Influence of preparation method and alumina content on crystallization and morphology of porous yttria stabilized zirconia

Influence of addition of alumina and preparation methods (sol-gel synthesis and mechanochemical preparation) on crystallization and morphology of yttria stabilised zirconia was examined. Presence of alumina was found to delay crystallization of zirconia, the effect being more pronounced at higher alumina content. The two oxides form easily distinguished separate phases. Milling lowers the crystallization temperatures of the sol-gel derived powders since nuclei are formed during the milling and smaller particle size allows easier removal of residual organic components. The milling results in crystallization of some monoclinic zirconia, both in sol-gel derived powders and in case of mechanochemical processing. There are no significant differences between the preparation methods in pore size and relative density of sintered tablets: powders obtained by mechanochemical processing and milled sol-gel derived powders both give tablets with homogeneous morphology. The advantage of sol-gel process is preparation of pure tetragonal zirconia phase without traces of monoclinic phase.     

Influence of Tetragonality on Tetragonal-to-Monoclinic Phase Transformation during Hydrothermal Aging in Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The influence of tetragonality, which is defined as the lattice parameter ratio c / a , on the tetragonal-to-monoclinic phase transformation during hydrothermal aging was investigated in yttria-stabilized zirconia coatings. The yttria content was adjusted in the range of 4–8 mass% (denoted as x YZ, where x = 4–8 and YZ represents the yttria-stabilized zirconia). The tetragonality of the zirconia in the as-sprayed coatings was less than that in the powders. To change the tetragonality for each yttria content, the coatings were annealed at 1273 K before aging. Without annealing, the phase transformation was prevented only in 8YZ. When annealing was applied, an increase of the tetragonality (i.e., recovery of the tetragonality) was observed, and transformation during hydrothermal aging was also suppressed in 6YZ. It was concluded that the increase in tetragonality that occurred without a change in the yttria content was suggested to be caused by the lattice relaxation of the tetragonal phase, and this relaxation is believed to cause a reduction of the critical yttria concentration, thus preventing the phase transformation.     

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.     

Grain boundary evolution on sintering in yttria (8 mol%)-stabilized zirconia assisted by one or two driving forces

Sintering of yttria (8 mol%)–zirconia ceramics assisted by one or two driving forces was performed using nanosized precursors hydrothermally synthesized.Two monophasic precursors, both containing 8 mol% of yttria, were hydrothermally synthesized from amorphous zirconia–yttria co-precipitate and from a mechanical mixture of amorphous zirconia with crystalline yttria, respectively.Two diphasic precursors, characterized by the same yttria content (8 mol%), were obtained mixing in an equal molar ratio two hydrothermally synthesized powders with yttria content of 2 and 14 mol%, respectively.By applying a same pressureless heating schedule, the different precursors, previously pressed by cold isostatic pressing, were sintered and characterized.The diphasic precursors assisted on sintering by the double driving force, the surface area and the composition gradient, show an exaggerated formation of the grain boundary interfaces in comparison to that of the monophasic precursors sintered in the presence of the single driving force, the surface area.     

Low temperature degradation resistant nanostructured yttria-stabilized zirconia for dental applications

When used in prosthetic dentistry, zirconia encounters severe durability issues due to low temperature degradation: exposure to humidity results in a transition from tetragonal to monoclinic phase, associated to disruptive integrity loss. Recently it has been shown that size-induced stabilization helps maintaining zirconia in tetragonal form, when the grain size is reduced to the nano-range. Objective of this work is to demonstrate the applicability of High Pressure Field Assisted Sintering (HP-FAST) to the preparation of dense, nanostructured samples of tetragonal yttria stabilized zirconia, with yttria content between 0.5 and 3 mol% and showing resistance to low temperature degradation. The yttria stabilized zirconia nanopowders were prepared by a hydrothermal method. Sintering by HP-FAST was performed at 900 °C in 5 min, under a pressure of 620 MPa. Resistance to low temperature degradation was tested at 134 °C, under vapor pressure, for up to 40 h. Both pristine and aged samples were characterized by X-ray diffraction, high-resolution scanning electron microscopy and nanoindentation tests in continuous stiffness measurement mode. The sintered samples presented a grain size between 20 and 30 nm and low or null monoclinic content. Both parameters resulted unaffected by ageing. The best results in terms of phase composition and mechanical properties have been obtained with the material containing 1.5 mol% of yttria. These results induce to reconsider the use of yttria stabilized zirconia as material for dental prosthetic systems requiring long-term durability.     

