1. Introduction

Long persistent phosphors (LPPs), which can absorb and store photoelectron under excitation, emit light in the violet (UV), visible (VIS) or near-infrared (NIR) spectral regions for minutes, hours or even days after ceasing excitation. In initial civil applications, LPPs are commonly used in concealed lighting, i.e. decoration, safety displays, equipment nameplates, etc., and they also have a wide range of applications in advanced scientific fields, such as clinical medicine, biomedicine, life sciences, energy and environment engineering [1–6]. It is not a new concept to incorporate AM techniques into long afterglow materials. A conventional process is based on fused deposition modeling (FDM) using the fibers which commonly prepared by melting polymer fibers, mixing with LPPs and extruding organic/inorganic fibers as references [7–9]. FDM has limitation in the production of fine structures with high precision. Stereolithography (SLA) as the most versatile method with high accuracy and precision can provide potential new solution for incorporating persistent phosphors into 3D objects. In addition, fabrication of parts through SLA with resin slurry containing LPPs reduces the energy needed to produce fibers containing LPPs when parts are fabricated by FDM. What’s more, the laser intensity required to initiate photo-polymerization during SLA is rather low, typically between 10 mW ~1 W, which is economically attractive. This work demonstrates the synthesis of resin slurry containing LPPs and its use for stereolithography. The viscosity of the slurry, photoluminescence (PL) spectra, CIE phosphorescent chromaticity coordinates, afterglow decay properties, thermal stability and temperature sensing performance were systematically investigated.

2. Experimental demonstration and details

2.1 Materials

Most raw materials were obtained from Aladdin Corporation (Shanghai, China). Main reactants of the prepolymerization are isophorone diisocyanate (mixture of isomers) (IPDI), 2-hydroxyethyl methacrylate (HEMA), polyethylene glycol (PEG, average Mn 300) and 2-hydroxyethyl acylate (used as diluent). Ditin butyl dilaurate (DBTDL) was used as a catalyst which allowed the prepolymerization to take place at a rapid rate at lower temperatures. 2,6-di-tert-butyl-4-methylphenol (BHT) was used as polymerization inhibitor and 2-hydroxy-2-methylpropiophenone (photo-initiator 1173) acted as the initiator of the light curing. Main reactants of LPPs are Al₂O₃, SrCO₃, CaS, Eu₂O₃, Dy₂O₃, Nd₂O₃ and CaCO₃. H₃BO₃ was added as a flux to lower the reaction temperature in synthesis process of LPPs. Tinuvin 1130 is an UV light absorbent obtained from BASF Corporation (Florham Park, NJ, USA) to absorb excess UV light and control extra solidification during stereolithography. All materials were used as-received without further purification.

2.2 Synthesis of long persistent phosphors (LPPs)

The long phosphors CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ were synthesized using solid state reaction reported previously with minor modification [10–12]. In a typical process for the synthesis of SrAl₂O₄: Eu²⁺, Dy³⁺, raw materials with stoichiometric ratio were wet mixed with moderate amount of ethanol, and ground homogeneously by a planetary ball mill at an operating speed of 200 rpm for 5 h. The dried mixture was placed in an aluminum crucible in a muff furnace under ordinary atmospheres, then heated to 800 °C for 10 h, and finally sintered at 1300 °C in a mild reducing atmosphere (5% H₂ and 96% N₂). The samples were cooled down to the room temperature spontaneously and crushed to fine powder. A focused ion beam scanning electron microscope (SEM) was used in characterizing the powder morphology of these LPPs. The SEM was operated at an acceleration voltage of 3 kV. The powder of LPPs was scattered onto a carbon tape and sputtered with gold particles prior to analysis. Comparatively, the powder particles of SrAl₂O₄: Eu²⁺, Dy³⁺ in Fig. 1 are uniform and have an average size of around 10 μm. The CaS: Eu²⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ particles both show irregular shapes, with size scattered between 10 μm and 100 μm. Before being mixed with IPDI, the powder particles were ground and sifted with 400 mesh sieve several times to get high homogeneity and uniform distribution.

 

Fig. 1 SEM images of long persistent phosphors of CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ respectively.

