WO2016200349A1 - LONG-LASTING YELLOWISH-GREEN EMITTING PHOSPHORESCENT PIGMENTS IN THE STRONTIUM ALUMINATE [SrAl2O4] SYSTEM - Google Patents

Abstract

Long lasting yellowish-green emitting phosphorescent pigments in the strontium aluminate (SrAl₂O₄) system. This invention is about pigments with phosphorescence property, having long lasting luminescence of bluish-green colour in the dark and wide usage areas requesting optimum specifications in the product obtained.

Description

Long-Lasting Yellowish-Green Emitting Phosphorescent Pigments in The Strontium Aluminate [SrAl2O4] System

This invention is related with pigments, having phosphorescence property, that can glow longterm in the dark, glowing yellowish-green, having optimum characteristics for optained product and production method and having a wealth of usage areas.

The Known Related Technical Condition

Phosphorescence (or afterglow) refers to the light emissionof an insulator that persists at room temperature after stoppingexcitation (usually UV irradiation). This delayed light

emission arises from the fact that charge carriers (i.e.,electrons and/or holes) generated by the excitation are trappedat certain defect sites, and their detrapping is thermallyactivated. Phosphorescence event, besides luminescence center, requires the presence of discrete layers in forbidden bandgap related with chemical and physical defects (contributions and gaps) in the main lattice. Some electrons and gaps occuring with excitation under UV emission are entrapped at these kind of layers. According to these defects and dimensional split between luminescence centers or more precisely direct re-combination has low possibility in the lack of orbital overlaps. As a result, the trapped charge carriers stay at metastable state unless there is not enough energy to trigger of reunification.

Phosphorescent pigments that absorb a certain wavelength of light emit to surrounding when the light source is removed.

In recent years the strontium aluminate hosts such as Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ and SrAl₂O₄: Eu²⁺, Dy³⁺ had been studied intentionally as their advantages of high quantum efficiency, long lasting phosphorescence and stability.

SrAl₂O₄, SrAl₁₂O₁₉, Sr₂Al₆O₁₁ and Sr₄Al₁₄O₂₅ phases in the SrO-Al₂O₃ system are well known 4 main crystals. The green emission from SrAl₂O₄ crystal phasethat Eu²⁺ and Dy³⁺ ions are used as co-dopants and obtained by re-cyrstallizing of utilizing melting property of B₂O₃ is known as long-afterglow phosphor. The phosphorescence characteristics of SrAl₂O₄:Eu²⁺, Dy³⁺ system are explained as a mechanism which occurs with thermal emissions of charge carriers and traps at room temperature where Dy³⁺ ion is functioned as trappingcenter and Eu²⁺ ion is emission center. It is also observed that B₂O₃ improved the long lasting luminescence of phosphor system. B₂O₃ is used as high temperature melter in reducing atmosphere to quicken the growth of particles of strontium aluminate. This had increased the penetration of trapping centers in ceramics. Improvement of brightness and long term luminescence is provided by doping the SrAl₂O₄:Eu²⁺, Dy³⁺ lattice with univalent ions such as K⁺ and Na⁺ or bivalent ions like Mg²⁺ and Zn²⁺. This kind of balancing decreases charge defects by taking place of trivalent rare earth ions in alkaline earth ion areas inside aluminate. In the studies performed by Han et al. the optimum concentrations were set as 2, 4, 6 mole % of Eu²⁺ , Dy³⁺and B₂O₃ respectively for optimum phosphorescent properties in SrAl₂O_4. Apart from this 4 moles % Mg²⁺ ion doping had provided the improvement in the luminescence. The improvement of photoluminescence and long lasting luminescence properties with doping B₂O₃had been based to liquid phase sintering at 1350 ^oC. The presence of charge stabilizer Mg²⁺ is for the decrease of the defects of interstitial oxygen which lead to the reduction in phosphorescence.

