One-Step Microwave Synthesis of New Hybrid Phosphor (CSSC) for White Light-Emitting Diodes

Abstract

The possibility of synthesizing a new hybrid phosphor CSSC (mixture of 0.5 CaSrSiO₄:Eu²⁺: 0.29 Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺: 0.21 Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) using a one-step microwave synthesis method is demonstrated. The concentrations of europium and calcium in the synthesized phosphors were optimized at 1 and 10 mol. %, respectively, to achieve maximum brightness and color rendering index. The optimal conditions for the synthesis of phosphors in a microwave furnace were determined as 750 °C for 10 min. The resulting phosphor exhibited a wide luminescence spectrum that covered the entire visible region, resulting in a high color rendering index and a warm white luminescence when used as a light source. It is shown that the sol–gel method for preparing the charge mixture for the new phosphor allows for a 35% higher luminescence brightness compared to the solid-phase method, due to a more uniform distribution of the activator.

1. Introduction

In today’s world, a significant challenge is the widespread adoption of white light-emitting diodes (WLEDs) with a broad luminescence spectrum, minimal short-wavelength radiation, and high color rendering index in lighting products. The most commonly used phosphor for WLEDs is cerium-doped yttrium aluminum garnet (YAG: Ce) [1,2]. However, this phosphor has drawbacks such as a strong emission band in the blue region of the spectrum, which suppresses the secretion of melatonin [3,4]—a hormone involved in regulating the circadian rhythm. Additionally, the red emission intensity in this phosphor is low, and the spectrum width is insufficient. Various other approaches, including classical phosphors and new systems like quantum dots, perovskites, layered perovskite oxides, etc., have been explored [5,6]. Generally, the spectrum of WLEDs significantly differs from that of sunlight, resulting in a low color rendering index. Phosphors with a spectrum similar to that of sunlight do not currently exist, necessitating the use of mixtures of phosphors with different spectral characteristics [7,8,9]. In this case, each component of the mixture is synthesized separately and then mixed in the desired proportions.

Therefore, it is of interest to develop a one-pot synthesis technology for hybrid phosphors—synthesizing a mixture of phosphors in a single step. This approach would be a cost-effective solution, producing a WLED luminescence spectrum that closely mimics solar radiation.

For this purpose, a suitable option is a hybrid phosphor based on silicates and chlorosilicates doped with Eu²⁺ ions. Silicate and chlorosilicate phosphors exhibit good chemical and thermal stability, durability, and luminescence intensity, as well as a high quantum yield [10]. The luminescence bands are sufficiently broad, and a combination of phosphors can achieve a high color rendering index for WLEDs. The same set of starting chemicals can be used for their synthesis, making it possible to simultaneously synthesize the desired hybrid phosphor in one step. By varying the synthesis conditions, hybrid phosphors with the required spectral characteristics can be obtained.

The material CaSrSiO₄:Eu²⁺ has been extensively investigated [11,12,13]. Originally, it was studied for its application in mercury gas discharge lamps [13], and subsequently, it has been examined for its potential use in white light generation with near-UV LEDs [11,12]. However, the luminescence of this material is not white, as depicted in Figure 1.

To enhance its performance, it can be combined with Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺ and Ca₁₀Si₆O₂₁Cl₂:Eu²⁺ phosphors (Figure 1). The addition of a halogen ion to the silicate matrix causes a shift of excitation and luminescence spectra towards longer wavelengths due to the coordination effects of the halogen ion. Chlorosilicates exhibit high chemical and thermal stability, making them promising materials for phosphor bases [14,15].

By combining these phosphors, it is possible to achieve light sources with a broad emission spectrum.

Typically, silicate phosphors are synthesized in a muffle furnace under a layer of coal [16] or in a hydrogen atmosphere [17]. Our previous works have demonstrated that microwave synthesis offers several advantages, such as reduced time, temperature, and energy costs during the synthesis process [18,19,20,21,22]. Additionally, the application of microwave energy to radio-absorbing materials, by means of the ponderomotive effect and electrodiffusion, enhances the characteristics of the phosphors [23,24,25,26,27,28].

Thus, the objective of this research is to develop a cost-effective “one pot” technology for the production of europium-activated hybrid (mixture of silicates and chlorosilicates) white phosphor, which exhibits a WLED luminescence spectrum closely resembling solar radiation, with a high color rendering index.

