Synthesis of Nano-sized Yttria via a Sol-Gel Process Based on Hydrated Yttrium Nitrate and Ethylene Glycol and Its Catalytic Performance for Thermal Decomposition of NH_4ClO_4

Nano-sized yttria particles were synthesized via a non-aqueous sol-gel process based on hydrated yttrium nitrate and ethylene glycol. The effects of the molar ratio of ethylene glycol to yttrium ion and calcination temperature on crystallite size of the products were studied. The catalytic performance of the as-prepared yttria for the ammonium perchlorate （AP） decomposition was investigated by differential scanning calorimetry （DSC）. The results indicate that the nano-sized cubic yttria particles with less than 20 nm in average crystallite size can be obtained after 2 h reflux at 70℃, dried at 90 ℃, forming xerogel, and followed by annealing of xerogel for 2 h, and that the addition of the nano-sized yttria to AP incorporates two small exothermic peaks of AP in the temperature ranges of 310 - 350 ℃ and 400 - 470 ℃ into a strong exothermic peak of AP and increases the apparent decomposition heat from 515 to over 1110 J·g^- 1. It is also clear that the temperature of AP decomposition exothermic peak decreases and the apparent decomposition heat of AP increases with the increase of the amount of nano-sized yttria. The fact that the addition of the 5 % nano-sized yttria to AP decreases the temperature of AP exothermic peak to 337.7℃ by reduction of 114.6℃ and increases the apparent decomposition heat from 515 to 1240 J·g^-1, reveals that nano-sized yttria shows strong catalytic property for AP thermal decomposition.

EFFECT OF PRECIPITANT ON THE PROPERTIES OF ULTRAFINE YTTRIA POWDER PRODUCED BY PRE CIPITATION METHOD

Using yttriu m nitrate as the m other salt , synthesis of ultrafine yttria po w der through w et che mical route w as investigated . Choice of precipitant has dra m atic effects on co m position ,particle size distribution and particle m orphology of the precipitates . When a m m onia solutionw as used as the precipitant , the precursor precipitate w as m ainly Y2( O H) 5 14( N O3) 0 86· H2 O with co m paratively large particle size , broad size distribution and co m plex particleshapes . When sodiu m hydroxide solution w as used , roughly spherical α Y( O H)3·3 H2 Opre cipitate w ith sm all particle size and narro w size distribution w as obtained . The transfor m a tion sequence of dry α Y( O H)3 ·3 H2 O gel during calcination w as determ ined to be α Y( O H) 3·3 H2 O→ Y O O H→ Y2 O3 . After calcining at 600 ℃ for 1 hour , both Y2( O H) 5 14( N O3) 0 86· H2 O and α Y( O H) 3·3 H2 O transfor m to well crystallized Y2 O3 pow ders ,w ith particle sizes of 50 ～3000 n m and 20 n m , respectively .

Fabrication of Transparent Polycrystalline Yttria Ceramics by Combination of SPS and HIP

Transparent polyerystalline yttria is a promising optical ceramics with excellent physical and chemical properties. A commercial yttria powder with a mean particle size of 1.0μm and narrow size distribution was selected as the starting material. Transparent polyerystalline yttria ceramics without any additives were successfully prepared by hot isostatic pressing (HIP) at 1700℃for 2 h under the pressure of 200 MPa in Ar following spark plasma sintering (SPS). The as-prepared specimens consist of uniform grains of ~ 40μm. Scanning electron microscopy (SEM) images show their pore-free structure. The influences of the yttria powder and sintering process on the properties of the yttria ceramics, including the microstructure and optical properties, were further investigated in our study.

Cathodoluminescent properties of Tb^(3+)-doped yttria nanocrystallites

The Tb3＋-doped Y2O3 nanopowders were synthesized using the modified Pechini method. The average size of nanocrystaUites was controlled by different sintering temperatures. The structure and morphology of obtained nanopowders were examined using the XRD and SEM analyses. The Cr：Al2O3 was mixed with Th3＋：Y2O3 powders and its normalized emission was used. to measure a relative intensity of Tb3＋：Y2O3. The mixtures were electrophoretically deposited on ITO-glass slides. The cathodoluminescence spectra of obtained layers were recorded and analysed. The discussion over an influence of average grains size on phosphor efficiency was presented.

