Synthesis of spherical yttrium aluminum garnet via a mixed solvothermal method

Abstract

Yttrium aluminium garnet (YAG) powders were synthesized by a mixed solvothermal method. The crystal structure of the YAG powders was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The impacts of the mixed solvents, the synthesis temperature and the polymer additive on the spherical grain formation and the crystal structure of the YAG has been researched by changing the synthetic conditions. The results show that spherical YAG powders were obtained using ethylenediamine–ethanol as mixed solvents with polyethylene glycol as additive at 220 °C for 20 h. Pure spherical YAG particles of 100–200 nm in diameter are formed directly in the process of mixed solvothermal reaction. The degree of crystallization and the crystal structure of the YAG phase have been improved with the increase of the synthesis temperature. The particles tend to be spherical and the distribution of particle size is homogeneous as expected. Finally, in this paper we investigated the formation mechanism of spherical YAG particles.

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Introduction

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) as an important functional material attracts the focus of the world.¹ YAG doped with a transition metal or lanthanide ions acts as an important solid state laser material which is widely used in luminescence systems,² window materials for a variety of light sources,^3,4 and for fiber optic telecommunication systems.⁵ The YAG has been considered as an ideal candidate for phosphors in projection cathode-ray tube systems,⁶ field emission,⁷ vacuum fluorescents,⁸ electroluminescent displays⁹ and as scintillators in X-ray and positron emission tomographs.¹⁰ YAG:Nd single crystal, used as solid state laser material, was variously applied in medicine, materials processing, military and research. YAG transparent ceramics prepared from YAG ultra-fine particles exhibit many advantages in material properties.^11–13 YAG:Tb is a characteristic narrowband phosphor suitable for contrast-enhanced display applications in high ambient illumination conditions. It is important to develop phosphors with high quantum efficiency, controlled morphology and small particle sizes for enhancing the brightness and resolution of these displays.^14,15 Phase-pure YAG powders with optimum morphology, mono-dispersed spherical shape, suitable average size and narrow size distribution are necessary to achieve the application requirements. However, obtaining YAG in a single phase has not been a simple task even if strictly do according to the stoichiometric ratio of ingredients, because of the three well-known intermediate compounds in the Y₂O₃–Al₂O₃ binary system of the crystal: yttrium aluminum monoclinic (Y₄Al₂O₉, YAM), yttrium aluminum perovskite (YAlO₃, YAP), and Y₃Al₅O₁₂ (YAG).¹⁶ YAG is normally synthesized by the conventional solid-state reaction at high temperatures (>1400 °C) between aluminum oxide and yttrium oxide. However, the conventional process has some disadvantages such as inappropriate particle sizes, stoichiometry impurities, formation of undesirable phases, etc. While high temperature firing is necessary for crystallization, it can introduce defects and impurities.^17–19 Various synthesis methods have been developed to fabricate the YAG-based materials for overcoming the drawbacks of the solid-state reaction process, such as hydrothermal synthesis,¹⁴ sol–gel method,^20,21 spray-pyrolysis synthesis,²² microwave-induced combustion process²³ and the hydroxide coprecipitation method.^24–27

YAG powders can be obtained by hydrothermal or solvothermal synthesis method at lower temperature and pressure using water or organic solvent, that can avoid the disadvantages described above.^28–30 YAG powders, with well-dispersed fine spherical grains and a relatively narrow grain size distribution, can be synthesized in the ethanol–water mixed solvent at lower temperature and pressure from inexpensive starting materials and precursor by Zhang et al.³¹ Wu et al.³² prepared the sub-micron sized YAG powders by the solvo-thermal method under mild conditions with ethylenediamine solution as the solvent at the temperature of 200 °C for 5 h.

YAG particles have been synthesized via mixed solvothermal method process by using ethylenediamine–ethanol as mixed solvents. Optimizing the synthesis conditions and adding additives (polyvinyl alcohol, arable gum, polyethylene glycol) can improve the synthetic performances of YAG powders. YAG crystal formation mechanism is discussed in this paper to provide the basis for better application of YAG powders.

Experimental

The yttrium and aluminum sources for YAG synthesis were Y₂O₃ (powder, 99.99%, Sinopharm Chemical Reagent Co., Ltd) and Al(NO₃)₃·9H₂O (99.99%, Tianjin Bodi Chemical Co., Ltd), respectively. Polyvinyl alcohol (1750 ± 50), arable gum, polyethylene glycol (2000) (Sinopharm Chemical Reagent Co., Ltd) as additives; ammonia–ethanol, n-butylamine–ethanol, ethylenediamine–ethanol (Sinopharm Chemical Reagent Co., Ltd) as mixed solvents were used in this work. At first, Y(NO₃)₃ solution was obtained by dissolving Y₂O₃ in nitric acid, and adding distilled water to make Y(NO₃)₃ solution whose concentration was 0.3 mol L⁻¹. Then appropriate amount of Al(NO₃)₃·9H₂O was dissolved in the above solution, in which the mol ratio of the Y³⁺, and Al³⁺ was maintained as 3 : 5. The ammonium hydrogen carbonate (AHC) solution was prepared by adding 10 g of NH₄HCO₃ (Shanghai no. 4 Reagent & H.v Chemical Co., Ltd.) into 100 mL of distilled water, and its pH value was 7.7. At last, the precipitate was prepared by adding the precursor salt solution at a speed of 0.5 mL min⁻¹ into the above AHC mixture solution under vigorous agitation at room temperature. The precipitate was washed twice with ethanol. The 22.5 mL solution of precipitate which dispersed in mixed solvent of ammonia–ethanol, n-butylamine–ethanol, ethylenediamine–ethanol respectively was placed in an 25 mL autoclave. The fill-factor of the autoclave was 90%. The autoclave was heated at 210–250 °C and heating rate of 2 °C min⁻¹ for 15–20 h. After cooling to room temperature, the products in the autoclave were washed then dried at 120 °C in the air for further study. The experimental conditions are shown in Table 1. The grain size was calculated according to Scherrer equation.³³

Table 1 All experimental conditions

	Sample name	Temperature/°C	Holding time/h	Precursor/g	Mixed solvent/mL	Polymer additive
S1	Ammonia–ethanol = 20 : 1	220	20	4.5	18	Without additive
S2	n-Butylamine–ethanol = 20 : 1	220	20	4.5	18	Without additive
S3	Ethylenediamine–ethanol = 20 : 1	220	20	4.5	18	Without additive
S4	Ethylenediamine–ethanol = 20 : 1	180	15	4.5	18	Without additive
S5	Ethylenediamine–ethanol = 20 : 1	210	15	4.5	18	Without additive
S6	Ethylenediamine–ethanol = 20 : 1	230	15	4.5	18	Without additive
S7	Ethylenediamine–ethanol = 20 : 1	250	15	4.5	18	Without additive
S8	Ethylenediamine–ethanol = 20 : 1	220	20	4.5	18	Polyethylene glycol
S9	Ethylenediamine–ethanol = 20 : 1	220	20	4.5	18	Arable gum
S10	Ethylenediamine–ethanol = 20 : 1	220	20	4.5	18	Polyvinyl alcohol

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The structure of the powders was analyzed by X-ray diffraction (XRD, PAnalytical X'Pert PRO, Cu Kα, λ = 0.15418 nm). The morphology, size and dispersion of powder were observed by a transmission electron microscope (TEM; Hitachi model H-800) and a scanning electron microscope (SEM; Quanta200ESEM FElco-Holland). The size distribution of the powders was observed using a laser scattering particle size distribution analyzer (Model LA-950, Horiba, Japan). FT-IR measurements were performed with a KBr wafer to evaluate the chemical composition of the samples. Infrared spectra were recorded in the region 4000–400 cm⁻¹ (Model Tensor 27, Switzerland, Bruker).

