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Sintering kinetics of disperse ultrafine equiaxed α-Al2O3 nanoparticles
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Sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles Wenbin Cao, Xuan Mao, Yuan Yuan, Lu Li, Libin Zhao, Jiangong Li ∗ Institute of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 13 December 2016 Received in revised form 8 March 2017 Accepted 28 March 2017 Available online xxx Keywords: Alumina (˛-Al2O3) Sintering Nanocrystalline materials Kinetic analysis Kinetic window
a b s t r a c t The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine ˛-Al2 O3 nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of ˛-Al2 O3 and almost fully dense ˛-Al2 O3 nanocrystalline ceramic obtained also has the finest grain so far. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Alumina ceramics have wide applications as electrical insulations, translucent sodium vapor lamp envelopes, and structural materials due to their unique physicochemical properties [1–4]. However, their fatal weakness is brittleness which limits their wider applications. CaF2 and TiO2 nanocrystalline ceramics with a mean grain size of 8 nm exhibit large plastic deformations at 80 and 180 ◦ C, respectively [5]. ˛-Al2 O3 nanocrystalline ceramic with fine grains may show low temperature ductility as well. Many efforts have been made to prepare ˛-Al2 O3 nanocrystalline ceramics. Liu et al. obtained fully dense ˛-Al2 O3 ceramic with a mean grain size of 2.2 m from the particles with a mean size of 1 m by spark plasma sintering [6]. Zuo et al. reported that ˛-Al2 O3 ceramic with a mean grain size of 1.4 m and a relative density of 95% was obtained from submicron particles by microwave sintering [7]. Chang et al. obtained fully dense ˛-Al2 O3 ceramic with a mean grain size of 0.61 m from the particles with a mean size of 57 nm by hot pressing at 60 MPa [8]. Hesabi et al. [9] prepared ˛-Al2 O3 ceramic with a mean grain size of 0.5 m and a relative density of 98% by a simple two-step pressureless sintering [10] from the starting particles with a mean size of 150 nm. Li et al. [11] obtained ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 70 nm and a relative density of 95% by two-step sintering from the ˛-Al2 O3 particles with a mean size of 10 nm which has vermicular agglomeration. The vermicular agglomeration would lead to differential densifica-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Li).
tion and result in a low final density in sintering [11–13]. Highly dense (>99%) ˛-Al2 O3 nanocrystalline ceramics are difficult to prepare because disperse ˛-Al2 O3 nanoparticles are extremely difficult to synthesize and the ˛-Al2 O3 particles obtained usually have a submicron size or vermicular agglomeration [6–9,11,14–17]. Liao et al. prepared ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 49 nm and a relative density of 98.2% by high pressure (8 GPa)/low temperature sintering from -Al2 O3 nanoparticles with a mean grain size of 18 nm [18]. In our previous work, we prepared disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with different mean particle sizes below 10 nm by a mechanochemistry − selective corrosion − fractionated coagulation separation approach; and an ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 60 nm and a relative density of 99.5% was sintered from such ˛-Al2 O3 nanoparticles with a mean size of 7.9 nm and a size distribution of 4–14 nm by two-step sintering [19]. In our another previous work, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 55 nm and a relative density of 99.6% was sintered from the ˛-Al2 O3 nanoparticles with a mean size of 9 nm [20]. The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains. However, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles below 10 nm has not been systematically explored so far. In our present work, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration, obtained by mechanochemistry − selective corrosion − fractionated coagulation separation, was studied. Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering
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Fig. 1. The TEM images (a and b), size distribution histogram (c), and XRD pattern (d) of the ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm.
(heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). 2. Experimental procedure The ˛-Al2 O3 nanoparticles were prepared by mechanochemistry − selective corrosion − fractionated coagulation separation [19]. At first, Fe2 O3 and Al powders mixed stoichiometrically according to Fe2 O3 + 2Al = 2Fe + Al2 O3 were ball-milled using a high-energy planetary ball mill to obtain the composite of ˛-Al2 O3 nanoparticles embedded in Fe matrix. In order to remove Fe and other impurities in the composite, the composite powders were corroded with 12 mol/L hydrochloric acid (HCl) at room temperature for 10 h and then corroded with 4 mol/L HCl in a sealed hydrothermal synthesis reactor at 120 ◦ C for 10 h. Then the ˛-Al2 O3 nanoparticles were separated using HCl as coagulating agent by fractionated coagulation separation at different HCl concentrations. Finally, ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm and narrow size distributions were obtained. The ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were uniaxially pressed at 500 MPa into green compacts of 8 mm diameter and 0.5 mm thickness. The green compacts were sintered by normal sintering and two-step sintering. For normal sintering, the green compacts were heated at a rate of 10 ◦ C/min to different temperatures without hold and immediately cooled at 10 ◦ C/min down to room temperature. For two-step sintering, the green compacts were heated at
10 ◦ C/min to first-step sintering temperatures T1 without hold, cooled at 10 ◦ C/min down to second-step sintering temperatures T2 with hold, and then cooled at 10 ◦ C/min down to room temperature. X-ray diffraction (XRD) analysis of the ˛-Al2 O3 nanoparticles and sintered ceramic samples was conducted on a Rigaku D/max2400 diffractometer using Cu K˛ radiation in the 2 = 20–80◦ range. The purity of the ˛-Al2 O3 nanoparticles were determined by inductively coupled plasma-atomic emission spectrometry (ICPAES) on a Thermo Jarrell Ash IRIS Advantage spectrometer. The morphology of the ˛-Al2 O3 nanoparticles was analyzed by transmission electron microscopy (TEM) on an FEI Tecnai G2 F30 electron microscope operating at an accelerating voltage of 300 kV. The microstructure of the sintered samples with low densities was analyzed by TEM on the samples ground slightly in an agate mortar. The sintered samples were fractured and then thermally etched for 2 h at the temperature 50–200 ◦ C lower than the sintering temperature for scanning electron microscopy (SEM) observations on a Tescan MIRA 3 electron microscope operating at an accelerating voltage of 15 kV. The grain (or particle) sizes of more than 500 grains (or particles) observed in different areas of the initial ˛-Al2 O3 nanoparticles and the sintered samples by TEM or SEM were measured. The grain (or particle) size distribution histograms, mean grain (or particle) sizes, and standard deviations of the initial ˛-Al2 O3 nanoparticles and the sintered samples were statistically determined from the sizes and numbers of the grains (or particles). The relative densities of
Please cite this article in press as: W. Cao, et al., Sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.066
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Fig. 2. The relative density and mean grain size of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles as a function of sintering temperature without hold.
the sintered samples were calculated according to the equation [13,21]
= 0 1 −
L L0
Fig. 3. The mean grain size of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles as a function of relative density (I: the early sintering stage; II: the late sintering stage).
−3 ,
(1)
where 0 is the relative density of the green compact and L/L0 is the shrinkage of the sintered sample determined by the SEM observations. The relative densities of the sintered samples were also measured by the Archimedes’ method. 3. Results and discussion The level of impurity left in the ˛-Al2 O3 nanoparticles obtained by ball-milling and selective corrosion was analyzed by ICP-AES. The contents of Fe and Cr impurities in the ˛-Al2 O3 nanoparti-
cles are 0.27% and 0.08% (mass percent), respectively. The Fe and Cr mainly come from the raw materials and stainless steel balls and vials. After the fractionated coagulation separations, ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm and narrow size distributions were obtained. The ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm were characterized by TEM and XRD. The TEM images in Fig. 1a and b show that the ˛-Al2 O3 nanoparticles are disperse without any agglomeration, ultrafine in size, and equiaxed in shape. The size distribution histogram of these ˛-Al2 O3 nanoparticles in Fig. 1c reveals that the ˛-Al2 O3 nanoparticles have a narrow size distribution of 2–8 nm. The mean particle size of the ˛-Al2 O3 nanoparticles is 4.5 nm with a standard deviation of 1.1 nm. If averaged over the masses of the nanoparticles,
Fig. 4. The TEM images of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles sintered at 700 (a), 900 (b), 1000 (c), 1050 (d), 1100 (e), and 1150 ◦ C (f) without hold.
