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Effects of multicomponent catalyzer on preparation of ultrafine α-Al2O3 at low sintering temperature
Advanced Powder Technology 20 (2009) 542–547
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original research paper
Effects of multicomponent catalyzer on preparation of ultrafine a-Al2O3 at low sintering temperature Su-peng *, Guo-xueyi 1, Ji-shujun 1 School of Metallurgical Science and Engineering, Chamber 226, Central South University, Changsha 410083, Hunan Province, China
a r t i c l e
i n f o
Article history: Received 31 May 2009 Received in revised form 25 July 2009 Accepted 28 July 2009
Keywords: a-Al2O3 Precursor Catalyzer Thermal decomposition Phase transformation
a b s t r a c t a-Al2O3 powder, with a purity of 99.95% and an average particle size of 80 nm, was prepared via the thermal decomposition of AACH precursor. During the AACH preparation, OP-10 (alkylphenol ethoxylates) was used as a dispersant and a deflocculating agent, and a self-confected multicomponent catalyzer (MC) was used to prevent powder agglomeration. Our experiments illustrated that: apart from the necking and agglomeration-preventing effect during the sintering process, MC had potential promotive effects on the reduction of a-Al2O3 phase transformation temperature by maintaining the powder particles in a comparatively dynamic-sintering state, thus significantly improving the dispersibility of the sintered powder. BET and ICP analyses indicated that MC was propitious for increasing the specific surface area of the formed a-Al2O3 powder and induced very tiny impurities in the sintered products. Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ultrafine alumina, as an important function material, has been widely used in the fields of ceramics, metallurgy, electronics and catalysts due to its high heat-resistance, high rigidity, high corrosion-resistance, high abrasion resistance, and good electrical resistivity and catalytic abilities [1–4]. Conventional methods for preparing ultrafine alumina involve mechanical milling [5,6], vapor phase reaction [7,8], sol–gel method [9–13], thermal decomposition [14–16] and combustion synthesis [17,18]. Mechanical milling synthesis of ultrafine alumina is time consuming and easy to introduce impurities. Vapor phase reaction demands special equipments which are of high unit cost. As for the sol–gel method, the biggest problem is the high price of metal alkoxides used as raw materials and the long gelation periods. The combustion synthesis method has a shortage of hard agglomeration and broad particle size distribution. Comparatively speaking, the recently developed thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), as an improvement of the thermal decomposition of ammonium aluminum sulfate to produce ultrafine a-Al2O3 powders [16,19–21], is a relatively simple and efficient method. Apart from the advantages of no serious sulfur dioxide environmental contamination and no self-dissolution during thermal
* Corresponding author. Tel.: +86 0731 88877863; +86 15947126682 (mobile); fax: +86 0731 88836207. E-mail addresses: [email protected] (Su-peng), [email protected] (Guo-xueyi), [email protected] (Ji-shujun). 1 Tel.: +86 0731 88877863; fax: +86 0731 88836207.
decomposition, gases generated from the thermal decomposition of AACH play an important role in reducing particle agglomeration. Nevertheless, reduction of the particle agglomeration and sintering temperature remains a big problem in the AACH decomposition process. During the sintering of AACH precursor, amorphous alumina initially is converted into metastable transition phases such as c-Al2O3, d-Al2O3, and h-Al2O3 [22], and is ultimately transformed into stable a-Al2O3 phase with temperature increasing. During this course, especially from h phase to a phase, crystal matrix rearrangement takes place, involving the shifting of large ionic radius O2, which requires high energy consumption, and the temperature is usually over 1200 °C [23]. At such a high temperature, solid sintering starts and hard agglomeration with vermicular morphology easily emerges, due to the necking between the particles, once a-Al2O3 phase has been formed [24,25]. Yang-ye et al. [26] have added a-Al2O3 seeds and NH4NO3 to the AACH precursor, and discovered that due to the decomposition of NH4NO3 at 320 °C, multiform gases are released, reducing the stacking density of the transition phases, and resulting in large amounts of highpowered defects and lattice deformations which were propitious to the reduction of a-phase transformation temperature and particle agglomerations, but the decomposition of NH4NO3 went along tempestuously, releasing environment-effecting NOx. Bai-shihe et al. [27] have studied the effect of NH4Cl on the a-phase transformation of ultrafine alumina, and discovered that NH4Cl addition to the AACH can restrain the agglomerations and can get the average grain size reduced, but having no notable reducing effect on phase transformation temperature, and the shortage of this method is the increase of dendritic crystal. Wu-yucheng et al. [28] have studied
0921-8831/$ - see front matter Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2009.07.004
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the effect of colloid TiO2 addition to the a-phase transformation of alumina and found that TiO2 addition can evidently reduce the phase transformation temperature, but meanwhile introducing more impurities. Other researchers [28,29] have tried the addition of mineralizers such as ZnF2 and AlF3 but having the same problem as TiO2. In this paper, self-made multicomponent catalyst (MC), containing mainly aluminum sulfate, ammonium nitrate, a-Al2O3 seeds, and a small amount of cerium dioxide and magnesium nitride, was used to prevent particle agglomeration and to reduce the phase transformation temperature of a-Al2O3 in the sintering process of AACH precursor. Ultrafine a-Al2O3 with good dispersibility and homogeneous size distribution was successfully synthesized. The decomposition and phase transformation processes are characterized and the mechanisms action of the MC are analysed and discussed. 2. Experimental 2.1. Preparation of MC Table 1 shows the composition of the MC with all the agents being analytically pure. The components were weighed respectively according to the mixture ratio given in Table 1 using an electronic balance, and then they were poured into a planetary ball milling machine with polyethylene inner liner. Using absolute ethanol as a dispersant and / 6–15 mm corundum balls as a milling medium, the mixture was ball milled for 2 h under the rev of 380 r/min and was heated in a vacuum drying oven at 80 °C for 4 h to remove the absolute ethanol.
