Photoluminescence properties of thermographic phosphors YAG:Dy and YAG:Dy, Er doped with boron and nitrogen

Abstract

This paper investigates Dy³⁺-doped and Dy³⁺, Er³⁺-co-doped yttrium aluminum garnets (YAG) with the admixture of boron nitride with the aim of using them as efficient thermographic phosphors at high temperatures. The phosphors were synthesized using a conventional high-temperature solid-state method. The influence of two fluxes, B₂O₃ and LiF/NH₄F, and the effect of activator and coactivator concentrations were investigated. Additionally, the effect of B³⁺ and N³⁻ substituting for Al³⁺ and O²⁻ ions, respectively, in the YAG:Dy³⁺ co-doped with Er³⁺ was studied for the first time. The changes in the host lattice led to a much stronger photoluminescence compared with the samples without B³⁺ and N³⁻ substitution. The admixture of BN also improves the thermal sensitivity of the YAG:Dy and YAG:Dy, Er thermographic phosphors.

1 Introduction

Phosphor thermometry is based on the correlation of photoluminescence (PL) properties of phosphors with temperature. This technique has a number of advantages compared to standard IR thermography. By using phosphor thermometry, temperature can be analyzed with non-contact measurement extensively and instantaneously with high sensitivity and accuracy. This technique has, therefore, been used for surface thermometry in gas turbines [1, 2] and engines [3] as well as for temperature analyses on gaseous flows [4–8]. In several cases, measurable signals and high chemical stability of phosphors at temperatures higher than 1200 K are required. One of the best phosphors for the high temperature range is Y₃Al₅O₁₂ (YAG) garnet doped with rare earth (RE³⁺) ions [1], especially YAG:Dy [15, 19], which according to Cates et al. [13], shows measurable luminescence at 1705 °C.

Generally, the host lattice Y₃Al₅O₁₂ is well suited for the development of optical materials for different applications. Various rare earth dopants including Ce, Tb, Eu, Nd and Dy have been incorporated into YAG to control the phosphor luminescence. Single crystals and single crystalline films (SCF) of YAG, doped with Ce³⁺ and Pr³⁺ ions, are used as cathodoluminescent screens [9] and scintillators for visualization of X-ray images [10]. Crystals and SCF of Nd³⁺-, Er³⁺-, and Yb³⁺-doped YAG are well-known laser media with laser action in the infrared range [11]. Dy³⁺-, Sm³⁺-, and Tm³⁺-doped YAG phosphors have been used for thermographic applications [12–16]. Thermographic phosphors (TP) have to survive in high-temperature environment, and their temperature-dependent fluorescence characteristics such as emission wavelength, peak intensity ratio, or decay time can be calibrated for temperature measurements [17–20]. Modification of the host matrix allows to improve the thermographic properties. For example, Hansel et al. [21] substituted 50 % of the aluminum in YAG:Ce ((Y_1−x Ce _x )₃Al₅O₁₂) with gallium to produce a low-temperature TP, i.e., YAGG:Ce (Y_1−x Ce _x )₃(Al_1−y Ga _y )₅O₁₂, where x = 0.01, 0.02 and y = 0, 0.5), using a solution combustion synthesis. They found that shortening of the luminescence lifetime of YAGG:Ce occurred at a lower temperature than in YAG:Ce, which shifted the useful measurement range. Wang et al. [16] substituted the Al³⁺ and O²⁻ ions at tetrahedral sites in YAG:Ce with B³⁺ and N³⁻ ions to produce YABNG:Ce (Y_3−x Ce _x Al_5−y B _y N _y O_12−y ) using solid-state synthesis. The emission intensity was enhanced over the temperature range of 293–573 K, and the onset of thermal quenching was delayed from 323 to 523 K. Kwong et al. [14] demonstrated a new phosphor produced by substituting tetrahedral site Al³⁺–O²⁻ in YAG:Dy with B³⁺–N³⁻ to produce YABNG:Dy to investigate the thermal response of YAG:Dy and YABNG:Dy (Y_3−x Dy _x Al_5−y B _y N _y O_12−y ) for high-temperature laser-induced phosphorescence (LIP) thermometry. Tests of the thermal response showed that the YABNG:Dy had much stronger phosphorescence emission than did YAG:Dy and the beginning of thermal quenching was delayed by approximately 100 K for YABGN:Dy compared to YAG:Dy. However, the mechanism causing the observed change in thermal quenching is currently not well understood.

