四方ZrO_2的X射线衍射鉴定特征及定量分析中的几个问题

单斜ＺｒＯ２中加入不同的稳定剂如ＭｇＯ、Ｙ２Ｏ３、ＣｅＯ、ＣａＯ等，经过一定的工艺处理能转化成室温下存在的四方ＺｒＯ２。由于各种稳定剂的质量吸收系数不同，对Ｘ射线定量分析结果带来一定的差异。本文探讨了常用的几种稳定剂如ＭｇＯ、Ｙ２Ｏ３、ＣａＯ对定量分析的影响；并应用经验公式和标准曲线、外标法三种定量方法进行比较；并指出稳定剂不同、工艺条件不同、定量方法上也应有所选择，否则会给定量分析带来较大误差。     

Lifetime determination of tetragonal zirconia under static loading using the Constant Stress Rate method

Subcritical crack growth is a phenomenon which limits service time of a ceramic material. It is especially prevalent for oxides, because this phenomenon is attributed to the activity of water at the crack tip of the material and can be caused even by water present as a humidity in the air (Salem and Jenkins, 2002; Michalske and Freiman, 1983). It is very important to determine its lifetime at the setting loads with high probabilities of survival. The Constant Stress Rate method gives results that are sufficient for lifetime predictions. Estimations are based on n parameter which results from the slope of the strength vs. stress rate dependence. Only a conversion from dynamic to static conditions has to be done (Wojteczko et al., 2016). The attempts were made at different stress rates on sintered samples with pre-existing flaws and in two environments − air and water. Tetragonal zirconia was the tested material. Biaxial loading method was used for strength measurements. Microstructural and fractographic observations were made using the scanning electron microscope.     

The influence of zirconia fibre on ablative composite materials

ABSTRACT

Zirconia fibres have excellent high temperature ablation resistance and have been widely used in ablative materials. In this paper, zirconia fibre was used for reinforcing the ablative composite materials to study the influence of zirconia fibre had upon the mechanical properties and the high temperature ablation properties of such composites. The results showed that the bending strength of the material was also good and reached a maximum of 13.05?MPa. After sintering at 1400°C, the bending strength was also great which could reach 13.05?MPa. In addition, the corrosion resistance of the composites was excellent and the oxygen-acetylene line ablation rate was 0.03?mm?s^?1 when the fibre content was 30?wt-%.     

The effective mass,vibration and electromagnetic properties of tetragonal iron aluminide FeAl2

MoSi₂-type tetragonal iron dialuminide (space group I4/mmm) have been successfully synthesized and determined in a previous stud by Tobita et al. In this work, the electronic structure, effective mass, vibration, and electromagnetic properties of t-FeAl₂ were predicted using the density functional theory (DFT). First, the predicted crystal lattice parameters reproduced the previously reported experimental data. Second, the electronic structure shows that t-FeAl₂ is a narrow-gap semiconductor material with an indirect gap (0.059 eV), and the effective masse of the electron (m_e*) and hole (m_h*) is 0.35 m₀ and 0.2 m₀, respectively. Third, the electromagnetic wave absorption characteristics such as the frequency-dependence of the relative complex permittivity, dielectric loss tangent tgδ, electrical conductivity, quality factor Q × f and reflection loss are calculated. Besides, using the linear response theory, the phonon dispersion curves and vibrational properties are calculated. The phonon dispersion curves do not have imaginary phonon frequencies, confirming the dynamic stability of t-FeAl₂. Finally, the vibrational properties are investigated, and the infrared (IR)-active and Raman-active modes are also assigned and briefly discussed.     

Synthesis of high-purity zircon,zirconia, and silica nanopowders from local zircon sand

High-purity zircon (ZrSiO₄) nanopowder was successfully produced from Indonesian natural zircon sand using a low-cost purification approach via magnetic separation, immersion in HCl, and reaction with NaOH, followed by a top-down nanosizing process using wet ball-milling for 10?h and annealing at 200?°C for 2?h. Furthermore, polymorph zirconia (ZrO₂ – amorphous, tetragonal, and monoclinic) and silica (SiO₂ – amorphous and cristobalite) nanopowders were also successfully derived from the purified zircon powder using a bottom-up method via alkali fusion and co-precipitation processes followed by calcination. The crystallite size of the powders was estimated from X-ray diffraction (XRD) data analysis to give 40, 31, 61, and 149?nm, respectively, for the zircon, tetragonal- and monoclinic-zirconia, and cristobalite. Microstructural characteristics of the zircon, silica, and zirconia nanopowders were revealed in transmission electron microscopy (TEM) images which confirmed that the average sizes of the particles were in a good agreement with the XRD estimated values.     

