Phase Formation and Grain Growth Kinetics of High-Tc Superconducting Tl-1223 Ceramics

The phase formation and grain growth of (Tl,Pb,Bi)(Sr,Ba)₂Ca₂Cu₃O_9?δ ceramics were investigated in the temperature range between 860°C and 1000°C. It was found that partial melting seems to improve the texture of the samples. The phase purity, however, is limited due to an incomplete re-precipitation process due to the formation of a liquid phase, which is already present above 885°C. The resulting grain growth exponent n, however, appears to differ from the classical exponent of 1/3 for the coexistence of a liquid phase in the temperature range between 885°C and 920°C. For the first time, the activation energy for the high-T_c Tl-1223 phase has been determined showing faster grain growth kinetics than for Y-123 and Bi-2223.     

Evolution of the microstructure of undoped and Nb-doped SrTiO3

Undoped and Nb-doped SrTiO₃ specimens with excess titania compositions were prepared by sintering in air at 1420 or 1480 °C. Large grains due to liquid-phase sintering were obtained for undoped specimens containing 0.6 mol % excess titania and fired at 1480 °C. On the other hand uniform fine grains were observed for samples fired at 1420 °C, resulting from grain-growth inhibition due to exsolved TiO₂ second phase. The solubility of excess titania seemed less than 0.2 mol% under our experimental conditions. The microstructural behaviour of Nb-doped SrTiO₃ could be explained well by the Sr-vacancy compensation model. According to this model, the solubility of excess titania in SrTiO₃ increased with Nb₂O₅ dopant concentration. Thus, for specimens which had high excess titania compositions and were sintered at 1480 °C, large grains were observed when the Nb content was low enough to retain sufficient excess titania-forming liquid phase. For specimens having the same compositions and fired at 1420 °C, uniform fine grains were obtained due to grain growth inhibition by the exsolved TiO₂ second phase, when the Nb content was low. If the excess titania was less than the solubility determined by the amount of Nb dopant, Ruddlesden-Popper-type phases were believed to be formed and resulted in poor densification. Although excess titania was the major factor in determining the grain size of the specimens, the niobium dopant enhanced grain growth.     

Microstructure and permittivity of sintered BaTiO3: influence of particle surface chemistry in an aqueous medium

The influence of changes in the surface chemistry and surface composition of colloidal BaTiO₃, due to its dissolution and adsorption/precipitation of Ba²⁺ in an aqueous medium, on the microstructure and permittivity of sintered powder compacts was investigated. For BaTiO₃ powder with Ba-deficient (Ti-excess) surface prepared at pH 3, grain growth was enhanced at 1350 °C (above the eutectic) and permittivity was reduced (relative to stoichiometric BaTiO₃ prepared at pH 9) with increasing sintering temperature due to the liquid phase formed at grain boundaries. This same sample showed minimal grain growth and moderate enhancement of sinterability at 1300 °C (below the eutectic) attributed to sliding of the Ti-excess surface phase. BaTiO₃ powder treated at pH 3 and subsequently adjusted to pH 10 results in a core-shell structure with a varying near-surface stoichiometry, and produced abnormal grain growth for the compact sintered at 1350 °C. Permittivity of this sample was significantly reduced at 1350 °C due to the formation of the liquid phase, while exhibiting a similar permittivity to that of the stoichiometric sample when sintered at 1300 °C, despite significant microstructural coarsening. We conclude that changes in the surface-phase Ba/Ti ratio of particulate precursors, due to dissolution, adsorption and precipitation reactions in aqueous media, are as significant in determining the mechanical and electronic properties of the sintered material as are variations in the bulk stoichiometry of BaTiO₃.     

Metastable states of the 2223 phase in the Bi(Pb)SrCaCuO system

By the magnetically modulated microwave absorption method (MAMMA) we observed the modifications induced by different calcination temperatures (between 830°C and 870°C) on the 2223 phase formed in a system sintered at (855±5)°C with the starting composition Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y . The presence of the 2223 phase in almost distinct states (consequently, multiple 2223 phases) in the same sample was observed. As the calcination temperature was increased up to 850°C, the highest temperature state of the 2223 phase intensified. Higher calcination temperatures resulted in the enhancement of other lower-temperature states. The homogeneity of the 2223 phase was greatly improved by annealing the samples at, 800°C for 5 min in a flowing nitrogen atmosphere. We labeled as metastable the lower-temperature states having excess oxygen, which, by easily losing the supplementary oxygen under the above annealing procedure, were shifted to higher temperatures.     

