Grain Boundary Shortening in CuTl-1234 Superconductor by the Addition of ZnO Nanoparticles

The superconducting properties of Cu_0.5Tl_0.5 Ba₂Ca₃Cu₄O_12?x are studied after the inclusion of ZnO nanoparticles. The ZnO nanoparticles prepared by a sol-gel method were incorporated during the second stage of the synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12?x phase in y =?0, 3.0, 5.0, and 7.0 wt%. It is observed that the structure, the morphology, and the superconductivity properties are greatly influenced by the inclusion of ZnO nanoparticles. The lattice parameters of the orthorhombic phase of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12?x superconductor are decreased with the increase of x. Similarly, the grain morphology has been changed from needle-like to spherical grains. One of the major benefits of the inclusion of ZnO nanoparticles is the increase in critical temperature, critical magnetic fields, and critical current density as observed from the theoretical calculations of fluctuation-induced conductivity analysis.     

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi₂O₃-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are = 38, J _L = 5.55 × 10^–6 A/cm² and V _bk = 600 V/mm, whereas the intrinsic parameters of the materials are _b = 2.84 eV, N _d = 1.85 × 10²⁴ m^–3 and N _s = 7.02 × 10¹¹ cm^–2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, S) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (N _s) and the potential barrier height (_b), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.     

Interface reactions in the system sialon-Cu-Cu2O

The interface reactions between an /gb-sialon ceramic and Cu, Cu₂O or a Cu-Cu₂O mixture have been studied. A fully dense sialon ceramic material prepared by pressureless sintering at 1775 ° C with 6 wt% Y₂O₃ as sintering aid, were coldpressed together with Cu, Cu₂O or Cu-Cu₂O mixtures into cylindrical tablets. These samples were heat treated at 700, 850 and 1000 ° C in evacuated silica tubes. The reaction zones formed between the sialon and the powder compacts were studied in a SEM equipped with an EDS system. No reaction between copper and sialon ceramic could be detected in spite of prolonged heat treatment at 1000 ° C. Cu₂O reacted with the ceramic at 850 and 1000 ° C to form a glass containing copper and all the other sialon components. The interaction between the sialon material and the Cu-Cu₂O powder compacts was characterized by a redox reaction. The sialon was thus oxidized to SiO₂ and N₂ while Cu₂O was reduced to copper. A glass phase containing silicon, aluminium, yttrium and copper was also formed in the reaction.     

The microstructure of a ZnO varistor material

The microstructure of a ZnO varistor material has been investigated by a combination of X-ray diffractometry and analytical electron microscopy (SEM, TEM, STEM, EDX). The material was found to consist of: ZnO grains (doped with manganese, cobalt and nickel); smaller spinel grains which hinder the growth of ZnO grains during sintering; intergranular Bi-rich phases (namely -Bi₂O₃, pyrochlore and an amorphous phase); and a small proportion of ZnO-ZnO interfaces which did not have any intergranular film but to which bismuth had segregated. The intergranular microstructure is largely a result of processes which occur during liquid phase sintering and subsequent cooling to room temperature.     

Antibacterial activity of Cu2O and Ag co-modified rice grains-like ZnO nanocomposites

In this work, a series of Cu₂O-Ag/ZnO, Cu₂O/ZnO and Ag/ZnO nanocomposites with various compositions were prepared via a hydrothermal method followed by chemical modification, and their antibacterial performance was investigated in detail. X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy results confirmed that 31?nm Cu₂O and 30?nm Ag nanoparticles are well-dispersed on 202?nm ZnO grains to form a Cu₂O/ZnO and Ag/ZnO heterojunction, respectively. The bi-heterojuction structure in the Cu₂O-Ag/ZnO provided a synergistic effect on antibacterial activity, and the (Cu₂O)_0.04Ag_0.06ZnO_0.9 nanocomposites showed the highest antimicrobial activity of all samples with minimum inhibitory concentration and minimum bactericidal concentration against Escherichia coli and Staphylococcus aureus as low to 31.25?μg/mL, 250?μg/mL, 125?μg/mL and 500?μg/mL, respectively. This is the first report of the antibacterial activities of Cu₂O and Ag co-modified ZnO nanocomposites.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Precipitation of β particles in a fully lamellar Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si (at.%) alloy

