YTTRIUM ALUMINATE CERAMIC FIBERS VIA PRE-CERAMIC POLYMER AND SOL-GEL ROUTES

Yttrium aluminum garnet (YAG - Y₃Al₅O₁₂) fibers have been prepared by dry spinning solutions of yttrium and aluminum carboxylate polymers (precursor route) and by dry spinning aqueous oxide sols (sol-gel route). Fibers from aqueous diphasic gels are prepared by mixing a colloidal alumina sol containing 50-nm hydrous alumina with a colloidal yttria sol containing 10-nm yttrium oxide, using polyvinylpyrrolidone as a spinning aid. Fibers by the precursor route are made from spinnable THF solutions of yttrium isobutyrate and aluminum isobutyrate or from aqueous solutions of polymeric aluminum formate and yttrium acetate.

The isobutyrate materials decompose between 200-400^°C to an amorphous residue. Crystallization occurs abruptly between 875°C and 900^°C, forming the YAG phase directly. The formate-acetate also decomposes to amorphous residues, which form YAG at 900^°C. In the diphasic gel, YAG forms gradually between 1000 and 1200^°C, with intermediate products YAP (YalO₃ perovskite) and/or YAM (Y₄Al₂O₉ monoclinic). At 1500^°C, single phase YAG is obtained as pore-free fibers with 500 nm grains.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.     

Fluidized-Bed Nitridation of Stainless Steel Powder

An attempt was made to produce high-nitrogen stainless steel powder, that can be consolidated into a solid compact with uniform, high-nitrogen concentration. The nitridation of 316L stainless steel powder of average particle size of 70 μn with a minimum fluidization velocity of 4.5 cms^-1, was nitrided in a 55 mm I.D. fluidized-bed reactor at temperatures ranging from 550^°C to 750^°C using NH₃/H₂ gas mixtures. NH₃ concentration and temperature had pronounced effects on the progress of nitridation while the effect of H₂ was minor. Nitrogen contents reached as high as about 10% by mass, completely converting Cr and Fe into CrN and Fe₄N, at temperatures below 600^°C. Only Cr was nitrided to form CrN at 750^°C, reaching a saturated level of 6% nitrogen by mass. A nitrided shell was always developed on the particle, resulting from the precipitation of CrN, that progressed inward.     

SURFACE CHARGE OF SOL-GEL DERIVED ALUMINAS AS A FUNCTION OF CALCINATION TEMPERATURE

The sol-gel route for preparing alumina is thought to yield a unique molecular structure. Thus the powder characteristics of sol-gel derived aluminas are expected to be different from alumina powders derived by traditional processing methods. In this study wet alumina gels, after the sol-gel reaction was completed, were dried and calcined at 25^°C, 300^°C, 500^°C, 800^°C, 1100^°C, and 1400^°C following the methods of Yoldas and Clark and Lannutti. The initial points of zeta potential reversal (PZR) of the calcined powders determined from electrophoretic mobilities are 9.4, 10.2, 8.6, 10.6, 4.2 and 8.3, respectively for the above calcination series. The PZR of the alumina powders increased with calcination temperature up to 800^°C, with the exception of the alumina calcined at 500^°C. The PZR's of powders calcined between 800-1400^°C decreased. The anomalous PZR of the alumina calcined at 500^°C is due to catalytically formed carboxylates from the residual sec-butano1. The general trend of PZR change for other phases of gel derived alumina was compared with previously reported electrolcinetic behavior of traditional alumina powders and no significant differences were found.     

High-Density Infrared Cladding of Ta on Steel

The addition of tantalum to the inside diameter of a gun barrel would reduce erosion during firing of medium and large caliber guns. In this work, chemical vapor deposited (CVD) Ta was bonded to A723 Steel. High-density infrared (HDI) heating was employed to bond Ta to steel at 1440^°C while maintaining bulk steel temperatures below the 357^°C threshold for retaining beneficial compressive stresses (autofrettage). Through-thickness temperature evolution modeling was performed. Metallographic evaluation of claddings is reported. Characterization of the interface showed that metallurgical bonding occurred while keeping bulk temperatures low.     

80% Ni-20% Cr Powdered Metal Heating Element

80% Ni and 20% Cr powder mixture was admixed with some minor components. These powder mixtures were cold isostatically pressed, and sintered at 1200^°C or 1300^°C in various atmospheres for one hour. Compacts sintered in neutral or reducing atmospheres could be rolled to high density, while those sintered in air were too brittle to be rolled. The microslructure of the rolled sample was homogeneous solid solution. Fusion tests was carried out on the commercial Nichrome wire and showed that this material began to sag at 1200^°Cand sagging grew and broke at 1400^°, while the rolled sample of the present work, when heated, kept straight, untii it broke at 1400^°C. The microstructure of the sample unable to be rolled composed of some pure nickel islands embedded in a matrix of oxide. This sample, when heated, kept straight, until it broke at a temperature of higher than 1700^°C.     

