Effect of dopants on the properties of metal-doped zirconia prepared by the glycothermal method

The effects of dopants on zirconia prepared via the glycothermal method were investigated by XRD to determine the crystal structure and crystallite size. Morphologies of products were observed by SEM. The basic sites of zirconia were studied by CO₂-temperature programmed desorption (CO₂-TPD). The functional group in the samples was determined using IR. The intensity of Zr³⁺, characterized by ESR, could be described as the oxygen coordinatively unsaturated Zr sites. The results suggest that doping elements can modify the surface chemistry of ZrO₂ to form hydroxyl groups and surface energies depending on the structure (cubic, tetragonal) in different dense phase. ESR peaks of Pb- and Bi-doped zirconia are different from the others, which showed high intensity of Zr³⁺.     

Synthesis and characterization of nanocrystalline zirconia powder by simple sol–gel method with glucose and fructose as organic additives

The present study has devised the sol–gel method using glucose and fructose as two organic additives so as to synthesize zirconia nanoparticles. The presence of these organic additives has produced some positive effect on the phase transition from tetragonal to monoclinic and played an important role in the morphology and crystallite size of the nanoparticles. Fourier transform infrared (FT-IR) spectra have shown Zr–O–Zr bond. Crystal phase and crystallite size have been determined by X-ray Diffraction (XRD) analysis. Besides, the morphology of the samples has been investigated by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The optical properties of the samples have been analyzed using photoluminescence (PL) spectroscopy, too. All the analyses consistently have shown fairly uniform nanoparticles with small size, containing both tetragonal and monoclinic phases with crystallite size between 10 and 30 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.     

Ce_(0.7)Zr_(0.3)/γ-Al_2O_3纳米固溶体的制备及其结构

以氯氧化锆、硝酸铈、硝酸铝为原料,按n(Ce)∶n(Zr)=0.7∶0.3的比例,采用化学共沉淀法与有机物共沸蒸馏法,将CeO2、ZrO2分散到γ-Al2O3表面上使其形成Ce0.7Zr0.3O2/γ-Al23固溶体。用XRD考察纳米固溶体在不同温度下焙烧后的相结构。结果表明:Ce0.7Zr0.3O2/γ-Al2O3纳米固溶体为立方晶型,且随着焙烧温度的升高,样品的衍射峰依次变强,峰宽变窄。     

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.     

Solution combustion synthesis and physicochemical characterization of ZrO2-MoO3 nanocomposite oxides prepared using different fuels

A series of MoO₃-ZrO₂ nanocomposite oxides with MoO₃ content in the range of 2-20 mol% were prepared by solution combustion method. Three different fuels namely urea, glycine and hexamethylenetetramine (HMTA) were used for the preparation of MoO₃-ZrO₂ oxides. For the sake of comparison, the MoO₃-ZrO₂ composite oxides were also prepared by impregnation of zirconia with molybdenum salt precursor and subsequent heat treatment. The synthesized nanomaterials were characterized by XRD, SEM, TEM and UV-vis spectroscopic technique. XRD study indicated selective stabilization of the tetragonal phase of zirconia in the presence of MoO₃. The method of preparation was found to be crucial for the phase composition of zirconia in the composite oxide. The crystallite size and rms stain were calculated from the Fourier line shape analysis of the broadened X-ray diffraction profiles. With increase in the MoO₃ content, the crystallite size of the tetragonal zirconia phase was found to be decreased. TEM study indicated the presence of small nanoparticles with size in the 5-10 nm range. UV-vis study of the composite oxide materials revealed well dispersion of the molybdenum oxide component in the form of monomer, dimers and nanoclusters in the zirconia matrix. The nature of fuel was found to be crucial in determining the morphology and shape of the particles.     

