Non-congruence of high-temperature mechanical and structural behaviors of LaCoO3 based perovskites

This paper presents the mechanical behavior of LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics under four-point bending in which the two cobaltites are subjected to a low stress of ∼8 MPa at temperatures ranging from room temperature to 1000 °C. Unexpected stiffening is observed in pure LaCoO₃ in the 700–900 °C temperature range, leading to a significant increase in the measured Young’s modulus, whereas La_0.8Ca_0.2CoO₃ exhibits softening from 100 °C to 1000 °C, as expected for most materials upon heating. Neutron diffraction, X-ray diffraction and micro-Raman spectroscopy are used to study the crystal structure of the two materials in the RT–1000 °C temperature range. Despite a detailed study, there is no conclusive evidence to explain the stiffening behavior observed in pure LaCoO₃ as opposed to the softening behavior in La_0.8Ca_0.2CoO₃ at high temperatures (above 500 °C).     

Temperature dependence of Young’s modulus and damping of partially sintered and dense zirconia ceramics

Young’s modulus and damping of partially sintered and almost fully dense zirconia ceramics (tetragonal zirconia polycrystals with 3 mol.% yttria), obtained by firing to different temperatures (range 1000–1400°C), have been determined via impulse excitation, and the evolution of Young’s modulus and damping of partially sintered zirconia with temperature has been monitored from room temperature to 1400°C and back to room temperature. The room-temperature Young’s modulus of the partially sintered materials obeys the Pabst-Gregorová exponential prediction, which is relatively unusual for partially sintered materials. With increasing temperature Young’s modulus decreases, until the original firing temperature is exceeded and sintering (densification) continues, resulting in a steep Young’s modulus increase. During heating and cooling the temperature dependence obeys a master curve with a typical inflection point at approximately 200 °C, the temperature where damping (internal friction) exhibits a maximum. The reasons for this characteristic behavior of doped zirconia are recalled.     

Young’s modulus evolution during heating,re-sintering and cooling of partially sintered alumina ceramics

The Young modulus of partially and fully sintered alumina ceramics, obtained by firing to different temperatures (range 1200–1600°C), has been determined via impulse excitation, and the evolution of Young’s modulus of partially sintered alumina with temperature has been monitored from room temperature to 1600°C. As expected, the room-temperature Young modulus of the partially sintered materials is lower than all theoretical predictions. With increasing temperature Young’s modulus decreases, until the original firing temperature is exceeded and sintering (densification) continues, resulting in a steep Young’s modulus increase. During heating and cooling the temperature dependence obeys a master curve for alumina, unless the temperature of the original firing is excessively low.     

A case study of mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This study has investigated mechanical properties of perovskite-structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) oxygen transport membrane. The Young’s modulus and fracture toughness are determined by both macroscopic-scale and microscopic-scale methods. Both three-point and ring-on-ring bending tests as macroscopic-scale methods produce broadly similar results with a Young’s modulus, which is lower than that measured from micro-indentation method under a 10 N load. Young’s modulus and fracture toughness of BSCF show strongly dependent of the porosity. However, the fracture toughness of BSCF is independent of grain size. The fracture toughness determined by macroscopic-scale method is similar with that measured by microscopic-scale method. The crack shape of BSCF under a 10 N load is determined to be a median-radial mode. The intrinsic Young’s modulus and fracture toughness are determined to be 105.6 GPa and 1.49 MPa m^0.5, respectively, according the Minimum Solid Area (MSA) model. Annealing decreases the fracture toughness of BSCF between RT and 800 °C.     

Synthesis,properties and thermal decomposition of the Ta4AlC3 MAX phase

The present work describes a synthesis route for bulk Ta₄AlC₃ MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta₂H, Al and C powder mixtures in the 1200–1650 °C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity α-Ta₄AlC₃ was obtained by hot pressing at 1500 °C for 30 min at 30 MPa. The β-Ta₄AlC₃ allotrope was observed in the samples produced by SPS. The Young’s modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta₄AlC₃ into TaC_x and Al vapour in high (˜10⁻⁵ mbar) vacuum at 1200 °C and 1250 °C was also investigated, as a possible processing route to produce porous TaC_x components.     