2. Materials and Methods

The charge mixtures for the synthesis were prepared via two different methods: mechanical mixing and sol–gel synthesis.

During mechanical mixing, the powders of the initial SiO₂, SrCl₂ 6H₂O, Ca(OH)₂, and Eu₂O₃ were mixed in a calculated ratio (SiO₂/SrCl₂·6H₂O/Ca(OH)₂/Eu₂O₃ = 20/58/8/14 wt.%). The mixture was placed in a container and homogenized in a mixer for 4 h. These mixtures were used for the synthesis of phosphors.

Phosphor synthesis was carried out using two different heating methods. The first method involved using a muffle furnace at a temperature of 950 °C for 150 min, under an atmosphere of N₂:H₂ (5 vol.%). The second method involved using a specially fabricated microwave furnace at a temperature of 750 °C for a dwell time of 10 min, with a frequency of 2.45 GHz and power of 450 W.

The microwave heating apparatus used in the second method consisted of a resonator chamber, magnetron, circulator, and resonance sensors (Figure 2). The resonator chamber was made from a rectangular waveguide, with one end having a smaller cross-section aperture and the opposite end having a movable wall (plunger). The energy was delivered along the longitudinal axis of the chamber. The main functional elements of the chamber were as follows:

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The body of the working chamber was made of a piece of copper rectangular waveguide with a standard cross-section of 90 mm × 45 mm. This waveguide had the highest figure of merit for passing electromagnetic energy with a frequency of 2.45 GHz and power of 450 W. The length of the waveguide (235 mm) was selected to be a multiple of the length of the half-wave of the microwave radiation, ensuring that there were an integer number of zones of maximum intensity of the electromagnetic field (in this case, three bundles) formed in the chamber. The upper wall of the chamber had a loading hole in the zone of the middle beam for installing the thermostat with the object to be heated. There was also a hole with a diameter of 4 mm in the side wall of the thermostat and waveguide for temperature control using a pyrometer.

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The purpose of the aperture is to allow microwave energy to enter the chamber and create a standing wave through the superposition of incoming and reflected waves from the opposite wall. The size of the aperture in the diaphragm is matched to the standing wave coefficient. However, when a dielectric load is placed in the chamber and heated, it can alter the standing wave coefficient and affect the level of matching between the chamber and the generator, causing an increase in the amount of reflected energy from the diaphragm.

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The short-circuited movable wall, known as the plunger, is a control element used to adjust the length of the chamber. This adjustment can partially or completely align the phases of the waves and form energy bundles of a specific intensity.

The chamber-resonator is designed to achieve maximum matching between the generator, resonator, and load in the ANSYS program; see Figure 3.

In a waveguide-type chamber, there are three zones where the electromagnetic field strength is locally increased (known as beams). These beams occur when the chamber length exceeds three times the wavelength at a frequency of 2.45 GHz. The beam zones are symmetrically located along the motion of the electromagnetic wave and occupy the same volume. Each beam zone has an elliptical cross-section, and the electric field vector E has a “column” shape along the entire height of the chamber, with maximum intensity in the center (up to 10 V/m). The samples were placed in the zones of maximum electromagnetic field intensity.

Microwave heating theoretically has no upper limit for temperature, but the need to retain heat in the product requires the use of thermostats with special properties. Fibrous corundum thermostats, which are transparent to microwave radiation, were used during heating to preserve the heat generated by the samples within the working chamber.

Figure 2. The custom-made microwave furnace: 1—power supply unit and magnetron, 2—reaction chamber, 3—sensor of supplied microwave energy, 4—sensor of reflected microwave energy, 5—circulator, 6—load, 7—optical radiometer. Arrows—the direction of movement of electromagnetic waves.

Figure 2. The custom-made microwave furnace: 1—power supply unit and magnetron, 2—reaction chamber, 3—sensor of supplied microwave energy, 4—sensor of reflected microwave energy, 5—circulator, 6—load, 7—optical radiometer. Arrows—the direction of movement of electromagnetic waves.

Figure 3. Model of the reaction chamber: 1—diaphragm, 2—movable wall, 3—antinodes of the electric field energy.

