Monoclinic ZrO2 Phase Research Articles

The effect of vanadium content and speciation on the activity of VOx/ZrO2 catalysts in the conversion of ethanol to acetaldehyde

Catalytic conversion of ethanol to acetaldehyde under oxidizing conditions over VOx/ZrO2 catalysts with various surface VOx densities has been investigated. Vanadium complexes anchored on the surface of zirconia oxide support are characterized in detail and their catalytic properties reported. The catalysts were prepared by incipient wetness impregnation from a peroxidic solution of ammonium metavanadate. The texture and crystallinity of the catalysts were investigated by N2 physisorption and XRD analysis. The speciation and redox properties of the vanadium complexes were studied by dispersive Raman spectroscopy and the temperature programmed reduction (TPR) method. The effects of vanadium species distribution and ZrO2 support crystallinity on the catalytic activity in the oxidative dehydrogenation of ethanol to acetaldehyde were analyzed and discussed in a broad context. A comparison of the catalytic results with details of the distribution of vanadium complexes has led to the conclusion that vanadium speciation has a great impact on catalytic behavior and that small oligomeric vanadium complexes dominate in the catalysts with vanadium content slightly above half of monolayer capacity, exhibiting the highest catalytic activity and very high selectivity to acetaldehyde (better than 93%). A comparison of the catalytic behavior of VOx supported on tetragonal and monoclinic ZrO2 phases has not revealed any differences in the activity or selectivity of the ethanol ODH reaction. Long-term catalytic experiments have shown good stability of the catalysts (stable conversion and selectivity for more than 600 h).

The peculiarities of light absorption and light emission in Cu-doped Y-stabilized ZrO2 nanopowders

The influence of doping with copper and annealing temperature on the characteristics of yttrium-stabilized ZrO2 nanopowders was investigated using photo- and cathodoluminescence as well as diffuse reflectance and electron paramagnetic resonance methods. After annealing at temperatures below 800 °С, both Cu-doped and Cu-free powders demonstrate light emission in the blue–red spectral range that are characteristic of the tetragonal ZrO2 phase. In the excitation spectra of blue–orange photoluminescence it was found a maximum in the range of light absorption by oxygen vacancies, indicating that the corresponding emitting centers are vacancy related complexes. A red luminescence band was found to be excited only by the light from band-to-band ZrO2 absorption. It was observed that the increase of annealing temperature up to 1000 °C stimulates the appearance of additional luminescence bands at ~ 510 and 580 nm correlated with the formation of monoclinic ZrO2 phase. The corresponding emitting center is considered to be a substitutional copper ion, the excitation of which can occur both via the capture of free carriers and via the energy transfer from oxygen vacancies.

The role of carbon nanotubes on the stability of tetragonal zirconia polycrystals

The effect of single walled carbon nanotubes (SWNT) at zirconia grain boundaries on the stability of a tetragonal zirconia polycrystalline matrix has been explored in as–sintered composites and after low–temperature hydrothermal degradation (LTD) experiments. For this purpose, highly–dense 3 mol% Y2O3–doped tetragonal zirconia polycrystalline (3YTZP) ceramics and SWNT/3YTZP composites were prepared by spark plasma sintering (SPS). Quantitative X–ray diffraction analysis and microstructural observations point out that an increasing amount of well–dispersed SWNT bundles surrounding zirconia grains decreases the metastable tetragonal phase retention in the ceramic matrix after sintering. In contrast, the tetragonal ceramic grains in composites with SWNTs are less sensitive to the presence of water, i.e. to undergo a martensitic transformation under LTD conditions, than monolithic 3YTZP ceramics. The SWNT incorporation diminishes micro–cracking due to tetragonal to monoclinic ZrO2 phase transformation in the composites.

Hot corrosion behavior of plasma sprayed powder-solution precursor hybrid thermal barrier coatings

In recent times, plasma sprayed powder-solution precursor hybrid composite thermal barrier coatings have been developed to harness the dual benefits of both conventional atmospheric plasma spraying (APS) and solution precursor plasma spraying (SPPS) processes. In this study, hot corrosion behavior of plasma sprayed powder-solution precursor composite (PSP-SPC) YSZ TBCs in molten salt mixtures of 90 wt.% Na2SO4 + 5 wt.% V2O5 + 5 wt.% NaCl and 50 wt.% Na2SO4 + 50 wt.% V2O5 at 900 °C was investigated. The employed coating showed a bimodal microstructure comprising coarse splats derived from the powder feedstock as in the APS process and fine splats resulting from the solution precursor as typical of SPPS process. The PSP-SPC coatings showed a significantly higher resistance to spallation than APS, SPPS and EB-PVD coatings in both the salt environments. These coatings showed shorter life in vanadate environment compared to that of the chloride environments as the former promotes the formation of monoclinic ZrO2 and YVO4 phases more than the latter.

