Superplasticity-like Deformation of Nanocrystalline Monoclinic Zirconia at Elevated Temperatures

The deformation behavior of nanocrystalline monoclinic ZrO₂ polycrystals (nanocrystalline MZP) was studied at 1273–1373 K in compression tests. The deformation of nanocrystalline MZP was characterized by stress exponent ( n = 2.5), grain-size exponent ( p = 2.5), and apparent activation energy ( Q = 350 kJ/mol). The values of n and p were similar to the superplasticity of high-purity Zn-22% aluminum alloy. The strain rate of nanocrystalline MZP was faster than that of Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) at temperatures lower than the monoclinic-tetragonal transition temperature. The strain rate of MZP gradually approached to that of Y-TZP as the temperature increased to the transition temperature. The comparison of present data with published data suggested that trace amount of impurities affected the deformation behavior of MZP.     

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.     

Reactions of Zirconia and Titanium at Elevated Temperatures

The reactions between zirconia and titanium were studied by (1) weight change and bulk density determinations, (2) chemical analysis, (3) metallographic and petrographic analyses, (4) microhardness determinations, (5) lattice parameter measurements, (6) electron probe analysis, and (7) thermal conductivity determinations. These studies were made on specimens which had been fired in vacuum in the temperature range 1200° to 2000°C. Experimental data indicated that titanium contents up to 4 atomic % were retained at room temperature as a substitutional solid solution in zirconia. In titanium-rich binary alloys, zirconium and oxygen were retained in solid solution in titanium for zirconia contents up to approximately 10 mole %. In this case, the zirconium entered the titanium lattice substitutionally and the oxygen went into interstitial positions.     

纳米颗粒粒径对等离子喷涂法制备氧化锆纳米涂层的影响

利用扫描电镜(SEM)、场发射扫描电镜(FESEM)、透射电镜(TEM)、比表面积吸附法(BET)等分析测试技术,研究了3种不同纳米尺寸氧化锫粉粒的造粒性能、沉积效率以及对等离子喷涂涂层晶粒大小、涂层熔融性能、结合强度的影响.结果表明:纳米氧化锆粉体一次颗粒粒径大小显著影响纳米粉体的喷雾造粒性能、沉积效率、涂层表面粗糙度、涂层晶粒粒径和结合强度大小.本试验中,利用颗粒一次粒径范围为50～70 nm的纳米氧化锆粉体,等离子喷涂制备了晶粒粒径范围为80～120 nm,沉积效率为43%,涂层表面粗糙度为5.92 μm,结合强度为27 MPa的纳米结构氧化锆涂层.     

Synthesis of Niobium(V)-Stabilized Tetragonal Zirconia Nanocrystalline Powders

The polymer precursor method is very useful to prepare Nb⁵⁺-stabilized nanocrystalline powders of t -ZrO₂. The precursor solution is composed of zirconium oxalate, niobium tartrate, and poly(vinyl alcohol), which help to form a network matrix to disperse the metal ions homogeneously. Nb⁵⁺ is an effective agent to stabilize t -ZrO₂, and ease of formation of the tetragonal phase increases with increasing dopant concentration. Thermal stability of t -phase is found up to 1700°C having 15 mol% Nb⁵⁺, prepared at 600°C with particle sizes of 35 ± 5 nm.     

Crystallite and Grain-Size-Dependent Phase Transformations in Yttria-Doped Zirconia

In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the "equilibrium" monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal → monoclinic phase transformation is examined more broadly across the yttria–zirconia system. Using dilatometry and high-temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal → monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases.     

氧化锆陶瓷弯曲强度的尺寸效应

利用流延方法制备不同厚度的氧化锆陶瓷(3YSZ)片状试样,然后经过300℃脱脂,1520℃烧结,分别测试其抗弯强度及烧结密度,发现其抗弯强度与试样的厚度尺寸有关,随着试样的厚度减小,试样的抗弯强度不断增加,其弯曲弹性模量也增大,表明陶瓷材料厚度的减小,有利于试样内部气孔的排除,从而使材料强度得到提高.     

