High‐temperature stability of YSZ and MSZ ceramic materials in CaF2–MgF2–MgO molten salt system

The high‐temperature stability of YSZ and MSZ specimens was investigated in CaF₂–MgF₂–MgO molten salt at 1200°C. YSZ was mostly composed of m‐ZrO₂ and a small part of YF₃ in the early stages. The formation of YF₃ was attributed to the chemical reaction between Y₂O₃ and MgF₂, which can lead to the leaching of Y₂O₃ from YSZ. With an increase in exposure time, the degraded surface was coarser, and considerable amount of cracks, pores, and spallations were formed. Furthermore, no Y₂O₃ was found up to 120 μm of the YSZ bulk in the early stages. MSZ was composed of t‐ZrO₂ after 24 hours. However, the volume fraction of m‐ZrO₂ was 72% after 72 hours, and CaZrO₃ was formed by the chemical reaction between CaO and ZrO₂ after 168 hours. In addition, the volume fraction of m‐ZrO₂ was 60% in 2.5 wt% MgO and 49% in 10 wt% MgO. In 5 wt% MgO, CaZrO₃ was formed. We demonstrate that the high‐temperature stability of MSZ was better than that of YSZ, and that 10 wt% MgO was much more stable than the other concentrations of MgO.     

Oxidation resistance and thermal stability of the SiC-ZrB2 composite ceramic fibers

In this study, continuous SiC-ZrB₂ composite ceramic fibers were synthesized from a novel pre-ceramic polymer of polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam cross-linking, pyrolysis, and finally sintering at 1800°C under argon. The ZrB₂ particles with an average grain size of 30.7 nm were found to be uniformly dispersed in the SiC with a mean size of 59.7 nm, as calculated using the Scherrer equation. The polycrystalline fibers exhibit dense morphologies without any obvious holes or cracks. The tensile strength of the fibers was greater than 2.0 GPa, and their elastic modulus was ~380 GPa. After oxidation at 1200°C for 1 hour, the strength of the fibers did not decrease despite a small loss of elastic modulus. Compared to the advanced commercial SiC fibers of Tyranno SA, the fibers exhibited improved high-temperature creep resistance in the temperature range 1300-1500°C.     

Thin water films and particle morphology evolution in nanocrystalline MgO

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X‐ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing‐induced morphology changes on powders of MgO nanocubes in different gaseous H₂O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H₂O) = 10^?5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two‐dimensional solvent for Mg²⁺ ion hydration. In addition, we obtained direct evidence for hydroxylation‐induced stabilization of (110) faces and step edges of the grain surfaces.     

Phase Equilibria in the ZrO2–MgO–MnOx System

Phase equilibria were experimentally investigated in the MgO–MnO_x and the ZrO₂–MgO–MnO_x systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnO_x system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO₂–MgO–MnO_x phase diagram were constructed for air conditions ( = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for = 10^?4 bar. The β‐spinel and halite phases of the MgO–MnO_x system were found to dissolve up to 2 and 5 mol% ZrO₂. A continuous c‐ZrO₂ solid solution forms between the boundary ZrO₂–MnO_x and ZrO₂–MgO systems. It stabilizes in the ZrO₂–MgO–MnO_x system down to at least 1613 K in air and down to 1506 K at = 10^?4 bar.     

Thermal stability of plasma-sprayed La2Ce2O7/YSZ composite coating

A composite coating composed of La₂Ce₂O₂ (LCO) and yttria-stabilized zirconia (YSZ) in a weight ratio of 1:1 was deposited by the plasma spraying using a blended YSZ and LCO powders, and the stability of the LCO/YSZ interface exposed to a high temperature was investigated. The LCO/YSZ deposits were exposed at 1300 °C for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ scanning electron microscopy assisted by X-ray energy-dispersive spectrum analyses and X-ray diffraction measurements. At an exposure temperature of 1300 °C, the grain morphology of LCO splats in contact with YSZ splats changed from columnar grains to quasi-axial grains with interface healing, and some grains tended to disappear during the thermal exposure. The results indicate that the phases in LCO–YSZ composite coating are not stable at 1300 °C. The element La in the LCO splat diffused towards the adjacent YSZ splat during the exposure, generating the reaction product layers composed of La₂Zr₂O₇ between the LCO and YSZ splats. After exposed for 200 h, the composite coating consisted of a mixture of mainly La₂Zr₂O₇ and CeO₂ and a minor amount of YSZ, accounting for the unusual decrease in the thermal conductivity at the late stage of exposure.     

