Method SNEHT

Method of measurement of the spectral normal emissivity at high temperature

Components of the measurement system

Detection system
Nicolet 6700 FTIR spectrometer
Reference radiation source
Laboratory blackbody Omega BB-4A
Laser system of sample heating
400W fiber laser, scan head
System for sample surface temperature measurement
FLIR A320 infrared camera (thermocouple measurement system Adam 4018)
Opto-mechanical components
Sample holder, rotary parabolic mirror, motorized and manual micro-positioners, motorized shutter, alignment lasers, thermocouples, optical apertures, filter unit, optical box

Description of the measurement system

Schematic view of the SNEHT method

The reference radiation source and the sample are placed against each other outside of the spectrometer. In the middle between them, there is inserted a rotary parabolic mirror, which collects radiation alternately from the two sources while preserving the same optical path. The collected radiation from the source is incident through an external port (can be closed by motorized shutter) and spectrometer sample space (apertures) on the detector. The optical path is covered by an optical box. The optical box temperature is controlled at two places (near the laser scan head for the sample heating and at a point between the sample and the infrared camera) by unsheathed calibrated thermocouples.

The measured sample is placed in a holder made of ceramic fiber insulation, which is mounted on an optical table so that it is possible to precisely position it by linear translation stages and a mounting rod. Heating of the sample is realized by the fiber laser with the scanning head. The desired sample temperature is achieved by controlling the laser power. The desired homogeneous temperature field on the front side of the sample is achieved by an appropriate choice of time-space laser beam movement across the back side of the sample.

Surface temperature of the sample is measured by several methods. The first method uses an infrared camera and a reference coating with a known effective emissivity coated on one half of each sample. Another method is based on a contact temperature measurement by a thermocouple welded to the surface of each sample. The last method combines Christiansen effect with contact and non-contact temperature measurement systems.

Gallery

Temperature ranges

250 to 1000°C, depending on the measured sample (shape, size, thermal conductivity)

Wavelength ranges

The maximum range of 1.38 to 26 μm (MIR) depending on temperature

300°C2.5 to 26 μm
400°C2.0 to 26 μm
500°C1.8 to 26 μm
600°C1.66 to 26 μm
> 700°C1.38 to 26 μm

Measured quantity

  • normal spectral emissivity, notation ελ (λ, θ, T), θ=0°

Computed quantities

  • band normal emissivity, notation ε̅ (θ, T), θ=0°
  • total normal emissivity, notation ε (θ, T), θ=0°
  • spectral normal absorptivity, notation Aλ (λ, θ, T), θ=0°
  • band normal absorptivity, notation A̅ (θ, T), θ=0°
  • integrated normal absorptivity, notation Aint (θ, T, Ts), θ=0°

Analyzed materials

  • opaque coatings (thickness of tens to hundreds of micrometers) on a bulk substrate
  • semitransparent coatings (thickness of tens to hundreds of micrometers) on an opaque bulk substrate (measured is emissivity of coating - substrate system)
  • ceramics
  • metals
  • insulators

Dimensions and shapes of samples

Specification of samples for the SNEHT method

Standard samples

A1
circular samples with a diameter of 25 mm and a thickness of 5 mm (standard substrates used - refractory steel CSN 41 7153, blasted on the front side)

Non-standard samples

B1
circular samples with a diameter of 18 to 30 mm and a thickness of 0.5 to 5 mm
B2
square samples with an edge size of 18 to 30 mm and a thickness of 0.5 to 5 mm
B3
square samples with a small curvature and edges with size of 18 to 30 mm and a thickness of 0.5 to 5 mm
B4
individual dimensions and shapes of samples

Additional requirements for samples

  • analyzed bulk materials and substrates must not transmit radiation with a wavelength of 1064 nm and must withstand the required thermal load
  • maximum roughness of the part of the sample, on which will be applied the reference coating is Ra = 5 μm, Rz = 35 μm

Time-temperature measurement modes

Temperature modes of the SNEHT method
mode 1
step heating of one sample - increasing of the temperature from 300°C to 1000°C with a temperature step of 100°C
mode 2
heating of one sample with temperature cycling - cooling to room temperature before raising the temperature to the next temperature level
mode 3
heating of new sample at each temperature level - a new sample is heated from room temperature directly to the next temperature level
mode 4
individual time and temperature mode – on demand defined sequence of time and temperature steps of sample heating and emissivity measurement

Outputs of the method

Example of a graph result from the SNEHT method
  • spectral normal emissivity - graphical display of spectral emissivity distribution (separately for each measurement) and data in electronic form; spectrum is influenced by atmospheric absorption (higher uncertainty) in the bands of 2.5 to 2.95 μm, 4.17 to 4.5 μm, 4.8 to 7.9 μm and from 13.2 to 17.2 μm

Form of the results

basic

  • measurement protocol in electronic form in PDF format or in printed form - example
  • XLSX file that contains a measurement protocol in electronic form and emissivity values including measurement uncertainty

extended

  • technical report in electronic form in PDF format or in printed form; the report contains description of method and measurement procedure, measured samples, summary of results of all measurements, discussion of the results and conclusions of the research
Petra Honnerová

Responsible person

Ing. Petra Honnerová, Ph.D.
Phone: +420 377 634 719
Mobile: +420 735 713 911
Email: petrahon@ntc.zcu.cz

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