Heat treatable low emissivity coatings for architectural glass application

Udo Schreiber 1, Ingo Wegener 1, Sven Stille 1, Dr. Jutta Trube 1, Dr. Stephan Küper 1

1 Leybold Optics GmbH, Alzenau, Germany

udo.schreiber@leyboldoptics.com

Keywords

Large area coating, heat treatable low emissivity coating, heat isolation, safety glass manufacturing

Abstract

Heat treatable low emissivity layer stacks available in the market are often proprietary and involve all of metallic, oxidic and nitridic layers. This paper describes an alternative set of layers stacks which comprises only metallic and oxidic layers and which can be obtained with Leybold Optics’ coaters. The stacks consist of a silver layer contained in an appropriate arrangement of different blocking and dielectric layers with a high reflectance in infrared spectra range and high transmittance for visual light. It withstands heat treatment requirements for safety glass production. The described layer stack provides a transmittance of more than 80% at a sheet resistance of less than 3?/□. By splitting the silver into two or three different layers with dielectric films in between it is possible to reduce the sheet resistance to 1 ?/□ at a transmittance of 76 %. Compared to other temperable layer stacks the solution described here consists of metals and oxides only which contributes to fast and reliable process set up.

1 Introduction

Low emissivity coating (low-e) is a standard application in architectural glass industry to reduce heat exchange of buildings through window glass. It reduces heat loss in winter and heating-up in summer whereby the energy costs for heaters and air conditioning facilities are reduced significantly.

Heat treatable low emissivity coatings are designed for efficient production of thermally toughened safety glass. Due to the fact that tempered glass cannot be cut, a low-e coating that withstands the tempering process for safety glass manufacturing is coated on standardized large area glass panes. As it can be produced with high throughput and high coating yield it is a cost efficient way to provide low-e coatings on tempered architectural glass.

2 Layer stack design

2.1 Single low-e

The basic layer stack design shown in the first figure consists of a high reflective silver layer in combination with several dielectric layers. Those are arranged in an appropriate way to achieve good infrared reflection in combination with high visible transmittance.

In order to protect the silver layer from oxidation during tempering by diffusing oxygen atoms it is important to choose proper working points for the dielectric layers as well as for the blocker ZnO:Al.

[Figure 1] Schematic drawing of basic layer stack

2.2 Further layer stack embodiments

The presented basic layer stack can be modified by splitting the silver into two or three different layers, each of them embedded between AZO as seed layer and ZnO:Al as blocker. Splitting silver into several layers makes the spectrum transition from visual to infrared wavelength range steeper and realizes a lower sheet resistance at high visual transmittance.

3 Optical and electrical properties

3.1 Transmittance

Figure 2 shows typical transmittance spectra for single (SLE), double (DLE) and triple silver low-e (TLE) with its maxima around 550nm and different slopes between visual and infrared wavelength range. Due to splitting the silver layer the ability of blocking NIR light improves significantly.