氧化锆/氧化铝复相陶瓷显微组织缺陷分析

针对5Y-ZrO2/Al2O3复相陶瓷出现的晶粒异常长大和晶粒开裂问题展开研究。以ZrO2和Al2O3为主要原料,采用常压烧结工艺制备陶瓷样品,利用SEM观察显微组织。分析表明:MgO对抑制Al2O3晶粒异常长大有重要影响,MgO的加入量应随着Al2O3加入量的变化而改变;烧结温度的改变将导致异常长大的Al2O3晶粒细化。当烧结温度较低时,Al2O3晶粒将在短轴方向逐渐断开成段;当温度较高时,则沿着长轴方向逐渐开裂成条状。ZrO2晶粒的断裂主要与烧结温度有关:在1630℃以上烧结时,出现裂纹并贯穿晶粒;晶粒开裂的原因是:烧结温度较高时,陶瓷中形成了t-ZrO2,在降温过程中大颗粒的t相发生t→m相变,而小颗粒t相则无法变成m相,引起局部体积变化不均匀,从而产生相变应力导致晶粒穿晶断裂。     

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.     

Fabrication and characterization of novel excellent thermal-protection Gd2Zr2O7/ZrO2 composite ceramic fibers with different proportions of Gd2Zr2O7

Three kinds of Gd₂Zr₂O₇/ZrO₂ (GZC) composite fibers with different proportions of Gd₂Zr₂O₇ were prepared by electrospinning method through changing the amount of Gd³⁺ in precursor solutions. The thermal decomposition, crystallization process, high temperature stability and heat-conducting properties of GZC fibers were fully characterized. The results showed that there were three crystalline phases, tetragonal phase ZrO₂, cubic phase ZrO₂ and defect fluorite phase Gd₂Zr₂O₇ in all the GZC fibers. The content of Gd₂Zr₂O₇ increased gradually with the increase of Gd³⁺ in precursor solutions which led to the gradual slowing down of grain growth rate, the decrease of thermal conductivity and the increase of high temperature stability of the obtained composite fibers. The thermal conductivities of all the GZC fiber sheets were lower than that of 7YSZ fiber sheet. The sheets of all the GZC fibers could keep the high temperature stability up to 1300 °C.     

Manipulation of ZrO2/CaZrO3 fibrous membrane with high thermal stability and NIR reflectivity

Grain size control is an important aspect to manipulate the microstructure and high temperature stability of ceramic fibers. In the present work, phase competition mechanism was proposed to regulate the grain size of ZrO₂/CaZrO₃ composite fibers. Calcium precursor with different molar ratio of Ca/Zr was introduced to the zirconia system not only for phase stabilization of ZrO₂ but also for phase competition between solid-solution Ca_xZr_1-xO_2-x and new phase CaZrO₃. Effects of Ca/Zr molar ratio on the thermal pyrolysis, crystallization, solid reaction of precursor fibers were fully explored. The change of grain size of fibers with various Ca/Zr molar ratio was characterized and discussed. Furthermore, the near-infrared reflectivity and high-temperature stability of fibrous membranes were also presented. The results indicated that ZrO₂/CaZrO₃ fibrous membrane has promising applications in high-temperature for the excellent thermal stability and high near infrared (NIR) reflectivity.     

Improvement of thermal stability of alumina by addition of zirconia

To maintain a large surface area at elevated temperatures, zirconia was added to transition alumina. The addition of a small amount of zirconia resulted in a marked suppression of phase transformation from θ- to α-alumina. After heating at 1200°C, ZrO₂‐containing alumina exhibited a large surface area of 50 m₂/g. UV‐VIS and XRD measurements indicated that zirconia existed in a high dispersion state after calcining at 800°C. XPS measurement also showed that zirconia existed as monolayer. Zirconia monolayers are concluded to cover the alumina surface and the interaction between them may be the cause for the suppression of phase transformation and also for the maintenance of the large surface area at elevated temperatures. The interaction remains up to 1200°C, therefore, θ phase remained at 1200°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of La2O3 addition and sintering mode on the mechanical properties and microstructural evolution on an 8YSZ ceramic alloy

