Crystallization of acicular platelet particles of W-type hexagonal strontium ferrite for magnetic recording applications

Acicular platelets of the W-type hexagonal strontium ferrite (SrFe₁₈O₂₇) have been crystallized from the borate glass system with the aid of 1 mol% SiO₂ and Bi₂O₃. It was found that particle size and morphology of the system can be effectively altered by proper use of the additives and the thermal treatment of the samples. Addition of SiO₂ induces growth of ferrite as fine grains (average size 0.1m), while Bi₂O₃ with the acicular-shaped particles (which are platelets) with an aspect ratio of 10 are formed. The system also exhibits a relatively sharp particle size distribution compared to that for the samples containing no additive. Detailed magnetic and microstructural measurements of all the useful samples were carried out to characterize them for applications in magnetic recording devices.Visiting Scientist.     

Pseudo-binary system Bi2O3-TeO2 in air

The phase equilibria in the pseudo-binary system Bi₂O₃-TeO₂ at 600° 950° C in air were examined by solid-state reaction techniques and X-ray powder diffraction method. Four pseudo-binary compounds appeared, i.e., -Bi₂O₃ type solid solution having a compositional range of (1-x)Bi₂O₃·xTeO₂ wherex=0 0.4 a new compound Bi₆Te₂O₁₅ which has an orthorhombic cell of a=2.27(4) nm, b=1.06(1) nm and c = 0.539(8) nm, 2Bi₂O₃ · 3TeO₂, and an unidentified phase Bi₂O₃·2TeO₂. The formation of the phase Bi₆Te₂O₁₅, in which all the Te ions are hexavalent, was confirmed by the thermogravimetry and by the Mössbauer spectra. The liquidus curves for whole system were determined by DTA method.     

Fabrication of Bi2Sr2CaCu2Ox films on metal substrates by the chemical process using metal alkoxides

The formation behaviour of Bi₂Sr₂CaCu₂O_x from compounds prepared by hydrolysis of metal alkoxides was studied and Bi₂Sr₂CaCu₂O_x films on metal substrates were fabricated using a metal alkoxide solution. Bi₂Sr₂CaCu₂O_x was formed through intermediate phases such as Bi₂Sr₂Cu₁O_x, Bi₂CuO₄, SrCO₃, CaCO₃ and CuO. Bi₂CuO₄ was initially formed with SrCO₃, CaCO₃ and CuO, and then reacted with SrCO₃ to form Bi₂Sr₂Cu₁O_x. Bi₂Sr₂Cu₁O_x reacted with CaCO₃ and CuO to give Bi₂Sr₂CaCu₂O_x. Bi₂Sr₂CaCu₂O_x films were successfully fabricated on nickel substrates using the metal alkoxide solution at the nominal composition of BiSrCaCu=2223. Bi₂Sr₂CaCu₂O_x was precipitated on Ni substrates at firing temperature of 770 °C or above, and a sharp T _c was obtained at the firing temperature of 800 °C.     

Synthesis of Silver Nanoparticles by a Laser–Liquid–Solid Interaction Technique

Silver nanoparticles of high chemical homogeneity have been synthesized by a novel laser–liquid–solid interaction technique from a solution composed of silver nitrate, distilled water, ethylene glycol, and diethylene glycol. Rotating nickel, niobium, stainless steel, and ceramic Al₂O₃ substrates were irradiated using a continuous-wave CO₂ laser and Q-switched Nd–YAG laser ( = 1064 and 532 nm). The silver nanoparticles were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe x-ray microanalysis (EPMA). The shape of silver particles was dependent on the chemical composition and laser parameters. The synthesis mechanism of silver nanoparticles has been proposed to occur primarily at the laser–liquid–substrate interface by a nucleation and growth mechanism.     

A high-resolution transmission electron microscope study of a zinc oxide varistor

Investigations were made of varistor microstructure, the morphology of Bi₂O₃ at multiple ZnO grain junctions, Bi₂O₃/ZnO grain boundaries and ZnO/ZnO grain boundaries (especially whether Bi₂O₃ is present or not at the ZnO/ZnO grain boundary) by means of high-resolution transmission electron microscopy and X-ray microanalysis in the scanning transmission electron microscope. Bi₂O₃ at multiple ZnO grain junctions consists of small particles of 0.1m in diameter, and they are vitrified to some extent. It is suggested that bismuth ions dissolve into ZnO grains over a 30 nm range from a Bi₂O₃/ZnO grain boundary; however, there is no bismuth at ZnO/ZnO grain boundaries.     

