nHALO Hot Aerosol Layering Operation - A New Method to Coat Glass using Atmospheric Coating Technique by Beneq

Topics covered

Introduction
The Working Principle
Versatile and economical
Coloring Glass with nHALO
Equipment for Production
Conclusion

Introduction

The glass industry of today is a highly competitive field of innovation driven development. Producing plain flat glass no longer suffices for economic growth, and hence producers are relentlessly fostering added value products, especially in the shape of special coatings on glass - and the need is ever rising. Today, more than half of all flat glass (float) is coated with one or many, more or less functional coatings. Properties like color, low emissivity, solar control and self-cleaning have become daily bread in any glass producer's vocabulary. Several means have evolved for applying these properties and functional coatings to glass, and so currently a variety of coating methods are used by the industry today, e.g., chemical vapor deposition (CVD), spray pyrolysis, physical vapor deposition (PVD), sol-gel etc. These techniques are either implemented as on-line (continuous) or off-line (batch) coating operations.

With Hot Aerosol Layering Operation (nHALO®), Beneq is introducing an atmospheric coating technique, which is applicable to both on-line and off-line coating solutions. The method can either be used to modify the surface layer of the glass, by allowing particles to diffuse into the glass matrix, or to produce a coating by depositing particles on the top surface only. Applications offered include, but are not limited to, solution and colloidal coloring, Low-e, self-cleaning and enhanced surface hardness.

nHALO was originally developed for coloring objets d'art (see Fig. 1) and tableware. The main incentive of the early research was to lower manufacturing costs by surface coloring glass pieces during forming, thus eliminating the need for color-dedicated furnaces in small-scale glassworks. The development work, however, eventually lead to a robust and viable technique for producing nanoparticulate matter, one application of which was the aforementioned glass coloring. Patenting the unique method followed and further implementation as a new way of doping optical fiber preforms with rare earth elements.

The Working Principle

Basically, nHALO is an atmospheric flame-based particle synthesis process. The flame, which predominantly is a turbulent hydrogen-oxygen flame, is fed with precursor chemicals in a liquid, vaporous or gaseous form. In the intense heat of the flame, up to 2700°C, the precursor undergoes thermo-chemical and -physical reactions, ultimately leading to the synthesis of particulate matter (see Fig. 2). The particles produced by nHALO principally exhibit a size distribution ranging from 10 to 100 nm, depending on the precursor composition and process parameters. The nanoparticles are then directed either to collection or direct deposition, as is the case for glass coloring and coating. The synthesis route of the particles follows the process steps of conventional nucleation-condensation controlled gas-to-particle synthesis. nHALO as a means for particle generation is widely documented in the literature, to which the reader is referred for a more comprehensive description.

Versatile and Economical

nHALO offers a wide array of materials that can be used as precursors. Any material that can be obtained in a liquid or vaporous form can be used as a precursor for nHALO. The simplest raw material is an inorganic compound, e.g., cobalt nitrate, which is dissolved in an alcohol or de-ionized water. Sulfates, alkoxides and halide vapors are also viable raw materials. The resulting list of possible elements is extensive, ranging from the alkali and alkaline earth metals (e.g., Na, Mg and Sr), through the transition and other metals (e.g., Ti, Ni, Zn and Al) to the rare earth elements (e.g., Er, Nd, Pr, Yb and Eu), additionally including metalloids (e.g., Si) and some non-metals (e.g., Se). The solvent, in the case a metal salt is used, is an alcohol or de-ionized water.

The product species of nHALO are single component metal oxides or noble metal nanoparticles (e.g., TiO₂ and Ag), multicomponent metal oxide or noble metal nanoparticles (e.g., Y₂O₃-ZrO₂, Ag-Pd) and metal oxide-noble metal nanoparticles (e.g., TiO₂-Ag). Combinations of elements can be obtained simply by mixing them in the liquid state and feeding them as a true solution to the process.

With the possibilities to produce various metal oxides, multicomponent oxides, nanostructured noble metals and combinations thereof in one step, nHALO creates totally new possibilities for the production of doped nanostructured coatings. These coatings can be used for producing color in glass or functional properties. Dopants, such as in SnO2:F, can also be added to the precursor solution.

