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At least 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Advanced polymer impregnation and pyrolysis (PIP) of SiOC-based ceramic matrix composites (CMCs)

Silicon carbide (SiC) ceramic matrix composites (CMCs) are valued for their high-temperature properties, making them ideal for harsh environments. However, conventional polymer impregnation and pyrolysis (C-PIP) often result in porous composites and require numerous cycles for densification. This study introduces the advanced PIP (A-PIP) method using a crosslinked polycarbosiloxane (PCS) precursor to enhance densification efficiency. Five SiC CMCs were fabricated using C-PIP and A-PIP, with variations in fiber reinforcement and fiber coatings. A-PIP achieved up to 19 % higher density and 78 % lower porosity in 79–86 % less processing time compared to C-PIP. Moreover, SiC-SiOC composites with boron nitride-coated fibers showed significant improvements in tensile strength (11 MPa to 134 MPa) and strain at maximum strength (0.01–0.11 %), underscoring the role of weak fiber-matrix interfaces. In conclusion, these results demonstrate A-PIP's potential to produce dense, low-porosity SiC CMCs more efficiently, significantly reducing manufacturing time and costs.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Siloxane-modified polycarbosilane flexible Prepregs for fabrication of ceramic matrix composites via compression molding and PIP densification

For this work, the development of ceramic matrix composites (CMCs) using 5 harness satin carbon fiber fabric impregnated with commercial polycarbosilane precursor plasticized by siloxane copolymer was investigated aiming to improve wetting behavior and shape conformability. The polycarbosilane precursor and polysiloxane plasticizer were mixed at varying weight ratios to obtain flexible preceramic resin prepreg cloths. 13 plies of preceramic polymer prepreg cloths were stacked in 0/90° layup and cured by compression molding, followed by densified via PIP process. The CMCs were densified with eight PIP cycles followed by final crystallization at 1600 °C. The CMCs made with 10 wt% polysiloxane loading showed lower viscosity of the prepreg resin as well as higher strength and displacement to failure compared to those fabricated from unplasticized polycarbosilane prepolymer. In contrast, at higher concentration of plasticizer, viscosity of the prepreg resin increased, and the CMC became more brittle; however, it exhibited considerably higher thermal conductivity.

Carbosilane↗
DOE OSTI2014

Development of iron phosphate ceramic waste form to immobilize radioactive waste solution

The objective of this research was to develop an iron phosphate ceramic (IPC) waste form using converter slag obtained as a by-product of the steel industry as a source of iron instead of conventional iron oxide. Both synthetic off-gas scrubber solution containing technetium-99 (or Re as a surrogate) and LiCl-KCl eutectic salt, a final waste solution from pyrochemical processing of spent nuclear fuel, were used as radioactive waste streams. The IPC waste form was characterized for compressive strength, reduction capacity, chemical durability, and contaminant leachability. Compressive strengths of the IPC waste form prepared with different types of waste solutions were 16 MPa and 19 MPa for LiCl-KCl eutectic salt and the off-gas scrubber simulant, respectively, which meet the minimum compressive strength of 3.45 MPa (500 psi) for waste forms to be accepted into the radioactive waste repository. The reduction capacity of converter slag, a main dry ingredient used to prepare the IPC waste form, was 4,136 meq/kg by the Ce(IV) method, which is much higher than those of the conventional Fe oxides used for the IPC waste form and the blast furnace slag materials. Average leachability indexes of Tc, Li, and K for the IPC waste form were higher than 6.0, and the IPC waste form demonstrated stable durability even after 63-day leaching. In addition, the Toxicity Characteristic Leach Procedure measurements of converter slag and the IPC waste form with LiCl-KCl eutectic salt met the universal treatment standard of the leachability limit for metals regulated by the Resource Conservation and Recovery Act. This study confirms the possibility of development of the IPC waste form using converter slag, showing its immobilization capability for radionuclides in both LiCl-KCl eutectic salt and off-gas scrubber solutions with significant cost savings.

