Temperature dependence of the elastic moduli of multiferroic PbFe2/3W1/3O3 ceramics

Elastic properties of multiferroic PbFe_2/3W_1/3O₃ (PFW) ceramics have been studied in a temperature range of 4.2–400 K, which contains the regions of existence of the ferroelectric relaxor and antiferromagnetic phases. The longitudinal, shear, and bulk elasticity moduli, Young’s modulus, and Poisson’s ratio of PFW ceramics have been determined for the first time. Regions of temperature stability of the elastic properties extending over several dozen degrees have been found.     

High-temperature elastic moduli of thermoelectric SnTe1±x – y SiC nanoparticulate composites

In waste heat recovery applications, thermoelectric (TE) generators are subjected to thermal gradients and thermal transients, creating mechanical stresses in the TE legs. Such stresses are functions of the elastic moduli of the TE material. For SnTe_1±x matrices (where x = 0.0 or 0.016) composite specimens with 0–4 vol% SiC nanoparticle (SiC_NP) additions, the elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured by resonant ultrasound spectroscopy from room temperature (RT) to 663 K. The effects of matrix composition and the SiC_NP additions on the RT intercepts and the slopes of the elastic modulus as a function of temperature are also discussed.     

Orthotropic properties of loblolly pine (Pinus taeda) strands

Orthotropic properties of loblolly pine strands were measured from growth ring numbers 1–10 and 11–20 using digital image correlation (DIC). Eight elastic properties (E _L , E _R , E _T , G _LR , G _LT, υ_LR, υ_LT, and υ_RT) and five strength values (σ_L, σ_R, σ_T, τ_LR, and τ_LT) of loblolly pine strands were measured. Compliance matrices were derived and used to calculate the maximum normal and shear strains. With the increment of growth ring number, elastic modulus and ultimate tensile strength (UTS) generally increased, whereas shear strength decreased. Statistical comparison showed that elastic moduli, Poisson’s ratio, and UTS were highly dependent upon orientation of strands with loading direction. Only elastic modulus and UTS from the longitudinal-radial plane strands were significantly different by the two growth ring positions. More distinctive orthotropic properties of loblolly pine strands from growth ring numbers 11–20 were found associated with more consistent failure modes.     

CVD法制备SiC先进陶瓷材料研究进展

SiC陶瓷材料具有许多优异的性能如高比强度、高比模量、低密度、高导热系数，低的热膨胀系数、耐腐蚀、抗氧化等，从而被广泛用作高温结构部件，CVD工艺灵活，制备的SiC陶瓷具有很高纯度和致密度，因而是制备先进SiC陶瓷的最有希望的工艺之一。对CVD法制备SiC涂层和SiC基复合材料的研究及应用进行了综述。     

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al₂O₃ and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al₂O₃ content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al₂O₃, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.     

Formation mechanism and high temperature mechanical property characterization of SiC depletion layer in ZrB2/SiC ceramics

When ZrB₂/SiC ceramics are exposed to high temperatures, a SiC depletion layer will appear near the material surface. Due to the degradation of mechanical properties for the SiC depletion layer, the ZrB₂/SiC ceramics might fail initially at the SiC depletion layer. Based on chemical reaction analysis and microstructural observation, a pore evolution model is presented to characterize the formation mechanisms of the SiC depletion layer, which can calculate the variation in the volume fraction of each constituent during the oxidation process. The micromechanical models are developed to analyze the mechanical properties of the SiC depletion layer during high temperature oxidation. It is found that relative modulus and relative strength of the SiC depletion layer initially decrease and then gradually rise with the increase of oxidation time. As the SiC content of ZrB₂/SiC ceramics increases, the porosity of SiC depletion layer increases as well, whereas the mechanical properties significantly decrease.     

