Possible data sources: evitherm, CINDAS

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Are the data relevant to my application?

Below are comments from the evitherm project team giving some guidance on selecting data that are suitable for the material and the application of interest, addressing each thermal property in turn.


Thermal conductivity and diffusivity of glasses

Heat transfer in glasses is determined by the solid thermal conductivity and radiative heat transfer. Most glasses are semitransparent for thermal radiation. Radiative heat transfer is significant at higher temperatures and increases with the third power of temperature. The solid thermal conductivity/diffusivity of glasses depends on their chemical composition. For the determination of the thermal conductivity/diffusivity of glasses the radiative properties of the glass has to be taken into account to yield reliable results.

Expansivity and density

Thermal expansion behaviour up to 150 °C below the glass transition temperature should be repeatable, but at higher temperatures the fictive condition of the glass, i.e. the state of annealing and the hold time below the annealing temperature, may determine the characteristics. On heating through the glass transition temperature there is usually a significant increase in expansivity as the material becomes more liquid-like. In mechanical dilatometers, when the viscosity has dropped below about 10¹⁰ Nsm^-2 the small compressive force applied by the pushrod will result in an apparent peak followed by a fall in length as viscous deformation occurs. The glass transition temperature is often determined from the temperature of the intersections of tangents drawn to the apparent expansion curve below and above the transition region.

Emissivity and other infrared optical properties

Glass has a high transmissivity in the visible and near-infrared region and is non-transparent (opaque) in the mid-infrared and far infrared region. Additionally, glass has a high emissivity in the infrared region. A reflection peak at about 10 µm is characteristic for glass. Glass is a non-scattering medium. Incoming radiation is partly reflected at the interface between air and glass, partly absorbed within the glass and partly transmitted through the glass.

In many applications glass consists of more substances than silica or SiO2, and this may change the infrared-optical properties. Additionally, glasses are often coated to optimise their properties. Anti-reflection coatings, for example, reduce reflectivity at certain wavelengths, mainly in the visible spectral range, to get a better transparency. Low-e coatings decrease the emissivity in the infrared region, to reduce heat loss due to thermal radiation. Mostly, the low-e coatings are selective, which means that they are transparent in the visible wavelength region and highly reflecting in the infrared wavelength region. Thus, in a building a window would be good at allowing light in but preventing heat (infrared) from being radiated out through the window.

Specific heat capacity
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