A comparison study of the Born effective charges and dielectric properties of the cubic, tetragonal, monoclinic, ortho-I, ortho-II and ortho-III phases of zirconia

Using density-functional perturbation theory, we systematically investigate the Born effective charges and dielectric properties of cubic, tetragonal, monoclinic, ortho-I (Pbca), ortho-II (Pnma) and ortho-III (Pca21) phases of ZrO2. The magnitudes of the Born effective charges of the Zr and oxygen atoms are greater than their nominal ionic valences (+4 for Zr and −2 for oxygen), indicating a strong dynamic charge transfer from Zr atoms to O atoms and a mixed covalent-ionic bonding in six phases of ZrO2. For all six phases of ZrO2, the electronic contributions εij∞ to the static dielectric constant are rather small (range from 5 to 6.5) and neither strongly anisotropic nor strongly dependent on the structural phase, while the ionic contributions εijion to the static dielectric constant are large and not only anisotropic but also dependent on the structural phase. The average dielectric constant ε¯0 of the six ZrO2 phases decreases in the sequence of tetragonal, cubic, ortho-II (Pnma), ortho-I (Pbca), ortho-III (Pca21) and monoclinic. So among six phases of ZrO2, the tetragonal and cubic phases are two suitable phases to replace SiO2 as the gate dielectric material in modern integrated-circuit technology. Furthermore, for the tetragonal ZrO2 the best orientation is [100].

Cubic phase stability, optical and magnetic properties of Cu-stabilized zirconia nanocrystals

By means of experimental and ab initio investigations, we report on the cubic phase stability of Cu doped zirconia (ZrO2) at room temperature, and further characterize its structural, optical and magnetic properties. Various compositions of Zr1−xCuxO2 (0.01 ⩽ x ⩽ 0.25) nanocrystallites of average size ∼16 nm were synthesized using co-precipitation technique. Thermal analysis and kinetics of crystallization revealed that the cubic phase at ambient temperature can be stabilized by using a critical calcination temperature of 500 °C for 8 h in air and a critical composition of . For x < xc, some undigested monoclinic phase of ZrO2 exists together with the cubic structure. However, for x > xc, the monoclinic CuO emerges as a secondary phase with shrinkage of unit-cell volume with increasing the Cu content. At x = 0.05 and 500 °C calcination temperature, we observe a high degree of cubic crystallinity which breaks down into monoclinic phase with increasing calcination temperature beyond 550 °C. Electron magnetic resonance studies provide evidence for the substitution of Cu2+ (2D5/9,3d9) ions at Zr4+ sites with g, g and average ga = ( + 2)/3 ∼ 2.1. The temperature dependence of magnetic susceptibility measurements from 2 K to 300 K exhibits Curie–Weiss behaviour whose analysis using ga = 2.1 and spin S = 1/2 yields x = 0.028 and x = 0.068 for the nominal x = 0.05 and x = 0.20 samples, respectively. This magnetic analysis confirms the findings from x-ray diffraction that only a part of Cu is successfully doped into cubic phase of Cu-doped ZrO2. The optical bandgap decreases with increasing x, which is due to the emergence of Cu-d states at Fermi-level near the valence bands, thus making Cu-doped zirconia a hole doped (p-type) semiconductor.

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (∼4.0 mol% Y2O3–ZrO2, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200 °C. Here we report an excellent zirconia system co–doped with 16 mol% CeO2 and 4 mol% Gd2O3 (16Ce–4Gd) presenting nontransformable feature up to 1500 °C, in which no detrimental monoclinic (m) ZrO2 phase formed on partitioning. It also exhibits a high fracture toughness of ∼46 J m−2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500 °C.

White and yellow light emission from ZrO2:Dy3+ nanocrystals synthesized by a facile chemical technique

White and yellow light were obtained from ZrO2:Dy3+ nanocrystals which were synthesized by the solvent evaporation technique. The crystalline structure was studied by X-ray diffraction, resulting in a tetragonal and monoclinic mixture phases of ZrO2 when the powders were annealed at 600 °C and the zirconia monoclinic phase when they were thermal treated at 1000 °C. By means of atomic force microscopy images was observed that the synthesized powders are constituted by nanocrystals about 20 nm for the samples annealed at 600 °C, whereas samples annealed at 1000 °C were constituted by crystals about 135 nm, these features were confirmed by TEM images. Luminescence properties were analyzed by means of photo and cathodoluminescence; exhibiting emissions in the white light region of the chromatic diagram. In the case of photoluminescence white-warm color (x = 0.35, y = 0.37) was observed, which is close to pure white color; while for cathodoluminescence the emission was yellowish with coordinates (x = 0.39, y = 0.39) in the chromatic diagram. PL decay time measurements were carried out; a lifetime of 0.66 ms was found. In addition, the PL quantum efficiency was measured; the obtained value was as high as 45%.