Fluidized bed chemical vapor deposition of pyrolytic carbon-III. Relationship between microstructure and mechanical properties

Pyrolytic carbon (PyC) coatings with different densities were produced by fluidized bed chemical vapor deposition under different deposition conditions. Their Young’s modulus and hardness were measured by nano-indentation, whereas the deformation behavior was studied through analyzing the force–displacement curves of the indentations. The deformation mechanism of PyC under indentation is attributed to the slip of the graphene planes, and its reversibility is discussed in terms of the defects of the microstructure. We observed a linear relationship between the density of PyC’s and their Young’s modulus and hardness, for densities lower than 1.9 g/cm³. Above this value, the mechanical properties were controlled by the amount of interstitial defects. Samples were also heat treated at 1800 °C and 2000 °C, and their changes in microstructure, hardness and Young’s modulus are discussed as a function of density.     

Effects of UV exposure and thermal history on properties of a preimidized photosensitive polyimide

The structure and properties of a preimidized photosensitive polyimide (Probimide PSPI, a copolyimide of benzophenonetetracarboxylic dianhydride with alkyl groups substituted aromatic diamines) were studied with variations of UV exposure energy and bake temperature by means of wide angle X-ray diffraction, dynamic mechanical thermal analysis, stress-strain analysis, and residual stress analysis. The X-ray diffraction patterns patterns indicate that the PSPI is amorphous in the solid state. The T_g was 378°C ～ 410°C, depending upon the thermal history over the range of 350°C ～ 400°C. At the glass transition region, the dynamic storage modulus E′ was very sensitive to both i-line exposure energy and thermal history. However, the mechanical stress-strain behavior at room temperature was primarily dependent on the thermal history. The mechanical properties were 2.6 GPa ～ 2.9 GPa Young's modulus, 131 MPa ～ 168 MPa tensile strength, 10% ～ 12% yield strain, and 16% ～ 74% elongation at break, depending upon the baking or annealing. These dynamic and static mechanical properties indicate that on the PSPI backbone, crosslinks are formed thermally as well as photochemically. The thermal crosslinks might be formed through thermal liberation of the labile alkyl groups of aromatic diamine moieties and subsequent coupling of the radicals. The thermal degradation was also evidenced in the mechanical properties degraded by baking above 375°C or annealing above 350°C. In addition, during baking and cooling, the residual stress was dynamically measured on Si wafers as a function of temperature. The stress at room temperature was 48 MPa ～ 52 MPa for the PSPI films baked at 350°C or 400°C, regardless of i-line exposure.     

Processing temperature dependent mechanical response of a thermoplastic elastomer with low hard segment

The mechanical responses including monotonic and cyclic tensile responses have been investigated on a microphase-separated poly (styrene-isoprene-styrene) triblock copolymer (SIS). The specimens were injection-molded by using different melt temperatures to acquire different microphase structures. As a result of temperature-dependent segregation driving force, the specimens with reduced microphase separation can be obtained by increasing processing melt temperature from 180 °C to 240 °C. On the basis of stress-strain behavior, Young's modulus was found to increase with increasing PS domain continuity in the order of disorder state to disordered spheres to body-cubic-centered (BCC) spheres to oriented cylinders morphology. Meanwhile, cyclic hysteresis decreases with reduced microphase separation and with decreasing the applied predetermined maximum tensile strain. In addition, the Mooney–Rivlin phenomenological approach was used to evaluate and explore the relationship between the polymer topological networks and the rubber elasticity of thermoplastic elastomers.     

Mechanical properties of solid oxide fuel cell glass-ceramic sealants in the system BaO/SrO-MgO-B2O3-SiO2

Planar solid oxide fuel cells (p-SOFCs) require materials that can satisfy the high mechanical demands related to their utilization in stationary and, especially, in mobile applications. Two suitable glass-ceramic sealants based on the system BaO/SrO-MgO-B₂O₃-SiO₂ have been characterized with respect to their mechanical properties such as hardness, Young’s modulus, flexural strength at room and elevated temperature, fracture toughness as well as creep behavior at relevant operation temperatures (800 °C). Fracture toughness was calculated from crack opening displacements (COD) and the results were compared with fracture toughness measured by bending tests of notched bar samples. The mechanical behavior has been discussed regarding different thermal aging times of the glass-ceramics and their microstructural evolution. The glass-ceramics containing SrO revealed a better mechanical behavior than glass-ceramics with BaO. In particular, several superior properties were found in comparison to previously reported materials for this application.     