Figure 3. Model of the reaction chamber: 1—diaphragm, 2—movable wall, 3—antinodes of the electric field energy.

The phosphors that were obtained underwent a washing process using distilled water to eliminate any excessive chloride.

The composition of the samples that were obtained was determined through X-ray phase analysis, utilizing a Rigaku Smart Lab 3 device. The photoluminescence spectra were measured using an AvaSpec-3648 spectrofluorometer, with an excitation wavelength of 395 nm, which corresponds to the wavelength of the ultraviolet (UV) light-emitting diode (LED) utilized in white light-emitting diode (WLED) fabrication. The brightness of the photoluminescence was quantified using an IL 1700 radiometer. The color rendering index, chromaticity coordinates, and correlated color temperature were measured employing a TKA-VD/01 spectrocolorimeter. The microstructure analysis of the phosphors was conducted using a Quanta 200 scanning electron microscope (SEM).

3. Results and Discussion

3.1. Selection of Thermostat Design for Synthesis in Microwave Furnace

Materials are classified into two types based on their level of microwave absorption:

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Dielectrics (SiO₂, Al₂O₃, MgO) have a low level of microwave absorption at room temperature, which increases as the temperature rises;

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Materials with semiconductor properties (SiC, TiO₂, CuO) start absorbing microwaves at room temperature [29,30].

Mixtures used for synthesizing phosphors belong to the first type of materials in terms of microwave absorption. These mixtures are heated using susceptors, which are materials that actively absorb microwave radiation when heated. The first fibrous corundum thermostat design, shown in Figure 4a, includes a charge mixture placed in a quartz crucible surrounded by four quartz crucibles containing a susceptor (SiC). The use of an oxidizing atmosphere in this design resulted in the synthesis of a red luminescent phosphor.

To obtain a white phosphor, it is necessary to convert Eu³⁺ to Eu²⁺. This can be achieved by synthesizing in a reducing atmosphere. Carbon black was added to the structure to create a reducing atmosphere for microwave synthesis. It was either placed between the charge layers or mixed with the charge itself (Figure 4b,d,e).

The use of carbon black in designs b, c, and d allowed the reaction volume to be heated without SiC. Carbon black serves two functions: a current collector for heating the system and a reducing agent.

During microwave fusion, the third thermostat design produced samples with the highest brightness, as shown in Figure 5. Therefore, this design was used for all subsequent experiments.

3.2. Choice of Synthesis Technology (Muffle or Microwave Furnace)

The selection of synthesis technology, either a muffle or microwave furnace, results in the formation of a mixture of phosphors. These phosphors include the following: CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺. X-ray phase analysis confirms that the synthesized phosphors possess a multiphase structure, consisting of CaSrSiO₄:Eu²⁺ (card 72–2260), Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺ (card 43–85), and Ca₁₀Si₆O₂₁Cl₂:Eu²⁺ (card 48–827). In the phosphors synthesized using the muffle furnace, the CaSrSiO₄:Eu²⁺ phase is predominant. On the other hand, synthesis in the microwave furnace leads to a substantial increase in the yield of chlorosilicate phases. This difference is attributed to the more pronounced interaction between strontium chloride and other reagents, as well as the improved diffusion of chlorine ions into the mixed luminophore structure during synthesis in the microwave furnace.

After the synthesis of luminophores in muffle and microwave furnaces, photoluminescence spectra were taken from the samples. The photoluminescence spectrum of luminophores is decomposed into constituent bands in Figure 6 and

Table 1. The bands found in the hybrid phosphor (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺).

№ of the Band	Wavelength, nm	Electron Transitions	Phases
1	421	5d-4f Eu2+	SrCl2
2	452	5d-4f Eu2+	Ca10Si6O21Cl2
3	486	5d-4f Eu2+	Ca6Sr4Si6O21Cl2
4	516	5d-4f Eu2+	CaSrSiO4
5	562	5d-4f Eu2+	Ca6Sr4Si6O21Cl2
6	643	5d-4f Eu2+	CaSrSiO4
7–18	597–702	4f-4f Eu3+	CaSrSiO4

.