During studies performed, SrAl₂O₄, CaAl₂O₄ and BaAl₂O₄ had been chosen as different crystal stucture materials, dopped with rare earth elements (Eu, Dy) as activator and sub-activator. Generally MAl₂O₄: Dy (M: Ca, Sr, Ba) developed phosphors are synthesized by traditional methods. Studies have shown that prepared MAl₂O₄: Dy (M: Ca, Sr, Ba) phosphors have not exhibited luminescence behavior. This situation supports the approach that occurs related to the 5d → transition of Eu²⁺ ions of main emission peaks. Actually, CaAl₂O₄, SrAl₂O₄ and BaAl₂O₄ main crystals have tridimit form and main structure containing AlO₄ ^5- tetrahedras and M²⁺ (Ca²⁺, Sr²⁺, Ba²⁺) ions which support the charges in the holes. Emission spectrum depends on the structure of main lattice and alkali ion selection. If it is necessary to submit an example of MAl₁₂O₁₉:Eu (M: Ca, Sr, Ba) phosphors, the main emission peaks are 410, 395 and 443 nm. MAl₂O₄ main crystals consist of three dimensional structures of AlO₄ tetrahedras shared from corners each one of oxygen is shared by two aluminium ion and each one of tetrahedron has a negative charge. The charge balance is completed by bivalent large cations (Ca²⁺, Sr²⁺ and Ba²⁺) which hold interstitial sites in the tetrahedral structure and tetrahedral structure having tridymite structure. Therefore, the emission spectrums are similar to each other.

According to XRD patterns of MAl₂O₄: Eu, Dy (M: Ca, Sr, Ba) phosphors, SrAl₂O₄ has two phases as high temperature hexagonal (β-phase) and low temperature monoclinic (α-phase) forms. Phase transition temperature is 650 ˚C. The crystal structure of β-SrAl₂O₄ is very similar to that of BaAl₂O₄. Because the ionic radius of Ba²⁺ ion (0.135 nm) is almost the same with Sr²⁺ (0.127 nm) and O^2- (0.135 nm) ions. Ba²⁺ and Sr²⁺ ions are convenient for packing of O^2- ions. Ca²⁺ ion is quite small which results deterioration in the structure. It’s a known fact that the Eu²⁺ ions settle in M²⁺ locations in MAl₂O₄main crystals. The radius of Ba²⁺ and Sr²⁺ ions are almost the same with that of Eu²⁺, but the radius of Ca²⁺ ion (0.112 nm) is 85 % smaller than that of Eu²⁺ ion (0.130). Therefore, the crystallographic deteriorations will affect the crystal area in Eu²⁺ location. As a function of crystal area resistance (Δ), the schematic representation of the energy level of Eu²⁺ ion in MAl₂O₄ main crystal is as shown in Figure 1.

This figure shows the change in emission peaks of different crystal structures.

According to the research of Abbruscato, the SrAl₂O_4: Eu phosphors photocariers form with irregular UV irradiation and Halt effected mesaurements have shown that holes are conductive while under UV excitation. This situtation arises from two different centers as emission center of Eu²⁺ and Dy³⁺ traps. Therefore, phosphorescence mechanism of phosphors system can be explained the excitation of UV, 4f→5d direct transition and production of many holes, respectively. Some of the free holes thermally released to valence band and at the same time are trapped by Dy³⁺as being carried to valence band. Eu¹⁺ becomes metastable [(Eu¹⁺)] when excitation source is removed. The trapped holes release thermally to valence band and combine with residual electrons in metastable [(Eu¹⁺)] location which move together. Schematic representation of luminescence mechanism is given in Figure 2.

As a result of studies performed, Dy³⁺ trap depths in MAl₂O₄: Eu, Dy phosphors are listed in the order of BaAl₂O₄ main crystal > CaAl₂O₄ main crystal > SrAl₂O₄ main crystal (Figure 3). In this case, if releasing of trapped holes and occured traps are deeper in the main crystal, it is more difficult for them to reunite. The initial luminescence intensity and persistent property of SrAl₂O₄: Eu, Dy between these three phosphors in MAl₂O₄: Eu, Dy system are much more than others. The reasons can be more appropriate level of Dy³⁺ trap depthsand location of Eu²⁺ ion in SrAl₂O₄ main crystal.

