Direct Vapor Deposition Research Articles

Morphology and thermal conductivity of yttria-stabilized zirconia coatings

An electron beam directed vapor deposition method was used to grow 7 wt.% Y 2O 3–ZrO 2 (7YSZ) coatings and the effects of substrate rotation upon the coating porosity, morphology, texture, and thermal conductivity were explored. As the rotation rate was increased, the texture changed from 〈1 1 1〉 to 〈1 0 0〉. Under stationary deposition, the coatings were composed of straight columns, while low-frequency rotation resulted in wavy columns. Increases in rotation rate resulted in a gradual straightening and narrowing of the growth columns. The pore fraction slowly decreased as the rotation rate increased. The thermal conductivity was found to be inversely related to the pore fraction. The structural and thermal conductivity alterations are a result of changes to flux shadowing associated with specimen rotation in a gas jet-entrained vapor plume. The minimum thermal conductivity at a low rotation rate is 0.8 W/(m K), well below that of conventionally deposited coatings.

INFLUENCE OF VAPOR DEPOSITION ON WILD BLUEBERRY WATER REQUIREMENTS IN A HUMID COASTAL CLIMATE

Growers need recommendations to improve both the timing and amount of irrigation water applied to wild blueberries (Vaccinium angustifolium) in the humid coastal region of Maine where direct vapor deposition may also supply water to the crop. This study was initiated to quantify the rates and timing of vapor deposition in relation to rates of evapotranspiration (ET). Weighing lysimeters were used to determine rates of net water vapor deposition (VD) or vapor uptake (VU) during hours when rainfall and drainage were not occurring. Vapor deposition occurred throughout the evening, night, and early morning during the fruit bearing year of a two year cropping cycle for the period June 11 to October 8, 2003. The mean values of daily VD, VU, and ET were 0.075 cm/d, 0.335 cm/d, and 0.27 cm/d with a coefficient of variability of 71%, 44%, and 71%, respectively. Thus, VD accounted for around 22% of total water taken up by the plants and amounted to 28% of ET. Under these conditions, classical approaches to irrigation scheduling based solely on rainfall measurements may result in over application of water by failing to account for vapor deposition.

Vapor deposition of platinum alloyed nickel aluminide coatings

Platinum-doped NiAl coatings are widely used to increase the oxidation resistance of superalloys. These coatings are usually synthesized by a solid state reaction-diffusion process conducted at high temperature. It requires the chemical vapor deposition of aluminum on a nickel rich superalloy substrate that has been pre-coated with several microns of electrodeposited platinum. Here, we show that an electron beam directed vapor deposition (EB-DVD) technique can be used to deposit well bonded, structurally and chemically homogeneous NiAlPt bond coats of any composition onto superalloy substrates. The approach utilized a high voltage, rapid scan frequency electron beam to independently heat elemental nickel, aluminum and platinum melt pools to create three closely spaced vapor plumes. These vapor plumes were then entrained in an inert gas jet flow, which mixed and directed them to a substrate. By adjusting the electron beam current applied to each elemental source, homogeneous, dense, Pt alloyed β-phase NiAl coatings could be synthesized at substrate temperatures of 1050 °C. The width of the substrate–coating interdiffusion zone was controlled by the deposition temperature and time.

Direct Synthesis of a Metalloporphyrin Complex on a Surface

We demonstrate that well-defined monolayers of a metal complex on a surface can be prepared by direct vapor deposition of the metal atoms on monolayers of the ligand. In particular, ordered monolayers of adsorbed tetraphenylporphyrin (2H-TPP) on a silver surface were exposed to cobalt vapors, resulting in the complexation of the metal by the porphyrin. The formation of the metal complexes was monitored by means of X-ray photoelectron spectroscopy (XPS), which reveals that this metalation reaction leads to a chemical equivalence of all four nitrogen atoms. The described in situ metalation provides a convenient way to produce adsorbed monolayers of more reactive (e.g., air- or solvent-sensitive) or thermally unstable metalloporphyrins that are difficult to evaporate or even to obtain as pure compounds at room temperature.

