The Luminous mechanism of Eu 2+ and Dy 3+ co-doped long persistent luminous fiber

Abstract

Fiber-forming polymer polyethylene terephthalate chips were blended with Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄ (SAOED) to afford luminous fiber with long and persistent afterglow. A dynamical model was set up to study the afterglow process in order to correlate the afterglow characteristics with the trap levels of SAOED and luminous fiber. The results indicated that the illustration of initial afterglow for luminous fiber was obviously lower than that of SAOED, but its decay process was moderately slow and therefore longer than that of SAOED. Compared with SAOED, the thermo-luminescence peak of the fiber shifted to the higher temperature, and its intensity was lower than that of SAOED. With the time extension of delay time after excitation, the depth of trap level for luminous fiber in our studies did not show any significant change. The afterglow decay behavior can be best fit by using I = I₀/(1 + bt)²; the fitting showed that the afterglow decay process followed the second order dynamics.

Keywords

luminous fiber preparation process afterglow mechanism trap level

Luminescent fiber is a novel photoluminescent material with nontoxic, non-radiative, recyclable emitting properties and functions.¹ It can emit visible light in darkness for as long as hours upon excitation. As shown in Figure 1, luminous fiber can emit warm red, yellow, blue and green in the dark. Responsible for the afterglow is the presence of the Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄ (SAOED) evenly dispersed in the fiber. With a long-lasting afterglow with good intensity, such materials are widely used in the fiber industry.^2–4 This luminescent fiber has been industrialized. The energy level transition of the rare-earth ions have resulted in the different colors of long-lasting luminescence of materials. It has been used preliminarily in developing products such as plush toys, embroidery, ornamental paintings, products for daily life, costuming, etc. Luminescent fiber also has potential industrial applications, such as aviation navigation, defense industry applications, architectural decoration, transportation, night work, etc.

Figure 1.

Long persistent luminescent fiber: (a) with light; (b) without light.

The mechanism possibly responsible for the luminescence of rare-earth aluminate phosphors^5–7 has been extensively studied. For luminous fiber, efforts have been concentrated on the preparation of such materials^8–11; there is as yet no detailed mechanistic study of the afterglow mechanism. Various models for understanding the luminescence have been proposed, including the cavity transfer model,^12,13 bitwise coordinate model,^14–16 electron trap model¹⁷ and Dorenbos model,¹⁸ out of which the cavity transfer model has been commonly accepted. It was questioned, however, whether the energy of the luminescent center Eu²⁺ ground state between the cavity and the valence band showed a large difference, which needs a large energy to complete the transfer. However, it cannot provide the above-mentioned energy to prove the long afterglow phenomenon at room temperature. Furthermore, the existence of the unusual Eu⁺ and Dy⁴⁺ oxidation states has yet to be proved experimentally. On the other hand, although the bitwise coordinate model provides a good understanding of the long afterglow, it fails to explain the photoconduction and pyroconductivity in the materials. The model only stated the capture and release of electrons, but the existence of a cavity and its effect on the long afterglow were not considered.

We have been interested in investigating the mechanism possibly responsible for the afterglow in luminous fibers. Ge and his coworkers^19,20 studied the changes in the structure, composition, physical properties and fiber spinning feasibility of the fibers when phosphors were added. However, the change of trap-level number, depth, electron concentration and mechanism of luminescent phosphor applied to the fiber has not been researched systematically and therefore remains unclear. As an initial step in tackling the mechanistic problem, we in this work report the preparation of a luminous fiber formed by melting spinning of the composite containing SAOED in the polyethylene terephthalate (PET) polymer matrix. The afterglow decay curves and the thermoluminescence spectra of the phosphor and the corresponding fiber were measured to illustrate the effect of luminescent materials’ trap level on the afterglow properties of the fiber. Setting up the dynamical model was a good way to understand and explain the process and mechanism of afterglow decay.

