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Materials of the ZrO2-SiO2 system were obtained by the sol-gel method applying two different types of ZrO2 precursors: zirconium (IV) n-propoxide Zr(OC3H7)4 and zirconium (IV) acetate Zr(OOC2H3)4 (organic acetic acid salt) while commonly used tetraethoxysilane TEOS was selected as SiO2 introducing one. ZrO2 concentration in synthesized samples varied from 20% to 50% (mol.). After drying for 28 days, all gels were annealed at 500 °C, 1000 °C, and 1200 °C in air. FTIR spectroscopy together with XRD diffraction was selected as the two main structure research methods. SEM microscopy was applied to analyze the local chemical compositions of samples and to observe the morphology of gels’ surfaces. The analysis of FTIR spectra and XRD diffraction patterns allowed us to recognize different ZrO2 polymorphs which appeared in the samples depending strongly as well on ZrO2 precursor type as on the temperature of annealing. Samples synthesized by using the zirconium (IV) n-propoxide contained both cubic and tetragonal zirconia phases in general but showed the tendency of the increasing t-ZrO2 content in gels richer in ZrO2 and heated up to 1200 °C. However, in materials obtained applying zirconium (IV) acetate, the first detected at 500 °C phase was t-ZrO2 which was then conversing to m-ZrO2 form with the increasing temperature in case of samples rich in ZrO2. Meanwhile, t-ZrO2 was the predominant phase in samples of the lower content of ZrO2 but annealed at higher temperatures. By the analysis of changes in band profiles and positions, one can draw conclusions that the structure of studied samples is mostly built up of an amorphous silica matrix, in which different types of zirconia polymorphs create their own crystal lattice. The presence of the particular polymorph depends strongly on the type of zirconia precursor and the temperature of annealing.

The characterization of XRD, N2 sorption, Raman, Py‐IR, O2‐TPD, NH3‐TPD, SEM‐EDX and TEM were utilized to investigate the influence of titanium species on the ordered mesoporous structure of V‐doped titanium and zirconium composite metal oxides (VTZ) catalysts prepared by EISA method. The characterization results showed that the introduction of titanium species would increase the catalyst crystallization temperature resulting in pure t‐ZrO2 phase formation with uniform distribution for all elements and more oxygen vacancies owing to the inhibition of its crystal transformation, which would be beneficial to the promotion of specific surface area and stronger Brønsted acidity. The catalytic performance in the liquid‐phase oxidation of naphthalene with hydrogen peroxide was also studied. The 12V15TZ‐550 catalyst with the molar ratio of vanadium and titanium species to the total titanium and zirconium species of 12 and 15%, respectively, calcined at 550 °C showed higher catalytic performance than other catalysts, where the naphthalene conversion of 17.3% and the 1,4‐naphthoquinone selectivity of 71.0% can be obtained. The catalytic stability would be kept well after repeated use for 5 times. [ABSTRACT FROM AUTHOR]

The performance of K and Fe loaded ZrO 2 species (Fe K/ZrO 2) for CO 2 hydrogenation was relevant to the concentration of oxygen vacancies due to the differences of ZrO 2 crystal phases. The K and Fe were deposited on the surface of monoclinic zirconia (m-ZrO 2) and tetragonal zirconia (t-ZrO 2) with the Fe loading of 0–20 wt% and the K loading of 1 wt% by impregnation methods. The ZrO 2 served as both a support and a co-catalyst for CO 2 hydrogenation, exhibiting 40.54% CO 2 conversion and 14.98% light olefins selectivity over the 10Fe 1K/m-ZrO 2 with 10 wt% Fe and 1 wt% K, at 2.0 MPa and 340 °C. Such CO 2 conversion were almost 200% higher than that of 10Fe 1K/t-ZrO 2. The better activity of 10Fe 1K/m-ZrO 2 is mainly attributed to the higher concentration of oxygen vacancies on the surface of m-ZrO 2 after the reduction in H 2 /N 2. However, covering the surface of m-ZrO 2 , excess surface Fe species lowered the concentration of surface oxygen vacancies for reduced 10Fe 1K/t-ZrO 2 , which inhibited the catalytic performance of ZrO 2 for CO 2 conversion. These investigations may propel the utilization of Zr-based catalysts on CO 2 hydrogenation to various value-added chemicals. Image 1 • CO 2 conversions over 10F 1K/m-ZrO 2 are almost 200% higher than that of 10Fe 1K/t-ZrO 2. • The reduction of catalysts with H 2 promotes the generation of oxygen vacancies. • Surface oxygen vacancies promote the catalytic CO 2 hydrogenation. [ABSTRACT FROM AUTHOR]

The structural and optical features of zirconia toughened alumina (ZTA) due to the assorted range of Eu3+ substitutions are demonstrated. The characterization studies affirm the pivotal role of Eu3+ on the improved structural stability of ZTA and associated tetragonal zirconia (t‐ZrO2) → cubic zirconia (c‐ZrO2) transformation. Eu3+ prefers accommodation at the lattice sites of ZrO2 and their gradual accumulation induces t‐ → c‐ZrO2 transition. Beyond the substitution limit, Eu3+ reacts with Al2O3 to form EuAlO3. Optical studies validate typical Eu3+ emissions, and further, the emission spectrum also predicts the symmetry of Eu3+ coordination at the ZrO2 lattice. Uniform distribution of ZrO2 and Al2O3 grains throughout the microstructures are evident from the morphological analysis. Further, the influence of Eu3+ on the enhanced mechanical stability of ZTA is ensured from indentation technique. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1170–1179, 2019. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Al 2 O 3 modified mesoporous ZrO 2 is achieved from one-pot EISA strategy. • The t -ZrO 2 and m -ZrO 2 crystalline phase could be accurately controlled. • M-ZrAl material has excellent thermal stability and textural property. • M-ZrAl material is taken as support for MoO 3 /M-ZrAl solid acid catalyst. • The catalyst with t -ZrO 2 crystalline phase shows a higher catalytic performance. Abstract In this paper, a series of Al 2 O 3 modified mesoporous ZrO 2 (M-ZrAl) materials are designed and achieved through a one-pot EISA strategy. As proved by different characterizations, the introduced Al 2 O 3 species exist as highly dispersed states, and the crystalline phase (t -ZrO 2 and m -ZrO 2) of M-ZrAl material could be accurately controlled through adjusting the content of Al 2 O 3 species. Moreover, the M-ZrAl-10 material exhibits excellent textural properties (specific surface area (56 m2·g−1), pore size (8.2 nm) and pore volume (0.13 cm3·g−1)) even treated at 800 °C. The obtained materials are employed as support for MoO 3 /M-ZrAl solid acid catalyst, and the relationship between catalytic performance (Friedel-Crafts alkylation, acetalization and esterification) and crystalline structure of support is specially explored. The result shows that the support with t -ZrO 2 phase is beneficial for the formation of acid sites and enhancement of catalytic performance for the MoO 3 /M-ZrAl solid acid catalyst. [ABSTRACT FROM AUTHOR]

The mullite-zirconia composites were prepared by reaction-sintering of alumina and zircon powder. Besides, the slip casting method was employed for fabrication of these composites. Then, the effect of nano-TiO2 addition on the properties of these composites was investigated and the results were compared with micro-sized TiO2. Hence, the physical properties, phase composition, flexural strength and microstructure of these composites after firing at 1600°C were studied. The results showed that the flexural strength of composite tends to increase with the addition of 0.5 wt.% nano-TiO2. It is attributed to the formation of larger size rod-like ZrO2 and enhanced ceramic bonding between them.