2.3. Sintering processes Samples 1 and 2 were sintered in a muffle furnace in air atmosphere. The highest temperatures of sample 1 were 1000 °C, 1100 °C, 1150 °C and 1200 °C, respectively. For sample 2, the highest sintering temperatures were 800 °C, 900 °C and 1000 °C, respectively. For all the sintering systems, the heating rate was 5 °C/min, and the holding time at highest temperature was 2 h. 2.4. Characterization DSC–TGA analysis of sample 1 was carried out using a SDT Q600 Simultaneous DSC–TGA instrument (TA Corporation, USA) at a constant heating rate of 10 °C/min from room temperature to 1200 °C in air. The product of each sintering temperature system was identified by XRD analysis using a Japanese Rigaku D/max2550VB+18KW rotating target X-ray diffractometer. Microstructure analysis was carried out using a AIS2100 scanning electron microscope (SEM) and grain size analysis was performed for the digitized SEM photographs using image analysis software (Scion Corporation, USA). Specific surface area analysis for the sintered powder was carried out by BET method in nitrogen gas atmosphere with a Monosorb specific surface area analyser (Quantachrome Corporation, USA). Impurity element analysis of the sintered products was carried out by means of Inductively Coupled Plasma (ICP). 3. Results and discussion 3.1. Synthesis reaction for AACH When aluminum sulfate and ammonium carbonate are dissolved in water, hydrolysis reactions may occur as follows:
2.2. Preparation of precursors
3þ
2.2.1. Sample 1 Aluminum sulfate solution of 0.3 mol/l and ammonium carbonate solutions of 2 mol/l were, respectively, prepared using 0.1 mol A12(SO4)318H2O(AR) and 0.4 mol (NH4)2CO3(AR). Heated up to 60 °C in a water bath kettle, the aluminum sulfate solution was slowly added to the rapidly stirred (1400 r/min) ammonium carbonate solution. About 0.3–0.4 wt% OP-10 (alkylphenol ethoxylates) as a dispersant was simultaneously dropped into the mixture to prevent flocculation. While adjusting the pH value in the range of 11.0–11.6 using a strong solution of ammonia, the slurry gained was aged for 4 h. Then the suspension was filtered and washed repeatedly for three times with deionised water until no precipitate formed. Afterwards, the filter cake was mixed with normal-butanol and dehydrated by heterogeneous azeotropic distillation processing [30]. At last, the blending was filtered again and the filter cake gained was dried at 110 °C for 8 h in a vacuum drying oven. 2.2.2. Sample 2 Sample 2 was prepared by adding 5 wt% MC into the oven-dried sample 1. After co-mixed ball milling (the same process as for sample 1) for 30 min, the blending was dried at 110 °C for 8 h in a vacuum drying oven to get sample 2. Except for this step, all the processes fore-and-aft are identical with the counterparts of sample 1.
þ 3SO2 4 þ ðNH4 Þ2 CO3 ¼ 2NH4 þ CO3 CO2 3 þ H2 O ¼ HCO3 þ OH 3þ Al þ 4OH ¼ AlOðOHÞ2 þ Al2 ðSO4 Þ3 ¼ 2Al
ð1Þ ð2Þ ð3Þ H2 O
ð4Þ
According to Lin-yuanhua [20] and Shazo Kato [31]: when the con centrations of [NHþ 4 ], [AlOðOHÞ2 ] and [HCO3 ] reach a certain extent, pure NH4Al(OH)2CO3 deposits as follows:
NHþ4 þ AlOðOHÞ2 þ HCO3 ¼ NH4 AlðOHÞ2 CO3 # þOH
ð5Þ
And the overall reactions can be expressed as:
Al2 ðSO4 Þ3 þ 4ðNH4 Þ2 CO3 þ 2H2 O ¼ 2NH4 AlðOHÞ2 CO3 # þ 3ðNH4 Þ2 SO4 þ 2CO2 "
ð6Þ
Thus it can be concluded that the key to obtain pure AACH is maintaining the concentration of AlOðOHÞ 2 to a certain extent and simultaneously increasing the concentrations of NHþ 4 and as much as possible. With concentration of reactants and HCO 3 pH values altering, different products can be generated. According to Shazo Kato [31]: when the pH value is kept in range of 6.0–7.6, amorphous Al(OH)3 forms as follows:
Al2 ðSO4 Þ3 þ 3ðNH4 Þ2 CO3 þ H2 O ¼ 2AlOOH # þ 3ðNH4 Þ2 SO4 þ 3CO2 "
ð7Þ
Table 1 Compositions of the MC. Substances
Aluminum sulfate (A12(SO4)318H2O)
Ammonium nitrate (NH4NO3)
Cerium oxide (CeO2)
Magnesium nitride (Mg3N2)
White corundum powder (aAl2O3)
Content (wt%)
65
24
0.1
0.1
9.8
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Fig. 1. XRD pattern of precursor: (a) sample 1, and (b) sample 2.
And for the pH range of 7.6–8.5, the products can be Al(OH)3 and NH4Al(OH)2CO3, and for the pH value over 8.5, pure NH4Al(OH)2CO3 may deposit. In this study we chose the pH range 11.0–11.6, because in this pH range, according to the research of Yan-guojin et al. [32], one can get the best dispersing effect of slurry. 3.2. XRD and DSC–TGA analyses of precursor The XRD patterns of sample 1 and sample 2 before sintering are shown in Fig. 1. The position, intensity and the Bragg reflection semi-broadening for the diffraction peaks corresponded totally to the PDF standard card of AACH, indicating that the precursor obtained was ammonium aluminum carbonate hydroxide (AACH). Fig. 2 shows the DSC–TGA curves of sample 1, it can be seen that the TG curve descends sharply before 630 °C. The value of mass loss before 180 °C is about 0.57, and after 180 °C, the value is in range of 0.075–0.085, and the total mass loss is about 0.65, These are totally in agreement with the theoretical mass loss value of the AACH decomposition reactions as follows:
NH4 AlðOHÞ2 CO3 ¼ AlOOH þ CO2 " þNH3 " þH2 O "
ð8Þ
2AlOOH ¼ Al2 O3 ðamorphousÞ þ H2 O "
ð9Þ
From these two reactions, we can calculate that the theoretical mass loss values are 0.568 for reaction (8) and 0.065 for reaction (9) and that the total loss is 0.633. So the TG weight loss (0.65) is very close to the theoretical value of the complete decomposition of AACH, and the out profile may be due to the evaporation of the residual normal-butanol and water in the precursor. The aforementioned course altogether can testify that the precursor is AACH. When the temperature exceeds 630 °C, the TG curve is nearly horizontal, indicating that the mass loss stage of powder
has ended, and amorphous alumina generated from reaction (9) begins to transform into other transitional phases such as cAl2O3, h-Al2O3, and finally a-Al2O3. The DSC curve shown in Fig. 2 has one sharp endothermic peak at about 180 °C, and this is attributed to the thermodecomposition reaction of AACH, from which water gives away in form of vapor and absorbs a great deal of energy, as well as gases are released (CO2 and NH3), also absorbing part of the energy. Although a large amount of energy is released from the thermodecomposition reaction, the total is negative after counteracting the parts consumed, so the DSC curve presents a sharp endothermic peak. The two exothermic peaks at about 80–100 °C and 280 °C may be respectively associated with the decomposition of the OP dispersant, and the thermodecomposition reaction of AACH. While the inflexion at about 870 °C may be due to the phase transformation of alumina from amorphous Al2O3 to c-Al2O3.
3.3. Phase transformation of the samples The XRD patterns of samples 1 and 2 sintered at various temperatures are shown in Figs. 3 and 4, respectively. As shown in Fig. 3, after sintering to 1000 °C and holding for 2 h, the XRD patterns mainly illustrated are diffraction peaks of h-Al2O3 and a few faint peaks of c-Al2O3. After sintering to 1100 °C, the diffrac-
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20
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30
40
•
50
° Fig. 2. DSC–TGA patterns of the precursor of sample 1.
Fig. 3. XRD patterns of sintered products of sample 1.
