Phase Transformations in Calcium‐Substituted Lanthanum Ferrite

Phase equilibria in the La_1?xCa_xFeO_3?δ (LCF) system were assessed at temperatures below 1350°C in both simulated air and argon atmospheres using a combination of differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, and high‐temperature X‐ray diffraction. The solubility limit of Ca in the perovskite structure was determined to be 38% A‐site substitution. A high‐temperature orthorhombic to rhombohedral transition was identified and the dependence on oxygen partial pressure and effect on thermal expansion were characterized. A partial, pseudobinary LaFeO₃–CaFeO_2.5 phase diagram is presented that is based on these analyses combined with data available in the open literature.     

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.     

Stability and Decomposition of Ca‐Substituted Lanthanum Ferrite in Reducing Atmospheres

Calcium‐substituted lanthanum ferrites (La_1?xCa_xFeO_3?δ x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized in air and subsequently decomposed in reducing atmospheres. The partial pressure of oxygen () was controlled by varying the H₂/H₂O ratio by bubbling hydrogen/argon mixtures through water baths at controlled temperatures. Three regions of mass loss were identified as the was reduced, two of which were determined to be associated with decomposition reactions. Calcium was shown to decrease the thermal stability of the perovskite compound, but rather than incrementally increasing the required for decomposition proportional to calcium concentration, all samples partially decomposed at a single . The extent of the partial decomposition was dependent on the amount of calcium substitution and temperature. The perovskite phase remaining after the partial decomposition was found to fully decompose at the same oxygen partial pressure as pure lanthanum ferrite.     

钙掺杂对铬酸镧热膨胀系数的影响

利用固相法合成的铬酸镧粉制备铬酸镧样品。研究了钙掺杂量对铬酸镧样品热膨胀系数的影响。结果表明：当钙掺杂铬酸镧时,Ca^2＋于A位取代La^3＋,引起A位离子半径减小,晶胞参数发生变化,同时Cr^3＋变价形成Cr^4＋,引起了O^2--O^2-和Cr^3＋-O^2-键长、键角和键能相应改变,从而对铬酸镧样品的热膨胀系数和相变温度造成影响;从室温到1000℃,铬酸镧的热膨胀系数随钙掺杂量增加而增大,由正交结构向菱形结构转变的相变温度随钙掺杂量增加呈线性提高。     

Thermal Expansion of Rare‐Earth Pyrosilicates

The use of RE₂Si₂O₇ materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE₂Si₂O₇. High‐temperature X‐ray diffraction (HTXRD) patterns were collected on different RE₂Si₂O₇ polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE₂Si₂O₇ compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE₂Si₂O₇ compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A‐RE₂Si₂O₇ polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REO_x polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE₂Si₂O₇ compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A‐RE₂Si₂O₇ compounds, with an average of 12.1 × 10^?6 K^?1, whereas the lowest values are those of β‐ and γ‐RE₂Si₂O₇, which showed average ABCTE values of ~4.0 × 10^?6 K^?1.     

Thermal Properties of Hf‐Doped ZrB2 Ceramics

The effect of Hf additions on the thermal properties of ZrB₂ ceramics was studied. Reactive hot pressing of ZrH₂, B, and HfB₂ powders was used to synthesize (Zr_1?x,Hf_x)B₂ ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room‐temperature heat capacity values decreased from 495 J·(kg·K)^?1 for a Hf content of 0.01 at.% to 423 J·(kg·K)^?1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K)^?1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB₂ ceramics. Furthermore, the results indicated that reported thermal properties of ZrB₂ ceramics are affected by the presence of impurities and do not represent intrinsic behavior.     

锶钴掺杂铁酸镧修饰的直接碳固体氧化物燃料电池银基阳极的催化性能

直接碳固体氧化物燃料电池(DC-SOFC)银基阳极具有高化学稳定性和抗硫抗积碳性能,然而受制于较差的催化性能,使得DC-SOFC的电化学性能一直无法让人满意.锶钴掺杂铁酸镧(LSCF)是一种催化性能佳,且兼具电子电导和离子电导的电极材料.通过LSCF修饰DC-SOFC的银基阳极(银-钆掺杂的氧化铈,Ag-GDC),显著...     

Thermal Expansion of Tungsten Carbide at Low Temperature

The thermal expansion of WC from 20 to 900 K was measured in the a and c axial directions using high-resolution neutron powder diffraction. Third-order polynomials have been calculated from the expansion curves. The results complement and are compared with prior data in the 293 to 900 K region.     

Chemical Expansion and Change in Lattice Constant of Y‐Doped BaZrO3 by Hydration/Dehydration Reaction and Final Heat‐Treating Temperature

Knowledge of thermal behavior of electrolyte is important for fuel cell fabrication. In this study, using high‐temperature X‐ray diffraction analysis (HT‐XRD) and thermo‐mechanical analysis (TMA), a systematic investigation of lattice constants was performed on Y‐doped BaZrO₃, which is a promising candidate for electrolyte in protonic ceramic fuel cells. The results revealed that a chemical expansion was observed between 300°C and 450°C during the heating process in HT‐XRD, and was attributed to the dehydration of BZY. Furthermore, it was found that the lattice constants of the samples doped with Y, Sm, Eu, and Dy were larger for the ones finally heat‐treated at 1600°C for sintering than those heat‐treated at 1300°C for synthesizing. The similar behavior was not observed in Sc‐doped samples.     