Sintering mechanisms of 0.99 SnO2-0.01 CuO mixtures

The densification kinetics of 0.99 SnO₂-0.01 CuO molar mixtures have been studied between 850 and 1150 °C. Both experimental analyses and theoretical modelling show the role of a liquid phase for sintering temperatures T _s 940 °C: sintering is controlled by a liquid phase after a fast shrinkage due to grain rearrangement. The analytical formulation of the shrinkage behaviour suggests that dissolution at the solid-liquid interface is the limiting process when T _s 1000 °C and diffusion is the limiting step at higher temperatures.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3Oy Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Influence of the additives and processing conditions on the characteristics of dense SnO2-based ceramics

Three different SnO₂-based powder mixtures, containing 2 wt% CuO as sintering aid and Sb₂O₃ in amounts from 0 to 4 wt% as activator of the electrical conductivity, were sintered to high density at temperatures in the range 1000–1400°C and soaking times from 1 to 6 h. Densification behaviour and microstructure development are strongly dependent on the presence of CuO, that gives rise to a liquid phase, and on Sb₂O₃ that retards the liquid phase formation and hinders grain growth. Cu and Sb cations can enter s.s. in the SnO₂ network with different oxidation states and in different positions, depending on the sintering conditions. The characteristics of the grain boundary phase, of the SnO₂ solid solutions and their modification depending on thermal treatments were analyzed. The electrical resistivity values varied in a wide range from 10^–1 to 10⁴ cm, depending on starting composition and processing conditions: in terms of the final density and of the electrical conductivity, the optimal sintering conditions were found to be 1200°C, for 1–3 h. The electrical resistivity was related to the microstructural features, particularly to the characteristics of the resulting SnO₂-based solid solutions.     

Study on microstructure and magnetic domain structure in sputtered (Ni66Fe22Co12) x C1−x nanocomposite films

Composition and annealing temperature dependence of microstructure and magnetic domain structure in sputtered (Ni₆₆Fe₂₂Co₁₂) _x C_1–x nanocomposite films with x=10–75 at % were studied by X-ray diffraction and magnetic force microscopy (MFM). Films with x20 at % showed amorphous structures, and no domain structure could be found due to the disappearance of magnetocrystalline anisotropy. For the films with x=30–55 at %, face-centered cubic (fcc) NiFeCo nanocrystals encapsulated in graphite-like carbon could be found in the samples annealed beyond 400 °C, and stripe domains with typical dimension of 120–150 nm were observed. For the films with x62 at %, the as-deposited films went through a meta-stable stage at which a rhombohedral Ni₃C phase and fcc NiFeCo co-existed after annealing to a temperature between about 300–400 °C (dependent on composition). Upon further annealing to a sufficiently high temperature between about 350–500 °C, the carbide phase decomposed into fcc NiFeCo and graphite. While short-range domain structures were observed in the samples before the formation of carbide phase, long-range domain structure with dispersed domains in the meta-stable stage were observed. After the decomposition of carbide, large domains with typical size of 500–700 nm were observed due to the formation of large grain aggregators.     

Material properties and growth mechanism of CuInSe2 prepared by H2Se treatment of metallic alloys