To gain a deeper insight into physical processes and to achieve a higher sensitivity of the phosphors, we performed a comprehensive study of Y_(3−x)Al₅O₁₂:xDy³⁺, Y_(3−x−y)Al₅O₁₂:xDy³⁺, yEr³⁺ and the same compositions doped by boron and nitrogen. The samples were prepared by a conventional high-temperature solid-state reaction process. The influence of two different fluxes (B₂O₃ and NH₄F/LiF) and the effect of doping concentration of activator ions on the phase formation and on the intensity of photoluminescence (PL) were investigated by PL excitation and emission spectroscopy. The effect of B³⁺ and N³⁻ incorporation in the host lattice of YAG doped with 2 % Dy³⁺ and YAG:2 % Dy³⁺ co-doped with 1 % Er³⁺ on the PL properties was studied. The effect of temperature in the range of 293–1023 K on the PL spectra was investigated as well. The intensity ratio I _{(450–460 nm)}/I _{(490–500 nm)} as a function of temperature, which is useful for temperature measurements, was determined.

2 Experimental procedures

Y_(3−x)Al₅O₁₂: x % Dy³⁺ and Y_(3−x−y)Al₅O₁₂:x % Dy³⁺, y % Er³⁺ powder phosphors with the addition of BN were fabricated by conventional high-temperature solid-state method. The expressions of Y_(3−x)Al₅O₁₂:xDy³⁺ and Y_(3−x−y)Al₅O₁₂:xDy³⁺, yEr³⁺ in the following sections are abbreviated as YAG:x % Dy³⁺, y % Er³⁺, where x is 1, 2, and 3 % dopant concentration (x = 0.03; 0.06; 0.09) and y−1 % dopant concentration (y = 0.03). B³⁺ and N³⁻ ions in these compositions were provided by BN (6 % boron substitution of Al atoms). Two compounds were chosen as fluxes in order to decrease the sintering temperature and to improve the PL efficiency of the phosphor: B₂O₃ and LiF/NH₄F.

Stoichiometric amounts of Y₂O₃, Al₂O₃, Er₂O₃, BN, B₂O₃, LiF, NH₄F (all, 99.99 %, Alfa Aesar) Dy₂O₃ (99.9 %, Reacton) were used as starting materials in the present work. The starting powder was mixed and ground in an agate mortar by hand. Subsequently, it was fired in an alumina crucible at 1673 K for 15 h in air to produce the final samples. After firing, samples were cooled to room temperature in the furnace and then ground to powder for subsequent use.

The crystal structure of the phosphors was examined by using a diffractometer (Siemens D500 X-ray) with a Cu-Kα radiation source (λ = 1, 5406 Å) at room temperature. Morphology and elemental analysis of the phosphors were done with a scanning electron microscope (SEM) (Philips ESEM XL30) equipped with a field emission gun operated at 20 kV. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of powders at room temperature were measured with a spectrofluorometer (JASCO FP-8500). Optical measurements were conducted under identical conditions for all samples with a spectral resolution of 5 nm of both, excitation and emission monochromators. For the temperature-dependent PL tests, a thin layer of phosphor powder was deposited onto the surface of a stainless-steel sample holder and measured in temperature range of 293–1023 K with a fiber-coupled diode laser operating at 405 ± 2 nm wavelength (NovaPro Fiber 405-75-SM, by RGB Lasersysteme GmbH, Germany). An electric heating pad which can withstand temperatures up to 1273 K was used to heat the powder. Two thermocouples were connected to the sample holder right next to the phosphor to monitor the surface temperature. Luminescence spectra were recorded by a spectrometer sensitive in the range from 340 to 860 nm (ULS2048L by Avantes BV, the Netherlands; slit size 25 μm, resolution 1 nm) at different temperatures.