Low-temperature processing of porous SiC ceramics

Porous silicon carbide ceramics were fabricated from SiC, polysiloxane, and polymer microbead (as a pore former) at a temperature as low as 800 °C by a simple pressing and heat-treatment process. The effects of polysiloxane and template contents on the porosity and strength of the ceramics were investigated. During heat treatment, the polysiloxane transformed to an amorphous SiOC phase, which acted as the bonding material between SiC particles, and the polymer microbeads decomposed into gases and left pores. The porosity of porous SiC ceramics could be controlled within a range of 26–56 % with the present set of processing variables. The porous SiC ceramics showed a maximal porosity of 56 % when 10 μm SiC particles and 16 % polysiloxane were used with 20 % polymer microbeads. Flexural strength generally increased with increasing polysiloxane content and decreased with increasing polymer microbead content. Typical flexural strength of the porous SiC ceramics was 53 MPa at 42 % porosity.     

Correlation of elastic properties of melt infiltrated SiC/SiC composites to in situ properties of constituent phases

The ability to correlate the elastic properties of melt infiltrated SiC/SiC composites to properties of constituent phases using a hybrid Finite Element approach is examined and the influence of material internal features, such as the fabric architecture and intra-tow voids, on such correlation is elucidated. Tensile testing was carried out in air at room temperature and 1204 °C. Through-thickness compressive elastic modulus utilizing the stacked disk method was measured at room temperature. In situ moduli of constituent materials were experimentally evaluated using nano-indentation techniques at room temperature. A consistent relationship is observed between constituent properties and composite properties for in-plane normal and shear moduli and Poisson’s ratio at room temperature. However, experimental data for through-thickness compressive elastic modulus is lower than the calculated value. It is hypothesized that the existence of voids inside the fiber tows and their collapse under compressive loads is the cause of such discrepancy. Estimates for the change in elastic moduli of constituent phases with temperature were obtained from literature and used to calculate the elastic properties of the composites at 1204 °C. A reasonable correlation between the in-plane elastic moduli of the composite and the in situ elastic properties of constituent phases is observed.     

Effect of on-axis tensile loading on shear properties of an orthogonal 3D woven SiC/SiC composite

The present study examines in-plane and out-of-plane shear properties of an orthogonal 3D woven SiC fiber/SiC matrix composite. A composite beam with rectangular cross-section was subjected to a small torsional moment, and the torsional rigidities were measured using an optical lever. Based on the Lekhnitskii’s equation (Saint–Venant torsion theory) for a orthotropic material, the in-plane and out-of-plane shear moduli were simultaneously calculated. The estimated in-plane shear modulus agreed with the modulus measured from ±45° off-axis tensile testing. The effect of on-axis (0°/90°) tensile stress on the shear stiffness properties was also investigated by the repeated torsional tests after step-wise tensile loading. Both in-plane and out-of-plane shear moduli decreased by about 50% with increasing the on-axis tensile stress, and it is mainly due to the transverse crack propagation in 90° fiber bundles and matrix cracking in 0° fiber bundles. It was demonstrated that the torsional test is an effective method to estimate out-of-plane shear modulus of ceramic matrix composites, because a thick specimen is not required.     

Mechanical characterization of graphite/epoxy nanocomposites by multi-scale analysis

Mechanical properties of nanocomposites consisting of epoxy matrix reinforced with randomly oriented graphite platelets were studied by the Mori–Tanaka approach in conjunction with molecular mechanics. Elastic constants of graphite nanoplatelets, which are the inclusion phase in the micromechanical model, were calculated based on their molecular force field. The calculated elastic constants compared well with both experimental data and other published theoretical predictions. The results of the Mori–Tanaka micromechanical analysis, using the graphite platelet moduli calculated by molecular mechanics, were found to be insensitive to the variation of out-of-plane modulus E₃ and Poisson’s ratio ν₁₃. However, the nanocomposite modulus is sensitive to the in-plane modulus E₁ and out-of-plane shear modulus G₁₃ of the graphite platelets and less sensitive to the in-plane Poisson’s ratio ν₁₂ for its small range of variation under consideration. The calculations confirm that the modulus of the nanocomposites studied here is strongly dependent on the aspect ratio of the reinforcing particles, but not on their size. The predicted moduli compare favorably with experimental results of several nanocomposites with graphite particles of various aspect ratios and sizes.     

