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Examines the role of isovalent dopants in the high-temperature deformation behavior of gallium arsenide. Details of the experimental procedure; Summary of work on both undoped and boron and indium-doped crystals; Results of the solute hardening analysis.

Presents a study which examined the deformation behavior of undoped and indium (In)-doped gallium arsenide (GaAs) single crystals. Measurement of the temperature dependence of undoped and In-doped GaAs; Experimental procedures used; Values for the upper yield as a function of temperature.

Presents a study which measured high-temperature hardness of Ga[sub1-x]In[subx]As. Experimental details; Results and discussion; Conclusion.

Fracture toughness of La0.6Sr0.4Co0.2Fe0.8O3-δ ( LSCF) in both bulk and film forms after sintering at 900°C to 1200°C was measured using both single-edge V-notched beam ( SEVNB) 3-point bending and Berkovich indentation. FIB/ SEM slice-and-view observation after indentation revealed the presence of Palmqvist radial crack systems after indentation of the bulk materials. Based on crack length measurements, the fracture toughness of bulk LSCF specimens was determined to be in the range 0.54-0.99 MPa·m1/2 (depending on sintering temperature), in good agreement with the SEVNB measurements (0.57-1.13 MPa·m1/2). The fracture toughness was approximately linearly dependent on porosity over the range studied. However, experiments on films showed that the generation of observable indentation-induced cracks was very difficult for films sintered at temperatures below 1200°C. This was interpreted as being the result of the substrate having much higher modulus than these films. Cracks were only detectable in the films sintered at 1200°C and gave an apparent toughness of 0.17 MPa·m1/2 using the same analysis as for bulk specimens. This value is much smaller than that for bulk material with the same porosity. The residual thermal expansion mismatch stress measured using XRD was found to be responsible for such a low apparent toughness. [ABSTRACT FROM AUTHOR]

Powders composing of La2 Zr2 O7 ( LZ) and ( Zr0.8 Y0.2) O1.9 (10 YSZ) phases (volume ratio = 1:1) were synthesized by using a sol-spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ-10 YSZ powders were investigated. XRD results showed that the grain size of LZ and 10 YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single-phase LZ and 10 YSZ powders were in the range 9%-36% and 37%-86%. The activation energy for grain growth of LZ and 10 YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single-phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10 YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10 YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10 YSZ phases in the composite. [ABSTRACT FROM AUTHOR]

A liquid desiccant solvent extraction technique is introduced to significantly reduce the amount of warping during drying in thermoreversibly gelcast alumina samples by eliminating capillary stresses during the initial solvent removal stage. This process is combined with a secondary drying step in a vacuum oven with an in-line solvent trap, allowing for the complete recovery of the alcohol solvent used in producing gels. Optimal concentrations and immersion times for poly( tert-butyl acrylate)-based and water-based liquid desiccants are determined, along with the osmotic stresses and equilibrium solvent concentrations of gels immersed in these desiccants. Solvent loss during the liquid desiccant solvent extraction step, volumetric shrinkage during drying and sintering, and sintered porosity are measured. It is determined that a ~10 wt% poly( tert-butyl acrylate)/90 wt% isopropyl alcohol liquid desiccant is optimal. [ABSTRACT FROM AUTHOR]

A catalytic technique to enhance graphite formation in nongraphitizing carbons was adapted to work with three-dimensional wood-derived scaffolds. Unlike many synthetic graphite precursors, wood and other cellulosic carbons remain largely disordered after high temperature pyrolysis. Using a nickel nitrate liquid catalyst and controlled pyrolysis conditions, wood-derived scaffolds were produced showing similar graphitic content to traditional pitch-based graphite while retaining the high-aspect ratio pores of the precursor wood microstructure. Graphite formation was studied as a function of processing time and pyrolysis temperature, and the resulting carbons were analyzed using x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, and electron microscopy techniques. [ABSTRACT FROM AUTHOR]

