sintered TiO2: Ceramic materials often have a low dislocation density due to the nature of their ionic and covalent bonds. Dislocations appear in ceramics during high-temperature deformations. However, a high density of dislocations and stacking faults were observed in flash-sintered TiO2 before any deformation (
To examine point defects in flash-sintered specimens, atomic-resolution TEM experiments were conducted to investigate the flash-sintered TiO2.
Defect-assisted room-temperature plasticity in flash-sintered TiO2: Most ceramics, including TiO2, are brittle especially when tested at low temperatures. Few ceramics have shown limited deformability at ambient temperatures, such as single crystal SrTiO3 and MgO, and polycrystalline ZrO2 and YSZ. MgO can be plastically deformed at room temperature as dislocations in MgO are mobile at relatively low stresses. ZrO2 exhibits superelasticity because of the martensitic phase transformation. Prior studies show that bulk TiO2 has no plasticity unless deformed above 600° C.
The micropillar compression studies of this disclosure showed that the zero-field and the conventional sintered TiO2 fractured at an average strain of about 2% at room temperature, and at about 3% strain when tested at 400° C. (
In ductile metallic materials, plasticity is accommodated by the nucleation and migration of dislocations. As the lattice friction stress is low in metallic materials, dislocations can be highly mobile and improve the plasticity of metallic materials. However, the corresponding friction stress in ceramic materials is typically high due to the strong directional interatomic bonding. What makes the situation worse is the resolved shear stress required for dislocation nucleation in ceramics is exceptionally high, on the order of the theoretical shear strength for perfect crystals under athermal conditions. Hence the high friction stress and resistance to dislocation nucleation lead to the brittle behavior in most ceramics at low to intermediate temperatures. Nanotwinned metals have shown high strength and plasticity as twin boundaries can significantly increase the work hardening ability of metals. However twin boundaries are generally absent in ceramic materials, and twin boundary or stacking faults dominated plasticity is scarce in ceramics. In comparison, flash-sintered TiO2 already contains abundant pre-existing defects, such as dislocations and short segments of stacking faults, which may foster plastic deformation at room temperature. Moreover, O vacancies and the pre-existing defects could also facilitate the nucleation and propagation of defects, such as the high-density of stacking faults and nanotwins formed during deformation at room temperature.
The grain size and porosity also affected the deformability of TiO2. The grain size of flash-sintered and conventional sintered TiO2 was about 10 and about 50 respectively, and about 1 μm for zero-field sintered TiO2. It was likely that the micropillars (diameters of about 3 μm) fabricated from the flash-sintered and conventional sintered TiO2 specimens contained one or few grains, while the pillars from zero-field sintered TiO2 contained multiple grains. Although smaller grain size often improves the plasticity of ceramics, the flash-sintered TiO2 with large grains has significant plasticity at room temperature compared to the brittle fracture of zero-field sintered TiO2. It is also worth mentioning that the existence of nano/micropores inside the materials may also have affected the mechanical behavior under compression. The pores inside conventional sintered and zero-field sintered TiO2 were dominated by intergranular pores, and the shapes of these pores were usually triangular or irregular. In comparison, the pores inside flash-sintered TiO2 were usually intragranular pores, and their shapes were either circular or faceted as shown in
DFT calculations showed that the stacking fault energy of TiO2 was low (comparable to some metals like Ag and Cu), 30-40 mJ/m2, but the unstable stacking fault energy, which gives an approximation of the barrier to dislocation nucleation and motion, was very high, about 1.9 J/m2, as shown in
Temperature-dependent deformation behavior in flash-sintered TiO2: In general, conventional sintered TiO2 will fracture before yielding at room temperature, and hence no yield strength values were reported for TiO2 at room temperature. The yield stress of TiO2 tested at 500° C. may be about 800 MPa based on a prior high-temperature test of TiO2 single crystal specimen. It has also been reported that the flow stress of TiO2 was sensitive to test temperature. At high temperatures (greater than 600° C.), the primary slip systems in TiO2 were
The difference between the continuously serrated stress flows at room temperature (
At 600° C., no slip bands were observed in flash-sintered TiO2, and the stress-strain curves appeared smooth with minor serrations. Post-compression TEM studies showed that a high density of dislocations and stacking fault segments formed after compression at 600° C. (
It has been shown that conventional TiO2 can sustain certain plastic strain when deformed above 600° C. Post compression TEM studies of conventional TiO2 in the current study showed that there were few scattered stacking faults and twin boundaries accompanied by numerous intragranular cracks.