Fabrication and Electromechanical Characterization of Silver-Clad BSCCO Tapes

The powder-in-tube technique consisting of industrial processes such as wire drawing and rolling has been widely used to fabricate superconducting tapes. In the present investigation a novel technique was adopted to fabricate BSCCO 2223 tapes. Instead of wire drawing, the silver billet was reduced in size by groove rolling. Stress conditions during groove rolling were analyzed and appropriate changes were incorporated to optimize the deformation process. Subsequent thermomechanical treatment resulted in tapes with average critical current densities of 18,000 A/cm². Phase development and microstructural evolution during the thermomechanical treatment were studied using XRD, SEM, and TEM. The electromechanical properties of monofilament and composite BSCCO tapes were evaluated by subjecting them to in situ tensile tests. The strain tolerance of the composite was found to be better than that of the monofilament BSCCO tape.     

Fatigue of a Reinforced High Temperature Superconducting Tape

High‐cycle fatigue‐life behavior of commercial multifilamentary BSCCO 2223 powder‐in‐tube tape and reinforced tape was evaluated. Mechanical fatigue at room temperature and 77 K, and the onset of significant superconductivity degradation at 77 K were evaluated under tension perpendicular to the flat faces of the tapes. No fatigue behavior was observed in this transverse configuration for unreinforced tape, but fatigue–life behavior associated with delamination of the reinforcement was found for reinforced tape. Fatigue tolerance of the reinforced tape is very high, about 24 MPa, to at least 100.000 cycles.     

SINTERING AND COATING OF CERAMICS USING CARBON DIOXIDE LASER

Laser sintering and ceramic coating techniques for synthesis of new ceramics have been investigated. The laser sintering method is useful instead of furnace treatment for the densification of materials having high melting points. The new ceramics, ZrO₂-V₂O_-HfO₂ which have a melting point of 2850^°C and a hardness of about 1850 kgf/cm have been sintered. The single crystal of AI₂O₃3WO₃ was found to form under nonequilibrium states in the Al₂O₃-WO₃ system. A ceramic coating that forms a solid electrolyte layer of a fuel cell was also developed. The laser beam was parallel to the substrate surface on which melted ceramics were deposited by a powder gun. The properties of the coated layer are discussed with the melted particles passing through the laser beam     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Preparation and Dielectric Properties of Sr(Ti, Nb)03 Ceramic from Sol-Gel Route

IR, DSC/TGA and XRD were used to study the formation of Nb₂O₅-doped SrTi0₃ powder. The rates of hydrolysis and polycondensation of titanyl acylate precursors prepared by chelating of titanium isopropoxide with acetic acid were slow. Therefore, the particle size of the powder obtained was finer and the particle size distribution was more uniform. The powder obtained was about 0.1 um of primary particle size, and the powder was readily sintered. The activation energy of grain growth for the ceramics calculated by the Arrhenius' formula is 121 KJ mol^-1. The dielectric constant and dissipation factor of the bulk powders were also determined at 1300 to 1450^°C.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

Tribological Evaluation of Plasma Sprayed Coatings for High Temperature Sliding Surface Against Cordierite

A study has been carried out to develop a high temperature sliding seal for the ceramic gas turbine engine's (CGT) regenerator. The solid-lubricant coatings, which consisted of the following combinations: NiO, Ni/Cr, Cr₃ Cr₂,/NiCr, ZrO₂,/ CaO, CaF₂, BaF₂, Ag₂O and graphite, were synthesized by low pressure plasma spraying. Friction and wear characteristics were evaluated at temperatures from 50 to 1000^°C by rubbing the coatings against a cordierite disk. After the sliding tests, the worn surfaces were analyzed by SEM-EDX and XRD. The analytical results showed that the tribological properties of coatings were significantly related to the surface layers. Formation of a CaF₂/BaF rich layer led to a low friction and a low wear property. On the other hand, formation of solid-state reaction product (Mg_0.4Ni_0.6O) between the coating and the cordierite increased the friction coefficients, and the transfer of the cordierite debris caused a seizure and/or a severe wear. Among the sprayed coatings, the Cr₃Cr₂/NiCr based coating containing CaF₂/BaF₂ and Ag₂O showed excellent tribological properties, which should satisfy the specifications for the basic design of a CGT engine.     

Structural and Physical Properties of Nd Substituted Bismuth Cuprates Bi1.7 Pb0.3−x Nd x Sr2Ca3Cu4O12+y

BiPb-2234 bulk samples with nominal composition of the compound Bi_1.7Pb_0.3−x Nd _x Sr₂Ca₃Cu₄O_12+y (BSCCO) (0.025≤x≤0.10) have been prepared by the melt-quenching method. The effects of Nd substitution on the BSCCO system have been investigated by electrical resistance (R–T), scanning electron microscopy (SEM), X-ray diffraction (XRD) and magnetic hysteresis measurements. It has been the BSCCO (2212) low-T _c phase is formed for all the substitution levels, together with the BSCCO (2223) high-T _c phase. The results obtained suggest that with increasing Nd³⁺ doping for Pb²⁺ the (2223) phase existing in undoped BSCCO gradually transforms into the (2212) phase and hence all of the samples have a mixed phase formation. The R–T result of the samples show two-step resistance transition; first transition occurs at 100 K and second in an interval of 80–90 K, depending on the Nd concentration. We have found that the magnetization decreases with increasing temperature in agreement with the general characteristic of the high-T _c materials. The samples exhibit weak field dependence particularly after 2 T and changes on the magnetic hysteresis, M–H curve rather small compared to the conventional superconducting materials. The maximum critical current density, J _c, value was calculated to be 8.51×10⁵ at 4.2 K and J _c decreases with increasing temperature and the substitution level.      

