(Gd,Lu)AlO₃:Dy³⁺ and (Gd,Lu)₃Al₅O₁₂:Dy³⁺ as high-temperature thermographic phosphors

The phosphor powders used in the present study were prepared by the conventional high temperature solid-state method such that they had the following overall compositions: (Gd_0.8Lu_0.2)_2.97Dy_0.03Al₅O₁₂, (Gd_0.8Lu_0.2)_2.94Dy_0.06Al₅O₁₂, (Gd_0.8Lu_0.2)_2.91Dy_0.09Al₅O₁₂, Gd_0.98Dy_0.02AlO₃, (Gd_0.78La_0.2)Dy_0.02AlO₃ and (Gd_0.78Lu_0.2)Dy_0.02AlO₃. The amount of Lutetium (Lu³⁺) was chosen based on the study of Li et al [22], who observed decreasing intensities of both the Gadolinium (Gd³⁺) and Dysprosium (Dy³⁺) excitation bands for amounts of Lu³⁺ higher than 20%, due to the higher electronegativity of Lu³⁺ ($\chi = 1.27$) than Gd³⁺ ($\chi = 1.20$). For comparability, a Lanthanum (La³⁺) stabilized host was synthesized using the same amount of La³⁺ as Lu³⁺ . The main starting materials for the solid-state synthesis were Gd₂O₃, Lu₂O₃, La₂O₃, Al₂O₃ (all 99.99%, Alfa Aesar) and Dy₂O₃ (99.9%, Reacton), which were mixed according to the stoichiometric ratio. Lithium fluoride (LiF) was selected as flux material in order to decrease the sintering temperature and to improve the phosphor efficiency. The atomic radius of Dy³⁺ (105.2 pm) is slightly lower than that of the Gd³⁺ (107.8 pm) atom, which can cause lattice site defects. The incorporation of smaller ions such as Li⁺ and F⁻ can work as a compensator in the crystal lattice. The addition of 10 wt.% of LiF to the stoichiometric compositions supports the homogeneous distribution of Dy³⁺ ions in the lattice and promotes both a higher crystallization degree and reduced surface defects. Generally, the synthesis was done for 1 g of the final product and all starting materials were mixed and ground thoroughly in an agate mortar. After grinding, powders were heated in an alumina crucible in air for four hours at a heating temperature of 1473 K. Subsequently, the samples were cooled to room temperature and ground again to fine powder. In contrast to the GAP samples, the fabrication of GAG phosphors is not trivial due to its structural metastability, and special care must be taken during synthesis. The crystalline structure of the samples was characterized by x-ray diffractometry (Panalytical EMPYREAN XRD Powder/SAXS Diffractometer) with a Cu-K$\alpha$ radiation source ($\lambda=1.5406~\mathring{\rm A}$) at room temperature. The morphology and the size of the phosphor powders was imaged with a scanning electron microscope (SEM, JEOL JSM-7610F) equipped with a field emission gun operated at 5 kV. The optical properties of the powders at room temperature were measured using a spectrofluorometer (Jasco FP-8500) equipped with a Xe arc lamp as the excitation source. Photoluminescence excitation and emission spectra were recorded under identical conditions for all samples with a spectral resolution of 5 nm on both, excitation and emission side. The luminescence emission spectra were measured for excitation wavelengths of 275 nm and 352 nm, respectively.