Results and discussion

Effect of mixed solvents on synthesis reaction

YAG powders were synthesized by three different kinds of mixed solvents at 220 °C for 20 h to explore the influence of mixed solvents on synthesis reaction. The XRD patterns of YAG powders with different mixed solvents as shown in Fig. 1. The powders prepared in ammonia–ethanol (S1) are composed of Y(OH)₃, YOHO and AlO(OH) phases in Fig. 1a, and the AlO(OH) particles had mainly fibrous and thin layers shape as shown in Fig. 2b. The powders are amorphous when using the n-butylamine–ethanol (S2) as a solvent for synthesis of samples (Fig. 1b). While the mixed solvents are ethylenediamine–ethanol (S3), pure YAG phase without any impurity phase is obtained as shown in Fig. 1c. The YAG powders are mainly composed of well dispersed and nearly spherical fine grains (100–250 nm) (Fig. 2c and d).

Fig. 1 The XRD patterns of YAG powder prepared with different mixed solvents ((a) ammonia–ethanol (S1), (b) n-butylamine–ethanol (S2), (c) ethylenediamine–ethanol (S3)).

Fig. 2 TEM images of precursor precipitate (a) AlO(OH) (S1) (b) and YAG particles obtained at 220 °C (S3) (c) and SEM images of YAG particles at 220 °C (S3) (d).

The results indicate that being different composition and performance, different solvents have different adaptability and dissolving ability to the precursor in the process of reaction, which led to different nucleation and material transfer, and have different effects on synthesis reaction of YAG. Ethylenediamine is one kind of strong polar solvent and alkaline, which has strong dissolving and complexing ability and is easy to react with inorganic ions for crystal nucleation and growth.^34,35 Therefore, compared with other solvents, ethylenediamine–ethanol acted as mixed solvents can provide the formation of YAG with the appropriate reaction conditions to enhance solubility, diffusion and crystallization, which lead to the synthesis of pure phase YAG powders in low temperature. It is mild enough to make molecular building blocks to participate in a solid-state YAG phase.

Effect of temperature on synthesis reaction

Our previous research results show that the suitable synthetic reaction time range of the solvothermal method was 15–20 h and longer reaction time is advantageous to the synthetic reaction. But, the reaction time being too long can lead to high temperature and abnormal grain growth. So we chose 15 h to study the effect of temperature. Fig. 3 shows the XRD patterns of the powders prepared at different temperature for 15 h with ethylenediamine–ethanol as mixed solvents. The YAG powders prepared at 180 °C (S4) are amorphous in Fig. 3a. Pure YAG powders were obtained at 210 °C (S5), but the characteristic peak intensity of YAG powders is weak and the widening of diffraction line profile appears (Fig. 3b). The powder grain size is about 160 nm accompanying with agglomerative phenomenon (Fig. 4a). The diffraction peaks of YAG phase become acute and the crystallinity of the YAG powders is enhanced as the synthesis temperature reaches to 230 °C (S6) (Fig. 3c). The particle size of powders prepared at 230 °C is in a range between 150 nm and 500 nm. The particle size distribution of the powders is heterogeneous, and the shape of particles is irregular (Fig. 4b). When the reaction temperature reaches to 250 °C (S7), the characteristic peak intensity and crystallinity of YAG powders are the largest (Fig. 3d). The particle size of powders prepared is about 1.4 μm, and the particle size distribution tends to be uniform with the spherical shape of particles (Fig. 4c).

Fig. 3 XRD patterns of synthetic powder at [a] 180 °C (S4), [b] 210 °C (S5), [c] 230 °C (S6) and [d] 250 °C (S7) for 15 h with ethylenediamine–ethanol as mixed solvents.

Fig. 4 SEM photograph of synthetic powder prepared at [a] 210 °C (S5), [b] 230 °C (S6), [c] 250 °C (S7) for 15 h with ethylenediamine–ethanol as mixed solvents.

It is noticed that the crystallinity is enhanced and particle size of YAG powders has increased with temperature increasing, and the particles have a higher regularity. The distribution of particle size has become more uniform. Because the pressure of reaction system rises with increasing reaction temperature, the driving forces of grain nucleation and grain growth are enhanced, and the rate of grain nucleation and mass transfer speed up meanwhile. Large amounts of grain can grow up in a relatively short period of time evenly for a larger size and integrity structure.

