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Final Report: Replacement of Organic Solvents by Carbon Dioxide for Forming Aerosols in Coating Processes
EPA Grant Number: R824728Title: Replacement of Organic Solvents by Carbon Dioxide for Forming Aerosols in Coating Processes
Investigators: Sievers, R. E. , Miles, Barbara A.
Institution: University of Colorado
EPA Project Officer: Karn, Barbara
Project Period: October 1, 1995 through September 30, 1997
Project Amount: $200,000
RFA: Technology for a Sustainable Environment (1995)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:The manufacture of thin films has many important applications in our lives, from protective coatings, to membrane separations, to the manufacture of semiconductor devices. Traditionally, one of the best techniques for thin film deposition of advanced materials (single or multicomponent materials designed for specific applications) has been chemical vapor deposition (CVD). The conventional CVD delivery method relies on the transport of volatile metal precursors to a deposition chamber. Precursors that are thermally stable and volatile enough to transport are oftentimes highly toxic. An alternative approach to CVD, spray pyrolysis, relies on the transport of one or more precursors in the form of an aerosol to a heated surface. In this technique, mass transport does not depend on the equilibrium vapor pressure of the precursors; compounds with low volatility and low thermal stability can be used provided that they are soluble in an acceptable solvent. Unfortunately, transport does require the dissolution of the metal precursors that are, in many cases, only appreciably soluble in organic or halogenated solvents that may adversely impact the environment and pose serious worker health issues. In collaboration with Ford researchers, our objective was to develop a new process for the deposition of thin films that does not require the use of highly toxic volatile metal precursors or the use of halogenated organic solvents. Summary/Accomplishments (Outputs/Outcomes):
We have successfully developed a new supercritical fluid-assisted aerosolization process that can employ the use of water-soluble precursors, thereby circumventing the need for either toxic volatile metal precursors or organic solvents. Water is the solvent of choice, provided that fine droplet dense aerosols can be formed. The new process utilizes the unique properties of supercritical carbon dioxide, gaseous CO2 that has been pressurized above its critical pressure (1070 psi) and heated above its critical temperature (31°C), to assist in the production of aerosols of aqueous solutions of metal precursors. Aerosolization is accomplished by pumping the aqueous solution containing the metal precursors into one side of a low-dead-volume tee, while supercritical or near-critical CO2 is pumped into a second arm of the tee. A micro-emulsion is formed inside the tee, which is fitted with a small diameter (<100 m) restrictor nozzle. Upon decompression of the emulsion through the restrictor, a dense aerosol plume is formed, and this is directed towards a heated glass substrate surface. Water evaporation, hydrolysis, oxidation, and surface reactions can then take place in tandem, and deposition of fine grained thin films occurs. This process was successfully used to deposit a variety of metal oxide thin films on glass and other substrates from water-soluble metal nitrates and metal acetates. These high-quality films were chemically identical to films produced by conventional CVD methods.
Accomplishments and Research Results
The deposition of metal oxide thin films was investigated using supercritical CO2-assisted aerosolization. Several system configurations were explored to optimize the deposition process and the quality of the thin films produced. A diagram of the final system setup appears in Figure 1. In this system, aqueous metal salt precursors were pumped into one side of a low-dead-volume tee by an ISCO syringe pump, while supercritical CO2 was pumped into another inlet on the tee using a separate ISCO syringe pump. This two-syringe pump configuration prevents the slight aerosol pulsing (attributed to the reciprocating type pumps used in the production of fine particles) that would be detrimental to the deposition of smooth, uniform films. This configuration generates a steady aerosol plume that may be sustained for hours. The aerosol plume was directed toward heated substrates where pyrolysis and oxidation of the metal precursors could occur to form the desired metal oxide thin film.
Early experiments showed the necessity of maintaining a constant temperature at the substrate to produce uniform films of the desired composition. Thick soda lime glass substrates tended to crack upon rapid heating and cooling. During deposition, the surface of the glass could be up to 100?C cooler than the temperature-regulated ceramic block heater. For these reasons, a 1-inch diameter tube furnace was used to uniformly heat the glass substrates, reducing the large thermal gradient associated with the ceramic block heater. The aerosol plume was directed through the horizontally mounted tube furnace across the heated substrate. This configuration provided the best quality thin films deposited onto glass substrates. Silicon and sapphire substrates were more thermally resilient and could withstand the rapid heating and cooling on a ceramic hot plate. Silicon and sapphire substrates also were significantly thinner than the soda lime glass substrates and were more uniformly heated by the ceramic block heater. This configuration allowed for the manual rastoring of the spray tip to provide a uniform deposit over the entire substrate surface.