converter slag↗
DOE OSTI2022

Irradiation-induced chemical disordering in ceramics: The case of SiC

Chemical disordering is one feature of damage that occurs in irradiation environments and is common in ceramic compounds. This paper reviews irradiation-induced chemical disordering in SiC-based materials at elevated temperatures. Here the results were obtained using advanced analytical tools to characterize atomistic defects and to evaluate the chemistry of defect clusters and homonuclear bond structures. Finally, this paper demonstrates that chemical disordering is crucial to understanding microstructural stability and establishing a mechanistic modeling of microstructural evolution of irradiated SiC.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Layer-structured triple-conducting electrocatalyst for water-splitting in protonic ceramic electrolysis cells: Conductivities vs. activity

Electron, proton and oxygen-triple-conducting materials are becoming the dominant steam electrode candidate to break the rate limit on the water-splitting reaction that throttles the performance of protonic ceramic electrolysis cells (PCECs). In this study, based on Pr 2 NiO 4+δ Ruddlesden-Popper phase, we manipulate these conductivities by Pr-site Ba substitution to probe the correlation of each conductivity with the kinetics of the elementary reaction steps. It is found that the proton conductivity is vital to sustain an extended active surface area for faster adsorption of reactants and desorption of products. The effect of oxygen conductivity is surprisingly found insignificant in the water-splitting reaction. On the contrary, surface oxygen removal is discovered as the most rate-limiting process. The electronic conductivity is not a direct limiting factor. However, an electron transfer process between the current collector and the electrode junction could introduce extra resistance that is perceptible at a high operating temperature range. The best water-splitting activity is obtained on a proton conductivity/oxygen surface desorption capability well-balanced sample after Ba substitution. As a result, a water-splitting reaction resistance of 0.022 Ωcm 2 , a current density of 1.96 A/cm 2 at 700 °C is achieved on Pr 1.7 Ba 0.3 NiO 4+δ , one of the best performances for PCECs.

25 ENERGY STORAGE↗
DOE OSTI2012

Effect of Gd substitution on the structure and magnetic properties of YFeO{sub 3} ceramics

High quality Gd-doped YFeO{sub 3} polycrystalline samples were synthesized by solid-state reaction method. The crystal structure of the samples can be described by the space group of Pbnm (No. 62). {sup 57}Fe Moessbauer spectra show that only the high spin Fe{sup 3+} exists and the samples are magnetic ordering at room temperature. The spontaneous magnetization at room temperature was observed and the paramagnetic contribution from Gd{sup 3+} ions was confirmed by the significant enhancement of the magnetization. Temperature-dependent magnetization data correlate with the Gd concentration x and follow the Curie-Weiss law above the ferromagnetic transition temperature. Magnetic measurements suggest that the substitution of Y{sup 3+} ions by Gd{sup 3+} ions effectively enhance the magnetization of YFeO{sub 3}. This is achieved by the additional Gd-Gd interactions, Gd-Fe interactions and the variation of Fe-O-Fe superexchange bond caused by distortion of the crystal structure. - Graphical abstract: The crystallographic structure of YFeO{sub 3} and the magnetic hysteresis loops of Y{sub 1-x}Gd{sub x}FeO{sub 3} (x=0, 0.1, 0.5, and 1) samples at room temperature. Highlights: Black-Right-Pointing-Pointer Y{sub 1-x}Gd{sub x}FeO{sub 3} (0{<=}x{<=}1) ceramics were synthesized by solid state reaction method. Black-Right-Pointing-Pointer Y{sub 1-x}Gd{sub x}FeO{sub 3} (0{<=}x{<=}1) samples are ferromagnetic ordering at room temperature. Black-Right-Pointing-Pointer Results of {sup 57}Fe Moessbauer spectra are typical for Fe{sup 3+} high spin state. Black-Right-Pointing-Pointer Substitution of Y{sup 3+} ions by Gd{sup 3+} ions enhances the magnetization of YFeO{sub 3} obviously.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Crystal structures and microwave dielectric properties of Zn,W co-substituted BaTiO{sub 3} perovskite ceramics