Coloring Glass with nHALO

For coloring glass, nHALO has already proven its potential for the flat glass industry, because it produces a durable and chemically stable color, which is inside the glass, not on it. In other terms, the color will last as long as the glass itself. Furthermore, nHALO enables nearby instantaneous change of color or coating material during production. This enables a considerable reduction in the amount of lost glass and labor when, e.g., changing the color of conventionally body-tinted glass. For other applications, such as Low-e, solar control and self-cleaning, nHALO offers the advantage of an alternative method to achieve these market-driven properties.

nHALO is currently being used to color both float (Fig. 3) and cast (Fig. 4) flat glass. The range of colors at present is different hues of blue, yellow, bronze and grey, with more colors under development. The blue color in Figs 3 and 4 is a solution color achieved with oxides of Co. Grey is also a solution color, whereas the brilliant yellow color in Fig. 3 is colloidal, derived from elemental silver. Coloring with nHALO is easy, because the chemistry of the color can be the same as in conventional body-tinted, only the coloring instant and method are different.

Figure 3. nHALO float glass color palette. The colors are (clockwise from top): blue, yellow, grey and bronze.

Figure 4. nHALO colored cast glass.

Recent research reveals the coloration characteristics of blue nHALO colored float glass compared to those of body tinted. In the research, transmission spectra and depth profiles of the colorant for blue, cobalt, is measured. The depth to which the colorants penetrate is a function mainly governed by; process temperature, i.e., temperature of glass prior to, during and after coating; and dwell time, i.e., time for the colorants to diffuse into the glass. In the case of the nHALO colored blue samples in the referenced work, the process temperature is 675°C and the dwell time a matter of minutes.

When comparing the transmission spectrum of the nHALO colored glass (see Fig. 5) with that of a calculated absorption spectrum of Co₃O₄-coloured soda-lime glass, it is seen, that the measured transmission spectrum can easily be interpreted using documented characteristics of Co₃O₄-coloured glass, even when the colorant is introduced to the surface layer only.

The concentration of the colorant material in the nHALO sample was determined by measuring the concentration profile of cobalt in the glass matrix with laser ablation mass spectrometry (LA-MS). Due to the nature of nHALO coloring, the colorant concentration is highest just below the surface of the glass and lowest at a depth of some tens or even hundreds of microns. Figure 6 presents the measured LA-MS signal from a cross-sectional scan of an nHALO colored blue sample. The first 4 µm of sample surface have been screened by the laser beam diameter, and are thus omitted from the analysis. The LA-MS count is reliable until about 14 µm, where the accuracy becomes restricted by low count levels. The solid line shows an erfc (complementary error function) type fit with a characteristic diffusion length of about 4 µm.

By assuming an erfc-type concentration distribution of cobalt in the colored glass, we can cross-check the obtained profile numerically by dividing the erfc-type profile into 1.6 µm slices and integrating over the slices in order to replicate the ablation scans. The results of this operation are shown in Figure 7. A surface concentration that yields the best fit to the measured results is found to be around 32 mg/g, or 3.2 wt-%, for the sample.

As shown in Figure 5, the nHALO colored glass surface replicates well the transmission spectrum of traditionally cobalt colored glass. The measured and calculated concentration levels and diffusion profiles correlate well with calculated values.

Equipment for Production

Beneq offers industrial equipment for applying nHALO to glass production operations. The Functional Coating System (see Fig. 8) FCS 4000F is designed for surface modification (including coatings and coloring) of float glass. The modular system can be adapted as an integral part of the tin bath or as a post-tin bath coating station prior to the annealing section.

All Beneq's products represent proven and reliable technology. The gas delivery, control and burner systems are designed to meet the harshest of operational and work environment requirements. Environmental aspects, especially those related to nanoparticulate matter and exhaust emissions, are handled with uttermost care and expertise.

Conclusion

nHALO is an enabling atmospheric coating technique. It has obvious benefits in conventional applications, such as coloring, but also unprecedented opportunities in the field of functional coatings and surface modifications. With the versatility of raw materials, the realm of properties on glass is virtually unlimited. This technique is now available from Beneq.

This article was originally published in Glass Worldwide, issue 8, Nov/Dec, 2006.

Source: Beneq

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