t-ZrO2 Phase Research Articles

Nanophase-Separated Copper–Zirconia Composites for Bifunctional Electrochemical CO2 Conversion to Formic Acid

A copper-zirconia composite having an evenly distributed lamellar texture, Cu#ZrO2, was synthesized by promoting nanophase separation of the Cu51Zr14 alloy precursor in a mixture of carbon monoxide (CO) and oxygen (O2). High-resolution electron microscopy revealed that the material consists of interchangeable Cu and t-ZrO2 phases with an average thickness of 5 nm. Cu#ZrO2 exhibited enhanced selectivity toward the generation of formic acid (HCOOH) by electrochemical reduction of carbon dioxide (CO2) in aqueous media at a Faradaic efficiency of 83.5% at -0.9 V versus the reversible hydrogen electrode. In situ Raman spectroscopy has revealed that a bifunctional interplay between the Zr4+ sites and the Cu boundary leads to amended reaction selectivity along with a large number of catalytic sites.

Phase equilibria and non-transformable tetragonal zirconia in ZrO2-RETaO4 systems and their stabilization mechanism: Experiments and calculations

The non-transformable tetragonal zirconia (t-ZrO2) in ZrO2-RETaO4 systems offers significant promise in the development of next generation thermal barrier coatings. This work focuses on the effects of rare-earth dopants on phase equilibria and phase stability of tetragonal zirconia in ZrO2-RETaO4 systems. Precursor-derived oxides were heat treated at 1500 °C to elucidate phase constitution by X-ray diffraction. The composition ranges of non-transformable t-ZrO2 in ZrO2-YbTaO4, ZrO2-DyTaO4 and ZrO2-GdTaO4 are measured to be about 30–41, 22–29 and 16–19 mol% RE0.5Ta0.5O2, respectively. However, non-transformable t-ZrO2 cannot be found in ZrO2-NdTaO4 and ZrO2–LaTaO4. Formation abilities of t-ZrO2 in ZrO2-RETaO4 systems are determined to be inversely proportional to the ionic radius of RE3+. Based on our calculations, stabilization mechanisms of non-transformable t-ZrO2 were considered to be related to degree of strain energy release and high-temperature decomposed reactions of the doped t-ZrO2. Firstly, RE-Ta pair dopants could relieve the strain energy of the strained tetragonal zirconia structure, which would prevent the displacive transformation of t-ZrO2→m-ZrO2. Secondly, the solid solubilities of RE-Ta dopants should be large enough to ensure the doped t-ZrO2 phase cannot decompose before inhibiting t-ZrO2→m-ZrO2. For large ionic radius of Nd3+ and La3+ dopants, the decomposed reactions of doped t-ZrO2 can take place even at very low dopant concentration, which explains the observations that t-ZrO2 cannot be detected in their whole compositions. The emerging results are useful to the development of appropriate thermal barrier coatings.

Microstructure, mechanical properties, friction and wear performance, and cytotoxicity of additively manufactured zirconia-toughened alumina for dental applications

Zirconia-toughened alumina (ZTA) ceramic has potential dental restoration and repair applications due to its high mechanical strength, toughness, wear resistance, chemical durability, and biocompatibility. Nevertheless, there is a great deal of crack initiation caused by stress accumulation and also material wastage after computer-aided design/computer-aided manufacturing of ZTA ceramics in conventional clinical processing. In this study, we prepared a ZTA green compact using Stereolithography (SLA) technology followed by sintering at high temperatures ranging between 1450 °C and 1650 °C. The microstructures, mechanical properties, friction and wear performance, and cytotoxicity of the ZTA ceramic were investigated. The results indicate that all ZTA samples were composed of α-Al2O3, m-ZrO2, and t-ZrO2 phases before and after sintering. With increasing sintering temperatures from 1450 °C to 1650 °C, the density, mechanical properties, hardness, and wear performance of the ZTA samples were gradually improved. The ZTA sample sintered at 1650 °C with a relative density of 96.65% showed the best mechanical performance, with a flexural strength of 572.0 MPa, a Vickers hardness of 16.2 GPa, and a fracture toughness of 7.4 MPa m1/2. The sintered ZTA sample exhibited a good cell viability toward MC3T3-E1 cells.

Microstructural Characteristics of 3Y-TZP Ceramics and Their Effects on the Flexural Strength

This work evaluates the effects of grain growth and tetragonality of the t-ZrO2 phase on the mechanical properties of 3Y-TZP ceramics. Samples were sintered at 1475 °C for 2 h, and at 1600 °C for 2, 12, or 24 h. After sintering, the tetragonal ZrO2 polytypes t and t′ were observed under all sintering conditions, while a residual content of monoclinic ZrO2 was detected in samples sintered at 1600 °C for 24 h. The average grain size was found to vary from 0.65 ± 0.10 to 2.20 ± 0.35 μm. Moreover, zirconia ceramics sintered at 1475 °C for 2 h exhibit higher flexural strength (1210 ± 85 MPa), while samples sintered at 1600 °C for 24 h exhibit the lowest flexural strength (910 ± 90 MPa). These results were related to the progressive formation of Y3+-rich grains (t′-ZrO2) due to the grain boundary segregation-induced phase (GBSIPT) mechanism. Due to the high stabilizer concentration in the solid solution, these grains present lower tetragonality, being highly stable at room temperature. Consequently, the observed strength reduction of samples sintered at 1600 °C for 24 h is related to the presence of the t′-ZrO2 phase, which is less prompt to the phase transformation toughening process, limiting shielding zones’ effectiveness at the crack tip.

High temperature abradable sealing coating for SiCf/SiC ceramic matrix composites

A study of porous YSZ abradable sealing coating (ASC) plasma-sprayed onto SiCf/SiC ceramic matrix composites (CMC) through the compatibility of intermediate layers is reported. The multilayer Si/Yb2Si2O7/LaMgAl11O19 thermal-environmental barrier coating (T-EBC) is served as intermediate layers in consideration of its ability to protect the CMC from recession and ease the misfit of the thermal expansivity. Isothermal exposure and thermal shock tests were conducted at 1200°C and led to the decomposition of t'-ZrO2 phase to t-ZrO2 and c-ZrO2 phases in YSZ topcoat, the formation of mud-cracks throughout the entire coating structure and thermally grown oxide (SiO2), with following an Yb2Si2O7 reaction layer. The measured bond strength of the coated samples was 5.47 ± 0.85 MPa, and the fracture position mainly happened inside the CMC substrate. The Superficial Rockwell Hardness (HR15Y) considered to be an important factor in abradability increased by only 1.34% after 1200°C isothermal exposure for 100 h, showing excellent high temperature hardness stability. The abradability of the ASC was investigated by a sliding wear test, the fatigue wear mainly occurred in worn scar when encountering Si3N4 ceramic ball with high hardness and low thermal conductivity, while adhesive wear occurred when GCr15 steel ball with low hardness and high thermal conductivity are encountered.

Comparison of in-situ oxidation behavior of Zr by micro-arc oxidation and selective laser melting

Al2O3-ZrO2 ceramic coatings were prepared on 6061 Al alloy by Micro Arc Oxidation (MAO) and Selective Laser Melting (SLM). Coatings prepared by MAO exhibit a porous structure and a progressive oxidation of the substrate as a function of processing time. Those made by SLM are denser, although the presence of cracks is observed throughout their surface. XRD results suggest that the ceramic coating consists of m-ZrO2, t-ZrO2 and γ-Al2O3 phases for the MAO process, while a Zr3O phase is also present in SLM. The hardness of the MAO Al2O3-ZrO2 coating is higher than that obtained by SLM. Young’s modulus of the MAO coating is lower, due to higher level of porosity observed in the coating prepared by MAO. Moreover, in-situ oxidation, heating and solidification processes are all discussed and compared within the context of MAO and SLM peculiarities. The processes and transformation mechanisms involved in the processing methods produce diverse microstructures.

