Y-stabilized ZrO2 Research Articles

Design and Scalable Synthesis of Thermochromic VO2-Based Coatings for Energy-Saving Smart Windows with Exceptional Optical Performance.

We report strongly thermochromic YSZ/V0.855W0.018Sr0.127O2/SiO2 coatings, where YSZ is Y-stabilized ZrO2, prepared by using a scalable deposition technique on standard glass at a low substrate temperature of 320 °C and without any substrate bias voltage. The coatings exhibit a transition temperature of 22 °C with an integral luminous transmittance of 63.7% (low-temperature state) and 60.7% (high-temperature state) and a modulation of the solar energy transmittance of 11.2%. Such a combination of properties, together with the low deposition temperature, fulfills the requirements for large-scale implementation on building glass and has not been reported yet. Reactive high-power impulse magnetron sputtering with a pulsed O2 flow feedback control allows us to prepare crystalline W and Sr codoped VO2 of the correct stoichiometry. The W doping of VO2 decreases the transition temperature, while the Sr doping of VO2 increases the luminous transmittance significantly. A coating design utilizing second-order interference in two antireflection layers is used to maximize both the integral luminous transmittance and the modulation of the solar energy transmittance. A compact crystalline structure of the bottom YSZ antireflection layer further improves the VO2 crystallinity, while the top SiO2 antireflection layer provides also the mechanical and environmental protection for the V0.855W0.018Sr0.127O2 layer.

Structure and Chemical Reactivity of Y-Stabilized ZrO2 Surfaces: Importance for the Water-Gas Shift Reaction.

The surface structure and chemical properties of Y-stabilized zirconia (YSZ) have been subjects of intense debate over the past three decades. However, a thorough understanding of chemical processes occurring at YSZ powders faces significant challenges due to the absence of reliable reference data acquired for well-controlled model systems. Here, we present results from polarization-resolved infrared reflection absorption spectroscopy (IRRAS) obtained for differently oriented, Y-doped ZrO2 single-crystal surfaces after exposure to CO and D2O. The IRRAS data reveal that the polar YSZ(100) surface undergoes reconstruction, characterized by an unusual, red-shifted CO band at 2132 cm-1. Density functional theory calculations allowed to relate this unexpected observation to under-coordinated Zr4+ cations in the vicinity of doping-induced O vacancies. This reconstruction leads to a strongly increased chemical reactivity and water spontaneously dissociates on YSZ(100). The latter, which is an important requirement for catalysing the water-gas-shift (WGS) reaction, is absent for YSZ(111), where only associative adsorption was observed. Together with a novel analysis Scheme these reference data allowed for an operando characterisation of YSZ powders using DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). These findings facilitate rational design and tuning of YSZ-based powder materials for catalytic applications, in particular CO oxidation and the WGS reaction.

Grazing-incidence synchrotron radiation diffraction studies on irradiated Ce-doped and pristine Y-stabilized ZrO2 at the Rossendorf beamline.

In this work, Ce-doped yttria-stabilized zirconia (YSZ) and pure YSZ phases were subjected to irradiation with 14 MeV Au ions. Irradiation studies were performed to simulate long-term structural and microstructural damage due to self-irradiation in YSZ phases hosting alpha-active radioactive species. It was found that both the Ce-doped YSZ and the YSZ phases had a reasonable tolerance to irradiation at high ion fluences and the bulk crystallinity was well preserved. Nevertheless, local microstrain increased in all compounds under study after irradiation, with the Ce-doped phases being less affected than pure YSZ. Doping with cerium ions increased the microstructural stability of YSZ phases through a possible reduction in the mobility of oxygen atoms, which limits the formation of structural defects. Doping of YSZ with tetravalent actinide elements is expected to have a similar effect. Thus, YSZ phases are promising for the safe long-term storage of radioactive elements. Using synchrotron radiation diffraction, measurements of the thin irradiated layers of the Ce-YSZ and YSZ samples were performed in grazing incidence (GI) mode. A corresponding module for measurements in GI mode was developed at the Rossendorf Beamline and relevant technical details for sample alignment and data collection are also presented.

Characterization of Ca-doped YCoO3 Perovskite-type oxide as cathode for solid oxide fuel cells

Solid oxide fuel cell (SOFC) uses solid oxide as an electrolyte and has a high-power generation efficiency. One of the major problems in the development of SOFC is the side reaction that occurs at the interface between the electrode and the electrolyte. (Y, Ca)CoO[Formula: see text] materials were evaluated as a new air electrode material to replace the conventional (La, Sr)CoO3 materials. Y[Formula: see text]Ca[Formula: see text]CoO[Formula: see text] has good compatibility with Y-stabilized ZrO2 (YSZ) electrolyte. In addition, the power generation performance was equal to or better than the conventional SOFC cells.

