As the landscape of warfare changes, the US Army must shift its approach to protection strategies to maintain its warfare dominance. New technological advances in manufacturing have enabled the dismounted Soldier to tailor their body armor material needs to the demand of the environment. Traditional manufacturing of ceramics present limitations of long lead times, fabrication of complex geometries, and cost-expensive components. The additive manufacturing of ceramics offers engineering-grade ceramic components in approximately 90% less time than traditional ceramics. Typical turnaround can be as little as five days depending on complexity of the part. This not only allows for faster time to market, but also allows for more iterations during the design process resulting in a better end product. Additionally, 3-D printed parts can be fabricated with a higher degree of complexity for weight reduction while saving on the cost of the part because of the reduction in material used. Over the last few years, the US Army Research Laboratory investigated the integrity of 3- D printed ceramics for protection systems. This paper discusses the technical challenges that were discovered and provides opportunities to the ceramic community to create robust solutions.
The effects of control mode and test rate on the measured fracture toughness of ceramics were evaluated by using chevron-notched flexure specimens in accordance with ASTM C1421. The use of stroke control gave consistent results with about 2% (statistically insignificant) variation in measured fracture toughness for a very wide range of rates (0.005 to 0.5 mm/min). Use of strain or crack mouth opening displacement (CMOD) control gave similar to 5% (statistically significant) variation over a very wide range of rates (1 to 80 mu m/m/s), with the measurements being a function of rate. However, the rate effect was eliminated by use of dry nitrogen, implying a stress corrosion effect rather than a stability effect. With the use of a nitrogen environment during strain controlled tests, fracture toughness values were within about 1% over a wide range of rates (1 to 80 mu m/m/s). CMOD or strain control did allow stable crack extension well past maximum force, and thus is preferred for energy calculations. The effort is being used to confirm recommendations in ASTM Test Method C1421 on fracture toughness measurement.
Fracture toughness is a critical structural design parameter and an excellent metric to rank structural materials. It determines fracture strength by way of the flaws, both inherent and induced, and defines the endpoint of the slow crack growth (SCG) curve. The fracture toughnesses of glasses and structural and optical ceramics as measured by several techniques is compared. When good metrology is employed, the results are very comparable with two exceptions: materials exhibiting crack growth resistance (R-curve) and those with low SCG exponents. For materials exhibiting R-curves, the result is a function of extension and can be minimized with short cracks. For materials with low SCG exponents, such as glasses, elimination of the corrosive media and/or increasing the stress intensity rate produces consistent results. A summary of values is given for optical materials and glasses.
Glass-free gas seals for high temperature operation are attractive for SOFC systems because they enable us to shorten set-up time of SOFC systems and to obtain excellent thermal cycle durability. Mica is usually a candidate for SOFC compression seals. However, it requires relatively large compression loads, and it easily brings about gas leakage through the interface between swelled or rough substrate and the seals. We thus investigated vermiculite/talc composite seals to overcome the interfacial gas leakage issues. Vermiculite irreversibly expands in c-axis direction by firing at high temperatures and calcined vermiculite can be expected to show exceptional compression properties. Vermiculite calcined at moderate temperatures would expand further while SOFC cells or modules are heated at a working temperature. This self-expansion would settle the interfacial leakage issues. The expansion rate of the composite seals estimated by heating from room temperature to 700°C much depended on calcination temperature and particle size of vermiculite. The seals showed anisotropic expansion behavior. The vertical expansion of the seals was 0.4-10% being dependent on the pre-treatment of vermiculite whereas the horizontal was less than 1.7%. An adequate vertical self-expansion would decrease the interfacial gas leakage.
Corrosion and chromium evaporation from commonly used metallic interconnects in humidified oxidizing environment during SOFC operation remains an issue. We have studied the role of pretreatment of AISI 430 alloy with or without nickel coating on the oxidation resistance and formation of interfacial compounds at 650-800 degrees C in moisture-saturated air. It is found that electro deposited nickel coated AISI 430 alloy pretreated in dry hydrogen (Ar-3% H2) improves the oxidation resistance but not pre-oxidized alloy. The underlying mechanisms for improved oxidation resistance are discussed using structural and microstructural analysis.
Boron carbide is a ceramic of great interest due to its high strength, low density, high hardness, and of course, its tendency to undergo amorphization during failure. Commercial boron carbide commonly contains a variety of defects including pores, carbon inclusions, and aluminum nitride and boron nitride inclusions, but what role do the defects play? Certainly they can be fracture initiation sites, but do they influence the formation of amorphous material and do the nitride phases undergo phase changes which influence the failure process? In order to begin to address these questions, we have performed high resolution XCT (X-ray computed tomography) scans to quantify the internal defect structure of samples of commercial boron carbide. Select portions of this data were converted into a form where mesoscale simulations of shock wave propagation could be conducted to explore the role that defects play in the failure process. These simulations provide insight into the effect of these defects on the local stress state during shock wave propagation and their role in contributing to or diminishing the formation of amorphous material.
