Kai Lau

Aluminum and alumina coatings on copper by chemical vapor deposition in fluidized bed reactors

Abstract High temperature Knudsen cell mass spectrometry was used to study the reactions of Al(c) with HCI(g) and with HCl(g) + 0 2 (g) over the range 127–627°C (400–900 K) to gain information pertinent to the chemical modeling of aluminum coating on copper by chemical vapor deposition in a fluidized bed reactor (CVD-FBR). In the Al−H−Cl system the measured pressures of AlCl 3 and AlCl were in remarkably good agreement with the values predicted by thermochemical calculations when the system is at equilibrium in the temperature range 377–623°C (650–900 K). Neither AlH x Cl3 − x ( x =1–3) nor AlCl 2 species were observed within the entire investigated temperature range. An upper limit of about − 218 ± 20 kJ mol −1 was estimated for the enthalpy of formation of AlCl 2 required to bring the observed and predicted pressures into agreement. The revised database was used to model aluminum coating on copper from a halide system by CVD-FBR. All the experimental results arid thermochemical calculations show that AlCl gas is an important reactive precursor in the aluminium-coating process. Using the most favorable conditions determined from these experiments and the thermodynamic calculations, a thin aluminum coating was deposited on a copper wire immersed for 30 min in a fluidized bed of aluminium particles at 487°C (760 K), using argon containing 0.12% HCl and 7% H 2 as the fluidizing gas. This coating significantly improves the corrosion resistance of copper exposed to air.

A study of the role of alkali metal salts as char gasification catalysts by Knudsen cell mass spectrometry

Abstract Knudsen cell mass spectrometry has been used to determine the gaseous species in thermodynamic equilibrium with admixtures of carbon and alkali metal salts. Both halides and carbonates were used as additives. In addition to measurements of equilibrium vapor pressures, the effect of added steam or carbon dioxide on the composition of the Knudsen cell effusate was examined. With the carbonates, the equilibrium pressure of the alkali metal over the solid phase is at least twice that of the decomposition pressure of the pure salt, but orders of magnitude below that appropriate to the carbothermic reduction of the carbonate. By way of contrast, the alkali halide-carbon admixtures exhibit no free metal vapor up to 1000 K, a temperature in the range employed for gasification. Apparently, the carbonates react with the carbon to form a stable phase with alkali metal activity substantially below unity. Inferences on the composition of such a phase and its role in promoting the carbon gasification rate are discussed.

WSi, FeSi, and CuSi interfaces

The formation and properties of metal-silicon interfaces are of great technological interest in semiconductor, high temperature material, and corrosion applications. Si can be deposited and diffused on metal surfaces to increase corrosion resistance in aqueous acidic environments, oxidation resistance at high temperatures, and erosion resistance. Siliconization of steel to increase its oxidation resistance is performed above 900 C to permit diffusion of Si into the metal. However, the deposition of Si on metals has not been studied in depth. Recently, the authors developed a new coating technique based on chemical vapor deposition (CVD) that allows deposition of Si at much lower temperatures than conventional CVD or pack siliconization techniques. The authors have been able to deposit silicon on W, Fe, and Cu from 350 to 850 C. This paper describes some of the preliminary findings on the metal-Si interfaces formed at these low temperatures.

Low-cost solar-grade silicon: Purification and consolidation of silicon fines from wafering

More than 90% of commercial solar cells are produced using mono- and poly-crystalline silicon, with estimated about 15,000 MT of silicon feedstock used in 2008. Future silicon use is estimated to grow proportionally with the solar industry (30%/y). Substantial amounts of highly refined polycrystalline silicon material are wasted in the final stages of producing ingots and in wafer slicing. Recycling of wasted silicon in the form of fines generated in deposition reactors and in slicing operation can lower the production cost of solar-grade silicon feedstock to less than

Coatings for corrosion protection of steel used in reinforced concrete

10/kg, and substantially shorten energy payback time. The SRI International developed technology is based on the prepurification and simultaneous melt-consolidation and further purification of silicon fines from wafering operations. The consolidated product in form of granules and ingots with a resistivity range of 1 to 5 Ω·cm can be used in the melt growth of both single-crystal and polysilicon ingots for use in photovoltaic cells.

