Yttria-stabilized Hafnia Research Articles

First Demonstration of Yttria-Stabilized Hafnia-Based Long-Retention Solid-State Electrolyte-Gated Transistor for Human-Like Neuromorphic Computing.

Electrolyte-gated transistors have strong potential for high-performance artificial synapses in neuromorphic bio-interfaces owing to their outstanding synaptic characteristics, low power consumption, and human-like mechanisms. However, the short retention time is a hurdle to overcome owing to the natural diffusion of protons. Here, a novel modulation technique of ionic conductivity is proposed with yttria-stabilized hafnia for the first time to enhance the retention characteristic of a solid-state electrolyte-gated transistor-based artificial synapse. With the optimization of the ionic conductivity in yttria-stabilized hafnia, a high retention time of over 300s and remarkable synaptic characteristics are accomplished by regulating channel conductance with precise modulation of the strength of the proton-electron coupling intensity along the input signals. Furthermore, pattern recognition simulation is conducted based on the measured synaptic characteristics, exhibiting 94.41% of operation accuracy, which implies a promising solution for neuromorphic in-memory computing systems with a high operation accuracy and low power consumption.

Effect of yttria on thermal transport and vibrational modes in yttria-stabilized hafnia

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y2O3 and HfO2. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y2O3, and then reaches a limiting value (∼2.1 W m−1K−1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y2O3 exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y2O3. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory.

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf0.84Y0.16O1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m-1 K-1 at as-sprayed state to 1.6 W m-1 K-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.

Single crystal growth and luminescent properties of YSH:Eu scintillator by optical floating zone method

Single crystal of europium doped yttria stabilized hafnia (YSH) was grown by optical floating zone method. The structure of as grown sample was determined through X-ray diffraction. Its transmission spectrum was recorded. Its photoluminescence (PL) and scintillation properties, including PL, PL decay, scintillation luminescence efficiency and afterglow level, were measured and compared with another heavy scintillator Lu2O3:Eu. The reasons for the PL and scintillation difference between YSH:Eu and Lu2O3:Eu were discussed in this paper.

Mechanisms of Current Transport and Resistive Switching in Capacitors with Yttria-Stabilized Hafnia Layers

The peculiarities of resistive switching in capacitors with yttria-stabilized hafnia layers were studied. The characteristics of current transport in the initial state and after electroforming and resistive switching at different temperatures were examined. The parameters of a small-signal equivalent circuit of a capacitor were determined for switching into low- and high-resistance states. These parameters suggest that the resistance of filaments changes after each successive switching. This provides an opportunity to use such measurements to determine the nature of resistive switching and verify the reproducibility of its parameters. The contribution of electron traps to switching was revealed. Ion migration polarization was observed at temperatures above 500 K, and the activation energy of ion migration and the ion concentration were determined. The effect of resistive switching under the influence of temperature was observed and interpreted for the first time.

Механизмы токопереноса и резистивного переключения в конденсаторах со слоями стабилизированного иттрием диоксида гафния

AbstractThe peculiarities of resistive switching in capacitors with yttria-stabilized hafnia layers were studied. The characteristics of current transport in the initial state and after electroforming and resistive switching at different temperatures were examined. The parameters of a small-signal equivalent circuit of a capacitor were determined for switching into low- and high-resistance states. These parameters suggest that the resistance of filaments changes after each successive switching. This provides an opportunity to use such measurements to determine the nature of resistive switching and verify the reproducibility of its parameters. The contribution of electron traps to switching was revealed. Ion migration polarization was observed at temperatures above 500 K, and the activation energy of ion migration and the ion concentration were determined. The effect of resistive switching under the influence of temperature was observed and interpreted for the first time.

