The afterglow characteristics and trap level distribution of chromatic rare-earth luminous fiber

Materials

Materials such as SrCO₃(AR), Al₂O₃(GR), Eu₂O₃(99.99%) and Dy₂O₃(99.99%) and H₃BO₃(AR) were all purchased from SINOPHARM Chemical Reagent Co. Ltd. PET chips with different shape section were purchased from Wuxi Taiji Industry Co. Ltd. (Wuxi, China). Transparent inorganic pigments (including iron oxide red, pigment green and cobalt blue) and functional additives (including titanate coupling agent, octadecanamide and PE-WAX) were supplied by Jiangsu Guoda Complete Wiring Equipment Co. Ltd.

Preparation of luminescent materials

The samples of strontium aluminate materials were prepared by solid-state reaction. The Sr_0.95Al₂O₄: Eu²⁺_0.02, Dy³⁺_0.03 was prepared by mixing the powder materials, SrCO₃, Al₂O₃, Eu₂O₃ and Dy₂O₃, according to the mole ratios of the elements and adding 5% of H₃BO₃ as a flux, which were dissolved in approximately 100 ml of absolute ethanol and followed by ultrasonic dispersion with 25 kHz for 20 minutes in order to get a homogeneous mixture. Subsequently, the mixtures were dried at 90℃ for 24 hours, ground in a planetary high-energy ball mill for 1 hour and sintered at the 1300℃ with a heating rate of 10℃ min⁻¹ in a powered carbon reduction atmosphere. The sintered products were re-milled and sieved to get the desired samples, SrAl₂O₄: Eu²⁺, Dy³⁺ (SAOED).

Preparation of luminescent fiber

Luminescent fibers were prepared by the melt-spinning process. The fiber-forming polymer PET chips were dried in an oven at 110℃ for 24 hours and mixed with scheduled weights of strontium aluminate, inorganic pigment and functional additives in a high-speed mixer. Then, each mixture was extruded in a twin-screw master batch producer at 270–290℃ to get master batches for the spinning application. After drying at 110℃, the master batches were melted and spun to obtain the white (no pigments added), green, yellow, blue and red luminescent PET fiber with a spinning temperature of 270℃, spinning speed of 3000 m min⁻¹ and draw ratio of 3.

Since the amount of fibers and residual brightness in the fiber will affect the accuracy of results in the process of testing, the samples under test must be made accurately. The fiber sample weights were 5 ± 0.1 g, and were wound evenly on a 6 cm × 6 cm homemade “I shaped model to ensure that the difference of the samples’ initial brightness was less than ±0.01 Cd m⁻² under the same excitation conditions. Before the testing, samples were placed in the cassette for more than 15 hours to be certain that the afterglow brightness had been attenuated completely.

Characteristic analysis

Crystal structures of samples were checked by means of a D8 Advance X-ray diffractometer (Bruker AXS, Germany) using Cu Kα radiation (λ = 0.15406 nm) at a voltage of 40 kV and a current of 30 mA. Samples were scanned over a range of diffraction angles, from 10° to 70°, with a scan speed of 4° min⁻¹ at room temperature.

The afterglow decay curves were measured by a PR-305 Long Rays Fluorescence Tester (Zheda Sensing Instruments Co., Ltd) with simulated daylight of 1000 lx and tested 2 seconds after light excitation stopped.

The thermoluminescence spectra were obtained using a FJ27A-I TL Dosimeter (CNNC Beijing Nuclear Instrument Factory) with a heating rate of 1℃ s⁻¹ and the temperature ranging from room temperature to 200℃. The testing was carried out under the following conditions: a Philips cold light lamp of 25 W (D65) as the light resource, an excitation illumination of 1000 lx and an excitation time of 15 minutes, and the samples were tested after 120 minutes of excitation as well.

Phase analysis

The X-ray diffraction (XRD) patterns of the SAOED phosphors and chromatic luminescent PET fibers are shown in Figure 1. From the figure it can be seen that the diffraction peaks of SAOED appeared multimodal and sharp. In contrast to the Joint Committee on Powder Diffraction Standards data file (No.34-0379), the sample was a the α-SrAl₂O₄ phase belonging to a monoclinic system phosphorus quartz crystal structure and showed good crystallization degree and no impurity phase existed. In addition, the pattern of white luminescent fiber was the simple superposition of the sharp peaks of the inorganic luminescent materials and the macromolecular smooth structure without new peaks. ¹⁵ The pattern of blue luminescent fiber was basically the same as that of the white fiber, which indicated that the phase structures of the fiber were not affected or destroyed during the complex manufacturing process. However, due to adding the red inorganic pigment, the lattices of the inorganic materials of the red fiber were disturbed, making the diffraction peaks broader. The afterglow properties of red fiber decreased as well. OPEN IN VIEWER

