Cold Spraying: the Future for Architectural Applications?

CenterLine-GlassInt-June2008-QK 10/6/08 12:05 Page 1

Cold spray technology

Cold spraying: The future for architectural applications?

From plain glass of the past to today’s heated and photovoltaic glass, innovative technologies have always played a key role. Dr Julio Villafuerte* explains the impact of cold spray technology in the fabrication of modern glass for architectural applications.

lass is the amorphous state of pattern of electrically conductive ceramics, metals or polymers material (silver-based frit) is applied to Gobtained when liquids are cooled the surface by screenprinting faster than the material would require (silkscreening). The pattern is to transform into a more connected to busbars, which are also thermodynamically stable crystalline screenprinted along the two opposite structure. Common glass is obtained by edges of the glass sheet.

cooling a blend of molten silica (SiO2), One of the drawbacks of

limestone (CaCO3/CaMgCO3) and screenprinting is that it often requires

sodium carbonate (Na2CO3). For multiple surface treatment steps, centuries, the properties of common including heating at elevated glass combined with the widespread temperatures. Many of these secondary availability of the raw materials have processes often limit the conditions made glass the choice material for a under which a given glass sheet can be vast selection of products. Modern windows are made from flat continued » glass produced by forming glass sheets over a bed of molten tin in large float Fig 1. Thin-film heated glass. Photo courtesy of tanks. Over the last few decades, the Thermique Technologies, LLC: window industry has witnessed www.thermiquetechnologies.com innovative changes as a result of many improvements in the window design Fig 2. Array of thin-film photovoltaic panels (12kW) and construction process. From the made by First Solar LLC, powering an entire simple glass of the past to today’s building at the University of Toledo Campus, Toledo, Ohio. Photo courtesy of the University of Toledo: insulated glazing units and www.utoledo.edu/research sophisticated thin film deposition techniques, architectural glass has become a remarkable product that provides the highest levels of comfort, safety and efficiency.

Screenprinting for heated glass Heated glass is created to avoid fogging and icing. The latter occurs when the temperature of the glass surface gets below the dew point of the surroundings, especially in windows exposed to extreme temperature differentials. Applications include refrigerator doors and rear windows for

automobiles. To create heated glass, a www.glassmediaonline.com 1 Glass International June 2008 CenterLine-GlassInt-June2008-QK 10/6/08 12:05 Page 2

Cold spray technology

processed. The printed lines are visible and interfere with vision. The process lacks flexibility and the costs associated with the process can only be justified for significantly high volume production. Soldering of metal lead-outs to screenprinted busbars usually involves stringent techniques to avoid damaging the printed pattern.

Thin-film deposition Fig3. Thin-film photovoltaic glass that Today, modern heated glass uses polycrystalline silicon crystallised by a heating process. Photo courtesy of CSG manufacturing takes advantage of Solar: www.cgsolar.com thin-film deposition techniques, such as physical vapour deposition (PVD) or chemical vapour deposition Fig 4. Robotic low pressure cold spray system creating busbars on architectural (CVD). These techniques not only glass. Photo courtesy of SST, a division of allow the deposition of materials that CenterLine (Windsor). provide electrical conductivity but also other materials that provide a photoelectric effect displayed by a cadmium telluride (CdTe) and copper-

wide range of optical and thermal number of semiconductor materials. indium-diselenide (CuInSe2), which are properties to the glass. The additional Conventional solar cells are made claimed to reach conversion functionalities become available from 200-400μm thick wafers of pure efficiencies as high as 12%. New without compromising transparency monocrystalline or polycrystalline methods to improve conversion and aesthetics. silicon, providing energy conversion efficiencies include the use of Some of the thin films used for efficiencies up to 18%. These cells are amorphous silicon deposited by heated glass include fluorine-doped normally opaque, but by substituting chemical vapour deposition onto tin oxide, indium-tin oxide and the carrier material with glass as well as textured glass surfaces, which is then combinations of tin oxides and silver spacing the solar cells further apart, converted into polycrystalline silicon metal. These films are transparent, semi-transparent modules can be by controlled heating (see fig 3). electrically conductive and of low obtained. Yet the high manufacturing In photovoltaic windows, a semi- emissivity (low-E). Thin-films are cost of pure silicon combined with the transparent photovoltaic composite either thermally deposited during the relatively low energy conversion film is incorporated on the exterior manufacture of the glass itself or by efficiency has limited the widespread glass of glass units. Similar to heated PVD/CVD onto pre-cut sheets of glass. use of conventional solar cells. glass, electrical connectivity can be The fast growing market for low-E Furthermore, it has been observed that attained by creating busbars from glass is credited by high demands for only a few microns into the exposed which individual panels are low-cost tin oxide and metallic silver. surface of a solar cell are effective in interconnected to others, building up Thin-film heated glass is very reliable, converting solar energy. an entire system. Transparency can be as the generated heat is rather evenly Subsequently, thin-film deposition of increased by selectively removing diffused over a large surface than silicon has evolved as a low-cost narrow strips of the film using a laser concentrated in a small area. The alternative to conventional solar cell beam, allowing as much as 12% of the uniform radiation - especially at making (see fig 2). The drawback of this incident light to pass through. moderate temperatures - is one of this method is that thin-film deposition Even though the cost of this technology’s strengths, often requiring often yields amorphous silicon, which technology is still high, savings in less overall energy for equivalent displays conversion efficiencies in the building materials, electricity, and performance. The benefit is the heated range of 4% to 6%; much lower than cooling loads are expected to offset the glass that adds comfort, condensation crystalline silicon. These low cost of photovoltaic glass. While this control, frost control, energy conversion efficiencies, however, have technology is already available in efficiency and attractive aesthetics (see not discouraged the massive Europe, it is still carving its way into fig 1). implementation of thin-film techniques the US market through pilot to produce solar cells for many solar programmes developed by the Photovoltaic glass powered commodities such as US Department of Energy. Another common application of thin- calculators, watches and toys. film deposition is in the creation of Today, the quest for higher solar Low pressure cold spraying photovoltaic glass [1] used in solar energy conversion efficiencies has Representing an innovative step cells. The technology of capturing resulted in alternative semiconductor forward from the old screenprinting of solar energy and converting it into materials and deposition methods.

www.glassmediaonline.com solar power relies on the well-known Some of these materials include continued » 2 Glass International June 2008 CenterLine-GlassInt-June2008-QK 10/6/08 12:06 Page 3

Cold spray technology

Fig 5. Schematic of low-pressure cold spray process.

silver frit, today’s low pressure cold spray technology offers an economical and reliable solution for producing busbars on heated and photovoltaic glass (see fig 4). In contrast to all other techniques used in architectural glass production, the low pressure cold spray process is characterised by simplicity and reliability, features that bid for widespread metals, cermets and other material Minimum setup is required. implementation of cold spray for combinations. With the conductive Low-cost materials. architectural glass manufacturing. materials being sprayed at very low No pre- or post-treatment of glass Cold spray is an emerging solid state temperatures, the cold spray process is required. spraying process in which the coating does not affect the glass, nor does it No masking of glass is necessary. materials are not melted. Instead, the require any special surface preparation. High flexibility and easy kinetic energy of supersonically Robotic low pressure cold spray has changeover. accelerated solid particles is converted already been successfully used to Consistent bonding properties. into interfacial heat upon impact with create busbars on thin-film heated No residual stresses in the coating. the substrate. During low pressure cold glass (see fig 6) using a proprietary Low porosity of the busbar. spraying (see fig 5), pressurised air or blend of non-ferrous materials. Cold Low oxygen content. nitrogen (60-140 psi) is heated and sprayed busbars exceed the bond Can be used on any glass surface directed to a convergent-divergent strength and electrical resistance profile. (DeLaval) nozzle. At the divergent part requirements set forth by the industry. No special procedure required for of the nozzle, the expanding gas Since cold spray does not require any lead-free soldering of lead-outs. accelerates to supersonic speeds while special pre- and/or post-treatment, its temperature sharply decreases to less cost savings associated with this The author acknowledges than 100ºC. Powder feedstock is process have sparked interest from discussions about heated glass injected downstream at the diverging glass manufacturers. with Jim Olzak, President, section of the nozzle [2] where it gains Some of the advantages of cold spray Nextronex, Toledo, USA (Email: velocities of up to 600m/s. Particles over traditional screenprinting [email protected]). Contributions charged with this kinetic energy processes for busbars creation are: by CenterLine-Supersonic Spray impact the substrate, producing a Technologies, Thermique combination of metallurgical and Technologies LLC, CSG Solar and [3] mechanical bonding . Heat the University of Toledo are very Cold spray is capable of much appreciated. producing thick coatings that exhibit References extremely low [1] Nath, P, Singh, A, ‘Photovoltaic porosity (<1%), Window Assembly’, US Patent with avoidance of 5,228,925, United Solar Systems, Co., oxidation, phase July 20, 1993. transformations and [2] Kashirin, AI, Klyuev, OF, and tensile residual stresses Buzdygar, TV (2002), ‘Apparatus for for a wide selection of Gas-Dynamic Coating’, US Patent 6,402,050, June 11, 2002. [3] Assadi, H, Gartner, F, Stoltenhoff, Electrodes by T, and Kreye, H (2003) ‘Bonding Cold Spray Mechanism in Cold Gas Spraying’, Acta Materialia, (51) 15, September 3, 2003, pp 4379-4397.

Thin Conductive * Dr Julio Villafuerte, Director of Coating Research and Development, CenterLine (Windsor), Canada. Email: Fig 6. Schematic of a finished heated-glass unit. [email protected] Website: www.cntrline.com www.glassmediaonline.com 3 Glass International June 2008