Nitrate fusion synthesis and two-step sintering of nanocrystalline yttria stabilized hafnia powders

Bulk quantities of nanocrystalline yttria stabilized hafnia (YSH) powders with crystallite size ranging from 8 to 15nm were successfully prepared for the first time through nitrate fusion synthesis at a temperature as low as 673K. The yttrium content was varied from 6 to 30mol%. The dependence of the properties of the final product on the quantity of the dopant was investigated. Microstructural investigations were carried out with scanning electron microscopy and transmission electron microscopy. A maximum relative sintered density of 98.2 ± 0.3% T.D (theoretical density) was obtained for YSH containing 10mol% yttrium by using “two-step sintering” at a final temperature of 1773K. Anisotropic shrinkage factor (0.70–0.95) was found to vary linearly with the compaction pressure. SEM investigations reaffirmed that the sintered pellets comprised uniform distribution of faceted grains and elemental mapping revealed that yttrium is distributed uniformly in these sintered YSH monoliths.

Physical properties of metal–insulator–semiconductor structures based on n-GaAs with InAs quantum dots deposited onto the surface of an n-GaAs layer

The properties of metal–insulator–semiconductor (MIS) structures based on n-GaAs in which silicon oxide and yttria-stabilized zirconia and hafnia are used as the insulator containing InAs quantum dots, which are embedded at the insulator/n-GaAs interface, are investigated. The structures manifest the resistive switching and synaptic behavior.

Enhanced stability of hafnia based coatings in a hot gas environment

Yttria stabilized hafnia (YSH) coatings have been fabricated by magnetron sputtering, a physical vapor deposition (PVD) method. Coatings of a mixed composition of hafnia (HfO2) and zirconia (ZrO2) (YSHZ) stabilized by yttria (Y2O3) were also fabricated in order to compare and contrast the resulting properties. The composition of the material was varied by varying the ratio of HfO2 and ZrO2 (4:1, 2:1, 1:1, 1:2 and 1:4) while keeping the Y2O3 stabilizer content constant at 7.5 mol%. Thermal and chemical stability along with the durability of the YSH and YSHZ coatings were evaluated by exposing the coatings to hot gases in a combustor rig. A nanoindentation technique was used to evaluate the mechanical properties. A diamond tipped, sharp nanoindenter with known geometry and mechanical properties was forced into the sample while both the force and indentation depth were recorded. Hardness (H) and Young's modulus were estimated for the YSH and YSHZ coatings. Atomic force microscopy was utilized to provide imagery of the indentation area on the sample surface. Residual stress analysis was performed using X-ray diffraction (XRD). The results from nano-indentation indicate that the YSH sample possesses values of hardness and Young's modulus as high as 18 GPa and 220 GPa, respectively. The residual stress estimated using XRD indicates very high compressive stress within the coatings. The stability and durability tests demonstrate the enhanced stability of YSH coatings in a hot gas environment created by burning natural gas with oxygen.

Microstructure and thermal oxidation behavior of yttria-stabilized hafnia nanostructured coatings deposited on alumina

Nanostructured yttria-stabilized hafnia (YSH) coatings were grown on α-Al2O3 substrates with variable coating thickness in a wide range of ~50nm to 1μm. Microstructure and thermal oxidation behavior of the grown YSH coatings were studied employing X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and isothermal furnace oxidation testing. The effect of coating thickness on the crystal structure, surface/interface morphology and thermal oxidation was investigated. X-ray diffraction analyses revealed the formation of monoclinic phase for relatively thin coatings (<100nm) indicating that the interfacial phenomena play a dominant role in phase stabilization. The evolution towards stabilized cubic phase with increasing coating thickness is observed. The SEM results indicate the dense, columnar structure of YSH coatings as a function of thickness. Thermal oxidation measurements indicate the enhanced high-temperature oxidation resistance of cubic YSH coatings.