Phosphorescence observations are done for monoclinic phase of SrAl₂O₄: Eu²⁺ phosphors prepared in thin layers or in crystallized form by solid-state reactions (sintering for couple of hours at 1300 ˚C), sol-gel methods (at 1150 ˚C), microwave method and combustion method. It goes in to europium as Eu³⁺ (Eu₂O₃) which is in the form of oxidizing and phosphorescence observed after this reduction process is performed. The luminescence mesaurements showed that europium is mainly reduced (Eu²⁺) after this reduction process. But according to Mössbauer measurements, approximately 5-10 mole % of Eu³⁺ took place in the system (residual Eu³⁺). The Dy³⁺ co-dopant is stabilized for XANES measurements.

There are identical coordination numbers (as 6+1) of Sr²⁺, two different crystallographic locations which have similar average Sr – O internal (2.695 Å and 2.667 Å) and each one of them has similar average Sr – O internal (Wyckoff 2a position). This two media are seperated from each other only by a little distortion of square planes. Size of Sr²⁺ and Eu²⁺ ions are very similar to one another (1.20 and 1,21 Å respectively). Apart from this, when crystallographic location of Sr²⁺ is occupied by Eu²⁺ ions, it will have a very similar local distortion. Therefore, Eu²⁺ ions settled in two different Sr²⁺locations.

Locations of dopants and co-dopants of SrAl₂O₄ are determined according to ionic radius. Eu²⁺ (1.20 Å), Eu³⁺ (1.01 Å) and Dy³⁺ (0.97 Å) ions can be easly substituted Sr²⁺ (1.21 Å) ions. Two different locations of Sr²⁺ are different from each other in crystallograpy. Therefore, it is expected from Eu²⁺ ions to be available in both locations. EPR measurements have supported this expectation. Also, it is a known fact that it is possible to show the ionic radius of Sr²⁺ and Eu²⁺ ions which are very close to each other when the reduction of Eu³⁺ ions as Eu²⁺ in Sr²⁺ locations. Moreover, the B³⁺ co-dopant ions (0.11 Å) occupy locations of Al³⁺ ions and this was supported by IR and NMR measurements. But, due to the difference between ionic radius, replacement of B³⁺ ions with Al³⁺ ions will cause strong local stresses. Later on, these stresses needs a partial release by occurence of triangular planar BO₃ units according to IR and NMR measurements.

So far, different mechanisms have been proposed to explain the phosphorescence of co-doped SrAl₂O₄:Eu²⁺. One of them studied by Matsuzawa et al. developed for SrAl₂O₄: Eu²⁺system depends on photoconductivity works of powder samples in the SrAl₂O₄: Eu²⁺ system. This event shows that UV irradiation causes photoconductivity in hole type, so establishes the presence of hole trap (or held) (Figure 4).

Excitation and emission spectrums are related with Eu²⁺ ion;
[Chem. 1]
Eu ^{2 +} (4f ⁷ ) + hv Eu ^{2 + *} (4f ⁶ 5d ¹ )
[Chem. 2]
Eu ^{2 + *} (4f ⁶ 5d ¹ ) Eu ^{2 +} (4f ⁷ ) hv’
transformations indicates this event.

According to Matsuzawa et al. origin of the holes is capturing of electrons from the valence band by the Eu^2+* ions, and the co-dopants Dy³⁺ trap these holes.
[Chem. 3]
Dy ^{3 +} (4f ⁹ ) + h ⁺ Dy ^{4 +} (4f ⁸ )

The transformation above depicts the best way and the return of trapped holes for the distorted emission to Eu location is started by the supports of electrons thermally applied to Dy⁴⁺ from highest valence band. It is also mentioned by other researches that trapped holes are created by cationic holes. Matsuzawa et. al submitted two important assumptions on their mechanisms; top or bottom level of 4f⁷ arrangements of Eu²⁺ is in a similar energy to valence band (0.06 eV) and Eu²⁺ ion excited by irradiation transform to Eu¹⁺ by catching an electron;
[Chem. 4]
Eu ^{2 +} (4f ⁶ 5d ¹ ) + e ^- Eu ⁺ (4f ⁷ 5d ¹ )