Albedo of the south pole on Mars determined by topographic forcing of atmosphere dynamics

The nature of the martian south polar cap has remained enigmatic since the first spacecraft observations. In particular, the presence of a perennial carbon dioxide ice cap, the formation of a vast area of black 'slab ice' known as the Cryptic region and the asymmetric springtime retreat of the cap have eluded explanation. Here we present observations and climate modelling that indicate the south pole of Mars is characterized by two distinct regional climates that are the result of dynamical forcing by the largest southern impact basins, Argyre and Hellas. The style of surface frost deposition is controlled by these regional climates. In the cold and stormy conditions that exist poleward of 60 degrees S and extend 180 degrees in longitude west from the Mountains of Mitchel (approximately 30 degrees W), surface frost accumulation is dominated by precipitation. In the opposite hemisphere, the polar atmosphere is relatively warm and clear and frost accumulation is dominated by direct vapour deposition. It is the differences in these deposition styles that determine the cap albedo.

NiAl bond coats made by a directed vapor deposition approach

Intermetallic, nickel aluminide alloys are widely used as bond coat materials in thermal barrier coating systems applied to nickel base super alloy components. They are usually synthesized by a solid-state reaction–diffusion heat treatment following the chemical vapor deposition of aluminum on the nickel rich substrate. Here, an electron beam directed vapor deposition (EB-DVD) technique is used to simultaneously evaporate nickel and aluminum and then reactively deposit NiAl bond coats that are structurally and chemically homogeneous and well bonded to the superalloy substrate. The approach utilizes individual nickel and aluminum sources placed within a rarefied helium gas jet with flow conditions that promote vapor phase intermixing. By adjusting the electron beam current applied to each elemental source, we show that the coating layer composition can be precisely controlled. Fully dense, homogeneous, β-phase NiAl coating layers with a relatively sharp compositional interface with the substrate have been deposited. The extent of the substrate interdiffusion zone is controlled by the deposition conditions.

Determination of Evapotranspiration and Drainage in Lowbush Blueberries (Vaccinium angustifolium) Using Weighing Lysimeters

SUMMARY Lowbush blueberry growers need recommendations for irrigation scheduling. This study was conducted to determine irrigation water use potential. Initial data were collected from the end of April through early August 2002 using weighing lysimeters to measure evapotranspiration and drainage. High rainfall and drainage rates near the end of April indicated a period of rapid leaching when chemical applications could be beneficially postponed. Nighttime increases in lysimeter weight were observed indicating that direct vapor deposition is a significant factor in the water balance. Results from this ongoing study will be used for scheduling irrigation water applications to improve water use and production efficiency.

Modelling alkali chloride superheater deposition and its implications

Alkali chlorides are often involved in superheater tube fouling and corrosion problems, especially during biofuel combustion. We have carried out a modelling study on fume particle and condensable alkali chloride vapour behaviour in superheater tube boundary layers. Two models of boundary layer aerosol behaviour are presented: one is a general computational fluid dynamics (CFD)-based model and the other one is a one-dimensional boundary layer theory-based model for the stagnation point. Simulations of NaCl behaviour in recovery boiler panel heat exchangers indicated that both (1) boundary layer condensation and subsequent particle thermophoretic deposition and (2) direct vapour deposition are significant deposition mechanisms for NaCl. In the first panel tube boundary layer, NaCl is clearly supersaturated, while further in the panel section boundary layer, it seems to be closer to the thermodynamic equilibrium. As the surface temperature of the ash deposit layer increases during a soot-blowing cycle from 480 to 670 °C, the overall deposition rate of NaCl remains fairly constant (increases only by 52%) due to boundary layer aerosol dynamics. Homogeneous nucleation within boundary layers may take place at certain conditions. Additionally, we have made estimates of the subsequent behaviour of deposited alkali chlorides. The rates of deposit sulphation and generation of HCl(g) do not seem to be sensitive to alkali chloride deposition rates provided that SO 2 levels are sufficiently low.