Zn,W co-substituted BaTiO{sub 3} perovskite ceramics with compositions of Ba[Ti{sub 1-x}(Zn{sub 1/2}W{sub 1/2}){sub x}]O{sub 3} (x=0.40-1.00) were synthesized by the conventional solid-state reaction method and studied by X-ray diffraction and dielectric measurements. Their structures transferred from cubic (x=0.40) with space group Pm3{sup Macron }m to hexagonal (x=0.55-0.60) with P6{sub 3}/mmc, and then to cubic double pervoskite (x=1.00) with Fm3{sup Macron }m. The refinement results of sample (x=0.60) revealed that the negative and positive charge centers are not consistent with each other in the face-sharing oxygen octahedra. Compared with the cubic perovskites, the hexagonal ones had the increased quality factor (Q Multiplication-Sign f), the decreased dielectric constant ({epsilon}{sub r}), and especially the near-zero temperature coefficient of resonant frequency ({tau}{sub f}), whose absolute values were less than 5 ppm/ Degree-Sign C. The relationships between structures and properties were discussed. - Graphical abstract: The departure of the negative and positive charge centers in the face-sharing oxygen octahedra of hexagonal unit cell. Highlights: Black-Right-Pointing-Pointer A phase transition from simple cubic to hexagonal and then to cubic double pervoskite occurred. Black-Right-Pointing-Pointer Refinement results prove that spontaneous dipoles present in face-sharing oxygen octahedra. Black-Right-Pointing-Pointer Hexagonal perovskites had higher Q Multiplication-Sign f, nearer-zero {tau}{sub f}, and lower {epsilon}{sub r} than the cubic ones. Black-Right-Pointing-Pointer On the presence of the hexagonal perovskite, |{tau}{sub f}| was less than 5 ppm/ Degree-Sign C.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of synthesis methods on the Ca{sub 3}Co{sub 4}O{sub 9} thermoelectric ceramic performances

Three different synthesis methods producing nanometric grain sizes, coprecipitation with ammonium carbonate, oxalic acid, and by attrition milling have been studied to produce Ca{sub 3}Co{sub 4}O{sub 9} ceramics and compared with the classical solid state route. These three processes have produced high reactive precursors and all the organic material and CaCO{sub 3}·have been decomposed in a single thermal treatment. Coprecipitation leads to pure Ca{sub 3}Co{sub 4}O{sub 9} phase, while attrition milling and classical solid state produce small amounts of Ca{sub 3}Co{sub 2}O{sub 6} secondary phase. Power factor values are similar for all three samples, being slightly lower for the ones produced by attrition milling. These values are much higher than the obtained in samples prepared by the classical solid state method, used as reference. The maximum power factor values determined at 800 °C (∼0.43 mW/K{sup 2} m) are slightly higher than the best reported values obtained in textured ones which also show much higher density values. - Graphical abstract: Impressive raise of PF in Ca{sub 3}Co{sub 4}O{sub 9} thermoelectric materials obtained from nanometric grains. - Highlights: • Ca{sub 3}Co{sub 4}O{sub 9} has been produced by four different methods. • Precursors particle sizes influences on the final performances. • Coprecipitation methods produce single Ca{sub 3}Co{sub 4}O{sub 9} phase. • Power factor reaches values comparable to high density textured materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Rietveld refinement, electronic structure and ionic conductivity of Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}F{sub 2} and Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}O ceramics

In this paper, we report the effect of the tunnel anions on the ionic conductivity of Strontium-Lanthanum silicate apatites. The Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}F{sub 2} and Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}O ceramics were prepared by the solid state reaction method. X-ray diffraction, NMR spectroscopy and Raman measurements were performed to investigate the crystal structure and vibrational active modes. Moreover, the electronic structures of the crystals were evaluated by the first-principles quantum mechanical calculation based on the density functional theory. Finally, the ionic conductivity was studied according to the complex impedance method. - Graphical abstract: The relaxed primitive unit cell for Sr{sub 4}La{sub 6}Fap. Display Omitted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Segmentation, statistical analysis, and modelling of the wall system in ceramic foams

