Impulsive excitation of mechanoluminescence in europium activated strontium ortho-silicate phosphor

Abstract

Europium doped strontium ortho-silicate (Sr₂SiO₄:Eu²⁺) phosphor was prepared by the traditional high temperature solid state reaction method. The crystal structure of sintered phosphor was consistent with the orthorhombic crystallography with space group Pmna. The surface of the prepared phosphor was not found to be uniform and particle distribution in different size. An energy dispersive X-ray spectroscopy (EDS) spectrum confirms the presence of elements in sintered phosphor. The mechanoluminescence (ML) intensity of Sr₂SiO₄:Eu²⁺ phosphor increases linearly with increasing impact velocity of the moving piston which suggests that these phosphors can be used as sensors to detect the stress of an object. Thus, the present investigation indicates the piezo-electricity is responsible to produce ML in prepared Sr₂SiO₄:Eu²⁺ phosphor. The time of the peak ML intensity and the decay rate did not change significantly with respect to increasing impact velocity of the moving piston and peak ML intensity increases linearly.

1 Introduction

Mechanoluminescence (ML) is photoemissions from some special materials due to an applied mechanical stimulation. These mechanical actions could include friction, pressure, bending, erasing or rubbing, fracture, and shocking etc. ML phenomena produced under different mechanical actions are sometime given specific names, such as tribo-luminescence, piezo-luminescence, elastico-luminescence, plastico-luminescence and fracto-luminescence respectively [1, 2]. It is defect related phenomenon, associated with a trap involved process, in which electrons or holes dwell in the trap for some time and then recombine with the luminescence center either by travelling in the conduction band (or valence band) or by electron (or holes) tunneling. This phenomenon has been observed in many kinds of solids including ionic crystals, semiconductors, metals, glasses and organic crystals [3, 4]. Although ML has been a phenomenon known for centuries, no practical applications have been made due to weakness of ML intensity in the past.

In recent years, ML materials with high intensity have been developed. ML has found various important applications such as impact sensors in spacecrafts (the emission intensity can be used to determine the kinetic energy of impact), fracture sensor, damage sensor, stress sensor etc. Thus, many researchers have been focused on the investigation of phosphors with high ML [5]. Until now, some phosphors with high ML, such as (red phosphor) BaTiO₃–CaTiO₃:Pr, (green phosphor) SrAl₂O₄:Eu, (yellow phosphor) ZnS:Mn, and (blue phosphor) CaYAl₃O₇:Eu etc., have been developed. However, the requirement of application for ML sensors still is not satisfied with the development of ML materials. At the same time, the high stabilities, such as resistance of water, thermal stability are also very important for the application of ML. More ML phosphors with strong ML intensity and high stability are needed [6–8].

Phosphors containing rare earth ions have received increasing attention in recent years due to their technological importance. It is known that host materials and intentionally added impurity play an important role in the luminescence process. Alkaline earth silicate is an important luminescent material because of its excellent chemistry and thermal stabilization and cheap raw material (SiO₂). It is well known that Eu²⁺ activated alkaline earth silicate such as Sr₂MgSi₂O₇:Eu²⁺; Sr₂Al₂SiO₇:Eu²⁺; Ca₂Al₂SiO₇:Eu²⁺ and Ca₂MgSi₂O₇:Eu²⁺; SrCaMgSi₂O₇:Eu²⁺ phosphors have also been largely investigated [9–13].

In recent years, silicate based phosphors have attracted researcher’s attention due to the advantages of stable crystal structure and high thermal stability. Among them, the rare earth ion doped (such as Dy³⁺, Pr³⁺, Sm³⁺, and Eu²⁺) strontium ortho-silicate phosphor (Sr₂SiO₄) seems to have the potential for the application of various optical devices. In the present paper, we report the synthesis, structural characterization and luminescence properties of Sr₂SiO₄:Eu²⁺ phosphor by solid state reaction method. Investigation on the crystal structure and elemental analysis of sintered phosphor was determined by the X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Luminescence properties were also investigated on the basis of mechanoluminescence (ML) properties.

2 Experimental

Europium doped strontium ortho-silicate namely Sr₂SiO₄:Eu²⁺ phosphor was prepared by the high temperature solid state reaction method. The raw materials were strontium carbonate [SrCO₃ (99.90 %)], silicon di-oxide [SiO₂ (99.99 %)] and europium oxide [Eu₂O₃ (99.99 %)], all of analytical grade (A. R.), were employed in this experiment. Boric acid (H₃BO₃) was added as flux. Initially, raw materials were weighed according to the nominal compositions of Sr₂SiO₄:Eu²⁺ phosphor. Then the powders were mixed and milled thoroughly for 2 h using the mortar and pestle. The chemical reaction used for stoichiometric calculation is:

$$ 2{\text{SrCO}}_{3} + {\text{SiO}}_{2} + {\text{Eu}}_{2} {\text{O}}_{3} \mathop{\longrightarrow}\limits^{{1300^{\; \circ } {\text{C}}}} {\text{Sr}}_{2} {\text{SiO}}_{4}{:}{\text{Eu}}^{2 + } + \, 2{\text{CO}}_{2} \uparrow $$