The behaviour of ceramics, and more generally of brittle materials, depends strongly upon the microstructural features that determine the initiation and propagation of cracks. The difficulties in the direct observation of crack initiation propagation at the microscopic level, especially under dynamic loading conditions, as well as the complexity in the manufacturing materials with defined features, hinders the research of the microstructural configuration with increased performance under specific loading conditions. Numerical modelling methods contributed to increase the understanding of some of the mechanisms involved in the deformation and failure of ceramic materials at the grain level, but the currently available computational power has hindered the direct application of micromechanical models to macroscopic structures. The adoption of multiscale methods, however, allows to overcome the computational limits by only refining the volumes of the domain of interest. The method presented in this paper, in particular, allows to refine on-the-fly parts of macroscopic domain with representative polycrystalline microstructures discretised with hexahedral elements of arbitrary size, thus allowing for the adaptive concurrent simulation of large structures under dynamic loading conditions at the grain scale.
National Institute of Advanced Industrial Science and Technology (AIST), Japan is developing anode-supported microtubular solid oxide fuel cells (SOFCs) since 2005. We found that the nickel-gadolinia-doped ceria (Ni-GDC) has high durability against carbon deposition during butane utilization. In this paper, microtubular SOFC stacks were developed, and prototype portable SOFC systems were demonstrated using commercially available LPG cartridges. In the future, portable SOFC systems will be expected as power sources for mobile applications such as electric vehicles, robots and drones.
3C-SiC, 6H-SiC and a-Si phases were observed in a reaction bonded SiC/Si composite (RBSC). Microstructural characterization indicates that Si phases form a network surrounding SiC particles and there are residual a-Si distributed at the vicinity of the grain boundaries in addition to single crystalline Si. Desired nano-scale tensile specimens of SiC-Si were successfully prepared by Ga focused ion beam (FIB) and a homebuilt in-situ tensile test device was accommodated inside an FE-SEM. Interfacial mechanical properties of RBSC were for the first time measured and the average SiC-Si interfacial strength was found to be 1588 MPa, consistent with the strength of amorphous Si. Localized amorphous interfacial structure was further confirmed by HRTEM.
The erosion behavior of ceramic matrix composites (CMCs) is markedly different from traditional monolithic ceramic materials due to their unique architectural configurations. As CMCs are further implemented into gas turbine engines the need to characterize their erosion behavior arises as these materials can be subject to erosive environments. The erosion behavior of an N720/alumina CMC was characterized at ambient temperature as a function of particle type, size, and velocity. The erosion damage was quantified with respect to the erosion rate, and the damage was characterized via SEM and optical microscopy along with 3D image mapping. Post-erosion residual strength testing was performed in flexure to quantify the severity of the overall erosion damage.
Solid-state batteries with non-flammable inorganic solid electrolytes are one candidate to replace the currently-used liquid electrolyte. The inorganic ceramic solid electrolyte with garnet structure Li7La3Zr2O12, known as LLZO, has been developed, which has several superior advantages, such as high ionic conductivity (10(-4)similar to 10(-3)S cm(-1)) at room temperature, wide electrochemical window (similar to 6V), good chemical stability against Li metal anode. However, the total resistance of thick LLZO solid electrolyte is still too large to be applied in the industry. In order to reduce the resistance and improve the power density of LLZO solid electrolyte, ceramic membranes with micron thickness should be prepared. The sol-gel method was adopted to synthesize LLZO ultrafine precursor powders and tape-casting method is proposed to prepare LLZO green film with excellent machinability. DTA-TG was used to test thermal properties. The crystal structure, microstructure and electrochemical property of LLZO membrane electrolyte were investigated by XRD, SEM and EIS. The use of Plasma activated sintering (PAS) technology allows rapid densification of the ceramic membranes. The thickness of the ceramic membrane is about 130 mu m and its conductivity is 3.38x10(-5)S/cm at room temperature.
