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Piezoelectrically-induced stress-luminescence phenomenon in CaAl2O4:Eu2+
Journal of Alloys and Compounds 646 (2015) 86e89
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Piezoelectrically-induced stress-luminescence phenomenon in CaAl2O4:Eu2þ Yongbin Wei a, Zheng Wu b, *, Yanmin Jia a, *, Yongsheng Liu c a
Department of Physics, Zhejiang Normal University, Jinhua 321004, China College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China c Department of Physics, Shanghai University of Electric Power, Shanghai 200090, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 25 May 2015 Accepted 26 May 2015 Available online 10 June 2015
Piezoelectrically-induced stress-luminescence in the CaAl2O4:Eu2þ was investigated. Blue light that was visible to the naked eye could be observed in the dark when a pulse force of ~7.7 kN was applied to the sample. The intensity of the stress-luminescence strongly depended on the magnitude of the applied force during a pulse cycle. The intensity decreased with repetitive application of pulse stress and was completely recovered after irradiation with ultraviolet light. It is suggested that the stress-luminescence effect in CaAl2O4:Eu2þ arises from the piezoelectrically-induced de-trapping of the charge carriers. A CaAl2O4:Eu2þ ceramic that exhibits a stress-luminescence effect has potential applications in smart stress optically-sensing devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Luminescence Piezoelectric CaAl2O4:Eu2þ Stress-luminescence
1. Introduction Recently, materials that exhibit a rapid luminescence response to an electric field (electroluminescence), to a chemical reaction (chemiluminescence), photoexcitation (photoluminescence), or to sound (sonoluminescence) have been intensively studied and widely used in practical applications such as in light-emitting diodes and color cathode ray tubes [1e4]. Relatively few investigations have been done on stress-luminescence, which could be utilized in smart mechanical-optical sensor devices [5]. Stress-luminescence, also known as the piezoluminescence effect, is the light output from a material under stress. This effect was first reported in the sixteenth century [6]. To date, stressluminescence has been observed in many inorganic salts and organic molecular solids including sugars, molecular crystals, alkali halides, SiO2 and ferroelectric polymers [7e12]. However, in most of the stress-luminescent materials reported, the coupling between stress and the light output is very weak and the stressluminescence was destructive and not reusable. Hence, it cannot be used for practical applications. Investigation of a material that exhibits non-destructive stress luminescence with a high emission
* Corresponding authors. E-mail addresses: [email protected] (Z. Wu), [email protected] (Y. Jia). http://dx.doi.org/10.1016/j.jallcom.2015.05.159 0925-8388/© 2015 Elsevier B.V. All rights reserved.
intensity and repeatability are desired for applications such as in smart stress sensors. As one of the most important long-afterglow luminescent materials, the Eu2þ activated alkaline earth aluminates have drawn intense attention in recent years because of their high quantum efficiency in the visible region and good physical and chemical stability [13,14]. The rare earth doped alkaline aluminates are also promising stress-luminescence materials and G. Yun et al. reported the repeatable stress-luminescence effect in rare earth Eu2þ doped aluminate ceramics [15]. In this study, we report the piezoelectrically-induced stressluminescence in the CaAl2O4:Eu2þ ceramic. It was found that the intensity of the stress-luminescence was drastically decreased by repetitive application of pulse stress and could be completely recovered after irradiation by ultraviolet (UV) light. 2. Experimental The 1 mol.% Eu2þ doped CaAl2O4 ceramic powders were synthesized by a solid-state reaction method using reagent grade CaCO3, a-Al2O3 and Eu2O3 as raw materials. The raw materials were weighted in the stoichiometric ratio and ball milled 12 h with a small amount of ethanol. After separating the balls, the mixture was preheated at 900 C for 1 h in air and then sintered at 1300 C for 4 h in a reduction atmosphere (95%Ar þ 5%H2). The sintered