Comparison of the oxidation behavior of a zirconium nitride coating in water vapor and air at high temperature

The oxidation behavior of zirconium nitride coating in high-temperature water vapor and air environments was studied. The parabolic rate constant of ZrN oxidizing in the water vapor environment at 600 °C was approximately 100 times faster than that in air, due to the larger pores and greater number of cracks that were formed across ZrO2 oxide layer formed during the water vapor oxidation process than during the air oxidation process. A bilayer-structure ZrO2 with tetragonal ZrO2 near the ZrN/ZrO2 interface and monoclinic ZrO2 approaching the outer ZrO2 surface were formed in both cases. The lateral cracks across the ZrO2 scale were caused by volume expansion from the tetragonal ZrO2 phase to the monoclinic ZrO2 phase transition.

Ag-(Mo-W)/ZrO2 catalysts for the production of propylene oxide: Effect of pH in the preparation of ZrO2 support

Direct oxidation of propylene to propylene oxide was carried out over Ag-(Mo-W)/ZrO2 (pH X) catalysts prepared by changing the pH of Zr precursor solution from 3 to 14 while preparing the ZrO2 support. It was found that the fraction of monoclinic phase and the ratio of acidity to basicity in the catalysts showed a volcano-shaped trend with respect to pH of Zr precursor solution. Such characteristics were well correlated with the selectivity of reaction towards propylene oxide. Ag-(Mo-W)/ZrO2 (pH 10) with the highest fraction of monoclinic phase of ZrO2 and ratio of surface acidity to basicity showed the highest selectivity (68%) for propylene oxide.

Improving Corrosion Resistance of 316L Austenitic Stainless Steel Using ZrO2 Sol-Gel Coating in Nitric Acid Solution

In this study, thin-film coating of zirconium oxide (ZrO2) was prepared by sol-gel method and subsequent heat treatment process. The sol was prepared by controlled hydrolysis of zirconium tetrapropoxide using acetic acid and ethanol/acetylacetone mixture as catalyst and chelating agent, respectively, and finally deposited onto the 316L austenitic stainless steel (316L SS) using dip coating method in order to improve its corrosion resistance in nitric acid medium. The composition, structure, and morphology of the coated surface were investigated by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The obtained results from XRD and FTIR state the formation of tetragonal and monoclinic ZrO2 phase. Also, the obtained results from surface morphology investigation by SEM and AFM indicate the formation of smooth, homogeneous and uniform coatings on the steel substrate. Then, the corrosion behavior of stainless steel was investigated in a 1 and 10 M nitric acid solutions using electrochemical impedance spectroscopy and linear polarization test. The obtained results from these tests for ZrO2-coated specimens indicated a considerable improvement in the corrosion resistance of 316L stainless steel by an increase in corrosion potential and transpassive potential, and a decrease in passive current density and corrosion current density. The decrease in passive current density in both the concentration of solutions was two orders of magnitude from bare to coated specimens.

Structural and photoluminescent properties of ZrO2:Tb3+ coatings formed by plasma electrolytic oxidation

Tb3+ doped ZrO2 coatings were formed on zirconium substrate by plasma electrolytic oxidation (PEO) in electrolyte containing Tb4O7 powder. The evolution morphology, chemical composition, and crystalline structure of formed coatings as a function of PEO time are probed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The concentration of Tb incorporated into coatings increases with the time of PEO processing. Obtained coatings are crystallized and composed of monoclinic and tetragonal phases of ZrO2. The ratio of monoclinic and tetragonal phases, as well as crystallite size, is controlled by PEO time and concentration of Tb in coatings. Photoluminescence (PL) emission spectra of Tb doped ZrO2 coatings feature two distinct regions. The first region is related to ZrO2 PL band with a maximum positioned at about 490nm, while the second region features several sharp emission bands which can be attributed to f-f transitions of Tb3+ from excited level 5D4 to lower levels 7FJ (J = 2,3,4,5, and 6). PL excitation spectra of Tb doped ZrO2coatings are characterized by broad band region from 250nm to 350nm with a maximum at around 280nm originating from 4f8 → 4f75d1 transition of Tb3+ ions. The evolution of PL emission spectra shows that with increasing PEO time, i.e. increasing concentration of Tb in coatings, sharp emission bands of Tb3+ increase, while PL intensity of broad emission band of ZrO2 host decreases. These results indicate the existence of energy transfer from ZrO2 host to Tb3+ dopant.