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Thermodynamics for formation of each stable single phase in BSCCO thin films

High quality BSCCO thin films have been fabricated by means of an ion beam sputtering at various substrate temperatures, T_sub, and ozone gas pressures, PO₃. The correlation diagrams of the BSCCO phases appeared against T_sub and PO₃ are established in the 2212 and 2223 compositional films. In spite of 2212 compositional sputtering, Bi2201 and Bi2223 phases as well as Bi2212 one come out as stable phases depending on T_sub and PO₃. From these results, the thermodynamic evaluations of H and S, which are related with Gibbs' free energy change for single Bi2212 or Bi2223 phase, are performed.     

Plasma Spraying of Al2O3 and ZrSiO4 to Form ZrO2- Mullite Composites

Zirconia is effective in improving fracture toughness of a number of ceramics when introduced as a reinforcement either in the form of participates, dispersed phase or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO₂, reinforced mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon and alumina are injected onto a D.C. plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon, and alumina. It was found that fine grained, even amorphous and chemically homogeneous composite powders could be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1OOO^°C in plasma sprayed powders. This value is more than 500^°C lower than the formation of mullite in asmilled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000^°C in the sprayed coatings.     

FLUIDIZATION BEHAVIOR OF STICKY PARTICLES

To obtain fundamental Information on the fluidization behavior of sticky particles, a visually observable fluidized bed, which can be operated under conditions in which agglomeration of the bed materials occurs, was designed and constructed. Polyethylene particles with a softening temperature of 117.2^°C and melting temperature of 124.4^°C, in sizes from 850 to 50 μm, were chosen as the bed material. Two types of fluidizing gas distributors were used: a porous plate and a porous plate with an independently fed jet at the center. The agglomeration rate was measured at different jet and bed temperatures, particle sizes, and residence times in the fluidized bed.

The amount of agglomerates generated increased linearly with residence time and increased exponentially with either auxiliary air or jet air temperature. The agglomerates were formed from the whole range of particle sizes, but fine particles enhanced the agglomeration rate. A set of operating conditions must be properly chosen; otherwise, excessive agglomeration can lead to catastrophic sintering and defluidization.     

Nb3Al AND Nb3(Al,Ge) SUPERCONDUCTING TAPES BY LASER BEAM ANNEALING

Nb₃Al AND Nb₃(Al,Ge) tapes with excellent superconducting properties have been fabricated using high energy density laser beam irradiation, irradiations were carried out continuously on Nb-25at%AI and Nb-20at%Al -5at%Ge tapes prepared by powder metallurgical processing. With the high power density and short irradiation time, the tape could be heated and cooled much faster than a tape heat treated by conventional methods. The results were fine structure and large critical current density J_c values at high magnetic fields. J_c at 23 Tesla was over 10⁴ A/cm². This value was much greater than that obtained from a commercial Nb₃Sn superconductor. New continuous irradiation systems have been developed for scale-up development. Nb₃Al tapes 10 m in length were successfully fabricated. There was some scatter in J_c along the tape length. Small coil tests in magnetic fields indicated that the tape had excellent stability against a flux jump and mechanical strain.     

Numerical Studies of Thermally Induced Residual Strain/Stress in Bi2Sr2Ca2Cu3O x /Ag/Ag Alloy Composite Tapes and the Dependence of Material Properties on the Temperature

Thermally induced residual strain/stress in Bi₂Sr₂Ca₂Cu₃O _x (Bi2223)/Ag/Ag alloy composite tapes and the dependence of material properties on the temperature have been studied numerically. Based on both the straight and bending 3D tape models, and with the temperature dependence on material properties (especially the coefficient of thermal expansion) among the constituents (Bi2223, Ag and Ag alloy sheath) of Bi2223 multifilament composite tapes, the residual strain accumulation and the distribution of the residual stress have been obtained. We found that by taking account of the temperature dependence on material properties of Bi2223 composite tapes the residual strain in the current transportation direction is up to 15 % larger than that without taking temperature dependence into account. Furthermore, by considering the distribution of the stress induced from the changing temperature, we analyzed the mechanical strength of Bi2223 composite tapes and concluded that the initial mechanical failure due to large temperature circle (intrinsically induced from the mismatch of the coefficient of thermal expansion of each constituent in composite tapes) comes from the following aspects: (i) the tensile fracture in the Bi2223 filaments occurring at the center of the tape and (ii) the delamination most likely arising at the interface between the Bi2223 filaments and Ag matrix near both edges of the cross-section of the tape, which originates at the Bi2223 side of the interface.