Flame Pyrolysis Spraying of Alumina-Yttria Ceramic Coating

Alumina yttria composite oxide coating was obtained on the stainless steel through the pyrolysis of aluminium nitrate and yttrium nitrate by flame spraying in the open atmospheric environment. The isothermal oxidation behaviour of the specimens coated with alumina yttria was investigated in air at the temperature of 1273 K. The results show that coating can promote the selective oxidation of chromium in the alloy and improve the high temperature corrosion resistance of the stainless steel.

Thermal stability and sputtering resistance under irradiation of yttria dispersed ferrum films

Oxide dispersion strengthened （ODS） steels are considered as potential candidates for high temperature applications in fusion reactors be-cause of their excellent thermal creep behavior. In the present work, the double-target magnetron co-sputtering method was recommended to prepare yttria dispersed ferrum films. Vacuum annealing and ion irradiation were carried out to study the surface topography and structural features of the prepared yttria dispersed ferrum samples. Experiments proved that while the yttria doping ratio in the ferrum film increases, the recrystallization temperature of the film will be enhanced and the sputtering damage by Xenon ion irradiation will be lowered. The sput-tering resistance of the obtained films would be improved with the growing of grains under vacuum annealing.

Industrial growth of yttria-stabilized cubic zirconia crystals by skull melting process

We reported the development of a Ф100 cm growth apparatus for skull melting growth of yttria-stabilized cubic zirconia(YSZ) crystals and more than 1000 kg crystals have been grown in the furnace each time.The growth conditions were optimized and the structure of the as-grown crystals was characterized by X-ray diffraction.The transmittance of 15 mol.% yttria-stabilized cubic zirconia crystal was nearly 80% in the range of 400–1600 nm.The refractive indices were measured and fitted the Sellmeier equation whi...

Catalytic properties of Ni/ceria-yttria electrode materials for partial oxidation of methane

The catalytic properties of electrode materials Ni/Ce1-xYxO2-δ （x = 0.05, 0.10, 0.15 and 0.20） were investigated for partial oxidation of methane （POM）. The CeO2-Y2O3 solid solutions were prepared by co-precipitaion method. The Ni-based catalysts supported on the solid solutions were obtained using the impregnation method. Structural, surface and redox characteristics of the prepared catalysts were system- atically examined by means of X-ray diffraction （XRD）, N2 adsorption-desorption （Brunauer-Emmet-Teller BET method）, H2 temperature- programmed reduction （H2-TPR） and X-ray photoelectron spectroscopy （XPS） methods. The results indicated that yttria doped in the ceria system, forming a good solid solution, readily induced more defects and oxygen vacancies that favored the improvement of catalytic activity and coking resistance. In the temperature range of 600-850 ℃, Ni/Ce0.90Y0.10O1.950 catalyst exhibited the best catalytic activity among the four tested catalysts, with the CH4 conversion, CO selectivity and H2 selectivity of 78.8%, 90.6% and 89.8%, respectively, at 850 ℃. And the H2/CO molar ratio in products of Ni/Ce0.90Y0.10O1.950 catalyst was closer to the theoretical value of 2.0. The excellent coking resistant behaviors for all catalysts were clearly manifested by thermal analysis.

SURFACE ELECTRICAL PROPERTIES AND STRUCTURE OF YTTRIA-PARTIALLY-STABILIZED ZIRCONIA IMPLANTED WITH ^(57) Fe IONS

Sintered plates of 5mol% yttria-partially-stabilized zirconia have been implanted at room temperature with 5 ×1015to 2 ×1017 Fe+ ions/cm2 at 140 KeV. Electrical measurement, Rutherford backscattering spec-troscopy(RBS), Raman spectroscopy and X-ray photoelec-tron spectroscopy(XPS) have been used to study the surface electrical properties and the structure of the implanted layer before and after thermal annealing treatment in N2.

