Phase-transition toughening of high-T c superconducting ceramics

The effect of the superconducting transition (SCT) on the plane-strain fracture toughness K _1c of YBCO and BSCCO is studied by the method of three-point bend loading at room temperature and at 77 K. Toughening is observed and related to variation of the Gibbs free energy through the SCT, and hence to the condensation energy of a Cooper-pair gas. This is explained by the additional energy needed to excite superconducting electrons to normal ones in crack surfaces owing to magnetic field penetration. The superconducting gap parameters of YBCO and BSCCO are estimated and the results are reasonable.     

ZrO2陶瓷的强韧化技术

介绍了ZrO2 陶瓷的增韧机理及各种增韧技术。     

Fabrication and characterization of SiC-AIN alloys

SiC-AIN alloys were prepared by the carbothermal reduction of silica and alumina, derived from an intimate mixture of silica, aluminium chloride and starch. The resulting single-phase SiC-AIN powder was hot-pressed without additives to a high density. The dense bodies had a fine-grained uniform microstructure. The Young's elastic modulus, microhardness, fracture toughness, thermal expansion and thermal conductivity were measured as functions of composition. The creep behaviour of the SiC-AIN alloy was compared with that of silicon carbide.     

The stiffness and strength of transformation toughening ceramics with misoriented microcracks

Theoretical studies on misoriented transformation particles and microcracks in transformation toughening ceramics are presented using the Eshelby equivalent inclusion method. The stress field, stiffness and strength were calculated. Experiments were done by the three-point bend method using Al₂O₃/ZrO₂ ceramics and the stiffness and strength were also measured. Comparison between theoretical and test results confirmed the important role of microcracks.     

Transformation criterion of phase transformation toughening ceramics with misoriented microcracks

Phase transformation criterion is the key to investigating the toughness of phase transformation ceramics. In this paper, the modified equivalent inclusion theory by the authors is employed to study the interaction between microcracking and transformation in ceramics. The transformation criterion is derived. The influence of microcracks and transformation particles on the critical transformation load is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

A micromechanics theory for the transformation toughening of two-phase ceramics

Summary Based on the principle of thermodynamic equilibrium, the condition of stress-induced phase transformation in a two-phase ceramic is established. The development makes use of the change of potential energy that was calculated with a mean-field approach. In this process the elastic heterogeneity of the constituent phases, and the shape and volume concentration of the randomly oriented metastable ellipsoidal inclusions, are fully accounted for. Both the transformation heightH of the process zone with a steadily growing crack and the fracture toughness increment K of the transforming system are derived. The derived theory is then used to address the effect of inclusion shape and elastic inhomogeneity on the transformation toughening of two-phase ceramics. By considering the metastable ellipsoidal inclusions as phase 1 and the stable matrix as phase 0, it is found that, when ₁/₀>1, flat-like discs always provide a larger transformation-height while spherical ones provide the smallest, and vice versa. As the ratio of ₁/₀ increases, the size of the process zone also increases. For the toughness increment, the results indicate that thin-disc inclusions are again the most effective toughening medium. It is further found that Poisson's ratio of the constituent phases also has a significant effect; the combination ofv ₁0.5 for the inclusions andv ₁0 for the matrix has the best enhancement for fracture toughness. But whenv ₁, the toughness increment K all approaches to an asymptotic value regardless of the values of Poisson's ratios. Some explicit solutions of toughness change for several distinctive shapes of inclusions are also derived for the first time.     

固溶体的价电子结构与固溶强化

为在价电子结构层次研究代位固溶体与间隙固溶体强化差异的本质,建立了代位固溶体含溶质晶胞点阵常数的计算公式,以余(瑞璜)氏固体与分子经验电子理论为基础,按BLD(键距差分析方法)分析方法计算了代位固溶体含溶质晶胞的价电子结构.结果表明:代位固溶体中含溶质原子晶胞的nA值与不含溶质原子晶胞的nA值相差无几,而间隙固溶体中含溶质原子晶胞的nA值远高于不含溶质原子晶胞的nA值.这-信息从价电子结构的角度深刻揭示了两种固溶体强化程度存有严重差异的本质,从而把固溶体强化的机制追溯到固溶体的价电子结构.     

ZrO_2增韧Al_2O_3陶瓷的力学性能和增韧机制

对通过热压烧结法制备的3种陶瓷99.5vol%Al2O3(AD995)、ZrO2(15vol%)/Al2O3和ZrO2(25vol%)/Al2O3的力学性能和增韧机制进行了实验和理论研究。基于复合材料细观力学理论并考虑ZrO2的相变特性,建立了描述ZrO2/Al2O3陶瓷力学性能的本构模型。结果表明:ZrO2的加入细化了基体Al2O3晶粒,ZrO2/Al2O3陶瓷的致密性得到提高;3种陶瓷试件的破坏呈现小变形到脆性破坏的特点,压缩加载下试件应力-应变曲线近似为线性关系;AD995陶瓷的断裂韧性为5.65 MPa·m1/2,ZrO2(25vol%)/Al2O3陶瓷的断裂韧性为8.42 MPa·m1/2,提高了近50%;随ZrO2增韧相含量的增加,ZrO2/Al2O3陶瓷的弹性模量降低而断裂韧性增加,这一变化趋势与实验结果有良好的一致性。     

Transformation toughening behavior of two edge cracks emanating from a circular hole in zirconia ceramics

The growth and fracture toughening behavior of two edge cracks emanating from a circular hole in zirconia toughened ceramics is analyzed in this paper. The supercritical model is adopted in this study to simulate the stress-induced phase transformation. The two edge cracks are of equal length and located symmetrically at the edge of the circular hole. The initial transformation zone is assessed with the model proposed by David. M. Stump. Then the growth of the two edge cracks in supercritical zirconia ceramics is simulated with computational methods. Numerical examples on the enhanced fracture toughness due to phase transformation are given under different material and geometry parameters.     

