Metal contact quenching mechanism of sintered SrAl₂O₄:Eu²⁺, Dy³⁺ composite coating

Ten-millimeter cube 45 # steel substrates were ultrasonically degreased and with alcohol as solvent cleaning. Phosphor SrAl₂O₄:Eu²⁺, Dy³⁺ were purchased commercial spherical fluorescent powder, select 270–360 mesh particle size (particle size 40–53 μm), which has shown irregular grains and free of other impurities. Cu-14A1-X powder was prepared by gas atomization method; the main component of the powder is shown in Table 1. Copper powder, nickel powder, iron powder, manganese powder are commercial electrolytic powder.

The powder samples of various metals with 30 vol% phosphor and a certain amount of liquid paraffin as a type agent were mixed for 120 min in three-dimensional vortex mixer. After that, the powder mixtures were put into a graphite die lined with a flexible graphite foil. Powder sintering was proceeded in a RYJ-2000Z type hot pressing (HP) furnace at different temperatures, different pressures. Finally, the hot press chamber was cooled down naturally at an average rate of 10 °C/min. Experimental program and sintering process is shown in Table 2. Before testing, the samples were mechanically polished to 2000 grit Sic paper, and then cleaned ultrasonically with ethyl alcohol.

Phase identification of the coatings was carried out by X-ray diffract meter (D/MAX 2500PC), using Cu a radiation over the 2θ range of 10°–90° with a step width of 6^°/min. The accelerating voltage and emission current were 35 kV and 200 mA, respectively. The optical spectra were measured by the fluorescence spectrometer (F-97) with Xe lamp as excitation source at the room temperature.

Figure 1 shows XRD patterns of Cu-14Al-X/SrAl₂O₄: Eu²⁺, Dy³⁺ composite coatings produced at different sintered temperatures. The XRD profiles indicate that the main phases are Cu₉Al₄ and Cu₃Al and a small amount of AlFe₃ phase also exists in Cu-14Al-X/SrAl₂O₄:Eu²⁺, Dy³⁺ composite coatings. In addition, with the increase of sintering temperature, the intensity of strontium aluminates peak gradually stronger which means the crystallization of SrAl₂O₄:Eu²⁺ become better. In this paper, there are five kinds of metal matrixes (Cu, Fe, Ni, Mn and Cu-14Al-X) coupled with fluorescent particles (SrAl₂O₄:Eu²⁺, Dy³⁺) and get five different matrix composite coatings respectively. XRD analysis indicated that the composite coatings with different matrixes are all composed by metal phase and SrAl₂O₄. The fluorescent particles do not react with the metal and no impurity phases were observed in sintered coatings as shown in Fig. 2.

Figure 3 shows the temperature dependent PL spectra of Cu-14Al-X/SrAl₂O₄:Eu²⁺, Dy³⁺ coatings which were excited with a xenon lamp at a wavelength of 365 nm. The luminous intensity of the coating increases with sintered temperature increase in temperature range of 650–850 °C. This phenomenon is different from the thermal quenching because the crystallization of SrAl₂O₄:Eu²⁺, Dy³⁺ in the coating becomes better, so the luminous enhanced [13].The emission spectrum of the Cu-14Al-X/SrAl₂O₄:Eu²⁺, Dy³⁺ coatings includes a broad band peak at 515 nm corresponding to the green emission. This is the characteristic emission of Eu²⁺ which is attributed to the transition from 4f⁶5d to the ground state 4f⁷. The decay curves of the coatings were shown in Fig. 4, and it is found the initial luminescent intensity is slightly enhanced with increasing of temperature. Those clearly illustrates that the sintered temperature is not major quenching factor for SrAl₂O₄:Eu²⁺, Dy³⁺ in the composite coatings.

The emission spectrum of different matrix composite coatings is shown in Fig. 5. Green emission is similar to the coatings at different sintering temperatures. The maximum intensity is Ni/SrAl₂O₄:Eu²⁺, Dy³⁺ coating and Mn/SrAl₂O₄:Eu²⁺, Dy³⁺ coating has the lowest intensity. Figure 6 shows the decay curve of composite coatings with different metals power. The initial emission intensity of different matrix composite coatings is showed by the order: Ni > Fe  >  Cu > Cu-14A1-X > Mn. Although the initial brightness of the Fe/SrAl₂O₄:Eu²⁺, Dy³⁺composite coating is high, but fell sharply in the subsequent decay process.

The trap initial electron density of SrAl₂O₄:Eu²⁺, Dy³⁺ in coating is a crucial factor for the quenching, fluorescent particles coupled with different matrix powers show different quenching tendency. If the trap initial electron density is low, a small number of electrons are needed when excited electrons in the trap come back to the excited state, so initial electron density are stored only in trap energy level and afterglow phenomenon is not easy to be observed in coating [14, 15]. Therefore, the trap initial electron density of SrAl₂O₄:Eu²⁺, Dy³⁺ is investigated.

The trap initial electron density of SrAl₂O₄:Eu²⁺, Dy³⁺ is investigated to explore the potential quench mechanism. The persistent luminescence for these metal composite coating has been analyzed by curve fitting. Samples afterglow decay curves satisfy binomial exponential decay equation, which can be described as follow [16]:where I is the phosphorescence intensity, I ₀ , I ₁ , I ₂ are luminous intensity constants, t is decay time, and τ ₁ and τ ₂ are the decay times for fast and slow exponential components respectively. The fitting results of composite coatings are tabulated in Table 3.

In the strontium aluminate structure, Dy³⁺ substitute Sr²⁺ formed the point defect \(\left( {\text{D}{\text{y}_{\text{Sr}}}^{{{**}}}}\right)\) which can serve as an effective electron trap having appropriate depth for persistent luminescence. But the electron trap \({\text{V}}_{\text{O}}^{{{**}}}\) (inherent oxygen ion vacancies) does not arouse the obvious change of persistent luminescence [17]. In coatings the intensity of the recombination luminescence and time can be also expressed by the following formula:

Suppose afterglow decay curves fast decay (I ₂ , τ ₂ ) from the electronic release in the \({\text{V}}_{\text{O}}^{{{**}}}\) trap, and slow decay(I ₁ , τ ₁ ) was generally related to the electronic release of \(\left( {\text{D}{\text{y}_{\text{Sr}}}^{{{**}}}}\right)\) trap, by the above formula and the available data in the Table 3 can be obtained corresponding the initial concentration of electrons in the trap n₀₁, n₀₂, respectively [18].

As shown in Table 3, the initial electron concentration of pure SrAl₂O₄:Eu²⁺, Dy³⁺ is higher than that of different matrix composite coatings. It also can be seen that the initial electron concentration in decreasing order of Ni > Cu  > Cu-14A1-X > Mn > Fe. This is in line with the emission intensity spectrum of the different matrix composite luminescent coatings except Fe. Some electrons promoted to the 5d levels may get trapped at the \({\text{V}}_{\text{O}}^{{{**}}}\) oxygen defects and the trapped electrons lead to occupation of the 5d levels, which results more phosphor valence electron were excited and enhanced luminescence by magnetic energy. The anomalously n₀₂ values in Fig. 7 is evident. It is generally accepted that Fe display a wide variety of magnetic behaviors result an oriented rearrangement process of Eu²⁺ during the transition. In the magnetic field, emission peak position is not shifted, but the intensity changes.

A rudimentary energy level scheme is constructed to explain the contact quenching mechanism of Fig. 8. When the coating after excitation, the luminescent center Eu²⁺ is excited electrons, as well as valence and conduction bands also produce large amounts of electrons and holes which were captured by Eu²⁺. The contact metal can be as a defect level and dissipation center of electron transition during the energy transfer process, which can capture electrons and reduce the probability of transition of them. Due to the high work function of the contact metal, it can be as a defect level and dissipation center of electron transition during the energy transfer process, which can capture electrons and reduce the probability of transition of them. For different metal composite metals, the work function of different metals is in order: Ni > Fe > Cu > Cu-14A1-X > Mn. The metal is distributed at different energy levels, so there was also a contact energy barrier between metal particles and SrAl₂O₄:Eu²⁺, Dy³⁺. For Mn/SrAl₂O₄:Eu²⁺, Dy³⁺ coating, the electronics in conduction band and excited state of Eu²⁺ from the SrAl₂O₄:Eu²⁺, Dy³⁺ fluorescent particles easily transferred to Mn and filled with the energy level of metal. On the contrary, the large contact barrier between Ni and SrAl₂O₄:Eu²⁺, Dy³⁺ makes the electrons difficult to cross this barrier and the probability of electrons being captured is small, so the Ni/SrAl₂O₄:Eu²⁺, Dy³⁺ composite coating has the highest luminous intensity. This result is consistent with the emission spectrum.