The precipitation of (B2) particles at intermediate temperatures, between 760°C and 1050°C, is investigated in a fully lamellar TiAl-WMoSi alloy. The particles, having a thin-plate shape, usually precipitate on the ₂ side of ₂/ interfaces at low aging temperatures in an uneven two-dimensional growth mode. While those formed at higher aging temperatures, growing extensively within the ₂ plate and into the adjacent lamellae, have an ellipsoid shape. The growth of particles at low aging temperatures yields an activation energy of about 366 kJ/mol. It is suggested that at low aging temperatures the growth of particles proceeds via an ₂ precipitation process controlled by diffusion of W and Mo along the /₂ and / interfaces.     

Ceramic system based on ZnO-CuO-glass

Mixtures of powders containing ZnO, (CH₃COO)₂Cu·H₂O and glass frit were obtained by freeze-drying with compositions of 99 wt.% [(100 − x) mol% ZnO + x mol% Cu²⁺ (x = 0.05, 0.5, 5.0)] + 1 wt.% G, where G is a glass powder containing 26% SiO₂, 62% PbO, 7% B₂O₃, 5% ZnO in weight. Pellets were sintered in air at 950 °C/1 h. Ceramic system based on ZnO-CuO-glass originated during heating above 425 °C. Liquid-phase sintering occurred above 850 °C. Results showed that the breakdown electric field, the nonlinearity coefficient (α), average grain size and grain boundary voltage increased; at the same time that the leakage current decreased, densification was enhanced as the Cu-contents increased from 0.05 to 5. An excess Cu-containing segregate phase was observed in ceramics containing 5.0 mol% Cu²⁺. Samples presented densities above 95% of the theoretical density of ZnO and α > 200.     

Electrical properties and longitudinal modulus of superconductor/polymer NdBa2Cu3O7−δ/PVC composites

High temperature superconductor NdBa₂Cu₃O_7–/poly(vinyl chloride) (Nd123/PVC) composites have been prepared by cold press and hot press methods. Addition of PVC resulted in increase of the electrical resistivity with percolation occurring between 0.3 and 0.5 Nd123 volume fraction for both preparation methods. Although the samples showed some form of magnetic levitation at liquid nitrogen temperature, they do not show any zero-resistance temperature indicating the lack of effective superconducting percolative path. The density deviates from the linear calculated value with increasing Nd123 content at 0.3 volume fraction for the cold press and at 0.6 for the hot pressed samples due to increasing porosity. X-ray powder diffraction patterns suggest that the Nd123 crystals tend to align when the composites are prepared by hot press method. A resistance anomaly is observed at 90 K in the hot press composites indicating improved electrical contact between the aligned Nd123 grains. The longitudinal modulus increases as Nd123 content is increased.     

Influence of microstructure on strength and fracture toughness ofβ-Sialon

-Sialon (Si_6–z Al _z O _z N_8–z ) withz = 0.5 was fabricated by hot-pressing of a spray dried mixture of –Si₃N₄ and aluminium-isopropoxide solution. Phase composition, flexural strength and microstructure of a sintered body were investigated. Phases identified by XRD were -Sialon and a small amount of O-Sialon. The flexural strength (three-point bending) was about 1500 MN m^–2. This value, about three times higher than that of -Sialon fabricated from -Si₃N₄ and -Al₂O₃ powder, was mainly due to the homogeneous microstructure without large defects such like clusters of large grains. -Sialon was heat treated at 2000 °C for 2 h in 4 M Pa N₂ to develop elongated -Sialon grains with high aspect ratio. Microstructure, flexural strength and fracture toughness (K _Ic) of it were investigated. Both strength andK _Ic were lower than those of hot-pressed sample, even though an elongated microstructure was achieved. This fact showed that the toughening of -Sialon with elongated grains could not be achieved without grain boundary phase which resulted in a crack deflection.     