Kinetics and extent of low-temperature crystallization of ethylene–vinyl acetate copolymers

The enthalpies of fusion of two types of EVA copolymers containing 9 and 16% of vinyl acetate, respectively, were investigated by DSC. After melting, the samples were cooled down and held at 10, 15, 20, and 25°C for different periods of time from 15 min to 2.5 months (only at 20°C). The enthalpy of fusion increased over the 2.5-month period for 9.3 and 11.3 J/g, respectively. There was a new small melting peak on the endotherm of the aged sample whose position and size depended on aging temperature and aging time. During 2.5 months, the peak shifted toward higher temperature for 8°C. The enthalpy of fusion and corresponding degree of crystallinity changed linearly with the logarithm of time, as is the case in high-temperature annealing or secondary crystallization at high temperatures. The rate and the extent of low-temperature crystallization of ethylene copolymers depend on the comonomer content, sequence length distribution, and temperature. © 1996 John Wiley & Sons, Inc.     

聚乙二醇／聚丙烯酰胺共混物的制备及性能研究

通过微波加热法制备聚丙烯酰胺,将其与聚乙二醇共混制备聚乙二醇／聚丙烯酰胺（PEG,PAM）共混物,采用傅立叶变换红外线光谱（FTIR）,差示扫描量热仪（DSC）,X射线粉末衍射仪（XRD）,熔点偏光显微镜等测定共混物的性能。结果显示共混物中PEG与PAM形成氢键,共混物的升降温过程中具有吸热和放热峰,相变温度和相变焓随共混物中PAM含量的增加,逐渐降低。PAM的存在对PEG的结晶过程造成了破坏,共混使PEG难以形成均匀球晶,结晶粒子变大。     

Polymorphic Mixes of Zirconia from a Borate Glass Melt

Yttria-stabilized zirconia crystals were produced by devitrification within a sodium borate glass containing zirconia and yttria. Yttria (3, 6, or 10 mol%) in zirconia was added as part of the starting material of the glass batch. Phase separation and crystallization occurred during the cooling and subsequent heat treatment of the glasses. The zirconia powders obtained after leaching away the sodium borate had surface areas between 16 and 48 m²/g. X-ray diffraction traces revealed a mixture of monoclinic, tetragonal, and cubic phases, the phase ratios of which depended on the composition of the glass.     

Investigation of Annealing Induced Yttria Segregation in Sputtered Yttria‐Stabilized Zirconia Thin Films

8 mol% yttria‐stabilized zirconia (8YSZ) is an extensively studied solid electrolyte. But there is no consistency in the reported ionic conductivity values of 8YSZ thin films. Interfacial segregation in YSZ thin films can affect its ionic conductivity by locally altering the surface chemistry. This article presents the effects of annealing temperature and film thickness on free surface yttria segregation behavior in 8YSZ thin film by Angle Resolved XPS and its influence on the ionic conductivity of sputtered 8YSZ thin films. Surface yttria concentration of about 32, 20, and 9 mol% have been found in 40 nm 8YSZ films annealed at 1273, 1173, and 1073 K, respectively. Yttria segregation is found to increase with increase in annealing temperature and film thickness. Ionic conductivities of 0.23, 0.16, and 0.08 Scm^?1 are observed at 923 K for 40 nm 8YSZ films annealed at 1073, 1173, and 1273 K, respectively. The decrease in conductivity with increase in annealing temperature is attributed to the increased yttria segregation with annealing. Neither segregation nor film thickness is found to affect the activation energy of oxygen ion conduction. Target purity is found to play a key role in determining free surface yttria segregation in 8YSZ thin films.     

Crystalline and thermal behavior of poly(ethylene terephthalate)/polyphenoxy blends

Poly(ethylene terephthalate) (PET)/polyphenoxy blends were prepared by melt blending. Crystalline and thermal behaviors of PET/polyphenoxy blends were verified by use of DSC. The experiment results show that the initial temperature, peak temperature, and ending temperature of cold crystallization increase with increasing phenoxy content. On the contrary, the onset melting temperature, finishing melting temperature, and peak temperature in the first heating and the secondary heating processes decrease with increasing phenoxy content. The crystallization enthalpy and melting enthalpy, as well as the crystallization rate, decrease with increasing phenoxy content. Avrami exponents of the blends are slightly higher than that of pure PET and almost independent of phenoxy content. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 878–885, 2005     

Calorimetric study of nanocomposites of multiwalled carbon nanotubes and isotactic polypropylene polymer