The ground sample were placed in an alumina crucible and subsequently fired at 1300 °C for 3 h in weak reducing atmosphere. The weak reducing atmosphere was generated with the help of activated charcoal. The work of reducing atmosphere is to convert Eu³⁺ to Eu²⁺ ions. At last the nominal compounds were obtained after the cooling down of programmable furnace and product was finally ground into powder for characterizing the phosphor. Solid state reaction method is widely used to prepare silicate based phosphors because samples prepared using this method has good luminescence and very good morphology.

2.1 Phosphor measurement techniques

The crystal structure of the prepared Sr₂SiO₄:Eu²⁺ phosphor was characterized by powder XRD analysis. Powder XRD pattern has been obtained from Bruker D8 advanced X-ray powder diffractometer and the data were collected over the 2θ range 10°–80°. The X-rays were produced using a sealed tube (CuKα) radiation source and the wavelength of X-ray was 1.54060 Å. The X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector). The crystal structure of the sample was verified with the help of Joint Committee of Powder Diffraction Standard Data (JCPDS) file. The morphological image of prepared phosphor was collected by the FESEM. The sample was coated with a thin layer of gold (~10 nm) and then the surface morphology of prepared phosphor was observed by FESEM; ZIESS Ulta Plus-55 operated at the acceleration voltage of 10 kV. An EDS spectrum was used for the elemental (qualitative and quantitative) analysis of the prepared phosphor. The ML measurement was observed by the home made lab system comprising of an RCA-931A photomultiplier tube (PMT). All measurements were performed at the room temperature.

2.2 Experimental setup for mechanoluminescence (ML) measurement

The experimental set up used for the impulsive deformation of ML was shown in Fig. 1. A load of particular mass and shape was dropped from different heights for striking the prepared Sr₂SiO₄:Eu²⁺ phosphor at different impact velocities. In this experiment, the mass of the dropping load was 400 gm and shape of the load was cylindrical. The phosphor under study was placed on the upper surface of a transparent lucite plate and it was then covered with a thin aluminum foil and fixed with an adhesive tape. The foil reflects light and prevents scattering of the fragments during the impact of a moving piston onto the prepared phosphor. This arrangement eliminates the error in the ML measurement due to the scattering of the crystallite fragments during the impact of the load onto the phosphor. The housing is made up of thick soft iron to provide shielding from light and magnetic field. The slit arrangement at the window is provided to adjust the size of the window according to the incident beam. When the phosphor placed on the lucite plate was crushed by impact of the load, light is emitted [14].

Fig. 1

Schematic diagram of the experimental setup for ML measurement

In the Fig. 1, 1—Stand; 2—Pulley; 3—Metallic wire; 4—Load; 5—Guiding cylinder; 6—Aluminium foil; 7—Phosphor; 8—Transparent Lucite plate; 9—Wooden block; 10—Photomultiplier tube (PMT); 11—Storage oscilloscope; 12—Iron base mounted on a table.

By changing the distance between the piston to be dropped and the sample placed on the lucite plate, the impact velocity (υ₀) of the load could be changed from 98 to 328 cm/s, since the pulley and the guiding cylinder used were of negligible friction, the impact velocity (υ₀) was taken as $\sqrt {2gh}$, where “g” is the acceleration due to gravity and “h” is the height through which the load is dropped freely. An RCA 931A photomultiplier tube (PMT) was placed below the transparent lucite plate. The PMT was run at 750 V. The output of PMT was connected to the phosphorescent screen oscilloscope (Scientific 300 MHz, SM 340). The ML glow curve can be plotted with the help of SM-340 application software installed in a computer attached with the storage oscilloscope [15].

3 Results and discussions

3.1 XRD analysis

In order to determine the crystal structure, the powder XRD analysis has been carried out. XRD patterns of the Sr₂SiO₄:Eu²⁺ phosphor was shown in Fig. 2. It has been reported that the crystal structures of α-Sr₂SiO₄ (orthorhombic) and β-Sr₂SiO₄ (monoclinic) are very similar, and the XRD peaks are therefore also similar in the JCPDS cards of both phases, and the both structures can co-exist. From Fig. 2, all peaks can be attributed to the pure phase of α-Sr₂SiO₄ (JCPDS # 39-1256) [16] without any impurities. No peaks of un-reacted SrCO₃ and SiO₂ are observed in XRD patterns, indicating that the reaction of raw materials is complete. Incorporation of europium has not changed the XRD pattern which confirms that doping is proper has not distorted the structure of strontium orthosilicate. Since no impurity peaks were observed, it is feasible to suggest that prepared sample are in single phase of α-Sr₂SiO₄ with the space group of Pnma according to PDF card no. 39-1256, revealing that single-phase α-Sr₂SiO₄ phosphor was obtained.