The flexural strength of gypsum is reported for freestanding single crystals in three-point bending carried with a nanoindenter. The elastic modulus, splitting tensile strength, and fracture toughness of monolithic gypsum consisting of interlocking needle-like microcrystals are also reported as functions of porosity and accelerator addition. This study shows that geometric configurations, in addition to porosity, affect the mechanical properties of gypsum. The properties are improved by 50–100% when the crystal network changes from needle aggregates to one made up of homogeneous randomly oriented single crystals. An Ashby geometric model for open-cell foams is adopted to link the properties of the individual crystals and the bulk properties. The lower and upper bounds of the measured elastic modulus are in accordance with bending-dominated behavior and stretch-dominated behavior predicted by the model, respectively. However, the strength of gypsum is much lower than values predicted by the model, which is based failure on fracture of individual crystals, suggesting that the strength of monolithic gypsum may be instead controlled by the failure of weak intercrystalline contacts. [ABSTRACT FROM AUTHOR]

Thermoreversible gelcasting (TRG) is an attractive net-shape powder-based processing technique which relies on the temperature-driven gelation of a polymer solution. This study uses the TRG of alumina to investigate the implications of triblock copolymer design (block length, endblock fraction, and midblock chemistry) on rheological and green body properties. The liquid-to-solid transition and relaxation time in the gel state are controlled by the polymer's endblock length while the total polymer length controls the viscosity at high temperature. Although triblock design and concentration do not affect the green body porosity or sintered density, they do have significant effects on green body behavior. Triblocks with a high fraction of rubbery midblock behave as elastomers and confer significant toughness to the green bodies. In contrast, those with glassy midblocks increase the strength of the body but also behave in a brittle manner. Green body strength increases with increasing triblock concentration and is well described by a model for the strength of ceramic bodies with the binder localized at the particle necks. [ABSTRACT FROM AUTHOR]

The microstructures of beech wood and of beech wood-derived carbon, silicon carbide (SiC), and an aluminum/SiC composite were studied using both scanning electron microscopy (SEM) and synchrotron X-ray micro-computed tomography (µCT). As opposed to traditional two-dimensional imaging techniques, the µCT data allowed nondestructive evaluation of relatively large sample volumes. Nondestructive three-dimensional data analysis led to the observation of microstructural features, such as varying pore-wall topographies not previously seen in SEM, calculations of the volume fraction of porosity and characterization of the interconnectivity of porosity in the SiC material. [ABSTRACT FROM AUTHOR]

Silicon-based ceramics (SiC, Si3N4) are promising materials systems for high-temperature structural applications in gas turbine engines. However, the silica layer that forms on these materials is susceptible to attack from water vapor present in combustion environments. To protect against this degradation, environmental barrier coatings (EBCs) have been developed to shield the underlying substrate and prevent degradation. Here we report on elastic and thermal properties, as well as internal stresses of candidate multilayer coatings, as measured in situ using microfocused high-energy X-rays in a transmission diffraction geometry. Doped aluminosilicate coatings were investigated for their stability on a SiC/SiC melt-infiltrated substrate. The coatings consisted of a Ba1− xSr xAl2Si2O8 topcoat with a mullite or mullite+SrAl2Si2O8 interlayer, and a silicon bond coat. A numerical model was used to compare the stress results with an ideal coating system. Experiments were carried out on as-sprayed and heat-treated samples in order to analyze the strain and phase evolution as a function of multilayer depth and temperature. The phase transformation of the topcoat promoted healing of cracks in the EBC and reduced stresses in the underlying layers and the addition of SAS to the interlayer reduced stresses in thermally cycled coatings, but did not stop cracks from forming. [ABSTRACT FROM AUTHOR]

The development of texture in barium hexaferrite by templated grain growth was studied as a function of the Fe2O3/BaCO3 ratio, B2O3 additions in the starting materials, and sintering temperature. A magnetic field was used to orient the template particles during the gelcasting process. Excess BaCO3 resulted in abnormal grain growth and maximized texture, while B2O3 additions promoted coarsening, but no abnormal grain growth. [ABSTRACT FROM AUTHOR]

Biomorphic silicon carbide (bioSiC), a novel porous ceramic derived from natural wood precursors, has potential applicability at high temperatures, particularly when rapid temperature changes occur. The thermal conductivity of bioSiC from five different precursors was experimentally determined using flash diffusivity and specific heat measurements at temperatures ranging from room temperature to 1100°C. The results were compared with values obtained from object-oriented finite-element analysis (OOF). OOF was also used to model and understand the heat-flow paths through the complex bioSiC microstructures. [ABSTRACT FROM AUTHOR]