Effects of Yttria Addition on Microstructure,Mechanical Properties,Wear Resistance and Corrosive Wear Resistance of TiNi Alloy

TiNi alloy has a high resistance to wear and could be an excellent candidate for various tribological applications. But studies show that oxygen active elements can improve properties of some alloys, markedly. Yttrium is one of the oxygen active elements. In this paper, the effects of yttria addition on properties of TiNi have been studied via micro-indentation, hardness, wear and corrosive wear tests. It is demonstrated that by addition of yttria to 5%, TiNi alloy can own improved mechanical properties and resistance to wear and corrosive wear.

Solid reaction mechanism of CaHPO_4·2H_2O + CaCO_3 with and without yttria

The effect of yttria on the solid reaction mechanism of a CaHPO4·2H2O ＋ CaCO3 system at different temperatures was experimentally stud-ied. The samples with and without yttria were subjected to thermogravimetric/differential scanning calorimetry measurement. The samples were heat treated at the temperatures corresponding to the peaks on the DSC spectra, and the resulted phase compositions were identified by X-ray diffraction. The transformation mechanism was deduced by comparing the phases obtained at different temperatures. The results show that the transformations at below 1073 K are not affected by yttria, but all those at above 1073 K are completely altered. The formation tem-perature of hydroxyapatite decreases by 134 K, and the decomposition temperature increases by 38 K. The polymorphous transformation of Ca3（PO4）2 from β phase to α phase increases by 47 K. The thermodynamic properties of the transformations at above 1073 K are also modi-fied by the addition of yttria; that is, the endothermal peaks are substituted by exothermal peaks.

Yttria promoted metallic nickel catalysts for the partial oxidation of methane to synthesis gas

A metallic Ni catalyst was prepared with nickel sponge, followed by acid treatment. It was further promoted with yttria by an impregnation method. The catalysts were characterized by SEM, BET, XRD, TPR, XPS, etc., and studied in the partial oxidation of methane to syngas. The characterization results showed that the yttria promoted metallic Ni catalysts had high specific surface area and more NiO. The reaction results showed that the yttria promoter increased the CH4 conversion and the selectivity for H2 and CO.

Yttria Promoted Nickel Nanowire Catalyst for the Partial Oxidation of Methane to Synthesis Gas

A yttria promoted nickel nanowire catalyst was prepared by a hard templating method, and characterized by transmission electron microscopy (TEM) and N2 physical adsorption. The catalytic properties of the yttria promoted nanowire catalyst in the partial oxidation of methane to syngas were compared with a metallic Ni catalyst which was prepared with nickel sponge. The characterization results showed that the yttria promoted nickel nanowire catalyst had high specific surface area and there was more NiO phase in the nickel nanowire catalyst than in the metallic Ni catalyst. The reaction results showed that the yttria promoted nickel nanowire catalyst had high CH4 conversion and selectivities to H2 and CO.

Dielectric Properties of Yttria Ceramics at High Temperature

Based on Clausius-Mosotti equation and Debye relaxation theory, the dielectric model of yttria ceramics was developed according to the dielectric loss mechanism. The dielectric properties of yttria ceramics were predicted at high temperature. The temperature dependence and frequency dependence of dielectric constant and dielectric loss were discussed, respectively. As the result, the data calculated by theoretical dielectric model are in agreement with experimental data.

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy(TEM), scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.

Isothermal and athermal type martensitic transformations in yttria doped zirconia