Y. Wei et al. / Journal of Alloys and Compounds 646 (2015) 86e89
powders were then ball milled 12 h again with the addition of ethanol to obtain homogeneous ceramic powders. X-ray diffraction (XRD) was employed to study the crystal structure of the ceramic powders. A FLS920 spectrophotofluorometer (Edinburgh, UK) was used for the measurements of the PL and photoluminescence excitation (PLE) spectra. In order to reduce the fracture of the CaAl2O4:Eu2þ ceramic, CaAl2O4:Eu2þ powders were mixed with an optical epoxy resin (G-2011) evenly by using mechanical stirring. The volume fraction of the ceramic powders in epoxy resin is ~9.1%. The mixture was injected into a cylinder mode with the dimensions of 25 mm in diameter and 15 mm thick and then was put into a vacuum drying oven (DZF6021, Shanghai Jinghong Laboratory Instrument Co., Ltd., China) and curred in a vacuum environment (~133 Pa) at room temperature for 24 h. The intensity of the stress-luminescence was recorded using a photon counting system that consisted of a photomultiplier tube (PMTH-S1-CR131, Zolix Instruments CO., LTD, China) and an oscilloscope (TDS 2002, Tektronix Crop., U.S.A.). The pulse stress was applied on the sample using a universal testing machine (WDW-30, Jinhua Julong Co., Ltd., China). The sample was irradiated under an UV light (ZF-1, power of~30 W, wavelength of 365 nm, Shanghai Jihui Scientific Instrument Co., Ltd., China) for 3 min before the measurement of the stress-luminescence.
3. Results and discussion The XRD pattern indicated that the sample possesses a pure monoclinic phase, as shown in Fig. 1. All the diffraction peaks of the XRD pattern matched perfectly with what reported for CaAl2O4 (JCPDS 70-0134) and no peak of any other phase was detected, which means that the Eu2þ ions have diffused into the CaAl2O4 host. Considering that the effective ionic radii of Eu2þ, Ca2þ and Al3þ are 1.20, 1.06 and 0.39 Å, respectively, the Eu2þ doping is expected to occupy the Ca2þ sites. The PL and PLE spectra of the CaAl2O4:Eu2þ ceramic powders are shown in Fig. 2. Excitation spectra of the ceramic powders monitored at an emission wavelength (lem) of 448 nm had an intense and broad peak corresponding to the Eu2þ ion in the UV region that is associated with 4f7e4f65d1 electronic transitions [16,17]. Excited at an excitation wavelength (lex) of 338 nm, the sample shows a peak centered at 448 nm, which corresponds to the transition of Eu2þ from an excited state with a configuration of 4f65d1 to the ground state 8S7/2 with 4f7 configuration [18]. No obvious emission
Fig. 1. XRD patterns of the CaAl2O4:Eu2þ ceramic powder.
87
Fig. 2. The PL and PLE spectra of the CaAl2O4:Eu2þ ceramic powders. The dash lines show the three decomposed individual Gaussian bands of the PLE spectrum.
peak of the trivalent europium ion was observed, indicating that there were little europium impurities in the sample. The Stokes Shift, DES, which is the difference in energy between the absorption and emission peaks, the HuangeRhys factor, S, and the effective phonon energy,Zu, can provide quantitative characterizations of the electron-vibrational interaction [19]. The values of DES and Zu can be estimated using [19]:
DES ¼ ð2S 1ÞZu
(1)
1=2 Zu FWHMðTÞ ¼ 2:35 Zu S coth 2kT
(2)
