Effect of Sintering Temperature on the Microstructural and Flux Pinning Characteristics of Bi1.6Pb0.5Sr1.8La0.2Ca1.1Cu2.1O8+δ Ceramics

We evaluated the microstructure, superconducting, and flux pinning properties of La-doped (Bi,Pb)-2212 bulk sample by varying its sintering temperature ( T _sinter) between 846° and 860°C. Significant variations in microstructure, self- and in-field J _Cs at 64 K and flux pinning properties have been observed for La-substituted samples with respect to T _sinter. The sample sintered at 858°C shows best self-field J _C while that sintered at 846°C shows the best in-field J _C due to the changes in microstructure. The activation energy of flux motion (pinning potential U _o) is estimated from the field dependent resistivity–temperature curves, and the flux pinning force ( F _P) from the field-dependent J _C values. It is found that the La-doped samples, sintered at 846°C show maximum F _P and U _o of 463 kN/m³ and 0.380±0.001 eV as against 93.6 k/Nm³ and 0.140±0.001 eV, respectively, for the sample sintered at 858°C. The undoped (Bi,Pb)-2212 shows a maximum F _P and U _o of 12.7 k/Nm³ and 0.074±0.001 eV, respectively. The origin of these enhanced properties is mainly from the normal-like defects introduced by optimum La-doping at the Sr-site, at an optimum sintering temperature.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Preparation and Electrical Properties of New Oxide Ion Conductors Ce6−xGdxMoO15−δ (0.0≤x≤1.8)

A series of oxide ion conductors Ce_{6− x} Gd _x MoO_15−δ (0.0≤ x ≤1.8) have been prepared by the sol–gel method. Their properties were characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Raman, IR, X-ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The XRD patterns showed that the materials were single phase with a cubic fluorite structure. The conductivity of Ce_{6− x} Gd _x MoO_15−δ increases as x increases and reaches the maximum at x =0.15. The conductivity of Ce_4.5Gd_1.5MoO_15−δ is σ_t=3.6 × 10⁻³ S/cm at 700°C, which is higher than that of Ce_4.5/6Gd_1.5/6O_2−δ (σ_t=2.6 × 10⁻³ S/cm), and the corresponding activation energy of Ce_4.5Gd_1.5MoO_15−δ (0.92 eV) is lower than that of Ce_4.5/6Gd_1.5/6O_2−δ (1.18 eV).     

Hall Effect and Charge Carrier Generation in Y1Ba2Cu3Ox

The Hall coefficient, and hence the charge carrier density, of the system of Y₁Ba₂Cu₃O_x was measured at room temperature as a function of oxygen content, x , in its entire homogeneity range (6 < x < 7). The cuprate is essentially of the p-type (σ_p/σ_n > 70) and of extrinsic origin for x > ∼6.37, below which the electron contribution grows appreciably, and hole generation mechanisms differ depending on crystal structure: one hole is generated for every oxygen atom added in the orthorhombic structure ( x > ∼6.5) and for every two oxygen atoms added in the tetragonal structure ( x < ∼6.5). The redox reactions, 1/2O₂( g ) = O_i'+ h^. and O₂( g ) = (O_i,O_i)'+ h^., respectively, are proposed as responsible for the hole generation, and their equilibrium constants are evaluated with the reaction enthalpies of 0.75 ± 0.05 and 2.7 ± 0.2 eV, respectively. Based on these data, predictions about such properties as the oxygen potential dependences of isothermal conductivity are compared with reported results, and the numerical values for hole mobility are extracted. Other proposed redox reactions are discussed in light of the present findings.     

Densification Energy during Nanoindentation of Silica Glass

Load/unload displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5–1.2 μm using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, H _u, and elastic recovery, E _R, at the penetration depth, h _t, of 1.0 μm are H _u= 4.1 GPa and E _R= 0.7. The following energies for total deformation, U _t, elastic deformation, U _e, and plastic deformation (i.e., densification during loading), U _p, are obtained: U _t=190, U _e=135 and U _p= 55 kJ/mol at h _t= 0.5 μm and U _t= 139, U _e= 96 and U _p= 43 kJ/mol at h _t=1.0 μm. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38–55 kJ/mol for 0.5 < h _t < 1.2 μm are very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass, but are very small compared with the single bond strength (443 kJ/mol for Si—O bond) of SiO₂.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

K/Na Ratio Dependence of the Electrical Properties of [(KxNa1−x)0.95Li0.05](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K _x Na_{1− x} )_0.95Li_0.05](Nb_0.95Ta_0.05)O₃ (K _x NLNT) ( x= 0.40–0.60) lead-free piezoelectric ceramics were prepared by conventional solid-state sintering. The effects of K/Na ratio on the dielectric, piezoelectric, and ferroelectric properties of the K _x NLNT ceramics were studied. The experimental results show that the electrical properties strongly depend on the K/Na ratio in the K _x NLNT ceramics. The K _x NLNT ( x =0.42) ceramics exhibit enhanced properties ( d ₃₃∼242 pC/N, k _p∼45.7%, k _t∼47%, T _c∼432°C, T _o−t =48°C, ɛ_r∼1040, tanδ∼2.0%, P _r∼26.4 μC/cm², E _c∼10.3 kV/cm). Enhanced electrical properties of the K _x NLNT ( x =0.42) ceramics could be attributed to the polymorphic phase transition near room temperature. These results show that the K _x NLNT ( x =0.42) ceramic is a promising lead-free piezoelectric material.     