Reactive vapor deposition of metal oxide coatings

The reactive deposition of metal oxide coatings has been achieved using an electron beam directed vapor deposition (DVD) approach. In this approach, a transonic helium carrier gas jet has been combined with electron beam evaporation to create and efficiently transport metal vapor to a substrate. Metal oxide coatings were then produced by adding oxygen to the carrier gas. The synthesis of multi-component oxide coatings could be achieved using either an alloy source or by simultaneous evaporation of metals from two or more sources. In the latter, the reactive carrier gas jet was used to facilitate vapor phase mixing of the evaporated fluxes. The preferred approach was found to be dependent on the vapor pressures of the individual source components. When large differences between the component vapor pressures exist, multi-source evaporation is preferred. Results are reported for the reactive deposition of zirconia from a single metal source and reactive deposition of yttria stabilized zirconia from two metal sources (Y and Zr). The phase and morphology are similar to those found in similar coating compositions created by the evaporation of a pre-compounded oxide. Very good compositional uniformity in the binary metal oxide films has been observed. Vapor transport modeling is used to explore the origin of the composition uniformity.

Synthesis of open-cell metal foams by templated directed vapor deposition

Low-density, open-cell nickel base superalloy foams have been synthesized by a high-rate, electron beam-directed vapor deposition process and their mechanical properties evaluated. The deposition process uses an open-cell polymer foam template upon which is deposited a metal alloy coating. The electron beam evaporated flux was entrained in a rarefied transonic gas jet and propagated along the flow stream lines through the polymer structure. After vapor deposition, the polymer template was removed by low-temperature thermal decomposition. The resultant ultralightweight metal foams consisted of a three-dimensional open cell, reticulated structure possessing hollow triangular ligaments with relative densities of <3%. Their mechanical integrity was increased by either pressureless or transient liquid phase sintering. The mechanical properties of these ultralightweight metal foams were comparable to theoretical predictions for open-cell, reticulated foams.

Low thermal conductivity vapor deposited zirconia microstructures

Low thermal conductivity yttria stabilized zirconia (YSZ) coatings have been grown using a low-vacuum (0.20 Torr) electron beam directed vapor deposition process. In this approach, a transsonic helium jet was used to entrain and transport an evaporated YSZ flux to a substrate. The interaction of the helium jet with the coating surface resulted in many of the evaporated species making oblique angles of contact with the substrate. This resulted in the formation of a highly porous, columnar microstructure without substrate rotation. When the substrate was positioned perpendicular to the axis of the jet, coatings with intercolumnar pores normal to the substrate surface were formed. The ambient temperature thermal conductivity of a coating grown in this arrangement was 1.9 Wm/K, comparable to that of conventional, high-vacuum electron beam coatings. When the column and pore orientation was inclined (by tilting the substrate) the thermal conductivity was observed to fall. By alternating the inclination angle as growth progressed, coatings containing zig-zag columns and pores could be synthesized. Using this technique, YSZ coatings with thermal conductivities as low as 0.8 W/m K were obtained. The observed thermal conductivity reduction arises from the longer thermal diffusion path of the zig-zag pore micro-structures.

Electron Beam - Directed Vapor Deposition of Multifunctional Structures

ABSTRACTMultifunctional structures are those that combine load bearing support in addition to supplemental functions such as actuation, electrochemical energy storage or thermal management. Electron beam - directed vapor deposition (EB- DVD) technology has been used for the deposition of templated cellular structures for micro heat-pipe structures and porous electrode coatings for rechargeable nickel - metal hydride cells. In addition to load bearing support, the tem- plated cellular structures exhibit enhanced thermal management characteristics and the electrochemical cells can be integrated into the load bearing supports of linear and truss based structures leading to their multifunctionality. Dur- ing EB-DVD, the electron beam evaporated vapor flux is encompassed by a rarefied transonic inert gas jet, entraining the vapor in a non-reactive gas flow and transporting it onto a polymer or metal template structure. Here, EB-DVD technology has been used to synthesize copper based templated cellular structures for thermal management systems and porous nickel coatings for the positive electrode of rechargeable nickel - metal hydride cells.