Effects of oriented growth on properties of Ag/Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>/p-Si heterostructure prepared by Sol-Gel method with rapid thermal annealing techniques

Ag/Bi₄Ti₃O₁₂/p-Si heterostructures were fabricated by Sol-Gel method with rapid thermal annealing techniques. The effects of oriented growth on ferroelectric characteristics and electrical properties of Bi₄Ti₃O₁₂ film were investigated. Bi₄Ti₃O₁₂ films on bare p-Si exhibit preferred c-axis-orientation with the increase of annealing temperature, which would impair the ferroelectric properties but help to drop down the leakage current density of Bi₄Ti₃O₁₂ films. The Polarization-Voltage curves and the electrical characteristics curves show that the Bi₄Ti₃O₁₂ films annealed at 650 C for 5 min have good ferroelectric and electrical properties with a remanent polarization of 8.3 C/cm² and a leakage current density of < 5 × 10^–9 A/cm² at 6 V, which demonstrate that the Ag/Bi₄Ti₃O₁₂/p-Si heterostructure by Sol-Gel method with rapid thermal annealing techniques is a promising configuration for MFS-FETs applications.     

Some details of the ternary system Bi2O3-CaO-CuO

Before investigating the ternary system Bi₂O₃-CaO-CuO, a revision of the binary bounding system Bi₂O₃-CaO was first necessary. In the range between 20 and 70 mol % CaO the solid solutions and '₁ and the stoichiometric compounds Bi₂CaO₄, Bi₆Ca₄O₁₃, and Bi₂Ca₂O₅ were found to exist for 675 °CT780 °C. Above 780 °C a new high temperature compound with the formula Bi₆Ca₅O₁₄ has been identified. The X-ray powder diffraction data, unit cell dimensions, as well as the space group, have been reported. The ternary system contains no intermediate compounds and also no solubility was found for the binary bounding phases. Below 780 °C all the Bi-Ca oxides mentioned above are in equilibrium with CuO. At 820 °C, a wide liquidus field is dominating so that these quasibinary equilibria disappear. For 780 °C6Ca₅O₁₄ forms an equilibrium with Ca₂CuO₃ and CuO. Other equilibria are deduced. For a fixed ternary composition the reaction path of the sintering process was investigated as a function of time and temperature.     

Fabrication of CdO–Bi2O3 Ceramics

The phase formation in the CdO-rich part (10 wt % Bi₂O₃) of the CdO–Bi₂O₃ system was studied by x-ray diffraction, IR spectroscopy, electron microscopy, and other techniques. The results indicate that the grinding of the starting powder mixtures in a planetary mill leads to an inhomogeneous Bi₂O₃ distribution and a high defect density in the CdO and Bi₂O₃ particles. Sintering in the range 910–990 K gives rise to the formation of the _h phase, whose content depends strongly on the mixture composition and sintering conditions. At sintering temperatures above its melting point, the _h phase acts as a binder of CdO grains.     

Phase equilibrium relations in the binary system Bi2O3-ZnO

Phase equilibria in the binary system Bi₂O₃-ZnO were studied by quenching technique. Heat-treated compositions were subjected to X-ray diffraction for phase identification, and differential thermal analysis, optical and scanning electron microscopy were used to determine the solid-liquid equilibria occurring in this system. The data thus obtained revealed that incorporation of a small amount of ZnO to the high-temperature face-centered cubic lattice of Bi₂O₃ leads to the formation of a body-centered cubic solid solution (-Bi₂O₃), which extends up to a composition of 2.2 mol% ZnO at a temperature near 750°C. On cooling, the -Bi₂O₃ solid solution undergoes a eutectoid transformation at a temperature of 710°C to yield the low-temperature monoclinic polymorph of Bi₂O₃ (-Bi₂O₃) and Bi₃₈ZnO₅₈. The eutectoid occurs at a composition of 1.8 mol% ZnO. The compound Bi₃₈ZnO₅₈ has a crystal structure analogous to the body-centered cubic -Bi₂O₃ solid solution and melts incongruently at a temperature near 753 ± 2°C to yield -Bi₂O₃ and liquid. A binary eutectic occurs between Bi₃₈ZnO₅₈ and ZnO at a composition near 25 ± 1.0 mol% ZnO with a melting temperature of 738 ±2°C. Based on the data obtained in this study, a revised phase diagram of the binary system Bi₂O₃-ZnO is proposed.     