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The phase transformation temperature reduction can be contributed more to the addition of NH4NO3 to the MC. Comparing the XRD pattern of sample 1 (Fig. 3) with that of sample 2 (Fig. 4) it can be seen that few h-Al2O3 diffraction peaks have been formed for the latter of which the sintering temperature is much lower. That is to say, most of the c-Al2O3 converts into a-Al2O3 directly without undergoing the transition phase of h-Al2O3 at a much lower temperature. This fits into the study by Yang-ye et al. [26] and Gu-tangsheng [36] that thermodecomposition of NH4NO3 at about 320 °C enhances the adjustment of bonding length and bonding angle of non-crystalline alumina, resulting in high energetic defections among the transitional-phase Al2O3, hindering the phase transformation from c-Al2O3 to h-Al2O3, and thus promoting the nucleation of a-Al2O3. Besides the temperature reducing effect, the intensive emission of NOx and NH3 gases from the decomposition reaction of NH4NO3 can keep the precursor in a fluffy state, enlarging the distance between the particles, and reducing the powder agglomeration. The phase transformation promoting function is more notable for the MC component of aluminum sulfate ((A12(SO4)318H2O)), the decomposition of which can be shown as follows [37]: Al2 ðSO4 Þ3 18H2 O
87—250 C
!
Al2 ðSO4 Þ3 18H2 Oþ15H2 O " ðweight loss 40:5%Þ
ð10Þ
Fig. 4. XRD patterns of sintered products of sample 2.
Al2 ðSO4 Þ3 18H2 O
tion peaks of c-Al2O3 vanish and peaks of a-Al2O3 appear. As the sintering temperature reaches 1150 °C, the intensity of diffraction peaks of a-Al2O3 is evidently strengthened. Well after sintering to 1200 °C, all the diffraction peaks vanish except the peaks of aAl2O3, indicating that a-Al2O3 with complete crystallization is synthesized. Therefore, it can be concluded that the sequence of phase transformation of AACH in the sintering stage is AACH ? AlOOH ? amorphous Al2O3 ? c-Al2O3 ? h-Al2O3 ? a-Al2O3. As shown in Fig. 4, diffraction peaks of a-Al2O3 firstly occur after sintering to 800 °C and holding for 2 h, and the phase transformation of a-Al2O3 is completed after sintering to 1000 °C. These two temperatures are about 300 °C and 200 °C lower than the counterparts of sample 1, respectively. The reduction of phase transformation temperature in this test is mainly associated with the addition of MC, and mechanism can be described as follows: c-Al2O3 and h-Al2O3 have similar defective spinelle structure. As sintering temperature increases, the regularity of Al3+ in cubic crystalline matrix of O2 in Al2O3 is enhanced to a certain extent compared to that in c-Al2O3, so the phase transformation from c-Al2O3 to h-Al2O3 does not require too much energy and can be realized at a relatively low temperature. Well, according to Igor Levin and David Brandon [33], phase transformation of a-Al2O3 is related to the arrangement of O2 from fcc framework to hcp framework, which is a kind of reconstruction of crystal lattice and a nucleation growth process requiring large amounts of energy to overcome the barrier energy for nucleation, so in the case of no external interference, the phase transformation temperature is usually over 1200 °C. According to Bagwell [34] and Kumagai [35]: if there exist some a-Al2O3 particles as nucleation seeds, the activation energy for nucleation of a-Al2O3 can be decreased, while the effect of a-Al2O3 seeds upon reducing phase transformation temperature is not great. In this test, white corundum powder is one of the components of the MC, and after ball milling by corundum balls, the particle size of the corundum was reduced remarkably from about 15 lm to less than 100 nm. As inducible seeds, they increase the density of nucleation sites and reduce the energy barrier of phase transformation process, and to some extent lower the phase transformation temperature.
545
Al2 ðSO4 Þ3
250—414 C
770—1120 C
!
!
Al2 ðSO4 Þ3 þ3H2 O " ðweight loss 8:1%Þ
Al2 O3 ðamorphousÞþ3SO3 " ðweight loss 36%Þ
ð11Þ ð12Þ
It can be seen from reaction (10)–(12) that the decomposition reaction of aluminum sulfate is accompanied by the whole decomposition and sintering process of AACH. In the low temperature range under 414 °C, the reactant losses nearly half of its weight and large amount of water is released in vapor due to the dehydration of aluminum sulfate. This vapor, together with the vapor and gases released from AACH decomposition, makes the precursor inflate tempestuously, breaking the linking and aggregation among the particles. Even more importantly, the high temperature (770–1120 °C) decomposition of aluminum sulfate occurs almost simultaneously with the alumina phase transformation from c-Al2O3 to h-Al2O3, so the amorphous Al2O3, produced during the decomposition reaction, which has much more crystal lattice defects, acts as the activated nucleation center of a-Al2O3, leading to activation energy reduction of phase transformation as well as the reduction of phase transformation temperature. On the other hand, the SO2 released acts as a relaxant of the reactants and is more effective to hinder the powder agglomeration. As for the other two components in the MC: CeO2 and Mg3N2, their main effects lie in activating crystal lattice and grain refining. Ce, as we have known, has the atomic structure of 4f15d16s2, and when element Ce changes from metal to ion, the 4f orbit is still surrounded by the outer electron cloud 5s25p6, which can easily lose two 6s electrons and one 5d or 4f electron to form the electronic structure of 4fx5s25p6. This variable valence of CeO2 can induce the formation of fluorite type hypoxia phase with anion vacancy, such as CenO2n-2 (n = 4, 6, 7, 9, 10, 11). The decrease of oxygen content leads to lattice deformation and weak ion bond, thus altering the sensitivity of the structure [38], which makes it act as an activating center of nucleation. On the other hand, the radius of Ce4+ is much larger than that of Al3+, and it is hard to be embedded into the gaps of Al2O3 lattice to form sosoloid. So the CeO2 particles mainly gather on the grain boundaries, and due to their great volume, the grain boundary migration rate of alumina particles in the structure is reduced. This is conducive to inhibit grain growth to achieve the purpose of grain refinement.
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And for Mg3N2, when the temperature surpasses 800 °C, decomposition reaction takes place as follows: 800 C
Mg3 N2 þ ð1:5 þ 2xÞO2 ! 3MgO þ 2NOx "
ð13Þ
The amorphous MgO produced from above reaction can attach to the surface of the formed a-Al2O3 to form spinel (MgAl2O4) or MgO-Al2O3 sosoloid due to the similar ionic radius of Mg2+ and Al3+. The inter-diffusion reaction between Mg2+ and Al3+ is exother1 mic (DG900K ¼ 52:135kJ mol ) and the pinning effect caused by these MgAl2O4 or sosoloid keeps the interfacial energy much lower, thus reducing the diffusion rate between the a-Al2O3 particles, and hindering the necking and growth of the particles. Summarized from the above analysis, the addition of the MC may keep the powder in a comparatively dynamic-sintering state which has a great synergistic effect on the dynamics of phase transformation of alumina, by increasing the density of nucleation sites and reducing the energy barrier of nucleation [39], and thus accelerating the phase transformation and lowering the phase transformation temperature. 3.4. Morphology of sintered products Fig. 5 shows the SEM images of samples 1 and 2 sintered at 1200 °C and 1000 °C, respectively. As shown in Fig. 5(a), the leading crystal form for sample 1 is spindle like, accompanied with a slight amount of spherical particles. Simultaneously, vermicular morphology rigid agglomeration is formed due to the extensive neckings and reagglomeration of the secondary particles. Whereas, for particles of sample 2 shown in Fig. 5(b), the crystal form is a homogenously spherical structure. Compared with sample 1, which has an average particle size of about 300 nm and a broad particle size distribution, sample 2 has an average particle size of only about 80 nm and a homogeneous size distribution, and much less conspicuous neckings or agglomeration can be observed. The distinction of grain size and particle size distribution between the two samples can be contributed to the variation of sintering temperature. Rigid agglomerations are more likely to form during the sintering process of ultrafine a-Al2O3 powders, when the phase transformation temperature is near 1200 °C, a-Al2O3 grain grows rapidly once its nucleation occurs, resulting in neckings between neighboring particles and finally vermicular morphology rigid agglomeration. With the addition of MC, the sintering temperature is reduced from 1200 °C to 1000 °C, and the average grain size is reduced drastically due to the low grain growth velocity.
Fig. 6. The specific surface areas of the sintered products at different temperatures.
Table 2 Content of impurities (mass fraction, %). Sample Sample Sample Sample Sample
1–1 1–2 2–1 2–2
Na
Ca
Fe
Pb
Ce
Mg
0.0072 0.0073 0.0071 0.0072
0.0046 0.0045 0.0043 0.0044
0.0051 0.0051 0.0051 0.0052
0.0043 0.0044 0.0042 0.0043
– – 0.0043 0.0043
– – 0.0033 0.0032
sis. As shown in Fig. 6, the specific surface areas of samples 1 and 2 all decrease sharply with a rise in the sintering temperature. This is attributed to the intensive volume shrinkage (about 15%) when metastable c-Al2O3 and h-Al2O3 are transformed into stable a-Al2O3 at a high temperature. Well for sample 2, the specific surface areas at different sintering temperatures are much higher than those of sample 1, and when the phase transformation of a-Al2O3 is nearly complete at 1000 °C and 1200 °C for sample 2 and sample 1, respectively, the specific surface area of the former is almost 6.5 times higher than that of the latter. This is because the addition of MC not only accelerates the phase transformation (comparing Fig. 4 with Fig. 3), but also effectively hinders the particle agglomeration and leaves large amounts of pores inside the particles by releasing more gases during the high temperature decomposition reaction of its components.
3.5. Specific surface area of sintered products
3.6. Impurity element analysis of sintered products
Fig. 6 shows the specific surface area for sintered products of samples 1 and 2 at different sintering temperatures by BET analy-
Table 2 shows the results of the ICP analysis of the impurity elements of the sintered products. As shown in Table 2, contents of
Fig. 5. SEM images of sintered samples: (a) sample 1, 1200 °C/2 h; (b) sample 2, 1000 °C/2 h.
Su-peng et al. / Advanced Powder Technology 20 (2009) 542–547
impurities in the sintered products of sample 2 are nearly identical with the counterparts of sample 1 except for small quantities of Ce and Mg, indicating that the addition of MC has a faint effect on the impurity introduction that can be overlooked. And by content calculation from Table 2, the Al2O3 purity of sample2 reaches 99.95%. 4. Conclusions (1) Using low-cost aluminum sulfate and ammonium carbonate as raw materials, the precursor AACH was synthesized. (2) The phase transformation sequence of AACH precursor in the sintering process with no MC added is AACH ? AlOOH ? amorphous Al2O3 ? c-Al2O3 ? h-Al2O3 ? a-Al2O3. The a-Al2O3 particles obtained after sintering at 1200 °C have a broad size distribution with an average grain size of about 300 nm and a high density of neckings and vermicular morphology rigid agglomeration. (3) The addition of MC to the AACH precursor can effectively prevent agglomerations of precursor in sintering process as well as has potential synergistic effects on the reduction of the phase transformation temperature of a-Al2O3, by keeping the powder particles in a comparatively dynamic-sintering state, therefore the trends of necking and rigid agglomerations are remarkably decreased, so the dispersity of powder particles is significantly improved and the average particle size is about 80 nm. (4) BET and ICP analyses indicate that MC is fit for increasing the specific surface area of a-Al2O3 powder, and has a very tiny effect on the introduction of impurity to the sintered products.
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[13] Hui-Ling Wen, Fu-Su Yen, Growth characteristics of boehmite-derived ultrafine theta and alpha-alumina particles during phase transformation, Journal of Crystal Growth 208 (2000) 696–708. [14] Chi-Cheng Ma, Xue-Xi Zhou, Xin Xu, et al., Synthesis and thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), Materials Chemistry and Physics 72 (2001) 374–379. [15] Ji-Young Park, Seong-Geun Oh, Ungyu Paik, et al., Preparation of aluminum oxide particles using ammonium acetate as precipitating agent, Materials Letters 56 (2002) 429–434. [16] I.N. Bhattacharya, P.K. Gochhayat, P.S. Mukherjee, et al., Thermal decomposition of precipitated low bulk density basic aluminium sulfate, Materials Chemistry and Physics 88 (2004) 32–40. [17] S. Bhaduri, E. Zhou, S.B. Bhaduri, Auto ignition processing of nanocrystalline aAl2O3, Nanostructured Materials 7 (1996) 487–496. [18] J.C. Toniolo, M.D. Lima, A.S. Takimi, Synthesis of alumina powders by the glycine-nitrate combustion process, Materials Research Bulletin 40 (2005) 561–571. [19] Ai-fei Zhang, Ji-ping Liu, A new precipitation method for the preparation of alumina nanopowders, Inorganic Chemicals Industry 35 (2003) 27–28 (in Chinese). [20] Lin yuanhua et al., Preparation of high pure ultrafine a-Al2O3 powder by means of pyrolytic reaction of precursor, Journal of the Chinese Ceramic Society 28 (2000) 268–271 (in Chinese). [21] K. Morinaga, T. Torikai, K. Nakagawa, et al., Fabrication of fine a-alumina powder by thermal decomposition of ammonium alumina carbonate hydroxide (AACH), Acta Materialia 48 (2000) 4735–4741. [22] Igor Levin, David Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, Journal of the American Ceramic Society 81 (1998) 1995–2012. [23] Yu-cheng Wu, Zhen-ya Song, Ye Yang, et al., Mechanism and control of aphase transformation of alumina, Chinese Journal of Rare Metals 28 (2004) 1043–1048 (in Chinese). [24] F.W. Dynys, J.W. Halloran, Alpha alumina formation in alum-derived gamma alumina, Journal of the American Ceramic Society 65 (1982) 442–448. [25] F.S. Yen, H.L. Wen, Y.T. Hsu, Crystallite size growth and the derived dilatometric effect during h- to a-phase transformation on nano-sized alumina powders, Journal of Crystal Growth 233 (2001) 761–773. [26] Yang ye et al., Preparation of ultrafine a-Al2O3 powder by thermal decomposition of AACH at low temperature, The Chinese Journal of Process Engineering 8 (2002) 325–329. [27] Bai shihe et al., Effect of NH4CI on the phase transformation of Al2O3 derived from ammonium aluminum carbonate hydroxide, Rare Metal Materials and Engineering 8 (2007) 112–114. [28] S. Rajendran, Production of ultrafine alumina powder and fabrication of fine grained strong ceramics, Journal of Materials Science 29 (1994) 5664–5672. [29] Yi-Quan Wu, Yu-Feng Zhang, Xiao-Xian Huang, Preparation of nano-sized alumina powders at low temperatures, Journal of Inorganic Materials 16 (2) (2001) 349–352 (in Chinese). [30] Xiao-jin, Qin-qi, Wan-ye, et al., Preparation of ultrafine a-Al2O3 using precipitation-azeotropic distillation method, Journal of Hunan University of Science & Technology (Natural Science Edition) 22 (2) (2007) 35–39. [31] Shazo Kato, Preparation of ammonium alumina carbonate hydroxide, Journal of the Japan Ceramic Society 84 (5) (1976) 215–219. [32] Yan-guojin et al., Preparation of high pure ultrafine alumina powder by means of pyrolytic process of ammonium aluminium carbonate, Diomand and Abrasives Engineering 156 (6) (2006) 781–785 (in Chinese). [33] Igor Levin, David Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, Journal of the American Ceramic Society 81 (8) (1998) 1995–2012. [34] R.B. Bagwell, G.L. Messing, Effect of seeding and water vapor on the nucleation and growth of Al2O3 from c-Al2O3, Journal of the American Ceramic Society 82 (4) (1999) 825–832. [35] M. Kumagai, G. Messing, Controlled transformation and sintering of a boehmite sol–gel by a-alumina seeding, Journal of the American Ceramic Society 68 (9) (1985) 500–505. [36] Gu-tangsheng, Lin-guangming, Phase transformation and infrared absorption spectra of amorphous and crystalline of nano Al2O3 powder, Journal of Inorganic Materials l2 (6) (1997) 840–844. [37] Wu-yan, Li-laishi, Zhai-yuchun, Thermal behavior and decomposition kinetics of Al2(SO4)318H2O, Journal of Molecular Science 23 (6) (2007) 380– 384. [38] Wang-Zhongjun, Rare earth and ceramics, Hebei Ceramics 28 (2) (2000) 29–30. [39] Masato Kumagai, Gary L. Messing, Controlled transformation and sintering of a boehmite sol–gel by a-alumina seeding, Journal of the American Ceramic Society 68 (9) (1985) 500–505.
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original research paper
Effects of multicomponent catalyzer on preparation of ultrafine a-Al2O3 at low sintering temperature Su-peng *, Guo-xueyi 1, Ji-shujun 1 School of Metallurgical Science and Engineering, Chamber 226, Central South University, Changsha 410083, Hunan Province, China
a r t i c l e
i n f o
Article history: Received 31 May 2009 Received in revised form 25 July 2009 Accepted 28 July 2009
Keywords: a-Al2O3 Precursor Catalyzer Thermal decomposition Phase transformation
a b s t r a c t a-Al2O3 powder, with a purity of 99.95% and an average particle size of 80 nm, was prepared via the thermal decomposition of AACH precursor. During the AACH preparation, OP-10 (alkylphenol ethoxylates) was used as a dispersant and a deflocculating agent, and a self-confected multicomponent catalyzer (MC) was used to prevent powder agglomeration. Our experiments illustrated that: apart from the necking and agglomeration-preventing effect during the sintering process, MC had potential promotive effects on the reduction of a-Al2O3 phase transformation temperature by maintaining the powder particles in a comparatively dynamic-sintering state, thus significantly improving the dispersibility of the sintered powder. BET and ICP analyses indicated that MC was propitious for increasing the specific surface area of the formed a-Al2O3 powder and induced very tiny impurities in the sintered products. Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ultrafine alumina, as an important function material, has been widely used in the fields of ceramics, metallurgy, electronics and catalysts due to its high heat-resistance, high rigidity, high corrosion-resistance, high abrasion resistance, and good electrical resistivity and catalytic abilities [1–4]. Conventional methods for preparing ultrafine alumina involve mechanical milling [5,6], vapor phase reaction [7,8], sol–gel method [9–13], thermal decomposition [14–16] and combustion synthesis [17,18]. Mechanical milling synthesis of ultrafine alumina is time consuming and easy to introduce impurities. Vapor phase reaction demands special equipments which are of high unit cost. As for the sol–gel method, the biggest problem is the high price of metal alkoxides used as raw materials and the long gelation periods. The combustion synthesis method has a shortage of hard agglomeration and broad particle size distribution. Comparatively speaking, the recently developed thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), as an improvement of the thermal decomposition of ammonium aluminum sulfate to produce ultrafine a-Al2O3 powders [16,19–21], is a relatively simple and efficient method. Apart from the advantages of no serious sulfur dioxide environmental contamination and no self-dissolution during thermal
* Corresponding author. Tel.: +86 0731 88877863; +86 15947126682 (mobile); fax: +86 0731 88836207. E-mail addresses: [email protected] (Su-peng), [email protected] (Guo-xueyi), [email protected] (Ji-shujun). 1 Tel.: +86 0731 88877863; fax: +86 0731 88836207.
decomposition, gases generated from the thermal decomposition of AACH play an important role in reducing particle agglomeration. Nevertheless, reduction of the particle agglomeration and sintering temperature remains a big problem in the AACH decomposition process. During the sintering of AACH precursor, amorphous alumina initially is converted into metastable transition phases such as c-Al2O3, d-Al2O3, and h-Al2O3 [22], and is ultimately transformed into stable a-Al2O3 phase with temperature increasing. During this course, especially from h phase to a phase, crystal matrix rearrangement takes place, involving the shifting of large ionic radius O2, which requires high energy consumption, and the temperature is usually over 1200 °C [23]. At such a high temperature, solid sintering starts and hard agglomeration with vermicular morphology easily emerges, due to the necking between the particles, once a-Al2O3 phase has been formed [24,25]. Yang-ye et al. [26] have added a-Al2O3 seeds and NH4NO3 to the AACH precursor, and discovered that due to the decomposition of NH4NO3 at 320 °C, multiform gases are released, reducing the stacking density of the transition phases, and resulting in large amounts of highpowered defects and lattice deformations which were propitious to the reduction of a-phase transformation temperature and particle agglomerations, but the decomposition of NH4NO3 went along tempestuously, releasing environment-effecting NOx. Bai-shihe et al. [27] have studied the effect of NH4Cl on the a-phase transformation of ultrafine alumina, and discovered that NH4Cl addition to the AACH can restrain the agglomerations and can get the average grain size reduced, but having no notable reducing effect on phase transformation temperature, and the shortage of this method is the increase of dendritic crystal. Wu-yucheng et al. [28] have studied
0921-8831/$ - see front matter Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2009.07.004
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the effect of colloid TiO2 addition to the a-phase transformation of alumina and found that TiO2 addition can evidently reduce the phase transformation temperature, but meanwhile introducing more impurities. Other researchers [28,29] have tried the addition of mineralizers such as ZnF2 and AlF3 but having the same problem as TiO2. In this paper, self-made multicomponent catalyst (MC), containing mainly aluminum sulfate, ammonium nitrate, a-Al2O3 seeds, and a small amount of cerium dioxide and magnesium nitride, was used to prevent particle agglomeration and to reduce the phase transformation temperature of a-Al2O3 in the sintering process of AACH precursor. Ultrafine a-Al2O3 with good dispersibility and homogeneous size distribution was successfully synthesized. The decomposition and phase transformation processes are characterized and the mechanisms action of the MC are analysed and discussed. 2. Experimental 2.1. Preparation of MC Table 1 shows the composition of the MC with all the agents being analytically pure. The components were weighed respectively according to the mixture ratio given in Table 1 using an electronic balance, and then they were poured into a planetary ball milling machine with polyethylene inner liner. Using absolute ethanol as a dispersant and / 6–15 mm corundum balls as a milling medium, the mixture was ball milled for 2 h under the rev of 380 r/min and was heated in a vacuum drying oven at 80 °C for 4 h to remove the absolute ethanol.