3 Results and discussion

3.1 Phase identification and morphology

The X-ray diffraction measurements revealed well-defined Bragg reflections of pure crystalline powder samples, indicating that the materials under study are well crystallized in a cubic phase, without notable changes due to the Dy³⁺ and Dy³⁺, Er³⁺ doping. These patterns have been indexed to the Y₃Al₅O₁₂ (YAG) garnet structure. Perfect garnets are usually described in terms of a 160 atom body-centered-cubic unit cell (80-atom primitive cell), which contains 8 f.u. of A₃B₂′B₃″O₁₂, where A, B′, and B″ are cations in different symmetry sites. In YAG, A≡Y is eightfold coordinated in a distorted cubic D ₂ local site; this site is traditionally labeled as the dodecahedral site because the distorted cube produces a polyhedron with 12 triangular faces. B′≡Al is in an octahedral environment and B″≡Al in a tetrahedral environment. Idealized cubic YAG belongs to the Ia3¯d (230) space group with Y in 24(c) sites, Al_oct in 16(a) sites, Al_tet in 24(d) sites, and the remaining 96 oxygen atoms in (h) sites. Due to ionic size considerations in the YAG lattice, the RE³⁺ ions are expected to predominantly enter into the distorted dodecahedral sites by replacing the Y³⁺ ions and to be coordinated by eight O²⁻ ions [22].

X-ray diffraction was performed to verify the successful synthesis of YAG:x % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ (including samples synthesized with an admixture of BN) in the form of pure crystalline powder. There were no additional or missing reflections which could imply the presence of unknown byproducts. The observed diffraction patterns are shown in Fig. 1. Also, the results revealed that some of the samples contained a minor secondary yttrium orthoborate YBO₃ phase as side product. The presence of YBO₃ phase can be explained by the limited solubility of the fluxes B₂O₃ and BN in Y₃Al₅O₁₂.

Fig. 1

XRD patterns of the YAG:2 % Dy³⁺ (a), YAG:2 % Dy³⁺, BN (b), YAG:2 % Dy³⁺, 1 % Er³⁺ (c) and YAG:2 % Dy³⁺, 1 % Er³⁺, BN (d) phosphors

A SEM study was carried out to investigate the surface morphology and crystallite sizes of the synthesized phosphors, at both low (2500) and high (10,000) magnification. Figure 2 shows the SEM morphology of the phosphors prepared by conventional high-temperature solid-state method at 1673 K. It can be clearly seen from Fig. 2 that the size of crystallite varies from few to several micrometers. The SEM images indicate that the particles are conglomerated due to the flux used during synthesis. Considering the different fluxes B₂O₃ and LiF/NH₄F applied, there are no morphological changes due to them.

Fig. 2

Particle morphology of the YAG:2 % Dy³⁺(a), YAG:3 % Dy³⁺ (b), YAG:2 % Dy³⁺, 1 % Er³⁺ (c)—(B₂O₃ flux); YAG:3 % Dy³⁺ (d), YAG:2 % Dy³⁺, BN (e), YAG:2 % Dy³⁺, 1 % Er³⁺, BN (f)—(NH₄F/LiF flux) phosphors

