Microwave synthesis of nanosized particles of a complex oxide system with a garnet structure

The possibility of applying the method of microwave synthesis for the fabrication of nanosized particles of complex oxides has been demonstrated. The effect of fabrication conditions on the size of nanoparticles with a garnet structure has been established on the example of the Gd–Ga–Al–O system. The concentration of the organic solvent for obtaining particles of an average size of 50 nm has been determined experimentally. The phase composition of thermally treated samples has been investigated. The optimal conditions of producing nanosized powders of garnets, Gd₃(Ga,Al)₅O₁₂, have been determined.     

Combustion synthesis and structural characterization of YAG: Influence of fuel and Si doping

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) was synthesized by low-temperature combustion synthesis (CS) with different fuels such as urea, glycine, and ammonium acetate. It has been observed that combination of urea and glycine fuels results in the formation of YAG with some impurity phase. The effect of Si incorporation in the process was studied. The combustion synthesis from mixed fuels and silica was found superior for low-temperature synthesis of pure YAG at without further heat treatment. Phase evolutions and results of flame temperature measurements are reported. Rietveld refinement and analytical calculation of different structural parameters were performed to get proper notion about Si substitution in host matrix. The thermoluminescence (TL) of materials caused by UV irradiation was used to elucidate the nature of traps. The TL analysis revealed the presence of shallow traps whose amount grew with Si doping.     

Synthesis of GAGG:Ce3+ powder for ceramics using mechanochemical and solution combustion methods

Gadolinium aluminum gallium garnet doped with cerium (Gd_2.99Al_2.00Ga_3.00O_12.00:Ce_0.01, or GAGG:Ce) powders has been synthesized with high‐energy ball milling (HEBM) and solution combustion synthesis (SCS) techniques. The structures and morphologies of the powders were characterized through use of powder X‐ray diffraction (PXRD), photoluminescence (PL) measurements, dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV‐decay time measurements. Changes in the mechanochemically produced powders were monitored over time, while the solution combustion synthesis of GAGG:Ce was conducted using various ratios and types of fuels. In both production methods, particle sizes on the order of microns gave evidence of agglomeration.     

Cooperative cation substitution facilitated construction of garnet oxynitride MgY2Al3Si2O11N:Ce3+ for white LEDs

Structural modification is an important means to induce redshift of Ce³⁺ emission in garnet phosphor. We intend to design and synthesize garnet oxynitride compounds which combine attributes of rigidity inherited from garnet structure and of high covalence characteristic of oxynitride compounds. However, impurity phase usually occurs in the nitridation of garnet phosphor, due to the low solubility of nitrogen in oxides. We herein exploit the cooperative cation substitution strategy to facilitate the incorporation of nitrogen in Y₃Al₅O₁₂. It is found that partial substitution of Y³⁺‐Al_tet³⁺ pairs by Mg²⁺‐Si⁴⁺ pairs can diminish the phase instability caused by the replacement of Al_tet³⁺‐O^2? by Si⁴⁺‐N^3?. A novel pure garnet phase oxynitride phosphor MgY₂Al₃Si₂O₁₁N:Ce³⁺ with a higher substitution content of N has been obtained and the successful incorporation of N in the garnet phosphor is confirmed by the Rietveld refinements of XRD, XPS, and TEM. The emission and excitation spectra indicate that the blue‐light‐excitable MgY₂Al₃Si₂O₁₁N:Ce³⁺ phosphor exhibited a bright yellow‐orange emission peaking at 570 nm, which is redshifted by 28 nm when compared to YAG:Ce³⁺. The garnet oxynitride phosphor exhibit excellent thermal stability with high quantum efficiency and is a promising candidate for warm white LED.     

Sintering and microstructure evolution of Er1.5Y1.5-xScx+yAl5-yO12 garnet ceramics with scandium in dodecahedral and octahedral sites

The sintering behavior of scandium-containing garnet materials within the Y₂O₃-Er₂O₃-Sc₂O₃-Al₂O₃ system was investigated. Single-phase ceramic powders and optical ceramics’ samples of Er_1.5Y_1.5-xSc_xSc_yAl_5-yO₁₂ (x = 0.2 ÷ 1.5; y = 0.2 ÷ 0.8) were prepared. An increase of scandium content in the dodecahedral garnet site has led to a temperature decrease of the shrinkage onset and the maximum shrinkage rate, as well as vacuum sintering temperatures narrowing. The decomposition of solid solutions with a high scandium dodecahedral content, accompanied by an optical quality deterioration due to mechanical mixtures formation, has been detected. According to the SEM data, the decomposition processes are accompanied by abnormal grains growth. Optical ceramics’ samples transmission coefficients were about 80% in the visible and near-IR ranges. An unique “SAG:Er” material (Er_1.5Sc_1.7Al_4.8O₁₂), characterized by the minimum dodecahedral radius and the unit cell parameter compared to the known erbium-doped garnet matrixes has been obtained.     

In-doped LiCa2.98MgV3O12 rare-earth-free phosphor with a high photoluminescence quantum yield of 67.4%

Rare earth resources applied in numerous novel materials, are increasingly scarce in worldwide. Meanwhile, great attention has been focused on the blue rare-earth-free fluorescent phosphors as an excitation source of near-ultraviolet (NUV) chips, which are available in white-light-emitting diodes (WLEDs) that combined with yellow commercial phosphors. Many solutions have been found to improve the photoluminescence quantum yield (PLQY) of phosphor. In this study, we effectively increased PLQY by introducing indium ion to vanadate garnet phosphor forming a series of LiCa_3−XMgV₃O₁₂: xIn³⁺ (x = 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10). The resultant In-doped vanadate garnet phosphors exhibit a relative increase in PLQY, and the optimal phosphors is LiCa_2.98MgV₃O₁₂: 0.02In³⁺ with a PLQY as high as 67.4%. These In-doped vanadate garnet phosphors have a broad emission at 480 under 320 nm where near UV-light excitation. It is revealed that the distortion of V-V and V-O bond length in [VO₄] crystal structure and unit cell volume data of phosphors have an extensive influence on PLQY. Therefore, these novel vanadate garnet phosphors can be the essential blue color choice for WLED devices application in illumination.     

Formation of YAIO3 with Garnet structure

Amorphous material, which yields a YAlO₃ powder, has been prepared by the simultaneous hydrolysis of yttrium and aluminum alkoxides. A phase with a garnet structure is formed as an intermediate product. The formation process of YAlO₃ is described.     

Temperature-related changes in the structure of YSAG:Yb garnet solid solutions with high Sc3+ concentration

The paper presents the results of the Sc³⁺ solubility limits estimation in dodecahedral and octahedral garnet sites for 1100–1700 °C temperature range. The region of existence of solid solutions with the garnet structure in the Y₂O₃-Al₂O₃-Sc₂O₃ and Yb₂O₃-Al₂O₃-Sc₂O₃ systems narrows with increasing synthesis temperature. The Sc³⁺ solubility in the dodecahedral site of the YbSAG and YSAG garnets decreases with increasing temperature, and at 1700 °C it is about 64 at. %. In the octahedral site of YbSAG garnet, the Sc³⁺solubility decreases from 89.5 to 66.5 at. % with increasing temperature, and in the YSAG system it is about 97.5 at.%. The overall Sc³⁺ solubility with the simultaneous introduction into both garnet sites is lower than with doping on one of the sites and also decreases with increasing temperature. The thermal stability of the compositions belonging to single-phase regions was confirmed by vacuum sintering up to 1850 °C.     

Formation enthalpy and magnetic properties of Bi-YIG powders

The formation enthalpy and magnetic properties of bismuth-doped yttrium iron garnet powders were investigated. The formation enthalpy and the crystallization temperature both decreased with increasing bismuth substitution for Bi_xY_3−xFe₅O₁₂ (0.25 ≦ x ≦ 1.25) powders prepared by the coprecipitation process. Bi substitution for Y can significantly reduce crystallization temperature for bismuth-doped yttrium iron garnet powders, and the magnetic properties (saturation magnetization, remanence, and coercive force) are independent of Bi substitution amounts. The average particle size has been determined by the specific surface area. As Bi substitution for Y increased, the average particle size also increased, while the specific surface area decreased.     

Effect of synthesis methods and aging on synthesis of uvarovite garnet by ceramic and sol–gel processes

Abstract

The uvarovite garnet (CaO)₃ (Cr₂O₃ )(SiO₂ )₃ has been synthesised (mineralised with borax to facilitate diffusion of precursors) by several sol–gel methods. Two routes for uvarovite formation have been observed: where CaCrO₄ forms as an intermediate phase; and where metastable pseudowollastonite (α-CaSiO₃ ) forms as an intermediate phase. Synthesis via CaCrO₄ appears to be more reactive in unaged samples. The reactivity of samples can be directly related to the chemical homogeneity of raw powders, and two methods of synthesis, Pechini and alkoxide, were found to stand out for their reactivity. Aging of raw powders for 6 months inhibits nucleation. Consequently, samples without nucleating agents (i.e. chlorides), such as gels from alkoxides, lead to the stabilisation of amorphous material or metastable phases (α-CaSiO₃ ) and, as a result, hinder uvarovite formation. In contrast, aged samples involving heterogeneous nucleation agents, as in the Pechini method with CaCl₂ as precursor, enhance reactivity.     

The enhancement of the Ce-solubility limit and saturation magnetization in the Ce0.25BixPryY2.75-x-yFe5O12 garnet synthesized by the conventional ceramic method

This paper has focused on the structural and magnetic properties of Ce_0.25Bi_xPr_yY_2.75-x-yFe₅O₁₂(x = 0.15, 0.20, 0.25) and (y = 0.25, 0.5) prepared by the solid-state method. The structural and microstructural evaluations were carried out by the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Raman Spectroscopy (RS) and Fourier-transform infrared spectroscopy (FTIR). Furthermore, the magnetic properties were studied through a vibrating sample magnetometer (VSM). As it can be obtained from the XRD patterns, a single garnet phase with high purity has been formed in all samples in the sintering condition. Also, the FTIR analysis confirmed the formation of the garnet phase and related inorganic bonding. Raman spectroscopy reveals the dominant garnet phase with good incorporation of the Bi, Pr and Ce dopants into the garnet structure. XPS confirmed the presence of Ce⁴⁺ and also its value was measured from the peak areas. The results showed that the amount of Ce⁴⁺ in all the samples is lower than 10%. According to the results, by increasing the Bi content from x = 0.15 up to x = 0.25, saturation magnetization (M_s) increased from 29.31 to 37.79 emu/g, and by enhancing the Pr concentration from x = 0.25 to x = 0.5, this magnetic characteristic enhanced from 29.31 to 33.16 emu/g. Furthermore, the Ce-solubility limit was enhanced from x = 0.2 to x = 0.25 by the charge compensation mechanism. An increase in the Ce-solubility limit and also the saturation magnetization led to an enhancement in the magneto-optical and magnetic properties of YIG for telecommunication applications and memory storages, respectively.     

Cation and polyhedron substitution strategies: Effects on local crystal structure and on Bi3+ and Eu3+ co-doped inverse garnet phosphors’ luminescence property

Following the rapid growth of lightning technology, the development of red-emitting phosphors is effective for improving color temperature and color rendering index for w-LEDs devices. Herein, a single phased garnet phosphor with cation and polyhedron substitution modification was firstly prepared. For Mg₃Gd₂Ge₃O₁₂: Bi³⁺, Eu³⁺, the intensity has been remarkably improved by about 16% compared to the one without Bi³⁺ sensitization. The energy transfer mechanism is identified in this work. Based on cation and polyhedron substitution strategies, novel phosphors with different compositions were obtained and further modified the PL properties. With Lu³⁺ substitution, the bond lengths between Bi³⁺ ion and anion ligands are decreased and the site symmetry has been strengthened, which leads to a 21 nm blue shift when Lu³⁺ totally replaced Gd³⁺ ions. In addition, Lu³⁺ and [SiO₄] substitution strategies both effectively increased symmetric rigid structure, which leads to a significant improvement in thermal stability, indicating the samples own great potential in optical applications This work provides a new insight to synthesis red-emitting phosphors for warm white-LEDs.     

High-entropy transparent ceramics: Review of potential candidates and recently studied cases

High-entropy ceramics have been widely explored and extensively studied since the first demonstration of the configuration entropy stabilized reversible transitions between multiple and single phases by Rost et al. in 2015. Most of the current research on high-entropy ceramics has focused on properties like thermal conductivity, thermoelectricity, structures, and others. Some recent studies have extended the high-entropy concept to the field of transparent ceramics. We reviewed these papers and proposed four potential ceramics groups for high-entropy transparent ceramics including fluoride ceramics, fluorite-deficient and/or ordered pyrochlore A₂B₂O₇ ceramics, garnet ceramics, and sesquioxide ceramics. In this article, we review ceramic powder synthesis, the fabrication of transparent ceramics, high-entropy ceramics, and limited cases of high-entropy transparent ceramics for each category. High-entropy transparent ceramics with diverse compositions and structures will provide more possibilities for functional transparent ceramics in the future.     

Rapid synthesis of Y3Al5O12 powders via plasma electrolysis

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) is an important functional material. However, the strict and complicated preparation has limited its wide application. This study aimed to rapidly synthesize Y₃Al₅O₁₂ by plasma electrolysis for the first time. The prepared powder was studied from topography, structure and elements by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The powder had a good crystal form with a spherical shape. The single kind of diffraction peak of Y₃Al₅O₁₂ in XRD revealed the high purity of the synthesized powders. The study of the relationship between the applied voltage and the synthesized powder revealed a threshold voltage of 210 V under the present condition. The higher voltage led to the damage of the electrode due to excessive heat. The synthesis of the YAG powder had a melt-quench process. The two processes were carried out at the same time.     

Combustion synthesis of compounds in the Y2O3-Al2O3 system

In the Y₂O₃-Al₂O₃ system, compounds Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG), YAlO₃ (yttrium aluminum perovskite, YAP), and Y₄Al₂O₉ (yttrium aluminate monoclinic, YAM) are well known. YAG and YAP are of considerable technological importance. Conventional solid-state reaction techniques require high sintering temperatures (above 1800°C) to prepare phase-pure compounds. Though several soft chemical routes have been explored for synthesis of YAG, YAP and YAM, most of these methods are complex. Moreover, phase-pure materials are not obtained in one step and prolonged annealing at temperatures around 1000°C is necessary. In this paper, one-step combustion synthesis of these compounds is reported using a modified procedure and employing mixed (glycine + urea) fuel. Phosphors based on Ce³⁺ activation were also prepared and exhibited characteristic photoluminescence.     

Phase evolution and chemical stability of Nd-doped natural garnet waste form by microwave sintering

Advanced materials and processes are needed to immobilize fission products in the form of chemically durable waste. Nd-doped Ca₃Zr₂Fe₂SiO₁₂ (GZG) ceramics with different sintering temperatures (1100–1300 °C), sintering times (0.5–6 h) and solid solutions (0 ≤ x ≤ 2.5) were prepared by microwave sintering. The results show that by adjusting a series of key parameters, natural garnet structures with excellent chemical stability are obtained. XRD shows that a single phase of solid solution can be obtained by increasing the sintering temperature and controlling the actinide solid solution below the solid solution limit. The sintering temperature only affects the compactness of the solid solution. Thermodynamic calculations show that high temperature is favorable for the synthesis of GZG, and it is positive and irreversible. The normalized leaching rate of Nd³⁺ in the GZG structure is approximately ～ 10^?6 g m^?1 d^?1. The release form of Nd in aqueous solution is inner diffusion. The experimental results show that the naturally occurring garnet structure is an excellent waste form.     

Modern high-density oxide ceramics with a controlled microstructure. Part VI. Fabrication of light-transmitting oxide materials

Results obtained in fabrication of light-transmitting ceramics based on yttrium-aluminum garnet are presented. The best optical properties were exhibited by ceramics with HfO₂, ZrO₂, or Sc₂O₃ additives introduced through salts by coprecipitation. The properties of light-transmitting ceramics made of yttrium-aluminum garnet and materials based on them are described and the ceramics are shown to have good prospects in various fields of engineering.     

Rapid synthesis of YAG phosphor by facile mechanical method

In this study, synthesis of yttrium aluminum garnet (YAG):Ce³⁺ phosphor powders for white light emitting diodes was investigated by mechanical method using the attrition-type mill with no external heating and no flux in dry phase. High mechanical energy input to the starting powder mixture of Y₂O₃, Al₂O₃ and CeO₂ achieved the synthesis of YAG:Ce³⁺ without any flux materials. X-ray diffraction patterns of the processed powders after 5 min processing revealed the peaks of YAG were clearly identified. The maximum temperature of the mill chamber during the processing was 240℃. The YAG phosphor obtained by the mechanical method revealed the internal quantum yield of 65% in the case of the sample mechanically processed under a reducing atmosphere. The synthesized powder showed granule structure consisting of submicron size of YAG particles, which is better handling for the fabrication of light emitting diode devices.     

Combining Soft Chemistry and Spark Plasma Sintering to Produce Highly Dense and Finely Grained Soft Ferrimagnetic Y3Fe5O12 (YIG) Ceramics

We report the synthesis of yttrium iron garnet (YIG) combining soft chemistry route, namely the polyol process, and spark plasma sintering (SPS) technique. The polyol process produced an intermediary amorphous phase containing both iron and yttrium cations in the desired ratio. They were annealed at 400°C in air to decompose the organic content of the reaction (polyol and acetate). To achieve the garnet phase, the polyol‐obtained precursor was subjected to reactive SPS treatment at a temperature of 750°C, far below the typical temperatures (1350°C) used in the classic solid‐state reaction process. In 15 min pure and high‐density Y₃Fe₅O₁₂ ceramic, with about 100 nm sized crystalline grains, was obtained. We report as well the characterization of the initial amorphous phase and the obtained YIG by X‐ray diffraction, scanning electron microscopy, Fourier‐transform infrared spectroscopy, ⁵⁷Fe Mössbauer spectrometry, and magnetization measurements.