Modulated differential scanning calorimetry (MDSC) was used to measure the complex specific heat of the crystallization and melting transitions of nanocomposites of isotactic polypropylene (iPP) and carbon nanotubes (CNT) as function of CNT weight percent and temperature scan rate. In the last few years, great attention has been paid to the preparation of iPP/CNT nanocomposites due to their unique thermal and structural properties and potential applications. As the CNT content increases from 0 to 1 wt %, heterogeneous crystal nucleation scales with the CNT surface area. Above 1 wt %, nucleation appears to saturate with the crystallization temperature, reaching ～8 K above that of the neat polymer. Heating scans reveal a complex, two‐step, melting process with a small specific heat peak, first observed ～8 K below a much larger peak for the neat iPP. For iPP/CNT samples, these two features rapidly shift to higher temperatures with increasing ?_w and then plateau at ～3 K above that in neat iPP for ?_w ≥ 1 wt %. Scan rates affect dramatically differently the neat iPP and its nanocomposites. Transition temperatures shift nonlinearly, while the total transition enthalpy diverges between cooling and heating cycles with decreasing scan rates. These results are interpreted as the CNTs acting as nucleation sites for iPP crystal formation, randomly pinning a crystal structure different than in the neat iPP and indicating complex transition dynamics. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Chemical synthesis and structural characterization of nanocrystalline powders of pure zirconia and yttria stabilized zirconia (YSZ)

Nanocrystals of pure zirconia and yttria stabilized zirconia (YSZ) are obtained by a simple chemical synthesis route using sucrose, polyvinyl alcohol (PVA) and metal nitrates. The reaction mixture on pyrolysis and calcination gives nanocrystals. These are characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The size of the nanocrystallites for pure zirconia is in the range of about 7.0–45.0 nm and for yttria stabilized zirconia, is in the range of about 5.0–24.0 nm at 200°C and above, according to the preparative condition. At 200°C, pure zirconia forms cubic phase and this cubic phase is stable up to 600°C and then slowly transformed into monoclinic form. For yttria stabilized zirconia, the crystals are tetragonal in the temperature range from 200 to 1200°C.     

Low-Temperature Sintering and Mechanical Property Evaluation of Nanocrystalline 8 mol% Yttria Fully Stabilized Zirconia

Fully stabilized zirconia containing 8 mol% yttria (8Y-FSZ) in nanocrystalline form has been synthesized by the coprecipitation method. The formation of an easily filterable hydroxide is facilitated by the addition of ammonium sulfate and polyethylene glycol during precipitation. The precipitate is then calcined to produce nanocrystalline powder. Using this powder, it has been possible to obtain a sintering density of more than 95% at a temperature as low as 1150°C by following a conventional sintering schedule. Adoption of a two-stage sintering schedule, in which heat treatment of the powder compact has been carried out initially to a high temperature, followed by a long holding at a lower temperature, resulted in further lowering of the sintering temperature. Hardness and toughness values have been found to be dependent on the microstructure in low-temperature-sintered samples.     

齐聚物对PET结晶性能的影响

运用差示扫描量热法研究了不同齐聚物含量的聚对苯二甲酸乙二酯(PET)的非等温结晶过程。结果表明:随齐聚物含量的增加,PET的熔融结晶温度向低温偏移,过冷度增大,结晶热焓增加,结晶度先增大后减小;在相同的结晶温度下,随着齐聚物含量的增加,PET的结晶温度区域逐渐变宽,半结晶周期增大,结晶速率降低。     

The activity and selectivity of oxygen atoms adsorbed on a Ag/α-Al2O3 catalyst in ethene epoxidation

The bonding of the oxygen species held on a Ag/-Al₂O₃ catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/-Al₂O₃ catalyst was produced by the thermal decomposition of a Ag oxalate/-Al₂O₃ precursor. Oxygen desorbs from this Ag/-Al₂O₃ catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol^–1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol^–1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/-Al₂O₃ catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO₂ evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO₂ shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.     

Kinetics of nucleation and crystallization in poly(butylene succinate) nanocomposites

Differential fast scanning calorimetry (DFSC) was employed on poly(butylene succinate) nanocomposites containing silver nanoparticles and multi-walled carbon nanotubes (MWCNT), in order to identify the temperature range of heterogeneous nucleation caused by both nanofillers. The fast scanning rates also allow investigating self-nucleation by recrystallization experiments approaching the crystallization temperature from low temperatures. The recrystallization behavior of PBSu and its nanocomposites is distinct from all other polymers studied so far as only the previously crystallized part of the material is able to recrystallize, independently on the available large number of nuclei. Since full melting of small crystals at low temperatures is observed this highlights the importance of ordered structures remaining in the polymer melt. On cooling from the melt the neat polymer did not crystallize at rates higher than 70 K/s, while the nanocomposites needed rates of 500 K/s and 300 K/s for silver and MWCNT, respectively. Below 280 K the crystallization kinetics of the matrix was almost the same with the nanocomposite samples. The nucleation mechanism changes at 280 K from heterogeneous to homogeneous. The study further confirms that below the glass transition nucleation and crystallization appears only after approaching the enthalpy value of the extrapolated supercooled liquid by enthalpy relaxation.     

