YSZ Thin Films Research Articles

Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

AbstractThe crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr0.85Y0.15O1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.

High power density thin film SOFCs with YSZ/GDC bilayer electrolyte

Bilayer electrolytes composed of a gadolinium-doped CeO 2 (GDC) layer (∼6 μm thickness) and an yttria-stabilized ZrO 2 (YSZ) layer with various thicknesses (∼330 nm, ∼440 nm, and ∼1 μm) were deposited by a pulsed laser deposition (PLD) technique for thin film solid oxide fuel cells (TFSOFCs). The bilayer electrolytes were prepared between a NiO–YSZ (60:40 wt.% with 7.5 wt.% carbon) anode and La 0.5Sr 0.5CoO 3–Ce 0.9Gd 0.1O 1.95 (50:50 wt.%) composite cathode for anode-supported single cells. Significantly enhanced maximum power density was achieved, i.e., a maximum power density of 188, 430, and 587 mW cm −2 was measured in a bilayer electrolyte single cell with ∼330 nm thin YSZ at 650, 700, and 750 °C, respectively. The cell with the bilayer electrolyte (YSZ ∼330 nm) doubles the overall power output at 750 °C compared to that achieved in the GDC single layer cell. This signifies that the YSZ thin film serves as a blocking layer for preventing electrical current leakage in the GDC layer and also provides chemical, mechanical, and structural integrity in the cell, which leads to the overall enhanced performance.

Effects of the oxygen pressure on the crystalline orientation and strains of YSZ thin films prepared by E-beam PVD

Yttria−stabilized zirconia, YSZ, thin films were prepared by E-beam physical vapor deposition (PVD) at 200 °C under oxygen pressure of 1 × 10 −3∼1 × 10 −5 Torr. Observations by Field Emission Scanning Electron Microscope (FESEM) proved that different oxygen pressures influenced the thickness of interfacial SiO x layer formed between the YSZ thin films and Si(100)-substrate. X-ray diffraction (XRD) patterns were used to determine the crystalline structure and calculate the surface grain size of deposited YSZ thin films. XRD patterns also showed that the peaks corresponding to planes (111), (200), (220), and (311) were found and the YSZ thin films revealed the fluorite structure. At lower oxygen pressure (1 × 10 −5∼1 × 10 −4 Torr) YSZ thin films revealed the (111) preferred orientation and at higher oxygen pressure (5 × 10 −4∼1 × 10 −3 Torr) YSZ thin films revealed the (200) preferred orientation. The effects of oxygen pressure on the lattice constants and the internal strains of YSZ thin films were also investigated.

Influence of initial powder particle size on yttrium stabilized zirconium thin films formed by e-beam technique

Yttrium stabilized zirconium (YSZ – (Y 2O 3) 0,08(ZrO 2) 0,92) thin films were formed by e-beam evaporation technique. The powders of different surface areas were used to prepare thin ceramic films. The influence of the initial powder material on formed thin films electrical and microstructure properties was studied. YSZ thin films were deposited on two types of substrates: optical quartz (SiO 2) and Alloy 600 (Fe–Ni–Cr) substrates. Deposition rate was changed from 2 to 16 Å/s to test its influence on thin film formation and properties. Electrical parameters of YSZ thin ceramics were investigated in the frequency range from 0.1 to 1.0 MHz in temperature range from 473 to 873 K. The conductivity of the electrolyte YSZ-8 thin films was determined by impedance spectroscopy. The formed YSZ films repeat the crystallographic orientation of the initial powder material and do not depend on the powder grain size. It was determined that the deposition rate has influence on the crystallite size. Initial powder grain size and deposition rate influence the relative density of formed YSZ thin films. The ionic conductivity of formed films depends on the grain size of initial powder, also.

Suppression of Ni agglomeration in PLD fabricated Ni-YSZ composite for surface modification of SOFC anode

The suppression of Ni agglomeration in Ni-yttria stabilized zirconia (Ni-YSZ) nano-composite thin films deposited by pulsed laser deposition (PLD) has been investigated by varying post-annealing temperatures at a range of 800–1200 °C. Grain growth to a certain extent appears to be necessary to obtain a stable Ni-YSZ composite microstructure by suppressing massive Ni agglomeration. The microstructurally stable and uniform nano-porous Ni-YSZ thin film was obtained by 1200 °C post-annealing and reduction of the NiO–YSZ thin film, and it was applied as the surface modification layer of the bulk anode support used in conventional solid oxide fuel cells (SOFCs). By this approach, we were able to successfully realize a thin film electrolyte SOFC exhibiting the open cell voltage (OCV) higher than 1 V with a 1-μm thick film electrolyte on the porous anode support.