The observed spectra exhibited slight variations and were analyzed using referenced literature data [11,12,13,14,15,16,17,18,19,20,21,22,31]. These differences included the presence of bands at 452, 485, and 517 nm, situated to the left of the main peak and associated with the 5d-4f transitions of Eu²⁺ in the predominant phases of the hybrid phosphor, as well as the band at 618 nm, located to the right of the main peak and corresponding to the 5D0-7F2 transition of Eu³⁺. In the muffle furnace synthesis, the bands at 452 and 485 nm are more pronounced due to an increase in crystallinity and the average size of the coherent scattering region of the chlorosilicate phases. Conversely, in the microwave furnace synthesis, the band at 517 nm is more intense as a result of an increase in crystallinity and the average size of the coherent scattering region of the silicate phase. The band at 618 nm is more pronounced in the muffle furnace synthesis due to the oxidation of a greater number of europium ions.

Table 2. The main characteristics of hybrid phosphors (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺).

Characteristics	Synthesis Conditions
	Microwave Furnace, 10 min, 750 °C	Muffle Furnace, N2:H2 2.5 h, 950 °C
Phase composition (CaSrSiO4:Eu2+, Ca6Sr4Si6O21Cl2:Eu2+, Ca10Si6O21Cl2:Eu2+), %	40, 27, 33	45, 26, 29
Particle size, µm	1–5	1–10
FWHM, nm	155	160
Intensity, a.u.	3010	2135
Correlated color temperature, K	3700	3440
Color coordinates	x = 0.401	x = 0.417
Color rendering index	y = 0.408	y = 0.418

presents the main characteristics of the phosphors synthesized in muffle and microwave furnaces, including phase composition, particle size, intensity, correlated color temperature, color coordinates, color rendering index, etc.

Both samples exhibit luminescence within the “warm” white light region (Figure 7) and share similar characteristics. However, the photoluminescence intensity is significantly higher in samples synthesized in a microwave furnace, presumably due to a more uniform distribution of europium in the phosphors resulting from microwave effects, such as the ponderomotive effect and electrodiffusion.

Electron microphotographs (Figure 8) reveal particle sizes ranging from 1 to 5 microns for microwave synthesis and up to 10 microns for synthesis in a muffle furnace. The smaller particle size in the case of microwave furnace synthesis is attributed to lower synthesis temperatures. The particle sizes of the obtained samples meet the requirements for phosphors used in white LEDs.

3.3. Selection of Microwave Synthesis Parameters

Given that the synthesis carried out in a microwave furnace oven demonstrated enhanced luminophore performance, it is imperative to determine the ideal temperature and duration for synthesis. The samples containing 5.6 mol. % Eu²⁺ were subjected to incubation at temperatures of 650, 700, and 750 °C for a duration of 10 min. Additionally, incubation at a temperature of 700 °C for 5, 10, and 15 min was conducted. Figure 9 depicts the relationship between the brightness of the hybrid phosphor (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) and the synthesis time and temperature in the microwave furnace. It is noteworthy that the brightest phosphor was obtained after synthesizing at 700 °C for 10 min.

3.4. Selection of Charge Preparation Method

The impact of the charge preparation method (mechanical mixing, sol–gel) on the properties of the blended phosphors was investigated. The mixture that was mechanically mixed with reagents and prepared using the sol–gel method was then synthesized in a microwave furnace at 750 °C for 10 min. The main characteristics of the luminophores are presented in

Table 3. Characteristics of samples.

Mixing Method	Intensity, a.u.	Full Width at Half Maximum, nm	Correlated Color Temperature, K
Mechanical mixing	700	111	3800
Sol–gel synthesis	945	111	4200

.

The brightness of the sample acquired using the sol–gel method of charge preparation exhibits a 35% increase in comparison to samples with the same half-width of the photoluminescence spectrum. Additionally, the correlated color temperature of the sol–gel-prepared sample is 400 K higher. Consequently, it is concluded that utilizing sol–gel synthesis in charge preparation enables a 35% enhancement in the brightness of mixed luminophores’ photoluminescence when compared to mechanical mixing. This is attributed to a more uniform distribution and improved integration of the activator into the phosphor base, thereby making the sol–gel method of charge preparation more favorable in the synthesis of mixed luminophores.

3.5. Selection of Chemical Composition in Microwave Synthesis

To manipulate the photoluminescence spectrum of the specimens, the level of carbon black was doubled to prevent the introduction of trivalent europium and decrease the proportion of Ca²⁺ in the charge. Consequently, the 597 to 702 nm bands associated with Eu³⁺ transitions in CaSrSiO₄ disappeared from the spectrum, and the spectrum itself shifted towards shorter wavelengths, a consequence of increased CaSrSiO₄:Eu²⁺ content in the phosphor mixture (Figure 10). The optimum concentration of Ca²⁺ in the charge was determined to be 10 mol. %, resulting in a color rendering index of 93 (

Table 4. Hybrid SCSC phosphors’ (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) characteristics with different Ca²⁺ concentrations.

Hybrid Phosphor Characteristics	Ca2+ Concentration in Charge Mixture
	8 mol. %	10 mol. %	12 mol. %	15 mol. %
Phase composition (CaSrSiO4:Eu2+, Ca6Sr4Si6O21Cl2:Eu2+, Ca10Si6O21Cl2:Eu2+), %	57, 33, 10	50, 29, 21	42, 28, 30	40, 27, 33
Color rendering index	87	93	85	83

).

It is observed that the color rendering index of the synthesized hybrid phosphors achieves the highest value at a Ca²⁺ concentration of 10 mol. % in the charge mixture, attributable to the alteration of the phosphor’s phase composition. Therefore, the optimal phase composition, CaSrSiO₄:Eu²⁺: Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺: Ca₁₀Si₆O₂₁Cl₂:Eu²⁺ = 50:29:21, is suitable for use in UV-excited white light-emitting diodes with wide emission spectra and a high color rendering index.

Subsequently, the optimal concentration of Eu²⁺ in the phosphor was determined by synthesizing all samples at 700 °C for 10 min. The most luminous samples of the hybrid phosphor were obtained at a europium concentration of 1 mol. % (Figure 11). Increasing the activator quantity beyond 1 mol. % resulted in a decrease in brightness due to concentration quenching. The spectral characteristics of the samples were unchanged.

Table 4 presents the characteristics of hybrid phosphors (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) that were synthesized from charge mixtures with varying Ca²⁺ concentrations.

It is established that the ionic radius of Eu²⁺ is larger than that of Ca²⁺ but approximately equal to that of Sr²⁺ [32]. As a result, one can hypothesize that Eu²⁺ is more likely to substitute Sr²⁺ rather than Ca²⁺ in phosphor matrices (CaSrSiO₄ and Ca₆Sr₄Si₆O₂₁Cl₂).

The cathodoluminescence of the synthesized mixed phosphor was studied. The brightness dependence on the voltage at the anode is linear (Figure 12), and the color of the luminescence is also a “warm” white, which allows for the use of the hybrid phosphor (CaSrSiO₄:Eu²⁺, Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) in cathodoluminescent light sources.

Thus, a methodology was devised for the amalgamation of hybrid phosphors (0.5 CaSrSiO₄:Eu²⁺, 0.29 Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺, 0.21 Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) utilizing a microwave furnace, enabling attainment of the traits of specimens that are typically only achievable over an extended duration under elevated temperatures in a muffle furnace (i.e., elevated luminosity and elevated color rendering index). The emission spectrum of a white light-emitting diode (WLED) fabricated using the aforementioned phosphor synthesis technique more closely approximates the solar spectrum in comparison to a conventional WLED (refer to Figure 13).

4. Conclusions

• A one-step method has been developed for synthesizing a new hybrid CSSC phosphor (0.5 CaSrSiO₄:Eu²⁺: 0.29 Ca₆Sr₄Si₆O₂₁Cl₂:Eu²⁺: 0.21 Ca₁₀Si₆O₂₁Cl₂:Eu²⁺) using microwave heating. The phosphor synthesized under optimal conditions exhibits a wide luminescence spectrum similar to sunlight, resulting in a high color rendering index and warm white luminescence color in WLEDs based on it.
• The sol–gel method for preparing the charge mixture for the hybrid phosphor (0.5 CaSrSiO₄:Eu²⁺, 0.29 Ca₁₀Si₆O₂₁Cl₂:Eu²⁺, 0.21 Ca₆Sr₄(Si₂O₇)₃Cl₂:Eu²⁺) allows for 35% higher luminescence brightness compared to the solid-phase method, due to a more uniform distribution of the activator.
• A CSSC phosphor can be used in cathodoluminescent white light sources.