Structure and Thermal Conductivity of Nanostructured Hafnia-Based Thermal Barrier Coating Grown on SS-403

Yttria-stabilized hafnia (YSH) coatings were grown onto stainless steel 403 (SS-403) and Si substrates. The deposition was made at various growth temperatures ranging from room temperature (RT) to 500 °C. The microstructure and thermal properties of the YSH coatings were evaluated employing grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and photoacoustic measurements. GIXRD studies indicate that the coatings crystalize in cubic structure with a (111) texturing. Well-grown triangular dense morphology was evident in SEM data. EDS analysis indicates the composition stability of YSH coatings. The grain size increases with the increasing growth temperature. Thermal conductivity measurements indicate lower thermal conductivity of YSH coatings compared to either pure hafnia or yttria-stabilized zirconia.

Theoretical prediction of ion conductivity in solid state HfO2

A theoretical prediction of ion conductivity for solid state HfO2 is carried out in analogy to ZrO2 based on the density functional calculation. Geometric and electronic structures of pure bulks exhibit similarity for the two materials. Negative formation enthalpy and negative vacancy formation energy are found for YSH (yttria-stabilized hafnia) and YSZ (yttria-stabilized zirconia), suggesting the stability of both materials. Low activation energies (below 0.7 eV) of diffusion are found in both materials, and YSH's is a little higher than that of YSZ. In addition, for both HfO2 and ZrO2, the supercells with native oxygen vacancies are also studied. The so-called defect states are observed in the supercells with neutral and +1 charge native vacancy but not in the +2 charge one. It can give an explanation to the relatively lower activation energies of yttria-doped oxides and +2 charge vacancy supercells. A brief discussion is presented to explain the different YSH ion conductivities in the experiment and obtained by us, and we attribute this to the different ion vibrations at different temperatures.

Yttria-stabilized hafnia: Thermochemistry of formation and hydration of nanoparticles

Abstract

Growth, Structure, and Thermal Conductivity of Yttria-Stabilized Hafnia Thin Films

Yttria-stabilized hafnia (YSH) films of 90 nm thickness have been produced using sputter-deposition by varying the growth temperature (T(s)) from room-temperature (RT) to 400 °C. The effect of T(s) on the structure, morphology, and thermal conductivity of YSH films has been investigated. Structural studies indicate that YSH films crystallize in the cubic phase. The lattice constant decreases from 5.15 to 5.10 Å with increasing T(s). The average grain size (L) increases with increasing T(s); L-T(s) relationship indicates the thermally activated process of the crystallization of YSH films. The analyses indicate a critical temperature to promote nanocrystalline, cubic YSH films is 300 °C, which is higher compare to that of pure monoclinic HfO(2) films. Compared to pure nanocrystalline hafnia, the addition of yttria lowers the effective thermal conductivity. The effect of grain size on thermal conductivity is also explored.

Structure and thermal conductivity of yttria-stabilized hafnia ceramic coatings grown on nickel-based alloy

Yttria-stabilized hafnia (YSH) coatings were produced by sputter-deposition onto nickel (Ni) based super alloy substrates. The deposition was made by varying the growth temperature from room-temperature (RT) to 500°C. The microstructure and thermal properties of the YSH coatings were evaluated employing grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and phot-acoustic measurements. GIXRD studies indicate that the coatings grown at RT are amorphous while those grown at 300°C and higher are crystalline. The crystalline YSH coatings crystallize in cubic hafnia phase and exhibit slightly enhanced lattice parameter compared to pure hafnia. The average grain size increases with increasing growth temperature. Thermal measurements indicate an effective reduction in thermal conductivity of YSH coatings compared to pure hafnia.

High-Temperature Radiative Properties of an Yttria-Stabilized Hafnia Ceramic

The normal spectral emittance of a partially yttria-stabilized hafnia (YSH) ceramic has been measured by infrared emittance spectroscopy in a spectral range going from 60 to 17 500 cm−1 (0.075–2.2 eV), and up to 2350 K. Using these experimental data, the temperature dependence of the absorption coefficient and the total emittance have been calculated. The physical mechanisms explaining the spectral features of the whole set of spectra are exposed qualitatively. The spectral weight of each contribution is then discussed for interpreting the influence of temperature on the total normal emittance in the range of temperatures (1200–2500 K) where YSH ceramics can be used for their refractory properties (thermal barrier coating, nuclear reactor, thermal protection systems, etc.).