Thermal Expansion Mismatch Analysis of Nano‐bonded Refractory Castables

A mathematical model to predict the thermomechanical stresses was used to understand the expansion mismatch damage in castables, evaluated by hot elastic modulus measurements. The results indicated that via elastic modulus evaluation and the tensile stress calculation, the critical interactions among the particles leading to the formation of flaws were identified. To assure the materials' integrity, it is fundamental to keep a low Δα between the components and not only a low overall linear thermal expansion coefficient. For multicomponent compositions, a second‐phase raw material with different properties should be added as finer particles to minimize the thermomechanical stresses generated.     

Viscosity and Fragility of Alkaline‐Earth Sodium Boroaluminosilicate Liquids

Understanding the composition and temperature dependence of viscosity of silicate liquids is of the highest importance not only for geological processes but also for production of industrial glass. In this work, we have determined the temperature dependence of equilibrium liquid viscosity of 36 alkaline‐earth sodium boroaluminosilicate liquids as a function of the Si/Al ratio and the type of alkaline‐earth oxide (MgO, CaO, SrO, or BaO). We demonstrate that the isokom temperature at 10¹² Pa s (i.e., the glass transition temperature) generally increases with increasing Si/Al ratio, whereas the isokom temperatures at 10⁴ and 10^1.5 Pa s exhibit a decrease with increasing [Al₂O₃] in the peraluminous regime. The isokom temperatures decrease with increasing alkaline‐earth size in the peralkaline regime, whereas they increase with increasing alkaline‐earth size in the peraluminous regime. The liquid fragility index m exhibits a minimum value at an intermediate Si/Al ratio, with the position of the minimum increasing to a higher value of [Al₂O₃] with increasing alkaline‐earth size. We have discussed our findings in terms of the underlying structural and topological changes as a function of composition and temperature.     

Eu‐Doped β‐SiAlON Phosphors: Template‐Assistant Low Temperature Synthesis,Dual Band Emission,and High‐Thermal Stability

A hard template route has been successfully developed for synthesis of β‐SiAlON:Eu phosphors at low temperatures. The synthesis utilizes mesoporous silica (SBA‐15) skeleton as an active Si source, combined with the carbothermal reduction and nitridation method. It has been shown that the additional driving force from high surface area and porosity of SBA‐15 enables β‐SiAlON:Eu (with compositions of Si_6?zAl_z?xO_z+xN_8?z?x: xEu, x = 0.010–0.200 and z = 1.000) phosphors to be formed as a dominant phase at low temperature of 1400°C. The resultant β‐SiAlON:Eu phosphor powders exhibit a typical rod‐like morphology and a well dispersed state. By tailoring the Eu²⁺ concentration in the phosphors, a continuous change in emission band can be realized, that is a blue emission dominated for low Eu²⁺ concentrations and a green emission dominated for high Eu²⁺ doping concentrations. Furthermore, the resultant phosphors exhibit a small thermal quenching up to high temperature of 250°C. Therefore, the developed method is beneficial to synthesize LED phosphors of oxynitride systems at lower temperatures.     

Investigation on the Thermal Expansion and Theoretical Density of 1,3,5‐Trinitro‐1,3,5‐Triazacyclohexane

The CTE and the theoretical density are important properties for energetic materials. To obtain the CTE and the theoretical density of 1,3,5‐trinitro‐1,3,5‐triazacyclohexane (RDX), XRD, and Rietveld refinement are employed to estimate the dimensional changes, within the temperature range from 30 to 170 °C. The CTE of a, b, c axis and volume are obtained as 3.07×10⁻⁵ K⁻¹, 8.28×10⁻⁵ K⁻¹, 9.19×10⁻⁵ K⁻¹, and 20.7×10⁻⁵ K⁻¹, respectively. Calculated from the refined cell parameters, the theoretical density at the given temperature can be obtained. The theoretical density at 20 °C (1.7994 g cm⁻³) is in close match with the RDX single‐crystal density (1.7990 g cm⁻³) measured by density gradient method. It is suggested that the CTE measured by XRD could perfectly meet with the thermal expansion of RDX.     

Dopant Site Occupancy and Chemical Expansion in Rare Earth‐Doped Barium Zirconate

Rare earth‐doped BaZrO₃ is a very attractive material in electrochemical applications due to its proton conductive property. In this work, powder X‐ray diffraction patterns of BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) were collected using synchrotron radiation, and also using characteristic X‐ray of CuKα in dry and wet atmospheres at high temperature. Then, a combined interpretation of the diffraction patterns was established by using Rietveld refinement. The results revealed that an obvious lattice expansion was observed for BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) in wet O₂ compared with the case in dry condition, indicating a chemical expansion effect on lattice volume by incorporating water into lattice. Eu, Sm, and Dy cations occupied both A‐ and B‐sites of BaZrO₃ crystalline lattice, whereas Sc cations were determined to occupy B‐site only. These results indicate clearly an increasing tendency toward A‐site occupation for the rare earth cations in BaZrO₃ with an increasing radius.     