Zirconia nanoparticles (ZrO2) were prepared by a simple electrochemical method and characterized using X-ray diffraction, transmission electron microscopy, Brunnauer–Emmett–Teller surface area analysis, and ultraviolet-visible diffuse reflectance spectroscopy. In this study, tetragonal ZrO2(t-ZrO2) particles were stabilized at a lower calcination temperature without the addition of a dopant in the preparation step. It was suggested that the use of tetraethylammonium perchlorate as the supporting electrolyte was responsible for the stabilization oft-ZrO2by preventing the agglomeration of metal clusters with undesired powders in the electrolysis system, resulting in a decrease in ZrO2crystal size. The photoactivity of the catalyst was optimized using central composite design with response surface methodology and the optimum values of the parameters were a solution pH of 11, contact time of 4 h, catalyst dosage of 1.0 g L−1, and calcination temperature of 523 K. This resulted in 84.9% decolorization of methylene blue (MB) obtained from the predicted model, which fitted well with the laboratory results of 83.6%. The kinetics study demonstrates that the reaction followed pseudo first-order kinetics, and the rate constantsKR= 0.09 mg L−1 h−1andKLH= 0.43 L mg−1were determined using the Langmuir–Hinshelwood model. The mineralization of MB was measured by chemical oxygen demand removal, BOD5/COD, and TOC/TOC0ratio analyses with values of 78.5%, 1.48, and 0.21, respectively, after 4 h of contact time. The regeneration study shows that the catalyst could be maintained with a slight decrease in decolorization (<10%) after five cycling runs. Furthermore,t-ZrO2facilitated good photoactivity towards MB decolorization under UV light in a batch reactor compared with commercialt-ZrO2(58.7%) and Degussa P25 TiO2(64.5%). [ABSTRACT FROM AUTHOR]

• The synthesis of the zirconia- silica gels applying three different SiO 2 precursors. • The influence of SiO 2 precursor on the structure of dried and annealed gels. • The presence of the silica matrix in gels of higher silica content. • The formation of t- ZrO 2 in all annealed at 1000 °C and above gels regardless the silica precursor type. Zirconia–silica materials are well known and studied for a long time. Application of the sol-gel method allows to expand their properties by introducing new type of components from the appropriate precursors used in the process. Simultaneously, facilitates fabrication materials, first of all because of the decrease the temperature of their synthesis. In this work, the main focus was put not on zirconia precursors (as in most articles), but on SiO 2 introducing ones. Three different types of silica precursors were applied: TEOS – tetraethoxysilane, Si(OC 2 H 5) 4 , DDS – dimethyldiethoxysilane, Si(OC 2 H 5) 2 (CH 3) 2 and Aerosil™ (silica powder). As ZrO 2 introducing component, zirconium (IV) propoxide, Zr(OC 3 H 7) 4 was exclusively applied. As methods of structural investigations, two complementary methods were selected: FTIR spectroscopy and X-Ray diffraction (XRD) in the standard configuration. To obtain additional data, thermal methods: thermogravimetry (TG) and differential scanning calorimetry (DSC) were applied. According to XRD studies, all the obtained samples remained amorphous after annealing up to 500 °C. At higher temperatures, t- ZrO 2 and m- ZrO 2 began to crystallize. The temperature of their formation depended on SiO 2 precursor type and SiO 2 concentration in the system. In the samples richer in ZrO 2 , crystallization of t- ZrO 2 began only at 1000 °C but the tendency of its crystallization was much faster than in the samples richer in SiO 2 which was confirmed by comparison of mean crystallite sizes of t- ZrO 2 in the samples synthesized with the use of different SiO 2 precursors. FTIR spectra of the samples of higher silica concentration, despite the type of silica precursor, showed progressive formation of silica matrix, in which zirconium atoms were located. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] PEO of Zircaloy-2 is investigated in different concentrations of aluminate. [•] “Soft” sparks enhance the growth of the inner coating layer in dilute aluminate. [•] Coatings with superior wear resistance are formed in concentrated aluminate. [•] Large amount of Al2O3 besides t-ZrO2 provides the superior wear resistance. [ABSTRACT FROM AUTHOR]