To determine the temperature-dependent luminescence characteristics, the phosphor samples were heated from room temperature up to 1600 K. Each phosphor sample was placed into a corundum ceramic sample carrier and was reproducibly positioned inside a high temperature oven (Supertherm HT 18/10) with a reproducible regulating inaccuracy of 5 K. The bulk density was kept nearly constant for all samples, analysing the same mass of powder in a container with constant volume. A type B thermocouple (shielded to avoid radiation effects, 0.75% inaccuracy) was used to monitor the temperature in the proximity of the phosphor sample. The third harmonic of a pulsed Nd:YAG laser (Quantel Q-smart 450, pulse duration: 6 ns, repetition rate: 10 Hz) with a laser fluence of 0.03 J cm⁻² per pulse was used for excitation. The chosen laser fluence allowed to obtain sufficient signal at high temperatures but it was low enough to minimize heating effects in the sample. Excitation and detection were carried out along the optical access of the furnace, using a 355 nm dichroic mirror. The subsequently emitted luminescence was imaged onto the detector by two spherical lenses positioned just outside of the furnace, which collimated and focused the luminescence signal. A 355 nm longpass edge filter blocked the backscattered laser light ahead of the detector. The luminescence lifetime was recorded with a high-speed photo diode (Thorlabs, DET210) in combination with an oscilloscope (PicoScope 6, real-time sampling rate of 200 ${\rm \mu}$s div⁻¹) and averaged over 51 shots. A bandpass filter (494 $\pm$ 10 nm) was placed in front of the photo diode to measure the specific lifetime of the ⁴F_9/2 $\rightarrow$ ⁶H_15/2 transition of Dy³⁺ . An Ocean Optics Flame-S spectrometer covering a wavelength range of 400 nm to 700 nm was employed for spectral emission analysis. The spectrometer was equipped with a 16 bit 2048 Pixel CCD detector, a slit width of 25 ${\rm \mu}$m and a grating of 1200 lines mm⁻¹. To avoid strong interference of black-body radiation, an integration duration of 2 ms was chosen. All spectra were corrected for instrumental response, and each measurement is an average of 300 single shots. The spectral and lifetime measurements were done separately, and all optical measurements were conducted under identical conditions for all samples, thus minimizing the influence of the measurement equipment.

X-ray diffration was performed to verify the successful synthesis of (Gd,Lu)AG:Dy³⁺ , GAP:Dy³⁺ , (Gd,La)AP:Dy³⁺ and (Gd,Lu)AP:Dy³⁺ phosphors in the form of pure garnet and orthorhombic phase, respectively, without appreciable changes due to the Dy³⁺ doping. The observed diffraction patterns are shown in figure 1 for (Gd_0.8Lu_0.2)₃Al₅O₁₂ and (Gd_0.8Lu_0.2)AlO₃ doped with 2 mol% of Dy³⁺ . It has been shown before that both the (Gd,Lu)AP and the (Gd,Lu)AG sample have marginally smaller lattice constants than the non-stabilized GdAlO₃ (space group: Pbnm [20]) and Gd₃Al₅O₁₂ (space group: Ia3d [22]). This can be explained by the smaller ionic radius of Lu³⁺ than Gd³⁺ . No additional or missing reflections were observed which could imply the presence of unknown byproducts.

Zoom In Zoom Out Reset image size

Examples of the surface morphology and the crystallite sizes of the phosphor powders are shown for (Gd_0.8Lu_0.2)₃Al₅O₁₂ and (Gd_0.8Lu_0.2)AlO₃ doped by 2 mol% of Dy³⁺ in figure 2. The SEM images show that the overall morphology is comparable for all garnet samples. The phosphor particles form irregularly shaped agglomerates due to the flux used during the sintering process, and the size of the crystallites is in the range of about two to seven micrometers. The GAP phosphors have a higher degree of agglomeration, and the addition of Lu³⁺ or La³⁺ to the perovskite compound does not change the particle shape.

Zoom In Zoom Out Reset image size

Figure 3 shows the room temperature photoluminescence excitation (PLE) spectra of the different GAP and GAG phosphors, monitored at emission wavelengths of 583 nm and 584 nm, respectively. The monitored wavelength was chosen based on the maximum emission peak, which differed marginally between the perovskite and garnet hosts. Four major bands arise in the longer wavelength region centered at  ∼325 nm, ∼352 nm, ∼366 nm and  ∼386 nm, which are assigned to the f–f transitions of the Dy³⁺ ion from the ⁶H_15/2 ground state (GS) to higher levels ⁴M_17/2, ⁴M_15/2, ⁶P_7/2, ⁴I_11/2, ⁶P_5/2 ⁴I_13/2, and ⁴F_7/2, respectively. As marked in the figure, other weaker bands were also observed, which slightly differ for the perovskite and the garnet host. Besides, the PLE spectra of all samples exhibit a strong excitation band at  ∼275 nm corresponding to the ⁸S_7/2 $\rightarrow$ ⁶I_J transition and a weaker line at  ∼312 nm for the ⁸S_7/2 $\rightarrow$ ⁶P_J intra-f–f transition of Gd³⁺ . The presence of these Gd³⁺ -lines in the excitation spectrum of Dy³⁺ examined at 584 nm provides reliable evidence of an efficient energy transfer from the Gd³⁺ to the Dy³⁺ activators.