Time‐Evolution of TATB‐Based Irreversible Thermal Expansion (Ratchet Growth)

TATB is an insensitive high explosive, attractive for use because of its safety aspects. TATB compactions, with or without binder, undergo irreversible volume expansion (or ratchet growth) upon thermal cycling. In the past, experimental elucidation of this phenomenon has focused on irreversible expansion as a function of the number of thermal excursions over a given temperature range, where growth is asymptotic with increasing cycle number. In this paper, we demonstrate that ratchet growth also occurs as a function of time at constant temperature, and that growth is substantial at elevated temperatures. We have measured strain response in PBX 9502, a TATB‐based composite, by performing thermal‐cycling tests with different durations at high temperature. Irreversible growth arises from the thermal ramps themselves (increasing and decreasing), as well as from the subsequent isotherms. PBX 9502 specimens with previously‐identified TATB texture/orientation were used in order to eliminate and/or evaluate texture as a variable. Measurements were also performed on dry‐pressed TATB (no binder) to confirm that expansion as a function of time (constant temperature) is not caused by the binder. A simple analysis of the time‐response data demonstrates consistency in the results. We propose that the primary driving force for irreversible expansion is the proximity of the current strain value (due to thermal history) to the strain saturation point of the current cycle (i.e. strain at infinite high‐temperature hold times or an infinite number of cycles). Such tests should aid in the understanding and modeling of ratchet growth response in these materials.     

High‐Temperature Isothermal Oxidation of Ultra‐High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB₂ + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE‐TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C–1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C–1600°C. The mass gain increased from ~5 to 45 g/m² and parabolic rate constants increased from 0.037 to 2.2 g²/m⁴·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass‐ceramic surface layer as rate limiting.     

Thermal Expansion,Heat Capacity,and Thermal Conductivity of Nickel Ferrite (NiFe2O4)

Nickel ferrite (NiFe₂O₄) is a major constituent of the corrosion deposits formed on the exterior of nuclear fuel cladding tubes during operation. NiFe₂O₄ has attracted much recent interest, mainly due to the impact of these deposits, known as CRUD, on the operation of commercial nuclear reactors. Although advances have been made in modeling CRUD nucleation and growth under a wide range of conditions, the thermophysical properties of NiFe₂O₄ at high temperatures have only been approximated, thereby limiting the accuracy of such models. In this study, samples of NiFe₂O₄ were synthesized to provide the thermal diffusivity, specific heat capacity, and thermal expansion data from room temperature to 1300 K. These results were then used to determine thermal conductivity. Numerical fits are provided to facilitate ongoing modeling efforts. The Curie temperature determined through these measurements was in slight disagreement with literature values. Transmission electron microscopy investigation of multiple NiFe₂O₄ samples revealed that minor nonstoichiometry was likely responsible for variations in the Curie temperature. However, these small changes in composition did not impact the thermal conductivity of NiFe₂O₄, and thus are not expected to play a large role in governing reactor performance.     

Ultra‐Broad Temperature Stability Obtained with Ce‐Doped BaTiO3‐Based Ceramics

Ce‐doped BaTiO₃‐based ceramics were prepared and studied to satisfy ultra‐broad temperature stability (from ?55°C to 300°C, capacitance variation rate based on C_20°C is within ±15%). The sample with 0.6 mol% CeO₂ succeeds to achieve this performance with a remarkably high ceiling temperature of 300°C. Meanwhile, the sample has good dielectric and electrical properties at room temperature (ε_r = 1667, tanδ = 1.478%, ρ_V = 5.9 × 10¹² Ω·cm). Ce ion can substitute for Ti ion as Ce⁴⁺ or Ba ion as Ce³⁺. The substitution decreases the spontaneous polarization of BaTiO₃, and then weakens the ferroelectricity of BaTiO₃. As a result, the temperature stability of samples is improved obviously. Besides, CeO₂ addition promotes the formation of exaggerated grains, which are consisting of Ba₆Ti₁₇O₄₀.     

Thermal Properties of Transparent Yb‐Doped YAG Ceramics at Elevated Temperatures

This work presents the thermal properties of ytterbium‐doped yttrium aluminium garnet (Yb:YAG) transparent ceramics at elevated temperatures in dependence on the dopant concentration and on temperature. Transparent polycrystalline Yb:YAG ceramics were prepared by solid‐state reaction of oxide powders sintered under high vacuum. The dopant amount varied from 0 to 20 at.% of Yb. Thermal diffusivity of the sintered samples was measured by the laser and xenon flash methods at temperatures ranging from room temperature to 900°C. Both the thermal diffusivity and thermal conductivity values decreased with increasing dopant content, and until 500°C a decrease was observed also with increasing temperature. When available, the measured values were compared to data published in literature, and were found to be in good agreement. Based on the measured values, empirical relations in the form of shifted power laws are proposed for the temperature dependence of thermal diffusivity.