It is observed that mechanism of Matsuzawa et. al and all phosphorescence mechanisms mentioned in the literature are not compatible with each other according to the experimental and theoric observations.A new phophorescence mechanism development became significant being consistent with each other by experimental and theoric observations for the SrAl₂O₄ system, with or without adding Dy³⁺ and/or B³⁺ co-dopants. The Eu²⁺concentration reduces under UV excitation in the new mechanism seen in Figure 5.This mechanism depends on the arguments that phosphorescence samples consist of Eu³⁺ ion and it is infact d orbitals of Eu²⁺ settles on near the bottom conduction band of SrAl₂O₄ as imposibility of reduction of all Eu³⁺ ions to Eu²⁺ in synthetic conditions.

Electrons under UV irradiation take position from occupied level to empty 5d level of Eu²⁺ ion and from top valence band to unoccupied level of remained Eu³⁺.While holes created in valence band can trap in V_Sr or V_Al level, the electrons are risen to level 5d can trap in V_O defects which settle arround Eu³⁺ cations. Remained Eu³⁺ ion is reduced to Eu²⁺ while Eu²⁺ is oxidizing to Eu³⁺ during this trapping process, thermal energy causes release of trapped electrons to 5d level of Eu³⁺ ions.By this way, green phosphorescence occurs by transition of 4f⁶5d¹ 4f⁷ (⁸S_7/2). The blue emission in 450 nm wavelength which only observed at low temperature (less than 150 K) possibly occurs because of the charge transfer to valence band from main level of 4f⁷ arrangements of Eu²⁺ and related to hole releasing mechanism.

The luminescence mechanisms to explain long persistent phosphorescence of SrAl₂O_4: Eu²⁺, Dy³⁺ system and others are not able to provide some experimental and teoric observation. The basic reality is; the Eu²⁺ ion occurs as a result of its d orbitals to settle near the bottom of conducting band, reducing of Eu²⁺ concentration under UV excitation.Trace quantity of Eu³⁺ always remains in these compounds.Phosphorescence mechanism iscreated by oxidizing Eu²⁺ ions to Eu³⁺.Reunification of electrons which trapped in photoproductive Eu³⁺ locations with 520 nm emission of phosphorescence occurs and electrons trap in oxygen holes around photoproductive Eu³⁺. One of the other emission of SrAl₂O₄: Eu²⁺, Dy³⁺ system at low temperature at 450 nm is a fact related to UV irradiation, occuring during charge transfer to Eu³⁺ ion from oxygen and hole trap in holes of Sr²⁺. These studies showed that it is an important fact to phosphorescence mechanismthat Dy³⁺ the co-dopant increases the number of electron traps and depth around Eu³⁺ ion.

Dy³⁺ application in phosphorescence process causes lots of trap levels occurance in the structure. As it is seen in Figure 6; Eu²⁺ ions excite by UV irradiation and electron hole pairs occur. Thus, electron transforms to excitation state (Transition 1). Later on, the electron relaxes to metastable state fastly (Transition 2). Apart from this, Transition 4 constitution which causes reduction of phosphorescence strength might occur. It is being thought that Eu²⁺ Eu¹⁺ transformation also occurs in Transition 3. As a result, it occurs by release of visible light hole and re-unification with electrons (Transition 5).

The studies indicated that, as Dy³⁺ the co-dopant increases electron trap amount in Eu³⁺ neighborhood and deepths and it is an important fact in phosphorescence progress.

The researches on phosphorescence development process by attraction of Eu²⁺ and Dy³⁺ added SrAl₂O₄ and Sr₄Al₁₄O₂₅ phosphors emphasized on various addition effects, molar rates of components and preparation methods. During these studies it was found out that shape and size of phosphor particles affect phosphorescence specifications. New properties are invented as emission intensity transforms to blue when particle size reaches nano level. Phosphor particles are expected to create high density compact as a result of orientation and better light absorbtion when the particles are well shaped and in a plate-like structure. This situation provides an obtain of higher luminescence strength. Particle size and shape of phosphorescent powders can be depend on the type of consisted crystal, particle size of begining materials and so preparation process.