Effect of the Y2O3 amount on the oxidation behavior of ZrB2-SiC-based coatings for carbon/carbon composites

In this paper, the effect of Y2O3 addition on the oxidation resistance of ZrB2-SiC-Y2O3 coating for the SiC coated carbon/carbon composites was investigated. Results confirmed the great benefits of adding Y2O3 to the oxidation-resistant properties of the original coating from different aspects. The dense structure of surface and cross-section and formation of yttria stabilized zirconia, Y2Si2O7, t-ZrO2 phases were observed after adding Y2O3. Additionally, according to the TEM results, yttrium silicate existed in the form of nanoparticles, while yttria stabilized zirconia existed in the form of agglomeration within the SiO2 liquid phase. This superior oxidation resistance was attributed to the following reasons: (i) the formed Zr-Si-Y-O glass barrier layer blocked the oxygen diffusion and healed the cracks; (ii) the reduced m-ZrO2 content weakened the volume expansion of the coating and avoided spallation; (iii) Y2Si2O7 served as a pinning phase which modified the stability of liquid SiO2 at elevated temperatures.

Microstructural evolution of ceramic nanocomposites coated on 7075 Al alloy by plasma electrolytic oxidation

Alumina-zirconia nanocomposite coatings were prepared on 7075 Al alloy by plasma electrolytic oxidation (PEO) at current densities 0.1–0.3 A/cm2 in a Zr-containing electrolyte under DC galvanostatic mode. Coatings were investigated by X-ray photoelectron spectroscopy (XPS), X-ray and electron diffraction, and high-resolution transmission electron microscopy (HRTEM). The coatings developed at current density higher than 0.2 A/cm2 consist of γ-, t- and α-alumina and tetragonal zirconia and were composed of four sub-layers: (i) interface nanocomposite layer close to the substrate with a dense structure of heterogeneously distributed α-Al2O3 and t-Al2O3 nanocrystals (2–5 nm in size) embedded in an amorphous matrix, followed by (ii) a pure amorphous Al2O3 sublayer, (iii) a dense zirconia encapsulated alumina sublayer and (iv) a top amorphous sublayer with dispersed globular alumina and zirconia nanoparticles. The zirconia encapsulated alumina sublayer is composed of large t-Al2O3 and γ-Al2O3 grains bordered by fine t-ZrO2 nanoparticles. The t-ZrO2 phase was nucleated in an atomically coherent manner at Al2O3 grain boundaries. The microstructure evolution in the coating revealed alumina formation at the early stages of the process while the oxidation, crystallization and incorporation of zirconia occurred at the last stages of the PEO process. The phenomenology of the PEO Al2O3/ZrO2 nanocomposite coating formation is presented to account for the present experimental observations.

Preparation and microstructural evolution of cellular submicrocrystal Al2O3/TZP powders

Cellular submicrocrystal Al2O3/TZP powders (CSP) were successfully prepared by Al-Al(NO3)3 combustion synthesis assisted rapid atomization and water cooling (CS-RAWC). CSP mainly contains intracellular metastable Al2O3 phase with the grain size of ∼ 250 nm and intercellular t-ZrO2 supersaturated solid solution (SS) phase with the thickness of ∼ 30 nm. The t-ZrO2(SS) phases are t-Zr1-x-yAlxCeyO2-x/2 and t-Zr1-x-yAlxYyO2-x/2-y/2, due to phase transition stabilizers of CeO2, Y2O3 and Al2O3. There are also some t-ZrO2(SS) phases in the small-sized cellular submicro-Al2O3 crystals. After CSP heat-treated at 1450 °C for 1 h, it still retains cellular microstructure. Meanwhile, high-density nano- and supra-nano-ZrO2 particles by in-situ nanoprecipitation are evenly distributed in intercellular ZrO2 and intracellular Al2O3 grains. The special microstructures of the CSP before and after heat-treatment are difficult to be realized by previous methods. Therefore, the CSP approach has played an important guiding role in the preparation of cellular crystal powders and nanocomposite ceramics.

