www.ptcer.pl/mccm
1. Introduction
Although a history of white light emitting diodes (wLEDs) is a few years old and wLEDs found a permanent place in the daily life as a source of light, there is still a need to improve them. Phosphors used in wLEDs are responsible for conver-sion of a UV/blue emisconver-sion of a semiconductor to yellow light [1]. The matrix of the phosphors can be made of oxides, oxy-nitrides or oxy-nitrides doped by rare earth ions [2–3]. Oxynitride phosphors like Ca-α-SiAlON doped with Eu2+ are promising blue light convertors in the white light emitting diodes due to strong absorption in the UV area and a broad emission band in the VIS region [4–5]. The α-SiAlON ceramics are usu-ally manufactured via the reactive sintering of a mixture of nitrides and relevant oxide under the nitrogen overpressure [6]. This method cannot be simply transferred to manufactur-ing α-SiAlON powders since an amount of the liquid phase should be limited and sintering of powder particles should be avoided. Consequently, the solid state reaction method, an attractive one for the industry application, must be modifi ed in order to obtain the α-SiAlON powder with particles of a
de-sired shape and size, the repeatable chemical composition of the solid solution and properties. Among other ways of this high-temperature synthesis improvement, the application of the mechanochemical processing (MCP) of the initially mixed powders before the thermal treatment, could be the alternative one. The MCP should allow reduction of the syn-thesis temperature, better homogenization of powders along with a more effi cient distribution of RE cations [7]. The MCP is a well-known method of the powder particles refi nement and particle deformation, but it also can initiate the chemical reactions between particles during milling [8]. On the other hand, application of high-energy ball milling to the nitride compounds will result in their oxidation, even if performed in the protective atmosphere. Moreover, a signifi cant damage of the silicon nitride crystal structure will affect its ability to volatization in a reducing atmosphere at high temperatures [9] thus possible changes in Si:Al ratio could occur. There is a report on application of the MCP process to formation of Ca-α-SiAlON and resultant optical properties [10], but apart from enhanced ability for SiAlON crystallization at lower tem-perature and more homogeneous europium ion distribution,
D
ANIELM
ICHALIK1*, T
OMASZP
AWLIK1, M
AŁGORZATAS
OPICKA-L
IZER1, R
ADOSŁAWL
ISIECKI21Silesian University of Technology, Department of Materials Science, Krasińskiego 8, 40-019 Katowice, Poland 2Institute of Low Temperature and Structure Research, Okólna 2, 50–422 Wrocław, Poland
*e-mail: daniel.michalik@polsl.pl
The effect of mechanochemical processing
on formation of Ca-α-SiAlON:Eu
2+
powders
Abstract
The Ca-α-SiAlON:Eu2+ powders were developed by combining mechanochemical processing (MCP) of the initial powders with the sub-sequent solid state reaction in the reducing atmosphere of a graphite furnace. The photoluminescence (PL) spectra of the mono-phase Ca-α-SiAlON:Eu2+ phosphor powder were compared to the mono-phase powders produced without the MCP step. Specimens were synthesized in the temperature range of 1450–1650 °C for 2 h under a nitrogen fl ow. Mechanochemical processing of the initial mixture of nitrides led to their surface oxidation and formation of a larger amount of the oxynitride liquid. On the other hand launching the Eu2+ ions among additives during MCP and the subsequent Ca-Al-Si-Eu-O-N eutectic liquid formation during the thermal treatment is responsible for a higher optical performance of the resultant phosphor.
Keywords: Sialon, Oxynitrides, Phosphor, Mechanochemical processing
WPŁYW OBRÓBKI MECHANO-CHEMICZNEJ NA SYNTEZĘ I WŁAŚCIWOŚCI PROSZKÓW Ca-α-sialon:Eu2+
W badaniach uzyskano luminescencyjne proszki Ca-α-sialon:Eu2+ dwuetapową metodą obróbki mechano-chemicznej (MCP) prekur-sorów sialonowych i ich następczej syntezy metodą reakcji w stanie stałym w piecu grafi towym z przepływem azotu. Uzyskany materiał charakteryzował się budową jednofazową. W artykule porównano właściwości luminescencyjne proszków sialonowych, wytworzonych metodą wspomaganą procesem MCP, z proszkiem pozyskanym metodą konwencjonalną. Badane próbki syntezowano w temperatu-rach z zakresu 1450–1650 °C w czasie 2 godzin w przepływie azotu. Proces MCP zastosowany do prekursorów sialonowych prowadził do częściowego powierzchniowego utlenienia się proszków i tworzenia większej ilości fazy ciekłej. Z drugiej strony MCP w układzie Ca-Al-Si-Eu-O-N sprzyja tworzeniu się eutektyki ułatwiającej proces dyfuzji jonów europu, co jest odpowiedzialne za polepszenie właści-wości optycznych badanego luminoforu.