Liquid phase benzoylation of arenes over iron promoted sulphated zirconia

The present work undertakes the preparation and physico-chemical characterisation of iron promoted sulphated zirconia (SZ) with different amounts of iron loading and their application to Friedel–Crafts benzoylation of benzene, toluene and xylene under different experimental conditions. XRD and laser Raman techniques reveal the stabilisation of the tetragonal phase of zirconia and the existence of iron in highly dispersed form as Fe₂O₃ on the catalyst surface. The surface acidic properties were determined by ammonia temperature programmed desorption (TPD) and perylene adsorption. The results were supported by the TGA studies after adsorption of pyridine and 2,6-dimethylpyridine (2,6-DMP). Strong Lewis acid sites on the surface, which are evident from TPD and perylene adsorption studies, explain the high catalytic activity of the systems towards benzoylation. The experimental results provide evidence for the truly heterogeneous nature of the reaction. The studies also establish the resistance to deactivation in the metal incorporated sulphated systems.     

Fabrication and characterization of tetragonal yttria-stabilized zirconia single-crystalline thin film

Tetragonal yttria-stabilized zirconia thin film was successfully fabricated by a pulsed laser deposition method. The thin film grew heteroepitaxially with the orientation relationship of ZrO₂‖Al₂O₃. Energy dispersive X-ray spectroscopy mapping revealed that Y³⁺ ions were distributed homogeneously without local segregations. X-ray and electron-diffraction analysis confirmed a single crystalline structural feature of the film. On the other hand, high-resolution scanning transmission electron microscopy observations show that this film contains small-angle tilt grain boundaries, which is composed of the periodic array of dislocations with the Burgers vector .     

Tensile behavior of tetragonal zirconia micro/nano-fibers and beams in situ tested by push-to-pull devices

The tensile mechanical behavior of tetragonal zirconia micro/nano-fibers and beams was studied with push-to-pull (PTP) devices equipped in an in situ nanoindenter. The small-volume ceramics generally experienced linear elastic deformation before fracture. Polycrystalline and oligocrystalline micro/nano-fibers exhibit a tensile strength of ∼0.9–1.4 GPa, while single-crystal beams exhibit a much higher tensile strength (∼2.1–3.2 GPa). The tensile strength of the small-volume zirconia is found comparable to the corresponding compressive strength, which indicates the large discrepancy between the tensile and compressive strength observed in bulk zirconia becomes insignificant at micro/nano-scales. No martensitic transformation induced shape memory strain was detected in the zirconia fibers and beams. Further variation in dopant concentration and crystal orientation was explored for single-crystal beams and their significance in controlling the tensile strength was discussed. Our work offers a new insight into the mechanical behavior of tetragonal zirconia-based ceramics at small scales.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

Microstructure and toughening mechanisms of Al2O3/(W,Ti)C/graphene composite ceramic tool material

To toughen the Al₂O₃ matrix ceramic materials, Al₂O₃/(W, Ti)C/graphene multi-phase composite ceramic materials were fabricated via hot pressing. The effects of the graphene nanoplates (GNPs) content on microstructure and mechanical properties were investigated. Results showed that the fracture toughness and flexural strength of the composite added with just 0.2?wt% GNPs were markedly improved by about 35.3% (~ 7.78?MPa?m^1/2) and 49% (~ 608.54?MPa) respectively compared with the specimens without GNPs while the hardness was kept about 24.22?GPa. However, the mechanical properties degrade with the further increase of GNPs’ content owing to the increased defects caused by agglomeration of GNPs. Synergistic toughening effects of (W, Ti)C and GNPs played an essential role in improving the fracture toughness of composites. By analyzing the microstructures of fractured surface and indentation cracks, besides GNPs pull-out, crack deflection, crack bridging, crack branching and crack arrest, new toughening mechanisms such as break of GNPs and crack guiding were also identified. Furthermore, interface stress can be controlled by means of stagger distributed strong and weak bonding interfaces correlated with the distribution of GNPs.     