颗粒分布、比表面积、化学组成对水泥强度的影响

水泥的化学组成、细度决定着水泥的力学性能.根据已有的关于水泥颗粒的粒度分布和比表面积的数学模型,对4个不同水泥厂的水泥数据进行了分析,发现这些数据与模型之间没有很好的对应.利用120组数据建立了水泥颗粒的粒度分布与比表面积新的数学模型及28 d抗压强度与4种矿物组成的数学关系式,并把水泥的抗压强度用水泥颗粒大小、比表面积和化学组成表示出来了.     

Prevention of Low-Temperature Surface Transformation by Surface Recrystallization in Yttria-Doped Tetragonal Zirconia

The low-temperature (100° to 400°C) tetragonal to monoclinic transformation in yttria-doped tetragonal zirconia (Y-TZP) can be inhibited by a postsintering grinding and annealing treatment. The surface region so treated contains fine tetragonal grains which have recrystallized from the severely damaged ground surface. The various features of the recrystallized surface that may affect the lowtemperature transformation are analyzed.     

NOVA1000e型比表面积和孔径分析仪测定高纯纳米氧化铝比表面积

主要讲述了使用NOVA1000e型比表面积和孔径分析仪测定高纯纳米氧化铝的比表面积,仪器操作方法和测试过程中样品处理条件.把NOVA1000e型比表面积和孔径分析仪与ST-03型比表面积分析仪进行性能对比.     

Structural and Mechanical Property Changes in Toughened Magnesia-Partially-Stabilized Zirconia at Low Temperatures

The mechanical properties of high-toughness magnesia-partially-stabilized zirconia were found to be dramatically altered by a single cooling cycle between room temperature and − 196°C. Raman spectroscopy and X-ray diffraction were used to correlate the changes in mechanical properties with structural changes that occur at temperatures below ∼− 100°C. Most of the tetragonal precipitates that are responsible for toughening transformed to an orthorhombic phase with unit-cell volume intermediate between those of the tetragonal and monoclinic phases. The orthorhombic phase was stable with heating to 300°C, but it transformed back to the tetragonal structure when heated to 400°C. Surprisingly, the orthorhombic phase was not readily transformable by stress, with the consequence that, after the cooling cycle, most of the high-toughness properties of the original tetragonal-containing material were lost.     

Preparation and Structural Characterization of Ultrafine Zirconia Powders

Factors concerning the stabilization of the tetragonal and cubic phases, metastable at room temperature, with respect to the monoclinic stable phase in ultrafine zirconia powders are studied. The importance of Na⁺ ions in the initial zirconia amorphous gel in obtaining a cubic phase has been confirmed. By an X-ray diffraction study using a new peak profile fitting procedure, the amount of the crystalline phases and their microstructural properties (crystallite size and lattice distortions) have been obtained.     

Synthesis of Thermally Stable Tetragonal Zirconia with Large Surface Area and Its Catalytic Activity in the Skeletal Isomerization of 1-Butene

Thermally stable zirconia with a large BET surface area was prepared by using zirconium atrane derivatives and commercial alkyltrimethylammonium bromide. The zirconia samples were characterized by wide-angle X-ray diffraction, thermal analysis and N₂ adsorption/desorption analysis. The results showed that the present zirconia existed as a thermally stable tetragonal crystallite and consisted of mesoporous material with microporous contribution. The BET surface area was found larger than 240 m²/g even after calcination at 600 °C. The present zirconia after sulfation was applied to the skeletal isomerization of 1-butene, and its catalytic activity was found superior to that of the conventional sulfated zirconia.     

Effect of Grain Size on the Sliding Wear and Friction of Alumina at Elevated Temperatures

The sliding friction and wear of three different grain-size aluminas were studied from room temperature through 1000°C. The coefficient of friction revealed two distinct regions of decrease with increased temperature, with a transition at ∼700°C. Below 700°C, the coefficient of friction decreased rapidly with increased temperature (∼10^-3/°C). However, above 700°C, the decrease was more gradual (∼10^-5/°C). This was believed to be related to a brittle-to-ductile transition at the wear surface. The coefficient of friction was only weakly dependent on grain size, because the largest grain sizes exhibited slightly higher friction coefficients. However, the specific wear loss of the aluminas increased with increased grain size at room temperature and at 600°C, both below the 700°C transition. The primary mechanism of wear was ascertained to be brittle microfracture along grain boundaries. At 1000°C, above the 700°C transition, the specific wear loss was significantly decreased and appeared to be independent of the alumina grain size. At 1000°C, the wear surfaces developed a thin layer of fine grains formed by dynamic recrystallization. The grain size within the thin layer was in agreement with the previously reported grain-size/Zener-Hollomon parameter relationship.     