improvement of its optical properties is hardly noticed. It is believed that restraining parameters of the high-energy mill-ing to the homogenization rather than to the destruction of Si3N4 crystal lattice could be benefi cial for the phosphor syn-thesis and europium incorporation into the SiAlON matrix.
In this work Ca-α-SiAlON:Eu2+ was prepared using the MCP method with rather mild energy in order to evaluate the infl uence of MCP on formation, crystallization and fi nally the optical properties of the resultant α-SiAlON phosphor.
2. Experimental
The following commercial powders were used for the preparation of Ca- SiAlON phosphor powders: AlN (Sig-ma-Aldrich, 99.96%), α-Si3N4 (UBE, SN-E10, >98%), CaCO3 (POCH, >99%), Eu2O3 (Treibacher, 99.99%). α-The chemical composition of prepared SiAlONs was Eu0.04Ca0.76Si9.6Al2.4O0.8N15.2 (5 at% of Ca is replaced by Eu;
m = 1,6 and n = 0,8 in the general α-SiAlON formula). The
weighed out relevant amounts of the powders were ground using a Fritsch Pulverisette 7 Premium line planetary mill. A milling rate and a total milling time was 1000 rpm and 60 minutes, respectively. Silicon nitride grinding jars and grind-ing media were used. Two different ball-to-powder (B:P) ra-tios were applied: 4:1 (PM specimens) and 10:1 (AM speci-mens). The milling process was conducted with the 2 ml addition of ethanol as a control agent. The powder without the milling step was also prepared as a reference speci-men (BM). The reference specispeci-men was mixed in an agate mortar for 10 minutes in acetone. The powder mixtures were then dried at 60 °C for 24 h. In order to check the effect of milling on sintering ability, both the powders and pellets were manufactured for the subsequent heat treatment. The pelletized specimens were prepared by uniaxial pressing under a pressure of 120 MPa. The powders or pellets were sintered in a graphite furnace at the temperature range of 1450–1650 °C for 2 h under a nitrogen fl ow.
Particle size distributions of the reference and milled pow-ders were measured using the laser diffraction technique (Malvern Mastersizer 2000) and water as a dispersant. The phase analysis of the phosphor powders was obtained by a Philips X’pert pro diffractometer in the range of 10–60° using a 3 kW Cu lamp with a nickel fi lter. A microstructure of the ceramic pellets and morphology of the powders were observed by using a Hitachi S-3400N scanning electron mi-croscope at 15 kV. Porosity was calculated on the basis of
microscopic images using image binarization and calcula-tions of pixels. Measurements of the optical properties were performed on the powders after hand-grinding in a mortar. Excitation and emission spectra of the phosphor powders were taken with an Optron DongWoo monochromators with 750 mm focal length and detected by a Hamamatsu R-955 photomultiplier. The emission spectra were recorded in the regions of 460–700 nm, excited at 450 nm. All measure-ments were carried out at room temperature.
3. Results and discussion
Table 1 shows the results of particle size distribution measurements for samples after milling (PM and AM) and compared with the starting mixture which was homogenized in the agate mortar (BM specimen). After 60 minutes of mill-ing, the values of particle diameters of the fi nal powders exceed average diameter of particles in the starting mix-ture. A larger increase of particle diameter was observed for the AM powder where the higher B:P ratio was applied. Obviously, such a behaviour is related to the formation of agglomerates during the milling process.
X-ray diffraction patterns of the received powder speci-mens are shown in Fig. 1. The presented results show XRD patterns of three types of specimens: BM, PM, and AM after synthesis at the temperature of 1450 °C, 1550 °C and 1650 °C. Ca-α-SiAlON of the chemical formula Ca0.68Si9.96Al2.04O0.68N15.32 (ICDD 00–042–0252) was used as a reference one. Pure Ca-α-SiAlON was obtained after synthesis at 1650 °C regardless of the MCP application.
The presence of impurities such as β-SiAlON (ICDD 00–48–1616) and α-Si3N4 (ICDD 00–004–6602) was ob-served in specimens synthesized at 1450 °C and 1550 °C and presented in Fig. 1. This trend is observed also for the samples after milling. The main phase of Ca-α-SiAlON was formed at 1450 °C, and it coexisted with still unreacted
Table 1. Particle size distribution of the initial powder mixture (BM) and the powders after 60 minutes milling with the B:P ratio of 4:1 (PM), and 10:1 (AM). BM PM AM D10 [μm] 0.808 0.709 1.133 D50 [μm] 3.046 3.273 6.098 D90 [μm] 14.857 25.672 28.092 a) b) c)
Morphology of the powders after synthesis at 1450 °C is shown in Fig. 2. The powders obtained at this temperature showed the most signifi cant differences in their morphology. In the non-milled BM-1450 specimen (Figs. 2a and 2b) the large agglomerates consist of fi ne (1–2 μm) particles with a diameter close to the initial silicon nitride powder. Some effects of sintering the particles inside agglomerates can be observed, when the initial mixture was milled with lower energy (B:P ratio 4:1) as depicted in Figs. 2c and 2d. The ag-glomerates look denser, and the partial sintering of primary particles is visible inside the agglomerates. On the contrary, morphology of the AM-1450 °C specimens (Figs. 2e and 2f) showed sintered agglomerates in the form of large and hard particles with sharp edges and a particle size of 2–30 μm. If the synthesis was performed at higher temperature the differences among the tested powders were less detect-able. However, after grinding, some dissimilarities among powders could have been noticed, mostly in the shape of cracked edges.
An increase in sintering ability of the milled powders was confi rmed when the pellets were reactive sintered. Fig. 3 shows cross sections of the investigated BM, PM, and AM pellets sintered at 1650 °C.
The microstructure of the cross-sectioned samples con-fi rms the increased ability for sintering of the initial mixture of powders when milled. The pellets prepared from the non-milled powders (BM-1650), shown in Fig. 3a, contain roughly 8.5 vol.% of open porosity with an average pore size of 46 μm. Apart from the large pore population, the small holes of diameter close to 1 μm are also visible, but they were not taken for calculation. The cross-section of the PM-1650 pellet (Fig. 3b) illustrates infrequent volume defects, which were not observed in the AM-1650 pellets originated from the powders milled with higher energy (Fig. 3c). Those results demonstrate various ability for densifi ca-α-Si3N4. The β-SiAlON phase appears after the synthesis at
1550 °C. The shifting of Ca-α-SiAlON peaks was observed with increase of the synthesis temperature. It seems that the composition of newly created α-SiAlON was enriched with the Al-O content. With increase of the synthesis tempera-ture the content of Si-N bond also increases since α-Si3N4 disappears, and the peaks of the main phase are shifted to higher angles. The single Ca-α-SiAlON was observed after synthesis at the temperature of 1650 °C in all the tested specimens thus the infl uence of milling on its formation was not obvious. Besides, the milling process did not infl uence the temperature of the Ca-α-SiAlON formation. However, more detailed studies of the XRD results (Table 2) show differences in positions of the two chosen peaks of speci-mens synthesized at 1650 °C. They are clearly related to the changes in chemical composition of the Ca,Eu-α-SiAlON solid solution. This may be due to a higher oxygen content in the specimens milled with the higher energy (the higher B:P ratio), and to the subsequent change in the well de-scribed process of α-SiAlON formation: formation of the transient liquid phase, solution, diffusion, and precipitation from a saturated liquid [11–12]. The FWHM of peaks of the studied XRD patterns increases proportionally with milling energy. The smallest crystallite size (the largest broadening of the peaks) are observed in the AM specimens milled with the highest energy.
Table 2. FWHM of two peaks of specimens synthesized at 1650 °C.
Specimens BM PM AM 2Θ position of (201) peak [°] 30.548 30.569 30.575 FWHM of (201) peak [°] 0.166 0.191 0.278 2Θ position of (210) peak [°] 34.848 34.888 34.919 FWHM of (210) peak [°] 0.195 0.244 0.293 a) c) e) b) d) f)
Fig. 2. SEM images of morphology of powder specimens after synthesis at 1450 °C: a) and b) without milling, c) and d) low-energy milling, e) and f) high-energy milling.
tion versus milling energy, expressed by the ball-to-powder ratio during the high-energy milling.
