5.3 Liquid phase deposition

Introduction

Silicon dioxide (silica, SiO ₂ ) has been the most researched chemical compound apart from water. Silica has been used throughout history, for example, flint, which when sharpened formed one of humanities first tools. Crystalline silica, or sand, was melted into glass as early as 5000 B.C., birthing a technology that has gained sophistication in modern times. Silicon is the second most plentiful element in the Earth’s crust, the most plentiful being oxygen. It is thus surprising that it was not until 1800 that silica was named a compound by Sir Humphry Davy. He, however, failed to isolate its components via electrolysis, and it is Jöns Jacob Berzelius who is thus credited with discovering silica in 1824. He heated potassium fluorosilicate with potassium metal and, after purifying the product of this reaction with water, produced amorphous silica powder.

The most common forms of silica employed in industry include α-quartz, vitreous silica, silica gel, fumed silica and diatomaceous earth. Synthetic quartz is hydrothermally grown from a seed crystal, with aqueous NaOH and vitreous SiO ₂ , at 400 °C and 1.7 kbar. Because it is a piezoelectric material, it is used in crystal oscillators, transducers, pickups and filters for frequency control and modulation. Vitreous silica is super cooled liquid silica used in laboratory glassware, protective tubing sheaths and vapor grown films. Silica gel is formed from the reaction of aqueous sodium silicate with acid, after which it is washed and dehydrated. Silica gel is an exceptionally porous material with numerous applications including use as a dessicant, chromatographic support, catalyst substrate and insulator. Pyrogenic or fumed silica is produced by the high temperature hydrolysis, in an oxyhydrogen flame, of SiCl ₄ . Its applications include use as a thickening agent and reinforcing filler in polymers. Diatomaceous earth, the ecto-skeletons of tiny unicellular marine algae called diatoms, is mined from vast deposits in Europe and North America. Its primary use is in filtration. Additional applications include use as an abrasive, insulator, filler and a lightweight aggregate.

Methods of colloidal growth and thin film deposition of amorphous silica have been investigated since 1925. The two most common and well-investigated methods of forming SiO ₂ in a sol or as a film or coating are condensation of alkoxysilanes (known as the Stober method) and hydrolysis of metal alkoxides (the Iler or dense silica [DS] process).

Liquid phase deposition (lpd)

LPD is a method for the “non-electrochemical production of polycrystalline ceramic films at low temperatures.” LPD, along with other aqueous solution methods [chemical bath deposition (CBD), successive ion layer adsorption and reaction (SILAR) and electroless deposition (ED) with catalyst] has developed as a potential substitute for vapor-phase and chemical-precursor systems. Aqueous solution methods are not dependent on vacuum systems or glove boxes, and the use of easily acquired reagents reduces reliance on expensive or sensitive organometallic precursors. Thus, LPD holds potential for reduced production costs and environmental impact. Films may be deposited on substrates that might not be chemically or mechanically stable at higher temperatures. In addition, the use of liquid as a deposition medium allows coating of non-planar substrates, expanding the range of substrates that are capable of being coated. Aqueous deposition techniques have not reached the level of maturation that vapor-phase techniques have in respect to a high level of control over composition, microstructure and growth rates of the resulting films, but their prospect makes them attractive for research.