A fundamental understanding of the mechanism of growth of CuInSe₂ is essential for the production of device quality material. In this contribution, the growth kinetics of thin film CuInSe₂ are investigated in the special case of H_2Se/Ar treated copper-indium metallic alloys. A systematic study was conducted in which the evolution of surface morphologies by scanning electron microscopy (SEM), formation of crystalline X-ray diffraction (XRD) and variation in film composition, energy dispersive spectrometry (EDS) were evaluated during various stages of selenization. SEM and XRD studies revealed a dramatic improvement in crystalline quality with increasing selenization temperature. SEM studies indicated a substantial increase in grain size (0.2m 1m) when the reaction temperature was increased from 150 °C to 450 °C. XRD studies revealed the presence of mostly binary phases (i.e. Cu₁₁In₉, InSe, In₆Se₇ and CuSe) at selenization temperatures up to 250 °C. CuInSe₂ was found to be the dominant phase at 350 °C and the film was almost completely converted to single phase material at 450 °C. The composition of the selenized films remained virtually unchanged in the temperature range between 150 °C and 350 °C. However, reaction of the metallic alloys to H_2Se/Ar at temperatures around 450 °C resulted in a significant loss of indium from the films and subsequently to an increase in the Cu/In atomic ratio. The variation in crystalline quality of the films during various stages of selenization was also clearly reflected by low temperature photoluminescence (PL) studies. Virtually no PL response was detected from samples selenized at low temperatures below 350 °C, compared to rather strong emissions from samples selenized at higher temperatures around 450 °C. Furthermore, a significant difference in PL response was detected from samples selenized at 350 °C and 450 °C, respectively. Comparative studies indicated the presence of a free-to-bound transition (at 0.992 eV) only in the case of samples selenized at 450 °C, which indicated that these specific point defects (V_ln) are created at high selenization temperatures. This observation is consistent with EDS results, indicating a substantial loss of In from samples selenized in this high temperature range. PL spectra from samples selenized at 350 °C were also characterized by a broad peak close to the band gap value, which was attributed to the presence of point defects associated with In-rich secondary phases. The improvement in crystalline quality with increased selenization temperatures and reaction periods was also clearly reflected by the reduction in the FWHM values of the PL peaks. The information gained from this study played an important role in the production of high quality films in our laboratories.     

In-situ growth of oriented PbTiO3 thin films by low temperature metalorganic chemical vapor deposition

Oriented PbTiO₃ thin films were successfully grown on (200)-oriented Pt/SiO₂/Si by metalorganic chemical vapor deposition at low temperature range from 350°C to 400°C, using -diketonate complex of Pb(tmhd)₂ and titanium isopropoxide as source precursors. Dependences of orientation and formation of crystalline PbTiO₃ phase on Pb/Ti ratio and substrate temperature was investigated. Crystalline phases and preferred orientations were determined by X-ray diffraction technique, and surface morphology was identified with scanning electron microscopy. As the deposition temperature was raised from 350°C to 400°C at two fixed Pb/Ti ratios of 3.3 and 5.0, structures of PbTiO3w₃ films transformed from amorphous to polycrystalline and preferred orientation changed from random to [100] parallel to the surface. Similar results were also observed in the films deposited at 400°C with the increase of Pb/Ti ratio from 1.1 to 5.0. As the Pb/Ti ratio increased, the dielectric constant and current density increased due to crystallization of the PbTiO₃ films. It is found that the control of excess Pb precursor amount through Pb/Ti ratio change is the key process parameter for the formation of crystalline PbTiO₃ phase in the low temperature MOCVD process.     

Effects of doping with La and Mn on the crystallite growth and phase transition of BaTiO3 powders

The effects of La and Mn dopants on the crystallite growth and the phase transformation of BaTiO₃ powders were studied. The barium titanate powders were obtained by calcining barium titanyl oxalate tetrahydrate in the temperature range 800 to 1200 °C. Crystallite growth of BaTiO₃ powders was promoted by the use of Mn dopant due to the increase of oxygen vacancies. The dissolution of La dopant into BaTiO₃ structure may decrease the oxygen vacancies so that the growth of BaTiO₃ crystallites is inhibited at high temperature ( 900 °C). When the crystallite size is small, the barium titanate can exist as a cubic phase due to the manifestation of the surface energy. Undoped cubic BaTiO₃ powders can be stable at a size < 30 nm. Doping with La and Mn would bring the crystallite size for the cubic-to-tetragonal phase transformation to 100 nm, resulting from the presence of cation or oxygen vacancies.     

Influence of powder characteristics on gas pressure sintering of Si3N4

The sintering behaviours of four kinds of Si₃N₄ powders were investigated by dilatometry in 10 atm N₂ at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si₃N₄ grains and reprecipitation as elongated -Si₃N₄ grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth.     

Surface Modification of Sericite Using TiO2 Powders Prepared by Alkoxide Hydrolysis: Whiteness and SPF Indices of TiO2-Adsorbed Sericite

Titania (TiO₂) powders have been prepared from the 0.025-M titanium isopropoxide/ethanol solution and the 0.5-M distilled water/ethanol solution. The prepared TiO₂ powders showed an anatase phase and a rutile phase after heat treatment at 500°C for 2 h and 1000°C for 2 h, respectively. The heterocoagulation adsorption between TiO₂ powder and sericite surface in water was achieved in the range of pH 3.63.7 (where this pH range shows a maximum Zeta-potential difference for two powders). On the other hand, an anomalous transformation behavior appeared in the TiO₂-adsorbed sericite after heat treatment at 1000°C. The surface modification of sericite through the TiO₂-adsorption improved the whiteness as well as the SPF (Sun Protection Factor) indices.     