where u is the frequency of the active vibrational mode, k is the Boltzmann constant, and T is a given absolute temperature. Eq. (2) describes the temperature variation of the full width at halfmaximum (FWHM). The values of the FWHM and DES can be extracted from the experimental emission and excitation spectra. It should be noted that the wavelength lab of absorption wavelength peak related to the lowest level of the 4f65d1, is usually not resolved at room or higher temperatures. Here we can only give an estimation value of DES. The wide excitation band is made up of three individual Gaussian bands centered at 268 nm [16], 335 nm and 383 nm as shown by the dashed lines in Fig. 1. Using the first absorption band at 383 nm (26109 cm1) and the emission band maximum at 448 nm (22321 cm1), DES is about 3778 cm1. The FWHM value of the CaAl2O4:Eu2þ is about 51 nm, which is smaller than that of YAG:Ce3þ (~120 nm) or SrSi2O2N2:Eu2þ (~82 nm) [18,20]. This smaller FWHM value means that there is a higher color purity of the synthesized Eu2þ-doped CaAl2O4 than those other materials [21]. Fig. 3(a) shows the applied pulse pressure and Fig. 3(b) shows the stress-luminescence of CaAl2O4:Eu2þas a function of time. Intense blue light (~0.81 lux) that was visible to the naked eye was emitted from the sample when a pulse force of ~7.7 kN (corresponding to a pressure of ~15.8 MPa) was applied to the sample. The luminescent intensity became lower and lower as the cycle of the applied pulse stress went up. The applied pressure and stressluminescence of CaAl2O4:Eu2þ as a function of time in the first cycle of the pulse stress is depicted in the inset of Fig. 3(b). It can be seen that the luminescence output responded to the applied stress quickly. In a pulse cycle, the intensity of the stress-luminescence
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Y. Wei et al. / Journal of Alloys and Compounds 646 (2015) 86e89
Fig. 4. The logarithm of stress-luminescence intensity as a function of the inverse square root of applied pressure. Fig. 3. The applied pulse pressure (a) and the stress-luminescence of CaAl2O4:Eu2þ (b) as a function of time. The inset of Fig. 3(b) shows the applied pressure and stressluminescence of CaAl2O4:Eu2þ as a function of time in the first cycle of the pulse stress.
firstly increased with the increase of the applied stress, then decreased with the decrease of stress. For the electroluminescent materials, the intensity of luminescence (I) as a function of the applied electric field strength (E) can be expressed as following [5,22]:
.pffiffiffi E I ¼ a0 f 1=2 exp b0
(3)
where a0, b0 (a0 > 0, b0 > 0) are constants, which depend on the environmental temperature and the chemical compositions of the luminescent material, f is the frequency. For stress-luminescence materials, the electric field E induced by piezoelectric effect under the applied pressure (P) can be further written as E ¼ gP on basis of the piezoelectric constitutive equations [23]. Here g is the piezoelectric voltage constant of the material. Then the stress-luminescence can be given as Eq. (4):
.pffiffiffiffiffiffi gP I ¼ a0 f 1=2 exp b0
(4)
On basis of Eq. (5), the stress-luminescent intensity increases with the increase of the applied stress. The theoretical analysis agrees well with our experimental results. Taking the logarithm of Eq. (4), we get:
.pffiffiffiffiffiffi pffiffiffi gP ln I ¼ ln a0 f b0
(5)
On basis of Eq. (5), for the stress-luminescence materials, LnI has a linear response to the inverse square root of the applied pressure. Fig. 4 plots the logarithm of stress-luminescence intensity as a function of the inverse square root of the applied pressure (corresponding to the pressure P ¼ 15.8, 15.3, 14.4, 12.1, 8.9, 7.1, 4.9 MPa, respectively). The linear fitting result was noted by the red straight line. The excellent linear relationship between the logarithm of stress-luminescence intensity and the inverse square root of the applied pressure in Fig. 4 agrees with our theoretical prediction (Eq. (5)) very well. Fig. 5 shows the dependence of the peak intensity of the stressluminescence of the CaAl2O4:Eu2þ ceramic on the numbers of the applied stress cycle. The luminescence intensity decreased with
Fig. 5. The dependence of the stress-luminescent peak intensity of the CaAl2O4:Eu2þ on the numbers of the applied pulse stress cycle.
