Nitrogen-Containing Tetragonal Zirconia

The present paper gives evidence for the replacement of oxygen by nitrogen when partially stabilized tetragonal zirconia is reacted with nitrogen above 1400°C to give a tetragonal product but in the t ' (nontransformable) form. No transformation toughening is therefore expected from this nitrogencontaining tetragonal phase. These results may explain why less-effective toughening is obtained in nitrogen-containing ceramic-based composites than in equivalent zirconia-alumina and other oxide-based systems.     

Hot Isostatic Pressing of a Presintered Yttria-Stabilized Zirconia Ceramic

The effect of hot isostatic pressing on the bend strength of ZrO₂–3 mol% Y₂O₃ was critically dependent on the presintering process. Optimally sintered bodies contained no open porosity and exhibited large increases in strength following hot isostatic pressing. When open porosity of as little as 0.3% persisted after sintering, hot isostatic pressing increased the bulk density, but little or no increase in strength was realized. Two-parameter Weibull analysis of the strength data was used to quantify the strength improvement obtained following hot isostatic pressing. Typical fracture-initiating flaws were identified through optical and electron microscopy.     

Effects of In Vitro Aging of Magnesia-Stabilized Zirconia

The intent of this project was to evaluate any changes in the flexure strength of magnesia-partially-stabilized zirconia when aged in distilled water for time periods of 6, 12, and 18 mos. The aging conditions were autoclaved specimens in air (AA) or distilled water (AW) and nonautoclaved specimens in distilled water (NW) at stressing rates of 10.0, 1.0, or 0.1 mm/min. The statistical analysis showed no significant difference between the aging times. The aging-time data were then pooled and a statistical analysis run between the different aging conditions (AA, AW, and NW) and stressing rates. The specimens tested in air were significantly stronger than those tested in water autoclaved or nonautoclaved. For the pooled data, all stressing rates were statistically significant from each other.     

氰基胍作为PVC热稳定剂的应用

用刚果红法和烘箱变色法比较研究了以干混料和塑化料为试样时氰基胍(CG)、硫醇辛基锡(OTM)、硬脂酸钙(CaSt2)和硬脂酸锌(ZnSt2)对PVC的热稳定性能,结果表明:以干混料和塑化料为试样的测试结果存在明显差别,也就是说,以干混料为试样得到的热稳定性能测试结果不能反映实际应用条件下的表现,要获得对实际应用具有指导意义的热稳定性能结果,应以塑化料作为试样进行测试。根据以塑化料为试样的测试结果,CG单独使用时效果不明显,与CaSt2、环氧大豆油(ESBO)并用存在明显的协同效应;CG的热稳定性能与1,3-二甲基-6-氨基尿嘧啶(DMAU)相比还存在较明显的差距,电绝缘性能相当,但透明性能则优于后者。     

New Route for the Extraction of Crude Zirconia from Zircon

A commercial grade of zircon (ZrSiO₄) concentrate was mechanically milled with MgO for up to 100 h in a laboratory-scale mill. The resultant powders were subjected to thermal processing, chemical leaching, and X-ray diffraction (XRD). There was no direct evidence of reaction during the milling step, with no new phases evident from XRD. Leaching of the powder showed that a reaction had occurred, and increased solubility with milling time was attributed to the formation of a nanostructured Mg-Zr-Si oxide, which dissolved congruently. Heating the powders resulted in a number of thermal events, including the formation/crystallization of ZrO₂ and Mg₂SiO₄. Thermal treatment of the milled powders allowed selective chemical leaching of the magnesium and silicon, leaving a powder containing ∼90% ZrO₂.     

Analysis of Hot Isotatic Pressing of Presintered Zirconia

Hot isostatic pressing (HIP) of presintered Y-TZP was studied at 1100° to 1400°C, 5 to 200 MPa, for 0.5 to 4 h. The effects of process variables of HIP and the characteristics of the presintered specimens on the densification behavior of HIP were examined. The microstructural development after HIP was also examined. Grain growth occurred during the densification in HIP. Empirically, a linear relation having a rather constant slope was found between the logarithms of porosities and grain sizes, for each starting condition. Provided this relationship was taken into account, the Ashby's model for HIP could express the densification process for this system satisfactorily.     

苯基脲作为PVC热稳定剂的应用

用刚果红法和烘箱变色法比较研究以干混料和塑化料为试样时苯基脲(PU)、硫醇辛基锡(OTM)、硬脂酸钙( CaSt2)和硬脂酸锌(ZnSt2)时PVC的热稳定性能,结果表明:以干混料和塑化料为试样的测试结果存在明显差别,也就是说,以干混料为试样得到的热稳定性能测试结果不能反映实际应用条件下的表现,要获得对实际应用具有指导意义的热稳定性能结果,应以塑化料作为试样进行测试.根据以塑化料为试样的测试结果,PU具有类似于ZnSt2的热稳定特性,属于初效型主热稳定剂,与CaSt2环氧大豆油(ESBO)并用存在明显的协同效应;苯基脲的热稳定性能与1,3-二甲基-6-氨基尿嘧啶(DMAU)相比还存在较明显的差距,但透明性能和电绝缘性能则明显优于后者.     