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Fig. 5. The second-step temperatures T2 and mean grain sizes used in second-step sintering from relative densities of 83–84% (a) and 89–90% (b) after first-step sintering. The starting ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were used. The solid symbols represent full density without grain growth in second-step sintering at T2 . The open symbols above the upper border lines represent full density with grain growth, and the open symbols below the lower border lines represent no full density achieved in second-step sintering at T2 . If the data for relative densities of 83–84% in (a) and 89–90% in (b) are plotted together, the plot (c) results.
the mean particle size is 5.2 nm. The XRD pattern of these ˛-Al2 O3 nanoparticles in Fig. 1d is a typical diffraction pattern of ˛-Al2 O3 (JCPDS No. 75-1862) [22], indicating that the obtained nanoparticles are pure ˛-Al2 O3 . The mean crystallite size of the ˛-Al2 O3 nanoparticles calculated using the Scherrer’s equation from the diffraction peak broadenings [23] is 4.8 nm, close to the mean particle size averaged over the masses (5.2 nm). The disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without agglomeration were used to explore the sintering kinetics of ˛-Al2 O3 nanoparticles. The green compacts pressed from the 4.5 nm ˛-Al2 O3 nanoparticles at 500 MPa have a relative density of 48%. The green compacts were heated to different temperatures
without hold and immediately cooled down to room temperature. Fig. 2 shows the relative density and mean grain size of the sintered samples as a function of sintering temperature. It reveals that the densification occurs above about 700 ◦ C, and the relative density reaches to 98.8% and 99.8% at 1200 and 1300 ◦ C without hold, respectively. The relative density increases rapidly with temperature from 700 to 1200 ◦ C and has the maximal densification rate at about 1125 ◦ C. Echeberria et al. explored the dependence of relative density on temperature (at heating rate of 10 ◦ C/min without hold) using the ˛-Al2 O3 particles with mean particle sizes of 0.2 and 0.4 m [24]. They found that the densification occurs above 900 ◦ C (700 ◦ C in our present work), and the maximal densification rates for the 0.2 and 0.4 m particles appear at 1320 and 1425 ◦ C, respectively, much higher than 1125 ◦ C in our present work. Li et al. [11] studied the dependence of relative density on sintering temperature (with a 1 h hold) for ˛-Al2 O3 nanoparticles agglomerated from primary particles of 10 nm in size and found that the maximal densification rate appears in the 1350–1400 ◦ C temperature range, much higher than 1125 ◦ C in our present work. So the 4.5 nm disperse ˛-Al2 O3 nanoparticles with a narrow size distribution in our present work exhibit a much higher sintering activity than the nanoparticles with agglomeration or larger particle sizes [11,24,25]. The grain growth occurs above 800 ◦ C. The mean grain size increases obviously with temperature increasing from 800 to 1050 ◦ C. The grain growth slows down with temperature increasing from 1050 to 1150 ◦ C. The mean grain size increases extremely fast with temperature increasing from 1200 to 1300 ◦ C. The rate of grain growth usually increases with increasing relative density and the variation of mean grain size with relative density usually has a concave shape in the sintering of nanoparticles or submicron particles [9,10,26–32]. In our present work, the variation of mean grain size with relative density deduced from the data in Fig. 2 is plotted in Fig. 3. When the relative density is less than 75% (corresponding to the sintering temperature below 1050 ◦ C), the mean grain size increases rapidly with increasing relative density or increasing temperature (Fig. 2). The mean grain size is 7 times the mean particle size of the starting ˛-Al2 O3 nanoparticles when the relative density reaches to 75%. But the mean grain size increases obviously slowly with increasing relative density in the relative density range of 75–95% (corresponding to the temperature range of 1050–1150 ◦ C). When the relative density increases from 75% to 83% and to 95%, the mean grain size only increases from 30 to 36 nm and to 43 nm, respectively. The variation of mean grain size with relative density can be regarded as a convex shape in the early sintering stage (relative densities less than 95%), different from a concave shape usually found for nanoparticles or submicron particles in literature [9,10,26–32]. In the late sintering stage (relative densities larger than 95%), the rate of grain growth increases dramatically with increasing relative density. When the relative density reaches 95%, the pores become closed and decrease with increasing relative density, some grain boundaries break away from the pinning of the closed pores, which lead to obvious grain growth [26]. In order to further explore the variation of mean grain size with relative density in the early sintering stage, the samples sintered at 700, 900, 1000, 1050, 1100, and 1150 ◦ C without hold were analyzed by TEM (Fig. 4). When the green compact was sintered at 700 ◦ C, the particle/grain sizes do not change visibly compared with the starting ˛-Al2 O3 nanoparticles. After sintered at 900 ◦ C, most grains are less than 10 nm, but some grains larger than 20 nm (even larger than 30 nm) appear. After sintered at 1000 ◦ C, most grains range from 10 to 35 nm in size. When the green compact was sintered at 1050 ◦ C, the grains less than 15 nm almost disappear, and most grains are in the size range of 20–40 nm. Compared with the
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Fig. 6. The relative density and mean grain size as a function of holding duration at the second-step sintering temperatures T2 of 900 (a), 950 (b), and 1000 ◦ C (c) for the samples from the 4.5 nm ˛-Al2 O3 nanoparticles after a first-step sintering at T1 = 1100 ◦ C without hold.
grain size of the sample sintered at 1050 ◦ C, no remarkable grain growth for the samples sintered at 1100 and 1150 ◦ C was observed. Compared with the sintering behaviors of nanoparticles or submicron particles reported in literature [9,10,26–32], the sintering behavior of the 4.5 nm ˛-Al2 O3 nanoparticles in the early sintering stage in our work is different. The 4.5 nm ˛-Al2 O3 nanoparticles are disperse without any agglomeration, ultrafine in size, and narrow in size distribution. They have small surface curvature radii and high specific surface area. ˛-Al2 O3 has a higher surface energy than -Al2 O3 and becomes thermodynamically unstable at specific surface areas greater than 100 m2 /g at room temperature (75 m2 /g at 527 ◦ C), corresponding to spherical particle sizes less than 15 nm at room temperature (20 nm at 527 ◦ C) [33]. Considering the above factors, the diffusion inside the sample would be active and rapid above 700 ◦ C, which will lead to the growth of larger grains at the expense of smaller grains and the formation of non-equilibrium grain boundaries with small curvature radii [26,34]. Non-equilibrium grains and grain boundaries with small curvature radii have a high driving force and low activation energy of diffusion which will lead to rapid diffusion and grain growth of
5
a few grains corresponding to Fig. 4b [34]. So grain boundary slip, rotation of grains, and the decrease of distance between the centers of two contiguous grains occur, which leads to rapid densification along with rapid grain growth [26]. When the samples were heated to the temperature range of about 700–1050 ◦ C, a large number of smaller non-equilibrium grains (about 10 nm) with small curvature radii (having high driving force and low activation energy of diffusion) are present in the samples to maintain rapid diffusion rate which causes rapid densification and rapid grain growth, as shown in Figs. 2–4. In the temperature range of 1050–1150 ◦ C, as the grains smaller than 15 nm almost disappear, a sharp increase in the curvature radii of grain boundaries occurs and the driving force of grain boundary migration decreases. So the grain growth rate slows down, but high grain boundary diffusion can maintain rapid densification in the temperature range of 1050–1150 ◦ C, as shown in Figs. 2 and 3. So the variation of mean grain size with relative density shows a convex shape in the early sintering stage, different from a concave shape usually found for nanoparticles or submicron particles in literature [9,10,26–32]. In general, nanoparticles or submicron particles used as starting particles for sintering reported in literature usually have hard agglomeration or larger particle sizes (much larger than 4.5 nm) and thus have more stable structure and lower thermodynamic driving force of sintering in the early sintering stage. The Fe and Cr impurities in our ˛-Al2 O3 nanoparticles are 0.27% and 0.08% (mass percent), respectively. Drahus et al. found that compared with undoped ˛-Al2 O3 , the addition of 1000 ppm Fe3+ reduces the densification rate and simultaneously retards the grain growth [35]. Ji et al. reported that in comparison with undoped ˛Al2 O3 , the addition of 5 vol.