Corrosion kinetics and mechanisms of ZrC1-x ceramics in high temperature water vapor.

The corrosion kinetics and mechanisms of ZrC1−x ceramics in water vapor between 800 and 1200 °C were investigated. The results showed that there was only cubic ZrO2 phase in the corrosion layer when corroded at 800 °C, while a scale layer consisted of a mixture of cubic and monoclinic ZrO2 phases when corroded at 1000 °C and 1200 °C. A series of crystallographic relationships at the ZrC/c-ZrO2 interface were detected. The c-ZrO2 formed near the interface retained some crystallographic orientations of the initial ZrC before corrosion, presenting an “inheritance in microstructure” between c-ZrO2 and ZrC. The corrosion behavior mainly followed a parabolic relationship. The incremental rate of weight gain increased with increased corrosion temperature and decreased C/Zr ratio and the carbon vacancy was passive to the decrease of corrosion rate. The main corrosion controlling mechanism changed from phase boundary reactions to surface diffusion and then to grain boundary diffusion with increased temperature.

Effect of Cooling Rate on Dopant Spatial Localization and Phase Transformation in Cu‐Doped Y‐Stabilized ZrO2 Nanopowders

The effect of calcination temperature (TC = 500–1000 °C) and cooling rate on the dopant distribution in Cu‐doped Y‐stabilized ZrO2 nanopowders is studied. The powders are produced by co‐precipitation technique and investigated by attenuated total reflection, UV‐vis diffuse reflectance, electron paramagnetic resonance, and transmission electron microscopy methods. The cooling rate is found to affect the amount of Cu substances on grain surface, the powders subjected to fast cooling (quenching) showed higher amount of Cu‐related complexes on the grains’ surface than their counterparts cooled with furnace after calcination. It is observed that Cu impurities diffuse inside ZrO2 grains from Cu‐related surface substances when TC &lt; 800 °C. This process is accompanied by the enhancement of 275‐nm absorption band caused by oxygen vacancies formation. For TC &gt; 800 °C, outward migration of Cu dopants takes place. Simultaneously, the intensity of 275‐nm absorption band decreases, the monoclinic ZrO2 phase forms and its contribution rises with TC. It is proposed that monoclinic phase formation is caused by the replacement of Cu atoms from lattice sites to interstitials leading to an appearance of the channels for Y out‐diffusion via cation vacancies and destabilization of ZrO2 tetragonal phase.

Structural Infrared Analysis of Ultra Nanosized Crystals of Yttria‐Stabilized Zirconia

AbstractThis paper describes the characterization of ultra nanosized crystals as monoclinic, tetragonal, cubic or mixed phases of ZrO2. Nanopowders are produced using the microwave method, which allows control of the ratio of zirconium to yttrium to yield various crystallographic structures. The synthesized ultra nanosized particles have very broad XRD peaks. In particular, the similarity of the XRD peaks of tetragonal and cubic nanocrystals makes it difficult to distinguish between these two crystals. In this paper, tetragonal and cubic nanocrystals were clearly distinguished using infrared spectroscopy (IR). An infrared spectroscopic method to quantitatively analyse the percentage distribution between the two crystal phases was proposed. The results showed that pure zirconium dioxide was composed of 60 % monoclinic and 40 % tetragonal phases, while 8 % yttria‐stabilized zirconia (YSZ) contained 50 % tetragonal and 50 % cubic phases, and 14 % YSZ contained 100 % cubic phase.

Fabrication and characterization of microarc oxidation films on Ti-39Nb-6Zr alloy at different voltages in KOH electrolyte

The TiO2/Nb2O5/ZrO2 ceramic films were successfully prepared on Ti-39Nb-6Zr alloy by microarc oxidation (MAO) at different voltages in KOH electrolyte. Their morphology, phase structure and composition were characterized by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Their conducting properties were analyzed by impedance spectroscopy and Mott–Schottky test. Meanwhile, their corrosion behaviors were evaluated by electrochemical impedance spectroscopy (EIS). The MAO films as n-type materials were 8 μm–15.8 μm thick with fine micropores on the surface. These films consisted of anatase TiO2, rutile TiO2, Nb2O5, monoclinic ZrO2 (m-ZrO2) and ZrTiO4 phases. Their conductivity gradually increased with the frequency from 4.07 × 10−15 S cm−1 at 0.01 Hz to 1.61 × 10−6 S cm−1 at 107 Hz. Their flat band potential was in the range of 0.34 V–0.77 V, and doping density varied between 3.50 × 1017 cm−3 and 5.25 × 1018 cm−3 which decreased with increasing positive voltage of MAO process. The Ti-39Nb-6Zr alloy coated at +400 V/−60 V had good corrosion resistance with lower corrosion current density and higher impedance.