Synthesis,microstructure and mechanical properties of high-entropy (VNbTaMoW)C5 ceramics

High-entropy ceramics (HEC) with a fixed composition of (VNbTaMoW)C₅ were prepared by spark plasma sintering (SPS) from 1500 °C to 2200 °C. XRD, TEM, HRTEM, SAED and EDX were used to investigate effects of the sintering temperatures on compositional homogeneity, constituent phases and microstructure of the HECs. The results showed that single-phase HEC formed at a temperature as low as 1600 °C while ultimate elemental distribution homogeneity could be obtained at 2200 °C. Elemental distribution homogenization was accompanied by microstructural coarsening and oxide impurities aggregating at grain boundaries as temperature increased. SPS at 1900 °C for 12 min could yield uniform HECs (VNbTaMoW)C₅ with Vickers hardness, nanohardness, fracture toughness and Young’s modulus reaching 19.6 GPa, 29.7 GPa, 5.4 MPa m^1/2 and 551 GPa, respectively. The resultant HECs showed excellent wear resistance when coupled with WC at room temperature.     

Young׳s modulus,hardness and thermal expansion of sintered Al2W3O12 with different porosity fractions

Owing to their unusual thermal expansion properties ceramic phases from A₂M₃O₁₂ family have potential for applications as thermal expansion controlling fillers inside soft matrices or as materials with high thermal shock resistance when prepared in monolithic forms. In spite of this, the consolidation routes for achieving bulk forms with adequate microstructure and their mechanical and thermal properties are scarcely known and rarely studied. A prelaminar study on sinterability of Al₂W₃O₁₂, a low thermal expansion phase, was accomplished for the temperature range between 850 °C and 1000 °C. Sintered samples with the porosity fraction between 0.1 and 0.25 were produced and their Young׳s moduli, hardness and thermal expansion studied through nanoidentation and dilatometry. Acoustic emission was employed for studying of microcrack formation during heating and cooling of sintered samples. Sintering study showed that the temperatures higher than 1150 °C may lead to the decomposition of tungstate due to WO₃ evaporation, while the sintering at the temperature of 850 °C provokes only small changes over grain size distribution. Hardness and Young׳s modulus decrease linearly in porosity range between 0.1 and 0.25. Young׳s modulus for fully dense Al₂W₃O₁₂ was calculated to be 70 GPa, illustrating that the phases from A₂M₃O₁₂ family are considerable softer than traditional ceramics. Microcrack formation was observed on cooling and heating, as well, causing the discrepancy between the intrinsic coefficient of thermal expansion (CTE), measured in powder form, and the CTE measured in bulk form. The key feature for future development of A₂M₃O₁₂ phases for thermal shock resistance applications is the better understanding of sintering processes in order to improve microstructure and reduce influence of microcracks over mechanical and thermal properties.     

Ultrasonic measurement of Young’s modulus MgO/C refractories at high temperature

The paper deals with the elastic behavior of MgO/C refractories used in BOF at temperatures up to 1400°C in air or inert atmosphere. Measurements have been made by the way of a high temperature ultrasonic technique. Heating-cooling cycles and long time aging in the range 700–1400°C show strong variations of Young’s modulus which have been interpreted with the aid of XRD analysis, SEM observations and EDS analysis. Carbon oxidation and sintering of MgO particles are found to be responsible of the major parts of the measured evolutions. ©     

Influence of cerium doping on mechanical properties of tetragonal scandium-stabilized zirconia

Mechanical properties of tetragonal scandium-stabilized zirconium oxide (ScSZ) co-doped with cerium oxide are studied before and after a complete reduction of Ce⁴⁺ to Ce³⁺. Samples of 6Sc1CeSZ (i.e., 6 mol% Sc₂O₃, 1 mol% CeO₂), 6Sc2CeSZ, and 4Sc1CeSZ are produced by pressure-less sintering in air at 1450 °C. Test pieces are annealed at 1200 °C in an atmosphere of forming gas (5% H₂, 95 % N₂). Vickers hardness, Young’s modulus, 4-point bending strength, and fracture toughness are measured at room temperature. Values of hardness and Young’s modulus are similar for different compositions of ScSZ:Ce and stay practically unaffected by the reduction. Both the fracture toughness and strength significantly decrease and the largest drop is measured for the 6Sc2CeSZ, having the highest concentration of cerium. The observed degradation of mechanical properties suggests that cerium-free electrolytes should be preferred for the fabrication of the electrolyte-supported planar SOFCs.     