The phase transformation from the high temperature tetragonal phase to the low temperature monoclinic phase of zirconia had been long considered to be a typical athermal martensitic transformation until it was recently identified to be a fast isothermal transformation. The isothermal nature becomes more apparent when a stabilizing oxide, such as yttria, is doped, by which the transformation temperature is reduced and accordingly the transformation rate becomes low. Thus it becomes easy to experimentally establish a C-curve nature in a TTT （Time-Temperature-Transformation） diagram. The C-curve approaches that of well known isothermal transformation of Y-TZP （Yttria Doped Tetragonal Zirconia Polycrystals）, which typically contains 3mol% of Y2O3.In principle, an isothermal transformation can be suppressed by a rapid cooling so that the cooling curve avoids intersecting the C-curve in TTT diagram. Y-TZP is the case, where the stability of the metastable tetragonal phase is relatively high and thus the tetragonal phase persists even at the liquid nitrogen temperature. On the other hand, the high temperature tetragonal phase of pure zirconia can never be quenched-in at room temperature by a rapid cooling; instead it always turns into monoclinic phase at room temperature. This suggests the occurrence of an athermal transformation after escaping the isothermal transformation, provided the cooling rate was fast enough to suppress the isothermal transformation. Thus, with an intermediate yttria composition, it would be possible to obtain the tetragonal phase which is not only metastable at room temperature but athermally transforms into the monoclinic phase by subzero cooling. The objective of the present work is to show that, with a certain range of yttria content, the tetragonal phase can be quenched in at room temperature and undergoes isothermal transformation and athermal transformation depending on being heated at a moderate temperature or under-cooled below room temperature. Because both of the product phases are essentially the same monoclinic phase, both transformations are regarded as martensitic transformation, i.e. isothermal and athermal martensite. In some steels such as Fe-Mn-Ni and Fe-Ni-C, the occurrence of both isothermal and athermal martensitic transformations has been reported. However, in these cases, the isothermal transformation occurs at temperatures slightly above the Ms （Martensite start） temperatures, and thus these transformations are considered to conform the same C-curve. On the other hand, the Ms temperature of the present material is well below the C-curve, which suggests that completely different mechanisms are controlling the kinetics of these two modes of transformations. Other aspects on these transformations are also to be reported..

Preparation of alumina-doped yttria-stabilized zirconia nanopowders by microwave-assisted peroxyl-complex coprecipitation

Alumina-doped yttria-stabilized zirconia (ADYSZ) nanopowders were prepared by microwave-assisted peroxyl-complex coprecipitation (MAPCC) using ZrOCl2·8H2O, Y2O3 and AlCl3·6H2O as starting materials, NH3·H2O as precipitant and H2O2 as complexant. The effects of adding H2O2 and microwave drying on the preparation and properties of ADYSZ were investigated. The precursors and nanopowders were studied by EDX, XRD, SEM and TEM techniques. The results show that the uniformity of component distribution within ADYSZ nanopowders is improved by adding appropriate dosage of H2O2. Complexing reaction between H2O2 and Zr4+ ion restrains the hydrolyzation and precipitation of Zr4+ ion. With the addition of H2O2, Al3+, Y3+ and Zr4+ ions can be precipitated synchronously in a relatively narrow range of pH value. H2O2 also improves the filterability of the wet precipitate. The highly hydrophilic precipitates can be quickly and effectively separated from aqueous solution. During microwave drying process, the moisture of wet precursors is selectively heated. Quick expansion of steam vapor within the wet colloidal particles causes the aggregations burst into numerous tiny lumps. Compared with oven drying, microwave drying can not only shorten drying time but also reduce aggregation intensity of the resultant ADYSZ nanopowders.

Y_(2)O_(3)对氧化物结合Ti(C,N)复合材料性能的影响

为探究稀土氧化物对Ti(C,N)复合材料性能的影响,以TiO_(2)粉、石墨粉和Si粉为原料,Y_(2)O_(3)粉为烧结助剂,采用碳热还原氮化法经1600℃高温烧成制备氧化物结合Ti(C,N)材料。研究Y_(2)O_(3)添加量(加入质量分数分别为1%、2%、4%)对Ti(C,N)复合材料物相组成、显微形貌及物理性能的影响。结果表明,高温烧后试样的主物相均为Ti(C,N)、Ti_(3)O_(5)、α-石英及Y_(2)Si_(2)O_(7)。随Y_(2)O_(3)添加量的增大,试样中液相生成量增多,使Ti(C,N)晶粒粗化,Y_(2)Si_(2)O_(7)物相分布不均匀,试样的力学性能受损。当Y_(2)O_(3)添加量(w)为1%时,Ti(C,N)晶粒的平均尺寸约为0.76μm,复合材料具有较高致密度及优异的力学性能,其显气孔率、常温抗折强度和弹性模量分别为0.8%、242.2 MPa和230.3 GPa。