Sintering of Sb2O3-doped ZnO

The effect of Sb₂O₃ on the densification of ZnO was studied and compared with that for additives such as ZnSb₂O₆,-Zn₇Sb₂O₁₂, and Sb₂O₄. Addition of up to 2.0 mol% Sb₂O₃ raised the densification temperature of ZnO from 600° C to 1000° C regardless of doping level and the particle size of Sb₂O₃. Addition of other antimony-additives, however, slowed down the densification of ZnO, and the densification temperature increased with increasing amount of additives. Results of TG, XRD and SEM showed that the observed densification characteristics in Sb₂O₃-doped ZnO are explained by the volatile nature of Sb₂O₃. Thus, Sb₂O₃ evaporates during oxidation at about 500° C and condenses on the ZnO particle surfaces as a noncrystalline phase of an unknown composition, which checks the material transport across ZnO particles resulting in the retarded densification.     

Green synthesis of ZnO and Mg doped ZnO nanoparticles,and its optical properties

The Zinc oxide nanoparticles (ZnO NPs) and Magnesium doped ZnO nanoparticles (Mg doped ZnO NPs) are synthesized by Psidium guajava leaf extract. X-ray diffraction studies confirmed that, synthesized nanoparticles were retained the wurtzite hexagonal structure. In FESEM and HRTEM image analysis, ZnO and Mg doped ZnO NPs morphology were trigonal and spherical shape. Elemental compositions were identified by EDAX analysis. From FTIR result, the Zn–O stretching was observed at 453 and 448 cm^?1 for both ZnO samples. In Raman spectra, the high intensive E₂ ^high mode observed for 438 cm^?1 for ZnO NPs. But Mg doped ZnO NPs intensity of E₂ ^high mode decreased as compared to the pure ZnO NPs, it is due to the Mg²⁺ ion in to ZnO lattice site. The photoluminescence measurements revealed that the broad emission was composed of seven different bands due to zinc vacancies, oxygen vacancies and surface defects.     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Electrical characteristics and reheat-treatment effects in a ZnO varistor fabricated by two-stage heat-treatment

Conventional ZnO varistors are generally fabricated by sintering ZnO powder mixed with additives such as Bi₂O₃, Sb₂O₃, Cr₂O₃, Co₂O₃, and MnO₂. To reduce abnormal grain growth and change in electrical characteristics in the conventional ZnO varistors caused by volatilization of Bi₂O₃, the ZnO powder with all additive oxides except Bi₂O₃ was pressed into disc form and sintered. The disc was then painted with metal oxide paste containing Bi₂O₃ and again fired. The ZnO varistor fabricated by this process, i.e. a two-stage heat-treatment process, showed typical non-linearI-V characteristics with higher breakdown voltage exceeding 800 V mm^–1. It was also observed that the non-linearI-V coefficient change rate, , in the ZnO varistor due to reheat-treatment is almost linearly proportional to the sintered density.     

Influence of the high temperature treatment of zinc phosphate conversion coatings on the corrosion protection of steel

An anhydrous -Zn₃(PO₄)₂ phase converted by the dehydration of hydrous zinc phosphate, Zn₃(PO₄)₂·2H₂O, crystal coatings in air at a temperature of approximately 300 °C, significantly enhances the corrosion resistance of steel, and also reduces the susceptibility of the crystals to alkaline dissolution. A subsequent phase transition at approximately 500 °C results in a poor protection behaviour, because of the formation of numerous microcracks on the crystal faces.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Dielectric behaviour of spray dried Pb0.98(La1?x/3Nax)0.02(Zr0.53Ti0.47)0.9950O3 system