Internal Friction, Dielectric Loss, and Ionic Conductivity of Tetragonal ZrO2-3% Y2O3 (Y-TZP)

The defect structure in 3 mol% Y-TZP was studied by correlated internal friction, dielectric loss, and ionic conductivity experiments. A prominent mechanical and dielectric loss peak occurs in the temperature range between 380 and 550 K that depends on the frequency of measurement. The relaxation parameters were determined as H_m = 90 ± 3 kJ·mol⁻¹, τ_∞= (1.0_+1.5^−0.6) × 10₋₁₄ s for the mechanical relaxation and H_d = 84 ± 3 kJ·mol⁻¹, τ_∞= (1.6_+1.7^−0.9) × 10^–13 s for the dielectric relaxation. The ionic conductivity below 790 K is controlled by an activation enthalpy of H _σ= 89 ± 3 kJ·mol⁻¹; at higher temperatures H _σ= 60 ± 3 kJ·mol^–1. An atomistic model is presented which assumes that oxygen vacancies are trapped by yttrium ions forming anisotropic complexes which—by reorientation—cause anelastic and dielectric relaxation. At higher temperatures (>790 K) these complexes are dissociated, which leads to the reduced activation enthalpy for ionic conductivity.     

The Crystal Structure of LaAlO3-Stabilized La2/3TiO3 Ceramics: An HRTEM Investigation

LaAlO₃-stabilized La_2/3TiO₃ (LT) ceramics were prepared by the conventional mixed oxide route. Small amounts of manganese oxide were added to eliminate Ti⁴⁺ reduction. The powders were calcined at 1150°C and sintered at 1400°–1500°C for 4 h and cooled at rates of 900°–15°C/h. The products were high density and single phase, with an average grain size of 6 μm. The LaAlO₃-stabilized LT ceramics exhibited a relative permittivity (ɛ_r) of 64, a positive temperature coefficient of resonant frequency (τ_f) of 84, and dielectric Q value × resonant frequency ( Q × f ) values of 16 400 GHz. The crystal structure and microstructures have been investigated using high-resolution transmission electron microscopy (HRTEM) in conjunction with X-ray diffraction (XRD). One candidate crystal structure, a ≈2 a _p (where a _p is the lattice parameter of the high-temperature form of the cubic perovskite), b ≈2 a _p, and c ≈2 a _p with a space group Cmmm (65), has been confirmed by XRD, electron diffraction, and lattice imaging techniques. Microtwins, with twin boundaries parallel to the {100} planes, were observed in the microstructures.     

Preparation and Crystallographic Properties of A2+B22+O4, Type Calcium and Strontium Scandates

The compounds CaSc₂O₄ and SrSc₂O₄ were synthesized by solid-state reaction of the component oxides at temperatures ranging from 950° to 1400°C. Both calcium and strontium monoscandates are orthorhombic and analogous to CaFe₂O₄. Lattice parameters for CaSc₂O₄a re a _o= 9.461, b _o= 11.122, c _o= 3.143 and for SrSc₂O₄, a _o= 9.698, b _o= 11.302, c _o= 3.185. Based on four molecules per unit cell, the X-ray density for CaSc₂O₄ is 3.89 g per cm³ and that for SrSc₂O₄ is 4.59 g per cm³.     

Investigation of the Phase Diagram for the Pseudobinary System Li2SO4–La2(SO4)3 and Its Ionic Conductivity

The phase diagram of the pseudobinary system Li₂SO₄–La₂(SO₄)₃ has been investigated by means of X-ray diffraction and differential thermal analysis. LiLa(SO₄)₂ is formed by a peritectic reaction in this system; the peritectic temperature is 653±3°C. The eutectic reaction of Li₂SO₄ and LiLa(SO₄)₂ occurs at 553±3°C; the composition at the eutectic point is 17 mol% La₂(SO₄)₃. LiLa(SO₄)₂ is monoclinic with a=1.375 nm, b=0.6744 nm, c=0.7068 nm, and β=105.4°. The ionic conductivity of LiLa(SO₄)₂ has been studied from room temperature to 350°C and is found to be relatively low at room temperature or at lower temperatures. Its activation energy is 0.66 eV. Thus it is not suitable as a fast ion conductor.     

Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Open-circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO₂. The open-circuit emf data were used to delineate the low- p o₂ ionic domain boundary for "pure" ThO₂, which is presented as a log P_θ line on a log Po₂-1/ T diagram. In addition the ionic conductivity, σ_ion, and the high-Po₂ log P_θ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po₂^I/4 dependence of σ_p, the p-type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 10³/4.575T, log σ_ion= 1.9−44.3×10³/4.575T, and log P_θ=−1.0−31.4 × 10³/4.575T; for the second batch, log Pθ=11.2−219.7 × 10³/4.575T, log σ_ion= 1.7−41.6 × 10³/4.575T, and log Pθ=0.6−40.4 × 10³/4.575T. The oxygen permeability of ThO₂ tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.     

Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°C.     

Electrical Conductivity of TiO2-V2O5-P205 Glasses

The dc conductivities (σ) of V₂O₅-P₂O₅ glasses containing up to 30 mol% TiO₂ were measured at T=100° to ∼10°C below the glass-transition temperature. Dielectric constants from 30 to 10⁶ Hz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism was considered to be small polaron hopping between V ions, as previously reported for V₂O₅-P₂O₅ glass. The temperature dependence of σ was exponential with σ = σ₀ exp(-W/kT ) in the high-temperature range. When part of the P₂O₅ was replaced by TiO_2,σ increased and W decreased. The hopping energy depended on the reciprocal dielectric constant which, in this case, increased with increasing TiO₂ content.     

Phase Structure and Electrical Properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 Lead-Free Piezoelectric Ceramics

(1− x )(K_0.48Na_0.52)(Nb_0.95Ta_0.05)O₃– x LiSbO₃ [(1− x )KNNT− x LS] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases was identified in the composition range of 0.03< x <0.05. The ceramics near the MPB exhibit a strong compositional dependence and enhanced electrical properties. The (1− x )KNNT– x LS ( x =0.04) ceramics exhibit good electrical properties ( d ₃₃=250 pC/N, k _p=45.1%, k _t =46.3%, T _c=348°C, T _{o − t} =74°C, P _r=25.9 μC/cm², E _c=10.7 kV/cm, ɛ_r∼1352, tan δ∼3%). These results show that (1− x )KNNT– x LS ceramic is a promising lead-free piezoelectric material.     

Neutron Powder Diffraction Study of a Phase Transition in La0.68(Ti0.95Al0.05)O3

Crystal structures and structural changes of the compound La_0.68(Ti_0.95Al_0.05)O₃ have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25° to 592°C. The Rietveld profile-fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low-temperature phase of La_0.68(Ti_0.95Al_0.05)O₃ is orthorhombic Cmmm (2 a _p× 2 a _p× 2 a _p; p: pseudo-cubic perovskite). The unit-cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25°, 182°, and 286°C, and with the tetragonal P 4/ mmm ( a _p× a _p× 2 a _p) for intensity data obtained at 388° and 592°C. The P 4/ mmm -to- Cmmm phase transition was found to be induced by tilting of the (TiAl)O₆ octahedron. The tilt angle decreased with increasing temperature, reaching 0° at the Cmmm – P 4/ mmm transition temperature.     

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.     

Transient Thermal Stress Behavior in ZrO2-Toughened Al2O3

The initial strength of (σ_i) and thermal shock resistances (Δ T_c and σ_r/σ_i), as determined by quench tests, of Al₂O₃-ZrO₂ composites are increased by increasing amounts of tetragonal ZrO₂ second phase for contents of up to ∼15 vol%. For composites with ≤9 vol% ZrO₂ the increases in σ_r and Δ T_c reflect the increase in γ_IC with addition of ZrO₂ However, for ZrO₂contents >9 vol%, the thermal shock resistances (Δ T_c and σ_r/σ_i) and σ_i are also affected by machining-induced microcracking in the surface of the samples. For ZrO₂ contents >14 vol%, bulk microcracking can become extensive and result in a degradation of σ_i and Δ T_c .     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

High-Temperature Thermophysical Properties of Uranium Monocarbide

Thermal conductivity ( k ), electrical resistivity ( p ), total hemispherical emittance (ε_t), and normal spectral emittance (ε_0.65μ) of dense, arc-cast uranium monocarbide (5.3 wt % total carbon) were measured in the temperature range 1150° to 2050°K. The results were as follows: k , 0.057 cal/sec-cm-deg, 1200° < T < 2050°K, probable error ± 0.002; p, 20.4 × 10⁻⁶+ 114.8 T × 10⁻⁹ ohm-cm, 1175° < T < 2050°K, probable error ± 1.7 × 10⁻⁶; ε_t0.42, 1250° < T < 1980°K, probable error ± 0.02; ε_0.65 0.539 – 0.02 T × 10⁻³ 1150° < T < 1890°K, probable error ± 0.02. Experimental methods are discussed and error sources are analyzed. Uranium monocarbide exhibited typical metallic behavior in its thermophysical properties.