1. Thin Spinel Films
Spinel films, which are rich in cobalt oxide, are useful for passive solar shading due to their combined good optical, physical, and chemical properties. We attempted to form thin spinel films, composed of cobalt-iron-chromium oxides, in the discrete molar ratio of 70:18:12, respectively, using our supercritical CO2-assisted aerosolization technique. Metal precursors for the spinel films were chosen based on their solubility in water, their ability to thermally decompose and/or oxidize to the desired metal oxides, the nature of the decomposition by-products, commercial availability, and cost effectiveness. The metal nitrates and acetates of cobalt, iron, and chromium were found to meet these criteria and have been shown to decompose to the desired metal oxides at 600?C by thermogravimetric analysis (TGA).
An aqueous 0.1 M solution of the metal nitrates (premixed with the correct molar ratio of the metals in the desired product) was sprayed at atmospheric pressure onto the surface of a glass substrate heated at 500?600?C. During 5 minutes of pyrolysis/deposition time, a highly reflective, transparent thin film was deposited onto the glass substrate. The films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersion spectroscopy (EDS), inductively coupled plasma-atomic absorption (ICP-AA), and X-ray diffraction (XRD). The SEM data indicated that these films were uniformly deposited and had a very fine grained structure, which also was confirmed by AFM. The grain size was found to be approximately 50?100 nm by AFM. EDS confirmed the presence of cobalt, iron, and chromium, while ICP indicated that these metals were present in the molar ratio of 70:18:12, indicating that the stoichiometry of the premixed aqueous solution had been retained. XRD analysis showed that the films have a crystalline structure. These films passed a series of durability tests performed by the Ford Glass Division. Similar results were obtained using an aqueous metal acetate precursor solution.
2. Deposition of Other Materials
As the Ford Glass Division entered a restructuring period, Sandy Vong, the Ford researcher with whom we worked most closely, retired. Our collaborative research on the utility of this new method continued, with the production of other metal oxide thin films that currently require the use of environmentally harmful solvents and toxic metal precursors.
Deposition of Thin Films of Zinc Oxide: Thin films of zinc oxide (ZnO) are important materials for solar cells, gas sensors, ultrasonic oscillators, transducers, and display phosphors. We attempted to form thin films of this material using the supercritical CO2-assisted aerosolization technique. Good quality thin films of ZnO were deposited from aqueous precursor solutions onto soda lime glass and single crystal silicon substrates. Out of the aqueous Zn2+ precursors investigated (Zn(CH3COO)2, Zn(NO3)2?6H2O and ZnCl2), the best quality ZnO films were observed using 0.01 M zinc acetate as the precursor. A few drops of glacial acetic acid were added to the precursor solution to prevent precipitation. Typical parameter settings for the deposition of ZnO thin films are given in Table 1.
Table 1. Typical conditions for the deposition of ZnO
|
Parameter |
Typical Range |
|
CO2 Pressure |
1100?1500 psi |
|
Precursor Flow Rate |
0.1?1.0 mL/min |
|
Deposition Temperature |
350?550 ?C |
|
Restrictor Inner Diameter |
50?100 µm |
|
Distance from Restrictor Tip to Substrate |
8 cm |
|
Deposition Time |
2?10 min |
The flow rates and restrictor sizes reported are those required to maintain a consistent aerosol spray for the duration of the deposition. The surface of the substrate must be maintained above 350?C to continuously vaporize the droplet solvent and pyrolyze/oxidize the metal precursor upon contact with the surface. If the substrate temperature is too high, the radiative heating will vaporize the aerosol droplets in flight, promoting the formation of individual particles. Soda lime glass substrates were heated in a quartz tube furnace, while the silicon substrates were heated on a ceramic block heater.