Zoom In Zoom Out Reset image size

The luminescence emission spectra of GAP phosphors obtained at UV excitation at 275 nm and 352 nm are shown in figure 4. The spectra of GAP: Dy³⁺ , (Gd,Lu)AP: Dy³⁺ and (Gd,La)AP: Dy³⁺ have similar shapes for 352 nm excitation. Two dominant emission peaks appear at 480 nm (blue) and 580 nm (yellow) belonging to the ⁴F_9/2 $\rightarrow$ ⁶H_15/2 and the ⁴F_9/2 $\rightarrow$ ⁶H_13/2 transition of Dy³⁺ , respectively. Weaker emission bands are observed at 670 nm and 760 nm due to the ⁴F_9/2 $\rightarrow$ ⁶H_11/2 and ⁴F_9/2 $\rightarrow$ ⁶H_9/2 transitions. The integrated yellow emission is more intense than the blue one in all cases. For the excitation at 275 nm an additional unidentified broad emission band appears between 650 nm to 800 nm. This effect might be due to an unknown interaction between the host and the Dy³⁺ ions, or the appearance of some defect-induced host emission. The luminescence intensity at room temperature of GAP:Dy³⁺ with Lu³⁺ substitution is doubled for an excitation wavelength at 275 nm compared to GAP:Dy³⁺ and the intensity is more than three times higher for excitation wavelengths above 310 nm. The introduction of La³⁺ into the host further increases the luminescence intensity. Generally, for all samples the luminescence intensity is the highest for excitation at 275 nm.

Zoom In Zoom Out Reset image size

Figure 5 compares the emission spectra of (Gd,Lu)AG: Dy³⁺ prepared at various concentrations of Dy³⁺ (1–3 mol%) for excitation at 275 nm and 352 nm. Similar to the perovskite phosphors, the strongest emission bands occur at 480 nm and 580 nm with lower emission at 670 nm and 760 nm, attributed to the transition from the excited ⁴F_9/2 state to the ⁶H_15/2, ⁶H_13/2, ⁶H_11/2 and ⁶H_9/2 states, respectively. Comparable to the GAP host, an excitation at 275 nm produces a broad emission band starting from 730 nm, which can likewise be associated with the influence of the (Gd,Lu)AG host. Depending on the excitation wavelength, maximum intensity values occurred for Dy³⁺ concentrations of 1% ($\lambda$_ex  =  275 nm) or 2% ($\lambda$_ex  =  352 nm), an aspect that also manifests itself in the excitation spectrum. This suggests that concentration quenching is more significant in the case of energy transfer from Gd³⁺ to Dy³⁺ . The luminescence intensity of the garnet (Gd,Lu)AG:Dy(2%) as can be seen from figure 5 is almost doubled at room temperature compared to (Gd,Lu)AP:Dy(2%) perovskite. No garnet samples were synthesized with La³⁺ substitution, since La³⁺ ions are bigger than Gd³⁺ ions, and thus a stabilization of the GAG garnet matrix is not possible.