Long lasting luminescence of phosphor with small particle size can be explained by 4f-5d transition of excited Eu²⁺ from after exciting by light source and creation of large amount of space near valence band. Some of the free-state holes are thermally released to valence band, immigrate towards valence band and trapped by Dy³⁺-borate complex. The trapped spaces are released to valence band thermally, immigrate towards excited Eu²⁺ and at the end reunification which provides long persistent luminescence occurs (Figure 7). It is being understood that long persistent luminescence depends on the number of held holes which depend on the concentration of Dy-borate complexes and trap depths. As long as the particle size reduces, transformation to blue appears by trap depths, and this provides improvement of persistent luminescence.

Technical Problems and Solutions

Until a few years ago, the only practically known phosphorescence compound was copper and cobalt-doped zinc sulfate (ZnS: Cu⁺, Co²⁺). Phosphorescence applications remained limited due to the large number of defects. There are two undesired properties in the ZnS: Cu⁺, Co²⁺ material; short decay time (nearly 1 hour) and sensitivity to humidity. These two properties have brought limitation seriously. But, this material is still used in watches and wall clocks to show the time in the dark. Tens of years ago, radioactive materials such as promethium, mezotorium and trithium were attached to the main lattice which consists of ZnS to eliminate the loss of luminescence intensity in a short time. On those days, Swiss producers managed to isolate microtubes consisting of a very little amount of gas tritium in glass. Inner wall of microtube was coated with ZnS which diffuses green light, doped and undoped. The tritium amount used was as little as it can be neglected and radioactive beta particles were not able to go out from the glass they locked in.

Explanation of Images

The figures to support the phosphorescence pigments which have luminescence in yellowish-green colour description are given below:
In the yellowish-green phosphorescence system, four different recipes (Y-01, Y-03, Y-05, Y-07) are examined when spectroscopic specifications of Y₂O₃ in 0.001-0.007 molar rate by changing the molar amount in yttrium:
XRD graphs are given in Figure 8.
Emission graphichs (excitation wavelength ~255 nm) which belong to yellowish-green phosphorescent pigments consisting of different Y₂O₃ amounts are exhibited in Figure 9.
Graphics belonging to the mesaurement of afterglow property for powders are given in Figure 10.

Sample Recipes

The yellowish-green phosphorescence pigment recipe in SrAl2O4:Eu, Dy, Y system are studied with 4 different Y2O3 molar rates:

Recipes	SrCO 3 (g)	H 3 AlO 3 (g)	Dy 2 O 3 (g)	Eu 2 O 3 (g)	Nd 2 O 3 (g)	H 3 BO 3 (g)
Recipe 10.001 mole Y2O3	4.82	4.83	0.18	0.1	0.02	0.19
Recipe 20.003 mole Y2O3	4.80	4.80	0.15	0.07	0.05	0.17
Recipe 30.005 mole Y2O3	4.75	4.72	0.12	0.04	0.1	0.12
Recipe 40.007 mole Y2O3	4.70	4.65	0.10	0.01	0.15	0.1

According to XRD analysis results (Figure 8), it can be seen that different Y₂O₃ amounts do not affect consisted main phase of structure and, SrAl₂O₄ occurs as a main crystal in 4 different recipes consistingY₂O₃. In the prepared recipes SrAl₄O₇ phase begin to disappear by increasing Y₂O_3, in compounds of Y-05 and above it disappeared completely.

It can be clearly seen from the emission graphs (Figure 9) belonging to yellowish-green phosphorescence pigments consisting different amount of Y₂O_3, each one of all the prepared pigments diffuses in green wavelength rate. Some amounts of Y₂O₃ are added to structure of aluminate crystal and the rest to structure of secondary phase. It can be thought that the yttrium contributes the support of creation of carrier traps or stability-enhancing carrier traps meaning, directly increases brightness of phosphorescence, and second phase has a role in stabilizing carrier traps by arranging aluminate crystal. As a result brightness of phosphorescent pigments can be increased and even excitation time to reduce half the way.

