Superplastic behaviour of YBa2Cu3O7−x /Ag superconductors

High-temperature deformation characteristics of YBa₂Cu₃O_7–x oxide (YBCO) and YBa₂Cu₃O_7–x /Ag composite (YBCO/Ag) in uniaxial compression have been investigated. A compression test was carried out at temperatures from 780–930°C at initial strain rates between 10^–6 and 10^–4 s^–1. YBCO/Ag composites with fine, dense and equiaxed grains were compressed over 120% with no indication of failure at higher temperatures, and the strain-rate sensitivity exponent, m, was found to be about 0.42–0.46 between 890 and 930°C. They are considered to be one indication of superplasticity. The activation energy for deformation was 500–580 KJ mol^–1. The specimens suffered grain growth slightly during the deformation at 930°C and the majority of growth might be a function of exposure time, temperature and silver content, but each grain maintained the equiaxed shape after extensive superplastic deformation. This is consistent with a grain-boundary sliding mechanism. The silver at grain boundaries acts to decrease the activation energy for deformation and promote the grain-boundary sliding.     

Some elevated temperature mechanical properties of Magnox AL80

The influence of grain-boundary hydride (MgH₂) precipitation on the mechanical properties of Magnox AL80 has been investigated by carrying out slow tensile and fatigue tests. The presence of hydride precipitates at some grain boundaries was found to have a detrimental effect on the slow tensile ductilities at the lower end (20 to 40% h^–1) of the range of strain rates used (20 to 2000% h^–1) at 250° C and at 20% h–1 at 300° C. Grain-boundary cavities caused by the dissociation of the hydride precipitates were introduced either byin situ thermal treatment prior to testing at 250° C or by testing at higher temperatures and also resulted in lower tensile ductilities at slow strain rates. The fatigue lives of Magnox AL80 were not significantly influenced by the presence of grain-boundary hydride precipitates or cavities due to the dissociation of the precipitates. This is due to the majority of the boundaries being unsuitably oriented to experience high shear stresses. Long-term ageing at 450° C was found to reduce the subsequent fatigue strength at that temperature and this effect is associated with the boundaries becoming more prone to migration during fatigue cycling.     

HRTEM study of [001] low-angle tilt grain boundaries in fiber-textured BaTiO3 thin films

Low-angle tilt grain boundaries in [001] fiber-textured BaTiO₃ thin films were investigated by high-resolution transmission electron microscopy. Extensive observation revealed a very high density of low-angle tilt grain boundaries in the film. The low-angle tilt grain boundaries can be described as periodical arrays of dislocations on {100} and {110} boundary planes. The boundaries with (100) plane on {100} planes are composed of perfect dislocations with Burgers vectors b = a < 100 > (a = lattice constant of BaTiO₃: 0.3992 nm), while the boundaries with (110) plane on {110} planes are composed of the dissociated dislocations with Burgers vectors a/2 < 110 >. It was thus found that the difference in the boundary plane leads to different dislocation structures along the low-angle grain boundaries.     

The relationship between the microstructure and microwave dielectric properties of zirconium titanate ceramics

Zirconium titanate (ZrTiO₄) ceramics have been prepared by the mixed oxide route using small additions of ZnO, Y₂O₃ or CuO. Specimens were sintered mainly at 1400 °C and cooled at various rates: water-quench, air-quench, 300 °C h^–1, 120 °C h^–1, 6 °C h^–1 and 1 °C h^–1. Products prepared with additives exhibited densities of at least 93% of the theoretical value. As the cooling rate after sintering was decreased, the length of the lattice parameter in the b direction was reduced and transmission electron diffraction revealed superlattice reflections associated with cation ordering. For specimens cooled at 1 °C h^–1, electron diffraction patterns exhibited features consistent with an incommensurate superstructure in the a direction. The dielectricQ value of rapidly cooled (air-quenched) ceramics was 2000 at 5 GHz. With an increase in the degree of cation ordering theQ value increased to a maximum of 4400 for specimens cooled at 6 °C h^–1. For specimens cooled at the slowest rate (1 °C h^–1) theQ value fell to 2000 due in part to the presence of microcracks and exsolved ZrO₂. Diffusion of trivalent impurities (yttria) into the host ZrTiO₄ grains also led to a lowering of theQ values.The microwave dielectric properties of zirconium titanate ceramics are sensitive to processing conditions and mircrostructural features. The highestQ values (lowest loss) should be achieved in homogeneous specimens, free of trivalent impurities and lattice defects, in which lowQ-value second phases, microcracks and pores are eliminated.     

