It is known from theory that growing oxide layers on metal surfaces effect interferences of the spectral emissivities.
These interferences can strongly change with increasing thickness of the oxide layer, because their maximum and
minimum values shift to longer wavelengths and their intensities become larger or smaller. The changes of spectral
emissivities also lead to changes of the band-emissivities used as values for emissivity correction of pyrometers or IRcameras.
Primarily, these effects depend only on emissivity and can occur without changes of metal's temperature. For
the working ranges of the optical temperature instruments follows, that the values of band-emissivities may change
constantly with growing oxide layers. This will result in problems for an accurate temperature measurement.
At the University of Duisburg-Essen it is now possible to carry out in-situ measurements of the spectral emissivities of
growing oxide layers on metals. These investigations can be done under technically relevant conditions of heating time,
and temperature. Low alloyed steel with a growing oxide layer was investigated. The steel sample was heated up to
1150°C within a period of 2 minutes and, further, tempered for 1 minute more at this temperature. During this period,
the oxide layer was steadily growing and the changes of spectral emissivities were measured at several increments of
time. It was found, that the theoretically predicted interference effects, their changes and shifts in the spectral properties
are readily seen and could be measured.
Results will be presented of the spectral emissivities from 0.7 to 25 μm and of based on it calculated band-emissivities
between 0.7 to 4.5 μm, which are in correlation with the working ranges of three chosen pyrometers. Strong and
relatively fast changes of the spectral emissivities can be achieved with values between 0.6 and 0.9. Therefore, during
the growth of the oxide layer accurate temperature measurements with optical instruments make no sense because of
the permanent change in the values of band-emissivities. An exact determination of the temperature under these
circumstances is not possible. Only weak influence on the values of band-emissivities were found when correlated with
the spectral response functions of the chosen pyrometers.
The thermal radiation properties of different metals are taken over a large temperature range because they are needed as
material data for construction and process control of thermal technical plants and for pyrometric temperature
measurements. This includes non-ferrous metals and steel for heat up and heat treatment technologies as well as
construction metals for high temperature installations and units. Metals are generally used for technical applications as
specially alloyed materials, for which the thermo-physical properties in most cases are not sufficient known. That
concerns also the radiative properties within the range of higher temperatures, which therefore have to be determined.
Based on the treatment and application of metals in higher temperature ranges the emittance is the most qualified
radiative property for use in heat transfer calculations and for adjustment of temperature measuring instruments. At the
university of Duisburg-Essen measurements of spectral emissivities for a large number of metals are carried out. The
measuring systems, based on a monochromator and a FT-IR-spectrometer are described. A survey of results for
different metals is given. The influences of temperature and roughness are investigated. Other results lead to the
existence of a so called X-point. The oxidation of metals show drastically changes of emissivities. For oxidizing steel
the theoretical expected interferences of spectral emissivities could shown by measurements.
In technical heat treatment processes - e.g. the sintering process - the kiln as well as the heat treated material need to have a fairly homogeneous temperature distribution. In the kiln the heat is transferred from the furnace walls to the kiln aids and from the aids to the heat treated material. An example for such processes is the sintering of ceramics, where the heat is transferred from sintering aids to the sintering ceramic material. To guarantee an optimized heat treatment the details of the heat transfer need to be known. Many heat treatment procedures operate at such high temperatures that heat transfer is mainly dominated by radiation, but the emissivities of typical refractory materials of kiln and sintering aids in general are hardly known. Therefore, the spectral emissivities of some typical kiln materials and sintering aids were measured in the temperature range from 200 to 1200°C and in the spectral range from 0.8 to 25 μm at the radiation measurement device of the University Duisburg-Essen. These data were used to compute the temperature-dependent total emissivities. The paper describes the equipment for radiation measurement, the measured temperature-dependent spectral emissivities as well as the computed temperature-dependent total emissivities. It was found out, that different materials have different temperature-dependent spectral and total emissivities which may significantly influence the heat transfer in high temperature processes.
Ceramic materials, in particular high temperature technical ceramics and refractories, are used in a wide range of temperature. Band and total emissivities of ceramics are needed for non-contact temperature measurements and for modeling of radiation heat transfer. If the spectral emissivities are known in a sufficient range of wavelength and temperature, the band and total emissivities for most applications can be calculated. At the University Duisburg-Essen an experimental setup for measuring of temperature-dependent spectral emissivities was modernized with a FT-IR-spectrometer. A remarkable quality improvement of the measurements could be reached. The spectral and temperature ranges of the measuring device are now 0.8 to 25 μm and 100 to 1200°C. In the paper a description of the measuring device will be given and selected examples of measuring possibilities of the spectrometer are shown. By the example of high temperature ceramics the obtained improvements of the device are presented for measured temperature-dependent spectral emissivities.
Ceramics are used as construction materials for buildings and thermal technical plants. Depending on the fields of its application between ambient temperature and more than 1000 °C there are different ceramic materials in use. For the temperature measurements with pyrometers and infrared cameras band emissivities are needed as settings. Pyrometers and infrared cameras have different spectral work ranges. Therefore, for different devices different emissivities are needed for one and the same material. Selectivity of the spectral emissivities like with ceramic materials can lead thereby to larger differences between the emissivities of a material, and furthermore to temperature dependence of the band emissivities of a material. Examples of different temperature-dependent spectral, band, and total emissivities are shown. These emissivities for different work ranges of pyrometers and infrared cameras were computed based on measured spectral emissivities. The investigation leads to a selection of suitable band emissivities for radiation thermometry of ceramics.