Crystallization and properties of Li-Al-B-Ti-Zn-silicate system glass-ceramic fibres

Phase separation, nucleation, crystallization and micro-crack extension and their affects on the tensile strength and alkaline resistance of TiO₂ nucleating Li₂O-Al₂O₃-B₂O₃-ZnO-SiO₂ system glass-ceramic fibres are studied by DTA, XRD, TEM and SEM. Phase separation, temperature range of nucleation, T _g, sequence and kinetics of crystallization, sizes of microstructure and surface microcracks, tensile strength and weight loss of alkali corrosion of glass-ceramic fibres are also studied. The mechanism of crystallization and the process of microcrack extension during the preparation of glass-ceramic fibres are discussed in detail. The glass prepared for glass-ceramic fibres should be characterized by the temperature of phase separation, nucleation and crystallization of the glass ought to be low, as near T _g as possible, corresponding to its basic properties and the rate of dense bulk crystallization must be closely controlled. The microstructure of small and concentrated crystallites, about, 25 nm in size while the diameter of the glass-ceramic fibres is 16 m, produced in the glass-ceramic fibres increases its tensile strength and alkaline resistance. A suitable coupling agent covering the surface of the glass-ceramic fibres and tensile stress exerted on them during heat treatment benefit its mechanical and chemical properties.     

Study on the Physical and Electrical Properties of Bi2 ? δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

We have fabricated a series of glass-ceramic (Bi_{2 –} Zn) Sr₂Ca₂Cu₃O_{10 + y} , where = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the = 0 sample, the T _c and T _zero was obtained at 110 K and 107 K, respectively. The J _c values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis–Bennett. The calculated activation energy, E _a, of the system was found to be in the range of 258–336 kJ/mol.     

Devitrification and vitrification of tellurite glasses

The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 10²² eV mol^–1. The glass transformation, T _g, starting crystallization, T _x, crystallization, T _c and melting temperatures, T _m, have been reported for binary tellurite glasses of the form (1 – x) TeO₂–xA_nO_m [A_nO_m = MnO₂, Co₃O₄ and MoO₃]. Among many different parameters of the glass forming potential the two-thirds rule, T _g/T _m, the glass stabilization range, T= T _x – T _g, and the glass forming tendency, K _g= (T _c– T _g)/(T _m – T _c), are reported for the first time for tellurite glasses.     

Crystallization behavior of calcium phosphate glass powder

The crystallization behavior of calcium phosphate glass powder with the molar ratio [CaO]/[P₂O₅] = 0.88, to which 6.38 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was investigated. The results indicate complex crystallization behavior which depends on the powder particle size and the crystallization temperature. In the crystallization temperature range T _c < 900°C the surface mechanism of crystallization dominates in the case of all particle sizes ranging from 0–1 mm and the -Ca₂P₂O₇ phase is formed. At very long annealing times volume crystallization occurs and the TiP₂O₇ and AlPO₄ phases are formed. In the temperature interval T _c > 900°C the dominant crystallization mechanism depends on the particle size. In the size range 0.15–0.5 mm the surface mechanism of crystallization is replaced by the volume one. In the range >0.5 mm the volume mechanism of crystallization is dominant. The phases -Ca₂P₂O₇, TiP₂O₇ and AlPO₄ are formed in that temperature interval for all particle sizes. The additives TiO₂ and Al₂O₃ influence the nucleation and formation of TiP₂O₇ and AlPO₄ but do not influence the formation of -Ca₂P₂O₇.     

Synthesis of melt-quenched Bi1.7V0.3Sr2Ca2Cu3O10+y superconducting glass-ceramics

A glass in the BSCCO system with Bi₁₇V_0.3Sr₂Ca₂Cu₃O_10+y nominal composition was prepared by the melt-quenching (glass) method. The suitability of the glass ceramic method has been assessed in terms of physical and electrical properties. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization has been obtained. The result obtained indicated that substitution of vanadium for bismuth increased the activation energy compared to the unsubstituted BSCCO system but did not enhance the superconducting phase formation. The activation energy for crystallization of glass has been found, E _a=355 kJ mol^–1. The crystal structure was found to differ from that in the unsubstituted BSCCO system. Most importantly, the HT _c phase was formed by reaction between the constituent phases at lower temperatures and not directly from the glass material. The best electrical properties were obtained at T ₀=75 K and J _c=12 × 10³A cm^–2 at 4.2 K.     

Synthesis Conditions and Superconductivity in 123-Type Superconductors

We have synthesized REBa₂Cu₃O_7–, where RE = Y, Sm, Gd, and SM(123):Ag under the identical conditions and characterized R – T and SEM–EDAX measurements by XRD. It is observed for sample SmBa₂Cu₃O_7– with = 0.3 that substitution of 10% Ag in place of Cu reduces T _c by 13 K along with the enhancement of normal-state resistivity by almost 200 times. When the T _c of the samples were optimized, it was found that the normal-state resistivity decreases by the substitution of Ag along with a decrease in T _c.     

Preparation of high-T c superconducting film in the system of Bi-Sr-Ca-Cu-O from sol-gel process and its properties

High-T _c superconducting Bi₂Sr₂CaCu₂O_x films withT _c ^off =80 K were prepared by the dipping method of sol-gel processing using inorganic salts. The influence of the preparation conditions on the superconducting properties of the derived material is reviewed. Bi, Sr, Ca and Cu nitrates were used as raw materials. Glycerol was used as solvent. The thickness of films made by the dip method was about 0.5 m. The films were crystallized by heat-treatment at 830°C for 10 min.T _c ^off of films was 80 K andJ _c at 77 K was more than 8 kA cm^–2. Synthesis of high-T _c superconducting films was very easy and the crystallization of films was possible with a relatively low heat-treatment temperature.     

Crystallization of leucite as the main phase in glass doped with fluorine anions

The crystallization of a multicomponent glass containing 1.63 wt% of F^– anions was studied. The results show in powder glass with particle sizes less than 0.15 mm, that surface crystallization is dominant, whereby two phases: leucite and dioside are formed. In glass powder or particle size about 0.15 mm, three phases, phlogopite, diopside and leucite, are formed, accompanied by an abrupt decrease in the resistance of the glass to crystallization. If the particle sizes are in the range 0.15 to 0.45 mm, both surface and volume crystallization are significant, while with particle sizes >0.45 mm, volume crystallization is dominant. Two nucleation temperatures, T _n1 = 655°C and T _n2 = 675°C, were determined in the temperature range of 600–710°C. These temperatures satisfy the condition that T _n T _g . Crystallization of bulk glass occurs in the temperature range of T _c = 870–1100°C, the crystal phases appearing in the sequence: phlogopite, followed by diopside, followed by leucite. Kinetical and microstructural studies show that the crystallization process is controlled by volume diffusion.     

Thermal annealing-induced ageing and structural effects in YBa2Cu3O7−δ superconductors

The YBa₂Cu₃O_7– superconductor loses part of its oxygen by thermal annealing, effectively at annealing temperaturesT _a 400 °C, under reduced air pressure. The release of the oxygen decreases monotonically as a function of time, at givenT _a, and presents two different slopes; fast at the beginning and slow later. The initial slopes (dM/dt) of the isothermal mass loss curves, measured at differentT _a, follow a straight line, as In (dM/dt) is plotted against theT _a ^–1 (K^–1), suggesting the release of the oxygen to be a kinetic process with an activation energyQ ₁=0.45 eV. The oxygen vacancies probably reside along the central-cage Cu-O chains, replacing O(4) ions, which break up spinal Cu-O linkages, and consequently the highT _c superconductivity properties suffer. A YBa₂Cu₃O₇ sample annealed for 30 min at 600 °C (at 10 mbar air pressure) thus gives a considerably reducedT _c at 50 K, compared to 91 K without annealing. However, annealing, peculiarly at a higherT _a 700 °C, for a short period of 10 min, has little effect on the highT _c appearing at 70 K. We believe that the oxygen vacancies possibly populate on O(1) sites along the side Cu-O chains, at this particular temperature, and become positively charged on trapping the hole, thus helping the superconductivity.     

The crystallization of calcium phosphate glass with the ratio [CaO]/[P2O5] < 1

The phase evolution in a calcium phosphate glass with the molar ratio [CaO]/[P₂O₅] < 1, to which 6.4 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was studied. The results indicate that the primary phase -Ca₂P₂O₇ is formed during surface nucleation and crystallization which are dominant at T < 930°C. No presence of metastable calcium phosphates was registered during the formation of the primary phase. The growth of -Ca₂P₂O₇ crystals occurs on the faceted crystal/glass interface with dendritic morphology at a crystal growth rate that is independent of time. The kinetics of -Ca₂P₂O₇ crystal growth are characterised by a growth activation energy of E _a = 426 ± 15 kJ/mol. The parameters of the unit cell of the -Ca₂P₂O₇ phase decrease with increasing temperature. The secondary TiP₂O₇ and AlPO₄ phases are formed by volume nucleation and crystallization. The temperature of the maximum nucleation rate was determined to be T _n = 690°C, and it is higher than the transformation temperature T _g. In the temperature interval T < 930°C secondary phases appear during long annealing times. At T > 930°C glass volume crystallization is dominant.     

Pseudoelastic behavior in Y1−xPrxBa2Cu3Oy compounds

Tensile tests show that ferroelastic loops always occur at 100–150 K, 200–250 K and room temperature in the Y_1–xPr_xBa₂Cu₃O_y samples withx=0 and 0.1 for whichT _c is 92 K and 82.5 K respectively, and the shape memory effect is always observed in the compact sample near 130 K, just similar to that of thermal elastic martensitic alloys. The loss fraction (W/W) which is proportional to the area of ferroelastic loop as a function of temperature shows that there always exist static hysteresis (W/W) peaks at 130 K and 110 K which are attributed to the phaselike transition (PLT) characterized by the jump of lattice parameters. No ferroelastic loops and shape memory effect are observed in the range of 100 K to 150 K for the lowerT _c samples withx=0.3, 0.4 and 0.6.     

Non-isothermal kinetic studies for binary TeO2-P2O5 glass system

A study of TeO₂–P₂O₅ glass system has been carried out by Differential Thermal Analysis (DTA) to elucidate the kinetics of crystallization for these glassy samples. The results of DTA performed at different heating rates are discussed. The values of the glass transition temperature, T _g , as well as the glass crystallization temperature, T _c , are found to be dependent upon the heating rate. From this dependence, the values of activation energy for both the glass transition and crystallization are evaluated and discussed     

Sintering and crystallization phenomena in Silceram glass

An investigation has been carried out on the feasibility of employing a CaO-MgO-Al₂O₃-SiO₂ glass-ceramic (known as Silceram) as a matrix for a fibre composite produced by a powder route. Some important properties of the parent glass, e.g. surface energy, viscosity, have been measured as well as the kinetics and structural aspects of the sintering and crystallization processes. It was found that the crystallization of the main phase, diopside, occurred from the surface of individual particles and internally with activation energies of 392 to 452 kJ mol^–1 and 258 to 323 kJ mol^–1 respectively. However, the surface crystallization was only dominant when the glass powder was fine (<>m). The work has demonstrated that this glass-ceramic shows promise as the matrix component of a composite because

500–600 K Dilatation Anomaly and Tc Increase in YBa2Cu3Ox

YBa₂Cu₃O _x bulk ceramics have been simultaneously investigated by the thermal expansion and the acoustic emission methods in the 500–600 K temperature range. An error of the differential dilatometer does not exceed 10^–7. At T = 555 K, the dilatation anomaly accompanied by the acoustic emission has been observed and measured. Based on this measured anomaly, a value of c = –0.003 m has been determined. And based on this c size the stoichiometric coefficient x = 7.01 has been calculated too. The possible mechanism for T _c increase in YBa₂Cu₃O _x as a result of closing b axis by the absorbed oxygen atoms during the samples' heating through 500–600 K has been discussed.     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

A study of the crystallization process of Na1.6Zn0.8Si1.2O4 glass and the ionic conductivity properties of partially crystallized samples

The crystallization process of Na_1.6Zn_0.8Si_1.2O₄ glass was studied by means of differential scanning calorimetry, X-ray powder diffraction and the platinum/carbon replication technique. Partially crystallized samples were made by rapidly cooling samples from elevated temperatures using the DSC apparatus, and the ionic conductivity of the materials was determined by means of impedance measurements conducted at lower temperatures where the crystallization rate was negligible. The glass was found to crystallize at 830 K by precipitation and three-dimensional grain-growth of a crystobalite-type phase with the same composition as the glass. The overall activation energy for the crystallization process was determined from isothermal DSC measurements to be 340 kJ mol^–1. The bulk ionic conductivity for partially crystallized samples increases smoothly from 9.3 x 10^–5( cm)^–1 at 600 K for the glass to 2.4 x 10^–3( cm)^–1 for the crystallized material.     

Microstructure and properties of an infra-red transmitting chalcogenide glass-ceramic

A chalcogenide glass-ceramic (0.3 PbSe-0.7 Ge_1.5 As_0.5 Se₃) which transmits in the infra-red region between 8 and 12 m was produced from a phase separated parent glass. The glass transition temperature (T _g) was increased from 280 to 340°C by crystallizing the phase with the lowerT _g. Further heat-treatment produced a glass-ceramic that was up to 60% crystalline and contained PbSe, PbSe₂ and GeSe₂ crystals with a gran size of 0.5 m. The infra-red transmission of the glass-ceramic decreased with increased crystallinity. The glass-ceramic modulus of rupture (38 MN m^–2) was increased to as much as twice that of the glass and the Vickers hardness increased by 30% to 280 kg mm^–2.     

Phase Relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O System

The phase relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O system were studied near Bi₂Sr₂CaCu₂O_{8 +} (Bi-2212) and (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} (Bi-2223) between 850 and 930°C. The introduction of Sc led to the formation of a new compound Sr₂ScBiO₆, which coexisted with Bi-2212 and Bi-2223. Using crystallization from a peritectic melt at different cooling rates, we obtained Bi-2212 matrix composites containing finely dispersed Sr_1.9Ca_0.1ScBiO₆inclusions, with T _cattaining 89 K. The T _cof the Bi-2223–Sr_1.9Ca_0.1ScBiO₆superconducting ceramic prepared by solid-state sintering of a Bi–(Pb)–Sr–Ca–Cu–Sc–O precursor was 108.5 K.     

A crystallization study of the Ge25Sb20S55 glassy alloy

Differential scanning calorimetry analysis was used to gain insight on the thermal stability and calorimetric behaviour on heating of the Ge₂₅Sb₂₀S₅₅ glassy alloy. The as-quenched glass shows a glass transition atT _g=570 K. From the variation ofT _g with the heating rate a value of h=240 kJ mol^–1 was obtained for the activation enthalpy of the mean relaxation time. On further heating an exothermic process appears ranging from 725 to 800 K giving rise to glassy GeS₂ and needle-like crystals of antimony. The microstructure of the crystallization product was obtained from X-ray diffraction and scanning electron microscopy. The resulting material shows two glass transitions. The crystallization process is explained correctly by the Johnson-Mehl-Avrami-Erofe'ev equation with a kinetic exponent ofn=1.6. The apparent activation energy of crystallization is evaluated asE=286 kJ mol^–1.