Sodium modified PLZT, Pb_0.98(La_{1 – x/3}Na_x)_0.02(Zr_0.53Ti_0.47)_0.9950O₃ (PLNZT) with x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, ceramics have been synthesized by spray drying technique. Rhombohedral phases of PLNZT ceramics were confirmed by XRD. Pellets prepared from the calcined powders were sintered at 1100°C. Micro-structural studies show the grains were spherical and homogeneous. Dielectric behaviour of these compounds has been studied as a function of temperature at 10 KHz frequency. Diffusivity of these compounds calculated from -T plots were found to lie between 1 and 2, which suggests that the compounds have diffuse phase transition (DPT). The dielectric constant () as found to increase initially upto x = 1.5 and then to decrease with increasing Na—concentrations.     

Growth of YBa2Cu3O7 – δ Single Crystals in “Ba3Cu5O8”/Y2BaCuO5 and (“Ba3Cu5O8” + 0.2BaCuO2)/Y2BaCuO5 Diffusion Couples

YBa₂Cu₃O_{7 –} single crystals were grown in Ba₃Cu₅O₈/Y₂BaCuO₅ and (Ba₃Cu₅O₈ + 0.2BaCuO₂)/Y₂BaCuO₅ diffusion couples at temperatures between 1170 and 1270 K under optimized conditions. Copper nonstoichiometry was shown to have a significant effect on the superconducting properties of YBa₂Cu₃O_{7 –} crystals subjected to thermal cycling.     

Eu Atomic Motion in EuSr2Cu3O7?δ and EuBa2Cu3O7?δ: A Comparative M?ssbauer Study

We present a comparative Mössbauer study of the ¹⁵¹Eu motions in the nonsuperconducting and superconducting cuprates EuSr₂Cu₃O_7– (ESCO) and EuBa₂Cu₃O_7– (EBCO), respectively, with the aim of investigating atomic-motion anharmonicities in these materials. In this study, a comparison with the results of a similar analysis of powdered samples of EBCO having oriented grains is also performed. We show that the Eu ion in ESCO, as well as in the common EBCO cuprates, does not move in a parabolic potential, but in a potential with a flat bottom. Moreover, for ESCO the flat part is about 1.5 times larger than for EBCO.     

Effect of Er2O3 addition on the microstructure, electrical properties, and stability of Pr6O11-based ZnO ceramic varistors

The microstructure, electrical properties, and stability of Pr₆O₁₁-based ZnO varistors, which are composed of ZnO-Pr₆O₁₁-CoO-Er₂O₃ systems, were investigated with Er₂O₃ additive content. The density of ceramics was in the range of 84–88% of TD at 1300 °C and 93–98% of TD at 1350 °C, and greatly affected the stability. Most of the added-Er₂O₃ were segregated at nodal points. The varistors with 0.5 mol% Er₂O₃ sintered at 1300 °C exhibited the best nonlinear current-voltage characteristics, which the nonlinear exponent is 52.8 and the leakage current is 9.8 A. All the varistors sintered at 1300 °C, even under relatively weak stress, exhibited the thermal runaway within short time in order of high leakage current. On the contrary, the stability of varistors sintered at 1350 °C exhibited far higher stability than that at 1300 °C. Particularly, the varistors with 0.5 mol% Er₂O₃ exhibited not only relatively good nonlinear current-voltage characteristics, which the nonlinear exponent is 34.8 and the leakage current is 7.4 A, but also excellent stability, which the variation rates of varistor voltage, nonlinear exponent, and leakage current are below 1%, 3%, and 3%, respectively, even under more severe stress such as (0.80 V _{1 mA}/90 °C/12 h) + (0.85 V _{1 mA}/115 °C/12 h) + (0.90 V _{1 mA}/120 °C/12 h).