successive repetitive application of stress and became stable at about 5% of the initial intensity. After the sample was irradiated under UV light for 3 min, the intensity of the luminescence recovered completely, which means that the stress-luminescence was produced by a nondestructive deformation of CaAl2O4:Eu2þ. Also, the recovery of the stress-luminescence intensity after UV irradiation suggests that traps occur in the CaAl2O4:Eu2þ sample that are filled by UV irradiation [24]. The CaAl2O4:Eu2þ ceramic can also be used as an energy storing material that absorbs UV light from sunlight and then gradually releases the absorbed energy in the dark in the form of visible light [25,26]. A schematic of the stress-luminescence emission in CaAl2O4:Eu2þ is presented in Fig. 6. As previously reported, the band gap of CaAl2O4 is ~6.7 eV and the 5d levels of doped Eu2þ are partly in the conduction band (CB) of CaAl2O4 [27,28]. The lowest of the 5d levels was ~0.3 eV below the host CB [29]. After UV irradiation, the electrons are excited from the 4f ground level of Eu2þ to the 5d levels. Some electrons can escape directly to the CB from a 5d level located within the CB with the aid of thermal energy or photoexcitation energy. Then the electrons in the CB are captured by oxygen vacancies that are close to bottom of CB. The electrons migrate from one trap to another by releasing and acquiring
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Acknowledgments This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LY14E020003), the National Natural Science Foundation of China (No. 11374204), “Shu Guang” project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG52), and the Science and Technology Commission of Shanghai Municipality (Nos. 12JC1404400, 14520501000).
References
Fig. 6. Energy level schematic for the piezoelectrically-induced stress-luminescence emission in CaAl2O4:Eu2þ.
thermal energy. The applied stress produces a piezoelectric field in the noncentrosymmetric CaAl2O4:Eu2þ. This would reduce the trap depth and lead to a number of electrons migrating from the traps to the CB and then to the 5d levels. The electrons rapidly relax to the lowest 5d level via a nonradiative transition and finally return to the ground state by emitting a light at about 448 nm.
4. Conclusions In conclusion, the strong piezoelectrically-induced stressluminescence effect in the CaAl2O4:Eu2þ ceramic was investigated. The stress-luminescence intensity strongly depends on the applied stress magnitude in each cycle of the applied pulse stress. As the number of cycles of the applied pulse stress increases, the stressluminescence intensity decreases. The intensity of the luminescence can be recovered completely after irradiation under UV light for 3 min. It is suggested that the stress-luminescence in CaAl2O4:Eu2þ arises from the piezoelectric field induced detrapping of the charge carriers. The strong stress-luminescence effect in CaAl2O4:Eu2þ ceramic has potential applications in smart stress optically-sensing devices.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Piezoelectrically-induced stress-luminescence phenomenon in CaAl2O4:Eu2þ Yongbin Wei a, Zheng Wu b, *, Yanmin Jia a, *, Yongsheng Liu c a
Department of Physics, Zhejiang Normal University, Jinhua 321004, China College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China c Department of Physics, Shanghai University of Electric Power, Shanghai 200090, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 25 May 2015 Accepted 26 May 2015 Available online 10 June 2015
Piezoelectrically-induced stress-luminescence in the CaAl2O4:Eu2þ was investigated. Blue light that was visible to the naked eye could be observed in the dark when a pulse force of ~7.7 kN was applied to the sample. The intensity of the stress-luminescence strongly depended on the magnitude of the applied force during a pulse cycle. The intensity decreased with repetitive application of pulse stress and was completely recovered after irradiation with ultraviolet light. It is suggested that the stress-luminescence effect in CaAl2O4:Eu2þ arises from the piezoelectrically-induced de-trapping of the charge carriers. A CaAl2O4:Eu2þ ceramic that exhibits a stress-luminescence effect has potential applications in smart stress optically-sensing devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Luminescence Piezoelectric CaAl2O4:Eu2þ Stress-luminescence
1. Introduction Recently, materials that exhibit a rapid luminescence response to an electric field (electroluminescence), to a chemical reaction (chemiluminescence), photoexcitation (photoluminescence), or to sound (sonoluminescence) have been intensively studied and widely used in practical applications such as in light-emitting diodes and color cathode ray tubes [1e4]. Relatively few investigations have been done on stress-luminescence, which could be utilized in smart mechanical-optical sensor devices [5]. Stress-luminescence, also known as the piezoluminescence effect, is the light output from a material under stress. This effect was first reported in the sixteenth century [6]. To date, stressluminescence has been observed in many inorganic salts and organic molecular solids including sugars, molecular crystals, alkali halides, SiO2 and ferroelectric polymers [7e12]. However, in most of the stress-luminescent materials reported, the coupling between stress and the light output is very weak and the stressluminescence was destructive and not reusable. Hence, it cannot be used for practical applications. Investigation of a material that exhibits non-destructive stress luminescence with a high emission
* Corresponding authors. E-mail addresses: [email protected] (Z. Wu), [email protected] (Y. Jia). http://dx.doi.org/10.1016/j.jallcom.2015.05.159 0925-8388/© 2015 Elsevier B.V. All rights reserved.
intensity and repeatability are desired for applications such as in smart stress sensors. As one of the most important long-afterglow luminescent materials, the Eu2þ activated alkaline earth aluminates have drawn intense attention in recent years because of their high quantum efficiency in the visible region and good physical and chemical stability [13,14]. The rare earth doped alkaline aluminates are also promising stress-luminescence materials and G. Yun et al. reported the repeatable stress-luminescence effect in rare earth Eu2þ doped aluminate ceramics [15]. In this study, we report the piezoelectrically-induced stressluminescence in the CaAl2O4:Eu2þ ceramic. It was found that the intensity of the stress-luminescence was drastically decreased by repetitive application of pulse stress and could be completely recovered after irradiation by ultraviolet (UV) light. 2. Experimental The 1 mol.% Eu2þ doped CaAl2O4 ceramic powders were synthesized by a solid-state reaction method using reagent grade CaCO3, a-Al2O3 and Eu2O3 as raw materials. The raw materials were weighted in the stoichiometric ratio and ball milled 12 h with a small amount of ethanol. After separating the balls, the mixture was preheated at 900 C for 1 h in air and then sintered at 1300 C for 4 h in a reduction atmosphere (95%Ar þ 5%H2). The sintered
Y. Wei et al. / Journal of Alloys and Compounds 646 (2015) 86e89
powders were then ball milled 12 h again with the addition of ethanol to obtain homogeneous ceramic powders. X-ray diffraction (XRD) was employed to study the crystal structure of the ceramic powders. A FLS920 spectrophotofluorometer (Edinburgh, UK) was used for the measurements of the PL and photoluminescence excitation (PLE) spectra. In order to reduce the fracture of the CaAl2O4:Eu2þ ceramic, CaAl2O4:Eu2þ powders were mixed with an optical epoxy resin (G-2011) evenly by using mechanical stirring. The volume fraction of the ceramic powders in epoxy resin is ~9.1%. The mixture was injected into a cylinder mode with the dimensions of 25 mm in diameter and 15 mm thick and then was put into a vacuum drying oven (DZF6021, Shanghai Jinghong Laboratory Instrument Co., Ltd., China) and curred in a vacuum environment (~133 Pa) at room temperature for 24 h. The intensity of the stress-luminescence was recorded using a photon counting system that consisted of a photomultiplier tube (PMTH-S1-CR131, Zolix Instruments CO., LTD, China) and an oscilloscope (TDS 2002, Tektronix Crop., U.S.A.). The pulse stress was applied on the sample using a universal testing machine (WDW-30, Jinhua Julong Co., Ltd., China). The sample was irradiated under an UV light (ZF-1, power of~30 W, wavelength of 365 nm, Shanghai Jihui Scientific Instrument Co., Ltd., China) for 3 min before the measurement of the stress-luminescence.
3. Results and discussion The XRD pattern indicated that the sample possesses a pure monoclinic phase, as shown in Fig. 1. All the diffraction peaks of the XRD pattern matched perfectly with what reported for CaAl2O4 (JCPDS 70-0134) and no peak of any other phase was detected, which means that the Eu2þ ions have diffused into the CaAl2O4 host. Considering that the effective ionic radii of Eu2þ, Ca2þ and Al3þ are 1.20, 1.06 and 0.39 Å, respectively, the Eu2þ doping is expected to occupy the Ca2þ sites. The PL and PLE spectra of the CaAl2O4:Eu2þ ceramic powders are shown in Fig. 2. Excitation spectra of the ceramic powders monitored at an emission wavelength (lem) of 448 nm had an intense and broad peak corresponding to the Eu2þ ion in the UV region that is associated with 4f7e4f65d1 electronic transitions [16,17]. Excited at an excitation wavelength (lex) of 338 nm, the sample shows a peak centered at 448 nm, which corresponds to the transition of Eu2þ from an excited state with a configuration of 4f65d1 to the ground state 8S7/2 with 4f7 configuration [18]. No obvious emission
Fig. 1. XRD patterns of the CaAl2O4:Eu2þ ceramic powder.
