Application of smart glass at low and very low temperatures

"Smart glass" is gradually but indomitably invading our lives. The area of its application is growing day by day. Numerous types of smart glass find their application in various spheres. First of all, it is glazing of buildings and structures, both exterior and interior, starting from shop windows and ending with office partitions. Transportation is not bypassed, and it is not only cars, but also air transport, it is enough to remember the ultramodern Boeing Dreamliner. In marine transportation, smart glass is widely used not only for ships used in tropical regions, where the sun is abundant all year round, it is also in demand in Arctic zones, where protection from blinding sunlight reflected from ice and snow in summer is required. And there is no need to mention the numerous filters for control, surveillance, and augmented reality systems. In other words, smart glass finds application wherever glass as such is used at all. But there are many variants of smart glass, and each has its own niche.

For example, glass or film with Polymer Dispersed Liquid Crystal (PDLC) technology is widely used in applications where privacy is required but there is no requirement to change the light transmission value directly. PDLC glass can become opaque through dispersion - it will still let in light, but you can't see what's happening on the other side of it, at the snap of your fingers - the switching time is a few tenths of a second, faster than the human eye can catch. Another type of smart glass that uses the principle of "nanoblinds" distributed in a polymer matrix: SPD (Suspended particle device) technology is similar to PDLC, but it is used where it is necessary to tint or untint the glass when required, controlling the amount of insolation it provides. SPD and PDLC are excellent examples of well commercialized smart glass technologies that can be found in unexpected places. For example, SPD-based glass has been used extensively to protect rare works of art and cultural heritage that are so dilapidated that even the bright light of showrooms is harmful to them. Such objects are placed in isolated boxes with a constant microclimate behind tinted glass, and if a visitor needs to look at a rare object, he presses a button and the glass become untinted, presenting the original of the rarest thing of the past. Glass with PDLC-technology, in turn, has found application, for example, in the creation of glass walls separating the living area from the sanitary area in small hotel rooms. When there is no one in the bathroom, the wall becomes transparent, which visually increases the modest area of the hotel room, and if it is necessary to take a shower, at the push of a button the glass wall becomes cloudy and opaque, and the bathroom space is isolated from the rest of the room.

But the smart glass market is not limited to SPDs and PDLCs. In addition to these, for example, there is liquid crystal (LC) technology that has the speed of PDLC, but also provides tinting and untinting capabilities for the translucent devices in which it is embedded. In other words, it provides the speed of PDLC and the tinting capability of SPD. But LC does not exhaust the full range of smart glass technologies either. Electrochromic systems (EC) operate on a slightly different principle - through electrochemical processes of oxidation, reduction and/or simultaneous oxidation and reduction of some of their components, or through their intercalation (introduction) by ions of the donor substance, from which the whole system is colored in one or another color, changing light transmission or, in other words, tinting the glass. EC glass has the greatest market potential due to its unique characteristics - for example, the range of achievable light transmission thresholds (so-called "contrast"), when the glass in the lightest state can have a light transmission of more than 75%, and in the darkest tinted to zero light transmission.

The above-mentioned types of smart glass are electrically controlled, they change their characteristics under the action of electric current: light transmission for SPD, LC and EC, haze level for PDLC. But there are other types of systems on the market. They change their properties under the action of temperature or electromagnetic radiation, such as photochromic systems that tint when exposed to ultraviolet light and are widely used in eyeglasses; or thermochromic systems that change coloration when the temperature changes, and some others. Various combinations of several types of smart glass within a single device are quite possible, as well as the emergence of new types with similar or even better characteristics than existing solutions.

But all dreams of wide application of smart glass are significantly slowed down when it comes to natural operating conditions. First and foremost, the problem lies in low ambient temperatures. All existing smart glass technologies significantly change their behavior at low air temperatures, up to complete inoperability, and some types of devices can be damaged altogether, sometimes irreversibly, when frozen. The problem is not only relevant for means of transportation, be it road, rail, water or air transport, but also for architectural solutions for exterior glazing. In addition, besides architecture and transportation, smart glass is also being applied in other areas of technology related to its exterior operation. A good example: controllable filters for optical sights. In this case, the filter has to work both in a hot desert and on a glacier in the mountains. And if in a car the smart glass heats up from the heat of the interior, and the problem is not that serious, then the filter of a sight has nothing to heat up from, and it must work.