% Cr also reduces the densification rate and retards the grain growth [36]. As the Fe and Cr impurities in ˛-Al2 O3 reduce the densification rate and retard the grain growth simultaneously during sintering, their effect on the relative density and grain sizes of sintered ˛-Al2 O3 nanocrystalline ceramic may be unobvious. In order to avoid grain growth in the final stage, Chen et al. developed a two-step pressureless sintering method [10]. At first, the green compacts were heated to a higher temperature T1 (firststep sintering temperature) without hold to reach a critical density. Above the critical density, pores become subcritical and unstable, which will provide driving force for subsequent densification. Then the compacts were cooled down to a lower temperature T2 (secondstep sintering temperature) with hold. In the second-step sintering at T2 , due to the difference in kinetics between grain boundary diffusion and grain boundary migration, the grain boundary diffusion is maintained to achieve full density, but the grain boundary migration is suppressed to prevent grain growth. In our work, when a relative density of 83–84% is achieved for the samples at T1 without hold, the temperatures T2 and the mean grain sizes used for the second-step sintering are shown in Fig. 5a. The ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were used. The solid symbols in Fig. 5a represent the temperatures at which the samples can reach full density (at least 99% in this paper) without grain growth in the second-step sintering. They are bordered by two lines which delineates the temperature range for achieving full density without grain growth in the second-step sintering called a kinetic window [10]. The open symbols above the upper border line in Fig. 5a represent the temperatures at which the samples reached full density but with a grain growth in the second-step sintering. The open symbols below the lower border line in Fig. 5a indicate the temperatures at which no grain growth occurs but densification is exhausted in the second-step sintering. The upper and lower border lines of the kinetic window go upward with increasing grain size. When the samples have a relative density of 89–90% after sintered at T1 without hold, the kinetic window is depictured in Fig. 5b, which is similar as in Fig. 5a. If the difference in relative density
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Fig. 7. The SEM images and corresponding grain size distribution histograms of the samples heated to 1100 ◦ C without hold and then cooled to 900 (a, d), 950 (b, e), and 1000 ◦ C (c, f) with a 40 h hold.
for the samples sintered at T1 (83–84% for Fig. 5a and 89–90% for Fig. 5b) is neglected for describing the kinetic window, similar as Chen et al.’s works [10,30,32], the kinetic windows in Fig. 5a and b may be combined into one kinetic window as shown in Fig. 5c. However, as indicated by dashed lines in Fig. 5c, there are no definite borders between solid symbols and open symbols in Fig. 5c which describe the temperature range for the second-step sintering to achieve full density without grain growth. So the relative density achieved in the first-step sintering should be considered as a factor affecting a kinetic window. Namely, different from Chen et al.’s works [10,30,32], the different relative densities obtained at T1 (for example, 83–84% and 89–90%) will result in different temperature ranges for T2 to achieve full density without grain growth or different kinetic windows, as shown in Fig. 5a and b. The detailed data including relative densities of green compacts, sintering parameters used for first-step and second-step sintering, and relative densities and mean grain sizes after first-step and secondstep sintering are listed in Table 1 (corresponding to the data in Fig. 5). It also shows that the relative density of the green compacts increases with increasing size of the starting ˛-Al2 O3 nanoparticles, but the lowest densities for different starting ˛-Al2 O3 nanoparticles after first-step sintering are similar and have a value of about 83% below which full density without grain growth cannot be achieved in second-step sintering (not shown in Table 1). So a relative density should reach a critical value of about 83% after first-step sintering to achieve full density without grain growth in the second-step sintering. Fig. 6 shows the variation of relative density and mean grain size versus holding duration at different T2 . All samples were first heated to T1 = 1100 ◦ C without hold and have a mean grain size of 36 nm and a relative density of 83%. Then they were held at different T2 for different holding durations. As the holding duration at T2 = 900 ◦ C increases from 0 to 40 h, the grain size maintains 36 nm, but the relative density increases only to 97%, as shown in Fig. 6a.
This implies that T2 = 900 ◦ C is too low to maintain boundary diffusion and achieve full density. As the holding duration at T2 = 950 ◦ C increases from 0 to 40 h, the grain size also maintains 36 nm, however, the relative density of the sample increases to 99.7%, as shown in Fig. 6b. So T2 = 950 ◦ C is high enough for activating boundary diffusion but still too low to activate gain boundary migration. When held at T2 = 1000 ◦ C for 20 h, the sample reaches a relative density of 99.5% without obvious grain growth, as shown in Fig. 6c. When the holding duration at T2 = 1000 ◦ C was prolonged to 40 h, the relative density increases to 99.6%, but the mean grain size increases to 48 nm. Namely, an obvious grain growth occurred when the holding duration at T2 = 1000 ◦ C was prolonged to 40 h. Fig. 7 shows the SEM images and corresponding grain size distribution histograms of the samples first heated to T1 = 1100 ◦ C without hold and then cooled down to 900, 950, and 1000 ◦ C with a 40 h hold, respectively. They reveal clear grain boundaries, equiaxed grains, and narrow grain size distributions for the three samples. The mean grain sizes of the three samples are 36, 36, and 48 nm and their grain size distributions are 10–85, 15–85, and 20–100 nm, respectively. The XRD pattern of the ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% in Fig. 8 reveals that this nanocrystalline ceramic is pure ˛-Al2 O3 . In order to determine the mechanisms of densification during the second-step sintering, the samples heated to 1100 ◦ C without hold and then cooled down to different T2 for different holding durations were analyzed. During holding, the linear shrinkage L/L and holding duration t satisfy the equation [37,38] 1 L = A (T ) t n , L0
(2)
where A(T) can be regarded as a constant depending on T and n is a characteristic exponent. When n is 2.5, the sintering is controlled by volume diffusion. When n is 3, the sintering is controlled by grain boundary diffusion. According to Eq. (2), the dependence of linear shrinkage on holding duration is plotted in Fig. 9. The values
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Table 1 Two-step sintering of ˛-Al2 O3 (4.5, 7.3, and 62 nm nanoparticles). Relative densities of green compacts (0 ), sintering parameters used for first-step and second-step sintering, and relative densities and mean grain sizes after first-step and second-step sintering are listed. Sample
0 (%)
˛-Al2 O3 −1 ˛-Al2 O3 −2a ˛-Al2 O3 −3a ˛-Al2 O3 −4a ˛-Al2 O3 −5a ˛-Al2 O3 −6a ˛-Al2 O3 −7a ˛-Al2 O3 −8a ˛-Al2 O3 −9a ˛-Al2 O3 −10a ˛-Al2 O3 −11a ˛-Al2 O3 −12b ˛-Al2 O3 −13b ˛-Al2 O3 −14b ˛-Al2 O3 −15b ˛-Al2 O3 −16b ˛-Al2 O3 −17b ˛-Al2 O3 −18b ˛-Al2 O3 −19b ˛-Al2 O3 −20b ˛-Al2 O3 −21b ˛-Al2 O3 −22b ˛-Al2 O3 −23b ˛-Al2 O3 −24c ˛-Al2 O3 −25c ˛-Al2 O3 −26c ˛-Al2 O3 −27c ˛-Al2 O3 −28c ˛-Al2 O3 −29c ˛-Al2 O3 −30c ˛-Al2 O3 −31c ˛-Al2 O3 −32c ˛-Al2 O3 −33c ˛-Al2 O3 −34c ˛-Al2 O3 −35c ˛-Al2 O3 −36c ˛-Al2 O3 −37c a
a b c
48 48 48 48 48 48 48 48 48 48 48 50 50 50 50 50 50 50 50 50 50 50 50 58 58 58 58 58 58 58 58 58 58 58 58 58 58
After first-step sintering without hold
After second-step sintering
T1 (◦ C)
1 (%)
G1 (nm)
T2 (◦ C)
t2 (h)
2 (%)
G2 (nm)
1100 1100 1100 1100 1100 1100 1125 1125 1125 1125 1125 1125 1125 1125 1125 1125 1125 1150 1150 1150 1150 1150 1150 1250 1250 1250 1250 1250 1250 1250 1275 1275 1275 1275 1275 1275 1275
83 83 83 83 83 83 89 89 89 89 89 84 84 84 84 84 84 90 90 90 90 90 90 84 84 84 84 84 84 84 89 89 89 89 89 89 89
36 36 36 36 36 36 40 40 40 40 40 43 43 43 43 43 43 49 49 49 49 49 49 75 75 75 75 75 75 75 86 86 86 86 86 86 86
900 925 950 975 1000 1025 900 925 950 1000 1050 900 925 950 975 1025 1050 900 925 950 1000 1050 1075 950 1000 1050 1100 1125 1150 1200 950 975 1025 1050 1125 1175 1225
75 75 40 40 20 10 75 75 40 10 10 75 75 75 40 20 10 75 75 40 40 10 10 75 75 75 20 20 20 10 75 75 75 40 40 20 10
97 98.7 99.7 99.7 99.5 99.3 98 99.2 99.7 99.5 99.5 93 95 98.8 99.6 99.6 99.4 95 98 99.2 99.7 99.3 99.5 92 95 99 99.4 99.4 99.5 99.3 93 95 99.1 99.1 99.5 99.4 99.2
36 36 36 37 38 44 40 40 40 41 49 43 43 43 43 45 52 49 49 49 49 54 58 75 75 76 75 77 78 86 86 86 86 86 88 90 97
4.5 nm nanoparticles. 7.3 nm nanoparticles. 62 nm nanoparticles.