Structure evolution of AlSi6.5Cu2.8Mg alloy in semi-solid remelting processing

The transformation from solid to liquid and the structure evolution of primary crystals of AlSi6.5Cu2.8Mg alloy were investigated in semi-solid remelting condition. Results show that the structure evolution of the alloy consists of three stages: liquid increasing, composition change of liquid from eutectic to hypoeutectic, and uniformity of hypoeutectic liquid. The crystals grow in forms of the coalescence and the collision-amalgamation in the first and second stages and in the form of Ostwald ripening in the third stage. It was found from statistical analysis that 1) the power exponent in the parabolic function relation between crystal diameter and remelting time changes with remelting temperatures, 2) the exponents with respect to remelting temperatures of 573°C, 578°C and 584°C are 4.004, 2.38 and 1.84, respectively, and 3) convection intensity in the semi-solid melt is a main factor to influence these exponents. It is found in this investigation that there is another mechanism of coalescence growth, which is responsible for the high exponent at low temperature. And at high temperature, the minimum value of the exponent is 1 (due to) Ostwald ripening growth.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

Annealing behaviour of amorphous Fe80B20 on continuous heating

Resistance measurements during direct heating of Fe₈₀B₂₀ amorphous alloys indicate phase changes occur at 395, 500, 720 and 840° C. Samples heated to these temperatures, and maintained for five minutes in a neutral atmosphere, show that a hardness maximum occurs at the crystallization temperature of 395° C and that annealing at 500° C produces a material with the same hardness. Above 500° C the microhardness is seen to drop below that of the amorphous alloy. Saturation magnetization measurements show a steady increase following each anneal, up to a temperature of 720° C, and the rate of increase is seen to drop in the range of 720 to 840° C. X-ray diffraction studies show that only a small fraction of the matrix is crystallized following the anneal at 395° C and the transformed phases are -Fe and Fe₃B. Following annealing at 500° C, an increased proportion of -Fe and Fe₃B are observed with complete crystallinity while samples heattreated at 720° C are seen to consist of a three-phase mixture of -Fe, Fe₂₃B₆ and Fe₂B. Annealing at 840° C is seen to produce an equilibrium phase mixture of -Fe and Fe₂B phases. Only in the sample annealed at 395° C is a fraction of the amorphous phase seen to persist, indicating that a 5 min anneal is not sufficient, at this temperature, to induce complete crystallization. These structural features are corroborated by field ion microscope analyses, made at liquid nitrogen temperature in a medium of pure neon, and scanning electron microscopy, and are also consistent with our earlier study involving the isothermal annealing, for various times, of Fe₈₀B₂₀ alloy at 780° C.     

Lateral grain growth during reactive isothermal solidification in the Ni-Sn system

In this research lateral growth of Ni₃Sn₄ grains in the Ni-Sn system during isothermal solidification was investigated. Experiments involved multi-layered samples heat-treated for different durations at temperatures in the range 235–600C. During solidification Ni₃Sn₄ was the only intermediate phase found to grow as a layer at the solid/liquid interface. SEM and optical microscopy cross-sectional views of samples heat-treated at 500C and at 600C reveal that lateral grain growth occurs, the mean lateral grain size being proportional to t(t-the duration of the heat treatment). This process was found to be connected with solution reprecipitation process between adjacent grains in order to equate the local curvature of the solid/liquid interface at groove zone. Rotation of the grain boundary groove due to this mass transfer induces a curvature of the grain boundary, which is the controlling mechanism for its migration. The process was numerically formulated, which provides quantitative fitting to the observed kinetics. The formulation includes a contribution due to the good wetting of the grain boundaries by the liquid phase supporting the observed enhancement in growth rate.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

The influence of ball milling and subsequent calcination on the formation of LiFeO2

The influence of ball milling and subsequent calcination of a 1:1 molar mixture of -Fe₂O₃ and Li₂CO₃ on the formation of LiFeO₂ has been investigated. Pre-milling was found to lower the temperature of ferrite formation by ca. 200°C and a thermally stable -LiFeO₂ phase was found to form in the temperature range 500–600°C. Slow cooling of the pre-milled mixture calcined at higher temperatures resulted in the formation of some LiFe₅O₈.