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2.3 Synthesis of resin containing LPPs

Figure 2 illustrates the slurry synthesis process, stereolithography and long afterglow of the printed samples systematically. Phosphors were doped into the IPDI in the first step. Then HEMA was mixed to form short chains containing -NCO- and PEG was added to get long chains through polymerization between -OH- and -NCO- groups. The above two reactions are exothermic. To get the prepolymer with uniform length, HEMA and PEG have to be added dropwise under constant stirring to control the reacting temperature between 60 °C and 70 °C. Constant stirring allows the LPPs particles to be grafted by polymer chains that lead to homogeneous dispersion of particles without agglomeration in the resin slurry. So, it is not advisable to add LPPs after the resin synthesis process. LPPs with a variety of weight ratios were added to make the slurry, and the results indicate that the optimum concentration is 3 wt%. Higher contents of LPPs powder led to precipitation and lower contents affect the luminescence intensity.

 

Fig. 2 Schematic illustration of slurry synthesis, the stereolithography process, long afterglow of the printed samples.

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2.4 Viscosity

Viscosity which reflects the flow resistance generated by the interaction between the liquid molecules is an important parameter to ensure smooth printing process of SLA and good quality of SLA-printed parts. The viscosity of the slurry was measured using a German HAAKE RS 6000 rotational rheometer at the temperature of 30 °C. Figure 3 illustrates the rheological characterization of the slurries, GP is the shear rate, Eta is the viscosity. The red, green and blue lines represent the results for slurries containing CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺, CaAl₂O₄: Eu²⁺, Nd³⁺ respectively. The organic/inorganic slurries show obvious non-Newtonian fluid behavior at the temperature of 30 °C and is also pseudoplastic fluid because the viscosity of all three decreases with the increase of shear rate. Incorporation of different LPPs does not significantly change the viscosity of the resin slurry and the viscosity of the three slurries is all smaller than 2 Pa·s which is suitable for SLA, screen, dispenser and stencil printing methods. The slurry containing SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors has the smallest viscosity which is related with smaller size and rounder shape of SrAl₂O₄: Eu²⁺, Dy³⁺ powder particles. The relationship between the viscosity and shear rate of slurry containing CaS: Eu²⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ phosphors is similar. The resin slurries are not sensitive to oxygen, and there is no need to store or use them in an inert or nitrogen atmosphere. Therefore, stereolithography can be done in ambient atmosphere.

 

Fig. 3 Viscosity of the three slurries incorporated with 3 wt% long persistent phosphors of CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ respectively.

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3. Optical characterization

3.1 Stereolithography and the photoluminescence (PL) emission spectra

SLA is based on a liquid resin photo-curing process. The liquid resin is placed in a reservoir, a positionally programmed laser scans over the resin surface to initiate photo-polymerization, and this cures the resin and converts it to solid via chemical crosslinking [13]. The SLA machine employs the bottom-up approach and the laser (355 nm) scanning controlled by a computer can activate the photo-initiator 1173 and trigger the polymerization. We adjusted the laser power to 250 mw and laser scanning speed to 100 mm/s. The z-axis precision of printing was 0.1 mm. Cubic samples were 3D-printed with a size of 10 mm*10 mm*2 mm. The photoluminescence (PL) emission spectra of the resin samples incorporated with CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ phosphors were recorded using a spectrometer (Edinburgh FLS-920) under the excitation of 468 nm, 399 nm, 398 nm, which are the absoption peaks of phosphors respectively. As shown in Fig. 4, the spectra all have wide emission bandwidths. The emission peaks of the three resin samples are located at 653 nm, 520 nm, 470 nm which give their red, green and blue color under excitation. The corresponding CIE color coordinates calculated from the emission spectra are shown in the right corner of Fig. 4. Resin samples containing SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ LPPs with green and blue emissions based on alkaline earth aluminates exhibit stronger afterglow performance than the resin containing CaS: Eu²⁺ LPPs with red emission when observed with naked eyes. To confirm the LPPs are homogenously mixed with the resin slurry, square samples with the size of 10 mm*10 mm*10 mm were 3D-printed. Luminescence intensity of 10 points (with diameter of 500 μm) which were randomly selected on each surface was detected when the long persistent phosphors have reached saturation under the irradiation of xenon lamp. The data indicates less than 5% difference in luminescence intensity among these points.