Microstructural properties of Pt-doped YBa2Cu3O7−x high-T c superconductor prepared by melting method

We have studied the effect of platinum addition on the superconducting properties of YBa₂Cu₃O_7–x (123) compound and elucidated from the metallurgical point of view the mechanism of formation of the fine dispersion of Y₂BaCuO₅ (211) particles in YBa₂Cu₃O_7–x superconductor prepared by a melting method. In this study, the amounts of BaCuO₂ and CuO-rich phases unreacted during the peritectic reaction were markedly decreased by the 211 powder addition. The 211 particles of Pt-free sintered samples were 8–10 m in size, but in 1 wt% Pt-added samples 211 particles were finely dispersed in the 123 matrix and the size of 211 particle was about 1–2 m. The critical temperature (T _c,zero) of Pt-doped samples was 91.5 K and the transport critical current density (J _c) of Pt-doped samples was much more than 10⁴A cm^–2. The high J _c and fine dispersion of 211 particles of Pt-doped YBa₂Cu₃O_7–x superconductor are attributed to Ba₄CuPt₂O₉ compounds formed during the partial melting, which were considered as nucleation sites of 211 particles, rather than Pt itself.     

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.     

Synthesis of new compounds of the pseudo-ternary system SrO-Bi2O3-TeO2 in air

The phase equilibria of the pseudo-ternary system SrO-Bi₂O₃-TeO₂ at 600 to 850 C were examined in air by solid-state reaction techniques and X-ray powder diffraction. Three pseudo-ternary compounds were found: a hexagonal solid solution Bi₂O₃ · 2xSrO · 5xTeO₂ (x = 0.02 to 0.20,a = 0.394(1) to 0.399(5),c = 1.89(6) to 1.86(7) nm), a rhombohedral phase Bi₄Sr₃Te₅O₁₉ (a = 0.405(2),c = 2.71 (5) nm in hexagonal terms) and a cubic phase Bi₂SrTeO₇ (a = 1.086(9) nm). The Mössbauer spectra of¹²⁵Te in the compounds indicated that the tellurium atoms were mostly in the state of Te⁺⁴ in the former two compounds but were exclusively Te⁺⁶ in Bi₂SrTeO₇. The electrical conductivity of the first two were about 10^–2–1 cm^–1 at 600 C. However, Bi₂SrTeO₇ was an electrical insulator.     

Synthesis of high-T c superconducting Bi-Pb Sr-Ca-Cu-O ceramics prepared by an ultrastructure processing via the oxalate route

The oxalate precursor yielded by a sol-gel processing can be pyrolysed and oxidized at 840 ° C for 40h into the desired high-T _c superconductor in the lead-doped Bi-Sr-Ca-Cu-O system withT _c(onset) = 140 K andT _c(zero) = 104K. The mechanisms of the sol-gel reaction were monitored by analysis of Fourier transformation infrared (FTIR) spectroscopy. Moreover, identification of the various phases existing on the specimens after firing at selected temperatures, was made by analysis by FTIR spectroscopy and X-ray diffractometry. Based on the experiment results, the mechanisms of forming Bi₂Sr₂CaCu₂O₈₊ (phase 2212) might be identified as the reaction of Bi₂Sr₂CuO⁶⁺, Ca₂CuO₃ and CuO at 800 ° C. Furthermore, Bi₂Sr₂Ca₂Cu₃O₁₀₊ (phase 2223) might be synthesized by the reaction of phase 2212, Ca₂CuO₃ and CuO at 840 ° C. Ca₂PbO₄ found in lead-doped materials may catalyse the formation reaction of Ca₂CuO₃ at 650 ° C and play a role of a flux to fuse Bi₂Sr₂CaCu₂O₈₊, CuO and residual Ca₂CuO₃ and then improve the further synthesis of Bi₂Sr₂Ca₂Cu₃O₁₀₊ at 840 C.     