Phase stability and thermal conductivity of nanostructured tetragonal yttria–stabilized zirconia thermal barrier coatings deposited by air–plasma spraying

High performance thermal barrier coatings (TBCs) produced from nanocrystalline 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder show significant thermal and mechanical property advantages compared with current benchmark products. The objective of the present study was to investigate the phase stability and thermal conductivity of the nanostructured 4YSZ coatings plasma–sprayed from the sol–spray pyrolysis powder. Characterization results indicated that the as–sprayed 4YSZ coatings were chemically very homogeneous and had a completely tetragonal–prime (t′) structure. Such a uniform composition was almost entirely consistent with that of the feedstock powder. The grain size of as–sprayed 4YSZ coatings was in the range of 35 ~ 75 nm, which resulted in an extremely low thermal conductivity of 0.8–1.2 W m–1 K–1 in the coatings. The high–temperature aging test showed that no monoclinic ZrO2 (m–ZrO2) phase present at all of the 4YSZ coatings after a heat treatment at 1473 K for 1–650 h. Furthermore, the phase composition of the 4YSZ coating kept t′–ZrO2 structure after 1500 thermal cycles at a peak surface temperature of 1373 K. The relative phase stability was attributed to the highly pure t′ phase composition and uniform constituents in the 4YSZ coatings. These results suggested that the development of high performance 4YSZ ceramic powders was benefit to improve the coating properties for TBC application.

Synthesis of mesoporous sulfated zirconia nanoparticles with high surface area and their applies for biodiesel production as effective catalysts

Sulfated zirconia (SZ) was employed as solid acid catalyst for biodiesel production via the transesterification of soybean oil with methanol. The effects of synthesis method, template/sulfonating agent and calcination temperature were investigated. Compared to conventional two-step route, one-step liquid-crystal template (LCT) method achieved the successful synthesis of mesoporous SZ nanoparticles (MNSZ) with larger surface area, favored the more acid sites, stronger acidity and smaller tetragonal ZrO2 (t-ZrO2). Sodium dodecyl sulfate (SDS), both as template/sulfonating agent, was superior to Hexadecylsulfonic acid sodium salt (CSS) in retarding the grain growth of t-ZrO2, formation of monoclinic ZrO2 phase and loss of sulfated groups. High calcination temperature resulted in the serious loss of acid sites, decrease of surface area and increase in crystal size of t-ZrO2, accelerated the transformation of active t-ZrO2 into monoclinic ZrO2, thus leaded to severe decline in catalytic activity. MNSZ, prepared via one-step LCT method at 550°C using SDS as template/sulfonating agent, was found to be the best catalyst. Over this catalyst of 4wt%, approximately 100% of soybean oil conversion and FAME yield of 94.9% could be achieved at 120°C in 4h, using the feedstock with methanol/soybean oil molar ratio of 12:1.

Highly increased capacitance and thermal stability of anodic oxide films on oxygen-incorporated Zr-Ti alloy

Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm−3 ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO2 phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m−2, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO2 phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.

Synthesis and Crystallization Behavior of 3 mol% Yttria Partically Stabilized Zirconia (3Y-PSZ) Nanopowders by Microwave Pyrolysis Process

A crystalline Nano powders of 3 mol% yttria-partially stabilized (3Y-PSZ) has been synthesized using ZrOCl2 and Y (NO3)3 as raw materials by microwave pyrolysis with a TE666 resonant mode at 700-900°C. The frequency of the microwave was 2.45 GHz with the maximum power of 10 KW, and a hybrid heating structure was used with insulation of porous mullite and SiC aided heaters. For comparison, conventional heating was performed in air at 750°C for 20 min. The as-synthesized products were characterized by SEM and TEM images, XRD patterns. It was found that microwave energy promotes the conversion of tetragonal ZrO2 (t-ZrO2) to monoclinic ZrO2 (m-ZrO2) phase compared with conventional pyrolysis. TEM images showed that highly dispersed 3Y-ZrO2 powders with ~23 nm in size were obtained by microwave pyrolysis at 750°C for 20 min.