Novel glasses and glass ceramics with different combinations of garnets doped with rare earth and 3d metal ions for temperature-resistant phosphors in high-power white LEDs – NOGACE

Project leader Monika Skruodiene

Agreement No 1.1.1.2/16/I/001

Research application No. 1.1.1.2/VIAA/3/19/480

There is a strong demand for the development of new inorganic luminescent materials to be used in different applications, for example lighting or image displays. Among variety of different inorganic materials, glass ceramics, combining the properties of glass and crystals, possess several advantages over traditional luminescent materials, like optical transparency, chemical and mechanical stability, high luminescence efficiency and others. The synthesis of such materials is linked with a number of technological challenges and some of them are still to be solved.

The goal of the present research work is to enhance the efficiency of recently developed active materials of modern LEDs using two alternatives:

(1) synthesis of the monodisperse nano-scale powders of new garnet crystal structure compounds of Z₃X_1.5Q_3.5O₁₂:M (Z = Mg, Ca, Sr; X = V, Ta; Q = Al, Cr; M = lanthanide or transition metal) by environmentally friendly sol-gel technique.

(2) development of fabrication technique for the incorporation of new garnets into the borate-silicate glasses and glass ceramics.

The improvement of quantum yield, thermal and radiation stability in novel glasses and glass ceramics with different combinations of garnets doped with rare earth and transition metal ions will be achieved.

The project will be implemented at the Institute of Solid State Physics, University of Latvia from 01.06.2020 until 31.05.2023. The total cost of the project is 133’805.88 EUR.

Project progress

31.05.2023.

Fabrication of prototype

A white LED prototype based on YAG:Ce (yttrium aluminum garnet doped with cerium) is a specific type of white light-emitting diode that utilizes this phosphor material to generate white light. YAG:Ce is one of the most used phosphors in white LEDs, known for its high efficiency and good color rendering properties.

In this type of prototype, a blue or ultraviolet LED chip is used as the light source. When the LED chip emits blue or ultraviolet light, the YAG:Ce phosphor coating absorbs a portion of this light and converts it into longer-wavelength yellow light. The combination of the blue or ultraviolet light emitted by the LED chip and the yellow light emitted by the phosphor creates a broad spectrum of light that appears as white to the human eye.

Fig. 1. Images of the prototype. YAG:Ce@SiO₂ glass

28.02.2023.

Scanning electron microscopy analysis

Scanning electron microscopy (SEM) is a powerful imaging technique that can be used to analyze samples at a high resolution. When examining glass samples using SEM, several important considerations should be considered. SEM imaging provides valuable insights into the microstructure, surface features, and elemental composition of glassy samples. By analyzing the SEM images, researchers can observe characteristics such as surface defects, cracks, grain boundaries, and the morphology of glassy structures. These observations contribute to a better understanding of the sample's properties and behavior, aiding in various fields, including materials science, geology, and manufacturing.

Fig. 1. SEM images of the samples in different magnifications. 0.5%YAG:0.5%Ce@SiO₂ (A I,II), 1.0%YAG:0.5%Ce@SiO₂ (B I,II), 1.5%YAG:0.5%Ce@SiO₂ (C I,II) and 2.0%YAG:0.5%Ce@SiO₂ (D I,II)

30.11.2022.

X-Ray Diffraction analysis. X-ray diffraction (XRD) is a powerful analytical technique used to identify the composition of a material by analyzing its crystalline structure. XRD produces a pattern of diffracted X-rays that is characteristic of the material, and is used to identify the crystalline structure, the phase, and the orientation of the material. It can be used to study the structure of crystals to identify the phases of a material.

For phase identification at room temperature, the XRD data was collected using Rigaku MiniFlexII diffractometer. The synthesized samples were characterized by the XRD method to evaluate their phase composition and purity. In Figure 1A two main phases are observed – amorphous and crystal. The reflexes were assigned to the garnet phase (COD ID # 1529037), meaning that SiO₂ doped YAG samples possess phase with cubic crystal structure, which corresponds to Ia3d (#230) space group. Figure 1B demonstrates diffraction patterns of YAG:0.5%Ce, the reflexes were assigned to the single phase.

Fig. 1. XRD patterns of YAG:0.5%Ce and YAG:0.5%Ce@SiO₂

31.08.2022.

SiO₂ synthesis by sol-gel technique. Sol-gel synthesis is a method of producing materials from a solution of precursors. It is mostly used to produce materials such as silica (SiO₂). In the sol-gel synthesis of SiO₂, a solution of silica precursors (such as tetraethyl orthosilicate, TEOS) is mixed with water and an acid catalyst. The acid catalyst speeds up the hydrolysis reaction which causes the TEOS to break down and form silica nanoparticles. These nanoparticles can then be processed further to produce a variety of materials ranging from thin films, thick coatings to glasses and glass ceramics.

Glass ceramic materials, such as SiO₂ doped with yttrium aluminum garnet doped with cerium, at low temperatures and with high purity. In this method, a solution of precursors is formed by mixing them in a solvent such as water or alcohol. This solution is then subjected to hydrolysis, a process whereby the precursors react with the solvent to form a gel. The gel is then heated to remove the solvent and the remaining material is sintered, or baked, to produce the desired ceramic material. This process can be used to produce a variety of ceramic materials, including SiO₂ doped with cerium. The sol-gel synthesis method is beneficial because it allows to produce materials with high purity and low-temperature sintering. Additionally, it is possible to tailor the properties of the resulting material by adjusting the reaction conditions, such as the concentration of the reactants, the reaction time, and the temperature.

The main goal was optimization of synthesis of SiO₂ glass doped with yttrium aluminum garnet doped with cerium to produce the highest quality product. The goal is to create a synthesis process which is as efficient and cost-effective as possible while still achieving the desired outcome.

Fig. 1. Proposed schematic representation of the steps involved in the sol-gel route

31.05.2022.

Project progress

Luminescence properties. The emission spectra – were measured under λ_Ex = 330 and 395 nm excitation. The emission spectra (Fig. 2) demonstrate peaks at 589 nm, 593 nm (⁵D₀ → ⁷F₁), 608 nm, 614 nm, 618 nm (⁵D₀ → ⁷F₂), 697 nm, 703 nm, and 709 nm (⁵D₀ → ⁷F₄). The first obvious indication is that YVO exhibits higher emission compared to YAG irrespective to the excitation wavelength. Secondly, there are some changes in the emission spectra of the samples if the emission is monitored for Eu³⁺ under 395 nm excitation in YAG compared to the emission Eu³⁺ in YVO. Two additional bands with maxima ca. 589 and 709 nm appear. They are typical bands for Eu³⁺ in the YAG matrix and remains in the emission spectra of any composite under 395 nm excitation. The addition of vanadium to the YAG structure has a staggering impact on the emission resulting in the appearance of dominant peaks at the positions characteristic of YVO under 330 nm or 395 nm excitation and remaining YAG peaks under 395 nm excitation. Note, that even the size of YVO particle is larger in the composites obtained by mechanical method, the integrated emission intensity of chemically obtained composites is slightly higher.

Fig. 2. Emission spectra of the samples.

The publication was submitted in Journal Materials Letters: “Modifying Optical Properties of Yttrium Aluminum Garnet with the Yttrium Vanadate Impurity Phase”.

02.28.2022.

Scanning electron microscopy analysis. SEM analysis was obtained to analyze and compare the morphology and the particle size of synthesized and mechanically mixed powders. Figure 1A represents YAG:Eu. It possesses well-shaped irregular sphere-like morphology. The pores, clearly visible in Fig. 1A and Fig. 1C, could be formed due to the escaping gasses during the decomposition of the organic components and residual nitrates. It can be observed that most of the particles are in the range between 200 – 800 nm. Meanwhile, YVO₄:Eu possess irregular, slightly angular, sphere-like morphology with clear boundaries between each particle (Fig. 1B). The particle size is much bigger, about 1.7-6 μm. Figures 1C and 1D represents YAG:Eu_1.0V synthesized and YAG:Eu_1.0V mechanically mixed samples, respectively. From the figures it is clearly visible two groups of particles of different sizes and shapes. Note, that in the chemically way obtained composite 0.5 – 4 μm size particles of YVO₄ are located on the surface and it seems they are sticked and even sunk in the particles of garnet (Fig. 1C), meanwhile the particles of each component of the composite are separate in Fig. 1D.

A	B
C	D

Fig. 1. SEM images of powder the samples of YAG:1%Eu (A), YVO₄:1%Eu (B), Y₃V₁Al₄O₁₂: 1% Eu synthesized (C) and Y₃V₁Al₄O₁₂: 1% Eu mechanically mixed (D).

30.11.2021.

The synthesized samples were characterized by the XRD method in order to evaluate their phase composition and purity. The main reflexes were assigned to the garnet phase (COD ID # 1529037). Meaning that all co-doped Y₃V_xAl_5-xO₁₂ samples (x = 0.1 – 0.4) possess a major phase with cubic crystal structure, which corresponds to Ia3d (#230) space group. It have to be mentioned, that for these compounds, small additional peaks were observed at around 2θ = 25°, indicating the formation of an impurity phase. This phase corresponds to YVO₄ (COD ID # 9011137). This phase increases with increasing x value.

Fig. 1. XRD patterns of Y₃V_xAl_5-xO₁₂: 1% Eu, when x = 0.1 – 3.0, synthesized via Sol-Gel assisted Molten-Salt route, annealed at 1300 °C in KCl, in air.

The publication was accepted and published in Journal of Alloys and Compounds: “Sol-Gel Assisted Molten-Salt Synthesis of Novel Single Phase Y_3-2xCa_2xTa_xAl_5-xO₁₂:1%Eu Garnet Structure Phosphors”.

Participation in the conference “Chemistry and Chemical Technology 2021 Vilnius” on September 24, 2021. Poster Presentation.

04.06.2021.

The more detailed structural analysis of the synthesized samples was performed using Rietveld refinement. Rietveld refinement results confirmed the formation of the secondary phase as mentioned before in co-doped samples synthesized via Sol-Gel route. The monoclinic Ta₂O₅ phase with I121 space group in the samples with x = 0.01, 0.05, 0.1 accounts for a fraction of 3.5 %, 5.6 % and 2.2 % of the total volume, respectively. With the increase of calcium and tantalum concentrations, the cell parameter a is also increasing, confirming our initial observation, even if the secondary phase starts to form (Table 1. Sol-Gel route). These results further confirm our initial assessment of Sol-Gel method being unfit for the preparation of such compounds. Meanwhile, in the case when samples were prepared by Sol-Gel assisted Molten-Salt route, Rietveld refinement analysis confirmed that there is no evidence of the secondary phase formation in the samples. With the increase of calcium and tantalum concentrations, cell parameter a is also gradually increasing (Table 1. Molten-Salt route), indicating that all of the added tantalum is successfully incorporated into the garnet structure, to the best of our best knowledge, for the first time. Note, the values of the lattice parameter in the table show trends rather than absolute values. Although the lattice parameters of the compounds synthesized by Sol-Gel method are higher than those of the synthesized by Sol-Gel assisted Molten-Salt method, an error is also possible due to the impurity phase.

Table 1 represents the structural phases and unit cell parameters of synthesized samples.

Table 1. Crystallographic data of synthesized samples.

The publication prepared during the previous reporting period was submitted: “Sol-Gel Assisted Molten-Salt Synthesis of Novel Single Phase Y_3-2xCa_2xTa_xAl_5-xO₁₂:1%Eu Garnet Structure Phosphors”

Participation in the Researchers' Night on the Virtual Exhibition/Conference Platform on April 30, 2021.

25.02.2021.

All the investigated phosphors were synthesized by the citric acid sol-gel method. Yttrium aluminum garnet was doped at different concentrations of Ca²⁺, Ta⁵⁺ and Eu³⁺ ions.

Scanning electron microscopy (SEM) analysis. SEM analysis revealed the morphology of the surface of the synthesized powders. All analyzed samples possess well-shaped irregular sphere-like morphology. During the annealing process, the partly molten nanosheets become cross-linked causing highly agglomerated particles. The presence of pores is clearly visible, which could be formed due to the escaping gasses during the burning of organic components and residual nitrates.

It is clear that doping does not affect the morphology of samples. In Figure 1 the YAG:1%Eu is presented. There are no significant differences in the SEM pictures for the samples with the different content of dopant ions as seen in Figure 2 – 4. The micro-size agglomerates maintain well-shaped irregular sphere-like morphology regardless the chemical composition.

The micrographs of the Y₃Al₅O₁₂:1%Eu, Y_2.98Ca_0.02Ta_0.01Al_4.99O₁₂:1%Eu, Y_2.9Ca_0.1Ta_0.05Al_4.95O₁₂:1% Eu and Y_2.8Ca_0.2Ta_0.1Al_4.9O₁₂:1% Eu samples are represented in Figure 1, Figure 2, Figure 3 and Figure 4 respectively.

10.12.2020.

All investigated phosphors were synthesized by the citric acid sol-gel method. All the samples were doped with optical active ions (Ce³⁺ and Eu³⁺).

Sol-gel synthesis parameters, to obtain single-phase materials, were determined. The synthesized samples were analyzed by the XRD analysis and optical properties were analyzed (excitation and emission spectra were recorded).

XRD patterns of synthesized powders, annealed at different temperatures/time intervals are presented below. It is clearly visible that the samples with low concentrations of tantalum and calcium sintered in reducing atmosphere at 1500 °C observe peaks matching the standard XRD data (JCPDS file 33–40), which means that YAG:Ta,Ca:Ce/Eu samples are single phase and possess a cubic garnet crystal structure.

09.09.2020.

All investigated phosphors were synthesized by the citric acid sol-gel method.

Necessary sol-gel synthesis parameters (sintering time and temperature, concentration of dopants (Ta, Ca) to obtain single-phase materials were determined. The synthesized samples were analyzed by the XRD analysis.

XRD patterns of synthesized powders, annealed at different temperatures/time intervals are presented below. It is clearly visible that the samples with low concentrations of tantalum and calcium sintered at 1200 – 1500 °C observe peaks matched to the standard XRD data (JCPDS file 33–40), which means that YAG:Ta,Ca samples are single phase and possess a cubic garnet crystal structure.