Fig. 4 and Table 3 show the emission spectra of the re-ceived phosphor powders. The samples were excited at 450 nm wavelength. The relative intensity was calculated in correlation to the highest emission intensity among all the measured specimens, i.e. the AM specimen from the1650 °C synthesis temperature.
All emission spectra show the broad emission band typical for f-d transition of Eu2+. The milling of the initial mixture substantially changed the quality of the emission spectra, and it also modifi ed the course of the phosphor development at elevated temperatures. The reference specimen shows gradually increasing emission intensity with the increase of synthesis temperature. Very similar behaviour was observed for the specimen milled at the lower ball-to-powder ratio (PM). Both specimens demon-strate the same intensity and position of the wavelength maximum after heat treatment at 1650 °C. The only differ-ence between both specimens could be seen when the phosphors obtained at lower temperatures were excited. Those from the milled powders showed noticeably lower emission intensities and their maximum wavelength was shifted to the slightly lower value. The noteworthy chang-es were visible if the powders were milled with the higher energy: the blue shift and a very low emission intensity in specimens synthesized at lower temperatures. On the other hand, the twofold growth of the emission intensity occurred when this phosphor was synthesized at 1650 °C. Thus the development of the emission with the increas-ing synthesis temperature was similar in both phosphors obtained from the milled powders, but it was more evident in the powders milled with the higher ball-to-powder ratio. Such a course of the phosphor photoluminescence prog-ress must be related to the various evolution of the phase
assemblage during the high-temperature treatment. XRD data showed some differences in the phase composition if the milled powders were heat treated at lower tempera-tures. It must be remembered that α-SiAlON is a solid so-lution in the Me-Si-Al-O-N system, and it is formed as a re-sult of dissolution and precipitation from the liquid phase [11–12]. Consequently, the fi nal chemical composition of α-SiAlON derived from the same mixture of the initial pow-ders could be different if some amount of the liquid phase did not crystallize. The morphology of the low-temperature derived phosphor powders depicted clearly differences in an amount of the liquid phase in different specimens. On the other hand, the obvious blue shift of the emission in the milled powders indicates the changes of the Eu2+ chemical environment in the fi rst coordination sphere. Since the chemical bond of the Eu2+ cation with oxygen is more ionic than with nitrogen we could deduce that Ca,Eu-α-SiAlON derived from the milled powders is oxy-gen richer than the reference phosphor. This observation is consistent with the observed larger amount of the liquid phase in the milled powders (Fig. 2). The intensive milling of the nitride powders could increase their specifi c surface area thus some oxidation of nitrides particles could occur. On the other hand, a larger amount of the transient liquid phase during synthesis at 1650 °C increased the means of ions transport thus it must have been benefi cial for incorporation of Eu2+ cation into the α-SiAlON structure since the emission intensity grew considerably comparing to the reference phosphor.
4. Conclusions
Application of high energy milling of the initial powders mixture affected their ability for formation of Ca,Eu-α-SiAlON, and signifi cantly improved sintering capability. It has
a) b) c)
Fig. 3. SEM images of cross-section of pellets reactive-sintered at 1650 °C: a) BM, b) PM, and c) AM.
a) b) c)
been found that the emission intensity of the phosphors from the milled powders was meaningfully higher in comparison to the reference ones and those milled with lower ball-to-powder ratio. The blue shift of emission wavelength in the milled powders proves changes in the chemical environment of Eu2+ optical activator, and it indicates the key role of an oxygen amount in the formation of Ca,Eu-α-SiAlON crystal structure. Nevertheless, the thermal treatment temperature of 1650 °C is essential for synthesis of the Ca-α-SiAlON:Eu2+ phosphor with or without milling.
Acknowledgements
The fi nancial support by NCN under the project No: 2011/01/B/ST8/07480 is gratefully acknowledged.
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Received 21 October 2016, accepted 10 November 2016. Table 4. Relative intensity of emission and peak position of Ca-α-SiAlON:Eu2+.
Temperature [°C]
Specimen
BM PM AM
Rel. int. Wavelength
[nm] Rel. int. Wavelength [nm] Rel. int. Wavelength [nm] 1450 0.18 572 0.09 570 0.05 565 1550 0.34 572 0.23 570 0.08 565 1650 0.57 578 0.57 578 1 575