In this research, the influence of La₂O₃ addition on the microstructure, phase stability and mechanical properties of 8?mol% yttria stabilized zirconia (8YSZ) was studied. 8YSZ with La₂O₃ (9, 12 and 15?wt%) ceramics were fabricated by microwave and conventional sintering at 1400?°C/ 20?min and 1400?°C/ 5?h, respectively. Irrespective of the sintering technique, the relative sintered density was found to decrease with increasing amount of La₂O₃. The grain growth of 8YSZ was enhanced significantly by the addition of La₂O₃. The XRD results demonstrated that addition of La₂O₃ up to 15?wt% did not disrupt the cubic 8YSZ phase regardless of sintering technique; additionally evolution of pyrochlore phase, La₂Zr₂O₇ was observed in all sintered specimens. Vickers hardness of 8YSZ ceramic compacts were also found to decrease with increasing amount of La₂O₃.     

Phase stability and microstructure evolution of yttria-stabilized zirconia during firing in a reducing atmosphere

The phase transition and microstructure evolution of yttria-stabilized zirconia (YSZ) when firing in a reducing atmosphere are investigated in the present study. As the sintering temperature is lower than 1400 °C, the phase observed for the YSZ specimens is the first tetragonal (t) phase with a grain size of only 0.4 μm in average. When the sintering temperature is increased above 1450 °C, the second tetragonal (t′) phase appears together with a rapid grain growth rate. A surface layer with grains larger than 20 μm is observed for the YSZ specimens sintered at 1600 °C. It is found that the addition of metallic nickel particles has little influence on the phase transition of YSZ; however, the growth of t′-phase grains is reduced with the addition of Ni particles.     

Effects of water vapor on the crystallization and microstructure manipulation of MgO ceramic fibers

Magnesium oxide (MgO) ceramic fibers are a promising candidate material in high temperature insulating area, supporting area, adsorption area and catalytic area, and most of the properties are determined by the microstructure. In the present work, MgO precursor fibers were fabricated by the centrifugal-spinning combined with sol-gel method. The thermal decomposition and crystallization process of the ceramic fibers were fully characterized. Different atmosphere preheat treatment results suggested that water vapor promoted the thermal decomposition and crystallization of precursor fibers at a lower temperature. Three kinds of particles, including round particles, polyhedron particles to plate particles in the microstructure of the fibers could be manipulated by adjusting the water vapor preheat treatment. The change in textural properties (BET surface area, pore size and pore volume) of the MgO fibers, heated at different temperatures in air after pre-heated treatment in water vapor, was analyzed. Furthermore, the formation mechanisms of the microstructures of the fibers were also presented. The easy manipulation of the microstructures of MgO ceramic fibers may make it a promising material in various areas.     

添加ZrO_2或Y_2O_3对氧化镁高温真空稳定性的影响

为提高氧化镁在高温真空环境下的稳定性,在粒度≤0.074 mm、w(Mg O)=98.70%的轻烧氧化镁粉中分别外加质量分数为0、0.5%、1%和2%的Zr O2或Y2O3后,混合、成型的试样经1 750℃保温6 h煅烧。将煅烧后试样在真空度为1.5×10-2Pa、于1 600℃保温1 h的条件下进行真空稳定性试验,然后对试样的质量损失、物相组成进行分析,并进行晶体学计算。结果表明:添加Zr O2或Y2O3可引起氧化镁的晶格畸变,Zr4+、Y3+部分取代Mg O晶格中Mg2+位置,形成晶格畸变,同时产生阳离子空位,对周围原子起到束缚作用,降低了原子振动频率,从而减少逸散原子的数量,提高了氧化镁在高温真空条件下的稳定性。     

Stable and Metastable Phase Relationships in the System ZrO2–ErO1.5

Stable and metastable phase relationships in the system ZrO₂–ErO_1.5 were investigated using homogeneous samples prepared by rapid quenching of melts and by arc melting. The rapidly quenched samples were annealed in air for 48 h at 1690°C or for 8 months at 1315°C. Two tetragonal phases ( t - and t '-phases) were observed after quenching samples heated at 1690°C to a room temperature, whereas one t -phase and cubic ( c -) phase were found in those treated at 1315°C. Since the t '-phase is obtained through a diffusionless transformation during cooling from a high-temperature c -phase, t - and c -phases can coexist at high temperature. The t - and c -phases field spans from 4 to 10 mol% ErO_1.5 at 1690°C and from 3 to 15 mol% ErO_1.5 at 1315°C. The equilibrium temperature T ^t-m ₀ between the t - and monoclinic ( m -) phases estimated from A_s and M_s temperatures decreased with increasing ErO_1.5 contents.     