of n corresponding to different T2 of 900, 950, and 1000 ◦ C are 3.01, 2.96, and 2.88, respectively. So grain boundary diffusion should be the main mechanism of densification at T2 in the 900–1000 ◦ C range and a weak contribution of volume diffusion to densification occurs at T2 of 1000 ◦ C. As mentioned above, the upper and lower border lines for kinetic windows describe the temperature ranges for T2 to achieve full density without grain growth. Grain boundary migration is suppressed to prevent grain growth below the upper border line but activated to trigger grain growth above the upper border line in the second-step sintering. Similar as Chen et al.’s works [10,30,32], the upper border lines are upward in our kinetic windows (Fig. 5a and b). This may be due to the decrease in the driving force of grain boundary migration with increasing grain size. As the grain size increases, the temperature T2 for activating grain boundary migration has to be increased. Densification will be exhausted below the lower border line but maintained to achieve full density above the lower border line in the second-step sintering. The lower border lines are likewise upward in our kinetic windows (Fig. 5a and b) whereas they are downward in Chen et al.’s works [10,30,32]. This implies that a higher temperature T2 is required to maintain densification and achieve full density in the second-step sintering for a larger grain size. According to Eq. (2) and the deduced characteristic exponent values, grain boundary diffusion is the main mechanism of densification in the second-step sintering at a lower T2 (corre-
Fig. 8. The XRD pattern of the ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7%.
sponding to T2 around the lower border line). With increasing grain size, the amount of grain boundaries decreases and grain boundary diffusion weakens, which will lower the densification rate. So
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Fig. 9. The dependence of linear shrinkage on holding duration at different secondstep sintering temperature T2 of 900, 950, and 1000 ◦ C (the samples were heated to 1100 ◦ C without hold and then cooled down to different T2 for different holding durations).
a higher temperature T2 is required to maintain grain boundary diffusion with increasing grain size around the lower border line in our kinetic windows. In Chen et al.’s works, a lower temperature T2 can also maintain densification and realize full density for a larger grain size [10,30,32]. The dislocations have a threshold nucleation energy or stress which is inversely proportional to grain size and rather substantial than capillary pressure when the grain size is less than 100 nm [10]. So the dislocation pipe diffusion should be suppressed at the grain sizes less than 100 nm and the suppression should diminish at larger grain sizes [10]. In Chen et al.’s works, the grain sizes in the second-step sintering are mainly in the 100–1000 nm range, so the effect of dislocations should be obvious and dislocation pipe diffusion can occur to achieve full density at lower T2 with increasing grain size though grain boundary diffusion weakens with increasing grain size [10,30,32]. In our present work, the grain sizes in the second-step sintering range from 36 to 86 nm, so the effect of the dislocation pipe diffusion can be ignored. As grain boundary diffusion is the mechanism of densification, the linear shrinkage L/L and activation energy of grain boundary diffusion Q follow the equation [39]
L Q = B (T ) exp − L0 3RT
,
(3)
where B(T) can be regarded as a constant depending on T, R is the gas constant, and T is the absolute temperature. Using the data of the samples heated to T1 = 1100 ◦ C without hold and then cooled down to different T2 for a 2 h hold, the dependence of linear shrinkage on temperature is shown in Fig. 10. The activation energy of grain boundary diffusion Q calculated by Eq. (3) is 199 kJ/mol, which is much lower than those reported in literature (about 400–900 kJ/mol) [40–43]. In our work, almost fully dense ˛Al2 O3 nanocrystalline ceramic can be obtained by grain boundary diffusion at a T2 of 950 ◦ C more than 200 ◦ C lower than the temperatures of second-step sintering of nanoparticles with agglomeration or larger particle sizes [10,11,44]. It should be due to the smaller activation energy of grain boundary diffusion in our work. Compared with ˛-Al2 O3 ceramics sintered by pressureless sintering, or pressure sintering, or electrically assisted sintering [6–9,11,16–20], our almost fully dense (99.7%) ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm obtained by two-step pressureless sintering have the finest grain reported so far. The sintering temperature is also the lowest for pressureless
Fig. 10. The dependence of linear shrinkage on second-step sintering temperature T2 (the samples were heated to 1100 ◦ C without hold and then cooled down to different T2 with a 2 h hold).