Instrumented indentation testing provides the means to measure many mechanical properties and characteristics of materials. One such mechanical property that can be ascertained from indentation testing is a material's yield stress, the stress corresponding to the onset of permanent deformation. In brittle materials such as ceramics and glasses, traditional testing methods fall short of precisely establishing the yield stress. A new technique to estimate the yield stress is under development and described here. It utilizes the sensing and interpreting of the initiation of a residual surface impression through the change of the instantaneous contactstiffness(3)/load (S-3/P) as measured by load and depth-sensing indentation with spherical indenters. Several brittle materials (borosilicate, soda-lime silicate, and bulk metallic glass) are evaluated, and the test method and manner of interpreting the yield-like response through S-3/P are described.
A series of experiments and simulations were conducted to investigate the effects of hardness and toughness of ceramic on the ballistic performance of ceramic armour modules. The experiment was conducted using ceramics from 3M technical ceramic, using the tiles Grade T, T+, F and F+ with dimensions of 100 mm X 100 mm X 20 mm. These ceramic tiles were assessed using a simplified ceramic armour module consisting of the ceramic tile sandwiched between a cover plate and a backing plate. The test samples were made of either single tile module or multiple tile modules. They were tested against tungsten alloy long rod projectiles at a nominal velocity of 1250 m/s. Experimental results showed that ballistic performance of the ceramic correlated with hardness but not fracture toughness, based on mass efficiency. Fracture toughness correlated instead with the damage radius in the ceramic. Simulation using LS-DYNA revealed that improved ballistic performance of the ceramic tiles was due to long dwell time. High fracture toughness of ceramics could suppress damage by tensile stress wave generated.
The electrochemical reaction efficiency for an SOFC is determined mainly at the interfaces between electrodes and electrolyte. The cell performance is influenced by a variety of parameters, such as the particle size, porosity, and microstructure. Microstructural tuning in the electrodes hence provides the performance elevation by increase the effective interfacial reaction sites. In this study, we use traditional materials to form the cells constructed with NiO-YSZ (Yttria stabilized zirconia) as anode supported substrate, YSZ as electrolyte, and YSZ-La0.8Sr0.2Mn0.3 (LSM) vertical bar LSM as functional and cathode layers. In the anode, two different cermet structures with powder refinement approach have been adopted to increase the oxygen exchange rate. On the other hand, introducing LSM-YSZ composite as functional layer is essential for the limited ion conductivity of LSM, which will be beneficial for power output. The cell performance elevation and microstructure analysis allows us to verify functions of sub-micron cermet at anode during the cell operation. The composition effect in cathode may also favor the enhancement. Three typical cells were prepared, and the maximum power density is 280 mWcm(-2) for cell with original materials used. The power density increases to 395 mWcm(-2) for the cell with cermet particle refinement. It further improves to 520 mWcm(-2) for cell with cathode functional layer modification. The small grain size of several tens nano-meter structure in the interfaces exists in a local region near interfaces. It is evidenced that the nano-sized electrode materials which may be attributed to the high current loading can increase the triple phase boundary (TPB) density.
Solid oxide cells (SOC) are electrochemical devices that can operate efficiently both in fuel cell (solid oxide fuel cell, SOFC) and electrolysis mode (solid oxide electrolysis cell, SOEC). However, long-term performance degradation hinder the widespread commercialization of this technology. Nickel coarsening is a major cause of the decrease of the cells' performance. Therefore, investigating and quantifying effects of nickel coarsening on the microstructural evolution in SOCs is crucial to understand the degradation processes occurring during operation. Focused-ion-beam scanning electron microscopy (FIB-SEM) tomography and phase field (PF) modelling are used to investigate the microstructure evolution of Ni/Yttria-Stabilized Zirconia (YSZ) SOC fuel electrodes. A cell, tested as part of a 25 cells stack for 9000 hours, and a reference cell (never operated) are reconstructed using FIB-SEM tomography. Microstructural parameters were calculated on the two cells showing that the percolated triple phase boundary (TPB) length in the cell decreases from 1.85 mu m/mu m(3) for the reference cell to only 1.01 mu m/mu m(3) for the long-term tested one. Phase field simulations were run on the reference cell geometry and microstructural parameters such as particle size distribution (PSD), TPB length and surface areas are computed and quantified on the simulated volumes. A trend of decreasing percolated TPB length with time is observed in the simulations. The numerical results are used to investigate the effects of nickel coarsening as well as to obtain information on the kinetics of the phenomenon.
As for the fuel cladding in the light-water reactor, silicon carbide fiber reinforced silicon carbide composite (SiC/SiC composite) is one of the promising candidates as a replacement of zircaloy due to many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the caulking method was employed as the method for sealing the end cap of SiC/SiC composite tube by zircaloy tube where the titanium micro-powder was inserted between two tubes. The diode or fiber laser was circumferentially irradiated on the outer surface of zircaloy tube, and the insert method of titanium powder was varied by changing the cutting method of SiC/SiC composite tube. The results using the diode laser irradiations with the screw cutting indicated that the wider width of irradiation line seems to be inadequate because the shrinkage of zircaloy would break the thread of SiC/SiC composite. In addition, the examinations about the slit shape effect for the flat cutting suggested that the hook slit is considered to be the best cutting method for holding the titanium powder during the laser irradiation where the width of slit should be narrower than that of laser irradiation line.