Fabrication of Dense Zirconia Electrolyte Films for Tubular Solid Oxide Fuel Cells by Electrophoretic Deposition

Electrophoretic deposition (EPD) is used to produce zirconia electrolyte films for tubular solid oxide fuel cells. A simple suspension chemistry (8-mol%-yttria-stabilized zirconia particles in acetic acid) yields films of similar quality to those from conventional approaches (such as electrochemical vapor deposition), but at potentially much less expense. A key factor in obtaining high-density, adherent films via the EPD approach is the application of a thin fugitive phase (carbon in this study) on the porous, doped lanthanum manganite cathode tubes prior to zirconia deposition.     

State of Magnesia in Magnesia (10.4 mol%)-Doped Zirconia Powder Prepared from Coprecipitation

MgO (10.4 mol%)-doped ZrO₂ powder, which was prepared by coprecipitation and calcination at 750°C, was leached and milled, respectively. The powders were characterized by Auger electron scanning microprobe and XRD technologies. Only the 2.3 mol% MgO was soluble in ZrO₂; it stabilized ZrO₂ as the metastable tetragonal phase (crystallite size ∼45 nm) at room temperature. The rest of the MgO (8.1 mol%) existed on the surface of the ZrO₂ grains in the prepared powder.     

Strength and Toughness of Tape-Cast Yttria-Stabilized Zirconia

The biaxial flexural strength and fracture toughness of tape-cast yttria-stabilized zirconia, for application as the electrolyte in solid oxide fuel cells, have been measured at room temperature and at a typical operating temperature of 900°C. The flexural strength was measured in ring-on-ring loading and decreased from 416 MPa at room temperature to 265 MPa at 900°C. The fracture toughness was measured using two different techniques: indentation fracture and double-torsion loading. The latter was more reliable and gave a fracture toughness of 1.61 ± 0.12 MPa·m^1/2 at room temperature and 1.02 ± 0.05 MPa·m^1/2 at 900°C. The flexural strength and fracture toughness were quantitatively consistent with fracture being initiated at the observed surface defects. The lower fracture toughness at 900°C is partly due to a reduction in elastic modulus and partly due to a reduction in the work of fracture.     

Reaction of Uranium Hexafluoride Gas with Alumina and Zirconia at Elevated Temperatures

Alumina and zirconia are considered for potential use in uranium hexafluoride (UF₆) based nuclear gaseous-core reactors. The corrosion reactions of these oxide ceramics in high-temperature UF₆ environment were investigated. Chemical thermodynamic analysis of the reactions was performed by using a computer data base and code package, FACT, to identify stoichiometric products. An alumina reaction tube in a flowing loop test unit was used to expose alumina and calcia-stabilized zirconia samples to UF, at temperatures from 873 to 1473 K and pressures of about 20.0 to 20.7 k Pa. Chemical reaction rates were measured by a discontinuous gravimetric method. X-ray diffraction and EDS analyses and SEM microstructural examination identified corrosion products and the morphology of the exposed surfaces. Results indicate that reactions were not inhibited or slowed down for either oxide. At 1273 K and below, alumina samples formed stable AIF₃ scales and showed acceptable compatibility with UF₆. The rate of UF₆ reaction with alumina and weight loss dramatically increased at 1473 K. Zirconia samples failed to resist degradation and reacted rapidly with UF₆ at 1073 K. The maximum service temperatures for alumina and zirconia in UF₆ environment do not seem to exceed 1273 and 973 K, respectively.     

Thin Yttrium-Stabilized Zirconia Electrolyte Solid Oxide Fuel Cells by Centrifugal Casting

A centrifugal casting technique was developed for depositing thin 8-mol%-yttrium-stabilized zirconia (YSZ) electrolyte layers on porous NiO-YSZ anode substrates. After the bilayers were cosintered at 1400°C, dense pinhole-free YSZ coatings with thicknesses of ∼25 μm were obtained, while the Ni-YSZ retained porosity. After La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-Ce_0.9Gd_0.1O_1.95 (GDC) or La_0.8Sr_0.2MnO₃ (LSM)-YSZ cathodes were deposited, single SOFCs produced near-theoretical open-circuit voltages and power densities of ∼1 W/cm² at 800°C. Impedance spectra measured during cell tests showed that polarization resistances accounted for ∼70%–80% of the total cell resistance.     

The Characterization of Tungsten-Oxide-Modified Zirconia Supports for Dual Functional Catalysis

The results of catalytic titration measurements indicated that WO _x /ZrO₂ catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WO _x /ZrO₂ contains 0.004 meq-H⁺/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Brønsted acid sites on the surface of these catalysts. The concentration of strong Brønsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Brønsted–Lewis site. Pt/WO _x /ZrO₂ undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WO _x /ZrO₂ (16% W) catalyst, consistent with the presence of bulk WO₃ in this material as observed by TPR.     

Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y₂O₃-stabilized ZrO₂ (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.     