水热法合成Ce_（0.9）Gd_（0.1）O_（1.95）及其光催化性质研究

采用水热法合成了花状Ce0.9Gd0.1O1.95纳米粉,通过X射线衍射、红外光谱、场发射扫描电子显微镜测试手段对产物进行了表征,探讨了其生长机制,以亚甲基蓝的光降解为模型反应,研究其光催化性能。结果表明,以甲酸为介质,110℃的水热条件下合成的纳米Ce0.9Gd0.1O1.95颗粒经800℃煅烧后具有萤石结构,平均晶粒尺寸为21.6 nm,花状形貌;由于Gd3＋的掺杂,纳米Ce0.9Gd0.1O1.95对亚甲基蓝紫外光催化性能强于未掺杂的CeO2。该合成方法简单易行,对纳米Ce0.9Gd0.1O1.95的形貌控制起到了启示作用,所得的花状纳米Ce0.9Gd0.1O1.95在光催化领域有着重要应用价值。     

Mesoporous high surface area Ce0.9Gd0.1O1.95 synthesized by spray drying

Mesoporous gadolinium doped cerium dioxide with high surface area was produced by spray drying using Pluronic 123 as surfactant. The powder, when calcined at 400 °C, had a BET surface area of 136 m² g⁻¹ and was polycrystalline as confirmed by XRD and TEM. XEDS confirmed Ce, Gd and O, as the only elements present in the powders. Nitrogen adsorption measurements showed that the material was mesoporous with pore diameters of approximately 10 nm, which was later confirmed by TEM studies. TEM-EELS were used to confirm that Ce was in the +4 oxidation state. SEM studies showed particles of 10 nm diameter, corresponding to the crystallite size calculated from XRD data. The similar size range of the mesopores and the observed crystallite size indicates that the porosity is partly formed from intergranular mesoporosity. Using the spray drying method of a surfactant assisted liquid precursor solution it can be possible to manufacture mesoporous gadolinium doped ceria in large quantities in a continuous production.     

Energy Crossovers in Nanocrystalline Zirconia

The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse-grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high-temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO₂ was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m². The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m², for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/surface area gave surface enthalpies of 4.2 J/m² for the monoclinic form and 0.9 J/m² for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO₂ was estimated to be 10±1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol.     

微乳法制备纳米氧化锆及其结构分析

以四水合硫酸锆为原料,通过油酸/正丁醇/氨水体系制备纳米ZrO2。运用热重-差热分析、红外光谱分析、X射线衍射、激光粒度分布、透射电子显微镜等手段对产物的结构和形态进行了表征。实验结果表明,所合成的ZrO2为四方晶型,产物的形状为球形,粒径在20 nm左右。红外光谱测试结果表明,随着煅烧温度的升高,Zr─O─Zr键的吸收频率产生红移;随着微乳体系中氨水浓度的增大,所制备样品的Zr─O─Zr键的吸收频率产生红移,ZrO2的粒径增大。     

Modified Sol–Gel Based Nanostructured Zirconia Thin Film: Preparation,Characterization, Photocatalyst and Corrosion Behavior

In this work, AISI 316L stainless steel was coated by nanostructured zirconia using the sucrose assisted sol–gel dip-coating route. Then, the effect of different calcination temperatures and the thickness of the coating on the corrosion protection of 316L stainless steel was investigated. Here, Zr(acac)₄ and sucrose were used as starting materials and gelation agents, respectively. Thermogravimetry and differential thermal analysis, X-ray powder diffraction (XRD), Fourier transform infrared, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the coatings. XRD revealed that the pure tetragonal phase of zirconia was obtained at the calcination temperature of 300–500 °C. However, the mixture of monoclinic (m) and tetragonal (t) phase found in the zirconia coating calcined at 650 °C. Also, by increasing the calcination temperature from 300 to 650 °C, the mean of the crystallite size of structures was increased from 7 to 27 nm. AFM result show that the average roughness value of the sample calcined at 300 °C is 10.5 nm and the dimensions of the particles on the surface of this sample smaller than 50 nm. The potentiodynamic polarization and electrochemical impedance spectroscopy results revealed that the as-synthesized nanostructured sol–gel zirconia coatings exhibited a barrier property for the protection of the substrate. However, the highest corrosion resistance was obtained by the zirconia coating calcined at 300 °C. This was as a result of the desirable compromise of good adhesion, low defect density, and high barrier behaviour. Furthermore, zirconia nanoparticles were synthesized by calcination of the gel at the different temperature. The photocatalytic activity of samples was tested for degradation of methyl orange solutions. It is found that ZrO₂ nanoparticles calcined at 500 °C have higher photocatalytic activity than the other samples under UV light.     

Investigating the effect of metal oxide additives on the properties of Cu/ZnO/Al2O3 catalysts in methanol synthesis from syngas using factorial experimental design

The Cu/ZnO/Al₂O₃ catalysts, prepared by co-precipitation method, have been modified by adding small amount of Mn, Mg, Zr, Cr, Ba, W and Ce oxides using design of experiments (1/16 full factorial design). The structure and morphology of catalysts were studied by X-ray diffraction (XRD) and BET. Performance of the prepared catalysts for CO/CO₂ hydrogenation to methanol was evaluated by using a stainless steel fixed-bed reactor at 5 MPa and 513 K. The oxide additives were found to influence the catalytic activity, dispersion of Cu, Cu crystallite size, surface composition of catalyst and stability of catalysts during their operations. The results showed that the Mn and Zr promoted catalysts have high performance for methanol synthesis from syngas.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Fabrication and characterization of boron doped yttria‐stabilized zirconia nanofibers

Boron doped and undoped poly(vinyl) alcohol/zirconium‐yttrium acetate (PVA/Zr‐Y) nanofibers were prepared by electrospinning using PVA as a precursor. The effect of boron doping was investigated in terms of solution properties, morphological changes and thermal properties. The effect of boron doping on calcined yttria stabilized zirconia (YSZ) fibers was evaluated by X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy analysis. XRD analysis revealed varying amounts of monoclinic and tetragonal zirconia present in the undoped fibers calcined at 800°C. The average crystallite sizes of the undoped YSZ were increased from 9.28 to 22.79 nm with calcining temperature increasing from 250 to 800°C. The crystallite size was enhanced with boron doping. The systematic evolution of morphological features in the spun and the processed fibers were employed by scanning electron microscopy. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Low-Temperature Aging of t'-Zirconia: The Role of Microstructure on Phase Stability

Polycrystalline, tetragonal ( t ') zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t ' materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t ' materials. Even then, the t ' materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t ' materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.     

Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: I, Trivalent Dopants

Local atomic structures of Zr and dopant cations in zirconia solid solutions with Fe₂O₃, Ga₂O₃, Y₂O₃, and Gd₂O₃ have been determined. The Zr ions in both partially stabilized tetragonal and fully stabilized cubic zirconia have their own characteristic structures which are dopant-independent. The dopant cations substitute for Zr ions despite severe local distortions necessitated by the large difference in dopant–O distance ana Zr─O distance. Dopant ionic size determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants (Y and Gd) are located as nearest neighbors to Zr atoms, leaving 8-fold oxygen coordination to dopant cations. Undersized dopants (Fe and Ga) compete with Zr ions for the oxygen vacancies in zirconia, resulting in 6-fold oxygen coordination and a large disturbance to the surrounding next nearest neighbors. Since oxygen vacancies associated with Zr can provide stability for tetragonal and cubic zirconia, these results suggest an explanation for the observation that oversized trivalent dopants are more effective than undersized trivalent dopants in stabilizing cubic and tetragonal phases.     

Effect of ceria content on the sintering of ZrO2 based ceramics synthesized from a polymeric precursor

Zirconia-ceria powders with ceria concentration varying from 0 to 12 mol% were synthesized using a polymeric precursor route based on the Pechini process. Powder characteristics were evaluated with regard to the crystallite size, BET surface area, phase distribution, nitrogen adsorption/desorption behavior, and agglomeration state. Sintering was studied considering the shrinkage rate, densification, grain size, and phase evolution. It was demonstrated that the synthesis method is effective to prepare nanosized powders of tetragonal zirconia single-phase. Sinterability mainly depended on the agglomeration state of powders and the monoclinic phase content. Fully tetragonal zirconia ceramic, with grain size of 2.4 μm, was obtained after addition of at least 9 mol% ceria and sintering at 1500°C for 4 h.     