A single-step catalyst-free CVD technique has been used for producing an array of mullite nanofibers covered by several layers of graphene-like nanostructures. The hybrid fibers have been then exploited as electroconductive fillers to alumina and yttria stabilized zirconia matrixes. The electrically conductive additives do not substantially change the mechanical properties. However, the resistivity of the composites undergoes a considerable drop turning the dielectric oxides into conductive composites by addition of 2 vol% of fillers which corresponds to 0.2 and 0.38 vol% of carbon content in alumina and zirconia, respectively. Three-dimensional Monte Carlo simulation of systems, exploiting the critical path based tunnelling-percolation model, gives a tunnelling length-scale of 2. 23 nm for the materials under consideration.
A Density Functional Theory (DFT) model for the (0001) oriented boron suboxide surface is developed. The effect of surface termination (boron- vs oxygen-terminated) and icosahedral stacking (alpha-ABCABC vs beta-ABAB) on the structural and electronic reconstructions of the surface were determined. Icosahedral stacking sequence did not change the electronic structure of the surface and therefore both alpha and beta boron suboxide phases are expected to behave chemically similar. The adsorption of yttrium atoms is highly favorable for both oxygen and boron terminated surfaces with a 0.57 eV preference for the oxygen termination. Three unique adsorption sites for yttrium atoms were found. Site 1 is a bridging site over the interstitial site between three surface icosahedra on a boron terminated surface. Site 2 is a bridging site between two surface icosahedra on an oxygen terminated surface. Site 3 is tetrahedrally bonded to a surface icosahedra on an oxygen terminated surface. Using HAADF-STEM imaging, we were able to experimentally identify yttrium occupying a site 2 configuration along a (0001) oriented grain boundary interface in hot-pressed boron suboxide which had been doped with Y2O3:SiO2 powder. The spacing between yttrium atoms along the grain boundary as viewed in projection was 4.74 angstrom which compared well to the DFT prediction of 4.67 angstrom.
In SOFC stacks the electrical contact between the ceramic cathode layer and the metallic interconnector (IC) has to be adjusted carefully to minimize the contact resistance and slow down degradation mechanisms. In JULICH, ICs made of Crofer22APU are coated with a MnCo1.9Fe0.1O4 spinel (MCF), applied by atmospheric plasma spraying (APS), which successfully prevents the diffusion of volatile Cr species from the steel into the cell. Materials for the cathode contact layer have to be sufficiently good electronic conductors, stable in oxidizing atmosphere, chemically stable and compatible with the adjacent layers, and show a thermal expansion behavior similar to MCF and the cathode material, i. e. La0.58Sr0.4Co0.2Fe0.8O3-x(LSCF). To obtain an optimal contact layer, different aspects such as material properties, microstructure, and processing have to be considered. Here we compare various materials regarding their electrical conductivity and compatibility with each other and discuss different processing routes for the application of the cathode contact layer. DC conductivity measurements and scanning electron microscopy (SEM) imaging give information on the correlation of material properties and microstructure. We conclude that an LSCF contact layer with coarse porosity on top of the fine structured LSCF cathode is the most suitable contacting for JULICH SOC stacks.
Bay0.2Ce0.7Zr0.1O3-δ(BYCZ) is a promising candidate as electrolyte for a proton-conducting SOFC (P-SOFC). In this work, the ceramic powder of BYCZ was fabricated by using glycine nitrate combustion (GNC) process. Characterizations of the powders were executed by X-ray diffractometer (XRD), field-emission scanning electron microscopy (FE-SEM), thermal expansion coefficients (TECs) measurement as well as the surface area analyses by Brunauer-Emmett-Teller (BET) method. The results reveal that the single phase of perovskite structure of BYCZ powder occurs at calcination temperature of 1550 °C and the average value of the thermal expansion coefficient (TEC) is 12.6 × 10-6 K-1 at the temperature range of room temperature to 700 °C. In addition, the relative density of BYCZ powder can be achieved near 100% at 1550°C. The result indicates that the crystal phase of BYCZ is cubic perovskite structure at the calcination temperature of 1550°C. Further application on the BYCZ-based P-SOFC cell performance evaluation will be conducted.