The effect of an added seed on the phase transformation and the powder properties in the fabrication of Al2O3 powder by the sol-gel process

-Al₂O₃ powder was produced by the sol-gel process. The prepared sol-gels were seeded with 1.5 wt% powder (0.12m). The phase transformation of Boehmite into -Al₂O₃ and also the particle size distribution of the transformed -Al₂O₃ were strongly influenced by seeding and the heating rate during calcination. -Al₂O₃ seed particles have been shown to act as a nuclei for the transition of - to -Al₂O₃ and also to increase the driving force of the phase transformation, which consequently lowers the transformation temperature by about 200 °C. The particles derived from the seeded sol-gels retarded the formation of vermicular microstructures and were finer than those in the unseeded case. The seeding and the control of the heating rate during calcination could inhibit the grain growth due to transformation into -Al₂O₃. Fine particles which are homogeneous and have a high sinterability at lower temperatures could be obtained.     

Synthesis and dielectric properties of pyrochlore solid solutions in the Bi2O3-ZnO-Nb2O5-TiO2 system

Phase studies of solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ revealed extended regions of pyrochlore formation in the Bi₂O₃-ZnO-TiO₂-Nb₂O₅ system. At room temperature and 1 MHz (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ has a high permittivity ( = 200), low dielectric loss (tan<1·10^–4) and a temperature coefficient of the permittivity, , = –1300ppm/K. Pyrochlore solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be formed with (Bi_2–xZn_x)(Ti_{2– x} Nb_x)O₇ (0.35x1.0) and with (Bi_1.5 Zn_0.5–y/3Ti_1.5+yNb_0.5–2y/3) O₇ (–1.5y0.75). Investigations of the dielectric characteristics showed that the high temperature dependence of the permittivity of (Bi_1.5 Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be significantly modified by changing the composition of the pyrochlore within these regions of solid solubility. Below room temperature several of these compositions also exhibit a diffuse frequency dependent maximum in their permittivity characteristic of a transition to a relaxor type ferroelectric state. A third region of high permittivity pyrochlores with (Bi_1.5+2zZn_0.25–z Ti_2.25–z)O₇ (0.0z<0.15) was also identified in the Bi₂O₃-ZnO-TiO₂ sub system.     

Preparation of nano-structured ceramics using nanosized Al2O3 particles

This study is established with the theory that toughness of ceramics can be improved by reducing the grain sizes of alumina (Al₂O₃) ceramics. For nano-structured Al₂O₃ ceramics, nano-sized Al₂O₃ particles can be synthesized by MOCVD (metal organic chemical vapor deposition) method with Al(CH₃)₃. The synthesized particle sizes were 5.6, 11.2 and 22.4 nm, and these particles were used as initial materials. The grain sizes in nano-structured ceramics were controlled by both sintering temperature and holding time. They transformed dramatically from -Al₂O₃ to -Al₂O₃ when sintered above 1223 K for two hours. The transformation temperature was lowered by 250 K in comparison the temperature of the phase transformation in bulk. The grain size of nano-structured alumina ceramics grew with increasing sintering temperature. At the sintering temperature of 1173 K, it is necessary of an incubation time for grain growth. The incubation time became longer while particle size decreased. After the incubation time, the rate of grain growth was steeply increased. Above 1273 K, the grain growth directly occurred without any incubation time.     

The effect of secondary phases on electrical properties of ZnO-based ceramic films prepared by a sol–gel method

The ZnO-based ceramic films doped with Bi₂O₃, Sb₂O₃, MnO, Co₂O₃ and Cr₂O₃ were prepared for use as film varistors by a sol–gel method. The formation and the changes of the phases in the films doped with different dopants and annealed at different temperatures were investigated via X-ray diffraction analysis. Three secondary phases, i.e., Bi₂O₃, Zn₇Sb₂O₁₂ spinel and ZnCr₂O₄, were detected in the films when the annealing temperature was above 550 °C. The lattice constants of ZnO and Zn₇Sb₂O₁₂ spinel phase changed with dopants and the annealing temperature, indicating that the diffusion of the ions into the crystals of ZnO and spinel phase had taken placed. The redistribution of the ions changed the constituents of the intergranular phases and the relevant defect species in ZnO grains, and affected intensively the electrical properties of the films, which were used as film varistors. The highest nonlinear coefficient () with the lowest leakage current was achieved when the film, which was doped with Bi₂O₃, Sb₂O₃, MnO and Cr₂O₃, was annealed at 750 °C.     

LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> particles prepared by spray pyrolysis from spray solution with citric acid and ethylene glycol

Spinel LiMn₂O₄ particles with fine sizes and regular morphologies were successfully synthesized by ultrasonic spray pyrolysis at the severe preparation conditions from a spray solution with citric acid and ethylene glycol. The as-prepared particles with spherical shapes, porous structures and micron sizes turned into LiMn₂O₄ particles with submicron size and narrow size distribution at the post-treatment temperature of 800 °C. The discharge capacities of the particles prepared from the spray solution with citric acid and ethylene glycol changed from 90 to 127 mAh/g when the post-treatment temperature was changed from 700 to 1,000 °C. The LiMn₂O₄ particles had maximum discharge capacities at the post-treatment temperature of 800 °C. The discharge capacity of the LiMn₂O₄ particles dropped from 127 to 108 mAh/g by the 30th cycle.     

Solution route synthesis and characterization of nanocrystalline Bi<Subscript>2</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> for usage in resistor paste application

In this paper, we report synthesis of Bi₂Ru₂O₇ by solution route following the usual procedure, wherein, RuCl₃ solution was added drop by drop to a solution of bismuth nitrate while maintaining the pH of reaction at 6.75 with the help of dilute HCl and NaOH. Various physico-chemico-thermal analysis of these as -precipitated powders reveal that Bi & Ru precipitate as complex hydroxides with particle size ranging between 100–200 nm. The XRD analyses of the vacuum-dried (250°C) samples indicate conversion to Bi₂Ru₂O₇ (BRO) phase with low crystallinity having some RuO₂ impurity. Further heat treatment at 600°C for 4 h in an air atmosphere produces highly crystalline Bi₂Ru₂O₇ powder with a trace impurity of RuO₂. Resistor pastes were made using these BRO powders by intimately mixing them with other ingredients such as standard lead alumino-boro-silicate glass frit and polymeric thixotropic agents like ethyl cellulose and organic solvents like butyl cellosolve and -Terpinol. For all the paste samples, the inorganic part of the paste contained 43% glass and 57% BRO. These pastes were screen-printed and fired onto alumina substrates with pre-fired in-house Pd-Ag conductor terminations. The fired resistors had sheet resistance of 35–75 and hot TCR between 150 to 480 ppm/°C and zero cold TCR for resistors No.10 & 18 and for other resistors the cold TCR deviated from zero due to low scale processing of resistor paste.     

Electrical characteristics of SrxBi2.4Ta2O9 thin film and Pt/Sr0.85Bi2.4Ta2O9/Al2O3/Si structure

Sr _x Bi_2.4Ta₂O₉ (0.7 x 1.3) thin films were processed by metalorganic decomposition and their ferroelectric characteristics were investigated. The Sr-deficient Sr _x Bi_2.4Ta₂O₉ films exhibited well-developed ferroelectric hysteresis curves compared to those of the Sr-excess films, and Sr_0.85Bi_2.4Ta₂O₉ film had the optimum electrical characteristics among Sr _x Bi_2.4Ta₂O₉ films. Electrical characteristics of the Pt/SBT/Al₂O₃/Si structure using Sr_0.85Bi_2.4Ta₂O₉(SBT) film were investigated for metalferroelectric-insulator-semiconductor field-effect-transistor (MFIS-FET) applications. Memory window of C-V hysteresis characteristics of the Pt/SBT/Al₂O₃/Si structure became large with decreasing the Al₂O₃ thickness, and the Pt/SBT(400 nm)/Al₂O₃ (10 nm)/Si structure gave memory window of 2.2 V at sweeping voltages of ±5 V. The Pt/SBT/Al₂O₃/Si structure can be proposed for MFIS-FET applications.