Fig. 2

XRD patterns of Sr₂SiO₄:Eu²⁺ phosphor

3.2 Field emission scanning electron microscopy (FESEM)

It is known that the luminescence characteristics of phosphor particles depend on the morphology of the particles, such as size, shape, size distribution, defects, and so on. The morphologies of prepared Sr₂SiO₄:Eu²⁺ phosphor was also observed by means of FESEM in Fig. 3. The surface morphology of the particles has shown irregular which means the distribution of the crystallite sizes is not homogeneous. The morphological images of prepared Sr₂SiO₄:Eu²⁺ phosphor displays that the particles are aggregated tightly with each other due to the high temperature synthesis method. From the FESEM image, it can be observed that the prepared sample consists of particles with different size distribution.

Fig. 3

FESEM micrograph of Sr₂SiO₄:Eu²⁺ phosphor

3.3 Energy dispersive X-ray spectroscopy (EDS)

The chemical composition of the powder sample has been measured using an EDS spectrum. Figure 4 shows the EDS spectra of Sr₂SiO₄:Eu²⁺ phosphor. Table 1 shows the compositional elements of Sr₂SiO₄:Eu²⁺ phosphor, which is compare with the standard element. An EDS is a standard procedure for identifying and quantifying elemental composition of sample area as small as a few nanometers. There appeared no other elements apart from strontium (Sr), silicon (Si), oxygen (O) and europium (Eu) in EDS spectra of the sample. In the spectrum intense peaks are present which confirm the presence of elements in Sr₂SiO₄:Eu²⁺ phosphor.

Fig. 4

EDS spectra of Sr₂SiO₄:Eu²⁺ phosphor

Table 1 Chemical composition of Sr₂SiO₄:Eu²⁺ phosphor

3.4 Mechanoluminecsnce (ML)

In the present ML studies, an impulsive deformation technique has been used. During the deformation of a solid, a great number of physical processes may occur within very short time intervals, which may excite or stimulate the process of photon emission. When a moving piston is dropped to the phosphor, initially the ML intensity increases with time, attains a peak value and then decreases with time. Such a curve between the ML intensity and deformation time of phosphors is known as the ML glow curve. It is seen that when moving piston is dropped at particular height, then the ML emission also takes place. The prepared phosphor was fracture without any pre-irradiation such as X-ray, β-rays, γ-rays, UV, etc. Every time for the ML measurement, the quantity of Sr₂SiO₄:Eu²⁺ phosphor was kept constant (8 mg) and it was placed onto the upper surface of a transparent. The ML was excited impulsively by dropping a load onto the sample from different heights (h = 10, 20, 30, 40, 50 cm). The phosphor was fracture via dropping a load [moving piston] of particular mass (400 gm) and cylindrical shape on the Sr₂SiO₄:Eu²⁺ phosphor. The velocity of the moving piston, holding the impact mass, could be changed (from 140 to 313 cm/s), by changing the height through which it was dropped. Figure 5 shows that the characteristics glow ML curve between ML intensity versus time for different heights. When the moving piston is dropped onto the prepared phosphor at different height, light is emitted. The photon emission time is nearly 2 ms, when prepared Sr₂SiO₄:Eu²⁺ phosphor fractures. In these ML measurements, maximum ML intensity has been obtained for the 50 cm dropping height and ML intensity increases with the dropping height of the moving piston [17].

Fig. 5

ML intensity versus time curve of Sr₂SiO₄:Eu²⁺ phosphor (inset ML intensity vs. impact velocity curve of Sr₂SiO₄:Eu²⁺ phosphor)

Figure 5 (inset) shows that the characteristics curve between ML intensity versus impact velocity of Sr₂SiO₄:Eu²⁺ phosphor. The ML intensity increases linearly with the increases the dropping height of the moving piston; that is, the ML intensity depends upon the impact velocity of the moving piton [υ₀ = $\sqrt {2gh}$ (where “g” is the acceleration due to gravity and “h” is the height through which the load is dropped freely)]. The ML intensity of Sr₂SiO₄:Eu²⁺ phosphor increases linearly with increasing the mechanical stress. Increasing impact velocity causes more number of Eu²⁺ ions to get excited to the higher energy level, subsequently de-excitation of more Eu²⁺ ions occurs. This gives rise to increase in ML intensity [18].