Experimental details

Materials

SrCO₃ (AR), Al₂O₃ (GR), Eu₂O₃ (99.99%), Dy₂O₃ (99.99%) and H₃BO₃ (AR) were purchased from Shanghai National Chemical Reagents Corporation. PET slides were obtained from Wuxi Taiji Corporation. The functional additives (including titanate coupling agent, octadecanamide and polyethylene (PE) wax) were supplied by Jiangsu Guoda Wiring Equipment Co. Ltd.

Preparation of luminescent materials

Sr_0.95Al ₂O₄:Eu²⁺_0.02,Dy³⁺_0.03 was mixed with 10% (molar ratio) of H₃BO, and the resulting mixture was ground in a mortar for 2 hours to ensure complete mixing. Absolute ethanol was then added, and the mixture was dried at 60℃ for 1 hour. After sieving with a 300-mesh sieve, the powder sample collected was placed in an alumina boat and heated in a tube furnace to 1300℃ at a rate of 10℃/min using carbon powder as a protecting gas atmosphere. The solid so obtained, SAOED, was ground and used as the phosphor for the preparation of the luminous fiber.

Preparation of luminescent fibers

The samples of luminescent fibers were prepared by the melt-spinning process. Slides of the fibrous polymer PET were dried at 100℃ for 24 hours prior to its mixing with SAOED and the facilitating agents. The mixture containing the inorganic luminous materials and the polymers was extruded in a twin-screw master batch producer at 270–290℃ to afford the parent luminous particles. After drying at 110℃, the master batches were melted and spun at 270℃ at the spinning speed of 3000 m/min and a draw ratio of 3. The content of SAOED was 5 wt%. The luminescent fibers are termed SAOED-PET.

Characteristic analysis

The afterglow decay curves of samples were measured after excitation illumination using a PR-650 (Photo Research Co. of America) spectral scanning colorimeter in standard light (powder: 8.3 W, lamp: Xenon, excitation illumination: 1000 lx, excitation time: 2 minutes).

The thermoluminescence spectra were measured by a FJ27A-I thermo-luminescent fluorimeter (CNNC Beijing Nuclear Instrument Factory, increasing temperature rate 1℃/s from room temperature to 200℃; light source: 25 W D65 Philips cold light with an excitation intensity of 1000 lx and an excitation duration of 15 minutes).

All measurements were carried out at room temperature. To ensure the accuracy of the test results, it is important to ensure all samples were prepared consistently. Specifically, the samples each containing 6.0 ± 0.1 g of fiber were spun evenly onto an I-shaped mold. All samples were checked under the same excitation conditions to make sure the difference of the samples’ initial illumination of the luminescence was less than ±0.2 lx. Prior to any repeated tests, samples were set in a cassette for more than 24 hours to ensure the afterglow had been fully quenched.

Results and discussion

Analysis of the afterglow characteristics

The afterglow decay patterns of the SAOED phosphors and SAOED-PET fiber are shown in Figure 2. It can be seen that the afterglow brightness declined quickly at the initial stage, followed by a slower decay as the process progressed. The afterglow intensity of SAOED was higher than that of SAOED-PET. Although the initial afterglow intensity and decay speed were different, the phosphor and the corresponding fiber showed similar decay behaviors.

Figure 2.

The afterglow decay curves of SrAl₂O₄:Eu²⁺,Dy³⁺ (SAOED) and luminescent fiber. PET: polyethylene terephthalate.

Figure 3.

The afterglow decay fitting curves of (a) SrAl₂O₄:Eu²⁺,Dy³⁺ (SAOED) and (b) luminescent fiber. PET: polyethylene terephthalate.