莫来石对Y-TZP陶瓷摩擦磨损性能的影响

用环-块摩擦磨损试验机在室温下研究了莫来石弥散钇稳定四方氧化锆多晶陶瓷(mullite dispersed yttria stabillized tetragonal zirconia polycrystal,MDZ)与高铬铸铁(high chromium cast iron,HCCI)摩擦副在2％(质量分数)SiO2磨粒的5％NaOH溶液中的摩擦磨损性能。结果表明：载荷在100-500N范围内,含15％(质量分数,下同)莫来石的15 MDZ复合陶瓷的耐磨性明显优于20 MDZ(含有20％莫来石)。载荷500N下,虽然3Y-TZP(yttria stabillized tetragonal zirconia polycrystal,含3％摩尔分数氧化钇)中弥散15％莫来石后力学性能下降,但耐磨性提高。柱状莫来石对耐磨性的主要贡献是：具有承载作用;阻碍裂纹扩展;折断的柱状莫来石的“滚针轴承”作用减轻磨擦副之间的摩擦磨损。MDZ复合陶瓷的主要磨损机制为微观犁削和柱状莫来石晶粒脱落。     

Phase Evolution of SiO2–Al2O3–ZnO–CaO–ZrO2–TiO2‐Based Glass with Added Y‐PSZ Particles

Yttria partially stabilized zirconia Y‐PSZ/glass‐ceramic composites were prepared by reaction sintering using powder mixtures of a SiO₂–Al₂O₃–ZnO–CaO–ZrO₂–TiO₂‐based glass and yttria partially stabilized zirconia (Y‐PSZ). The glass crystallized during sintering at temperatures of 1173, 1273, and 1373 K to give a glass‐ceramic matrix for high‐temperature protecting coatings. With the increasing firing time, the added zirconia reacted with the base glass and a glass‐ceramic material with dispersed zircon particles was prepared in situ. Furthermore, the added zirconia changed the crystallization behavior of the base glass, affecting the shape, amount, and distribution of zircon in the microstructure. The bipyramid‐like zircon grains with imbedded residual zirconia particles turned out to have two growth mechanisms: the inward growth and the outward growth, and its rapid growth was mainly dominated by the later one. For comparison, the referenced glass‐ceramic was prepared by sintering using exclusive glass granules and its crystallization behavior at 1173–1373 K was examined as well. Scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) were used to characterize the crystallization behavior of the base glass and the phase evolution of the Y‐PSZ/glass‐ceramic composites.     

Non-isothermal crystallization kinetics of reactive microgel/nylon 6 blends☆

The non-isothermal crystal ization kinetics of reactive microgel/nylon 6 blends was investigated by differential scanning calorimetry (DSC). The Mo equation was employed to analyze the non-isothermal crystallization data. The crystallization activation energies were also evaluated by the Kissinger method. The results show that the crystallization onset temperature (Tonset) and crystallization peak temperature (Tp) decrease with the increase of the content of reactive microgel, whileΔT (Tonset–Tp), the crystallization half-time (t1/2) and the crystal ization enthalpy (ΔHc) increase. The required cooling rates of blends are higher than that of neat nylon 6 in order to achieve the same relative crystallinity in a unit of time. The crystallization activation energies of the reactive microgel/nylon 6 blends are greater than those of the neat nylon 6. When the content of reactive microgel is 30%, the relative crystallinity (Xt) reaches the maximum.     

Direct synthesis of copper faujasite

Copper containing faujasite has been successfully prepared for the first time using a direct synthesis method. Faujasite type zeolite can be prepared in the presence of copper species by tuning the synthesis conditions. Ammonium hydroxide was used to form a copper complex that was later mixed with the reacting gel. Sodium is required to obtain copper faujasite. The complete elimination of sodium ions from the starting gel produces amorphous material. Crystallization took place at 358 K for 11 days. Crystallization temperature of 373 K produces ANA type zeolite as an impurity. Increasing by two times the amount of copper complex added to the reacting gel increases the crystallization time of Cu-FAU from 11 to 20 days (the crystallization rate decreases). The copper containing faujasite obtained was characterized by XRD, FESEM, EDX, EPR, FT-IR, TPR, and BET. According to the XRD pattern only FAU type zeolite was obtained. According to TPR experiments, the reduction temperature for Cu²⁺ ions present in Cu-FAU prepared by direct synthesis was 70 K more than for Cu-FAU prepared by ion-exchange. This difference can be due to the different location of the copper ions in the supercages or in the sodalite cages of the faujasite.