Production of low-cost solar-grade silicon by reduction of SiF 4 gas with sodium: Technical and industrial developmental status

Abstract Silicon and silicon-titanium protective coatings were deposited on steel rebars, wires and fibers using a novel chemical vapor deposition technique that combines the low cost of pack metallization with the advantages of subhalide chemistry and with the high heat and mass transfer of a fluidized bed reactor. The steel samples were immersed in a bed of silicon or silicon-titanium particles fluidized by an argon-0.1% HCl mixture and kept at temperatures ranging from 400 to 750°C. Diffusion coatings were obtained in all cases. Coating rates of over 1 μm min−1 were obtained at the highest temperatures. Selected coated samples were tested for corrosion resistance by chemical and electrochemical techniques. In general, silicon provided some corrosion protection as expected. A.c. impedance measurements in acidic chloride solutions indicated that very thin and very thick silicon coatings were as protective as coatings 1–5 μm thick. The best coatings were obtained when silicon and titanium were codeposited at temperatures around 550°C. These coatings increased the corrosion resistance by more than an order of magnitude with respect to the uncoated sample.

Polycrystalline silicon film and solar cells by FBR-CVD

To meet increasing demand for electrical power using solar photovoltaics, millions of tons of solar-grade silicon costing <

Production of solar-grade silicon: A comparison study of the reduction of SiF 4 with Na, Mg, or Al metals

20/kg will be needed. Low-cost solar-grade silicon can be mass produced by burning Na metal in an atmosphere of pure SiF4 gas—an exothermic, fast, and complete reaction. SRI International routinely uses reactors that can produce at rates >20 metric tons per year (mty), and has licensed the technology to several companies. The Si product is completely separated from the by-product NaF by leaching or melt separation. With the direct use of industrially available Na, B and P levels in the Si product are <1 ppm. Using purified Na results in Si with dopant levels as low as 20 ppb. Experimental ingots grown have resistivity of 1 to 30 Ω·cm, with >100 Ω·cm when purified reactants were used. The purity, electronic parameters, and solar cell efficiency of 15% indicate that this silicon is of solar grade and can be a key contributor in developing solar power markets.

Progress on crystalline silicon thin film solar cells by FBR-CVD: Effect of substrates and reactor design

Solar industry growth and a silicon feedstock shortage have spurred interest in thin silicon film photovoltaic (PV) technology. To reduce PV panel and electricity cost, technologies are needed with high deposition rates of high-quality Si film, scalability to large areas and integrated cell and panel fabrication. A new SRI International deposition technology based on fluidized bed reactor-chemical vapor deposition (FBR-CVD) takes advantage of the high heat and mass transfer in a FBR, and combines it with subhalide CVD chemistry with highly reactive species created in the reactor. The SRIs FRB design minimizes boundary layer thickness to achieve deposition rates as high as several microns per minute and good coating uniformity. The resulting silicon films are highly crystalline with 10–100 µm grain sizes over 5 cm2; with in-depth homogeneous resistivity, typically 0.1–5 Ω·cm, but up to 1000 Ω·cm obtained under some deposition conditions; and with bulk diffusion length >200 µm. The reactor configuration can be used for continuous and integrated cell/panel fabrication.

Development of Metallic Coatings for Corrosion Protection of Steel Rebars

The process of producing solar-grade silicon by direct reduction of SiF4 with Na metal is well established and demonstrated to pilot-plant scale (50 MTY). The silicon obtained contains most impurities, including boron and phosphorus, at levels below 1 ppm. Such silicon has been used to produce solar cells with efficiencies over 18% using commercial manufacturing lines and processes. Recently, we have experimented using Mg and Al as reducing elements. We have shown that Mg may also be a good candidate, and its use may appreciably reduce material costs and total energy required for the solar-grade silicon production. The reduction with Al, although thermodynamically favored, takes place only at high temperatures with a relatively low rate and extent of reaction. In addition, this reaction is complicated by several chemical vapor transport mechanisms. The conclusion is that Al is not attractive reductant for solar silicon production. A comparison study of thermodynamics, the reaction behaviors, and the purity of silicon will be presented.