87
Fig. 2. The PL and PLE spectra of the CaAl2O4:Eu2þ ceramic powders. The dash lines show the three decomposed individual Gaussian bands of the PLE spectrum.
peak of the trivalent europium ion was observed, indicating that there were little europium impurities in the sample. The Stokes Shift, DES, which is the difference in energy between the absorption and emission peaks, the HuangeRhys factor, S, and the effective phonon energy,Zu, can provide quantitative characterizations of the electron-vibrational interaction [19]. The values of DES and Zu can be estimated using [19]:
DES ¼ ð2S 1ÞZu
(1)
1=2 Zu FWHMðTÞ ¼ 2:35 Zu S coth 2kT
(2)
where u is the frequency of the active vibrational mode, k is the Boltzmann constant, and T is a given absolute temperature. Eq. (2) describes the temperature variation of the full width at halfmaximum (FWHM). The values of the FWHM and DES can be extracted from the experimental emission and excitation spectra. It should be noted that the wavelength lab of absorption wavelength peak related to the lowest level of the 4f65d1, is usually not resolved at room or higher temperatures. Here we can only give an estimation value of DES. The wide excitation band is made up of three individual Gaussian bands centered at 268 nm [16], 335 nm and 383 nm as shown by the dashed lines in Fig. 1. Using the first absorption band at 383 nm (26109 cm1) and the emission band maximum at 448 nm (22321 cm1), DES is about 3778 cm1. The FWHM value of the CaAl2O4:Eu2þ is about 51 nm, which is smaller than that of YAG:Ce3þ (~120 nm) or SrSi2O2N2:Eu2þ (~82 nm) [18,20]. This smaller FWHM value means that there is a higher color purity of the synthesized Eu2þ-doped CaAl2O4 than those other materials [21]. Fig. 3(a) shows the applied pulse pressure and Fig. 3(b) shows the stress-luminescence of CaAl2O4:Eu2þas a function of time. Intense blue light (~0.81 lux) that was visible to the naked eye was emitted from the sample when a pulse force of ~7.7 kN (corresponding to a pressure of ~15.8 MPa) was applied to the sample. The luminescent intensity became lower and lower as the cycle of the applied pulse stress went up. The applied pressure and stressluminescence of CaAl2O4:Eu2þ as a function of time in the first cycle of the pulse stress is depicted in the inset of Fig. 3(b). It can be seen that the luminescence output responded to the applied stress quickly. In a pulse cycle, the intensity of the stress-luminescence
88
Y. Wei et al. / Journal of Alloys and Compounds 646 (2015) 86e89
Fig. 4. The logarithm of stress-luminescence intensity as a function of the inverse square root of applied pressure. Fig. 3. The applied pulse pressure (a) and the stress-luminescence of CaAl2O4:Eu2þ (b) as a function of time. The inset of Fig. 3(b) shows the applied pressure and stressluminescence of CaAl2O4:Eu2þ as a function of time in the first cycle of the pulse stress.