 

Fig. 4 Emission spectra of organic/inorganic 3D-printed composites containing 3 wt% CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ LPPs. The excitation wavelengths are 468 nm, 399nm and 398 nm respectively. The inset picture in the right corner shows the CIE color coordinates calculated from the emission spectra.

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3.2 The afterglow of printed parts

It is well known that cation or anion vacancies can produce local potentials able to serve as traps for electrons or holes, which play major roles in the initial intensity and persistent time in LPPs. The afterglow is governed by the slow liberation of trapped charge carriers by thermal stimulation [14]. The excitation wavelengths are 468 nm, 399 nm, 398 nm for the samples incorporated with CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ respectively. We charged the printed composites containing LPPs to saturation. Then we recorded the intensity change in peak position of 653 nm, 520 nm and 470 nm over time after turning off the excitation light because the decay time plays a vital role in LPPs. Theoretically, white afterglow resin samples can be obtained by co-doping of currently available LPPs with red, green and blue emissions. However, LPPs with different colors seldom have very similar decay times. Moreover, slight composition change can allow for facile tuning of CIE color coordinates [15, 16]. As shown in Fig. 5, the luminescence intensity decreases greatly over time in the darkness and the decay rate in afterglow intensity is very high for the initial 30 min. The composites containing SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ LPPs based on alkaline earth aluminates show long afterglow duration for more than 2 h and the composite with SrAl₂O₄: Eu²⁺, Dy³⁺ has the best afterglow performance, while the composite containing CaS: Eu²⁺ has the shortest afterglow for around 2000 s. It has to be noted here that there is a six-fold difference in the decay time. Therefore, it is difficult to fabricate such a composite sample that retains the white persistent luminescence, because composite samples containing different LPPs particles change colors over time after the cease of the excitation. The inset pictures in Fig. 5 a-c are the images of square resin samples with the size of 10 mm*10 mm*2 mm containing LPPs of CaS: Eu²⁺, SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ photographed in darkroom after 10-minutes exposure to excitation light.

 

Fig. 5 The afterglow decay curves of the composites containing 3 wt% phosphors: (a) CaS: Eu²⁺, (b) SrAl₂O₄: Eu²⁺, Dy³⁺ and (c) CaAl₂O₄: Eu²⁺, Nd³⁺. The vertical axis is log10 of the emission intensity. The excitation wavelengths are 468 nm, 399nm and 398nm and the recorded wavelengths are 653 nm, 520 nm and 470 nm, respectively. The inset pictures are the composites photographed after 10-minutes exposure to their excitation light.

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4. Incorporating LPPs into slurry for stereolithography

4.1 A gear wheel, a dog and a hollow birdcage hanging decoration

In order to investigate the feasibility for the slurry containing LPPs to be used for functional parts in industrial application, complex-shaped parts were printed by SLA. It will take more time, energy and money to prepare these structures through traditional casting, which requires sophisticated molds that need repair and maintenance. The optical images of 3D-printed parts in daytime and in darkroom after ceasing the excitation were photographed by a digital camera. As shown in Fig. 6, each part is of good quality and has a uniform color on the whole, which confirms that the LPPs particles are homogeneously distributed. The low transparency can be attributed to the high scattering of LPPs which have larger size and higher refractive index relative to the resin polymer. The part containing CaS: Eu²⁺ phosphors appears light pink in daytime, while parts containing SrAl₂O₄: Eu²⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ appear white, as shown in Fig. 6(a)-(c). After charging by a xenon lamp for 5 min, strong afterglow with red, green and blue wavelengths can be observed with the naked eye for a long time. In addition, the charging-discharging process of the composite parts containing LPPs can be repeated for many times without degradation of afterglow properties. The resin has a negative effect on the afterglow brightness of long persistent phosphors, but printed parts has considerable luminescence intensity and attractive afterglow decay time. Therefore, the slurry doped with LPPs can be used for exquisite crafts, customized toys and night indicators with high precision.

 

Fig. 6 Optical images of the complex-shaped parts printed by SLA in daylight photographed by a digital camera: (a) A gear wheel printed with resin slurry containing CaS: Eu²⁺ phosphors; (b) A hollow dog printed with resin slurry containing SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors; (c) a hollow birdcage hanging decoration printed with resin slurry containing CaAl₂O₄: Eu²⁺, Nd³⁺ phosphors. Optical images photographed in the darkroom right after 5-minutes excitation by a xenon lamp. (d) A gear wheel containing CaS: Eu²⁺ phosphors. (e) A dog containing SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors. (f) A hollow birdcage hanging decoration containing CaAl₂O₄: Eu²⁺, Nd³⁺ phosphors.

