The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature ( T ₀) at which the tetragonal and monoclinic phases of either ZrO₂ or HfO₂ coexist is generally defined by the middle temperature of A _s (the onset transformation temperature on heating) and M _s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T ₀ is important for the prediction of A _s and M _s in ZrO₂ or HfO₂-based materials. In this work, the ZrO₂–HfO₂ system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T ₀ temperatures obtained for ZrO₂ and HfO₂ are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO₂−HfO₂ phase diagram is presented.     

Experimental Determination and Thermodynamic Calculation of the Zirconia–Calcia–Magnesia System at 1600°, 1700°, and 1750°C

Phase relations in the ternary system ZrO₂–CaO–MgO were experimentally established at 1600°, 1700°, and 1750°C. The investigation was based on powder X-ray diffractometry, scanning electron microscopy–energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO₂–CaZrO₃ phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO₂–CaO–MgO.     

Phase Diagram of the Alumina–Zirconia–Samaria System

The phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system was constructed in the temperature range 1250°–2800°C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this ternary system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1680°C and it corresponds to the ternary eutectic Al₂O₃+F-ZrO₂+SmAlO₃. The solidus surface projection, the schematic of the alloy crystallization path, and the vertical sections present the complete phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system.     

Experimental Investigation and Thermodynamic Calculation of the Titanium–Silicon–Carbon System

The 1100°C isothermal section and the isopleths at 5, 10, and 15 at.% C in the Ti–Si–C system were determined by DTA and XRD methods. Five invariant reactions (L (liquid) = Si + SiC + TiSi₂ at 1330°C, L = TiSi + TiSi₂+ Ti₅Si₃C _x at 1485°C, L + Ti₅Si₃C _x = Ti₃SiC₂+ TiSi₂ at 1485°C, L + Ti₃SiC₂= TiSi₂+ SiC at 1473°C, and L + TiC = bcc-(Ti) + Ti₅Si₃C _x at 1341°C) were observed. The transition temperature for L + TiC = Ti₃SiC₂+ SiC was measured by the Pirani technique. Optimized thermodynamic parameters for the Ti–Si–C system were then obtained by means of the CALPHAD (calculation of phase diagrams) method applied to the present experimental results and reliable literature data. The calculations satisfactorily account for most of the experimental data.     

Stable and Metastable Phase Relations in the System Alumina–Zirconia–Yttria

The Al₂O₃ZrO₂Y₂O₃ system was studied in the range of temperatures 1600°–2800°C by methods of X-ray analysis at 20°C, petrography, DTA in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis. The stable and metastable phase diagrams were constructed. The liquidus and solidus projections, crystallization paths for the alloys, and polythermal sections are presented in the article. The structure of the restricting systems defines the phase equilibria in the ternary system. No ternary compounds were found. The metastable phase relations were caused by the ambivalent behavior of Y₃Al₅O₁₂ during crystallization.     

Twinning and Microcracking Associated with Monoclinic Zirconia in the Eutectic System Zirconia–Mullite

Crystallography and morphology of twins and microcracks in the eutectic system mullite–zirconia are discussed in view of the tetragonal-to-monoclinic transformation and associated toughening mechanisms. Specific twin relationships were observed in monoclinic ZrO₂. Highly symmetric and crystallographically well-defined microcracks were observed at the mullite–ZrO₂ interface. Microdiffraction revealed closely related crystallography of monnoclinic twins and microcracks. The number of twin variants depend on the monoclinic ZrO₂ particle size. A method to calculate twinning shear strain using microcrack morphology is suggested. This parameter is essential in several fracture-mechanics calculations.     

Effect of Additives on the Hydration Resistance of Materials Synthesized from the Magnesia–Calcia System

The effect of a variety of doping additives on the hydration resistance of calcined materials in the MgO–CaO system was investigated. Samples were prepared from lightly calcined flotation magnesite that was mixed with dolomite, as well as from analytically pure CaO and MgO; then, the samples were doped with additives that contained cations with various valences (monvalent to tetravalent). Both the hydration rate and the powdering rate were measured. The important role of the higher-valence cations in improving the hydration resistance of the MgO–CaO materials was revealed. This behavior is believed to be due to the formation of vacancies in solid solutions of CaO or MgO with higher-valence cations. The Ti⁴⁺ cation forms a solid solution with CaO, which reduces the Ca²⁺ concentration and leads to the improved hydration resistance of calcined materials from the MgO–CaO system.     

