Anisotropic Fracture Behavior in Ferroelectric Relaxor PZN–4.5%PT Single Crystals

The anisotropic fracture behavior in unpoled and poled (1− x )Pb(Zn_2/3Nb_1/3)− x PbTiO₃ ( x =0.045) (PZN–4.5%PT) single-crystal relaxor ferroelectrics cut along the crystal planes, [010] and [001], was characterized. The crack tip toughness ( K _tip) determined from Vickers indentations was compared with the R -curve behavior measured using the single-edge V-notch beam (SEVNB) method. Several of the SEVNB fracture experiments resulted in cracks forming and propagating under mixed mode loading ( K _I and K _II) along the [110] crystal plane. Other specimens formed cracks at 0° along the [010] plane. To assess the anisotropic fracture behavior, the local and global critical energy release rates were determined using Stroh's formalism.     

Mixed-Mode Fracture of Ceramics in Asymmetric Four-Point Bending: Effect of Crack-Face Grain Interlocking/Bridging

The mixed-mode fracture of a large-grain-size alumina ceramic and a soda-lime glass is investigated. These ceramics are tested using straight-through precracked or notched specimens. The straight-through precrack is introduced by the single-edge-precracked beam method. Precracked or notched specimens are subjected to combined mode I/II or pure mode II fracture, under asymmetric four-point bending, and pure mode I fracture, under symmetric four-point bending. A pure mode II fracture is never achieved in the precracked polycrystalline alumina by the crack-face friction inevitably induced by grain interlocking/bridging. The crack-face friction in sliding mode reduces the local mode II stress intensity factor in the crack-tip region and produces a sizable amount of mode I deformation. Accounting for the contribution of the crack-face friction to the crack-tip local stress intensity factors, K _I and K _II, in mixed-mode fracture tests, the experimental results of the K _I/ K _{I c} versus K _II/ K _{I c} envelope and the initial angle of noncoplanar crack extension are in good agreement with the theoretical predictions of the maximum hoop-stress theory.     

Determination of Fracture Toughness at High Temperatures after Subcritical Crack Extension

As a consequence of R -curve behavior, ceramic materials may exhibit increased fracture toughness ( K _Ic) following slow crack extention. In this investigation, the effect of crack propagation on fracture toughness is studied in static bending tests. For the calculation of stress intensity factors ( K _I) the stress distribution must be known at the moment of fracture. As a consequence of creep, this stress distribution must deviate from the linear distribution. The corresponding stress intensity factors are computed using the fracture mechanical weight function. Experimental results for fracture toughness are communicated for a 2.5%-MgO-doped hot-pressed Si₃N₄ at 1300°.     

Slow Crack Growth from Controlled Surface Flaws in Hot-Pressed Si3N4

The elevated-temperature slow-crack-growth behavior of HS-130 Si₃N₄ was studied by extending "controlled" surface cracks in bars loaded in 4-point bending. Several such nonin-teracting cracks were produced on the tensile surfaces of bend bars by Knoop microhardness indentation. The stress and dimensions of the subcritically growing cracks were used to calculate the stress-intensity factor, K₁ , from fracture-mechanics formulas for semielliptical surface cracks in bending. The crack-growth velocity, v, was obtained by dividing crack extension by loading time interval. The data indicated very large scatter in measured velocities for given K₁ values, which was interpreted as due to the interaction of the small cracks with local material heterogeneities. No simple functional relation between K _I and v could be established for HS-130 Si₃N ₄ from the v − K ₁ data.     

Direct Measurement of Crack Tip Stresses

Raman microprobe spectroscopy was used to measure the crack tip stress distribution in single-crystalline silicon using a wedge-loaded double cantilever beam (dcb) specimen. The wedge was advanced until the crack just propagated. After the crack arrested, the stresses were measured between 3 and 20 μm directly ahead of the crack. An average value of –0.43 was obtained for the slope of log stress vs log distance from the crack tip, rather than the theoretical value of –0.5. The value of K _I was determined from (1) the intercept of this curve and (2) the slope of stress against     . The values were in good agreement. The average experimental value of K _I was determined to be 0.71 MPa · m^1/2, compared to literature values for the K _IC ranging from silicon of 0.8 to 0.94 MPa · m^1/2. The value measured with the Raman is the arrest value of K _I and is expected to be lower from kinetic energy considerations associated with wedge-loaded dcb specimens.     