Pure Ta2O5 has been investigated as a possible environmental barrier coating (EBC) material for silicon nitride-based ceramics, but was found to be inadequate because of localized buckling and the formation of microcracks. To address these issues, alloying additions were explored to improve some of the properties of Ta2O5 for this application. Aluminum oxide (Al2O3) was investigated to stabilize the low-temperature β-Ta2O5 phase and reduce grain growth in the coating through the formation of a solid solution. Lanthanum oxide (La2O3) was investigated as a second phase former designed to pin grain-boundaries and reduce grain growth. High-brilliance X-rays at the Advanced Photon Source at Argonne National Laboratory were used to evaluate residual stress and phase evolution in the alloyed coatings. Scanning electron microscopy was used to study microstructural changes and grain growth. Al2O3 was successful at stabilizing β-Ta2O5 and the Al2O3 and La2O3 significantly reduced grain growth in the coatings. Significant microcracking was observed in the coatings alloyed with Al2O3 after thermal cycling in air. Coatings alloyed with Al2O3 and La2O3 showed no evidence of microcracking. However, some of the alloyed coatings which were first thermally cycled in air and subsequently exposed to high-temperature steam completely delaminated from the AS800 silicon nitride substrates, a highly undesirable result for an EBC. [ABSTRACT FROM AUTHOR]

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta2O5) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta2O5 coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta2O5. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic. [ABSTRACT FROM AUTHOR]

Alumina bilayers of different relative thicknesses and densities were produced with a strong interface using a modified gel-casting technique. Tolerance to surface damage is examined using biaxial flexure of disks damaged with a single Vickers indentation at various loads. The greatest surface flaw tolerance is seen in bilayers consisting of a thin porous layer on the tensile surface coupled to a thick dense layer. Here, the modulus mismatch causes redistribution of the applied stress, and fracture initiates at the internal porous-dense interface rather than at the surface from the introduced indentation flaw. [ABSTRACT FROM AUTHOR]

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 μm, 19 μm and 32 μm alumina; the resulting effective lamella diameters were 10 μm, 28 μm, and 55 μm, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 μm powder exhibited greater moduli than tubes sprayed from either 19 or 32 μm powders. This difference was attributed to the greater percentage of unmelted α-Al2O3 in the coating. [ABSTRACT FROM AUTHOR]

Multilayer coatings were prepared using small-particle plasma spray to investigate the effect of interfaces on thermal conductivity and phase stability. Monolithic and multilayer alumina and yttria partially-stabilized zirconia coatings, with 0, 3, 20, and 40 interfaces in 200–380 μm thick coatings were studied. Thermal conductivity was determined for the temperature range 25 °C to 1200 °C using the laser flash method and differential scanning calorimetry. Thermal conductivity of the multilayer coatings was accurately modeled by a series heat transfer equation, indicating that interfacial resistance plays a negligible role in heat transfer in the direction perpendicular to the coating plane. Powder X-ray diffraction results indicate that identical phase transitions occur in all the coatings. Independent of coating microstructure (i.e. layer thickness), as-sprayed γ-Al2O3 transforms to α-Al2O3 after 100 hours at 1200°C; as-sprayed metastable t′–ZrO2 converts to a mixture of t–ZrO2 and c–ZrO2 after 100 hours at 1300 °C. Thus, the results indicate that the interfaces do not aid in stabilizing the as-sprayed phases after prolonged severe heat treatments. [ABSTRACT FROM AUTHOR]

Presents a study on progressive fiber debonding in steel fiber/cementitious matrix composites using a single fiber pullout test. Pullout testing procedures and interferometry technique; Stress analysis; Discussion on interfacial sliding stresses.

The mechanical behavior [i.e., stiffness, strength, and toughness (KIC)] of SiC Al-Si-Mg metal-ceramic composites (50:50 by volume) was studied at temperatures ranging from 25 to 500 °C. The SiC phase was derived from wood precursors, which resulted in an interconnected anisotropic ceramic that constrained the pressure melt-infiltrated aluminum alloy. The composites were made using SiC derived from two woods (sapele and beech) and were studied in three orthogonal orientations. The mechanical properties and corresponding deformation micromechanisms were different in the longitudinal (LO) and transverse directions, but the influence of the precursor wood was small. The LO behavior was controlled by the rigid SiC preform and the load transfer from the metal to the ceramic. Moduli in this orientation were lower than the Halpin-Tsai predictions due to the nonlinear and nonparallel nature of the Al-filled pores. The LO KIC agreed with the Ashby model for the KIC contribution of ductile inclusions in a brittle ceramic. [ABSTRACT FROM AUTHOR]