Self-assembly preparation of popcorn-like colloidal silica and its application on chemical mechanical polishing of zirconia ceramic

Traditional mobile phone backplane materials are difficult to meet the requirements of the 5G era, and zirconia ceramic is one of the most promising backplane materials. However, its precision machining is difficult due to the hard and brittle nature. In this work, a novel popcorn-like colloidal silica was prepared by the self-assembly growth of nanoparticles for chemical mechanical polishing of the yttria-stabilized tetragonal zirconia ceramic sheets. The surface of the popcorn-like colloidal silica particles has a noticeably uneven shape, and the particle size distribution is uniform. The chemical mechanical polishing results show that the material removal rate of the prepared popcorn-like colloidal silica is increased by about 50% compared with the spherical colloidal silica, and the surface morphology is also obtained improvement. In the process of chemical mechanical polishing, the particles form multi-point contact with the ceramic sheet, resulting in an increase in the coefficient of friction, which is beneficial to the tribochemical reaction. In addition, multi-point contact can distribute the load, make the indentation shallower, and help reduce mechanical scratches. In general, the expected results are expected to provide experimental basis for the optimization of the structure of chemical mechanical polishing abrasive particles.     

Iron and manganese promoted sulfated zirconia: acidic properties and n-butane isomerization activity

Sulfated zirconia catalysts promoted with Fe and Mn have been synthesized and the acidity characterized by IR spectroscopy of adsorbed pyridine. The catalytic isomerization of n-butane was investigated in a fixed bed reactor operated at low conversion at atmospheric pressure and 250 °C. The main effect of the promoters was to change the ratio Brönsted:Lewis acidity of the samples. The catalytic activity was found to be correlated with the number of strong Brønsted acid sites.     

Effect of PyC interface phase on the cyclic ablation resistance and flexural properties of two-dimensional Cf/HfC composites

Composites based on hafnium carbide and reinforced with continuous naked carbon fiber with and without PyC interface were prepared at low temperature by precursor infiltration and pyrolysis and chemical vapor deposition method. The microstructure, mechanical property, cyclic ablation and fiber bundle push-in tests of the composites were investigated. The results show that after three times ablation cycles, the bending strength of samples without PyC interface decreased by 63.6 %; the bending strength of samples with PyC interface only decreased by 37.8 %. The force displacement curve of the samples with PyC interface presented a well pseudoplastic deformation state. The mechanical behavior difference of two kinds of composites was due to crucial function of PyC interface phase including protection of fiber and weakening of fiber/matrix interface.     

Influence of additives on the purity of tetragonal phase and grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP)

A comprehensive study on the influence of typical additives on zirconia (ZrO₂) crystallization was presented. Zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) and cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) were employed as reagents, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) or glycerol were adopted as additives, and ammonia water was adopted as pH regulator. The ZrO₂ powders were prepared by hydrothermal method. The crystal phase purity, grain size and micro morphology of the ZrO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) to investigate the influence of EDTA-2Na, glycerol and Ce⁴⁺ content on the purity of tetragonal phase and the grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). It was found that EDTA-2Na could decrease the purity of tetragonal phase and alter the grain size of Ce-TZP nonlinearly, while glycerol could not decrease the purity of tetragonal phase and the grain size of Ce-TZP, and the grain size was not linear with the amount of glycerol; Doping Ce⁴⁺ could increase the purity of tetragonal phase of zirconia but could not decrease the grain size, and the grain size was not linear with the Ce⁴⁺ content; In addition, it was indicated that EDTA-2Na and glycerol could not improve the distribution uniformity of Ce⁴⁺. This study is expected to have provided a novel path to achieve tailored ZrO₂ crystals with reduced low-temperature degradation.     

Tetragonal structure, anionic vacancies and catalytic activity of SO4-ZrO2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO₄²⁻-ZrO₂ with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO₄²⁻-ZrO₂ catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H₂ was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work.

A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO₄²⁻-ZrO₂ is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.     