Properties of Zirconia–Ceria–Hafnia Alloys

Tetragonal zirconia alloys containing up to 18 mol% ceria and 30 mol% hafnia were prepared by several wet-cnemical techniques, sintered to high density, and their microstructural and physical properties studied. Toughening in such alloys depends on the retention of the metastable tetragonal phase and the stress required for its transformation to the monoclinic phase. The degree of metastability of the tetragonal phase in these ternary zirconia alloys was a function of the relative amounts of ceria and hafnia present. Such alloying, along with control of grain size, provides an ability to modify the fracture toughness and the temperature range over which the toughness is optimized.     

Mechanically Stable Monoclinic Zirconia–Nickel Composite

A dense (>98% theoretical density), residual-stress-free m-ZrO₂/40 vol% nickel composite with K I C ∼ 5.4 MPa·m^1/2 and ς_f∼ 225 MPa has been obtained using a simple wet processing route and subsequent sintering at 1430°C in a 90% argon/10% hydrogen atmosphere. The mechanism for release of internal stresses by the composite developed during the t → m transformation on cooling is explained in terms of plastic flow in the infinite nickel cluster formed at the percolation threshold.     

BaO–SiO_2二元系相图的热力学优化与计算

Ba O–Si O2体系是Ba O–Lu2O3–Si O2三元系相图的一个重要的边二元系。基于实验相图和热力学数据,采用双亚晶格模型(Ba2+)P(O2–,Si O44–,Si O20)Q描述高温液相的Gibbs自由能,对Ba O–Si O2二元系相图进行了热力学优化与计算。计算获得了Ba2Si O4,Ba Si2O5,Ba Si O3,Ba2Si3O8,Ba3Si5O13,Ba3Si O5,Ba5Si8O21七个中间相的Gibbs自由能,这些数据很好地重现了实验相图值;在富Si O2端,获得了与实验数据更加吻合的液相线;计算的Si O2活度曲线与实验数据吻合良好,Ba O的活度曲线与实验值在Ba O的摩尔分数x(Ba O)为50%~80%范围时有差别,可能是实验误差所致;同时给出了液相混合Gibbs自由能。获得的自洽一致的相图与热力学数据可用于新型单相基质荧光粉及相关冶金体系的研究。     

Mechanical Properties of Alumina–Zirconia–Silver Composites

Either ceramic inclusions or metallic inclusions can be used to enhance the mechanical properties of ceramics. In the present study, both silver inclusions and zirconia agglomerates have been added to alumina. The presence of the inclusions inhibits the grain growth of the alumina matrix. The strength of AI₂O₃-ZrO₂-Ag composites is increased by microstructural refinement. Together with the plastic deformation of silver inclusions and the phase transformation of tetragonal zirconia agglomerates, the toughness of the composites is enhanced. Because silver inclusions and zirconia agglomerates are attached after sintering, the toughness increase for the Al₂O₃-ZrO₂-Ag composites is less than the sum of the toughness increments for Al₂O₃-Ag and Al₂O₃-ZrO₂ composites.     

Zirconia–Silica–Carbon Coatings on Ceramic Fibers

Precursors for zircon–carbon mixtures were made to coat fibers for ceramic-matrix composites. Precursors were characterized using XRD, TGA, and DTA. Zircon formed from vanadium- or lithium-doped precursors after heat treatments at ≥900°C in air, but it did not form at 1200°–1400°C in argon when large amounts of carbon were added. Some precursors were used to coat Nextel™ 720 and Hi-Nicalon™ fibers. The coatings were characterized using SEM and TEM, and coated-fiber tensile strengths were measured. Although zircon formed in powders, only tetragonal-zirconia–silica mixed phases formed in fiber coatings at 1200°C in air. Loss of vanadium oxide flux to the fibers may have caused the lack of conversion to zircon. The strengths of the coated fibers were severely degraded after heat treatment at ≥1000°C in air, but not in argon. The coated fibers were compared with zirconia–carbon-coated fibers made using similar methods. Mechanisms for fiber strength degradation are discussed.     