The surface of both glass and silicon substrates showed concentric interference fringes when viewed perpendicularly to a white light source after deposition. The films on glass were quite transparent and optical data were obtained on a PE Lambda 9 UV/Vis/NIR spectrophotometer. The visible spectrum indicates 80?90 percent optical transparency in the region between 500 and 800 nm. A sharp decrease in transmission occurs at 380 nm, indicative of a band gap of approximately 3.27 eV. This value is consistent with the literature value reported for the band gap of ZnO. The XRD powder pattern of ZnO (Figure 2) on silicon is consistent with JPCDS-ICDD card number 36?1451 for zinc oxide. Deposition rates were approximately 100 ? per minute, as determined by SEM of thin films on silicon substrates that were cleaved and mounted "edge on" in the microscope. Films deposited on silicon for less than 10 minutes ( <1 m in thickness) were featureless in the SEM (Figure 3). Increasing the overall deposition time produced films up to 10 microns thick (Figure 4) with some surface features observable at high magnification (Figure 5). AFM indicated that the grain size of the ZnO films produced by this process was between 50 and 100 nm.
Deposition of Phosphor Thin Films: With the increasing demand for high resolution, low cost, portable displays, we investigated the deposition of thin films of luminescent materials using supercritical CO2-assisted aerosolization. We initially focused on the common red phosphor, yttrium oxide, doped with europium oxide. Some of this work was supported also by the Defense Advanced Research Projects Agency (DARPA), in a joint project with Professor Aron Vecht and his colleagues at the University of Greenwich. The metal nitrates of both yttrium and europium were chosen based on the same criteria as for the spinel films. An aqueous solution of yttrium nitrate and europium nitrate (95:5 mole percent) was sprayed onto glass and sapphire substrates heated between 250 and 300?C; pyrolysis of the fine aerosol at the substrate surface yielded thin phosphor films of europium-doped yttrium oxide over 20?30 minutes of deposition time. These films were analyzed by SEM, ICP, and cathodoluminescence. The SEM analysis indicated the formation uniform thin films with very fine grain sizes. The ratio of yttrium to europium of the thin film was shown by ICP to be well within experimental error (0.2 percent) of the precursor solution. Cathodoluminescence measurements indicated that upon electron impact, the film had a primary emission at 610 nm, which is characteristic of the europium-doped yttrium oxide phosphor material prepared by other methods commonly used in cathode ray tubes.
Deposition of Various Metal Oxides: Other materials, such as Al2O3, Cr3O4, CuO, and YBa2Cu3O7-x, also were successfully deposited by the CO2-assisted aerosolization method. These films were produced from the corresponding metal-acetate or metal-nitrate aqueous solutions.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
| Other project views: | All 8 publications | 3 publications in selected types | All 2 journal articles |
| Type | Citation | ||
|---|---|---|---|
|
|
Sellers SP, Miles BA, Sievers RE, Halverson W. The production of thin metal oxide films by spray pyrolysis using supercritical CO2-assisted aerosolization of aqueous solutions. KONA 2000;18:74-80. |
R824728 (Final) |
not available |
|
|
Sievers RE, Karst U, Milewski PD, Sellers SP, Miles BA, Schaefer JD, Stoldt CR, Xu CY. Formation of aqueous small droplet aerosols assisted by supercritical carbon dioxide. Aerosol Science and Technology 1999, Volume: 30, Number: 1 (JAN), Page: 3-15. |
R824728 (Final) |
not available |
thin film deposition, supercritical carbon dioxide-assisted aerosolization of metal nitrate and metal acetate aqueous solutions, deposited reflective, transparent spinel films on glass and quartz substrates. , Toxics, Sustainable Industry/Business, Scientific Discipline, RFA, Technology for Sustainable Environment, Sustainable Environment, Environmental Engineering, cleaner production/pollution prevention, Environmental Chemistry, New/Innovative technologies, VOCs, Economics and Business, aerosols, process modification, worker safety, chemical processing, cleaner production, CO2 - based systems, in-process changes, Volatile Organic Compounds (VOCs), waste reduction, toxic organic solvents, coating processes, industrial ecology, Methylene Chloride, organic coatings, solvents, water-based coatings, reduced emissions, waste minimization, spinel films, industrial innovations, industrial process, alternative materials, source reduction, Production/Pollution Prevention, innovative technology, emission controls
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