Afterglow timegraphics of phosphorescent powders are given in Figure 10. When results are examined, it is seen that the emission and decay of Y-03 and Y-05 coded phosphorescence pigments are longer comparing to the others.

Explanation of The Invention

Earth alkaline aluminate systems activated with rare earth elements are phosphorescence pigments with luminescence ability. The most important feature of these systems are strong light absorbtion by courtesy of their crystal structure, ability for storageand dissemination and as a result showing long-lasting phosphorescence with high brightness. According to the light source (generally room light) which they are under influence, they provide emission for more than 12 hours after the light source was removed. Their brightness and delay time are ten times more than that of very known zinc sulphate. Earth alkaline aluminate systems do not show any harmfull effects to health as they do not contain radioactive contribution. Apart from this, they are stable and resistant to atmospheric effects unlike zinc sulphate system. Light excitation and emission drive continuously.

In this invention, the yellowish-green colour emitting phosphorescent pigments from strontium aluminates ( SrAl₂O₄: Eu, Dy, Y) which are most common and accomplished in the earth alkaline aluminate systems were developed under the lights of previous studies reported in the literature.

1. When we characterize the long lasting yellowish-green color emitting phosphorescent pigments MAl₂O₄: R x, y, z, t, k, l, m, r, a, b, c, d is valid:
M: Strontium (Sr) element
R: Europium (Eu) as emission center, one or more from Dysprosium (Dy), Yttrium (Y) or Neodymium (Nd) Yttrium (Y), Praseodymium (Pr), Ytterbium (Yb), Erbium (Er), Gadolinium (Gd), Cerium (Ce), Samarium (Sm), Hafnium (Hf), Thulium (Tm) occur as co-dopant in the structure.

2. In the phosphorescent pigments havinglong lasting bluish-green emission mentioned in the claim 1 the molar ranges of rare earth elements; Eux, Dyy, Ndz, Yt, Prk, Ybl, Erm, Gdr, Cea, Smb, Hfc, Tmd which can be added to SrAl2O4, the main phase are as follows:

SrAl 2 O 4 : Eu x , Dy y , Nd z , Y t , Pr k , Yb l , Er m , Gd r , Ce a , Sm b , Hf c , Tm d , B e

0.0001≤ x, y, z, t, k, l, m, r, a, b, c, d ≤0.1

3. The phosphorescent pigments withlong lasting yellowish-green emission mentioned in the claim 1-2 have been characterized by synthesizing via solid-state reaction method under reductive nitrogen (N₂) – hydrogen (H₂) gas atmosphere.

4. Claim 3 indicates the synthesizing method and its specification consists of the relevant steps as follows:
- Mentioned gas atmosphere is nitrogen (N₂) – hydrogen (H₂) mixture. 90-98.5 % N₂ and 1.5-10 H₂ % is used.
- Appropriate raw materials are chosen for solid-state reaction method of the phosphorescence pigment (in oxide, carbonate, hydroxide etc. forms). The determined raw materials are selected with the proper purities (>99.5 %).
- To get long-afterglow phosphorescent pigment by the synthesizing method mentioned wet milling is applied for appropriate time by planetary mill (30 minutes – 6 hours).
- In the wet milling process mentioned the proper wet medium (ethanol, propanol-2 pure water, etc.), appropriate milling and mixing media, aluminum and zirconuim oxide balls in the diameter of 1mm-, and their jars are provided.
A) In the synthesizing method mentioned to get long-afterglow phosphorescent pigment a proper gas flow rate (0.1-0.50 lt/min) is set in the sintering furnace.
B) In the synthesizing method mentioned to obtain Sr₄Al₁₄O₂₅ phase, a proper sintering temperature (1250-1600 ^oC) and time (30 minutes-6 hours) are set in the furnace.
- In the synthesizing method mentioned the wet-milled and then dried batch is loaded in proper crucibles. The conditions indicated in A and B are followed:
- In the synthesizing method mentioned to get long-afterglow phosphorescent pigment dry milling to the bulk phosphor is performed under proper conditions for certain time to obtain particle sizes previously determined depending on the application field of the pigment.