Polymorphism of Tricalcium Silicate and Its Solid Solutions

In tricalcium silicate and its solid solutions six modifications were established: one rhombohedral ( R ), two monoclinic ( M _I, M _II), and three triclinic ( T _I, T _II, T _III). All these forms are rhombohedral or pseudorhombohedral and extremely similar; the variations in the X-ray diagrams and the transformation enthalpies are minute. Only three of the five transitions were observed by both X rays and DTA: T _I- T _II (600°), T _III– M _I (980°), and M _I– M _II (990°); one was established by DTA ( T _II– T _III, 920°) and one by X rays only ( M _II– R , 1050°C). This scheme of polymorphism is compared critically with results of other authors, and the nature of the various displacive transformations is discussed. X-ray and DTA investigations of solid solutions of Ca₃SiO₅ with Al₂O₃ and ZnO established the phase relations as functions of temperature and composition. With Al₂O₃, two modifications ( T _I and T _II) were stabilized; and with ZnO, five polymorphs ( T _I T _II, M_I, M _II, and R ). With both pure C₃S and the solid solutions, the results are consistent and have a close bearing on the constitution of alites in portland cement clinker.     

Power Generation of a p-Type Ca3Co4O9/n-Type CaMnO3 Module

Thermoelectric modules composed of eight pairs of p -type Ca_2.7Bi_0.3Co₄O₉ (Co-349) and n -type CaMn_0.98Mo_0.02O₃ (Mn-113) bulks were constructed using Ag electrodes and paste including powder of the n -type oxide. The former bulks were prepared by hot pressing. On the other hand, the latter were densified using a cold isostatic pressing technique and sintered in atmospheric pressure. Dimensions of both oxide legs were 5 mm wide and thick and 4.5 mm high. An alumina plate was used as a substrate, and there was no alumina plate on the other side of the modules. When the substrate side was heated, the module could generate up to 1.0 V and 0.17 W of open circuit voltage ( V _O) and maximum power ( P _max), respectively, at a hot-side temperature of 1273 K (furnace temperature as a heat source) and a cold-side temperature of 298 K (circulated water temperature) in air. But internal resistance R _I reached a value of 1.5 Ω, which is about six times higher than the calculated one from resistivity of both p - and n -type bulks. When the substrate side was cooled, V _O and P _max reached 0.7 V and 0.34 W of V _O and P _max, respectively, at a furnace temperature of 1273 K.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Alignment of YBa2Cu3O7-x and Ag─YBa2Cu3O7-x Composites at ∼930°C by Eutectic Formation

The present work describes a new technique to synthesize aligned YBa₂Cu₃O_{7- x} and Ag─YBa₂Cu₃O_{7- x} superconducting composites from Ba- and Cu-deficient compositions (relative to YBa₂Cu₃O_{7- x} ) plus BaCuO₂. For YBa₂Cu₃O_{7- x} , high transition temperature midpoint T_c (91 K), temperature of zero resistivity T ₀ (90 K), and critical current density J_c (>3000 A°Cm⁻² at 77 K) were achieved by using this technique. This procedure provides the potential for using a reliable and reproducible densification and alignment technique alternative to partial or full melting. The composite is highly aligned, with an average grain size of ∼1 to 2 mm and domains of width greater than 5 mm. The initial phase assemblage consists of YBa₂Cu₃O_{7- x} (123) as the major phase plus YBa₂CuO₅ (211) CuO as minor phases. The BaCuO₂ is added to the Ba- and Cu-deficient starting composition in order to assist in the formation of a CuO-rich liquid as well as to compensate for the Ba and Cu deficiences in 123. Since the liquid forms at ∼900°C and is compatible with 123, it can be used to facilitate alignment of 123 at ∼930°C. The addition of Ag to the system results in eutectic formation with the (solidified) liquid, substantial filling of the pores during sintering, and improved alignment.     

Enhancement of Tc(0) by Substitution of Gallium in the Bismuth-Based High-Tc Superconducting Material

We report the enhancement of the zero resistivity T _c(0) by 5.5 K i.e. from 104 to 109.5 K by substitution of gallium 1.34% of copper in the bismuth 2223 compound. A series of Ga-containing compounds Bi₂Pb_0.4Sr₂Ca₂Cu_{3− x} Ga _x O _y ( x =0.00, 0.02, 0.04, 0.06, and 0.08) are synthesized by the solid-state reaction method. The samples are characterized by measurements of their dc electrical resistivity and ac magnetic susceptibility and by the powder X-ray diffraction analysis. It is noted that the high- T _c (2223) phase increases from 57.55% in an undoped sample to 92.99% in samples containing a low concentration of gallium i.e. x ≤0.04.     