Characteristics and Catalytic Properties of Alumina–Zirconia Mixed Oxides Prepared by a Modified Pechini Method

A modified Pechini method was used to prepare alumina–zirconia mixed oxides at three different molar ratios. For comparison, pure alumina and pure zirconia were prepared using the same method. The mixed oxides were characterised by the BET method for surface area, X-ray diffraction, CO₂ and NH₃ temperature-programmed desorption. Elimination of 2-propanol was used as a probe reaction to characterise the surface of the mixed oxides. The modified Pechini preparation resulted in a poor acid–base strength of alumina surface resulting in high acetone selectivity where imperfect crystal structure of the tetragonal zirconia favoured high propylene production in 2-propanol elimination at 200 °C.     

卫星电池用高性能氧化锆纤维布膜的研制

以聚砜微孔膜为增强体制备增强的氧化锆纤维隔膜材料,可使布膜的强度由原来的102g/cm(宽)提高到现在的198g/cm(宽),吸碱速率由原来的2cm/min提高到现在的4cm/min,而其它的性能如厚度、孔隙率及面电阻等基本不受影响,研制的氧化锆纤维碱性电池隔膜材料基本满足工程化应用的要求.     

Ultrasonic Spray Pyrolysis for Synthesis of Spherical Zirconia Particles

This paper presents new findings on ultrasonic spray pyrolysis of zirconium hydroxyl acetate precursor drops whose sizes were precisely measured using laser light diffraction technique. Precursor concentration plays a predominant role in determination of product particle size. At 0.01 wt% precursor concentration, conventional spray pyrolysis at 750°C using precursor drops 5–8 μm in diameter, generated by an ultrasonic nebulizer at 2.66 MHz, yielded uniform spherical yttria-stabilized zirconia (YSZ) particles 73 nm in diameter measured by scanning electron microscopy. The YSZ particle diameters were much smaller than those predicted by the one-particle-per-drop mechanism. Under similar reaction conditions, the high-throughput ultrasound-modulated two-fluid (UMTF) spray pyrolysis of larger precursor drops (28-μm peak diameter) also yielded spherical dense particles; they were significantly smaller in size than those produced by the low-throughput conventional ultrasonic spray pyrolysis of smaller drops (6.8-μm peak diameter).     

Enhanced Phase Stability for Tetragonal Zirconia in Precipitation Synthesis

Tetragonal ZrO₂ nanocrystallites—with or without yttria (3 mol%) doping—have been synthesized via a precipitation process in which the hydrous oxide precipitate reacts with hexamethyldisilazane (HMDS) vapor before calcination. The nanocrystallites are formed and retain a tetragonal structure for hours after calcination at temperatures of 300°–1100°C. The enhanced structural metastability has been attributed to the combined effect of suppressed grain growth and reduced surface energy that results from the HMDS treatment.     

添加Li_2O对YSZ电解质性能影响

8%（摩尔分数,下同）Y2O3稳定的ZrO2（8YSZ）是固体氧化物燃料电池（SOFC）中最常用的电解质材料,本文研究了在8YSZ基体中加入n%Li2O（n=0,0.25,0.50,1.00,1.50,1.70,2.00,2.50,3.00）后（记为n%Li2OYSZ）对其晶相结构、晶格常数、烧结性能、微观形貌、电导率及其作为SOFC电解质性能的影响。结果表明,Li2O中的Li＋可以固溶到YSZ的晶格内使其晶格常数减小;Li2O的加入量n〈1.70时,瓷体在烧结过程中不会发生相变。加入少量的Li2O（n=0.25,0.50）可以提高YSZ的致密度和电导率,0.25%Li2OYSZ和0.50%Li2OYSZ样品800℃的电导率分别高达0.030 2 S/cm和0.027 6 S/cm,分别是纯YSZ的1.35和1.24倍;当Li2O含量n≥1.00时,相同条件下烧结体致密度随Li2O加入量的增大而逐渐减小;当n≥1.70时,样品在烧结过程中虽然出现相变,但在高于1400℃可以烧结致密,并得到纯立方相YSZ。将1250℃烧结制得的0.25%Li2OYSZ和0.50%Li2OYSZ作为SOFC电解质的单电池,800℃时的开路电压高于1.0V,说明YSZ中没有出现电子电导,具有比纯YSZ为电解质的单电池更高的性能输出,表现出了良好的应用前景。     

Fabrication of Highly Porous Yttria-Stabilized Zirconia by Acid Leaching Nickel from a Nickel-Yttria-Stabilized Zirconia Cermet

Porous Y₂O₃-stabilized ZrO₂ (YSZ) samples were synthesized by preparing NiO/YSZ composites by tape casting and calcining at 1800 K, reducing the NiO to nickel in H₂ at 973 K, and finally leaching the nickel out of the structure with 2.2 M HNO₃ at 353 K. Porous YSZ was prepared from NiO/YSZ composites containing 0, 20, 40, and 50 wt% NiO. Complete removal of the nickel was demonstrated by XRD, weight changes, and porosity increases. Porosities >75% could be achieved without structural collapse of the YSZ phase. Finally, the method was applied to the fabrication of a solid oxide fuel cell with a copper-based anode operating on H₂ and n -butane.