3.2 PL properties of YAG doped with Dy³⁺ and co-doped with Er³⁺ at room temperature

Optical measurements of photoluminescence spectra of the YAG:x % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ phosphors were conducted under identical conditions at room temperature by using a spectrofluorometer at an excitation wavelength of 405 nm. The excitation at this wavelength is relevant for the PL thermometry applications, as the 405-nm laser diodes are readily available, are inexpensive, and lead to less safety issues compared with the standard UV excitation. The absorption cross section of Dy³⁺ in YAG, however, is relatively small at this wavelength, and it was one of the goals of this work to clarify the feasibility of the 405-nm excitation. The weak absorption at 405 nm for Dy³⁺ in YAG is partly compensated by the thermal shift of Dy³⁺ absorption to longer wavelengths. Figure 3 shows the schematic diagram of the relevant energy levels of Dy³⁺ and the processes of thermal distribution of electrons between the excited states. After excitation to the higher states, a non-radiative and radiative relaxation of Dy³⁺ occurs. At low temperatures, the luminescence intensity at 458 nm, corresponding to the ⁴I_15/2 → ⁶H_15/2 transition is low, if compared to the intensity of the 497-nm line, corresponding to the ⁴F_9/2 → ⁶H_15/2 transition. At elevated temperatures, the intensity of 458 nm increases compared to that of 497 nm. This is due to the increasing population of the ⁴I_15/2 state, according to the Boltzmann distribution [23].

Fig. 3

Schematic energy diagram of partial energy levels of Dy³⁺ and the possible energy transfer processes. Black wiggly arrows represent non-radiative de-excitation mechanisms (phonons) and straight arrows represent radiative de-excitation mechanisms (photons), i.e., luminescence

Luminescence intensities of those levels for YAG:2 % Dy³⁺ are shown in Fig. 4. The dependence of luminescence intensity on dopant and co-dopant ion (Dy³⁺; Dy³⁺, Er³⁺) concentration is shown in Figs. 4, 5, 6, and 7. The shape of the emission spectrum does not vary with the Dy³⁺ and Dy³⁺, Er³⁺ concentration, but the luminescence intensity changes significantly. Figure 5 shows that the emission intensity of Dy³⁺ increases with an increase in dopant ion concentration, reaching a maximum at 3 %, a further increase in dopant concentration leads to decreasing intensity due to concentration quenching [24, 25]. Concentration quenching may occur because the excitation energy migrates between a large numbers of Dy³⁺ ions and can easily undergo a non-radiative relaxation at a defect site [26–28]. Non-radiative energy transfer from one Dy³⁺ to another or to a co-dopant may take place via exchange interaction, radiation re-absorption, or electric multipolar interaction. The exchange interaction needs a large direct or indirect overlap between wave functions of the donor and acceptor. Re-absorption of radiation comes into effect only when the emission and excitation spectra show a strong overlap. Thus, the small spectral overlap for the YAG:Dy³⁺ phosphors indicates that radiation re-absorption can be ignored in this case. As a result, the energy transfer of Dy³⁺ ions in YAG:Dy³⁺ would be due to an electric multipolar interaction [29].

Fig. 4

a Excitation and b emission spectra of the YAG:2 % Dy³⁺ phosphor synthesized with different fluxes, B₂O₃ and NH₄F/LiF. The 497-nm line was monitored for recording the excitation spectra. Excitation at 405 nm was used for the emission spectra

Fig. 5

a Excitation and b normalized emission spectra of 1 % Dy³⁺, 2 % Dy³⁺, 3 % Dy³⁺ and 5 % Dy³⁺ doped YAG. The 497-nm line was monitored for recording the excitation spectra. Excitation at 405 nm was used for the emission spectra. The inset of a shows the PL intensity at 497 nm as a function of Dy³⁺ concentration

Fig. 6

a Excitation and b normalized emission spectra of 2 % Dy³⁺ and 2 % Dy³⁺, 1 % Er³⁺ doped YAG. The 497-nm line was monitored for recording the excitation spectra. Excitation at 405 nm was used for the emission spectra

Fig. 7

a Excitation and b normalized emission spectra of 2 % Dy³⁺; 2 % Dy³⁺, 1 % Er³⁺ and BN-doped 2 % Dy³⁺; 2 % Dy³⁺, 1 % Er³⁺ doped YAG. The 497-nm line was monitored for recording the excitation spectra. Excitation at 405 nm was used for the emission spectra