High-performance thermochromic YSZ/[formula omitted]/YSZ coatings for energy-saving smart windows

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2) makes VO2-based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. We report on a scalable sputter deposition technique for fast preparation of strongly thermochromic YSZ/V0.986W0.014O2/YSZ coatings, where YSZ denotes Y-stabilized ZrO2, on conventional soda-lime glass at a relatively low substrate surface temperature (350 °C) and without any substrate bias voltage. The thermochromic V0.986W0.014O2 layers and the antireflection YSZ layers were deposited using a controlled high-power impulse magnetron sputtering of a single V–W and Zr–Y target, respectively. A coating design utilizing a second-order interference in the YSZ layers was applied to increase both the integral luminous transmittance (Tlum) and the modulation of the solar energy transmittance (ΔTsol). We present the phase composition (X-ray diffraction) and microstructure (high-resolution transmission electron microscopy) of the coatings and their optical properties (spectrophotometry and spectroscopic ellipsometry). The YSZ/V0.986W0.014O2/YSZ coatings exhibit a transition temperature of 33–35 °C with Tlum = 64.5% and ΔTsol = 7.8% for a V0.986W0.014O2 thickness of 37 nm, and Tlum = 46.1% and ΔTsol = 13.2% for a V0.986W0.014O2 thickness of 67 nm. The results constitute an important step to a cost-effective and high-rate preparation of large-area thermochromic VO2-based coatings for future smart-window applications.

Detailed pathway for a fast low-temperature synthesis of strongly thermochromic W-doped VO2 films with a low transition temperature

We report the discharge characteristics, and population of chosen atomic neutral (V, O and Ar) and ionic (V+, V2+, O+ and Ar+) species in the discharge during low-temperature (350 °C) depositions of W-doped VO x films onto a 170 nm thick Y-stabilized ZrO2 layer on a 1 mm thick conventional soda-lime glass. The depositions were performed using reactive deep oscillation magnetron sputtering with a pulsed O2 flow control and to-substrate O2 injection into a high-density plasma in front of the sputter V-W target. The pre-selected critical values of the oxygen partial pressure were in the range from 82 mPa to 92 mPa. Optical emission spectroscopy proved a relatively high density of O atoms in front of the substrate, which is important for production of the W-doped VO2 films. It was found that setting of the amount of O2 in the gas mixture determines not only the elemental and phase composition of the films and their deposition rate but also the crystallinity of the thermochromic VO2 phase in them. The crystalline W-doped VO2 films with a low transition temperature of 28 °C exhibited a high modulation of the solar energy transmittance of 8.9%. The results are important for further improvement of this new scalable sputter deposition technique for a fast (53 nm min−1 at a target-substrate distance of 100 mm in this work) low-temperature synthesis of high-performance durable thermochromic VO2-based multilayer coatings designed for smart-window applications.

Y‐Stabilized ZrO2 as a Promising Wafer Material for the Epitaxial Growth of Transition Metal Dichalcogenides

Y‐stabilized ZrO2 (YSZ) as a promising single‐crystal wafer material for the epitaxial growth of transition metal dichalcogenides applicable for both physical (PVD) and chemical vapor deposition (CVD) processes is used. MoS2 layers grown on YSZ (111) exhibit sixfold symmetry and in‐plane epitaxial relationship with the wafer of () MoS2 || () YSZ. The PVD‐grown submonolayer thin films show nucleation of MoS2 islands with a lateral size of up to 100 nm and a preferential alignment along the substrate step edges. The layers exhibit a strong photoluminescence yield as expected for the 2H‐phase of MoS2 in a single monolayer limit. The CVD‐grown samples are composed of triangular islands of several micrometers in size in the presence of antiparallel domains. The results represent a promising route toward fabrication of wafer‐scale single‐crystalline transition metal dichalcogenide layers with a tunable layer thickness on commercially available wafers.

Impact of iso-structural template layer on stabilizing pyrochlore Bi2Rh2O7

We present an epitaxial stabilization of pyrochlore Bi2Rh2O7 on the Y-stabilized ZrO2 (YSZ) (111) substrate by inserting a pyrochlore Eu2Ti2O7 template layer, otherwise Bi-based layered structures being formed directly on the YSZ (111) substrate. This result reveals that “iso-structural crystal phase” plays an important role in the interfacial phase control. The Bi2Rh2O7 film exhibits p-type electrical conduction with the lowest longitudinal resistivity (ρxx) among the reported Rh pyrochlore oxides. Such parameters as ρxx, carrier density, and mobility show almost no temperature dependence in the measured range of 2–300 K.