2.3. Sintering processes Samples 1 and 2 were sintered in a muffle furnace in air atmosphere. The highest temperatures of sample 1 were 1000 °C, 1100 °C, 1150 °C and 1200 °C, respectively. For sample 2, the highest sintering temperatures were 800 °C, 900 °C and 1000 °C, respectively. For all the sintering systems, the heating rate was 5 °C/min, and the holding time at highest temperature was 2 h. 2.4. Characterization DSC–TGA analysis of sample 1 was carried out using a SDT Q600 Simultaneous DSC–TGA instrument (TA Corporation, USA) at a constant heating rate of 10 °C/min from room temperature to 1200 °C in air. The product of each sintering temperature system was identified by XRD analysis using a Japanese Rigaku D/max2550VB+18KW rotating target X-ray diffractometer. Microstructure analysis was carried out using a AIS2100 scanning electron microscope (SEM) and grain size analysis was performed for the digitized SEM photographs using image analysis software (Scion Corporation, USA). Specific surface area analysis for the sintered powder was carried out by BET method in nitrogen gas atmosphere with a Monosorb specific surface area analyser (Quantachrome Corporation, USA). Impurity element analysis of the sintered products was carried out by means of Inductively Coupled Plasma (ICP). 3. Results and discussion 3.1. Synthesis reaction for AACH When aluminum sulfate and ammonium carbonate are dissolved in water, hydrolysis reactions may occur as follows:
2.2. Preparation of precursors
3þ
2.2.1. Sample 1 Aluminum sulfate solution of 0.3 mol/l and ammonium carbonate solutions of 2 mol/l were, respectively, prepared using 0.1 mol A12(SO4)318H2O(AR) and 0.4 mol (NH4)2CO3(AR). Heated up to 60 °C in a water bath kettle, the aluminum sulfate solution was slowly added to the rapidly stirred (1400 r/min) ammonium carbonate solution. About 0.3–0.4 wt% OP-10 (alkylphenol ethoxylates) as a dispersant was simultaneously dropped into the mixture to prevent flocculation. While adjusting the pH value in the range of 11.0–11.6 using a strong solution of ammonia, the slurry gained was aged for 4 h. Then the suspension was filtered and washed repeatedly for three times with deionised water until no precipitate formed. Afterwards, the filter cake was mixed with normal-butanol and dehydrated by heterogeneous azeotropic distillation processing [30]. At last, the blending was filtered again and the filter cake gained was dried at 110 °C for 8 h in a vacuum drying oven. 2.2.2. Sample 2 Sample 2 was prepared by adding 5 wt% MC into the oven-dried sample 1. After co-mixed ball milling (the same process as for sample 1) for 30 min, the blending was dried at 110 °C for 8 h in a vacuum drying oven to get sample 2. Except for this step, all the processes fore-and-aft are identical with the counterparts of sample 1.
þ 3SO2 4 þ ðNH4 Þ2 CO3 ¼ 2NH4 þ CO3 CO2 3 þ H2 O ¼ HCO3 þ OH 3þ Al þ 4OH ¼ AlOðOHÞ2 þ Al2 ðSO4 Þ3 ¼ 2Al
ð1Þ ð2Þ ð3Þ H2 O
ð4Þ
According to Lin-yuanhua [20] and Shazo Kato [31]: when the con centrations of [NHþ 4 ], [AlOðOHÞ2 ] and [HCO3 ] reach a certain extent, pure NH4Al(OH)2CO3 deposits as follows:
NHþ4 þ AlOðOHÞ2 þ HCO3 ¼ NH4 AlðOHÞ2 CO3 # þOH
ð5Þ
And the overall reactions can be expressed as:
Al2 ðSO4 Þ3 þ 4ðNH4 Þ2 CO3 þ 2H2 O ¼ 2NH4 AlðOHÞ2 CO3 # þ 3ðNH4 Þ2 SO4 þ 2CO2 "
ð6Þ
Thus it can be concluded that the key to obtain pure AACH is maintaining the concentration of AlOðOHÞ 2 to a certain extent and simultaneously increasing the concentrations of NHþ 4 and as much as possible. With concentration of reactants and HCO 3 pH values altering, different products can be generated. According to Shazo Kato [31]: when the pH value is kept in range of 6.0–7.6, amorphous Al(OH)3 forms as follows:
Al2 ðSO4 Þ3 þ 3ðNH4 Þ2 CO3 þ H2 O ¼ 2AlOOH # þ 3ðNH4 Þ2 SO4 þ 3CO2 "
ð7Þ
Table 1 Compositions of the MC. Substances
Aluminum sulfate (A12(SO4)318H2O)
Ammonium nitrate (NH4NO3)
Cerium oxide (CeO2)
Magnesium nitride (Mg3N2)
White corundum powder (aAl2O3)
Content (wt%)
65
24
0.1
0.1
9.8
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Fig. 1. XRD pattern of precursor: (a) sample 1, and (b) sample 2.