When the load or piston makes an impact on the crystal with an initial velocity v₀, the former decelerates and after a particular time its velocity becomes zero. The time dependence of the velocity of the piston may be written as

$$ v = v_{0} \exp \left( { - \beta v_{0} t} \right) $$

(1)

where β is a constant, Eq. (1) can be written as

$$ \frac{dx}{dt} = v_{0} \exp \left( { - \beta v_{0} t} \right) $$

(2)

where dx is the compression of the crystal during the time interval dt.

Integrating Eq. (2), we have

$$ x = \frac{1}{\beta }\exp \left( { - \beta v_{0} t} \right) + C $$

(3)

x = 0 for t = 0, therefore, Eq. (3) may be written as

$$ x = \frac{1}{\beta }\left[ {1 - \exp \left( { - \beta v_{0} t} \right)} \right] $$

(4)

The prepared phosphor is in powder form and the impact velocities compress it to a certain extent, but this does not change significantly with increasing impact velocity. Equation (4) shows that impact time remains mostly unchanged with increasing impact velocity because there is no significant change in compression, which is expressed by ‘x’ in Eq. (4). This may be one possible reason why the time that corresponds to the peak ML intensity does not change significantly with increasing impact velocity [19]. Figure 6 shows the time corresponds to ML signal peak with impact velocity of Sr₂SiO₄:Eu²⁺ phosphor.

Fig. 6

Time corresponds to ML signal peak with impact velocity of Sr₂SiO₄:Eu²⁺ phosphor

The relationship between semi-log plot of ML intensity versus (t − t_m) for Sr₂SiO₄:Eu²⁺ phosphor is shown in Fig. 7, and the lines were fitted using the Eq. (5) with Origin Pro 8.0

$$ \tau = \frac{1}{slop\;of\;straight\;line} $$

(5)

Fig. 7

Semi-log plot of ML intensity versus (t − t_m) for Sr₂SiO₄:Eu²⁺ phosphor

Curve fitting results show that decay constant (τ) varies from 1.47 to 0.85 ms. The ML decay constant value was decreases with the impact velocities, and maximum for the minimum impact velocities (see Table 2). In order to further clarification of the ML decay mechanism in Sr₂SiO₄:Eu²⁺ phosphor, more experimental and theoretical studies are needed.

Table 2 Calculation of ML decay constant

When a mechanical stress, such as compress, friction, and striking, and so on, was dropped on the sintered Sr₂SiO₄:Eu²⁺ phosphors, piezo-electric field can be produced. Therefore, in such phosphor the ML excitation may be caused by the local piezoelectric field near the impurities and defects in the crystals. During the impact on the material, one of its newly created surfaces gets positively charged and the other surface of the crack gets negatively charged (Fig. 8). Thus, an intense electric field in the order of 10⁶–10⁷ V/cm is produced [20]. Under such order of electric field, the ejected electrons from the negatively charged surface may be accelerated and subsequently their impact on the positively charged surfaces may excite the luminescence center. Thus, depending on the prevailing conditions, recombination luminescence may be produced. For the impact velocity, the impact pressure P₀ will be equals to, P₀ = Zυ₀, where Z is a constant. With increasing value of impact velocity, the trap depth will decrease, therefore, for the trap depth beyond a particular pressure the traps will be unstable and they will be de-trapped, in which the number of de-trapped electrons will increases with the increasing impact velocity. Thus, the ML intensity will increase proportionally with increasing value of impact velocity [21].

Fig. 8

Langevin model for the piezo-electrification induce phosphor

As the impact velocity increases, the impact pressure also increases leading to the increase in the electric field at local region which causes the decrease in trap depth. Hence the probability of de-trapping increases. From Fig. 6 (inset), it can be seen that with increasing impact velocity, ML intensity also increases linearly i.e., the ML intensity of Sr₂SiO₄:Eu²⁺ phosphors are linearly proportional to the magnitude of the impact velocity. When the surface of an object was coated with the ML materials, the stress distribution in the object beneath the layer could be reflected by the ML brightness and could be observed. Based on the above analysis these phosphors can also be used as sensors to detect the stress of an object [22].

4 Conclusion

In summary, europium doped strontium ortho-silicate (Sr₂SiO₄:Eu²⁺) phosphor was prepared by the traditional high temperature solid state reaction method. The orthorhombic crystal structure of prepared Sr₂SiO₄:Eu²⁺ phosphor was confirmed by the XRD. Decay rates for different impact velocities were also calculated using curve fitting technique. Time of ML peak and rate of decay did not change largely with respect to increasing impact velocity of the load and peak ML intensity varied linearly. It is worthy to note that the dependence between ML intensity of Sr₂SiO₄:Eu²⁺ phosphor and the impact velocity of the moving piston is close to linearity, which suggests these phosphors can also be used as sensors to detect the stress of an object.