Engineers developing smart glass technologies and the products themselves are trying to invent methods to improve product performance in harsh winter conditions. The most obvious solution for electrically controlled technologies is to change the nature of the power supply to the glass. As a rule, all electrically controlled smart glass technologies are made of materials whose electrical conductivity decreases with temperature, and the lower the temperature, the higher the voltage loss across all elements of the device. Eventually the voltage sag becomes critical, and the device's performance changes to the point where this fact becomes noticeable to the user. The device starts to switch slowly, the coloring of the device may change, the characteristics may not change over the entire area of the glass, and so on. But changing the power supply does not completely solve the problem of very low temperatures, and for non-electrically controlled types of smart glass such a method of solution is not applicable.

The crux of the problem is basically that smart glass technologies rely on the mobility of the medium in one way or another: charge carriers move in the electrolyte; nano shutters are oriented along the force lines of electric fields; being encapsulated in mobile polymer droplets, liquid crystals are formed or rotated in a dense medium. When the temperature drops, all these environments thicken and prevent the device from functioning properly. What should be done in such a case?

Octoglass engineers have found a solution to level out the harmful effects of low temperatures on smart glass. It is about additional heating of smart glass.

Since the beginning of the use of ordinary glass, mankind has encountered an unpleasant moment associated with the relatively high thermal conductivity of the glass itself. The glass is covered with frost, water freezes on it and ice crust is formed, after a snowfall there can be a whole snowdrift on the glass, which will frost and reliably freeze, and even at non-negative (on the Celsius scale) temperatures, when water remains liquid, it tries to condense on the surface of the glass, and the glass loses its transparency, fogging. To solve this problem, people have developed different methods. For example, can be used multi-chamber multi-glazed window, which has a significant level of thermal insulation, not allowing the outer glass to be too warm, and the inner - too cold. But not everywhere can a multi-glazed unit with gas or vacuum interlayers be used to provide adequate thermal insulation. In some cases, such as in automobiles, the shape of the glass, its weight and thickness does not allow the application of a full multi-glazed unit. In addition to ways to increase thermal insulation can be used additionally applied hydrophobic coating, preventing the adhesion of water to the surface of the glass. But such coatings are not durable, easily damaged. By and large, the only option left, perhaps, is forced heating.

Heating in a simple and cheap way solves the problem of icing and condensation and can be done in a lot of well-known ways. The simplest is installing an infrared heater in front of the glass or constantly blowing warm air on the glass. Slightly more technologically advanced methods involve integrating the heating system directly with the glass. The most common method is the application of a grid of electrically conductive material on the glass surface: when voltage is applied to the grid due to the relatively high ohmic resistance of the material, the grid heats up the glass itself. In this way, for example, rear windows in passenger cars are heated. But such heating cannot be used on the windshield, as the mesh tracks are quite thick and limit the visibility for the driver. Therefore, some manufacturers have decided to integrate a thin wire in the form of a sparse spiral, usually made of a refractory material with high electrical resistance, such as nichrome, directly into the structure of the glass. For this purpose, laminated glass is used, where the thin and extremely vulnerable to mechanical influences heating spiral is safely encapsulated inside the laminated glass unit. Such heating operates over the entire area of the glass, while itself does not attract attention, as the spiral of extremely small diameter wire is used, and you can notice it only if you look closely. For example, many modern cars, such as Ford, Rolls-Royce, Land Rover, Lincoln and some others, can boast of such a windshield.

The integration of spring heating inside the glass is quite labor intensive, so some other manufacturers, especially those making bullet resistant glazing, use a slightly different approach. Bullet resistant glass is laminated glass, where multiple layers of glass are joined by plastic. With high degrees of protection, the thickness of such glass can reach a dozen or more centimeters. It is impossible to heat such glass with a blower or a small infrared heater. And integrated springs will not give a tangible result, because bullet-proof or laminated safety glass of great thickness is required to protect against ice from the outside and fogging from the inside, and a large number of springs will make the glass opaque. But even in such a difficult task manufacturers have found a way out. One or several layers of glass with conductive coating of metal oxides - so-called TSO-coating (Transparent Conductive Oxides) are used. Such glass is produced by several companies, is not so expensive, but has excellent light transmission characteristics. A prime example of TCO-coated glass is the TEC line of glass from NSG (which owns the Pilkington brand).

Due to the large thickness of bullet-resistant glass, one or more such panes are used as part of a laminated insulating glass unit; if necessary, an electric current is applied to them and the glass is heated evenly over the area and volume. As an alternative to glass with conductive coating can be successfully used polymer film with a similar coating. The film is laminated into a laminated glass unit in the same way as glass and heats up when an electric current is applied to the transparent conductive layer of the TCO coating. There are no springs or other elements in the visible area of the glass, while maintaining excellent optical quality and strength characteristics.