And for the pH range of 7.6–8.5, the products can be Al(OH)3 and NH4Al(OH)2CO3, and for the pH value over 8.5, pure NH4Al(OH)2CO3 may deposit. In this study we chose the pH range 11.0–11.6, because in this pH range, according to the research of Yan-guojin et al. [32], one can get the best dispersing effect of slurry. 3.2. XRD and DSC–TGA analyses of precursor The XRD patterns of sample 1 and sample 2 before sintering are shown in Fig. 1. The position, intensity and the Bragg reflection semi-broadening for the diffraction peaks corresponded totally to the PDF standard card of AACH, indicating that the precursor obtained was ammonium aluminum carbonate hydroxide (AACH). Fig. 2 shows the DSC–TGA curves of sample 1, it can be seen that the TG curve descends sharply before 630 °C. The value of mass loss before 180 °C is about 0.57, and after 180 °C, the value is in range of 0.075–0.085, and the total mass loss is about 0.65, These are totally in agreement with the theoretical mass loss value of the AACH decomposition reactions as follows:
NH4 AlðOHÞ2 CO3 ¼ AlOOH þ CO2 " þNH3 " þH2 O "
ð8Þ
2AlOOH ¼ Al2 O3 ðamorphousÞ þ H2 O "
ð9Þ
From these two reactions, we can calculate that the theoretical mass loss values are 0.568 for reaction (8) and 0.065 for reaction (9) and that the total loss is 0.633. So the TG weight loss (0.65) is very close to the theoretical value of the complete decomposition of AACH, and the out profile may be due to the evaporation of the residual normal-butanol and water in the precursor. The aforementioned course altogether can testify that the precursor is AACH. When the temperature exceeds 630 °C, the TG curve is nearly horizontal, indicating that the mass loss stage of powder
has ended, and amorphous alumina generated from reaction (9) begins to transform into other transitional phases such as cAl2O3, h-Al2O3, and finally a-Al2O3. The DSC curve shown in Fig. 2 has one sharp endothermic peak at about 180 °C, and this is attributed to the thermodecomposition reaction of AACH, from which water gives away in form of vapor and absorbs a great deal of energy, as well as gases are released (CO2 and NH3), also absorbing part of the energy. Although a large amount of energy is released from the thermodecomposition reaction, the total is negative after counteracting the parts consumed, so the DSC curve presents a sharp endothermic peak. The two exothermic peaks at about 80–100 °C and 280 °C may be respectively associated with the decomposition of the OP dispersant, and the thermodecomposition reaction of AACH. While the inflexion at about 870 °C may be due to the phase transformation of alumina from amorphous Al2O3 to c-Al2O3.
3.3. Phase transformation of the samples The XRD patterns of samples 1 and 2 sintered at various temperatures are shown in Figs. 3 and 4, respectively. As shown in Fig. 3, after sintering to 1000 °C and holding for 2 h, the XRD patterns mainly illustrated are diffraction peaks of h-Al2O3 and a few faint peaks of c-Al2O3. After sintering to 1100 °C, the diffrac-
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•
20
•
30
40
•
50
° Fig. 2. DSC–TGA patterns of the precursor of sample 1.
Fig. 3. XRD patterns of sintered products of sample 1.
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The phase transformation temperature reduction can be contributed more to the addition of NH4NO3 to the MC. Comparing the XRD pattern of sample 1 (Fig. 3) with that of sample 2 (Fig. 4) it can be seen that few h-Al2O3 diffraction peaks have been formed for the latter of which the sintering temperature is much lower. That is to say, most of the c-Al2O3 converts into a-Al2O3 directly without undergoing the transition phase of h-Al2O3 at a much lower temperature. This fits into the study by Yang-ye et al. [26] and Gu-tangsheng [36] that thermodecomposition of NH4NO3 at about 320 °C enhances the adjustment of bonding length and bonding angle of non-crystalline alumina, resulting in high energetic defections among the transitional-phase Al2O3, hindering the phase transformation from c-Al2O3 to h-Al2O3, and thus promoting the nucleation of a-Al2O3. Besides the temperature reducing effect, the intensive emission of NOx and NH3 gases from the decomposition reaction of NH4NO3 can keep the precursor in a fluffy state, enlarging the distance between the particles, and reducing the powder agglomeration. The phase transformation promoting function is more notable for the MC component of aluminum sulfate ((A12(SO4)318H2O)), the decomposition of which can be shown as follows [37]: Al2 ðSO4 Þ3 18H2 O
87—250 C
!
Al2 ðSO4 Þ3 18H2 Oþ15H2 O " ðweight loss 40:5%Þ
ð10Þ
Fig. 4. XRD patterns of sintered products of sample 2.
Al2 ðSO4 Þ3 18H2 O
tion peaks of c-Al2O3 vanish and peaks of a-Al2O3 appear. As the sintering temperature reaches 1150 °C, the intensity of diffraction peaks of a-Al2O3 is evidently strengthened. Well after sintering to 1200 °C, all the diffraction peaks vanish except the peaks of aAl2O3, indicating that a-Al2O3 with complete crystallization is synthesized. Therefore, it can be concluded that the sequence of phase transformation of AACH in the sintering stage is AACH ? AlOOH ? amorphous Al2O3 ? c-Al2O3 ? h-Al2O3 ? a-Al2O3. As shown in Fig. 4, diffraction peaks of a-Al2O3 firstly occur after sintering to 800 °C and holding for 2 h, and the phase transformation of a-Al2O3 is completed after sintering to 1000 °C. These two temperatures are about 300 °C and 200 °C lower than the counterparts of sample 1, respectively. The reduction of phase transformation temperature in this test is mainly associated with the addition of MC, and mechanism can be described as follows: c-Al2O3 and h-Al2O3 have similar defective spinelle structure. As sintering temperature increases, the regularity of Al3+ in cubic crystalline matrix of O2 in Al2O3 is enhanced to a certain extent compared to that in c-Al2O3, so the phase transformation from c-Al2O3 to h-Al2O3 does not require too much energy and can be realized at a relatively low temperature. Well, according to Igor Levin and David Brandon [33], phase transformation of a-Al2O3 is related to the arrangement of O2 from fcc framework to hcp framework, which is a kind of reconstruction of crystal lattice and a nucleation growth process requiring large amounts of energy to overcome the barrier energy for nucleation, so in the case of no external interference, the phase transformation temperature is usually over 1200 °C. According to Bagwell [34] and Kumagai [35]: if there exist some a-Al2O3 particles as nucleation seeds, the activation energy for nucleation of a-Al2O3 can be decreased, while the effect of a-Al2O3 seeds upon reducing phase transformation temperature is not great. In this test, white corundum powder is one of the components of the MC, and after ball milling by corundum balls, the particle size of the corundum was reduced remarkably from about 15 lm to less than 100 nm. As inducible seeds, they increase the density of nucleation sites and reduce the energy barrier of phase transformation process, and to some extent lower the phase transformation temperature.
545
Al2 ðSO4 Þ3
250—414 C
770—1120 C
!
!