Implementation of The Invention to Industry

Phosphorescent pigments are used in coating of products surfaces and can also be mixed with plastic, elastic, polyvinyl chloride (PVC), other synthetic resins and glass.

They find themselves a wide usage area in traffic safety signs, traffic control gloves, reflection plates of vehicles, reflection flags, highway signs, tyres, shoes, trench coats, telephone keypad overlays, watches, stair edges, emergency exit signs, the surface of the fire extinguisher cylinder, toys, writing tools. Apart from these, ceramic glazes doped with phosphorescence pigment have potential usage in apartments, especially as skirting (ceramic production used in area where the wall and ground intersect). It can be a practical solution in the case of sudden electric cut, afterwards in emergency when the phosphorescent signs are placed beside stairs. Ceramic and glass products with phosphorescence pigment can be used as decor. As a border, glow stone, overlays in kitchen, bathroom or pool, ceramics with phosphorescent pigments containing glazes also provide visual richness which could be of ceramic artists interest.

Claims (4)

1. When we characterize the long lasting yellowish-green color emitting phosphorescent pigments MAl₂O₄: R x, y, z, t, k, l, m, r, a, b, c, d is valid:
   M: Strontium (Sr) element
   R: Europium (Eu) as emission center, one or more from Dysprosium (Dy), Yttrium (Y) or Neodymium (Nd) Yttrium (Y), Praseodymium (Pr), Ytterbium (Yb), Erbium (Er), Gadolinium (Gd), Cerium (Ce), Samarium (Sm), Hafnium (Hf), Thulium (Tm) occur as co-dopant in the structure.
2. In the phosphorescent pigments havinglong lasting bluish-green emission mentioned in the claim 1 the molar ranges of rare earth elements; Eu_x, Dy_y, Nd_z, Yt, Pr_k, Yb_l, Er_m, Gd_r, Ce_a, Sm_b, Hf_c, Tm_d which can be added to SrAl₂O₄, the main phase are as follows:
   Reference---Table 2
3. The phosphorescent pigments withlong lasting yellowish-green emission mentioned in the claim 1-2 have been characterized by synthesizing via solid-state reaction method under reductive nitrogen (N₂) – hydrogen (H₂) gas atmosphere.
4. Claim 3 indicates the synthesizing method and its specification consists of the relevant steps as follows:
   - Mentioned gas atmosphere is nitrogen (N₂) – hydrogen (H₂) mixture. 90-98.5 % N₂ and 1.5-10 H₂ % is used
   - Appropriate raw materials are chosen for solid-state reaction method of the phosphorescence pigment (in oxide, carbonate, hydroxide etc. forms). The determined raw materials are selected with the proper purities (>99.5 %).
   - To get long-afterglow phosphorescent pigment by the synthesizing method mentioned wet milling is applied for appropriate time by planetary mill (30 minutes – 6 hours).
   - In the wet milling process mentioned the proper wet medium (ethanol, propanol-2 pure water, etc.), appropriate milling and mixing media, aluminum and zirconuim oxide balls in the diameter of 1mm-, and their jars are provided.
   C) In the synthesizing method mentioned to get long-afterglow phosphorescent pigment a proper gas flow rate (0.1-0.50 lt/min) is set in the sintering furnace.
   D) In the synthesizing method mentioned to obtain Sr₄Al₁₄O₂₅ phase a proper sintering temperature (1250-1600 ^oC) and time (30 minutes-6 hours) are set in the furnace.
   - In the synthesizing method mentioned the wet-milled and then dried batch is loaded in proper crucibles. The conditions indicated in A and B are followed:
   In the synthesizing method mentioned to get long-afterglow phosphorescent pigment dry milling to the bulk phosphor is performed under proper conditions for certain time to obtain particle sizes previously determined depending on the application field of the pigment.

Images