Actually, the afterglow decay process of most luminescent materials was complex. It was found that the afterglow decay process of SAOED included three stages, which referred to the first fast decay stage corresponding to the emission of Eu²⁺, the second moderate decay stage rooted from the captured Eu²⁺ by the trap center and the third slow decay stage caused by releasing electrons from the Dy³⁺ deeper trap center.² In order to explain the decay of the afterglow process, the curves of afterglow decay for SAOED and SAOED-PET were carried out to fit exponential decay (second order, third order) by origin procedure in Figure 3. It was clearly shown that the third-order curve fitted best and coincided better with the experimental data than the second. The afterglow illumination was evaluated based on the decay time with a three-component exponential curve, as shown in Equation (1)²¹

I = I_{0} + I_{1} e^{- \frac{t - t_{0}}{τ_{1}}} + I_{2} e^{- \frac{t - t_{0}}{τ_{2}}} + I_{3} e^{- \frac{t - t_{0}}{τ_{3}}}

(1)

In the equation, I is the afterglow intensity at any time t, and I₀ and t₀ are modified constants. I₁, I₂, I₃ represent the initial afterglow intensity of the three individual stages of decay, corresponding respectively to a fast, moderate and a slow decay process, while τ₁, τ₂, τ₃ are the time needed to complete each of these three stages.

The parameters of I₁, I₂, I₃, τ₁, τ₂, τ₃ and correlation index R induced from the fitting are listed in Table 1.

Table 1.

The fitting results with each afterglow decay stage of the samples

Sample	Fast		Moderate		Slow		R² (%)
Sample	I₁ (lx)	τ₁ (min)	I₂ (lx)	τ₂ (min)	I₃ (lx)	τ₃ (min)	R² (%)
SAOED	6.16	0.18	1.59	0.85	0.38	5.37	99.99
SAOED-PET	2.08	0.31	0.66	1.42	0.19	7.43	99.98

SAOED: Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄; PET: polyethylene terephthalate.

The R² value for the fitting data of the third order compared with the experimental data is more than 99.9%; the starting afterglow intensity of the fast decay stage was much higher than that of the slow decay stage, but the decay time of the slow decay stage was significantly longer than that of the fast decay stage. The initial afterglow intensity of SAOED-PET was obviously lower than that of SAOED, but its decay time at each stage was greater than that of SAOED. It can be understood that the higher the τ value, the longer the decay time, and the slower the decay speed.

The afterglow characteristics of SAOED-PET mainly depended on SAOED dispersed in the fiber. However, the afterglow decay characteristics of SAOED were related to the types, depth and electron concentration of the trap originating from the crystal defect. Upon excitation, electrons in the shallow traps rapidly escaped back to the excited state, and relaxed from the excited state to the ground state to constitute the fast decay stage. The number of electrons in the moderate and deeper trap levels reduced and had a low probability of escape, which corresponded to the moderate and slow decay stages. The reason for the low afterglow intensity and long decay time of SAOED-PET was that the emission of SAOED in fiber was hindered by the polymer substrate PET, which reduced the amount of excitation energy in the process of excitation and thus led to the low number of captured electrons that subsequently transited to the excited state. Therefore, the initial afterglow intensity of SAOED-PET was lower. Meanwhile, part of the light from the excitation of SAOED in the fiber was selectively absorbed by the polymer matrix, and its excitation energy was exhausted. Another part of the optical propagation encountered the luminescent particles that continued to emit, back and forth, prolonging the transmission of light and delaying the decay speed. It can be seen that SAOED-PET had a long duration.

Analysis of the trap level

In order to identify the effects of the polymer substrate on the afterglow decay and trap level of the luminescent materials in the fiber, the thermo-luminescent curves (the curves of the luminescent intensity with the change of temperature) of SAOED and SAOED-PET were measured under the same conditions (Figure 4). The samples both presented broadband spectra. The thermo-luminescent peaks of SAOED and SAOED-PET were at 436 and 440 K, respectively, from which it can be concluded that the thermo-luminescent peak moved a little to the higher temperature, and the activation energy of the charge carrier increased as well after adding SAOED to the polymer substrate. Observing the shape of thermoluminescence spectra, the peak intensity of SAOED was much higher than that of SAOED-PET, and its half width was broad. The lower amount of charge carrier released in the fiber may be related to the lower content of luminescent materials in the fiber.

Figure 4.