Figure 4 shows the PLE (a) and PL (b) spectra of YAG:2 % Dy³⁺ phosphor samples synthesized by using different fluxes (B₂O₃ and NH₄F/LiF). The amount of fluxes was chosen 5 wt%, since increasing of flux concentration from 1 to 5 wt% further increase the intensity of PLE and PL spectra. Intensity of YAG:2 % Dy³⁺ luminescence is slightly higher in case of B₂O₃ flux in comparison with intensity obtained by using NH₄F/LiF flux. The PL spectrum was measured under 405-nm laser excitation. Three main emission bands each containing multiple Dy³⁺ emission lines were observed in the 455–460 and 470–500 nm blue and 570–600 nm yellow regions corresponding to ⁴I_15/2 → ⁶H_15/2, ⁴ F_9/2 → ⁶H_15/2, and ⁴F_9/2 → ⁶H_13/2 ground-state transitions in the 4f levels of Dy³⁺ ions, respectively. One additional unidentified line was observed at 693 nm for samples synthesized with B₂O₃ flux which is not typical for YAG structure. A possible explanation for this peak could be the emission of Dy³⁺ in YBO₃ phase which presents in this sample up to 5 %. However, the YBO₃ phase was detected in samples which were synthesized with NH₄F/LiF flux as well and no emission at 693 nm (14,430 cm⁻¹) and 702 nm (14,240 cm⁻¹) was detected in these samples. Therefore, we believe that these lines correspond to the electronic multiplet transition ⁴I_15/2 → ⁶H_9/2 in Dy³⁺ (14,430, 14,240 and 13,850 cm⁻¹). For the PLE spectra, the examined excitation wavelength ranged from 300 to 480 nm by monitoring the emission line at 497 nm. This line was selected as an emission from the lowest sublevel of the ⁴F_9/2 multiplet and as the best separated one from this set of lines. Multiple PLE (Fig. 4a) peaks were observed with the highest peak located at about 352 nm, which is attributed to the excitation of electrons from ⁶H_15/2 to ⁴M_15/2, ⁶P_7/2 levels in the 4f band of Dy³⁺ ions. Figure 4b demonstrates that the luminescence of YAG:2 % Dy³⁺ can well be excited by the 405-nm diode laser.

In order to improve the luminescence intensity, we synthesized YAG:x % Dy³⁺ by using B₂O₃ flux and the concentration of Dy³⁺ doping was varied in a range of 1–5 mol%. It was found that the YAG:3 % Dy³⁺ phosphor sample showed the maximum PL peak intensity as displayed in Fig. 5, which shows excitation (a) and normalized emission (b) intensities at 1–3 and 5 mol% Dy³⁺ doping concentrations. With concentration increasing from 1 to 3 %, the PL intensity increased continuously, and at higher concentrations, the intensity is reduced due to concentration quenching.

Due to normalization of the spectra in Fig. 5b, it is easy to see that the relative intensity of the 458-nm line is strongly reduced at higher Dy³⁺ concentrations. The intensity ratio of the two emission lines of interest (458 and 497 nm) is changed by five times. Moreover, the lines which are attributed to the electronic multiplet transition ⁴I_15/2 → ⁶H_9/2 in Dy³⁺ (the lines 693, 702, and 722 nm, i.e., 14,430, 14,240, and 13,850 cm⁻¹) and which are very sensitive to the B₂O₃ content, show a similar sensitivity to the increasing Dy³⁺ concentration. A possible explanation to this behavior is the cross-relaxation between neighboring Dy³⁺ ions, which reduces the population of the ⁴I_15/2 state.

Co-doping of YAG:x % Dy³⁺ phosphor was performed with 1 mol% Er³⁺ as coactivator. For this experiment, 2 mol% concentration of Dy³⁺ was chosen to prevent the concentration quenching. We observed increased intensities in Er³⁺-co-doped YAG:2 % Dy³⁺ (Fig. 6a). Two lines were observed at 542 and 556 nm with Er³⁺ as a coactivator. They correspond to the ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions. Photons with this energy have been observed in the Er³⁺ luminescence in several garnets [30, 31].