sintering of ˛-Al2 O3 . It is mainly due to the excellent sintering activity of the disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm. 4. Conclusions The sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration obtained by mechanochemistry − selective corrosion − fractionated coagulation separation was studied. In the early sintering stage (relative densities less than 95%), the variation of mean grain size with relative density shows a novel convex shape, different from a concave shape usually found for nanoparticles or submicron particles. The 4.5 nm ˛-Al2 O3 nanoparticles with narrow size distribution and without any agglomeration have a non-equilibrium structure, smaller surface curvature radii, lower surface activation energy, and larger thermodynamic driving force of diffusion. A rapid grain growth occurs in the late sintering stage for relative densities larger than 95%. Using a two-step sintering method, the kinetic windows which describe the temperature ranges of the second-step sintering for densification without final grain growth were determined for the ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm. The different relative densities (83–84% and 89–90%) obtained at first-step sintering temperatures T1 will result in different temperature ranges for the second-step sintering temperatures T2 to achieve full density without grain growth or different kinetic windows. Different from the downward lower border lines in literature, the lower border lines in our kinetic windows are upward, which may be due to the different grain size ranges used in the second-step sintering (36–86 nm in our work and 100–1000 nm in literature) and different diffusion mechanisms in the secondstep sintering. The main mechanism of densification in second-step sintering in the 900–1000 ◦ C range in our work is grain boundary diffusion. The activation energy of grain boundary diffusion is 199 kJ/mol, much lower than those reported in literature (about 400–900 kJ/mol). Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of ˛-Al2 O3 and the obtained almost fully dense ˛-Al2 O3 nanocrystalline ceramic has the finest grain so far. The detailed exploration of densification,
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Sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles Wenbin Cao, Xuan Mao, Yuan Yuan, Lu Li, Libin Zhao, Jiangong Li ∗ Institute of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 13 December 2016 Received in revised form 8 March 2017 Accepted 28 March 2017 Available online xxx Keywords: Alumina (˛-Al2O3) Sintering Nanocrystalline materials Kinetic analysis Kinetic window
a b s t r a c t The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine ˛-Al2 O3 nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of ˛-Al2 O3 and almost fully dense ˛-Al2 O3 nanocrystalline ceramic obtained also has the finest grain so far. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Alumina ceramics have wide applications as electrical insulations, translucent sodium vapor lamp envelopes, and structural materials due to their unique physicochemical properties [1–4]. However, their fatal weakness is brittleness which limits their wider applications. CaF2 and TiO2 nanocrystalline ceramics with a mean grain size of 8 nm exhibit large plastic deformations at 80 and 180 ◦ C, respectively [5]. ˛-Al2 O3 nanocrystalline ceramic with fine grains may show low temperature ductility as well. Many efforts have been made to prepare ˛-Al2 O3 nanocrystalline ceramics. Liu et al. obtained fully dense ˛-Al2 O3 ceramic with a mean grain size of 2.2 m from the particles with a mean size of 1 m by spark plasma sintering [6]. Zuo et al. reported that ˛-Al2 O3 ceramic with a mean grain size of 1.4 m and a relative density of 95% was obtained from submicron particles by microwave sintering [7]. Chang et al. obtained fully dense ˛-Al2 O3 ceramic with a mean grain size of 0.61 m from the particles with a mean size of 57 nm by hot pressing at 60 MPa [8]. Hesabi et al. [9] prepared ˛-Al2 O3 ceramic with a mean grain size of 0.5 m and a relative density of 98% by a simple two-step pressureless sintering [10] from the starting particles with a mean size of 150 nm. Li et al. [11] obtained ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 70 nm and a relative density of 95% by two-step sintering from the ˛-Al2 O3 particles with a mean size of 10 nm which has vermicular agglomeration. The vermicular agglomeration would lead to differential densifica-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Li).
tion and result in a low final density in sintering [11–13]. Highly dense (>99%) ˛-Al2 O3 nanocrystalline ceramics are difficult to prepare because disperse ˛-Al2 O3 nanoparticles are extremely difficult to synthesize and the ˛-Al2 O3 particles obtained usually have a submicron size or vermicular agglomeration [6–9,11,14–17]. Liao et al. prepared ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 49 nm and a relative density of 98.2% by high pressure (8 GPa)/low temperature sintering from -Al2 O3 nanoparticles with a mean grain size of 18 nm [18]. In our previous work, we prepared disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with different mean particle sizes below 10 nm by a mechanochemistry − selective corrosion − fractionated coagulation separation approach; and an ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 60 nm and a relative density of 99.5% was sintered from such ˛-Al2 O3 nanoparticles with a mean size of 7.9 nm and a size distribution of 4–14 nm by two-step sintering [19]. In our another previous work, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 55 nm and a relative density of 99.6% was sintered from the ˛-Al2 O3 nanoparticles with a mean size of 9 nm [20]. The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains. However, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles below 10 nm has not been systematically explored so far. In our present work, the sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration, obtained by mechanochemistry − selective corrosion − fractionated coagulation separation, was studied. Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering
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Fig. 1. The TEM images (a and b), size distribution histogram (c), and XRD pattern (d) of the ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm.
(heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). 2. Experimental procedure The ˛-Al2 O3 nanoparticles were prepared by mechanochemistry − selective corrosion − fractionated coagulation separation [19]. At first, Fe2 O3 and Al powders mixed stoichiometrically according to Fe2 O3 + 2Al = 2Fe + Al2 O3 were ball-milled using a high-energy planetary ball mill to obtain the composite of ˛-Al2 O3 nanoparticles embedded in Fe matrix. In order to remove Fe and other impurities in the composite, the composite powders were corroded with 12 mol/L hydrochloric acid (HCl) at room temperature for 10 h and then corroded with 4 mol/L HCl in a sealed hydrothermal synthesis reactor at 120 ◦ C for 10 h. Then the ˛-Al2 O3 nanoparticles were separated using HCl as coagulating agent by fractionated coagulation separation at different HCl concentrations. Finally, ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm and narrow size distributions were obtained. The ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were uniaxially pressed at 500 MPa into green compacts of 8 mm diameter and 0.5 mm thickness. The green compacts were sintered by normal sintering and two-step sintering. For normal sintering, the green compacts were heated at a rate of 10 ◦ C/min to different temperatures without hold and immediately cooled at 10 ◦ C/min down to room temperature. For two-step sintering, the green compacts were heated at
10 ◦ C/min to first-step sintering temperatures T1 without hold, cooled at 10 ◦ C/min down to second-step sintering temperatures T2 with hold, and then cooled at 10 ◦ C/min down to room temperature. X-ray diffraction (XRD) analysis of the ˛-Al2 O3 nanoparticles and sintered ceramic samples was conducted on a Rigaku D/max2400 diffractometer using Cu K˛ radiation in the 2 = 20–80◦ range. The purity of the ˛-Al2 O3 nanoparticles were determined by inductively coupled plasma-atomic emission spectrometry (ICPAES) on a Thermo Jarrell Ash IRIS Advantage spectrometer. The morphology of the ˛-Al2 O3 nanoparticles was analyzed by transmission electron microscopy (TEM) on an FEI Tecnai G2 F30 electron microscope operating at an accelerating voltage of 300 kV. The microstructure of the sintered samples with low densities was analyzed by TEM on the samples ground slightly in an agate mortar. The sintered samples were fractured and then thermally etched for 2 h at the temperature 50–200 ◦ C lower than the sintering temperature for scanning electron microscopy (SEM) observations on a Tescan MIRA 3 electron microscope operating at an accelerating voltage of 15 kV. The grain (or particle) sizes of more than 500 grains (or particles) observed in different areas of the initial ˛-Al2 O3 nanoparticles and the sintered samples by TEM or SEM were measured. The grain (or particle) size distribution histograms, mean grain (or particle) sizes, and standard deviations of the initial ˛-Al2 O3 nanoparticles and the sintered samples were statistically determined from the sizes and numbers of the grains (or particles). The relative densities of
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Fig. 2. The relative density and mean grain size of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles as a function of sintering temperature without hold.
the sintered samples were calculated according to the equation [13,21]
= 0 1 −
L L0
Fig. 3. The mean grain size of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles as a function of relative density (I: the early sintering stage; II: the late sintering stage).
−3 ,
(1)
where 0 is the relative density of the green compact and L/L0 is the shrinkage of the sintered sample determined by the SEM observations. The relative densities of the sintered samples were also measured by the Archimedes’ method. 3. Results and discussion The level of impurity left in the ˛-Al2 O3 nanoparticles obtained by ball-milling and selective corrosion was analyzed by ICP-AES. The contents of Fe and Cr impurities in the ˛-Al2 O3 nanoparti-
cles are 0.27% and 0.08% (mass percent), respectively. The Fe and Cr mainly come from the raw materials and stainless steel balls and vials. After the fractionated coagulation separations, ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm and narrow size distributions were obtained. The ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm were characterized by TEM and XRD. The TEM images in Fig. 1a and b show that the ˛-Al2 O3 nanoparticles are disperse without any agglomeration, ultrafine in size, and equiaxed in shape. The size distribution histogram of these ˛-Al2 O3 nanoparticles in Fig. 1c reveals that the ˛-Al2 O3 nanoparticles have a narrow size distribution of 2–8 nm. The mean particle size of the ˛-Al2 O3 nanoparticles is 4.5 nm with a standard deviation of 1.1 nm. If averaged over the masses of the nanoparticles,
Fig. 4. The TEM images of the bodies sintered from the 4.5 nm ˛-Al2 O3 nanoparticles sintered at 700 (a), 900 (b), 1000 (c), 1050 (d), 1100 (e), and 1150 ◦ C (f) without hold.
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Fig. 5. The second-step temperatures T2 and mean grain sizes used in second-step sintering from relative densities of 83–84% (a) and 89–90% (b) after first-step sintering. The starting ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were used. The solid symbols represent full density without grain growth in second-step sintering at T2 . The open symbols above the upper border lines represent full density with grain growth, and the open symbols below the lower border lines represent no full density achieved in second-step sintering at T2 . If the data for relative densities of 83–84% in (a) and 89–90% in (b) are plotted together, the plot (c) results.
the mean particle size is 5.2 nm. The XRD pattern of these ˛-Al2 O3 nanoparticles in Fig. 1d is a typical diffraction pattern of ˛-Al2 O3 (JCPDS No. 75-1862) [22], indicating that the obtained nanoparticles are pure ˛-Al2 O3 . The mean crystallite size of the ˛-Al2 O3 nanoparticles calculated using the Scherrer’s equation from the diffraction peak broadenings [23] is 4.8 nm, close to the mean particle size averaged over the masses (5.2 nm). The disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without agglomeration were used to explore the sintering kinetics of ˛-Al2 O3 nanoparticles. The green compacts pressed from the 4.5 nm ˛-Al2 O3 nanoparticles at 500 MPa have a relative density of 48%. The green compacts were heated to different temperatures
without hold and immediately cooled down to room temperature. Fig. 2 shows the relative density and mean grain size of the sintered samples as a function of sintering temperature. It reveals that the densification occurs above about 700 ◦ C, and the relative density reaches to 98.8% and 99.8% at 1200 and 1300 ◦ C without hold, respectively. The relative density increases rapidly with temperature from 700 to 1200 ◦ C and has the maximal densification rate at about 1125 ◦ C. Echeberria et al. explored the dependence of relative density on temperature (at heating rate of 10 ◦ C/min without hold) using the ˛-Al2 O3 particles with mean particle sizes of 0.2 and 0.4 m [24]. They found that the densification occurs above 900 ◦ C (700 ◦ C in our present work), and the maximal densification rates for the 0.2 and 0.4 m particles appear at 1320 and 1425 ◦ C, respectively, much higher than 1125 ◦ C in our present work. Li et al. [11] studied the dependence of relative density on sintering temperature (with a 1 h hold) for ˛-Al2 O3 nanoparticles agglomerated from primary particles of 10 nm in size and found that the maximal densification rate appears in the 1350–1400 ◦ C temperature range, much higher than 1125 ◦ C in our present work. So the 4.5 nm disperse ˛-Al2 O3 nanoparticles with a narrow size distribution in our present work exhibit a much higher sintering activity than the nanoparticles with agglomeration or larger particle sizes [11,24,25]. The grain growth occurs above 800 ◦ C. The mean grain size increases obviously with temperature increasing from 800 to 1050 ◦ C. The grain growth slows down with temperature increasing from 1050 to 1150 ◦ C. The mean grain size increases extremely fast with temperature increasing from 1200 to 1300 ◦ C. The rate of grain growth usually increases with increasing relative density and the variation of mean grain size with relative density usually has a concave shape in the sintering of nanoparticles or submicron particles [9,10,26–32]. In our present work, the variation of mean grain size with relative density deduced from the data in Fig. 2 is plotted in Fig. 3. When the relative density is less than 75% (corresponding to the sintering temperature below 1050 ◦ C), the mean grain size increases rapidly with increasing relative density or increasing temperature (Fig. 2). The mean grain size is 7 times the mean particle size of the starting ˛-Al2 O3 nanoparticles when the relative density reaches to 75%. But the mean grain size increases obviously slowly with increasing relative density in the relative density range of 75–95% (corresponding to the temperature range of 1050–1150 ◦ C). When the relative density increases from 75% to 83% and to 95%, the mean grain size only increases from 30 to 36 nm and to 43 nm, respectively. The variation of mean grain size with relative density can be regarded as a convex shape in the early sintering stage (relative densities less than 95%), different from a concave shape usually found for nanoparticles or submicron particles in literature [9,10,26–32]. In the late sintering stage (relative densities larger than 95%), the rate of grain growth increases dramatically with increasing relative density. When the relative density reaches 95%, the pores become closed and decrease with increasing relative density, some grain boundaries break away from the pinning of the closed pores, which lead to obvious grain growth [26]. In order to further explore the variation of mean grain size with relative density in the early sintering stage, the samples sintered at 700, 900, 1000, 1050, 1100, and 1150 ◦ C without hold were analyzed by TEM (Fig. 4). When the green compact was sintered at 700 ◦ C, the particle/grain sizes do not change visibly compared with the starting ˛-Al2 O3 nanoparticles. After sintered at 900 ◦ C, most grains are less than 10 nm, but some grains larger than 20 nm (even larger than 30 nm) appear. After sintered at 1000 ◦ C, most grains range from 10 to 35 nm in size. When the green compact was sintered at 1050 ◦ C, the grains less than 15 nm almost disappear, and most grains are in the size range of 20–40 nm. Compared with the
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Fig. 6. The relative density and mean grain size as a function of holding duration at the second-step sintering temperatures T2 of 900 (a), 950 (b), and 1000 ◦ C (c) for the samples from the 4.5 nm ˛-Al2 O3 nanoparticles after a first-step sintering at T1 = 1100 ◦ C without hold.
grain size of the sample sintered at 1050 ◦ C, no remarkable grain growth for the samples sintered at 1100 and 1150 ◦ C was observed. Compared with the sintering behaviors of nanoparticles or submicron particles reported in literature [9,10,26–32], the sintering behavior of the 4.5 nm ˛-Al2 O3 nanoparticles in the early sintering stage in our work is different. The 4.5 nm ˛-Al2 O3 nanoparticles are disperse without any agglomeration, ultrafine in size, and narrow in size distribution. They have small surface curvature radii and high specific surface area. ˛-Al2 O3 has a higher surface energy than -Al2 O3 and becomes thermodynamically unstable at specific surface areas greater than 100 m2 /g at room temperature (75 m2 /g at 527 ◦ C), corresponding to spherical particle sizes less than 15 nm at room temperature (20 nm at 527 ◦ C) [33]. Considering the above factors, the diffusion inside the sample would be active and rapid above 700 ◦ C, which will lead to the growth of larger grains at the expense of smaller grains and the formation of non-equilibrium grain boundaries with small curvature radii [26,34]. Non-equilibrium grains and grain boundaries with small curvature radii have a high driving force and low activation energy of diffusion which will lead to rapid diffusion and grain growth of
5
a few grains corresponding to Fig. 4b [34]. So grain boundary slip, rotation of grains, and the decrease of distance between the centers of two contiguous grains occur, which leads to rapid densification along with rapid grain growth [26]. When the samples were heated to the temperature range of about 700–1050 ◦ C, a large number of smaller non-equilibrium grains (about 10 nm) with small curvature radii (having high driving force and low activation energy of diffusion) are present in the samples to maintain rapid diffusion rate which causes rapid densification and rapid grain growth, as shown in Figs. 2–4. In the temperature range of 1050–1150 ◦ C, as the grains smaller than 15 nm almost disappear, a sharp increase in the curvature radii of grain boundaries occurs and the driving force of grain boundary migration decreases. So the grain growth rate slows down, but high grain boundary diffusion can maintain rapid densification in the temperature range of 1050–1150 ◦ C, as shown in Figs. 2 and 3. So the variation of mean grain size with relative density shows a convex shape in the early sintering stage, different from a concave shape usually found for nanoparticles or submicron particles in literature [9,10,26–32]. In general, nanoparticles or submicron particles used as starting particles for sintering reported in literature usually have hard agglomeration or larger particle sizes (much larger than 4.5 nm) and thus have more stable structure and lower thermodynamic driving force of sintering in the early sintering stage. The Fe and Cr impurities in our ˛-Al2 O3 nanoparticles are 0.27% and 0.08% (mass percent), respectively. Drahus et al. found that compared with undoped ˛-Al2 O3 , the addition of 1000 ppm Fe3+ reduces the densification rate and simultaneously retards the grain growth [35]. Ji et al. reported that in comparison with undoped ˛Al2 O3 , the addition of 5 vol.% Cr also reduces the densification rate and retards the grain growth [36]. As the Fe and Cr impurities in ˛-Al2 O3 reduce the densification rate and retard the grain growth simultaneously during sintering, their effect on the relative density and grain sizes of sintered ˛-Al2 O3 nanocrystalline ceramic may be unobvious. In order to avoid grain growth in the final stage, Chen et al. developed a two-step pressureless sintering method [10]. At first, the green compacts were heated to a higher temperature T1 (firststep sintering temperature) without hold to reach a critical density. Above the critical density, pores become subcritical and unstable, which will provide driving force for subsequent densification. Then the compacts were cooled down to a lower temperature T2 (secondstep sintering temperature) with hold. In the second-step sintering at T2 , due to the difference in kinetics between grain boundary diffusion and grain boundary migration, the grain boundary diffusion is maintained to achieve full density, but the grain boundary migration is suppressed to prevent grain growth. In our work, when a relative density of 83–84% is achieved for the samples at T1 without hold, the temperatures T2 and the mean grain sizes used for the second-step sintering are shown in Fig. 5a. The ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm were used. The solid symbols in Fig. 5a represent the temperatures at which the samples can reach full density (at least 99% in this paper) without grain growth in the second-step sintering. They are bordered by two lines which delineates the temperature range for achieving full density without grain growth in the second-step sintering called a kinetic window [10]. The open symbols above the upper border line in Fig. 5a represent the temperatures at which the samples reached full density but with a grain growth in the second-step sintering. The open symbols below the lower border line in Fig. 5a indicate the temperatures at which no grain growth occurs but densification is exhausted in the second-step sintering. The upper and lower border lines of the kinetic window go upward with increasing grain size. When the samples have a relative density of 89–90% after sintered at T1 without hold, the kinetic window is depictured in Fig. 5b, which is similar as in Fig. 5a. If the difference in relative density
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Fig. 7. The SEM images and corresponding grain size distribution histograms of the samples heated to 1100 ◦ C without hold and then cooled to 900 (a, d), 950 (b, e), and 1000 ◦ C (c, f) with a 40 h hold.
