G.Y. Meng

Deposition and characterization of YSZ thin films by aerosol-assisted CVD

Thin films of ionic-conducting electrolyte yttria-stabilized zirconia (YSZ) have been prepared on Si(111) substrates by aerosol-assisted chemical vapor deposition (AACVD), using the β-diketonate precursors zirconium acetylacetonate, Zr(acac)4, and yttrium acetylacetonate, Y(acac)3, in the substrate temperature range of 550–700°C. YSZ thin films were homogeneous and transparent with a cubic fluorite structure. The compositions of the thin films are very close to those of mixed solution precursors at deposited temperatures. The electrical property investigation demonstrated that the ionic conductivity in the surface layer of the as-deposited film is much higher, and its activation energy is slightly less than that of the YSZ single crystal material.

Aerosol-assisted MOCVD growth of Gd2O3-doped CeO2 thin SOFC electrolyte film on anode substrate

Abstract Aerosol-assisted metal-organic chemical vapor deposition (AA-MOCVD) with β-diketonate precursors was employed in depositing thin films of solid oxide fuel cells (SOFCs) electrolyte material, gadolinia-doped ceria (GDC), on anode (NiO+YSZ) substrates. The films have been successfully synthesized in a wide temperature range (300–600 °C) and characterized by X-ray powder diffraction, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The sheet resistance measurements were carried out between 350 and 850 °C to study the effects of adsorbed oxygen on film ionic conductivity.

Aerosol-assisted MOCVD deposition of YDC thin films on (NiO + YDC) substrates

Abstract Aerosol-assisted metallo-organic CVD (MOCVD) method (spray pyrolysis) has been employed to deposit thin films of solid electrolyte onto dense (NiO + YDC) substrates in our laboratory. The β-diketonate precursors Ce(tmhd) 4 and Y(tmhd) 3 were chosen as the source materials for deposition of yttria-doped ceria (YDC) thin films in the temperature range 500–700°C. Scanning electron microscopy (SEM) observation revealed the YDC films to have uniform and nanometric grains, with thickness ranging from 0.18 to 1.2 μm with different deposition times. X-ray diffraction (XRD) analyses showed that the films possessed a single phase with a fluorite cubic structure. X-ray photoelectron spectroscopy (XPS) showed that the elemental ratio Y/Ce of the film was close to that of the mixed solution precursor at a deposition temperature 600°C. After being reduced in an H 2 atmosphere at 600°C for 10 h, the (NiO + YDC) substrate was converted into Ni + YDC. The YDC thin film was found to be N 2 leak tight up to the pressure of 0.65 MPa. AC impedance analyses showed that the ionic conductivity of YDC thin film on (Ni + YDC) substrate was slightly less than that of YDC prepared by sintering, but higher than that of yttria-stabilized zirconia (YSZ). These results suggest that the YDC thin film obtained by aerosol-assisted MOCVD is a potential solid electrolyte alternative to YSZ, at intermediate operating temperatures, for solid oxide fuel cell (SOFC) applications.

Palladium membranes supported on porous ceramics prepared by chemical vapor deposition

Abstract Palladium films on porous alumina supports were prepared by the chemical vapor deposition (CVD) technique of reducing PdCl2 with hydrogen. The phase composition and morphology of Pd composite membranes were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning tunnelling microscopy (STM). A well-crystallized palladium layer was deposited on porous α-alumina supports. The thickness and grain size of the palladium layer were estimated to be about 4 μm. The palladium grains consisted of columnar crystallites of 20–40 nm in length and 5–8 nm in width. The penetration depth of palladium into the α-alumina support was as large as 20 μm. An opposite-diffusion CVD technique is suggested for preparing a dense, thin palladium membrane with high hydrogen permeability and good mechanical stability.

Progress in ion-transport inorganic membranes by novel chemical vapor deposition (CVD) techniques

Abstract Novel metal–organic chemical vapor deposition (MOCVD) techniques using metal β-diketonate chelates as volatile precursors and some technical modifications, have been developed in the last decade in order to prepare thin films or layers of functional materials, particularly the multi-component ion-transport inorganic membranes. This article gives an overview on the progress in these aspects, mainly based on the research activities at the authors laboratory.

