Technology fundamentals

Outline

The sensor coatings developed by STS are based on integrating phosphor materials into functional coatings. The principle of phosphorescence is covered here. Essentially it is a process whereby light is used to excite the material and as a result light of a different wavelength (colour) is emitted. The emitted light can be analysed to give important information about the state of the material.
These types of materials, often called phosphors, can be found in energy-efficient lighting, plasma screens and LEDs.

How does luminescence work?

Phosphors are a particular type of ceramic materials which emit light after being excited by an external light source. They are usually white inorganic crystalline materials, synthetically manufactured. The material is not to be confused with phosphorous. The external light source can be a laser or an LED for example, which emits UV light.
The optically active components within the phosphor are either rare earth or transition metal elements. When excited with UV-light, these dopants absorb the energy (directly or indirectly), which promotes their electrons to higher energetic states (figure: A > B).

These higher energetic states are unstable, and the electrons eventually fall back to their stable ground. However, the transition back can only occur according to quantum mechanics selection rules. The rules determine the time it takes for the electrons in the luminescent material to fall back to their ground state by emitting light (figure 1: C  D). This time is characterised by the lifetime decay, which is an important parameter we measure.

Figure 1 configurational coordinate diagram [1]

Electrons are only allowed to exist at specific discrete energy levels. For trivalent rare-earth dopants (RE3+), the energy levels are given by the Dieke diagram [2]. Such a diagram (an extract is shown in figure 2) can be used to estimate the emission spectra of a phosphor doped with RE3+ elements.

Figure 2 Energy levels of some trivalent rare-earth elements [3]

An important parameter for the choice of the dopant for the application is the energy band gap, illustrated in figure 2 by the grey bands. The larger the energy band gap, the wider its temperature dynamic range.

What do we measure?

The optical properties that are measured are of spectral or temporal nature. The later only can be obtained if the excitation light is pulsed.
Spectral measurements involve the measurement of the emission spectrum, the intensity of certain emission lines or the intensity ratio between different emission lines of the phosphor. This can be obtained under both continuous and pulsed UV excitation.
Temporal measurements involve the measurement of the light emitted from the phosphor as function of the time. The rise time characterises the emission of the phosphor as function of the time when the light excitation starts to excite the phosphor. The decay time characterises the emission of the phosphor as function of the time when the light excitation ends.

Figure 3 Excitation and emission in a coated layer [3]

How is the measured luminescence related to temperature?

With increasing temperatures, the optical properties of the phosphor change reversibly. When the phosphor is put back to room temperature, the optical properties are restored.
The lifetime decay of the luminescence decreases with increasing temperature, which is illustrated in figure 4 (left). This is due to the increase of the phonons (quantised vibrations) within the phosphor. These quench the luminescence, when their number and energy is sufficiently high. The decrease of the lifetime decay starts at a certain temperature, dependent on the host and the dopant used. This “starting” temperature is sometimes called the knee temperature.
Southside Thermal Sciences have characterised several phosphors for temperature measurements. Figure 4 (right) shows the calibration curves for lifetime decays of several phosphors that can be used for thermal sensing.

Figure 4 (left)Luminescence decay after a pulsed excitation. The decay is exponential and dependent on temperature [4] (right) The lifetime decay of several phosphors have been calibrated against temperature [5]

Another parameter which changes with temperature is the emission intensity, which reduces with increasing temperature. This is due to increased vibrational energy within the phosphor, which quenches the luminescence. The emission intensity can be calibrated with temperature and used as indicator of temperature.
In some phosphors, the ratio between two or more lines in the emission spectrum can change with the temperature, such that it can be calibrated and used as indicator of temperature (see figure 5). The advantage of this technique over the measurement of the emission intensity alone is that the ratio is independent of the lighting conditions, the excitation strength and angle of illumination and observation.

Figure 5 Normalized emission intensity spectra of a phosphor at different operating temperatures [3]

What other parameters can we measure, apart from the temperature?

The ”online” temperature is not the only parameter we can measure with these intelligent coatings. We have identified other parameters for which there is a change in emission properties, and thus a possibility of using for measurement purposes.

• Thermal history
  
  Some of the phosphors we are using have the capability of changing irreversibly when exposed to high temperatures. After cooling down the phosphor to room temperature, the changes remain and can be interrogated at room temperature. The changes occurring in the phosphor might be of various physical and chemical natures. The changes are reflected in the luminescence properties and can thus be used as a measure of the thermal history. The phosphor then acts as a tool that remembers the thermal conditions it has been exposed to for a certain time.

Figure 6 Thermal history sensing [4]

• Ageing
  
  Phase changes occur in the coating material when it is exposed to high temperatures for extended periods of time. These phase changes cause subtle changes to the luminescence characteristics of the material which can be calibrated and quantitatively related to the percentage of the new phase. Furthermore the effect of aging does not affect the temperature measurement capability which is outlined in the previous section, as such both detection methods can be conducted using the same coating.

Figure 7 Aging Intensity Ratio [6]

• Erosion
  
  The emission properties of the phosphorescence are primarily dependent on the dopant used. As such different dopants in the same host emit light at different wavelengths (colours). Therefore if different dopants are applied in layers in a coating, as the coating is eroded the different colour layers are revealed. The structure of the coating is known so a quantitative measurement of erosion can be made simply by imaging the luminescence across the surface.

Figure 8 Rainbow Coating for erosion detection [6]

• Corrosion
  
  During corrosion the chemical composition of the coating material changes whereby certain materials attack the coating. For example, vanadium attack, typical during hot corrosion, causes a significant change to the emission spectrum which can be detected using optical equipment.

Figure 9 Corrosion detection [6]

References: