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yttria nanocomposite powders using different precipitants
Materials Chemistry and Physics xxx (2016) 1e9
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A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants G. Kafili a, B. Movahedi a, *, M. Milani b a b
Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 81746-73441, Iran Faculty of Advanced Materials and Renewable Energy Research Center, Tehran, Iran
h i g h l i g h t s Urea proved to be an appropriate precipitant for obtaining a core-shell alumina/yttria nanocomposite. Alumina/yttria nanocomposite powders with more appropriate morphology and highly sinterability. A fine-grained YAG ceramic was obtained by SPS of alumina-yttria nanocomposite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 28 July 2016 Accepted 6 August 2016 Available online xxx
Alumina/yttria nanocomposite powder as an yttrium aluminum garnet (YAG) precursor was synthesized via partial wet route using urea and ammonium hydrogen carbonate (AHC) as precipitants, respectively. The products were characterized using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and energy dispersive spectroscopy. The use of urea produced very tiny spherical Y-compounds with chemical composition of Y2(CO3)3$nH2O, which were attracted to the surface of alumina nanoparticles and consequently, a coreshell structure was obtained. The use of ammonium hydrogen carbonate produced sheets of Y-compounds with chemical composition of Y(OH)CO3 covering the alumina nanoparticles. A fine-grained YAG ceramic (about 500 nm), presenting a non-negligible transparency (45% RIT at IR range) was obtained by the spark plasma sintering (SPS) of alumina-yttria nanocomposite synthesized in the urea system. This amount of transmission was obtained by only the sintering of the powder specimen without any colloidal forming process before sintering or adding any sintering aids or dopant elements. However, by spark plasma sintering of alumina-yttria nanocomposite powder synthesized in AHC system, an opaque YAG ceramic with an average grain size of 1.2 mm was obtained. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramics Nanostructures Optical materials Sintering
1. Introduction Yttrium aluminum garnet (YAG) is an oxide ceramic with outstanding mechanical properties such as high-temperature strength and low-creep rate. YAG crystallizes with cubic symmetry so it does not exhibit any birefringence effects at the grain boundaries which lead to its high in-line transparency [1]. It is well known as an excellent IR transparent window material and is widely used as a photoluminescence material [2]. Properties such as high-purity, fine particle size and narrow size distribution, lowagglomeration and high-sinterability are essential for fabricating of transparent polycrystalline YAG ceramic [3]. Traditional solid-state
* Corresponding author. E-mail address: [email protected] (B. Movahedi).
method for preparing YAG powders requires prolonged mechanical mixing as well as calcination at high-temperatures and also obtained powders have an uncontrollable morphology problem and introduced impurities are unavoidable during ball milling [4]. Hence, many kinds of wet chemical routes such as sol-gel [5], coprecipitation with ammonium hydrogen carbonate (AHC), urea or ammonia water as precipitants [6e8], microwave irradiation [9,10], solvothermal [11], supercritical water synthesis [12], spray pyrolysis [13], combustion synthesis [14] and homogeneous precipitation [15] have been used for synthesis of YAG powders. Unfortunately, most of the chemical methods suffer from complex and time-consuming procedures and possible mismatch in the solution behavior of the constituents. Although, sol-gel processing and co-precipitation methods were widely used for powder synthesis, one main disadvantage of these two methods is that
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ultrafine particles of the gel-like precursors underwent severe agglomeration during drying, causing poor-sinterability of the resultant YAG powders [16]. Recently, partial wet routes have been used for synthesis of YAG powders via alumina/Y-compound core-shell structure, in which a layer of Y-compound nanoparticles smoothly covers the alumina core. Sang et al. [17e19] by calcining of the alumina/Y-compound core-shell at 1250 C could achieve the single-phase of YAG powders and by vacuum sintering of them at 1780 C for 4 h, transparent YAG ceramic was obtained. It should be considered that most techniques used to produce YAG powders, whether they are solid state reactions or chemical synthesis, are conducted at hightemperature (>1000 C). However, it is known that hightemperature processing is harmful to microstructure and in-turn the properties due to the non-uniform grain growth [20]. Therefore, low-temperature synthesis and calcination process of nanocrystalline powders of alumina/yttria nanocomposites should be investigated in detail. In this circumstance, the synthesis of alumina/yttria nanocomposites with the stoichiometric ratio of YAG composition seems to have the capacity of creating a new way for the fabrication of YAG precursors. On the other hand, the uniform distribution of yttria in the alumina matrix could lead to higher transparency of the obtained YAG ceramic, even without adding any sintering aids, dopants or colloidal forming processes before sintering. Hence, the first objective of this study was to synthesize of alumina/yttria nanocomposites via a partial wet route as the YAG precursors. This work describes the influence of urea and AHC as the precipitating agents on the morphology and the sinterability of the alumina/yttria nanocomposites. The second objective was to use the spark plasma sintering (SPS) as an effective sintering technique for obtaining transparent YAG ceramic with high-density in a very short-cycle. 2. Experimental procedure 2.1. Synthesis method Yttria powder (Merck, 99.9%) was dissolved in a high-purity nitric acid up to a transparent solution of Y(NO3)3 was obtained. Briefly, Y(NO3)3 and urea (Merck, 99.5%) were dissolved in distilled water to make a solution. Urea was applied as a precipitant agent at a ratio of 10:1, 20:1 and 33:1 urea: Y3þ. The a-alumina (US-Nano research, 99.9%, 50 nm) was added into the mixed solution, and dispersed using ultra-sonication. The mixed turbid liquid with a ratio of Y/Al ¼ 3/5, was homogenized using a hot-plate magnetic stirrer at 90 C for 3 h. After cooling down naturally, the resulting precursor material was collected via suction filtration and byproducts were removed by washing the particles with deionized water three times via suction filtration. After rinsing with anhydrous ethanol, the particles were dried in an oven at 100 C for 24 h and then calcined at 550 C for 3 h. To evaluate a slow-reaction rate during precipitation, NH4HCO3 solution with concentration of 2 mol/L was obtained by dissolving NH4HCO3 in deionized water and (Sigma Aldrich, 99%) at a ratio of 3:1 and 13:1 AHC to Y3þ was used as a precipitant by adding it to the above alumina suspension in the droplet form. Separate Y-compound particles were also synthesized by homogeneous precipitation with urea and AHC, in order to study the nature of these particles. The specimen No. and different synthesis conditions of the samples are listed in Table 1. The terminology of U for urea and A for AHC was used as a precipitating agent in the specimen No. 2.2. Preparation of YAG ceramic The alumina-yttria nanocomposite powders were poured in a
Table 1 The name and conditions of different samples. Specimen no.
[Precipitant]/[Y3þ]
Calcination (temperature-time)
Sintering process
U1 A1 U2 U3 U4 A2 A3 U5 A4
33 13 10 20 33 3 13 33 13
N/A N/A 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3
N/A N/A N/A N/A N/A N/A N/A SPS SPS
h h h h h h h
graphite die of the SPS apparatus (SPS 60-10). The temperature was measured by a pyrometer focused on the upper graphite punch. The main process parameters used for heating and holding at the sintering temperature, are shown in Fig. 1. The sintering temperature is 1450 C and the pressure loading starts from 1 kN and increases before heating to 22 kN, corresponding to a maximal external pressure of 80 MPa. The soaking time for achieving transparency is about 10 min. 2.3. Characterization methods X-ray diffraction (XRD) patterns were recorded on the Bruker D8 Advance X-ray diffractometer with Cu Ka (l ¼ 0.15406 nm). The average crystallite size was calculated from Rietveld method using Maud software (version: 2.49). The composition and impurity contents of the synthesized powders were determined using an inductively coupled plasma (ICP- Model OES 730) chemical analysis. A Tescan Mira (III) field emission scanning electron microscopy (FESEM) was used to characterize the morphology of the samples. The chemical composition of the synthesized powders was evaluated via the energy-dispersive X-ray spectroscopy (EDX). A Philips cm30 transmission electron microscopy (TEM) was employed to study the core-shell structure. For powder preparation, the powder particles were dispersed in an ethanol using an ultrasonic stirrer and an adequate amount of dispersed solution was applied to a carbon-coated copper grid. The Fourier transform infrared (FTIR) spectroscopic studies were performed on a JASCO FTIR-6300 spectrometer. After sintering process, the relative density of the ceramic was determined by Archimedes's method (the theoretical density of YAG was taken as 4.545 g/cm3) [21]. The sintered specimen was polished using a diamond powder and the transparency of it was determined by inline transmission measurements using a FTIR spectroscopy (Shimadzu- 3500S) within the IR range. 3. Results and discussion 3.1. Phase evaluations and crystallite sizes measurement The phase evaluations of the different nanocomposite powders (U2, U3, U4, A2 and A3) are shown in Fig. 2. The diffraction peaks of the powders can be identified as corundum (a-Al2O3, JCPDS card No. 01-071-1128) with hexagonal symmetry and cubic Y2O3 (JCPDS card No. 41-1105) phases which indicate that the powders are the mixture of these two phases. No peaks of impurities were observed, confirming the formation of pure products. Based on the results, Alumina/Yttria nanocomposite with both urea and AHC precipitating agents is obtained. The average crystallite size of the yttria phase in the alumina/ yttria nanocomposite in both urea and AHC systems has been calculated from the results of XRD measurements by the Rietveld
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
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Fig. 1. The temperature and pressure regimes followed in the course of the SPS treatment.
Fig. 2. XRD patterns of a) U2, b) U3, c) U4, d) A2 and e) A3 nanocomposite powders.
method (Table 2). It is shown that the average size of yttria crystallite has an uptrend by increasing the [U]/[Y3þ] ratio. But in AHC system, by increasing the [AHC]/[Y3þ] ratio, the average crystallite size of yttria phase has decreased. Generally, the crystallite size of yttria in the urea system was smaller than that in the AHC system. To evaluate the weight percentage of alumina and yttria phases in the urea and AHC system, the MAUD software was used. These results are presented in Table 3. Accordingly, the highest efficiency in the urea system belongs to U4 nanocomposite and in the AHC
Table 2 Average crystallite size of yttria phase in alumina/yttria nanocomposites calculated by the Rietveld method. Specimen no.
U2
U3
U4
A2
A3
Average Crystallite size
6 nm
10 nm
15 nm
31 nm
22 nm
Table 3 Calculation of weight percent of different phases by quantitative analysis of XRD peaks using MAUD software. Specimen no.
%wt alumina
%wt yttria
U2 U3 U4 A2 A3
54.57 49.19 42.98 58.04 43.03
45.43 50.81 57.02 41.96 56.96
system belongs to A3 nanocomposite. In these molarity ratios, the value of precipitating agent is sufficient to use the entire yttrium precursors and form Y-compounds. 3.2. Morphological observations The morphologies of Y-compound nanoparticles in the urea
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system and as-calcined U4 and A3 nanocomposites with the elemental mapping images are shown in Fig. 3. As shown in Fig. 3a, the particle size of Y-compound nanoparticles in the urea system is less than 50 nm, which is smaller than the Y-compounds synthesized by Qin et al. [22] but both are spherical in shape. Furthermore, the self-assembly of the very tiny Y-compound nanoparticles can be seen, which is the characteristic feature of these particles and is affirmed by sang et al. [19]. Fig. 3b demonstrates the a-alumina particles covered with yttria nanoparticles (small white spots on the darker alumina particles). The particle size of the U4 nanocomposite is around 100e200 nm (arrow indicated in the highmagnification image) and the surface is covered with yttria crystallites in average sizes of about 15 nm as analyzed from the XRD measurements (Table 2). As evident in Fig. 3b the surface of U4 nanocomposite powder is rough, which is consistent with the surface morphology of the alumina/Y-compound particles, reported by Sang et al. [18]. Fig. 3c shows typical low and high-magnification FESEM images of A3 nanocomposite powders. Sheet forms of Y-compounds covering the spherical alumina nanoparticles in the AHC system, can be seen. Close observation reveals that the Y-compounds in the AHC system possess a rough surface and have an irregular sheet like shapes. With reference to Fig. 3b and c, the aggregation in A3 nanocomposite powders is greater than U4 nanocomposite
powders. The EDS mapping results (inserts in Fig. 3b and c) shows the distribution of yttrium and aluminum elements in the FESEM images of alumina/yttria nanocomposite powders. As shown, there is a uniform distribution of Al and Y in both nanocomposites, but the amount of them is different in the mapping images. For better investigating the situation, TEM image of U4 nanocomposite was checked out. Fig. 4 shows the high resolution transmission electron microscopy (HRTEM) images of the a-Al2O3 and the U4 nanocomposite powders. The HRTEM image of pure Al2O3 confirmed the good dispersibility of the Al2O3 nanoparticles (Fig. 4a). This implied that the Al2O3 nanoparticles were suitable for use as templates. As evident in Fig. 4b, core-shell structure of U4 nanocomposite could be seen, in which, the yttria shell has covered the whole surface of the alumina nanoparticles. The average diameter of Al2O3 core was 50 nm, and according to the HRTEM images (Fig. 4b) as well as quantitative XRD analysis (Table 2), it is possible to estimate that the thickness of the yttria shell is less than 20 nm. The crystallinity of the U4 sample is high as shown in the selected area electron diffraction pattern (SAEDP), which confirmed the polycrystalline structure. Based on HRTEM and FESEM images, owing to forming alumina/ yttria core-shell structure in the U4 nanocomposite, yttria has
Fig. 3. FESEM images of a) Y-compound in urea system, b) U4 and c) A3 nanocomposite powder and EDS mapping data for Al þ Y (blue spots: Al, red spots: Y). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
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Fig. 4. TEM and SADP images of a) pure a-Al2O3 nanoparticles b) U4 nanocomposite powder.
covered the entire surface of alumina cores and in the EDS mapping results, Y element is more recognized (Fig. 3b). However, in A3 nanocomposite, the surface of alumina nanoparticles is covered by yttria sheets and therefore, in some areas, it is not fully covered by yttria; hence, in Fig. 3c, both elements of Y and Al are seen.
3.3. Analytical investigations The FTIR analysis is presented in order to further study of the nanocomposite powders composition in both AHC and urea systems. Fig. 5 shows the FTIR spectra of pure a-alumina particles, the A1 and U1 nanocomposite powders. The broad band at 3431 cm1 corresponded to water absorption [23]. It is due to the absorption of moisture either from atmosphere by nanocomposite powders or from KBr pellet used as a standard reference sample [24]. Absorption bands in the range of 800e400 cm1, attributed mainly to the metal-oxygen lattice vibration, were identified in the FTIR spectra of the pure a-alumina (curve a). Bands appearing at 439, 475 and 585 cm1 are assigned to characteristic AleO vibrations [19]. As shown in Fig. 5, curve b and c, the two strong peaks at 1401 and 1521 cm1 are attributed to y3 mode of carbonate in the as-dried powder. The other bands at 1082, 843, and 751-688 cm1 are assigned to y1, y2, and y4 modes of carbonate, respectively [25]. On
the other hand, the FTIR peaks of A1 are similar to those of U1 nanocomposite. Furthermore, two more peaks at 3220 and 3028 cm1 are probably attributed to the structural OeH bands in the A1 nanocomposite [26]. Urea is a very weak bronsted base (pKb ¼ 13.8) highly soluble in water, which decomposes homogenously at the temperature above 83 C. The main reactions in the formations of YOHCO3 and Y2(CO3)3. nH2O (n ¼ 2e3) can be expressed as follows, i.e, equations (1)e(5) [25].
COðNH2 Þ2 þ H2 O/CO2 þ 2NH3
(1)
NH3 þ H2 O4NHþ 4 þ OH
(2)
þ CO2 þ H2 O4CO2 3 þ 2H
(3)
Y3þ þ OH þ CO2 3 /YOHCO3
(4)
2Y3þ þ 3 CO2 3 þnH2 O/Y2 ðCO3 Þ3 $nH2 O
(5)
As shown by FTIR results, no peak attributed to structure of OH was in Fig. 5 (curve c), which affirms the formation of
Fig. 5. FTIR spectra of a) a-Alumina, b) A1 and c) U1 nanocomposite powders.
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Y2(CO3)3$nH2O in the urea system. The composition of the precursor prepared by AHC will be the result of the competition between OH and the carbonate species generated by the following chemical reactions (equations (6)e(9)) during combining with metal cations:
NH4 HCO3 þ H2 O4NH4 OH þ H2 CO3
(6)
NH4 OH4NHþ 4 þ OH
(7)
H2 CO3 4Hþ þ HCO 3
(8)
2 þ HCO 3 4H þ CO3
(9)
In AHC system, Y3þ may most likely precipitate as normal carbonate of [Y2(CO3)3$nH2O (n ¼ 2e3)] or basic carbonate [Y(OH) CO3] from the present carbonate anions containing AHC solution [16]. In this study, concerning Fig. 5 (curve b), OeH band in the A1 sample shows the possibility of forming Y(OH)CO3 in the AHC system. 3.4. Fabrication of YAG ceramic In most of the cases, the commercial nano sized or coprecipitated YAG powders have been used as a precursor of the SPS process [2,27,28]. On the contrary, in this study the alumina/ yttria nanocomposites (U4 and A3) were used as the precursors and the ceramic pellets were fabricated using spark plasma sintering process. The XRD patterns corresponding to U5 and A4 ceramics are shown in Fig. 6. The obtained peaks of both ceramics (Fig. 6a and b) are compared with the standard JCPDS card No. 33-0040. All the peak positions of the obtained ceramics match with the standard YAG diffraction reference pattern and no intermediate phases such as yttrium aluminum monoclinic (YAM) or yttrium aluminum perovskite (YAP) were observed. Hence, it seems that the yield of the synthesizing process with both AHC and urea precipitants was so high, that the pure YAG phase was obtained by SPS of the alumina/yttria nanocomposite powders. It is interesting to note that, the diffusion between alumina and yttria phases has occurred in just 10 min and the single phase ceramic has been obtained, which probably could not be occurring in a short time by other sintering techniques [2,27]. The fracture surface micrographs of U5 and A4 samples that were obtained by FESEM are shown in Fig. 7. Both micrographs put in
evidence the intergranular fracture features. The U5 ceramic sample displays a fine sub-micron microstructure with mean grain size of about 0.5 mm (Fig. 7a), while the A4 ceramic sample has a slightly coarser structure with mean grain size of about 1.2 mm (Fig. 7b), which both are 10 times smaller than the grain sizes of vacuum sintered specimens [1,29]. It shows the possibility of obtaining finegrained ceramics with the spark plasma sintering process. The Archimedes density of U5 and A4 ceramics was 99.3% and 92.4%, respectively. The presence of residual porosity in the sample fabricated by A3 nanocomposite powder is visible in Fig. 7b. As shown in Fig. 8, the transmittance of U5 ceramic after annealing treatment in the infrared range is about 47%, which is similar to the transmittance reported by Suarez et al. [30] for SPS of co-precipitated YAG powder. The apparent transparency of U5 and A4 ceramics after annealing treatment at 1250 C for 3 h are also demonstrated in Fig. 9. The annealing thermal treatment led to a faint color for U5 and opaque color for A4 ceramics. The results of ICP analyses of the U4 powder show that the amount of Na, Ca and K impurity introduced by distilled water is not negligible (Table 4). In order to further evaluate, the EDX analysis of the fracture surface of U5 ceramic (Fig. 7a, “A” area), shows that amounts of K and Ca impurities in grain boundaries. It can be guessed that, these impurities have segregated to grain boundaries during the annealing treatment, leading to a reduction in the transparency in both U5 and A4 ceramics. Moreover, it could be due to the evaporation of carbon particles, leaving behind tiny pores in the bulk, or to the microstructural changes during annealing, leading to new or larger defects, as suggested by Spina et al. [31]. The in-line transmittance depends not only on the porosity but also is very sensitive to the pore diameter. When the pore size becomes of the same order of magnitude of the incident light wavelength, the transmission reaches a minimum [31]. Hence, the larger size of pores in the A4 than U5 ceramics can explain the loss of transmittance in IR and visible ranges. Moreover, it probably seems that the more aggregated powder morphology of A3 compared with U4 (Fig. 3b and c), leads to its less sinterability, lower density and more residual porosities (Fig. 7a and b). Based on TEM image (Fig. 4b), the nanocomposite synthesized by urea as a precipitant, consists of spherical core-shell nanoparticles and this shape of particles is appropriate for using as a powder precursor for sintering. On the contrary, as shown in FESEM images (Fig. 3c), the nanocomposite synthesized by AHC as a precipitant, consists of spherical alumina nanoparticles covered by the sheet like yttria particles. It can be guessed that, this shape of particles is not useful for sintering process and usually leads to an
Fig. 6. XRD patterns of SPS samples a) U5, b) A4.
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
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Fig. 7. FESEM images of the YAG fracture surface after SPS for 10 min a) U5, b) A4 ceramics, and c) EDX results of the “A” area of the fracture surface image of U5, insert: quantitative results of EDX.
Fig. 8. Optical transmittance of YAG specimen (U5 sample with 1 mm thickness) in IR part of spectrum.
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
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Fig. 9. a) U5, b) A4 sample of 1 mm thick disc fabricated by the SPS process.
Table 4 The results of ICP analyses of U4 alumina/yttria nanocomposite powder.
[2]
Element
Na
K
Ca
Al
Y
Quantity (ppm)
324
1201
1457
>10%
>10%
opaque YAG ceramic. It is well known that the optimization of SPS process, using colloidal forming processes and also adding different sintering aids such as tetraethyl orthosilicate (TEOS), MgO and LiF play a key role in enhancing the optical characteristics of YAG ceramics, in order to obtain a material with a small grain size, high theoretical density and transparency in both the visible and the infrared range. Hence, we are studying on the effect of these parameters, which will be mentioned later in our future works.
[3]
[4]
[5]
[6]
[7] [8]
4. Conclusions [9]
In this framework of this approach, the alumina/yttria nanocomposite powders were produced via partial wet route using urea and AHC as precipitants, respectively. Urea exceeds AHC for the production of an alumina/yttria nanocomposite with an appropriate morphology of YAG precursor. The use of urea produced very tiny Y-compounds, which were attracted to the surface of the alumina nanoparticles and formed a core-shell structure. Therefore, the morphology of the alumina/yttria nanocomposite powder in the urea system was spherical as a result of spherical alumina particles being covered by a uniform yttria shell layer. The sheet like Y-compound was synthesized using ammonium hydrogen carbonate as a precipitant. Consequently, in the AHC system, alumina nanoparticles were covered by Y-compound sheets, hence; the whole surface of alumina nanoparticles was not uniformly covered by yttria. Urea proved to be an appropriate precipitant for obtaining alumina/yttria nanocomposite powders with appropriate morphology and highly sinterability, which leads to a transparent YAG ceramic, while only opaque YAG ceramic was obtained from alumina/yttria nanocomposite powder synthesized in the AHC system. Our study demonstrated that in addition to YAG precursors synthesized via different methods, the alumina/yttria nanocomposite powders with more appropriate morphology and highly sinterability could be used as a powder precursor for fabricating transparent YAG ceramics, even without adding any sintering aid materials. References [1] L. Wen, X. Sun, Z. Xiu, S. Chen, C.-T. Tsai, Synthesis of nanocrystalline yttria
[10] [11]
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A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants G. Kafili a, B. Movahedi a, *, M. Milani b a b
Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 81746-73441, Iran Faculty of Advanced Materials and Renewable Energy Research Center, Tehran, Iran
h i g h l i g h t s Urea proved to be an appropriate precipitant for obtaining a core-shell alumina/yttria nanocomposite. Alumina/yttria nanocomposite powders with more appropriate morphology and highly sinterability. A fine-grained YAG ceramic was obtained by SPS of alumina-yttria nanocomposite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 28 July 2016 Accepted 6 August 2016 Available online xxx
Alumina/yttria nanocomposite powder as an yttrium aluminum garnet (YAG) precursor was synthesized via partial wet route using urea and ammonium hydrogen carbonate (AHC) as precipitants, respectively. The products were characterized using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and energy dispersive spectroscopy. The use of urea produced very tiny spherical Y-compounds with chemical composition of Y2(CO3)3$nH2O, which were attracted to the surface of alumina nanoparticles and consequently, a coreshell structure was obtained. The use of ammonium hydrogen carbonate produced sheets of Y-compounds with chemical composition of Y(OH)CO3 covering the alumina nanoparticles. A fine-grained YAG ceramic (about 500 nm), presenting a non-negligible transparency (45% RIT at IR range) was obtained by the spark plasma sintering (SPS) of alumina-yttria nanocomposite synthesized in the urea system. This amount of transmission was obtained by only the sintering of the powder specimen without any colloidal forming process before sintering or adding any sintering aids or dopant elements. However, by spark plasma sintering of alumina-yttria nanocomposite powder synthesized in AHC system, an opaque YAG ceramic with an average grain size of 1.2 mm was obtained. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramics Nanostructures Optical materials Sintering
1. Introduction Yttrium aluminum garnet (YAG) is an oxide ceramic with outstanding mechanical properties such as high-temperature strength and low-creep rate. YAG crystallizes with cubic symmetry so it does not exhibit any birefringence effects at the grain boundaries which lead to its high in-line transparency [1]. It is well known as an excellent IR transparent window material and is widely used as a photoluminescence material [2]. Properties such as high-purity, fine particle size and narrow size distribution, lowagglomeration and high-sinterability are essential for fabricating of transparent polycrystalline YAG ceramic [3]. Traditional solid-state
* Corresponding author. E-mail address: [email protected] (B. Movahedi).
method for preparing YAG powders requires prolonged mechanical mixing as well as calcination at high-temperatures and also obtained powders have an uncontrollable morphology problem and introduced impurities are unavoidable during ball milling [4]. Hence, many kinds of wet chemical routes such as sol-gel [5], coprecipitation with ammonium hydrogen carbonate (AHC), urea or ammonia water as precipitants [6e8], microwave irradiation [9,10], solvothermal [11], supercritical water synthesis [12], spray pyrolysis [13], combustion synthesis [14] and homogeneous precipitation [15] have been used for synthesis of YAG powders. Unfortunately, most of the chemical methods suffer from complex and time-consuming procedures and possible mismatch in the solution behavior of the constituents. Although, sol-gel processing and co-precipitation methods were widely used for powder synthesis, one main disadvantage of these two methods is that
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ultrafine particles of the gel-like precursors underwent severe agglomeration during drying, causing poor-sinterability of the resultant YAG powders [16]. Recently, partial wet routes have been used for synthesis of YAG powders via alumina/Y-compound core-shell structure, in which a layer of Y-compound nanoparticles smoothly covers the alumina core. Sang et al. [17e19] by calcining of the alumina/Y-compound core-shell at 1250 C could achieve the single-phase of YAG powders and by vacuum sintering of them at 1780 C for 4 h, transparent YAG ceramic was obtained. It should be considered that most techniques used to produce YAG powders, whether they are solid state reactions or chemical synthesis, are conducted at hightemperature (>1000 C). However, it is known that hightemperature processing is harmful to microstructure and in-turn the properties due to the non-uniform grain growth [20]. Therefore, low-temperature synthesis and calcination process of nanocrystalline powders of alumina/yttria nanocomposites should be investigated in detail. In this circumstance, the synthesis of alumina/yttria nanocomposites with the stoichiometric ratio of YAG composition seems to have the capacity of creating a new way for the fabrication of YAG precursors. On the other hand, the uniform distribution of yttria in the alumina matrix could lead to higher transparency of the obtained YAG ceramic, even without adding any sintering aids, dopants or colloidal forming processes before sintering. Hence, the first objective of this study was to synthesize of alumina/yttria nanocomposites via a partial wet route as the YAG precursors. This work describes the influence of urea and AHC as the precipitating agents on the morphology and the sinterability of the alumina/yttria nanocomposites. The second objective was to use the spark plasma sintering (SPS) as an effective sintering technique for obtaining transparent YAG ceramic with high-density in a very short-cycle. 2. Experimental procedure 2.1. Synthesis method Yttria powder (Merck, 99.9%) was dissolved in a high-purity nitric acid up to a transparent solution of Y(NO3)3 was obtained. Briefly, Y(NO3)3 and urea (Merck, 99.5%) were dissolved in distilled water to make a solution. Urea was applied as a precipitant agent at a ratio of 10:1, 20:1 and 33:1 urea: Y3þ. The a-alumina (US-Nano research, 99.9%, 50 nm) was added into the mixed solution, and dispersed using ultra-sonication. The mixed turbid liquid with a ratio of Y/Al ¼ 3/5, was homogenized using a hot-plate magnetic stirrer at 90 C for 3 h. After cooling down naturally, the resulting precursor material was collected via suction filtration and byproducts were removed by washing the particles with deionized water three times via suction filtration. After rinsing with anhydrous ethanol, the particles were dried in an oven at 100 C for 24 h and then calcined at 550 C for 3 h. To evaluate a slow-reaction rate during precipitation, NH4HCO3 solution with concentration of 2 mol/L was obtained by dissolving NH4HCO3 in deionized water and (Sigma Aldrich, 99%) at a ratio of 3:1 and 13:1 AHC to Y3þ was used as a precipitant by adding it to the above alumina suspension in the droplet form. Separate Y-compound particles were also synthesized by homogeneous precipitation with urea and AHC, in order to study the nature of these particles. The specimen No. and different synthesis conditions of the samples are listed in Table 1. The terminology of U for urea and A for AHC was used as a precipitating agent in the specimen No. 2.2. Preparation of YAG ceramic The alumina-yttria nanocomposite powders were poured in a
Table 1 The name and conditions of different samples. Specimen no.
[Precipitant]/[Y3þ]
Calcination (temperature-time)
Sintering process
U1 A1 U2 U3 U4 A2 A3 U5 A4
33 13 10 20 33 3 13 33 13
N/A N/A 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3 550 C-3
N/A N/A N/A N/A N/A N/A N/A SPS SPS
h h h h h h h
graphite die of the SPS apparatus (SPS 60-10). The temperature was measured by a pyrometer focused on the upper graphite punch. The main process parameters used for heating and holding at the sintering temperature, are shown in Fig. 1. The sintering temperature is 1450 C and the pressure loading starts from 1 kN and increases before heating to 22 kN, corresponding to a maximal external pressure of 80 MPa. The soaking time for achieving transparency is about 10 min. 2.3. Characterization methods X-ray diffraction (XRD) patterns were recorded on the Bruker D8 Advance X-ray diffractometer with Cu Ka (l ¼ 0.15406 nm). The average crystallite size was calculated from Rietveld method using Maud software (version: 2.49). The composition and impurity contents of the synthesized powders were determined using an inductively coupled plasma (ICP- Model OES 730) chemical analysis. A Tescan Mira (III) field emission scanning electron microscopy (FESEM) was used to characterize the morphology of the samples. The chemical composition of the synthesized powders was evaluated via the energy-dispersive X-ray spectroscopy (EDX). A Philips cm30 transmission electron microscopy (TEM) was employed to study the core-shell structure. For powder preparation, the powder particles were dispersed in an ethanol using an ultrasonic stirrer and an adequate amount of dispersed solution was applied to a carbon-coated copper grid. The Fourier transform infrared (FTIR) spectroscopic studies were performed on a JASCO FTIR-6300 spectrometer. After sintering process, the relative density of the ceramic was determined by Archimedes's method (the theoretical density of YAG was taken as 4.545 g/cm3) [21]. The sintered specimen was polished using a diamond powder and the transparency of it was determined by inline transmission measurements using a FTIR spectroscopy (Shimadzu- 3500S) within the IR range. 3. Results and discussion 3.1. Phase evaluations and crystallite sizes measurement The phase evaluations of the different nanocomposite powders (U2, U3, U4, A2 and A3) are shown in Fig. 2. The diffraction peaks of the powders can be identified as corundum (a-Al2O3, JCPDS card No. 01-071-1128) with hexagonal symmetry and cubic Y2O3 (JCPDS card No. 41-1105) phases which indicate that the powders are the mixture of these two phases. No peaks of impurities were observed, confirming the formation of pure products. Based on the results, Alumina/Yttria nanocomposite with both urea and AHC precipitating agents is obtained. The average crystallite size of the yttria phase in the alumina/ yttria nanocomposite in both urea and AHC systems has been calculated from the results of XRD measurements by the Rietveld
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Fig. 1. The temperature and pressure regimes followed in the course of the SPS treatment.
Fig. 2. XRD patterns of a) U2, b) U3, c) U4, d) A2 and e) A3 nanocomposite powders.
method (Table 2). It is shown that the average size of yttria crystallite has an uptrend by increasing the [U]/[Y3þ] ratio. But in AHC system, by increasing the [AHC]/[Y3þ] ratio, the average crystallite size of yttria phase has decreased. Generally, the crystallite size of yttria in the urea system was smaller than that in the AHC system. To evaluate the weight percentage of alumina and yttria phases in the urea and AHC system, the MAUD software was used. These results are presented in Table 3. Accordingly, the highest efficiency in the urea system belongs to U4 nanocomposite and in the AHC
Table 2 Average crystallite size of yttria phase in alumina/yttria nanocomposites calculated by the Rietveld method. Specimen no.
U2
U3
U4
A2
A3
Average Crystallite size
6 nm
10 nm
15 nm
31 nm
22 nm
Table 3 Calculation of weight percent of different phases by quantitative analysis of XRD peaks using MAUD software. Specimen no.
%wt alumina
%wt yttria
U2 U3 U4 A2 A3
54.57 49.19 42.98 58.04 43.03
45.43 50.81 57.02 41.96 56.96
system belongs to A3 nanocomposite. In these molarity ratios, the value of precipitating agent is sufficient to use the entire yttrium precursors and form Y-compounds. 3.2. Morphological observations The morphologies of Y-compound nanoparticles in the urea
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system and as-calcined U4 and A3 nanocomposites with the elemental mapping images are shown in Fig. 3. As shown in Fig. 3a, the particle size of Y-compound nanoparticles in the urea system is less than 50 nm, which is smaller than the Y-compounds synthesized by Qin et al. [22] but both are spherical in shape. Furthermore, the self-assembly of the very tiny Y-compound nanoparticles can be seen, which is the characteristic feature of these particles and is affirmed by sang et al. [19]. Fig. 3b demonstrates the a-alumina particles covered with yttria nanoparticles (small white spots on the darker alumina particles). The particle size of the U4 nanocomposite is around 100e200 nm (arrow indicated in the highmagnification image) and the surface is covered with yttria crystallites in average sizes of about 15 nm as analyzed from the XRD measurements (Table 2). As evident in Fig. 3b the surface of U4 nanocomposite powder is rough, which is consistent with the surface morphology of the alumina/Y-compound particles, reported by Sang et al. [18]. Fig. 3c shows typical low and high-magnification FESEM images of A3 nanocomposite powders. Sheet forms of Y-compounds covering the spherical alumina nanoparticles in the AHC system, can be seen. Close observation reveals that the Y-compounds in the AHC system possess a rough surface and have an irregular sheet like shapes. With reference to Fig. 3b and c, the aggregation in A3 nanocomposite powders is greater than U4 nanocomposite
powders. The EDS mapping results (inserts in Fig. 3b and c) shows the distribution of yttrium and aluminum elements in the FESEM images of alumina/yttria nanocomposite powders. As shown, there is a uniform distribution of Al and Y in both nanocomposites, but the amount of them is different in the mapping images. For better investigating the situation, TEM image of U4 nanocomposite was checked out. Fig. 4 shows the high resolution transmission electron microscopy (HRTEM) images of the a-Al2O3 and the U4 nanocomposite powders. The HRTEM image of pure Al2O3 confirmed the good dispersibility of the Al2O3 nanoparticles (Fig. 4a). This implied that the Al2O3 nanoparticles were suitable for use as templates. As evident in Fig. 4b, core-shell structure of U4 nanocomposite could be seen, in which, the yttria shell has covered the whole surface of the alumina nanoparticles. The average diameter of Al2O3 core was 50 nm, and according to the HRTEM images (Fig. 4b) as well as quantitative XRD analysis (Table 2), it is possible to estimate that the thickness of the yttria shell is less than 20 nm. The crystallinity of the U4 sample is high as shown in the selected area electron diffraction pattern (SAEDP), which confirmed the polycrystalline structure. Based on HRTEM and FESEM images, owing to forming alumina/ yttria core-shell structure in the U4 nanocomposite, yttria has
Fig. 3. FESEM images of a) Y-compound in urea system, b) U4 and c) A3 nanocomposite powder and EDS mapping data for Al þ Y (blue spots: Al, red spots: Y). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. TEM and SADP images of a) pure a-Al2O3 nanoparticles b) U4 nanocomposite powder.
covered the entire surface of alumina cores and in the EDS mapping results, Y element is more recognized (Fig. 3b). However, in A3 nanocomposite, the surface of alumina nanoparticles is covered by yttria sheets and therefore, in some areas, it is not fully covered by yttria; hence, in Fig. 3c, both elements of Y and Al are seen.
3.3. Analytical investigations The FTIR analysis is presented in order to further study of the nanocomposite powders composition in both AHC and urea systems. Fig. 5 shows the FTIR spectra of pure a-alumina particles, the A1 and U1 nanocomposite powders. The broad band at 3431 cm1 corresponded to water absorption [23]. It is due to the absorption of moisture either from atmosphere by nanocomposite powders or from KBr pellet used as a standard reference sample [24]. Absorption bands in the range of 800e400 cm1, attributed mainly to the metal-oxygen lattice vibration, were identified in the FTIR spectra of the pure a-alumina (curve a). Bands appearing at 439, 475 and 585 cm1 are assigned to characteristic AleO vibrations [19]. As shown in Fig. 5, curve b and c, the two strong peaks at 1401 and 1521 cm1 are attributed to y3 mode of carbonate in the as-dried powder. The other bands at 1082, 843, and 751-688 cm1 are assigned to y1, y2, and y4 modes of carbonate, respectively [25]. On
the other hand, the FTIR peaks of A1 are similar to those of U1 nanocomposite. Furthermore, two more peaks at 3220 and 3028 cm1 are probably attributed to the structural OeH bands in the A1 nanocomposite [26]. Urea is a very weak bronsted base (pKb ¼ 13.8) highly soluble in water, which decomposes homogenously at the temperature above 83 C. The main reactions in the formations of YOHCO3 and Y2(CO3)3. nH2O (n ¼ 2e3) can be expressed as follows, i.e, equations (1)e(5) [25].
COðNH2 Þ2 þ H2 O/CO2 þ 2NH3
(1)
NH3 þ H2 O4NHþ 4 þ OH
(2)
þ CO2 þ H2 O4CO2 3 þ 2H
(3)
Y3þ þ OH þ CO2 3 /YOHCO3
(4)
2Y3þ þ 3 CO2 3 þnH2 O/Y2 ðCO3 Þ3 $nH2 O
(5)
As shown by FTIR results, no peak attributed to structure of OH was in Fig. 5 (curve c), which affirms the formation of
Fig. 5. FTIR spectra of a) a-Alumina, b) A1 and c) U1 nanocomposite powders.