Al2 ðSO4 Þ3 þ3H2 O " ðweight loss 8:1%Þ
Al2 O3 ðamorphousÞþ3SO3 " ðweight loss 36%Þ
ð11Þ ð12Þ
It can be seen from reaction (10)–(12) that the decomposition reaction of aluminum sulfate is accompanied by the whole decomposition and sintering process of AACH. In the low temperature range under 414 °C, the reactant losses nearly half of its weight and large amount of water is released in vapor due to the dehydration of aluminum sulfate. This vapor, together with the vapor and gases released from AACH decomposition, makes the precursor inflate tempestuously, breaking the linking and aggregation among the particles. Even more importantly, the high temperature (770–1120 °C) decomposition of aluminum sulfate occurs almost simultaneously with the alumina phase transformation from c-Al2O3 to h-Al2O3, so the amorphous Al2O3, produced during the decomposition reaction, which has much more crystal lattice defects, acts as the activated nucleation center of a-Al2O3, leading to activation energy reduction of phase transformation as well as the reduction of phase transformation temperature. On the other hand, the SO2 released acts as a relaxant of the reactants and is more effective to hinder the powder agglomeration. As for the other two components in the MC: CeO2 and Mg3N2, their main effects lie in activating crystal lattice and grain refining. Ce, as we have known, has the atomic structure of 4f15d16s2, and when element Ce changes from metal to ion, the 4f orbit is still surrounded by the outer electron cloud 5s25p6, which can easily lose two 6s electrons and one 5d or 4f electron to form the electronic structure of 4fx5s25p6. This variable valence of CeO2 can induce the formation of fluorite type hypoxia phase with anion vacancy, such as CenO2n-2 (n = 4, 6, 7, 9, 10, 11). The decrease of oxygen content leads to lattice deformation and weak ion bond, thus altering the sensitivity of the structure [38], which makes it act as an activating center of nucleation. On the other hand, the radius of Ce4+ is much larger than that of Al3+, and it is hard to be embedded into the gaps of Al2O3 lattice to form sosoloid. So the CeO2 particles mainly gather on the grain boundaries, and due to their great volume, the grain boundary migration rate of alumina particles in the structure is reduced. This is conducive to inhibit grain growth to achieve the purpose of grain refinement.
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And for Mg3N2, when the temperature surpasses 800 °C, decomposition reaction takes place as follows: 800 C
Mg3 N2 þ ð1:5 þ 2xÞO2 ! 3MgO þ 2NOx "
ð13Þ
The amorphous MgO produced from above reaction can attach to the surface of the formed a-Al2O3 to form spinel (MgAl2O4) or MgO-Al2O3 sosoloid due to the similar ionic radius of Mg2+ and Al3+. The inter-diffusion reaction between Mg2+ and Al3+ is exother1 mic (DG900K ¼ 52:135kJ mol ) and the pinning effect caused by these MgAl2O4 or sosoloid keeps the interfacial energy much lower, thus reducing the diffusion rate between the a-Al2O3 particles, and hindering the necking and growth of the particles. Summarized from the above analysis, the addition of the MC may keep the powder in a comparatively dynamic-sintering state which has a great synergistic effect on the dynamics of phase transformation of alumina, by increasing the density of nucleation sites and reducing the energy barrier of nucleation [39], and thus accelerating the phase transformation and lowering the phase transformation temperature. 3.4. Morphology of sintered products Fig. 5 shows the SEM images of samples 1 and 2 sintered at 1200 °C and 1000 °C, respectively. As shown in Fig. 5(a), the leading crystal form for sample 1 is spindle like, accompanied with a slight amount of spherical particles. Simultaneously, vermicular morphology rigid agglomeration is formed due to the extensive neckings and reagglomeration of the secondary particles. Whereas, for particles of sample 2 shown in Fig. 5(b), the crystal form is a homogenously spherical structure. Compared with sample 1, which has an average particle size of about 300 nm and a broad particle size distribution, sample 2 has an average particle size of only about 80 nm and a homogeneous size distribution, and much less conspicuous neckings or agglomeration can be observed. The distinction of grain size and particle size distribution between the two samples can be contributed to the variation of sintering temperature. Rigid agglomerations are more likely to form during the sintering process of ultrafine a-Al2O3 powders, when the phase transformation temperature is near 1200 °C, a-Al2O3 grain grows rapidly once its nucleation occurs, resulting in neckings between neighboring particles and finally vermicular morphology rigid agglomeration. With the addition of MC, the sintering temperature is reduced from 1200 °C to 1000 °C, and the average grain size is reduced drastically due to the low grain growth velocity.
Fig. 6. The specific surface areas of the sintered products at different temperatures.
Table 2 Content of impurities (mass fraction, %). Sample Sample Sample Sample Sample
1–1 1–2 2–1 2–2
Na
Ca
Fe
Pb
Ce
Mg
0.0072 0.0073 0.0071 0.0072
0.0046 0.0045 0.0043 0.0044
0.0051 0.0051 0.0051 0.0052
0.0043 0.0044 0.0042 0.0043
– – 0.0043 0.0043
– – 0.0033 0.0032
sis. As shown in Fig. 6, the specific surface areas of samples 1 and 2 all decrease sharply with a rise in the sintering temperature. This is attributed to the intensive volume shrinkage (about 15%) when metastable c-Al2O3 and h-Al2O3 are transformed into stable a-Al2O3 at a high temperature. Well for sample 2, the specific surface areas at different sintering temperatures are much higher than those of sample 1, and when the phase transformation of a-Al2O3 is nearly complete at 1000 °C and 1200 °C for sample 2 and sample 1, respectively, the specific surface area of the former is almost 6.5 times higher than that of the latter. This is because the addition of MC not only accelerates the phase transformation (comparing Fig. 4 with Fig. 3), but also effectively hinders the particle agglomeration and leaves large amounts of pores inside the particles by releasing more gases during the high temperature decomposition reaction of its components.
3.5. Specific surface area of sintered products
3.6. Impurity element analysis of sintered products
Fig. 6 shows the specific surface area for sintered products of samples 1 and 2 at different sintering temperatures by BET analy-
Table 2 shows the results of the ICP analysis of the impurity elements of the sintered products. As shown in Table 2, contents of
Fig. 5. SEM images of sintered samples: (a) sample 1, 1200 °C/2 h; (b) sample 2, 1000 °C/2 h.
Su-peng et al. / Advanced Powder Technology 20 (2009) 542–547
impurities in the sintered products of sample 2 are nearly identical with the counterparts of sample 1 except for small quantities of Ce and Mg, indicating that the addition of MC has a faint effect on the impurity introduction that can be overlooked. And by content calculation from Table 2, the Al2O3 purity of sample2 reaches 99.95%. 4. Conclusions (1) Using low-cost aluminum sulfate and ammonium carbonate as raw materials, the precursor AACH was synthesized. (2) The phase transformation sequence of AACH precursor in the sintering process with no MC added is AACH ? AlOOH ? amorphous Al2O3 ? c-Al2O3 ? h-Al2O3 ? a-Al2O3. The a-Al2O3 particles obtained after sintering at 1200 °C have a broad size distribution with an average grain size of about 300 nm and a high density of neckings and vermicular morphology rigid agglomeration. (3) The addition of MC to the AACH precursor can effectively prevent agglomerations of precursor in sintering process as well as has potential synergistic effects on the reduction of the phase transformation temperature of a-Al2O3, by keeping the powder particles in a comparatively dynamic-sintering state, therefore the trends of necking and rigid agglomerations are remarkably decreased, so the dispersity of powder particles is significantly improved and the average particle size is about 80 nm. (4) BET and ICP analyses indicate that MC is fit for increasing the specific surface area of a-Al2O3 powder, and has a very tiny effect on the introduction of impurity to the sintered products.
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