Effect of polymer additive on synthesis reaction

Fig. 5 shows the X-ray diffraction patterns of the products synthesized with or without additive at the 220 °C for 20 h using ethylenediamine–ethanol as mixed solvents. The X-ray diffraction patterns of the products synthesized with polyvinyl alcohol (S10) (Fig. 5b) and polyethylene glycol (S8) (Fig. 5d) all show that XRD characteristic peak of YAG powders weakens after adding additive, namely YAG crystallinity decreased than that of the products synthesized without additive (S3) (Fig. 5a). It can be considered that the polymer adsorbed on the different surface of grains, where changed the surface free energy and hindered the growth of the crystal surface to a certain extent. The growth habit of YAG cubic phase has changed, and inhibited recrystallization of crystal particle with the crystallinity decreased. Arable gum additive (S9) has weaker influence on the crystallinity of powder than polyvinyl alcohol and polyethylene glycol (Fig. 5c).

Fig. 5 X-ray diffraction pattern of the products synthesized with or without additive at 220 °C for 20 h with ethylenediamine–ethanol as mixed solvents ((a) without additive (S3), (b) polyvinyl alcohol/18 mmol L⁻¹ (S10), (c) arable gum/0.06 mmol L⁻¹ (S9), (d) polyethylene glycol/4 mmol L⁻¹ (S8)).

Furthermore, by comparing with TEM and SEM images of YAG powders synthesized with (Fig. 6) and without (Fig. 2c) additive, the results show that different additives have different effects on particle size, size distribution and degree of sphericity of YAG powders. Fig. 6a and b show that the average particle size and size distribution of YAG particles prepared with polyvinyl alcohol (18 mmol L⁻¹) (S10) as additive become smaller than that in Fig. 2c. Fig. 6c and d are TEM morphologies of YAG powders prepared with arable gum (0.06 mmol L⁻¹) (S9) as additive. By contrast, the morphology of YAG particles is nearly spherical, the size distribution has become narrow, and the particle size decreases to 100 nm after adding arable gum in the ethylenediamine–ethanol mixed solvents, but the sphericity goes are bad. Fig. 6e is SEM image of YAG powders prepared with polyethylene glycol (4 mmol L⁻¹) (S8) as additive. The resulting particles have a good spherical shape and well dispersion. The grain size distribution of the powders synthesized with polyethylene glycol (S8) in Fig. 6f shows that the grain size range is about 100–200 nm. Comparing with the S3 sample in Fig. 2c, the grain size of YAG powders (S8) decreases obviously. The polymer molecules were adsorbed on crystal surface, which would reduce the surface energy, inhibit the aggregation of powder particles, and limit the growth of grains because of the dangling bond saturation on the surface of grain. This could be one reason which leads to the above phenomenon.

Fig. 6 Morphology characterization of the powder synthesized with different polymer additives heated at 220 °C for 20 h with ethylenediamine–ethanol as mixed solvents. (a and b) TEM images of the powder synthesized with polyvinyl alcohol/18 mmol L⁻¹ (S10), (c and d) TEM images of the powder synthesized with arable gum/0.06 mmol L⁻¹ (S9), (e) SEM images of the powder synthesized with polyethylene glycol/4 mmol L⁻¹ (S8), (f) grain size distribution of the powder synthesized with polyethylene glycol (S8).

Fig. 7 is the illustration of the possible function of polymer additive in controlling the growth of YAG powders. The polymer additive adsorbed onto crystal surfaces, changed the relative surface free energies of the surfaces and blocked sites essential to the incorporation of new solute into the crystal lattice, which may cause changes in growth kinetics and habit modification of the YAG cubic phase. The polymer was adsorbed on crystal surface which reduces the critical crystal nucleus, reduction of the nuclear power and improves the nucleation rate. The additive also promoted the YAG cubic phase formation, decreased the surface energy or the edge energy of the nuclei by saturation of dangling bonds on the crystal surface, which will decrease the crystallization of the YAG cubic phase. At the same time, the polymer additive could inhibit the aggregation of small particles.³⁶

Fig. 7 Illustration of the possible function of polymer additive in controlling the growth of YAG powders.