Closed walls in otherwise open foam structures may have a great impact on macroscopic properties of the materials. In this paper, we present two algorithms for the segmentation of such closed walls from micro-computed tomography images of the foam structure. The techniques are compared on simulated data and applied to tomographic images of ceramic filters. This allows for a detailed statistical analysis of the normal directions and sizes of the walls. Finally, we explain how the information derived from the segmented wall system can be included in a stochastic microstructure model for the foam.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Influence of surface crystallinity on the surface roughness of different ceramic glazes

Surface smoothness is an important characteristic of ceramic glazes as it influences chemical resistivity, glossiness and stainability of glazes. Surface crystallinity of zirconia-based and titania-based glazes (as two common opaque glazes), calcium-zinc based matte glaze and also two transparent glazes with different compositions were studied in this work. Degree of surface crystallinity measured by stereological methods using scanning electron microscopy (SEM) images showed that transparent glaze has the minimum degree of crystallinity (2.1%) and the maximum amount (25.4%) was obtained for matte glaze. In addition, X-ray diffraction (XRD) was used to characterize the glaze mineralogy. Optical surface profiler showed that the smoothest surface belongs to titania-based opaque glaze (R{sub a} = 0.0157 µm) and transparent glaze (R{sub a} = 0.0168 µm). In contrast, the roughest glaze surface corresponded to the matte glaze (R{sub a} = 0.2772 µm). The results suggest that surface roughness is influenced by crystallinity of surface, but degree of surface crystallinity is not the only parameter that can influence the surface roughness. The most important factor is the morphology of the crystals grow and their protrusion from the surface. - Highlights: •Degree of crystallinity of glazed surfaces was measured using stereological methods. •Crystallinity of the glazed surfaces is not the main reason of surface roughness. •The morphology of crystal grow and its surface protrusion can affect the roughness.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Coalescence growth mechanism of inserted tin dioxide belts in polycrystalline SnO2-based ceramics

Highlights: • Grain growth in SnO{sub 2}-based varistors was studied. • The growth of 1D SnO{sub 2} belts in the SnO{sub 2}-CoO-Cr{sub 2}O{sub 3}-Nb{sub 2}O{sub 5} system occurs by coalescence process. • Cross-Section and Tomography images showed tridimensional and pore free “giant grains”. • TEM was used to prove the alignment of the grains towards the SnO{sub 2} belt. • The coalescence growth just occurs with 1D structure and grains having the same crystal structure. - Abstract: SnO{sub 2}-based varistors have been considered promising technological devices. However their practical application is usually stated as limited to high voltage circuits based on the high breakdown electric field exhibited by these ceramics. Recently, authors have shown that the insertion of one-dimensional (1D) SnO{sub 2} belts allows overcoming this limitation. In this work, we present a detailed study of the growth mechanism of the belts inside varistors using electron microscopy techniques. We were able to show that mass transport has an intrinsic dependence on the sintering time and requires similar crystalline structure between the belts and the matrix. Dual beam and high-resolution transmission electron microscopy techniques permitted determining that 3D growth of belts occurs by coalescence.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tailoring the interfacial microstructure and mechanical strength of SiC ceramic joints using joining temperature and interlayer thickness