firstly increased with the increase of the applied stress, then decreased with the decrease of stress. For the electroluminescent materials, the intensity of luminescence (I) as a function of the applied electric field strength (E) can be expressed as following [5,22]:
.pffiffiffi E I ¼ a0 f 1=2 exp b0
(3)
where a0, b0 (a0 > 0, b0 > 0) are constants, which depend on the environmental temperature and the chemical compositions of the luminescent material, f is the frequency. For stress-luminescence materials, the electric field E induced by piezoelectric effect under the applied pressure (P) can be further written as E ¼ gP on basis of the piezoelectric constitutive equations [23]. Here g is the piezoelectric voltage constant of the material. Then the stress-luminescence can be given as Eq. (4):
.pffiffiffiffiffiffi gP I ¼ a0 f 1=2 exp b0
(4)
On basis of Eq. (5), the stress-luminescent intensity increases with the increase of the applied stress. The theoretical analysis agrees well with our experimental results. Taking the logarithm of Eq. (4), we get:
.pffiffiffiffiffiffi pffiffiffi gP ln I ¼ ln a0 f b0
(5)
On basis of Eq. (5), for the stress-luminescence materials, LnI has a linear response to the inverse square root of the applied pressure. Fig. 4 plots the logarithm of stress-luminescence intensity as a function of the inverse square root of the applied pressure (corresponding to the pressure P ¼ 15.8, 15.3, 14.4, 12.1, 8.9, 7.1, 4.9 MPa, respectively). The linear fitting result was noted by the red straight line. The excellent linear relationship between the logarithm of stress-luminescence intensity and the inverse square root of the applied pressure in Fig. 4 agrees with our theoretical prediction (Eq. (5)) very well. Fig. 5 shows the dependence of the peak intensity of the stressluminescence of the CaAl2O4:Eu2þ ceramic on the numbers of the applied stress cycle. The luminescence intensity decreased with
Fig. 5. The dependence of the stress-luminescent peak intensity of the CaAl2O4:Eu2þ on the numbers of the applied pulse stress cycle.
successive repetitive application of stress and became stable at about 5% of the initial intensity. After the sample was irradiated under UV light for 3 min, the intensity of the luminescence recovered completely, which means that the stress-luminescence was produced by a nondestructive deformation of CaAl2O4:Eu2þ. Also, the recovery of the stress-luminescence intensity after UV irradiation suggests that traps occur in the CaAl2O4:Eu2þ sample that are filled by UV irradiation [24]. The CaAl2O4:Eu2þ ceramic can also be used as an energy storing material that absorbs UV light from sunlight and then gradually releases the absorbed energy in the dark in the form of visible light [25,26]. A schematic of the stress-luminescence emission in CaAl2O4:Eu2þ is presented in Fig. 6. As previously reported, the band gap of CaAl2O4 is ~6.7 eV and the 5d levels of doped Eu2þ are partly in the conduction band (CB) of CaAl2O4 [27,28]. The lowest of the 5d levels was ~0.3 eV below the host CB [29]. After UV irradiation, the electrons are excited from the 4f ground level of Eu2þ to the 5d levels. Some electrons can escape directly to the CB from a 5d level located within the CB with the aid of thermal energy or photoexcitation energy. Then the electrons in the CB are captured by oxygen vacancies that are close to bottom of CB. The electrons migrate from one trap to another by releasing and acquiring
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Acknowledgments This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LY14E020003), the National Natural Science Foundation of China (No. 11374204), “Shu Guang” project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG52), and the Science and Technology Commission of Shanghai Municipality (Nos. 12JC1404400, 14520501000).
References
Fig. 6. Energy level schematic for the piezoelectrically-induced stress-luminescence emission in CaAl2O4:Eu2þ.
thermal energy. The applied stress produces a piezoelectric field in the noncentrosymmetric CaAl2O4:Eu2þ. This would reduce the trap depth and lead to a number of electrons migrating from the traps to the CB and then to the 5d levels. The electrons rapidly relax to the lowest 5d level via a nonradiative transition and finally return to the ground state by emitting a light at about 448 nm.
4. Conclusions In conclusion, the strong piezoelectrically-induced stressluminescence effect in the CaAl2O4:Eu2þ ceramic was investigated. The stress-luminescence intensity strongly depends on the applied stress magnitude in each cycle of the applied pulse stress. As the number of cycles of the applied pulse stress increases, the stressluminescence intensity decreases. The intensity of the luminescence can be recovered completely after irradiation under UV light for 3 min. It is suggested that the stress-luminescence in CaAl2O4:Eu2þ arises from the piezoelectric field induced detrapping of the charge carriers. The strong stress-luminescence effect in CaAl2O4:Eu2þ ceramic has potential applications in smart stress optically-sensing devices.
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