Abstract: Plasma electrolytic oxidation of Zircaloy-2 at constant rms current is examined in silicate and aluminate electrolytes, revealing significantly different behaviors in the growth kinetics and properties of the coatings. Coatings thicken continuously in the silicate electrolyte, while in the aluminate electrolyte, the thickness reaches a relatively constant value. The latter coincides with changing appearances of discharges and detachment of an outer coating layer. Dissolution of zirconium is faster in the silicate electrolyte in the early stage of PEO, but is faster in the aluminate electrolyte following coating breakdown. The pre-spallation coating formed in the aluminate electrolyte shows superior wear resistance, which can be ascribed to its relative compactness, associated with the presence of tetragonal zirconia stabilized by aluminium species. [Copyright &y& Elsevier]

Abstract: The thermodynamic properties of tetragonal-ZrO2 (t-ZrO2) were investigated at high pressures and temperatures using plane-wave pseudopotential method within generalized gradient approximation (GGA) and the quasi-harmonic Debye model. The variations of the volume, bulk modulus, the heat capacity, entropy, Debye temperature, the Grüneisen parameter and thermal expansion coefficient of t-ZrO2 with pressures and temperatures were obtained and compared with the available results. The thermodynamic properties of t-ZrO2 have been predicted using the quasi-harmonic Debye model for the first time and the obtained results are in agreement with the previous experimental and theoretical data. [Copyright &y& Elsevier]

The mullite-zirconia composites were prepared by reaction-sintering of alumina and zircon powder. Besides, the slip casting method was employed for fabrication of these composites. Then, the effect of nano-TiO2 addition on the properties of these composites was investigated and the results were compared with micro-sized TiO2. Hence, the physical properties, phase composition, flexural strength and microstructure of these composites after firing at 1600 °C were studied. The results showed that the flexural strength of composite tends to increase with the addition of 0.5 wt.% nano-TiO2. It is attributed to the formation of larger size rod-like ZrO2 and enhanced ceramic bonding between them. [ABSTRACT FROM AUTHOR]

Abstract: Plasma electrolytic oxidation was undertaken on Zircaloy-4 in alkaline silicate and pyrophosphate electrolytes, with a square waveform AC current regime. The resultant coatings were examined using scanning electron spectroscopy, X-ray diffraction and nanoindentation. The coatings formed in silicate electrolyte comprised mainly a porous inner layer and a more compact outer layer, with characteristic solidification structures being evident following prolonged treatment. The coatings contained monoclinic and tetragonal ZrO2, the latter being mainly present in the outer layer, which was of hardness up to ∼8GPa. In contrast, extensively cracked coatings resulted from use of pyrophosphate electrolyte; the coating integrity was improved by the addition of silicate to the pyrophosphate electrolyte. The different morphologies of the coatings appeared to be related to the differing nature of the microdischarges and to the incorporation of silicon species that enhanced the formation of t-ZrO2. [Copyright &y& Elsevier]

Abstract: Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20nm) and the size distribution was very narrow (3.4nm). Systematic structural characterization revealed the t-ZrO2 and indexed for its tetragonal structure (a =3.5975Å and c =5.1649Å). Calculated microstrain in most of the plane indicated the presence of compressive stress (65–288MPa) along various planes of the particles. Observed space group (P42/nmc) revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16h positions (0.25, 0.25, 0.4534). The metal-oxide (Zr–O) band observed at the low wavenumber region further confirmed the phase purity of the as-prepared ZrO2 nanopowders. Peaks at the binding energy positions 2.042 and 0.525keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia. The particle size estimated by TEM was in good agreement with the results obtained through X-ray line broadening (20.81nm) measurements. The nanopowders were sintered to above 98% of the theoretical density by using vacuum sintering technique at a relatively low temperature of 1300°C. Stable tetragonal ZrO2 experimentally yield the permittivity value of about 28 at 10MHz. [Copyright &y& Elsevier]