Effect of Pulsed Electrical Field on Deposition of YSZ Thin Films in an Aqueous Solution

In this study we investigated YSZ thin film deposition from an aqueous solution by applying constant electrical field and the effect of a pulsed electrical field upon the growth condition of films. The precursor was an aqueous solution of Zr(NO2)3-2H2O, Y(NO3)3-6H2O, and 0.5 vol% NH3(aq). The thin film was deposited on the minus electrode side of the glass substrate, which was placed above the minus electrode with a gap distance of 48–530 μm. By applying the electrical field, the thin film was effectively deposited on glass substrates under an applied voltage of 2.5 V for 300 s at room temperature. The as-deposited film was amorphous, and a crystalline phase with a transparent and smooth surface can be obtained after annealing at 773 K for 3 h in air. When a pulse bias is applied to the electrical field, the film thickness, surface defects, and roughness were changed with the frequency. At 2 Hz, the film was fabricated effectively, and a thick film was obtained, but films with smoother and fewer defect surfaces were obtained in a range of about 10–100 Hz.

Electrolyte and Cathode Studies for Micro-Solid Oxide Fuel Cell

AbstractSputter deposited YSZ thin films were studied for the application as electrolyte membrane in micro solid oxide fuel cells. A new micro-machined test structure was developed to test 200 μm fuel cell membranes that are integrated onto silicon substrates. The membranes are liberated by means of deep silicon dry etching from the backside, and both contacts are situated on the front side of the wafer. Annular Pt electrodes provide contacts to specific anode and cathode layers. Preliminary tests are reported for the situation without specific electrodes. At 450 °C, an OCV of 570 mV, and a maximal power of about 0.55 mW/cm2 is obtained. LaSrMnO electrodes deposited by pulsed laser deposition were evaluated by electrical impedance spectroscopy. The dense films yielded too high ASR values. Interestingly, these could be reduced by applying a DC bias voltage.

Metal-Supported SOFC Development at JPOWER

JPOWER has been developing the low-cost and high-reliability tubular type metal-supported SOFC in collaboration with LBNL (Lawrence Berkeley National Laboratory, USA). In our collaboration, JPOWER is mainly focusing on stacking the metal-supported thin-film YSZ SOFC cells developed by LBNL, while LBNL is focusing on developing the cells. The cells are composed of 5-layer in which the cell elements (Anode/Electrolyte/Cathode) are sandwiched between porous stainless steel current collectors, so it is easy to stack and/or bundle the cells by the existing technology of metal joining. JPOWER designed the segment-in-series type simplified cell stacking structure and verified the proof-of-concept by testing a single cell and 2-cells stacking structure. In this paper, JPOWER's activities for metal-supported SOFC development are discussed.

Synthesis of a Thin-Layered Ionic Conductor, CeO2−Y2O3, by Atomic Layer Deposition in View of Solid Oxide Fuel Cell Applications

Thin films of yttria-doped ceria (YDC) electrolyte were processed for the first time by atomic layer deposition (ALD) for solid oxide fuel cell (SOFC) applications. The β-diketonate complexes Y(thd)3 and Ce(thd)4 were used as yttrium and cerium precursors, respectively, whereas ozone was used as the oxygen source. Four-hundred-nanometer thick YDC films were deposited at 300 °C on soda lime, Si(100), stainless steel (430), La0.8Sr0.2FeO3 (LSF) cathodes, and on Ni-YSZ anodes. Thickness, crystallinity, and morphology of YDC films were determined by UV−vis spectroscopy, XRD, and SEM/EDX, respectively. Electrical properties of the thin films in the range of 250−475 °C were characterized by impedance spectroscopy. The deposited layers were crystalline as deposited, without any need for annealing. SEM observations showed that the films were homogeneous and well-covering. Their conductivities were significantly lower than those reported in the literature for bulk YDC. Nevertheless, electrical properties of YDC thin films seem to be superior to those of the YSZ thin films, characterized under the same conditions, in the temperature range of 500−750 °C. Thus, YDC by ALD represents a new and promising SOFC generation.

Shape forming of ceramics with controllable microstructure by drying-free colloidal casting

A new drying-free colloidal casting method was developed for shape forming of ceramics with controllable microstructure. A polymerizable solvent such as furfuryl alcohol (FA) was employed to disperse ceramic particles to form a colloidal ceramic suspension with good fluidity. After casting, the solvent was polymerized into a polymer, resulting in solidification of the colloidal suspension. For the purpose of demonstrating this casting method, three types of ceramic powders (NiO/YSZ, Al2O3 and YSZ) were used to fabricate a NiO/YSZ ceramic tube, an Al2O3 ceramic part with a hexagonal screw nut shape, and a YSZ thin film. Our study shows that the microstructure (pore size and porosity) of ceramics can be readily tuned by adjusting the amount of FA, and the drying-free casting method offers better controllability over the shape forming of a wide variety of ceramic materials.

氧分压和沉积速率对YSZ薄膜残余应力的影响

氧分压和沉积速率对YSZ薄膜残余应力的影响

Thin film YSZ coatings on functionally graded freeze cast NiO/YSZ SOFC anode supports

Intermediate temperature (600–800 °C) solid oxide fuel cell (SOFC) technology is often limited by inadequate gas transport in electrodes, and high ion transport resistance electrolytes. In this study, large area filtered arc deposition (LAFAD) and hybrid filtered arc-assisted e-beam physical vapor deposition (FA-EBPVD) technologies, in combination with freeze-tape-casting, were used to fabricate SOFC anode/electrolyte bi-layers with functionally graded porous anode microstructures and thin film electrolytes favorable for both gas transport and low resistance. Traditionally-processed NiO/YSZ in addition to freeze-tape-cast NiO/YSZ anode substrates were fabricated and subsequently coated with thin film (<1–20 μm) YSZ via LAFAD and FA-EBPVD. LAFAD was found to be effective in applying thin (~1 μm) dense YSZ films on porous substrates at ~400 °C. FA-EBPVD produced relatively thick (~10–20 μm) dense YSZ coatings on porous substrates, with columnar morphology and nano-metrical grain size. A ~10 μm FA-EBPVD YSZ coating was observed to bridge NiO/YSZ surface pores of ~10 μm, which typically requires pre-filling prior to conventional thin film coating processes. Coated substrates exhibited negligible curvature, yielding flat anode/electrode bi-layers up to 2.5 cm in diameter. These results are presented with conderations for future SOFC development discussed.

Substrate effect on the electrical properties of sputtered YSZ thin films for co-planar SOFC applications

The physical and electrical properties of sputtered YSZ thin films on various substrates were investigated. The in-plane electrical properties of the films were measured for evaluating YSZ thin film for co-planar SOFC electrolytes. The conductance measured on YSZ over Si substrates was significantly affected by the buffer layer thickness and exhibited higher values than that of YSZ on sapphire. This indicates that electrical leakage occurred through the substrate when Si substrates were utilized. Nevertheless, pure ionic conduction was observed in YSZ/sapphire regardless of the film thickness. It implies that much care should be taken for the selection of substrate materials in measuring or utilizing in-plane conductivity, especially for high temperature applications.

Y 2O 3 stabilized ZrO 2 thin films deposited by electron-beam evaporation: Optical properties, structure and residual stresses

This paper describes the preparation and the characterization of Y 2O 3 stabilized ZrO 2 thin films produced by electric-beam evaporation method. The optical properties, microstructure, surface morphology and the residual stress of the deposited films were investigated by optical spectroscopy, X-ray diffraction (XRD), scanning probe microscope and optical interferometer. It is shown that the optical transmission spectra of all the YSZ thin films are similar with those of ZrO 2 thin film, possessing high transparency in the visible and near-infrared regions. The refractive index of the samples decreases with increasing of Y 2O 3 content. The crystalline structure of pure ZrO 2 films is a mixture of tetragonal phase and monoclinic phase, however, Y 2O 3 stabilized ZrO 2 thin films only exhibit the cubic phase independently of how much the added Y 2O 3 content is. The surface morphology spectrum indicates that all thin films present a crystalline columnar texture with columnar grains perpendicular to the substrate and with a predominantly open microporosity. The residual stress of films transforms tensile from compressive with the increasing of Y 2O 3 molar content, which corresponds to the evolutions of the structure and packing densities.

Preparation of YSZ thin films for intermediate temperature solid oxide fuel cells by dip-coating method

By studying the effects of different solvents, dispersants and solid loading amount on the suspension stability from sedimentation and viscosity experiments, a simple, effective and highly stable YSZ particle suspension based on MEK/EtOH was developed for dip-coating anode supported electrolyte films for intermediate temperature solid oxide fuel cells (IT-SOFCs). The morphologies of the prepared YSZ thin films from different dip-coating times were studied by scanning electron microscopy (SEM), and a film with a thickness of 16 μm by twice dip-coating was determined to be homogeneous, crack-free and well adherent to the anode substrate. The single cell assembled with this film presents an open-circuit voltage (OCV) of 1.01 V, and a maximum power density of 262 mW cm −2 under H 2 as the fuel at 800 °C.