Effects of rare-earth luminescent materials on the afterglow characteristics of luminescent fiber

Effects of grain size on the afterglow characteristics of luminescent fiber

The grain size of the luminescent phosphors used in the fiber has an effect on the afterglow brightness of the fiber to some extent; when the grain size of the phosphors was larger, the brightness of the fiber was higher, and the samples met the requirements of the spinning process as well. Three samples with different grain sizes obtained by sieving strontium aluminate were used to prepare luminescent fiber based on a PET matrix. Table 1 shows the initial afterglow brightness of the white luminescent PET fiber when different grain sizes of strontium aluminate phosphors were added. It can be seen from the table that when the strontium aluminate particles are larger, the initial afterglow brightness of the fiber is higher. However, filament breakage often occurred in the process of spinning when the luminous particles size was 14–15 µm. From the experimental data, it can be seen that the grain size of 5–6 µm of strontium aluminate in the fiber showed a good luminescent effect and met with the requirement of spinning.

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Table 1.

Initial afterglow brightness of luminescent PET fiber with adding different diameter of phosphors

Grain size of luminescent materials (µm)	Initial afterglow brightness of the fiber (Cd m−2)
1–2	0.376
5–6	0.854
14–15	1.294

Effect of polymer matrix on the afterglow characteristics of luminescent fiber

The afterglow decay curves of the SAOED phosphors and the luminescent PET fiber excited for 15 minutes are recorded in Figure 3. The content of SAOED in the fiber was 5 wt%, named as White-PET-SAOED Fiber. It can be seen that the luminescent intensities were dramatically decreased at the start of luminescence and were followed by a long decay of the afterglow, which indicated that the initial luminescent brightness and decay speed of the afterglow existed differently, but the curve trend of luminescent PET fiber was in accordance with that of SAOED. It included three regimes: a fast initial decay corresponding to the intrinsic life of Eu²⁺, an intermediate decay due to Eu²⁺ ions trapped by shallow centers and a long decay of the afterglow arising from ions in deep trapping centers, such as Dy³⁺.^9,16 As shown in the figure, the initial afterglow brightness of the SAOED phosphor was much higher than that of the luminescent PET fiber. OPEN IN VIEWER

To the best of our knowledge, the matrix of the fiber-forming polymer PET had no fluorescence emission in the range of 450–600 nm, and the luminescence was attributed to the strontium aluminate in the fiber. When excited by the exciting light, the electrons of each energy level in the Eu²⁺ ion absorbed a certain wavelength energy and began to transit from the 4f ground state to the 5d excited state. Some electrons went back to the ground state because of energy imbalance, and meanwhile each electron gave up energy and emitted it in the form of light. After reviewing the XRD patterns, it was determined that the phase structures of the fiber were not affected or destroyed during the complex manufacturing process. Therefore, it was deemed that the low afterglow brightness of the fiber was attributed to the existence of the polymer matrix and it hindered the light travel during the process of luminescence.

In order to explain the effect of the polymer matrix on the luminescence of SAOED in the fiber, we simulated and analyzed its luminescence process in an ideal state. Firstly, we discussed the excitation process of the light. When an exciting light hit the air–fiber surface, it was reflected, scattered, absorbed and refracted by the polymer matrix. Then, only the refracted light entered the fiber and began to propagate within it, which had two possible outcomes. One part of the light encountered the SAOED particles and excited them directly, and the other part of the light which reached the fiber–air surface directly without encountering any particles, reflected and refracted again. Secondly, we discussed the emission light generated from the SAOED particles in the polymer matrix. One part of the emission light reached the fiber–air surface directly and was reflected and refracted on this surface, which suggested that the reflected light continued to travel within the fiber and began to a new cycle of excitation and emission, and the refracted light passed through the fiber and gave out light. The other part of the emission light encountered the SAOED particles and began a new excitation process. The light of excitation and emission were reflected, absorbed and refracted for many times, and then formed the last luminescence of the fiber. Therefore, the exciting light was not absorbed by the fiber efficiently during the excitation process, and the emission light generated from the luminescence particles in the PET matrix was consumed in some degree during the emission process in the fiber because of the existence of the fiber-forming polymer.