Synthesis and Characterization of Zirconia Nanorods

ZrO₂ nanorods are prepared by annealing precursor powders produced in the novel inverse microemulsion system. The length and diameter of ZrO₂ nanorods are a few micrometers and 40–100 nm, respectively. The microstructure of the resultant nanorods are studied by XRD, TEM, selected area electron diffraction, and Raman spectroscopy. The ZrO₂ nanorods are single crystalline and have monoclinic structure. The formation of ZrO₂ nanorods is discussed.     

Crystallite Size and Microstrain of Thermally Aged Low-Ceria- and Low-Yttria-Doped Zirconia

The final phases of zirconia powders depend on the synthesis method employed, and the amounts of stabilizer present. In this study, ceria- and yttria-doped zirconia powders were prepared by urea hydrolysis and subsequent hydrothermal treatment. The amount of tetragonal ( t ) vs monoclinic ( m ) phase in the powders increased with increasing stabilizer content, while the tetragonal phase size decreased and the microstrain of t crystals remained unchanged. The thermal degradation behavior of the metastable t phase in zirconia containing a low CeO₂ or YO_1.5 doping level was explored during aging treatment by means of X-ray line profile broadening analysis. Both ultrafine yttria- and ceria-doped zirconia powder pellets exhibit isothermal t → m transformation after aging at 900°C for various times. It is argued that a crystallite size effect, rather than the dopant valence, dictates the occurrence of the t → m transformation in ultrafine zirconia powders. The change in crystallite sizes of both t and m phases during aging depends significantly on the amount of stabilizer, aging time, and mechanism of t → m phase transformation. However, the change of microstrain in both t and m phases is related to the amount of stabilizers present and the matrix constraints.     

纳米ZrO2前驱物表面电性能的研究

利用3000HS型ζ-电位分析仪研究了ZrOCl2浓度、pH值、分散剂等因素对纳米氧化锆前驱物表面ζ-电位的影响.结果表明:低浓度ZrOCl2溶液生成的ZrO2前驱物的ζ-电位对pH值变化更加敏感;添加2%(质量分数,下同)的PEG6000和PEG20000使ZrO2前驱物的表面ζ-电位有所降低,添加2%的PEG100对前驱物ζ-电位的影响更为显著.     

Synthesis and Colloidal Processing of Zirconia Nanopowder

Nanosized tetragonal 3 mol% Y₂O₃-doped ZrO₂ powder was produced by hydrothermal precipitation from metal chlorides and urea sol followed by a washing–drying treatment and calcination. The effects on powder properties of powder washing by water and ethanol with subsequent centrifuging, with possible deagglomeration using microtip ultrasonication, were experimentally shown. Ultrasonic irradiation induced pressure waves, which generated cavities that could violently collapse, producing intense stress. This induced stress was effective in minimizing secondary particle size, deagglomerating the powder, redispersing the ZrO₂ after all the washing–centrifuging cycles, and minimizing mean aggregate size after final calcination. A uniformly aggregated tetragonal ZrO₂ nanopowder with a mean secondary particle size of ∼45 nm and without hard agglomerates was prepared. The properties of the nanopowders produced by colloidal processing and CIP were studied. Determination of the best suspension parameters allowed for low-temperature sinterability, which resulted in a nanograined ∼95 nm ceramic.     

Effect of Preparation Methods and Condition of Precursors on the Phase Composition of Yttria-Stabilized Zirconia Powders

An yttria-stabilized zirconia powder, free of monoclinic phase, may be prepared by an oxalate method in an ethanol solution at strong acidity. This study demonstrates that the control of pH in the preparation of precursors has a significant effect on the ability of precursors to crystallize and hence plays an important role in determining the formation and fraction of various crystalline phases in the resulting yttria-stabilized zirconia powder. With the increase of pH, a precursor with a certain crystalline form may be transformed into an amorphous precursor, and a monoclinic phase appers in the phase composition of the resulting powder. The results of XRD analysis and Raman spectroscopy are discussed.