Formation mechanism of YAG

Both precursor precipitate and YAG powder are further investigated using Fourier transform infrared (FTIR) spectroscopy as shown in Fig. 8. The broad band at 3411 cm⁻¹ is ascribed to the O–H stretching vibration of water, 1620 cm⁻¹ to O–H bends of the crystal water and absorbed water as shown in Fig. 8a. There are four absorption bands at 1521, 1417, 1088 and 846 cm⁻¹, which are stretching vibration of the CO₃²⁻ vibration. The bands at 3411 cm⁻¹ and 1620 cm⁻¹ weaken in Fig. 8b and c, indicating that there was the dehydration reaction in the synthesis. The bands at 1521 cm⁻¹, 1417 cm⁻¹, 1088 cm⁻¹ weaken, implying that CO₃²⁻ is decomposing in the process of reaction. The IR spectra show that the bands at low frequency had splitting and new bands at 786, 723, and 499 cm⁻¹ appeared (indicating that Y–O, Al–O bond formation, and began to form aluminum oxygen tetrahedron and yttrium oxide polyhedron with YAG crystallization) at 210 °C for 15 h as shown in Fig. 8b. The bands at 3411 cm⁻¹, 1620 cm⁻¹, 1521 cm⁻¹, 1417 cm⁻¹ disappear with the temperature reaching to 250 °C for 15 h (Fig. 8c), which reveal the dehydration reaction has finished, and CO₃²⁻ decomposed completely. The bands at 786, 723, and 499 cm⁻¹ are enhanced with the formation of a large number of Al–O tetrahedral and octahedral, which indicated Al–O and Y–O bond strength has been enhanced with reaction finished mostly.³⁷

Fig. 8 FTIR spectra of precursor precipitate (a) and YAG powder at 210 °C (S4) (b) and 250 °C (S7) (c) for 15 h.

Combined with the XRD patterns of the powders prepared at different temperatures (Fig. 3), formation mechanism of YAG would be speculated. In the initial reaction period, the solubility of A1(OH)₃ and Y₂(CO₃)₃·nH₂O is quiet low in ethylenediamine–ethanol solvents. With the increase of reaction temperature, the union between mixture is broken, and evenly dispersed in the mixed solvents. Precursors dissolve in ethylenediamine–ethanol mixed solvents and generate metal complex Al(en)₂(OH)²⁺, Y(Ce)(en)₂(OH)²⁺ which is called growth units of crystal. The concentration of Al³⁺ and Y³⁺ in solvent reaches supersaturation state with dehydration reaction, and forms a large number of Al–O tetrahedron and octahedron. With increasing of the concentration of molecular groups, the collision between groups increases gradually, and forms four, five and even six coupled Al–O molecular groups. YAG three-dimensional skeleton was formed by interconnection of Al–O polyhedron. Y in dodecahedron gap of three-dimensional skeleton was changed from original four ligand into eight ligand and forming Y–O–Al bonds followed. The polyhedron arrange according to the rules of YAG crystal structure (YAG phase nucleate) meanwhile. As the extension of heat preservation time, the dissolved metal complexing ions gather nucleation and transport from dissolving zone to growing region. Complexation ion crystallizes on the growth surface of the seed crystal. With the crystallization process, dissolving process of the precursors continued because of the concentration of complex metal ions aggregate is lower than the precursor of solubility until the precursor runs out. Grain balanced with dissolving-crystallization in the solvent. Aggregation and recrystallization of powder grains occur with the extension of soaking time. To reduce the total surface free energy of system, YAG particles tend to be spherical. A single phase of YAG crystal with good performance was obtained finally.

Conclusion

Types of solvents, reaction temperature and additives have different effects on the synthesis of YAG and powder properties. The pure spherical YAG particles with the size of around 100–200 nm are formed directly via a low temperature method of the mixed solvothermal reaction by using ethylenediamine–ethanol as solvents with polyethylene glycol (4 mmol L⁻¹) as polymer additive at 220 °C for 20 h. The crystallization temperature of the derived materials was much lower than that used in the conventional solid-state reaction (>1400 °C). The pure spherical YAG particles prepared by this method show a good spherical shape, well dispersion, less agglomerated and narrow particle distribution properties. This synthetic method can be used in the synthesis of YAG:Ce fluorescent powders and YAG:Nd transparent ceramics powders with good potential applications.

Acknowledgements

The authors thank Natural Science Foundation of China (Grant no. 51172132, 51272144 and 51472127) for the financial support.

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