The thermo-luminescent curve: (a) SrAl₂O₄:Eu²⁺, Dy³⁺ (SAOED); (b) luminescent fiber. PET: polyethylene terephthalate.

The trap-level depth of materials can be obtained by the thermoluminescence curves. Based on a general dynamic model, Chen²² deduced the expression of the trap level by the peak shape method as follows

E = C_{τ} (\frac{{kT}_{m}^{2}}{τ}) - B_{τ} (2 {kT}_{m})

(2)

C_{τ} = 1.51 + 3.0 (μ_{g} - 0.42)

(3)

B_{τ} = 1.58 + 4.2 (μ_{g} - 0.42)

(4)

μ_{g} = δ / ω

(5)

δ = T_{2} - T_{m}

(6)

ω = T_{2} - T_{1}

(7)

τ = T_{m} - T_{1}

(8)

In the formulae, E is the trap-level 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 constant. The exact parameter and results are shown in Table 2.

Table 2.

The trap-level parameters of the samples

Sample	T_m (K)	T₁ (K)	T₂ (K)	$μ_{g}$	$C_{τ}$	$B_{τ}$	E (eV)
SAOED	436	398	501	0.63	2.14	2.46	0.74
SAOED-PET	440	397	484	0.51	1.78	1.96	0.54

SAOED: Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄; PET: polyethylene terephthalate.

Analyzing the results of trap-level depth, it can be seen that the activation energy of the charge carrier for SAOED blending with increasing polymer substrate PET increased, which indicated that the charge carrier in the trap was difficult to release, and the decay time was prolonged. In addition, the electron concentration of SAOED-PET was lower than that of SAOED, which resulted in the decrease of the charge carrier captured and released by the trap in the fiber. When the energy E of the electrons released from the trap was lower, the luminescence probability of the electrons escaping from the trap and combining with the luminescent center was higher.

Analysis of the afterglow dynamics

After the light excitation was stopped, the process of the trap capturing and releasing the charge carrier had different effects on the analysis of the afterglow characteristics with the different time periods. While the afterglow brightness was exhausted finally, it remained unknown whether there were charge carriers not released completely in the trap. Based on this, in the different time periods (after the light excitation stopped), the thermo-luminescent curves of SAOED-PET were tested to obtain the thermoluminescence spectra. The dynamics model of the afterglow decay process for SAOED-PET was deduced and set up through the theory of afterglow dynamics.

Factors including the light excitation time, delay time (the time intervals from the light excitation stopping to the beginning of thermoluminescence heating), the intensity of light excitation and heating rate were considered. With other conditions unchanged, the thermoluminescence curves of SAOED-PET were measured with different waiting times to analyze the releasing charge carrier of the trap systematically. The delay time was set as 0.25, 0.5, 1, 2, 4 and 24 hours. The results are shown in Figure 5.

Figure 5.

The different thermoluminescence curves with different delay times.

With the delay time prolonged, the intensities of the thermoluminescence peak declined drastically and shifted to the higher temperature areas simultaneously. The intensity and temperature of the thermoluminescence peak for SAOED-PET with different delay times are shown in Figures 6 and 7, respectively. The electron trap of luminescent materials dispersed in fiber was caused by the unequal replacing of Sr²⁺ with doped Dy³⁺, which determined the character of the trap level. As soon as samples were irradiated, the electrons of Eu²⁺ transitioned from the ground state to the excitation state with a large number of electron hole pairs appearing; thereupon, some of the electrons or holes moved on the valence band or conduction band and were captured by the trap level. While the light excitation stopped, under the condition of less delay time, an increment of the charge carrier captured and stored in the trap occurred. With the occurrence of thermal disturbance, more charge carriers escaped from the trap, resulting in a higher thermoluminescence intensity. With the prolongation of the delay time, the electrons stored in the trap were gradually released in the process of afterglow attenuation. The longer the delay time, the fewer the number of unreleased electrons in the trap, which necessitates a higher temperature of disturbance to release the rest of the electrons. Hence, the temperature of the thermoluminescence peak shifted in the direction of higher temperature with the prolongation of delay time. The depth of the trap level mainly depended on the lattice structure of the matrix and the electronic structure of substitution ion, so the delay time barely affected the depth of the trap level.