Oxygen Displacement through the Ferroelectric Phase Transition of Barium Titanate—High-Temperature 17O NMR

Perovskite-structured BaTiO₃ undergoes a ferroelectric tetragonal-to-cubic phase transition at 130°C. High-temperature ¹⁷O MAS and static NMR line shapes and relaxation times were examined to elucidate the nature of the anion motions associated with the transition. ¹⁷O MAS spectra show a gradual shift of the two oxygen peaks toward one another with increasing temperature, and a sudden collapse to one peak at the transition, which is consistent with its first-order character. ¹⁷O T _I data are relatively imprecise below the transition, but appear to decrease above it, indicating an increase in low-frequency motions, suggesting a dynamically averaged high-temperature phase.     

Domain Boundary and Domain Switching in a Ceramic Rare-Earth Orthoniobate LaNbO4

By using TEM and X-ray diffraction, the domain structure in ceramic LaNbO₄ has been studied. It has been found that the domains have two different orientations, separated by a boundary with_irrational Miller indexes between (20     )_I/ (402)_II and (20     )_I/(602)_II lattice planes, which is in good agreement with the boundary indexes calculated from elasticity theory and spontaneous strain tensor. The orientation relationship between the domains has been determined as a rotation of 95.6° about the [010] axis. A model for domain boundary configuration, a diffuse boundary with a transition zone, has been proposed according to TEM diffraction patterns and HRTEM images. Based on this domain boundary model, a domain switching mechanism is suggested, and the corresponding driving force for domain switching is considered.     

Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and III Loading

Fracture and subcritical crack-growth characteristics under combined Modes I and III loading were studied using the modified compact tension (CT) specimens of soda-lime glass. The combined mode load was applied to the specimen in the direction β with respect to the initial crack. By superposition of Mode III, the advancing crack begins to rotate at an angle Ψ to the initial crack plane, which nearly maximizes the Mode I stress intensity factor K _I(Ψ), and the crack continues to propagate in the same direction. In this case, unlike combined Modes I and II, the crack breaks into multiple partial fronts, and ligamentary bridging forms fracture lances when these segmented cracks are held together. The crack velocity d a /d t was plotted versus the maximum Mode I stress intensity factor K _I(Ψ) for combined Modes I and III loading. The d a /d t values are initially high, and the crack growth tends to be discontinuous compared with the result for pure Mode I. The subcritical crack growth seems to occur when the K _I value for the initial crack reaches a certain value. The d a /d t - K _I(Ψ) curves for combined Modes I and III lie roughly on the same curve as that for pure Mode I as the crack growth increases.     

Characterization of a Large-Scale Nondoped YBa2Cu3O7−x Superconductor Prepared by Plastic Forming Without High-Pressure Molding

A slurry containing YBa₂Cu₃O_{7− x} particles and a fine YBa₂Cu₃(OH) _x colloid solution was prepared, and a large-scale bulk YBa₂Cu₃O_{7− x} superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa₂Cu₃O_{7− x} structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T _c) was 88.7±1.4 K. The critical current density ( J _c) measured at 77 K was about 440 A/cm², the value of J _c was improved by the heat treatment under an oxygen atmosphere, and J _c=1.6 × 10³ A/cm² was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.     

K/Na Ratio Dependence of the Electrical Properties of [(KxNa1−x)0.95Li0.05](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K _x Na_{1− x} )_0.95Li_0.05](Nb_0.95Ta_0.05)O₃ (K _x NLNT) ( x= 0.40–0.60) lead-free piezoelectric ceramics were prepared by conventional solid-state sintering. The effects of K/Na ratio on the dielectric, piezoelectric, and ferroelectric properties of the K _x NLNT ceramics were studied. The experimental results show that the electrical properties strongly depend on the K/Na ratio in the K _x NLNT ceramics. The K _x NLNT ( x =0.42) ceramics exhibit enhanced properties ( d ₃₃∼242 pC/N, k _p∼45.7%, k _t∼47%, T _c∼432°C, T _o−t =48°C, ɛ_r∼1040, tanδ∼2.0%, P _r∼26.4 μC/cm², E _c∼10.3 kV/cm). Enhanced electrical properties of the K _x NLNT ( x =0.42) ceramics could be attributed to the polymorphic phase transition near room temperature. These results show that the K _x NLNT ( x =0.42) ceramic is a promising lead-free piezoelectric material.     

Effect of Praseodymium on Superconductivity and Magnetism in Y1–xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that most samples of Y_1-x Pr_xBa₂Cu₄O₈ examined in the present study contained a single YBa₂ Cu₄O₈ (1-2-4) superconductive phase for x<0.7.Lattice parameters a and b increased with Pr concentration, suggesting that most of the Pr is trivalent in Y_1-x Pr_x-Ba₂Cu₄O₈. The zero-resistance temperature, T _co, decreases monotonically from 80 K at x=0 to 12 K at x=0.65, and superconducting transition widths tend to broaden for x>0. The room-temperature resistivity changes linearly until x=0.7 and increases abruptly at x=-0.75. The critical concentration, x_cr, thus was estimated to be 0.7. The effective magnetic moments of Pr in Y _1-x Pr_xBa₂Cu₄O₈ were 3.63., 3.35, and 3.23, μ_B for x=0.2, 0.4 and 0.6, respectively. In the R_0.8 Pr_0.2Ba₂Cu₄O₈ system, the depression of T_c weakly depends on the ionic radius of rare-earth elements. Similarities and differences between Y _1-x Pr_xBa₂Cu₄O₈ and Y_1-xPr_x-Ba₂Cu₃O_7-y also were noted and are discussed in this paper.     

Methane Oxidation on Perovskite-Type Ca(Mn1−xTix)O3−δ

X-ray photoelectron spectroscopy was performed to elucidate the catalytic activity of CH₄ oxidation on perovskite-type Ca(Mn_{1− x} Ti _x )O_3−δ synthesized at 1173 K in a flow of oxygen from a gel with citric acid and ethylene glycol. The Mn ion content decreases and the ratio of the Mn³⁺ ion in the Mn ion increases with increases in x . Ca(Mn_{1− x} Ti _x )O_3−δ has a high catalytic activity of CH₄ oxidation at x =0.4. These results indicate that the catalytic activity strongly depends on the Mn³⁺ ion content of the surface.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

Ferroelectric and Piezoelectric Properties of Na1−xBaxNb1−xTixO3 Ceramics

Piezoelectric ceramics Na_{1− x} Ba _x Nb_{1− x} Ti _x O₃ with low BaTiO₃ concentrations x have been prepared by the solid-state reaction method, and their ferroelectric and piezoelectric properties have been studied. The ceramics are classic ferroelectrics when x ≤0.10, and the ferroelectric–paraelectric phase transition becomes diffusive when x ≥0.15. A low doping level of BaTiO₃ changes the NaNbO₃ ceramics from antiferroelectric to ferroelectric. With the increase in BaTiO₃ doping level, the Curie temperature of ceramics decreases linearly and the remnant polarization and coercive field also decrease, while their dielectric constant increases. Na_0.9Ba_0.1Nb_0.9Ti_0.1O₃ ceramics show the largest piezoelectric constant d ₃₃ (147 pC/N) and good sinterability, suggesting that it is a good candidate for lead-free piezoelectric ceramics.     

Preparation and Characterization of a Large-Scale YBa2Cu3O7−x Superconductor Prepared by Plastic Forming without a High-Pressure Molding: Effect of Polyvinyl Alcohol (PVA) Addition on the Superconducting Properties

The preparation of large-scale YBa₂Cu₃O_{7− x} superconductor samples was investigated. This method is based on plastic forming using a slurry consisting of YBa₂Cu₃O_{7− x} particles and a sol solution made up of multimetallic hydroxide particles (YBa₂Cu₃(OH) _x colloidal particles) and poly(vinyl) alcohol (PVA). The effects of adding PVA on the product, the crystallinity, and the superconducting properties of the sample were investigated. It was found that PVA acted as a protective colloid in the sol solution and stabilized YBa₂Cu₃(OH) _x colloidal particles, and that the role of PVA changed from a thickener to a flocculant during drying so that the formability/workability of the green sheet sample was improved and large samples (about 80 mm × 80 mm × 3 mm) without large cracks were obtained after firing. The samples became superconducting at 91.5±0.5 K ( T _con) and the full transition temperature ( T _coff) was 88.5±1.5 K. The critical current density ( J _c) of the sample prepared from the slurry containing 1 wt% PVA was 713±150 A/cm² at 77 K. This J _c value was improved to 2300 A/cm² by heat treatment at 773 K under an oxygen atmosphere.