Study of t-ZrO2 : Eu3+ nanophosphor obtained by laser vaporisation using a cw CO2 laser

ZrO2 : Eu3+ nanophosphor has been fabricated by vaporisation of a ceramic target of specified composition using a CO2 laser, with subsequent vapour condensation in an argon flow. Nanoparticles with spherical morphology have been synthesised at a pressure of 0.1 atm. The t-ZrO2 phase is found to be dominant (98 %). According to the transmission electron microscopy data, the nanoparticle size is 10 ± 4 nm. The luminescence properties of fabricated nanoparticles have been studied. It is shown that the strong red luminescence of the samples is mainly due to the bands peaking at 591 and 606 nm, which are related, respectively, to the 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+ ion in the t-ZrO2 structure. It is shown also that europium is uniformly distributed over the ZrO2 matrix during laser synthesis. A comparative study of the t-ZrO2 : Eu3+ nanophosphor and the initial coarse-grained target has been performed. It is concluded that the proposed method of laser vaporisation is promising for synthesising t-ZrO2-based nanophosphors.

Nano-agglomerated powder and thermal shock cycling property of 8YSZ nano-structured thermal barrier coating

Based on the excellent thermodynamic and service performances of nano-structured coatings, 8YSZ nano-agglomerated spraying powder and its nano-structured thermal barrier coating (TBC) were successfully prepared by the integrated application of nanotechnology and atmospheric plasma spraying. The influences of spraying granulation process (solid content of slurry and inlet air temperature) on the spraying powder characteristics and the water quenching-thermal shock cycling behavior at 1050 °C of 8YSZ nano-structured TBC were investigated, respectively. The results showed that the nano-agglomerated powders exhibit apple-like, hollow spheroids with relatively uniform particle size when the spraying granulation process involved a 40% solid content of slurry, a 240 °C inlet air temperature, and a 35 Hz atomizer rotation frequency. The fluidity and apparent density were 57.53 s/50 g and 1.39 g/cm3, respectively. The nano-agglomerated 8YSZ spraying powders met the requirements of plasma-spraying technology. The plasma-sprayed 8YSZ nano-structured coating was composed of a single t-ZrO2 phase, had a layered structure and a certain amount of nanostructures, and presented a special “bimodal microstructure”. The average porosity of the coating was 7.49%. 8YSZ nano-structured TBC failed layer-by-layer after water quenching-thermal shock 264 cycles at 1050 °C. The main reasons for the failure of 8YSZ nano-structured TBC were the large thermal stress and cracks in the substrate caused by the water quench-thermal shock cycle, and the mismatch of thermal expansion between the metal bond coat and the ceramic coat.

Determination of the ATZ ceramics with different SiO-=SUB=-2-=/SUB=- contents tensile strength by the "brazilian test" method

The possibility of using the "Brazilian test" method to determination the tensile strength sigmat of composite zirconia ceramics small-size samples were demonstrated and the reliability of the obtained sigmat values were confirmed. It was found that the dependence of the tensile strength of alumina toughened zirconia (stabilized by calcium oxide) with silica addition (CaO-ATZ + SiO2 ceramics) on SiO2 concentration in them has a maximum (sigma_t=450 MPa, at CSiO_2=5 mol.%). The observed toughened is explained by an increase in the transformability of the tetragonal t-ZrO2 phase and, accordingly, by an enhanced role of transformation toughening when SiO2 is adding into the ATZ ceramics. Keywords: ATZ ceramic, tensile strength, "Brazilian test", phase transformations, transformation toughening.

Диагностика предела прочности на растяжение ATZ-керамики с различным содержанием SiO-=SUB=-2-=/SUB=- методом "бразильского теста"

The possibility of using the "Brazilian test" method to determination the tensile strength σt of composite zirconia ceramics small-size samples were demonstrated and the reliability of the obtained σt values were confirmed. It was found that the dependence of the tensile strength of alumina toughened zirconia (stabilized by calcium oxide) with silica addition (CaO-ATZ+SiO2 ceramics) on SiO2 concentration in them has a maximum (σt = 450 MPa, at SiO2 = 5 mol.%). The observed toughened is explained by an increase in the transformability of the tetragonal t-ZrO2 phase and, accordingly, by an enhanced role of transformation toughening when SiO2 is adding into the ATZ ceramics.