Highlights: • Bonding temperature could be decreased from 1600 °C to 1300 °C by inserting the Ti foil between SiC and Ta-5W. • The initial thickness interlayer and the number of interlayer had a significant influence on interface bonding. • Layered reaction layers consisting of carbides and silicide formed at the SiC/Ti/Ta-5W interfaces. • The joint strength depended mainly on the reaction layer thickness, especially the thickness of silicide layer. - Abstract: Silicon carbide ceramic joints solid state diffusion bonded by spark plasma sintering with Ti/Ta-5W/Ti multi-interlayer in the temperature range from 1250 °C to 1500 °C were investigated in detail in terms of interfacial microstructure, elemental diffusion, phase constituents, shear strength, and fracture morphology. Layered reaction layers consisting of carbides (TiC, Ti{sub 3}SiC{sub 2}, (Ta,Ti,W)C), silicide ((Ta,Ti,W){sub 5}Si{sub 3}) and Ta-rich solid solution (Ta,Ti,W) formed at the SiC/Ti/Ta-5W interfaces. It was found that joining temperature could be decreased from 1600 °C to 1300 °C by inserting the Ti foil between SiC and Ta-5W. Nevertheless, an attempt to further decrease the joining temperature to 1250 °C failed even using a thin Ti foil. The initial thickness interlayer and the number of interlayer had an influence on the interfacial diffusion bonding at SiC/Ti interface. Higher joining temperature was needed to achieve a tough interface joining for a thick interlayer compared with a thin interlayer. The increase of interlayer number also required higher joining temperature. The shear strength reached 123.5 ± 25.3 MPa for the joint diffusion bonded at 1300 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2012

The characterisation of lead-free thick-film resistors on different low temperature Co-fired ceramics substrates

Graphical abstract: Display Omitted Highlights: ► Lead free thick film resistors based on ruthenium oxide were developed. ► The compatibility of resistors with different LTCC substrates was evaluated. ► The interactions between resistors and glassy LTCC substrates were not detected. ► Electrical characteristics were comparable with commercial thick film resistors. -- Abstract: Lead-free thick-film resistors were synthesised and investigated. The thick-film resistor materials with nominal sheet resistivities from 50 ohm/sq. to 50 kohm/sq. were prepared using combinations of two lead-free glasses with reflow temperatures at 940 °C and 1240 °C, respectively, and two RuO{sub 2} powders (fine-grained and coarse-grained RuO{sub 2}). The thick-film resistors were printed and fired on alumina and on low temperature co-fired ceramics substrates and fired at 850 °C and 950 °C. The fired resistors were investigated by X-ray powder diffraction, by scanning electron microscopy and by energy dispersive X-ray analysis. The sheet resistivities, temperature coefficients of resistivity, gauge factors and noise indices were measured.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Tm{sup 3+}-sensitized up- and down-conversions in nano-structured oxyfluoride glass ceramics

Graphical abstract: Display Omitted Highlights: ► UC and DC are achieved in a single-ion sensitized phosphor for PV applications. ► The most intense fractions of AM 1.5G spectrum below/above c-Si bandgap are absorbed. ► The converted photons match the optimal spectral response of c-Si. -- Abstract: Tm{sup 3+}-sensitized up-conversion and down-conversion are studied in transparent glass ceramics embedded with β-YF{sub 3} nanocrystals. Upon excitation at 1220 nm, where crystalline silicon (c-Si) solar cells no longer absorb, the sub-bandgap photons could be converted to the higher-energy ones via up-conversion. In addition, under excitation at 468 nm (a wavelength close to the peak in the air-mass 1.5 global solar spectrum), one blue photon might be split in up to two near-infrared ones via down-conversion. In both cases, the frequency-converted photons match the spectral response of c-Si solar cell well. Hopefully, the investigated luminescent materials may act as the spectral conversion layers to reduce the sub-bandgap transmission and charge carrier thermalization losses of c-Si solar cells, and in turn, enhance the energy efficiency.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Fracture modes in tubular LSFCO ceramic membranes under graded reducing conditions