Zoom In Zoom Out Reset image size

The temperature-dependent luminescence properties of phosphors are the central parameter to determine the application field of the material for phosphor thermometry. The spectral emission behaviour of (Gd,Lu)AP: Dy³⁺ for temperatures from 300 K to 1100 K is shown in figure 6, all spectra being normalized to the peak at 480 nm. At room temperature, two emission bands at 480 nm and 580 nm are prominent, which can be attributed to the transition from the ⁴F_9/2 excited state to the ⁶H_15/2 and ⁶H_13/2 state of Dy³⁺ , respectively. A strongly temperature-dependent peak appears at 458 nm due to the ⁴I_15/2 $\rightarrow$ ⁶H_15/2 transition. The ⁴F_9/2 and ⁴I_15/2 energy levels of Dy³⁺ are separated only by about 1000 cm⁻¹ and at low temperatures the upper level remains nearly unpopulated. With rising temperatures the population of the ⁴I_15/2 state gradually increases according to the thermal equilibrium process, which consequently leads to significantly increased emission intensities around 458 nm. However, for temperatures above 1100 K luminescence signal intensities of GAP:Dy³⁺ dropped to levels that were too low for reliable signal detection. The integral luminescence emission in the yellow (580 nm) is higher by a factor of five than the emission in the blue (480 nm), and the ratio between the two emission bands increases with temperature. No significant change in the shape and position of the emission bands occurs for the substitution of Gd³⁺ by La³⁺ or Lu³⁺.

Zoom In Zoom Out Reset image size

Figure 7 shows the luminescence emission spectrum of (Gd,Lu)AG:Dy(2%) from 300 K to 1600 K, normalized to the peak at 497 nm. Similar peaks appear for the garnet phosphors as compared to the perovskite (Gd,Lu)AP. In contrast to the perovskite materials, (Gd,Lu)AG:Dy(2%) has slightly sharper and more defined peaks, especially in the range from 475–500 nm, which can be an indicator for a lower number of defects in the garnet hosts or stronger electron-phonon coupling. Lattice vibration leads to a broadening of the peaks for increasing temperature, but no shift in the peak position is observed. The yellow-to-blue ratio of (Gd,Lu)AG:Dy(2%) is lowered by a factor of two as compared to (Gd,Lu)AP:Dy(2%). The increased emission in the blue part of the spectrum is an advantage of the garnet materials with regard to thermometry applications, since the influence of black-body radiation is lower in the blue part of the wavelength spectrum.

Zoom In Zoom Out Reset image size

The luminescence intensity ratio between the temperature dependent band around 458 nm corresponding to the ⁴I_15/2 $\rightarrow$⁶H_15/2 transition and the temperature-independent emission centered at 480 nm due to the ⁴F_9/2$\rightarrow$ ⁶H_15/2 transition is given in figure 8. In the present study, the intensity ratio is calculated based on the integrated intensity of two wavelength bands, one from 445 nm to 465 nm and the other from 480 nm to 497 nm. This approach corresponds to the method actually applied in thermometry applications. The emitted intensities of these two levels I_i are proportional to the population of each energy level and the intensity ratio R from two thermally coupled energy levels, denoted by 1 and 2, is given by [28, 29]

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle R = \frac{I_2({}^{4}I_{15/2}\rightarrow ^{6}H_{15/2})}{I_1({}^{4}F_{9/2}\rightarrow ^{6}H_{15/2})} = \frac{c_2 \, g_2 A_2 \,h \nu_2}{c_1 \, g_1 A_1 \, h \nu_1} \mathrm{exp} \left(\frac{-\Delta E_{21}}{kT} \right) \mathrm{,} \label{eq:IR} \nonumber \end{align} \tag{ 1 }$$

where the parameters g_i, A_i and $h\nu_i$ describe the degeneracy of the thermalized level, the total spontaneous radiative emission rate, and the average phonon energy of each transition, respectively; c_i is a pre-exponential factor accounting for the response of the detection system and considered equal to unity since the emission spectra have been corrected for the instrumental response. $\Delta E_{21}$ is the energy gap between the two levels, k is Boltzmann's constant, and T the temperature.

Zoom In Zoom Out Reset image size

The three curves for GAP:Dy³⁺ , (Gd,Lu)AP:Dy³⁺ and (Gd,La)AP:Dy³⁺ show a similar trend with similar temperature sensitivity. The luminescence signal intensity for GAP:Dy³⁺ was high enough up to 900 K to calculate the intensity ratio. The Lu³⁺ and La³⁺ substitution increased the absolute luminescence intensity, thereby extending the measurement range up to 1150 K and 1100 K, respectively.