Morphology and structure of tantalum oxide deposit prepared by KrF excimer laser CVD

Tantalum oxide films have been deposited on substrates by decomposing Ta(OCH₃)₅ photolytically in the beam of a KrF excimer laser under various conditions of laser fluence (200–450 J m^–2), repetition rate (20–120 Hz), supply rate of Ta(OCH₃)₅ (50–400 mg h^–1) and substrate temperature (403–723 K). The deposits were highly oriented when produced at laser fluences of 350 and 450 J m^–2. Their XRD patterns suggested the formation of -Ta₂O₅. The (1 1 1 0) planes were preferentially oriented parallel to the substrate surface when produced at lower repetition rates, higher supply rates of Ta(OCH₃)₅, and lower substrate temperatures; whereas (1 1 1 1) planes were similarly oriented when the conditions were reversed. The preferred orientation may be explained in terms of supersaturation. The deposits produced at a fluence of 200 J m^–2 were, however, rather amorphous.     

Aerobic composting of tobacco industry solid waste—simulation of the process

Solid waste accumulated during the processing of tobacco for cigarette manufacture mostly contains tobacco particles and flavoring agents. Its main characteristics are a high content of nicotine (2,000 mg per kg of total solids), which is a toxic compound, and high value of total organic carbon of the aqueous extract (12,620.0 mg l^–1). Because of this fact tobacco waste cannot be disposed of with urban waste.The aim of this work was to stabilize tobacco solid waste by aerobic composting. The experiments were carried out in closed thermally insulated column reactors (1.0 l and 25 l) under adiabatic conditions and at an airflow rate of 0.9 l min^–1 kg^–1 of volatile solids for 16 days. During the process, temperature changes in the reactor, CO₂ production and the numbers of mesophilic and thermophilic organisms in the mixed microbial culture were closely monitored. Nicotine concentration in the samples was analyzed at the start and at the end of process. It was estimated that at the end of composting the volume and mass of total solids in the tobacco waste were reduced by about 50% and those of nicotine by 80%. A simple empirical model was used to simulate the biodegradation rate of the organic fraction of the solid waste. It was found that the selected model describes aerobic composting fairly well, although only two kinetic parameters (k₀ and n) were estimated.List of symbols c_pS specific heat capacity of the substrate, kJ kg^–1 K^–1 - c_pz specific heat capacity of air, kJ kg^–1 K^–1 - F_Ku and F_Ki molar airflow at the reactor inlet and outlet, mol h^–1 - H_r reaction enthalpy, kJ kg^–1 of dry substrate - k specific rate, Eqs. (5) and (9), h^–1 - k_o constant in Eq. (9), day^–1 - m_o initial mass of the substrate, kg - m_S mass of dry substrate, kg - n order of the reaction in Eq. (5) - n_K molar amount of oxygen, mol - Q_v airflow volume, m³ h^–1 - r_K oxygen depletion rate, mol kg^–1 h^–1 - r_S degradation rate, kg kg^–1 h^–1 - _z air density, kg m^–3 - SD mean square deviation - t time, h - T temperature in reactor, °C - T_o temperature of substrate at the beginning of reaction, °C - T_K temperature of compost at the end of reaction, °C - T_u temperature of air at the reactor inlet - space time, day - w_S mass fraction of compost, m_sm_o^–1, kg kg^–1     

Rapid electroplating of photocatalytically highly active TiO2-Zn nanocomposite films on steel

Nanocomposite films consisting of TiO₂ and Zn with thickness of 10–15 m (TiO₂-Zn) have been electrodeposited on steel plates by rapid plating from a ZnSO₄-based bath (I _d > 10 A dm^–2). Upon addition of NH₄NO₃ to the bath (<0.3 g L^–1), the uptake of TiO₂ in the film significantly increased. Glow discharge optical emission spectrometry clarified that TiO₂ particles were incorporated throughout the film and the loaded amount increased near the surface. The first-order rate constant (k/h^–1) for gas-phase CH₃CHO oxidation was employed as an indicator of the photocatalytic activity. The k value for the TiO₂-Zn film prepared at I _d = 12 A dm^–2 (0.20 h^–1) was comparable to that for the sample from a ZnCl₂-based bath at I _d = 4 A dm^–2 (0.27 h^–1). X-ray diffraction measurements indicated that a TiO₂-ZnO nanocomposite layer was generated on the surface by the heat treatment in air at 673 K for 6 h. Consequently, the photocatalytic activity was further improved (k = 0.29 h^–1); this effect was explained in terms of the synergy of TiO₂ and ZnO in photocatalysis.     

Observations on crystal growth mechanisms in the directionally solidified high temperature superconductor Y1Ba2Cu3O7−δ

The growth behaviour of Y₁Ba₂Cu₃O_7–(Y1-2-3) crystals has been studied by horizontal directional solidification of samples heated into the liquid (L) plus Y₂Ba₁Cu₁O₅(Y2-1-1) phase field. Cylindrical samples of initially stoichiometric Y1-2-3 composition were solidified at rates ranging from 1 to 10 mm h^–1 using temperature gradients between 2.5 and 5 K mm^–1. A novel method was employed to support the L+Y2-1-1 semi-solid and minimize reaction with the crucible. Selected samples were quenched during solidification so that growth mechanisms could be studied. Quasi-single crystals of Y1-2-3 formed, providing that the growth rate did not exceed 1 mm h^–1 and that the temperature gradient was > 3.5 K mm^–1. The quasi-single crystals contained particles of Y2-1-1 as well as Ba-Cu-rich bands as secondary phases and had a preferred orientation of [001] at 45° to the growth axis when grown from a polycrystalline seed crystal. No preferred orientation developed when samples were grown without a seed. In quenched samples, macroscopic growth steps were observed on the (001) plane, and within the cylindrical sample the faces of the growing crystal were found to be mutually perpendicular planes. During solidification liquid was found to be lost from the L+Y2-1-1 semi-solid region of the sample. This occurred both by wetting of the support bars and by liquid migration into the seed crystal region.     

Eutectic solidification of the pseudo binary system of polyethylene and 1, 2, 4, 5-tetrachlorobenzene

This paper describes a study on the solidification of the pseudo binary eutectic system, consisting of unfractionated linear polyethylene and the faceted growing diluent 1, 2, 4, 5-tetrachlorobenzene. Crystallization under eutectic conditions resulted in very fine-grained structures, which were found to depend on the growth rate. This rate of solidification was varied by pulling the polymer solutions through a fixed temperature gradient of 3° C mm^–1 at different speeds ranging from 1 to 216 mm h^–1. Fibrillar polymer crystals with lateral dimensions of about 0.5 m remained after removal of the solid diluent. At rates of solidification in the region of 2 to 20 mm h^–1, the fibrils appeared to be aggregated in domains of well oriented structures, closely resembling the complex regular structures of the eutectic Sn-Bi system. At higher speeds the fibrillar crystals formed an irregular three-dimensional network. The polymeric structures grown from more dilute mixtures were characterized by rectangular holes originating from the growth of faceted primary diluent crystals. Despite the complexity of the crystallization of the highly entangled polymer solutions there appears to be quite some similarities between the eutectic polymeric system investigated and faceted/non-faceted atomic or small molecular eutectics.     

The effect of CuO on the grain growth of ZnO

The grain growth kinetics in the 1, 2, 3 and 4 wt.% CuO doped ZnO was studied using the simplified phenomenological grain growth kinetics equation Gⁿ – G₀ⁿ = K₀ t exp (– Q/RT) together with microstructure properties of the sintered samples. The grain growth exponent value (n) was found to be 3 for 1, 2 and 3 wt.% CuO doped ZnO and 5.5 for 4 wt.% CuO doped ZnO. The apparent activation energy was decreased with CuO doping up to 3 wt.% from 250 kJ/mol to 150 kJ/mol but it was not changed significantly (155 kJ/mol) by 4 wt.% CuO doping. CuO doping up to 3 wt.% promoted the grain growth of ZnO whereas 4 wt.% CuO doping inhibited the grain growth of ZnO because of formation of Cu-rich secondary phase in the grain boundaries.     