Figure 6.

The different thermo-luminescent peak intensities with different delay times.

Figure 7.

The different thermoluminescence peak temperatures with different delay times.

Two possibilities may be responsible for the shift to the higher temperature of the thermoluminescence peak with the prolongation of delay time. The first is the different distribution of trap-level energy, that is, when the loss of trap-level energy increased and the half width of thermoluminescence peak decreased. As shown in Table 3, the half width of the thermoluminescence peak was almost the same, implying that although the trap-level energy changed to some extent, which was not the main reason causing the shift of the peak position. The other possibility is the reason for the dynamics of the charge carriers in the trap level. Chen and Mckeever²³ considered that the process of the trap level capturing and releasing charge carriers is in accordance with the second-order dynamics (two times of the capturing process by trap level after the charge carriers are released), then the position of the thermo-luminescent peak will shift to the higher temperature as the time is extended. It is necessary to discuss the first-order and second-order dynamics process of SAOED-PET.

Table 3.

The trap-level parameters of luminescent fiber with different delay times

Delay time (h)	T_m (K)	δ (K)	τ (K)	ω (K)	μ _g
0.25	349	25	24	49	0.51
0.5	361	23	22	45	0.51
1	363	23	20	43	0.53
2	366	22	24	46	0.48
4	373	26	23	49	0.53
24	390	26	25	51	0.51

The trap-level depth of SAOED-PET was 0.5 eV. Although the thermoluminescence peak shifted with the prolongation of delay time, the depth of the trap was not changed. According to the research of Chen,²² the expression between the initial concentration of charge carriers n₀ in the trap and T_m is as shown in Equation (9)

β / T_{m}^{2} = (sk / E) [1 + (l - 1) \frac{2 {kT}_{m}}{E}] exp (- \frac{E}{{kT}_{m}})

(9)

In the equation, β is the heating rate, which is 1 K/s. s is frequency factor and $s'$ is the constant. E is the depth of the trap level, k is the Boltzmann constant and l is the kinetic order. Equation (10) can be inferred from Equation (9) as follows

n_{0} = {\frac{E exp (\frac{E}{{kT}_{m}})}{s' {kT}_{m}^{2} [1 + (l - 1) \frac{2 {kT}_{m}}{E}]}}^{\frac{1}{l - 1}}

(10)

The E value of 0.5 eV was substituted into Equation (10), and the results are shown in Equation (11)

n_{0} = [\frac{C_{1} exp (\frac{5801}{T_{m}})}{T_{m}^{2} (1 + \frac{C_{2} T_{m}}{2900})}]^{\frac{1}{C_{2}}}

(11)

On the basis of the constant $C_{1} = E / s' k$ , $C_{2} = l - 1$ , the pattern between the initial concentration of charge carriers n₀ in the trap and temperature of the thermoluminescence peak T_m is as shown in Figure 8. After fitting, C₁ and C₂ were 499.0 and 1.0, respectively, from which we can deduce that l was 2.0, which demonstrated that the second-order dynamics was reasonable for the thermoluminescence process of SAOED-PET. In addition, the geometrical factor of the thermoluminescence peak μ _g was in the range of 0.42–0.53. The μ _g value of 0.42 reflected the thermoluminescence process of materials consistent with the first-order dynamic model, but its value of 0.53 indicated accordance with the second-order dynamic model. It can be seen obtained from Table 3 that the μ _g value was mostly near 0.51, so in the thermoluminescence process of SAOED-PET the second-order dynamics is more relevant. Meanwhile, analyzing the afterglow curves and according to the theory of the first-order dynamic, the afterglow intensity can be concluded by Equation (12)

I (t) = - \frac{dn}{dt} = sn exp (- \frac{E}{kT})

(12)

Figure 8.