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Y2(CO3)3$nH2O in the urea system. The composition of the precursor prepared by AHC will be the result of the competition between OH and the carbonate species generated by the following chemical reactions (equations (6)e(9)) during combining with metal cations:
NH4 HCO3 þ H2 O4NH4 OH þ H2 CO3
(6)
NH4 OH4NHþ 4 þ OH
(7)
H2 CO3 4Hþ þ HCO 3
(8)
2 þ HCO 3 4H þ CO3
(9)
In AHC system, Y3þ may most likely precipitate as normal carbonate of [Y2(CO3)3$nH2O (n ¼ 2e3)] or basic carbonate [Y(OH) CO3] from the present carbonate anions containing AHC solution [16]. In this study, concerning Fig. 5 (curve b), OeH band in the A1 sample shows the possibility of forming Y(OH)CO3 in the AHC system. 3.4. Fabrication of YAG ceramic In most of the cases, the commercial nano sized or coprecipitated YAG powders have been used as a precursor of the SPS process [2,27,28]. On the contrary, in this study the alumina/ yttria nanocomposites (U4 and A3) were used as the precursors and the ceramic pellets were fabricated using spark plasma sintering process. The XRD patterns corresponding to U5 and A4 ceramics are shown in Fig. 6. The obtained peaks of both ceramics (Fig. 6a and b) are compared with the standard JCPDS card No. 33-0040. All the peak positions of the obtained ceramics match with the standard YAG diffraction reference pattern and no intermediate phases such as yttrium aluminum monoclinic (YAM) or yttrium aluminum perovskite (YAP) were observed. Hence, it seems that the yield of the synthesizing process with both AHC and urea precipitants was so high, that the pure YAG phase was obtained by SPS of the alumina/yttria nanocomposite powders. It is interesting to note that, the diffusion between alumina and yttria phases has occurred in just 10 min and the single phase ceramic has been obtained, which probably could not be occurring in a short time by other sintering techniques [2,27]. The fracture surface micrographs of U5 and A4 samples that were obtained by FESEM are shown in Fig. 7. Both micrographs put in
evidence the intergranular fracture features. The U5 ceramic sample displays a fine sub-micron microstructure with mean grain size of about 0.5 mm (Fig. 7a), while the A4 ceramic sample has a slightly coarser structure with mean grain size of about 1.2 mm (Fig. 7b), which both are 10 times smaller than the grain sizes of vacuum sintered specimens [1,29]. It shows the possibility of obtaining finegrained ceramics with the spark plasma sintering process. The Archimedes density of U5 and A4 ceramics was 99.3% and 92.4%, respectively. The presence of residual porosity in the sample fabricated by A3 nanocomposite powder is visible in Fig. 7b. As shown in Fig. 8, the transmittance of U5 ceramic after annealing treatment in the infrared range is about 47%, which is similar to the transmittance reported by Suarez et al. [30] for SPS of co-precipitated YAG powder. The apparent transparency of U5 and A4 ceramics after annealing treatment at 1250 C for 3 h are also demonstrated in Fig. 9. The annealing thermal treatment led to a faint color for U5 and opaque color for A4 ceramics. The results of ICP analyses of the U4 powder show that the amount of Na, Ca and K impurity introduced by distilled water is not negligible (Table 4). In order to further evaluate, the EDX analysis of the fracture surface of U5 ceramic (Fig. 7a, “A” area), shows that amounts of K and Ca impurities in grain boundaries. It can be guessed that, these impurities have segregated to grain boundaries during the annealing treatment, leading to a reduction in the transparency in both U5 and A4 ceramics. Moreover, it could be due to the evaporation of carbon particles, leaving behind tiny pores in the bulk, or to the microstructural changes during annealing, leading to new or larger defects, as suggested by Spina et al. [31]. The in-line transmittance depends not only on the porosity but also is very sensitive to the pore diameter. When the pore size becomes of the same order of magnitude of the incident light wavelength, the transmission reaches a minimum [31]. Hence, the larger size of pores in the A4 than U5 ceramics can explain the loss of transmittance in IR and visible ranges. Moreover, it probably seems that the more aggregated powder morphology of A3 compared with U4 (Fig. 3b and c), leads to its less sinterability, lower density and more residual porosities (Fig. 7a and b). Based on TEM image (Fig. 4b), the nanocomposite synthesized by urea as a precipitant, consists of spherical core-shell nanoparticles and this shape of particles is appropriate for using as a powder precursor for sintering. On the contrary, as shown in FESEM images (Fig. 3c), the nanocomposite synthesized by AHC as a precipitant, consists of spherical alumina nanoparticles covered by the sheet like yttria particles. It can be guessed that, this shape of particles is not useful for sintering process and usually leads to an
Fig. 6. XRD patterns of SPS samples a) U5, b) A4.
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
G. Kafili et al. / Materials Chemistry and Physics xxx (2016) 1e9
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Fig. 7. FESEM images of the YAG fracture surface after SPS for 10 min a) U5, b) A4 ceramics, and c) EDX results of the “A” area of the fracture surface image of U5, insert: quantitative results of EDX.
Fig. 8. Optical transmittance of YAG specimen (U5 sample with 1 mm thickness) in IR part of spectrum.
Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011
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G. Kafili et al. / Materials Chemistry and Physics xxx (2016) 1e9
Fig. 9. a) U5, b) A4 sample of 1 mm thick disc fabricated by the SPS process.
Table 4 The results of ICP analyses of U4 alumina/yttria nanocomposite powder.
[2]
Element
Na
K
Ca
Al
Y
Quantity (ppm)
324
1201
1457
>10%
>10%
opaque YAG ceramic. It is well known that the optimization of SPS process, using colloidal forming processes and also adding different sintering aids such as tetraethyl orthosilicate (TEOS), MgO and LiF play a key role in enhancing the optical characteristics of YAG ceramics, in order to obtain a material with a small grain size, high theoretical density and transparency in both the visible and the infrared range. Hence, we are studying on the effect of these parameters, which will be mentioned later in our future works.
[3]
[4]
[5]
[6]
[7] [8]
4. Conclusions [9]
In this framework of this approach, the alumina/yttria nanocomposite powders were produced via partial wet route using urea and AHC as precipitants, respectively. Urea exceeds AHC for the production of an alumina/yttria nanocomposite with an appropriate morphology of YAG precursor. The use of urea produced very tiny Y-compounds, which were attracted to the surface of the alumina nanoparticles and formed a core-shell structure. Therefore, the morphology of the alumina/yttria nanocomposite powder in the urea system was spherical as a result of spherical alumina particles being covered by a uniform yttria shell layer. The sheet like Y-compound was synthesized using ammonium hydrogen carbonate as a precipitant. Consequently, in the AHC system, alumina nanoparticles were covered by Y-compound sheets, hence; the whole surface of alumina nanoparticles was not uniformly covered by yttria. Urea proved to be an appropriate precipitant for obtaining alumina/yttria nanocomposite powders with appropriate morphology and highly sinterability, which leads to a transparent YAG ceramic, while only opaque YAG ceramic was obtained from alumina/yttria nanocomposite powder synthesized in the AHC system. Our study demonstrated that in addition to YAG precursors synthesized via different methods, the alumina/yttria nanocomposite powders with more appropriate morphology and highly sinterability could be used as a powder precursor for fabricating transparent YAG ceramics, even without adding any sintering aid materials. References [1] L. Wen, X. Sun, Z. Xiu, S. Chen, C.-T. Tsai, Synthesis of nanocrystalline yttria
[10] [11]
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
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Please cite this article in press as: G. Kafili, et al., A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.08.011