Preparation of Ni–YSZ thin and thick films on metallic interconnects as cell supports. Applications as anode for SOFC

In this work, we propose the preparation of a duplex anodic layer composed of both a thin (100 nm) and a thick film (10 μm) with Ni–YSZ material. The support of this anode is a metallic substrate, which is the interconnect of the SOFC unit cell. The metallic support limits the temperature of thermal treatment at 800 °C to keep a good interconnect mechanical behaviour and to reduce corrosion. We have chosen to elaborate anodic coatings by sol–gel route coupled with dip-coating process, which are low cost techniques and allow working with moderate temperatures. Thin films are obtained by dipping interconnect substrate into a sol, and thick films into an optimized slurry. After thermal treatment at only 800 °C, anodic coatings are adherent and homogeneous. Thin films have compact microstructures that confer ceramic protective barrier on metal surface. Further coatings of 10 μm thick are porous and constitute the active anodic material.

Oxide ion conduction of yttria-stabilized zirconia films deposited by atomic layer deposition

AbstractYttria-stabilized zirconia (YSZ) is one of the most common electrolytes in high temperature solid oxide fuel cell (SOFCs). We utilize atomic layer deposition (ALD) to fabricate the electrolyte of SOFC, which may potentially improve several fundamental characteristics of SOFC. Recently, our group demonstrated that ultra-thin ALD YSZ SOFSs can deliver high power density at low temperatures [1]. These SOFCs demonstrated not only reduction of Ohmic loss, but also enhancement of surface kinetics.The focus of this work is to investigate the surface and bulk conduction characteristics of YSZ films produced by ALD. In plane conductivity was measured as a function of film thickness and temperature dependence. YSZ thin films were deposited on standard 4 ” quartz substrates with thicknesses ranging from 8 nanometers to 55 nanometers. Micro-electrodes were patterned on top of the ALD YSZ layer by standard photolithography process. The impedances of the YSZ thin films with different thicknesses were measured. We have observed higher conductivities for thinner films which were attributed to higher oxide ion conductivity in the vicinity of the surface, and similar phenomenon was observed with YSZ films produced by electron beam evaporation [2].

Microstructures of CGO and YSZ Thin Films by Pulsed Laser Deposition

AbstractYttrium‐stabilized zirconia (YSZ) and cerium gadolinium oxide (CGO) thin films were prepared by pulsed laser deposition (PLD) under different ambient pressures and substrate temperatures. The microstructures of the obtained films are compared with existing structural zone models for thin‐film growth. The model developed by Thornton (1974) for metallic sputtered films is applied to the metal oxide films grown by PLD and gives a good description for the growth process and the dependence of the microstructure on deposition temperature and pressure. A map of formed microstructures is compiled as a function of background pressure and substrate temperature, and the conditions to obtain dense or porous films for YSZ and CGO are established. The two materials show the same dependence on temperature and pressure. Higher background pressure yields more porous films and a fully dense structure cannot be achieved for substrate temperatures in the range from room temperature to 800 °C. Of special interest for us was the realization and characterization of dense films processed at low temperature (&lt; 500 °C) for the preparation of free‐standing membranes to be used in a micro solid oxide fuel cell. We could achieve such films by processing at 400 °C in 0.026 mbar of oxygen. In‐plane electric conductivities of the films were measured and correlated with the microstructures.

박막공정의 융합화를 통한 초소형 고체산화물 연료전지의 제작: I. Spray Pyrolysis법으로 증착된 Ni 기반 음극과 스퍼터링으로 증착된 YSZ 전해질의 다층구조

Physical properties of sputtered YSZ thin film electrolytes on anode thin film by spray pyrolisis has been investigated to realize the porous electrode and dense electrolyte multilayer structure for micro solid oxide fuel cells. It is shown that for better crystallinity and density, YSZ need to be deposited at an elevated temperature. However, if pure NiO anode was used for high temperature deposition, massive defects such as spalling and delamination were induced due to high thermal expansion mismatch. By changing anode to NiO-CGO composite, defects were significantly reduced even at high deposition temperature. Further research on realization of full cells by processing hybridization and cell performance characterization will be performed in near future.

Fabrication of YSZ thin film by electro-chemical deposition in an aqueous solution

Fabrication of YSZ thin film by electro-chemical deposition in an aqueous solution