Pattern for trapped carrier concentration and thermoluminescence peak temperature.

In the equation, I(t) is the afterglow intensity and n is the carrier concentration in the trap. The carrier lifetime in the trap was fitted for Equation (13) that substituted into Equation (12); thus, Equation (14) was obtained

τ = s^{- 1} exp (\frac{E}{kT})

(13)

I (t) = - \frac{dn}{dt} = \frac{n}{τ}

(14)

The result of I(t) was the exponential function, as shown in Equation (15)

I (t) = I_{0} exp (- \frac{t}{τ})

(15)

It can be concluded from Equations (14) and (15) that the initial intensity of the thermoluminescence curve made up the function with the delay time, from which we can obtain the afterglow decay curves of SAOED-PET, as shown in Figures 9 and 10. The curves were fitted by Equation (15), but they cannot be fitted well with the experimental data. The afterglow decay rule did not suit for the first-order dynamic. As for the second-order dynamic, the afterglow intensity was in direct proportion to the square of the carrier concentration to get Equation (16)

I (t) = - \frac{dn}{dt} = {cn}^{2}

(16)

Figure 9.

The different initial afterglow intensities with different delay times.

Figure 10.

The decay curve of trapped carrier concentration with different delay times.

In the equation, c is the constant. Equation (17) was calculated from Equation (16) as follows

n = \frac{n_{0}}{1 + at}

(17)

In the equation, a is the constant. According to Equations (16) and (17), the afterglow intensity was in accordance with Equation (18)

I = \frac{I_{0}}{(1 + bt)^{2}}

(18)

I₀ is the initial afterglow intensity and b is the constant implying the speed of afterglow decay. This function can fit the curve of afterglow decay well, and therefore the rule of afterglow decay was in accordance with the second-order dynamic.

The afterglow intensity and the decay time of luminous fiber were affected by the excitation resource, the constitution of the fiber (polymer matrix, pigment, additives, etc.) and the forming process of the fiber. The 4f electrons of rare-earth ions absorbed certain wavelength energy corresponding to its energy level and then began to transit from the ground state to the excited state. The decay processes of samples were caused by the different depths of trap level in the materials. The shallower the depth of the trap level, the easier it is for electrons in the traps to escape, showing a fast initial decay corresponding to the shallow trap and a long-lasting decay corresponding to the deep trap. Therefore, different decay processes corresponded to the different depths of the trap.

Conclusion

The afterglow intensity and decay rate were both different, but showed a similar decay rule. The initial afterglow intensity of SAOED-PET was lower than that of SAOED, but the decay time of each stage was greater than that of SAOED. For SAOED-PET, the start afterglow intensity of the fast decay stage was far higher than that of the slow decay stage, but the decay time of the slow decay stage was significantly longer than that of the fast decay stage.

The thermoluminescence curves of SAOED-PET and SAOED both presented broadband spectra. The thermoluminescence peak shifted slightly to a higher temperature after adding SAOED to the polymer substrate, which demonstrated that the afterglow decay time of SAOED-PET was much longer, and this was accorded with the fitting results of the afterglow decay process. The thermoluminescence peak intensity of SAOED was much higher than that of SAOED-PET, and its half width was broad. This implied that the carrier concentration of SAOED was higher than that of SAOED-PET, and consequently its afterglow intensity was greater than that of SAOED-PET.

With the delay time being prolonged, the intensities of the thermoluminescence peak declined drastically and shifted to higher temperature areas simultaneously. The depth of the trap was not changed. The second-order dynamics was more appropriate for the thermoluminescence and afterglow process of SAOED-PET, which was the two times of capturing process by the trap level after most of the charge carriers had been released. This process helped extend the afterglow time.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work was supported by the China Postdoctoral Science Foundation (No. 2015M581860), the Jiangsu Province Postdoctoral Science Funded Project (No. 1501062A), Jiangsu QingLan Project Young Academic leaders and the Jiangsu 333 Science Funded Project (No. BRA2016444).