Sulfate-vanadate hot corrosion of neodymium cerate/yttria stabilized zirconia composite coating

Neodymium cerate (Nd₂Ce₂O₇) was laboratory synthesized by solid-state reactions of neodymia and ceria and spray dried to get the feedstock for plasma spraying. Hot corrosion behavior of air plasma sprayed 10% Nd₂Ce₂O₇/yttria stabilized zirconia (YSZ) topcoat was studied in the presence of V₂O₅ and Na₂SO₄ salts at 950°C for 10-60 hours. It was observed that due to the presence of relatively higher basic compounds Nd₂O₃ and CeO₂ (form Nd₂Ce₂O₇) than Y₂O₃ in YSZ changed the reaction dynamics during hot corrosion and the major part of corrosive (NaVO₃) was consumed by Nd of Nd₂Ce₂O₇, making NdVO₄. The Y₂O₃ of YSZ contributed in the corrosive reaction partially, relieving majority tetragonal zirconia (t-ZrO₂) in topcoat (YSZ). No remarkable cracking was developed in the topcoat even after 60 hours of accelerated hot corrosion, which was attributed to the retention of in-relief t-ZrO₂ in YSZ.     

Study on the interaction between SiO2 and ZrO2 in the chemical mechanical polishing of zirconia ceramic with colloidal silica

Colloidal silica is usually used for the chemical mechanical polishing of zirconia ceramic wafer in industry, but the process is often optimized only through experience without a precise understanding of the polishing mechanism. There are still many theoretical and technical issues, especially the material removal mechanism and the effect of polishing on the phase transformation, have not been studied in depth. In this study, the effect of the abrasive concentration, polishing pressure and slurry pH on the material removal rate was analyzed. It is found that the removal rate tends to be stable when the concentration exceeds 30 wt%; the influence of pressure on the polishing rate conforms to the Preston formula. When the pH of the slurry is 6, the removal rate is the highest, but polishing under acidic conditions will leave corrosion pits due to the dissolution of the stabilizer. Through X-ray photoelectron spectroscopy analysis of the residue on the wafer surface, it was found that Si-O-Zr bonds were formed, but it was uncertain whether the residue was zirconium silicate. Through X-ray diffraction analysis, it is found that polishing will not affect the crystal structure of zirconia. The Zr-O-Si bond formed by tribochemical action on the ceramic surface prevents the deep migration of surface hydroxyl groups. At the same time, kinetic factors will cause internal hydroxyl groups to transfer to the surface for recovery oxygen vacancies, thereby stabilizing the tetragonal phase.     

铌掺杂介孔氧化锆复合催化材料的合成与表征

以氧氯化锆（ZrOCl2·8H2O）为锆前驱体,乙酰丙酮（ACAC）作为水解抑制剂,十六烷基三甲基溴化铵（CTAB）为模板剂,用无水乙醇稀释的氨水调节溶液的pH,采用溶胶-凝胶法于溶剂无水乙醇中合成介孔氧化锆。在此基础上以草酸铌为铌前驱体,将铌元素掺杂到介孔氧化锆体系,制备介孔Nb2O5-ZrO2复合氧化物催化剂。采用低温氮气吸附-脱附、X射线粉末衍射（XRD）、Hammett指示剂法对样品进行表征。结果显示,将铌掺入介孔氧化锆后,可制备出介孔 Nb2O5-ZrO2复合氧化物,当n（Nb）∶n（Zr）＝0.05时,其产品BET表面积为122.42 m2/g,孔径为3.68 nm,与未掺杂介孔氧化锆的样品相比,虽然并没有产生新的酸位,但总酸量有所增加。     

Effects of twin boundaries and pre-existing defects on mechanical properties and deformation mechanisms of yttria-stabilized tetragonal zirconia

In annealing of yttria-stabilized tetragonal zirconia (YSTZ), {011}-specific twins and sub-surface defects are often observed, however their effects on the martensitic phase transformation and deformation behavior of YSTZ have never been investigated. In this work, the roles of twin boundaries (TBs) and pre-existing defects in determining the mechanical properties and subsequent deformation mechanisms of YSTZ nanopillars are studied. Using large-scale molecular dynamics simulations, we show that Young’s modulus and strength of YSTZ decrease with the increase of TB density, but the ductility of YSTZ pillars increases. Phase transformation behavior is found to be correlated to TB density. The sensitivity of mechanical responses of twinned structures to pre-existing defects is also studied. A competitive mechanism between TB-induced phase transformation and void-induced phase transformation is observed. When the diameter of a pre-existing void is smaller than a critical value, only TB-induced phase transformation occurs, which leads to void-insensitive mechanical properties.