Strengthening of Zirconia–Alumina During Cyclic Fatigue Testing

Recent fatigue results of advanced ceramics are reviewed. It is pointed out that current statements of fatigue properties often relate to specimens with long artificial cracks; in this study, a Y-ZrO₂+Al₂O₃ ceramic is tested under cyclic stresses using specimens with natural flaws. Cyclic fatigue is restricted to a narrow range of stresses near the ultimate strength. Survivor specimens reveal increased residual strength, possibly resulting from a higher ZrO₂ phase transformation encouraged by long-term cyclic stresses.     

Thermodynamic Assessment of the Silver–Oxygen System

An assessment of the silver–oxygen system has been made, and a consistent set of thermodynamic parameters has been optimized. The calculated thermodynamic properties and phase relations are in good agreement with the experimental data. Ag₂O is the only phase that is commonly found within the system. In air, it decomposes to silver and oxygen gas at 420 K. There is a eutectic between silver and Ag₂O at a temperature of 804 K, an oxygen partial pressure ( P _O2) of 526 bar (5.26 × 10⁷ Pa), and an oxygen mole fraction in the liquid phase of 0.25. Uncertainties remain on the Ag₂O liquidus for P _O2 > 10⁸ Pa. An ionic two-sublattice model has been used to describe the liquid phase. This work is part of a study of interactions between compounds from the bismuth-strontium-calcium-copper-oxygen system and silver.     

Phase Transformations of Plasma-Sprayed Zirconia–Ceria Thermal Barrier Coatings

Phase constituents and transformations of plasma-sprayed thermal barrier coatings (TBCs) with CeO₂-stabilized ZrO₂ (CSZ; 16–26 wt% CeO₂) have been investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The as-coated CSZ coatings with 16 and 18 wt% CeO₂ consisted only of the nonequilibrium tetragonal ( t ') phase. A mixture of the t ' and the nonequilibrium cubic ( c ') phases was observed for the as-coated CSZ coatings containing 20–26 wt% CeO₂. During 65 min cyclic oxidation at 1135°C (45 min hold time) in air, the t ' or the mixture of the t ' and the c ' phases decomposed to the equilibrium tetragonal ( t ) and the equilibrium cubic ( c ) phases. Some of the t phase transformed to the monoclinic ( m ) phase on cooling. More m phase was observed to develop in the CSZ coating containing 16 wt% CeO₂ than in the other coatings. More m phase was observed on the top surface than on the bottom surface of the CSZ coating. Spalling of the plasma-sprayed CSZ coating during thermal cycling occurred after 230 cycles for the CSZ coating containing 16 wt% CeO₂, whereas the lifetime of the CSZ coatings with 18–26 wt% CeO₂ ranged between 320 and 340 cycles.     

LNG热物性计算

天然气在其液化或液态状态下输送过程中,其物性参数随着温度和压力的变化而不断改变,因此液化天然气物性计算中不能按理想状态处理。根据实际工程需要,使用LKP方程及其关联式对密度、焓和熵值进行计算;通过对比态原理和Lohrenz等关联式计算了比热容和粘度。计算结果和实验数据对比,表明其计算精度满足工程实际需求。     

Evidence of Nearest-Neighbor Ordering in Wet-Processed Zirconia–Nickel Composites

Monolithic zirconia–nickel (ZrO₂/Ni) cermets have been prepared by a wet-processing method with nickel volume concentrations of 16%–40%. Microstructural analysis performed on scanning electron microscopy images has revealed evidence of a partial ordering of metallic particles inside the ceramic matrix. This ordering does not appear in mullite/molybdenum cermets. Complex impedance measurements have shown that the percolation threshold of ZrO₂/Ni cermets appears at a filling factor ( f _c) of 0.34, exceeding the theoretical value ( f _c= 0.16), as a consequence of its microstructural order. Electrical measurements display the expected increase of capacity near the percolation threshold. These results open the possibility to design new devices with the appealing electric, magnetic, and mechanical properties that are predicted by the percolation theory.