High performance UV photodetectors based on W doped δ -Ta2O5 single crystalline films

Amorphous or polycrystalline tantalum pentoxide (Ta2O5) films with high resistance have been widely used in semiconductor devices as insulating dielectric layers. In this work, tungsten (W) doped δ-Ta2O5 (0001) monocrystal films were deposited on Y-stabilized ZrO2 (111) substrates using a pulsed laser deposition system. The lattice structure, heteroepitaxial relationship, and electrical properties of the films were analyzed in detail. The carrier concentration, Hall mobility, and resistivity of the 2% W doped δ-Ta2O5 film are 6.61 × 1015 cm−3, 65.2 cm2/V s and 14.5 Ω cm, respectively. High performance metal–semiconductors–metal ultraviolet (UV) detectors based on the W doped δ-Ta2O5 films were fabricated. The UV detector based on a 2% W doped δ-Ta2O5 film exhibits a high photo responsivity of 10.32 A/W and a photocurrent-to-darkcurrent ratio of 1.3 × 104. The performances of the UV detectors in this work are so high, which indicates that the W doped δ-Ta2O5 films can be applied in UV detectors as an active layer.

Epitaxial growth, optical and electrical conductivity of the metallic pyrochlore Bi2Ru2O7 on Y-stabilized ZrO2 substrate

Epitaxial heterostructures composed of complex correlated metal oxides, grown along specific crystallographic orientations, offer a route to investigating emergent phenomena such as topological states and spin liquids through geometrical lattice engineering. A2Ru2O7 pyrochlore ruthenates, in particular, exhibit a metal–insulator transition with varying A cation, whose mechanism is not fully understood. We report on the epitaxial growth, and structural and electrical properties of metallic pyrochlore bismuth ruthenate heterostructures, grown along both the [001] and [111] directions. Ordered pyrochlore thin films were obtained with a highly oriented texture along the [001] and [111] crystallographic directions. Density functional theory calculations of the electronic band structure and density of states indicated that Bi2Ru2O7 is semimetallic and that hybridization of the Ru 4d and Bi 6p orbitals via the anion network at the Fermi energy was responsible for the metallicity. Electrical conductivity measurements confirmed that the compound is weakly metallic, in agreement with the reported conductivity for the stoichiometric bulk compound. The carrier concentration and mobility of the electrons compared favorably with previous reports on bulk material and indicate strong electron–electron interactions. The measured and computed optical conductivities were found to share coincident spectral features and confirm the electronic correlation. Comparison of the electrical and optical properties of the two distinct orientations indicates differences that cannot be attributed to differences in crystalline quality or dislocations and may indicate anisotropy in the electronic structure of Bi2Ru2O7. This study will enable access to the kagome lattice arising naturally in the 111 planes of the pyrochlore B cation sublattice, which may be used to uncover emergent topological properties.

Low-content and highly effective zoned Rh and Pd three-way catalysts for gasoline particulate filter potentially meeting Euro 7

The next-generation Euro 7 standard proposed much lower pollutant limits from gasoline vehicles, specifically for CO and NOx, which would be challenging for the three-way catalysts (TWCs) utilized commercially to eliminate these pollutants. TWCs with reductive (Rh) and oxidative (Pd) active components on gasoline particulate filters (TWC on GPF) play importantly auxiliary roles in the remediation of CO and NOx downstream the close coupled TWCs to meet their emission targets. Here, a low-content Rh-based TWC (0.17 wt%) zoned with a less expensive Pd-based TWC (0.29 wt%) for GPF applications (cGPF) is reported using improved colloidal deposition method. The supporting of Rh on Y-stabilized ZrO2 rather than on CeO2–ZrO2 inhibits the formation of inactive Ce rhodate species, while Pd on CeO2–ZrO2 not only guarantees the high oxygen storage capacity (OSC) but also enhances catalytic activity. The layout of the front one-fifth in volume being 0.29 wt% Pd on Ce0.43Zr0.57O2 and the rear four-fifths being 0.17 wt% Rh on Zr0.85Y0.15O2 prevents the possible alloying of Rh with Pd. The highly effective zoned Rh and Pd TWCs show synergistic three-way activity before and after severe hydrothermal aging at 1000 °C with 10% water for 24 h, which could be potential choices for close coupled GPF application to satisfy the upcoming stringent emission standards, such as Euro 7 and China 6b.