for the samples sintered at T1 (83–84% for Fig. 5a and 89–90% for Fig. 5b) is neglected for describing the kinetic window, similar as Chen et al.’s works [10,30,32], the kinetic windows in Fig. 5a and b may be combined into one kinetic window as shown in Fig. 5c. However, as indicated by dashed lines in Fig. 5c, there are no definite borders between solid symbols and open symbols in Fig. 5c which describe the temperature range for the second-step sintering to achieve full density without grain growth. So the relative density achieved in the first-step sintering should be considered as a factor affecting a kinetic window. Namely, different from Chen et al.’s works [10,30,32], the different relative densities obtained at T1 (for example, 83–84% and 89–90%) will result in different temperature ranges for T2 to achieve full density without grain growth or different kinetic windows, as shown in Fig. 5a and b. The detailed data including relative densities of green compacts, sintering parameters used for first-step and second-step sintering, and relative densities and mean grain sizes after first-step and secondstep sintering are listed in Table 1 (corresponding to the data in Fig. 5). It also shows that the relative density of the green compacts increases with increasing size of the starting ˛-Al2 O3 nanoparticles, but the lowest densities for different starting ˛-Al2 O3 nanoparticles after first-step sintering are similar and have a value of about 83% below which full density without grain growth cannot be achieved in second-step sintering (not shown in Table 1). So a relative density should reach a critical value of about 83% after first-step sintering to achieve full density without grain growth in the second-step sintering. Fig. 6 shows the variation of relative density and mean grain size versus holding duration at different T2 . All samples were first heated to T1 = 1100 ◦ C without hold and have a mean grain size of 36 nm and a relative density of 83%. Then they were held at different T2 for different holding durations. As the holding duration at T2 = 900 ◦ C increases from 0 to 40 h, the grain size maintains 36 nm, but the relative density increases only to 97%, as shown in Fig. 6a.
This implies that T2 = 900 ◦ C is too low to maintain boundary diffusion and achieve full density. As the holding duration at T2 = 950 ◦ C increases from 0 to 40 h, the grain size also maintains 36 nm, however, the relative density of the sample increases to 99.7%, as shown in Fig. 6b. So T2 = 950 ◦ C is high enough for activating boundary diffusion but still too low to activate gain boundary migration. When held at T2 = 1000 ◦ C for 20 h, the sample reaches a relative density of 99.5% without obvious grain growth, as shown in Fig. 6c. When the holding duration at T2 = 1000 ◦ C was prolonged to 40 h, the relative density increases to 99.6%, but the mean grain size increases to 48 nm. Namely, an obvious grain growth occurred when the holding duration at T2 = 1000 ◦ C was prolonged to 40 h. Fig. 7 shows the SEM images and corresponding grain size distribution histograms of the samples first heated to T1 = 1100 ◦ C without hold and then cooled down to 900, 950, and 1000 ◦ C with a 40 h hold, respectively. They reveal clear grain boundaries, equiaxed grains, and narrow grain size distributions for the three samples. The mean grain sizes of the three samples are 36, 36, and 48 nm and their grain size distributions are 10–85, 15–85, and 20–100 nm, respectively. The XRD pattern of the ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% in Fig. 8 reveals that this nanocrystalline ceramic is pure ˛-Al2 O3 . In order to determine the mechanisms of densification during the second-step sintering, the samples heated to 1100 ◦ C without hold and then cooled down to different T2 for different holding durations were analyzed. During holding, the linear shrinkage L/L and holding duration t satisfy the equation [37,38] 1 L = A (T ) t n , L0
(2)
where A(T) can be regarded as a constant depending on T and n is a characteristic exponent. When n is 2.5, the sintering is controlled by volume diffusion. When n is 3, the sintering is controlled by grain boundary diffusion. According to Eq. (2), the dependence of linear shrinkage on holding duration is plotted in Fig. 9. The values
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Table 1 Two-step sintering of ˛-Al2 O3 (4.5, 7.3, and 62 nm nanoparticles). Relative densities of green compacts (0 ), sintering parameters used for first-step and second-step sintering, and relative densities and mean grain sizes after first-step and second-step sintering are listed. Sample
0 (%)
˛-Al2 O3 −1 ˛-Al2 O3 −2a ˛-Al2 O3 −3a ˛-Al2 O3 −4a ˛-Al2 O3 −5a ˛-Al2 O3 −6a ˛-Al2 O3 −7a ˛-Al2 O3 −8a ˛-Al2 O3 −9a ˛-Al2 O3 −10a ˛-Al2 O3 −11a ˛-Al2 O3 −12b ˛-Al2 O3 −13b ˛-Al2 O3 −14b ˛-Al2 O3 −15b ˛-Al2 O3 −16b ˛-Al2 O3 −17b ˛-Al2 O3 −18b ˛-Al2 O3 −19b ˛-Al2 O3 −20b ˛-Al2 O3 −21b ˛-Al2 O3 −22b ˛-Al2 O3 −23b ˛-Al2 O3 −24c ˛-Al2 O3 −25c ˛-Al2 O3 −26c ˛-Al2 O3 −27c ˛-Al2 O3 −28c ˛-Al2 O3 −29c ˛-Al2 O3 −30c ˛-Al2 O3 −31c ˛-Al2 O3 −32c ˛-Al2 O3 −33c ˛-Al2 O3 −34c ˛-Al2 O3 −35c ˛-Al2 O3 −36c ˛-Al2 O3 −37c a
a b c
48 48 48 48 48 48 48 48 48 48 48 50 50 50 50 50 50 50 50 50 50 50 50 58 58 58 58 58 58 58 58 58 58 58 58 58 58
After first-step sintering without hold
After second-step sintering
T1 (◦ C)
1 (%)
G1 (nm)
T2 (◦ C)
t2 (h)
2 (%)
G2 (nm)
1100 1100 1100 1100 1100 1100 1125 1125 1125 1125 1125 1125 1125 1125 1125 1125 1125 1150 1150 1150 1150 1150 1150 1250 1250 1250 1250 1250 1250 1250 1275 1275 1275 1275 1275 1275 1275
83 83 83 83 83 83 89 89 89 89 89 84 84 84 84 84 84 90 90 90 90 90 90 84 84 84 84 84 84 84 89 89 89 89 89 89 89
36 36 36 36 36 36 40 40 40 40 40 43 43 43 43 43 43 49 49 49 49 49 49 75 75 75 75 75 75 75 86 86 86 86 86 86 86
900 925 950 975 1000 1025 900 925 950 1000 1050 900 925 950 975 1025 1050 900 925 950 1000 1050 1075 950 1000 1050 1100 1125 1150 1200 950 975 1025 1050 1125 1175 1225
75 75 40 40 20 10 75 75 40 10 10 75 75 75 40 20 10 75 75 40 40 10 10 75 75 75 20 20 20 10 75 75 75 40 40 20 10
97 98.7 99.7 99.7 99.5 99.3 98 99.2 99.7 99.5 99.5 93 95 98.8 99.6 99.6 99.4 95 98 99.2 99.7 99.3 99.5 92 95 99 99.4 99.4 99.5 99.3 93 95 99.1 99.1 99.5 99.4 99.2
36 36 36 37 38 44 40 40 40 41 49 43 43 43 43 45 52 49 49 49 49 54 58 75 75 76 75 77 78 86 86 86 86 86 88 90 97
4.5 nm nanoparticles. 7.3 nm nanoparticles. 62 nm nanoparticles.