It should be noted that although flash-sintered TiO2 contains a small number of preexisting stacking faults, deformation at room temperature induces a significant increase of stacking fault density separated with a spacing of a few nanometers. A majority of these stacking faults may arise from the migration of partials from preexisting dislocations. As the density of deformation induced stacking faults is high, the stress-strain curve appears relatively smooth with small serrations. In addition, the effect of crystal orientation on the mechanical behavior of flash-sintered TiO2 in this study shall be briefly discussed. In general, the mechanical behavior of ceramics was largely determined by the crystal orientation. In this study, as the average grain size of the flash-sintered TiO2 was about 5 μm, most flash-sintered pillars have single-crystal or bicrystal-like nature, and these pillars may have different crystal orientation depending on the position at which the pillars were made from the flash-sintered specimens. One may expect that the reproducibility of stress-strain curves should be poor in this study if the crystal orientation plays a significant role in the plasticity of pillars. However, the reproducibility was in general good at all test temperatures. Such a phenomenon suggested that crystal orientation plays a relatively insignificant role in determining the mechanical behavior of flash-sintered TiO2. The high-density of pre-existing defects may have a major impact on the mechanical behavior of the flash-sintered TiO2. In addition, residual stress due to orientation dependent coefficient of thermal expansion (CTE) mismatch or defect annihilation if occurred during the pillar fabrication may add some uncertainty on the reproducibility of micropillar compression studies. However, given the comparable large grain size in both conventional and flash-sintered TiO2, the difference of residual stress between the two types of specimens may be insignificant. Also, the reasonable reproducibility of stress-strain curves for TiO2 pillars tested at various temperatures suggested that the microstructures developed in flash-sintered TiO2 may be similar in these pillars, containing high-density defects. For instance, preexisting defects can be readily identified in the lower (largely undeformed) portion of the pillar in
It should also be noted that non-uniform grain and pore size distributions exist in the current flash-sintered samples, preventing reliable macroscopic mechanical testing. Continuous efforts have been devoted to explore the flash-sintering mechanisms and to achieve better control on the sample uniformity and quality. Promising results have been demonstrated by AC field sintering, controlled current ramping, SPS-flash combined process, and others. Given the potentials in microstructure and defect density control offered by the flash sintering process, flash-sintered ceramics hold great potentials in demonstrating improved ductility in macroscopic specimens under moderate temperatures or room temperature, compared to conventional sintered samples.
Based on the investigations reported above, in situ micropillar compression tests showed that flash-sintered TiO2 exhibits unexpected significant plasticity at room temperature, sustaining 10% strain without noticeable cracks. The high density of pre-existing defects and O vacancies introduced during the non-equilibrium flash sintering process facilitated the nucleation of dislocations. Room temperature deformation was dominated by the formation of nanoscale stacking faults, followed by the creation of nanotwins when tested at 200-400° C. Dislocation glide takes over the deformation mechanism at about 600° C. The investigations strongly suggested that the flash sintering method of this disclosure presents great potential in promoting deformation plasticity in a range of ceramic materials.
Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described, as other implementations may be possible. Therefore, the scope of the invention is to be limited only by the following claims.
This application claims the benefit of U.S. Provisional Application Nos. 62/815,738 filed Mar. 8, 2019, and 62/843,406 filed May 4, 2019. The contents of these prior patent applications are incorporated herein by reference.
This invention was made with government support under Contract No. N00014-17-1-2087 and N0014-16-2778 awarded by Office of Naval Research. The government has certain rights in the invention.
Number | Date | Country | |
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62815738 | Mar 2019 | US | |
62843406 | May 2019 | US |