Effect of Pulse Frequency on Phase Composition and Properties of Amorphous Zirconium Alloy Micro-arc Oxidation Coating

The micro-arc oxidation coating was prepared on the surface of the amorphous zirconium sheet, and the influence of frequency on the micro-arc oxidation coating of the Zr-based amorphous alloy was studied. The results show that the change of the frequency of the electrical parameters of the micro-arc oxidation in the silicate micro-arc oxidation bath will promote the transformation of the t-ZrO2 phase to the m-ZrO2 phase in the coating; the frequency affects the gap, uniformity and thickness of the coating. When the frequency is 400 Hz, the coating surface has small voids and no cracks, and the thickness reaches a maximum of 15 μm. With the increase of frequency, the weight loss of the amorphous zirconium micro-arc oxidation coating first decreases and then increases. The wear resistance can be improved.

Thermally tunable dielectric performance of t-ZrO2 stabilized amorphous Si(Pb,Zr)OC ceramic nanocomposites

The study reports in-situ nanocrystallization of Pb stabilized t-ZrO2 in an amorphous Si(Pb,Zr)OC matrix (PZSiOC) derived from lead zirconate modified poly(hydridomethylsiloxane) with thermally tunable dielectric behavior. The stabilization of t-ZrO2 phase in the amorphous silicon oxycarbide (SiOC) matrix is ascribed to the critical crystallite size effect and incorporation of Pb ion into the t-ZrO2 lattice. The dielectric behavior as a function of temperature (−50 °C–300 °C) and frequency (100 Hz–10 MHz) is comprehensively investigated. It is observed that these ceramic nanocomposites exhibit high permittivity and low loss in contrast to SiOC at all measured temperatures and reached colossal values (ε’> 104) above critical temperature with an S-shaped frequency dependent dielectric loss. Thermally activated interfacial-charge polarization process can be reasoned out for this interesting behavior. The study paves the way for the attainment of cost-effective, tailored as well as highly functional PZSiOC ceramic nanocomposites with colossal permittivity for a wide array of applications. The dielectric permittivity results exemplify the huge potential of PZSiOC ceramic nanocomposites in energy storage devices. Studies are in progress to assess the suitability of PZSiOC ceramic nanocomposites in supercapacitors.

Evolution of the phase composition of ZrB2-35MoSi2-15Al composite coating at annealing

In this article, new composite coatings ZrB2-35MoSi2-15Al were deposited on the surface of a carbon-carbon composite using a multi-chamber detonation accelerator. The evolution of the phase composition of ZrB2-35MoSi2-15Al coatings was analyzed with differential scanning calorimeter (DSC) and X-ray diffractometry (in situ HT-XRD) at temperatures from room temperature (∼ 25°C) to 1400°C (normal atmosphere and pressure). The coating before annealing according to X-ray diffractometry data is tetragonal (t-ZrO2) and monocline (m-ZrO2) zirconium dioxide, monocline silicon oxide (m-SiO2), hexagonal zirconium diboride (h-ZrB2), tetragonal molybdenum disilicide (t-MoSi2), cubic aluminum (c-Al) and cubic yttrium oxide (c-Y2O3). It was found that the coating crystallizes in m-ZrO2 at 460ºC, and then mullite with rhombic crystal lattice appears at 960ºC. When the temperature reaches 1050ºC the m-ZrO2, ZrSiO4, t-ZrO2, m-SiO2 and mullite phases formed in the coating. At 1400 º C, cubic zirconium dioxide appears in the coating. Experimental results can become the basis for the application of ceramic coating ZrB2-35MoSi2-15Al, which can improve the properties of carbon-carbon composites in an oxygen-containing environment at elevated temperatures.

Low-temperature degradation resistance and plastic deformation of ATZ ceramics stabilized by CaO

In this work, the phase composition (relative fractions of monoclinic m-ZrO2, tetragonal t-ZrO2, and cubic c-ZrO2 phases) and mechanical properties (hardness, fracture toughness, compressive strength) of alumina toughened zirconia (ATZ) ceramics, with an addition of silica were investigated. Calcium oxide was used as a stabilizer for the zirconia tetragonal phase. It was shown that CaO-ATZ+SiO2 ceramics demonstrate increased resistance to low-temperature degradation. The plasticity signs at room temperature were found due to the SiO2 addition to CaO-ATZ ceramics. A yield plateau appears in the uniaxial compression diagram at 5 mol. % SiO2 concentration. It is hypothesized that discovered plasticity is due to the increased t→m transformability.