Effect of microstructure on electrical and mechanical properties of La5.4WO12-δ proton conductor

The relationships between microstructural characteristics and electrical as well as mechanical properties of La_5.4WO_12-δ (LWO54) materials were studied. Polycrystalline LWO54 samples revealed identical transport mechanisms regardless of the sample microstructure. The studied samples show predominately proton conductor behaviour below 800?°C and become predominant n-type and oxygen ion conductors above this temperature. The magnitude of the total conductivity is enhanced with larger grain size and lower porosity. Young’s modulus decreased by 20% with increasing temperature up to 1000?°C regardless of grain size and atmosphere. Fracture strength was determined via ring-on-ring bending tests, yielding values that strongly depended on microstructural characteristics and homogeneity of the microstructure. Elevated temperature deformation studies revealed that creep is governed by cation diffusion mechanism.     

Mechanical Behavior of Poly(Ethylene 2,6-Naphthalene-Dicarboxylate) (PEN) Fibres near the Glass-Rubber Transition Temperature

Abstract

The mechanical properties of as-spun poly(ethylene 2,6-naphthalene-dicarboxylate) (PEN) fibres were studied in order to characterize this relatively new material near its glass-rubber transition.

Tensile tests were carried out on amorphous (low-speed spun) PEN filaments. The temperature range of 90°C up to 160°C was covered using increments of 10°C. A transition from necking and cold drawing to rubber-like behavior was observed in the stress-strain relationship. Dynamic mechanical experiments were carried out on PEN yarns spun at speeds from 500 to 4000 m min^?1. Both temperature and frequency were varied. The maxima in loss modulus depend on spinning speed. Tensile behavior and dynamic mechanical behavior of PEN fibres demonstrate that the glass-rubber transition temperature of PEN is approximately 125°C.     

Direct ink writing of 3Y-TZP ceramics using PEG-Laponite® as additive

This study investigated a new colloidal ink, containing Laponite® nanoclay as additive, to develop ZrO₂-based ceramic prototypes by Direct Ink Writing (DIW). The ink developed contained 31 vol% 3Y-TZP as solid load and 69% v/v of gel containing 7.5% (w/w) Laponite®, polyethylene glycol (PEG), and dibutyl phthalate. The rheologyical behavior of this ceramic ink was analyzed and its extrudability was optimized. The samples were DIW 3D-printed at a speed of 10 mm/s with nozzle diameter of 0.41 mm. After drying for 24 h, the zirconia samples underwent debinding and were pre-sintered at 1100 °C under a heating rate of 1 °C/min. After pre-sintering, the samples were sintered at 1550 °C for 2 h. Subsequently, the sintered samples were characterized by Scanning Electron Microscopy and X-ray diffractometry. Fracture toughness was measured by Vickers indentation, whereas nano-hardness and Young’s modulus were assessed by dynamic Vickers nanoindentation (250–1960 mN loads). A relative density of 89.3 ± 0.5% was found. The following crystalline phases were observed: monoclinic-ZrO₂ (34%), tetragonal-ZrO₂ (31%), and cubic-ZrO₂ (35%). This behavior is probably due to the segregation of Y⁺³ between the zirconia grains, favored by the presence of Laponite® nanoclay during the sintering process. The microstructure of the prototypes showed two distinct populations of ZrO₂ grains with average particle sizes of 1 μm (monoclinic and tetragonal phases) and >5 μm (cubic phase). Vickers hardness (HV_1000gF) was 952 ± 40 HV and fracture toughness was 4.6 ± 1.1 MPa m^1/2. A Young’s modulus of ∼200 GPa and hardness values of 1837.9 and 1943.3 HV were found for loads of 250 and 1960 mN, respectively.     

Structural and mechanical properties of polybutadiene-containing polyurethanes

A series of segmented polyurethanes based on hydroxylterminated polybutadienes (HTPBD) and their hydrogenated derivatives (HYPBD) has been synthesized. Thermal, mechanical, and spectroscopic studies were carried out over a wide temperature range to elucidate the structure-property relationships existing in these polymers. Both thermal and dynamic mechanical response showed a soft segment T_g at ?74°C for the unsaturated polyurethanes and at ?69°C for the hydrogenated samples. In addition, two hard segment transitions are observed by differential scanning calorimetry (DSC) at 40 and 75°C and a softening region by thermal mechanical analysis (TMA) at 190°C. The low T_g, very close to that of the free HTPBD and HYPBD and independent of hard segment content, indicated that these polymers were well phase separated. Results of infrared analysis revealed that at room temperature, 90-95 percent of the urethane N-H groups formed hydrogen bonds. Since hydrogen bonding resides only within the hard segment domain in these butadiene-containing polyurethanes the extent of H-bonding served as additional evidence for nearly complete phase segregation. From dynamic mechanical studies, the plateau modulus above the soft segment T_g and stress-strain behavior depended upon the concentration of hard segments. A slight increase in the modulus, a moderate increase in stress (σ_b), and decrease in elongation accompanied a higher hard segment content. The thermal and mechanical response of these polyurethanes appears to be consistent with behavior observed for other phase segregated systems. Variations in behavior resulting from hydrogenation of the precursor prepolymer are discussed.     