It is easy to see in the normalized PL spectra in Fig. 6b that the lines at 693, 702, and 722 nm assigned to the electronic multiplet transition ⁴I_15/2 → ⁶H_9/2 and which are sensitive to the doping levels of Dy³⁺, are significantly reduced due to co-doping by Er³⁺. Moreover, the “working” intensity ratio 458/497 nm was significantly increased by this doping.

3.3 PL properties of BN-doped YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ at room temperature

Additionally, the influence of addition of BN on PL properties of phosphors was investigated. Ionic radius of B³⁺ is small (0.23 Å), so we suppose that the Al³⁺ (0.51 Å) ions in tetrahedral positions are mainly substituted by B³⁺. Most probable is that B³⁺ and N³⁻ ions replaced the Al³⁺ and O²⁻ ions which are part of the tetrahedral ligands in the host lattice of YAG.

$${\text{B}}^{3 + } \to {\text{Al}}^{3 + } ;{\text{ N}}^{3 - } \to {\text{O}}^{2 - }$$

For charge compensation vacancies of O²⁻ are necessary:

$$3\left[ {{\text{V}}_{\text{O}}^{2 - } } \right] \leftrightarrow 2\left[ {{\text{B}}^{3 + } {-}{\text{N}}^{3 - } } \right]$$

As host matrix affects significantly the luminescence behavior of the thermographic phosphor, YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ were synthesized with an admixture of BN (6 % boron substitution of Al atoms) to improve the emission intensity.

Figure 7 shows the PLE (a) and the normalized PL (b) spectra of YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ doped with BN phosphor samples synthesized by using NH₄F/LiF flux. The intensity of the co-doped YAG:2 % Dy³⁺, 1 % Er³⁺ phosphor is slightly higher in comparison with the intensity of YAG:2 % Dy³⁺. The addition of BN to YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ phosphors led to a great improvement in intensity. The lines observed at 542 and 555 nm are common for all the samples co-doped with Er³⁺ [30, 31]. Further investigations are necessary to clarify the reasons for the increased PL intensity in the samples with admixed BN. Possible reasons are strengthening of the corresponding optical transitions due to impurity-distorted lattice sites, more efficient incorporation of Dy³⁺ ions in the samples with BN admixture, and better morphology of the crystallites and weaker influence of surface defects. Wang et al. [16] held the improved morphology responsible for the increased PL intensity in their study. In our case, the morphology, reflected by the SEM images in Fig. 2, is not correlated with the PL properties, and we believe that the stronger luminescence is mainly due to improved incorporation of Dy³⁺ ions.

3.4 PL properties of Y₃Al₅O₁₂:Dy³⁺ and Y₃Al₅O₁₂:Dy³⁺, Er³⁺ with BN-doped at high temperature

Luminescence from most thermographic phosphors originates from multiple excited states. The population distribution of atoms among the energy levels of the system follows the Boltzmann’s law; an increased number of high energy levels are available for transitions at higher temperatures. This leads to a significant variation in thermal dependence of intensities of each optical transition line. This thermal effect is enhanced in rare earth ions because of their closely spaced high energy levels. From Boltzmann’s law, the ratio of intensities (from two transition lines) can be used to determine the sample temperature. In YAG:Dy, the best choice for the high-temperature thermometry are the transitions ⁴I_15/2 → ⁶H_15/2 and ⁴ F_9/2 → ⁶H_15/2—at 458 and 497 nm, respectively, with an energy separation of about 1000 cm⁻¹. The thermal energy kT for the temperature of about 1200 K is 800 cm⁻¹. This leads to a significant sensitivity of the ⁴I_15/2 state population to the temperature and is the main principle behind the spectral ratio technique used in luminescence-based thermometry [13, 19, 32]. PL spectra of YAG:2 % Dy³⁺; YAG:3 % Dy³⁺; and YAG:2 % Dy³⁺, 1 % Er³⁺ (doped with BN) phosphor samples were measured at 293–1023 K in the 430- to 700-nm region. Phosphor samples synthesized by using B₂O₃ flux—YAG:3 % Dy³⁺ and YAG:2 % Dy³⁺ co-doped with 1 % Er³⁺ showed higher sensitivity and a higher overall signal intensity compared to YAG:2 % Dy³⁺ in the studied temperature range. Figure 8 represents the emission lines at room temperature compared to 1023 K. Figure 9 shows intensity ratio I _{(450–460 nm)}/I _{(490–500 nm)} as a function of temperature for YAG:2 % Dy³⁺, YAG:3 % Dy³⁺, and YAG:2 % Dy³⁺, 1 % Er³⁺.