Microstructure, electrical properties, and degradation behavior of praseodymium oxides-based zinc oxide varistors doped with Y2O3

The microstructure, electrical properties, and degradation behavior of Pr-based zinc oxide varistors, which are composed of Zn-Pr-Co-Cr-Y oxides were investigated according to Y₂O₃ additive content in the range 0.5–4.0 mol%. The majority of the Sadded Y₂O₃ were segregated at the multiple ZnO grain junctions and grain boundaries. The average grain size was markedly decreased in the range 27.3–8.6 m with increasing Y₂O₃ additive content. Y₂O₃ acted as an inhibitor of grain growth. Additions of Y₂O₃ increased the varistor voltage in the range 36.90–686.58 V/mm, increased the nonlinear exponent in the range 3.75–87.42, decreased the leakage current in the range 115.48–0.047A, increased the barrier height in the range 1.06–2.16 eV, and decreased the donor concentration in the rang 1.87 × 10¹⁸–0.19 × 10¹⁸ cm^–3. Y₂O₃ acted as an acceptor, as a result of the decrease of donor concentration. All Pr-based ZnO varistors doped with Y₂O₃ exhibited very predominant degradation characteristics, which show a nearly symmetric I-V after the stress. In particular, since 4.0 mol% Y₂O₃-added ZnO varistor has not only very excellent non-ohmicity, but also very stable degradation behavior, it is estimated to be sufficiently used to various application fields.     

Grain growth in (Ca1−x,Mg x )Zr4(PO4)6 ceramics

Grain growth in (Ca_1–x ,Mg _x )Zr₄(PO₄)₆ (CMZP) ceramics in the final stage of sintering has been investigated. The grain growth in CMZP ceramics obeys the isothermal grain-growth kinetics with time exponent,n, lying between 1.8 and 2.4 which depends on magnesium content, indicative of a change in grain-growth rate. The time exponent for the grain growth of CMZP can be taken as 2.0 which implies that a normal grain growth develops in the CMZP ceramics. The apparent activation energy for grain growth demonstrates a maximum atx = 0.0 and a minimum atx = 0.1, with 103.2 and 39.4 kcal mol^–1, respectively, indicating that a small amount of magnesium promotes grain-boundary migration. The critical grain size for initiating microcracks in the CMZP increases with increasing magnesium and reaches 9–12 m whenx = 0.4.     

Density and deposition rate of chemical-vapour-deposited boron nitride

A study is made on the density and deposition rate characteristics of chemical-vapour-deposited boron nitride (CVD-BN) plates synthesized by use of the BCl₃-NH₃-H₂ system at a deposition temperature (T _dep) of 1200 to 2000°C and a total gas pressure (P _tot) of 5 to 60 torr. At aP _tot of 5 torr, all the CVD-BN plates synthesized at eachT _dep above 1300°C had a density greater than 2.O g cm^–3, and thus showed no noticeable dependence onT _dep. Over theP _tot range from 10 to 60 torr. on the other hand, the density of the plates reached the maximum of 2.08g cm^–3 at aT _dep of 2000° C. AsT _dep was lowered, the density decreased down to a minimum of 1.40 g cm^–3 The deposition rate varied with bothT _dep andP _tot and showed a maximum value under a certainP _tot at a givenT _dep. The value ofP _tot where the deposition rate becomes maximum changed depending on theT _dep. The maximum deposition rate was 0.6 mm h^–1 for the CVD-BN plates when the density was less than 2.0 g cm^–3, and 0.4 mm h^–1 when the density was above 2.0 g cm^–3 The effects of deposition conditions on the characteristics of density and deposition rate are discussed in terms of the structure and deposition mechanism.     

Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current–voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current–voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current–voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current–voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent Σ1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

Effects of pulling speed and hot zone temperature on the directional growth of YBa2Cu3Ox superconductor

The effects of pulling speed and hot zone temperature on the microstructure and the superconducting properties of YBa₂Cu₃O_x sample textured using directional growth of Y₂BaCuO₅, BaCuO₂, and CuO powder mixture were studied. The grain size, the alignment, and the critical current density of the sample were increased as the pulling speed decreased. The sample grown directionally at 1.5 mm h^–1 pulling speed consisted of a single grain. The zero resistivity temperature and the critical current density of the sample increased as the hot zone temperature increased up to 1120°C beyond which the sample consisted of Y₂O₃ and impurities and showed resistivity at 77 K. The sample grown directionally at 1.5 mm h^–1 pulling speed and at 1120 °C hot zone temperature showed sharp resistivity transition of 91 K zero resistivity and over 6000 Acm^–2. The sample showed well aligned microstructure. Compared to data from another study, the hot zone temperature required to produce maximum critical current density is lower due to low liquid forming temperature.     

A kinetic study of the silver-mercury contact reaction

The solid-liquid diffusion couple technique was employed to determine the interdiffusion coefficient of the gamma phase in the Ag-Hg contact reaction. Diffusion coefficients were calculated with the aid of an equation given by Wagner. The composition range of the gamma phase was determined to be between 55.3 and 57.5 at% by electron microprobe analysis, and values for the average interdiffusion coefficient of the gamma phase were found to beD _av(cm²sec^–1)=3.181×10^–5exp (–32539 (J mol^–1)/RT) in the temperature range 40 to 115° C. The amalgamation reaction between silver and liquid mercury proceeded with the formation of gamma phase and a solid solution of Ag-Hg. The growth of gamma phase followed a parabolic rate law. The penetration of liquid mercury into grain boundary of the gamma phase caused the gamma to be crumbled off. The possibility of short-circuit diffusion is discussed.     

Growth and structural studies of multifilamentary Nb3Sn formed by solid state diffusion

Growth of Nb₃Sn layer formed in a multifilamentary composite of bronze matrix by solid state diffusion has been investigated and its T_c values are measured after different time periods of diffusion anneal. The grain boundary structures are also systematically studied with scanning electron microscopy. Results obtained beyond reasonable doubt conclude that the rate controlling process for growth of Nb₃Sn is via diffusion through grain boundaries of Nb₃Sn layer, and grain growth significantly affects the layer growth. Both T_c and the transition width ΔT_c change in a regular way as annealing progresses, and the variation of these parameters is related to changes in the grain structure and internal strains of the sample.     

Effects of trace zirconium on the initiation and propagation behavior of fatigue cracks in Cu–6Ni–2Mn–2Sn–2Al alloy

The effects of trace Zr on the fatigue behavior of Cu–6Ni–2Mn–2Sn–2Al alloy were studied through the initiation and growth behavior of a major crack. When stress amplitude was less than σ_a = 350 MPa, the fatigue life of Zr-containing alloys was about 2 times larger than that of alloy without Zr. When σ_a = 400 MPa, the effects of Zr addition on fatigue life disappeared. Increased fatigue life due to Zr addition resulted from an increase in crack initiation life and microcrack growth life. Zr addition generated strengthened grain boundaries (GBs) that developed from the precipitation of SnZr compounds. Strengthened GBs contributed to the increase in crack initiation life. The effects of Zr addition on fatigue behavior were discussed with relation to the behavior of microcracks.     

The effect of boron addition on brittle-to-ductile transition temperature and its strain rate sensitivity in gamma titanium aluminide

Temperature dependence of tensile properties ofTi–47Al–2Mn–2Nb–0.8TiB₂ alloy was investigated andbrittle-to-ductile transition temperature (T _BD) wasevaluated accordingly within the strain rate range from 10^–5 to10^–1 s^–1. T _BD and its strain rate sensitivity inTi-47Al-2Mn-2Nb-0.8TiB₂ alloy were compared withthose in Ti-47Al-2Mn-2Nb alloy. It is found that theminor addition of 1.0 at% boron reduces T _BD by more than100 K and that T _BD in both alloys shows a positivesensitivity to the strain rate. But the B-doped alloy has a lower BDTactivation energy (256 kJ/mol) than that of B-free alloy(324 kJ/mol). The effect of boron on T _BD and its strain ratesensitivity is attributed to the reduction in the grain size.