Epitaxial synthesis of unintentionally doped p-type SnO (001) via suboxide molecular beam epitaxy

By employing a mixed SnO2 + Sn source, we demonstrate suboxide molecular beam epitaxy (S-MBE) growth of phase-pure single-crystalline metastable SnO (001) thin films on Y-stabilized ZrO2 (001) substrates at a growth rate of ∼1.0 nm/min without the need for additional oxygen. These films grow epitaxially across a wide substrate temperature range from 150 to 450 °C. Hence, we present an alternative pathway to overcome the limitations of high Sn or SnO2 cell temperatures and narrow growth windows encountered in previous MBE growth of metastable SnO. In situ laser reflectometry and line-of-sight quadrupole mass spectrometry were used to investigate the rate of SnO desorption as a function of substrate temperature. While SnO ad-molecule desorption at TS = 450 °C was growth-rate limiting, the SnO films did not desorb at this temperature after growth in vacuum. The SnO (001) thin films are transparent and unintentionally p-type doped, with hole concentrations and mobilities in the range of 0.9–6.0 × 1018 cm−3 and 2.0–5.5 cm2 V−1 s−1, respectively. These p-type SnO films obtained at low substrate temperatures are promising for back-end-of-line (BEOL) compatible applications and for integration with n-type oxides in pn heterojunctions and field-effect transistors.

Trends in bandgap of epitaxial A2B2O7 (A = Sn, Pb; B = Nb, Ta) films fabricated by pulsed laser deposition

Pyrochlore oxides A2B2O7 have been a fruitful playground for condensed matter physics because of the unique geometry in the crystal structure. Especially focusing on the A-site tetrahedral sub-lattice, in particular, pyrochlore oxides A2B2O7 (A = Sn, Pb and B = Nb, Ta), recent theoretical studies predict the emergence of the “quasi-flat band” structure as a result of the strong hybridization between filled A-ns and O-2p orbitals. In this work, we have established the growth conditions of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7 films by pulsed laser deposition on Y-stabilized ZrO2 (111) substrates to elucidate their optical properties. Absorption-edge energies, both for direct and indirect bandgaps, increase in the order of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7. This tendency can be well explained by considering the energy level of the constituent elements. A comparison of the difference between direct and indirect bandgaps reveals that Pb2B2O7 tends to have a less dispersive valence band than Sn2B2O7. Our findings are consistent with the theoretical predictions and are suggestive of the common existence of the hybridized states in this class of compounds.

As-Sintered Manganese-Stabilized Zirconia Ceramics with Excellent Electrical Conductivity

As promising electrolyte materials in intermediate-temperature solid oxide fuel cells (IT-SOFCs), Sc-stabilized ZrO2 (ScSZ) and Y-stabilized ZrO2 (YSZ) electrolytes continue to be plagued by high cost and low intermediate conductivity. To mitigate these problems, Mn has been chosen as a new stabilization element for the synthesis of Mn-stabilized ZrO2 ceramics (MnSZ) through solid state sintering. Microstructures and electrical properties of micron-crystalline Zr1−xMnxO2−δ (x = 0.15, 0.20 and 0.25) ceramics electrolytes for IT-SOFCs have been systematically evaluated. Within the applied doping content, Mn2+ ions can enter the ZrO2 crystal lattice, leading to the formation of single cubic phase samples. Electrical conductivity measurements in the temperature range between 400 °C and 1000 °C show a sharp increase in conductivity due to Mn doping. The highest conductivity is obtained from the Zr0.75Mn0.25O2−δ samples, being 0.0144 S/cm at 600 °C and 0.182 S/cm at 1000 °C. The electrical conductivity at 600 °C is twice higher than that of the YSZ and two orders of magnitude higher than that of the ScSZ. These properties can fulfill the conductivity requirement (∼1 × 10−2 S/cm) for the electrolyte. Therefore, based on this study, we propose that Mn stabilized ZrO2 is a promising candidate as a solid electrolyte for IT-SOFCs.

Single crystalline Sn2Nb2O7 films with Ti-doping fabricated by pulsed laser deposition

Sn2Nb2O7, a pyrochlore oxide, is expected to have a peculiar flat band structure that can be a source of intriguing physical properties. We have prepared Sn2Nb2O7 thin films by pulsed laser deposition on Y-stabilized ZrO2(111) substrates by finding a growth window at low growth temperature (∼450 °C) and higher laser fluence, possibly due to the rather high volatility of stannous oxide. By Ti-doping, the lattice constant shrinks due to the smaller Ti4+ ion compared with Nb5+. Optical measurements capture an absorption edge at ∼2.6 eV originating from the transition from the Sn-5s and O-2p hybridized valence band to the Nb-4d conduction band, which is not observed in LiNbO3. Opposed to the expectation from the Burstein–Moss shift, Ti-doping results in reduction of the bandgap, indicating that the doping results in the formation of in-gap states and does not contribute to injecting mobile carriers.

Synthesis and characterization of single-crystalline δ-Ta2O5 epitaxial films on Y-stabilized ZrO2 (111) substrates

Hexagonal tantalum pentoxide (δ-Ta2O5) epitaxial films were successfully prepared on Y-stabilized ZrO2 (YSZ) (111) substrates by metalorganic chemical vapor deposition. The crystal structure and optical characteristics of the deposited films as a function of the substrate temperature were investigated. High quality stoichiometric single-crystalline δ-Ta2O5 film was gained at the optimum epitaxial growth temperature of 800 °C and the full-width at half-maximum of the ω rocking curve of δ-Ta2O5 (0001) was only 0.066°. The heteroepitaxial relationship between the δ-Ta2O5 film and YSZ (111) substrate was identified as δ-Ta2O5 (0001)‖YSZ (111) with δ-Ta2O5 [21¯1¯0‖YSZ [101‾] and was illustrated in a schematic diagram. This proposed film makes adequate preparations for developing a range of ultra wide band gap semiconductor devices.

New Paramagnetic Center in Cu-Doped Y-Stabilized ZrO2

The results of electron paramagnetic resonance (EPR) of Cu-doped Y-stabilized ZrO2 powders calcined at 800 °C–1000 °C are presented. It was found that the calcination of the samples at high temperatures leads to dramatic modification of EPR spectrum and appearance of new Cu-related signal. Computer simulations of both X- and Q-band spectra allowed to determine the following parameters of the copper centers: S = 1/2, I = 3/2, gx = 2.021, gy = 2.026, gz = 2.168, Axx ∼ 40 × 10−4 cm−4, Ayy ∼ 35 × 10−4 cm−4, Azz ∼ 186 × 10−4 cm−4, Axz ∼ 10 × 10−4 cm−4, Ayz ∼ 5 × 10−4 cm−4 for the 63Cu isotope. The analysis of the experimental data allows proposing center model as CuZr2+ in the monoclinic ZrO2 lattice. The comparison with photoluminescent data allows assumption that EPR center investigated participate in the light emission process.

Direct-methane solid oxide fuel cells with an in situ formed Ni–Fe alloy composite catalyst layer over Ni–YSZ anodes

Coking on Ni surfaces limits the direct application of methane-based fuels in SOFCs with Ni-cermet anodes. Loading an anodic catalytic layer with a high catalytic activity for CH4 conversion can effectively protect the Ni-based anode from coking and increase the cell durability. In this work, a Ni–Fe alloy composite catalyst was prepared by reducing perovskite La0.7Sr0.3Fe0.8Ni0.2O3-δ (LSFN) and then evaluating its catalytic activity in the partial oxidation of CH4. The catalyst was applied on a conventional Ni–8 mol.% Y-stabilized ZrO2 (YSZ) anode for methane SOFCs using two methane-containing fuels (97% CH4–3% H2O and 30% CH4–70% air). The catalyst-modified cells showed much higher performances and durability than the conventional cell using a Ni–YSZ anode, indicating the potential application for direct-methane SOFCs.

Long-term electrocatalytic N2 fixation by MOF-derived Y-stabilized ZrO2: insight into the deactivation mechanism

A MOF-derived carbon/Y-stabilized ZrO2 nanocomposite (C@YSZ) works as an efficient and long-term electrocatalyst for N2 fixation.

Effective Ce-based catalysts deposition on ceramic open cell foams

In this work, ceramic powder dispersion was achieved by using a glycerol/water solution as dispersion medium and polyvinyl alcohol as binder and rheology modifier. Five different powders were dispersed and they were deposited onto open cell ceramic foams: the bare carrier (i.e. cerium oxide) and four catalytically active powders (i.e. nickel, cobalt, iron and copper oxides supported on ceria). Catalysts powders have been extensively characterized and were demonstrated to be suitable samples candidates of model powders for the deposition onto open-cell foams. Rheological properties were found to be related to the dispersed powder, thus directly affecting washcoat load. Multiple depositions were performed by means of dip-coating on 20, 30 and 40 PPI Y-stabilized ZrO2 open cell foams. Depending on formulation and foam pore density, coating loads in the 6–30 % wt. range were obtained. Washcoat layers were found to be quite homogeneous and well adherent therefore suitable for catalytic applications, such as for instance the methane steam reforming synthesis for Ni-based catalysts.