A Sonochemical‐Assisted Synthesis of Pure Nanocrystalline Tetragonal Zirconium Dioxide Using Tetramethylethylenediamine

Pure nanocrystalline zirconia has been successfully synthesized by sonochemical‐assisted route from zirconyl nitrate and tetramethylethylenediamine (TMED) as starting materials. Here, TMED for the first time was used as precipitating agent to generate nanocrystalline tetragonal zirconia via a sonochemical‐assisted route. The effect of solvent and concentration of TMED on the morphology and crystallite size was studied. It was found that these parameters have significant influence on the morphology and crystallite size of the products. The products were characterized by SEM, UV–vis absorption and TEM images, XRD patterns, PL, FTIR, and EDS spectroscopy.     

Effects of SO2/redox exposure on the microstructure of cerium–zirconium mixed metal oxides

Cerium–zirconium solid solutions were prepared and characterized to determine the effects of SO₂/redox exposure on the microstructure of the crystallites. The mixed oxides were prepared via co-precipitation of cerium(IV) ammonium hydroxide and zirconium oxynitrate using ammonium hydroxide. The oxides were characterized prior to and after SO₂ exposure to discern the effects of temperature and SO₂ on crystallite properties. The samples were treated at 673 and 1073 K with a flow of 100 ppm SO₂/balance N₂ with a concomitant redox pulse of 5% H₂/balance N₂ and 5% O₂/balance N₂ on 10 s intervals. The cubic crystalline structure (CaF₂) was observed and maintained for compositions ranging from 100 to 25 at.% cerium without indication of a separate tetragonal phase. While the cubic structure was maintained, the addition of zirconium in the cubic lattice reduced the cubic lattice parameter (a₀) and the crystallite grain size. Under identical redox conditions and temperatures, exposure to SO₂ resulted in smaller grain sizes, as calculated by X-ray diffraction (XRD) and confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In addition, higher exposure temperatures resulted in larger crystallite grain sizes without altering the cubic lattice parameter. Auger electron spectroscopy (AES) confirmed the absence of surface sulfur species, indicating sulfur dioxide alters the microstructure of the crystallites under redox conditions without the formation of cerium–zirconium chemisorbed sulfur species.     

A comparative study of (Ce) and (Gd) doping influence on the superconducting properties of YBCO ceramics

This work is devoted to investigate the structural and electrical properties of the Ce, Gd-doped YBCO superconductors bulk ceramics. YBa_2-xRE_xCu₃O_7−δ (x = 0, 0.01, 0.05, 0.1) (RE = Gd, Ce) samples were prepared by means of conventional solid-state reaction. X-ray diffraction analysis was carried out to identify the present phases in the as-prepared samples followed by the determination of their lattice parameters. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify the functional groups. Furthermore, the morphology and the surface roughness of the studied samples were characterized using Scanning Electronic Microscopy (SEM) and Atomic Force Microscopy (AFM). Vickers Micro-hardness of the as-prepared samples was examined. Besides, the electrical resistivity measurements were achieved to determine the critical transition temperature T_C and the critical current density J_C.The effect of Ce and Gd additions is clearly noticed in the obtained results, where all the prepared samples are superconductors with the presence of Y123 as a major polycrystalline phase. From the XRD patterns, the intensities of the Y123 corresponding peaks decrease with further increasing the Ce and Gd contents. In addition, the variation of the cell parameters was significant after additions of both Ce and Gd, which affect the grain size and the oxygen content of the YBa_2-xRE_xCu₃O_7−δ system. An improvement of the structure and surface roughness is observed on SEM and AFM images. Likewise, Vickers micro-hardness has increased after the Ce and Gd additions. Although, the critical transition temperature T_C was not further increased upon Ce or Gd additions compared to the undoped YBCO samples. Nevertheless, an exception has been recorded with an increase of T_C for YBa_2-xRE_xCu₃O_7−δ with (RE = Gd, x=0.01) to reach 88 K. In contrary, an improvement of the deduced critical current density J_C was achieved for all Ce-doped YBCO samples unlike those of Gd-doped samples.