Sculptured thin films—II. Experiments and applications

A report on the preparation and potential applications of sculptured thin films (STFs) is presented. STFs are nano-engineered columnar thin films in which the columnar direction can made to change easily and often during growth. STFs of virtually any material can be prepared through directional vapor deposition onto any surface under low adatom-mobility conditions. Columnar shapes, such as zig-zag, C-, S-, helicoidal and superhelical, can be engineered in any sequence and with controlled density profiles. Although oblique deposition and the resulting anisotropic properties have been known for over a century, the general recognition that such nanoengineered morphologies can lead to unique and predictable optical, mechanical, electrical, magnetic, thermal, and biological properties has occured only recently.

Electron beam directed vapor deposition of thermal barrier coatings

Electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) are currently applied to gas turbine engine components to improve engine durability but, in the future, may be used to increase engine efficiency. These coatings are costly to apply in part because of the low evaporated materials utilization efficiency (a few percent) and the relatively slow rate of deposition (∼5–10 μm min−1). Here, an emerging electron beam directed vapor deposition (EB-DVD) approach is explored as a method for producing these TBCs. The EB-DVD technique combines low vacuum (10−3–10 Torr) electron beam evaporation with a carrier gas jet to rapidly create and efficiently transport evaporant to a deposition surface. Studies of the deposition of yttria partially stabilized zirconia (YSZ) and reactively formed zirconia coatings have been performed and the composition, microstructure, phase content and crystallographic texture of deposited layers are reported. The results indicate EB-DVD’s ability to form compositionally controlled YSZ and zirconia coatings, with a 50% or greater materials utilization efficiency, at a rate in excess of 50 μm min−1. The EB-DVD films have a columnar microstructure, phase content and crystalline texture similar to that of the EB-PVD coatings used for high performance gas turbine applications.

Silicon Dioxide Thin Films Prepared from Silicon Tetraacetate Using ArF Excimer Laser by Chemical Vapor Deposition

Silicon dioxide thin films were prepared from silicon tetraacetate by low-pressure direct photochemical vapor deposition (photo-CVD) using an ArF excimer laser. The film prepared by vertical irradiation of an unheated substrate using a laser showed the lowest relative dielectric constant which is comparable to that of the film prepared by thermal-CVD at the substrate temperature of 300°C. The relative dielectric constant was approximately proportional to the absorbance of Si–O· and Si–OH bond at about 950 cm-1, indicating that ionic polarization due to the nonbridging oxygen was closely related to the relative dielectric constant of film prepared by decomposition of silicon tetraacetate at a low substrate temperature. Low-temperature annealing in air decreased the amount of nonbridging oxygen and resulted in a denser film.

Silicon dioxide thin films prepared by ArF excimer laser chemical vapor deposition from silicon tetraacetate

Silicon dioxide thin films were prepared from silicon tetraacetate by low-pressure direct photochemical vapor deposition (photo-CVD) using an ArF excimer laser. The film prepared by a vertical irradiation of laser to an unheated substrate showed the lowest relative dielectric constant which is comparable to that of the film prepared by thermal-CVD at a substrate temperature of 300 °C. The relative dielectric constant was approximately proportional to the absorbance of Si-O · bond at about 950 cm −1. This indicates that an ionic polarization due to the nonbridging oxygen is closely connected with the relative dielectric constant of the film prepared by the decomposition of silicon tetraacetate at a low substrate temperature. Low-temperature annealing in air decreased the dangling bond and made the film denser.