Abstract: For the first time, single reverse microemulsion-assisted direct precipitation route has been successfully used to synthesize tetragonal zirconia nanoparticles in narrow size range. The synthesized powder was characterized using FT-IR, XRD and HRTEM techniques. The zirconia nanoparticles obtained were spherical in shape and has narrow particle size distribution in the range of 13–31nm and crystallite size in the range of 13–23nm. [Copyright &y& Elsevier]

A series of MoO3/ t-ZrO2 catalysts was prepared by slurry deposition. Structural studies revealed that the presence of MoO3 had no influence on the phase composition of zirconia support. IR/RS and EPR results confirmed that both isolated and polymeric (2D) oxomolybdenum species occurred on catalysts surface. Adsorption of NO resulted in the reduction of molybdenum centers with simultaneous formation of surface nitrates. [ABSTRACT FROM AUTHOR]

Abstract: This manuscript describes the surface nitriding of yttria (Y2O3) stabilized tetragonal zirconia (t-ZrO2) by using the pulsed Nd-YAG laser irradiation in the nitrogen atmosphere. The results show modification of the crystallographic structure between the original tetragonal zirconia and the treated one. Nitrogen atoms enter in zirconia substituting oxygen atoms in the network. X-ray diffraction and Raman spectra show an efficient transformation of the t-ZrO2 at the surface that exhibit typical yellow-gold color of ZrN with high hardness and wear resistance. The X-ray diffraction results evidenced the new diffraction lines [c-ZrN(200)] that appear on the nitrided sample with alteration and shift in peaks as well as change in d-spacing or lattice parameter, whereas the Raman spectra revealed the two characteristics band related to acoustic part of low frequency in the 150–260cm−1 region and optical part at high frequency in the 450–750cm−1 region. Nitriding is accompanied by the formation of a few micron thick gradient layer of ZrN structure on the top via dissociative process: 2ZrO2 +N2 =2ZrN+2O2 and a partial transformation of t-ZrO2 into cubic zirconia (c-ZrO2) beneath deep in the bulk due to nitrogen concentration gradient. Some new features on the physical characteristics such as hardness and roughness of the cut sections of t-ZrO2 in the N2 and O2 atmosphere are discussed. [Copyright &y& Elsevier]

Tetragonal (t-)ZrO2 rods and spheres were prepared by simple and time-saving low-temperature synthesis and characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The morphology of the product can be changed from rods to spheres by changing the reaction conditions slightly. The t-ZrO2 rods (0.5–0.1mm approximately) were obtained by topotactic reaction. The work illustrates the utility of Raman spectroscopy as a powerful characterization tool when XRD is not able to conclusively identify the phases. [Copyright &y& Elsevier]

Abstract: Surface nitriding of yttria (Y2O3) stabilized tetragonal zirconia (t-ZrO2) has been performed using the pulsed Nd-YAG laser irradiation in the nitrogen atmosphere. The results show modifications of the crystallographic structure between the original tetragonal zirconia and the treated one. Nitrogen atoms enter in the zirconia substituting to oxygen atoms in the network. X-ray diffraction and Raman spectra show an efficient transformation of the t-ZrO2 at the surface that exhibit the typical yellow-gold color of ZrN with high hardness and wear resistance. The hardness measured with Vickers indenter bears a high value of about 1409 HV. Nitriding is accomplished by the formation of a few micron thick gradient layer of ZrN structure on the top and a partial transformation of t-ZrO2 into cubic zirconia (c-ZrO2) beneath deep in the bulk due to nitrogen concentration gradient. [Copyright &y& Elsevier]

Zr&z.sbnd;Si&z.sbnd;N coatings were deposited on steel and silicon substrates by reactive sputtering of a composite Zr&z.sbnd;Si target. The coatings were oxidised in air in the 600–750 °C temperature range. As-deposited and oxidised films were characterised by X-ray diffraction, micro-Raman spectroscopy, X-ray photoemission spectroscopy and glow discharge optical emission spectroscopy. The oxidation behaviour of Zr&z.sbnd;Si&z.sbnd;N coatings was compared to that of ZrN ones. It was demonstrated that addition of silicon in the 3–5 at.% range into ZrN-based coatings promotes the onset of oxidation by nearly 100 °C. The structure of the oxide layer was strongly dependent on the film’s silicon content: monoclinic zirconia for ZrN films, a mixture of monoclinic and tetragonal zirconia for intermediate silicon concentration and tetragonal zirconia for high silicon content. Finally, the stabilisation of the tetragonal form is discussed taking into account the occurrence of doping elements in the oxide layer, the intrinsic stresses in the oxidised films and the mean crystal size of zirconia grains. [Copyright &y& Elsevier]

Transition of zirconium alloys during uniform corrosion is not expected but inevitable, since it accelerates the degradation of corrosion property. Up to now, proposed transition mechanisms have not clearly illustrated the critical behavior of interfacial t-ZrO 2 and other oxide features under the critical transition condition. Therefore, study on the under-transition sample was carried out. Microscopically, the monoclinic stress increases after transition, strongly supporting the occurrence of transformation from t-ZrO 2 to m-ZrO 2. A sudden stress decrease in t-ZrO 2 occurs in pre-transition region of the under-transition sample. This decrease might be attributed to formation of permeable paths which induce consumption of interfacial oxygen vacancies. This critical behavior possibly destabilizes the grown t-ZrO 2 grains. And the post-transition regions will propagate after t-ZrO 2 transformation. The transition region in the oxide consists of lateral cracks propagated transgranularly and large numbers of interconnected pores, both of which may originate from t-ZrO 2 transformation. Current works suggest that three critical behaviors of t-ZrO 2 induce oxide fracture and corrosion transition. The growth nature and evolution mechanism of t-ZrO 2 are further illustrated. Even though t-ZrO 2 layer similarly evolve as the ZrO suboxide layer, the unstable nature of t-ZrO 2 indicates that it is severely affected by grain size, stress value and chemical composition. [ABSTRACT FROM AUTHOR]

The investigation underline the in situ formation of ZrO 2 /ZnFe 2 O 4 composites and the resultant structural, morphological, mechanical and magnetic properties. The characterization results ensured the crystallization of tetragonal ZrO 2 (t -ZrO 2) and ZnFe 2 O 4 phases at 900 °C. Depending on Zn2+/Fe3+ content, the composite system revealed a gradual increment in the phase yield of ZnFe 2 O 4. The significance of monoclinic ZrO 2 (m -ZrO 2) is also evident in all the systems at 900 °C; however, the incremental heat treatment to 1300 °C indicated its corresponding loss, thus indicating the reverse m - → t -ZrO 2 transition. The crystallization of ZnFe 2 O 4 as a secondary phase in the t -ZrO 2 matrix is also affirmed from the morphological analysis. Mechanical studies accomplished good uniformity in all the investigated compositions despite the variation in the phase content of ZnFe 2 O 4 in composite system. All the t -ZrO 2 /ZnFe 2 O 4 composites ensured strong ferrimagnetic features and moreover better biocompatibility and non-toxicity characteristics were displayed from in vitro tests. • In situ formation of ZrO 2 /ZnFe 2 O 4 composites with assorted composition ratios • Zn2+/Fe3+ dependent evolution of monoclinic ZrO 2 at low temperatures • Gradual monoclinic to tetragonal ZrO 2 transition dependent upon Zn2+/Fe3+ content and progressive heat treatments • Better mechanical and in vitro compatibility displayed by ZrO 2 /ZnFe 2 O 4 composites • Magnetic features determined by the ZnFe 2 O 4 content in composites [ABSTRACT FROM AUTHOR]