Non-destructive characterization of zirconia-toughened alumina ceramics using ultrasonics

Ultrasonics has been used for the characterization of sintered zirconia-toughened alumina (ZTA) ceramics. The variation of ultrasonic velocity with volume fraction of zirconia in an alumina matrix was studied. Variation of ultrasonic velocity with volume fraction of ZrO₂ content was analysed in terms of a second-degree polynomial in the volume fraction of ZrO₂. The elastic moduli and Poisson's ratio evaluated from the longitudinal and shear velocities have also been described in terms of a similar equation. The observed variation in velocity and moduli with porosity has been compared with the predicted values based on elasticity theory.     

Elastic moduli and crosslinking of some tellurite glass systems

Tellurite glass systems in the form 80(TeO₂)–5(TiO₂)–(15 − x)(WO₃)–(x)A_nO_m have been prepared by the melt quenching technique. The A_nO_m oxide was Nb₂O₅ or Nd₂O₃ or Er₂O₃ and x ≤ 5 mol%. Density and Molar volume have been determined for the prepared glasses. Both longitudinal and shear ultrasonic velocities were measured in different compositions of the glass system by using the pulse-echo method at 5 MHz frequency and at room temperature. Ultrasonic velocity and density data have been used to calculate elastic moduli (longitudinal modulus L, shear modulus G, Young's modulus E, Bulk modulus K), Poisson's ratio σ, and Debye temperature θ_D. Quantitative analysis of elastic moduli based on the number of bonds per unit volume, average crosslinks and number of vibrating atoms per unit volume has been achieved.     

Room temperature elastic moduli and Vickers hardness of hot-pressed LLZO cubic garnet

Cubic garnet Li_6.24La₃Zr₂Al_0.24O_11.98 (LLZO) is a candidate material for use as an electrolyte in Li–Air and Li–S batteries. The use of LLZO in practical devices will require LLZO to have good mechanical integrity in terms of scratch resistance (hardness) and an adequate stiffness (elastic modulus). In this paper, the powders were fabricated by powder processing of cast ingots. All specimens were then densified via hot pressing. The room temperature elastic moduli (Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio) and hardness were measured by resonant ultrasound spectroscopy, and Vickers indentation, respectively. For volume fraction porosity, P, the Young’s modulus was 149.8?±?0.4?GPa (P?=?0.03) and 132.6?±?0.2?GPa (P?=?0.06). The mean Vickers hardness was 6.3?±?0.3?GPa for P?=?0.03 and 5.2?±?0.4 for P?=?0.06.     

A nanoindentation analysis of the effects of microstructure on elastic properties of Al2O3/SiC composites

Nanoindentation tests were carried out to investigate certain elastic properties of Al₂O₃/SiC_p composites at microscopic scales (nm up to μm) and under ultra-low loads from 3 mN to 250 mN, with special attention paid to effects caused by SiC particles and pores. The measured Young’s modulus depends on the volume fraction of SiC particles and on the composite porosity and it can compare with that of alumina. The Young’s modulus exhibits large scatters at small penetrations, but it tends to be constant with lesser dispersion as the indentation depth increases. Further analysis indicated that the scatter results from specific microstructural heterogeneities. The measured Young’s moduli are in agreement with predictions, provided the actual role of the microstructure is taken into account.     

Preparation and properties of mullite-bonded porous SiC ceramics using porous alumina as oxide

Mullite-bonded porous silicon carbide ceramics were prepared by an in situ reaction bonding technique and sintering in air with SiC, porous Al₂O₃, and graphite as starting materials. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al₂O₃. The surface of SiC was oxidized to SiO₂ at high temperature. With a further increase in temperature, SiO₂ reacted with Al₂O₃ to form mullite. The reaction-bonding characteristics, phase composition, open porosity, mechanical strength as well as the microstructure of porous SiC ceramics were investigated.     

Influence of silver nanoparticle addition,porosity, and processing technique on the mechanical properties of Ba0.3Co4Sb12 skutterudites

The thermoelectric skutterudite Ba_0.3Co₄Sb₁₂ is a promising candidate for waste heat recovery applications. Recently, it was demonstrated that the addition of silver nanoparticles (Ag_NP) to Ba_0.3Co₄Sb₁₂ increases both the thermoelectric figure of merit and electrical conductivity. This study is the first to examine the effect of Ag_NP addition on the material’s mechanical properties. This study also found that the Young’s modulus, E, shear modulus, G, and bulk modulus, B, decreased linearly with increasing volume fraction porosity, P. Resonant ultrasound spectroscopy was employed to measure the elastic moduli, and Vickers indentation was used to determine the hardness, H, and fracture toughness, K _C. Trends in the mechanical properties as a function of grain size, porosity, and the Ag_NP are discussed in terms of the pertinent literature. While K _C was independent of Ag_NP addition, porosity, and grain size, both E and H decreased linearly with increasing porosity. In addition, this study is the first to identify (i) the Ag₃Sb phase formed and (ii) the enhanced densification that occurs when the Ag_NP is sintered with Ba_0.3Co₄Sb₁₂ powders, where both effects are consistent with the eutectic and peritectic reactions observed in the binary phase diagram Ag–Sb. These eutectic/peritectic reactions may also be linked to the enhancement of electrical conductivity previously observed when Ag is added to Ba_0.3Co₄Sb₁₂. Also, similar beneficial eutectic/peritectic reactions may be available for other systems where conductive particles are added to other antimonides or other thermoelectric systems.     

Open pore description of mechanical properties of ceramics

A dependence of Young's modulus of elasticity on open porosity in ceramics is derived from an open-porosity model, which in the literature, is applied to salinity conductivity and fluid permeability in rocks. A random distribution of grain and pore size is assumed. The relation developed,E(p)=E _o(1–"p)^m, whereE is the modulus of elasticity of the porous ceramic,E _o is the theoretical elastic modulus,p is the porosity andm is an exponent dependent on the tortuosity of the structure of the ceramic, adequately describes the dependence of the modulus of elasticity on porosity. The model is applied to the experimental data from several ceramics such as alumina, silicon nitride, silicon carbide, uranium oxide, rare-earth oxides, and YBa₂Cu₃O_7– superconductor, and the value ofm is obtained for each case. We have shown thatm has a value of nearly 2 for sintered ceramics, unless sintering aids or hot pressing have been used during fabrication of the ceramic. Such additional procedures approximately double the magnitude ofm. On joint appointment with: Materials Laboratory, Physics Department, University of the West Indies, Mona, Kingston 7, Jamaica,     

Shear coupling effects on stress and strain distributions in wood subjected to transverse compression

The mechanical behaviour of a wood board subjected to transverse compression is relevant to the performance of glulam beams and solid wood structures. The wood material can be described as polar orthotropic, due to the annual ring structure and to the differences in moduli in different directions in the radial–tangential plane. Strain measurements are performed on single wood boards using a whole-field digital speckle photography technique. Finite element analysis is performed and compared with experimental data. Good agreement in terms of strain fields and apparent moduli is observed between predictions and data. The experimental data show strong variations in local strain due to the polar orthotropic behaviour of wood in this plane, and the extremely low value for shear modulus G_rt as compared with the other moduli. This leads to shear coupling effects resulting in large local shear deformation and correspondingly low effective stiffness under transverse global loading.     

Mechanical and microstructural properties of cordierite-bonded porous SiC ceramics processed by infiltration technique using various pore formers

Cordierite-bonded porous SiC ceramics were prepared by air sintering of cordierite sol infiltrated porous powder compacts of SiC with graphite and polymer microbeads as pore-forming agents. The effect of sintering temperature, type of pore former and its morphology on microstructure, mechanical strength, phase composition, porosity and pore size distribution pattern of porous SiC ceramics were investigated. Depending on type and size of pore former, the average pore diameter, porosities and flexural strength of the final ceramics sintered at 1400 °C varied in the range of ~ 7.6 to 10.1 µm, 34–49 vol% and 34–15 MPa, respectively. The strength–porosity relationship was explained by the minimum solid area (MSA) model. After mechanical stress was applied to the porous SiC ceramics, microstructures of fracture surface appeared without affecting dense struts of thickness ~ 2 to 10 µm showing restriction in crack propagation through interfacial zone of SiC particles. The effect of corrosion on oxide bond phases was investigated in strong acid and basic salt medium at 90 °C. The residual mechanical strength, SEM micrographs and EDX analyses were conducted on the corroded samples and explained the corrosion mechanisms.