When a large area of glass is to be heated, the conductive surface is divided into sectors to ensure uniformity of heating: the laser cuts tracks that isolate small conductive areas with supply tracks. By the way, a similar scheme with cutting tracks is used in glass interior heaters made of the same type glass. By the way, cutting tracks on the conductive surface helps in radio communication, as it leaves a small loophole for the penetration of radio waves. After all, a solid conductive coating of metal shields radio waves quite well, and in some cases it may be impossible to use radio communication at certain frequencies.

In practice, when it is necessary to manufacture smart glass for operation in a wide range of temperatures, including low temperatures, we at Octoglass use heating integrated into the glass unit from one or more substrates - glass or polymer film with an applied conductive layer. The final insulating glass unit is laminated in a way suitable for the use of smart glass. If the heated part is located closer to the outer side of the glass, the one facing the interior, this heating will be used to thaw ice and snow on the outside of the glass and maintain the optimum temperature of the smart glass layer. If the heating layer is located closer to the interior facing part of the glass, such heating is used to prevent condensation on the glass from the interior side and also to maintain the optimum temperature of the smart glass layer. Since ordinary smart glass is multi-layered, but does not go beyond the thickness of ten millimeters, it is enough one heating layer, it is able to heat the entire volume of glass. Well, in the case of integration of the "smart" layer in the protective bullet-resistant double-glazed window, which has a significant thickness, it is appropriate to use several heating layers. In general, there are many variations and combinations of manufacturing, it all depends on the solution of specific tasks and requirements for the final product.

But in addition to the purely technological task of integrating heating into smart glass, we should not forget about heat management. After all, high temperatures can be disastrous for smart glass, as well as for the entire laminated glass structure. Heating can be powered in a variety of ways, it can be supplied with direct or alternating current, can use pulse current, can be used potentiostatic or galvanostatic power supply schemes, special algorithms of voltage supply and other kinds of ways, and in certain cases even exotic multipolar schemes. But the task of temperature control can be carried out both by a trivial temperature sensor and by changing the electrical characteristics of the heated layer. After all, as the temperature of the layer itself changes, its resistance also changes. But, since the main specialization in "Octoglass" is smart glass, we already control the "smart" part of the glass by means of a specialized controller. And we integrate the controller of the "smart" part of the glass with the controller of its heated part. In this way, it is possible, firstly, to automate the heating control if necessary, and secondly, to ensure optimal working conditions of the "smart" layer in the glass. As a result, the optimal temperature mode of smart glass operation is created and maintained during its operation in severe cold climate conditions.

Of course, adding heating not only complicates the design of the final product, but also increases its cost. We are often approached by customers who are concerned about how our smart glass will work in winter on a car operated in the harsh conditions of Siberia, where temperatures of minus 40 Celsius and below are not uncommon. Where spit manages to freeze into ice as it flies to the ground. We warn customers that in minus 40 and below you will not use smart glass, because, except for cases, if the car is stored in a warm garage or is not turned off during parking, it will not start itself. And if it does, it will not be able to move, because the transmission and rubber freeze to such an extent that it is impossible to use them. And with the engine running and the interior warmed up, the temperature of the smart glass is already in optimal operating conditions. But for some customers this is not enough, and for them variants with additional heating are realized. Of course, there are cases when the integration of heating is simply necessary, for example, in railway transport when glazing the driver's cabin. In this case, the issue is safety, and other arguments such as additional cost and complication of the design are not taken into account.

So, there are different types of smart glass in the world, made on the basis of different technologies, each type has its own main areas of application depending on the key characteristics. Somewhere the speed of response is important, somewhere the highest contrast, somewhere maximizing privacy. All types of smart glass have problems in one way or another when operating in a wide range of temperatures, most notably low and very low ambient temperatures. Low temperatures not only degrade the performance of the final product, but can also lead to partial or complete degradation of the smart glass. One of the most effective solutions to combat low temperatures is, of course, to heat the smart glass to its normal operating temperature. In turn, there are several ways to heat the glass, ranging from blowing them with warm air to integrating heated elements directly into the glass. In our opinion, the most optimal way of heating smart glass is the use of conductive coatings based on metal oxides, which can be made both on the glass and on the polymer film. Electric current is supplied to the conductive coating, and the glass is heated to the required temperature, which, in turn, is maintained and regulated either by primitive feedback controllers or by complex devices based on microprocessors that analyze many parameters and use unique algorithms to power the conductive coatings. In this version of the heating design, the appearance of the final product does not suffer, the optical properties are not degraded, and overall reliability is significantly higher than alternative heating methods. Moreover, the glass or film with conductive coating can be easily integrated into protective, including bullet-resistant glass, without revealing its true thickness to an unauthorized observer, which is especially important when it is used for hidden armor of objects such as cars or facade glass.

May, 2024