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5. Temperature-detecting

5.1 Thermal properties

The thermal stability of the organic-inorganic composite containing SrAl₂O₄: Eu²⁺, Dy³⁺ was evaluated by TGA and DSC using ME-51140728 from METTLER TOLEDO instruments, and the results are shown in Fig. 7. According to the TGA curve, it remains stable at temperature up to 200 °C with minor loss around 1.037% which is ascribed to the desorption of air moisture and the decomposition of unreacted components contained in the sample. Moreover, the baseline of DSC in this part is smooth which indicates that its specific heat capacity did not change from room temperature to 200 °C. With continuous increase of the temperature, the composite begins to experience weight loss and resin totally decomposes when the temperature rises to 700 °C. The glass transition temperature obtained from DSC is about 329 °C which is enough for daily use as amorphous thermoplastic.

 

Fig. 7 TGA and DSC curves of the resin composites incorporated with SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors.

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5.2 Temperature sensing

Typical fiber optical sensors are Bragger grating fibers, Fabry Perot interferometer and Mach-Zehnder Interferometer [17–23]. They are based on electrically passive operation and they are immune to EMI (electro-magnetic interference), while they are not suitable for temperature detection of moving objects. It has been reported that optical devices can be fabricated in optical fiber ends using optical lithography [24]. In addition, optical temperature detectors based on emission spectra are quite sensitive and free of power cord and signal connection, making them highly suitable for remote temperature sensing needed in certain circumstances. They also feature small volume and strong anti-interference ability. As a proof-of-concept experiment, we show here the emission spectra recorded at temperatures between 40 °C and 70 °C of the composite containing SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors under the excitation at 399 nm. As shown in Fig. 8, the emission spectra consisted of a continuous band ranging from 460 nm to 600 nm with one emission peak at 520 nm. The 520 nm emission originates from the Eu²⁺ 4f⁶5d¹ →4f⁷(⁸S_7/2) transitions associated with the Eu²⁺ ions randomly distributed at the two different Sr²⁺ sites [14]. The relative intensity of the emission spectra decreases gradually with the increase of temperature. It can be seen that the dependence of emission intensity on temperature is roughly linear. The temperature dependence can be generally rationalized by taking the photon-assisted process into account which increases in probability with rise of temperature. The intensity (I) can be expressed as I = nT + C, and the slope (the value of n) obtained is approximately −256.87 at 520 nm between 40 °C and 70 °C. The results suggest that the 3D-printed parts incorporated with LPPs have potential application as remote optical thermometers with high accuracy and sensitivity.

 

Fig. 8 Luminescence spectra of resin composite containing SrAl₂O₄: Eu²⁺, Dy³⁺ recorded at different temperatures. The excitation wavelength is 399 nm. The inset picture shows the dependence of emission intensity on sample temperature at 520 nm.

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6. Conclusion

In summary, resin slurry containing long persistent phosphors suitable for stereolithography was successfully synthesized in our experiment. The afterglow was investigated and the results indicated that their strong intensity, long decay time and multi-color emission after ceasing the excitation are reliable. The printed composite parts incorporated with SrAl₂O₄: Eu²⁺, Dy³⁺ and CaAl₂O₄: Eu²⁺, Nd³⁺ LPPs particles show longer and stronger afterglow compared with the parts incorporated with CaS: Eu²⁺ LPPs. Exquisite and sophisticated 3D architectures which exhibit long persistent luminescence and have considerable afterglow lifetime have been fabricated by stereolithography, suggesting that this method can be used in high precision manufacturing. We also showed that the photoluminescence of the composite containing SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors has a large temperature coefficient when the temperature is from 40 °C and 70 °C, implying its potential application in optical thermometers. We also expect that, besides temperature sensing and artworks, the incorporation of long persistent phosphors into resin slurry and the utilization of stereolithography technique will extend their applications to diverse fields, such as bioimaging and solar cells in the dark.

Disclosures

The authors declare that there is no conflict of interest.