Thermal conductivity of yttria-stabilized zirconia–hafnia solid solutions

The thermal conductivity of the yttria-stabilized zirconia–hafnia solid solution series as a function of temperature is reported. For all the compositions studied, the thermal conductivity of the fully dense materials is almost temperature independent up to 1000 °C, the limit of our measurements. A strong effect of solid solution alloying is reported with the mixed zirconia–hafnia compositions having lower thermal conductivity than either yttria-stabilized zirconia or yttria-stabilized hafnia end members of the solid solution series. The compositions with the lowest thermal conductivity are approximately equimolar zirconia–hafnia, and these have a thermal conductivity approximately 25% lower than that of the standard 8YSZ composition currently used in thermal barrier coatings. Although solid solutions generally have lower thermal conductivity at low temperatures, where the conductivity is temperature dependent, this is the first report of solid solution alloying lowering the value of the minimum thermal conductivity, the high-temperature limit of thermal conductivity. The effect is attributed to mass disorder on the cation sublattice of zirconia in which Zr 4+ ions are replaced and mixed with Hf 4+ ions, decreasing the phonon mean free path. This is in addition to the disorder in the anion sublattice created by the vacancies introduced by the Y 3+ stabilizer.

Enthalpy of formation of cubic yttria-stabilized hafnia

The enthalpy of formation of cubic yttria-stabilized hafnia from monoclinic hafnia and C-type yttria was measured by oxide melt solution calorimetry. The enthalpies of formation fit a function independent of temperature and quadratic in composition. The enthalpies of transition from m-HfO2 and C-type YO1.5, to the cubic fluorite phase are 32.5 ± 1.7 kJ/mol and 38.0 ± 13.4 kJ/mol, respectively. The interaction parameter in the fluorite phase is strongly negative, -155.2 ± 10.2 kJ/mol, suggesting even stronger short range order than in ZrO2–YO1.5. Regular solution theory or any other model assuming random mixing on the cation and /or anion sublattice is not physically reasonable. A more complex solution model should be developed to be consistent with the new calorimetric data and observed phase relations.

New sterically hindered Hf, Zr and Y β-diketonates as MOCVD precursors for oxide films

New Zr, Hf and Y 2,7,7-trimethyl-3,5-octanedionates (tod) as well as the corresponding Cu β-diketonate have been synthesized and characterized by FT-IR, elemental analyses, TGA, and 1H NMR or ESR. The molecular structure of Hf(tod)4 has been determined by single-crystal X-ray diffraction, the metal displays a square antiprismatic geometry. Zr, Hf and Y β-diketonates were investigated as precursors for MOCVD deposition of ZrO2, HfO2 and yttria-stabilized hafnia thin films. The films were characterized by XRD, XPS, EDS and AFM. Highly textured and in-plane oriented films were deposited on sapphire by pulsed liquid injection MOCVD. Zr(tod)4 and Hf(tod)4 precursors lead to higher growth rates of ZrO2 and HfO2 films at lower temperatures than conventional Zr(thd)4 and Hf(thd)4 precursors (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) and are attractive new precursors for oxide films.

High-temperature infrared reflectivity of yttria-stabilized hafnia single crystals

Infrared reflectivity spectra of cubic yttria-stabilized hafnia were measured in the frequency range 30–5000 cm −1 at temperatures from 300 to 1100 K. A heavily damped oscillator has been observed in the far infrared, its parameters being essentially constant in the whole temperature range. This term is attributed to vibrations of clusters of defects, in accord with previous studies on other fluorite-type ionic conductors.