Crack Resistance and Fatigue of Transforming Ceramics: II, CeO2-Stabilized Tetragonal ZrO2

Crack resistance characteristics and fatigue properties have been investigated in Ce-TZP ceramics with different grain sizes. The relatively low critical transformation stress allows the development of larger transformation zones (≦200 μm), leading to flaw-tolerant behavior. However, autocatalytic transformation processes are found to be bound to grain sizes beyond a critical value; transformation is then very limited in finer microstructures. Fatigue as a specific cyclic effect is more pronounced in microstructures with larger grains. Thus, damaging processes in the course of extensive t–m transformation are suspected to be intensified during cyclic loading.     

Metastable Phase Selection and Partitioning in ZrO2—MgO Processed from Liquid Precursors

Aqueous mixtures of either zirconium acetate or zirconium nitrate and magnesium nitrate were dried and subsequently pyrolyzed at fast heating rates (upquenching) to form metastable crystalline phases of ZrO₂ with various degrees of MgO supersaturation. The crystallization temperature was determined to be 380°C for the zirconium acetate, and 270°C for the zirconium nitrate at a heating rate of 5°C/min. The crystalline structures were characterized as a function of MgO content and thermal history for specimens containing 0 to 30 mol% MgO. Upquenching to 900°C, where monoclinic ( m ) ZrO₂ and MgO are the equilibrium phases, yielded single-phase tetragonal ( t ) ZrO₂ (<8 mol% MgO), single-phase cubic ( c ) ZrO₂ (9 to 17 mol% MgO), and two-phase c -ZrO₂+ MgO structures (>17 mol% MgO). The composition for which T ₀( t/c ) = 900°C was estimated as 9 ± 1 mol% MgO. Compositions crystallizing as metastable t -ZrO₂ (<8 mol% MgO) partitioned at higher temperatures and/or longer times into two-phase mixtures, following the general sequence t → t + m → m + MgO. Similarly, compositions forming metastable c -ZrO₂ (10 to 30 mol% MgO) partitioned in the following sequence: c → c + t + MgO → t + MgO → t + m + Mgo → m + Mgo. The initial phase selection and subsequent partitioning sequence are discussed in light of phase hierarchies predicted from thermodynamic concepts and kinetic constraints which are introduced by the solute partitioning required to achieve equilibrium.     

Role of oxygen on the phase stability and microstructure evolution of CaCu3Ti4O12 ceramics

Phase stability and microstructure evolution of polycrystalline CaCu₃Ti₄O₁₂ (CCTO) ceramics were studied by controlling the partial pressure of oxygen (from a poor to an oxygen rich atmosphere) during the sintering process at high temperatures. The samples were analyzed by X-ray powder diffraction, scanning electron microscopy and X-ray energy dispersive spectroscopy. Our results show that the oxygen partial pressure during the sintering process is an important parameter that controls the phase stability, non-stoichiometry, and decomposition process of the CCTO phase as well as the densification and grain growth mechanisms on these polycrystalline ceramics. These results provided us further insight into the important role of copper reduction and copper/oxygen diffusion on the crystalline structure and morphological characteristics of polycrystalline CCTO ceramics.     

Stability of Monoclinic and Orthorhombic Zirconia: Studies by High-Pressure Phase Equilibria and Calorimetry

The stability fields of two high-pressure polymorphs of ZrO₂ (ortho I and ortho II) were determined by both calorimetry and phase equilibrium experiments. Enthalpies of transition were measured by transposed temperature drop calorimetry. The entropies of transition and slopes of phase boundaries were calculated using the measured enthalpies and free energies calculated from the results of phase equilibrium experiments. From the thermodynamic measurements, it is seen that the entropy increases and the volume decreases during the monoclinic–ortho I transition, whereas both the entropy and the volume decrease during the ortho I–ortho II transition. Accordingly, the gradient of phase boundaries, dP/dT , is negative in the former and positive in the latter. These trends are consistent with those of phase equilibrium experiments.