of n corresponding to different T2 of 900, 950, and 1000 ◦ C are 3.01, 2.96, and 2.88, respectively. So grain boundary diffusion should be the main mechanism of densification at T2 in the 900–1000 ◦ C range and a weak contribution of volume diffusion to densification occurs at T2 of 1000 ◦ C. As mentioned above, the upper and lower border lines for kinetic windows describe the temperature ranges for T2 to achieve full density without grain growth. Grain boundary migration is suppressed to prevent grain growth below the upper border line but activated to trigger grain growth above the upper border line in the second-step sintering. Similar as Chen et al.’s works [10,30,32], the upper border lines are upward in our kinetic windows (Fig. 5a and b). This may be due to the decrease in the driving force of grain boundary migration with increasing grain size. As the grain size increases, the temperature T2 for activating grain boundary migration has to be increased. Densification will be exhausted below the lower border line but maintained to achieve full density above the lower border line in the second-step sintering. The lower border lines are likewise upward in our kinetic windows (Fig. 5a and b) whereas they are downward in Chen et al.’s works [10,30,32]. This implies that a higher temperature T2 is required to maintain densification and achieve full density in the second-step sintering for a larger grain size. According to Eq. (2) and the deduced characteristic exponent values, grain boundary diffusion is the main mechanism of densification in the second-step sintering at a lower T2 (corre-
Fig. 8. The XRD pattern of the ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7%.
sponding to T2 around the lower border line). With increasing grain size, the amount of grain boundaries decreases and grain boundary diffusion weakens, which will lower the densification rate. So
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Fig. 9. The dependence of linear shrinkage on holding duration at different secondstep sintering temperature T2 of 900, 950, and 1000 ◦ C (the samples were heated to 1100 ◦ C without hold and then cooled down to different T2 for different holding durations).
a higher temperature T2 is required to maintain grain boundary diffusion with increasing grain size around the lower border line in our kinetic windows. In Chen et al.’s works, a lower temperature T2 can also maintain densification and realize full density for a larger grain size [10,30,32]. The dislocations have a threshold nucleation energy or stress which is inversely proportional to grain size and rather substantial than capillary pressure when the grain size is less than 100 nm [10]. So the dislocation pipe diffusion should be suppressed at the grain sizes less than 100 nm and the suppression should diminish at larger grain sizes [10]. In Chen et al.’s works, the grain sizes in the second-step sintering are mainly in the 100–1000 nm range, so the effect of dislocations should be obvious and dislocation pipe diffusion can occur to achieve full density at lower T2 with increasing grain size though grain boundary diffusion weakens with increasing grain size [10,30,32]. In our present work, the grain sizes in the second-step sintering range from 36 to 86 nm, so the effect of the dislocation pipe diffusion can be ignored. As grain boundary diffusion is the mechanism of densification, the linear shrinkage L/L and activation energy of grain boundary diffusion Q follow the equation [39]
L Q = B (T ) exp − L0 3RT
,
(3)
where B(T) can be regarded as a constant depending on T, R is the gas constant, and T is the absolute temperature. Using the data of the samples heated to T1 = 1100 ◦ C without hold and then cooled down to different T2 for a 2 h hold, the dependence of linear shrinkage on temperature is shown in Fig. 10. The activation energy of grain boundary diffusion Q calculated by Eq. (3) is 199 kJ/mol, which is much lower than those reported in literature (about 400–900 kJ/mol) [40–43]. In our work, almost fully dense ˛Al2 O3 nanocrystalline ceramic can be obtained by grain boundary diffusion at a T2 of 950 ◦ C more than 200 ◦ C lower than the temperatures of second-step sintering of nanoparticles with agglomeration or larger particle sizes [10,11,44]. It should be due to the smaller activation energy of grain boundary diffusion in our work. Compared with ˛-Al2 O3 ceramics sintered by pressureless sintering, or pressure sintering, or electrically assisted sintering [6–9,11,16–20], our almost fully dense (99.7%) ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm obtained by two-step pressureless sintering have the finest grain reported so far. The sintering temperature is also the lowest for pressureless
Fig. 10. The dependence of linear shrinkage on second-step sintering temperature T2 (the samples were heated to 1100 ◦ C without hold and then cooled down to different T2 with a 2 h hold).
sintering of ˛-Al2 O3 . It is mainly due to the excellent sintering activity of the disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm. 4. Conclusions The sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration obtained by mechanochemistry − selective corrosion − fractionated coagulation separation was studied. In the early sintering stage (relative densities less than 95%), the variation of mean grain size with relative density shows a novel convex shape, different from a concave shape usually found for nanoparticles or submicron particles. The 4.5 nm ˛-Al2 O3 nanoparticles with narrow size distribution and without any agglomeration have a non-equilibrium structure, smaller surface curvature radii, lower surface activation energy, and larger thermodynamic driving force of diffusion. A rapid grain growth occurs in the late sintering stage for relative densities larger than 95%. Using a two-step sintering method, the kinetic windows which describe the temperature ranges of the second-step sintering for densification without final grain growth were determined for the ˛-Al2 O3 nanoparticles with mean particle sizes of 4.5, 7.3, and 62 nm. The different relative densities (83–84% and 89–90%) obtained at first-step sintering temperatures T1 will result in different temperature ranges for the second-step sintering temperatures T2 to achieve full density without grain growth or different kinetic windows. Different from the downward lower border lines in literature, the lower border lines in our kinetic windows are upward, which may be due to the different grain size ranges used in the second-step sintering (36–86 nm in our work and 100–1000 nm in literature) and different diffusion mechanisms in the secondstep sintering. The main mechanism of densification in second-step sintering in the 900–1000 ◦ C range in our work is grain boundary diffusion. The activation energy of grain boundary diffusion is 199 kJ/mol, much lower than those reported in literature (about 400–900 kJ/mol). Finally, ˛-Al2 O3 nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 ◦ C without hold and then cooled down to 950 ◦ C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of ˛-Al2 O3 and the obtained almost fully dense ˛-Al2 O3 nanocrystalline ceramic has the finest grain so far. The detailed exploration of densification,
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Please cite this article in press as: W. Cao, et al., Sintering kinetics of disperse ultrafine equiaxed ˛-Al2 O3 nanoparticles, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.066