Effect of inorganic pigment on the afterglow characteristics of luminescent fiber

The afterglow decay curves of the luminescent PET fiber with the addition of several different transparent inorganic pigments are shown in Figure 4. The content of SAOED in the samples was 5 wt% and excitation time was 15 minutes. The decay trend of the luminescent fibers with inorganic pigments were similar and close to that of the white fiber without any pigment as a whole, but the initial afterglow brightness of the samples were different to some degree, which when listed in a declining order were white fiber, green fiber, yellow fiber, blue fiber and red fiber. In addition, we observed a step in the curves when the afterglow brightness reached approximately 0.1 Cd m⁻²; this point is when a substantial decrease in afterglow was exhibited. This phenomenon may be related to the patterns of the fast initial decay and the long decay process. ⁹ The decrease occurred in white fiber at 97 seconds, green fiber at 79 seconds, yellow fiber at 62 seconds, blue fiber at 59 seconds and red fiber at 48 seconds. After 60 minutes, the level of the afterglow brightness still remained in the same declining order: white fiber, green fiber, yellow fiber, blue fiber and red fiber, and the values of brightness approached to each other. OPEN IN VIEWER

It was considered that different afterglow characteristics of the samples were related to the influence of the inorganic pigments on the color of the light emitted by the luminescent materials. It is well known that when light meets an object, some photons with a certain wavelength will be easily absorbed by the object if it has the same wavelength; other wavelengths will pass through the object to form the object’s hues. The luminescence of the luminescent PET fiber was attributed to the rare-earth strontium aluminates that distributed on the surface or inner portions of the fiber, with almost no particles agglomeration existing. ⁹ When the luminescent fiber was excited by the visible light, most of the strontium aluminate in the fiber, hindered by the polymer matrix and the inorganic pigments from obtaining excitation energy indirectly, generated photon transmissions, to which it was deduced that some of the photons inevitably encountered pigments that were absorbed selectively. Therefore, for the same polymer matrix, inorganic pigments had more influence on the transmission of photons emitted by the strontium aluminates. In the process of excitation, the white luminescent fiber without pigments obtained the most excitation energy and lost the least compared with the other fibers, which led to the highest initial afterglow intensity and the longest afterglow life. However, in the process of emission, photons from the white fiber were scarcely encountered, so the start of the luminescence exhibited a more rapid rate of decay. This is because the hue of the yellow-green color light emitted by the strontium aluminate is closer to that of the yellow and green pigments which transmit the yellow-green color light selectively. Thus, the afterglow brightness of the green and yellow luminescent fiber was slightly higher than that of the blue and red luminescent fiber.

Effect of light excitation conditions on the afterglow characteristics of luminescent fiber

Figure 5 shows the afterglow decay curves of the white luminescent fiber with different excitation times, 5, 15 and 30 minutes, in the same excitation light and with the same intensity, and records their attenuation conditions with a duration time of 900 seconds. It shows that the afterglow brightness of the fibers were different with different excitation times. It can be seen that the initial brightness of the fiber with excitation a time of 30 minutes was the maximum and that the excitation time of 5 minutes was the minimum. Moreover, the initial brightness of the fiber excited for 5 and 15 minutes were approximately at the extreme and showed similar decay characteristics. After about 100 seconds, the three curves flattened out gradually and the final brightness of the samples were almost the same, which showed that the afterglow brightness of the fiber with the excitation time of 5, 15 and 30 minutes successively became lower and were 0.00607 Cd/m², 0.0058 Cd/m² and 0.00568 Cd/m², respectively, at an attenuation time of 900 seconds. Moreover, we tested the afterglow brightness of the fiber with an excitation time of 10 minutes for a longer time. It was observed that the residual brightness of 0.32 mCd m⁻² was present at about 8 hours, which could apply practically in some areas of stage performance and night decoration and in some engineering applications. OPEN IN VIEWER

The above results suggested that the excitation time influenced the initial afterglow brightness and the attenuation time of the fiber to some extent but scarcely changed the law of afterglow decay. Strontium aluminate, absorbing the photons excited by the light, reached the saturation conditions, which indicated that prolonging the excitation time cannot stimulate more electrons. The state of the afterglow time mainly depended on the depth of the trap level. When the fiber was lit, the electrons that obtained enough energy moved from the valence band to the conduction band, in the process of which part of the electrons in the excited state generated fluorescence combined with a luminescent center in a short time, while the other electrons were captured by the trap level. The deeper trap first captured the electrons with the shorter excitation time, which equated to the result that when the excitation time was shorter, the duration time was longer. Meanwhile, the shallower trap captured the excess electrons with the longer excitation time, due to the shallower trap level corresponding to the faster decay process, and, as a result, the initial afterglow brightness of the fiber with the longer excitation time was relatively higher.