Highlights: ► Microstructural evolution in LSFCO membranes under graded environment is reported. ► The role of chemically induced stresses and oxygen deficiency is evaluated. ► The stress distribution is modeled by a point defect model. - Abstract: Chromium (III) oxide (Cr{sub 2}O{sub 3})-doped LaSrFeO{sub 3} perovskite, La{sub 0.2}Sr{sub 0.8}Fe{sub 0.8}Cr{sub 0.2}O{sub 3−δ} (LSFCO), is being considered as a potential material for applications in solid oxide fuel cells, gas separation membranes, and electrochemical reactors because of its high electro-catalytic activity. Similar to other perovskites, the performance and mechanical strength of LSFCO materials are significantly affected by environment and temperature. Here, we report a fracture gradient phenomenon in tubular C-ring-shaped LSFCO ceramic membranes under graded reducing conditions. The graded reducing condition was produced by flushing N{sub 2} on the outer side of the C-ring membranes at 1000 °C while keeping the inner side untreated. The rings were then diametrically compressed to fracture, and the resultant fracture morphology was analyzed with a scanning electron microscope (SEM). A fracture gradient with three distinct regions across the thickness of the membranes was identified on the split surfaces. In the outer region of the C-ring specimen exposed to N{sub 2}, a mixed inter/transgranular fracture with a predominant intergranular pattern was observed. In the middle section of the fracture surface, a characteristic transgranular fracture of the perovskite grains was found. At the inner region of the ring, a mixed inter/transgranular fracture with a predominant transgranular pattern occurred. The mechanism of gradient fractures was attributed both to chemically induced stresses caused by oxygen diffusion and to the formation of a separate phase of oxygen-deficient perovskite in the parent perovskite. The stresses generated were modeled by a point defect model. This work provides significant information on microstructure evolutions of tubular LSFCO membranes under graded reducing atmospheres.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ceramic thick film humidity sensor based on MgTiO{sub 3} + LiF

Graphical abstract: - Highlights: • The fabricated sensor based on MgTiO{sub 3} + LiF materials used the spin coating technology. • The response time is 70 s to detect variation between 5 and 95% relative humidity. • The addition of Scleroglucan controls the viscosity and decreases the roughness of thick film surface. • This humidity sensor is a promising, low-cost, high-quality, reliable ceramic films, that is highly sensitive to humidity. - Abstract: The feasibility of humidity sensor, consisting of a thick layer of MgTiO{sub 3}/LiF materials on alumina substrate, was studied. The thermal analysis TGA-DTGA and dilatometric analysis worked out to confirm the sintering temperature. An experimental plan was applied to describe the effects of different parameters in the development of the thick film sensor. Structural and microstructural characterizations of the developed thick film were made. Rheological study with different amounts of a thickener (scleroglucan “sclg”), showing the behavior variation, as a function of sclg weight % was illustrated and rapprochement with the results of thickness variation as a function of angular velocity applied in the spin coater. The electrical and dielectric measurements confirmed the sensitivity of the elaborated thick film against moisture, along with low response time.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of neodymium substitution on structural and ferroelectric properties of BNT ceramics

Graphical abstract: - Highlights: • First time, we synthesized (Bi{sub 1−x}Nd{sub x}){sub 0.5}Na{sub 0.5}TiO{sub 3} system by semi wet technique. • Grain size reduced with doping and Nd acts as an inhibitor in the grain growth. • Specimen with x = 0.02 exhibits excellent ferroelectric properties at RT. • P–E loops show the co-existence of polar and non polar regions around ‘T{sub d}’. - Abstract: Polycrystalline specimens of (Bi{sub 1−x}Nd{sub x}){sub 0.5}Na{sub 0.5}TiO{sub 3} (BNNT) ceramic system with compositions x ≤ 0.04 were synthesized by semi-wet technique using ethylene glycol precursor. Structural and electrical properties were investigated in detail to observe the effect of neodymium (Nd) substitution in BNT system. XRD patterns for all the specimens showed single phase formation with rhombohedral structure. Field emission scanning electron micrographs (FE-SEM) revealed that the grain growth was inhibited significantly with Nd content. The temperature dependence behaviour of dielectric constant revealed that the depolarisation temperature ‘T{sub d}’ decreases whereas temperature of maximum dielectric constant ‘T{sub m}’ increases with Nd concentration. The piezoelectric charge coefficient (d{sub 33}) showed maxima at x = 0.02 and well defined ferroelectric behaviour was observed for all the samples.

36 MATERIALS SCIENCE↗