Young's Modulus of Elasticity of Carbon‐Bonded Alumina Materials up to 1450°C

Investigating Young's modulus at elevated temperatures supports the understanding of microstructural changes as a function of application temperature. A sintered alumina and three carbon‐bonded alumina materials with carbon contents of 20 and 30 wt% and alumina grain size of 0.6–3 mm were investigated. Young's modulus was measured in a temperature range from 25°C to 1450°C by the impulse excitation technique. The Young's modulus of carbon‐bonded materials increases up to 140% at 1450°C. After one cycle, a decrease of the Young's modulus up to 50% is registered at room temperature. There is a strong hysteresis behavior during one cycle. Thermal expansion measurements show highest expansion for the highest graphite content material. The expansion of alumina grains and graphite flakes, resulting in microcrack generation during cooling and microcrack healing during heating, is reflected in the registered values of the Young's modulus as a function of the temperature. It is assumed, that higher graphite amounts as well as coarse grains lead to lower sintering effects of the microstructure at elevated temperatures and as a result lower values of the Young's modulus have been registered.     

Sealing Glass‐Ceramics with Near‐Linear Thermal Strain,Part II: Sequence of Crystallization and Phase Stability

The sequence of crystallization in a recrystallizable lithium silicate sealing glass‐ceramic Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO was analyzed by in situ high‐temperature X‐ray diffraction (HTXRD). Glass‐ceramic specimens have been subjected to a two‐stage heat‐treatment schedule, including rapid cooling from sealing temperature to a first hold temperature 650°C, followed by heating to a second hold temperature of 810°C. Notable growth and saturation of Quartz was observed at 650°C (first hold). Cristobalite crystallized at the second hold temperature of 810°C, growing from the residual glass rather than converting from the Quartz. The coexistence of quartz and cristobalite resulted in a glass‐ceramic having a near‐linear thermal strain, as opposed to the highly nonlinear glass‐ceramic where the cristobalite is the dominant silica crystalline phase. HTXRD was also performed to analyze the inversion and phase stability of the two types of fully crystallized glass‐ceramics. While the inversion in cristobalite resembles the character of a first‐order displacive phase transformation, i.e., step changes in lattice parameters and thermal hysteresis in the transition temperature, the inversion in quartz appears more diffuse and occurs over a much broader temperature range. Localized tensile stresses on quartz and possible solid‐solution effects have been attributed to the transition behavior of quartz crystals embedded in the glass‐ceramics.     

XPS Characterization of reduced LaCoO3 perovskite

Three samples of LaCoO₃ were prepared by two different methods and calcined at 800 or 1000 °C. They had BET areas of 1, 12, and 16 m²/g and all of them showed pure perovskite X-ray diffraction patterns with identical unit cell dimensions. In a series of experiments the oxide, having larger surface area, was stepwise reduced in hydrogen at temperatures between 60 and 500 °C. The XPS spectra, taken at room temperature after evacuation at 400 °C at each reduction level, showed that the surface concentration of Co° was very low up to 300 °C but increased sharply between 300 and 350 °C (9–75%). This concentration further increased to 100% after 10 min reduction at 450 °C, but upon heating in hydrogen for an additional 10 min at 500 °C it decreased again to 75%. In another series of experiments the mixed oxide was reoxidized after each reduction. A fresh sample was reduced to 350 and 400 °C by contacting with hydrogen for 1 hr and evacuated at temperatures between 400 and 500 °C. Both high evacuation temperatures and reduction at 400 °C during 1 hr produced a sharp decrease in Co° surface concentration. These results are consistent with the catalytic behavior of this perovskite reported earlier by E. A. Lombardo et al. (4–7). A model is proposed to interpret the reduction of LaCoO₃.