Formation and spectra of clathrate hydrates of methanol and methanol-ether mixtures

Infrared spectra of mixed clathrate hydrates, with either ethylene oxide (EO) or tetrahydrofuran (THF) and methanol molecules as the guest species, have been obtained from thin films prepared by vapor deposition of D2O mixtures in the 115–130 K range. Although methanol acts as a suppressant to the direct vapor deposition of a type I clathrate with EO, nearly complete conversion of 115 K amorphous codeposits, to the crystalline mixed clathrate, occurs upon warming near 150 K. By contrast, the type II clathrate of THF shows an increased crystalline quality when methanol is included in the vapor deposits of the mixed clathrate hydrate at 130 K. The observation of the OD stretch-mode band of weakly bonded CD3OD near 2575 cm−1 is part of the evidence that the methanol molecules are encaged. However, as shown theoretically by Tanaka, the clathrate hydrates of methanol, even when mixed with an ether help gas, are not stable structures but form at low temperatures because of kinetic factors, only to decompose in the 140–160 K range. Attempts to prepare a simple type I or type II clathrate hydrate of methanol have produced mixed results. Limited amounts of clathrate hydrate form during deposition but annealing does not result in complete conversion to crystalline clathrates, particularly for host : guest ratios of 17 : 1.

Functionally graded materials synthesis via low vacuum directed vapor deposition

The spatially distributed microstructures needed to implement many functionally graded material (FGM) designs are difficult to realize affordably with today's materials synthesis/processing technologies. To address this need, a new directed vapor deposition (DVD) technique has been developed and explored as a potential FGM synthesis tool. The technique exploits supersonic helium jets in combination with electron beam/resistive evaporation under low vacuum (10 −3 -10 Torr) conditions to atomistically spray deposit a wide variety of monolithic and composite materials. Two of the most important processing parameters (the carrier gas velocity and the deposition chamber pressure) that control deposition are identified, and their effect upon deposition efficiency for flat and fiber substrates is explored systematically. Under certain conditions, the DVD approach is found to deposit vapor onto fibers with a significantly higher efficiency than traditional high vacuum line-of-sight vapor deposition techniques. It can even deposit material onto surfaces that are not in the line-of-sight of the source. A computational fluid dynamics model has been used to interpret the experimental observations and to identify the role of carrier gas dynamics in controlling deposition efficiency and spatial distribution.

Interface Electronic Properties between Silicon and Silicon Nitride Deposited by Direct Photochemical Vapor Deposition

Interface electronic properties of silicon nitride (SiNx)/crystalline silicon (c-Si) are studied. The interface electronic properties are improved by changing NH3/SiH4 flow ratios (F NH3/F SiH4) of a thin SiNx layer (≃6 nm in thickness) between c-Si and a thick SiNx layer (≃65 nm) deposited with F NH3/F SiH4=30, where both SiNx layers are formed by direct photochemical vapor deposition (photo-CVD). In the case of F NH3/F SiH4=120, the minimum interface-trap density and hysteresis obtained from the capacitance-voltage (C–V) characteristics of Al/thick SiNx/thin SiNx/c-Si diodes are 3 ×1010 cm-2 \\cdotp eV-1 and 0.1 V, respectively. In the deposition of thin SiNx, the interface electronic properties are degraded by exposing SixHy or NxHy species to a c-Si surface, where these species are produced by photodecomposition of NH3/SiH4 gas mixtures. Injected carriers, which cause hysteresis of the C–V characteristics, are discussed in terms of the barrier height for tunneling and the number of interface traps.

The properties of SiO 2 films using direct photo-chemical vapor deposition on strained SiGe layers

Low temperature silicon-dioxide (SiO 2) layers were deposited on strained SiGe by using direct photo-chemical vapor deposition (DPCVD) with a deuterium lamp as the excitation source. It was found that the deposition rate increases linearly with the chamber pressure. The Auger electron spectroscopy profile shows that neither was Ge rejected nor was a Ge-rich layer formed after devices were fabricated. The capacitance-voltage ( C- V) measurements show that the flat-band voltage is about −1.1 V with an effective oxide charge of about 7 × 10 10 cm −2 and interface trap density of 3 × 10 11 cm −2 eV −1. At room temperature, the leakage current is about 3 × 10 −9 A/cm 2 under a 2 × 10 6 V/cm electric field. The breakdown field can reach over 16 MV/cm at 1 μA/cm 2 for these SiGe metal-oxide semiconductor (MOS) diodes.