The trap level distribution of the luminescent PET fiber with different colors

As for the long afterglow materials, doped foreign ions replaced the ions of the host crystal’s lattice to make the matrix lattice defective and formed the trap level with a different depth that captured the excitation electrons from the conduction band. It is well known that in SrAl₂O₄:Eu, Dy phosphor, Eu²⁺ ions are the luminescent centers. The photo-excited luminescence is considered to be due to the transition from the 5d level to the 4f level of the Eu²⁺ and the holes in the traps are responsible for the long afterglow. In the present system, Dy³⁺ act as hole traps. ³ For the deeper trap, heating the luminescent materials can release the electrons from the trap and emit light. Normally the thermoluminescent method is employed to discuss the trap distribution of materials. Also rare-earth strontium aluminate in the fiber is a special thermoluminescent material, the thermoluminescent curves (the curves of the luminescent intensity with the change of temperature) can be analyzed to study the trap level distribution of the chromatic luminescent fiber. The afterglow characteristic is induced by the trap level; consequently, the investigation of the traps of materials may provide an appropriate explanation for the afterglow experimental phenomena. Figure 6 shows the thermoluminescent curves of the luminescent PET fibers for the five different colors: white (W), green (G), blue (B), yellow (Y) and red (R). OPEN IN VIEWER

As can be seen in the figure, the thermoluminescent intensity of the samples declined in the order of white, green, yellow, blue and red luminescent fiber, and the intensity of the red fiber was almost approaching that of the blue fiber. The thermoluminescent peaks of the samples were in the 393–403 K temperature range, which was not in accordance with the previous report that rare-earth strontium aluminates had thermoluminescent peaks at two temperatures. ¹⁷ This was the result because the thermoluminescent curves are related to the composition of the fiber, the preparation processing, the test conditions, etc. The depth of the trap level of the fiber can be deduced from the thermoluminescent curves. Chen ¹⁸ calculated the expression of trap level by the peak shape method based on a general dynamic model as follows

E = C τ ( KT m 2 τ ) - B τ ( 2 KT M )

(1)

C τ = 1 . 51 + 3 . 0 ( μ g - 0 . 42 )

(2)

B τ = 1 . 58 + 4 . 2 ( μ g - 0 . 42 )

(3)

The formulas including μ g = δ / ω , δ = T 2 - T m , ω = T 2 - T 1 and τ = T m - T 1 are used to calculate the trap level parameters of the luminescent PET fiber with different colors as shown in Table 2. E is the trap depth, T_m is the temperature at the maximum of the band, T₁ is the temperature corresponding to the half width at the low temperature side of the band and T₂ is the temperature corresponding to the half width at the high temperature side of the band. K is the Boltzmann’s constant.

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Table 2.

The trap level parameter of luminescent PET fiber with different colors

Samples	Tm/K	T1/K	T2/K	μ g	C τ	B τ	E/ev
W	401	359	445	0.51	1.78	1.96	0.45
B	402	362	448	0.53	1.84	2.04	0.50
G	399	359	445	0.53	1.84	2.04	0.49
R	397	357	447	0.56	1.92	2.2	0.50
Y	395	353	443	0.53	1.84	2.04	0.45

As shown in Table 2, the trap depth of the blue, red and green fibers were a little closer and higher than the trap depth of the white and yellow fibers of 0.45 eV. The thermoluminescent phenomenon was due to the trap level existing in a forbidden band of the captured electrons and holes. With an increase in the temperature of the fiber, the release probability of the electrons and the holes increased, and the luminescence was strengthened. After reaching a certain temperature, the luminescent intensity of the fiber began to weaken. The electrons and the holes of the traps decreased gradually because of their constant release, which created a peak in the pattern of the thermoluminescent spectra. When the temperature was higher, corresponding trap level was deeper.

When the energy E of the electrons released from the trap was lower, the luminescent probability of the electrons escaping from the trap and combining with the luminescence center was higher. According to the temperature of the thermoluminescent peaks in the figure, it can be estimated that the trap level of the fibers preliminarily belonged to a deeper trap. The results from Table 2 show that the E values of the white, yellow and green luminescent fibers are slightly less than that of the blue and red fibers, which led to the electrons in the trap releasing easily and obtained the high luminescent brightness. This result can be verified from the afterglow decay curves of the luminescent PET fibers with different colors. It was reported that the temperature of the thermoluminescent peak located in the range of 323–383 K, corresponding to the trap, was suitable for generating the phenomenon of long afterglow. For this reason, suitable trap depth played an important role in acquiring excellent long afterglow characteristics for the fiber.