Phase separation mechanism of rubber-modified epoxy

The phase separation mechanism during the cure reaction of a liquid rubber-modified epoxy resin was investigated by light scattering, light microscopy, torsional braid analysis, electron microscopy, and differential scanning calorimetry. The binary mixture of epoxy oligomer (diglycidyl ether of bisphenol A) and carboxyl-terminated butadiene-acrylonitrile copolymer (liquid rubber) exhibited the upper critical solution temperature-type phase behaviour. The mixture loaded with curing agent was a single-phase system in the early stage of curing. When the cure reaction proceeded, phase separation took place via the spinodal decomposition induced by the increase in the molecular weight of epoxy. This was supported by the characteristic change of light scattering profile with curing time. Electron microscopy revealed that, in cured resin, the spherical rubber domains are dispersed somewhat regularly in an epoxy matrix. The regular domain arrangement seems to result from a specific situation; the competitive progress of the spinodal decomposition and polymerization; i.e. the coarsening process to irregular domain structure seems to be suppressed by network formation in the epoxy phase. It was also shown that curing at higher temperatures resulted in the suppression at an earlier stage of spinodal decomposition, and hence, shorter interdomain spacing.     

Phase separation and ordering in the Fe-Co system

The Fe₅₀Co₅₀, Fe₆₅Co₃₅, and Fe₃₅Co₆₅ alloys heat-treated between 500 and 1300°C were characterized by x-ray diffraction and transmission electron microscopy. The surface layer of the alloys was found to phase-separate in the range 900–1300°C, leading to the formation of alternating α and γ zones on the order of 0.5 μm in thickness. Phase separation in the bulk of the alloys led to precipitation of an fcc phase, with a volume fraction no greater than 0.1. In the ordering region, the volume fraction of the B2 phase was also no greater than 0.1. The phase-separation region was shown to be divided from the ordering region by a solid-solution phase. Below 550°C, the alloys undergo a phase transformation accompanied by a slight increase in lattice parameter. Comparison with a similar transformation of the Fe₆₀Cr₃₀Co₁₀ alloy suggests that, under certain conditions, this transformation may lead to a significant rise in coercivity. The results are represented in a modified Fe-Co phase diagram.     

Phase morphology of nanofibre interlayers: Critical factor for toughening carbon/epoxy composites

Electrospun thermoplastic nanofibres were employed to toughen carbon/epoxy composites by direct deposition on carbon fibre fabrics, prior to resin impregnation and curing. The toughening mechanism was investigated with respect to the critical role of phase morphology on the toughening effect in carbon/epoxy composites. The influences of solubility in epoxy and melting characteristics of thermoplastics were studied towards their effects on phase structure and delamination resistance. For the three different thermoplastic nanofibre interlayers used in this work, i.e. poly(ε-caprolactone) (PCL), poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) nanofibre interlayers, only PCL nanofibres produced toughening. Although cylinder-shaped fibrous macrophases existed in all three interlayer regions, only PCL nanofibres had polymerisation-induced phase separation with epoxy, forming ductile thermoplastic-rich particulate microphases on the delamination plane. These findings clearly show that the polymerisation-induced phase separation is critical to the interlayer toughening by thermoplastic nanofibres. An optimal concentration (15 wt.%) of PCL solution for electrospinning was found to produce composites with enhanced mode I interlaminar fracture toughness (G_IC), stable crack growth and maintained flexural strength and modulus.     

Phase transition and PTCR effect in erbium doped BT ceramics

In this work the dielectric behaviour and main features of the phase transition of BaTiO₃ and Ba_0.99Er_0.01TiO₃ ceramics were carefully investigated. The temperature and frequency dependences of the dielectric properties of erbium doped BaTiO₃ ceramics were measured in the 25–225 °C and 100 Hz to 10 MHz ranges, respectively. From this study, a dielectric anomaly in the ferroelectric–paraelectric phase transition of the Ba_0.99Er_0.01TiO₃ ceramic was observed. The features of the samples phase transition were analysed by using Curie–Weiss, Santos–Eiras’ and order parameter local phenomenological models. In the BaTiO₃ system, all models showed a normal phase transition, while was not possible to establish the character of the phase transition in the Ba_0.99Er_0.01TiO₃ system. The relaxation parameters of conductive processes for the study ferroelectric materials, analysed in the time domain, did not show any influence on the ferroelectric–paraelectric phase transition. Finally, it was demonstrated that the anomaly observed on the phase transition of the erbium doped BaTiO₃ ceramics is associated with the processes that results in the PTCR effect.     

Phase transitions in Na2TeO4 ceramics

Polycrystalline samples of NaTeO₄ were prepared by conventional solid-state reaction technique at low temperature (600° C). X-ray powder diffraction (XRD) technique was used to check the formation of single phase NaTeO₄ compound with cell parametersa = 10·602(1) åb = 70·622(1) å andc = 8·506(1) å in orthorhombic crystal system. Detailed studies of dielectric constant (ε) (and loss tangent (tan δ) as a function of frequency (400 Hz-10 kHz) and temperature (?120°C–260°C) show that the compound has two phase transitions in the ferroelectric phase.