Fig. 8

Emission (405-nm excitation wavelength) spectra of 2 % Dy³⁺; 3 % Dy³⁺ and 2 % Dy³⁺, 1 % Er³⁺ doped YAG. a 293 K; b 1023 K

Fig. 9

I _{(450–460 nm)}/I _{(490–500 nm)} lines of signal intensity ratio as a function of temperature (YAG:2 % Dy³⁺, YAG:3 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺)

Phosphor samples, which were synthesized by using NH₄F/LiF flux—YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ co-doped with BN, showed an enhancement of signal intensity compared with the YAG:2 % Dy³⁺ in the studied temperature range. In both cases, intensity increased and the maximum of absolute intensity was reached by doping of YAG:2 % Dy³⁺ with BN.

Figure 10 shows emission lines at room temperature and at 1023 K. Figure 11 shows intensity ratio I _{(450–460 nm)}/I _{(490–500 nm)} as a function of temperature of YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ co-doped with BN.

Fig. 10

Emission (405-nm excitation wavelength) spectra of 2 % Dy³⁺; 2 % Dy³⁺, 1 % Er³⁺ and BN-doped 2 % Dy³⁺; 2 % Dy³⁺, 1 % Er³⁺ doped YAG a 293 K; b 1023 K

Fig. 11

I _{(450–460 nm)}/I _{(490–500 nm)} lines of signal intensity ratio as a function of temperature (YAG:2 % Dy³⁺; YAG:2 % Dy³⁺, 1 % Er³⁺ and BN-doped YAG:2 % Dy³⁺; YAG:2 % Dy³⁺, 1 % Er³⁺)

A comparison of luminescence intensities and intensity ratios of two emission lines is shown in Table 1. By far, the highest intensity was reached for BN-doped YAG:2 % Dy³⁺. The intensity is more than doubled compared with standard YAG:2 % Dy³⁺. Additionally, intensity ration I _{(450–460 nm)}/I _{(490–500 nm)} is slightly larger and closer to one, which is a preferable situation for measurement equipment. Therefore, YAG:2 % Dy³⁺ phosphor doped by BN was determined as the material best suited as thermographic phosphor in the temperature range.

Table 1 Intensity I _{(450–460 nm)} and intensity ratio I _{(450–460 nm)}/I _{(490–500 nm)} for different compositions at 1023 K

4 Conclusions

Photoluminescence excitation and emission spectra of YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ and the same compositions doped by BN were investigated at room temperature as well as at high temperatures. Phosphors were synthesized using conventional high-temperature solid-state method. Applying two fluxes, B₂O₃ and LiF/NH₄F, did not result in significant differences in the phase formation, size of the particles and photoluminescence intensity. However, incorporation of BN in the host compositions revealed a strong enhancement of the luminescence intensity of YAG:2 % Dy³⁺ and YAG:2 % Dy³⁺, 1 % Er³⁺ compared to compositions without substitution of Al³⁺ and O²⁻ by the B³⁺ and N³⁻ ions in the studied temperature range (293–1023 K). Luminescence intensity could be more than doubled at 1023 K. Furthermore, the intensity ratio of the 457- and 497-nm lines of the investigated samples was slightly increased by the admixture of BN (from around 0.28–0.31), which improves the thermal sensitivity of these phosphors. In summary, the admixture of BN was found to improve strongly the luminescence properties of YAG:Dy and YAG:Dy, Er used as a high-temperature thermographic phosphors.