Doped ceria-chloride composite electrolyte for intermediate temperature ceramic membrane fuel cells

A kind of oxide-salt composite electrolyte, gadolinium-doped ceria (GDC)-LiCl-SrCl2, prepared with hot-press technique, shows superior ionic conductivity, which is 2-10 times higher than that of GDC itself at the temperature range of 400-600 degreesC. More interestingly, not like the GDC electrolyte, which has some extent of electronic conduction under reducing atmosphere, the composite electrolyte is almost a pure ionic conductor, evidenced by the fuel cells (FC) open circuit voltage (OCV) close to the theoretical one. The fuel cells based on this composite electrolyte show excellent power density output even at temperature as low as 500 degreesC (240 mW cm(-2)) in spite of the relatively thick electrolyte (0.4 mm). Such high performance, in combination with its low cost in both raw materials and fabrication process, make this kind of composite electrolyte a good candidate electrolyte material for future ultra-low-cost intermediate temperature ceramic membrane fuel cells (IT-CMFCs).

Preparation and characterization of Pd and Pd-Ni alloy membranes on porous substrates by MOCVD with mixed metal β-diketone precursors

A novel metal-organic chemical vapor deposition (MOCVD) process with a mixed metal {beta}-diketone precursor to successfully prepare Pd and Pd-Ni alloy composite membranes is described. A palladium phase deposit consisting of agglomerates of tiny crystallites with a dimension of around 30 x 10 nm was formed on porous Al{sub 2}O{sub 3} substrates, when using a single Pd(AcAc){sub 2} precursor. With a mixed precursor of Pd(AcAc){sub 2} + Ni(AcAc){sub 2}, uniform and dense Pd-Ni alloy membranes (consisting of primary crystallites with a dimension of 40 x 15 nm) were reproducibly obtained on porous alumina supports. N{sub 2} permeation tests indicated that the Pd-Ni alloy layer was gastight and crack-free. XPS studies revealed that no oxygen nor carbon impurities were incorporated into the bulk membranes. The Pd/Ni ratio on the surface of Pd-Ni alloy layer was very close to that of the mixed precursor, but much less Ni content (<1/150) was detected in the deposit body.

Rate-limiting process and growth kinetics of AlN thin films by microwave plasma CVD with AlBr3-NH3-N2 system

Abstract Microwave plasma enhanced CVD with AlBr 3 -NH 3 -N 2 system is developed for the preparation of AlN thin films. The growth rate and morphology of the films on Si(111) substrate as a function of the operation parameters were systematically studied. (002) oriented AlN thin films have been obtained. The growth rate vs. temperature showed a two-regime behavior with activation energies of 3.12 kJ/mol and 15.6 kJ/mol, respectively. The dependence of the growth rate on AlBr 3 precursor temperature well fitted the prediction from thermodynamic analysis of the CVD system, indicating the deposition rate limited by the mass feeding process. A surface kinetic model was proposed to interpret the (002) oriented growth behavior.

Properties characterization of Ce(DPM)4 served as precursor for MOCVD

Abstract High purity Ce(DPM) 4 (DPM=2,2,6,6-tetramethyl-3,5-heptanedionato) powder was successfully synthesized from Ce(NO) 3 and HDPM followed by reduced pressure distillation and recrystallization from toluene. This metal β-diketonate complex used as precursor for metal–organic chemical vapor deposition (MOCVD) of CeO 2 film has been characterized by elemental analyses, X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG–DTA) analysis, 1 H -NMR spectroscopy, and infrared spectroscopy. The structure of this complex is monocline and the sublimation temperature is 145°C. This product was sufficiently stable after it was exposed to air for 30 days, as only a few impurities such as Ce 2 (CO 3 ) 3 ·8H 2 O, Ce(OH) 3 , and H 2 O can be identified. Ce(DPM) 4 decomposes during evaporation, the chemical bonds dissociate in the sequence of CC(CH 3 ) 3 >CH>CO and CC by heating. At 280°C this complex thoroughly decomposes to CeO 2 .

Aerosol and plasma assisted chemical vapor deposition process for multi-component oxide La0.8Sr0.2MnO3 thin film

Abstract It has been of increasing interest to develop a chemical vapor deposition (CVD) process to prepare thin films of multi-component oxides, such as fluorite and perovskite type materials, which exhibit unique functional properties and potential applications. The lack of the proper volatile precursors and the difficulty in composition control are the major barriers to sluggish the achievement of this purpose. Recently, we developed an aerosol assisted CVD method to deposit perovskite-type oxide La 1− x Sr x MnO 3 , a mixed oxygen ion/electron conductor. Furthermore, microwave plasma activation to the aerosol phase is applied to promote the deposition process. This paper reports briefly the preliminary results from this novel technique.