References

Ge MQ and Yu GW. Color and color light of rare-earth luminescent fiber and preparation methods. Patent 1584138, China, 2005.

Jia

. Charging curves and excitation spectrum of long persistent phosphor SrAl₂O₄:Eu²⁺, Dy³⁺. Opt Mat 2003; 22: 65–69.

Haranath

Virendra

Harish

, et al. Studies on the decay characteristics of strontium aluminate phosphor on thermal treatment. Mat Chem Phys 2002; 78: 6–10.

Niittykoski

Aitasalo

Hölsä

. Effect of boron substitution on the preparation and luminescence of Eu²⁺ doped strontium aluminates. J Alloys Compd 2004; 374: 108–111.

Matsuzawa

Nabae

Katsumata

, et al. Effects of composition on the long phosphorescent SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor crystals. J Electrochem Soc 1997; 144: 243–245.

Luan

Guo

Huang

. Effect of Al/Sr ratio on properties of strontium aluminate long lasting phosphor. J Inorg Mat 2009; 24: 53–56.

Zhu

Zheng

Zeng

, et al. Luminescence enhancing encapsulation for strontium aluminate phosphors with phosphate. Mat Chem Phys 2009; 113: 721–726.

Shimizu Y, Ogasawara K, Sakakura H, et al. High luminance luminous fiber and process for producing the same. Patent 6162539, USA, 2000.

Miyazaki S and Sekkai S. Polyester fibers containing luminescent substances and their manufacture. Patent 285806, Japan, 2003.

10.

Mishra

Revaprasadu

, et al. Strontium aluminate/polymer composites: morphology, luminescent properties and durability. J Appl Polym Sci 2009; 112: 3347–3354.

11.

Siggel A, Pltrawa T and Langheim H. Preparation of luminescent fibers useful in security marking of fiber-containing materials. DE Patent 19802588, 1998.

12.

Kamada

Murakami

Ohno

. Excitation spectra of a long-persistent phosphor SrA1₂O₄: Eu, Dy in vacuum ultraviolet region. J Lumin 2000; 87-89: 1042–1044.

13.

Zhang

Yin

. Study on long persistent luminescence mechanism of Eu²⁺,Dy³⁺ co-actived strontium aluminate phosphors. J Synth Cryst 2004; 2: 67–70.

14.

Zhang

Wang

. Luminescent properties of MAl₂O₄:Eu²⁺, Re³⁺. Chin J Lumin 1999; 20: 170–174.

15.

Qiu

Miura

Inouye

, et al. Blue emission induced in Eu²⁺-doped glasses by an infrared femtosecond laser. J Non Cryst Solid 1999; 244: 185–188.

16.

Qiu

Hirao

. Long lasting phosphorescence in Eu²⁺-doped calcium aluminoborate glasses. Solid State Commun 1998; 106: 795–798.

17.

Chen

Wang

, et al. Modification on luminescent properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor by Yb³⁺ ions doping. J Lumin 2008; 128: 1180–1184.

18.

Dorenbos

. Scintillation mechanisms in Ce³⁺ doped halide scintillators. Phys Status Solidi A 2005; 202: 195–200.

19.

Guo

Zhao

. Photochromic properties of rare-earth strontium aluminate luminescent fiber. Fibers Polym 2011; 12: 875–879.

20.

Guo

. The afterglow characteristics and trap level distribution of chromatic rare-earth luminous fiber. Text Res J 2013; 83: 1263–1272.

21.

Lin

Zhang

Tang

. The characterization and mechanism of long afterglow in alkaline earth aluminates phosphors co-doped by Eu₂O₃ and Dy₂O₃. Mater Chem Phys 2001; 70: 156–159.

22.

Chen

. Glow curves with general order kinetics. J Electrochem Soc 1969; 116: 1254–1257.

23.

Chen

Mckeever

SWS

. Theory of thermoluminescence and related phenomena, Singapore: World Scientific Published Co., 1997.