For the GAG samples, a variation of the dopant concentration from 1 to 3 mol% of Dy³⁺ does only marginally influence the trend of the intensity ratio curve and the temperature sensitivity. All three garnet phosphors offered measurement capabilities up to temperatures of 1600 K, because of a great enhancement of the luminescence intensity of (Gd,Lu)AG:Dy³⁺ . Note that the maximum temperatures were limited by the heating capabilities of the oven.

The temperature dependence of the absolute luminescence intensities of the phosphor samples are shown in figure 9 for the perovskite hosts and in figure 10 for the garnet samples. The absolute luminescence values are determined based on the integrated intensities in the wavelength region from 440 nm to 520 nm. Signal levels for GAP:Dy³⁺ are strongly inferior compared to the samples with Lu³⁺ and La³⁺ substitution over the whole temperature range. For the (Gd,Lu)AP:Dy³⁺ and (Gd,La)AP:Dy³⁺ phosphors, intensities start dropping strongly above 800 K, and no acceptable signal can be detected above 1100 K. The intensities of the (Gd,Lu)AG:Dy³⁺ at various dopant concentrations differ by a factor of almost 2 at room temperature compared to (Gd,Lu)AP:Dy³⁺ , and they are about two orders of magnitude higher at 1000 K. A remarkable increase in signal intensities with temperature for all three (Gd,Lu)AG:Dy³⁺ samples is observed. For the thermographic phosphor YAG:Dy³⁺ this distinct increase in signal intensity was not visible [30].

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The luminescence emission lifetimes of the ⁴F_9/2 $\rightarrow$ ⁶H_15/2 transition are shown in figure 11 for all studied phosphor samples. The lifetime $\tau$ of the emission is determined by the rates of the radiative and non-radiative transition probability from the excited state. While the probability of the radiative process is usually temperature-independent, non-radiative processes such as vibrational relaxation, internal conversions and thermal quenching become more likely with increasing temperature. Assuming a single-exponential decay time, the decay of the population of the excited state is directly proportional to the measurable decrease of the phosphorescence intensity

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle I(t) = I_0 \exp \left(- \frac{t}{\tau} \right) \mathrm{,} \nonumber \end{align} \tag{ 2 }$$

where I₀ is the initial signal intensity.

Zoom In Zoom Out Reset image size

Comparing the temperature-dependent lifetime characteristics of the garnet phosphors with different concentrations of Dy³⁺ , all samples show a similar trend. The lifetime remains nearly constant up to a quenching temperature of nearly 1200 K, above which the lifetime continuously decreases. This temperature region from 1200 K up to 1600 K can be used for phosphor thermometry based on the lifetime method. However, even though general trends are similar, the absolute lifetime strongly varies with dopant concentrations. At room temperature, it decreases from 750 ${\rm \mu}$s for the lowest Dy³⁺ concentration of 1 mol% over 600 ${\rm \mu}$s for GAG:Dy(2%) and down to 450 ${\rm \mu}$s for the sample with 3 mol% Dy³⁺ . The decreasing lifetime values can be explained by stronger concentration quenching for higher amounts of activator ions, although this increased non-radiative transition rate is compensated by a higher luminescence signal from a larger quantity of optically active ions. The lifetime of GAG:Dy(2%) at room temperature is close to that (606 ${\rm \mu}$s) reported for YAG(BN):Dy(2%), but is significantly lower than observed for YAG:Dy(2%) (800 ${\rm \mu}$s) [30].

The decay time and maximum temperature of the phosphors GAP:Dy³⁺ , (Gd,La)AP:Dy³⁺ and (Gd,Lu)AP:Dy³⁺ are significantly lower compared to those of (Gd,Lu)AG:Dy³⁺ . These perovskite materials have the same quenching point at 650 K, and the decay time is about 400 ${\rm \mu}$s at room temperature. The signal of GAP:Dy³⁺ is almost completely quenched above 900 K. Since the hosts with either Lu³⁺ or La³⁺ substitution have higher luminescence intensity, the measurement range based on the lifetime approach can be extended by 100 K or 50 K, respectively. Compared to YAP:Dy³⁺ perovskite with a lifetime of about 600 ${\rm \mu}$s and a quenching temperature of 1400 K [30], the high temperature measurement capabilities of GAP:Dy³⁺ and its variations are significantly limited.