Thiolation behaviors of methanol catalyzed by bifunctional ZSM-5@t-ZrO2 catalyst

The catalytic properties of bifunctional ZSM-5@t-ZrO2 catalyst for methanol thiolation were revealed by using methanol and H2S as probe molecules. Taking the ZSM-5/t-ZrO2 physically-blended catalyst, pure ZSM-5 and t-ZrO2 as control catalysts, we investigated the characteristics of adsorption and transformation of different reaction molecules on or over different catalysts by combination of in-situ diffuse reflectance Fourier transform infrared spectroscopy (Drifts) and a variety of techniques including XRD, XPS, XRF, FT-IR, N2 adsorption-desorption, CO2/NH3-TPD, and DSC. The results showed that the synergistic effect of acid sites between ZSM-5 phase and t-ZrO2 phase in the bifunctional catalyst enhanced the adsorption and dissociation of methanol molecules, while the base sites in t-ZrO2 phase were mainly responsible for the adsorption and dissociation of H2S molecules. Due to the small specific surface area of pure t-ZrO2 catalyst, precursor species for sulfur deposition and carbon deposition were easily formed. Presulfurization could improve the initial activity of methanol thiolation and shorten the induction period. The dissociation of H2S at the base sites was the rate-determining step of the bimolecular reaction, and the appropriate increase of base sites in the bifunctional components could construct the matching formation rate of sulfhydryl and methoxy groups. At the same time, the special composite structure and meso-microporous system of ZSM-5@t-ZrO2 could effectively reduce the formation rate of carbon and sulfur deposits.

Ablation behavior of (Zr,Ta)B2-SiC coating on carbon/carbon composites at 2300 °C

Zr-Ta-B-SiC multiphase coating was prepared on carbon/carbon composites by pack cementation and slurry method. The (Zr,Ta)B2 solid solution was found to be the key antiablative component. The ablation behavior suggested that the synthesized Zr0.70Ta0.30B2-SiC coating exhibited the best ablation resistance at 2300 °C. It is asserted that the micromorphology and distribution of Zr-Ta-O in the coating played a vital role in the evolution of the protective layer and ablation performance. A coating with ∼30 % Ta was beneficial for stabilizing the t-ZrO2 phase and alleviating the volume change at high temperature. The formation of a thermal barrier and low volatility of Zr-Ta-O layer were responsible for the excellent ablation resistance.

Microstructure evolution and toughening mechanism of Al2O3/YSZ directionally solidified eutectic ceramic

Al2O3/YSZ directionally solidified eutectic ceramics (DSECs) with a diameter of 10 mm were prepared via the induction heating zone melting (IHZM), and effects of the pulling rate of the crucible on the microstructure and mechanical properties were investigated. The eutectic components included α-Al2O3, t-ZrO2 and m-ZrO2 phases. The DSECs’ microstructure evolved from a mixture of discontinuous lamellar and round colony structure to the round colony structure, then to the faceted colony structure, and finally to a cell structure with increasing the pulling rate. The formation kinetics and growth process of the round colony structure were described and discussed in detail. The reason that the faceted colony structure formed could attributed to the large increase of solid-liquid interface curvature. When the pulling rate was 300 mm/h, the actual growth rate vg = 81.9 ± 17.5 mm/h was calculated from λ2v=37.7 corresponding to the intracolony; or vg = 99.5 ± 58 mm/h was calculated from with λ2v=631 corresponding to the intercolony. The DSECs’ hardness and fracture toughness increased with the pulling rate, and got the maximum values of 18.6 GPa and 9.39 MPa m1/2 at 300 mm/h, respectively, which could be attributed to the refinement of phases and the decrease of m-ZrO2 phase fraction.