Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Formation of Pores and Y2BaCuO5-Free Regions during Melt Processing of Y1.6 Ba2.3Cu3.3Ox

The formation of spherical pores and regions free of Y₂BaCuO₅ (2-1-1) has been studied by melt processing Y_1.6Ba_2.3Cu_3.3O _x: in two different atmospheres (air and oxygen). When the sintered Y_1.6Ba_2.3Cu_3.3O _x specimens are melted at 1050°C, many spherical pores form in the melted specimens. During the subsequent cooling, the pores are filled by liquid flow and finally solidified to Y₂BaCuO₅-free regions. Melt processing in an oxygen atmosphere produces more pores and regions free of 2-1-1 than in air. Because peritectic melting of YBa₂Cu₃O_7-y in an oxygen atmosphere produces more oxygen gas than that in air, the formation of the pores and Y₂BaCuO₅-free regions is suggested to be attributed to the oxygen evolution during the peritectic melting of YBa₂Cu₃O_7−y     

Preparation of Superconductor/Polymer Composite with Resistive Transition Temperature [Tc) above 77 K

A technique for preparing Y₁, Ba₂Cu₃O_7-x/polymer composites showing superconducting resistive transition above liquid-nitrogen temperature has been developed. A sintered Y₁Ba₂Cu₃O_7-x disk was impregnated with a low-viscosity 2-ethylhexyl acrylate monomer containing benzoyl peroxide and dicumyl peroxide as polymerization catalysts. The impregnation of sintered disks was carried out under moderate vacuum for 30 min followed by an overnight soaking. The polymerization treatment consisted of a heat treatment at 60°C for 24 h followed by a second heat treatment at 80°C for 8 to 10 h. The superconductorlpo1ymer specimens exhibited superconducting resistive transition at ≃80 K. The scanning electron micrograph reveals excellent welting, infiltration, and polymerization of the monomer into the superconducting material.     

Effect of Praseodymium on Superconductivity and Magnetism in Y1–xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that most samples of Y_1-x Pr_xBa₂Cu₄O₈ examined in the present study contained a single YBa₂ Cu₄O₈ (1-2-4) superconductive phase for x<0.7.Lattice parameters a and b increased with Pr concentration, suggesting that most of the Pr is trivalent in Y_1-x Pr_x-Ba₂Cu₄O₈. The zero-resistance temperature, T _co, decreases monotonically from 80 K at x=0 to 12 K at x=0.65, and superconducting transition widths tend to broaden for x>0. The room-temperature resistivity changes linearly until x=0.7 and increases abruptly at x=-0.75. The critical concentration, x_cr, thus was estimated to be 0.7. The effective magnetic moments of Pr in Y _1-x Pr_xBa₂Cu₄O₈ were 3.63., 3.35, and 3.23, μ_B for x=0.2, 0.4 and 0.6, respectively. In the R_0.8 Pr_0.2Ba₂Cu₄O₈ system, the depression of T_c weakly depends on the ionic radius of rare-earth elements. Similarities and differences between Y _1-x Pr_xBa₂Cu₄O₈ and Y_1-xPr_x-Ba₂Cu₃O_7-y also were noted and are discussed in this paper.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics

Purified air is passed over a specimen of YBa₂Cu₃O_{7– x} at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10⁻⁵ Pa for BaO( g ), 1.2 × 10⁻⁴ Pa for Cu( g ), and 2.2 × 10⁻⁵ Pa for CuO( g ) are obtained under 2.1 × 10⁴ Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

Stability Diagram for the System Yba2Cu3O7–x

The dependence of the degree of nonstoichiometry of YBa₂Cu₃0_7–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10^–6 to 1 atm (1 atm = 10⁵ Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10^–6 to 10^–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y₂BaCuo₅ (211) + BaCuO₂° Cu₂O→ 211 ° BaCuO₂° BaCu₂O₂→ 211 ° YBa₃Cu₂O_x (132) ° BaCu₂O₂→ 211 ° BaCu₂O₂°BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10^–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

Superconductive Properties of the YBa2Cu4O8 Superconductor

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa₂Cu₄O₈ (124) at 850°C in flowing oxygen at 1 atm (≅10⁵ Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were characterized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be ∼5 × 10⁴ A/cm², and the superconducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase.     

Characterization of a Large-Scale Nondoped YBa2Cu3O7−x Superconductor Prepared by Plastic Forming Without High-Pressure Molding

A slurry containing YBa₂Cu₃O_{7− x} particles and a fine YBa₂Cu₃(OH) _x colloid solution was prepared, and a large-scale bulk YBa₂Cu₃O_{7− x} superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa₂Cu₃O_{7− x} structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T _c) was 88.7±1.4 K. The critical current density ( J _c) measured at 77 K was about 440 A/cm², the value of J _c was improved by the heat treatment under an oxygen atmosphere, and J _c=1.6 × 10³ A/cm² was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.     

Hot Deformation of YBa2Cu3O7-δ and Composite YBa2Cu3O7-δ/Ag

The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa₂Cu₃O_7-δ and composite YBa₂Cu₃O₇-δ₆/Ag were tested at constant true strain rates from 10^-6 to 10^-3 s^-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa₂Cu₃O_7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10^-6 to 10^-4 S^-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.     

Phase Compatibilities in the System Y2O3-BaO-CuO at 950°C

We report the compatibility relationships between compounds, including the newly discovered superconducting Y₁Ba₂Cu₃O_x phase, in the Y₂O₃-BaO-CuO system at 950°C. In addition to the previously reported ternary compounds, there is a new compound with a composition Y_lBa₃Cu₂O_x. The new compound is a perovskite 'space group P4mm) with lattice parameters a =4.078 Å (0.4078 nm) and c =4.01 Å (0.401 nm). There are also at least two structurally distinguishable binary phases between barium oxide and the known BaCuO₂, but they appear to be hygroscopic and are beyond our current capabilities of analysis.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

Intrinsic Kinetics of Carbon Dioxide Degradation of YBa2Cu3O7-x

The intrinsic kinetics, unaffected by diffusional and masstransfer effects, of the CO₂ degradation of superconducting particles have been determined using a nonisothermal technique. Below 900°C, the carbonization of YBa₂Cu₃O_{7- x} leads to formation of BaCO₃, Y₂Cu₂O₅, CuO, and Cu₂O. A further increase in temperature results in formation of BaCuO₂ from BaCO₃ and CuO. The carbonization rate shows the 1.5th-order dependence on the amount of unreacted YBa₂Cu₃O_{7- x} for the temperature range of 550° to 815°C. The activation energy of carbonization was determined to be 95.1 kJ · mol⁻¹.     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity